Synthesis and characterization of manganese(i), ruthenium(ii), and rhodium(i) CNS-type complexes derived from n-heterocyclic carbenes

Abstract

An efficient method was developed for the synthesis of a wide range of hemilabile NHC sulfur-containing pyridine-based ligands (CNHCNPyS-type ligands) with different substituents at the imidazole (Me, tBu, Ph, Mes) and sulfur (Et, Ph) positions. The prepared ligands were subjected to metallation reactions, yielding a series of novel Rh(I), Ru(II), and Mn(I) complexes, [(CNS)Ru(H)(CO)(PPh3)]Cl, (CNS)Rh(Cl)(CO), and [(CNS)Mn(CO)3]Br, which were fully characterized by FTIR and NMR spectroscopy. The hemilabile behavior of the CNS ligand of [(CNS)Mn(CO)3]Br as a model complex was assessed using density functional theory (DFT) methods, showing exergonic (by 8.5 kcal/mol) dissociation of the S-containing ligand arm. The stoichiometric reactivity of [(CNS)Mn(CO)3]Br towards hydride reagents (NH3BH3, LiEt3BH) showed the feasibility of the formation of manganese-hydride species, potentially active in hydrogenation catalysis. Preliminary studies of the catalytic activity of [(CNS)Ru(H)(CO)(PPh3)]Cl and [(CNS)Mn(CO)3]Br in the transfer hydrogenation of acetophenone, formates, and organic carbonates were conducted. The[(CNS)Ru(H)(CO)(PPh3)]Cl-catalyzed transfer hydrogenation of ethylene carbonate in ethylene glycol revealed the formation of 18% of methanol. Since organic carbonates can be considered CO2-trapping products, this process, upon optimization, can lead to the development of novel catalytic systems for the "indirect" conversion of carbon dioxide to methanol. Further research will focus on investigating the scope of hydrogenation reactions and optimizing hydrogenation reaction conditions.