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-N,N--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 fac-RuHP2 and a mer-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 fac-RuHP2 is lower in energy than mer-RuHP2. The complexes were additionally applied in the transfer hydrogenation of CO2 to produce formic acid with 2-propanol.