Received 27 August 2013
The title compound, 2C3H7N6+·C6H7NO62-·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-HN interactions. These tapes extend parallel to the  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-HO and N-HO 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 . The two twin components were present in the ratio 0.5918:0.4082 (14).
For compounds of melamine with simple carboxylic acid, see, for example: Froschauer & Weil (2012); Eppel & Bernstein (2009); Perpétuo & Janczak (2002). For those with tricarboxylic acids, see: Eshtiagh-Hosseini et al. (2010); Huczynski et al. (2009); Perpetuo & Janczak (2003). For assignment of protonation on the grounds of bond angle and bond length, see: Childs et al. (2007) and Hingerty et al. (1981), respectively. An introduction to graph-set theory may be found in Etter et al. (1990).
Data collection: X-AREA (Stoe & Cie, 2002); cell refinement: X-AREA; data reduction: X-RED (Stoe & Cie, 2002); program(s) used to solve structure: SHELXS86 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: ZL2565 ).
KH thanks the University of Hull for the award of a PhD studentship.
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