Synthesis and crystal structure of the adduct between 2-pyridylselenyl chloride and isobutyronitrile

The reaction between 2-pyridylselenenyl chloride and isobutyronitrile results in the formation of the corresponding cationic pyridinium-fused 1,2,4-selenodiazole, namely, 3-(propan-2-yl)-1,2,4-[1,2,4]selenadiazolo[4,5-a]pyridin-4-ylium chloride, C9H11N2Se+·Cl−, in high yield (89%). The bifurcated Se⋯Cl−⋯H—Cl chalcogen-hydrogen-bonding interactions were analysed by DFT followed by a topological analysis of the electron-density distribution.

Here we report the preparation and structural characterization of a cationic pyridinium-fused 1,2,4-selenodiazole, which was prepared via reaction of 2-pyridylselenenyl chloride with isobutyronitrile (reagent ratio of 1:1).The reaction was carried out under stirring at room temperature in CH 2 Cl 2 / Et 2 O over 24 h, which led to the formation of a white suspension.Isolation and purification gave a crystalline solid of the target compound in a yield of 89%.

Supramolecular features and QTAIM analysis
The crystal packing is shown in Fig. 2. The molecules of the title compound are packed in layers parallel to the ac plane.Each row of 1,2,4-selenodiazolium salts in the layer is located antiparallel to the adjacent one.In addition to Se� � �Cl À contacts (Table 1), the anions form C-H� � �Cl À contacts (Table 2) that link the cations and anions both within the layers and between them.

Figure 1
Molecular structure of one of the four conformational isomers in the title compound.

Figure 2
View along the a axis of the crystal packing of the title compound.
Hirshfeld surface analysis for the crystal structure reveals that crystal packing is determined primarily by intermolecular contacts involving hydrogen atoms.Interestingly, the title compound did not form supramolecular dimers via Se� � �N contacts.To obtain a deeper understanding of the nature and quantify the strength of the bifurcated Se� � �Cl À � � �H-C chalcogen-hydrogen bonding in the title compound, single-point DFT calculations based on the experimental X-ray geometry were performed at the B97XD/6-311++G** level of theory using the dispersioncorrected hybrid functional !B97XD using GAUSSIAN09 (Frisch et al., 2009) with the 6-311++G** basis sets used for all atoms, followed by a topological analysis of the electrondensity distribution.
A QTAIM analysis of the model structure demonstrates the presence of bond critical points (3, À 1) for short contacts Se� � �Cl À and C-H� � �Cl À in the formed 1,2,4-selenodiazole (Table 3 and Fig. 4).The low magnitude of the electron density, positive values of the Laplacian of the electron density and zero or very close to zero values of energy density in these bond critical points (3, À 1) and estimated strength for appropriate short contacts are typical for weak, purely non-covalent interactions (Espinosa et al., 2002).Note that the nature of the discussed non-covalent contacts are similar to those weak interactions in closely related chemical systems (Grudova et al., 2022a,b).

Synthesis and crystallization
General remarks.All manipulations were carried out in air and all reagents used in this study were obtained from commercial sources (Aldrich, TCI-Europe, Strem, ABCR).
The Bondi (1966) van der Waals radii for the H, Se, and Cl atoms are 1.20, 1.90, and 1.75 A ˚, respectively.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. H atoms were positioned geometrically (C-H = 0.95-1.00A ˚) and refined as riding with U iso (H) = 1.2-1.5Ueq (C).The remaining positive and negative residual electron density close to the Se1, Se2, Se3 and Se4 atom positions (1.71A ˚À 3 at 0.94 A ˚from Se4, 1.67 A ˚À 3 at 1.05 A ˚from Se2, 1.58 A ˚À 3 at 1.03 A ˚from Se3, 1.54 A ˚À 3 at 1.06 A ˚from Se4 and À 1.53 A ˚À 3 at 1.06 A ˚from Se4) suggests the possible presence of a small twin component as well.

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.

Figure 4
Figure 4 Contour line diagram of the Laplacian of electron density distribution r 2 r(r), bond paths, and selected zero-flux surfaces (left panel), visualization of electron localization function (ELF, center panel) and reduced density gradient (RDG, right panel) analyses for bifurcated chalcogen-hydrogen bonding Se� � �Cl À � � �H-C in sample (for Se� � �Cl À 2.900 A ˚and C-H� � �Cl À 2.609 A ˚). Bond critical points (3, À 1) are shown in blue, nuclear critical points (3, À 3) in pale brown, ring critical points (3, +1) in orange, bond paths are shown as pale-brown lines, length units are A ˚and the colour scale for the ELF and RDG maps is presented in a.u.

Table 4
Experimental details.