Robust Supramolecular Dimers Derived from Benzylic-Substituted 1,2,4-Selenodiazolium Salts Featuring Selenium⋯π Chalcogen Bonding

The series of benzylic-substituted 1,2,4-selenodiazolium salts were prepared via cyclization reaction between 2-pyridylselenyl chlorides and nitriles and fully characterized. Substitution of the Cl anion by weakly binding anions promoted the formation supramolecular dimers featuring four center Se2N2 chalcogen bonding and two antiparallel selenium⋯π interactions. Chalcogen bonding interactions were studied using density functional theory calculations, molecular electrostatic potential (MEP) surfaces, the quantum theory of atoms-in-molecules (QTAIM), and the noncovalent interaction (NCI) plot. The investigations revealed fundamental role of the selenium⋯π contacts that are stronger than the Se⋯N interactions in supramolecular dimers. Importantly, described herein, the benzylic substitution approach can be utilized for reliable supramolecular dimerization of selenodiazolium cations in the solid state, which can be employed in supramolecular engineering.


Introduction
A bottom-up approach to the synthesis of functional materials with programmable structures and therefore properties is a long-standing challenge. In the past decades, coordination chemistry has been successfully employed as a tool to create such materials, which resulted in the emergence of metal-organic frameworks (MOFs) [1,2]. In analogous fashion, supramolecular organic frameworks (SOFs) [3,4] have recently emerged, which utilize noncovalent interactions as a tool for a programmable self-assembly. Among SOFs, hydrogen-bonded organic frameworks (HOFs) [4][5][6][7][8] represent a promising class of materials. However, halogen (XB) and chalcogen bonding (ChB) have recently appeared as powerful alternatives to the hydrogen bonding due to their directionality and tunability . In spite of their potential advantages, XB and ChB have received a very limited attention for the creation of extended materials analogous to HOFs [31].

Results and Discussion
Benzylic-substituted 1,2,4-selenodiazolium chlorides 2-4 were prepared by the oxidation of diselenide 1 with PhICl 2 and sequential cyclization of in situ generated 2-pyridylselenyl chloride with corresponding aryl acetonitriles in 78-93% yields (Scheme 1). The NMR and mass spectrometry for data for 2-4 were in accord with the proposed structures.
tunability . In spite of their potential advantages, XB and ChB have received a very limited attention for the creation of extended materials analogous to HOFs [31].
Here, we describe the synthesis of benzylic-substituted 1,2,4-selenodiazolium salts. When halide anions were substituted by the weakly binding anions, we always observed the formation Se2N2 squares, supported by the pair of antiparallel selenium···arene ChB interactions. These interactions have been further studied and characterized using density functional theory (DFT) calculations, molecular electrostatic potential (MEP) surfaces, the quantum theory of atoms-in-molecules (QTAIM), and the noncovalent interaction (NCI) plot.

