Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase.

Binding of N by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 and 4 H equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(μ-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron μ-carbyne complex (PArC)Fe(μ-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(μ-C)(μ-H)Fe model complexes linked by a 2 transfer which bear some resemblance to a pair of E and E states of nitrogenase. Both species populate = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the μ-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe(μ-H)} state of FeMoco (E(4H)), the data and analysis presented herein suggest that the hydride ligands in E(4H) bridge isovalent (most probably Fe) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(PArC)Fe(μ-H)] and [(PArC)Fe(μ-H)] by pulse EPR revealed that redox chemistry induces significant changes in Fe-C covalency (-50% upon 2 reduction), a conclusion further supported by X-ray absorption spectroscopy, Fe Mössbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.