In Situ Synthesis and Applications for Polyinterhalides Based on BrCl

Abstract The use of neat BrCl in organic and inorganic chemistry is limited due to its gaseous aggregate state and especially its decomposition into Cl2 and Br2. The stabilization of BrCl in form of reactive ionic liquids via a novel in situ synthesis route shifts this equilibrium drastically to the BrCl side, which leads to safer and easier‐to‐handle interhalogenation reagents. Furthermore, the crystalline derivatives of the hitherto unknown [Cl(BrCl)2]− and [Cl(BrCl)4]− anions were synthesized and characterized by single‐crystal X‐ray diffraction (XRD), Raman and IR spectroscopy, as well as quantum chemical calculations.


Introduction
After the discovery of attractive interactions between dihalogens and halide anions in 1819, [1] Chattaway and Hoyle carried out first systematic investigations of polybromides and -chlorides in 1923. [2] These reports marked the startingp oint of the vast chemistry of polyhalogen compounds. In the last years not only our knowledge of the structurald iversity of several polyhalogen anions, but also the possible applications of polyhalogen compoundsi ncreased continuously. [3] In general, interhalogena nionsc an be separated into classical interhalides, which possess an electropositive centert hat is surrounded by more electronegative halogen atoms, for example, [ICl 4 ] À , [4] [IF 6 ] À , [5] and nonclassical interhalides. An onclassicali nterhalide can be best described as ac entral halide ion X À ,w hich is surrounded by dihalogen Y 2 (e.g.,[ Cl(I 2 ) 4 ] À [6] )o ri nterhalogen molecules XY/YZ (e.g.,[ Br(IBr) 2 ·2 IBr] À ). [7] Lately first examples of extraordinarily large polyhalogen anions of the lighter halogens such as the octahedrally coordinated monoanions [Cl 13 ] À [8] and [Cl(BrCl) 6 ] À [9] as well as the highly reactive [Br 2 F 7 ] À and [Br 3 F 10 ] À anions [10] were reported. Besides the detailed structural investigation of polyhalides, their applications as halogenation reagents, [11] as electrolytes in dye-sensitized solar cells [12] or (redox flow) batteries [13] are well established. Recently further applicationsa sr eactive ionic liquidsw hich are able to dissolve noble metals and alloys were reported. These reactive ionic liquids showpromise for applications in metal recycling. [14] The formation of polyhalides can be explained by the concept of halogen bonding. According to this concept, the electrostatic potentialo fadihalogen molecule is anisotropic and can be divided into two regions:a na rea of higher electron density,w hich forms ab elt perpendicular to the molecule's bondinga xis, and ar egion of am ore positive electrostatic potential, which is situated on the bonding axis, the so-called shole. [15] While fors ymmetrical dihalogens such as Br 2 and Cl 2 , the s-hole is symmetricalo nb oth halogen atoms, ford iatomic interhalogens such as IBr and BrCl, the s-hole is more pronounced at the more electropositive halogen atom, see Figure 1. Owing to their polarized bond and pronounced shole, interhalides are expected to form more stable anionsa nd their tendency to form extended polyhalogen networks is lowered. Figure 1. Electrostatic potential in the range of À0.01au( red) to 0.06 au (blue)f or the molecules Cl 2 ,Br 2 and BrCl (view of the Cl or Br atom,isosurface value 0.0035a u);calculated at the B3LYP-D3/def2-TZVPP levelo f theory.

