Quantitative Assessment of Substitution NMR Effects in the Model Series of o-Carborane Derivatives: α-Shift Correlation Method

Article abstract of DOI:10.1021/acs.inorgchem.7b01023

The principles of a new α-shift correlation (ASC) NMR method are demonstrated on a model series of substituted derivatives of o-carborane for which reliable NMR data are available. This graphical method revealed an acceptable linear correlation between α(¹¹B) or α(¹³C) shifts and those induced by substituents in unsubstituted (u) positions of the carborane cluster. The linearity holds for all nuclei involved in skeletal bonding: Δδ(N)_u = g × α (where N = ¹¹B, ¹³C, and ¹H). The factor g (slope of the correlation line × 10²) becomes an important measure of sensitivity of a given cage position to substituent changes. The β, γ, and δ = A (= antipodal) shifts can be therefore derived from the α-shift, are linearly proportional, and reflect additive character in double substitution. The ASC method appears to be an important tool for quantitative assessment of substituent NMR effects in all exo-substituted boron-cluster systems.

Full text of DOI:10.1021/acs.inorgchem.7b01023

Article
pubs.acs.org/IC
Quantitative Assessment of Substitution NMR Effects in the Model
Series of o‑Carborane Derivatives: α‑Shift Correlation Method
̌
Bohumil Stíbr,* Oleg L. Tok, and Josef Holub
̌
Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 250 68 Rez, Czech Republic
̌
S
* Supporting Information
ABSTRACT: The principles of a new α-shift correlation (ASC) NMR
method are demonstrated on a model series of substituted derivatives of o-
carborane for which reliable NMR data are available. This graphical method
revealed an acceptable linear correlation between α(11B) or α(13C) shifts and
those induced by substituents in unsubstituted (u) positions of the carborane
cluster. The linearity holds for all nuclei involved in skeletal bonding: Δδ(N)u
= g × α (where N = 11B, 13C, and 1H). The factor g (slope of the correlation
line × 102) becomes an important measure of sensitivity of a given cage
position to substituent changes. The β, γ, and δ = A (= antipodal) shifts can
be therefore derived from the α-shift, are linearly proportional, and reflect
additive character in double substitution. The ASC method appears to be an
important tool for quantitative assessment of substituent NMR effects in all
exo-substituted boron-cluster systems.
demonstrate graphically on a series of selected o-carborane
INTRODUCTION
■
derivatives some new aspects of NMR substituent effects
operating within the B-cluster area characterized by delocalized
bonding. From the surprisingly poor selection available, we
chose a few rare examples of compound series for which
reliable and comparable 11B and 13C NMR data have been
available.
NMR spectroscopy has become a key method of structural
research in the field of B-cluster chemistry that still awaits
major development.1 Despite nearly 2000 publications on o-
carborane chemistry, contiguous series of specifically sub-
stituted derivatives have been rare even in this most studied
area of cluster-borane chemistry. Regardless that the [11B−11B]-
2
3
1
COSY and H-{11B(selective)} techniques have been long
known, NMR data are mostly published without assignments;
complete 1H NMR data for BH units are almost unavailable. As
a result, the number of substituted borane clusters for which all
11B, 13C, and complete 1H NMR parameters have been
measured is still incredibly low. For example, even in the
most trivial closo-C2B10H11-9-X and closo-C2B10H10-9,12-X2
series (where X = halogens and other substituents), chosen
for this work as important model compounds for demonstrat-
ing substituent effects owing to the broad range of α-shifts,
RESULTS AND DISCUSSION
■
It has been generally established that an arbitrary substituent X
attached to any B-cluster4 molecule induces a considerable
NMR shift at the substituted site (α-shift), whose magnitude (α
= αs − αp, where the symbols s and p relate to the substituted
and parent compound, respectively) is dependent on the
substituent nature and type of the molecule. Moreover, X-
substitution also invokes shift changes in all unsubstituted (u)
positions, as exemplified by all figures below. It was reasonable
to suppose that these changes must be in some way
proportional to α-shifts. Indeed, the graphs presented in this
work show that this assumption was correct. An important
aspect emerging from the figures is that chemical shift changes in
unsubstituted BH and CH positions are, in principle, linearly
proportional to α-shifts. In other words, the α-shifts directly
regulate shift changes (Δδu) in other positisons:
some 13C and detailed H NMR data were missing, Therefore,
1
the compounds had to be reprepared to obtain a complete set
of reliable values under standard conditions (measured with
high resolution on the same equipment and in the same
solvent), even though a number of miscellaneous incomplete
data have already been reported.1
Trends in the 11B NMR shielding effects for some exo-
substituted borane and carborane derivatives were reviewed by
11
Δδ(N)u = g × α( B or 13C) + c
(1)
̌ ́
Herman
ek in 1992.4 These included α (ipso), neighboring (β),
where N = 11B, 13C, and 1H; u relates to unsubstituted
positions, and c = constant of no relevance for NMR
assessments.
