Synthesis and Characterization of the Tetrafluoridochlorates(III) A[ClF4] (A = K, Rb, Cs)

: Single-crystalline tetrafluoridochlorates(III) A [ClF 4 ] ( A = K, Rb, Cs) were synthesized from solvolysis reactions of alkali metal fluorides in liquid chlorine trifluoride. The structures were examined by means of single-crystal X-ray diffraction. K[ClF 4 ] crystallizes in the K[BrF 4 ] structure type, whereas the Rb and Cs compounds crystallize in the Li[AuF 4 ] structure type.


Introduction
[3] These results were disputed at a meeting of the Gesellschaft Deutscher Chemiker in 1950 and later revised by the same authors, who showed that the products contained alkali metal fluoridochlorates(III) as well as the alkali metal fluorides. [2,4][13] This approach is also commonly used for the syntheses of tetrafluoridobromates(III), see Equation ( 1). [14,15][20][21][22] Thus far, the [Br 3 F 10 ] -anion has the highest MF 3 content, exhibiting a The compounds were further characterized by Raman and IR spectroscopy.Solid-state quantum-chemical calculations with hybrid density functional methods reproduced the experimental structures and enabled the interpretation of the experimental Raman and IR spectra.
(1) central μ 3 -bridging F atom, and was obtained as the Rb and Cs salts. [20]The possibility of anions of the composition [M 4 F 13 ] - for Au and Br has been the subject of a theoretical study. [23]ere, we report on the syntheses, structure determinations, vibrational spectroscopic characterizations and hybrid density functional theory calculations of the tetrafluoridochlorates(III), A[ClF 4 ] (A = K, Rb, Cs).

Results and Discussion
The tetrafluoridochlorates(III) of K, Rb and Cs were synthesized by a solvolysis reaction of the respective alkali metal fluoride in liquid ClF 3 , see Equation ( 2).

EurJIC
European Journal of Inorganic Chemistry the shape of a distorted square-antiprism.A projection of the crystal structure of K[ClF 4 ] is shown in Figure 1.
The [ClF 4 ] -anions are isolated and are superimposed when viewed along the a axis.Along the c axis, the anions are oriented at 90°towards one another.The fluorine atoms of the [ClF 4 ] -anion each coordinate to two potassium atoms.The Cl atoms are each surrounded by eight K atoms in the shape of a cube and vice versa, being similarly arranged like the Cs and Cl atoms in CsCl. [26]he salts Rb[ClF 4 ] and Cs[ClF 4 ] are isotypic and crystallize in the monoclinic space group C2/c (No. 15) with four formula units per unit cell, Pearson code mS24 and Wyckoff sequence 15, f 2 ed.Therefore, the compounds are isotypic to Li[AuF 4 ]. [16]ee Table 1 for selected crystallographic data and details of the structure determinations.Atom coordinates and isotropic displacement parameters are given in Table 3 and Table 4. Anisotropic displacement parameters are reported in the Supporting Information.The chlorine atom occupies the Wyckoff posi-  The Rb/Cs atom occupies the Wyckoff position 4e (site symmetry 2) and is coordinated by twelve fluorine atoms in the shape of a distorted anti-cuboctahedron.The crystal structure of Rb[ClF 4 ] is shown in Figure 2. The fluorine atoms of the [ClF 4 ] -anion each coordinate to three different alkali metal cations.The Cl atoms are surrounded by six alkali metal cations in the shape of a distorted octahedron and vice versa.The Rb/Cs and Cl atoms thus have a similar arrangement to the Na and Cl atoms in NaCl. [27]The lattice parameters and atomic positions of the three compounds reported here were fully optimized with the DFT-PBE0 method using the CRYSTAL17 program package. [28,29]The lattice parameters and the xyz coordinates of the optimized structures are given in the Supporting Information.The cell volumes of the optimized structures are bigger than the experimentally obtained ones: 4 % for K[ClF 4 ], and 6.

