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The asymmetric unit of the title compound, C 5 H 7 N 2 O + ·C 4 H 4 NO 4 S - , contains one cation and one anion. The 6-methyl-2,2,4-trioxo-2 H ,4 H -1,2,3-oxa-thia-zin-3-ide anion adopts an envelope conformation with the S atom as the flap. In the crystal, the anions and cations are held together by N-H⋯O, N-H⋯N, O-H⋯O and C-H⋯O hydrogen bonds, thus forming a three-dimensional structure. The Hirshfeld surface analysis and fingerprint plots reveal that the crystal packing is dominated by O⋯H/H⋯O (43.1%) and H⋯H (24.2%) contacts., (© Kansiz et al. 2020.)

The crystal structure determination of the dinicotinamidium squarate salt, 2C(6)H(7)N(2)O(+).C(4)O(4)(2-), is reported, with the squarate dianion residing on an inversion centre and the unique cation in a general position. Salt formation occurs by donation of two H atoms from squaric acid to the nicotinamide base. The crystal packing is derived from three types of hydrogen bonding. The primary hydrogen bond involves a squarate anion O atom and an H atom of the protonated pyridine group of the nicotinamide, with an N.O distance of 2.5760 (13) A. The second hydrogen bond involves a second anion O atom and an amide H atom, with an N.O distance of 2.8374 (14) A. Thirdly, an intermolecular interaction between two coplanar nicotinamide moieties occurs between an amide O atom and a symmetry-related amide H atom, with an N1.O3 distance of 2.8911 (15) A. These hydrogen bonds are also responsible for the planarity of the nicotinamide moiety in the salt.