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Three new compounds, namely, 5‐fluorocytosine–urea (2/1), 2C4H4FN3O·CH4N2O, (I), 5‐fluorocytosine–5‐fluorocytosinium 3,5‐dinitrosalicylate–water (2/1/1), 2C4H4FN3O·C4H5FN3O+·C7H2N2O7−·H2O, (II), and 2‐amino‐4‐chloro‐6‐methylpyrimidine–6‐chloronicotinic acid (1/1), C6H4ClNO2·C5H6ClN3, (III), have been synthesized and characterized by single‐crystal X‐ray diffraction. In compound (I), 5‐fluorocytosine (5FC) molecules A and B form two different homosynthons [R22(8) ring motif], one formed via N—H...O hydrogen bonds and the second via N—H...N hydrogen bonds. In addition to this interaction, a sequence of fused‐ring motifs [R21(6), R33(8), R22(8), R43(10) and R22(8)] are formed, generating a supramolecular ladder‐like hydrogen‐bonded pattern. In compound (II), 5FC and 5‐fluorocytosinium are linked by triple hydrogen bonds, generating two fused‐ring motifs [R22(8)]. The neutral 5FC and protonated 5‐fluorocytosinum cation form a dimeric synthon [R22(8) ring motif] via N—H...O and N—H...N hydrogen bonds. On either side of the dimeric synthon, the neutral 5FC, 5‐fluorocytosinium cation, 3,5‐dinitrosalicylate anion and water molecule are hydrogen bonded through N—H...O, N—H...N, N—H...OW and OW—HW...O hydrogen bonds, forming a large ring motif [R1010(56)], leading to a three‐dimensional supramolecular network. In compound (III), 2‐amino‐4‐chloro‐6‐methylpyrimidine (ACP) interacts with the carboxylic acid group of 6‐chloronicotinic acid via N—H...O and O—H...O hydrogen bonds, generating an R22(8) primary ring motif. Furthermore, the ACP molecules form a base pair via N—H...N hydrogen bonds. The primary motif and base pair combine to form tetrameric units, which are further connected by Cl...Cl interactions. In addition to this hydrogen‐bonding interaction, compounds (I) and (III) are further enriched by π–π stacking interactions. [ABSTRACT FROM AUTHOR]

Four salts, namely, 2,4,6‐triaminopyrimidinium 6‐chloronicotinate dihydrate, C4H8N5+·C6H3ClNO2−·2H2O, (I), 2,4,6‐triaminopyrimidinediium pyridine‐2,6‐dicarboxylate dihydrate, C4H9N52+·C7H3NO42−·2H2O, (II), 2,4,6‐triaminopyrimidinediium sulfate monohydrate, C4H9N52+·SO42−·H2O, (III), and 2,4,6‐triaminopyrimidinium 3,5‐dinitrobenzoate dihydrate, C4H8N5+·C7H3N2O6−·2H2O, (IV), were synthesized and characterized by X‐ray diffraction techniques. Proton transfer from the corresponding acid to the pyrimidine base has occurred in all four crystal structures. Of the four salts, two [(I) and (IV)] exist as monoprotonated bases and two [(II) and (III)] exist as diprotonated bases. In all four crystal structures, the acid interacts with the pyrimidine base through N—H...O hydrogen bonds, generating an R22(8) ring motif. The sulfate group mimics the role of the carboxylate anions. The water molecules present in compounds (I)–(IV) form water‐mediated large ring motifs. The formation of water‐mediated interactions in these crystal structures can be used as a model in the study of the hydration of nucleobases. Water molecules play an important role in building supramolecular structures. In addition to these strong hydrogen‐bonding interactions, some of the crystal structures are further enriched by aromatic π–π stacking interactions [(I) and (II)]. [ABSTRACT FROM AUTHOR]