The Bromine-Chlorine Interhalides [Br3Cl5]2–, [Br4Cl4]2– and [Br6.56Cl1.44]2–

Article abstract of DOI:10.1002/ejic.202000478

The bromine-chlorine dianionic octahalide-containing salts [C₃(NEt₂)₃]₂[Br₄Cl₄] (1c) and [C₃(NEt₂)₃]₂[Br₃Cl₅] (2d) were prepared using the triaminocyclopropenium cation as a template. The salt 1c was prepared in two steps, firstly, either by addition of BrCl to the triaminocyclopropenium chloride salt [C₃(NEt₂)₃]Cl or addition of Cl₂ to the bromide salt [C₃(NEt₂)₃]Br to form the trihalide [BrCl₂]^– salt, secondly, addition of half an equivalent of Br₂ gave the desired product upon crystallization at low temperature. Salt 2d is prepared by addition of excess BrCl to [C₃(NEt₂)₃]Cl. Both salts are best described as two trihalides bridged by a dihalogen. When the tetraethylammonium cation is used, a salt of the formula [NEt₄]₂[Br_4.56Cl_0.44][ClBr₂] (4), containing disordered [ClBr₂]^– and a chain of [ClBr₄]^– and [Br₅]^– pentahalides (in a ratio of 0.44 to 0.56, respectively). These interhalides were characterized by X-ray crystallography, computational studies, Raman and Far-IR spectroscopy, as well as by TGA and melting point.

Full text of DOI:10.1002/ejic.202000478

European Journal of Inorganic Chemistry
Accepted Article
Title: The bromine-chlorine interhalides [Br3Cl5]2–, [Br4Cl4]2– and
[Br6.56Cl1.44]2–
Authors: Mohammed S. Abdelbassit, Owen John Curnow, Samuel J.
Brooke, and Mark R. Waterland
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000478
Link to VoR: https://doi.org/10.1002/ejic.202000478
01/2020

10.1002/ejic.202000478
European Journal of Inorganic Chemistry
FULL PAPER
The bromine-chlorine interhalides [Br3Cl5]2–, [Br4Cl4]2– and
[Br6.56Cl1.44]2–
Mohammad S. Abdelbassit,[a] Owen J. Curnow,*[a] Samuel J. Brooke,[b] and Mark R. Waterland*[b]
Abstract: The bromine-chlorine dianionic octahalide-containing salts
[C3(NEt2)3]2[Br4Cl4] (1c) and [C3(NEt2)3]2[Br3Cl5] (2d) were prepared
using the triaminocyclopropenium cation as a template. The salt 1c
was prepared in two steps, firstly, either by addition of BrCl to the
triaminocyclopropenium chloride salt [C3(NEt2)3]Cl or addition of Cl2 to
Classical species such as IF3, [BrF4]– and [ICl4]– contain a central
less-electronegative halogen coordinated by more
electronegative halogens, typically Cl or F. Non-classical
examples are formed via halogen bonding and are an extension
of the polyhalides discussed above. As with the polyhalides then,
the bromide salt [C3(NEt2)3]Br forms the trihalide [BrCl2]– salt, secondly, the number of non-classical interhalides would be expected to
addition of half an equivalent of Br2 gave the desired product upon
crystallization at low temperature. Salt 2d is prepared by addition of
excess BrCl to [C3(NEt2)3]Cl. Both salts are best described as two
trihalides bridged by a dihalogen. When the tetraethylammonium
cation is used, a salt of the formula [NEt4]2[Br4.56Cl0.44][ClBr2] (4),
containing disordered [ClBr2]– and a chain of [ClBr4]– and [Br5]–
pentahalides (in a ratio of 0.44 to 0.56, respectively). These
interhalides were characterized by X-ray crystallography,
computational studies, Raman and Far-IR spectroscopies, as well as
by TGA and melting point.
decrease as we go up the group. However, such species are still
quite rare: There are less than dozen iodine-bromine
a
interhalides[2,3,10,11] and less than half a dozen iodine-chlorine
interhalides.[2,3,12,13] The number of structurally-characterized
bromine-chlorine interhalide species was until recently limited to
–[14–17]
–
just the trihalides BrCl2
and ClBr2 (which has been
structurally characterized just once).[18] Spectroscopic evidence
has also been provided for [BrCl4]–.[19] Riedel and coworkers
initially reported three examples of chloride coordinated to BrCl
dihalogens via the Br atoms: [Cl(BrCl)3]–, [Cl(BrCl)5]– and
[Cl(BrCl)6]– consist of chloride ions with distorted trigonal
pyramidal, square pyramidal and octahedral coordination
environments, respectively.[20] See reference 21 for examples of
[Cl(BrCl)2]– and [Cl(BrCl)4]– that were published subsequent to
submission of our manuscript. Along with the bromine-chlorine
interhalogens BrCl and BrCl3, Nelson and Pimentel reported
spectroscopic evidence for BrCl2Br and BrClBr2 in an inert gas
matrix at 20 K.[22]
Introduction
The number of polyhalides of iodine is very large[1] and there is
also a significant number of polybromides.[2,3] The number of
polyhalides decreases dramatically to chlorine (including the salt
[Et4N]2[(Cl3)2·Cl2] which contains a polychloride network with Cl2-
bridged octachloride fragments)[2,4,5,6] and fluorine, for which only
–
–
F3 and F5 have been observed in cryogenic matrices.[7,8] Poly-
and interhalogen anions have been very recently reviewed.[3] The
major reason for the increase in the number of polyhalides down
group 17 is that the less electronegative halogens have an
increasingly larger electrophilic “σ hole” trans to the X–X bond,
along with a corresponding nucleophilic region in a ring on the
halogen atom around the bond axis.[9] Halogen bonds are then
formed between the electrophilic regions of halogens and the
nucleophilic regions of neighbouring halogens, halides or
polyhalides.
