Die vollständige Charakterisierung eines reduzierten biomimetischen [2 Fe‐2 S]‐Clusters

Abstract

Den Zuständen auf den Grund gegangen: Ein biomimetischer [2 Fe‐2 S]‐Cluster wurde im [Fe3+Fe3+]‐ und [Fe2+Fe3+]‐ Zustand strukturell charakterisiert. Der [2 Fe‐2 S]‐Kerns bleibt bei der Reduktion nahezu unverändert. Die gemischtvalente Form hat einen S=1/2‐Grundzustand, wobei das ungepaarte Elektron teilweise über den Clusterkern delokalisiert ist. Die experimentell bestimmte effektive Kopplungskonstante ermöglicht die Vorhersage der Lage der Intervalenzbande im IR‐Bereich.

A Functional Model for the Rieske Center: Full Characterization of a Biomimetic N‐Ligated [2Fe‐2S] Cluster in Different Protonation States

“…Furthermore, the lack of peripheral N atoms in 1 2− precluded the investigation of the coupling of proton and electron transfer that is key to the function of bc ‐type Rieske proteins. For related bis(benzimidazolate) ligated clusters, it was then possible to isolate for the first time, in crystalline form, a synthetic [2Fe‐2S] analogue 2 3− in the mixed‐valent state, which allowed for a detailed spectroscopic investigation of this Fe II Fe III species 8. 9 Even a super‐reduced Fe II Fe II cluster 2 4− could be isolated for this system 10.…”

“…In line with the structural data, magnetic susceptibility measurements (SQUID; see the Supporting Information) revealed the same strong antiferromagnetic coupling of the ferric ions with an S =0 ground state for both 2 2− and 2 H 2 ( J =−179 cm −1 and J =−180 cm −1 , respectively, in a −2 JS 1 ⋅ S 2 model) 18. Zero‐field Mössbauer spectra recorded at 80 K showed a slight change of the isomer shift from δ =0.24 ( 2 2− )8 to 0.28 mm s −1 ( 2 H 2 ; Figure S13 in the Supporting Information). Differences in the quadrupole splitting (Δ E Q =0.87 mm s −1 for 2 2− versus Δ E Q =1.18 mm s −1 for 2 H 2 ) likely result from the more acute N‐Fe‐N angles in the protonated form.…”

“…These results are intriguing, because the nonprotonated mixed‐valent cluster 2 3− has shown quite different properties 8. The spectrum at low temperature (Figure 7, bottom left) reveals an enhanced mixing of ferrous and ferric character for the individual sites compared to the protonated cluster 2 H 2 − ( δ (A)=0.47 mm s −1 , Δ E Q (A)=1.41 mm s −1 , δ (B)=0.69 mm s −1 , Δ E Q (B)=2.90 mm s −1 ; the difference of the isomer shift Δ δ is only 0.22 mm s −1 ).…”

“…For related bis(benzimidazolate) ligated clusters, it was then possible to isolate for the first time, in crystalline form, a synthetic [2Fe-2S] analogue 2 3À in the mixed-valent state, which allowed for a detailed spectroscopic investigation of this Fe II Fe III species. [8,9] Even a super-reduced Fe II Fe II cluster 2 4À could be isolated for this system. [10] Herein, we report the effect of protonation of 2 2À and 2 3À on its electronic and redox properties and the observation of proton-coupled electron transfer (PCET) akin to the biological Rieske cluster.…”

“…These results are intriguing, because the nonprotonated mixed-valent cluster 2 3À has shown quite different properties. [8] The spectrum at low temperature (Figure 7 (Figure 7, bottom right). Protonation at the peripheral ligands apparently led to pronounced valence localization in the [2Fe-2S] core.…”