Six dinuclear vanadium(V) complexes have been synthesized: NH₄[(VO₂)₂(^HLH)] (NH₄[1]), NH₄[(VO₂)₂(^t-BuLH)] (NH₄[2]), NH₄[(VO₂)₂(^ClLH)] (NH₄[3]), [(VO₂)(VO)(^HLH)(CH₃O)] (4), [(VO₂)(VO)(^t-BuLH)(C₂H₅O)] (5), and [(VO₂)(VO)(^ClLH)(CH₃O)(CH₃OH/H₂O)] (6) (where ^HLH₄ = 1,5-bis(2-hydroxybenzaldehyde)carbohydrazone, ^t-BuLH₄ = 1,5-bis(3,5-di-tert-butyl-2-hydroxybenzaldehyde)carbohydrazone, and ^ClLH₄ = 1,5-bis(3,5-dichloro-2-hydroxybenzaldehyde)carbohydrazone). The structures of NH₄[1] and 4–6 have been determined by X-ray diffraction (XRD) analysis. In all complexes, the triply deprotonated ligand accommodates two V ions, using two different binding sites ONN and ONO separated by a diazine unit −N–N–. In two pockets of NH₄[1], two identical VO₂⁺ entities are present, whereas, in those of 4–6, two different VO₂⁺ and VO³⁺ are bound. The highest oxidation state of V ions was corroborated by X-ray data, indicating the presence of alkoxido ligand bound to VO³⁺ in 4–6, charge density measurements on 4, magnetic susceptibility, NMR spectroscopy, spectroelectrochemistry, and density functional theory (DFT) calculations. All four complexes characterized by XRD form dimeric associates in the solid state, which, however, do not remain intact in solution. Compounds NH₄[1], NH₄[2], and 4–6 were applied as alternative selective homogeneous catalysts for the industrially significant oxidation of cyclohexane to cyclohexanol and cyclohexanone. The peroxidative (with tert-butyl hydroperoxide, TBHP) oxidation of cyclohexane was performed under solvent-free and additive-free conditions and under low-power microwave (MW) irradiation. Cyclohexanol and cyclohexanone were the only products obtained (high selectivity), after 1.5 h of MW irradiation. Theoretical calculations suggest a key mechanistic role played by the carbohydrazone ligand, which can undergo reduction, instead of the metal itself, to form an active reduced form of the catalyst.

Template condensation of 3,5-di-tert-butyl-2-hydroxybenzaldehyde S-methylisothiosemicarbazone with pentane-2,4-dione and triethyl orthoformate at elevated temperatures resulted in metal complexes of the type M^IIL, where M = Ni and Cu and H₂L = a novel tetradentate ligand. These complexes are relevant to the active site of the copper enzymes galactose oxidase and glyoxal oxidase. Demetalation of Ni^IIL with gaseous hydrogen chloride in chloroform afforded the metal-free ligand H₂L. Then by the reaction of H₂L with Zn(CH₃COO)₂·2H₂O in a 1:1 molar ratio in 1:2 chloroform/methanol, the complex Zn^IIL(CH₃OH) was prepared. The three metal complexes and the prepared ligand were characterized by spectroscopic methods (IR, UV−vis, and NMR spectroscopy), X-ray crystallography, and DFT calculations. Electrochemically generated one-electron oxidized metal complexes [NiL]⁺, [CuL]⁺, and [ZnL(CH₃OH)]⁺ and the metal-free ligand cation radical [H₂L]^+• were studied by EPR/UV−vis−NIR and DFT calculations. These studies demonstrated the interaction between the metal ion and the phenoxyl radical.

Mononuclear nickel(II), copper(II), and manganese(III) complexes with a noninnocent tetradentate Schiff base ligand containing a disiloxane unit were prepared in situ by reaction of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with 1,3-bis(3-aminopropyl)tetramethyldisiloxane followed by addition of the appropriate metal(II) salt. The ligand H₂L resulting from these reactions is a 2:1 condensation product of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with 1,3-bis(3-aminopropyl)tetramethyldisiloxane. The resulting metal complexes, NiL·0.5CH₂Cl₂, CuL·1.5H₂O, and MnL(OAc)·0.15H₂O, were characterized by elemental analysis, spectroscopic methods (IR, UV–vis, X-band EPR, HFEPR, ¹H NMR), ESI mass spectrometry, and single crystal X-ray diffraction. Taking into account the well-known strong stabilizing effects of tert-butyl groups in positions 3 and 5 of the aromatic ring on phenoxyl radicals, we studied the one-electron and two-electron oxidation of the compounds using both experimental (chiefly spectroelectrochemistry) and computational (DFT) techniques. The calculated spin-density distribution and localized orbitals analysis revealed the oxidation locus and the effect of the electrochemical electron transfer on the molecular structure of the complexes, while time-dependent DFT calculations helped to explain the absorption spectra of the electrochemically generated species. Hyperfine coupling constants, g-tensors, and zero-field splitting parameters have been calculated at the DFT level of theory. Finally, the CASSCF approach has been employed to theoretically explore the zero-field splitting of the S = 2 MnL(OAc) complex for comparison purposes with the DFT and experimental HFEPR results. It is found that the D parameter sign strongly depends on the metal coordination geometry.