Structural Organization of Dibromodiazadienes in the Crystal and Identification of Br···O Halogen Bonding Involving the Nitro Group

Nitro functionalized dibromodiazadiene dyes were prepared and fully characterized including X-ray single crystal analysis. Electron deficient dibromodiazadienes were found to be able to act as donors of halogen bonding (XB), while the nitro group acted as an acceptor of the XB. Depending on the substituents, the Br···O XB competed with other weak interactions, and for some of the dyes, they even outcompeted the XB involving the nitro group. However, the nitro functionalized dibromoalkenes 6a and 10a, which had only the nitro moiety as the most plausible acceptor of the XB, reliably formed 1D chains via Br⋯O XB. Experimental work was supported by the DFT calculations and topological analysis of the electron density distribution within the framework of Bader’s theory (QTAIM method).

Recently, Nenajdenko et al. discovered a remarkable carbon-carbon bond-forming reaction between aryl hydrazones and polyhaloalkanes, induced by the copper catalyst, and leading to halogenated diazabutadienes (Scheme 1) [17]. Furthermore, we demonstrated that the CCl2 moiety in easily polarizable dichlorodiazadienes can act as donors of XB [18,19]. We showed that the Hal⸱⸱⸱Hal interactions dictate a packing preference for halogenated dichlorodiazadienes, a newly discovered class of dyes.
In the course of our exploration of the novel Cu-catalyzed reaction between hydra-Scheme 1. Reaction between aryl hydrazones and CCl 4 , induced by the copper catalyst. Furthermore, we demonstrated that the CCl 2 moiety in easily polarizable dichlorodiazadienes can act as donors of XB [18,19]. We showed that the Hal···Hal interactions dictate a packing preference for halogenated dichlorodiazadienes, a newly discovered class of dyes.
In the course of our exploration of the novel Cu-catalyzed reaction between hydrazones and polyhaloalanes [18][19][20], and following our interest in non-covalent interactions [21][22][23], here we describe the coupling between CBr 4 and nitro-functionalized hydrazones, which results in the formation of the mixture of dibromodiazadienes and dibromoalkenes via N 2 extrusion. Multiple XB in the solid state for both dibromodiazadienes and dibromoalkenes were studied theoretically by means of DFT calculations and topological analysis of the electron density distribution within the formalism of Bader's theory (QTAIM method).

Results and Discussion
Dibromodiazadienes 1-15 were prepared employing CBr 4 (Scheme 2), in a similar fashion as earlier described dichlorodiazadienes [17,18]. Dibromo dyes were isolated in high yields (50-63%) as red solids. Interestingly, for the coupling of primary hydrazones with CBr 4 we observed the formation of dibromoalkenes in a significant amount (19-27%, Scheme 2). Furthermore, we demonstrated that the CCl2 moiety in easily polarizable dichlorodiazadienes can act as donors of XB [18,19]. We showed that the Hal⸱⸱⸱Hal interactions dictate a packing preference for halogenated dichlorodiazadienes, a newly discovered class of dyes.
In the course of our exploration of the novel Cu-catalyzed reaction between hydrazones and polyhaloalanes [18][19][20], and following our interest in non-covalent interactions [21][22][23], here we describe the coupling between CBr4 and nitro-functionalized hydrazones, which results in the formation of the mixture of dibromodiazadienes and dibromoalkenes via N2 extrusion. Multiple XB in the solid state for both dibromodiazadienes and dibromoalkenes were studied theoretically by means of DFT calculations and topological analysis of the electron density distribution within the formalism of Bader's theory (QTAIM method).
According to what we expected, the nitro group in the dibromo-dyes' backbone acted as an acceptor of the XB involving C=CBr2 fragment and had a dramatic impact on the packing in the crystal. Compound 1, featuring o-nitrophenyl substituent by the C=C double bond, exhibited type 1 Br···Br contacts ( Figure 1). Additionally, the nitro group formed Br···O XB with one of the bromine atoms of the C=CBr2 fragment ( Figure 1).
The identity and purity of 1a, 6a, 10a and 1-15 was confirmed by the 1 H and 13 C NMR spectroscopies and single crystal X-ray diffraction analysis for 1, 8, 13, 15, 6a and 10a (Figures 1-7). Bond lengths and angles are similar to what was observed earlier for relevant diazabutadienes and azocompounds [17,19,24,25]. Interestingly, the introduction of the fluorine in the para position of the aryl group by the azo functionality had no impact on the dyes self-assembly in the solid state: akin 1 compound 4 featured Br···Br and Br···O XB in the crystal, while the F atom was not involved in any XB ( Figure 2).  brown, red, light gray and gray spheres represent nitrogen, bromine, oxygen, carbon and hydrogen atoms, respectively.
