1,2-, 1,7- And l,12-dicarba-closo-dodecaborane(12) derivatives revisited by13C NMR spectroscopy and DFT calculations. First observation of isotopeInduced chemical shifts1?10/11B( 13C), and the signs and magnitud

Article abstract of DOI:10.1002/zaac.200900022

The origin of broadening of ¹³C(carborane) NMR signals of 1,2-, 1,7- and l,12-dicarba-claso-dodecaboranes(12) and several diphenylsilyl derivatives has been examined in detail and could be traced only partially to unresolved ¹³C-

Full text of DOI:10.1002/zaac.200900022

ARTICLE
DOI: 10.1002/zaac.200900022
1,2-, 1,7- and 1,12-Dicarba-closo-dodecaborane(12) Derivatives Revisited by
13C NMR Spectroscopy and DFT Calculations. First Observation of Isotope-
Induced Chemical Shifts 1Δ10/11B(13C), and the Signs and Magnitudes of
1
1
Coupling Constants J(13C,13C) and J(13C,11B)
[a]
[a]
[a]
[a]
´
´
Bernd Wrackmeyer,* Zureima Garcıa Hernandez, Julian Lang, and Oleg L. Tok
Dedicated to Professor Wilhelm Preetz on the Occasion of His 75th Birthday
Keywords: Carboranes; NMR spectroscopy; Isotope-induced chemical shifts; Coupling constants; Density functional
calculations
Abstract. The origin of broadening of 13C(carborane) NMR signals
of 1,2-, 1,7- and 1,12-dicarba-closo-dodecaboranes(12) and several
diphenylsilyl derivatives has been examined in detail and could be
traced only partially to unresolved 13CϪ11B spin-spin coupling.
Other contributions to the line widths arise from 13CϪ1H dipole-
dipole interactions and, in particular, from isotope-induced chemi-
cal shifts 1Δ10/11B(13C), observed here for carboranes for the first
time. In the case of 1-diphenylsilyl-1,2-dicarba-closo-dodecaborane-
natural abundance of 13C. The small value of this coupling constant
and its negative sign is predicted by calculations based on op-
timised structures [B3LYP/6-311ϩG(d,p) level of theory] of the
parent carboranes and 1-silyl-1,2-dicarba-closo-dodecaborane(12)
1
as a model compound [calcd. J(13C,13C) ϭ Ϫ10.5 Hz]. Calculated
coupling constants 1J(13C,11B) are small (<7 Hz), in contrast to
1
published assumptions, and of either sign, whereas J(11B,11B) are
all positive and range up to 15 Hz.
1
(12), the coupling constant J(13C,13C) ϭ 9.3 Hz was measured in
measured so far. However, in substituted carboranes, the
broadening appears to be less serious [11, 25Ϫ27] as a result
of relatively fast quadrupole-induced 11B nuclear spin relax-
ation. Therefore, the 13C(carborane) NMR signals are read-
ily observed and deserve attention with respect to structural
assignments. Moreover, they offer an almost unique oppor-
tunity to observe isotope-induced chemical shifts [28]
1Δ10/11B(13C) which are difficult to determine for most
other boronϪcarbon compounds.
Introduction
There is a wealth of derivatives of the three isomers of
dicarba-closo-dodecaboranes(12) („ortho-” 1a, “meta-” 2a
and “para-carborane” 3a) bearing substituents at one or
both carbon atoms [1Ϫ11], and many of these compounds
were characterised by NMR spectroscopic methods. From
the beginning of carborane chemistry, 11B NMR played an
1
important role [12Ϫ17] accompanied by routine H NMR
Here we report the first example of 1J(13C,13C) for a
1-silyl derivative of 1a, and also the first examples of
1Δ10/11B(13C) for ortho-, meta- and para-carboranes bearing
silyl groups at the carbon atoms (Scheme 1). We also point
out that these effects are most likely observable for many
other similar carborane derivatives.
