Carba-closo-dodecaborates – Synthesis, Structure, and Energetics

Article abstract of DOI:10.1002/ejic.201800219

A full set of analytical data of salts (e.g., Me₃NH⁺, Cs⁺, Ag⁺) bearing nido-[B₁₁H₁₄]^–, closo-[CHB₁₁H₁₁]^–, and chlorinated congener [CHB₁₁Cl₁₁]^– is reported. Structures and energetics of [CHB₁₁H_11–nX_n]^– and [B₁₂X_mH_12–m]^2– (n = 5, 11; m = 0, 12; X = halogen, CH₃, CF₃) and their protonated and silylated forms are studied. A direct correlation between theoretical calculated NICS values, gas-phase acidities, and experimentally determined ¹³C NMR shifts is discussed.

Full text of DOI:10.1002/ejic.201800219

A Journal of
Accepted Article
Title: Carba-Closo-Dodecaborates - Synthesis, Structure and
Energetics
Authors: Alexander Hepp, Rene Labbow, Fabian Reiss, Axel Schulz,
and Alexander Villinger
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10.1002/ejic.201800219
European Journal of Inorganic Chemistry
FULL PAPER
Carba-Closo-Dodecaborates – Synthesis, Structure and
Energetics
Alexander Hepp,[a] René Labbow,[b] Fabian Reiß,[c] Axel Schulz,*[b,c] and Alexander Villinger[b]
Abstract: A full set of analytical data of salts (e.g. Me3NH+, Cs+, Ag+)
bearing the nido-B11H14, closo-[CHB11H11]− anion and its chlorinated
congener [CHB11Cl11]− is reported. Structures and energetics of
Carborane anions have been studied both experimentally and
analytically, as they offer an almost inexhaustible variety with
regard to feasible substitution- and constitution pattern. For
example, a direct correlation between computed aromaticity
(NICS values, NICS = nucleus independent chemical shift) and
stability depending on the size, symmetry and substitution
pattern of carborane species was reported.[22]
It was our intention to investigate in particular halogenated
carborate ions with regard to physical properties/correlations
such as a correlation between theoretically determined NICS
values, experimentally measured 13C NMR shifts and gas phase
acidities as well as [Me3Si]+ affinities. Moreover, we report here
an optimized and detailed synthesis procedure for
perchlorinated carba-closo-dodecaborates.
[CHB11H11-nXn]− and [B12XmHm-12 2− (n = 5, 11; m = 0, 12; X = halogen,
]
CH3, CF3) and their protonated and silylated forms are studied. A
direct correlation between theoretical calculated NICS values, gas
phase acidities and experimentally detected 13C NMR shifts are
discussed.
Introduction
Closo-borates and carba-closo-borates have been known for
more than hundred years. The development of boron cage
chemistry started in the early 20th century with Alfred Stock who
was one of the pioneers in borane chemistry.[1] In 1976 William
N. Lipscom was honored with the Nobel prize for his
investigations on the stereochemistry of boron cages.[2] In the
course of the discovery of carba-closo-boranes they and their
corresponding anions were classified as a spherical and
expensive obscurity.[3] In recent years a lot of work was carried
out with such cage molecules which are now appreciated for
their low basicity, low nucleophilicity, as well as their (electro)-
chemical and thermal robustness.[4–6] Borane based cage
molecules found a wide range of applications in medicine,[7,8]
catalysis[9,10] or even as weakly coordinating anion (WCA) for the
stabilization of highly reactive cations in solution and the solid
state if none of the well-known WCAs work (e.g. stable
[Cat][CHB11Cl11] versus unstable [Cat][A]; [Cat]+ = H+, [Alk3E]+;
Alk = Me, Et, t-Bu; E = Group 14, [A]− = [BF4]−, [PF6]−, [B(C6F5)4]−,
[Al(ORf )4]−.[11–16] Josef Michl summarized in his review articles of
carba-closo-dodecaboranes the development of such chemistry
in the last decades.[17,18] Halogenated, arylated and alkylated
derivatives of carba-closo-boranes and -borates (e.g.
p-CH2B10H10, [B12F11X]−, [CHB11H5Cl6]−, X = NR3) are useful
irreplaceable building blocks in nowadays chemistry.[3,7,8,19–21]
Results and Discussion
Synthesis and characterization of perchlorinated carba-
closo-dodecaborates. A common precursor for the synthesis of
salts containing carba-closo-dodecaborates are salts of the nido-
[B11H14]− ion (1). There are different ways of synthesizing
[B11H14]− salts using simple starting materials like BF3·OEt2 and
Na[BH4] (Scheme 1, eq. 1) or neutral higher borane-clusters like
decaborane(10) or pentaborane(9) in combination with
appropriate hydride sources (Scheme 1, eq. 2 and 3).[23–25]
Utilizing the first reaction path (eq. 1) with a subsequent cation
metathesis reaction, we were able to isolate salts of 1 as
analytically pure [Me3NH]+-salt, [Me3NH]1, in yields up to
32 %.[24,26] With regard to the atom economy, which plays an
essential role in borate syntheses, this constitutes a rather
excellent yield.
diglyme, 105°C
17 Na[BH4]
+
20 BF3 OEt2
2 Na[B11H14
]
20 H2 (1)
+
-Et2O -Na[BF4]
dioxane, 90°C
B10H10
M[B11H14
]
]
+
+
2 H2
(2)
M[BH4]
MH
+
DME, 85°C
+ 2.2 B5H9
M[B11H14
3.4 H2 (3)
Scheme 1. Synthesis routes for salts containing nido-[B11H14]
− with M = Na, K.
