Molecular "compasses" and "gyroscopes." III. Dynamics of a phenylene rotor and clathrated benzene in a slipping-gear crystal lattice

Article abstract of DOI:10.1021/ja025753v

Samples of 1,4-bis(3,3,3-triphenylpropynyl)benzene 3 were prepared by Pd(0)-catalyzed coupling of 3,3,3-triphenylpropyne (1) and 1,4-diiodobenzene. The structure of compound 3 is such that the central phenylene can play the role of a gyroscope wheel, whil

Full text of DOI:10.1021/ja025753v

Published on Web 06/05/2002
Molecular “Compasses” and “Gyroscopes.” III. Dynamics of a
Phenylene Rotor and Clathrated Benzene in a Slipping-Gear
Crystal Lattice
Zaira Dominguez, Hung Dang, M. Jane Strouse, and Miguel A. Garcia-Garibay*
Contribution from the Department of Chemistry and Biochemistry, The UniVersity of California,
Los Angeles, California 90095-1569
Received January 30, 2002
Abstract: Samples of 1,4-bis(3,3,3-triphenylpropynyl)benzene 3 were prepared by Pd(0)-catalyzed coupling
of 3,3,3-triphenylpropyne (1) and 1,4-diiodobenzene. The structure of compound 3 is such that the central
phenylene can play the role of a gyroscope wheel, while the alkyne bond and trityl groups can act as an
axle and shielding frameworks, respectively. Crystals grown from benzene and dichloromethane were
13
characterized by X-ray diffraction, variable-temperature C CPMAS NMR, quadrupolar echo solid-state
2
H NMR, and thermal analyses. The rotational dynamics of benzene molecules and phenylene groups
were characterized in terms of 6-fold rotation and 2-fold flipping models, respectively. The possibility of a
gearing mechanism between adjacent benzene molecules and phenylene groups suggested by the clathrate
structure was investigated. However, it was found that 6-fold rotation of benzene molecules at 300 K occurs
in the gigahertz regime (or higher) and 2-fold flipping of phenylene groups in the kilohertz range in a structure
that can be described as a slipping-gear lattice. The rotational dynamics of the phenylene group in the
solvent-free structure were remarkably similar to those in the clathrate, and both are among the fastest
known for phenylene rotation in solids. The results presented here provide a valuable starting point for the
design and analysis of crystalline solids with correlated molecular motions.
1. Introduction
1
Numerous developments in X-ray diffraction and solid-state
2
NMR have paved the way for the study of internal molecular
motions in crystalline solids. We recently became interested in
the rational design of crystalline solids that can support fast
molecular motions. We reasoned that crystals built with
molecules that experience volume-conserving conformational
motions should have many interesting physicochemical proper-
ties. In particular, we are interested in electrooptic materials
based on dipolar units that can reorient under the influence of
electric, magnetic, and optical stimuli. The desired molecular
structures must permit the reorientation of a polar group while
maintaining a static framework that supports a very robust
crystal lattice. In analogy with macroscopic objects and to
suggest some of their properties and functions, we refer to these
structures as molecular compasses and molecular gyroscopes.
Figure 1. Molecular compasses and gyroscopes based on 1,4-bis-
triarylpropynyl)benzenes. The central difluorobenzene should be shielded
from contacts with neighboring molecules and solvent of crystallization.
Shielding may be accomplished by bridges across the two trityl groups (1)
or by bulky tert-butyl groups (2).
(
One of several promising molecular architectures currently
explored in our group for the synthesis of molecular compasses
and gyroscopes is based on 1,4-bis(triarylpropynyl)benzenes
(Figure 1). We expect that sterically guarded 1,4-diethynylphen-
ylenes should be able to reorient in the solid state, while the
triarylmethyl (trityl) framework remains static in the crystal
lattice. Rotation of aryl groups attached to triple bonds should
*
To whom correspondence should be addressed. E-mail: mgg@
chem.ucla.edu.
(
1) (a) B u¨ rgi, H. B. Annu. ReV. Phys. Chem. 2000, 51, 275-296. (b) Dunitz,
J. D.; Maverick, E. F.; Trueblood, K. N. Angew. Chem., Int. Ed. Engl.
3
be essentially frictionless in the gas phase, and steric shielding
1
8
988, 27, 880-895. (c) Gavezzotti, A.; Simonetta, M. Chem. ReV. 1982,
2, 1-13.
will be accomplished with groups bridging across the two triaryl
methanes, such as structure 1, and with bulky substituents such
as the tert-butyl groups in compound 2.
Working toward the synthesis of compounds 1 and 2, we
recently reported a simple procedure for the preparation of 4-bis-
(
2) (a) Fyfe, C. A. Solid State NMR for Chemists; CFC Press: Guelph, Ontario,
1
1
1
983. (b) Lyerla, J. R.; Yannoni, C. S.; Fyfe, C. A. Acc. Chem. Res. 1982,
5, 208-216. (c) Blumich, B.; Spiess, H. W. Angew. Chem., Int. Ed. Engl.
988, 27, 1655-1672. (d) Wendeler, M.; Fattah, J.; Twyman, J. M.;
Edwards, A. J.; Dobson, C. M.; Heyes, S. J.; Prout, K. J. Am. Chem. Soc.
1
997, 119, 9793-9803.
10.1021/ja025753v CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 7719-7727
9
7719

A R T I C L E S
Dominguez et al.
Scheme 1
Figure 2. Space-filling models and line formulas illustrating the packing
arrangement of rotor 3 and the benzene dimer. Trityl groups, 1,4-phenylenes,
and benzene are colored in blue, red, and purple, respectively. A fourth
molecule in the forward plane that closes a four-pillar cage was omitted
for clarity. The angle formed by the phenylene 1,4-axis and benzene 6-fold
axis is 59.7°.
grown from benzene and a new desolvated crystal form obtained
from dichloromethane. The dynamic behavior of the phenylene
groups and benzene molecules in the crystal lattice of the
(
3,3,3-triphenylpropynyl)benzene 3 and a preliminary study of
4,5
its solid-state dynamics. Crystals grown from benzene contain
13
benzene clathrate was investigated by variable-temperature
C
one molecule of 3 (Figure 2, in blue and red) and two molecules
2
CPMAS NMR measurements and quadrupolar echo H NMR
line-shape analyses between 200 and 330 K. Results clearly
show that 2-fold flipping of the phenylene group at ambient
temperatures occurs in the kilohertz regime, while rotation of
benzene molecules about their 6-fold axis occurs with frequen-
cies that are above 100 MHz. Although results from these
experiments suggest a description of the benzene clathrate of 3
as a slipping-gear lattice, they provide us with a starting point
for the design of crystals with correlated rotation. Rapid 2-fold
flipping was also observed for isolated phenylene groups in
solvent-free crystals grown from dichloromethane.
