A Search for Predictable Hydrogen-Bonding Synthons in Cocrystallization of Unusual Organic Acids with a Bent Dipyridine

A bent dipyridyl compound, 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-bpo), was employed to crystallize with
1,4-cyclohexanedicarboxylic acid (H2chdc), hydroquinone, citric acid (H4ca·H2O), barbituric acid, and ferrocene dicarboxylic acid
(H2Fc) under general conditions, affording binary cocrystals [(H2chdc)·(4-bpo)] (1), [(hydroquinone)·(4-bpo)2] (2), [(H4ca)·(4-bpo)]
(3), [(barbituric acid)·(4-bpo)] (4), and [(HFc)·(H-4-bpo)] (5) in high yield. X-ray single-crystal structures of these compounds
reveal that a supramolecular synthon [

(7)] containing classical O−H···N and weak C−H···O interactions, usually observed in
organic cocrystals of carboxylic acid and heterocyclic base, is again involved in constructing the hydrogen-bonding networks of 1,
3, and 5. Its ionic analogous synthon [consisting of strong N−H···O and weak C−H···O,

(7)] also appears in the partly charge-transfer salt 5. For 2, only hydroxyl of the hydroquinone component can act as the hydrogen-bonding contributor, giving a strong
O−H···N bond with 4-bpo. Multiple hydrogen-bonded donating/accepting sites in compound 3 fulfill the formation of new
supramolecular patterns. For 4, another heterosynthon ring [containing strong N−H···N and weak C−H···O,

(7)] is formed due
to the presence of the imide functional group. As a consequence, robust hydrogen-bonding interactions in these compounds afford
diverse 2-D waved or planar layers (1 or 4), 1-D extended tape (2), 3-D net (3), and 1-D molecular-box type (5) supramolecular
architectures. Thermal stability of these compounds has been investigated by thermogravimetric analysis (TGA) of mass loss.