Molecular spur gears comprising triptycene rotators and bibenzimidazole-based stators

Article abstract of DOI:10.1021/ja2063346

Dynamic gearing of molecular spur gears, the most common type of mechanical gear, is elucidated. Molecular design and conformational analysis show that derivatives of 4,4-bis(triptycen-9-ylethynyl)bibenzimidazole represent suitable constructs to investigate gearing behavior of collateral triptycene (Tp) groups. To test this design, DFT calculations (B97-D/Def2-TZVP) were employed and the results suggest that these molecules undergo geared rotation preferentially to gear slippage. Synthesis of derivatives was carried out, providing a series of molecular spur gears, including the first desymmetrized spur gear molecules, which were subsequently subjected to stereochemical analysis.

Full text of DOI:10.1021/ja2063346

Article
pubs.acs.org/JACS
Molecular Spur Gears Comprising Triptycene Rotators and
Bibenzimidazole-Based Stators
Derik K. Frantz, Anthony Linden, Kim K. Baldridge,* and Jay S. Siegel*
Organic Chemistry Institute, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
S
* Supporting Information
ABSTRACT: Dynamic gearing of molecular spur gears, the most common type of mechanical gear, is elucidated. Molecular
design and conformational analysis show that derivatives of 4,4-bis(triptycen-9-ylethynyl)bibenzimidazole represent suitable
constructs to investigate gearing behavior of collateral triptycene (Tp) groups. To test this design, DFT calculations (B97-D/
Def2-TZVP) were employed and the results suggest that these molecules undergo geared rotation preferentially to gear slippage.
Synthesis of derivatives was carried out, providing a series of molecular spur gears, including the first desymmetrized spur gear
molecules, which were subsequently subjected to stereochemical analysis.
consequences8 (vide infra) of dynamic gearing in derivatives
INTRODUCTION
■
of di(triptycen-9-yl)methane (1)9 and di(triptycene-9-yl) ether
(2),10 respectively. The energy barriers for correlated
disrotation (geared rotation) in derivatives of 1 and 2 have
been calculated to be 1−2 kcal/mol; correlated conrotation of
the Tp groups, the process of gear slippage, requires highly
strained transition structures whose activation energies have
been experimentally determined to lie in the range of 32−
45 kcal/mol.11 Analogs of 1 and 2 include isostructural
derivatives 3,12 in which the Tp groups are linked by a variety
of atoms or groups, and a tightly intermeshed vinylene-linked
system 4.13 A molecular gear that incorporates a clutch function
by silane−silicate interconversion 5 has recently been
synthesized and studied.14
The hard-sphere model of a molecule evokes an image of
a mechanical object,1 within which the molecular rotator2−4 is a
fundamental mechanical component. Coupling two or more
molecular rotators, such that their motions are correlated,
corresponds to gearing, a principal mechanical mechanism. At
the molecular level, dynamic gearing5 has been demonstrated in
bevel gears comprising intermeshed triptycene (Tp) groups.6
To date, no extensive investigations into the function of
molecular representations of spur gears (Figure 1) have been
Molecules with three secured rotator components have
been synthesized in linear (6)15 or cyclic (7, 8)16 arrangements,
the former exhibit smooth rotation (Ea < 2 kcal/mol) as a
correlated triplet, whereas the parity requirement of disrotatory
motion locks the latter in a frustrated state (Ea of rotation =
20 kcal/mol). Each of these arrangements has stereochemical
consequences: system 6 is dynamic and exhibits phase
isomerism;17 7 is static and achiral (C3h) whereas 8 is static
and chiral (C3).
Figure 1. Macroscopic bevel and spur gears. Photographs courtesy of
Emerson Power Transmission Corp. (www.emerson-ept.com).
reported, though these are the most common gear arrange-
ments found in macroscopic machinery. This report presents
the stereochemical analysis, design, calculations, syntheses, and
dynamic NMR studies of molecular spur gears.
Inspired by Oki’s investigations into the rotational barriers
̅
of 9-substituted derivatives of Tp,7 Mislow and Iwamura
independently synthesized and studied the stereochemical
Received: July 15, 2011
Published: January 4, 2012
© 2012 American Chemical Society
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few kcal/mol lower in energy than their barriers to gear
slippage can still function as efficient gears.
