Crystal structure of 2-(aza-niumylmeth-yl)pyridinium bis(hydrogen squarate).

The asymmetric unit of the title compound, C6H10N22+·2C4HO4-, comprises two hydrogen squarate (Hsq-; systematic name: 2-hy-droxy-3,4-dioxo-cyclo-butano-late) anions and a 2-(aza-niumylmeth-yl)pyridinium dication. The squaric acid mol-ecules each donate an H atom to the N atoms of the pyridine ring and the amino-methyl units of a 2-(amino-meth-yl)pyridine mol-ecule, forming the 1:2 salt. The Hsq- anions are linked by strong O-H⋯O hydrogen bonds and an N-H⋯O hydrogen bond links the 2-(aza-niumylmeth-yl)pyridinium cation to one of the squaric acid anions. The crystal structure features additional N-H⋯O and O-H⋯O hydrogen bonds, π-π stacking and unusual weak C-O⋯π(ring) inter-actions.