Molecular shuttling of a compact and rigid H-shaped [2]rotaxane

Article abstract of DOI:10.1002/anie.201108488

Push-button control: A new templating motif for mechanically interlocked molecules involving benzimidazolium axles and dibenzo[24]crown-8 wheels was used to create [2]rotaxane molecular shuttles with a compact and rigid, H-shaped molecular structure for c

Full text of DOI:10.1002/anie.201108488

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Angewandte
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DOI: 10.1002/anie.201108488
Interlocked Molecules
Molecular Shuttling of a Compact and Rigid H-Shaped [2]Rotaxane**
Kelong Zhu, V. Nicholas Vukotic, and Stephen J. Loeb*
Bistable [2]rotaxanes (molecular shuttles) are mechanically
interlocked molecules (MIMs) that can be switched between
different translational co-conformations in response to an
external stimulus.[1] Chemical, electrochemical, and photo-
chemical inputs have been used to control the actions of these
systems in solution and some very efficient and sophisticated
examples are known, including those that occupy non-
equilibrium states and function using a ratchet-type mecha-
nism.[2] Since most [2]rotaxane molecular shuttles are large,
flexible molecules with multiple functional groups they
operate with efficiency in solution where the interlocked
components enjoy a high degree of structural and dynamic
freedom. The size and flexibility of the molecular structure
are rarely a problem,[3] however, these attributes make them
much less practical for highly condensed phases where the
size, positional orientation and degree of aggregation of the
MIM components are crucial to optimum switching effi-
ciency.[4] To incorporate efficient molecular switches into
materials and devices with a high density of functional
components, a compact structure with a rigid axle and short,
linear track for translational motion would be most desir-
able.[5]
H][BF4] was synthesized by reacting 4,7-dibromo-2-phenyl-
1H-benzimidazole[8] with 2 equivalents of 4-fluorophenylbor-
onic acid under Suzuki coupling conditions followed by
protonation with HBF4 (see the Supporting Information). The
1H NMR spectrum of a solution comprised of equimolar
amounts of [2-H]+ and DB24C8 (1.0 ꢀ 10À3 m, CD3CN, 298 K)
shows the efficient formation of [2]pseudorotaxane [2-
HꢀDB24C8]+ (Figure 1). The association constant of 1.0 ꢀ
We report herein a compact MIM-based switch with well-
defined geometry, rigid backbone, and efficient switching
characteristics for future application in highly dense media.
Highlights include 1) the development of a new recognition
template for [2]pseudorotaxane formation built around a 24-
membered crown ether wheel and a benzimidazolium axle
with extended aromatic substituents, 2) a high yielding,
modular synthesis of [2]rotaxane molecular shuttles with
a compact and rigid, H-shaped molecular structure, 3) access
to three distinct molecular states and corresponding molec-
ular shuttling rates through acid–base chemistry, and 4) con-
trol of the rate of molecular shuttling through lithium ion
“ferrying” between neutral binding sites.[6]
Figure 1. 1H NMR spectrum of an equimolar solution of [2-H][BF4] and
DB24C8 showing formation of [2]pseudorotaxane [2-HꢀDB24C8]+ (the
NH resonance for the uncomplexed axle is very broad). Conditions:
1.0ꢀ10À3 m in CD3CN at 298 K. Blue labels=complexed [2-H]+ axle,
red labels=complexed DB24C8 wheel, black labels=uncomplexed
components.
103 mÀ1 is two orders of magnitude larger than for simple
imidazolium or benzimidazolium derivatives. Significant
shifts to higher frequency were observed for the NH
(+ 0.73 ppm) and proton a resonances (+ 0.37 ppm) indicat-
ing hydrogen-bonding interactions while shifts to lower
frequency for peaks b and c on the axle as well as e and f
on the wheel (À0.31 to À0.63 ppm) are the result of efficient
p-stacking. While [2]pseudorotaxane formation between
DB24C8 and axles containing two benzimidazolium groups
linked by an ethane chain was reported previously,[9,10] this
new templating motif shows much stronger association and
the modular synthesis should allow for the incorporation of
a wide variety of functionalized aromatics into the molecular
scaffold.[11]
The X-ray structure of [2-HꢀDB24C8][BF4] was consis-
tent with the non-covalent interactions observed in solution.
