Novel cationic and neutral Pd(II) complexes bearing imidazole based chelate ligands: synthesis, structural characterisation and catalytic behaviour

A variety of neutral and cationic methyl palladium(II) complexes [Pd(CH3)(Cl)(L–L)] (3) and [Pd(CH3)(CH3CN)(L–L)]BF4 (4), containing known and new N-methylimidazole-2-yl (mim) and N-methylbenzimidazole-2-yl (bmim) based chelate ligands (L–L) have been synthesised. The dichloro complexes [Pd(Cl)2{(mim)2CO}] (2b) and [Pd(Cl)2(mim)PPh2] (2e) have also been synthesised. The crystal structures of [Pd(Cl)2{(mim)2CNPh}] (2d) and [Pd(CH3)(Cl){(bmim)2CO}] (3c) have been determined and show a square planar geometry at palladium(II) with an appreciable diminished bite angle (85.6°) in 3c compared to 2d (88.5°). The molecular structure of 2d and 3c show both bisimidazole ligands coordinated via the ring nitrogen donors and inclination of the chelate ring bridge to the coordination plane (boat conformation). Low temperature NMR spectra indicate boat-to-boat inversion of the chelate ring in [Pd(CH3)(CH3CN){(mim)2CO}]+ (4b). Examples of the neutral and cationic complexes were tested in the Heck reaction and CO/ethylene copolimerisation respectively. The CO bridged bisimidazole complex (3b) exhibited high activity (TON of 100 000) in the Heck reaction whereas the closely related benzimidazole complex 3c was about half as active (TON of 53 000). Low activities were found for the cationic complex 4b in the CO/ethylene copolymerisation (TON 449).