Vibrational Spectroscopy
The experimental Raman and IR spectra of the tetrafluoridochlorates(III) are shown in Figure 3. Values and band assignments are given in Table 5 and Table 6.The Raman and IR spectra of the compounds were also calculated with the DFT-PBE0 method and the calculated band positions, assignments and figures are given in the Supporting Information.In the Raman spectra, the symmetric in-phase Cl-F stretching modes, vs(ClF 4 ), are observed at slightly above 500 cm -1 and the symmetric out-of-phase Cl-F stretching modes, vs(ClF 4 ), at approximately 415 cm -1 .The values shift towards lower energy with increasing cation size.The scissoring vibration of the [ClF 4 ] -ion is observed in the range of 290-260 cm -1 , showing no dependency on the cation size.Below 150 cm -1 , further deformation vibrational modes -twisting, umbrella bending and rocking -of the [ClF 4 ] -ion are observed,  541 436 [30]   478 425 [32]   595, 556, 538, 523 424, 420 DFT-PBE0 which are coupled with the respective cation.These values nicely agree with our calculated values and Raman spectra that have been reported thus far. [10,30,31]s was previously mentioned, the reported IR data of the tetrafluoridochlorates(III) are not as consistent as the Raman data. [15]In most studies Pyrex glass Schlenk lines were used for the handling of ClF 3 or the products were not handled under inert conditions, leading to impure compounds. [8,13,30,32]We also observed impurity bands in the IR spectra, especially when the ClF 3 used was not free from traces of HF.These bands are observed above 1000 cm -1 and are caused by HF-adducts of the alkali metal fluorides, [AF(HF) n ], (A = K, Rb, Cs; n being usually 1-3). [33,34]This highlights the fact that HF is a stronger Lewis-base than ClF 3 and thus displaces it from the tetrafluoridochlorates(III). [35]evertheless, in the IR spectra two broad bands are seen in the region of 600 to 400 cm -1 which can be attributed to the [ClF 4 ] -ion.The first one, with a maximum at approximately 500 cm -1 , can be assigned to the asymmetric Cl-F stretching mode, v as (ClF 4 ), and the latter one at circa 420 cm -1 to the umbrella bending vibration of the [ClF 4 ] -ion.The calculated frequencies for the stretching modes are somewhat overestimated by the DFT-PBE0 method that was employed.The calculated Raman and IR spectra otherwise nicely agree with our experimental results.

EurJIC
European Journal of Inorganic Chemistry The Raman and IR spectra could be interpreted on the basis of quantum-chemical solid-state calculations.

Experimental Section
General: All operations were performed on a Monel metal Schlenk line, which was passivated with 100 % fluorine and chlorine trifluoride at various pressures before use.Moisture-sensitive compounds were stored and handled in an Ar-filled glove box (MBraun).Reaction vessels were made out of fluoropolymer (perfluoroalkoxy alkanes, PFA) and passivated with fluorine before use.Preparations were carried out in an atmosphere of dry and purified argon (5.0, Praxair).Chlorine trifluoride was stored over NaF to remove traces of HF.Caution!Fluorine, chlorine trifluoride, and tetrafluoridochlorates(III) must be handled using appropriate protective gear with ready access to proper emergency treatment procedures in the event of contact.The aforementioned are potent oxidative fluorinators that are only stable under the rigorously anhydrous conditions employed in the experimental procedures outlined in the Experimental Section.They react vigorously to explosively upon hydrolysis or contact with organic materials.The utmost precautions must be taken when disposing of these materials and their derivatives.

Synthesis: K[ClF 4 ]:
A PFA reaction vessel was loaded with 12.3 mg KF (0.218 mmol) inside a glove box and attached to a stainless-steel valve.An excess of ClF 3 (approximately 0.35 g, 3.8 mmol) was then condensed onto the solid at 77 K.The resulting suspension was gently heated in an air bath to approximately 50 °C for two hours.The remaining ClF 3 was then slowly pumped off and the colorless product (21.2mg isolated, calculated: 31.87 mg) was transferred into the glove box.Rb[ClF 4 ]: A PFA reaction vessel was loaded with 28.1 mg RbF (0.269 mmol) inside a glove box and attached to a stainless-steel valve.An excess of ClF 3 (approximately 0.40 g, 4.3 mmol) was then condensed onto the solid at 77 K.The resulting suspension was gently heated in an air bath to approximately 50 °C for one hour.The remaining ClF 3 was then slowly pumped off and the colorless product (43.2mg isolated, calculated: 52.97 mg) was transferred into the glove box.Cs[ClF 4 ]: A PFA reaction vessel was loaded with 20.9 mg CsF (0.138 mmol) inside a glove box and attached to a stainless-steel valve.An excess of ClF 3 (approximately 0.25 g, 2.7 mmol) was then condensed onto the solid at 77 K.The resulting solution was kept at room temperature for several hours.The remaining ClF 3 was then slowly pumped off and the colorless product (28.3mg isolated, calculated: 33.61 mg) was transferred into the glove box.The mass balances of the three reactions indicate that a full conversion of the alkali metal fluorides to the respective tetrafluoridochlorates(III) was not achieved under the chosen conditions.Also, not all of the solid product could be isolated from the reaction vessels.
Single-Crystal X-ray Diffraction: Crystals of the moisture-sensitive compounds were selected under dried perfluorinated oil (Fomblin YR1800, Solvay, stored over molecular sieves 3 Å) and mounted on a MiTeGen loop.Intensity data of suitable crystals were recorded with a D8 Quest diffractometer (Bruker) or an IPDS2T diffractometer (STOE).The diffractometers were operated with monochromatized Mo-K α radiation (0.71073 Å, multi layered optics (D8 Quest) or graphite monochromator (IPDS2T)) and equipped with a PHOTON 100 CMOS detector (D8 Quest) or an image plate detector (IPDS2T).Evaluation, integration and reduction of the diffraction data was carried out with the APEX3 software suite (D8 Quest) or the X-Area software suite (IPDS2T). [36,37]The diffraction data was corrected for absorption utilizing the multi-scan method of SADABS within the APEX3 software suite (D8 Quest) or the integration method with the modules X-Shape and X-Red32 of the X-Area software suite (IPDS2T).The structures were solved with dual-space methods (SHELXT) and refined against F 2 (SHELXL). [38,39]For Rb[ClF 4 ] one systematic absence violation ([I > 3σ(I)] is observed for space group C2/c (No. 15).Solution and refinement in the subgroups Cc (No. 9) and C2 (No. 5) showed correlation for some atoms.Subsequently, space group C2/c was chosen, which was also indicated by the Addsym function in PLATON. [40,41]The highest residual electron densities after the final refinements were the following: K[ClF 4 ]: 0.68 Å distant from atom Cl(1); Rb[ClF 4 ]: 1.58 Å distant from atom F(2); Cs[ClF 4 ]: 1.05 Å distant from atom Cs (1).Representations of the crystal structures were created with the Diamond software. [42]DC

