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

Organopalladium(II) complexes of general formula [PdR(N-O)L] [R = Ph, N-O = pyridinecarboxylate (pyca), L = P(C 6 H 11 ) 3 ; R = Ph, N-O = 6-methylpyridinecarboxylate (mpyca), L = PPh 3 , PMePh 2 , P(C 6 H 11 ) 3 or PEt 3 ; R = Ph, N-O = 4-nitropyridinecarboxylate (npyca), L = P(C 6 H 11 ) 3 ; R = Ph, N-O = 6-methyl-4-nitropyridinecarboxylate, L = P(C 6 H 11 ) 3 ; R = Me, N-O = mpyca, L = PPh 3 , P(CH 2 Ph) 3 or P(C 6 H 11 ) 3 ; R = Me, N-O = npyca, L = PPh 3 , PMePh 2 or P(C 6 H 11 ) 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(C 6 H 11 ) 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)(PPh 3 )] 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 interactions betwen the σ-methyl ligand and the methyl group on the N-O ligand.