Figure 2 shows views emphasizing the primary NH···O hydro-
gen bonding (2.90 ꢁ, 1638; 2.83 ꢁ, 1648) between benzimi-
dazolium NH groups and DB24C8 ether oxygen atoms and
N+···O ion–dipole interactions (2.96–3.29 ꢁ) as well as the p-
The interaction of dibenzo[24]crown-8 (DB24C8) with the
imidazolium (Ka = 8mÀ1 [7] or phenylbenzimidazolium, [1-H]+
)
(Ka = 5.0 ꢀ 101mÀ1) cation is weak but this can be drastically
increased by adding aromatic groups to the 4- and 7-positions
of the benzimidazolium unit. The benzimidazolium salt [2-
[*] Dr. K. Zhu, V. N. Vukotic, Prof. S. J. Loeb
Department of Chemistry and Biochemistry, University of Windsor
Windsor, Ontario, N9B 3P4 (Canada)
E-mail: loeb@uwindsor.ca
[**] This work was supported by a NSERC of Canada Discovery grant to
S.J.L. V.N.V. is grateful to NSERC of Canada for the awarding of an
Alexander Graham Bell Graduate Doctoral Scholarship and to the
International Center for Diffraction Data for a Ludo Frevel
Crystallography Scholarship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108488.
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Figure 2. Ball-and-stick representations of the single-crystal X-ray struc-
ture of the cationic [2]pseudorotaxane [2-HꢀDB24C8]+. Left: emphasiz-
ing the primary NH···O hydrogen bonds. Right: emphasizing the p-
stacking and additional CH···O hydrogen bonds to the aromatic rings.
F=yellow, O=red, N=blue, C=black, H=white, axle=gold bonds,
wheel=silver bonds.
Figure 3. 1H NMR spectra from top to bottom: the neutral axle in
stacking of the benzimidazolium ring between the catechol
rings of DB24C8. Four additional CH···O interactions (3.38–
3.76 ꢁ, 155–1618) are observed between DB24C8 oxygen
atoms and aromatic protons reinforcing the premise that the
addition of these groups contributes significantly to the
increase in association between axle and wheel. The C-shape
conformation adopted by the DB24C8 macrocycle allows
efficient p-stacking interactions by clamping around the
electron-poor benzimidazolium ring (3.44–4.40 ꢁ).
Utilization of this new [2]pseudorotaxane templating
motif to prepare a permanently interlocked [2]rotaxane was
accomplished by taking advantage of the mild conditions used
to prepare the original benzimidazole unit through conden-
sation of a diamine with an aldehyde followed by catalytic
oxidation.[12]
Substituting terephthalaldehyde for benzaldehyde in the
axle synthesis allowed formation of a [2]pseudorotaxane
similar to [2-HꢀDB24C8]+, but with an exposed aldehyde
functional group (Ka = 9.7 ꢀ 103 mÀ1 in 10% CH3NO2/CHCl3).
In a one-pot reaction, this [2]pseudorotaxane was condensed
with an equivalent of 1,2-diamino-3,6-di(4’-fluorophenyl)ben-
zene[13] and then oxidized with a catalytic amount of ZrCl4 to
produce a [2]rotaxane which was isolated as the monoproto-
nated species [3-HꢀDB24C8][BF4] in 93% yield. This
methodology also provides a facile method to produce
[2]rotaxane molecular shuttles without nuisance of [3]rotax-
ane side-products.[6,14,15]
CD2Cl2; the neutral [2]rotaxane, [3ꢀDB24C8] in CD2Cl2; the monopro-
tonated [2]rotaxane, [3-HꢀDB24C8]+ in CD3CN; and the diprotonated
[2]rotaxane, [3-H2ꢀDB24C8]2+ in CD3CN. Blue labels=complexed
portion of the axle, red labels=complexed DB24C8 wheel, black
labels=uncomplexed portion of the axle.
and reduces the degree of hydrogen bonding between axle
and wheel. A comparison of the chemical shifts of the NH
group in the neutral axle, 3 (d = 9.65 ppm), and the neutral
[2]rotaxane, [3ꢀDB24C8] (d = 10.53 ppm), shows that resid-
ual hydrogen bonding between the neutral benzimidazole NH
group and crown ether oxygen atoms remains.
