Single-Component, Photocatalytic H2 Production Through Acceptorless Alcohol Dehydrogenation: Atypical Metal-Ligand Cooperativity in a Ruthenium(II)-PN/P Complex

Abstract

Metal-ligand cooperativity (MLC) involving reversible aromatization/dearomatization of pyridine-derived pincer ligands is considered important in acceptorless (de)hydrogenation for dihydrogen generation and storage. Dehydrohalogenation of pyridine-derived pincer ruthenium complexes often leads to dearomatized moieties. Thus, we were surprised to find an aromatized kappa-3-NCP binding mode in [{LutP`}Ru(CO)(H)(PPh3)] (2) upon dehydrohalogenation of the lutidine-derived PN/P complex [{LutP}Ru(CO)(Cl)(H)(PPh3)] (1) with KOtBu. The reaction of H2 with 2 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 that we leveraged to demonstrate a unique strategy for unsensitized single-component photocatalytic H2 production via acceptorless alcohol dehydrogenation. Although labeling studies implicate MLC processes during the photocatalytic reaction, they may be off-path intermediates, emphasizing that aromatization/dearomatization may not be necessary for acceptorless transformations.

Reversible Photoisomerization in a Ru cis-Dihydride Catalyst Accessed through Atypical Metal–Ligand Cooperative H2 Activation: Photoenhanced Acceptorless Alcohol Dehydrogenation

“…We also tested Milstein’s Ru-PNN trans -dihydride 3Py and obtained TON = 1–3 with or without light, but 3Py is known to spontaneously lose H 2 on standing in solution at rt, and we found that even the chlorido precursor to 3Py (i.e., 1Py ) produces hydrogen on irradiation in anhydrous benzene; thus, the comparison is rather inconclusive . We also note that irradiation of Ru-MACHO and 3Py with broad-band light resulted in solutions containing a multitude of hydride resonances in the 1 H NMR spectrum and speciation by 31 P­{ 1 H} NMR was complicated.…”