Sarcoplasmic reticulum calcium ATPase is inhibited by organic vanadium coordination compounds: pyridine-2,6-dicarboxylatodioxovanadium(V), BMOV, and an amavadine analogue

The general affinity of the sarcoplasmic reticulum (SR) Ca (2+)-ATPase was examined for three different classes of vanadium coordination complexes including a vanadium(V) compound, pyridine-2,6-dicarboxylatodioxovanadium(V) (PDC-V(V)), and two vanadium(IV) compounds, bis(maltolato)oxovanadium(IV) (BMOV), and an analogue of amavadine, bis( N-hydroxylamidoiminodiacetato)vanadium(IV) (HAIDA-V(IV)). The ability of vanadate to act either as a phosphate analogue or as a transition-state analogue with enzymes' catalysis phosphoryl group transfer suggests that vanadium coordination compounds may reveal mechanistic preferences in these classes of enzymes. Two of these compounds investigated, PDC-V(V) and BMOV, were hydrolytically and oxidatively reactive at neutral pH, and one, HAIDA-V(IV), does not hydrolyze, oxidize, or otherwise decompose to a measurable extent during the enzyme assay. The SR Ca (2+)-ATPase was inhibited by all three of these complexes. The relative order of inhibition was PDC-V(V) > BMOV > vanadate > HAIDA-V(IV), and the IC 50 values were 25, 40, 80, and 325 microM, respectively. Because the observed inhibition is more potent for PDC-V(V) and BMOV than that of oxovanadates, the inhibition cannot be explained by oxovanadate formation during enzyme assays. Furthermore, the hydrolytically and redox stable amavadine analogue HAIDA-V(IV) inhibited the Ca (2+)-ATPase less than oxovanadates. To gauge the importance of the lipid environment, studies of oxidized BMOV in microemulsions were performed and showed that this system remained in the aqueous pool even though PDC-V(V) is able to penetrate lipid interfaces. These findings suggest that the hydrolytic properties of these complexes may be important in the inhibition of the calcium pump. Our results show that two simple coordination complexes with known insulin enhancing effects can invoke a response in calcium homeostasis and the regulation of muscle contraction through the SR Ca (2+)-ATPase.     

Schiff base complexes of vanadium(III, IV, V) as catalysts for the electroreduction of O2 to H2O in acetonitrile

Fifteen Schiff base ligands were synthesized and used to form complexes with vanadium in oxidation states III, IV, and V. Electrochemical and spectral characteristics of the complexes were evaluated and compared. In acidified solutions in acetonitrile the vanadium(IV) complexes undergo reversible disproportionation to form V(III) and V(V) complexes. With several of the ligands the V(III) complexes are much more stable in the presence of acid than is the previously studied complex with salen, an unelaborated Schiff base ligand (H(2) salen = N,N'-ethylenebis(salicylideneamine)). Equilibrium constants for the disproportionation were evaluated. The vanadium(III) complexes reduce dioxygen to form two oxo ligands. The reaction is stoichiometric in the absence of acid, and second-order rate constants were evaluated. In the presence of acid some of the complexes investigated participate in a catalytic electroreduction of dioxygen.     

Vanadium(IV,V) complexes of D-saccharic and mucic acids in aqueous solution

The vanadium(IV,V) complexes formed with two aldaric acids (D-saccharic or D-glucaric acid, and mucic or galactaric acid) in aqueous solution were characterised by employing pH-potentiometry, EPR, multinuclear NMR and UV-VIS spectroscopy. The stoichiometry and stability constants of the complexes formed were determined at 25 degrees C and ionic strength I= 0.2 mol dm(-3)(KCl). The spectral measurements revealed that vanadium(IV,V) coordinates first at the terminal COO(-) functions, forming mononuclear complexes. At pH > 3, through the metal ion-induced deprotonation and coordination of the neighbouring alcoholic functions, (COO(-), O(-)) coordinated dinuclear complexes are formed, which predominate in the pH range 4-8. In the basic pH range, the ligand molecules are displaced and binary metal hydroxo and oxo complexes are present. EPR measurements at room temperature and at 140 K proved that formation of the VO(iv) dimers is more enhanced at room temperature, but at 140 K their dissociation is favoured. An interesting pH-dependent cis-trans isomeric equilibrium was assumed and analysed by EPR and molecular modelling in the case of the complexes [(VO)(2)L(2)H(x)](x=-2 and -4). Joint evaluation of the pH-potentiometric and (51)V NMR measurements revealed that both aldaric acids are able to bind an excess of vanadium(V), through the formation of oligomeric 2:1 and 3:2 species, besides the 2:2 species formed with VO(IV).     

