Photoactivation of water by Cp2'Mo and photochemical studies of Cp2MoO.: Investigation of a proposed water-splitting cycle and preparation of a water-soluble molybdocene dihydride

Irradiation (lambda > 350 nm) of Cp-2'MoH2 (Cp' = eta(5)-C5H4CH3) dissolved in 3:5 H2O-CH3CN (v/v) results in the quantitative formation of Cp-2'MoO and 2 equiv. of H-2. In light of this result, the photochemistry of the Cp-2'MoO and Cp-2'MoO 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), {(eta-C5H5)(mu-[eta(1):eta(5)C(5)H(4)])Mo}(2) (C20H18Mo2) and Cp2MoPPh3 (in the presence of PPh3), but gas chromatographic and mass spectroscopic analyses showed that free O-2 was not a product. Variations in the temperature, pH of the solution, and wavelength of the irradiating light did not yield any O-2. Experiments showed that O-2 reacted with Cp2MoO to form [Cp2MoO2(MoO2)(2)]. A sensitive apparatus was therefore built to remove and quantitate any O-2 generated in solution (as little as 0.02 mu mol of O-2) before it could react with Cp2MoO, but no O-2 was detected in experiments using this apparatus. It is concluded that O-2 is not produced by irradiation of Cp2MoO. Electron spin resonance experiments in the presence of alpha-phenyl-tert-butylnitrone, a radical spin trap, demonstrated that Cp* radicals form when Cp2MoO is irradiated, and it is proposed that this photoprocess may be responsible for the observed photochemistry. (C) 2000 Elsevier Science S.A. All rights reserved.