2-Pyridylselenenyl versus 2-Pyridyltellurenyl Halides: Symmetrical Chalcogen Bonding in the Solid State and Reactivity towards Nitriles

The synthesis of 2-pyridyltellurenyl bromide via Br2 oxidative cleavage of the Te–Te bond of dipyridylditelluride is reported. Single-crystal X-ray diffraction analysis of 2-pyridyltellurenyl bromide demonstrated that the Te atom of 2-pyridyltellurenyl bromide was involved in four different noncovalent contacts: Te⋯Te interactions, two Te⋯Br ChB, and one Te⋯N ChB contact forming 3D supramolecular symmetrical framework. In contrast to 2-pyridylselenenyl halides, the Te congener does not react with nitriles furnishing cyclization products. 2-Pyridylselenenyl chloride was demonstrated to easily form the corresponding adduct with benzonitrile. The cyclization product was studied by the single-crystal X-ray diffraction analysis, which revealed that in contrast to earlier studied cationic 1,2,4-selenadiazoles, here we observed that the adduct with benzonitrile formed supramolecular dimers via Se⋯Se interactions in the solid state, which were never observed before for 1,2,4-selenadiazoles.


Introduction
The field of noncovalent interactions has experienced rapid growth and constitutes one of the most intensely studied areas of current chemistry. Noncovalent interactions allow the design and construction of supramolecular materials and control of their ultimate architectures and symmetry [1,2]. Importantly, the properties of supramolecular aggregates are different from the sum of the constituent molecules [1,[3][4][5][6][7][8][9]. Recently, chalcogen bonding (ChB) has emerged as a powerful tool for the creation of such materials. In contrast to halogen bonding (XB) or hydrogen bonding (HB), usage of ChB in crystal engineering, preparative chemistry, sensing, etc., is still emerging [10]. contrast to halogen bonding (XB) or hydrogen bonding (HB), usage of ChB in cry gineering, preparative chemistry, sensing, etc., is still emerging [10].
We have recently showcased that the addition of 2-pyridylselenenyl halides ple CN bond of unactivated nitriles resulted in the formation of novel cationic 1, nadiazoles [11]. Moreover, we showed that the Se atom in the adducts of 2-pyri nenyl halides and nitriles could provide two σ-holes and act as a donor ChB (Figu It is worth mentioning that chalcogen diazoles are appealing research object their utilization in the preparation of soft materials with tunable physical paramet 15]. ChB allows modulation of the self-assembly and, therefore, fine-tuning of transport within these heterocycles.
Here we describe the synthesis and crystal structure of 2-pyridyltellurenyl b compare its self-assembly in the solid-state with structurally similar selenium co 2-pyridylselenenyl chloride, and compare the reactivity of these two 2-pyridy genenyl halides towards benzonitrile. Interestingly, while 2-pyridylselenenyl readily forms an adduct with PhCN, the Te analog does not react with benzonitril other simple nitrile tested in the framework of the current study.

Materials and Methods
General remarks. All manipulations were carried out in air, unless specified specified, chemicals were purchased from the commercial sources. NMR data tained on a Bruker Avance neo 700; chemical shifts are given in ppm, coupling co in Hz. C, H, S, and N elemental analyses were performed on Euro EA 3028HT C Py2Se2 was prepared as reported earlier [16].
X-ray crystal structure determination. The single-crystal X-ray diffraction data for 15 and 16 were obtained on a thr Bruker D8 Venture(Kurnakov Institute of General and Inorganic Chemistry, RA men, Germany) or Bruker D8 QUEST PHOTON-III CCD (Zelinsky Institute of Chemistry, RAS, Bremen, Germany) diffractometers using φ and ω scan mode. fraction data were processed using the SAINT program [17] and an absorption co based on equivalent reflections was applied with the SADABS program [18]. Crys details of data collection, and results of structure refinement are summarized in T The structures were solved by the direct method and refined on F2 with anisotro placement parameters for non-hydrogen atoms. The hydrogen atoms in all com were placed in calculated positions and refined within the riding model with fi tropic displacement parameters (Uiso(H) = 1.5Ueq(C) for the CH3-groups and 1.2 for the other groups). All calculations were carried out using the SHELXTL progr and OLEX2 program package [20].
Crystallographic data for all investigated compounds have been deposited w Cambridge Crystallographic Data Center, CCDC 2113480 and 2113481. Copies of formation may be obtained free of charge from the Director, CCDC, 12 Union Roa It is worth mentioning that chalcogen diazoles are appealing research objects due to their utilization in the preparation of soft materials with tunable physical parameters [12][13][14][15]. ChB allows modulation of the self-assembly and, therefore, fine-tuning of charge transport within these heterocycles.
