Cocrystallization of Trimesic Acid and Pyromellitic Acid with Bent Dipyridines

Bent dipyridyl bases, 2,5-bis(3-pyridyl)-1,3,4-oxadiazole (3-bpo) and its 4- N -donor analogue 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-bpo), were applied to crystallize with trimesic acid (benzene-1,3,5-tricarboxylic acid, H 3 TMA) and pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid, H 4 BTA), affording new binary molecular cocrystals [(H 3 TMA)·(3-bpo)]·DMF ( 1 ), [(H 3 TMA)·(4-bpo)] ( 2 ), [(H 4 BTA)·(3-bpo)]·2H 2 O ( 3 ), and a partly charge-transfer salt [(H 2 BTA) 1/2 ·(H-4-bpo)·(H 4 BTA) 1/2 ]·H 2 O ( 4 ) under general conditions. An unexpected compound 5 (pyromellitic dihydrazide) was obtained during the attempt to attain a cocrystal of 4-bpo and H 4 BTA by hydrothermal synthesis. X-ray single-crystal diffraction studies reveal that supramolecular synthon I [ (7)] containing classical O−H···N and weak C−H···O interactions is again testified to be involved in constructing the binary hydrogen-bonding networks 1 − 4, in which the heteromeric intermolecular interactions are often utilized as effective synthetic tools to provide a significant portion of the stabilization energy of molecular crystals. In compound 1, 1-D acid/base zigzag chains formed by synthon I are extended to a 2-D layered architecture via additional C−H···O interactions, the cavities of which are occupied by guest solvents ( N, N -dimethylformamide, DMF). Compound 2 displays fascinating 1-D triple-helical motifs via synthon I (formation of the single strand helix) and C−H···O interactions, which are further expanded to a 3-D supramolecular array by multiple hydrogen bonds. Self-recognition via hydrogen bonding has been evidently embodied in this unique supramolecular system. In compounds 3 and 4, water moieties are present in both resultant assemblies and play different roles in the formation of 3-D supramolecular networks. For 3, each water molecule acts as a 3-connected node, linking three 1-D acid/base double chains resulting from the combination of synthon I and C−H···N bonds, to afford a 3-D sandwich network. For 4, water molecules only behave as guests to further stabilize the robust 3-D hydrogen-bonding host framework. Compound 5 has a 2-D sheet structure via a pair of strong head-to-tail N−H···O interactions. The thermal stability of cocrystals 1 − 4 has been investigated by thermogravimetric analysis of mass loss.