o-Carborane derivatives for probing molecular polarity effects on liquid crystal phase stability and dielectric behavior

Article abstract of DOI:10.1039/c5tc02046h

A series of mesogenic derivatives of o-carborane was synthesized, their properties were analyzed by thermal, optical and XRD methods, and results were compared with those of isostructural p-carborane and benzene analogues. Comparative analysis revealed lower nematic phase stability and enhanced smectic behavior, including SmC, in the o-carborane derivatives relative to the isosteric p-carborane analogues. The effect of o-carborane on the electrooptical properties was assessed for biphenyl derivative 1[B]a in the 6CHBT nematic host giving the extrapolated Δε = 11.0, and a moderate increase of the elastic constants K_ii. Complete analysis of the dielectric results for o-carborane and p-carborane analogues 1[B]a and 1[A]a was performed using the Maier-Meier formalism augmented by DFT computational methods.

Full text of DOI:10.1039/c5tc02046h

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o-Carborane derivatives for probing molecular
polarity effects on liquid crystal phase stability
and dielectric behavior†
Cite this:DOI: 10.1039/c5tc02046h
Jacek Pecyna,ab Aleksandra Jankowiak,a Damian Pociechac and Piotr Kaszynski*abde
´
A series of mesogenic derivatives of o-carborane was synthesized, their properties were analyzed by
thermal, optical and XRD methods, and results were compared with those of isostructural p-carborane
and benzene analogues. Comparative analysis revealed lower nematic phase stability and enhanced smectic
behavior, including SmC, in the o-carborane derivatives relative to the isosteric p-carborane analogues. The
effect of o-carborane on the electrooptical properties was assessed for biphenyl derivative 1[B]a in the
6CHBT nematic host giving the extrapolated De = 11.0, and a moderate increase of the elastic constants Kii.
Complete analysis of the dielectric results for o-carborane and p-carborane analogues 1[B]a and 1[A]a was
performed using the Maier–Meier formalism augmented by DFT computational methods.
Received 7th July 2015,
Accepted 28th September 2015
DOI: 10.1039/c5tc02046h
www.rsc.org/MaterialsC
Introduction
During the past two decades para-carborane,1 [closo-1,12-C2B10H12]
(A, Fig. 1), has become an attractive linear structural element of
liquid crystals,2–4 molecular construction sets,5,6 and pharmaco-
logical compounds,7–9 owing primarily to its geometry, symmetry,
and chemical reactivity. In contrast, the two isomers of A, ortho-
carborane (B) and meta-carborane (C) have received much less
attention, even though they have additional properties that are of
particular interest for certain molecular and functional designs.
For instance, in contrast to A, ortho- and meta-carboranes have
ground-state dipole moments of 4.53 D and 2.85 D, respec-
tively,10,11 which are oriented at about 301 to the 1,12-axis for B
and nearly parallel to the 2,9-axis in C (Fig. 1). The different
magnitude and orientation of the molecular dipole moment in
Fig. 1 The structures of three isomeric carboranes [closo-C2B10H12], A, B
and C, and their disubstituted derivatives IA, IB, and IC, respectively. Each
vertex represents a BH fragment and the sphere is a carbon atom. The arrow
represents the electric dipole vector of the cluster (values from ref. 10).
the three essentially isosteric carboranes are of interest for dipole in smectic phase induction and liquid crystalline phase
fundamental studies of the liquid crystal phenomenon and for stabilization in general.14,15 A comparison of the properties of
development of materials for LCD applications.12,13 One such a isosteric mesogenic derivatives IA–IC would provide an opportu-
fundamental question relates to the role of the molecular electric nity for such investigation, and also for the development of new
polar materials for electrooptical applications.
a Department of Chemistry, Vanderbilt University, Nashville, TN 37235, USA.
E-mail: piotr.kaszynski@vanderbilt.edu; Tel: 1-615-322-3458
b Department of Chemistry, Middle Tennessee State University, Murfreesboro,
TN 37132, USA
Recently, we have reported practical access to isomerically
pure 1,12-difunctionalized derivatives of o-carborane,16 which
opened up the door to investigation of new classes of liquid
crystals of type IB. In this context two such derivatives, 1[B]a
and 2[B]e (Fig. 2), were prepared and their preliminary studies
indicated a significant smectogenic character,16 which warrants
further detailed investigation. Here we describe the synthesis
and characterization of a series of isostructural derivatives 1
and 2 (Fig. 2) in which isosteric carborane derivatives of types
IA and IB are compared to those of benzene analogues. The
smectic phases are analyzed by powder XRD and selected
c Department of Chemistry, University of Warsaw, Zwirki i Wigury 101,
02-089 Warsaw, Poland
d
´ ´
´ ´
Faculty of Chemistry, University of Łodz, Tamka 12, 91-403 Łodz, Poland
e Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences,
´ ´
Sienkiewicza 112, 90-363 Łodz, Poland
† Electronic supplementary information (ESI) available: Details of thermal and
dielectric analysis of binary mixtures, additional XRD data, partial results of DFT
calculations, archive of equilibrium geometries for 1[A]a and 1[B]a. See DOI:
10.1039/c5tc02046h
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transhalogenation of the analogous bromide17 3[A] (Hal = Br,
n-BuLi followed by I2).
Dimethylene-linked derivatives 2f were obtained by Pd-
catalyzed hydrogenation23 of the previously reported stilbenes
2e,16,23 as shown for 2[B]f in Scheme 2.
The preparation of aniline derivative of p-carborane 4[A] is
shown in Scheme 3. The first step is a one-pot alkylation–
arylation24 process, which gave the desired nitrophenyl deriva-
tive 6[A] in 20% yield after careful chromatographic separation
from several by-products. Subsequent catalytic reduction of
6[A], as described for 6[B],16 gave aniline 4[A].
Fig. 2 The structures of investigated compounds. Definitions of rings A
and B are shown in Fig. 1.
The nitroso ester 5 was prepared by partial oxidation of the
known25,26 4-aminobenzoate ester 7 with Oxones, according to
a general literature procedure27 (Scheme 4).
derivatives are investigated by dielectric methods as low con-
centration additives in a nematic host. Experimental results
are augmented using DFT calculations and dielectric data are
analyzed by the Maier–Meier formalism.
Thermal analysis
Transition temperatures and enthalpies of compounds 1 and 2
were determined by differential scanning calorimetry (DSC).
Phase structures were assigned by optical microscopy in polarized
light and confirmed by powder XRD measurements. The results
are shown in Tables 1 and 2 and Fig. 3–5.
Results
Synthesis
Biphenyls 1a were obtained from appropriate haloarenes 3 in
C–C cross-coupling reactions with 4-C8H17OC6H4M (Scheme 1):
Suzuki for 1[A]a (Hal = Br,17 M = B(OCH2)2),18 Negishi for 1[B]a
(Hal = I, M = ZnCl),16 and Kumada for 1[Ph]a (Hal = I,19 M = MgBr),18
and their details are described elsewhere. The iodoarenes 3 (Hal = I)
were also used for a modified Sonogashira20 coupling reaction
with 4-C8H17OC6H4CCH21,22 and for the preparation of tolanes
1b (Scheme 1); the corresponding bromides were ineffective in
this reaction. The reaction was conducted at ambient tempera-
ture in the presence of anhydrous non-nucleophilic Hunig’s base
to avoid amine- and hydroxide-promoted deboronation of
o-carborane derivatives.
