Biphenyl-2,2′-dicarboxylic acid and tripropylamine, which were purchased from Aladdin Corporation, were mixed with a molar ratio of 1:4 and dissolved in the water/ethanol solvent (1:2 v/v). The mixture was vigorously stirred for 1 h at room temperature to get a yellow solution. After setting aside for 1 week at room temperature, yellow block crystals of the title compound were obtained.

In the crystal structure, non-hydrogen atoms were refined with anisotropic displacement parameters, and all H atoms bonded to C atoms were positioned at calculated positions, riding on the parent atom. The H atoms bonded to O and N atoms were located in difference maps and refined with the riding model of the fixed O—H distance of 0.86 ± 0.01 Å and N—H of 0.93 ± 0.01 Å.

As an aromatic acid with two carboxyl groups, biphenyl-2,2′-dicarboxylic acid can be used to interact with different compounds to obtain various crystal structures [1], [2], [3]. However, the crystal structure of biphenyl-2,2′-dicarboxylic acid and tripropylamine has not been reported till now. Herein we report the related title crystal structure, which can help to understand the crystallography of biphenyl-2,2′-dicarboxylic acid and the deprotonated anions under basic conditions. Due to its two carboxyl groups, biphenyl-2,2′-dicarboxylic acid tends to form the related anions [2, 3].

In the asymmetric unit of the title compound, there are one molecule of biphenyl-2,2′-dicarboxylic acid, two biphenyl-2,2′-dicarboxylate monoanions and two tripropyl-ammonium cations. The two anions display very similar configurations with the similar interplanar angles of 81.7° and 83.5° and the torsion angles of 51.7° and 56.0° between the carboxyl groups and the related benzene rings. The neutral molecule shows a smaller interplanar angle of 65.5° and the carboxyl groups are almost coplanar with the adjacent ring with angles of 6.1° and 10.2° between the planes. Obviously, the deprotonation of biphenyl-2,2′-dicarboxylic acid result in very different configurations between the neutral molecule and the related anions. It can be observed that the neutral molecule mentioned above can link two related anions with intermolecular O—H⋯O hydrogen bonding to yield a butterfly-like motif, in which the molecule can be regarded as the ‘body’ of the ‘butterfly’ and the two anions are just like two ‘wings’. Furthermore, there exist intramolecular O—H⋯O hydrogen bonds in these two anions to stabilize their spacial configurations. Then the two counterions of tripropylazanium interact with the two anions to generate the final structure with strong N—H⋯O hydrogen bonds, in which the cations are just like two ‘streamers’ decorated on the two ‘wings’. Undoubtedly, various O—H⋯O and N—H⋯O hydrogen bonding are crucial factors in forming the crystal structure of the title compound.

We thank for the support from Henan University of Traditional Chinese Medicine.