Electron Transfer and Proton-Coupled Electron Transfer Reactivity and Self-Exchange of Synthetic [2Fe–2S] Complexes: Models for Rieske and mitoNEET Clusters

This report describes the thermochemistry, proton-coupled electron transfer (PCET) reactions and self-exchange rate constants for a set of bis-benzimidazolate-ligated [2Fe-2S] clusters. These clusters serve as a model for the chemistry of biological Rieske and mitoNEET clusters. PCET from [Fe2S2((Pr)bbim)((Pr)bbimH)](2-) (4) and [Fe2S2((Pr)bbim)((Pr)bbimH2)](1-) (5) to TEMPO occurs via concerted proton-electron transfer (CPET) mechanisms ((Pr)bbimH2 = 4,4-bis-(benzimidazol-2-yl)heptane). Intermolecular electron transfer (ET) self-exchange between [Fe2S2((Pr)bbim)2](2-) (1) and [Fe2S2((Pr)bbim)2](3-) (2) occurs with a rate constant of (1.20 ± 0.06) × 10(5) M(-1) s(-1) at 26 °C. A similar self-exchange rate constant is found for the related [2Fe-2S] cluster [Fe2S2(SArO)2](2-/3-), SArO(2-) = thiosalicylate. These are roughly an order of magnitude slower than that reported for larger [4Fe-4S] clusters and 1 order of magnitude faster than that reported for N-ligated high-spin iron complexes. These results suggest that the rate of intermolecular ET to/from [Fe-S] clusters is modulated by cluster size. The measured PCET self-exchange rate constant for 1 and 4 at -30 °C is (3.8 ± 0.7) × 10(4) M(-1) s(-1). Analysis of rate constants using the Marcus cross-relation suggests that this process likely occurs via a concerted proton-electron transfer (CPET) mechanism. The implications of these findings to biological systems are also discussed, including the conclusion that histidine-ligated [2Fe-2S] clusters should not have a strong bias to undergo concerted e(-)/H(+) transfers.