Scheme 2 Synthesis of liquid crystal 2[B]f. Reagents and conditions: (i) H2
(50 psi), Pd/C (10%), THF, rt.
Anilines 4[A], 4[B],16 and 4[Ph]5 served as suitable precursors
to Schiff bases 1c and azobenzene derivatives 1d obtained by
acid-catalyzed condensation with 4-C8H17OC6H4CHO and 4-nitro-
sobenzoate ester 5, respectively (Scheme 1). Aniline 4[B] was also
converted to the corresponding iodide 3[B] (Hal = I) by a modified
Scheme 3 Synthesis of aniline 4[A]. Reagents and conditions: (i) 1.
n-BuLi, THF, ꢀ78 1C - 0 1C; 2. n-C5H11I, ꢀ78 1C - rt; 3. DME, n-BuLi,
ꢀ78 1C - rt; 4. CuI, 0 1C; 5. Pyridine; 6. 4-IC6H4NO2, reflux; (ii) H2 (40 psi),
Sandmeyer reaction,16 while iodide 3[A] (Hal = I) was obtained by Pd/C (10%), THF, rt.
Scheme 1 Synthesis of 1. Reagents and conditions: (i) [A]: Hal = Br, M = B(OCH2)2, Pd(PPh3)4, Na2CO3, benzene, reflux (ref. 18); [B]: Hal = Br, M = ZnCl,
Pd(dba)2, PChx3, THF, reflux (ref. 16); [Ph]: Hal = I, M = MgBr, NiBr2, THF, reflux (ref. 18); (ii) Hal = I, (i-Pr)2EtN, Pd(PPh3)2Cl2, CuI, THF, rt; (iii) EtOH, cat.
AcOH, reflux; (iv) [A] and [C]: CH2Cl2, cat. AcOH, rt; [B]: AcOH, rt; (v) [B], Hal = I: (1) [NO]+[PF6]ꢀ and (2) [Bu4N]+[I]ꢀ, MeCN, 0 1C (ref. 16).
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Scheme 4 Synthesis of nitroso ester 5. Reagents and conditions: (i) (S)-2-
methylbutan-1-ol, pyridine; (ii) H2 (40 psi), Pd/C (10%), THF, rt; (iii) Oxones
CH2Cl2/H2O, rt.
,
All carborane derivatives in series 1 form a nematic phase
with clearing temperatures in a range of 78–177 1C, while
smectic phases were found only in 1[A]a, 1[B]a–1[B]c (Table 1,
Fig. 3 and 4). Compounds 2[B]e and 2[B]f have the lowest TNI in
the series and are the only compounds with monotropic behavior.
Surprisingly, however, crystalline polymorphs obtained from the
melt have significantly lower melting points (63 1C16 and 69 1C,
respectively) and the nematic phase becomes enantiotropic for
both compounds. Such a large difference in melting temperatures
of crystalline polymorphs (DT = 58 K for 2[B]e) is rarely observed
for mesogens and has never before been observed for carborane
derivatives.
Comparative analysis demonstrates that o-carborane deriva-
tives 1[B] have lower clearing temperatures (Tc) and higher
smectogenic character than the p-carborane analogues 1[A]. For
the first four members of the series, 1a–1d, the difference in
Tc is small (average DTc = 7 ꢁ 2 K; Table 1, Fig. 5), while
the difference for the pairs 2e and 2f is significantly greater
Fig. 3 DCS trace for 1[B]b; heating rate 5 K minꢀ1. The inset shows a
magnified portion of the trace with the SmA–N and N–I transitions.
(34 K and 45 K, respectively; Table 2). This decrease in phase
stability upon substitution of o-carborane for p-carborane
indicates the importance of lateral dipole–dipole interactions
and arising from them the relative difficulties in molecular
packing in the mesophase particularly for 2[B]e and 2[B]f in which
the bulky carborane unit is at the center of the rigid core. This
corroborates with the observed significant increase of smecto-
genic behavior upon replacement of p-carborane with the isosteric
o-carborane. Thus, a SmA phase is more stable by 76 K in 1[B]a
than in the p-carborane analogue 1[A]a, and in compounds 1b
and 1c this stabilization is at least 45 K (Table 1). Moreover, a
monotropic SmC phase was also detected in 1[B]a (Fig. 4c).
Table 1 Transition temperatures (1C) for 1a
R
a
b
c
–C6H4–OC8H17
–CQCC6H4–OC8H17
–NQCHC6H4–OC8H17
Cr 71 (SmA 45) N 149 Ib
Cr 90 N 176 I
Cr 111 N 177 I
Cr 58 (SmC 38) SmA 121 N 144 Ic
Cr 114 SmA 140 N 168 I
Cr 101 SmA 157 N 171 I
Cr 97 E 194 SmB 211 SmA 222 Ib
Cr 96 Cr 156 B 177 N 206 I
Cr 95 (H? 90) G 138 SmF 150 SmI 166 SmC
191 SmA 198 N 214 I
d
–NQNC6H4–COOC5H11
*
Cr 94 N* 160 I
Cr 124 N* 151 I
Cr o25 E 115 SmB 136 SmA* 193 I
a
b
c
Enthalpies are reported in the ESI. Cr-crystal, N-nematic, Sm-smectic, soft-crystalline phases B, E and G. Ref. 18. Ref. 16.
Table 2 Transition temperatures (1C) for 2a
L
e
f
–CHQCH–
–CH2CH2–
Cr 124 N 135 Ib
Cr 94 N 123 Ib
Cr 121 (N 101) Ic
Cr 81 (N 78) I
Cr 72 X 209 G 254 SmC 258 SmA 281 N 285 Ib
Cr 118 Bd 173 Ib
a
d
b
c
Enthalpies are reported in the ESI. Cr-crystal, N-nematic, Sm-smectic, soft-crystalline phases B and G, X-unidentified phase. Ref. 23. Ref. 16.
Previously an E phase was reported.
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Fig. 6 Equilibrium geometry for 1[A]a (upper) and 1[B]b (lower) at the
B3LYP/6-31G(2d,p) level of theory.
Fig. 4 Optical textures obtained by cooling from the isotropic phase of
1[B]a: (a) N (135 1C), (b) SmA (100 1C) and (c) SmC phase (30 1C), and of
1[Ph]d: (d) E phase (100 1C).
was significantly stabilized in the o-carborane derivatives 1[B]b
and 1[B]c relative to 1[B]a.
