Structure of a 4-Nitroso-5-aminopyrazole and Its Salts: Tautomerism, Protonation, and <i>E</i>/<i>Z</i> Isomerism

The structures of 1-benzyl-4-nitroso-5-aminopyrazole (1) and its hydrochloride (1H+) have been determined in the solid state and in solution in DMSO, methanol, and ethanol. The free base exists in solution as a mixture of amino/nitroso tautomers 2a and 2b rather than in the imino/oxime tautomers 3. The conjugated cation 1H+ results from the protonation of the nitroso group. X-ray crystallography showed that both amino hydrogen atoms of 2a form NH···ON hydrogen bonds: one is intramolecular, the other links adjacent molecules in an infinite chain.