Halogen Bonding in the Complexes of Brominated Electrophiles with Chloride Anions: From a Weak Supramolecular Interaction to a Covalent Br–Cl Bond

: The wide-range variation of the strength of halogen bonds (XB) not only facilitates a variety of applications of this interaction, but it also allows examining the relation (and interconversion) between supramolecular and covalent bonding. Herein, the Br ... Cl halogen bonding in a series of complexes of bromosubstituted electrophiles (R-Br) with chloride anions were examined via X-ray crystallographic and computational methods. Six co-crystals showing such bonding were prepared by evaporation of solutions of R-Br and tetra-n-propylammonium chloride or using Cl − anions released in the nucleophilic reaction of 1,4-diazabicyclo[2.2.2]octane with dichloromethane in the presence of R-Br. The co-crystal comprised networks formed by 3:3 or 2:2 halogen bonding between R-Br and Cl − , with the XB lengths varying from 3.0 Å to 3.25 Å. Analysis of the crystallographic database revealed examples of associations with substantially longer and shorter Br ... Cl separations. DFT computations of an extended series of R–Br ... Cl − complexes conﬁrmed that the judicious choice of brominated electrophile allows varying halogen Br ... Cl bond strength and length gradually from the values common for the weak intermolecular complexes to that approaching a fully developed covalent bond. This continuity of halogen bond strength in the experimental (solid-state) and calculated associations indicates a fundamental link between the covalent and supramolecular bonding.


Introduction
Halogen bonding, R-X···D, is a directional attraction of electron-rich species D to electrophilic halogen atoms, X (covalently bonded to a group or atom R) [1,2]. Many experimental and computational studies demonstrated that its strength can be varied considerably by changing the nature of XB donors and acceptors [1][2][3][4][5][6][7]. The directionality of halogen bonding and the possibility of modulation of its strength open the way to a variety of applications of this intermolecular interaction in areas ranging from crystal engineering of solid-state materials and molecular recognition, to catalysis and rational drug design [1,2,8]. Besides practical applications, the XB variability provides an opportunity to explore the nature of supramolecular bonding and the boundary between such bonding and the covalent bond [6].
Most frequently, halogen bonding (XB) is explained using an electrostatic model which relates this interaction primarily to the attraction of electron-rich nucleophiles to the areas of positive potentials (σ-hole) on the surfaces of the covalently bonded halogen atoms [9]. These areas are formed along the extension of the covalent R-X bonds, and the positive potentials are enhanced by the presence of electron withdrawing substituents in R, as well as by high polarizability of the large bromine and iodine CBr 3 NO 2 and 12 mg (0.011 mmol) of DABCO in 5 mL of dry dichloromethane in a Schlenk tube. The resulting solution was carefully layered with 2 mL of a 1:1 dichloromethane/hexane mixture, and then with hexane (20 mL), and kept at −35 • C. Slow diffusion of hexane into the dichloromethane resulted in the formation of colorless crystals suitable for single crystal X-ray measurements. Co-crystals of (DABCO-CH 2 Cl)Cl·CBr 3 CONH 2 were obtained in a similar way using 30 mg (0.010 mmol) of tribromoacetamide instead of tribromonitromethane.
