Ring-alkyl connecting group effect on mesogenic properties of p-carborane derivatives and their hydrocarbon analogues

Article abstract of DOI:10.3762/bjoc.5.83

The effect of the phenyl-alkyl connecting group on mesogenic properties of several series of isostructural compounds containing p-carborane (A and B), bicyclo[2.2.2]octane (C), and benzene (D) was investigated using thermal and optical methods. Results de

Full text of DOI:10.3762/bjoc.5.83

Ring-alkyl connecting group effect on mesogenic
properties of p-carborane derivatives and their
hydrocarbon analogues
Aleksandra Jankowiak1, Piotr Kaszynski*1, William R. Tilford1,
Kiminori Ohta2, Adam Januszko1, Takashi Nagamine2 and Yasuyuki Endo2
Full Research Paper
Open Access
Address:
Beilstein Journal of Organic Chemistry 2009, 5, No. 83.
1Organic Materials Research Group, Department of Chemistry,
Vanderbilt University, Box 1822 Station B, Nashville, TN 37235, USA,
Phone/Fax: (615) 322-3458 and 2Tohoku Pharmaceutical University,
4-4-1, Komatsushima, Aoba-ku, Sendai 981-8558, Japan
doi:10.3762/bjoc.5.83
Received: 11 August 2009
Accepted: 16 December 2009
Published: 30 December 2009
Email:
Piotr Kaszynski* - piotr.kaszynski@vanderbilt.edu
Guest Editor: S. Laschat
* Corresponding author
© 2009 Jankowiak et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
p-carborane; liquid crystals; structure-property relationship
Abstract
The effect of the phenyl–alkyl connecting group on mesogenic properties of several series of isostructural compounds containing
p-carborane (A and B), bicyclo[2.2.2]octane (C), and benzene (D) was investigated using thermal and optical methods. Results
demonstrated that mesophase stability in the series containing A–D follows the order (Alk)CH2CH2– < (Alk)OOC– < (Alk)CH2O–
< (Alk)COO–. Surprisingly, the connecting groups (Alk)CH2CH2– and (Alk)OOC– destabilize the mesophase significantly
stronger for carboranes (A and B) than for carbocyclic derivatives (C and D). Analysis indicates that this effect may have quadru-
polar and conformational origin.
Introduction
During the past decade, we have been investigating mesogenic unusual molecular symmetry and size affect bulk properties of
derivatives of p-carboranes A and B (Figure 1) in the context of mesogens. Through extensive comparison of isostructural
fundamental and applied studies of liquid crystals and develop- mesogenic derivatives of p-carboranes (A and B),
ment of new materials for electrooptical applications [1-23]. bicyclo[2.2.2]octane (C), and benzene (D), we have been
p-Carboranes belong to an extensive family of closo-boranes probing fundamental aspects of structure-property relationships
and are characterized by 3-dimensional σ-aromaticity and high- in liquid crystals such as the effect of conformational properties
order symmetry axis [23]. Therefore, it is of interest to under- [1,2], the structure of the linking group [5], and tail fluorination
stand how the electronic properties of the two clusters and their [18,19] on mesophase stability, and also the effectiveness of
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Beilstein Journal of Organic Chemistry 2009, 5, No. 83.
13 the effect of incorporation of the O atom as the connecting
group is practically the same for all ring systems [15].
A recently developed series of isostructural mesogens allows to
analyze the effect of the replacement of an alkoxy in 14[6] with
an ester group in 15[6]. The CH2O→OOC exchange dramatic-
ally destabilized the nematic phase for the 10- and 12-vertex
p-carborane derivatives, while a much smaller effect was
observed for the carbocycles [25]. Series 14 and 15 [25] and
Figure 1: The molecular structures of 1,12-dicarba-closo-
dodecaborane (12-vertex p-carborane, A) and 1,10-dicarba-closo-
decaborane (10-vertex p-carborane, B). Each vertex corresponds to a
BH fragment and the sphere represents a carbon atom.
shielding of a lateral substituent [8,16,20] and chirality transfer also diesters 17 [2] provide an opportunity for further investiga-
phenomena [17]. Results of these studies are important for the tion of this interesting phenomenon. Therefore, we focused on
design of new materials and optimizing of their properties for series 14–16 to investigate the CH2O, COO, OOC connecting
applications.
groups, and also on series 17–20 to study the CH2O, CH2CH2,
COO, OOC groups.
