Efficient preparation of separable pseudo[n]rotaxanes by selective threading of oligoalkylammonium salts with cucurbit[7]uril

Article abstract of DOI:10.1002/chem.200900251

We describe the efficient preparation of well-defined, homogeneous pseudo[n]rotaxanes (n=2, 3, 4, 5) by the selective threading of oligoalkylammonium salts with cucurbit[7]uril (CB[7]) and the subsequent simple separation of the pure pseudo[n]rotaxanes fr

Full text of DOI:10.1002/chem.200900251

DOI: 10.1002/chem.200900251
Efficient Preparation of Separable Pseudo[n]rotaxanes by Selective
Threading of Oligoalkylammonium Salts with Cucurbit[7]uril
Jun Yin, Chunyan Chi, and Jishan Wu*[a]
Abstract: We describe the efficient
preparation of well-defined, homoge-
neous pseudo[n]rotaxanes (n=2, 3, 4,
5) by the selective threading of oligoal-
kylammonium salts with cucurbit[7]uril
(CB[7]) and the subsequent simple sep-
aration of the pure pseudo[n]rotaxanes
from the complicated system. A series
of well-defined oligoalkylammonium
salts were prepared by stepwise synthe-
sis and their molecular recognition and
self-assembly with a solution of CB[7]
in CF3CO2D/D2O (1:1 v/v) were inves-
tigated. As a result of the high affinity
of CB[7] to the p-xylene diammonium
units (-NH2+CH2C6H4CH2NH2+-) in
the oligoalkylammonium salts, the
CB[7] rings threaded onto these cation-
ic threads to form pseudorotaxane
structures. Interestingly, due to the re-
pulsive dipole–dipole electrostatic in-
teractions between the CB[7] units and
steric hindrance, the threading process
simple counterion exchange with
NH4PF6 and the pure pseudo[n]rotax-
anes are stable and partially soluble in
normal organic solvents. Such facile
preparation and separation are mainly
ascribed to the high binding constant
between CB[7] and the oligoalkylam-
monium salts and the unique solubility
of CB[7] in protonic acid. As most
pseudorotaxanes reported in the litera-
ture exist as a dynamic mixture and
their separation is usually difficult, our
research represents one good example
in which pure pseudo[n]rotaxanes (n=
2, 3, 4, 5) can be obtained by a simple
“threading-followed-by-precipitation”
method.
exhibited
a high selectivity and a
unique self-assembling mode was dis-
covered in which two CB[7] units
cannot bind the same ammonium site.
The as-formed well-defined pseudo[n]-
rotaxanes can be easily separated as
pure compounds from the mixture by
Keywords: counterion exchange
·
·
cucurbiturils
self-assembly
chemistry
· pseudorotaxanes
·
supramolecular
been prepared by using a template-directed clipping reac-
tion,[8] threading-followed-by-stoppering,[1e,9] slipping reac-
tion[10] and other methods.[1h,11] Unique properties were
found in polyrotaxanes compared with those of conventional
covalent polymers.[12] Pseudorotaxane is a similar structure
to rotaxane, in that bulk stoppers do not exist and the inter-
locked structures are solely stabilized by molecular recogni-
tion between macrocycles and the recognition sites on the
thread. Usually pseudorotaxanes are easily formed by
simply threading the thread with suitable macrocycles, and
in most cases, a large excess of macrocycles has to be used
to reach a maximum degree of threading. This is mainly be-
cause the binding between the macrocycles and the thread is
not strong enough and a dynamic threading–dethreading
process takes place; this leads to a complicated mixture of
different pseudo[n]rotaxanes coexisting with excess macro-
cycles in the solution. All these make the separation of
pseudorotaxanes more challenging than the purification of
rotaxanes.[13] To date, although the synthesis of higher order
rotaxanes has been quite successful,[8] the efficient prepara-
tion and separation of pseudorotaxanes still lags behind. A
Introduction
Interlocked molecules, such as rotaxanes, catenanes, and
molecular knots,[1] comprise a major research field of supra-
molecular chemistry and have been worthy of steadily in-
creasing attention in recent decades[2] because they allow
one to construct functional molecular devices of high sophis-
tication.[1f,g,2h,3] For example, rotaxanes have been used as
molecular switches within nanoelectronics,[4] artificial mus-
cles,[5] macroscopic liquid transport,[6] and mesoporous-
silica-mounted nanovalves.[7] Polyrotaxanes, in which cyclic
rings and a linear dumbbell-like species carrying recognition
sites are mechanically interlocked with each other, have
[a] Dr. J. Yin, Dr. C. Chi, Prof. Dr. J. Wu
Department of Chemistry
National University of Singapore
3 Science Drive 3, 117543 (Singapore)
Fax : (+65)6779-1691
E-mail: chmwuj@nus.edu.sg
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900251.
