New Ru(II) N′NN′-type pincer complexes: synthesis, characterization and the catalytic hydrogenation of CO2 or bicarbonates to formate salts

[RuCl(L1)(MeCN)(2)]Cl (1) and [RuCl(L2)(MeCN)(2)]Cl (2) complexes were prepared through the reaction of [RuCl2(p-cymene)](2) with 2,6-bis(benzimidazole-2-yl)-4-hydroxy-pyridine (L1) or 2,6-bis(benzimidazole-2-yl) pyridine (L2) in acetonitrile, respectively. The treatment of [Ru(OTf)(L2)(MeCN)(2)] OTf (3) with 1 equivalent of PPh3 in ethanol resulted in the formation of [Ru(L2(-1))(MeCN)(PPh3)(2)] OTf (4), in which one of the N-H moieties of L2 is deprotonated to give an anionic ligand (L2(-1)). It was found that complex 1 can catalyze the hydrogenation of CO2 to formate salts, producing sodium formate in 34.0% yield with a turnover number (TON) of 407 under the optimized conditions. Further investigations revealed that complexes 1-4 can efficiently catalyze the hydrogenation of sodium bicarbonate to sodium formate, and the catalytic activity follows the order 4 > 1 > 2 approximate to 3. In particular, sodium formate was obtained in good yield (77%) with a high TON (1530) when complex 4 was used as the catalyst. The present results illustrate a new example of Ru(II) complexes bearing a rigid N'NN' framework for the efficient hydrogenation of CO2 to formate salts in a homogeneous system.