Up to this point, all detailed studies of molecular gear
constructs have focused on bevel gears. A precise depiction of a
molecule as a machine requires that it not only structurally
resembles, but also functionally act as, a mechanical component;
from this perspective, no investigations of molecular spur gears
have yet been reported. Before discussing the details of our
study, let us discuss the conformational analysis necessary to
motivate such molecular designs.
CONFORMATIONAL ANALYSIS AND MOLECULAR
DESIGN
■
Design of molecular gears requires a detailed understanding of
conformational analysis and steric shape. Simple valence bond
graphs attract one to notions of gearing that are immediately
seen as untenable from a space-filling representation.21 For
example, in 1,9-dimethyltriptycene (11) (Figure 2), the energy
Several gear-like constructs have been reported without
stereochemical investigations of labeled derivatives. Represen-
tatives of this type include a metallocene-based gearing proto-
type 918 with a 3 × 4 gearing ratio and di(triptyceno) crown
ethers 10a and 10b, whose shapes resemble spur gears.19
Figure 2. Gear-meshed and gear-clashed conformations of 1,9-
dimethyltriptycene.
minimum ground state was originally assumed22 to be a gear-
meshed Cs conformation in which the methyl groups undergo
correlated rotation, but was later calculated23 to prefer a
buttressed, gear-clashed Cs conformation that does not exhibit
correlated rotation of the methyl groups. These studies
demonstrate that the geometry of a methyl group closely
resembles a triangle-based pyramid (tetrahedron); its effective
steric size is conformation-dependent and cannot be best
represented simply by its hydrodynamic radius.24
With the caveat in mind that any serious discussion of
molecular gearing warrants a rigorous geometric analysis, let us
consider a framework that holds two identical three-bladed
rotators, with hydrodynamic radii of r, in a collateral arrange-
ment. Depending on the nature of the blades, the rotators
could be approximated by several models: one extreme could
render the rotators as three-toothed propellers of negligible
thickness, a moderate estimation could approximate the
components as triangular prisms, and the other extreme case,
where the rotators’ steric bulk is well approximated by its
hydrodynamic radius, may be represented by cylinders (Figure 3).
Positioning these models in gearing (C2 and Cs) and gear-
clashed (C2v and C2v*) orientations,25 with respect to the
molecular framework, and examining their minimum interaxle
distances establish their effective steric size.
In any imaginable orientation, the cylinders’ minimum
interaxle distance is 2r. In contrast, the minimum axle−axle
distances of the propeller or triangular prism approximations
depend on their conformations: 0.86r (C2), r (Cs), and r (C2v)
for the propeller and 1.15r (C2), 3r/2 (Cs), and r (C2v) for the
triangular prism.
Assuming a hard-surface model, full rotation can only occur
in the absence of surface clashing. This condition can be met
by increasing the interaxle distance to a point where one
Given the development of molecular gears and the general
interest in molecular machines, it would be useful to have some
parameters that characterize gearing efficiency. Gearing fidelity
(Fgear) and gearing temperature (Tgear) are terms to address this
issue.20 The expression Fgear = kgeared rotation/kgear slippage provides a
measure of gearing fidelity at a given temperature (298 K, for
example) as the average number of geared rotations per gear
slip. Gearing temperature is the temperature below which the
rate of gear slippage remains slower than a certain value (1 s−1
when not otherwise stated) and is expressed as Tgear# where # is
the log of the number of slips/second (0 when not otherwise
stated).
In molecules 1−6, the efficiency of gearing is outstanding,
giving high values of Fgear and Tgear; if 2 kcal/mol were the
barrier to geared rotation and 35 kcal/mol the barrier to gear
slippage, Fgear would be on the order of 1023 and Tgear would be
584 K. Because a small difference in ΔG‡geared
and
rotation
ΔG‡gear
has a significant effect on the rates of the
slippage
processes, systems whose barriers to geared rotation are only a
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A macro-mechanical analysis considers gears to be hard
shapes that fit together with shape complementarity defined
by steric exclusion.26 An equivalent molecular model would
consider then only steric exclusion when the conformational
energy is assessed.
