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Doctoral Thesis

Synthesis and Properties of Acylphosphines

Author(s): Schrader, Erik

Publication Date: 2018

Permanent Link: https://doi.org/10.3929/ethz-b-000280388

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ETH Library DISS. ETH NO. 25041

Synthesis and Properties of Acylphosphines

A thesis submitted to attain the degree of DOCTOR OF SCIENCES of ETH ZURICH (Dr. sc. ETH Zurich)

presented by ERIK SCHRADER MSc ETH (Chemistry), ETH Zurich

born on 26.01.1990 citizen of Germany

accepted on the recommendation of

Prof. Dr. Hansjörg Grützmacher Prof. Dr. Antonio Togni

2018

“So once you do know what the question actually is, you’ll know what the answer means.” Douglas Adams

Acknowledgements This work would not have been possible without the help and support of a lot of people. First and foremost I am very thankful, that Prof. Dr. Hansjörg Grützmacher allowed me to perform my PhD studies in his group. I was able to overcome some rough patches in different projects through his support and constant encouragement and when results came through, he was able to further unlock their potential. I would also like to thank Prof. Dr. Antonio Togni – not only for being my co-examiner, but also for giving me the opportunity to get a taste of teaching chemistry during my Masters studies as a teaching assistant in his lecture. Special thanks goes to my two mentors, who taught me the tricks of the trade in a chemical laboratory: Dr. Aaron Tondreau and Dr. Riccardo Suter. Without these two, the chemistry presented in this thesis would not have been achieved. Without Dr. Suter a lot of days in the lab would not have been as enjoyable and the evenings would have been much more dull. He is also credited with performing the DTF-calculations in Chapter 4 and proofreading a lot of this thesis. Dr. Michael Wörle of the Small Molecule Crystallography Center at ETH Zurich has contributed immensely to this work, not only by helping with difficult structures but also by keeping everything up and running and giving the freedom to measure crystals, that maybe were not perfect all the time. The same holds true for Dr. René Verel and Matt Baker, who were always helping with NMR-related problems or fixing the machines, when they could not cope with the workload. A big thank you goes to all members of the Grützmacher group, which supported me during the years: First of all – Christine Rüegg, whithout whom the whole group likely would have been disbanded by now. I was also very glad to have the support of other group members, such as Dr. Mark Bispinghoff, Dr. Robert Gilliard, Dr. Jaap Borger, Dr. Thomas Gianetti, Bruno Pribanic, Johanna Scoul, Fabian Müller, Jan Bloch, Andreas Beil, Riccardo Conti and Pascal Jurt. I would like to thank my students Jacqueline Waldvogel, Grégoire le Corré, Lukas Wassermann, Yannick Kürsteiner and Stefan Banz, for doing what I could not or did not want to do. My collaborators in the Kovalenko group, Georgian Nedelcu and Andriy Stelmakh, contributed to important part of this project and allowed to give fundamental studies a fascinating application. An important part of my time at ETH were my friends, who went to lunch with me on countless occassions. They were an unending source of morale, sometimes simply due to the fact, that others had it as bad me… In the end, I want to thank my family. Without my parents and grandparents, I would not be where I am today. They supported me the whole time and gave me the freedom and confidence to pursue the path I wanted to take. I am very grateful to my partner Andrea, who always supported me and gave me a place to come home to, where chemistry did not matter for a while.

Table of Contents TABLE OF CONTENTS ...... 7 ABSTRACT ...... 11 ZUSAMMENFASSUNG ...... 15 PRECEEDING REMARKS ...... 19 1 INTRODUCTION ...... 21

1.1 SYNTHESIS OF ACYLPHOSPHINES ...... 21 1.2 PROPERTIES OF ACYLPHOSPHINES ...... 22 1.3 THE PHOSPHORUS-ACYL BOND ...... 24 1.4 AIM OF THIS THESIS ...... 25 2 SYNTHESIS OF PRIMARY ACYLPHOSPHINES BASED ON THE REACTION OF SODIUM DIHYDROPHOSPHIDE WITH ORGANIC ESTERS ...... 27

2.1 INTRODUCTION ...... 27 2.2 AIM OF THIS PROJECT ...... 30 2.3 RESULTS AND DISCUSSION ...... 30 2.4 SUMMARY AND OUTLOOK ...... 37 3 REACTIVITY OF MESITOYLPHOSPHINE ...... 39

3.1 INTRODUCTION ...... 39 3.1.1 The Phospha-Michael Addition ...... 39 3.1.2 Synthesis of Diphosphenes ...... 40 3.2 AIM OF THIS WORK ...... 42 3.3 RESULTS AND DISCUSSION ...... 43 3.3.1 Phospha-Michael Addition ...... 43 3.3.2 Synthesis of Diphosphenes ...... 47 3.4 SUMMARY AND OUTLOOK ...... 53 4 SYNTHESIS OF MONO(ACYL)PHOSPHOLE OXIDES ...... 55

4.1 INTRODUCTION ...... 55 4.2 AIM OF THIS WORK ...... 57 4.3 RESULTS AND DISCUSSION ...... 58 4.4 CONCLUSION AND OUTLOOK ...... 64 5 SYNTHESIS OF TETRAKIS(ACYL)DIPHOSPHINES ...... 65

5.1 INTRODUCTION ...... 65 5.2 AIM OF THIS WORK ...... 67 5.3 RESULTS AND DISCUSSION ...... 67 5.4 SUMMARY AND CONCLUSION ...... 72 6 SYNTHESIS OF TRIS(ACYL)- AND TRIS(CARBAMOYL)PHOSPHINES ...... 75

6.1 INTRODUCTION ...... 75 6.2 AIM OF THIS PROJECT ...... 79 6.3 RESULTS AND DISCUSSION ...... 79 6.4 CONCLUSION AND OUTLOOK ...... 90 7 THERMAL PROPERTIES OF ACYLPHOSPHINES AND THEIR APPLICATION IN THE SYNTHESIS OF INP NANOPARTICLES ...... 93

7.1 INTRODUCTION ...... 93 7.2 AIM OF THIS WORK ...... 95

7.3 RESULTS AND DISCUSSION ...... 96 7.3.1 Cyclotetraphosphines ...... 96 7.3.2 Tetrakis(acyl)diphosphines ...... 99 7.3.3 Bis(acyl)phosphines and Tris(acyl)phosphines ...... 100 7.3.4 A Potential Precursor for AsP3 ...... 104 7.4 SUMMARY AND OUTLOOK ...... 108 8 SODIUM FORMPHOSPHIDE – SYNTHESIS, STRUCTURE AND REACTIVITY ...... 111

8.1 INTRODUCTION ...... 111 8.2 AIM OF THIS WORK ...... 113 8.3 RESULTS AND DISCUSSION ...... 115 8.3.1 Isolation and Solid-state Structure of Na(HPCHO) ...... 115 8.3.2 Reactivity of Na(HPCHO) ...... 119 8.3.3 Thermal decomposition of Na(HPCHO) ...... 122 8.4 SUMMARY AND OUTLOOK ...... 125 9 GENERAL OUTLOOK ...... 127

9.1 THE SYNTHESIS OF 1,2-AZAPHOSPHOLES ...... 127 9.2 THE PHOSPHA-MICHAEL ADDITION OF PRIMARY ACYLPHOSPHINES ...... 128 9.3 ACYLPHOSPHOLE OXIDES ...... 128 9.4 ACYLPHOSPHINES IN THE SYNTHESIS OF PHOSPHIDE-NANOPARTICLES ...... 129 10 EXPERIMENTAL ...... 131

10.1 GENERAL REMARKS ...... 131 10.2 COMPOUNDS PREPARED IN CHAPTER 2 ...... 132 10.2.1 Synthesis of Sodium bis(ethoxycarbonyl)phosphide (3) ...... 132 10.2.2 Synthesis of 1-methyl-3-((trimethylsilyl)oxy)-1H-benzo[d][1,2]azaphosphole (TMS-8) ...... 133 10.2.3 Synthesis of Primary Acylphosphines ...... 134 10.3 COMPOUNDS FROM CHAPTER 3 ...... 136 10.3.1 Synthesis of Mesitoyl-2,6-diphenylphosphinanone (26) ...... 136 10.3.2 Synthesis of Mesitoyl-2,6-diphenylphosphinanone Oxide (27) ...... 137 10.3.3 Synthesis of Tetrakis(mesitoyl)cyclotetraphosphine (30) ...... 138 10.3.4 Synthesis of Mesitoyl-N,N-dimethylimidazoliumphosphinidene (32) ...... 139 10.3.5 Synthesis of Tetrakis(ortho-phenylbenzoyl)cyclotetraphosphine (34) ...... 140 10.4 COMPOUNDS FROM CHAPTER 4 ...... 141 10.4.1 Synthesis of Mesitoyl-2,5-diphenylphosphole (37) ...... 141 10.4.2 Synthesis of Mesitoyl-2,5-diphenylphosphole Oxide (38) ...... 142 10.4.3 Synthesis of Mesitoyl-2,3,4,5-tetraethylphosphole oxide (39) ...... 143 10.5 COMPOUNDS FROM CHAPTER 5 ...... 144 10.5.1 Synthesis of Tetrakis(acetyl)diphosphine (41) ...... 144 10.5.2 Synthesis of Tetrakis(mesitoyl)diphosphine (43) ...... 145 10.5.3 Synthesis of Tetrakis(benzoyl)diphosphine (46) ...... 145 10.6 COMPOUNDS FROM CHAPTER 6 ...... 146 10.6.1 Synthesis of Tris(benzoyl)phosphine (47) ...... 146 10.6.2 Synthesis of Tris(pivaloyl)phosphine (48) ...... 147 10.6.3 Synthesis of Tris(acetyl)phosphine (49) ...... 148 10.6.4 Synthesis of (Bis(benzoyl)phosphido)dichlorobis(tetrahydrofurano)titanium(III) (53) ...... 149 10.6.5 Synthesis of Tris(benzoyl)phosphine oxide (50) ...... 150 10.6.6 Synthesis of Tris(dimethylcarbamoyl)phosphine (54) ...... 151 10.6.7 Synthesis of Tris(diphenylcarbamoyl)phosphine (55) ...... 152 10.6.8 Synthesis of Tris(4-morpholinecarbonyl)phosphine (56) ...... 153 10.7 COMPOUNDS FROM CHAPTER 7 ...... 154 10.7.1 Synthesis of Tris(bis(benzoyl)phosphido)arsine (61) ...... 154 10.8 COMPOUNDS FROM CHAPTER 8 ...... 155 10.8.1 Synthesis of Sodium Formphosphide (62) ...... 155 10.8.2 Synthesis of Disodium 2,3-diphosphabuten-1,4-olate triphenylborane adduct (66)...... 157 11 LITERATURE ...... 158 12 LIST OF ABBREVIATIONS ...... 163 13 CURRICULUM VITAE ...... 164 14 LIST OF PUBLICATIONS ...... 165 15 CRYSTALLOGRAPHIC DETAILS ...... 166

Abstract Acylphosphines are important precursors for the potentially photoactive acylphosphine oxides. However, most syntheses of these compounds reported in literature are based on tris(trimethylsilyl)phosphine (P(TMS)3) as phosphorus source, which is an expensive, toxic and pyrophoric liquid. Based on previous reports by the Grützmacher group, this work documents the development of synthetic pathways to mono-, bis- and tris(acyl)phosphines and derivatives thereof. These are based on sodium dihydrophosphide (Na(PH2)) and disodium hydrophosphide (Na2(PH)) as phosphorus sources, which can be prepared in situ from red phosphorus, elemental sodium and tert-butanol. The reaction of these phosphorus sources with organic esters allows for the synthesis of sodium mono(acyl)phosphides, which can subsequently be converted into various acylphosphines (see Scheme 0.1).

Scheme 0.1 Synthesis of mono-, bis- and tris(acyl)phosphines from sodium hydrophosphides. A variety of different aryl substituted mono(acyl)phosphines was prepared. These compounds are isolable and storable, contradicting previous reports, which described primary acylphosphines as unstable towards disproportionation. It could be shown that such a disproportionation is much more likely to occur under basic conditions. Limitations to the substrate scope arose when organic esters bearing leaving groups (-CN) or anthranoyl substituents were used (see Scheme 0.2).

Scheme 0.2 Reactions of Na(PH2) with organic esters not leading to sodium mono(acyl)phosphides.

Using mesitoylphosphine as a model compound, the reactivity of primary acylphosphines was explored. Hydrophosphination of dibenzylideneacetone (DBA) in a phospha-Michael addition led to formation of mesitoyl-2,6-diphenylphosphinanone, while oxidative coupling using C2Cl6/NEt3 led to formation of tetrakis(mesitoyl)cyclotetraphosphine (see Scheme 0.3)

Scheme 0.3 Mesitoylphosphine reacts in a phospha-Michael reaction (left) and in an oxidative coupling (right). Acyl substituted phosphole oxides were investigated in Chapter 4. Despite the fact that these compounds are acylphosphine oxides they are poor photoinitiators. DTF-studies on one of the compounds (mesitoyl-2,5-diphenylphosphole oxide) revealed that this is due to a n-π* interaction between the carbonyl group and the endocyclic C-C double bonds. This inhibits the interaction of the carbonyl group with the phosphorus-oxygen moiety, which is usually responsible for the good performance of acylphosphine oxides as photoinitiators. This n-π* interaction does however lower the HOMO-LUMO gap significantly compared to a phenylsubstituted analogue. This shows that acylation can be a promising new methodology for the tuning of phosphole-oxide-based optic materials. The methodology used for the oxidative coupling of mesitoylphosphine (see Scheme 0.3) was transferred to the oxidative coupling of bis(acyl)phosphines. This allowed for the synthesis of tetrakis(acyl)diphosphines. Cyclovoltammetric studies on these compounds revealed the oxidative coupling to be reversible at reductive potentials, which allows the bis(acyl)phosphine dimers to be used as precursors for bis(acyl)phosphides.

Tris(acyl)phosphines were prepared from Na2(PH) as depicted in Scheme 0.1, allowing for a unified synthesis of three literature known acylphosphines (R = Me, tBu, Ph). The reactivity of this class of compounds was explored using tris(benzoyl)phosphine as a model compound (see Scheme 0.4).

Scheme 0.4 Reactivity of tris(benzoyl)phosphine. 12

The chemically more stable tris(carbamoyl)phosphines were also found to be accessible via this route and thermal analysis of these compounds revealed that upon thermal decomposition elemental phosphorus is released. Thermal analysis of the acylphosphines described in the previous chapters revealed that elemental phosphorus is formed upon decomposition of a variety of different phosphines (see Scheme 0.5). The mechanism of this decomposition could not be elucidated completely. It could however be shown that for all but the smallest substituents (–Me. –NMe2) thermal decomposition likely proceeds via a disproportionation of the acyl groups, which causes the formation of a wide array of decomposition products.

Scheme 0.5 Different phosphines, which were identified as thermal sources for P4. Based on these findings, acylphosphines were tested as precursors for InP nanoparticles (in collaboration with Prof. Kovalenko – ETH Zurich) and transition metal phosphides (in collaboration with Prof. Sanchez – CNRS Paris). The synthesis of InP nanoparticles is best performed with a mixture of bis(benzoyl)phosphine and tris(benzoyl)phosphine. Investigation of this precursor system revealed that the reactive phosphorus species in this case is most likely not P4, but PH3. It is liberated from the acylphosphines by reaction with the added surfactant (oleylamine) and subsequent disproportionation of the formed mono(acyl)phosphine. Synthesis and thermal analysis of tris(bis(benzoyl)phosphido)arsine, showed that tris(bis(acyl)phosphido)arsines are potential thermal precursors of AsP3. The last chapter concerns itself with sodium formphosphide. This compound was structurally characterized for the first time in the solid state. Isolation of pure Na(HPCHO) allowed for the investigation of its reactivity, which was found to be dominated by decarbonylation reactions. The only exception to this is the oxidative P-P coupling of Na(HPCHO) with triphenylborane and Na(HMDS), which causes the formation of the triphenylborane adduct of the formed dianionic dimer.

The thermal decomposition of Na(HPCHO) was investigated as well. It could be shown that the concentration of Na(HPCHO) in the solid state affects the ratio of the observed decomposition products. The gaseous products were found to be methylphosphine and derivatives thereof, while the solid residue consists of phosphates or phosphonates. This indicates that the decomposition of Na(HPCHO) proceeds via a disproportionation reaction, similar to those observed for the thermal decomposition of acylphosphines.

Zusammenfassung Acylphosphine sind wichtige Ausgangsstoffe für die potentiell photoaktiven Acylphosphinoxide. Die meisten Literaturvorschriften zur Synthese dieser Verbindungen basieren jedoch auf Tris(trimethylsilyl)phosphin (P(TMS)3) als Ausgangsstoff, einer teuren, giftigen und pyrophoren Flüssigkeit. Ausgehend von bisherigen Resultaten der Grützmacher- Gruppe, wird in dieser Arbeit die Entwicklung synthetischer Routen zu Mono-, Bis- und Tris(acyl)phosphinen beschrieben. Diese basieren auf Natriumdihydrophosphid (Na(PH2)) und Dinatriumhydrophosphid (Na2(PH)) als Phosphorquellen, welche in situ aus rotem Phosphor, elementarem Natrium und tert-Butanol hergestellt werden können. Die Reaktion dieser Phosphorquellen mit organischen Estern erlaubt die Synthese von Natriummono(acyl)phosphiden, welche wiederum in verschiedene Acylphosphine konvertiert werden können, (siehe Schema 0.1)

Schema 0.1 Synthese von Mono-, Bis- und Tris(acyl)phosphinen ausgehend von Natriumhydrophosphiden. Verschiedene arylsubstituierte Mono(acyl)phosphine wurden hergestellt. Diese Verbindungen sind isolier- und lagerbar. Dies widerspricht bisherigen Berichten, die primäre Acylphosphine als instabil gegenüber Disproportionierung beschreiben. Es konnte gezeigt werden, dass eine solche Disproportionierung leichter unter basischen Bedingungen vonstattengeht. Organische Ester, welche durch eine Abgangsgruppe (-CN) oder Anthranylgruppen substituert sind, reagieren in einer unterschiedlichen Art und Weise (siehe Schema 0.2)

Schema 0.2 Reaktionen von Na(PH2) mit organischen Estern, die nicht zu Natriummonoacylphosphiden führen.

Mesitoylphosphin wurde als Modellverbindung verwendet um die Reaktivität von primären Acylphosphinen zu untersuchen. Hydrophosphinierung von Dibenzylidenaceton (DBA) in einer Phospha-Michaelreaktion führte zur Bildung von Mesitoyl-2,6-diphenylphosphinanon, während die oxidative Kopplung unter Verwendung von C2Cl6/NEt3 die Bildung von Tetrakis(mesitoyl)cyclotetraphosphin bewirkte (siehe Schema 0.3).

Schema 0.3 Mesitoylphosphin reagiert in einer Phospha-Michaelreaktion (links) und in einer oxidativen Kopplung (rechts). Acylsubstituierte Phospholoxide wurden in Kapitel 4 untersucht. Obwohl diese Verbindungen Acylphosphinoxide darstellen, sind sie keine guten Photoinitiatoren. DTF-Studien an einer der Verbindungen (Mesitoyl-2,5-diphenylphospholoxid) zeigten, dass dies der Fall ist aufgrund einer n-π*-Interaktion zwischen der Carbonylgruppe und den endozyklischen C-C Doppelbindungen. Diese behindert die Interaktion der Carbonylgruppe mit der PO-Gruppe, welche normalerweise verantwortlich ist für die hohe Effizienz von Acylphosphinoxiden als Photoinitiatoren. Die n-π*-Interaktion verringert jedoch den HOMO-LUMO Abstand deutlich im Vergleich zur analogen, phenylsubstituierten Verbindung. Dies zeigt, dass Acylierung eine vielversprechende neue Methode zur Modifikation phospholoxidbasierter optischer Materialien darstellt. Das Verfahren, welches zur oxidativen Kopplung von Mesitoylphosphin verwendet wurde (siehe Schema 0.3), wurde auf die oxidative Kopplung von Bis(acyl)phosphinen übertragen. Dies erlaubte die Synthese von Tetrakis(acyl)diphosphinen. Cyclovoltammetrische Untersuchungen dieser Verbindungen zeigten dass die oxidative Kopplung bei reduktiven Potentialen umkehrbar ist. Dadurch können Bis(acyl)phosphindimere als Vorläufer von Bis(acyl)phosphiden verwendet werden.

Tris(acyl)phosphine wurden wie in Schema 0.1 beschrieben, ausgehend von Na2(PH) hergestellt. Dies ermöglichte die Vereinheitlichung der Synthese dreier bekannter Tris(acyl)phosphine (R = Me, tBu, Ph). Die Reaktivität dieser Verbindungsklasse wurde untersucht, unter Verwendung von Tris(benzoyl)phosphin als Modellverbindung (siehe Schema 0.4).

Schema 0.4 Reaktivität von Tris(benzoyl)phosphin. Die chemisch stabileren Tris(carbamoyl)phosphine sind über die gleiche Route zugänglich und die thermische Analyse dieser Verbindungen ergab, dass bei der thermischen Zersetzung dieser, elementarer Phosphor freigesetzt wird. Die thermische Analyse der Acylphosphine, welche in den vorgängigen Kapiteln hergestellt wurden, ergab dass elementarer Phosphor bei der thermischen Zersetzung einer Reihe verschiedener Phosphine freigesetzt wird (siehe Schema 0.5). Der Mechanismus dieser Zersetzung konnte nicht vollständig aufgeklärt werden. Es konnte jedoch gezeigt werden, dass mit Ausnahme der kleinsten Substituenten (R = Me, NMe2) die Zersetzung über eine Disproportionierung der Acylgruppen verläuft, welche eine große Bandbreite von Zersetzungsprodukten verursacht.

Schema 0.5 Verschiedene Phosphine, welche als thermische Quellen von P4 identifiziert wurden. Aufgrund dieser Erkenntnisse wurden Acylphosphine als Ausgangsstoffe getestet, für die Synthese von InP-Nanopartikeln (in Zusammenarbeit mit Prof. Kovalenko – ETH Zürich) und Übergangsmetallphosphid-Nanopartikeln (in Zusammenarbeit mit Prof. Sanchez –

CNRS Paris). Die Synthese von InP-Nanopartikeln verläuft am besten unter Verwendung eines Gemisches von Bis(benzoyl)phosphin und Tris(benzoyl)phosphin. Untersuchung dieser Ausgangsstoffmischung zeigten, dass in diesem Fall die reaktive Phosphorspezies höchstwahrscheinlich nicht P4 ist, sondern PH3. Dieses bildet sich aus den Acylphosphinen durch Reaktion mit dem zugegebenen Oleylamin und nachträglicher Disproportionierung des gebildeten Mono(acyl)phosphins. Synthese und thermische Analyse von Tris(bis(benzoyl)phosphido)arsin ergab, dass Tris(bis(acyl)phosphido)arsine potentielle thermische Quellen von AsP3 sind. Das letzte Kapitel beschäftigt sich mit Natriumformphosphid. Diese Verbindung konnte das erste Mal als Feststoff strukturell charakterisiert werden. Isolierung von reinem Na(HPCHO) ermöglichte die Untersuchung seiner Reaktivität, welche von Decarbonylierungsreaktionen dominiert wird. Die Einzige gefundene Ausnahme ist die oxidative P-P Kopplung von Na(HPCHO) mittels Triphenylboran und Na(HMDS), welche die Bildung des Triphenylboranaddukts des dianionischen Dimers bewirkt. Die thermische Zersetzung von Na(HPCHO) wurde ebenfalls untersucht. Es konnte gezeigt werden, dass die Konzentration von Na(HPCHO) im Feststoff, das Verhältnis der beobachteten Zersetzungsprodukte beeinflusst. Die gasförmigen Produkte sind Methylphosphin und dessen Derivate, während der feste Rückstand aus Phosphaten oder Phosphonaten besteht. Dies weisst darauf hin, dass die thermische Zersetzung von Na(HPHCO) in Form einer Disproportionierung vonstattengeht, ähnlich der die für Acylphosphine beobachtet wurde.

Preceeding Remarks In order to avoid confusion or needless repetition, the following statements are made to apply to the whole thesis: . Solid-state structures are depicted with thermal ellipsoids shown at the 50% probability level. . In solid-state structures, hydrogen atoms are depicted as spheres of an arbitrary radius. . Unless stated otherwise, hydrogen atoms, non-coordinating ions and solvent molecules have been omitted for clarity in solid-state structures. . Solutions and plots of solid-state structures have been obtained using OLEX2.[1] . A complete list of used abbreviations can be found on page 163. . The depicted TGA curves have been corrected for buoyancy effects either by a correction measurement or using mathematical corrections.

1 Introduction Organophosphorus compounds have found widespread application, for instance as: . Herbicides, most notably in form of glyphosate[2] . Flame retardant coatings and additives[3] . Plasticizers[4] . Pharmaceutical reagents combating diseases such as osteoporosis and cancer[5] . Photoinitiators for radical-initiated polymerization reactions.[6] The last application mentioned relies on a specific class of organophosphorus compounds – acylphosphine oxides. The application of acylphosphine oxides as photoinitiators was first patented in 1979, in the form of mono(acyl)phosphine oxides.[7] Bis(acyl)phosphine oxides were later developed to further increase performance.[8] The high performance of acylphosphine oxides as photoinitiators is attributed to an interaction of the carbonyl group with the phosphorus-oxygen moiety. Therefore the energy required to excite the ππ* and nπ* electronic transitions is lowered, which causes a batochromic absorption shift and a more facile homolytic bond cleavage of the phosphorus acyl bond upon irradiation.[9] Acylphosphine oxides are generally prepared by oxidation of the corresponding acylphosphines. The synthesis of acylphosphines does however impose several challenges, as outlined in the next section. 1.1 Synthesis of Acylphosphines The most common methodology for P-C bond formation is the reaction of a halophosphine with a nucleophile, such as a Grignard reagent.[10] Nucleophilic transfer of an acyl group does however rely on a Seebach-type Umpolung using carbanions of 1,3-dithianes.[11] To avoid this, the synthesis of acylphosphines has generally been performed using nucleophilic phosphorus [12] [13] [14] sources, such as PH3 , P(TMS)3 and alkali metal dihydrophosphides, such as K(PH2) [6] and Na(PH2) (see Scheme 1.1).

Scheme 1.1 Some of the first reported syntheses of acylphosphines using nucleophilic P-sources.

While PH3 is not commonly used due to it being a toxic, pyrophoric gas, P(TMS)3 has more often been employed for syntheses on laboratory scale, since it can be isolated and stored as a [15] liquid. P(TMS)3 can be prepared from either red phosphorus or PCl3. It is however toxic and

21 pyrophoric. Even though this compound can be handled in research laboratories, problems arise on scale-up, since the cost associated with P(TMS)3 is significant. While the price for tris(trimethylsilyl)phosphine varies depending on quality and supplier, one publication quotes it as 232.5 € per gram.[16] The high price, paired with the pyrophic nature of this compound makes synthetic processes that replace P(TMS)3 with cheaper or easier to handle phosphorus sources very attractive for academic as well as industrial application.

A possible alternative to P(TMS)3 exists in the form of alkali dihydrophosphides. Although potassium dihydrophosphide has already been used in the synthesis of acylphosphines, by Liotta and co-workers in 1981 (see Scheme 1.1)[14], alkali metal dihydrophosphides did not represent a convenient phosphorus source at that time. This was due to the fact that they needed to be prepared from PH3 and the corresponding alkali metal in liquid ammonia and exhibit low solubility in organic solvents, when prepared this way. These problems were alleviated in 2000 in form of the synthesis of sodium dihydrophosphide by Brandsma and co-workers, which relied on the reduction of red phosphorus by elemental sodium in DME and subsequent protonation using tert-butanol.[17] Prepared in this manner, the phosphide forms the so-called “Brandsma-complex” with the formed sodium tert-butoxide, causing the complex to be more soluble in organic solvents (see Scheme 1.2).

Scheme 1.2 Synthesis of the Brandsma-complex. The use of this complex in the synthesis of acylphosphines was pioneered in the Grützmacher group and led to the filing of a patent together with BASF.[6] Until recently this methodology was mainly used in the synthesis of bis(mesitoyl)phosphine and derivatives thereof. However, investigations by M.Bispinghoff have shown that synthesis of bis(acyl)phosphines bearing [18] different acyl groups is possible from Na(PH2). His studies also revealed that bis(acyl)phosphines are accessible from PH3 in presence of Lewis acids. 1.2 Properties of Acylphosphines

Silylated mono- and bis(acyl)phosphines (generally prepared from P(TMS)3), possess a trimethylsiloxy group and therefore reside in the so-called enol tautomer instead of the keto tautomer.[13] Trimethylsilylated mono(acyl)phosphines have been used in the synthesis of phosphaalkynes by elimination of bis(trimethylsilyl)ether (see Scheme 1.3).[19]

Scheme 1.3 Synthesis of tert-butylphosphaalkyne from silylated pivaloylphosphine. Protonated acylphosphines exhibit a more dynamic keto-enol tautomerism, which has been extensively studied in bis(acyl)phosphines (see Scheme 1.4), with the ratio of keto to enol form depending on the acyl substituent and the used solvent (see Table 1.1).[20]

Scheme 1.4 Keto-enol tautomerism present in bis(acyl)phosphines.

Table 1.1 Keto-enol ratios of bis(acyl)phosphines depending on solvent and acyl substituent. R Solvent Keto Enol [20] Me CHCl3 66% 34% [20] Me C6H6 55% 45% [21] tBu CHCl3 12% 88% [21] tBu C6H6 10% 90% Cy[22] Toluene 36% 64% [9] Mes CH2Cl2 18% 82% [18] Ph CHCl3 0% 100%

The keto-enol tautomerism in bis(acyl)phosphines resembles the one present in acetylacetonates.[23] Subsequently, the reactivity of bis(acyl)phosphines mimics that of carbon- based enolate system. They are commonly substituted at the central phosphorus atom using alkyl halides[6] or Michael acceptors such as acrylates.[24] Coordination to transition metal centers occurs via both oxygen atoms, similar to acac-type ligands (see Scheme 1.5). [21] [25] [26]

Scheme 1.5 Reported coordination complexes incorporating bis(acyl)phosphines. Acylphosphines have been reported to be sensitive towards oxygen, with bis(mesitoyl)phosphine[9] and bis(benzoyl)phosphine[18] being the only reported exceptions, since they are stable towards oxidation in the solid state. However, in solution even these two compounds also oxidize in presence of oxygen. Recently this sensitivity towards oxidation has been used in the synthesis of bis(acyl)phosphinic acids - a new type of bis(acyl)phosphine oxide based photoinitiators (see Scheme 1.6).[18, 27]

Scheme 1.6 Synthesis of bis(acyl)phosphinic acids. 1.3 The Phosphorus-Acyl Bond Although acylphosphines can be regarded as the heavier analogues of amides, the bonding situation in acylphosphines differs significantly from that present in amides. The C-N bond in amides exhibits partial double bond character due to delocalization of the nitrogen lone pair over the amide bond, leading to a shortening of the bond and hindered rotation around it.[28] Computational studies have shown that the opposite is the case for the P-C bond in acylphosphines. Due to hyperconjugative interactions between the n(O) and the σ*(P-C) orbitals, the P-C bond is elongated and has partial ionic character, with no significant contribution to the bonding situation by the lone pair on the phosphorus (see Scheme 1.7).[29]

Scheme 1.7 Dominant mesomeric contributions to the bonding situation in amides (left) and acylphosphines (right). Although there has been one report in literature about hindered rotation around the phosphorus- acyl bonds in tris(benzoyl)phosphine[30], this report has later been called into question.[31] There have been no further reports of hindered rotation around P-acyl bonds, further corroborating the bonding situation depicted in Scheme 1.7. This decrease in bond order affects the reactivity of acyl groups bound to phosphorus and they have been shown to undergo attack by nucleophiles quite easily.[32] In the case of mono(acyl)phosphines a nucleophilic attack was reported to take place even in pure compounds, such as benzoyl-[14] and acetylphosphine[20] (see Scheme 1.8).

Scheme 1.8 Disproportionation of mono(acyl)phosphines. The sensitivity towards nucleophilic attack is somewhat alleviated by the introduction of a sterically demanding substitutent such as a mesitoyl group, which enabled the success of bis(mesitoyl)phosphine derivatives in a variety of reactions.[9, 24]

1.4 Aim of this Thesis As outlined in this introductory chapter, different synthetic pathways towards acylphosphines have already been established. Most of these do however rely on the use of tris(trimethyl)silylphosphine, which is not only difficult to handle, but also very expensive. This makes preparation of acylphosphines on a gram-scale unachievable for research groups, which do not enjoy significant funding. Replacing P(TMS)3 with a cheaper phosphorus source, such as Na(PH2) in form of the “Brandsma-complex” potentially lowers the cost of laboratory- scale preparation of most acylphosphines significantly. Since Na(PH2) has recently been shown to be a suitable precursor for the preparation of acylphosphines other than bis(mesitoyl)phosphine[18], it is only reasonable to attempt to increase the substrate scope for the acylation of Na(PH2) even further. Therefore, the goal is to establish Na(PH2) as a suitable precursor for the synthesis of mono-, bis- and tris(acyl)phosphines, preferably in form of simple one-pot syntheses, which do not require complex work-ups. Since acylphosphines should be much easier prepared using this methodology, larger scale syntheses can be performed, allowing for the testing of these compounds in applications such as photoinitiation or flame retardancy. A property of acylphosphines, which has so far been mostly unexplored is the thermal behavior of the phosphorus-acyl bond. The thermal decomposition of the structurally related carbamoylphosphines has been reported to result in the formation of elemental phosphorus.[33] The study of the thermal behavior of acylphosphines is enabled by the acquisition of new analytical equipment in the Grützmacher group. Compounds can now be studied using Themogravimetric Analysis (TGA) paired with Differential Scanning Calorimetry (DSC) or Differential Thermal Analysis (DTA). While TGA reveals any change of mass upon temperature change, DSC/DTA shows if the sample takes up energy or releases it during the heating process. Coupling to the TGA/DSC apparatus via heated transfer lines allows for simultaneous analysis of the fragments lost upon decomposition by MS and IR spectroscopy (see Figure 1.1).

Figure 1.1 Netzsch Jupiter II TGA/DSC set-up coupled to mass spectrometry and IR-spectroscopy.

2 Synthesis of Primary Acylphosphines based on the Reaction of Sodium Dihydrophosphide with Organic Esters 2.1 Introduction The first syntheses of primary acylphosphines were published by Becker and co-workers in 1981.[34] [20] They are based on the reaction of trimethylsilyl phosphines with acyl chlorides under loss of trimethylsilyl chloride. Liberation of the primary phosphine was achieved by alcoholysis as depicted in Scheme 2.1.[35]

Scheme 2.1 Synthesis of primary acylphosphines from trimethylsilyl phosphines. In 1984 Liotta and co-workers reported a different approach, reacting potassium dihydrophosphide and benzoic esters in the presence of crown ether to form primary phosphines and phosphaindoles (Scheme 2.2).[14] [36]

Scheme 2.2 Synthesis of acylphosphines from K(PH2). The primary acylphosphines prepared by Becker and Liotta were reported to be unstable towards disproportionation, to form PH3 and bis(acyl)phosphines, as shown in Scheme 2.3. In the case of benzoylphosphine, complete decomposition was reported within 6 hours at room temperature. In solution, the decomposition was reported to be slower, but still observable even at low temperatures (20% decomposition within 3 days at –78 °C).[14]

Scheme 2.3 General scheme of the disproportionation of primary acylphosphines. An expansion of the scope of the reaction between alkali dihydrophosphides and organic esters was published by Becker and co-workers in 1992. Li(PH2) reacts with ethyl formate or dimethylcarbonate under formation of lithium formphosphide (1) or lithium phosphaethynolate (2) as described in Scheme 2.4.[37]

Scheme 2.4 Reactions of LiPH2 with ethyl formate and dimethyl carbonate.

[38] Solutions of LiPH2 are readily prepared by lithiation of phosphine gas with n-butyllithium. The heavier alkali metal analogues of Li(PH2) are not accessible via an organometallic metalation route. They are however accessible from Na3P. Brandsma and co-workers reported in the year 2000, that Na3P can be prepared from red phosphorus and elemental sodium. Na3P can be protonated using tert-butanol, forming the sodium tert-butoxide complexed Na(PH2) depicted in Scheme 2.5.[17]

Scheme 2.5 Formation of the Brandsma-reagent from Na3P.

Due to the co-crystallized tert-butoxide and DME molecules, [Na(PH2)∙NaOtBu2∙DME2/3] is commonly referred to as “organic Na(PH2)” - in contrast to the “inorganic” Na(PH2), which is prepared in pure form from phosphine and elemental sodium in liquid ammonia. The “organic Na(PH2)” shows a comparatively high solubility in organic solvents. It is also safer to handle than pyrophoric PH3 or P(TMS)3. For these reasons the “organic Na(PH2)” was used in the Grützmacher group for a variety of syntheses and is referred to as Na(PH2) from now on. The first unification of the results of Liotta and Brandsma was published by Grützmacher and [39] co-workers in 2009. Na(PH2) reacts readily with diethyl phthalate at room temperature, forming the sodium analogue of the phosphaindole reported by Liotta (Scheme 2.2)[36]. The [40] reaction of NaPH2 with methyl mesitoate affords sodium mesitoylphosphine , which upon protonation by etheric HCl, gave rise to free mesitoylphosphine.[9] Syntheses of monoacylphosphines in the coordination sphere of transition metals have been reported as well. The acylation of an iron phosphine complex[41] as well as of an osmium phosphide complex[42] led to the formation of the compounds depicted in Scheme 2.6. Liberation of the phosphine ligands was however not reported.

