Chiral-phosphate anion-containing polymers with expanded π-conjugation systems derived from through-space interactions in the piperazinium ring

Eco-friendly synthesis of ionic helical polymers and their chemical properties and reactivity

Reaction of N-(2,4-dinitrophenyl)pyridinium chloride (salt(Cl⁻)) with sodium dicyanamide (Na(CN)₂N) resulted in anion exchange between Cl⁻ and (CN)₂N⁻ to yield a new Zincke salt, salt((CN)₂N⁻). Reactions of salt((CN)₂N⁻) with piperazine, specifically (R)-(−)- or (S)-(+)-2-methylpiperazine under eco-friendly conditions, such as in aqueous solution, in the absence of a catalyst, and at room temperature, resulted in pyridinium ring opening to yield ionic high-molecular-weight polymers with 5-2,4-dienylideneammonium dicyanamide units or chiral 5-(2-methylpiperazinium)-2,4-dienylideneammonium dicyanamide units, namely, polymer(H;(CN)₂N⁻), polymer(R-Me;(CN)₂N⁻), and polymer(S-Me;(CN)₂N⁻). UV-Vis measurements revealed that the π-conjugation system expanded along the polymer chain due to the orbital interaction between the electrons on the two nitrogen atoms of the piperazinium ring. Circular dichroism (CD) measurements revealed a helical conformation of the main chain in polymer(R-Me;(CN)₂N⁻) and polymer(S-Me;(CN)₂N⁻). The reaction of polymer(H;(CN)₂N⁻) with p-phenylenediamine (PDA) caused recyclization of the 2,4-dienylideneammonium unit and resulted in depolymerization to yield N-(4-aminophenyl)pyridinium dicyanamide. Cyclic voltammetry analysis suggested that the polymers obtained in this study undergo electrochemical oxidation and reduction.

Reactions of salt((CN)₂N⁻) with (R)-(−)- or (S)-(+)-2-methylpiperazine under eco-friendly conditions resulted in a yield of ionic helical polymers.