Synthesis, characterization and structure of nickel and copper compounds containing ligands derived from keto-enehydrazines and their catalytic application for aerobic oxidation of alcohols

Abstract

Ligand precursors HL^R,Ph (R = Me, Ph) were synthesised by condensation of acetylacetone and the corresponding N,N-substituted hydrazines and were characterised spectroscopically and structurally. Both in the solid state and in solution they behave as (Z)-keto-enehydrazines and this was confirmed by DFT calculations which showed that this form was the most stable of their possible tautomers. The reaction of HL^R,Ph compounds with copper acetate and nickel acetate in EtOH afforded the corresponding complexes [M(L^R,Ph)₂] (M = Cu, Ni; R = Me, Ph). The methyl-substituted derivatives were structurally characterised by X-ray methods. A four-coordinate environment around the metal centre, where the two L^Me,Ph ligands act as bidentate N,O-chelators and lie in a pseudo-trans conformation, was found for both compounds. The dihedral angle between the two six-membered metallacycles M(L^Me,Ph) was 0° for nickel, a typical square planar coordination, meanwhile it was 23° for copper, a square planar slightly distorted to pseudotetrahedral coordination. Copper complexes [Cu(L^R,Ph)₂] were tested as catalysts, in combination with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical), for the aerobic oxidation of 4-nitrobenzyl alcohol as the model reaction. An almost complete conversion to the corresponding aldehyde was observed after 1 h at 60 °C and 1 bar of dioxygen, in toluene as the solvent. Importantly, air at atmospheric pressure was also observed to be appropriate for the oxidation, although longer reaction times were required. 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 yield was found for 1-octanol.