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

Abstract

Binding of N_2 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 which reversibly bind CO in redox states Fe^(II)_4 through Fe^(II)Fe^(III)_3. 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 (10^5-10^(22) 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 N_2 and CO by FeMoco, which may occur on an oxidized edge of the cofactor.