2-(4-Aminobenzenesulfonamido)-4,6-dimethylpyrimidin-1-ium 2-carboxy-4,6-dinitrophenolate

In the structure of the phenolate salt of the sulfa drug sulfamethazine with 3,5-dinitrosalicylic acid, C12H15N4O2S+·C7H3N2O7 −, the dihedral angle between the pyrimidine and benzene rings of the cation is 59.70 (17)°. In the crystal, cation–anion hydrogen-bonding interactions involving pyrimidine–carboxy N+—H⋯O and amine–carboxy N—H⋯O pairs give a cyclic R 2 2(8) motif while secondary N—H⋯O hydrogen bonds between the aniline group and both sulfone and nitro O-atom acceptors give a two-dimensional structure extending in (001).

In the structure of the phenolate salt of the sulfa drug sulfamethazine with 3,5-dinitrosalicylic acid, C 12 H 15 N 4 O 2 S + Á-C 7 H 3 N 2 O 7 À , the dihedral angle between the pyrimidine and benzene rings of the cation is 59.70 (17) . In the crystal, cation-anion hydrogen-bonding interactions involving pyrimidine-carboxy N + -HÁ Á ÁO and amine-carboxy N-HÁ Á ÁO pairs give a cyclic R 2 2 (8) motif while secondary N-HÁ Á ÁO hydrogen bonds between the aniline group and both sulfone and nitro O-atom acceptors give a two-dimensional structure extending in (001).

Comment
The drug sulfamethazine (or sulfadimidine) [4-amino-N-(4,6-dimethylpyrimidin-2-yl)benzenesulfonamide] (O′Neil, 2001) has been used as a model for co-crystal formation (Caira, 2007;Ghosh et al., 2011), commonly forming 1:1 adducts with carboxylic acids, predominently the benzoic analogues but including some amides. The structures of a number of these have been reported, e.g. anthranilic acid and 4-aminobenzoic acid (Caira, 1991), 2,4-dinitrobenzoic acid (Lynch et al., 2000), as well as benzamide, 4-hydroxybenzamide and picolinamide (Ghosh et al., 2011). In all of these cocrystals, heterodimers are formed through a cyclic intermolecular hydrogen-bonding motif [graph set R 2 2 (8) (Bernstein et al., 1995)], involving amine N-H···O carboxyl and carboxylic acid O-H···N pyrimidine pairs. However, there are no examples of the structures of proton-transfer salts of sulfamethazine with carboxylic acids so we looked at the products from the 1:1 stoichiometric reactions with some strong acids. Crystalline materials were obtained from the 5-nitrosalicylic acid and picric acid reactions, namely the anhydrous (1:1) carboxylate and picrate salts, respectively (Smith & Wermuth, 2013). With 3,5-dinitrosalicylic acid (DNSA), the poorly-formed anhydrous 1:1 salt of the title compound, C 12 H 15 N 4 O 2 S + C 7 H 3 N 2 O 7 -, was obtained, and the structure is reported herein. DNSA has been particularly useful in providing crystalline proton-transfer salts with both aliphatic and aromatic amines, the majority of which have been picrates, in which an anti-related acidic proton is retained on the carboxylic acid group rather than on the phenolic group (Smith et al., 2003).
With the title salt, the phenolate anion is found (Fig. 1), providing a variant of the R 2 2 (8) cation-anion hydrogenbonding interaction as found in the non-transfer co-crystal structures, the difference arising from the presence of the transferred acid proton on the pyrimidine nitrogen (N1A). The slight asymmetry in the N1A···O and N2A···O hydrogen bond distances [2.622 (5) and 2.732 (4) Å] (Table 1) is comparable with those in the non-transfer co-crystals. In the DNSA anion, the anti-related acid proton forms the usual intramolecular hydrogen bond with the phenolate O-atom (Smith et al., 2003). Both H-atoms of the aniline group of the cation participate in intermolecular N-H···O hydrogenbonding interactions with both sulfone and nitro O-atom acceptors, giving extensions along the a and b axes respectively, giving a two-dimensional structure lying along (001) (Fig. 2).
Partial evaporation of the solvent gave poorly-formed yellow crystal plates (m.p. 457-458 K) from which a specimen was cleaved for the X-ray analysis.

Refinement
Hydrogen atoms potentially involved in hydrogen-bonding interactions were located by difference methods but their positional and isotropic displacement parameters were subsequently allowed to ride in the refinement with U iso (H) = 1.2U eq (N) or 1.5U eq (O). Other H atoms were included at calculated positions [C-H (aromatic) = 0.93 Å or C-H (methyl) = 0.96 Å] and also treated as riding, with U iso (H) = 1.2U eq (C) aromatic or 1.5U eq (C) methyl .   The two-dimensional network structure viewed down c, showing hydrogen-bonding associations as dashed lines. Nonassociative H atoms are omitted. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.010 Δρ max = 0.87 e Å −3 Δρ min = −0.51 e Å −3 Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.