Nitrogen–nitrogen-functionalized N-heterocyclic carbene ruthenium(ii) complexes realized efficient CO2 hydrogenation to formate

Three novel Ru(ii)–CNN complexes, namely, [RuH(CO)(PPh3)2(κ2-NimN)]Cl2 (1), [fac-RuH(CO)(PPh3)(κ3-CNHN)]Cl (2) and [fac-RuH(CO)(PPh3)(κ3-CNMeN)]Cl (3), have been synthesized for efficient CO2 hydrogenation to formate. Complex 2 bearing a CNHN ligand exhibits excellent activity and a long catalytic lifetime of 400 h at 170 °C. Notably, the addition of PPh3 leads to increased activity and gives a high TON of 6.5 × 105 after 400 h.