Transition Metal Complexes in Cooperative Lewis Pairs for Small Molecule Activation

  • Krishna Mistry

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The development of transition metal complexes as a component of Lewis acid/base pairs for use in small molecule activation is presented. This concept stems from the nascent field of frustrated Lewis pair (FLP) chemistry.
A Pt(0)-diphosphine monocarbonyl complex, [Pt(CO)(L)] was shown to act as the Lewis base component of a cooperative Lewis pair with tris(pentafluorophenyl)borane (B(C6F5)3). The synthesis of [Pt(CO)(L)] with the two diphosphine ligands, 1,2-bis(di-tert-butylphosphino)xylene (L1) and 1,2-bis(di-tert-butylphosphinooxy)benzene (L2), was achieved with L2 being a stronger π-acceptor than L1. The cooperative Lewis pair system [Pt(CO)(L)]/B(C6F5)3 successfully activated H2, ethene, phenylacetylene, THF, H2O and CO¬2. The difference in the electronics of the two systems and the use of isotopically labelled [Pt(13CO)(L)] led to a proposed pathway of activation for the small molecules tested. NMR spectroscopy was used to observe the binuclear Pt(I) cation, part of the ion pair [Pt2(µ-H)(µ-CO)(L)2][HC(O-B(C6F5)3)B(C6F5)2], as a key intermediate in the pathway of dihydrogen activation of the system.
Preliminary investigations into the modification of the Pt(0)/B Lewis pair system were made through variation of the ligand diphosphine backbone and it was found that a sufficiently bulky phosphine substituent was necessary to stabilise a Pt(0)-monocarbonyl complex. A range of Lewis acids were substituted for B(C6F5)3 and were tested for dihydrogen activation. Primarily in these cases, oxidative addition of H2 to [Pt(CO)(L)] was observed followed by cooperative action of the Lewis acid to form cation
[Pt2(µ-H)3(L)2]+, observable by NMR spectroscopy.
A new series of pendant amine diphosphine ligands (R’PCH2N(R)PR’ (PR’NRPR’), R’P = s¬-PhobP (9-phosphabicyclononane) or CgP (1,3,5,7-tetramethyl-2,4,8-trioxa-6-phosha-adamantane cage), R = Me, iPr, Bn) and the corresponding manganese complexes [Mn(CO)3(PNP)][BArF4] were synthesised as an intramolecular cooperative Lewis pair system. H2 activation was only successful when using PCgNRPCg with slower rates observed for larger N-substituents. Initial results for their use in the electrocatalytic oxidation of H2 is also presented.
Date of Award23 Jan 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorDuncan F Wass (Supervisor) & Paul G Pringle (Supervisor)

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