Halogenated closo-Dodecaborate Anions Stabilize Weakly Bound [(Me3NH)3X]2+ (X = Cl, Br) Dications in the Solid State

Article abstract of DOI:10.1002/ejic.201700620

Cocrystallization of [Me₃NH]₂[B₁₂Y₁₂] (Y = Cl, Br, I) with 1 equiv. of [Me₃NH]X (X = Cl, Br) from acetonitrile/diethyl ether solution produced large colorless single crystals of [(Me₃NH)₃X][B₁₂Y₁₂]. Four different compounds of this type were prepared and structurally characterized. The compounds consist of weakly coordinating perhalogenated closo-dodecaborate [B₁₂Y₁₂]^2– dianions and unprecedented weakly bound discrete [(Me₃NH)₃X]²⁺ dications. The [(Me₃NH)₃X]²⁺ dications are built up from a central halide anion and three [Me₃NH]⁺ cations bound by N–H···X hydrogen bonds. The cations have a pyramidal structure, as determined by X-ray diffraction, which is in contrast to quantum-chemical calculations, which predict a trigonal-planar structure. While the [(Me₃NH)₃Cl]²⁺ and [(Me₃NH)₃Br]²⁺ cations were easily prepared, attempts to synthesize the corresponding [(Me₃NH)₃F]²⁺ and [(Me₃NH)₃I]²⁺ cations failed. Three further crystal structures containing the angulated [(Me₃NH)₂X]⁺ cation stabilized by [B₁₂Cl₁₂]^2–, [B₁₂Br₁₂]^2–, or [Me₃NB₁₂Cl₁₁]^– are reported as well.

Full text of DOI:10.1002/ejic.201700620

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Weakly Bound Dications | Very Important Paper |
Halogenated closo-Dodecaborate Anions Stabilize Weakly
Bound [(Me3NH)3X]2+ (X = Cl, Br) Dications in the Solid State
Christoph Bolli,[a] Janis Derendorf,[a] Carsten Jenne*[a] and Mathias Keßler[a]
Dedicated to Professor Rüdiger Mews on the occasion of his 75th birthday
Abstract: Cocrystallization of [Me3NH]2[B12Y12] (Y = Cl, Br, I) cations bound by N–H···X hydrogen bonds. The cations have a
with 1 equiv. of [Me3NH]X (X = Cl, Br) from acetonitrile/diethyl pyramidal structure, as determined by X-ray diffraction, which
ether solution produced large colorless single crystals of is in contrast to quantum-chemical calculations, which predict
[(Me3NH)3X][B12Y12]. Four different compounds of this type a trigonal-planar structure. While the [(Me3NH)3Cl]2+ and
were prepared and structurally characterized. The compounds [(Me3NH)3Br]2+ cations were easily prepared, attempts to
consist of weakly coordinating perhalogenated closo-dodeca- synthesize the corresponding [(Me3NH)3F]2+ and [(Me3NH)3I]2+
borate [B12Y12]2– dianions and unprecedented weakly bound cations failed. Three further crystal structures containing the
discrete [(Me3NH)3X]2+ dications. The [(Me3NH)3X]2+ dications angulated [(Me3NH)2X]+ cation stabilized by [B12Cl12]2–,
are built up from a central halide anion and three [Me3NH]+ [B12Br12]2–, or [Me3NB12Cl11]– are reported as well.
Introduction
stabilized by [B12Y12] (Y = Cl, Br, I)[14] and [K2(OH2)n]2+ (n = 2,4)
dications stabilized by [B12F12]2– were reported as well.[15] Thus,
perhalogenated dodecaborate anions [B12Y12]2– (Y = F, Cl, Br, I)
are able to lattice-stabilize weakly bound dications in the solid
state that otherwise would dissociate into two singly charged
cations.
