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

Abstract

Addition of β-cyclodextrin (β-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 _a pp, of the -N ⁺₂ > and -NO₂ groups. β-CD hinders the reduction of the nitro group since E _a pp(-NO₂) is shifted towards more negative values, in contrast with the E _a pp shift observed for the one-electron reduction of the -N ⁺₂ > group to form the arenediazenyl radical, ArN ^·₂ >, which is shifted towards more positive values and thus favouring the reduction of PNBD. Either the decrease in peak currents or E _a pp shifts are interpreted in terms of the formation of an inclusion complex between β-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 β-CD were estimated from the electrochemical measurements.

Acknowledgements. Financial support from the following institutions is acknowledged: MCYT of Spain (BQU2000-0239-C02), Xunta de Galicia (XUGA 38301A92 and XUGA 38305A94) and Universidad de Vigo. B.F.-C. thanks Universidad de Vigo for a research training grant.