Photocatalytic Hydrogen Production Using Models of the Iron-Iron Hydrogenase Active Site Dispersed in Micellar Solution

Iron-thiolate complexes of the type [Fe-2(mu-bdt)(CO)(6-x)P(OMe3)(x)] (bdt=S2C6H4=benzenedithiolate, x <= 2) are simplified models of iron-iron hydrogenase enzymes. Recently, we have shown that these water-insoluble organometallic complexes, when included into micelles formed by sodium dodecyl sulfate (SDS), are good catalysts for the electrochemical production of hydrogen in aqueous solutions at pH<6. We herein report that the all-CO derivative [Fe-2(mu-bdt)(CO)(6)] (1), owing to its comparatively low reduction potential, is also a robust molecular catalyst for visible-light-driven production of H-2 in aqueous SDS solutions at pH 10.5. Irradiation at lambda=455 nm of a system consisting of complex 1, Eosin Y as a sensitizer, and triethylamine as an electron donor produced up to 0.86 mL of H-2 in 4.5 h, corresponding to a turnover number of 117 mol of H-2 per mol of catalyst. In the presence of a large excess of sensitizer, the production of H-2 lasted for more than 30 h, stressing the relative stability of complex 1 under the photocatalytic conditions used herein. Thermodynamic considerations and UV/Vis spectroscopy experiments suggest that the catalytic cycle begins with the photo-driven reduction of complex 1. The reduced intermediate reacts with a proton source to yield iron hydride. Subsequent reduction and protonation steps produce H-2, regenerating the starting complex. As a result, the iron-thiolate complex 1 is a versatile proton reduction catalyst that can utilize either solar or electrical energy inputs, providing a starting point for the construction of noble metal-free molecular systems for renewable H-2 production.