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)(C l)(L-L)] (3) and [Pd(CH3)(CH3CN)(L-L)]BF4 (4), containing known and new N-m ethylimidazole-2-yl (mim) and N-methylbenzimidazole-2-yl (bmim) based chela te ligands (L-L) have been synthesised. The dichloro complexes [Pd(Cl)(2){( mim)(2)CO}] (2b) and [Pd(Cl)(2)(mim)PPh2] (2e) have also been synthesised. The crystal structures of [Pd(Cl)(2){(mim)(2)C=NPh}] (2d) and [Pd(CH3)(Cl){ (bmim)(2)CO}] (3c) have been determined and show a square planar geometry a t palladium(II) with an appreciable diminished bite angle (85.6 degrees) in 3c compared to 2d (88.5 degrees). The molecular structure of 2d and 3c sho w both bisimidazole ligands coordinated via the ring nitrogen donors and in clination of the chelate ring bridge to the coordination plane (boat confor mation). Low temperature NMR spectra indicate boat-to-boat inversion of the chelate ring in [Pd(CH3)(CH3CN){(mim)(2)CO}](+) (4b). Examples of the neut ral and cationic complexes were tested in the Heck reaction and CO/ethylene copolimerisation respectively. The C=O bridged bisimidazole complex (3b) e xhibited high activity (TON of 100 000) in the Heck reaction whereas the cl osely 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/ethyl ene copolymerisation (TON 449). (C) 2000 Elsevier Science S.A. All rights r eserved.