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](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, C(4)H(7)N(2) (1), glutarimi­dine as 6-imino-3,4,5,6-tetra­hydro­pyridin-2-amine methanol monosolvate, C(5)H(9)N(3)·CH(3)OH (2), and the corresponding hydrolyzed imino–imide com­pounds as amino–amides 5-amino-3,4-di­hydro-2H-pyrrol-2-one, C(4)H(6)N(2)O (3), and 6-amino-4,5-di­hydro­pyridin-2(3H)-one, C(5)H(8)N(2)O (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), C(4)H(8)N(3) (+)·Cl(−)·C(4)H(7)N(3)·H(2)O (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(2)) 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.