Interhalides and interhalogens (i.e. those consisting of at
least two different halogen atoms) can be classified as either
classical or non-classical, as defined by Haller and Riedel.[2]
Scheme 1. Syntheses of selected iodine-bromine,[10] iodine-chlorine[12] and
iodine-bromine-chlorine[23] octahalide salts. The [C3(NEt2)3]+ cation is not shown.
We have recently reported the rational synthesis of an
extensive family of octahalide dianions templated by the
tris(diethylamino)cyclopropenium cation.[10,12,23] CH---X hydrogen
bonding and halogen to C–N π-hole interactions appear to be a
key ingredient in the formation of the octahalides. The general
procedure involves the crystallization of the desired product from
a stoichiometric mixture of the requisite trihalide and dihalogen.
Consideration of the halogen oxidation states is critical to
ensuring that the desired product can be formed. In the iodine-
[a]
[b]
Mohammed S. Abdelbassit and A/Prof. Owen J. Curnow
School of Physical and Chemical Sciences
University of Canterbury, Private Bag 4800, Christchurch,
New Zealand
E-mail: owen.curnow@canterbury.ac.nz
https://www.canterbury.ac.nz/science/contact-us/people/owen-
curnow.html
Samuel J. Brooke and A/Prof. Mark R. Waterland
Institute of Fundamental Sciences,
Massey University, Private Bag 11-222, Palmerston North 4442,
New Zealand
chlorine series, only [C3(NEt2)3]2[I4Cl4] (1a), [C3(NEt2)3]2[I3.6Cl4.4
]
E-mail: M.Waterland@massey.ac.nz
and [C3(NEt2)3]2[I3Cl5] (2a) can be formed,[12] whereas in the
iodine-bromine series, [C3(NEt2)3]2[I4Br4] (1b), [C3(NEt2)3]2[I3Br5]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.202000478
European Journal of Inorganic Chemistry
FULL PAPER
(2b), [C3(NEt2)3]2[I2Br6] (3a) and [C3(NEt2)3]2[Br8] are able to be
formed (Scheme 1).[10] Ternary octahalides have also been
X-ray structures
–
presented: Two octahalides based on ICl2
trihalides,
The salts 1c and 2d readily crystallize from CH2Cl2/diethylether
solutions at low temperature. As with the other octahalides that
we’ve reported, both salts form in the monoclinic C2/m space
group with one cation and half a dianion in the asymmetric unit.
Structure refinement data are summarized in Table S1.
[C3(NEt2)3]2[I3BrCl4] (2c) and [C3(NEt2)3]2[I2Br2Cl4] (3b) (Scheme
1), and three based on BrICl– trihalides containing the dianions
[I4Br2Cl2]2–, [I3Br3Cl2]2– and [I2Br4Cl2]2–.[23] We now report on the
use of this template to investigate the synthesis, structure and
properties of the non-classical bromine-chlorine octahalides
[Br4Cl4]2– and [Br3Cl5]2– which contain the first interhalides based
Like the isotypic [C3(NEt2)3]2[I4Cl4] and [C3(NEt2)3]2[I4Br4]
salts 1a and 1b, respectively,[10,12] 1c is also well-ordered. Figure
1 shows the arrangement of the dianion with the two most
associated cations (see the supplementary information for the
cation labelling scheme). The octahalide sits on a crystallographic
inversion centre and lies in a mirror plane. Each octahalide is
sandwiched by two cations in addition to weaker interactions with
–
on BrCl2 .
Results and Discussion
–
other cations. The structure can be described as two BrCl2
Synthesis
trihalides bridged by a Br2 dihalogen. There are strong hydrogen-
bonding interactions between each internal Cl atom (Cl1) and two
methylenes (C11) of a diethylamino group; the Cl---H distance is
2.8706(11) Å and the Cl1---C11 distance is 3.634(4) Å. The next
closest Cl1---C distances are two at 4.416 Å whereas Cl2 has
eight Cl2---C distances between 3.8–4.2 Å. The Br2---C1 and Br2-
--N1 distances are 3.528(5) and 3.636(5) Å, respectively, which
are similar to the distances to the central Br2 in [C3(NEt2)3]2[I2Br6]
at 3.602(13) Å and 3.598(16) Å, respectively.[10] Interestingly, that
structure contained a disordered central Br2 and is intermediate
between an asymmetric pentahalide/trihalide combination and a
octahalide structure, whereas [Cu(dafone)3](IBr4)(I2Br6)0.5·CH3CN
contains a symmetric octahalide.[24] The Br–Br distance in 1c, at
2.3471(9) Å, unsurprisingly, is closer to that of the symmetric
As with our previous work to prepare octahalides, the syntheses
were carried out in situ in two steps. Firstly, the desired trihalide
unit is prepared and, secondly, the desired dihalogen is added.
