University of Groningen
Formazanate as redox-active, structurally versatile ligand platform Chang, Mu-Chieh
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record
Publication date: 2016
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA): Chang, M-C. (2016). Formazanate as redox-active, structurally versatile ligand platform: Zinc and boron chemistry. University of Groningen.
Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license. More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne- amendment.
Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.
Download date: 07-10-2021
Formazanate as a Redox-Active,
Structurally Versatile Ligand Platform
Zinc and Boron Chemistry
Mu-Chieh Chang
The research described in this thesis was carried out at the Stratingh Institute for Chemistry, University of Groningen, The Netherlands.
The work was financially supported by the Netherlands Organization for Scientific Research (NWO).
Cover designed by Mu-Chieh Chang Printed by Ipskamp Drukkers, The Netherlands
ISBN: 978-90-367-8573-0 eISBN: 978-90-367-8572-3
Formazanate as a Redox-Active, Structurally Versatile Ligand Platform
Zinc and Boron Chemistry
PhD thesis
to obtain the degree of PhD at the University of Groningen on the authority of the Rector Magnificus Prof. E. Sterken and in accordance with the decision by the College of Deans.
This thesis will be defended in public on
Monday 18 January 2016 at 14.30 hours
by
Mu-Chieh Chang
born on 23 March 1985 in Tainan, Taiwan
Supervisor Prof. W.R. Browne
Co-supervisor Dr. E. Otten
Assessment Committee Prof. J.G. Roelfes Prof. J.G. de Vries Prof. F. Meyer
Contents
Chapter 1 Introduction ………………………………………………………………………… 1 1.1 General Introduction ………………………………………………………………… 2 1.2 Redox-Active Ligands in Nature ……………………………………………………. 3 1.3 Redox-Active Ligands in Laboratories ……………………………………………… 4 1.3.1 Dioxygen Activation by ZrIV Complex Bearing Diimine Ligands ……………….. 4 1.3.2 Catalytic Reactions Based on Redox-Active Ligands ……………………………. 5 1.3.2.1 Catalytic Cyclization of Enynes and Dienes …………………………………... 5 1.3.2.2 Catalytic Nitrene Transfer Based on ZrIV Complex ……………………………. 6 1.4 Formazanate Ligands: Nitrogen Rich Analogues of -Diketiminate Ligands ……… 7 1.4.1 Redox-Active Nature of Formazanate Ligands …………………………………... 8 1.4.2 Redox-Active Nature of -Diketiminate Ligands ………………………………... 9 1.4.3 Metal Complexes Bearing Fromazanate Ligands ………………………………… 11 1.5 Overview of Thesis ………………………………………………………………….. 11 1.6 References …………………………………………………………………………… 13
Chapter 2 Formazan Synthesis ………………………………………………………………... 15 2.1 Introduction ………………………………………………………………………….. 16 2.1.1 Method A1: Formazan Synthesis from Diazonium Salt and Arylhydrazone ………... 16 2.1.2 Method A2: Formazan Synthesis from Diazonium Salt and Active Methylene Group .. 18 2.1.3 Structure of Formazans and Formazanate Anions …………………………………... 18 2.2 Formazan Synthesis …………………………………………………………………. 19 2.2.1 Mono-Formazan Ligand Synthesis ………………………………………………….. 20 2.2.2 Phenylene-Linked di-Formazan Ligand Synthesis ………………………………….. 25 2.3 Conclusion …………………………………………………………………………... 26 2.4 Experimental Section ………………………………………………………………... 26 2.5 References …………………………………………………………………………… 36
Chapter 3 (Formazanate)Zinc Complexes ……………………………………………………. 37 3.1 Introduction ………………………………………………………………………….. 38 3.2 (Formazanate)Zinc Methyl complexes ……………………………………………… 39 3.2.1 Zinc Methyl Complex with Phenylene-Linked Diformazanate Ligand …………….. 40 3.2.2 Quantitative Description of Trigonal Pyramidal Zinc Center ……………………….. 42 3.3 Bis(Formazanate)Zinc Complexes ………………………………………………….. 44 3.3.1 Synthesis and Coordination Chemistry of Bis(Formazanate)Zinc Complexes ……... 44 3.3.2 UV-Vis Spectroscopy of Bis(Formazanate)Zinc Complexes ……………………….. 51 3.3.3 Cyclic Voltammetry of Bis(Formazanate)Zinc Complexes …………………………. 52 3.4 Chemical Reduction of Bis(Formazanate)Zinc complexes …………………………. 56 3.4.1 Synthesis and Characterization of 1-Electron Reduction Products …………………. 56 3.4.2 Synthesis and Characterization of 2-Electron Reduction Products …………………. 58 3.4.3 UV-Vis Spectroscopy of Reduced Products ………………………………………… 59 3.4.4 DFT Calculations ……………………………………………………………………. 60 3.4.4.1 DFT Calculations of 1-Electron Reduction Compound (5–) ……………………….. 60 3.4.4.2 DFT Calculations of 2-Electron Reduction Compound (5–2) ……………………….. 61 3.5 Conclusion …………………………………………………………………………... 62 3.6 Experimental Section ………………………………………………………………... 63 3.7 References …………………………………………………………………………… 72
Chapter 4 (Formazanate)Boron Difluoride Complexes Formation via Zinc to Boron Transmetallation …………………………………………………... 75 4.1 Introduction ………………………………………………………………………….. 76 4.2 Formazanate Transfer from Zinc to Boron ………………………………………….. 77 4.2.1 Formation of (Formazanate)Boron Difluoride via Transmetallation ………………... 77 4.2.2 Isolation of a Six-Coordinated Zinc Complex as a Key Intermediate ………………. 78 4.2.3 Proposed Mechanism of Transmetallation Reaction ………………………………... 81
4.2.4 Reaction of Heteroleptic Complex 5aj with BF3·Et2O ……………………………… 82 4.2.5 1,2,3-Triazole Formation ……………………………………………………………. 85 4.3 Conclusion …………………………………………………………………………... 86 4.4 Experimental Section ………………………………………………………………... 87 4.5 References …………………………………………………………………………… 89
Chapter 5 (Formazanate)Boron Complexes ………………………………………………….. 91 5.1 Introduction ………………………………………………………………………….. 92 5.2 Synthesis of (Formazanate)Boron Complexes ……………………………………… 94 5.2.1 (Formazanate)Boron Difluoride …………………………………………………….. 94 5.2.2 (Formazanate)Boron Diphenyl and (Formazanate)Boron Dihydride ……………….. 96 5.3 Characterization of Formazanate Boron Complexes ………………………………... 97 5.3.1 X-ray Crystallographic Analysis …………………………………………………….. 97 5.3.2 Absorption and Emission Spectroscopy …………………………………………….. 101 5.3.3 Redox Chemistry ……………………………………………………………………. 103 5.4 Reduced Products: mono-Formazanate Ligands ……………………………………. 106 5.4.1 Synthesis and Crystal Structures ……………………………………………………. 106 5.4.2 Absorption and Emission Spectroscopy …………………………………………….. 107 5.4.3 EPR Spectra and DFT Calculations …………………………………………………. 108 5.5 Reduced products: Phenylene-Linked Diformazanate Ligands …………………….. 110 5.5.1 Synthesis and Crystal Structures ……………………………………………………. 110 5.5.2 DFT calculations and VT-EPR Studies ……………………………………………… 112 5.6 Conclusion …………………………………………………………………………... 114 5.7 Experimental Section ………………………………………………………………... 114 5.8 References …………………………………………………………………………… 124
Chapter 6 Reduction of (Formazanate)Boron Difluoride Provides Evidence for an N-Heterocyclic B(I) Carbenoid Intermediate ……………………………………. 127 6.1 Introduction ………………………………………………………………………….. 128 6.2 Synthesis and Characterization ……………………………………………………… 129 6.2.1 Synthesis ……………………………………………………………………………………………………… 129 6.2.2 X-ray Crystallography ………………………………………………………………. 132 6.2.3 NMR Spectroscopy and UV-Vis Analysis …………………………………………... 134 6.2.4 Reduction Chemistry ……………………………………………………………………………………. 136 6.3 DFT Calculations …………………………………………………………………………………………... 140 6.4 Thermal Stability ……………………………………………………………………. 143 6.5 Trapping (Formazanate)Boron(I) by Acetylene …………………………………….. 146 6.6 Chemical Oxidation …………………………………………………………………. 148 6.7 Conclusion …………………………………………………………………………... 150 6.8 Experimental Section ………………………………………………………………... 150 6.9 References …………………………………………………………………………… 157
Chapter 7 Intramolecular Hydride Transfer Reactions in (Formazanate)Boron Dihydride Complexes ……………………………………………………………… 159 7.1 Introduction ………………………………………………………………………….. 160 7.2 Synthesis of (Formazanate)Boron Dihydride Complexes …………………………... 161 7.3 Thermally Induced Intramolecular Hydride Transfer ……………………………….. 161
7.3.1 [PhNNC(p-tolyl)NNPh]BH2 (10a) ………………………………………………….. 161
7.3.2 [PhNNC(p-tolyl)NNMes]BH2 (10c) ………………………………………………… 166
7.3.3 [C6F5NNC(p-tolyl)NNMes]BH2 (10f) ………………………………………………. 169 7.4 Kinetic Study and Proposed Mechanism ……………………………………………. 170
7.4.1 Kinetic Study of the Thermolysis of [PhNNC(p-tolyl)NNPh]BH2 (10a) …………… 170 7.4.2 Proposed Mechanism and DFT Calculations ……………………………………….. 171 7.5 Discussion …………………………………………………………………………… 173 7.6 Conclusion …………………………………………………………………………... 175 7.7 Experimental Section ………………………………………………………………... 175 7.8 References …………………………………………………………………………… 181
English Summary ……………………………………………………………………. 183 Nederlandse Samenvatting ………………………………………………………….. 189 Prospective ………………………………………………………………………….. 195 Acknowledgements …………………………………………………………………. 199 誌謝 …………………………………………………………………………………. 202
Chapter 1
Introduction Chapter 1
Chapter1 Introduction 1.1 General Introduction In the field of homogeneous catalysis, the bond breaking and the bond forming steps are often two-electron processes. In other words, the catalyst needs to donate or accept two electrons to or from the substrates in the catalytic cycle. For example, palladium-catalyzed coupling reactions have become very popular and useful catalytic reactions both in industry and academia, and the Nobel Prize in Chemistry was awarded in 2010 to Heck, Negishi, and Suzuki, who pioneered the development of these reactions.1 The key elementary steps of these palladium-catalyzed coupling reactions involve oxidative addition and reductive elimination (Scheme 1.1).2 In the catalytic cycle, the substrates are activated by oxidative addition in which the Pd center donates two electrons to the substrate to form new chemical bonds resulting in an increase in the oxidation state of the Pd center by 2. Subsequently, a second substrate is introduced by transmetallation or ligand exchange. The last step is product release and catalyst regeneration by reductive elimination, which results in a decrease in the oxidation state of the Pd center by 2.
R1 R 0 LnPd Reductive elimination RXOxidative R addition L PdII n R1 R L PdII n X MX
1 Transmetallation R M Scheme 1.1 The general mechanism of palladium-catalyzed coupling reactions In order to support the two-electron redox processes, most of the catalysts that are used today are based on precious metals.3 This is because precious metals, in general, have relatively stable and easily accessible (n) and (n+2) oxidation states. However, precious metals are usually scarce, expensive and toxic; most importantly, the natural abundance of precious metals is limited. All of these factors make the use of precious metals in catalysis increasingly expensive and this motivates chemists to develop more sustainable (catalytic) methods for organic synthesis and energy applications, in which the precious metal component of catalysts is replaced by more abundant elements (base metal or main group elements).
2
Introduction
A major challenge of using base metal catalysts is that the base metal is usually suitable for one-electron redox processes due to its relatively stable (n) and (n+1) oxidation states. In other words, the challenge of replacing precious metals by systems based on base metals is how to impart two-electron processes to these base metal complexes. A possible solution to meet this challenge is utilizing redox-active ligands to develop selective two-electron redox reactions using base metal catalysts.4 In this scenario, the redox-active organic ligand framework can actively participate in the redox changes required during chemical transformations by donating or accepting electrons to or from substrates. Utilizing this special feature, metal centers coordinated by redox-active ligands avoid reaching unstable oxidation states during chemical redox transformations. Thus, such complexes can perform catalytic redox reactions that are not possible with conventional ligands.
1.2 Redox-Active Ligands in Nature
Nature is definitely the pioneer of using redox-active ligands in chemical transformations or in catalysis.5 One of the famous examples from Nature is the active site of cytochrome P450 (Chart 1.1), which have been identified in all domains of life6 and is a catalyst for a monooxygenase reaction. The active site of cytochrome P450 comprises a heme cofactor constituted by a Fe+2 center and a porphyrin ligand.7 In 2010, a highly reactive intermediate, which is usually referred to as P450 Compound I (P450-I), of the catalytic cycle was isolated by Green and co-workers.8 The P450-I is an iron(IV)-oxo species bearing a singly oxidized porphyrin radical ligand and capable of conversion of hydrocarbons to alcohols. The P450-I promoted hydrocarbons to alcohols conversion is a 2-electron process, in which the redox equivalents are provided by the iron(IV) center and singly oxidized porphyrin radical ligand resulting in a formation of an iron(III) center and a porphyrin ligand.
Chart 1.1 Active site of Cytochrome P450 (left) and compound I (right)
3
Chapter 1
Another example of the redox-active ligand from Nature is an intradiol oxidative cleavage of catechols by catechol dioxygenase (Scheme 1.2).9 The first step of the reaction is the formation of a catecholate Fe(III, HS) complex from catechol and high-spin Fe(III) enzyme. Subsequently, the o-semiquinone Fe(II, HS) complex is generated via intramolecular electron transfer from the catecholate to the iron center. The o-semiquinone Fe(II, HS) complex then reacts with dioxygen generating muconic acid as the product.