spectra, mainly focusing on the organic moieties of sub-
stituents at the carbon atoms. In contrast, 13C NMR spec-
troscopy dealing with the carbon atoms in the carborane
cage has received less attention [18Ϫ22]. The substantial
broadening observed for the 13C NMR signals of the parent
carboranes C2B10H12 [18Ϫ22] has been interpreted as a re-
sult of scalar relaxation of the second kind [23], owing to
partially relaxed 13CϪ11B spin-spin coupling (11B: I ϭ 3/2)
[24] and might have been somewhat discouraging for more
detailed studies. Thus, indirect nuclear 13CϪ13C spin-spin
coupling [1J(13C,13C)] in 1 or its derivatives has not been
* Prof. Dr. B. Wrackmeyer,
Fax: ϩ49-921-552157
E-mail: b.wrack@uni-bayreuth.de
[a] Laboratorium für Anorganische Chemie
Universität Bayreuth
Scheme 1. Dicarba-closo-dodecacarboranes(12) and derivatives
95440 Bayreuth, Germany
studied here by 13C NMR spectroscopy.
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ARTICLE
although some cross peaks were not observed. In these
experiments, it was assumed that the magnitude of
1J(13C,11B) was in the order of 70 Hz [22]. This assumption
is in contrast with the appearance of the one-dimensional
(1D) 13C{1H} NMR spectra, for which one would then ex-
pect to observe a fine structure owing to partially resolved
13CϪ11B spin-spin coupling, taking into account the
T1(11B) values determined by the same authors [22]. We
have approached this problem in two ways: (i) In the case
of boranes, the NMR parameters such as 11B chemical
shifts [31] and coupling constants [32Ϫ34], can be calcu-
lated reasonably well by using various ab-initio methods
and also by DFT methods (vide infra); (ii) the origin of the
broadening of the 13C(carborane) NMR signals needs to be
elucidated more precisely.
Results and Discussion
The carborane derivatives 1Ϫ3 (Scheme 1) were prepared
by mono-lithiation of the parent carboranes, using butyl-
lithium, followed by reaction with the corresponding chlo-
rosilanes. Because the lithiation affords mixtures containing
mainly the desired C mono-lithiated carborane along with
the dilithiated carborane and the respective parent carbor-
ane [29, 30], the reaction mixture, in general, also contains
the parent carboranes and small amount of disubstituted
carboranes, When dichloro(diphenyl)silane was used, in
order to prepare 1d, the mixture of soluble materials also
contained 1a and 1c. All carborane derivatives 1Ϫ3 are
fairly soluble and stable in C6D6 or CDCl3, and their com-
position is readily established by 1H, 11B, 13C and 29Si
NMR spectroscopy.
The literature on 13C NMR studies of the parent carbor-
anes [18Ϫ22] points out the particular feature of broad 13C
NMR signals which, at a first glance, is not surprising con-
sidering the neighbourhood of the 13C nuclei to the quadru-
Broadening of 13C(Carborane) NMR Signals
In principle, 11B decoupling should remove the broaden-
polar 11B and/or 10B nuclei. The 11B NMR signals of the ing effect of 11B nuclei on the line widths of 13C NMR sig-
parent carboranes are rather sharp, indicating only moder- nals [35]. In the cases of 1aϪ3a, this leads to significant
ately fast 11B quadrupole-induced relaxation, confirmed by sharpening of the 13C NMR signals, as expected. However,
measurements of the relaxation times T1(11B) [22]. Unre- the line widths of the signals are still in the order of 2 to
solved 11B-11B and 11B-10B spin-spin coupling will also con- 5 Hz, depending on the field strength (B0 ϭ 4.6 or 11.5 T)
tribute significantly to the line widths of the 11B NMR sig- of the spectrometer, the signals being slightly sharper at
nals [13]. A two-dimensional (2D) heteronuclear shift corre- lower field strength. Since the molecular shape of the parent
lation 13C/11B has been carried out in order to explore the carboranes rules out anisotropic behaviour in solution,
potential of this experiment for establishing 13CϪ11B con- there must be other reasons. One reason for residual broad-
nectivities [22]. The experiment seemed to be successful, ening could be efficient relaxation by 13CϪ1H dipole-dipole
1
Figure 1. 125.8 MHz 13C{1H} NMR spectrum of a mixture containing 1d along with 1a and 1c, 29Si satellites for J(29Si,13C) ϭ 59.1 Hz
are marked by asterisks (expansion for 1d).
