A Photoswitchable Ru cis-Dihydride Catalyst Accessed through Atypical Metal-Ligand Cooperative H2 Activation: Photo-Enhanced Acceptorless Alcohol Dehydrogenation

Abstract

Dehydrohalogenation of pyridine-derived pincer ruthenium complexes often lead to dearomatized moieties, such as in Milstein's PNN-Ru(CO)(Cl)(H) (1Py) catalyst. Thus, we were surprised to find an aromatized k 3 -N,C,P binding mode in the lutidine-derived bidentate analog [{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 formation of a cis-dihydride [{LutP}Ru(CO)(H)2( PPh3)] (3) and labeling studies confirm cooperative metal-ligand activation. 3 exhibits reversible photochemistry, forming another cis-dihydride isomer (4). The lability of 4 toward ligand substitution was leveraged to demonstrate a unique example of photoswitchable 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 based on 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.