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 KO^tBu 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.