Synthesis and solid-state rotational dynamics of molecular gyroscopes with a robust and low density structure built with a phenylene rotator and a tri(meta -terphenyl)methyl stator

Article abstract of DOI:10.1021/cg200373g

Recent studies suggest that the rotational dynamics in crystals of molecular gyroscopes become more favorable (i.e., faster) when the packing coefficient of the corresponding lattice is decreased by increasing the steric bulk of the stator, as expected fo

Full text of DOI:10.1021/cg200373g

ARTICLE
pubs.acs.org/crystal
Synthesis and Solid-State Rotational Dynamics of Molecular
Gyroscopes with a Robust and Low Density Structure Built with
a Phenylene Rotator and a Tri(meta-terphenyl)methyl Stator
Zachary J. O’Brien, Arunkumar Natarajan, Saeed Khan, and Miguel A. Garcia-Garibay*
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States
S Supporting Information
b
ABSTRACT: Recent studies suggest that the rotational dy-
namics in crystals of molecular gyroscopes become more
favorable (i.e., faster) when the packing coefficient of the
corresponding lattice is decreased by increasing the steric bulk
of the stator, as expected for structures with high protuberances
or deep cavities. In an effort to explore the effects of increased
stator size on the solid-state dynamics of these crystalline
models for molecular machines, molecular gyroscope 4 with
an “exploded” bis(tri(meta-terphenyl)methyl) stator was
synthesized. Single crystal X-ray diffraction analysis revealed a
packing structure with two crystallographically distinct gyroscope molecules and four ethyl acetate molecules per unit cell. Although
a relatively low packing coefficient of 0.68 was determined for the corresponding packing motif, we noticed that rotators at the two
sites have significantly different environments. The solid state rotational dynamics of the two central phenylenes in an ethyl acetate
clathrate of 4 were explored by variable-temperature 13C NMR with cross-polarization with magic angle spinning (13C CPMAS
NMR) and by quadrupolar echo 2H NMR measurements with isotopically labeled samples. It was found that the increased stator
size does indeed allow for more free-volume and faster rotational dynamics as compared to molecular gyroscopes with smaller or
more globular stators. However, the two crystallographic sites experience different rotational dynamics, suggesting that the average
density available from the packing coefficient is a very crude indicator of solid-state dynamics.
’ INTRODUCTION
reasonable correlation between the rate of rotational exchange
and the barriers to rotation determined by solid-state NMR,
and with the packing coefficients calculated from the X-ray
structures.8 However, it should also be pointed out that molec-
ular crystals with very low packing densities tend to be unstable
and collapse into more densely packed forms, sometimes becom-
ing amorphous. It is common for low CK structures to crystallize
with solvent molecules, and it is normal for these crystals to
collapse when a certain amount of the included solvent escapes.
Thus, while compound 3 was shown to form an open structure
conducive to fast rotation at ambient temperature,5c crystals were
found to lose solvent and collapse when variable temperature
solid state NMR measurements were attempted, providing just a
lower rotational exchange limit and an upper bound for the
activation energy.5c As this observation suggests the need of
crystals that have low CK values that are structurally robust, we
decided to explore a structural modification that would give a
lower packing coefficient and a more robust crystal lattice.
Recognizing that the globular tert-butyl groups on the periphery
of the trityl stator in 3 fail to lock molecules in place, as their
Recent literature on artificial molecular machinery has ad-
dressed the function and dynamics of molecules supported on
surfaces1,2 and within bulk solids,3 as it has been suggested that
molecular machines may find important applications in materials
science.4 Studies in our group have been based primarily on
molecular gyroscopes with 1,4-bis(triarylpropynyl)benzene
architectures5 and related 1,4-bis(9-triptycylethynyl)benzene
structures,6 with much work focused on the exploration of
different molecular topologies and their effects on crystal packing
and rotational dynamics. A trend shown by structures 1ꢀ3 in
Figure 1 is that increasing the steric bulk of the stator from trityl
(1) to tris(3,5-di-tert-butylphenyl)methyl (3) results in crystals
with greater rotational exchange rates, suggesting the bulkier
stators offer more effective steric shielding of the central rotator
component in the solid state.
