Synthèse, Réactivité Électrochimique Et Structure Électronique De 1-R-Métallatranes (M=Si,Sn) Yu Wang

Synthèse, réactivité électrochimique et structure électronique de 1-R-métallatranes (M=Si,Sn) Yu Wang

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Yu Wang. Synthèse, réactivité électrochimique et structure électronique de 1-R-métallatranes (M=Si,Sn). Autre. Université Rennes 1, 2012. Français. NNT : 2012REN1S149. tel-00806250

HAL Id: tel-00806250 https://tel.archives-ouvertes.fr/tel-00806250 Submitted on 29 Mar 2013

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. N° d’ordre : ANNÉE 2012

THÈSE / UNIVERSITÉ DE RENNES 1 sous le sceau de l’Université Européenne de Bretagne

pour le grade de DOCTEUR DE L’UNIVERSITÉ DE RENNES 1 Mention : Chimie

Ecole doctorale : Science de la Matière (sdlm)

présentée par Yu WANG

Préparée à l’unité de recherche UMR CNRS 6226 ISCR Chimie et Photonique Moléculaire UFR Structure et Propriétés de la Matière

Thèse soutenue à Rennes Synthesis, le 30 Novembre 2012 electrochemical devant le jury composé de : Christa GROGGER reactivity and Professeur Université Technique de Graz, Autriche/ rapporteur electronic structure Mohamed MELLAH Maître de conférence-HDR of 1-R-metallatranes Université Paris sud / rapporteur Floris CHEVALLIER (M = Si, Sn) Maître de conférence-HDR Université Rennes1 / membre Viatcheslav JOUIKOV Professeur Université Rennes1 / directeur de thèse Acknowledgement

During these three years, I received plenty of help from many people. I know I can‟t mention all of them here but I still would like to express my sincere gratitude to very persons who supported me at all times or through any condition throughout my thesis. First and foremost, I am extremely grateful to my thesis supervisor Pr. Viatcheslav Jouikov, who not only chose me for this thesis but also gave me a lot of valuable guidance and consistent encouragement, which I received throughout the research work. Special thanks are due to Pr. Christa GROGGER and Dr. Mohamed MELLAH for accepting to be the reporters of my thesis and for the careful review and many suggestions that they provided on the original manuscript. I would like to thank Dr. Floris CHEVALLIER for being the member of my jury and providing many useful advices on my manuscript. I would like to offer special thanks to laboratory “Chimie et Photonique Moléculaires” UMR 6226 for providing me very good working conditions and help at all time. I would like to thank my boyfriend Mr. Jan Recho for being with me through all three years during my thesis and offering me encouragement and help anytime. I want to tell him „without you I couldn‟t have achieved it‟. Also very sincere thanks to my mum Mrs. Liu Ying who always supported my decision and accompanied me during the last three months of my thesis by taking very good care of my life. I am so lucky to have you as my family. I am even luckier to have my friends by my side with no conditions. First I would like to thank one of my best friends Lena Driehaus for helping me, jogging with me every day, laughing or crying with me and preparing an extraordinary „french-chinese pot‟ for me after my viva. I want to thank the couple Liu Ming and Bai Xiao for helping me to find all the solutions I needed to get used to the software LYX that I used for writing my thesis, as well as Yi Xiaohui for drawing the crystal figure for my thesis. Grateful thanks to my colleagues in the laboratory Aline Vanryckeghem, Nada Marquise, Aude Sourdon, Nicolas Richy and Jonathan Daniel for helping the little foreigner from China to improve my poor French with „nice‟ jokes and expressions. It is such a pleasure working with you as well as having coffee in the „fume club‟. Summary

Metallatranes (M = Si, Ge, Sn) have been studied intensively during 50 years and have many applications in various domains namely physics, biology, medicine and so on. However their electrochemical properties were poorly studied up to date. In the present work, new metallatranes have been synthesized and characterized using dierent approaches and techniques (organic synthesis, cyclic voltammetry, preparative electrolysis, EPR, spectroelectrochemistry, cyclic voltammetry coupled with EPR, NMR, GC- MS). Several novel structures of metallatranes have been obtained and validated by XRD. Electrochemical measurements show that the electrooxidation of metallatranes occurs via electrochemically reversible one- electron transfer resulting in the corresponding cation radicals (CRs), detected by cyclic voltammetry, UV-vis spectroscopy and real-time EPR spectroscopy. The spin delocalization in these paramagnetic species has been characterized and supported by DFT calculations. We focused on the electronic properties of metallatranes and on various bonding systems between pentacoordinated silicon and N atom including the substituents. We also considered the issue of non-coordinating, non-nucleophilic supporting salts for electrochemistry as they provide good ionic conductivity in the solvents of low polarity and allow carrying out voltammetry and EPR experiments on unstable electrogenerated electrophilic and radical species.

Résumé

Les métallatranes (M=Si, Ge, Sn) ont fait l'objet d'un intérêt important de la communauté scien- tique durant les 50 dernières années et ont des applications dans de nombreux domaines, de la physique à la biologie en passant par la médecine, etc. . . Cependant, leurs propriétés électrochimiques n'ont été étudiées que de façon très incomplète à ce jour. Dans le travail présenté ici, nous avons réalisé la synthèse de nouveaux métallatranes et les avons caractérisés, et ce par plusieurs approches et techniques diérentes (synthèse organique, voltammétrie cyclique, électrolyse préparative, EPR, spectroélectrochimie, voltam- métrie cyclique couplée à un EPR, RMN, GC-MS). Plusieurs nouveaux métallatranes ont été obtenus, et leurs structures validées par diraction RX. Les mesures électrochimiques montrent que l'électrooxidation des métallatranes a lieu via des transferts électroniques réversibles, et les cations-radicaux (CRs) qui résultent de leur oxydation ont été détectés par voltammétrie cyclique, spectroscopie UV-visible, et spectroscopie EPR en temps réel. La délocalisation de spin dans ces espèces paramagnétiques a été caractérisée et soutenue par des calculs de DFT. Nous avons concentré nos eorts sur les propriétés électroniques des métallatranes et la nature des diverses liaisons entre l'atome de silicium penta-coordonné des silatranes d'une part, l'atome d'azote de la structure atrane, et les atomes de diérents substituants d'autre part. Nous avons également étudié le problème posé par les sels du fond pour l'électrochimie, qui doivent être non-coordinants et non-nucléophiles, et doivent fournir une bonne conductivité ionique dans des solvants peu polaires. Ils permettent ainsi de mener des expériences de voltammétrie et de la RPE sur des espèces électrogénérées instables, notamment électrophiles ou des radicaux libres.

Key words

1. Silatranes 5. Cyclic voltammetry 2. Hypervalent silicon species 6. DFT calculations 3. Cation radicals 7. EPR spectroscopy 4. Electrochemistry 8. Non-cooridating electrolyte

1 Contents

1 Glossary 3

2 Introduction 5

3 Bibliography 9 3.1 Silatranes, their structure and speciﬁc intramolecular bonding and their electrooxidation ...... 9 3.1.1 Transannular bonds to Si ...... 11 3.1.2 Electrooxidation of silatranes ...... 13 3.2 Electrooxidation of germatranes ...... 16 3.3 Stannatranes ...... 21 3.4 Radical cations of metallatranes (theoretical considerations) ...... 23 3.5 Supporting electrolytes ...... 25

4 Experimental Section 27 4.1 Solvents, supporting electrolytes ...... 27 4.2 NMR spectroscopy ...... 27 4.3 Gas chromatography (GC-MS) ...... 27 4.4 X-ray crystallography (XRD) ...... 27 4.5 Electrochemical techniques ...... 27 4.5.1 Cyclic voltammetry and chronoamperometry ...... 28 4.5.2 Electrolyses ...... 28 4.5.3 Electron Paramagnetic Resonance (EPR) spectroscopy ...... 28 4.6 Syntheses of silatranes and related products ...... 29 4.6.1 Silatranes ...... 29 4.6.2 Bis-(silatranyl)alkanes ...... 33 4.6.3 Bi-silatranes ...... 35 4.7 Electrosynthesis of di-stannatrane ...... 41

5 Results and discussion 43 5.1 Simple 1-R-silatranes ...... 46 5.1.1 Introduction ...... 46 5.1.2 Structure ...... 47 5.1.3 Cyclic voltammetry ...... 50 5.1.4 EPR spectroscopy ...... 57 5.1.5 UV-vis spectroscopy ...... 61 5.1.6 DFT calculations ...... 62

1 5.2 Bis-(silatranyl)alkanes ...... 63 5.2.1 Cyclic voltammetry ...... 64 5.2.2 DFT calculations ...... 65 5.2.3 6 center-8 electron hypervalent bonding ...... 68 5.3 Bi-silatranes ...... 68 5.3.1 Structure ...... 69 5.3.2 DFT calculations ...... 73 5.3.3 Cyclic voltammetry of bi-silatranes and of the model tris(1,3-dihydroxy- 2-propyl)- amine ...... 74 5.4 1-R-Stannatranes ...... 78 5.4.1 Cyclic voltammetry ...... 78 5.4.2 EPR-spectroelectrochemistry ...... 81 5.4.3 DFT calculations ...... 82 5.5 Distannatrane from electroreduction of halostannatranes ...... 86 5.5.1 Cyclic voltammetry of electroreduction of halostannatranes . . . . . 86 5.5.2 Electroreduction of halostannatranes at transition metal cathodes . . . 90 5.5.3 Oxidation of distannatrane ...... 93 5.6 New non-coordinating ﬂuoride encapsulating silsesquioxane-based supporting electrolyte ...... 95

6 Conclusion 103

2 1 Glossary

The abbreviations commonly used in the present manuscript: 3c-4e: 3 centers, 4 electrons bond (5c-6e: 5 centers 6 electrons bond, 6c-8e, etc...) ACN: Acetonitrile Bu: Butyl C: concentration CCDB: Cambridge Crystallographic DataBase DDM: Dimethyl diazomalonate DFT: Density functionnal theory DMF: Dimethylformamide a Ep : Anodic peak potential c Ep: Cathodic peak potential EPR: Electron paramagnetic resonance F: Faraday’s constant (96 485 C.mol−1) Fc: Ferrocene G: Gauss (unit of magnetic ﬁeld strength) GC: Glassy carbon GC-MS: Gas Chromatography, coupled with Mass Spectrometry detector HOMO: Highest occupied molecular orbital

ip: Peak current J: NMR coupling constant between 2 nuclei K: equilibrium constant of a reaction LAH: Lithium aluminum hydride LUMO: Lowest unoccupied molecular orbital NBO: Natural Bonding Orbital method NMR: Nuclear magnetic resonance ppm: Part per million p-TsOH: p-toluenesulfonic acid R-: Any functional group R: Ideal gas constant (8,3145 J·mol−1·K−1) S: Entropy SCE: Saturated calomel electrode SN: Nucleophilic substitution

T8: Octahedral silsesquioxanes TBA: Tetrabutylammonium TBAP: Tetrabutylammonium perchlorate THF: Tetrahydrofurane

3 TMS: tetramethoxysilane v : Scan rate, in cyclic voltammetry experiments oC: Degree centrigrade α: Transfer coefﬁcient of an electrode reaction χ: electronegativity δ: Chemical shift (NMR, ppm)

INTRODUCTION 2 Introduction

Since 1960, when a new class of organosilicon compounds, triptych-siloxazolidines, was firstly synthesized by Frye et al. [1], metallatranes, -the name that has been coined by Voronkov,- have been studied intensively over 50 years not only because of their unique transannular interactions between nitrogen of the atrane cage and the metal (Si, Ge and Sn) but also because of many applications in medicine and biology they found [2, 3, 4]. However the electrochemical properties of metallatranes did not catch enough attention and have been poorly studied [5] up to date. Analysis of the literature on physical chemistry of metrallatranes prompts one to believe that they have big potential namely in the electrochemical aspect. First of all, in the cation radicals of this family, the hypervalent bond between N and the metal (Si, Ge and Sn) atom limits the spatial range of spin localization; protected by the three lateral branches of the atrane cage, the spin is trapped inside and is naturally isolated. Second, the atrane structure has axial symmetry and is in principle allowing the formation of long linear systems in this direction. Normal σ-conjugated polysilane chains are either chemically unstable or have poor electron conductivity because of poor overlap of silicon orbital. Bonding element of the extended metallatrane structures occupies the axial position and is intrinsically isolated. Now if an electron transfer can induce a positive charge to these linear systems, one might expect to obtain a new kind of conductive molecular wires (Figure 1) which are chemically stable, conformation- independent and allow electronic conjugation throughout their whole length. This would be a remarkable development compared to the normal σ-conjugated polysilane chains which are either not chemically stable or have poor electron conductivity because of poor overlap of silicon orbital. Thus we consider it is a completely different new concept for conducting chain molecules in which the electrons are able to be transferred along the chain. In addition, the cation radicals of metallatranes, at least those already studied, show another remarkable feature: the distance between N and Si varies depending on the charge on N atom. Using this axial flexibility of the atrane system, we can modulate the chain length and realize different electrochemical functions, for instance an electrochemically addressable molecular muscle. For the above reasons, we decided to focus the present work on the electrochemical behavior of metallatrane-based systems. To achieve this goal, the work presented in this manuscript was carried out starting from the synthesis of metallatranes and their structural characterization, to their electrochemical study and quantum chemical calculations. This manuscript is composed of four essential chapters. The first chapter provides bibliography and contains a historical overview of metallatranes, their structure and behavior as well as that of their cation radicals; some non-coordinating supporting electrolytes that have similar pentacoordinating structure of incorporated Si atoms were also considered.

5 Si Si Si Si Si Si Si Si Si Si Si Si n n

a b

O O O O O O O O

Si N Si Si N Si Si N Si n O O O O O O O O O O

Figure 1: Different types of silicon chain molecules. a. σ-conjugated molecular chain with good electronic conductivity but poor chemical stability; b. Silicon chain molecule with good chemical stability but poor molecular orbital overlap; c. New molecular chain with atrane structure as elementary unit, which is supposed to provide efﬁcient electron transfer and good chemical stability.

The second chapter concerns the experimental details, including chemical and electrochemical synthesis, physical and chemical characterization of the prepared compounds considered afterwards in other chapters of the manuscript. Two main chemical reactions as well as one electrochemical process were used for the synthesis of the metallatranes. NMR spectroscopy, GC-MS and XRD were used for the structural characterization. Cyclic voltammetry, chronoamperometry and EPR spectroscopy were used for analyzing electrochemical reactivity. The following chapter includes the obtained results and discussions; it is divided into five subsections according to the metallatranes considered and finally one last section concerning new supporting electrolytes. The manuscript begins with simple 1-R-silatranes with substituents providing different electronic effects to the reaction center of the electron transfer. Then, bis-(silatranyl)alkanes and bi-silatranes are considered. These molecules with the atrane moiety doubled in different ways represent the first step of constructing a silylated chain allowing long-distance conjugative interactions. This discussion is followed by 1-R-stannatranes in which the metal of the atrane is changed from Si to Sn. Finally, we will present new non- coordinating silsesquioxane-based supporting electrolytes for the large use in electrochemistry

6 and organometallic chemistry. The last chapter, based on the experiment, analysis and above discussion, gives a conclusion for the whole research on metallatranes. References and appendix are given at the end of the manuscript.

BIBLIOGRAPHY 3 Bibliography

3.1 Silatranes, their structure and speciﬁc intramolecular bonding and their electrooxidation

To a great extent, it is due to their unique structure (see Figure 2) that silatranes are so attractive systems for researchers since they were ﬁrst synthesized in 1960 by Frye et al [1]. Since then, silatranes have been very well studied in many aspects. Numerous silatranes were synthesized and characterized by different methods: molecular structure [6, 7, 8], dipole moments [9, 10, 11, 12], vibrational spectra [1, 9, 13, 14, 15], UV spectra [16, 17, 18], NMR spectra [9, 19, 20, 21], mass spectra [22, 23, 24, 25] and the results of other physical methods were reported. In addition, cleavage and retention of the silatrane ring were studied [26, 27] as well as other chemical reactions. A great deal of attention was paid to their biological activity.

First of all, the series of silatranes have common distinctive structure of the atrane fragment: three alkyl branches all connected with a nitrogen atom, on one end, and connected back together with a O3SiR group on the other. Their cage can also be seen as three heterocyclic rings sharing two transannular atoms, silicon and nitrogen. The interaction between Si and N atoms is fascinating because the lone electron pair of the nitrogen acts as an electron-donor to silicon atom and is usually not available for the interactions with external Lewis acids, like

BF3. Thus silatrane could be considered as a compound bearing a pentacoordinate Si.

O O O Si

Figure 2: Silatrane structure

A large number of reports highlight the existence of the intermolecular N→Si bonding by means of single crystal diffraction analysis [28, 29]. Table 1 is a part of the large available crystallographic data, showing that the distance between Si and N ranges from 2.1Å to 2.7Å. This distinctly differs from either the usual Si-N bond length (1.7-1.8Å) or the sum of van der Waals radii of N and Si atoms (∼ 3.5Å).

9 Table 1: Crystal data of selected silatranes [28, 29]

R Si-N Si-R Si-O Cl 2.02 2.15 1.65 CH3 2.17 1.87 1.67 C2H5 2.21 1.88 1.66 ClCH2 2.12 1.91 1.67 Cl(CH2)3 2.18 1.88 1.66 3-O2NC6H4 2.12 1.91 1.66 CH3O 2.22 1.67 1.66

N 2.088 1.909 1.667

N N 2.088 1.898 1.664

N 2.089 1.902 NR N N N 2.089 1.908 1.659

N 2.097 1.897 NR

N O O 2.102 1.910 NR

N 2.110 1.910 NR N N 2.118 1.663 1.913

Silatranes might exist in two main configurations of N atom: exo and endo. Without the valence molecular angles and bonds twisting, the interatomic N–Si distance of endo and exo structures is limited in a narrow range, 2.35-2.45 and 2.9-3.0 Å, respectively. According to the calculations, as the example of the 1-methylsilatrane molecule, the unstable exo form should transform naturely to the endo form which is relatively stable [7]. The endo form of silatranes is more stable due to whole set of factors related to configurational, dipole-dipole (electrostatic) and covalent interactions [29, 30]. For instance, from the molecular bonding orbital shown in Figure 3, it is obvious not only the atomic orbital of N and Si was necessarily involved but also the atomic orbital of oxygen participated into the formation of molecular bonding as well. According to the features of their frontier molecular orbital: highest occupied molecular orbital (HOMOs) in endo configuration is lower than exo configuration; and lowest unoccupied molecular orbital (LUMOs) is rather higher which also support the stability of endo configuration. Additionaly, the energy gap between HOMO and LUMO decreases in

10 going from structure endo to exo. All the information indicates that endo structure of silatrane is more stable than exo form [31]. The quantum-chemical calculations also support that the electronic density “transfer” from the nitrogen atom is about 0.25 eV for the endo form of silatranes. Thus, the peculiar features of the silatrane structure are associated with the N→Si interaction [29]. View Online

head-to-head pairs of crystallographically independent molecules of a and b modifications. The indole moieties of a and b molecules are virtually parallel to each other, with oppositely directed pyrrole rings.40–43 A comparison of crystal structure of 47 (Fig. 5) with 48 revealed an identical inductive effect of both heterocyclic substituents on the silatranyl moiety. Molecular packing specifies the presence of four independent molecules per asymmetric unit which are arranged in a head-to-tail fashion and planar benzotriazole rings run parallel to each other. These molecules are separated by van der Waals distances

Fig. 2 MO32 orbital of methylsilatrane showing contribution of N, Si and packed in such a fashion that the crystal structure and O atomic orbitals. 44–46 Figure 3: MO32 orbital of methylsilatrane showing contribution of N, Si and O atomicattains orbital maximum stability. In addition, the structures of [28]. 56 and 57 (Fig. 6) indicates a slightly distorted trigonal Sometimes solvation may cause disorder in crystal packing bipyramidal coordination polyhedron for the silicon atom.48

which makes the depiction of structure more difficult, e.g.,the Regardless of the length of the (CH2)n bridge between 3.1.1 Transannularstructure bonds of the to hexasilatrane Si clathrochelate Fe[(AtrS)2Gm]3(BPh)2 dimethylpyrazole and the silatranyl moiety, average values is not deduced in detail due to involvement of a large number of N–C, Si–O and C–O bond lengths in the coordina- The interaction betweenof parameters metal andsuch nitrogen as crystal atoms packing in metallatranes effects and disorder to this of day cannottion be polyhedron per- of the silicon atom are virtually the 35 fectly explained bysolvate a single CHCl theory.3 molecules. Good results are obtained by the use of severalsame. theories, The X-ray crystal structure of 44 highlights leveling of bond In the crystal structure of 58 (Fig. 7) the five-membered each theory modelisinglengthsa of different the pyrrole part ring of which the molecule. confirms increased The hybridization aromaticity theorysuccinimide claims that heterocycle attached to the silatrane molecule is 3 silicon in silatranesof is the pentacoordinated, ring due to coordination meaning valence with the orbital silatranylmethyl are hybridized asflatsp withd. LikeC2v symmetry and only one type of molecule is found in other moleculesgroup. with Whereas,sp3d hybridized in silatranes silicon, having the geometry indole (43) of and the carbazole Si in silatranesinis a a unit trigo- cell whereas the asymmetric unit of 59 (Fig. 8) (45), this leveling is observed in the benzene ring in spite of the contains two types of molecules (a and b) which differ by an nal bipyramid with the silicon atom located in the center, and three oxygen atoms, one nitrogen pyrrole ring, which shows enhanced aromaticity of benzene opposite turn of methylene bridges in the atrane fragment atom and one atomring from (Fig. the 3 substituent and 4). The connected order of dipole to Si located moment in observed the corners in of eachrelative trigonal to the pseudo-threefold symmetry axis. Bond lengths pyramid (Figure 4).silatranes having heterocyclic rings (1-pyrrolyl o 1-indolyl- o of corresponding Si–N bonds in a and b molecules are 9-carbazolyl 2-methyl-1-indolyl moiety) is attributed to virtually equal.50 On the other hand, electron-withdrawing Three oxygen atoms and theo substituent R form four σ bonds with Si whereas N and Si electronic interaction and steric effects of molecules. Crystal effects of the imidazole moiety shortens the Si–N bond distance form a dative bondpacking with showsthe lone eight electron molecules pair of of43 thein aN unit atom cell as forming a donor four element.in the Although crystal structures of 60 and 6152,53. this theory seems to explain the molecular geometry of silatrane well, it ignores the fact that the three heterocyclic rings in the silatrane structure will cause electron density to be different Downloaded by Bibliotheque de L’Universite Rennes I on 13 June 2012

in the dativePublished on 20 December 2010 http://pubs.rsc.org | doi:10.1039/B925899J N-Si bond and in the covalent bond to the substituent on the other side of silicon atom. Due to this change in electron density, the silicon atom is not on the plan of the three oxygen atoms so the geometry of silatrane is not a perfect trigonal bipyramid. The drawback of hybridization theory alone is that it presumes the involvement of d-orbital in the chemical bond. The 3d-orbital of the silicon atom contains comparatively high energy electrons, rather

Fig. 3 X-Ray crystal structures of 43* (a and b) (Reprinted with permission from ref. 41 (* this structure may be erroneous).

1810 Chem. Soc. Rev., 2011, 40, 1791–1840 This journal is c The Royal Society of Chemistry 2011 unﬁt to hybridize. So, rather, 4s- and 4p-orbital of silicon atom must be considered to explain the electron conﬁguration in the dative N\rightarrow Si bond. In this case, the orbital hybridization should not be sp3d anymore but turns into a sp3d4s hybridization (Figure 5).

O Si O O

Figure 4: Structure of a silatrane Si atom as predicted by hybridization theory

To circumvent these issues, attempts to explain silatrane geometry using the molecular orbital theory (MO) were undertaken. In this theory the silatrane is considered as a hypervalent molecule. This is a theory first defined by Jeremy I. Musher in 1969 [32]. It involves a 3-centers-4-electrons (3c-4e) bond, which is described as a combination of three molecular orbital: a half filled p-orbital from the central silicon atom, one fully filled orbital from the lone electron pair of the nitrogen atom, and one half filled valence orbital from the substituent. The combination of these electrons results in a filled bonding orbital, a filled non-bonding orbital (HOMO) and an empty anti-bonding orbital (LUMO). In this theory, the 3s and the remaining 2p orbital of the silicon atom hybridize into sp2 orbital and form three σ bonds with three oxygen atoms in the structure. As follows from the electrochemical studies and DFT modelling of metallatranes [33, 34], the N-M-X 3-centers system behaves as a single orbital with electron availability from both sides, which thus remains open to further conjugative interactions. Now, extrapolating this 3c-4e bond (Figure 5), we expect to apply the same theory to imagine a 5c-6e bond with 2 metallatrane structures enabling a more extended conjugated electron system. In this 5c-6e bond, the central and terminal positions should be occupied by more electronegative elements, with less electronegative elements at the other two positions. Farnham et al. called the 5c-6e bond an extended hypervalent σ bond [35], because it is constructed by the combination of the ∗ two hypervalent np ··· σ 3c-4e metallatrane-like interactions through the central np orbital. ∗ ∗ Such stablilized 5c-6e system can then be presented as σ ··· np ··· σ extended hypervalent bonding. For instance, in bis(silyl)hydride niobocene complex Cp2Nb(SiClMe2)2H bearing an electron-withdrawing chloride substituent at silicon [36], direct interligand interaction causes the rehybridization at the silicon atom: the silicon orbital taking part in the five-center

12 six-electron Cl ··· Si ··· H ··· Si ··· Cl bond develop more p character, and thus more s character is left for the bonding with other substituents, thus accounting for the shortening of the

Nb-Si bond and the lengthening of the Si-Cl bond in Cp2Nb(SiClMe2)2H. An approximate molecular orbital model which summarizes this 5c-6e idea developed by Nakanishi [37, 38] is shown in ﬁgure 6 below.

LUMO

anti-bonding ψ3

∗ ∗ σ σ HOMO p non-bonding ψ2 z

HOMO-1 σ σ bonding ψ1

R Si N

Figure 5: Molecular orbital diagram for 3c-4e bonding scheme

Cl Si H Si Cl ψ5

ψ4

E ψ3

ψ2

ψ1

Figure 6: Approximate molecular orbital model of the 5c-6e bond

3.1.2 Electrooxidation of silatranes

To obtain a better understanding of the transannular dative bonding interaction between the silicon and the bridged nitrogen atom, the silatranyl radical cation is bound to attract attention. The silatranyl radical cation is expected to be formed when a silatrane loses one electron which would induce the electron density changes in this system and speciﬁcally at the level of transannular interaction between Si and N.

13 203

(a)

1 I~A (b)

J J

1.0 1 2 1.4

−3 Figure 7: Cyclic voltammogramFig. 1. Cyclic of 10 voltammogramsM solution of of 10 silatrane -3 M solution with R of = compoundC2H5 in Ic ACN in ACN+TBAP + on platinum TBAP on platinum electrodeelectrode (scan (scan rate 200rate 200 mV/s) mV ats- 201) atoC different[5]. temperatures: (a) 200C; (b) - 10"C.

The detection of the radicalchemistry cation developed by cyclic voltammetry recently provides is very easygood to achieve.facilities In in their this regard, enabling 1993 paper, Broka et al.unstable report [5] transients the electrochemical produced oxidation electrochemically of silatranes to using be cyclicdetected and chemically irreversible waves to be converted to reversible waves [17]; therefore, it seemed to voltammetry and a rotating ring-disc electrode. A series of silatranes with different subsituents be worth applying this method in our study of silatranes. were studied by their methodCV of cyclicexperiments voltammetry. were Thecarried oxidation out peakfor the potentials compounds are in thel(b, c, h, j) at various range 1.42-1.85 V (vs. SCE).temperatures The process isfrom diffusion-controlled 20 to -30°C. andThe proceeds best results by one-electron were obtained at -10°C, transfer. The results showshowing that all silatranesreversible exhibit cyclic irreversible voltammograms electro-oxidation of electro-oxidation signals are in the case of silatranes undergoing the process partially reversibly at ordinary temperature (Fig. at scan rates below 100 mV/s. When the scan rate increases, the process of oxidation for l(b)). Further lowering of the temperature caused an increased difference between silatranes with R = CH3, Cthe2H 5anodic, CH2= CHand, Ccathodic6H5 and ppeaks-Cl-C 6(forH4 becomes instance, partially E a -E reversiblec = 100 mV at T-- -30°C). at the scan rate of 200 mV/sOne and roomcan suggest temperature that (Figurethis is 7).due This to suggestsstructural that changes cation radicals that accompany electron are created as the primarytransfer, products ofi.e. electro-oxidation from the endo-form and have in relativelythe neutral low stability.molecule This to the exo-form in the cation radical and vice versa. At lower temperature (T < -10°C), this process is process is coupled with chemical reactions with moderately low rate constants, which explains rather slow, therefore limiting the rate of the electrochemical reaction. the irreversibility at low scanQuantum rates: chemical calculations and X-ray structure analysis studies have revealed that the structure of neutral silatrane molecules corresponds to the endoconformation [18,19]. Owing to the donor-acceptor interaction, this form is more N stable than the exo-form. We suggest Nthat, on releasing an electron, the distance between the_ silicone and nitrogen atoms should be increased, resulting in the O Si transformationO from the endo- toO the Siexo-form.O O As mentioned above, the valuesO of n are higher in the presence of water in the R aprotic solvent. A gradual increase ofR n with the amount of water added to the solution was observed. Moreover, the process of oxidation under these conditions Schemewas 1: completely Formation ofirreversible radical cations for all of silatranesthe silatranes studied. This observation suggests the participation of H20 molecules in the electrochemical reactions. Obviously, The potentials of electro-oxidationwater interacts are affectedwith cation signiﬁcantly radicals by formed the electronic in the course effects ofof the electro-oxidation. This silane substituents (Table 2). Electron-donating substituents, by stabilizing the cation radical, facilitate electrooxidation, whereas electron-withdrawing subsituents shift the potentials of the

14 silatranes towards higher values.

Table 2: Electrochemical oxidation peak potentialsa of silatranes [5]

Substituents of silatrane (R) Ep(V ) H 1.70 CH3 1.43 C2H5 1.42 CH2Cl 1.85 OCH3 1.53 OC2H5 1.53 Cl not oxid CH2=CH 1.52 HC≡C 1.80 C6H5 1.55 p-C6H4Cl 1.60 2-furyl 1.45b 2-thienyl 1.60b 3-furyl 1.60b 3-thienyl 1.55b a Experiments were carried out in ACN + 0.1 M TBAP. Working electrode was glassy carbon. All potentials vs. SCE. b Voltammograms are ill-deﬁned.

O O

Si N

Figure 8: HOMO (atrane cage) and HOMO-1 (phenyl ring) orbital of silatranes (R = aryl)

Good correlations between the oxidation potentials and NMR data, as well as the pro- ∗ nounced effect of substituents (the value of ρπ = 0.4 V shows that the reaction site might be close to the substituent), could be regarded as an argument that the nitrogen atom is the probable reaction site [5]. However, when considering the impact of the electronic effects of the substituent on the stability of cation radicals, it should be carefully taken into account that the p-orbital of aryl substitutents have no effect whatsoever on the 3c-4e silatrane bond. Indeed, p-orbital are oriented perpendicularly to the components of this dative bond, and therefore cannot have any interaction with them. In other words, mesomeric effects are not the actors

15 213th ECS Meeting, Abstract #538, © The Electrochemical Society

ELECTROCHEMICAL REACTIVITY atrane was oxidized at a GC electrode in CH3CN AND ANODIC CYANATION OF BENZYL containing 0.1 M Bu4NCN as supporting salt and the GERMATRANES source of CN- anion. Controlled potential electrolyses (E = 0,8-1.2 V) were carried out until ca. 1.5-1.8 F/mol of Saida Soualmi1, Lubova Ignatovich2, Edmunds Lukevics2, electricity were passed to yield α-cyanated germatrane 1 3 in such systems and can be ignored.Ali Ourari In this, context, Viatcheslav an aryl substitutentJouikov would have a very and silatrane, respectively. CN similar effect as that213th of ECS an alkylMeeting,1 substitutent Abstract #538, (see © The Figure Electrochemical 8). Society N Laboratory of Electrochemistry, -2e -H+ RM(OCH2CH2)3N Molecular Engineering and Redox Catalysis, University CN- O O M of Setif, 19000 Setif, Algeria O 2 3.2 Electrooxidation Institute of germatranesofELECTROCHEMICAL Organic Synthesis, REACTIVITY Latvian Academy ofatrane was oxidized at a GC electrode in CH3CN R AND ANODIC CYANATION OF BENZYL containing 0.1 M Bu4NCN as supporting salt and the Sciences,GERMATRANES LV-1006 Riga, Latvia source of CN- anion. Controlled potential electrolyses (E M=Ge;R=m-BrC H CH - 3 6 4 2 Some previous work done UMR in our laboratory 6510 CPM, must University be mentioned of as Rennes it set the directionI, = 0,8-1.2 of my V) were carried out until ca. 1.5-1.8 F/mol of M=Si;R=H2C=CH- Saida Soualmi1, Lubova Ignatovich2, Edmunds Lukevics2, electricity were passed to yield α-cyanated germatrane work. Germanium derivativesCampus are another deAli Beaulieu, Ourari large1, Viatcheslav family 35042 of metallatranes.Jouikov Rennes,3 France They were and ﬁrst silatrane, syn- respectively. CN thesized in 1965 by Mehrotra and Chandra1 [39] and were widely studied in many aspects. N Laboratory of Electrochemistry, -2e -H+ 5.0 RM(OCH2CH2)3N The electrochemical reactivity of benzyl germatranes - - Similarly to the parent silatranes,Molecular the intramolecularEngineering and Redox coordination Catalysis, between University N and Ge atoms of CN O O 4.0M the germatranesrecently caught many synthesized attentions asof Setif,tricyclic the subject 19000 Setif,germanium of studies, Algeria by structural derivatives characterization, of O 2 Institute of Organic Synthesis, Latvian Academy of R atrane family showing high neurotropic activity [1] - was 3.0 Sciences, LV-1006 Riga, Latvia A NMR, mass-spectroscopy, UV, dipole moments measurements and other physico-chemical M=Ge;R=m-BrC6H4CH2- 3 M=Si;R=Hµ C=CH-2.0 , 2

UMR 6510 CPM, University of Rennes I, I properties determinationsstudied by as wellcyclic as synthesis voltammetry and chemical and supported reactivity of germatranes.by density Campus de Beaulieu, 35042 Rennes, France 1.0 functional B3LYP/6-311G calculations and large-scale 5.0 The electrochemical reactivity of benzyl germatranes - 0.0 electrolysis. 4.0 recently synthesized tricyclic germanium derivatives of atrane family showing high neurotropic activity [1] - was 3.0 -1.0 N A µ 2.0 , studied by cyclic voltammetry and supported1 R = Hby density I 0.0 0.5 1.0 1.5 functional B3LYP/6-311G calculations and large-scale 1.0 O 2 R = o-Br E, V vs. Ag/0.1 M AgNO electrolysis.O Ge 0.0 3 O 3 R = m-Br -1.0 N 1 R = H -1 4 R = p-Br 0.0Figure 0.5 1. 1.0 Cyclic 1.5 voltammogram of 1 (C = 0.8 mmol L ) O 2 R = o-Br E, V vs. Ag/0.1 M AgNO O Ge R 3 O 3 R = m-Br in CH3CN / 0.1M Bu4NPF6 at a 0.7 mm GC electrode. v = 4 R = p-Br Figure 1. Cyclic voltammogram-1 of 1 (C = 0.8 mmol L-1) R 32 V s ; T = 22 °C. in CH3CN / 0.1M Bu4NPF6 at a 0.7 mm GC electrode. v = -1 For all compounds of this reaction series, the 32 V s ; T = 22 °C. Figure 9: Germatranes 1-4. oxidation peak Forcurrents all compounds i are of linearthis reaction with series, concentration the Electrochemical behavior, the oxidation oxidation peak currentsp ip are linear with concentration Electrochemical behavior, the oxidation and, for vand, > 0.5-1for v > 0.5-1 V s V-1 ,s -1with, with thethe square square root of root the scan of the scanmechanism, the resultsmechanism, of electrolysis andthe DFT results B3LYP/6- of electrolysis and DFT B3LYP/6- -1 -1/2 311G modeling of germatranes 1-4 and their cation rate (i C-1ratev-1/2 (ip C= vconst), = const), suggesting suggesting that that the process the processis is 311G modeling of germatranes 1-4 and their cation A great deal of the reportedp under worksdiffusional confirm control; that the then value most is close important to unity practical for propertiesradicals show of that though turned inside the atrane cage -1 towards Ge and affected by stereoelectronic interactions under diffusionalv ≥ 0.2-0.5 V control; s . the n value is close to unity for radicals show that though turned inside the atrane cage germatranes are certainly those related-1 to their versatile and high biological activity [2,with 3, C-C 4]. Aand Ge-C σ-bond orbitals and with n-electrons v ≥ 0.2-0.5 V s Oxidation. potentials of 1-4 show a distinct towards Ge and affected by stereoelectronic interactions series of ring-substituted bromobenzyl germatranes were studied in our laboratory.of The O atoms, elec- it is the lone pair (n-electrons) of nitrogen dependence on the substitution at the aromatic ring of that brings main contributionwith C-C to the and HOMO Ge-C of these σ -bond orbitals and with n-electrons trooxidation of the germatranesbenzyl (1-4 and) are (Figure well described 9) and ofby thea two-parameter model compounds model: 6 and compounds.7 (Figure Oxidation potentials of 1-4 show a distinct of O atoms, it is the lone pair (n-electrons) of nitrogen 10) in CH CN and DMF solutions was studied [33].+ All four* germatranes studied show a 3 dependence on Ethep = 1.209 substitution + 0.11 σ + at 0.016 the σ aromatic ring of Contrary tothat what bringswas supposed main earlier, contribution endo- to the HOMO of these distinct oxidation peak (Figure 11). Peak currents ip for 1-4 are linear with the germatraneconformation of the cation radicals of germatranes is benzyl andwhere are σ +well is an electrophilic described Hammett’ by a two-parameterconstant and σ* is a model: compounds. 1/2 more favorable than exo: the geometry around Ge atom in concentration and, for v > 0.5-1 unified V/s, inductive with constant the square of the root substituent of the calculated scan rate (ip/v =these const species., is closer to trigonal bipyramid and the + * according to (∆χi is the difference of electronegativities N→Ge distance is shorter than in the neutral molecule. Figure 12), thus suggestingE diffusionalp = 1.209 control + 0.11 of the σ process.The + 0.016 electronσ stoichiometry for Contrary to what was supposed earlier, endo- between i-th atom and the reaction center; Ri is the i-th The comparison of these conformers by B3LYP/6-311G +• -1 the first step of oxidation inatom all covalent cases wasradius determined and ri its distance either from from the reaction direct comparisonhas shown of the that the endoconformation-form of 1 is ≅ 6.96of the kcal molcation radicals of germatranes is + * +• limiting currentswhere of the germatraneσ centeris an [2]): electrophilic and ferrocene, takingHammett’ no account constant of the differenceand σ more is in a stable their than exomore-1 . Harmonic favorable frequency than analysis exo : the geometry around Ge atom in provided the change of entropy, ∆S(1 → 1+•) ≅ -0.6 kcal 2 1/2 -1 -1 diffusion coefficientsunifiedD, orinductive combiningσ* =constant voltammetry7.84 Σ{∆χi (Rofi/r parameterithe) } substituent ip/v withcalculated the CottrellK mol slope showing thatthese neither species symmetry isnor closer number ofto trigonal bipyramid and the vibrations change upon oxidation. Thus somewhat slow obtained from chronoamperometryaccording to (∆χ ati theis the same difference electrode and of theelectronegativities same solution [40]. Beyond N→Ge distance is shorter than in the neutral molecule. As follows from the behavior of Ep plotted electron transfer kinetics of the process is mostly due to the lowest scan ratesbetween (v > 0iagainst.-th2 − atom0 lg(C).5 V/s) andand both lg(v), the methods bimolecular reaction provide self-reactions center; the n valuesRof i is the close i-th tothe 1. solvent reorganizationThe followingcomparison remarkable of chargethese conformers by B3LYP/6-311G electrogenerated cation radicals should be ruled out in redistribution in the cation radical and not to a flip- +• -1 atom covalent radius and ri its distance from the reaction favor of deprotonation, like in the case of model flopping of N betweenhas exo shown- and endo that- configurations. the endo -form of 1 is ≅ 6.96 kcal mol 16 +• center [2]):compounds Et3N and (HOCH2)3N. The deprotonation is more stable than exo-1 . Harmonic frequency analysis also in agreement with the behavior of ip at low scan rates Anodic α-cyanation of m-bromobenzyl +• and disappearance of the oxidation signal of 1 in the germatrane and vinylsilatraneprovided shows the that change this process of can entropy, ∆S(1 → 1 ) ≅ -0.6 kcal 2 -1 -1 σpresence* = 7.84 of CF 3ΣCOOH.{∆χi (RThei /rratei) of} deprotonation of the provide a convenientK toolmol for the functionalizationshowing that of neither symmetry nor number of cation radical of 1, determined by cyclic voltammetry, is atrane-type compounds. 3 -1 k = 3.98×10 s . Intramolecular N→Ge trans-annular vibrations change upon oxidation. Thus somewhat slow Ascoordinating follows interactions from thein benzyl behavior germatranes of shiftEp plottedtheir electronReferences transfer kinetics of the process is mostly due to against lg(C)Ep towards and more lg(v), anodic bimolecular potentials but the self-reactions oxidation of mechanism remains the same as for tertiary amines (EC [1] E. Lukevics, L. theIgnatovich, solvent T.Shul'ga, reorganization O. Mitchenko, following remarkable charge electrogeneratedprocess). cation radicals should be ruled out in S. Belyakov, J. Organomet.redistribution Chem. 2002, in 659 the, 165. cation radical and not to a flip- favor of deprotonation, like in the case of model [2] R.A. Cherkasov, V.I. Galkin, A.R. Cherkasov, Rus. Deprotonation of α-carbon in the cation Chem. Rev. 1996, 65,flopping 641. of N between exo- and endo- configurations. compoundsradicals Et3 ofN atranes and was(HOCH used to 2perform)3N. The the oxidative deprotonation is functionalization of sila- and germatranes by anodic also in agreementsubstitution. Typically,with the 1.5 behavior mmol of the correspondingof ip at low scan rates Anodic α-cyanation of m-bromobenzyl and disappearance of the oxidation signal of 1 in the germatrane and vinylsilatrane shows that this process can presence of CF3COOH. The rate of deprotonation of the provide a convenient tool for the functionalization of cation radical of 1, determined by cyclic voltammetry, is atrane-type compounds. 3 -1 k = 3.98×10 s . Intramolecular N→Ge trans-annular coordinating interactions in benzyl germatranes shift their References Ep towards more anodic potentials but the oxidation mechanism remains the same as for tertiary amines (EC [1] E. Lukevics, L. Ignatovich, T.Shul'ga, O. Mitchenko, process). Downloaded on 2012-08-07 to IP 2.25.247.227 address. Redistribution subject to ECS licenseS. or Belyakov, copyright; see www.esltbd.org J. Organomet. Chem. 2002, 659, 165. [2] R.A. Cherkasov, V.I. Galkin, A.R. Cherkasov, Rus. Deprotonation of α-carbon in the cation Chem. Rev. 1996, 65, 641. radicals of atranes was used to perform the oxidative functionalization of sila- and germatranes by anodic substitution. Typically, 1.5 mmol of the corresponding

