Mechanistic Study on the Photogeneration of Hydrogen by Decamethylruthenocene

Detailed studies on hydrogen evolution by decamethylruthenocene ([Cp* 2 Ru II ]) highlighted that metallocenes are capable of photoreducing hydrogen without the need for an additional sensitizer. Electrochemical, gas chromatographic, and spectroscopic (UV/Vis, 1 H and 13 C NMR) measurements corroborated by DFT calculations indicated that the production of hydrogen occurs by a two‐step process. First, decamethylruthenocene hydride [Cp* 2 Ru IV (H)] + is formed in the presence of an organic acid. Subsequently, [Cp* 2 Ru IV (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* 2 Ru III ] + is further reduced with a second release of hydrogen by deprotonation of a methyl group of [Cp* 2 Ru III ] + . Experimental and computational data show spontaneous conversion of [Cp* 2 Ru II ] to [Cp* 2 Ru IV (H)] + in the presence of protons. Calculations highlight that the first reduction is endergonic (Δ G 0 =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* 2 Ru II ] was also considered.