In molecular systems, the thickness of the gear blade might
be estimated by the rough van der Waals surface. Bringing the
Tp groups in these C2, Cs, and C2v orientations together as
closely as their van der Waals surfaces27 reveals that the
minimum axle−axle distances are 7.0, 8.1, and 7.2 Å,
respectively (Figure 4). Thus, the Cs conformation is required
Figure 4. Space-filling representations of Tp groups in C2 (left), Cs
(center), and C2v (right) conformations, indicating the minimum
axle−axle distances.
to have a longer axle−axle distance than the C2v conformation
by almost 1 Å. In this molecular model, forcing the rotators
closer together would increase the energy required for geared
rotation more than for gear slippage. Focusing only on van der
Waals repulsion by exclusion leaves little hope for this design to
result in an effective gear system.
Fortunately, molecular interactions are more varied than
simple hard sphere repulsion. Attractive dispersive terms,
along with various electrostatic terms, enrich the analysis and
offer the possibility that gearing mechanisms can prevail; for
example, van der Waals face-to-face and edge-to-face π−π
interactions may stabilize the C2 and Cs conformations and
H−H repulsions or a pocket of vacuum between the Tp
groups may destabilize the C2v conformation. If such were the
case, geared rotation could be preferred over gear slippage,
albeit unlikely to reach the robust character seen in bevel
gears 1−6. Properly addressing such scenarios exceeds the
abilities of simple qualitative models and commitment to full
quantum mechanical (QM) analysis is thus called for (see
below).
Despite the need for rigorous QM computations to assign
an accurate energy ranking of the conformations, the basic
conformational analysis sets rough target dimensions that
should allow the Tp groups to interconvert between C2 and
Cs conformations with minimal strain. The stator must
project the two axles in parallel and separate them by an
appropriate distance that allows for interaction of the
rotators. Our recent work on 1,1′-bridged derivatives of
4,4′-disubstituted 2,2′-bibenzimidazole (BBI)28 provides
synthetic access to a stator framework that meets these
requirements. Substituents at the 4- and 4′-positions extend
in parallel at a distance of roughly 8 Å,29 accommodating a Cs
conformation. Acetylene groups extend linearly and provide
a sufficient distance between the stator and rotator, and their
flexibility was expected to allow for a favorable C2 confor-
mation to be reached with minimal strain. The combination
Figure 3. Geometrical analysis of two three-bladed rotators,
approximated as propellers, triangular prisms, and cylinders, in C2,
Cs, C2v, and C2v* orientations. The distance from the axle to the blade
is r, and the minimum distance between axles is given as a function
of r.
rotator can rotate by 2π without clashing with its static
partner, as is the case for the Cs structure in rotators
resembling triangular prisms. Alternatively, clashing can be
avoided by correlated motion of both rotators (gearing) or by
gear slippage.
Assuming there is a spring-like tension holding the rotators
together, then Hooke’s law indicates that the relative interaxle
distance r is a measure of potential energy. From this
assumption, the relative potential energy of each conformer
can be assessed from its interaxle distance. In a system with
cylinders, all orientations have the same energy and the groups
rotate freely and independently of one another. In the case of
triangular prisms, the C2v orientation is predicted to be the
lowest energy conformation. In the propeller system, the C2
orientation would suffer the least repulsion due to steric
exclusion, whereas Cs and C2v orientations are higher in energy
but not distinguished.
Adding a thickness to the blades requires a larger separation
for Cs orientations compared to C2v orientations. Thus, the
conclusion from the simple mechanical model would then be
that Cs orientations would suffer more steric repulsion due to
exclusion than C2v orientations at close distances.
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of these rotator, axle and stator components, leads to the
generic structure 12.
which differ in the saturation of the pyrazine ring at the center
of the stator.
Molecular geometries were fully optimized in C2, Cs, and
C2v conformations from which their relative energies, including
effects of zero point energy, were calculated (Figure 6). For
The final aspect of the design concerns the location and type of
labeling groups that must be added to the rotators to provide
desymmetrized derivatives, potentially giving rise to phase isomers.
Dynamic gearing of molecular rotators can be established by the
observation of phase isomers in desymmetrized derivatives under
conditions at which, on the time scale of observation, gear slippage
is slow. In three-bladed gears, with one blade labeled, three phase
isomers are expected: two enantiomers and one meso isomer
(Figure 5).30 The phase isomers exist as gearing circuits comprising
Figure 5. Schematic representation of phase isomers. Benzene rings
containing substituents are red.
