Unusually labile organoplatinum complexes: a mechanistic study of insertion–deinsertion into platinum–alkyl complexes of pyridine-2-carboxylic acid

Abstract

Platinum–alkyl complexes of the general formula [PtR(pyca)L][R = Me, Hpyca = pyridine-2-carboxylic acid, L = PPh₃, P(CH₂Ph)₃, P(C₆H₁₁)₃, pyridine or 4-methylpyridine; R = Et, L = PPh₃ or P(CH₂Ph)₃] and the complex [PtMe(hoqu)(PPh₃)](Hhoqu = 8-hydroxyquinoline) have been prepared and their carbonylation activity studied. The complexes show a large variation in carbonylation behaviour, which is markedly dependent on the ligands attached to the platinum centre. The donor capacity of L and the rigidity of the chelating ligand are major factors. For complexes containing pyca as the chelating ligand; where R = Me or Et and L = PPh₃, acyl complexes are readily prepared; where R = Et and L = P(CH₂Ph)₃, a mixture of two products is formed, the expected acyl complex [Pt(COEt)(pyca){P(CH₂Ph)₂}] and a compound with a terminal CO ligand in which the pyca ligand is monodentate [Pt(CO)(COEt)(pyca){P(CH₂Ph)₃}]; where L = P(C₆H₁₁), pyridine or 4-methylpyridine, no carbonylation occurs. With the complex containing the rigid hoqu ligand no carbonylation occurs. The complexes [Pt(COMe)(pyca)-(PPh₃)] and [PtEt(pyca)(PPh₃)] are unusually labile, eliminating CO or ethylene when warmed, giving [PtMe(pyca)(PPh₃)] or platinum–hydride complexes respectively. The behaviour of these complexes and the carbonylation behaviour of the platinum–alkyl complexes has been studied in detail and a mechanism for the insertion–deinsertion reactions is proposed.