Scheme 2.6 Monoacylphosphine complexes prepared by direct acylation. The synthesis of a structural motive similar to primary acylphosphines from Na(OCP) was published by Goicoechea and co-workers in 2013.[43] Reaction of Na(OCP) with ammonium salts affords carbamoylphosphines as illustrated in Scheme 2.7. The substrate scope of this

28 reaction has been shown to include, in addition to the parent ammonium salt, primary (ethyl, tert-butyl and cyclohexyl) and secondary (diethyl) ammonium salts.[44]

Scheme 2.7 Formation of carbamoylphosphines from Na(OCP). For R = H, R’ = Et, tBu, Cy – for R = Et, R’ = Et. Phosphinecarboxamides exhibit a much higher stability towards disproportionation compared to primary acylphosphines. Formation of the bis(carbamoyl)phosphine and PH3 only occurs upon addition of 0.5 equivalents of base. The disproportionation proceeds via a nucleophilic attack of the deprotonated phosphorus atom at the carbonyl carbon of a free phosphinecarboxaminde as shown in Scheme 2.8.[44]

Scheme 2.8 Formation of bis(cyclohexylcarbamoyl)phosphine. The increased stability towards disproportionation is attributed to a significant donation of the nitrogen lone pair into the CO π*-orbital, analogous to the mesomeric stabilization of amides (Scheme 2.9).[43] This leads to increased electron density on the carbonyl carbon, causing a nucleophilic attack on the carbonyl carbon to be less favorable.[28]

Scheme 2.9 Important resonance structures of phosphinecarboxamide. In 2017 Protasiewicz and co-workers published the first insertion of Na(OCP) into a metal- carbon bond. Hydrolysis of the formed complex liberated benzoylphosphine (Scheme 2.10).[45]

Scheme 2.10 Synthesis of benzoylphosphine from Na(OCP). The benzoylphosphine provided via this route was reported to be stable indefinitely in solution and only slightly contaminated with bis(benzoyl)phosphine. No further investigation of this unprecedented stability was reported.

2.2 Aim of this project

The preparation of Na(PH2) has been optimized in the Grützmacher group over several years and is considered a routine reaction, which can be performed on large scale. The precursors for this reaction – red phosphorus and elemental sodium – are very inexpensive chemicals, as are many organic esters. The reaction between Na(PH2) and organic esters has therefore a potential, which has already been realized in the synthesis of Na(OCP) – to generate versatile phosphorus-containing small molecules in an inexpensive reaction on large scale. To our best knowledge, there have been no reports about the substrate scope of the reaction of benzoic esters with alkali dihydrophosphides. Investigation of the tolerance of steric hindrance as well as π-donating and -withdrawing substituents on the benzoic moiety should give a first insight into the versatility of the formation of sodium mono(acyl)phosphides as depicted in Scheme 2.2.

The reactivity of Na(PH2) towards small organic esters, like formates, acetates and urethanes is of interest due to the potential to form reactive small phosphorus-containing molecules. Such small building blocks are of importance in the synthesis of functional materials.[46] Formed sodium mono(acyl)phosphides will be protonated in order to generate primary acylphosphines. Preparation and isolation of these primary acylphosphines is expected to afford a better insight into their stability and their disproportionation. This will allow to elucidate the conflicting reports about the chemical stability of primary acylphosphines.[14, 45] 2.3 Results and Discussion

Reaction of Na(PH2) with urethane exclusively led to the formation of PH3. The same observation can be made, when employing Na3P as phosphorus source. This is likely due to the high basicity of alkali phosphides compared to amides.

31 As evidenced by P–NMR spectroscopy, ethyl acetate and formate react with Na(PH2) under formation of sodium acetylphosphide and sodium formphosphide. Since protonation of these compounds only formed PH3, their chemistry will be discussed in different chapters. (see Chapters 5 and 8).

The reaction of Na(PH2) with ethyl cyanoformate led to the formation of Na(OCP) 31 31 (δ( P) = -388.6 ppm) and NaP(COOEt)2 (3, δ( P) = –31.1 ppm), due to the fact, that the cyano substituent can act as a leaving group similar to alkoxides. The ratio of Na(OCP) to 3 was estimated to be 3:2 based on the 31P-NMR spectrum of the reaction mixture, which measured after stirring for 3 hours (see Figure 2.1).

31 Figure 2.1 P-NMR of the reaction between Na(PH2) and ethyl cyanoformate Na(OCP) can be precipitated by addition of 1,4-dioxane and removed by filtration, allowing for the isolation of 3 in the form of colorless crystals. The obtained solid-state structure is depicted in Figure 2.2.

Figure 2.2 Solid-state structure of 3 – for selected structural parameters see Table 2.1. Complex 3 forms a coordination polymer in the solid state consisting of the tetrameric repeating unit (3)4(Et2O)2. The repeating units are linked via sodium atoms, which coordinate to either O(1) and O(2) or to P(1) and O(6). The formation of a coordination polymer has not [47] been observed in the otherwise structurally similar compound LiP(COOMe)2 (4).

Table 2.1 Comparison of selected averaged structural parameters of 3 and 4.

[47] 3∙(Et2O)0.5 4∙DME1.5 d(P–C) 1.81(1) Å 1.80(1) Å d(C=O) 1.23(1) Å 1.22(1) Å d(C–O) 1.35(1) Å 1.37(1) Å ⦟(C–P–C) 99.1(1) ° 98.8(1) ° ⦟(O=C–P) 131.7(1) ° 132.5(1) °

Comparison of the structural parameters of the solid-state structures of 3 and 4 (see Table 2.1) shows that neither the change from methoxy to ethoxy substituents nor the exchange from lithium to sodium as a counter ion has a significant impact on the structural core of the molecule in the solid state.

Reacting Na(PH2) with either methyl anthranilate (5) or methyl N-methyl anthranilate (6) caused an immediate gas evolution. In the 31P-NMR spectrum of the reaction mixtures, peaks were observed at δ = 138.9 ppm for 5 and δ = 171.5 ppm for 6. These values correspond to much higher frequencies than deprotonated acylphosphines, which would be expected to exhibit signals around 5 ppm.[14] Coupling to 1H-nuclei can also be observed: A doublet with a JPH coupling constant of 29.5 Hz for 5 and a quartet with a JPH coupling constant of 6.5 Hz for 6. These spectral data indicate that a P-N bond formation takes place in order to form Na-7 and Na-8, as illustrated in Scheme 2.11.

Scheme 2.11 Formation of Na-7 and Na-8 from NaPH2 and methyl anthranilates. A neutral benzannulated 1H-1,2-azaphosphole has been prepared by Regitz and co-workers via flash vacuum thermolysis (FVT) in 1989 (see Scheme 2.12).[48] The 31P-NMR chemical shift of this compound has been reported as 194.4 ppm, which is higher than those of Na-7 and Na- 8. The difference most likely stems from the methyl group ortho to the phosphorus, which is missing in Na-7 and Na-8. The interaction between this methyl group and the phosphorus is 2 significant as indicated by the JPC coupling constant of 12.7 Hz.

Scheme 2.12 Benzannulated 1,2-azaphosphole prepared by FVT. Na-7 and Na-8 could not be isolated via crystallization in pure form. Addition of an excess of trimethylsilyl chloride to the reaction crude containing Na-8 allowed purification of 8 in the form of TMS-8 as shown in Scheme 2.13.

Scheme 2.13 Formation of TMS-8 via silylation. TMS-8 is soluble in hexane and was therefore separated from the reaction mixture by extraction. The yellow liquid TMS-8 can be purified by distillation under reduced pressure. In the case of Na-7, competing silylation of the nitrogen inhibited further purification via this route. The isolation of TMS-8 allowed for conformation of the structure by assignment of characteristic 1H–NMR, 13C–NMR and 29Si–NMR chemical shifts, as shown in Table 2.2.

Table 2.2 Assignment of characteristic NMR-shifts and coupling constants of TMS-8.

Nucleus δ [ppm] JPX [Hz] C1 182.4 34.7 C2 33.7 22.7 C3 0.0 4.0 H(C2) 3.60 8.46 H(C3) 0.26 0.62 Si 24.2 –

29 [49] The Si-NMR shift is in agreement with a trimethylsilyl ether group , whereas a P-SiMe3 [50] 1 group would be expected at lower frequencies and also expected to show a significant JPSi coupling. The absence signals in the 13C-NMR spectrum above 200 ppm, indicates the absence 13 of a phosphorus bound acyl group. The frequency of the signal assigned to C1 in the C-NMR 1 spectrum paired with the large JPC coupling constant of 34.7 Hz supports the presence of a [51] double bond between P and C1. The decline in J-coupling constants to the phosphorus with increasing distance is in good agreement with the depicted structure of TMS–8. The small change in the 31P-NMR chemical shift between Na–8 (δ = 171.5 ppm) and TMS-8 (δ = 165.5 ppm) indicates no significant structural change during the silylation. The connectivity of Na–7 and Na–8 is therefore corroborated indirectly by the spectroscopic evidence for the connectivity of TMS-8. In order to further probe the nature of this formal dehydrocoupling only one equivalent of tert- butanol was used in the protonation of Na3P. The “Na2(PH)” prepared this way was used in situ. In the reaction with 6, Na-8 was formed, as evidenced by 31P-NMR spectroscopy, under formation of a deep-red by-product, likely the deprotonated form of 6.

Scheme 2.14 Annulation under hydrogen deficient conditions using 6.

In the case of the more acidic 5, the double-deprotonated mono(acyl)phosphine Na2-9 (see Scheme 2.15) was formed as indicated by the signals of the E- and Z-isomer in the 31P-NMR spectrum (δ = 10.6 ppm, δ = 4.5 ppm).

Scheme 2.15 Formation of Na2-9 from Na2(PH) and 5. Upon addition of one equivalent of formic acid, gas evolution is observed. In the 31P-NMR spectrum resonances belonging to Na-7 are observed, supporting Na2-9 as the deprotonated form of an intermediate in the formal dehydrocoupling forming Na-7. A similar sodium mono(acyl)phosphide is also the most likely intermediate in the formation of Na-8.

Similar annulations have not been observed in the reaction of Na(PH2) with other organic esters. Instead the reaction of Na(PH2) and benzoic esters was found to form the sodium monoacylphosphides according to Scheme 2.16.

Scheme 2.16 General synthesis of sodium monoacylphosphides (for R, R’ see Table 2.3). The formation of the sodium mono(benzoyl)phosphides is evidenced by 31P–NMR spectroscopy (see Table 2.3). Assignment of the E- and Z-isomer has been performed in analogy to the assignment performed by Becker in the case of 1.[37] This is justified, since the signals follow the same pattern as those for 1 – the signal with the lower chemical shift showing 1 a lower JPH-coupling constant. Sodium benzoylphosphide (Na-10) is the only outlier in this 1 observation. The JPH coupling constant for Na-14 could not be determined due to low solubility in DME.

31 1 Table 2.3 Sodium mono(acyl)phosphides: P–NMR chemical shifts δ and JPH coupling constants in DME.

1 1 R’ δ [ppm] JPH [Hz] R’ δ [ppm] JPH [Hz] H (Na-10) 5.6 (E) 146.2 2,6-OMe (Na-14) 29.7 (E) – -9.4 (Z) 151.2 15.1 (Z) – 2,4,6-Me (Na-11) 12.9 (E) 148.8 2-ONa (Na2-15) 5.4 (E) 141.5 6.2 (Z) 130.4 3.4 (Z) 136.3 2-Me (Na-12) 8.2 (E) 154.9 4-NMe2 (Na-16) -1.1 (E) 144.6 2.4 (Z) 136.3 -21.9 (Z) 133.3 2-Ph (Na-13) 17.5 (E) 154.5 4-CN (Na-17) 16.2 (E) 152.9 4.1 (Z) 137.6 -0.7 (Z) 150.1

The solid state structure of Na2-15 exhibits two pseudo-cubic subunits incorporating four molecules of sodium phenoxide (formed from phenyl salicylate used in the synthesis of [52] Na2-15) and two phosphine atoms chelating two sodium atoms as shown in Figure 2.3.

Figure 2.3 Solid-state structure and schematic representation of Na2-15. The first goal in the optimization of the synthesis of primary mono(acyl)phosphines was the replacement of the expensive etheric HCl as proton source.[9] Protonation using formic acid had no adverse effect on the formation of the primary acylphosphines. Due to the exothermic nature of the protonation reaction, the acid needs to be added dropwise while applying external cooling to the reaction vessel (ice bath). Successful formation of the primary acylphosphine was evidenced by triplet-signals in the 31P-NMR spectrum, collapsing into singlets upon 1H-decoupling. Signals for each substituent are listed in Table 2.4. In the case of mesitoylphosphine (11), the enol form (doublet) as well as the keto form (triplet), are observable via 31P-NMR spectroscopy.[9] The keto form is the only observable species for the other primary acylphosphines listed in Table 2.4.

31 1 Table 2.4 Primary acylphosphines: P–NMR chemical shifts δ and JPH coupling constants in DME.

1 1 R δ [ppm] JPH [Hz] R δ [ppm] JPH [Hz] H (10) -109.9 220.1 2,6-OMe (14) –94.1 212.7 2,4,6-Me (11) –96.3 216.8 2-OH (15) -107.0 223.7 2-Me (12) -99.3 220.6 4-NMe2 (16) -115.2 216.1 2-Ph (13) -100.9 213.2 4-CN (17) -107.5 223.5

The influence of the substituents on the chemical shift in Table 2.4 is diminished in comparison 1 to Table 2.3 and is almost negligible with regards to the JPH-coupling constant. This indicates that the electronic structure of primary monoacylphosphines is predominantly influenced by the two mesomeric structures shown in Scheme 2.17 – in contrast to phosphacarboxamides where the phosphorus lone pair has a less significant impact on the overall electronic structure (Scheme 2.9).[43]

Scheme 2.17 Important mesomeric contributions to the electronic structure of primary acylphosphines. The primary acylphosphines are soluble in hexane, allowing for a facile separation from the reaction mixture by extraction. All products synthesized via this route show small contamination by the corresponding bis(acyl)phosphines. No increase of contamination was

35 observed by 31P–NMR spectroscopy upon prolonged storage at 4 °C under protective atmosphere. Upon heating the compounds were determined to undergo disproportionation. In the case of 11 the boiling and decomposition point was determined to be 120 °C at 0.01 mbar. Therefore distillation was ruled out as a method of purification for all the compounds shown in Table 2.4. Compound 14 is exceptional, because it is isolable in form of colorless crystals, which can be grown by cooling a hexane solution of 14 to -78 °C. Compound 14 can therefore be isolated free from contamination by the bisacylphosphine. 14 is more prone to decomposition and requires storage at –30 °C under protective atmosphere.

When reacting two equivalents of Na(PH2) with dimethyl phthalate the disodium salt Na2-18 is formed. The formation was evidenced by 31P–NMR spectroscopy (δ = –1.8 ppm (s, broad)). Addition of formic acid causes an annulation to the phosphaindole 19 as illustrated in Scheme 2.18. Using 31P–NMR spectroscopy to monitor the reaction in DME, signals for the keto form 1 of 19 (δ = 46.6 ppm, JPH = 169.0 Hz) as well as the enol form (δ = 27.8 ppm) can be observed in a ratio of approximately 1:1.

Scheme 2.18 Formation of 19 from Na2-18 upon protonation. This annulation reaction takes most likely place, due to an intramolecular attack after the first protonation of Na2-18. The carbonyl group of the formed acylphosphine is most likely attacked by the remaining phosphide moiety, as described in the case of carbamoylphosphines (see Scheme 2.8).[44] This is inhibited by changing the organic diester to dimethylisophthalate. The disodium salt Na2-20 is formed via the same route as Na2-18. Protonation led in this case to the formation of the diphosphine 20 (see Scheme 2.19). Due to the different positioning of the acylphosphide groups in Na2-20, an intramolecular attack as in the formation of 19 cannot occur upon protonation.

Scheme 2.19 Protonation of Na2-20 proceeds without significant disproportionation. The rotation of the carbonyl groups of 20 seems to be hindered as evidenced by the appearance 31 1 of two triplet signals in the P-NMR spectrum in hexane (δ1 = -111.0 ppm, JPH = 217.5 Hz, 1 δ2 = -111.3 ppm, JPH = 216.4 Hz).

Introduction of a pyridine substituent at the organic ester leads to unstable primary acylphosphines. Upon protonation of sodium nicotinoylphosphine – prepared from Na(PH2) and methyl nicotinate – disproportionation took place and bis(nicotinoyl)phosphine and PH3 were the only products observed by 31P-NMR spectroscopy. This disproportionation is likely caused by the internal pyridine base and is in accordance with the disproportionation of carbamoylphosphines under basic conditions.[44] This indicates that the instability observed by Liotta and co-workers[14] might have been caused by a basic impurity. This hypothesis is also corroborated by the instability of 14, which possesses protons which likely interact with the electron-rich methoxy substituents on the benzoyl substituent. This allows for a rationalization of the contamination of primary acylphosphines listed Table 2.4 with bis(acyl)phosphines. Due to the slow addition of acid to the reaction mixture, formed acylphosphine can undergo nucleophilic attack by sodium mono(acyl)phosphide still present in the solution. This leads to formation of bis(acyl)phosphine until the reaction mixture is protonated completely. The formation of 19 can be considered an extreme case of this scenario, since a second protonation is impossible, due to the much faster intramolecular attack of the remaining phosphide moiety. 2.4 Summary and Outlook

The reaction between Na(PH2) and organic esters was studied, probing the substrate scope of the reaction.

Cyanide acts as a leaving group in the reaction between Na(PH2) and ethyl cyanoformate, forming Na(OCP) and 3. Although 3 is a new compound, a more straightforward preparation can most likely be achieved using the synthesis developed by Mark Bispinghoff, based on the [18] reaction of PH3 with dialkylcarbonates in presence of sodium tert-butoxide.

Reacting the methyl anthranilates 5 and 6 with Na(PH2) gave rise to the 1H-1,2- azaphospholates Na-7 and Na-8. Silylation of Na–8 leads to the formation of the isolable species TMS-8. The connectivity of TMS–8 was confirmed by 1H- and heteronuclear NMR- spectroscopy. The synthesis of N-Methyl-1H-1,2-azaphospholes has first been reported by Schmidpeter and co-workers in 1986.[53] However, the synthesis of such azaphospholes bearing a functionalizable group, such as an alcoholate, has not been reported. For benzannulated 1H-1,2-azaphospholes the only reported synthesis is based on flash vacuum thermolysis. The synthesis of TMS-8 provides a simple access to a new class of substrates with the potential of further functionalization. Preliminary tests have shown that TMS-8 can be desilylated using tetrabutylammonium fluoride, potentially allowing for further functionalization.

The formation of sodium mono(acyl)phosphides from Na(PH2) and benzoic esters has been shown to tolerate substrates with varying steric demand (phenyl, tolyl, mesityl), electron- withdrawing substituents (4-CN) as well as electron-donating substituents (4-NMe2, 2,6-OMe). The protonation of sodium mono(acyl)phosphides allowed for the synthesis the corresponding primary acylphosphines. The immediate disproportionation of nicotinoylphosphine upon preparation gave insight into the instability of primary acylphosphines reported in literature.[14] The internal basic functionality most likely catalyzes the disproportionation. This observation is similar to the 37 base-induced disproportionation of carbamoylphosphines (see Scheme 2.8).[44] This makes the absence of basic impurities the most likely source of stability against disproportionation of the primary acylphosphines described in this chapter. Although the majority of the obtained primary acylphosphines were not isolated in pure form, their storability allows for further exploration of their reactivity, part of which will be elucidated using compound 11 in the next chapter.

3 Reactivity of Mesitoylphosphine 3.1 Introduction This chapter focusses on two important reactions reported for primary phosphines: the phospha-Michael addition and the synthesis of diphosphenes. 3.1.1 The Phospha-Michael Addition The phospha-Michael addition is an important metal-free hydrophosphination reaction, widely employed in organic chemistry, e.g. in the synthesis of heterocycles.[54] It was first used by Day and co-workers in the synthesis of phosphinanones 21 and 22 by reaction with either phorone or dibenzylideneacetone (DBA) at high temperatures (see Scheme 3.1).[55]

Scheme 3.1 Synthesis of phosphinanones at high temperatures. Conformational analysis of 22 by Berlin and co-workers, revealed that the thermodynamically favored product of this synthesis is the rac-isomer, in which the two carbon bound phenyl groups are situated trans to each other.[56] In 2006, two publications described improved syntheses of phosphinanones. The first one by Pringle and co-workers describes the synthesis of 22 in acetonitrile under acid catalyzed conditions, leading to a mixture of enantiomers (see Scheme 3.2). This synthesis was reported to avoid side-products formed in the original synthesis by Day and co-workers.[57]

Scheme 3.2 Modified synthesis of 22 by Pringle and co-workers. The second publication - by Capretta and co-workers - describes the synthesis of derivatives of 21 bearing different substituents at the phosphorus.[58] These derivatives were subsequently reduced in a Wolff-Kishner reduction to the corresponding phosphorinanes (see Scheme 3.3). The first step of this synthesis represents a modification of the procedure reported by Day and co-workers[55], since the phosphine is added dropwise to hot phorone, instead of heating up the compounds together.

Scheme 3.3 Synthesis of phosphinanones and subsequent reduction. Bis(acyl)phosphines act as Michael donors in phospha-Michael additions under much milder conditions than those mentioned before. Hydrophosphination of activated double bonds can be achieved under base catalytic conditions at room temperature using bis(mesitoyl)phosphine (MesBAP-H) as Michael donor (see Scheme 3.4).[24]

Scheme 3.4 Phospha-Michael addition using MesBAP-H in presence of base (EWG = -COOMe, -CN). The only hydrophosphination using primary acylphosphines has been reported using mesitoylphosphine (11) in the synthesis of an acylphosphine oxide as shown in Scheme 3.5.[9]

Scheme 3.5 Hydrophosphination of methyl acrylate using 11 and subsequent oxidation. 3.1.2 Synthesis of Diphosphenes The first synthesis of a stable diphosphene was reported in 1981 by Yoshifuji and co-workers. Bis(2,4,6-tris(tert-butyl)phenyl)diphosphene (Mes*P=PMes*) was prepared in a Wurtz-type * [59] homocoupling, by reacting (Mes PCl2) with magnesium. Only symmetric diphosphenes are accessible via this synthetic route. The first syntheses of asymmetric diphosphenes were reported in 1983 by the groups of Cowley[60] and Yoshifuji[61] based on the reaction of * (Mes PH2) with dichlorophosphines in the presence of 1,8-diazabicyclo[5.4.0]undec-7-en (DBU) (see Scheme 3.6). This methodology represents the first syntheses of diphosphenes from primary phosphines.

* Scheme 3.6 Synthesis of stable diphosphenes from Mes PH2. With the exception of the phenyl-derivative prepared by Yoshifuji and co-workers, the diphosphenes were described as stable and isolable due to the steric protection imposed by the super-mesityl group on one of the phosphorus atoms. In 1986, Satgé and co-workers published a synthesis of diphosphenes based on the homocoupling of trichlorogermyl hydrophosphines. The precursors are prepared by reacting primary phosphines with GeCl4, followed by homocoupling presence of DBU (see Scheme [62] * 3.7). Stable diphosphenes are obtained for bulky substituents (R = Mes or CH(SiMe3)2). However, smaller substituents (R = Ph, Mes, tBu) mainly formed cyclic oligomers of the general formula [(RP)n] with n ranging from 3 to 5.

* Scheme 3.7 Synthesis of diphosphenes from primary phosphines and GeCl4 (R = Mes or CH(SiMe3)2). Diphosphenes are prone to undergo dimerization reactions to cyclotetraphosphines in [2+2] cycloadditions. In the case of a bis(trimethylsilyl)methyl substituted diphosphene dimerization could be monitored at room temperature (see Scheme 3.8) due to the half-life of the diphosphene being roughly one week.[63]

Scheme 3.8 Dimerization of dibisyldiphosphene. Formation of cyclotetraphosphines has also been observed in coupling reactions of other mono(alkyl)- or mono(aryl)phosphines. Tetrakis(adamantyl)cyclotetraphosphine can be either prepared via a Wurtz-type coupling of adamantyldibromophosphine or by reacting adamantylphosphine with phosgene as illustrated in Scheme 3.9.[64]

Scheme 3.9 Coupling of adamantylphosphines leads to formation of the cyclotetraphosphine.

Zirconium catalyzed dehydrocoupling of naphthylphosphine, yielding the corresponding cyclotetraphosphine was reported by Stephan and co-workers. Dehydrocoupling of phenyl-, mesityl- and cyclohexylphosphine afforded the cyclic pentamers.[65] Tetrakis(pivaloyl)cyclotetraphosphine (23) is the only acyl substituted cyclotetraphosphine reported in literature. However, only the solid-state structure of this compound has been reported by Barth and co-workers.[66] The preparation of this compound is not reported in the thesis cited by the authors. Recent work in the Grützmacher group suggests that the structurally similar carbamoyl [67] substituted diphosphenes are accessible from Na(OCP). Na(OCP) reacts with (iPr2N)2PCl to form an unstable phosphaketene, which rearranges to diphosphene 24. The transient diphosphene itself is unstable and dimerizes to the cyclotetraphosphine 25 or can be trapped with an additional equivalent of Na(OCP) as depicted in Scheme 3.10.[68]

Scheme 3.10 Formation of the transient diphosphene 24 and subsequent dimerization or trapping. 3.2 Aim of this work Mesitoylphosphine (11) was chosen as a model compound to investigate primary acylphosphines as Michael donors as well as precursors in the synthesis of diphosphenes. Earlier work in the group has shown that compound 11 reacts at room temperature under basic conditions as Michael donor (see Scheme 3.5).[9] These conditions are much milder than the ones reported so far in the synthesis of phosphinanones (see Scheme 3.1). The target of this investigation is to apply this reactivity in the synthesis of mesitoyl substituted phosphinanones under mild, base catalyzed conditions. Oxidation of the formed acylphosphinanones provides access to yet unknown acylphosphinanone oxides, which may have a potential application as photoinitiators for radical initiated polymerization reactions, since similar bicyclic systems have already been reported for this application.[69] The steric bulk of the mesitoyl substituent of 11, qualifies it as a precursor for the synthesis of diphosphenes, since syntheses of diphosphenes or derivatives thereof have been achieved by [60] *[59] employing other sterically demanding substituents at the phosphorus (CH(SiMe3)2 , Mes , adamantyl[64], naphthyl[65]). Synthesis of bis(mesitoyl)diphosphene will upon dimerization provide access to the mesityl analogue of the structure reported by Barth and co-workers.[66] Based on the rich chemistry of the carbamoyl substituted diphosphene 24,

42 bis(acyl)diphosphenes are expected to exhibit a versatile reactivity and provide access to cycloaddition products. 3.3 Results and Discussion 3.3.1 Phospha-Michael Addition Compound 11 reacts at room temperature with DBA under base catalyzed conditions to form phosphinanone 26 (see Scheme 3.11).

Scheme 3.11 Formation of 26 from 11 and DBA. Using triethylamine as base, the reaction is completed within two days, as evidenced by 31P- NMR spectroscopy. Using the more basic N,N-tetramethylguanidine (TMG) as catalyst caused full conversion after 15 minutes to be observable via 31P-NMR spectroscopy. Only the diastereomer depicted in Scheme 3.11 is visible in the 31P-NMR spectrum at a chemical shift of δ = 28.3 ppm. The oxidation of 26 using tert-butyl hydroperoxide as depicted in Scheme 3.12 proceeds at room temperature as well. If the synthesis of 26 is carried out in non-etheric solvents, the oxidation can be performed in situ without prior isolation of 26.

Scheme 3.12 Oxidation of 26 – forming 27. Both 26 and 27 are only formed in the rac-isomer as evidenced by their solid-state structures (see Figure 3.1). Since only one signal is observed for either 26 or 27 via 31P-NMR spectroscopy, it is reasonable to assume that the isomers depicted in Figure 3.1 are the only ones present in solution. This assumption is supported by the report of Pringle and co-workers, in which the different chemical shifts of the diastereomers of the related compound 22 (Scheme 3.1) are reported.[57]

Figure 3.1 Solid-state structures of 26 (left) and 27 (right), for structural parameters see Table 3.1. Whereas 27 resides in the twist-boat conformer, 26 adopts a chair conformation in the solid state with the phenyl substituent at C(2) in axial and the one at C(3) in equatorial position. The P-phenyl derivatives 22 and its P-oxidized version 22-O adopt the same conformations. The conformation of 22 has been confirmed in solution[56], while that of 22-O was postulated to be a twist conformer by Wingad and co-workers based on the structures of similar diphenyl phosphinanone oxides.[57] The only reported solid state structure suitable for comparison is that of the sulfide of 22 (22-S), which adopts a chair conformation.[70]

Table 3.1 Selected interatomic distances and angles of 26, 27 and 22–S. 26 27 22-S[70] d(P(1)-C(1)) 1.879(4) Å 1.877(6) Å 1.805(2) Å d(P(1)-C(2)) 1.862(3) Å 1.836(5) Å 1.853(2) Å d(P(1)-C(3)) 1.868(3) Å 1.836(5) Å 1.840(2) Å d(C(1)-O(1)) 1.217(4) Å 1.226(6) Å – d(P(1)-O/S) – 1.481(4) Å 1.957(2) Å ⦟(C(1)-P(1)-C(2)) 98.6(1) ° 107.4(3) ° 108.0(1) ° ⦟(C(1)-P(1)-C(3)) 105.8(2) ° 104.1(3) ° 109.0(1) ° ⦟(C(2)-P(1)-C(3)) 100.1(2) ° 100.4(3) ° 100.4(1) ° ⦟(C(2)-P(1)-O/S) – 116.8(2) ° 114.3(1) ° ⦟(C(3)-P(1)-O/S) – 113.8(2) ° 111.3(1) ° ⦟(CO-PO/S) – 174.2(4) ° – sum of angles around P 304.5(2) ° 311.9(3) ° 317.4(4) °

Compounds 27 and 22-S are structurally very similar, with the only difference being the bond length P(1)-C(1), which is elongated in 27 compared to that in 22-S, as expected for an acylphosphine compared to an arylphosphine. Compound 26 exhibits a long bond between P(1) and C(1) as well. When comparing 26 and 27, the difference in bond lengths between the bonds of P(1) to C(2)/C(3) and the one of P(1) to C(1) gets more pronounced upon oxidation. This contraction of the backbone is also observable in the 31P-NMR spectra of 26 and 27. The 31P- 2 NMR spectrum of 26 shows no resolvable JPH coupling, whereas the signal of 27 is a signal 2 of higher order caused by the JPH coupling to the inequivalent protons on C(2) and C(3). The

44 signal of 27 in the 31P-NMR spectrum collapses into a singlet upon 1H-decoupling. The second observable difference is the orientation of the acyl group. In 26 the mesityl substituent is oriented towards the equatorial phenyl ring at C(3). The shortest distance between the mesityl and the phenyl ring in 26 is 3.309(4) Å, indicating an intramolecular π-π interaction between the two rings.[71] In 27 on the other hand the carbonyl group is oriented almost perfectly aligned opposite to the bond P(1)-O(2), as shown by the O(1)-C(1)-P(1)-O(2) dihedral angle of 174.2 °. This causes the mesityl group to be situated roughly equidistant to both phenyl rings. The UV/VIS spectrum of 27 in acetonitrile (see Figure 3.2) shows weak absorption bands between 425 nm and 360 nm. Irradiation of a solution of 27 with ultraviolet light for 30 min, caused partial decomposition. However, irradiation of a 27 in presence of monomers such as methyl acrylate or hexanediole diacrylate, did not cause polymerization. Irradiation of 27 with ultraviolet light does therefore not cause the formation of sufficiently reactive radicals to initiate polymerization.

Figure 3.2 UV/VIS spectrum of 27 (left) and magnification of the absorption between 450 nm and 350 nm (left). Compound 27 is not a viable photoinitiator. Its synthesis however represents a new synthetic pathway to phosphinanones and phosphinanone oxides under much milder conditions than previously reported. However the use of compound 11 as Michael donor is likely to impose restrictions on this reaction. Reaction of 11 with phorone did not proceed under basic conditions; neither at room temperature nor at elevated temperatures (see Scheme 3.13). This is likely due to the steric repulsion of the methyl groups on the phorone and 11.

Scheme 3.13 Hydrophosphination of phorone does not proceed using 11. Dehydrogenation of the backbone of 27 was attempted, in order to probe the influence of the backbone on the photochemical properties of 27. Reaction with DDQ in refluxing 1,4-dioxane

45 leads to partial formation of the phosphinine oxide 28 (see Scheme 3.14) as evidenced by 31P- 3 NMR spectroscopy, forming a triplet at δ = 3.63 ppm with a JPH-coupling of 31.3 Hz.

Scheme 3.14 Equilibrium between 28 and 27 in the reaction with DDQ.

The reaction is incomplete likely due to the partial solubility of DDQH2 in 1,4-dioxane at high temperatures. Due to this partial solubility the hydrogenation of 28 takes place as well. This leads to the formation of other diastereomers of 27, evidenced by multiple overlapping signals in the 31P-NMR spectrum of the crude reaction between 29 ppm and 31 ppm, as well as monodehydrogenated species evidenced by overlapping peaks between 18 ppm and 19 ppm. Separation of the different reaction products could not be achieved via selective crystallization. Column chromatography over silica gel caused decomposition. A higher concentration of reagents or an excess of DDQ did not cause an increase in formation of 28, but rather caused a cleavage of the phosphorus acyl bond, as evidenced by 13C-NMR spectroscopy (disappearance of dublets at chemical shifts δ > 190 ppm). A different pathway to a phosphininone is the reaction of 11 with bis(phenylacetylenyl)ketone. The reaction however proceeds without added base in an unselective manner leading to an inseparable mixture of products. Therefore, bis(phenylacetylenyl)ketone is too reactive to provide an access to 28 via a phospha-Michael addition (see Scheme 3.15).

O O Ph P Ph + H2P O Ph Ph

Scheme 3.15 Synthesis of a phosphinine is not possible using 11 and bis(phenylacetylenyl)ketone. The synthesis of a phosphorus heterocycle with an unsaturated backbone bearing an acyl group is not possible via a phospha-Michael addition of 11. Investigations to allow the preparation unsaturated phosphorus ring systems bearing acyl groups is however continued in Chapter 4 in the form of the synthesis of acylphosphole oxides.

3.3.2 Synthesis of Diphosphenes The synthesis of a diphosphene from 11 was attempted using transition metal mediated dehydrocoupling, similar to the work by Stephan and co-workers.[65] The dehydrocoupling of 11 was attempted using Cp2Ti and Cp2Zr prepared in situ from the corresponding metallocene dichlorides and n-butyl lithium at -78 °C. Both reactions did not lead to isolable diphosphene or derivatives thereof. From the reaction of 11 with Cp2Ti however, crystals of the anionic titanium complex 29 could be isolated (see Figure 3.3).

Figure 3.3 Solid-state structure, without counter-ion (left) and schematic representation (right) of 29 - for selected structural parameters see Table 3.2.

Table 3.2 Selected interatomic distances and angles of 29 (Cp = calculated centroid of the plane formed by the cyclopentadienyl ligand). d(P(1)-C(1)) 1.725(2) Å d(C(1)-O(1)) 1.296(2) Å d(P(3)-C(2)) 1.730(2) Å d(C(2)-O(2)) 1.296(2) Å d(P(1)-P(2)) 2.204(1) Å d(O(1)-Ti(1)) 2.078(1) Å d(P(2)-P(3)) 2.204(1) Å d(O(2)-Ti(2)) 2.092(1) Å d(P(2)-Ti(1)) 2.642(1) Å d(Ti(1)-Cp) 2.064(1) Å d(P(2)Ti(2)) 2.643(1) Å d(Ti(2)-Cp) 2.066(1) Å ⦟(P(1)-P(2)-Ti(1)) 98.8(1) ° ⦟(P(2)-Ti(1)-O(1)) 82.6(1) ° ⦟(P(3)-P(2)-Ti(2)) 98.0(1) ° ⦟(P(2)-Ti(2)-O(2)) 81.2(1) ° ⦟(P(1)-P(2)-P(3)) 95.2(1) ° ⦟(Ti(1)-P(2)-Ti(2)) 136.2(1) ° ⦟(P(1)-C(1)-O(1)) 128.8(2) ° ⦟(Cp-Ti(1)-Cp) 133.8(1) ° ⦟(P(3)-C(2)-O(2)) 128.2(2) ° ⦟(Cp-Ti(2)-Cp) 133.6(1) °

The length of bonds P(1)-C(1) and P(3)-C(2) are elongated compared to an average P-C double bond length (1.67 Å)[72]. They are however still shorter than delocalized P-C double bonds, such as those in MesBAP-H (d(P-C) = 1.784(2) Å). The carbon oxygen bond on the other hand is elongated, in agreement with the single bond character of this bond. The titanocene backbone [73] shows no significant differences when compared to that of Cp2Ti(acac). Compound 29 cannot be crystallized cleanly from the reaction mixture. Single crystals of 29 could be isolated by manually removing crystals, from the oily residue which formed by cooling a THF-solution of the reaction crude. The unclean reaction is likely due to the unknown stoichiometry of the reaction. Different molar ratios of phosphine to titanocene did not improve

47 the selectivity of the reaction. Since the only successful method of isolation was the picking of crystals under a microscope, no further analysis of compound 29 was performed. The formation of 29 is likely to proceed via loss of an acyl group. In order to avoid this, an alternative to the metal mediated dehydrocoupling was deemed necessary. Mono-chlorination of 11 may lead to an intermediate chlorohydrophosphine, which would subsequently dimerize by reacting with itself under loss of HCl. The general procedure for preparation of diphosphenes of Satgé and co-workers[62] proceeds via such an intermediate. However, the use of GeCl4 as chlorinating agent is too expensive on a reasonable preparative scale. An alternative method of chlorination has been reported by Weferling and co-workers. They reported the use of hexachloroethane in presence of triethylamine in the cyclization of 1,3- bis(methylphosphanyl)propane forming a phosphorus-phosphorus bond under formation of [74] NEt3∙HCl, likely via a monochlorinated intermediate.

Compound 11 does not react with C2Cl6 at room temperature in THF or DCM. Upon addition of NEt3 an exothermic reaction takes place and a white precipitate (NEt3∙HCl) is formed immediately. Reaction control via 31P-NMR spectroscopy revealed that two equivalents of base are needed for complete consumption of the starting material. This suggests the formation of bis(mesitoyl)diphosphene as depicted in Scheme 3.16.

Scheme 3.16 Formation of a transient diphosphene from 11. This diphosphene formation is supported by 31P-NMR spectroscopic analysis of the reaction mixture, which reveals two sets of signals. One main singlet at δ = – 44.7 ppm and three minor signals that show JPP coupling (δ1 = 143.3 (d, JPP = 320 Hz), δ2 = 85.8 (d, JPP = 313 Hz), 1 δ3 = 18.3 (dd, JPP = 319Hz, JPP = 313 Hz) ppm). No couplings to H-nuclei could be observed. After removal of NEt3∙HCl by filtration, tetrakis(mesitoyl)cyclotetraphosphine 30 (see Scheme 3.17) could be crystallized from the reaction mixture. The signal in the 31P-NMR spectrum at δ = –44.7 ppm could be assigned to 30, by measurement of the pure compound. The three remaining signals most likely belong to a five-membered ring containing a chain of three inequivalent phosphorus atoms. This can be rationalized as the product of a reaction of the diphosphene with unreacted mesitoyl phosphine, likely forming the heterocycle 31 as depicted in Scheme 3.17.

