Synthesis and carbonylation studies on methylpalladium(II) complexes containing chelating pyridinecarboxylate ligands: dynamic behaviour of the ligands and implications for the carbonylation mechanism

Abstract

Methylpalladium complexes of the general formula [PdMe(pyca)(L)][pyca = pyridine-2-carboxylate, L = PPh₃, PMePh₂, PMe₂Ph, P(C₆H₄Me-p)₃, P(CH₂Ph)₃, P(C₆H₁₁)₃, PPrⁱ₃, P(OMe)₃, pyridine, 4-methylpyridine or NMe₂Ph] were synthesised and the dynamic behaviour of the ligands pyca and L studied in detail. The ligands were found to undergo readily a variety of exchange reactions, clearly demonstrating their lability under miid conditions. The ligand exchange behaviour and data on rates of carbonylation for the palladium complexes led to a dissociative mechanism, requiring lability in both the ligand L and the chelate, being proposed for the CO insertion process. The methylpalladium complexes exist in solution as a mixture of cis and trans isomers. However the acyl complexes were found to occur only in the geometry with the phosphine cis to the acyl group. X-Ray crystal structures were obtained for the complexes [Pd(COMe)(pyca){P(CH₂Ph)₃}] and [Pt(COMe)(pyca)(PPh₃)]. The complexes have distorted square-planar geometry: for the palladium complex, which is highly distorted, P–Pd–O(21) 97.3(1), P–Pd–C(01) 88.9(2), N–Pd–O(21) 78.4(2) and N–Pd–C(01) 95.9(2)° and for the platinum complex P–Pt–O(21) 93.9(5), P–Pt–C(01) 95.1(9), N–Pt–O(21) 80.4(7) and N–Pt–C(01) 91(1)°.