Scheme 1.2 Intradiol cleavage of catechols by catechol dioxygenase 1.3 Redox-Active Ligands in Laboratories
Taking inspiration from the systems in Nature mentioned above, several redox-active ligands have been developed. Classical examples are given by oxygen-based catecholate type ligands10, or its nitrogen- or sulfur-analogue11, and the porphyrin type ligands (Chart 1.2).12 Besides the bio-inspired redox-active ligand systems, several artificial redox-active ligand systems have been reported. In the following paragraphs, some progress in the design and application of redox-active ligands will be presented.
Chart 1.2 General structure of catecholate type ligands (left) and porphyrin type ligands (right)
1.3.1 Dioxygen Activation by ZrIV Complex Bearing Diimine Ligands
Molecular oxygen is a very attractive oxidant for chemists due to its ready availability (from the air). In 2007, the multi-electron activation of dioxygen by a ZrIV complex, which was coordinated by two diamido ligands, leading to a stable ZrIV bisperoxo bis(diimine) complex 13 IV was reported by Abu-Omar and co-workers (Scheme 1.3). The (diamido)2Zr complex was synthesized from 2 equivalents of -diimine ligand, magnesium metal, and ZrCl4. The
4
Introduction
IV spectroscopic data and the crystal data shown that the isolated product is a (diamido)2Zr complex, in which each diamido ligand stores two electrons that come from the magnesium metal. The key features of this dioxygen activation reaction are that the electrons used to active the dioxygen were stored in the redox-active diamido ligands first, and the oxidation state of the ZrIV center did not change during the reaction with dioxygen. The reaction proves an important concept that redox equivalents can be stored in the redox-active ligand first and subsequently used for a chemical transformation (in this case, oxygen reduction). Importantly, the metal center is a binding site of substrates and does not change its oxidation state.
IV Scheme 1.3 O2 activation of (diamido)2Zr complex 1.3.2 Catalytic Reactions Based on Redox-Active Ligands
In a catalytic reaction, the catalyst must be regenerated at certain step(s) to close the catalytic cycle and to start the next turnover. The regeneration of the catalyst is one of the greatest challenges of designing catalytic reactions based on redox-active ligands. In the field of precious metal catalysts, the catalytic cycle is closed by reductive elimination resulting in the two-electron reduction of the metal center. In the field of redox-active ligand, the ligand must be reduced after the reductive elimination step to close the catalytic cycle. In the next two sections, two catalytic reactions are presented to illustrate the concept that base-metal complexes containing an artificial redox-active ligand are capable of catalyzing a multi- electron synthetic transformation.
1.3.2.1 Catalytic Cyclization of Enynes and Dienes
Chirik and co-workers reported several examples of iron catalyzed hydrogenative cyclization of enynes (Scheme 1.4) and diynes.10,14 These are beautiful examples showing the concept that base metal catalysts can catalyze multi-electron processes by the help of redox-active ligands. The catalyst they used is a Fe(PDI)(N2) complex (PDI: pyridine(diimine)), which has a FeII center coordinated by a triplet diradical [PDI]2- ligand and a dinitrogen molecule. In the presence of enynes, the dinitrogen molecule is replaced resulting in metallacycle formation and construction of a C-C bond. The enyne cyclization event forms an S = 1 iron compound
5
Chapter 1 with formal one-electron oxidation events occurring both at the pyridine(diimine) ligand ([PDI]2-→[PDI]1-) and iron center (FeII→FeIII). Upon hydrogenation, an iron(III) alkyl hydride complex is formed. The last step to close the catalytic cycle is the reductive elimination of the product and the regeneration of active catalyst FeII(PDI)2- followed by the coordination of new substrate. Even though this is a FeII/FeIII catalytic cycle, the iron complex still can catalyze the 2-electron process by the help of redox-active pyridine(diimine) ligand.
Scheme 1.4 Catalytic Cycle for the Fe-catalyzed Hydrogenative Cyclization of Enynes 1.3.2.2 Catalytic Nitrene Transfer Based on ZrIV Complex
The nitrene transfer reaction catalyzed by the ZrIV complex bearing the redox-active ligand bis(2-isopropylamido-4-methoxyphenyl)amide ([NNNcat]3-) was reported by Heyduk and co- workers in 2011 (Scheme 1.5).15 The [NNNcat]3- ligand is a nitrogen analogue of the catecholate type tridentate. Like the catecholate ligand, the [NNNcat]3- ligand can be oxidized by two electrons to its quinonate form ([NNNq]-). The active catalyst of the nitrene transfer was generated by ligand dissociation to open the binding site of the metal center. The second step is the transfer of nitrene from the organic azide to the zirconium(IV) center. In order to compensate the formation of the ZrIV-N multiple-bond, the [NNNcat]3- ligand is oxidized by two electrons to the [NNNq]- form. The next two steps are migratory insertion and reductive elimination leading to the formation of the carbodiimide C=N bond and the reduction of the [NNNq]- ligand back to the [NNNcat]3-. In this example, the oxidation state of ZrIV center does
6
Introduction not change in the catalytic cycle, and the redox equivalents of the catalytic cycle are provide by the [NNNcat]3- ligand.
Scheme 1.5 Heyduk’s ZrIV system of catalytic nitrene transfer 1.4 Formazanate Ligands: Nitrogen Rich Analogues of -Diketiminate Ligands
Even though the examples mentioned above show great potential of utilizing redox-active ligands in the field of small molecules activation and multi-electron (catalytic) processes, the diversity of redox-active ligands is still very limited. This is mainly due to a redox-inert nature of most organic compounds. In literature, reported examples of bidentate ligands having redox-active property are also very limited. In addition to the catecholate type ligands (Chart 1.2) mentioned above, other reported redox-active bidentate ligands are diimine (Scheme 1.3) or pyridine-imine type ligands.16 Therefore, there is a clear need for developing new types of redox-active bidentate ligands. From our point of view, formazanate ligands,
7
Chapter 1 which are deprotonated formazans, are good candidates for a new redox-active ligand platform due to the structural similarity between formazanate ligands and -diketiminate ligands, and an easily accessible redox-active nature of formazanate ligands.
Formazans, organic compounds that contain the N=N-C=N-N fragment, have a long history as dye molecules.17 In modern chemical research, formazans not only show application in the field of biochemical research18 but also are precursors towards heterocycles such as verdazyl radicals (Scheme 1.6).19 Verdazyls are an unusual class of organic compounds because they are radicals that have high thermodynamic stability without having to rely on bulky substituents. The stability of verdazyl radicals is due to their low-energy SOMO that is a * orbital that is delocalized over four nitrogen atoms (Chart 1.3). The verdazyl radicals are good building blocks for constructing multi-radical systems, such as di-, tri- or tetra-radicals (Chart 1.3).20 In addition, the verdazyl radicals are also used as ligand platforms for metal complexes synthesis due to their nitrogen-rich structure (Chart 1.3).21
Scheme 1.6 Verdazyl radical synthesis
Chart 1.3 SOMO of verdazyl radical (left), examples of di-radicals based on the verdazyl unit (middle), and metal complex bearing verdazyl radical ligands (right)
1.4.1 Redox-Active Nature of Formazanate Ligands The redox-active nature of formazanate ligands was first reported by Hicks and co-workers in 22 2007. They prepared a (formazanate)boron diacetate (LB(OAc)2) complex (Scheme 1.7), which shows one quasi-reversible22 and one irreversible reduction23 in cyclic voltammetry experiments. The singly reduced product ([LB(OAc)2][Cp2Co]) was synthesized by reacting
8
Introduction
LB(OAc)2 with Cp2Co resulting in a green powder. The ligand-based reduction of
[LB(OAc)2][Cp2Co] was confirmed by EPR and UV-Vis spectra. Later on the same group reported that a heteroleptic Pd complex (Scheme 1.7), which is coordinated by a formazanate and an acetylacetonate ligand, also shows a formazanate-based reduction, albeit irreversibly.24 These examples not only show the redox-active nature of formazanate ligands but also indicate that the ligand-based reduction of formazanate ligand is easily accessible by using mild reducing agents (such as Cp2Co in the first example).
Scheme 1.7 Synthesis and chemical reduction of the (formazanate)B(OAc)2 complex from Hicks and co-workers (left); and the structure of heteroleptic Pd complex (right) 1.4.2 Redox-Active Nature of -Diketiminate Ligands
Formazanate ligands are organic compounds structurally similar to well-known - diketiminate ligands (or NacNac-, Chart 1.4), which are nitrogen-based monoanionic bidentate ligands and have been studied extensively during the past decades.25 The - diketiminate ligands have found widespread use as a versatile auxiliary ligand. The large tunability of the steric parameters or electronic properties of the -diketiminate ligands is its major advantage in chemical research. By using different starting material, all the substituents (R1-R5) of -diketiminate ligands can be changed. In addition to the rich chemistry of the - diketiminate ligands, the redox-active property of -diketiminate ligands has been established in the past few years.
Chart 1.4 General structure (left), HOMO (middle), and LUMO (right) of -diketiminate ligands
9
Chapter 1
The ligand-based oxidation of the -diketiminate ligands was first reported by Khusniyarov, Wieghardt and co-workers based on a bis(-diketiminate)NiII complex (Scheme 1.8).26 The ligand-based oxidation of -diketiminate ligands was supported by EPR experiments and DFT calculations. The first crystallographic study of the singly oxidized bis(- diketiminate)NiII was reported by Itoh and co-workers in 2014.27 The metrical parameters of II + ligand backbone of the ([L]2Ni ) do not show much difference compared to the parent II complex [L]2Ni . This is due to the nonbonding character of the HOMO of -diketiminate ligands (Chart 1.4).26,27
Scheme 1.8 Ligand-based oxidation of bis(-diketiminate)Ni complex The ligand-based reduction of -diketiminate ligands was first mentioned and structurally characterized in 2002 by Lappert and co-workers in systems of Sm and Yb complexes.25c The - crystal structure of the reduced bis(-diketiminate)Sm complex ([L2Sm] ) clearly shows elongations of N-C bond lengths of one of the -diketiminate ligand indicating a dianionic ligand backbone (L-2, Scheme 1.9). In the next year, the ligand-based 2-electron reduction, which results in a trianionic ligand backbone (L-3, Scheme 1.9), was reported by the same group based on ytterbium and lithium complexes.28 The elongations of N-C bond lengths in reduced bis(-diketiminate) metal complexes are due population of orbitals that have anti- bonding character between C and N (the LUMO in Chart 1.4). Generally speaking, very strong reducing agents, such as Li metal or Yb-naphthalene complex, are necessary to synthesize reduced products of -diketiminate metal complexes.
Scheme 1.9 Consecutive two-electron reduction of a -diketiminate ligand
10
Introduction
1.4.3 Metal Complexes Bearing Fromazanate Ligands Even though -diketiminate ligands are very good supporting ligands, utilization of their redox-active properties for (catalytic) reactions is difficult, which is mainly due to the low stability of the oxidized forms26 and poor accessibility of reduced forms (which are only observed at very negative potentails)28. In the case of formazanate ligands, the four nitrogen atoms at the ligand backbone make its reduction chemistry occurs at much more accessible potentials and stabilizes the resulting products (similar with the case of the verdazyl radicals). In addition, the structural similarity with -diketiminate ligands suggests that formazanate ligands could also be good supporting ligands. These two features make formazanate ligands an attractive candidate for the development of a new redox-active ligand platform; however, the coordination chemistry of formazanate ligand is relatively unexplored and most of the reported literature focused on the late transition metal complexes such as Pd, Co, Fe, Ni, and Cu.29 Besides the limited examples of formazanate metal complexes, there are only few documented examples reporting chemical transformations based on formazanate metal 30 29b,31 complexes such as H2O2 decomposition, and ethylene oligomerization and none of them utilize (or describe) the redox-active nature of formazanate ligands.
1.5 Overview of Thesis The goal of the research described in this thesis is to establish the redox-active feature of formazanate ligands. To achieve this aim, synthetic procedures and characterization methods of the free ligand and metal complexes are developed. The redox properties of formazanate ligands were established by electrochemical methods, and the first examples of isolated and well-characterized ligand-based reduction products are described in this thesis.
In Chapter 2, the general method of formazan synthesis is introduced. The method was applied to synthesize a series of formazan ligands, which contain a large diversity of steric and electronic properties of the substituents. A synthetic procedure for the di-formazan system was developed, in which the two formazan fragments are linked by a phenylene linker at meta and para position.
In Chapter 3, mono- and bis(formazanate)zinc complexes are synthesized and characterized. The solid-state and solution-state studies of the zinc complexes reveal that formazanate ligands show a high flexibility in coordination chemistry. At room temperature, the equilibrium between six- and five-membered chelate rings was established. The bis(formazanate)zinc complexes show four steps of one-electron reduction in cyclic
11
Chapter 1 voltammetry experiments. The crystal structures and EPR spectra of the reduced (formazanate)zinc radical (or diradical) compounds conclusively establish formazanate ligands as a novel class of redox-active ligands.
In Chapter 4, an unexpected zinc-to-boron transmetallation from bis(formazanate)zinc complexes to mono(formazanate)boron difluoride complexes is described. The key intermediate of the transmetallation reaction, which is a six-coordinated zinc complex having 1 3 5 a composition of [R NNC(R )NN(BF3)R ]2Zn, was isolated and fully characterized. A key step of the reaction is an isomerization from six-membered chelate ring to five-member chelate ring of formazanate ligand to open space around the zinc center to accommodate incoming substrates (BF3 in this case).
In Chapter 5, the synthetic procedures for (formazanate)boron complexes (LBX2; X = F, Ph, and H) are described. All the LBX2 show two (quasi)reversible redox couples in their cyclic ·- voltammetry experiments. The 1-electron reduction products [LBF2] were isolated and characterized by EPR spectroscopy and x-ray crystallography. In addition, the absorption and emission spectra of the neutral (formazanate)boron complexes show that these are a new class of fluorescent dyes (structurally related to BODIPYs) that show strong absorption and large Stokes shifts but are only weakly emissive (low quantum yield).