Scheme 3.17 Formation of the major product 30 and the minor product 31 from bis(mesitoyl)diphosphene.

The signals of 31 observed via 31P-NMR spectroscopy are similar to those obtained in the trapping of diphosphene 24 with Na(OCP). Formation of 30 and 31 has already been observed in minor quantities by 31P-NMR spectroscopy in mechanistic studies on the formation of mesitoylphosphaketene.[67] Compound 31 was only a minor product and not isolated. Compound 30 is a pale yellow solid and stable towards water and air. It does not react with 0 common reducing agents such as Li or KC8 and is also unreactive towards oxidants such as PhIO or tBuOOH. Even when dissolved in molten sulfur 30 did not get oxidized. The structure of 30 shows similar structural properties compared to that of 25[67] (see Figure 3.4).

Figure 3.4 Solid-state structures of 30 (left) and 25 (right)[67] – symmetry generated atoms are starred. For structural parameters, see Table 3.3. Another structure suitable for comparison is that of tetrakis(pivaloyl)cyclotetraphosphine 23.[66] The structural parameters of 30, 25 and 23 are summarized in Table 3.3.

Table 3.3 Selected interatomic distances and angles of 30, 25 and 23. Data for 23 was obtained from the crystallographic information file obtained from the CCDC (#1268493). 30 25[67] 23[66] d(P(1)-P(2)) 2.208(1) Å 2.334(1) Å 2.196(2) Å d(P(1)-P(1)*) 2.216(2) Å 2.231(1) Å 2.212(2) Å d(P(2)-P(2)*) 2.229(2) Å 2.231(1) Å 2.212(2) Å d(P(1)-C(1)) 1.887(3) Å 1.894(2) Å 1.895(5) Å d(P(2)-C(2)) 1.892(3) Å – 1.893(4) Å d(C(1)-O(1)) 1.213(3) Å 1.224(2) Å 1.198(6) Å d(C(2)-O(2)) 1.204(3) Å – 1.198(6) Å d(P(1)-P(2)*) 3.069(1) Å 2.958(2) Å 3.069(1) Å d(P(2)-P(1)*) 3.069(1) Å 3.065(1) Å 3.069(1) Å ⦟(P(1)*-P(1)-P(2)) 87.8(1) ° 86.7(1) ° 90.3(1) ° ⦟(P(1)-P(2)-P(2)*) 87.5(1) ° 83.0(1) ° 89.5(1) ° ⦟(P(1)-C(1)-O(1)) 120.4(2) ° 117.9(1) ° 117.6(4) ° ⦟(P(2)-C(2)-O(2)) 122.8(2) ° – 117.5(4) ° ⦟(C(1)-P(1)-P(2)) 98.8(1) ° 98.6(1) ° 100.6(2) ° ⦟(C(2)-P(2)-P(1)) 100.5(1) ° – 96.7(2) ° sum of angles around P 284.8(1) ° 292.3(1) ° 292.4(2) °

Both 30 and 25 exhibit a kinked structure of the central ring. In the solid-state structure of 30 a fold angle of 32.5(1) ° is observed, while the fold angle for 25 is even higher with 47.2(1) °. The structure of 23 on the other hand differs significantly, with the central ring being almost

49 planar (fold angle = 173.2(2) °). All three structures exhibit the characteristically long phosphorus carbonyl bonds.[29] The structure of 30 exhibits a more pronounced asymmetry compared to 25 and 23. This is likely due to a co-crystallized molecule of THF, causing an asymmetry of the repeating unit of the crystal structure. P-P bond cleavage of aryl and alkyl substituted cyclotetraphosphines by N-heterocyclic carbenes has been reported by Riegel and co-workers.[75] Reaction of 30 with a carbene generated in situ from N,N-dimethylimidazolium iodide and potassium tert-butoxide led to formation of the NHC-phosphinidene adduct 32 as shown in Scheme 3.18.

Scheme 3.18 Formation of 32 by reaction of 30 with an N-heterocyclic carbene. The solubility of 32 in organic solvents is lower than that of 30 and it readily crystalized from THF. The solid-state structure of 32 is shown in Figure 3.5.

Figure 3.5 Solid-state structure (left) and schematic representation (right) of 32. For structural parameters, see Table 3.4. The low solubility of 32 can be rationalized by hydrogen bonding observed in the solid-state structure. The distances of the oxygen atom to the protons bound to C(12), C(13) and C(15) are 2.66(3) Å, 2.28(3) Å and 2.47(4) Å respectively (all protons were located individually on the difference Fourier map and independently refined). This is within the sum of the van der Waals radii of a carbon and a hydrogen atom, which is 2.80 Å.[71] The structure of 32 is best compared to the bis(diisopropylphenyl)-NHC stabilized acylphosphinidenes reported by M. Bispinghoff.[18] The para-fluorobenzoyl derivative 33a and the diphenylacetyl derivative 33b are shown in Scheme 3.19.

Scheme 3.19 Structures of known acylphosphinidines 33a and 33b – DIPP = diisopropylphenyl. Numbering was modified to allow for comparison with 32.

Table 3.4 Selected interatomic distances and angles of 32, 33a and 33b. 32 33a[18] 33b[18] d(P-C(1)) 1.790(3) Å 1.783(2) Å 1.779(4) Å d(P-C(11)) 1.813(3) Å 1.813(2) Å 1.800(3) Å d(C(13)-C(14)) 1.347(4) Å 1.334(3) Å 1.333(5) Å ⦟(C(11)-P-C(1)) 99.5(2) ° 95.8(1) ° 95.8(2) ° ⦟(N-C(11)-N) 105.7(1) ° 105.3(1) ° 104.6(3) °

The similarity of the structural parameters of 32 to those of 33a and 33b suggests that 32 is best described by the resonance structure NHC+–P=C(O–)R, as it is the case for 33a and 33b.[18] This is in agreement with the shortened bond length P–C(1) compared to 30. The bond length P–C(1) is of similar length compared to the phosphorus acyl bond length reported for the enol form of bismesitoylphosphine (1.784(2) Å)[9], further corroborating the partial double bond character of the bond between P and C(1). Using compound 30 as a precursor for further reactions required the preparation of larger amounts of this compound. This was problematic, since the isolated yield of 30 is unfortunately only 15% based on 11. Closer examination of the 31P-NMR spectrum of the crude reaction mixture revealed a broad resonance between 0 ppm and 30 ppm, which upon exposure to air transforms into large, sharp peaks between 0 ppm and 10 ppm. This broad signal indicates that the low yield may be caused by undesired side reactions forming oligomers of 11. Changing the order of addition or variation of reaction temperature had no beneficial effect on the yield of 30. At lower temperatures the reaction proceeded slower giving the same yield, while at higher temperatures the yield was diminished. A change of solvent did also not affect the yield. The reaction proceeded in THF, DCM, DME, chloroform and toluene. Varying the concentration of the reactants had no influence on the isolated yield as well. To probe the influence of the oxidant on the yield, a variety of different oxidants were used in the synthesis of 30 and the isolated yield compared (see Table 3.5).

Table 3.5 Oxidants used in the synthesis of 30 and corresponding yields: oxidants marked with a star (*) have † been used in presence of NEt3, yields marked with a dagger ( ), indicate little or no consumption of 11. oxidant yield [%] oxidant yield [%] * * C2Cl6 15 TsCl 14 * * Ph3PCl2 0 CCl4 15 * DMSO/(COCl)2 0 I2 7 * * SOCl2 0 NCS 15 * * TEMPO-BF4 0 NBS 10 BrCN* 0 DDQ 12 * * † 1,2-dibromoethane 0 C2Cl4 0 i † Al(O Pr)3/acetone 0 benzoquinone 0

Table 3.5 indicates that the influence of the oxidant is mainly dependent on the leaving group in the reaction shown in Scheme 3.16. Softer leaving groups like bromide and iodide lower the yield. DDQ is the only oxidant which is unlikely to proceed via the route shown in Scheme 3.16. It likely causes P-P bond formation via dehydrocoupling and formation of the diphosphene via a second dehydrogenation by a second equivalent of DDQ. Dehydrocoupling of 11 with DDQ was also performed under UV-irradiation, leading to no increase in yield. i Dehydrocoupling in an Oppenauer reaction (entry: “Al(O Pr)3/acetone”) did not proceed. Since a change of oxidant had no positive effect on the yield, the effect of the base was probed next. Potassium tert-butoxide was unsuitable as base, since no signal for 30 could be observed via 31P-NMR spectroscopy. Using pyridine, 1-picoline or DBU as base caused an increase in reaction time, but no increase or decrease in yield. An amine base is therefore likely needed, to form a reactive phospha-ammonium species but does not seem to have an impact on the yield of the reaction. The rate of the reaction is likely to depend on the reactivity of these phospha- ammonium species, which in turn is likely to be influenced by the employed base. The last parameter to modify was the substituent on the acylphosphine. Therefore, the reaction conditions of the synthesis of 30 were tested with the acylphosphines listed in Table 2.4 (see Chapter 2) and the reaction crude was analyzed by 31P-NMR spectroscopy. While the acylphosphine was consumed in all cases, only broad peaks between -20 ppm and +50 ppm could be observed for all but one entry. The only exception is represented by ortho- biphenylacylphosphine (13), which formed the tetrakis(acyl)cyclotetraphosphine 34 as shown in Scheme 3.20.

Scheme 3.20 Formation of 34 by oxidative coupling of 13.

Formation of 34 was validated by 13C-NMR and 31P-NMR spectroscopy and likely proceeds via the same mechanism as the formation of 30. The biphenyl derivative of 31 is visible in the 31 P-NMR spectrum of the reaction crude as by-product in the synthesis of 34 (δ1 = 140.2 (d, JPP = 312 Hz), δ2 = 87.8 (d, JPP = 309 Hz), δ3 = 22.0 (dd, JPP = 312Hz, JPP = 309 Hz) ppm). The isolated yield of 34 is however significantly higher, being 50% based on 13. The yellow powder 34 is chemically less stable compared to 30, requiring storage under protective atmosphere. Notably, the solubility of 34 in organic solvents is much higher compared to 30. Although formation of 30 and 34 is likely to proceed via an intermediate diphosphene, the exact mechanism could not be proven. Trapping of the intermediate diphosphene via a cycloaddition with 2,3-dimethylbutadiene has been performed for 24. However, performing the synthesis of 30 or 34 at low temperatures in 2,3-dimethylbutadiene did not yield cycloaddition products as evidenced by 31P-NMR spectroscopy. The steric bulk of the acyl group has an observable influence on the cyclotetraphosphine formation, which might indicate a transition state for the [2+2] cycloaddition forming 34, which is lower in energy compared to the [2+2] cycloaddition forming 30. However based on two positive results, it is not possible to determine a clear correlation between steric bulk of the substituent and yield of the cyclotetraphosphine formation. 3.4 Summary and Outlook Hydrophosphination of DBA by mesitoylphosphine (11) proceeds under base catalyzed conditions at room temperature and gives rise to mesitoyl-2,6-diphenylphosphinanone (26). This represents the first synthesis of a phosphinanone at ambient temperature. Therefore synthesis of phosphinanones derived from temperature sensitive dienones should be possible by this route in the future. Although hydrophosphination of phorone was unsuccessful using 11, less sterically demanding primary acylphosphines could still be viable substrate for a phospha-Michael addition using sterically demanding dienones. Oxidation of 26 proceeds readily forming 27, which decomposes slowly under ultraviolet light. It is however not able to initiate a polymerization of methyl acrylate or hexanediol diacrylate under UV light, which indicates that UV-irradiation does not produce radicals reactive enough to initiate polymerization reactions. A dehydrogenation of the backbone of 27 could lead to a better absorption of irradiation and therefore, a more efficient cleavage of the phosphorus acyl bond. Dehydrogenation of 27 using DDQ did only proceed partially and the different products have not been separated. Therefore, investigation of unsaturated heterocyclic acylphosphine oxides is continued in Chapter 4, in the form of acylphosphole oxides. Dehydrocoupling of 11 to the corresponding diphosphene was unsuccessful using titanocene or zirconocene. However in the case of titanocene, complex 29 could be isolated in low yield. Formation of 29 indicates the partial cleavage of the phosphorus acyl bonds in 11 during the reaction. Since the yield of the synthesis of 29 could not be improved by varying the ratio of phoshine to metallocene, this compound was not further scrutinized. However, it does show that formation of the diphosphene of 11 via metal induced dehydrocoupling is likely inhibited by interaction of the oxygen of the acyl group with the metal center.

Dehydrocoupling of 11 using C2Cl6 in the presence of triethylamine proceeded readily at room temperature. The resulting diphosphene appeared to be transient and dimerized to form cyclotetraphosphine 30. Compound 30 is chemically very inert and the only reactivity that

53 could be observed, was P-P bond cleavage induced by an N-heterocyclic carbene forming 32. This reaction represents an alternative to the acylation of NHC-stabilized phosphinidenes in the synthesis of NHC-stabilized acylphosphinidenes. The usefulness of 30 as a precursor for further reactions is severely inhibited by the low yield of 15%. The yield could not be improved by change of: . order of addition of reagents . concentration of reagents . reaction temperature . employed base. The only improvement in yield could be observed when changing the primary acylphosphine to the ortho-biphenyl derivative 13. Reaction of 13 with C2Cl6 in presence of NEt3 leads to formation of the cyclotetraphosphine 34 in a yield of 50%. Although 34 is chemically less stable compared to 30, they both exhibit highly interesting thermal properties. These thermal properties are discussed in Chapter 7. The synthesis of 30 still needs to be improved. A possible route would be the trapping of the intermediate diphosphine and subsequent slow release, as it is possible in the case of 24, which can be thermally released from the OCP-adduct depicted in Scheme 3.10.[68] A method of dehydrocoupling of phosphines unexplored in this chapter is the use of Lewis acids such as B(p-HC6F5)3, which was used by Stephan and co-workers in the catalytic dehydrocoupling of phenylphosphines.[76]

The oxidative coupling of acylphosphines using C2Cl6 under basic conditions can also be employed in the synthesis of tetrakis(acyl)diphosphines, which will be discussed in Chapter 5.

4 Synthesis of Mono(acyl)phosphole Oxides 4.1 Introduction Phospholes have found widespread application in the synthesis of functional materials, mainly in the synthesis of organic π-conjugated polymers.[77] The phosphorus atom is however not part of the aromatic system in phospholes[78], as evidenced by its pyramidal bonding geometry. This geometry causes phospholes to be an electron-accepting building block, due to a σ*-π* interaction between the phosphole backbone and the exocyclic substituent (see Scheme 4.1).

P P

Scheme 4.1 Phospholes: general structure (left) and σ*-π* interaction (right). Due to their electron-withdrawing nature, π-conjugated polymers incorporating phospholes provide an excellent scaffold for n-type conjugated materials.[77] The electronic structure phospholes can be easily modified, either by modification of the substituents on the backbone or oxidation of the phosphorus using different chalcogenides.[79] This allows for a tuning of the HOMO-LUMO gap of phospholes, which plays a significant role in their absorption and emission properties. These photoproperties are not only limited to polymeric compounds. For instance, monomeric phosphole based chromophores have recently been developed for stimulated emission depletion (STED) microscopy.[80] The majority of syntheses of phospholes fall into one of the two following categories[78]: a) McCormack-type reactions of dihalophosphines with a 1,3-dienes[81] b) Fagan-Nugent-type reactions of dihalophosphines with metallated 1,3-dienes[82] The most common P-substituent reported for phospholes is a phenyl ring, likely due to the commercial availability and low cost of dichlorophenylphosphine compared to the alkyl analogues as well as the chemical inertness of the phenyl substituent. P-phenyl substituted phospholes can however further be functionalized using strong reducing agents, such as elemental alkali metals. Reductive cleavage of the P-phenyl bond allows for subsequent functionalization of the resulting anion using electrophiles.[78] Pathway a) has mainly been employed in the synthesis of small phospholes, such as 3,4- dimethyl-1-phenylphosphole (35 - see Scheme 4.2).[83]

Scheme 4.2 Synthesis of 35 from 2,3-dimethylbutadiene and dihalophenylphosphines (X = Cl, Br). This synthetic methodology is not very versatile, since the availability of different butadienes is limited and the reaction requires long reaction times. The long reaction times are especially

55 pronounced when dichlorophosphines are used. Synthesis of 35 from dibromophenylphosphine requires eight days reaction time for the formation of the intermediate adduct, while the formation of the analogous adduct from dichlorophenylphosphine requires twelve days.[83] These reaction times are drastically reduced (10 h for dibromophenylphosphine) at high pressure (4-5 kbar).[84] The only reported exception to these long reaction times is the synthesis of 1,2,5-triphenylphosphole 36, which can be performed at higher temperatures in the absence of solvent (see Scheme 4.3) and is therefore completed in less than three hours.[85]

Scheme 4.3 Synthesis of 36 from 1,4-diphenylbutadiene and PhPCl2 at elevated temperatures. Pathway b) on the other hand proceeds via the formation of an in situ formed metallacycle and subsequent reaction with a dihalophosphine. Two of the most common metalation reagents are i [86] [87] Ti(O Pr)4 and Cp2ZrCl2 , which react with alkynes to form the required intermediates, which then react with chlorophosphines to form phospholes as depicted in Scheme 4.4.

i Scheme 4.4 Synthesis of phospholes using Ti(O Pr)4 (top, R = COOEt) or Cp2ZrCl2 (bottom). The synthesis of phospholes via this pathway has the advantage of shorter reaction times and a wider range of available acetylenes, either commercially available or accessible from common organic reactions. However, the metalation reactions have to be performed at low temperatures, due to the instability of the reduced metal centers, and the metalation reagent has to be used in a stoichiometric fashion and is usually not recovered. These factors cause pathway b) to be a high-cost synthetic method, especially with regard to potential industrial application. Despite the well-established chemistry of phospholes, there have only been few reports of acyl substituted phospholes or phosphole oxides. The reported acylphospholes do however indicate a high lability of acyl groups bound to the phosphorus atom of a phosphole moiety. The first synthesis of an acylphosphole and its oxide was reported by Lindner and Frey in 1980.[88] Dibenzophosphole reacts with trifluoroacetic anhydride under formation of an acylphosphole, which can be oxidized using molecular oxygen (see Scheme 4.5).

Scheme 4.5 The first reported synthesis of an acylphosphole and its oxide (R = H, F). The products depicted in Scheme 4.5 were described to be very sensitive towards hydrolysis, reacting with water to under loss of either acetic acid or trifluoroacetic acid. Acyl-substituted phosphafluorene oxides similar to those depicted in Scheme 4.5 have been patented as photoinitiators in light curable resin compositions in 2007.[89] Deprotonation of in situ prepared acylphospholes and subsequent sigmatropic rearrangement was reported in 2005 by Mathey and co-workers (see Scheme 4.6).

Scheme 4.6 Synthesis of 3-acylphospholes via sigmatropic rearrangement (R = Ph, Me, 2-thienyl). Le Floch and co-workers describe a similar sigmatropic rearrangement in their synthesis of a 2-carboxyphosphole.[90] The sigmatropic rearrangement is initiated by thermal formation of KOtBu from KOBoc (itself a byproduct of the acylation step), which subsequently deprotonates the phosphole ring (see Scheme 4.7).

Scheme 4.7 Synthesis of a 2-carboxyphosphole via sigmatropic rearrangement. 4.2 Aim of this work Acylphosphole oxides are potential photoinitiators, as evidenced by a patent from 2007.[89] Furthermore the observed lability of the acyl group in acylphospholes, indicates a weak phosphorus acyl bond, which should allow for a facile photocleavage. In order to exclusively probe the influence of the phosphole backbone, acylphosphole oxides will be prepared from small phospholes such as 35. Compound 36 however will not be ruled out as a precursor, since it is by far the easiest to prepare on a laboratory scale. Prepared acylphosphole oxides will be tested under polymerization conditions, to determine if they are efficient photoinitiators for radical induced polymerization reactions.

4.3 Results and Discussion Reaction of 35 with elemental lithium led to the cleavage of the P-phenyl bond, forming the anionic phospholide. Reaction of this anion with mesitoyl chloride however did not yield an acylphosphole (see Scheme 4.8).

Scheme 4.8 Unsuccessful acylation of 3,4-dimethylphosphole. Rather than acylation, decomposition was observed in form of a large number of signals in the 31P-NMR spectrum of the reaction mixture between –60 ppm and +60 ppm. Decomposition also occurred upon reaction of the phospholide with other alkyl and aryl substituted acyl chlorides. The cause of this decomposition is unknown, but it is reasonable to suggest, that it could be caused by a sigmatropic shift of the acyl group due to a lack of substituents in ortho position to the phosphorus atom, similar to those depicted in Schemes 4.7 and 4.8,. In order to assess this hypothesis, compound 36 was chosen as alternative phospholide precursor, due to its phenyl-substituents in 2- and 5-position. Cleavage of the P-phenyl bond of 36 proceeded upon reaction with elemental sodium, as reported in literature.[85] The prepared phospholide was then reacted in situ with mesitoyl chloride, forming acylphosphole 37 as depicted in Scheme 4.9, corroborating the hypothesis, that substituents in 2- and 5-positions are required for the synthesis of acylphospholes.

Scheme 4.9 Preparation of 37 from 36. Compound 37 was isolated as a yellow powder. Subsequent reaction with one equivalent of hydrogen peroxide yielded the oxide 38, which was isolated as an orange powder. Single crystals of both compounds could be obtained by slow crystallization at low temperature. This allowed for analysis of both compounds by single crystal X-ray diffraction (see Figure 4.3). Having all solid-state structures in hand, compounds 37 and 38 can be compared to their phosphinanone analogues 26 and 27, in order to assess the influence of the phosphole backbone.

Figure 4.1 Solid-state structures of 37 (left) and 38 (right) – for structural parameters see Table 4.1.

Table 4.1 Selected interatomic distances and angles for 37, 38, 26 and 27. 37 38 26 27 d(P(1)-C(1)) 1.815(3) Å 1.813(3) Å 1.862(3) Å 1.836(5) Å d(P(1)-C(4)) 1.809(3) Å 1.821(3) Å 1.868(3) Å 1.836(5) Å d(P(1)-C(5)) 1.895(3) Å 1.897(3) Å 1.879(4) Å 1.877(6) Å d(C(5)-O(1)) 1.206(3) Å 1.213(4) Å 1.217(4) Å 1.226(6) Å d(P(1)-O(2)) – 1.483(2) Å – 1.481(4) Å ⦟(C(1)-P(1)-C(4)) 92.4(2) ° 94.1(2) ° 100.1(2) ° 100.4(3) ° ⦟(C(1)-P(1)-C(5)) 101.9(2) ° 104.3(2) ° 105.8(2) ° 107.4(3) ° ⦟(C(4)-P(1)-C(5)) 104.9(2) ° 101.2(2) ° 100.1(2) ° 104.1(3) ° ⦟(C(1)-P(1)-O(2)) – 120.6(2) ° – 116.8(2) ° ⦟(C(4)-P(1)-O(2)) – 118.4(2) ° – 113.8(2) ° ⦟(CO-PO) – 152.8(4) ° – 174.2(4) ° sum of angles around P 299.1(2) ° 299.6(2) ° 304.5(2) ° 311.9(3) °

Comparison of the phosphorus-carbon bond lengths listed in Table 4.1 shows that the phosphorus-acyl bonds in 37 and 38 are slightly elongated compared to those in 26 and 27. Upon oxidation of 37 the endocyclic P-C bonds are slightly elongated, while oxidation of 26 causes a contraction of these P-C bonds. The change in phosphorus-carbon bond length upon oxidation is however much less pronounced between 37 and 38, when compared with 26 and 27. As it is the case for 26, the mesityl group of 37 is rotated towards one of the phenyl rings of the backbone (bound to C(1)) and the short distance between both aromatic rings (3.183(4) Å) indicates intramolecular π- π interactions.[71] As evidenced by the sum of angles, the geometry around the phosphorus atom is slightly more pyramidal in the case of the phospholes, when compared to the phosphinanones, likely due to the σ*-π* interaction depicted in Scheme 4.1. The angle between the P-O bond and the carbonyl group however differs significantly, when comparing 38 and 27. The lower angle for 38 is likely caused by the much more rigid backbone of the phosphole ring, which causes stronger steric interaction between the mesityl ring and the two phenyl rings. Analysis of the UV/VIS-spectra, confirmed the batochromic absorption shift upon oxidation of 37 to 38, which was indicated by the change of color from yellow to orange (see Figure 4.2).

Figure 4.2 Overlay of UV/VIS-spectra of 0.05 M solutions of 37 and 38 in MeCN. Despite the promising absorption properties of 38, the compound is not a viable photoinitiator for radical initiated polymerization reactions. While decomposition does occur slowly upon irradiation of solutions of 38 in C6D6, mixtures of a monomer such as hexanediol diacrylate and 38 do not polymerize upon irradiation with either ultraviolet or blue (λ = 415 nm) light. To assess whether this effect is caused by the phosphole backbone or the aroyl substituent on the phosphorus, different acyl chlorides were reacted with 2,5-diphenylphospholide (see Scheme 4.10)

Scheme 4.10 Synthesis of derivatives of 38 (R = (4-NMe2)Ph, furyl, thionyl, (4-OMe)Ph). The reaction depicted in Scheme 4.10 is a modification of the synthesis of 38, using water-free tert-butylhydroperoxide (dissolved in decane) instead of H2O2. This modification was necessary, since oxidation using hydrogen peroxide caused a cleavage of the phosphorus-acyl bond for all acyl substituents listed in Scheme 4.10. This is likely due to the diminished steric protection of the carbonyl group, due to the loss of both ortho-methyl groups on the aryl substituent. While there is an influence of the aroyl substituent on the absorption spectra of the acylphosphole oxides depicted in Scheme 4.10, no significant shift of the absorption band was observed (see Figure 4.3).

Figure 4.3 Overlay of the UV-spectra of derivatives of 38, depicted in Scheme 4.10. As expected from the small influence of the aroyl substituent on the absorption properties, none of the derivatives depicted in Scheme 4.10 exhibited an improved photolability of the acyl group. Polymerization tests in hexanediole diacrylate were unsuccessful with all four derivatives. To exclude an inhibiting influence of the phenyl substituents on the phosphole backbone, synthesis of an alkylsubstituted acylphosphole was performed. This synthesis employed tetraethylphospholide, prepared in situ from the corresponding chlorophosphole[87] as depicted in Scheme 4.4. Reaction of the phospholide with mesitoyl chloride and subsequent oxidation using tert-butylhydroperoxide gave rise to compound 39 (see Scheme 4.11).

Scheme 4.11 Synthesis of 39 from sodium tetraethylphospholide. The exchange of the phenyl substituents with ethyl groups led to a significant hypsochromic absorption shift compared to 38, as evidenced by the UV/VIS-spectrum of 39 (see Figure 4.4).

Figure 4.4 UV/VIS-spectrum of 39. Compound 39 does not decompose in solution, when irradiated with either blue (λ = 415 nm) or ultraviolet light. This suggests that the phenyl substituents on the phosphole ring have a positive effect on the absorption properties of acylphosphole oxides. Since none of the derivations of 38 showed an increased photoactivity, the next step was to determine why 38 is not a useful photoinitiator. Therefore, DFT calculations were performed. The structure of 38 has been optimized in the gas phase on the b3lyp/6-311+G(d,p) level of theory and is depicted in Figure 4.5.

Figure 4.5 Structure of 38 optimized on the RPBEPBE/6-311+G(D,P) level of theory. The main features do not differ significantly from the solid-state structure depicted in Figure 4.1, which was used as optimization origin. The acyl group is oriented trans to the PO group in both cases and attempts to rotate the CO bond yielded structures of higher energy. Time depended DFT (rb3lyp td-fc/6-311+G(d,p)) calculations revealed a major absorption in the UV/VIS-spectrum to be located at 456.4 nm. This absorption mainly consists of the excitations HOMO → LUMO and HOMO → LUMO+1. The molecular orbitals involved in the absorption are depicted in Figure 4.6.

Figure 4.6 Selected molecular orbitals of 38 and respective energies. Orbitals shown with an isovalue of 0.04. As Figure 4.6 shows, the HOMO and LUMO of 38 are not localized on the exocyclic phosphorus carbon bond. The LUMO+1 is partially localized on this bond, but does reside over 0.4 eV higher in energy, causing its contribution to the absorption band to be diminished. This leads to the conclusion, that the absorption at low energies visible in the UV-spectrum of 38 (see Figure 4.2) does likely not cause the cleavage of the phosphorus-acyl bond necessary for the formation of the required radical species. The majority of the excitation processes are localized on the π*-orbital of the phosphole backbone and have no effect on the P-acyl moiety. The excited state does therefore not lead to P-C bond cleavage but rather to the emission of light in a broad wavelength range between 500 nm and 800 nm (see Figure 4.7).

Figure 4.7 Emission spectrum of 38 (excitation wavelength = 430 nm). When comparing the HOMO-LUMO gap of 38 (2.03 eV) with that of pentaphenylphosphole oxide (3.44 eV - B3LYP/6-31G(d)[91]), a significant lowering of the HOMO-LUMO gap can be observed by the acylation. This together with the shape of the molecular orbitals indicates a n-π* interaction of the acyl substituent with the phosphole backbone of 38, which likely suppresses a possible interaction with the PO moiety.

The photoproperties of phosphole oxides have been studied extensively so far, but manipulation of absorption and emission properties is usually achieved by modification of the phosphole backbone, while the third substituent on the phosphorus remains a phenyl group.[92] The observed strong influence of an acyl group on the phosphole system could therefore enable a new route for the modification of the optic properties of phosphole oxides. In order to achieve viability as building blocks for optoelectronic materials, the slow decomposition of acylphosphole oxides upon irradiation must however be supressed entirely. 4.4 Conclusion and Outlook Acylsubstitued phosphole oxides have been prepared by reaction of metallated phospholes with acyl chlorides and subsequent oxidation. Despite the presence of the CO-PO moiety commonly found in acylphosphine oxide photoinitiators, acylphosphole oxides do not initiate polymerization of monomers upon irradiation. Variation of the substituents on the phosphole ring and/or the acyl group did not lead to an increase in performance in this regard. TD-DFT calculations on mesitoyl-2,5-diphenylphosphole oxide (37) revealed the first UV/VIS-absorption to be dominated by the HOMO → LUMO transition, with both orbitals being localised on the carbonyl group and the phosphole backbone. The HOMO-LUMO gap is lowered by about 40% (1.41 eV) compared to a similar P-phenyl substituted phosphole oxide. These observations indicate a strong influence of the acyl group on the absorption and emission properties of acylphosphole oxides. This could lead to a new methodology in the design of materials with potential applications in optoelectronics, since modification of phosphole oxides at the phosphorus atom instead of the backbone is not well-established.[92]

5 Synthesis of Tetrakis(acyl)diphosphines 5.1 Introduction The dimer of bis(acetyl)phosphine (40) is tetrakis(acetyl)diphosphine (41). Compound 41 is the first dimer of a bis(acyl)phosphine described in literature. It was detected in a distillation fraction in the purification of tris(acetyl)phosphine, prepared from P(TMS)3 and acetyl bromide.[35] The compound was not isolated and was only identified by its 1H-NMR signal as well as cryoscopic molar mass determination. The formation of 41 in this reaction was attributed to partial decomposition, due to insufficient cooling. The preparation of diphosphines bearing a single acyl substituent has been reported in 1990 in form of a reaction of a deprotonated acylalkylphosphine with secondary chlorophosphines.[93] In 1991 the dimerization of acylalkylphosphines was reported by Filipov and co-workers as a side reaction in their chlorination by tetrachloromethane (see Scheme 5.1).[94]

Scheme 5.1 Formation of bis(acyl)bis(alkyl)diphosphines in the reaction of secondary phosphines with CCl4. Formation of a tetrakis(acyl)diphosphine was reported as well by D. Stein in 2006.[40] It was prepared by titrating a solution of sodium bis(mesitoyl)phosphide (Na-42) in DME with a solution of iodine in DME forming tetrakis(mesitoyl)diphosphine (43) (see Scheme 5.2).

Scheme 5.2 Preparation of 43 from Na-42 by oxidative coupling. In order to cleanly form 43, Na-42 needs to be isolated beforehand, due to the tert-butoxide ions present in solution due to the use of Na(PH2) as precursor in the synthesis of Na-42 (see Chapter 2). Na-42 reacts with potassium tert-butoxide upon addition of iodine under oxidative P-O bond formation forming 44 (see Scheme 5.3).

Scheme 5.3 Alkoxidation of Na-42 using iodine and potassium tert-butoxide.

For compounds 41 and 43 NMR-spectroscopic data has been reported. For 41 only the 1H- NMR data has been given as a triplet at δ = 2.53 ppm with a coupling constant of 5.3 Hz.[35] The triplet signal was rationalized as a higher order spectrum caused the two coupling constants 3 4 JPH and JP’H being of a similar value. Compound 43 does also show triplet signals caused by coupling to both phosphorus nuclei. In case of 43 triplets arise for the 13C-NMR signals of the 1 2 carbonyl carbon (δ (ppm) = 210.8, t, JCP + JCP = 26.7 Hz) and the ipso-carbon nucleus 1 2 [40] adjacent to it (δ = 139.4 (t, JCP + JCP = 15.2 Hz) ppm). These triplets are however unlikely to be caused by similar coupling constants, but rather by virtual J-coupling.[95] Virtual J-coupling is a phenomenon observed, if two nuclei are strongly coupled to each other, but have little or no difference in chemical shift.[96] If this strong coupling is larger than the coupling to other nuclei, this weaker coupling will appear to be with both of the strongly coupled nuclei. This is illustrated using three generic 1H nuclei in Figure 5.1.

Figure 5.1 Generic representation of virtual coupling. Picture used with permission of Prof. Hans J. Reich, University of Wisconsin. In the case of tetrakis(acyl)diphosphines both phosphorus nuclei are magnetically equivalent as evidenced by the singlet signal of 43 in the 31P-NMR at 47.6 ppm.[40] The strong coupling of the two phosphorus nuclei causes the signals of all other coupled nuclei to appear as triplets. Potential precursors in the synthesis of tetrakis(acyl)diphosphines are bis(acyl)phosphines. The first reports on the synthesis of bis(acyl)phosphines reported preparations based on tris(trimethylsilyl)phosphine.[13] Recent investigations of the Grützmacher group however focused on the preparation of bis(acyl)phosphines from Na(PH2). The most successful being [97] the synthesis of bis(mesitoyl)phosphine 42 from Na(PH2) and mesitoyl chloride. Studies by M. Bispinghoff have shown that for the synthesis of bis(acyl)phosphines bearing substituents other than mesitoyl, a different pathway is beneficial. The synthesis of bis(benzoyl)phosphine (45) proceeds by reacting Na(PH2) with methyl benzoate forming Na-10, which is reacted in situ with benzoyl chloride forming Na-45 (see Scheme 5.4).[18]

Scheme 5.4 Synthesis of Na-45 starting from NaPH2. Compound Na-45 is protonated in situ by addition of formic acid, giving rise to 45.

5.2 Aim of this work The synthesis of 42 and 45 provides promising starting reagents for the synthesis of tetrakis(mesitoyl)diphosphine (43) and tetrakis(benzoyl)diphosphine (46). Based on the results in the synthesis of tetrakis(mesitoyl)cyclotetraphosphine described in Chapter 3 and the results [94] obtained by Filipov and co-workers , the reaction of 42 and 45 with C2Cl6 in presence of a base should lead to an oxidative P-P bond formation. These conditions would represent a much milder alternative to the reported synthesis of 43 (see Scheme 5.2)[40] and possibly not require titration of the reaction. Based on these results, a synthesis of 41 should be possible as well. Using the route developed by M. Bispinghoff (see Scheme 5.4)[18] synthesis of Na-40 and/or 40 should be possible and provide suitable precursors for the synthesis of 41. A possible obstacle for this synthesis is the reported instability of 40, which has been reported to decompose over the course of days.[20] The final goal of this chapter is the reversion of the oxidative P-P bond formation. Cleavage of phosphorus-phosphorus bonds by alkali metals is known for other diphosphines, such as tetraphenyldiphosphine[98] or tetramethyldiphosphine[99]. Reaction of the prepared tetrakis(acyl)diphosphines with alkali metals should therefore lead to a reductive cleavage of the phosphorus-phosphorus bond and liberation of the alkali bis(acyl)phosphide. 5.3 Results and Discussion

The bis(acyl)phosphines 42 and 45 react with C2Cl6 in presence of a base under formation of 43 and 46 respectively. The employed base however had a large impact on the conversion of the bis(acyl)phosphines. Using one equivalent of triethylamine as base in the reaction of 42 31 with C2Cl6 led only to partial conversion to 43 as determined by P-NMR spectroscopy. However employing one equivalent of TMG as base led to a much cleaner reaction. The opposite is observed in the case of 45. The diphosphine is formed much more cleanly when triethylamine is employed as base, while TMG led to a large amount of 45 still visible in the 31P-NMR spectrum of the reaction mixture. These results are summarized in Scheme 5.5.

Scheme 5.5 Oxidative coupling of 42 (R = Mes) and 45 (R = Ph) using C2Cl6. Using potassium tert-butoxide or an excess of amine base caused diminished formation of diphosphine for both substituents. The formation of both 43 and 46 proceeds in toluene allowing for a facile work-up by aqueous extraction under protective atmosphere.

As already mentioned in Chapter 2, Na(PH2) reacts with ethyl acetate under formation of sodium acetylphosphide. Addition of acetyl chloride to the reaction mixture led to formation of Na-40. The alkoxides present in solution also partially reacted with the added acyl chloride. It is therefore beneficial to use only one equivalent of tert-butanol to protonate Na3P, leading to the formation of Na2(PH) as precursor and causing less alkoxides to be present in solution.