and remote (γ and δ = A = antipodal5−7) shifts for which effects
invoked by various substituents in the skeletal B-positions were
reported, but attempts at quantitative assessment of relations
among these parameters have not been done to date, except
perhaps for nonlinear correlations between chemical shifts and
halogen electronegativities.8−11 In this work we would like to
Received: April 23, 2017
© XXXX American Chemical Society
A
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The parameter g (gradient = the slope of the correlation line
×102 quoted in parentheses) varies for β, γ, and A positions and
becomes one of the specific NMR characteristics for a given
type of cluster and site of substitution that can be taken as a
measure of sensitivity in specific positions of a given structure to
substituent changes. We suggest that positive g factors indicate
the prevailing trend for shielding (on going from X = I to X =
F); negative values stand for deshielding.
Figures 1−4 illustrate the acceptable accuracy of the ASC
method when the measurements were performed under equal
Figure 2. A plot of Δδ(11B)u (black lines) and Δδ(13C)u (red lines)
shift changes vs α(11B) shifts for unsubstituted (u) cluster positions in
9-X-o-carboranes (X = EWG substituents, g × 102 values in
parentheses). Data from refs 4 and 13.
Figure 1. A plot of Δδ(11B)u (black lines) and Δδ(13C)u (red lines)
shift changes vs α(11B) shifts for unsubstituted (u) cluster positions in
9-X-substituted o-carboranes (X = halogens, bottom scale, this work).
Data for X = RC6H4 derivatives (upper scale, dotted lines) from ref 12
(mean g × 102 values in parentheses).
1
conditions with high resolution (500 and 600 MHz for H). It
was also verified that the linear behavior holds even for 13C and
1H nuclei involved in the skeletal bonding (experimental NMR
shifts for haloderivatives are in Figure S1; for selection of
authentic NMR spectra see Figures S2−S19). To recognize the
1
subtle changes in the H NMR shifts and their assignments to
individual BH vertexes, the use of high-resolution [11B−1H]
correlation techniques seems necessary. The elusive 9,12-F2
derivative has not yet been isolated (the reported 11B data14
with two 1:1 singlets are incompatible with the 9,12-F2
structure).
Figure 3. A plot of Δδ(11B)u (black lines) and Δδ(13C)u (red lines)
shift changes vs α(11B) shifts for unsubstituted (u) cluster positions in
9,12-dihalo o-carboranes (mean Δδ/g × 102 values in parentheses).
Data for substituted RC6H4-groups (upper scale, dotted lines) from ref
12.
