Investigating the effect of macrocycle size in anion templated imidazolium-based interpenetrated and interlocked assemblies

Article abstract of DOI:10.1039/c2ob26237a

The effect of varying the size of the macrocycle component on the formation of anion templated imidazolium interpenetrated assemblies is investigated. Two different approaches to reducing the macrocycle size are undertaken and the stabilities of the resul

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ARTICLE TYPE
Investigating the Effect of Macrocycle Size in Anion Templated
Imidazolium–Based Interpenetrated and Interlocked Assemblies
Graeme T. Spence, a Nicholas G. Whitea and Paul D. Beer*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The effect of varying the size of the macrocycle component on the formation of anion templated
imidazolium interpenetrated assemblies is investigated. Two different approaches to reducing the
macrocycle size are undertaken and the stabilities of the resulting pseudorotaxanes incorporating
substituted imidazolium threading components studied using 1H NMR spectroscopy. Novel imidazolium
10 axle containing interlocked rotaxane host structures are synthesised using chloride anion templated amide
condensation and ‘stoppering’ methods, and the anion recognition properties of the ‘stoppered’ rotaxane
investigated.
Introduction
Results and Discussion
The positively charged imidazolium heterocycle is capable of
Macrocycle Synthesis
15 effective anion complexation via strong electrostatic and
hydrogen bonding interactions, and hence has been widely
employed in a variety of acyclic and macrocyclic anion receptor
and sensing systems.1-9
Through careful design, the anion templated synthesis of
20 mechanically interlocked structures can result in the construction
of potent and selective anion receptors possessing three
dimensional host cavities, with recent systems exploiting a
variety of anion binding groups.10-15
45
Two different design strategies for reducing the size of
macrocycle 3 were undertaken, as shown in Fig. 1. Firstly, in the
case of macrocycle 4, the linker between the isophthalamide
binding cleft and the electron-rich aromatic groups, in this case
phenolic rather than hydroquinone, was shortened, reducing the
50 macrocycle size by four atoms. Secondly, for macrocycle 5,18
three atoms were removed from the polyether chain of 3.
In a previous investigation,16 substituted imidazolium threads
25 with contrasting hydrogen bond donor arrangements,
4,5-dimethylimidazolium 1·Cl and 2-methylimidazolium 2·Cl
(Fig. 1), were demonstrated to form stable chloride anion
templated pseudorotaxane assemblies with isophthalamide
macrocycle 3 via favourable, cooperative amide–halide hydrogen
30 bonding and donor–acceptor interactions between the
electron-rich macrocycle hydroquinone groups and electron-
deficient imidazolium motifs. Furthermore, the corresponding
imidazolium axle containing rotaxane host systems displayed
selectivity for halide guest species over larger, more basic
35 oxoanions.17
In an effort to ultimately further improve the anion recognition
properties of such interlocked hosts, we describe herein an
investigation into the effect of reducing the size of the
macrocycle component on anion templated pseudorotaxane
40 assembly stability and in the construction of novel imidazolium–
based rotaxanes.‡
Fig. 1 4,5-Dimethyl- and 2-methyl-midazolium threads 1·Cl and 2·Cl and
macrocycles 3, 4 and 5.
55 Whilst similar macrocycle derivatives to 4 have been reported,19-
21
this system is novel due to the tert-butyl functionalisation of
the isophthalamide motif, which is necessary for direct
comparison with macrocycles 3 and 5. Condensation of bis-amine
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620 with 5-(tert-butyl)isophthaloyl dichloride in the presence of
2.5 equivalents of Et3N gave macrocycle 4 in 13% yield
(Scheme 1).§
35
5
Scheme 1 Synthesis of macrocycle 4.
Crystals of macrocycle 4 suitable for single-crystal X-ray
diffraction structural analysis were obtained by recrystallisation
from MeCN and analysed using synchrotron radiation at
Diamond Light Source beamline I19 (Fig. 2). In the solid-state,
10 the amide groups of 4 are approximately syn-anti and form an
extended hydrogen bonding network with the amide and
polyether groups of adjacent macrocycles.
Fig. 3 Partial 1H NMR spectra (500 MHz) in CDCl3 at 293 K of
40 a) macrocycle 4, b) macrocycle 4 plus 1.0 equivalent of thread 1·Cl, and
c) thread 1·Cl.
15 Fig. 2 Crystal structure of macrocycle 4. Protons and lower occupancy
positions of disorder are omitted for clarity.
Pseudorotaxane Investigations
1
Initially, H NMR pseudorotaxane assembly studies were carried
out in CDCl3 using macrocycle 4 and imidazolium threading
20 components 1·Cl and 2·Cl. The spectra for macrocycle 4,
4,5-dimethylimidazolium thread 1·Cl and an equimolar solution
of both components are shown in Fig. 3, with the observed
chemical shift changes indicative of the formation of anion
templated pseudorotaxane 1·4·Cl. Specifically, polarisation of the
25 templating chloride anion away from the thread and towards the
macrocycle hydrogen bond donors is responsible for downfield
shifts in the signals corresponding to macrocycle protons c and d,
and an upfield shift in imidazolium thread proton 1. Additionally,
macrocycle phenolic protons g are shifted significantly upfield
30 upon pseudorotaxane formation, due to favourable donor–
acceptor interactions with the electron-deficient imidazolium
thread. Similar shifts were also observed for the formation of the
analogous 2-methylimidazolium pseudorotaxane 2·4·Cl (Fig. 4).