The single crystal structures (except (DABCO-CH 2 Cl)Cl·CBr 3 NO 2 ) were examined on a Bruker Quest diffractometer with a fixed chi angle, a sealed tube fine-focus X-ray tube, single crystal curved graphite incident beam monochromator, a Photon100 area detector and an Oxford Cryosystems low-temperature device (all X-ray instrumentation was from Bruker AXS Inc., Madison, WI, USA). Data were collected at 150 K. Examination and data collection were performed with Mo Kα radiation (λ = 0.71073 Å). A single crystal of (DABCO-CH 2 Cl)Cl·CBr 3 NO 2 was analyzed with a Bruker Kappa APEX CCD area detector diffractometer with (microsource) Cu Kα radiation (λ = 1.54178 Å) at 100 K. Reflections were indexed and processed, and the files were scaled and corrected for absorption using APEX3 [27]. The space groups were assigned using XPREP within the SHELXTL suite of programs [28], the structures were solved by direct methods and refined by full matrix least squares against F 2 with all reflections using Shelxl2018 [29,30] using the graphical interface Shelxle [31]. If not specified otherwise, H atoms attached to carbon and nitrogen atoms were positioned geometrically and constrained to ride on their parent atoms, with C-H bond distances of 1.00, 0.99 and 0.98 Å for aliphatic C-H, CH 2 and CH 3 moieties and N-H distances of 0.88 Å, respectively. Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. U iso (H) values were set to a multiple of U eq (C) with 1.5 for CH 3 , and 1.2 for C-H and N-H units, respectively. X-ray structural data were presented using the Mercury 2020.2.0 program [32].
Complete crystallographic data, in CIF format, have been deposited with the Cambridge Crystallographic Data Centre. CCDC 2020744-2020749 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.

Computations
Quantum-mechanical calculations were carried out using the Gaussian 09 suite of programs [33]. Geometries of electrophiles and halogen-bonded R-Br . . . Cl complexes were optimized without constraints in the gas phase and in dichloromethane via DFT calculations with the M06-2X functional and the 6-311+G(d,p) basis set [34]. Previous theoretical analyses indicated that such commutations provide excellent geometries and energies at a reasonable computational cost [35][36][37]. Importantly, our earlier studies demonstrated that characteristics of the XB complexes between R-Br electrophiles with halide or pseudohalide anions obtained via such computations were in good agreement with experimental data [15,21,38,39]. Since previous works showed that the solid-state environment is best modelled using calculations with a moderately polar solvents as the medium [15], the geometry optimizations in the current work were done via computations in the gas phase and in dichloromethane (the latter were carried out using a Polarizable Continuum Model [40]). The absence of imaginary vibrational frequencies confirmed that the optimized structures represent true minima. Values of halogen-bond energies ∆E were determined as: ∆E = E comp − (E R-Br + E Cl ) + BSSE, where E comp , E R-Br and E Cl are sums of the electronic and ZPE of the optimized complex, R-Br and chloride, respectively, and BSSE is a basis set superposition error [41]. Since formation of complexes involve deformation of R-Br, these values can be expressed as ∆E = ∆E strain + ∆E int , where ∆E int is an interaction energy between deformed reactants [42]. The ∆E int values follow the same trends as those of ∆E (see Figure S3 in the Supporting Information). Energies and atomic coordinates of the calculated complexes are listed in the Supporting Information.

Results and Discussion
3.1. Crystallization and X-ray Structural Characterization of the Solid-State Associates of R-Br with Cl − Evaporation of a solution containing a mixture of tribromofluoromethane and Pr 4 NCl (1:1 molar ratio) in acetone resulted in formation of colorless block-like crystals containing both the organic molecule and the chloride salt. X-ray structural analysis of these monoclinic co-crystals 1 (Pn space group) revealed the presence of quasi-2D networks formed by brominated electrophiles and chloride anions. Each CBr 3 F molecule in these networks is coordinated (through its three bromine substituents) with three Cl − anions. In turn, each chloride is bonded to three CBr 3 F molecules. Such 3:3 CBr 3 F/Cl − bonding led to formation of zigzag networks consisting of twelve-membered rings. Since C-Br . . . Cl − fragments are close to linear, these rings resemble cyclohexane in a chair conformation (Figure 1a).