Molecular modeling
For a better understanding of the powder XRD results and the
impact of substitution of p-carborane with o-carborane on
the molecular dimensions, molecular structures of selected
compounds were fully optimized at the B3LYP/6-31G(2d,p) level
of theory. Analysis of the results demonstrated that the struc-
turally analogous o-carborane and p-carborane derivatives are
essentially isosteric with the total length of about 31 Å (for 1a)
or 33 Å (for 1b and 1c) in the most extended conformations
(Fig. 6).28 The benzene analogues are about 0.7 Å shorter than
the corresponding carborane derivatives.
Fig. 5 Trends in mesophase stability (N–I or SmA–I transition tempera-
tures) as a function of the linking group L in series 1.
Powder XRD data
Analysis of the smectic phases formed by o-carborane (B)
derivatives revealed typical behavior: the layer thickness in the
SmA phase slightly increases with decreasing temperature due to
diminishing thermal motion (e.g. k = ꢀ9.65 ꢁ 0.08 pm Kꢀ1 for
1[B]b), while the SmC layer in 1[B]a contracts rapidly due to the
progression of the molecular tilt.28 The observed interlayer spacing
in the SmA phase is about 2 Å shorter than the calculated
molecular lengths.
XRD analysis of benzene derivatives 1[Ph]a–1[Ph]c con-
firmed their rich smectic polymorphism, as shown for 1[Ph]c
in Fig. 7. Thus, below the SmA and SmC phases, there are more
organized tilted smectic (SmI, SmF) and soft crystalline phases
with transitions well marked by the changes in the intensity of
the diffraction signal related to the layer thickness. Phase
identification was confirmed by analysis of the wide angle
range of XRD patterns.28 The transition from the SmC having
liquid-like layers to a hexatic SmI is accompanied by a relatively
large enthalpy of transition (3.6 kJ molꢀ1), which is consistent
with increasing positional correlations of molecules within the
smectic layers. The measured layer spacing in the SmA phase is
32.3 Å, which correlates well with the calculated molecular
length of 32.4 Å. Similar analysis was performed for other
benzene derivatives and details are listed in the ESI.†
Although o-carborane derivatives have relatively strong ten-
dency to the formation of lamellar phases, this behavior is far
from the smectogenic character of the benzene analogues
1[Ph]. For instance, the SmA phase is more stable by over
100 K in 1[Ph]a than in 1[B]a, and Schiff base 1[Ph]c and stilbene
1[Ph]e exhibit rich smectic polymorphism (Tables 1 and 2). This
observed difference in smectogenic behavior is related to the
shape of the carborane and benzene and, consequently, to
molecular packing of their derivatives in lamellar phases, as
it was already postulated for p-carborane derivatives.2 Interest-
ingly, chiral azo ester 1[Ph]d does not crystallize and exhibits an
E phase at ambient temperature (Fig. 4d).
Series of 1a–1c also permitted an analysis of the impact of
the linking group L on the mesophase stability. Thus, insertion
of a two-atom fragment between the benzene rings in the
terphenyl derivative 1[Ph]a lowered the mesophase stability in
1[Ph]b and 1[Ph]c by nearly 20 K (Fig. 5) presumably due to
increased flexibility of the core and lower packing fraction. In
contrast, the analogous transformation of carborane derivatives
1[A]a and 1[B]a stabilized the nematic phase by about 25 K in
the corresponding derivatives 1b and 1c. Also the SmA phase
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Fig. 7 Layer spacing d and intensity of the related XRD signal for 1[Ph]c as
a function of temperature.
Fig. 9 Dielectric parameters of binary mixtures of 1[A]a (black) and 1[B]a
(blue) in 6CHBT as a function of concentration.
XRD analysis of 2[Ph]f revealed a soft crystalline B phase
instead of an E phase, which was previously reported23 on the
basis of microscopic observations.
phase of p-carborane 1[A]a is stabilized by nearly 30 K in the
host relative to the pure compound. Additional stabilization of
the nematic phase is also apparent for 1[B]a at higher concen-
trations (Fig. 8); however at concentrations o10 mol% the
mixture exhibits nearly ideal behavior.
Dielectric analysis of the binary mixtures in 6CHBT at 1 kHz
revealed linear dependence of dielectric parameters on concen-
tration (Fig. 9), which, after extrapolation, established the dielec-
tric values shown in Table 3. As expected, the polar derivative
1[B]a has a substantial longitudinal dielectric permittivity com-
ponent eJ = 17.5 and, consequently, a significant positive dielectric
anisotropy De = 11.0. In contrast, the p-carborane analogue 1[A]a
has a small negative De = ꢀ1.1, resulting from an unrealistically
low extrapolated eJ = 0.2.
The carborane additives also affect elastic constants of the
host: both compounds systematically increase splay and twist
constants K11 and K22, although the effect is smaller for the
o-carborane derivative 1[B]a. For instance, both values are higher
by about 30% for 10 mol% solutions. Further analysis indicates
that the p-carborane derivative 1[A]a lowers the rotational viscosity
of the host, while the results for the 1[B]a analogue are ambiguous.
Binary mixtures
To assess the impact of the molecular dipole moment of
o-carborane on electrooptical properties, two isostructural
biphenyls 1[A]a and 1[B]a were investigated as low concen-
tration additives to 6CHBT,28–30 which is an ambient tempera-
ture nematic characterized by a positive De = +8.06.
Thermal analysis of the binary mixtures demonstrated a
linear dependence of the nematic–isotropic transition on
concentration for the p-carborane derivative 1[A]a, while the
correlation for the o-carborane analogue 1[B]a deviates from
linearity above 10 mol% (Fig. 8). Linear extrapolation of the
mixture’s N–I transition peak temperatures to the pure additive
gave the virtual N–I transition temperatures [TNI] of 187 ꢁ 3 1C
for 1[A]a and 148 ꢁ 3 1C for 1[B]a in 6CHBT. A comparison of
the clearing temperatures for pure compounds (Table 1) with
the extrapolated values [TNI] demonstrates that the nematic
Analysis of dielectric data
Dielectric parameters extrapolated for pure additives were ana-
lyzed using the Maier–Meier relationship (eqn (1)),31,32 which
connects molecular and phase parameters. Using experimental
eJ and De values (Table 3) and DFT-calculated parameters m, a,
and b (Table 4), eqn (2) and (3) permitted the calculation of the
apparent order parameter Sapp and the Kirkwood factor g = meff2/m2,
as shown in Table 3. The effect of the additive was neglected and
the field parameters F and h in eqn (2) and (3) were calculated
using the experimental dielectric and optical data for the pure
6CHBT host.28–30
ꢀ
ꢁ
2
À
Á
NFh
Fmeff
2kBT
De ¼
Da ꢀ
1 ꢀ 3 cos 2b
S
(1)
e0
2Dee0
Sapp
¼
2
2
ꢀ
ꢀ
NFh½2Da þ 3að1 ꢀ 3 cos bÞꢂ ꢀ 3ðe ꢀ 1Þe0ð1 ꢀ 3 cos bÞ
Fig. 8 A plot of peak temperature of the N–I transition for binary mixtures
of 1[A]a (circles) and 1[B]a (diamonds) in 6CHBT.
