Vanadium(III) Complexes with Picolinic Acid and Dipicolinic Acid in Aqueous Solution

The complex species formed in aqueous solution (25 ^∘C, I = 3.0 mol-dm⁻³ KCl ionic medium) between V³⁺ cation and the ligands: picolinic acid (Hpic, HL) and dipicolinic acid (H₂dipic, H₂L), have been studied potentiometrically and by spectrophotometric measurements. The application of the least-squares computer program LETAGROP to the experimental emf (H) data, taking into account the hydrolytic species of V³⁺ ion, indicates that under the employed experimental conditions, the formation of the complexes [VL]²⁺, [V(OH)L]⁺, [VL₂]⁺, [VL₃], [V₂OL₄] with picolinic acid and the complexes [VL]⁺, [V(OH)L], [V(OH)₂L]⁻, [V(HL)(L)], and [VL₂]⁻ with dipicolinic acid were observed. The stability constants of the complexes formed were determined by potentiometric measurements, and spectrophotometric measurements were done in order to perform a qualitative characterization of the complexes formed in aqueous solution.     

Speciation of the Vanadium(III) Complexes with 1,10-Phenanthroline, 2,2′-Bipyridine,and 8-Hydroxyquinoline

The complex species formed between vanadium(III) and 1,10-phenanthroline (phen), 2,2′-bipyridine (bipy), and 8-hydroxyquinoline (8hq) were studied in aqueous solution by means of electromotive forces measurements, emf(H), at 25 °C with 3.0 mol⋅dm⁻³ KCl as the ionic medium. The potentiometric data were analyzed using the least-squares computational program LETAGROP, taking into account the hydrolytic vanadium(III) species formed in solution. Analysis of the vanadium(III)–phen system data shows the formation of [VHL]⁴⁺, [V(OH)L]²⁺, [V₂OL₂]⁴⁺ and [V₂OL₄]⁴⁺ complexes. In the vanadium(III)–bipy system the [VHL]⁴⁺, [V(OH)L]²⁺, [V₂OL₂]⁴⁺ and [V₂OL₄]⁴⁺ complexes were observed, and in the vanadium(III)–8hq system the complexes [V(OH)L]⁺, [V(OH)₂L], [VL₂]⁺ and [VL₃] were detected.     

Na-Li-[V3O8] insertion electrodes: Structures and diffusion pathways

The potential insertion-electrode compounds Na_1.2[V₃O₈] (NaV) and Na_0.7Li_0.7[V₃O₈] (NaLiV) were synthesized from mixtures of Na₂CO₃, Li₂CO₃ and V₂O₅, which were melted at 750° and subsequently cooled to room temperature. The structures of NaV and LiV contain sheets of polymerized (VO_n) polyhedra, which are topologically identical to the sheet of polymerized polyhedra in Li_1.2[V₃O₈] (LiV). Vanadium occurs in three different coordination environments: [2+3] V(1), [2+2+2] V(2) and [1+4+1] V(3). Calculated bond-valence sums indicate that V⁴⁺ occurs preferentially at the V(3) site, which agrees with the general observation that [6]-coordinated V⁴⁺ prefers [1+4+1]-rather than [2+2+2]-coordination. The M-cations Na and Li occur at three distinct sites, M(1), M(2) and M(3) between the vanadate sheets. The M(1)-site is fully occupied and has octahedral coordination. The M(2) sites are partly occupied in NaV and NaLiV, in which they occur in [4]- and [6]-coordination, respectively. Li partly occupies the M(3) site in NaLiV, in which it occurs in [3]-coordination. The M(2) and M(3) sites in NaLiV occur closer to the vanadate sheets than the M(2) sites in NaV and LiV. The shift in these cation positions is a result of the larger distance between the vanadate sheets in NaLiV than in LiV, which forces interstitial Li to move toward one of the vanadate sheets to satisfy its coordination requirements. Bond-valence maps for the interstitial cations Na and Li are presented for NaV, NaLiV and LiV. These maps are used to determine other potential cation positions in the interlayer and to map the regions of the structure where the Na and Li have their bond-valence requirements satisfied. These regions are potential pathways for Na and Li diffusion in these structures, and are used to explain chemical diffusion properties of Na and Li in the Na-Li-[V₃O₈] compounds.     