Figure 6. B97-D/Def2-TZVP calculated C2, Cs, and C2v structures and
energetics of 12a and 12b rendered as electrostatic potential maps
(top, contour level = 0.005) and HOMOs (bottom, contour level =
0.02) in each case. Energies relative to the respective C2 conformations
are given beneath the structures. Values in parentheses indicate the
estimated energies for the transition state structures for gearing (Cs)
and gear slippage (C2v) in 12a, calculated by subtracting the energetic
cost of planarity in the stator/axle unit (see text).
six energy minimum conformations that, in the absence of gear
slippage, may interconvert only through the individual phases of
the circuit. Barriers to gear slippage can be calculated by
determining the rates at which the phase isomers interconvert.
Introduction of labeling groups at the 4-positions of the Tp
groups avoids steric hindrances between the two Tp rotators and
between the rotators and the stator. In the enantiomeric phase
isomers, the labeling groups come in close contact in three of the
conformations; in the meso phase isomer, they remain distant from
one another, a difference that was expected to induce chemical shift
differences between the meso and enantiomeric phase isomers in
NMR spectroscopy. Trifluoromethyl (CF3) groups were chosen, as
they permit 19F-NMR experiments and were expected to enhance
31
both 12a and 12b, the C2 conformation is the lowest energy
ground state minimum. Hessian analysis confirms the Cs
conformation as a transition structure for geared rotation and
the C2v conformation as a transition state to gear slippage, for
12b. In 12a, the Cs and C2v conformations each exhibit two
imaginary frequencies, one matching the motion and magnitude
of the frequencies in the analogous structures of 12b, and a much
higher mode arising from out-of-plane twisting of the ethylene
bridges. This result derives from the central 2,3-dihyropyrazine
moiety’s preference to avoid the eclipsed conformation and
adopt a twisted/staggered conformation.32 The planarity of this
ring, as required by symmetry in the Cs and C2v conformations,
thereby uniformly raises the energies of these conformations to
1
the chemical shift differences between phase isomers in H NMR
spectra. Methyl groups were also chosen, as the behavior of the
1H NMR signal of this methyl group could indicate the presence
of phase isomers at low temperatures.
RESULTS AND DISCUSSION
■
Calculations. Dispersion-enabled DFT, B97-D/Def2-
TZVP, was applied to spur gear molecules 12a and 12b,
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levels that are higher than those of the genuine transition states.
Analysis of molecule 12c indicated that in the absence of gear
interactions the twisted/staggered C2 conformation is 3.67 kcal/
mol lower in energy than the C2v structure. Subtraction of this
value from the energies of the Cs and C2v conformations of 12a
provides an estimated value for the energies of the genuine
transition state conformations (shown in parentheses in Figure 6).
Hessian analysis of 12c in the C2v conformation showed a single
imaginary mode essentially identical in motion and frequency to
the higher mode found for 12a. The high consistency among
these computed structures (12a−c) provides greater confidence in
the accuracy of the gearing model and analysis.
Scheme 1. Retrosynthesis of Derivatives of Molecular Spur
Gears 12a and 12b
As expected from Figure 4, the distances between the axles of
rotation33 in the Tp groups increase from C2 to C2v to Cs in 12a
and 12b. Distances of 6.84 Å (12a) and 6.85 (12b) for C2
conformations and 7.98 (12a) and 7.99 (12b) for Cs
conformations are ∼0.10−0.15 Å shorter than those shown
in Figure 2, due to inward bending of the Tp groups. In the
C2 conformations, the proximal benzene rings display close
face-to-face and edge-to-face arene distances the correlative of
which places the Tp groups close together, with the axles
bending inward. Analysis of the HOMOs shows in-phase
orbital interactions between the two Tp groups.
anthracene-9-carboxaldehyde (18) and 3-substituted ben-
zynes (19).
Bibenzimidazole 15 was synthesized in six steps from 17
(Scheme 2). Protection of the amino group in 17 with an acetyl
Scheme 2. Synthesis of Stator Intermediate 15
The inward bending of the Tp groups in the Cs
conformations and their lower energies compared to the C2v
conformations suggests that a slight attraction may exist
between the Tp groups. As a consequence of the molecular
symmetry, the Cs structures exhibit nearly degenerate HOMOs,
each containing significant density on one of the Tp groups
(one of which is shown for each in Figure 6).