In Chapter 6, attempts to synthesize 2-electron reduction products of (formazanate)boron difluoride (LBF2) complexes are described. While these products appear relatively stable based on electrochemical methods, the chemical reaction of 2 equivalents of Na/Hg and the
LBF2 complex results in a series of BN-heterocycles which were characterized by NMR and X-ray crystallography. The formation of the BN-heterocycles is shown to go through a reactive (formazanate)B(I) intermediate, which is stabilized by the low-lying * orbital of the formazanate ligand. The isolated BN-heterocycles can be chemically oxidized back to the
LBF2 complex by reacting with XeF2. The regeneration of LBF2 suggests that the (formazanate)B moiety that is incorporated in the BN-heterocycles shows reactivity that derives from a B(I) fragment.
In Chapter 7, the thermolysis reaction of the (formazanate)boron dihydride (LBH2) is discussed. Heating up the LBH2 complexes results in a series of intramolecular hydride transfer products. A series of intermediates was characterized by 1D and 2D NMR spectroscopy. One of the identified intermediates contains a cyclohexadiene substituent, which indicates that first step of the reaction is an unusual hydride transfer to the ortho
12
Introduction position of the N-Ar ring on the ligand. In the case of unsymmetrical formazanate ligands, the distribution of isomers of each intermediate is affected by the substituents of the formazanate ligand.
1.6 References (1) Colacot, T. J. Platinum Metals Rev., 2011, 55, 84–90. (2) Chen, X.; Engle, K. M.; Wang, D. H.; Yu, J. Q. Angew. Chem. Int. Ed., 2009, 48, 5094–5115. (3) (a) Spargo, P. L. Transition Metals for Organic Synthesis; 2nd Ed; WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany; 2004. (b) Diederich, F.; Stang, P. J. Metal-catalyzed cross-coupling reactions; 2nd Ed; WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany; 2008. (4) (a) Dzik, W. I.; van der Vlugt, J. I.; Reek, J. N. H.; de Bruin, B. Angew. Chem. Int. Ed., 2011, 50, 3356–3358. (b) Chirik, P. J.; Wieghardt, K. Science, 2010, 327, 794–795. (c) Lyaskovskyy, V.; de Bruin, B. ACS Catal., 2012, 2, 270–279. (d) van der Vlugt, J. I. Eur. J. Inorg. Chem., 2011, 2012, 363– 375. (5) Kaim, W.; Schwederski, B. Coord. Chem. Rev., 2010, 254, 1580–1588. (6) Lamb, D. C.; Lei, L.; Warrilow, A. G. S.; Lepesheva, G. I.; Mullins, J. G. L.; Waterman, M. R.; Kelly, S. L. J. Virol., 2009, 83, 8266–8269. (7) Poulos, T. L.; Finzel, B. C.; Howard, A. J. J. Mol. Biol., 1987, 195, 687–700. (8) Rittle, J.; Green, M. T. Science, 2010, 327, 933–937. (9) (a) Ito, M.; Que, L. Angew. Chem. Int. Ed., 1997, 36, 1342–1344. (b) Lin, G.; Reid, G.; Bugg, T. D. H. J. Am. Chem. Soc., 2001, 123, 5030–5039. (c) Borowski, T.; Bassan, A.; Siegbahn, P. E. M. Chem. Eur. J., 2004, 10, 1031–1041. (d) Borowski, T.; Bassan, A.; Siegbahn, P. E. M. Biochemistry, 2004, 43, 12331–12342. (e) Borowski, T.; Siegbahn, P. E. M. J. Am. Chem. Soc., 2006, 128, 12941–12953. (10) (a) Pierpont, C. G. Inorg. Chem., 2011, 50, 9766–9772. (b) Henthorn, J. T.; Lin, S.; Agapie, T. J. Am. Chem. Soc., 2015, 137, 1458–1464. (c) Kramer, W. W.; Cameron, L. A.; Zarkesh, R. A.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem., 2014, 53, 8825–8837. (11) (a) Machata, P.; Herich, P.; Lušpai, K.; Bucinsky, L. Organometallics, 2014, 33, 4846–4859. (b) Petrenko, T.; Ray, K.; Wieghardt, K. E.; Neese, F. J. Am. Chem. Soc., 2006, 128, 4422–4436. (c) Cappillino, P. J.; Pratt, H. D.; Hudak, N. S.; Tomson, N. C.; Anderson, T. M.; Anstey, M. R. Adv. Energy Mater., 2014, 4, 1300566. (d) Lippert, C. A.; Arnstein, S. A.; Sherrill, C. D.; Soper, J. D. J. Am. Chem. Soc., 2010, 132, 3879–3892. (e) Lippert, C. A.; Hardcastle, K. I.; Soper, J. D. Inorg. Chem., 2011, 50, 9864–9878. (f) Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A. F. J. Am. Chem. Soc., 2008, 130, 2728–2729. (12) Blusch, L. K.; Craigo, K. E.; Martin-Diaconescu, V.; McQuarters, A. B.; Bill, E.; Dechert, S.; DeBeer, S.; Lehnert, N.; Meyer, F. J. Am. Chem. Soc., 2013, 135, 13892–13899. (13) Stanciu, C.; Jones, M. E.; Fanwick, P. E.; Abu-Omar, M. M. J. Am. Chem. Soc., 2007, 129, 12400– 12401. (14) (a) Sylvester, K. T.; Chirik, P. J. J. Am. Chem. Soc., 2009, 131, 8772–8774. (b) Bart, S. C.; Chłopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc., 2006, 128, 13901–13912. (c) Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc., 2006, 128, 13340–13341. (d) Russell, S. K.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc., 2011, 133, 8858–8861. (15) Nguyen, A. I.; Zarkesh, R. A.; Lacy, D. C.; Thorson, M. K.; Heyduk, A. F. Chem. Sci., 2010, 2, 166- 169. (16) Myers, T. W.; Berben, L. A. Chem. Commun., 2013, 49, 4175–4177. (17) (a) Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA, Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2000. (b) Bamberger, E.; Wheelwright, E. Ber. Dtsch. Chem. Ges., 1892, 25, 3201-3213. (c) Friese, P. Ber. Dtsch. Chem. Ges., 1875, 8, 1078– 1080. (18) (a) Moodley, S.; Koorbanally, N. A.; Moodley, T.; Ramjugernath, D.; Pillay, M. J. Microbiological Methods., 2014, 104, 72–78. (b) Martín, A.; Morcillo, N.; Lemus, D.; Montoro, E.; da Silva Telles, M. A.; Simboli, N.; Pontino, M.; Porras, T.; León, C.; Velasco, M.; Chacon, L.; Barrera, L.; Ritacco, V.; Portaels, F.; Palomino, J. C. Int. J. Tuberc. Lung Dis., 2005, 9, 901-906. (19) (a) Buzykin, B. I. Chem. Heterocycl. Comp., 2010, 46, 379–408. (b) Kuhn, R.; Munzing, W. Chem. Ber.-Recl. 1953, 86, 858–862. (c) Kuhn, R.; Neugebauer, F. A.; Trischmann, H. Monatsh. Chem., 1966, 97, 525–553. (d) Kuhn, R.; Trischmann, H. Monatsh. Chem., 1964, 95, 457–479. (e) Hausser, I.; Jerchel, D.; Kuhn, R. Chem. Ber., 1949, 82, 515–527.
13
Chapter 1
(20) Fico, R. M., Jr; Hay, M. F.; Reese, S.; Hammond, S.; Lambert, E.; Fox, M. A. J. Org. Chem., 1999, 64, 9386–9392. (21) (a) McKinnon, S.; Patrick, B. O.; Lever, A. Chem. Commun., 2010, 46, 773-775. (b) Barclay, T. M.; Hicks, R. G.; Lemaire, M. T.; Thompson, L. K. Inorg. Chem., 2003, 42, 2261–2267. (c) Brook, D. J R.; Fornell, S.; Stevens, J. E.; Noll, B.; Koch, T. H.; Eisfeld, W. Inorg. Chem., 2000, 39, 562-567. (d) Johnston, C. W.; McKinnon, S. D. J.; Patrick, B. O.; Hicks, R. G. Dalton Trans., 2013, 42, 16829– 16836. (e) McKinnon, S. D. J.; Patrick, B. O.; Lever, A. B. P.; Hicks, R. G. J. Am. Chem. Soc., 2011, 133, 13587–13603. (f) Sanz, C. A.; Ferguson, M. J.; McDonald, R.; Patrick, B. O.; Hicks, R. G. Chem. Commun., 2014, 50, 11676–11678. (g) Frolova, N. A.; Vatsadze, S. Z.; Stash, A. I.; Rakhimov, R. D.; Zyk, N. V. Chem. Heterocycl. Comp., 2006, 42, 1444–1456. (h) Myers, T. W.; Chavez, D. E.; Hanson, S. K.; Scharff, R. J.; Scott, B. L.; Veauthier, J. M.; Wu, R. Inorg. Chem., 2015, 54, 8077-8086. (22) Gilroy, J. B.; Ferguson, M. J.; McDonald, R.; Patrick, B. O.; Hicks, R. G. Chem. Commun., 2007, 43, 126–128. (23) Gilroy, J. B.; University of Victoria (Canada). The design, synthesis, and chemistry of stable verdazyl radicals and their precursors; ProQuest, 2008. (24) Gilroy, J. B.; Ferguson, M. J.; McDonald, R.; Hicks, R. G. Inorg. Chim. Acta., 2008, 361, 3388–3393. (25) (a) Tsai, Y.-C. Coord. Chem. Rev., 2012, 256, 722–758. (b) Asay, M.; Jones, C.; Driess, M. Chem. Rev., 2011, 111, 354–396. (c) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev., 2002, 102, 3031–3066. (26) Khusniyarov, M. M.; Bill, E.; Weyhermueller, T.; Bothe, E.; Wieghardt, K. Angew. Chem. Int. Ed., 2011, 50, 1652–1655. (27) Takaichi, J.; Morimoto, Y.; Ohkubo, K.; Shimokawa, C.; Hojo, T.; Mori, S.; Asahara, H.; Sugimoto, H.; Fujieda, N.; Nishiwaki, N.; Fukuzumi, S.; Itoh, S. Inorg. Chem., 2014, 53, 6159–6169. (28) (a) Eisenstein, O.; Hitchcock, P. B.; Khvostov, A. V.; Lappert, M. F.; Maron, L.; Perrin, L.; Protchenko, A. V. J. Am. Chem. Soc., 2003, 125, 10790–10791. (b) Avent, A. G.; Hitchcock, P. B.; Khvostov, A. V.; Lappert, M. F.; Protchenko, A. V. Dalton Trans., 2004, 33, 2272–2280. (29) (a) Sigeikin, G. I.; Lipunova, G. N.; Pervova, I. G. Russ. Chem. Rev., 2006, 75, 885-900. (b) Siedle, A. R.; Pignolet, L. H. Inorg. Chem., 1980, 19, 2052–2056. (c) Zaĭdman, A. V.; Khasbiullin, I. I.; Belov, G. P.; Pervova, I. G.; Lipunov, I. N. Pet. Chem., 2012, 52, 28–34. (d) Zaidman, A.; Pervova, I.; Vilms, A.; Belov, G.; al, E. Inorg. Chim. Acta., 2011, 367, 29-34. (e) Gok, Y.; Senturk, H. Dyes and Pigments. 1991, 15, 279–287. (30) Gorbatenko, Y. A.; Pervova, I. G.; Lipunova, G. N.; Maslakova, T. I.; Lipunov, I. N.; Sigeikin, G. I. Russ. J. Appl. Chem., 2005, 78, 936–939. (31) (a) Rezinskikh, Z. G.; Pervova, I. G.; Lipunova, G. N.; Maslakova, T. I.; Gorbatenko, Y. A.; Lipunov, I. N.; Sigeikin, G. I. Russ. J. Coord. Chem., 2008, 34, 659–663. (b) Pavlova, I. S.; Pervova, I. G.; Belov, G. P.; Khasbiullin, I. I. Pet. Chem., 2013, 53, 127-133.
14
Chapter 2
Formazan Synthesis
The synthetic procedures for a series of formazan ligands, which have different electronic and steric properties, are described. The results show that suitable reaction conditions and isolation procedures are different for each formazan derivative. Besides the mono-formazan ligand, the synthesis of di-formazan compounds that are linked via a phenylene spacer is also described in this chapter. In some cases where formazan synthesis was unsuccessful, we were able to isolate and characterize other products from these reactions. Analysis of their structure provides additional insight in potential complications during formazan synthesis.
Parts of this chapter have been published:
M.-C. Chang, P. Roewen, R. Travieso-Puente, M. Lutz, and E. Otten* ”Formazanate Ligands as Structurally Versatile, Redox-Active Analogues of β-Diketiminates in Zinc Chemistry” Inorg. Chem., 2015, 54, 379-388.
R. Travieso-Puente, M.-C. Chang and E. Otten* “Alkali metal salts of formazanate ligands: diverse coordination modes as a result of the nitrogen-rich [NNCNN] ligand backbone” Dalton Trans., 2014, 43, 18035-18041. Chapter 2
Chapter 2 Formazan Synthesis 2.1 Introduction
Formazans, organic compounds that contain the N=N-C=N-N fragment, have a long history as dye molecules.1 The first formazan was synthesized more than a century ago by von Pechmann.2 Since then, several synthetic procedures for formazan synthesis have been developed.3 The most common synthetic procedure used today is using diazonium salts (Method A) to form the C-N bond of formazans. The formazan synthesis can be divided into two subgroups based on the starting materials (Method A1: arylhydrazone; Method A2: compounds having active methylene groups).
