Reported herein are alkyne and alkene adducts of synthetic [Fe₄S₄]⁺ clusters that model intermediates and inhibitor-bound states in enzymes involved in isoprenoid biosynthesis. Treatment of the N-heterocyclic carbene-ligated cluster [(IMes)₃Fe₄S₄(OEt₂)][BAr^F₄] (IMes = 1,3-dimesitylimidazol-2-ylidene; [BAr^F₄]⁻ = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) with phenylacetylene (PhCCH) or cis-cyclooctene (COE) results in displacement of the Et₂O ligand to yield the corresponding π complexes, [(IMes)₃Fe₄S₄(PhCCH)][BAr^F₄] and [(IMes)₃Fe₄S₄(COE)][BAr^F₄]. EPR spectroscopic analysis demonstrates that both clusters are doublets with g_iso > 2 and thus are spectroscopically faithful models of the analogous species characterized in the isoprenoid biosynthetic enzymes IspG and IspH. Structural and Mössbauer spectroscopic analysis reveals that both complexes are best described as [Fe₄S₄]⁺ clusters in which the unique Fe site engages in modest back-bonding to the π-acidic ligand. Paramagnetic NMR studies show that, even at room temperature, the alkyne/alkene-bound Fe centers harbor minority spin and therefore adopt an Fe²⁺ valence. We propose that such valence localization could likewise occur in Fe–S enzymes that interact with π-acidic molecules.

中文翻译：

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合成 [Fe4S4]+ 簇的炔烃和烯烃加合物的价态定位

本文报道的是合成 [Fe ₄ S ₄ ] ⁺簇的炔烃和烯烃加合物，这些簇模拟参与类异戊二烯生物合成的酶中的中间体和抑制剂结合状态。N-杂环卡宾连接簇的处理[(IMes) ₃ Fe ₄ S ₄ (OEt ₂ )][BAr ^F ₄ ] (IMes = 1,3-二甲基咪唑-2-亚基；[BAr ^F ₄ ] ⁻ = tetrakis （3,5-双（三氟甲基）苯基）硼酸盐）与苯乙炔 (PhCCH) 或顺式环辛烯 (COE) 导致 Et _2的置换O配体产生相应的π配合物，[(IMes) ₃ Fe ₄ S ₄ (PhCCH)][BAr ^F ₄ ]和[(IMes) ₃ Fe ₄ S ₄ (COE)][BAr ^F ₄ ]。EPR 光谱分析表明，这两个簇都是g _iso > 2 的双峰，因此是类异戊二烯生物合成酶 IspG 和 IspH 中表征的类似物种的光谱忠实模型。结构和穆斯堡尔光谱分析表明，这两种配合物最好描述为 [Fe ₄ S ₄ ] ⁺簇中独特的 Fe 位点与 π- 酸性配体适度反向键合。顺磁核磁共振研究表明，即使在室温下，炔烃/烯烃结合的 Fe 中心也具有少数自旋，因此采用 Fe ²⁺价态。我们提出这种价态定位同样可能发生在与 π 酸性分子相互作用的 Fe-S 酶中。