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.