From structure to function: tunable electrical and catalytic properties in rare Mo(vi)-thiophene-2-carboxylic acid hydrazone complexes obtained mechanochemically

Abstract

This study focuses on the synthesis of molybdenum complexes coordinated with a thiophene-2-carboxylic acid hydrazone-type ligand (H₂L), obtained via mechanochemical reaction condensation of 2-hydroxybenzaldehyde with thiophene-2-carboxylic acid hydrazide. The Mo complexes were prepared via a green mechanochemical route, [MoO₂(L)(MeOH)] and [MoO₂(L)]_n, while the complex [MoO₂(L)(H₂O)] was obtained by classic solution-based synthetic pathway. All obtained compounds were characterized using attenuated total reflectance infrared spectroscopy (IR-ATR), elemental analysis (EA), and thermogravimetric analysis (TGA). The crystal and molecular structures of the ligand H₂L and the complexes [MoO₂(L)(MeOH)] and [MoO₂(L)(H₂O)]·(CH₃)₂CO were elucidated by means of single-crystal X-ray diffraction (SCXRD) analysis. Catalytic studies revealed that complex [MoO₂(L)(MeOH)] efficiently promoted the oxidation of benzyl alcohol under mild conditions, employing hydrogen peroxide as a green oxidant. Solid-state impedance spectroscopy (ss-IS) confirmed the semiconducting behavior, with DC conductivities of ∼10⁻¹² (Ω cm)⁻¹ and activation energies of ∼60–63 kJ mol⁻¹, consistent with electronic transport. Dielectric insights revealed frequency-dependent polarization processes dominated by Maxwell–Wagner interfacial effects, with dielectric constants of ∼11–13. The novelty of this work lies in the green development of rare Mo-based materials that uniquely combine structural, electrical, and catalytic features. Importantly, this study establishes correlations between these properties, representing, to the best of our knowledge, one of the first systematic investigations of molybdenum coordination complexes incorporating thiophene–carbohydrazide ligands.