[Br4F21]− – a unique molecular tetrahedral interhalogen ion containing a μ4-bridging fluorine atom surrounded by BrF5 molecules

The reaction of [NMe4][BrF6] with an excess of BrF5 leads to the compound [NMe4][Br4F21]·BrF5. It features molecular [(μ4-F)(BrF5)4]− anions of tetrahedron-like shape containing central μ4-bridging F atoms coordinated by four BrF5 molecules. It is the most BrF5-rich fluoridobromate anion by mass. Quantum-chemical calculations showed that the μ4-F–Br bonds within the anion are essentially ionic in nature. The compound is the first example where F atoms bridge μ4-like neither to metal nor to hydrogen atoms. It was characterized by Raman spectroscopy and by single-crystal X-ray diffraction. The latter showed surprisingly that its crystal structure is related to the intermetallic half-Heusler compound and structure type MgAgAs.


Table of contents
Table of contents  Electronic Supplementary Material (ESI) for Chemical Science.This journal is © The Royal Society of Chemistry 2024 Collection of currently known homo-and hetero polyhalide anions and polyhalogen cations, "mononuclear" and "oligonuclear" species Table S1.Currently known polyhalide anions.

Experimental
All operations were performed on a stainless steel (316L) Schlenk line, which was passivated with fluorine and ClF 3 at various temperatures and pressures before use.Reaction vessels were made out of fluoropolymer (perfluoroalkoxy alkanes, PFA or perfluorinated ethylene propylene copolymer, FEP) and sealed with either a PFA needle valve (Swagelok).The vessels were baked out in vacuum (~10 -3 mbar) at circa 393 K for several times and passivated with diluted prepared according to literature. [84,189,190]ution!F 2 , BrF 5 and the fluoridobromates(V) are highly toxic and very strong oxidizers and therefore must be handled with proper protective equipment and with appropriate emergency treatment procedures available in the event of contact.The utmost precautions must be taken when disposing of these materials and their derivatives.As soon as BrF 5 starts to condense in the left reaction vessel, it starts reacting.A bright flash (second frame) followed by an orange-yellow flame shooting upwards through the vessel (third frame) can be seen.After the reaction, the vessel is still intact and only covered by a dense layer of soot (fourth frame) that can be wiped off again.equipped with a PHOTON III C14 detector.Evaluation, integration and reduction of the diffraction data was carried out with the APEX3 software suite. [191]The diffraction data was corrected for absorption utilizing the numerical absorption correction method of SADABS within the APEX3 software suite.The structure was solved with dual-space methods (SHELXT) in space group P2 1 3 (No.198), [192] and refined against F 2 using the SHELXL program within the ShelXle suite as a merohedral twin by introducing the following twin instruction: TWIN 0 -1 0 -1 0 0 0 0 1. [193,194] Because of the BrF 5 moieties as well as the [NMe 4 ] + ion located at 4a positions with .3.

Single crystal X-ray structure determination
symmetry, those are affected by disorder.Alternatively, a structural model in the orthorhombic subgroup P2 1 2 1 2 1 (No.19), without threefold rotation axis, was considered.For this purpose, the detwinned data were first extracted from the fcf-file created with the LIST 8 option of SHELXL using the HKLF5Tools program, [195] which allowed to introduce the additional threefold twin instruction (TWIN 0 0 1 1 0 0 0 1 0 3) for the following refinement in space group P2 1 2 1 2 1 .However, a stable refinement in space group P2 1 2 1 2 1 was only possible with hard constraints such as rigid bonds or fixed atomic positions and fixed batch scale factors, and still resulted in ill-behaved or non-positive atomic displacement parameters, strong correlations between atom coordinates, and therefore worse R-values than for the refinement in space group P2 1 3, which therefore is the superior choice.The description of the structure as disordered is also reasonable because the recorded reflection intensities decrease sharply at higher diffraction angles, which would not be expected if the structure were merely twinned by classical pseudomerohedry.
Representations of the crystal structures were created with the Diamond software. [196]Nonhydrogen atoms were refined with anisotropic displacement parameters.Anisotropic displacement parameters of fluorine atoms affected by disorder were restrained to approximate isotropic behavior by introducing appropriate ISOR instructions when necessary.H atom positions of the disordered methyl groups were calculated and refined using a riding model (AFIX) with isotropic displacement parameters 1.5 times U eq of the associated carbon atom.
The highest residual electron density after the final refinement was 0.45 e•Å -3 , 0.89 Å distant from atom F2E.CCDC 2311055 contains the supplementary crystallographic data for this paper.These data are provided free of charge by The Cambridge Crystallographic Data Centre.