2.1.1 Method A1: Formazan Synthesis from Diazonium Salt and Arylhydrazone
Method A is the most common synthetic procedure of formazan synthesis and a majority of the known formazans were synthesized by this method, especially for triarylformazans or macrocyclic formazans.3ef,4 A key reagent used in Method A to synthesize formazans is an arylhydrazone (Method A1, Scheme 2.1), which can be synthesized by the condensation reaction of aldehyde and arylhydrazine in high yield (> 95%). The coupling reaction of the diazonium salt and arylhydrazone to generate a formazan occurs under basic reaction conditions. Pyridine, sodium hydroxide, sodium acetate and triethylamine are the common sources of the base of the reaction. This also means that for different combinations of starting materials, which includes arylhydrazone and diazonium salt, and solvents of the reaction, different sources of the base might be needed. In some of the case, a basic buffer system is used.5
Scheme 2.1 General reaction scheme of Method A1
The mechanism of the coupling reaction of the diazonium salt and the arylhydrazone is still not very clear, but likely mechanisms are shown in Scheme 2.2.3a The first step of the coupling reaction is the deprotonation of hydrazone NH by the base leading to the formation of a resonance stabilized azaenolate. The terminal nitrogen of the azaenolate anion will be a
16
Formazan Synthesis nucleophile attacks the diazonium cation forming the compound A. After the rearrangement of A, the desired formazan is formed. This mechanism was further supported by the isolation of the unstable compound B, which can isomerize to a formazan, by Busch in 1931.6
Scheme 2.2 Mechanisms of Method A1
Another possible mechanism after the deprotonation is that the diazonium cation attacks at the -carbon position of the azaenolate anion leading to a di-azo compound (C). The formazan can be isolated from C by deprotonation and protonation (tautomerization). The second pathway was supported by the isolation of compound D (Scheme 2.2).7 The importance of the hydrazone NH was proved by the work of Pechmann8 and Bush9. They used disubstituted hydrazone (E) as the starting material for the coupling reaction. Under a similar reaction condition with previous cases, the desired formazan was not isolated or detected but an unexpected azohydrazone (F) was identified (Scheme 2.3). While the experimental data on the mechanism of formazan formation using Method A1 does not lead to a conclusive picture (in fact could well differ from substrate to substrate), this represents the most versatile method for the synthesis of formazans today.
Scheme 2.3 Reaction of disubstituted hydrazone (E) with diazonum salt
17
Chapter 2
2.1.2 Method A2: Formazan Synthesis from Diazonium Salt and Active Methylene Group
The second commonly used strategy for formazan synthesis is a coupling between diazonium salts with compounds having active methylene groups, the activation of which is usually due to a carboxyl or nitro group on the -carbon atom. In this class of reaction, several functional groups, such as carboxyl, acetyl, ethoxycarbonyl and benzoyl substituents, can be replaced by diazonium cations under basic condition. The reaction of acetoacetic acid with diazonium cation is a beautiful example of this type of reaction (Scheme 2.4). Upon treatment of acetoacetic acid with one, two, or three equivalents of diazonium salt, the mono-, di- or tri- substitution product can be isolated, respectively,10 in which the di- and tri-substitution products contain the N=N-C=N-N formazan moiety.
Scheme 2.4 Reaction of acetoacetic acid with diazonium cation (Method A2)
2.1.3 Structure of Formazans and Formazanate Anions
The structures of formazans, which are shown in Chart 2.1, have two alternating double bonds in the backbone. The two double bonds in the ligand backbone makes that formazans can exist in four possible isomeric forms: syn, s-cis (closed form); syn, s-trans (open form); anti, s-cis; and anti, s-trans (linear form).11 Experimentally, only closed, open and linear forms have been observed, and they can be easily distinguished by 1H NMR spectroscopy. For example, the 1H NMR resonance of the NH group of the closed, open and linear form is located at 15-16, 12-10 and 10-8 ppm, respectively.3g Generally speaking, the closed form is the most usual and stable isomer of formazans; the open or linear form usually appears when the R3 substituent is a cyano or methylthio group.3g,12
The four nitrogen atoms of the formazan backbone and the four possible isomers lead to highly flexible coordination chemistry of formazanate ligands. The formazanate ligand can coordinate with metal ions in three different ways forming six-, five-, or four-membered chelating rings13 (Chart 2.1), which are not possible for the -diketiminate ligand system. In
18
Formazan Synthesis addition, the free formazanate anion prefers the linear isomer to reduce the repulsion between two lone pairs on the two terminal nitrogen atoms.12,13
Chart 2.1 Isomers of formazan, formazanate metal complex and formazanate anion
Despite that Method A1 shows great potential for synthesizing unsymmetrical formazans (R1 ≠ R5), the synthetic procedure has remained relatively little explored. In this chapter, several synthetic methods for unsymmetrical formazans, which contains steric and electronic asymmetry, were developed. Beside simple mono-formazan compounds (1a-1j), two examples of phenylene-linked di-formazan ligand systems (1k and 1l) were also achieved.
2.2 Formazan Synthesis
In this chapter, the formazans were synthesized by using Method A1 and Method A2 (Scheme 2.5). In Method A1, condensation of a monosubstituted hydrazine with an aldehyde generates a hydrazone, which then reacts with a diazonium salt under basic conditions to give the desired formazan. This synthetic approach can be applied to a broad range of substituents. By varying the substitution pattern on the starting materials, all the substituents on the resulting formazans can be changed. The potential limitations of this method are the accessibility of starting materials and stability of diazonium salts. In Method A2, the synthesis of formazan is achieved by treatment of acidic methylene compounds such as cyanoacetic acid with aryldiazonium salts under basic conditions. By using this method, formazans with sterically demanding N-aromatic groups can be prepared.3g One of the limitations of this method is that it produces only symmetrical formazan derivatives (R1 = R5). For Method A1, the major challenge is to find the suitable reaction conditions, which include the base source,
19
Chapter 2 temperature, and solvent mixture, to promote the desired coupling reaction and to slow down the competing decomposition of the diazonium salt.
Scheme 2.5 Formazan synthesis
2.2.1 Mono-Formazan Ligand Synthesis
As a starting point, we synthesized the known formazan PhNNC(p-tolyl)NNHPh (1a) in a biphasic reaction medium via the procedure published by Hicks and co-workers (Method A1).14 The coupling of trimethylacetaldehyde-phenylhydrazone with phenyldiazonium chloride afforded the bis(phenyldiazenyl)methane compound PhNNCH(tBu)NNPh (1b), which does not spontaneously tautomerize to the corresponding formazan.7 Deprotonation of 1b, however, is facile and results in a delocalized formazanate anion that may be reprotonated to give the formazan tautomer. Attempts to prepare the somewhat more sterically demanding asymmetric derivative MesNNC(p-tolyl)NNHPh (1c) by treatment of the hydrazone (p- + tolyl)C=N-NHPh with MesN2 (either prepared in situ as the chloride or isolated as the BF4- salt)15 resulted in intensely colored reaction mixtures from which we were unable to isolate the formazan. Changing the solvent in which the reaction was conducted to acetone/water with NaOH as the base yielded the product MesNNC(p-tolyl)NNHPh (1c) in moderate yield
20
Formazan Synthesis upon crystallization from CH2Cl2/MeOH. The symmetrical derivative MesNNC(p- tolyl)NNHMes (1d) required yet another solvent mixture: this sterically demanding formazan could be obtained in low yield (11%) from coupling of MesN2Cl with (p-tolyl)C=N-NHMes in MeOH with NaOH/NaOAc.5 Using similar synthetic procedures, electron-poor formazans with C6F5 substituents either at the terminal N atoms (PhNNC(p-tolyl)NNH(C6F5) (1e) and
MesNNC(p-tolyl)NNH(C6F5) (1f) or the backbone C atom (PhNNC(C6F5)NNHMes (1g) and
(C6F5NNC(C6F5)NNHMes (1h) were obtained. Formazan derivatives with an electron- withdrawing cyano group on the central carbon atom can be isolated from the direct coupling of cyanoacetic acid with two equivalents of aryl diazonium salts under basic conditions (Method A2).3a Using this strategy we prepared the formazan PhNNC(CN)NNHPh (1i) and MesNNC(CN)NNHMes (1j). In related -diketiminate chemistry, 2,6-disubstituted aromatic rings have become popular since they provide steric protection above and below the coordination plane; we anticipate that the mesityl groups in our formazanate ligands will behave similarly.
Scheme 2.6 Synthesis of 1j-B(C6F5)3
The cyano group in 1i and 1j is a potential site to tune the electronic properties of the ligand by coordination of a neutral Lewis acid. In order to prove this concept, 1j was reacted with tris(pentafluorophenyl)borane (B(C6F5)3) in toluene at room temperature (Scheme 2.6). The red crystalline material 1j-B(C6F5)3 can be isolated from DCM/hexane mixture with high 19 yield (77 %). In the F NMR spectrum, 1j-B(C6F5)3 shows three resonances at -133, -156 and
-163 ppm (in CDCl3), which are in a good agreement with the reported CH3CN-B(C6F5)3 16 system (-135, -155, -163 ppm, in C6D6), suggesting an acid-base interaction between the cyano group and B(C6F5)3. This acid-base interaction was further confirmed by single crystal X-ray crystallography (Figure 2.1, metrical parameters in Table 2.1). The B1-N5 bond length is
1.578(2) Å, which is similar to the reported B(C6F5)3-adduct of the cyano-substituted (- 17 diketiminate)ZrCl2Cp complex (1.586(4) Å) and shorter than the CH3CN-B(C6F5)3 system
21
Chapter 2
(1.616(3) Å).16 Comparing with 1j, the bond lengths of N3-N4 (1.309(2) Å) and C10-C11
(1.426(2) Å) of 1j-B(C6F5)3 are shorter, and the bond length of N3-C10 (1.321(2) Å) is elongated. The change of the bond lengths before and after the coordination of B(C6F5)3 to 1j is similar to the reported (-diketiminate)ZrCl2Cp complex (Figure 2.1 and metrical parameters in Table 2.1). The bond length of N1-N2, N2-C10, and C11-N5 of 1j-B(C6F5)3 do not change much compared to 1j. The bond lengths distribution described above suggest the contribution from different resonance structures (i and ii, Scheme 2.7).17b The resonance structure ii shows double bond character for the N3-N4 and C10-C11bonds and single bond character for the N3-C10 bond, which explains the shorter N3-N4 and C10-C11 bond and longer N3-C10 bond of 1j-B(C6F5)3 than 1j. In addition, the resonance structure ii has extra negative charge at N5, which makes N5 a better electron donor resulting in a relatively short N5-B1 bond.
Figure 2.1 Crystal structures of 1j-B(C6F5)3 (left) and (-diketiminate)ZrCl2Cp complex (right); Structures are showing 50% probability ellipsoids (all hydrogen atoms, chloride atoms and solvent molecules omitted for clarity).
a b Table 2.1 Selected bond lengths (Å) of 1j , 1j-B(C6F5)3, (nacnac)ZrCl2Cp and (nacnac)ZrCl2Cp- b B(C6F5)3 1j 1j-B(C6F5)3 (nacnac)ZrCl2Cp (nacnac)ZrCl2Cp-B(C6F5)3 N1-N2 1.269(2) 1.271(2) N1-C2 1.301(3) 1.296(4) N2-C10 1.392(2) 1.390(2) C2-C3 1.463(5) 1.478(5) N3-C10 1.304(2) 1.321(2) C3-C4 1.425(6) 1.447(5) N3-N4 1.325(2) 1.309(2) C4-N5 1.323(4) 1.310(3) C10-C11 1.444(3) 1.426(3) C3-C7 1.437(4) 1.409(4) C11-N5 1.137(3) 1.139(2) C7-N8 1.139(4) 1.135(4) N5-B1 - 1.578(2) N8-B1 - 1.586(5) a: Reported by Gilroy and co-workers.3g b: Reported by Rojas and co-workers.17
22
Formazan Synthesis
Scheme 2.7 Resonances structures of 1j-B(C6F5)3
Scheme 2.8 Synthesis of 1m and 2
Introducing a functional group at the para position of R3 substituents is a way to tune the electronic properties without influencing the steric properties of formazan. Therefore, 3-(4- bromophenyl)formazan (1m) was prepared (Scheme 2.8). A potential application of 1m is that other functional groups or fragments can be introduced at the bromo substituent to expand the research scope of the formazanate ligand.18 Compound 1m was synthesized by the same method as 1a. After recrystallization from DCM/MeOH mixture, two different crystalline products can be identified in the isolated product. One of them is the desired formazan (1m), which is a red crystal; the other one is a bis(diazo) compound (2), which is a colorless crystal. The formation of these two product was confirmed by single crystal X-ray crystallography (Figure 2.2). The N-N, N-C and C-C bond lengths of 2 are 1.239(2), 1.480(2), and 1.537(2) Å, respectively. The N-N bond length is very close to the normal azo compound, and the C-C bond length is close to normal C-C single bond. These metrical parameters suggest that 2 is the C-C coupling product of benzyl diazene radicals. The NNCCNN backbone of 2 has been reported in the 1960’s and 1970’s, but most of the compounds with this general atom connectivity that are known to date are di-hydrazone compounds instead of di-azo compounds.19 Compound 2 could have potential as being a bidentate ligand or photoswitchable molecule.
23
Chapter 2
Figure 2.2 Crystal structures of 1m (left) and 2 (right); Structures are showing 50% probability ellipsoids (all hydrogen atoms and solvent molecules omitted for clarity).
N N N + - PhNN Cl PhNN+Cl- N N N N NH N base NH base NH
1n 3 (not observed) Scheme 2.9 Synthesis of 3
In order to introduce an extra coordination site of the formazan ligand, we attempted to synthesize 3-(2-pyridyl)formazan from 2-Pyridinecarboxaldehyde. The desired hydrazone can be isolated with almost quantitative yield (97%), but the synthesis of PhNNC(2-pyridyl)NNPh (1n) by using the same reaction conditions as those employed in the synthesis of 1a was not successful (Scheme 2.9). Compound 3 was isolated after the reaction of hydrazone with phenyl diazonium salt in pyridine/methanol/acetic acid mixture (16%). The 1H-NMR of 3 clearly shows two distinct groups of resonances from phenyl groups and a singlet of an NH group at 7.79 ppm, which is outside the normal range of formazans (Figure 2.3). The formation of 3 was further confirmed by single crystal X-ray crystallography (Figure 2.3). The formation of 3 was also reported by Nikolay V. Zyk in 2006,20 in which a solvent mixture of pyridine, water and acetic acid was used.
