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Synthesis, Properties, and Coordination Chemistry of T-Bu-Xantphos Ligands

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Synthesis, Properties, and Coordination Chemistry of T-Bu-Xantphos Ligands Synthesis, Properties, and Coordination Chemistry of t-Bu-Xantphos Ligands by Melanie Ruth Maria Nelson A thesis submitted to the Victoria University of Wellington in fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry. Victoria University of Wellington 2015 Abstract This thesis provides an account of research into a group of diphosphine ligands with a rigid xanthene backbone and tert-butyl substituents on the phosphorus atoms. The three ligands have different groups in the bridgehead position of the backbone (CMe2, SiMe2, or S) which change the natural (calculated) bite-angle of the ligand. The coordination chemistry of these t-Bu-xantphos ligands with late-transition metals has been investigated with a focus on metal complexes that may form in catalytic reactions. The three t-Bu-xantphos ligands were synthesised by lithiation of the backbone t using sec-butyllithium/TMEDA and treatment with P Bu2Cl. The natural bite- angles of the Ph-xantphos (111.89–114.18◦) and t-Bu-xantphos (126.80–127.56◦) ligands were calculated using DFT. The bite-angle of the t-Bu-xantphos ligands is larger due to the increased steric bulk of the tert-butyl substituents. The elec- tronic properties of the t-Bu-xantphos ligands were also investigated by synthesis 1 of their phosphine selenides. The values of J PSe (689.1–698.5 Hz) indicate that the t-Bu-xantphos ligands have a higher basicity than Ph-xantphos between PPh2Me and PMe3. The silver complexes, [Ag(t-Bu-xantphos)Cl] and [Ag(t-Bu-xantphos)]BF4 were synthesised with the t-Bu-xantphos ligands. In contrast to systems with phenyl phosphines, all species were monomeric. [Rh(t-Bu-xantphos)Cl] complexes were κ synthesised, which reacted with H2, forming [Rh(t-Bu-xantphos- P,O,P’)Cl(H)2] complexes, and with CO, forming [Rh(t-Bu-xantphos)(CO)2Cl] complexes. The [Rh(t-Bu-xantphos)Cl] species are air-sensitive readily forming [Rh(t-Bu-xant- 2 2 phos)Cl(η -O2)] complexes. The crystal structure of [Rh(t-Bu-xantphos)Cl(η -O2)], contained 15% of the dioxygen sites replaced with an oxo ligand. This is the first crystallographic evidence of a rhodium(III) oxo complex, and only the third rhodium oxo species reported. The coordination chemistry of the ligands with platinum(0) and palladium(0) showed some differences. [Pt(t-Bu-xantphos)(C2H4)] complexes were synthe- sised for all three ligands. However, reaction with [Pt(nb)3] produced a mixture of [Pt(t-Bu-xantphos)] and [Pt(t-Bu-xantphos)(nb)] for t-Bu-sixantphos and t-Bu- thixantphos. Although few examples of isolable [Pt(PP)] complexes with diphos- phines have been reported [Pt(t-Bu-thixantphos)] was isolated by removal of the norbornene. t-Bu-Xantphos formed small amounts of [Pt(t-Bu-xantphos)] ini- tially, which progressed to [Pt(t-Bu-xantphos)H]X. The analogous reactions with [Pd(nb)3] gave [Pd(t-Bu-xantphos)] and [Pd(t-Bu-xantphos)(nb)] complexes in all cases. [Pt(t-Bu-thixantphos)(C2H4)] and [M(t-Bu-thixantphos)] (M = Pd, Pt) react 2 with oxygen forming [Pt(t-Bu-thixantphos)(η -O2)], which reacts with CO to give [Pt(t-Bu-thixantphos-H-κ-C,P,P’)OH] through a series of intermediates. [M(t-Bu-xantphos)Cl2] (M = Pd, Pt) complexes