Copper(ii) and iron(iii) complexes with arylhydrazone of ethyl 2-cyanoacetate or formazan ligands as catalysts for oxidation of alcohols

Abstract

The aquasoluble [Cu(1κN,O²:2κO-HL¹)(S)]₂ [S = CH₃OH (1), (CH₃)₂NCHO (2)] and [Cu(κN-HL¹)(en)₂]·CH₃OH·H₂O (3) Cu^II complexes were prepared by the reaction of Cu^II nitrate hydrate with the new ligand (E/Z)-4-(2-(1-cyano-2-ethoxy-2-oxoethylidene)hydrazinyl)-3-hydroxybenzoic acid (H₃L¹), in the presence (for 3) or absence (for 1 and 2) of ethylenediamine (en), while the Fe^III complex [Fe(κN³-HL²)₂] (4) was isolated by treatment of iron(III) chloride hexahydrate with the new ligand (1E,1E)-N′,2-di(1H-1,2,4-triazol-3-yl)diazenecarbohydrazonoyl cyanide (H₃L²), and characterized by elemental analysis, IR spectroscopy and single crystal X-ray diffraction. Cooperative E,Z → E isomerization of H₃L¹, induced by coordination and ionic interactions, occurs upon interaction with Cu^II in the presence of en. Complexes 1–4 act as catalyst precursors for the solvent-free microwave (MW) assisted selective oxidation of primary or secondary alcohols and diols to the corresponding aldehydes, ketones and diketones, respectively, with yields in the 5–99% range (TONs up to 4.96 × 10²) after 60 min of MW irradiation at 120 °C. The influence of temperature, time and organic radicals was studied and also the regioselective oxidation of the catalytic systems involving the primary and secondary alcohols.