Synthesis, Characterization, Electrochemistry, and Computational Studies of Ferrocenyl-Substituted Siloles

Ferrocenylsiloles of the type 2,5-Fc2-3,4-Ph2-cC4SiR2 (3a, R = Me; 3b, R = Ph) have been prepared by reductive cyclization from diethynylsilanes, followed by ferrocenylation using the Negishi C,C cross-coupling protocol with the silole ring serving as either the vinyl halogenide species or as the zinc organic component and the complementary functionality introduced on the ferrocenyl moiety. The electrochemical behavior of these silacyclic-bridged bis­(ferrocenyl) complexes was investigated by cyclic and square wave voltammetry, and the nature of the redox products was studied by in situ UV–vis–near-IR spectroelectrochemical measurements. 3a,b each undergo two sequential ferrocenyl-based redox processes, the separation of which (ΔE°′ = ΔE2°′ – ΔE1°′ = 300 mV (3a), 280 mV (3b)) is in the range of structural similar systems such as 2,5-diferrocenyl-1-phenyl-1H-phosphole (280 mV) and 2,5-diferrocenylfuran (290 mV). Interestingly, the more electron rich silole 3b, in comparison to 3a, shows a modestly lower redox separation between the individual ferrocenyl oxidation processes, which may be due to the capacity of this group to shield the effect of an adjacent positive charge. An intervalence charge transfer (IVCT) absorption was found in the in situ NIR spectra of [3a]+ and [3b]+, the analysis of which is consistent with a moderate electronic interaction between the iron atoms through the cis-diene-like fragment of the silole bridge, allowing their description as Robin and Day class II mixed-valence systems. These conclusions are supported by results from quantum chemical calculations, which together with NMR studies of 3b, also reveal the likely presence of a range of molecular conformations in solution.

本研究通过二乙炔基硅烷的还原环化反应，结合根岸（Negishi）C,C交叉偶联策略，以硅咯（silole）环作为乙烯基卤代物或有机锌组分，并在二茂铁基部分引入互补官能团，成功合成了结构为2,5-Fc₂-3,4-Ph₂-cC₄SiR₂的二茂铁基硅咯类化合物（3a：R = Me；3b：R = Ph，其中Fc为二茂铁基，Ph为苯基）。随后采用循环伏安法与方波伏安法，对这类硅环桥联双二茂铁基配合物的电化学行为进行了研究；同时通过原位紫外-可见-近红外光谱电化学测试，分析了其氧化还原产物的本质。3a与3b均会发生两步连续的基于二茂铁基的氧化还原过程，其氧化还原电势差（ΔE°′ = ΔE₂°′ – ΔE₁°′）分别为300 mV（3a）与280 mV（3b），该数值处于结构类似体系的区间内，例如2,5-二茂铁基-1-苯基-1H-磷咯（280 mV）与2,5-二茂铁基呋喃（290 mV）。值得注意的是，相较于3a，富电子硅咯3b的分步二茂铁基氧化过程的电势差略低，这可能源于该基团对相邻正电荷的屏蔽作用。在[3a]⁺与[3b]⁺的原位近红外（NIR）光谱中，观测到了区间电荷转移（IVCT）吸收峰；分析结果表明，两个二茂铁单元之间通过硅咯桥实现了中等强度的电子耦合，因此这类配合物可被归类为罗宾-戴（Robin and Day）II类混合价体系。上述结论得到了量子化学计算结果的佐证，结合针对3b的核磁共振（NMR）研究，还揭示了该类化合物在溶液中可能存在多种分子构象。