Preparation and X-ray Structural Study of Dibenzobromolium and Dibenzochlorolium Derivatives

Various five-membered cyclic dibenzobromolium salts (dibenzo[b,d]bromol-5-ium chloride, nitrate, hydrogen sulfate, dihydrogen phosphate, trifluoroacetate, and tetrafluoroborate) were prepared by diazotization–cyclization of 2′-bromo-[1,1′-biphenyl]-2-amine in solution of appropriate acids. The chlorolium analogues (iodide, trifluoroacetate, and tetrafluoroborate) were obtained by a similar procedure. Additional dibenzohalolium derivatives (dibenzo[b,d]bromol-5-ium and dibenzo[b,d]chlorol-5-ium azides, bis(trifluoromethanesulfonyl)imidates, thiocyanates, and trifluoromethanesulfonates) were prepared by anion exchange. Structures of ten of these dibenzohalolium derivatives were established by X-ray analysis. Bond distances and angles for the halogen atoms in different dibenzohalolium derivatives were summarized and discussed.


■ INTRODUCTION
Cyclic diaryliodonium salts, which are mainly represented by the five-membered dibenzoiodolium derivatives, have found wide application in organic synthesis and pharmaceutical research. 1he analogous dibenzobromolium and dibenzochlorolium derivatives have been less investigated despite been known since 1952. 2 Very recently, several important synthetic applications of dibenzobromolium salts have been reported, such as halogen-bonding organocatalysts, 3 reagents for crosscoupling reactions with anionic nucleophiles, 4 as aryne precursors in cycloaddition reactions, 5 and reagents for one pot synthesis of benzo [c]-cinnolines and azobenzenes. 6ynthetic utilization of dibenzochlorolium salts as aryne precursors has recently been reported. 7In general, arylchloronium and arylbromonium derivatives have higher reactivity toward nucleophilic organic substrates compared to aryliodonium salts, which justifies synthetic interest in these compounds.It should be noted that previously prepared dibenzobromolium and dibenzochlorolium derivatives were limited to tetrafluoroborate, 3a,4,9 hexafluorophosphate, 9 iodide, 2a chloride, 2a triflate, 3a,5,7 tosylate, 6 and tetraarylborate 3a salts.It is well known that there is a significant secondary bonding between the iodine atom in aryliodonium salt and the counteranion, which has a strong effect on the reactivity of aryliodonium species. 1 It can be expected that the nature of the counterion in dibenzobromolium and dibenzochlorolium derivatives also affects the structure and reactivity of these important reagents.In the present paper, we report convenient experimental procedures for the preparation of dibenzobromolium and dibenzochlorolium salts with various anions and X-ray structural analysis of these compounds.

