Melaminium 2,4,6-trihydroxybenzoate dihydrate

In the title compound, C3H7N6 +·C7H5O5 −·2H2O, the melaminium and benzoate ions are approximately planar (r.m.s. deviation of the non-hydrogen atoms is 0.093 Å) and there is a strong C 2 2(8) hydrogen-bonding embrace between them. The centre of symmetry generates a second acid–base pair which is bound to the first by a C 2 2(8) (N—H⋯N) embrace common between melamine molecules in similar compounds. Further extensive hydrogen bonding assembles the components into a three-dimensional hydrogen-bonded network.

In the title compound, C 3 H 7 N 6 + ÁC 7 H 5 O 5 À Á2H 2 O, the melaminium and benzoate ions are approximately planar (r.m.s. deviation of the non-hydrogen atoms is 0.093 Å ) and there is a strong C 2 2 (8) hydrogen-bonding embrace between them. The centre of symmetry generates a second acid-base pair which is bound to the first by a C 2 2 (8) (N-HÁ Á ÁN) embrace common between melamine molecules in similar compounds. Further extensive hydrogen bonding assembles the components into a three-dimensional hydrogen-bonded network.
RLK thanks the University of Hull for the award of a studentship.
The title compound crystallizes in the centrosymmetric space group P2 1 /n with four formula units in the unit cell. The 2,4,6-trihydroxybenzoic acid molecule is deprotonated at the carboxylic acid function. One of the nitrogen atoms of the triazine ring of melamine is protonated. This acid-base pair forms a complementary C 2 2 (8) embrace illustrated in Figure   1. Details of the hydrogen bonding within this structure are given in Table 1. A second acid-base pair is generated by the inversion centre. This forms a pair of strong hydrogen bonds to the first through the two melaminium ions. This melaminemelamine C 2 2 (8) embrace is observed in many other compounds involving melamine. This four molecule unit (illustrated in Figure 2) can be thought of as the repeat unit in an infinite chain. These links are held together by weaker, non-classical C-H···O hydrogen bonds between the C6-(H6) and the hydroxyl group (O4) of another acid unit. This is illustrated in  (Allen, 2002) reveals that the distances and geometry displayed here are in good agreement with those previously reported.
One similar example is reported by Bouvet et al. (2007).
Infinite chains formed from this repeat unit are arranged in stacks. The vertical separation between chains is 3.3496 (5) Å. These stacks of chains are arranged along the c axis. The chains within adjacent stacks are alternately parallel to the [120] and [120] directions. The angle between chains parallel to [120] and [120] is 33.242 (8) °. A view of part of the structure down the crystallographic c axis is shown in Figure 4. Between these stacks a large number of classical hydrogen bonds are formed. These are reinforced by the presence of the two water molecules. Full details are given in Table 1.

Experimental
A solution of melamine and 2,4,6-trihydroxybenzoic acid (0.012 mol dm -3 in each component) was prepared in deionized water. 5 mL portions of this solution were allowed to evaporate at room temperature in air from suitably sized vials. After a period of approximately two weeks, good sized colourless crystals were obtained.

Refinement
The data were of sufficient quality to allow identification of all the hydrogen atoms within a difference Fourier map once the heavier atoms had been located. The positions and displacement parameters of the hydrogen atoms were refined independently subject to soft restraints that chemically equivalent bonds should have similar lengths.      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 Rfactors(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.