Polybromide salts of tetraalkyl and N-heterocyclic cations: New entries into the structural library

Article abstract of DOI:10.1016/j.ica.2017.10.008

Reactions between Br₂ dissolved in HBr and salts of various organic cations resulted in formation of a series of polyhalide salts: (IsoquinolH)Br₃ (1), (2-BrPyH)Br₃ (2), (H₂(4,4′-bipy))(Br₃)₂ (3), Bu₄NBr₃ (4), (Collidinium){(Br₃)(Br₂)} (5) and Et₄N{(Br₃)(Br₂)₂} (6). All compounds were characterized by X-ray diffractometry; the role of supramolecular Br?Br contacts is discussed.

Full text of DOI:10.1016/j.ica.2017.10.008

Accepted Manuscript
Research paper
Polybromide salts of tetraalkyl and N-heterocyclic cations: new entries into the
structural library
Igor D. Gorokh, Sergey A. Adonin, Maxim N. Sokolov, Pavel A. Abramov, Ilya
V. Korolkov, Evgeniy Yu. Semitut, Vladimir P. Fedin
PII:
S0020-1693(17)30620-5
https://doi.org/10.1016/j.ica.2017.10.008
ICA 17934
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
20 April 2017
5 September 2017
8 October 2017
Please cite this article as: I.D. Gorokh, S.A. Adonin, M.N. Sokolov, P.A. Abramov, I.V. Korolkov, E.Y. Semitut,
V.P. Fedin, Polybromide salts of tetraalkyl and N-heterocyclic cations: new entries into the structural library,
Inorganica Chimica Acta (2017), doi: https://doi.org/10.1016/j.ica.2017.10.008
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Polybromide salts of tetraalkyl and N-heterocyclic cations: new entries into the structural library
Igor D. Gorokha,b, Sergey A. Adonina,b,* Maxim N. Sokolova,c, Pavel A. Abramov,a,b Ilya V. Korolkov,a,b
Evgeniy Yu. Semituta,d and Vladimir P. Fedina,b
aNikolaev Institute of Inorganic Chemistry SB RAS, 630090, 3 Lavrentiev av., Novosibirsk, Russia
bNovosibirsk State University, 630090, Pirogova st. 2, Novosibirsk, Russia
cKazan Federal University, Alexander Butlerov Institute of Chemistry, Lobachevskogo St. 1/29,
420008, Kazan, Russia
d National Research Tomsk Polytechnic University, 30 Lenin Ave., 634050 Tomsk, Russia
*Corresponding author: Dr. S.A. Adonin, E-mail: adonin@niic.nsc.ru
Abstract. Reactions between Br2 dissolved in HBr and salts of various organic cations resulted in
formation of a series of polyhalide salts: (IsoquinolH)Br3 (1), (2-BrPyH)Br3 (2), (H2(4,4`-bipy))(Br3)2 (3),
Bu4NBr3 (4), (Collidinium){(Br3)(Br2)} (5) and Et4N{(Br3)(Br2)2} (6). All compounds were characterized
by X-ray diffractometry; the role of supramolecular Br···Br contacts is discussed.
Keywords: polybromides / polyhalides / X-ray diffractometry / supramolecular contacts
Introduction
Polyhalide compounds constitute a field of modern chemistry which attracts much attention [1].
Although the first example of such substances were reported almost 200 years ago [1], new exciting
compounds continue to be reported. The most recent results are concentrated on the generation of
non-iodide polyhalide units of higher nuclearity (usually more than 5). For example, very recently,
Riedel at al. reported formation of stable [Cl8]2- [2] and a 2D polychloride network [3]. It is known that
the tendency to form polyhalide anions decreases dramatically in the row I >> Br > Cl > F, and,
therefore, polyodides remain the best studied class [4-5].
Interest in polybromides is caused by both fundamental reasons and their potential use in several
application areas. In particular, polybromides may be utilized as a form of “capturing” of Br2 in Zn/Br
redox flow batteries, decreasing their ability to spontaneous discharge [6-10]. A number of

