Improved physical properties and rotational dynamics in a molecular gyroscope with an asymmetric stator structure

Article abstract of DOI:10.1021/ol060894d

A molecular gyroscope consisting of a 1,4-diethynylphenylene rotator linked to trityl and triptycyl groups (3) showed significantly improved physical properties and faster rotational dynamics than analogous symmetric bis(trityl) (1) or bis(triptycyl) (2)

Full text of DOI:10.1021/ol060894d

ORGANIC
LETTERS
2006
Vol. 8, No. 16
3417-3420
Improved Physical Properties and
Rotational Dynamics in a Molecular
Gyroscope with an Asymmetric Stator
Structure
Steven D. Karlen, Carlos E. Godinez, and Miguel A. Garcia-Garibay*
Department of Chemistry and Biochemistry, UniVersity of California,
Los Angeles, California 90095-1569
mgg@chem.ucla.edu
Received April 12, 2006
ABSTRACT
A molecular gyroscope consisting of a 1,4-diethynylphenylene rotator linked to trityl and triptycyl groups (3) showed significantly improved
physical properties and faster rotational dynamics than analogous symmetric bis(trityl) (1) or bis(triptycyl) (2) structures. An activation energy
of 7.9 kcal/mol for 3 was determined by 2H NMR. This is ca. 4
−6 kcal/mol lower than that of compound 1. The different dynamics of the three
compounds can be qualitatively understood in terms of their different packing coefficients.
Recent interest in the field of artificial molecular machines
has been focused on the creative design of molecular
analogues to macroscopic objects. These systems have been
studied in solutions,1 on surfaces,2 and in polymers.3 With
the purpose of engineering molecular motions in crystals,4
we have recently investigated a series of compounds with
structural attributes analogous to those of macroscopic
gyroscopes.5 As illustrated in Figure 1, many of these
structures consist of a para-diethynylphenylene acting as the
rotator and axle (shown in red) and two bulky triptycyl
(X ) CH)6 or triarylmethyl (X ) H,H,H)7 groups acting as
a shielding stator (shown in blue).8
(1) (a) Mislow, K. Chemtracts: Org. Chem. 1988, 2, 151-174. (b)
Bedard, T. C.; Moore, J. J. Am. Chem. Soc. 1995, 117, 10662-10671. (c)
Kelly, T. R. Acc. Chem. Res. 2001, 34, 514-522. (d) Balzani, V.; Credi,
A.; Venturi, M. Molecular DeVices and Machines - A Journey into the
Nano World; Wiley-VCH: Weinheim, Germany, 2003. (e) Fletcher, S. P.;
Dumur, F.; Pollard, M. M.; Feringa, B. L. Science 2005, 310, 80-82. (f)
Leigh, D. A.; Wong, J. K. Y.; Dehez, F.; Zerbetto, F. Nature 2003, 242,
174-179. (g) Collin, J.-P.; Dietrich-Buchecker, C.; Gavin˜a, P.; Jimenez-
Molero, M. C.; Sauvage, J.-P. Acc. Chem. Res. 2001, 34, 477-487.
(2) (a) Hernandez, R.; Tseng, H.-R.; Wong, J. W.; Stoddart, J. F.; Zink,
J. I. J. Am. Chem. Soc. 2004, 126, 3370-3371. (b) Steuerman, D. W.; Tseng,
H.-R.; Peters, A. J.; Flood, A. H.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart,
J. F.; Heath, J. R. Angew. Chem., Int. Ed. 2004, 43, 6486-6491. (c) van
Delden, R. A.; ter Wiel, M. K. J.; Pollard, M. M.; Vicario, J.; Koumura,
N.; Feringa, B. L. Nature 2005, 437, 1337-1340. (d) Shirai, Y.; Osgood,
A. J.; Zhao, Y.; Kelly, K. F.; Tour, J. M. Nano Lett. 2005, 05.
(4) (a) Karlen, S. D.; Garcia-Garibay, M. A. Amphidynamic Crystals:
Structural Blueprints for Molecular Machines. In Molecular Machines;
Kelly, T. R., Ed.; Topics in Current Chemistry Series V. 262; Springer-
Verlag: Berlin, 2005; pp 179-227. (b) Garcia-Garibay, M. A. Proc. Natl.
Acad. Sci. U.S.A. 2005, 102, 10771-10776.
(5) For elegant synthetic work on molecular gyroscopes, see also: (a)
Shima, T.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2004, 43,
5537-5540.
(6) Godinez, C. E.; Zepeda, G.; Garcia-Garibay, M. A. J. Am. Chem.
Soc. 2002, 124, 4701-4707.
(7) (a) Dominguez, Z.; Dang, H.; Strouse, M. J.; Garcia-Garibay, M. A.
J. Am. Chem. Soc. 2002, 124, 2398-2399. (b) Dominguez, Z.; Khuong,
T.-A. V.; Dang, H.; Sanrame, C. N.; Nunez, J. E.; Garcia-Garibay, M. A.
J. Am. Chem. Soc. 2003, 125, 8827-8837. (c) Dominguez, Z.; Dang, H.;
Strouse, M. J.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2002, 124, 7719-
7727.
(3) (a) Marsella, M. J.; Reid, R. J.; Wang, L.-S. Polym. Mater. Sci. Eng.
2001, 85, 474. (b) Hugel, T.; Holland, N. B.; Cattani, A.; Moroder, L.;
Seit, M.; Gaub, H. E. Science 2002, 296, 1103-1106.
10.1021/ol060894d CCC: $33.50
© 2006 American Chemical Society
Published on Web 07/12/2006