(2)
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Table 3 Extrapolated experimental (upper) and predicted (lower) dielec-
tric data and results of Maier–Meier analysis for 1[A]a and 1[B]aa
The experimental data in Table 3 show that the dielectric
parameters for 1[B]a are smaller than expected, especially the eJ
component (17.5 instead of 26.9). Consequently, the calculated
apparent order parameter Sapp is lower than that for the host,
and the Kirkwood factor g is unusually small (0.28) at concen-
trations o10 mol%; the latter parameter may indicate signifi-
cant aggregation in the solution, while the Sapp suggests poor
alignment of the molecules (presumably aggregates) in the
nematic phase. The dielectric results for the p-carborane ana-
logue 1[A]a are more difficult to rationalize: while the value of
Sapp is small but acceptable, the negative g factor is physically
unrealistic, although consistent with the unrealistically low
extrapolated eJ = 0.2 (predicted 5.0, Table 3). Similar results
were obtained for solutions of another p-carborane derivative in
6CHBT,19 which presumably reflect molecular dynamics at 1
kHz and specific solute–solvent interactions. In all cases, the
unusually low extrapolated eJ results in excessively negative
extrapolated De values.
Compd
eJ
e>
De
Sapp
g
1[A]a
Exp
Theory
Exp
0.2
1.3
ꢀ1.1
1.5b
0.45
0.67c
0.60
0.67c
ꢀ2.06
0.45c
0.28
5.0b
17.5
3.5b
6.5
1[B]a
11.0
Theory
26.9b
7.9b
19.0b
0.45c
a
For details see the text and the ESI. Typical error of extrapolated
b
dielectric parameters is ꢁ0.20. Calculated value assuming host’s
c
order parameter S = 0.67 and g = 0.45. Assumed values.
Table 4 Calculated molecular parameters for 1[A]a and 1[B]aa
Compd
mJ/D
m>/D
m/D
bb/1
Da/Å3
a
avrg/Å3
1[A]a
1[B]a
1.63
8.24
1.12
2.93
1.98
8.75
34.5
19.6
48.2
49.9
84.6
85.3
a
Obtained at the B3LYP/6-31+G(2d,p)// B3LYP/6-31G(2d,p) level of
theory in the 6-CHBT dielectric medium. For details see the text and
b
the ESI. Angle between the net dipole vector m and mJ.
ÂÀ
Á
Ã
Discussion
2
3
ꢀ
e ꢀ 1 e0 ꢀ aNFh ꢀ DaNFhSapp 3kBT
k
Â
Ã
g ¼
(3)
The recent availability16 of isomerically pure derivatives 8 and 9
has opened the way to a new class of polar mesogens based on
o-carborane (Fig. 11). In general, these currently available
precursors permit incorporation of motifs I and II into the
molecular structure as either a terminal or a central part of the
rigid core. Thus far, the B(12)–I group was used for B-alkylation
(Negishi pentylation of 8) and B-arylation (pentyloxyphenyla-
tion of 9) reactions, and the vinyl group in 9 was arylated in
Heck coupling;16 however, other C–C coupling reactions and
synthesis of other intermediates are possible to expand the
structural variety and fine-tuning of molecular and bulk proper-
ties of the materials. The first series of o-carborane-containing
mesogens presented here was obtained using mainly 4-
halophenyl derivatives 3[B] and aniline 4[B], which can serve
as versatile precursors to other similar mesogenic derivatives.
Results for the handful of available mesogenic o-carborane
derivatives demonstrate that compounds containing fragment I
(series 1[B]) exhibit a greater propensity for the formation of
smectic phases than those in series 2[B]. This is presumably
because of the position of the bulky carborane unit in the
molecule: in 1[B] it resides at the edge of the rigid core, which is
more favorable for the formation of lamellar phases further
enhanced by lateral dipole–dipole interactions.
NF2hm2 1 ꢀ ð1 ꢀ 3 cos 2bÞSapp
The molecular electric dipole moment m and the polariz-
ability a required for the Maier–Meier analysis of dielectric
results were obtained at the B3LYP/6-31+G(2d,p)//B3LYP/6-
31G(2d,p) level of theory in the dielectric medium of the host.28
The data in Table 4 demonstrate that the replacement of p-
carborane with o-carborane increases particularly strongly the
longitudinal dipole moment component. The modest dipole
moment of the p-carborane derivative 1[A]a, m = 1.98 D, results
from a combination of mainly two local dipole moments
associated with the weakly polar alkoxyphenyl group and
polarization from the electron-donating alkoxy group to the
electron withdrawing carborane unit (sp = 0.12).33 The resulting
net dipole is oriented at 34.51 relative to the molecular axis of
inertia. Substitution of the o-carborane for p-carborane in 1[A]a
increases the polarization of the rigid core in 1[B]a due to a
stronger electron-withdrawing character of the cluster (sp
=
0.43)34 and also introduces a reinforcing additional local dipole
associated with the cage itself (Fig. 10). This results in a
substantial longitudinal dipole moment component, mJ = 8.24
D, and a smaller angle b E 201 (Table 4). Thus, on the basis of
the calculated molecular dipole moment it is expected that the
p-carborane derivative 1[A]a will exhibit a small positive value of
dielectric anisotropy, De = 1.5, while the o-carborane 1[B]a will
possess a significant De = 19.0, assuming the host’s order
parameter S = 0.67 and a reasonable Kirkwood factor g = 0.45
(Table 3). Experimental dielectric values are, however, signifi-
cantly smaller.
Fig. 11 Precursors 8 and 9 and derived from them molecular fragments
I and II.
Fig. 10 Major local dipole moments in 1[B]a.
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oriented along the main axis, which results in sizable dielectric
anisotropy (De = 11.0 in the 6CHBT host at 1 kHz) and makes
such derivatives attractive for electrooptical applications. Also
the combination of smectogenic character and the inherent
polarity of o-carborane derivatives is of interest for the devel-
opment of materials for FLC and AFLC applications. Initial
results also suggest that derivatives of type 1 may have a
relatively low cross-over frequency fc and thus may be attractive
for dual-frequency devices. These results warrant further
exploration of mesogenic carborane derivatives as additives to
nematic and smectic hosts.
Fig. 12 A pseudo-Newman projection of a conformational minimum of
1-phenyl-o-carborane (ref. 35). The bar represents the benzene ring plane
and the spheres are the carbon atoms.
