Melaminium hydrogen malonate

The melaminium (2,4,6-triamino-1,3,5-triazin-1-ium) cation in the title compound, C3H7N6 +·C3H3O4 −, is essentially planar, with a r.m.s. deviation of the non-H atoms of 0.0085 Å. Extensive hydrogen bonding of the types N—H⋯N and N—H⋯O between cations and cations and between cations and hydrogen malonate (2-carboxyethanoate) anions leads to the formation of supramolecular layers parallel to (1-2-1). An intramolecular O—H⋯O hydrogen bond in the single deprotonated malonate anion also occurs.

The X-ray centre of the Vienna University of Technology is acknowledged for financial support and for providing access to the single-crystal diffractometer.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: CV5323).

Melaminium hydrogen malonate Barbara Froschauer and Matthias Weil Comment
Melamine is a weak base with three different pK a values which decline with decreasing protonation status. The first (pK a = 5.10) is slightly above the pK a of acetic acid (4.75), the second and third (0.20 and -2.10, respectively) are significantly below the most organic carboxylic acids. Since the difference between the pK a values during an acid-base reaction corresponds to the free energy of reaction, stable products can only be expected for acids with a pK a value significantly below 5.10, whereas organic acids with acidities in the range of 5.10 or above can be expected to yield mixtures of unreacted melamine, free acid and melaminium salts. Depending on the acid valency and strengths, mono and disalts can be formed by simply heating the components or their respective solutions.
The pK a values of 2.82 and 5.69 for the first and second deprotonation step of malonic acid led to a single deprotonated anion in the title compound, melaminium hydrogen malonate, C 3 H 7 N 6 +. C 3 H 3 O 4 -. The protonation of melamine takes place at one of the triazine N ring atoms (Fig. 1) as observed for all other single protonated melaminium salts listed above.
Both the melaminium cation and the hydrogenmalonate anions are essentially planar with r.m.s. deviations of 0.0085 Å (cation) and 0.061 Å (anion) for the non-H atoms. The angle between the two least-squares planes is 6.61 (8) °, making it possible to set up supramolecular layers held together by strong to medium hydrogen bonds of the type N-H···O and N -H···.N between cations and cations and cations and anions ( Fig. 2; Table 1). The motif for the hydrogen-bonded assembly of two melaminium cations is observed in many other melamine or melaminium structures as reported previously by Prior et al. (2009). In the crystal, the supramolecular layers are arranged parallel to (121) (

Refinement
The proton at the triazine ring of the melaminium cation was clearly discernible from a difference Fourier map (like all other H atoms). For refinement, the H atoms attached to C or N atoms were set in calculated positions and treated as riding on their parent atoms with C-H = 0.97 Å and N-H = 0.86 Å and with U iso (H) = 1.2U eq (C,N). The remaining proton of the carboxyl group was refined freely.

Figure 1
The molecular components of the title compound with displacement parameters drawn at the 90% probability level. H atoms are displayed as spheres with an arbirtary radius.   The assembly of supramolecular layers in the crystal parallel to (121).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.