51V NMR Signals Research Articles

Structural properties of alumina-doped lithium borovanadate glasses and glass-ceramics

This study elucidates the role of Al2O3 in the lithium borovanadate (LBV) system, Li2O-B2O3-V2O5, in improving the glass-forming region of LBV glass. This increased glass-forming tendency is likely due to formation of [AlO4]− units. 11B, 27Al, and 51V nuclear magnetic resonance measurements suggest an importance of V-O-Al linkages and distribution of Li for charge compensation of [AlO4]− and [BO4]− units. Intense 51V NMR signals and resolved hyperfine coupling patterns in the electron paramagnetic resonance (EPR) spectra indicate that the majority of vanadium is pentavalent (V5+) and minor amounts of paramagnetic vanadium (V4+) are present. The EPR interaction parameters suggest that the vanadyl-like environments can be described as tetragonally compressed distorted octahedra. Based on V-2p3/2 X-ray photoelectron spectroscopy, the V4+ concentration is estimated at <3%. These vanadate glasses are attractive for energy storage applications, as they have higher specific capacities than commercial lithium-ion battery cathodes.

Characterization of V–Mo–W Mixed Oxide Catalyst Surface Species by 51V Solid-State Dynamic Nuclear Polarization NMR

The investigation of V–Mo–W mixed oxide catalysts by 51V DNP MAS NMR is reported. It is shown that the 51V NMR signal from surface near vanadium nuclei was enhanced by a factor of ≈50 by direct polarization transfer from unpaired electrons to 51V nuclei. The dependency of the DNP enhancement of the samples on different polarizing mixtures is investigated and complemented by EPR studies. V–Mo–W mixed oxide catalysts with different stoichiometry (V2Mo8W0.5Oy, V2Mo8W1Oy, and V2Mo8W1.5Oy) are compared, and a suitable assignment of 51V signals to different phases that are present near to the catalysts surfaces is given. Finally, the applicability of DNP solid-state NMR for the characterization of V–Mo–W mixed oxide catalysts is discussed in terms of the synthesis conditions (i.e., pH-value).

51V-NMR study of charge order induced by cation order in δ-Ag2/3V2O5

51V-NMR experiments have been performed to investigate the local magnetic and electronic properties of the mixed-valence oxide δ-Ag2/3V2O5, which shows the novel V4+/V5+ charge ordering trigged by Ag ion ordering. We have observed the abrupt loss of 51V-NMR signal coming from the non-magnetic V5+-like ions above Tc = 225 K. It indicates that the majority of V ions are magnetic above Tc and that the phase transition is accompanied by the charge separation and the charge ordering of 3d electrons on V sites. In the low-temperature phase below Tc, the nuclear spin-lattice relaxation rate 1/T1 shows the thermal activation-type temperature dependence with the activation energy of about 130 K. It has been clarified that the ground state of this material is the charge ordered one with the spin singlet of 3d electrons on V4+-V4+ pair from a microscopic point of view.

51V-NMR study of low-temperature phase in δ-Ag2/3V2O5

51V-NMR experiments have been performed to investigate the local magnetic and electronic properties on the phase transition in the mixed-valence oxide δ-Ag2/3V2O5. We have observed the abrupt loss of 51V-NMR signal coming from the non-magnetic V5+-like ions above Tc = 225 K. It indicates that the majority of V ions are magnetic above Tc and that the phase transition is accompanied by the charge separation and charge ordering of 3d electrons on V sites. In the charge ordered state below Tc, the magnetic V4+-like ions are located at both V1 and V2 sites with equal distribution.

