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Title: Cobalt PCP pincer and rhodium diphosphine complexes as methanol homologation catalysts
Author: Kent, Mark Alexander
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2012
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The first two examples of cobalt phosphine PCP pincer complexes, [Co{2,6- (CH2PPh2-KPhC6H3-K[1}(COh] and [Co{2,6-(CH2PPh2-KPhC6H3-KC1}(PMe3h] have been synthesised from the precursors [CoCl(PMe3)3] and [CO(CH3) (PMe3)4]. Both pincer complexes possess pseudo-trigonal bipyramidal geometry with the coordinating pincer P atoms in equatorial sites and the pincer C atom in an axial site. A route to these pincer complexes has been discovered which involves a rearrangement with loss of PMe3 from the cyclometallated complex [Co{2-(CH2PPh2-KP)-4-(CH2PPh2)C6H3-K[1}(PMe3)3]. The mechanism for this rearrangement has been probed by both kinetic studies and deuterium-labelling studies, which suggests that two mechanisms operate, one of which involves a series of intramolecular C-H oxidative additions and reductive eliminations to effectively 'shuttle' protons around the complex. The other mechanism is suggested to involve a proton source, which may be the solvent, THF. The preparation of some rare examples of cobalt phosphinite PCP pincer complexes based on the diphosphinites 1,3-bis(diphenylphosphinooxy)benzene and 1,3-bis(diphenylphosphinooxy)-4,6-(di-tert-butyl)benzene is also reported. The synthesis of these pincers proceeds via an isolated cobalt(III) pincer complex with hydride, chloride and PMe3 ancillary ligands. The subsequent reduction to cobalt(l) pincer complexes proceeds using NEt3 or LiAlH4 as reducing agents. The new propane-backboned diphosphines 1,3-bis(SH-benzo[b ]phosphindol-S- yl)propane and 1,3-bis(di-2-naphthylphosphino)propane were synthesised. These diphosphines were complexed with [{RhCl(COh}Z] to afford the corresponding dinuclear 12-membered metallacycles [{RhCl(CO)(Il-diphosphine)}2], the latter of which possesses two rotational isomers which involves the exchange of the positions of one Cl and one CO ligand. The diphosphines 1,3-bis(SH- benzo [b] phosphindol-S-yl) propane and o-di phenyl phosphino benzyldi phenyl- phosphine were complexed with [Rh(CODh]BF4 to afford cis-chelating mononuclear Rh(l) complexes. The application of these phosphines and complexes as catalysts for methanol reductive carbonylation and homologation has been investigated. The catalyst involving the tetrahydrochloride salt of 1,3-bis( di-2-pyridylphosphino )propane exhibits methanol reductive carbonylatiori/homologation selectivity superior to the best previously reported literature system. Also, the catalyst involving the diphosphine 1,3-bis(SH-benzo[b ]phosphindol-S-yl)propane exhibits promising stability and selectivity to acetic acid under reductive carbonylation conditions, signalling a potential application as a methanol carbonylation catalyst.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available