Computations
Quantum-mechanical calculations were carried out using the Gaussian 09 suite of programs [33]. Geometries of electrophiles and halogen-bonded R-Br … Cl complexes were optimized without constraints in the gas phase and in dichloromethane via DFT calculations with the M06-2X functional and the 6-311+G(d,p) basis set [34]. Previous theoretical analyses indicated that such commutations provide excellent geometries and energies at a reasonable computational cost [35][36][37]. Importantly, our earlier studies demonstrated that characteristics of the XB complexes between R-Br electrophiles with halide or pseudohalide anions obtained via such computations were in good agreement with experimental data [15,21,38,39]. Since previous works showed that the solid-state environment is best modelled using calculations with a moderately polar solvents as the medium [15], the geometry optimizations in the current work were done via computations in the gas phase and in dichloromethane (the latter were carried out using a Polarizable Continuum Model [40]). The absence of imaginary vibrational frequencies confirmed that the optimized structures represent true minima. Values of halogen-bond energies ΔE were determined as: ΔE = Ecomp − (ER-Br + ECl) + BSSE, where Ecomp, ER-Br and ECl are sums of the electronic and ZPE of the optimized complex, R-Br and chloride, respectively, and BSSE is a basis set superposition error [41]. Since formation of complexes involve deformation of R-Br, these values can be expressed as ΔE = ΔEstrain + ΔEint, where ΔEint is an interaction energy between deformed reactants [42]. The ΔEint values follow the same trends as those of ΔE (see Figure S3 in the Supporting Information). Energies and atomic coordinates of the calculated complexes are listed in the Supporting Information.

Crystallization and X-ray Structural Characterization of the Solid-State Associates of R-Br with Cl −
Evaporation of a solution containing a mixture of tribromofluoromethane and Pr4NCl (1:1 molar ratio) in acetone resulted in formation of colorless block-like crystals containing both the organic molecule and the chloride salt. X-ray structural analysis of these monoclinic co-crystals 1 (Pn space group) revealed the presence of quasi-2D networks formed by brominated electrophiles and chloride anions. Each CBr3F molecule in these networks is coordinated (through its three bromine substituents) with three Cl − anions. In turn, each chloride is bonded to three CBr3F molecules. Such 3:3 CBr3F/Cl − bonding led to formation of zigzag networks consisting of twelve-membered rings. Since C-Br … Cl − fragments are close to linear, these rings resemble cyclohexane in a chair conformation (Figure 1a).  The CBr 3 F molecules and Cl − anions propagated along a axes. The Pr 4 N + counter-ions are located in the channels of the XB network ( Figure 1b).
Analogous co-crystallizations of Pr 4 NCl with tetrabromomethane produced a salt co-crystal 2 which comprised hybrid networks formed by a 3:3 halogen-bonded association between CBr 4 and Cl − anions. They consisted of twelve-membered rings similar to that in the co-crystals with CBr 3 F ( Figure 2). The CBr3F molecules and Cl − anions propagated along a axes. The Pr4N + counter-ions are located in the channels of the XB network ( Figure 1b).
. The three crystallographically non-equivalent Br … Cl halogen bond lengths of 3.0650 (7) Å, 3.1769 (7) Å and 3.2431 (7) Å in these co-crystals were in the same range as those in the crystals with CBr3F. The fourth bromine atom in CBr4 formed a symmetric contact referred to as Type I halogen-halogen contact [1] with a symmetry equivalent CBr4 bromine atom from another layer ( Figure 2). The Br⋅⋅⋅Br distance is 3.3711 (7) Å and both C-Br⋅⋅⋅Br angles are 151.62 (7) o . The Pr4N + counter-ions in these crystals filled the channels formed by stacking of the XB layers propagating along the b axis. In should be mentioned in this respect that previous works showed the existence of two types of close halogenhalogen contacts with a clear geometric and chemical distinctions. All R-Br⋅⋅⋅Cl − interactions described herein follow description of Type II contacts, which occur between a nucleophile and the positively-charged areas on the surface of XB donor. These XB contacts are characterized by the R-Br⋅⋅⋅C − angles of approximately 180 o . In comparison, Type I contacts represent interactions between identical areas on the surfaces of two covalently-bonded halogens in the two R-X molecules, with the equal R-X⋅⋅⋅X angles (usually in the 150 o -160 o range). Resnati, Metrangolo et al. pointed out [1] that (symmetrical) Type I contacts are related to close-packing requirements and they are not halogen bonding (which occur between electrophilic and nucleophilic sites according to the IUPAC definition).