During our investigation of structurally related series of meso-
gens containing rings A–D (Figure 2), it became apparent that Here we describe the preparation of an isostructural series of
the benzene ring–alkyl chain connection has a distinctly diesters 16[6] and 18B, and also tetraesters 19 and 20, and a
different impact on phase stability in derivatives of p-carborane detailed comparative analysis focusing on the impact of the
(A) than in their isostructural carbocycles. For instance, a larger connecting group on mesogenic properties. The analysis is
stabilization of the nematic phase, upon CH2→O replacement, aided by molecular modeling of the pertinent molecular frag-
was observed in p-carborane mesogens relative to benzene ments. In addition, we report two homologues of 16A[6],
analogues. Thus, in series 1–4, the nematic phase is stabilized diesters 16A[5] and 16A[7].
by about 14 K more for the pairs 1A/2A and 3A/4A, than for
Results
terphenyl (D) and bicyclo[2.2.2]octane (C) analogues
(Figure 2). Similarly high values for phase stabilization of about Synthesis
34 K are observed in pairs of alkyl and alkoxy dioxane deriva- Diesters 16[n] were prepared from diphenols 21 and appro-
tives 5[n] and 6[n] [4], as compared to similar benzene meso- priate carboxylic acid chlorides in the presence of a base as
gens [24]. Also in series 7–11 the introduction of the connecting shown in Scheme 1. The requisite diphenols 21 were obtained
oxygen atom gives a larger increase in mesophase stability by in nearly quantitative yields by treating the corresponding
an average of 6 ± 2 K for the 12-vertex p-carborane derivatives dimethoxy derivatives 14[0] with BBr3. This procedure repres-
than for their benzene analogues [5]. However, in series 12 and ents a significant improvement to the original preparation of
Figure 2: The molecular structures of derivatives 1–20.
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Beilstein Journal of Organic Chemistry 2009, 5, No. 83.
Scheme 1: Preparation of diesters 16[n].
1,12-bis(4-hydroxyphenyl)-p-carborane (21A) from 14A[0] appropriate dicarboxylic acid 22 and phenols 23 and 24,
[26] and 1,4-bis(4-hydroxyphenyl)benzene (21D) [27]. respectively. p-Carborane-1,12-dicarboxylic acid 22A and tere-
The preparation of diphenol 21C will be described elsewhere phthalic acid (22D) were converted to the corresponding acid
[25]. The dimethoxy carborane derivative 14A[0] was obtained chlorides using PCl5 and then reacted with phenols in the pres-
using the Wade’s arylation procedure [28] of p-carborane (A) ence of a base. The previously described method [2] for the
with 4-iodoanisole as described before [26]. The 10-vertex preparation of esters of bicyclo[2.2.2]octane-1,4-dicarboxylic
analogue 14B[0] was prepared in a similar way and will be acid (22C) was unsuccessful and the desired esters 19C and
described elsewhere [25]. 4,4”-Dimethoxyterphenyl 14D[0] was 20C were obtained from the diacid and appropriate phenol
prepared in 84% yield from 1,4-dibromobenzene and using the classical Mitsunobu procedure [34]. A similar
(4-methoxyphenyl)boronic acid using the Suzuki coupling procedure was used for the preparation of tetraester 20B, while
procedure [29]. This method is comparable to other Pd(0)- 19B was prepared more efficiently using the acid chloride
assisted methods for the synthesis of 14D[0] [30-32].
method. Ester 19D has been reported in the literature [35].