6
of benzene (shown in purple) per unit cell. The triclinic packing
arrangement (space group P 1h ) can be described in terms of cages
formed by four adjacent molecular rotors incarcerating a parallel-
7
displaced benzene dimer. Phenylene groups and benzene
molecules experience gear-like, edge-to-edge contacts with their
1
,4- and 6-fold axes, respectively, at an angle of 59.7°.
Using a simple coalescence analysis of the 13C CPMAS
NMR, we were able to establish that phenylene group undergoes
2
-1
rapid 2-fold flipping with a rate of 1.3 × 10 s at -18 °C.
From a sharp signal in the 13C CPMAS NMR spectrum, we
deduced that benzene molecules are also in a state of rapid
rotation. Knowing that phenylene groups and benzene molecules
are within close distances, the question arose whether they could
behave as a dynamically geared system. Although the transfer
of momentum from six-cogged to two-cogged gears should be
inefficient, the prospect of observing geared crystalline lattices
with angular momentum transfer from benzene molecules to
phenylene units in a concerted fashion is a very intriguing
possibility.
2. Results and Discussion
2
.1. Synthesis. Samples of 1,4-bis(3,3,3-triphenylpropynyl)-
benzene 3 and its deuterated analogue, 1,4-bis(3,3,3-triphenyl-
propynyl)-d4-benzene d4-3, were synthesized by a simple two-
step procedure from triphenylmethyl chloride (trityl chloride)
6
as illustrated in Scheme 1.
Substitution of the chloride by ethynylmagnesium bromide
in refluxing THF under an argon atmosphere by the method of
Masson et al. gave 3,3,3-triphenylpropyne 7 in 80% yield. The
Many specific mechanisms must exist for redistribution of
thermal energy in crystalline solids, and the concerted rotation
of molecular segments in a crystal lattice may be one possibility.
The simplest feature of a supramolecular geared system, such
as the benzene clathrate of 3, would entail the rate of rotation
of the phenylene group about its 1,4-axis to be commensurate
to the rate of rotation of benzene molecules along their 6-fold
axis. Looking for a simple qualitative answer, we have analyzed
the solid-state structure and thermal properties of crystals of 3
8
formation of triphenylmethane 8 as a side product, and formation
of triphenylmethyl peroxide in the presence of oxygen, suggest
the formation of trityl radicals as a side reaction. The second
and final step was accomplished by Pd(0)-catalyzed coupling
of 2.5 equiv of alkyne 7 with 1,4-diiodobenzene or 1,4-dibromo-
d4-benzene in refluxing piperidine to yield rotors 3 and d4-3,
respectively. There was no significant difference in reaction
times and yields when either of the two aryl halides were used.
Analysis of the crude reaction mixtures by GC (carbowax
(
3) (a) Saebo, S.; Almolof, J.; Boggs, J. E.; Stark, J. G. J. Mol. Struct. 1989,
2
00, 361-373. (b) Abramenkov, A. V.; Almenningen, A.; Cyvin, B. N.;
1
column, 300 °C) and H NMR indicated a crude reaction yield
Cyvin, S. J.; Jonvik, T.; Khaikin, L. S.; Rommingin, C.; Vilkov, L. V.
Acta Chem. Scand. 1988, A42, 674-678. (c) Sipachev, V. A.; Khaikin, L.
S.; Grikina, O. E.; Nikitin, V. S.; Traettberg, M. J. Mol. Struct. 2000, 523,
of ca. 80% with the mono-coupling product 4 and alkyne dimer
9
5
in ca. 5% each. Column chromatography in hexanes:C6H6:
1
-22. (d) Seminario, J.; Zacarias, A. G.; Tour, J. M. J. Am. Chem. Soc.
000, 122, 3015-3020. (e) Miteva, T.; Palmer, L.; Kloppenburg, L.; Neher,
2
CH2Cl2 ) 90:5:5, and recrystallization gave pure samples of 3
and d4-3 in nonoptimized yields of 13 and 11%, respectively.
The structures of 3 and d4-3 were confirmed by 13C and H
NMR, high-resolution mass spectrometry, and single-crystal
X-ray diffraction data.
D.; Bunz, U. H. F. Macromolecules 2000, 33, 652-654.
4) Dominguez, Z.; Dang, H.; Strouse, M. J.; Garcia-Garibay, M. A. J. Am.
Chem. Soc. 2002, 124, 2398-2399.
(
(
1
5) Molecular compasses and gyroscopes based on triptycene frameworks have
also been investigated in our group: Godinez, C. E.; Zepeda, G.; Garcia-
Garibay, M. A. J. Am. Chem. Soc. 2002, 124, in press.
(
6) Compound 3 is structurally related to molecules known to form so-called
wheel-and-axle clathrates. For leading references, please see: McNicol,
D. D.; Toda, F.; Bishop, R. ComprehensiVe Supramolecular Chemistry;
Pergamon: Oxford, 1996; Vol. 6.
(8) Masson, J.-C.; Quan, M. L.; Cadiot, P. Bull. Soc. Chim. Fr. 1968, 3, 1085-
1088.
(7) Hobza, P.; Slezle, H. L.; Schag, E. W. J. Am. Chem. Soc. 1994, 116, 3500-
(9) Hart, H.; Lin, L. T. W.; Ward, D. L. J. Am. Chem. Soc. 1984, 106, 4043-
3
506.
4045.
