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

Chloro- and hydroxo-palladium(II) complexes with an imidazole-based scorpionate ligand bis(1-methylimidazolyl)methylphenylborate, [B(ImN–Me)2MePh]– (= LPh), have been synthesized. Reaction of a lithium salt of LPh with [PdCl2(NCPh)2] yields a miscellaneous mixture involving a homoleptic complex [Pd(LPh)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(LPh)(py)] with a square-planar palladium(II) center supported by the bidentate LPh ligand. Treatment of 1 with NaOH yields non-pyridine-containing hydroxo species with LPh such as [Pd(OH)(LPh)(H2O)] and [(PdLPh)2(µ-OH)2]. The hydroxo species 2 can convert into active species for catalytic secondary alcohol oxidation with O2 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 LPh is essential to the formation of a putative palladium–alkoxide intermediate.