INCLUSION AND STABILIZATION OF ELECTROCHEMICALLY GENERATED ANION RADICALS OF DICYANOBENZENES BY CYCLODEXTRINS IN WATER

The interaction between cyclodextrins (CyDs) (α-, β-, γ-, and 2,6-O-dimethyl-β-CyDs) and the electrogenerated-intermediate anion radicals of 1,2-(PN), 1,4-(TPN), and 1,3-dicyanobenzenes in aqueous solution has been investigated by cyclic voltammetry and in situ electrochemical electron spin resonance (ESR) spectroscopy. Most strikingly, the addition of 2,6-O-dimethyl-β-CyD (DMβ-CyD) changes an irreversible two-electron-reduction wave of PN or TPN to a reversible one-electron wave, yielding its stable anion radical (PN⋅- or TPN⋅-), at a slow scan rate. The cyclic voltammetric and absorption spectral data have revealed a 1:1 inclusion complex formation of CyDs with the anion radicals as well as with the parent dicyanobenzenes, where the anion radicals are included more tightly than the parents and stabilized in a microscopic environment of the CyD cavity with respect to the consumptive homogeneous chemical reaction. Digital simulation analysis of the voltammograms points to the relatively aprotic property of the CyD cavity. The ESR spectral behavior of PN⋅- and TPN⋅- in the presence of DMβ-CyD indicates that PN⋅- penetrates the cavity with its benzene ring first at the 3-hydroxyl side of the CyD and TPN⋅- penetrates the cavity with its cyano group first. The unusual stronger inclusion of the anion radicals can be explained in terms of their dipole-induced dipole interaction. The dependence of the formation constants on the dicyanobenzene isomers and the CyDs is interpreted in view of the steric specificity. © 1990, American Chemical Society. All rights reserved.