Design of Co-crystals/Salts of Aminopyrimidines and Carboxylic Acids through Recurrently Occurring Synthons

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 R22(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 ΔpKa for both MP and MOP molecular complexes. The failure to explain MOP using ΔpKa also suggests that the use of Pka values in predicting co-crystals/salts could vary from system to system.