Detailed studies on hydrogen evolution by decamethylruthenocene ([Cp*2 RuII ]) 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 RuIV (H)]+ is formed in the presence of an organic acid. Subsequently, [Cp*2 RuIV (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 RuIII ]+ is further reduced with a second release of hydrogen by deprotonation of a methyl group of [Cp*2 RuIII ]+ . Experimental and computational data show spontaneous conversion of [Cp*2 RuII ] to [Cp*2 RuIV (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*2 RuII ] was also considered.
Keywords: hydrogen evolution reaction; metallocenes; photochemistry; reaction mechanisms; ruthenium.
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