On-column complexation and simultaneous separation of vanadium(IV) and vanadium(V) by capillary electrophoresis with direct UV detection

An on-column complexation method has been developed for the simultaneous determination of V(IV) and V(V). Vanadium species were chelated with aminopolycarboxylic acids to form anionic complexes which were separated by capillary zone electrophoresis (CZE) with direct UV detection. Ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentacetric acid (DTPA), nitrilotriacetic acid (NTA), and N-2-hydroxyethylethlendiaminetriacetric acid (HEDTA) were investigated as both ligand and running electrolyte. Of the ligands studied the complexes of EDTA with V(IV) and V(V) resulted in the highest selectivity and UV response.The conditions used for on-column complexation and separation, including pH, and electrolyte ligand concentration, were examined to achieve reasonable separation selectivity and detection sensitivity. The optimum separation of the anionic forms of V(IV) and V(V) was obtained by use of CZE with UV detection at 185 nm and an electrolyte containing 5 mmol L(-1) EDTA at pH 4.0. Linear calibration plots were obtained in the concentration range10-300 micro mol L(-1); detection limits were 3 micro mol L(-1) for V(IV) and 1 micro mol L(-1) for V(V). The proposed method was demonstrated for the determination of vanadium in groundwater spiked with V(IV) and V(V).     

Liquid anion-exchange separation of vanadium from malonate media

Summary Vanadium(IV) and (V) can be quantitatively extracted with 0.2 mol/l Amberlite LA-2 in xylene at pH 3.0 from 0.02 mol/l malonic acid, stripped with 0.5 mol/l hydrochloric acid, and determined spectrophotometrically. Five other liquid anion exchangers (Amberlite LA-1, Primene JM-T, Aliquat 336S, TOA and TIOA) were examined as possible extractants. The extraction of vanadium(IV) was found to be quantitative only with Amberlite LA-2, while that of vanadium(V) was quantitative with Amberlite LA-1 and LA-2, Primene JM-T and Aliquat 336S. Eight common solvents were tested as diluents; of these hexane, cyclohexane, benzene, and xylene were found to be satisfactory. Vanadium was separated from elements that do not form anionic complexes with malonic acid by selective extraction, from those that form weak complexes by washing the organic extract with water, and from metals that form strong malonato complexes by selective stripping with hydrochloric, nitric, or sulphuric acid. The method has been applied to the determination of vanadium in steel, coal fly ash and fuel oil. The precision of measurement is within ±5% and the detection limit of the method for vanadium is 0.5 mg/kg.     

4-amino- and 4-nitrodipicolinatovanadium(V) complexes and their hydroxylamido derivatives: synthesis, aqueous, and solid-state properties

A number of 4-substituted, dipicolinatodioxovanadium(V) complexes and their hydroxylamido derivatives were synthesized to characterize the solid state and solution properties of five- and seven-coordinate vanadium(V) complexes. The X-ray crystal structures of Na[VO2dipic-NH2].2H2O (2) and K[VO2dipic-NO2] (3) show the vanadium adopting a distorted, trigonal-bipyramidal coordination environment similar to the parent coordination complex, [VO2dipic]- (1), reported previously as the Cs+ salt. The observed differences in the chemical shifts of the complexes both in the 1H (ca. 0.7-1.4 ppm) and 51V (ca. 1-11 ppm) NMR spectra were consistent with the electron-donating or electron-withdrawing properties of the substituent groups, respectively. Stoichiometric addition of a series of hydroxylamine ligands (H2NOH, MeHNOH, Me2NOH, and Et2NOH) to complexes 1-3 led to the formation of seven-coordinate vanadium(V) complexes. The X-ray crystal structure of [VO(dipic)(Me2NO)(H2O)].0.5H2O (1c) was found to be similar to the previously characterized complexes [VO(dipic)(H2NO)(H2O)] (1a) and [VO(dipic)(OO-tBu)(H2O)]. While only slight differences in the 1H NMR spectra were observed upon addition of the hydroxylamido ligand, the signals in the 51V NMR spectra change by up to 100 ppm. The addition of the hydroxylamido ligand increased the complex stability of complexes 2 and 3. Evidence for a nonstoichiometric redox reaction was found for the monoalkyl hydroxylamine ligand. The reaction of an unsaturated five-coordinate species with a hydroxylamine to form a seven-coordinate vanadium complex will, in general, dramatically increase the amounts of the vanadium compound that remain intact at pH values near neutral.     

