Ligand Modification of Cyclometalated Ruthenium Complexes in the Aerobic Oxidative Dehydrogenation of Imidazolines

New cyclometalated ruthenium­(III) complexes bearing 2-phenylpyridine derivatives were synthesized and characterized. Chemical modification of the cyclometalating ligand affected its σ-donor character and resulted in regulation of the redox potential of the ruthenium metal center, which was elucidated by X-ray crystallography and cyclic voltammetry. The increase in the electron-donating ability of the cyclometalating ligand improved the catalytic activity of the ruthenium complexes in the aerobic oxidative dehydrogenation of 2-phenylimidazoline, and enabled the catalytic dehydrogenation of 2-phenylimidazoline in air at room temperature. The effect of the ligand structure on the catalytic activity was also elucidated by density functional theory (DFT) calculations and titration experiments.