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FULL PAPER
general consideration for the synthesis and separation of
pure pseudorotaxanes is that the binding constant between
the recognition sites and the macrocycles must be high
enough so that the as-formed pseudorotaxane does not un-
dergo a dethreading process even with the absence of excess
macrocycles.
xylene diammonium salts can form a stable 1:1 complex
with CB[6] or CB[7] under suitable conditions and they can
be prepared by using reasonable synthetic methods. As rep-
resented in Scheme 1, an interesting question is whether
every p-xylene unit can be encircled by one CB[7] ring
during the threading of the oligoalkylammonium salts with
excess CBs. Our detailed studies disclosed that the threading
process showed high selectivity and only a limited number
of CB units can be located at very specific sites on the cat-
ionic thread. At the same time, by taking advantage of the
high binding constant between the CBs and the p-xylene di-
ammonium units[14–18] and the unique solubility of CBs in
protonic acid,[19] we found that these well-defined pseudo[n]-
rotaxanes could also be separated as pure and stable com-
pounds from the complex systems by simple counterion ex-
change.
Cucurbit[n]urils (CB[n]s, n=5,6,7,8,10) as a kind of im-
portant supramolecular host have been well known to form
host–guest complexes with various guests carrying ammoni-
um sites due to their high affinity and highly selective bind-
ing.[14–18] Furthermore, cucurbiturils have also been used in
the preparation of polyrotaxanes and pseudopolyrotax-
anes[19] by diversified approaches as reported by Kim and
co-workers,[20] Steinke and Tuncel,[21] Liu,[22] and others.[23]
One general method is to utilize the unique click chemistry
of alkyne and azide in the inner cavity of cucurbituril. A
(pseudo)rotaxane is then formed after the reaction. The
second general procedure is to thread polymeric alkylammo-
niums or polyviologens with CBs. Statistic pseudopolyrotax-
anes are then formed. In the latter case, excess CBs have to
be used to reach a high degree of threading and thus how to
purify and separate these CB-based pseudopolyrotaxanes is
a challenging problem. There are a few reports on the pu-
rification and separation of crown ether and cyclodextrin-
based rotaxanes by methods such as gel permeation chroma-
tography (GPC)[24] or high-performance liquid chromatogra-
phy (HPLC),[1h,25] however, these techniques are only suita-
ble for separation of a small quantity of compounds and are
only applicable to specific systems. For CB-based polyrot-
axanes and pseudopolyrotaxanes, there are so far no good
methods to obtain pure compounds. In addition, although
the self-assembling mode of the crown ether with polyam-
monium salts is well known,[8d,13] the detailed binding mode
of CBs with the cationic polymer thread is still unclear.[17b,26]
To better understand the threading process and the self-as-
sembling mode, we are interested in investigating the
threading of well-defined oligoalkylammonium salts with
CBs. The strong binding between the CBs and the oligoalky-
lammonium salts also makes it possible to prepare well-de-
fined pseudo[n]rotaxanes in pure form.
Results and Discussion
Synthesis of oligoalkylammonium salts: The stepwise syn-
thesis of the linear oligoalkylammonium salts is shown in
Scheme 2. The p-xylene diammonium salt 1·ACHTNUGTRNEG(UN PF6)2 was
easily obtained by protonation of commercially available p-
xylene diamine with trifluoroacetic acid (TFA) and followed
by counterion exchange with aqueous ammonium hexafluor-
ophosphate (NH4PF6). The triammonium salt 2·ACHTUNGTRENNUNG(PF6)3,
which contains two p-xylene units, was synthesized from the
known bisazide compound 6.[27] The azide groups in com-
pound 6 were reduced by lithium aluminum hydride to give
amines. Subsequent deprotection of the tert-butoxycarbonyl
(Boc) group with excess TFA and counterion exchange with
NH4PF6 provided the desired oligoalkylammonium salts 2·
A
ACHTUNGTRENNUNG(PF6)n–5·
AHCTUNGTRENNUNG
À
À
À
well-defined number of -Ph CH2
NACTHNGUTERN(NUG Boc) CH2- repeating
units and two terminal aldehyde groups according to the re-
ported procedures.[8d] Condensation reaction of compounds
8a–c with the 2-(trimethylsilyl)ethylcarbamate (Teoc) mo-
noprotected xylene diamine 7[8d] in toluene gave the corre-
sponding imines 9a–c in nearly quantitative yields. Reduc-
tion of the imine bonds in compounds 9a–c with NaBH4 af-
forded the free amines 10a–c in high yields. Compounds
10a–c had high polarity and
In this experiment, we synthesized a series of well-defined
oligoalkylammonium salts 1·ACHTUNTRGENNU(G PF6)n–5·ACHTUNGTERN(NUGN PF6)n in which the
ammonium ions are linked by p-xylene units (Scheme 1).