■ RESULTS AND DISCUSSION
A typical synthetic approach to dibenzobromolium (2) and dibenzochlorolium (4) salts involves thermal decomposition of the corresponding diazonium salts 1 and 3 with a neighboring bromine or chlorine atom (Scheme 1).2a,9 This approach requires the isolation of potentially explosive diazonium salts and affords dibenzohalolium salts 2 and 4 in a relatively low yield.
We have developed a simple and convenient experimental procedure for direct preparation of several dibenzobromolium and dibenzochlorolium salts directly from 2′-bromo-[1,1′biphenyl]-2-amine 5 or 2′-chloro-[1,1′-biphenyl]-2-amine 6 by the diazotization−cyclization sequence in the presence of appropriate acids (Scheme 2).In this procedure, the corresponding diazonium salts were generated from amines 5 and 6 and sodium nitrite in aqueous solution in the presence of strong acids HY at 0 °C.The resulting solution was then heated to reflux until completion of gas evolution, cooled to 0 °C, and white precipitates of products were collected by filtration.This straightforward procedure allows the preparation of products 2, 4, 7, and 8 in better yields (up to 77%) and with wide selection of counteranions.
Dibenzobromolium and dibenzochlorolium salts 2, 7b, and 8b can be further converted to several other derivatives by anion exchange reactions with appropriate inorganic salts (NaN 3 , LiNTf 2 or AgNTf 2 , KSCN, LiOTf or AgOTf) in aqueous methanol solution (Scheme 3).It is noteworthy that the anion exchange approach allows the preparation of novel azides 7f, 8c and thiocyanates 7h, 8f which contain highly nucleophilic anions in their structure.Thermal stability of these derivatives is an unexpected result because arylchloronium and arylbromonium derivatives have exceptionally high reactivity toward nucleophiles, even compared to the highly reactive aryliodonium salts. 8Dibenzoiodolium azide and thiocyanate were previously reported. 10ll products 7 and 8 were obtained as thermally stable, white, microcrystalline solids, and fully characterized by NMR and IR spectroscopy and ESI mass spectrometry.Structures of ten of these products were established by single-crystal X-ray diffraction.The bond and close contact distances and angles for the halogen atoms in these structures are summarized in Table 1.The two halogen−carbon bonds were found to be symmetric throughout the series of dibenzobromolium and dibenzochlorolium compounds.The average distances of the shorter and longer C−X distance are 1.928 and 1.935 Å for the 7 series and 1.779 and 1.789 Å for the 8 series, respectively.The average difference between the longer and shorter halogen− carbon bonds was found to be within 0.02 Å for the 7 and 8 series compounds.The average C1−Br−C12 and C1−Cl1− C12 bond angles of 7 and 8 are 87 and 92°, respectively, with compounds in each series being within about 1°.The close contact distances are discussed using normalized contact (Nc, defined here as the observed distance divided by the sum of the Bondi van der Waals radii).Two close contacts with the halogen Scheme 1. Classic Synthetic Approach to Dibenzobromolium and Dibenzochlorolium Salts 2a,9 Scheme 2. Synthesis of Dibenzobromolium and Dibenzochlorolium Salts of Strong Acids ACS Omega atom form the coordination sphere consistent with halogen bonding (XB). 11The extended four-coordinate environment of 7 and 8 was consistent with the distorted square planar environment.The average Nc of the two close contacts that completed the four-coordinate geometry was 0.85 for 7 and 0.90 for 8 indicating a moderate attractive interaction, with the 7 series interacting stronger than the 8 series.The stronger interactions of the 7 series were also observed by the close contact forming an angle closer to 180°with the halogen and carbon atom compared to a smaller angle for the 8 series.Structures of 7f and 8f are shown as examples of the families of compounds (Table 1, Figures 1, and 2).Additionally, three Br species, 7a, 7b, and 7g, contain bifurcated halogen bonding from a bidentate ligand, but are still consistent with the assignment. 11 survey of several dibenzoiodolium salts resulted in an average Nc of ∼0.81 of close contacts with anions of − OTs, − NTf 2 , Cl − , − SCN, and N 3 − . 7,10,11The trend of average Nc for the halogen bonding in the dibenzohalolium series provides the basis of a structure−function hypothesis for the counteranion-reactivity dependence to be I (0.81) > Br (0.85) > Cl (0.90), with the average Nc value in parentheses.
It is interesting to compare the coordination pattern in structures 7f and 8f with the previously reported X-ray structures of their iodolium analogues, dibenzoiodolium azide, and thiocyanate. 10The coordination spheres of halogen atoms in dibenzoiodolium azide 10 and dibenzobromolium azide 7f are very similar to the I•••N distance 2.749−2.753Å (Nc 0.78) and the Br•••N distance 2.724−2.730Å (Nc 0.81), respectively.Both compounds have a dimeric structure with a square-planar geometry of the iodine or bromine centers.The coordination of halogen atoms in dibenzoiodolium thiocyanate 10 and dibenzochlorolium thiocyanate 8f is different.Dibenzoiodolium thiocyanate has a polymeric structure formed by two short intermolecular contacts between iodine center and two thiocyanate anions.One of these contacts (∼2.79 Å, Nc 0.79) is between iodine and nitrogen, while the second (∼3.11Å, Nc 0.82) is between iodine and sulfur atoms. 10In contrast, dibenzochlorolium thiocyanate 8f has a dimeric structure with a square-planar geometry of the chlorine centers coordinated on nitrogen with Cl•••N distances 2.981−3.133Å (Nc 0.90−0.95).No significant chlorine−sulfur interaction is observed in structure 8f.

■ CONCLUSIONS
In summary, we have developed a convenient experimental procedure for the preparation of dibenzobromolium and dibenzochlorolium salts by the diazotization−cyclization sequence of the appropriate precursors in the presence of strong acids.Additional dibenzohalolium derivatives, most notably azides and thiocyanates, were prepared by anion exchange.Structures of key products, including the first known examples of stable dibenzobromolium azide and dibenzochlorolium thiocyanate, have been established by single-crystal X-ray diffraction analysis.