An average measure of the free volume that exists within a
crystal can be obtained by calculating the packing coefficient, CK.
The packing coefficient was defined by Kitaigorodskii in terms of
the van der Waals volume of all the molecules within the unit cell
(Vvdw) relative to the total volume (Vcell) of the unit cell, CK =
Vvdw/Vcell.7 For most molecular crystals, CK varies from 0.64 to
0.77, and it generally correlates with the stability of the lattice.
One can see from compounds 1ꢀ3 in Figure 1 that there is a
Received: March 23, 2011
Revised:
April 14, 2011
Published: April 15, 2011
r
2011 American Chemical Society
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Figure 1. Examples of molecular gyroscopes with triarylmethyl stators
showing a correlation between the rates (sꢀ1) of rotational exchange
(kr) at 298 K, their activation energies (kcal/mol), and the calculated
packing coefficients. The data for compounds 1 and 2 correspond to the
benzene clathrates of the two while the data for compounds 3 and 4
correspond to the dichloromethane and ethyl acetate clathrates, respec-
tively. The rotational exchange of compound 3 could not be measured as
a function of temperature because the crystal lattice collapses as
dichloromethane escapes. The CK is listed as 0.68, which corresponds
to complete dichloromethane inclusion from the X-ray crystal structure,
but the effective packing coefficient of the sample used for NMR analyses
is unknown because as the dichloromethane escapes, the lattice collapses
to a crystal form with unknown unit cell parameters.
Figure 2. Ethyl acetate clathrate crystals of 4 obtained from an ethyl
acetate/methanol mixture viewed through a polarizing microscope
(10ꢁ magnification).
Scheme 1
intermolecular interactions are weak, we decided to compare a
substitution with less globular, disk-shaped, phenyl groups with
the expectation that the stronger edge-to-face and πꢀπ stacking
interactions would yield a more robust crystal lattice. The
required m-terphenyl unit has been used as a tecton in crystal
engineering, along with other polyphenylbenzenes, in order to
instill low density, rigidity, and high crystallinity.9 We report here
the synthesis of compound 4 and its isotopologue 4-d4, which
have the same general 1,4-bis(triarylpropynyl)benzene architec-
ture with 12 phenyl groups on the periphery of the stator. As
described in detail below, compound 4 has a strong tendency to
crystallize with ethyl acetate in a low-density structure with a
packing coefficient of 0.68, with two crystallographically distinct
gyroscope molecules in the lattice. Crystals in the form of long,
beautiful needles (Figure 2) are stable over a wide temperature
range, losing solvent of crystallization at 78 °C and decomposing
without melting at 374 °C. Variable temperature solid-state
2H NMR studies between ꢀ73 and 35 °C confirmed that
phenylene rotation is different at each of the two sites, with
one of them experiencing a rotational exchange that is ca. 1000
times faster than the other, with activation energies of ca. 5.0 and
7.0 kcal/mol, respectively.
give the bulky triaryl methanol 6. Treatment of 6 with acetyl
chloride, followed by reaction with ethynylmagnesium bromide,
yielded alkyne 7. Sonogashira coupling of 7 with 0.5 equiv of 1,4-
diiodobenzene afforded compound 4. The isotopologue 4-d4
with a deuterated rotator for 2H NMR analysis was synthesized
under similar conditions with 1,4-dibromobenzene-d4. All struc-
tures were confirmed with solution 1H NMR, 13C NMR,
attenuated total reflectance (ATR) FT-IR (Supporting In-
formation), and MALDI-TOF mass spectrometry.