The 10-vertex p-carborane diester 18B was obtained from the Phenol 24 was prepared by acylation of 4-benzyloxyphenol
corresponding dicarboxylic acid 22B [33] and 4-pentylphenol with butyryl chloride followed by removal of the protective
(Scheme 2) according to a recently described procedure [2]. The benzyl group under reductive conditions as described in the lit-
two series of tetraesters 19 and 20 were prepared from the erature [36] (Scheme 3).
Scheme 2: Preparation of esters 18–20.
Scheme 3: Preparation of phenol 24.
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Beilstein Journal of Organic Chemistry 2009, 5, No. 83.
Mesogenic properties
Table 2: Transition temperatures (°C) for 16A[n].a
Transition temperatures and enthalpies of the newly prepared
compounds were determined by differential scanning calorime-
try (DSC). The phase types were assigned by comparison of
microscopic textures observed in polarized light with those
published for reference compounds [37-39]. Results for these
and also their structural analogues are shown in Table 1 and
Table 2.
n
Transition temperatures
5
6
7
Cr1 66 Cr2 120 N 155 I
Cr 108 N 132 I
Cr1 76 Cr2 92 N 124 I
All p-carborane derivatives in series 14–20 exhibit exclusively
the nematic phase. Similar nematic behavior is observed for
carbocycles in series 17–20 with the exception of 19D, which
exhibits a SmA phase in addition to a N phase. In contrast, most
carbocyclic derivatives in series 14[6]–16[6] display only
aObtained on heating; Cr: crystal, N: nematic, I: isotropic. Transition
enthalpies are listed in the SI.
smectic and soft crystalline polymorphs. The 16D[6] is shown in Figure 3, and representative textures of its
bicyclo[2.2.2]octane derivative 16C[6] is the only exception mesophases are presented in Figure 4. The tilted phases in both
and exhibits a narrow range nematic phase above a soft crystal- terphenyl compounds were identified by the appearance and
line phase designated as L or E on the basis of viscosity, ability subsequent characteristic changes of the Schlieren textures in
to supercool, and optical textures. In general, the homeotropic regions of the SmA phase upon cooling.
bicyclo[2.2.2]octane derivatives 14C[6]–16C[6] exhibit ortho-
gonal phases (SmA and SmB), while the terphenyl analogues In general, the order of phase stability for all five series follows
display a rich smectic polymorphism involving mainly tilted A ~ B < C < D. Derivatives of both p-carboranes A and B
phases. The terphenyl derivatives 14D[6] and 16D[6] exhibit exhibit similar stability of the nematic phase, with the excep-
the most interesting polymorphism in the series with 4 smectic tion of 15[6] and 19 for which the monotropic nematic phase of
phases and possibly a soft crystalline modification such as a G the 10-vertex carborane derivatives is significantly less stable
phase below the SmF phase in the latter. A DSC trace for (<20 K) than that of the 12-vertex analogues.
Table 1: Transition temperatures (°C) for selected liquid crystals.a
*
A
B
C
D
X
14[6]
–CH2O–(Ph)
Cr 96 N 98 Ib
Cr 73 N 105 Ib
Cr 98 SmB 161 SmA 179 Ib Crc 182 SmF 218 SmI 219
SmC 232 SmA 235 Ib
15[6]
16[6]
–OOC–(Ph)
–COO–(Ph)
Cr 112 (N 31) Ib
Cr 65 (N 11) Ib
Cre 102 N 136 I
Crd 114 SmA 148 Ib
Crf 102 X 205 N 207 I
Cr 134 SmC 143 SmA 183 Ib
Cr 108 N 132 I
Cr 66 X 96 SmF 226 SmI 232
SmCg 250 SmA 251 I
17
18
19
20
–CH2O–(Ph)
Cr 137 N 182.6 Ih
Cri 111 N 183.4 Ih Cr 112 N 229.5 Ih
Cr 189 N 235 Ij
–CH2CH2–(Ph) Cr 106 N 118 Ik
Cr 85 N 110 I
Cr 160 (N 128) I
Cr 120 N 234 I
Cr 98 N 173 Il
Cr 121 N 195 I
Cr 133 N 275 I
Cr 155 N 181 Im
–OOC–(Ph)
–COO–(Ph)
Cr 203 (N 139) Ig
Cr 130 SmA 207 N 221 In
Cr 230 N 287 I
Cr 133 N 230 I
aObtained on heating; Cr: crystal, Sm: smectic, N: nematic, I: isotropic, X: unidentified phase. Transition enthalpies for new compounds are listed in the
SI. bRef. [25]. cCr – Cr transition at 108 °C. dCr – Cr transition at 100 °C. eCr – Cr transition at 73 °C (14.9 kJ/mol). fCr – Cr transition at 33 °C (11.6 kJ/
mol). gOptical determination obtained on cooling .hRef. [2]. iCr – Cr transition at 70 °C. jRef. [40]. kPreviously reported Cr 104 N 114 I, ref. [21]. lRef.
