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

The title salt, C(16)H(18)N(6)O(2)(2+)center dot 2ClO(4)(-)center dot 2H(2)O, was obtained unintentionally as a major product in the reaction of Zn(ClO(4))(2)center dot 6H(2)O with the N',N'(2)-bis[(1E)-1-(2-pyridyl)ethylidene]ethanedihydrazide (H(2)L) ligand. The (H(4)L)(2+) cation lies across a centre of inversion. The pyridiniumimine fragments of (H(4)L)(2+) adopt syn orientations. Intramolecular N-H center dot center dot center dot N and N-H center dot center dot center dot O hydrogen bonds lead to the formation of S(5) motifs. In the crystal, neighbouring cations are connected by pi-pi interactions between pyridinium units with a centroid-centroid distance of 3.600 (1) angstrom. Moreover, the crystal components are assembled into two-dimensional layers via N-H center dot center dot center dot O and O-H center dot center dot center dot O hydrogen bonds, with no direct hydrogen-bonding interactions between cations.