Structure of an Organorhodium Intermediate in the Production of Solar Fuel

The reduction of protons to dihydrogen, the reductive half of water splitting, requires a catalyst to proceed at a reasonable rate. In molecular catalysis for hydrogen generation, reduced metal complexes react with protons to generate hydrides that can be further reduced or react to generate dihydrogen. Progress in understanding the electronic structure of these reduced metal complexes, however, has been challenging due to their high reactivity and sensitivity to air and moisture. In this project, Schlenk and glovebox techniques have been used to isolate rhodium(I) complexes that are model intermediates in proton reduction catalysis by group 9 metals. Specifically, low-valent rhodium complexes of Cp* (Cp* is pentamethylcyclopentadienyl) with a bidentate chelate ligand (1,10-phenanthroline (phen), 5,6-dimethyl-1,10-phenanthroline (Me₂phen), 4,4’-dimethyl-2,2’-bipyridyl (Me₂bpy), or 4,4’-diphenyl-2,2’-bipyridine (Ph₂bpy)) were isolated by treatment of rhodium(III) precursors with sodium amalgam or thallium formate. The isolated complexes were characterized by ¹H and ¹³C NMR spectroscopy. X-ray crystallography was also used to determine the structure of Cp*Rh(phen). The nature of these complexes will be discussed, as well as preliminary findings regarding their electronic structure as explored with electronic absorption spectroscopy.