7720 J. AM. CHEM. SOC.
9
VOL. 124, NO. 26, 2002

Molecular “Compasses” and “Gyroscopes”
A R T I C L E S
Table 1. Unit Cell Parameters of the Two Crystalline Structures
for Molecular Rotor 3
benzene clathrate
solvent-free
temp (K)
habit
298(2)
prisms
100(2)
prisms
100(2)
prisms
system
space group
a (Å)
b (Å)
c (Å)
triclinic
P1h
triclinic
P1h
triclinic
P1h
8.6601(15)
9.6711(17)
14.600(2)
76.245(4)
76.679(4)
72.968(3)
1118.5(3)
1
8.5157(19)
9.547(2)
14.467(3)
77.315(4)
76.469(4)
72.970(4)
1078.4(4)
1
9.1500(14)
10.3067(16)
10.5004(17)
61.736(3)
85.731(3)
72.477(3)
829.0(2)
1
R (deg)
â (deg)
γ (deg)
Figure 4. Stereoview of the packing structure of molecular rotor 3 viewed
down the c-axis.
3
vol (Å )
Z
3
dens (Mg/m )
1.139
1.181
1.223
Figure 3. Ortep diagram of the X-ray structure determined from diffraction
data measured at 298 K of trityl rotor 3 recrystallized from benzene. Thermal
ellipsoids drawn at the 30% probability level.
Figure 5. Clathrate structure of rotor 3 with trityl groups removed to
illustrate the two closest H-H distances in Å between each phenylene group
(
A-D) and benzene molecules (B1 and B2). Edge-to-edge distances which
2
.2. X-ray Analysis: A Potential Crystalline Gear Chain.
could lead to dynamic gearing (B1-C and B2-B) are noted in bold face.
X-ray quality prisms of rotor 3 were obtained by slowly cooling
a saturated hot benzene solution. X-ray diffraction data were
acquired at 298 K, and the structure was solved in the triclinic
space group P 1h . Diffraction data acquisition and structure
refinement parameters are listed in Table 1. With coincident
molecular and crystallographic inversion centers, the unit cell
consists of only one molecule of 3 and two molecules of
benzene. The molecular structure of 3 and its relation with a
benzene molecule are shown in Figure 3. The alkyne groups
deviate only slightly from linearity. The angle between C2, the
center of the C4-C5 bond, and C6 is only 3°. The triphenyl-
methyl groups adopt an anti-conformation with the plane of the
center-to-center distances of 4.88 Å. It is also noteworthy that
each benzene molecule experiences an aromatic C-H-π interac-
1
0
tion with a hydrogen from a neighboring trityl group directed
to the center of the benzene molecule at a distance of 2.94 Å.
Regarding the possible transfer of angular momentum from
rotating benzene molecules to phenylene groups, it is known
that benzene can undergo rapid in-plane rotation about its 6-fold
axis, while rotation of the phenylene groups is restricted along
their 1,4-axis. A close inspection of the clathrate structure is
facilitated by analysis of a caged dimer in Figure 5, where the
trityl groups have been removed for clarity. Benzene molecules
in the figure are labeled B1 and B2 and phenylene groups with
letters A-D. Each benzene molecule is in close proximity with
the other one in the dimer, and with four surrounding molecular
rotors. There are no benzene-to-phenylene distances shorter than
the sum of the van der Waals radii of two hydrogen atoms11
(2 × 1.2 Å ) 2.4 Å). However, as indicated in the table in
Figure 5, the hydrogen atoms in each benzene molecule are
relatively close to hydrogen atoms in three of the four sur-
rounding phenylenes (dH-H ) 2.67-3.48 Å). From the three
closest phenylene rotors, there is only one that experiences edge-
to-edge interactions with a given benzene. These edge-to-edge
contacts involve molecules labeled B1-C and B2-B. With edge-
to-edge distances as close as 2.67 Å, one may speculate that
1
,4-phenylene nearly parallel to the vector given by the bond
between the methane (C6) and ipso carbons (C7) corresponding
to one of the three aryl groups. The twisting of the phenyl groups
in each trityl substituent gives rise to chiral propeller conforma-
tions with dihedral angles Calkyne-Cmethane-Cipso-Cortho of
(29.8, (50.5, and (45.1. The different sense of rotation in
each trityl group renders them enantiomeric, as required by the
molecular center of inversion.
As expected for a rigid, rodlike structure, all molecules of 3
are aligned in the same direction with complementary face-to-
edge contacts between enantiomeric trityl propellers. A stereo-
view of the packing diagram down the c-axis in Figure 4
illustrates a cage formed by four molecules of 3 encapsulating
the benzene molecules in a parallel-displaced dimeric arrange-
ment. Benzene dimers possess an interplanar distance of 3.59
Å, which is nearly identical to that calculated for the lowest
energy parallel-displaced gas-phase dimer. However, a transla-
tion of 3.31 Å in the clathrate is significantly longer than a
(
10) (a) Takahashi, H.; Tsuboyama, S.; Umezawa, Y.; Honda, K.; Nishio, M.
Tetrahedron 2000, 56, 6185-6191. (b) Umezawa, Y.; Tsuboyama, S.;
Takahashi, H.; Uzawa, J.; Nishio, M. Tetrahedron 1999, 55, 10047-10056.
(
c) Suezawa, H.; Yoshida, T.; Hirota, M.; Takahashi, H.; Umezawa, Y.;
Honda, K.; Tsuboyama, S.; Nishio, M. J. Chem. Soc., Perkin Trans. 2 2001,
053-2058. (d) Nishio, M.; Umezawa, Y.; Hirota, M.; Takeushi, Y.
Tetrahedron 1995, 51, 8665-8701.
(11) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
2
1
.64 Å translation calculated in the gas phase. This results in
J. AM. CHEM. SOC.
9
VOL. 124, NO. 26, 2002 7721

A R T I C L E S
Dominguez et al.
Figure 6. ORTEP diagram of rotor 3 in desolvated crystals with thermal
ellipsoids drawn at a 30% probability level. Crystals were grown from
CH2Cl2.
in-plane rotation of a benzene molecule may gear with rotation
of the phenylene group along the 1,4-axis. Clockwise rotation
of B1 would result in anticlockwise rotation of C. However,
transfer of motion from a six-cogged benzene gear to a two-
cogged phenylene may lead to frequent slipping. It is interesting
to speculate that additional pathways for transfer of (linear and)
angular momentum through contacts with neighboring rotors,
and from one benzene to another, may follow dynamic pathways
with high degree of order restricted by lattice symmetries.
