Structure of 3-amino-4,5-dihydropyrazoles in acid media: X-ray structure of 3-amino-1-phenyl-4,5-dihydropyrazol-2-ium picrate and the origin of broad signals in 1H NMR spectroscopy

The origin of the broadening of NMR signals in 3-amino-4,5-dihydropyrazoles has been studied in the 1-phenyl series. It has been proved that the broadening is due to the presence of acid in some solvents, like deuteriochloroform, by reproducing it by addition of trifluoroacetic acid. To determine the protonation site of 3-amino-1-phenyl-4,5-dihydropyrazole 1, the crystal structure of its picrate 5 was determined. Contrary to other dihydropyrazoles, 1 protonates on N2, like an amidine.