Crystallization from a solution containing a mixture of Pr4NCl and hexabromoacetone resulted in formation of co-crystals 3 showing quazi-2D layers formed by 3:3 halogen bonding between the brominated electrophile and the anion ( Figure 3). In contrast to the layers described above, they consisted of two types of cells. The smaller cells were formed by pairs of CBr3COCBr3 and pairs of Cl − . The larger cells comprised four XB donors linked by four chloride anions ( Figure 3). The three crystallographically non-equivalent Br . . . Cl halogen bond lengths of 3.0650 (7) Å, 3.1769 (7) Å and 3.2431 (7) Å in these co-crystals were in the same range as those in the crystals with CBr3F. The fourth bromine atom in CBr 4 formed a symmetric contact referred to as Type I halogen-halogen contact [1] with a symmetry equivalent CBr 4 bromine atom from another layer ( Figure 2). The Br···Br distance is 3.3711 (7) Å and both C-Br···Br angles are 151.62 (7) • . The Pr 4 N + counter-ions in these crystals filled the channels formed by stacking of the XB layers propagating along the b axis. In should be mentioned in this respect that previous works showed the existence of two types of close halogen-halogen contacts with a clear geometric and chemical distinctions. All R-Br···Cl − interactions described herein follow description of Type II contacts, which occur between a nucleophile and the positively-charged areas on the surface of XB donor. These XB contacts are characterized by the R-Br···C − angles of approximately 180 • . In comparison, Type I contacts represent interactions between identical areas on the surfaces of two covalently-bonded halogens in the two R-X molecules, with the equal R-X···X angles (usually in the 150 • -160 • range). Resnati, Metrangolo et al. pointed out [1] that (symmetrical) Type I contacts are related to close-packing requirements and they are not halogen bonding (which occur between electrophilic and nucleophilic sites according to the IUPAC definition).
Crystallization from a solution containing a mixture of Pr 4 NCl and hexabromoacetone resulted in formation of co-crystals 3 showing quazi-2D layers formed by 3:3 halogen bonding between the brominated electrophile and the anion (Figure 3). In contrast to the layers described above, they consisted of two types of cells. The smaller cells were formed by pairs of CBr 3 COCBr 3 and pairs of Cl − . The larger cells comprised four XB donors linked by four chloride anions ( Figure 3). Crystals 2020, 10, x 6 of 15      Diffusion of hexane into a dichloromethane solution containing 1,4-diazabicyclo[2.2.2]octane (DABCO) and either tribromoacetamide or tribromonitromethane resulted in formation of co-crystals 5 and 6 containing R-Br, Cl − anions and the DABCO-CH 2 Cl + counter-ions. Such crystallization utilized the Menshutkin reaction between DABCO and CH 2 Cl 2 (producing Cl − ) in the presence of the XB donors [26]. Co-crystals 5 of CBr 3 NO 2 with Cl − anions and DABCO-CH 2 Cl + counter-ions obtained in this way showed 2D layers ( Figure 5). The layers consisted of chair-like cells formed by 3:3 bonding of Cl − and CBr3NO2 (similar to that obtained with CBR3F or CBr4 with Pr4NCl), with Br … Cl − bond lengths of 3.1420 (17) Å, 3.1728 (16) Å and 3.1715 (16) Å. Stacking of the XB layers created channels that are filled with DABCO-CH2Cl + counter-ions (which form also hydrogen bonds with the chloride anions, Figure S1 in the Supporting Information).