Reduction of [VO2(ma)2]- and [VO2(ema)2]- by ascorbic acid and glutathione: kinetic studies of pro-drugs for the enhancement of insulin action

To shed light on the role of V(V) complexes as pro-drugs for their V(IV) analogues, the kinetics of the reduction reactions of [VO2(ma)2]- or [VO2(ema)2]- (Hma = maltol, Hema = ethylmaltol), with ascorbic acid or glutathione, have been studied in aqueous solution by spectrophotometric and magnetic resonance methods. EPR and 51V NMR studies suggested that the vanadium(V) in each complex was reduced to vanadium(IV) during the reactions. All the reactions studied showed first-order kinetics when the concentration of ascorbic acid or glutathione was in large excess and the observed first-order rate constants have a linear relationship with the concentrations of reductant (ascorbic acid or glutathione). Potentiometric results revealed that the most important species in the neutral pH range is [VO2(L)2]- for the V(V) system where L is either ma- or ema-. An acid dependence mechanism was proposed from kinetic studies with varying pH and varying maltol concentration. The good fits of the second order rate constant versus pH or the total concentration of maltol, and the good agreement of the constants obtained between fittings, strongly supported the mechanism. Under the same conditions, the reaction rate of [VO2(ma)2]- with glutathione is about 2000 times slower than that of [VO2(ma)2]- with ascorbic acid, but an acid dependence mechanism can also be used to explain the results for the reduction with glutathione. Replacing the methyl group in maltol with an ethyl group has little influence on the reduction rate with ascorbic acid, and the kinetics are the same no matter whether [VO2(ma)2]- or [VO2(ema)2]- is reduced.     

Separation via flotation, spectrophotometric speciation, and determination of vanadium(IV) in wastes of power stations.

1-(2-Hydroxy-4-methoxybenzophenone)-4-phenylthiosemicarbazone (HMBPT) was investigated as a new reagent for the flotation of vanadium(IV). At pH approximately 1.5, vanadium(IV) forms a 1:1 pale-violet complex with HMBPT in aqueous solution. An intense clear violet layer was formed after flotation, by adding an oleic acid (HOL) surfactant. The composition of the float was 1:1 [V(IV)]:[HMBPT]. A highly selective and sensitive spectrophotometric procedure was proposed for the determination of microamounts of V(IV) as its floated complex. The molar absorptivities of the V(IV)-HMBPT and V(IV)-HMBPT-HOL systems were 0.4 x 10(4) and 0.12 x 10(5) L mol(-1) cm(-1) at 560 nm, respectively. The formation constants of the species formed in the presence and absence of HOL were 4.6 x 10(7) and 8.7 x 10(5) L mol(-1), respectively. Beer's law was obeyed up to 1 x 10(-4) mol L(-1) in the aqueous layer as well as in the oleic acid layer. The HMBPT-V(IV) complexes formed in the aqueous solution and scum layer were characterized by elemental analysis, infrared and UV spectrophotometric studies. The mode of chelation between V(IV) and HMBPT is proposed to be due to a reaction between the protonated bidentate HMBPT ligand and V(IV) through the S=C and N=C groups. Interferences from various foreign ions were avoided by adding excess HMBPT and/or Na2S2O3 as a masking agent. The proposed flotation method was successfully applied to the analysis of V(IV) in synthetic mixtures, wastes of power stations, simulated samples and in real ores. The separation mechanism is discussed.     

N,N'-ethylenebis(pyridoxylideneiminato) and N,N'-ethylenebis(pyridoxylaminato): synthesis, characterization, potentiometric, spectroscopic, and DFT studies of their vanadium(IV) and vanadium(V) complexes