Syntheses and characterization of a new class of vanadia precursors of oxime-modified oxovanadium(V) isopropoxide, crystal and molecular structure of [VO{ONC10H16}3

Modification of [VO(OPri)3] with oximes in different molar ratios, yielded new class of vanadia precursors, [VO{OPri}3−n{L}n] {where, n = 1–3 and LH = C9H16C=NOH (1–3) and (CH3)2C=NOH (4–6)}.All the products are yellow in colour. (1) and (2) are liquid/viscous liquid, while others are solids. Molecular weight measurements of all these derivatives and the ESI-mass spectral studies of (1), (2), (3) and (5) indicate their monomeric nature. 1H and 13C{1H} NMR spectra suggest that the oximato moieties are monodentate in solution which was further confirmed by the 51V NMR signals, appeared in the region expected for tetra-coordinated oxo-vanadium atoms. On ageing, a disproportionation reaction occurs in (1) and some crystals appeared. Single crystal X-ray diffraction analyses of the crystals obtained from (1) as well as from (3) were found to be the same and indicate the presence of side-on {dihapto η2-(N, O)} binding modes of the oximato ligands, leading to the formation of seven coordination environment around the vanadium atom. Thermogravimetric curve of (1) exhibits multi-step decomposition with the formation of V2O5 as the final product at ~850 °C. Sol–gel transformation of (3) yielded (a) VO2 sintered at 300 °C and (b) V2O5 at 600 °C. Similarly, sol–gel transformations of (1) and (2) yielded V2O5 (c) and (d) at 600 °C, respectively. Formation of monoclinic phase in (a) and orthorhombic phase in (b), (c) and (d) were confirmed by powder XRD patterns.

Recent advances into vanadyl, vanadate and decavanadate interactions with actin

Although the number of papers about "vanadium" has doubled in the last decade, the studies about "vanadium and actin" are scarce. In the present review, the effects of vanadyl, vanadate and decavanadate on actin structure and function are compared. Decavanadate (51)V NMR signals, at -516 ppm, broadened and decreased in intensity upon actin titration, whereas no effects were observed for vanadate monomers, at -560 ppm. Decavanadate is the only species inducing actin cysteine oxidation and vanadyl formation, both processes being prevented by the natural ligand of the protein, ATP. Vanadyl titration with monomeric actin (G-actin), analysed by EPR spectroscopy, reveals a 1:1 binding stoichiometry and a K(d) of 7.5 μM(-1). Both decavanadate and vanadyl inhibited G-actin polymerization into actin filaments (F-actin), with a IC(50) of 68 and 300 μM, respectively, as analysed by light scattering assays, whereas no effects were detected for vanadate up to 2 mM. However, only vanadyl (up to 200 μM) induces 100% of G-actin intrinsic fluorescence quenching, whereas decavanadate shows an opposite effect, which suggests the presence of vanadyl high affinity actin binding sites. Decavanadate increases (2.6-fold) the actin hydrophobic surface, evaluated using the ANSA probe, whereas vanadyl decreases it (15%). Both vanadium species increased the ε-ATP exchange rate (k = 6.5 × 10(-3) s(-1) and 4.47 × 10(-3) s(-1) for decavanadate and vanadyl, respectively). Finally, (1)H NMR spectra of G-actin treated with 0.1 mM decavanadate clearly indicate that major alterations occur in protein structure, which are much less visible in the presence of ATP, confirming the preventive effect of the nucleotide on the decavanadate interaction with the protein. Putting it all together, it is suggested that actin, which is involved in many cellular processes, might be a potential target not only for decavanadate but above all for vanadyl. By affecting actin structure and function, vanadium can regulate many cellular processes of great physiological significance.

Synthesis and Characterization of Chloride-incorporated Dodecavanadate from Dicopper Complex of Macrocyclic Octadecavanadate

Abstract A chloride-incorporated dodecavanadate, [V12O32(Cl)]5− (1) has been synthesized from a dicopper macrocyclic complex, [Cu2V8O24]4− (2). By using hydrogen cyanide with a chloride guest anion, two copper cations are removed and the presence of chloride anion template affords a new bowl-shaped dodecavanadate, 1, with chloride anion at the center. The 1:1 mixture of 1 and [V12O32(CH3CN)]4− (3) in acetonitrile shows no 51V NMR signal scrambling at room temperature suggesting the tight binding of the chloride anion inside the anionic bowl molecule. The dissociation of the chloride anion was only observed at higher temperatures.

