Electrochemical determination of the stability constant of an aryl radical with β-cyclodextrin

Addition of beta-cyclodextrin (beta-CD) to an aqueous acid solution of p-nitrobenzenediazonium (PNBD) tetrafluoroborate causes a substantial decrease in peak currents and shifts in opposite directions the apparent peak potential values, E-app, of the -N-2(+) and -NO2 groups. beta-CD hinders the reduction of the nitro group since E-app(-NO2) is shifted towards more negative values, in contrast with the E,PP shift observed for the one-electron reduction of the -N-2(+) group to form the arenediazenyl radical, ArN2, which is shifted towards more positive values and thus favouring the reduction of PNBD. Either the decrease in peak currents or Eapp shifts are interpreted in terms of the formation of an inclusion complex between beta-CD and PNBD, with the -NO2 group inserted into the CD cavity. Such specific spatial configuration contrasts with that observed when crown ethers and calix-[n]arenes are employed as host molecules. Values for the association constants of the parent arenediazonium ions and of the electrochemically-generated arenediazenyl radicals with beta-CD were estimated from the electrochemical measurements.