Acid-induced opening of [closo-B10H10]2- as a new route to 6-substituted nido-B10H13 decaboranes and related carboranes

Article abstract of DOI:10.1021/ic3014267

Protonation of the polyhedral anion [closo-B₁₀H ₁₀]^2- under superacidic conditions apparently generates an electrophilic intermediate, [B₁₀H₁₃]⁺, that forms 6-R-nido-B₁₀H₁₃ (R = aryl, alkyl, triflate) derivatives by electrophilic aromatic substitution, C-H bond activation, or ion-pair collapse, respectively. The proposed mechanism of formation of the 6-R-nido-B₁₀H₁₃ derivatives via the boranocation [B ₁₀H₁₃]⁺ is discussed. The synthesis of carboranes, starting from 6-R-nido-B₁₀H₁₃ decaboranes, and single-crystal X-ray diffraction analyses of several 6-R-nido-B ₁₀H₁₃ decaboranes and carboranes are described.

Full text of DOI:10.1021/ic3014267

Article
pubs.acs.org/IC
Acid-Induced Opening of [closo-B10H10]2− as a New Route to
6‑Substituted nido-B10H13 Decaboranes and Related Carboranes
Oleg Bondarev, Yulia V. Sevryugina, Satish S. Jalisatgi, and M. Frederick Hawthorne*
International Institute of Nano & Molecular Medicine, School of Medicine, University of Missouri, 1514 Research Park Drive,
Columbia, Missouri 65211, United States
S
* Supporting Information
ABSTRACT: Protonation of the polyhedral anion [closo-
B10H10]2− under superacidic conditions apparently generates an
electrophilic intermediate, [B10H13]+, that forms 6-R-nido-
B10H13 (R = aryl, alkyl, triflate) derivatives by electrophilic
aromatic substitution, C−H bond activation, or ion-pair
collapse, respectively. The proposed mechanism of formation
of the 6-R-nido-B10H13 derivatives via the boranocation [B10H13]+ is discussed. The synthesis of carboranes, starting from 6-R-
nido-B10H13 decaboranes, and single-crystal X-ray diffraction analyses of several 6-R-nido-B10H13 decaboranes and carboranes are
described.
acid in hexane.14 Sneddon et al. reported an efficient synthesis
INTRODUCTION
■
of 6-X-nido-B10H13 (X = Cl, Br, I) using the cage-opening
Since its first reported synthesis from B10H12(ligand)2,1 the
reactions of closo-(NH4)2B10H10 with superacidic hydrogen
[closo-B10H10]2− ion has been the subject of numerous studies
halides in the presence of an ionic liquid.15
pertaining to its chemical properties and reactivity.2−7 It can be
The 6-R-nido-B10H13 decaboranes are attractive compounds
conveniently prepared by the reaction of B10H12(ligand)2 with 2
in both decaborane and substituted carborane syntheses. The
equiv of base.1,8 The geometric change of the open arachno-
syntheses of carboranes by reacting acetylenic compounds with
B10H12(ligand)2 structure to the bicapped square-antiprismatic
decaborane−Lewis base mixtures are well-known.16,17 As a part
[closo-B10H10]2− ion structure involves the removal of the
of our continued interest in the development of strategies for
bridging hydrogen atoms as protons from arachno-
carborane syntheses, we report here an extension of our
B10H12(ligand)2. The resulting filled two-center orbitals act as
previous work7 on the triflic acid induced opening of [closo-
internal nucleophiles to displace the ligands from the 6 and 9
B10H10]2− to 6-R-nido-B10H13 (compounds 1−3) and its
positions of the arachno-B10H12(ligand)2 structure.6 Additional
subsequent conversion to carboranes (compounds 4−6,
research illustrates the cage-opening reaction of [closo-
respectively).
B10H10]2− with 2 equiv of strong acid and 2 equiv of ligand
to produce arachno-B10H12(ligand)2.9 Monoprotonation of
RESULTS AND DISCUSSION
[closo-B10H10]2− with a strong acid, such as trifluoroacetic
■
acid, produces a [B10H11]− anion.10,11 The initial X-ray
6-Substituted nido-B10H13 Decaborane Derivatives.
Our previously reported synthesis of 6-substituted decaborane
diffraction studies12 of [Ph4P]+ and [Ph3P(Et)]+ salts of
by a triflic acid induced cage opening of [closo-B10H10]2− has
been extended to other arene systems. These reactions proceed
through a triflic acid induced opening of [closo-B10H10]2− to
form a [closo-B10H13]+ intermediate that functions as an
electrophile in the presence of an aromatic substrate, such as
toluene, mesitylene, chlorobenzene, and α,α,α-trifluorotoluene,
to give the corresponding 6-substituted decaboranes in 12−
92% yield (Scheme 1).
The reaction of closo-Cs2B10H10 in chlorobenzene at room
temperature with 5 equiv of triflic acid under anhydrous
conditions rapidly produced 6-(ClC6H4)-nido-B10H13 in 77%
yield (1a) as a mixture of predominantly ortho and para
isomers. The 11B, 1H, and 13C{H} NMR spectra are consistent
with the 6-(ClC6H4)-nido-B10H13 structure. The phenyl proton
[B10H11]− did not locate the 11th hydrogen atom associated
with the [B10H11]− cluster. However, a recent X-ray structural
analysis of [PPh3(benzyl)][B10H10] shows that the B10 core of
[B10H11]− is similar in shape to that of [closo-B10H10]2− and the
11th hydrogen atom symmetrically caps an apical face of the
[closo-B10H11]− cluster.13 It is possible that this is the first stage
in the cage-opening process but that a superacid might be
required to complete the process.
