Bis(melaminium) succinate succinic acid monosolvate dihydrate

The asymmetric unit of the solvated title salt, 2C3H7N6 +·C4H4O4 2−·C4H6O4·2H2O, contains one essentially planar melaminium (2,4,6-triamino-1,3,5-triazin-1-ium) cation (r.m.s. deviation of the non-H atoms = 0.0097 Å), one-half of a succinate anion, one-half of a succinic acid solvent molecule and one water molecule of crystallization; full molecules are generated by inversion symmetry. Supramolecular layers parallel to (12-1) are formed through extensive intermolecular hydrogen bonding of the types O—H⋯O, N—H⋯N and N—H⋯O between the components.

The pK a values of 4.21 and 5.72 for the first and second deprotonation step of succinic acid (the pK a of the first deprotonation step of melamine is 5.10) led to a doubly deprotonated anion in the title compound, bis-melaminium succinate succinic acid solvate dihydrate, 2(C 3 H 7 N 6 ) +. C 4 H 4 O 4 -. C 4 H 6 O 4 . 2(H 2 O). However, besides lattice water, there is also one succinic acid solvent molecule present in the unit cell. The succinic acid molecule and the succinate anion are located with their central C-C bond on inversion centres. As observed for all single protonated melaminium cations, the protonation of melamine takes place at one of the triazine N ring atoms (Fig. 1).
The melaminium cation is essentially planar with a r.m.s. deviation of 0.0097 Å. Likewise, the anion (r.m.s. deviation 0.039 Å) and the succinic acid molecule (r.m.s. deviation 0.060 Å) can be considered as planar. The angles between the least-squares planes of 6.59 (9) ° and 5.76 (12) ° between the anion and the cation and the succinic acid molecule, respectively, lead to the formation of supramolecular layers where cations are arranged in rows alternating with rows of anions, succinic acid solvent and lattice water molecules (Fig. 2). Extensive intermolecular hydrogen bonding of the types O-H···O, N-H···N and N-H···O between the molecular components is present. Details are reported in Table 1.
The motif for the hydrogen-bonded assembly of two melaminium cations in such a layer is the same as in the hydrogenmalonate salt and other melaminium salts (Froschauer & Weil, 2012). 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 proton of the carboxy group of the succinic acid solvent molecule was refined with a distance restraint O-H = 1.00 (2) Å; H atoms of the water molecule were likewise refined with a distance restraint of O-H = 0.88 (2) Å.

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.