Experimental and Theoretical Mechanistic Investigation on the Catalytic CO2 Hydrogenation to Formate by a Carboxylate-Functionalized Bis(N‑heterocyclic carbene) Zwitterionic Iridium(I) Compound

The bis-imidazolium salt, 1,1-bis­(N-methylimidazolium) acetate bromide, is a convenient precursor for the synthesis of zwitterionic iridium­(I) [Ir­(cod)­{(MeIm)2CHCOO}] and cationic iridium­(III) [IrH­(cod)­{(MeIm)2CHCOO}]+ compounds (MeIm = 3-methylimidazol-2-yliden-1-yl) having a carboxylate-functionalized bis­(NHC) ligand. The [Ir­(cod)­{(MeIm)2CHCOO}] compound catalyzes the hydrogenation of CO2 to formate in water using NEt3 as base, reaching turnover numbers of approximately 1500. Reactivity studies have shown that activation of the catalyst precursor involves the reaction with H2 in a multistep process that under catalytic conditions results in the formation of a dihydrido iridium­(III) octahedral [IrH2(H2O)­{(MeIm)2CHCOO}] species stabilized by the κ3-C,C′,O coordination of the ligand. DFT studies on the mechanism were carried out to elucidate two possible roles of the base. In the first one, NEt3 neutralizes only the produced formic acid, whereas in the second it assists the proton transfer in heterolytic cleavage of the H2 molecule. Although this base-involved mechanism is more favorable in that it exhibits a lower energy span for the overall reaction, the energy barrier obtained from kinetic experiments suggests that both mechanisms could be operative under the experimental reaction conditions.