The tertiary alcohol in compound 6 was identified by a sharp
1H NMR resonance at 3.08 ppm and a sharp OꢀH stretching
band at 3566 cmꢀ1. The carbinol carbon in the 13C NMR
spectrum resonates at 82.7 ppm. Alkyne 7 has the characteristic
terminal alkyne 1H NMR signal at 2.91 ppm and the correspond-
ing IR signals at 3294 cmꢀ1 and 2245 cmꢀ1, indicating sp C—H
and CtC stretches, respectively. The 13C NMR signals of the
terminal alkyne occur at 89.7 ppm and 74.8 ppm, and a peak at
56.3 ppm corresponds to the quaternary carbon. Compounds 4
and 4-d4 have similar proton spectra, except that the central
’ RESULTS AND DISCUSSION
Synthesis and Characterization. Preparations of 1,4-bis-
(30,30,30-tri(meta-terphenyl)propynyl) benzene (4) and its deut-
erated analog 4-d4 were accomplished via the 4-step synthesis
shown in Scheme 1. Samples of the 3,5-diphenylbromobenzene
derivative 5 were prepared via Suzuki cross coupling according to
the procedure published by Matsumoto and co-workers.10
A
Grignard reagent was prepared by reaction of 5 with magnesium
1
ribbon and then reacted with /3 equiv of diethylcarbonate to
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ARTICLE
Figure 4. Close up views of the rotators (red capped sticks) corre-
sponding (a) to the site with no close contacts and (b) to the site with six
close contacts and more steric hindrance. The stator portion at the front
was removed for clarity, and close neighbors are represented as space
filling models. Molecules of 4 are green and yellow, and ethyl acetate
molecules are purple and pink.
molecular crystals fall in the range 0.64ꢀ0.77,12 so that crystals
of 4 are indeed toward the lower-density end of normal molec-
ular crystals and are more robust than crystals of 3, confirming
our hypothesis that this design with larger terphenyl-containing
stators would yield a relatively open structure. Considering that
there are two distinct gyroscope molecules in the crystal lattice,
we were interested to see whether they have similar or different
environments near the central phenylene rotators. Figure 4
shows space-filling model views with a close up of each of the
two rotator environments, with neighboring molecules repre-
sented in green and yellow and solvent molecules in pink and
purple. With the rotator in each view represented in red capped
sticks, one can see that the less hindered site represented in
Figure 4a has a more spherical cavity than that of the more
hindered site in Figure 4b. While the rotator in the less hindered
site has no neighboring atoms at a distance shorter than the sum
of their van der Waal’s radii, the rotator in the more hindered site
has close contacts that conform to the rectangular cross section
of the phenylene rotator.13
To further illustrate the differences in the environments
surrounding the two rotators, a Connolly surface14 of the crystal
structure was calculated using a 1.3 Å diameter probe in
Cerius2.15 A model was built with the rotators and axles removed,
leaving only the stators visible. The resulting surfaces correspond
to the space available to the rotator moieties, which are repre-
sented in red and blue for each of the two crystallographically
distinct sites. It is evident from Figure 5 that the rotators that
occupy the blue regions are sterically more restricted and likely to
have a higher activation energy for internal rotation. In contrast,
the rotators occupying the red regions can accommodate larger
amplitude motions and should have a lower activation energy.
Solid-State CPMAS 13C NMR. It has been shown that rotary
dynamics in the solid state can be determined by taking
advantage of line shape analysis of variable temperature CPMAS
13C NMR spectra.16 This method requires the sites involved in
the dynamic process to be magnetically nonequivalent and their
exchange to occur at frequencies that fall within the range of their
chemical shift differences, usually 50ꢀ1000 Hz. The solid-state
CPMAS 13C NMR spectrum of crystalline 4 was obtained
between 236 and 317 K with a magic angle spinning (MAS)
rate of 10 kHz. We noticed that the spectrum acquired between
236 and 268 K showed signals corresponding to ethyl acetate at
15, 20, and 60 ppm but that loss of solvent occurred under fast
sample spinning between ca. 285 and 295 K. The loss of ethyl
acetate was accompanied by changes in the spectrum that
included a redistribution of intensities in the aromatic region
Figure 3. Packing view of the molecular gyroscope 4 (hydrogens
omitted for clarity) viewed down the b-axis. The molecules shown in
gold have the triarylmethyl groups with a slight conformational disorder,
which is not depicted in the figure.
phenylene signal of 4 at 7.55 ppm is not present in 4-d4. Similarly,
the two carbon spectra are also very similar, except that the
deuterated rotator appears as a triplet at 131.4 ppm, slightly
upfield of the corresponding rotator signal in the 13C NMR
spectrum of 4 (131.8 ppm).