The analogous reaction between DABCO and CH2Cl2 in the presence of tribromoacetamide led to formation of (DABCO-CH2Cl)Cl·CBr3CONH2 crystals 6. They showed multiple types of halogen and hydrogen bonding ( Figure 6). Most relevant for the current work was 2:2 bonding between bromine substituents of CBr3CONH2 and Cl − anions in which each chloride is bonded with two electrophiles and two substituents of the latter that are bonded to two Cl − . The lengths of these bonds were 3.0457 (12) Å and 3.2770 (12) Å. The CBr3CONH2 molecules are also hydrogen-bonded (via the NH2 group) with another Cl − anion ( Figure 6). Besides, co-crystals showed a halogen Cl … Cl − bond of 3.2479 (16) Å between chlorine substituent of DABCO-CH2Cl + and Cl − anions, as well as two hydrogen bonds between the cations and chlorides.   Figure S1 in the Supporting Information).
The analogous reaction between DABCO and CH 2 Cl 2 in the presence of tribromoacetamide led to formation of (DABCO-CH 2 Cl)Cl·CBr 3 CONH 2 crystals 6. They showed multiple types of halogen and hydrogen bonding ( Figure 6). Most relevant for the current work was 2:2 bonding between bromine substituents of CBr 3 CONH 2 and Cl − anions in which each chloride is bonded with two electrophiles and two substituents of the latter that are bonded to two Cl − . The lengths of these bonds were 3.0457 (12) Å and 3.2770 (12) Å. The CBr 3 CONH 2 molecules are also hydrogen-bonded (via the NH 2 group) with another Cl − anion ( Figure 6). Besides, co-crystals showed a halogen Cl . . . Cl − bond of 3.2479 (16) Å between chlorine substituent of DABCO-CH 2 Cl + and Cl − anions, as well as two hydrogen bonds between the cations and chlorides. The layers consisted of chair-like cells formed by 3:3 bonding of Cl − and CBr3NO2 (similar to that obtained with CBR3F or CBr4 with Pr4NCl), with Br … Cl − bond lengths of 3.1420 (17) Å, 3.1728 (16) Å and 3.1715 (16) Å. Stacking of the XB layers created channels that are filled with DABCO-CH2Cl + counter-ions (which form also hydrogen bonds with the chloride anions, Figure S1 in the Supporting Information).
It should be noted, however, that the Br . . . Cl distances vary substantially in the solid-state associations of Cl − anion with the same R-Br electrophile (Table 1). For example, the co-crystals of CBr 4 with chloride show Br . . . Cl distances from 3.07 Å to 3.24 Å, and those of CBr 3 NO 2 show distances from 2.99 Å to 3.27 Å. Apparently, such bonding is characterized by a shallow energy minimum, and its length can vary considerable in response to crystal forces. Furthermore, each XB donor in these solid-state networks is bonded with several Cl − anions and each anion is bonded to several electrophiles. Such multiple bondings can noticeably affect the XB length in the solid state [46]. Thus, to verify variations in the XB length derived from the X-ray crystallographic data, we carried out computational analysis of 1:1 XB complexes. focused on the minima showing single halogen bonding between reactants (with the X-Br-Cl angles in the range between 170 • and 180 • , which is common for halogen bonding). The calculated interaction energies, ∆E, and the Br . . . Cl − separations, d Br···Cl , in the XB complexes calculated in dichloromethane are listed in Table 2. The energies of the complexes and individual components, X-Br-Cl angles and X-Br bond length, as well as the corresponding characteristics of the complexes obtained from the calculations in vacuum and atomic coordinates of all complexes are presented in the Supporting Information. For comparison, Table 2 also presents characteristics of the BrCl molecule, which could be formally considered as a result of interaction between a Br + electrophile and a Cl − nucleophile.  [37]. 4 Ratio of X ·· Br bond lengths (where X = C, N, Cl, F or Br) in the separate and halogen-bonded R-Br electrophile. 5 Charge on the halogen-bonded Cl atom, from NBO analysis [47].