Scheme 3. Preparation of Dibenzobromolium and Dibenzochlorolium Derivatives by Anion Exchange Reactions with Inorganic Salts
■ EXPERIMENTAL SECTION General Information.All reactions were performed in open air with a stopper and oven-dried glassware.All commercial reagents were of ACS grade and were used without further purification.NMR spectra were recorded using an Oxford 500 and 300 MHz and a Bruker 400 MHz NMR spectrophotometer ( 1 H NMR, 13 C NMR, and 19     PerkinElmer Spectrum 1600 series FT-IR spectrometer.X-ray crystal analysis was performed by Rigaku RAPID II XRD Image Plate using graphite-monochromated Cu or Mo Kα radiation (λ = 1.54187 or 0.71073 Å) at 125 or 173 K. See the CIF file for more detailed crystallography information.2′-Bromo-[1,1′biphenyl]-2-amine 5 and 2′-chloro-[1,1′-biphenyl]-2-amine 6 were prepared according to the reported procedures.2a General Procedure for the Synthesis of Cyclic Bromonium Salts 2, 7a−e and Cyclic Chloronium Salts 4, 8a,b.A solution of 2′-bromo-[1,1′-biphenyl]-2-amine 5 (1.0 equiv) or 2′-chloro-[1,1′-biphenyl]-2-amine 6 (1.0 equiv) in the respective acid (3.0 equiv) was stirred overnight at room temperature.Then, an additional amount of 10% aqueous acid was added and the reaction continued to stir at reflux until the precipitate completely dissolved.After that, the reaction mixture was cooled to 0 °C and sodium nitrate aqueous solution (2 equiv of 0.75 M solution) was added to the reaction mixture and stirred at 0 °C for 1 h.Urea (2.3 equiv) was then added, and the reaction mixture was stirred at 0 °C for another 1 h, then the reaction mixture was heated to reflux until completion of gas evolution.After the completion of the reaction, the reaction mixture was filtrated, washed with a hot 10% solution of the acid used, and the combined solution was concentrated and then kept at 0 °C until the product precipitated out.Then, the precipitate was collected, and after washing with water and ether, the solid was dried in vacuum to obtain final products.
Dibenzo[b,d]bromol-5-ium Chloride 2. 12 Reaction with 5 (820 mg, 3.3 mmol) and HCl according to the general procedure afforded 376 mg (47%) of product 2, isolated as an off-white solid: mp 238.Single crystals of product 7a suitable for X-ray crystallographic analysis (Figure 3) were obtained by slow crystallization from MeOH−H 2 O solution.X-ray diffraction data for 7a were collected on a Rigaku RAPID II Image Plate system using graphite-monochromated Cu Kα radiation (λ = 1.54187Å) at 123 K.The structure was solved by SIR2004 and refined using SHELXL-2014/7.Crystal data for 7a C 12 H 10 .5    Single crystals of product 8b suitable for X-ray crystallographic analysis (Figure 6) were obtained by slow crystallization  Reaction with cyclic bromolium salt 2 (200 mg, 0.747 mmol) and AgNTf 2 (334 mg, 0.860 mmol) in MeOH (14 mL) according to the general procedure afforded analytically pure product 7g; 353 mg (92%) isolated as a light-yellow solid identical to the sample from the previous experiment.
Single crystals of product 7g suitable for X-ray crystallographic analysis (Figure 8) were obtained by slow crystallization from MeOH−H 2 O solution.X-ray diffraction data for 7g were collected on a Rigaku RAPID II Image Plate system using graphite-monochromated Cu Kα radiation (λ = 1.54187Å) at 123 K.The structure was solved by Sir 2011 and refined using SHELXL-2014/7. 13Crystal data for 7g Significant disorder was modeled in structure 7g.Three independent molecules of interest and three independent counteranions were found in the unit cell with one pair on an inversion center.Two of the three anions and molecules of interest were modeled with significant disorder.One counteranion with the "B" series label was modeled over an inversion center with 50% occupancy with the Part −1 command and the 1−2 and 1−3 lengths between atoms were restrained with the SAME command to be the same as the counterion that was modeled over one position.Furthermore, the atoms neighboring by the inversion center were constrained to have the same  anisotropic displacement parameters with the EADP command and nonpositive definite atoms were restrained to be more isotropic with the ISOR command.
The second counteranion (labeled "A" and "C" series) was modeled over two positions and were restrained to have the same 1−2 and 1−3 atom distances and constrained to have the same anisotropic displacement parameters using the SAME and EADP command as mentioned previously and the atom.The occupancy of the two positions refined to an occupancy of 61− 39%.
One of the disordered molecules of interest (labeled "A" and "C" series) was modeled over two positions and the occupancy refined to a 59−41%.The anisotropic displacement parameters of neighboring atoms were constrained with the EADP command.The 1−2 and 1−3 atom distances were restrained to be the same as the molecule of interest that was modeled in a single position using the SAME command.Both clockwise and anticlockwise directions of the 1−2 and 1−3 distances were used due to molecular symmetry.
The second disordered molecule of interest (labeled "B" and "D" series) was modeled over two positions and an inversion center using the PART −1 command.Interestingly, the four positions refine to a 0.24:0.24:0.26:0.26occupancy ratio.The 1−2 and 1−3 atom distances were restrained to be the same as the molecule of interest that was modeled as a single position using the SAME command.Both clockwise and anticlockwise directions of the 1−2 and 1−3 distances were used due to molecular symmetry.Finally, the atoms neighboring by the inversion center and disorder were constrained to have the same anisotropic displacement parameters with the EADP command and nonpositive definite atoms were restrained to be more isotropic with the ISOR command.
Single crystals of product 7i suitable for X-ray crystallographic analysis (Figure 9) were obtained by slow crystallization from MeOH−H 2 O solution.X-ray diffraction data for 7i were collected on a Rigaku RAPID II Image Plate system using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 173 K.The structure was solved by Superflip and refined using SHELXL-2014/7.Crystal data for 7i
X-ray data for compounds 7 and 8 (CIF), and copies of NMR spectra for all compounds (PDF) ■