Thermal Analysis. Visual melting point observations indi-
cated that 4 decomposes before melting at 374 °C. Differential
scanning calorimetric (DSC) analysis of crystals of 4 showed a
sharp endotherm at 78 °C on the heating curve, corresponding to
the loss of ethyl acetate, and a broad endotherm starting at
374 °C that was associated with the sample decomposition
before melting.
Single X-ray Crystal Structure Analysis of 4. Good quality
single crystals were obtained by slowly cooling a boiling 9:1 ethyl
acetate/methanol solution of 4 down to room temperature.
X-ray diffraction data were collected at 100 K, and the structure
was solved in the triclinic space group P1 with two crystal-
lographically distinct half molecules of 4 and two molecules of
ethyl acetate per asymmetric unit. As required by the space group
symmetry, the unit cell contains two distinct molecules of 4,
shown in gold and green in Figure 3, with coincident molecular
and crystallographic centers. The molecular gyroscopes shown in
gold display rotational disorder with four of the six terphenyl
phenyl groups adopting two slightly different conformations.
The two structures are characterized by having the two triar-
ylmethyl groups assuming anti conformations with the alkyne
axles deviating from linearity, as measured by an angle of 175°
from the ipso carbon of the rotator to the center of the alkyne
bond to the trityl central carbon. The solvent molecules are
localized between the two distinct rotor molecules (shown in
pink and purple in Figure 3), and they are disordered. One ethyl
acetate molecule has all of the carbon and oxygen atoms in a
coplanar conformation and is disordered over two sites related
by 180° rotation in the plane with occupancies in an 80:20 ratio.
The other ethyl acetate molecule has the ethyl group rotated by
93° out of the plane of the acetate group and is also disordered
over two sites in the ratio of 55:45.
Further analysis shows that the packing coefficient for this
structure is 0.68,11 indicating that the unit cell contains 32%
“empty” space. Typical values of packing coefficients for
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Crystal Growth & Design
ARTICLE
Figure 6. Simulated and experimental solid state 2H quadrupolar echo
NMR spectra of the ethyl acetate clathrate of 4-d4: (a) spectra simulated
with a model that considered 2-fold flipping exchange in two distinct
sites; (b) experimental spectra; (c) spectra simulated with a single 2-fold
flipping exchange. The exchange frequencies used in the simulations and
the temperature of the experimental spectra are indicated.
Figure 5. Connolly surfaces calculated by removing the rotator and axle
moieties of 4 and then using a 1.3 Å diameter probe. The red and blue
regions correspond to the space occupied by the phenylene rotators of
gyroscope molecules in the two crystallographic sites.
are the sum of the two frequencies that gave the best visual
approximation to the experimental spectra. The differences
between the one and two component simulations are significant
and more strongly manifested in the central feature and the side
shoulders. For example, fitting the spectrum collected at 200 K
with any single frequency proved impossible, as it could not
reproduce the combination of high shoulders and flat center.
Shown at the bottom of Figure 6c, the most reasonable match
was obtained at 700 kHz, yet it is highly unsatisfactory. In
contrast, the high-shoulder and flat-top appearance of the
experimental spectrum was simulated reasonably well with a
model that includes a 1:1 sum of slow (5 kHz) and intermediate
exchange components (2 MHz). Similar results were obtained at
225 and 248 and 268 K, with the spectrum collected at 308 K,
largely in the fast exchange regime, simulated reasonably well by
both the one and two frequency models. The top spectrum in
Figure 6a was obtained with 2-fold flipping in the fast (>108 Hz)
and intermediate exchange (500 kHz) exchange regimes, with
the former undergoing a ( 20° libration consistent with the
oscillation of the rotator at the bottom of its energy well. The
corresponding spectrum in Figure 6c was obtained with a single
frequency fast-exchange model with a slightly smaller librational
amplitude ((15°). It should be recognized that there is a large
uncertainty in the simulations in Figure 6a and that the suggested
results are primarily qualitative. However, the frequencies used to
model every spectrum were selected based on good visual
matching and an internally consistent Arrhenius-type behavior.