polybromides with various molecular structures have been reported to date [11-16]; the synthetic
approaches towards these compounds may include the use of ionic liquids [17].
During our experiments aiming at the synthesis of novel Bi(III) polybromide complexes [18], we have
noticed that in many cases reactions “Br2 + HBr + [BiBr6]3-“ do not lead to the formation of a desired
product. Instead, there forms a mixture of bromobismuthate and polybromide (most commonly,
tribromide) salts of an organic cation. Considering the fact that XRD and PXRD are the main methods
for identification of the products in reactions of this type, information on the crystal structures of
such salts is needed to allow their rapid identification. In this work, we describe the crystal structures
of six novel polybromides of various organic cations containing different amounts of “captured”
dibromine per one Br-: (IsoquinolH)Br3 (1), (2-BrPyH)Br3 (2), (H2(4,4`-bipy))(Br3)2 (3), Bu4NBr3 (4),
(Collidinium){(Br3)(Br2)} (5) and Et4N{(Br3)(Br2)2} (6).
Experimental section
All reagents were obtained from commercial sources (Sigma-Aldrich) and used without any additional
purification. Initially, samples of 1-6 were precipitated from the cooled (6°C) mixtures of two
solutions: (1) Bi2O3 + excess of Br2 and (2) bromide of corresponding cation (both in 2M HBr). Further,
the same general approach was applied: solutions of Br2 and corresponding cation (generated in situ
by protonation or taken as bromide salt), both in aqueous 2M HBr, were mixed together and cooled
down to 6°C. In initial non-optimized experiments, the cation/Br2 ratio was 1:1 (with great excess of
bromide anion in solution). Crystals of 1-6, suitable for XRD, were obtained within 1-2 days (see SI).
Crystallographic data and refinement details for 1-6 are given in Table 1. The diffraction data were
collected on a New Xcalibur (Agilent Technologies) diffractometer with MoKα radiation (λ = 0.71073)
by doing φ scans of narrow (0.5°) frames at 130 K. Absorption correction was done empirically using
SCALE3 ABSPACK (CrysAlisPro, Agilent Technologies, Version 1.171.37.35 (release 13-08-2014
CrysAlis171 .NET) (compiled Aug 13 2014,18:06:01)). Structure was solved by direct method and
refined by full-matrix least-squares treatment against |F|2 in anisotropic approximation with SHELX
2014/7 [19] in ShelXle program [20]. Hydrogen atoms were refined in geometrically calculated
positions. Cifcheck from http://journals.iucr.org/services/cif/checkcif.html gives Alert B-type
ADDSYM suggests Possible Pseudo/New Space Group P-1 for the crystal structure of 1. But the
structure was solved in P-1 and checked with PLATON [21]. The ADDSYM procedure from PLATON
does not find a new space group and suggests check pseudo translations. Checking frames for
violations didn’t detect any results. Analyzing of all facts we suggest to submit to CCDC the crystal

structure of 1 as it is. Solution the crystal structure of 4 in space group P21 as racemic twin with BASF
value 0.54 gives R 3.8%. Refinement in P21/n lead to disordering of TEA cation over two positions
with occupancy around 0.5/0.5 and R value increases to 7.23%. According to this crystal structure of
4 is an asymmetric racemic twin with low domination of one part. The main geometrical parameters
are summarized in Table S1 (see SI). The crystallographic data have been deposed in the Cambridge
Crystallographic Data Centre under the deposition codes CCDC 1544201-1544206.
Table 1. Details of crystallographic experiments for 1-6
1
2
3
4
Chemical formula
C9H8Br3N
369.89
C10H9Br5N2
556.74
C10H10Br6N2
637.66
C16H36Br3N
482.19
Mr
Crystal system,
space group
Triclinic, P¯1
Triclinic, P¯1
Monoclinic, P2/n
Triclinic, P¯1
a, b, c (Å)
7.5644 (2),
10.3668 (4),
14.5156 (4)
8.3073 (5),
8.4831 (5),
11.3016 (7)
6.9211 (4), 9.5152 8.9920 (3), 9.9237
(6), 12.6423 (10) (3), 12.5044 (6)
α, β, γ (°)
96.664 (2),
92.227 (2),
108.708 (3)
87.164 (5),
71.941 (5),
80.975 (5)
90, 103.236 (7), 90 109.539 (4), 90.649
(3), 97.727 (3)
V (Å3)
Z
1067.36 (6)
747.83 (8)
2
810.45 (10)
1040.12 (7)
4
2
2
μ (mm-1)
11.29
13.42
14.85
5.81
Crystal size (mm)
0.12 × 0.12 × 0.26 × 0.12 × 0.08 0.30 × 0.12 × 0.12 0.12 × 0.10 × 0.08
0.06
Tmin, Tmax
0.332, 1.000
0.153, 1.000
0.236, 1.000
0.933, 1.000
No. of measured,
independent and
observed [I > 2σ(I)]
reflections
9648, 5070, 5772, 3492, 2643 3973, 1927, 1575
3775
8090, 4863, 3768
Rint
0.032
0.055
0.019
0.018
θ values (°)
θmax = 29.6, θmin θmax = 29.5, θmin = θmax = 29.6, θmin = θmax = 29.6, θmin = 3.4
= 3.3
3.3
3.3
(sin θ/λ)max (Å-1)
0.695
0.694
0.695
0.695
Range of h, k, l
h = -9ꢀ10, k = - h = -10ꢀ11, k = -
h = -9ꢀ7, k = -
12ꢀ10, l = -
11ꢀ17
h = -12ꢀ11, k = -
12ꢀ13, l = -17ꢀ13
13ꢀ12, l = -
14ꢀ18
11ꢀ10, l = -
15ꢀ15
R[F2 > 2σ(F2)],
wR(F2), S
0.039, 0.084,
0.98
0.045, 0.097, 0.033, 0.084, 1.27 0.074, 0.230, 1.09
0.98

