A Phosphine-Oxide Cobalt(II) Complex and Its Catalytic Activity Studies toward Alcohol Dehydrogenation Triggered Direct Synthesis of Imines and Quinolines.

Herein, a new pincer-like amino phosphine donor ligand, , and its phosphine-oxide analog, , were synthesized. Subsequently, cobalt(II) complexes and were synthesized by the reaction of anhydrous Co(II)Cl with ligands and respectively. The ligands and complexes were fully characterized by various physicochemical and spectroscopic characterization techniques. Finally, the identity of the complexes and was confirmed by single crystal X-ray structure determination. The phosphine ligand containing complex was converted to the phosphine oxide ligand containing complex in air in acetonitrile solution. Both complexes and were investigated as precatalysts for alcohol dehydrogenation-triggered synthesis of imines in air. The phosphine-oxide complex was more efficient than the phosphine complex . A wide array of alcohols and amines were successfully reacted in a mild condition to result in imines in good to excellent yields. Precatalyst was also highly efficient for the synthesis of varieties of quinolines in air. As in has side arms that can be deprotonated, we investigated complex for its base (KOBu) promoted deprotonation events by various spectroscopic studies and DFT calculations. These studies have shown that mono deprotonation of the amine side arm attached to the pyridine is quite feasible, and deprotonation of complex leads to a dearomatized pyridyl ring containing complex . The mechanistic investigations of the catalytic reaction, by a combination of experimental and computational studies, have suggested that the dearomatized complex, acted as an active catalyst. The reaction proceeded through the hydride transfer pathway. The activation barrier of this step was calculated to be 26.5 kcal/mol, which is quite consistent with the experimental reaction temperature under aerobic conditions. Although various pincer-like complexes are explored for such reactions, phosphine oxide ligand-containing complexes are still unexplored.