Hydrogen Activation with Ru-PN3P Pincer Complexes for the Conversion of C1 Feedstocks

The hydrogenation of C1 feedstocks (CO and CO2) has been investigated using ruthenium complexes RuHCl(CO)(PN3P) as the catalyst. PN3P pincer ligands containing amines in the linker between the central pyridine donor and the phosphorus donors with bulky substituents (tert-butyl (1) or TMPhos (2)) are required to obtain mononuclear single-site catalysts that can be activated by the addition of KOtBu to generate stable five-coordinate complexes RuH(CO)(PN3P–H), whereby the pincer ligand has been deprotonated. Activation of hydrogen takes place via heterolytic cleavage to generate RuH2(CO)(PN3P), but in the presence of CO, coordination of CO occurs preferentially to give RuH(CO)2(PN3P–H). This complex can be protonated to give the cationic complex RuH(CO)2(PN3P)+, but it is unable to activate H2 heterolytically. In the case of the less coordinating CO2, both ruthenium complexes 1 and 2 are highly efficient as CO2 hydrogenation catalysts in the presence of a base (DBU), which in the case of the TMPhos ligand results in a TON of 30,000 for the formation of formate.