Highly efficient hydrogenation of carbon dioxide to formate catalyzed by iridium(iii) complexes of imine–diphosphine ligands† †Electronic supplementary information (ESI) available: Experimental procedures; spectral data for all new compounds. See DOI: 10.1039/c5sc00248f Click here for additional data file. Click here for additional data file.

Carbon dioxide (CO2), an economical, safe, environmentally friendly, and renewable carbon source, is an ideal one-carbon building block for organic chemicals, including carbohydrates and fuels. However, its thermodynamic and kinetic stability presents a fundamental obstacle to the use of CO2 in both academia and industry. High-energy reagents, harsh reaction conditions, and special activation mechanisms are typically required for transforming CO2 into other chemicals. Hydrogen is a green high-energy material that can be used to convert CO2 to valuable secondary energy carriers, such as methane, methanol, and formic acid. Formic acid is widely used in agriculture and in the leather and dye industries, as well as in fuel cells and synthetic chemistry. Homogeneous catalytic hydrogenation of CO2 into formic acid catalyzed by transition-metal complexes based on rhodium, ruthenium, iridium, iron, and cobalt has been extensively investigated since it was rst reported in 1976 by Inoue et al., who used Wilkinson's catalyst, RhCl(PPh3)3, to accomplish the reaction. In the early 1990s, Graf and Leitner achieved turnover numbers (TONs) as high as 3400 for CO2 hydrogenation reactions catalyzed by rhodium phosphine complexes. Noyori et al. found that with RuH2(PMe3)4 as a catalyst, the reaction was more efficient in supercritical CO2 than in traditional organic media. Half-sandwich iridium(III) complexes containing a strong electron donor ligand such as 4,40-dihydroxy-2,20-bipyridine or 4,7-dihydroxy-1,10-phenanthroline were used for the hydrogenation of CO2 by Himeda et al., who reported a TON of 2.2 10. Subsequently, a signicant breakthrough was made by Nozaki et al., who discovered that an iridium(III) trihydride complex with a