Closing the Gap: Structural Evidence for the Missing Hexabromide Dianion [Br6]2?

Article abstract of DOI:10.1002/chem.201705912

The formation and experimental characterization of the first hexabromide dianion is presented. This dianion fills the last remaining gap in the series of polybromides from the tribromide [Br₃]^? to the undecabromide [Br₁₁]<

Full text of DOI:10.1002/chem.201705912

DOI: 10.1002/chem.201705912
Communication
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Halogens |Hot Paper|
Closing the Gap: Structural Evidence for the Missing
Hexabromide Dianion [Br6]2À
Karsten Sonnenberg,[a] Patrick Prçhm,[a] Carsten Mꢀller,[b] Helmut Beckers,[a]
Simon Steinhauer,[a] Dieter Lentz,[a] and Sebastian Riedel*[a]
smaller polybromides, there is one gap left to fill: namely the
hexabromide dianion [Br6]2À, see Figure 1. Herein, we report on
the structure and the unique bonding situation in this dianion.
Abstract: The formation and experimental characteriza-
tion of the first hexabromide dianion is presented. This
dianion fills the last remaining gap in the series of poly-
bromides from the tribromide [Br3]À to the undecabro-
mide [Br11]À. The experimental results are compared to
quantum-chemical calculations. These calculations pre-
dict—based on electrostatic interactions—a T-structure for
the hexabromide dianion, while halogen–halogen bond-
ing favors the hockey-stick-like structure experimentally
found in the crystal structure. The hexabromide is built of
two tribromide moieties, one of which is highly asymmet-
ric. The classification of this unique anion as hexabromide
dianion is discussed. The counter ion [C5H10N2Br]+ stabiliz-
es the hexabromide dianion by additional s-hole interac-
tions. The compound is fully characterized by mass spec-
trometry, NMR-, IR and single crystal Raman spectroscop-
ies as well as single-crystal X-ray diffraction.
Figure 1. Visualization of known polybromides between [Br3]À and [Br11]À.
Polybromides have been investigated intensively over the last
years due to their interesting structures as well as their appli-
cation possibilities.[1] They can be used as bromine-storage ma-
terials and brominating agents to avoid the problematic han-
dling of elemental bromine. The high conductivity of poly-
bromides[2] has also led to investigations of their applicability
in electrochemistry similar to polyiodides for example, in dye-
sensitized solar cells.[3] Furthermore, very recently such poly-
bromides have been evaluated as electrolytes in batteries.[4]
So far, ten types of polybromides are experimentally known:
Recently, halogenoamidinium halides have been discussed
regarding s-hole interactions and halogen bonding.[14,15] In our
previous work, we were able to stabilize the first polychloride
dianion [Cl8]2À by chloroamidinium cations.[16] Therefore, we
were interested in investigating the analogous bromoamidini-
um bromides. Surprisingly, the reaction of the urea derivative
C5H10N2O with oxalyl bromide yields [C5H10N2Br]2[Br6] 1 in con-
trast to the analogous reaction with oxalylchloride where
[C5H10N2Cl][Cl] is obtained in quantitive yield, see Scheme 1.
The compound [C5H10N2Br]2[Br6] crystallizes in the triclinic
[Br3]À,[5] [Br4]2À,[6] [Br5]À,[7] [Br7]À,[1] [Br8]2À,[8] [Br9]À,[9] [Br10]2À [10]
,
[Br11]À,[11] [Br20]2À[12] and [Br24]2À.[13] Also, polybromide networks
exist. They can be broken down into the mono- and dianions
mentioned above. Evidently, among the well-characterized
¯
space group P1, see Figure 2. When compound 1 is treated
with allylbenzene the bromide [C5H10N2Br]Br 2 is formed which
crystallizes in the monoclinic space group P21/n. In 2 a short
Br···Br contact of 321.8 pm between the cation and anion is ob-
served, see Figure 3. This distance and the almost linear C5-
[a] K. Sonnenberg, P. Prçhm, Dr. H. Beckers, Dr. S. Steinhauer, Prof. Dr. D. Lentz,
Prof. Dr. S. Riedel
Fachbereich Biologie, Chemie, Pharmazie
Institut fꢀr Chemie und Biochemie-Anorganische Chemie
Fabeckstr. 34/36, 14195 Berlin (Germany)
E-mail: s.riedel@fu-berlin.de
[b] Dr. C. Mꢀller
Fachbereich Biologie, Chemie, Pharmazie
Institut fꢀr Chemie und Biochemie-Theoretische Chemie
Takustr. 3, 14195 Berlin (Germany)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under https://doi.org/10.1002/
chem.201705912.
Scheme 1. Synthesis of 2-bromoimidazolinium bromides.
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cation–anion interactions in the solid state. There are no hy-
drogen bridges found between the cations and anions. The
dianion itself consists of the two almost linear tribromides Br6-
Br2-Br8 (175.78) and Br7-Br1-Br4 (177.48). The latter shows
bond lengths of 258.5 pm (Br1ÀBr4) and 251.4 pm (Br1ÀBr7)
whereas the former is even more asymmetric with bond
lengths of 267.3 pm (Br2ÀBr6) and 246.8 pm (Br2ÀBr8). The
closest contact between the two tribromide moieties is
359.3 pm (Br7ÀBr8) which is below the sum of the Van-der-
Waals radii (370 pm), see Table 1.
