Jacqueline Canny

Metal ion complexes derived from the α1 isomer of (P2W17O61)10−: comparison with the corresponding α2 species

Abstract The lacunary α 1 - and α 2 -[P 2 W 17 O 61 ] 10− isomers derived from the Dawson molecule [αP 2 W 18 O 62 ] 10− have been used as ligands for the following metal cations: Ca II , V V , Mn II , Fe III , Co II , Ni II , Cu II , Zn II , and Mo VI , respectively. We have succeeded in obtaining pure samples of stable compounds of the α 1 series. The cyclic voltammograms of these lacunary precursor species as well as those of the substituted complexes have been run in a pH 3 medium with the aim of finding out distinctive features from one series to the next. Several compounds of the α 2 series are known to be stable and can, therefore, serve eventually as references to distinguish the two series. For the α 2 compounds, the first two two-electron processes are followed by a set of two, presumably one-electron steps. and this fingerprint is observed throughout the series, except for the copper and the molybdenum derivatives. These last two complexes can, however, be identified by other specific features. As another general observation on shifting from the α 2 to the α 1 series, it is found also that the peak potential of the first tungsten wave is driven in the negative direction. Finally, a simple rationale is offered for the whole set of observations.

Electrochemical and catalytic behaviour of Dawson-type complexes derived from [(1),2,3-P2Mo2W15O61]10− and first transition metal ions

The acid-base and electrochemical behaviour of the lacunary Dawson-type heteropolyanion α-[(1)-2,3-P2Mo2W15O61]10− and of its derivatives with VIV, MnII, FeIII, CoII, NiII, ZnII, and CuII has been studied as a function of pH. Essentially two groups can be distinguished. In the first group, constituted by the VIV, MnII, CoII, NiII, ZnII-substituted complexes, the redox activity, if any, of the heterometallic cations occurs far from the potential location of that of the molybdenum moieties. At pH 3, the molybdenum waves are very close to each other and to that of the precursor lacunary complex. For higher pH values the apparent slowness observed for the first wave of the lacunary compound becomes less pronounced in the substituted complexes, but is modulated differently by each substituent. For the FeIII and CuII substituted complexes, the redox activity of these cations can mix with that of the molybdenum centres, depending on the pH of the solution. Spectroelectrochemical experiments have helped to clarify the conditions of the separation of redox activities in the case of the FeIII-substituted complex. It has been possible to obtain an overall three electron process on the first wave of this compound. In contrast, copper deposition is observed with the CuII substituted complex in appropriate conditions. All the complexes show electrocatalytic properties on their respective first wave for the reduction of NO. Owing to the very high stability of the VIV-substituted complex throughout the whole pH domain, it has been possible to study the electrocatalytic oxidation of NADH to NAD+ at pH 8.

Ligand and electrolyte effects in the electroreduction of copper-substituted heteropolyanions

Abstract Focus is put on the electrochemical behaviours of the Cu 2+ centre in several copper-substituted heteropolyanions. These behaviours are found to depend clearly on the structure and composition of the precursor lacunary oxometalates. In sulfate medium, complete demetallation is easily observed upon reduction. However, quantitative recovery of the complex is obtained by reoxidation. Therefore, after the deposition process, the lacunary heteropolyanions retain their original properties and can be used again to capture metal ions in solution. Various behaviours of the copper deposition process are observed depending on the presence of chloride in the supporting electrolyte.

Mixed-metal sandwich complexes [MII2(H2O)2FeIII2(P2W15O56)2]14− (MII = Co, Mn): Synthesis and stability. The molecular structure of [MII2(H2O)2FeIII2(P2W15O56)2]14−

Two new mixed-metal sandwich complexes [M(II)2(H2O)2Fe(III)2(P2W15O56)2]14- (abbreviated [M2Fe2P4W30], M(II) = Co(II), Mn(II)) were obtained at pH 3 by addition of M2+ to [Na2(H2O)2Fe(III)2(P2W15O56)2]16- (abbreviated [Na2Fe2P4W30]) without substitution in the alpha-[P2W15O56]12- (abbreviated [P2W15]) units. Their X-ray structures are reported. At lower pH, back conversion to [Na2Fe2P4W30] was followed by 31P NMR, electrochemistry and UV-visible spectroscopy. The preparation and the characterization in solution of the lacunary intermediate [NaCo(II)(H2O)2Fe(III)2(P2W15O56)2]15- (abbreviated [NaCoFe2P4W30]) is also described.

Synthesis, Characterization and Electrochemistry of Complexes Derived from [(1),2,3-P2Mo2W15O61]10– and First Transition Metal Ions

The complexes formed by [(1)-2,3-P2Mo2W15O61]10– with vanadium(IV), manganese(II), cobalt(II), nickel(II), copper(II), and zinc(II) were synthesized. They were characterized by elemental analysis, IR, UV/Vis and 31P NMR spectroscopy. Their electrochemical studies are complemented by those of the previously synthesized iron(III)- and Mo(VI)-substituted complexes. Among the main conclusions from the electrochemistry of all these compounds, it can be pointed out that only the iron(III) and the copper(II) ions within the complexes are reducible in the same potential domain as the molybdenum(VI) moieties. In the case of the iron(III) complex, it has been possible to obtain a reversible overall three-electron process on the first wave of the oxometalate by varying the pH of the solution. In contrast, copper can be deposited from the corresponding complex. Substitution of [(1)-2,3-P2Mo2W15O61]10– with vanadium(IV), manganese(II), iron(III), cobalt(II), nickel(II), copper(II), zinc(II) and molybdenum(VI) gives a series of complexes in which the trimetallic moiety constituting one of the ‘‘caps’’ of the polyoxometalate contains cations different from the remaining tungsten skeleton. The electrochemical properties of these complexes are modulated by this particular constitution.