That is, we assumed that the high and low frequency motions
follow the thermal behavior expected from two distinct activation
barriers. As a criterion to support this assumption, we noticed
that the best simulations resulted in reasonable pre-exponential
and a large displacement of the alkyne sp carbons from 82 and
100 ppm to 87 and 95 ppm, respectively (Figure SI-17 of the
Supporting Information). In addition to problems associated
with the loss of ethyl acetate under fast sample spinning,
the strongly overlapping trityl aromatic signals centered at
124 ppm and 140 ppm conceal the signals of the phenylene
rotator, expected at ca. 130 ppm. Based on these observations, we
decided to study the site exchange of the phenylene rotator of 4
2
by taking advantage of low-temperature quadrupolar echo H
NMR using phenylene-deuterated static samples.
2
Variable Temperature Quadrupolar Echo H NMR Mea-
surements of 4-d4. Variable temperature 2H NMR is a powerful
method of studying the internal dynamics of static powder
samples over the range ca. 103ꢀ108 Hz.17 The strength of this
method arises from the orientation-dependent coupling of the
2
nuclear electric quadrupole with the H nuclear spin, which
2
results in a broad H spectrum known as the Pake or powder
pattern. The method is based on the fact that the shape of the
powder pattern changes in a predictable manner as a function of
2
changes in the reorientation of the Cꢀ H bond vector with respect
to the external magnetic field.18 Crystalline samples with well-
defined periodic processes display spectra that change as a function
of temperature in such a manner that the spectra can be simulated
with a model that considers the trajectories and frequencies of
motion. In the case of phenylene rotation, the trajectory of the
2
Cꢀ H bonds typically involves an exchange process between sites
related by 180° rotation, also known as 2-fold flips.
The 2H NMR experiments were undertaken with a standard
2H quadrupolar echo sequence using samples freshly crystallized
from a 9:1 ethyl acetate/methanol solution. Spectra were col-
lected at temperatures ranging from 200 to 308 K (Figure 6). The
2H labels on the central phenylene represent only 0.5% of the
total mass of the sample, so the spectra required the averaging of
2048 transients collected with a 20-s recycle delay. Considering
that there are two crystallographically distinct gyroscope mol-
ecules with rather different environments surrounding the rota-
tors, it seemed reasonable that we would detect two different
factors of ca. 9.7 ꢁ 1011 sꢀ1 and 4.3 ꢁ 1010
s
ꢀ1, which are within
the range of values observed in other crystalline phenylene
rotators.19 The corresponding activation energies have values
of 5 and 7 kcal/mol for the faster and slower rotators. While these
values are somewhat tentative, as we cannot use data below and
above the low (<103 sꢀ1) and fast exchange (>108 sꢀ1) regimes,
there is no question that rotational exchange at ambient tem-
perature occurs with frequencies that are greater than 100 MHz.
