Photoactivation of water by Cp′2Mo and photochemical studies of Cp2MoO. Investigation of a proposed water-splitting cycle and preparation of a water-soluble molybdocene dihydride

Irradiation (λ>350 nm) of Cp′2MoH2 (Cp′=η5-C5H4CH3) dissolved in 3:5 H2O–CH3CN (v/v) results in the quantitative formation of Cp′2MoO and 2 equiv. of H2. In light of this result, the photochemistry of the Cp′2MoO and Cp2MoO complexes was re-examined to determine the feasibility of using these molybdocene complexes as sensitizers in a photochemical water-splitting scheme. The metal-containing products formed by irradiation of Cp2MoO were [Cp2MoO2(MoO2)]2, {(η-C5H5)(μ-[η1:η5-C5H4])Mo}2 (C20H18Mo2) and Cp2MoPPh3 (in the presence of PPh3), but gas chromatographic and mass spectroscopic analyses showed that free O2 was not a product. Variations in the temperature, pH of the solution, and wavelength of the irradiating light did not yield any O2. Experiments showed that O2 reacted with Cp2MoO to form [Cp2MoO2(MoO2)]2. A sensitive apparatus was therefore built to remove and quantitate any O2 generated in solution (as little as 0.02 μmol of O2) before it could react with Cp2MoO, but no O2 was detected in experiments using this apparatus. It is concluded that O2 is not produced by irradiation of Cp2MoO. Electron spin resonance experiments in the presence of α-phenyl-tert-butylnitrone, a radical spin trap, demonstrated that Cpradical dot radicals form when Cp2MoO is irradiated, and it is proposed that this photoprocess may be responsible for the observed photochemistry.