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

The combination of 2-hydroxy­propane-1,2,3-tricarboxylic acid (H(3)hypta, 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, 1.5C(10)H(8)N(2)·C(6)H(8)O(7). The asymmetric unit contains one and a half 4,4′-bipy units, with one lying across a centre of inversion, and one H(3)hypta 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 H(3)hypta [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.