[41]. mRef. [42]. nRef. [35].
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Beilstein Journal of Organic Chemistry 2009, 5, No. 83.
b)
a)
c)
d)
Figure 3: Partial DSC trace for 16D[6]. Heating rate 5 K/min.
Figure 4: Optical textures of 16D[6] obtained for the same region of
the sample upon cooling: (a) SmA growing from isotropic (251 °C), (b)
focal-conic texture of SmC (242 °C), (c) SmI (229 °C), and (d) broken
focal-conic texture of SmF (211 °C). Magnification × 60.
Analysis of three homologues 16A[n] demonstrated the
decreasing stability of the nematic phase with increasing chain
length from TNI of 155 °C for n = 5 to 124 °C for n = 7
(Table 2). Investigation of the 4,4″-dimethoxyterphenyl 14D[0] In contrast, the difference in TNI is larger by 10 K for
revealed a high temperature nematic phase (Cr 277 N 295 I), p-carborane A and 18 K for p-carborane B (Figure 5 and
which is in disagreement with the original literature report [43]. Figure 6). These results are consistent with earlier findings for
pairs 1/2, 3/4, and 5/6 (Figure 5) in which a particularly large
Comparative Analysis
impact of the –OCH2– → –CH2CH2– substitution on TNI is
Mesogenic properties of structurally analogous pairs were observed for the rigid biphenyl derivatives 1–4.
compared, and the results are presented in Figure 5 and
Figure 6.
The –OCH2– → –OOC– and –OCH2– →–COO–
substitution
The –OCH2– → –CH2CH2– substitution
Data in Table 1 demonstrate that the replacement of the
A comparison of TNI for compounds in series 17 [2] versus their heptyloxy group with heptanoyloxy in 14[16]/16[6] and butoxy
isostructural analogues 18, in which the linking oxygen atom is with butanoyloxy in 17/20 results in an increase of the TNI by
replaced with –CH2–, demonstrates that the presence of the about 30 K and 45 K, respectively, for all structural units A–D.
oxygen atom increases the phase stability by about 55 K (or The only exception is the pair 14D[6]/16D[6] for which the
27 K per alkoxyphenyl group) for the carbocyclic compounds. change in TNI is only 16 K. The larger change of TNI for pairs
Figure 5: The change in the clearing temperature ΔTc upon substitu-
tion –OCH2– → –CH2CH2– in selected pairs of compounds. The lines
are guides for the eye.
Figure 6: The change in the clearing temperature ΔTc upon replacing
of the –OCH2– connecting group with another in selected pairs of com-
pounds. The lines are guides for the eye.
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Beilstein Journal of Organic Chemistry 2009, 5, No. 83.
