Acetoxime coordinates to palladium(II) and is catalytically hydrolysed to acetone†

Abstract

Several new palladium(II) complexes containing acetoxime as a unidentate ligand were synthesized from cis-[Pd(en)(solv)2]2+1a and cis-[Pd(dtod)(solv)2]2+1b, in which the displaceable ligand solv is water or acetone, en is ethane-1,2-diamine, and dtod is 3,6-dithia-1,8-octanediol. The acetoxime complexes are characterized by UV-visible spectrophotometry and 1H and 13C NMR spectroscopy in solution. Acetoxime in the mono-oxime complexes cis-[Pd(en){N(OH)C(CH3)2}(solv)]2+2a and cis-[Pd(dtod){N(OH)C(CH3)2}(solv)]2+2b undergoes hydrolysis to acetone and hydroxylamine. The proposed mechanism involves internal attack of a PdII-bound hydroxo ligand at the coordinated acetoxime. This palladium(II)-catalysed hydrolysis is at least 104 times faster than hydrolysis in the absence of a catalyst. The rate enhancement arises from polarization of acetoxime upon coordination to palladium(II), the availability of the nucleophilic hydroxo ligand, and close proximity of these two species. The complex [Pd(dien){N(OH)C(CH3)2}]2+, which contains the tridentate diethylenetriamine ligand, is almost unreactive toward hydrolysis because it lacks a PdII-bound aqua or hydroxo ligand, so that the reaction occurs via the less-favorable external attack of solvent water. Acetoxime in the bis-acetoxime complex cis-[Pd(dtod){N(OH)C(CH3)2}2]2+3b hydrolyses very slowly because this complex also lacks aqua or hydroxo ligands. Therefore, this complex was crystallized and its structure determined by X-ray crystallography.