IR and Raman spectroscopy:
The IR spectra were recorded on a Bruker alpha FT-IR spectrometer using an ATR Diamond module with a resolution of 4 cm -1 .The spectrometer was located inside a glovebox (MBraun) under argon atmosphere.The spectra were processed with the OPUS software package. [43]The Raman spectra were measured at room temperature with a Monovista CRS+ confocal Raman microscope (Spectroscopy & Imaging GmbH) using a 532 nm solid-state laser and either a 300 grooves/mm (low-resolution mode, FWHM: <4.62 cm -1 ) or a 1800 grooves/mm (high-resolution mode, FWHM: <0.368 cm -1 ) grating.
Density Functional Calculations: Periodic quantum-chemical calculations were carried out for the A[ClF 4 ] (A = K, Rb, Cs) salts using the PBE0 density functional method (DFT-PBE0). [28,29]A triple-zetavalence + polarization (TZVP) level basis set was applied for F and Cl and split-valence + polarization (SVP) level basis sets were applied for K, Rb and Cs.[46][47] All calculations were carried out with the CRYSTAL17 program package. [48]The reciprocal space for the salts was sampled using the following Monkhorst-Pack-type k-point grids: K[ClF 4 ]: 4×4×4, Rb[ClF 4 ]: 5×5×3, Cs[ClF 4 ]: 5×5×3.For the evaluation of the Coulomb and exchange integrals (TOLINTEG), tight tolerance factors of 8, 8, 8, 8, 16 were used for all calculations.Both the atomic positions and lattice parameters were fully optimized within the constraints imposed by the space group symmetry.Default DFT integration grids and optimization convergence thresholds were applied in all calculations.
The xyz coordinates used in the calculations of each salt are given in the Supporting information.[51][52] The Raman intensities were calculated for a polycrystalline powder sample (total isotropic intensity in arbitrary units).The Raman spectra were obtained by using a pseudo-Voigt band profile (50:50 Lorentzian:Gaussian) and an FWHM of 8 cm -1 .The Raman spectra were simulated taking into account the experimental setup (T = 298.15K, λ = 532 nm).For the IR spectra, a Lorentzian lineshape and an FWHM of 8 cm -1 was used.The band assignments were carried out by visual inspection of the normal modes using the J mol program package. [53]

( 2 )
Single crystals of K[ClF 4 ] and Rb[ClF4 ] were obtained by carefully heating suspensions of the alkali metal fluorides in ClF 3 up to 50 °C in an air bath.In the case of Cs[ClF 4 ], single crystals were obtained by slowly removing excess ClF 3 from solutions in vacuo.
5 % for both Rb[ClF 4 ] and Cs[ClF 4 ].Thus, the interaction between the alkali metal cations and the [ClF 4 ] -anions is probably somewhat underestimated by the DFT-PBE0 method that was employed.The calculated Cl-F distances are only slightly longer than the experimental ones: 1.82 Å in K[ClF 4 ]; 1.81 and 1.83 Å in Rb[ClF 4 ]; 1.82 Å in Cs[ClF 4 ].

Table 1 .
for selected Selected crystallographic data and details of the structure determinations of the compounds A[ClF 4 ] (A = K, Rb, Cs).

Table 3 .
Atomic coordinates and equivalent isotropic displacement parameters U iso of Rb[ClF 4 ].

Table 4 .
Atomic coordinates and equivalent isotropic displacement parameters U iso of Cs[ClF 4 ].
1980748 (for Cs[ClF 4 ]), 1980749 (for Rb[ClF 4 ]), and 1980750 (for K[ClF 4 ]) contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.