The Complete Characterization of a Reduced Biomimetic [2Fe-2S] Cluster
The Complete Characterization of a Reduced Biomimetic [2Fe-2S] Cluster
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DOI：
10.1002/anie.201100727
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发表时间：
2011-01-01
影响因子：
16.6
通讯作者：
Meyer, Franc
中科院分区：
文献类型：
--
作者：
Albers, Antonia;Demeshko, Serhiy;Meyer, Franc
铁还蛋白结合型和Rieske型铁-硫簇，包括双核[2Fe-2S]中心，是普遍存在的生物辅因子。[1]它们的主要功能是电子转移，其中Fe/S核穿梭于[2Fe-2 S]2+和[2Fe-2S]1+态之间。[2]硫醇配位的[2Fe-2S]核的合成类似物在生物无机化学中已经很好地建立了，[3]但与其他末端配体的特征良好的[2Fe-2S]配合物仍然相对较少；[4，5]最近才发表了含有一组异构性配体的Rieske型簇合物的模型络合物。[6]大多数已报道的[2Fe-2S]位的类似物都是在全铁状态下合成的，而混价的[2Fe-2 S]1+态如果有的话，Gibson和Beardwood报道了一些原位化学还原生成的[2Fe-2 S]簇，并在溶液中进行了研究。[8]此外，他们还使用双齿双(苯并咪唑)末端配体分离和表征了一个处于[2Fe-2S]1+态的物种；用穆斯堡尔谱对其进行了研究，提出了该体系具有部分离域混合价的S=1/2基态[9].然而，还没有报道混合价[2Fe-2S]1+态的[2Fe-2S]团簇的晶体结构，也没有对这种还原团簇进行磁性测量，这种测量可以证实基于Mçssbauer数据的提议.本文报道了一个合成的混价[2 Fe-2 S]簇合物的首次晶体结构和SQUID磁化率数据，并对其进行了完整的光谱表征。我们还决定使用双齿双(苯并咪唑)配体，因为这些杂环系统已被证明足以稳定混合价形式。在配基主链上添加了苯基，以改善溶解性和结晶(图1)。在标准的配体交换反应中，通过向(Net4)2[Fe2S2Cl4]中加入过量的去质子化配体，合成了均质全铁簇合物1。在MeCN/0.1M nBu4NPF6中，室温下用循环伏安法研究了化合物1的氧化还原性质(图2，上)。在E_1/2=±1.14V和E_1/2=±2.10V与FC/F_c+对(F_c=[(C_5H_5)_2Fe])处，两个可逆单电子还原过程被指定为混合价物种和全铁物种的形成。与相关的硫代配位络合物相比，[2Fe-2S]簇合物的势移到了更大的正值，这是由于双(苯并咪唑)封端配体的吸电子特性所致。半胱氨酸配位的[2Fe-2S]铁氧还蛋白的电化学势通常在±1.05V到±0之间。[11]两个半波电位之间的大间隔显示混价物种2(Kc=1.7 1016)显著稳定，这一事实允许1(使用十甲基钴)的化学还原和2的分离，事实证明即使在室温下氮气下几个小时，2也是稳定的。1的UV/Vis光谱(图2，底部)在411 nm(ε=7340m±1 cm？1)和528 nm(ε=7020m±1 cm？1)处有两个显著的谱带，在
Protein-bound iron–sulfur clusters of the ferredoxin and Rieske types, including binuclear [2Fe-2S] sites, are ubiquitous biological cofactors.[1] Their dominant function is electron transfer, where the Fe/S core shuttles between the [2Fe-2 S] 2+ and [2Fe-2S] 1+ states.[2] Synthetic analogues for thiolato-coordinated [2Fe-2S] cores are well established in bioinorganic chemistry,[3] but well-characterized [2Fe-2S] complexes with other terminal ligands are still relatively rare;[4, 5] a model complex for Rieske-type clusters comprising a heteroleptic set of ligands was published only recently.[6] Most reported analogues of [2Fe-2S] sites have been synthesized exclusively in the all-ferric state, while the mixed-valent [2 Fe-2 S] 1+ state could, if at all, only be accessed by electrochemical methods.[7] Gibson and Beardwood reported some reduced [2 Fe-2 S] clusters that were generated by chemical reduction in situ and investigated in solution.[8] Furthermore they were able to isolate and characterize one species in the [2Fe-2S] 1+ state using a bidentate bis (benzimidazolato) terminal ligand; it was studied by Mçssbauer spectroscopy, and an S= 1/2 ground state with partially delocalized mixed valence has been proposed for the system.[9] However, neither has a crystal structure of any [2Fe-2S] cluster in the mixed-valent [2Fe-2S] 1+ state been published nor have magnetic measurements of such reduced clusters been performed; such measurements could confirm the proposals based on Mçssbauer data. Herein we report the first crystal structure and SQUID magnetic susceptibility data of a synthetic mixed-valent [2 Fe-2 S] cluster, together with its complete spectroscopic characterization. We decided to also use a bidentate bis (benzimidazolato) ligand since these heterocyclic systems have proven to sufficiently stabilize the mixed-valent form. A phenyl group was appended at the ligand backbone to improve solubility and crystallization (Figure1). The homoleptic all-ferric cluster 1 was synthesized in a standard ligand-exchange reaction by addition of an excess of the deprotonated ligand to (NEt4) 2 [Fe2S2Cl4].[10]Redox properties of 1 were studied by cyclic voltammetry in MeCN/0.1 m nBu4NPF6 at room temperature (Figure2, top). Two reversible one-electron reduction processes are assigned to the formation of the mixed-valent species at E1/2= À1. 14 V and the all-ferrous species at E1/2= À2. 10 V vs. the Fc/Fc+ couple (Fc=[(C5H5) 2Fe]). The potentials are shifted to more positive values compared to related thiolato-coordinated complexes and are relatively high for synthetic [2Fe-2S] clusters, a result of the electron withdrawing character of the bis (benzimidazolato) capping ligands. Electrochemical potentials of cysteine-coordinated [2Fe-2S] ferredoxines usually lie in the range À1. 05 V to À0. 75 V versus the Fc/Fc+ couple.[11] The large separation between the two halfwave potentials revealed significant stabilization of the mixed-valent species 2 (Kc= 1.7 1016), a fact that allowed the chemical reduction of 1 (using decamethylcobaltocene) and isolation of 2, which proved to be stable in the solid state under nitrogen even at room temperature for several hours. The UV/Vis spectrum (Figure2, bottom) of 1 shows two prominent bands at 411nm (ε= 7340mÀ1 cmÀ1) and 528 nm(ε= 7020mÀ1 cmÀ1) and a minor band at