Solvent effect on reductive bond cleavage of 1-chloro-10-methyltribenzotriquinacene: Change from the concerted to the stepwise mechanism

Electrochemical reduction of 1-chloro-10-methyltribenzotriquinacene in benzonitrile, acetonitrile and N,N-dimethylformamide (DMF) is compared. A two-electron DISP1 mechanism was found in C6H5CN at glassy carbon and mercury electrodes with C–Cl bond cleavage as the rate determining step. In CH3CN and DMF the hydrogen atom abstraction from solvent molecules by a radical formed in bond cleavage results in the one-electron process. In the above-mentioned solvents an analysis of a potential dependence of the transfer coefficient, obtained from convoluted voltammetric curves at mercury electrode, showed the concerted mechanism. A change from the stepwise process in C6H5CN to the concerted one in CH3CN and DMF is explained by acceleration of bond cleavage in the radical anion due to stronger solvation of product chloride anions by CH3CN and DMF.