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13C NMR and DFT Study of Dicarba-closo-dodecaborane(12) Derivatives
interactions. This question can be answered by studying splitting due to isotope effects is hidden in the broadened
mono-substituted carboranes. In such compounds, the signal. It is likely that the isotope effects can be detected
relaxation behaviour of the boron nuclei should have the for many carborane derivatives, if 13C NMR spectra are
same effects on both the substituted and unsubstituted 13C carefully measured observing conditions for best field
nuclei. At the same time, the increasing molecular size of homogeneity and sufficient digital resolution. This is shown
the carborane derivative causes faster 11B nuclear spin in Figure 3 for two examples of 1,2-disubstuted ortho-
relaxation, and 13C{11B,1H} experiments become unnecess- carborane derivatives which have previously been measured
ary. Figure 1 shows the 13C(carborane) NMR signals of a under routine conditions [36]. Improved conditions clearly
mixture containing 1a, 1c and 1d. The 13C(CH) NMR sig- reveal the isotope effects.
nals of 1c and 1d are somewhat broad, whereas the 13C(SiC)
NMR signals are sharper and split into several lines.
13C؊11B and 13C؊13C Spin-Spin Coupling in Carboranes
An inspection of the relative intensities of these lines
clearly indicates additive isotope-induced chemical shifts
The qualitative analysis of the origin of broad lines of
1Δ10/11B(13C), and their positions are a function of the num- 13C(carborane) NMR signals has shown that partially re-
ber of 10B atoms adjacent to the 13C nucleus in question. laxed 13C-11B spin-spin coupling plays a role which how-
In Figure 2, a comparison of these patterns is shown for ever, is not in agreement with the proposed [22] large magni-
the different isomers 1b, 2b and 3b. There are four boron tudes (ഠ 70 Hz) of these coupling constants. Although it is
atoms adjacent to each carbon atom in ortho-carboranes, difficult to obtain the accurate line widths of the 13C(car-
and five boron atoms are adjacent to the carbon atoms in borane) NMR signals, as outlined above, one can estimate
1
both meta- and para-carboranes, leading to characteristic [37] that a magnitude of J(13C,11B)Η < 10 Hz is much more
calculated intensity patterns, very close to those observed correct. This is in reasonable agreement with calculated
experimentally. Apparently, the influence stemming from data (vide infra).
boron atoms non-adjacent to carbon atoms is extremely
small.
A major challenge in carbon compounds is the determi-
1
nation of coupling constants J(13C,13C) at natural abun-
The isotope effects are resolved solely for the 13C NMR dance of 13C [38, 39]. In the case of the ortho-carborane
signals of the substituted carbon atoms, because residual derivatives, this task is even more difficult, considering the
broadening owing to strong 13CϪ1H dipolar interactions is broadening of the 13C NMR signals. Nevertheless, it should
absent. This is in contrast to the 13CϪ1H unit, where the be possible to overcome this problem by using more time
Figure 2. 125.8 MHz 13C{1H} NMR spectra of the SiϪCcarb resonances of the carborane isomers 1b, 2b and 3b, showing the pattern due
1
to the isotope-induced chemical shifts Δ10/11B(13C) together with the calculated intensity distributions for the neighbourhood of carbon
to four (1b) and five boron atoms (2b, 3b).