The L-shaped anions [Br6]2À are arranged in layers (AB) in the
crystal with no obvious mutual interactions. Layers A and B are
separated by 345 pm. Along the crystallographic a-axis the
consecutive L-shaped anions are rotated by 1808. Therefore,
two neighboring hexabromides (AB) resemble a rectangle, see
Figure 4.
Figure 2. Structure of [C5H10N2Br]2[Br6] 1 in the solid state, ellipsoids are
shown at the 50% probability level.
Figure 3. Structure of [C5H10N2Br]Br 2 in the solid state, ellipsoids are shown
at the 50% probability level.
Br1-Br2 angle of 177.88 are in good agreement with a halogen
bond between the cation and anion.
Figure 4. Arrangement of [Br6]2À in [C5H10N2Br]2[Br6]. View along the a-axis,
Commonly, polybromides are formed by the interaction of
the Lewis base BrÀ and elemental bromine, Br2. The Lewis base
BrÀ donates electron density into the LUMO of the Br2 mole-
cule, resulting in an elongated BrÀBr bond length and a corre-
sponding shift of the stretching mode in the Raman spectra of
the coordinated Br2.[17] However, in the case of the title com-
pound we observe an interaction between two asymmetric
[Br3]À units in a hockey-stick-like structure based on halogen-
bonding interactions, see Figure 2. The formed dianion shows
two short contacts to the counterions [C5H10N2Br]+. The dis-
tance between Br5 and Br6 is 345.4 pm with a Br5-Br6-Br2
angle of 72.88 and the distance between Br3 and Br4 is
339.9 pm with a Br3-Br4-Br1 angle of 76.68, which underlines
ellipsoids are shown at the 50% probability level.
Since the tribromide moiety Br6-Br2-Br8 is highly asymmet-
ric, one might also consider it as a bromide anion (Br6) coordi-
nated by a bromine molecule Br2ÀBr8 which in turn bridges to
the tribromide Br7-Br1-Br4. The potential energy curve of an
isolated tribromide anions has been investigated by Pichierri
et al.[18] According to this study a BrÀBr bond elongation or
compression at the equilibrium distance of 255 pm by 10 pm
requires an energy of less than 5 kJmolÀ1, which can easily be
accomplished.
Table 1. Bond lengths and angles from crystal structure [C5H10N2Br]2[Br6], see Figure 2.
Bond Length [pm]
Bond Length [pm]
Angle [8]
Br3ÀBr4
Br4ÀBr1
Br1ÀBr7
Br7ÀBr8
339.9
258.5
251.4
359.3
Br8ÀBr2
Br2ÀBr6
Br5ÀBr6
Br1ÀH[a]
246.8
267.3
345.4
298.8
Br3-Br4-Br1
Br1-Br7-Br8
Br5-Br6-Br2
76.6
87.7
72.8
[a] Closest contact between hydrogen atom of the cation and a bromine atom of the anion.
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DFT calculations of isolated [Br3]À anions predict large nega-
tive atomic charges at the two terminal atoms and a small
negative charge for the central one (À0.45 and À0.10 a.u., re-
spectively). Moreover, we performed calculations based on the
structure of the dianion in the solid state. In the crystal of 1,
atomic charges of À0.36 and À0.28 a.u. are predicted for Br7
and Br8, respectively, see Figure 2. Hence the hockey-stick–like
structure seems unfavorable due to Coulomb repulsion. How-
ever, the calculations also predict a weak s-hole and a corre-
sponding cylindrical negative charge accumulation around the
main axis of a [Br3]À subunit (Figure 5). This favors a hockey-
The presence of the cation can also be observed by NMR
spectroscopy and mass spectrometry. Since the cation is the
1
same for 1 and 2, they both show identical H NMR spectra,
where two signals are observed. One singlet at about d=
4.2 ppm is due to the ring CH2-groups and another singlet at
3.3 ppm is found for the methyl groups. The 13C NMR shows
signals at d=51 and 36 ppm for the CH2 and the CH3 groups,
respectively. In addition, the 13C NMR of the bromide 2 shows
a weak signal for the quarternary carbon C5 at d=151 ppm,
see Figures S3–S6 in the Supporting Information. The IR spec-
trum of the bromide salt 2 is shown in Figure S2. The CÀBr
stretching band emerged at 590 cmÀ1. The mass spectrum (+
ESI) of [C5H10N2Br]2[Br6] reveals two major signals at m/z 177
and 179 in a roughly 1:1 intensity ratio, see Figure S7. This pat-
tern corresponds to the isotopic abundance of a single bro-
mine atom. A signal observed at m/z 115 corresponds to a side
product probably formed by hydrolysis during the measure-
ment preparation.