Earlier, we reported that [closo-B10H10]2− could be opened to
selectively form 6-R-nido-B10H13 (R = triflate, phenyl, cyclo-
hexyl) if treated with triflic acid in noncoordinating solvents
such as benzene and cyclohexane, respectively.7 We suggested
that the formation of a boranocation intermediate, [closo-
B10H13]+, occurred through a facile protonation under non-
coordinating conditions.
On the basis of this discovery, 6-(HO)-nido-B10H13 was
synthesized from closo-(NH4)2B10H10 by treatment with sulfuric
Received: July 2, 2012
Published: August 27, 2012
© 2012 American Chemical Society
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terminal B−H bonds of the cage is seen at 2574 cm−1 in the IR
spectrum.
Scheme 1. Triflic Acid Induced Opening of [closo-B10H10]2−
in the Presence of Aromatic Hydrocarbons
The acid-induced opening of closo-Cs2B10H10 in triflic acid in
the presence of aromatic compounds with electron-withdrawing
groups, such as α,α,α-trifluorotoluene, showed that the
electrophilic substitution occurred slowly and with formation
of the byproduct 6-(CF3SO3)-nido-B10H13 (3). Compound 3
was independently synthesized by the reaction of closo-
Cs2B10H10 with triflic acid and without additional solvent in
92% yield.5 The desired 6-(CF3C6H4)-nido-B10H13 (1c) was
isolated mostly as the meta isomer with a 12% yield because the
CF3 group is strongly deactivating and meta-directing. Seven
peaks with relative areas of 1/2/1/2/2/1/1 constitute the
11B{H} NMR spectrum, in which one of the singlets is
attributed to B6 substituted with an α,α,α-trifluorotolyl group.
The 1H NMR shows the characteristic meta-substituted
aromatic system: δ 7.9 (s), 7.53 (d), 7.44 (d), 6.99 (t). The
resonance in the 1H NMR spectra, the two sets of signals in the
13C{H} NMR, and a two-dimensional (2D) HMQC (hetero-
nuclear multiple quantum correlation) experiment confirmed
the characteristic phenyl-substitution pattern for the ortho and
para isomers. Gas chromatography/mass spectrometry (GC/
MS) analysis showed the ratio of ortho and para isomers as
35.7/63.5. However, GC/MS analysis showed the presence of
1
13C{H} and H NMR spectra are in accordance with the 1c
structure. GC/MS analysis revealed the presence of a minor
para isomer, with the ortho/meta/para products showing a 0/
92/8 distribution.
Only one isomer was observed for 6-[2,4,6-(CH3)3C6H2]-
nido-B10H13 (1d). Three peaks due to the ring carbon atoms
and two peaks due to the methyl carbon atoms were seen in the
13C{H} NMR spectrum. The carbon atom attached to the
boron atom was not observed in the 13C{H} NMR spectrum
due to quadruple splitting of the boron atom. The aromatic
protons were seen at δ 6.68, while the methyl group protons at
δ 2.17 and 2.11 were observed with an integration of 2/6/3 in
1
the meta isomer (0.8%), which was not evident in the H and
13C NMR spectra.
With the advent of the 2D 11B−11B NMR technique,
structural elucidation of substituted polyhedral boranes using
11B NMR has received additional attention. Several reports have
demonstrated the potential of this powerful new NMR tool.18
The 11B{H} and 11B−11B correlated spectroscopy (COSY)
NMR for 1a provided ample information for making the
structural assignments (see the Supporting Information). We
could observe the cross peak for the B6 atom of para and ortho
isomers connected to the B2 atom. However, the second cross
peak connecting B6 to B5 and B7 was not observed because of
the bridging hydrogen atoms across the B5 and B7 atoms.
Thus, the following assignments can be made based on the
contour plot: δ 22.1 (p-B6), 19.5 (o-B6), 10.4 (B1,3), 9.2 (B9),
0.9 (B8,10), −1.2 (o-B5,7), −4.1 (p-B5,7), −32.2 (p-B2), −33.8
(o-B2), −37.6 (B4).
1
the H NMR spectrum.
The use of cyclohexane rather than arenes in the cage-
opening reaction gave 6-(C6H11)-nido-B10H13 (2) in 8% yield,
accompanied by 6-(CF3SO3)-nido-B10H13 derivative 3 (Scheme
2). Compound 2 was previously prepared by reacting
Scheme 2. Triflic Acid Induced Opening of [closo-B10H10]2−
in the Presence of Cyclohexane
The reaction of closo-Cs2B10H10 in toluene at room
temperature with 5 equiv of triflic acid under anhydrous
conditions rapidly produced 6-(CH3C6H4)-nido-B10H13 (1b) as
a mixture of ortho-, meta-, and para-substituted isomers with a
1
92% combined yield. The isomers were confirmed by H and
13C{H} NMR and a 2D HMQC experiment. GC/MS analysis
showed the distribution of ortho/meta/para isomers to be 5.5/
14.0/80.5. Brown and Nelson19 have shown that the level of
meta-substituted product formed in the electrophilic aromatic
substitution of monosubstituted benzene such as toluene can
be used as a measure of the reactivity of the electrophile used in
the reaction. Using this analogy, the formation of 14% meta-
substituted product will place the [B10H13]+ electrophile next to
the methanesulfonium cation (15% meta substitution) in
reactivity. The low percentage of the ortho isomer may be
due to steric hindrance. As in compound 1a, a 11B−11B COSY
NMR experiment was helpful to identify various 11B
resonances, which were assigned as follows: 23.6 (m/p-B6),
22.1 (o-B6), 10.2 (B1,3), 9.0 (B9), 1.1 (B8,10), −2.2 (o-B5,7),
−4.7 (m/p-B5,7), −31.7 (m/p-B2), −33.0 (o-B2), −37.7 (B4).