Results and Discussion
Considering the excellenthalogenbondingp roperties, it is surprising that larger nonclassical interhalides based on bromine monochloride (BrCl) have been reported only recently. [9] The interhalide BrCl exists in an equilibrium with Br 2 and Cl 2 ,s ee Scheme1. [16,17] This hampers its use as ar eagent due to side reactions. An equilibrium ratio of approximately 60 %f or BrCl, and 20 %f or Cl 2 and 20 %B r 2 was experimentally determined at room temperature. [16] This in good agreement with quantumc hemical calculations at B3LYP-D3(BJ)/def2-TZVPP [18]- [28] and SCS-MP2/def2-TZVPP [29,30] level of theory.T his equilibrium makes the stoichiometric usage of neat BrCl nearly impossible. Due to the different vapor pressures [31] of Br 2 ,C l 2 and BrCl, first mainly chlorine boils off ab atch of BrCl while bromine is enriched in the remainingl iquid.D ue to the pronounced s-hole, the halogen bondingp roperties of BrCl are enhanced in comparison to Br 2 and Cl 2 .T hus, an addition of ah alogen bond acceptor,f or example,achloride salt,y ielding BrCl based polyinterhalides such as the pentahalide [Cl(BrCl) 2 ] À ,s hould result in as ignificant shift of the equilibrium to the BrCl side, see Scheme2. Our calculations show that the equilibrium is almost entirely located on the side of the BrCl based interhalides (> 99.99 %, calculated from DG8). Therefore, the stabilization of BrCl in form of polyinterhalide compoundsp rovides ac onvenient BrCl source.
Herein;w er eport an ew synthetic route for polyinterhalides based on BrCl as well as the preparation and characterization of hitherto unknown BrCl interhalide monoanions, which complete the set of possible coordination numbers. These compoundsh ave been characterized by single crystal X-ray diffraction (XRD), Raman and IR spectroscopy as well as quantum chemicalcalculations.
An ew synthetic approach generates the interhalide BrCl in situ, see Scheme 3. In this approach, ac hloride salt was provided in ar eaction flask and afterwards elemental chlorine and bromine were condensed onto the salt at À196 8C.
Condensing Br 2 and Cl 2 as starting materialsg uarantees the exact stoichiometry compared to the usage of ap reviously prepared BrCl batch. Warming up to ambient temperature leads to the formation of BrCl, which immediatelyr eacts with the chloride to afford the desired polyinterhalide species. Due to this in situ generation of BrCl the undesired impurities of Br 2 and Cl 2 are minimized and consequently ac lean product is obtained. These interhalide compounds offer themselves as an easy-to-handle and safer alternative to neat BrCl for applications in organic and inorganic chemistry.
To demonstrate the advantages of the new synthesis route, ac omplete set of the possible BrCl interhalides with one to six coordinating BrCl molecules was synthesized and fully characterized, see Figure 2.
The addition of 0.85 equivalents of the dihalogens Br 2 and Cl 2 to [NEt 4 ]Cl in DCM leads to the formation of the hitherto unknown pentahalide[ Cl(BrCl) 2 ] À .T he anion possessesaslightly distorted V-shaped structure, see Figure S38. In the crystal structure ap artial substitution of the coordinating BrCl molecules by Br 2 is observed. Therefore, as mall excesso fc hlorine (0.85 equivB r 2 ,1 .1 equiv Cl 2 )w as used resulting in crystals of a non-disordered [Cl(BrCl) 2 ] À anion and an additional, slightly unsymmetrical [Cl(BrCl)] À ,s ee Figure S2. In contrastt ot he 3c-4e bond of trihalides,t he bondings ituation of the higher poly(inter)halides can be best described as ad onor-acceptor interaction. The central halide ion acts as Lewis base and donates electron density into the LUMO (s*) of the Lewis acids, namely the coordinating dihalogen molecules. The charget ransfer results in ab ond weakening and elongation of the coordinating molecules, which can also be observed experimentally. [3] In case of the non-disordered pentahalide 2 the bond lengths of the coordinating BrCl molecules are elongated by 10.5(2)-14.2(2) pm compared to free BrCl (213.6(1) pm). [38] Adding more equivalents of Br 2 and Cl 2 to the chloride salt, anions with higher coordination numbersc an be obtained. Consequently the heptahalide [Cl(BrCl) 3 ] À (3)c rystallized with [NEt 4 ] + as the counter ion. The heptahalide wasa lready reported with [AsPh 4 ] + as the cation. In the reported structure of [AsPh 4 ] [Cl(BrCl) 3 ]t wo crystallographically independent heptahalides, which slightly interactw ith each other,c an be observed in the asymmetricu nit. [9] The [Cl(BrCl) 3 ] À presented here in [NEt 4 ] [Cl(BrCl) 3 ]c onsists of just one discrete anion,w hich shows no interactions to other halogen atoms and is arranged in ad istorted trigonal-pyramidal structure.
