Electron-Transfer Processes in 3,4-Diferrocenylpyrroles: Insight into a Missing Piece of the Polyferrocenyl-Containing Pyrroles Family

3,4-Diiodo-1-(triisopropylsilyl)-1H-pyrrole (1), 3,4-diferrocenyl-1-(triisopropylsilyl)-1H-pyrrole (2), and 3,4-diferrocenyl-1H-pyrrole (3) were prepared and characterized using spectroscopic methods and X-ray crystallography. UV–vis spectra of 2 and 3 were correlated with their density functional theory (DFT)-calculated electronic structures as well as theoretically predicted by the time-dependent (TD) DFT-calculations vertical excitation energies. Redox properties of 2 and 3 were investigated using cyclic voltammetry, differential pulse voltammetry, and spectroelectrochemical approaches. Ferrocene-centered oxidation processes in 2 and 3 were found to be separated by ∼180 and ∼300 mV in DCM/TBAP and DCM/(NBu4)­[B­(C6F5)4] systems, respectively. Stepwise spectroelectrochemical oxidation of 2 and 3 allowed us to obtain spectroscopic signatures of the mixed-valence [2]+ and [3]+ cations. Hush analysis of the intervalence charge-transfer band in [2]+ and [3]+ is suggestive of class II (in Robin and Day classification) mixed-valence behavior. Electronic structures of neutral and spin-localized/delocalized single-electron oxidized mixed-valence cations of 2,5-di-, 3,4-di-, and 2,3,4,5-tetraferrocenylpyrroles were investigated by DFT calculations to resolve current uncertainties regarding the first oxidation process of tetraferrocenylpyrrole.

本研究合成并通过光谱法及X射线晶体学表征了3,4-二碘-1-(三异丙基硅基)-1H-吡咯(1)、3,4-二茂铁基-1-(三异丙基硅基)-1H-吡咯(2)以及3,4-二茂铁基-1H-吡咯(3)。将化合物2与3的紫外-可见（UV–vis）光谱与其密度泛函理论（Density Functional Theory，DFT）计算得到的电子结构，以及经含时（TD）DFT计算得到的垂直激发能的理论预测值进行了关联。本研究采用循环伏安法、差分脉冲伏安法及光谱电化学法对化合物2与3的氧化还原性质进行了研究。研究发现，在二氯甲烷（Dichloromethane，DCM）/四丁基高氯酸铵（Tetrabutylammonium perchlorate，TBAP）与二氯甲烷/四丁基铵四(五氟苯基)硼酸盐体系中，化合物2与3的二茂铁基氧化电位分别相差约180 mV与约300 mV。通过对化合物2与3进行分步光谱电化学氧化，本研究成功获取了混合价态[2]+与[3]+阳离子的光谱特征信号。对[2]+与[3]+中的区间电荷转移带进行哈什（Hush）分析后，结果表明其符合罗宾-戴（Robin and Day）分类中的第II类混合价态行为特征。为厘清当前关于四二茂铁基吡咯首次氧化过程的现存不确定性，本研究通过DFT计算，对2,5-二茂铁基吡咯、3,4-二茂铁基吡咯以及2,3,4,5-四二茂铁基吡咯的中性态、自旋定域/自旋离域单电子氧化混合价态阳离子的电子结构展开了研究。