pound rotates by 180° and flips at a rate of ca. 1.3 MHz at
65 ° C while the phenylene group of the bis(triptycyl)
analogue 2 is static.
Considering the remarkable differences in physical and
dynamic properties of 1 and 2 and knowing that a hybrid
structure would severely compromise the packing motif of
either component, it was of interest to investigate molecular
gyroscope 3 with a mixed trityl-triptycyl stator (Figure 2).
Figure 1. Molecules with structural attributes reminiscent of
macroscopic gyroscopes may be constructed with trityl (X )
H,H,H) and or triptycyl (X ) CH) groups.
With studies centered on variations on the triptycyl or trityl
groups, we have shown that changes in the structure of the
stator may have a dramatic effect on the physical properties
and rotary dynamics. For example, the parent bis(trityl) (1)
and bis(triptycyl) (2) structures (Scheme 1) have a tendency
Figure 2. Molecular gyroscopes consisting of a 1,4-diethynylphen-
ylene rotator linked to trityl and triptycyl groups (3), symmetric
bis(trityl) (1), and bis(triptycyl) (2) stators.
We reasoned that a hybrid structure could have the desirable
sturdiness of 2, the good solubility of 1, and the rapid
dynamics potentially associated with a lower-density crystal.
The desired molecular gyroscope 3 was prepared in two
steps (Scheme 1) by Pd(0)-coupling reactions among di-
iodobenzene, 1,1,1-triphenylpropyne 4, and 9-triptycyl acet-
ylene 6. We opted for two sequential coupling steps rather
than one to prevent a statistical mixture of products. To
minimize the formation of 1 during the preparation of
compound 5, 2 equiv of diiodobenzene was used per equiv
of 4. As expected, the hybrid structure 3 possessed the
solubility associated with 1, being readily soluble in common
organic solvents such as methylene chloride, chloroform,
benzene, etc. The 1H and 13C NMR spectra of 3 in solution
closely resemble a combination of the spectra from the two
symmetric structures, including a central phenylene charac-
terized by an A2B2 1H spin system and four easily distin-
guishable alkyne carbons in the 13C NMR. Molecular
gyroscope 3 formed crystals with a visually determined
melting point of 305 °C, which is close to the melting point
of the bis(trityl) analogue 1 of 316 °C.7 Although compound
3 has a tendency to form microcrystalline solids from several
solvents, we were able to obtain X-ray quality single crystals
by slow evaporation from chloroform. These crystals turned
out to be a solvent clathrate (Figure 3), as it is commonly
observed for the structures of both 1 and 2.4 Thermogravi-
metric (TG) and differential scanning calorimetric (DSC)
analysis of the clathrate showed the gradual loss of chloro-
form between 50 and 100 °C followed by melting at 305
°C. The broad range of chloroform loss, extending to
temperatures well above its boiling point (61 °C), suggests
that solvent molecules are held tight within the crystal lattice.
The structure of the chloroform clathrate of 3 was solved
in the triclinic space group P1h.11 The asymmetric unit
Scheme 1
to crystallize with solvent, and having rigid, rod-shaped
structures, all molecules align in the same direction in low-
symmetry crystals. However, molecular gyroscope 1 is
soluble in most organic solvents and melts at 316 °C whereas
compound 2 dissolves only in hot aromatic solvents (e.g.,
xylenes and chlorobenzene) and forms crystals that decom-
pose without melting above 400 °C.
The crystal structures of 1 and 2 also differ by their
packing coefficients, a measure of the amount of empty
volume in the structure. The packing coefficient Ck is given
by the volume of all the molecules in the unit cell relative
to the total cell volume (Ck ) Vmol/Vcell).9,10 For organic
compounds, they typically vary between 0.65 and 0.80. With
Ck ) 0.69 and Ck ) 0.85 for 1 and 2, respectively, it is not
surprising that the phenylene group of the bis(trityl) com-
(8) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. ReV. 2005,
105, 1281-1376.
(9) Kitaigorodskii, A. I. Molecular Crystals and Molecules; Academic
Press: New York, 1973.
(10) Molecular volumes were calculated using the group increment
approach with the values reported in: Gavezzotti, A. J. Am. Chem. Soc.
1983, 105, 5220-5225.
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Org. Lett., Vol. 8, No. 16, 2006