Halogenated closo-dodecaborate anions [B12Y12]2– (Y = F, Cl, Br,
I) have become important weakly coordinating anions with
high chemical, thermal, and electrochemical stability.[1] Their
fluoro, chloro, and iodo derivatives have been frequently used,
because they can be prepared by simple procedures in large
amounts[2] or are commercially available. They were shown to
stabilize reactive cations,[3] to act as counteranions in cataly-
sis,[4] and to be part of solid superacids,[5] host–guest com-
plexes,[6] and low-melting salts.[7] They are interesting materials
for electrochemical applications,[8] as anions in solid-state ion
conductors,[9] and in biochemistry,[10] and they show unprece-
dented behavior in the gas phase.[11] The closo-dodecaborates
have a charge of –2, which results in higher lattice energies of
the respective salts compared to those of salts of singly charged
weakly coordinating anions. The higher lattice energy of salts
containing dianions allows very weakly bound dications to be
stabilized in the solid state.[12] Some time ago we reported the
synthesis and crystal structure of the weakly bound [Li2(SO2)8]2+
dication.[13] This dication is lattice-stabilized by the weakly coor-
dinating [B12Cl12]2– dianion in the solid state. This means that
the Coulomb explosion of [Li2(SO2)8]2+ to give two [Li(SO2)4]+
cations, which is thermodynamically favorable in the gas phase,
is suppressed in the solid state by the high lattice energy of the
dication/dianion salt. In line with this, [Cs2(NCMe)2]2+ dications
In the course of our investigations of the [B12Cl12]2– dianion
as a weakly coordinating anion, we attempted to crystallize
[Me3NH]2[B12Cl12], which proved to be difficult. However, suit-
able crystals for X-ray diffraction are easily obtained from aceto-
nitrile/diethyl ether solution, when an excess of [Me3NH]Cl
is present. The crystal structure determination showed
that the structure contains the discrete hydrogen-bonded
[(Me3NH)3Cl]2+ dication (1b). Surprisingly, no data is available in
the literature on this and related simple [(R3NH)3X]2+ cations.
Herein we report the syntheses and crystal structures of sev-
eral salts containing the previously unknown [(Me3NH)3Cl]2+
(1b) and [(Me3NH)3Br]2+ (1c) dications. Compounds containing
the related [(Me3NH)2X]+ cations [X = Cl (2b), Br (2c)] were ob-
tained as well. All cations are stabilized by perhalogenated
closo-dodecaborates [B12Y12]2– (Y = Cl, Br, I)[1] or the recently
reported [Me3NB12Cl11]– anion.[16]
Results and Discussion
[a] Fakultät für Mathematik und Naturwissenschaften, Anorganische Chemie,
Bergische Universität Wuppertal,
Synthesis
Gaussstr. 20, 42119 Wuppertal, Germany
E-mail: carsten.jenne@uni-wuppertal.de
http://www.molchem.uni-wuppertal.de/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201700620.
Cocrystallization of [Me3NH]2[B12Y12] with 1 equiv. of [Me3NH]X
from acetonitrile/diethyl ether solution produced large colorless
single crystals of the title compounds [Equation (1)] that were
suitable for single-crystal X-ray diffraction analysis.
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cation with an overall charge of +2. The dication 1b has a py-
ramidal structure and forms one additional very weak C–H···Cl
contact to an acetonitrile molecule of solvation (Figure 1). The
[(Me3NH)3X]2+ cations presented herein are unprecedented, and
no other [(R3NH)3X]2+ (R = alkyl) cations are known to the best
of our knowledge. However, the pyramidal (N–H···)3Cl structural
motif has been seen before in the chloride complex of proto-
nated tris(pyrazolyl)hydroborate.[18] The crystal structure of tri-
methylammonium chloride was obtained in 1968,[19] and that
of the [(Me3NH)2Cl]+ cation (2b) was reported by Mootz and
co-workers in 1997.[20]
(1)
Addition of a second equivalent of [Me3NH]X led to crystals
containing the [(Me3NH)2X]+ cation (2) [Equation (2)].
(2)
In
an
analogous
manner
cocrystallization
of
[Me3NH][Me3NB12Cl11][16] with 1 equiv. of [Me3NH]Cl gave
[(Me3NH)2Cl][Me3NB12Cl11] containing the same [(Me3NH)2Cl]+
cation (2b) [Equation (3)]. Addition of further [Me3NH]Cl did not
lead to the formation of the [(Me3NH)3Cl]2+ dication (1b).
(3)
Figure 1. Part of the crystal structure of [(Me3NH)3Cl][B12Br12]·H3CCN. The
H···Cl distances are given in pm, and carbon-bonded hydrogen atoms are
omitted for clarity.
While numerous attempts to crystallize simple trimethyl-
ammonium salts of [B12Y12]2– did not yield single crystals of
suitable quality, addition of 1 or 2 equiv. of trimethylammonium
halide led to large crystals of very good quality in a short time.