The interaxle distances in the C2v structures [7.71 Å (12a),
7.74 Å (12b)], though shorter than their Cs counterparts, are
∼0.5 Å longer than would be required from steric exclusion
alone. This widening suggests a destabilizing repulsion between
the blades of the Tp groups, which is further supported by the
higher energy of the C2v conformations. The HOMO orbital
also shows antibonding interactions between the Tp groups.
The computations clearly indicate that gear-meshed
conformations C2 and Cs are favored over the gear-clashed
C2v conformation and provide computed (12b) and estimated
group afforded acetanilide 20, which underwent nitration to
generate 21. Deprotection yielded nitroaniline 22 and
subsequent bromination yielded 16.35 Reduction of the nitro
group with SnCl2 afforded bromodiamine 23. Treatment of 23
with 0.5 molar equiv of methyl 1,1,1-trichloroacetimidate and a
catalytic amount of HCl, followed by basification with K2CO3,
provided bibenzimidazole 15.36
(12a) values for ΔG⧧
and ΔG⧧gear slippage. From these
geared rotation
data, gearing efficiency in terms of Fgear and Tgear can be
assessed. The computations thus predict Fgear values of 1300
(12a) and 1100 (12b) and Tgear values of 138 K (12a) and
133 K (12b).34 The results from the calculations support the
molecular design and provided motivation for the chemical
synthesis and investigation of BBI-based molecular spur gears.
Synthesis of Molecular Spur Gears. Retrosynthetic
analysis of 12a and 12b leads to stator component 4,4′-dibro-
mo-BBI (13) and rotator/axle component 9-ethynyltriptycene
(14) (Scheme 1). To increase the solubility of the synthetic
targets, 4,4′-dibromo-6,6′-di-tert-butyl-BBI (15) was substi-
tuted for 13. Bibenzimidazole 15 can be disconnected to
bromonitroaniline (16), which could be prepared from com-
mercially available 4-tert-butylaniline (17). 9-Ethynyltriptycene,
and 4-substituted derivatives thereof, can be disconnected from
Rotator/axle components 9-ethynyltriptycene 14a and
4-labeled derivatives 14b,c were prepared by slightly modifying
a published synthesis37 (Scheme 3). Aldehyde 18 was
converted to cyclic acetal 24, which was treated with derivatives
of benzyne, generated in situ from derivatives of anthranilic acid
to yield triptycene 25. Deprotection gave aldehyde 26, which
could be converted to ethynyltriptycenes 14a−c by a Corey−
Fuchs38 homologation via dibromoalkenes 27a−c. An alternate
synthesis employing a Sonogashira cross-coupling is known39
and also provided access to 14a, but the Corey−Fuchs
method was preferable in our hands. Reaction of 24 with
3-methylbenzyne gave the 1-methyl and 4-methyl derivatives of
25. These isomers were not separated but carried on in the
synthesis as a mixture until the Wittig step of the Corey−Fuchs
methodology, which selectively reacted with the anti isomer
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Scheme 3. Synthesis of Rotator/Axle Components 14a−c
Scheme 5. Synthesis of Dioxopyrrole-Fused Spur Gears
32a−c
imately two molecules of chlorobenzene, which are distributed
across three partially occupied sites with high disorder. The unit
and allowed for straightforward purification of 27c by column
chromatography.
With compounds 14a−c and 15 in hand, ethylene-bridged
molecular spur gears 28a−c were synthesized (Scheme 4).
cell contains two molecules of 28a and the space group is P1,
̅
with the center of inversion located at a point elevated above
the plane of the heterocyclic stator component.
The molecular structure, though unsymmetrical, closely
resembles the calculated Cs conformation of 12a (Figure 7).
Scheme 4. Synthesis of Ethylene-Bridged Molecular Spur
Gears 28a−c
The 1- and 1′-positions of 15 were protected with Boc groups,
affording 29, which was treated with alkynes 14a, 14b, or 14c
under standard Sonogashira cross-coupling conditions to yield
30a−c. Subsequent deprotection, followed by treatment with
an excess of 1-bromo-2-chloroethane, afforded 28a−c in yields
ranging from 62% to 64%.
