Mechanistic Study on the Photogeneration of Hydrogen by Decamethylruthenocene

Detailed studies on hydrogen evolution by decamethylruthenocene ([Cp*2RuII]) highlighted that metallocenes are capable of photoreducing hydrogen without the need for an additional sensitizer. Electrochemical, gas chromatographic, and spectroscopic (UV/Vis, 1H and 13C NMR) measurements corroborated by DFT calculations indicated that the production of hydrogen occurs by a two‐step process. First, decamethylruthenocene hydride [Cp*2RuIV(H)]+ is formed in the presence of an organic acid. Subsequently, [Cp*2RuIV(H)]+ is reversibly reduced in a heterolytic reaction with one‐photon excitation leading to a first release of hydrogen. Thereafter, the resultant decamethylruthenocenium ion [Cp*2RuIII]+ is further reduced with a second release of hydrogen by deprotonation of a methyl group of [Cp*2RuIII]+. Experimental and computational data show spontaneous conversion of [Cp*2RuII] to [Cp*2RuIV(H)]+ in the presence of protons. Calculations highlight that the first reduction is endergonic (ΔG0=108 kJ mol−1) and needs an input of energy by light for the reaction to occur. The hydricity of the methyl protons of [Cp*2RuII] was also considered. No sensitizer required: Elucidation of the mechanism of the photoproduction of hydrogen by nonsacrificial donor decamethyruthenocene in the presence of protons revealed that metallocenes are capable of photoreducing hydrogen without the need for an additional sensitizer. Electrochemical, gas chromatographic, and spectroscopic (UV/Vis, 1H and 13C NMR) measurements corroborated by DFT calculations indicated that the production of hydrogen occurs by a two‐step process (see figure).