The dimerization of 2,5-diaryl-1,4-dithiin radical cations

The electrochemical oxidation of five 2,5-diaryl-1,4-dithiins with aryl=p-anisyl (BAD), p-tolyl (BTD), phenyl (DPD), p-chlorophenyl (BCD) and p-nitrophenyl (END) has been studied in MeCN-CH2Cl2 (1:1) by cyclic voltammetry (CV), derivative cyclic voltammetry (DCV) and linear sweep voltammetry (LSV) as well as constant current coulometry and product analyses. All compounds were found to undergo two quasi-reversible one-electron transfers to the radical cations and the dications. The formal potential of the first redox couple and the life-time of the radical cation were found to decrease when the aryl group became more electron donating. The radical cation of END was non-reactive on the timescale of slow-scan CV, whereas the radical cation of BAD was so reactive that it was impossible to outrun the follow-up reaction even at a scan rate of 1000 V s(-1). The number of electrons determined in the presence of 2,6-lutidine, added to prevent acid-catalyzed conversion of substrate, was between 1.1 and 1.5 except for END, the radical cation of which catalyzed the oxidation of some unknown species in the solution. The major products from preparative electrolysis were the corresponding 2,2'-dimers, which were isolated in yields up to 20%. A detailed mechanistic and kinetic analysis of the dimerization, in the presence of 2,6-lutidine, involving the simultaneous fitting of theoretical data and experimental data obtained by LSV and DCV on a timescale covering several orders of magnitude, demonstrated that the process was of the radical cation-radical cation type and allowed for the determination of rate and equilibrium constants, k(1) and K-1. In addition, the values of the heterogeneous electron transfer rate constant, k(8), and the transfer coefficient, alpha, could be determined this way. The values of k(1) were found to vary between 2.2x10(6) M(-1) s(-1) (BAD) and 8.7 x 10(2) M(-1) s(-1) (BCD). The Hammett-plot, log k(1) vs. sigma(+), was linear indicating the importance of resonance stabilization for the dicationic transition state. The formal potentials for the reversible oxidation of the 2,2'-dimers to the radical cations were 70-120 mV higher than those for the monomers although AMI calculations predicted the ionization potentials of the dimers to be slightly lower than those for monomers. It is suggested that differences in solvation more than counterbalance the purely electronic effects.