Downloaded on 2012-08-07 to IP 2.25.247.227 address. Redistribution subject to ECS license or copyright; see www.esltbd.org S. Soualmi et al. / Journal of Organometallic Chemistry 693 (2008) 1346–1352 1347

13 the study undertaken in order to elucidate their redox (t, 4H, CH2–N), 2.43 (t, H, CH–CN, J = 6.9 Hz); C properties, the role of intramolecular N ? Ge coordinating (CDCl3): 139.65, 129.49 (vinyl), 119.52 (CN), 62.58 (C– interactions in their electrochemical reactivity and an O), 55.84, 52.65 (C–N); LC–MS (CI): 227 (M+H)+. attempt to use electrochemistry for the functionalization Product of cyanation of 3 (47% NMR yield). 1H NMR of these compounds. (CDCl3, TMS), d ppm: 7.73 (s, 1H, o-Br), 6.68, 6.90, 7.43 (3H, Ar), 3.67 (d, 2H, CH2–OCN, J = 11.4 Hz), 3.3–3.45 (4H, CH2–O), 2.66 (t, H, CH–CN, J = 11.4 Hz), 2.47 13 N (4H, CH2–N), 2.10 (s, 2H, CH2–Ge); COH (CDCl3): 136.52, 132,40, 132.01, 130.37, 125.47 (Ar), 114.53 (CN), O N O Ge O 58.76 (C–O), 55.92 (C–N), 48.95 (CH2–Ge). O O Si3. Results and discussionN R O N HO OH 1 R = H Anodic oxidation of benzyl germatranes 1–4 and of the 2 R = o-Br model compounds – vinylsilatrane (5), triethylamine (6) 3 R = m-Br 4 R = p-Br and triethanolamine (7) was studied in CH3CN and 56 7 DMF solutions. Upon oxidation in 0.1 M Bu4NPF6 solution in these solvents, all four germatranes show a distinct 2. Experimental peak (Fig.Figure 1) whose 10: The reproducibility model compounds depends5, 6 onand the7. compound and varies with the solvent and the electrode mate- 2.1. Instrumentation rial. In general, the voltammograms obtained at glassy carbon (GC) electrode are in most cases better than those Voltammetric and chronoamperometric experiments at Pt. Currents are better reproducible in DMF, probably were performed using a PAR 2273 and a EG&G Model because the basicity of germatranes is lower in this solvent 362 scanning potentiostats. Glassy carbon discs of 0.7 and the adsorptional interactions with the electrode are and 3 mm diameter or a 0.5 mm Pt disc were used as work- weaker. There is also a second oxidation signal at about ing electrode and, for this purpose, carefully polished Ep ffi 2 ...2.1 V (Fig. 1), observed only in CH3CN, which before each run. The counter electrode was a glassy carbon is much less reproducible and is usually seen as a whole In the publication, the authors also pointed out [33] that a reverse peak appears on the rod and the reference electrode was Ag/0.1 M AgNO3 in only in the first scan. This peak is tentatively attributed CH3CN, separated from the solution by anvoltammograms electrolytic to of the the non-substituted ensuing oxidation1 and, of theto a primary lesser extent, cation of radicals.p-Br benzylgermatrane (4) bridge filled with CH3CN/0.1 M Bu4NPF6. DMF and Peak currents ip for 1–4 are linear with the concentration at higher scan rates (Figure 13). BecauseÀ1 the oxidation peaks of both compounds are too large CH3CN containing 0.1 M Bu4NPF6 or Bu4NBF4 were used and, for v > 0.5–1 V s , with the square root of the scan (Ea − Ec = 118 and 110 mV1/2 for 1 and 4) to correspond to a perfectly reversible redox couple, as supporting electrolytes. IR-compensation facilityp ofp the rate (ip/v = const., Fig. 2), thus suggesting diffusional potentiostat was always used to account forthe ohmic process drops can rathercontrol be of characterized the process. as a quasi-reversible one. in the solution. To correct the experimental potentials, vol- The electron stoichiometry for the first step of oxidation tammograms of ferrocene were traced under similar condi- (Table 1) in all cases was determined either from direct 0 tions and EðFcþ=FcÞ ¼ 0:158 vs. SCE was used as the comparison of the limiting currents of the germatrane reference. Unless otherwise stated, the temperature was and ferrocene, taking no account the difference in their dif- controlled at 20 °C. fusion coefficients D, or combining voltammetry parameter 1/2 The electrolyses have been performed under the inert ip/v with the Cottrell slope obtained from chronoampe- atmosphere in a 15 mL coaxial three-electrode three-compartment electrochemical cell, with a 15 2 30 mm glassy carbon plate as working electrode.

2.2. Reagents and solutions

Syntheses of germatranes [9] and of vinylsilatrane [22] were described previously. Analytical grade CH3CN (sds) and DMF (Acros) were twice distilled under Ar atmosphere from CaH2 and BaO, respectively. Electrochemical grade tetrabutylammonium hexafluorophosphate, tetra- fluoroborate (Fluka) and cyanide (Aldrich) were dried in a desiccator over P2O5 and used without further purification. Product of cyanation of vinylsilatran, 5a (65% yield). 1H NMR (CDCl3, TMS), d ppm: 5.78–6.00 (3H, vinyl), 3.90 À1 Fig. 1. Cyclic voltammogram of 3 (C = 2 mmol L )inCH3CN/0.1 M (d, 2H, CH –O , J = 6.9 Hz), 3.83 (t, 4H, CH –O), 2.87 À1 2 CN Figure2 11: CyclicBu voltammogram4NPF6 at a 0.7 mm of glassy3 ( carbonC = 2 electrode. mmol/L)v =10Vs in CH3CN; T =22/0.1°C.M Bu4NPF6 at a 0.7 mm glassy carbon electrode. v = 10 V/s; T = 22 oC.

17 1348 S. Soualmi et al. / Journal of Organometallic Chemistry 693 (2008) 1346–1352

90 5.0

80 4.0 s -1 70 3.0 L V -1 A

60 μ 2.0 , I

A mol 5 μ 1.0 , 50

1/2 0.0 /Cv p 40 i

-1. 0 30

0 5 10 15 20 25 30 35 40 45 50 55 0.0 0.5 1.0 1.5 -1 v, V s E, V vs. Ag/0.1 M AgNO3

Fig. 2. Normalized oxidation peak currents i CÀ1vÀ1/2 of: (s)–1;( )–2; À1 p Fig. 3. Cyclic voltammogram of 1 (C = 0.8 mmol L )inCH3CN/0.1 M ($)–3;(h)–4;(j)–5. GC disc electrode, CH1/2CN/0.1 M Bu NBF . À1 Figure 12: Normalized oxidation peak currents ip/Cv 3 of: (◦)-1;(4 •)-42;(O)-Bu3;(4NPF)-6 at4;( a 0.7) mm GC electrode. v =32Vs ; T =22°C. -5. Voltammetry at GC disc electrode, in CH3CN/0.1 M Bu4NPF4. rometry at the same electrode and the same solution [24]. the dative intramolecular N ? Ge bonding, the surface of TriethylamineBeyond (6) and the triethanolamine lowest scan (rates7) as (v comparisonsP 0.2–0.5 V also sÀ1) were both studiedthe electrode by cyclic showed a remarkable trend to passivate, espe- voltammograms andmethods compared provide with the fourn value germatranes. close to 1. In 6 and 7, as is reflectedcially by the in CH trend3CN. On this reason, it was hard to cover a At higher scan rates, a reverse peak appears on the vol- large span of concentrations for the oxidation of 1–4 to in Ep (Table 3), the basicity of N is higher than in germatranes for the reason of N→Ge inter- À3 À1 tammograms of the non-substituted 1 and, to a lesser study their Ep À lg(C) behavior: at C > 2 10 mol L action in the latter,extent, thus theof p protonation-Br benzylgermatrane rate of 1-4 (4is)( slower.Fig. 3). Also, Though slightly the higherthe adsorption oxidation starts deforming the shape of the voltam- potentials of the germatranesoxidation peaks versus ofE bothof silatranes compounds [5] are indicate the narrowest larger involvement in mograms. of the lone However, a careful examination of Ep of 1 in p a c À4 6 6 the reaction series (Ep À Ep/2 in Table 1), the Ep À Ep differ- the interval of concentrations 5 10 C 2 pair of N in N→Geence relative (118 andto N→ 110Si mV dative for interaction;1 and 4, respectively) consequently, for this these site is10 lessÀ3 mol available LÀ1 with the increment of 2.5 10À4 mol LÀ1 1348 S. Soualmi et al. / Journalfor of oxidation Organometallic andpeaks protonation.Chemistry is somewhat 693 (2008) too 1346–1352 large to correspond to a perfectly does not show any visible dependence of Ep on C thus rul- reversible redox couple so the process can rather be charac- ing out bimolecular self-reactions of electrogenerated cat- 90 terized as a quasi-reversible one. ion radicals [26]. This is also in agreement with the slopes À1 Compared5.0 to silatranes [23], the compounds 1–4 show of linear Ep À lg(v) dependence for 1–4 at v < 10–20 V s 80 less reversible voltammograms: only small cathodic signal (Table 1). appears4.0 for 1 (Fig. 3)atv 30 V sÀ1, which does not grow At v < 0.1 V sÀ1, the normalized oxidation peak current s ffi -1 upon further increasing the scan rate. Instead, the DE / i /v1/2 decreases (Fig. 2) instead of expected increasing due 70 3.0 p p

L V Dlg(v) slope, that tends to 30 mV at lower scan rates, to the growing contribution of the spherical diﬀusional -1

A À1 60 increasesμ 2.0 at v ffi 30–250 V s as electron transfer kinetics flow. Using larger electrodes in order to respect the semi- , becomesI the limiting step. infinite diffusion conditions at low scan rates, the n value A mol 5 μ 1.0 v , 50 Though chemical grafting to the electrode during the was shown to tend to 0.5 at ? 0 thus corresponding to 1/2 oxidation of tertiary amines is usually not observed [25] the case of hidden limiting currents of third type (reaction 0.0 /Cv and the germatranes are supposedly even less prone to it of electrochemically produced species with the starting p 40 i because of the involvement of the lone pair of nitrogen into molecule [27]). This fact might reflect a slow protonation -1. 0 30

Table 1 0 5 10 15 20 25 30 35 40 45 50 55 0.0 0.5 1.0 1.5 Parameters of anodic oxidation of germatranes 1–4 and of the model compounds at a GC disc electrode v, V s-1 E, V vs. Ag/0.1 M AgNO a 3 o b c *d Cmpdyr Ep (V) Ep À Ep/2 (V) DEp/Dlg(v) (mV) nE(V) ks a IP (eV) r Fig. 2. Normalized oxidation peak currents i CÀ1vÀ1/2 of: (s)–1;( )–2; À1 p Fig. 3. CyclicCH voltammogram3CN DMF of 1 (C = CH 0.83CN mmol L DMF)inCH3 CHCN/0.13CN M DMF ($)–3;(h)–4;(j)–5. GC disc electrode, CH CN/0.1 M BuFigureNBF 13:. Cyclic voltammogram of 1 (C = 0.8 mmol/L)À in1 CH CN/0.1 M Bu NPF at a 0.7 3 4 4 Bu4NPF6 at a 0.7 mm GC electrode. v =32Vs ; T =223 °C. 4 6 1 1.209o 59 37 1.1 1.149 11.1 0.80 6.016 1.526 mm glassy carbon2 electrode.1.285v = 32 V/s; T = 22 101C. 45 1.0 1.237 4.1 0.57 6.238 1.994 3 1.276 109 39 1.0 1.225 2.0 0.60 6.225 1.703 rometry at the same electrode and the same solution [24]. the4 dative intramolecular1.236 1.364 N ? Ge 83 bonding, 74 the surface 33 of 34 0.9 1.202 4.4 0.74 6.105 1.660 The results of cyclic voltammetry combined with DFT B3LYP/6-311G calculations, have Beyond the lowest scan rates (v P 0.2–0.5 V sÀ1) both the5 electrode0.774 showed a 0.795 remarkable 169 trend to 124 passivate, 36 espe- 41 1.0 methods provide the n value close to 1. cially6 in CH0.582CN. On 0.888this reason, 193 it was 145 hard to 64 cover a 0.9 7 1.1263 18 497 98 0.9 At higher scan rates, a reverse peak appears on the vol- largea span of concentrationsÀ1 for the oxidation of 1–4 to Peak potential at v =1Vs vs. Ag/0.1 M AgNO3 in CH3ÀCN.3 À1 studyb their2 E À1lg(C) behavior: at C > 2 10 mol L tammograms of the non-substituted 1 and, to a lesser ks 10 in cmp À s . c extent, of p-Br benzylgermatrane (4)(Fig. 3). Though the the adsorptionTransfer coefficient starts estimated deforming as a mean the of a shapefrom DE ofp/D thelg(v) = voltam- 29.6/a and Ep À Ep/2 = 1.85 RT/aF [38]. d Group inductive constants calculated according to Eq. (1) [33]. oxidation peaks of both compounds are the narrowest in mograms. However, a careful examination of Ep of 1 in a c À4 6 6 the reaction series (Ep À Ep/2 in Table 1), the Ep À Ep differ- the interval of concentrations 5 10 C 2 ence (118 and 110 mV for 1 and 4, respectively) for these 10À3 mol LÀ1 with the increment of 2.5 10À4 mol LÀ1 peaks is somewhat too large to correspond to a perfectly does not show any visible dependence of Ep on C thus rul- reversible redox couple so the process can rather be charac- ing out bimolecular self-reactions of electrogenerated cat- terized as a quasi-reversible one. ion radicals [26]. This is also in agreement with the slopes À1 Compared to silatranes [23], the compounds 1–4 show of linear Ep À lg(v) dependence for 1–4 at v < 10–20 V s less reversible voltammograms: only small cathodic signal (Table 1). appears for 1 (Fig. 3)atv ffi 30 V sÀ1, which does not grow At v < 0.1 V sÀ1, the normalized oxidation peak current 1/2 upon further increasing the scan rate. Instead, the DEp/ ip/v decreases (Fig. 2) instead of expected increasing due Dlg(v) slope, that tends to 30 mV at lower scan rates, to the growing contribution of the spherical diffusional increases at v ffi 30–250 V sÀ1 as electron transfer kinetics flow. Using larger electrodes in order to respect the semi- becomes the limiting step. infinite diffusion conditions at low scan rates, the n value Though chemical grafting to the electrode during the was shown to tend to 0.5 at v ? 0 thus corresponding to oxidation of tertiary amines is usually not observed [25] the case of hidden limiting currents of third type (reaction and the germatranes are supposedly even less prone to it of electrochemically produced species with the starting because of the involvement of the lone pair of nitrogen into molecule [27]). This fact might reflect a slow protonation

Table 1 Parameters of anodic oxidation of germatranes 1–4 and of the model compounds at a GC disc electrode a o b c *d Cmpd Ep (V) Ep À Ep/2 (V) DEp/Dlg(v) (mV) nE(V) ks a IP (eV) r

90 5.0

80 4.0 s -1 70 3.0 L V -1 A

60 μ 2.0 , I

A mol 5 μ 1.0 , 50

1/2 0.0 /Cv p 40 i

-1. 0 30

0 5 10 15 20 25 30 35 40 45 50 55 0.0 0.5 1.0 1.5 -1 v, V s E, V vs. Ag/0.1 M AgNO3

rometry at the same electrode and the same solution [24]. the dative intramolecular N ? Ge bonding, the surface of Beyond the lowest scan rates (v P 0.2–0.5 V sÀ1) both the electrode showed a remarkable trend to passivate, espe- methods provide the n value close to 1. cially in CH3CN. On this reason, it was hard to cover a At higher scan rates, a reverse peak appears on the vol- large span of concentrations for the oxidation of 1–4 to À3 À1 tammograms of the non-substituted 1 and, to a lesser study their Ep À lg(C) behavior: at C > 2 10 mol L extent, of p-Br benzylgermatrane (4)(Fig. 3). Though the the adsorption starts deforming the shape of the voltam- shownoxidation that donor peaks activity of of both the nitrogen compounds atom is substantially are the narrowest reduced because in mograms. of the dative However, a careful examination of Ep of 1 in a c À4 6 6 N→Gethe bond reaction coordination series compared(Ep À Ep/2 to Etin 3TableN and (HOCH 1), the 2ECHp À2)E3Np.differ- the interval of concentrations 5 10 C 2 ence (118 and 110 mV for 1 and 4, respectively) for these 10À3 mol LÀ1 with the increment of 2.5 10À4 mol LÀ1 Further,peaks Soualmi is somewhatet al [34] too reported large electrochemical to correspond oxidation to a perfectlyof 1-substituteddoes germatranes not show any visible dependence of Ep on C thus rul- 1, 4,reversible8, 9, 10 and redox11 (Figure couple 14) sooccurring the process via an electrochemicallycan rather be charac- reversible electroning out trans- bimolecular self-reactions of electrogenerated cat- fer andterized resulting as a in quasi-reversible germatrane cation radicals one. that were detected by cyclic voltammetryion radicals and [26]. This is also in agreement with the slopes E v v À1 studied byCompared real time CW to EPR silatranes - spectroelectrochemistry.[23], the compounds 1–4 show of linear p À lg( ) dependence for 1–4 at < 10–20 V s less reversible voltammograms: only small cathodic signal (Table 1). appears for 1 (Fig. 3)atv ffi 30 V sÀ1, which does not grow At v < 0.1 V sÀ1, the normalized oxidation peak current The involvement of n(pz) electrons of the atrane nitrogen into intramolecular N→Ge dative upon further increasing the scan rate. Instead, the DE / i /v1/2 decreases (Fig. 2) instead of expected increasing due coordination substantially reduces the ease of electron withdrawal duringp the oxidationp of Dlg(v) slope, that tends to 30 mV at lower scan rates, to the growing contribution of the spherical diffusional germatranesincreases (by at aboutv ffi 500-60030–250 mV) V s comparedÀ1 as electron to parent transfer tertiary amines kinetics and by aboutflow. 70-100 Using larger electrodes in order to respect the semi- mV comparedbecomes to the silatranes limiting so far step. studied by cyclic voltammetry. infinite diffusion conditions at low scan rates, the n value Though chemical grafting to the electrode during the was shown to tend to 0.5 at v ? 0 thus corresponding to oxidation of tertiary amines is usually not observed [25] the case of hidden limiting currents of third type (reaction and the germatranes are supposedly even less prone to it of electrochemically produced species with the starting because of the involvement of the lone pair of nitrogen into molecule [27]). This fact might reflect a slow protonation

Table 3: Parameters of anodic oxidation of germatranes 1-4 and of the model compounds at a GC discTable electrode 1 Parameters of anodic oxidation of germatranes 1–4 and of the model compounds at a GC disc electrode a o b c *d Cmpd Ep (V) Ep À Ep/2 (V) DEp/Dlg(v) (mV) nE(V) ks a IP (eV) r

CH3CN DMF CH3CN DMF CH3CN DMF 1 1.209 59 37 1.1 1.149 11.1 0.80 6.016 1.526 2 1.285 101 45 1.0 1.237 4.1 0.57 6.238 1.994 3 1.276 109 39 1.0 1.225 2.0 0.60 6.225 1.703 4 1.236 1.364 83 74 33 34 0.9 1.202 4.4 0.74 6.105 1.660 5 0.774 0.795 169 124 36 41 1.0 6 0.582 0.888 193 145 64 0.9 7 1.126 497 98 0.9 a o À1 b c ∗d Peak potential at vn=1VsE (V)vs. Ag/0.1ks Mα AgNOIP (eV)3 in CHσ3CN. b 2 À1 ks 10 in cm s 1.1. 1.149 11.1 0.80 6.016 1.526 c Transfer coeﬃcient1.0 estimated 1.237 as a4.1 mean 0.57 of a from 6.238DEp/D 1.994lg(v) = 29.6/a and Ep À Ep/2 = 1.85 RT/aF [38]. d Group inductive1.0 constants 1.225 calculated 2.0 according 0.60 6.225 to Eq. (1) 1.703 [33]. 0.9 1.202 4.4 0.74 6.105 1.660 1.0 0.9 0.9 a Peak potential at v = 1 V/s vs. Ag/0.1 M AgNO3 in CH3CN. b 2 ks × 10 in cm/s. c Transfer coefﬁcient estimated as a mean of α from ∆Ep/∆lg(v) = 29.6/α and Ep − Ep/2 = 1.85RT/αF [41]. d ∗ P 2 Group inductive constants calculated according to σ = 7.84 {∆χi(Ri/ri) } [42].

19 N

O O Ge O

R = o-F-C6H4- (8), o-Br-C6H4- (9), p-Et2N-C6H4- (10), p-Me2N-C6H4- (11)

Figure 14: 1-Substituted germatranes 8-11.

The values of nitrogen hfc constants in their EPR spectra are aN = 18 − 19 G, this result shows a remarkable feature of aryl germatranes in the form of cation radicals that the nitrogen atom of atrane is practically planar. This is very typical for planar N-centered radicals. There is a difference of about 40 times of axial and equatorial α-methylene protons in the cation radicals in their response to the magnetic ﬁeld. Neither is any contribution from Ge nor from the aromatic ring seen in the spin delocalization. While the N atom remains ﬂat, the Ge atom S. Soualmi, L. Ignatovich and V. Jouikov in the cation radical of aryl germatranes is no more on but out of the Ge(−O)3 plane.

Figure 2. EPR spectrum of the cation radical of o-bromo-phenyl germa- +• −1 trane, 4 , electrogenerated in CH3CN–0.1 mol L Bu4NPF6 at E = 0.92 V; modulation amplitude a = 0.4G;T= 253 K.

Another remarkable feature of 4+• is that the contribution of the aromatic moiety into spin delocalization is not seen in this 33503370 3390 CR, or if there is any, the hfc constants from the aromatic protons H, Gauss +• are below the resolution of the spectrum. This means that N pz Figure 3. EPR spectrum of the cation radical of benzyl germatrane 1 generated in CH CN–0.1 mol L−1 Bu NPF at E = 0.85 V: (a) experimental orbital (the lone pair of nitrogen), providingFigure main electron 15: EPR density spectrum of the cation3 radical of4 benzyl6 germatrane generated in CH3CN–0.1 to the HOMO of germatranes, and hence losing one electron and (b) simulated using the parameters in the text. Modulation amplitude Bu NPF E a = 0.4G;T = 233a K. b upon oxidation, is still axially oriented insidemol/L the silatrane4 cage6 at = 0.85 V: ( ) experimental and ( ) simulated using the parameters in the when becoming SOMO – the orbital carryingtext. unpaired Modulation electron. amplitude a = 0.4 G; T = 233 K [34]. The conjugation of this orbital with the lateral fragments (α- parameters. The spectrum is centered at g = 2.0035, indicating [28] and β-CH2 groups), especially with the spatially nearest orbitals that it is not a Ge-localized radical. Nitrogen hfc constant of three axial σ(C–H) bonds, stabilizes the CR.For Thus the the cation space radicalsis rather of benzyl small now, germatranes,a(N) = 4.337 weG only, found but quite in contrast a different to spin distribution +• +• occupied by unpaired electron is hiddenas inside shown the atranefollowed: cage thethe aromatic CRs 3 systemand 4 , altogether a large contribution with an“electron from the benzyl bridge” σ(C–Ge) orbital and protected from intermolecular radical interactions by lateral fragment in spin-orbital interactions is seen: a(p-H) = 8.426 G, aliphatic branches. Besides this, there mustaccommodate be another effectthe major2 × parta(o-H) of= the2.907 spin G, 2 density× a(m-H) while= 0.972 only G for a smallaromatic part ring of it is residing on N providing an additional stabilization to this species because 4+• protons and 2 × a(H) = 0.31 G for the protons of benzylic has a substantially longer lifetime than the CR of parent Et3N methylene group. This feature outlines a quite different spin (both CRs undergo first-order deprotonation[18,26] and have similar delocalization, mostly on the aromatic20 ring with only very little of +• carbon environment around N). Indeed, in Et3N free rotation the unpaired electron density remaining on the atrane nitrogen. about N–C and C–C bonds is possible and therefore the same spin The benzylic methylene group in this system plays the key role +• +• stabilization is enabled as in 4 [EPR spectrum of Et3N shows of an electronic bridge. Its σ(Csp3-Ge) orbital, affected by three- different pattern: a(N) = 20.8 G and 6 × a(α-H) = 19 G[29]], but this center N–Ge–C bonding and whose orientation is favorable for CR is not detectable by EPR under these conditions. conjugation with the π pz electrons of the aromatic system, allows A very similar spectrum was observed for the CR of o-F-phenyl thespindensitytobetransmittedfromNatom(wherefreeelectron germatrane 3+•, electrogenerated at E = 1.22 V under the same appears upon withdrawal of one electron from the doublet of N) to conditions. Its g-factor is slightly smaller, g = 2.0042, probably the phenyl group so as to form the delocalized thermodynamically because of the electron-acceptor effect of F in the aromatic ring. more stable spin configuration. In 1-aryl substituted germatranes The general pattern (nine rays) is similar to and the spectral width 3 and 4 such a ‘bridge’ is absent since σ(Csp2-Ge) orbital is is also practically the same as for 4+•, = 160 G. Nitrogen and orthogonal to the orbitals of aromatic π-system which therefore proton hfc constants are quasi-identical in both spectra, therefore cannot accommodate spin density. showing a similar feature of interactions of the unpaired electron This mechanism involves non-negligible reorganization energy +• +• with α-andβ-CH2 protons of the atrane cages in 3 and 4 . (both internal/structural and external/solvent related) accompa- Similarly, no visible contribution of the aromatic ring to the spin nying the formation of this CR from the neutral molecule; for this delocalization is seen for 3+•. reason, electron transfer during the oxidation of benzyl germa- Less stable than the CRs of aryl germatranes, the cation radical tranes has rather quasi-reversible character, in good agreement of benzyl germatrane 1+• needed slightly lower temperature for with the results of cyclic voltammetry. its stabilization and detection by EPR spectroscopy (Fig. 3). The The EPR spectrum of the cation radical 5+•, electrogenerated at first remarkable feature of the EPR spectrum of electrogenerated E = 0.35 V in the CH3CN/0.1 M Bu4NPF6 solution, is quite different CR of this germatrane is that it is substantially narrower than from those observed for the CRs of aryl germatranes 3 and 4. those of o-Br- and o-F-phenyl germatranes (end-to-end width is For the CR of 1-[4-(dialkylamino)phenyl]-germatrane, the isotropic ≈ 27–28 G only) and it does not show large hfc constants g-factor of 2.003 is still close to those of the CRs of 1–4;also, characteristic for the aryl substituted CRs. The spectrum is a the characteristic pattern from one N atom is present. However little noisy but its Fourier transform filtering and reconstruction in contrast to the germatranes of similar structures 3 and 4,the using Bruker SimFonia software allowed extraction of the essential EPR spectral signature of 5+• also shows spin coupling with the

wileyonlinelibrary.com/journal/aoc Copyright c 2010 John Wiley & Sons, Ltd. Appl. Organometal. Chem. (2010) atom. Neither EPR spectroscopy (Figure 15) result nor DFT calculations congregate in that the atrane nitrogen in the cation radical adopts an exo-configuration. On the other hand, a remarkable molecular motion around Ge atom occurs which provides it much flatter and more similar to trigonal bipyramid. This is on the contrary to what was found for the cation radicals of aryl germatranes. For that we could summary that the main driving force determining the geometry of these species is that the 3c-4e bond N–Ge–Csp2 upon oxidation and favored metal pentacoordination in the cation radicals of germatranes because of the importance of the interactions of Ge with its environment and with all three O atoms. A distonic cation radical could be formed when nitrogen atom of atrane part 3c-4e bond (Ge–C fragment) has no possibility for lateral overlapping by its conjugative preferences and remains the unpaired electron. Because the conjugation is cut off by orthogonal orientation of the susceptible to conjugative interactions orbital of the substituent. If this N–(Ge–C) center is available for further conjugation, spin delocalization will find a new configuration which is more stable and spin density will shift to the aromatic system and only very small amount of it still seeing the atrane N atom. Thus the formation of distonic cation radicals observed for aryl germatranes might reflect a typical feature for the germatranes substituted with the groups ”ending” electron transmission by the absence of further conjugation: aliphatic, vinyl and ethynyl substituents. If so, electrochemistry will provide a new insight into the electronic interactions in these compounds and open possibilities for designing molecular electronic de- vices with specific electronic, electromechanic and optical properties [34].

3.3 Stannatranes

In contrast to silatranes or germatranes, stannatranes have attracted much less attention than their lighter congeners in the family of metallatranes. Compared to Si or Ge atoms, Sn has evident strong metal properties and much larger atomic diameter. However, the presence of the transannular bond beween N and Sn in stannatranes was well established; moreover, the bond strength even slightly increases in the order Si < Ge < Sn [43].

toluene Sn(O-t-Bu)4 + N(CH2CMe2OH)3 O Sn O -3 t-BuOH O

t-BuO

Scheme 2: Synthesis of t-butoxystannatrane

Whithin the cooperation with professor K. Jurkschat and his team at the department of inorganic chemistry in Technical University of Dortmund, a series of the stannatrane deriva-

21 tives of the type N(CH2CMe2O)3SnX have been synthesized and studied by XRD and cyclic voltammetry. Notably in this series of stannatranes, there is no tin-carbon bond as the atom connecting tin atom with the substituent is either 15 group element (O or S) or halogen. This feature is responsible for the nontoxicity of these compounds. Inorganic stannatranes of this type that lack any tin−carbon bond are scarce [44, 45, 46, 47] so it is interesting to investigate their structure and electrochemical properties.

The starting precursor of stannatrane was tin tetra-tertbutoxide, Sn(O−tBu)4, which, reacting with tris(2-hydroxy-2-methyl-propyl)-amine, N(CH2CMe2OH)3, gave the product t- butoxystannatrane as the starting material for the synthesis of a variety of inorganic stannatranes (Scheme 2).

N R O Sn O toluene O O Sn O + X (a) O - t-BuOH

t-BuO XH 12, X = O, R = t-Bu 13, X = S, R = Me R

N N

toluene O Sn O + RX O Sn O (b) O - RO-t-Bu O X t-BuO 14, X = Cl, R = CH3C(O) 15, X = I, R = Me3Si 16, X = Br, R = Me3Si

Scheme 3: Synthesis of stannatranes 12-16.

The t-butoxystannatrane is highly sensitive to moisture, well soluble in dichloromethane, tetrahydrofurane, and diethylether and show moderate solubility in toluene and benzene. Thus, in an acid−base type reaction the treatment of the t-butoxystannatrane with p-tBu phenol, 4- methyl thiophenol gave the stannatranes 12 and 13, respectively, in high yields (Scheme 3 (a)). The synthesis of the halogenido-substituted stannatranes 14-16 was achieved by the reaction of t-butoxystannatrane with acetyl chloride, CH3C(O)Cl, and trimethylhalogenido silanes,

Me3SiX (X = Br, I), respectively (Scheme 3 (b)). The ﬁve stannatranes are colorless or yellowish crystalline materials. Notably, on expo-

22 sure to air and light the color of the iodido-substituted stannatrane 15 changed to yellow. All compounds are soluble in benzene, toluene, dichloromethane and tetrahydrofurane, but 5698 I.S Ignatyev et al. / Journal of Organometallic Chemistry 692 (2007) 5697–5700 at room temperature the chlorido and bromido derivatives 14 and 16 are only well soluble in * Table 1 the nitrogen atom into the antibondingdichloromethaner M–X orbital or chloroform. [16]. As a result the N–M bond strength depends on the Geometry parameters of the silatrane and germatrane cage of FM(O– electronegativity of the axial substituent X [13]. CH2–CH2)3N (M = Si, Ge) and corresponding cations, obtained at the B3LYP/cc-pVDZ level (bond lengths in A˚ , bond angles in ) However, recently it was assumed that the Coulomb interaction of the oppositely charged germanium3.4 Radical and nitro- cationsParameters of metallatranes M = Si (theoretical M = Ge considerations) gen atoms gives the major contribution to N–Ge bonding rg exp [20] F-atrane Cation F-atrane Cation in germatranes [19]. Metallatranyl cationsM–N are of peculiar 2.318 interest 2.305 because 1.897 they help 2.329 to better 1.998 understand the molec- N–C 1.479 1.472 1.515 1.474 1.510 To discuss the nature of the transannularular structures bond in atr- and hypervalent bonding in the molecules of metallatranes. Belyakov et al. in anes it is quite important to address the structure of the O–C 1.390 1.409 1.452 1.412 1.453 M–O 1.651 1.694 1.652 1.813 1.775 corresponding cations. Atranyl cations were1999 not [48] character- pointed outC–C that the attractive 1.512 interaction 1.534 N-M(X)1.544 1.535 in atranes 1.543XM(Y–CH2–CH2)3N ized experimentally, although the quantumoriginates chemical from pre- the donationN–M–O of the 81.3 lone pair 81.7 of the 96.5nitrogen 81.6 atom into 93.5 the antibonding σ∗(M– diction of the structure of silatranyl and germatranyl M–O–C 123.7 121.5 109.5 116.8 107.2 cations was recently published [21–23].X) Here,orbital. we discuss This claimM–N–C would imply 103.2 that the 102.7 N–M bond 102.8 strength 102.8 to depend 102.5 on the electronega- the electronic structure of the germatranyl cation. By com- C–C–O 117.0 109.6 108.3 110.5 110.0 tivity of the axial substituentC–C–N X104.5 [30]. However, 107.3 there 107.6 is another 108.4 approach, 108.8 which states that the paring its structure with those of halogermatranes we try to C–N–C 115.3 115.3 115.2 115.2 115.5 analyze the changes in the transannularCoulomb N–Ge bond interaction on O–M–O of the oppositely 117.8 charged 117.9 germanium 118.7 117.9and nitrogen 119.6 atoms gives the major going from cation to a neutral molecule.contribution It should be noted to N–Geq M bonding in germatranes [49]. Hence 2.24 ﬁguring 2.24 out the structure of the however that our description refers to isolated molecules, qN À0.63 À0.69 in condensed phases, where M–N bondcorresponding lengths are sub- cationsqO would help to have a further knowledgeÀ0.88 aboutÀ the0.84 interaction of M–X. stantially shorter, the picture of interactions may diﬀer.

1.775 2. Computational details 1.652 O O O O Ge 1.452 Si 1.453 O O C C Theoretical calculations were performed using GAUSS- C 1.897 C 1.998 IAN 03 [24] on the Sun Fire 25 K computers at the Finnish 1.544 1.542 C IT centre for science (CSC). Becke’s three-parameter C N C C N C 1.515 1.510 exchange functional (B3) [25] was used jointly with the C C Lee, Yang and Parr’s correlation functional (LYP) [26]. The Dunning’s-cc-pVDZ correlation-consistent basis set [27,28] was employed. All stationary points were characterized by vibrational frequency calculations in the theoretical C method used for optimization. NBO method [29] was used C 1.599 O 1.723 as implemented in Gaussian codes. OO O Si C Ge C 3. Results and discussion O O C C Since the main goal of this study was to analyze changes in the strength and bond length of transanular X...N interaction on going from cations to F-atranes, the correct Fig. 1. Equilibrium structures obtained at the B3LYP/cc-pVDZ level of Figure 16: Equilibriumtheory structures of silatranyl obtained and germatranyl by DFT cations calculations and corresponding at the classical B3LYP/cc-p-VDZ level reproduction of the experimental X...N interatomic dis- cations. tance (obtained in the gas phase only forof F-silatrane theory of silatranyl[20]) and germatranyl cations and corresponding classical cations [50]. is of the main importance. Practically in all previous theoretical studies of silatranes and germatranes [16–23]SundiusPople’set al. in 2007The cation [50] carried M becomes out a tetracoordinated series of calculations and M–N on the bond equilibrium structures split valence basis sets were employed. Our predicted value formally becomes one of the sp3 hybrid bonds. This inver- (Table 1) of the Si...N interatomic distanceof silatranyl in F-silatrane and germatranylsion of the cations configuration as well at as M of demonstratescorresponding itself fluoroatranes in the using DFT cal- obtained with the correlation-consistentculations double-zeta at B3LYP/cc-p-VDZ zeta increase of the level N–M–O of theory, bond and angle analyzing which becomes ongoing larger changes in the bonding gives a better agreement with the experimental value [20] than 90. Note, however, that this effect is less pronounced than even that achieved with the largest offrom the germatranylPople’s fam- cationin germatranyl to a neutral cation. molecule M–O bonds by using become the stronger NBO method. due to In the publication ily basis set used, i.e. the 6-311++G** setof[23] this. group,Moreover, the electronicthe disappearance structure of of the the fluorine germatranyl lone pair cation contribution is discussed. to By comparing our experience evidences that the use of cc-pVnZ sets pro- the antibonding M–O orbitals. vides the convergence to experimental valuesits structure even for DFT with thoseTo of understand halogermatranes the differences they try in the to analyze structure the of atranyl changes in the transannu- methods [30]. cations with ‘‘classical’’ cations it is useful to compare them lar N–Ge bond induced by the evolution from neutral+ molecule to+ a cation [50]. From the The main equilibrium geometry parameters of F-silatra- with the structures of (CH3O)3Si and (CH3O)3Ge cat- ne, silatranyl cation, F-germatrane, and germatranyl cation ions (Fig. 1). The main difference is the substantially are presented in Table 1 and Fig. 1. For both atranes with lengthened M–O bonds in atrane23 cations. M = Si and M = Ge the loss of fluorine leads to a substan- In ‘‘classical’’ cations the shortening of M–O inter- tial shortening of M–N and M–O interatomic distances. atomic distances may be explained by the charge transfer Electrochemical Reactivity and Anodic Cyanation of Germatranes

S. Soualmia, L. Ignatovichb, E. Lukevicsb, A. Ouraria, V. Jouikovc1 a Laboratory of Electrochemistry, Molecular Engineering and Redox Catalysis, University of Setif, 19000 Setif, Algeria b Institute of Organic Synthesis, Latvian Academy of Sciences, LV-1006 Riga, Latvia c UMR 6510 CPM, University of Rennes I, 35042 Rennes, France

The electrooxidation of germatranes with different R groups at Ge was studied in CHtheoretical3CN and calculations,DMF solutions. it is shown The results that loss of of cyclic fluorine substituent shortens the interatomic voltammetry supporteddistances by of M–NDFT and B3LYP/6-311G M–O (Figure 16). calculations The cation M becomes tetracoordinated and the M– suggest that donor activity of the N atom is substantially reduced N bond formally becomes one of the sp3 hybrid bonds. This inversion of the configuration at by the dative N→Ge coordination compared to parent M demonstrates itself in the increase of the N–M–O bond angle which becomes larger than trialkylamino derivatives, Et3N and (HOCH2CH2)3N. The electro chemical reactivity90 of◦ [50]. these compounds is well described by the + + generalized additiveBy inductive comparison model with “classic” including cations mesomeric ((CH3O)3Si and (CH3O)3Ge , for which bond interactions. The lengthsoxidation are shown of germatranes in figure 16), M–Ofollows bonds classical in atrane cations are substantially lengthened scheme for tertiary amines - reversible electron transfer with the (Figure 16). The shortening of M–O interatomic distances in “classic” cations may be ex- ensuing deprotonation of α-carbon atom; last reaction was used for anodic cyanation ofplained m-bromobenzyl by the charge germatrane. transfer from oxygen lone pairs to the vacant p-orbital of M atom. On the other hand, the vacant p-orbital in the atrane cations is involved in N–Si bonding. This significantlyIntroduction decreases the stabilizing interaction between the oxygen lone pair and the vacant p-orbital of the metal [50]. The research interest for germatranes,Sundius - etgermanium al. also considered derivatives the change of the of atrane natural family, bond orbital - is and their interactions upon in great part due to their rich and various biological activities (1). Their unique physico- the transition from the germatranyl cation to the 1-fluorogermatrane which were revealed by chemical properties arise from the specific trans-annular N→Ge intramolecular bond inducing the pentacoordinationthe of NBO Ge analysis.and reducing The result the availab showedility that Ge–Nof nitrogen bonding lone displays pair. a larger covalent character The redox properties of germatranes,in the cation which and it may are explain often theplaying identity an of NBOimportant as well role as Mulliken in positive charges on M biological activity, are practicainlly the germatranylunexplored cation(2). Meanwhile and in the 1-fluorogermatrane: their silicon homologues, the increase of positive charge on M silatranes, were shown to possessupon interesting the formation redox of a bondactivity with (2-5). fluorine Silatrane is compensated moiety, byacting the increase of electron density as substituent, exhibits electron-donorin a M–N bond. properties as witnessed by the decrease of the redox potential of silatrane-substituted ferrocene. With this, own oxidation of the silatranes is more difficult comparedThe study to trie ofthylamine orbital interactions and other in tert theiary germatrane amines cation picturing radicals has been carried out in the lack of electron density onour th laboratorye lone pair previously of N that using acts DFT as reaction B3LYP/ 6-311Gcenter of calculations electron on a series of germatranes transfer during the oxidation. [34]. R : Continuing our study on electrooxidation of 1a PhCH - germatranes (6) in order to provide a deeper 2 b o -Br-C H CH - insight into their electrochemical reactivity, 6 4 2 N c m -Br-C6H4CH2- the role of intramolecular N→Ge dative d p -Br-C6H4CH2- R = p-EtO(O)C-C H - (17), p-F-C H CC- (18) interactions and of the substituent R, we O 2 p-Et2N-C6 64H4- 6 4 O Ge 3 p-EtO(O)C-C H - report here the results on the anodic O 6 4 4 o-F-C6H4- S oxidation of recently prepared germatranes (19) 5 p-F-C6H4CC- 1-7 substituted at Ge atom (7), showing R S 6 S various and high biological activity. S

Figure 17: Germatranes7 S 17-19.