A comparison of the mesogenic derivatives of o-carborane
with the isosteric p-carborane analogues provides an excellent
opportunity for a better understanding of the fundamental
issue of the role of a molecular dipole moment in phase
stability. In recent years, we have demonstrated that replace-
ment of the non-polar C–C bond in carborane derivatives with
the isosteric polar B–N bond introduces a substantial long-
Computational details
itudinal dipole moment of 12 D in 10-vertex and 10 D in Quantum-mechanical calculations were carried out using the
12-vertex derivatives.14,15 This replacement generally increases Gaussian 09 suite of programs.40 Geometry optimizations for
the nematic phase stability by up to 55 K, and, surprisingly, does unconstrained conformers of 1a with the most extended mole-
not induce smectic polymorphism. In the present work, we cular shapes were undertaken at the B3LYP/6-31G(2d,p) level of
compare isomeric carborane derivatives, which result from the theory using default convergence limits. Vibrational frequencies
exchange of positions of cluster’s B and C atoms. The results show were used to characterize the nature of the stationary points and to
that phases formed by the polar o-carborane derivatives are less obtain exact polarizabilities in vacuum. The alkyl groups were in
thermodynamically stable and smectic polymorphism is more all-trans conformation. No conformational search was attempted.
pronounced, when compared to the weakly polar p-carborane Final coordinates for each molecular model are provided in
analogues. The compounds are essentially isosteric but the the ESI.†
conformational properties of the carborane–aryl(alkyl) bond differ
Dipole moments and exact electronic polarizabilities of 1a
somewhat due to the relatively short C–C bond in o-carborane. used in the Maier–Meier data analysis were obtained in the
This is evident from the analysis of molecular models for 1[B] 6CHBT dielectric medium using the B3LYP/6-31+G(2d,p)//
and consistent with experimental data for 1-phenyl-o-carborane35 B3LYP/6-31G(2d,p) method and the PCM solvation model41
(Fig. 12).
requested with SCRF (Solvent = Generic, Read) keywords and
Dielectric data for the two carborane derivatives 1[A]a and ‘‘eps = 6.70’’ and ‘‘epsinf = 2.4623’’ parameters (single point
1[B]a show smaller than expected eJ components (hence lower calculations). Exact polarizabilities were obtained using the
De values), presumably due to specific polar solvent–polar POLAR keyword. The reported values for the dipole moment
solute interactions,36 molecular aggregation,37 and/or molecular components and dielectric permittivity tensors are at Gaussian
dynamics; the former depend on the host, while the dynamics standard orientation of each molecule (charge based), which is
depend on the frequency used in the measurement. If the low eJ close to the principal moment of inertia coordinates (mass
value is mainly due to the frequency of the oscillating electric based), and are listed in the ESI.†
field (which is plausible especially for the weakly polar 1[A]a),
then both compounds may exhibit a low cross-over frequency fc
(where De changes the sign) just above the measuring frequency
Experimental section
(1 kHz). In such a case, compounds of this type, in which there is
a large heavy substituent (the carborane cage) at the end of the
rigid core with a significant moment of inertia, are potentially
suitable for formulation of materials for dual-frequency addres-
sing of LC displays.38,39 This warrants further detailed investiga-
tion of such derivatives using dielectric spectroscopy tools.
General
NMR spectra were obtained at 400 MHz (1H), 125 MHz (13C),
and 128 MHz (11B), using a 9.4 T Bruker or a JOEL ECX 500
instrument in CDCl3, unless specified otherwise. 1H NMR
spectra were referenced to the solvent and 11B NMR chemical
shifts to an external boric acid sample in CH3OH that was set to
18.1 ppm. Thermal analysis was obtained using a TA Instru-
ments DSC 2920 using small samples of about 0.5–1.0 mg. IR
spectra were recorded in KBr pellets using ATI Mattson Infinity
FTIR 60. HRMS data were obtained on a Bruker micrOTOF II
instrument. MALDI-TOF mass data were acquired using a
Voyager Elite instrument.
Summary and conclusions
A strategy for a potentially broad series of liquid crystalline
derivatives of the polar o-carborane has been developed and
has been demonstrated for a handful of derivatives. These
derivatives exhibit somewhat lower clearing temperatures and
higher smectic phase stability than the analogous derivatives of
Binary mixtures preparation
p-carborane. The inherent dipole moment of the o-carborane Solutions of carborane derivatives 1a in the 6CHBT host
cage gives rise to a substantial molecular dipole moment (15–20 mg of the host) were prepared in an open vial. A mixture
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of the compound and the host in CH2Cl2 was heated for 2 h at (t, J = 6.6 Hz, 2H), 6.86 (d, J = 8.8 Hz, 2H), 7.41–7.48 (m, 6H);
60 1C to remove the solvent and was evacuated (10ꢀ2 Torr). The MALDI-TOF (CHCA) m/z 516.5–521.5 (max at 519.5, [MH]+).
resulting binary mixtures were analyzed by polarized optical Anal. calcd for C29H46B10O: C, 67.14; H, 8.94. Found: C, 67.07;
microscopy (POM, PZO Bipolar) to ensure homogeneity. The H, 8.93.
R
mixtures were then allowed to stand for 2 h at room tempera-
ture before thermal and dielectric measurements.
4-(4-C5H11C6H4)C6H4C CC6H4OC8H17 (1[Ph]b). White solid:
1H NMR (400 MHz, CD2Cl2) d 0.90 (t, J = 7.0 Hz, 3H), 0.91 (t, J =
6.8 Hz, 3H), 1.23–1.40 (m, 12H), 1.46 (quint, J = 7.6 Hz, 2H), 1.65
(quint, J = 7.6 Hz, 2H), 1.79 (quint, J = 7.5 Hz, 2H), 2.65 (t, J =
Electrooptical measurements
Dielectric properties of solutions of selected compound in 7.7 Hz, 2H), 3.98 (t, J = 6.6 Hz, 2H), 6.88 (d, J = 8.8 Hz, 2H), 7.27
6CHBT were measured by a Liquid Crystal Analytical System (d, J = 8.2 Hz, 2H), 7.46 (d, J = 8.8 Hz, 2H), 7.53 (d, J = 8.1 Hz, 2H),
(LCAS – Series II, LC Vision, Inc.) using GLCAS software version 7.56 and 7.59 (AB, J = 8.5 Hz, 4H); 13C NMR (125 MHz, CDCl3) d
0.13.14, which implements literature procedures for dielectric 14.0, 14.1, 22.5, 22.7, 26.0, 29.19, 29.22, 29.3, 31.2, 31.5, 31.8,
constants. The instrument was calibrated using a series of 35.6, 68.1, 88.0, 90.0, 114.5, 115.1, 122.2, 126.75, 126.79, 128.9,
capacitors. The homogenous binary mixtures were loaded into 131.8, 133.0, 137.7, 140.5, 142.5, 159.2; IR (KBr) n 2212 (CRC),
ITO electro-optical cells by capillary forces with moderate 1604, 1510, 1246 cmꢀ1; MALDI-TOF (ANP) m/z 452.3 ([M]+) and
heating supplied by a heat gun. The cells (about 10 mm thick, 453.3 ([M + 1]+). Anal. calcd for C33H40O: C, 87.56; H, 8.91.
electrode area 1.00 cm2 and anti-parallel rubbed polyimide Found: C, 87.26; H, 8.88.
layer) were obtained from LC Vision, Inc. The filled cells were
heated to an isotropic phase and cooled to rt before measuring
Preparation of azomethine derivatives 1c. General procedure
the dielectric properties. Default parameters were used for A mixture of appropriate aniline 4 (0.15 mmol), 4-octyloxybenz-
measurements: triangular shaped voltage bias ranging from aldehyde42 (0.15 mmol) and cat. amounts of AcOH in EtOH (2 mL)
0.1–20 V at 1 kHz frequency. The threshold voltage Vth was was refluxed for 1 h. The solvent was evaporated and the crude
measured at a 5% change. For each mixture the measurement product was recrystallized repeatedly from hexane.
