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

0376013 - ÚFCH JH 2013 RIV US eng J - Journal Article
Fortage, J. - Peltier, C. - Perruchot, Ch. - Takemoto, Y. - Teki, Y. - Bedioui, F. - Marvand, V. - Dupeyre, G. - Pospíšil, Lubomír - Adamo, C. - Hromadová, Magdaléna - Ciofini, I. - Lainé, P. P.
Single-Step versus Stepwise Two-Electron Reduction of Polyarylpyridiniums: Insights from the Steric Switching of Redox Potential Compression.
Journal of the American Chemical Society. Roč. 134, č. 5 (2012), s. 2691-2705. ISSN 0002-7863. E-ISSN 1520-5126
Institutional research plan: CEZ:AV0Z40400503
Keywords : FUNCTIONALIZED TERPYRIDYL LIGANDS * GATED PHOTOINDUCED PROCESSES * ONE-ELECTRON REDUCTION
Subject RIV: CG - Electrochemistry
Impact factor: 10.677, year: 2012

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.
Permanent Link: http://hdl.handle.net/11104/0208529