[a]
[b]
[c]
Alexander Hepp
Institut für Anorganische und Analytische Chemie
Universität Münster
Correnstraße 28-30, 48149 Münster, Deutschland
René Labbow, Axel Schulz,* Alexander Villinger
Institut für Chemie, Abteilung für Anorganische Chemie
Albert-Einstein-Straße 3a, 18059 Rostock, Deutschland
E-Mail: axel.schulz@uni-rostock.de
Closing the nido-boron vertex in [B11H14]− (Figure 1, left) by
carbene insertion utilizing the well-known dichlorocarbene led to
the formation of the carba-closo-dodecaborane anion
[CHB11H11]− (2) (Figure 1, middle) in rather poor yields of 9 % in
a laboratory scale (Scheme 2, eq. 4). This procedure was
initially introduced by the group of Michl.[27] Other possible
(known) routes include the use of decaborane(14) and alkali
cyanides or iso-cyanides (Scheme 2, eq. 5 and 6).[28–31] For
safety and handling reasons (e.g. solvent mixtures of
decaborane(14) are able to explode instantaneously, acidic work
up procedures for cyanide containing borates, carcinogenic
activity of methanal), we chose the more comfortable but low
Fabian Reiß, Axel Schulz*
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Straße 29a, 18059 Rostock, Deutschland
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800219
European Journal of Inorganic Chemistry
FULL PAPER
yield synthesis of the carba-closo-dodecaborane anion (2)
employing the in situ generated dichlorocarbene (Scheme 2,
eq. 4).
more comfortable reaction pathway when using glacial acetic
acid and chlorine gas as halogenation agent leading to
[CHB11H5Cl6]− (4) (Scheme 3, eq 8).[5]
1) NaH
2) CHCl3
3) C2H5OH
ICl
SO2Cl2
-196°C reflux
1) [Me3NH]Cl
2) CsOH
Cs[CHB11H11
]
Cs[CHB11HnCl11-n
]
Cs[CHB11Cl11]
(7)
(8)
HOTf, 200°C
autolcave, 5 d
[Me3NH][B11H14
]
Na[CHB11H11
]
Cs2
(4)
THF, 0°C
H2O
H+
Cl2
B10H14
+
NaCN
Na2[NCB10H13
]
H3NCB10H12
Me2SO4
Cs[CHB11H11
]
Cs[CHB11H5Cl6]
H2O
ion-exchange
CH3COOH,
anhydrous
(5)
20%
−
Na
Et3NH BH3
180°C
CsCl
Scheme 3. Synthesis routes for closo-[CHB11Cl11
]
(3) and closo-
Cs2
Cs[CHB10H11
]
Na[CHB10H11]
Me3NCB10H12
[CHB11H5Cl6]− (4), with n = 2-5).
THF, reflux
Me2S BH3
B10H14
+
HCHO + CsOH
Cs[H2CB9H12
]
Cs2
(6)
Carba-closo-borates are well known for their (electro)-chemical
and thermal robustness,[44,45] e.g. the truncated icosahedral
species [Me3NH]1 decomposes at a temperature of 237 °C.
Closing the cage by carbene-insertion as in [Me3NH]2 raises the
decomposition temperature by more than 100 °C up to 357 °C. A
cation exchange to the alkali metal Cs (Cs2: 517 °C) and the
H2O
ClCH2CH2Cl,
reflux
66%
Scheme 2. Selected common synthesis routes for closo-[CHB11H11]
− (2).
The carbon capped icosahedron of 2 offers now a new
electronic situation, e.g. a three-dimensional σ-aromaticity was
discussed in this diatropic closed shell inorganic cage
molecule.[22,32] The electronic situation can be changed by
replacing the hydrogen atoms directly bonded to the cage boron
atoms by proper atoms/groups like halogens or alkyl groups
(Figure 2).
chlorination of the boron vertex led to
a
remarkable
melting/decomposition point of >600 °C for Cs3. With regard to
νBH stretching modes it was observed that a closed cage
displays a slight blue-shift of 30 cm−1 which even can be
extended by the introduction of electron withdrawing halogens to
more than 40 cm−1 (cf. νBH 1: 2554 cm−1, 2: 2580−1 cm−1, 4:
2593 cm−1, Table 1).
H
Y
C
Y
Y
Y
B
meta-belt B2-B6
B
B
B
Y
Figure 1. ORTEP representation of the molecular structures of the anions of
[Me3NH]1, Cs2 and Cs3 in the crystal. Thermal ellipsoids correspond to 50%
probability at 173 K. Cations and solvents are omitted for clarity.
B
B
B
B
X
X
ortho-belt B7-B11
antipodal B12
B
Depending on time, solvent, pressure, halogenation,
respectively, alkylation reagent it is feasible to get selectively
six- and elevenfold substitution. Even a mixed substitution
pattern with regard to ortho- and meta-belt substituents is
possible (Figure 2).[5,14,33–40]Moreover, substitution on the carbon
atom is also feasible. Eleven-fold chlorination [CHB11Cl11]− (3)
can be achieved by different routes. Ozerov et al. reported of a
complete chlorination under reflux conditions, which only worked
with a special batch of SO2Cl2 under atmospheric conditions.[38]
H. Poleschner and co-workers optimized a synthesis also
reported by Ozerov taking SbCl5 as chlorination reagent.[41]
Unfortunately, we were not able to reproduce their results, when
taking a prechlorinated carborane anion as starting material.
Only recently, new attempts using SO2Cl2 as (single) chlorination
agent under pressurized or photochemical conditions were
reported.[42–44] We found it necessary for safety and handling
purposes (see Supporting Information) to use SO2Cl2 as
prechlorination agent and treated afterwards the partly
halogenated vertex ([CHB11HnCl11-n]− with n = 2-5) with ICl and
triflic acid in an autoclave (V4A) equipped with a teflon beaker at
200 °C (Scheme 3, eq. 7). Splitting up the reaction procedure
led to analytically pure Cs3 (Figure 1, right) in reproducible
yields of 40 %. Six-fold chlorination of 2 can be achieved by a
B
X
X
X
B
X
Figure 2. Numbering scheme of closo-[CHB11XnY11-n]− (n = 6, 11; X, Y = H, F,
Cl, Br, I, CH3, CF3).
Forming closo-cage anions 2, 3 or 4 from nido-species 1 led to a
contraction of all B-B bond lengths within the cage, especially
the distances between the antipodal boron atom and the upper
ortho-belt were affected (cf. Table S80, Σrcov (B-B) = 1.76 Å, (C-
B) = 1.61 Å, see also Figure 2).[46] However, in comparison with
the calculated gas phase structures, these structural changes
were less pronounced or not observed. Hence, it can be
assumed that anion-cation interactions or packaging effects
might be the origin of these structure differences.
Gas phase DFT (Density Functional Theory) calculations
(pbe0/aug-cc-pwCVDZ, see below) agree well with data
obtained from solid state X-ray structure elucidations. The solid
state structural data of all investigated anions 1-4 have already
been reported, however, in combination with other cations e.g.