Conversely, the diprotonated species [3-H2ꢀDB24C8]2+
contains two strongly binding recognition sites each reminis-
cent of that observed for the [2]pseudorotaxane. The
1H NMR spectrum shows two sets of peaks that can be
clearly assigned as complexed and uncomplexed axle under-
going slow exchange at 298 K. The monoprotonated [2]rotax-
ane, [3-HꢀDB24C8]+, contains both protonated and neutral
1
recognition sites and the H NMR spectrum appears to be
static showing only resonances due to a protonated site which
is complexed and a neutral site which is uncomplexed.
Single-crystal X-ray structures were determined for the
neutral [3ꢀDB24C8] and diprotonated [3-H2ꢀDB24C8]2+
[2]rotaxanes. For both structures, the rigid axle measures
14 ꢁ between F atoms on the same benzimidazole group (up-
right of the H) and 12 ꢁ between centroids of the benzimi-
dazole phenyl rings (crossbar of the H).
Figure 4 (top) shows that for the diprotonated form the
arrangement of axle and wheel are almost identical to that
observed for the [2]pseudorotaxane in which the DB24C8
wheel clamps around one of the benzimidazolium recognition
sites. The primary NH···O hydrogen bonding (2.78 ꢁ, 1678;
2.82 ꢁ, 1658), N+···O ion–dipole (3.03–3.82 ꢁ), p-stacking,
and CH···O interaction parameters are similar. The structure
of the neutral [2]rotaxane [3ꢀDB24C8] shown in Figure 4
(bottom) is very different. Since the benzimidazole units are
no longer charged, there are no ion–dipole interactions and
no significant driving force for p-stacking. The DB24C8
macrocycle adopts a more open conformation and only
The presence of two benzimidazole recognition sites on
a single axle means that there are three potential forms of the
[2]rotaxane accessible by acid–base chemistry: neutral,
monocation, and dication. Each of these forms might be
expected to adopt a different structure and undergo molec-
ular shuttling at a different rate. Figure 3 shows the 1H NMR
spectra of the neutral axle (no crown), the neutral [2]rotaxane
[3ꢀDB24C8], the monoprotonated [2]rotaxane
[3-HꢀDB24C8]+, and the diprotonated [2]rotaxane [3-
H2ꢀDB24C8]2+. The spectrum of the neutral species is the
simplest as both ends of the axle appear to be equal due to fast
exchange between complexed and uncomplexed species. This
is because the benzimidazole units are not protonated which
eliminates both the ion–dipole and p-stacking interactions
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Figure 5. Four states of a new compact and rigid, H-shaped [2]rotax-
ane molecular shuttle are accessible. The rate of translational motion
(shuttling) along the short, linear axle between benzimidazole recog-
nition sites can be controlled by facile addition and removal of either
protons or lithium ions. Rates shown are those in measured in CD2Cl2
solution with DG° at 298 K.
Figure 4. Ball-and-stick representations of single-crystal X-ray struc-
tures. Top: [3-H2ꢀDB24C8]2+; left, showing primary NH···O hydrogen
bonds; right, the p-stacking. Bottom: [3ꢀDB24C8]; left, showing
hydrogen bonds; right, the additional CH···p hydrogen bonds.
F=yellow, O=red, N=blue, C=black, H=white, axle=gold bonds,
wheel=silver bonds.
further investigation into other methods to affect the rate of
shuttling in this system. Since the structure of the neutral
[2]rotaxane contains a residual donor nitrogen atom on the
benzimidazole ring in close proximity to several oxygen atoms
from the DB24C8 macrocycle, it was reasoned that this
convergence of donor atoms might be an effective chelating
site for metal ions.[21] Given the donor set and compact nature
of the site, Li+, Na+ and Zn2+ were tested for coordination.