The Schiff base N,N'-ethylenebis(pyridoxylideneiminato) (H(2)pyr(2)en, 1) was synthesized by reaction of pyridoxal with ethylenediamine; reduction of H(2)pyr(2)en with NaBH(4) yielded the reduced Schiff base N,N'-ethylenebis(pyridoxylaminato) (H(2)Rpyr(2)en, 2); their crystal structures were determined by X-ray diffraction. The totally protonated forms of 1 and 2 correspond to H(6)L(4+), and all protonation constants were determined by pH-potentiometric and (1)H NMR titrations. Several vanadium(IV) and vanadium(V) complexes of these and other related ligands were prepared and characterized in solution and in the solid state. The X-ray crystal structure of [V(V)O(2)(HRpyr(2)en)] shows the metal in a distorted octahedral geometry, with the ligand coordinated through the N-amine and O-phenolato moieties, with one of the pyridine-N atoms protonated. Crystals of [(V(V)O(2))(2)(pyren)(2)].2 H(2)O were obtained from solutions containing H(2)pyr(2)en and oxovanadium(IV), where Hpyren is the "half" Schiff base of pyridoxal and ethylenediamine. The complexation of V(IV)O(2+) and V(V)O(2) (+) with H(2)pyr(2)en, H(2)Rpyr(2)en and pyridoxamine in aqueous solution were studied by pH-potentiometry, UV/Vis absorption spectrophotometry, as well as by EPR spectroscopy for the V(IV)O systems and (1)H and (51)V NMR spectroscopy for the V(V)O(2) systems. Very significant differences in the metal-binding abilities of the ligands were found. Both 1 and 2 act as tetradentate ligands. H(2)Rpyr(2)en is stable to hydrolysis and several isomers form in solution, namely cis-trans type complexes with V(IV)O, and alpha-cis- and beta-cis-type complexes with V(V)O(2). The pyridinium-N atoms of the pyridoxal rings do not take part in the coordination but are involved in acid-base reactions that affect the number, type, and relative amount of the isomers of the V(IV)O-H(2)Rpyr(2)en and V(V)O(2)-H(2)Rpyr(2)en complexes present in solution. DFT calculations were carried out and support the formation and identification of the isomers detected by EPR or NMR spectroscopy, and the strong equatorial and axial binding of the O-phenolato in V(IV)O and V(V)O(2) complexes. Moreover, the DFT calculations done for the [V(IV)O(H(2)Rpyr(2)en)] system indicate that for almost all complexes the presence of a sixth equatorial or axial H(2)O ligand leads to much more stable compounds.     

Formation and structure of 1:1 complexes between aryl hydroxamic acids and vanadate at neutral pH

Although aryl hydroxamic acids are well-known to form coordination complexes with vanadate (V(V)), the nature of these complexes at neutral pH and submillimolar concentrations, the conditions under which such complexes inhibit various serine amidohydrolases, is not well established. A series of qualitative and quantitative experiments, involving UV/vis, (1)H NMR, and (51)V NMR spectroscopies, established that both 1:1 and 1:2 vanadate/hydroxamate complexes form at pH 7.5, with the former dominating at submillimolar concentrations. Formation constants for the complexes of several aryl and alkyl hydroxamic acids were determined; for example, for benzohydroxamic acid, the stepwise formation constants of the 1:1 and 1:2 complexes were 3000 and 400 M(-1), respectively. The (51)V chemical shift of the 1:1 4-nitrobenzohydroxamic acid complex was -497 ppm, and that of its unsubstituted analogue was -498 ppm. A (1)H-(15)N HSQC spectrum of the 4-nitrobenzo-(15)N-hydroxamic acid/vanadate complex indicated the presence of an N-H group with (15)N and (1)H chemical shifts of 115 and 5.83 ppm, respectively. A (13)C NMR spectrum of the complex of 4-nitrobenzo-(13)C-hydroxamic acid with vanadate displayed a resonance at 170.1 ppm and thus a coordination-induced shift (CIS) of +3.8 ppm. In contrast, the CIS value of an established 1:2 complex, thought to contain chelated hydroxamic acid ligands, was +11.9 ppm. These spectral data led to the following structural picture of 1:1 complexes of vanadate and aryl hydroxamic acids. They contain penta- or hexa-coordinated vanadium. The ligand is in the hydroxamate rather than hydroximate form. The ligand is presumably bound to vanadium through the hydroxamic hydroxyl oxygen, but the hydroxamic acid carbonyl oxygen interacts weakly with vanadium. These species are the most likely candidates for the inhibitors of serine amidohydrolases found in vanadate/hydroxamic acid mixtures.     