The obtained compounds contain the unprecedented
[(Me3NH)3X]2+ (X = Cl, Br) dication (1) or the [(Me3NH)2X]+ cat-
ion (2). Note that only two crystal structures of trialkylammo-
nium salts of the type [R3NH]2[B12Y12] with halogenated do-
decaborates as counteranions have been published so far, that
is, they have little tendency to crystallize.[11a,17]
Figure 2 shows the [(Me3NH)3X]2+ (1) cations in the crystal
structures of 1b[B12Cl12]·H3CCN, 1b[B12Br12]·H3CCN, 1b[B12I12]·
H3CCN, and 1c[B12Cl12]·H3CCN. Table 1 compiles selected aver-
aged distances and angles for the [(Me3NH)3X]2+ cations. The
cations have a pyramidal structure, and the sum of the N–X–N
angles varies only slightly, between 330.1 and 338.3°, depend-
ing on the closo-dodecaborate counteranion. The Cl···H and
N···Cl distances are within the expected range for strong N–
H···Cl hydrogen bonds.[21] In comparison to the [(Me3NH)2Cl]+
cation (2b), the slightly longer distances suggest weaker
hydrogen bonds in 1b than in 2b. The bromide-centered cation
1c has a structure similar to that of 1b. The N···Br distances are
longer (av. 323.3 pm), in accord with the larger radius of the
bromine atom. The sum of the N–Br–N angles is smaller than
that in 1b; thus, 1c is more pyramidal.
Crystal Structures
Molecular Structures of the [(Me3NH)3X]2+ Dications (1)
The salts 1b[B12Y12]·H3CCN (Y = Cl, Br, I) and 1c[B12Cl12]·H3CCN
crystallize isostructurally in the orthorhombic space group
Pnma. In the Supporting Information selected features of the
The obtained structures are almost independent of the halo-
crystal packing are visualized. Exemplarily, part of the crystal gen substituents on the weakly coordinating [B12Y12]2– anions.
structure of 1b[B12Br12]·H3CCN is shown in Figure 1. The struc- However, a closer look at the values in Table 1 reveals a clear
ture consists of [B12Br12]2– dianions and [(Me3NH)3Cl]2+ (1b) di- trend. A longer H···Cl contact goes along with a shorter N···Cl
cations. In cation 1b a central chloride ion forms three N–H···Cl distance and a smaller N–H···Cl angle. This might be caused by
contacts to three cationic [Me3NH]+ units resulting in a discrete packing effects.
Table 1. Selected averaged distances and angles in the [(Me3NH)3X]2+ cation (1).
Av. X···H distance
Av. N···X distance
Sum of the N–X–N angles
[°]
Av. N–H–X angle
[°]
[pm]
[pm]
1b[B12Cl12]·H3CCN
1b[B12Br12]·H3CCN
1b[B12I12]·H3CCN
1c[B12Cl12]·H3CCN
235.7
228.0
221.8
240.7
311.8
312.3
315.0
323.3
330.1
332.6
338.3
320.3
161.6
169.8
171.7
173.5
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cal range for strong N–H···Cl hydrogen bonds,[21] and are
slightly shorter than those of the [(Me3NH)2Cl]+ cation in the
crystal structure of 2b[HCl2] (308.8 and 309.9 pm).[20] The H···Cl
distances show a larger deviation and vary between 204.6 and
227.1 pm. The N···Br distances in 2c are longer (319.6 and
320.4 pm) than the N···Cl distances in 2b, in accord with a larger
radius of the bromine atom.
Comparison of 1 and 2 with the Parent Halonium Ions
[H3X]2+ and [H2X]+
The [(Me3NH)3X]2+ (1) and [(Me3NH)2X]+ (2) cations are formally
derived from the dicoordinate and tricoordinate halonium ions
[H2X]+ and [H3X]2+ by formal replacement of the protons by
trimethylammonium cations (Scheme 1).[23] While the parent
halonium ions are experimentally unknown in condensed
phase, several noncyclic dicoordinate chloronium compounds
were synthesized and structurally characterized. Prominent re-
cent examples are the dialkylchloronium ions [R2Cl]+ (R = Me,
Et),[24] the bis(trimethylsilyl)chloronium ion [(Me3Si)2Cl]+,[25] and
the neutral Me2(B12Cl12) molecule.[26] Much less is known about
Figure 2. Visualization of the [(Me3NH)3X]2+ (1) cations in the crystal structures
of 1b[B12Cl12]·H3CCN, 1b[B12Br12]·H3CCN, 1b[B12I12]·H3CCN, and 1c[B12Cl12]·
H3CCN. The H···X distances are given in pm, and carbon-bonded hydrogen
atoms are omitted for clarity.
the respective tricoordinate dicationic chloronium ions [R3Cl]2+
.
