PNN-Pincer Ruthenium Complexes Catalyzed Acceptorless Dehydrogenation of Alcohols and Amines.

We have developed a series of new ruthenium hydride complexes bearing PNN pincer ligands with a pyridine-based backbone and an NH spacer in the phosphine arm, showing competent activity in the catalytic acceptorless dehydrogenation of alcohols to ketones and double dehydrogenation of primary amines to nitriles under oxidant- and base-free conditions. Hammett analysis and DFT calculations suggest that an outer-sphere pathway is energetically more favorable in alcohol dehydrogenation. In amine dehydrogenation, possible intermediates, including a neutral PNNRu-NH2Bn complex coordinated by benzylamine trans to the hydride ligand, were isolated and characterized by 1H NMR and X-ray crystallography, demonstrating comparable activity to the PNNRu precursor in catalytic amine dehydrogenation. Notably, control experiments using a related N-methylated analogue of the PNNRu complex and DFT calculations highlight the vital role of NH in the ligand scaffold for the dehydrogenation process.