Inspection of Figure 2 shows considerably steeper correlation
lines for a group of electron-withdrawing group (EWG)
substituents (RNH, OH, and F)4,13 exhibiting higher g values
than those for halogen and phenylene functions. Both figures
also reveal relatively high g values for antipodal (A) shieldings,
enhanced in the 9,12-substituted series by cumulative effects
due to disubstitution that also leads to remarkable shielding of
all positions, when compared to the monosubstituted counter-
part. The linearity is preserved even when a much more
detailed α-scale is used to outline shieldings for a group of
phenylene substituents.12 The graphs suggest, too, that different
substituent groups exhibit different shielding behavior and
correlation lines.
The next Figure 5 shows another interesting feature: the
α(11B) scale can be replaced by the “alkane” α(13C) scale,15
which is then used quite conveniently as a fixed reference scale
(αref) for ASC purposes, as it eliminates the sudden break in the
continuity of the correlation line for EWG substituents, so that
B
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disubstituted series derive from the combined A + β effect in
the C1,2 vertexes. In addition, the shift diagrams can be used
for accurate predictions of NMR shifts for all nuclei involved
(see the dotted lines for the elusive 9,12-F2 derivative and the
predicted spectrum in Figures 5 and S1).
The disubstituted o-carborane series was supplemented in
this work by the preparation of new 9,12-Cl,X o-carborane
derivatives (where X = Br and I) that contain two different
halogens in antipodal sites with respect to CH1,2. The
diagrams in Figure 6 reflect shift accumulation effects invoked
by the two different halogens at unsubstituted positions.
Figure 4. A plot of Δδ(1H)u (600 MHz) shift changes (blue lines) vs
α(11B) shifts for unsubstituted (u) cluster positions in 9-X- and 9,12-
X2-o-carboranes (g × 102 values in parentheses).
Figure 6. A plot of Δδ(11B)u (black lines) and Δδ(13C)u (red
lines)shift changes vs αref shifts for unsubstituted cluster positions in
closo-1,2-C2B10H10-9,12-ClX compounds; g × 102 values in paren-
theses. Measured in CDCl3; for Δδ values see Supporting Information,
Figure S1.
The supplementary Figure 7 demonstrates the validity of
linear behavior for 9-R substituted o-carborane derivatives (R =
Figure 5. A plot of Δδ(11B)u (192.6 MHz, black lines), Δδ(13C)u
(150.9 MHz, red lines), and Δδ(1H)u (600.2 MHz, blue lines) shift
changes vs αref shifts for unsubstituted (u) cluster positions for
halogenated closo-1,2-C2B11H11-9-X and 1,2-C2B11H10-9,12-X2 com-
pounds (g × 102 values in parentheses). Measured in CDCl3; for Δδ
values see Supporting Information, Figure S1.
all data points, inclusive of those for F, lie on the line. The
reason is that the αref scale reflects only electron-withdrawing
effects at the substituted site and thus removes the variances
that α(11B) shifts exhibit for individual B cages and sites of
substitution4 due to back-donation of substituent nonbonding
electrons to the cluster. As a result, both experimental α(11B) or
α(13C) and αref scales can be used for the ASC assessment of
substituent effects.
Furthermore, comparison of the g factors for mono- and
disubstituted compounds in Figure 5 clearly suggests that the
sensitivity in all unsubstituted sites of the o-carborane cage
approximately doubles after introducing one more substituent,
in agreement with earlier proposed shift additivity rules.4,16−22
It is also obvious that the resulting A(13C and 1H) effects in the
Figure 7. A plot of Δδ(11B)u shift changes vs α(11B) shifts for
unsubstituted (u) cluster positions in closo-1,2-C2B11H11-9-R com-
pounds (g × 102 values in parentheses). Measured in CDCl3; 64.18
MHz 11B data from ref 4.
C
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Me, SH, and SMe) bearing other than halogen substituents on
C1 atom, which must be taken with caution, as the authors did
not quote the Δδ shifts in acetone-d6.
The ASC correlation diagrams for the less-explored 3-R
derivatives (R = I, Br, alkyls, and aryls24−26) are illustrated in
Figure 9. Except for the 3-I derivative,25 no 11B assignments
4
1
the cage. Unfortunately, 13C and complete H parameters still
remain unknown, and it is therefore impossible to define the
1
A(13C) shift and H behavior.