Formation of Na-40 is evidenced by a singlet in the 31P-NMR spectrum of the crude reaction at δ = 69.1 ppm. Addition of C2Cl6 to this mixture led to formation of 41 as well as two new species, exhibiting singlet signals at high chemical shifts in the 31P-NMR spectrum (δ1 = 143.2 ppm and δ1 = 125.4 ppm) compared to the singlet signal of 41 (δ = 29.4 ppm). These species are likely alkoxylated derivatives of 40. This assumption is based on compound 44 (see Scheme 5.3), which exhibits a similarly high chemical shift in the 31P-NMR spectrum (δ = 110.5 ppm).[40] Addition of one or two equivalents of formic acid leads to no observable change in the 31P- NMR spectrum of the reaction mixture. This indicates, that protonation of NaOtBu and NaOEt present in solution is favored over the protonation of Na-40, which in turn indicates a high acidity of 40. Addition of a third equivalent of formic acid leads to formation of 40 as evidenced 31 by the P-NMR signals of its enol form (δenol = 71.5 (s) ppm) and its keto form (δketo = -2 (d, 1 JPH = 245.5 Hz) ppm), which is in agreement with the spectroscopic data reported by Becker and co-workers.[20]

Addition of C2Cl6 to in situ formed 40 leads to formation of 41 with no evidence of formation of alkoxylated species, leading to the overall reaction depicted in Scheme 5.6.

Scheme 5.6 One-pot synthesis of 41 from Na2(PH). Compound 41 could be isolated in crystalline form after extraction of the reaction mixture with n-hexane and subsequent filtration at low temperature. Formation of the tetrakis(acyl)diphosphines leads to loss of electronic delocalization over the acyl groups. In the 31P-NMR spectrum this leads to a shift towards lower frequencies compared to the enol form of the precursor, due to the loss of the partial double bond character between phosphorus and carbon. The loss of delocalization is also evidenced by a hypsochromic absorption shift. Compound 40 has been described as a yellow oil[20], whereas crystals of 41 are colorless. The yellow solid 42 is converted to the colorless solid 43, while the orange solid 45 is converted to the yellow solid 46. Another consequence of this loss of delocalization is reduced chemical stability compared to the bis(acyl)phosphine precursors. While 42 and 45 can be stored in the solid state under air, their dimers 43 and 46 require storage under protective atmosphere. The solid-state structure of 43 has already been reported.[40] The solid-state structures of 41 and 46 on the other hand were so far unknown and are shown in Figure 5.2.

Figure 5.2 Solid-state structures of 46 (left) and 41 (right). For selected structural parameters, see Table 5.1.

Table 5.1 Interatomic distances and angles – average values ⟨in brackets⟩. 41 43[40] 46 d(P-P) 2.171(1) Å 2.171(1) Å 2.176(1) Å d(P(1)-C(1)) 1.878(2) Å 1.879(2) Å 1.901(2) Å d(P(1)-C(2)) 1.894(2) Å 1.885(3) Å 1.888(2) Å d(P(2)-C(3)) 1.892(2) Å 1.884(3) Å 1.891(2) Å d(P(2)-C(4)) 1.879(2) Å 1.891(2) Å 1.903(2) Å d(C(1)-O(1)) 1.215(2) Å 1.211(3) Å 1.213(3) Å d(C(2)-O(2)) 1.200(2) Å 1.211(3) Å 1.215(3) Å d(C(3)-O(3)) 1.213(2) Å 1.214(3) Å 1.213(2) Å d(C(4)-O(4)) 1.205(2) Å 1.217(3) Å 1.209(3) Å ⟨d(C-R)⟩ 1.498 Å 1.498 Å 1.488 Å ⦟(P(2)-P(1)-C(1)) 99.7(1) ° 107.0(1) ° 105.5(1) ° ⦟(P(2)-P(1)-C(2)) 101.6(1) ° 98.2(1) ° 99.4(1) ° ⦟(P(1)-P(2)-C(3)) 101.0(1) ° 104.7(1) ° 106.9(1) ° ⦟(P(1)-P(2)-C(4)) 102.4(1) ° 103.5(1) ° 95.7(1) ° ⦟(C(1)-P(1)-C(2)) 105.0(1) ° 110.5(1) ° 97.9(1) ° ⦟(C(3)-P(2)-C(4)) 104.0(1) ° 100.3(1) ° 96.9(1) ° ⟨⦟(P-C-O)⟩ 119.2 ° 120.0 ° 117.5 ° angle sum around P 306.8(2) ° 312.1(2) ° 301.2(2) °

Table 5.1 shows that the bond lengths in the structural core of tetrakis(acyl)diphosphines vary on average very little upon variation of the acyl substituents. The difference in distance between the carbonyl carbon and the aryl ring in 43 and 46 is present in the free bis(acyl)phosphines as well[18] and is likely due to the high steric demand of the mesityl group. More variation is visible upon analysis of the bond angles. The bond angle ⦟(C-P-C) for 46 is much smaller when compared to those of 41 and 43. This is also visible in the Newman projections of the three compounds. The three compounds adopt a gauche conformation, with the dihedral angle for 46 being approximately half the size of those of 41 and 43, while the angle of the other two substituents to each other stays close to 180° for all three compounds (see Figure 5.3).

Figure 5.3 Newman projections along the P-P bond for 41, 43 and 46. The reason for this distortion is likely due to π-π interactions in the crystal structure of 46, which are depicted in Figure 5.4.

Figure 5.4 Solid-state structure of 46 with π-π interactions as dashed lines (left) and schematic representation, illustrating the position of these dashed lines (right). The planes formed by the two interacting phenyl rings depicted in Figure 5.4 are situated in a distance of 3.285(2) Å towards each other, which is well within the sum of the van der Waals radii of two carbon atoms (3.40 Å[71]). The other phenyl rings exhibit this type of interaction as well, leading to the formation of two-dimensional sheets within the crystal structure. In the solid-state structure of 43, the molecules are arranged in the same geometry, but due to steric repulsion of the methyl groups on the mesityl ring the distance between the π-systems is much larger. In the case of 43 the same plane-to-plane distance as depicted in Figure 5.4 has a value of 3.615(3) Å and therefore does not qualify for π-π interactions. Although compounds 41 and 43 exhibit similar parameters in their solid-state structures, their signals in solution state 31P-NMR spectroscopy differ significantly. The chemical shift of 43 (δ = 44.7 ppm) is at significantly higher frequencies than those of 41 (δ = 29.5 ppm) and 46 (δ = 28.1 ppm). All three compounds show virtual coupling observable by 13C-NMR spectroscopy. Virtual coupling to both 31P nuclei causes the signals for the carbonyl carbon nucleus C1 and the adjacent carbon nucleus C2 to be split into triplets, regardless of magnetic field (see Table 5.2).

Table 5.2 Triplet signals observable by 13C-NMR spectroscopy, caused by virtual coupling to both 31P nuclei. 41 43[40] 46 δ(C1O) 212.8 ppm 210.8 ppm 207.0 ppm 1 JPC 49.4 Hz 26.7 Hz 19.4 Hz δ(C2) 34.8 ppm 139.4 ppm 139.7 ppm 2 JPC 17.5 Hz 15.2 Hz 18.1 Hz

Table 5.2 shows that the only parameter significantly affected by the change of substituent is the coupling constant between C1 and the phosphorus nucleus, which is much larger for 41, when compared to those of 43 and 46. The nature of this increased coupling could not be identified, it could be either caused by the increased electron density around C1 or a less hindered rotation, due to the lower steric demand of the methyl substituents. As expected, compounds 41, 43 and 46 react with metallic sodium or lithium in THF within 2 hours. Monitoring the reaction by 31P-NMR spectroscopy shows clean formation of the corresponding sodium bis(acyl)phosphides. In order to gain more insight into the redox properties of the system formed by bis(acyl)phosphides and their dimers, cyclic voltammograms of 41, 43 and 46 were recorded (see Figure 5.5).

Figure 5.5 Cyclic voltammograms of 41 (top left), 43 (top right) and 46 (bottom) vs Fc/Fc+ (scan speed: 300 mV/s). The oxidative waves depicted in Figure 5.5 cannot be isolated and can only be observed if reductive potentials have been applied first, indicating that the observed oxidation is that of the products formed in the reduction of the respective tetrakis(acyl)diphosphine. Based on the observation of reductive P-P bond cleavage upon reaction with alkali metals, these redox potentials are interpreted as the potential of reductive cleavage of the P-P bond and the potential of oxidative dimerization of the formed bis(acyl)phosphides (see Scheme 5.7).

Scheme 5.7 Redox reactivity of bis(acyl)phosphines and corresponding potentials. In the case of 41 only single waves can be observed for oxidative and reductive potentials. For the other two compounds more complex reductive waves can be observed. In the case of 43 two reductive waves are observed. Since the both waves are irreversible and P-P bond cleavage is likely the first reaction to take place, the first wave was assigned to this reaction and the second one to a possible reduction of the formed bis(acyl)phosphide. Reduction of 46 shows formation of a plateau before the peak potential, which could not be observed isolated from the main wave. This might indicate other reductive processes taking place, before the bond P-P cleavage, which cannot be easily elucidated using cyclic voltammetry. The measured potentials are summarized in Table 5.3.

Table 5.3 Peak reduction potentials (ERed) and peak oxidation potentials (EOx) of bis(acyl)phosphine dimers. 41 43 46 ERed -2.25 V -2.06 V -2.32 V EOx 0.20 V 0.39 V 0.38 V Δ(E) 2.45 V 2.45 V 2.70 V

While the chemistry upon reduction of tetrakis(acyl)diphosphines could not be elucidated completely using CV, it does reveal that the reductive cleavage does indeed require strong reducing agents, such as alkali metals and cannot potentially be achieved using milder reducing agents like Zn0 (E0 = -0.76 V[100]). The oxidative formation of 41 from its monomer occurs at a significantly lower potential than the formation of the aroyl substituted dimers 43 and 46. This has implications for the use of bis(acyl)phosphides as ligands for transition metal complexes. According to the measured oxidation potentials, bis(mesitoyl)phosphine (42) and bis(benzoyl)phosphine (45) should be able to coordinate to electronegative metal centers such as Ru3+ ( E0 = 0.25 V[100]), without undergoing oxidative coupling reactions. This is not the case for bis(acetyl)phosphide (40), which is more prone to this type of reactivity and requires coordination to more electropositive metal centers such as Fe3+ ( E0 = -0.04 V[100]) or Co2+ (E0 = -0.28 V[100]). 5.4 Summary and Conclusion Oxidative coupling of bis(acyl)phosphines has been achieved using hexachloroethane in presence of a base. This method is milder than that reported for the synthesis of 43.[40] Reaction of in situ generated Na-40 or 40 with C2Cl6 leads to formation of 41. This allows for a one-pot synthesis of 41 starting from red phosphorus and sodium. Solid 41 is stable over long periods of time under protective atmosphere. Reaction with elemental sodium as well as cyclic voltammetry of the prepared tetrakis(acyl)diphosphines, shows that the dimerization is reversible at reducing potentials. This reductive P-P bond cleavage allows for reformation of bis(acetyl)phosphine (40) from 41. Therefore 41 is an isolatable and storable form of 40. 40 has so far only been synthesized by fractional condensation after alcoholysis of tris(acetyl)phosphine and has been described as unstable if stored over several days.[20]

Compound 41 being a stable source of 40 could for instance allow for preparation of transition metal complexes incorporating the parent “phosphorus-acac” ligand (see Scheme 5.8).

Scheme 5.8 Potential formation of “phosphorus-acac” complexes from 41. Complexation of a metal with ligand 40 could potentially allow for the formation of bimetallic complexes, by coordination of one metal center to both oxygen atoms as well as coordination of a second metal center to the phosphorus moiety.

6 Synthesis of Tris(acyl)- and Tris(carbamoyl)phosphines 6.1 Introduction Tris(acyl)phosphines were some of the first acylphosphines reported in literature. The first report – by Płażek and co-workers - dates to 1959, reporting the synthesis of tris(benzoyl)phosphine (47) by bubbling PH3 through a solution of benzoyl chloride in pyridine.[12] A report of the extension of the substrate scope of this reaction was published by the same authors in 1961 (see Scheme 6.1).[32] This second publication is a German translation, which unfortunately does not include the experimental details, which were only published in the Polish language.[12, 101]

O Ar O pyridine PH3 + 3 Ar P O Cl Ar -pyridine.HCl O Ar

Ar = Ph, p-Tol, m-Tol, Napht, -Napth

Scheme 6.1 Synthesis of tris(aroyl)phosphines from PH3 by Płażek and co-workers. The prepared compounds were characterized by IR-spectroscopy as well as their reactivity. The compounds are described to have a strong acylating character. Compound 47 is reported to react with alcohol, amines, aqueous ammonia and aqueous sodium hydroxide solutions under formation of PH3 and the respective esters, amides or acids. At the same time as the synthesis of the first tris(acyl)phosphines, the synthesis of the structurally closely related tris(carbamoyl)phosphines was reported. In 1959, Buckler reported that PH3 reacts with isocyanates under base catalyzed conditions to tris(carbamoyl)phosphines (see Scheme 6.2).[33]

Scheme 6.2 Formation of tris(carbamoyl)phosphines from PH3 and isocyanates (R = H, Cl, NO2). The tris(carbamoyl)phosphines depicted in Scheme 6.2 were reported to be thermally stable in the solid state up to 200 °C and more stable towards hydrolysis than the primary acylphosphines and acylphosphonates known at that time. The compounds were stable in boiling water for 16 hours. The thermal decomposition was studied further for R = NO2. Decomposition of the neat sample was found to evolve a mixture of CO and CO2. In boiling DMF or nitrobenzene 4,4’-dinitroanilide and elemental phosphorus is formed (see Scheme 6.3).[33]

Scheme 6.3 Thermal decomposition of tris(4-nitrophenylcarbamoyl)phosphine in solution. After its synthesis, the spectroscopic properties of 47 were subject to discussion in literature. In 1978 Mastalerz and co-workers reported hindered rotation of one acyl group of 47.[30] This was based on an observation in the 13C-NMR spectrum of 47. In the 13C-NMR spectrum of 47 two signals at high frequencies were observed at a temperature of –80 °C, one at a chemical 1 shift of δ = 207.0 ppm with a JPC coupling constant of 32.5 Hz and a second one at 1 δ = 199.9 ppm with a JPC coupling constant of 57.4 Hz. The two signals were observed in a ratio of 2:1 and reported to coalesce at 49.5 °C. These two signals were attributed to a hindered rotation of one acyl group based on a resonance structure with a P-C double bond (see Scheme 6.4).

Scheme 6.4 Resonance structure responsible for hindered rotation in 47 proposed by Mastalerz and coworkers. Mislow and co-workers refuted this claim in 1979.[31] The signal at 199.9 ppm in the 13C-NMR spectrum was observed by Mislow as well, but in different ratios, depending on the method of preparation. Also no coalescence of these two signals was observed at any temperature. According to Mislow, the signal in the 13C-NMR spectrum at δ = 199.9 ppm correlates with the appearance of an impurity at δ = 22 ppm in the 31P-NMR spectrum of 47, which was either present from the beginning or appeared over time, upon storage of the sample in an NMR tube. Based on a patent on the synthesis of tris(aroyl)phosphine oxides from 1996, the species corresponding to the signal in the 31P-NMR spectrum at δ = 22 ppm is most likely tris(benzoyl)phosphine oxide 50.[102] The publication of Mastalerz and co-workers has however not been retracted or corrected significantly.[30] An application of 47 as a photoinitiator was patented by Rettig in 1971.[103] The patent states that tris(aroyl)phosphines can be used as photoinitiators for radical initiated polymerization reactions. The polymerizations could be performed using visible light of different wavelengths (λ = 366 nm, 405 nm, 435 nm). The irradiation times were however quite long (20 min) and the yields comparatively low (19.6% polymerization yield for methyl methacrylate). A spectroscopic study on 47 has been published by Robert and co-workers in 1980.[104] By measuring the 13C- and 31P-NMR spectra of triple 13C-labelled 47 in the nematic phase of a liquid crystal, the C-P-C bond angle was determined to be 95.9 ° ± 0.2 °. In 1977 Becker and co-workers reported the formation of tris(pivaloyl)phosphine (48) from [13] P(TMS)3 and pivaloyl chloride. Compound 48 was reported as a side product in the synthesis of trimethylsilylated bis(pivaloyl)phosphine (see Scheme 6.5).

Scheme 6.5 Formation of 48 from P(TMS)3 and pivaloyl chloride. The structure of 48 was confirmed by single crystal X-ray diffraction. However, only the connectivity could be modelled, due to a disorder present in the crystal. The preparation of 47 from P(TMS)3 and benzoyl chloride on the other hand was not possible and led to the formation of polymeric products of unknown composition.[25]

Tris(acetyl)phosphine 49 was first prepared by Schneider and co-workers by acylation of PH3 coordinated to a manganese complex (see Scheme 6.6).[105]

Scheme 6.6 Synthesis of 49 by acylation of PH3 coordinated to [CpMn(CO)2]. The final complex was not obtained in pure form and no decoordination of 49 was reported. The synthesis of free 49 was reported by Becker and co-workers in 1981 in a manner similar [35] to that of 48 from P(TMS)3. P(TMS)3 and acetyl chloride react under loss of trimethylsilyl chloride to form 49 at room temperature. Compound 49 was reported as an extremely temperature and air sensitive liquid – decomposing immediately if heated above 60 °C or exposed to air. Tris(adamantoyl)phosphine was reported to be accessible via the same route in [106] 1993 by Schmutzler and co-workers. P(TMS)3 reacts with adamantoyl chloride yielding tris(adamantoyl)phosphine in 9.2% yield.

Preparation of tris(acyl)phosphines from P(TMS)3 and various ortho-substituted aroyl chlorides was reported as well in a patent of Ciba-Geigy from 1996.[102] The tris(acyl)phosphines were subsequently oxidized using hydrogen peroxide. The formed tris(acyl)phosphine oxides were employed as photoinitiators for the photopolymerization of ethylenically unsaturated systems. One reported preparation is that of tris(ortho- methylbenzoyl)phosphine 51, which is oxidized to form tris(ortho-methylbenzoyl)phosphine oxide 52 (see Scheme 6.7).

O O O O P(TMS)3 + 3Cl O P + H2O2 O P

O O

51 52 Scheme 6.7 Formation of 52 via 51 patented by Ciba-Geigy.

In 2016, Crossley and co-workers reported the synthesis of 52 together with its solid state structure.[107] However the spectral data given for 52 is in direct conflict with those reported by Ciba-Geigy.[102] The authors acknowledged the discrepancy of the data, but suggest their data to be correct, due to the coherence of analytical data obtained. This is rather daring, when taking into account the data reported by Mislow and co-workers for 47 and its impurity 50*, which is presumed to be 50 - the oxide of 47[31] (see Table 6.1).

Table 6.1 Comparison of reported NMR-spectral data for 47, 50* and 52. 50* is an impurity observed by Mislow and co-workers, presumed to be 50. 52 52 47 50* (Crossley) (Ciba) (Mislow) (Mislow) δ(31P) 67.2 ppm 26.9 ppm 54.5 ppm 23.0 ppm 13 δ( CCO) 208.8 ppm – 206.0 ppm 199.3 ppm 1 JPC 34.8 Hz – 33 Hz 57 Hz

Table 6.1 shows that the data reported by Crossley and co-workers is much more similar to that reported for 47. If 50* is indeed compound 50 as indicated by the report from Ciba-Geigy[102], then the data by Crossley and co-workers is very likely to correspond to the unoxidized phosphine 51 instead of 52. Compound 51 was likely oxidized after the recording of the spectra by Crossley and co-workers, leading to all further analyses indicating the formation of 52. In the Grützmacher group the synthesis of tris(acyl)phosphines from bis(mesitoyl)phosphine 42 was investigated by S. Christen.[108] It could be shown, that the steric demand of the mesitoyl substituents shields the phosphorus atom partially from electrophilic attacks by acyl chlorides. Acylation at one of the oxygen atoms was observed instead. Reaction with sterically demanding acyl chlorides lead almost exclusively to O-acylation. Sterically less demanding acyl chlorides allowed partial P-acylation as determined by 31P-NMR spectroscopy (see Scheme 6.8).

Scheme 6.8 Reaction of 42 with acyl chlorides and product ratios determined by 31P-NMR spectroscopy. In 2016, Mezailles and co-workers claimed the synthesis of tris(mesitoyl)phosphine from [109] 31 Li(PH2)(BEt3)2. The product however was only characterized by P-NMR spectroscopy. This is somewhat risky, since distinguishing between tris- and bis(acyl)phosphines by 31P- NMR spectroscopy is difficult. This difficulty is evidenced by the small difference in the 31P- NMR chemical shifts between bis(pivaloyl)phosphine (δ = 51 ppm in benzene)[21], the DME- complex of lithium bis(pivaloyl)phosphide (δ = 52.6 ppm in Benzene/DME)[21] and tris(pivaloyl)phosphine (δ = 51 ppm in benzene)[13]. Compound 47 was also prepared from

31 Li(PH2)(BEt3)2. The P-NMR chemical shift reported for 47 (δ = 64.6 ppm) does however differ from that reported by Mislow and co-workers (see Table 6.1). 6.2 Aim of this project

Tris(acyl)phosphines have not been prepared from Na(PH2) so far. However the results by M. Bispinghoff have shown that the synthesis of less sterically shielded bis(acyl)phosphines is [18] [108] possible from Na(PH2). Based on the observations of S. Christen these sterically more accessible bis(acyl)phosphines should be easier to acylate at the phosphorus atom than 42, allowing for the synthesis of P-acylated tris(acyl)phosphines. The already reported compounds 47, 48 and 49 are obvious first synthetic targets, due to their known spectroscopic properties and the commercial availability of the corresponding esters and acyl chlorides. Preparation of 47 will allow to further settle the conflict between the publications by Mastalerz and co- workers[30] and Mislow and co-workers[31]. It will also allow for a determination of the 31P- NMR chemical shift of 47 about which there have also been conflicting reports.[31, 109] The oxidation of 47 to form 50 will allow to determine whether or not the impurity 50* observed by Mislow and co-workers is in fact the oxide of 47. Preparation of 50 also should allow clarification about the conflicting results reported by Ciba-Geigy[102] and Crossley and co- workers[107] regarding the NMR-spectral data of the structurally similar compound 52. Investigation of the reported photochemistry[103] of 47 is of interest, due to its lack of a PO bond. Acylphosphines usually need to be oxidized to become photoactive, since their photochemistry relies on interactions of the carbonly group with the P=O moiety.[9] Irradiation experiments will allow to determine the cause of the reported low yields of polymerization. It would also be of interest to determine if tris(carbamoyl)phosphines can also be prepared from Na(PH2). Carbamoyl chlorides of the general formula R2NC(O)Cl are commercially available and represent possible precursors for the synthesis of novel tris(carbamoyl)phosphines. These compounds would differ from those prepared by Buckler[33], due to the absence of an NH functionality. It is therefore of interest to study the thermal properties of the prepared tris(carbamoyl)phosphines in order to compare them to those prepared by Buckler. They should also be chemically more stable than tris(acyl)phosphines, based on the observations of Goicochea and co-workers for mono- and bis(carbamoyl)phosphines.[44] As mentioned in Chapter 2, the stability of the carbamoylphosphines reported by Goicochea was attributed to a significant donation of the nitrogen lone pair into the CO π*-orbital, analogous to the mesomeric stabilization of amides (Scheme 6.9).[43]

Scheme 6.9 Important resonance structures of phosphinecarboxamide. 6.3 Results and Discussion Bis(benzoyl)phosphine (45) reacts in presence of triethylamine with benzoyl chloride under formation of 47. By 31P-NMR spectroscopy only conversion of 45 (δ = 60.1 ppm) to 47 (δ = 53.7 ppm) is observable, with no signals corresponding to the formation of O-acylated species being observable. The 31P-NMR shift is in good agreement with the data reported by Mislow and co-workers[31], while 31P-NMR chemical shift reported by Mezailles and co-

79 workers for 47 (δ = 64.6 ppm) could not be observed at any point during the synthesis, even when the synthesis is performed in the same solvent (THF). Since the acylation to from 47 proceeded so cleanly, a one-pot synthesis of 47 was performed, starting from red phosphorus. Na-10 was formed in situ by reacting of Na2(PH) and methyl benzoate. Addition of an excess of benzoyl chloride to the reaction mixture led to formation of 47 as depicted in Scheme 6.10.

Scheme 6.10 Synthesis of 47 from NaPH2, methyl benzoate and benzoyl chloride.

Na2(PH) was chosen as precursor instead of Na(PH2) to limit the amount of alkoxides present in solution, which stem from the protonation of Na3P by tert-butanol. These alkoxides react with acyl chlorides upon addition, which causes the need for the addition of an excess of acyl chloride. The same methodology could be applied in the one-pot synthesis of compounds 48 and 49. Compounds 47 and 48 were isolated by crystallization, while 49 required fractional distillation using a diffusion pump. During the determination of the solid-state structure of 48, the same problems were encountered, which had already been reported by Becker in 1977.[13] The compound seemingly crystallizes in the space group R3m. The central phosphorus atom is situated on the three fold rotational axis. The phosphorus atom, the carbonyl group and one carbon of the tert-butyl group are situated on the mirror plane. This leads to a distortion of the molecule by symmetry generation. With the help of Michael Wörle of the Small Molecule Crystallography Center (SMoCC) at ETH Zurich, the correct solid-state structure of 48 could be modelled. Compound 48 crystallizes in the space group R3, while being systematically twinned. This twinning likely mimicked a mirror plane in the diffraction pattern and led to prior determination of the wrong space group. Such problems were not encountered during the determination of the solid-state structure of 47, which crystallizes in the space group P21/c. This lower symmetry is caused by the crystallographic inequivalence of the acyl substituents of 47 in the solid state. Both structures are depicted in Figure 6.1.

Figure 6.1 Solid-state structures of 47 (left) and 48 (right) – starred (*) atoms generated by symmetry. For selected structural parameters, see Table 6.2. 80

Table 6.2 Selected average interatomic distances and angles for 47 and 48 - for non-averaged values for 47 see Table 6.4. 47 48 d(P-C) 1.892 Å 1.901(2) Å d(C-O) 1.219 Å 1.204(2) Å d(C-R) 1.487 Å 1.533(2) Å ⦟(P-C-O) 120.2 ° 120.9(1) ° ⦟(C-P-C) 97.6 ° 95.3(1) ° angle sum around P 292.9(2) ° 285.8(5) °

The structural core of both structures is very similar, as illustrated in Table 6.2. In both structures the acyl groups are oriented almost perpendicular to each other, which corroborates the bond angle measured by Robert and co-workers obtained in a liquid crystal.[104] Examining the 13C-NMR spectroscopic data of 47, the results by Mislow and co-workers could [31] be reproduced. A sample of pure 47 in C6D6 shows only one signal corresponding to the carbonyl carbon nucleus in the 13C-NMR spectrum at 25 °C at a chemical shift of 1 δ = 205.6 ppm with a JPC coupling constant of 33.0 Hz. In terms of reactivity, the acylating character of 47 reported by Tyka and co-workers[32] could also be reproduced. Compound 47 reacts with dilute solutions of alcohols or amines under formation of benzoylphosphine (10) and the corresponding esters or amides. This lability of Ph the benzoyl substituents was employed in the synthesis of the photoinitiator BAPO-ONBu4, which was first reported by M.Bispinghoff as an isolatable form of bis(benzoyl)phosphinic [18] acid. Compound 47 reacts with NBu4OH under to form the tetrabutylammonium salt of 45, Ph which can be oxidized using peracetic acid to form BAPO-NBu4 (see Scheme 6.11).

Ph Scheme 6.11 Synthesis of BAPO-NBu4 from 47. This synthesis is obviously less atom efficient than the synthesis by M. Bispinghoff.[18] It does however exemplify the synthetic usefulness of the lability of the benzoyl substituents on 47. This lability even extends into reactions with organometallic compounds. Reacting 47 with TiCl3(THF)3 leads to the formation of the titanium-bis(acyl)phosphido complex 53 under loss of benzoyl chloride (see Scheme 6.12). The formation of benzoyl chloride was evidenced by 13C-NMR spectroscopy.

Ph O Ph O O Cl O O P Ph +TiCl3(THF)3 P Ti + O Cl Ph O Cl Ph Ph O

Scheme 6.12 Formation of 53 and benzoyl chloride from 47 and TiCl3(THF)3.

Complex 53 forms deep green crystals, which could be analyzed by single crystal X-ray diffraction. The determined solid-state structure of 53 is depicted in Figure 6.2.

Figure 6.2 Solid-state structure of 53. For selected structural parameters, see Table 6.3. Co-crystallized THF molecule omitted for clarity. The structure of 53 is best compared to those of 45 and the aluminum complex of 45 Ph [18] [Al( BAP)3], which were both reported by M. Bispinghoff.

Ph Table 6.3 Average interatomic distances and angles for 53, 45 and Al( BAP)3.

[18] Ph [18] 53 45 Al( BAP)3 (X = Ti) (X = H) (X = Al) d(P-C) 1.786 Å 1.783 Å 1.77 Å d(C-O) 1.274 Å 1.283 Å 1.26 Å d(C-R) 1.487 Å 1.480 Å 1.48 Å d(O-X) 1.989 Å 1.22 Å 1.87 Å ⦟(C-P-C) 100.8 ° 98.2 ° 101 ° ⦟(P-C-O) 126.9 ° 124.6 ° 128 ° ⦟(O-X-O) 83.2 ° 162 ° 91 °

Table 6.3 shows, that there is no significant change in the ligand backbone upon coordination Ph to either aluminum or titanium. While the complex [Al( BAP)3] exhibits an octahedral coordination geometry around the central atom, the coordination geometry around the titanium center in 53 forms a distorted octahedron. Atoms O(1), O(2), Cl(1), Cl(2) and Ti(1) reside on one plane (RMSD = 0.016 Å). The oxygens of the THF-ligands however are kinked towards ligand 45 (⦟(O(3)-Ti(1)-O(4)) = 174.1(1) °), causing a decrease in symmetry of the complex. The bis(acyl)phosphide ligand itself is kinked towards O(4), likely to increase the distance to a co-crystallized THF molecule present in the crystal. The EPR-spectrum of 53 was recorded at –40 °C in THF. A linefit simulation was performed using easyspin[110], which revealed a g-value of 1.9525 as well as a hyperfine splitting of 14.11 G due to interaction with 47Ti (see Figure 6.3).

Figure 6.3 EPR-spectrum of 53 at –40 °C (black) and line-fit simulation (blue). The absence of hyperfine splitting due to coupling to the phosphorus nucleus in Figure 6.3 indicates that there is little to no delocalization of the unpaired electron of the Ti(III) nucleus over the bis(acyl)phosphide ligand. In order to further assess the nature of the impurity of 47 observed via 31P-NMR spectroscopy by Mislow and co-workers[31], the oxidation of 47 was attempted in order to form 50. With hydrogen peroxide or organic peroxides the major product formed was bis(benzoyl)phosphinic acid, as evidenced by the corresponding 31P-NMR shift, reported by M. Bispinghoff.[18] Molecular oxygen however proved to be a suitable oxidant for the formation of 50 (see Scheme 6.13), although overoxidation still partially took place.

Scheme 6.13 Reactivity of 47 towards different oxidants. Compound 47 is converted to 50 within two hours, when dissolved in dry THF and stirred 31 under an atmosphere of dry oxygen. The P-NMR chemical shift of 50 in CDCl3 was determined as δ = 22.0 (s) ppm, while the 13C-NMR signal corresponding to the carbonyl 1 carbon nucleus is observable at δ = 199.6 (d, JPC = 55.6 Hz) ppm. These data show that the

83 impurity observed by Mislow and co-workers, which led to the false assumption of hindered rotation in 47 by Mastalerz and co-workers[30], is indeed compound 50. They also corroborate the assumption that the NMR-spectral data reported for 52 by Crossley and co-workers[107] are likely those of the unoxidized phosphine 51, which in turn corroborates the data reported for 52 by Ciba-Geigy in 1996.[102] Compound 50 forms pale yellow crystals and its solid-state structure is depicted in Figure 6.4.

Figure 6.4 Solid-state structure of 50 – for selected structural parameters see Table 6.4. Comparison of the solid-state structure of 50 with those of 47 and 52[107] elucidates the structural impact of the oxidation (in the case of 47) and the ortho methyl substituents on the phenyl ring (in the case of 52) respectively (see Table 6.4).

Table 6.4 Selected interatomic distances and angles of 50, 47 and 52 - average values ⟨in brackets⟩. 50 47 52[107] d(P(1)-C(1)) 1.891(2) Å 1.890(3) Å 1.897(2) Å d(P(1)-C(2)) 1.884(2) Å 1.901(3) Å 1.892(2) Å d(P(1)-C(3)) 1.883(2) Å 1.885(3) Å 1.986(2) Å d(P(1)-O(4)) 1.481(2) Å – 1.474(2) Å d(C(1)-O(1)) 1.214(3) Å 1.220(3) Å 1.213(3) Å d(C(2)-O(2)) 1.215(3) Å 1.213(3) Å 1.213(3) Å d(C(3)-O(3)) 1.215(3) Å 1.224(3) Å 1.216(3) Å ⟨d(C-R)⟩ 1.473 Å 1.487 Å 1.478 Å ⦟(C(1)-P(1)-C(2)) 100.1(1) ° 97.1(2) ° 97.9(1) ° ⦟(C(2)-P(1)-C(3)) 100.6(1) ° 96.7(2) ° 100.1(1) ° ⦟(C(3)-P(1)-C(1)) 100.6(1) ° 99.1(2) ° 99.4(1) ° ⦟(C(1)O-PO) 136.8(1) ° – 124.2(2) ° ⦟(C(2)O-PO) 142.2(1) ° – 135.2(2) ° ⦟(C(3)O-PO) 126.1(1) ° – 137.3(2) °

As evidenced by Table 6.4, oxidation does not affect the P-acyl bond length significantly, as predicted by computational studies.[29] The P-acyl bond lengths are unaffected as well by the methyl groups in ortho position in 52. Oxidation does however cause a slight flattening of the

84 cone formed by the phosphorus atom and the three acyl groups, likely due to the lower steric demand of the P-O double bond, compared to the lone pair in 47. In order to probe the reported photoactivity of 47[103] and 50[102], both compounds were analyzed by UV/VIS spectroscopy. The UV/VIS-spectra of 47 and 50 reveal a hypsochromic absorption shift upon oxidation of 47 to 50 as depicted in Figure 6.5.

Figure 6.5 UV/VIS-spectra of 47 and 50 between 330 nm and 450 nm.

The absorption maximum λmax shifts from λmax(47) = 381 nm to λmax(50) = 385 nm with a shoulder between 400 nm and 405 nm present for 50. This shift in absorption properties manifests itself, when solutions of 47 and 50 are irradiated with light (λ = 405 nm). Compound 50 decomposes within 5 min of irradiation, with a variety of decomposition products observable by 31P-NMR spectroscopy in a range of chemical shifts between -10 ppm and 50 ppm. Compound 47 decomposes slower, but forms a defined product. After 90 minutes of irradiation 47 is converted completely to the bis(acyl)phosphine dimer 46 and benzyl (see Scheme 6.14)

Scheme 6.14 Photolytic formation of 46 from 47.

The reaction depicted in Scheme 6.14 likely proceeds via a homolytic bond cleavage of the P- acyl bond. The formed benzoyl radicals then dimerize to benzil, while the bis(benzoyl)phosphine radicals form the photo-inactive compound 46. Since compound 47 does not exhibit an absorption maximum or shoulder above 400 nm, it is likely that the irradiation with λ = 405 nm is not the most efficient radiation to induce the homolytic bond cleavage. The slower decomposition upon irradiation with non-UV irradiation is likely the cause of the low polymerization yields reported by Rettig when using 47 as photoinitiator for radical initiated polymerization reactions.[103] While 47 immediately decomposes upon contact with air, 50 can be handled briefly under air. Compound 50 does however decompose upon longer exposure to air and moisture– likely due to hydrolysis of the phosphorus acyl bond. This is a significant drawback compared to other acylphosphine oxides, such as 27 (see Chapter 3), 38 (see Chapter 4), 52[102], PhBAPO- [18] Mes [111] ONBu4 and BAPO-OH , which have been reported to be storable at ambient conditions.

Due to the success of the synthesis of tris(acyl)phosphines from Na2(PH), the synthesis of tris(carbamoyl)phosphines was attempted directly from Na2(PH). Na2(PH) reacts with an excess of carbamoyl chloride to form tris(carbamoyl)phosphines as depicted in Scheme 6.15.

Scheme 6.15 Reaction of NaPH2 with carbamoyl chlorides. The excess carbamoyl chloride likely reacts with sodium tert-butoxide present in solution from the preparation of Na2(PH), as discussed earlier in case of acyl chlorides. The tris(carbamoyl)phosphines depicted in Scheme 6.15 are colorless solids and their solid state structures are depicted in Figure 6.6.

Figure 6.6 Solid-state structures of 55 (left), 54 (middle) and 56 (right), for structural parameters see Table 6.5.

