Crystal structure of melaminium cyanoacetate monohydrate

The crystal of the melaminium salt C3H7N6 +·NCCH2COO−·H2O was produced by mixing melamine with cyanoacetic acid in aqueous solution. The melaminium cations are interconnected by N—H⋯N hydrogen bonds, forming tapes. These tapes of melaminium cations develop a three-dimensional network through multiple donor–acceptor hydrogen-bonding interactions between the cyanoacetate anions and water molecules.


Chemical context
Melamine (systematic name: 2,4,6-triamino-1,3,5-triazine), a trimer of cyanamide, has many industrial applications. The cross-linked resins of melamine with formaldehyde have applications in adhesive coatings, laminations and flame retardants (Billmeyer, 1984). In the past, various organic melamine salts were tested as potential melamine substitutes for melamine urea formaldehyde resins (Weinstabl et al., 2001). In general, protonation of melamine with organic and inorganic acids has been found to yield compounds with extensive hydrogen-bonding networks involving both N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds. This paper is a part of our investigation of the chemistry of cyanoacetate with nitrogen-based cations and their potential application as flame retardants since cyanoacetic acid is an analogue to polyacrylonitrile. It is well known that polyacrylonitrile is used in industry to manufacture carbon fibers because of its ability to produce carbon char (Bacon & Hoses, 1986). Cyanoacetic acid has a nitrile group and also can act as acid source, both of which could enhance the flame-retarding properties.
In the anion, both O atoms of the carboxylate group are involved in hydrogen bonds to amino groups of adjacent melaminium ions. The nitrile group has a bond length of 1.145 (2) Å that is typical of a nitrile (Kanters et al., 1978). The angle at the nitrile carbon, N C-C, is 179. 30 (19) which is close to the theoretical value of 180 . The O atom of the water molecule acts as a lone-pair donor to the protonated nitrogen of the melaminium ion that is present in the same eightmembered ring. The presence of the water molecule in the structure of melaminium cyanoacetate can be expected to contribute to fire retardancy as its release and evaporation will provide cooling.

Figure 1
Molecular structure of the title compound, showing 50% probability displacement ellipsoids and the atom-numbering scheme. Hydrogen atoms are shown as spheres of arbitrary radius and hydrogen bonds as dashed lines. symmetry codes as in Table 1) to form a tape-like structure propagating along [110] and running between the cyanoacetate anions. Three N-HÁ Á ÁO hydrogen bonds (N7-H7AÁ Á ÁO13 i , N8-H8AÁ Á ÁO11 and N9-H9AÁ Á ÁO13 iii ; Table 1) link the cation with three different cyanoacetate anions. Furthermore, the cation is also connected with a water molecule via an N-HÁ Á ÁO hydrogen bond (N2-H2Á Á ÁO1S) between the protonated imine and the water O atom. Finally, the cation is linked with the nitrile group of the anion via an N-HÁ Á ÁN hydrogen bond (N7-H7BÁ Á ÁN16 ii ; Table 1). There also exist O-HÁ Á ÁO (O1S-H1SAÁ Á ÁO11 and O1S-H1SBÁ Á ÁO11 vi ) hydrogen bonds between the water molecule and the anion. In addition, a C-HÁ Á ÁO hydrogen bond between the methylene H and water O atoms is observed as the C-H group is activated because of the electron-withdrawing cyano group adjacent to it. Altogether, these hydrogen bonds existing between the cations, anions and water molecules generate a three-dimensional network (Fig. 2).

Synthesis and crystallization
A solution of cyanoacetic acid (1.7g, 20 mmol) in 100 ml of deionized water was added to a solution of melamine (2.5 g, 20 mmol) in 100 ml of deionized water. The reaction mixture was heated to 353 K for 3 h. The resulting clear solution was cooled to room temperature and then was allowed to slowly evaporate. Single crystals of the title compound formed after several days.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were initially determined by geometry (C-H = 0.99 Å ) and were refined using a riding model, with U iso (H) = 1.2U eq (C). H atoms bonded to N and O were located in a difference map, and their positions were refined freely, with U iso (H) = 1.2U eq (N or O).  Computer programs: SMART and SAINT (Bruker, 2007), SHELXT (Sheldrick, 2015a), SHELXL2018/3 (Sheldrick, 2015b), SHELXTL (Sheldrick, 2008) and CrystalMaker (Palmer, 2014 Data collection: SMART (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008) and CrystalMaker (Palmer, 2014); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.