A linear trend can be also traced in the NMR spectra of C-
substituted 1-R-closo-1,2-C2B10H11 compounds, which is
certainly the most studied field of o-carborane chemistry.
However, key derivatives were prepared in early stages of B
chemistry, and the data could not be considered. It was really
difficult to find a suitable set of structurally similar compounds
the NMR data of which have been collected under comparable
conditions; results from different laboratories cannot be often
compared, too. Among the plethora of various derivatives we
found two sufficiently long series of diverse simple substitution
on the carbon vertex. The ASC correlation picture is illustrated
in Figure 8. The lower diagram shows the A-shift/αref
Figure 9. (upper) A plot of Δδ(11B)u (black line) and Δδ(13C)u (red
line) shift changes vs α(11B)exp-shifts (CDCl3) for unsubstituted (u)
cluster positions in closo-1,2-C2B10H11-3-R-compounds (mean Δδ/g ×
102 values in parentheses). (bottom) Diagrams for a group of EWG
substituents.
Figure 8. (upper) A plot of Δδ(11B)u (163.5 MHz, black lines) and
Δδ(13C)u (125.75 MHz, red line) shift changes vs α(13C)exp-shifts
(acetone-d6) for unsubstituted (u) cluster positions in 1-R-closo-1,2-
C2B10H11 compounds23 (g × 102 values in parentheses). (bottom) A
plot of Δδ(11B)u (64.18 MHz, CDCl3, black line) shift changes4 vs αref
shifts for A-positions for various 1-R substituents.
were reported, and therefore we dared to assign tentatively only
some of the resonances. The 11B spectra for halogen and alkyl/
aryl-substituted compounds exhibit a standard A-shift behavior
(g = 17.0), but there is a different diagram for EWG-
substitution13 (F and amino derivatives; see the more detailed
view in the bottom diagram). Of interest is the exceptionally
low sensitivity and the unusualy mild trend for deshielding in
the otherwise strongly shielded antipodal B10 sites for EWG (g
= −4.8) together with a strong γ-shielding trend for B6 atoms
(g = 44.0) and a strong β-deshielding at B8 vertexes (g =
−12.7). The diagram also allows for predicting the δ(13C) shift
= 57.0 ppm for the 3-F compound. Nevertheless, a more
reliable insight into the 3-substituted family would be provided
if the 11B spectra were definitely assigned.
correlation for compounds equipped with functionalities of
broadly different character, which may perhaps account for the
apparent dispersity (max ≈ 2 ppm) around the correlation line.
1
No 13C or H measurements in this series have been reported
by the author.4 The upper diagram (note the fine 13C scale)
relates to the derivatives of 1-Me o-carborane23 that exhibit a
narrow span of α values and a good linear behavior on
correlation with the experimental α(13C) shifts. An unusual
feature is the β(13C) deshielding trend (g = −180, relative to
CDCl3 standard) for C2 vertexes neighboring the substituted
The 4-substituted o-carborane derivatives have so far been
very rare,27 and a contiguous series suitable for ASC treatment
has not been available.