2
rotational rates in the H spectra. The experimental spectra
’ CONCLUSIONS
shown in Figure 6b were simulated using both single exchange
frequencies in Figure 6c, and with a model that includes two
different frequencies in Figure 6a. The simulations in Figure 6a
We have shown that a molecular gyroscope with a triaryl-
methyl stator based on relatively flat 3,5-diphenyl substituents
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yields a more open and robust packing motif than the analogous
one observed in a molecular gyroscope with tert-butyl groups in
the corresponding 3,5-positions. However, a structure with two
crystallographically distinct gyroscope molecules in the lattice
highlights the limitations of an average quantity, such as the
packing coefficient, given that internal rotation depends on the
local environment, rather than the unit cell average. Using
3,3,3-Tri(50-m-terphenyl)propyne (7). A dry 500-mL round-
bottom flask was charged with a magnetic stir bar and 6 (2.708 g, 3.777
mmol) and equipped with a reflux condenser. Acetyl chloride (30 mL)
was added, and the mixture was set to reflux for 60 min. Excess acetyl
chloride was removed in vacuo, and the residue was taken up in 30 mL of
dry benzene. The solvent was removed again, and 250 mL of dry
benzene was added. The solution was allowed to stir for 15 min before a
0.5 M solution of ethynylmagnesium bromide in THF (20 mL, 10
mmol) was added. The mixture was set to reflux for 4 h and was
quenched with saturated ammonium chloride, and the organics were
extracted with diethyl ether and dried over magnesium sulfate. The
crude product was purified by column chromatography (1 L of 100%
hexanes followed by 1 L of 9:1 hexanes/diethyl ether) to afford 2.07 g
(75.5% recovered yield) of 7 as a white solid. Analysis: melting point:
decomposes above 175 °C. 1H NMR (500 MHz, CDCl3) δ 7.79 (t, 3H,
J = 1.5 Hz), 7.74 (d, 6H, J = 1.5 Hz), 7.61 (dd, 12H, J = 1.0, 7.0 Hz), 7.44
2
variable temperature H NMR, we found evidence for two
different rotational exchange rates that we assign to the two
crystallographically independent gyroscope molecules in the
lattice. The activation energies of ca. 5 and 7 kcal/mol estimated
from the temperature-dependence of the rotational exchange are
among the lowest observed for phenylene rotators linked to
triarylmethyl stators.
(ddd, 12H, J = 1.0, 7.5, 7.5 Hz), 7.36 (t, 6H, J = 7.5 Hz), 2.91 (s, 1H). 13
C
’ EXPERIMENTAL SECTION
1
NMR (125 MHz, CDCl3) δ 145.7, 141.7, 141.2, 128.9, 127.6, 127.5,
127.2, 125.3, 89.7, 74.8, 56.3. FTIR (HATR, cmꢀ1): 3294, 3035, 2246,
1593, 1497, 1412. MALDI-TOF MS, C57H40: calcd, 724.3130; found,
724.3208.
IR spectra were obtained with an HATR-FTIR instrument. H and
13C NMR spectra were acquired on a spectrometer at 500 and 125 MHz,
respectively with CDCl3 as the solvent. Solid-state 13C CPMAS NMR
spectra were obtained at 75 MHz. 2H quadrupolar echo measurements
were acquired at 46.07 MHz with a 90° pulse width of 2.5 μs and
spinꢀecho and refocusing delays of 50 μs, and a recycle delay of 20 s.
DSC analyses were recorded between room temperature and the
melting/decomposition temperature as determined by a standard melt-
ing point apparatus.
3,5-Diphenylbromobenzene (5). A 500-mL three-neck round-
bottomed flask equipped with a reflux condenser was charged with 1,3,5-
tribromobenzene (13.83 g, 43.90 mol) and tetrakistriphenylphosphine
palladium (0) (1.26 g, 1.10 mmol) under argon. A 125-mL suspension of
phenylboronic acid (10.71 g, 87.80 mmol) in toluene was sparged with
argon for 30 min and added to the flask. A 100-mL solution of aqueous
1 M sodium carbonate was also sparged with argon and added to the
flask. The mixture was stirred vigorously and set to reflux for 48 h.
Reaction progress was monitored by thin layer chromatography. The
solvent was removed in vacuo, and the residue was taken up in 200 mL of
a 1:1 ethyl acetate/water mixture. The water layer was washed with ethyl
acetate (3 ꢁ 100 mL), and the combined organic layers were dried over
MgSO4. The solvent was removed, and the crude product was purified
by column chromatography (100% hexanes) to afford 8.20 g (60.4%
isolated yield) of 5 as a white solid. Analysis: melting point: 103 °C. 1H
NMR (500 MHz, CDCl3): δ 7.72 (s, 3H), 7.62 (dd, 4H, J = 1.5, 7.5 Hz),
7.48 (ddd, 4H, J = 1.5, 7.5, 7.5 Hz), 7.40 (tt, 2H, J = 1.5, 7.5 Hz). 13C NMR
(125 MHz, CDCl3) δ 143.6, 139.8, 128.9, 128.0, 127.2, 124.8, 123.2. FTIR
(solid HATR, cmꢀ1): 3084, 3060, 3035, 1595, 1560, 1498, 754, 696.