[Ph3PH]1,[47] [Bpy-H]2,[48] [Ph3C]3[49] and [Ph3C]4,[49] therefore we
abstain here from a detailed structural discussion.
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10.1002/ejic.201800219
European Journal of Inorganic Chemistry
FULL PAPER
Table 1: Physical properties and spectroscopic details of selected carba-
C. Reed and co-workers were able to determine the X-ray
crystallographic structure for H[CHB11Cl11].[12] With these results
in hand, we calculated the minimum structures for all six- and
eleven-fold halogenated carborates, as well as the dianionic
closo-borates. Furthermore, the minimum structures of the
protonated and trimethylsilylated congeners were computed.
Each of the more than 200 structures were determined as
minima and checked for zero imaginary frequencies (see
closo-borates.
mp.
[°C]
νCH
νBH
δ13
C
[cm−1
]
[cm−1
]
exp.
-
[Me3NH]1
[Me3NH]2
Cs2
237[c]
350[c]
517
-
2518/2554
3027
2519/2549
51.5
51.4
47.6
47.6
47.4
32.8
supporting information Tables S14-S16, S64-S113).
A
3048/3059
3022
2580
H
H/Y
H
Y
Y
Y
C
C/B
C
1
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
>600°C[d]
196[c]
207[c]
230[c]
-
B
B
B
5
5
5
Cs3
B
B
B
B
B
B
B
B
B
4
6
B
B
4
6
B
B
4
6
B
B
B
B
B
B
2
3
2
3
11
B
2
3
X
X
X
X
X
X
B
B
7
11
B
7
7
11
B
Ag3[a]
3022
-
8
10
8
10
8
10
X
B
B
X
X
B
B
X
X
B
B
X
9
9
9
X
X
X
12
12
12
H
H
[Ph3C]3[b]
[Ph3C]4[b]
3022/3043[e]
3058[e]
-
H
X
X
X
A
B
C
2593
H
X
X
X
H
H
H
X
Y
X
X
B
1
C
B
B
B
B
B
1
5
1
5
X
X
X
Y
X
X
X
X
X
Y
Y
X
X
X
B
5
5
Y
H
[a] ⅓ · CH3CN solvate adduct; [b] taken from ref. [49]; [c] decomposition; [d] no
changes during DSC measurement until 635 °C; [e] superimposed by CH-
phenyl stretching mode.
B
B
4
B
B
B
B
B
B
B
4
X
X
6
B
B
6
B
B
4
6
B
B
4
6
B
B
B
B
B
B
B
B
2
3
2
3
11
B
2
3
2
3
X
X
X
X
X
X
X
B
B
7
111
B
7
7
11
B
7
11
B
B
B
8
B
8
10
8
10
10
8
10
X
B
9
X
X
B
B
X
X
B
B
X
X
B
B
X
9
9
9
H
X
X
X
X
B
12
12
12
12
H
H
X
X
X
X
A
Computational details. Gas phase DFT calculations at the
pbe0/aug-cc-pwCVDZ level of theory were carried to elucidate
structural and electronic effects of differently substituted halo-
closo-carborates ([Cat][CHB11H11−nXn]; n = 6, 11; X = H, F, Cl, Br,
I; Me, and CF3; [Cat]m[B12X12]m−2 X = H, halogen, m = 1, 2; Cat =
H+, [Me3Si]+). To study the electronic situation of substituted
halo-closo-carborates in detail we computed 13C NMR shifts,
NICS values (Nucleus-independent chemical shift), and
trimethylsilyl and proton affinities.[50–56] Additionally, the
partial charges (NBO = natural bond orbital) and a NLMO
calculation (Natural Localized Molecular Orbital) were
calculated.[57–59]
D
E
F
G
Figure 3. Optimized gas phase structures of different proton binding modes,
A: X = Y = H, B: H[CHB11XY]: X = halogen / Y = H; X = Y = halogen and
H[B12X12]
− X = Y = Cl, Br, I, C: X = CH3 / Y = H; X = Y = CH3, CF3, D: X = CF3,
Y = H, E: X = H, F: X = F, G: X = Cl, Br, I, protons highlighted by arrows.
As expected, all halogenated carba-closo-borates of the type
[CHB11H5X6]– and [CHB11X11]– feature a C5v symmetric structure
as global minimum in the gas phase, while only C1 symmetry is
found for the minima of [CHB11H5R6]– and [CHB11R11]– (R = CH3,
CF3). Dianionic borates of the type [B12X12]2– display the
expected ideal Ih symmetry. All optimized structures can be
found in the supporting information (see S64-S113).
For the mono and diprotonated species, the situation is rather
complex (Table S15/S16) since in most cases different isomers
with similar energy are found displaying different binding modes
as depicted in Figure 3. There are six (Figure 3 A - G) major
Structural effects. I. Koppel et al. and C. Jenne et al. already
investigated different protonation sites for fluorinated and
chlorinated B12-closo-borates, as well as some other carba-
closo-dodecaborates.[60–62]
Table 2: Selected values of halogenated closo-borates.
δ(13C)
C-B12-X12 [°]
E [kcal·mol−1 [c]
δ(13C)
C-B12-X12 [°]
E [kcal·mol−1 [c]
compound
qC [e][a]
NICS[b]
compound
qC [e][a]
NICS[b]
calc.
]
calc.
]
[CHB11H5F6]−
7.8
19.9
-
−0.779
−0.936
-
−34.8
−33.3
−35.7
−32.2
−34.4
−37.2
−27.7
177.6 / 39.5[d]
[CHB11H5Br6]−
37.6
54.4
-
−0.806
−30.6
−32.0
−34.3
−28.0
−28.5
−30.9
−27.8
171.5 / 112.9
−
−
[CHB11F11
]
177.6 / 42.8
[CHB11Br11
]
−0.978
171.8 / 131.7
2−
2−
[B12F12
]
-
[B12Br12
]
-
-
[CHB11H5Cl6]−
32.2
51.3
-
−0.801
−0.947
-
172.3 / 71.6
[CHB11H5I6]−
49.6
59.6
-
−0.816
171.7 / 106.9
−
−
[CHB11Cl11
]
171.6 / 73.5
[CHB11 11
I
]
−1.028
172.1 / 133.5
2−
2−
[B12Cl12
]
-
-
[B12 12
I
]
-
-
-
-
−
2−
[CHB11H11
]
52.8
−0.812
[B12H12
]
-
[a] NBO charge; [b] GIAO magnetic shield tensor at the boron cage center; [c] C-B12-X12 angle of distortion by coordination of [Me3Si]+ for
[Me3Si][CHB11H11-nYn] / Gain of energy (E) for building a hydrogen bridge (X-H-X) determined by a NLMO calculation for H[CHB11H11-nYn]; [d] sum of three
equivalent hydrogen bridges.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800219
European Journal of Inorganic Chemistry
FULL PAPER
different binding modes for the singly and doubly protonated
(car)borate anions (Figure 3). However, it should be noted that
13 different structures are located (see Table S15) for the
monoprotonated species while 38 structures are found for the
doubly protonated species (see Table S16).