No. of reflections,
parameters,
restraints
5070, 235, 0
3492, 154, 0
1927, 84, 0
4863, 181, 0
Weighting scheme w = 1/[σ2(Fo ) + w = 1/[σ2(Fo ) +
w = 1/[σ2(Fo ) +
w = 1/[σ2(Fo ) +
2
2
2
2
(0.0291P)2]
where P = (Fo + where P = (Fo +
(0.0361P)2]
(0.0136P)2 +
4.678P]
(0.0823P)2 +
18.1006P]
2
2
2
2
2
2
2Fc )/3
2Fc )/3
where P = (Fo +
where P = (Fo +
2
2Fc )/3
2
2Fc )/3
Δmax, Δmin (e Å-3)
0.86, -0.91
0.98, -1.11
0.69, -0.78
2.83, -2.83
5
6
Chemical formula
C8H12Br5N
521.74
C8H20Br7N
689.62
Mr
Crystal system, space
group
Monoclinic, P21/c
Monoclinic, P21
a, b, c (Å)
8.7294 (4), 7.4739 (4), 22.0451
(9)
9.0935 (5), 10.3722 (5), 10.2994 (6)
α, β, γ (°)
V (Å3)
90, 91.662 (4), 90
1437.68 (12)
4
90, 107.880 (6), 90
924.52 (9)
Z
2
μ (mm-1)
Crystal size (mm)
Tmin, Tmax
13.94
15.17
0.36 × 0.08 × 0.08
0.231, 1.000
7446, 3195, 2640
0.20 × 0.08 × 0.08
0.448, 1.000
4243, 3233, 2743
No. of measured,
independent and
observed [I > 2σ(I)]
reflections
Rint
0.027
θmax = 29.0, θmin = 3.3
0.682
0.018
θmax = 29.6, θmin = 3.6
0.694
θ values (°)
(sin θ/λ)max (Å-1)
Range of h, k, l
h = -9ꢀ11, k = -7ꢀ9, l = -
27ꢀ28
h = -12ꢀ7, k = -14ꢀ10, l = -11ꢀ13
R[F2 > 2σ(F2)], wR(F2),
0.034, 0.061, 1.08
0.038, 0.074, 1.05
3233, 146, 1
S
No. of reflections,
parameters,
restraints
3195, 127, 0
2
2
Weighting scheme
w = 1/[σ2(Fo ) + (0.0186P)2 +
0.2622P]
w = 1/[σ2(Fo ) + (0.0207P)2 + 1.8624P]
2
2
where P = (Fo + 2Fc )/3

2
where P = (Fo + 2Fc )/3
2
Δmax,Δmin (e Å-3)
0.76, -0.72
1.09, -0.77
Results and discussion
As mentioned above, polybromides 1-6 were obtained during attempted syntheses of Bi(III)
polybromide complexes. In most cases, we observed the formation of crystals of two sorts: light
yellow (obviously bromobismuthate complexes; the presence of Bi was confirmed by elemental
analysis) and orange/red. Once the absence of bismuth in the isolated orange/red crystals was
established by XRD, we decided to focus on polybromide products all experiments were reproduced
according to the abovementioned scheme (the nature of crystalline samples was confirmed by
comparison of measured cell parameters). It can be seen that, although the initial stoichiometry was
the same (or very similar), the cation/Br2 ratio in the products varies.
Compounds 1-4 belong to the most widespread type, the tribromides [22-33]. In this case, no
-
additional contacts between the Br3 anions are observed. Crystal packings in 1-4 are shown in
-
Figures 1 and 2. The Br-Br distances in Br3 fit within the usual range: 2.4353(5)-2.6829(5), 2.5317(8)-
2.5589(8), 2.429(1)-2.681(1) and 2.470(2)-2.468(2) Å in 1, 2, 3 and 4, respectively. The crystal
structure of 2 features the presence of specific supramolecular contacts between the Br atoms of 2-
-
bromopyridinium cation and terminal Br of Br3 (Br···Br = 3.4588(9) Å) (Fig. 3). In all cases where N-
protonated bases act as cation, there are also short NH···Br contacts.
Figure 1. Crystal packings in 1 and 2. Br blue