1 Corresponding author, E-mail: [email protected] analysis of the electrochemical reactivity, DFT calculations and the results of electrol-

ysis show that the reaction center of the oxidation of the germatranes 1 and 17, 8, 9 and 18 is localized on the atrane N atom. Its n-electrons bring the main contribution to the HOMO of these compounds, though they are pointed towards Ge and hidden inside the atrane cage (Figure 18). The cation radicals of these germatranes exist in endo-conﬁguration, the geometry around Ge atom being closer to trigonal bipyramid than in the neutral molecule. The

24 change of entropy ∆S, noticed using harmonic frequency analysis of 1 and 1+•, has shown that the molecular geometry itself does not undergo substantial structural reorganization upon Thiophene fragments in 6 and 7 also have lower ionization energy (εΗΟΜΟ = – 5.524 and oxidation.-6.0014 Thus, eV for the 6 somewhatand 7, respectively) slow electron than transferthe lone kineticspair of the of theatrane oxidation nitrogen of ( germatranesε ≅ – 6.2 is mostlyeV) hence due to the the electron solvent withdr reorganizationawal during following the oxidation remarkable prim chargearily affects redistribution the thienyl in the substituent. Germatranes 6 and 7 show the same trend as was pointed out in (11): the Ep cation radicals and not to a flip-flopping of N between exo- and endo- configurations. of 2,2’-substituted thiophene is noticeably higher (Table 1) than Ep of 2,2’-bithiophene.

Figure 3. HOMO of 1a, 2 and 6 according to B3LYP/6-311G. Figure 18: HOMO of 1, 10 and 19 according to B3LYP/6-311G

The oxidation potentials of 1-7 are distinctly related with the nature of substitution at IfGe a atom substituent (Table 1) with though low the own analysis ionization of their energy electrochemical (Et2NC6H reactivity4, thienyl) is isobviously present not in the germatranea straightforward molecule, task. the reaction Previously center we offound oxidation a good is localizedtwo-parameter on this correlation substituent of andthe the oxidation potentials of benzylgermatranes 1a-d with inductive and mesomeric (σ+) atraneconstants moiety (6). only However, acts as a it weak is difficult electron-donor to homogenize substituent the (Figuremesomeric 18). contribution When the atraneof moietysubstituents itself undergoes in the whole the electron reaction withdrawal,series 1-7 so theonly germatranes the role of i follownductive an interactions electrochemically was 2 reversibleconsidered oxidation as a withfirst approach a fast deprotonation. The inductive of the constants cation radical,that provided with the the kinetics best fit ofof this experimental potentials of benzylgermatranes (6) were based on the additive model processproposed being by modulated Cherkasov by (9). the The electronic same unified properties constants, of the derived substituents from direct at Ge. interaction At higher scanof rates, the concerned however, atom the electron and the reac transfertion center kinetics were becomes therefore the used limiting for 1-7 factor (Table impeding 1, fig. 4) the and were calculated individually for each compound using the equation σ* = 7.84 observation of cation2 radicals. The deprotonation of the α-carbon atom of the atrane cage Σ{∆χi(Ri/ri) } (where ∆χi is the difference of electronegativities between i-th atom and allows anodic substitution at this position, in particular with CN− anion [33]. the reaction center; Ri is the i-th atom covalent radius and ri is its distance from the reaction center obtained from B3LYP/6-311G optimization).

3.5 Supporting electrolytes

In our work, we considered a not so related to metallatrane chemistry, but very important for organometallic electrochemistry, issue of non-cooridinating, and non-nucleophilic supporting salts for electrochemistry. Here is a short exposition of this problem. The importance of supporting electrolytes in electrochemistry is well recognized and needs not to be advocated [51]. Major requirements for these systems usually follow from their role in the electrochemical processes (but not limited to): to be not electroactive (within the range of potentials used), to provide an ionic dissociation and conductivity much larger than those due to the analyzed electroactive species added to the electrolyte and to have good solubility, chem- Figure 4. Oxidation potentials Ep of 1-7 versus additive inductive constants σ*. ical andNumbering electrochemical corresponds inertness, to row thenumbers ease ofin separationTable 1. from the products of electrolyses and, in many cases, speciﬁc interactions with the electroactive substrates, electrogenerated interme- + + diates2 Which, or with given the a limited electrode efficiency (Li of, Methe overlap4N and of HOMO-forming higher teraakylammonium orbitals of the germatrane salts, ionicmoiety liquids, − 2 − − − Ph4Bwith, p perﬂuorinatedz π-orbitals of C(sp arylborates) carbon atoms(C attached6F5)4B to Ge,and is [(CFnot so 3coarse.)2C6H 3]4B , ((CF3)3CO)4Al etc)

25 [52, 53, 54, 55]. Stabilization and observation of electrophilic electrogenerated intermediates is often only possible using the supporting salt with an appropriate non-nucleophilic anion. In the case of weak interactions of the anions of supporting electrolyte with the analyte species of large size and delocalized charge, ion pairing occurs; this phenomenon was widely explored by cyclic voltammetry through the modiﬁcation of ∆E(E2 − E1) potentials of successive electron transfers [56, 57, 58]. When electrophilic center is more localized, coordinating and even covalent interactions occur. Silicon is less electronegative than carbon (Figure 19) and due to this virtue, organosilanes are almost always bearing a positive partial charge +δ centered on the silicon atom, which makes it very vulnerable to coordinating effects or nucleophilic substitution 2 (SN2) and like other attacks from the nucleophilic species. This is even truer for siliconium cations and related species, often produced in anodic reactions of silicon organic compounds. For this reason, even more than for most organic compounds, the electrochemistry of organosilanes requires the choice of a non-coordinating electrolyte; speciﬁcally, the choice of an appropriate non-nucleophilic counterion is often crucial [59].

χO = 3.44 O C χC = 2.55 +δ Si χSi = 1.9 O χ O = 3.44 O χO = 3.44

Figure 19: Electronegativity of Si center and other atoms in organosilanes (χ is the electronegativity using the Pauling scale) [60].

In particular, the sensitivity of alkoxysilanes to fluorides is well known. Fluoride contain- − − − ing anions like BF4 , PF6 , SbF6 actually exist in an equilibrium between said anion and free − − fluoride with the corresponding tri- or penta-coordinated neutral species, e.g. PF6 PF5 + F . Given the very strong affinity of Si for fluoride anions, the use of such anions should therefore be avoided with organosilanes in most cases (except, naturally, when Si-F bond formation is the very effect the electrochemist wants to induce). We therefore looked for an alternative non- nucleophilic ionic system than might serve as supporting electrolyte under these conditions.

EXPERIMENTAL SECTION 4 Experimental Section

4.1 Solvents, supporting electrolytes

All the solvents that we used for either chemical syntheses or electrochemical tests were of analytical reagent grade purity (more than 99%). Acetonitrile (99.99%, Fisher Scientiﬁc), dichloromethane (> 99.95%, SDS), toluene (99.5%, VWR) and DMF (99.8%, VWR) dried by stirring them with calcium hydride followed by distillation under argon, and stored over molecular sieves 3Å, 8 to 12 mesh (ACROS); THF (99.9%, VWR) was distilled from sodium benzophenone ketyl.

TBAPF6 (98%, ACROS) was used as the supporting electrolyte in cyclic voltammetry and chronoamperometry. It was dried by stirring the powder for 1h under high vacuum at 60 oC then stored under argon.

4.2 NMR spectroscopy

The 1H and 13C NMR spectra were recorded on Bruker ARX (300 MHz and 500 MHz) spec- trometers. TMS (0.00 ppm) was used as internal standard for 1H NMR chemical shifts in all solvents. The 119Sn NMR spectra was recorded at the Regional Center of Physical Measure- ments of the West (CRMPO) on a Bruker AM 400 WB spectrometer. Me4Sn was used as the internal standard reference (0.00 ppm). In 13C NMR, chemical shifts were reported in ppm relative to the central line of the triplet of deuterated chloroform (CDCl3, 77.16 ppm).

4.3 Gas chromatography (GC-MS)

The GC-MS spectra were recorded on a Hewlett-Packard 5890 Series II gas chromatograph, coupled to a Hewlett-Packard 5972 Series mass selective detector (70 KeV ionizing voltage).

4.4 X-ray crystallography (XRD)

X-ray crystallography was performed on Bruker SAINT [61]. The cell reﬁnement was carried on Bruker SMART [61]; the structure was solved with the program(s) SIR97 [62] and was reﬁned with the program(s) SHELXL97 [63] and the molecular graphics was processed with ORTEP-3 for Windows [64]. All the X-ray experiments were done by Thierry Roisnel and Vincent Dorcet (Institut des Sciences Chimiques de Rennes UMR 6226).

4.5 Electrochemical techniques

The electrochemical properties of the metalatranes were studied by cyclic voltammetry (CV), chronoamperometry, large-scale electrolysis and electron paramagnetic resonance (EPR) cou-

27 pled with cyclic voltammetry.

4.5.1 Cyclic voltammetry and chronoamperometry

Voltammetric and chronoamperometric experiments were performed using a PARSTAT 2273 potentiostat controlled by the PowerSuite sofware. A glassy carbon disc of 3 mm diameter or a 1 mm Pt disc was used as working electrodes. To assure the reproductivity and preci- sion of the measurements, the working electrode was carefully polished and washed with acetone before each measurement. The counter electrode was platinum plate with 1 cm2 surface area. The 0.1 M tetrabutylammonium hexaﬂuorophosphate (TBAPF6) solutions in acetonitrile CH3CN, dichloromethane CH2Cl2 and DMF, were used as supporting electrolytes. The reference electrode was a stainless steel rod with a deposited layer of polypyrrole which was put into an electrolyte bridge ﬁlled with an inert electrolyte TBAPF6 solution (0.1 M). The polypyrrole deposited reference electrode provides a stable and precise potential value [65]. IR-compensation facility of the potentiostat was always used to account for ohmic drops in the solution. To correct the experimental potentials, at the end of every experiment we added fer- 0 rocene and plotted its voltammogram. E(F c+/F c) = 0.247 V vs. SCE was used as the reference [66]. All the experiments were carried out under argon atmosphere at the room temperature of 20 oC.

4.5.2 Electrolyses

The electrolyses have been performed under argon atmosphere, at room temperature, in 50 ml cell equipped with a magnetic stirrer (Figure 20). The working electrode was made of glassy carbon fiber as it has the maximum working surface and a platinum wire electrode was used as reference. The counter electrode was put into small chamber with an electrolyte bridge at the bottom, which itself was placed in another bigger chamber with an electrolyte bridge as well. The glassy carbon fiber and platinum wire were fixed outside the bigger filter chamber and plunged into the 50 ml glass container mentioned above. The counter electrode was separated from the cathodic compartment by two electrolyte bridges in order to avoid migration of the product from the anodic reaction to the catholyte; the space between two chambers provided a static environment, mass transfer in which was effected only by slow natural diffusion.

4.5.3 Electron Paramagnetic Resonance (EPR) spectroscopy

EPR spectra were registered on a Bruker EMX (X-band) spectrometer with a Gunn diode operating at the working frequency 9.46 GHz, coupled with a standard rectangular cavity. The modulation frequency was 100 kHz, g-factors were corrected using DPPH as standard.

28 Figure 20: Electrolytic cell.

Microwave power was adjusted to 2-8 mW to maintain the EPR signal intensity below power saturation. EPR capillary spectroelectrochemical cell was a three-electrode version of the cell described in [67]. Post-processing and simulation of the EPR spectra were carried out using WINEPR System and WINEPR SimFonia software provided by Bruker [68][69].

4.6 Syntheses of silatranes and related products

4.6.1 Silatranes

To obtain the 1-R-substituded silatranes, we used a typical procedure reacting triethanolamine with the corresponding organotrichlorosilane. The Si–Cl bond is very easy to hydrolyze, hence this reaction goes very quickly even at room temperature. However, the main drawback of chlorosilanes as starting material is that they require extremely anhydrous conditions, since water, even in trace quantities, would be a much more potent nucleophile than alco- hols. Triethanolamine was added to trichlorosilane under argon atmosphere and the reaction performed, leading to formation of our product and hydrochloric acid. For this reason, triethylamine was added as well, to neutralize said hydrochloric acid (Scheme 44.5.2). Using this method, tolyl, 1-naphthylmethyl, t-butyl and 2-cyanoethyl silatranes were synthesized using dichloromethane as a solvent. Tolylsilatrane. In a 50 ml Schlenk flask flushed with argon, p-tolyltrichlorosilane (2.26 g, 10 mmol) was dissolved in 30 ml of dry dichloromethane. In another Schlenk flask, triethanolamine (1.49 g, 10 mmol) and triethylamine (3.04 g, 30 mmol) were dissolved as

29 N OH- RSiCl3 + N(CH2CH2OH)3 + 3 N(CH2CH3)3 HCl O Si O 3 N(CH2CH3)3 O R

R = tolyl, 1-naphthylmethyl, t-butyl, 2-cyanoethyl Scheme 4: Condensation of trichlorosilane and triethanolamine well in 10 ml of dichloromethane under argon; the latter solution was then added slowly into the p-tolyltrichlorosilane solution under vigorous stirring. The reaction was exother- mic, HCl was formed. The solution was stirred overnight (more than 12 h). A side product

N(CH2CH3)3 · HCl precipitated at the end of the reaction and filtered off on Celite; the solvent was then evaporated from the filtrate. The residue was purified by column chromatography

(eluent: CH3CN/CH2Cl2, 2 : 1 v/v) to give lightly colored tolylsilatrane in 70% yield. The product was recrystallized from acetonitrile to afford lightly colored, needle-shaped crystals. 1 H NMR (300 MHz, CDCl3, 298K): δH : 7.596 (2H, d, J = 7.934 Hz, 2(CH3CCH)

14,16), 7.069 (2H, d, J = 7.422 Hz, 2(SiCCH) 13, 17), 3.880 (6H, t, J = 5.873 Hz, 3(OCH2) 13 3, 7, 10), 2.904 (6H, t, J = 5.877 Hz, 3(NCH2) 4, 6, 9), 2.275 (3H, s, CH3 18); C NMR (75

MHz, CDCl3, 298K): δC : 138.0, 137.2, 134.0, 128.1, 57.8, 51.2, 21.4;

7 8 14 13 O 6 1 5 18 H3C Si N 15 12 9 11 O 4 O 16 17 2 10 3

1-Naphthylmethylsilatrane. The procedure is the same as described for tolylsilatrane, with 1-napthylmethyltrichlorosilane (2.76 g, 10 mmol), triethanolamine (1.49 g, 10 mmol) and triethylamine (3.04 g, 30 mmol) as starting materials. Crystallization from CH3CN and gave 2.14 g of 1-naphthylmethylsilatrane (68% yield) as colorless needle-shaped crystals. 1 H NMR (300 MHz, CDCl3, 298K): δH : 8.296 (H, m, CH 22), 7.760 (H, m, CH 19), 7.526 (H, m, CH 16), 7.371 (4H, m, 4(CH) 14, 15, 20, 21), 3.712 (6H, t, J = 5.843 Hz,

3(OCH2) 3, 7, 10), 2.751 (6H, t, J = 5.841 Hz, 3(NCH2) 4, 6, 9), 2.46 (2H, s, CH2Si 12); 13 C NMR (75 MHz, CDCl3, 298K): δC : 140, 133.7, 132.5, 128, 126.4, 126.2, 125.4, 124.6, 124.1, 123.8, 57.7, 51.2, 22.9;

30 15 16 14 7 17 8 19 O 6 13 18 1 5 20 Si N 12 9 22 11 O 4 21 O 2 10 3

t-Butylsilatrane. Same procedure as described for tolylsilatrane, with t-butyltrichlorosilane (0.29 g, 1.51 mmol), triethanolamine (1.10 g, 7.4 mmol) and triethylamine (2.24 g, 22.2 mmol) as starting materials. Crystallization from CH3CN gave 0.23 g of t-butylsilatrane (65% yield). 1 H NMR (300 MHz, CDCl3, 298K): δH : 3.707 (6H, t, J = 5.730 Hz, 3(OCH2) 3, 7, 10), 13 2.745 (6H, t, J = 5.720 Hz, 3(NCH2) 4, 6, 9), 0.895 (9H, s, C(CH3)3 13, 14, 15); C NMR

(75 MHz, CDCl3, 298K): δC : 58.5, 51.8, 27.7, 19.4;

7 13 8 O H3C 6 12 1 5 14 H3C Si N 9 H C 11 O 4 3 O 15 2 10 3

2-Cyanoethylsilatrane. The procedure is the same as described above, with starting materials 2-cyanoethyltrichlorosilane (0.61 g, 3.24 mmol), triethanolamine (0.48 g, 3.24 mmol) and triethylamine (0.98 g, 9.72 mmol). Crystallization from CH3CN gave 0.47 g of the product 2-cyanoethylsilatrane (63.5% yield) as light yellow platy crystals. 1 H NMR (300MHz, CDCl3, 298K): δH : 3.758 (6H, t, J = 5.875 Hz, OCH2), 2.828 (6H, 13 t, J = 5.873 Hz, NCH2), 2.360 (2H, m, CH2CN), 1.225 (2H, t, J = 7.109 Hz, SiCH2); C

NMR (75 MHz, CDCl3, 298K): δC : 123.2, 57.3, 51.0, 12.8, 8.4.

7 8 O 15 N 13 6 14 5 1 Si N 12 9 11 O 4 O 2 10 3

For some compounds, no trichlorosilyl derivatives were available commercially. To obtain these silatranes, we synthesized them by transesterification from trialkoxylsilane and triethanolamine. Silaphilic fluoride ions were added in catalytical quantities in the form of tetrabutylammonium fluoride because they are well-known reactants for the cleavage of organosilanes (typically for deprotection of alcohol groups); they were used here to trigger transesterification. The fluoride ion attacks the silicon atom, creating a penta-coordinated Si center. Thus activated silicon quickly undergoes transesterification with triethanolamine (Scheme 5).

31 N

[CH3(CH2)3]4NF ' R Si(OR)3 + N(CH2CH2OH)3 + 3 ROH Reflux O Si O O

R = CH3, CH2CH3 R'

NH2 O

R' = H2C

Scheme 5: Synthesis of silatranes by transesteriﬁcation.

This method was applied to synthesize p-aminophenyl, p-methoxylphenyl and benzyl silatrane using acetonitrile as solvent. p-Aminophenylsilatrane. In a 50 ml Schlenk flask flushed by argon atmosphere, p- aminophenyltrimethoxysilane (0.21 g, 1 mmol) was dissolved in 50 ml CH3CN and 1% (0.3 ml) tetrabutylammonium fluoride (1 M in THF) was added to the solution above. The Schlenk flask was put into the oil-bath and heated to 90 oC. Triethanolamine (0.15 g, 1 mmol) was added into the solution above drop wise and the whole solution was refluxing overnight (more than 12 h). The solution was purified using silica column chromatography (eluent:

CH3CN/CH2Cl2, 2 : 1 v/v) and recrystallized from acetonitrile to afford 0.1 g light yellow p-aminophenylsilatrane crystals (yield 40%). 1 H NMR (300 MHz, CDCl3, 298K): δH : 7.512 (2H, d, J = 8.392 Hz, 2(NH2CCH) 14,

16), 6.618 (2H, d, J = 8.392 Hz, 2(SiCCH) 13, 17), 3.876 (6H, t, J = 5.833 Hz, 3(OCH2) 3, 13 7, 10), 3.577 (2H, s, NH2 19, 20), 2.888 (6H, t, J = 5.837 Hz, 3(NCH2) 4, 6, 9); C NMR

(75 MHz, CDCl3, 298K): δC : 154.8, 135.3, 126.0, 114.6, 57.9, 51.2;

7 8 14 13 O 19 6 18 5 N 1 Si N 12 15 9 11 O 4 20 O 16 17 2 10 3

p-Methoxylphenylsilatrane. The procedure is the same as described above for p-aminophenylsilatrane, with p-methoxylphenyltrimethoxysilane (0.23 g, 1 mmol) and triethanolamine

(0.15 g, 1 mmol) as starting materials. Crystallization from CH3CN gave 0.22 g of p-methoxyphenylsilatrane (80% yield) as colorless platy crystals. 1 H NMR (300 MHz, CDCl3, 298K): δH : 7.625 (2H, d, J = 8.658 Hz, 2(CH3OCCH) 14,

16), 6.795 (2H, d, J = 8.663 Hz, 2(SiCCH) 13, 17), 3.870 (6H, t, J = 5.858 Hz, 3(OCH2) 3, 13 7, 10), 3.753 (3H, s, OCH3 19), 2.885 (6H, t, J = 5.865 Hz, 3(NCH2) 4, 6, 9); C NMR (75

MHz, CDCl3, 298K): δC : 159.4, 135.4, 133.0, 112.9, 57.7, 54.9, 51;

32 7 8 14 13 O 6 18 1 5 19 H3C O Si N 12 15 9 11 O 4 O 16 17 2 10 3

Benzylsilatrane. The procedure is the same as described, with benzyltriethoxysilane (0.38 g, 1.5 mmol) and triethanolamine (0.22 g, 1.5 mmol) as starting materials. Crystallization from

CH3CN gave 0.26 g of benzylsilatrane (65% yield) as colorless needle-shaped crystals. 1 H NMR (300 MHz, CDCl3, 298K): δH : 7.197 (4H, m, Ph 2, 3, 5, 6), 6.983 (1H, m, Ph

4), 3.733 (6H, t, J = 5.833 Hz, 3(OCH2) 3, 7, 10), 2.772 (6H, t, J = 5.838 Hz, 3(NCH2) 4, 13 6, 9), 1.962 (2H, s, SiCH2 12); C NMR (75 MHz, CDCl3, 298K): δC : 143.1, 129.0, 127.4, 123.0, 57.6, 51.1, 26.2.

3 4 2 7 8 O 6 5 1 5 1 Si N 6 12 9 11 O 4 O 2 10 3

4.6.2 Bis-(silatranyl)alkanes

For the synthesis of bis-(silatranyl)alkanes, we had considered using the same method as the one previously used for silatrane synthesis. However, it didn’t work very well. The product tended to polymerize when it was mixed with triethanolamine, and the resulting polymer was in mixture with the triethylammonium hydrochloride which could not be separated thereafter. We therefore looked for another method in the hope that transesterification might work well in this case. So instead of using a chlorosilane, we first transformed the chlorosilane into corresponding acetylsilane, which was then reacted with triethanolamine to obtain the desired bis-silatrane (Scheme 6). The side product, acetic acid, would be removed easily by evaporation under vacuum [70]. Bis(silatranyl)methane. Bis(trichlorosilyl)methane (1.83 mL, 10 mmol), acetic anhydride (6.2 g, 60 mmol) and sodium acetate (200 mg) were placed in a two-necked flask fitted with a pressure equalising dropping funnel and a Vigreaux column with a distillation unit. The reaction mixture was heated to 60 oC while stirred for 4-5 h. After removal of acetylchlo- ride formed during the reaction, the temperature was raised to 150 oC when unreacted acetic anhydride distilled over as well. The reaction mixture was cooled down to 0 oC then 20 mL

33 Cl Cl OO AcO OAc O X NaOAc X Cl Si Si Cl + 6 AcO Si Si OAc + 6 60 0C O Cl Cl Cl AcO OAc

O Si O AcO OAc O X O CH2Cl2 AcO Si Si OAc + 2 N X + 6 0 0C OH 3 O AcO OAc OH O Si O

X : CH2 (CH2CH2)3 (CH2CH2)4 Scheme 6: Synthesis of bis-silatranes. of dichloromethane was added into the mixture. Triethanolamine (2.65 g 20 mmol) dissolved in 10 mL of dichloromethane was drop wise added with vigorous stirring overnight (12 h). The white solid present in suspension was then removed by ﬁltration and the ﬁltrate was concentrated by rotary evaporator, affording a light colored solid bis(silatranyl)methane (2.5 g, 70%).

NMR (300 MHz, CDCl3, 298K): δH : 3.798 (12H, d, J = 4.878 Hz, OCH2 3, 7, 9, 14, 18, 13 20), 2.890 (12H, t, J = 4.897 Hz, NCH2 2, 8, 10, 15, 17, 22), 1.253 (2H, s, SiCH2Si 23); C

NMR (75 MHz, CDCl3, 298K): δC : 57.605, 53.744, 3.825;

18 3 19 2 O O 9 4 17 10 O 11 N 16 12 Si Si 1 5 N 22 23 6 15 O 21 O 20 13 8 O 14 7

Bis(silatranyl)hexane. The procedure is the same as described for bis(silatranyl)methane, with bis(trichlorosilyl)hexane (2.65 mL, 10 mmol) and triethanolamine (2.65 mL, 20 mmol) as starting materials and obtained 2.25 g of bis(silatranyl)hexane (52% yield) as light yellow solid. 1 H NMR (300 MHz, CDCl3, 298K): δH : 3.745 (12H, d, J = 5.787 Hz, OCH2 3, 7, 9, 14,

18, 20), 2.772 (12H, t, J = 5.784 Hz, NCH2 2, 8, 10, 15, 17, 22), 1.25 (8H, m, 2(CH2CH2) 13 24-27), 0.419 (4H, m, 2(SiCH2) 23, 28); C NMR (75 MHz, CDCl3, 298K): δC : 58.005, 53.447, 30.996, 16.952, 12.095;

34 3 2 O 9 10 O 4 11 28 Si 1 5 N 18 19 26 O 24 27 6 17 8 O 25 N 16 12 Si 7 23 22 O 21 15 O 20 13 14

Bis(silatranyl)octane. The procedure is the same as described for bis(silatranyl)methane, with bis(trichlorosilyl)octane (2.97 mL, 10 mmol) and triethanolamine (2.65 mL, 20 mmol) as starting materials and obtained 3.18 g of bis(silatranyl)octane (69% yield) as light pink solid. 1 H NMR (300 MHz, CDCl3, 298K): δH : 3.751 (12H, d, J = 5.789 Hz, OCH2 3, 7, 9, 14,

18, 20), 2.779 (12H, t, J = 5.783 Hz, NCH2 2, 8, 10, 15, 17, 22), 1.245 (12H, m, 3(CH2CH2) 13 24-29), 0.415 (4H, m, 2(SiCH2) 23, 30); C NMR (75 MHz, CDCl3, 298K): δC : 57.985, 51.200, 31.588, 22.655, 14.126.

3 2 O 9 10 O 4 18 30 11 O 19 28 17 24 26 Si 1 5 N 29 6 N 16 12 Si 27 25 O 22 8 23 15 7 O 21 O 20 13 14

4.6.3 Bi-silatranes

Tris(1,3-dihydroxy-2-propyl)amine In order to prepare these compounds, we followed the synthesis reported in [71]. Based on this method, we adjusted some steps in the way more adapted to our later synthesis. The 1,3-dihydroxyacetone dimer (starting product) can better be seen, as the name indicates, as the hemiacetal of two molecules of 1,3-dihydroxyacetone (a in Scheme 7). The addi- + tion of ammonia NH4 on this ketone will form an imine (dehydratation rection b in Scheme 7). − The addition of the hydride H from the borohydride BH2CN allows to reduce the imine back to an amine (c in Scheme 7), which can then react with the C=O group of a second molecule of 1,3-dihydroxyacetone to form the branched imine (d, e in Scheme 7). After the ﬁrst reaction, we obtained a white sticky liquid residue which was dissolved in water and has been passed through an ion-exchange column (Amberlite, IR-120, H+).

35 OH

O OH O base HO 2 H2O 2 MeOH (a) HO acetal hydrolysis O HO HO

O NH HO imine formation HO NH3 (b)

HO HO

NH NH 2 HO HO imine reduction BH2CN (c)

HO HO

N NH O 2 imine formation HO OH HO HO (d)

HO OH HO HO

N H N HO OH imine reduction HO OH BH2CN (e) HO OH HO OH

Scheme 7: Synthesis of intermediate tetrahydroxy diisopropylamine

Before usage, the Amberlite resin was twice soaked in 7% HCl solution for 24h, then packed in the column and washed with water until the exit flow turns neutral. We then injected our product (dissolved, in water) and eluted it with water until all impurities were flushed out, and the outlet flow turns neutral once more. Finally, we then passed a solution of aqueous ammonia (1.00 M) to wash out the pure product from the column. This tetrahydroxy diisopropylamine was dissolved in methanolic HCl to form the corresponding ammonium chloride. Then the solvent was removed under vacuum, which is a very crucial point to the whole reaction. The H2O must be absolutely removed from the product to let the reaction be able to start in an anhydrous environment. Under argon, the ammonium chloride was dissolved in anhydrous DMF, before 2,2-dimethoxypropane (DMP) and the catalyst p-toluenesulfonic acid monohydrate (p-TsOH) were added. The solution was stirred at least 12 h until the solution became very turbid. The solution was neutralized with triethylamine. The mixture was concentrated in vacuum, the residue redissolved with triethylamine and ethyl acetate. The ammonium salts were simply filtered off, and the solvent evaporated to afford an orange colored viscous liquid. Recrystalization in hexane followed: however, even with heating, the viscous liquid cannot be dissolved completely into hexane. The hot 2-phases solution was put to the refrigerator immediately. On cooling, white crystals formed right at the interface between hexane and the orange colored viscous liquid. This partial recrystalization

36 step was repeated several times to obtain 25% yield of bis(4,4-dimethyl-3,5-dioxanyl)amine (Scheme 8).

HO HO HO HO HCl H DMP, DMF O O NH MeOH N Cl p-TsOH, rt H O O N HO HO H HO HO

Scheme 8: Hydroxy group protection

The next step was to introduce the third branch to the tetrahydroxy diisopropylamine, and we did it by inserting an N–H bond to the secondary amine with a carbene or carbenoid generated from a diazocarbonyl compound, which is the key step in the synthesis of the target tertiary amine. Dimethyl diazomalonate (DDM) was chosen as the source of carbenoid. DDM was synthesized by a diazo transfer reaction (Scheme 9).

SO2Cl SO2N3

+ (n-Bu)4N Br + NaN3 CH2Cl2

NHCOCH3 NHCOCH3

SO2N3 SO2NH2

CO2Me CO2Me Et3N ++N2 CH3CN CO2Me CO2Me

NHCOCH3 DDM NHCOCH3

Scheme 9: Synthesis of dimethyl diazomalonate (DDM)

To insert DDM into the NH bond of bis(4,4-dimethyl-3,5-dioxanyl)amine, dirhodium tetraac- etate was used as a catalyst of the decomposition of DDM since it is well-known to form strong complexes with Lewis bases. This would lead to the tertiary amine-dimethyl 2-(N,N-bis(4,4- dimethyl-3,5-dioxanyl)amino)malonate. However, according to Porter and his coworkers [72,

73], the catalytic activity of Rh2(OAc)4 is totally inhibited by primary amines (Scheme 10). If the secondary bis(4,4-dimethyl-3,5-dioxanyl)amine has some impurities containing primary amine, the catalyst will be poisoned immediately when added. The solution would then become red-brown instead of the normal green color. Hence, bis(4,4-dimethyl-3,5-dioxanyl)amine must be very pure when used. Fortunately, we obtained very white pure crystals from the previous step. Therefore, a slight excess of DDM and 8 mol% of catalyst Rh2(OAc)4 were added into a toluene solution of acetonide amine at room temperature. The mixture was heated to

37 reﬂux (110 oC) for one night. The solvent was removed by rotary evaporator and the product was puriﬁed by chromatography passing it on a silica column with ethyl acetate: hexane (1:3 / v:v) as an eluent to afford the tertiary amine dimethyl 2-(N,N-bis(4,4-dimethyl-3,5- dioxanyl)amino)malonate.

Lithium aluminum hydride (LiAlH4) was then used to reduce the ester groups. 6 Equiva- lents of LAH were dissolved in 20 ml of THF to give a gray suspension which was injected drop wise into 10 ml of tertiary amine in THF solution under argon at 0 oC. The whole suspension was kept stirring overnight at room temperature. Then were added sequentially water (0.9 ml), 15% sodium hydroxide (0.9 ml) and water (3×0.9 ml). The mixture was ﬁltered and the ﬁltrate was evaporated to obtain a light yellow transparent liquid, 2-(N,N-bis(4,4-dimethyl- 3,5-dioxanyl)amino)-1,3-propanediol. Yield 50%.

N2 O O O O MeO OMe O O O O LiAlH4 O O O O N THF, rt N O O MeO OMe N Rh2(OAc)4, PhH H reflux O O OH OH

Scheme 10: Synthesis of 2-(N,N-bis(4,4-dimethyl-3,5-dioxanyl)amino)-1,3-propanediol

Trifluoroacetic acid in aqueous THF was employed to cleave the acetal groups of 2-(N,N- bis(4,4-dimethyl-3,5-dioxanyl)amino)-1,3-propanediol, affording deprotected tertiary amine as trifluoroacetate salt. This mixture was flushed through an ion-exchange resin column, Am- berlite, IR-120, H+. The column was eluted with water first, and then with a solution of 1 M aqueous ammonia. A yellow oil was obtained. The oil was kept under high vacuum overnight, and it became a yellow solid tris(1,3-dihydroxy-2-propyl)amine (Scheme 11). The product was obtained in 72% yeild. Tris(1,3-dihydroxy-2-propyl)amine 1 13 H NMR (300 MHz, D2O, 298K): δH : 3.521 (12H, m, CH2O), 3.146 (3H, m, NCH); C

NMR (75 MHz, D2O, methanol, 298K): δC : 57.1, 61.3.

O O Ion O O HO CF3COO exchange HO N CF3COOH resin NH N H O, THF rt 2 HO 3 HO 3

OH OH

Scheme 11: Synthesis of tris(1,3-dihydroxy-2-propyl)amine.

38 Using this tris(1,3-dihydroxy-2-propyl)amine, we synthesized 3 new silatranes which have two atrane structures sharing the same nitrogen atom and three adjacent carbon atoms. The silatranes were synthesized by transesterification of tris(1,3-dihydroxy-2-propyl)amine with the corresponding organotrialkoxysilane. The transesterification method is wildly used in silatrane synthesis. In the thesis of Yuan Pingjie [71], it was mentioned that they obtained two triptych-silatranes, di(phenyl)- and di(methyl)-bi-silatrane using this method. In our case, we used mixed solvents DMF and toluene instead of single solvent DMF. DMF was used as a good solvent to dissolve tris(1,3-dihydroxy-2-propyl)amine and toluene was used to provide a lower reflux temperature, thus preventing the product from thermal decomposition (Scheme 12).

R O Toluene, DMF O Si O HO Si 110 oC N + 2 R O N 3 HO 3 Overnight O Si O O R

R = CH2CH3, C6H5CH2, C6H5

Scheme 12: Synthesis of bisilatranes.

Di(phenyl)-bi-silatrane. Phenyltriethoxysilane (0.49 mL, 2.016 mmol) was added to the solution of tris(1,3-dihydroxy-2-propyl)amine (80 mg, 0.336 mmol) in DMF (1 mL) and toluene (4 mL). Under argon, the solution was heated (oil bath 110 °C) for 12 h. TLC was employed to follow the reaction. At the beginning of the reaction there were two new spots on the TLC plates. The relatively polar spot of the two gradually disappeared. Finally, there was only one new spot left. The solvent and excess of phenyltriethoxysilane were removed under reduced pressure and the residue was puriﬁed by recrystallization from CH3CN to afford white crystal (126 mg, 85%).

1 H NMR (300 MHz, CDCl3, 298K): δH : 7.704 (4H, m, C6H5 24, 25, 29, 30), 7.380 (6H, m, C6H5 23, 27, 26, 28, 31, 32), 3.813 (6H, dd, J = 4.568, 11.58, CH2 3, 7, 11, 13, 15, 18), 13 3.744 (6H, t, J = 11.520, CH2 3, 7, 11, 13, 15, 18), 3.509 (3H, m, CH 4, 6, 9); C NMR (75

MHz, CDCl3, 298K): δC : 134.3, 131.9, 130.5, 128.0, 61.1, 57.6.

39 18 15 13 7 O 10 28 16 26 29 O14 27 O 9 6 O 8 21 30 Si 17 N 5 1 Si 25 22

O 2 24 31 32 12 O 23 4 11 3

Di(ethyl)-bi-silatrane. The procedure is the same as described for di(phenyl)-bi-silatrane, with ethyltriethoxysilane (0.43 mL, 2.016 mmol) and tris(1,3-dihydroxy-2-propyl)amine (80 mg, 0.336 mmol) as starting materials. Crystallization from hexane gave 60 mg of di(ethyl)- bi-silatrane (51% yield) as colorless needle-shaped crystals. 1 H NMR (300 MHz, CDCl3, 298K): δH : 3.664 (6H, dd, J = 4.375, 11.48, CH2 3, 7, 11,

13, 15, 18), 3.524 (6H, t, J = 11.146, CH2 3, 7, 11, 13, 15, 18), 3.283 (3H, m, CH 4, 6, 9), 13 0.967 (6H, t, J = 7.887,CH3 21, 22), 0.582 (4H, q, J = 7.94, 15.76, SiCH2 19, 20); C NMR

(75 MHz, CDCl3, 298K): δC : 60.62, 57.17, 6.63, 3.90;

18 15 13 7 O 10 16 O14 O 8 9 6 O 21 20 Si 17 1 Si N 5 19 22 O 2 12 O 4 11 3

Di(benzyl)-bi-silatrane. The procedure is the same as described above, with benzyltriethoxysilane (0.52 mL, 2.016 mmol) and tris(1,3-dihydroxy-2-propyl)amine (80 mg, 0.336 mmol) as starting materials. Crystallization from toluene gave 116 mg di(benzyl)-bi-silatrane (78% yield) as colorless crystals. 1 H NMR (300 MHz, CDCl3, 298K): δH : 7.125 (10H, m, C6H5 23-32), 3.637 (6H, dd, J

= 4.263, 11.49, CH2 3, 7, 11, 13, 15, 18), 3.483 (6H, t, J = 11.219, CH2 3, 7, 11, 13, 15, 18), 13 3.238 (3H, m, CH 4, 6, 9), 2.129 (4H, s, SiCH2 19, 20); C NMR (75 MHz, CDCl3, 298K):

δC : 137.71, 128.83, 128.10, 124.56, 60.70, 57.18, 21.42.

18 26 15 13 7 O 10 27 25 16 O14 O 8 9 6 O 21 28 20 24 Si 17 N 5 1 Si 29 19 23 22 O 2 12 O 30 32 4 31 11 3

40 4.7 Electrosynthesis of di-stannatrane

In the two-compartment electrolytic cell described above, 0.049 g of stannatrane chloride and 0.48 g of lithium perchlorate were dissolved into 30 mL of acetonitrile under argon. A tension of -1.94 V was then applied to the system for 1 hour and the current changes were recorded. At the end of the electrochemical reaction, the solvent was evaporated on a rotary evaporator. The resulting product was re-dissolved in hexane, creating a suspension of LiClO4 that was ﬁltered off. The remaining impurities were puriﬁed by column chromatography (eluent: hexane) to afford pure di-stannatrane. In the series of repeated electrolyses, the material yield was between 5% and 30%. Bi-stannatrane. 1 H NMR (500 MHz, CDCl3, 298K): δH : 1.608 (12H, s, 2 × 3(NCH2) 4, 6, 11, 15, 17, 13 22), 1.345 (36H, s, 2 × 6(CH3) 23-34); C NMR (125 MHz, CDCl3, 298K): δC : 72.075, 119 69.197, 31.104, 29.263; Sn NMR (186 MHz, CDCl3, 298K): δSn: -639.72.

