Linear hydrogen-bonded molecular tapes in the cocrystals of squaric acid with 4,4'-dipyridylacetylene and 1,2-bis(4-pyridyl)ethylene.

The title compounds, 4,4'-(ethyne-1,2-diyl)dipyridinium bis(squarate), C(12)H(10)N(2)(2+).2C(4)HO(4)(-), and 4,4'-(ethene-1,2-diyl)dipyridinium bis(squarate), C(12)H(12)N(2)(2+).2C(4)HO(4)(-), are isomorphous and crystallize in space group P-1. The cocrystals contain linear hydrogen-bonded molecular tape structures along the [120] direction. The squarate monoanions form a ten-membered dimer linked by two intermolecular O-H.O hydrogen bonds. Each component molecule forms a segregated stack along the c axis. The bond lengths of the squarate monoanion indicate delocalization of the enolate anion.