were synthesised, showing ex- clusive trans coordination of the diphosphine ligands. The X-ray crystal struc- ◦ ture of [Pt(t-Bu-thixantphos)Cl2] has a bite-angle of 151.722(15) . This is the ◦ first [PtCl2(PP)] complex with a bite-angle between 114 and 171 . In polar sol- vents a chloride ligand dissociates from the [Pt(t-Bu-xantphos)Cl2] complexes producing [Pt(t-Bu-xantphos-κP,O,P’)Cl]+. The analogous [Pd(t-Bu-xantphos- κP,O,P’)Cl]+ complexes were formed by reaction of the dichlorides complexes κ + with NH4PF6. The [Pt(t-Bu-xantphos- P,O,P’)Me] pincer complexes were the only product from reaction with [Pt(C6H10)ClMe], with the stronger trans influ- ence of the methyl ligand promoting loss of the chloride. The formation of the pincer complexes was further explored using DFT. κ + The values of J PtC for the methyl carbons in the [Pt(t-Bu-xantphos- P,O,P’)Me] complexes, and J RhH for the hydride trans to the oxygen atom in the [Rh(t-Bu- κ xantphos- P,O,P’)Cl(H)2] complexes were largest for t-Bu-sixantphos, then t-Bu- thixantphos, then t-Bu-xantphos. The trans influence of the t-Bu-xantphos oxy- gen donor follows the trend t-Bu-sixantphos < t-Bu-thixantphos < t-Bu-xant- phos. Acknowledgements As always there are numerous people that have helped me over the years. My supervisor, Prof. John Spencer, thank you for your guidance, advice and support, even coming in to University after a major earthquake to rescue our reaction. Dr. Matthias Lein, thank you for being an excellent supervisor while John was on research and study leave and for all of your assistance with DFT. Thanks to both of you for reading through various sections while on holiday. The past and present members of the JLS research group: Almas, Brad, Chris, David, Kathryn, Lia, Rosie, Sarah and Teresa. Thank you for getting excited with me over the minutiae of NMR spectra, and for all of the good times and commis- erations that we’ve shared. The other postgraduate students in SCPS, particularly my office mates in AM204, thanks for your friendship over the years. The general staff of SCPS are amazingly supportive towards the research in the school and many things would be impossible without them. Particular thanks to the lab technicians Teresa, Jamie-Ann and Jackie, for letting me borrow glassware and other equipment. Thanks to Peter Northcote, Ian Vorster and John Ryan for helping with NMR. I am grateful to Rob Keyzers and Yinrong Lu for help with mass spectrometry and to Jan Wikaira, Christ Fitchett and Matt Polson at the University of Canterbury for X-ray crystallography. Thank you to the members of Disarray, and all of my non-chemistry friends for giving me something else to think about when I just needed a break. My family have been extremely supportive and caring, as they always are. Finally to my husband, Stephen, these last 10 years have been wonderful, thank you for all of your love and understanding especially over these last few months. I would also like to acknowledge Victoria University of Wellington for funding through the PhD, submission, and Curtis-Gordon scholarships. iii iv Contents Contents v List of Figures ix List of Schemes x List of Tables xiii Acronyms xv 1 Introduction 1 1.1 TertiaryPhosphineLigands . 1 1.1.1 ElectronicandStericProperties . 2 1.1.2 DiphosphineLigands . 3 1.1.3 WideBite-AngleDiphosphineLigands . 6 1.2 PincerLigands............................... 