Results and Discussion
Benzylic-substituted 1,2,4-selenodiazolium chlorides 2-4 were prepared by the oxidation of diselenide 1 with PhICl2 and sequential cyclization of in situ generated 2pyridylselenyl chloride with corresponding aryl acetonitriles in 78-93% yields (Scheme 1). The NMR and mass spectrometry for data for 2-4 were in accord with the proposed structures. Compounds 2-4 could be recrystallized from MeOH to give single crystals suitable for X-ray structural analysis, which confirmed their structures ( Figure 1).
Selenodiazolium cations expectedly interacted with the Cl anions via a pair of "chelating" Se· · · Cl and H· · · Cl non-covalent interactions. Such chelation was observed earlier for all structurally characterized selenodiazolium chlorides so far [40,42,43]. Compounds 2 and 3 did not feature Se 2 N 2 dimers in the solid state. Instead, one of the Se σ-holes in 2 was involved in Se· · · Cl ChB interaction with co-crystallized CH 2 Cl 2 molecule, while in 3 this position was found vacant (Figure 1). In contrast, compound 4 formed Se 2 Cl 2 dimers featuring bridging Cl anions involved in two ChB interactions.
Further, we switched to salts with weakly coordinating anions, i.e., ClO 4 − , AuCl 4 − , and PF 6 − . The compounds 5-9 were simply prepared by the addition of saturated methanolic solutions of NaAuCl 4 , NBu 4 PF 6 , or LiClO 4 to 2-4 in MeOH, which resulted in the precipitation of desired compounds in 48-67% yields (Scheme 1). Analytical data (see Experimental Part) suggested the formation of selenodiazolium salts 5-10. Selenodiazolium cations expectedly interacted with the Cl anions via a pair of "chelating" Se⋯Cl and H⋯Cl non-covalent interactions. Such chelation was observed earlier for all structurally characterized selenodiazolium chlorides so far [40,42,43]. Compounds 2 and 3 did not feature Se2N2 dimers in the solid state. Instead, one of the Se σ-holes in 2 was involved in Se⋯Cl ChB interaction with co-crystallized CH2Cl2 molecule, while in 3 this position was found vacant (Figure 1). In contrast, compound 4 formed Se2Cl2 dimers featuring bridging Cl anions involved in two ChB interactions.
Importantly, 5, 6, 8, 9, and 10 formed supramolecular dimers in the solid state via two antiparallel Se· · · N and Se· · · π ChB interactions ( Figure 2). The dimerization occurred for both benzyl and naphthylmethyl derivatives and did not depend on the nature of weakly coordinating anion.
The Se-π interactions resembled η 6 metal-arene coordination complexes in terms of their geometry. To obtain more insights into Se· · · π ChB in 5, 6, 8, 9, and 10, we performed theoretical studies starting from the calculations of the molecular electrostatic potential (MEP) surfaces using the salts to have neutral systems. The MEP surfaces are gathered in Figure 3. The values at both σ-holes (labeled a,c) and at the middle of both σ-holes (labeled as b) are indicated. In all cases, the σ-hole labeled as "c" is deeper (ranging 57 to 71 kcal/mol) than the other one ("a" ranging 35 to 49 kcal/mol) due to the overlap with the positive and adjacent H-atom of the pyridyl ring. The MEP values at the middle of both σ-holes are also positive ("b" ranging 29 to 43 kcal/mol), thus suggesting that the directionality of the Se· · · π interaction is not crucial. Regarding the aromatic rings, the MEP surfaces reveal that the values are negative, ranging from −8.5 in 5 to −21 kcal/mol in 10. The larger negative MEP values in the aromatic rings of 9 and 10 are due to the closer proximity of the counterion. The Se-π interactions resembled η 6 metal-arene coordination complexes in terms of their geometry. To obtain more insights into Se⋯π ChB in 5, 6, 8, 9, and 10, we performed , and 10, demonstrating supramolecular dimerization via two Se· · · N and Se· · · π ChB interactions. Grey and light-grey spheres represent carbon and hydrogen, respectively.
The combined QTAIM/NCIplot analyses of the self-assembled dimers of the compounds shown in Figure 3 were computed and are represented in Figure 4 along with the dimerization energies. These dimerization energies were computed considering the salt as the monomer. Moreover, only intermolecular interactions were represented in the plots. It can be observed that in all cases the Se· · · N ChBs are characterized by a bond critical point (CP) and bond path connecting the N to the Se atoms. The bond CPs are represented as red spheres and bond paths as orange lines in Figure 4. Moreover, the reduced density gradient (RDG) isosurfaces were represented overlapped with the QTAIM analysis. The NCIplot analysis shows the existence of blue disk-shaped RDG isosurfaces that further characterize the Se· · · N contacts, thus confirming the attractive nature of the interaction. Interestingly, a yellow RDG isosurface is also observed connecting the blue disks that characterize the Se· · · N bonds, as is detailed in the enlargement given in Figure 4f, thus revealing a N· · · N repulsion (blue and green colors are used for attractive interactions and yellow and red for repulsive). In fact, in compound 8, where the N· · · N distance is shorter, a bond CP and bond path interconnect both nitrogen atoms, further confirming the existence of a N· · · N interaction. The yellow color of the isosurface evidences the repulsive contribution of this contact that is obviously compensated by the Se· · · N attraction. Interestingly, the existence of the Se· · · π ChBs is also confirmed by both the QTAIM