D
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Table 1. Mean Shift (Δδ)/Sensitivity (g)/Max Deviation (ppm) Values for Unsubstituted Positions for Groups of Specifically
Substituted o-Carborane Derivatives
series
β-shift
γ-shift
A-shift
a
b
a
a
1-R alkyls
1-R others
3-R
2.8/33.0/0.2 8.3/−180.0/0.3
−0.08/7.6/0.2
0.9/39.0/0.2
a
−6.0/14.2/2.0
−2.0/17.0/0.3
−3.8/−4.8/0.3
−4.0/44.0/0.1 0.05/0.8/0.01
−5.5/15.0/0.1
b
a
4.0/7.0/0.3
a
b
a
a
3-R EWG
9-X halogens
9-R aryls
0.0/−12.7/0.2 1.8/−19.7/0.2
0.5/3.5/0.1 0.35/0.4/0.02
−0.5/44.0/0.3
a
c
a
b
c
c
b
c
−0.9/6.6/0.1 0.0/19.0/0.0 0.22/0.3/0.01 0.10/1.0/0.02
b
b
−1.5/9.5/0.1
−1.0/5.0
a
a
9-R others
9-R EWG
9,12-X2 halogens
0.5/11.0
a
b
a
b
−0.7/11.0/0.1 −4.0//28/0.2
1.5/9.5/0.2 0.55/1.6/0.02
−2.3/14.0/0.1
−12.0/63.0/0.3
−5.0/38.0/0.2 0.3/3.0/0.01
−4.5/40.0/0.0
a
c
a
c
c
b
c
−1.5/15.1/0.1 0.45/1.0/0.02 −0.05/0.1/0.0
a
b
b
9,12-X2 aryls
1.5/6.0/0.1
−1.5/15.0/0.1
a
b
c
11B-shift. 13C-shift. 1H-shift.
Table 1 shows that mean Δδ shifts (defined by positioning of
changes, apart from the fact that they take shift additivity effects
into account, too.
the center of the correlation line) usually exhibit shielding
growing in the series β < γ < A, depending on the substitution
site and type of the substituent. Maximum deviations from the
correlation line are in most cases within the ranges of 0.3 ppm
The ASC method becomes an important tool for quantitative
(numerical) assessment of substituent NMR effects that is
generally applicable to all exo-substituted boron-cluster systems.
Nowadays, when substitution reactions dominate B-cluster area
and NMR methods have become absolutely necessary for
structural investigations, any contribution to the recognition of
substitution phenomena might be helpful; extensions of the
ASC approach to other B-clusters are in progress in our
laboratories. Insights into the background of the ASC
phenomenon might perhaps call attention of theoretical
chemists as well.
1
for 11B and 13C and of 0.02 ppm for H nuclei. Comparison
of absolute g values in Table 1 is in accord with the prevailing
hierarchy of sensitivity of substituent shifts toward substituent
changes: A ≫ β ≥ γ. The A-effect is thus the most important
feature of the unsubstituted sphere, being typically of similar
magnitude for comparable derivatives discussed in this work (g
≈ 15−40 for 11B; 40−80 for 13C), though there are exceptions
for EWG substitution. Moreover, the shielding trends invoked
in unsubstituted positions are in most cases inversed to those
observed at substituted sites. For example, in the 3- and 9-
substituted series the shielding order in A- and γ-positioned
vertexes is inversed to that observed in α-positions.
The concept behind the ASC phenomenon may consist in
synergic activation of the + I and the + R resonance effects,4
which is in some respects analogical to substituent effects on
the benzene ring. The + I effect seems to operate mostly at the
α-site, while the + R effect is associated with electron donation
from the substituent electron pair to the substituted atom and
afterward to the skeleton as a whole, the + R effect being
enhanced by electrodeficiency at the cage α-vertex. Both of
these effects seem to be transmitted via cluster B−B, C−B, and
C−C bonds on the surface of the electron-deficient
pseudoaromatic carborane core.28
EXPERIMENTAL SECTION
■
Materials and Methods. All the reactions associated with the
synthesis of the haloderivatives of o-carborane were performed under
argon atmosphere. Dichloromethane and hexane were dried over
CaH2 and freshly distilled before use. Other conventional chemicals
were of reagent or analytical grade and were used as purchased. High-
resolution NMR spectroscopy was performed in the CDCl3 solvent
(296 K) at field strength of 14.1 T (600 Mz for 1H, 192.6 Hz for 11B,
and 150.9 Mz for 13C), inclusive of 11B HMQC and [11B−11B] COSY2
pulse sequences, which led to complete assignments of all resonances
to individual cage BH units. Chemical shifts in Figure S1 are given in
parts per million to high-frequency (low field) of Ξ = 32.083 971 MHz
(nominally F3B × OEt2 in CDCl3) for 11B (quoted 0.5 ppm), Ξ =
25.144 MHz for 13C (quoted 0.5 ppm), and Ξ = 100 MHz for H
1
(quoted 0.05 ppm), Ξ is defined as in ref 29, and the solvent
resonances were used as internal secondary standards. The g factors
relating to individual cluster sites were determined either graphically or
calculated on the basis of Δδ values given in Figure S1 (for
haloderivatives) and those calculated on the basis of literature data.