6 1.3 Xantphos.................................. 9 1.3.1 Alkyl-SubstitutedXantphosLigands . 12 1.3.2 t-Bu-Xantphos .......................... 16 1.4 ResearchObjectives............................ 20 2 Ligand Synthesis and Properties 23 2.1 LigandSynthesis ............................. 24 2.2 Bite-AngleCalculations . 29 2.3 Basicity................................... 33 2.4 Selenides.................................. 38 2.5 Summary.................................. 45 3 Coordination Complexes with Silver 47 v vi Contents 3.1 SilverChlorideComplexes. 49 3.2 Reactions with Silver Tetrafluoroborate . ... 56 3.2.1 ReactionswithLiCCPh . 60 3.3 Summary.................................. 61 4 Coordination Complexes with Rhodium 63 4.1 Synthesis of [Rh(κP,O,P’)Cl]Complexes . 65 4.2 ReactionwithHydrogen . 71 4.3 RhodiumCarbonylComplexes . 78 4.4 DioxygenandOxoComplexes . 87 4.5 Summary.................................. 98 5 Coordination Complexes with Platinum(0) and Palladium(0) 101 5.1 Reactions of Ph-thixantphos with Platinum(0) Precursors ................................. 103 5.2 Reaction of t-Bu-xantphos Ligands with [Pt(nb)3] ..........112 5.3 Reaction of t-Bu-thixantphos with [Pt(cod)2] .............116 5.4 Reaction of t-Bu-xantphos Ligands and [Pt(C2H4)3] .........116 5.5 FormationofPlatinumDioxygenComplexes . 119 2 5.6 Reactions of [Pt(t-Bu-thixantphos)(η -O2)]...............125 5.7 ReactionswithPalladium(0)Precursors . 130 5.7.1 Reaction of [Pd(t-Bu-thixantphos)] with Oxygen . 132 5.8 ComputationalResults . 133 5.9 Summary..................................135 6 Coordination Complexes with Platinum(II) and Palladium(II) 141 6.1 Reactions with Platinum Dichloride Starting Materials ....... 144 6.1.1 Reactions with [PtCl2(hex)] ................... 155 6.2 Reactions with [Pd(cod)Cl2].......................161 κ 6.2.1 Formation of [Pd(t-Bu-xantphos- P,O,P’)Cl]PF6 Complexes 163 6.3 ComputationalResults . 164 6.4 Reactions with Platinum Dimethyl Starting Materials . ...... 166 6.5 Reactionswith[PtCl(hex)Me] . 168 6.5.1 ComputationalResults. 174 6.6 Summary..................................178 7 Conclusion 181 Contents vii 8 Experimental 185 8.1 GeneralProcedures . .. .. .. .. .. .. .. .. 185 8.2 LigandsandNon-Transition MetalDerivatives . 186 8.3 SilverComplexes ............................. 192 8.4 RhodiumComplexes .. .. .. .. .. .. .. .. 196 8.5 PlatinumComplexes . 204 8.6 PalladiumComplexes . 221 Bibliography 227 viii Contents List of Figures 1.1 Selectionofdiphosphineligands . 4 1.2 Thebiteangle ............................... 5 1.3 Widebite-anglediphosphineligands . .. 6 1.4 Trans-spanningdiphosphineligands . .. 7 1.5 Namingofpincerligands .. .. .. .. .. .. .. 7 1.6 Firstreportedpincerligands. 8 1.7 Generalrepresentationofpincercomplexes . .... 8 1.8 Selectionofxantphosderivatives . .. 10 2.1 Comparison of the natural and crystallographic bite-angles. 30 2.2 1H NMR spectra for the [(t-Bu-xantphos)H]+ ions .......... 35 2.3 VT-NMR for [(t-Bu-thixantphos)H]+ .................. 37 2.4 VT 1H-coupled 31P NMR for [(t-Bu-thixantphos)H]+ ......... 38 2.5 X-ray crystal structure of [(t-Bu-thixantphos)H]CPh(SO2CF3)2 ... 39 3.1 1:1silverdiphosphinecomplexes . 48 3.2 SilverPh-xantphoscomplexes. 48 3.3 X-ray crystal structure of AgBr(Ph-xantphos) . ..... 49 3.4 X-ray crystal structure of Ag(Ph-xantphos)(HpyS)Br .
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