One barbiturate and two solvated thiobarbiturates containing the triply hydrogen‐bonded ADA/DAD synthon,plus one ansolvate and three solvates of their coformer 2,4‐diaminopyrimidine

A path to new synthons for application in crystal engineering is the replacement of a strong hydrogen‐bond acceptor, like a C=O group, with a weaker acceptor, like a C=S group, in doubly or triply hydrogen‐bonded synthons. For instance, if the C=O group at the 2‐position of barbituric acid is changed into a C=S group, 2‐thiobarbituric acid is obtained. Each of the compounds comprises two ADA hydrogen‐bonding sites (D = donor and A = acceptor). We report the results of cocrystallization experiments of barbituric acid and 2‐thiobarbituric acid, respectively, with 2,4‐diaminopyrimidine, which contains a complementary DAD hydrogen‐bonding site and is therefore capable of forming an ADA/DAD synthon with barbituric acid and 2‐thiobarbituric acid. In addition, pure 2,4‐diaminopyrimidine was crystallized in order to study its preferred hydrogen‐bonding motifs. The experiments yielded one ansolvate of 2,4‐diaminopyrimidine (pyrimidine‐2,4‐diamine, DAPY), C₄H₆N₄, (I), three solvates of DAPY, namely 2,4‐diaminopyrimidine–1,4‐dioxane (2/1), 2C₄H₆N₄·C₄H₈O₂, (II), 2,4‐diaminopyrimidine–N,N‐dimethylacetamide (1/1), C₄H₆N₄·C₄H₉NO, (III), and 2,4‐diaminopyrimidine–1‐methylpyrrolidin‐2‐one (1/1), C₄H₆N₄·C₅H₉NO, (IV), one salt of barbituric acid, viz. 2,4‐diaminopyrimidinium barbiturate (barbiturate is 2,4,6‐trioxopyrimidin‐5‐ide), C₄H₇N₄⁺·C₄H₃N₂O₃⁻, (V), and two solvated salts of 2‐thiobarbituric acid, viz. 2,4‐diaminopyrimidinium 2‐thiobarbiturate–N,N‐dimethylformamide (1/2) (2‐thiobarbiturate is 4,6‐dioxo‐2‐sulfanylidenepyrimidin‐5‐ide), C₄H₇N₄⁺·C₄H₃N₂O₂S⁻·2C₃H₇NO, (VI), and 2,4‐diaminopyrimidinium 2‐thiobarbiturate–N,N‐dimethylacetamide (1/2), C₄H₇N₄⁺·C₄H₃N₂O₂S⁻·2C₄H₉NO, (VII). The ADA/DAD synthon was succesfully formed in the salt of barbituric acid, i.e. (V), as well as in the salts of 2‐thiobarbituric acid, i.e. (VI) and (VII). In the crystal structures of 2,4‐diaminopyrimidine, i.e. (I)–(IV), R₂²(8) N—H…N hydrogen‐bond motifs are preferred and, in two structures, additional R₃²(8) patterns were observed.