Synthesis and structure of nickel and copper complexes containing the N-allyl-o-hydroxyacetophenoniminato ligand and the application of copper complex as catalyst for aerobic alcohol oxidations

Abstract

The ligand precursor 2-(1-(allylimino)ethyl)phenol, HL, was synthesized by condensation of o-hydroxyacetophenone and allylamine, in the absence of solvent, and was analytically and spectroscopically characterized. The reaction of compound HL with nickel acetate and copper acetate in the presence of NEt afforded the corresponding complexes [M(L)] (M = Ni, 1; Cu, 2), which were structurally characterized by X-ray diffraction methods. A four-coordinate square planar environment around the metal centre was found for both compounds, in which the two N-allyl-o-hydroxyacetophenoniminato ligands, L, act as bidentate N,O-chelators and lie in a pseudo-trans conformation. Copper complex [Cu(L)], 2, was tested as catalyst, in combination with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical), for the aerobic oxidation of benzyl alcohol as model reaction. Importantly, by using air at atmospheric pressure and room temperature, good yields of the corresponding aldehyde were observed after 18 h in toluene as solvent. After the optimization of the reaction conditions, the study was extended to other alcohol substrates and good catalytic activity was found for benzylic-type alcohols, while low-medium yield was found for 1-octanol. Interestingly, the use of ionic liquids [Cmim]PF and [Cmim]PF (Cmim = 1-butyl-3-methylimidazolium; Cmim = 1-octyl-3-methylimidazolium) as reaction solvents gave comparable aldehyde yields in the Cu/TEMPO-catalyzed oxidation of benzylic-type alcohols and this allowed the catalyst recycling. In fact, benzaldehyde yields remained approximately constant after three catalytic cycles in [Cmim]PF, but from fourth cycle a small drop in activity was detected.