Bis(2,4,6-triaminopyrimidin-1-ium) sulfate pentahydrate

The asymmetric unit of the title salt, 2C4H8N5 +·SO4 2−·5H2O, contains four 2,4,6-triaminopyrimidinium (TAPH+) cations, two sulfate anions and ten lattice water molecules. Each two of the four TAPH+ cations form dimers via N—H⋯N hydrogen bonds between the amino groups and the unprotonated pyrimidine N atoms [graph-set motif R 2 2(8)]. The (TAPH+)2 dimers, in turn, form slightly offset infinite π–π stacks parallel to [010], with centroid–centroid distances between pyrimidine rings of 3.5128 (15) and 3.6288 (16) Å. Other amino H atoms, as well as the pyrimidinium N—H groups, are hydrogen-bonded to sulfate and lattice water O atoms. The SO4 2− anions and water molecules are interconnected with each other via O—H⋯O hydrogen bonds. The combination of hydrogen-bonding interactions and π–π stacking leads to the formation of a three-dimensional network with alternating columns of TAPH+ cations and channels filled with sulfate anions and water molecules. One of the sulfate anions shows a minor disorder by a ca 37° rotation around one of the S—O bonds [occupancy ratio of the two sets of sites 0.927 (3):0.073 (3)]. One water molecule is disordered over two mutually exclusive positions with an occupancy ratio of 0.64 (7):0.36 (7).

The asymmetric unit of the title salt, 2C 4 H 8 N 5 + ÁSO 4 2À Á5H 2 O, contains four 2,4,6-triaminopyrimidinium (TAPH + ) cations, two sulfate anions and ten lattice water molecules. Each two of the four TAPH + cations form dimers via N-HÁ Á ÁN hydrogen bonds between the amino groups and the unprotonated pyrimidine N atoms [graph-set motif R 2 2 (8)]. The (TAPH + ) 2 dimers, in turn, form slightly offset infinitestacks parallel to [010], with centroid-centroid distances between pyrimidine rings of 3.5128 (15) and 3.6288 (16) Å . Other amino H atoms, as well as the pyrimidinium N-H groups, are hydrogenbonded to sulfate and lattice water O atoms. The SO 4 2À anions and water molecules are interconnected with each other via O-HÁ Á ÁO hydrogen bonds. The combination of hydrogenbonding interactions andstacking leads to the formation of a three-dimensional network with alternating columns of TAPH + cations and channels filled with sulfate anions and water molecules. One of the sulfate anions shows a minor disorder by a ca 37 rotation around one of the S-O bonds [occupancy ratio of the two sets of sites 0.927 (3):0.073 (3)]. One water molecule is disordered over two mutually exclusive positions with an occupancy ratio of 0.64 (7):0.36 (7).
Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT; program(s) used to solve structure: SHELXS2013 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2013 and SHELXLE (Hü bschle et al., 2011); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2013 and publCIF (Westrip, 2010 Pyrimidine and its derivatives are a class of heteroaromatic compounds with exceptional importance for biology, pharmacology and live sciences. An immensely large number of naturally occurring compounds are based on the pyrimidine skeleton, including three of the four DNA and RNA nucleobases, cytosine, thymine, and uracil. The pyrimidine ring, while less basic than equivalent pyridines, can nonetheless be protonated with relative ease, at either one of the two nitrogen ring atoms (double protonation is hampered by near complete loss of basicity upon the first protonation), or at the ring carbon atom (Demeter & Wéber, 2004;Németh et al., 2006). It can also act as a Lewis base in metal complex formation (Zamora et al., 1997;Louloudi et al., 1997;Jolibois et al., 1998;Katritzky et al., 1984), or as an acceptor for strong hydrogen bonding interactions. These three properties of pyrimidines are the key aspects to the wide range of their biological and pharmacological functionalities, both natural as well as synthetic.
Amino-substituted pyrimidines are of interest due to their similarity to the nucleic acids cytosine, adenine and guanine.
Multi-amino substituted derivatives have recently attracted intense attention due to their similarity with melamine, which had been added to dairy products and other food to give a false appearance of a higher protein level. Exposure to high levels of melamine in food can however lead to melamine-induced kidney failure, and adulterated food had been the cause for several severe outbreaks of nephrolithiasis in pets ("2007 pet food recall") and humans ("2008 Chinese milk scandal" with more than 50,000 infant hospitalizations and six deaths) (Wei & Liu, 2012). The cause for the melamineinduced kidney stone formation was found to be a highly insoluble co-crystalline precipitate of melamine and uric acid, a hydrolysis product of melanine itself (Dobson et al., 2008). The insolubility of the melamine-uric acid co-crystal can be largely traced back to dense π-π stacking interactions and the formation of a network of strong N-H···O and N-H···N hydrogen bonds (Whitesides et al., 1991). The propensity of melamine and its derivatives to form tightly hydrogenbonded insoluble networks is thus of great interest.
One such multi-amino substituted pyrimidine derivative is 2,4,6-triaminopyrimidine, which finds, for example, use as an internal standard in testing for melamine in food. In this communication we present the structure of the sulfate salt of 2,4,6-triaminopyrimidine, in the form of its pentahydrate, (C 4 H 8 N 5 ) 2 + SO 4 2-. 5H 2 O, (I).
The asymmetric unit of compound (I) consists of four mono-protonated 2,4,6-triaminopyrimidine cations (TAPH + ), two sulfate anions and ten water molecules (Fig. 1). All four TAPH + cations are protonated at one of the pyrimidine ring nitrogen atoms (N1 atoms in molecules A, B, C and D). As it is common for pyridyl derivatives, the bond angles at the protonated nitrogen are slightly larger than those at the unprotonated nitrogen atoms (Krygowski et al., 2005). methylpyrimidinium, C 6 H 10 N 3 +. HSO 4 -, with an angle at the protonated nitrogen of 122.3 (1)° and of 117.6 (1)° at the unprotonated nitrogen atom, respectively (Hemamalini et al., 2005). S-O bonds lengths of the tetrahedral sulfate anions are in the range from 1.449 (4) Å to 1.471 (4) Å, indicating delocalized SÛO bonds rather than distinct single and double bonds. The amino groups, which are not protonated, are sp 2 hybridized and the NH 2 groups are coplanar with the pyrimidinium rings.
The packing of the molecules is dominated by a mixture of π-π-stacking and hydrogen bonding interactions. The primary packing motif formed by the TAPH + cations are hydrogen-bonded dimers. Pairs of N-H···N hydrogen bonds between the amino NH 2 groups and the unprotonated pyrimidine nitrogen atoms of each two of the four TAPH + cations form dimers (graph set motif R 2 2 (8); Etter et al., 1990). The dimers, formed between cations A and C and B and D, respectively, have local pseudoinversion symmetry. The thus formed (TAPH + ) 2 dimers are in turn forming slightly offset π-π-stacks that stretch parallel to [010] (Fig. 2). Centroid-centroid distances between individual pyrimidine rings are between 3.5128 (15) and 3.6288 (16) Å. Interplanar distances are, due to the offset between the stacked dimers, substantially shorter and range between 3.2456 (11) and 3.2847 (11) Å.
The thus formed columns of TAPH + cations make up about half of the unit cell volume (Fig. 3). The remainder of the The combination of π-stacking interactions and hydrogen bonding leads to the formation of a tightly interconnected three-dimensional network with alternating columns of TAPH + cations and channels filled with sulfate anions and water molecules (Fig. 4).