Q
4-(12-C5H11-p-carboran-1-yl)C6H4N CHC6H4OC8H17 (1[A]c).
was repeated 10 times manually for two cells. The results were
averaged to calculate the mixture’s parameters. Results are White solid, (80% yield): 1H NMR (400 MHz, CDCl3) d 0.84
provided in the ESI† and extrapolated values for pure additives (t, J = 7.3 Hz, 3H), 0.89 (t, J = 6.9 Hz, 3H), 1.05–1.40 (m, 14H),
are shown in Table 3.
1.50–4.0 (m, 10H), 1.46 (quint, J = 7.4 Hz, 2H), 1.65 (br t, J =
8.4 Hz, 2H), 1.80 (quint, J = 7.1 Hz, 2H), 4.01 (d, J = 6.6 Hz, 2H),
6.94 (d, J = 8.9 Hz, 2H), 6.95 (d, J = 8.8 Hz, 2H), 7.19 (d, J = 8.7 Hz,
Preparation of derivatives of tolanes 1b. General procedure
To a solution of 4-octyloxyphenylacetylene21,22 (0.35 mmol), 2H), 7.78 (d, J = 8.7 Hz, 2H), 8.27 (s, 1H); 11B NMR (128 MHz,
iodoarene 3 (0.30 mmol) in dry THF (3 mL) and Hunig’s base CDCl3) d ꢀ12.3 (d, J = 164 Hz); IR (KBr) n 2606 (B–H), 1696
(0.2 mL) were added at rt under an Ar atmosphere, followed by (NQC), 1601, 1509, 1256 cmꢀ1; MALDI-TOF (CHCA) m/z 520.6–
Pd(PPh3)2Cl2 (0.03 mmol) and CuI (0.015 mmol). The mixture 524.6 (max at 523.6, [MH]+). Anal. calcd for C28H47B10NO: C,
was stirred at rt for 2–3 h, 5% HCl was added, organic products 64.45; H, 9.08; N, 2.68. Found: C, 64.64; H, 9.19; N, 2.71.
Q
4-(12-C5H11-o-carboran-1-yl)C6H4N CHC6H4OC8H17 (1[B]c).
were extracted (CH2Cl2), extracts dried (Na2SO4), and the solvents
were evaporated. The resulting crude product was purified on a White solid, (78% yield): 1H NMR (400 MHz, CDCl3) d 0.73
silica gel plug (CH2Cl2/hexane, 1 : 5) followed by repeated recrys- (t, J = 7.3 Hz, 2H), 0.87 (t, J = 7.0 Hz, 3H), 0.89 (t, J = 6.7 Hz, 3H),
tallization (hexane and EtOH/EtOAc).
1.22–1.41 (m, 14H), 1.50–4.0 (m, 9H), 1.47 (quint, J = 7.1 Hz, 2H),
R
4-(12-C5H11-p-carboran-1-yl)C6H4C CC6H4OC8H17 (1[A]b). 1.81 (quint, J = 7.1 Hz, 2H), 3.90 (br s, 1H), 4.02 (d, J = 6.6 Hz, 2H),
White solid: 1H NMR (400 MHz, CDCl3) d 0.84 (t, J = 7.3 Hz, 6.97 (d, J = 8.8 Hz, 2H), 7.09 (d, J = 8.7 Hz, 2H), 7.50 (d, J = 8.7 Hz,
3H), 0.88 (t, J = 6.8 Hz, 3H), 1.08–1.38 (m, 8H), 1.45 (quint, 2H), 7.81 (d, J = 8.8 Hz, 2H), 8.31 (s, 1H). Anal. calcd for
J = 7.4 Hz, 2H), 1.50–4.0 (m, 10H), 1.65 (br t, J = 8.4 Hz, 2H), 1.78 C28H47B10NO: C, 64.45; H, 9.08; N, 2.68. Found: C, 64.39; H,
(quint, J = 7.1 Hz, 2H), 3.96 (t, J = 6.6 Hz, 2H), 6.84 (d, J = 8.8 Hz, 9.11; N, 2.65.
Q
4-(4-C5H11C6H4)C6H4N CHC6H4OC8H17 (1[Ph]c). Off-white
2H), 7.15 (d, J = 8.6 Hz, 2H), 7.27 (d, J = 8.6 Hz, 2H), 7.40 (d, J =
8.8 Hz, 2H); 13C NMR (125 MHz, CDCl3) d 13.9, 14.1, 22.2, 22.6, solid, (80% yield): 1H NMR (400 MHz, CDCl3) d 0.90 (t, J =
26.0, 29.16, 29.22, 29.33, 29.7, 31.2, 31.8, 37.9, 68.1, 80.5 (br), 7.0 Hz, 3H), 0.91 (t, J = 6.8 Hz, 3H), 1.24–1.41 (m, 12H), 1.48
81.6 (br), 87.0, 90.7, 114.5, 114.8, 123.7, 127.2, 130.9, 133.0, (t, J = 7.4 Hz, 2H), 1.66 (quint, J = 7.6 Hz, 2H), 1.82 (quint, J =
135.9, 160.0; 11B NMR (128 MHz, CDCl3) d ꢀ12.3 (d, J = 164 Hz); 7.1 Hz, 2H), 2.65 (t, J = 7.8 Hz, 2H), 4.03 (t, J = 6.6 Hz, 2H), 6.98
IR (KBr) n 2607 (B–H), 2217 (CRC), 1516, 1247 cmꢀ1; MALDI- (d, J = 8.8 Hz, 2H), 7.24–7.28 (m, 4H), 7.53 (d, J = 8.2 Hz, 2H),
TOF (ANP) m/z 516.5–521.5 (max at 519.5, [M]+). Anal. calcd for 7.61 (d, J = 8.5 Hz, 2H), 7.85 (d, J = 8.7 Hz, 2H), 8.44 (s, 1H); 13
C29H46B10O: C, 67.14; H, 8.94. Found: C, 67.15; H, 9.05. NMR (125 MHz, CDCl3) d 14.0, 14.1, 22.6, 22.7, 26.0, 29.16,
C
R
4-(12-C5H11-o-carboran-1-yl)C6H4C CC6H4OC8H17 (1[B]b). 29.23, 29.34 31.2, 31.5, 31.8, 35.6, 68.2, 114.7, 121.3, 126.7,
White solid: 1H NMR (400 MHz, CDCl3) d 0.73 (br t, J = 127.6, 128.8, 130.7 (br), 138.0, 138.5, 142.0, 159.4, 162.0; IR
6.6 Hz, 2H), 0.87 (t, J = 6.6 Hz, 3H), 0.89 (t, J = 7.4 Hz, 3H), (KBr) n 1606, 1252 cmꢀ1; MALDI-TOF (CHCA) m/z 456.4 ([MH]+)
1.20–1.39 (m, 14H), 1.50–4.0 (m, 9H), 1.45 (quint,
J
=
and 457.4 ([MH + 1]+). Anal. calcd for C32H41NO: C, 84.35; H,
7.3 Hz, 2H), 1.77 (quint, J = 7.1 Hz, 2H), 3.90 (br s, 1H), 3.97 9.07; N, 3.07. Found: C, 84.14; H, 9.18; N, 3.11.
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Preparation of azobenzene derivatives 1d. General procedure
29.02, 34.6, 39.2, 59.9, 67.7, 68.1, 68.6 (br), 113.6, 114.8, 129.1,
130.5, 133.6, 158.0, 158.8; 11B NMR (160 MHz, CDCl3) d ꢀ11.4
(m, 4B), ꢀ7.9 (d, J = 158 Hz, 3B), ꢀ1.2 (d, J = 151 Hz, 2B), +5.8
(br s, 1B). ESI-HRMS, calcd for C26H44B10O2ꢃNa [M ꢀ Na]+ m/z
521.4169; found m/z 521.4185.
