Cationic rhodium complexes with ortho-Aryl substituted DPEphos ligands and rhodium-diphosphine catalysed phosphine-borane dehydropolymerisation

Thesis

Transition metal-phosphine chemistry is the underpinning theme of this thesis. Chapter One briefly introduces the subject, with particular focus on ortho-aryl substituted phosphines, the DPEphos ligand and agostic and anagostic metal-hydrogen interactions. A comprehensive presentation of the developments in the field of phosphine-borane dehydropolymerisation is then provided.

Chapter Two describes the synthesis, characterisation and structural studies of a range of Schrock-Osborn type complexes containing ortho-aryl substituted DPEphos ligands of varying steric bulk, namely [Rh(o-R-DPEphos)(NBD)][BArF4] [R = H, Me, OMe and iPr, NBD = norbornadiene and ArF= 3,5-(CF3)2C6H3]. These complexes were all shown to contain a Rh···H–C anagostic interaction with an ortho-aryl proton, which were studied experimentally and computationally. The reactivity of the Schrock-Osborn complexes, upon hydrogenation of the alkene fragment in acetone, resulted in a range of Rh(I) and Rh(III) products, depending on the phosphine ortho-aryl substituent. In the bulkiest, oiPr version, C–H activation is observed at the isopropyl group, which is shown to be reversible through reactivity with CO and D2. This reactivity was used to form the first reported complex that contains both an anagostic interaction and an agostic bond.

Within Chapter Three, the coordination chemistry of the di-ortho-fluorine analogue of DPEphos, o-F,F-DPEphos, is described in comparison with the parent o-H-DPEphos and the bulkier o-MeDPEphos. Rh···F interactions are observed in the Schrock-Osborn complex of o-F,F-DPEphos [Rh(oF,F-DPEphos)(NBD)][BArF4]. The hydrogenation products in 1,2-F2C6H4 solvent are also compared, revealing the formation of a Rh dimeric complex Rh(H)(fac-κ3-P,O,P-µ-CH-DPEphos)]2[BArF4]2 with a bridging C–H agostic interaction for the parent DPEphos. The coordination chemistry of bimetallic gold complexes with these ligands is also explored, including an example of through-space F-F interactions.

The final chapter details an in depth study of the dehydropolymerisation of primary phosphineboranes using a [Rh(dppe)]+ catalyst. The mechanism is probed using in-situ NMR spectroscopy, stoichiometric reactivity and polymer molecular weight variation studies. Exploiting the mechanistic insight, the synthesis and characterisation of the first block copolymer of poly(phosphinoboranes) is described, which is also shown to self-assemble in solution to form spherical micelles.

Authors: Race, JJ

• DOI: 10.5287/ora-mvv4nvr58
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English