Theoretical studies of the oxidative addition of azolium salts to a model Wilkinson's catalyst.
The oxidative addition of 1,3-dimethylimidazolium to a model Wilkinson's catalyst (RhCl(PH(3))(3)) has been studied with density functional calculations (B3LYP). According to our free energy calculations, the octahedral rhodium carbene hydride product forms from initial predissociation of a phosphine molecule to subsequently form a 5 ligand intermediate; however, results indicate that a six ligand, associative route with a concerted three-centred transition structure may also be competitive. Exchange of the phosphine molecule on the metal centre with trimethylphosphine had a significant effect in lowering the barrier to oxidative addition and decreasing the endothermicity of the reaction. Solvation was found to have a moderate effect on the overall reaction. Bulk solvent calculations reflected a relative stabilisation of reactants for both pathways, resulting in an endothermic overall reaction. A study of alternative azolium salts revealed the saturated 1,3-dimethyl-4,5-dihydroimidazolium resulted in little change to the reaction geometries or energies, while the use of 3-methylthiazolium salt significantly reduced the barrier to addition and increased the exothermicity of the reaction considerably.
Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF10 3TB, Wales, UK.
Kirsty J
KJ
Hawkes
David S
DS
McGuinness
Kingsley J
KJ
Cavell
Brian F
BF
Yates