ELECTRON-TRANSFER BOND-BREAKING PROCESSES - AN EXAMPLE OF NONLINEAR ACTIVATION DRIVING FORCE RELATIONSHIP IN THE REDUCTIVE CLEAVAGE OF THE CARBON SULFUR BOND

The heterogeneous (electrode) and homogeneous electron transfer (ET) to triphenylmethyl p-cyanophenyl sulfide in N,N'-dimethylformamide is shown to involve the irreversible reductive cleavage of the C(alkyl)-S sigma-bond through a sequential charge-transfer/bond-breaking pathway. The intermediate formation of the sulfide anion radical, whose lifetime has been determined to be ca. 10 ns, evidences the outer-sphere nature of the initial ET. The kinetics of this step has been analyzed as a function of the reaction free energy. Heterogeneous rate constants have been determined as a function of the applied potential from voltammetric curves and their convolution. The kinetics of the homogeneous ET from a series of substituted azobenzene anion radicals has been investigated by voltammetric redox 'catalysis. Both the homogeneous and the heterogeneous reactions show a quadratic dependence of the activation free energy on the driving force. The rate constant data have been treated in terms of classical ET theories in order to obtain the corresponding thermodynamic and kinetic parameters. Noteworthy, the values of the standard potential for the sulfide/sulfide anion radical redox couple, derived in the treatment of the two independent sets of homogeneous and heterogeneous data, are practically coincident. Structural parameters, from semiempirical MO calculations on azobenzene and its anion radical have been used to evaluate the azobenzene contribution to the cross ET activation energy and consequently to estimate, by comparison with the experimental results, the electron-exchange activation energy for the sulfide. The transition from the neutral sulfide to its anion radical involves relevant variations of the internal coordinates, in agreement with the very fast anion radical fragmentation.