45
Fig. 4 Partial 1H NMR spectra (500 MHz) in CDCl3 at 293 K of
a) macrocycle 4, b) macrocycle 4 plus 1.0 equivalent of thread 2·Cl, and
c) thread 2·Cl.
2
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Quantitative 1H NMR titration experiments were then undertaken,
with aliquots of threading components 1·Cl and 2·Cl added to
CDCl3 solutions of macrocycle 4. In addition, control titrations
were carried out using hexafluorophosphate threads 1·PF6 and
2·PF6, and tetrabutylammonium (TBA) chloride. The macrocycle
phenolic protons f and g were monitored throughout all of the
titration experiments and the shifts observed are shown in
Fig. S11 (Supplementary Information). The minor shifts in the
signal corresponding to protons g during the control titrations
5
10 (||≤ 0.07 ppm) indicate that the significant upfield shifts
observed upon addition of 1·Cl and 2·Cl (= − 0.44 and
− 0.43 ppm respectively) are not simply due to the macrocycle
sequestering the anion, and that pseudorotaxane formation
requires the presence of the halide template. Further evidence for
15 pseudorotaxane formation was obtained from 2D 1H-1H NMR
ROESY spectroscopy, with several intercomponent correlations
observed for both systems, as shown in Fig. S7 and S8.
Moreover, crystals of 2·4·Cl suitable for single-crystal X-ray
diffraction structural analysis were obtained by slow diffusion of
20 iPr2O into an equimolar CDCl3 solution of thread 2·Cl and
macrocycle 4. These were found to contain two independent
pseudorotaxane assemblies in the asymmetric unit, however each
have very similar geometries (with only slight differences in
atomic distances) and only one is shown in Fig. 5. In contrast to
25 the analogous pseudorotaxane containing macrocycle 3,17 in
which strong inter-assembly electrostatic interactions resulted in
the imidazolium thread existing in two contrasting orientations
within the macrocycle, only one co-conformation is observed for
2·4·Cl, in agreement with the solution phase orientation of the
30 2-methylimidazolium thread (depicted in Fig. 4). The halide
template forms three hydrogen bonds to one of the imidazolium
C-H protons (3.467(8) or 3.465(9) Å for the two assemblies in the
asymmetric unit) and to the two macrocycle N-H donor atoms
(3.254(7)–3.370(11) Å). Hydrogen bonds between the
35 imidazolium methyl protons of the thread and several polyether
oxygen atoms of the macrocycle are also observed, with one such
close contact shown in Fig. 5 (C···O distance of 3.228(14) or
3.175(15) Å).
To calculate the relative stabilities of assemblies 1·4·Cl and
40 2·4·Cl, macrocycle amide protons d were monitored throughout
the titration experiments (Fig. S12), and WinEQNMR222 analysis
determined the pseudorotaxane association constants shown in
Table 1. The amide protons were chosen for consistency with the
previously reported pseudorotaxane studies using macrocycle 3,
45 the results of which are also shown in Table 1.16 It should be
noted that these values are apparent 1:1 association constants for
ternary complexes. However, as discussed in the Supplementary
Information, the other equilibria present are insignificant
compared with the intended association, indicating that these
50 values do indeed reflect the relative stabilities of the anion
templated pseudorotaxane assemblies.
Fig. 5 Crystal structure of pseudorotaxane 2·4·Cl. One of the two
independent pseudorotaxane assemblies in the asymmetric unit is shown.
60 H···Cl and H···O close contacts are displayed using dashed black lines.
Protons omitted for clarity where appropriate.
Table 1 Association constants, Kapp (M-1), for pseudorotaxane formation
using macrocycles 3, 4 and 5 and threading components 1·Cl and 2·Cl in
CDCl3 at 293 K. Obtained by monitoring macrocycle amide protons d.
65 Errors are given in parentheses.
Guest
1·Cl
Macrocycle 316 Macrocycle 4
Macrocycle 5
101 (6)
97 (3)
644 (24)
2·Cl
245 (5)
3287 (177)
258 (9)
Importantly, for macrocycle 4, both the 4,5-dimethyl- and
2-methyl-imidazolium assemblies, 1·4·Cl and 2·4·Cl, exhibit
much greater stability when compared with the analogous
70 pseudorotaxane assemblies containing macrocycle 3, which
suggests a higher degree of complementarity exists in these
interpenetrated structures. Furthermore, the difference between
the systems containing contrasting imidazolium motifs, with the
2-methyl- being significantly more stable than the
75 4,5-dimethyl-imdazolium, is much more pronounced with
macrocycle 4 than 3.