H2[B12F12] (Figure 3 F) displaying two doubly coordinated
protons (by F8, F9 / F1, F4). A similar situation is found for all
H2[B12X12] species (X = Cl,[67] Br,[67] I), however, here the second
doubly coordinated proton sits between X8 and X12 (Figure 3 G).
H[CHB11H11] displays a rather interesting binding mode, which is
best described as a side-on coordination of a H2 molecule to
boron atom B12 (H–H 0.846 Å, vide supra). As depicted in
Figure 3A, for H[CHB11H11] protonation is favored at the
antipodal B12 atom, in contrast to the situation found for
H[CHB11H5X6], H[CHB11X11] and H[B12X’12]– (X’ = Cl, Br, and I)
for which the proton is observed between X12 (attached to B12)
and X8 (attached to B8) with two distinct different X…H distances
(Tables S13). By this intramolecular hydrogen bonding, C5v
symmetry of the deprotonated species is decreased to C1.
Interestingly, in case of H[CHB11I12] identical d(H…I) distances
are computed. According to NBO analysis data, the donor-
acceptor energies for LP(X)  *(X-H) interactions (LP = lone
pair) lie in the range between 40 – 134 kcal/mol (Table 2).
A further complexation mode of the proton is shown in Figure 3C
displaying a 3-coordinated H+ between X8, X9 and X12. Besides
four other, less favored, minimum structures (Figure S45-S47,
Table S15) such a 3-coordinated H+ species are found for
H[CHB11H5Me6], H[CHB11Me11], H[CHB11(CF3)11], H[B12X’12]– (X’
= H, F). This intramolecular hydrogen bonding situation arises
from the fact that the alkyl groups form a pocket which provides
enough space for the proton, leading to rather short H–B
distances (1.387 – 1.431; cf. ∑rcov(B-H) = 1.17 Å)[63] and
significantly longer H–CR3 distances (2.189 – 2.254 Å). While for
H[CHB11X’12] all three H–B distances are identical, for the alkyl
substituted species one short and two longer distances are
found (Table S13). The origin of the unusual trigonal prismatic
coordination is reasoned by an increased steric demand of the
alkyl groups. Interestingly, the carbon atoms of the methyl
groups possess a large negative charges (–0.95 - –1.00e) while
the carbon atoms in the perfluorinated species are positively
charged (+0.91 - +0.95e). Hence a complex situation of van der
Waals interactions with all atoms forming the “pocket” for the
proton can be expected.[64]
A
Me3Si
H/Y
X
Y
X
C/B1
B5
B1
B5
Y
Y
Y
X
X
X
B6
B8
B4
Y
B6
B8
B4
X
B2
B7
B3
B11
B10
B2
B7
B3
B11
B10
X
X
X
X
X
B9
X
B9
X
X
X
X
B12
B12
X
X
SiMe3
SiMe3
A
B
A
H
H
A
Y
Y
C
C
Me3Si
Y
Y
Y
Y
Y
Y
B5
B5
B6
B8
B4
B3
Y
B6
B8
B4
B3
Y
B2
B7
B2
B7
X
X
X
X
B11
B10
B11
B10
B9
X
X
R2C
B9
X
X
X
B12
B12
Me3Si-R
X
X
A
C
D
Figure 4. Optimized gas phase structures of different [Me3Si]+ binding modes,
A: [Me3Si][CHB11XY]:
X = halogen / Y = H; X = Y = halogen and
− X = Y = H, halogen; B: [Me3Si]2[B12X12] X = Y = H, halogen; C:
[Me3Si][B12X12
]
X = CH3, CF3, Y = H; D: X = Y = CH3, CF3, R = F, H; [Me3Si]-groups
highlighted by arrows.
In a second series of computations, the proton is formally
substituted by the Me3Si moiety which can be regarded as a “big
proton”.[68–72] Interestingly, stable isomers for the monosilylated
are only found for those species which provide B-X (X = halogen,
H) bonds. In all other cases [Me3Si][CHB11R11] (R = CH3, CF3),
contrary to the H+ species, decomposition into Me3Si–H/F and
(H/F)2C=R are observed (Figure 4 D). As long as there are B–X
and B–H bonds present, the highly reactive Me3Si group forms
Me3SiXB 4-electron-3-center bonds ([Me3Si][B12F12]–: e.g.
d(Si–F) = 1.78 Å, cf. ∑rcov(Si–F) = 1.8 Å, d(Si–F) = 1.753(9) Å in
The six-fold (trifluoro)methylated carborate shows an exception
probably due to an increased sterical stress between the
trifluoromethyl groups (Figure 3 D). Here structure type A with a
side-on bonded dihydrogen at one boron atom of the meta-belt
(d(H–H) = 0.87 Å, ∡(H-B-H) = 37.5°, Σrcov (H-H) = 0.64 Å) is
found to be favored by 11.1 kcal·mol−1 over isomer C (see
Table S15). H[CHB11H5(CF3)6] favors protonation at one of the
meta-belt boron atoms (Figure 3 D) with one distinct H-B bond
(d(H-B) = 1.33/1.36 Å, d(H-F) = 1.86, d(H-H) = 0.866 Å).
[Me3Si–F–SiMe3]+).[69]
For
all
[Me3Si][CHB11X11]
and
[Me3Si][B12X12]– species silylation is always favored at the
antipodal boron atom (Figure 4 A, Table S14). This phenomena
is not known in literature. Published solid state X-ray structures
revealed a trimethylsilylation at the ortho-belt (Figure 2, cf.
