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

Reactions of N-(2,4-dinitrophenyl)-4-arylpyridinium chlorides (aryl = H (1), phenyl (Ph); 2, and 4-pyridyl (Py); 3) with R-(−)-, S-(+)-, or racemic 1,1′-binaphthyl-2,2′-diyl sodium phosphates (R-, S-, or rac-BINAP-PO4Na) resulted in anion exchange between Cl− and R-, S-, or rac-BINAP-PO4- to yield Zincke salts with chiral anions, namely 1R, 2R, 3R, 1S, 2S, 3S, 1rac, 2rac, and 3rac. Reactions of these Zincke salts with 2-methylpiperazine resulted in pyridinium ring opening to yield ionic polymers with 5-(2-methylpiperazinium)-3-aryl-penta-2,4-dienylideneammonium 1,1'-binaphthyl-2,2′-diyl phosphate units, namely, polymer(rac-Me;Ar;R-BINAP-PO4-) (Ar = H, Ph, and Py), polymer(rac-Me;Ar;S-BINAP-PO4-) (Ar = H, Ph, and Py), and polymer(rac-Me;Ar;rac-BINAP-PO4-) (Ar = H, Ph, and Py). CD measurements revealed a partially helical conformation of the main chain in polymer(rac-Me;H;R-BINAP-PO4-) and polymer(rac-Me;H;S-BINAP-PO4-). The 2-methylpiperazinium rings of the polymers converted from the boat to the chair form via a half-chair intermediate in solution. This conformational change was accompanied by a decrease in π-conjugation length. This decrease corresponded to a shift in the electrochemical oxidation peaks of the polymers to higher potentials when the polymer solution was left standing in air.