Communications
Heteropolyhalogen Cations
The Interhalogen Cations [Br2F5]+ and [Br3F8]+
Sergei I. Ivlev, Antti J. Karttunen, Magnus R. Buchner, Matthias Conrad, and Florian Kraus*
Abstract: The synthesis and characterization of unique poly-
halogen cations containing m-bridging fluorine atoms are
reported. The [Br2F5]+ cation features a symmetric [F2Br–m-
F–BrF2] bridge, whereas the [Br3F8]+ contains asymmetric m-F
bridges. These fluoronium ions, obtained as [SbF6]ꢀ salts, were
investigated using Raman and 19F NMR spectroscopy, as well
as single-crystal X-ray diffraction. Quantum chemical calcu-
lations were carried out for the gas-phase cations as well as for
the solid-state compounds. Population analyses show the m-F
atoms to possess the most negative partial charge within the
cations.
Due to the peritectic points in the phase diagram we have not
yet been successful in preparing both compounds in phase-
pure form from the two-component “melt”.
The compound [Br2F5][SbF6], m-fluoridobis(difluorido-
bromine(III)) hexafluoridostibiate(V), melts at approxi-
mately 308C. It crystallizes in the monoclinic crystal system,
space group P21/c, with a = 10.111(2), b = 9.0433(18), c =
9.986(2) ꢀ, b = 94.16(3)8, V= 910.8(3) ꢀ3, Z = 4 at T=
100 K. Further crystallographic details are available in the
Supporting Information. The compound contains the [Br2F5]+
cation (Figure 1), which may be theoretically disassembled
T
he chemistry of polyhalide anions is experiencing a renais-
sance and excellent reviews describe the rapid progress in the
field.[1–8] A closer look on the currently known polyhalide
anions (Table S1) shows that interhalogen anions containing
m-bridging F atoms are still rare. Only recently, anions
containing m2- and m3-bridging F atoms were unambiguously
reported.[9,10] In comparison to the polyhalide anions, far
fewer polyhalogen cations are known (Table S2) and no
multiply charged heteropolyhalogen cations seem to exist.
Also, not a single cationic species containing m-bridging F
atoms has been reported so far. In other words, fluoronium
ions are unknown for these systems.
Figure 1. The [Br2F5]+ cation of the crystal structure of [Br2F5][SbF6].
Anisotropic displacement parameters are shown at the 70% probabil-
ity level at 100 K. Selected atom distances [ꢀ] and angles [8]: Br(1)–
F(1)ax 1.738(3), Br(2)–F(5)ax 1.738(3), Br(1)–F(2)eq 1.708(3), Br(2)–
F(4)eq 1.708(3), Br(1)-m-F(3) 2.041(3), Br(2)-m-F(3) 2.042(3); Br-m-F-Br
164.36(15)8, Fax-Br-m-F 175.63(12) and 177.14(14)8.
We report here on the first F-bridged heteropolyhalogen
cations, [Br2F5]+ and [Br3F8]+. The phase diagram of the two-
component system BrF3/SbF5 (Figure S1) has been investi-
gated to some detail previously. However, the existing phases
have not been characterized, with the exception of the
congruent melting phase [BrF2][SbF6].[11] The compounds
[Br2F5][SbF6] and [Br3F8][SbF6] are obtained according to
Eequations (1) and (2). Details of the syntheses are available
in the Supporting Information.
into two symmetrically m-F-bridged [BrF2]+ cations of the
type [F2Br–m-F–BrF2]+. Both Br atoms are coordinated in
a trigonal-planar manner which may be expected according to
the VSEPR theory (y-trigonal bipyramidal due to the two
lone pairs on each Br atom). The shape of the lone pairs is of
course s- and p-orbital-like (no hybridization), as shown by
quantum chemical calculations (see below). Also, the VSEPR
approach fails for the prediction of the Br–m-F–Br angle,
which should be more bent.
The distances between the Br atoms and the terminal
bound Fatoms F(2) and F(4), which will be called Feq atoms in
the following to make use of the simple picture of the y-
trigonal bipyramid, are 1.708(3) ꢀ. The distances between the
Br atoms and the Fatoms F(1) and F(5), called Fax atoms from
now on, are 1.738(3) ꢀ. Thus, the Br–Fax distance is longer
than the Br–Feq distance. This finding is in accordance with
the expectation for the distances between a central atom and
homoleptic ligands in a trigonal-bipyramidal coordination
mode. The observed Br–F distances compare nicely with
those determined in compounds containing “isolated” [BrF2]+
cations. So far, only [BrF2][AuF4],[12] [BrF2][SbF6],[13] and
[BrF2]2[GeF6][14] have been characterized by single-crystal X-
ray diffraction. The Br–F distances are approximately 1.69-
(2) ꢀ in these cases. As may be expected from the higher
coordination number of the m-F atom, the m-F–Br distances
(2.041(3) and 2.042(3) ꢀ) are longer than the axial and
terminal Br–F bonds. The m-F–Br distances are identical
within the standard deviation. This clearly shows that the
½BrF2ꢁ½SbF6ꢁ þ BrF3 ! ½Br2F5ꢁ½SbF6ꢁ
½BrF2ꢁ½SbF6ꢁ þ 2 BrF3 ! ½Br3F8ꢁ½SbF6ꢁ
ð1Þ
ð2Þ
Both syntheses yielded slightly yellow crystals, which were
investigated using X-ray diffraction and Raman spectroscopy.
[*] Dr. S. I. Ivlev, Dr. M. R. Buchner, Dr. M. Conrad, Prof. Dr. F. Kraus
Fachbereich Chemie, Philipps-Universitꢁt Marburg
Hans-Meerwein-Strasse 4, 35032 Marburg (Germany)
E-mail: f.kraus@uni-marburg.de
Prof. Dr. A. J. Karttunen
Department of Chemistry and Materials Science
Aalto University
00076 Aalto (Finland)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
Angew. Chem. Int. Ed. 2018, 57, 1 – 6
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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