Such oligomers were chosen because it is known that the p-
their purification by column
chromatography was difficult.
To obtain high-purity oligo-
mers, the free amine groups in
compounds 10a–c were pro-
tected by Boc by reaction with
Boc2O and the obtained com-
pounds 11a–c could be easily
purified by routine column
chromatography. The pure
oligomers 11a–c were then
Scheme 1. The schematic representation of pseudo[n]rotaxane formation by threading of oligoalkylammonium
salts with CB[7].
treated with excess TFA to
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J. Wu et al.
mitted for threading studies
with different amount of
CB[n]s. It was found that these
oligoalkylammonium salts had
poor solubility in frequently
used solvent systems such as
aqueous hydrochloride acid or
formic acid, even when the
counterions are CF3COOÀ
(i.e., in the form before coun-
terion exchange). However,
they were soluble in mixed sol-
vents of TFA and water (1:1 v/
v). Therefore, all the threading
processes were followed by
using 1H NMR spectroscopy
on samples in CF3COOD/D2O
(1:1) by varying the ratio of
the CB[n]s to the ammonium
salts. We found that upon addi-
tion of CB[6] into the solution
of 1·
icant change in the 1H NMR
spectra of either 1·(PF6)2 or
ACHTUGNTRENUN(GN PF6)2, there was no signif-
AHCTUNGTRENNUNG
CB[6], which suggests that
there was no host–guest bind-
ing in such a solvent system.
However, when we used the
larger CB[7] instead of CB[6],
obvious
changes
in
the
1H NMR spectra were ob-
served. Upon addition of one
equivalent of CB[7], the reso-
nances of the aromatic protons
and methylene protons of 12+
were shifted upfield by d=0.79
and 0.22 ppm, respectively,
which suggests that
CB[7]–12+ complex (i.e.,
pseudo[2]rotaxane)
a
1:1
a
was
Scheme 2. Synthetic route of the oligoalkylammonium salts.
formed (Figure S1 in the Sup-
porting Information). The up-
remove both the Boc and Teoc groups, and at the same
time, all the amine groups were protonated. For the higher
order oligomers such as 11b and 11c, it was necessary to
use neat TFA to remove all protective groups. After coun-
terion exchange by adding saturated aqueous NH4PF6 to the
field shift of the resonance for the guest molecules was at-
tributed to the shielding effect of the CB[7] host molecule,
and at the same time, a slight change to the resonance of
the CB[7] was also observed, further supporting such a bind-
ing process. The absence of an exchange equilibrium of
complexed and uncomplexed species in such a solvent
system also indicated a high binding constant (>104 mÀ1),
which, however, cannot be directly determined by NMR
spectroscopic techniques.[14b,16c,d,17b,26,28] Interestingly, such a
1:1 complex can be easily separated from the mixture solu-
tion by the addition of excess saturated NH4PF6 solution,
and after counterion exchange, white precipitate was collect-
ed, washed with water, and dried in air to give the pure
solution, pure oligoalkylammonium salts 3·
ACHTUTNGRENG(NU PF6)4, 4·ACHTUNGTREN(NUGN PF6)6,
+
and 5·(PF6)8 containing a well-defined number of -CH2NH3
ACHTUNGTRENNUNG
and -CH2NH2+CH2- units were obtained in nearly quantita-
tive yields. All of the intermediate and final compounds
were well characterized by standard spectroscopic tech-
niques such as NMR spectroscopy and mass spectrometry
(see the Supporting Information).