The preparation of the [Br4Cl4]2– salt therefore involves the
–
synthesis of BrCl2 followed by addition of half an equivalent of
Br2. We found two routes to prepare the trihalide: (i) oxidation of
the bromide salt [C3(NEt2)3]Br with Cl2; and (ii) addition of the
interhalogen BrCl to the chloride salt [C3(NEt2)3]Cl (Scheme 2).
The latter route was found to give the highest yield of the
octahalide [C3(NEt2)3]2[Br4Cl4] (1c) after addition of Br2 to the
trihalide followed by crystallization (63% versus 46%). Traces of
water from the generation of Cl2 is not thought to significantly
affect the reaction and the products were obtained in very good
yields. Note also that [BrCl2]– can be prepared in an aqueous
medium.[14]
octahalide
(2.3692(12)
Å)
than
the
asymmetric
pentahalide/trihalide (2.557(5) Å). This short Br–Br distance is
–
consistent with weak halogen bonding between BrCl2 and Br2.
The Br–Cl bonds in the trihalide fragment differ by 0.11 Å, which
is intermediate compared to other BrCl2– trihalides (Table 1), and
indicates the presence of some halogen bonding. There are only
–
three other structures containing BrCl2 : a [PPh4]+ salt, a [NEt4]+
salt and a salt with a Re complex cation (two of these salts contain
two independent anions).[14–16] We prepared and collected an
improved data set for [Et4N]BrCl2 (see supplementary
information). Figure 2 plots the Br–Cl distances (long versus
short) which shows that the BrCl2– units in 1c (and 2d, see below)
are similar to the structures of the discrete BrCl2– trihalides. Table
1
provides Cl–Br distances in published bromine-chlorine
Scheme 2. Syntheses of [Br4Cl4]2– (1c) and [Br3Cl5]2– (2d).
interhalides. The long trihalide–dibromine (Cl1---Br2) distance of
3.0196(11) Å can only be compared to the related Br---Br2
distances in [I2Br6]2– and [Br8]2–. The only other structure with a
The synthesis of the [Br3Cl5]2– salt 2d can be done in one step by
addition of 1.5 equivalents of BrCl to the chloride salt, with BrCl2
being formed along the way (Scheme 2). The orange-brown
colour of 2d is significantly lighter than that of 1c.
We also attempted to prepare the pentahalide [Cl(Br2)2]– by
treatment of [Et4N]Cl with two equivalents of Br2, however, X-ray
diffraction (below) revealed a product that can best be formulated
as [Et4N]2[Cl0.44/Br0.56(Br2)2][ClBr2] (4) containing pentahalide and
trihalide units in a 2D layer of weak halogen bonds. The presence
chloride-dibromine interaction, that of ClBr2 , is disordered.[18] Br–
–
–
---Br2 distances range from 2.91–3.17 Å,[10] indicating that the
larger Br atoms are better donors to Br2 since the distance is
essentially the same. The angle at the internal Cl atom of
118.41(5)° is typical for octahalides (80–150°).
of
some
bromide
suggests
some
chloride-induced
–
disproportionation of Br2 into Br– and BrCl2 , as is similarly found
in the reaction of chloride with iodine.[12]
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10.1002/ejic.202000478
European Journal of Inorganic Chemistry
FULL PAPER
dihalogen Cl---Br distance in 2d is shorter than in 1c, and the Cl-
--Cl distance is much longer than either of those (3.359(19) Å).
The central Br–Cl distance is intermediate between that of free
BrCl and the distances in the trihalide fragments, which also
indicates very weak halogen bonding. Also, the very similar
internal and terminal Br–Cl distances in the trihalide fragments
indicates weak halogen bonding: an only slightly larger difference
of 0.139 Å in the BrCl2– fragment in the “pentahalide” than in the
trihalide (0.098 Å). This compares to differences of 0.181/0.043 Å
in [I3Br5]2– and 0.17/0.03 Å in [I3Cl5]2–, which are both much more
like a pentahalide-trihalide combination.
Figure 1. Illustration of the cation-anion interactions in 1c and the halogen
Table 1. Summary of Br–Cl distances in bromine-chlorine interhalides.
labelling scheme (distances in Å).