Z. Anorg. Allg. Chem. 2009, 1087Ϫ1093
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ARTICLE
Figure 3. 125.8 MHz 13C{1H} NMR spectra of two examples of 1,2-disubstituted ortho-carboranes [36] recorded and processed in order to
show the isotope-induced chemical shifts 1Δ10/11B(13C) typical of ortho-carboranes. 77Se satellites for 1J(77Se,13C) are marked by asterisks.
for the measurement. The next problem however, concerns
the approximate magnitude of this coupling constant. Thus,
“normal” single pulse 1D 13C{1H} NMR spectra of the
compounds 1b and 1c did not give useful results in spite of
excellent signal-to-noise ratios. In the case of 1d, the differ-
ence in chemical shifts Δδ13C(1,2) was too small. Different
experiments with the shortest INADEQUATE pulse se-
quence [40], in order to suppress the central line, using vari-
1
ous delays according to assumed values of J(13C,13C) fi-
nally gave the desired result for 1b (Figure 4). The coupling
constant 1J(13C,13C) ϭ 9.3 Hz is rather small and therefore,
satellite signals would be invisible, covered by the central
line, using single pulse 13C{1H} experiments.
DFT Calculations
The gas-phase geometries of the parent carboranes 1a,
Figure 4. 100.4 MHz 13C{1H} NMR spectra of the Cxarb-H signal
of 1b. The upper trace shows the result of the normal single pulse
experiments after 4000 transients, and the lower trace shows the
13C satellites as the result of an INADEQUATE experiment [40],
assuming 1J(13C,13C) ഠ10 Hz (relaxation delay 5 s, after 12 h, 5000
transients). The residual intensity of the central line is marked by
a filled circle.
2a, 3a and of the model compound 1-H3Si-1,2-C2Ba0H11
(1M) were optimised at the B3LYP/6-311ϩG(d,p) level of
theory [41, 42], and the coupling constants 1J(11B,11B),
1
1J(13C,11B) and J(13C,13C) were calculated [43Ϫ45] at the
same level of theory. Selected data are given in Table 1. The
calculated values for 1J(13C,11B) are small and of either
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13C NMR and DFT Study of Dicarba-closo-dodecaborane(12) Derivatives
Table 1. Selected calculated coupling constants /Hz for the carbora- icosahedral carboranes is reflected by spin-spin coupling in-
nes 1a, 1M, 2a, and 3a.
volving those nuclei forming the polyhedral frame. This is
J(13C,11B)
Ϫ0.4 (1Ϫ3) Ϫ5.0 (1Ϫ2) ϩ9.5 (3Ϫ4)
J(13C,13C)
1J(11B,11B)
a first attempt in this field to combine the knowledge about
1
1
1
coupling constants J(11B,11B), J(13C,11B) and J(13C,13C)
1,2-C2B10H12 (1a)
both by experiment and calculations, and this may well be
ϩ2.9 (1Ϫ4)
ϩ8.5 (4Ϫ5)
ϩ0.1 (1Ϫ12)
ϩ14.5 (7Ϫ12) extended to other nuclei being part of polyhedral clusters.