Threading of oligoalkylammonium salts with CBs: The pure
pseudo[2]rotaxane CB[7]–1·
ACHTUNGTREN(NNUG PF6)2. This simple and conven-
oligoalkylammonium salts 1·ACTHNUTRGENNUG(PF6)n–5·CAHUTGNTRNEN(UGN PF6)n were then sub-
ient method was performed to obtain the pure pseudo[2]ro-
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Separable Pseudo[n]rotaxanes
FULL PAPER
taxane as a result of the following: 1) the pseudorotaxane
with PF6 has poor solubility in the water phase and thus
tion mass spectrum (HR-ESIMS) of CB[7]–1·
tonitrile clearly showed three peaks that can be assigned to
ACHTUGNTRNEN(UNG PF6)2 in ace-
À
À
À
À
precipitates from the solution; 2) the high binding constant
is advantageous in preventing the pseudorotaxane from
dethreading during the counterion exchange process and
also to stabilize the interlocked structure in suitable organic
solvents, even with the absence of excess CB[7]; and 3) any
excess CB[7] will remain in the solution due to its high affin-
ity to protonic acid. Such a simple method was also found to
be very efficient in preparing pure higher order pseudo[n]-
rotaxanes as will be discussed later. The pseudo[2]rotaxane
was soluble in normal organic solvents such as acetonitrile
and allowed us to characterize it by using standard spectro-
scopic techniques. The high-resolution electrospray ioniza-
the [MÀ2PF6 ]2+, [MÀHPF6ÀPF6 ]+, and [M+H+ÀPF6 ]2+
species, in which M is the 1:1 complex (Figure 1A). The lack
of peaks for uncomplexed CB[7] and ammonium salts indi-
cates that a tightly bonded pseudo[2]rotaxane complex was
formed instead of the existence of a fast exchange equilibri-
1
um. In addition, the H NMR spectrum was recorded for the
complex in CD3CN and it clearly showed a pure 1:1 com-
plex with the proposed pseudo[2]rotaxane structure (Fig-
ure 2A). The simple NMR spectrum again suggested a tight
binding between compound 1·ACHTNUGTRNEUNG(PF6)2 and CB[7] in acetoni-
trile. However, we also observed that in DMSO an obvious
slow exchange equilibrium between the complexed and un-
Figure 1. The high-resolution ESI mass spectra of pseudo[n]rotaxanes: A) CB[7]–1·
E) CB[7]–5·(PF6)8.
ACHTGNUNRETN(UNG PF6)2, B) CB[7]–2·CAHTUNGTRNEGU(N PF6)3, C) CB[7]–3·AHCUTNGERTGN(NUN PF6)4, D) CB[7]–4·ACHTUNGTRENNUNG(PF6)6, and
ACHTUNGTRENNUNG
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J. Wu et al.
ces of 23+ were split into two
sets of resonance peaks, with
one set showing an upfield
shift and the second set dis-
playing a downfield shift (Fig-
ure 3C). The upfield shift of
the resonance peaks for aro-
matic protons (Hb, Hc) and
methylene protons (Ha, Hd)
suggested that one of the
xylene units was encircled with
CB[7], and the resonance as-
signed to the uncomplexed
xylene unit showed a down-
field shift because of the effect
of adjacent carbonyl oxygen
atoms on CB[7] (see detailed
assignment
in
Fig-
ure 3C).[16,28,29]
Interestingly,
addition of more CB[7]
(2 equiv) did not change the
spectrum, and the approximate
1:1 integration ratio of the res-
onances for the complexed and
uncomplexed phenyl rings indi-
Figure 2. 1H NMR spectra (500 MHz, CD3CN, 298 K) of pseudo[n]rotaxanes: A) CB[7]–1·
B) CB[7]–3·(PF6)4, C) CB[7]–4·(PF6)6, and D) CB[7]–5·(PF6)8 (resonances labeled by “ꢂ” indicate impurities
in the solvents).
ACHTUGNTERN(NUNG PF6)2,
G
N
ACHTUNGTRENNUNG
complexed species existed, suggesting that the binding was
also solvent-dependent.
cated that only one CB[7] ring threaded onto the 2·ACHTUNGTRENNUNG(PF6)3 to
form a pseudo[2]rotaxane. Addition of an excess of aqueous
NH4PF6 to the solution resulted in precipitation of the as-
The threading of 2·
under similar conditions. The 1H NMR spectrum of com-
pound 2·(PF6)3 in CF3COOD/D2O (1:1) showed simple reso-
ACHTUGNTERN(NUNG PF6)3 with CB[7] was then conducted
À
formed pseudo[2]rotaxane with PF6 as counterions, where-
G
as the excess CB[7] still remained in the acidic solution.