–
Salt
BrCl
fragments
BrCl2
ClBr2
/
Cl–---BrX
Reference
–
fragments
25
BrCl(g)
2.1388
2.518(3)
2.277(3)
[Re(tacnCO)(CO)2Br]
[BrCl2]
14
2.545(3)
2.280(3)
2.4060(8)
2.3885(8)
[Et4N]BrCl2
This work
2.4175(7)
2.3590(7)
2.4264(15)
2.4265(15)
[Ph4P]BrCl2
15
18
[(Me3NC3H6)2]
[Br2Cl]2
2.5670(10)
Disordered
2.4518(11)
2.3450(11)
2.470(13)
2.331(12)
1c
2d
3.0196(11)
2.774(19)
This work
Figure 2. Br–Cl distances in BrCl2– trihalides (blue circles) and the octahalides
in 1c and 2d (red triangles).[14,15]
2.235(5)
This work
2.432(14)
2.334(8)
2.31(2)
2.70(1)
2.81(1)
2.92(1)
4
2.40(3)
The structure of 2d has, as expected, a dianion that is
disordered over two positions (Figure 3). Symmetry dictates that
the central BrCl dihalogen must have 50:50 occupancy; it is
reasonable that the occupancy of the disordered BrCl2– units are
dictated by the orientation of the BrCl. Our choice of atoms for the
trihalide units was not clear-cut since the possible Br–Cl distances
are all very similar (2.47–2.31 Å), which also indicates that, rather
[Cl(BrCl)2]–
2.207(4)–
2.278(1)
2.517(1)–
2.664(2)
21
2.189(2)–
2.237(2)
2.603(2)–
2.801(3)
[Cl(BrCl)3]–
[Cl(BrCl)4]–
[Cl(BrCl)5]–
20, 21
21
2.213(2)
2.786(1)
2.180(2)–
2.2379(17)
2.811(2)–
2.986(2)
20, 21
20, 21
than
being
a
pentahalide-trihalide
combination
like
[C3(NEt2)3]2[I3Cl5] (2a) and [C3(NEt2)3]2[I3Br5] (2b),[10,12] this is
more like two trihalides bridged by a dihalogen. It should be noted
that, due to the disorder, the uncertainty in the distances in 2d is
greater than in 1c. From Table 1, three types of species based on
Br–Cl distances can be clearly distinguished: (i) Distances less
than 2.24 Å correspond to BrCl dihalogen fragments; (ii) distances
longer than 2.60 Å correspond to halogen bonding from a chloride
or bromine dichloride to a dihalogen; and (iii) in between (2.27–
2.55 Å) are the bromine dichloride fragments which generally
exhibit a short and a long distance, depending on the degree of
associated halogen bonding to other species.
2.7933(5)–
2.9125(4)
[Cl(BrCl)6]–
2.1539(10)–
2.1694(9)
The intermolecular distance of the Br atom to the CN π-hole of
the cation in 2d (N1---Br2 = 3.558(6) Å; C1---Br2 = 3.513(6) Å) is
slightly shorter than in 1c, while the Cl to CN π bond is similar in
distance but more asymmetric (N1---Cl3 = 3.640(8) Å; C1---Cl3 =
3.446(8) Å). The methylene-chloride hydrogen bonding distances
are similar to 1c (Cl1---H11b = 2.888(14) Å; Cl1---C11 = 3.629(13)
Å and Cl1a---H11b = 2.822(14) Å; Cl1a---C11 = 3.600(13) Å).
The largest sigma hole on BrCl is on the Br atom, and this
is also larger than that of Br2.[20] Consequently, the trihalide-
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10.1002/ejic.202000478
European Journal of Inorganic Chemistry
FULL PAPER
pentahalide (a new bromine-chlorine interhalide) is almost
symmetric with approximately equal Cl---Br distances (2.70(1)
and 2.81(1) Å), whereas the [Br5]– pentahalide is asymmetric
(2.664(4) and 2.939(3) Å) and could therefore be better described
as a trihalide-dihalogen combination. Slightly weaker halogen
bonds from the terminal Br5 atom to the central atom of a
neighboring pentahalide (Cl1---Br5 = 2.92(1) Å; Br1---Br5 =
3.064(3) Å) then forms a chain of pentahalides. The Br–Br
distances in the dibromine units (2.3769(7) Å and 2.3924(9) Å)
are slightly longer than the dibromine distance in 1c (2.3471(9) Å).
Interestingly, the Cl atom in the pentahalide is within 2.70–2.92 Å
of three Br2 units. These bonds are only slightly longer than the
Cl–BrCl distances in [Cl(BrCl)3]– of 2.60–2.80 Å,[20,21] despite the
larger σ hole on BrCl compared to Br2.
It should be pointed out that the weak interactions between
the trihalides and the pentahalides (3.71–3.81 Å) mentioned
earlier are similar to the van der Waals distance of 3.70 Å between
Br atoms. Therefore, they should not be considered as bonding
distances (they are not shown in Figure 5). The distances
between the pentahalides, however, are much shorter, are well
within the van der Waals distance, and support a chain of
pentahalides as indicated in Figure 5.
Figure 3. The disordered octahalide in 2d (top) and one of the asymmetric
octahalides (bottom).
