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 1H- or a 3H-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, (Ia), 2,6-diaminopyrimidin-4-one dimethylacetamide monosolvate, C(4)H(6)N(4)O·C(4)H(9)NO, (Ib), 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, (Ic). The 2,6-diaminopyrimidin-4-one molecules exist only as 3H-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(3H)-one-2-amino-6-methylpyrimidin-4(1H)-one-dimethylacetamide (1/1/1), C(5)H(7)N(3)O·C(5)H(7)N(3)O·C(4)H(9)NO, (IIa), and 2-amino-6-methylpyrimidin-4(3H)-one-2-amino-6-methylpyrimidin-4(1H)-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, (IIb). In both structures, a 1:1 mixture of 1H- and 3H-tautomers is present, which are linked by three hydrogen bonds similar to a Watson-Crick C-G base pair.