2,2′-{1,1′-[2,2′-Oxalylbis(hydrazin-2-yl-1-ylidene)]diethylidyne}dipyridinium bis(perchlorate) dihydrate

The title salt, C16H18N6O22+·2ClO4−·2H2O, was obtained unintentionally as a major product in the reaction of Zn(ClO4)2·6H2O with the N′,N′2-bis[(1E)-1-(2-pyridyl)ethylidene]ethanedihydrazide (H2L) ligand. The (H4L)2+ cation lies across a centre of inversion. The pyridiniumimine fragments of (H4L)2+ adopt syn orientations. Intramolecular N—H...N and N—H...O hydrogen bonds lead to the formation of S(5) motifs. In the crystal, neighbouring cations are connected by π–π interactions between pyridinium units with a centroid–centroid distance of 3.600 (1) Å. Moreover, the crystal components are assembled into two-dimensional layers via N—H...O and O—H...O hydrogen bonds, with no direct hydrogen-bonding interactions between cations.