A mixture of appropriate aniline 4 (0.15 mmol), (S)-2-methylbutyl
4-nitrosobenzoate (5, 0.15 mmol) and catalytic amounts of AcOH
in CH2Cl2 (1 mL) was stirred at rt under Ar for 24 h. Reaction with
aniline 4[B] (o-carborane) was conducted in AcOH (1 mL). Solvents
and AcOH were removed under reduced pressure and the orange
crude product was purified on a silica gel plug (hexane/CH2Cl2,
3 : 1) and then recrystallized repeatedly from EtOH.
1-(4-Iodophenyl)-12-pentyl-p-carborane (3[A], Hal = I). To a
solution of bromophenyl derivative17 3[A] (Hal = Br, 100 mg,
0.27 mmol) in THF (10 mL), n-BuLi in hexanes (0.33 mmol) was
added at ꢀ78 1C. After 1 h solid I2 was added and the mixture
was stirred for 1 h. The solvent was evaporated, and the residue
was passed through a silica gel plug (hexanes). The crude
product (90 mg, containing some dehalogenated derivative)
was purified by recrystallization (hexane, ꢀ10 1C) giving the
iodide 3[A] Hal = I as a white solid: 1H NMR (400 MHz, CDCl3) d
0.84 (t, J = 7.3 Hz, 3H), 1.05–1.27 (m, 6H), 1.50–3.50 (m, 10H),
1.64 (br t, J = 8.3 Hz, 2H), 6.93 (d, J = 8.7 Hz, 2H), 7.49 (d, J =
Q
4-(12-C5H11-p-carboran-1-yl)C6H4N NC6H4COOC5H11* (1[A]d).
Orange solid: 1H NMR (400 MHz, CDCl3) d 0.85 (t, J = 7.2 Hz, 3H),
0.95 (t, J = 7.4 Hz, 3H), 1.03 (d, J = 6.7 Hz, 3H), 1.06–1.34 (m, 6H),
1.50–3.50 (m, 10H), 1.66 (br t, J = 8.5 Hz, 2H), 1.89 (sext,
J = 7.1 Hz, 1H), 4.16 (dd, J1 = 10.7 Hz, J2 = 6.6 Hz, 1H), 4.24
(dd, J1 = 10.7 Hz, J2 = 6.0 Hz, 1H), 7.37 (d, J = 8.8 Hz, 2H), 7.73
(d, J = 8.8 Hz, 2H), 7.90 (d, J = 8.6 Hz, 2H), 8.17 (d, J = 8.6 Hz, 2H);
11B NMR (128 MHz, CDCl3) d 12.3 (d, J = 164 Hz); IR (KBr) n 2609
(B–H), 1716 (CQO), 1272 (C–O) cmꢀ1; MALDI-TOF (CHCA) m/z
506.5–512.5 (max at 510.5, [MH]+). Anal. calcd for C25H40B10N2O2:
C, 59.03; H, 7.93; N, 5.51. Found: C, 59.09; H, 7.85; N, 5.44.
8.7 Hz, 2H); 11B NMR (128 MHz, CDCl3) d ꢀ12.4 (d, J = 165 Hz).
+
ESI-HRMS calcd for C8H14B10I [M ꢀ C5H11
]
m/z 347.1078;
found m/z 347.1053.
1-(4-Aminophenyl)-12-pentyl-p-carborane (4[A]). The nitro-
phenyl derivative 6[A] (250 mg, 0.75 mmol) in THF (10 mL)
was hydrogenated (40 psi) in the presence of 10% Pd/C (25 mg)
for 2 h. The catalyst was filtered off, the solvent was evaporated,
and the resulting crude product was purified by SiO2 plug
(CH2Cl2) giving 180 mg (79% yield) of amine 4[A] as a white
solid: mp 85.4–86.0 1C; 1H NMR (400 MHz, CDCl3) d 0.83
(t, J = 7.3 Hz, 3H), 1.05–1.27 (m, 6H), 1.50–3.50 (m, 10H), 1.63
(br t, J = 8.4 Hz, 2H), 4.00 (br s, 2H), 6.46 (d, J = 8.7 Hz, 2H), 6.97
(d, J = 8.7 Hz, 2H); 13C NMR (125 MHz, CDCl3) d 13.9, 22.2, 29.2,
31.2, 37.8, 80.4 (br), 81.2 (br), 114.6, 127.2, 128.2, 145.5; 11B NMR
(128 MHz, CDCl3) d ꢀ12.4 (d, J = 165 Hz); MALDI-TOF (CHCA)
m/z 303.4–307.4 (max at 306.4, [M]+). Anal. Calcd for C13H27B10N: C,
51.11; H, 8.91; N, 4.59. Found: C, 51.58; H, 9.03; N, 4.63.
Q
4-(12-C5H11-o-carboran-1-yl)C6H4N NC6H4COOC5H11* (1[B]d).
1
Orange solid: H NMR (400 MHz, CDCl3) d 0.75 (br t, J = 7.8 Hz,
2H), 0.88 (t, J = 6.6 Hz, 3H), 0.98 (t, J = 7.4 Hz, 3H), 1.04 (d, J =
6.8 Hz, 3H), 1.22–1.36 (m, 14H), 1.50–4.0 (m, 9H), 1.50–1.62
(m, 2H), 1.81 (sext, J = 7.1 Hz, 1H), 3.98 (br s, 1H), 4.17 (dd, J1 =
10.7 Hz, J2 = 6.6 Hz, 1H), 4.25 (dd, J1 = 10.7 Hz, J2 = 6.0 Hz, 1H),
7.66 (d, J = 8.8 Hz, 2H), 7.88 (d, J = 8.8 Hz, 2H), 7.95 (d, J =
8.6 Hz, 2H), 8.20 (d, J = 8.6 Hz, 2H); 13C NMR (125 MHz, CDCl3)
d 11.2, 14.0, 16.5, 22.6, 26.3, 29.5, 29.7, 34.3, 35.1, 58.9, 69.3,
67.0, 122.9, 123.2, 128.8, 130.6, 132.8, 136.1, 152.8, 154.8, 166.0;
11B NMR (160 MHz, CDCl3) d ꢀ10.7 (m, 5B), ꢀ7.5 (d, J = 146 Hz,
2B), ꢀ0.7 (d, J = 148 Hz, 2B), 8.5 (br s, 1B); MALDI-TOF (CHCA) m/z
507.5–512.5 (max at 510.5, [MH]+). Anal. calcd for C25H40B10N2O2:
C, 59.03; H, 7.93; N, 5.51. Found: C, 59.39; H, 7.99; N, 5.50.