[Me3Si][CHB11F11][14]. For those species with an alkylated lower
boron belt (as in [Me3Si][CHB11H5R6] R = CH3, CF3), stable
isomers are only computed with Me3Si…H…B (attached to a meta
belt boron atom, Figure 2) bonds. Again silylation at the alkyl
groups results in decomposition yielding Me3Si–X and X2C=R (X
= H, F). Double silylation for all [B12X12]2– species, which gives
[Me3Si]2[B12X12], affords para-silylated species as the most
favorable species (Figure 4 B, Table S14) displaying Si…X…B 4-
electron-3-center bonds. This is also in contrast to literature
Upon double protonation as in H2[B12H12] the second proton is
directly attached to the H–B12 unit (Figure 3 E) again in a side-on
H2 coordination mode with H–H distances of 0.848 Å (cf.
∑rcov(H-H) = 0.64 Å), while the first proton as in H[B12H12]– (see
above) is 3-coordinated by one B3H3 moiety (Figure 3 E, d(H···H)
= 1.82-1.85 Å). Such H2-side-on binding mode is also discussed
+
for CH5 for which calculations hinted at a very high degree of
fluxionality owing to the unusual two-electron–three-center
bonding.[65,66] A completely different situation was found for
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10.1002/ejic.201800219
European Journal of Inorganic Chemistry
FULL PAPER
known structures of borates. The attachment of two Et3Si groups
were found in 1,7 position (meta-silylation, Figure 2) in the case
of [B12Cl12]2-[73] and [Me3NB12Cl11]-.[74]
Hence, [Me3Si][CHB11H5(CF3)6] represents the best silylation
species and most “naked” Me3Si+ species within the considered
species. Nevertheless within [Me3Si–F–SiMe3]+ “[Me3Si]+” is less
strong bound. Silylation at the para position is favored for
[CHBH5X6]– and [CHB11X11]– while silylation in [CHB11H5Alk6]–
only occurs at the meta protons otherwise in case of a para or
ortho silylation the formation of Me3Si–H and Me3Si–F is
predicted (vide supra, Figure 4 D). In all other case silylation at
the ortho belt is less favored by 1 – 5 kcal·mol−1. The small
energy differences might indicate a moveable TMS group on the
carborane surface known for the protonated species (vide supra).
As discussed before solid state structures revealed a silylation in
ortho-position, which might be favored due to crystal packing.[14]
The TMSA difference between [CHBH5X6]– and [CHB11X11]– is
roughly 10 kcal·mol−1. A similar situation is found for the
silylation of the dianions. The TMSA values decreases along the
series H (193.6) > F (167.0) > Cl (151.5) > Br (149.4) > I
(146.7 kcal·mol−1) for the first step, while for the second silylation
step much smaller TMSA values are computed (H (119.1) > F
(108.2) > Cl (99.2) > Br (98.9) > I (98.6 kcal·mol−1), which is
within the range of the TMSA values of the [Me3Si][carborate]
series (vide supra, Figure 5).
Proton and TMS affinities. The gas phase proton affinity (PA)
for the reaction An + H+  [AH]n+1(g) (n = –1, –2; An = borate
(g)
(g)
ion) is defined as the negative of the reaction enthalpy at
298.15 K.[75] Trimethylsilyl affinity (TMSA) can be defined in
similar way as the negative of the reaction enthalpy at 298.15 K
[69,76]
for the reaction An + [Me3Si]+  [A(Me3Si)]n+1
.
(g)
These
(g)
(g)
PA values are usually discussed for [AH]n+1(g) and [A(Me3Si)]n+1
(g)
species that is for the reverse reaction. The proton and
trimethylsilyl affinities of all considered species are listed in
Table S12. The proton affinities lie in the range between
366.9 kcal/mol (formation of H[B12H12]–) and 184.9 kcal/mol
(formation of H[CB11(CF3)11], (cf. PA [kcal·mol−1]: A = F− 368.5,
OH− 390.1, [ClO4]− 301.1, [CF3SO3]− 299.7). The calculated data
are in agreement with already known literature data. An
exchange of the computational method HF, DFT (PBEO,
B3LYP) or the perturbation theory MP2 in combination with
different basis sets aug-cc-pwCVDZ, def2-TZVPP, 6-31+G*, 6-
311+G** partly displays a large influences on the the absolute
PA values. Deviations of up to 14.2 kcal mol–1were found (see
H[CHB11H5F6]). Moreover, depend on the utilized method/basis
set different global minima were found on the PES in some
cases (see H2[B12F12] Isomer 22 (Figure S56). For a detailed
comparison see supporting information (Tables S81-S84).
A proton transfer reaction is best possible with H[CHB11(CF3)11],
which represents the strongest acid amongst all considered
species. However, also all other species represent very strong
acids compared to common acids (cf. PA [kcal·mol−1]: A =
[HSO4]− 311.1, H2O 164.3, [NO3]− 322.6, Et2O 196.9). Within the
series [CHB11H5X6]– ions the smallest PA value was found for
H[CHB11H5F6] (241.6 kcal·mol−1) and the largest value for
H[CHB11H5I6] (257.7 kcal·mol−1, Figure 5). The same trend is
found for all [CHB11X11]– species with the largest PA determined
for H[CHB11I11] (250.5 kcal·mol−1) and the smallest for the
fluorine species H[CHB11F11] (226.4 kcal·mol−1, Figure 6). For all
these species protonation is favored between BX12 and BX8
(Figure 3 B). Comparison of the PAs between H[CHB11X11] and
H[CHB11H5X6] displays that the PAs of H[CHB11H5X6] are larger
by 7-15 kcal·mol−1. With increasing degree of alkylation, the PA
values increase (e.g. H[CHB11(Me)11]: 271.4 kcal·mol−1), while in
case of the H[CHB11(RF)11] (RF = fluorinated alkyl group) the
opposite is observed (cf. PA H[CHB11(CF3)11] = 184.9 kcal·mol−1).
Usually, protonation is favored between B12X12 and B8X8/B9X9
with the exception of [CHB11H5(CF3)6]– for which H2 formation is
found upon protonation at the meta-belt B atom (Figures 3 C/D).
