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

摘要

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)_(2)CHCOO}] and cationic iridium(III) [IrH(cod){(MeIm)_(2)CHCOO}]~(+) compounds (MeIm = 3-methylimidazol-2-yliden-1-yl) having a carboxylate-functionalized bis(NHC) ligand. The [Ir(cod){(MeIm)_(2)CHCOO}] compound catalyzes the hydrogenation of CO_(2) to formate in water using NEt_(3) as base, reaching turnover numbers of approximately 1500. Reactivity studies have shown that activation of the catalyst precursor involves the reaction with H_(2) in a multistep process that under catalytic conditions results in the formation of a dihydrido iridium(III) octahedral [IrH_(2)(H_(2)O){(MeIm)_(2)CHCOO}] species stabilized by the κ~(3)- 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, NEt_(3) neutralizes only the produced formic acid, whereas in the second it assists the proton transfer in heterolytic cleavage of the H_(2) 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.