Clarifying the structures of imidines: using crystallographic characterization to identify tautomers and localized systems of π-bonding.

Nitrogen heterocycles are a class of organic compounds with extremely versatile functionality. Imidines, HN[C(NH)R], are a rare class of heterocycles related to imides, HN[C(O)R], in which the O atoms of the carbonyl groups are replaced by N-H groups. The useful synthesis of the imidine compounds succinimidine and glutarimidine, as well as their partially hydrolyzed imino-imide congeners, was first described in the mid-1950s, though structural characterization is presented for the first time in this article. In the solid state, these structures are different from the proposed imidine form: succinimidine crystallizes as an imino-amine, 2-imino-3,4-dihydro-2H-pyrrol-5-amine, CHN (1), glutarimidine as 6-imino-3,4,5,6-tetrahydropyridin-2-amine methanol monosolvate, CHN·CHOH (2), and the corresponding hydrolyzed imino-imide compounds as amino-amides 5-amino-3,4-dihydro-2H-pyrrol-2-one, CHNO (3), and 6-amino-4,5-dihydropyridin-2(3H)-one, CHNO (4). Imidine 1 was also determined as the hydrochloride salt solvate 5-amino-3,4-dihydro-2H-pyrrol-2-iminium chloride-2-imino-3,4-dihydro-2H-pyrrol-5-amine-water (1/1/1), CHN·Cl·CHN·HO (1·HCl). As such, 1 and 2 show alternating short and long C-N bonds across the molecule, revealing distinct imino (C=NH) and amine (C-NH) groups throughout the C-N backbone. These structures provide definitive evidence for the predominant imino-amine tautomer in the solid state, which serves to enrich the previously proposed imidine-focused structures that have appeared in organic chemistry textbooks since the discovery of this class of compounds in 1883.