Multi-vanadium substituted polyoxometalates as efficient electrocatalysts for the oxidation of l-cysteine at low potential on glassy carbon electrodes

The electrochemical behaviours of the sandwich-type complex [As2W18(VO)3O66]11- were studied in a pH 7 medium and compared with those of the three following Dawson-type vanadium-substituted complexes: [P2V2W16O62]8- (P2V2W16), [P2MoV2W15O62]8- (P2MoV2W15) and [P2V3W15O62]9- (P2V3W15). Electrochemistry shows that the sandwich-type POM contains 2 VIV centers and one VV center and must be formulated As2V2IVVW18, in agreement with titration, elemental analysis and magnetic measurements on this element.. All the POMs of this work proved efficient for the oxidation of L-cysteine. Comparison of the present results with those of mono-Vanadium substituted POMs indicates that accumulation of vanadium atoms in the POM framework is beneficial in the electrocatalytic process. In addition, the present work highlights the important influence of the POM structure in the electrocatalytic oxidation of L-cysteine. The remarkable outcome of this work is that the potential for the oxidation of L-cysteine in the presence of the selected POMs has been substantially driven in the negative direction compared to the case of glassy carbon alone, a feature which is associated with faster kinetics. The stability of the systems must also be pointed out

Photoactive Polyoxometalate/DASA Covalent Hybrids for Photopolymerization in the Visible Range

International audienc

Studies of the Effectiveness of Bisphosphonate and Vanadium-Bisphosphonate Compounds In Vitro

Leishmaniasis is a disease that is a significant problem for people, especially in tropical regions of the world. Current drug therapies to treat the disease are expensive, not very effective, and/or of significant side effects. A series of alkyl bisphosphonate compounds and one amino bisphosphonate compound, as well as alendronate and zoledronate, were tested as potential agents against Leishmania tarentolae. Also, two polyoxometalates (POMs) with nitrogen-containing bisphosphonate ligands, vanadium/alendronate (V5(Ale)2) and vanadium/zoledronate (V3(Zol)3), were tested against L. tarentolae and compared to the results of the alendronate and zoledronate ligands alone. Of the compounds evaluated in this study, the V5(Ale)2 and V3(Zol)3 complexes were most effective in inhibiting the growth of L. tarentolae. The V5(Ale)2 complex had a larger impact on cell growth than either alendronate or orthovanadate alone, whereas zoledronate itself has a significant effect on cell growth, which may contribute to the activity of the V3(Zol)3 complex

On the possibility of magneto-structural correlations: detailed studies of di-nickel carboxylate complexes

A series of water-bridged dinickel complexes of the general formula [Ni<sub>2</sub>(μ<sub>2</sub>-OH<sub>2</sub>)(μ2- O<sub>2</sub>C<sup>t</sup>Bu)<sub>2</sub>(O<sub>2</sub>C<sup>t</sup>Bu)2(L)(L0)] (L = HO<sub>2</sub>C<sup>t</sup>Bu, L0 = HO<sub>2</sub>C<sup>t</sup>Bu (1), pyridine (2), 3-methylpyridine (4); L = L0 = pyridine (3), 3-methylpyridine (5)) has been synthesized and structurally characterized by X-ray crystallography. The magnetic properties have been probed by magnetometry and EPR spectroscopy, and detailed measurements show that the axial zero-field splitting, D, of the nickel(ii) ions is on the same order as the isotropic exchange interaction, J, between the nickel sites. The isotropic exchange interaction can be related to the angle between the nickel centers and the bridging water molecule, while the magnitude of D can be related to the coordination sphere at the nickel sites

Trapping the δ isomer of the polyoxometalate-based Keggin cluster with a tripodal ligand

We report the synthesis, structural, and electronic characterization of the theoretically predicted, but experimentally elusive δ-isomer of the Keggin polyanion. A family of δ-Keggin polyoxoanions of the general formula, (TEA)HpNaq [H2M12(XO4)O33(TEA)].rH2O where p, q, r = [2,3,8] for 1 and [4,1,4] for 2 were isolated by the reaction of tungstate(VI) and vanadium(V) with triethanolammonium ions (TEAH), acting as a tripodal ligand grafted to the surface of the cluster leading to the entrapment and stabilization of the elusive polyanhionic δ Keggin archetype. The δ-Keggin species were characterized by single-crystal X-ray diffraction, FT-IR, UV-vis, NMR and ESI-MS spectrometry. Electronic structure and structure-stability correlations were evaluated by means of DFT calculations. The compounds exhibited multi-electron transfer and reversible photochromic properties by undergoing single-crystal-to-single-crystal (SC-SC) transformations accompanied with colour changes under ligh

