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

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 μ²-C atoms and four pendant S donors.The [L₂Fe₂] (L = {[Ph₂P(S)]₂C}^2-) cluster is isolable in two oxidation levels, all-ferrous Fe₂^II and mixed-valence Fe^IIFe^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 Fe₂^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 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.