A molecular-ion peak at m/z 212.2054 in the APCI mass
spectrum confirmed the molecular formula of
[B10H13C6H4CH3]− for 1b. The strong absorption due to the
B10H13MgBr with cyclohexyl fluoride.20 The 11B{H} NMR
spectrum of 2 shows seven peaks consistent with the B6-
substituted decaborane structure. Two sets of broad peaks
arising from the cyclohexyl protons resonate at δ 1.67 and 1.18
1
in the H NMR spectrum. The 13C{H} NMR spectrum of the
cyclohexyl carbon atoms shows four peaks at δ 32.5, 28.1, 27.4,
and 26.2. A molecular-ion peak at m/z 204.2909 in the APCI
spectrum confirms the molecular formula of 2.
Proposed Mechanism of the Acid-Induced Cage
Opening of [closo-B10H10]2−. A remarkable array of reaction
products is available from [closo-B10H10]2− and a strong protic
acid, such as CF3SO3H.
The regiospecificity seen in these apparent electrophilic
substitution reactions and the formation of the triflate
derivative (vertex 6 in B10H14) suggest that the arylation,
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alkylation, and triflation reactions proceed from a common
Carboranes. Sneddon et al.15 have recently reported a new
high-yield route to carboranes through a decaborane dehydro-
genative alkyne-insertion reaction in a biphasic ionic liquid/
toluene mixture. Using this insertion procedure, we synthesized
1-phenyl-4-mesityl-1,2-dicarba-closo-dodecaborane (4) by treat-
ing 1d with phenylacetylene in the presence of a catalytic
amount of ionic liquid BMIMCl acting as a Lewis base in
toluene (Scheme 4). The reaction was complete in 3 h. Product
purification by flash chromatography and recrystallization from
hexane gave the desired carborane (4) in 67% yield.
high-energy boranocation [B10H13]+ intermediate (Scheme 3).
Scheme 3. Protonation of [closo-B10H10]2− Giving a
+
Hypothetical B10H13 Electrophile
Scheme 4. Synthesis of 4
Not unexpectedly, this high-energy intermediate is produced
only in noncoordinating solvents. It is presumed that the
[B10H13]+ cation is topologically similar to decaborane and may
have numerous canonical structures. Scheme 3 shows one of
many possible resonance structures that convey the picture of
decaborane bonding.21
We previously postulated that the addition of three protons
to [closo-B10H10]2− might produce an open [B10H13]+ cage
having 22 skeletal electrons (such as the A isomer shown in
Scheme 3), which could possibly rearrange to an isomer B by
converting an empty localized skeletal orbital to an empty
terminal boron orbital by internal hydride migration.7
The B3LYP/6-311G(d)-optimized22 geometry for [B10H13]+,
structure B illustrated in Figure 1, shows distinctions from the
Alternatively, carborane 4 was also synthesized in 38% yield
by heating a solution of 1d and phenylacetylene in toluene at
120 °C for 72 h in the presence of N,N-dimethylaniline (DMA)
acting as a Lewis base16 (Scheme 4).
The 11B{H} NMR spectrum of carborane 4 contained seven
peaks in the δ −0.2 to −14.0 range that were distributed in a
ratio of 1/1/1/1/2/2/2. The proton-coupled spectrum shows a
singlet at δ −0.2 for the B4 atom, while all of the other signals
appear as doublets (JBH = 148−170 Hz). These results are
consistent with a 4-substituted closo-1,2-carborane structure. In
the 1H NMR spectrum, the C2H proton resonates at δ 3.98 and
the mesityl methyl groups resonate at δ 2.67 and 2.17. A strong
molecular-ion peak at m/z 338.2414 in the TIS-MS also
confirms the structure of carborane 4.
The conversion of decaborane to carborane derivatives
strongly depends on the nature of the acetylene compounds
used.16 In the case of electron-withdrawing acetate groups
attached to the acetylene moiety, the yields were significantly
higher. Thus, treating 6-(C6H5)-nido-B10H13 with propargyl
acetate in the presence of DMA for 7 days gave a mixture of
two isomers, 1-(acetoxymethyl)-4-phenyl-1,2-dicarba-closo-do-
decaborane (5a) and 1-(acetoxymethyl)-7-phenyl-1,2-dicarba-
closo-dodecaborane (5b), with a 47% combined yield (Scheme
5, procedure A). Although the isomers were not distinguishable
in the 11B{H} NMR spectra, the 13C{H} and 1H NMR spectra
showed isomers 5a and 5b with an isomer ratio of 3/1. Mass
spectral analysis of the products agrees with structures 5a/5b.
Alternatively, this reaction in the presence of an ionic liquid
BMIMCl/toluene mixture for 30 min at 120 °C gave 69% yield
of predominantly isomer 5b, with an isomer 5a/5b ratio of 1/9
(Scheme 5, procedure B). Recrystallization from the benzene/
hexane mixture gave a single regioisomer 5b in 4% yield.
To avoid the complexity of isomer formation in the
carborane synthesis, we used the symmetrical 2-butyne-1,4-
diol diacetate as the alkyne. Thus, 1,2-bis(acetoxymethyl)-4-
phenyl-1,2-dicarba-closo-dodecaborane (6) was synthesized by
reacting 6-(C6H5)-nido-B10H13 with 2-butyne-1,4-diol diacetate
in the presence of an ionic liquid BMIMCl/toluene mixture for
1 h at 120 °C (Scheme 6). Purification by flash chromatog-
Figure 1. DFT B3LYP/6-311G(d)-optimized geometry of the
[B10H13]+ isomer B and its LUMO density surface.