Quantum-Mechanical
Comparison with the experimental data in Table 1 demonstrates that the Br . . . Cl distances in the complexes calculated in dichloromethane are mostly within the range of the Br . . . Cl separations observed in the solid-state associations. The correlation between calculated and average experimental bond length in these complexes is characterized by an R 2 of 0.94 (see Figure S2 in the Supporting Information). The Br . . . Cl distances in the calculated gas-phase complexes are, on average, approximately 0.25 Å shorter than that in dichloromethane. As such, they are also substantially shorter than that measured for the corresponding solid-state associations (Table 1). Further, ∆E values for the XB complexes of Cl − anions with the same R-Br molecule are more negative for the pairs calculated in vacuum than for the corresponding pairs calculated in dichloromethane. These results are consistent with the data reported earlier showing a higher bonding strength of the XB complexes involving an anionic XB acceptor when calculated in the gas-phase [15,22]. Further, while most of the complexes calculated in dichloromethane and all complexes calculated in the gas phase are characterized by the negative ∆E values (i.e., they are thermodynamically stable (Table S1 in the Supporting Information), the complexes with the weakest R-Br electrophiles calculated in dichloromethane are characterized by the positive ∆E values (i.e., they are only kinetically stable).
Most notably, the data in Table 2 and Table S1 in the Supporting Information show a gradual change in the XB Br . . . Cl length from the values which are close to the sum of the van der Waals radii of these atoms (R~1) to those which are 40% shorter than these separations. The latter are within 10% of the covalent Br-Cl bond (2.14 Å). The decrease in the Br . . . Cl distances is accompanied by a progressive increase in the interaction strength ( Table 2 and Table S1 in the Supporting Information). Besides, in the stronger complexes, the shortening of the Br . . . Cl distance is accompanied by an elongation of the X-Br bond length (see the d X-Br com /d X-Br sep ratios in Table 2 and in Table S1 in the Supporting Information). Importantly, the appropriate choice of R-Br electrophiles in the series of the complexes in Tables 1 and 2 (and Table S1 in the Supporting Information) allowed eliminating any large changes between subsequent entries. As such, the energies and interatomic distances in these series cover the whole range between weak van der Waals complexes and the covalently-bonded BrCl molecule leaving no substantial gaps between subsequent entries (Figure 7).
Crystals 2020, 10, x 10 of 15 calculated in vacuum than for the corresponding pairs calculated in dichloromethane. These results are consistent with the data reported earlier showing a higher bonding strength of the XB complexes involving an anionic XB acceptor when calculated in the gas-phase [15,22]. Further, while most of the complexes calculated in dichloromethane and all complexes calculated in the gas phase are characterized by the negative ΔE values (i.e., they are thermodynamically stable (Table S1 in the  Supporting Information), the complexes with the weakest R-Br electrophiles calculated in dichloromethane are characterized by the positive ΔE values (i.e., they are only kinetically stable).