The proton affinities of H[B12X12]– (formation of the dianions
[B12X12]2–) are significantly larger and follow the trend as
observed above F (318.6) > Cl (314.2) ≈ Br (314.5) ≈ I
(314.6 kcal·mol−1), while the PA of [B12H12]2– is largest of all
considered species with 366.9 kcal·mol−1. The PA values for the
second protonation drop into the range 230.4 (F) to
268.7 kcal·mol−1 (H). The TMSA values lie all within the range
Figure 5. NICS values (blue triangles) and gas phase acidity (green
diamonds) / trimethylsilyl affinity (red crosses) each in dependency to the 13C
NMR shift of closo-[CHB11H5X6]− with X = F, Cl, Br, I.
Charge transfer, 29Si / 13C NMR shifts and NICS values. To
get insight into the electronic situation of the protonated and
silytated species, the charge transfer (QCT(H+) and QCT(Me3Si+),
Table S13) is calculated by NBO analysis describing the amount
of electrons transferred from the borate anion to the H+ and
Me3Si+ ion upon protonation and silylation, respectively. The
monoprotonated species display a charge transfer between
0.96 e, which means the formation of a formal “H0” as in
[CHB11I11]–, and 0.37 e in the fluorine species [CHB11F11]–,
[CHB11H5F11]– or [B12F12]2–. As expected the observed trends
follow the electronegativities of the substituents. Interestingly,
there are almost no differences between the charge transfers of
the [CHB11X11]–, [CHB11H5X11]– and [B12X12]2– species. The
methylated species are in the range of the chlorine species
(0.71-0.75 e). It should be noted that also the charge transfer of
the CF3 substituted species lies in the same range (0.71-0.74 e).
A much smaller charge transfer is found for the silylated
compared to the protonated species ranging between 0.17 e
([CHB11F11])– and 0.63 e ([B12I12]2–). Again the trends agree with
those of the protonated species. It is noteworthy to mention that
bond-dissociation energies (BDE) of H-X and Si-X[77] following
71.5
[Me3Si][CHB11H5(CF3)6]
and
109.8
kcal·mol−1
[Me3Si][CHB11H11] (cf. [Me3Si–F–SiMe3]+ 34.8 kcal·mol−1).[69]
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10.1002/ejic.201800219
European Journal of Inorganic Chemistry
FULL PAPER
the same trend, the more electronegative the substituent the
larger the BDE (ΔHH/Si-X F > Cl > Br > I).
Interestingly, a six fold substitution of the carborate vertex
reveals a greater highfield shift than full chlorination with a shift
difference of 14.8 ppm. This might be reasoned in a bond
shortening inside the boron scaffold due to the charge
polarization, which directly affects the 13C NMR shift of the
anions.
Figure 6. NICS values (blue triangles) and gas phase acidity (green
diamonds) / trimethylsilyl affinity (red crosses) each in dependency to the
]
13C NMR shift of closo-[CHB11X11 − with X = F, Cl, Br, I.
The computed NICS values are all negative ranging between
−25 ([CHB11H5(CF3)6]–) - –37 ([B12Cl12]2–) indicating aromaticity
for all considered species. The computed 29Si NMR shifts are
found between +52.3 ([B12H12]2–) and +135.4 ppm ([CHB11F11]–).
In accord with the above 29Si NMR data, the charge transfer is
smaller and the TMSA values are larger the higher the 29Si NMR
shift. Hence, [Me3Si][CHB11F11] contains the most less stabilized
[Me3Si]+ ion amongst the considered species. Experimental
evidence for a three-coordinate silyl cation was first provided by
the crystal structure of [(Mes)3Si][CHB11Me5Br6] (where Mes is
2,4,6-trimethyl-phenyl).[78] Here ortho-methyl groups of the
mesityl substituents shield the silicon atom. The downfield 29Si
shift in the solid state (226.7 ppm) was found to be almost
identical to that in “gas phase” calculations, indicating that three-
coordination can be preserved in all phases. The resonance of
naked [Me3Si]+ is computed at 385 ppm, indicating for our
species still significantly stabilized [Me3Si]+ species in accord
with the computed TMSA values.
Figure 7. 1H NMR spectra (top) and 1H{11B} NMR spectra (bottom) of
[Me3NH]1 both in THF-[D8]. A = NH, B = NCH3, C = B12H, D = B2-6H, E =
B7-11H, F = H12-14
.
All computed 13C NMR shifts lie between 7.8 ([CHB11H5F6]–) and
67.9 ppm ([CHB11(CH3)11]−). With respect to the accuracy of the
computed 13C NMR shifts, the average deviation between
experiment and theory amounts to 3.4 ppm, while the largest
deviation is found for the [CHB11(CH3)11]− anion with –7.4 ppm.
The full set of both the experimental and theoretical NMR data
are listed in Table S12.
As negative NICS values are found for all carborates, lying
between –25.1 ([CHB11(CH3)11]–) and –34.8 [CHB11H5F6]–, a
three-dimensional aromaticity[51,80,81] can be assumed (e.g. 1:
−25.7, 2: −27.2, 3: −34.4, 4: −32.2, [B12H12]2-: −27.7, [B12Cl12]2-:
−37.2). Already Paul Ragué von Schleyer, investigated the
stability and established three-dimensional aromaticity for
unsubstituted mono/di-carba-closo-borates.[22] With increasing
symmetry and an increasing number of halogen atoms, the more
Until now there are only four X-ray crystallographic structures of
nido-cage compounds (1) known. The authors of these structural
reports assumed bridging H atoms on the upper meta-boron-belt
in highly disordered anions (Figure 1, left).[47,48] In the liquid
negative NICS values are found displaying
a
better
1
phase a dynamic behavior of the protons is observed. H NMR
delocalization upon halogen substitution (Table 2, Table S12),
an effect also known for planar perchlorinated Hückel aromatic
heterocycles (see Table S18). The aromaticity of closo-borane
hydride clusters was also studied by other methods such as
ECSA method (ECSA = electronic confined space analogy).[82,83]
analysis (Figure 7, Figures S1/S2, S6-S9) show a broad signal
strongly highfield shifted for three chemically equal protons
attached to the meta belt at −3.60 ppm (Δν1/2 = 100 Hz).
