Electrochemical oxidation in different media of bis(p-phenylene crown ether) as a symmetrical molecule bearing two identical redox centers

The oxidation of the long chain polyether p-disubstituted benzene derivativ e (BO5O5) and the related macrocyclic paracyclophane (BBO5O5) have been inv estigated by cyclic voltammetry at different concentrations (10(-3)-3 x 10( -2) M) and potential scan rates (0.05-250 V s(-1)). At about 10(-3) M in CH 3CN or CH2Cl2 containing a perchlorate salt as the supporting electrolyte, BO5O5 was oxidized reversibly into moderately stable radical cations. The f ormal potential E degrees' corresponding to this first oxidation step was f ound to be shifted towards more positive values when the cation was changed from Bu4N+ to Na+, and this variation may be ascribed to the coordination of the alkali cation by the polyether chains. At higher concentrations of t he reactant and in a thoroughly dried medium, no deposit of a polymeric mat erial onto the electrode surface was electrogenerated. At low scan rates an d under the same conditions, the oxidation of BBO5O5 gave rise to two close ly spaced, irreversible processes. The radical cations involved in each ste p were thought to be highly reactive towards nucleophilic species present i n solution. At high scan rates, a single reversible one-electron transfer w as observed, the formal potential of which varied with the nature of the el ectrolyte cation in the same way as for BO5O5. The heterogeneous charge-tra nsfer rate constants estimated for BO5O5 and BBO5O5 were compared with the values predicted by the Marcus theory. As for BO5O5, attempts to electropol ymerize BBO5O5 were unsuccessful. (C) 1999 Elsevier Science S.A. All rights reserved.