Design of vanadium complexes with a thiophene-based hydrazone ligand for selective benzyl and salicyl alcohol oxidation

Abstract

Two vanadium complexes incorporating a thiophene-2-carboxylic acid hydrazone ligand (H2L) were synthesized and structurally characterized. The ligand was obtained by mechanochemical condensation of 2-hydroxybenzaldehyde with thiophene-2-carboxylic acid hydrazide, while the vanadium complexes were prepared via a solution-based synthetic approach. Crystallization from methanol afforded compound [VO(L)(OMe)(MeOH)] (1), whereas crystallization from dichloromethane or acetone yielded compound [V2O3(L)2] (2). The ligand and complexes were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), elemental analysis (EA), and thermogravimetric analysis (TGA). Additionally, the crystal structures of the vanadium complexes were determined by single-crystal X-ray diffraction (SCXRD). Structural stability of the material later employed as a catalyst, the methanol-derived complex [VO(L)(OMe)(MeOH)] (1), was investigated through temperature-dependent FTIR spectroscopy (TD-FTIR) and temperature-dependent powder X-ray diffraction (TD-PXRD). Catalytic investigations of [VO(L)(OMe)(MeOH)] (1) and [V2O3(L)2] (2) demonstrated that both vanadium complexes efficiently catalyzed the oxidation of benzyl alcohol and salicyl alcohol under mild reaction conditions. The combined structural, thermal, and catalytic studies provide insight into the relationships among the coordination environment, structural stability, and catalytic performance of vanadium complexes containing thiophene-2-carbohydrazide-derived ligands.