New thiosemicarbazide and dithiocarbazate based oxidovanadium(iv) and dioxidovanadium(v) complexes. Reactivity and catalytic potential

Abstract

The Schiff bases {H₃dfmp-(smdt)₂} (I), {H₃dfmp-(sbdt)₂} (II) and {H₃dfmp-(tsc)₂} (III) are synthesized by reaction of 2,6-diformyl-4-methylphenol (H₃dfmp) and S-methyldithiocarbazate (smdt), S-benzyldithiocarbazate (sbdt) and thiosemicarbazide (tsc), respectively. Addition of [V^IVO(acac)₂] to solutions of these compounds in methanol leads to the formation of the oxidovanadium(IV) complexes [V^IVO{Hdfmp-(smdt)₂(CH₃OH)}] (1), [V^IVO{Hdfmp-(sbdt)₂(CH₃OH)}] (2) and [V^IVO{Hdfmp-(tsc)₂(CH₃OH)}] (3). All these V^IVO-compounds can be oxidized to the corresponding dioxidovanadium(V) (V^VO₂) complexes in methanolic solution upon aerial oxidation in the presence of KOH. The isolated compounds are K[V^VO₂{Hdfmp-(smdt)₂}] (4), K[V^VO₂{Hdfmp-(sbdt)₂}] (5) and K[V^VO₂{Hdfmp-(tsc)₂}] (6). The Cs⁺ salts of these complexes i.e. Cs[V^VO₂{Hdfmp-(smdt)₂}] (7), Cs[V^VO₂{Hdfmp-(sbdt)₂}] (8) and Cs[V^VO₂{Hdfmp-(tsc)₂}] (9) are prepared similarly in the presence of CsOH. All these compounds are characterized by various spectroscopic techniques like FT-IR, UV-visible, and ¹H and ⁵¹V NMR and thermal studies. IR spectral data confirm the coordination of ligands through the azomethine nitrogen, the sulphur and the phenolic oxygen atoms to the metal. These complexes show excellent catalytic activity and selectivity for the oxidation of benzyl alcohol and ethylbenzene in the presence of H₂O₂ as an oxidant. Various parameters such as the amount of catalyst and oxidant, reaction time, reaction temperature and solvent were taken into consideration to optimize these catalytic oxidations. Compound 7 was also remarkably efficient and selective in the catalytic oxidation of primary and secondary alcohols to the corresponding aldehyde/ketone, as well as of several aromatic compounds such as toluene, benzene, cumene and tetralin.