1-SiH3-1,2-C2B10H11 (1M) Ϫ1.7 (1Ϫ3) Ϫ10.5 (1Ϫ2) ϩ10.3 (3Ϫ4)
Ϫ0.6 (1Ϫ4) [exp. 9.3 (1b)] ϩ9.3 (4Ϫ5)
ϩ1.2 (2Ϫ3)
ϩ2.1 (2Ϫ7)
ϩ13.8 (7Ϫ12)
Experimental Section
1,7-C2B10H12 (2a)
ϩ6.2 (1Ϫ2) ϩ0.2 (1Ϫ7) ϩ2.5 (2Ϫ3)
The preparative work required at various stages exclusion of oxy-
gen and moisture, and the handling of the compounds was done
with great care to prepare samples of high quality for the NMR
experiments. The parent carboranes 1a, 2a and 3a (Katchem),
butyllithium in hexane (1.6 m) and the chlorosilanes (Aldrich) were
used as commercial products. NMR spectra were recorded at 23 °C
with Bruker WP 200, Bruker ARX 250, Bruker DRX 500 and
Varian Inova 400 spectrometers, all equipped with multinuclear
units, using C6D6 solutions (ca. 5Ϫ10 % v/v) in 5 mm tubes. The
instruments Bruker WP 200 and Bruker DRX 500 were equipped
with hardware for performing the 13C{1H,11B} heteronuclear
triple resonance experiments. Chemical shifts are given relative to
SiMe4 [δ1H (C6D5H) ϭ 7.15; δ13C (C6D6) ϭ 128.0; δ29Si ϭ 0 for
Ξ(29Si) ϭ 19.867184 MHz], BF3ϪOEt2 [δ11B ϭ 0 for Ξ(11B) ϭ
32.083971 MHz]. 29Si NMR spectra were measured using the re-
ϩ1.1 (1Ϫ4)
ϩ2.3 (1Ϫ6)
ϩ9.5 (3Ϫ4)
ϩ8.2 (4Ϫ5)
ϩ2.1 (8Ϫ12)
ϩ14.2 (9Ϫ12)
1,12-C2B10H12 (3a)
ϩ4.2 (1Ϫ2) Ϫ0.2 (1Ϫ12) ϩ7.3 (2Ϫ3)
ϩ16.6 (2Ϫ7)
sign. In organoboranes or organoborates with 2e/2c BϪC
bonds, the coupling constants 1J(13C,11B) are generally
>40 Hz and possess a positive sign [24], and the agreement
between experimental and calculated data is excellent [33,
34d]. Thus, multi centre bonding reduces expectedly the
BϪC s orbital overlap integral and gives negative contri-
butions to the dominant Fermi contact term in the spin- focused INEPT pulse sequence [based on 1J(29Si,1H) ഠ250 Hz (1b,
2b, 3b) or 3J(29Si,1HPh) ഠ 8 Hz (1c, d)] with 1H decoupling [47].
spin coupling mechanism. This is more pronounced for
Isotope-induced chemical shifts 1Δ10/11B(13C) are given in ppb with
electronegative elements such as nitrogen in aza-closo-
a negative sign indicating a shift of the 13C NMR signal of the
more heavy isotopomer to lower frequency.
dodecaborane NB11H12 [46] or carbon in the carboranes
discussed here than for the electropositive boron, where all
coupling constants 1J(11B,11B) possess positive signs and
The calculations were performed using the Gaussian 03 program
their calculated magnitudes are in the order expected and
experimentally determined for BϪBϪB multicentre bond-
ing [13, 16, 46].
package [48]. Frequencies were calculated analytically to character-
ise the stationary points of the optimised geometries as minima by
the absence of imaginary frequencies. The Fermi contact term is
dominant in the spin-spin coupling mechanism. Rather small con-
tributions were calculated for the paramagnetic spin-orbital term
(PSO) with less than 3 % to the coupling constants 1J(11B,11B),
1J(13C,11B) and 1J(13C,13C). The contributions from the spin-dipole
term (SD) and the diamagnetic spin-orbital term (DSO) were small
(<< 1 Hz) in all cases studied.