B10H14 structure. The B−B distance between B6 and B9 is
shorter (3.28 Å) than that in B10H14 (3.57 Å). Similarly, the B−
B distance between B2 and B6 is also considerably shorter
(1.63 Å) than that in B10H14 (1.71 Å). Also, the distances of the
bridging hydrogen bonds between B6 and B7 and between B6
and B5 are shorter (1.25 Å) than those in B10H14. These shorter
distances are most probably due to the cationic nature of the B6
atom. The lowest unoccupied molecular orbital (LUMO)
density surface plot of B shows localization on B6, indicating
the strong electrophilic nature of B6. The B6 cation in structure
B is stabilized by two adjacent bridging hydrogen atoms, H11
and H12. The B6−H11 and B6−H12 distances have shortened
to 1.26 Å (from the normal distance of 1.34 Å in decaborane).
1
raphy gave the carborane 6 in 70% yield. The 11B{H}, H, and
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Scheme 5. Synthesis of 5a and 5b
Figure 2. ORTEP representation of 1b, drawn at the 40% probability
level.
13C NMR spectra and MS analysis are consistent with
compound 6.
X-ray Crystallography. The crystallographic determina-
tions of 1b and 2 confirmed their previously proposed
structures, in which the tolyl and cyclohexyl groups are bonded
at the terminal positions on the B6 atom of the decaborane
open face (Figures 2 and 3). The B6···B9 distance of 3.629(3)
Å in both 1b and 2 is longer than that in B10H14 (3.57 Å). The
B2−B6−C1 angles of 131.01(17)° in 1b and 134.31(9)° in 2
compare with the 132° angle in B10H14. The observed B6−C1
bond length of 1.555(3) Å in 1b is comparable to that reported
earlier for 6-(C6H5)-nido-B10H13 [1.563(14) Å]; however, it is
significantly shorter than that in 2 [1.5772(15) Å]. This
difference may be related to the improved π-type back-donation
of the electron density from the π-delocalized aromatic system
to the vacant LUMO p orbital centered on the B6 atom.23 The
B−B distances around B6 in 1b and 2 are significantly
elongated compared to those around B9 and to those that have
been reported for unsubstituted B10H14 (Table 1). All of the
other B−B distances are comparable to the B−B distances that
have been reported for B10H14. These results indicate a slight
distortion of the decaborane framework upon substitution at
B6. Also, as shown in Figure 3, the six-membered cyclohexyl
ring in 2 adopts a chair conformation.
One of the interesting features of the packing motif in 1b is
that the open face of decaborane is aligned with the aromatic
ring of the tolyl group of the neighboring molecule, with an
Hbridge···centroid separation in the 3.194−3.582 Å range (Figure
4). This structural motif is absent in the previously determined
structure of 6-(C6H5)-nido-B10H13.7
The structures of carboranes 4 and 5b were confirmed by
single-crystal X-ray crystallography (Figures 5 and 6). Both
compounds were crystallized as racemic mixtures. The
carborane cage in 5b (Figure 6) displays the B−B and B−C
bond lengths of 1.765(3)−1.793(3) and 1.692(2)−1.723(2) Å,
Figure 3. ORTEP representation of 2, drawn at the 40% probability
level.
Table 1. Selected Bond Distances (Å) in Selected
Decaboranes
6-R-B10H13, where R =
c
24
C6H5
p-CH3(C6H5) (1b)
C6H11 (2)
B10H14
a
B1−B2
B1−B3
B1−B5
B2−B5
1.786(16)
1.794(18)
1.755(20)
1.789(19)
1.998(15)
1.738(21)
1.735(19)
1.809(17)
1.779(19)
1.563(14)
1.780(3)
1.779(3)
1.755(3)
1.793(4)
1.984(3)
1.735(3)
1.713(3)
1.803(3)
1.790(4)
1.7835(17)
1.7774(16)
1.7541(16)
1.7921(16)
1.9831(17)
1.7456(16)
1.7276(17)
1.8071(16)
1.7922(17)
1.5772(15)
1.778
1.772
1.756
1.786
1.973
1.715
a
a
b
b
B5−B10
B6−B2
B9−B4
b
B6−B5
1.775
B9−B10
B6−C1
1.555(3)
a
b
Averaged assuming C2v symmetry. Averaged assuming Cs symmetry.
c
Averaged for two crystallographically independent molecules.
Scheme 6. Synthesis of the Symmetrical Carborane 6
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Figure 4. Fragment of the packing alignment in the a direction for
compound 1b. The selected intermolecular Hbridge···centroid separa-
tions are in 3.194−3.582 Å range.
Figure 6. ORTEP representation of the major (95%) orientation of 1-
(CH2COOCH3)-7-(C6H5)-C2B10H10 (5b), drawn at the 40%
probability level. The selected interatomic distance is C1−C3 =
1.523(2) Å.
1.68−1.72 Å). The observed Bcarborane−Caromatic bond lengths
are 1.602(2) Å in 4 and 1.575(2) Å in 5b.
An analysis of the solid-state packing (Figure 7) of 4 shows
that the intermolecular hydrogen bonding is dominated by the
Figure 5. ORTEP representation of 1-(C6H5)-4-[2,4,6-(CH3)3C6H2]-
C2B10H10 (4), drawn at the 40% probability level. The enantiomers of
4 were cocrystallized as a racemic mixture. The selected interatomic
distances are C1−C3 = 1.5089(19) Å and B4−C9 = 1.602(2) Å.
