Oxidovanadium(V) complexes with tridentate hydrazone ligands as oxygen atom transfer catalysts

Four isostructural oxovanadium(V) complexes with hydrazone ligands have been synthesised, characterised, and evaluated as epoxidation and sulfoxidation catalysts. The reactions between [VO(acac)₂] (acac^– = acetylacetonate) and H₂Lⁿ (n = 1–4), precursors for monoanionic tridentate hydrazone ligands, afford complexes formulated as [VO(Lⁿ)(bzh)·MeOH] (1–4) when bidentate benzohydroxamic acid (Hbzh) is included as a co-ligand. Single crystal X-ray structure analyses showed that complexes 1–3 have a distorted octahedral coordination geometry with an O₅N coordination environment. Cyclic voltammetry showed that all complexes undergo two quasi-irreversible reduction peaks and a single irreversible oxidation peak. The bonding in 1 has been investigated by electronic structure calculations, and these data are discussed with respect to the electrochemical results. Complexes 1–4 were tested as catalysts for the epoxidation of cis-cyclooctene at 50 °C and sulfoxidation of methyl-p-tolylsulfide at room temperature using tert-butyl hydroperoxide (tBuOOH) and aqueous H₂O₂ as the terminal oxidants.