24
Formazan Synthesis
1 Figure 2.3 H NMR spectrum of 3 (CDCl3, 400 MHz, left); crystal structure of 3 (right) showing 50% probability ellipsoids (solvent molecules omitted for clarity).
2.2.2 Phenylene-Linked di-Formazan Ligand Synthesis
Scheme 2.10 Synthesis of phenylene-linked di-formazan ligands (1k and 1l)
In order to study bimetallic systems incorporating formazan or formazanate ligands, two di- formazan ligands were prepared (Scheme 2.10).21 The synthesis of di-hydrazones is very similar to a mono-hydrazone system; condensation of a terephthalaldehyde or isophthalaldehyde with phenyl hydrazine affords the desired di-hydrazone derivatives. In the next step, reactions of di-hydrazone with phenyl diazonium chloride were conducted in pyridine/DMF mixture due to the low solubility of the di-hydrazones in more conventional solvents. The desired di-formazan ligands were further purified by recrystallization from 1 CH2Cl2/MeOH mixture to give the pure products in 15% (1k) and 50% (1l) yield. The H NMR spectra of 1k and 1l are shown in Figure 2.4. In both spectra, a peak at ~15 ppm that corresponds to 2 protons confirms the formation of a di-formazan compound. In the case of 1k, the highly symmetrical structure, which has D2h point group, results in 4 peaks with integrations 4:8:8:4 in the aromatic region. In the case of 1l, the singlet at 9.04 ppm, which corresponds to 1 proton, is a strong evidence for the presence of a 1,3-disubstutited benzene structure as the linker.
25
Chapter 2
1 Figure 2.4 H NMR spectra of 1k (THF-d8, 400 MHz, top) and 1l (CDCl3, 400 MHz, bottom)
2.3 Conclusion
In this chapter, several procedures for the synthesis of sterically or electronically unsymmetrical formazan ligands were successfully developed. Two examples of di-formazan compounds were also prepared and characterized. The results here show the generality of Method A for formazan synthesis. But the formation of 2 and 3 indicates that using Method A to synthesize formazan ligand is not straightforward all the time and competing side reactions can lead to significant amounts of other products through radical pathways. Even though in these cases the substituent we introduced is far away from the site where the desired C-N or N-N bond formation should take place (to give formazans), our results indicate that these remote positions can have a large influence on the products that are isolated.
2.4 Experimental Section
General Consideration. All manipulations were carried out under air atmosphere.
Deuterated solvents (CDCl3 Aldrich and C6D6 Apollo), pentafluorophenylhydrazine (Aldrich, 97%), 4-bromobenzaldehyde (Aldrich, 97%), 2-pyridinecarboxaldehyde (Acros 99%), terephthalaldehyde (Aldrich, 99%), isophthalaldehyde (Aldrich, 97%), NaOH (Acros), 2,4,6-
26
Formazan Synthesis trimethylaniline (Aldrich, 98%), aniline (Sigma-Aldrich, 99%), tetrabutylammonium bromide (Sigma-Aldrich, 99%), cyanoacetic acid (Aldrich, 99%), sodium nitrite (Sigma-Aldrich, 99%), sodium carbonate (Merck), phenylhydrazine (Aldrich, 97%), p-tolualdehyde (Aldrich, 97%) and tert-butyl nitrite (Aldrich, 90%) were used as received.
The compounds 1a,14 1b,7 1i,3g and 1j3g were synthesized according to published procedures. NMR spectra were recorded on Mercury 400 or Varian 500 spectrometers. The 1H and 13C NMR spectra were referenced internally using the residual solvent resonances and reported in ppm relative to TMS (0 ppm); J is reported in Hz. Assignment of NMR resonances was aided by gradient-selected COSY, NOESY, HSQC and/or HMBC experiments using standard pulse sequences. Elemental analyses were performed at the Microanalytical Department of the University of Groningen.
Hydrazone synthesis
[PhNNC(pTol)H]. Phenylhydrazine (2.70 g, 25 mmol) was combined with p-tolualdehyde (3.00 g, 25 mmol) and ethanol (40 mL). After the mixture was stirred at RT for 30 min at which time a yellow precipitate had formed, water (100 mL) was added and the mixture was stirred for additional 10 min. The light yellow solid was collected by filtration and washed 1 with ether (3 × 5 mL), yield 4.80 g (22.8 mmol, 92 %). H NMR (400 MHz, C6D6, 25 °C): 7.59 (d, 2H, J = 8 Hz, p-tolyl CH), 7.24 (t, 2H, J = 8 Hz, Ph m-CH), 7.11 (d, 2H, J = 8 Hz, Ph o-CH), 7.05 (d, 2H, J = 8 Hz, p-tolyl CH), 6.86 (t, 1H, J = 7 Hz, Ph p-CH), 6.80 (s, 1H, 13 N=CH), 6.69 (bs, 1H, NH), 2.12 (s, 3H, p-tolyl CH3). C NMR (100 MHz, C6D6, 25 °C):
145.7, 138.7, 137.8, 133.8, 130.0, 129.9, 126.9, 120.5, 113.4, 21.7 (p-tolyl CH3) ppm.
[C6F5NNC(pTol)H]. Pentafluorophenylhydrazine (3.96 g, 20 mmol) was combined with p- tolualdehyde (2.40 g, 20 mmol) and ethanol (20 mL). After the mixture was stirred at RT for 30 min at which time a yellow precipitate had formed, the reaction mixture was cooled slowly to -30 °C for 1 day. The light yellow solid was collected by filtration and washed with cold 1 ethanol (5 mL), yield 4.40 g (14.7 mmol, 73 %). H NMR (400 MHz, C6D6, 25 °C): 7.49 (d, 2H, J= 8 Hz, p-tolyl CH), 6.98 (d, 2H, J= 8 Hz, p-tolyl CH), 6.66 (s, 1H, N=CH), 6.40 (s, 1H, 13 NH), 2.07 (s, 3H). C NMR (100 MHz, C6D6, 25 °C): 143.4, 140.1, 138.9 (dm, J= 246 Hz), 138.5 (dm, J= 244 Hz), 135.7 (dtt, J= 245, 14, 4 Hz), 132.4, 130.0, 127.2, 121.5 (tm, J= 11 19 Hz), 21.6 (p-tolyl CH3). F NMR (375 MHz, C6D6, 25 °C): -156.8 (d, 2F, J= 23 Hz, C6F5 o-CF), -164.4 (td, 2F, J= 22, 5 Hz, C6F5 m-CF), -168.9 (tt, 1F, J= 22, 5 Hz, C6F5 p-CF) ppm.
Anal. calcd for C14H9F5N2: C, 56.01; H, 3.02; N, 9.33. Found: C, 55.64; H, 3.00; N, 9.23.
27
Chapter 2
[PhNNC(C6F5)H]. 2,3,4,5,6-pentafluorobenzaldehyde (1.96 g, 10 mmol) and phenylhydrazine (1.08 g, 10 mmol) were stirred at room temperature in ethanol (30 mL) for 3 hours. After the reaction 60 mL water was added to the reaction mixture, and stirred for 1 hour. The light yellow solid that precipitated was collected and washed with water and hexane.
After drying under reduced pressure overnight 2.8 g light yellow solid of PhNHNC(C6F5)H 1 (0.96 mmol, 96 %) was obtained. H NMR (C6D6): δ 7.18 (t, 2H, J= 8 Hz, Ph m-H), 7.04 (d, 2H, J= 8 Hz, Ph o-H), 6.84 (t, 1H, J= 8 Hz, Ph p-H), 6.75 (s, 1H, NH), 6.62 (s, 1H, NHNCH). 13 C NMR (C6D6): δ 144.8 (dm, J= 252 Hz, C6F5), 144.3 (Ph i-C), 140.3 (dtt, J= 252, 14, 5 Hz,
C6F5 p-F), 138.2 (dm, J= 245 Hz, C6F5), 130.0 (Ph m-C), 123.6 (q, J= 3 Hz, NHNC), 121.9 19 (Ph p-C), 113.6(Ph o-C), 111.5 (td, J= 12, 4 Hz, C6F5 i-C). F NMR (C6D6): δ -144.1 (dd, 2F,
J= 22, 8 Hz, C6F5 m-F), -157.0 (t, 1F, J= 21 Hz, C6F5 p-F), -163.6 (td, 2F, J= 21, 8 Hz, , C6F5 o-F) ppm. Anal. calcd for C13H7N2F5: C, 54.56; H, 2.47; N, 9.79. Found: C, 54.59; H, 2.44; N, 9.75.
[C6F5NNC(C6F5)H]. Pentafluorophenylhydrazine (991 mg, 5 mmol) was combined with pentafluorobenzaldehyde ( 980 mg, 5 mmol) and ethanol (15 mL). 100 mL of deionized water was added after the mixture was stirred at room temperature (RT) for 3 hours. The light yellow solid was collected by filtration and washed with water and hexane,yield 1.779 g (4.73 1 13 mmol, 94 %). H NMR (400 MHz, C6D6, 25 °C): d 6.55 (s, 1H, N=CH), 6.45 (s, 1H, NH). C
NMR (100 MHz, C6D6, 25 °C): 145.2 (dm, J= 254 Hz), 141.3 (dtt, J= 255, 14, 5 Hz), 138.2 (dm, J= 248 Hz), 136.8 (dtt, J= 248, 14, 4 Hz), 129.4 (q, J= 3 Hz), 119.9 (tm, J= 10 Hz), 110.1 (td, J= 12, 4 Hz), In the range of 140.5-139.8 and 137.9-137.3 are two groups of 19 resonances containing two carbon. F NMR (375 MHz, C6D6, 25 °C): d -142.71(dd, 2F, J=
21, 8 Hz, C6F5 o-CF), -154.5 (t, 1F, J= 22 Hz, C6F5 p-CF), -155.7 (d, 2F, J= 23 Hz, C6F5 o-
CF), -163.0 (td, 2F, J= 21, 7 Hz, C6F5 m-CF), -163.7 (td, 2F, J= 22, 5 Hz, C6F5 m-CF), -166.0
(t, 1F, J= 22 Hz, C6F5 p-CF) ppm. Anal. calcd for C13H2F10N2: C, 41.51; H, 0.54; N, 7.45. Found: C, 41.37; H, 0.53; N, 7.46.
[PhNNC(4-BrC6H4)H]. Phenylhydrazine (1.1 mL, 11 mmol) was combined with 4- bromobenzaldehyde (1.96 g, 11 mmol) and ethanol (150 mL). After the mixture was stirred at RT for overnight at which time a yellow precipitate had formed, water (100 mL) was added and the mixture was stirred for additional 10 min. The light yellow solid was collected by filtration and washed with water and hexane, yield 2.63 g (9.6 mmol, 90 %). 1H NMR (400
MHz, C6D6, 25 °C): 7.27 (d, 2H, J = 8 Hz, Br-C6H4 CH), 7.22-7.15 (m, 4H, Ph o-CH and m-CH), 6.99 (d, 2H, J = 8 Hz, Br-C6H4 CH), 6.82 (t, 1H, J = 8 Hz, Ph p-CH), 6.65 (bs, 1H,
28
Formazan Synthesis
13 NH), 6.48 (s, 1H, N=CH) ppm. C NMR (100 MHz, C6D6, 25 °C): 144.5, 135.3, 134.6, 131.6, 129.2, 127.3, 121.9, 120.2, 112.8 ppm.
[PhNNC(C5H4N)H]. Phenylhydrazine (2.0 g mL, 18.5 mmol) was combined with 2- pyridinecarboxaldehyde (2.0 g, 18.7 mmol) and ethanol (50 mL). After the mixture was stirred at RT for 3 hours at which time a yellow precipitate had formed, water (100 mL) was added and the mixture was stirred for additional 30 min. The light yellow solid was collected by filtration and washed with water and hexane, yield 3.58 g (18.2 mmol, 97 %). 1H NMR
(400 MHz, CDCl3, 25 °C): 8.54 (d, 1H, J = 5 Hz, Py CH), 8.10 (s, 1H, NH), 8.01 (d, 1H, J = 8 Hz, Py CH), 7.82 (s, 1H, N=CH), 7.69 (td, 1H, J = 8, 2 Hz, Py CH), 7.29 (t, 2H, J = 8 Hz, Ph m-CH), 7.20-7.17 (m, 1H, Py CH), 7.15 (d, 2H, J = 8 Hz, Ph o-CH), 6.91 (t, 1H, J = 7 Hz, 13 Ph p-CH) ppm. C NMR (100 MHz, C6D6, 25 °C): 154.4, 148.7, 144.0, 136.9, 136.5, 129.3, 122.5, 120.8, 119.7, 113.0 ppm.
[(PhNNCH)2(para-C6H4)]. Phenylhydrazine (4.8 g, 44.7 mmol) was combined with terephthalaldehyde (3.0 g, 22.3 mmol) and ethanol (250 mL). After the mixture was stirred at RT for 3 hrs at which time a yellow precipitate had formed. The light yellow solid was collected by filtration and washed with ethanol, yield 5.87 g (19.7 mmol, 88 %). 1H NMR
(400 MHz, DMSO, 25 °C): 10.41 (s, 2H, NH), 7.88 (s, 2H, CH=N), 7.67 (s, 4H, C6H4), 7.25 (t, 4H, J = 7 Hz, Ph m-CH), 7.11 (d, 4H, J = 8 Hz, Ph o-CH), 6.77 (t, 2H, J = 7 Hz, Ph p-CH) 13 ppm. C NMR (100 MHz, DMSO, 25 °C): 145.7 (Ph i-C), 136.6 (C=N), 135.8 (C6H4 o-C),
129.6 (Ph m-C), 126.3 (C6H4 CH), 119.3 (Ph p-C), 112.5 (Ph o-C) ppm.
