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†
Abstract
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 Npy^S/Se bidentate coordination whereas L5 shows an Npy^Npz 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 Npz^Npy^X (X = S or Se) coordination mode in their respective cationic palladium complexes (1b–4b) whilst L5 in complex 5b adopts a Npy^Npz 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.