25 26 29 23 24 30 9 3 18 19 O 2 11 4 17 10 O O

N 16 12 Sn Sn 1 5 N

O 21 6 15 22 O 13 O 20 8 7 14

31 28 32 34 33 27

RESULTS AND DISCUSSION 5 Results and discussion

Analysis of bonding and electronic interactions in aryl, benzyl and alkyl silatranes allows us to outline several conjugative schemes involving transannular interactions (both axial and lateral) which might be found in metallatrane-based reaction series. Main building block in these systems is the 3c-4e unit. As it was revealed above, most interesting feature of the 3c-4e system in silatranes is that upon one-electron withdrawal (providing a 3c-3e system), it forms an odd-electron system with axial symmetry in which the unpaired electron occupies internal axial orbital (mostly, pz(N)). Extension of such structure by further axial (/lateral) conjugation (/hyperconjugation) can open the route to formation of more conjugated systems like 4c-6e, 5c-6e, 6c-8e etc. Their efficiency of conjugation and stability might be quite different but those identified as stable energy minima can then be envisaged to be used for designing molecular systems with long-term transmission of electronic effect, i.e. a limiting nc-(n+1)e scheme which might start a new generation of molecular wires. From this standpoint, we considered several systems and attempted their synthesis and study of their electrochemical behavior. Selected structures and their bonding schemes are shown in figure 21.

43 44 45 Figure 21: Selected metallatrane-based structures and their bonding schemes.

5.1 Simple 1-R-silatranes

5.1.1 Introduction

Though the family of known 1-R-silatranes is extremely large, it has been very poorly studied by electrochemistry [74, 5]. Comparison of the few available data on the electrooxidation of this class of compounds with the works on electrochemistry of homologous germatranes [49, 33, 34] allowed to suppose that electronic structure on the cation radicals of Si-substituted aromatic and aliphatic derivatives on one hand, and benzylic derivatives on the other, should have different nature and therefore, would result in a different electrochemical behavior. This difference is intrinsically related to the nature of bonding interactions in neutral silatranes.

CH2 NH R = 2 N C

O Si O p-aminophenyl 1-naphthylmethyl t-butyl tolyl O H2 C R C N H2 O p-methoxyphenyl benzyl 2-cyanoethyl

Figure 22: 1-R-silatranes

In order to elucidate the speciﬁc features of these effects and the possibility of incorpora-

46 tion of a simple 3c-4e system into more extended 5c-6e or higher (5c-6e)n electronic systems, we ﬁrst synthesized and studied several model silatranes (Figure 22).

5.1.2 Structure

Seven 1-R-silatranes structures, studied in the next section were prepared according to previously reported methodes, as described in the experimental section. Although they all are already known and their crystal structure was even released in several publications [1, 28, 29], we found that these structures were neither deposited at CCDB, nor the details on their structure were really published. So we found it useful to consider more this missing aspect. The crystal structures of four silatranes were obtained by X-ray single crystal diffraction. The structure of p-aminophenylsilatrane, 1-naphthylmethylsilatrane, p- methoxyphenylsilatrane and benzylsilatrane are the ﬁrst we characterized. Their molecular structures are shown in ﬁgures 23-26, the crystallographic data are listed in the table 4 and the selected bond lengths and angles are listed in table 5.

Table 4: Selected crystallographic data and details of reﬁnement for p-aminophenylsilatrane, 1-naphthylmethylsilatrane, p-methoxyphenylsilatrane and benzylsilatrane.

R p-aminophenyl 1-naphthylmethyl p-methoxyphenyl benzyl

Empirical formula C12H18N2O3Si C17H21NO3Si C13H19NO4Si C13H18NO3Si Formula weight 266.37 315.44 281.38 265.38 Crystal system Orthorhombic Orthorhombic Monoclinic Monoclinic

Space group P c21n Pbca P 21/n P 21/n a (Å) 6.6141 (4) 13.8928 (5) 10.4799 (8) 10.3529 (2) b (Å) 11.8122 (8) 14.5060 (6) 10.4219 (8) 11.5350 (2) c (Å) 16.1752 (11) 15.0774 (6) 12.5058 (10) 11.0310 (2) α(o) β(o) 99.275 (4) 102.224 (1) γ(o) 3 V (Å ) 1263.72 (14) 3038.5 (2) 1348.03 (18) 1287.46 (4) Z 4 8 4 4 3 Dcalcd(Mg/m ) 1.400 1.379 1.386 1.369 Radiation (MoKα) λ = 0.71073Å λ = 0.71073Å λ = 0.71073Å λ = 0.71073Å T (K) 150 150 423 150 θRange (o) 2.5–27.5 2.4–27.5 2.4–27.5 2.7–27.4 Total reflections 14009 46767 10522 10430 Unique reflections 2496 3488 3091 2919 Parameters 163 199 173 163 a R1, wR2 (I > 2σ(I)) 0.023, 0.064 0.045, 0.119 0.037, 0.097 0.032, 0.087 Goodness-of-fit on F 2 1.04 1.06 1.06 1.05 a P P P 2 2 2 P 2 2 1/2 R1 = ||F0| − |Fc||/ |F0|; wR2 = { [w(F0 − Fc ) ]/ [w(F0 ) ]}

47 The silatrane structure can be seen as a distorted trigonal bipyramid with N atom and C atom in the axial positions, three oxygen atoms occupying the equatorial sites and the Si atom in the body center. The N-Si-C atoms are almost aligned (177.61(7)–179.29(6)◦) in all these compounds. The values of the distance between Si and N in these four molecules lie within the typical range for silatranes (1.964–2.420 Å) which clearly veriﬁes the existence of Si–N transannular bond [75].

Table 5: The main geometrical parameters for p-aminophenylsilatrane, 1- naphthylmethylsilatrane, p-methoxyphenylsilatrane and benzylsilatrane

o Compound d(Si ··· N) (Å) d (Si–C) (Å) d Si–O (Å) ∠N–Si–C ( ) 1.6710(11) p-aminophenylsilatrane 2.1665(13) 1.8831(14) 1.6718(10) 179.29(6) 1.6727(11) 1.6663(13) 1-naphthylmethylsilatrane 2.1393(14) 1.9020(17) 1.6676(13) 177.61(7) 1.6698(12) 1.6702(11) p-methoxyphenylsilatrane 2.1523(13) 1.8958(15) 1.6713(10) 178.20(6) 1.6631(11) 1.6685(9) benzylsilatrane 2.1215(11) 1.8990(13) 1.6700(9) 178.50(5) 1.6733(9)

d Si1··· (O4–O7–O10) (Å) d N1··· (C2–C5–C8) (Å) 0.2031 0.3782 0.1884 0.3786 0.1897 0.3735 0.1761 0.3821

The silicon atom is displaced from the plane formed by three oxygen atoms towards the carbon atom on the apical position, whereas the nitrogen atom is displaced from the plane of three carbon atoms in α-positions inward the atrane cage thus adopting an endo-configuration. Concerning the five-membered side branches of the rings of silatrane skeleton in these four compounds, they adopt an “envelope”-like conformation, the carbon atoms in α-positions to the N atom being like the “flap” sites. As shown in the table 5, in the atrane structure N atom is not on the plane of the three C atoms (C2, C5 and C8) but is sunk inward by a distance around 0.37-0.38 Å. According to the VSEPR (valence shell electron pair repulsion) theory, the lone-pair-electron of N and the other three valence electrons should repel each other as much as possible to reach a stable status. From this theory it is as well proved that the lone pair electrons of N are pointed right to the

48 Si atom. The Si atom is ∼0.18 Å off the plane formed by O4, O7 and O10 which is also explained by VSEPR theory. We notice that the distance between Si and three oxygen atoms has a slight difference. This could be caused by the substituents because the angle ∠N–Si–C in those four silatranes is not 180o (Table 5). For the silatranes shown in figure 23 and figure 25, the aromatic substituents are connected to the silicon atom directly. As the carbon atoms in the aromatic substituent have sp2 hybridization, the delocalized π bonds are on the both sides of the aromatic plane and are perpendicular to the orbital of Si and N atoms in the atrane structure. Thus there is not interaction between the N→Si bond and the π-system of aromatic substituents. On the other side, there are amino- and methoxy- groups at the para-position of the aromatic substituent (Figure 23 and figure 25, respectively). The lone pair electron on the N and O atomPage participate1 of 1 in the formation of delocalized π bond and the conjugated system even if they are not exactly on the same plane.

Figure 23: ORTEP drawing of the molecular structure of p-aminophenylsilatrane.

Figure 24: ORTEP drawing of the molecular structure of 1-naphthylmethylsilatrane.

For 1-naphthylmethylsilatrane and benzylsilatrane (Figure 24 and ﬁgure 26), the Si atom 3 connected the CH2 bridge by a σ bond (with C in the sp hybridization). The σ bond is symmetrical with respect to rotation about the bond axis which cause the angle ∠Si–C21–C22 (Figure 24) to be equal to 118.37o and Si–C11–C12 (Figure 26) equal to 116.07o. On the file:///C:/Users/rainy/Desktop/Bureau/manipulation/Crystallographica/aminophe∠ nylsilatrane/VJ21_150K_ortep.gif 2012/8/23

49 other hand, the carbon atoms C22 (Figure 24) and C12 (Figure 26) of the aromatic substituent, either phenyl or naphthyl, are in sp2 hybridization. The electrons from this σ bond and the adjacent C atom π∗ orbital can interact via hyperconjugation mechanism which leads to the shortening of the single bond length (d (C11–C12) = 1.5057 Å and d (C21–C22) = 1.518(2) Å). For naphthyl substituent, the conjugated system is weaker than in phenyl,Page 1 henceof 1 the hyperconjugation is not that notable which makes the single bond length much closer to the normal C–C length (∼1.520 Å).

Figure 25: ORTEP drawing of the molecular structure of p-methoxyphenylsilatrane

Figure 26: ORTEP drawing of the molecular structure of benzylsilatrane

5.1.3 Cyclic voltammetry

The oxidation of seven Si-substituted silatranes was studied in 0.1 M tetrabutylammonium hexaﬂuorophosphate (TBAPF6) acetonitrile (CH3CN) solutions at a Pt electrode. A Pt mi- crodisk electrode wasfile:///C:/Users/rainy/ usedDesktop/Bureau/manipulation/Crys as the workingtallographica/Meoph/VJ22_150K_ortep.gif electrode as it yielded voltammograms2012/8/23 better shaped than those obtained with other electrodes (glassy carbon was also tried). The oxidation peak potentials of these compounds are in the range 0.78-1.45 V (vs. SCE), depending on the nature of substituent R (Table 6). Upon oxidation, three out of seven silatranes show a distinct reversible peak (Figure 27). We observed direct proportionality between the oxidation peak current (ip) and the concentration of the electroactive substance. Direct proportionality was 1/2 observed as well between the peak current and the square root of the scan rate (ip/v = const.) at sufﬁciently fast scan rates (v > 0.5 V/s). These two results suggest a diffusion

50 control of the process. The number of electrons transferred at this oxidation step has been determined using the method proposed by Malachesky [40] which is based on the following relations. Peak current ip for a linear sweep voltammogram of a reversible redox system can be expressed as follows:

5 3/2 1/2 1/2 ip = 2.68 × 10 n ACD v

On the other side, for potentiostatic chronoamperometry [66]:

it1/2 = nF ACD1/2π−1/2

Combining these two equations, one obtains the following relationship:

i /v1/2 R = p = 4.92n1/2 it1/2 which allows one direct evaluation of the absolute electron stoichiometry. This method is free of the uncertainty due to the different diffusion coefﬁcient D, concentration and electrode surface, contrary to the method based on the comparison of ip with a standard compound like ferrocene. Now applying this relationship to the oxidation of sever silatranes, we found that 0 all of them undergo mono-electronic process at Ep (Table 6).

Table 6: Parameters of electrochemical oxidation of silatranes at a platinum disk electrode at v = 1V/sa

Ea − Ea ∆Ea/∆lg(V ) R Ea(V) Ec(V) Eo(V) n p p/2 p αb p p (mV) (mV) p-aminophenyl 0.78 – – 1 93 14 0.50 1-naphthylmethyl 1.10 – – 1 88 28 0.50 t-butyl 1.22 1.14 1.18 1 91 10 0.50 tolyl 1.35 1.26 1.30 1 72 14 0.65 p-methoxyphenyl 1.37 – – 1 66 40 0.72 benzyl 1.42 – – 1 150 40 0.32 2-cyanoethyl 1.45 1.30 1.38 0.9 81 23 0.58 a Cyclic voltammetry was carried out in CH3CN / 0.1 M TBAPF6. All the voltammograms are brought to the SCE scale. b Transfer coefﬁcient estimated as a mean of α from Ep − Ep/2 = 1.85RT/αF [41].

As mentioned previously, the electronic effects of the substituents R affect signiﬁcantly the

51 oxidation potentials of the silatranes [5]. Amongst seven silatranes, t-butyl and tolyl silatranes show a distinct reversible electrooxidation signal even at low scan rate (v = 0.5 V/s). Tolyl- silatrane has an oxidation peak at Ep =1.35 V (vs. SCE), with the corresponding cathodic counterpart at 1.26 V (vs. SCE). As for the t-butyl derivative, it is a substituent with a strong electron-donating inductive effect which leads to a relatively easy oxidation of this silatrane (1.22 V vs. SCE).

6 a b c 5

3 A

µ 2

, I

- 1

- 2 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 E , V v s F c

Figure 27: Cyclic voltammetry of a, t-butylsilatrane; b, tolylsilatrane; c, 2-cyanoethylsilatrane o (C = 0.1 mmol/L) in CH3CN/0.1M Bu4NPF6 at a 1 mm Pt electrode. v = 1 V/s; T = 25 C.

Quite surprisingly at ﬁrst sight but perfectly in agreement with our hypothesis on electronic interactions as main factor of the stability of cation radicals of metallatranes, 2-cyanoethly- silatrane exhibits reversible oxidation at Ep = 1.45 V vs. SCE (Figure 27). The corresponding cathodic peak is less pronounced compared to t-butylsilatrane and formal E0 is anodically shifted (Table 6). Both facts supposedly arise from the electron-withdrawing inductive effect of the CN group. The other four silatranes (Figure 28) showed a different electrochemical behavior under the same conditions. Their oxidation peaks (Table 6) are irreversible even at higher scan rates (v = 10 V/s). In order to rationalize these data, electronic structure and possible mechanisms of transmission of electronic effects in these systems were considered in more details. For the methoxyphenyl silatrane, phenyl substituent is in principle a mesomeric π-donor, but it does not have the expected effect on Ep and stability of cation radicals because the π orbital system of the phenyl ring is perpendicular to the σ orbital of the bond between Si and C atoms (Figure 30). Thus the inductive effect of the substituent must be considered in this case. According to Hammett equation [76]:

K log = σρ Ko 52 For a given equilibrium reaction with substituent R, K is relating the equilibrium constant and the reference Ko constant when R is a hydrogen atom to the substituent constant σ which depends only on the specific substituent R and the reaction constant ρ which depends only on the type of reaction but not on the substituent used. Here the constant σ reflects all the combination of field, inductive and resonance effects influencing the reaction rates. However, in our case, we only consider the inductive influence of the substituent. Thus we use σI constants (Taft’s modification) indicating the electronic effect of a substituent (Table 7).

Table 7: Substituent ﬁeld/inductive parameters for selected substituents

∗ c σI = 0.45σ b σI Taft R a σF Taft CH2X Taft (F NMR) C2H5 -0.05 0.00 0.06 C6H4Me 0.11 0.12 0.14 H 0.00 0.00 0.00 C6H4OMe 0.14 0.13 0.08 Cl 0.47 0.45 0.43 C2H4CN 0.33 0.17 0.53 C(CH3)3 -0.09 0.00 0.09 a [77, 78]; b [79]; c Calculated from the meta-substituted ﬂuorobenzene F NMR shift (felative to ﬂuorobenzene) [80].

a c d 8 b

4 A µ

, I

- 2 0 . 0 0 . 4 0 . 8 1 . 2 1 . 6 E , V v s F c

Figure 28: Cyclic voltammetry of a, p-aminophenylsilatrane; b, naphthylsilatrane; c, p- −1 methoxyphenylsilatrane; d, benzylsilatrane (C = 0.1 mmol L ) in CH3CN/0.1M Bu4NPF6 at a 1 mm Pt electrode. v = 1 V/s; T = 25 oC.

It can be seen that the oxidation peak Ep of t-butyl, tolyl, methoxyphenyl and 2-cyanoethylsilatrane correlate well with inductive σI constants (Figure 29), with a slope of 1.78 (R-square

53 = 0.97). The fact that inductive effect affects the oxidation peak is not surprising in itself; most important is that the points for aromatic subsituents ﬁt the same correlation as aliphatic groups. This is perfectly in agreement with the 3c-4e character of the HOMO in these compounds.

0 . 4 4

0 . 3

0 . 2

σ 0 . 1 3

0 . 0 1

- 0 . 1

1 . 2 0 1 . 2 5 1 . 3 0 1 . 3 5 1 . 4 0 1 . 4 5

E p

Figure 29: Oxidation peak of selected silatranes vs. σI constants. 1. t-butylsilatrane; 2. tolylsilatrane; 3. p-methoxyphenylsilatrane and 4. 2-cyanoethylsilatrane.

N N

3c-4e Si Si HOMO 90o σ π

ϕas ϕsym. π no interaction (Ph) -I O O effect

Figure 30: Molecular orbital of methoxyphenylsilatrane and inductive effect in this system (the atrane cage is not shown).

From the table we can see that the electron-withdrawing effect of phenyl is stronger than that of akyl substituents which makes methoxyphenyl group a σ-acceptor; in accordance with this, this silatrane has a relatively higher Ep = 1.37 V vs. SCE (Table 6). In fact comparing to Csp3 in akyl substituents with Csp2 linking atom in phenyl, the latter has stronger electron- withdrawing properties. Moreover, Pauling electronegativity of Si atom (1.90) is less than

54 that of C atom attaching the substituent. Aryl substituents therefore, are simply σ-acceptors with respect to the silatrane moiety, leading to the lowering of the HOMO of such compounds which is another reason why they have a high electrooxidation potential. Inductive effect fades out very rapidly with distance (Figure 30); so there must not be a big difference in an overall inductive effect of phenyl and anisyl groups in the corresponding silatranes (Table 7). The a a p-methoxyphenylsilatrane, on its part, has a very sharp oxidation peak (the value Ep − Ep/2= 66 mV is close to those for purely Nernstain systems), and might therefore become reversible for v → ∞ (Figure 31).

1 µA

0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 1 . 6 E

Figure 31: Cyclic voltammogram of p-methoxyphenylsilatrane oxidation at v = 10 V/s.

One can see that methoxyphenylsilatrane has an oxidation peak potential Ep =1.37 V vs. SCE, (Table 6) higher than that of the naphtylmethylsilatrane (Ep =1.10 V). Indeed, 1- naphtylmethylsilatrane -contrary to tolyl- and anisyl derivatives- exhibits an overall π-donor effect of the naphtyl group. Remarkably, π-system from naphtyl and the σ(C–Si) fragment from the 3c-4e bond are in the hyperconjugation which leads to the electron delocalization (Figure 32).

H2C

Figure 32: Hyperconjugation of 3c-4e bond with the π-system in 1-naphtylmethylsilatrane.

55 Same conjugative mechanism also allows spin delocalization in the cation radical from N atom to naphthyl π-system. The unpaired electron, localized on N atom and isolated inside the atrane cage in the case of alkyl or aryl silatranes, becomes visible from outside by external radicalophilic species due to its presence on naphtyl rings, primarily at 4-position. Thus the observed lowering of the oxidation potential of this silatrane is due to two factors: electron donating thermodynamic contribution of the substituent, and fast ensuing reaction of electrogernerated cation radicals (kinetic shift of Ep). Therefore, it unsurpisingly has one of the lowest oxidation peak potentials in Table 6, at Ep =1.10 V vs. SCE. The benzylsilatrane is very similar to 1-naphtylmethylsilatrane. However, it has a more positive oxidation peak Ep = 1.42 V vs. SCE (Table 6), which supposedly results from the hyperconjugation interplay of the same factors as in 1-naphtylmethylsilatrane, plus -probably- some contribution of heterogeneous interactions with the electrode. Indeed, different from other voltammograms, benzyl silatrane exhibits a rather large irreversible oxidation peak. Al- though the oxidation of benzylsilatrane begins at close potentials (foothill part of the voltammogram, ﬁgure 28), the peak potential is located at 1.42 V vs. SCE. The peak width, Ep −Ep/2 of benzylsilatrane is 150 mV, which is remarkably larger than for the other compounds of the reaction series (Table 6). Since its oxidation is supposed to be electrochemically reversible, as in general is expected in the silatrane family, one could think of adsorptional interactions of its aromatic system with Pt as a possible reason of this abnormally large peak. However, naphtyl silatrane, which has a similar (and even more developed) aryl moiety, does not show this feature which thus remains quite unclear.

NH2 NH2

0.78 V vs. SCE e

O O O Si O O Si O

N N

Scheme 13: p-Aminophenylsilatrane oxidation.

Finally, p-aminophenylsilatrane has the less anodic oxidation potential (0.78 V vs. SCE) which falls out of the silatrane electrooxidation range. This might be explained by the ease with which the amino substituent itself can be oxidized. The oxidation peak at 0.78 V would then arise from the oxidation of the aniline fragment (Scheme 13), since the pz(N) electrons, contributing the most to the HOMO of this compound have lowest ionization potential and

56 therefore this atom would be the ﬁrst that would become electrooxidized. The voltammogram of the aminophenylsilatrane might become reversible at high scan rates (Figure 33).

1 µA I

0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 E

Figure 33: Cyclic voltammogram of p-aminophenylsilatrane oxidation at higher scan rate v = 10 V/s

Compared to the voltammograms of triethylamine (Ep = 0.582 V) and triethanolamine

(Ep = 1.126 V [33]) in which the nitrogen atom is not involved in any coordinating interactions, silatranes have higher oxidation potentials. This anodic shift is explained by the transannular interaction between nitrogen and silicon atoms. This effect causes a strong transmission of electron density from N to Si atom which could be considered as an internal donor-acceptor charge transfer. This anodic shift is clearly seen in the results shown in Table 6. In general, the potentials of electrooxidation of the silatranes were 0.52-0.87 V higher than that of triethylamine. Thus, the potential of electrooxidation of silatranes reﬂects the combined effects of the N→Si transannular bonding and the electronic effects of the substituent R attached to the silicon atom.

5.1.4 EPR spectroscopy

Reversibility of the oxidation of phenylsilatrane (reverse peak of reduction of the cation radicals is seen already at v = 0.2 V/s allowed us to study this silatrane by EPR-spectroelectrochemistry. When phenylsilatrane was oxidized at microelectrode in an EPR cell, a signal of electrogenerated odd-electron species appeared at E = 0.7 V. The spectrum has a characteristic feature 9

57 group of repeating pattern, the separation between them going to the baseline. EPR spectrum of the cation radical of phenylsilatrane (Figure 34) has a large end-to-end width (~160 G) and consists of more than 184 well-resolved lines. Given its large spectral span, the high field part of this spectrum shows some distortion by second-order effects, which makes it difficult to reconstruct it with the simple tools included into Bruker Symphonia package. Nevertheless, the following parameters have been extracted from low-field part allowing α a 99.8%-quality reconstruction of this spectrum: g = 2.0037; aN = 18.41 G; 3×a Hax = 37.93 α β G; 3 × a Hlat = 0.23 G; 6 × a H = 1.8 G. This spectrum corresponds to a species with practically planar N atom, carrying the major part of the free electron density, which has strong interactionEnd with-to-end three width axially-oriented of this spectrumα -protons.is ~40 G  The too C-Hsmall bonds for a simple of the silatrane… other three (mustα-protons be are~160 practically G). No comments, orthogonal just: to Figure the axes X. ofEPR this spectrum molecule from so the that oxidation their interaction of t-BuSiTRan with in magnetic field**/0.1M is about ** at 160 a Pt times microspiral weaker electrode. compared E = to 1.3 that V, ofT = axial 234 αK.-protons.

Figure R. EPR spectrum of the cation radical of phenyl silatrane. Pt microplate electrode. FigureMicrowave 34: EPR frequency spectrum f = 9.46 of theGHz; cation modulation radical amplitude of phenylsilatrane. a = 0.25 G. T = Pt 223 microplate K. E = 0.7 electrode. V. Microwave frequency f = 9.46 GHz; modulation amplitude a = 0.25 G. T = 223 K. E = 0.7 V. EPR spectrum of the cation radical of phenyl silatrane (figure R) has a large end-to-end width (~160 G) and consists of more than 184 well-resolved lines. Given its large spectral span, theThe high oxidation field part ofof p-thismethoxyphenylsilatrane spectrum shows some distortion (Figure by 35) second under-order similar effects, conditions which makes also re- sultedit difficult in an EPR to reconstruct spectrum it of with electrogenerated the simple tool cation included radical into though Bruker less Symphonia resolved package. than in the Nevertheless, the following parameters have been extracted from low-field part allowing a case of phenylsilatrane (Figure 35). The spectrum is also split into 9 groupsα with the same 99.8%-quality reconstruction of this spectrum: g = 2.0037; aN = 18.41 G; 3a Hax = 37.93 G; α β integral3a Hlat intensities = 0.23 G; and6a hasH = a1.8 similar G. This end-to-end spectrum corresponds width. These to a species facts indicate with practically similar planar spin dis- tributionN atom, in carrying these two the species.major part It of is tothe be free noted electron that density, very similar which spectra has strong were interaction observed with for the three axially-oriented -protons. The C-H bonds of the other three -protons are practically cationorthogonal radicals to the of 1-organogermatranesaxes of this molecule so [34],that their attesting interaction that inwith these magnetic two familiesfield is about of metalla- 160 tranes,times theweaker unpaired compared electron to that is of mostly axial  residing-protons. on N-atom, with relatively strong spin-orbital interactions with the elements (C–H bonds) parallel to the axes of the atrane cage. Upon the oxidation of 1-naphtylsilatrane at Pt electrode in the EPR spectroelectrochemical

58 cell, the signal of paramagnetic species-cation radicals produced by 1e at Ep = 1.25 V was observed (Figure 36). The spectrum of the cation radical of naphthylsilatrane was reconstructed

using the following parameters: g = 2.0051, hfc aN = 6.152, 3xaH−α = 3.633, 2xaH(CH2) = 2.326, ∼ aH−p = 2.595, ∼2xaH = 1.509, ∼ aH = 0.34. LW = 0.28 G. In the whole it corresponds to a radical strongly delocalized over naphthyl moiety with only small contribution from N atom and from three α-protons of the silatrane cage. Figure X. EPR spectrum of electrogenerated cation radicals of napht-Sitran. In situ oxidation in **/0.1M ** at a Pt microspiral electrode. E = 1.25 V, T = 224 K.

FigureFigure 35: F. EPR EPR spectrum spectrum of electrogenerated of electrogenerated cation radicalscation radicals of p-methoxyphenylsilatrane. of MeOPh-Sitran. Oxidation in **/0.1MOxidation ** inatCH a Pt3CN microspiral/0.1M TBAPF electrode.6 at a Pt microspiral E = 1.35 electrode.V, T = 224 E = K. 1.35 V, T = 224 K.

The spectrum of the cation radical of napht-Sitran was reconstructed using the following parameters:Oxidation g = 2.0051, of p-aminophenylsilatrane hfc aN = 6.152, 3x ina theH- cavity = 3.633, of the 2x EPRaH(CH spectrometer2) = 2.326, also ~ allowedaH-p = 2.595, ~2xaHdetecting = 1.509, the ~ electrogeneratedaH = 0.34. LW cation = 0.28 radicals. G. In The the spectrum whole (ﬁgureit corresponds 37) was then to reconstructed a radical strongly delocalizedusing the over following naphthyl parameters: moiety with g = 2.003, only aaN small= 11.73 contribution G, ao−H = 7.309from G,N aatomm−H =and 0.65 from three - protonsG, ofaNH the= 5.718silatrane G. The cage.g-factor corresponds to a purely organic radical, apparently with no contributionAnysilsilatrane from the (fig. Si atom. F) donne The coupling vrais metallatrane with only one-based nitrogen cation is present radicals which, : based9 groups of signalson … own similar ionization to that potentials of Ph- ofsilatrane aminophenyl epr… and silatranyl fragments, was assigned to the

NH2-group nitrogen. The spectrum of this cation radical is therefore, the one of the silatranyl- substituted aniline cation radical. Because comparing to the aniline, atrane fragment has higher

HOMO and needs more energy to be oxidized. Therefore, the oxidation potential Ep of p- aminophenylsilatrane has approximate value as aniline as well as similar spin EPR spectrum.

This observation is perfectly in line with the results of DFT calculations of this and NEt2- substituted compounds.

Figure 36: EPRFigure spectrum X. EPR spectrum of electrogenerated of electrogenerated cation cation radicals radicals of napht naphthylsilatrane.-Sitran. In situ a. experimental;oxidationb. Simulated. in **/0.1M Oxidation ** at a Pt inmicrospiralCH3CN electrode./0.1M TBAPF E = 1.25 V,6 atT = a 224 Pt microspiralK. electrode. E = 1.25 V, T = 224 K.

Figure F. EPR spectrum of electrogenerated cation radicals of MeOPh-Sitran. Oxidation in **/0.1M ** at a Pt microspiral electrode. E = 1.35 V, T = 224 K.

The spectrum of the cation radical of napht-Sitran was reconstructed using the following parameters: g = 2.0051, hfc aN = 6.152, 3xaH- = 3.633, 2xaH(CH2) = 2.326, ~aH-p = 2.595, ~2xaH = 1.509, ~aH = 0.34. LW = 0.28 G. In the whole it corresponds to a radical strongly delocalized over naphthyl moiety with only a small contribution from N atom and from three - protons of the silatrane cage. Anysilsilatrane (fig. F) donne vrais metallatrane-based cation radicals : 9 groups of signals … similar to that of Ph-silatrane epr…

Figure 37:Fufure EPR spectraN. EPR of spectrap-aminophenylsilatrane. of p-NH2-phenyl sila Uppertran. –Upper experimental, – experimental, lower – lower simulated. – Oxidationsimulated. in OxidationCH3CN/0.1M in **/0.1MTBAPF **6 atat aa PtPt microspiralmicrospiral electrode.electrode. EE == 0.70.7 V,V, TT == 234234 K.K.

Oxidation of p-NH2-phenyl silatrane in the cavity of the EPR spectrometer allowed detecting the electrogenerated cation radicals. The spectrum (figure N) was then reconstructed using the following parameters: g = 2.003, aN = 11.73 G, ao-H = 7.309 G, am-H = 0.65 G, aNH = 5.718 G. The g-factor corresponds to a purely60 organic radical, apparently with no contribution from the Si atom. The coupling with only one nitrogen is present which, based on own ionization potentials of aminophenyl and silatranyl fragments, was assigned to the NH2-group nitrogen. The spectrum of this cation radical is therefore, the one of the silatranyl-substituted aniline cation radical. This observation is perfectly in line with the results of DFT calculations of this and NMe2-substituted compounds.

Figure Y. Solvent-subtracted UV spectra of t-Bu-silatrane (bleu) and of its electrogenerated cation radical (black). DME/AN (1:2 v/v) + 0.1 M TBAPF6. Ew = 1.2 V, Pt micro grid electrode. T = 270 K. Figure 38: HOMO and HOMO-1 of diethylanilinesilatrane by DFT B3LYP/Lanl2DZ calculations.

5.1.5 UV-vis spectroscopy

Voltammetric stability of the electrogenerated cation radicals of silatranes are allowed to study their UV absorbance using in situ UV-vis spectroelectrochemistry the spectrum of neutral t- butylsilatrane and of its cation radicals are shown in ﬁgure 40. Compared to phenylsilatrane (Figure 39), the maximal absorbance of the cation radicals of t-butylsilatrane lies at longer wavelenths, λ = 362 nm. Donor inductive effect of three Me groups of the t-Bu substituent at Si rise the level of (C-Si) component of the 3c-4e bond thus reducing the LUMO-SOMO gap in the cation radical compared to that in the cation radical of methyl or phenyl silatrane. This effect accounts to the bathochromic shift of the maximum of UV-absorbance of this species (sf. figure U).

FigureFigure 39: Solvent-subtracted U. Solvent-subtracted UV spectraUV spectra of phenyl of phenyl silatrane silatranea. and(a) and ofits of electrogeneratedits electrogeneratedcation radical (dotted cation line,radicalb). (dottedCH3CN line,/0.1M b). TBAPFAN / 0.16 .ME TBAPFw = 1.26 V,. E Ptw = micro 1.2 V, grid Pt micro electrode. grid electrode.λmax = 295 max nm. = 295 The nm. star The shows star ashows UV transition a UV transit whichion iswhich symmetry-forbidden is symmetry-forbidden in the neutralin the neutralmolecule molecule and which and which becomes becomes allowed allowed in the cationin the cation radical. radical.

HOMO of aryl silatrane (Ar = Ph) and N-localized SOMO of its cation radical by DFT B3LYP/Lanl2DZ calculations

Fufure N. EPR spectra of p-NH2-phenyl silatran. Upper – experimental, lower – simulated. Oxidation in **/0.1M ** at a Pt microspiral electrode. E = 0.7 V, T = 234 K.

Oxidation of p-NH2-phenyl silatrane in the cavity of the EPR spectrometer allowed detecting the electrogenerated cation radicals. The spectrum (figure N) was then reconstructed using the following parameters: g = 2.003, aN = 11.73 G, ao-H = 7.309 G, am-H = 0.65 G, aNH = 5.718 G. The g-factor corresponds to a purely organic radical, apparently with no contribution from the Si atom. The coupling with only one nitrogen is present which, based on own ionization potentials of aminophenyl and silatranyl fragments, was assigned to the NH2-group nitrogen. The spectrum of this cation radical is therefore, the one of the silatranyl-substituted aniline cation radical. This observation is perfectly in line with the results of DFT calculations of this and NMe2-substituted compounds.

FigureFigure 40: Solvent-subtracted Y. Solvent-subtracted UV spectraUV spectra of t-butylsilatrane of t-Bu-silatrane (blue) (bleu) and and of its of electrogenerated its electrogeneratedcation radical (black).cation radical DME/AN (black). (1:2 DME/AN v/v) + 0.1 (1:2 M v/v) TBAPF6. + 0.1 M EwTBAPF = 1.26. E V,w Pt= 1.2 micro V, Pt grid microelectrode. grid electrode. T = 270 K.T = 270 K.

Donor inductive effect of three Me groups of the t-butylsilatrane substituent at Si rises the level of σ(C–Si) component of the 3c-4e bond thus reducing the LUMO-SOMO gap in the cation radical compared to that in the cation radical of methyl or phenyl silatrane. This effect accounts to the bathochromic shift of the maximum of UV-absorbance of this species (sf. Figure 40).

5.1.6 DFT calculations

The geometry and electron density of phenyl- and benzylsilatrane as well as their cation radicals were adjusted ﬁrst at HF/6-311G level then the structures were optimized by DFT B3LYP/Lanl2DZ level for long-term and delocalizing electronic interactions. The same level was used for NBO analysis and to check the optimized structures for the absence of imaginary vibrations. In a total agreement with the results of EPR and qualitative orbital considerations, DFT calculations reveal that the HOMO of the “proper” silatranes (i.e. those not carrying an easy to

oxidize substituent, like H2NC6H4−) is mainly built of the n(N) orbital at the atrane nitrogen (Figure 41) upon withdrawal of one electron, transforming a 3c-4e system into a 3c-3e, the localization and symmetry of the highest (singly) occupied molecular orbital is conserved:

main contribution to the SOMO still comes from the pz(N) orbital. Axial C–H bonds, parallel to the main spin-carrying orbital, participate in spin stabilization in these species and determine the speciﬁc pattern of the EPR spectra of “proper” metallatranes.

Donor inductive effect of three Me groups of the t-Bu substituent at Si rise the level of (C-Si) component of the 3c-4e bond thus reducing the LUMO-SOMO gap in the cation radical compared to that in the cation radical of methyl or phenyl silatrane. This effect accounts to the bathochromic shift of the maximum of UV-absorbance of this species (sf. figure U).

Figure U. Solvent-subtracted UV spectra of phenyl silatrane (a) and of its electrogenerated cation radical (dotted line, b). AN / 0.1 M TBAPF6. Ew = 1.2 V, Pt micro grid electrode. max = 295 nm. The star shows a UV transition which is symmetry-forbidden in the neutral molecule and which becomes allowed in the cation radical.

HOMO of aryl silatrane (Ar = Ph) and N-localized SOMO of its cation radical by DFT Figure 41:B3LYP/Lanl2DZ HOMO of calculations phenylsilatrane and N-localized SOMO of its cation radical by DFT B3LYP/Lanl2DZ calculations.

Figure 42:Figure SOMO T. SOMO of of the the cation cation radical of ofbenzyl benzylsilatrane silatrane (DFT byB3LYP/Lanl2DZ DFT B3LYP/Lanl2DZ.)

As was shown by DFT B3LYP/Lanl2DZ calculations, spin density in the cation radical of The silatranesbenzyl silatrane with benzyl- is mostly and delocalized 1-naphthyl over benzyl substituents fragment, have with only similar small HOMO part still (mainresiding contri- on N atom. The contribution of the (C-Si) orbital, playing the key role in 3c-4e bonding and in the bution from"spinn(N)-leakage" electrons) mechanism but their is clearly cation seen. radicals Supposedly, show this a quitefeature different must be typical structure for all of highest frontier orbitalbenzylic (Figure (CH2- 42)aryl) Astype issubstituents. shown in ﬁgure 42 by DFT B3LYP/Lanl2DZ calculations, spin density in the cation radical of benzylsilatrane is mostly delocalized over benzyl fragment, with only small part still residing on N atom (For HOMO of this type of molecules, see ﬁgure 32 above). The contribution of the σ(C-Si) orbital, playing the key role in 3c-4e bonding and in the "spin-leakage" mechanism is clearly seen. Supposedly, this feature must be typical for all benzylic (CH2-aryl) type substituents.

5.2 Bis-(silatranyl)alkanesFigure M. A DFT B3LYP/Lanl2DZ-optimized geometry of neutral 1,2-bis(methyl)bisilatrane. Lateral and axial views. In this chapter we will present the results of three products bis-(silatranyl)methane, bis- (silatranyl)hexane and bis-(silatranyl)octane including the cyclic voltammetry and DFT calculations analyses.

Figure K. HOMO of 1,2-bis(methyl)bisilatrane from DFT B3LYP/Lanl2DZ calculations. Axial 5c-4e bonding system is clearly seen.

5.2.1 Cyclic voltammetry

Three bis-(silatranyl)alkanes were studied in acetonitrile (CH3CN) solutions containing

0.1 M tetrabutylammonium hexaﬂuoro- phosphate (TBAPF6). Several drops of dichloromethane were added to help dissolving these products. A glassy carbon (GC) working electrode was used to carry out the voltammetry of these compounds because it gave better peak shape than Pt in this case. All these compounds of this reaction series exhibit either one or two oxidation peaks (Figure 43). The oxidation peak current has a good relation with measured scan rate 1/2 (ip/v = const.) which implies the electrooxidation reactions follow a diffusion controlled process. In the same electrolyte the chronoamperometry was measured, then this result was combined with the value of the peak current to calculate the absolute electron stoichiometry.

2 5 0

2 0 0

1 5 0 c A

µ 1 0 0

, I 5 0 a b 0

- 5 0 0 . 0 0 . 5 1 . 0 1 . 5 E , V v s . S C E

Figure 43: Cyclic voltammetry of a, bis-(silatranyl)hexane; b, bis-(silatranyl)methane; c, bis- (silatranyl)octane (C = 1 mmol/L) in CH3CN/0.1 M Bu4NPF6 (several drops of CH2Cl2 was added to help dissolving the product) at a 2 mm GC electrode. v = 1 V/s; T = 25 oC.

Table 8: Parameters of Electrochemical Oxidation of Silatranes at a GC Disk Electrode at v=1V/s

c a o b R Ep(V) Ep (V) E (V) n Ep − Ep/2 (mV) ∆Ep/∆lgV (V) α bis-(silatranyl)methane 0.85 – – 1 117.13 0.18 0.41 bis-(silatranyl)hexane 0.78 – – 1 114.63 0.08 0.21 bis-(silatranyl)octane 1.30 1.14 1.22 2 87.44 – 0.24

The cyclic voltammetry was carried out in acetonitrile, with 0.1 M of TBAPF6 as the supporting electrolyte. All the voltammograms are corrected by SCE. Transfer coefﬁcient estimated as a mean of α, from Ep − Ep/2 = 1.85RT/αnF

In this series of three compounds, the oxidation peak of bis-(silatranyl)hexane is the low- c est, at Ep = 0.78 V, and the peak is irreversible at low scan rate. Similarly the peak of bis-

64 c (silatranyl)methane is very close to bis-(silatranyl)hexane Ep = 0.85 V as well as the first peak c of bis-(silatranyl)octane (Ep = 0.79 V). If it is the atrane moiety that gets oxidized, the oxidation potential should be higher than this value (less than 1 V). Moreover the measured value of the peak current should be much larger, at such concentrations as the ones presented in the conditions, and at the respective scan rate. On the other hand, this potential corresponds to the oxidation of one of the reactants, namely triethanolamine. It is therefore credible that the peak observed at less than 1 V for the three products represents the oxidation of triethanolamine. Apart from the first peak shown on its cyclic voltammograms, the bis-(silatranyl)octane displays peaks showing a reversible reaction (Figure 43), at Eo = 1.22 V even at low scan rate. It shows a normal peak current value, compared to the previous peak, each recorded in the same conditions respectively. Bis-(silatranyl)octane displays a reversible peak even at low scan rate (v = 0.5 V/s) and has the oxidation peak (Ep − Ep/2 = 87.44 mV) (Table 8), but the apparent peak width is too large to correspond to a perfect reversible redox process. As briefly mentionned above, during the experiment, several drops of dichloromethane were added to the solution to help dissolve the compound. However, the quantity of added CH2Cl2 was somewhat different for the three compounds. Compared to acetonitrile, dichloromethane is less polar (ε = 9 vs. 37 for acetonitrile) so the voltammograms in such binary solvent are subject to higher distortion due to uncompensated ohmic drops, iR. This could lead to the observed deformation, a broadening and anodic shift of the voltammogram.

