Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase
Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase
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DOI：
10.1021/jacs.0c05920
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发表时间：
2020-11-04
影响因子：
15
通讯作者：
Agapie, Theodor
中科院分区：
文献类型：
--
作者：
Arnett, Charles H.;Bogacz, Isabel;Agapie, Theodor
固氮酶的FeMo-辅因子与N-2的结合被认为是在4 e和4 Fr当量转移到活性位点后发生的。虽然脉冲EPR研究表明存在两个Fe-（μ-H）-Fe部分，这种混合价中间体的结构和电子特征仍然知之甚少。为了更好地理解这种生物有机金属簇，我们在这里报告，二铁μ-碳炔复合物（P6 ArC）Fe-2（μ-H）可以被氧化和还原，允许第一次光谱表征的两个EPR活性的Fe（μ-C）（μ-H）Fe模型复合物连接的2 e（-）转移承担一些相似的一对E-n和En+2，固氮酶的状态。这两个物种填充S = 1/2状态在低温下，和价（去）本地化的μ-氢化物配体的光谱特征的影响进行了评估脉冲EPR研究。与FeMoco（E-4（4 H））的{Fe-2（μ-H）}（2）态的类似数据相比，本文提供的数据和分析表明，E-4（4 H）中的氢化物配体桥接等价（最可能是Fe-III）金属中心。虽然电子转移涉及金属定域轨道，但通过脉冲EPR对[（P6 ArC）Fe-2（mu-H）]（+1）和[（P6 ArC）Fe-2（mu-H）]（-1）的研究表明，氧化还原化学引起Fe-C共价的显著变化（在2 e（-）还原时为-50%），这一结论得到X射线吸收光谱、Fe-57穆斯堡尔研究和DFT计算的进一步支持。结合，我们的研究表明，共价缓冲剂的变化对积累的过量的电荷密度的金属部分重新分配到桥接碳，从而促进多电子转换。
Binding of N-2 by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 e and 4 Fr equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(mu-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron mu-carbyne complex (P6ArC)Fe-2 (mu-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(mu-C)(mu-H)Fe model complexes linked by a 2 e(-) transfer which bear some resemblance to a pair of E-n and En+2, states of nitrogenase. Both species populate S = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the mu-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe-2(mu-H)}(2) state of FeMoco (E-4(4H)), the data and analysis presented herein suggest that the hydride ligands in E-4(4H) bridge isovalent (most probably Fe-III) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(P6ArC)Fe-2(mu-H)](+1) and [(P6ArC)Fe-2 (mu-H)](-1) by pulse EPR revealed that redox chemistry induces significant changes in Fe-C covalency (-50% upon 2 e(-) reduction), a conclusion further supported by X-ray absorption spectroscopy, Fe-57 Mossbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.