C O MMU N I C A T I O N S
Figure 1. Ortep diagram of open rotor 3 shown with 50% probability.
Colors are consistent with the packing diagram in Figure 2 and match the
NMR assignment in Figure 3.
2
Figure 5. Experimental (right) and simulated (left) solid state H NMR of
desolvated samples of 3-d4. The rotation rate constants used for fitting, from
6
-1
bottom to top are (×10 s ): 0.015, 0.4, 1.3, 2.2, and 3.8.
In conclusion, we have shown that molecular frames based on
alkyne linkages and trityl groups can facilitate rapid rotation of
phenylene groups along their 1,4-axis. Molecular dynamics and
dielectric measurements with samples prepared with polar phenyl-
enes in molecular compasses and gyroscopes will be reported soon.
Figure 2. Left: Cluster of molecules showing rotor 3 in blue and red and
benzene molecules in purple. Right: View of the same cluster with benzene
molecules removed.
Acknowledgment. We thank NSF grants DMR9988439,
CHE9871332 (X-ray), and DMR9975975 (NMR). Z.J.D. thanks
CONACYT and UC-Mexus.
Supporting Information Available: Stereoscopic views and X-ray
data tables for rotor 3, DSC, and TGA traces (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(
1) (a) Liu, C.-Y.; Bard, A. J. Acc. Chem. Res. 1999, 32, 235-245. (b) Wang,
S. J.; Oldham, W. J.; Hudack, R. A.; Bazan, G. C. J. Am. Chem. Soc.
2
000, 122, 5695-5709. (c) Guymon, G. A.; Dougan, L. A.; Martens, P.
J.; Clarck, N. A.; Walba, D. M.; Bowman, C. N. Chem. Mater. 1998, 10,
2378-2388.
Figure 3. 13C CPMAS NMR spectra of (top) rotor 3 crystallized fom C6H6,
(2) Rotation about alkynyl single bonds is nearly frictionless: (a) Saebo, S.;
Almolof, J.; Boggs, J. E.; Stark, J. G. J. Mol. Struct. (THEOCHEM) 1989,
(middle) rotor 3 crystallized from C6D6, and (bottom) deuterium-labeled
2
00, 361-373. (b) Sipachev, V. A.; Khaikin, L. S.; Grikina, O. E.; Nikitin,
rotor 3-d4 crystallized from C6H6.
V. S.; Traettberg, M. J. Mol. Struct. 2000, 523, 1-22.
(
3) Rotation about aryl-alkyne bonds in the excited state may be subject to
significant barriers: Levitus, M.; Garcia-Garibay, M. A. J. Phys. Chem.
2
000, 104, 8632-8637.
(
4) For a review on related clathrate-forming structures see: McNicol, D.
D.; Toda, F.; Bishop, R. ComprehensiVe Supramolecular Chemistry;
Pergamon: Oxford, 1996; Vol. 6.
(5) Masson, J.-C.; Quan, M. L.; Cadiot, P. Bull. Soc. Chim. Fr. 1968, 3, 1085-
1088. Yields reported by these authors were much lower.
(
6) This two-step sequence works well with several trityl chlorides and
aromatic halides. Several examples with full experimental details will be
described in the full paper.
(
7) Compound 3: C48
H34‚2C H , MW ) 766.97, triclinic, space group P 1h , a
6 6
o
)
8.5157(19) Å, b ) 9.547(2) Å, c ) 14.467(3) Å, R ) 77.315(4) , â )
o
o
3
7
6.469(4) , γ ) 72.970(4) , V ) 1078.8(4) Å , Z ) 1, Fcald ) 1.181 Mg/
3
-
1
m , F(000) ) 406, λ ) 0.71073 Å, µ(Mo KR) ) 0.067 mm , T ) 100-
Figure 4. Variable-temperature 13C CPMAS NMR with rotor 3.
3
(2) K, crystal size ) 0.2 × 0.2 × 0.1 mm , of the 6884 reflections collected
(
1.47 e θ e 28.25°), 4725 [R(int) ) 0.0197] were independent reflections;
-3
max/min residual electron density 0.192 and -0.176 e Å , R1 ) 0.0409
constant for rotation of 7.7 ms and an approximate barrier of 12.8
kcal/mol were calculated.
(I > 2σ(I)) and wR2 ) 0.0885 (all data).
(8) Fyfe, C. A. Solid State NMR for Chemists; C.F.C Press: Guelph, Ontario,
2
1983.
Phenylene rotation was also analyzed by quadrupolar echo H
(
9) Evidence for rotation may be inferred from X-ray derived atomic
displacement parameters (ADP) and will be analyzed in a later paper.
For a description of the method see: Dunitz, J.; Maverick, E. F.;
Trueblood, K. N. Angew. Chem., Int. Ed. Engl. 1988, 27, 880-895.
NMR by using a model that involves 180° flips about the 1,4-
axis.1
1,12
Measurements with the benzene clathrate below 295 K
fell within the slow exchange regime, given by an upper limit of
(
10) Intermolecular cross polarization depends on the proximity between donnor
and acceptor: Cizmeciyan, D.; Sonnichsen, L. B.; Garcia-Garibay, M.
A. J. Am. Chem. Soc. 1997, 119, 184-188.
11) Cholli, A. L.; Dumais, J. J.; Engel, A. K.; Jelinski, L. W. Macromolecules
1984, 17, 2399-2404.
4
-1
k
rot < 10 s . Measurements at higher temperatures were compli-
13
cated by the loss of benzene. However, line-shape analysis of
spectra measured with desolvated samples between 297 K and 385
K confirmed a two-fold flipping motion with rotation rates ranging
(
(
12) Spectra were obtained at 46.07 MHz with a 20 µs 90° pulse, echo delays
of 30 and 20 µs, and 60 s between scans. Spectra were fit assuming QCC
) 180 kHz and η ) 0 with the program Turbopowder (Wittebort, O.;
Olejniczak, T. T.; Griffin, R. G. J. Chem. Phys. 1987, 86, 5411-5420).
13) Desolvation occurs as a first-order phase transition at 100 °C and is closely
followed by in situ recrystallization. Melting of the desolvated phase occurs
at 315 °C and thermal decomposition above 400 °C.
4
6 -1
between ca. 1.5 × 10 and 3.8 × 10 s (Figure 5). The activation
energy barrier for phenylene rotation calculated from this data is
(
2
ca. 14.6 kcal/mol (r ) 0.96). This is about 2 kcal/mol higher than
the barrier estimated in the benzene clathrate, suggesting only minor
collapse of the initially porous lattice.
JA0119447
J. AM. CHEM. SOC.
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VOL. 124, NO. 11, 2002 2399