17/20 than for 14[6]/16[6] is consistent with attenuation of the Discussion and Conclusion
substitution effect by the shorter alkyl chain in the latter (–C3H7 Results presented in Table 1 are in agreement with general
vs –C6H13).
trends [47,48] and demonstrate that the replacement of –OCH2–
with –OOC– increases TNI, while replacement with –CH2CH2–
In contrast, replacement of the oxymethylene linking group or –COO– decreases TNI. Overall, the effectiveness of the
with a carboxy group of reversed orientation relative to the core connecting group in mesophase stabilization follows the order:
(pairs 14[6]/15[6] and 17/19) leads to significant destabiliza- –OOC(Alk) > –OCH2(Alk) > –COO(Alk) > –CH2CH2(Alk).
tion of the mesophase, and the magnitude of the effect markedly The magnitude of the effect for the –OCH2– → –OOC–
depends on the nature of the central structural element replacement is practically independent of the central structural
(Figure 6). Thus, for derivatives of carbocycles C and D, Tc element A–D. In contrast, the decrease in TNI upon substitution
decreases less than 55 K for 14[6]/15[6] and less than 40 K for of –OCH2– with –CH2CH2– or –COO– is stronger for
17/19, while for the p-carboranes the decrease is larger, p-carborane derivatives (A and B) than for their carbocyclic
reaching a value of 94 K for the pair 14B[6]/15B[6].
analogues. This effect is observed for compounds in which the
p-carborane is connected directly to the substituted benzene ring
Data in Table 1 also allow for assessment of the impact of the (1–4, 14–16) or through a spacer (5, 6, 17–20).
orientation of the connecting carboxyl group on TNI as a func-
tion of the central structural element. Thus, in pairs 16[6]/15[6] The origin of the observed relative effectiveness of the
and 20/19, the change of carbonyloxy to oxycarbonyl leads to a connecting groups (–OOC(Alk) > –OCH2(Alk) > –COO(Alk) >
marked phase destabilization for all structural elements A–D. –CH2CH2(Alk)) is unclear at the moment. In general, the phase
Consistent with our previous analysis, the effect is much stability is related to packing fraction, and for more compact
stronger for p-carboranes (>90 K) than for carbocycles (<80 K) anisometric molecules (high packing fraction) the clearing tem-
with the typical order: D, C < A < B.
perature is higher [49]. Thus, it can be expected that com-
pounds with substituents preferring coplanar orientation with
the aryl ring (–OR and –COOR) would exhibit higher meso-
Molecular Modeling
For a better understanding of the terminal substituent’s impact phase stability than those with non-coplanar orientation (–CH2R
on the conformational ground state of the molecules, four and –OOCR). While this simple steric argument is consistent
benzene derivative models, 25–28, were optimized at the with data for pairs –CH2R/–OR, the effect of orientation of the
B3LYP/6-31G(d) level of theory, and their equilibrium geomet- carboxy group on mesophase stability is opposite. Therefore,
ries are presented in Figure 7. Results show that the replace- steric arguments alone cannot explain the observed trend for
ment of the oxygen atom with a –CH2– group reorients the these analogues.
terminal chain from co-planar, in the conformational ground
state of ethoxybenzene (25), to the orthogonal position relative
to the benzene ring plane in propylbenzene (26). Replacement
of the –CH2O– fragment with the –COO– group leads to an
increase of angle θ between the ring and substituent planes to
about 50° in phenyl acetate (27). Reversing the connectivity of
the ester group (–COO– → –OOC–) results in return to the
co-planar orientation of the substituent in benzoate 28. The
computational results are consistent with experimental findings
for anisole [44] and ethylbenzene [45], and solid-state struc-
tures for compounds containing fragments 25–28 [46].
Overall, the interplanar angle θ between the ring and substituent
increases in the series 25, 28 < 27 < 26 or –OR, –C(O)OR <
–OOCR < –CH2R.
Further analysis of the computational results demonstrates that
Figure 7: Equilibrium ground state geometries (B3LYP/6-31G(d)) for
benzene derivatives: ethoxybenzene (25), propylbenzene (26), phenyl
acetate (27), and methyl benzoate (28) and pertinent molecular para-
molecular dipole moment μ increases in the following order: 26
< 25 < 27 < 28. The quadrupole moment tensor Qxx perpendic-
meters: dihedral angle θ, dipole moment μ, and quadrupole moment
tensor QXX perpendicular to the ring plane.
ular to the benzene ring is larger for the esters than for alkyl or
alkoxy derivatives.