includes one molecule of 3 and one molecule of chloroform.
The diethynylphenylene of 3 has a relatively sharp bend
(Figure 3), with an angle of 170.76° from the quaternary
Figure 3. ORTEP diagram of molecular gyroscope 3 with thermal
ellipsoids shown at 50% probability.
carbon of the trityl group to the center of the phenylene to
the bridgehead carbon of the triptycene. As in the case of 1
and 2, molecular gyroscope 3 packs in a parallel alignment,
as illustrated in Figure 4. With a feature reminiscent of the
packing of 2, a face-to-face interaction between the phen-
ylene rotator of one molecule and the benzene of a neighbor-
ing triptycene leads to a close contact (3.376 Å) between
alkyne and triptycene carbons (Figure 4, top). When viewed
along the normal of the (0,1,-1) plane, molecules of 3 pack
with trityl and triptycene groups aligned in alternating
directions (Figure 4, bottom). There are several close contacts
in the structure. Among these, the chloroform hydrogen is
positioned near the center of one of the benzene rings of a
triptycene, with distances to four of the benzene carbons
varying between 2.533 and 2.842 Å (Figure 4, bottom).
The stability of the solvate was first evaluated under the
conditions of cross polarization-magic angle spinning (CP-
MAS) NMR measurements at ambient temperature. The
solid-state 13C CP-MAS NMR spectra agreed qualitatively
well with the spectrum obtained in solution. As shown in
the top of Figure 5, signals corresponding to the quaternary
triptycyl carbons at 53 ppm are well resolved from the
quaternary trityl carbon at ca. 56 ppm. However, chloroform
and alkyne signals between 75 and 100 ppm are not
consistent with a pure crystallographic phase and a unique
molecule per asymmetric unit, suggesting that structural
heterogeneity arises as a result of desolvation. Complete
conversion of the CHCl3 clathrate to a solvent-free form was
accomplished by keeping the former at 110 °C for 2 days.
The 13C CP-MAS NMR spectrum obtained after this
Figure 4. Molecular packing of chloroform clathrate of 3 (top)
viewed normal to the (1,0,0) plane and (bottom) normal to the
(0,1,-1) plane. Close contacts between neighboring molecules are
indicated with dotted lines.
treatment revealed the loss of solvent signals at 75 ppm along
with major changes in every region of the spectrum. We were
able to confirm that the resulting solvent-free phase is
crystalline by a sharp X-ray powder diffraction (Supporting
Information).
Figure 5. (Top) 13C CP-MAS NMR spectrum of a freshly
prepared CHCl3 clathrate of 1. (Bottom) Solvent-free sample of 1
after removal of CHCl3 by heating the sample to 110 °C for 2 days.
(11) Selected crystallographic information for chloroform clathrate of
compound 3: C49H32‚C1H1Cl3, MW ) 740.11, triclinic, space group P1h,
a ) 8.0460(14) Å, b ) 14.953(3) Å, c ) 16.805(3) Å, R ) 80.884(8)°,
â ) 76.657(8)°, γ ) 75.271(8)°, V ) 1891.8(6) Å3, Z ) 2, Fcald ) 1.299
Mg/m3, F(000) ) 768, λ ) 0.71073 Å, µ(Mo KR) ) 0.067 mm-1, T )
120(2) K, crystal size ) 0.40 × 0.15 × 0.10 mm3; of the 16 000 reflections
collected (1.42 e θ e 28.35°), 8752 [R(int) ) 0.0400] were independent
Given the relative instability of the solvent clathrate and
the impossibility of reliably measuring its NMR spectrum
as a function of variable temperature, we decided to analyze
the solid-state rotational dynamics of 3 in a desolvated
sample. As in previous studies, measurements were carried
out by quadrupolar echo 2H NMR, one of the most sensitive
reflections; max/min residual electron density 0.413 and -0.512 e Å-3
R1 ) 0.0556 (I > 2σ(I)) and wR2 ) 0.1455 (all data).
,
Org. Lett., Vol. 8, No. 16, 2006
3419