The unit cell volumes of 1c (2125.44(11) Å3) and 2d (2111.89(13)
Å3) are very similar, as expected based on the small change in
molecular weight. These are the smallest cell volumes of the
[C3(NEt2)3]+ octahalides that we’ve characterized. On the other
hand, [I4Br4]2– (2322.25(16) Å3) appears to be the upper size limit
for the isotypic octahalide series based on the [C3(NEt2)3]+ cation
(we were unable to prepare an octaiodide).[10]
Structure refinement data for 4 are given in Table S1. The
structure was found to have alternating layers of [Et4N]+ cations
and polyhalide anions. Figure 4 shows one asymmetric unit with
the atomic labelling scheme and Figure 5 illustrates a layer of
polyhalide which is best described as a combination of a trihalide
–
and a chain of pentahalides. The ClBr2 trihalide is disordered
over two positions. The disorder is not symmetric, with Cl2A-Br7-
Br6 having an occupancy of 0.59 and Cl2-Br7-Br6A an occupancy
of 0.41, presumably because the halogen bonding to the rest of
the polyhalide network is not symmetric. We considered the
Figure 4. The asymmetric unit of 4 with the atomic labelling scheme.
–
–
possibility that this is a mixture of Br3 and BrCl2 , however, the
occupancies rule this out and the Br–Br distances (2.545(17) Å
and 2.489(5) Å) are relatively short and appear to be more
–
consistent with ClBr2 . The Br–Cl distances are quite different
(2.31(2) and 2.40(3) Å), which is consistent with a soft Cl---Br2
halogen bond. The infrared spectra of 1c, 2d and [Et4N]BrCl2 all
have a very strong band for the asymmetric BrCl2– stretch at 190–
200 cm–1 which does not appear in the infrared spectrum of 4 (see
below). Similarly, whereas 1c, 2d and [Et4N]BrCl2 all have a
–
strong band for the symmetric BrCl2 stretch at 250–280 cm–1 in
the Raman, there are no strong bands in this region of the Raman
spectrum of 4 (see below). The central Br7 atom is not disordered
and appears to be locked into place by weak halogen bonding to
Br3 (3.7374(8) Å). The terminal Br atoms of the pentahalides have
additional weak halogen bonding to the trihalides (Br3---Br6A =
3.810(17) Å; Br5---Br6 = 3.713(6) Å). This halogen bonding also
–
explains the bent ClBr2 geometry (Cl-Br-Br = 171.3(9)° and
175.2(7)°). The rest of the structure contains a 0.44/0.56 mixture
of [ClBr4]– and [Br5]– pentahalides, respectively. The [ClBr4]–
Figure 5. A polyhalide layer of 4.
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10.1002/ejic.202000478
European Journal of Inorganic Chemistry
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Melting point and thermal decomposition studies
Melting points and thermal decomposition processes of 1c, 2d, 4
and [NEt4]BrCl2 were studied by coupled DSC/TGA (Table 2).
Salts 1c and 2d show similar decomposition profiles (Figure 6).
Since the boiling point of BrCl is only 5°C, some mass loss occurs
while the salts are still in crystalline form. A short burst of mass
loss occurs upon melting which would effectively correspond to
the release of trapped BrCl(g). At this point, the melt of 1c will
–
–
consist of an equilibrium mixture of Cl–, BrCl2 , ClBr2 , and the
dihalogens BrCl, and Br2, and possibly some Cl2 (BrCl in
equilibrium with Cl2 + Br2), whereas 2d will consist largely of Cl–,
–
BrCl2 and BrCl. Both liquids show a steady mass loss
corresponding to two equivalents of the more volatile BrCl. A
second decomposition process then occurs at about 180°C
corresponding to loss of one equivalent of Br2 or BrCl,
respectively, to leave the chloride salt which decomposes shortly
after with Tonset of 290°C to leave what is proposed to be a
graphitic-like material formulated as [C5N]Cl. The decomposition
of 4 and [Et4N]BrCl2 similarly show a burst of mass loss upon
melting due to trapped dihalogen, but most of the dihalogen is lost
at a similar temperature to that of the second decomposition
process seen for 1c and 2d. The remaining [Et4N]Cl or [Et4N]Br
decomposes very shortly after and the two processes are difficult
to separate.
Figure 6. TGA at 10°C min–1 of 2d (calculated percentages).
Far-IR and Raman spectroscopy
Salts 1c, 2d, 4 and [Et4N]BrCl2 were studied by both Far-IR and
Raman spectroscopy in the solid state; the results are
summarized in Table 3 and illustrated in Figures 7 and 8. Band
assignments are given in Table 3. Band assignments have been
made by comparison to the calculated spectra (Figures S6 and
S8). The normal modes of the octahalide species can be
described as combinations of the stretching modes of the terminal
trihalide and central dihalide units. The phase relationships
between the oscillations of the structure are sufficient to
distinguish the modes. A full description of the modes can be
found in ref. 10. Table 3 also notes the relationships between the
normal modes in the iodine-bromine and the bromine-chlorine
octahalides. The normal mode sequence differs between the
series as both atomic masses and bond strength (or more
generally the force constant matrix) determine the coupling
between the internal coordinates used to describe the normal
modes. We note that all of the Raman modes are observed in the
spectra of 1c and 2d. For 1c, it is also possible to distinguish the
modes in terms of localized pentahalide and trihalide modes
Table 2. Summary of TGA and DSC results at 10°C min–1
.
