Dual Electron Uptake by Simultaneous Iron and Ligand Reduction in an N-Heterocyclic Carbene Substituted [FeFe] Hydrogenase Model Compound

An N-heterocyclic carbene containing [FeFe]H2ase model complex,
whose X-ray structure displays an apical carbene, shows an
unexpected two-electron reduction to be involved in its electrocatalytic dihydrogen production. Density functional calculations
show, in addition to a one-electron Fe−Fe reduction, that the aryl-substituted N-heterocyclic carbene can accept a second electron
more readily than the Fe−Fe manifold. The juxtaposition of these
two one-electron reductions resembles the [FeFe]H2ase active site
with an FeFe di-iron unit joined to the electroactive 4Fe4S cluster.