Formation and structure in aqueous solution of complexes between vanadium(V) and aminohydroxamic acids that potentiates vanadium’s insulinomimetic activity: l-glutamic γ-hydroxamic and l-aspartic-β-hydroxamic acids

The full speciation of the vanadium(V) complexation systems with two aminohydroxamic acids, aspartic-β- and glutamic-γ-hydroxamic acid, has been determined using potentiometric and spectroscopic techniques. Formation constants were calculated in a systematic study at different ligand to metal molar ratios and the coordination types are proposed. An almost constant value of the 51V NMR signal in neutral medium can be attributed to two (1:1 and 1:2 metal to ligand ratios) similar structures, both of which can be either protonated or deprotonated. The two ligands have a carboxylic group in the structure and show comparable biological activities. In this work analogous complexation behavior at physiological conditions was found despite the presence of two or three methylenic groups between the amino and hydroxamate groups. The carboxylic groups are quite distant from the hydroxamic groups and are not involved directly in the coordination process. Therefore the coordination structures are related to that found in the vanadium(V)-β-alaninehydroxamic acid in which there is not a carboxylic group.

DFT-NMR Investigation and 51V 3QMAS Experiments for Probing Surface OH Ligands and the Hydrogen-Bond Network in a Polyoxovanadate Cluster: The Case of Cs4[H2V10O28]·4H2O

This work shows that the combination of first-principles calculations and (51)V NMR experiments is a powerful tool to elucidate the location of surface hydroxyl groups and to precisely describe the hydrogen bond network in the complex decavanadate cluster Cs(4)[H(2)V(10)O(28)].4H(2)O, enhancing the strength of NMR crystallography. The detailed characterization of H-bond networks for these kinds of inorganic compounds is of primary importance and should benefit from the DFT-NMR predictions by considering explicitly the periodic boundary conditions. The determination of the Cs(4)[H(2)V(10)O(28)].4H(2)O structure by single-crystal X-ray diffraction was not sufficiently accurate to provide the location of protons. From available diffraction data, five different protonated model structures have been built and optimized using DFT-based methods. The possible interconversion of two decavanadate isomers through a proton exchange is evaluated by calculating the energy barrier and recording variable-temperature (1)H MAS NMR spectra. First-principles calculations of (51)V NMR parameters clearly indicate that these parameters are very sensitive to the local intermolecular hydrogen-bonding interactions. Considering the DFT error limits, the fairly good agreement between calculated and experimental NMR parameters arising from the statistical modeling of the data allows the unambiguous assignment of the five (51)V NMR signals and, thus, the location of OH surface ligands in the decavanadate cluster. In particular, first-principles calculations accurately reproduce the (51)V quadrupolar parameters. These results are fully consistent with (51)V 3QMAS NMR spectra recorded with and without (1)H decoupling. Finally, correlations are established between local octahedral VO(6) deformations and (51)V NMR parameters (C(q) and Deltadelta), which will be useful for the characterization of a wide range of chemical species containing vanadium(V).

What Is Inside a Nonionic Reverse Micelle? Probing the Interior of Igepal Reverse Micelles Using Decavanadate

The interiors of reverse micelles formed using nonionic surfactants to sequester water droplets in a nonpolar environment have been investigated using the decavanadate molecule as a probe. Chemical shifts and line widths of the three characteristic signals in the 51V NMR spectrum of decavanadate, corresponding to vanadium atoms in equatorial peripheral, equatorial interior, and axial locations, measure the local proton concentration and characteristics of the reverse micellar interior near the decavandate probe. All samples investigated indicate deprotonation of the vanadate probe in the reverse micelle environment. However, the relative mobility of the decavanadate molecule depends on the reverse micellar components. Specifically, the 51V NMR signals of the decavandate in reverse micelles formed using only the Igepal CO-520 surfactant display sharp signals indicating that the decavandate molecule tumbles relatively freely while reverse micelles formed from a mixture of Igepal CO-610 and -430 present a more viscous environment for the decavanadate molecule; the nature of the interior of the nonionic reverse water pool varies significantly depending on the specific Igepal. The 51V NMR spectra also indicate that the interior core water pool of the reverse micelles is less acidic than the bulk aqueous solution from which the samples were created. Together, these data provide a description that allows for a comparison of the water pools in these different nonionic reverse micelles.