Interestingly, the introduction of the fluorine in the para position of the aryl group by the azo functionality had no impact on the dyes self-assembly in the solid state: akin 1 compound 4 featured Br···Br and Br···O XB in the crystal, while the F atom was not involved in any XB ( Figure 2). However, compound 12, which is an isomer of 4 and contains a nitro group in a para position, did not exhibit Br···O XB ( Figure 3). In this case, other weak interactions outcompeted the formation of the contact between the nitro group and XB donating Br atom. Like 1 and 4, 12 also featured Br···Br XB, but they were rather type 2 contacts ( Figure 3).  Interestingly, switching from the F to the Cl or Br substituents had a dramatic impact on the dyes' self-assembly in the solid state. The neighboring dibromodiazadiene molecules in the crystal of 13 or 14 featured Br···Cl and Br···Br contacts, respectively, and a remarkable combination of "chelating" Br⋯N and Br⋯H non-covalent interactions (Figure 4). The latter type of supramolecular structural motif was not observed for the earlier described dichlorodiazadienes [18], and was arguably related to the larger size and softness of the Br atom in the dibromo dyes. No XB involving the nitro group was observed for 13 or 14. According to what we expected, the nitro group in the dibromo-dyes' backbone acted as an acceptor of the XB involving C=CBr 2 fragment and had a dramatic impact on the packing in the crystal. Compound 1, featuring o-nitrophenyl substituent by the C=C double bond, exhibited type 1 Br···Br contacts ( Figure 1). Additionally, the nitro group formed Br···O XB with one of the bromine atoms of the C=CBr 2 fragment ( Figure 1).
Interestingly, the introduction of the fluorine in the para position of the aryl group by the azo functionality had no impact on the dyes self-assembly in the solid state: akin 1 compound 4 featured Br···Br and Br···O XB in the crystal, while the F atom was not involved in any XB ( Figure 2). However, compound 12, which is an isomer of 4 and contains a nitro group in a para position, did not exhibit Br···O XB ( Figure 3). In this case, other weak interactions outcompeted the formation of the contact between the nitro group and XB donating Br atom. Like 1 and 4, 12 also featured Br···Br XB, but they were rather type 2 contacts ( Figure 3).  Furthermore, switching from the para (14) to the meta (8) nitro substitution had some interesting implications to the dyes' self-assembly in the solid state. It was found that 8 also featured Br···Br contacts via one of the Br atoms of the CBr2 fragment. The second Br atom of the dibromoalkene fragment was involved in the Br⋯O XB with the nitro functionality ( Figure 5). In this case, Br⋯O XB outcompeted the formation of "chelating" Br⋯N and Br⋯H non-covalent interactions. Finally, when the Me group was in the para position of the aryl substituent by the azo fragment (compound 15), only one of the Br atoms of the dibromodiazadiene was involved in the XB, "chelating" Br⋯N and Br⋯H interactions; the structural motif which was already found for 13 and 14 ( Figure 6). No Br⋯O XB with the nitro functionality was observed for 15. Finally, when the Me group was in the para position of the aryl substituent by the azo fragment (compound 15), only one of the Br atoms of the dibromodiazadiene was involved in the XB, "chelating" Br⋯N and Br⋯H interactions; the structural motif which was already found for 13 and 14 ( Figure 6). No Br⋯O XB with the nitro functionality was observed for 15. Figure 6. Ball-and-stick representations of 15 demonstrating "chelating" Br⋯N and Br⋯H interactions in the crystal. Blue, brown, red, light gray and gray spheres represent nitrogen, bromine, oxygen, carbon, and hydrogen atoms, respectively.
