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

Abstract

Nitrogen heterocycles are a class of organic compounds with extremely versatile functionality. Imidines, HN[C(NH)R]2, are a rare class of heterocycles related to imides, HN[C(O)R]2, 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, C4H7N2 (1), glutarimidine as 6-imino-3,4,5,6-tetrahydropyridin-2-amine methanol monosolvate, C5H9N3·CH3OH (2), and the corresponding hydrolyzed imino–imide compounds as amino–amides 5-amino-3,4-dihydro-2H-pyrrol-2-one, C4H6N2O (3), and 6-amino-4,5-dihydropyridin-2(3H)-one, C5H8N2O (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), C4H8N3 +·Cl−·C4H7N3·H2O (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—NH2) 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.