Reversible Photoisomerization in a Ru cis-Dihydride Catalyst Accessed through Atypical Metal-Ligand Cooperative H₂Activation: Photoenhanced Acceptorless Alcohol Dehydrogenation

Dehydrohalogenation of pyridine-derived pincer ruthenium complexes often leads to dearomatized moieties, such as in Milstein's PNN-Ru(CO)(Cl)(H) (1Py) catalyst. Thus, we were surprised to find an aromatized κ3-N,C,P binding mode in the lutidine-derived bidentate analogue [{LutP′}Ru(CO)(H)(PPh3)] (2), instead of a dearomatized compound, upon dehydrohalogenation of [{LutP}Ru(CO)(Cl)(H)(PPh3)] (1). The reaction of 2 with H2 results in the formation of the cis-dihydride [{LutP}Ru(CO)(H)2(PPh3)] (3), and labeling studies confirm cooperative metal-ligand activation. 3 exhibits reversible photoisomerization, forming another cis-dihydride isomer (4) upon irradiation. The lability of 4 toward ligand substitution was leveraged to demonstrate photoenhanced H2 production via acceptorless alcohol dehydrogenation. Labeling studies implicate metal-ligand cooperative (MLC) processes during the photocatalytic reaction, but they appear to be off-path processes on the basis of our mechanistic study of the system. The latter emphasizes that aromatization/dearomatization may not be necessary for acceptorless transformations, which is generally consistent with several contemporary studies on analogous Ru catalysts.