Trithiacyanuric acid: a second triclinic polymorph

The title compound, C3H3N3S3, is a triclinic modification. The other reported modification crystallizes with just one molecule in the asymmetric unit, [Guo et al. (2006 ▶). Cryst. Growth Des. 6, 846–848] and was solved by power X-ray diffraction data. The present modification has Z′ = 2. In the crystal, molecules are linked by strong intramolecular N—H⋯S hydrogen bonds with set graph-motif R 2 2(8). In both molecules, all of the N atoms and two of the S atoms are involved in hydrogen bonding, with an average H⋯S distance of 2.58 Å and N—H⋯S angles in the range 163–167°. π–π stacking interactions are not observed. In the solid state, the molecules exist in the thione form. The molecular and supramolecular properties are similar in both polymorphs.

The title compound, C 3 H 3 N 3 S 3 , is a triclinic modification. The other reported modification crystallizes with just one molecule in the asymmetric unit, [Guo et al. (2006). Cryst. Growth Des. 6, 846-848] and was solved by power X-ray diffraction data. The present modification has Z 0 = 2. In the crystal, molecules are linked by strong intramolecular N-HÁ Á ÁS hydrogen bonds with set graph-motif R 2 2 (8). In both molecules, all of the N atoms and two of the S atoms are involved in hydrogen bonding, with an average HÁ Á ÁS distance of 2.58 Å and N-HÁ Á ÁS angles in the range 163-167 .stacking interactions are not observed. In the solid state, the molecules exist in the thione form. The molecular and supramolecular properties are similar in both polymorphs.

Comment
Trithiocyanuric acid and its trisodium salt are widely applied in industry, analytical chemistry and biochemistry. For example its trisodium salt is used as a precipitating agent for many heavy metals from contaminated water (Henke et al., 2000).
Moreover, it was found that the acid inhibits the Toxoplasma gondii uracil phosphoribosyltransferase enzyme in vitro better than 5-fluorouracil and emimcin compounds showing an antitoxoplasmal activity (Iltzsch et al., 1993(Iltzsch et al., , 1994. The title compound bearing three N,S donor sets can display a great versatility of coordination As a matter of fact it can use from one to all the six of its donor atoms (Clegg et al., 1998;Yamanari et al., 1993) to form polynuclear complexes (Bailey et al., 2001;Hunks et al., 1999). Its capability to act as a bridging ligand is also shown in polymeric compounds (Tzeng et al., 1997). The structure of compound (II) (Guo et al., 2006) was solved by powder X-ray diffraction using the direct-space genetic algorithm technique for structure solution followed by Rietveldt refinement. The authors were unable to obtain single-crystals due to the title compound having a strong propensity to form co-crystals (solvates) in crystallization experiments from the types of solvents in which it is readily soluble. They reported that their modification crystallized with just one molecule in the asymmetric unit, (Z=2) from density considerations.
We are particularly interested in the utility of the title compound due its great versatility for the fabrication of different coordination polymers. We report here the structure of a new polymorph of (I) isolated during attempts to synthetize coordination polymers between (I) and PdCl 2, Fig 1. The present modification has Z'=2. The bond lengths C-S and C-N are 1.658 (7)Å and 1.355 (9) Å. The two molecules in the asymmetric unit are linked by two strong N-H···S intramolecular hydrogen bonds with set graph-motif R 2 2 (8) (Bernstein et al., 1995), Fig 2. In both molecules of the asymmetric unit all of the nitrogen atoms and two of the sulfur atoms are involved in hydrogen bonding with an average H-S distance of 2.58 Å and N-H-S angle ranging from 163-167° (Table 1). π-π stacking interactions were not observed. In the solid state the title compound exists in the thione form.
The common feature of both polymorphs is that the crystal structure comprises sheets of molecules. In (I) these sheets are parallel to (100) and in (II) parallel to (1-20) planes (consistent with the fact that the PXRD pattern has a peak of dominant intesity, indexed as (1-20)). Within the sheets, there is extensive N-H···S hydrogen bonding. Each N-H bond is a donor in one N-H···S hydrogen bond, but the S atoms in the molecule differ in their behavior as hydrogen bond acceptors. Thus, one S atom (in both molecules of the aymmetric unit of (I)) accepts two N-H···S hydrogen bonds, one S atom accepts one N-H···S hydrogen bond, and the other S atom is not involved in any hydrogen bonding (Table 1). In the hydrogen bonding network groups of six molecules are arranged in a cyclic manner, at the center of which four S atoms (including two S atoms not involved in hydrogen bonding) are in van der Waals contact (S···S 3.40-3.90Å for (I) and 3.37-3.52Å for (II)).In general the molecular and supramolecular properties are similar in both polymorphs.

Refinement
All H atoms were placed in idealized positions with d(N-H) = 0.88Å and refined using a riding model with U iso (H) fixed at 1.2 U eq (N). Fig. 1. The two molecules in the asymmetric unit of the title compound with displacement ellipsoids at the 50% probability level. 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.