Increasing the amount of Br 2 and Cl 2 to 2.5equivalents and changing the cation to [NPr 4 ] + leads to the formation of the nonahalide [Cl(BrCl) 4 ] À ,w hich is the first reported BrCl based nonahalide, see Figure 3.
The compound crystallizes in the tetragonals pace group I 4 and its structure can be best described as ad istorted tetrahedron. The obtained polyinterhalide compounds is isostructural to the already published [NPr 4 ][Br(Br 2 ) 4 ] (Figure 3). [39] In comparison to the [Br(Br 2 ) 4 ] À ,w hose intermolecular distances are about 30 pm shorter than the sum of the van der Waals radii (370 pm), [40] the [Cl(BrCl) 4 ] À anion is more discrete. The intermolecular distances of 4 are just 10 pm shorter than the sum of the van der Waals radii( 360 pm), [40] which again showst he lower tendency of BrCl based interhalides to be stabilized by secondary halogen-halogen interactions. Furthermore,t he hypothetical square-planar structure( D 4h )h as been quantumchemically investigated at B3LYP-D3(BJ)/def2-TZVPP and SCS-MP2/def2-TZVPP level. Both optimized structures show at ransition state for the interconversion of the tetrahedral structure which are 14.6 and 7.2 kJ mol À1 higher in energy than the T d structure.
From ar eaction mixture of an excesso fB r 2 and Cl 2 (3.5 equiv each) with [NEt 4 ]Cl, single crystalso ft he undecainterhalide [NEt 4 ][Cl(BrCl) 5 ]w ere obtained (5). As imilar undecahalide structure was already published earlierw ith [CCl(NMe 2 ) 2 ] + as the counter ion. With this cation the structural parameter t (t = (bÀa)/608, b and a are the largest angles in the coordination sphere) was determined to be t = 0.25, which indicated ar ather square-pyramidal arrangement. [9,41] With [NEt 4 ] + as the cation, the structural parameter becomes larger (t = 0.38). This underlines the strong influence of the cation on the structure of the anion. Another reason for the distorted structure of the undecahalide is the bridging between the [Cl(BrCl) 5 ] À units, which leads to an octahedral coordination sphere for the central chloride Cl1, see Figure 4.
As mentioned above,t he tendency to form extended networks is lowered for BrCl due to its asymmetrical s-hole in comparison to the symmetrical Br 2 .T he solid state structureo f [Cl(BrCl) 5 ] À (5)p ossesses one disordered coordinatingd ihalogen unit (Br5-Cl6/Br6), whichi sb ridging between two anions, see Figure 4. The interaction of the bromine with the two central chloride ions leads to the formation of ac hain.
It was possible to synthesize and crystallize interhalides based on BrCl with tetraalkylammoniuma st he cation with coordination numbers going from one up to five. The hexa-coordinated tridecainterhalide could not be isolated with tetraalkylammonium cations, even an excesso fB rCl leads to the formation of the [Cl(BrCl) 5 ] À anion. This underlines the great influ-  ence of the counter ions size on the obtained anion.U sing the large bis(triphenylphosphoranylidene)iminium ([PNP] + )a st he cation leads to the formation of the tridecainterhalide, which was already published. [9] In this series of polyinterhalides the Br-Cl bond length of the terminal BrCl molecules is decreasing with the increase of BrCl molecules coordinating to the central atom.
