Pseudopolymorphs of 2,6‐diaminopyrimidin‐4‐one and 2‐amino‐6‐methylpyrimidin‐4‐one: one or two tautomers present in the same crystal

The derivatives of pyrimidin‐4‐one can adopt either a 1 H ‐ or a 3 H ‐tautomeric form, which affects the hydrogen‐bonding interactions in cocrystals with compounds containing complementary functional groups. In order to study their tautomeric preferences, we crystallized 2,6‐diaminopyrimidin‐4‐one and 2‐amino‐6‐methylpyrimidin‐4‐one. During various crystallization attempts, four structures of 2,6‐diaminopyrimidin‐4‐one were obtained, namely solvent‐free 2,6‐diaminopyrimidin‐4‐one, C 4 H 6 N 4 O, (I), 2,6‐diaminopyrimidin‐4‐one–dimethylformamide–water (3/4/1), C 4 H 6 N 4 O·1.33C 3 H 7 NO·0.33H 2 O, (I a ), 2,6‐diaminopyrimidin‐4‐one dimethylacetamide monosolvate, C 4 H 6 N 4 O·C 4 H 9 NO, (I b ), and 2,6‐diaminopyrimidin‐4‐one– N ‐methylpyrrolidin‐2‐one (3/2), C 4 H 6 N 4 O·1.5C 5 H 9 NO, (I c ). The 2,6‐diaminopyrimidin‐4‐one molecules exist only as 3 H ‐tautomers. They form ribbons characterized by R 2 2 (8) hydrogen‐bonding interactions, which are further connected to form three‐dimensional networks. An intermolecular N—H...N interaction between amine groups is observed only in (I). This might be the reason for the pyramidalization of the amine group. Crystallization experiments on 2‐amino‐6‐methylpyrimidin‐4‐one yielded two isostructural pseudopolymorphs, namely 2‐amino‐6‐methylpyrimidin‐4(3 H )‐one–2‐amino‐6‐methylpyrimidin‐4(1 H )‐one–dimethylacetamide (1/1/1), C 5 H 7 N 3 O·C 5 H 7 N 3 O·C 4 H 9 NO, (II a ), and 2‐amino‐6‐methylpyrimidin‐4(3 H )‐one–2‐amino‐6‐methylpyrimidin‐4(1 H )‐one– N ‐methylpyrrolidin‐2‐one (1/1/1), C 5 H 7 N 3 O·C 5 H 7 N 3 O·C 5 H 9 NO, (II b ). In both structures, a 1:1 mixture of 1 H ‐ and 3 H ‐tautomers is present, which are linked by three hydrogen bonds similar to a Watson–Crick C–G base pair.