Figure 7. Fragment of the packing alignment in the bc plane for the
structure of 4.
respectively. These bond lengths are within the expected range
for the carborane cage.25 Although a wide variety of o-
carboranes have been reported in the literature, compound 4
(Figure 5) is the second that has been structurally characterized
as 1,4-substituted o-carborane. The first compound was 1,4-
[1,2-dicarbadodecaborane(12)-1,4-diyl]-2-butanone, in which
C1 and B4 are linked through the exocage cyclohexenone
ring.26 The icosahedron in 4 is not severely distorted by
substitution, and the majority of the B−B and B−C distances
are within the expected ranges for the carborane cage
[1.764(2)−1.791(2) and 1.693(2)−1.747(2) Å, respectively].
However, the 1,4-substitution in 4 does influence the structure
in the vicinity of the B4 atom, which can be seen from the bond
lengths around B4 being slightly longer [B−B = 1.797(2)−
1.822(2) Å and B−C = 1.7793(19) Å] than the respective
bonds in a typical o-carborane cage (B−B = 1.79 Å and B−C =
C−H···π interactions formed by the acidic C−H of the
carborane and mesitylene ring of the neighboring molecule [the
CCb···centroid separation is 3.230(2) Å and the CCb−H···π
angle 164(2)°]. The solid-state packing also displays very weak
intermolecular π···π interactions formed between the two
slipped-parallel benzene groups in adjacent carborane mole-
cules [the intercentroid distance is 4.4923(9) Å, the dihedral
angle between planes is 0°, and the slippage is 2.723 Å]. In
addition, there are weak intramolecular π···π interactions of
4.7884(8) Å [the dihedral angle between the planes is
60.11(7)°] between the benzene and mesitylene groups at
the B1 and B4 atoms. The intermolecular Ccarborane−H···π
contacts have been described elsewhere as weak nonclassical
C−H···π hydrogen bonds27 and are common in supramolecular
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assemblies of carboranes.28 The solid-state packing motif of 5b
is mainly represented by C−H···O hydrogen bonds between
the acidic C−H of carborane and the CO of the
acetoxymethyl group [the C···O bond is 3.135(2) Å, and the
C−H···O angle is 140°].
under vacuum. The residue was chromatographed over silica gel with
toluene to afford 1d as a colorless solid (253 mg, 92% yield). 11B NMR
(160.4 MHz, C6D6): δ 23.6 (m/p-B6, s), 22.1 (o-B6, s), 10.2 (B1,3),
9.0 (B9), 1.1 (B8,10), −2.2 (o-B5,7), −4.7 (m/p-B5,7), −31.7 (m/p-
1
B2), −33.0 (o-B2), −37.7 (B4) (all doublets). H NMR (500 MHz,
C6D6): δ 7.60 (o-2H, d, J = 7.2 Hz), 7.48 (p-2H, d, J = 7.8 Hz), 7.43
(m-2H, s), 7.36 (m-2H, d, J = 7.8 Hz), 7.10 (m+o-4H, m), 6.99 (p-2H,
d, J = 7.8 Hz), 6.96 (o-2H, d, J = 7.6 Hz), 5.5−0.5 (9H, br), 2.20 (m-
3H, s), 2.11 (p-3H, s), 2.17 (o-3H, s), −1.19 (2H, s), −2.51 (2H, s).
13C{H} NMR (125.8 MHz, C6D6): δ 140.5, 133.2, 128.9, 21.1. MS
CONCLUSION
■
The protonation reactions of the polyhedral anion [closo-
B10H10]2− give valuable insight into the versatility of this anion’s
ability to form a series of substituted decaboranes. Depending
on the Lewis basicity of the solvent, mono- or diprotonation of
[closo-B10H10]2− can easily be achieved. Furthermore, in
noncoordinating solvents, the addition of the third proton
was shown to occur, generating a boranocation [B10H13]+ that
acts as an electrophile in the electrophilic aromatic substitution
reaction to give the corresponding 6-substituted decaboranes.
The density functional theory (DFT) calculations support the
formation of 6-substituted nido-B10H13 products. Subsequent
reactions with acetylenic substrates in the presence of a Lewis
base gave corresponding substituted carboranes.
(APCI). Calcd for [B10H13C6H4CH3]−: m/z 212.2566. Found: m/z
212.2054. IR (KBr): ν 2574 (B−H), 3046, 2918, 1614, 1496, 1456,
1001, 821, 808, 686 (aromatic) cm−1.
Preparation of 6-(C6H4CF3)-nido-B10H13 (1c) from closo-
Cs2B10H10. closo-Cs2B10H10 (1.0 g, 2.6 mmol) was suspended in 50
mL of α,α,α-trifluorotoluene, and an excess of triflic acid (1.15 mL, 13
mmol) was added using an Eppendorf pipet with constant stirring.
The reaction mixture was then stirred for an additional 2 h. The
residue left in the reaction mixture was filtered in air, and the solvent
was evaporated under vacuum. The residue was chromatographed over
silica gel using hexanes to afford 1c as a colorless solid (84 mg, 12%
yield). 11B NMR (160.4 MHz, C6D6): δ 22.4 (B6, s), 10.9 (B1,3), 9.9
(B9), 1.0 (B8,10), −3.2 (B5,7), −32.4 (B2), −37.1 (B4) (all doublets).
1H NMR (500 MHz, C6D6): δ 7.90 (1H, s), 7.53 (1H, d, J = 5.9 Hz),
7.44 (1H, d, J = 5.9 Hz), 6.99 (1H, t, J = 5.9 Hz), 4.5−0.5 (9H, br),
−1.40 (2H, s), −2.37 (2H, s). 13C{H} NMR (125.8 MHz, C6D6): δ
136.4, 130.2 (q, J = 32.6 Hz), 128.6, 129.2, 126.7, 124.7. 19F{H} NMR
(C6D6): δ −62.4. MS (APCI). Calcd for [B10H13C6H4CF3]−: m/z
266.2284. Found: m/z 266.2138. IR (KBr): ν 2582 (B−H), 2947,
1608, 1505, 1330, 1121, 1004, 956, 904, 804, 729, 661 (aromatic)
cm−1.