The synthesis of haloderivatives outlined below is based on
modification of previously reported procedures.14,30
closo-1,2-C2B10H11-9-X and closo-1,2-C2B10H10-9,12-X2 Deriv-
atives. A solution of o-carborane (150 mg, 1.04 mmol) in CCl4 (10
mL) was treated with anhydrous AlCl3 (∼10 mg), and the mixture was
stirred for 24 h at room temperature. After this time, monitoring by
11B NMR revealed complete disappearance of the B9 doublet signal of
1 and indicated a formation of a mixture of 9-Cl and 9,12-Cl2
derivatives. The mixture was carefully treated with water (5 mL)
and CH2Cl2 (20 mL). The organic layer was dried over MgSO4,
filtered, and evaporated with ∼5 g of silica gel. The material remaining
after evaporation of the solvent was mounted onto the top of a silica
gel packed liquid chromatography (LC) column (∼2.5 × 30 cm),
which was developed with 15% CH2Cl2−hexane to collect several
fractions. These were checked by 11B NMR for purity. Fractions
containing pure products were combined, evaporated to dryness, and
subjected to vacuum sublimation at ∼100−120 °C (bath) to isolate
CONCLUSIONS
■
The ASC method provides best results when NMR experi-
ments are done under identical, standard conditions, with
highest possible resolution. Evidently, the ASC parameters
differ for individual cluster shapes and sites, in particular, for
specific substituent groups (halogens, alkyls, aryls, amines, etc).
As demonstrated on a selected series of simple o-carborane
derivatives, the ASC approach has unambiguously revealed
inter-relations among chemical shifts operating within the
cluster space that hold for all nuclei involved in the skeletal
bonding. The main feature of the method is the trend for linear
correlation between α-shifts (αref, α(13C)exp or α(11B)exp) and
those induced in unsubstituted positions. Simply, all Δδ
changes in all cluster sites derive from the α-shift and can be,
for the first time, assessed quantitatively (see Table 1). The
easily obtainable g factors resulting from the method reflect the
measure of sensitivity of a given cage position to substituent
E
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̌
̌ ́ ̌ ̌
(6) Hermanek, S.; Gregor, V.; Stíbr, B.; Plesek, J.; Janousek, Z.;
∼60% and 15% of 9-Cl and 9,12-Cl2. The 9-Br and 9,12-Br2
compounds were isolated in a similar manner from the room-
temperature bromination with Br2 (∼1.5 molar excess) in CS2 (yields
∼52 and 17%, respectively) in the presence of AlCl3. The 9-I and 9,12-
I2 compounds were obtained in a similar way from the AlCl3-catalyzed
reaction with elemental I2 in refluxing benzene. All the compounds
were characterized by NMR (see Supporting Information, Figure S1;
for selected NMR measurements see Figures S2−S19).
Antonovich, V. A. Antipodal and Vicinal Shift Effects in 11B, 13C, and
1H NMR Spectra of Substituted Dicarba-closo-dodecaboranes(12).
Collect. Czech. Chem. Commun. 1976, 41, 1492−1499.
(7) Stanko, V. I.; Babushkina, T. A.; Klimova, T. P.; Goltiapin, Yu. V.;
Klimova, A. I.; Vasilev, A. M.; Alimov, A. M.; Khrapov, V. V.