Synthesis of analogs of unsaturated gear 12b presented a
different challenge. Several initial attempts to introduce a
vinylene bridge to compound 15 failed, but deprotonation
with NaH, followed by treatment with N-benzyl-2,3-dibromo-
maleimide,40 successfully introduced an unsaturated C−C bond
at the desired position, affording dibromoimide 31 (Scheme 5).
The yield of this reaction was routinely ∼10%, lower than the
reported yield of 36% for the same reaction with unsubstituted
BBI.41 Copper-free cross-coupling reactions42 with alkynes
14a−c afforded spur gears 32a−c. Standard Sonogashira
conditions using CuI failed, presumably due to complexation
of the Cu(I) by 31.
Figure 7. (Top left) The X-ray molecular structure of 28a [50%
thermal ellipsoids; hydrogen atoms removed for clarity. (Top right and
bottom) An overlay of experimental structure of 28a (green) and the
calculated Cs structure of 12a (red).
The plane of the inward-facing, wedged benzene ring and the
planes of the flanking, proximal benzene rings of the
neighboring Tp group intersect at angles of 56.6(2)° and
67.2(2)°. The plane of this wedged benzene ring and the plane
of the distal benzene ring on the other Tp group are close to
parallel, intersecting at an angle of 10.4(2)°. The distances
between the (9−9′) and (10−10′) bridgehead atoms of the Tp
groups are 8.420(4) and 8.257(4) Å, respectively, longer than
those found in the calculated Cs structures 12a and 12b.
The molecular structure in the crystal, considered in the
context of the calculations, provides further evidence for the
preference of the Cs conformation over the C2v conformation.
The similarity of the experimental structure to a Cs con-
formation illustrates that the transition state to geared rota-
tion is a favorable conformation of relatively low energy
and provides additional validation to the calculated structures.
X-ray Structure of Molecular Spur Gear 28a. Single
crystals of 28a were obtained from chlorobenzene, and the
crystal structure was determined by X-ray crystallography. The
asymmetric unit contains one molecule of 28a and approx-
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Deviation from the C2 conformation, the most stable
conformation according to calculations, is attributable to crystal
packing forces.
envisioned and 13C-labeled derivatives thereof would allow one
to exploit the high resolution of 13C NMR spectroscopy.
More advanced targets (34) include gears with differentially
labeled Tp groups, in which one Tp group contains a CSA
inducer and the other Tp group contains an NMR-active
reporter substituent. Potential CSA inducers include alkynes
and nitriles, strongly anisotropic aryl or arylethynyl groups, or
possibly stable radicals. NMR-active reporter substituents could
include the methyl (1H NMR) and trifluoromethyl (19F-NMR)
groups described in this study as well as a variety of 13C-labeled
substituents (13C NMR).
VT-NMR Experiments. Although the energy barriers for
gear slippage were calculated to be in the range
7−8 kcal/mol (at the detection limit for VT-NMR studies)
compounds 28b, 28c, 32b, and 32c were probed in an attempt
to uncover phase isomerism at low temperature.43 Determi-
nation of coalescence temperatures for the interconversion of
meso and enantiomeric phase isomers could experimentally
confirm the molecular preference for geared rotation over gear
slippage and quantitatively establish the barrier to gear slippage.
1H NMR Samples were measured in the low-freezing solvent
CDCl2F44 at temperatures as low as 150 K.45
The VT-1H NMR signals of all of the studied molecules
significantly broaden upon cooling but show no clear signs of
decoalescence (see Supporting Information). 19F-NMR spectra
of 28b and 32b in CD2Cl2 were measured at temperatures as
low as 180 K and also showed broadening, but no splitting, of
the signals.
The absence of decoalescence in VT-NMR experiments
precludes the determination of the barrier to gear slippage and
experimental demonstration that the rotators prefer to undergo
geared rotation over gear slippage. Several explanations could
account for the absence of observable decoalescence:
CONCLUSIONS
■
Encompassing molecular design, density functional theory
calculations, chemical synthesis, structural examination, and
dynamic stereochemical analysis, this work illustrates a rigorous
investigation into molecular spur gears. Computational results
regarding 12a and 12b, combined with structural data of 28a,
strongly suggest that the Tp rotators on these molecules
preferentially undergo geared rotation over gear slippage,
although VT-NMR studies did not experimentally confirm the
presence of phase isomers in 28b,c and 32b,c. The syntheses
and investigations reported here form the basis for future work
in the field of molecular rotors and machinery. An emphasis is
placed on the importance of engineering molecular features not
attributable to the classical hard sphere/mechanical model.