Salt[a]
m.p.
(°C)
Compound(s) lost
Tonset
(°C)
Mass %
(Theory)
Mass %
(Exptl)
1c
68
2BrCl
36
23.9
16.5
48.3
11.3
25.0
12.5
50.6
11.9
100.0
41.0
59.0
24
16
48
11
25
12
51
12
97.5
–
Br2
179
290
350
52
C25H60N5Cl
[C5N]Cl
2BrCl
2d
65
BrCl
187
290
350
175
175
225
–
(calculated as [Br2Cl3]– and BrCl2 ) respectively.
C25H60N5Cl
[C5N]Cl
C16H40Br6.56Cl1.44N2
BrCl
The Raman spectra of 1c and 2d are dominated by a single
strong band at 250 cm–1 for 1c (largely the Br–Br stretch) and 277
cm–1 for 2d (the symmetric BrCl2– stretch). The octahalide salt 1c
shows a medium intensity band at 193 cm–1 that is not observed
for the less symmetric 2d, but which can be attributed to
symmetrically-coupled bending modes. Similar intensity patterns
were observed in the iodine-bromine series suggesting that
symmetry largely determines the observed intensity patterns in
the vibrational spectra of octahalides. The interpretation of the
Far-IR spectra is complicated by a strong background signal. The
calculated spectra of the cations and anions suggest that the
background is not coming from localized cation modes, however,
we cannot rule out phonon-like modes as the source of the
background. Characteristic bands for 1c are observed at 282 and
4
73
64
[Et4N]BrCl2
C8H20NCl
–
192 cm–1 (coupled terminal Br–Cl stretches and coupled BrCl2
–
bending modes, respectively). For 2d, characteristic bands
appear at 355, 296, 283 and 190 cm–1 (pentahalide stretches for
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10.1002/ejic.202000478
European Journal of Inorganic Chemistry
FULL PAPER
the three high-energy bands and coupled trihalide–pentahalide
bending modes at lower energy). Salt 2d also shows a strong
band at 220 cm–1 that we assign to the asymmetric stretch of
BrCl2– (see below).
Table 3. Summary of Far-IR and Raman spectra for 1c, 2d, 4 and
[Et4N]BrCl2.
Salt
Raman
(cm–1
Infrared
(cm–1
Calculated[a]
(cm–1
)
Assignment[b]
)
)
1c
322 (w)
310 (vw)
272
234
268
229
218
139
110
80
18 (= 18)[b]
16 (= 15)[b]
17 (= 17)[b]
15 (= 14)[b]
14 (= 16)[b]
13 (= 13)[b]
10,11,12
9
282 (w)
252 (vw)
250 (s)
209 (mw)
193 (m,br)
164 (w)
192 (s, br)
132 (s)
77
58
8
6
2d
356 (vw)
305
280
208
192
243
264
99
18 = 9(p)
17 = 8(p)
14 = 7(p)
13 = 6(p)
15 = 2(t)
16 = 3(t)
9
355 (vw)
296 (w)
283 (w)
277 (vs)
215 (vw)
133 (w)
220 (s)
190 (vs,br)
133 (vs)
99–107
106
9,10,12
11 = 1(t)
4
265 (w)
230 (mw)
189 (m)
158 (vs)
295 (vw)
278 (vw)
217 (m)
164 (vs)
135–110
(m,br)
[Et4N]BrCl2
275 (vs)
241
262
2
3
201 (vs)
Figure 7. Raman spectra of 1c, 2d, 4 and [Et4N]BrCl2.
220 (vw)
184 (w)
[Br2Cl]–
[Br2Cl]–
1
179 (s)
132 (ms)
99
[a] Vibrational frequencies were calculated using B3LYP with the GD3BJ
empirical dispersion correction and SDD basis set.
[b] p = pentahalide, t = trihalide; [b] equivalent mode from reference 10.
The assignment of the bands in 4 has not been attempted
due to the disorder and extended structure in this compound. We
note below that the Raman and IR spectra of 4 are adequately
reproduced by the pentahalide, [Br4Cl]–.
The Raman spectrum of [NEt4]BrCl2 shows a strong mode
at 275 cm–1 and this may be assigned to the symmetric stretch of
the linear BrCl2– anion. A similar energy for this band was reported
by Riedel.[20] The IR-allowed antisymmetric stretch is observed at
201 cm–1. The Raman spectrum shows some weaker bands that
–
cannot be attributed to BrCl2 . Meazza has reported bands in the
Raman spectrum of Br2Cl– at approximately 215 cm–1 and 190
cm–1 (as a shoulder).[18] It is likely that the bands we observe at
221 cm–1 and 184 cm–1 are due to trace amounts of Br2Cl–. Note
that the synthesis involves oxidation of Br– by Cl2 which would
initially give Br2 and Cl–.