Kinetics and Mechanism of the Electro-oxidation of Gold in Thiocarbamide Solutions

The kinetics and mechanism of the electrooxidation of gold and thiocarbamide in sulfuric acid solutions of thiocarbamide (TC) have been investigated. The potentials for the oxidation of gold in TC solutions to [Au(TC)₂]⁺ _ads and [Au(TC)₃]³⁺ _ads are 0.132 and 0.561 V (relative to the standard silver chloride electrode). The electrooxidation of thiocarbamide in sulfuric acid solution is characterized by two maximums on voltammograms at 0.983 V (formation of formamidine disulfide, FAD) and 1.437 V (oxidation of FAD, sulfides and hydrosulfides of gold(I)). The calculated rate constants for the electrosolution of gold at the maximum current of the voltammogram is k ₁ = 1.15·10^–5 cm/s and at the minimum current is k ₂ = 3.13·10^–6 cm/s in sulfuric solutions of TC. A mechanism is proposed for the electrosolution of gold and TC in sulfuric acid solutions of thiocarbamide.     

Polymerization of acrylonitrile by V5+–lactic acid system in aqueous sulfuric acid

Kinetics of polymerization of acrylonitrile by the redox system V⁵⁺–lactic acid in sulfuric acid at 20–35°C was studied. Oxidation of lactic acid by V⁵⁺ in the absence of monomer was also carried out under identical conditions. The rates of polymerization, V⁵⁺ disappearance and the chainlengths of polyacrylonitrile were measured. From the results it is concluded that the polymerization reaction is initiated by an organic free radical arising from the V⁵⁺–lactic acid reaction with termination by V⁵⁺ ions. Mutual termination of active polymer radicals does not appear to operate under the conditions studied. The various rate parameters were evaluated.     

Redox polymerization of acrylonitrile. Kinetics of the reaction initiated by the redox system tartaric acid–V5+

Kinetics of vinyl polymerization of acrylonitrile initiated by the redox system tartaric acid–V⁵⁺ have been investigated in aqueous sulfuric acid in the temperature range 30–40°C. The rates of polymerization and V⁵⁺ disappearance and the chain lengths of polyacrylonitrile were measured. From the results it is concluded that the polymerization reaction is initiated by an organic free radical arising from the V⁵⁺–tartaric acid reaction with termination by V⁵⁺ ions. A suitable kinetic scheme has been proposed, and the various rate and energy parameters were evaluated.     

Ternary Complex Formation between Vanadium(III), Dipicolinic Acid and Picolinic Acid in Aqueous Solution

Ternary complex species formed by the V³⁺ cation with the picolinic acid (Hpic, HL) and dipicolinic acid (H₂dipic, H₂L) ligands in aqueous solutions have been studied potentiometrically (25 °C, I=3.0 mol⋅dm⁻³ KCl ionic medium) and by spectrophotometric measurements. Application of the least-squares computer program LETAGROP to the experimental emf (H) data, taking into account the hydrolytic V(III) species and the binary V³⁺–picolinic acid and V³⁺–dipicolinic acid complexes, shows that under the investigated conditions the following ternary complexes are formed: [V(dipic)(pic)], [V(dipic)(pic)(OH)]⁻ and [V(dipic)(pic)₂]⁻. The stability constants of the ternary complexes were determined by potentiometric measurements whereas the spectrophotometric measurements were done in order to obtain a qualitative characterization of the complexes formed in aqueous solution.     

Electron Transfer and Switching in Rigid [2]Rotaxanes Adsorbed on TiO2 Nanoparticles

