Hydrogen produced from hydrohalic acid solutions by a two-electron mixed-valence photocatalyst

Energy conversion cycles are aimed at driving unfavorable, small-molecule activation reactions with a photon harnessed by a transition metal complex. A challenge that has occupied researchers for several decades is to create molecular photocatalysts to promote the production of hydrogen from homogeneous solution. We now report the use of a two-electron mixed-valence dirhodium compound to photocatalyze the reduction of hydrohalic acid to hydrogen. In this cycle, photons break two R-II-X bonds of a LRhO-(RX2)-X-II core in the presence of a halogen trap to regenerate the active LRhO-Rh-O catalyst, which reacts with hydrohalic acid to produce hydrogen.