Bis(2,4,6-triamino-1,3,5-triazin-1-ium) 2-[bis(carboxylatomethyl)azaniumyl]acetate trihydrate

The title compound, 2C3H7N6 +·C6H7NO6 2−·3H2O, was obtained by mixing melamine and nitrilotriacetic acid in aqueous solution. There is proton transfer from the nitrilotriacteic acid to melamine to produce two melaminium cations and an internal proton transfer to generate the [HN(CH2COO)]2− zwitterion. The melaminium cations are arranged in hydrogen-bonded tapes formed by N—H⋯N interactions. These tapes extend parallel to the [010] direction and are stacked parallel to the a axis at a mean separation of 3.3559 (11) Å. Between these tapes lie the anions and lattice water molecules. Further O—H⋯O and N—H⋯O hydrogen bonds exist between the water molecules, the anions, and the melaminium cations, generating a three-dimensional array. The crystal examined was found to be twinned by a twofold rotation about the direct lattice direction [100]. The two twin components were present in the ratio 0.5918:0.4082 (14).


Comment
Melamine is an organic base with a pronounced tendency to form hydrogen-bonded architectures in combination with carboxylic acids. Many of these compounds display common structural features and normally these are best classed as salts with full proton transfer to form melaminium cations. These melaminium cations then form hydrogen-bonded tapes composed of N-H···N hydrogen bonds. The tapes have a propensity to assemble into π-stacks and between these stacks are located the carboxylates. Several examples with simple alkyl carboxylic acids (see for example Perpétuo & Janczak, 2002) and dicarboxylic acids are known (such as Froschauer andWeil, 2012 &Eppel andBernstein, 2009). Those with three carboxylic acid groups are less common (Huczynski et al., 2009;Eshtiagh-Hosseini et al., 2010;Perpetuo and Janczak, 2003). We set out to explore the effect of introducing multiple carboxylic acid groups with a flexible geometry through the use of nitrilotriacetic acid, N(CH 2 COOH) 3 .
The title compound, 1, crystallizes in the triclinic space group P1. The asymmetric unit (illustrated in Figure 1) contains two crystallographically independent melaminium cations, the nitrilotriacetate anion and three water molecules. The crystal examined was found to be twinned by a 2-fold rotation about [100] corresponding to the twin law (1 0 0 0.422 -1 0 0.051 0 -1). It was possible to index the diffraction images using two twin domains. The integration procedure employed both twin domains. The structure was refined using all observed data using the HKLF5 formalism within SHELXL97. The relative amount of the two components was refined to be 0.5918: 0.4082 (14).
The assignment of hydrogen bonding between different components was greatly facilitated by the identification of hydrogen atoms with final difference Fourier maps. Examination of hydrogen locations reveals that crystallization of this compound is associated with proton migration from acid to the base. All three of the carboxylic acid groups are deprotonated. Two carboxylic protons migrate to one of the endocyclic N atoms in each melamine moiety giving rise to two melaminium cations. The third carboxylic acid proton migrates to the central nitrogen in the nitrilotriacetic acid moiety to generate an anion correctly called 2,2′,2′′-ammoniotriacetate. As in similar cases full proton transfer means these compounds are best described as salts rather than co-crystals. The assignment of the carboxylates is in line with work of Childs et al. (2007)  The two melaminium cations form hydrogen bonded tapes sustained by N-H···N interactions, common to many other similar compounds. Pairs of N-H···N interactions between adjacent melaminium cations form embraces with graph set notation R 2 2 (8) (Etter et al., 1990). There are two symmetry independent embraces of this type which assemble the melaminium cations into tapes that extend parallel to the crystallographic [010] direction. These tapes are stacked parallel to the a axis at a mean separation of a/2 (3.3559 (11) Å) suggestive of π-stacking. A single tape is illustrated in Figure 2.
Between the tapes lie the [HN(CH 2 COO) 3 ] 2anions and water molecules. Each carboxylate acts as a hydrogen bond acceptor to amino groups of the melaminium cations. In this way the anions are involved in N-H···O hydrogen bonds between different tapes within the same stack and between different stacks. Additional N-H···O and O-H···O hydrogen bonds formed by the water molecules present further help to knit together the cations and anions. Full details of the hydrogen bonds present are contained within Table 1. The crystal packing within 1 is represented in Figure 3.
The crystal structures of melamine with monocarboxylic acids form a well established family of compounds. Similarly linear dicarboxylic acid salts of melamine are well illustrated. These have similar structural features (the π-stacking of tapes of melaminium held together by R 2 2 (8) embraces). The structure of 1 shows analogous features to those of simpler carboxylates.

Experimental
Melamine (0.334 g, 2 mmol) and nitrilotriacetic acid (0.249 g,1 mmol) were dissolved in 50:50 ethanol:water (20 ml) and stirred for 15 min. The solution was left unperturbed for slow solvent evaporation in suitably sized vials. After approximately 3 days, colourless needle-shaped crystals were obtained.

Refinement
The crystal examined was found to be twinned by a 2-fold rotation about [100]. The twinning was apparent in the diffraction images. The twin law was identified by inspection of reciprocal space within X-AREA (Stoe & Cie, 2002). It was possible to index the diffraction images using two twin domains. The integration procedure employed both twin domains. The structure was refined using all observed data using the HKLF5 formalism within SHELXL97. The relative amount of the two components was refined to be 0.5918: 0.4082 (14). Refinement using a single twin domain was not satisfactory as approximately 40% of the spots from the first domain were overlapped by those from the second domain.
Hydrogen atoms were located in difference Fourier maps. Those on the organic components were placed with a riding model with U iso set to 1.2 times the U eq of the atom on which they ride. Hydrogen atoms attached to water were refined subject to sensible bond length and bond angle restraints with U iso values set at 1.5 times the U eq of the central oxygen atom.

Special details
Experimental. a face indexed abosorption correction was applied. this utilised the Tompa method implmented within Stoe X-Area. 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.7947 (