As mentioned before, the [Cl(BrCl)] À anion exhibits the longest BrCl bond due to the 3c-4e bond. From the pentahalide to the tridecahalide the bonds of the coordinating BrCl molecules are less weakened and therefore less elongated compared to the trihalide and free BrCl (213.6(1) pm). [38] The bonding situation of higher polyinterhalides can be described as donor/acceptor interaction between the centralc hloride ion, the Lewis base, and the surroundingB rCl molecules, which functionasL ewis acids.
The higher the coordination number of the central chloride the less electron density can be donated into each LUMO (s*) of each coordinating BrCl molecule. This leads to less weakening of the BrCl bond accompanied with shorter bond lengths, see Figure 2. Quantum chemical calculations for the minima structures in the gas phase of the polyinterhalide species( at the B3LYP-D3(BJ)/def2-TZVPP and SCS-MP2/def2-TZVPP level of theory) are in good agreement with the experimental data and verify the described trend, see Figure 5.
Due to their strong Raman scattering, Ramans pectroscopy is highly instructive for further analysiso fp olyinterhalide compounds.T herefore, low temperature Raman spectra of single crystalso fe ach polyinterhalide salt were recorded and compared with quantum chemical calculations, see Figure 6. The trend of decreasing bond weakening of the BrCl units leads to as hift of the BrCl stretching frequencies to higherw avenumbers in the corresponding Raman spectra. Due to the almost inversion-symmetrical structure of trihalides, only the symmetric stretching vibrationp ossesses significant Raman intensity. In the spectrum of [NEt 4 ] 2 [Cl(BrCl)] 2 ,t wo bands can be observed at 285 and 271 cm À1 respectively.T hese two bands can be explained by two symmetric stretching vibrations of two crystallographically independent [Cl(BrCl)] À anions, which differ in bond lengths and angles (241.8(1), 235.7(1) pm, 174.8(1)8; 240.4(1), 239.1(1) pm, 179.1(1)8). These bands are most shifted to lower wavenumbers in comparison to free BrCl (434 cm À1 ) [42] due to the 3cÀ4e bond in the trihalide. Quantum chemical calculations for the trihalide in D 1h symmetry at the SCS-MP2/ def2-TZVPP level of theory predicto ne band for the symmetrical stretching vibration at 270 cm À1 (A 1g ), whicha greesw ell with the experimental results. As describeda bove, the bond weakeningd ecreases with increasingn umber of coordinating BrCl molecules. The corresponding bands shift to higherw avenumbers and almost reach the frequency of free BrCl. In case of the larger polyinterhalide compounds two major bands in the Br-Cl stretching region can be observed, the symmetric and asymmetric stretch of the coordinatingB rCl molecules.
In the spectra of the three and four times coordinated interhalogen compounds, an additional band at 281 cm À1 is observed, which can be assigned to a[ Cl(BrCl)] À species, that results from residual mother liquor.T he stretching frequency of  the bridging Br 2 molecule in the solid state structure of the undecainterhalide 5 is observeda t2 90 cm À1 .I nt he Ramans pectrum of the pentainterhalide 2,t he intensities of the symmetric and asymmetric stretching modesa re inverted with respect to the calculations. This can be explained by interactions in the solid state, as shown by Ramans pectra of compound 2 in a DCM solutiona nd in bulk, see Figure S42. All experimental spectra are in good agreement with quantum-chemical calculations,see Figure 6.
Additionally,r oom-temperature ionic liquids (RT-ILs) containing 1-3 equiv of BrCl were prepared and analyzed by IR spectroscopy( Figure S12). The described trend of bond weakening and shift of vibrational frequencies is also visible in the IR spectra, however Raman spectroscopy is more suited to characterize the polyinterhalide species due to the superior signal to noise ratio.
The usage of poly(inter)halide compounds in organic and inorganic synthesis is growing over the last years.