Bistable [2]rotaxanes have been attached through a bulky tripodal linker to the surface of titanium dioxide nanoparticles and studied by cyclic voltammetry and spectroelectrochemical methods. The axle component in the [2]rotaxane contains two viologen sites, V₁ and V₂, interconnected by a rigid terphenylene bridge. In their parent dication states, V₁²⁺ and V₂²⁺ can both accommodate a crown ether ring, C, but are not equivalent in terms of their affinity towards C and have different electrochemical reduction potentials. The geometry and size of the tripodal linker help to maintain a perpendicular [2]rotaxane orientation at the surface and to avoid unwanted side‐to‐side interactions. When the rigid [2]rotaxane or its corresponding axle are adsorbed on a TiO₂ nanoparticle, viologen V₂²⁺ is reduced at significantly more negative potentials (?0.3 V) than in flexible analogues that contain aliphatic bridges between V₁ and V₂. These overpotentials are analysed in terms of electron‐transfer rates and a donor–bridge–acceptor (D–B–A) formalism, in which D is the doubly reduced viologen, V₁⁰, adjacent to the TiO₂ surface (TiO₂–V₁⁰), B is the terphenylene bridge and A is viologen V₂²⁺. We have also found that, in contrast with earlier findings in solution, no molecular shuttling occurs in rigid [2]rotaxane adsorbed at the surface. The observations were explained by the relative position of the viologen stations within the electrical double layer, screening of V₂²⁺ by the counterions and high capacity of the medium, which reduces the mobility of the crown ether. The results are useful in transposing of solution‐based molecular switches to the interface or in the design and understanding of the properties of systems comprising electroactive and/or interlocked molecules adsorbed at the nanostructured TiO₂ surface.     

Pentavalent Vanadium-Ascorbic Acid Redox System Initiated Polymerization of Acrylonitrile

Kinetics of polymerization of acrylonitrile initiated by the redox system V⁵⁺ascorbic acid were investigated in aqueous sulfuric acid in the temperature range of 35–50°C, and the rates of polymerization, of V⁵⁺/disappearance, etc. were measured. From the result it was concluded that the polymerization reaction is initiated by an organic free radical arising from the V⁵⁺ - ascorbic acid complex formation followed by subsequent decomposition and terminated by V⁵⁺ions. A suitable kinetic scheme was proposed and the various rate parameters were evaluated.     

Vinyl polymerization initiated by ceric ion reducing agent systems in sulfuric acid medium

Polymerization of the monomers, methyl acrylate (MA) and methyl methacrylate (MMA) was carried out in sulfuric acid medium at 15°C. With the redox initiator system, ceric ammonium sulfate–malonic acid. There was no induction period, and a steady state was attained in a short time. There was found to be no polymerization even after 1 hr. in the absence of the reducing agent R. The initiation was by the radical produced from the Ce⁴⁺–malonic acid reaction. The rate of monomer disappearance was proportional to [M]^1.5, [R]^0.5, and [Ce⁴⁺]^0.3–0.5, and the rate of ceric disappearance was directly proportional to [R] and [Ce⁴⁺]. Chain lengths of the polymers were directly proportional to [M] and inversely to [R]^1/2 and [Ce⁴⁺]^1/2. The experimental results were explained by a kinetic scheme involving the following steps: (a) oxidation of the substrate to give the primary radical which reacts with Ce⁴⁺ to give the products, (b) initiation by the primary radical, (c) propagation, and (d) termination of the growing polymer radicals by the mutual type. For the polymerization of acrylonitrile (AN) by the redox system, ceric ammonium sulfate–cyclohexanone (CH), in sulfuric acid at 15°C., the scheme was modified to include linear type of termination by Ce⁴⁺, along with the mutual termination to explain the results especially under conditions with [Ce⁴⁺] ≥ [CH].     

Aqueous polymerization of methacrylic acid initiated by Ce4+ -glycolic acid redox pair

The aqueous polymerization of methacrylic acid (MAA) initiated by a Ce⁴⁺ -glycolic acid (GA) system was observed in a sulfuric acid medium at 35 ± 0.2°C in a nitrogen atmosphere. The rate of monomer disappearance was proportional to [MAA]¹ and the rate of ceric ion disappearance was proportional to [GA][Ce⁴⁺]. An increase in the reaction temperature from 30 to 45°C raised the rate and the overall activation energy was 63 kJ/mol. The molecular weight increased with a rise in [MAA] and a reduction in [Ce⁴⁺]. The effect of varying [H₂SO₄] was also studied.     