Again, we found that the obtained pseudo[2]rotaxane was a
stable complex in acetonitrile, even with the absence of the
excess CB[7]. The HR-ESIMS recorded in acetonitrile dis-
closed a peak at m/z 709.65 for
nances at d=7.50 and 4.24 ppm (Figure 3B), which can be
assigned to the aromatic and methylene protons, respective-
ly. Upon addition of one equivalent of CB[7], the resonan-
À
[MÀHPF6À2PF6 ]2+ with well-
resolved isotope distribution.
This further confirmed the for-
mation of a pseudo[2]rotaxane
(Figure 1B). Unfortunately, the
very poor solubility of the
pseudo[2]rotaxane in acetoni-
trile limited detailed NMR
spectroscopic analysis. These
results suggested that the
placement of two CB[7] mole-
cules at two neighboring p-
xylene units was not favorable.
Such a unique self-assembling
mode also agreed well with
Steinkeꢁs mode[21b,23f] and can
be attributed to significant re-
pulsive dipole–dipole electro-
static interactions of the polar
carbonyl-rimmed portals be-
tween two CB[7] molecules
Figure 3. 1H NMR spectra (300 MHz, CF3COOD/D2O=1:1, 298 K) of A) CB[7], B) 2·
ACHTNUGTRNEUNG(PF6)3, C) CB[7]/2·
ACHTUNGTRENNUNG(PF6)3 =1:1, and D) CB[7]/2·AHCTUNTGREN(NUGN PF6)3 =2:1.
when they bind with the same
ammonium site. In addition,
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Separable Pseudo[n]rotaxanes
FULL PAPER
the steric interactions will also prevent two CB[7] rings from
threading onto two adjacent p-xylene units. If this self-as-
sembling mode is also applicable to the higher order oligoal-
kylammonium salts, one can expect that at the maximum
degree of threading, pseudo[3]rotaxane, pseudo[4]rotaxane,
ure 4D). The pure pseudo[3]rotaxane again was obtained by
simple counterion exchange with NH4PF6, and it was also in-
teresting that the sample had a higher solubility in acetoni-
trile than the pseudo[2]rotaxane obtained from 2·ACHTUNGTRENNUNG(PF6)3,
probably because there were two CB[7] units encircling the
1
and pseudo[5]rotaxane will be formed for 3·
and 5·(PF6)8, respectively.
To further confirm this assumption and to understand the
exact self-assembling mode, threading of 3·(PF6)4, 4·(PF6)6,
and 5·(PF6)8 with CB[7] was conducted using a similar ap-
A
N
cationic thread. H NMR spectra of the pure pseudo[3]ro-
N
taxane in acetonitrile clearly supported such a self-assem-
bling mode (Figure 2B). In addition, the HR-ESIMS exhib-
À
A
U
ited three peaks at m/z 676.10 for [MÀ4PF6 ]4+, 949.83 for
À
À
E
[MÀ3PF6 ]3+ and 1497.08 for [MÀ2PF6 ]2+ (Figure 1C),
further confirming the proposed structure. Based on these
results, it will be natural to expect that the threading of 4·
proach. In all cases, a gentle heating of the solution was
needed to obtain clear solutions. Upon addition of various
amounts of CB[7], the resonances of 34+ experienced a simi-
lar upfield and downfield shift (Figure 4). When four equiv-
ACHUTNGRENU(NG PF6)6 and 5·AHCTUNGTERN(NGUN PF6)8 with excess CB[7] will lead to the forma-
tion of pseudo[4]rotaxane and pseudo[5]rotaxane with the
CB[7] rings located at specific sites. In fact, our experimen-
tal data did support this assumption. The threading of 4·
(PF6)8 with CB[7] was followed by 1H NMR
ACHUTNRGEN(NUG PF6)6 and 5·ACHTUGNTRENNUGN
spectroscopic measurements (Figures S2 and S3 in the Sup-
porting Information). In both cases, a large excess of CB[7]
was required to reach a maximum degree of threading, and
finally, simple NMR spectra were obtained that can be as-
signed to the highly symmetric pseudo[4]rotaxane and pseu-
do[5]rotaxane, respectively. It was clear that for 4·
three CB[7] molecules were located at the two terminal and
one central xylene units. Similarly for 5·(PF6)8, four CB[7]
ACHTUNGTRENNUNG(PF6)6,
AHCTUNGTRENNUNG
molecules are located along the cationic thread with one un-
complexed xylene unit bridged between neighboring CB[7]
rings (see the models in Figures 1 and 2). Such a self-assem-
1
bling mode was further proved by the H NMR spectroscop-
ic and HR-ESIMS measurements of the pure pseudo[n]ro-
taxanes, which were obtained by similar counterion ex-
1
change. As shown in Figure 2, simple H NMR spectra were
obtained for the pseudo[4]rotaxane in CD3CN and it was in
agreement with the proposed structure (Figure 2C). The col-
lection of the 1H NMR spectra of the pseudo[5]rotaxane
was a little more challenging because of its poor solubility.