Crystal Structure of the VV Dimer [V2O2(μ‐O)(dmpp)2(OCH3)2] and Its Equilibrium with the VV Trimer [V3O3(μ‐O)3(dmpp)3(H2O)](H2O)2 in Methanol/Water Solutions

AbstractThe behaviour of the cyclic trimeric VV complex [V3O3(μ‐O)3(dmpp)3(H2O)](H2O)2, V3L3, (L = Hdmpp = 3‐hydroxy‐1,2‐dimethyl‐4‐pyridinone) was studied in methanol and methanol/water solutions by using 51V and 1D‐ and 2D 1H NMR spectroscopy. Red crystals, isolated from a highly concentrated methanol solution of the trimeric complex, were analysed by X‐ray crystallography. The solid‐state structure of the compound showed the presence of a new dinuclear VV cluster and allowed for its formulation as a [V2O2(μ‐O)(dmpp)2(OCH3)2] complex, V2Y2L2 (Y = OCH3). This complex crystallises in the monoclinic system: P21/c, a = 8.4573(11) Å, b = 15.034(2) Å and c = 15.849(2) Å, β = 105.300(2)°, V = 1943,7(4) Å3, Z = 2, and R1(wR2) = 0.0492(0.1706). The trimer V3L3 complex dissolved in a dry methanol solution fully decomposes, as shown by the 51V NMR signals at –388, –450 and –551 ppm, which are assigned to a monomer complex [VO(OMe)(dmpp)2] (VYL2), the dimer V2Y2L2 and the monomethyl ester of monovanadate, V1Y (V1 = monovanadate; Y = OCH3), respectively. In methanol/water solutions, a new 51V NMR signal appears at δ = –492 ppm, which is assigned to the [VO2(dmpp)(H2O)2] (VL) complex. When the percentage of water in the mixture increases, the relative intensities of the V2Y2L2 and V1Y signals decrease sharply, and a broad signal at –488 ppm appears, corresponding to the original VV trimer complex, which is the only species present in 94 % water. A temperature‐dependent 1H NMR study of a CD3OD solution of V3L3 confirmed the presence, at room temperature, of the dinuclear V2L2 complex and the VL2 species. At temperatures below 0 °C down to –50 °C, the appearance of new signals reflects the presence of isomers for the V2Y2L2 and VYL2 species with different stabilities and symmetries. 2D 1H homonuclear NMR exchange experiments (EXSY) allowed us to establish the isomeric equilibria that take place in solution, and indicates intramolecular exchange between the two ligands of the major isomer of VYL2 and intermolecular exchange between the major and minor isomers of species of different nuclearity, V2Y2L2 and VYL2. However, no evidence was found for intermolecular exchange between the major isomers and between the minor isomers of species of different nuclearity or between isomers of species of the same nuclearity. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

Magnetic Resonance Investigation of Vanadia and Vanadium−Molybdenum Gels Synthesized with Peroxovanadate Precursors

Vanadia gels and vanadium-molybdenum oxide gels were investigated using the magnetic resonance techniques, EPR spectroscopy and (51)V MAS NMR spectroscopy. The vanadium oxide gels were derived from the reaction of H(2)O(2) and V(2)O(5), and the vanadium-molybdenum oxide (VMoO) gels were derived from the reaction of peroxovanadates with an ammonium molybdate solution. EPR spectroscopy was utilized to determine quantitative information about the concentration of V(4+) paramagnetic species present in the samples and additional structural information about the V(4+) coordination environment. (51)V MAS NMR spectroscopy was used to elucidate the V(5+) electronic environment and how it changes as a function of molybdenum content. The observed line broadening of the (51)V NMR signal with increasing molybdenum content was correlated with an increase in the concentration of paramagnetic species as monitored by EPR spectroscopy. The evolution of various vanadium sites during thermal treatment was also investigated. This work provides further support for the hypothesis that the selectivity of VMoO catalysts in the oxidation of 1,3-butadiene to maleic anhydride is due to the presence of paramagnetic V(4+) sites.