EXPERIMENTAL SECTION
■
General Procedures and Materials. Standard Schlenk-line
techniques were employed for all manipulations of air- and
moisture-sensitive compounds. The benzene, toluene, mesitylene,
chlorobenzene, α,α,α-trifluorotoluene, cyclohexane, trifluoromethane
sulfonic acid, phenylacetylene, propargyl acetate, 2-butyne-1,4-diol
diacetate, and N,N-dimethylaniline (DMA) were distilled before use.
closo-Cs2B10H10 was precipitated from an aqueous solution of closo-
(Et3NH)2B10H10 using CsOH, filtered, washed with EtOH and Et2O,
and dried under vacuum. 1-Butyl-3-methylimidazolium chloride
[BMIMCl] (Fluka) and poly(methylhydrosiloxane) [PMHS] (Al-
drich) were used as received.
Preparation of 6-[2,4,6-(CH3)3C6H2)]-nido-B10H13 (1d) from
closo-Cs2B10H10. closo-Cs2B10H10 (0.5 g, 1.3 mmol) was suspended in
50 mL of mesitylene, and an excess of triflic acid (0.57 mL, 6.5 mmol)
was added using an Eppendorf pipet with constant stirring. The
reaction mixture was then stirred for an additional 30 min. The residue
from the reaction mixture was filtered in air, and the solvent was
evaporated under vacuum. The resulting residue was chromatographed
over silica gel using toluene to give 280 mg of 1d as a colorless solid
(90% yield). 11B NMR (160.4 MHz, C6D6): δ 22.1 (B6, s), 9.9 (B1,3),
7.7 (B9), 2.4 (B8,10), −3.1 (B5,7), −32.4 (B2), −37.8 (B4) (all
The 11B NMR spectra (160 MHz) were obtained on a Bruker AM-
500 spectrometer and referenced to external BF3·Et2O. The chemical
shifts for 1H and 13C NMR spectra were referenced to
tetramethylsilane (TMS) and were measured with respect to the
residual protons and carbon, respectively, in deuterated solvents. The
timed ion selector (TIS) and atmosphere-pressure chemical ionization
(APCI) MS spectra were recorded by operating in negative-ion mode
on an ABI Mariner mass spectrometer. The IR spectra were recorded
on a Nicolet Nexus 470 FT-IR spectrometer using KBr pellets.
Preparation of 6-(ClC6H4)-nido-B10H13 (1a) from closo-
Cs2B10H10. closo-Cs2B10H10 (0.50 g, 1.3 mmol) was suspended in 50
mL of chlorobenzene, and excess triflic acid (0.57 mL, 6.5 mmol) was
added using an Eppendorf pipet with constant stirring. The reaction
mixture was then stirred for an additional 1 h. The residue from the
reaction mixture was filtered in air, and the solvent was evaporated
under vacuum. The residue was chromatographed over silica gel using
toluene to give 1a as a white solid (232 mg, 77% yield). 11B NMR
(160.4 MHz, C6D6): δ 22.1 (p-B6, s), 19.5 (o-B6, s), 10.4 (B1,3), 9.2
(B9), 0.9 (B8,10), −1.2 (o-B5,7), −4.1 (p-B5,7), −32.2 (p-B2), −33.8
(o-B2), −37.6 (B4) (all doublets). 1H NMR (500 MHz, C6D6): δ 7.64
(o-1H, dd, J = 7.4 and 1.7 Hz), 7.18 (p-2H, d, J = 8.4 Hz), 7.36 (p-2H,
d, J = 8.4 Hz), 7.03 (o-1H, dd, J = 7.6 and 1.3 Hz), 6.84 (o-2H, m),
5.5−0.5 (9H, br), −1.21 (2H, s), −2.50 (2H, s). 13C{H} NMR (125.8
MHz, C6D6): δ 136.6 (ortho) 137.4 (para), 135.1 (para), 132.1
(ortho), 129.2 (para), 127.7 (ortho). MS (APCI). Calcd for
[B10H13C6H4Cl]−: m/z 232.2022. Found: m/z 232.1587. IR (KBr):
ν 2577 (B−H), 2932, 1585, 1490, 1089, 823, 754, 684 (aromatic)
cm−1.
1
doublets). H NMR (500 MHz, C6D6): δ 6.68 (2H, s), 2.17 (6H, s),
2.11 (3H, s), 4.5−0.5 (9H, br), −0.86 (2H, s), −2.08 (2H, s). 13C{H}
NMR (125.8 MHz, C6D6): δ 142.3, 139.6, 129.3, 23.6, 20.8. MS
(APCI). Calcd for [B10H13C9H10]−: m/z 239.2802. Found: m/z
239.2714. IR (KBr): ν 2575 (B−H) cm−1.
Preparation of 6-(C6H11)-nido-B10H13 (2) from closo-
Cs2B10H10. closo-Cs2B10H10 (1.0 g, 2.6 mmol) was suspended in 100
mL of cyclohexane. The suspension was warmed to 60 °C, and excess
triflic acid (1.15 mL, 13 mmol) was added using an Eppendorf pipet
with constant stirring. The reaction mixture was then stirred for an
additional 2 h. The resulting light-yellow solution was filtered, and the
solvent was evaporated under vacuum. The resulting residue was
chromatographed over silica gel using hexane to give 42 mg (8% yield)
of compound 2. 11B NMR (160.4 MHz, C6D6): δ 27.2 (B6, s), 10.8
(B1,3), 8.8 (B9), 0.9 (B8,10), −3.2 (B5,7), −34.3 (B2), −38.2 (B4)
(all doublets). 1H NMR (500 MHz, C6D6): δ 1.67 (5H, m), 1.18 (6H,
m), 4.5−0.5 (9H, br), −2.04 (2H, s), −2.46 (2H, s). 13C{H} NMR
(125.8 MHz, C6D6): δ 32.5, 28.1, 27.4, 26.2. MS (APCI). Calcd for
[B10H24C6]−: m/z 204.2879. Found: m/z 204.2909. IR (KBr): ν 2594
(B−H) cm−1.
