Reaction between [VO(acac)(2)] and the ONN donor Schiff base Hsal-ambmz (I) (Hsal-ambmz = Schiff base obtained by the condensation of salicylaldehyde and 2-aminomethylbenzimidazole) resulted in the formation of the complexes [V(IV)O(acac)(sal-ambmz)] (1), [V(V)O(2)(acac-ambmz)] (2) (Hacac-ambmz = Schiff base derived from acetylacetone and 2-aminomethylbenzimidazole), and the known complex [V(IV)O(sal-phen)] (3) (H(2)sal-phen = Schiff base derived from salicylaldehyde and o-phenylenediamine). Similarly, [V(IV)O(acac)(sal-aebmz)] (7) has been isolated from the reaction with Hsal-aebmz (II) (Hsal-aebmz derived from salicylaldehyde and 2-aminoethylbenzimidazole). Aerial oxidation of the methanolic solutions/suspensions of 1 and 7 yielded the dioxovanadium(V) complexes [V(V)O(2)(sal-ambmz)] (4) and [V(V)O(2)(sal-aebmz)] (8), respectively. Reaction of VOSO(4) with II gave [{V(IV)O(sal-aebmz)}(2)SO(4)] (9) and [V(IV)O(sal-aebmz)(2)] (10), along with 3 and 8. Under similar reaction conditions, I gave only [{V(IV)O(sal-ambmz)}(2)SO(4)] (5) and 3 as major products. Treatment of 1 and 7 with benzohydroxamic acid (Hbha) yielded the mixed-chelate complexes [V(V)O(bha)(sal-ambmz)] (6) and [V(V)O(bha)(sal-aebmz)] (11). The crystal and molecular structures of 2, 3.1/2DMF, 7.1/4H(2)O, 8, 9.2H(2)O, 10, and 11 have been determined, confirming the ONN binding mode of the ligands. In complex 10, one of the ligands is coordinated through the azomethine nitrogen and phenolate oxygen only, leaving the benzimidazole group free. In the dinuclear complex 9, bridging functions are the phenolate oxygens from both of the ligands and two oxygens of the sulfato group. The unstable oxoperoxovanadium(V) complex [V(V)O(O(2))(sal-aebmz)] (12) has been prepared by treatment of 7 with aqueous H(2)O(2). Acidification of methanolic solutions of 7 and 10 lead to (reversible) protonation of the bemzimidazole, while 8 was converted to an oxo-hydroxo species. Complexes 2, 4, and 8 catalyze the oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide and methyl phenyl sulfone, a reaction mimicking the sulfideperoxidase activity of vanadate-dependent haloperoxidases. These complexes are also catalytically active in the oxidation of styrene to styrene oxide, benzaldehyde, benzoic acid, and 1-phenylethane-1,2-diol.

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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(IV)O(acac)2] and the H3dfmp(L)2 in methanol the V(IV)O-complexes [V(IV)O{Hdfmp(inh)2}(H2O)] (1), [V(IV)O{Hdfmp(nah)2}(H2O)] (2) and [V(IV)O{Hdfmp(bhz)2}(H2O)] (3) were obtained. Upon their aerial oxidation in methanol [V(V)O(OMe)(MeOH){Hdfmp(inh)2}] (4), [V(V)O(OMe)(MeOH){Hdfmp(nah)2}] (5) and [V(V)O(OMe)(MeOH){Hdfmp(bhz)2}] (6) were isolated. In the presence of KOH, oxidation of 1-3 results in the formation of [V(V)O2{H2dfmp(inh)2}]n·5H2O (7), K[V(V)O2{Hdfmp(nah)2}] (8) and K[V(V)O2{Hdfmp(bhz)2}] (9). All compounds are characterized in the solid state and in solution, namely by spectroscopic techniques (IR, UV-Vis, EPR, (1)H, (13)C and (51)V NMR), and DFT is also used to calculate the V(IV) hyperfine coupling constants of V(IV)-compounds and (51)V NMR chemical shifts of several V(V)-species and assign them to those formed in solution. Single crystal X-ray analysis of [V(V)O(OMe)(MeOH){Hdfmp(bhz)2}] (6) and [V(V)O2{H2dfmp(inh)2}]n·5H2O (7) confirm the coordination of the ligand in the dianionic (ONO(2-)) 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(V)O2-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, (51)V NMR and DFT studies.

As a contribution to the development of novel vanadium complexes with pharmacologically interesting properties, two neutral dioxovanadium(V) complexes [VO2(Hpydx-sbdt)] (1) and [VO2(Hpydx-smdt)] (3) [H2pydx-sbdt (I) and H2pydx-smdt (II) are the Schiff bases derived from pyridoxal and S-benzyl- or S-methyldithiocarbazate] have been synthesized by the reaction of [VO(acac)2] and the potassium salts of the ligands in methanol followed by aerial oxidation. Heating of the methanolic solutions of these complexes yields the oxo-bridged binuclear complexes [{VO(pydx-sbdt)}2mu-O] (2) and [{VO(pydx-smdt)}2mu-O] (4). The crystals and molecular structures of 1, 3 x 1.5H2O, and 4 x 2CH3OH have been determined, confirming the ONS binding mode of the dianionic ligands in their thioenolate form. The ring nitrogen of the pyridoxal moiety is protonated in complexes 1 and 3. Acidification of 1 and 3 with HCl dissolved in methanol afforded oxohydroxo complexes, while in a methanolic KOH solution, the corresponding dioxo species K[VO2(pydx-sbdt/smdt)] are formed. Treatment of 1 and 3 with H2O2 yields (unstable) oxoperoxovanadium(V) complexes, the formation of which has been established spectrophotometrically. In vitro antiamoebic activities (against HM1:1MSS strain of Entamoeba histolytica) were established for all of the dioxo- and oxovanadium(V) complexes. The complexes 1, 2, and 4 were more effective than metronidazole, a commonly used drug against amoebiasis, suggesting that oxovanadium(V) complexes derived from thiohydrazones may open a new dimension in the therapy of amoebiasis.