A dimeric tethered π-coordinated-phenoxy ruthenium precatalyst has been used for the base-free hydrogenation of CO 2 in DMSO. The same precatalyst was also efficient in the reverse dehydrogenation of formic acid yet under base-free conditions. An unprecedented base-free cycle consisting in hydrogen storage and release was successfully implemented with the same precatalyst. The latent property of the catalyst has been introduced in the concept of hydrogen storage/transportation/release.

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Diverse synthetic routes of tethered η 5 -oxocyclohexadienyl ruthenium complexes, which are the extended version of the η 4 -cyclopentadienone Shvo-type active catalyst, are described and their reactivity studies are reported. An original dimeric η 5 -oxocyclohexadienyl compound has been isolated and its solid-state structure was established by X-ray analysis. DFT computational data suggested that this dimeric compound is prone to generate a coordinatively unsaturated mononuclear Lewis acid metal complex bearing a Lewis basic π-coordinated η 5oxocyclohexadienyl ligand. Preliminary studies in (transfer)hydrogenation of acetophenone confirmed that the dimeric η 5oxocyclohexadienyl complex behaves as a bifunctional catalyst with metal−ligand cooperativity. Promising results were obtained under base-free conditions using dihydrogen, iPrOH, and formic acid as hydrogen donors, the latter being active in water, hence highlighting the robustness and stability of the catalyst and opening perspectives toward sustainable catalytic transformations.