probes for dynamics in crystalline solids and rigid media.12
2
In brief, the H NMR method relies on the orientation
dependence of the quadrupolar coupling and its modulation
by molecular motions. Although powder samples with slowly
moving or rigid molecules give rise to broad symmetric
spectra known as a Pake pattern, molecular motion and
reorientation give rise to spectral variations that depend on
the rate and type of the deuteron motion. The dynamic range
covered by line shape analysis techniques varies from ca.
104 to 108 s-1, which correspond to the slow and fast
exchange limits of the method, respectively. For our studies,
we take advantage of the well-characterized spectral changes
occurring upon rotation of phenylene groups about their 1,4-
axis, typically by 180° angular displacement, which are
commonly referred to as phenylene flips.13,14
An isotopically labeled sample of 3-d4 for dynamic NMR
was prepared as shown in Scheme 1 using 1,4-dibromoben-
zene-d4. The spectra were acquired with a quadrupole echo
pulse sequence15 with a 90° pulse of 2.25 µs, with echo and
refocusing delays of 50 and 40 µs, respectively. The delay
between pulses was 30 s. Exchange rates were determined
by visually matching the experimental data to spectra
simulated using the program reported by Nishikiori et al.14
The simulated spectra were obtained using a quadrupolar
coupling constant (QCC) measured from spectra acquired
in the slow exchange regime and a model that considers site
Figure 6. (Left) Experimental (thin line) and simulated (thick line)
wide-line 2H NMR spectrum of 1 in a solvent-free structure. (Right)
Arrhenius plot of the exchange data.
Although we do not know the structure of solvent-free 3,
we note that this interesting result is consistent with the
smaller packing coefficient for the solvated crystals of hybrid
structure 3 (0.69) as compared to those of the trityl compound
1 (0.74) and the static triptycyl compound 2 (0.85). Interest-
2
exchange of the H nuclei by 180° rotations. The only
adjustable parameter used in the simulation was the site-
exchange rate krot, which was varied from 104 to 108 Hz.
The experimental and simulated spectra for the solvent-
free sample are shown in Figure 6 for data recorded between
300 and 448 K. The best visual agreement was obtained using
rates of rotation between 0.5 × 106 and 50 × 106 s-1.
Arrhenius analysis of the data gave a barrier for phenylene
flipping of 7.9 ( 1.6 kcal/mol and a preexponential factor
of 2.75 × 1011 Hz.16 The relatively linear plot suggests that
the solvent-free structure is a single phase and not a mixture
of polymorphs. Notably, the barrier in 3 is 6 kcal/mol lower
than that previously measured by the same method for
solvent-free samples of 1 within a similar temperature range.7
The lower barrier results in faster rotation rates, which are
desirable to control the orientation of dipolar analogues with
external electric and magnetic fields.
2
ingly, although an attempt to measure the H NMR of the
solvate gave a heterogeneous spectrum with two or more
crystalline components, we made a qualitative observation
that the rates of phenylene motion in the solvate are slower
than those of the solvent-free structure, suggesting that
desolvation occurs without collapse of the lattice (Supporting
Information).
In conclusion, molecular gyroscope 3 with an asymmetric
triptycyl-trityl stator structure retains the relatively high
melting point of the two symmetric structures, 1 or 2, but it
displays largely improved solubility and dynamic properties.
The packing structure of the CHCl3 clathrate retains the
parallel alignment observed in both 1 and 2, with phenylene-
triptycene intermolecular contacts similar to those present
in 2. We believe that compound 3 represents an interesting
lead that highlights the potential properties of solid-state
molecular gyroscopes with asymmetric structures.
(12) Hoatson, G. L.; Vold, R. L. NMR Basic Princ. Prog. 1994, 32, 1-67.
(13) (a) Cholli, A. L.; Dumais, J. J.; Engel, A. K.; Jelinski, L. W.
Macromolcules 1984, 17, 2399-2404. (b) Rice, D. M.; Wittebort, 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, 7707-7710. (c) Rice,
D. M.; Meinwald, Y. C.; Scheraga, H. A.; Griffin, R. G. J. Am. Chem. Soc.
1987, 109, 1636-1640. (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, 2, 255-270.
Acknowledgment. This work was supported by the
National Science Foundation through grants DMR0307028
and DGE-0114443 (NSF IGERT: Materials Creation
Training Program (MCTP)).
(14) Nishikiori, S.; Soma, T.; Iwamoto, T. J. Inclusion Phenom. 1997,
27, 233-243.
Supporting Information Available: Synthesis and char-
2
(15) The 2H NMR spectra were acquired on a Bruker Avance spectrom-
eter at a frequency of 46.073 MHz with a single channel solenoid probe
containing a 5 mm insert. Measurements were made from 300 to 448 K
with the temperature calibrated using a 195Pb shift standard.
(16) The uncertainty in the barrier was estimated from simulations with
the highest and lowest exchange rates that reasonably matched the
experimental spectra at each temperature value.
acterization of compounds 3 and 5, TGA traces, H NMR
of partially desolvated samples, and crystallographic infor-
mation (CIF) for 3. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL060894D
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