Synthesis and characterization of polymer-bound selenium coronands: enhancing the stability of reactive dications by restricting intermolecular interactions

Abstract

The stepwise synthesis of 1,5,9,13-tetraselenacyclohexadecan-3-ol 7 from the reaction of sodium propane-1,3-bisselenolate-2-ol with 4,8-diselenaundecan-1,11-di-p-toluenesulfonate 5 in THF and ethanol in 62% yield is described. Compound 5 is obtained from the corresponding diol 3, which is obtained, in turn, from the reaction of sodium propane-1,3-bisselenolate with two equivalents of 3-chloropropanol. 1,5-Diselenacyclooctan-3-ol 9 and 1,5,9,13-tetraselenacyclohexadecan-3,11-diol 10 are obtained in yields of 43% and 11%, respectively, from the one-step reaction of sodium propane-1,3-bisselenolate-2-ol with 1,3-dibromopropane. Polymers with pendant selenium coronands, 1,5-diselenacyclooctane (8Se2) and 1,5,9,13-tetraselenacyclohexadecane (16Se4), have been prepared by free radical polymerization of the corresponding selenium coronands 9 and 7, derivatized as their acryloyl esters 11 and 12. Copolymers of methyl acrylate and 3,7-diselenacyclooctyl acrylate 11 have also been prepared in analogous fashion. The polymers have been characterized by 1H and 13C NMR, IR, and UV–visible spectroscopy. The redox chemistry of polymer-bound selenium coronands in solution, and in the form of thin films coated on electrodes, has been determined by electrochemical methods. Selenium coronands attached to the polymer chain exhibit enhanced oxidative stability compared to monomeric analogues, presumably because restrictive motion of polymer-bound moieties inhibits inter- and intra-molecular degradation reactions. Electroactivity of polymer-modified electrodes is restricted to a few monolayers close to the electrode surface. Spatial separation of coronands along the chain, achieved by copolymerization with electro-inert methyl acrylate, gave copolymer-modified electrodes that possessed even greater redox stability than their corresponding homopolymers. Furthermore, by blending electro-inert poly(methyl acrylate) with electroactive selenium coronand copolymers, polymer-modified electrodes that possessed superior electrochemical stability were obtained. Key words: selenium coronands, polymers, electrochemistry, dications.