Specific ion-molecule interactions of imidazole and 1-methylimidazole in nitrobenzene

Abstract

We have studied, by conductivity measurements, the formation of hydrogenbonded complexes between imidazoles and ions in the three systems triethylammonium picrate (Et₃NHPic)+imidazole (Im), triethylammonium bromide (Et₃NHBr)+Im, and Et₃NHPic+1-methylimidazole (1-MeIm) in nitrobenzene in order to specify the importance of the two functions of the imidazole molecule, the tertiary nitrogen N₃, and the imino group N₁-H. While 1-MelIm forms only a single complex with the cationic species Et₃NH⁺, imidazole enters into specific interactions as well with the cations through its basic site N₃ and with the anions through its imino group. The complexing of the anions by imidazole, always weaker than the complexing of the cations, is more effective for Br⁻ than for Pic⁻. Moreover, if imidazole is used as ligand, a 1:2 complex is formed between the cation and the imidazole, in which the second molecule of imidazole is bonded to the N-H group of the first by a hydrogen bond at the tertiary N atom. We did not observe a correlation between the equilibrium constants K ⁺₁ for the complexing of the cation Et₃NH⁺ by imidazole and pyridines (k ⁺₁ for pyridine, 3–4 dimethylpyridine, and imidazole are 8, 24, and 165, respectively) and the pK _a values of these ligands due to the fundamental difference in the structure of the imidazole and pyridine molecules, although both are considered as aromatic nitrogen bases.