Fourteen crystals involving various substituted organic acid molecules and 2-amino-4,6-dimethylpyrimidine (MP)/2-amino-4,6-dimethoxypyrimidine (MOP) were prepared and characterized by using single crystal X-ray diffraction. Among the 14 crystals, proton transfer from the carboxylic acid to pyrimidine base has occurred only in 5 crystals while the remaining were co-crystals. It is obvious that the R 2 2(8) ring motif occurs in co-crystals/salts when an acid interacts with a pyrimidine base. The present work has been focused on this aspect, confining the study to MP and MOP, where the results surely indicate that there are three main synthons, which regularly occur, linear heterotetramer (LHT), cyclic heterotetramer (CHT), and heterotrimer (HT). Among the whole lot, in both the present study and the literature, LHT is dominant as it occurs in 30 out of the existing 54 structures, while CHT and HT correspond to 9 each with the remaining structures forming newer synthons. Our results are also consistent with a recent paper in Crystal Growth and Design (2008, 8, 4031–4044), where LHT is predicted to be more stable among other synthons. Furthermore in the present study, attempts have been made to correlate the formation of salts/co-crystals using ΔpK a for both MP and MOP molecular complexes. The failure to explain MOP using ΔpK a also suggests that the use of Pka values in predicting co-crystals/salts could vary from system to system.

A series of 17 co-crystals involving 2-amino-4-chloro-6-methyl pyrimidine/2-amino-4,6-dimethyl pyrimidine with various carboxylic acids, C7H5O2-R (where R = H, Cl, CH3, NO2, OCH3, COOH) and C9H8O2, have been prepared in which the analogues differ only with respect to chlorine and methyl groups. The basis of this work is the formation of isostructural co-crystals based on the chloro/methyl interchange. The results show good success in the harvest of isostructural compounds. Isostructurality calculations have been carried out in order to substantiate these results. Apart from this some of the compounds have been identified to show good resemblance in their structure, yet do not have any relation based on chloro/methyl interchange. This is explained by the fact that all the co-crystals in the present study tend to form one common supermolecule: a linear heterotetramer. This supermolecule is known to be the most stable and reliable heterotetramer formed between an aminopyrimidine and benzoic acid, according to a recent report, which is believed to be responsible for the isostructurality other than chloro/methyl exchange.

No abstract

Key indicators: single-crystal X-ray study; T = 293 K; mean (C-C) = 0.002 Å; R factor = 0.053; wR factor = 0.174; data-to-parameter ratio = 23.5. organic compounds o516 Ebenezer and Muthiah

In the title co-crystal C6H9N3O2·C10H9NO2, the 4,6-dimeth­oxy­pyrimidin-2-amine mol­ecule inter­acts with the carboxyl group of the 2-(1H-indol-3-yl)acetic acid mol­ecule through N—H⋯O and O—H⋯N hydrogen bonds, forming a cyclic hydrogen-bonded R 2 2(8) motif, which is further linked by an N—H⋯N hydrogen bond, forming a supra­molecular chain along the c axis. Neighboring chains are inter­linked via C—H⋯O hydrogen bonds, forming a supra­molecular ladder