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投稿者: Yuma Morimoto

Article

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

† Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
Inorg. Chem., Article ASAP
DOI: 10.1021/ic403131p
Publication Date (Web): March 4, 2014
Copyright © 2014 American Chemical Society

Synopsis

[Fe2S2(Prbbim)(PrbbimHx)]y- clusters have been generated in multiple redox and protonation states. Their PCET and ET thermochemistry and reactivity are described. The PCET self-exchange reaction occurs by concerted e−/H+ exchange, and the ET self-exchange barriers for different clusters are shown to scale with [Fe−S] cluster size. The implications of these results for the reactivity of biochemical imidazole-ligated clusters is discussed.

Abstract

Abstract Image
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(Prbbim)(PrbbimH)]2– (4) and [Fe2S2(Prbbim)(PrbbimH2)]1– (5) to TEMPO occurs via concerted proton–electron transfer (CPET) mechanisms (PrbbimH2 = 4,4-bis-(benzimidazol-2-yl)heptane). Intermolecular electron transfer (ET) self-exchange between [Fe2S2(Prbbim)2]2–(1) and [Fe2S2(Prbbim)2]3– (2) occurs with a rate constant of (1.20 ± 0.06) × 105 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–, SArO2– = 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) × 104 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.
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