Martin Ebel

Synthesis, characterisation, reactivity and in vitro antiamoebic activity of hydrazone based oxovanadium(IV), oxovanadium(V) and µ-bis(oxo)bis{oxovanadium(V)} complexes

Binuclear, mu-bis(oxo)bis{oxovanadium(V)} complexes [(VOL)2(mu-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-mu-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(mu-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 microM, respectively vs 1.81 microM 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.

Vanadium(IV and V) Complexes Containing SNO (Dithiocarbonylhydrazone; Thiosemicarbazone) Donor Sets

The VO2+ complexes [VOCl(ONS)] (3) and [VO(ONS)′] (4), and the VO3+ complexes [VO(OEt)(ONS)′] (5) {ONS = (R)-salicylaldehyde thiosemicarbazonate(1−) R = 5,6-C4H4 (3a) or 3-OMe (3b); (ONS)′ = (R)-salicylaldehyde[benzylmercaptothiocarbonylhydrazonate(2−)], R = H, (4a/5a) or 3-OMe (4b/5b)} have been prepared and characterised by IR, EPR, 1H-, and 51V-NMR spectroscopy. In (S)-sBuOH, 4a converts to [VO{(S)-OsBu}(ONS)′] (5c) and [VO(OH)(ONS)′] (or a condensation product thereof). Solutions of 5c show three 51V NMR signals, two of which are due to two diastereomers. The EPR spectra of 3 and 4 in THF reveal the presence of octahedral species in solution. The crystal and molecular structures of complexes 3a·OCMe2, 5a, and 5b have been obtained, revealing basically a tetragonal pyramid, and coordination of the sulfur function in the thiocarbonyl (3) or enethiolate mode (5). The relevance of the compounds to bioinorganic aspects is addressed.

Vanadium complexes with enamines having tyrosine constituents

Abstract Four new oxovanadium(IV)-complexes of Schiff-base ligands with tyrosine constituents have been synthesised and characterised. The crystal and molecular structures of [A-VO(nap-R-tyr)(H2O)]·MeOH (nap is derived from 2-hydroxy-naphthalene-1-carbaldhyde) have been determined. The biological significance of vanadium–tyrosine interaction is addressed.

Molecular assembly of novel hetero-metal clusters: [(O=MoS 3 Cu 2 ) 2 (μ-Sn 2 S 6 )] 4− and [(S=MoS 3 Cu 2 ) 3 (μ 3 -S) 2 ] 4−

Abstract Two mixed molybdenum–copper–sulfur clusters containing two or three {E=MoS3Cu2} (E=O, S) fragments have been assembled around the nucleophiles μ2-Sn2S64− or μ3-S2−, viz. [(MoES3Cu2)2(μ-Sn2S6)]4− (E=0.7 O + 0.3 S) and [(MoS4Cu2)3(μ3-S)2]4−.

Synthesis, characterisation and catalytic potential of hydrazonato-vanadium(V) model complexes with [VO]3+ and [VO2]+ cores

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}2mu-O](H2L = I: 2, H2L = II: 5). These dinuclear complexes slowly convert, in excess methanol, to [VO(OMe)(MeOH)L](H(2)L = I: 9, H(2)L = 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.

Modeling the active site structures of vanadate-dependent peroxidases and vanadate-inhibited phosphatases

The active center of vanadate-dependent peroxidases (VPOs) is represented by vanadate covalently attached to a histidine, with vanadium in a trigonal-bipyramidal environment. Protein phosphatases and kinases are inhibited by the phosphate analog vanadate [VVO2(OH)2- and VIVO(OH)3-], which can be related to the coordination of vanadium to histidine or a hydroxide function as provided by tyrosinate or serinate. The vanadium centers in these proteins have been modeled by employing chiral ONO ligands. The penta-coordinated chiral complexes [VO(OMe)(L1)] (H2L1 = substituted diethanolamine) are distorted trigonal-bipyramidal with the methoxy group and the amine-N in the axial positions. These structural models of VPO also mimic the sulfide-oxidation activity of the peroxidases. The complexes [VO(H2O)L3] (H2L3 = Schiff-base ligands based on salicylaldehyde derivatives (o-vanillin; 2-hydroxy-naphthylaldehyde) and L- or D,L-tyrosine, or D,L-serine are tetragonal-pyramidal; the OH functions of the amino acid moieties are not directly coordinated to vanadium; they are involved, however, in complex hydrogen-bonding networks. The oxo/peroxo anion [VO(O2)(L2)2]3- (H2L2 = 2,5-dipicolinic acid) contains a slightly asymmetrically bonded O22-, featuring structural characteristics of the peroxo/hydroperoxo intermediates of the peroxidases. XD structure results are reported for the following complexes: R,S- and R,R-[VO(OMe)(L1)], K3[VO(O2)(L2)2].4.5H2O, the Tyr derivatives L-[VO(H2O)L3].MeOH and D,L-[VO(H2O)L3].H2O, and the Ser derivative D,L-[VO(H2O)L3].2H2O.