Effect of chalcogens on CO insertion into the palladium-methyl bond of [(N∧N∧X)Pd(CH₃)]⁺ (X = O, S, Se) and on CO/ethylene copolymerisation

Neutral chloromethylpalladium(ii) complexes, [Pd(Cl)(CH₃)(L)] (1a-5a) with ligands κ²-N∧S-2-((3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl)-6-(phenylthiomethyl)pyridine (L1), κ²-N∧S-2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-6-(phenylthiomethyl)pyridine (L2), κ²-N∧Se-2-((3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl)-6-(phenylselanylmethyl)pyridine (L3), κ²-N∧Se-2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-6-(phenylselanylmethyl)pyridine (L4), and κ²-N∧N-2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-6-(phenoxymethyl)pyridine (L5) have been synthesised and characterised by various spectroscopic techniques. Ligands L1-L4 exhibit N_py∧S/Se bidentate coordination whereas L5 shows an N_py∧N_pz bidentate coordination mode in their corresponding neutral palladium complexes. Abstraction of chloride in neutral palladium complexes with NaBAr₄ (BAr₄ = tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate) resulted in the formation of the cationic palladium complexes 1b-5b, in which L1-L4 adopt a tridentate N_pz∧N_py∧X (X = S or Se) coordination mode in their respective cationic palladium complexes (1b-4b) whilst L5 in complex 5b adopts a N_py∧N_pz bidentate coordination mode and the palladium centre is stabilized by the weakly coordinating acetonitrile. Compounds 1b-5b readily undergo CO insertion into the Pd-CH₃ bond to form Pd-acyl that determines their ability to catalyse CO/ethylene copolymerisation to polyketones.