Experimental
Crystals of the title compound were isolated as an unintended by-product of the reaction of 2,4,6-triaminopyrimidine with phenylisothiocyanate when attempting to synthesize diaminopyrimidinyl-phenylthiourea. 2,4,6-Triaminopyrimidine, TAP, (0.36 g, 2.88 mmol) was dissolved in 40 cm 3 of ethanol at 333 K. Phenylisothiocyanate (0.35 ml, 2.93 mmol) was added and the mixture was stirred for 1 h. The resulting clear solution was filtered and left to evaporate at room temperature.
The crystalline material that formed upon standing for several days was filtered off and dried in vacuo (yield 0.4 g). A crystal was selected from the material and subjected to single-crystal structure analysis. No attempts were made to further analyze the remainder of the material.

Refinement
H atoms bonded to C and N atoms were constrained to ride on their parent atoms with C-H bond lengths of 0.93 Å for aryl C-H and N-H bond lengths of 0.86 Å with U iso (H) = 1.2U eq (C and N). All H atoms bonded to O atoms were located in a difference Fourier map and were refined isotropically. U iso values of water hydrogen atoms were constrained to 1.5 times the U eq value of their oxygen carrier atom. The structure exhibits two independent types of disorder. the disordered water molecule, O···H distances were restrained based on hydrogen bonding considerations (2.10 (2) Å for O2 i ···H18F and 2.20 (2) Å for O2 i ···H18D (symmetry operator (i): 1 + x, +y, +z).

Figure 1
The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level. The minor disordered moieties are omitted for clarity.

Figure 2
Part of the crystal structure of (I) showing intermolecular N-H···N hydrogen bonds (blue dashed lines) between TAPH + cations and π-π-stacks parallel to [010].  The packing structure of the title complex viewed down [010].

Figure 4
Part of the crystal structure showing formation of a tightly interconnected three-dimensional network with alternating columns of TAPH cations and channels filled with sulfate anions and water molecules.