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Beilstein Journal of Organic Chemistry 2009, 5, No. 83.
Electronic effects also cannot sufficiently explain the observed This dramatic effect has been attributed to conformational prop-
trend in mesophase stability and the poor performance of the erties of molecules in the condensed phase.
carboxyl group. Thus, the observed trend is inconsistent with
the order of dipole moments calculated for the relevant A more complete understanding of the impact of structural
molecular fragments 25–28 (Figure 7). According to the modification on bulk properties will emerge through further
computational results, esters 27 and 28 have similar, and also research on structure-property relationships and studying of
the largest molecular dipole and quadrupole moments. There- other examples of structurally similar mesogens containing the
fore, compounds containing these fragments would be expected four ring systems A–D.
to exhibit both similar and high mesophase stability. The data
show otherwise and a large difference in TC is observed for the Experimental
isomeric esters (e.g. for 19B/20B ΔTNI = 80 K; for other Optical microscopy and phase identification were performed
examples see LiqCryst database [50]).
using a PZO “Biolar” polarized microscope equipped with a
HCS400 Instec hot stage. Thermal analysis was obtained using
The origin of the observed excessive mesophase destabilization a TA Instruments 2920 DSC. Transition temperatures (onset)
in p-carborane derivatives by the –COO(Alk) and and enthalpies were obtained using small samples (1–2 mg) and
–CH2CH2(Alk) substituents is even more puzzling. Data in a heating rate of 5 K min−1 under a flow of nitrogen gas. For
Table 1 show that mesophase of p-carborane derivatives DSC and microscopic analyses, each compound was addition-
containing electron rich benzene rings (with the –OR and ally purified by dissolving in CH2Cl2, filtering to remove
–OOCR substituents) is excessively stabilized relative to those particles, evaporating and recrystallization typically from
containing either weakly donating (–CH2CH2R) or electron- hexanes or toluene/heptane mixture. The resulting crystals were
withdrawing (–COOR) substituents. This suggests that inter- dried in vacuum overnight at ambient temperature. 10- and
molecular quadrupolar interactions between p-carborane and 12-vertex p-carboranes (A and B) were purchased from
benzene ring may be responsible for the observed phase stabil- Katchem s. r. o. (Prague, Czech Republic).
ization. Support for this hypothesis is provided by the finding
that the connecting group affects bulk properties whether
Supporting Information
p-carborane is connected to the benzene ring directly or through
a spacer. The observed larger effect of the –CH2CH2– →
Synthetic procedures for compounds 14D[0], 16[n], 18B,
–OCH2– replacement in pairs 1A/2A and 3A/4A as compared
19, 20, 24, and analytical details are provided.
to 17A/18A suggests a role for the molecular dipole moment in
Supporting Information File 1
phase stabilization. Since p-carboranes are moderately electron
withdrawing substituents, the alkoxy derivatives have a larger
dipole moment than the alkyl derivatives [16]. Alternatively, the
effect can be due to higher rigidity of 1-4, which attenuates the
effect as compared to the more conformationally flexible
diesters.
General methods and synthetic procedures
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-5-83-S1.pdf]
Acknowledgements
Overall, the analysis cannot distinguish one particular factor We are grateful to Mr. Bryan Ringstrand for the photomicro-
responsible for the impact of structural elements (A–D) on graphs of 16D[6]. Financial support for this work was received
phase stabilization. Instead, a combination of conformational from the National Science Foundation (DMR-0606317 and
properties of structural elements A–D and substituents, their DMR-0907542) and from the Ministry of Education, Culture,
relative sizes [51], and electronic properties of the benzene ring Sports, Science and Technology, Japan (Grant-in-Aid for
bearing the substituent dictate mesogenic properties.
Scientific Research (B) No. 13470468).
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