Computational studies
As with our calculated structures of the iodine-bromine and iodine-
chlorine octahalide series,[10,12] our calculated structures are
generally about 6% longer than the experimental distances. The
geometry optimisation calculated the second derivative matrix
Figure 8. Far-IR spectra of 1c, 2d, 4 and [Et4N]BrCl2 (top to bottom).
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10.1002/ejic.202000478
European Journal of Inorganic Chemistry
FULL PAPER
(Hessian) at each step. Geometry optimisations were started from
approximately planar structures with C1 symmetry and found, for
the anions for 1c and 2d, non-planar optimised geometries. The
degree of non-planarity is characterised by a Br2-Cl3-Cl6-Br7
torsion angle for 1c of 142.7o (approximately C2 symmetric) and
for 2d of 134.9o. Stable geometries were only obtained with a
dispersion-corrected B3LYP functional (B3LYP-D3 with the
GD3BJ empirical correction and SDD basis set), due to the ability
of the dispersion correction to mitigate the effects of the strong
Coulomb repulsions present in silico in the absence of the lattice
cations. A solvent model was included as an approximation for
the dielectric background due to the crystal lattice. The solvent
parameters for water were used as these gave stable geometries
(presumably due to the Coulomb screening effect of the high
relative permittivity of water). The non-planar calculated
structures for the isolated anions quite possibly arise from the
same Coulomb effects.
structural motif. The other (4) contains an interesting layer
–
structure with a chain of halogen-bonded [ClBr4]– and Br5
pentahalides along with a disordered ClBr2– trihalide. The [ClBr4]–
pentahalide is
interhalides.
a new structural motif in bromine-chlorine
Experimental Section
All the experimental work and sample preparation were carried out under
nitrogen atmosphere using standard Schlenk techniques. Br2 (BDH),
CH2Cl2 (Merck), chloroform (Fischer), diethylether (Sigma-Aldrich), HCl
36% (Ajax Finechem). Solvents were dried using an in-house solvent
purification system. Br2 was used as received. [C3(NEt2)3]Cl,[26]
[16]
[C3(NEt2)3]Br[10] and [Et4N]BrCl2
were prepared as described in the
literature.
TGA/DSC data were collected on dried samples using a TA Instruments
SDT Q600 at 10°C min–1. The sample tube was kept in an ice/salt (1:3)
bath at –20 °C and transferred to a platinum crucible for TGA. Samples
turn into a liquid after a few hours at room temperature. Far-IR data were
collected at room temperature by using a Vertex 70 FT-IR spectrometer
from Bruker (Germany), operating with a Platinum ATR unit with a diamond
crystal. A resolution of 4 cm–1 and 16 scans were taken. The region from
680 to 30 cm–1 was covered by a DTGS detector. Raman spectra were
recorded with a custom-built Raman microscope using near infrared (785
nm) excitation. Near infrared excitation was required due to the
photoinduced decomposition of polyhalide compounds with visible (532
nm) excitation. Approximately 7 mW of 785 nm excitation was directed into
a 20 x magnification, N. A. = 0.2 microscope objective by a custom-built
785 nm edge filter (Iridian Spectral Technologies) and subsequently
focussed onto a crystalline sample of each compound contained in either
a 5 mm NMR tube or microwell plate. Back-scattered Raman light was
collected with the same objective, Rayleigh scattering was rejected by the
785 nm edge filter, and the Raman scattering was focussed into a fibre-
optic coupled to a Princeton Instruments LS785 lens spectrometer, with
PIXIS 400b CCD detector. Spectral data was acquired using LightField 6.1
software. Typical acquisition time was 20 seconds per sample.
Microanalyses were performed by Campbell Microanalytical Laboratory,
University of Otago, Dunedin.
–
The calculated structure of [Br3Cl5]2– shows a clear BrCl2
unit with essentially equal Br–Cl distances (2.56 Å) and a
[Cl(BrCl)2]– pentahalide with short terminal Br–Cl distances
(2.46/2.47 Å) and longer internal Br–Cl distances (2.70/2.66 Å).
–
For this reason, we also performed calculations on BrCl2 and
[Cl(BrCl)2]– in which we obtained very similar distances to that of
the fragments in the octahalide. In the calculated structure of
–
[Br4Cl4]2–, the terminal Br–Cl distance of the BrCl2 units is
shortened (2.52 Å) while the internal distance is lengthened (2.60
Å), as is found in the X-ray structure. Along with the Br–Br
distance (2.57 Å), these are all lengthened by about 0.15–0.20 Å
in the calculated structure compared to the X-ray structure. The
halogen bond distance is slightly shorter (ca. 0.1 Å) and generally
the soft halogen bonds show greater variability in the calculated
structures.
The Raman and IR spectra of 4 were best simulated by a
pentahalide structure [Br4Cl]– with C2v symmetry, as suggested by
the disordered structure shown in Figure 5. An attempt to simulate
the spectra of 2d as the superposition of spectra of independent
anions of [Br2Cl3]– and BrCl2– gave reasonable agreement for the
highest frequency modes (which are localised to the pentahalide
and trihalide fragments) but failed to correctly reproduce the
region below 100 cm–1. The calculated Raman spectrum for the
octahalide [Br3Cl5]– contains a mode at 41 cm–1 (mode 6) that
agrees with a shoulder in the experimental spectrum at 35 cm–1.
