Molecular Hydrogen Production from Formic Acid by Cationic Phenanthroline Ruthenium Complexes: Experimental and DFT Mechanistic Insights

A series of new monocationic Ru complexes containing phenanthroline derivatives were developed. The monometallic complexes [Ru-(κ²-OAc)-(dppb)-(N,N)]-OAc derivatives were synthesized in high yield via the reaction between [Ru-(κ²-OAc)₂dppb] and the corresponding N,N ligand. Additionally, dinuclear [(dppb)-(κ²-OAc)-(Ru-(μ-<i>N</i>,<i>N</i>--C,N)-Ru-(κ²-OAc)-(dppb)]-OAc complexes were synthesized from equimolar amounts of the appropriate monometallic complex and [Ru-(κ²-OAc)₂dppb]. All complexes were characterized by NMR, FTIR, UV-vis spectroscopy, and cyclic voltammetry. These precatalysts display selective catalytic activity toward dehydrogenation of formic acid for H₂ production, with the dinuclear systems demonstrating superior performance, achieving up to 100% conversion under optimized conditions. The dinuclear system maintained consistent TOF₅₀ values through several catalytic cycles, demonstrating excellent stability. Mechanism investigations revealed the formation of two Ru-monohydride species, showing a <i>fac</i>-RuHP₂ and a <i>mer</i>-RuHP₂ arrangement, respectively, formed via substitution of a κ²-OAc by a κ²-O₂CH followed by a β-elimination, where both are involved in the mechanisms. DFT calculations of the species involved in the mechanism showed that <i>fac</i>-RuHP₂ is lower in energy than <i>mer</i>-RuHP₂. The complexes were additionally applied in the transfer hydrogenation of CO₂ to produce formic acid with 2-propanol.