The significant interactions of the two tribromide anions
forming a hexabromide dianion is also supported by IR and
single-crystal Raman spectroscopy, see Figure 7. In the region
Figure 5. Electrostatic potential in the range of À0.16 a.u. (red) to À0.13 a.u.
(blue) around [Br3]À mapped on an isosurface of the electron density value
0.001 a.u.
stick like structure for two [Br3]À anions, although this s-hole is
much weaker than for example, in Br2. The structure optimiza-
tions for two interacting [Br3]À anions indeed yield a T-shaped
minimum structure induced by the electrostatic interactions,
see Figure 6. The arrangement corresponding to the crystal
Figure 7. Far-IR spectrum and single-crystal Raman spectrum of
[C5H10N2Br]2[Br6].
of the BrÀBr stretching bands four Raman bands are observed
at 205, 181, 165 and 139 cmÀ1. In agreement with other tribro-
mides, the strongest Raman band at 165 cmÀ1 is assigned to
the symmetric stretching mode of the almost linear Br4-Br1-
Br7 tribromide unit.[19] The corresponding asymmetric stretch-
ing mode is observed at 181 cmÀ1. Both bands also appear in
the IR spectrum at 182 and 163 cmÀ1, in which the asymmetric
stretch is stronger than the symmetric one. The Raman band
at 205 cmÀ1 is associated with the short Br2ÀBr8 bond
(246.8 pm). The stretching vibration due to the larger Br2ÀBr6
bond (267.3 pm) is assigned to a band observed at 139 cmÀ1
in the Raman and at 137 cmÀ1 in the far infrared spectrum. The
vibrational spectra are also in good agreement with the calcu-
lated one for the T-shaped [Br6]2À minimum structure, as for
both structures the calculated bond distances are very similar.
The T-shape structure is most probably prevented, and there-
fore the force constant, by packing effects and steric hindrance
Figure 6. Comparison of the experimentally observed L-shaped [Br6]2À in
comparison to the calculated T-shaped minimum on the RI-MP2/aug-cc-
pVTZ, COSMO e=100 level of theory.
structure of 1 is about 9 kJmolÀ1 less favorable but prevails
due to additional interactions in the solid state with the bulky
cations, see Figure 2. The angle between the two interacting
tribromides is about 87.78, very close to the 908 expected for
s-hole interactions. A DFT based atoms-in-molecules (AIM)
analysis yields a bond critical point (BCP) with insignificantly
low electron density between atoms Br7 and Br8, see Figure 2.
In accordance with the intra-molecular bond distances in
[Br6]2À this ion should be considered as two halogen bonded
[Br3]À units, which can formally be described as an [Br6]2À dia-
nion.
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as well as the close contacts between cations and anions, see
Figure 2.
tational time. K.S. thanks the BMBF (project IL-RFB, FKZ:
03SF0526A) for financial support while C.M. thanks the DFG-
GRK 1582 “Fluorine as a Key Element” for financial support.
In conclusion, the first hexabromide dianion [Br6]2À is found
in [C5H10N2Br]2[Br6]. The compound is analyzed by X-ray diffrac-
tion and additionally by mass spectrometry, NMR- and single
crystal Raman and IR spectroscopy. Experimentally, the hexa-
bromide dianion exhibits a hockey-stick structure, while for the
isolated dianion a T-shaped minimum structure is calculated to
be 9 kJmolÀ1 lower in energy. However, the hockey-stick struc-
ture is observed due to cation–anion interactions and packing
effects in the solid state. The molecular structure is explained
by halogen–halogen bonding within the [Br6]2À as well as by
additional stabilizing halogen–halogen interactions with the
bromine substituent of the cation. The novel hexabromide dia-
nion completes the known series of polybromide mono- and
dianions from the tribromide [Br3]À to the undecabromide
[Br11]À.
Conflict of interest
The authors declare no conflict of interest.
Keywords: dianions
·
halogen bonding
·
halogens
·
polybromides · quantum-chemical calculations
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Acknowledgements
Manuscript received: December 13, 2017
Accepted manuscript online: December 15, 2017
Version of record online: && &&, 0000
We gratefully acknowledge the Zentraleinrichtung fꢀr Daten-
verarbeitung (ZEDAT) of the Freie Universitꢂt Berlin for compu-
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COMMUNICATION
&
Halogens
K. Sonnenberg, P. Prçhm, C. Mꢀller,
H. Beckers, S. Steinhauer, D. Lentz,
S. Riedel*
&& – &&
Closing the Gap: Structural Evidence
for the Missing Hexabromide Dianion
[Br6]2À
For a long time, the missing hexabro-
mide dianion [Br6]2À left a gap in the
series of polybromides between the tri-
bromide [Br3]À and the undecabromide
[Br11]À. Finally, we were able to stabilize
the interaction of two tribromide moiet-
ies forming the hexabromide found in
[C5H10N2Br]2[Br6]. The compound is fully
characterized by mass spectrometry,
NMR-, IR and single crystal Raman spec-
troscopy as well as single-crystal X-ray
diffraction.
Chem. Eur. J. 2018, 24, 1 – 5
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