Mechanistic study of the photo‐generation of hydrogen by decamethylruthenocene

Hydrogen evolution by decamethylruthenocene (Cp2RuII) was studied in detail highlighting that metallocenes are capable of photo‐reducing hydrogen without the need of an additional sensitizer. Electrochemical, gas chromatographic and spectroscopic (UV/vis, 1H and 13C NMR) measurements corroborated by density functional theory (DFT) calculations indicate that the production of hydrogen occurs by a two‐step process. First, the decamethylruthenocene hydride ([Cp2RuIV(H)]+) is formed in the presence of an organic acid. Subsequently, [Cp2RuIV(H)]+ is reversibly reduced via a heterolytic reaction with one‐photon excitation leading to a first release of hydrogen. Thereafter, the resultant decamethylruthenocenium ion ([Cp2RuIII]+) is further reduced leading to a second release of hydrogen by deprotonation of a methyl group of [Cp2RuIII]+. Experimental and computational data show the spontaneous conversion of [Cp2RuII] to [Cp2RuIV(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 [Cp2RuII] was also considered.