Bis(1,2,3-benzotriazolium) sulfate dihydrate

In the asymmetric unit of the title hydrated salt, 2C6H6N3 +·SO4 2−·2H2O, there are two independent sulfate ions, one lying on a twofold axis, and the other in a general position. There are three independent benzotriazolium cations and three independent water molecules. The sulfate ion in a general position forms hydrogen-bonded chains of stoichiometry SO4 2−·3H2O in the b-axis direction. The sulfate on the twofold axis is unhydrated and accepts hydrogen bonds from four surrounding benzotriazoles. The benzotriazolium cations form two types of stacks along b. One stack contains only one type of independent cation, related by inversion centers. The other stack contains two alternating independent cations and no symmetry. The two types of stacks have orientations which are rotated by about 79° in the ac plane. 12 symmetrically distinct hydrogen bonds of type N—H⋯O(sulfate), N—H⋯O(water), O—H⋯O(sulfate) and O—H⋯O(water), with donor–acceptor distances in the range 2.5490 (13)–2.7871 (12) Å, form a three-dimensional array.

In the asymmetric unit of the title hydrated salt, 2C 6 H 6 N 3 + ÁSO 4 2À Á2H 2 O, there are two independent sulfate ions, one lying on a twofold axis, and the other in a general position. There are three independent benzotriazolium cations and three independent water molecules. The sulfate ion in a general position forms hydrogen-bonded chains of stoichiometry SO 4 2À Á3H 2 O in the b-axis direction. The sulfate on the twofold axis is unhydrated and accepts hydrogen bonds from four surrounding benzotriazoles. The benzotriazolium cations form two types of stacks along b. One stack contains only one type of independent cation, related by inversion centers. The other stack contains two alternating independent cations and no symmetry. The two types of stacks have orientations which are rotated by about 79 in the ac plane. 12 symmetrically distinct hydrogen bonds of type N-HÁ Á ÁO(sulfate), N-HÁ Á ÁO(water), O-HÁ Á ÁO(sulfate) and O-HÁ Á ÁO(water), with donor-acceptor distances in the range 2.5490 (13)-2.7871 (12) Å , form a three-dimensional array.
Data collection: APEX2 (Bruker, 2006); cell refinement: SAINT (Bruker, 2006); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXL97. Benzotriazole is used mainly as a corrosion inhibitor in aqueous based industrial cooling systems. In the preparation of benzotriazole from o-benzenediamine, the product benzotriazole is generally heavily contaminated with dark-colored impurities. Mention of such impurities can be found in the preparations and purifications of benzotriazole in Damschroder & Peterson (1955), Miller & Schlaudecker (1958), Howard & Popplewell (1967), and Spatz & Evans (1973). In a recently developed methodology for the purification of crude product aryltriazoles, it has been discovered that the sulfate salt of benzotriazole can be precipitated from acidic aqueous solution as a pure white hydrate, see Belter (2013). To determine the extent of hydration and to validate the stoichiometry of the salt, we crystallized the title benzotriazolium sulfate dihydrate, (1), (C 6 H 6 N 3 + ) 2 . SO 4 -2. 2H 2 O, from water at ice-bath temperature.
Upon melting point determination (flat stage), the crystals were observed to go through several transitions before melting. Crystals become opaque at >35°C, clarified at 64°C and softened at 72°C before melting at 114-116°C.
For a hydrated benzotriazolium salt (HClO 4 -. H 2 O) the packing is dictated by the clustering of anion-water units of 2 ClO 4 -. 2 H 2 O, which are further hydrogen bonded into columns. The cations crosslink the columns by hydrogen bonds, to water on one side, and to perchlorate on the other.
Our compound, 1, as a hydrate, displays similar clustering of anions and water. In this case, sulfates S1 are found as clusters containing 2 SO 4 2and 6 H 2 O, which are stacked to form infinite columns in the b direction. "Anhydrous" sulfates S2 lie on twofold axes and are isolated from other sulfates and water molecules, accepting hydrogen bonds only from cations.
In the current structure, the benzotriazolium cations bridge between anions and anion-water columns, as seen in the hydrogensulfate and perchlorate structures, having elements of both hydrated and anyhydrous types. Cation N1, N2, N3 bridges between S1 sulfate and water, cation N4, N5, N6 bridges between S1 and S2 sulfates, and cation N7, N8, N9 bridges water and S2 sulfate. H 2 SO 4 in an erlenmeyer flask whereupon the entirety dissolved. The mixture was allowed to cool several hours in an icewater bath whereupon precipitation had occurred. The precipitate was vacuum filtered to yield a white filter cake which proved to be 11.6% by weight of water, determined by Karl-Fisher titration. A second crop of precipitate was collected as crystals after continued icing of the mother liquor. One of these crystals was selected for X-ray analysis.

Refinement
H atoms on C were placed in idealized positions with C-H distances 0.95 Å and thereafter treated as riding. Coordinates of the NH and water hydrogen atoms were refined, with all N-H distances restrained to be equal and all O-H distances also restrained to be equal. U iso for H were assigned as 1.2 times U eq of the attached atoms (1.5 for water).

Figure 1
The asymmetric unit, also including symmetry-related (1 -x, y, 3/2 -z) O atoms on sulfate ion S2. Ellipsoids are drawn at the 50% level.  The unit cell, viewed down the b axis. Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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 R-factors(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.