HRMS (MALDI) C18H13Br: calcd, 308.0201; found, 308.0211.
1,4-Bis(30,30,30-tri(500-m-terphenyl)propynyl)benzene (4).
A 100-mL three-neck round-bottom flask was charged with a magnetic
stir bar, 7 (0.446 g, 0.615 mmol), 1,4-diiodobenzene (0.101 g, 0.306
mmol), THF (40 mL), and triethylamine (20 mL). The mixture was
sparged with argon for 1 h. Bis(triphenylphosphine) palladium chloride
(0.053 g, 0.075 mmol) and copper(I) iodide (0.0254 g 0.133 mmol)
were added, and the mixture was set to reflux for 20 h. The reaction
mixture was washed with saturated ammonium chloride, and the
organics were extracted with dichloromethane and dried over magne-
sium sulfate. The crude product was purified by column chromatogra-
phy (hexanes/DCM = 4:1) and recrystallized from a hexanes/DCM
mixture to afford 0.408 g (87.4% recovered yield) of 4 as a white solid.
1
Analysis: melting point: decomposes above 374 °C. H NMR (500
MHz, CDCl3) δ 7.78 (d, 6H, J = 1.5 Hz), 7.76 (d, 12H, J = 1.5 Hz), 7.60
(d, 24H, J = 7.5 Hz), 7.55 (s, 4H), 7.42 (dd, 24H, J = 7.5, 7.5 Hz), 7.34
(t, 12H, J = 7.5 Hz). 13C NMR (125 MHz, CDCl3) δ 146.1, 141.8, 141.2,
131.8, 129.0, 127.6, 127.5, 127.3, 125.4, 123.4, 97.4, 86.1, 57.0. FTIR
(HATR, cmꢀ1): 3031, 1590, 1575, 1497, 1428, 1412. MALDI-TOF MS,
C120H82: calcd, 1522.6417; found, 1522.4879.
1,4-Bis(30,30,30-Tri(500-m-terphenyl)propynyl) d4-Benzene
(4-d4). A glass tube was charged with a magnetic stir bar, 7 (0.224 g,
0.309 mmol), and 1,4-dibromobenzene-d4 (0.0366 g, 0.153 mmol),
THF (10 mL), and diisopropylamine (5 mL), and the mixture was
sparged with argon for 1 h. Bis(triphenylphosphine)palladium(II)
chloride (0.025 g, 0.036 mmol) and copper(I) iodide (0.0087 mg,
0.046 mmol) were added, and the tube was then sealed and the mixture
was heated to 75 °C and stirred for 24 h. The reaction was cooled to
room temperature and washed with saturated ammonium chloride, and
the organics were extracted with dichloromethane and dried over
magnesium sulfate. The purification was the same as that for 4 and
yielded 116.2 mg (49.7% recovered yield) of 4-d4 as a white solid.
Tri(50-m-terphenyl)methanol (6). A 250-mL three-neck round-
bottom flask was charged with a magnetic stir bar and magnesium ribbon
(0.7821 g, 32.18 mmol) and equipped with a reflux condenser. The
entire setup was flame-dried and allowed to cool under argon. A 100-mL
solution of 1 (6.395 g, 20.68 mmol) was added, and the mixture was
stirred at reflux until the magnesium ribbon disappeared. Diethyl
carbonate (0.83 mL, 6.9 mmol) was then added dropwise, and the
mixture was set to reflux for 4 h. The reaction was quenched with
saturated ammonium chloride, and the organics were extracted with
diethyl ether and dried over magnesium sulfate. The crude product was
purified by column chromatography (hexanes/CH2Cl2 = 2:1) to afford
3.34 g (68.1% isolated yield) of 6 as a white solid. Analysis: melting
point: 169ꢀ171 °C. 1H NMR (500 MHz, CDCl3): δ 7.87 (t, 3H, J = 1.5
Hz), 7.72 (d, 6H, J = 1.5 Hz), 7.61 (dd, 12H, J = 1.50, 7.5 Hz), 7.42 (dd,
12H, J = 7.5, 7.5 Hz), 7.34 (tt, 6H, J = 1.0, 7.5 Hz), 3.08 (s, 1H). 13C
NMR (125 MHz, CDCl3) δ 147.9, 141.7, 141.2, 128.9, 127.6, 127.5,
126.0, 125.6, 82.7. FTIR (HATR, cmꢀ1): 3566, 3031, 1595, 1426.
