Spectroscopic and electrochemical examination of the dark and photo-catalysed redox reactions that occur at the interface between solid alpha-[Hex(4)N](4)[S2Mo18O62], solid triphenylphosphine and water

Solid alpha-[Hex(4)N](4)[S2Mo18O62] was found to be reduced by solid triphe nylphosphine when the two species were ground together as powders and left for periods of several days in the presence of atmospheric water vapour. El ectrospray mass spectrometry confirmed that the final product formed by oxi dation of the phosphine was PPh3O, while experiments in the presence of (H2 O)-O-17 demonstrated that the oxygen source was atmospheric water vapour ra ther than [S2Mo18O62](4-). Voltammetry and UV/visible spectroscopy of sampl es of reacted solids dissolved in MeCN imply that the two-electron, two-pro ton reduced species [Hex(4)N](4)[H2S2Mo18O62] was the major product formed upon reaction of an equimolar solid mixture of PPh3 and [Hex(4)N](4)[S2Mo18 O62]. The rate of this process is accelerated by irradiation with 300-400 n m light, corresponding to the wavelength of an absorption band of [S2Mo18O6 2](4-). When PPh3 was in excess, three-electron and four-electron reduced f orms of [S2Mo18O62](4-) were detected by EPR spectroscopy and voltammetry, respectively. Direct evidence for the photooxidation of PPh3 was obtained t hrough solid-state photovoltammetric experiments in which a phototransient response for the oxidation of reduced forms of [S2Mo18O62](4-) was recorded for solid state mixtures which were mechanically attached to a pyrolytic g raphite electrode surface in contact with an aqueous medium.