5.2.2 DFT calculations

DFT calculations were carried on with the molecule bis-(silatranyl)methane as an example. Compared to the other bis-(silatranyl)alkanes with long chains, bis-(silatranyl)methane has only one CH2 between two atrane moieties. The CH2 between the two atrane structures forms

2 σ bonds, with the Csp3 atom connected with each of the two Si atoms. Taking into account the lone electron pairs on two N atoms, this structure can be seen as two 3c-4e bonds, one on each side of the atrane molecule.

Si O

N Si C

Figure 44: Molecular orbital of bis-(silatranyl)methane lateral σ(Si–O)-σ(Si–C) conjugation.

Figure B. Geometry of 1,2-(silatranyl)acetylene obtained from DFT B3LYP/Lanl2DZ optimization.

Compared to the silicon atom, each of the two N atoms on the terminal positions and the C atom on the center are more electronegative elements. This is the right condition to form a

5c-6e bond. However, the geometry of the central CH2 means the two σ bonds form an angle between two atrane parts, instead of being aligned whereas the orbital between Si and O atoms would approach to the two H atoms from CH2 and form a W shape moleculare orbital which is demostrated in the ﬁgure 44. This is supported by the DFT result (Figure 45). Either the Figure B. Geometry of 1,2-(silatranyl)acetylene obtained from DFT B3LYP/Lanl2DZ neutraloptimization. orFigure the B. cation SOMO radical of the cation form radical shows of 1,2 clearly-(silatranyl)acetylene that the carbon from DFT atom B3LYP/Lanl2DZ connecting two atrane o forms ancalculations angle with. Two the 3c two-4e bonding silicon fragments atoms. are This overlapping angle ∠ Si–C–Si’through the ≈-component118 (Table of C 9).C bridge. Note that none of acetylene -orbitals is involved.

Figure B. SOMO of the cation radical of 1,2-(silatranyl)acetylene from DFT B3LYP/Lanl2DZ calculations. Two 3c-4e bonding fragments are overlapping through the -component of CC bridge. Note that none of acetylene -orbitals is involved.

Figure D. Geometry of neutral (left panel) and cation radical (right panel) of 1,2- Figure(silatranyl)methane 45: Geometry optimized of neutral by DFT (left B3LYP/Lanl2DZ. panel) and cation radical (right panel) of bis- (silatranyl)methane optimized by DFT B3LYP/Lanl2DZ

The result ofthe DFT calculations of the SOMO of bis-(silatranyl)methane when it forms a cation radical is shown ﬁgure 46. It is clearly seen that the two parts of the atrane molecule present 2 independent 3c-4e hypervalent bonds instead of one 5c-6e bond. From all the evidences and analyses above, it can be concluded that the hypervalent structure of bis-(silatranyl)methane is established by 2 3c-4e bonds, and not 5c-6e bond, because sp3 hybridization of the central carbon atom and the attraction force between the central hydrogen atom CH2 and the molecu- Figure D. Geometry of neutral (left panel) and cation radical (right panel) of 1,2- lar orbital(silatranyl)methane between Si and optimized O. by DFT B3LYP/Lanl2DZ.

Figure 46: SOMO of the cation radical of bis-(silatranyl)methane from DFT B3LYP/Lanl2DZ calculations. Two crossing 3c-4e moieties are well seen.

66 Table 9: Selected geometrical parameters (bond lengths in Å, angles in degreeso) and charges in bis-(silatranyl)methane and its cation radical according to DFT B3LYP/Lanl2DZ calculations.

Mulliken (NBO)charges l(Si–N) l(Si–C) Si–C–Si’ ∠ N Si C O O’ Neutral 2.708 1.862 118.43 -0.219 1.623 -1.297 -0.616 -0.625 2.737 1.863 -0.212 1.656 -0.612 -0.624 -0.605 -0.602 Cation 3.031 1.853 118.44 -0.094 1.629 -1.273 -0.619 -0.628 radical 3.046 1.853 -0.089 1.646 -0.636 -0.638 -0.604 -0.600

Table 10: Selected geometrical parameters (bond lengths in Å, angles in degreeso) and charges in bis-(silatranyl)acetylene and its cation radical according to DFT B3LYP/Lanl2DZ calculations.

Mulliken (NBO)charges l(Si–N) l(Si–C) l(C≡C) ∠Si–N–Si’ N Si Csp O O’ 67 Neutral 2.669 1.839 1.204 180 -0.839 2.093 -0.408 -0.936 -0.936 2.668 1.839 -0.839 2.093 -0.408 -0.951 -0.964 -0.964 -0.951 Cation 3.008 1.814 1.229 180 -0.092 1.582 -0.342 -0.628 -0.615 radical 3.008 1.814 -0.092 1.582 -0.342 -0.615 -0.628 -0.599 -0.599

Table 11: Selected geometrical parameters (bond lengths in Å, angles in degreeso) and charges in di(methyl)bisilatrane and its cation radical according to DFT B3LYP/Lanl2DZ calculations.

Mulliken (NBO)charges l(Si–N)a l(Si–C ) l(N–C) Si–N–Si’ P( C–N–C’) O–Si–O’ Me ∠ ∠ ∠ N Si C(Me) O Neutral 3.026 1.850 1.471 180 360 108.17 -0.807 2.374 -1.027 -1.015 3.027 1.850 (-0.004) (2.359) (-1.073) (-0.996) Cation 3.025 1.850 1.471 180 360 108.17 -0.423 2.321 -1.015 -0.984 radical 3.027 1.850 (-0.037) (2.321) (-1.065) (-0.975)

aTotal Si↔Si separation is 6.054 Å in both species. 5.2.3 6 center-8 electron hypervalent bonding

Both bis-(silatranyl)hexane and bis-(silatranyl)octane have long alkyl chains linking the two atrane moieties. This leads us to think that the ﬂexibility displayed by such a long chain would allow two atrane moieties to approach each other when it forms the cation radicals and estab- lish a new hypervalent bonding: 6c-7e. However, for these two compounds, even the length of (CH2)8 chain is not enough to allow the required axial arrangement of the two terminal silatranyl groups, which ﬁnally form two independent 3c-4e bonds (Figure 47). Probably, if the number of carbons on the alkyl chain was higher than 12, such interamolecular arrangment could be observed (Figure 48).

O O N Si N O O O N Si N O

O Si O O Si O O O

(CH2)6 (CH2)8

Figure 47: Two independent 3c-4e bonds in molecule bis-(silatranyl)hexane and bis- (silatranyl)octane.

-e O O O X Si N N Si X 6c-7e

O O O δ− δ+

Figure 48: 6c-7e hypervalent bonding in cation radicals when number of carbon on the alkyl chain is more than 12.

5.3 Bi-silatranes

In the previous section we saw the hypervalent bonding in bis(silatranyl)methane. In fact, in classical 5c-6e bond, the central and terminal ligands are highly electronegative with respect to Si. In the case of bis(silatranyl)methane, these positions (the central carbon atom and two

68 terminal atoms N) are indeed occupied by more electronegative elements. In this section, a series of compounds with two merged silatrane structures were also synthesized, different from bis(silatranyl)methane. In a way, nitrogen atom is now the central atom in the 5c-6e bond (perfectly linear in these molecules), and the two carbon atoms from the substituents act as the terminal atoms. The electrochemical behavior of these new compounds will be discussed in the following section.

5.3.1 Structure

Two novel structures of bi-silatranes were obtained and analyzed by XRD. To complete our research, a known structure di(phenyl)-bi-silatrane was synthesized as well but its structure is not shown in this section. We started by characterizing the structure of the newly prepared di(ethyl)- and di(benzyl)- bi-silatranes. Their molecular structures are shown in ﬁgures 50 and ﬁgure 52, the crystallographic data is listed in table 12 and the selected bond lengths and angles are listed in table 13.

Table 12: Crystallographic data and details of structure reﬁnement for di(benzyl)-bi-silatrane

Crystal di(benzyl)-bi-silatrane

Empirical formula C23H29NO6Si2 Formula weight 471.65 Crystal system Monoclinic

Space group P 21/c a (Å) 12.3388(3) b (Å) 10.6408(2) c (Å) 18.0121(4) α(o) β(o) 99.0570(10) γ(o) 3 V (Å ) 2335.41(9) Z 4 3 Dcalcd(Mg/m ) 1.341 Radiation (MoKα) λ = 0.71073Å T (K) 150 θRange (o) 2.99–27.47 Total reflections 17784 Unique reflections 5275 Parameters 319 a R1, wR2 (I > 2σ(I)) 0.0563, 0.1465 Goodness-of-fit on F 2 1.055 a P P P 2 2 2 P 2 2 1/2 R1 = ||F0| − |Fc||/ |F0|; wR2 = { [w(F0 − Fc ) ]/ [w(F0 ) ]}

69 The structure of bi-silatranes can be seen in two ways. First, two atrane parts can be considered like two distorted bipyramids with two Si atoms in the body center with three O atoms, one C atom from the substituent and the sharing N atom on the vertexes (Figure 49 a). Second, bi-silatrane can be seen as one distorted bipyramid formed by N atom in the body center with three adjacent C atoms and two Si atoms on the vertexes (Figure 49 b).

Si N Si Si N Si

a b

Figure 49: Structure of bi-silatranes a. two distorted bypyramids with two Si atoms in the body center; b. one non-distorted bipyramid with N atom in the body center.

Notably, in the structure of bi-sialtrane, the nitrogen atom is located on the plane formed by the three carbon atoms in α-positions to the nitrogen atom (as can be seen since the sum of these C–N–C angles is 360◦). This is very different from the classic silatranes that we studied until now where nitrogen in endo-conﬁguration always kept some tetrahedrality due to its sp3 hybidization. Both silatrane moieties in this molecule are perfectly symmetric in relation to a plane containing the central N atom and the 3 carbon atoms around it. The Si1–N–Si2 axes is close to linear (Table 13) in both compounds.

Table 13: Main geometrical parameters for di(ethyl)-bi-silatrane and di(benzyl)-bi-silatrane

Compound d(Si ··· N) (Å) d(Si–C) (Å) d(Si–O) (Å) 1.583(3) 1.643(9) 2.9766 1.985(1) di(ethyl)-bi-silatrane 1.643(7) 1.712(2) 2.8408 1.964(4) 1.656(9) 1.600(6) 1.617(2) 1.576(3) 2.9214 1.860(2) di(benzyl)-bi-silatrane 1.621(3) 1.649(2) 2.8996 1.858(2) 1.665(2) 1.668(3)

o o ∠N–Si–C ( ) ∠Si1–N–Si2 ( ) 169.927 176.01 169.362 179,486 179.91 178,427

From these two ways it is obvious that all three atoms Si1, N and Si2 are contorted to an extracoordinating geometry so we can analyze its structure like coordination complex. Appar- ently being the center atom in a pentacoordination the atom should be electron pair acceptor.

70 However in the second propose of pentacoordination structure (Figure 49 b) that N atom being the center atom of pentacoordinating molecule is not very suitable, thus it is not very interesting for us. On the other hand, being pentacoordinating center Si atom is perfect because not only it is electron pair acceptor but also in ﬁrst structure propose (Figure 49 a) the atoms on the vertexes are more electronegative elements. The substituent of the C atom is positioned on the axial positions, three oxygen atoms at each atrane structure are occupying the equatorial sites and the two Si atoms are in the body centers of each of two pyramidal bodies. This proposal has similar geometry as simple silatrane structure.

For the di(ethyl)-bi-silatrane, the complete crystallographic data could not be obtained suc- cessfully since there are more than one molecular structure in one unit lattice - in fact, twelve - and they all point to different directions (Figure 53). This would be caused by the conforma- tionnally ﬂuctuating ethyl substituent on the both sides of this molecule (Figure 51). The Si–C σ bonds of the ethyl fragments allows the rotation about it leading to different orientations of substituents and cause a variety of molecular structures with different mutual orientations of the Et groups (Figure 53).

Figure 50: ORTEP drawing at the molecular structure of di(ethyl)-bi-silatrane. H atoms are omitted for clarity.

The silicon atom (Si1 and Si2 from figure 50) is displaced from the plane established by three oxygen atoms towards the carbon atom (C2 and C12 from figure 50) on the apical substituents. The distance between N atom and two Si atoms is practically the same on the both sides though there is a slight difference of 0.1358 (Å) (4.5%) for di(ethyl)-bi-silatrane. This difference does not seem meaningful in terms of pentacoordination of N but rather relates to the inhomogeneity caused by crystal field during the package of the elementary blocks.

Figure I. SOMO of 1,2-bis(methyl)bisilatrane. Left: one-electron populated (-orbital) part; right: empty level (-orbital). DFT UB3LYP/Lanl2DZ calculations.

Unrestricted mode of consideration of the electron configuration of 1,2-bis(methyl)bisilatrane allows one to visualize that main contribution to the HOMO of this molecule is constituted of n- electrons of N atom: after the electron removal, its empty part (seen as -orbital, figure I) is mostly on N pz-orbital, whereas remaining unpaired electron (seen as -orbital) is delocalized over the axial 5c-6e system (which became 5c-5e in the cation radical). A remarkable participation of parallel elements of the bi-atrane cage (C-C and C-O bonds -orbitals) is also seen. O O

Si N Si

O O O O

Figure 51: Conformational ambiguity of di(ethyl)-bi-sialtrane depending on the orientation of ethyl substituents.

Figure 52: ORTEP drawing of the molecular structure of di(benzyl)-bi-silatrane. Thermal ellipsoids are drawn with 50% probability.

Figure 53: A unit cell molecular structure of di(ethyl)-bi-silatrane.

The crystallographic data for di(benzyl)-bi-silatrane is listed in table 13. It has the similar structure on the atrane part. The difference of the distances between N1 to Si1 and to Si2 is 0.0218 Å (0.7%) which is not large enough to be meaningful so we can consider the atom N1 is in the middle of Si1 and Si2.

5.3.2Fig DFTure T. calculationsSOMO of the cation radical of benzyl silatrane (DFT B3LYP/Lanl2DZ )

The geometryAsFig wasure of T. theshown SOMO bi-silatranes by ofDFT the B3LYP/Lanl2DZ cation as radical well of as benzylcalculations, their silatrane cation spin ( radicalsDFT density B3LYP/Lanl2DZ in were the cation optimized radical) of using by a benzyl silatrane is mostly delocalized over benzyl fragment, with only small part still residing on combinedN atom. treatment: The contributionAs was ﬁrstly, shown of geometricthe by  DFT(C-Si) B3LYP/Lanl2DZ orbital, parameters playing were calculations,the key done role atinspin 3c HF/6-311G -density4e bonding in the and level, cation in the andradical then of the "spin-leakage"benzyl silatranemechanism is mostly is clearly delocalized seen. Supposedly, over benzyl this fragment, feature must with beonly typical small for part all still residing on structures and NBO analysis were optimized at the DFT B3LYP/LANL2DZ level. benzylicN (CH atom.2-aryl) The typecontribution substituents. of the (C-Si) orbital, playing the key role in 3c-4e bonding and in the "spin-leakage" mechanism is clearly seen. Supposedly, this feature must be typical for all benzylic (CH2-aryl) type substituents.

FigureFigure 54: M. A A DFT DFT B3LYP/Lanl2DZ-optimizedB3LYP/Lanl2DZ-optimized geometry geometry of neutral of1,2- neutralbis(methyl)bisilatrane. di(methyl)-bi-silatrane. Lateral and axial views. Lateral andFigure axial M. views. A DFT B3LYP/Lanl2DZ-optimized geometry of neutral 1,2-bis(methyl)bisilatrane. Lateral and axial views.

Figure K. HOMO of 1,2-bis(methyl)bisilatrane from DFT B3LYP/Lanl2DZ calculations. Axial 5c-4e bonding system is clearly seen. Figure 55: HOMOFigure K. ofHOMO di(methyl)-bi-silatrane of 1,2-bis(methyl)bisilatrane from from DFT DFT B3LYP/Lanl2DZ B3LYP/Lanl2DZ calculations. calculations. Axial Axial 5c-4e bonding system is clearly seen. 5c-4e bonding system is clearly seen.

Figure 56:Figure SOMO I. SOMO of di(methyl)-bi-silatrane.of 1,2-bis(methyl)bisilatrane. Left: Left: one one-electron-electron populated populated (-orbital) (α part;-orbital) right: part; empty level (-orbital). DFT UB3LYP/Lanl2DZ calculations. right: empty level (β-orbital). DFT UB3LYP/Lanl2DZ calculations. Unrestricted mode of consideration of the electron configuration of 1,2-bis(methyl)bisilatrane allows one to visualize that main contribution to the HOMO of this molecule is constituted of n- Unrestrictedelectrons modeof N atom: of consideration after the electron of removal, the electron its empty conﬁguration part (seen as -orbital, of di(methyl)-bi-silatrane figure I) is allows onemostly to visualize on N pz-orbital, that mainwhereas contribution remaining unpaired to the electron HOMO (seen of as this -orbital) molecule is delocalized is constituted of over the axial 5c-6e system (which became 5c-5e in the cation radical). A remarkable n-electronsparticipation of N atom: of parallel after elements the electron of the removal,bi-atrane cage its (C empty-C and partC-O (seenbonds  as-orbitals)β-orbital, is also ﬁgure 56) is mostlyseen. on N p -orbital, whereas remaining unpaired electron (seen as α-orbital) is delocal- z ized over the axial 5c-6e system (which became 5c-5e in the cation radical). A remarkable

participation of parallel elements of the bi-atrane cage (C–C and C–O bonds σ-orbital) is also seen.

5.3.3 Cyclic voltammetry of bi-silatranes and of the model tris(1,3-dihydroxy-2-propyl)- amine

For a better understanding of the electrochemical oxidation of the bi-silatranes, we carried out a series of cyclic voltammetry experiments on these bi-silatranes, and compared these results to the precursor tris(1,3-dihydroxy-2-propyl)amine. The cyclic voltammetry was carried out on the glassy carbon electrode either in acetonitrile (for bi-silatranes) containing 0.1M of

Bu4NPF6 or in the mixture of DMF and acetonitrile (for tris(1,3-dihydroxy-2-propyl)amine) containing 0.1M of Bu4NPF6. The peak currents for all four compounds are linear with 1/2 the square root of the scan rate (ip/v = const.), thus suggesting diffusional control of the process. The electron stoichiometry was determined by combining voltammetry parameter 1/2 ip/v with the Cottrell slope obtained from chronoamprometry at the same electronde and the same solution. The electrooxidation is proceeding by one-electron transfers in all four cases (Table 14). As is shown in ﬁgure 58, di(ethyl)- and di(phenyl)-bi-silatranes reveal a distinct reversible electrooxidation step even at low scan rate (v = 0.5 V/s). Di(ethyl)-bi-silatrane has an oxidation peak at 1.62 V (vs. SCE) with the cathodic counterpeak at E = 1.49 V (vs. SCE). Di(phenyl)- bi-silatrane has an electrooxidation peak at 1.68 V (vs. SCE) and cathodic peak at 1.59 V (vs.

SCE). The width (∆E = Ep − Ep/2 ≈ 65 − 80 mV) for these three compounds is somewhat larger than 58 mV, the theoretical value for an electrochemically reversible electron transfer.

For di(benzyl)- and di(ethyl)-bi-silatranes, the Ep increases with the scan rate slightly more than expected for a ﬁrst-order electrochemical follow up process of the cation radicals (ﬁrst- order increase with the slope of 30 mV per decade of the scan rate would be expected). The oxidation peak width of di(phenyl)-bi-silatrane is the smallest of the three compounds (Table

14) which implies that it is closer to a pure Nernstian system. Moreover, the ∆Ep/∆lg(v) slope is close to 20 mV instead of 30 mV which seems to reﬂect a reversible electron transfer and meaning that the cation radicals of this compound undergo a fast second-order reaction in contrast to the other compounds of this reaction series. Both di(ethyl)-bi-silatrane and di(phenyl)-bi-silatrane have reversible oxidation peak even at low scan rate (v = 0.5 V/s) which attests a good stability of the cation radicals. There is a slight difference between di(ethyl)-bi-silatrane and di(phenyl)-bi-silatrane in Ep. This slight difference is caused by the electronic properties of the substituents on two sides of bi-silatrane.

The Csp2 linking atom from phenyl has higher value of σI which makes it more electron- withdrawing as an electron acceptor than ethyl-substituent which has sp3 hybridization of linking C atom. This slight difference on the hybirdization was reﬂected in the peak potentials:

74 di(phenyl)-bi-silatrane has 60 mV higher Ep compared to di(ethyl)-bi-silatrane (Ep = 1.62 V).

5c-6e

σσ

H2C Si N Si H2C

ππ

Figure 57: Hyperconjugation effect in di(benzyl)-bi-silatrane

The oxidation peak of di(benzyl)-bi-silatrane is irreversible even at high scan rate (> 5 V/s) which means the cation radical of di(benzyl)-bi-silatrane is unstable at least at room c temperature. It has the lowest oxidation value (Ep = 1.45 V vs. SCE) in all three bi-silatranes which can be explained from two aspects. First of all, both sides of this molecule have the electron donating substituents which for the reasons discussed above for benzyl derivatives in metallatrane series, can manifest some of its π-donnor ability through hyperconjugative mechanism. In fact, similar to the 1-naphthylmethyl silatrane, the σ(C–Si) fragment from the 3c-4e bond and π-system from phenyl are boned by hyperconjugation. At the stage of the cation radical, this effect can cause the spin delocalization from N atom to the π-system of phenyl (Figure 57). Along with the increase HOMO energy (thermaldynamic facilitation of oxidation by lowering the Eo), the increased reactivity of electrogenerated cation radicals also contribute to the lowering of the apparent oxidation Ep through the kinetic shift of Ep with respect to Eo. Based on these two reasons, di(benzyl)-bi-silatrane has the lowest Ep = 1.45 V in three bi-silatranes.

Table 14: Cyclic voltametry data for the oxidation of bi-sialtranes. All samples are taken at v = 1V/s and potentials are corrected to SCE

c a o Compound Ep(V) Ep (V) E (V) n Ep − Ep/2(mV) Tris(1,3-dihydroxy-2-propyl)amine 0.84 0.69 0.76 1 72 di(benzyl)-bi-silatrane 1.45 – – 1 81 di(ethyl)-bi-silatrane 1.62 1.49 1.55 1 81 di(phenyl)-bi-silatrane 1.68 1.59 1.63 1 67

a ∆Ep/∆lgV (mV) α 34 0.7 33 0.6 36 0.6 18 0.7

75 a Transfer coefﬁcient estimated as a mean of α from Ep − Ep/2 = 1.85RT/αnF .

a b c d 1 0 0 1 0 0

5 0 A A µ

5 0

µ ,

I , I 0

0 - 5 0

0 . 8 1 . 0 1 . 2 1 . 4 1 . 6 1 . 8 0 . 8 1 . 0 1 . 2 1 . 4 1 . 6 1 . 8 E , V v s S C E E , V v s . S C E

Figure 58: Cyclic voltammetry of a, tris(1,3-dihydroxy-2-propyl)amine (C = 0.1 mmol/L) in CH3CN/DMF, 4:1 v/v 0.1M Bu4NPF6 at a 2 mm GC electrode; b, di(ethyl)-bi-silatrane; c, di(benzyl)-bi-silatrane; d, di(phenyl)-bi-silatrane (C = 0.1 mmol/L) in CH3CN/0.1M o Bu4NPF6 at a 2 mm GC electrode. v = 1 V/s; T = 25 C.

2 0 0

1 5 0

1 0 0 A µ

, I

5 0

0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 1 . 6 1 . 8 2 . 0 E , V v s . S C E

Figure 59: Cyclic voltammetry of the di(benzyl)-bi-silatrane (C = 0.1 mmol/L) in o CH3CN/0.1M Bu4NPF6 at a 2 mm GC electrode. v = 1 V/s; T = 25 C.

Thus one-electron oxidation of bi-silatranes results in the corresponding silatrane cation radicals. Potential-determining deprotonation of the cation radicals of di(benzyl)- and di(ethyl)- bi-silatranes can be limited at relatively slow scan rates, which creates favorable conditions for stabilizing these species at low temperatures and studying them by EPR spectroscopy. Cation radicals of di(phenyl)-bi-silatrane seem to follow a bimolecular way of reactivity, the feature more typical for delocalized or low energy intermediates.

1 2 0 0

1 0 0 0

8 0 0

6 0 0 A µ

, I 4 0 0

2 0 0

- 2 0 0 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 1 . 6 E , V v s . S C E

Figure 60: Cyclic voltammetry of di(benzyl)-bi-silatrane at a 2 mm GC electrode. v = 10 V/s; T = 25 oC.

For comparison, the voltammogram of tris(1,3-dihydroxy-2-propyl)amine was recorded under the similar conditions as the three bi-silatranes. The only difference was that the solvent was mixed with 20% DMF to help dissolving the tris(1,3-dihydroxy-2-propyl)amine completely. Obviously the molecule has only one atom which can be directly involved in electrooxidation (the nitrogen). In this sense, all these compounds have similar reaction center of electron transfer. It is seen (Figure 58) that bi-silatranes have 0.61-0.84 V higher oxidation potentials than tris(1,3-dihydroxy-2-propyl)amine. At the same time the oxidation potentials of the bi-silatranes are higher than those of homological simple silatranes: for instance, di(phenyl)-bi-silatrane has Ep = 1.68 V whereas tolylsilatrane and phenylsilatrane have oxidation peak potentials at 1.35 V and 1.55 V [5], respectively; similarly di(ethyl)-bi-silatrane has 200∼400 mV more anodic value (Ep = 1.62 V) comparing to t-butylsilatrane (Ep = 1.22

V) or ethylsilatrane (Ep = 1.42 V) [5]. This anodic shift would be correlated with the specific position of nitrogen atom. As the molecules are symmetric, N atom is located in the center of bi-silatrane. Unlike the structure of simple silatrane, N atom in bi-silatrane was on the plane of three carbon atoms on the α-position. From the point of view of electronic effects, there are two acceptors on both sides of N atom forming two axial bonds with two Si atoms and three 2p orbital of N atom as well as three equatorial σ-bonds from sp3 hybridized with carbon atoms on the α-position (Figure 61). N atom is forced to have a planar configuration so as its pz orbital (4-electrons) is equally available for donor-acceptor interaction with both Si-fragments. As a result, its energy is remarkably lowered, which is reflected in the increased oxidation potentials.

77 C σ sp3

Si N Si σ 3 σ sp sp3 C C

Figure 61: Bonding structure of N atom in bi-silatranes

5.4 1-R-Stannatranes

From the previous studies of metallatranes by various physical chemical methods there follows that dative N→M interactions are stronger when going down the group 14, in the order Si < Ge < Sn [43]. It is expected more than the availability of the lone pair of nitrogen must decrease in the same order and hence the electron withdrawal in the homological metallatranes must become more and more difﬁcult. No data on electrochemistry of stannatranes was reported so far, so we considered the oxidation of several selected stannatranes under the same conditions as described above.

5.4.1 Cyclic voltammetry

To obtain an insight into the intermolecular electronic interactions in the new stannatranes we worked with, their electrochemical behavior was considered. Depending on their solubility, cyclic voltammetry of compounds 12, 13, 14 and 15 (Figure 62) was carried out in acetonitrile

(CH3CN), CH2Cl2 or in a binary mixture (CH3CN/CH2Cl2, 1:1 v/v) containing Bu4NPF6 (0.1 M) as supporting salt. Contrarily to silatranes [74, 5] and germatranes [33], the stannatranes 12 and 13 show distinct oxidation signals only at a Pt electrode. The limiting currents ip of stannatranes 12 and 1/2 13 are both diffusion controlled (ip/v =const; lg(ip)/lg(v) = 0.45 and 0.46 for 12 and 13, 1/2 respectively) and, as was shown using ip/v ratio along with Cottrell slope from chronoamperometry at the same electrode [40], the number of electrons transferred at this step is n = 1 1 (Table 15). Temperature dependence of lg(ip) of 13 provides Ea = 2.46 kJ/mol (2.85 kJ/mol for second peak), corresponding to the activation energy of the diffusion ﬂow of solvent. The peak half-widths Ep − Ep/2 of 12 and 13 are too large for simple one-electron reversible processes, supposedly because of the adsorption interactions or substantial structural reorganization accompanying electron transfer.

In a less polar solvent such as CH2Cl2, the oxidation of 13 proceeds via two steps (Figure

78 63), with the second one being reversible already at v = 0.5 V/s. With the vertex potential set before the onset of the second oxidation peak (Ev = 1.45 V), the ﬁrst peak also starts showing cathodic counterpart (at v > 50 V/s). Thus, both oxidation signals arise from electrochemically reversible processes.

R: 12, s N 13, O Sn O O 14, Cl R 15, I

Figure 62: 1-R-stannatranes 12, 13, 14 and 15

Table 15: Parameters of electrochemical oxidation of stannatranes 12, 13, 14 and 15 at a platinum disk electrode

a Cpd Solvent Ep, V Ep − Ep/2, V ∆Ep/lg(v), mV n IP, eV 12 CH3CN 1.47 0.171 30 0.94 5.569 12 CH3CN/CH2Cl2 1.46 0.115 48 0.94 12 CH2Cl2 1.33 0.127 43 0.97 13 CH3CN 1.41 0.119 25 1.40 5.959 d 13 CH3CN 1.41(1.47) 0.098 49 1.18 13 CH3CN/CH2Cl2 1.40(1.53) 0.082 58 1.03 14 CH2Cl2 >2.2 7.338 15 CH2Cl2 >2.3

b c Cpd Solvent α ∆Ep/lg(v), mV 12 CH3CN 0.3 23 12 CH3CN/CH2Cl2 0.4 19 12 CH2Cl2 0.4 17 13 CH3CN 0.4 22 13 CH3CN 0.5 25 13 CH3CN/CH2Cl2 0.6 26 a b Peak potentials at v = 1 V/s. Formal transfer coefﬁcient, found from (Ep − Ep/2)/1.85 = RT/αF. c Two peaks merged. d Second of two peaks.

1 The i-t curve with the hold at Ep (Figure 63) allowed to subtract the diffusion limited

79 1 current of the ﬁrst peak ip (the baseline for the second peak [66] and to quantify the current 2 of the second oxidation step, ip. The latter was shown to have an electron stoichiometry of n = 0.4, caused by self-reactions involving cation radicals, as it was observed in the case of

oxidation of Ar2S or ArSM [81] with the HOMO localized on the ArS fragment. Thus, the

similarity in the oxidation pattern of 13 and of aryl sulﬁdes suggests that the p-CH3C6H4S fragment in 13 is most probably the reaction site that accounts for the ﬁrst oxidation peak. The

Ep of compound 13 is about 150-200 mV higher than those of diaryl or arylalkyl sulﬁdes [82], due to the electron acceptor effect of the stannatranyl substituent. The possibility of oxidation 2 of the stannatranyl moiety at Ep should obviously be ruled out because not only the HOMO but also the lower lying HOMO-1 in 13 is mostly built of sulfur atom orbital.

Moreover, considering orbital energies (as IP = −εHOMO in Koopman’s approach, by B3LYP/LANL2DZ) of HOMO-1 in 13 (6.655 eV) and of SOMO in 13+• (6.094 eV), it is more probable that a second electron withdrawal affects the cation radical, like in the case of ox aryl sulﬁdes. In contrast to the trend in Ep of diaryl and arylalkyl thioethers, which are easier

to oxidize than parent ethers [82], the order of Ep values for oxygen- and sulfur-containing stannatranyl derivatives is reversed (Table 15). This is supposedly because of stronger Sn–S

versus Sn–O bonding and a stronger acceptor effect of the stannatranyl moiety on the Ep of 13

compared to the Ep of 12. Inorganic Chemistry Article

− − −3 −1 Figure 14. Cyclic voltammograms for the oxidation of stannatranes. Left: (a) 4 and (b) 8 (10 mol L )inCH3CN/0.1 M Bu4NPF6 at a Pt − × −3 −1 − disk electrode;Figurev = 5 V/s. 63: Right: CyclicI E curve voltammograms of 8 (1.1 10 formol the L )inCH oxidation2Cl2/0.1 of stannatranes. M Bu4NPF6 at a Pt Left: disk electrode. (a) − 12 (a)and normal (b) scan; (b) I t −1 3 −1 ° curve after the− hold13 at(10Ep . (c)mol/L) diffusion in limitingCH3CN current/0.1 of M theBu second4NPF oxidation6 at a Pt step. diskv = electrode; 0.5 V s ; Tv=22= 5C. V/s. Right: I−E −3 curve of 13 (1.1 × 10 mol/L) in CH2Cl2/0.1 M Bu4NPF6 at a Pt disk electrode. (a) normal Contrary to silatranes44,45 and germatranes,46 the stanna- to oxidize than parent ethers,50 the order of E values for scan; (b) I−t curve after the hold at E . (c) diffusion limiting current of the second oxidation p tranes 4 and 8 show distinct oxidation signals only atp a Pt oxygen- and sulfur-containing stannatranyl derivatives is reversed step. v = 0.5 V s−1; T = 22 °C. − electrode. The limiting currents ip of stannatranes 4 and 8 are (Table 7). This is supposedly because of stronger Sn S versus 1/2 − both diffusion controlled (ip/v = const; lg(ip)/lg(v) = 0.45 Sn O bonding and a stronger acceptor effect of the stanna- and 0.46 for 4 and 8, respectively) and, as was shown using tranyl moiety on the Ep of 8 compared to the Ep of 4. 1/2 The stannatranes 14 and 15 did not show any distinct oxidation peaks up to the media lim- ip/v ratio along with Cottrell slope from chronoamperometry The stannatranes 13 and 15 did not show any distinct oxi- 47 at the same electrode,its. As wasthe pointed number out of by electrons Broka transferredet al. [5],the at chloro-substituteddation peaks up silatrane to the mediaN(CH limits.2CH As2O) was3SiCl pointed out by 1 45 this step is n = 1 (Table 7). Temperature dependence of lg(ip ) Broka et al., the chlorido-substituted silatrane N(CH2CH2O)3- of 8 provides Ea = 2.46 kJ/mol (2.85 kJ/mol for second peak), 80 SiCl cannot be oxidized. No such data exist for the chlorido- corresponding to the activation energy of the diffusion flow substituted germatrane N(CH2CH2O)3GeCl but one can − of solvent. The peak half-widths Ep Ep/2 of 4 and 8 are too expect even higher Ep since germatranes are more difficult to 46,51 large for simple one-electron reversible processes, supposedly oxidize than silatranes. From a simple correlation of Ep with because of the adsorption interactions or substantial structural calculated IPs of the stannatranes 4, 8, and 13, the oxidation reorganization accompanying electron transfer. potential of the chlorido-substituted derivative 13 should be In a less polar solvent such as CH2Cl2, the oxidation of 8 above 2.3 V which corroborates the results of its voltammetry. proceeds via two steps (Figure 14), with the second one being On the contrary, compound 13 shows a well-shaped reduc- −1 − reversible already at v = 0.5 V s . With the vertex potential set tion peak at Ep = 1.5 V; the reduction process results in − ∼ before the onset of the second oxidation peak (Ev = 1.45 V), elimination of Cl anions whose oxidation peak at 1Vis the first peak also starts showing cathodic counterpart (at v > detected by voltammetry during the second cycle. The peak at 50 V/s). Thus, both oxidation signals arise from electro- −1.5 V falls into the range of reduction potentials of known chemically reversible processes. The i−t curve with the hold at chloridostannanes52 and might have arisen from the dissociative 1 − Ep (Figure 14) allowed to subtract the diffusion limited current reduction of Sn Cl bond in 13. Similarly, the oxidation peak of 1 48 − of the first peak ip (the baseline for the second peak and to the iodide anion I appears after the reduction of 15. This is in 2 quantify the current of the second oxidation step, ip . The latter agreement with the conductivity measurements of 13 in CH3CN/ was shown to have an electron stoichiometry of n = 0.4, caused CH2Cl2 (50:50 v/v), which have only shown residual solvent by self-reactions involving cation radicals, as it was observed in conductance and no contribution from any ionic conductivity 49 − the case of oxidation of Ar2S or ArSM with the HOMO due to the possible Sn Cl dissociation prior to the reduction. localized on the ArS fragment. Thus, the similarity in the oxi- A practically identical conductivity curve was observed for the dation pattern of 8 and aryl sulfides suggests that the p-CH3C6H4S phenolato-substituted stannatrane 4. Therefore the chlorido- fragment in 8 is most probably the reaction site that accounts substituted stannatrane 13, just as 4, remains a covalent com- for the first oxidation peak. The Ep of compound 8 is about pound in solution, at least in this media. 150−200 mV higher than those of diaryl or arylalkyl sulfides,50 EPR-Spectroelectrochemistry. Since the oxidation of the due to the electron acceptor effect of the stannatranyl sub- phenolato-substituted stannatrane 4 shows partial reversibility stituent. The possibility of oxidation of the stannatranyl moiety (Figure 14), it was studied by real-time EPR-coupled electro- 2 −3 −1 at Ep should obviously be ruled out because not only the chemistry. The solution of 4 (10 mol L ) in AN/0.1 M − HOMO but also the lower lying HOMO 1in8 is mostly built Bu4NPF6, initially ERP-silent, has shown the signal of para- of sulfur atom orbitals. Moreover, considering orbital energies magnetic species when the potential 1.23 V was applied (Figure 15). −ε ’ Δ (as IP = HOMO in Koopman s approach, by B3LYP/LANL2DZ) The visible end-to-end span of the observed spectrum ( = of HOMO−1in8 (6.655 eV) and of SOMO in 8+• (6.094 eV), 23−24 G) corresponds to the coupling constants from 2 sets of it is more probable that a second electron withdrawal affects the equivalent protons and to some contribution from 117/119Sn cation radical, like in the case of aryl sulfides. In contrast to the nuclei. The g-factor (g = 2.0036) is slightly increased relative to ox trend in Ep of diaryl and arylalkyl thioethers, which are easier that of pure organic radicals because of the interaction with

1052 dx.doi.org/10.1021/ic202179e | Inorg. Chem. 2012, 51, 1041−1056 cannot be oxidized. No such data exist for the chloro-substituted germatrane N(CH2CH2O)3GeCl but one can expect even higher Ep since germatranes are more difﬁcult to oxidize than silatranes [33, 83]. From a simple correlation of Ep with calculated IPs of the stannatranes 12, 13 and 14, the oxidation potential of the chloro-substituted derivative 14 should be above 2.3 V which corroborates the results of its voltammetry.

On the contrary, compound 14 shows a well-shaped reduction peak at Ep = -1.5 V; the reduction process results in elimination of Cl− anions whose oxidation peak at ∼1 V is detected by voltammetry during the second cycle. The peak at -1.5 V falls into the range of reduction potentials of known chloridostannanes [84] and might have arisen from the dissociative reduction of Sn–Cl bond in 14. Similarly, the oxidation peak of the iodide anion I− appears after the reduction of 15. This is in agreement with the conductivity measurements of 14 in

CH3CN/CH2Cl2 (50:50 v/v), which have only shown residual solvent conductance and no contribution from any ionic conductivity due to the possible Sn–Cl dissociation prior to the reduction. A practically identical conductivity curve was observed for the phenolatosubsti- tuted stannatrane 12. Therefore the chloridosubstituted stannatrane 14, just as 12, remains a covalent compound in solution, at least in this media.

5.4.2 EPR-spectroelectrochemistry

Since the oxidation of the phenoxy-substituted stannatrane 12 shows partial reversibility (Figure 63), it was studied by real-time EPR-coupled electrochemistry. The solution of 12 −3 (10 mol/L) in CH3CN/0.1 M Bu4NPF6, initially ERP-silent, has shown the signal of paramagnetic species when the potential 1.23 V was applied (Figure 64). The visible end-to-end span of the observed spectrum (∆ = 23 − 24 G) corresponds to the coupling constants from 2 sets of equivalent protons and to some contribution from 117/119Sn nuclei. The g-factor (g = 2.0036) is slightly increased relative to that of pure organic radicals because of the interaction with oxygen and tin and falls into the range of the values for known phenoxyl radicals [85, 86]. The triplet of triplets pattern with 1:2:1 intensities is rather straightforward and ﬁts well with two pairs of practically equivalent protons (o,o´- and m,m´-) in the supposed phenoxyl species. Though the ends of the spectrum are not well enough resolved to allow extracting exact Sn117/119 coupling constants, its symmetry permits to suppose two values a119Sn = 6.7 G and a117Sn = 7.4 G. The set of 2H with the hfc constant of aHo = 6.22 G can be substituted with two large proton hfc constants (aHo−-= 6.27 G and 0 aHo− = 6.03 G), assigned to two slightly different ortho-protons of the phenyl ring (see Figure 63).

