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

Abstract

Binding of N 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 through FeFe. One electron redox events result in small changes in the affinity for (at most ∼400-fold) and activation of CO (at most 28 cm for ν). 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 (10-10 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), it provides a conduit for CO binding at an Fe center. Indeed, internal electron transfer from these remote sites accommodates binding of CO at an Fe, with a small energetic penalty arising from redox reorganization (∼2.6 kcal·mol). The ease with which these clusters redistribute electrons in response to ligand binding highlights a potential pathway for coordination of N and CO by FeMoco, which may occur on an oxidized edge of the cofactor.

Metabolomics of Escherichia coli Treated with the Antimicrobial Carbon Monoxide-Releasing Molecule CORM-3 Reveals Tricarboxylic Acid Cycle as Major Target

“…This suggests that Ru(II) species, at the concentrations used, are not toxic to E. coli cells and do not inhibit the iron-sulfur enzymes in general. The fact that CO is the active inhibitor of these enzymes is also consistent with the recently reported binding of CO to iron-sulfur clusters in different redox states (34,35).…”

Nitric oxide activation facilitated by cooperative multimetallic electron transfer within an iron-functionalized polyoxovanadate–alkoxide cluster

“…Similar flexibility in site-differentiated multimetallic clusters has recently been reported by Agapie and coworkers. 55 This report demonstrates the utility of electron transfer from a mixed-valent iron oxide base to a site-differentiated ferric ion upon coordination of CO, resulting in the formation of [LFe 3 O(PhIm)Fe(CO) n ] (L = 1,3,5-triarylbenzene ligand motif; PhIm = 1-phenyl imidazole) complexes. In both examples, electron transfer clarifies that the metalloligand plays an important part in substrate activation, providing electron density to a ferric centre to facilitate substrate coordination and reduction.…”

Localized Electronic Structure of Nitrogenase FeMoco Revealed by Selenium K-Edge High Resolution X-ray Absorption Spectroscopy

“…Recently, redox reorganization upon CO-binding was observed in an electronically localized synthetic [Fe 4 O] 8+ cluster, wherein an electron is transferred from a local Fe 2+ center to a formerly Fe 3+ center to facilitate CO binding. 72 We speculate that such a redox reorganization mechanism in FeMoco would likely be facilitated by the highly covalent carbide, suggesting a plausible role for the central atom during catalysis. Alternatively, a change in the relative coupling across Se3A and Se5A could also account for the observed differences in Se 3A/5A and CO-Se 3A/5A , as our TDDFT calculations on model systems have shown increased pre-edge intensity for antiferromagnetically coupled centers relative to ferromagnetic centers.…”