Mode 6 is predominantly due to relative displacement of the
pentahalide and trihalide fragments (i.e. Cl2–Cl3 stretch). The
presence of this mode in the calculated spectrum supports the
existence of a binding interaction between the pentahalide and
trihalide fragments, i.e. the calculation and Raman spectra
support the existence of octahalide [Br3Cl5]– moieties in the
structure.
Tris(diethylamino)cyclopropenium
[C3(NEt2)3]2[Br4Cl4] (1c).
tetrabromotetrachloride,
Procedure 1, from the chloride salt: Elemental bromine (0.416 g, 2.605
mmol) was dissolved in CH2Cl2 and diethyl ether (30 mL in a 1:2 ratio,
respectively) and cooled to –20°C. An excess of chlorine gas (15.869
mmol) was produced from the reaction between KMnO4 (1.00 g, 6.35
mmol) and HCl, 37% (5.0 mL, 60.2 mmol) and then bubbled slowly into the
bromine solution for 20 minutes until the colour of the solution changed
from red to the yellow of BrCl. [C3(NEt2)3]Cl (1.5 g, 5.211 mmol) was added
to the solution of BrCl and stirred for 1 h at –20°C. Half an equivalent of
elemental bromine (0.416 g, 2.61 mmol) was added and stirred for 1 h.
The yellow solution was stored under nitrogen at –21°C overnight to give
orange-brown needles (1.58 g, 63% yield).
Conclusions
Procedure 2, from the bromide salt: [C3(NEt2)3]Br (1.500 g, 4.514 mmol)
was dissolved in CH2Cl2 and diethyl ether (30 mL in a 1:2 ratio,
respectively) and cooled to 0°C. An excess of chlorine (15.9 mmol) was
produced from the reaction between KMnO4 (1.00 g, 6.35 mmol) and HCl,
37% (5.0 mL, 60.2 mmol) and then bubbled slowly into the [C3(NEt2)3]Br
solution. Half an equivalent of elemental bromine (0.361 g, 2.26 mmol) was
We have reported three new non-classical interhalides based on
bromine-chlorine. Two of them (1c and 2d) extend the number of
octahalides with [C3(NEt2)3]+ as the cation that we have reported
–
to 14 examples and include the first examples to contain a BrCl2
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10.1002/ejic.202000478
European Journal of Inorganic Chemistry
FULL PAPER
dissolved in CH2Cl2 (10 mL) and added dropwise at 0°C. The reaction
mixture was stored under at –21°C overnight to give orange-brown crystals
[5]
[6]
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(1.01 g, 46% yield). The salt is too unstable to obtain
microanalytical result.
a good
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C. Müller, H. Beckers, S. Riedel, Angew. Chem. Int. Ed. 2015, 54, 8279-
8283.
Tris(diethylamino)cyclopropenium
[C3(NEt2)3]2[Br3Cl5] (2d).
tribromopentachloride
[8]
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between KMnO4 (2.00 g, 12.7 mmol) and HCl, 37% (10 mL, 120 mmol)
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and stirred at 0°C. Elemental bromine (1.92 g, 12.07 mmol) dissolved in
dichloromethane (10 mL) was added gradually under nitrogen. The
solution was stirred for 1 h at 0°C and then stored at –21°C overnight to
give orange crystals (2.05 g, Yield 80.7 %). Elemental analysis calcd (%)
for C16H40Br6.56Cl1.44N2 – BrCl: C 26.67, H 5.59, N 3.94; found: C 27.08, H
6.30, N 4.00.
Crystal data
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2d and 4, respectively, as well as CCDC 1992819 for an updated structure
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Computational details
Vibrational frequencies and normal modes were calculated using the
Gaussian09 suite.
A dispersion-corrected B3LYP hybrid functional
(B3LYP-D3) was selected to minimise potential Coulomb explosion effects.
The SDD basis set was used throughout. The CPCM polarisable
continuum model was used to provide a dielectric background for the anion
using the dielectric properties of water, dichloromethane and benzene.
The solvent continuum had a strong effect on equilibrium geometries, with
the results obtained for water giving closest agreement with the observed
crystal structure data.
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Keywords: bromine • chlorine • interhalides• octahalides •
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Entry for the Table of Contents (Please choose one layout)
FULL PAPER
Interhalides
Mohammad S. Abdelbassit, Owen J.
Curnow,* Samuel J. Brooke, and
Mark R. Waterland*
Non-classical bromine-
chlorine octahalides and
pentahalides can be
prepared from Cl–,
Page No. – Page No.
–
The bromine-chlorine interhalides
BrCl2 ,BrCl and Br2 building
[Br3Cl5]2–, [Br4Cl4]2– and
blocks. A cyclopropenium
cation acts as a template for
the formation of octahalides
whereas [NEt4]+ generates a
2D layer structure containing
pentahalides and trihalides.
2–
[Br6.56Cl1.44
]
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