Only the smallest Li+ ion showed binding[22] as evidenced by
significant changes in the 1H NMR spectrum.
participates in hydroge-bonding to the neutral NH group
(3.24 ꢁ, 1608) and two adjacent CH groups on the central
phenyl ring (3.38 ꢁ, 1518; bifurcated 3.29 ꢁ, 1368; 3.33 ꢁ,
1368). Importantly, this demonstrates that even when the
templating charge is removed from the axle there remains
enough residual non-covalent interaction to allow the benzi-
midazole unit to act as a recognition site for the DB24C8
macrocycle.
Adding one equivalent of Li+ to a solution of the neutral
[2]rotaxane [3ꢀDB24C8] in CD2Cl2 showed chemical shift
changes consistent with strong binding of Li+ to the benzimi-
dazole nitrogen atom and adjacent crown ether oxygen atoms.
2D EXSY spectra showed that the addition of Li+ ions results
in a slowing of the shuttling (5.1 sÀ1, CD2Cl2) and raising of the
barrier to motion (16.5 kcalmolÀ1). However, unlike the
monoprotonated [2]rotaxane which showed no evidence of
translational motion, [3-LiꢀDB24C8]+ shuttles at a rate
intermediate between those observed for the neutral and
dicationic molecules. Since the rate of shuttling was deter-
mined to be independent of the concentration of Li+ ion (see
the Supporting Information), the shuttling process is best
described as an intramolecular “ferrying” of the cation
between benzimidazole sites utilizing residual Li+···O inter-
actions between the cation and the crown ether. Addition of
an equivalent of [12]crown-4 ether (12C4) completely seques-
ters the Li+ ion and completely reverts the 1H NMR spectrum
back to that of the neutral species [3ꢀDB24C8] (see the
Supporting Information).
Since these three [2]rotaxanes contain an axle with two
recognition sites but only a single macrocyclic wheel, each
should function as a molecular shuttle.[16] 2D EXSY[17] and VT
1H NMR spectroscopy were employed to determine the rate
of molecular shuttling at 298 K. The dicationic system [3-
H2ꢀDB24C8]2+ exhibits the lowest rate of shuttling (8.4 ꢀ
10À3 sÀ1, CD2Cl2; 3.8 ꢀ 10À1 sÀ1, CD3NO2) since the non-
covalent interactions between axle and wheel are the
strongest and thus the barrier (20.3 kcalmolÀ1, CD2Cl2;
17.8 kcalmolÀ1, CD3NO2) to motion is the highest
(Figure 5). Removal of the charges by deprotonation elimi-
nates the great majority of these non-covalent interactions
and the neutral species shows a high rate of shuttling (4.3 ꢀ
106 sÀ1, CD2Cl2)[18] and correspondingly lower barrier (8.4 kcal
molÀ1).[5c,19] When the [2]rotaxane is singly charged there is
the potential for competition between the charged benzimi-
dazolium and neutral benzimidazole recognition sites for the
wheel, but NMR experiments showed the crown ether is
essentially static (approximately 0.00 sÀ1, CD2Cl2) preferring
the charged state to such an extent that the rate of shuttling
appears to be negligible.
In summary, a new rigid and compact, H-shaped [2]rotax-
ane was prepared using a new templating motif and a new
high yield capping methodology. The [2]rotaxane design
provides an axle with an exactly linear shuttling trajectory
that is the shortest known (7.5 ꢁ) and goal-post style end
groups that shelter the macrocycle. The neutral form of the
molecular shuttle [3ꢀDB24C8] undergoes extremely rapid
Although the demonstration of facile acid–base control
between three states of molecular shuttling[20] was gratifying,
the access to a rare, neutral molecular shuttle motivated
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shuttling between two equivalent recognition sites on the
ends of the rigid axle. Addition of one equivalent of acid to
give [3-HꢀDB24C8]+ arrests the shuttling by biasing the
molecule in favor of a single co-conformer to the point that
the wheel interacts only with one end of the axle. Diproto-
nation produces [3-H2ꢀDB24C8]2+ containing two equivalent
recognition sites and the shuttling is resumed but at a much
lower rate due to an increased barrier to translational motion.
