Sulfur-Ligated [2Fe-2C] Clusters as Synthetic Model Systems for Nitrogenase

[Image: see text] Metal clusters featuring carbon and sulfur donors have coordination environments comparable to the active site of nitrogenase enzymes. Here, we report a series of di-iron clusters supported by the dianionic yldiide ligands, in which the Fe sites are bridged by two μ(2)-C atoms and four pendant S donors.The [L(2)Fe(2)] (L = {[Ph(2)P(S)](2)C}(2–)) cluster is isolable in two oxidation levels, all-ferrous Fe(2)(II) and mixed-valence Fe(II)Fe(III). The mixed-valence cluster displays two peaks in the Mössbauer spectra, indicating slow electron transfer between the two sites. The addition of the Lewis base 4-dimethylaminopyridine to the Fe(2)(II) cluster results in coordination with only one of the two Fe sites, even in the presence of an excess base. Conversely, the cluster reacts with 8 equiv of isocyanide (t)BuNC to give a monometallic complex featuring a new C–C bond between the ligand backbone and the isocyanide. The electronic structure descriptions of these complexes are further supported by X-ray absorption and resonant X-ray emission spectroscopies.