2.3. Thermal Stability of the Benzene Clathrate: DSC and
TG Analysis. To characterize the rotational dynamics of the
benzene clathrate by solid-state VT NMR experiments, we
needed to determine its thermal stability. Differential scanning
calorimetry (DSC) and thermal gravimetric analysis (TGA)
measurements were carried out on freshly crystallized samples
of compound 3. From the TGA measurement, a weight loss of
Figure 7. Packing diagrams of compound 3 in the clathrate (top, with
benzene molecules removed for clarity) and desolvated structures (bottom).
2
0% was observed between 100 and 130 °C, which is due to
the evaporation of benzene from the clathrate. DSC analysis
run with the same sample showed an endothermic transition at
1
00 °C followed by an exothermic peak at 105 °C and sharp
melting at 316 °C. The transitions at 100 and 105 °C correspond
to desolvation followed by a reconstructive solid-to-solid-phase
transition. Clathrate single crystals left at ambient temperature,
and pressure lost benzene and turned opaque within 2 to 3 days.
2.4. X-ray Structure of the Desolvated Crystals. Desolvated
single crystals obtained by slow evaporation from CH2Cl2 were
shown to be spectroscopically (FT-IR and NMR) and thermally
Figure 8. Space-filling view of the local environment of the phenylene
group (red) in desolvated crystals of 3 illustrating the “cage” formed by 10
phenyl groups from three trityl groups above (dark blue) and three below
(DSC) identical to those obtained upon loss of benzene from
(light blue).
the clathrate (vide infra). Diffraction data and structure refine-
ment parameters for the solvent-free crystals obtained at 100 K
are listed in Table 1. The structure was solved in a triclinic P1h
space group with one-half of a molecule per asymmetric unit
due to coincident molecular and crystallographic inversion
centers.
no significant modifications in the clathrate upon cooling from
93 to 100 K. Structures with and without solvent had different
unit cell dimensions but very similar packing topologies (Figure
) as expected for a first-order solid-to-solid-phase transition.
2
7
Small changes in unit cell angles and a modest expansion in
the a- and b-directions (7.4 and 8.0%, respectively) are
accompanied by a large 27.4% contraction along the c-axis. As
suggested in Figure 7, the loss of benzene from the clathrate to
the solvent-free crystal lattice is facilitated by a relatively small
molecular displacement. Although removal of benzene from the
clathrate to the solvent-free structure decreases the volume of
the unit cell by 23.2%, the change in density is only 3.6% (at
100 K). The local environment around phenylene groups in the
desolvated form is illustrated in Figure 8. Close interdigitation
of adjacent molecules results in the formation of a supramo-
lecular “cage” constituted by 10 phenyl groups from six
neighboring trityl groups.
The molecular structure of rotor 3 in the desolvated crystal
is similar to that observed in the benzene clathrate (Figure 6).
The trityl groups adopt enantiomeric propeller conformations
related by the molecular center of inversion. The diethynyl
benzene fragment has a greater deviation from linearity than
that in the benzene clathrate. It can be appreciated from the
structure in Figure 6 that all four alkyne and the two phenylene
ipso carbons lie outside an imaginary straight line subtending
the two methane carbons (C6 and C6A) and passing through
the center of the phenylene ring.
To compare changes in packing structure with and without
solvent, the structure of the benzene clathrate was also
determined at 100 K (Table 1). It is worth noting that there are
7722 J. AM. CHEM. SOC.
9
VOL. 124, NO. 26, 2002

Molecular “Compasses” and “Gyroscopes”
A R T I C L E S
Figure 9. 13C CPMAS NMR spectrum of the benzene clathrate of molecular
rotor 3. For signal labels, please see Figure 3.
2
.5. Solid-State 13C CPMAS NMR Chemical Shift As-
signments in the Benzene Clathrate. Solution-like spectra can
be measured with solid samples by taking advantage of the cross
polarization and magic angle spinning technique developed by
Pines and co-workers.1 The cross polarization (CP) method
relies on the transfer of magnetization from the highly abundant
and sensitive hydrogen atoms to the less sensitive and highly
2,13
1
3
diluted C nuclei. The experiment includes simultaneous high
1
power H-decoupling and fast sample spinning (5-20 kHz) at
the magic angle (54.7°) to remove the line broadening that
comes from static anisotropic interactions mediated by the
external magnetic field such as heteronuclear dipolar coupling
and chemical shift anisotropy. Although resolution in the solid
state is not as high as that observed in solution, chemical shifts
determined by the CPMAS method are generally comparable.
The 13C CPMAS NMR spectrum of the benzene clathrate is
shown in Figure 9. Signals were labeled in the same manner as
those in the ORTEP diagram shown in Figure 3. Specific
assignments were confirmed by interrupted decoupling experi-
ments and by taking advantage of partial isotopic labeling. It is
well known that interrupted decoupling after cross polarization,
and prior to acquisition, results in the loss of signals corre-
Figure 10. Variable-temperature 13C CPMAS NMR of the benzene
clathrate of 3. The signals corresponding to C1 and C3 are indicated with
a star.
experiment to experiment. Knowing that carbon atoms C1 and
C3 are crystallographically and magnetically nonequivalent, we
recognized that variations in the width and intensity of this signal
may reflect dynamic processes related to rotation about the
phenylene 1,4-axis.
sponding to strongly coupled protonated carbons.14 Signals
corresponding to methane (C6), alkyne (C4 and C5), and
aromatic ipso carbons (C7, C13, and C19) were partially
assigned in this manner. Aromatic signals corresponding to
phenylene rotor, benzene of crystallization, and the protonated
carbons of the trityl groups were assigned with the help of
2
.6. Variable-Temperature 13C CPMAS Measurements in
the Benzene Clathrate of 3. The results from experiments
carried out between 220 and 297 K are shown in Figure 10. As
expected, the most important changes in this temperature range
occur to the signal assigned to C1 and C3 at 131 ppm. At 297
K, the signal appears weak and broad, suggesting that coales-
cence of two signals corresponding to each of the two carbons
may occur near ambient temperature. However, an increase in
intensity and eventual split into two well-resolved singlets at
4
deuterium labeling experiments as described previously. The
number of signals in the solid-state spectrum is in good
agreement with expectations from the symmetry present in the
crystal lattice. Single peaks at 55, 83, and 99 ppm correspond
to the methane (C6) and alkyne carbons (C4 and C5) of the
two crystallographically equivalent molecular halves. Three
signals between 142 and 147 ppm correspond to the ipso carbons
in each of the three phenyl groups of the two equivalent
triphenyl methanes (C7, C13, and C19). Although these signals
are equivalent in solution with an average chemical shift of 145.2
ppm in CDCl3, they have different magnetic environments in
the solid state. A signal at 122 ppm corresponds to the equivalent
ipso carbons of the central phenylene (C2 and C2A), and a broad
signal at 131 ppm was assigned to the four protonated carbons
1
31.6 and 130.8 ppm was observed as the temperature was
lowered to 250 K. With a splitting of 60 Hz between C1 and
C3, and a coalescence temperature of ca. 255 K, a time constant
-
1
for rotation of 7.7 ms (rate of 130 s ) and an approximate
barrier of 12.8 kcal/mol were calculated.
