Protonation and Concerted Proton–Electron Transfer Reactivity of a Bis-Benzimidazolate Ligated [2Fe–2S] Model for Rieske Clusters

A model system for biological Rieske clusters that incorporates
bis-benzimidazolate ligands (Prbbim)2– has been developed (PrbbimH2 = 4,4-bis­(benzimidazol-2-yl)­heptane).
The diferric and mixed-valence clusters have been prepared and characterized
in both their protonated and deprotonated states. The thermochemistry
of interconversions of these species has been measured, and the effect
of protonation on the reduction potential is in good agreement to
that observed in the biological systems. The mixed-valence and protonated
congener [Fe2S2(Prbbim)­(PrbbimH)]­(Et4N)2 (4) reacts rapidly
with TEMPO or p-benzoquinones to generate diferric
and deprotonated [Fe2S2(Prbbim)2]­(Et4N)2 (1) and 1 equiv
of TEMPOH or 0.5 equiv of p-benzohydroquinones, respectively.
The reaction with TEMPO is the first well-defined example of concerted
proton–electron transfer (CPET) at a synthetic ferric/ferrous
[Fe–S] cluster.