Crystal structure and Hirshfeld surface analysis of 2-amino-3-hydroxypyridin-1-ium 6-methyl-2,2,4-trioxo-2H,4H-1,2,3-oxathiazin-3-ide

In the crystal of the title compound, C9H11N3O5, the 2-amino-6-hydroxypyridin-1-ium cations and 6-methyl-2,2,4-trioxo-2H,4H-1,2,3-oxathiazin-3-ide anions are held together through N—H⋯O, N—H⋯N, O—H⋯O and C—H⋯O hydrogen bonds.


Chemical context
Food additives are substances added intentionally to foodstuffs to perform certain functions such as to impart colour, to sweeten or preserve. They play an essential role in the modern food industry, supporting quality and safety. In this context, artificial sweeteners are widely used in food, beverage, confectionery and pharmaceutical products throughout the world (Clauss & Jensen, 1973;Ni et al., 2009). Oxathiazinone dioxide, systematic name 6-methyl-1,2,3-oxathiazin-4(3H)one 2,2-dioxide and also known as 6-methyl-3,4-dihydro-1,2,3oxathiazin-4-one 2,2-dioxide or acesulfame, has been widely used as a non-caloric artificial sweetener (Duffy & Anderson, 1998) since 1988, after the FDA (US Food and Drug Administration) granted approval. Many countries have approved the use of acesulfame-K in soft drinks, toothpaste, candies, mouthwash, cosmetics and pharmacological preparations (Mukherjee & Chakrabarti, 1997). The chemistry of acesulfame is of interest not only because of its biological importance but also in relation to its coordination properties, since the acesulfame anion offers different donor atoms to metal ions, namely the imino nitrogen, ring oxygen, one carbonyl and two sulfonyl oxygen atoms. To advance the knowledge of such compounds, we report the synthesis, single crystal structure determination and Hirshfeld surface analysis of the 2-amino-3-hydroxypyridinium acesulfamate salt (I).

Structural commentary
A view of the asymmetric unit of (I) with the atom-numbering scheme is shown in Fig. 1. In the acesulfamate anion, the bond ISSN 2056-9890 dimensions correspond to the given structural formula with double bonds C1 O4 and C2 C3 and a single bond C1-C2 (Table 1). A relatively short N1-C1 bond indicates strongconjugation in the N1-C1 O4 fragment. Overall, the bond lengths in this anion compare well with those observed in other acesulfamate salts known from the literature (Yıldırım et al., 2019;Kansız et al., 2019). The six-membered acesulfamate ring adopts an envelope conformation with atom S1 as the flap; its deviation from the basal plane is 0.555 (1) Å . The basal plane of the envelope is slightly twisted, with an O1-C3-C1-N1 torsion angle of 2.2 (2) . The cyclic bond lengths in the 2-amino-3-hydroxypyridinium cation agree well with its aromatic nature. The short N3-C5 distance indicates strong conjugation of the amino N3 atom with the acceptor -system of the pyridinium ring. The cation is almost planar, the largest deviation from the least-squares plane of 0.008 (2) Å is observed for atom C6. The least-squares planes through the cation and the basal atoms of anion form a dihedral angle of 6.47 (11) .

Supramolecular features
The acesulfamate anions are linked to the 2-amino-3-hydroxypyridinium cations by strong N-HÁ Á ÁN and N-HÁ Á ÁO hydrogen bonds, forming centrosymmetric aggregates each consisting of two cations and two anions (

Figure 1
A view of the asymmetric unit of the title compound with the atomnumbering scheme. Displacement ellipsoids are drawn at the 50% probability level. The N-HÁ Á ÁO hydrogen bond is shown as a dashed line.

Figure 3
A view of the crystal packing of the title compound showing the threedimensional system of hydrogen bonds. Methyl H atoms are omitted for clarity. Symmetry codes: These aggregates are linked into a three-dimensional structure by weak O-HÁ Á ÁO hydrogen bonds involving the sulfonyl groups and by C-HÁ Á ÁO contacts (

Database survey
A search of the Cambridge Structural Database (CSD, version 5.39; Groom et al., 2016) gave 54 hits for the oxathiazin moiety.

Hirshfeld surface analysis
In order to visualize the intermolecular interactions in the crystal of (I), Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) was carried out using CrystalExplorer17.5 (Turner et al., 2017). Fig. 4 shows the Hirshfeld surface and the intermolecular contacts of the title compound mapped over d norm in the range À0.5966 to +1.0568 a.u. The red regions (distances shorter than the sum of the van der Waals radii) are apparent around the oxygen atom O4, which participates in the N-HÁ Á ÁO contacts, and around the nitrogen atom N1, which participates in the N-HÁ Á ÁN contacts (Fig. 2, Table 2). The fingerprint plots for (I) are given in Fig. 5 A view of the three-dimensional Hirshfeld surface of the title compound plotted over d norm in the range À0.5966 to +1.0568 a.u.  NÁ Á ÁH/HÁ Á ÁN contacts appear as a pair of characteristic tips in the fingerprint plots; they contribute 10% to the Hirshfeld surface (Table 2).

Synthesis and crystallization
Potassium acesulfamate (1 mmol) was dissolved in 15 mL ethanol and heated to 348 K. To this solution 1 mmol of 2amino-3-hydroxypyridine in 15 mL of ethanol was added slowly under continuous stirring. After the addition, the solution was stirred for another 6 min at the same temperature. The compound thus formed was separated from the solution and then recrystallized from ethanol solution at room temperature. The red needle-shaped crystals obtained were filtered, washed with ethyl acetate and dried, yield 91%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All C-bound hydrogen atoms were placed in idealized positions and refined isotropically using a riding model, with U iso (H) = 1.5U eq (C) for methyl and with U iso (H) = 1.2U eq (C) for other C atoms, C-H = 0.96 Å for methyl and 0.93 Å for sp 2 -hybridized C atoms. All other H atoms were located from the difference map and refined freely.