2
2
.7. Variable-Temperature H Quadrupolar Echo Mea-
surements with the Benzene Clathrate of 3. Deuterium NMR
in the solid state is largely dominated by the interaction between
the nuclear spin and electric quadrupole moment at the nucleus.15
The orientation-dependence of the quadrupolar coupling char-
acterizes the spectra and makes it one of the most sensitive
probes for dynamics in crystalline solids and other rigid media.
(C1, C1A, C3, and C3A). We noticed that the width and
intensity of the signal at 131 ppm could change slightly from
(
12) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569-590.
13) For some general reviews of the CPMAS experiment, please see: (a)
Schaefer, J.; Stejskal, E. O. Top. Carbon-13 NMR Spectrosc. 1979, 3, 283-
(
2
3
24. (b) Fyfe, C. A. Solid State NMR for Chemists; CFC Press: Guelph,
The spectrum of a single crystal with only one type of C- H
Ontario, 1983. (c) Yannoni, C. S. Acc. Chem. Res. 1982, 15, 201-208. (d)
bond would consist of a doublet with a quadrupolar splitting
Lyerla, J. R.; Yannoni, C. S.; Fyfe, C. A. Acc. Chem. Res. 1982, 15, 208-
2
16.
∆ν that depends on the orientation angle â that the bond makes
(
14) (a) Alemany, L. B.; Grant, D. M.; Alger, T. D.; Pugmire, R. J. J. Am.
Chem. Soc. 1983, 105, 6697-6704. (b) Opella, S. J.; Frey, M. J. Am. Chem.
Soc. 1979, 101, 5854-5856. (c) Opella, S. J.; Frey, M. H.; Cross, T. A. J.
Am. Chem. Soc. 1979, 101, 5856-5857.
15,16
with respect to the external field.
(15) Hoatson, G. L.; Vold, R. L. NMR: Basic Princ. Prog. 1994, 32, 1-67.
J. AM. CHEM. SOC.
9
VOL. 124, NO. 26, 2002 7723

A R T I C L E S
Dominguez et al.
3
2
2
3
2
∆
ν ) / (e q Q/h)(3 cos â - 1) ) / QCC(3 cos â - 1)
4 zz 4
Q in the equation represents the electric quadrupole moment of
the deuteron, e and h are the electric charge and Planck constant,
respectively, and qzz is the magnitude of the principal component
2
of electric field gradient tensor, which lies along the C- H
2
bond. The splitting frequency for aromatic C- H bonds, with
QCC ) 180 kHz, changes from 270 kHz when â ) 0° and 135
2
kHz for â ) 90°, to 0 kHz when the C- H bond is oriented at
the magic angle, that is, â ) 54.7°. Although powdered samples
have molecules with all values of â represented, they are not
all equally populated. A collection of doublets in all possible
orientations gives rise to a broad symmetric spectrum with two
maxima and two shoulders known as a Pake or powder pattern.
Variations in the powder pattern of a solid sample occur when
2
the C- H bonds experience reorientations that reduce the range
of their magnetic interactions by dynamic averaging. The
dynamic range covered by line shape analysis techniques is
commensurate with the magnitude of the QCC (∼180 kHz) and
4
8
-1
goes from ca. 10 to 10 s .
For our study, we take advantage of very well-characterized
spectral changes occurring upon 2-fold phenyl flipping and free
Figure 11. Calculated (a, c) and experimental (b, d) quadrupolar echo 2H
NMR spectra of 3-d4 in C6H6 and 3 in C6D6 at 298 K. The spectra of 3-d4/
17
C6H6 (a and b) correspond to motion of the phenylene group in the slow
phenyl rotation about its 1,4-axis, as well as in-plane rotation
of benzene molecules.18 Measurements were carried out with a
quadrupolar echo sequence on static powdered samples. Spectra
were obtained at 46.07 MHz with 90° pulse width of 2.2 µs,
echo and refocusing delays of 30 and 20 µs, respectively, and
a 60 s delay between pulses (20 s for 3/C6D6). Phenylene
dynamics were explored with crystals of 3-d4 grown from C6H6.
Spectra were acquired in the range between 146 and 324 K to
examine benzene dynamics in the C6D6 clathrate. It was not
possible to decrease the temperature below 146 K due to the
probe limitations. Spectra measured with either of the two
labeled samples between 200 and 300 K showed no spectral
changes. In agreement with the VT 13C CPMAS measurements,
spectra from the phenylene group correspond to the slow
3
exchange regime (krot ) 6 × 10 ). The simulated spectra for samples of 3
containing C6D6 (c and d) correspond to in-plane rotation of C6D6 about
8
-1
its 6-fold axis in the fast exchange limit (krot ) 10 s ).
4
-1
exchange, or static limit (Figure 11, krot < 10 s ). The spectra
corresponding to benzene rotation are consistent with discrete
jumps rather then continuous rotation. The jump rate, even at
7
-1
2
00 K, occurred in the fast exchange limit (krot > 10 s ).
Figure 12. 13C CPMAS NMR spectra of benzene-grown crystal of 3
illustrating the growth of the desolvated phase by 363 K.
Although simulations involving 60° and 120° jumps are
2
18b
indistinguishable from each other, previous H NMR and
1
9
neutron scattering studies with crystalline benzene and a
benzene clathrate with cyclohexen-1,3-dione18a have shown that
to that of the benzene clathrate, it seemed possible that
phenylene rotation in solvent-free crystals may occur within the
nearest neighbor 60° jumps (6-fold rotation) are the most likely.