Study of the oxidation products of the VO(dmpp) 2 complex in aqueous solution under aerobic conditions: comparison with the vanadate–dmpp system

The redox behavior of the potential oral insulin mimetic compound bis[3-hydroxy-1,2-dimethyl-4-pyridinonate]oxovanadium(IV), VO(dmpp) 2, was studied in aqueous solution, under aerobic conditions and at different pH values and metal-to-ligand ratios, as well as the speciation of its V(V) oxidation products. Dissolution of this compound in water originates a violet solution with pH 4.7, which is partially oxidized to two V(V) species, VO 2L and [VO 2L 2] −, detected through their 51V NMR signals at −485 and −504 ppm. However, the VO(dmpp) 2 compound still remains to some extent in solution, as confirmed by EPR Spectroscopy. High pH values favor the oxidation of the V(IV) complex in solution, whereas at low pH the V(IV) species are predominant. Aqueous solutions containing vanadate–Hdmpp mixtures spontaneously yield V(IV) species, reflecting the reducing capacity of the ligand. This reduction process is favored by ligand excess and low pH. For a ligand excess of fivefold or higher, the [VO 2L 2] − complex, the only species present in solution, produces, at pH<7, the paramagnetic species, [VO(dmpp) 2(H 2O)] and [VO(H 2O) 5] 2+, as detected by EPR. At higher vanadate–hdmpp ratios [VO(dmpp)(H 2O) 2] + is also produced. The V(V) species formed in solution were studied through a parallel 51V and 1H NMR study of the vanadate–Hdmpp system, as a function of metal/ligand ratio and pH. The p K a values of the two main mononuclear V(V) octahedral complexes, VO 2L and [VO 2L 2] − (L=dmpp), were determined from the pH dependence of their 51V and 1H NMR chemical shifts. For VO 2L, p K a values of 5.88±0.03 and 3.30±0.07 were assigned, respectively, to protonation of [VO 2(dmpp)(OH)(H 2O)] − at the coordinated hydroxo ligand and at the ligand nitrogen atom. For [VO 2L 2] −, the three p K a values obtained, 3.96±0.06, 5.62±0.03, and 2.9±0.4, were attributed, respectively, to protonation of [VO 2(dmpp) 2] − at the nitrogen atom of one of the ligands, at one oxygen of the VO 2 + core, and at its resulting VOH group, with formation of the VO 3+ core. The stability constants of these two complexes (log K 1=10.48±0.05; log K 2=5.25±0.04; log β 2=15.73±0.05) were obtained from 51V NMR data, at pH 8.2, ionic strength 0.1 M, by changing the metal/ligand ratio at constant metal concentration.

A family of vanadate esters of monoionized and diionized aromatic 1,2-diols: synthesis, structure, and redox activity.