Raman Spectroscopy
The Raman spectra were recorded with a Monovista CRS+ confocal Raman microscope (Spectroscopy & Imaging GmbH) using a 488 nm solid-state laser for excitation and either a 300 grooves/mm (low-resolution mode, FWHM: <5.50 cm −1 ) or a 1800 grooves/mm (highresolution mode, FWHM: <0.443 cm

Figure S1 .
Figure S1.Four consecutive frames of a video of the reaction of [NMe 4 ]F with BrF 5 .The right vessel contains [NMe 4 ]F and is cooled in aWeinhold vessel filled with liquid nitrogen.On the right side is the vessel containing BrF 5 , which is connected to the left vessel via the Schlenk line.As soon as BrF 5 starts to condense in the left reaction vessel, it starts reacting.A bright flash (second frame) followed by an orange-yellow flame shooting upwards through the vessel (third frame) can be seen.After the reaction, the vessel is still intact and only covered by a dense layer of soot (fourth frame) that can be wiped off again.

− 1 )
grating.Sample preparation of [NMe 4 ][Br 4 F 21 ]•BrF 5 :Crystals of the compound were selected under extensive exclusion of air under dried, cooled perfluoropolyether (Galden LS/230, Solvay, stored over molecular sieves 3 Å) using an optical microscope.For the transfer to the Raman microscope, the crystals within the oil were cooled with liquid nitrogen and quickly transferred onto the pre-cooled microscopy stage (BCS196, Linkam) of the Raman microscope.During the measurement, the sample chamber was flushed with a stream of nitrogen.However, air contact cannot be completely prevented when transferring the sample, which can also be seen from the fact that ice crystals form during the measurement on the object slide carrying the sample.The spectrum was recorded at a temperature of -60 °C in order to avoid the increase of hydrolysis products.After the measurement, the crystals of [NMe 4 ][Br 4 F 21 ]•BrF 5 suspended in perfluoropolyether were allowed to slowly warm up on the cooled sample holder and their decomposition was visually observed under the light microscope.At about -30 °C the crystals began to shrink and droplets of a colorless liquid formed on the surfaces of the crystals.As the temperature further increased, the crystals began to dissolve more and more.A Raman spectrum of the remaining crystals recorded at 5 °C indicated that the compound was still present.At about 22 °C, the remaining crystals and the liquid that had formed began to react vigorously, forming a brownish liquid indicative of the formation of bromine.However, since the presence of moisture could not be completely excluded and the hydrolysis was occurring simultaneously, the decomposition temperature of [NMe 4 ][Br 4 F 21 ]•BrF 5 could not be reliably determined.
*ISOR restraints on Uij parameters of disordered F atoms.

Table S6 .
Selected experimentally observed and calculated equivalent atomic distances of [NMe 4 ][Br 4 F 21 ]•BrF 5in Å (DFT-PBE0/TZVP for solid-state and DFT-PBE0/def2-TZVP for single molecule).Note that the site symmetry of the disordered anion in the crystal structure is C 3 , while in the solid-state quantum-chemical calculation the point group is C 1 and S 4 in the molecular calculation.

Table S7 .
Selected experimentally observed and calculated angles for the [Br 4 F 21 ] -anion in °.Symmetry operations for generation of equivalent atoms: 'z, x, y.Note that the site symmetry of the disordered anion in the crystal structure is C 3 , while in the solid-state quantum-chemical calculation the point group is C 1 and S 4 in the molecular calculation.DFT-PBE0/TZVP level of theory for solid-state and DFT-PBE0/def2-TZVP for single molecule.