Reaction between [VO(acac)₂] and H₂L (H₂L are the hydrazones H₂sal-nah I or H₂sal-fah II; sal = salicylaldehyde, nah = nicotinic acid hydrazide and fah = 2-furoic acid hydrazide) in methanol leads to the formation of oxovanadium(IV) complexes [VOL·H₂O] (H₂L = I: 1, H₂L = II: 4). Aerial oxidation of the methanolic solutions of 1 and 4 yields the dinuclear oxo-bridged monooxovanadium(V) complexes [{VOL}₂µ-O] (H₂L = I: 2, H₂L = II: 5). These dinuclear complexes slowly convert, in excess methanol, to [VO(OMe)(MeOH)L] (H₂L = I: 9, H₂L = II: 10), the crystal and molecular structures of which have been determined, confirming the ONO binding mode of the dianionic ligands in their enolate form. Reaction of aqueous K[VO₃] with the ligands at pH ca. 7.5 results in the formation of [K(H₂O)][VO₂L] (H₂L = I: 3, H₂L = II: 6). Treatment of 3 and 6 with H₂O₂ yields (unstable) oxoperoxovanadium(V) complexes K[VO(O₂)L], the formation of which has been monitored spectrophotometrically. Acidification of methanolic solutions of 3 and 6 with HCl affords oxohydroxo complexes, while the neutral complexes [VO₂(Hsal-nah)] 7 and [VO₂(Hsal-fah)] 8 were isolated on treatment of aqueous solutions of 3 and 6 with HClO₄. These complexes slowly transform into 9 and 10 in methanol, as confirmed by ¹H, ¹³C and ⁵¹V NMR. The anionic complexes 3 and 6 catalyse the oxidative bromination of salicylaldehyde in water in the presence of H₂O₂/KBr to 5-bromosalicylaldehyde and 3,5-dibromosalicylaldehyde, a reaction similar to that exhibited by vanadate-dependent haloperoxidases. They are also catalytically active for the oxidation of benzene to phenol and phenol to catechol and p-hydroquinone.

Binuclear, µ-bis(oxo)bis{oxovanadium(V)} complexes [(VOL)₂(µ-O)₂] (2 and 7) (where HL are the hydrazones Hacpy-nah I or Hacpy-fah II; acpy = 2-acetylpyridine, nah = nicotinic acid hydrazide and fah = 2-furoic acid hydrazide) were prepared by the reaction of [VO(acac)₂] and the ligands in methanol followed by aerial oxidation. The paramagnetic intermediate complexes [VO(acac)(acpy-nah)] (1) and [VO(acac)(acpy-fah)] (6) have also been isolated. Treatment of [VO(acac)(acpy-nah)] and [VO(acac)(acpy-fah)] with aqueous H₂O₂ yields the oxoperoxovanadium(V) complexes [VO(O₂)(acpy-nah)] (3) and [VO(O₂)(acpy-fah)] (8). In the presence of catechol (H₂cat) or benzohydroxamic acid (H₂bha), 1 and 6 give the mixed chelate complexes [VO(cat)L] (HL = I: 4, HL = II: 9) or [VO(bha)L] (HL = I: 5, HL = II: 10). Complexes 4, 5, 9 and 10 slowly convert to the corresponding oxo-µ-oxo species 2 and 7 in DMF solution. Ascorbic acid enhances this conversion under aerobic conditions, possibly through reduction of these complexes with concomitant removal of coordinated catecholate or benzohydroxamate. Acidification of 7 with HCl dissolved in methanol afforded a hydroxo(oxo) complex. The crystal and molecular structure of 2·1.5H₂O has been determined, and the structure of 7 re-determined, by single crystal X-ray diffraction. Both of these binuclear complexes contain the uncommon asymmetrical {VO(µ-O)}₂ diamond core. The in vitro tests of the antiamoebic activity of ligands I and II and their binuclear complexes 2 and 7 against the protozoan parasite Entamoeba histolytica show that the ligands have no amoebicidal activity while their vanadium complexes 2 and 7 display more effective amoebicidal activity than the most commonly used drug metronidazole (IC₅₀ values are 1.68 and 0.45 µM, respectively vs 1.81 µM for metronidazole). Complexes 2 and 7 catalyse the oxidation of styrene and ethyl benzene effectively. Oxidation of styrene, using H₂O₂ as an oxidant, gives styrene epoxide, 2-phenylacetaldehyde, benzaldehyde, benzoic acid and 1-phenyl-ethane-1,2-diol, while ethyl benzene yields benzyl alcohol, benzaldehyde and 1-phenyl-ethane-1,2-diol.