The concerned diols (general abbreviation, H(2)L) are catechol (H(2)L(1)) and its 3,5-Bu(t)(2) derivative (H(2)L(2)). Esters of the type VO(xsal)(HL), 2, are obtained by reacting H(2)L with VO(xsal)(H(2)O) or VO(xsal)(OMe)(HOMe), where xsal(2-) is the diionized salicylaldimine of glycine (x = g), L-alanine (x = a), or L-valine (x = v). The reaction of VO(acac)(2) with H(2)L and the salicylaldimine (Hpsal) of 2-picolylamine has furnished VO(psal)(L), 3. In the structures of VO(gsal)(HL(1)), 2a, and VO(vsal)(HL(2)), 2f, the HL(-) ligand is O,O-chelated, the phenolic oxygen lying trans to the oxo oxygen atom. The xsal(2-) coligand has a folded structure and the conformation of 2f is exclusively endo. In both 2a and 2f the phenolic oxygen atom is strongly hydrogen bonded (O...O, 2.60 A) to a carboxylic oxygen atom of a neighboring molecule. In VO(psal)(L(2)).H(2)O, 3b, the diionized diol is O,O-chelated to the metal and the water molecule is hydrogen bonded to a phenoxidic oxygen atom (O.O, 2.84 A). The C-O and C-C distances in the V(diol) fragment reveal that 2 is a pure catecholate and 3 is a catecholate-semiquinonate hybrid. In solution each ester gives rise to a single (51)V NMR signal (no diastereoisomers), which generally shifts downfield with a decrease in the ester LMCT band energy. The V(V)/V(IV) and catecholate-semiquinonate reduction potentials lie near -0.75 and 0.35, and 1.10 and 0.70 V vs SCE for 2 and 3, respectively. Molecular oxygen reacts smoothly with 2 quantitatively furnishing the corresponding o-quinone, and in the presence of H(2)L the reaction becomes catalytic. In contrast, type 3 esters are inert to oxygen. The initial binding of O(2) to 2 is proposed to occur via hydrogen bonding with chelated HL(-).

Several aggregates of V2O2(μ-S)2 units. Syntheses, structures and properties of (Et4N)3{[V2S2O2(Et2dtc)2]2K}, (Et4N)2(NH4){[V2S2O2(C5H10dtc)2]2Na}, and (Et4N)3{[V2S2O2(Me2dtc)2]3Na3(H2O)} (dtc=dithiocarbamate)

Three aggregates of V2O2(μ-S)2 units, (Et4N)3{[V2S2O2(Et2dtc)2]2K} (1), (Et4N)2(NH4){[V2S2O2(C5H10dtc)2]2Na} (2), and (Et4N)3{[V2S2O2(Me2dtc)2]3Na3(H2O)} (3) together with a mononuclear complex vanadium tri-pentamethylenedithiocarbamate have been obtained from a reaction system consisting of (NH4)3VS4, R2dtcNa (K) and PPh3 in CH3CN and structurally characterized. All the [V2S2O2(R2dtc)2]2− moieties possess very similar structure parameters and link each other by the interaction between V2O2(μ-S)2 units and alkali metal ion, which exists in O and S coordination sphere with coordinate number of 4, 5 or 6, respectively, for the three aggregate complexes. Infra red spectra of these aggregates exhibit the additional coordination of the V=O group to alkali metal, leading to red shift of the V=O stretch frequency. The 1H NMR signals of α-CH2 in the R2dtc ligands were observed to be close to those of the R2dtcNa, indicating the spin–spin coupling between both V(IV) sites in the [V2S2O2(R2dtc)2]2− moiety. The 51V NMR signals were observed at approximately 350 ppm. The cyclic voltammogram displays quasi-reversible couple at −0.60 to −0.73 V and an irreversible oxidation at 0.5–0.73 V, responsible for 3-/2-couple redox and 2- to 1- oxidation, respectively, of the [V2S2O2(R2dtc)2]2− moieties. Variable-temperature magnetic susceptibility study exhibits strong antiferromagnetic exchange interactions existing between the two V(IV) sites in the V2O2(μ-S)2 units.

Vanadium(IV and V) Complexes ContainingSNO (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.

A comparative study of V species in ß zeolite by photoluminescence, diffuse reflectance UV-Visible and 51V NMR spectroscopies

In calcined and rehydrated VSis zeolite prepared following a method described earlier, three kinds of tetrahedral VV species can be clearly identified by photoluminescence spectroscopy even at low V content, in contrast to 51V NMR and diffuse reflectance UV-Visible spectroscopies which do not allow them to be distinguished. A comparative study by photoluminescence and 51V NMR spectroscopies leads to the suggestion that there is a correlation between the vibrational energy of the V=O bond and the corresponding chemical shift of the 51V NMR signal.