Formation Constants for the Ternary Complexes of Vanadium(III), 2,2′-Bipyridine, and the Amino Acids Histidine, Cysteine, Aspartic and Glutamic Acids

In this study we report the stability constants and the speciation of the ternary vanadium(III) complexes with 2,2??-bipyridine (Bipy) and the amino acids histidine (HHis), cysteine (H₂Cys), aspartic acid (H₂Asp) and glutamic acid (H₂Glu) by means of potentiometric titrations employing 3.0 mol?dm^?3 KCl as the ionic medium at 25?°C. The potentiometric data were analyzed taking into account the hydrolysis of the vanadium(III) cation and the respective stability constants of the binary complexes and the acid?Cbase reactions of the ligands, which were kept fixed during the analysis. The complexes detected in the different systems are: in the vanadium(III)?CBipy?CHHis system, [V(HBipy)(HHis)]⁴⁺ and [V(HBipy)(H₂His)]⁵⁺; in the vanadium(III)?CBipy?CH₂Cys system, [V₂O(Bipy)(Cys)]²⁺; in the vanadium(III)?CBipy?CH₂Asp system, [V(Bipy) (Asp)]⁺, [V₂O(Bipy)(Asp)]²⁺, and V₂O(Bipy)₂(Asp)₂; and finally in the vanadium(III)?CBipy?CH₂Glu system, [V(Bipy)(H₂Glu)]³⁺ and [V(Bipy)(Glu)]⁺. The respective stability constants were determined and the specie distribution diagrams as a function of pH are briefly discussed.     

Aqueous Polymerization of Acrylamide Initiated by Glycolic Acid/Ce4+ Redox System

The aqueous polymerization of acrylamide initiated by the glycolic acid/Ce⁴⁺ redox system was studied in sulfuric acid medium at 35 ± 0.2°C under a nitrogen atmosphere. The initiation was carried out by the free radical generated in the decomposition of the complex formed between the oxidant and the reductant. The monomer disappearance was found to be proportional to [GA]^0,89[Ce⁴⁺]^0.57[M]^1.0, and the rate of ceric ion disappearance was found to be directly proportional to [Ce⁴⁺] and [GA] but independent of [M]. The activation energy of the system was found to be 7.21 kcal/deg/mol. The molecular weight of polyacrylamide increased with increasing [monomer] and decreased with increasing [catalyst]. The effect of pH was also studied in the pH range 2.22 to 1.44.     

Electron transfer. 109. The reaction of dimeric molybdenum(V) with carboxylato-bound chromium(V). A third mechanistic variation

The bridged dimer of molybdenum(V), Mo₂O₄²⁺ (aq) is oxidized to Mo(VI) by carboxylato-bound chromium(V). Reaction of bis(chelated) Cr(V) with excess (Mo^V)₂ yields a chelated Cr(III) complex, but this conversion proceeds through a pink Cr(IV) intermediate, indicating that the oxidation of (Mo^V)₂ entails a series of le^? steps, passing through a reactive transient, the mixed valence complex, Mo^VMo^VI. When experiments are carried out in buffers of the ligating acid, 2-ethyl-2-hydroxybutanoic acid, two stages of ligation of (Mo^V)₂ by the ligand anion, characterized by rate constants near 10⁴ and 0.14 M^?1 s^?1 (19°C; pH 3.0; μ = 0.6 M) must be considered. In quick mixing experiments, the first, but not the second, of these proceeds before the redox reaction gets under way, and autocatalytic redox profiles are observed. If the slower ligation is allowed to reach completion before Cr(V) is added, reduction to Cr(IV) is greatly accelerated and conforms to the superposition of two processes, whereas the reduction of Cr(IV) to Cr(III) is slow and exhibits a rate independent of [Cr^IV]. A proposed sequence applicable to the latter conditions includes reductions of Cr(V) at two ligation levels, slow unimolecular conversion of (Mo^V)₂ to an activated form, and rapid reduction of the latter with Cr(IV). Here Cr(IV) has assumed the role of a scavenger for the reactive form of (Mo^V)₂.     