and NCIplot analyses. The shape and extension of the RDG isosurfaces in all compounds reveal that most of the π-cloud of the aromatic ring participates in the ChB. The dimerization energies range from −15.6 kcal/mol in 5 to −22.6 kcal/mol in compound 10, which confirms the importance of these interactions in the solid state of the 1,2,4-selenodiazolium salts reported herein. In an effort to estimate the relative contribution of the Se· · · π interaction, we also computed the dimerization energies using mutated dimers. The mutation consists of the replacement of the aromatic ring (either phenyl or naphtyl) by a hydrogen atom. As a consequence, the symmetrically equivalent Se· · · π interactions in the dimers are not established, and the binding energy is drastically reduced (see values in red in Figure 4). The values range from −4.8 kcal/mol in 5 to −9.3 kcal/mol in compound 10, thus revealing that the contribution of the Se· · · π is indeed dominant in these complexes, in line with the participation of the deeper σ-hole of selenium. theoretical studies starting from the calculations of the molecular electrostatic potential (MEP) surfaces using the salts to have neutral systems. The MEP surfaces are gathered in Figure 3. The values at both σ-holes (labeled a,c) and at the middle of both σ-holes (labeled as b) are indicated. In all cases, the σ-hole labeled as "c" is deeper (ranging 57 to 71 kcal/mol) than the other one ("a" ranging 35 to 49 kcal/mol) due to the overlap with the positive and adjacent H-atom of the pyridyl ring. The MEP values at the middle of both σ-holes are also positive ("b" ranging 29 to 43 kcal/mol), thus suggesting that the directionality of the Se···π interaction is not crucial. Regarding the aromatic rings, the MEP surfaces reveal that the values are negative, ranging from −8.5 in 5 to −21 kcal/mol in 10.
The larger negative MEP values in the aromatic rings of 9 and 10 are due to the closer proximity of the counterion. The combined QTAIM/NCIplot analyses of the self-assembled dimers of the compounds shown in Figure 3 were computed and are represented in Figure 4 along with the dimerization energies. These dimerization energies were computed considering the salt as the monomer. Moreover, only intermolecular interactions were represented in the plots. It can be observed that in all cases the Se···N ChBs are characterized by a bond critical point (CP) and bond path connecting the N to the Se atoms. The bond CPs are represented as red spheres and bond paths as orange lines in Figure 4. Moreover, the reduced density gradient (RDG) isosurfaces were represented overlapped with the QTAIM analysis. The NCIplot analysis shows the existence of blue disk-shaped RDG isosurfaces that further characterize the Se···N contacts, thus confirming the attractive nature of the interaction. Interestingly, a yellow RDG isosurface is also observed connecting the blue disks that characterize the Se···N bonds, as is detailed in the enlargement given in Figure 4f, thus revealing a N···N repulsion (blue and green colors are used for attractive interactions and yellow and red for repulsive). In fact, in compound 8, where the N···N distance is shorter, a bond CP and bond path interconnect both nitrogen atoms, further confirming the existence of a N···N interaction. The yellow color of the Further, we were interested how introduction of electron deficient aryl substituent would affect the self-assembly of the corresponding selenodiazolium salts with weakly coordinating anion. Electron-deficient arenes are very weak Lewis bases, and their η 6 metal complexes are very scarce. Thus, we expected that selenodiazolium cations carrying electron deficient aryls would not undergo supramolecular dimerization via two Se· · · N and Se· · · π ChB interactions. In order to verify this hypothesis, we prepared compound 7, whose structure in the solid state was established by means of single-crystal X-ray crystallography ( Figure 5). mutated dimers. The mutation consists of the replacement of the aromatic ring (either phenyl or naphtyl) by a hydrogen atom. As a consequence, the symmetrically equivalent Se···π interactions in the dimers are not established, and the binding energy is drastically reduced (see values in red in Figure 4). The values range from −4.8 kcal/mol in 5 to −9.3 kcal/mol in compound 10, thus revealing that the contribution of the Se···π is indeed dominant in these complexes, in line with the participation of the deeper σ-hole of selenium. Further, we were interested how introduction of electron deficient aryl substituent would affect the self-assembly of the corresponding selenodiazolium salts with weakly coordinating anion. Electron-deficient arenes are very weak Lewis bases, and their η 6 metal complexes are very scarce. Thus, we expected that selenodiazolium cations carrying electron deficient aryls would not undergo supramolecular dimerization via two Se⋯N and Se⋯π ChB interactions. In order to verify this hypothesis, we prepared compound 7, whose structure in the solid state was established by means of single-crystal X-ray crystallography ( Figure 5).  As we expected, 7 did not feature Se···π ChB. This further demonstrate the predictable behavior of benzylic-substituted selenodiazolium salts towards selfaggregation via two Se⋯N and Se⋯π ChB interactions in the solid state and supports the theoretical analysis above that demonstrates the dominant role of the Se···π ChB in the self-assembled dimers.