Calculations by Crabtree revealed a small energy gap for a free
rotation of the capping H atoms with an activation barrier of only
1.2 kcal·mol−1.[79] We calculated 1J(1H-11B) coupling constants for
this proton which are rather small and lie within the range of
40 Hz (Table S11). Crabtree determined the coalescence
temperature of 1 at 140 K which is rather low and does not allow
any resolution for further investigations. Furthermore, in the
range of 0.4-2.8 ppm a complex pattern with 1J(1H-11B) = 120-
140 Hz was detected. Decoupling of 11B nucleus led to the
collapse of this complex pattern into three sharp signals, still
displaying a broad coupling pattern with the 10B nuclei (Figure 7,
Figure S9). A closer look at the 13C NMR shifts of the capping
carbon atom shows a highfield shift (cf. δ13C Cs2 = 51.4, Cs3 =
47.6, [Ph3C]4 = 32.8 ppm, Table 1) upon chlorine substitution.
A
lower electronegativity of the halogen atom and an
unsymmetrical substitution (six-fold halogenation) results in a
polarization of the boron cage which is indicated by a high-field
shift of the cage carbon atom resonance by more than 10 ppm
compared to eleven- and six-fold substituted derivatives (cf.
δ[13Ccalc.] (δ[13Cexp.]) of [CHB11H11−nXn]− X = F,[84] Cl, Br, I, Me; n =
6: 7.8, 32.2 (32.8),[49] 37.6 (41.5),[85] 49.6 (55.8),[86] 42.1[84]; n =
11: 19.9, 51.3 (Cs3: 47.6), 54.4 (54.1),[35] 49.6 (55.5),[35]
67.9 (60.5);[37] 2 = 52.8 (51.4) ppm, Table 2, Table S12)
Interestingly, the fully methylated carborane anion exhibits the
largest low-field 13C shift, thus featuring the largest Lewis acidic
carbon center compared to the halogenated congeners.
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10.1002/ejic.201800219
European Journal of Inorganic Chemistry
FULL PAPER
Fluorinated carborates feature the smallest NICS values while
their 13C NMR shifts are most high field shifted. Comparing six-
fold and fully-halogenated carborates, the six-fold species show
a slightly smaller 13C NMR shift and smallest negative NICS
values. A similar trend is also found for halogenated benzene
derivatives (see supporting information, Table S18), with the
hexachlorobenzene featuring the largest negative NICS value.
Moreover, the fluorine derivatives display also larger negative
NICS values compared to the less electronegative species (e.g.
Cl, Br, I or H substituted species). A glance at the literature
reveals that the outstanding formal [R3Si]+ transfer properties of
[R3Si][CHB11H5Br6][15,85–87] and [R3Si]3[41,86,88,89] are already being
used unconsciously.
H or F atoms from the alkyl group results in the formation of a
neutral charged trimethylsilane and a carborane bearing a
formal X2C=BN group (X = H, F, N = 2, 8, 12, for positions see
Figure 2). In general, with an increase of the 13C NMR shift of
closo-carborates there is also an increase of the basicity of the
anion (Figures 5-6) while a decrease of the corresponding NICS
values occurs.
Experimental Section
Detailed information for the syntheses of [Me3NH]1 and [Me3NH]2 can be
found in the Supporting information.
Cs2. Cesium carba-closo-undecahydrododecaborate: To a clear and
colorless water boiling solution of [Me3NH]2 (1.75 g, 8.6 mmol)
CsOH·H2O (2.88 g, 17.2 mmol) was added. After stirring for 0.5 h the
solvent was removed and the product dried in vacuo for 2 h. The product
was redissolved in water (50 mL) and four times extracted with
diethylether (50 mL). The combined organic phases were dried in vacuo
for 6 h at 100 °C yielding in colorless Cs2 (0.93 g, 3.37 mmol, 39 %).
Elemental analysis for CH12B11Cs (275.93 g/mol) calc. (found), %: C,
4.35 (4.56); H, 4.38 (4.14). mp. 517 °C. 1H NMR (25 °C, [D6]-acetone,
300.13 MHz): δ = 0.59-2.59 (m, 12H, CH/BH). 11B NMR (25 °C, [D6]-
Conclusions
Herein, we report an optimized laboratory scaled reaction
procedure for the preparation of [CHB11Cl11]− starting from easy
to handle building blocks. To summarize, upon protonation the
proton becomes bound in either mono/di or even tri-coordinated
fashion: (i) The proton is attached to one of the boron or carbon
atoms, (ii) the proton sits between halogen atoms with one short
and one longer HX distance, and (iii) the proton interacts with
one B3X3 unit which forms kind of a pocket with different
1
acetone, 96.29 MHz): δ = –16.18 (d, B2-6H, J(11B-1H) = 150 Hz), −13.11
(d, B7−11H, 1J(11B-1H) = 140 Hz), –6.69 (d, B12H, 1J(11B-1H) = 140 Hz).
13C{1H} NMR (25 °C, [D6]-acetone, 75.47 MHz): δ = 51.4 (b, CH). IR
(ATR, 16 scans, 25 °C, cm–1): 3058 (w), 2927 (w), 2856 (w), 2510 (s),
2352 (w), 1309 (w), 1214 (w), 1145 (w), 1089 (m), 1064 (w), 1024 (s),
972 (w), 891 (w), 854 (w), 757 (w), 715 (s), 588 (w). Raman (473 nm,
6 mW, 40 s, 8 acc., 25 °C, cm–1): 3059 (1), 3048 (1), 2580 (5), 1144 (1),
1091 (1), 1025 (1), 986 (1), 945 (1), 926 (1), 890 (1), 860 (1), 761 (10),
715 (1), 676 (1), 632 (1), 586 (2), 570 (2), 547 (1), 497 (1), 338 (1), 190
(1). MS (CI–, m/z (%)): 128 (85), 133 (17), 143 (6) [CHB11H11]–, 149 (25),
215 (100). MS (CI+, m/z (%)): 69 (23), 75 (100), 81 (12). 85 (24).
H···B/H···X interactions. In the latter case of tri-coordination, the
proton is located inside a distorted trigonal B38,9,12X3
8,9,12
(see
Figure 3C) prism slightly above the boron triangle which is
formed of two ortho-belt and one antipodal boron atoms. While
the B–H distances are rather short (1.38 - 1.43 Å), the distance
to the (trifluoro)methyl-carbon atoms (d(H–CR3) = 2.19 - 2.25 Å)
are rather long. Obviously, the alkyl-substituted species are
unable to form classic hydrogen bonds on the surface of the
borate ion. For all species a highly dynamic behavior of the
proton at the cage surface can expected since the energy
difference between the lowest lying isomers is rather small
(< 10 kcal mol–1).
