Bromination of bis(pyridin-2-yl) diselenide in methylene chloride: the reaction mechanism and crystal structures of 1H-pyridine-2-selenenyl dibromide and its cycloadduct with cyclopentene (3aSR,9aRS)-2,3,3a,9a-tetrahydro-1H-cyclopenta[4,5][1,3]selenazolo[3,2-a]pyridinium bromide

Bromination of bis(pyridin-2-yl) diselenide in methylene chloride was carried out and the mechanism is proposed. The molecular and crystal structures of 1H-pyridine-2-selenenyl dibromide and its cycloadduct with cyclopentene were studied by X-ray diffraction.


Chemical context
Selenium-containing molecules have attracted significant attention from chemical and medicinal scientists because of their wide range of biological activities, such as antitumor effects, cardiovascular protection, antibacterial or antiviral effects (Banerjee & Koketsu, 2017;Zhang et al., 2017;Á lvarez-Pé rez et al., 2018;Miao et al., 2018). However, the chemistry of organoselenium compounds has not been sufficiently developed in comparison with that of organosulfur compounds because of the instability of most Se-containing compounds ISSN 2056-9890 (Ninomiya et al., 2011). Thus, the synthesis, isolation and structural characterization of selenium-containing substances is essential for the further development of potential medicines.
Earlier, the product of bromination of bis(pyridin-2-yl) diselenide in methylene chloride was described by Japanese researchers (Toshimitsu et al., 1984). This compound had a melting point of 388-390 K and was assigned as 2-pyridylselenenyl bromide based on the elemental analysis and IR spectroscopic data.
However, as a result of our multiple experiments on the bromination of bis(pyridin-2-yl) diselenide under similar conditions, a product with a melting point of 373-375 K was obtained. We isolated a compound with the same melting point as that previously obtained by the Japanese authors only after recrystallization from methanol. In our opinion, it is the lower melting point product that is the 2-pyridylselenenyl bromide 1*. The product having a higher melting point was isolated by us and then structurally characterized by X-ray analysis to be 1H-pyridine-2-selenenyl dibromide 1 (Fig. 1).

Figure 3
The addition reaction of hydrogen bromide to selenenyl bromide 1*.

Figure 5
Molecular structure of 1. Displacement ellipsoids are shown at the 50% probability level. H atoms are presented as small spheres of arbitrary radius.
Compound 2, C 10 H 12 NSeBr, is a salt containing a selenazolopyridinium cation and a bromide anion (Fig. 6). The fivemembered heterocycle of the cation adopts a flattened envelope conformation with the C3A carbon atom deviating by 0.274 (3) Å from the plane through the other ring atoms. The cyclopentane fragment has the usual envelope conformation with the C2 carbon atom deviating from the plane through the other ring atoms by 0.648 (4) Å . The dihedral angle between the basal planes of the two five-membered rings of the cation is 62.45 (11) . The selenium atom of the cation forms two additional non-covalent interactions with the bromide anions at distances of 3.2715 (4) Å [Se4Á Á ÁBr1(x, 1 + y, z)] and 3.5683 (3) Å [Se4Á Á ÁBr1(1-x, 1-y, -z)], affording a distorted square-planar coordination.
Cation 2 has two asymmetric C3A and C9A carbon atoms. The crystal of the compound is racemic with the following relative configurations of the centers -rac-3aSR,9aRS.

Supramolecular features
In the crystal of 1, molecules are linked by intermolecular N-HÁ Á ÁBr and C-HÁ Á ÁBr hydrogen bonds (Table 1) as well as by the non-covalent SeÁ Á ÁBr interactions (see above) into a three-dimensional framework (Fig. 7).

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.40; Groom et al., 2016) for zwitterionic molecules containing the T-shaped SeBr 2 fragment yielded 22 such compounds. In 16 of them, the hypervalent SeBr 2 fragments have asymmetric geometries, with the difference in the two Se-Br bond lengths more than or close to 0.1 Å , which is explained by intermolecular non-covalent interactions in the crystals. Moreover, 12 out of these 16 crystal structures revealed the presence of intermolecular non-covalent SeÁ Á ÁBr interactions with distances of 3.3374 (5) Table 1 Hydrogen-bond geometry (Å , ) for 1.

Synthesis and crystallization
2-Pyridineselenenyl bromide (1*). A solution of bromine (0.32 g, 2 mmol) in ethylene chloride (10 ml) was added to a solution of bis(pyridin-2-yl)diselenide (0.628 g, 2 mmol) in methylene chloride (10 ml (2). A solution of cyclopentene (0.034 g, 0.5 mmol) in ethyl acetate (5 ml) was added to a solution of 1 (0.159 g, 0.5 mmol) in ethyl acetate (10 ml) at room temperature. The reaction mixture was kept at room temperature for 24 h, then the solvent was removed under vacuum. The crude white solid was recrystallized from methylene chloride. Single crystals suitable for X-ray diffraction analysis were obtained by recrystallization from methylene chloride.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydrogen atom of the NHgroup in 1 was localized in the difference-Fourier map and refined isotropically with fixed displacement parameters [U iso (H) = 1.2U eq (N)]. The other hydrogen atoms in 1 and 2 were placed in calculated positions with C-H = 0.95-1.00 Å and refined using a riding model with fixed isotropic displacement parameters [U iso (H) = 1.5U eq (C) for the CH 3 -groups and 1.2U eq (C) for the other groups].

Dibromo(pyridin-1-ium-2-yl)selanide (1)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.37 e Å −3 Δρ min = −1.06 e Å −3 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Br1 0.26838 (4) 0.03212 (2)    where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.64 e Å −3 Δρ min = −1.05 e Å −3 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.