81 3355.0 3367.0 3379.0 3391.0

H, Gauss

Figure 64: EPR spectrum from the oxidation of stannatrane 12 in CH3CN/0.1 M Bu4NPF6 at a Pt microspiral electrode. Upper, experimental; lower, simulated using the parameters in the text. T = 233 K, E = 1.23 V.

This substitution provides a better fitting but in any case, this difference is at the level of the (possible) contribution from the protons of the t-Bu group. Two m-protons account for a smaller constant, aHm = 1.85 G (2H). It appears as there is no coupling with the atrane cage nitrogen atom or, if any, its contribution is very small. In general, the presence of d- metals often results in strong spin-orbital interactions [87] broadening the spectral lines and making it difficult to observe well resolved hyperfine structure. Thus the cation radical of stannatrane 12 has the spin distribution as an O-substituted t-Bu-phenoxy radical and not as a proper metallatrane, similarly to the cation radicals of germatranes in which the 1-substituent has lower own ionization potential than the atrane nitrogen atom [83]. 5.4.3 DFT calculations

The geometry of the stannatranes 12, 13 and 14 and of their cation radicals was optimized using a combined treatment: primary adjustment was done at HF/6-311G level, and then the structure was optimized at DFT B3LYP/ DGDZVP level, better accounting for long-term and delocalizing electronic interactions. The same level was used for NBO analysis and to check the optimized structures for the absence of imaginary vibrations. The N–Sn distance is affected very little by electron withdrawal. It becomes slightly longer in 12+•, slightly shorter in 13+• and remains practically unchanged in 14+• (Table 16). Mean- while, other geometrical parameters of these stannatranes show remarkable conﬁgurational change when forming the cation radical. Thus, for the neutral stannatrane 12, the dihedral

82 Inorganic Chemistry Article

DFT Calculations. The geometry of the stannatranes 4, 8, and 13 and of their cation radicals was optimized using a combined treatment: primary adjustment was done at HF/6-311G level, and then the structure was optimized at DFT B3LYP/ DGDZVP level, better accounting for long-term and delocalizing electronic interactions. The same level was used for NBO analysis and to check the optimized structures for the absence of imaginary vibrations. The N−Sn distance is very little affected by electron withdrawal. It becomes slightly longer in 4+•, slightly shorter in 8+• and remains practically unchanged in 13+• (Table 8). Meanwhile, ◦ ◦ angle ϕ(∠Co-Ar–Ci-Ar–O–Sn)other geometrical is 93.63 parameters, while of it these becomes stannatranes 3.81 showin the re- cation radical (Figure markable configurational change when forming the cation radical. ◦ 65) showing that the atraneThus, for moiety the neutral twists stannatrane around the4,O–Sn the dihedral bond angle by theφ right angle (55 for +• ∠ − − − ° ° the couple 13/13 ). ( Co‑Ar Ci‑Ar O Sn) is 93.63 , while it becomes 3.81 in the cation radical (Figure 16) showing that the atrane moiety twists

Figure 15. EPR spectrum from the oxidation of stannatrane 4 in AN/ 0.1 M Bu4NPF6 at a Pt microspiral electrode. Upper, experimental; lower, simulated using the parameters in the text. T =233K,E =1.23V. oxygen and tin and falls into the range of the values for known phenoxyl radicals.53,54 The triplet of triplets pattern with 1:2:1 intensities is rather straightforward and fits well with two pairs of practically equivalent protons (o,o′- and m,m′-) in the supposed phenoxyl species. Though the ends of the spectrum are not well enough resolved to allow extracting exact Sn119/117 coupling constants, its symmetry permits to suppose two values aSn119 = 6.7 G and aSn117 = 7.4 G. The set of 2H with the hfc constant of aHo = 6.22 G can be substituted with two large proton hfc constants (aH ‑ = 6.27 G o Figure 16. B3LYP/DGDZVP optimized geometry of stannatrane 4 and aH ′ = 6.03 G), assigned to two slightly different ortho- o ‑ Figure 65: B3LYP/DGDZVPand its cation optimized radical (see geometry text). of stannatrane 12 and of its cation radical protons of the phenyl ring (see Figure 14). This substitution provides a better fitting but in any case, this(see difference text). is at the around the O−Sn bond by the right angle (55° for the couple level of the (possible) contribution from the protons of the 8/8+•).Thedrivingforceofthisremarkabletwististheinteraction π t-Bu group. Two m-protons account for a smaller constant, of pz-orbitals of the O and S atoms with the -systems of the aHm = 1.85 G (2H). It appears as there is no couplingThe driving with the force ofaromatic this remarkable fragments, destabilizing twist is the the HOMOinteraction in the of neutralpz-orbital of the O and S atrane cage nitrogen atom or, if any, itsatoms contribution with theis veryπ−systemsmolecules of the and aromatic stabilizing fragments, the corresponding destabilizing CRs. the The HOMO in the neutral small. In general, the presence of d-metals often results in stabilization of the phenoxy system in 4+• is stronger than of strong spin−orbital interactions55 broadeningmolecules the spectral and lines stabilizingthe arylthio the corresponding fragment in 8+•: CRs. while theThe couple stabilization4/4+• clearly of the phenoxy system − − and making it difficult to observe well resolvedin hyperfine12+• is structure. stronger thanswitches of the between arylthio two mostfragment stable inorthogonal13+•: while and planar the couple 12/12+• clearly Thus the cation radical, hereafter referred to as CR, of stannatrane forms (both corresponding to global minima on the energy 4 has the spin distribution as an O-substitutedswitchest-Bu-phenoxy between twosurface), most stable the bulkiness forms, of orthogonal the S atom and the planar, poorer (both match corresponding in to global radical and not as a proper metallatrane, similarly to the cation the size of overlapping orbitals in 8+• force the latter to adopt a radicals of germatranes in which the 1-substituentminima has on lower the own correspondingcompromise energy half-eclipsed surface), configuration. the bulkiness It implies of the S that atom the and the poorer match ionization potential than the atrane nitrogenin atom. the51 size of overlappingnuclear orbital frames in of134 +and• force8, following the latter the redistribution to adopt a compromiseof the half-eclipsed

Table 8. Selected Geometrical Parametersconfiguration. (Distances in Å, It Angles implies in Degrees), that the Fermi nuclear Contact frames Terms of (FCT,12 and MHz),13, and following NBO the redistribution Charges for Stannatranes 4, 8, 13, andof Their the Cation electron Radicals density from uponDFT B3LYP/DGDZVP oxidation (electrochemical Calculations electron transfer is adiabatic by its nature), exist in two fixed geometries: thoseNSn of neutral and of the oxidized forms (12 vs 12+• compd l(Sn−N) l(Sn−X)a ∠N−Sn−X ∠X−Sn−Ob ∠O−Sn−Ob FCT q(NBO) FCT q(NBO) +• 4 2.240 2.030 171.66and 13 102.46vs 13 (89.07)). 126.77 (101.43) −0.348 1.296 • 4+ 2.265 2.121 177.84This 96.32 fact (98.93) precludes 118.73 the formation 0.0000 of pure− Nernstian0.342 −0.3056 reversible 1.307 redox systems for these 8 2.402 2.442 175.94 101.72 116.02 −0.326 1.171 8+• 2.273 2.582 173.82compounds 99.99 (91.72) which accounts 118.46 for small 0.0003apparent− transfer0.342 coefficient−2.6584α 1.196for 12 and 13 (Table 15). 13 2.374 2.373 179.97 100.60 116.69 −0.327 1.195 +• 13+• 2.376 2.332 164.89There is 105.72 no (90.45)contribution th126.34 of the(111.03) nitrogen 0.7022 atom to−0.327 the spin−orbital 104.5738 interactions 1.163 in 12 since the a b +• X is first atom of the 1-substituent, as in Scheme3c-4e 4. systemfor two Sn-symmetrical has lower energy Oeq atoms, than the value SOMO given (Figurein the parentheses 66) and is for is the doubly remaining populated. The 12 is nonsymmetrical O . eq thus a phenoxyl radical; it agrees very well with its EPR spectrum that has no characteristic 1053 dx.doi.org/10.1021/ic202179e | Inorg. Chem. 2012, 51, 1041−1056 pattern of a nitrogen-centered radical. Fermi contact couplings at the nitrogen and tin atoms (Table 16), and at the atoms of the phenoxy fragment, obtained from B3LYP/DGDZVP calculations, agree well with this feature. Contrary to cation radicals of germatranes, showing no unpaired spin density on the germanium atom and no coupling with it [34], the spectrum of the iodido-substituted stannatrane 15 shows the 117/119Sn satellites which is also consistent

83 with the nonzero FCT on the Sn atom (Table 16).

Table 16: Selected geometrical parameters (distances in Å, angles in degrees), fermi contact terms (FCT, MHz), and NBO charges for stannatranes 12, 13, 14 and their cation radicals from DFT B3LYP/DGDZVP calculations

a b b Cpd l(Sn–N) l(Sn–X) ∠N–Sn–X ∠X–Sn–O ∠O–Sn–O 12 2.240 2.030 171.66 102.46 (89.07) 126.77 (101.43) 12+• 2.265 2.121 177.84 96.32 (98.93) 118.73 13 2.402 2.442 175.94 101.72 116.02 13+• 2.273 2.582 173.82 99.99 (91.72) 118.46 14 2.374 2.373 179.97 100.60 116.69 14+• 2.376 2.332 164.89 105.72 (90.45) 126.34 (111.03)

N Sn Cpd FCT q(NBO) FCT q(NBO) 12 -0.348 1.296 12+• 0.0000 -0.342 -0.3056 1.307 13 -0.326 1.171 13+• 0.0003 -0.342 -2.6584 1.196 14 -0.327 1.195 14+• 0.7022 -0.327 104.5738 1.163 a b X is ﬁrst atom of the 1-substituent. for two Sn-symmetrical Oeq atoms, the value given in the parentheses is for the remaining nonsymmetrical Oeq.

Upon oxidation of the chlorido-substituted stannatrane 14, the tin atom also undergoes substantial reorganization: instead of a trigonal bipyramid it adopts the conﬁguration of the pyramid with a rhombic base, when one oxygen atom is at the apical position and the long diagonal of the base is formed by nitrogen and chlorine atoms (Figure 67); in general it resem- bles to Berry pseudorotation transition state. The calculated Sn–Oeq bond length is 1.994 Å ◦ in average, whereas the third Sn–Oeq distance (for which ∠Cl–Sn–Oeq = 90 ) is 2.244Å. The N–Sn–Cl angle being 180◦ in the neutral molecule is smaller by about 15◦, bringing closer the nitrogen and chlorine atoms (Table 16). The structure of the HOMO of the chloro-substituted stannatrane 14 is rather complex (Figure 67). Contrary to the HOMOs of lighter metallatranes (M = Si, Ge) the 3c-4e bonding system of which involves the easiest to ionize orbital (n-electrons of N) [33, 34, 88], the HOMO of 14 only contains it to a small extent (at least, at the B3LYP/DGDZVP theory level) suggesting that upon electron withdrawal, the whole orbital system involved must be very perturbed with no possibility of delocalizing stabilization of the cation radical.

84 Inorganic Chemistry Article

Inorganic Chemistry Article

+• Figure 17. UnpairedFigure electron 66: density Unpaired (SOMO) electron delocalization density (SOMO) on the phenoxy delocalization fragment on in the the cation phenoxy radical fragment4 by B3LYP/DGDZVP in the (a), and doubly populated (SOMO−1) orbital (b) with small contribution of 3c−4e N−Sn−O bonding. cation radical 12+• by B3LYP/DGDZVP (a), and doubly populated (SOMO−1) orbital (b) Figure 17. Unpairedwith electron small density contribution (SOMO) of delocalization 3c-4e N–Sn–O on the bonding. phenoxy fragment in the cation radical 4+• by B3LYP/DGDZVP (a), and doubly populated (SOMO−1) orbital (b) with small contribution of 3c−4e N−Sn−O bonding.

Figure 18. Geometry and FMOs for 13/13+• from B3LYP/DGDZVP optimization: (a) neutral, (b) cation radical, (c) HOMO of 13 and (d) SOMO of 13+•. The contribution of intramolecular N−Sn−O (3c-4e) bonding is clearly seen, though with moderate orbital coefficients.

+• Figureelectron 18. densityGeometry upon and FMOs oxidation for 13/13 (electrochemicalfrom B3LYP/DGDZVP electron+• optimization:suggesting (a) that neutral, upon (b) electron cation radical, withdrawal, (c) HOMO the ofwhole13 and orbital (d) SOMO of 13+•. TheFigure contribution 67: Geometry of intramolecular and FMOs N− forSn−14O/ (3c-4e)14 from bonding B3LYP/DGDZVP is clearly seen, though optimization: with moderate (a) neutral, orbital coefficients. transfer is adiabatic by its nature), exist in two fixed geometries: system+• involved must be very perturbed with no possibility of those of neutral and(b) of cation the oxidized radical, forms (c) HOMO (4 vs 4+ of• and14 and8 vs (d8)+• SOMO). ofdelocalizing14 . The stabilization contribution of of the intramolecular cation radical. The spin-bearing electron density upon oxidation (electrochemical electron suggesting that• upon electron withdrawal, the whole orbital This fact precludesN–S–O the formation (3c-4e) bonding of pure is Nernstian clearly seen, reversible though withorbital moderate in 13+ orbitalis mainly coefﬁcients. localized on the tin atom (sf. Table 8), transfer is adiabatic by its nature), exist in two fixed geometries: system involved must be very perturbed with no possibility of redox systems for these compounds which accounts+• for small+• and is neither sterically shielded nor involved into conjugation those of neutral and of the oxidized forms (4 vs 4 and 8 vs 8 ). delocalizing stabilization46,56 of the cation radical. The spin-bearing apparent transfer coefficient α for 4 and 8 (Table 7).+• There is as in germatranes.• On this reason it is probably much more This fact precludes theThe formation spin-bearing of pure orbital Nernstian in 14 reversibleis mainly localizedorbital in on13 the+ is tin mainly atom localized (sf. Table on 16), the tin and atom (sf. Table 8), no contribution of the nitrogen atom to the spin−orbital difficult to observe this species in solution, since it is closer to redox systems for+is• these neither compounds sterically which shielded accounts nor involved for small into conjugationand is neither as stericallyin germatranes58 shielded [33, nor 34]. involved On into conjugation interactions in 4 since the 3c-4e system has lower energy than R4Sn or R5Sn radicals46,56 than to known CRs of sila- or germa- apparent transfer coefficient α for 4 and 8 (Table 7).• There is as in germatranes. On this reason it is probably46,56,57 much more SOMO (Figure 17)this and reason is doubly it is probably populated. much The more4+ is difﬁcultthus a to observetranes with this N-centered species in solution, spin-carrying since orbitals. it is The im- no contribution of the nitrogen atom to the spin−orbital difficult to observe this species in solution, since it is closer to phenoxyl radical;+• it agrees very well with its EPR spectrum that portance of surrounding58 Sn Oeq-atoms in the HOMO suggests interactions in 4 closersince theto R 3c-4e4Sn or systemR5Sn hasradicals lower energy [89] than than to knownR4Sn cation or R5Sn radicals radicals of sila-than or to germatranesknown CRs of sila- or germa- has no characteristic pattern of a nitrogen-centered• radical. a strongly perturbed Sn environment upon electron46,56,57 removal SOMO (Figure 17) and is doubly populated. The 4+ is thus a tranes with N-centered spin-carrying orbitals. The im- Fermi contact couplingswith N-centered at the nitrogen spin-carrying and tin atoms orbital (Table [33, 8), 34]. Theleaving importance few chances of surrounding for observing Sn O sucheq-atoms CR under conventional phenoxyl radical; it agrees very well with its EPR spectrum that portance of surrounding Sn Oeq-atoms in the HOMO suggests and at the atomsin the of theHOMO phenoxy suggests fragment, a strongly obtained perturbed from Sn environmentconditions. It upon is then electron rather removal questionable leaving whether it is possible has no characteristic pattern of a nitrogen-centered radical. a strongly perturbed Sn environment upon electron removal B3LYP/DGDZVP calculations, agree well with this feature. to observe these species in solution, above the freezing point of Fermi contact couplingsfew chances at the nitrogen for observing and tin such atoms cation (Table radical 8), underleaving conventional few chances conditions. for observing It is suchthen ratherCR under conventional Contrary to CRs of germatranes, showing no unpaired spin common electrochemical solvents. Also, 13 has high value of and at the atoms of the phenoxy fragment, obtained from conditions. It is then rather questionable whether it is possible density on the germaniumquestionable atom whether and no it coupling is possible with to it,56 observethe thesethespecies ionization in potential solution, (Table above 7) the which, freezing using the approximate B3LYP/DGDZVP calculations, agree well with this feature. to observe− these species in solution, above the freezing− point of spectrum of thepoint iodido-substituted of common electrochemical stannatrane 15 solvents.shows the Also, 14EphasIP high correlation, value of leadsthe ionization to the Ep potentialof about 2.4 2.6 V, mean- Contrary to CRs of germatranes, showing no unpaired spin common electrochemical solvents. Also, 13 has high value of 117/119Sn satellites which is also consistent with the nonzero ing that the oxidation of this stannatrane could not be observed density on the germanium(Table 15) atom which, and using no coupling the approximate with it,56 theEp – IP correlation,the ionization leads potential to the (TableEp of 7)about which, 2.4–2.6 using the approximate FCT on the Sn atom (Table 8). under− the conditions employed. However, these− estimations are spectrum of theV, iodido-substituted meaning that the stannatrane oxidation of15 thisshows stannatrane the couldEp IP not correlation, be observed leads under to the theEp conditionsof about 2.4 2.6 V, mean- Upon oxidation of the chlorido-substituted stannatrane 13, based on the gas phase data and bonds polarization in the 117/119Sn satellites which is also consistent with the nonzero ing that the oxidation of this stannatrane could not be observed the tin atom alsoemployed. undergoes However, substantial these reorganization: estimations instead are based onsolution the gas might phase lower data and the bondsactual oxidation polarization potential, so a broad FCT on the Sn atom (Table 8). under the conditions employed.∼ − However, these estimations are of a trigonal bipyramid it adopts the configuration of the shoulder observed at 2.2 2.3 V for 13 and 15 in CH2Cl2 Upon oxidationin of the the solution chlorido-substituted might lower thestannatrane actual oxidation13, based potential, on the so a gas broad phase shoulder data and observed bonds atpolarization in the pyramid with a rhombic base, when one oxygen atom is at the might actually stem from this oxidation. In any case, high the tin atom also∼ undergoes2.2–2.3 V substantial for 14 and reorganization:15 in CH Cl insteadmight actuallysolutionoxidation stem might frompotential lower this of oxidation. the13 actualand 15 oxidation Inexcludes any case, potential, the use of so spin a broad traps apical position and the long diagonal of the base is2 formed2 by ∼ − of a trigonal bipyramid it adopts the configuration of the shoulderbecause many observed of them at 2.2 undergo2.3 V oxidation for 13 and much15 beforein CH these2Cl2 nitrogen and chlorinehigh oxidation atoms (Figure potential 18); of in3 and general15 excludes it re- the use of spin traps because many of them pyramid with a rhombic base, when one oxygen atom is at the mightpotentials. actually59 stem from this oxidation. In any case, high sembles to Berry pseudorotation transition state. The calculated oxidation potential of 13 and 15 excludes the use of spin traps apical position and the long diagonal of the base is formed by Sn−O bond length is 1.994 Å in average, whereas the third 85 because many of them undergo oxidation much before these nitrogeneq and chlorine atoms (Figure 18); in general it re- Sn−O distance (for which ∠Cl−Sn−O =90°) is 2.244 Å. potentials.■ CONCLUSION59 sembleseq to Berry pseudorotation transitioneq state. The calculated The N−Sn−Cl angle being 180° in the neutral molecule is Sn−O bond length is 1.994 Å in average, whereas the third In this manuscript we have demonstrated the high potential of smallereq by about 15°, bringing closer the nitrogen and chlorine Sn−O distance (for which ∠Cl−Sn−O =90°) is 2.244 Å. ■the CONCLUSIONt-butoxido-substituted stannatrane N(CH2CMe2O)3SnO-t- atomseq (Table 8). eq − The N−Sn−Cl angle being 180° in the neutral molecule is Bu (1) for the synthesis, in mostly acid base-type reactions, of The structure of the HOMO of the chlorido-substituted In this manuscript we have demonstrated the high potential of smaller by about 15°, bringing closer the nitrogen and chlorine a great variety of novel inorganic stannatranes of the type stannatrane 13 is rather complex (Figure 18). Contrary to the theN(CHt-butoxido-substitutedCMe O) SnX (X stannatrane= OR, SR, OSiMeN(CH2CMeC H2SiMeO)3SnO-OSn-t- atoms (Table 8). 2 2 3 − 2 6 4 2 HOMO’s of lighter metallatranes (M = Si, Ge) the 3c-4e Bu(OCMe (1) forCH the) synthesis,N, OC(O)R, in mostly SP(S)Ph acid, halogen).base-type Inreactions, fact, com- of The structure of the HOMO of the chlorido-substituted 2 2 3 2 bonding system of which involves the easiest to ionize orbital apound great1 varietyis a tin of tetraalkoxide novel inorganic the cage-type stannatranes structure of the including type stannatrane 13 is rather complex (Figure 18). Contrary to the (n-electrons of N),46,56,57 the HOMO of 13 only contains it to N(CHintramolecular2CMe2O) N3→SnXSn interaction(X = OR, of SR, which OSiMe induces2C6H high4SiMe reactivity2OSn- HOMO’s of lighter metallatranes (M = Si, Ge) the 3c-4e a small extent (at least, at the B3LYP/DGDZVP theory level) (OCMeof the axial2CH Sn2)3−N,O OC(O)R, bond only. SP(S)Ph Consequently,2, halogen). compound In fact,1 com-might bonding system of which involves the easiest to ionize orbital pound 1 is a tin tetraalkoxide the cage-type structure including 46,56,57 (n-electrons of N), the HOMO of 13 only contains it to 1054 intramolecular N→Sndx.doi.org/10.1021/ic202179e interaction of which| Inorg. induces Chem. high2012, 51, reactivity 1041−1056 a small extent (at least, at the B3LYP/DGDZVP theory level) of the axial Sn−O bond only. Consequently, compound 1 might

1054 dx.doi.org/10.1021/ic202179e | Inorg. Chem. 2012, 51, 1041−1056 undergo oxidation much before these potentials. 5.5 Distannatrane from electroreduction of halostannatranes

Because of their low lying HOMO, halostannatranes are hardly oxidizable. On the contrary, they have a LUMO localized on antibonding σ∗(Sn–X) orbital which can relatively easily accept an extra electron and therefore allow electroreduction. Alkyl and even aryl chlorostannanes have similar LUMO, therefore one can expect that the reduction of halostannatranes would follow a similar way. Electrochemical reduction of chlorostannanes was reported by several authors who provided a lot of information on this process, albeit quite contradictory [90, 91, 92]. Most important conclusion that one can draw from these studies, is that -whatever the exact mechanism- electroreduction of tin chlorides in aprotic media results in the corresponding distannane products. With this in mind, we examined cathodic reduction of the above halostannatranes (X = Cl, Br, I) in view of obtaining so far unknown distannatrane. This molecule, containing central Sn–Sn bridge, corresponds to the 4c-6e bonding scheme and therefore it would be very tempting to prepare it in view of exploring its behavior in electron-withdrawal processes.

5.5.1 Cyclic voltammetry of electroreduction of halostannatranes

Before creating the Sn–Sn bond by electrochemical reaction of halostannatranes, it is required to reveal the mechanism of the electrochemical reduction of these compounds. We studied electroreduction of the halo stannatranes (X = Cl, Br, I) and compared it with model tin compounds, Ph3SnCl and Bu3SnCl. All three halostannatranes, chloride, bromide and iodide have well-shaped reduction peaks (Figure 68, 69). Although the peak shape is somewhat deformed -probably by adsorptional interactions-, their reduction peak currents are linear with concentration and with square root √ from the scan rate (∆lg(i)/∆lg( v)); these tests conﬁrm the diffusional character of the process at these potentials. To determine the calculate the electron stoichiometry of electroreduction of these hoalstannatranes, we used three methods whose results are presented in table

17. First, absolute n was obtained by combination of ip from the linear sweep voltammetry and of Cottrell relationship. Second, known amount of ferrocene was added to the solution at the end of every experiment and the voltammogram was recorded with ferrocene; n was then calculated from the proportion between ip of the stannatrane and ip of ferrocene. Third, n was − − obtained from the comparison of ip of stannatranes with those of Cl or Br ions, added as 1 corresponding salts LiCl and Et4NBr. All these methods converge in that the process at Ep corresponds to the transfer of 1-electron.

1 0 0 1 0 0

0 0

- 1 0 0 - 1 0 0 A

A - 2 0 0 - 2 0 0

µ µ

, , I I - 3 0 0 - 3 0 0

- 4 0 0 - 4 0 0

- 5 0 0 - 5 0 0 - 2 . 0 - 1 . 5 - 1 . 0 - 0 . 5 0 . 0 - 2 . 5 - 2 . 0 - 1 . 5 - 1 . 0 E , V v s . S C E E , V v s . S C E

Figure 68: Cyclic voltammetry of electroreduction of left, stannatrane chloride; right, stannatrane bromide (C = 0.1 mmol/L) in CH3CN/0.1M Bu4NPF6 at a 2 mm GC electrode. v = 1 V/s; T = 25 oC.

5 5 1 0 0 0

- 5 0 - 5 A A A µ µ

- 1 0 µ

, I , - 1 0 I - 1 0 I - 1 5 - 1 5 - 2 0 a - 2 0 b c - 2 0 - 2 5 - 2 . 0 - 1 . 5 - 1 . 0 - 2 . 0 - 1 . 5 - 1 . 0 - 0 . 5 - 2 . 0 - 1 . 5 - 1 . 0 - 0 . 5 E , V v s . S C E E , V v s . S C E E , V v s . S C E

Figure 69: Cyclic voltammetry of electroreduction of a, stannatrane chloride; b, stannatrane bromide (C = 0.1 mmol/L) in CH3CN/0.1M Bu4NPF6 at a 1 mm Pd electrode; c, stannatrane iodide (C = 0.1 mmol/L) in CH3CN/0.1M Bu4NPF6 at a 1 mm Pt electrode. v = 1 V/s; T = 25 oC.

Table 17: Parameters of electrochemical reduction of stannatrane chloride, stannatrane bromide, tributyltin chloride and triphenyltin chloride at a GC electrode.

a b c d Cmpd Ep(V) ∆Ep − Ep/2(V) n n n Stannatrane chloride -1.94 0.19 1 1 1 Stannatrane bromide -2.04 0.15 1 1 1 Stannatrane iodide – – – – – Tributyltin chloride -2.14 0.16 2 1 - Triphenyltin chloride -2.55 0.11 2 1 - a b c Peak potentials (Ep) at v = 1 V/s; n is obtained from chronoampermetry; n is from compar- d − − ison with ip of ferrocene; n is from oxidation currents of Cl and Br ions, in CH3CN/0.1 o M Bu4NPF6 v = 1 V/s; T = 25 C.

Several articles have reported the reduction process of Ph3SnCl. A. Savall et al. claimed

87 Table 18: Parameters of electrochemical reduction of stannatrane chloride, stannatrane bromide, tributyltin chloride and triphenyltin chloride at a transition metal electrode.

1 2 1 1 Cmpd Ep (V) Ep (V) ∆Ep − Ep/2(V) n Stannatrane chloride -1.17a -1.90a 0.13a 2 Stannatrane bromide -1.20a -1.89a 0.16a 2 Stannatrane iodide -1.00b -1.90b 0.15b 2 Tributyltin chloride -1.60c -2.90c – 2 Triphenyltin chloride – – – – aThe values are measured at a 1 mm Pd electrode; b the values are measured at a 1 mm Pt electrode; c [93].

+ that the reduction of Ph3SnCl at -2.2 V vs. Ag/Ag is a two-electron process leading to − • Ph3Sn via Ph3Sn and thereafter to Ph3SnSnPh3 by chemical steps [91] whereas R. E. Dessy et al. [93] found that the reduction of triphenyltin chloride exhibits two well-deﬁned waves. The reduction step at -1.6 V vs. AgClO4/Ag, shown to require one electron per mole of triphenyltin chloride, yielded quantitatively hexaphenylditin. The second reduction wave at

-2.9 V vs. AgClO4/Ag could represent the uptake of two electrons per mole to lead directly − to Ph3Sn then to the dimer Ph3SnSnPh3 [93]. By the other side, Holm et al. [94] reported

• − • − E1/2 potentials of EPh3Sn /Ph3Sn and EBu3Sn /Bu3Sn , which are -0.54 V and -0.93 V vs. SCE respectively [94]. The discrepancy one can note in the bibliographic data stems on the reduction of Ph3SnCl is caused by the fact that the reduction has been carried out on cathodes made of different materials. More precisely, some electrodes were made of transition (Pt) or post-transition (Hg) metals while others were made of graphite or glassy carbon (GC). At a GC electrode, a proper reduction of the Sn–X bond whose σ∗ component corresponds to the (LUMO of the molecule) takes place. On the contrary, on a Pt or Hg electrodes, a preced- ing oxidative addition of the Sn–X compound to the surface-located metal atoms might occur.

Consequently, it is not a Sn–X but Sn–Pt–X or R3–Sn–X intermediates who undergo electroreduction under these conditions (Figure 69). The electron uptake to such metallo-organic species requires much less cathodic potentials than the direct reduction of cation radicals to ∗ the R3Sn–X σ orbital (LUMO). The reduction potentials of such R3Sn–M–X species are less • negative than those of known E1/2 of reductuion of Sn -centered radicals [94]. Therefore, it is not surprising that one-electron reduction of R3Sn–M–X surface-located intermediates is followed by second one-electron step at more negative potentials, corresponding to the formation − of R3Sn anions (the step being less cathodic and hence usually absorbed in an overall two electron reduction peak if the reduction is carried out at a GC electrode).

Difference in the observed n values for Ph3SnCl and Bu3SnCl on the one hand, and for halostannatranes on the other, might arise from different steric hindrance in these systems in

88 homobimolecular reactions like SN2 or radical dimerization: Ph3Sn > Bu3Sn > Stannatrane. Unlike parent Si or even Ge derivatives, stannatrane is bulky because of the inner d-orbital of Sn. Sn atom forms longer bonds with its environment and is much more available for external reagents steric hindrance from the phenyl groups is comparable with that of three i-Pr substituents; n-Bu, though normally, create more steric hindrance than three O-Alk groups in stannatrane, because the latter are attached to the back N-atom and ﬁxed in one direction.

Therefore, the rates of SN2 attack of stannatranyl anion on chlorostannatrane or of the radical dimerization of stannatranyl radicals are supposedly higher than for the corresponding

Ph3Sn-species. In the case of a two-electron process (n = 2), the mechanism could be the following:

- + e -Cl - R3SnCl + e R3Sn R3Sn

E1 E2 |E1| > |E2| - - -Cl R3Sn + R3SnCl R3SnSnR3 Scheme 15: Two-electron reduction process of chlorostannanes (R = Ph, Bu).

Halogenostannane has associated one electron and lost Cl− at the ﬁrst step, leading to the • radicals R3Sn ; then these unstable radicals were reduced (addition of another electron) and − formed R3Sn at the potential of their formation. − Then the R3Sn anions would attack the unreduced molecule R3SnCl to substitute one Cl− and to form the dimeric organotin (Scheme 15). If there is a proton source in the system, − HD, the R3Sn anions could be protonated to form R3SnH:

- - -D R3Sn + HD R3SnH

Therefore, QTotal in a large scale reduction must be 1 F/mol, since two electrons per mole − are needed for the reduction of one Sn–Cl bond (Scheme 15) but the R3Sn anions, reacting with neutral molecule R3SnCl, consume second equivalent of the chlorostannane thus leading to the overall stoichiometry of 2e/2 molecules = 1 F/mol. However, from the cyclic voltammetry of R3SnCl the electron stoichiometry is n = 2. This probably because of the slow rate of the nucleophilic displacement (Scheme 15) in a short time scale (t→0), which does not occur to a substantial extent during the voltammetric experiment but is accomplished during much longer preparative electrolysis. Thus there is a disagreement on the electron stoichiometry between the results of cyclic voltammetry and large scale reduction analysis. On the contrary, for halo-stannatranes the n value is unity even in the conditions of voltammetry. This is supposedly because of higher rate of nucleophilic substitution at these compounds for the steric reasons discussed above.

89 The order of Ep of halo-stannatranes is seemingly inversed with respect to the usual easi- ness of the series Cl < Br < I. However, the values of their ∆Ep−Ep/2(V) are quite large (Table

17), so the observed difference in Ep (stannatrane chloride) and Ep (stannatrane bromide) is not really meaningful.

5.5.2 Electroreduction of halostannatranes at transition metal cathodes

From the analysis above it is supposed that for stannatrane chloride, during the electroreduc- • tion at a transition metal working electrode (Pt, Pd or Hg), the radical intermediate R3Sn is formed at less negative potential:

(0) Pd (II) + e N(CH2CMe2O)3SnCl + e N(CHCMe2O)3Sn Pd Cl -Cl-

+ e - N(CH2CMe2O)3Sn N(CH2CMe2O)3Sn

- N(CH2CMe2O)3Sn + N(CH2CMe2O)3SnCl (N(CH2CMe2O)3Sn)2 -Cl-

To prove the existence of these intermediate stannatranyl radicals, spin trapping with α- phenyl-N-tert-butyl-nitrone (PBN) was applied. Because the reduction potential of PBN is more negative than the potential of forming atranyl radicals Sn•, this spin trap can be used under the condition of reduction of R3Sn–Pt–Cl intermediates:

Pd + e N(CH2CMe2O)3SnCl + e N(CH2CMe2O)3Sn Pd Cl -Pd, -Cl-

O H PBN N N(CH2CMe2O)3Sn

N(CH2CMe2O)3Sn

During the electroreduction of the corresponding halostannatranes (table 19) it is con- ﬁrmed the formation of Sn-centered radicals during the reduction process in this range of weakly cathodic potentials and hence the occurrence of a different mechanism of reduction at transition metal electrodes. Under these conditions, similar species were as well trapped with model chlorostannanes, Ph3SnCl and Bu3SnCl (Figure 70). Small proton hfc constants

90 in these adducts attest bulky character of the trapped radical addend and the values of aH are reciprocate to the steric demand of the concerned species: Ph3Sn > Bu3Sn > stannatrane. In the case of stannatrane iodide (Figure 73), only the nitrogen triplet spectrum was obtained, arising from the spin-adduct without α-proton. As was mentioned before, this stannatrane is light-sensitive, its decomposition probably involving radical species. In addition, it has higher Lewis acidity than the other stannatranes. Since the concentration of PBN in the catholyte is inferior to that of the iodostannatane, the nitrone was supposedly all reacted with these decomposition-induced radicals via hydrogen abstraction or simply via deprotonation.

Table 19: Spin trapping of electrogenerated stannyl radicals with PBNa

Halostannane g-factor hfc, G Ratio Ph3SnCl 2.0065 aN: 14.687, aH: 2.732 50% 2.0067 aN: 13.815, aH: 2.333 50% Bu3SnCl 2.0065 aN: 14.625, aH: 3.129 75% 2.0067 aN: 13.854, aH: 2.286 23% 2.0073 aN: 16.183 2% b Stannatrane chloride 2.0083 aN: 14.596, aH: 3.357 92% Stannatrane bromide 2.0065 aN: 14.543, aH: 2.760 79% 2.0067 aN: 13.846, aH: 2.329 18% Stannatrane iodide 2.0064 aN: 15.704 98% a b Electrolyses at a Pd electrode in CH3CN/0.1 M TBAPF6; Electrolysis in THF/0.1 M TBAPF6.

Figure E. EPR spectrum from ex-cell spin-trapping experiments with PBN during the reduction Figure 70: EPR spectrum from ex-cell spin-trapping experiments with PBN during the reduc- of Bu3SnCl. Electrolysis at a Pd cathode in AN/0.1 M Bu4NPF6, E = ** V. T = 293 K. tion of Bu3SnCl. Electrolysis at a Pd cathode in CH3CN/0.1 M TBAPF6, E = -0.9 V. T = 293 K.

Besides major spin-adduct, in all experiments run in acetonitrile, there is also a signal ∼ ∼ from an extra radical species (with g = 2.0067 and aN = 13.83 G, aH = 2.3 G), supposedly the

Figure W. EPR spectrum from ex-cell spin-trapping experiments with PBN during the reduction of Ph3SnCl. Electrolysis at a Pd cathode in AN/0.1 M Bu4NPF6, E = -0.9 V. T = 293 K.

Figure V. SOMO of the cation radical of 1,2-(silatranyl)methane from DFT B3LYP/Lanl2DZ calculations. Two crossing 3c-4e moieties are well seen.

Table P. Spin trapping of electrogenerated stannyl radicals with PBN a

Halostannane g-factor hfc, G Ratio Ph3SiCl 2.0065 aN: 14.687, aH: 2.732 50% 2.0067 aN: 13.815, aH: 2.333 50%

Bu3SiCl 2.0065 aN: 14.625, aH: 3.129 75% 2.0067 aN: 13.854, aH: 2.286 23% 2.0073 aN: 16.183 2%

b SnATRCl 2.0083 aN: 14.596, aH: 3.357 92%

SnATRBr 2.0065 aN: 14.543, aH: 2.76 79% 2.0067 aN: 13.846, aH: 2.329 18%

SnATRI 2.0064 aN: 15.704 98% a b Electrolyses at a Pt/Pd ?? electrode in AN/0.1 M Bu4NPF6, - or DMF ? Electrolysis in THF/0.1 M Bu4NPF6.

FigureFigure O. 71: EPR EPR spectrum spectrum from from ex ex-cell-cell spin spin-trapping-trapping experiments experiments with with PBN PBN during during the the reduction reduc- oftion chlorido of chlorido stannatrane. stannatrane. (a) Experimental, (a) Experimental, (b) baseline (b) baseline-corrected,-corrected, (c) simulated. (c) simulated. Electrolysis Electrol- at a Pd cathode in THF/0.1 M Bu NPF , E = -1.0 V. T = 293 K. ysis at a Pd cathode in THF/0.14 M6TBAPF6, E = -1.0 V. T = 293 K.

spin-adduct from the trapping of solvent-related radical by PBN. In similar experiments using

THF as solvent, this signal was not observed. In case of reduction of Bu3SnCl in the presence of PBN, trace amounts of a spin-adduct without α-proton (spectrum as simple triplet with

single nitrogen constant, g = 2.0073 and aN = 16.183 G) was registered, the species issued of • a two-step trapping, which involves H-abstraction by Bu3Sn and first stannyl-radical addition • providing an even-electron stannylated adduct which then acts as a trap for second Bu3Sn radical. Therefore we tried to obtain the dimer of stannatrane by electroreduction. The result was shown in the figure 74: a is the cyclic voltammogram of stannatrane chloride before the electrolysis and b is after 1 h electrolysis. It is obvious that the reduction peak of stannatrane chloride disappeared after the electrolysis which indicates that there is no more stannatrane chloride in the solution (distannanes are not reducible [93]). After separation purification, and we obtained dimer of stannatrane chloride. We did the same electrolyse with two model organotin chlorides (triphenyltin chloride and tributyltin chloride) and also obtained corresponding the dimers. Table 20 shows the chemical shifts of 119Sn NMR. There are large shifts to higher fields from the monomer to dimer (Table 20). The result of GC-MS also proved that the dimer was obtained after electrolysis in the case of triphenyl- and tributyl tin chloride. However it

FigureFigure A. 72: EPR EPR spectrum spectrum from from spin spin-trapping-trapping with with PBN PBN during during the the reduction reduction of of bromo bromo stannatrane. stanna- ElectrolysisFigure A. EPR at a spectrum Pd cathode from in AN/0.1spin-trapping M Bu 4withNPF 6PBN, E = during -0.9 V. the T =reduction 293 K. of bromo stannatrane. trane. Electrolysis at a Pd cathode in CH3CN/0.1 M TBAPF6, E = -0.9 V. T = 293 K. Electrolysis at a Pd cathode in AN/0.1 M Bu4NPF6, E = -0.9 V. T = 293 K.