Figure 2. Crystal packings in 3 and 4. Br blue. In 4, TBA cations are shown in wire-and-stick model for
clarity
Figure 3. Specific Br···Br contacts in 2
It must be mentioned that two polybromides containing TBA cation were reported earlier. The first
compound is another polymorph of known TBABr3, demonstrating higher symmetry (monoclinic vs
triclinic in 4) [34], while in the second case the number of {Br2} units is greater [16].
-
In the structure of more bromine-rich polybromide 5, the anionic part also consists of Br3 which are
supramolecularly connected to neutral {Br2} units (Br···Br = 3.3261(6) Å) (Fig. 4), forming therefore a
-
V-shaped Br5 fragment. Therefore, it cannot be considered as “true pentabromide” described earlier
[35]. It must be noted that, although such supramolecular ({Br3} + {Br2}) associates have been
reported [36-38], their number is very limited. Analyzing the relevant structures, we noticed that
there are two types of geometry, featuring different Br-Br···Br angles: either close to 120° or close to
90° (93.48(2)° in 5).

Figure 4. Crystal packing in 5. Br blue, {Br3}···{Br2} contacts dashed
Finally, the most “Br- saturated” compound 6 demonstrates the most sophisticated structure, which
-
may be considered as a two-dimensional supramolecular polymer. There are one Br3 and two {Br2}
-
units per one TEA cation (Fig. 5); each Br3 interacts with four {Br2} neighbors by its terminal Br atoms
(Br···Br = 3.206(2) – 3.286(2) Å). The structure reveals certain similarity with other heptabromide-
based frameworks [12] [27].
Figure 5. The system of Br···Br contacts in the structure of 6. Br blue; TEA cations are omitted for
clarity

Overall, it can be seen that polybromides containing different amounts of “captured” {Br2} may be
isolated from HBr/Br2 system. Earlier it was noted [39] that in some cases (e.g. conducting reactions
in gas phase) formation of polybromides may proceed in stoichiometric way. However, numerous
reports of such compounds synthesized in ionic liquids [11, 12] allow us proposing that this is not a
common situation. In our opinion, an important factor affecting the composition and structure of
forming polyhalides is a nature of cation used in synthesis. Recent theoretical evaluations of halogen
bonding energies in polyhalides [40] result in the values which are comparable with those of
electrostatic cation···anion interactions; therefore, the role of crystal packing may be crucial.
According to our observations, a mass loss for all samples (especially for 5 and 6) could be detected
within hours when kept on air. Therefore, general safety precautions (usual when working with
halogens) are recommended. As follows from the PXRD data, our numerous attempts to prepare
pure phases of 1-6 succeeded only in the case of 1 (see SI). In some cases, there form mixtures of 2-6
and other polybromide phases with the same cations reported earlier (in particular, for 6).
Surprisingly, although the 4 is a tribromide salt which should be stable, in all cases we detected other
known polymorphs of TBABr3 as predominant phases. This fact, together with the limited stability
(which is rather expectable for 5 and 6), prevented us from obtaining of any reliable TGA data for the
whole set of compound, as well as determination of reaction yields. However, we successfully applied
TGA in the case of 1 (see SI), and results are in good agreement with our expectations.
Conclusions
A series of six novel polybromides, featuring different Br-/Br2 ratio, were obtained and structurally
characterized. Considering that: (a) the formation of the mixture of different polyhalides is quite
common and (b) the range of physical methods which can be used for their characterization is
limited, X-ray diffractometry and PXRD analysis based thereupon remain the most reliable and
informative approach. It is worth stressing that the formation of polybromides with higher content of
{Br2} can occur when working in HBr/Br2 medium; this method is simple and straightforward, and it
may be considered as an alternative to the use of ionic liquids (or, at least, the compounds prepared
in this way may be utilized as precursors for further IL synthesis).
Acknowledgements
This work has been supported by Russian Science Foundation (Grant No. 14-23-00013). SAA thanks
the President of Russian Federation for a research fellowship. We thank one of the referees for
her/his valuable suggestions.

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- Six novel polybromides are structurally characterized
- Br2/Br- ratio varies from 1 to 3
-Simple preparation strategy was proposed