A Mononuclear Tungsten Photocatalyst for H₂ Production

We report herein a mononuclear, homogeneous photocatalyst for H₂ production with sunlight. The synthesis and characterization of a (pyridyl)-N-heterocyclic carbene tungsten tetracarbonyl complex W(pyNHC)(CO)₄ is described, and its application as a precatalyst for photocatalytic generation of H₂ is evaluated. Electrochemical and photophysical studies were used to characterize and evaluate the precatalyst and in situ generated catalyst [W(pyNHC)(CO)₃] for the visible-light-driven production of H₂ in the presence of triflic acid and decamethylferrocene without an additional photosensitizer. Under irradiation with a solar-simulated spectrum, a catalyst turnover number (TON) of >17 in 3 h of reaction time is observed for the production of H₂ with this system, which compares favorably to a prior reported (multinuclear) homogeneous photocatalyst using visible light (4 TON). Photonic energy was found to be necessary to access the active catalysts from the precatalyst and in the catalytic cycle. A mechanism is detailed on the basis of a combined photophysical and computational approach.