Table 6.5 Selected average interatomic distances and angles for 54, 55 and 56 - average values ⟨in brackets⟩. 54 55 56 d(P(1)-C(1)) 1.881(2) Å 1.907(2) Å 1.881(3) Å d(P(1)-C(2)) 1.885(2) Å 1.899(2) Å 1.880(3) Å d(P(1)-C(3)) 1.885(2) Å 1.881(2) Å 1.894(3) Å d(C(1)-O(1)) 1.228(2) Å 1.216(2) Å 1.231(3) Å d(C(2)-O(2)) 1.229(2) Å 1.217(2) Å 1.229(3) Å d(C(3)-O(3)) 1.227(2) Å 1.220(2) Å 1.228(3) Å d(C(1)-N(1)) 1.345(2) Å 1.372(2) Å 1.351(3) Å d(C(2)-N(2)) 1.346(2) Å 1.375(2) Å 1.345(3) Å d(C(3)-N(3)) 1.351(2) Å 1.365(2) Å 1.347(3) Å ⟨d(N-R)⟩ 1.457(2) Å 1.441(2) Å 1.464(4) Å ⦟(C(1)-P(1)-C(2)) 96.6(1) ° 89.8(1) ° 100.5(2) ° ⦟(C(2)-P(1)-C(3)) 94.7(1) ° 94.5(1) ° 96.7(2) ° ⦟(C(3)-P(1)-C(1)) 96.7(1) ° 94.6(1) ° 95.4(2) ° ⟨⦟(P-C-O)⟩ 119.9 ° 122.3(1) ° 119.7(2) ° ⟨⦟(P-C-N)⟩ 116.8 ° 114.4(1) ° 116.8(2) ° ⟨⦟(CO-NR2)⟩ 175.2 ° 168.5(2) ° 176.1(2) ° angle sum around P 288.1(1) ° 278.9(1) ° 292.6(2) ° ⟨angle sum around N⟩ 359.0(2) ° 360.0(1) ° 359.9(2) °

Comparing the bond lengths listed in Table 6.5 with those listed in Table 6.3 shows that the phosphorus carbon bond lengths do not differ significantly between tris(acyl)- and tris(carbamoyl)phosphines. The influence of the nitrogen atom is however visible in form of the electronic donation of the nitrogen lone pair into the CO π*-orbital (see Scheme 6.9). This is observable in the following aspects of the solid-state structure depicted in Figure 6.6: . Elongation of the C=O bond lengths, compared to 47 and 48 . C–N bond lengths in the range of acyclic, tertiary amides[112] . Planar geometry around nitrogen atoms, as evidenced by the angle sum around nitrogen . Torsion angles between carbonyl group and amine group close to 180 ° The rigidity of this partial C–N double bond causes a pronounced effect of the steric bulk of the carbamoyl substituents on the structure. While the steric demand of the substituents on 54 and 56 is comparably small, the two phenyl groups per substituent on 55 exert a large steric influence. This is observable in an elongated C–N bond length as well as a smaller angle sum around the phosphorus atom, leading to a steeper cone formed by the phosphorus atom and the carbamoyl substituents. The signals observed for 54, 55 and 56 by 31P-NMR spectroscopy are observed at a lower chemical shift range (32 – 26 ppm) compared to those of 47, 48, and 49 (63 – 51 ppm). The decrease in chemical shift is likely caused by the π-donation of the nitrogen atoms, which increases electron density at the phosphorus atom. The partial double bond character of the C-N bond, causes the rotation around this bond to be hindered. This phenomenon is well described for other carbamates, such as methyl-N,N-dimethylcarbarbamate.[113] This hindered rotation manifests itself in form of separate signals for the E- and Z-substituents on the nitrogen observable by 1H- and 13C-NMR spectroscopy. This is best observed for compound 54 due to the relatively simple spectra arising from the small dimethylamino substituents. In the 1H-NMR 3 spectrum, signals can be observed at δ = 2.97 (d, JPH = 2.0 Hz) ppm and δ = 2.88 (s) ppm. To

87 confirm that the nature of this coupling is indeed a J-type coupling, which would indicate the phosphorus-coupled signal to be the trans-CH3 group, and not a “through-space”-type coupling, which would indicate indicate the phosphorus-coupled signal to be the cis-CH3 group, a 31P-1H HMBC spectrum was recorded. Due to the presence of HMBC cross-peaks, a “through-space”-coupling mechanism could be excluded, confirming the P-coupled CH3-group to be situated trans to the phosphorus center. The stabilization by the nitrogen atoms leads to a remarkably higher stability of 54, 55 and 56, compared to 47, 48, and 49. While tris(acyl)phosphines are immediately oxidized upon contact with air, compounds 54, 55 and 56 do not exhibit any signs of oxidation after 1 hour under a stream of pressurized air. Dissolving the compounds in a 1:4 mixture of chloroform and either methanol or oleylamine, did not provide any evidence for the loss of carbamoyl groups at the phosphorus atom. For compounds 54 and 56, exposure to oleylamine led to the appearance of a small signal in the 31P-NMR spectrum (δ = 5.2 ppm for 54 and δ = 4.9 ppm for 56) with no observable coupling to 1H nuclei. These are likely signals of the oxidized phosphines. Compound 55 proves to be more resistant towards oxidation. Bubbling air through a solution of 55 in dichloromethane for 2 hours, does not lead to the formation of new signals observable by 31P-NMR spectroscopy. Clean formation of the corresponding tris(carbamoyl)phosphine oxides could however not be achieved using oxygen, hydrogen peroxides or organic peroxides as oxidants. All of these oxidants lead to a partial formation of the signals attributed to the oxidized phosphines, full conversion could however not be achieved, since decomposition was observed with increasing amount of oxidant. In order to assess the thermal properties of 54, 55 and 56, TGA measurements were performed on the three compounds, with the results summarized in Table 6.6.

Table 6.6 Thermogravimetric data for 54, 55 and 56. 54 55 56 P-content w/w 12.54% 5.00% 8.30% Mass loss 85.41% 75.73% 63.59% Onset 282.5 °C 359.7 °C 297.5 °C Inflection 316.8 °C 377.4 °C 308.2 °C End 327.4 °C 410.1 °C 312.2 °C

The mass losses documented in Table 6.6 are the only ones detected in the temperature range of the measurement (60 °C – 600 °C). This data shows that the stabilizing effect of the phenyl groups of 55 also extends to the thermal properties, with the onset of decomposition of 55 being at much higher temperatures, when compared to 54 and 56. The remaining mass of 54 is close to the phosphorus content of the compound, while that of 55 and 56 is much higher. This indicates that in the case of 54 elemental phosphorus and volatile compounds such as CO and HNMe2 could be the main decomposition products. In the case of the other two compounds less volatile products of decomposition are more likely, due to the larger organic substituents. Thermal decomposition in bulk was performed in order to assess whether decomposition of these compounds does in fact lead to formation of elemental phosphorus as described for N-monosubstituted tris(carbamoyl)phosphines.[33] The formation of elemental phosphorus could be evidenced by decomposing samples of 54 and 55 in NMR-tubes by heating the tube with a heatgun (~400 °C) for 5 minutes. Both samples decompose into a deep red tar and a

88 pressure build-up is observed in both tubes. The residues were layered with C6D6 and subsequently ultrasonicated for 15 minutes. While both samples show a signal for white phosphorus in the 31P-spectrum (δ = 520 ppm), the decomposition of 54 proceeds faster than that of 55 (see Figure 6.7). This is in agreement with the obtained thermogravimetric data (see Table 6.6), since the compound with a lower onset of decomposition does indeed decompose faster.

31 Figure 6.7 P-NMR spectrum (C6D6) of the decomposition products of 54 (left) and 55 (right). The spectra depicted in Figure 6.7 do not offer a complete picture of the decomposition products, since the residues of the decomposition of both compounds are not completely soluble in benzene-d6. Heating compound 55 for a prolonged period of time (30 min) does lead to a decreased signal of the tris(carbamoyl)phosphine, but no significant increase of the relative intensity of the signal corresponding to P4 (see Figure 6.8).

31 Figure 6.8 P-NMR spectrum (C6D6) of the decomposition products of 55 after prolonged heating. At the temperature of decomposition of 55 formation of red phosphorus from white phosphorus [114] takes place. It is therefore likely that the amount of P4 formed by decomposition does not increase due to the simultaneous formation of insoluble Pred from P4. The fate of the carbamoyl substituents upon decomposition is difficult to determine. The high decomposition points of the prepared compounds do not allow for thermal decomposition in solution as it was performed by Buckler.[33] Even though the thermal decomposition of tris(carbamoyl)phosphines could not be elucidated completely, it could be shown that increasing substitution at the nitrogen leads to an increase in thermal stability (compared to the compounds of Buckler[33]). These phosphines decompose under formation of elemental phosphorus and organic by-products. The obtained thermogravimetric data suggest that the decomposition point of tris(carbamoyl)phosphines is dependent on the carbamoyl substituents. A wide variety of different carbamoyl chlorides is commercially available, due to their application in a wide variety of industrial processes.[115] Synthesis of tris(carbamoyl)phosphines bearing different substituents should allow for a better understanding of the influence of the substituents on the thermal stability of these compounds. 6.4 Conclusion and Outlook

The synthesis of tris(acyl)phosphines from Na2(PH) does not only open up a new synthetic route to these compounds - it also allowed to settle conflicting reports in literature about the spectral properties of tris(aroyl)phosphines. The solid-state structure of tris(pivaloyl)phosphine (48) was determined, revealing a systematic twinning, which led to the erroneous space group determination which inhibited a successful structure determination by Becker and co-workers over 40 years ago. Exploring the reactivity of tris(benzoyl)phosphine (47) revealed the extent of the acylating character of 47 which was already described by Płażek and co-workers in the seminal publication describing 47.[32] Compound 47 reacts under loss of at least one acyl group with amines, alcohols, hydroxides and organic peroxides. Loss of one acyl group can even be observed upon irradiation with blue light (λ = 405 nm) and upon reaction with TiCl3(THF)3. The synthesis of tris(carbamoyl)phosphines was performed using the methodology developed in the synthesis of tris(acyl)phosphines. These compounds are significantly more stable and in 90 the case of tris(diphenylcarbamoyl)phosphine can be stored under ambient conditions. Tris(carbamoyl)phosphines bearing two substituents at the nitrogen are thermally more stable than their N-monosubstituted counterparts[33] and the decomposition points are dependent on the substituents at the nitrogen. Upon decomposition, formation of gaseous decomposition products and elemental phosphorus is observed. Since carbamoyl chlorides are commercially available and industrially established chemicals[115], variation of the substituents of tris(carbamoyl)phosphines can be easily accomplished. This would allow to tune the thermal properties of these compounds for applications, such as flame retardant additives for polymeric materials.[114] The formation of elemental phosphorus from tris(carbamoyl)phosphines raises the question of the thermal behavior of acylphosphines, which will be explored in Chapter 7.

7 Thermal Properties of Acylphosphines and their Application in the Synthesis of InP Nanoparticles 7.1 Introduction There have been very few reports on the thermal properties of acylphosphines, apart from the determination of boiling and melting points (see Table 7.1).

Table 7.1 Reported melting and boiling points of acylphosphines. For melting points of a variety of derivatives of 42, see [9] and [97]. Compound Property Value Ref. bis(acetyl)phosphine (40) b.p. 51-58 °C / 1 Torr [35] tris(acetyl)phosphine (49) b.p. 44-46 °C / 10-5 Torr [35] bis(mesitoyl)phosphine (42) m.p. 115 °C [9] bis(benzoyl)phosphine (45) m.p. 79-82 °C [18] tris(pivaloyl)phosphine (48) m.p. 133 °C [13] bis(pivaloyl)phosphine m.p. 56-58 °C [116]

Compound 49 has been described as very temperature sensitive, with complete decomposition being observed if the final distillation is performed at too high temperatures.[35] Studies on the thermal reactivity of the commercially available photoinitiators bis(mesitoyl)phenylphosphine oxide (57) and diphenyl(mesitoyl)phosphine oxide (58) were reported by T. Ott in 2008.[117] Both compounds underwent a reverse Michaelis-Arbuzov rearrangement, when heated in solution (see Scheme 7.1).

Scheme 7.1 Reverse Michaelis-Arbuzov rearrangement of 57 and 58. While the same reactivity as depicted in Scheme 7.1 was observed, when heating 58 in absence of solvent, compound 57 decomposes upon heating in the solid state to mesitoic anhydride an pentaphenylcyclopentaphosphine (see Scheme 7.2).

Scheme 7.2 Thermal decomposition of 57 in absence of solvent. Studies on the thermal properties of unoxidized acylphosphines have been reported by G. Müller [9] and M. Bispinghoff [18]. Müller reports the TGA/DTA-data for compound 42, which loses 25.86% of its mass in a first step at 201.8 °C. In a second decomposition step at 393.8 °C 42 loses 20.03%, 28.56% and 5.62% of its original mass in three consecutive mass losses. Bispinghoff reports that tris(mesitoyl)heptaphosphine (59) was analyzed by TGA/DSC. A first

93 decomposition step occurred at 258 °C, followed by a second step at 378 °C. This second decomposition step showed a significant signal for white phosphorus in the mass spectrum of the decomposition products (see. Scheme 7.3).

Scheme 7.3 Thermal formation of white phosphorus from 59. The first decomposition step of 59 is at a temperature, which would qualify 59 as an additive for polypropylene fibers, since these are processed at temperatures at temperatures below 260 °C. Polypropylene presents one of the most important polymers in terms of industrial production.[118] The thermal formation of white phosphorus as shown in Scheme 7.3 was deemed a promising characteristic for a possible application as flame retardant, since this property is already exploited in other types of heptaphosphine based flame retardants.[119] These heptaphosphine based flame retardants take a rather unique spot in literature, since the majority of reported phosphorus-based flame retardants incorporate an oxygenated phosphorus atom.[120] Phosphorus-based flame retardants can act in a variety of different modes to retard flames.[121] The release of white phosphorus upon heating is however most likely to enable a gas-phase radical capture mechanism – quenching hydrogen radicals and hydroxy radicals in the gas phase by formation of less reactive radicals or radical dimerization.[122] Next to flame retardant additives to polypropylene, flame retardant coatings on cotton textile are an important field of application for flame retardants.[123] An important test for the flame retardancy of a compound is the determination of the limiting oxygen index (LOI). The LOI is the concentration of oxygen “below which sustained burning cannot occur”[124]. For cotton textiles the determination of the LOI is performed with a concentration of the flame retardant of 3% phosphorus by mass on the cotton textile sample. Textile samples with an LOI ≤ 21 are considered very flammable, an LOI between 21 and 25 is considered moderately flammable and an LOI ≥ 25 indicates a flame retardant textile.[123] Research in the Grützmacher group has been focused on flame retardant coatings for cotton textiles, which recently led to the report of a flame retardants with LOI values above 30.[125] The thermal behavior of phosphines is not only relevant in the context of flame retardancy. An emerging topic is the synthesis of InP nanoparticles, which uses phosphines as phosphorus sources at high temperatures (230 °C – 300 °C). A variety of different phosphorus sources for the synthesis of InP nanoparticles have been reported, such as P(TMS)3, P(NMe2)3, PCl3 and [126] white phosphorus. The formation of InP nanoparticles from indium myristate and P(TMS)3 has been studied in detail[127] and is an industrially established process. Currently the synthesis of InP nanoparticles using P(TMS)3 as P-source can be considered state-of-the-art. The use of P(TMS)3 does also have drawbacks. At the high reaction temperature P(TMS)3 depletes rapidly in solution. This depletion can cause an increase in polydispersity and does cause the need for [126] multiple injections of P(TMS)3 in the synthesis of larger sized InP nanocrystals. In a chemical laboratory the use of P(TMS)3 is associated with numerous difficulties as outlined in Chapter 1.

7.2 Aim of this work Due to the absence of coherent data on the subject, the study of the thermal behavior of acylphosphines has an intrinsic academic value. As outlined in the introduction, two applications are based on the thermal properties of phosphines: flame retardants and phosphorus sources in the synthesis of InP nanoparticles. Flame retardants are certainly the industrially more relevant application of these two.[118] Generally, flame retardants need to be stable towards air and moisture, which is not the case for most of the acylphosphines reported in the previous chapters. Therefore, focus on the application as phosphorus sources in the synthesis of InP nanoparticles, promises better results, since these syntheses are usually carried out in dry solvents under protective atmosphere.[126] Since studies on 59 have shown, that thermal decomposition of acyl phosphines can cause the release of a reactive phosphorus species (in this case white phosphorus), acylphosphines could qualify as a slow-release P-source for InP nanoparticles. This slow release could be beneficial, since reported phosphorus sources deplete too rapidly at the high reaction temperatures.[126] The cyclotetraphosphines 30 and 34 (see Chapter 3) are the best potential candidates for the thermal formation of P4, since four of the six phosphorus-phosphorus bonds present in P4 are already present in 30 and 34. The only other acylphosphines reported with a similarly advantageous connectivity are the tris(bis(acyl)phosphido)phosphines (60a and 60b) reported by Jones and co-workers in 2002 (see Scheme 7.4).[128]

Scheme 7.4 Synthesis of tris(bisacylphosphido)phosphines by Jones and co-workers (R = tBu (60a), Ph(60b)).

The products depicted in Scheme 7.4 are missing only three bonds towards the P4 tetrahedron and were described as “thermally robust” due to decomposition points at high temperatures (195–197 °C for R = tBu, 278–280 °C for R = Ph). The compounds could however only be isolated in low yields (20% for R = tBu, 15% for R = Ph), likely due to the steric crowding around the central atom. This crowding could potentially be alleviated, by replacing the central atom by an element with a larger atomic radius – for instance arsenic. Replacement of the central phosphorus atom with an arsenic atom is of course not likely to lead to thermal precursors for P4, but rather for AsP3. The synthesis of AsP3 was published by Cummins and [129] co-workers in 2009, by reacting a niobium cyclo-P3 complex with AsCl3 (see Scheme 7.5).

P P P THF Nb + AsCl3 AsP3 ODipp - NbCl2(ODipp)3 DippO ODipp

Scheme 7.5 Preparation of AsP3 from a Niobium cyclo-P3 complex.

AsP3 was found to be thermally stable in molten form (m.p. = 71 – 73 °C) as well as in solution (in refluxing toluene for 1 week) and can also be sublimed at 105 °C in a dynamic vacuum.[129] Thermal decomposition to white phosphorus and metallic arsenic was however observed, when [130] heating AsP3 to 300 °C for 36 h. This temperature therefore marks the upper limit for a potential thermal formation of AsP3. A different goal is the thermogravimetric analysis of other acylphosphines, such as the bis(acyl)phosphine dimers, prepared in Chapter 5. This should allow for an insight into the effect of a phosphorus-phosphorus bond on the thermal properties of acylphosphines by comparing them to the thermal properties of monomeric bis(acyl)phosphines or the tris(acyl)phosphines reported in Chapter 6. Promising candidates for the synthesis of InP nanoparticles will be tested in collaboration with the Sanchez group at CNRS Paris and the Kovalenko group at ETH Zurich. 7.3 Results and Discussion 7.3.1 Cyclotetraphosphines Thermogravimetric analysis of compounds 30 and 34 revealed a first onset of decomposition at similar temperatures for both compounds (243.8 °C (30) and 224.9 °C (34) – see Figure 7.1).

Figure 7.1 TGA curves with mass loss of 30 (black) and 34 (blue). The TGA curve of 34 shows a second defined decomposition step at 376.5 °C, while the decomposition of 30 slowly continues over the whole temperature range afterwards. Analysis 96 by mass spectrometry of the fragments lost, shows that upon decomposition of both 30 and 34 white phosphorus is formed (see Figure 7.2).

Figure 7.2 Overlay of normalized TGA curves (dotted lines) and normalized MS signals for m/z = 124 (solid lines) for 30 (black) and 34 (blue).

As depicted in Figure 7.2, the thermal formation of P4 from 30 is observed at lower temperatures compared to 34. Compound 34 eliminates white phosphorus only in the second mass loss step, while 30 shows a much broader temperature range for P4-evolution. A MS- signal at m/z = 124 is indicative of the formation of white phosphorus, but does not prove it. Therefore, samples of 30 and 34 were heated with a heat gun (T ≥ 400 °C) under an atmosphere of argon in NMR tubes for 15 min. After cooling to room temperature, the residues were layered with C6D6 and ultra-sonicated for 10 min. Both samples showed white phosphorus as the only P-containing species present in solution as evidenced by the characteristic 31P-NMR chemical shift at δ = –525 ppm. The next goal was to quantify the amount of P4 present in solution. The aforementioned decomposition experiments were repeated using 25 mg of cyclotetraphosphine and 2.5 mg of triphenylphosphine were added to the C6D6 as an internal standard. In order to obtain a homogenous excitation over the broad spectral range, the 31P- NMR spectrum of the solution was measured with very short excitation pulses and a high number of scans and without 1H-decoupling. Integration of the signals reveals a rather high yield of white phosphorus in solution for 30 (72%) and a rather low yield for 34 (28.5%). This difference is likely due to the insolubility of the residue of the decomposition of 34, compared to the residue of decomposition of 30, which does mostly dissolve after ultrasonication. The fate of the acyl substituents upon decomposition is harder to determine. 1H- and 13C-NMR spectroscopy does indicate the formation of the corresponding aldehydes. Analysis of the C6D6 solution by GC-MS however shows a plethora of other products being formed. In the case of 30 the following products could be identified via GC-MS: . mesitylaldehyde . mesitoic acid . 1,3-dimethylbenzene . 1,3,5-trimethylbenzene . 1,2,3,5-tetramethylbenzene

At higher retention times, signals for high masses (m/z > 400) could be observed. This leads to the conclusion that disproportionation and oligomerization reactions are likely to occur with regards to the acyl substituents. This potentially explains the low solubility of the decomposition products of 34. The biphenyl groups can easily form larger polyaromatic compounds with low solubility. These oligomeric compounds form an organic matrix from which P4 is not easily extracted by the deuterated solvent. Only two compounds could be tested in this section, due to the synthetic problems arising in the synthesis of tetrakis(acyl)cyclotetraphosphines (see Chapter 3). This does make it hard to make generalizing statements on the dependence of the thermal properties on the acyl substituents. The only statement that can be made is that tetrakis(acyl)cyclotetraphosphines form white phosphorus upon heating to temperatures above 400 °C and that the morphology of the residue of decomposition is very dependent on the acyl substituents. As mentioned previously, a thermal release of white phosphorus can have a flame retardant effect. Since compound 30 is air stable, it was tested as a flame retardant on cotton textiles. Cotton samples were impregnated with a solution of 30 in CH2Cl2 and subsequently dried. These samples contained 3% phosphorus by mass, the LOI was determined to be 23.1% and the samples charred consistently, which means that less cotton was combusted (see Figure 7.3).

Figure 7.3 LOI measurement of 30 on cotton (left) and charring of samples after measurement (right). The measured LOI shows that compound 30 is a flame retardant, although not a very good one. Because of these rather underwhelming results, the investigation of the flame retardancy of 30 was not extended towards polypropylene fibers. Tests in the synthesis of InP nanoparticles on the other hand were successful. In the Kovalenko group InP nanoparticles could be synthesized using 30 as P-source and the Sanchez group was able to synthesize transition metal phosphide nanoparticles using 30. Compound 34 is currently also being tested in the Sanchez group.

7.3.2 Tetrakis(acyl)diphosphines Due to the success of 30 as P-source in the synthesis of InP nanoparticles, the role of the P-P bond for this application was investigated next. TGA measurements of compounds 41, 43 and 46 (see Chapter 5) were performed and the results are depicted in Figure 7.4.

Figure 7.4 TGA curves and mass losses of 41 (green), 43 (black) and 46 (blue). As depicted in Figure 7.4, the TGA curves of 41 and 46 are rather similar and exhibit two mass loss steps – one around 200 °C and one around 350 °C, with compound 46 showing a high residual mass at the end of the measurement (above 50% of the initial mass). The curve of 43 differs, exhibiting one sharp mass loss step at 238.7 °C and then showing slow decomposition until 500 °C. Analysis of the fragments released upon decomposition reveals, that only 41 forms white phosphorus upon decomposition (see Figure 7.5).

Figure 7.5 Overlay of the normalized TGA curve (dotted line) and MS-signals (solid line) of 41 for left: m/z = 15 (green), m/z = 28 (black) and m/z = 29 (violet) and right m/z = 124 (green).

Upon decomposition of 41 white phosphorus is evolved a rather high temperatures, similar to the decomposition of 34 (see Figure 7.2), while at lower temperatures gaseous fragments stemming from the acetyl groups are evolved. The mass loss steps of 43 and 46 at lower temperatures depicted in Figure 7.4 are more difficult to analyze by mass spectrometry. In these temperature ranges, no signals at m/z > 50 are observed. Also the signal at m/z = 40 (argon) as well as the total ion count (TIC) sharply decrease at the mass loss steps. This indicates the loss of fragments, which are not detectable by mass spectrometry, likely due to condensation or radical recombination in the gas phase before the fragments can reach the mass spectrometer. At higher temperatures, loss of aromatic fragments can be detected for 43 and 46 (see Figure 7.6).

Figure 7.6 Overlay of the normalized TGA curve (dotted line) and the normalized MS-signal (solid lines) of selected fragments for 43 (left) and 46 (right). The fate of the phosphorus upon decomposition is unclear from the obtained results. Compounds 41, 43 and 46 were all tested as P-sources in the synthesis of InP nanoparticles and no positive results could be obtained. Based on these results, a P-P bond was not found to be necessary for acylphosphine-based precursors in the synthesis of InP nanocrystals. Therefore investigation of the thermal decomposition of these compounds was not continued. 7.3.3 Bis(acyl)phosphines and Tris(acyl)phosphines Since compound 40 has been described as unstable, the studies of the thermal properties of bis(acyl)phosphines were focused on 42 and 45 (see Figure 7.7).

100

Figure 7.7 TGA curves and mass losses of 42 (black) and 45 (blue). Compound 42 decomposes in three defined decomposition steps, as previously described by G. Müller.[9] The phenyl substituted 45 constantly loses mass between 100 °C and 440 °C with no plateaus observable. The remaining mass percentage for both compounds is similar (28.0% for 42 vs. 30.7% for 45). Surprisingly and in contrast to their dimers, signals for m/z = 124 are observed for both compounds at high temperatures and in low intensity (see Figure 7.8).

Figure 7.8 Overlay of the normalized TGA curve (dotted line) and the normalized signal for m/z = 124 (solid) line of 42 (left, black) and 45 (right, blue). The low intensity of the signal for m/z = 124, can likely be attributed to the low phosphorus content of both compounds (9.5% w/w for 42 and 12.8% w/w for 45). Thermal decomposition of samples of 42 and 45 with a heatgun as described in Section 7.3.1, does not lead to formation of white phosphorus observable by 31P-NMR spectroscopy. No signals are observed at all in the 31P-NMR spectrum even after 30 minutes of ultra-sonication. In the earlier mass loss steps, signals for aromatic fragments similar to those depicted in Figure 7.6 can be observed for both samples. Compared to their dimers, the monomeric bis(acyl)phosphines decompose at lower temperatures, which is somewhat surprising, given their higher chemical stability in ambient conditions. The electronic delocalization over the enolate system which is the source of the

101 chemical stability of both bis(acyl)phosphines, seems to be detrimental with regards to thermal stability. This delocalization is also missing in tris(acyl)phosphines, therefore a higher thermal stability should also be expected in this substance class. This is indeed the case for 47, which is the best point of comparison, since the phosphorus in 47 is possesses only benzoyl substituents. Tris(acyl)phosphines bearing smaller substituents (48 and 49) decompose already at lower temperatures (see Figure 7.9).

Figure 7.9 TGA curves and mass losses of 45 (blue), 48 (red) and 49 (green). The early decomposition point of 49 is in accordance with the report by Becker.[35] The first decomposition step can be divided into two steps. This is evidenced by two separate endothermic peaks in the DSC curve of the decomposition as well as distinct signals for + + m/z = 15 (CH3 ), m/z = 28 (CO)) and m/z = 29 (CH3CH2 ) as shown in Figure 7.10. The baseline for CO is increased compared to the other two signals, since the transfer line to the mass spectrometer is not completely air-tight.

Figure 7.10 Overlay of TGA-curve (dotted line) and DSC-curve (left, solid line) and MS-signals (right) for m/z = 15 (green), m/z = 28 (black) and m/z = 29 (violet) for 49.

102

The residual mass after the first mass loss step is 18.72% of the original mass, which is close to the phosphorus content of 49 (19.36% w/w). However, formation of white phosphorus cannot be detected via MS at higher temperatures. The endothermic signals in the DSC measurement (see Figure 7.10) indicate that an increase in entropy is the driving force of the decomposition of 49. This endothermic decomposition cannot be observed for 47 and 48, which exhibit exothermic DSC-signals before each decomposition step (see Figure 7.11).

Figure 7.11 Overlay of TGA-curves (dotted line) and DSC-curves (solid line) for 47 (left, blue) and 48 (right, red). Melting point values indicated on the DSC-curves. As depicted in Figure 7.11 both compounds melt before exothermically decomposing. This indicates that decomposition is caused by an intermolecular reaction in the molten state, which then leads to a mass loss. This is in contrast to the decomposition of 49, which likely proceeds via thermal homolytic bond cleavage eliminating gaseous fragments such as CO, methane and ethane (see Figure 7.8). The intermolecular reaction preceding the mass loss steps complicates the prediction of lost fragments. All three compounds do however have in common, that no signal for m/z = 124 can be observed over the whole temperature range. Assessment of the bis(acyl)phosphines and tris(acyl)phosphines analyzed in this section by G. Nedelcu and A. Stelmakh from the Kovalenko group, revealed that compounds 45, 47 and 49 are suitable P-sources in the synthesis InP nanocrystals. These compounds are however unlikely to form white phosphorus at the reaction temperatures (~250 °C), since 47 and 49 do not exhibit any loss of fragments with m/z = 124 and 45 does only exhibit such a fragment at much higher temperatures. Therefore, these precursors are likely to exhibit different thermal behavior in the conditions of the InP nanoparticle synthesis compared to the solid state. Since the best precursor system was identified to be a mixture of compounds 45 and 47, these two compounds were analyzed under reaction conditions. Mixing either 45 or 47 with oleylamine, which is the surfactant in the synthesis of InP-nanoparticles, leads to the immediate formation of benzoylphosphine (10) and the corresponding carbamate as evidenced by 13C- NMR spectroscopy and 31P-NMR spectroscopy. Since 10 disproportionates upon heating to 45 and PH3 (see Chapter 2), phosphine gas is assumed to be the reactive phosphorus species, leading to the proposed mechanism depicted in Scheme 7.6.

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Scheme 7.6 Proposed mechanism for the formation of InP nanoparticles from 45 and 47 by chemical activation with oleylamine. This mechanism represents a different pathway for the thermal release of a reactive phosphorus species from stable precursors. In the mechanism depicted in Scheme 7.6 the more reactive precursor 10 is first formed by a chemical reaction and PH3 is released thermally from 10 via a disproportionation reaction, while in the case of the cyclotetraphosphines 30 and 34 white phosphorus can be released directly as reactive phosphorus species.

7.3.4 A Potential Precursor for AsP3 Based on the report of Jones and co-workers (see Scheme 7.4) and the success of the benzoyl group as a leaving group in the thermal formation of reactive phosphorus species (see Section 7.3.3), the functionalization of 45, using AsCl3 was performed.

Compound 45 reacts in presence of triethylamine with AsCl3 forming tris(bis(benzoyl)phosphido)arsine (61) as depicted in Scheme 7.7.

Scheme 7.7 Formation of 61 from 45 and AsCl3. Compound 61 forms yellow crystals and its solid-state structure is depicted in Figure 7.12.

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Figure 7.12 Solid-state structure of 61 – for selected structural parameters see Table 7.2. Compound 60b (see Scheme 7.4) is best suited for structural comparison, since the only difference between the two compounds is the central atom.[128]

Table 7.2 Selected interatomic distances and angles for 61 and 60b[128]. 61 60b[128] 61 60b[128] (E = As) (E = P) (E = As) (E = P) d(E-P(1)) 2.328(1) Å 2.182(1) Å ⦟(P(1)-E-P(2)) 105.4(1) ° 106.5(1) ° d(E-P(2)) 2.326(1) Å 2.176(1) Å ⦟(P(2)-E-P(3)) 100.1(1) ° 108.8(1) ° d(E-P(3)) 2.336(1) Å 2.182(1) Å ⦟(P(3)-E-P(1)) 88.5(1) ° 106.9(1) ° d(P(1)-C(1)) 1.909(3) Å 1.887(2) Å ⦟(C(1)-P(1)-C(2)) 98.6(2) ° 96.7(1) ° d(P(1)-C(2)) 1.908(3) Å 1.906(2) Å ⦟(C(3)-P(2)-C(4)) 94.6(2) ° 94.3(1) ° d(P(2)-C(3)) 1.893(3) Å 1.893(2) Å ⦟(C(5)-P(3)-C(6)) 96.1(2) ° 92.6(1) ° d(P(2)-C(4)) 1.883(3) Å 1.886(2) Å ⦟(C(1)O-C(2)O) 52.5(2) ° 65.3(2) ° d(P(3)-C(5)) 1.882(3) Å 1.900(2) Å ⦟(C(3)O-C(4)O) 82.2(2) ° 81.4(2) ° d(P(3)-C(6)) 1.886(3) Å 1.904(2) Å ⦟(C(5)O-C(6)O) 73.1(2) ° 81.4(2) ° ⟨d(C-O)⟩ 1.215 Å 1.214 Å ⟨⦟(E-P-C)⟩ 99.1 ° 101.7 °

The bonds to the central atom in 61 are elongated compared to those in 60b, which is consistent with the bigger van der Waals radius of arsenic compared to phosphorus.[71] The bis(acyl)phosphide substituents in both structures exhibit on average similar bond lengths and angles. The inequivalence of the substituents relative each other is however more pronounced in 61 compared to 60b as evidenced by the higher deviation of the angles of both carbonyl groups to each other (see Table 7.2). The change in central atom has a more pronounced effect on the isolated yields of both compounds. While 60b was isolated in 15% yield[128], compound 61 could be isolated in 60% yield. This increase in is likely caused by the reduced steric crowding around the central atom, due to the increased van der Waals radius of arsenic compared to phosphorus.[71]

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Analysis of the thermal properties of 61 revealed that the replacement of the central atom by arsenic causes decomposition to occur at lower temperatures compared to that of 60b [128] (Tdecomp(60b) = 278–280 °C ) as illustrated in Figure 7.13.

Figure 7.13 TGA-curve and mass losses for 61. Analysis of the fragments lost upon decomposition by mass spectrometry does not give rise to a signal of AsP3 (m/z = 168). However, strong signals can be observed for the loss of aromatic fragments (see Figure 7.14).

Figure 7.14 Overlay of the TGA-curve (dotted line) and the MS-signals of m/z = 77 (left) and m/z = 91 (right) for 61.

+ + In smaller intensities, signals can be observed for m/z = 62 (P2 ) and m/z = 75 (As ), which [129] have been reported as signals in the GC-MS spectrum of AsP3 (see Figure 7.15).

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Figure 7.15 Overlay of the TGA-curve (dotted line) and the MS-signals of m/z = 62 (left) and m/z = 75 (right) for 61. The signal for m/z can somewhat reliably assigned to As+, since this signal has not been observed in the decomposition of other benzoyl-substituted phosphines, such as 45, 46 and 47. + + Signals for fragments with m/z = 93 (P3 ) and m/z = 106 (AsP ) have been observed via GC- MS by Cummins and co-workers as well.[129] These fragments can be observed in low + intensities in the decomposition of 61, together with a small signal at m/z = 124 (P4 ) (see Figure 7.16).

Figure 7.16 Overlay of the TGA-curve (dotted line) and the MS-signals of m/z = 93 (left), m/z = 106 (middle) and m/z = 75 (right) for 61.

+ Due to the absence of an MS-signal for AsP3 , it is likely that this compound is not released as a volatile fragment during the decomposition of 61. This is likely due to the high temperature [130] of release, which is higher than the decomposition point of AsP3 (300 °C ). To assess whether or not AsP3 is formed at all during the decomposition of 61, a sample of 61 was heated in an NMR-tube to its decomposition point for about 1 second by means of a heatgun. The 31P- NMR spectrum of the residue suspended in THF revealed the decomposition to be incomplete, [130] however signals of P4, AsP3 and As2P2 can be observed at their reported chemical shifts (see Figure 7.17).

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31 Figure 7.17 P-NMR spectrum of decomposition products of 61 in THF (referenced to δ(P4) = -520 ppm).

The formation of a mixture of P4, AsP3 and As2P2 corroborates the assumption that the decomposition point of 61 is too high for a clean thermal formation of AsP3. At the decomposition temperature of 61 scrambling of arsenic and phosphorus is likely to occur, which will ultimately resolve in the formation of elemental arsenic and phosphorus as reported [130] for the thermal decomposition of AsP3 by Cummins and co-workers. However, the formation of these fragments indicates that AsP3 is an intermediate of the decomposition of 61. This in turn suggests that tris(bis(acyl)phosphidoarsines with decomposition points at lower temperatures could cause the formation of AsP3 in a cleaner manner. 7.4 Summary and Outlook Thermal analysis of different acylphosphines has been performed by TGA-MS measurements and these acylphosphines have been tested as P-sources in the synthesis of InP-nanoparticles. The decomposition point of acylphosphines is strongly depended on the acyl substituent, with aryl groups exhibiting a stabilizing effect and therefore shifting the decomposition point to higher temperatures. Despite their stability towards hydrolysis and oxidation, bis(acyl)phosphines 42 and 45 showed lower thermal stability their respective dimers 43 and 46 or the tris(acyl)phosphine 47. The cyclotetraphosphines 30 and 34 both release white phosphorus at high temperatures during the TG measurement as well as during decomposition in bulk in an NMR tube. The decomposition mechanism of acylphosphines could not be elucidated in detail. It is however reasonable to assume, that among the analyzed compounds only the acetyl substituted acyl phosphines decompose via thermal, hemolytic bond cleavage and subsequent release of gaseous fragments. Acylphosphines substituted by larger acyl groups are likely to decompose in a more complex manner, which includes disproportionation of the acyl groups, as evidenced by the formation of toluene from benzoyl substituents (see Figure 7.6) and tetramethylbenzene from mesitoyl substituents as evidenced by GC-MS. Compounds 30, 45, 47, and 49 were found to be suitable P-sources for the synthesis of InP nanocrystals, with a mixture of 45 and 47. Within this group of precursors two different modes of phosphorus delivery are likely to take place. While decomposition of 30 forms white phosphorus as reactive phosphorus species, compounds 45 and 47 react first with oleylamine (the surfactant) to form monoacylphosphine 10, which then releases PH3 as reactive phosphorus species in a disproportionation reaction. The last compound studied was the tris(bis(acyl)phosphido)arsine 61, which was prepared from AsCl3 and 45. The core of compound 61 can be regarded as an open AsP3 molecule, which

108 could potentially be reformed by elimination of the acyl groups on the phosphorus. Thermal decomposition of 61 releases fragments containing arsenic and phosphorus at temperatures between 350 °C and 500 °C. At these temperatures, AsP3 is unlikely to form, since 31 decomposition of AsP3 has been reported to occur at a temperature of 300 °C. The P-NMR spectrum of the decomposition products of 61 supports this assumption, since even at a very short heating time P4 and As2P2 are formed as well as AsP3, indicating an immediate decomposition of AsP3 at these temperatures. Based on the results in this chapter, the decomposition temperature of tris(bis(acyl)phosphido)arsines can potentially be lowered by changing the acyl groups on the phosphorus, ideally below the decomposition temperature of AsP3. Potentially viable compounds are depicted in Scheme 7.8.

Scheme 7.8 Possible precursors for the thermal formation of AsP3.

109

110

8 Sodium Formphosphide – Synthesis, Structure and Reactivity 8.1 Introduction The synthesis of the formphosphide (HPCHO–) anion was first reported by Becker and co- workers in 1991 together with the synthesis of its dehydrogenated analogue - the phosphaethynolate (OCP–) anion (see Scheme 8.1).[37]

Scheme 8.1 Formation of lithium phosphaethynolate (left) and lithium formphosphide (right) from LiPH2. The Grützmacher group reported the synthesis of the formphosphide anion on two occasions. [131] Na(HPCHO) (62) was synthesis by F. Puschmann from NaPH2 and ethyl formate while M. Bispinghoff reported its formation in the reaction of PH3 with carbon monoxide in the presence of sodium tert-butoxide [18] (see Scheme 8.2).