Most notably, the data in Table 2 and Table S1 in the Supporting Information show a gradual change in the XB Br … Cl length from the values which are close to the sum of the van der Waals radii of these atoms (R ~ 1) to those which are 40% shorter than these separations. The latter are within 10% of the covalent Br-Cl bond (2.14 Å). The decrease in the Br … Cl distances is accompanied by a progressive increase in the interaction strength ( Table 2 and Table S1 in the Supporting Information). Besides, in the stronger complexes, the shortening of the Br … Cl distance is accompanied by an elongation of the X-Br bond length (see the dX-Br com /dX-Br sep ratios in Table 2 and in Table S1 in the Supporting Information). Importantly, the appropriate choice of R-Br electrophiles in the series of the complexes in Tables 1 and 2 (and Table S1 in the Supporting Information) allowed eliminating any large changes between subsequent entries. As such, the energies and interatomic distances in these series cover the whole range between weak van der Waals complexes and the covalently-bonded BrCl molecule leaving no substantial gaps between subsequent entries (Figure 7). As the distance between Br and Cl decreased, substantial parts of the negative charge which was originally residing on the chloride anions were transferred onto the R-Br electrophile (the relation between the intermolecular distances and amount of charge transfer Δq can be described by the equation dBr…Cl= −0.303ln(Δq) + 2.2121 with R 2 of 0.98; see Figure S4 in the Supporting Information). Accordingly, the highest occupied molecular orbitals (HOMO) of the weak complexes are almost completely localized on the chloride anions, and their lowest unoccupied molecular orbital is on the R-Br electrophiles. The HOMOs and LUMOs of the strong complexes are delocalized over both counterparts ( Figure 8). As such, while the HOMO→LUMO electronic transition in the weak complexes represents essentially a charge transfer from the donor to acceptor, the analogous transition in the strong complexes does not involve a substantial charge redistribution between the reactants. Finally, due to substantial elongation of the N-Br bond in the complexes between Nbromopyrazinium and chloride and similar complexes in the bottom of Table 2, these associations As the distance between Br and Cl decreased, substantial parts of the negative charge which was originally residing on the chloride anions were transferred onto the R-Br electrophile (the relation between the intermolecular distances and amount of charge transfer ∆q can be described by the equation d Br . . . Cl = −0.303ln(∆q) + 2.2121 with R 2 of 0.98; see Figure S4 in the Supporting Information). Accordingly, the highest occupied molecular orbitals (HOMO) of the weak complexes are almost completely localized on the chloride anions, and their lowest unoccupied molecular orbital is on the R-Br electrophiles. The HOMOs and LUMOs of the strong complexes are delocalized over both counterparts ( Figure 8). As such, while the HOMO→LUMO electronic transition in the weak complexes represents essentially a charge transfer from the donor to acceptor, the analogous transition in the strong complexes does not involve a substantial charge redistribution between the reactants. Finally, due to substantial elongation of the N-Br bond in the complexes between N-bromopyrazinium and chloride and similar complexes in the bottom of Table 2, these associations can be described as an association between a group R and a BrCl molecule (Figure 8, right). This underlines the fact that as the Br . . . Cl distance decreases, the intermolecular bond is gradually transformed into a covalent bond and the intramolecular R-Br bond is converted into an intermolecular one. It is important to stress the continuous character of these changes, which underscore the direct link between the limiting types (intra-and intermolecular) of bonds.
can be described as an association between a group R and a BrCl molecule (Figure 8, right). This underlines the fact that as the Br … Cl distance decreases, the intermolecular bond is gradually transformed into a covalent bond and the intramolecular R-Br bond is converted into an intermolecular one. It is important to stress the continuous character of these changes, which underscore the direct link between the limiting types (intra-and intermolecular) of bonds.

Conclusions
Experimental and computational studies on a series of complexes formed by R-Br electrophiles and chloride anions showed a wide variation of the structural and thermodynamic features of these associations. At the one extreme, the intermolecular Br···Cl distances were within 5% of the sum of the van der Waals radii of bromine and chloride, and the interaction energies were close to zero. Halogen bonding within these complexes was accompanied by a very small charge transfer and such intermolecular interaction practically did not affect intramolecular bond length in the R-Br electrophiles. At the other extreme, the series comprised complexes in which the Br···Cl distances were within 10% of a covalent Br-Cl bond. Charge delocalization between R-Br and Cl − in these associations was accompanied by the elongation of the intramolecular X-Br bond. As such, these complexes could be considered as an association of a neutral or anionic residue R with a BrCl molecule. The series under study contained complexes with characteristics covering the whole range of values between the limiting cases, with no substantial gaps between successive entries. It showed that the characteristics of the Br···Cl bond can change gradually from the values typical for intermolecular associates to that of a covalent Br-Cl bond. Together with the similar series of complexes between R-Br and DABCO or between diiodine and heteroaromatic N-oxides [6,7], and other systems [48,49] this indicates a generality of the continuum between the limiting types of bonding.
