Melaminium sulfate

In the title compound, C3H8N6 2+·SO4 2−, the melaminium cations and sulfate anions are interconnected by N—H⋯N and N—H⋯O hydrogen bonds, forming a layer in the (101) plane. The layers are connected through multiple hydrogen bonds and π–π stacking interactions (centroid–centroid distance of about 3.4 Å).

In the title compound, C 3 H 8 N 6 2+ ÁSO 4 2À , the melaminium cations and sulfate anions are interconnected by N-HÁ Á ÁN and N-HÁ Á ÁO hydrogen bonds, forming a layer in the (101) plane. The layers are connected through multiple hydrogen bonds andstacking interactions (centroid-centroid distance of about 3.4 Å ).

Comment
Salts of melamine and its derivatives can develop supramolecular structures via hydrogen bonding by self assembly. Monoprotonated melaminium sulfate hydrate, (C 3 H 7 N 6 ) 2 SO 4 ·H 2 O, has been structurally investigated (Janczak & Perpétuo, 2001b). We present here the solid state structure of anhydrous diprotonated melaminium salt.  (Dewar et al., 1985) on the melaminium residue diprotonated at two ring N atoms, results in almost the same geometrical features as being found in the title compound. The distortion of the aromatic ring is quite similar to that reported for the melaminium diperchlorate monohydrate salt (Martin & Pinkerton, 1995), as well as for melaminium bis(4-hydroxybenzene-sulfonate) dihydrate (Janczak & Perpétuo, 2001a).
The melaminium residue is involved in eleven hydrogen bonds, two N-H···N bonds with the neighbouring melaminium residue and nine N-H···O bonds with six neighbouring SO 4 2− anions. Two of the SO 4 2− anions are acceptors of two and three hydrogen bonds, respectively, while the other four are acceptors of one hydrogen bond each (Fig. 2). The H atoms at the protonated N atoms of the melaminium residue are involved in almost linear N-H···O hydrogen bonds.
Each SO 4 2− ion is involved as an acceptor in nine hydrogen bonds connecting to six melaminium residues. The O1 and O2 atoms are the most interesting ones as they all accept three hydrogen atoms each. O3 forms two hydrogen bonds with melaminium residues via the H atoms of the amino groups, and O4 forms only one hydrogen bond via H5 atom at the protonated N atom of the melaminium residue.
The melaminium residues are interconnected by two almost linear N-H···N hydrogen bonds and five N-H···O hydrogen bonds. The distance between the centroids of the aromatic rings in adjacent layers (symmetry operator 2 − x, +y, 1.5 − z) is about 3.4 Å, which is much shorter than the maximum distance for the π-π stacking interaction (3.8 Å for centroid-centroid distance), indicating strong π-π stacking interactions. The two-dimensional layers are extensively interconnected by multiple hydrongen bonds with sulfate anions and π-π stacking interactions (Fig. 3).
Experimental 0.126 g (0.001 mol) of melamine was dissolved in 50 ml hot water. To this solution 4 ml 98% sulfate acid was slowly added.
After several days, colorless crystals of (I) appeared.

Refinement
The H atoms bonded to the ring N atoms were located in difference Fourier map and their positions and displacement parameters were refined freely. The amino H atoms were added geometrically and treated as riding, with N-H = 0.86 Å and U iso (H) = 1.2U eq (N). Fig. 1. View of the asymmetric unit of (I) showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented by circles of arbitrary size. x, y + 1, z; (ii) −x + 1, −y, −z + 1; (iii) −x + 1, −y + 1, −z + 1; (iv) −x + 1, y, −z + 1/2; (v) −x + 1, y + 1, −z + 1/2; (vi) −x + 3/2, y + 1/2, −z + 1/2; (vii) x, −y, z + 1/2; (viii) x, −y + 1, z + 1/2; (ix) x − 1/2, −y + 1/2, z + 1/2.   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 > 2sigma(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.