O H Na O CO Na(PH2) P PH3 H H NaOt-Bu 62 Scheme 8.2 Syntheses of Na(HPCHO) reported by F. Puschmann (left) and M. Bispinghoff (right).

Carbon monoxide does not react with alkoxide-free NaPH2 to Na(HPCHO), but rather to Na(OCP). Computational studies suggest that the activation barrier for the formation of Na(HPCHO) is 33 kJ/mol higher than that for the formation of Na(OCP). The presence of sodium tert-butoxide does however lower this activation energy and the use of t [NaPH2(NaO Bu)2(DME)2/3] in the reaction with CO does cause formation of Na(HPCHO) in a ratio of 70:30 relative to Na(OCP).[132] The conversion of HPCHO– to OCP– by dehydrogenation as well as the hydrogenation of OCP– to HPCHO– has not been reported.

Li(HPCHO) has only been isolated in form of its DME-complex [Li(HPCHO)∙DME]2. The formation of a dimeric complex has been suggested by Becker and co-workers based on the solid-state structures of other lithium acylphosphides.[37] Apart from its synthesis only spectroscopic data has been reported for Li(HPCHO). Via 31P-NMR spectroscopy both the E- isomer (δ = 38.6 ppm) and the Z-isomer (δ = 8.8 ppm) are observable.[37] Both isomers are observable by 1H- and 13C-NMR spectroscopy as well. The counterion has a strong influence on the 31P-NMR chemical shifts of the formphosphide anion, as evidenced by the chemical shifts reported by F. Puschmann for E-Na(HPCHO) (δ = 29.3 ppm) and Z-Na(HPCHO) (δ = -1.0 ppm). Both Li(HPCHO) and Na(HPCHO) have been reported as oils at room temperature, which explains why these compounds so far have not been analyzed by X-ray diffraction techniques. The reactivity of the formphosphide anion has not been explored so far, it has only been reported as sensitive towards protolysis and oxidation.[37] The phosphaethynolate anion – in the form of Na(OCP) on the other hand has become somewhat of a “hot topic” in

111 organophosphorus chemistry with the number of publications and citations steadily increasing over the last years (see Figure 8.1). Although the structure and reactivity of Na(OCP) and Na(HPCHO) are likely to differ, the reactions reported for Na(OCP) are reasonable starting points for the investigation of the reactivity of Na(HPCHO).

Figure 8.1 Publications (left) and citations (right) on the topic “phosphaethynolate” between 2007 and 2017 indexed in “ISI Web of Knowledge” (citation report retrieved on February 2nd 2018). One of the reasons of the success of Na(OCP), is its facile isolation as its 1,4-dioxane complex [Na(OCP)(dioxane)2.5]. The formation of this complex allows for preparation and isolation of [67] Na(OCP) on a large scale. [Na(OCP)(dioxane)2.5] has therefore become the most common form in which Na(OCP) is employed in chemical reactions, which is why Na(OCP) will be used as a shortened form of [Na(OCP)(dioxane)2.5] from now on. Only selected reactions reported for Na(OCP) will be discussed in this introduction. The first is the replacement of the counter ion of Na(OCP) by triorganyl compounds of the heavier group 14 elements. This has been reported to lead to the formation of phosphaketenes[133], while the reaction with a diorganylboron chloride led to trimerization to the phosphorus analogue of cyanuric acid[134] (see Scheme 8.3).

Scheme 8.3 Formation of phosphaketenes from Na(OCP) (left) and trimerization (right). Protasiewicz and co-workers reported the modification of the cationic moiety of Na(OCP) without covalently binding to either the phosphorus or the oxygen moiety. Reaction with dibenzo-18-crown-6-ether (db-18c6) leads to the formation of a monomeric complex, while [135] reaction with MgCl2 causes formation of a diphosphaethynolate complex (see Scheme 8.4).

Scheme 8.4 Counter ion exchange (left) and complexation (right) of Na(OCP).

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Stephan and co-workers report the Lewis acid induced dimerization of Na(OCP) by means of various different arylboranes (see Scheme 8.5).[136]

Scheme 8.5 Formation of different dimers of Na(OCP), depending on the used arylborane. Na(HPCHO) can however not only be regarded as the hydrogenated analogue of Na(OCP), it is also the sodium salt of formylphosphine. The synthesis of formylphosphine is unknown to literature, diaminophosphines bearing a formyl substituent have however been prepared by Bertrand and co-workers from their corresponding iminium salts (see Scheme 8.6).

KOH KOH O O THF DMSO THF R O R O N N P R R P R H P R = N(i-Pr)2 P R = N(i-Pr)2 R H R H R H NCy2 Scheme 8.6 Synthesis of formylphosphine and formylphosphine oxide from iminium salts. Apart from decarbonylation, which was observed upon irradiation (λ = 254 nm), no reactivity studies have been reported for the formylphosphines depicted in Scheme 8.6. 8.2 Aim of this work While the synthesis of Na(HPCHO) is well established, no procedure has been reported for a pure product. Becker and co-workers report the isolation of [Li(HPCHO)DME] as an oil, which “could not be purified further”[37], F. Puschmann reports for Na(HPCHO) impurities of DME and alkoxides[131] and M. Bispinghoff reports only 31P-NMR spectroscopic evidence for the formation of Na(HPCHO)[18]. Isolation of pure Na(HPCHO) is not only a prerequisite for the exploration of the reactivity of the formphosphide anion, it also increases the likelihood of crystallization of Na(HPCHO) and therefore analysis by single crystal X-ray diffraction. Even if Na(HPCHO) cannot be crystallized at room temperature, the Small Molecule Crystallography Center (SMoCC) at ETH Zurich provides access to specialized equipment, which allows for the picking and mounting of crystals at low temperatures (< -20 °C) under a protective nitrogen atmosphere (see Figure 8.2).

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Figure 8.2 Equipment by SMoCC for picking and mounting of crystals at low temperatures under N2. Comparison of the solid-state structure of Na(HPCHO) with that of Na(HPCHO) will allow for a comparison of the electronic structure of both atoms in the solid state. The electronic structure of the OCP– has been the target of computational investigation and it could been shown that the negative charge is localized on both the phosphorus and the oxygen atom.[133] The reactivity of Na(HPCHO) will likely be dominated by decarbonylation reactions. Substitution reactions by electrophiles on the phosphorus atom will likely cause the reformation of a carbon-phosphorus single bond and subsequent loss of carbon monoxide (see Scheme 8.7).

Scheme 8.7 Undesired decarbonylation reaction upon reaction of Na(HPCHO) with electrophiles. To suppress this decarbonylation, reactions should be attempted which preserve the P-C double bond in the HPCHO– moiety. Alternatively reactions with acyl chlorides could lead to delocalization over an acac-type system, which is also likely to inhibit the elimination of CO. Attack on the oxygen atom by oxophilic reagents e.g. as silyl chlorides and Lewis acids e.g.

114 triphenylboron should potentially also cause preservation of the P-C double bond and therefore preservation of the formphosphide moiety. The thermal decomposition of Na(HPCHO) is also of interest, due to the thermal reactivity of the nitrogen analogue of formylphosphine: formamide. Thermal decomposition of formamide at high temperatures under reduced pressure is a patented process for the industrial preparation of hydrogen cyanide.[137]

Scheme 8.8 Synthesis of hydrogen cyanide from formamide. HCP, the phosphorus analogue of hydrogen cyanide has first been prepared by Gier in 1961 by [138] reacting PH3 with graphite in a rotating electric arc. Different synthetic pathways to HCP [139] have been reported by Carrie and co-workers (flash vacuum thermolysis of CH3PCl2 ), [140] Denis and co-workers (dehydrohalogenation of Cl2CHPH2 ) and Mathey and co-workers (flash vacuum thermolysis of tris(allyl)phosphine[141]). These reactions all lack preparative viability. Cummins and co-workers reported a molecular precursor for HCP in 2016. HCP can be thermally released from a dibenzo-7-phosphanorbornadiene and be reacted in situ with an azide under formation of the 1,2,3,4-phosphatriazolate anion (see Scheme 8.9)

– Scheme 8.9 Thermal release of HCP from a dibenzo-7-phosphanorbornadiene and subsequent trapping with N3 . The thermal decomposition of Na(HPCHO) under loss of sodium hydroxide would represent an elegant and more atom efficient pathway to HCP. Investigations via TGA/DSC will allow to assess whether HCP can be thermally evolved from Na(HPCHO). 8.3 Results and Discussion 8.3.1 Isolation and Solid-state Structure of Na(HPCHO)

As described by F. Puschmann, Na(PH2) reacts with ethyl formate to form Na(HPCHO) (see Scheme 8.2), which was isolated as its DME-complex in form of a yellow oil. Repeated extraction with n-hexane and subsequent layering of the oil with n-hexane at low temperatures (-20 °C) caused crystals to grow into the hexane phase. These crystals melt at room temperature. Using the special equipment provided by SMoCC at ETH Zurich (see Figure 8.2), these crystals could be isolated and analyzed by single crystal X-ray diffraction, with the resulting solid-state structure depicted in Figure 8.3.

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Figure 8.3 Two different orientations of the solid-state structure of [Na(HPCHO)(DME)2] - disordered atoms indicated by indices, hydrogen atoms represented by spheres with an arbitrary radius – for structural parameters see Table 8.1. The formphosphide anion in the structure depicted in Figure 8.3 is disordered on both sides of the diamond shape. This disorder did not allow for the localization of the P-bound hydrogen atoms on the difference Fourier map. The structural core of [Na(HPCHO)(DME)2] (62(DME)2) [132] is similar to that of [Na(OCP)(DME)2] (63(DME)2) as evidenced by the structural parameters listed in Table 8.1.

[132] Table 8.1 Interatomic distances angles of 62(DME)2 and 63(DME)2 - average values ⟨in brackets⟩.

62(DME)2 63(DME)2 62(DME)2 63(DME)2 d(O(1)-Na(1)) 2.298(7) Å 2.349(3) Å ⦟(O(1)-Na(1)-O(2)) 89.2(2) ° 85.9(1) ° d(O(1)-Na(2)) 2.306(7) Å 2.344(3) Å ⦟(O(1)-Na(2)-O(2)) 88.6(2) ° 86.0(2) ° d(O(2)-Na(1)) 2.266(7) Å 2.336(3) Å ⦟(Na(1)-O(1)-C(1a)) 147.9(5) ° 132.6(3) ° d(O(2)-Na(2)) 2.279(7) Å 2.377(3) Å ⦟(Na(1)-O(1)-C(1b)) 118.2(2) ° – d(O(1)-C(1a)) 1.28(2) Å 1.203(4) Å ⦟(Na(2)-O(1)-C(1a)) 119.9(5) ° 133.0(3) ° d(O(1)-C(1b)) 1.29(3) Å – ⦟(Na(2)-O(1)-C(1b)) 147.0(2) ° – d(O(2)-C(2a)) 1.31(2) Å 1.214(4) Å ⦟(Na(1)-O(2)-C(2a)) 116.5(5) ° 132.1(3) ° d(O(1)-C(2b)) 1.34(2) Å – ⦟(Na(1)-O(2)-C(2b)) 155.5(9) ° – d(C(1a)-P(1a)) 1.67(2) Å 1.589(3) Å ⦟(Na(2)-O(2)-C(2a)) 150.6(5) ° 133.6(3) ° d(C(1b)-P(1b)) 1.69(3) Å – ⦟(Na(2)-O(2)-C(2b)) 107.4(8) ° – d(C(2a)-P(2a)) 1.71(2) Å 1.575(3) Å ⦟(O(1)-C(1a)-P(1a)) 130.6(7) ° 179.3(3) ° d(C(2b)-P(2b)) 1.69(2) Å – ⦟(O(1)-C(1b)-P(1b)) 126(5) ° – ⟨d(Na(1)-ODME)⟩ 2.43 Å 2.38 Å ⦟(O(2)-C(2a)-P(2a)) 125.6(7) ° 179.5(4) ° ⟨d(Na(2)-ODME)⟩ 2.43 Å 2.38 Å ⦟(O(2)-C(2b)-P(2b)) 120.4(2) ° –

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The effect of the formal hydrogenation of the OCP– anion to the HPCHO– anion is observable in a variety of structural parameters, such as: . Increase of the average C-O bond length (1.21 Å to 1.31 Å) . Increase of the average P-C bond length (1.58 Å to 1.69 Å) . Decrease of the average O-Na bond length (2.35 Å to 2.29 Å) . Decrease of the average P-C-O bond angle (179.4 ° to 125.7 °) The increase in P-C bond length is consistent with a decrease in bond order, while the increased C-O bond length can be interpreted as an increased single bond character. Together with the decreased O-Na bond length, it is reasonable to assume that the negative charge of the formphosphide anion is mostly localized on the oxygen atom, in contrast to the phosphaethynolate anion, which has a more ambivalent nature.[133] The P-C-O bond angle is consistent with an sp2-hybridization of the carbon atom in the HPCHO– ion, similar to that in [142] (NCy2)2PCHO (125.3(2) ° - see Scheme 8.6). Comparison with (NCy2)2PCHO does also corroborate the double bond character of the P-C bond in Na(HPCHO), due to the bond length being significantly shorter than that of the P-C single bond in (NCy2)2PCHO (1.841(2) Å).

The crystallization of [Na(HPCHO)(DME)2] at low temperatures is unfortunately not a viable method of purification on a preparative scale, since the crystals cannot be filtered off without melting. Addition of dibenzo-18-crown-6-ether (db-18c6) to a THF solution of crude Na(HPCHO) leads to the formation of its adduct with Na(HPCHO), as evidenced by the appearance of a new set of signals observable by 31P-NMR spectroscopy (δ(E) = 15.7 ppm, δ(Z) = -15.2 ppm). This adduct is solid at room temperature and its solid-state structure is depicted in Figure 8.4.

Figure 8.4 Solid-state structure of [Na(HPCHO)(db-18c6)(THF)] - disordered atoms indicated by indices, hydrogen atoms represented by spheres with an arbitrary radius - for structural parameters see Table 8.2.

Both the Z-isomer (HP(1a)) and the E-isomer (HP(1b)) of the can be observed in the structure depicted in Figure 8.4. The occupation parameters of both hydrogen atoms in the structure suggest the presence of 25(7)% E-isomer and 75(7)% Z-isomer in the crystal. The ratio of integrals of the signals of the formyl hydrogen atoms observed by 1H-NMR spectroscopy is

117 however 60:40, indicating that the ratio of diastereomers differs between solution and solid state. The effect of the crown ether complexation in the solid state can be assessed by comparison of the average structural parameters of 62(DME)2 with those of the db-18c6 adducts of Na(HPCHO) (62(db-18c6)THF) and Na(OCP) (63(db-18c6)THF[135]) (see Table 8.2).

[135] Table 8.2 Interatomic distances and angles for 62(DME)2, 62(db-18c6)THF and 63(db-18c6)THF – average values for 62(DME)2.

[135] 62(DME)2 62(db-18c6)THF 63(db-18c6)THF d(Na(1)-O(1)) 2.29 Å 2.281(3) Å 2.290(2) Å d(O(1)-C(1)) 1.31 Å 1.213(6) Å 1.207(4) Å d(C(1)-P(1)) 1.69 Å 1.700(5) Å 1.582(3) Å ⦟(P(1)-C(1)-O(1)) 125.7 ° 136.1(4) ° 179.4(3) ° ⦟(Na(1)-O(1)-C(1)) 132.9 ° 127.1(3) ° 138.1(2) ° ⦟(O(1)-Na(1)-O(O)) – 175.7(2) ° 169.8(9) °

While the coordination of the sodium ion by db-18c6 causes a decrease of the Na-O bond length in Na(OCP), this parameter is largely unaffected in Na(HPCHO). The opposite is the case for the C-O bond lengths of both compounds – a decrease is observed for Na(HPCHO), while no change is observed for Na(OCP). The P-C bond length of both anions is not affected by coordination of sodium to db-18c6. It is unclear if the change in bond lengths is best attributed to a change of electronic structure or rather due to steric repulsion between the crown ether and the anion. Due to the success of the complexation of Na(OCP) with 1,4-dioxane[132], the same reaction was performed with crude [Na(HPCHO)(DME)2]2. In contrast to Na(OCP), Na(HPCHO) cannot be precipitated out of solution with 1,4-dioxane. On the other hand, dissolving crude [Na(HPCHO)(DME)2]2 in 1,4-dioxane allows for precipitation of a pale yellow solid upon addition of n-hexane to the mixture. Unfortunately, this precipitate proves to be almost insoluble even in polar organic solvents, such as THF, DME, DMF or propylene carbonate. The limited solubility does still allow for the recording of NMR-spectra in THF-d8, but not for a reliable determination of the alkoxide content or the amount of coordinated dioxane, since the samples are always suspensions. Elemental analysis of the dioxane adduct reveals relatively low amount of carbon present in the sample. The measured weight percentages for carbon (25.8%) and hydrogen (4.57%) indicate ratio of formphosphide to dioxane of approximately one to three, which corresponds to a carbon content of 24.72% and hydrogen content of 4.15%. Analysis of the dioxane-adduct by solid-state MAS NMR reveals the presence of low amounts of alkoxides (see Figure 8.5), which likely explains the difference in carbon/hydrogen content. The low amount of 1,4-dioxane present in the sample, likely explains its low solubility. In the absence of coordinating ethers, the ions are likely tightly packed and/or form a coordination polymer in order to increase the coordination number of the sodium ions. The preference of Na+ to be coordinated by a large number of oxygen atoms is evidenced by the solid-state structures of [Na(HPCHO)(DME)2]2 (see Figure 8.3) and the solid-state structure of Na(HPCHO)db-18c6(THF) (see Figure 8.4).

118

13 Figure 8.5 Solid-state MAS (10kHz) C-NMR spectrum of Na(HPCHO)(dioxane)2, spinning sidebands starred.

8.3.2 Reactivity of Na(HPCHO) With isolated Na(HPCHO) available in form of the complexes with 1,4-dioxane and db-18c6, its reactivity could be explored. The first observation was the higher sensitivity of Na(HPCHO) towards air. While Na(OCP)dioxane2.5 can be briefly handled in air, Na(HPCHO)dioxane0.33 immediately catches fire upon contact with air. The db-18c6-adduct is not pyrophoric, but decomposition takes also place immediately upon contact with air. As expected, decarbonylation can be observed upon reaction with electrophiles such as alkyl 2 halides, acyl chlorides and carbamoyl chlorides, as evidenced by the loss of the JPH-coupling to the formyl group in the 31P-NMR spectrum. In these cases, a mixture of products was observed by 31P-NMR spectroscopy, which was not further analyzed. A defined product could be observed upon reaction with triphenyltin chloride, which upon decarbonylation led to the formation of tris(triphenylstannyl)phosphine, as evidenced by 31P-NMR (δ = -325 ppm, JPSn = 886 Hz). Reaction with silyl chlorides on the other hand leads to formation bis(siloxy)methylphosphines 64a, 64b, and 64c (see Scheme 8.10).

Scheme 8.10 Formation of bis(siloxy)methylphosphines from Na(HPCHO). The methyl groups of 64a, 64b, and 64c are likely to form by hydrogenation of the formyl group of Na(HPCHO), since in the formation of 64c no other methyl-sources are introduced by the silyl chloride. This in turn means that the stoichiometry of the reaction depicted in

119

Scheme 8.10 is not easily determined. This complicates the determination of the yield of these compounds. Since the formyl moiety is not preserved in this reaction, no further investigations were performed. Na(HPCHO) reacts with activated dienes such as phorone and DBA. In the case of phorone same hydrophosphination product (65) could be observed, which was reported by R. Suter in [143] the Michael reaction of Na(PH2) with phorone (see Scheme 8.11).

Scheme 8.11 Na(HPCHO) acting as masked NaPH2 in a phospha-Michael addition. The hydrophosphination of phorone using Na(HPCHO) proceeds within minutes, which is considerably faster than the hydrophosphination using Na(PH2) in DME (t = 16 h at 80 °C). The synthesis of 65 is however still likely better accomplished via the optimized synthetic route in DMSO, developed by R. Suter[143], due to the high sensitivity of Na(HPCHO) compared to Na(PH2). The only reaction of Na(HPCHO) that was found to preserve the formyl moiety is the reaction with triphenylborane. While addition of BPh3 without base leads to an observable broadening of the signals of Na(HPCHO) in the 31P-NMR, the addition of base causes the appearance of a new singlet at a chemical shift of δ = 156.8 ppm. This signal belongs to dimer 66, which is formed in an oxidative coupling reaction (see Scheme 8.12).

Scheme 8.12 Lewis-acid induced oxidative coupling of Na(HPCHO) by BPh3. Since an excess of formphosphide and triphenylborane is needed for full consumption of the base, it is reasonable to assume, that the reaction depicted in Scheme 8.12 is a formal disproportionation of the formphosphide anion to 66 and unidentified reduction products. Since the formation of precipitate is observed during the reaction, it is reasonable to assume that the reduced species cannot be observed by NMR-spectroscopy due to their low solubility in DME. Single crystals of 66 could be isolated from the reaction mixture by cooling and the obtained solid-state structure is depicted in Figure 8.6. The formed adduct seems to be metastable, since dissolving the solid residue in THF leads to reformation of Na(HPCHO). Such metastability has been observed for products of the reaction between Na(OCP) and triphenylborane.[136]

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Figure 8.6 Solid-state structure of 66, solvated sodium ions omitted for clarity. Atoms generated by symmetry are starred (*) - for selected structural parameters see Table 8.3. Compound 66 can be regarded as a 2,3-phosphabutadienenolate similar to the neutral, silylated phosphadienolate (67) prepared by Appel and co-workers in 1983 (see Scheme 8.13), as evidenced by the similar 31P-NMR chemical shifts of both compounds (δ(66) = 156.8 ppm, δ(67) = 132.2 ppm).[144] Both compounds adopt the same configuration, with both double bonds being in the Z-conformation.

Scheme 8.13 Sylilated diphosphadiene 67 reported by Appel.

Table 8.3 Selected interatomic distances and angles of 66 and 67 – average values for 67. 66 67[144] (M = B) (M = Si) d(P(1)-C(1)) 1.716(6) Å 1.69 Å d(C(1)-O(1)) 1.283(6) Å 1.36 Å d(O(1)-M) 1.587(6) Å 1.66 Å d(P(1)-P(1)*) 2.188(3) Å 2.171(5) Å ⦟(P(1)-C(1)-O(1)) 130.7(5) ° 127 ° ⦟(C(1)-O(1)-M) 119.8(5) ° 133 ° ⦟(P(1)*-P(1)-C(1)) 100.6(2) ° 100.5 °

The C-P bonds in compound 66 are slightly elongated compared to those in compound 67, while the C-O bonds are shortened. However, 66 can still be regarded as a diene system

121 incorporating two C-P double bonds, since the bond lengths are still well within the typical range of P-C double bonds.[72] The diene system does therefore not differ significantly between the two compounds, which indicates that compound 66 is likely to display the same hindered rotation around the P-P bond and subsequent unreactivity in pericyclic reaction, which has been observed for compound 67.[144] However, the ionic character of 66 opens up the possibility of substitution by electrophiles. 8.3.3 Thermal decomposition of Na(HPCHO)

Heating samples of Na(HPCHO)(dioxane)0.33 and Na(HPCHO)db-18c6(THF) leads to decomposition at temperatures around 100°C. However, the aspect of these decompositions differ significantly. While the crown ether adduct melts and turns orange, the dioxane adduct decomposes violently under gas and foam formation (see Figure 8.7). To probe the differences between the decomposition of these two compounds, both were analyzed via thermogravimetry (see Figure 8.8).

Figure 8.7 Before and after decomposition of Na(HPCHO)dioxane0.33 (left) and Na(HPCHO)db-18c6(THF) (right).

Figure 8.8 TGA curves of Na(HPCHO)DME0.33 (black) and Na(HPCHO)(db-18c6)THF (green).

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The first mass loss step of Na(HPCHO)db-18c6(THF) can be attributed to the loss of THF, as evidenced by the observation of the corresponding signal (m/z = 72) by mass spectrometry. Whether or not HCP is formed during the decomposition of Na(HPCHO) is difficult to determine using the TGA/DSC set-up, since the IR-frequency reported for HCP -1 [140] (νCP = 1267 cm ) is a frequency commonly observed for organic substances such as ethers -1 [145] (νCOC st as = 1310 - 1000 cm ) . The same holds true for the signal observable by mass spectrometry (m/z = 44), which also corresponds to a common fragment of cyclic ethers such +∙ [145] as 1,4-dioxane (C2H4O ). Therefore, the decomposition products were analyzed by NMR-spectroscopy instead. Condensation of the products released upon decompositionn in vacuo onto frozen CD2Cl2 allowed for such an analysis. While 1H-NMR spectroscopy allowed the detection of dioxane and db-18c6 in the decomposition products of the respective adducts, 31P-NMR spectroscopy allowed for identification of phosphorus containing species, which unfortunately did not contain a significant amount of HCP. However, as expected from the different aspect of the samples (see Figure 8.7), the products differ for both adducts. The decomposition of Na(HPCHO)(dioxane)0.33 yields methylphosphine, dimethylphosphine and phosphine as evidenced by 31P-NMR spectroscopy, while the decomposition of Na(HPCHO)db-18c6(THF) gives rise to additional products some of which only phosphirane could be identified by 31P-NMR spectroscopy (see Figure 8.9).

Figure 8.9 31P{1H}-NMR spectrum of the condensed decomposition products of Na(HPCHO)dioxane0.33 (top) and Na(HPCHO)db-18c6(THF) (bottom).

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In the 31P-NMR spectrum of the decomposition products of Na(HPCHO)db-18c6(THF) a signal can be observed at δ = -31.8 ppm, which is very close to the chemical shift reported for [139] 1 HCP (δ = -32.0 ppm in CDCl3 ). The corresponding signal in the H-NMR spectrum 2 [139] (δ = 2.80 (d, JPH = 44 Hz) ppm ) could however not be observed. Therefore the peak at δ(31P) = -31.8 ppm cannot conclusively be assigned to HCP. The residues of the decompositions were analysed by solid-state NMR spectroscopy. Interestingly, the solid-state 13C- and 31P-NMR of both residues exhibit similiar signals. The 13C-NMR spectra of both residues exhibit only broad signals, with the signal of the db-18c6 adduct being larger due to the higher carbon content. The 31P-NMR spectra on the other hand consist of a single peak at 13.4 ppm (see Figure 8.10).

Figure 8.10 Solid-state MAS-NMR spectra of the solid residues after decomposition of Na(HPCHO)dioxane0.33 (left, 10 kHz) and Na(HPCHO)db-18c6(THF) (right. 20 kHz). The analysis of the residues suggests that the residual phosphorus resides as a phosphate or phosphonate, due to the chemical shift close to zero[145] and the low amount of spinning sidebands observable, which indicates a high symmetry of the molecules. The formation of oxidized phosphorus species is corroborated by washing of the residues with D2O. The 31P-spectrum of these washings exhibits several peaks between 10 ppm and 0 ppm, indicating the formation of different phosphoric acid derivatives. Since both adducts of Na(HPCHO) exhibit the same residual decomposition product, it is reasonable to assume that the decomposition proceeds similarly in both adducts. The observed decomposition product, which is most similar to HPCHO- and therefore likely the first product formed, is methylphosphine. It can be regarded as a deoxygenated and fully hydrogenated form of HPCHO-. Since the ratio of gaseous decomposition products differ between the two adducts, their formation is likely depended on the concentration of formphosphide in the solid. A high

124 concentration of formphosphide leads to formation of methylphosphine and its disproportionation products (dimethylphosphine and PH3). The dibenzo-18-crown-6-ether adduct contains less HPCHO- and the presence of phosphirane in the decomposition products indicates that dehydrogenation of dimethylphosphine takes place, likely to compensate for the lack of HPCHO- as proton source. This in turn indicates that the gaseous products stem from a disproportionation of the formphosphide anion, forming the reduced methylphosphines and the oxidized phosphonates/phosphates. In order to exclude the complexing ether molecules as source of the hydrogen atoms, deuterium labelled formphosphide was prepared. Decomposition of the dioxane adduct of this deuterated species, yielded products with a high degree of deuteration, corroborating the hypothesis that disproportionation of the formphosphide anion takes place upon decomposition. The formation of other decomposition products could not be observed upon addition of additives to the solid adducts. Tested additives were Na(OH), MgO and Al2O3. In order to assess whether formation of HCP from formphosphide is possible at higher temperatures, a solution of crude Na(HPHCO) in DME was added dropwise onto a molten LiCl/KCl eutectic mixture (m.p. = 352 °C)[146], which led to the formation of the same decomposition products observed for the “regular” decomposition of Na(HPCHO)dioxane0.33 (see Figure 8.9). Change in decomposition products can be observed when thermally decomposing Na(HPCHO)dioxane0.33 in molten phenol, which led to exclusive formation of PH3, likely due to protonation of Na(HPHCO) prior to decomposition. In conclusion, the formation of HCP from Na(HPCHO) via a thermal pathway is not possible. The thermal decomposition of Na(HPCHO) does however highlight once more, that thermal decomposition rarely proceeds via homolytic bond cleavage and rather by disproportionation, similarly to the decomposition processes described in Chapter 7. 8.4 Summary and Outlook Sodium formphosphide was characterized in the solid state in form of an adduct with dimethoxyethane or dibenzo-18-crown-6 ether. The determined structural parameter are consistent with a decreased P-C bond order, when compared to Na(OCP). Complexation by 1,4-dioxane allowed for isolation of Na(HPCHO) in solid form, containing a low amount of complexing ether. Isolation of pure Na(HPCHO) allowed to probe its reactivity. Reaction with alkyl and acyl chlorides, triphenylstannyl chloride and activated dienes led to decarbonylation, causing Na(HPCHO) to act as a masked form of Na(PH2). Reaction with silyl chlorides leads to formation of methylbis(siloxy)phosphines, indicating deoxygenation and hydrogenation of formphosphide taking place. The only reaction, which was found to preserve the formyl moiety of Na(HPCHO) is the oxidative coupling by trihphenylborane in presence of Na(HMDS) as base. Thermal decomposition of Na(HPCHO) was found to proceed via disproportionation, releasing methylphosphine and derivatives thereof as well as phosphates and phosphonates. The ratio of decomposition products depends on the complexing ethers. However, the desired formation of HCP was not observed and could not be induced by additives during the decomposition or higher reaction temperatures.

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The reactions performed in this chapter do certainly not represent a complete investigation of the reactivity of Na(HPCHO). It does however provide an insight into the stability of the P-C double bond present in the HPCHO- anion. The results suggest that for retention of the formyl moiety a substitution at the phosphorus atom is not desirable. Based on the observed formation of compound 66, the stabilization using Lewis acids seems to be the most promising route towards functionalization of the formphosphide anion.[77]

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9 General Outlook The findings described in the preceeding chapters offers promising new possibilities in the field of acylphosphorus chemistry. The main achievement of this work is certainly the development of a scalable, robust and cost- effective synthetic route to acylphosphines bearing one, two or three acyl substituents. This route allows a skilled chemist to synthesize any desired acylphosphine in reasonable quantities without occurring the considerable cost connected to the use of tris(trimethylsilyl)phosphine.[16] This allows for the exploration of the reactivity of acylphosphines on a laboratory scale. However, on a larger-than-laboratory scale the reactions described in this thesis will require optimization. The formation of Na(PH2) from red phosphorus and elemental sodium is still a challenge on an industrial scale, which therefore hinders any industrial application of the syntheses reported in the previous chapters. Currently investigations on the scale-up of the synthesis of Na(PH2) from the elements are being undertaken by Riccardo Conti in the Grützmacher group. An improvement in this reaction step will require a reevaluation of the industrial of the syntheses of acylphosphines reported in this thesis. The explored reactivity of acylphosphines does offer a variety of aspects for further investigation. Reactions, such as the oxidative coupling of mesitoylphosphine (11) and the bis(acyl)phosphines 40, 42 and 45, have been studied in a rather detailed manner. There are however other topics, which can be explored in greater detail, such as: 9.1 The Synthesis of 1,2-azaphospholes The dehydrogenative P-N bond formation leading to the formation of 1,2-azaphospholates Na-7 and Na-8 can potentially be performed using other N-substituted derivatives of methyl anthranilate (5). Compound 5 is a readily available precoursor due to its application in crop protection[147] or as flavoring[148]. It is also easily modified as depicted in Scheme 9.1.[149] [150] [151]

Ph O HN

O Liao [149]

PhI Cu, 180°C Ph O NH2 O HN Ph Ph2CHBr O McKew [150] O EtN(i-Pr)2

5 O

Cy TMSCl O HN [151] NaBH4 O Breinbauer

Scheme 9.1 Examples of possible modifications of 5 leading to new precoursors for 1,2-azaphospholates.

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Bulkier substituents on 5 could potentially lead to easier to crystallize azaphospholates, which would allow for an easier method of purification compared to the silylation performed in the synthesis of TMS-8. Preliminary studies in this direction are currently being performed by Yannick Kürsteiner. 9.2 The phospha-Michael Addition of Primary Acylphosphines While the synthesis of the acylphosphinanone 26 from mesitoylphosphine and DBA has been optimized by changing the catalyst to TMG, there is a lot of room for exploration left concerning this reaction. The synthesis different monoacylphosphines, which represent possible Michael-donors has been described in Chapter 2. Michael-acceptors bearing substituents on the α-carbon atoms adjacent to the carbonyl group are of special interest - for examples, see Scheme 9.2.[152] [153] [154]

Scheme 9.2 Examples of α,α’-substituted dienones, representing possible Michael-acceptors. Phosphinanones derived from the Michael-acceptors depicted in Scheme 9.2 are potentially easier dehydrogenated using DDQ, due to the fact that structurally similar cyclohexanones are easier dehydrogenated using this reagent.[155] 9.3 Acylphosphole Oxides In order to further assess the viability of acylation as a method of modification for phosphole oxide based optoelectronic materials, polymeric compounds containing acylphosphole oxide units will have to be prepared. Phosphole based optoelectronic materials are commonly preferred by electropolymerization of thiol substituted phospholes.[77] Therefore, the next step towards an acylsubstitued polyphosphole compound is likely the replacement of the phenyl substituents on 38 by thiol substituents. The starting point for such a compound is likely the thiol substituted P-phenylphosphole reported by Réau and co-workers in 2000 (see Scheme 9.3)[156]

Scheme 9.3 Thiol substituted P-phenylphosphole reported by Réau and co-workers. Synthesis of an acylated derivative of the compound depicted in Scheme 9.3 should be feasible using the methodology used in the synthesis of 38. If the phosphorus-acyl bond is stable under the conditions used in the electropolymerization, an acylsubstitued polyphosphole oxide will be obtained. 128

9.4 Acylphosphines in the Synthesis of Phosphide-containing Nanoparticles The studies performed in collaboration with the groups of Prof. Kovalenko and Prof. Sanchez have shown the promising properties of acylphosphines in the synthesis of phosphorus- containing nanocrystalline compounds. However, the mechanism of these reactions needs to be understood in order to synthesize acylphosphines geared towards this application. Investigations into these mechanisms are currently underway in both the Kovalenko and the Sanchez group. Another important topic in the field of pnictogen-containing nanoparticles is the incorporation of arsenic instead of phosphorus. While the synthesis of acylarsines has been reported from [157] As(TMS)3 a synthesis using Na(AsH2) similar to those described in this thesis is unlikely to be practical. The preparation of Na(AsH2) from the elements was performed by Goicoechea and co-workers in the synthesis of Na(OCAs) and required much longer reaction times (three days at 70 °C) and addition of one equivalent of 18-crown-6 ether in order to solubilize the [158] formed Na(AsH2). These reaction conditions paired with the increased instability of [157] acylarsines compared to acylphosphines indicate that Na(AsH2) is not a convenient precursor for the synthesis of acylarsines. A more promising route is the synthesis of acylphosphorus compounds containing arsenic. Either by the formation of arsa-phosphacycles from Na(OCP) and bis(amino)chloroarsines, similar to compound 25 or by reacting acylphosphines with AsCl3 as described in the synthesis of 61. These topics are currently under investigation by Grégoire Le Corré and Fabian Müller in the group of Prof. Grützmacher. As summarized in this chapter, the chemistry of acylphosphines offers a lot of room for exploration, which is facilitated by the results of this thesis.

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10 Experimental 10.1 General Remarks All air- or moisture sensitive reactions were carried out under argon using either standard Schlenk techniques or an argon-filled glove box. The glassware was stored in an oven at 130 °C for at least 16 h and cooled under vacuum prior to use. Aqueous solutions were degassed by bubbling with a stream of argon or nitrogen. Solvents were purified using an Innovative Technology PureSolv MD 7 solvent purification system. Deuterated solvents were purchased from Cambridge Isotope Laboratories. THF-d8 and C6D6 were distilled from sodium benzophenone, whereas CD2Cl2 was dried over CaH2 before use. All reagents were used as received from commercial suppliers unless otherwise stated. Solution NMR spectra were recorded on Bruker 250 MHz, 300 MHz, 400 MHz and 500 MHz 1 13 spec-trometers. All H and C chemical shifts are reported in ppm relative to SiMe4 using the 1H and 13C shifts of the solvent as an internal standard. 31P-NMR shifts are reported relative to 85% H3PO4. Solid-state NMR spectra were recorded on Bruker 400 MHz and 500 MHz spectrometers equipped with 2.5 mm and 4 mm two-channel solid-state probe heads. The sample was contained in a 2.5 or 4 mm rotor and spun at 10 or 20 kHz. Melting points were determined as endothermic heat signatures using a Netzsch Jupiter II thermoanalytic set-up using either a TGA-DSC or TGA-DTA sample carrier. TGA measurements were performed on NETZSCH STA449F5 instrument connected to an EI-MS (AEOLUS III). Transmission IR experiments were recorded on a Bruker TENSOR II spectrometer. About 10 mg of the samples was heated from 40°C to 900°C at a rate of 10 K min-1 under argon flow. The line that transfers evolved gases from TGA to IR was maintained at 200°C, while the line that transfers from TGA to MS was maintained at 230°C. The FTIR spectra were recorded from 4000 cm-1 to 600 cm-1 at a resolution of 4 cm-1 with 32 scans per sample. The TGA data obtained during the measurement was analyzed using Netzsch Proteus – Thermal Analysis Software. Single crystals suitable for X-ray diffraction were coated with polybutene oil (Mn ≈ 920 g∙mol−1) in a glovebox and transferred to a Nylon loop. Diffraction studies were performed on an Rigaku Oxford Diffraction XCalibur S or a Bruker X8 APEX2 diffractometer, both equipped with a molybdenum X-ray tube (λ = 0.7107 Å). Preliminary data was collected to determine the crystal system. The data was processed using the corresponding diffractometer software (CrysAlisPro and Apex2) and corrected for absorption with a multi-scan method. The structures were solved using direct methods (ShelXS) or intrinsic phasing (ShelXT) and refined using least square procedures (ShelXL) in Olex2[1].