(S)-2-Methylbutyl 4-nitrosobenzoate (5). Following the general
procedure,27 to a solution of (S)-2-methylbutyl 4-aminobenzoate25,26
(7, 310 mg, 1.5 mmol) in CH2Cl2 (5 mL), a solution of Oxones
(1.85 g, 3.0 mmol) in water (18 mL) was added under Ar. The
mixture was stirred overnight at rt, the organic layer was
separated, dried (Na2SO4), and the solvent was evaporated. The
crude product was separated from unreacted starting materials
on a short silica gel plug (hexane/CH2Cl2, 5 : 1) to give 100 mg
(30% yield) of the nitroso ester 5 as a yellow-green viscous oil,
which was used immediately for the next transformation.
1H NMR (400 MHz, CDCl3) d 0.98 (t, J = 7.5 Hz, 3H), 1.04
(d, J = 6.8 Hz, 3H), 1.25–1.34 (m, 1H), 1.49–1.60 (m, 1H), 1.89
(sext, J = 6.6 Hz, 1H), 4.19 (dd, J1 = 10.8 Hz, J2 = 6.6 Hz, 1H), 4.27
(dd, J1 = 10.8 Hz, J2 = 6.0 Hz, 1H), 7.94 (d, J = 8.6 Hz, 2H), 8.30
(d, J = 8.6 Hz, 2H).
Q
4-(4-C5H11C6H4)C6H4N NC6H4COOC5H11* (1[Ph]d). Orange
1
solid: H NMR (400 MHz, CDCl3) d 0.92 (t, J = 6.7 Hz, 3H), 0.98
(t, J = 7.5 Hz, 3H), 1.05 (d, J = 6.7 Hz, 3H), 1.20–1.43 (m, 6H), 1.50–
1.62 (m, 2H), 1.67 (quint, J = 7.5 Hz, 2H), 1.89 (sext, J = 7.1 Hz,
1H), 2.67 (t, J = 7.7 Hz, 2H), 4.17 (dd, J1 = 10.7 Hz, J2 = 6.6 Hz, 1H),
4.25 (dd, J1 = 10.7 Hz, J2 = 6.0 Hz, 1H), 7.30 (d, J = 8.1 Hz, 2H),
7.60 (d, J = 8.1 Hz, 2H), 7.76 (d, J = 8.5 Hz, 2H), 7.97 (d, J = 8.4 Hz,
2H), 8.02 (d, J = 8.4 Hz, 2H), 8.20 (d, J = 8.5 Hz, 2H). Anal. calcd
for C29H34N2O2: C, 78.70; H, 7.74; N, 6.33. Found: C, 78.12; H,
7.69; N, 6.20.
4-(12-(4-C5H11OC6H4)-o-carboran-1-yl)CH2CH2C6H4OC5H11
(2[B]f). Styrene derivative16 2[B]e was reduced with H2 (50 psi)
in the presence of Pd/C (10%) in THF. The reaction mixture was
filtered, the solvent was evaporated and the resulting crude
product was passed through a silica gel plug (CH2Cl2/hexanes,
1 : 2) followed by recrystallization (pentane, ꢀ78 1C followed by
MeOH, ꢀ5 1C) giving the desired product as a white solid:
1H NMR (500 MHz, CDCl3) d 0.91 (t, J = 6.9 Hz, 3H), 0.93 (t, J =
6.9 Hz, 3H), 1.33–1.47 (m, 8H), 1.72–1.80 (m, 4H), 2.50 and 2.73
(A2X2, 4H), 3.64 (br s, 1H), 3.91 (t, J = 6.9 Hz, 2H), 3.92 (t, J =
1-(4-Nitrophenyl)-12-pentyl-p-carborane (6[A]). To a solution
of p-carborane (0.500 g, 3.50 mmol) in dry THF (10 mL), under
Ar at ꢀ78 1C, n-BuLi was added (3.8 mmol). After 0.5 h, the
mixture was warmed up to rt and stirred for 0.5 h, then cooled
to ꢀ78 1C and iodopentane was added (0.45 mL, 3.5 mmol), the
6.9 Hz, 2H), 6.76 (d, J = 8.6 Hz, 2H), 6.82 (d, J = 8.6 Hz, 2H), mixture was warmed up to rt, and stirred for 1 h. Dry DME
7.03 (d, J = 8.6 Hz, 2H), 7.27 (d, J = 8.6 Hz, 2H); 13C NMR (10 mL) was added, the mixture was cooled to ꢀ78 1C and
(125 MHz, CDCl3) d 14.0 (2C), 22.5 (2C), 28.20, 28.22, 28.96, n-BuLi (3.8 mmol) was added. After 15 min the mixture was
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14 B. Ringstrand and P. Kaszynski, J. Mater. Chem., 2010, 20,
9613–9615, and references therein.
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allowed to warm to rt, stirred for 0.5 h, cooled to 0 1C and dry
CuI (0.73 g, 3.8 mmol) was added. The mixture was stirred at rt
for 1 h and pyridine (2.1 mL, 26 mmol) was added followed by
4-iodonitrobenzene (0.872 g, 3.50 mmol) and the mixture was
gently refluxed for 20 h. 10% HCl was added, organic products
were extracted (CH2Cl2), extracts were dried (Na2SO4), solvents
were evaporated and the residue was separated by chromato-
graphy (SiO2, hexane/CH2Cl2, 5 : 1) to give 250 mg (20% yield) of
the desired product 6[A] as the first fraction: mp 83.0–83.5 1C;
1H NMR (400 MHz, CDCl3) d 0.84 (t, J = 7.3 Hz, 3H), 1.05–1.26
(m, 6H), 1.50–3.40 (m, 10H), 1.66 (br t, J = 8.3 Hz, 2H), 7.38
(d, J = 9.0 Hz, 2H), 8.02 (d, J = 9.0 Hz, 2H); 13C NMR (125 MHz,
CDCl3) d 13.8, 22.2, 29.1, 31.2, 38.0, 78.8 (br), 82.8 (br), 123.2,
128.4, 142.9, 147.6; 11B NMR (128 MHz, CDCl3) d ꢀ12.2
(d, J = 164 Hz); IR (KBr) n 2614 (B–H), 1598, 1514 (N–O), 1344
(N–O) cmꢀ1. Anal. Calcd for C13H25B10NO2: C, 46.55; H, 7.51; N,
4.18. Found: C, 46.79; H, 7.58; N, 4.27.
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Acknowledgements
This work was supported by the NSF grant DMR-1207585.
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