Relative Binding Affinity of Thiolate, Imidazolate, Phenoxide, and Nitrite Toward the {Fe(NO)₂} Motif of Dinitrosyl Iron Complexes (DNICs): The Characteristic Pre-Edge Energy of {Fe(NO)₂}⁹ DNICs

The synthesis, characterization, and transformation of the anionic {Fe(NO)2}9 dinitrosyl iron complexes (DNICs) [(NO)2Fe(ONO)2]− (1), [(NO)2Fe(OPh)2]− (2), [(NO)2Fe(OPh)(C3H3N2)]− (3) (C3H3N2 = imidazolate), [(NO)2Fe(OPh)(-SC4H3S)]− (4), [(NO)2Fe(p-OPhF)2]− (5), and [(NO)2Fe(SPh)(ONO)]− (6) were investigated. The binding affinity of ligands ([SPh]−, [−SC4H3S]−, [C3H3N2]−, [OPh]−, and [NO2]−) toward the {Fe(NO)2}9 motif follows the ligand-displacement series [SPh]− ∼ [−SC4H3S]− > [C3H3N2]− > [OPh]− > [NO2]−. The findings, the pre-edge energy derived from the 1s → 3d transition in a distorted Td environment of the Fe center falling within the range of 7113.4−7113.8 eV for the anionic {Fe(NO)2}9 DNICs, implicate that the iron metal center of DNICs is tailored to minimize the electronic changes accompanying changes in coordinated ligands. Our results bridging the ligand-substitution reaction study and X-ray absorption spectroscopy study of the electronic richness of the {Fe(NO)2}9 core may point the way to understanding the reasons for nature’s choice of combinations of cysteine, histidine, and tyrosine in protein-bound DNICs and rationalize that most DNICs characterized/proposed nowadays are bound to the proteins almost through the thiolate groups of cysteinate/glutathione side chains in biological systems.本研究对阴离子型{Fe(NO)₂}₉二亚硝基铁配合物（dinitrosyl iron complexes，DNICs）[(NO)₂Fe(ONO)₂]⁻（编号1）、[(NO)₂Fe(OPh)₂]⁻（编号2）、[(NO)₂Fe(OPh)(C₃H₃N₂)]⁻（编号3，其中C₃H₃N₂为咪唑负离子（imidazolate））、[(NO)₂Fe(OPh)(-SC₄H₃S)]⁻（编号4）、[(NO)₂Fe(p-OPhF)₂]⁻（编号5）以及[(NO)₂Fe(SPh)(ONO)]⁻（编号6）的合成、表征与转化进行了系统考察。各类配体[SPh]⁻、[−SC₄H₃S]⁻、[C₃H₃N₂]⁻、[OPh]⁻与[NO₂]⁻对{Fe(NO)₂}₉结构基元的结合亲和力遵循如下配体置换序列：[SPh]⁻ ≈ [−SC₄H₃S]⁻ > [C₃H₃N₂]⁻ > [OPh]⁻ > [NO₂]⁻。针对该类阴离子型{Fe(NO)₂}₉ DNICs，Fe中心处于畸变四面体（Td）环境中，其1s→3d电子跃迁对应的预边峰能量介于7113.4~7113.8 eV范围内，该结果表明DNICs的铁金属中心可通过适配配位配体的变化，最小化伴随配体改变产生的电子结构扰动。本研究将配体取代反应研究与{Fe(NO)₂}₉核电子结构的X射线吸收光谱研究相结合，可为理解自然界中蛋白质结合型DNICs选择半胱氨酸、组氨酸与酪氨酸组合的内在原因提供思路，同时可合理解释当前已表征或推测的绝大多数DNICs在生物体系中几乎均通过半胱氨酸盐/谷胱甘肽侧链的巯基与蛋白质结合这一现象。