1
After a long time, a relatively clear H NMR spectrum was
Figure 4. 1H NMR spectra (500 MHz, CF3COOD/D2O=1:1, 298 K) of
obtained, which again supported our expectation (Fig-
ure 2D).[30] The HR-ESIMS measurements further proved
the formation of these pseudo[n]rotaxanes (Figure 1D, E).
A) CB[7], B) 3·
G
N
ACHTUGNERTN(NUNG PF6)4 =2:1,
E) CB[7]/3·(PF6)4 =3:1, and F) CB[7]/3·ACTHUNGTRENNUNG
A
The mass spectrum of the pseudo[4]rotaxane formed from
À
alents of CB[7] were added, a simple 1H NMR spectrum
was obtained that clearly showed that a symmetric pseu-
do[3]rotaxane had formed, in which the two CB[7] rings en-
circled the two terminal xylene units with the central xylene
unit uncomplexed (Figure 4F). As a result, the resonances
for the complexed phenyl rings (Hb, Hc) showed an upfield
shift to d=6.89 (doublet) and 6.77 ppm (doublet), and the
resonance for the central uncomplexed phenyl rings (Hf)
displayed a downfield shift to d=8.02 ppm as a singlet. Sim-
ilar changes were observed for the methylene protons of 34+
and the integration ratio between the two sets of resonances
showed the expected value of 2:1. We also found that to
reach a maximum degree of threading, an excess of CB[7]
had to be added, as one can see that the addition of two
equivalents of CB[7] resulted in an equilibrium of both
pseudo[3]rotaxane and pseudo[2]rotaxane in solution (Fig-
4·
G
,
À
À
850.54 for [MÀ5PF6 ]5+, 1099.43 for [MÀ4PF6 ]4+ and
À
1514.23 for [MÀ3PF6 ]3+; the pseudo[5]rotaxane from 5·
À
A
,
808.17 for [MÀ7PF6 ]7+, 967.06 for [MÀ6PF6 ]6+, 1189.52
À
À
À
À
for [MÀ5PF6 ]5+ and 1524.24 for [MÀ4PF6 ]4+. All the
peaks gave well-resolved isotope distributions in agreement
with the calculated values.
Conclusions
In conclusion, a series of well-defined, higher order pseu-
do[n]rotaxanes (n=2, 3, 4, 5) have been prepared in pure
form by using a simple “threading-followed-by-precipita-
tion” approach. To the best of our knowledge, the obtained
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J. Wu et al.
Oxford, 1996; g) Molecular Catenanes, Rotaxanes and Knots: A
Journey through the World of MolecularTopology (Eds: J. P. Sauv-
age, C. Dietrich-Buchecker), Wiley-VCH, Weinheim, 1999; h) V.
Balzani, M. Venturi, A. Credi, Molecular Devices and Machines: A
Journey into the Nano World, Wiley-VCH, Weinheim, 2003.
pseudo[5]rotaxane represents the longest pure pseudorotax-
ane to date. By taking advantage of the tight binding prop-
erties between the CB[7] and xylene diammonium units,
well-defined pseuodorotaxanes are formed in solution by a
simple threading process, and the as-formed pseudo[n]rotax-
anes can be easily separated from the complicated mixture
by simple counterion exchange. Our studies also disclosed
an interesting self-assembling mode during the threading of
a series of oligoalkylammonium salts with CB[7], and this
suggested the importance of the interactions between the
macrocycles during the threading process. In this specific
case, the repulsion and steric hindrance between the CB[7]
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1
ing of 1·
ACHTUNGTRENNUNG(PF6)2, 4·ACHTUNGTRENNUNG(PF6)6, and 5·ACHTUNGTERN(NUGN PF6)8 with CB[7] are given in the Support-
ing Information.
Acknowledgements
This work was financially supported by the AcRF Tier 1 grant (R-143-
000-328-133) and the NUS Young Investigator Award (R-143-000-356-
101).
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Received: January 30, 2009
Published online: May 5, 2009
Chem. Eur. J. 2009, 15, 6050 – 6057
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6057