Along these lines, macro-engineering principles like those of
magnetic gearing may provide the insight necessary to achieve
high gearing fidelity at practical temperatures.
(1) The chemical shift differences between signals from the
enantiomeric and meso phase isomers may be so small as
to render them unobservable. Considering the common
peak broadening that results from shimming, tumbling
and other experimental issues at low temperature, it is
possible that independent signals could be hidden within
the broad peaks.
(2) The calculated values for the barriers to gear slippage
may be higher than those for the experimental systems
under the conditions of observation. If the barriers to
gear slippage for the experimental systems lie below
7 kcal/mol, rather than ∼8 kcal/mol, decoalescence would
not be observable even at temperatures as low as 150 K.
OUTLOOK
■
ASSOCIATED CONTENT
As technical issues did not allow for conclusive experimental
observation of the presence (or absence) of phase isomers, the
logical next step is to synthesize systems that would lead to
greater chemical dispersion of the NMR signals, increasing
the likelihood of observing phase isomers, should they exist.
The labeling groups remain distant from each other in the
meso phase isomer and come close in three of the six energy-
minimum conformations in the enantiomeric phase isomers.
This difference could be exploited in systems incorporating
strongly anisotropic units, including π-systems, which are
known to induce significant changes in the chemical shifts of
local nuclei.46
■
S
* Supporting Information
Full experimental synthesis procedures and characterization
1
data, including H-NMR and 13C-NMR spectra of all new
compounds; computational methods; VT-NMR spectra of 28b,
28c, 32b, and 32c; and CIF file of 28a. This material is available
free of charge via the Internet at http://pubs.acs.org/.
AUTHOR INFORMATION
■
Corresponding Authors
E-mail: kimb@oci.uzh.ch; jss@oci.uzh.ch
According to the Gutowsky−Holm approximation47 and the
Eyring equation, increasing Δν between decoalesced signals not
only makes observation of decoalesced signals easier to observe
but also increases the temperature of coalescence. By increasing
chemical shift dispersion, coalescence occurs at faster exchange
rates; therefore, Tc will be higher and hence more accessible.
Design of next-generation targets incorporates chemical shift
anisotropy (CSA) inducers. For example, a construct con-
taining triple bonds such as alkynes or nitriles (33) can be
Funding Sources
This work was supported by the Swiss National Science
Foundation (SNF).
ACKNOWLEDGMENTS
■
We thank Dr. Thomas Fox (Inorganic Chemistry Institute,
University of Zurich) for his assistance with the VT-NMR
measurements.
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Journal of the American Chemical Society
Article
Siegel, J. S. Angew. Chem., Int. Ed. 2000, 39, 2323). The structural dif-
ferences between corannulene, which is well represented as a circular bowl,
and 1,3,5,7,9-pentamethylcorannulene, which resembles a pentagon, also
illustrate this point (see: Bauert, T.; Merz., L.; Bandera, D.; Parschau, M.;
Siegel, J. S.; Ernst, K.-H. J. Am. Chem. Soc. 2009, 131, 3460).
(25) The point groups given in this discussion assume axles on one
side of the rotators that are connected by a stator. By this treatment,
a C2h orientation of lone rotators becomes C2-symmetric, D2v becomes
C2v, and C2v becomes Cs. The C2v* orientation positions the blades of
each rotator directly toward each other.
(26) Oberg, E.; Johnes, F. D.; Horton, H. L.; Ryffel, H. H.
Machinery’s Handbook; A Reference Book for the Mechanical Engineer,
Designer, Manufacturing Engineer, Draftsman, Toolmaker, and Machinist,
26th ed.; Industrial Press: New York, 2000.
(27) In Figure 3, the Tp groups are rendered according to the
following specifications: the thickness of a benzene ring is 3.4 Å, twice
the van der Waals radius of a carbon atom. The hydrogen atoms at
the tips of the benzene rings have a van der Waals radius of 1.2 Å. The
minimum distances between axles are calculated from van der Waals
contacts of inflexible Tp moieties.
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1535
dx.doi.org/10.1021/ja2063346 | J. Am. Chem.Soc. 2012, 134, 1528−1535