Extensive theoretical investigations revealed a C3v-symmetric
structure for the parent [H3Cl]2+ dication.[27] The chloronium
ions have short, covalent element–halogen bonds. In contrast,
the nature of the hydrogen–halogen bonds in cations 1 and 2
is significantly different, and they should be rather described as
typical N–H···X hydrogen bonds.[21,28]
Molecular Structures of the [(Me3NH)2X]+ Cations (2)
Figure 3 shows the [(Me3NH)2Cl]+ (2b) and [(Me3NH)2Br]+ (2c)
cations in the crystal structures of [2b]2[B12Cl12],
2b[Me3NB12Cl11]·H3CCN, and [2c]2[B12Cl12]·2H3CCN. All cations
have an angulated structure. The N–X–N angle is larger for 2b
(124.8 or 130.8°) than for 2c (110.2°). The N···Cl distances in 2b
vary between 302.7 and 307.5 pm, are significantly shorter than
the sum of the van der Waals radii (330 pm),[22] are in the typi-
Scheme 1.
To gain some insight, DFT calculations at the PBE0/def2-
TZVPP level of theory were performed. While the optimized
structures of [(Me3NH)3Cl]2+ (1b), [(Me3NH)3Br]2+ (1c), and
[(Me3NH)3I]2+ (1d) are planar, the structure of [(Me3NH)3F]2+ (1a)
was calculated to be pyramidal. Thus, the calculated structures
of 1b and 1c are in contrast to the experimentally observed
structures, which are pyramidal for [(Me3NH)3X]2+ (X = Cl, Br).
Similarly, the quantum-chemically optimized structures of the
[(Me3NH)2X]+ cations are linear, while the experimentally ob-
served structures are bent. In an extreme case, as in the parent
[H3X]2+ cations, a pyramidal structure is in accord with covalent
H–X bonds and an electron lone pair residing on the halogen
atom. At the other extreme, the trigonal-planar structure of the
cation can be viewed as a purely ionic adduct of a negatively
charged halide with three positively charged trimethylammo-
nium cations. In this case the positively charged trimethylam-
monium cations arrange in a trigonal-planar manner around
the central halide ion to minimize Coulomb repulsion. As dis-
cussed above, the N–H···X bond in the [(Me3NH)3X]2+ cations is
rather long and best described as a mainly ionic hydrogen
bond. There is no simple explanation why the experimentally
Figure 3. Visualization of the [(Me3NH)2X]+ (2) cations in the crystal structures
of [2b]2[B12Cl12], 2b[Me3NB12Cl11]·H3CCN, and [2c]2[B12Cl12]·2H3CCN. The
H···X distances are given in pm, and the carbon-bonded hydrogen atoms are
omitted for clarity.
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observed structures are pyramidal, while calculations predict rather than as an [(Me3NH)3F]2+ cation. Further support for the
planarity. Very likely the energy difference between planar and non-existence of the [(Me3NH)3F]2+ cation comes from the cal-
pyramidal structures in the [(Me3NH)3X]2+ cations is small, and culated geometrical parameters of the anions. The H···F distan-
packing effects play an important role.
ces in [(Me3NH)3F]2+ are very short (153 pm), in contrast to
those in the chlorine (213 pm) and bromine derivatives
(230 pm). This implies that there might be a steric problem. The
short H···F bond leads to compression of the molecule and
steric repulsion between the three NMe3 groups.
Do the [(Me3NH)3F]2+ (1a) and [(Me3NH)3I]2+ (1d) Cations
Exist?
Conversely, the non-existence of the [(Me3NH)3I]2+ cation
(1d) may be explained by the very weak H···I interaction. The
calculated frequency of the N–H stretching vibration for the
[(Me3NH)3I]2+ cation (3102 cm–1) is calculated to be the highest
in this series and indicates a weak H···I hydrogen interaction.
Along these lines the H···I bond in [(Me3NH)3I]2+ is calculated
to be the longest (253 pm). However, the weakness of the H···I
hydrogen bond cannot be the only reason for our failure to
prepare the [(Me3NH)3I]2+ cation, because Schleid et al. were
able to crystallize compounds containing [(NH4)3I]2+ cations.[30]
Another argument might be the N–X–N angle. According to the
trend of averaged experimental N–X–N angles in [(Me3NH)3X]2+
(1b: 110°; 1c: 106°), the N–I–N angle would be smaller than
106°, which may lead to steric problems.
