Ruthenium(II) Complexes Containing Lutidine-derived Pincer CNC Ligands: Synthesis, Structure and Catalytic Hydrogenation of C=N bonds

Abstract

A series of Ru complexes 3-5 containing lutidine-derived pincer CNC ligands have been prepared by transmetallation with the corresponding silver-carbene derivatives 2. Characterization of these derivatives shows both mer and fac coordination of the CNC ligands depending on the wingtips of the N-heterocyclic carbene fragments. Ru-CNC complexes 3-4 are active, in the presence of tBuOK, in the hydrogenation of a series of imines. In addition, these complexes also catalyze the reversible hydrogenation of phenantridine. Detailed NMR studies have shown the capability of the CNC ligand to be deprotonated and get involved in ligand-assisted activation of dihydrogen. More interestingly, upon deprotonation, Ru-CNC complex 5e(BF4) is able to add aldimines to the metal-ligand framework to yield amido complexes 10. Finally, investigation of the mechanism of the hydrogenation of imines has been carried out by means of DFT calculations. The calculated mechanism involves outer-sphere stepwise hydrogen transfer to the C=N bond assisted either by the pincer ligand or a second coordinated H2 molecule.