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10.2 Compounds prepared in Chapter 2 10.2.1 Synthesis of Sodium bis(ethoxycarbonyl)phosphide (3) Elemental sodium (2.4 g, 104 mmol, 3.2 equ.) was added in small chunks to a 250 mL Schlenk flask containing 100 mL dry DME and naphthalene (0.41 g, 3.2 mmol, 0.1 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (1.0 g, 32.3 mmol, 1 equ.) was added in one portion and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (6.1 mL, 4.8 g, 64.5 mmol, 2 equ.) was mixed with 10 mL dry DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. Ethyl cyanoformate (3.2 mL, 3.2 g, 32.3 mmol, 1 equ.) was added dropwise to the reaction mixture. After the addition was finished, the reaction mixture was stirred overnight at room temperature. The reaction mixture was filtered over a protective gas glass frit. The formed Na(OCP) was precipitated by addition of 100 mL dry 1,4-dioxane and removed via filtration. The filtrate was reduced in vacuo until ca. 50 mL of solvent remained. Addition of 100 mL of dry n-hexane caused the precipitation of the product, which was collected via filtration and dried in vacuo. Crystals suitable for X-ray diffraction were grown from a saturated DME solution at –30°C.

Yield: 1.8 g (27.9% based on Pred)

1 3 H-NMR (300 MHz, THF-d8): δ = 3.90 (q, JHH = 7.1 Hz, 4H, 3 13 1 CH2), 1.10 (t, JHH = 7.1 Hz, 6H, CH3) ppm. C{ H}-NMR 1 (75 MHz, THF-d8): δ = 196.5 (d, JPC = 66.5 Hz, CO), 54.4 ppm (d, 3 31 1 JPC = 7.6 Hz, CH2), 12.5 (s, CH3) ppm. P{ H}-NMR (121 MHz, THF-d8): δ = 34.8 (s) ppm.

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10.2.2 Synthesis of 1-methyl-3-((trimethylsilyl)oxy)-1H-benzo[d][1,2]azaphosphole (TMS-8) Elemental sodium (3.6 g, 156 mmol, 3.2 equ.) was added in small chunks to a 500 mL Schlenk flask containing 200 mL dry DME and naphthalene (0.62 g, 4.8 mmol, 0.1 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (1.5 g, 48.4 mmol, 1 equ.) was added in one portion and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (9.1 mL, 7.2 g, 96.8 mmol, 2 equ.) was mixed with 20 mL dry DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. Methyl N-methylanthranilate (6, 7.1 mL, 8 g, 48.4 mmol, 1 equ.) was added dropwise to the suspension. Immediate gas evolution took place. The excess pressure was relieved by means of the overpressure valve on the Schlenk line. After the addition was finished the reaction was stirred for 2 hours at which point no further gas evolution was observed. The completeness of the reaction was also observable by 31P-NMR spectroscopy. The only observable signal was that of Na-8 at δ = 171.5 ppm. The reaction mixture was cooled to 0°C and chlorotrimethylsilane (18.5 mL, 15.8 g, 145 mmol, 3 equ.) was added dropwise. After completion of the addition the reaction mixture was stirred for one hour. 100 mL of dry n- hexane was added to the reaction mixture, which was subsequently filtered over a protective gas glas filter frit. The volatiles of the filtrate were removed in vacuo. Should any precipitate occur after this step, n-hexane has to be added again to extract the product. The crude product is an orange oil, which can be purified by fractional distillation (b.p. 140°C at 0.1 mbar). It is advisable to remove any present naphthalene from the product by sublimation first. Otherwise naphthalene will clog the distillation apparatus. The product was collected as a yellow, air sensitive liquid. Yield: 7.5 g (65% based on P ) red TMSO 1H-NMR (300 MHz, CD Cl ): δ = 7.52 (dq, 3J = 8.15 Hz, 2 2 HH C C3 5 1 JHH = 0.80 Hz, 1H, C6H), 7.22-7.12 (m, 2H, Harom), 7.00-6.94 (m, C2 C4 3 P 2H, Harom), 3.60 (d, JPH = 8.46 Hz, 3H, NCH3), 0.26 (d, 13 1 C7 C5 JPH = 0.62 Hz, 9H, Si(CH3)3) ppm. C{ H}-NMR (75 MHz, N C 1 2 6 CD2Cl2): δ = 182.4 (d, JPC = 34.7 Hz, C1), 145.1 (d, JCP = 3.1 Hz, 2 3 CN C2), 129.3 (d, JCP = 3.0 Hz, C7), 125.5 (d, JCP = 5.7 Hz, Carom), 4 3 4 119.3 (d, JCP = 2.7 Hz, Carom), 118.2 (d, JCP = 8.7 Hz, Carom), 111.1 (d, JCP = 2.7 Hz, Carom), 2 29 1 33.7 (d, JCP = 22.7 Hz, CN), 0.0 (d, JCP = 4.0 Hz, CTMS) ppm. Si{ H}-NMR (50 MHz, 31 1 31 CD2Cl2): δ = 23.7 (s) ppm. P{ H}-NMR (121 MHz, CD2Cl2): δ = 165.5 (s) ppm. P-NMR 3 (121 MHz, CD2Cl2): δ = 165.5 (q, JPH = 8.5 Hz) ppm.

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10.2.3 Synthesis of Primary Acylphosphines Elemental sodium (2.3 g, 10 mmol, 3.1 equ.) was added to a 100 mL Schlenk flask containing 40 mL DME and naphthalene (0.41 g, 3.2 mmol, 0.1 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (1.0 g, 3.2 mmol, 1 equ.) was added in one portion and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (3.1 mL, 2.4 g, 3.2 mmol, 1 equ.) was mixed with 10 mL dry DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. The corresponding methyl benzoate (1 equ.) was added dropwise to the suspension, which was subsequently stirred for 2 hours at room temperature. Reaction control by 31P-NMR spectroscopy showed only the corresponding sodium benzoylphosphides present in solution (see Table 10.1).

31 1 Table 10.1 Sodium monoacylphosphides: P–NMR chemical shifts δ and JPH coupling constants in DME.

Formic acid (3.7 mL, 4.5 g, 9.7 mmol, 3 equ.) was added dropwise to the stirred suspension, which was subsequently stirred for 30 min. Reaction control by 31P-NMR spectroscopy showed only the corresponding benzoylphosphines present in solution (see Table 10.2.).

31 1 Table 10.2 Primary acylphosphines: P–NMR chemical shifts δ and JPH coupling constants in DME.

The reaction volume was reduced to roughly one third and n-hexane (50 mL) was added to precipitate any side products. All solids were removed by filtration and any volatiles present in the filtrate were removed in vacuo, leaving behind the crude primary acylphosphine as a yellow oil in a yield of 60-70%, which was usually used directly after isolation. The only exception is (2,6-dimethoxybenzoyl)phosphine (14), which was crystallized at -78°C as a colorless, temperature-sensitive solid.

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Primary Acylphosphines - Additional Spectral Data:

1 (2-phenylbenzoyl)phosphine (13): H-NMR (300 MHz, CDCl3): δ = 8.15 – 6.72 (m, 10H, 1 13 1 Harom), 3.70 (d, JPH = 213.2 Hz, 2H, PH2) ppm. C{ H}-NMR (CDCl3, 75.5 MHz): δ = 209.2 1 2 (d, JPC = 30.1 Hz, CO), 136.0 (d, JPC = 19.6 Hz, Cipso), 133.3 (s, Carom), 132.4 (s, Carom), 124.5- 118.3 (m, Carom) ppm.

1 (2,6-dimethoxybenzoyl)phosphine (14): H-NMR (300 MHz, CDCl3): δ = 7.23 (t, 3 4 3 4 JHH = 8.1 Hz, JHH = 1.7 Hz, 1H, Harom), 7.58 (dt, JHH = 8.0 Hz, JHH = 1.6 Hz, 1H, Harom), 1 13 1 7.42-7.30 (m, 2H, Harom), 4.05 (d, JPH = 212.7 Hz, 2H, PH2), ppm. C{ H}-NMR (CDCl3, 2 75.5 MHz): δ = 157.0 (s, CaromOMe), 131.9 (d, JPC = 24.5 Hz, Cipso), 104.3 (m, Carom), 56.0 (s, OCH3) ppm.

1 (2-hydroxybenzoyl)phosphine (15): H-NMR (300 MHz, CDCl3): δ = 11.31 (s. br, 1H, OHO), 3 4 3 4 77.23 (t, JHH = 8.1 Hz, JHH = 1.7 Hz, 1H, Harom), 7.58 (dt, JHH = 8.0 Hz, JHH = 1.6 Hz, 1H, 1 13 1 Harom), 7.42-7.30 (m, 2H, Harom), 4.14 (d, JPH = 223.7 Hz, 2H, PH2) ppm. C{ H}-NMR 1 3 (CDCl3, 75.5 MHz): δ = 217.8 (d, JPC = 31.0 Hz, CO), 160.0 (d, JPC = 3.5 Hz, CaromO), 131.7 2 (d, JPC = 9.7 Hz, Cipso), 129.9 (s, Carom), 119.4-117.6 (m, Carom) ppm.

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10.3 Compounds from Chapter 3 10.3.1 Synthesis of Mesitoyl-2,6-diphenylphosphinanone (26) DBA (1.2 g, 5 mmol, 1 equ.) and mesitoylphosphine (11, 1 g, 5 mmol, 1 equ.) were added to a 100 mL Schlenk flask containing 50 mL THF. Triethylamine (0.1 mL, 0.75 mmol, 0.15 equ.) was added and the solution was stirred for 1.5d*. Volatiles were removed under reduced pressure, leaving behind the crude product as a light yellow oil, which was recrystallized from Et2O/Hexane.

* The reaction can also be performed with TMG as base instead of NEt3, which lowers the reaction time to 30 min, this however renders the obtained product much harder to crystallize.

Crystals suitable for single crystal X-ray diffraction were grown from a saturated Et2O solution at -30°C. Yield: 1.42 g (68.6% based on 11)

1 H-NMR (CDCl3, 300 MHz): δ = 7.60-6.80 (13H, m, CHarom.), 3.65- 3.55 (2H, m, CH), 3.10-2.8.2 (4H, m, CH2), 2.13 (3H, s, CH3), 1.97 13 1 (6H, s, CH3) ppm. C{ H}-NMR (CDCl3, 75.5 MHz): δ = 226.4 (d, 1 JCP = 57.0 Hz, COMes), 209.9 (s, CO), 143-126 (m, Carom), 44.6 (d, 2 1 JCP = 33.5 Hz, CH2), 37.8 (d, JCP = 57.9 Hz, CH), 37.5 (d, 1 31 1 JCP = 65.0 Hz, CH), 21.0 (s, CH3) ppm. P{ H}-NMR (CDCl3, 31 121.5 MHz): δ (ppm) = 28.3 (s) ppm. P-NMR (CDCl3, 121.5 MHz): δ (ppm) = 28.3 (s) ppm.

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10.3.2 Synthesis of Mesitoyl-2,6-diphenylphosphinanone Oxide (27) Mesitoyl-2,6-diphenylphosphinanone (26,1 g, 2.3 mmol, 1 equ.) was added to a 50 mL Schlenk flask containing 20 mL Toluene and a solution of tert-butylhydroperoxide in decane (5.0-6.0 M, 0.5 mL, 1 equ.) was added dropwise. After stirring for 15 min, all volatiles were removed in vacuo and the product was precipitated by stirring the resulting oil with 10 mL of Et2O overnight. The colorless product was collected by filtration and dried in vacuo. Crystals suitable for single crystal X-ray diffraction were grown from a saturated THF solution at -30°C. Yield: 0.8 g (80.8% based on 26) Melting Point (DSC): 136.0°C

1 H-NMR (300 MHz, CDCl3): δ = 7.30-7.15 (m, 10H, Harom), 6.56 (s, 2H, Harom), 3.90 (m, 2H, CH2), 2.49 (m, 2H, CH2), 3.23 (m, 1H, CH), 2.99 (m, 1H, CH), 2.10 (s, 3H, p-CH3), 1.63 (s, 6H, o-CH3) ppm. 13 1 1 C{ H}-NMR (126 MHz, CD2Cl2): δ = 218.2 (d, JCP = 61.2 Hz, 4 COMes), 206.3 (d, JCP = 3.6 Hz, CO), 143-127 (m, Carom), 20.8 (s, p- 31 1 CH3), 19.0 (s, o-CH3) ppm. P{ H}-NMR (203 MHz, CD2Cl2): 31 δ = 28.6 (s) ppm. P-NMR (203 MHz, CD2Cl2): δ = 28.6 (m) ppm. FTIR: ν = 2921, 1720, 1653, 1608, 1495, 1450, 1424, 1377, 1297, 1255, 1205, 1176, 1143, 1078, 1033, 895, 849, 802, 765, 735, 696, 608, 580, 553, 533, 500, 480, 437 cm-1.

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10.3.3 Synthesis of Tetrakis(mesitoyl)cyclotetraphosphine (30) Mesitoylphosphine (11, 3.6 g, 20 mmol, 1 equ.) was added to a 250 mL Schlenk flask containing 100 mL THF. Hexachloroethane (4.73 g, 20 mmol, 1 equ.) was added in one portion. Triethylamine (5.6 mL, 4.04 g, 40 mmol, 2 equ.) was slowly added dropwise. The solution became turbid and formation of a white precipitate was observed. The suspension was stirred for 16 hours at room temperature before removing all volatiles in vacuo. Diethylether (50 mL) was added to the residue in order to precipitate the product as well as the formed Net3∙HCl. The solids were filtered of and washed twice with deionized water and once with diethylether. The product was isolated as a pale yellow solid, which was dried overnight in vacuo at 150°C. Crystals suitable for single crystal X-ray diffraction were grown from a saturated THF solution at -30°C.

Yield: 0.53 g (15% based on 11) O Mes Melting Point (DTA): 262.2°C (decomp.) Mes O 1 P P H-NMR (300 MHz, CDCl3): δ = 6.63 (s, 8H, CH), 2.25 (s, 13 1 12H, p-CH3), 1.92 (s, 24H, o-CH3) ppm. C{ H}-NMR (75 MHz, CDCl ): δ = 223.5 (m, CO), 138.7 (s, C ), 135.4 P P 3 para O Mes (m, Cipso), 132.7 (s, Cortho), 127.7 (s, CH), 20.2 (s, p-CH3), 18.8 (quin, J = 3.3 Hz, o-CH ) ppm. 31P{1H}-NMR (121 MHz, PC 3 Mes O 31 CDCl3): δ = -44.7 (s) ppm. P-NMR (121 MHz, CDCl3): δ = -44.7 (s) ppm. FTIR: ν = 2915, 1655, 1448, 1377, 1294, 1201, 1139, 1032, 955, 865, 843, 725, 676, 616, 543, 412 cm-1.

138

10.3.4 Synthesis of Mesitoyl-N,N-dimethylimidazoliumphosphinidene (32) N,N-dimethylimidazolium iodide (0.9 g, 4 mmol, 4 equ.) was added to a 50 mL Schlenk flask containing 30 mL THF. Potassium tert-butoxide (0.45 g, 4 equ.) was added in one portion and the reaction mixture was stirred for 30 min at room temperature. Tetrakis(mesitoyl)cyclotetraphosphine (30, 0.71 g, 1 mmol, 1 equ.) was added in one portion and the reaction was stirred overnight at room temperature. The product precipitates from THF, was isolated by filtration as a light brown solid and subsequently dried in vacuo. Yield: 0.8 g (81% based on 30)

1H-NMR (300 MHz, CDCl3): δ = 7.18 ppm (s, 2H, Harom), 6.85 (s, 2H, CHNHC), 4.02 ppm (s, 6H, NCH3), 2.48 (s, 3H, 13 1 o-CH3), 2.32 (s, 6H, p-CH3) ppm. C{ H}-NMR (75 MHz, 1 CDCl3): δ = 167.5 (d, JCP = 25.9 Hz, NCN), 137.9 (s, 1 Carom), 134.6 (s, Carom), 128.3 (d, JCP = 29.5 Hz, Cipso), 124.5 (s, Carom), 123.5 (s, Carom), 122,1 (s, CNHCH), 36.7 (s, NCH3), 21.1 (s, o-CH3), 19.7 (s, p- 31 1 31 CH3) ppm. P{ H}-NMR (121 MHz, CDCl3): δ = 0.0 (s) ppm. P-NMR (121 MHz, CDCl3): δ = 0.0 (s) ppm.

139

10.3.5 Synthesis of Tetrakis(ortho-phenylbenzoyl)cyclotetraphosphine (34) (Ortho-phenylbenzoyl)phosphine (13, 15.6 g, 73 mmol, 4 equ.) was added to a 500 mL Schlenk flask containing 200 mL Et2O. Hexachloroethane (17.3 g, 73 mmol, 4 equ.) was added in one portion. Triethylamine (20.5 mL, 15.0 g, 146 mmol, 8 equ.) was slowly added dropwise, which caused the formation of a white precipitate. After the addition was completed, the suspension was stirred overnight at room temperature. The precipitate was removed via filtration and the volatiles in the filtrate were removed in vacuo. The yellow, oily residue was taken up in a minimal amount of Et2O (~20 mL). n-Hexane (~80 mL) was added to the solution, which caused precipitation of a sticky lump of product, which turned into a powder upon stirring for 6 hours at room temperature. The product was collected via filtration and dried in vacuo. Yield: 7.5 g (50% based on 13) Melting Point (DSC): 75.8°C

1 H-NMR (300 MHz, CDCl3): 7.55-6.60 (m, 36H, Harom) ppm. 13 1 C{ H}-NMR (125 MHz, CDCl3): 217.7 (m, 4C, CO), 141- 31 1 139 (m, Carom), 132-126 (m, Carom) ppm. P{ H}-NMR 31 (121 MHz, CDCl3): δ = -39.0 ppm (s). P-NMR (121 MHz, CDCl3): δ = -39.0 ppm (s). FTIR: ν = 3056, 1633, 1589, 1577, 1470, 1303, 1252, 1191, 1100, 1073, 1024, 998, 890, 801, 769, 745, 698, 668, 643, 614, 532, 495, 429 cm-1.

140

10.4 Compounds from Chapter 4 10.4.1 Synthesis of Mesitoyl-2,5-diphenylphosphole (37) 1,2,5-triphenylphosphole[85] (3.1 g, 10 mmol, 1 equ.) was added to a Schlenk flask containing 50 mL THF and elemental sodium (0.4 g, excess). The stirred reaction mixture was refluxed for two hours. After the reaction had cooled to room temperature, tert-butyl chloride (1.1 mL, 0.92 g, 1 equ.) was added in one portion and the reaction mixture was stirred for 30 min. The formed NaCl was removed by cannula filtration. Mesitoyl chloride (1.8 g, 1.7 mL, 10 mmol, 1 equ.) was added to the stirred solution, which turned bright yellow at the end of addition. All volatiles were removed under reduced pressure and the residue was suspended in 50 mL toluene. The precipitated NaCl was removed by cannula filtration and the filtrate was reduced in vacuo to about 10 mL. Addition of n-hexane (40 mL) caused precipitation of the product, which was collected via filtration and dried in vacuo. Crystals suitable for single crystal X-ray diffraction were grown from a saturated solution in diethyl ether at -30°C. Yield: 2.5 g (65% based on 1,2,5-triphenylphosphole) Melting Point (DSC): 128.5°C

1 H-NMR (300 MHz, CDCl3): δ = 7.40-7.05 (m, 12H, Harom), 6.28 (s, 13 1 2H, CHMes), 1.95 (m, 9H, CH3) ppm. C{ H}-NMR (75 MHz, 1 CDCl3): δ = 214.1 (d, JPC = 47.0 Hz, CO), 143.0 (d, JCP = 1.8 Hz, Carom), 140 (d, JCP = 28.6 Hz, Carom), 138.7 (s), 136.2 (d, JCP = 16.6 Hz, Carom), 132.0 (s), 131.9 (d, JCP = 7.8 Hz, Carom), 128.3 (s, Carom), 128.0 (s, Carom), 127.4 (d, JCP = 8.4 Hz, Carom), 127.1 31 1 (s, Carom), 21 (s, p-CH3), 19.1 (d, JCP = 3.3 Hz, p-CH3) ppm. P{ H}-NMR (75 MHz, CDCl3): 31 41.0 (s) ppm. P-NMR (75 MHz, CDCl3): 41.0 (s) ppm. FTIR: ν = 2913, 1658, 1573, 1486, 1442, 1376, 1262, 1200, 1138, 1075, 1030, 909, 861, 838, 754, 721, 687, 671, 617, 579, 562, 531, 477, 422 cm-1.

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10.4.2 Synthesis of Mesitoyl-2,5-diphenylphosphole Oxide (38) Mesitoyl-2,5-diphenylphosphole (37, 1.9 g, 5 mmol, 1 equ.) was added to a 50 mL Schlenk flask containing 25 mL toluene. Hydrogen peroxide (30% in H2O, 0.5 mL, 0.55 g, 5 mmol, 1 equ.) was added dropwise and the solution was stirred overnight under exclusion of light. All volatiles were reduced under reduced pressure. The product was precipitated by addition of diethyl ether (30 mL) and collected via filtration under air. Yield: 1.4 g (70.3% based on 37) Melting Point (DSC): 115.2°C

1 H-NMR (300 MHz, CDCl3): δ = 7.40-7.05 (m, 12H, Harom), 6.28 (s, 13 1 2H, CHMes), 1.95 (m, 9H, CH3) ppm. C{ H}-NMR (75 MHz, 1 CDCl3): δ = 213.9 (d, JPC = 58.0 Hz, CO), 139.7 (s, Carom), 135.8 (s, Carom), 135.5 (d, JCP = 27.1 Hz, Carom), 129.2 (s, Carom), 128.9 (s, Carom), 128.7(s, Carom), 126.8 (d, JCP = 5.8 Hz, Carom), 20.3 (s, o-CH3), 19.6 (s, p-CH3) ppm. 31 1 31 P{ H}-NMR (75 MHz, CDCl3): 41.0 (s) ppm. P-NMR (75 MHz, CDCl3): 41.0 (s) ppm.

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10.4.3 Synthesis of Mesitoyl-2,3,4,5-tetraethylphosphole oxide (39) Zirconocene dichloride (4.4 g, 15 mmol, 1 equ.) was added to a 100 mL Schlenk flask containing 50 mL THF. The solution was cooled to -78 °C and a solution of nBuLi in hexane (1.6 M, 18.8 mL, 30 mmol, 2 equ.) was added dropwise through a septum via syringe. The mixture was stirred for 20 min at -78 °C before 3-hexyne (3.4 mL, 2.5 g, 30 mmol, 2 equ.) was added through the septum via syringe. The mixture was stirred for 10 min at -78 °C before removing the cooling bath and letting the reaction warm up over the course of 1 hour. The reaction mixture was cooled to 0 °C and PCl3 (1.3 mL, 2.1 g, 15 mmol, 1 equ.) was added via syringe. The reaction was allowed to warm up to room temperature and stirred for 30 min. All volatiles were removed in vacuo and the residue was suspended in 30 mL n-hexane. After removing all solids by filtration, NMR-spectroscopy confirmed the presence of tetraethyl-1- chlorophosphole as well as its dimer in solution. The solvent was therefore removed in vacuo and the residue dissolved in 50 mL THF. Lithium (0.5 g, 71.4 mmol, excess) was added to the solution, which was stirred for 1 hour. All solids were removed by cannula filtration and mesitoyl chloride (2.5 mL, 2,7 g, 15 mmol, 1 equ.) was added via syringe. The solution was stirred overnight. All volatiles were removed and the residue suspended in 30 mL toluene. All solids were removed via filtration. A solution of tert-butylhydroperoxide in decane (~5.5 M, 2.7 mL, 15 mmol, 1 equ.) was added via syringe and the solution was stirred for 1 h. Addition of 120 mL n-hexane caused precipitation of the product, which was collected via filtration and subsequently dried in vacuo.

Yield: 2.1 g (39.1% based on Cp2ZrCl2)

1 H-NMR (300 MHz, CDCl3): δ = 6.76 (s, 2H, CHarom), 2.26 (s, 6H, o-CH3, Mes), 2.20-2.12 (m, 8H, CH2), 1.93 (s, 3H, p-CH3, Mes), 1.02 3 3 (t, JHH = 7.4 Hz, CH3), 0.91 (t, JHH = 7.6 Hz, CH3) ppm. 13 1 1 C{ H}-NMR (75 MHz, CDCl3): δ = 216.0 (d, JPC = 61.4 Hz, 1 CO), 154.1 (d, JPC = 27.8 Hz, PC), 139-126 (m, Carom), 32-18 (m, 31 1 31 Calkyl) ppm. P{ H}-NMR (75 MHz, CDCl3): 41.3 (s) ppm. P- NMR (75 MHz, CDCl3): 41.3 (m) ppm.

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10.5 Compounds from Chapter 5 10.5.1 Synthesis of Tetrakis(acetyl)diphosphine (41) Elemental sodium (10.0 g, 433 mmol, 3.2 equ.) was added in small chunks to a 1 L Schlenk flask containing 500 mL dry DME and naphthalene (1.72 g, 13.3 mmol, 0.1 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (4.2 g, 136 mmol, 1 equ.) was added in two portions and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (12.8 mL, 10.1 g, 136 mmol, 1 equ.) was mixed with 50 mL dry DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. The suspension was again cooled to 0°C and ethyl acetate (13.2 mL, 11.9 g, 136 mmol, 1 equ.) was added dropwise. The reaction mixture was stirred for 30 min at room temperature, while turning greenish yellow. The suspension was again cooled to 0°C and acetyl chloride (14.5 mL, 16.0 g, 203 mmol, 1.5 equ.) was added dropwise. The reaction mixture was stirred for 30 min at room temperature. At this point, reaction control via 31P-NMR spectroscopy showed only sodium bis(acetyl)phosphide (Na-40) to be the only P-containing species present in solution (δ = 68.5 ppm (s)). The suspension was again cooled to 0°C and formic acid (15.3 mL, 18.7 g, 406 mmol, 3 equ.) was added dropwise. Addition of the acid first caused further precipitation within the suspension, which re-dissolved upon continued addition of acid. The suspension was stirred for 1 hour at room temperature. At this point, reaction control via 31P-NMR spectroscopy showed only bis(acetyl)phosphine (40) to be the only P- containing species present in solution (δ (keto) = 72.0 ppm (s), δ (enol) = 1.0 ppm 1 (d, JPH = 241.7 Hz)). The suspension was again cooled to 0°C and hexachloroethane (16.0 g, 67.7 mmol, 0.5 equ.) was added portionwise. The reaction was stirred for 30 min at room temperature and afterwards 400 mL of dry hexane were added. The suspension was filtered over celite and all volatiles were removed from the filtrate in vacuo (Attention! Do not heat above 50°C! Impurities of trisacetylphosphine decompose above this temperature, which cannot be removed afterwards!). The oily yellow residue was extracted three times with dry hexane and afterwards layered with dry hexane and stored in a freezer overnight. Colorless crystals formed, which could be isolated by removal of the cold hexane via filtration.

Yield: 7.1 g (44.7% based on Pred) Melting Point (DTA): 67.1°C

1 3 H-NMR (300 MHz, CD2Cl2): δ = 2.46 (t, JPH = 2.60 Hz) ppm. 13 1 1 C{ H}-NMR (75 MHz, CD2Cl2): δ = 212.7 (t, JPC = 49.4 Hz, CO), 2 31 1 34.8 (t, JPC = 17.5 Hz, CH3) ppm. P{ H}-NMR (121 MHz, CD2Cl2): 31 δ = 29.4 (s) ppm. P-NMR (121 MHz, CD2Cl2): δ = 29.4 (s). Note: All coupling constants are “virtual coupling constants” to both 31P-nuclei. FTIR: ν = 2975, 1690, 1663, 1406, 1379, 1344, 1246, 1215, 1103, 1081, 1066, 923, 858, 791, 674, 598, 489, 462 cm-1.

144

10.5.2 Synthesis of Tetrakis(mesitoyl)diphosphine (43) Bis(mesitoyl)phosphine (42, 3.3 g, 10 mmol, 2 equ.) was added to a 100 mL Schlenk flask containing 40 mL toluene. Hexachloroethane (1.2 g, 5 mmol, 1 equ.) was added in one portion. Dropwise addition of TMG (1.25 mL, 1.2 g, 10 mmol, 2 equ.) caused the solution to become turbid and a white precipitate formed. After stirring for 2 hours at room temperature, the organic phase was extracted with degassed, deionized water (40 mL). The organic phase was transferred into a separate 100 mL Schlenk flask and the aqueous phase was extracted once with toluene (20 mL). The combined organic phases were reduced to a volume of ~10 mL and n-hexane (30 mL) was added to precipitate the product, which was collected by filtration and subsequently dried in vacuo. The spectral data matched those reported in literature.[40] Yield: 2.4 g (74% based on 42) Melting Point (DSC): 197.2°C 10.5.3 Synthesis of Tetrakis(benzoyl)diphosphine (46) The same procedure as in the synthesis of 43 was performed, substituting 42 for bis(benzoyl)phosphine (45, 2.4 g, 10 mmol, 2 equ.) and substituting TMG for triethylamine (1.4 mL, 1.0 g, 2 equ.). Yield: 1.6 g (67% based on 46) Melting Point (DSC): 124.8°C

1 H-NMR (300 MHz, CDCl3): δ = 8.00-7.00 (m, 20H, Harom) ppm. 13 1 1 C{ H}-NMR (75 MHz, CDCl3): δ = 207.0 (t, JPC = 19.4 Hz, CO), 2 139.7 (t, JPC = 18.1 Hz, Cipso), 134.1 (s, Cpara), 129.2-128.2 (m, 31 1 31 Carom). P{ H}-NMR (121 MHz, CDCl3): δ = 27.7 (s) ppm. P- NMR (121 MHz, CDCl3): δ = 27.7 (s) ppm. FTIR: ν = 2974, 1662, 1637, 1594, 1577, 1446, 1407, 1379, 1360, 1302, 1246, 1204, 1166, 1112, 1066, 998, 944, 878, 858, 780, 769, 677, 650, 633, 623, 500, 465, 430, 417 cm-1.

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10.6 Compounds from Chapter 6 10.6.1 Synthesis of Tris(benzoyl)phosphine (47) From Bis(benzoyl)phosphine (45): Bis(benzoyl)phosphine (45, 2.4 g, 10 mmol, 1 equ.) was added to a 250 mL Schlenk flask containing 100 mL toluene. Triethylamine (1.4 mL, 1.0 g, 2 equ.) was added in one portion causing the suspension to turn a darker shade of red. Benzoyl chloride (1.2 mL, 1.4 g, 10 mmol, 1 equ.) was added dropwise to the stirred suspension, which was subsequently stirred for 2 hours at room temperature. The organic phase was extracted with degassed, deionized water (80 mL). The organic phase was transferred into a separate 100 mL Schlenk flask and the aqueous phase was extracted once with toluene (80 mL). The combined organic phases were reduced to a volume of ~40 mL, which already caused product to precipitate. Addition of n- hexane (50 mL) caused further precipitation of the product, which was collected by filtration and subsequently dried in vacuo. Yield: 3.0 g (86.7% based on 45)

From Na(PH2): Elemental sodium (24.6 g, 1.07 mol, 3.2 equ.) was added in small chunks to a 2 L Schlenk flask containing 800 mL dry DME and naphthalene (2.1 g, 16.7 mmol, 0.05 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (10.4 g, 334 mmol, 1 equ.) was added in several portions and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (31.2 mL, 24.8 g, 334 mmol, 1 equ.) was mixed with 100 mL DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. The suspension was again cooled to 0°C and methyl benzoate (41.7 mL, 45.5 g, 334 mmol, 1 equ.) was added dropwise. The reaction mixture was stirred for 30 min at room temperature, while turning greenish yellow. The suspension was again cooled to 0°C and benzoyl chloride (96.2 mL, 117 g, 835 mmol, 2.5 equ.) was added dropwise. The reaction mixture was stirred overnight at room temperature. THF (400 mL) was added to the suspension and all solids were removed by filtration. Removal of the solvent in vacuo caused the product to begin to precipitate. After the solvent volume was reduced to ~200 mL, n-hexane (400 mL) was added and the product collected by filtration.

Yield: 67.5 g (57.9% based on Pred) Melting Point (DSC): 159.7°C (decomp.)

1 H-NMR(300 MHz, C6D6): δ = 8.02-7.90 (m, 6H, o-CH), 7.12- 13 1 6.87 (m, 9H, Harom). C{ H}-NMR (75 MHz, C6D6): δ = 205.6 1 2 (d, JCP = 33.0 Hz, CO), 140.7 (d, JPC = 35.2 Hz, Cipso), 133.9 31 1 (m, Carom), 130.4 (m, Carom), 128.8 (s, Cpara). P{ H}-NMR 31 (121 MHz, C6D6): δ = 53.7 (s) ppm. P-NMR (121 MHz, C6D6): δ = 53.7 (s) ppm. FTIR: ν = 3062, 1787, 1721, 1632, 1589, 1577, 1486, 1444, 1314, 1277, 1201, 1170, 1111, 1072, 1025, 998, 930, 843, 768, 711, 678, 642, 632, 480 cm-1.

146

10.6.2 Synthesis of Tris(pivaloyl)phosphine (48) Elemental sodium (4.8 g, 209 mmol, 3.1 equ.) was added in small chunks to a 100 mL Schlenk flask containing 50 mL dry DME and naphthalene (0.5 g, 3.9 mmol, 0.19 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (2 g, 64.5 mmol, 1 equ.) was added in one portion and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (6 mL, 4.8 g, 64.5 mmol, 1 equ.) was mixed with 10 mL DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. The suspension was again cooled to 0°C and methyl pivalate (8.6 mL, 7.5 g, 64.5 mmol, 1 equ.) was added dropwise. The reaction mixture was stirred for 30 min at room temperature, while turning greenish yellow. The suspension was again cooled to 0°C and pivaloyl chloride (20.0 mL, 19.6 g, 162 mmol, 2.5 equ.) was added dropwise. The reaction mixture was stirred overnight at room temperature. All solids were removed by filtration. Removal of the solvent in vacuo caused the product to begin to precipitate. After the solvent volume was reduced to ~10 mL, n-hexane (40 mL) was added and the product collected by filtration. The spectral data matched those reported in literature.[13]

Yield: 7.5 g (40% yield based on Pred)

147

10.6.3 Synthesis of Tris(acetyl)phosphine (49) Elemental sodium (10.0 g, 433 mmol, 3.2 equ.) was added in small chunks to a 1 L Schlenk flask containing 500 mL dry DME and naphthalene (1.72 g, 13.3 mmol, 0.1 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (4.2 g, 136 mmol, 1 equ.) was added in two portions and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (12.8 mL, 10.1 g, 136 mmol, 1 equ.) was mixed with 50 mL dry DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. The suspension was again cooled to 0°C and ethyl acetate (13.2 mL, 11.9 g, 136 mmol, 1 equ.) was added dropwise. The reaction mixture was stirred for 30 min at room temperature, while turning greenish yellow. The suspension was again cooled to 0°C and acetyl chloride (29 mL, 32.0 g, 406 mmol, 3 equ.) was added dropwise. The reaction mixture was stirred for 2 h at room temperature. n-Hexane (300 mL) was added and the formed precipitate was removed by filtration. All volatiles of the filtrate were removed in vacuo (Attention: do not heat with anything but a luke-warm water bath). The oily yellow residue was transferred in to a 50 mL Schlenk flask and purified by bulb-to-bulb distillation using a diffusion pump (p = 10-5 mbar). The spectral data matched those reported in literature.[35]

Yield: 10.5 g (48% yield based on Pred) Boiling Point (DSC): 140.5°C (decomp.)

148

10.6.4 Synthesis of (Bis(benzoyl)phosphido)dichlorobis(tetrahydrofurano)titanium(III) (53)

Tris(benzoyl)phosphine (47, 0.35 g, 1 mmol, 1 equ.) and TiCl3(THF)3 (0.36 g, 1 mmol, 1 equ.) were suspended in 5 mL THF in a vial in a glovebox, shaken and left standing for 2 hours. The solution turned green and stored in a freezer -30 °C overnight. Green crystals formed overnight, which were collected by decanting the supernatant solution and subsequent drying in vacuo. Yield: 0.35 g (70% yield based on 47)

EPR: g = 1.9525, aTi = 14.1 G. Ph O O Cl P Ti O Cl Ph O

149

10.6.5 Synthesis of Tris(benzoyl)phosphine oxide (50) A solution of tris(benzoyl)phosphine (47, 1 g, 2.9 mmol, 1 equ.) in 20 mL THF was stirred in a 50 mL Schlenk flask under an atmosphere of oxygen, supplied by means of a balloon. Flushing the flask with O2 caused an immediate increase in temperature of the solution. After stirring for 30 min, all volatiles were removed in vacuo. The sticky residue was suspended in toluene (5 mL) and the product was precipitated by addition of hexane (20 mL). The product was collected via filtration and dried in vacuo. Yield: 0.45 g (43% based on 47)

1 H-NMR(300 MHz, CDCl3): δ = 8.29-7.86 (m, 6H, o-CH), 13 1 7.66-7.30 (m, 9H, Harom). C{ H}-NMR (75 MHz, CDCl3): 1 2 O δ = 199.6 (d, JCP = 55.6 Hz, CO), 135.6 (d, JPC = 46.8 Hz, O Cipso), 129.9 (m, Carom), 129.4 (m, Carom), 128.4 (s, Cpara). 31 1 O P P{ H}-NMR (121 MHz, CDCl3): δ = 22.0 (s) ppm. 31 P-NMR (121 MHz, CDCl3): δ = 22.0 (s) ppm. O FTIR: ν = 2955, 1786, 1720, 1655, 1640, 1591, 1577, 1448, 1223, 1178, 1158, 998, 931, 769, 699, 679, 647, 613, 502, 436 cm-1.

