Single-Step versus Stepwise Two-Electron Reduction of Polyarylpyridiniums: Insights from the Steric Switching of Redox Potential Compression

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• 作者：J茅r么me Fortage ; Cyril Peltier ; Christian Perruchot ; Yohei Takemoto ; Yoshio Teki ; Fethi Bedioui ; Val茅rie Marvaud ; Gr茅gory Dupeyre ; Lubom铆r Posp铆sil ; Carlo Adamo ; Magdal茅na Hromadov谩 ; Ilaria Ciofini ; Philippe P. Lain茅
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：February 8, 2012
• 年：2012
• 卷：134
• 期：5
• 页码：2691-2705
• 全文大小：774K
• 年卷期：v.134,no.5(February 8, 2012)
• ISSN：1520-5126

文摘

Contrary to 4,4鈥?dipyridinium (i.e., archetypal methyl viologen), which is reduced by two single-electron transfers (stepwise reduction), the 4,1鈥?dipyridinium isomer (so-called 鈥渉ead-to-tail鈥?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鈥搗is鈥揘IR 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).