Cyanide Docking and Linkage Isomerism in Models for the Artificial [FeFe]-Hydrogenase Maturation Process

Linkage isomerization of the cyanide on the [2Fe] subsite of the [FeFe]-H₂ase active site was reported to occur during the docking of various synthetic diiron complexes onto a carrier protein, apo-HydF, as the initial step for the artificial maturation of the [FeFe]-H₂ase enzyme (Berggren et al., Nature, 2013, 499, 66-70). An investigation of our triiron organometallic models (FeFe-CN/NC-Fe') revealed that, once a Fe-CN-Fe connection is formed, high barriers prevent such cyanide linkage isomerization ( Chem. Sci., 2016, 7, 3710-3719). To explore effects of variable oxidation states of the receiver unit, we introduce copper(I/II) fragments, precedented in Holm's models of cytochrome c oxidase to induce cyanide isomerization (Cu-CN/NC-Fe), to the diiron synthetic analogues of [FeFe]-H₂ase. For comparison, a zinc variant of the cytochrome c oxidase model is also examined. According to the oxidation state of copper, a cyanide flip was induced during the formation of both Zn-NC-Cu and FeFe-CN-Cu complexes. Density functional theory calculations are used to predict the mechanisms for such linkage isomerization and account for optimal conditions including oxidation states of metals, spin states, and solvation. These results on synthetic paradigms imply a role for oxidation state control of cyanide isomerization during hydrogenase active site assembly.