Here we describe the synthesis and crystal structure of 2-pyridyltellurenyl bromide, compare its self-assembly in the solid-state with structurally similar selenium congener, 2pyridylselenenyl chloride, and compare the reactivity of these two 2-pyridylchalcogenenyl halides towards benzonitrile. Interestingly, while 2-pyridylselenenyl chloride readily forms an adduct with PhCN, the Te analog does not react with benzonitrile or any other simple nitrile tested in the framework of the current study.

Materials and Methods
General remarks. All manipulations were carried out in air, unless specified. Unless specified, chemicals were purchased from the commercial sources. NMR data was obtained on a Bruker Avance neo 700; chemical shifts are given in ppm, coupling constants in Hz. C, H, S, and N elemental analyses were performed on Euro EA 3028HT CHNS/O. Py 2 Se 2 was prepared as reported earlier [16].
X-ray crystal structure determination. The single-crystal X-ray diffraction data for 15 and 16 were obtained on a threecircle Bruker D8 Venture(Kurnakov Institute of General and Inorganic Chemistry, RAS, Bremen, Germany) or Bruker D8 QUEST PHOTON-III CCD (Zelinsky Institute of Organic Chemistry, RAS, Bremen, Germany) diffractometers using ϕ and ω scan mode. The diffraction data were processed using the SAINT program [17] and an absorption correction based on equivalent reflections was applied with the SADABS program [18]. Crystal data, details of data collection, and results of structure refinement are summarized in Table S1. The structures were solved by the direct method and refined on F2 with anisotropic displacement parameters for non-hydrogen atoms. The hydrogen atoms in all compounds were placed in calculated positions and refined within the riding model with fixed isotropic displacement parameters (Uiso(H) = 1.5Ueq(C) for the CH3-groups and 1.2Ueq(C) for the other groups). All calculations were carried out using the SHELXTL program [19] and OLEX2 program package [20].
Crystallographic data for all investigated compounds have been deposited with the Cambridge Crystallographic Data Center, CCDC 2113480 and 2113481. Copies of this information may be obtained free of charge from the Director, CCDC, 12  with the help of the ORCA 4.2.1 program package [25]. The RIJCOSX approximation [26] has been utilized. The QTAIM analysis [27] has been performed by using the Multiwfn program (version 3.7) [28]. The Cartesian atomic coordinates for model supramolecular associates are presented in Table S1 and in attached xyz-files, Supplementary Materials. Synthesis of 15. 2-Pyridylselenenyl chloride (1 eq, 234 µmol, 45 mg) and PhCN (4.14 eq, 970 µmol, 100 µL) were stirred in Et 2 O (3 mL) at ambient temperature for 3 h. White solid precipitated, which was decantated, quickly washed with CH 2 Cl 2 (1 mL), Et 2 O (3 × 3 mL), and dried under vacuum. Yield: 60 mg (87%). Anal. Calcd for C 12
Interestingly, in the crystal packing of 16 each Te atom is involved in four different noncovalent contacts: Te· · · Te interactions, two Te· · · Br ChB and one Te· · · N ChB contacts forming 3D supramolecular symmetrical framework.
In contrast, the Se analog 1, which we described earlier, does not exhibit Se· · · Se interactions in the crystal but features analogous Se· · · N ChB and terminal Se· · · Cl ChB forming supramolecular dimers (Figure 3) [46]. Interestingly, in the crystal packing of 16 each Te atom is involved in four different noncovalent contacts: Te⋯Te interactions, two Te⋯Br ChB and one Te⋯N ChB contacts forming 3D supramolecular symmetrical framework.
In contrast, the Se analog 1, which we described earlier, does not exhibit Se⋯Se interactions in the crystal but features analogous Se⋯N ChB and terminal Se⋯Cl ChB forming supramolecular dimers ( Figure 3) [46].   Interestingly, in the crystal packing of 16 each Te atom is involved in four different noncovalent contacts: Te⋯Te interactions, two Te⋯Br ChB and one Te⋯N ChB contacts forming 3D supramolecular symmetrical framework.
In contrast, the Se analog 1, which we described earlier, does not exhibit Se⋯Se interactions in the crystal but features analogous Se⋯N ChB and terminal Se⋯Cl ChB forming supramolecular dimers (Figure 3) [46].  Further, we were interested whether 2-pyridyltellurenyl bromide 16 would react with nitriles in a similar fashion as the Se analogs 1 or 2, which we showed to easily react with a broad scope of nitriles. Surprisingly, 16 turned to be inert towards nitriles. 16 did not react with PhCN, CCl 3 CN, or MeCN in CH 2 Cl 2 at room temperature or at slight heating (60 • C).
Thus, switching from the Te to Se in 2-pyridyltellurenyl halides results in a dramatic impact on its reactivity towards nitriles.