Preparation of 6-(CF3SO3)-nido-B10H13 (3) from closo-
Cs2B10H10. Excess triflic acid (0.57 mL, 6.5 mmol) was added to a
well-stirred powder of Cs2B10H10 (0.50 g, 1.3 mmol). The reaction
mixture was then stirred for an additional 15 min. Vacuum distillation
into a −78 °C precooled flask yielded 325 mg (92%) of compound 3
as a white solid. 11B NMR (160.4 MHz, C6D6): δ 14.8 (B6, s), 12.2
(B9), 7.8 (B1,3), 1.3 (B8,10), −5.1 (B5,7), −35.0 (B2), −40.1 (B4)
(all doublets). 1H NMR (500 MHz, C6D6): δ 4.5−0.5 (9H, br), −1.21
(2H, s), −2.44 (2H, s). 13C{H} NMR (125.8 MHz, C6D6): δ 118.3 (q,
Preparation of 6-(CH3C6H4)-nido-B10H13 (1b) from closo-
Cs2B10H10. closo-Cs2B10H10 (0.50 g, 1.3 mmol) was suspended in 50
mL of toluene, and an excess of triflic acid (0.57 mL, 6.5 mmol) was
added using an Eppendorf pipet with constant stirring. The reaction
mixture was then stirred for an additional 30 min. The reaction
mixture residue was filtered in air, and the solvent was evaporated
9940
dx.doi.org/10.1021/ic3014267 | Inorg. Chem. 2012, 51, 9935−9942

Inorganic Chemistry
Article
J = 318.2 Hz). 19F{H} NMR (C6D6): δ −76.6. MS (APCI). Calcd for
X-ray Diffraction Studies. X-ray-quality crystals were obtained for
compounds 1b, 2, 4, and 5b as follows: 1b, by the slow evaporation of
a hexane mixture; 2, upon cooling of a toluene/hexane mixture; 4,
upon the slow evaporation of a chloroform mixture; 5b, upon the slow
evaporation of a chloroform/hexane mixture. Each crystal was coated
with paratone oil and mounted onto a MiTeGen MicroMount fiber.
Complete and redundant data were collected on a single flash-cooled
crystal (T = 100 K with an Oxford Cryostream LT device) using a
Bruker X8 Prospector Ultra X-ray diffractometer system with a three-
circle goniometer and an APEX II CCD area detector mounted on a
D8 platform and equipped with a Cu-IμS (λ = 1.54178 Å) microfocus
X-ray source operated at 30 W. The frames were collected with a scan
width of 0.5° in ω and an exposure time of 10 s/frame. The intensity
data sets consisted of ϕ and ω scans at a crystal-to-detector distance of
4.00 cm. The APEX 229 and SAINT30 software packages were used for
data collection and data integration. The data were corrected for
absorption effects using the SADABS empirical method.31 The
structures were solved and were refined by full-matrix least-squares
techniques based on F2 (SHELXL-97).32 All of the non-hydrogen
atoms were refined with anisotropic thermal parameters. The disorder
of the acetoxymethyl group in the crystal structure of 5b was modeled
with two orientations in a 0.95/0.05 ratio. All of the hydrogen atoms in
the crystal structures of 4 and 5b were included at geometrically
idealized positions. For 1b and 2, some of the hydrogen atoms were
located in difference Fourier maps and were refined individually; thus,
the refinement was mixed. The crystallographic data and details of the
data collection and structure refinements of 1b, 2, 4, and 5b are
provided in Table S1 in the Supporting Information.
[B10H13CF3SO3]−: m/z 270.1537. Found: m/z 270.1276.
Preparation of 1-Phenyl-4-mesityl-1,2-dicarba-closo-dodec-
aborane (4) from 6-[2,4,6-(CH3)3C6H2)]-nido-B10H13 (1c). Proce-
dure A: A solution of 1c (0.24 g, 1.0 mmol), phenylacetylene (0.10 g,
1.0 mmol), and DMA (0.24 g, 2.0 mmol) in 10 mL of toluene was
heated slowly until hydrogen evolution began. The temperature was
then steadily raised to 120 °C, and the solution was heated for an
additional 72 h. After cooling to room temperature, the volatiles were
removed under vacuum to leave a yellow oil. The residue was
chromatographed on silica gel with toluene as the eluent. The product
was additionally washed with hexane to give a colorless crystalline
product (4). The yield was 128 mg (38%).
Procedure B: Reacting 1c (0.24 g, 1.0 mmol) with 0.12 g (1.2
mmol) of phenylacetylene in a biphasic toluene (10 mL)/BMIMCl
(0.12 g, 0.69 mmol) mixture at 120 °C for 3 h gave, following toluene
elution from a silica gel column and washing with hexane, 226 mg
(67%) of 4 as a white solid. 11B NMR (160.4 MHz, CDCl31): δ −0.2
(s), −0.7, −4.7, −7.7, −10.1, −11.7, −13.0 (all doublets). H NMR
(500 MHz, CDCl3): δ 7.17 (3H, m), 7.06 (2H, m), 6.64 (2H, s), 3.98
13
(1H, s), 2.67 (6H, s), 2.17 (3H, s).
