Adaptive N-Mesoionic Ligands Anchored to a Triazolylidene for Ruthenium-Mediated (De)Hydrogenation Catalysis

A ruthenium cymene complex bearing a bidentate ligand composed of the N-mesoionic donor N-[1-methylpyridin-4(1H)-ylidene]-amide and the C-mesoionic donor 1,2,3-triazolylidene was prepared. Spectroscopic analyses including UV-vis, electrochemical, and NMR methods demonstrate that the pyridylideneamide ligand adapts to its environment and switches, depending on the solvent, between a formally anionic and a neutral donor. A mesoionic pyridinium-amidate structure predominates in polar solvents, whereas a neutral pyridylidene imine structure prevails in apolar solvents. The implications of these solvent-dependent electronic characteristics have been exploited in redox catalysis involving alcohol dehydrogenation and transfer dehydrogenation. The results provides a new approach to enhance catalytic performance. indicate that the ligand resonance flexibility provides a new approach to enhance catalytic performance.