A linear trinuclear mixed valence vanadium(V/IV/V) complex: synthesis, characterization, and solution behavior

The reaction between vanadium(III) acetylacetonate and N-hexanoylsalicylhydrazide (H3hshz) yields a linear trinuclear mixed valence vanadium(V/IV/V) complex, V3O3(hshz)2(OEt)2, 1 (where hshz3- is a triply deprotonated trianionic N-hexanoyl salicylichydrazidate), with a pseudo C2 symmetry. A V(IV)O2+ group is at the center of complex 1 and is spanned by two terminal vanadium(V) ions with a square pyramidal geometry bridged via hydrazido ligands. In the crystalline form, the oxo group of the central vanadium(IV) ion is weakly coordinated to one of the terminal square pyramidal vanadium(V) ions of the neighboring trinuclear complex to form a dimeric structure. These dimers are linked via bis mu-alkoxo bridges to form a one-dimensional zigzag chain structure. In chloroform or methylene dichloride, the weak linkages between the trinuclear complexes present in the crystalline form are broken, and only the mixed valence trinuclear complex can be identified. In dimethyl sulfoxide or dimethylformamide, the trinuclear complex partially dissociates, and the unligated ligands remain in equilibrium with the trinuclear complex.     

Hollow-fibre liquid phase microextraction for separation and preconcentration of vanadium species in natural waters and their determination by electrothermal vaporization-ICP-OES

For separation and determination of vanadium(IV/V) species, a fast and sensitive method by combining hollow-fibre liquid phase microextraction (HF-LPME) with electrothermal vaporization (ETV)-ICP-OES has been developed. Two vanadium species (V(IV) and V(V)) were separated by HF-LPME with the use of ammonium pyrrolidinecarbodithioate (APDC) as chelating agent for complexing with different V species and carbon tetrachloride as the extraction solvent, and the vanadium in the post-extraction organic phase was injected into the graphite furnace for ETV-ICP-OES detection, in which APDC was acted as the chemical modifier. At pH 5.0, both V(IV)-APDC and V(V)-APDC were extracted quantitatively into CCl₄ for determination of total V. For speciation studies, 1,2-cyclohexanediaminetetraacetic acid (CDTA) was added to the sample for masking V(IV), so that only V(V)-APDC was extracted and determined. The concentration of V(IV) was calculated by subtracting the V(V) concentration from the total concentration of V. Under the optimized experimental conditions, the enrichment factor was 74 and the detection limits for V(IV) and V(V) were 86 pg mL⁻¹ and 71 pg mL⁻¹, respectively. The proposed method has been applied to the speciation of V in environmental water samples, and the recovery was in the range of 94%-107%. The results show that V(V) is the dominant existence form in oxygenic water and V(IV) could not been detected. In order to validate the developed procedure, a NIES No.8 vehicle exhaust particulates certified reference material was analyzed, and the results obtained for total vanadium are in good agreement with the certified values. The proposed method is simple, rapid, selective, and sensitive and no oxidation/reduction is required, it is applicable to the speciation of vanadium in environmental samples with the complicated matrix.     

Preparation,properties and analytical application of silica with chemically grafted hydroxamic acid groups

The synthesis of silica chemically modified with propanohydroxamic acid groups and salicylhydroxamic acid is described. Sorption of 13 metal ions is studied as a function of pH. The composition of the sorbed complexes is determined and the apparent stability constants of the complexes in the sorbent phase are calculated. A correlation between the stabilities of the complexes in the sorbent phase and in aqueous solution is observed in some cases. Some analytical applications of the sorbents are demonstrated: trace amounts of elements can be concentrated from large volumes; molybdenum(VI) and zirconium(IV) can be separated from 10⁴-fold amounts of accompanying elements, and vanadium(V) can be determined in the sorbent phase by using diffuse reflection and photoacoustic spectrometric techniques.     

Speciation in Vanadium Bioinorganic Systems. 2. An NMR, ESR, and Potentiometric Study of the Aqueous H(+)-Vanadate-Maltol System