MALDI-TOF MS, C55H39þ: calcd, 699.3046; found, 699.3187.
1
Analysis: melting point: decomposes above 374 °C. H NMR (500
MHz, CDCl3) δ 7.81 (s, 6H), 7.79 (d, 12H), 7.63 (d, 24H, J = 7.5 Hz),
7.44 (dd, 24H, J = 7.5, 7.5 Hz), 7.36 (t, 12H, J = 7.5 Hz). 13C NMR (125
MHz, CDCl3) δ 146.2, 141.8, 141.2, 131.4 (t, 1JCD = 25.3 Hz), 129.0,
127.6, 127.5, 127.3, 125.4, 123.2, 97.4, 86.1, 57.0. FTIR (HATR, cmꢀ1):
3032, 1590, 1575, 1497, 1428, 1412. MALDI-TOF, C120H78D4: calcd,
1526.6668; found, 1526.7817.
’ ASSOCIATED CONTENT
S
Supporting Information. Spectroscopy data for com-
b
pounds 4 and 4-d4 and intermediates 5, 6, and 7; variable
2658
dx.doi.org/10.1021/cg200373g |Cryst. Growth Des. 2011, 11, 2654–2659

Crystal Growth & Design
ARTICLE
temperature 13C CPMAS NMR of compound 4 and Arrhenius
plot from the variable temperature 2H NMR of compound 4; and
cif data. This material is available free of charge via the Internet
at http://pubs.acs.org.
2007, 129, 4306. (c) Vergadou, V.; Pistolis, G.; Michaelides, A.;
Varvounis, G.; Siskos, M.; Boukos, N.; Skoulika, S Cryst. Growth Des.
2006, 6, 2486. (d) Moorthy, J. N.; Natarajan, R.; Venugopalan, P. J. Org.
Chem. 2005, 70, 8568.
(10) Matsumoto, K.; Hatano, K.; Umezawa, N.; Higuchi, T. Synthesis
2004, 13, 2181.
’ AUTHOR INFORMATION
(11) Kitaigorodskii, A. Molecular Crystals and Molecules; Academic
Press: New York, 1973.
(12) Dunitz, J. D.; Gavezotti, A. Acc. Chem. Res. 1999, 32, 677.
(13) Karlen, S. D.; Ortiz, R.; Chapman, O.; Garcia-Garibay, M. A.
J. Am. Chem. Soc. 2005, 127, 6554.
Corresponding Author
*E-mail: mgg@chem.ucla.edu.
(14) Connolly, M. L. Science 1983, 221, 709.
(15) Cerius2Modeling Environment, version 4.2; Molecular Simu-
lations Inc.: San Diego, CA, 1999.
’ ACKNOWLEDGMENT
Financial support by the National Science Foundation
(Grants DMR0605688 and CHE0551938 and creativity exten-
sion DMR0937243) is gratefully acknowledged. Z.O. acknowl-
edges the National Science Foundation IGERT MCTP Program
(DGE0114443). We thank Dr. Fernando Uribe-Romo for valu-
able discussions and advice regarding X-ray crystallography.
(16) (a) Fyfe, C. A. Solid State NMR for Chemists; C. F. C. Press:
Guelph, 1983. (b) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys.
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(17) Hoatson, G. L.; Vold, R. L. NMR 1994, 32, 1.
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dx.doi.org/10.1021/cg200373g |Cryst. Growth Des. 2011, 11, 2654–2659