C{H} NMR (125.8 MHz,
CDCl3): δ 145.1, 137.7, 132.4, 130.7, 129.5, 128.5, 127.4, 77.8, 61.0,
26.7. MS (APCI). Calcd for [B10C17H26]−: m/z 338.3041. Found: m/z
338.2414. IR (KBr): ν 2577 (B−H), 3046, 3048, 2922, 1605, 1447,
1414, 1069, 860, 846, 759, 687 (aromatic) cm−1.
Preparation of 1-(Acetoxymethyl)-4-phenyl-1,2-dicarba-
closo-dodecaborane (5a) and 1-(Acetoxymethyl)-7-phenyl-
1,2-dicarba-closo-dodecaborane (5b) from 6-(C6H5)-nido-
B10H13. Procedure A: A solution of 6-(C6H5)-nido-B10H13 (0.40 g,
2.0 mmol), propargyl acetate (0.30 g, 3.0 mmol), and DMA (0.48 g,
4.0 mmol) in 10 mL of toluene was heated slowly until hydrogen
evolution began. The temperature was then raised steadily up to 100
°C, and the solution was heated for an additional 7 days. After cooling
to room temperature, the volatiles were removed under vacuum,
leaving a yellow oil. The residue was chromatographed over silica gel
with toluene as the eluent. The product was additionally washed with
hexane to give a mixture of products 5a and 5b as a white solid. The
yield was 274 mg (47%).
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format, Cartesian coor-
■
S
+
dinates of a DFT-optimized B11H13 structure, crystallographic
details for compounds 1b, 2, 4, and 5b, the 11B−11B COSY
NMR spectrum of 1a, and the 11B{H} NMR spectrum of
compound 1b. This material is available free of charge via the
Internet at http://pubs.acs.org.
Procedure B: Reacting 6-(C6H5)-nido-B10H13 (0.40 g, 2.0 mmol)
with propargyl acetate (0.24 g, 2.5 mmol) in a biphasic toluene (10
mL)/BMIMCl (0.17 g, 0.95 mmol) mixture at 120 °C for 30 min gave,
following toluene elution from a silica gel column and washing with
hexane, 402 mg (69%) of mixture 5a/5b as a white solid.
Recrystallization from a benzene/hexane mixture gave 25 mg (4%)
of the single regioisomer 5b. 11B NMR (160.4 MHz, CDCl3): δ −1.9
(d), −2.6 (s), −3.6, −8.8, −9.7, −12.1, −13.1, −14.4 (all doublets).
The assignments of the NMR signals for 5a were made from the 5a/
5b mixture. 1H NMR (500 MHz, CDCl3) for 5a: δ 7.67 (2H, d, J = 6.3
Hz), 7.39 (3H, m), 4.41 (2H, d, J = 7.6 Hz), 4.04 (1H, s), 2.07 (3H,
s). 13C{H} NMR (125.8 MHz, CDCl3): δ 170.4, 135.6, 129.5, 128.3,
73.4, 65.4, 60.7, 21.1. 1H NMR (500 MHz, CDCl3) for 5b: δ 7.64 (2H,
d, J = 6.3 Hz), 7.43 (3H, m), 4.71 (2H, d, J = 4.3 Hz), 4.12 (1H, s),
2.24 (3H, s). 13C{H} NMR (125.8 MHz, CDCl3): δ 169.7, 134.8,
128.6, 127.9, 72.5, 64.6, 59.9, 20.4. MS (APCI). Calcd for
[B10C11H22O2]−: m/z 292.2467. Found: m/z 292.2531. IR (KBr): ν
2571 (B−H), 1749 (CO) cm−1.
Preparation of 1,2-Bis(acetoxymethyl)-4-phenyl-1,2-dicar-
ba-closo-dodecaborane (6) from 6-(C6H5)-nido-B10H13. The
reaction of nido-Ph-B10H13 (0.20 g, 1.0 mmol) with 0.17 g (1.0
mmol) of 2-butyne-1,4-diol diacetate in a biphasic toluene (10 mL)/
BMIMCl (0.12 g, 0.69 mmol) mixture at 120 °C for 1 h gave,
following toluene elution from a silica gel column, 254 mg (70%) of 6
as a colorless oil. 11B NMR (160.4 MHz, CDCl3): δ −0.4 (s), −1.9,
−9.4, −11.3 (all doublets). 1H NMR (500 MHz, CDCl3): δ 7.17 (2H,
d, J = 7.3 Hz), 7.38 (3H, m), 4.76 (2H, s), 4.62 (1H, d, J = 13.9 Hz),
4.43 (1H, d, J = 13.9 Hz), 2.15 (3H, s), 1.98 (3H, s). 13C{H} NMR
(125.8 MHz, CDCl3): δ 169.8, 169.7, 136.5, 129.9, 128.9, 77.4, 76.5,
63.5, 61.8, 21.2, 20.9. MS (APCI). Calcd for [B10C14H24O4]−: m/z
364.2680. Found: m/z 364.3026. IR (KBr): ν 2582 (B−H), 1757
(CO) cm−1.
AUTHOR INFORMATION
Corresponding Author
*E-mail: hawthornem@missouri.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank International Institute of Nano and Molecular
Medicine, University of Missouri, for financial support. Dr. Ilia
Guzei (University of WisconsinMadison) is thanked for the
in-house crystallographic program “ModiCIFer”.
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