Furthermore, addition of a single equivalent of Li+ to
[3ꢀDB24C8] yields [3-LiꢀDB24C8]+ in which a lithium
cation provides a braking action slowing the rapid shuttling
of the neutral [2]rotaxane to an intermediate rate by
participating in the ferrying of the lithium ion between
recognition sites. Conversion between these four states of this
new [2]rotaxane molecular shuttle is straightforward
(Figure 5). Control over the rates of translational motion
coupled with a rigid and compact design should find
application in condensed-phase materials. This system is
particularly well suited for inclusion into metal–organic
frameworks[23] where the size and rigidity of individual MIM
components and the nature of the translational pathway are
important for unperturbed motion and efficient switching.
Keywords: molecular shuttle · pseudorotaxane · rotaxane ·
supramolecular chemistry
.
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Experimental Section
DB24C8 was purchased from Aldrich Chemicals and used as
received. NMR experiments were recorded on a Bruker Avance
300 and 500 MHz NMR spectrometers. Details of the syntheses and
spectroscopic characterization of all new compounds can be found in
the Supporting Information.
X-ray data: [2-HꢀDB24C8][BF4]: C51H52BN3O8F6, M = 959.77,
ꢀ
colorless prisms (0.18 ꢀ 0.20 ꢀ 0.26 mm), triclinic, P1, a = 11.3489(14),
b = 12.4327(15), c = 17.633(2) ꢁ, a = 76.400(2), b = 79.367(2), g =
88.120(2)8, U = 2376.6(5) ꢁ3, Z = 2, 1calcd = 1.341 gcmÀ3
,
m =
[8] H. Akpinar, A. Balan, D. Baran, E. Unver, L. Toppare, Polymer
2010, 51, 6123.
0.105 mmÀ1
, min/max trans. = 0.8552, l(MoKa) = 0.71073 ꢁ, T=
173.0(2) K, 23201 total reflections (R(int) = 0.0826), R1 = 0.0777,
wR2 = 0.1773 [I > 2sI], R1 = 0.1469, wR2 = 0.2173 [all data],
GoF(F2) = 1.010, data/variables/restraints = 8309/623/0. X-ray data
for [3ꢀDB24C8]·(CHCl3)(CH3COOCH2CH3): C73H67Cl3F4O10N4,
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electron donating and withdrawing groups on the three aromatic
groups. The difluoro substituted axle reported herein is typical.
Full details of this study will be the subject of a future
publication.
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ꢀ
M = 1342.66, colorless prisms (0.30 ꢀ 0.26 ꢀ 0.20 mm), triclinic, P1,
a = 12.718(2), b = 17.456(2), c = 17.499(2) ꢁ, a = 106.241(2), b =
109.040(2), g = 106.716(2)8, U = 3203.8(7) ꢁ3, Z = 2, 1calcd
=
1.392 gcmÀ3, m = 0.220 mmÀ1, min/max trans. = 0.7905, l(MoKa ) =
0.71073 ꢁ, T= 173.0(2) K, 31108 total reflections (R(int) = 0.0796),
R1 = 0.0915, wR2 = 0.2543 [I > 2sI], R1 = 0.1334, wR2 = 0.2934 [all
data], GoF(F2) = 1.038, data/variables/restraints = 11235/847/12. X-
ray Data for [3-H2ꢀDB24C8][BF4]2: C68H60F12O8N4B2, M = 1310.82,
ꢀ
colorless prisms (0.34 ꢀ 0.24 ꢀ 0.18 mm), triclinic, P1, a = 11.182(2),
b = 16.755(3), c = 17.568(3) ꢁ, a = 86.127(2), b = 77.189(2), g =
74.007(2)8, U = 3085.1(8) ꢁ3, Z = 2, 1calcd = 1.411 gcmÀ3
,
m =
0.116 mmÀ1, min/max trans. = 0.7919, l(MoKa ) = 0.71073 ꢁ, T=
173.0(2) K, 28860 total reflections (R(int) = 0.0657), R1 = 0.0642,
wR2 = 0.1715 [I > 2sI], R1 = 0.0918, wR2 = 0.1906 [all data],
GoF(F2) = 1.066,
data/variables/restraints = 10772/847/0.
The
SHELXTL library of programs[24] was used for X-ray solutions and
figures were drawn with DIAMOND software.[25] CCDC 855552,
855553, and 855554 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Received: December 1, 2011
Published online: January 19, 2012
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