Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue Jean-Antoine de Baïf, 75013 Paris, France.
J Am Chem Soc. 2012 Feb 8;134(5):2691-705. doi: 10.1021/ja210024y. Epub 2012 Jan 26.
Contrary to 4,4'-dipyridinium (i.e., archetypal methyl viologen), which is reduced by two single-electron transfers (stepwise reduction), the 4,1'-dipyridinium isomer (so-called "head-to-tail" isomer) undergoes two electron transfers at apparently the same potential (single-step reduction). A combined theoretical and experimental study has been undertaken to establish that the latter electrochemical behavior, also observed for other polyarylpyridinium electrophores, is due to potential compression originating in a large structural rearrangement. Three series of branched expanded pyridiniums (EPs) were prepared: N-aryl-2,4,6-triphenylpyridiniums (Ar-TP), N-aryl-2,3,4,5,6-pentaphenylpyridiniums (Ar-XP), and N-aryl-3,5-dimethyl-2,4,6-triphenylpyridinium (Ar-DMTP). The intramolecular steric strain was tuned via N-pyridinio aryl group (Ar) phenyl (Ph), 4-pyridyl (Py), and 4-pyridylium (qPy) and their bulky 3,5-dimethyl counterparts, xylyl (Xy), lutidyl (Lu), and lutidylium (qLu), respectively. Ferrocenyl subunits as internal redox references were covalently appended to representative electrophores in order to count the electrons involved in EP-centered reduction processes. Depending on the steric constraint around the N-pyridinio site, the two-electron reduction is single-step (Ar = Ph, Py, qPy) or stepwise (Ar = Xy, Lu, qLu). This steric switching of the potential compression is accurately accounted for by ab initio modeling (Density Functional Theory, DFT) that proposes a mechanism for pyramidalization of the N(pyridinio) atom coupled with reduction. When the hybridization change of this atom is hindered (Ar = Xy, Lu, qLu), the first reduction is a one-electron process. Theory also reveals that the single-step two-electron reduction involves couples of redox isomers (electromers) displaying both the axial geometry of native EPs and the pyramidalized geometry of doubly reduced EPs. This picture is confirmed by a combined UV-vis-NIR spectroelectrochemical and time-dependent DFT study: comparison of in situ spectroelectrochemical data with the calculated electronic transitions makes it possible to both evidence the distortion and identify the predicted electromers, which play decisive roles in the electron-transfer mechanism. Last, this mechanism is further supported by in-depth analysis of the electronic structures of electrophores in their various reduction states (including electromeric forms).
与 4,4'-二吡啶鎓（即典型的甲紫精）不同，4,1'-二吡啶鎓异构体（所谓的“头到尾”异构体）通过两个电子转移（单电子转移）在明显相同的电势下进行（单步还原）。已经进行了一项理论和实验相结合的研究，以确定这种电化学行为也存在于其他聚芳基吡啶鎓染料中，这是由于源于大结构重排的电势压缩所致。已经制备了三个系列的支化扩展吡啶鎓（EP）：N-芳基-2,4,6-三苯基吡啶鎓（Ar-TP），N-芳基-2,3,4,5,6-五苯基吡啶鎓（Ar-XP）和 N-芳基-3,5-二甲基-2,4,6-三苯基吡啶鎓（Ar-DMTP）。通过 N-吡啶鎓芳基（Ar）苯基（Ph），4-吡啶基（Py）和 4-吡啶鎓（qPy）及其体积较大的 3,5-二甲基对应物二甲苯基（Xy），六甲基（Lu）和六甲基鎓（qLu）来调节分子内的空间位阻。为了计算 EP 中心还原过程中涉及的电子数，将二茂铁作为内部氧化还原参比物共价连接到代表性的电染料上。根据 N-吡啶鎓位点周围的空间位阻，两电子还原是单步（Ar = Ph，Py，qPy）或分步（Ar = Xy，Lu，qLu）。通过从头算建模（密度泛函理论，DFT）准确地解释了这种电势压缩的位阻切换，该模型提出了与还原偶联的 N（吡啶鎓）原子的三角化的机制。当该原子的杂化变化受到阻碍时（Ar = Xy，Lu，qLu），第一次还原是一个单电子过程。理论还表明，单步两电子还原涉及电子对异构体（电偶）的偶联，这些异构体同时显示出天然 EP 的轴向几何形状和双还原 EP 的三角化几何形状。这一图景通过紫外可见近红外光谱电化学和时间相关密度泛函理论的综合研究得到证实：原位光谱电化学数据与计算电子跃迁的比较使得可以证明这种扭曲并识别出预测的电偶，它们在电子转移机制中起着决定性作用。最后，通过深入分析各种还原态（包括电偶形式）的电染料的电子结构进一步支持了该机制。