The huge data set known for coupling constants
1J(13C,13C) [38, 39] contains very few examples with a nega-
tive sign. Again, the small CϪC s orbital overlap integral
and negative contributions to the Fermi contact term are
responsible in the case of the ortho-carboranes. Electroposi-
tive substtuents such as a silyl group at the carbon atom
apparently enhance these negative contributions. The calcu-
1-Diphenylsilyl-closo-1,2-dicarbadecaborane (1b): ortho-Carborane
1a (0.35 g, 2.39 mmol) was dissolved in THF (40 mL), and the solu-
tion was cooled to Ϫ78 °C and kept stirring. After addition of BuLi
in hexane (1.49 mL, 2.39 mmol), the mixture was warmed to room
temperature, cooled again to Ϫ78 °C, and the chloro(diphenyl)-
silane (0.53 g, 1.39 mmol) was added in one portion. Then the mix-
ture was stirred for 2 h at room temperature. All volatiles were re-
moved, the solid residue was taken up in hexane (20 mL), insoluble
materials were filtered off, hexane was removed, and the product
was characterised by NMR spectroscopy. 1H NMR (400 MHz): δ ϭ
1.6Ϫ3.4 (m, 10 H, BH), 3.04 (s, 1 H, CH), 5.11 [s, 1J(29Si,1H) ϭ
219.9 Hz, 1 H, SiH], 7.2Ϫ7.4 (m, 6 H, Ph), 7.65 (m, 4 H, Ph). 13C
NMR (125.8 MHz): δ ϭ 58.1 (Ccarb-H), 61.4 [1J(29Si,13C) ϭ
lated value 1J(13C,13C)
ϭ Ϫ10.5 Hz for 1-H3Si-1,2-
C2B19H11 (1M) is numerically very close to the experimen-
tal value of 9.3 Hz for 1-Ph2(H)Si-1,2-C2B10H11 (1b) and
therefore, we propose a negative sign for this coupling con-
stant.
Conclusions
Isotope-induced chemical shifts Δ10/11B(13C) in deriva-
1
tives of ortho-, meta- and para-carboranes C2B10H12 have
been observed for the first time. This type of NMR param-
eter depends in a complex manner on dynamic and elec-
tronic effects of molecular structures. Although the main
body of data [28] refers to the effects by exchanging hydro-
gen against deuterium [1Δ1/2H(X)], the availability of more
powerful NMR instruments with ever higher field strengths
opens a more convenient access to isotope-induced chemi-
50.1 Hz,
Ccarb-Si], 128.3 (p-Ph), 128.9 (m-Ph), 131.8 [i-Ph,
1J(29Si,13C) ϭ 54.8 Hz], 135.8 (o-Ph). 11B NMR (128.3 MHz): δ ϭ
Ϫ13.4 (2 B), Ϫ11.2 (4 B), Ϫ6.5 (2 B), Ϫ0.5 (2 B). 29Si NMR
(99.4 MHz): δ ϭ Ϫ7.1.
Bis(closo-1,2-dicarbadecaboran-1-yl)diphenylsilane (1d): The same
procedure as for 1b was applied, starting from 1a (0.35 g,
cal shifts in general. The unique electronic structure of 2.43 mmol) and dichloro(diphenyl)silane (0.154 g, 1.215 mmol).
Z. Anorg. Allg. Chem. 2009, 1087Ϫ1093
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ARTICLE
[21] O. A. Subbotin, T. V. Klimova, V. I. Stanko, Yu. A. Ustynyuk,
Zh. Obshch. Khim. 1979, 49, 415.
The soluble product obtained contained also a small amount of 1c
(see Figure 1). 1d: 1H NMR (400 MHz): δ ϭ 1.6Ϫ3.6 (br., 20 H,
BH), 3.03 (s, 2 H, CH), 7.1Ϫ7.7 (m, 10 H, Ph). 13C NMR
(100.5 MHz, C6D6): δ ϭ 63.3 (C-H), 63.2 [1J(29Si,13C) ϭ 58.1 Hz,
[22] T. Faecke, S. Berger, Magn. Reson. Chem. 1994, 32, 436.
[23] A. Abragam, The Principles of Nuclear Magnetism, Oxford
University Press, 1961, pp. 305Ϫ315.
C
carb-Si], 126.0 [1J(29Si,13C) ϭ 80.6 Hz, i-Ph], 129.2 (m-Ph), 133.1
[24] B. Wrackmeyer, Prog. Nucl Magn. Reson. Spectrosc. 1979, 12,
(p-Ph), 137.9 (o-Ph).
227.
[25] M. Herberhold, H. Yan, W. Milius, B. Wrackmeyer, Chem. Eur.