The reactions were performed in dichloromethane (DCM) at À78 8Ct op rovide the bromochlorinated product within 60 min reaction time in yields between 71 and 91 %. The interhalogenation of several alkenesa nd Michael systems was achieved even at low temperatures in very short reactiont imes and good yields.T oi nvestigate the long-term stability of these reactive interhalogenation reagents, Raman spectra of two BrCl based ionic liquids were measured continuouslyo ver ap eriod of one year,s ee Figures S36-37

Conclusions
In summary,w eh ave presented an ovel in situ synthesis route for BrCl based interhalides. The addition of ah alogen bond ac-ceptor,f or example, ac hloride salt, results in an almost entire shift of the equilibrium to the BrCl side (> 99.99 %). This stabilization of the reactive BrCl provides safer and easy-to-handle polyinterhalide ILs for furthera pplications such as interhalogenation reactions of alkenes, alkynesa nd Michael systemsi no rganic synthesis. Furthermore, we were able to synthesize via this new route ac omplete set of the possible BrCl interhalides with coordination numbers of one to six coordinatingB rCl molecules, showing that the averageC l-Br distances of the central chloride to the surrounding BrCl molecule correlate well with the coordination number.Within this set, the hitherto unknown V-shaped [Cl(BrCl) 2 ] À and the distorted tetrahedral [Cl(BrCl) 4 ] À anion were reported for the first time. All structures were characterized by single-crystal X-ray diffraction, singlecrystal Raman spectroscopy as well as quantum chemical calculations.H ence, our results not only complete the varietyo f BrCl based interhalides, they also provide an ew syntheses route to stabilize the gaseous BrCl for promising synthetic applications.

Experimental Section
Materials, instruments, and methods:A ll preparative work was carried out using standard Schlenk techniques. Chlorine (Linde, purity 2.8) was passed through calcium chloride before use to remove traces of water.B romine (purity > 99 %) was distilled and stored over activated molecular sieve. The [NR 4 ]Cl salts were obtained from commercial sources, dried for two days at 80 8Ca tr educed pressure and stored under inert conditions. Dichloromethane was stored over activated 3 molecular sieve. Raman spectra were recorded on aB ruker (Karlsruhe, Germany) MultiRAM II equipped with al ow-temperature Ge detector (1064 nm, 60 mW, resolution of 4cm À1 ). Spectra of single crystals were recorded at À196 8Cu sing the Bruker RamanScope III. ATR-IR spectra of the ILs were recorded with ar esolution of 4cm À1 on aT hermo Scientific Nicolet iS50 FT-IR with DTGS-polyethylene detector for the FIR range and for the halogenated alkene on aJASCO FT/IR-4100 spectrometer.N MR spectra were recorded at RT on aJ EOL Eclipse + 500 spectrometer.M ass spectroscopy was performed on an Agilent Technologies 6210 ESI-TOF spectrometer.X -ray diffraction data were collected on aB ruker D8 Venture CMOS area detector (Photon 100) diffractometerw ith MoKa radiation, see Ta ble S1-S2. Deposition numbers 1965314, 1965315, 1965317, 1965318, 1971174, 1984581, and 1984909 contain the supplementary crystallographic data for this paper.T hese data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.
Single crystals were coated with perfluoroether oil at low temperature (À40 8C) and mounted on a0 .2 mm Micromount. The structure was solved with the ShelXT [43] structure solution program using intrinsic phasing and refined with the ShelXL [44] refinement package using least squares on weighted F2 values for all reflections using OLEX2. [45] On the one hand, structure optimizations of the polyinterhalide anions were performed at DFT level using the RI-B3LYP hybrid functional with Grimme's dispersion correction D3 and BJ-damping together with the def2-TZVPP basis set. [18]- [28] On the other hand, further optimizations were performed at SCS-MP2 level using the def2-TZVPP basis set. [29,30] All calculations were carried out using the TURBOMOLE V7.3 program. [46] Minima on the potential energy