Synthesis and structure of 2-amino-4-methylpyridin-1-ium hydrogen squarate

The title salt (systematic name: 2-amino-4-methyl-pyridin-1-ium 2-hy-droxy-3,4-dioxo-cyclo-but-1-en-1-olate), C₆H₉N₂ ⁺·C₄HO₄ ^-, was obtained by the proton-transfer reaction between 2-amino-4-methyl-pyridine and squaric acid in aqueous solution: protonation occurs at the pyridine nitro-gen atom, while squaric acid is singly deprotonated. In the crystal, the cations and anions are linked by N-H⋯O, O-H⋯O, and C-H⋯O hydrogen bonds, forming infinite layers. These layers are consolidated by π-π stacking inter-actions, resulting in a columnar packing arrangement. Hirshfeld surface analysis reveals that O⋯H/H⋯O contacts dominate the inter-molecular inter-actions, consistent with the hydrogen-bonding network observed in the crystal structure.