Figure Q. EPR spectrum from spin-trapping experiments with PBN during the reduction of iodo stannatrane.FigureFigure Q. 73: EPR Electrolysis EPR spectrum spectrum atfrom a from Pd spin cathode spin-trapping-trapping in AN/0.1 experiments experiments M Bu 4withNPF with 6PBN, E PBN = during-0.7 during V. the T the =reduction 293 reduction K. of iodo of stannatrane.iodo stannatrane. Electrolysis Electrolysis at a Pd at cathode a Pd cathode in AN/0.1 in CH MCN Bu/0.14NPF M6, TBAPFE = -0.7 V., E T = = -0.7 293 V. K. T = 293 3 6 K. Spin trapping with -phenyl-N-tert-butyl-nitrone (PBN) during the electroreduction of the correspondingSpin trapping halo withstann atranes-phenyl (table-N-tert P)-butyl confirmed-nitrone the (PBN) formation during of the Sn -electroreductioncentered radicals of duringthe corresponding the reduction halo processstann atranesin this range(table ofP) weakly confirmed cathodic the formation potentials of and Sn -hencecentered the radicalsoccurrenc e ofduringcannot a different the detect reduction mechanism the dimer process ofof stannatranereduction in this range at chloride. transition of weakly metal cathodic electrodes. potentials Under and these hence conditions, the occurrenc e similarof a different species mechanism were as well of trappedreduction with at transitionmodel chlorostannanes, metal electrodes. Ph 3UnderSnCl and these Bu conditions,3SnCl (figure E)similar. Small species proton were hfc constantsas wellTable trapped in 20:these with dimer adducts model SnSn attest chlorostannanes, (Result bulky of charac GC-MS) terPh 3ofSnCl the andtrapped Bu3 SnClradical (figure addendE). Small andCompound proton the values hfc constants of aHδ 119are Snin reciprocate these(ppm) adductsE to(V) the attest steric bulky demand charac Product ofter the of concerned the trappedδ119 species:Sn radical(ppm) addend and the values of a are reciprocate to the steric demand of the concerned species: Ph3TriphenyltinSn > stannatrane chloride > Bu3Sn.H In -44.90 the case of -0.54 iodostannatrane, Tris(n-phenyl)distannane only the nitrogen triplet -143.27 spectrum Ph Sn > stannatrane > Bu Sn. In the case of iodostannatrane, only the nitrogen triplet spectrum was3 Tributyltinobtained, arising chloride from3 156.88(144)the spin-adducta without-0.93  Tris(-proton.n-butyl)distannane As was mentioned before, -82.66 this stannatranewasStannatrane obtained, is lightarising chloride-sensitive, from the its spin -243 decomposition-adduct -1.94without pro bably-proton. Bis-stannatrane involving As was radical mentioned species. before, -639.72 In addition, this it hasstannatranea[95, higher 96] Lewis is light acidity-sensitive, than theits decomposition other stannatranes. probably Since involving the concentration radical species. of PBN In in addition, the it catholytehas higher is Lewis inferior acidity to that than of thethe iodostannatane,other stannatranes. the Sincenitrone the was concentration supposedly ofall PBNreacted in thewith thesecatholyte decomposition is inferior -toinduced that of radicalsthe iodostannatane, or simply via the deprotonation. nitrone was supposedly all reacted with these5.5.3 decomposition Oxidation of-inducedIl distannatrane faut faire radicals un schema or simply de reductionvia deprotonation. … Besides major spinIl faut-adduct, faire un in schemaall experiments de reduction run in … AN (ou DMF?), there is also a signal Besides major spin-adduct, in all experiments run in AN (ou DMF?), there is also a signal fromBesides an extra the aboveradical proofs species obtained (with g = from 2.0067 chemical and aN analysis,  13.83 G, the a cyclicH  2.3 voltammetry G), supposedly of elec- the from an extra radical species (with g = 2.0067 and a  13.83 G, a  2.3 G), supposedly the spintrooxidation-adduct from of distannatrane the trapping of was solvent examined-related as wellradicalN as by compared PBN. In with Hsimilar the experiments oxidation of using dimer THFspin- asadduct solvent, from this the signal trapping was of not solvent observe-relatedd. In caseradical of reductionby PBN. In of similar Bu3SnCl experiments in the presence using of Bu SnSnBu PBN,THF3 astrace solvent, amounts3. this ofsignal a spin was-adduct not observe withoutd . In-proton case of (spectrum reduction as of simple Bu3SnCl triplet in the with presence single of nitrogenPBN,On trace theconstant, amounts anodic g branch= of 2.0073 a spin of -and voltammogramadduct aN =without 16.183 of G)-proton the was solutions registered, (spectrum electrolyzed theas simplespecies at triplet issued 1.34 with V of vs. a singletwo SCE,-step nitrogen constant, g = 2.0073 and a = 16.183 G) was registered, the species issued of a two-step trapping,new peaks which appeared, involves as H shown-abstraction in ﬁgureN by 75. Bu3 SinceSn and neither first stannyl stannatrane-radical chloride addition nor providing tributyltin an   eventrapping,-electron which adduct involves which H- thenabstraction acts as bya trap Bu 3forSn secondand first Bu stannyl3Sn radical.-radical addition providing an chloride are oxidizable at these potentials, one can suppose these peaks to arise from the even-electron adduct which then acts as a trap for second Bu3Sn radical. oxidation of the corresponding distannane products. However, other candidates for the oxidation at these potentials should be considered as

0 b

- 1 0 0 a

A - 2 0 0

, I - 3 0 0

- 4 0 0

- 3 . 0 - 2 . 5 - 2 . 0 - 1 . 5 - 1 . 0 - 0 . 5 0 . 0 E , V v s S C E

Figure 74: Voltammetric monitoring of the electrolysis of stannatrane chloride E = -1.94 V at GC electrode in CH3CN/0.1 M LiClO4 a. electroreduction before electrolysis; b. after 1 h

well. A very easy oxidation of Ph3SnH was reported, Ep = −0.3 V vs. SCE [97] or Ep = −0.4 V vs. SCE [98], though in more recent accounts it was confronted with much higher value of Ep = 0.8 V vs. SCE [99]. It was mentioned above that the E1/2 of the redox couple + • • − Ph3Sn /Ph3Sn was reported, E1/2 = 0.2 V vs. SCE [94]. (E1/2(Ph3Sn /Ph3Sn ) = −0.46 • − V in CH3CN vs. SCE and E1/2(Bu3Sn /Bu3Sn ) = -0.93 V in DMSO vs SCE).

4 0 0 b 3 0 0 a

2 0 0 A

I 1 0 0

- 1 0 0 0 . 5 1 . 0 1 . 5 E , V v s . S C E

Figure 75: Cyclic voltammetry of a bis-stannatrane (C = 0.1 mmol/L) in CH3CN/0.1M o LiClO4 at a 2 mm GC electrode v = 1 V/s T = 25 C; b tris(n-butyl)distannane (C = 0.1 o mmol/L) in CH3CN/0.1M TBAPF6 at a 2 mm GC electrode v = 1 V/s, T = 25 C.

Moreover it is reported that the oxidation dimer of Me3SnSnMe3 and Et3SnSnEt3 occurs at 1.28 V and 1.24 V vs. Ag/AgCl (Ag/AgCl = 1.99 V vs. NHE), respectively, in CH3CN at GC electrode [100] these values are close to those observed in our cyclic voltammetry expressions of dimers. Thus one can believe that the oxidation peaks at the potential 1.34 V and 1.33 V vs. SCE (Figure 75) are dimer Bu3SnSnBu3 (b) and distannatrane (a) respectively even though the peaks are not reversible and the values of peak width (Ep − Ep/2) are much larger to correspond to a perfect redox process.

94 5.6 New non-coordinating ﬂuoride encapsulating silsesquioxane-based supporting electrolyte

Weekly coordinating anions mostly used in supporting electrolytes can be arbitrarily divided Bu4N[B(C6F5)4] was prepared by metathesis of Li[B(C6F5)4] etherate (ABCR) with intoBu four4NClO groups4 in (Figuremethanol. 76): All those experiments with a Lewis were acid performed central atom under carrying Ar atmosphere. additional electron- Tetrabutylammonium salts of− octaorga− nosilsesquioxanes− − with encapsulated F- anion acceptor substituting atoms (a: BF4 , PF6 , AsF6 , SbF6 ), those with bulky delocalizing were prepared according− to the general method described by Bassindale et al (15) mixing organic groups (b: Ph4B ), those combining both features (c: bulky delocalizing groups THF solutions of the corresponding organotriethoxysilane RSi(OEt)3 (R = Ph, CH2CN, − − − withCH electron-withdrawing2CH2CN and CH2CH substituents,2CF3, all from(C ABCR)6F5)4B with, [(CF the 3stoechiometric)2C6H3]4B , [(CF amount3)3CO] of Bu4Al4NF), and(as relatively a 1 M solution recently in introduced THF with icosahedral 5% of water, carboranes Aldrich).CB After11 containing the overnight no πstirring,-orbital the (d: solvent− was vacuum evaporated and the obtained residues were washed with chloroform CB11H6X6 with X = Cl or Br). and filtered through a 5 cm layer of SiO2 or recrystallized, if necessary, from acetone. - 19 TheOn thepresence other of hand, F in manythe T8 "rattle-like"R8 cage was systemschecked (bye) areF NMR known, spectroscopy. in which an atom or an ion is entrapped inside of a hollow cavity, with no covalent bonding between them. Metals Results and Discussion • inside of fullerenes (M@C60), hydrogen atom in octahedral silsesquioxanes (H @T8), and ﬂuorideWeekly anion coordinating in zeolites provideanions mo suchstly examples. used in Recently,supporting Bassindale electrolytes et al can[101] be discoveredarbitrarily divided into four groups (figure 1): those −with a Lewis acid central atom carrying an a new class of silsesquioxane structures, with F anion encapsulated- - inside- of the- T8Ph8 cage. additional electron-acceptor substituting atom (a: BF4 , PF6 , AsF6 , SbF6 ), those with Negative charge in these systems is hidden in the- center of an inert siloxane box cutting off bulky delocalizing organic groups (b: Ph4B ), those combining both features (c: bulky - - directdelocalizing coordination groups with with external electr electrophileson-withdrawing in favor substituents, of purely electrostatic(C6F5)4B , [(CF interactions.3)2C6H3]4B , - - [(CF3)3CO]4Al ), and icosahedral carboranes CB11 containing no π-orbitals (d: CB11H6X6 with X = Cl or Br).

Figure 1. Different types of low-coordinating supporting electrolyte anions. Figure 76: Different types of low-coordinating supporting electrolyte anions On the other hand, many "rattle-like" systems (e) are known, in which an atom or an ion in entrapped inside of a hollow cavity, with no covalent bonding between them. Metals inside of fullerenes (M@C60), hydrogen atom in octahedral silsesquioxanes • (HIt was@T8 pointed), and fluoride out that anion such in compounds zeolites provide could such not exampl be preparedes. Recently, with non-acceptor Bassindale et sub- al - stituents(15) discovered at Si [101, a 102]. new Sinceclass of the silsesquioxane LUMO of T8 Rstructures,8 structures with lies F inside anion of encapsulated the T8 cage inside of the T Ph cage. Negative charge in these systems is hidden in the center of an [103], the electron-withdrawing8 8 substituents should raise its level, so if it worked with R = inert siloxane box cutting off direct coordination with external electrophiles in favor of Ph,purely then all electrostatic substituents interactions. whose inductive electron-withdrawing effect (Taft’s σR) is stronger than thatIt of was Ph (Figurepointed 77),out inthat principle such compounds must permit could the formationnot be prepared of such with anions. non-acceptor Based on thissubstituents consideration, (15,16). we prepared Since the TBA LUMO salts of with T8RF8 −structuresencapsulating lies insideT R ofcages the T choosing8 cage (17), the the electron-withdrawing substituents should raise its level, so if 8it worked8 with R = Ph, R groups bordering the silsesquioxane cage which would be potentially compatible with the then all substituents whose inductive electron-withdrawing effect (Taft's σR) is stronger electrochemicalthan that of Ph environment (fig. 2), in (Rprinciple = Ph, CH must2CN permit, CH2 CHthe2 formationCN, CH2CH of 2suchCF3 ).anions. Based on - this consideration, we prepared TBA salts with F encapsulated T8R8 cages choosing the R groups bordering the silsesquioxane cage95 which would be potentially compatible with the electrochemical environment (R = Ph, CH2CN, CH2CH2CN, CH2CH2CF3).

Figure 2. The substituents with Taft inductive constants σR suitable for the formation of - fluoride-encapsulated silsesquioxane salts TBA[F @T8R8].

Figure 2. The substituents with Taft inductive constants σR suitable for the formation of FigureIn 77: the The meantime substituents other with structures, Taft inductive all with constants the- substituentsσ suitable forR thesatisfying formation σR of > σPh fluoride-encapsulated silsesquioxane salts TBA[F @T8RR8]. − requirement,ﬂuoride-encapsulated have been silsesquioxane prepared: saltsR = p- TBA[R-Ph,F @T(CH8R2)82].(CF2)mCF3 (m = 0, 3, 5, 7 (18)), Vin, CH=CH2PhIn and the CH meantime2CH2(CF other2)nCF structures,3 (n = 0, 3,all 5) with (19). the substituents R satisfying σR > σPh Asrequirement, was shown have by been DFT prepared: B3LYP/6-311G(d,p) R = p-R-Ph, (CH 2calculations,)2(CF2)mCF3 (m there σ= 0, is3,σ 5,no 7 substantial(18)), Vin, In the meantime other structures, all- with the substituents R satisfying R > Ph require- chargeCH=CH transfer2Ph from and theCH 2centralCH2(CF 2F)n CFanion3 (n =to 0, the 3, 5)surrounding (19). atoms of the silsesquioxane ment, have been prepared:- R = p-R-Ph, (CH2)2(CF2)mCF3 (m = 0, 3, 5, 7 [104]), Vin, cage: NBO Ascharge was shownon F bychanges DFT B3LYP/6-311G(d,p)from -1 to -0.89 when calculations, passing there from is freeno substantial to the T8 - CH=CH2Ph and CH2CH2(CF2)- nCF3 (n =- 0, 3, 5) [105]. entrappedcharge form. transfer Given from that the F centralion can F enter anion and to theleave surrounding the T8 cage atoms (18,19), of the thesilsesquioxane use of polar - - Ascage: was shown NBO bycharge DFT on B3LYP/6-311G(d,p) F changes from -1 calculations, to -0.89 when there passing is no substantialfrom free chargeto the T8- solvents should rather be precluded,- as they will destabilize the F @T8 systems by entrapped form. Given− that -F ion can enter and leave the T8 cage (18,19), the use of polar solvationtransfer from of the centralcage-escapedF anion F to anion. the surrounding atoms of the silsesquioxane- cage: NBO solvents− should rather be precluded, as they will destabilize the F @T8 systems by chargeNegative on F changes charge, from mapped -1 to -0.89on - the when solvent passing ava fromilable free surface to the andT8-entrapped visible by form. external solvation of the cage-escaped F anion. - electrophiles,− is shown in figure 3 for F @T8R8 (with R = CH2CN and Ph), and is Given that FNegativeion can entercharge, and mapped leave the onT the8 cage solvent [104, ava 105],ilable- the surface use of polar and solventsvisible by should external - compared to several common anions. Whereas− free F anion is 100% available from all rather beelectrophiles, precluded, asis theyshown will in destabilizefigure+ 3+ for the F+F@T@8TR8 systems(with R by = solvationCH2CN and of the Ph), cage- and is sides, only small electrophiles (H , Li , Na ) can 8approach- the zones of a meaningful compared− to several common anions. Whereas free F anion is 100% available from all escaped F anion. - + - + + nucleophilicitysides, only in smallF @T electrophiles8(CH2CN)8 and(H , FLi@T, Na8(Ph)) can8. approach the zones of a meaningful - - nucleophilicity in F @T8(CH2CN)8 and F @T8(Ph)8.

- - - - Figure Figure3. Negative 3. Negative charge charge (dark (dark areas areas for − forF andF and BF− BF4 ,4 and, and those those markedmarked withwith thethe arrows arrows Figure 78: Negative charge (dark areas for F and BF4 , and those marked with the arrows for thethefor otherother the anions) otheranions) anions) mapped mapped mapped on solventon solventon availablesolvent avai avai surfacelablelable surface for surface low-coordinating for for low-coordinatinglow-coordinating anions (DFT anions anions (DFT B3LYP/6-311G(d,p) calculations);- F- and BF- - are given for comparison. (DFTB3LYP/6-311G(d,p) B3LYP/6-311G(d,p) calculations); calculations);F− and BF F− areand given BF4 for4are comparison. given for comparison. 4 - In aproticIn aprotic solvents solvents like likeCH CHCN3CN and and THF, THF, the the TBA(F TBA(F-@T@T8RR8)) saltssalts undergoundergo ionic ionic 3 8 8 λ dissociation. Limiting values of molar conductivity of these anions, λo, have been dissociation. Limiting values of molar conductivityλ 2 πηof these anions, o, have been estimated (table I) using Stokes equation o√ = NA2e /(6 r) and within the more advanced estimated ◦(table I) using Stokes equation2 λo = NAe /(6πηr)2 and within the1/2 more advanced modelλ = ofeF Kuznetsova x/rhη{0.5y (eq.(y/x 2)) [1taking + rS +into 4r Saccount2/(r +solventrS) (1 holes + M Wfluctuation,/MS) ]} radius of the model solvationof Kuznetsova sphere and(eq. frontal 2) taking resistance into accountto the movement solvent holesof a bulky fluctuation, ionic complex radius in of this the solvation sphere and frontal resistance to the movement of a bulky ionic complex in this media (20). 5.6.1 media (20). [2]

[2] Negative charge, mapped on the solvent available surface and visible by external elec-

96 − trophiles, is shown in ﬁgure 78 for F @T8R8 (with R = CH2CN and Ph), and is compared to several common anions. Whereas free F− anion is 100% available from all sides, only small electrophiles (H+, Li+, Na+) can approach the zones of a meaningful nucleophilicity in − − F @T8(CH2CN)8 and F @T8Ph8. − In aprotic solvents like CH3CN and THF, the TBA(F @T8R8) salts undergo ionic dis-

sociation. Limiting values of molar conductivity of these anions, λo, have been estimated 2 (table 21) using Stokes equation λo = NAe /(6πηr) and within the more advanced model of Kuznetsova (Eq 5.6.1) taking into account solvent holes ﬂuctuation, radius of the solvation sphere and frontal resistance to the movement of a bulky ionic complex in this media [106].

Here x is a mean distance between solvent molecules, rh and rS are “hole” and solvent

radii, y is the radius of the moving ionic complex, MW and MS are ion and solvent masses, respectively, and η is solvent viscosity.

− Table 21: Limiting molar conductivites of F @T8 anions and their TBA salts Here x is a mean distance between solvent molecules, rh and rs are “hole” and solvent radii, y is the radius− of the moving ionic complex, −MW and MS are ion and solvent M (F @T R ), a λ (F @T R ), R W η8 8 r(F−@T R ), Å o 8 8 Λ b, S cm2/mol masses, respectively,g/mol and is solvent viscosity.8 8 S cm2/mol o

Ph 1033- 6.70 27.15 (29.75) 69.45 (71.87) Table I. Limiting molar conductivities of F @T8 anions and their TBA salts (CH2)2CF3 1212- 6.40 29.75- (30.96) 72.05 (73.26) MW(F @T8R8), - λo(F @T8R8), a 2 -1 (CH2R) 2CN 867-1 r(F @T 6.34R ), Å 29.97 (31.25) 72.27Λ , S (73.55) cm mol g mol 8 8 S cm2 mol-1 o Ph 1033 6.70 27.15 (29.57) 69.45 (71.87) a From DFT B3LYP/6-311G(d, p) optimized geometry. (CH2)2CF3 1212 6.40 29.75 (30.96) 72.05 (73.26) b Λo = λo(F−@T R ) + λo(Bu N+); (CH2)2CN 8 8 867 4 in parenthesis, 6.34 the values 29.97 form (31.25) Stokes model 72.27 are given. (73.55) a ; in parentheses, the values form Stokes model are given.

ε = 7.6

- - Figure 5. Conductivity of THF solutions of: () TBA[F @T (CH− CH CF ) ], ({) TBA[F Figure 79: Molar conductivity of THF solutions of: ()TBA[F8 @T2 8(CH2 23CH8 2CF3)8], − - − @T(◦)TBA[F8(CH2CN)@T88],(CH (z)2 CN)TBA[F8], (•@T)TBA[F8Ph8], @T(U)8 PhTBABF8], (4)TBABF4, (V) TBAF.4, (5)TBAF .

97 ε = 37.5

ε = 16.6

- Figure 6. Conductivity of CH3CN solutions of: () TBA[F @T8(CH2CH2CF3)8], ({) - - TBA[F @T8(CH2CN)8], (z) TBA[F @T8Ph8], (U) TBABF4, (V) TBAF, () TMAF. ( ) - TBA[F @T8Ph8] in (CF3)2CHOH.

Here x is a mean distance between solvent molecules, rh and rs are “hole” and solvent radii, y is the radius of the moving ionic complex, MW and MS are ion and solvent masses, respectively, and η is solvent viscosity.

- Table I. Limiting molar conductivities of F @T8 anions and their TBA salts - - MW(F @T8R8), - λo(F @T8R8), a 2 -1 R -1 r(F @T R ), Å Λ , S cm mol g mol 8 8 S cm2 mol-1 o Ph 1033 6.70 27.15 (29.57) 69.45 (71.87) (CH2)2CF3 1212 6.40 29.75 (30.96) 72.05 (73.26) (CH2)2CN 867 6.34 29.97 (31.25) 72.27 (73.55) a ; in parentheses, the values form Stokes model are given.

ε = 7.6

− In THF solutions, the conductivity of TBA(F @T8R8) salts are about 10 times higher than

that of TBABF4 (Figure 79); in CH3CN it is slightly lower, even for the anion with CH2CN − substituent, mimicking acetonitrile itself. Interestingly, that in THF (ε = 7.8) the F @T8R8 anions all show a shallow minimum due to ion pairing, which occurs at the concentrations much below those practically used (C ∼= 5−10 M) beyond which the conductivity remains sta-

ble. For TBABF4, such minimum is observed at the values close to practical concentrations (C ∼ −10 − = 2 M). In perﬂuoroisopropanol, TBA(F @T8Ph8) shows a deep minimum corresponding to the formation of ion pairs and then the conductivity regains the same values as in CH CN 3 (Figure 80). - - Figure 5. Conductivity of THF solutions of: () TBA[F @T8(CH2CH2CF3)8], ({) TBA[F - @T8(CH2CN)8], (z) TBA[F @T8Ph8], (U) TBABF4, (V) TBAF.

ε = 37.5

ε = 16.6

- − { FigureFigure 80:6. Conductivity Molar conductivity of CH of3CHCN3 CNsolutionssolutions of: of: ( () )TBA[FTBA[F @T@T88(CH(CH22CHCH22CFCF33))88],], ( ) - − - − TBA[F(◦)TBA[F@T8@T(CH8(CH2CN)2CN)8], (8z],)( •TBA[F)TBA[F@T@T8Ph8Ph8],8 ](,U()4 TBABF)TBABF4,4 (,V(5) TBAF,)TBAF ,((♦))TMAF TMAF., ( ) - − TBA[F()TBA[F@T8Ph@T88]Ph in 8(CF] in(CF3)2CHOH.3)2CHOH .

In the same solvent, the difference of potentials between two consecutive electron transfers 1 2 of an electrochemically stable redex system, ∆E = Eo − Eo, is a good criterion for assessing the energy of ion pairing between the electrogenerated ionic species and the counter ion of the supporting electrolyte; this aspect was considered in details in many papers, both for reduction and oxidation processes, see e.g. [56, 57, 58]. We used the reversible one-electron oxidation of phenothiazine (Scheme 16), followed by formation of unstable dication, as a probe for com- − paring the ion-pairing ability of F @T8R8 anion with that of several common electrochemical anions.

98 S S S - e - e

N N N H H H Scheme 16: Reversible one-electron oxidation of phenothiazine

Table 22: Difference in potentials of ﬁrst and second oxidation peaks of phenothiazine with different supporting salt anions TBAX in CH3CN

1 a 1 b 2 2 1 X Eo , V ∆Eo , V Ep, V Ep − Eo , V − BF4 0.269 0 0.869 0.600 − PF6 0.281 0.012 0.925 0.644 − B(C6F5)4 0.325 0.056 1.188 0.863 − F @T8Ph8 0.325 0.056 1.316 0.991 a vs. Ag/0.1 M AgNO3 in CH3CN b 1 − vs. Eo (BF4 )

Table 23: Difference in potentials of ﬁrst and second oxidation peaks of phenothiazine with different supporting salt anions TBAX in CH2Cl2

1 a 1 b 2 2 2 1 X Eo , V ∆Eo , V Ep(Eo ), V Ep − Eo (∆Eo), V − BF4 0.258 0 1.018 0.760 − PF6 0.258 0 0.986(0.873) 0.728(0.615) − B(C6F5)4 0.319 0.061 1.148(0.967) 0.829(0.648) − F @T8(CH2CH2CF3)8 0.321 0.061 1.395 1.074 a vs. Ag/0.1 M AgNO3 in CH3CN b 1 − vs. Eo (BF4 )

The voltammograms of oxidation of phenothiazine with different anions are shown in ﬁg- ure 81, and the corresponding values of Eo and Ep are collected in Table 22. First oxidation − peak shows slight anodic shift upon going from BF4 anion, taken as a reference, to less co- − − 1 − ordinating PF6 and then to (C6F5)4B anion. Though Ep with F @T8Ph8 anion is more − positive than that with (C6F5)4B , the Eo values are similar in these two cases (Table 22). 2−1 2 1 Second oxidation peak shows more dramatic anodic shifts and the ∆E (Ep − Eo) value − is maximal for F @T8Ph8 anion attesting its least ion pairing with the dication of phenoth- 2−1 iazine. Similar trends were observed in CH2Cl2 (Table 23), the corresponding ∆E value − for F @T8(CH2CH2CF3)8 in this case is even greater.

-3 -1 Figure 7. Oxidation of phenothiazine (2×10−3 mol L ) at a GC disk electrode in CH3CN Figure 81: Oxidation of phenothiazine (2 × 10 mol/L) at a GC disk electrode in CH3CN using different supporting TBA salts. (a) TBABF4, (b) TBAPF6, (с) TBAB(С6F5)4, (d) using different- supporting TBA salts. (a) TBABF4,(b) TBAPF6,(c) TBAB(C6F5)4,(d) TBA(F− @T8Ph8). TBA(F @T8Ph8).

− The THF/0.07 M Bu4N(F @T8Ph8) system was used for EPR-spectroelectrochemical study of one-electron reduction (Scheme 17) of 9-haloﬂuorene [107]. Reconstruction of the observed spectrum (Figure 82) using Simphonia program (Bruker) was possible with the parameters (g = 2.0026; a9−H = 13.89 G, a1,1,−H = 3.92 G, a2,2,−H = 0.88 G, a3,3,−H = 3.81

G, a4,4,−H = 0.68 G) close to those reported for free ﬂuorenyl radical. It is supposed that

Ph groups of the silsesquioxane anion serve as a-3 kind of-1 aromatic matrix accommodating and Figure 7. Oxidation of phenothiazine (2×10 mol L ) at a GC disk electrode in CH3CN using different supporting TBA salts. (a) TBABF4, (b) TBAPF6, (с) TBAB(С6F5)4, (d) stabilizing ﬂuorenyl- radical. TBA(F @T8Ph8).

Figure 8. EPR spectra from the reduction of 9-iodo-fluorene in (propylene carbonate- - THF, 1:2 v/v)/0.07 M Bu4N(F @T8Ph8). (a) experimental, (b) high-frequency component, (c) same signal after FFT filtering, (d) simulated using the parameters in the text.

[4]

Figure 8. EPR spectra from the reduction of 9-iodo-fluorene in (propylene carbonate- - Figure 82: EPRTHF, 1:2 spectra v/v)/0.07 from M Bu the4N(F reduction@T- 8Ph8). of(a) 9-iodo-ﬂuoreneexperimental, (b) high-frequency (in propylene component, carbonate-THF The THF/0.07 −M Bu4N(F @T8Ph8) system was used for EPR-spectroelectro (1:2 v/v)/0.07(c) same M Bu signal4N( afterF @ FFTT8 filtering,Ph8). ((ad) simulated experimental, using the ( bparameters) high-frequency in the text. component, (c) chemical study of one-electron reduction (eq. 4) of 9-halofluorene (21). Reconstruction sameof signalthe observed after FFT spectrum ﬁltering, (figure (d) simulated 8) usin usingg Simphonia the parameters program in the (B text.ruker) was possible with the parameters (g = 2.0026; a9-H = 13.89 G, a1,1’-H = 3.92 G, a2,2’-H =[4] 0.88 G, a3,3’-H 100

- The THF/0.07 M Bu4N(F @T8Ph8) system was used for EPR-spectroelectro chemical study of one-electron reduction (eq. 4) of 9-halofluorene (21). Reconstruction of the observed spectrum (figure 8) using Simphonia program (Bruker) was possible with the parameters (g = 2.0026; a9-H = 13.89 G, a1,1’-H = 3.92 G, a2,2’-H = 0.88 G, a3,3’-H 3' 4' 4 3 I- + e 2' 2 - Br- - - I 9 1' 1

Br I Scheme 17: One-electron reduction of 9-haloﬂuorene.

N N Si - e O O Si Si O O Si Si O N + N O DME Si N - N TBA[F @T8] 1.27 V + e 0.92 V - e

+ e 2 N N O 0.92 V O Si Si

Scheme 18: Oxidation of the model disilane and two short-lived odd-electron species.

Electrooxidation of specially prepared ligand-stabilized 1, 1, 2, 2-tetramethyl-1, 2-bis

(quinolin-8-yloxy) disilane (the process is EPR-silent in CH3CN/0.05 M TBAPF6) in EPR − spectroelectrochemical cell using (DME-CH2Cl2, 3:1 v/v) as solvent and TBA(F @T8Ph8) as supporting electrolyte at 253 K allowed to detect two odd-electron species, supposedly short-lived cation radical of this disilane and the radical product of its cleavage (Scheme 18).

First species (g = 2.01, aSi = 39.2 G) is obviously a Si-centered radical, the integration of its 29Si satellites (≈9%) indicates the spin localization on two Si atoms. Second species is an organic radical, as follows from its g-factor (2.0033). Spin density in this radical is distinctly delocalized over the quinoline moiety (aN = 3.78 G, aH4 = 6.86 G, aH5 = 4.03 G, aH7 = 2.08 13 29 G, aH3 = 1.20 G, aH6 = 1.12 G) with an additional hfc constant, either from C or Si (a = 7.1 G), the quality of the spectrum edges not allowing a more precise assignment. − The TBA(F @T8R8) supporting salts are chemically stable and can be stored without any decomposition while solid; their solutions in the tested solvents are also stable, at least within the timeframe of typical electrochemical experiments. However, as was ﬁrst pointed out by Bassindale [104] and as follows from the above DFT consideration (Figure 78), they are sensitive to the presence of small cations in solution, namely of Li+ and H+. Figure 83 − shows that in CH3CN, if LiClO4 is added to the solution, the "leakage" of F anion from the − F @T8(CH2CN)8 cage to form LiF/TBAF and an empty T8(CH2CN)8 is practically accomplished within 9-10 hours. In a similar way, the protons liberated during 30 min experiments

101 = 3.81 G, a4,4’-H = 0.68 G) close to those reported for free fluorenyl radical. It is supposed that Ph groups of the silsesquioxane anion serve as a kind of aromatic matrix accommodating and stabilizing fluorenyl radical.

[5]

Electrooxidation of specially prepared ligand-stabilized 1,1,2,2-tetramethyl-1,2- bis(quinolin-8-yloxy)disilane (the process is EPR-silent in CH3CN/0.05 M TBAPF6) in - EPR-spectroelectrochemical cell using (DME-CH2Cl2, 3:1 v/v) as solvent and TBA(F @T8Ph8) as supporting electrolyte at 253 K allowed to detect two odd-electron species, supposedly short-lived cation radical of this disilane and the radical product of its cleavage (eq. 5). First species (g = 2.01, aSi = 39.2 G) is obviously a Si-centered radical, 29 ≈ the integration of its Si satellites ( 9%)− indicates the spin localization on two Si atoms. (voltammetrySecond species of methylanthracene)is an organic radical, in asF follows@T8(CH from2CH its2CF g-fa3)ctor8 in (2.0033). liquid SO Spin2, caused density the in ap- − 19 pearancethis radical of free is distinctlyF in the delo solution,calized though over the this quinoline process became moiety visible(aN = 3.78 in G,F NMR aH4 = only6.86 uponG, 4a hoursH5 = 4.03 of staying G, aH7 of = the2.08 used G, a solution.H3 = 1.20 Supposedly, G, aH6 = 1.12 the G) solvent with polarityan additional and the hfc efﬁciency constant, of either from 13C or 29Si (a = 7.1 G), the quality of the spectrum edges not allowing a more free F− solvation play here an important role, as pointed out above. precise assignment.

19 - Figure 9. 19F NMR-monitored leakage of F− anion from the T8R8 cage (R = CH2CN) in Figure 83: F NMR-monitored leakage of F anion from the T8R8 cage (R = CH2CN) in the the presence of LiClO4. Solvent: CH3CN, T =◦ 20 °C. presence of LiClO4. Solvent: CH3CN, T = 20 C.

- The TBA(F @T8R8) supporting salts are chemically stable and can be stored without any decomposition while solid; they solutions in the tested solvents are also stable, at least within the timeframe of typical electrochemical experiments. However, as was first pointed out by Bassindale (18), they are sensitive to the presence of small + + cations in solution, namely of Li and H . Figure 9 shows that in CH3CN, if LiClO4 is

102

CONCLUSION 6 Conclusion

Seven simple 1-R-silatranes have been synthesized. Four of their structures are characterized by X-ray diffraction (XRD). Electrochemical measurements show that electrooxidation of tolylsilatrane, 2-cyanoethylsilatrane and t-butylsilatrane are electrochemically reversible creating cation radicals by one electron transfer; these species can be detected by cyclic voltammetry, UV spectroscopy and real-time EPR spectroelectrochemistry. N and Si atoms from the atrane moiety and substituent atom at α-position to Si compose a 3c-4e conjugated system responsible for the redox properties of these compounds. The π-orbital of Ar substituents, are orthogonal to this system, so the electrooxidation of silatranes with Ar substituent are only affected by inductive effect of the substituent. Three bis-(silatranyl)alkanes have been synthesized but the structures were not diffractive by XRD because of the difficulty of obtaining the single crystals. However, electrochemical measurements show that anodic oxidation of bis-(silatranyl)octane is electrochemically reversible and results in relatively stable cation radicals. There are two electron transfers during the process. Bis-(silatranyl)methane has 7c-8e bonding system whereas bis-(silatranyl)hexane and bis-(silatranyl)octane have long carbon chains between two atrane structures which are flexible but not long enough to let the two atrane parts face each other in a line to form a 6c-8e bonding system. Three bi-silatranes have been synthesized. Their structure differs from other silatranes in that the N atom is now included in the plan formed by the 3 adjacent carbon atoms, as XRD spectroscopy has shown. In fact, the whole molecule is perfectly symmetrical in relation to this plane. This has consequences on the electrochemistry of these compounds. Though the electrooxidation is still proceeding via a one-electron step, the electrooxidation of these compounds is reversible (even at low scan rates for di(ethyl)-bi-silatrane and di(phenyl)-bi- silatrane). We have proved that the chemical structure, and most importantly the effect of the substituents, are the main factors explaining the electrochemical behavior of bi-silatranes. In line with the position of the N atom in the middle of the plan formed by its three carbon neigh- bors, we did record a noticeable anodic shift of the oxidation peak toward higher potentials. Electrochemical measurements show that anodic oxidation of p-aminophenylsilatrane, p- (tert-butyl)-phenylmethoxystannatrane and tolylsulfylstannatrane occurs via electrochemically reversible electron transfer resulting in corresponding cation radicals detected by cyclic voltammetry and real-time EPR spectroelectrochemistry. The own oxidation potential of the X-Ar substituent in these compounds is lower than that of the silatranyl and stannatranyl moiety, so p-aminophenylsilatrane, p-(tert-butyl)-phenylmethoxystannatrane and tolylsulfylstannatrane follow the oxidation pattern and form the cation radicals of the corresponding aromatic derivatives substituted with a silatranyl or stannatranyl group. Tetrabutylammonium salts with fluoride-encapsulating octaorganosilsesquioxane anions

103 − TBA(F @T8R8) met several electrochemical tests on their suitability as low-coordinating non-nucleophilic supporting electrolytes. They provide good ionic conductivity in the solvents of low polarity (THF, perfluoroisopropanol, liquid SO2) and fair conductivity in acetonitrile and allow one carrying out voltammetry and EPR experiments on unstable electrogenerated − electrophilic and radical species. Since F anion can leave the T8 cage and to appear in − the solution in free form, the use of the supporting salts with F @T8 anions is subject to the limitations imposed by the kinetics of this process. In the systems where the exohedral fluoride is thermodynamically more favorable than encapsulated endo-form, the interference of F− will occur. Fortunately, this process seems rather slow, leaving, at least in our experiments, more than 30 min working period before it becomes seen by means of voltammetry and NMR. In general, first results on the use of these new salts as supporting electrolytes are very promising, though more extensive studies on their advantages and limitations are necessary.

104

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111

RESUME 1 Introduction général

Depuis qu’en 1960, un nouvel organosilane, baptisé triptych-siloxazolidine, a été découvert pour la première fois par Frye et al. les métallatranes ont fait l’objet d’études et de recherches intensives pendant plus de 50 ans, non pas seulement en raison de son unique interaction transannulaire entre l’atome métallique (Si, Ge, ou Sn) et un atom d’azote, mais également en raison des nombreuses applications en médecine et en biologie qui utilisent ces composés. Cependant, les propriétés électrochimiques des métallatranes n’ont pas autant attiré l’attention des chercheurs, et ont été peu étudiées pendant une longue période. Les métallatranes ont pour- tant un potentiel très élevé en électrochimie en raison de leur structure particulière: l’atome métallique peut être vu comme pentacoordiné, avec une liaison hypervalente. En efet, pour commencer, la liaison hypervalente N-Métal limite la zone de probabilité de présence de la paire d’électron. Ensuite, en raison de la présence des trois branches alkoxysilanes, le doublet électronique est “piégé” dans la cage du métallatrane. Enﬁn, la structure atrane peut être éten- due linéairement par répétition du même motif. Les molécules formées d’une chaine silylée normale sont soit instables chimiquement, ou bien font preuve d’une mauvaise conduction électronique en raison du mauvais recouvrement des orbitales des atomes de silicium. En re- vanche, une chaine atrane possède une liaison Si-O forte, permettant de garantir la stabilité de la molécule, et la paire d’électrons libres de l’atom d’azote peut être partagée avec 2 atomes de silicium, permettant le transfert d’électrons le long de la chaine (Figure 1). Il s’agit d’un concept nouveau de chaine moléculaire, ou les électrons peuvent être transférés le long de la chaine.

Une autre caractéristique des métallatranes est dûe aux radicaux cations correspondants à leur forme oxydée. La distance entre l’atome d’azote et l’atome de métal dans le radical cation est différente de celle exhibée dans le métallatrane sous forme réduite, ce qui permet de changer la longueur de la molécule selon le potentiel. Cette ﬂexibilité des systèmes atranes peut-être utilisée et modulée en ajoutant différents substituants permettant la réalisation de différentes fonctions électrochimiques.

Pour toutes ces raisons, nous avons décidé de porter notre attention sur les comportements électrochimiques des métallatranes, et nous en avons fait le sujet de recherche traité dans cette thèse. Dans le cadre de cette étude, nous avons étudié les métallatranes depuis leur voie de synthèse jusqu’aux calculs de chimie quantique, en passant par la caractérisation structurale de ces molécules et l’étude de leur comportement électrochimique.

1 Si Si Si Si Si Si Si Si Si Si Si Si n n

a b

O O O O O O O O

Si N Si Si N Si Si N Si n O O O O O O O O O O

Figure 1: Différents types de chaines silylées. a. Chaine moléculaire possédant une bonne conductivité électrochimique, mais une forte instabilité; b. Chaine silylée stable chimiquement, mais exhibant de faibles recouvrements orbitalaires. c. Nouvelle chaine moléculaire incorporant la structure atrane, permettant une bonne stabilité moléculaire et des transferts électronqiues faciles.

2 1-R-silatranes

Si l’on se concentre sur la famille des 1-R-silatranes (métallatranes dont l’atome métallique est le silicium), bien qu’elle soit extrêmement large, elle a fait comme les autres familles d’atranes l’objet de peu d’études électrochimiques. La comparaison des quelques données disponibles sur l’électro-oxydation de cette classe de molécules avec les travaux effectués sur des germatranes similaires peut permettre de supposer que la structure électronique des radicaux cations des silatranes substitués par un groupement aliphatique ou aromatique, d’une part, et ceux substitués par un groupement benzilique d’autre part, auraient un comportement électrochimique différent. Cette différence est intrisèquement liée à la nature des interactions liantes entre les atomes des molécules de silatrane sous leur forme réduite (neutre). Aﬁn d’élucider les caractéristiques spéciﬁques de ces effets, et d’étudier la possibilité d’étendre un système électronique basé sur des liaisons à 3 centres et 4 électrons (3c-4e) à un système plus large, à 5 centres et 6 électrons (5c-6e), voire plus large encore (5c − 6e)n, nous avons commencé par réaliser la synthèse et l’étude de plusieurs “silatranes modèles”. 7 structures de 1-R-silatranes, étudiées dans la section suivante, ont été préparées selon des méthodes publiées.