These studies have shown 2-fold flip in the fast exchange limit
2
range covered by the H line shape analysis method. To explore
1
3
that, we decided to investigate the desolvation process by
C
above 140 K in the case of crystalline C6D6 and above 290 K
2
in the case of the cyclohexen-1,3-dione clathrate.18a
NMR and then to carry out a detailed H NMR analysis in the
desolvated samples. Desolvation analysis of the benzene clath-
2
.8. Desolvation Analysis by Solid-State NMR. Knowing
13
rate by C CPMAS was carried out with freshly grown crystals
of 3. The temperature was increased by 5° increments from 294
to 373 K. Representative spectra are illustrated in Figure 12.
In full agreement with the TGA and DSC experiments, a first-
order reconstructive phase transition can be observed as the
signals corresponding to the clathrate disappear and the signals
of the desolvated crystals grow-in sharply at 363 K (90 °C).
The evolution of the sample at that temperature is highlighted
in the figure by upward arrows corresponding to the new phase
and downward arrows that correspond to the disappearing
that the density of the desolvated molecular rotor was analogous
(
16) This equation assumes negligible asymmetry in the electric field gradient
tensor: η ) (qxx - qyy)/qzz ) 0. Deviations from this condition are generally
small for aromatic deuterons.
(17) (a) Cholli, A. L.; Dumais, J. J.; Engel, A. K.; Jelinski, L. W. Macromolecules
1
984, 17, 2399-2404. (b) Rice, D. M.; Witebort, R. J.; Griffin, R. G.;
Meirovich, E.; Stimson, E. R.; Meinwald, Y. C.; Freed, J. H.; Scheraga,
H. A. J. Am. Chem. Soc. 1981, 103, 7707. (c) Rice, D. M.; Meinwald, Y.
C.; Scheraga, H. A.; Griffin, R. G. J. Am. Chem. Soc. 1987, 109, 1636-
1
640. (d) Rice, D. M.; Blume, A.; Herzfeld, J.; Wittebort, R. J.; Huang, T.
H.; DasGupta, S. K.; Griffin, R. G. Biomol. Stereodyn., Proc. Symp. 1981,
, 255-270.
18) (a) Hoa, J.; Vodl, R. R.; Vold, R. L.; Etter, M. C. J. Phys. Chem. 1989, 93,
2
(
(
7
618-7624. (b) Boden, N.; Clark, L. D.; Hanlos, S. M.; Mortimer, M.
13
benzene clathrate. The C CPMAS spectrum of the desolvated
sample obtained in situ in Figure 12 (top spectrum) was identical
Faraday Symp. Chem. Soc. 1978, 109.
19) Fujara, F.; Petry, W.; Schnauss, W.; Silescu, H. J. Chem. Phys. 1988, 89.
7724 J. AM. CHEM. SOC.
9
VOL. 124, NO. 26, 2002

Molecular “Compasses” and “Gyroscopes”
A R T I C L E S
Scheme 2
2.10. Benzene, Phenyl, and Phenylene Rotation in the Solid
State. It is clear from our study that the rates of rotation of the
phenylene group and benzene molecules in the clathrate structure
of 3 are not correlated in a simple manner. The rate of the 6-fold
rotation for benzene molecules in the dimer falls in the fast
2
Figure 13. Experimental (right) and simulated (left) solid-state H NMR
of desolvated samples of 3-d4. The rotation rate constants used for fitting,
6
-1
from bottom to top, are (×10 s ) as follows: 0.015, 0.4, 1.3, 2.2, and
3
.8.
2
exchange regime of the H line shape analysis methods (krot )
8
-1
to spectra acquired with desolvated crystals grown from CH2Cl2
10 s ) under the conditions where the clathrate is thermally
stable (ca. 200-300 K). Although there are only a few examples,
it is known that benzene molecules in crystals reorient about
(not shown).
The spectrum of the desolvated phase was partially assigned
by analogy with the spectrum of the clathrate as well as by
their 6-fold axis with very low activation barriers and with very
1
4
18,19
dipolar dephasing and variable-temperature experiments. It is
primarily characterized by a larger chemical shift dispersion of
signals corresponding to the trityl group. While a group of 15
protonated trityl carbons in the clathrate occur between 126 and
high preexponential factors.
The rates of degenerate 6-fold
1
9
rotation in the solid state are affected in a relatively modest
manner as compared to the rates of isotropic rotation observed
in liquids. The rotational correlation time 2τc of benzene
-
12 20
1
29 ppm, they cover an expanded range between 126 and 132
ppm in the solvent-free phase. Two trityl signals at 131 and
32 ppm overlap with a signal from the phenylene rotor. The
molecules in liquid benzene at 298 K is 1.28 × 10
s. The
estimated rate of free rotation, determined by the principal
moment of inertia along the 6-fold axis at the same temperature,
1
1
2 -1 21
three nonequivalent ipso carbons in the clathrate at 144, 145,
and 146 ppm separate from each other to resonate at 141, 143,
and 150 ppm. Although the spectrum of desolvated samples is
not affected by heating to 378 K, sample cooling suggests that
phenylene rotation occurs in an analogous fashion to that
observed in the clathrate. This conclusion is primarily supported
by measurements carried out below 230 K, which revealed the
emergence of two phenylene signals at 130 and 132.5 ppm
is 1.7 × 10 s . The activation energy for 6-fold rotation of
benzene in benzene crystals is only 3.95 kcal/mol, and the
1
3
-1 19b
suggested preexponential factor is 2 × 10 s .
The
enclathration of benzene with 1,3-cyclexanedione, in a structure
that contains six aromatic C-H‚‚‚OdC hydrogen bonds, gives
rise to a slightly higher activation energy of 6 kcal/mol and a
1
3
-1 19a
preexponential factor of 8 × 10 s . Rates of benzene
rotation in these solids fall in the 10 s-1 range at ambient
temperature. A graphical summary of benzene, phenyl, and
phenylene rotational dynamics, with order of magnitude rates
in known crystals at ambient temperatures, is shown in
Scheme 2.
13
(
∆ν ≈ 187.5 Hz). Although limited resolution prevented us from
establishing a precise coalescence temperature, it appears that
phenylene rotation at ambient temperature in the solvent-free
phase also occurs in the kilohertz frequency range.
