Single and double bridged viologenes and intramolecular pimerization of their cation radicals

Abstract

As an extension of our studies dealing with reversible redox systems, six tetraquaternary salts have been synthesized. These compounds contain two 4,4'-bipyridinium units which are connected by either one or two rigid bridges. The single bridged systems (o-, m-3, p-3) contain one o-, m- or p-xylylene bridge. The double bridged systems (o,o-4, o,m-4 and m,m-4) contain two xylylene bridges and represent a new class of cyclophanes. A new and very simple high dilution technique is described for the synthesis of these compounds. Depending upon ring size, some of the systems show different internal mobility with regard to flipping of the bridges and rotation of the pyridinium rings. By voltammetry of compounds 3 and 4, only two or three of the expected four potentials are observed. This is probably due to the highly stabilized diradical dications 3SEM/SEM and 4SEM/SEM. From a study of the concentration and temperature dependent UV/VIS spectra of the cation radicals, the equilibrium constants K1 and K2 for intra- and intermolecular pimerization (CT complexation) together with their thermodynamic data are evaluated. The strongest intramolecular pimerization is observed with o-3SEM/SEM and o,o-4SEM/SEM, which exist exclusively as pimers.