Towards robust alkane oxidation catalysts: electronic variations in non-heme iron(II) complexes and their effect in catalytic alkane oxidation

A series of non-heme iron(II) bis(triflate) complexes containing linear and tripodal tetradentate ligands has been prepared. Electron withdrawing and electron donating substituents in the para position of the pyridine ligands as well as the effect of pyrazine versus pyridine and sulfur or oxygen donors instead of nitrogen donors have been investigated. The electronic effects induced by these substituents influence the strength of the ligand field. UV-vis spectroscopy and magnetic susceptibility studies have been used to quantify these effects and VT 1H and 19F NMR spectroscopy as well as X-ray diffraction have been used to elucidate structural and geometrical aspects of these complexes. The catalytic properties of the iron(II) complexes as catalysts for the oxidation of cyclohexane with hydrogen peroxide have been evaluated. In the strongly oxidising environment required to oxidise alkanes, catalyst stability determines the overall catalytic efficiency of a given catalyst, which can be related to the ligand field strength and the basicity of the ligand and its propensity to undergo oxidation

Redox behaviour, electrochromic properties and photoluminescence of potassium lanthano phosphomolybdate sandwich-type compounds

Sandwich-type phosphomolybdates with general formula K-n[Ln(III)(PMo11O39)(2)], where Ln(III) = Sm, Eu, Gd, Tb and Dy, were prepared and characterized by several techniques. The crystal structure of Sm(PMo11)(2) and Gd(PMo11)(2) were studied and showed that they crystallise in a P2(1)/c space group. All Ln(PMo11) 2 revealed four Mo-based electrochemical reduction processes with very similar E-1/2 values (approximate to 0.5, approximate to 0.3, approximate to 0.02 and approximate to-0.1 V) for all the Ln(III) atoms. The two more positive reduction processes correspond to pH independent one-electron reduction processes, whereas the two more negative processes correspond to pH dependent two-electron reduction processes. Electrolysis at two different potentials, 0.1 V - related to the two one-electron reduction processes - and -0.3 V - related to the two more negative two-electron reduction processes, confirmed the electrochromic properties of the Ln(PMo11)(2) species: their original yellow coloured solutions turned blue, corresponding to the appearance of four new electronic bands in the near UV-Vis-near IR region. These electronic bands were tentatively assigned based on their molar absorption coefficients (epsilon) and absorbance (Abs) variation as a function of electrolysis time: bands A (lambda(max) approximate to 855-870 nm) and B (lambda(max) approximate to 670-695 nm) were assigned to Mo-V -> Mo-VI intervalence charge transfer transitions, band C (lambda(max) approximate to 525 nm) to d-d transition due to d(1) configuration of the reduced addenda atom (Mo-V) and band D (lambda approximate to 310-315 nm) to an O -> Mo-V charge transfer transition (CTT). The emission features of the Eu(PMo11)(2) and Tb(PMo11)(2) samples reveal broad emission in the UV/vis spectral region resulting from d-d transition transitions. The Eu(PMo11)(2) also display the typical Eu3+ D-5(0) -> F-7(0-4) transitions, when excited through the O -> Eu-III and O -> Mo-VI CCTs

Directing a Non-Heme Iron(III)-Hydroperoxide Species on a Trifurcated Reactivity Pathway

The reactivity of [Fe-III(tpena)](2+) (tpena=N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N-acetate) as a catalyst for oxidation reactions depends on its ratio to the terminal oxidant H2O2 and presence or absence of sacrificial substrates. The outcome can be switched between: 1)catalysed H2O2 disproportionation, 2)selective catalytic oxidation of methanol or benzyl alcohol to the corresponding aldehyde, or 3)oxidative decomposition of the tpena ligand. A common mechanism is proposed involving homolytic O-O cleavage in the detected transient purple low-spin (S=1/2) [(tpenaH)(FeO)-O-III-OH](2+). The resultant iron(IV) oxo and hydroxyl radical both participate in controllable hydrogen-atom transfer (HAT) reactions. Consistent with the presence of a weaker sigma-donor carboxylate ligand, the most pronounced difference in the spectroscopic properties of [Fe(OOH)(tpenaH)](2+) and its conjugate base, [Fe(OO)(tpenaH)](+), compared to non-heme iron(III) peroxide analogues supported by neutral multidentate N-only ligands, are slightly blue-shifted maxima of the visible absorption band assigned to ligand-to-metal charge-transfer (LMCT) transitions and, corroborating this, lower Fe-III/Fe-II redox potentials for the pro-catalysts

Roles of Polyoxometalates in Photocatalytic systems

National audienc