[(PhNNCH)2(meta-C6H4)]. Phenylhydrazine (3.2 g, 29.6 mmol) was combined with isophthalaldehyde (2.0 g, 14.9 mmol) and ethanol (50 mL). After the mixture was stirred at RT for 3 hrs at which time a yellow precipitate had formed. The light yellow solid was collected by filtration and washed with ethanol, yield 4.37 g (13.9 mmol, 93 %). 1H NMR
(400 MHz, DMSO, 25 °C): 10.40 (s, 2H, NH), 7.92 (s, 2H, CH=N), 7.86 (s, 1H, C6H4 CH),
7.61 (d, 2H, J = 8 Hz, C6H4 CH), 7.41 (t, 1H, J = 8 Hz, C6H4 CH), 7.25 (t, 4H, J = 8 Hz, Ph m-CH), 7.12 (d, 4H, J = 8 Hz, Ph m-CH), 7.68 (t, 2H, J = 8 Hz, Ph p-CH) ppm. 13C NMR
(100 MHz, DMSO, 25 °C): 145.7 (Ph i-C), 136.7 (C=N), 136.7 (C6H4), 129.6 (Ph o-C),
129.4 (C6H4), 125.4 (C6H4), 123.5 (C6H4), 119.3 (Ph p-C), 112.5 (Ph m-C) ppm.
Formazan synthesis
PhNNC(p-tol)NNHMes 1c. PhNNC(p-tol)H (2.10 g, 10 mmol), sodium hydroxide (4.00 g,
100 mmol), water (105 mL) and acetone (105 mL) were mixed at 0 °C. MesN2BF4 (2.34 g, 10
29
Chapter 2 mmol) was slowly added into the solution. After stirring for 1h at 0 °C the reaction mixture was slowly warmed up to RT and stirred overnight. The crude product was extracted into
CH2Cl2 and the volatiles were removed on the rotavap. The desired formazan [PhNNC(p- tol)NNMes] was purified by recrystallization (CH2Cl2/MeOH) to give 1.6 g of product (4.5 1 mmol, 46 %). H NMR (400 MHz, CDCl3, 25 °C): 15.10 (s, 1H, NH), 7.97 (d, 2H, J= 7.6 Hz, p-tolyl CH), 7.56 (d, 2H, J= 8.0 Hz, Ph o-CH), 7.39 (t, 2H, J= 8.0 Hz, Ph m-CH), 7.23 (d, 2H, J= 8.0 Hz, p-tolyl CH), 7.149 (t, 1H, J= 7.2 Hz, Ph p-CH), 6.98 (s, 2H, Mes m-CH), 2.54 13 (s, 6H, Mes o-CH3), 2.39 (s, 3H, p-tolyl CH3), 2.33 (s, 3H, Mes p-CH3). C NMR (100 MHz,
CDCl3, 25 °C): 146.8 (Ph ipso-C), 144.7 (Mes ipso-C), 142.0 (NCN), 138.2 (Mes p-C), 137.4 (p-tolyl p-C), 135.3 (p-tolyl ipso-C), 131.4 (Mes o-C), 130.8 (Mes m-C), 129.6 (Ph m- C), 129.4 (p-tolyl CH), 125.9 (p-tolyl CH), 125.6 (Ph p-C), 117.4 (Ph o-C), 21.5 (p-tolyl p-
CH3), 21.3 (Mes p-CH3), 21.2 (Mes o-CH3) ppm. Anal. calcd for C23H24N4: C, 77.50; H, 6.79; N, 15.72. Found: C, 77.30; H, 6.76; N, 15.51.
MesNNC(p-tol)NNHMes 1d. Mesityl hydrazine·HCl (835 mg, 4.47 mmol) was dissolved in methanol (50 ml), and Et3N (480 mg, 4.74 mmol) and p-tolyl-aldehyde (535 mg, 4.45 mmol) were added. The resulting mixture was stirred for 30 minutes to give a yellow solution. NaOH (806 mg, 20.15 mmol) and NaOAc (524 mg, 6.39 mmol) were added and the solution was cooled to 0 °C and stirred for 30 minutes. Mesityl diazonium chloride was prepared in a separate flask by mixing mesityl aniline (623 mg, 4.61 mmol) with water (10 mL) and HCl
(1.25 ml) forming a white suspension that was cooled to 0 °C. NaNO2 (313 mg, 4.54 mmol) was added to this in small portions while everything went into solution, and the mixture was stirred for 30 minutes at 0 °C. The mesityl diazonium salt was added drop wise at 0 °C to the first flask containing the hydrazone solution. The color turned to red immediately and gas evolution was observed. After stirring for 2 hours at 0 °C a black oily precipitate was formed that was filtered off and purified by column chromatography over silica using DCM/hexane (1:5) as eluent (r = 0.54). The fractions were collected and subsequent removal of the solvent in vacuo afforded the product as a dark red solid (188 mg, 0.472 mmol, 11 % yield). 1H NMR
(C6D6, 400 MHz): δ 14.86 (1H, s, NH), 8.28 (2H, d, J = 7.8 Hz, Ph o-H), 7.18 (2H, d, J = 7.8
Hz, p-tol m-H), 6.76 (4H, s, Mes o-H), 2.37 (12H, s, Mes o-CH3), 2.18 (3H, s, p-tol p-CH3), 13 2.12 (6H, s, Mes p-CH3). C NMR (C6D6, 126 MHz): δ 144.05 (Mes i-C), 142.98 (p-tol i-C), 137.45 (p-tol p-C), 136.88 (Mes p-C), 136.31 (NNCNN), 130.97 (Mes o-C), 130.90 (Mes m-
C), 129.98 (p-tol o- C), 126.23 (p-tol m-C), 21.60 (p-tol p-CH3), 21.31 (Mes p-CH3), 20.35
30
Formazan Synthesis
(Mes o-CH3). Elemental analysis calculated for C26H30N4: C 78.35% H 7.59% N 14.06%; found C 78.18% H 7.60% N 13.96%.
C6F5NNC(p-tol)NNHPh 1e. Pentafluorophenylhydrazine (9.91 g, 50 mmol) was combined with p-tolualdehyde (5.90 mL, 50 mmol) and ethanol (200 mL) at RT. The mixture was stirred for 30 min at which time a light yellow solid had formed. CH2Cl2 (250 mL) and the aqueous solution containing sodium carbonate (20 g, 161.5 mmol) and tetrabutylammonium bromide (1.5 g, 4.7 mmol) was added to reaction mixture before being stirred at 0 °C for 1 h. A solution of diazonium salt made from stirring aniline (4.6 mL, 50 mmol), sodium nitrite (3.80 g, 54.5 mmol), water (25 mL), and hydrochloric acid (12.5 mL) for 30 min at 0 °C. The diazonium solution was added dropwise into the first solution at 0 °C. After stirring for 2 hours at RT the organic layer was collected and the water layer was extracted by CH2Cl2 (3 × 100 mL). The organic layer was combined and reduced by rotavapor. The desired formazan
[C6F5NNC(pTol)NNHPh] was purified by recrystallization (CH2Cl2/MeOH), yield 6.3 g (15.6 1 mmol, 32 %). H NMR (400 MHz, CDCl3, 25 °C): 15.08 (s, 1H, NH), 7.93 (d, 2H, J= 8.4 Hz, p-tolyl CH), 7.73 (d, 2H, J= 7.6 Hz, Ph o-CH), 7.47 (t, 2H, J= 8.0 Hz, Ph m-CH), 7.36 (t, 19 1H, J= 7.6 Hz, Ph p-CH), 7.22 (d, 2H, J= 8.0 Hz, p-tolyl CH), 2.39 (s, 3H, p-tolyl CH3). F
NMR (375 MHz, CDCl3, 25 °C): -152.3 (d, 2F, J= 16 Hz, C6F5 o-CF), -159.9 (t, 1F, J= 21 13 Hz, C6F5 p-CF), -162.5 (td, 2F, J= 21, 6 Hz, C6F5 m-CF). C NMR (100 MHz, CDCl3, 25 °C):
148.1 (Ph ipso-C), 143.7 (NCN), 139.7 (dm, J= 256 Hz, C6F5), 138.5 (dm, J= 252 Hz, C6F5 and C6F5), 138.4 (p-tolyl ipso-C), 133.7 (p-tolyl p-C), 129.8 (Ph m-C), 129.7 (Ph p-C), 129.5
(p-tolyl CH), 126.1 (p-tolyl CH), 123.2 (C6F5 ipso-C), 120.2 (Ph o-C), 21.5 (p-tolyl CH3) ppm.
Anal. calcd for C20H13F5N4: C, 59.41; H, 3.24; N, 13.86. Found: C, 58.98; H, 3.29; N, 13.49.
C6F5NNC(p-tol)NNHMes 1f. C6F5NNC(pTol)H (1.86 g, 6.2 mmol), sodium hydroxide (4.00 g, 100 mmol), water (100 mL) and acetone (100 mL) were mixed at 0 °C. MesN2BF4 (1.45 g, 6.2 mmol) was slowly added into the solution. After stirring for 1 hour at 0 °C the reaction mixture was slowly warmed up to RT. The crude product was extracted into CH2Cl2 and the volatiles were subsequently removed on the rotavap. The desired formazan [C6F5NNC(p- tol)NNMes] was purified by recrystallization (CH2Cl2/MeOH) to give 1.1 g of product (2.5 1 mmol, 41%). H NMR (400 MHz, CDCl3, 25 °C): 13.99 (s, 1H, NH), 7.88 (d, 2H, J= 8.0 Hz, p-tolyl CH), 7.21 (d, 2H, J= 8.0 Hz, p-tolyl CH), 6.99 (s, 2H, Mes m-CH), 2.48 (s, 6H, Mes o- 13 CH3), 2.38 (s, 3H, p-tolyl CH3), 2.34 (s, 3H, Mes p-CH3). C NMR (100 MHz, CDCl3, 25
°C): 144.9 (Mes ipso-C), 144.3 (NCN), 140.0 (Mes o-C), 139.4 (dm, J= 251 Hz, C6F5),
138.5 (dm, J= 251 Hz, C6F5), 138.4 (p-tolyl ipso-C), 137.6 (dm, J= 248 Hz, C6F5), 133.8 (p-
31
Chapter 2 tolyl p-C), 132.3 (Mes p-C), 130.9 (Mes m-CH), 129.4 (p-tolyl CH), 126.4 (p-tolyl CH),
122.7 (td, J= 9, 4 Hz, C6F5 ipso-C), 21.5(p-tolyl p-CH3), 21.4 (Mes p-CH3), 20.7 (Mes o-CH3). 19 F NMR (375 MHz, CDCl3, 25 °C): -153.2 (d, 2F, J= 18 Hz, C6F5 o-CF), -162.2 (t, 1F, J=
21 Hz, C6F5 p-CF), -162.8 (td, 2F, J= 21, 5 Hz, C6F5 m-CF) ppm. Anal. calcd for C23H19F5N4: C, 61.88; H, 4.29; N, 12.55. Found: C, 61.75; H, 4.25; N, 12.43.
PhNNC(C6F5)NNHMes 1g. A flask was charged with PhNHNC(C6F5)H (1.72 g, 6 mmol), sodium hydroxide (2.00 g, 50 mmol), water (100 mL) and acetone (160 mL) and the mixture + - cooled to 0 °C. At this temperature, [MesN2] [BF4] (1.40 g, 6 mmol) was added slowly with stirring. The reaction mixture was slowly warmed up to RT and stirred for an additional 30 mins. Acetic acid was added to the reaction mixture until pH = 7. The reaction mixture was stirred for another 2 hours. The crude organic product was extracted into CH2Cl2 and the solution was concentrated. The product was purified by recrystallization from CH2Cl2/MeOH 1 at -30 °C for 2 days to give 1.2 g of PhNNC(C6F5)NNHMes (2.7 mmol, 45%). H NMR (400
MHz, CDCl3, 25 °C) δ 12.21 (s, 1H, NH), 7.41- 7.35 (m, 4H, Ph o-H, Ph m-H), 7.31 (t, 1H,
J= 6.6, Ph p-H), 6.95 (s, 2H, Mes m-H), 2.41 (s, 6H, Mes o-CH3), 2.31 (s, 3H, Mes p-CH3). 19 F NMR (376.4 MHz, C6D6, 25 °C) δ -139.3 (dd, 2F, J= 23.3, 7.5, C6F5 m-F), -154.5 (t, 1F, 13 J= 20.9, C6F5 p-F), -162.9 (td, 2F, J= 23.0, 5.6, C6F5 o-F). C NMR (400 MHz, CDCl3, 25 °C)
δ 145.5 (dm, J= 251.8, C6F5), 145.2 (Ph i-C), 144.8 (Mes i-C), 141.4 (dm, J= 259.0, C6F5),
140.0 (Mes p-C), 137.8 (dm, J= 249.6, C6F5), 135.8 (NNCNN), 132.8 (Mes o-C), 130.8 (Ph m-C), 129.7 (Mes m-C), 125.3 (Ph p-C), 116.7(Ph o-C), 111.5-111.1 (m, C6F5), 21.4 (Mes p-
CH3), 20.6 (Mes o-CH3). Anal. calcd for C19H11N4F5: C, 61.11; H, 3.96; N, 12.96. Found: C, 61.23; H, 3.98; N, 12.80.
C6F5NNC(C6F5)NNHMes 1h. C6F5NNC(C6F5)H (1.13 g, 3 mmol), sodium hydroxide (1.00 g,
25 mmol), water (100 mL) and acetone (120 mL) were mixed at 0 °C. MesN2BF4 (0.7 g, 3 mmol) was slowly added into the mixed solution. The reaction mixture was slowly warmed up to RT after stirring for 1h at 0 °C. Then, the mixed solution was stirred overnight. The crude product was extracted into CH2Cl2 and the volatiles were removed on the rotavap. The desired formazan C6F5NNC(C6F5)NNHMes was purified by recrystallization (CH2Cl2/MeOH) 1 to give 0.89 g of product (1.7 mmol, 57%). H NMR (400 MHz, CDCl3, 25 °C): 10.62 (s,
1H, NH), 6.97 (s, 2H, Mes m-CH), 2.39 (s, 6H, Mes o-CH3), 2.33 (s, 3H, Mes p-CH3) ppm. 13 C NMR (100 MHz, CDCl3, 25 °C): 145.3 (dm, J= 249 Hz), 145.5 (bs), 142.7, 141.9 (dtt, J= 256, 13, 5Hz), 140.8 (bs), 139.0 (dm, J= 254 Hz), 139.0(overlapped) 138.8 (dm, J= 253 Hz),137.7 (dm, J= 253 Hz), 137.3 (dm, J= 250 Hz), 134.8, 131.2, 120.5 (bs), 109.2 (td, J= 19,
32
Formazan Synthesis
19 4 Hz), 21.6 (Mes p-CH3), 20.9 (Mes o-CH3) ppm. F NMR (375 MHz, CDCl3, 25 °C): -
138.1 (dd, 2F, J= 22, 6 Hz, C6F5 o-CF), -152.7 (t, 1F, J= 21 Hz, C6F5 p-CF), -154.9 (bs, 2F,
C6F5 o-CF), -162.3 - -162.6 (m, 4FC6F5 m-CF), -163.2 (bs, 1F, C6F5 p-CF) ppm. Anal. calcd for C22H12F10N4: C, 50.59; H, 2.32; N, 10.73. Found: C, 50.49; H, 2.25; N, 10.66.
