Carbonylation of hydrocarbylpalladium(II) complexes containing substituted pyridinecarboxylate chelating ligands. Steric and electronic manipulation of the CO-insertion mechanism

Abstract

Organopalladium(II) complexes of general formula [PdR(N–O)L][R = Ph, N–O = pyridinecarboxylate (pyca), L = P(C₆H₁₁)₃; R = Ph, N–O = 6-methylpyridinecarboxylate (mpyca), L = PPh₃, PMePh₂, P(C₆H₁₁)₃ or PEt₃; R = Ph, N–O = 4-nitropyridinecarboxylate (npyca), L = P(C₆H₁₁)₃; R = Ph, N–O = 6-methyl-4-nitropyridinecarboxylate, L = P(C₆H₁₁)₃; R = Me, N–O = mpyca, L = PPh₃, P(CH₂Ph)₃ or P(C₆H₁₁)₃; R = Me, N–O = npyca, L = PPh₃, PMePh₂ or P(C₆H₁₁)₃] have been prepared, and their carbonylation reactions studied in detail. Kinetic studies of the CO-insertion process have indicated that the rate of reaction decreases as the basicity of the phosphine, L, increases. Complexes containing the highly basic phosphine P(C₆H₁₁)₃ only undergo carbonylation if hemilability of the chelating ligand is promoted (by substitution of the N–O chelate). Substitution of the N–O ligand modifies the carbonylation pathway and provides an alternative route from that generally observed for palladium(II) and platinum(II) hydrocarbyl complexes of pyca. A mechanism for insertion of CO involving partial dissociation of the N–O chelate is proposed for these complexes. The crystal stucture of [PdMe(mpyca)(PPh₃)] has been determined. The complex has square-planar co-ordination with the nitrogen of pyca trans to the phosphorus. Considerable distortion of the inner co-ordination sphere is evident, caused by steric interaction between the σ-methyl ligand and the methyl group on the N–O ligand.