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

5 years ago

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

M. Victoria Jiménez, Raquel Puerta-Oteo, Vincenzo Passarelli, Walter Leitner, Jesús J. Pérez-Torrente, Markus Hölscher

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.

Publisher URL: http://dx.doi.org/10.1021/acs.organomet.7b00509

DOI: 10.1021/acs.organomet.7b00509