([1i]B(C6F5)3). 1i (200 mg, 0.6 mmol) and tris(pentafluorophenyl)borane (309 mg, 0.6 mmol) were stirred in toluene (15 mL) at room temperature for few hours. After which all the solvent was pumped out and the crude product was recrystallized from DCM/hexane to give 390 mg 1 of red crystalline product (0.46 mmol, 77 %). H NMR (400 MHz, CDCl3, 25 °C): 12.17 (bs,
1H, NH), 6.98 (s, 2H, Mes m-CH), 2.40 (s, 6H, Mes o-CH3), 2.34 (s, 3H, Mes p-CH3)ppm. 13 C NMR (100 MHz, CDCl3, 25 °C): 148.1 (dm, J= 244 Hz), 140.8, 140.3 (dm, J= 253 Hz), 137.2 (dm, J= 248 Hz), 132.5 (overlapped), 130.9, 122.6, 115.4 (bs), 111.2, 21.2, 19.8 ppm. 19 F NMR (375 MHz, CDCl3, 25 °C): -133.8 (dd, 2F, J= 22, 6 Hz, C6F5 o-CF), -156.6 (t, 1F,
J= 20 Hz, C6F5 p-CF), -163.7 (td, 2F, J= 22, 6 Hz, C6F5 m-CF) ppm.
Ph2-Ph-Ph2 1k. [(PhNNCH)2(para-C6H4)] (2g, 6.4 mmol) was dissolve in a solvent mixture containing pyridine (150 mL) and DMF (150 mL). A solution of diazonium salt made from stirring aniline (1.18g, 12.7 mmol), sodium nitrite (0.96 g, 13.8 mmol), water (20 mL), and hydrochloric acid (3.2 mL) for 30 min at 0 °C. The diazonium solution was added dropwise into the first solution at 0 °C. After stirring for few hours at RT the colour of solution turned to red and some deep red solid precipitated out from solution. Some water was added into reaction mixture to precipitate more solid from solution. The red solid was collected by filtration and washed with methanol, water and ether. The product was purified by recrystallization from DCM/MeOH at -30 °C, yield 497.3 mg (0.95 mmol, 15 %). 1H NMR 8 (400 MHz, THF-d , 25 °C): 15.26 (s, 2H, NH), 8.23 (s, 4H, C6H4), 7.82 (d, 8H, J = 8 Hz, Ph o-CH),7.48 (t, 8H, J = 7 Hz, Ph m-CH), 6.29 (t, 4H, J = 8 Hz, Ph p-CH) ppm. 13C NMR (100 8 MHz, THF-d , 25 °C): 149.3 (Ph i-C), 142.3 (NCN), 137.7 (C6H4 o-C), 130.4 (Ph m-C),
128.5 (C6H4 CH), 126.8 (Ph p-C), 119.9 (Ph o-C) ppm.
Ph2-mPh-Ph2 1l. The synthetic procedure of 1l is the same as 1k. [(PhNNCH)2(meta-C6H4)] (2g, 6.4 mmol), pyridine (150 mL), DMF (110 mL), aniline (1.18g, 12.7 mmol), sodium nitrite (0.96 g, 13.8 mmol), water (20 mL), and hydrochloric acid (3.2 mL) were used. After recrystallization from DCM/MeOH at -30 °C, 1.7 g (3.3 mmol, 52 %) of L12H2 can be 1 isolated. H NMR (400 MHz, CDCl3, 25 °C): 15.44 (s, 2H, NH), 9.04 (s, 1H, C6H4 CH),
8.15 (dd, 2H, J = 8, 2 Hz, C6H4 CH), 7.77 (d, 8H, J = 8 Hz, Ph o-CH), 7.53 (t, 1H, J = 8 Hz,
33
Chapter 2
13 C6H4 CH), 7.48 (t, 8H, J = 8 Hz, Ph m-CH), 7.31 (t, 4H, J = 7 Hz, Ph p-CH) ppm. C NMR
(100 MHz, CDCl3, 25 °C): 148.0 (Ph i-C), 141.0 (NCN), 137.5 (C6H4), 129.4 (Ph o-C),
128.6 (C6H4), 127.4 (Ph p-C), 124.9 (C6H4), 123.4 (C6H4), 118.8 (Ph p-C) ppm.
PhNNC(4-BrC6H4)NNPh 1m and PhNNCH(4-BrC6H4)CH(4-BrC6H4)NNPh 2. A flask was charged with PhNNC(4-BrC6H4)H (1.36 g, 5 mmol), CH2Cl2 (150 mL) and the aqueous solution (150 mL) containing sodium carbonate (2 g, 16.2 mmol) and tetrabutylammonium bromide (0.125 g, 0.37 mmol). A solution of diazonium salt made from stirring aniline (0.46 mL, 5 mmol), sodium nitrite (0.375 g, 5.45 mmol), water (5 mL), and hydrochloric acid (1.3 mL) for 30 min at 0 °C. The diazonium solution was added dropwise into the first solution at 0 °C. After stirring for 2 hours at RT the organic layer was collected and the water layer was extracted by CH2Cl2. The organic solution was conbined and concentrated. The product mixture of 1m and 2 can be isolated by recrystallization from CH2Cl2/MeOH at -30 °C to give total amount of 50.1 mg.
PhNNC(Py)Ph 3. [PhNNC(C5H4N)H] (1g, 5.1 mmol) was dissolved in a solvent mixture containing pyridine (8 mL), MeOH (4 mL) and acetic acid (1 mL). A solution of diazonium salt made from stirring aniline (0.47 g, 5.1 mmol), sodium nitrite (0.35 g, 5.1 mmol), water (6 mL), and hydrochloric acid (1.5 mL) for 30 min at 0 °C. The diazonium solution was added dropwise into the first solution at 0 °C. The reaction mixture was stirred at 60°C for 10 mins and then a 2M NaOH solution was added until pH = 9. After stirring over a weekend at RT, the crude product was collected by filtration and was recrystallizd from ether. After which 1 233 mg (0.85 mmol, 17 %) of red crystal was obtained. H NMR (400 MHz, CDCl3, 25 °C): 8.51 (d, 1H, J = 5 Hz, Py CH), 8.18 (d, 1H, J = 8 Hz, Py CH), 7.79 (s, 1H, NH), 7.71 (td, 1H, J = 8, 2 Hz, Py CH), 7.59 (t, 2H, J = 8 Hz, Ph m-CH), 7.51 (t, 1H, J = 7 Hz, Ph p-CH), 7.38 (d, 2H, J = 8 Hz, Ph o-CH), 7.27 (t, 2H, J = 8 Hz, Ph m-CH), 7.15 (ddd, 1H, J = 7, 5, 1 Hz, Py CH), 7.12 (d, 1H, J = 8 Hz, Ph o-CH), 6.89 (t, 1H, J = 7 Hz, Ph p-CH) ppm. 13C NMR (100
MHz, CDCl3, 25 °C): 156.5, 148.9, 144.1, 144.0, 135.9, 132.0, 129.5, 129.3, 129.2, 129.2, 122.1, 120.7, 120.6, 113.2 ppm.
Crystallographic data
Suitable crystals of 1j-B(C6F5)3, 1m, 2, and 3 were mounted on a cryo-loop in a drybox and transferred, using inert-atmosphere handling techniques, into the cold nitrogen stream of a Bruker D8 Venture diffractometer. The final unit cell was obtained from the xyz centroids of
9724 (1j-B(C6F5)3), 9915 (1m), 9950 (2) and 9906 (3) reflections after integration. Intensity data were corrected for Lorentz and polarisation effects, scale variation, for decay and
34
Formazan Synthesis
absorption: a multiscan absorption correction was applied, based on the intensities of symmetry-related reflections measured at different angular settings (SADABS).22 The structures were solved by direct methods using the program SHELXS.23 The hydrogen atoms were generated by geometrical considerations and constrained to idealised geometries and allowed to ride on their carrier atoms with an isotropic displacement parameter related to the equivalent displacement parameter of their carrier atoms. Structure refinement was performed with the program package SHELXL.23 Crystal data and details on data collection and refinement are presented in following table. Crystallographic data 1j-B(C6F5)3 1m 2 3 chem formula C38H23BF15N5 C19H15BrN4 C30Br2N4 C18H15N3 Mr 845.42 379.26 576.16 273.33 cryst syst monoclinic orthorhombic triclinic monoclinic color, habit orange, plate red, block colourless, plate yellow, block size (mm) 0.25 x 0.90 x 0.10 0.17 x 0.15 x 0.06 0.18 x 0.11 x 0.02 0.33 x 0.20 x 0.15 space group P21/n Pbca P-1 P21/c a (Å) 13.2170(9) 7.8048(3) 6.1295(3) 10.6972(12) b (Å) 11.3150(7) 18.9675(8) 9.3415(4) 8.6263(10) c (Å) 23.5257(16) 22.1335(10) 10.4730(5) 15.7962(18) (°) 83.9733(16) β (°) 90.469(2) 77.0380(17) 106.374(4) (°) 84.8992(15) V (Å3) 3518.2(4) 3276.6(2) 579.84(5) 1398.5(3) Z 4 8 1 4 -3 calc, g.cm 1.596 1.538 1.570 1.298 Radiation [Å] Mo Kα 0.71073 Mo Kα 0.71073 Mo Kα 0.71073 Mo Kα 0.71073 -1 µ(Cu K), mm -1 µ(Mo K), mm 0.151 2.516 3.516 0.079 F(000) 1704 1536 274 576 temp (K) 100(2) 100(2) 100(2) 100(2) range (°) 2.93-26.77 2.83-28.33 2.84-27.12 2.72-28.44 data collected (h,k,l) -16:16; -14:14; - -10:9; -25:25; -29:29 -7:7; -11:11; - -14:14; -11:11; - 29:29 13:13 20:21 min, max transm 0.6929, 0.7454 0.6515, 0.7457 0.6290, 0.7455 0.7104, 0.7457 rflns collected 114934 92987 17139 57374 indpndt reflns 7482 4087 2547 3514 observed reflns Fo 5696 3385 2333 2981 2.0 σ (Fo) R(F) (%) 4.14 2.88 2.39 4.07 wR(F2) (%) 8.56 6.70 5.39 10.3 GooF 1.034 1.039 1.066 1.028 weighting a,b 0.0382, 2.4179 0.034, 2.6501 0.0235, 0.4354 0.0579, 0.5229 params refined 542 221 145 194 min, max resid dens -0.258, 0.314 -0.228, 0.489 -0.346, 0.533 -0.212, 0.352
35
Chapter 2
2.5 References
(1) (a) Grychtol, K.; Mennicke, W. In Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: 2000. (b) Bamberger, E. and Wheelwright, E. Ber. Dtsch. Chem. Ges., 1892, 25, 3201-3213. (c) Friese, P. Ber. Dtsch. Chem. Ges., 1875, 8, 1078-1080. (2) v. Pechmann, H. Ber. Dtsch. Chem. Ges., 1892, 25, 3175-3190. (3) (a) Nineham, A. W. Chem. Rev., 1955, 55, 355–483. (b) Tezcan, H. Spectrochimica Acta Part A, 2008, 69, 971–979. (c) Ibrahim, Y. Tetrahedron, 1997, 53, 8507–8512. (d) Gok, Y. and Senturk, H. B. Dyes and Pigments. 1991, 15, 279–287. (e) Abbas, A. A. and Elwahy, A. H. M. ARKIVOC. 2009, 2009, 65– 70. (f) Katritzky, A. R.; Belyakov, S. A.; Durst, H. D. Synthesis, 1995, 1995, 577-581. (g) Gilroy, J. B.; Otieno, P. O.; Ferguson, M. J.; McDonald, R.; Hicks, R. G. Inorg. Chem., 2008, 47, 1279–1286. (4) (a) Turkoglu, G.; Berber, H.; Kani, I. New J. Chem., 2015, 39, 2728–2740. (b) Ibrahim, Y. A.; Abbas, A. A.; Elwahy, A. H. M. J. Heterocyclic Chem., 2004, 41, 135–149. (5) Di Zhu; Budzelaar, P. H. M. Dalton Trans., 2013, 42,11343–11354. (6) Busch, M.; Schmidt, R. J. Prakt. Chem., 1931, 131, 182-192. (7) Neugebauer, F. A.; Trischmann, H. Liebigs Ann. Chem., 1967, 706, 107-111. (8) Pechmann, H. V. Ber. Dtsch. Chem. Ges., 1894, 27, 1679-1693. (9) Busch, M.; Schmidt, R. Ber. Dtsch. Chem. Ges. A/B, 1930, 63, 1950-1952. (10) Bamberger, E. Ber. Dtsch. Chem. Ges., 1892, 25, 3547-3555. (11) Hausser, I.; Jerchel, D.; Kuhn, R. Chem. Ber., 1949, 82, 515-527. (12) Hutton, A. T.; Irving, H. M. N. H.; Nassimbeni, L. R. Acta Cryst., 1980, 36, 2071-2076. (13) Otten, E.; Travieso-Puente, R.; CHANG, M.-C. Dalton Trans., 2014, 43, 18035-18041. (14) Gilroy, J. B.; Ferguson, M. J.; McDonald, R.; Patrick, B. O.; Hicks, R. G. Chem. Commun., 2007, 126- 128. (15) Doyle, M. P.; Bryker, W. J. J. Org. Chem.,1979, 44, 1572-1574. (16) Jacobsen, H.; Berke, H.; Döring, S.; Kehr, G.; Erker, G. Organometallics, 1999, 18, 1724-1735. (17) (a) Rojas, R. S.; Peoples, B. C.; Cabrera, A. R.; Valderrama, M.; Fröhlich, R.; Kehr, G.; Erker, G.; Wiegand, T.; Eckert, H. Organometallics, 2011, 30, 6372-6382. (b) Cabrera, A. R.; Schneider, Y.; Valderrama, M.; Fröhlich, R.; Kehr, G.; Erker, G.; Rojas, R. S. Organometallics, 2010, 29, 6104-6110. (18) Leen, V.; Yuan, P.; Wang, L.; Boens, N.; Dehaen, W. Org. Lett., 2012, 14, 6150-6153. (19) Mirífico, M. V.; Caram, J. A.; Vasini, E. J. Tetrahedron Letters, 2006, 47, 6919-6922. (20) Frolova, N. A.; Vatsadze, S. Z.; Vetokhina, N. Y.; Zavodnik, V. E.; Zyk, N. V. Mendeleev Commun., 2006, 16, 251–254. (21) (a)Kuhn, R.; Neugebauer, F. A.; Trischmann, H. Monatshefte für Chemie und verwandte Teile anderer Wissenschaften. 1966, 97, 525-553. (b)Barbon, S. M.; Price, J. T.; Yogarajah, U.; Gilroy, J. B. RSC Adv., 2015, 5, 56316-56324. (22) Bruker. APEX2 (v2012.4-3), SAINT (Version 8.18C) and SADABS (Version 2012/1). Bruker AXS Inc., Madison, Wisconsin, USA. 2012. (23) Sheldrick, G. Acta Crystallographica Section A, 2008, 64, 112.
