A triclinic modification of 5,5′-dinitro-2,2′-dithiodipyridine

The asymmetric unit of the title compound, C10H6N4O4S2, contains two independent but similar molecules. The structure is a triclinic polymorph of the monoclinic structure reported previously [Brito, Mundaca, Cárdenas, López-Rodríguez & Vargas (2007 ▶). Acta Cryst. E63, o3351–o3352]. The most obvious difference between the two polymorphs is the C—S—S—C torsion angle [−80.13 (16), −79.8 (2) and 0° for the two molecules of the triclinic polymorph and the monoclinic polymorph, respectively]. The crystal structure of the title compound has two intramolecular C—H⋯S interactions with average H⋯S distances of 2.69 Å, whereas this kind of interaction is not evident in the monoclinic polymorph.

The asymmetric unit of the title compound, C 10 H 6 N 4 O 4 S 2 , contains two independent but similar molecules. The structure is a triclinic polymorph of the monoclinic structure reported previously [Brito, Mundaca, Cá rdenas, Ló pez-Rodríguez & Vargas (2007). Acta Cryst. E63, o3351-o3352]. The most obvious difference between the two polymorphs is the C-S-S-C torsion angle [À80.13 (16), À79.8 (2) and 0 for the two molecules of the triclinic polymorph and the monoclinic polymorph, respectively]. The crystal structure of the title compound has two intramolecular C-HÁ Á ÁS interactions with average HÁ Á ÁS distances of 2.69 Å , whereas this kind of interaction is not evident in the monoclinic polymorph. 362 parameters H-atom parameters constrained Á max = 0.33 e Å À3 Á min = À0.28 e Å À3 Table 1 Selected torsion angles ( ).
A triclinic modification of 5,5'-dinitro-2,2'-dithiodipyridine I. Brito, A. Mundaca, A. Cárdenas and M. López-Rodríguez Comment This paper forms part of our continuing study of the synthesis and structural characterization of divalent sulfur compounds (Brito et al., 2007 and references therein). We are particularly interested in the utility of the title compound of as flexible ligand, and its binding modes, for the fabrication of different coordination polymers topologies. We report here the structure of a new polymorph of 5,5'-Dinitro-2,2'-dithiodipyridine isolated during attempts to synthetize coordination polymers with silver trifluoromethanesulfonate of the ligand (Fig. 1, Table 1). The bond lenghts and the conformation are significantly different from those found in the monoclinic modification (Brito et al., 2007). The observed difference between (I) and the previously reported polymorph is in the torsion angle C-S-S-C [80.0 (2) and 0°, respectively]. A database survey of C-S-S-C fragments (Allen et al., 1987) found that S-S bond distances are bimodally distributed: for torsion angles in the ranges 75-105 and 0-20°, the mean S-S bond distances are 2.031 (15) and 2.070 (22) Å, respectively. The corresponding value in the title compound is 2.025 (2) Å and 2.0719 (11) Å for the previously polymorph placing it in the lower quartile for Allen's first set. In both polymorphs the torsion angles X-C-S-S (where X=N or C) are close to 0 or 180°a nd within the range found in other substituted aromatic disulfides with an equatorial conformation according to the Shefter classification (Shefter, 1970).
The molecular conformations are dominated by near orthogonality of the lone pairs on the two adjacent S atoms (Glidewell et al., 2000). The molecular packing in the title compound is completely different from that of the monoclinic polymorph. Only in the triclinic form the phenyl rings participant in significant intramolecular C-H···S interactions with average H···S distances of 2.69 Å. These interactions may stabilize the conformation adopted by the molecules in the solid state ( Fig. 1). The molecules are linked into chains by two intermolecular C-H···O hydrogen bond. Atoms C8 and C18 in the molecules at (x,y,z) acts as hydrogen bonds donor vía atom H8 and H18 to atoms O1 and O5 in the molecule at (x,-1 + y,z) respectively, so generating by translation two C(12) chains running parallel to [010] direction (Bernstein et al., 1995), Table 2). The triclinic modification is much less compact, as noted from the lower density (1.638 Mg m -3 compared with 1.725 Mg m −3 for the monoclinic form).

Experimental
All reactions were carried out under an atmosphere of purified nitrogen. Solvents were dried and distilled prior to use. 5,5'-dinitro-2,2'-dithiodipyridine and silver trifluoromethanesulfonate were purchased from Aldrich. The title compound was obtained as light yellow prismatic crystals, in an attempts to prepare coordination polymers with silver trifluoromethanesulfonate of the ligand. A mixture of 5,5'-dinitro-2,2'-dithiodipyridine (1 mmol, 310 mg) and silver trifluoromethanesulfonate (1 mmol, 256.9 mg) in methanol (20 ml) was refluxed for 8 h. After slow cooling of the reaction system to room temperature, light yellow prismatic crystals of (I) were formed that were filtered off and washed with cold diethyl ether. supplementary materials sup-2 Refinement H atoms were positioned geometrically. In the final cycles of the refinement, all H atoms were constrained to ride on their parent atoms, with C-H distances of 0.93 Å and with U iso (H) = 1.2 U eq (C).
The material was difficult to obtain in a suitable crystalline form and the best available specimen was lost during data collection, resulting in 95% completeness.  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 > 2sigma(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.