Structure and protonation study of the imidazo[2,1-b]-1,3,4-thiadiazole system: 1H nuclear magnetic resonance, crystal and molecular structure of 5,6-dimethylimidazo[2,1-b]-1,3,4-thiadiazole and its hydrobromide

Abstract

Crystal structure analysis of 5,6-dimethylimidazo[2,1-b]-1,3,4-thiadiazole (3) and its corresponding hydrobromide has shown that protonation occurs at N(7). Molecular properties, mainly aromaticity and basicity, of this system were interpreted on the basis of semi-empirically calculated ring-currents, which show that this molecule has a lower aromatic character than that of other fused heterocycles of imidazole with pyrimidine and thiazole rings. From ¹H n.m.r. measurements the protonation site in acidic solution could not be determined, but N(7) was found to be the site available for complex formation in the presence of a lanthanide shift reagent.