Abstract

The polyoxometalate [Co4(L)2(PW9O34)2]10−, where L is typically H2O, (1) is a much-studied homogeneous water oxidation catalyst that facilitates the conversion of water to dioxygen through a mechanism that remains poorly understood due to difficulty measuring electrochemical and acid–base properties in aqueous media. Studies in a non-aqueous, polar aprotic solvent such as acetonitrile, MeCN, are useful for probing the properties of this catalyst in the absence of reactive substrates or in the presence of competitive substrates. We report that in MeCN, 1 can be electrochemically and chemically oxidized to an unstable species, 1 ox , which reverts to 1 over time. Dioxygen formation is not observed in MeCN under catalytic conditions. In water, the presence of MeCN does not significantly affect kinetics of oxidation but significantly inhibits the yield of dioxygen. X-ray crystal structure determination shows that MeCN coordinates to the two external Co centers in the solid state; changes in the visible spectrum indicate that aqua and MeCN ligands on these Co centers exchange in the solution state. In agreement with these observations, acid–base titration behavior is shifted to reflect competitive binding and density-functional theory calculations show a 2.1 kcal mol−1 stronger interaction for MeCN than H2O. This competitive binding and the effects on water oxidation support the direct involvement of these two Co centers in binding substrate water and/or trapping of reactive intermediates by MeCN.

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