J. 2000, 6, 3026.
[26] M. Herberhold, H. Yan, W. Milius, B. Wrackmeyer, Organo-
metallics 2000, 19, 4289.
1c: 1H NMR (400 MHz): δ ϭ 1.6Ϫ3.6 (br., 10 H, BH), 3.11 (s, 1 H,
CH), 7.1Ϫ7.6 (m, 10 H, Ph). 13C NMR (100.5 MHz): δ ϭ 61.8
(Ccarb-H), 65.0 (Ccarb-Si).
[27] M. Herberhold, H. Yan, W. Milius, B. Wrackmeyer, Chem. Eur.
J. 2002, 8, 388.
[28] C. J. Jameson. In E. Buncel, J. R. Jones (Ed.), Isotopes in the
Physical and Medical Sciences, vol. 2, pp. 1Ϫ54, Elsevier,
Amsterdam 1991.
[29] A. Laromaine, F. Teixidor, R. Kivekäs, R. Sillanpää, R. Be-
nakki, B. Grüner, Cl. Vin˜as, Dalton Trans. 2005, 1785.
[30] H. Beall, in E. L. Muetterties (Ed.), Boron Hydride Chemistry,
Academic Press, London 1975, p. 316 and references cited ther-
ein.
1-Diphenylsilyl-1,7- and -1,12-dicarba-closo-dodecaboranes(12) (2b)
and (3b): The carborane derivatives 2b and 3b were prepared in the
same way as 1b, only at a smaller scale, starting from the respective
parent carborane (70 mg), sufficient to run the NMR spectra. 2b:
1H NMR (500 MHz): δ ϭ 1.6Ϫ3.7 (m, 10 H, BH), 2.67 (s, 1 H, C-
H), 5.03 [s, 1J(29Si,1H) ϭ 216.8 Hz, 1 H, Si-H], 7.4 (m, 6 H, Ph),
7.7 (m, 4 H, Ph). 13C NMR (100.5 MHz): δ ϭ 59.0 (C-H), 66.2
[1J(29Si,13C)
ϭ
52.6 Hz,
Ccarb-Si], 129.0 (m-Ph), 131.1
[1J(29Si,13C) ϭ 74.5 Hz, i-Ph], 131.5 (p-Ph), 136.4 (o-Ph). 29Si NMR
[31] a) M. Bühl, P. v. R. Schleyer, J. Am. Chem. Soc. 1992, 114,
477; b) M. Bühl, in P. v. R. Schleyer (ed.): Encyclopedia of
Computational Chemistry, vol. 3, pp. 1835Ϫ1845, Wiley, Chich-
ester, 1999.
1
(99.4 MHz): δ ϭ Ϫ6.6. 3b: H NMR (500 MHz): δ ϭ 1.7Ϫ3.4 (m,
10 H, BH), 2.35 (s, 1 H, C-H), 4.91 [s1J(29Si,1H) ϭ 214.3 Hz, 1 H,
Si-H], 7.2 (m, 6 H, Ph), 7.6 (m, 4 H, Ph). 13C NMR (100.5 MHz):
δ ϭ 68.3 (C-H), 76.0 [1J(29Si,13C) ϭ 53.1 Hz, Ccarb-Si], 128.8 (m-
Ph), 131.1 [1J(29Si,13C) ϭ 73.9 Hz, i-Ph], 131.3 (p-Ph), 136.4 (o-Ph).
29Si NMR (99.6 MHz): δ ϭ Ϫ5.9.
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Acknowledgement
This work was supported by the Deutsche Forschungsgemeinschaft.
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Z. Anorg. Allg. Chem. 2009, 1087Ϫ1093

13C NMR and DFT Study of Dicarba-closo-dodecaborane(12) Derivatives
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Received: January 21, 2009
Published Online: March 19, 2009
Z. Anorg. Allg. Chem. 2009, 1087Ϫ1093
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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