Pyridine-Based PCP-Ruthenium Complexes: Unusual Structures and Metal-Ligand Cooperation.

Metal-ligand cooperation (MLC) by dearomatization/aromatization provides a unique way for bond activation, which has led to the discovery of various acceptorless dehydrogenative coupling reactions. However, most of the studies are based on pincer complexes with a central nitrogen donor. Aiming at exploration of the possibility of MLC by PCP-type pincer complexes, we report herein the synthesis, characterization, structure, and reactivity of pyridine-based PCP-Ru complexes. X-ray structures and DFT calculations indicate a carbenoid character of quaternized pyridine-based PCP-Ru complexes. These complexes undergo dearomatization by direct deprotonation, and the dearomatized complex can react with hydrogen, alcohols, or nitriles to regain aromatization via MLC.