Atropisomeric 3,3′,4,4′,5,5′-Hexaferrocenyl-2,2′-bithiophene: Synthesis, Solid-State Structure, and Electrochemistry

3,3′,4,4′,5,5′-Hexaferrocenyl-2,2′-bithiophene
(4) has been prepared using a straightforward synthesis
protocol that follows the palladium-promoted Negishi cross-coupling
of 2-Br-3,4,5-Fc3-cC4S (2) with 3,4,5-Fc3-2-ZnCl-cC4S (3) (Fc = Fe­(η5-C5H4)­(η5-C5H5)). The electronic
and structural properties of 4 were investigated by UV–vis
spectroscopy and single-crystal X-ray diffraction studies. Comparison
of the appropriate bond distances in the 2,2′-bithiophene moiety
shows electron delocalization. Cyclic and square-wave voltammetry
and in situ UV–vis/NIR spectroelectrochemistry
highlight the electrochemical properties of 4. The ferrocenyls
in 4 can be reversibly and separately oxidized in dichloromethane
and anisole solutions using [NnBu4]­[B­(C6F5)4] as supporting
electrolyte. Generally, these studies prove electrostatic interactions
among the ferrocenyl termini as oxidation progresses. The atropisomerism
of 4 caused by the inner rotation barrier about the 2,2′-bithiophene
bond was investigated by dynamic 1H NMR spectroscopy.

3,3′,4,4′,5,5′-六二茂铁基-2,2′-联噻吩(4)采用简洁的合成路线制备得到，该路线通过钯催化的根岸交叉偶联（Negishi cross-coupling）反应，将2-Br-3,4,5-Fc₃-cC₄S(2)与3,4,5-Fc₃-2-ZnCl-cC₄S(3)进行偶联（其中Fc为二茂铁基，对应结构为Fe(η⁵-C₅H₄)(η⁵-C₅H₅)）。通过紫外-可见光谱（UV–vis spectroscopy）与单晶X射线衍射（single-crystal X-ray diffraction）分析，对化合物4的电子与结构性质展开了研究。对2,2′-联噻吩结构单元内相关键长的对比结果显示，该体系存在电子离域现象。循环伏安法（cyclic voltammetry）、方波伏安法（square-wave voltammetry）以及原位紫外-可见/近红外光谱电化学（in situ UV–vis/NIR spectroelectrochemistry）研究阐明了化合物4的电化学特性。在以[NⁿBu₄][B(C₆F₅)₄]（四正丁基铵四(五氟苯基)硼酸盐）为支持电解质（supporting electrolyte）的二氯甲烷与苯甲醚溶液中，4上的二茂铁基团可被可逆且分步氧化。整体而言，上述研究证实，随着氧化过程的推进，各二茂铁端基之间存在静电相互作用。针对由2,2′-联噻吩键的内旋转势垒所引发的化合物4的阻转异构（atropisomerism）现象，通过动态核磁共振氢谱（dynamic ¹H NMR spectroscopy）进行了探究。