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Transition Between Concerted and Stepwise Mechanisms of the Reductive Bond Cleavage of 1-Chloro-10-methyltribenzotriquinacene

Jan S. Jaworski 1Marek Cembor 

1. Uniwersytet Warszawski, Wydział Chemii, Warszawa, Poland

Abstract
Tribenzotriquinacenes belong to centropolyindanes, unusual polycyclic hydrocarbons, which are promising building blocs for molecular architecture [1]. However, their physicochemical properties are studied only scarcely. The title compound, RCl, undergoes the C-Cl bond cleavage induced by the electron transfer from electrodes [2] and because of its structure (a saturated carbon atom in the C-Cl bond and peripheral benzene rings) it looks interesting for investigations of a transition between the concerted (1) and stepwise (2) mechanisms of a reductive cleavage.

RCl + e → R + Cl- (1)

RCl + e = RCl- (2a)

RClR- → R + Cl- (2b)

Factors deciding on such transition were intensively examined [3]. In the present work a transition of a mechanism is analyzed for results obtained at the same conditions in different solvents. In benzonitrile at glassy carbon or mercury electrodes the stepwise process was found with the reversible electron transfer (2a) followed by the bond cleavage (2b) and the resulting radical R undergoes the disproportionation giving a two electrons transfer for the overall process. In acetonitrile and N,N-dimethylformamide (DMF) the one electron process was established, indicating the H atom abstraction by the radical R from solvent molecules. A detail analysis of potential dependence of the apparent transfer coefficient α = (RT/F) dlnk/dE, obtained from convoluted voltammetric curves in DMF and CH3CN, indicates the concerted process. According to the theory of the dissociative electron transfer [3] a slope dα/dE = 12.06/ΔGo is inversely proportional to the intrinsic activation barrier, ΔGo. A low slope dα/dE = 0.14 V-1 found for the concerted process is in agreement with the high intrinsic activation barrier, which includes a contribution form the bond dissociation energy (ca. 2.9 eV [2]). The transition from the stepwise process in C6H5CN to the concerted one in DMF and CH3CN can be explained by an acceleration of the bond cleavage in the radical anion (2b) due to a stronger solvation of the final product Cl- by CH3CN and DMF.

[1] D. Kuck, Pure Appl. Chem. 2006, 78, 749.

[2] J. S. Jaworski, M. Cembor, D. Kuck, Electrochim. Acta 2006, in press.

[3] S. Antonello, F. Maran, Chem. Soc. Rev. 2005, 34, 418.

 

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Presentation: poster at 18th Conference on Physical Organic Chemistry, Posters, by Jan S. Jaworski
See On-line Journal of 18th Conference on Physical Organic Chemistry

Submitted: 2006-05-29 09:50
Revised:   2009-06-07 00:44