Crystal structure of 3,6-bis(pyridin-2-yl)-1,4-dihydro-1,2,4,5-tetrazine

In the crystal structure, intermolecular N—H⋯N hydrogen bonds link the molecules into infinite ribbons extending along the [100] direction.


Structural commentary
Compound (I) crystallizes in the monoclinic space group P2 1 /n. The atomic labelling scheme is shown in Fig. 2. In (I), being a reduced form of (II), there are two hydrogen atoms at the 1 and 4 positions and two 2-pyridyl substituents at the 3 and 6 positions.
The C-C bond lengths are within the expected values known for aromatic systems (Allen et al., 1987). However, there is a fluctuation of bond distances involving nitrogen atoms. The N-N bonds within the central (A) ring are of almost equal length, being 1.4285 (15) (14) ]. In this conformation, hydrogen atoms are located in the equatorial positions of the ring and the N-H bonds are directed to the bottom of the boat (compare torsion angles in Table 1). The planes of the aromatic pirydyl rings (B and C) are not to parallel to each other. The dihedral angles between these rings and central tetrazine ring are 22.43 (7) (A and B) and 25.71 (6) (A and C). The dihedral angle between rings B and C is 27.13 (7) . The overall molecular structure could be recognized as a butterfly-like conformation as shown in Fig. 3.

Figure 2
The molecular structure of (I), showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level.

Figure 3
The butterfly-like molecular conformation of (I).

Figure 5
A view of the unit-cell packing, showing the ribbon-like arrangement of molecules. Short CÁ Á ÁN and CÁ Á ÁC intermolecular contacts between adjacent molecular ribbons are shown as dashed blue lines.

Synthesis and crystallization
Crystals suitable for X-ray measurements were obtained from a commercially available reagent (Aldrich Chemical Co.) and used without further purification. 0.5 mmol of 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine and 0.5 mmol of 2-mercaptopyridine Noxide (in a 1:1 molar ratio) were mixed in ethanol (4 ml). The resulting solution was warmed to 343 K and then kept at room temperature. Within two weeks, after slow evaporation of the solvent, two kinds of crystal were obtained in a crystallizer. X-ray studies confirmed that the pink crystals were of the known structure (II), while the yellow crystals were identified as being of a previously unreported structure, i.e. (I).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms of aromatic rings were introduced in calculated positions with idealized geometry and constrained using a rigid body model with isotropic displacement parameters equal to 1.2 the equivalent displacement parameters of the parent atoms. The H atoms of the NH groups, in 1,2,4,5-tetrazine ring, were located in a difference Fourier map and freely refined.

Funding information
Funding for this research was provided by: Narodowe Centrum Nauki (   Data collection: CrysAlis PRO (Rigaku OD, 2015); cell refinement: CrysAlis PRO (Rigaku OD, 2015); data reduction: CrysAlis PRO (Rigaku OD, 2015); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: WinGX (Farrugia, 2012); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b), PLATON (Spek, 2009) and publCIF (Westrip, 2010). 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.