Synthesis, Characterization and Aerobic Alcohol Oxidation Catalysis of Palladium(II) Complexes with a Bis(imidazolyl)borate Ligand

摘要

Chloro- and hydroxo-palladium(II) complexes with an imidazole-based scorpionate ligand bis(1-methylimidazolyl)methylphenylborate, [B(Im(N-Me))(2)MePh](-) (= L-Ph), have been synthesized. Reaction of a lithium salt of L-Ph with [PdCl2(NCPh)(2)] yields a miscellaneous mixture involving a homoleptic complex [Pd(L-Ph)(2)]. Replacement of the neutral ligand (benzonitrile) in the starting material with pyridine leads to the desired chloride complex 1. X-ray crystallographic analysis reveals the formula of 1 to be [PdCl(L-Ph)(py)] with a square-planar palladium(II) center supported by the bidentate L-Ph ligand. Treatment of 1 with NaOH yields non-pyridine-containing hydroxo species with L-Ph such as [Pd(OH)(L-Ph)(H2O)] and [(PdLPh)(2)(mu-OH)(2)]. The hydroxo species 2 can convert into active species for catalytic secondary alcohol oxidation with O-2 in the absence of extra base. The utility of 2 as a catalyst precursor and the inertness of chloride complex 1 suggest that the basic nature of the co-ligand (i.e. OH-) on the palladium center supported by L-Ph is essential to the formation of a putative palladium-alkoxide intermediate.