2
.9. Phenylene Rotational Dynamics in the Desolvated
While the small 60° jump angle and the highly symmetric
environment of benzene give rise to very large rates of in-plane
rotation, introduction of a single point of attachment slows down
the dynamics of phenyl groups in a dramatic manner. As
expected from space-filling environments and local symmetry
of the phenyl group, literature studies report a degenerate 2-fold
flipping process as the dominant mechanism. The 2-fold rotation
2
Sample by VT H Quadrupolar Echo Measurements. Ambi-
ent temperature phenylene rotation in the millisecond time scale
was confirmed by VT H quadrupolar echo measurements.
Measurements carried out with the desolvated sample between
2
2
97 and 385 K showed spectral changes consistent with a
dynamic process described by rotation of the phenylene group
about the 1,4-axis (Figure 13). As in previous experiments,
spectra were obtained with a 90° pulse width of 2.0 µs, echo
and refocusing delays of 30 and 20 µs, respectively. The delay
between pulses was chosen at 60 s. As illustrated in Figure 13,
spectra were suitably simulated with a model that assumes a
QCC ) 180 kHz and an anisotropy parameter of η ) 0. A 2-fold
flipping motion with rotation rates ranging between ca. 1.5 ×
2
of phenyl groups studied by H NMR in crystals of phenyl-
1
7,22
alanine and phenylalanine-containing peptides
has shown
4
5
-1
that phenyl flips tend to occur in the 10 -10 s range near
ambient temperatures. The reported activation energies range
from 10 to 20 kcal/mol (Scheme 2). In a recent study of
(
20) Wakai, C.; Nakahara, M. Bull. Chem. Soc. Jpn. 1996, 69, 853-860.
21) The rate constants of free rotation τFR were calculated with the moments
(
4
6 -1
1
0 at 297 K and 3.8 × 10 s at 385 K was suggested by the
of inertia I along a particular axis with the relation τ ) (2π/9)(I/kT)1/2
:
FR
Kawaski, A. Crit. ReV. Anal. Chem. 1993, 23, 459.
fitting results (Figure 13, right). An Arrhenius plot built with
the rate of calculated rotation yielded an activation energy of
(
22) Kamihira, M.; Naito, A.; Tuzi, S.; Saito, H. J. Phys. Chem. A 1999, 103,
3356-3363. (b) Hiraoki, T.; Kogame, A.; Norio, N.; Akihiro, T. J. Mol.
Struct. 1998, 441, 243-250. (c) Zhang, H.; Bryant, R. G. Biophys. J. 1997,
1
4.6 kcal/mol, which is only ∼2 kcal/mol higher than that
7
2, 372. (d) Naito, A.; Izuka, T.; Tuzi, S.; Price, W. S.; Hayamizu, K.;
estimated in the benzene clathrate.
Saito, H. J. Mol. Struct. 1995, 355, 55-60.
J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002 7725
9

A R T I C L E S
Dominguez et al.
phenylene rings and one with “facile” flipping motion.29 The
Scheme 3
“
mobile” polymorph was shown to belong to the space group
C2 with two independent half molecules per asymmetric unit.
As expected from its crystal symmetry, two dynamic environ-
ments were indeed observed. The most mobile phenylene was
6
-1
shown to undergo flipping motions at a rate of 5 × 10 s at
3
05 K. Phenylene motion in the static polymorph was slower
2
-1
2
than 10 s , as suggested by the H NMR data.
In the case of more complex polymeric samples, rotational
rate constants and activation energies reflect the structural and
dynamic heterogeneity of each material. Despite a range of
dynamics properties, a 2-fold flipping model has been proven
suitable in most cases. Deuterium line shape analyses with poly-
crystalline penicillin V dynamics by X-ray and 13C CPMAS
NMR, Wendeler and co-workers documented the rate of phenyl
flipping in various crystalline salts. Crystals of the free acid,
23
+
+
+
+
+
H , and with Li , Na , Rb , and Cs were shown to influence
the rate of phenyl rotation over 8 orders of magnitude at ambient
(
butylene therephthalate) (PBT) at 70 °C revealed a crystalline
2
-1
phase with very slow motion (krot < 10 s ), an amorphous
+
temperature. The rates of rotation for the Li salt were the
5
-1
phase with fast flips at a rate of krot ≈ 5.5 × 10 s , and a
-
1
+
slowest (∼0.1 s at 300 K), and the rates in the Rb salt were
5
-1
slower interphase with flipping rates of krot ≈ 1.2 × 10 s .
An activation energy of only 5.9 kcal/mol calculated for rotation
in the amorphous region suggests a fluidlike environment.
Dynamic studies with phenyl deuterated PET were interpreted
in terms of distributions of rate constants and narrow distribution
8
-1
the fastest (5 × 10 s ). Differences in activation energy for
phenyl flip in these crystals ranged by a factor of 2, between
1
2.3 and 26.5 kcal/mol. Interestingly, differences in flipping
rates in these cases arise primarily from differences in the
calculated preexponential factors which varied by as much as
3
-1
of flip angles giving rotational rate constants of <10 s at
ambient temperature. Rates of phenylene flip in the case of
14
18 -1 24
5
orders of magnitude (ca. 10 -10 s ). In another example
30
of phenyl flipping in crystals reported by Casarini and co-
bisphenol-containing polymers have also been measured in the
2
3 -1
workers, rates of rotation of 3.5 × 10 and 1.2 × 10 s were
calculated at ambient temperature for the two formally enan-
tiotopic phenyl groups of diphenyl sulfoxide.25
4
6
-1
31
1
0 -10 s range in the amorphous regions. Activation
energies measured in the amorphous phase of various polymers
range between 6 and 20 kcal/mol.