The Schiff base (H₂fsal-ohyba) derived from 3-formylsalicylic acid and o-hydroxybenzylamine has been covalently bonded to chloromethylated polystyrene cross-linked with 5% divinylbenzene (abbreviated as PS-H₂fsal-ohyba, I). Treatment of [VO(acac)₂] with PS-H₂fsal-ohyba in dimethylformamide (DMF) gave the oxovanadium(IV) complex PS-[VO(fsal-ohyba)·DMF] (1). Complex 1 can be oxidized into the dioxovanadium(V) species, PS-K[VO₂(fsal-ohyba)] (2) on aerial oxidation in the presence of KOH or into the oxoperoxo species, PS-K[VO(O₂)(fsal-ohyba)] (3) in the presence of H₂O₂ and KOH in DMF suspension. Similarly, PS-[MoO₂(fsal-ohyba)·DMF] (4) has been isolated by the reaction of [MoO₂(acac)₂] with PS-H₂fsal-ohyba. All these complexes have been characterised by various techniques. These complexes catalyse the oxidation of styrene, ethylbenzene and phenol efficiently. Styrene gives five reaction products namely styrene oxide, benzaldehyde, 1-phenylethane-1,2-diol, benzoic acid and phenylacetaldehyde, while ethylbenzene gives benzaldehyde, phenyl acetic acid, styrene and 1-phenylethane-1,2-diol. The oxidation products of phenol are catechol and p-hydroquinone. These catalysts are also able to catalyse the oxidative bromination of salicylaldehyde to 5-bromosalicylaldehyde with ca. 80% selectively in the presence of aqueous 30% H₂O₂/KBr, a reaction similar to that exhibited by vanadate-dependent haloperoxidases. Their corresponding neat complexes have also been prepared and their catalytic activities have been compared.

Binucleating hydrazones CH₂(H₂sal-bhz)₂ (I) and CH₂(H₂sal-fah)₂ (II), derived from 5,5′-methylbis(salicylaldehyde) and benzoylhydrazide or 2-furoylhydrazide, react with [V^IVO(acac)₂] to give dinuclear V^IVO-complexes [CH₂{V^IVO(sal-bhz)(H₂O)}₂] 1 and [CH₂{V^IVO(sal-fah)(H₂O)}₂] 4, respectively. In the presence of KOH or CsOH·H₂O, oxidation of 1 and 2 results in the formation of dioxidovanadium(V) complexes, K₂[CH₂{V^VO₂(sal-bhz)}₂]·2H₂O 2, K₂[CH₂{V^VO₂(sal-fah)}₂]·2H₂O 5, Cs₂[CH₂{V^VO₂(sal-bhz)}₂]·2H₂O 3 and Cs₂[CH₂{V^VO₂(sal-fah)}₂]·2H₂O 6. These complexes have also been prepared by aerial oxidation of in situ prepared oxidovanadium(IV) complexes 1 and 4. The compounds were characterized by IR, electronic, EPR, ¹H, ¹³C and ⁵¹V NMR spectroscopy, elemental analyses and thermogravimetric patterns. Single crystal X-ray analysis of 3 confirms the coordination of the ligand in the dianionic (ONO²⁻) enolate tautomeric form. The V^VO₂-complexes were used to catalyze the oxidative bromination of salicylaldehyde, therefore acting as functional models of vanadium dependent haloperoxidases, in aqueous H₂O₂/KBr in the presence of HClO₄ at room temperature. It is shown that the V^IVO-complexes [CH₂{V^IVO(sal-bhz)(H₂O)}₂] 1 and [CH₂{V^IVO(sal-fah)(H₂O)}₂] 4 are catalyst precursors for the catalytic oxidation of organic sulfides using aqueous H₂O₂. Plausible intermediates involved in these catalytic processes are established by UV-Vis, EPR and ⁵¹V NMR studies. The vanadium complexes along with ligands I and II are also screened against HM1:1MSS strains of Entamoeba histolytica, the results showing that the IC₅₀ values of compounds 3 and 6 are lower than that of metronidazole. The toxicity studies against human cervical (HeLa) cancer cell line also showed that although compounds 3 and 6 are more toxic than metronidazole towards this cell line, the corresponding IC₅₀ values are relatively high, the cell viability therefore not being much affected.

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.