A systematic study of the physiologically interesting vanadium-maltol (V-MaH) system has been performed in 0.150 M Na(Cl) at 25 degrees C, using NMR, ESR, and potentiometric techniques. Complexation occurs within a wide pH range, from around 1 up to 10.5. However, a pH-, concentration-, and time-dependent spontaneous reduction of vanadium(V) to vanadium(IV) occurs. From ESR spectra the conditions for this reduction are evaluated and discussed. From potentiometric (glass electrode) and quantitative (51)V NMR measurements, the full speciation in the H(+)-H(2)VO(4)(-)-MaH system was determined in the pH range 5-10.5. Data were evaluated with the computer program LAKE, which is able to treat combined emf and NMR data. The pK(a) value for MaH was determined to be 8.437 +/- 0.005. In the ternary system, three complexes are formed: VMa(2)(-), VMa(-), and VMa(2)(-), having log beta(0,1,2) = 7.02 +/- 0.03, log beta(0,1,1) = 2.66 +/- 0.05, and log beta(-)(1,1,1) = -7.37 +/- 0.21. The errors given are 3sigma. The VMa(2)(-) complex appears as the main species in a pH range from 4.5 to 8.5, whereas both mononuclear monoligand species are minor. Equilibrium conditions are illustrated in distribution diagrams, and the structures of the complexes formed are proposed.     

Speciation of vanadium (IV) and vanadium (V) with Eriochrome Cyanine R in natural waters by solid phase spectrophotometry

The separation and preconcentration of vanadium (IV) and vanadium (V) using Sephadex DEAE A-25 with Eriochrome Cyanine R has been studied, based on the preconcentration of vanadium (IV) in the first step and V(V) after reduction with ascorbic acid in the second step. Factors affecting the optimum fixation of the complex were investigated. The absorbance of the solid phase is measured directly at 563 nm for V(IV), at 585 nm for V(V) and at 750 nm for both. The proposed method provides a simple and specific procedure for the separation of vanadium in natural waters. The calibration graph is linear up to 150 ng/mL, with RSD of 4.7% for V(IV) and 4.0% for V(V). The detection limits are 1.6 and 1.4 ng/mL for V(IV) and V(V), respectively.     

Electrochemical properties of vanadium(III,IV,V)-salen complexes in acetonitrile. Four-electron reduction of O2 by V(III)-salen

The coordination chemistry and electrochemistry of complexes of vanadium(III,IV,V) with salen (H2 salen = N,N'-ethylenebis(salicylideneamine) were reexamined in an attempt to uncover the origin of puzzling results reported in a previous study (Inorg. Chem. 1994, 33, 1056). Microelectrodes were utilized to allow measurements in the absence of supporting electrolyte. The source of the puzzling results was identified and the modifications required in the previous interpretations are specified. Corrected values of formal potentials and diffusion coefficients are also given. The acid-induced disproportionation of V(IV)O(salen), as originally proposed by Bonadies et al. (J. Chem. Soc., Chem. Commun. 1986, 1218; Inorg. Chem. 1987, 26, 1218), was largely supported by the present results. The equilibrium constant for this disproportionation reaction was measured. The stoichiometry and kinetics of the reaction between O2 and the V(III)-salen complex were examined, and a possible mechanism for this four-electron reduction of O2 is suggested.     

Synthesis and characterization of vanadium(IV) complexes with cis-inositol in aqueous solution and in the solid-state

The complex formation of vanadium(IV) with cis-inositol (ino) and the corresponding trimethyl ether 1,3,5-trideoxy-1,3,5-trimethoxy-cis-inositol (tmci) was studied in aqueous solution and in the solid-state. With increasing pH, the formation of [VO(H-2L)], [(VO)2L2H-5]-, [VO(H-3L)]- (L = ino) or [(VO)2L2H-6]2- (L = tmci), [V(H-3L)2]2-, and [VO(H-3L)(OH)2]3- was observed. For the vanadium(IV)/ino system, [(VO)2L2H-7]3- was observed as an additional dinuclear species. The formation constants of these complexes were determined by potentiometric titrations (25 degrees C, 0.1 M KCl). In addition, the vanadium(IV)/ino system was investigated by means of UV-vis spectrophotometric methods. EPR spectroscopy and cyclic voltammetry confirmed this complexation scheme. EPR measurements indicated the formation of three distinct isomers of the non-oxo complex [V(H-3ino)2]2- in weakly basic solution. This type of isomerism, which is not observed for the vanadium(IV)/tmci system, was assigned to the ability of ino to bind the vanadium(IV) center with three alkoxo groups having either a 1,3,5-triaxial or an 1,2,3-axial-equatorial-axial arrangement. The structures of [V(H-3ino)2][K2(ino)2].4H2O (1) and [Na6V(H-3ino)2](SO4)2.6H2O (2) were determined by single-crystal X-ray analysis. In both compounds, the coordination of each ino molecule to the vanadium(IV) center via three axial deprotonated oxygen donors was confirmed. The centrosymmetric structure of the coordination spheres corresponds to an almost regular octahedral geometry with a twist angle of 60 degrees. The crystal structure of the potassium complex 1 represents an unusual 1:1 packing of [V(H-3ino)2]2- dianions and [K2(ino)2]2+ dications, in which both K+ ions have a coordination number of nine and are bonded simultaneously to a 1,3,5-triaxial and an 1,2,3-axial-equatorial-axial site of ino. In 2, the [V(H-3ino)2]2- complexes are surrounded by six Na+ counterions that are bonded to the axial alkoxo oxygens and to the equatorial hydroxy oxygens of the cis-inositolato moieties. The six Na+ centers are further interlinked by bridging sulfate ions. According to EPR spectroscopy, the D3d symmetric structure of the [V(H-3ino)2]2- anion is retained in H2O, in dimethylformamide, and in a mixture of CHCl3/toluene 60:40 v/v.     

