Protonation of pyridines by phenols, thiophenol, trifluoroacetic acid and methanesulfonic acid in aprotic solvents
At moderately low temperatures, 293–213 K, phenols and pyridines form complexes linked by hydrogen bridges as a way for proton migration: A—H⋯B ⇄ A–⋯H—B+. The enthalpy and entropy variations induced by this intracomplex proton transfer have been evaluated for the 2,6-dichloro-4-nitrophenol-3,5-lutidine complex from the temperature dependences of the phenol 13C chemical shifts. At lower temperatures, down to 110 K, 1H NMR results provided direct evidence for the homoconjugation equilibrium: A–⋯H—B++ B ⇄(A–)(B2—H+). Separate 1H signals of the B—H+ and B2—H+ species were recorded for solutions of pyridines with thiophenol, trifluoroacetic acid or methanesulfonic acid with molar excess of the base. The hydrogen-bridge protons in the homoconjugated cations were strongly deshielded, at ca. 20.5 ppm, and this observation suggests their central or near-central position in the bonds: δ+B⋯H⋯Bδ+. The thermodynamic quantities of the homoconjugation equilibrium were calculated for selected systems.