36
Chapter 3
(Formazanate)Zinc Complexes
The synthesis and characterization of mono(formazanate)ZnMe complexes (LZnMe; 4) and bis(formazanate)Zn complexes (L2Zn; 5) are presented. Single crystal structure determinations and VT-NMR studies of 5 show that the formazanate ligands have flexible coordination chemistry in both solid- and solution-state due to its nitrogen-rich structure. The cyclic voltammetry studies reveal that compounds 5 have five accessible oxidation states (50/- 1/-2/-3/-4), in which the first two reduced products ([5]- and [5]-2) were synthesized and characterized (5a-/-2 and 5b-/-2). The redox potentials of the compounds 5 can be altered in a straightforward manner over a relative wide range (~ 500 mV) by changing the steric or the electronic properties of the formazanate framework. The characterization data of the reduced products (5a-/-2 and 5b-/-2) prove the redox-active nature of the formazanate ligand.
Parts of this chapter have been published:
M.-C. Chang, T. Dann, Dr. D. P. Day, Dr. M. Lutz, Dr. G. G. Wildgoose and Dr. E. Otten* “The Formazanate Ligand as an Electron Reservoir: Bis(Formazanate) Zinc Complexes Isolated in Three Redox States” Angew. Chem. Int. Ed., 2014, 53, 4118–4122.
M.-C. Chang, P. Roewen, R. Travieso-Puente, M. Lutz, and E. Otten* ”Formazanate Ligands as Structurally Versatile, Redox-Active Analogues of β-Diketiminates in Zinc Chemistry” Inorg. Chem., 2015, 54, 379-388. Chapter 3
Chapter 3 (Formazanate)Zinc Complexes 3.1 Introduction
Despite the structural similarities between β-diketiminate,1 1,3,5-triazapentadienyl2 and formazanate, the coordination chemistry of formazanate ligands has remained relatively little explored. Most of the reported formazanate metal complex focused on late transition metals or noble metals. For example, Lipunov and co-workers reported bis(formazanate) nickel(II) complexes which were used to catalyze ethylene oligomerization.3 Poddel'sky and co-workers reported a heteroleptic cobalt complex, which is coordinated by a formazanate ligand and an o-semiquinonate radical anion, having a singlet ground state due to the antiferromagnetic exchange between low-spin cobalt(II) (S = 1/2) center and the o-semiquinonate radical anion (S = 1/2).4 Hicks and co-workers reported some well-characterized examples of late transition metal formazanate complexes,5 and boron compounds with formazanate ligands were shown to possess unusual redox properties.6
Unlike other late transition metal complex, the literature reports of (formazanate)zinc complexes are very limited and often poorly described. However, there are some reported zinc complexes bearing β-diketiminate ligands in the literature (Chart 3.1). Coates and co-workers reported a method for the synthesis of poly(ester-block-carbonate)s through a one-step, one- pot procedure with a (β-diketiminate)ZnX catalyst (A).7 Schaper and co-workers reported heteroleptic bis(β-diketiminate)zinc complexes (B) as the secondary building blocks of a 2D copper-zinc coordination polymer.8 The hydroamination of alkynes catalyzed by (β- diketiminate)ZnMe complexes (C) or bis(β-diketiminate)zinc complex (D) was reported by Roesky, Blechert and co-workers.9
Chart 3.1
NC Ar Py Py R3 R3 Ar N N N N N N Et N N Et Zn Zn Zn N Zn Et N Et N N Ar O O R2 R1 R1 R2 Ar N N
A B CD In this chapter, we synthesized a series of mono- and bis(formazanate) zinc complexes (compounds 4 and 5) to understand the coordination chemistry and redox-active property of the formazanate ligands. The reason why we chose Zn2+ complex as our starting point is because of the redox-inert nature of the Zn2+ ion, which has a stable +2 oxidation state and d10
38
(Formazanate)Zinc Complexes electron configuration. The stable +2 oxidation state (and lack of further Zn-based redox reactions) makes that any redox-chemistry that is observed in these compounds is due to the formazanate ligands. In addition, the d10 electron configuration results in a singlet ground state of the Zn complex, which allows us to use NMR to characterize the products or to follow the reactions.
3.2 (Formazanate) Zinc Methyl complexes
A preliminary study of metal complexation of formazanate ligands was carried out by synthesizing the mono(formazanate)ZnMe complexes (4). The compounds 4 were synthesized by reacting free formazan with 1equivlent of dimethyl zinc (Scheme 3.1). The 1H NMR spectrum of 4a shows a singlet resonance of the ZnMe group, which is located at -0.18 ppm, in addition to those expected for the formazanate ligand with a 1:1 ratio (Figure 3.1). Single crystals suitable for X-ray crystallography of 4a were obtained by slow diffusion of hexane into a solution of 4a in toluene (Figure 3.2, metrical parameters in Table 3.1). Even though 4a can be isolated as a crystalline material, 4a itself is unstable in the solution state. At room temperature, 4a establishes a Schlenk equilibrium with bis(formazanate)Zn complex (5a) in toluene or C6D6 solution with the color changing from deep purple to deep blue. The chemistry of bis(formazanate)Zn complexes (5) will be discussed later in this chapter.
Scheme 3.1 Synthesis of LZnMe (4) and formation of L2Zn (5)
1 Figure 3.1 H NMR spectrum of 4a (C6D6, 400 MHz)
39
Chapter 3
3.2.1 Zinc Methyl Complex with Phenylene-Linked Diformazanate Ligand
In the case of free di-formazan 1k, the corresponding zinc methyl complex 4k can be synthesized in high yield (93%) in THF solution using an excess of dimethyl zinc. The complex 4l can also be synthesized with the same method. Both compounds feature a 1 diagnostic resonance in the H NMR spectrum at -0.35 (4k, in THF-d8) and -0.08 (4l, in C6D6) ppm for the Zn-Me moiety. The crystal structure and metrical parameters of 4k are shown in Figure 3.2 and Table 3.1, respectively. Unlike 4a, the crystal structure of 4k shows two distorted trigonal pyramidal zinc centers, in which the formazanate and methyl groups occupy the equatorial positions, and one THF molecule sits at the axial position. All the metrical parameters in the formazanate ligand of 4k are very close to those in 4a. The Zn-N bonds of 4k are slightly longer than the Zn-N bonds of 4a. The difference is due to the different coordination number of zinc centers (4 for 4k and 3 for 4a). The Zn-O(thf) bond lengths of 4k are 2.190(3) and 2.220(3) Å, which are in a range (2.19-2.33 Å) of reported (- diketiminate)ZnEt(THF) complexes.10 The trigonal pyramidal metal center is very rare for the four coordinated complexes, in particular for the bidentate ligand system. In literature, only few of the reported (-diketiminate)ZnRX (R=alkyl; X=THF or pyridine) complexes show trigonal pyramidal structure.10,11 The common structures of four coordinated complexes are tetrahedral or square planar. In nature, trigonal pyramidal iron centers play a significant role in nitrogen fixation. For example, the active site of the Iron-Molybdenum cofactor of Nitrogenase is an iron belt constructed by six trigonal pyramidal iron centers (Chart 3.2).12 The vacant site of the trigonal pyramidal iron centers is a potential binding site for nitrogen molecule. Several iron complexes bearing -diketiminate ligands were prepared to mimic the trigonal pyramidal active site, but most of them show tetrahedral structure instead of the trigonal pyramidal structure. Another strategy to construct trigonal pyramidal metal centers is using specially designed tridentate ligands in which one of the donor atoms can occupy the axial position. For example, a trigonal pyramidal Ru(II) complex was synthesized by Turculet and co-workers by using a bis(phosphino)silyl ligand (Chart 3.2).13
40
(Formazanate)Zinc Complexes
Figure 3.2 Molecular structures of 4a (left) and 4k(THF)2 (right) showing 50% probability ellipsoids. The THF atoms except for O and all hydrogen atoms are omitted for clarity.
o Table 3.1 Selected bond length (Å) and bond angles ( ) of 4a and 4k(THF)2 4a 4k(THF)2 N1-N2 1.299(3) N1-N2 1.306(4) N5-N6 1.310(4) N2-C7 1.346(2) N2-C7 1.343(4) N6-C12 1.349(4) C7-N3 1.337(3) C7-N3 1.349(4) C12-N7 1.348(4) N3-N4 1.310(3) N3-N4 1.308(4) N7-N8 1.304(4) N1-Zn1 2.003(2) N1-Zn1 2.018(3) N5-Zn2 2.011(3) N4-Zn1 1.989(2) N4-Zn1 2.018(3) N8-Zn2 2.016(3) Zn1-C21 1.956(3) Zn1-C34 1.971(4) Zn2-C19 1.950(5) N1-Zn1-N4 90.80(7) N1-Zn1-N4 88.6(1) N5-Zn2-N8 89.0(1) N1-Zn1-C21 129.95(9) N1-Zn1-C34 136.3(1) N5-Zn2-C19 134.4(1) N4-Zn1-C21 138.62(9) N4-Zn1-C34 129.7(1) N8-Zn2-C19 126.3(1) Zn1-(N1N2N3N4)a 0.102 Zn1-( N1N2N3N4)a 0.355 Zn2-(N5N6N7N8)a 0.227 Zn1-(N1N4C21)b Zn1-(N1N4C34)b 0.256 Zn2-(N5N8C19)b 0.355 Zn1-O1 2.220(3) Zn2-O2 2.190(3) C34-Zn1-O2 103.4(1) C19-Zn1-O2 106.0(1) N1-Zn1-O2 90.7(1) N1-Zn1-O2 95.1(1) N4-Zn1-O2 95.6(1) N4-Zn1-O2 97.4(1) 0.72 0.57 S(T) 3.14 S(T) 2.45 S(TP) 3.41 S(TP) 3.67 S(bT) 2.12 S(bT) 1.42 S(bTP) 1.19 S(bTP) 1.73 a: Distance between Zn and N-N-N-N plane ; b: Distance between Zn and N-N-C(Me) plane.
Chart 3.2 Iron-Molybdenum cofactor of Nitrogenase (left) and bis(phosphino)silyl Ru(II) (right)
PCy2
Me Si H Si Cy2P Ru X Cy2P PCy2
41
Chapter 3
3.2.2 Quantitative Description of Trigonal Pyramidal Zinc Center
In order to quantitatively describe the rare trigonal pyramidal zinc centers of 4k, two parameters were used: angle-based parameter and continuous shape measures S(G).14
(i) Angle-based parameter :
In four coordinated complexes, is the normalized difference between the sum of the basal ligand-basal ligand angles and the sum of the basal ligand-axial ligand angles (Chart 3.3). The angle-based parameter is expressed in eq 3.1. The eq 3.1 is suitable to describe the structures that lie near the interconversion path between a perfect tetrahedron ( = 0) and a perfect trigonal pyramid ( = 1). The limitations of this parameter are that it can not describe the deviation from the C3 symmetry of the structures such as the bidentate ligand. In addition, it is not reliable in some extreme cases: the square planar ( = 0) and sawhorse ( = 1) structures. The of two Zn centers of 4k are 0.72 and 0.57, which indicate that the first zinc center is very close to the trigonal pyramidal structure and the second zinc center is sitting between the tetrahedral and trigonal pyramidal structure. It is worth pointing out that = 0.72 is the highest value we were able to find in the literatures.
Σ Σ 3.1 90
Chart 3.3
(ii) Continuous shape measures S(G):
The continuous shape measure was proposed by Avnir and co-workers to quantitatively describe the deviation of a set of atoms from a given ideal polyhedral shape G.14 The given ideal polyhedral shape is superimposed on the metal complexes to minimize the expression of eq 3.2. The vectors are the vectors constructed by the coordinates of N atoms of the investigated structure. The vectors are the vectors constructed by the coordinates for the perfect polyhedron closest in size and orientation. The vector is the coordinate vector of the geometrical center of the investigated structure. The value of S(G) is between 0 and 100;
42
(Formazanate)Zinc Complexes
S(G) = 0 means that the investigated structure is exactly the same as the given ideal polyhedral shape.