In addition, we obtained single crystals of dibromoalkenes 6a and 10a carrying the nitro group in the meta and para positions of the aryl substituent, respectively ( Figure 7). An electron deficient dibromoalkene fragment was expectedly involved in the XB. In these cases, we expected that the only possible acceptor of the XB could be the nitro group, and it was indeed found to form the XB with the Br atoms ( Figure 7). To prove the existence and approximately quantify the strength of intermolecular interactions of Br···NO2 in the obtained compounds, the DFT calculations followed by the topological analysis of the electron density distribution were carried out at the ωB97XD/6-311G* level of theory for model supramolecular associates (see Computational details and Table S1 in Supporting Information; note that inspection of the Cambridge Structural Database (CSD) reveals 10 examples of known X-ray structures featuring similar intermolecular interactions to Br···NO2, see Table S2). The existence of these non-covalent interactions was justified by the presence of bond critical points (3, −1) for appropriate intermolecular contacts and their lengths are shorter than the vdW radii sums of corresponding interacting atoms. Results of the QTAIM analysis are summarized in Table 1. The contour line diagrams of the Laplacian of electron density distribution  2 (r), bond paths, and selected zero-flux surfaces, visualization of electron localization function (ELF) and reduced density gradient (RDG) analyses for intermolecular interactions of Br···NO2 in the X-ray structures 6a and 15 are shown in Figures 8 and 9. Table 1. Values of the density of all electrons-(r), Laplacian of electron density- 2 (r) and appropriate λ2 eigenvalue, energy density-Hb, potential energy density-V(r), and Lagrangian kinetic energy-G(r) (a.u.) at the bond critical point (3, −1), corresponding to intermolecular interactions Br···NO2 in the obtained X-ray structures and estimated strength for these interactions Eint (kcal/mol).  Interestingly, switching from the F to the Cl or Br substituents had a dramatic impact on the dyes' self-assembly in the solid state. The neighboring dibromodiazadiene molecules in the crystal of 13 or 14 featured Br···Cl and Br···Br contacts, respectively, and a remarkable combination of "chelating" Br· · · N and Br· · · H non-covalent interactions (Figure 4). The latter type of supramolecular structural motif was not observed for the earlier described dichlorodiazadienes [18], and was arguably related to the larger size and softness of the Br atom in the dibromo dyes. No XB involving the nitro group was observed for 13 or 14.
Furthermore, switching from the para (14) to the meta (8) nitro substitution had some interesting implications to the dyes' self-assembly in the solid state. It was found that 8 also featured Br···Br contacts via one of the Br atoms of the CBr 2 fragment. The second Br atom of the dibromoalkene fragment was involved in the Br· · · O XB with the nitro functionality ( Figure 5). In this case, Br· · · O XB outcompeted the formation of "chelating" Br· · · N and Br· · · H non-covalent interactions.
Finally, when the Me group was in the para position of the aryl substituent by the azo fragment (compound 15), only one of the Br atoms of the dibromodiazadiene was involved in the XB, "chelating" Br· · · N and Br· · · H interactions; the structural motif which was already found for 13 and 14 ( Figure 6). No Br· · · O XB with the nitro functionality was observed for 15.
In addition, we obtained single crystals of dibromoalkenes 6a and 10a carrying the nitro group in the meta and para positions of the aryl substituent, respectively (Figure 7). An electron deficient dibromoalkene fragment was expectedly involved in the XB. In these cases, we expected that the only possible acceptor of the XB could be the nitro group, and it was indeed found to form the XB with the Br atoms (Figure 7).
To prove the existence and approximately quantify the strength of intermolecular interactions of Br···NO 2 in the obtained compounds, the DFT calculations followed by the topological analysis of the electron density distribution were carried out at the ωB97XD/6-311G* level of theory for model supramolecular associates (see Computational details and Table S1 in Supporting Information; note that inspection of the Cambridge Structural Database (CSD) reveals 10 examples of known X-ray structures featuring similar intermolecular interactions to Br···NO 2 , see Table S2). The existence of these non-covalent interactions was justified by the presence of bond critical points (3, −1) for appropriate intermolecular contacts and their lengths are shorter than the vdW radii sums of corresponding interacting atoms. Results of the QTAIM analysis are summarized in Table 1. The contour line diagrams of the Laplacian of electron density distribution ∇ 2 ρ(r), bond paths, and selected zero-flux surfaces, visualization of electron localization function (ELF) and reduced density gradient (RDG) analyses for intermolecular interactions of Br···NO 2 in the X-ray structures 6a and 15 are shown in Figures 8 and 9. Table 1. Values of the density of all electrons-ρ(r), Laplacian of electron density-∇ 2 ρ(r) and appropriate λ 2 eigenvalue, energy density-H b , potential energy density-V(r), and Lagrangian kinetic energy-G(r) (a.u.) at the bond critical point (3, −1), corresponding to intermolecular interactions Br···NO 2 in the obtained X-ray structures and estimated strength for these interactions E int (kcal/mol).  [26]. ** E int = 0.57G(r) (correlation developed specifically for noncovalent interactions involving bromine atoms) [27].  [26]. ** Eint = 0.57G(r) (correlation developed specifically for noncovalent interactions involving bromine atoms) [27].
