Characterisation of intermediates involved in the mer-W(CO)31-dpm)(η2-dpm) to mer-[W(CO)22-dpm)2]2+ (dpm=Ph2PCH2PPh2) oxidation process in acetone by steady-state and fast scan voltammetry at platinum and mercury microelectrodes

Alan M. Bond, Ray Colton, Roger S. Hutton

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Abstract

The oxidati of mer-W(CO)31-dpm)(η2-dpm) (dpm=Ph2PCH2PPh2) under conventional voltammetric conditions is known to involve an overall two-electron oxidation process to give [W(CO)32-dpm)2]2+ with no experimental evidence for postulated intermediates. Under steady-state or near steady-state conditions at platinum microelectrodes with radii greater than 25 μm the oxidation of mer-W(CO)31-dpm)(η2-dpm) is observed to still proceed via a single two-electron process whereas at a 3 μm radius microelectrode two one-electron processes are observed. This dependence upon electrode size is consistent with a mechanism involving single electron transfer steps and internal chelating reactions. The first electron transfer step is mer-W(CO)31-dpm)(η2-dpm)⇌mer-[W(CO)31-dpm)(η2-dpm)]+ +e- (reaction a). Following this initial oxidation there are two competing pathways to further oxidation: mer-[W(CO)31-dpm)(η2-dpm)]+→mer-[W(CO)32-dpm)2]+ (b) followed by mer-[W(CO)32-dpm)2]+→[W(CO)32-dpm)2]2+ +e- (c) or mer-[W(CO)31-dpm)(η2-dpm)]+⇌mer-[W(CO)31-dpm)(η2-dpm)]2+ +e- (d) followed by mer-[W(CO)31-dpm)(η2-dpm)]2+ → [W(CO)32-dpm)2]2+ (e). Under conventional voltammetric conditions reaction sequence (a), (b), (c) gives rise to a single apparent two-electron process. Two one-electron processes are observed when the internal chelation step is outrun by the short time scale achieved under steady-state conditions or by very fast transient voltammetry at platinum microelectrodes. The fast scan rate technique provides direct evidence for the fleeting existence of the seventeen-electron mer-[W(CO)31-dpm)(η2-dpm)]+ and the nineteen-electron [W(CO)32-dpm)2]+ species. The complementary nature of the steady-state and transient regimes of voltammetry at microelectrodes is therefore demonstrated by the present study. The mechanism of the electrochemical oxidation of mer-W(CO)31-dpm)(η2-dpm) at gold and carbon electrodes appears to be the same as at platinum. In contrast, at both conventional and microsized mercury electrodes weak interaction of the pendent phosphorus atom of mer-W(CO)31-dpm)(η2-dpm) with mercury effectively slows down the rate of chelation following oxidation. Thus voltammetry at a mercury electrode allows the two one-electron oxidation steps for mer-W(CO)31-dpm)(η2-dpm) to be observed under conventional voltammetric conditions. It is concluded therefore, that in the presence of a pendent phosphorus group, mercury electrode interaction provides another convenient method of studying unstable intermediates via what may be referred to as a surface trapping technique.

Original languageEnglish
Pages (from-to)671-677
Number of pages7
JournalInorganica Chimica Acta
Volume198-200
Issue numberC
DOIs
Publication statusPublished - 1 Jan 1992

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