150

10.6.6 Synthesis of Tris(dimethylcarbamoyl)phosphine (54) Elemental sodium (4.6 g, 200 mmol, 3.1 equ.) was added in small chunks to a 250 mL Schlenk flask containing 50 mL dry DME and naphthalene (0.5 g, 3.9 mmol, 0.19 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (2 g, 64.5 mmol, 1 equ.) was added in one portion and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (6 mL, 4.8 g, 64.5 mmol, 1 equ.) was mixed with 10 mL DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. The suspension was again cooled to 0°C and dimethylcarbamoyl chloride (23.7 mL, 25.8 g, 258 mmol, 4 equ.) was added dropwise. THF (100 mL) was added and the reaction mixture was stirred overnight at room temperature. All solids were removed by filtration. Removal of the solvent in vacuo caused the product to begin to precipitate. After the solvent volume was reduced to ~40 mL, n-hexane (40 mL) was added and the product collected by filtration.

Yield: 9.4 g (58.9% yield based on Pred) Melting Point (DSC): 158.0°C

1 H-NMR (300 MHz, CDCl3): δ = 3.02 (d, JPH = 2.0 Hz, 9H, E-CH3), 13 1 2.95(s, 9H, Z-CH3) ppm. C{ H}-NMR (75 MHz, CDCl3): δ = 170.3 31 1 (s, CO), 38.7 (d, JPC = 16.6 Hz, E-CH3), 35.3 (s, Z-CH3) ppm. P{ H}- 31 NMR (121 MHz, CDCl3): δ = 31.5 (s) ppm. P-NMR (121 MHz, CDCl3): δ = 31.5 (s) ppm. FTIR: ν = 2921, 1610, 1483, 1449, 1368, 1246, 1181, 1132, 1093, 1053, 906, 751, 687, 633, 518, 466, 440 cm-1.

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10.6.7 Synthesis of Tris(diphenylcarbamoyl)phosphine (55) Elemental sodium (7.6 g, 330 mmol, 3.1 equ.) was added in small chunks to a 1 L Schlenk flask containing 250 mL dry DME and naphthalene (0.68 g, 5.3 mmol, 0.05 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (3.3 g, 106 mmol, 1 equ.) was added in three portions and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (10 mL, 7.9 g, 32.3 mmol, 1 equ.) was mixed with 10 mL DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. The suspension was again cooled to 0°C and dimethylcarbamoyl chloride (98.6 g, 426 mmol, 4 equ.) dissolved in 150 mL THF was added dropwise. The reaction mixture was stirred overnight at room temperature. All solids were removed by filtration. Removal of the solvent in vacuo caused the product to begin to precipitate. After the solvent volume was reduced to ~100 mL, n-hexane (250 mL) was added and the product collected by filtration.

Yield: 55.3 g (83.8% based on Pred) Melting Point (DTA): 270.3°C

1 H-NMR (300 MHz, CDCl3): δ = 7.35-7.14 (m, 22H, Harom), 7.10- 13 1 7.02 (m, 3H, Harom), 6.89-6.79 (m, 6H, Harom) ppm. C{ H}-NMR (75 MHz, CDCl3): δ = 170.3 (s, CO), 142.4 (d, JPC = 1.0 Hz, Z- Cipso), 141.7 (d, JPC = 3.9 Hz, E-Cipso), 131.0 (d, JPC = 4.5 Hz. Z- CHortho), 129.0 (d, JPC = 7.2 Hz. E-CHortho), 128.3 (s, CHmeta), 126.5 31 1 31 (s, CHmeta), 126.3 (s, CHpara). P{ H}-NMR (121 MHz, CDCl3): δ = 32.1 (s) ppm. P-NMR (121 MHz, CDCl3): δ = 32.1 (s) ppm. FTIR: ν = 3063, 1725, 1663, 1634, 1587, 1487, 1450, 1313, 1259, 1143, 1071, 1026, 1002, 955, 822, 750, 692, 637, 606, 530, 514, 453, 436, 422 cm-1.

152

10.6.8 Synthesis of Tris(4-morpholinecarbonyl)phosphine (56) Elemental sodium (2.3 g, 100 mmol, 3.1 equ.) was added in small chunks to a 250 mL Schlenk flask containing 50 mL dry DME and naphthalene (0.4 g, 3.3 mmol, 0.1 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (1 g, 32.3 mmol, 1 equ.) was added in one portion and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (3 mL, 2.4 g, 6.4 mmol, 1 equ.) was mixed with 10 mL DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. The suspension was again cooled to 0°C and dimethylcarbamoyl chloride (19.3 g, 129 mmol, 4 equ.) dissolved in THF (100 mL) was added dropwise. The reaction mixture was stirred overnight at room temperature. All solids were removed by filtration. Removal of the solvent in vacuo caused the product to begin to precipitate. After the solvent volume was reduced to ~50 mL, n-hexane (100 mL) was added and the product collected by filtration.

Yield: 4.8 g (40.0% yield based on Pred) Melting Point (DTA): 170.4°C

1 H-NMR (300 MHz, CDCl3): δ = 3.75-3.55 (m, 18H, CH2), 13 1 3.50-3.35 (m, 6H, CH2) ppm. C{ H}-NMR (75 MHz, CDCl3): δ = 168.7 (s, CO), 66.5 (s, OCH2), 66.3 (s, OCH2), 31 1 49.8 (d, JPC = 15.7 Hz, E-CH2), 42.9 (s, Z-CH2) ppm. P{ H}- 31 NMR (121 MHz, CDCl3): δ = 26.2 (s) ppm. P-NMR (121 MHz, CDCl3): δ = 26.2 (m) ppm. FTIR: ν = 2975, 1604, 1406, 1360, 1301, 1267, 1247, 1215, 1104, 1081, 1063, 1014, 944, 859, 832, 791, 676, 652, 628, 581, 489, 468, 454, 428, 411 cm-1.

153

10.7 Compounds from Chapter 7 10.7.1 Synthesis of Tris(bis(benzoyl)phosphido)arsine (61) Bis(benzoyl)phosphine (45, 2.16 g, 9.6 mmol, 3.2 equ.) was added to a 100 mL Schlenk flask containing 30 mL toluene. Triethylamine (1.3 mL, 0.95 g, 9.6 mmol, 3.2 equ.) was added to the suspension, which darkened in color. Arsenic trichloride (0.25 mL, 0.54 g, 3.0 mmol, 1 equ.) was added dropwise to the stirred suspension. The solution turned yellow and a white precipitate formed. The solution was stirred with degassed water (30 mL) for 15 min. The organic phase was transferred into a 100 mL Schlenk flask by means of a canula. The water phase was extracted once with 15 mL toluene and the combined organic phases were reduced to a volume of ~10 mL in vacuo. Addition of n-hexane caused precipitation of the product as a yellow powder, which was isolated by filtration and subsequently dried in vacuo. Crystals suitable for X-ray diffraction were grown from a saturated toluene solution at -30°C.

Yield: 1.4 g (58.9% based on AsCl3) Melting Point (DTA): 148.1°C

1 H-NMR (400 MHz, CD2Cl2): δ = 7.89 (m, 6H, Harom), 7.56 13 1 (m, 3H, Harom), 7.41 (m, 6H, Harom) ppm. C{ H}-NMR (100 MHz, CD2Cl2): δ = 209.0 (m, CO), 139.7 (m, Cipso), 133.9 31 1 (s, Cpara), 128.7 (m, Carom), 128.6 (s, Carom) ppm. P{ H}-NMR (162 MHz, CD2Cl2): δ = 35.0 (s) ppm. FTIR: ν = 2975, 1603, 1443, 1407, 1379, 1360, 1303, 1248, 1215, 1168, 1114, 1081, 1066, 999, 947, 859, 766, 675, 646, 632, 597, 495, 466, 429 cm-1.

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10.8 Compounds from Chapter 8 10.8.1 Synthesis of Sodium Formphosphide (62) Elemental sodium (10.0 g, 433 mmol, 3.2 equ.) was added in small chunks to a 1 L Schlenk flask containing 500 mL dry DME and naphthalene (1.72 g, 13.3 mmol, 0.1 equ.). The reaction mixture was stirred until it had turned deep green. Red phosphorus (4.2 g, 136 mmol, 1 equ.) was added in two portions and the resulting suspension was stirred for 48 hours at room temperature. Leftover sodium was removed with tweezers and the black suspension was cooled to 0°C by means of an ice bath. Tert-butanol (12.8 mL, 10.1 g, 136 mmol, 1 equ.) was mixed with 50 mL dry DME in a dropping funnel and subsequently added dropwise to the stirred reaction mixture. The ice bath was removed and the suspension was stirred for 2 hours at room temperature. The suspension was again cooled to 0°C and ethyl formate (11.0 mL, 10.0 g, 136 mmol, 1 equ.) was added dropwise. The reaction mixture was stirred for 30 min at room temperature, while turning greenish yellow. All solids were removed via filtration over Celite and all volatiles present in the filtrate were removed in vacuo. The residue is crude Na(HPCHO)∙DME2. Complexation with 1,4-dioxane

Crude Na(HPCHO)∙DME2 was dissolved in 20 mL 1,4-dioxane and transferred into a 100 mL Schlenk by canula. The product is precipitated by addition of 60 mL n-hexane, isolated by filtration and subsequently dried in vacuo. Attention: The isolated adduct is highly pyrophoric!

Yield: 10.5 g (68.1% based on Pred, assuming Na(HPCHO)dioxane0.33).

1 2 H-NMR: (300 MHz, THF-d8): δ = 11.20 (dd, JPH = 11.4 Hz, 3 2 3 JHH = 5.1 Hz, CHcis), 9.82 (dd, JPH = 89.2 Hz, JHH = 13.3 Hz, CHtrans), 1 3 3.55 (s, OCH2), 2.70 (dd, JPH = 153.0 Hz, JHH = 5.1 Hz, PHcis) ppm. (PHtrans could not be observed due to the signals of 1,4-dioxane in the same region) 13C{1H}-NMR: (75 MHz, THF-d8): δ = 31.5 ppm (s, dioxane). (No other signals observed, 31 1 due to low solubility) P{ H}-NMR (121 MHz, THF-d8): δ = 24.5 (s, HPcis), -3.5 (s, 31 1 2 HPtrans) ppm. P-NMR: (121 MHz, THF-d8): δ = 24.5 (dd, JPH = 153.0 Hz, JPH = 11.4 Hz, 1 2 HPcis), δ = -3.5 (dd, JPH = 130.2 Hz, JPH = 89.2 Hz, HPtrans) ppm. Complexation with Dibenzo-18-crown-6-ether

Crude Na(HPCHO)∙DME2 was dissolved in 20 mL THF and transferred into a 100 mL Schlenk by canula. The product is precipitated by addition of dibenzo-18-crown-6-ether (24.5 g, 68 mmol, 0.5 equ.), isolated by filtration and subsequently dried in vacuo.

Yield: 24.g (34.2% based on Pred)

155

1H-NMR: (300 MHz, THF-d8): 2 δ = 11.54 (dd, JPH = 14.9 Hz, 3 JHH = 5.3 Hz, CHcis), 10.20 (dd, 2 3 JPH = 96.1 Hz, JHH = 13.0 Hz, CHtrans), 6.86-6.72 (m, CHarom), 4.10-3.95 (s, 1 OCH2), 3.42 (dd, JPH = 124.0 Hz, 3 JHH = 12.8 Hz, PHtrans), 2.40 (dd, 1 3 JPH = 142.2 Hz, JHH = 5.4 Hz, 13 1 PHcis) ppm. C{ H}-NMR: (75 MHz, 1 THF-d8): δ = 222.1 (d, JPC = 47.2 Hz, 1 CHcis), 214.0 (d, JPC = 70.4 Hz, CHtrans), 146.5 (s, OCarom), 118.6 (s, Carom), 67.2 (s, OCDB18c6), 31 1 65.9 (s, OCTHF), 23.5 (s, CTHF) ppm. P{ H}-NMR (121 MHz, THF-d8): δ = 15.7 (s, HPcis), - 31 1 2 15.2 (s, HPtrans) ppm. P-NMR: (121 MHz, THF-d8): δ = 15.7 (dd, JPH = 142.2 Hz, JPH = 1 2 14.9 Hz, HPcis), δ = -15.2 (dd, JPH = 124.0 Hz, JPH = 96.1 Hz, HPtrans) ppm.

156

10.8.2 Synthesis of Disodium 2,3-diphosphabuten-1,4-olate triphenylborane adduct (66)

Na(HPCHO)dioxane0.33 (62∙dioxane0.33, 226 mg, 2 mmol, 4 equ.), sodium hexamethyldisilazane (180 mg, 1 mmol, 2 equ.) and triphenylborane (360 mg, 1.5 mmol, 3 equ.) were suspended in 5 mL DME in a vial under inert atmosphere (glove box). The suspension was stirred overnight. The suspension was left standing for 24 h and the product crystallized together with an unknown precipitate. Since the compound partially decomposes upon redissolving, it was characterized in situ in DME. 11B-NMR(160 MHz, DME): δ = -2.9 (s) ppm. 13C{1H}-NMR (125 MHz, DME): δ = 211.5 (d, 1 JPC = 38.8 Hz, PC), 164.4 (m, broad, BC), 137.5- 134.5 (m, Carom), 126.5-121.2 (m, Carom) ppm. 31P{1H}-NMR (202 MHz, DME): δ = 157.7 (s) ppm. 31P-NMR (202 MHz, DME): δ = 157.7 (s) ppm.

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12 List of Abbreviations 18-c-6 18-crown-6-ether (1,4,7,10,13,16-hexaoxacyclooctadecane) b.p. boiling point Boc tert-butyloxycarbonyl Bz benzyl CCDC Cambridge Crystallographic Data Centre Cp cyclopentadienyl Cy cyclohexyl db-18c6 dibenzo-18-crown-6-ether DBA dibenzylideneacetone DBU 1,8-diazabicyclo[5.4.0]undec-7-en DCM dichloromethane DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone DFT Density Functional Theory dipp 2,5-diisopropylphenyl DME 1,2-dimethoxyethane DMF N,N-dimethylformamide DMSO dimethylsulfoxide DSC differential scanning calometry DTA differential thermal analysis FTIR Fourier-transform infrared FVT flash vacuum thermolysis GC gas chromatography HMBC heteronuclear multiple bond correlation HMDS hexamethyldisilazane HOMO highest occupied molecular orbital IR infrared LUMO lowest unoccupied molecular orbital LOI Limiting Oxygen Index m.p. melting point MAS magic angle spinning Mes mesityl (2,4,6-trimethylphenyl) Mes* supermesityl (2,4,6-tris(tert-butyl)phenyl) MS mass spectrometry NMR nuclear magnetic resonance RMSD root-mean-square deviation SMoCC Small Molecule Crystallography Center TD-DTF time-dependent Density Functional Theory TEMPO 2,2,6,6-tetramethylpiperidinyloxyl TGA thermogravimetric analysis THF tetrahydrofuran TIC total ion count TMS trimethylsilyl Ts tosyl (para-toluolsulfonyl) UV/ VIS ultraviolet/visible light

163

13 Curriculum Vitae Erik Schrader 26th January 1990 Dresden, Germany

Education since 02/2016 Teaching Diploma in Chemistry, ETH Zurich since 07/2014 Ph.D. studies (Prof. Hansjörg Grützmacher, ETH Zurich) „Synthesis and Properties of Acylphosphines“ 02/2012 – 03/2014 Master of Science in Chemistry, ETH Zurich 09/2009 – 09/2012 Bachelor of Science in Chemistry, ETH Zurich

Extracurricular Activities 03/2017 – 11/2017 Social Media Coordinator, Chemtogether Career Fair 11/2014 – 11/2016 Webmaster, Union of Doctoral Students in Chemistry (VAC) 11/2010 – 11/2011 Industry Relations, Union of Chemistry Students (VCS)

Industrial and Practical Experience 09/2014 – 04/2018 Teaching Assistant, ETH Zurich Supervision of students in lab courses and independent projects 11/2013 – 05/2014 Internship, BASF Performance Materials - Basel Switzerland

Awards 12/2017 Oral presentation award, ETH Inorganic Chemistry Christmas Symposium

164

14 List of Publications Scientific Publications R. J. Gilliard, R. Suter, E. Schrader, Z. Benkő, A. L. Rheingold, H. Grützmacher, J. D. Protasiewicz, Chem. Commun. 2017, 53, 12325-12328. Synthesis of P2C2O2 and P2CO via NHC-mediated coupling of the phosphaethynolate anion. R. Suter, H. Sinclair, N. Burford, R. McDonald, M. J. Ferguson, E. Schrader, Dalton Trans. 2017, 46, 7681-7685. Tris(2-pyridyl)phosphine as a versatile ligand for pnictogen acceptors.

Public Presentations 12/2017 ETH Inorganic Chemistry Christmas Symposium, Oral “Thermal Properties of Acylphosphines and their Application in the Synthesis of InP Nanoparticles” 03/2017 14th European Workshop on Phosphorus Chemistry, Poster “Bisacylphosphines as acac-Type Ligands for Mono- and Bimetallic Complexes” 03/3016 13th European Workshop on Phosphorus Chemistry, Poster “Synthesis of Phosphorus-containing Cyclic Structures from Acylphosphines” 03/2015 12th European Workshop on Phosphorus Chemistry, Poster “Low Valent Phosphorus-Acyl Species as Ligands for Transition Metal Complexes”

165

15 Crystallographic Details Sodium bis(ethoxy)phosphide (3)

Empirical formula C16H30Na2O9P2 Formula weight 474.32 Temperature/K 101.6 Crystal system triclinic Space group P-1 a/Å 9.1000(5) b/Å 12.1720(6) c/Å 12.2662(6) α/° 109.104(2) β/° 98.164(2) γ/° 106.176(2) Volume/Å3 1191.62(11) Z 2 3 ρcalcg/cm 1.322 μ/mm-1 0.260 F(000) 500.0 Crystal size/mm3 0.3 × 0.2 × 0.2 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 4.822 to 56.564 Index ranges -12 ≤ h ≤ 12, -16 ≤ k ≤ 16, -16 ≤ l ≤ 16 Reflections collected 41387 Independent reflections 5906 [Rint = 0.0658, Rsigma = 0.0465] Data/restraints/parameters 5906/0/268 Goodness-of-fit on F2 1.047 Final R indexes [I>=2σ (I)] R1 = 0.0405, wR2 = 0.0750 Final R indexes [all data] R1 = 0.0616, wR2 = 0.0808 Largest diff. peak/hole / e Å-3 0.35/-0.28

166

Mesitoyl-2,6-diphenylphosphinanone (26)

Empirical formula C50H56O2P2 Formula weight 750.88 Temperature/K 103.1 Crystal system monoclinic Space group P21/n a/Å 14.2130(14) b/Å 7.1710(8) c/Å 21.310(2) α/° 90 β/° 90.090(3) γ/° 90 Volume/Å3 2172.0(4) Z 2 3 ρcalcg/cm 1.148 μ/mm-1 0.138 F(000) 804.0 Crystal size/mm3 0.1 × 0.05 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.732 to 49.424 Index ranges -16 ≤ h ≤ 16, -8 ≤ k ≤ 7, -25 ≤ l ≤ 25 Reflections collected 11450 Independent reflections 2631 [Rint = 0.1048, Rsigma = 0.1049] Data/restraints/parameters 2631/0/274 Goodness-of-fit on F2 0.972 Final R indexes [I>=2σ (I)] R1 = 0.0569, wR2 = 0.1075 Final R indexes [all data] R1 = 0.1039, wR2 = 0.1229 Largest diff. peak/hole / e Å-3 0.28/-0.22

167

Mesitoyl-2,6-diphenylphosphinanone oxide (27)

Empirical formula C27H27O3P Formula weight 430.45 Temperature/K 103.3 Crystal system orthorhombic Space group Pbcn a/Å 14.3591(10) b/Å 11.5543(8) c/Å 27.2490(18) α/° 90 β/° 90 γ/° 90 Volume/Å3 4520.9(5) Z 8 3 ρcalcg/cm 1.265 μ/mm-1 1.281 F(000) 1824.0 Crystal size/mm3 0.2 × 0.05 × 0.05 Radiation CuKα (λ = 1.54178) 2Θ range for data collection/° 6.488 to 79.932 Index ranges -11 ≤ h ≤ 11, -9 ≤ k ≤ 9, -22 ≤ l ≤ 22 Reflections collected 17484 Independent reflections 1377 [Rint = 0.0553, Rsigma = 0.0226] Data/restraints/parameters 1377/0/283 Goodness-of-fit on F2 1.159 Final R indexes [I>=2σ (I)] R1 = 0.0453, wR2 = 0.1107 Final R indexes [all data] R1 = 0.0539, wR2 = 0.1150 Largest diff. peak/hole / e Å-3 0.16/-0.22

168

Tetrakis(mesitoyl)cyclotetraphosphine (30)

Empirical formula C40H44O4P4 Formula weight 712.63 Temperature/K 100.01(10) Crystal system monoclinic Space group C2/c a/Å 15.0614(5) b/Å 20.9482(6) c/Å 12.9787(4) α/° 90 β/° 91.366(2) γ/° 90 Volume/Å3 4093.7(2) Z 4 3 ρcalcg/cm 1.156 μ/mm-1 0.221 F(000) 1504.0 Crystal size/mm3 0.1 × 0.05 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 6.28 to 56.564 Index ranges -20 ≤ h ≤ 20, -27 ≤ k ≤ 27, -17 ≤ l ≤ 17 Reflections collected 34985 Independent reflections 5087 [Rint = 0.0769, Rsigma = 0.0551] Data/restraints/parameters 5087/0/223 Goodness-of-fit on F2 1.137 Final R indexes [I>=2σ (I)] R1 = 0.0730, wR2 = 0.1322 Final R indexes [all data] R1 = 0.0891, wR2 = 0.1385 Largest diff. peak/hole / e Å-3 0.37/-0.31

169

Mesitoyl-2,5-diphenylphosphole (37)

Empirical formula C26H23OP Formula weight 382.41 Temperature/K 106.4(2) Crystal system monoclinic Space group P21/n a/Å 8.2791(4) b/Å 15.2012(8) c/Å 16.2470(10) α/° 90 β/° 96.416(5) γ/° 90 Volume/Å3 2031.91(19) Z 4 3 ρcalcg/cm 1.250 μ/mm-1 0.149 F(000) 808.0 Crystal size/mm3 0.3 × 0.1 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.63 to 56.564 Index ranges -10 ≤ h ≤ 11, -20 ≤ k ≤ 20, -20 ≤ l ≤ 21 Reflections collected 15285 Independent reflections 5033 [Rint = 0.0770, Rsigma = 0.1145] Data/restraints/parameters 5033/0/256 Goodness-of-fit on F2 1.074 Final R indexes [I>=2σ (I)] R1 = 0.0831, wR2 = 0.1209 Final R indexes [all data] R1 = 0.1396, wR2 = 0.1387 Largest diff. peak/hole / e Å-3 0.41/-0.29

170

Mesitoyl-2,5-diphenylphosphole oxide (38)

Empirical formula C26H23O2P Formula weight 398.41 Temperature/K 106.1(3) Crystal system monoclinic Space group P21/c a/Å 12.6417(9) b/Å 18.8544(12) c/Å 8.8617(7) α/° 90 β/° 103.772(8) γ/° 90 Volume/Å3 2051.5(3) Z 4 3 ρcalcg/cm 1.290 μ/mm-1 0.154 F(000) 840.0 Crystal size/mm3 0.2 × 0.2 × 0.1 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 6.41 to 52.742 Index ranges -15 ≤ h ≤ 15, -23 ≤ k ≤ 23, -11 ≤ l ≤ 10 Reflections collected 16633 Independent reflections 4174 [Rint = 0.0741, Rsigma = 0.0729] Data/restraints/parameters 4174/0/265 Goodness-of-fit on F2 1.317 Final R indexes [I>=2σ (I)] R1 = 0.0999, wR2 = 0.1504 Final R indexes [all data] R1 = 0.1192, wR2 = 0.1571 Largest diff. peak/hole / e Å-3 0.47/-0.40

171

Tetrakis(acetyl)diphosphine (41)

Empirical formula C8H12O4P2 Formula weight 234.12 Temperature/K 100.01(10) Crystal system orthorhombic Space group P212121 a/Å 8.3114(2) b/Å 8.7169(2) c/Å 15.0529(4) α/° 90 β/° 90 γ/° 90 Volume/Å3 1090.58(5) Z 4 3 ρcalcg/cm 1.426 μ/mm-1 0.385 F(000) 488.0 Crystal size/mm3 0.2 × 0.2 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.598 to 59.15 Index ranges -11 ≤ h ≤ 11, -12 ≤ k ≤ 11, -20 ≤ l ≤ 20 Reflections collected 27114 Independent reflections 3046 [Rint = 0.0582, Rsigma = 0.0282] Data/restraints/parameters 3046/0/131 Goodness-of-fit on F2 1.097 Final R indexes [I>=2σ (I)] R1 = 0.0342, wR2 = 0.0845 Final R indexes [all data] R1 = 0.0363, wR2 = 0.0862 Largest diff. peak/hole / e Å-3 0.51/-0.31 Flack parameter -0.03(4)

172

Tetrakis(benzoyl)diphosphine (46)

Empirical formula C28H20O4P2 Formula weight 482.38 Temperature/K 100.00(11) Crystal system triclinic Space group P-1 a/Å 7.4709(3) b/Å 10.6048(5) c/Å 16.1091(8) α/° 90.081(4) β/° 102.756(4) γ/° 109.142(4) Volume/Å3 1172.18(10) Z 2 3 ρcalcg/cm 1.367 μ/mm-1 0.219 F(000) 500.0 Crystal size/mm3 0.3 × 0.1 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.898 to 54.424 Index ranges -9 ≤ h ≤ 9, -13 ≤ k ≤ 13, -20 ≤ l ≤ 20 Reflections collected 16499 Independent reflections 5232 [Rint = 0.0460, Rsigma = 0.0565] Data/restraints/parameters 5232/0/307 Goodness-of-fit on F2 1.119 Final R indexes [I>=2σ (I)] R1 = 0.0542, wR2 = 0.1039 Final R indexes [all data] R1 = 0.0690, wR2 = 0.1095 Largest diff. peak/hole / e Å-3 0.45/-0.40

173

Tris(benzoyl)phosphine (47)

Empirical formula C21H15O3P Formula weight 346.30 Temperature/K 147(60) Crystal system monoclinic Space group P21/c a/Å 4.6430(4) b/Å 10.4739(9) c/Å 34.711(3) α/° 90 β/° 92.145(7) γ/° 90 Volume/Å3 1686.8(2) Z 4 3 ρcalcg/cm 1.364 μ/mm-1 0.180 F(000) 720.0 Crystal size/mm3 0.3 × 0.05 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 6.1 to 55.434 Index ranges -6 ≤ h ≤ 6, -13 ≤ k ≤ 13, -45 ≤ l ≤ 45 Reflections collected 16574 Independent reflections 3937 [Rint = 0.0686, Rsigma = 0.0662] Data/restraints/parameters 3937/0/226 Goodness-of-fit on F2 1.194 Final R indexes [I>=2σ (I)] R1 = 0.0749, wR2 = 0.1240 Final R indexes [all data] R1 = 0.0951, wR2 = 0.1308 Largest diff. peak/hole / e Å-3 0.40/-0.30

174

Tris(pivaloyl)phosphine (48)

Empirical formula C15H27O3P Formula weight 286.34 Temperature/K 293(2) Crystal system trigonal Space group R3 a/Å 15.7756(2) b/Å 15.7756(2) c/Å 5.75150(10) α/° 90 β/° 90 γ/° 120 Volume/Å3 1239.60(4) Z 2.99997 3 ρcalcg/cm 1.151 μ/mm-1 0.169 F(000) 468.0 Crystal size/mm3 0.2 × 0.05 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 7.688 to 56.514 Index ranges -20 ≤ h ≤ 20, -20 ≤ k ≤ 20, -7 ≤ l ≤ 7 Reflections collected 8312 Independent reflections 1370 [Rint = 0.0199, Rsigma = 0.0112] Data/restraints/parameters 1370/1/64 Goodness-of-fit on F2 1.123 Final R indexes [I>=2σ (I)] R1 = 0.0151, wR2 = 0.0392 Final R indexes [all data] R1 = 0.0151, wR2 = 0.0392 Largest diff. peak/hole / e Å-3 0.14/-0.11 Flack parameter ?

175

Tris(benzoyl)phosphine oxide (50)

Empirical formula C21H15O4P Formula weight 362.30 Temperature/K 100.00(10) Crystal system triclinic Space group P-1 a/Å 7.1649(2) b/Å 8.1101(2) c/Å 16.2415(5) α/° 103.387(2) β/° 93.969(2) γ/° 93.616(2) Volume/Å3 912.87(4) Z 2 3 ρcalcg/cm 1.318 μ/mm-1 0.173 F(000) 376.0 Crystal size/mm3 0.2 × 0.2 × 0.2 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.718 to 55.754 Index ranges -9 ≤ h ≤ 9, -10 ≤ k ≤ 10, -21 ≤ l ≤ 21 Reflections collected 17706 Independent reflections 4360 [Rint = 0.0417, Rsigma = 0.0427] Data/restraints/parameters 4360/0/235 Goodness-of-fit on F2 1.289 Final R indexes [I>=2σ (I)] R1 = 0.0676, wR2 = 0.1121 Final R indexes [all data] R1 = 0.0778, wR2 = 0.1154 Largest diff. peak/hole / e Å-3 0.36/-0.40

176

Bis(benzoyl)phosphidodichlorobis(tetrahydrofurano)titanium(III) (53)

Empirical formula C26H34Cl2O5PTi Formula weight 576.30 Temperature/K 100.02(14) Crystal system orthorhombic Space group Pna21 a/Å 20.2474(3) b/Å 16.0952(3) c/Å 8.3930(2) α/° 90 β/° 90 γ/° 90 Volume/Å3 2735.16(9) Z 4 3 ρcalcg/cm 1.400 μ/mm-1 0.601 F(000) 1204.0 Crystal size/mm3 0.5 × 0.4 × 0.4 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.832 to 61.998 Index ranges -29 ≤ h ≤ 29, -23 ≤ k ≤ 23, -12 ≤ l ≤ 12 Reflections collected 65759 Independent reflections 8697 [Rint = 0.0337, Rsigma = 0.0204] Data/restraints/parameters 8697/1/316 Goodness-of-fit on F2 1.125 Final R indexes [I>=2σ (I)] R1 = 0.0396, wR2 = 0.1050 Final R indexes [all data] R1 = 0.0415, wR2 = 0.1062 Largest diff. peak/hole / e Å-3 0.89/-0.59 Flack parameter 0.005(6)

177

Tris(dimethylcarbamoyl)phosphine (54)

Empirical formula C9H18N3O3P Formula weight 247.23 Temperature/K 100.00(10) Crystal system monoclinic Space group Cc a/Å 14.4655(8) b/Å 8.8523(5) c/Å 9.9195(5) α/° 90 β/° 100.978(4) γ/° 90 Volume/Å3 1246.98(12) Z 4 3 ρcalcg/cm 1.317 μ/mm-1 0.218 F(000) 528.0 Crystal size/mm3 0.4 × 0.2 × 0.2 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.738 to 56.554 Index ranges -19 ≤ h ≤ 19, -11 ≤ k ≤ 11, -13 ≤ l ≤ 13 Reflections collected 6195 Independent reflections 3003 [Rint = 0.0175, Rsigma = 0.0257] Data/restraints/parameters 3003/2/151 Goodness-of-fit on F2 1.082 Final R indexes [I>=2σ (I)] R1 = 0.0227, wR2 = 0.0576 Final R indexes [all data] R1 = 0.0231, wR2 = 0.0577 Largest diff. peak/hole / e Å-3 0.22/-0.13 Flack parameter 0.03(3)

178

Tris(diphenylcarbamoyl)phosphine (55)

Empirical formula C39H30N3O3P Formula weight 619.63 Temperature/K 150.00(10) Crystal system triclinic Space group P-1 a/Å 9.3234(3) b/Å 11.2803(3) c/Å 19.3334(6) α/° 75.172(2) β/° 84.250(2) γ/° 69.277(3) Volume/Å3 1838.32(10) Z 2 3 ρcalcg/cm 1.119 μ/mm-1 0.112 F(000) 648.0 Crystal size/mm3 0.4 × 0.3 × 0.2 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.834 to 59.15 Index ranges -12 ≤ h ≤ 12, -15 ≤ k ≤ 15, -26 ≤ l ≤ 26 Reflections collected 53193 Independent reflections 10303 [Rint = 0.0352, Rsigma = 0.0300] Data/restraints/parameters 10303/0/415 Goodness-of-fit on F2 1.061 Final R indexes [I>=2σ (I)] R1 = 0.0405, wR2 = 0.0939 Final R indexes [all data] R1 = 0.0539, wR2 = 0.0998 Largest diff. peak/hole / e Å-3 0.35/-0.33

179

Tris(4-morpholinecarbonyl)phosphine (56)

Empirical formula C15H24N3O6P Formula weight 373.34 Temperature/K 100.00(10) Crystal system triclinic Space group P-1 a/Å 5.2515(2) b/Å 12.4718(5) c/Å 14.4568(6) α/° 108.260(4) β/° 99.745(4) γ/° 98.485(4) Volume/Å3 865.61(6) Z 2 3 ρcalcg/cm 1.432 μ/mm-1 0.197 F(000) 396.0 Crystal size/mm3 0.1 × 0.05 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.888 to 56.536 Index ranges -6 ≤ h ≤ 6, -16 ≤ k ≤ 16, -19 ≤ l ≤ 19 Reflections collected 17229 Independent reflections 4262 [Rint = 0.0634, Rsigma = 0.0702] Data/restraints/parameters 4262/0/226 Goodness-of-fit on F2 1.220 Final R indexes [I>=2σ (I)] R1 = 0.0762, wR2 = 0.1142 Final R indexes [all data] R1 = 0.0992, wR2 = 0.1208 Largest diff. peak/hole / e Å-3 0.45/-0.35

180

Tris(bis(benzoyl)phosphido)arsine (61)

Empirical formula C46H38AsO7P3 Formula weight 870.59 Temperature/K 100.01(10) Crystal system triclinic Space group P-1 a/Å 10.6639(3) b/Å 12.2631(3) c/Å 15.9189(4) α/° 86.020(2) β/° 85.470(2) γ/° 78.793(2) Volume/Å3 2032.57(9) Z 2 3 ρcalcg/cm 1.422 μ/mm-1 1.007 F(000) 896.0 Crystal size/mm3 0.05 × 0.05 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.636 to 56.564 Index ranges -14 ≤ h ≤ 14, -16 ≤ k ≤ 16, -21 ≤ l ≤ 21 Reflections collected 50933 Independent reflections 10084 [Rint = 0.0572, Rsigma = 0.0518] Data/restraints/parameters 10084/0/514 Goodness-of-fit on F2 1.199 Final R indexes [I>=2σ (I)] R1 = 0.0564, wR2 = 0.1070 Final R indexes [all data] R1 = 0.0666, wR2 = 0.1104 Largest diff. peak/hole / e Å-3 0.94/-0.49

181

Sodium formphosphide – DME complex (62∙dme2)

Empirical formula C18H42Na2O10P2 Formula weight 526.43 Temperature/K 293(2) Crystal system orthorhombic Space group Pna21 a/Å 20.4133(13) b/Å 12.9544(8) c/Å 11.3676(7) α/° 90 β/° 90 γ/° 90 Volume/Å3 3006.1(3) Z 4 3 ρcalcg/cm 1.163 μ/mm-1 0.214 F(000) 1128.0 Crystal size/mm3 0.2 × 0.2 × 0.1 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 3.144 to 51.356 Index ranges -24 ≤ h ≤ 24, -15 ≤ k ≤ 15, -13 ≤ l ≤ 13 Reflections collected 24986 Independent reflections 5710 [Rint = 0.0477, Rsigma = 0.0304] Data/restraints/parameters 5710/5/312 Goodness-of-fit on F2 1.133 Final R indexes [I>=2σ (I)] R1 = 0.1048, wR2 = 0.2685 Final R indexes [all data] R1 = 0.1174, wR2 = 0.2786 Largest diff. peak/hole / e Å-3 0.91/-0.37 Flack parameter -0.07(9)

182

Sodium formphosphide – db-18c6 complex (62(db-18c6)THF)

Empirical formula C25H34NaO8P Formula weight 516.48 Temperature/K 100.00(10) Crystal system orthorhombic Space group P212121 a/Å 9.5294(4) b/Å 10.0166(4) c/Å 27.9726(12) α/° 90 β/° 90 γ/° 90 Volume/Å3 2670.05(19) Z 4 3 ρcalcg/cm 1.285 μ/mm-1 0.164 F(000) 1096.0 Crystal size/mm3 0.1 × 0.05 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.826 to 56.564 Index ranges -12 ≤ h ≤ 12, -12 ≤ k ≤ 13, -37 ≤ l ≤ 35 Reflections collected 25265 Independent reflections 6634 [Rint = 0.0492, Rsigma = 0.0604] Data/restraints/parameters 6634/5/325 Goodness-of-fit on F2 1.033 Final R indexes [I>=2σ (I)] R1 = 0.0565, wR2 = 0.1218 Final R indexes [all data] R1 = 0.0878, wR2 = 0.1365 Largest diff. peak/hole / e Å-3 0.35/-0.26 Flack parameter 0.00(5)

183

Disodium 2,3-diphosphabuten-1,4-olate triphenylborane adduct (66)

Empirical formula C62H92B2Na2O14P2 Formula weight 1190.89 Temperature/K 100.01(10) Crystal system triclinic Space group P-1 a/Å 10.768(2) b/Å 11.742(3) c/Å 15.665(4) α/° 88.757(19) β/° 69.92(2) γ/° 64.22(2) Volume/Å3 1656.0(7) Z 1 3 ρcalcg/cm 1.194 μ/mm-1 0.138 F(000) 638.0 Crystal size/mm3 0.05 × 0.05 × 0.05 Radiation MoKα (λ = 0.71073) 2Θ range for data collection/° 5.602 to 50.052 Index ranges -12 ≤ h ≤ 10, -13 ≤ k ≤ 13, -18 ≤ l ≤ 18 Reflections collected 16711 Independent reflections 5834 [Rint = 0.1186, Rsigma = 0.1741] Data/restraints/parameters 5834/0/376 Goodness-of-fit on F2 0.968 Final R indexes [I>=2σ (I)] R1 = 0.0841, wR2 = 0.1720 Final R indexes [all data] R1 = 0.1815, wR2 = 0.2222 Largest diff. peak/hole / e Å-3 0.48/-0.46

184