If a single point of attachment reduces the rates of rotation
of phenyl groups in crystals, one may expect that the linkage
of a benzene ring along its 1,4-axis may restrict its rotational
3. Conclusions
1
3
An ideal crystalline geared lattice would be characterized by
the concerted rotation of individual molecular rotors, such that
they can influence each other over long distances by mechanisms
that transfer angular momentum from one to another in a
structurally predetermined manner. Extended correlated motions
in crystals would require precise rotational symmetry with
clockwise and anticlockwise rotors acting on each other without
interference. Parity rules that determine the motion of molecular
gears in a chain have been analyzed by Mislow,32 and the
transfer of angular momentum at the molecular level in
molecular gear trains of triptycyl ethers has been analyzed in
solution by Iwamura and co-workers. An ideal crystalline
geared lattice would “operate” under thermal equilibrium by
exchanging energy between molecular modes and extended
phonons. Unless activation energies and preexponential factors
are identical for different rotors, geared lattices would only
operate under a limited temperature range when their rotational
motions become isochronous or commensurate. Regarding the
benzene clathrate of compound 3, it is clear that in-plane rotation
of the benzene molecules and 2-fold phenylene flipping occur
with very different rates and very different activation energies.
dynamics even further. However, coalescence analysis by
CPMAS NMR in the case of 3 revealed rates of rotation of ca.
C
3
-1
1
0 s at ambient temperature, and an activation energy of only
2.8 kcal/mol was deduced. Although this analysis is of limited
1
precision, it is remarkable that rotation of the phenylene group
in desolvated samples, determined by deuterium line shape
analyses over a wider temperature range (Figure 13), has a
remarkably similar activation energy of only 14.6 kcal/mol.
Despite limitations in our data, a review of the literature
reveals that the phenylene group in compound 3 performs as
an excellent molecular rotor. There are fewer examples of
phenylene rotation in solids reported, and most of them relate
to phenylene-containing polymers. A few examples of phenylene
rotation in crystalline samples reveal a very slow process. In
one case, measurements carried out with drawn crystalline fibers
of poly(ethyleneterephthalate) (PET, R ) CH2-CH2-, Scheme
3
3
1
3
3
) measured by C exchange NMR give 2-fold flipping rate
-
1
26
constants of 1.5 s at 293 K. Phenylene flip in crystals of
,4-diphenoxy benzene, a model for poly(phenyl-O(-phenylene)
PPE), was shown to occur at a rate of 1.3 s-1 at 300 K.27
Similarly, studies with crystalline bisphenol phenyl carbonate
1
(
8
-1
While a lower limit of krot > 10 s was deduced for in-plane
(
BPPC, R ) -CO2Ph), a model for poly(bisphenyl carbonate)
28
(PBPC), revealed two crystalline forms, one with “immobile”
(
28) Enrichs, P. M.; Luss, H. R. Macromolecules 1988, 21, 860.
29) Enrichs, P. M.; Luss, H. R. Macromolecules 1988, 22, 2731.
(
(
23) Wendeler, M.; Fattah, J.; Twyman, J. M.; Edwards, A. J.; Dobson, C. M.;
Heyes, S. J.; Prout, K. J. Am. Chem. Soc. 1997, 119, 9793-9803.
(30) (a) Horii, F.; Kaji, H.; Ishida, H.; Kuwabara, K.; Masuda, K.; Tai, T. J.
Mol. Struct. 1998, 441, 303-311. (b) Kawaguchi, T.; Mamada, A.;
Hosokawa, Y.; Horii, F. Polymer 1998, 39, 2725-2732.
(24) Some of the suggested preexponential factors may be in error as they exceed
1
5
the limit of rotational inertia (Φ
0
) for a 2-fold-flip of ca. Φ
0
) 2.4 × 10
(31) (a) Shi, J.-F. I., P. T.; Jones, A. A.; Meadows, M. D. Macromolecules
1996, 29, 605-609. (b) Klug, C. A. W.; Jinhuang Xiao, C.; Yee, A. F.;
Schaefer, J. Macromolecules 1997, 30, 6301-6306.
-1
s
.
(
25) Casarini, D.; Lunazzi, L.; Mazzanti, A. Angew. Chem., Int. Ed. 2001, 40,
2
536-2540.
(32) Mislow, K. Chemtracts: Org. Chem. 1988, 2, 151-174.
(33) (a) Kawada, Y.; Iwamura, H. J. Am. Chem. Soc. 1983, 105, 1449-1459.
(b) Koga, N.; Kawada, Y.; Iwamura, H. Tetrahedron 1986, 42, 1679-
1686.
(
(
26) Wilhelm, M. S., H. W. Macromolecules 1996, 29, 1088-1090.
27) Reichert, D.; Hempel, G.; Zimmermann, H.; Schneider, H.; Luz, Z. Solid
State Nucl. Magn. Reson. 2000, 18, 17-36.
7726 J. AM. CHEM. SOC.
9
VOL. 124, NO. 26, 2002

Molecular “Compasses” and “Gyroscopes”
A R T I C L E S
benzene rotation (even at 200 K), rates of krot > 1012 s-1 at
ambient temperatures are likely. In contrast, phenylene rotation
by a 2-fold flipping mechanism at ambient temperature falls in
2 (Figure 1), will increase the rate of phenylene rotation and
will help us achieve the preparation of fast molecular compasses
and molecular gyroscopes.
3
4
-1
the range of 10 -10 s with activation energies of 12-15
kcal/mol. Although the 2-fold phenylene flip in compound 3 is
much slower than the rate of benzene rotation, it is much faster
than phenylene flips reported in most literature examples. We
speculate that crystalline geared lattices will require systems
with molecular rotors that experience steric contacts that
constitute each other’s main rotational barriers. Geared rotors
should also have matching cogs and rotational periods. Intrigued
by the benzene clathrate formed by compound 3, we are now
exploring the preparation of analogous clathrates with substituted
benzenes and molecular rotors with substituted phenylenes. We
believe that there is much to be learned in the construction and
analysis of crystalline solids with structurally programmed
motions. We also expect that phenylene flipping in molecular
rotors with intramolecular steric shielding, such as compound
Acknowledgment. This work was supported by NSF grants
DMR9988439, CHE9871332 (X-ray), and DMR9975975 (solid-
state NMR). Z.D. thanks CONACYT Mexico and the UC-
Mexus program for a postdoctoral fellowship.
Supporting Information Available: Experimental details on
1
13
the synthesis of compounds 3, 4, and 7, H and C NMR spectra
of compounds 3 and 4, 2-D HMBC analysis of 4, details of the
variable-temperature solid-state C CPMAS and 2H NMR
acquisition (PDF). Crystallographic information files (CIF) of
compound 3 grown from benzene and from methylene chloride.
This material is available free of charge via the Internet at
http://pubs.acs.org.
13
JA025753V
J. AM. CHEM. SOC.
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VOL. 124, NO. 26, 2002 7727