Base-free hydrogenation of CO2 to formic acid in water with an iridium complex bearing a N,N′-diimine ligand

Abstract

Hydrogenation of CO₂ to formic acid using homogeneous catalysts is an attractive approach for the conversion and utilization of CO₂. To date, the efficiency of this transformation in water without base remains rather low. Herein, we describe a highly efficient iridium catalyst, [Cp*Ir(N,N′)Cl]Cl (N,N′ = 2,2′-bi-1,4,5,6-tetrahydropyrimidine), for the direct hydrogenation of CO₂ to formic acid in water in the absence of a base, achieving an initial TOF of over 13 000 h⁻¹ at 80 °C and 5.0 MPa of H₂/CO₂ (1 : 1) and a TON of over 10 000 at 40 °C and 7.6 MPa. The in situ¹H NMR and reaction kinetics studies show that the reaction is limited in terms of turnover by the CO₂ insertion step. The kinetic isotope effect (KIE) study was applied to understand the effect of water on the reaction, and the inverse KIE results imply that the dissociation of the water molecule from the aqua iridium complex affects the kinetics of the catalytic cycle. This work extends the ligand type of the catalyst for CO₂ hydrogenation and provides a new strategy for catalyst design.