In analogy to Equation (1), experiments were carried out to
cocrystallize [Me3NH]F or [Me3NH]I with [Me3NH]2[B12Cl12].
While different salts of the [(Me3NH)3Cl]2+ and [(Me3NH)3Br]2+
cations were easily prepared, all attempts to synthesize
[(Me3NH)3F]2+ and [(Me3NH)3I]2+ cations were unsuccessful.
From the reactions with [Me3NH]I two different sorts of crystals
were obtained, which were identified as [(Me3NH)3Cl][B12Cl12]
and unconsumed [Me3NH]I. The presence of chloride in the
product can be traced back to a chloride impurity in the
hydrogen iodide used for the synthesis of [Me3NH]I (see Fig-
ure S2). No evidence for the formation of the iodide-centered
dication [(Me3NH)3I]2+ (1d) could be obtained.
From the reaction of [Me3NH]F with [Me3NH]2[B12Cl12], de-
signed to give the ([Me3NH]3F)2+ dication (1a), no single crystals
of the expected product were obtained. Instead, [Me3NH]F re-
acted in a manner similar to hydrogen fluoride with the glass
of the reaction vessel to form hexafluorosilicate anions. These
[SiF6]2– anions cocrystallized in the double salt [Me3NH]4-
[B12Cl12][SiF6]·1.5H3CCN. Interestingly, the [Me3NH]+ cations ex-
clusively form N–H···F contacts to the [SiF6]2– anion, while no
hydrogen bonding to the [B12Cl12]2– anion is observed. Thus,
the [SiF6]2– anion is more coordinating than the [B12Cl12]2– an-
ion in this case. The coordination sphere around the [SiF6]2–
anion is visualized in Figure 4.
In conclusion, compounds containing 1a or 1d could not be
prepared in this investigation under the selected physical and
chemical conditions. The reasons remain unclear, and we can-
not exclude that these cations may exist as well.
Importance of the Perhalogenated closo-Dodecaborates
for the Formation of Cations 1 and 2
It is intriguing that cations of the type [(Me3NH)3X]2+ (1) were
unknown prior to this work, and only one example[20] of cations
of the type [(Me3NH)2X]+ (2) was reported. It appears that the
weakly coordinating behavior of the perhalogenated closo-
dodecaborates [B12Y12]2– facilitates the formation of these
cations during the crystallization process. In contrast to other
classes of weakly coordinating anions, the closo-dodecaborates
carry a charge of –2, which causes a higher lattice energy of
the respective salts. The higher lattice energy leads to reduced
solubility and favors the formation of salts of the type
M2+[B12Y12]2–, which has been demonstrated before on several
occasions.[13–15] In addition, the spherical [B12Y12]2– anions tend
to form distorted close-packed structures with large holes,
which can be filled by suitable cations.
Figure 4. Part of the crystal structure of [Me3NH]4[B12Cl12][SiF6]·1.5H3CCN
showing the coordination sphere around the [SiF6]2– anion. Carbon-bonded Conclusions
hydrogen atoms are omitted for clarity.
We could show that the chlorinated dianion [B12Cl12]2– is able
Clearly, the formation of the fluoro and iodo derivatives of to stabilize the dication 1b as well as the monocation 2b in
the [(Me3NH)3X]2+ dication (1) is less favorable. We suggest that the solid state, depending on the stoichiometry of the starting
the strength of the H···X hydrogen bond might be crucial. Cal- materials. In contrast, all reactions with the very closely related
culated N–H stretching vibrations for the [(Me3NH)3X]2+ cations monoanion
[Me3NB12Cl11]–
led
to
crystals
of
show a significant difference between X = F (2612 cm–1) and 2b[Me3NB12Cl11]·H3CCN independently of the stoichiometry
the other halogens (3023–3102 cm–1). A lower calculated wave- used. Thus, the particular properties of the [B12Y12]2– dianions
number for the N–H stretch corresponds to lower bond energy seem to be crucial for the formation of the discrete weakly
of the N–H bond and thus implies a stronger H···X interac- bound
[(Me3NH)3X]2+
(1)
di-
tion.[29] Therefore, cation 1a can be viewed as [H3F]2+·(NMe3)3 cations.
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