Crystal structure and solution studies of the product of the reaction of β-mercaptoethanol with vanadate

Reaction of β-mercaptoethanol with vanadate under slightly alkaline conditions provided a crystalline complex that was characterized by X-ray diffraction and FTIR spectroscopy. The complex was dimeric in structure with a central [VO]2 core and a pentacoordinate, crudely trigonal bipyramidal arrangement about each vanadium atom with a sulphur occupying a pseudo-axial position. A single 51V NMR signal was observed for this complex when dissolved in water, chloroform or acetonitrile. A large influence of acetonitrile on the vanadium chemical shift suggested the possibility of reaction with acetonitrile. FTIR showed the presence of two complexes in acetonitrile solution but only one in chloroform or water. Mixed solvent studies were carried out in an effort to further characterize the solution complexes. Crystal structure of [{VO2(OC2H4S)}2][NEt4]2: monoclinic, space group P21/n,. a = 8.3451(17), b = 16.954(4), c = 10. 2064(25) Å; β = 101. 271(18)°; V = 1416.2 Å3; Z = 2; RF = 0.048 for 1355 data (Io 2..5σ (Io) and 147 variables.Key words: mercaptoethanol, vanadate, vanadium NMR, X-ray diffraction, FTIR, thiolate.

Crystal structure and solution studies of the product of the reaction of β-mercaptoethanol with vanadate

Reaction of β-mercaptoethanol with vanadate under slightly alkaline conditions provided a crystalline complex that was characterized by X-ray diffraction and FTIR spectroscopy. The complex was dimeric in structure with a central [VO]2 core and a pentacoordinate, crudely trigonal bipyramidal arrangement about each vanadium atom with a sulphur occupying a pseudo-axial position. A single 51V NMR signal was observed for this complex when dissolved in water, chloroform or acetonitrile. A large influence of acetonitrile on the vanadium chemical shift suggested the possibility of reaction with acetonitrile. FTIR showed the presence of two complexes in acetonitrile solution but only one in chloroform or water. Mixed solvent studies were carried out in an effort to further characterize the solution complexes. Crystal structure of [{VO2(OC2H4S)}2][NEt4]2: monoclinic, space group P21/n,. a = 8.3451(17), b = 16.954(4), c = 10. 2064(25) Å; β = 101. 271(18)°; V = 1416.2 Å3; Z = 2; RF = 0.048 for 1355 data (Io 2..5σ (Io) and 147 variables.Key words: mercaptoethanol, vanadate, vanadium NMR, X-ray diffraction, FTIR, thiolate.

Solvation, preferential solvation and complexation by the solvent of peroxovanadium complexes studied by 51V NMR spectroscopy. Correlations with the oxidative reactivity

The 51V NMR chemical shifts of four peroxovanadium complexes, namely [VO(O 2)pic] ( 1), [VO(O 2)pyraz] ( 2), [VO(O 2)aq 1] ( 3) and [VO(O 2) 2aq] ( 4), in twelve solvents have been measured. Excellent Kamlet-Taft-type correlations with both the polarity and the hydrogen bonding donor ability of the solvents have been found for complexes 1 and 2. Preferential solvation phenomena have been studied by measuring the 51V NMR chemical shifts of the four complexes in the mixed solvent H 2OCH 3CN. In such a medium only one 51V NMR signal is observed thus suggesting the occurrence of fast exchange processes between solvent molecules and ligands of vanadium. Preferential solvation plots indicate that CH 3CN may be considered a better solvent than H 2O for complexes 1–3 while for complex 4 the ability of two solvents is very similar. These results have been taken as an indication that CH 3CN displays a well characterized coordinating ability toward vanadium. In two other mixed solvents, i.e. H 2OCH 3OH and H 2O-DMF, under identical experimental conditions, two 51V NMR 51V NMR signals are observed. Also on the basis of the comparison with the behavior of a peroxovanadium complex containing a tridentate ligand, namely pyridine-2,6-dicarboxylic acid, it has been established that only the equatorial water molecular undergoes a slow exchange with the solvents. This appears to be related to the reactivity of peroxovanadium complexes as measured by their decomposition rate. In addition such a reactivity is also affected by the electron density on the metal caused by the nature of the ligands.