Paramagnetic Bridging Hydrides of Relevance to Catalytic Hydrogen Evolution at Metallosulfur Centers

Paramagnetic hydrides are likely intermediates in hydrogen-evolving enzymic and molecular systems. Herein we report the first spectroscopic characterization of well-defined paramagnetic bridging hydrides. Time-resolved FTIR spectroelectrochemical experiments on a sub-second time scale revealed that single-electron transfer to the mu-hydride di-iron dithiolate complex 1 generates a 37-electron valence-delocalized species with no gross structural reorganization of the coordination sphere. DFT calculations support and H-1 and H-2 EPR measurements confirmed the formation an S = 1/2, paramagnetic complex (g = 2.0066) in which the unpaired spin density is essentially symmetrically distributed over the two iron atoms with strong hyperfine coupling to the bridging hydride (A(iso) = 75.8 MHz).