School of Chemistry - Research Publications

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Subject: Transition Metal Chemistry; Expanding Knowledge in the Chemical Sciences × Author: Wedd, Anthony ×

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    Synthesis and redox properties of triarylmethane dye cation salts of anions [M6O19](2-) (M = Mo, W)
    Guo, S-X ; Xie, J ; Gilbert-Wilson, R ; Birkett, SL ; Bond, AM ; Wedd, AG (ROYAL SOC CHEMISTRY, 2011-01-01)
    Four salts have been isolated combining the triarylmethane dye cations pararosaniline (PR(+)) and crystal violet (CV(+)) with the hexametalates [M(6)O(19)](2-) (M = Mo, W). A new hexatungstic acid H(2)[W(6)O(19)]·4dma (dma = dimethylacetamide) was isolated and is a useful synthon for hexatungstate salts. Single-crystal X-ray diffraction confirmed the presence of PR(+) and [Mo(6)O(19)](2-) ions in [PR](2)[Mo(6)O(19)]·6dmf (dmf = dimethylformamide). A number of charge-assisted hydrogen bonds N-H···O exist between the cation -NH(2) functions and the anion oxygen atoms. Comparative cyclic voltammetry of salts [A]Cl (A = PR, CV), [Bu(4)N](2)[M(6)O(19)](2-) and A(2)[M(6)O(19)] was established in MeCN and Me(2)SO solutions and of solids in contact with the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [emim][tfsa]. In the molecular solvents, the reversible potential for the process [Mo(6)O(19)](2-/3-) is less negative than the first reduction processes of the dye cations. In contrast, that for [W(6)O(19)](2-/3-) is more negative. Spectro-electrochemistry and bulk electrolysis experiments reveal significantly different pathways in the two cases. In contrast, in the [emim][tfsa] medium, a positive shift in reduction potential of at least 400 mV is seen for the anion processes but relatively little change for the dye cation processes. This means that initial reduction of the anions always precedes that of the dyes, providing significant simplification of the complex voltammetric data. Chemically modified electrodes can be used in the ionic liquid because of slow dissolution kinetics. However, reduced anion salts dissolve rapidly, allowing dissolved phase electrochemistry to be examined. The electrochemistries of the oxidized salts A(2)[M(6)O(19)] are essentially those of the individual ions, although low level interaction of A(+) with reduced anions [M(6)O(19)](3-,4-) is evident. The work establishes protocols for synthesis and handling of intensely absorbing and relatively insoluble salts which can now be applied to systems containing more complex polyoxometalate anions.
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    Reactivity of one-, two-, three- and four-electron reduced forms of alpha-[P2W18O62](6-) generated by controlled potential electrolysis in water
    Bernardini, G ; Wedd, AG ; Bond, AM (Elsevier, 2011-08-01)
    One, two, three and four electron reduced forms of α-[P2W18O62]6− in aqueous acidic electrolyte media have been selectively generated by bulk electrolysis from a solution that has an initial pH of 3.6. The reactivities of the reduced polyoxometalate anions and identities of products formed in the presence and absence of dioxygen have been assessed via oxygen and hydrogen Clark-type electrodes, a pH electrode and rotating disk electrode voltammetry. [P2W18O62]7− is stable to protons but is slowly oxidized by dioxygen (timescale: hours to days) back to [P2W18O62]6−. In contrast, [P2W18O62]8− reacts more rapidly with O2 and slowly with H+, whereas generation of the [P2W18O62]9− and [P2W18O62]10− anion is accompanied by a large increase in pH and rapid reaction with O2 or, in its absence, with H+. Consequently, it is concluded that photocatalytic reactions based upon [P2W18O62]6− chemistry are only likely to be of significance if [P2W18O62]9− or more highly reduced species are generated and form part of the catalytic cycle.