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.