Transmission of Effect of Substituents in B-Chloro-ortho Carboranes,
B-Chloro meta Carboranes and B-Chloro para Carboranes. Zh. Obsh.
Khim. 1976, 46, 1071−1079.
(8) Smith, W. L.; Meneghelli, B. J.; Thompson, D. A.; Klymko, P.;
McClure, N.; Bower, M.; Rudolph, R. W. Directive Effects in the
Electrophilic Substitution of Deltahedral Boranes and Heteroboranes.
Deuteration and Halogenation of 1-Thiadecaborane(9) and Thiado-
decaborane(11). Inorg. Chem. 1977, 16, 3008−3012.
closo-1,2-C2B10H11-9-F. The literature procedure was employed,
except that n-hexane was used as a solvent (yield ≈ 35%).
closo-1,2-C2B10H10-9,12-ClX (where X = Br and I) Derivatives.
To the solution of 9-halocarboranes closo-1,2-C2B10H10-9-X (X = Br
and I, reaction scale ∼1 mmol) in dried CCl4 was added anhydrous
AlCl3 (∼0.1 mmol), and the mixture was stirred overnight at ambient
temperature. The colorless solution was decanted from the brown oil,
CCl4 was removed in vacuo, and the rest was purified by flash
chromatography on a silica gel substrate, using a benzene−hexane
(1:3) mixture as the mobile phase. The corresponding 9,12-ClX
dihalocarboranes were isolated as white crystalline solids in 85−90%
yields.
(9) Sprecher, R. F.; Aufderheide, B. E.; Luther, G. W., III.; Carter, J.
C. Boron-11 Nuclear Magnetic Resonance Chemical Shift Assign-
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Soc. 1974, 96, 4404−4410.
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(10) Ringstrand, B.; Bateman, D.; Shoemaker, R. K.; Janousek, Z.
Improved Synthesis of [closo-1-CB9H10]− Anion and New C-
Substituted Derivatives. Collect. Czech. Chem. Commun. 2009, 74,
419−431.
ASSOCIATED CONTENT
* Supporting Information
■
S
̌
(11) Janousek, Z.; Hilton, C. L.; Schreiber, P. J.; Michl, J. C-
Halogenation of the closo-[CB11H12]− Anion. Collect. Czech. Chem.
Commun. 2002, 67, 1025−1034.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01023.
(12) Fox, M. A.; Wade, K. Model Compounds and Monomers for
Phenylene Ether Carboranylene Ketone (PECK) Polymer Synthesis:
Preparation and Characterization of Boron-Arylated Ortho-Carboranes
Bearing Carboxyphenyl, Phenoxyphenyl or Benzoylphenyl Substitu-
ents. J. Mater. Chem. 2002, 12, 1301−1306.
Graphical representation of 11B, 13C, and 1H NMR
spectra for 9-X, 9,12-X2, and 9,12-ClX-substituted o-
carboranes with numerical chemical shift values.
Examples of authentic NMR measurements on closo-
1,2-C2B10H10-9,-X (where X = Cl and F) and closo-1,2-
C2B10H10-9,12-ClBr dicarbaboranes (PDF)
(13) Sevryugina, Y.; Julius, R. L.; Hawthorne, M. F. Novel Approach
to Aminocarboranes by Mild Amidation of Selected Iodo-carboranes.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: stibr@iic.cas.cz.
■
ORCID
̌
Bohumil Stíbr: 0000-0003-4010-4106
Notes
The authors declare no competing financial interest.
̌
̌ ̌
k, J.; Stíbr, B.; Fusek, J.; Mares,
(16) Herm
F. In Boron Chemistry; Herm
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̌
an
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ek, S.; Jelínek, T.; Plese
ACKNOWLEDGMENTS
The work was supported by the Grant Agency of the Czech
Republic (Project No. 16-01618S).
■
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anek, S., Ed.; World Scientific: Singapore,
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