Computational details: The single point calculations based on the experimental X-ray geometries have been carried out at the DFT level of theory using the dispersion-corrected hybrid functional ωB97XD [31] with the help of the Gaussian-09 [32] program package. The 6-311G* basis sets were used for all atoms. The topological analysis of the electron density distribution has been performed by using the Multiwfn program (version 3.7) [33]. The Cartesian atomic coordinates for model supramolecular associates are presented in Table S1, Supporting Information.
Computational details: The single point calculations based on geometries have been carried out at the DFT level of theory using t hybrid functional ωB97XD [31] with the help of the Gaussian-09 The 6-311G* basis sets were used for all atoms. The topological density distribution has been performed by using the Multiwfn pro The Cartesian atomic coordinates for model supramolecular asso Table S1, Supporting Information.

Synthesis of Dibromodiazadiens and Dibromoalkenes
A 20 mL screw neck vial was charged with DMSO (10 mL mmol), tetramethylethylenediamine (TMEDA) (295 mg, 2.5 mm mmol) and CBr4 (1 mmol). After 1-3 hours (until TLC analysis show tion of corresponding Schiff base) the reaction mixture was poure of HCl (100 mL, ~pH = 2), and extracted with dichloromethane (3 × organic phase was washed with water (3 ×50 mL), brine (30 mL) Na2SO4 and concentrated in vacuo. The residue was separated a chromatography on silica gel using appropriate mixtures of hexan (3/1-1/1). Computational details: The single point calculations based geometries have been carried out at the DFT level of theory usi hybrid functional ωB97XD [31] with the help of the Gaussian The 6-311G* basis sets were used for all atoms. The topologi density distribution has been performed by using the Multiwfn The Cartesian atomic coordinates for model supramolecular Table S1, Supporting Information.

Synthesis of Dibromodiazadiens and Dibromoalkenes
A 20 mL screw neck vial was charged with DMSO (10 mmol), tetramethylethylenediamine (TMEDA) (295 mg, 2.5 mmol) and CBr4 (1 mmol). After 1-3 hours (until TLC analysis s tion of corresponding Schiff base) the reaction mixture was po of HCl (100 mL, ~pH = 2), and extracted with dichloromethane organic phase was washed with water (3 ×50 mL), brine (30 m Na2SO4 and concentrated in vacuo. The residue was separate chromatography on silica gel using appropriate mixtures of he (3/1-1/1). Computational details: The single point calculations based on th geometries have been carried out at the DFT level of theory using the hybrid functional ωB97XD [31] with the help of the Gaussian-09 [3 The 6-311G* basis sets were used for all atoms. The topological an density distribution has been performed by using the Multiwfn prog The Cartesian atomic coordinates for model supramolecular assoc Table S1, Supporting Information.

Conclusions
In summary, here we report the synthesis and structural characterization of 11 dibromodiazadiene dyes carrying the nitro group in the backbone. An electron deficient and easily polarizable dibromodiazadiene fragment was involved in multiple XB interactions involving the Br atoms, the strength of which are comparable with energies of Br···Br noncovalent interactions in crystals of Sn(IV) (2.1-4.3 kcal/mol) [14], Bi(III) (1.4-2.5 kcal/mol) [17], and Au(III) (1.6 kcal/mol) [20] bromide complexes, Br···N halogen bonds in 2,5-dibromothiophenes (2.5-2.9 kcal/mol) [15], Br···O contacts in [{AgL} 2 Mo 8 O 26 ] 2− complexes (2.1 kcal/mol) [19], and Cl···Br halogen bonding in bromoaryl-substituted dichlorodiazabutadienes (1.2-1.8 kcal/mol) [13]. For some of the dyes, "chelating" Br· · · N and Br· · · H interactions were identified, which were not observed dichlorodiazadienes. The nitro group was involved in the XB for some cases; however, for some dyes, other weak interactions outcompeted the Br· · · O XB formation. In contrast, the nitro decorated dibromoalkenes 6a and 10a, which had only the nitro moiety as the most plausible acceptor of the XB, reliably formed 1D chains via Br· · · O XB. Experimental work was supported by the DFT calculations and topological analysis of the electron density distribution within the framework of Bader's theory (QTAIM method).