Cationic methylpalladium(II) complexes containing bidentate N–O ligands as catalysts for the copolymerisation of CO and ethylene. Identification and isolation of intermediates from the stepwise insertion reactions, and subsequent detailed mechanistic interpretation‡

Abstract

A series of cationic methylpalladium(II) complexes containing bidentate N–O ligands, of the general formula [PdMe(N–O)L]BF₄ (N–O = methyl picolinate, methyl 6-methylpicolinate, N,N-diisopropylpicolinamide, 6-methyl- N,N-diisopropylpicolinamide; L = PPh₃ or PCy₃) have been prepared and characterised. The solid-state structure of [PdMe(N–O)(PPh₃)]BF₄ (N–O = N,N-diisopropylpicolinamide), in comparison with that for the complex with N–O = methyl picolinate, indicates a significant lengthening of the Pd–P bond [Δ(Pd–P) = 0.018(3) Å] possibly due to the presence of the more strongly co-ordinating N–O ligand. Complexes with L = PPh₃ were found to be active for the copolymerisation of CO and ethylene to give polyketone. The complexes [PdMe(N–O)(PPh₃)]BF₄ (N–O = methyl 6-methylpicolinate or diisopropylpicolinamide) have the highest catalytic activities (80 g polymer per g Pd per hour and 58 g polymer per g Pd per hour respectively, at 20 °C). Examples of the complexes form simple acyl complexes when treated with CO at room temperature and pressure and the spectroscopic data of the resulting acetyl complexes are reported. The stepwise migratory insertion of CO and ethylene into the complex [PdMe(N–O)(PPh₃)]BF₄ (N–O = methyl picolinate) has been carefully monitored and the individual insertion products have been characterised. Insertion of ethylene into the Pd–acyl bond of [Pd(COMe)(N–O)(PPh₃)]BF₄ (N–O = methyl picolinate) affords one of the first examples of an isolable product from insertion of an unstrained alkene into a Pd–acyl bond. A detailed mechanism for the co-reaction of CO and ethylene catalysed by complexes containing chelate ligands with distinct donor groups is discussed and an explanation of the observed reaction behaviour provided. The proposed mechanism represents one of the most comprehensive interpretations of this important reaction.