A Thermodynamic Model for Redox-Dependent Binding of Carbon Monoxide at Site-Differentiated, High Spin Iron Clusters

Binding of N₂ and CO by the FeMo-cofactor of nitrogenase depends on the redox level of the cluster, but the extent to which pure redox chemistry perturbs the affinity of high spin iron clusters for π-acids is not well understood. Here, we report a series of site-differentiated iron clusters that reversibly bind CO in redox states Fe^II₄ through Fe^IIFe^III₃. One electron redox events result in small changes in the affinity for (at most ∼400-fold) and activation of CO (at most 28 cm^-1 for ν_CO). The small influence of redox chemistry on the affinity of these high spin, valence-localized clusters for CO is in stark contrast to the large enhancements (10⁵-10²² fold) in π-acid affinity reported for monometallic and low spin, bimetallic iron complexes, where redox chemistry occurs exclusively at the ligand binding site. While electron-loading at metal centers remote from the substrate binding site has minimal influence on the CO binding energetics (∼1 kcal·mol^-1), it provides a conduit for CO binding at an Fe^III center. Indeed, internal electron transfer from these remote sites accommodates binding of CO at an Fe^III, with a small energetic penalty arising from redox reorganization (∼2.6 kcal·mol^-1). The ease with which these clusters redistribute electrons in response to ligand binding highlights a potential pathway for coordination of N₂ and CO by FeMoco, which may occur on an oxidized edge of the cofactor.