NMR, Voltammetric, and Photoelectrochemical Studies on the Dark and Light-Catalyzed Reactions of α-[S2Mo18O62]4- with Aryl- and Alkylphosphines

Abstract

The polyoxometalate, α-[S2Mo18O62]4- is reduced by phosphines in the mixed-solvent medium CH3CN/H2O (95/5 v/v). Monitoring of reaction pathways by voltammetric and NMR (17O, 31P) techniques shows that the identity of the final products is determined by the basicity of the phosphine. Thus reaction with aryl- (Ph3P, Ph2PCH2PPh2, and Ph2PCH2CH2PPh2) and alkyl- (Et3P and nBu3P) phosphines leads to two- and one-electron reduced polyoxometalate products, respectively, for example: [S2Mo18O62]4- + Ph3P + H2O → [HS2Mo18O62]5- + Ph3PO + H+; 2[S2Mo18O62]4- + 3nBu3P + H2O → 2[S2Mo18O62]5- + nBu3PO + 2nBu3PH+. For each reaction, the primary electron-transfer step is believed to be [S2Mo18O62]4- + R3P → [S2Mo18O62]5- + R3P•+with R3P•+ then reacting with water to generate R3PO and protons. If R3P is relatively basic (R = nBu, Et), a 1:2 mixture of R3PO and R3PH+ is formed due to the protonation of R3P. However, if R3P (R = Ph) is a weak base, H+ preferentially initiates the following disproportionation reaction: 2[S2...