Stable isomers of the monosilylated species are only found for
those species which provide B-X (X = halogen, H) bonds. In all
other cases ([Me3Si][CHB11Alk11]) decomposition into Me3Si-H
and H2C=R is predicted (Figure 4 D).
The computed proton affinities, ranging between 366.9 and
184.9 kcal·mol−1, are characteristic for very strong acids such as
(PA [SO3CF3]− = 299.7 kcal·mol−1). The TMSA values are all
within the range 58.2 and 109.8 kcal·mol−1 therefore displaying
very good silylating reagents. [Me3Si][CHB11H5(CF3)6] is the best
silylation reagent and most “naked” Me3Si+ species within the
considered species.
Cs3. Cesium carba-closo-undecachlorododecaborate: SO2Cl2 (100 mL)
was condensed in vacuo on cooled (−196 °C) Cs2 (1.6 g, 5.8 mmol). The
reaction mixture was allowed to warm up in a pressure equalized flask
and refluxed additionally for 2 h. The prechlorinated carborate was dried
in vacuo for 1 h at 60 °C. The resulting crude product was dissolved in
water (50 mL) and four times extracted with diethyl ether (50 mL). The
combined ether phases were dried in vacuo for 1 h at 60 °C. This product
was transferred into a V4A stainless steel autoclave under a steam of
argon. ICl (48.7 g, 0.3 mol) and HOTf (67 g, 0.45 mol) were added
quickly and the autoclave was heated 5 d at 200 °C. The reaction mixture
was transferred into a flask and the product was dried in vacuo, by
removing excess ICl, I2 and HOTf. The resulting powder was dissolved in
boiling water filtered G3 through kieselguhr. After an extraction with
diethylether the combined organic layers were dried in vacuo resulting in
colorless Cs3 (1.52 g, 2.32 mmol, 40%). Elemental analysis for
CHB11Cl11Cs (654.83 g/mol) calc. (found), %: C, 1.83 (2.23); H, 0.15
(0.69). mp./dec. >600 °C. 1H NMR (25 °C, [D6]-acetone, 300.13 MHz): δ
= 4.33 (b, 1H, CH). 11B NMR (25 °C, [D6]-acetone, 96.29 MHz): δ =
−12.94 (s, B2-6Cl), –9.84 (s, B7-11Cl), –2.41 (s, B12Cl). 13C{1H} NMR
(25 °C, [D6]-acetone, 75.47 MHz): δ = 47.6 (s, CH). IR (ATR, 64 scans,
25 °C, cm–1): 3646 (w, νH2O), 3569 (w, νH2O), 3023 (m), 1608 (w), 1253
(w), 1116 (s), 1008 (s), 952 (s), 898 (m), 715 (m), 671 (m), 582 (m), 530
(s). Raman (473 nm, 6 mW, 10 s, 10 acc., 25 °C, cm–1): 3024 (2), 1081
(1), 898 (1), 490 (1), 319 (4), 305 (10), 164 (1), 132 (2). MS (CI–, m/z
(%)): 127 (59), 148 (34), 286 (100), 324 (11), 380 (12). MS (CI+, m/z
(%)): 75 (21), 173 (6), 276 (11), 520 (100) [CB11Cl11]+.
13C NMR shifts and NICS values might be used as indicators to
classify the reactivity of the considered borate towards (Lewis)
acids. While a large 13C NMR shift can be seen as an indicator
for an increased acidic character of the anion, a larger NICS
value might hint on
a
perturbed possibility of charge
delocalization inside the boron cage. This already favors an
electrophilic attack for instance by a [Me3Si]+ - ion with its empty
pz-orbital abstracting an exposed atom (with regard to disturbed
aromaticity) from an terminal alkyl group. The abstraction of an
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10.1002/ejic.201800219
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Ag3. Silver(I) carba-closo-undecachlordodecdecaborate: Cs3 (0.267 g,
0.41 mmol) was dissolved in hot (60 °C) water (40 mL). After addition of
a few drops HNO3 a solution of AgNO3 (0.16 g, 0.82 mmol) was added.
3 mL CH3CN was needed to precipitate the silver salt. After stirring in the
dark for 0.25 h the salt was extracted four times with toluene by
extensively shaking. The combined toluene layers were dried in vacuo at
70 °C for 12 h. The product was washed with a small amount of cold
toluene and dried again yielding in Ag3·⅓ CH3CN (0.1 g, 0.16 mmol,
40%). Elemental analysis for CHB11AgCl11·⅓ CH3CN (641.35 g/mol) calc.
(found), %: C, 3.11 (3.25); H, 0.31 (0.67); N, 0.73 (0.74). mp. 196 °C. 1H
NMR (25 °C, [D6]-acetone, 300.13 MHz): δ = 2.01 (m, CH3CN) 4.21 (b,
1H, CH). 11B NMR (25 °C, [D6]-acetone, 96.29 MHz): δ = –12.94 (s,
B2-6Cl), –9.83 (s, B7-11Cl), –2.44 (s, B12Cl). 13C{1H} NMR (25 °C, [D6]-
acetone, 75.47 MHz): δ = 47.61 (b, CH). IR (ATR, 8 scans, 25 °C, cm–1):
3022 (m), 2308 (w, ν(C-NCH3CN)), 2279 (w, ν(C-NCH3CN)), 1025 (sh, ν(C-
CCH3CN), 1118 (s), 1006 (s), 954 (m), 898 (m), 709 (m), 671 (m), 582 (m),
528 (s). Raman (784 nm, 17 mW, 15 s, 6 acc., 25 °C, cm–1): 320 (2), 306
(4), 294 (2), 128 (4), 71 (10). MS (CI–, m/z (%)): 106 (12), 128 (80), 245
(80), 350 (80), 450 (38), 828 (100). MS (CI+, m/z (%)): 77 (8), 160 (6),
260 (23), 520 (100) [CB11Cl11]+.
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Entry for the Table of Contents
FULL PAPER
Full check up. An
Carba-closo-borates
optimized synthesis
procedure for
Alexander Hepp, René Labbow,
Fabian Reiß, Axel Schulz,* and
Alexander Villinger
perchlorinated carba-closo-
borates starting from
inexpensive building blocks
is shown. Next to this a
detailed overview of
different quantum chemical
information is given.
Page No. – Page No.
Carba-Closo-Dodecaborates –
Synthesis, Structure and
Energetics
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