Vanadium(V) Complexes with Substituted 1,5-bis(2-hydroxybenzaldehyde)carbohydrazones and Their Use As Catalyst Precursors in Oxidation of Cyclohexane

Abstract

Six dinuclear vanadium(V) complexes have been synthesized: NH<sub>4</sub>[(VO<sub>2</sub>)<sub>2</sub>(<sup>H</sup>LH)] (NH<sub>4</sub>[<b>1</b>]), NH<sub>4</sub>[(VO<sub>2</sub>)<sub>2</sub>(<sup><i>t</i>‑Bu</sup>LH)] (NH<sub>4</sub>[<b>2</b>]), NH<sub>4</sub>[(VO<sub>2</sub>)<sub>2</sub>(<sup>Cl</sup>LH)]\n(NH<sub>4</sub>[<b>3</b>]), [(VO<sub>2</sub>)(VO)(<sup>H</sup>LH)(CH<sub>3</sub>O)] (<b>4</b>), [(VO<sub>2</sub>)(VO)(<sup><i>t</i>‑Bu</sup>LH)(C<sub>2</sub>H<sub>5</sub>O)]\n(<b>5</b>), and [(VO<sub>2</sub>)(VO)(<sup>Cl</sup>LH)(CH<sub>3</sub>O)(CH<sub>3</sub>OH/H<sub>2</sub>O)] (<b>6</b>) (where <sup>H</sup>LH<sub>4</sub> = 1,5-bis(2-hydroxybenzaldehyde)­carbohydrazone, <sup><i>t</i>‑Bu</sup>LH<sub>4</sub> = 1,5-bis(3,5-di-<i>tert</i>-butyl-2-hydroxybenzaldehyde)­carbohydrazone, and <sup>Cl</sup>LH<sub>4</sub> = 1,5-bis(3,5-dichloro-2-hydroxybenzaldehyde)­carbohydrazone).\nThe structures of NH<sub>4</sub>[<b>1</b>] and <b>4</b>–<b>6</b> have been determined by X-ray diffraction\n(XRD) analysis. In all complexes, the triply deprotonated ligand accommodates\ntwo V ions, using two different binding sites ONN and ONO separated\nby a diazine unit −N–N–. In two pockets of NH<sub>4</sub>[<b>1</b>], two identical VO<sub>2</sub><sup>+</sup> entities are present, whereas, in those of <b>4</b>–<b>6</b>, two different VO<sub>2</sub><sup>+</sup> and VO<sup>3+</sup> are bound. The highest oxidation state of V ions was corroborated\nby X-ray data, indicating the presence of alkoxido ligand bound to\nVO<sup>3+</sup> in <b>4</b>–<b>6</b>, charge density\nmeasurements on <b>4</b>, magnetic susceptibility, NMR spectroscopy,\nspectroelectrochemistry, and density functional theory (DFT) calculations.\nAll four complexes characterized by XRD form dimeric associates in\nthe solid state, which, however, do not remain intact in solution.\nCompounds NH<sub>4</sub>[<b>1</b>], NH<sub>4</sub>[<b>2</b>], and <b>4</b>–<b>6</b> were applied as alternative\nselective homogeneous catalysts for the industrially significant oxidation\nof cyclohexane to cyclohexanol and cyclohexanone. The peroxidative\n(with <i>tert</i>-butyl hydroperoxide, TBHP) oxidation of\ncyclohexane was performed under solvent-free and additive-free conditions\nand under low-power microwave (MW) irradiation. Cyclohexanol and cyclohexanone\nwere the only products obtained (high selectivity), after 1.5 h of\nMW irradiation. Theoretical calculations suggest a key mechanistic\nrole played by the carbohydrazone ligand, which can undergo reduction,\ninstead of the metal itself, to form an active reduced form of the\ncatalyst.