[1,4]Ditellurino[2,3-b:5,6-b′]dipyrazine

The title compound, C8H4N4Te2, is the first reported [1,4]tellura[2,3-b:5,6-b′]dipyrazine. Three independent unit-cell molecules are folded along their Te⋯Te axes, with an average angle φ = 57.9°. C—Te—C angles range from 91.48 (6) to 93.80 (6)°. Intermolecular N⋯Te bonding interactions between tellurium atoms of the central ring and the pyrazine N atoms of adjacent molecules result in a supramolecular helix motif.

The C-Te-C angles for the three independent molecules of the title compound range from 91.48 (6) to 93.80 (6) , similar to those of 95.3 and 95.9 , respectively, previously reported for telluranthrene (Dereu et al., 1981). C-Te bond lengths range from 2.1105 (16) Å to 2.1381 (17) Å , in good agreement with those predicted by their covalent radii.

Figure 3
A portion of the helical chain, side view.

Synthesis and crystallization
Preparation of [1,4]ditellurino[2,3-b:5,6-b 0 0 0 ]dipyrazine: a 100 ml round-bottom flask equipped with mechanical stirring and inert gas inlet was charged with tellurium power (200 mesh, 1.28 g, 10 mmol), sodium hydride (0.6 g of 60% emulsion in mineral oil, 15 mmol) and dry N-methyl-2-pyrrolidone (12 ml). The mixture was purged with nitrogen, placed in a Wood's metal bath and heated to 453 K with mechanical stirring for two hours. 2,3-Dichloropyrazine (1.49 g, 10 mmol) was then added, followed by continued stirring at 453 K. The mixture was allowed to cool and diluted with water (100 ml). Solids were collected by filtration and dried. They were subsequently extracted with 2 Â 10 ml of chloroform. The combined extracts were chromatographed on a 1.5 Â 10 cm column (silica gel, neutral, 200 mesh) using chloroform as mobile phase, followed by chloroform: acetonitrile (10:1 v/v). A yellow band eluted first and was identified as bis(pyrazin-2yl)tellurium by mass spectrometry. This was followed by a blue band, identified as bis(3-chloropyrazin-2-yl)ditellurium. The following yellow band contained the title compound. Crystallization from chloroform solution furnished yellow crystals, m.p. 413-415 K, yield 46 mg (2.2%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. In the later stages of refinement, a small amount of twinning was detected, by 180 rotation about the reciprocal 110 direction. Final refinement was as a twincomponent twin using an HKL5 file prepared by ROTAX (Parsons et al., 2003). The BASF parameter is 0.0250 (4).     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. Refinement. Refined as a two-component twin using an HKL5 file prepared by ROTAX (Parsons et al., 2003). The BASF parameter is 0.0250 (4). All H atoms were located in difference maps and then treated as riding in geometrically idealized positions with C-H distances 0.95 Å and with U iso (H) =1.2U eq for the attached C atom.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq