Mechanochemical synthesis, solvent-controlled coordination, and catalytic oxidation activity of furoic acid-based Mo(vi) complexes

Abstract

Polynuclear and mononuclear molybdenum(VI) complexes coordinated with water or methanol were synthesized using Schiff base ligands derived from the condensation of 2-furoic hydrazide with 2-hydroxybenzaldehyde (H₂L¹) or 2-hydroxy-5-nitrobenzaldehyde (H₂L²), emphasising a mechanochemical synthetic pathway. The complexes were characterized using comprehensive spectroscopic techniques, while single-crystal X-ray diffraction provided definitive structural elucidation for [MoO₂(L¹)(MeOH)] (1) and [MoO₂(L¹)(H₂O)] (3). Thermogravimetric analysis revealed insights into the thermal stability and decomposition pathways of the complexes. DFT calculations showed that solvent donor ability controls Mo(VI) coordination and aggregation, rendering μ-oxo dimer formation thermodynamically unfavourable. The catalytic performance of six Mo(VI) complexes was investigated for the oxidation of benzyl alcohol using tert-butyl hydroperoxide (TBHP) in aqueous medium, with systematic optimization of the oxidant-to-substrate ratio. To explore greener alternatives, H₂O₂ was also evaluated as an oxidant, and the influence of acetonitrile as a co-solvent and reaction temperature on catalytic efficiency was thoroughly studied. These results highlight the importance of ligand structure and solvent coordination in modulating catalytic activity. Overall, this study demonstrates that these Mo(VI) complexes serve as highly efficient and tunable catalysts for selective alcohol oxidation under mild and environmentally benign conditions. This work provides new insights into the design of molybdenum-based oxidation catalysts and emphasizes the potential of fine-tuning reaction parameters to achieve optimal catalytic performance.