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 FeII4 through FeIIFeIII3. 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 νCO). 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 FeIII center. Indeed, internal electron transfer
from these remote sites accommodates binding of CO
at an FeIII, 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.