Vanadium complexes having [VO]2+, [VO]3+ and [VO2]+ cores with hydrazones of 2,6-diformyl-4-methylphenol: synthesis, characterization, reactivity, and catalytic potential

Abstract

The Schiff bases H₃dfmp(L)₂ obtained by the condensation of 2,6-diformyl-4-methylphenol and hydrazones [L = isonicotinoylhydrazide (inh), nicotinoylhydrazide (nah) and benzoylhydrazide (bhz)] are prepared and characterized. By reaction of [V^IVO(acac)₂] and the H₃dfmp(L)₂ in methanol the V^IVO-complexes [V^IVO{Hdfmp(inh)₂}(H₂O)] (1), [V^IVO{Hdfmp(nah)₂}(H₂O)] (2) and [V^IVO{Hdfmp(bhz)₂}(H₂O)] (3) were obtained. Upon their aerial oxidation in methanol [V^VO(OMe)(MeOH){Hdfmp(inh)₂}] (4), [V^VO(OMe)(MeOH){Hdfmp(nah)₂}] (5) and [V^VO(OMe)(MeOH){Hdfmp(bhz)₂}] (6) were isolated. In the presence of KOH, oxidation of 1–3 results in the formation of [V^VO₂{H₂dfmp(inh)₂}]_n·5H₂O (7), K[V^VO₂{Hdfmp(nah)₂}] (8) and K[V^VO₂{Hdfmp(bhz)₂}] (9). All compounds are characterized in the solid state and in solution, namely by spectroscopic techniques (IR, UV-Vis, EPR, ¹H, ¹³C and ⁵¹V NMR), and DFT is also used to calculate the V^IV hyperfine coupling constants of V^IV-compounds and ⁵¹V NMR chemical shifts of several V^V-species and assign them to those formed in solution. Single crystal X-ray analysis of [V^VO(OMe)(MeOH){Hdfmp(bhz)₂}] (6) and [V^VO₂{H₂dfmp(inh)₂}]_n·5H₂O (7) confirm the coordination of the ligand in the dianionic (ONO²⁻) enolate tautomeric form, one of the hydrazide moieties remaining non-coordinated. In the case of 7 the free N(pyridine) atom of the inh moiety coordinates to the other vanadium center yielding a polynuclear complex in the solid state. It is also demonstrated that the V^VO₂-complexes are catalyst precursors in the oxidative bromination of styrene by H₂O₂, therefore acting as functional models of vanadium dependent haloperoxidases. Plausible intermediates involved in the catalytic process are established by UV-Vis, ⁵¹V NMR and DFT studies.