Redox Interconversions and Aqueous Solution Properties of the Cuboidal Complexes [Mo4S4(edta)2]m− (m = 4, 3, 2)

Paul W. Dimmock, Joseph McGinnis, Bee Lean Ooi, A. Geoffrey Sykes

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    Redox interconversions of the recently prepared Mo4S4 complexes present in the 4+, 5+, and 6+ states, and in this case with N,N,N′,N′-ethylenediaminetetraacetate (edta) as ligand, [Mo4S4(edta)2]m− (m = 4, 3, and 2, respectively), have been studied at 25 °C, I = 1.00 M (LiClO4). The most readily obtained complex is green [Mo4S4(edta)2]3−, the EPR spectrum of which (in H2O at 10 K) confirms one unpaired electron, gav value 2.48. From cyclic voltammetry in 0.5 M LiClO4, reduction potentials (vs NHE) are [Mo4S4(edta)2]3−/4− (-0.046 V) and [Mo4S4(edta)2]2−/3− (0.65 V). The [Mo4S4(edta)2]4− ion is air-sensitive, and in the presence of H+ and ClO4 is oxidized to [Mo4S4(edta)2]3−. Reactions studied were at pH >3 to limit also dechelation of the edta, except in the case of the fast [Fe(H2O)6]3+ oxidation of [Mo4S4(edta)2]3−, when it was necessary to work at higher [H+] in the range 0.075–0.30 M. The kinetics of the 1:1 cross-reactions with [Co(edta)] (0.37 V; 5.4 × 1033 M−1 s−1) and [Fe(edta)] (0.12 V; 2.4 × 106 M−1 s−1) as oxidants for [Mo4S4(edta)2]4− give a self-exchange rate constant for the [Mo4S4(edta)2]4−/3− couple of 1.5 × 107 M−1 s−1. Similarly with [Co(dipic)2] (0.747 V; 17.8 M−1 s) as the oxidant, a self-exchange rate constant of 7.7 × 105M−1 s−1 is obtained for the [Mo4S4(edta)2] 3−/2− couple. The reaction with [Fe(H2O)6]3+ as oxidant (0.77 V; 6.7 × 104 M−1 s−1 at 10 °C) gives no dependence on [H+] and is too fast to be other than outer-sphere. Both self-exchange reactions are clearly very favorable processes. The comproportionation reaction of [Mo4S4(edta)2] with [Mo4S4(edta)2] is too fast to measure directly by stopped-flow UV-vis spectrophotometry. A rate constant of 2.4 × 1010 M−1 s−1 calculated from the self-exchange values is close to or at the diffusion-controlled limit for reactants of radius ~7.4 Å. The two-electron oxidant [PtCl6]2− (0.726 V) oxidizes [Mo4S4(edta)2]−4 in one-electron steps. Interestingly, the self-exchange rate constant for [Mo4S4(edta)2]4−/3− is ~104 times more favorable than that for the [Mo4S4(H2O)12]4+/5+ exchange.

    Original languageEnglish
    Pages (from-to)1085-1089
    Number of pages5
    JournalInorganic Chemistry
    Issue number5
    Publication statusPublished - 1 Jan 1990


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