Chelation of vanadium(IV) by a natural and edible biopolymer poly(gamma-glutamic acid) in aqueous solution: structure and binding constant of complex

The naturally occurring edible biopolymer poly(gamma-glutamic acid) (gamma-PGA) is shown to be an efficient chelating agent of vanadium(IV). The structure of poly(gamma-glutamic acid)oxovanadium(IV) (VO-gamma-PGA) complex in solution has been analyzed by electron spin resonance and UV-visible absorption spectra. The equatorial coordination sphere of vanadium(IV) is proposed to be [2 x carboxylate (2O)-VO-(OH2)2]. The binding isotherm is determined for suspensions of gamma-PGA in vanadium(IV) oxide sulfate (VS) solutions of different concentrations, and the data have been adjusted to fit the modified Langmuir equation. The maximum amount of vanadium bound per gram of gamma-PGA is estimated to be 141 mmol . g(-1) with a binding constant of 22 L . g(-1) at pH 3.     

Speciation of vanadium (IV) and vanadium (V) with Eriochrome Cyanine R in natural waters by solid phase spectrophotometry

The separation and preconcentration of vanadium (IV) and vanadium (V) using Sephadex DEAE A-25 with Eriochrome Cyanine R has been studied, based on the preconcentration of vanadium (IV) in the first step and V(V) after reduction with ascorbic acid in the second step. Factors affecting the optimum fixation of the complex were investigated. The absorbance of the solid phase is measured directly at 563 nm for V(IV), at 585 nm for V(V) and at 750 nm for both. The proposed method provides a simple and specific procedure for the separation of vanadium in natural waters. The calibration graph is linear up to 150 ng/mL, with RSD of 4.7% for V(IV) and 4.0% for V(V). The detection limits are 1.6 and 1.4 ng/mL for V(IV) and V(V), respectively. Received: 21 November 1996 / Revised: 15 April 1997 / Accepted: 18 April 1997     

Vanadium(V) tartrato complexes: speciation in the H3O+(OH-)/H2VO4-/(2R,3R)-tartrate system and X-ray crystal structures of Na4[V4O8(rac-tart)2].12H2O and (NEt4)4[V4O8((R,R)-tart)2].6H2O (tart = C4H2O6(4-))

A study of the aqueous H3O+(OH-)/H2VO4-/(2R,3R)-tartrate system has been performed at 273 K in a 1.0 mol/L Na+(Cl-) ionic medium using 51V NMR spectroscopy. In this relatively complicated system, more than 12 different species were observed. Ligand concentration, vanadate concentration, and pH variation studies were carried out, particularly for the range of pH 5.8-8.0 and for pH 2.4. Chemical shifts, vanadium-ligand stoichiometry, and also composition and formation constants for some, but not all, species are given. Despite some reduction of vanadium(V) to vanadium(IV) in an acidic medium at pH approximately 2.4, the stoichiometries of the principal species in solution at this pH were determined. Electrospray ionization mass spectra for some solutions were obtained and were in accordance with the conclusions drawn from the speciation studies. A series of crystalline vanadium(V) tartrato complexes M4[V4O8(tart)2].aq were also prepared and characterized. X-ray diffraction studies of Na4[V4O8(rac-tart)2].12H2O (1) and (NEt4)4[V4O8((R,R)-tart)2].6H2O (2) revealed unique tetranuclear [V4O8(tart)2]4- ions for which the {V4O4} rings have boat conformations.