2 CH2 NH R = 2 N C

O Si O p-aminophenyl 1-naphthylmethyl t-butyl tolyl O H2 C R C N H2 O p-methoxyphenyl benzyl 2-cyanoethyl

Figure 2: 1-R-silatranes

Bien qu’ils s’agisse de composés déjà connus et que leur structure cristalline ait même déjà fait l’objet de quelques publications, Nous avons noté qu’elles n’avaient fait l’objet d’aucun dépôt dans la CCDB, et que les détails de ces structures, eux, n’avaient pas été publiés. Nous avons donc estimé utile de nous pencher sur ces aspects manquants. Les structures de 4 de ces silatranes ont été étudiées par diffraction des rayons X sur monocristal. La structure des p-aminophenylsilatrane, 1-naphthylmethylsilatrane, p-methoxyphenylsilatrane et benzylsilatrane ont été les premières que nous avons caractérisées. L’oxydation de 7 silatranes Si-substitués a été étudiée dans des solutions d’acétonitrile contenant 0.1M de tétrabutylammonium hexaﬂuorophosphate (TBAPF6), sur une électrode de platine. nous avons utilisé une électrode microdisque de Pt comme électrode de travail dans la mesure où cela nous a permis d’obtenir des voltammogrammes mieux formés que ceux obtenus avec d’autres types d’électrodes (des électrodes de carbone vitreux ont notamment aussi été essayées). Les potentiels des pics d’oxydation de ces composés se trouvent dans une fenêtre de 0.78-1.45V (vs. SCE), selon la nature du substituant R (table 1). Soumis à oxydation, 3 de ces 7 silatranes exhibent un pic réversible distinct. Nous avons observé l’existence d’une loi de proportionnalité entre le courant de pic (ip) et la concentration de substance électroactive. Une loi de proportionnalité a également été observée entre le courant 1/2 de pic et la racine carrée de la vitesse de balayage (ip/v = const.) pour des vitesses de balayage sufﬁsamment élevées (v > 0.5 V/s). Ces 2 résultats suggèrent une cinétique controlée par la diffusion. Le nombre d’électrons transférés durant l’étape d’oxydation a été déterminé par la méthode proposée par Malachesky, basée sur les relations suivantes.

Le courant de pic ip pour un voltammogramme à balayage linéaire d’un système rédox réversible peut-être exprimé par:

5 3/2 1/2 1/2 ip = 2.68 × 10 n ACD v

3 Parallèlement, en chronoampérométrie potentiostatique [?]:

it1/2 = nF ACD1/2π−1/2

La combinaison de ces 2 équations donne la relation suivante:

i /v1/2 R = p = 4.92n1/2 it1/2

Le qui permet d’évaluer directement la stoechiométrie électronique du mécanisme. Ce résultat est indépendant des incertitudes dues aux differents coefﬁcients de diffusion D, ainsi que de la valeur de la concentration en silatrane à la surface de l’électrode, contrairement à la méthode basée sur la comparaison des ip avec un composé standard comme le ferrocène. En appliquant cette relation au cas de l’oxydation des 7 silatranes, nous avons abouti à la 0 conclusion qu’ils suivent tous un mécanisme mono-électronique à Ep (Table 1).

Table 1: Paramètres de l’oxydation electrochimique des silatranes par une electrode à disque de platine, à v = 1V/sa

Ea − Ea ∆Ea/∆lg(V ) R Ea(V) Ec(V) Eo(V) n p p/2 p αb p p (mV) (mV) p-aminophenyl 0.78 – – 1 93 14 0.50 1-naphthylmethyl 1.10 – – 1 88 28 0.50 t-butyl 1.22 1.14 1.18 1 91 10 0.50 tolyl 1.35 1.26 1.30 1 72 14 0.65 p-methoxyphenyl 1.37 – – 1 66 40 0.72 benzyl 1.42 – – 1 150 40 0.32 2-cyanoethyl 1.45 1.30 1.38 0.9 81 23 0.58 a Voltammetrie cyclique réalisée dans un melange CH3CN / 0.1 M TBAPF6. Tous les voltammogrammes sont corrigès par SCE. b Coefﬁcient de transfert estimé par α avec Ep − Ep/2 = 1.85RT/αF .

Comme mentionné précédemment, les effets électroniques des substituants R affectent signiﬁcativement les potentiels d’oxydation des silatranes. Parmi les 7 silatranes testés, les t- butyl et tolyl-silatranes exhibent un signal distinct d’électrooxydation réversible même à faible vitesse de balayage (v = 0.5 V/s). Le tolylsilatrane a un pic d’oxydation à Ep =1.35 V (vs. SCE), avec un pic cathodique correspondant à 1.26V (vs. SCE). Quant au dérivé tert-butyl, il s’agit d’un substituant présentant un fort effet inductif donneur, ce qui conduit à une oxydation

4 aisée de ce silatrane (1.22 V vs. SCE). Les 4 autres silatranes montrent des comportements électrochimiques différents sous les mêmes conditions. Leurs pics d’oxydation (table 1) sont irréversibles même à des vitesses de balayage plus élevées (v = 10 V/s). Aﬁn de rationnaliser ces données, les structures électron- iques et les mécanismes possibles de transmission des effets électroniques de ces systèmes ont été étudiés en détail.

On peut noter que le méthoxyphénylsilatrane a un potentiel de pic d’oxydation Ep =1.37 V vs. SCE (table 1), soit un potentiel plus élevé que celui du pic d’oxydation du naphtylméthyl- silatrane (Ep =1.10 V). En effet, le1-naphtylméthylsilatrane (contrairement aux dérivés tolyl- et anisyl-) proﬁte d’un effet globalπ-donneur du groupe naphtyl. De façon remarquable, le système π du groupe naphtyl et la liaison σ(C–Si) de la liaison 3c-4e sont hyperconjugués, ce qui conduit à une délocalisation électronique (ﬁgure 3).

H2C

Figure 3: Hyperconjugaison de la liaison 3c-4e avec le système π du 1-naphtylméthylsilatrane.

Le même mécanisme de conjugaison permet la délocalisation électronique du radical cation dans le système π du naphtyl. L’électron libre, localisé sur l’atome d’azote et isolé dans la cage atrane des alkyl ou aryl silatranes, devient dans le cas des naphtylsilatranes, visible de l’extérieur de la cage pour des entités radicalophiles externes en raison de sa présence sur les cycles aromatiques naphtyl, principalement en position 4. De là, la diminution observée du potentiel d’oxydation de ce silatrane est dûe à deux facteurs: la contribution thermody- namique (donneur électronique) du substituant, et la rapidité de la réaction de consommation des radicaux cations électrogénérés qui s’ensuit (contribution cinétique provoquant un shift de la valeur de Ep). Par conséquent, ce composé a sans surprise l’un des potentiels de pic d’oxydation parmi les plus bas de la table 1, à Ep =1.10 V vs. SCE. Le caractère réversible de l’oxydation du phénylsilatrane (le pic retour de réduction du cation radical est détecté dès v = 0.2 V/s) nous permet d’étudier ce silatrane par spectroélec- trochimie EPR. Quand on oxide le phénylsilatrane sur une microélectrode placée dans une cellule EPR, le signal de l’électron libre de l’espèce générée apparait à E = 0.7 V. Le spectre exhibe un aspect caractéristique, 9 groupes d’un motif qui se répète, la séparation entre ces 9

5 groupes rejoignant la ligne de base. Le spectre EPR du radical cation d’un phénylsilatrane (ﬁgure 4) a une importante largeur de spectre (~160G), et consiste en plus de 184 bandes parfaitement résolues. Etant donnée la largeur importante du spectre, la partie de ce spectre située aux champs les plus hauts montre une distortion dûe aux effets de 2e ordre, ce qui rend difﬁcile sa reconstruction à l’aide de l’outil simple inclus dans le pack Brüker Symphonia. Néanmoins, les paramètres suivants ont été extraits de la partie situés aux champs bas, permettant une reconstruction de ce spectre avec 99.8% de qualité: g = 2.0037; aN = 18.41 α α β G; 3 × a Hax = 37.93 G; 3 × a Hlat = 0.23 G; 6 × a H = 1.8 G. Ce spectre correspond à des espèces dont l’atome d’azote est pratiquement plan. Celui ci porte la majorité de la densité électronique libre, et possède un forte interaction avec les 3 protons α orientés de façon axiale.

Les liaisonsEnd C-H-to-end avec width les of 3 autresthis spectrum protons is α~40sont G  presque too small orthogonales for a simple àsilatrane… l’axe de la (must molécule, be de~160 sorte G). que No leur comments, interaction just: Figure avec leX. champ EPR spectrum magnétique from the est oxidation près de of 160 t-BuSiTRan fois plus in faible que **/0.1M ** at a Pt microspiral electrode. E = 1.3 V, T = 234 K. celle des protons α axiaux.

Figure R. EPR spectrum of the cation radical of phenyl silatrane. Pt microplate electrode. FigureMicrowave 4: Spectre frequency EPR f = du 9.46 cation GHz; radical modulation du phénylsilatrane. amplitude a = 0.25 Electrode: G. T = 223 microplaqueK. E = 0.7 V. de Pt. Fréquence des micro-ondes f = 9.46 GHz; Modulation d’amplitude a = 0.25 G. T = 223 K. E = 0.7 V. EPR spectrum of the cation radical of phenyl silatrane (figure R) has a large end-to-end width (~160 G) and consists of more than 184 well-resolved lines. Given its large spectral span, the high field part of this spectrum shows some distortion by second-order effects, which makes it difficult to reconstruct it with the simple tool included into Bruker Symphonia package. Nevertheless, the following parameters have been extracted from low-field part allowing a α 399.8% Bis-(silatranyl)alkanes-quality reconstruction of this spectrum: g = 2.0037; aN = 18.41 G; 3a Hax = 37.93 G; α β 3a Hlat = 0.23 G; 6a H = 1.8 G. This spectrum corresponds to a species with practically planar NousN atom, allons carrying à présent the présentermajor part les of résultats the free deelectron voltammétrie density, which cyclique has destrong 3 bis-(silatranyl)alkanes: interaction with three axially-oriented -protons. The C-H bonds of the other three -protons are practically leorthogonal bis-(silatranyl)méthane, to the axes of this le bis-(silatranyl)hexane molecule so that their interaction et le bis-(silatranyl)octane with magnetic field qui is ontabout été 160 étudiés danstim deses weaker solutions compared d’acétonitrile to that of ( axialCH3 CN-protons.) contenant 0.1 M de tétrabutylammonium hexaﬂuoro-

6 phosphate (TBAPF6). Quelques gouttes de dichlorométhane ont été ajoutées à ces solutions pour permettre une bonne dissolution de ces produits. Tous les composés de cette série exhibent un pic d’oxydation, suivi d’une seconde étape d’oxydation. Ce second pic d’oxydation a été observé dans les mesures de voltammétrie cyclique de chacun des 3 composés. Les courants de pic d’oxydation de ces composés, mesurés à différentes vitesses de bal- 1/2 ayage, sont proportionnels à la racine carrée de la vitesse de balayage (ip/v = const.), ce qui implique que l’électrooxydation à ce potentiel suit un contrôle diffusionnel. Le nombre de moles d’électrons transférés durant la réaction d’électrooxydation a été calculé à partir de la √ valeur du courant de pic ip et de la racine carrée de la vitesse de balayage correspondante v, en utilisant le coefﬁcient de Cottrell issu des courbes i=f(t). Ces courbes i=f(t) ont été obtenues par chronoampérométrie à double saut de potentiel, et toutes les mesures ont été réalisées sur la même électrode et dans la même solution. Les premiers potentiels de pic d’oxydation de ces composés sont compris dans une fenêtre de 0.78-1.05 V (vs. SCE), selon la nature du fragment central. Dans cette série de 3 composés, le pic d’oxydation du bis-(silatranyl)hexane est le plus bas, c à Ep = 0.78 V, et ce pic est irréversible à faible vitesse de balayage. Le coefﬁcient apparent de transfert α du bis-(silatranyl)hexane, déterminé à partir de la largeur de pic, est de 0.21, ce qui n’est pas compatible avec le caractère réversible de l’oxydation et indique probablement que des interactions d’absorption déforment le voltammogramme. Le bis-(silatranyl)octane présente un pic réversible même à faible vitesse de balayage (v

= 0.5 V/s), et à le pic d’oxydation le plus étroit en largeur (Ep − Ep/2 = 117.20 mV) des 3 composés. Mais même pour ce composé, la largeur de pic apparente est trop large pour pouvoir correspondre à un processus rédox parfaitement réversible. Tous ces composés ont un second pic d’oxydation à des potentiels de valeur positive plus élevée, en particulier le bis-(silatranyl)octane, dont le second pic d’oxydation réversible se trouve à Ep = 1.30 V. Ceci suggère que le cation radical instable formé à l’issue de la première réaction a été par la suite réoxidé pour donner N+. Les bis-(silatranyl)hexane et bis-(silatranyl)octane ont tous deux une longue chaine alkyle faisant la liaison entre les deux groupes atrane. Ceci laisse penser que la flexibilité d’une chaine aussi longue pourrait permettre le rapprochement des deux groupes atrane, donnant lieu à une nouvelle liaison hypervalente: 6c-8e. Cependant, pour ces deux composés, même la longueur importante de la chaine (CH2)8 n’est pas suffisante pour permettre l’alignement axial requis des 2 groupes terminaux silatranyls, ce qui amène finalement à penser en l’existence de 2 liaisons 3c-4e indépendantes (figure 5). Il est probable qu’une chaine alkyle de plus de 12 atomes de carbone pourrait conduire à un tel arrangement intramoléculaire (figure 6). Dans une liaison 5c-6e classique, les 2 ligands central et terminal sont très électronégatifs

7 O O N Si N O O O N Si N O

O Si O O Si O O O

(CH2)6 (CH2)8

Figure 5: 2 liaisons indépendantes 3c-4e dans les molécules de bis-(silatranyl)hexane et de bis-(silatranyl)octane

-e O O O

X Si N N Si X 6c-8e

O O O δ− δ+

Figure 6: Liaison hypervalnte 6c-8e, pour un nombre d’atomes de carbones de la chaine alkyle supérieur à 12. par rapport au silicium. Dans le cas du bis-(silatranyl)méthane, par exemple, ces positions (l’atome de carbone central et les deux atomes d’azote terminaux) sont effectivement occupées par des éléments plus électronégatifs. Dans cette section, une série de composés possédant 2 structures silatranes fusionnées, ce qui les différencient du bis-(silatranyl)méthane, ont été synthétisées. D’un certain point de vue, l’atome d’azote peut maintenant être considéré comme l’atome central de la liaison 5c-6e (parfaitement linéaire dans ces molécules), et les deux atomes de carbone des substituants considérés, commes les atomes terminaux. Le comportement élec- trochimique de ces composés va être discuté dans la section qui suit.

4 Bi-silatranes

Deux nouvelles structures de bi-silatranes ont été synthétisées et analysées par DRX. La structure de ces bi-silatranes peut être vue de deux façons. Premièrement, les deux parties atranes peuvent être considérées comme deux bipyramides distordues, avec les deux atomes de silicium au centre et les 3 atomes d’oxygène, l’atome de carbone du substituant, et l’atome d’azote partagé formant les sommets (ﬁgure 7 a). Deuxièmement, le bi-silatrane peut être considéré comme une bipyramide non distordue

8 formée de l’atome d’azode au centre, les 3 atomes de carbone adjacents et les 2 atomes de silicium formant les sommets. (ﬁgure 7 b).

Si N Si Si N Si

a b

Figure 7: Structure des bi-silatranes. a. 2 bipyramides distordues avec les 2 atomes de silicium au centre; b. Une bipyramide non distordue, l’atome d’azote occupant le centre.

Pour une meilleure compréhension de l’oxydation chimique des bi-silatranes, nous avons conduit une série d’expériences de voltammétrie cyclique sur ces composés, et avons com- paré ces résulats à ceux obtenus avec un précurseur de ces molécules, la tris(1,3-dihydroxy- 2-propyl)amine. L’électrooxydation procède par transfert monoélectronique dans les 4 cas que nous avons considéré. Les diéthyl- et diphényl- bi-silatranes montrent distinctement une étape d’électrooxydation réversible même à faible vitesse de balayage (v = 0.5 V/s). Pour les di(benzyl)- et di(éthyl)-bi-silatranes, Ep augmente avec la vitesse de balayage légèrement plus qu’anticipé pour que le processus électrochimique suivant cette oxydation en cation-radical soit du premier ordre (un accroissement du premier ordre avec une pente de 30 mV par décade de la vitesse de balayage devrait avoir lieu). La largeur du pic d’oxydation du di(phenyl)-bi- silatrane est la plus faible de la série des trois composés, ce qui indique que l’on se rapproche d’un pur système Nernstien. De plus, la pente ∆Ep/∆lg(v) est proche de 20 mV plutôt que de 30 mV, ce qui semble indiquer un transfert électronique réversible et signiﬁe donc que ce composé subit une rapide réaction du deuxième ordre après oxydation, au contraire des autres composés de la série. Les di(éthyl)-bi-silatrane et di(phényl)-bi-silatrane ont tous deux un pic d’oxydation réversible même à faible vitesse de balayage (v = 0.5 V/s), ce qui atteste de la bonne stabilité de ces 2 radicaux cations. Par conséquent, l’oxydation mono-électronique de bi-silatranes aboutit au radical-cation correspondant.

5 1-R-stannatranes et distannatrane

D’après des études antérieures des métallatranes par diverses méthodes physico-chimiques, il ressort que l’interaction dative N→M est de plus en plus forte au fur et à mesure que l’on descend dans les atomes du groupe 14, dans l’ordre Si < Ge < Sn. Il est prévisible que la disponibilité du doublet libre de l’azote décroisse selon le même ordre, et donc que l’attaque électrophile du métallatrane soit de plus en plus difﬁcile. Aucune donnée sur l’électrochimie

9 Table 2: Resultats de voltammetrie cyclique pour l’oxydation de bi-sialtranes. Tous les echan- tillons sont pris à v = 1V/s et les potentials sont corrigès par SCE.

a ∆Ep/∆lgV (mV) α 34 0.7 33 0.6 36 0.6 18 0.7 a Coifﬁcient de transfert estimé avec α de Ep − Ep/2 = 1.85RT/αnF . des stannatranes n’a été rapportée à ce jour, à notre connaissance, et nous avons donc porté notre attention sur l’oxydation de quelques stannatranes que nous avons sélectionné, dans les mêmes conditions que celles décrites précédemment. Aﬁn d’obtenir un aperçu des interactions électroniques intramoléculaires dans les nouveaux stannatranes avec lesquels nous avons travaillé, leur comportement électrochimique a été observé. Selon leur solubilité, les expériences de voltammétrie cyclique sur les composés

12, 13, 14 et 15 (ﬁgure 8) ont été menées dans l’acétonitrile (CH3CN), le dichlorométhaneCH2Cl2 ou dans un mélange binaire (CH3CN/CH2Cl2, 1:1 v/v), contenant dans tous les cas 0.1M de

Bu4NPF6. o

R: 12, s N 13, O Sn O O 14, Cl R 15, I

Figure 8: 1-R-stannatranes 12, 13, 14 et 15

A l’opposé des silatranes et des germatranes, les stannatranes 12 et 13 n’exhibent un signal d’oxydation distinct que sur électrode de Pt. Les courants limitants ip des stannatranes

10 1/2 12 et 13 sont tous deux sous contrôle diffusionnel (ip/v =const; lg(ip)/lg(v) = 0.45 et 0.46 1/2 pour 12 et 13, respectivement), et comme on peut le voir en comparant le rapport ip/v à la pente de Cottrell obtenue à partir d’une expérience de chronoampérométrie sur la même électrode, le nombre d’électrons transférés au cours de cette étape est n = 1. La dépendance à 1 la température de lg(ip) pour 13 nous donne Ea = 2.46 kJ/mol (2.85 kJ/mol pour le deuxième pic), ce qui correspond à l’énergie d’activation du ﬂux de diffusion du solvant. Les largeurs

de demi-pics Ep − Ep/2 de 12 et 13 sont trop larges pour de simples procédés résersibles mono-électroniques, probablement en raison d’interactions d’absorption ou d’une réorganisa- tion structurelle substantielle accompagnant le transfert électronique.

Dans un solvant moins polaire comme CH2Cl2, l’oxydation de 13 procède en 2 étapes (ﬁgure 9), la seconde étant réversible dès v = 0.5 V/s. Si l’on ﬁxe le potentiel maximum avant

le début du second pic d’oxydation (Ev = 1.45 V), le premier pic commence aussi à exhiber un pic cathodique lui correspondant (à v > 50 V/s). Par conséquent, les deux signaux d’oxydation découlent de processus électrochimiques réversibles. Inorganic Chemistry Article

− − −3 −1 Figure 14. Cyclic voltammograms for the oxidation of stannatranes. Left: (a) 4 and (b) 8 (10 mol L )inCH3CN/0.1 M Bu4NPF6 at a Pt − × −3 −1 −3 − disk electrode;Figurev = 5 V/s. 9: Right: VoltammétrieI E curve of cyclique8 (1.1 10 sur lesmol composés L )inCH2Cl12,2/0.1 13 M. Gauche: Bu4NPF6 at (a) a Pt − disk12 et electrode. (b) − 13 (a)( normal10 scan; (b) I t 1 −1 ° curve after themol/L) hold at E inp .CH (c)3 diffusionCN/0.1 limiting M Bu current4NPF of6 theat a second Pt disk oxidation electrode; step. v v== 0.5 5 V V/s. s ; T Right:=22 C. I−E curve of 13 −3 (1.1 × 10 mol/L) in CH2Cl2/0.1 M Bu4NPF6 at a Pt disk electrode. (a) normal scan; (b) I−t Contrary to silatranes44,45 and germatranes,46 the stanna- to oxidize than parent ethers,50 the order of E values for curve after the hold at E . (c) diffusion limiting current of the second oxydation step. v = 0.5 p tranes 4 and 8 show distinct oxidationp signals only at a Pt oxygen- and sulfur-containing stannatranyl derivatives is reversed V s−1; T = 22 °C. − electrode. The limiting currents ip of stannatranes 4 and 8 are (Table 7). This is supposedly because of stronger Sn S versus 1/2 − both diffusion controlled (ip/v = const; lg(ip)/lg(v) = 0.45 Sn O bonding and a stronger acceptor effect of the stanna- and 0.46 for 4 and 8, respectively) and, as was shown using tranyl moiety on the Ep of 8 compared to the Ep of 4. 1/2 En raison d’une HOMO située à un faible niveau d’énergie, les halostannatranes sont très ip/v ratio along with Cottrell slope from chronoamperometry The stannatranes 13 and 15 did not show any distinct oxi- at the same electrode,difﬁciles47 àthe oxyder. number Ils of ont electrons au contraire transferred une atLUMOdation localisée peaks sur up une to orbitalethe media anti-liante limits. Asσ was∗(Sn– pointed out by 1 45 this step is nX), = 1 (Table qui peut 7).accepter Temperature relativement dependence facilement of lg(ip ) unBroka électron et al., supplémentaire,the chlorido-substituted et donc silatrane permettre N(CH2CH2O)3- of 8 provides Ea = 2.46 kJ/mol (2.85 kJ/mol for second peak), SiCl cannot be oxidized. No such data exist for the chlorido- leur électroréduction. corresponding to the activation energy of the diffusion flow substituted germatrane N(CH2CH2O)3GeCl but one can − of solvent. The peak half-widths Ep Ep/2 of 4 and 8 are too expect even higher Ep since germatranes are more difficult to Par conséquent, nous avons tenté d’obtenir un dimère de stannatrane46,51 par électroréduc- large for simple one-electron reversible processes, supposedly oxidize than silatranes. From a simple correlation of Ep with because of thetion. adsorption Le résultat interactions est montré or substantial ﬁgure 10:structural en a est représentécalculated IPs le ofvoltammogramme the stannatranes 4, cyclique8, and 13 du, the oxidation reorganization accompanying electron transfer. potential of the chlorido-substituted derivative 13 should be In a less polar solvent such as CH2Cl2, the oxidation of 8 11 above 2.3 V which corroborates the results of its voltammetry. proceeds via two steps (Figure 14), with the second one being On the contrary, compound 13 shows a well-shaped reduc- −1 − reversible already at v = 0.5 V s . With the vertex potential set tion peak at Ep = 1.5 V; the reduction process results in − ∼ before the onset of the second oxidation peak (Ev = 1.45 V), elimination of Cl anions whose oxidation peak at 1Vis the first peak also starts showing cathodic counterpart (at v > detected by voltammetry during the second cycle. The peak at 50 V/s). Thus, both oxidation signals arise from electro- −1.5 V falls into the range of reduction potentials of known chemically reversible processes. The i−t curve with the hold at chloridostannanes52 and might have arisen from the dissociative 1 − Ep (Figure 14) allowed to subtract the diffusion limited current reduction of Sn Cl bond in 13. Similarly, the oxidation peak of 1 48 − of the first peak ip (the baseline for the second peak and to the iodide anion I appears after the reduction of 15. This is in 2 quantify the current of the second oxidation step, ip . The latter agreement with the conductivity measurements of 13 in CH3CN/ was shown to have an electron stoichiometry of n = 0.4, caused CH2Cl2 (50:50 v/v), which have only shown residual solvent by self-reactions involving cation radicals, as it was observed in conductance and no contribution from any ionic conductivity 49 − the case of oxidation of Ar2S or ArSM with the HOMO due to the possible Sn Cl dissociation prior to the reduction. localized on the ArS fragment. Thus, the similarity in the oxi- A practically identical conductivity curve was observed for the dation pattern of 8 and aryl sulfides suggests that the p-CH3C6H4S phenolato-substituted stannatrane 4. Therefore the chlorido- fragment in 8 is most probably the reaction site that accounts substituted stannatrane 13, just as 4, remains a covalent com- for the first oxidation peak. The Ep of compound 8 is about pound in solution, at least in this media. 150−200 mV higher than those of diaryl or arylalkyl sulfides,50 EPR-Spectroelectrochemistry. Since the oxidation of the due to the electron acceptor effect of the stannatranyl sub- phenolato-substituted stannatrane 4 shows partial reversibility stituent. The possibility of oxidation of the stannatranyl moiety (Figure 14), it was studied by real-time EPR-coupled electro- 2 −3 −1 at Ep should obviously be ruled out because not only the chemistry. The solution of 4 (10 mol L ) in AN/0.1 M − HOMO but also the lower lying HOMO 1in8 is mostly built Bu4NPF6, initially ERP-silent, has shown the signal of para- of sulfur atom orbitals. Moreover, considering orbital energies magnetic species when the potential 1.23 V was applied (Figure 15). −ε ’ Δ (as IP = HOMO in Koopman s approach, by B3LYP/LANL2DZ) The visible end-to-end span of the observed spectrum ( = of HOMO−1in8 (6.655 eV) and of SOMO in 8+• (6.094 eV), 23−24 G) corresponds to the coupling constants from 2 sets of it is more probable that a second electron withdrawal affects the equivalent protons and to some contribution from 117/119Sn cation radical, like in the case of aryl sulfides. In contrast to the nuclei. The g-factor (g = 2.0036) is slightly increased relative to ox trend in Ep of diaryl and arylalkyl thioethers, which are easier that of pure organic radicals because of the interaction with

1052 dx.doi.org/10.1021/ic202179e | Inorg. Chem. 2012, 51, 1041−1056 chlorostannatrane avant électrolyse, et en b après 1h d’électrolyse. Il est évident que le pic de réduction du chlorostannatrane a disparu après électrolyse, ce qui indique qui n’y a plus de chlorostannatrane en solution (les distannatranes ne peuvent pas être réduits). Après séparation et puriﬁcation, nous avons de fait obtenu le dimère de chlorostannatrane. Nous avons réalisé une électrolyse similaire sur deux composés organostanniques util- isés comme modèles (chlorure de triphénylétain et chlorure de tributylétain), et nous avons également obtenu les dimères correspondants. La table 3 montre les déplacement chimiques observés en RMN du 119Sn. On constate un large déplacement du pic vers les champs élevés quand on passe du monomère au dimère (table 3). Les résultats d’analyses de GC-MS on également démontré la présence de dimère après électrolyse dans le cas du chlorure de triphényl- et de tributyl-étain.

0 b

- 1 0 0 a

A - 2 0 0

, I - 3 0 0

- 4 0 0

- 3 . 0 - 2 . 5 - 2 . 0 - 1 . 5 - 1 . 0 - 0 . 5 0 . 0 E , V v s S C E

Figure 10: Suivi voltammétrique de l’électrolyse du chlorure de stannatranne. E = -1.94 V sur électrode de carbone vitreux dans CH3CN/0.1 M LiClO4 a. Electroréduction avant électrolyse; b. Après 1 h d’électrolyse.

Table 3: Dimère SnSn (d’après analyse GC-MS) Compound δ119Sn (ppm) E (V) Product Q (F/mol) δ119Sn (ppm) Chlorure de triphénylétain -44.90 -0.54 Tris(n-phenyl)distannane 29.03 -143.27 Chlorure de tributylétain 156.88(144) -0.93 Tris(n-butyl)distannane 35.57 -82.66 Chlorure de stannatrane -243 -1.94 Bis-stannatrane 11.06 -639.72

6 Nouvel electrolyte supportant

Les anions faiblement coordinants utilisés dans les électrolytes de support peuvent arbitraire- ment être sépares en 4 groupes (ﬁgure 11) : ceux dont l’atome central, acide de Lewis lui- − − − − même, porte en plus des substituants électro-accepteurs (a : BF4 , PF6 , AsF6 , SbF6 ), ceux

12 Bu4N[B(C6F5)4] was prepared by metathesis of Li[B(C6F5)4] etherate (ABCR) with quiBu portent4NClO de4 in gros methanol. groupements All experiments organiques were permettant performed la under délocalisation Ar atmosphere. des charges élec- Tetrabutylammonium salts of octaorganosilsesquioxanes with encapsulated F- anion Ph B− troniqueswere prepared (b : according4 ), ceux to combinantthe general cesmethod 2 caractéristiques described by Bassindale en même tempset al (15) (c : mixing larges − − groupesTHF aromatiquessolutions of portantthe corresponding des substituants organotriethoxysilane électroattracteurs (C RSi(OEt)6F5)4B 3, (R[(CF = 3Ph,)2C 6CHH32]4CN,B − et [(CFCH2CH3)3CO]2CN4 Aland ),CH et2CH les2 carboranesCF3, all from icosaédriques ABCR) withCB the11 stoechiometricne contenant aucuneamount orbitaleof Bu4NFπ, (as a 1 M solution in THF with 5% of water, Aldrich). After the overnight stirring, the d’utilisation relativement récente (d: CB H X− avec X=Cl ou Br). solvent was vacuum evaporated and 11the 6obtaine6 d residues were washed with chloroform andD’autre filtered part, through plusieurs a 5 systèmes cm layer basés of SiO sur2 des or «recrystallized, pièges quantiques if necessary, » (e) sont from connus, acetone. dans - 19 lesquelsThe presence un atome of ou F un in ionthe estT8R piégé8 cage dans was une checked cavité by sans F qu’il NMR n’y spectroscopy. ait de liaison covalente entre la paroi de la cavité et l’espèce piégée. On peut penser à titre d’exemple aux métaux piégés Results and Discussion dans des fullerènes (M@C60), ou aux atomes d’hydrogène piégés dans des silsesquioxanes • octaédrauxWeekly (coordinatingH @T8), ainsi anions qu’à desmostly anions used ﬂuorures in supporting dans des electr zéolites.olytes Récemment, can be arbitrarily Bassin- divided into four groups (figure 1): those with a Lewis acid central atom carrying− an dale et al ont découvert une nouvelle classe de structures silsesquioxane,- - - dont l’anion- F est additional electron-acceptor substituting atom (a: BF4 , PF6 , AsF6 , SbF6 ), those with encapsulé dans une cage T8Ph8. La charge négative- de ce type de système est cachée au bulky delocalizing organic groups (b: Ph4B ), those combining both features (c: bulky - - centredelocalizing d’une boite groups siloxane with inerte, electr interdisanton-withdrawing toute coordinationsubstituents, directe(C6F5) avec4B , [(CF des électrophiles3)2C6H3]4B , - - extérieurs[(CF3)3 etCO] ne4Al permettant), and icosahedral que des interactions carboranes purement CB11 containing électrostatiques. no π-orbitals (d: CB11H6X6 with X = Cl or Br).

Figure 1. Different types of low-coordinating supporting electrolyte anions. Figure 11: Différents types d’anions faiblement coordinants pour utilisation dans des élec- trolytes supportOn the other hand, many "rattle-like" systems (e) are known, in which an atom or an ion in entrapped inside of a hollow cavity, with no covalent bonding between them. MetalsAinsi, commeinside of des fullerenes calculs de (M@C DFT B3LYP/6-311G(d,p)60), hydrogen atom l’ontin octahedral démontré, silsesquioxanes il n’y a pas de (H•@T ), and fluoride anion in zeolites provide such examples. Recently, Bassindale et al transfert de8 charge substantiel entre l’anion F− central et les atomes environnants de la cage (15) discovered a new class of silsesquioxane structures, with F- anion encapsulated − silsesquioxane.inside of the LaT8Ph charge8 cage. NBO Negative de F chargevarie dein these -1 à -0.89systems quand is hidden on passe in the de l’étatcenter libre of an à − l’étatinert piégé siloxane dans unboxT 8cutting. Etant off donné direct que coordination l’ion F peut with entrer external dans laelectroph cage duilesT8 etin enfavor sortir, of l’utilisationpurely electrostatic de solvants interactions. polaires est à bannir, dans la mesure où ils vont déstabiliser le système It was pointed out that such compounds could not be prepared with non-acceptor − − F @T8 par solvatation de l’ion F libre. substituents (15,16). Since the LUMO of T8R8 structures lies inside of the T8 cage (17), theLa electron-withdrawing charge négative, cartographiée substituents sur should la surface raise de its solvant level, so disponible if it worked et visible with R par = Ph, les then all substituents whose inductive electron-withdrawi− ng effect (Taft's σ ) is stronger électrophiles extérieurs est montrée ﬁgure 12 pour F @T8R8 (avec R= CH2CNR et Ph), et than that of Ph (fig. 2), in principle must permit the formation of such− anions. Based on est comparée à celle de plusieurs anions communs. Alors- que l’anion F libre est disponible this consideration, we prepared TBA salts with F encapsulated T8R8 cages choosing the à 100%R groups depuis bordering n’importe the quellesilsesquioxane direction, cage seuls whic lesh électrophiles would be potentially de taille trèscompatible réduite (withH+, Li+the, Na electrochemical+) peuvent approcher environment des zones (R =présentant Ph, CH2CN, un CH caractère2CH2CN, nucléophile CH2CH2CF signiﬁcatif3). dans − − F @T8(CH2CN)8 et F @T8Ph8. − Le système THF/0.07M Bu4N(F @T8Ph8) a été utilisé pour l’étude par spectroélec-

Figure 2. The substituents with Taft inductive constants σR suitable for the formation of - fluoride-encapsulated silsesquioxane salts TBA[F @T8R8].

In the meantime other structures, all with the substituents R satisfying σR > σPh requirement, have been prepared: R = p-R-Ph, (CH2)2(CF2)mCF3 (m = 0, 3, 5, 7 (18)), Vin, CH=CH2Ph and CH2CH2(CF2)nCF3 (n = 0, 3, 5) (19). As was shown by DFT B3LYP/6-311G(d,p) calculations, there is no substantial charge transfer from the central F- anion to the surrounding atoms of the silsesquioxane - cage: NBO charge on F changes from -1 to -0.89 when passing from free to the T8- - entrapped form. Given that F ion can enter and leave the T8 cage (18,19), the use of polar - solvents should rather be precluded, as they will destabilize the F @T8 systems by solvation of the cage-escaped F- anion. Negative charge, mapped on the solvent available surface and visible by external - electrophiles, is shown in figure 3 for F @T8R8 (with R = CH2CN and Ph), and is compared to several common anions. Whereas free F- anion is 100% available from all sides, only small electrophiles (H+, Li+, Na+) can approach the zones of a meaningful - - nucleophilicity in F @T8(CH2CN)8 and F @T8(Ph)8.

- - Figure 12:3. Negative Charges négatives charge (dark (zones areas sombres for pourF andF− BFand4 ,BF and−, etthose marquées marked par with des flèchesthe arrows for the other anions) mapped on solvent available surface4 for low-coordinating anions pour les autres anions), cartographiées sur la surface- de- solvant disponible pour des anions (DFT B3LYP/6-311G(d,p) calculations); F and BF4 are given− for comparison.− faiblement coordinants (selon calculs DFT B3LYP/6-311G(d,p)); F et BF4 sont donnés pour comparaison. - In aprotic solvents like CH3CN and THF, the TBA(F @T8R8) salts undergo ionic dissociation. Limiting values of molar conductivity of these anions, λo, have been trochimie EPR de la réduction mono-électronique (Schéma2 17) des 9-halofluorènes. La recon- estimated (table I) using Stokes equation λo = NAe /(6πηr) and within the more advanced modelstruction of du Kuznetsova spectre observé (eq. (figure 2) taking 13) grâce into auaccount programme solvent Simphonia holes fluctuation, (Bruker) a été radius réalisé, of the solvationconduisant sphere à des paramètres and frontal (g =resistance 2.0026; a 9to−H th=e 13movement.89 G, a1, 1of,− Ha =bulky 3.92 ionicG, a2, 2complex,−H = 0.88 in this mediaG, a3,3 ,(20).−H = 3.81 G, a4 , 4,−H = 0.68 G) proche de ceux rapportés pour le radical fluorenyl

-3 -1 libre. On peutFigure supposer 7. Oxidation que lesof phenothiazine groupes phényl (2×10 de mol l’anion L ) at silsesquioxanea GC disk electrode agissent in CH3CN comme une using different supporting TBA salts. (a) TBABF4, (b) TBAPF6, (с) TBAB(С6F5)4, (d) [2] sorte de matrice aromatique- stabilisant le radical ﬂuorenyl. TBA(F @T8Ph8).

Figure 8. EPR spectra from the reduction of 9-iodo-fluorene in (propylene carbonate- - Figure 13: SpectresTHF, 1:2 v/v)/0.07 EPR de M la Bu réduction4N(F @T8Ph du8). ( 9-iodoﬂuorenea) experimental, (b (dans) high-frequency un système component, propylene car- − bonate : THF(c) -same 1:2 signal - v:v after / 0.07M FFT filtering,Bu4N (dF) simulated@T8Ph using8). (a)the parameters : Expérimental. in the text. (b) : Section des hautes fréquences. (c) : Le même signal après traitement FFT. (d) : Simulé en utilisant les paramètres du texte. [4]

Les sels supports de TBA(F−@T R ) sont chimiquement stables, et peuvent être stockés 8 - 8 The THF/0.07 M Bu4N(F @T8Ph8) system was used for EPR-spectroelectro sous formechemical solide sansstudy crainteof one-electron de décomposition. reduction (eq. 4) of Leurs 9-halofluorene solutions (21). dans Reconstruction les solvants testés of the observed spectrum (figure 8) using Simphonia program (Bruker) was possible with the parameters (g = 2.0026; a9-H = 13.8914 G, a1,1’-H = 3.92 G, a2,2’-H = 0.88 G, a3,3’-H = 3.81 G, a4,4’-H = 0.68 G) close to those reported for free fluorenyl radical. It is supposed that Ph groups of the silsesquioxane anion serve as a kind of aromatic matrix accommodating and stabilizing fluorenyl radical.

[5] sont stables également, du moins pendant la durée typique d’une expérience d’électrochimie. Cependant, comme cela a été remarque pour la première fois par Bassindale et comme on peut le déduire des calculs DFT ci-dessus (figure 12), ils sont sensibles à la présence de petits cationsElectrooxidation en solution, spécifiquement of speciallyLi +preparedet H+. ligand-stabilized 1,1,2,2-tetramethyl-1,2- bis(quinolin-8-yloxy)disilane (the process is EPR-silent in CH3CN/0.05 M TBAPF6) in La figure 14 montre que dans l’acétonitrile, si on ajoute LiClO4 à la solution, l’anion- EPR-spectroelectrochemical cell using (DME-CH2Cl2, 3:1 v/v) as solvent and TBA(F − − F@Tfuit8Ph hors8) as desupporting la cage du electrolyte système F at @253T8 (CHK allowed2CN)8 topour detect former two LiF/TBAF, odd-electron et onspecies, obtient dessupposedly cages T8 (CHshort-lived2CN)8 entièrementcation radical vides of enthis 9-10 disilane heures an end pratique.the radical De product façon similaire,of its cleavage (eq. 5). First species (g = 2.01, a = 39.2 G) is obviously a Si-centered radical, les protons libérés au cours des 30 minutes d’expérienceSi (voltammètrie du méthylanthracène) the integration of its 29Si satellites (≈9%) indicates the spin localization on two Si atoms. − − dansSecond le cas species de F is@ anT 8organic(CH2CH ra2dical,CF3) 8asdans follows le SO from2 liquide its g-fa ontctor provoqué (2.0033). l’apparition Spin density de inF 19 librethis enradical solution, is distinctly bien que delo cecalized processus over nethe soit quinoline devenu moiety visible (a enN = RMN 3.78 G, du aH4F =qu’après 6.86 G, 4 aH5 = 4.03 G, aH7 = 2.08 G, aH3 = 1.20 G, aH6 = 1.12 G) with an additional hfc constant, heures de repos13 de la29 solution utilisée. Il est probable que la polarité du solvant et l’efficacité either from C or Si− (a = 7.1 G), the quality of the spectrum edges not allowing a more deprecise la solvatation assignment. des F libres aient joué un rôle essentiel, comme mentionné précédemment.

19 - Figure 9. F NMR-monitored− leakage of F anion19 from the T8R8 cage (R = CH2CN) in Figure 14: Fuite des anions F suivie par RMN du F hors d’une cage de T8R8 (R = CH2CN), the presence of LiClO4. Solvent: CH3CN, T = 20◦ °C. en présence de LiClO4. Solvant: CH3CN, T = 20 C.