Study of the Ternary Complex Formation Between Vanadium(III)-Picolinic Acid and the Amino Acids: Cysteine,Histidine, Aspartic and Glutamic Acids

The complex species formed between vanadium(III)-picolinic acid (HPic) and the amino acids: cysteine (H₂Cys), histidine (HHis), aspartic acid (H₂Asp) and glutamic acid (H₂Glu) were studied in aqueous solution by means of electromotive forces measurements emf(H) at 25 °C and 3.0 mol⋅dm⁻³ KCl as ionic medium. Data analysis using the least-squares program LETAGROP indicates the formation of ternary complexes, whose stoichiometric coefficients and stability constant were determined. In the vanadium(III)-picolinic acid-cysteine system the model obtained was: [V(Pic)(H₂Cys)]²⁺, [V(Pic)(HCys)]⁺, V(Pic)(Cys) and [V₂O(Pic)(Cys)]⁺. The vanadium(III)-picolinic acid-histidine system contained the following complexes: [V(Pic)(HHis)]²⁺, [V(Pic)(His)]⁺, V(Pic)(His)(OH) and [V(Pic)₂(HHis)]⁺. In the vanadium(III)-picolinic acid-aspartic acid system the model obtained was: V(Pic)(Asp), [V(Pic)(Asp)(OH)]⁻ and [V₂O(Pic)(Asp)]⁺ and finally, in the vanadium(III)-picolinic acid-glutamic acid system the complexes: V₂O(Pic)₂(HGlu)₂, V(Pic)(HGlu)₂ and V(Pic)₂(HGlu) were observed.     

Speciation of the Vanadium (III) Complexes with 6-Methylpicolinic Acid,Salicylic Acid and Phthalic Acid

The complex species formed in aqueous solutions (25 °C, I=3.0 mol⋅dm⁻³ KCl ionic medium) between the V(III) cation and the ligands 6-methylpicolinic acid (MePic, HL), salicylic acid (H₂Sal, H₂L) and phthalic acid (H₂Phtha, H₂L) have been studied by potentiometric and spectrophotometric measurements. Application of the least-squares computer program LETAGROP to the experimental emf(H) data, taking into account the hydrolytic species and hydrolysis constants of V(III), indicates that under the employed experimental conditions the complexes [VL]²⁺, [V(OH)L]⁺, [V(OH)₂L], [V(OH)₃L]⁻, [VL₂]⁺, [VL₃] and [V₂OL₄] form in the vanadium(III)–MePic system. Were observed the complexes [VL]⁺, [VL₂]⁻, [V(OH)L₂]²⁻ and [VL₃]³⁻ in the vanadium(III)–H₂Sal system, and the species [VHL]²⁺, [VL]⁺, [V(OH)L], [VHL₂], [VL₂]⁻, [V(OH)L₂]²⁻, [V(OH)₂L₂]³⁻ and [VL₃]³⁻ in the vanadium(III)–H₂Phtha system. The stability constants of these complexes were determined by potentiometric measurements, and spectrophotometric measurements were made in order to perform a qualitative characterization of the complexes formed in aqueous solution.     

Vanadium(V)-substituted Keggin-type heteropolyoxotungstophosphates as electron transfer and antimicrobial agents: oxidation of glutathione and sensitization of MRSA towards β-lactam antibiotics

Glutathione (GSH) undergoes facile electron transfer with vanadium(V)-substituted Keggin-type heteropolyoxometalates, [ \textPV^\textV \textW _{1 1} \textO _{4 0} ] ^{4 -} [ {\text{PV}}^{\text{V}} {\text{W}}_{ 1 1} {\text{O}}_{ 4 0} ]^{{ 4 { - }}} (HPA1) and [ \textPV^\textV \textV^\textV \textW _{1 0} \textO _{4 0} ] ^{5 -} [ {\text{PV}}^{\text{V}} {\text{V}}^{\text{V}} {\text{W}}_{ 1 0} {\text{O}}_{ 4 0} ]^{{ 5 { - }}} (HPA2). The kinetics of these reactions have been investigated in phthalate buffers spectrophotometrically at 25 °C in aqueous medium. One mole of HPA1 consumes one mole of GSH and the product is the one-electron reduced heteropoly blue, [ \textPV^\textIV \textW _{1 1} \textO ₄₀ ] ^5- [ {\text{PV}}^{\text{IV}} {\text{W}}_{ 1 1} {\text{O}}_{ 40} ]^{ 5- } . But in the GSH-HPA2 reaction, one mole of HPA2 consumes two moles of GSH and gives the two-electron reduced heteropoly blue [ \textPV^\textIV \textV^\textIV \textW ₁₀ \textO ₄₀ ] ^7- [ {\text{PV}}^{\text{IV}} {\text{V}}^{\text{IV}} {\text{W}}_{ 10} {\text{O}}_{ 40} ]^{ 7- } . Both reactions show overall third-order kinetics. At constant pH, the order with respect to both [HPA] species is one and order with respect to [GSH] is two. At constant [GSH], the rate shows inverse dependence on [H⁺], suggesting participation of the deprotonated thiol group of GSH in the reaction. A suitable mechanism has been proposed and a rate law for the title reaction is derived. The antimicrobial activities of HPA1, HPA2 and [ \textPV^\textV \textV^\textV \textV^\textV \textW ₉ \textO _{4 0} ] ^{6 -} [ {\text{PV}}^{\text{V}} {\text{V}}^{\text{V}} {\text{V}}^{\text{V}} {\text{W}}_{ 9} {\text{O}}_{ 4 0} ]^{{ 6 { - }}} (HPA3) against MRSA were tested in vitro in combination with vancomycin and penicillin G. The HPAs sensitize MRSA towards penicillin G.     