In an extension of our earlier works, here we demonstrate that benzonitrile also easily reacts with 1 forming cationic 1,2,4-selenadiazole 15 in excellent yield (Scheme 1). Single with nitriles in a similar fashion as the Se analogs 1 or 2, which we showed to easily react with a broad scope of nitriles. Surprisingly, 16 turned to be inert towards nitriles. 16 did not react with PhCN, CCl3CN, or MeCN in CH2Cl2 at room temperature or at slight heating (60 °C).
Thus, switching from the Te to Se in 2-pyridyltellurenyl halides results in a dramatic impact on its reactivity towards nitriles.
In an extension of our earlier works, here we demonstrate that benzonitrile also easily reacts with 1 forming cationic 1,2,4-selenadiazole 15 in excellent yield (Scheme 1). Single crystals of 15 were obtained from CH2Cl2, and X-ray analysis pointed to the generation of the adduct with PhCN ( Figure 4). Interestingly, the adduct 15 formed supramolecular dimers via Se⋯Se interactions in the solid-state, which we never observed before. For the earlier studied cationic 1,2,4selenadiazoles we observed supramolecular dimerization via four-center Se⋯N ChB, Se⋯Cl ChB and H⋯Cl interactions ( Figure 5), but never via Se⋯Se contacts. Interestingly, the adduct 15 formed supramolecular dimers via Se· · · Se interactions in the solid-state, which we never observed before. For the earlier studied cationic 1,2,4selenadiazoles we observed supramolecular dimerization via four-center Se· · · N ChB, Se· · · Cl ChB and H· · · Cl interactions ( Figure 5), but never via Se· · · Se contacts.
Inspection of the crystallographic data revealed the presence of several nontrivial noncovalent interactions in the crystal structures 1, 15, and 16. To understand the nature and quantify the strength of these noncovalent interactions, the quantum chemical calculations and QTAIM analysis [27] were carried out at the ωB97X-D3/Sapporo-DZP-2012 level of theory. For results of QTAIM analysis, see Table 1, and Figures 6-8 shown diagrams of the Laplacian of electron density distribution ∇ 2 ρ(r) as well as electron localization function (ELF) and reduced density gradient (RDG) analyses for these noncovalent interactions in the crystal structures of 1, 15 and 16. Table 1. Electron densities-ρ(r), electron density Laplacians-∇ 2 ρ(r) and appropriate λ 2 values, densities of energy-H b , potential energy densities-V(r), and Lagrangian kinetic energies-G(r) (a.u.) at the bond critical points (3, −1), associated with various nontrivial noncovalent interactions in the model supramolecular associates 1, 15, and 16, and estimated strength for these contacts E int (kcal/mol).  [47]. See xyz-files of model structures in Supplementary Materials for atoms numeration. ** E int ≈ −V(r)/2 [48]. Inspection of the crystallographic data revealed the presence of several nontrivial noncovalent interactions in the crystal structures 1, 15, and 16. To understand the nature and quantify the strength of these noncovalent interactions, the quantum chemical calculations and QTAIM analysis [27] were carried out at the ωB97X-D3/Sapporo-DZP-2012 level of theory. For results of QTAIM analysis, see Table 1, and Figures 6-8 shown diagrams of the Laplacian of electron density distribution ∇ 2 ρ(r) as well as electron localization function (ELF) and reduced density gradient (RDG) analyses for these noncovalent interactions in the crystal structures of 1, 15 and 16.     Laplacians of electron density distribution ∇ 2 ρ(r) (left panels), visualization of electron localization function (ELF, center panels) and reduced density gradient (RDG, right panels) analyses for noncovalent interactions Te· · · Br and Te· · · Te in the X-ray structure 16. Bond critical points (3, −1) are shown in blue, the color scale for the ELF and RDG maps is presented in a.u., length units-Å.
In conclusion, we described the synthesis and characterization of 2-pyridyltellurenyl bromide and attempted to perform its cyclization with nitriles. In contrast to the Se analogs, 2-pyridyltellurenyl bromide does not react with nitriles. We also performed structural characterization of 2-pyridyltellurenyl bromide by the single-crystal X-ray diffraction. Interestingly, the Te atom of 2-pyridyltellurenyl bromide was involved in four different noncovalent contacts: Te· · · Te interactions, two Te· · · Br ChB and one Te· · · N ChB contacts forming 3D supramolecular symmetrical framework. In contrast, 1 did not exhibit Se· · · Se interactions in the crystal but featured similar Se· · · N ChB and terminal Se· · · Cl ChB forming supramolecular dimers. Within this study, we also prepared and performed a structural investigation of the adduct of 2-pyridylselenenyl chloride with benzonitrile. In contrast to earlier studied cationic 1,2,4-selenadiazoles here we observed that the adduct 15 formed supramolecular dimers via Se· · · Se interactions in the solid-state, which we never observed before.