Synthesis of Guanamine-Based Ruthenium Pincer Complexes and Their Application in Catalytic (De)hydrogenation Reactions

Reaction of RuHCl(CO)(PPh3)3 with ligand N,N′-bis(diisopropylphosphino)-benzoguanamine leads to the formation of monomeric and dimeric ruthenium complexes. Investigations of the stability of these guanamine-based PNP-pincer complexes reveals that no interconversion between both species is taking place under reaction conditions employed. Hence, both species are tested in different model reactions and compared to state-of-the-art systems. The new complexes show good to excellent performance in the dehydrogenation of formic acid and the hydrogenation of CO2. Notably, in the former reaction, the monomeric complex is more active than all so far reported systems. Additionally, both complexes are utilized in the hydrogenation of different functional groups (ketone, nitrile, ester, N-heteroarene), important in organic synthesis. Generally, the monomeric species is more active than the dimeric one, which can be attributed to the difference in steric shielding of the metal center.