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 indicate that the ligand resonance flexibility provides
a new approach to enhance catalytic performance.