(NH4)3[VO2(SO4)2(H2O)2]·1.5H2O

The title compound, tri­ammonium cis‐di­aqua‐cis‐dioxo‐trans‐disulfatovanadate 1.5‐hydrate, was obtained by oxidizing V^IV to V^V in a 2 M sulfuric acid solution of vanadyl­ sulfate and adding ammonium sulfate. Here, the V atom is sandwiched by two sulfate groups by corner‐sharing to form a discrete [VO₂(SO₄)₂(OH₂)₂]^3? anion. The water mol­ecules occupy cis positions in the equatorial plane of the vanadium octahedron.     

Mechanism of oxidation of cyclohexanehexol (inositol) by quinquevalent vanadium

The oxidation of inositol by quinquevalent vandadium in acid medium is a first-order reaction both in vanadium (V) and inositol. The stoichiometry of the reaction is consistent with the use of two equivalents of vanadium (V) per mole of inositol with the formation of one mole of inosose. The reaction is catalyzed both by sulfuric and perchloric acid, but the rate is faster in sulfuric acid than in perchloric acid. In 1M–6M perchloric acid solutions the reaction has shown a variable order in H⁺, but in solutions of 2M–5M sulfuric and perchloric acid of constant ionic strength, the rate has a linear dependence on [H⁺]². There is also a linear correlation between the rate and bisulfate ions in sulfuric acid at constant hydrogen ion concentration. The energy of activation is found to be 19 kcal/mole and a negative entropy value of ? 14 e.u. A suitable mechanism, consistent with the kinetics in 2M–5M acid solutions, is suggested and the values of various rate constants are evaluated.     

Preparation,Properties, and ESCA Characterization of Vanadium Surface Compounds on Silicagel. II

Binding energy (BE) behavior and signal intensity as function of V conc. and ligands L were critically examined in Vⁿ/SiO₂ species (where n = +5, +4, and +3) and securing of optimized products and ESCA samples is described. The results moreover demonstrate conditions and approaches whereby ESCA can provide reliable information on highly diluted surface compound systems. From aquous media 1.0–1.2 wt. % V^V and 0.4–1.2 wt. % V^III proved to be preferred ranges whereas in non aqueous (metalorganic) preparates the surprisingly low concentration of 0.4–0.15 wt. % V^III gave usable spectra and even 0.4 wt. % V^V could be measured. ESCA data with the peroxo complex (?Si? O)₃V? (O₂) (I) confirmed these trends. Superiorly subdivided surface compounds (reflected in the quality of spectra) are obtainable from V[N(SiMe₃)₂]₃ impregnations where 0.4–0.5 wt. % V^III (or V^V) are maximum/optimum conc. limits. Evidence for formulating Vⁿ/SiO₂ as three legged surface compounds is summarized and diagnostic/predictive uses of the additivity principle for surface electron states illustrated. In the (?Si? O)₃V? L series, where L = none, ?O, and ? (O₂) positive ΔBE shifts for ?Si? O? V (0.7 eV); V?O (0.6 eV); and V? (O₂) (0.4 eV) were estimated. Tentative BE contributions of some donor ligands (and ? Cl) are also suggested. In (I) ESCA indicated a formal oxidation state of approx. +4. Certain limits (precautions) with ESCA are noted and extensions of the additivity relationship discussed.