General Remarks
All manipulations were carried out in air unless specified. All the reagents used in this study were obtained from the commercial sources (Aldrich, TCI-Europe, Strem, Figure 5. Ball-and-stick representation of crystal structure of 7, demonstrating Se· · · Cl ChB interactions. Grey and light-grey spheres represent carbon and hydrogen, respectively.
As we expected, 7 did not feature Se· · · π ChB. This further demonstrate the predictable behavior of benzylic-substituted selenodiazolium salts towards self-aggregation via two Se· · · N and Se· · · π ChB interactions in the solid state and supports the theoretical analysis above that demonstrates the dominant role of the Se· · · π ChB in the self-assembled dimers.

General Remarks
All manipulations were carried out in air unless specified. All the reagents used in this study were obtained from the commercial sources (Aldrich, TCI-Europe, Strem, ABCR). Commercially available solvents were purified by conventional methods and distilled immediately prior to use. NMR spectra were recorded on a Bruker Avance neo-700; chemical shifts (δ) are given in ppm and coupling constants (J) in Hz. C, H, and N elemental analyses were carried out on a Euro EA 3028HT CHNS/O analyzer. Di(2-pyridyl)diselenide was prepared as reported earlier [39,40].

X-ray Crystal Structure Determination
The single-crystal X-ray diffraction data were collected on three-circle Bruker Smart APEX-II (2, 3), Bruker D8 QUEST (4, 6, 8), and Bruker D8 Venture (9) diffractometers (graphite monochromator, w and j scanning mode) and on a four-circle Rigaku Synergy S diffractometer equipped with a HyPix6000HE area-detector (graphite monochromator, shutterless ω-scan mode) (5, 7, 10). For 2-4, 6, 8, and 9, the data were indexed and integrated using the SAINT program [53] and then scaled and corrected for absorption using the SADABS program [54]. For 5, 7, and 10, the data were integrated and corrected for absorption by the CrysAlisPro program. For details, see Table S1 (Electronic Supporting Information). The structures were determined by direct methods and refined by full-matrix least-squares technique on F 2 with anisotropic displacement parameters for non-hydrogen atoms. The hydrogen atoms in all compounds were placed in calculated positions and refined within 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). All calculations were carried out using the SHELXTL program [55].
One molecule of MeOH in the unit cell (V = 232 A 3 in one void) of 9 was found to be disordered over multiple positions and could therefore not be modelled satisfactorily. It was removed from the electron density map using the OLEX solvent mask command.
Crystallographic data for all investigated compounds were deposited with the Cambridge Crystallographic Data Center, CCDC 2220382-2220390. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CHB2 1EZ, UK (Fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www.ccdc.cam.ac.uk, accessed on 18 November 2022).

Conclusions
We have synthesized a series of benzylic-substituted 1,2,4-selenodiazolium salts with weakly binding anions that promote the formation self-assembled dimers with the recurrent Se 2 N 2 supramolecular motif. The dimers are further supported by two symmetrically equivalent selenium· · · arene ChB interactions. The Se· · · N and Se· · · π ChB interactions have been studied theoretically, and we disclosed the fundamental role of the Se· · · π interactions, which are stronger than the Se· · · N interactions observed in the Se 2 N 2 motifs. The DFT analysis demonstrates that the stronger nature of the Se-π interactions is due to the participation of the more positive σ-hole and to the existence of a repulsive N· · · N interaction in the Se 2 N 2 motif. The relevance of the Se· · · π interaction is further supported by the fact that compound 7 does not exhibit the Se· · · π interaction in the solid state due to the presence of electron withdrawing groups in the phenyl ring.
Importantly, the herein-described strategy can be employed for reliable supramolecular dimerization of 1,2,4-selenodiazolium cations in the solid state. This supramolecular synthon can be employed in supramolecular engineering for the creation of SOFs with programmable structures and properties.