a
Nc is defined as the observed contact length divided by the sum of the van der Waals radii (Bondi).b Structure of 7a also contains a bifurcated halogen bond from a second oxygen atom of the NO 3− at an Nc of 0.92 and a water oxygen atom at Nc 0.97.c Bond lengths C1−X and C12−X reported as the short and long bonds in the complex for comparison.d Structure 7b has one bifurcated halogen bond with an additional oxygen atom from the HSO 4 − at Nc 0.96.e Structure 7g has a Z′ = 3 and contains three different close contact coordination spheres.One close contact set contains bifurcated halogen bonds from two oxygen atoms, each pair from separate NTf 2 − counterions with the second oxygen atom from the NTf 2 − at Nc 0.89.A second close contact coordination sphere of approximately square planar with a single nitrogen and oxygen atoms (from separate NTf 2 − ) as close contacts was also observed.The third cation−anion set in 7g was badly disordered and not considered in this analysis.

Figure 3 .
Figure 3. Thermal ellipsoid plot of 7a drawn to the 50% probability level.One molecule of interest and nitrate ion are displayed for clarity.Water molecules and hydrogen atoms were removed for clarity.

Figure 4 .
Figure 4. Thermal ellipsoid plot of 7b drawn to the 50% probability level.Nonoxygen hydrogen atoms were removed for clarity.

Figure 5 .
Figure 5. Thermal ellipsoid plot of 8a•2H 2 O drawn to the 50% probability level.Nonoxygen hydrogen atoms were removed for clarity.

Figure 7 .
Figure 7. Thermal ellipsoid plot of 7f drawn to the 50% probability level.Hydrogen atoms were removed for clarity.

Figure 8 .
Figure 8. Thermal ellipsoid plot of 7g drawn to the 50% probability level.Hydrogen atoms were removed for clarity.Only one molecule and counterion of interest out of three are shown for clarity.

Figure 9 .
Figure 9. Thermal ellipsoid plot of 7i drawn to the 50% probability level.Hydrogen atoms were removed for clarity.

Figure 10 .
Figure 10.Thermal ellipsoid plot of 8d drawn to the 50% probability level.Hydrogen atoms were removed for clarity.

Figure 11 .
Figure 11.Thermal ellipsoid plot of 8e•LiOTf drawn to the 50% probability level.Nonoxygen hydrogen atoms were removed for clarity.

Figure 12 .
Figure 12.Thermal ellipsoid plot of 8f drawn to the 50% probability level.Nonoxygen hydrogen atoms were removed for clarity.

AUTHOR INFORMATION Corresponding Authors
Akira Yoshimura − Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812, United States; Faculty of Pharmaceutical Sciences, Aomori University, Aomori 030-0943, Japan; Email: ayoshimura@ aomori-u.ac.jp