It should be noted that the involvement of the weakly-covalent interactions in the formation of XB complexes represents a major topic of the ongoing discussions about the nature and properties of halogen bonding (and many other supramolecular interaction, e.g., hydrogen bonding [50,51] or multicenter π-bonding between ion-radicals [52,53]). Indeed, the electrostatic (σ-hole) model explains linear geometry and (if polarization is taken into account) energy variations in a large number of XB complexes [9,10]. Yet, many computational studies indicated that covalent (charge transfer) interactions represent an important part of the bonding in many such associates [11][12][13]. Experimental studies of the XB complexes also revealed many features which can be explained by involvement of charge transfer. For example, photoelectron spectra of XB complexes and chargedensity analysis directly demonstrated charge transfer from XB acceptor (nucleophile) to XB donor (electrophile) indicating covalency of halogen bonding [16,19]. Furthermore, appearance of a strong absorption bands in the UV-Vis spectra of complexes and structural characteristics of many such associates suggest that the molecular-orbital interactions between XB species play a vital role in their formation [15,18]. While such features are usually most pronounced in the strong XB complexes, the

Conclusions
Experimental and computational studies on a series of complexes formed by R-Br electrophiles and chloride anions showed a wide variation of the structural and thermodynamic features of these associations. At the one extreme, the intermolecular Br···Cl distances were within 5% of the sum of the van der Waals radii of bromine and chloride, and the interaction energies were close to zero. Halogen bonding within these complexes was accompanied by a very small charge transfer and such intermolecular interaction practically did not affect intramolecular bond length in the R-Br electrophiles. At the other extreme, the series comprised complexes in which the Br···Cl distances were within 10% of a covalent Br-Cl bond. Charge delocalization between R-Br and Cl − in these associations was accompanied by the elongation of the intramolecular X-Br bond. As such, these complexes could be considered as an association of a neutral or anionic residue R with a BrCl molecule. The series under study contained complexes with characteristics covering the whole range of values between the limiting cases, with no substantial gaps between successive entries. It showed that the characteristics of the Br···Cl bond can change gradually from the values typical for intermolecular associates to that of a covalent Br-Cl bond. Together with the similar series of complexes between R-Br and DABCO or between diiodine and heteroaromatic N-oxides [6,7], and other systems [48,49] this indicates a generality of the continuum between the limiting types of bonding.
It should be noted that the involvement of the weakly-covalent interactions in the formation of XB complexes represents a major topic of the ongoing discussions about the nature and properties of halogen bonding (and many other supramolecular interaction, e.g., hydrogen bonding [50,51] or multicenter π-bonding between ion-radicals [52,53]). Indeed, the electrostatic (σ-hole) model explains linear geometry and (if polarization is taken into account) energy variations in a large number of XB complexes [9,10]. Yet, many computational studies indicated that covalent (charge transfer) interactions represent an important part of the bonding in many such associates [11][12][13]. Experimental studies of the XB complexes also revealed many features which can be explained by involvement of charge transfer. For example, photoelectron spectra of XB complexes and charge-density analysis directly demonstrated charge transfer from XB acceptor (nucleophile) to XB donor (electrophile) indicating covalency of halogen bonding [16,19]. Furthermore, appearance of a strong absorption bands in the UV-Vis spectra of complexes and structural characteristics of many such associates suggest that the molecular-orbital interactions between XB species play a vital role in their formation [15,18]. While such features are usually most pronounced in the strong XB complexes, the data presented herein verify continuity of the bonding from the weak supramolecular associations to a covalent bond. This continuum implies an intrinsic link between the limiting types of bonding and, therefore, it suggests an onset of covalency in intermolecular interactions. The progressive increase in its input leads to the transformation of a weak supramolecular interaction into a covalent bond.