4,4′-Bipyridine–2-hydroxypropane-1,2,3-tricarboxylic acid (3/2)

The combination of 2-hydroxypropane-1,2,3-tricarboxylic acid (H3hypta, also called citric acid) and 4,4′-bipyridine (4,4′-bipy) in a 1:1.5 molar ratio leads to the formation of the title molecular cocrystal, 1.5C10H8N2·C6H8O7. The asymmetric unit contains one and a half 4,4′-bipy units, with one lying across a centre of inversion, and one H3hypta molecule. The significant differences in the C—O bond distances support the existence of the un-ionized acid molecule and confirm the formation of a cocrystal. There are π–π and C—H...π stacking interactions between the aromatic rings of 4,4′-bipy [with interplanar distances of 3.7739 (8) and 3.7970 (8) Å] and between CH groups of H3hypta [with an H...π distance of 2.63 Å]. In the crystal structure, intermolecular O—H...N hydrogen bonds occur and an O—H...O hydrogen bond occurs within the citric acid moiety.

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