In contrast, there is no significant difference between the
association constants obtained from the titrations with
macrocycle
5 compared with the previous studies with
80 macrocycle 3. Hence, shortening the polyether chain by three
atoms, and the associated effect on the hydrogen bonding and
donor–acceptor interactions as well as entropic considerations,
has had no effect on the overall stabilities of the respective anion
templated imidazolium pseudorotaxane assemblies.
85
To explain these findings, the shifts observed for imidazolium
methyl protons (2 and 2’, labelled in Fig. 3 and 4) during the
pseudorotaxane titrations were studied (Fig. S13 and S15). Whilst
the methyl protons 2 of 1·Cl shifted only slightly upon addition to
In addition, analogous titration experiments were undertaken
using macrocycle 5, with the observed shifts in agreement with
those seen in previous pseudorotaxane investigations.16, 18 Again,
55 the amide protons were monitored throughout these experiments
(Fig. S14) to give the association constants shown in Table 1.
macrocycle
4 ( = + 0.08 ppm after 10.0 equivalents,
90 Fig. S13a), the methyl protons 2’ of 2·Cl shifted significantly
downfield during the corresponding titration ( = + 0.70 ppm,
Fig. S13b). These shifts are due to hydrogen bonding between the
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23-25
thread methyl groups and the polyether chain of macrocycle 4,
systems.13,
Encouraged by the strong stability of
with the 2-methylimidazolium group of thread 2·Cl much more 40 pseudorotaxane assembly 2·4·Cl, the macrocyclisation reaction to
favourably aligned for these interactions than the
4,5-dimethylimidazolium groups of 1·Cl (as depicted in Fig. 3
and 4). This results in considerable secondary stabilisation of
2·4·Cl, explaining its increased stability compared to 1·4·Cl.
form 4 was carried out in the presence of one equivalent of
2-methylimidazolium thread 2·Cl (Scheme 2). Importantly, a
significant enhancement of the yield from 13% to 32% was
observed, thus the presence of 2·Cl in the reaction mixture
5
Smaller downfield shifts were observed for these protons in the 45 appears to favour [1+1]-macrocyclisation over higher order
pseudorotaxane systems containing the other macrocycles, 1·3·Cl
macrocyclic and oligomeric by-products.
and 2·3·Cl ( = − 0.01 and + 0.08 ppm respectively),16 and
10 1·5·Cl, and 2·5·Cl ( = + 0.03 and + 0.15 ppm respectively,
Fig. S15). These shifts suggest that the observed trend in the
pseudorotaxane stabilities of these systems, 2-methylimidazolium
greater than 4,5-dimethylimidazolium, is also due to the presence
of
these
hydrogen
bonding
interactions
in
the
15 2-methylimidazolium system. However, the much larger shifts
observed for assemblies 1·4·Cl and 2·4·Cl reflect the higher
degree of complementarity between the imidazolium threading
components and macrocycle 4. The hydrogen bonding
interactions between the imidazolium methyl groups and
20 macrocycle polyether chain do not contribute as much in the
4,5-dimethylimidazolium systems, but the stabilisation from the
donor–acceptor interactions is also expected to be increased due
to the shortened linker between the anion binding cleft and the
electron-rich aromatic groups.
Scheme 2 Templated synthesis of macrocycle 4.
In light of this result, the analogous rotaxane synthesis was
50 attempted using axle 7·Cl, bis-amine 6 and 5-nitroisophthaloyl
dichloride (Scheme 3). Rotaxane 8·Cl was isolated from the
reaction mixture by preparative silica gel thin layer
1
chromatography, but in a very low yield of 4%. The H NMR
25
Having demonstrated that reducing the macrocycle size in this
specific manner considerably favours chloride anion templated
imidazolium pseudorotaxane formation, attempts to synthesise
analogous rotaxane structures for anion recognition studies were
undertaken. As enhanced hydrogen bonding between the
spectrum for 2-methylimidazolium rotaxane 8·Cl is shown in
55 Fig. 6, with the signal corresponding to macrocycle phenolic
protons f appearing further downfield than those in macrocycle 4
(at 6.36 ppm compared with 6.69 ppm). This is in agreement with
the analogous shifts observed in the pseudorotaxane
investigations (Fig. 4), indicating interlocked structure formation.
60 Additionally, imidazolium methyl protons 2’ are considerably
shifted upfield compared to axle 7·Cl ( = − 1.08 ppm),
presumably due to the macrocycle phenolic ring currents. Further
characterisation of the rotaxane was provided by high resolution
electrospray mass spectrometry and 2D 1H-1H NMR ROESY
65 spectroscopy, with the interlocked nature of 8·Cl confirmed by a
number of intercomponent ROESY correlations (Fig. S9).
30 imidazolium methyl group of 2·Cl and the polyether chain of 4
contributed to the increased pseudorotaxane stabilities, it was
decided to synthesise interlocked systems incorporating
2-methylimidazolium axle components and retaining the five
oxygen polyether chain within the macrocycle.
35 Rotaxane Synthesis By Condensation
The synthesis of anion templated interlocked structures via a
novel amide condensation route has recently been established by
Beer et al. and applied to a number of pyridinium-based
NO2
O
O
N
N
a
NO2
O
O
Cl
b
NH
c
HN
O
O
d
7·Cl
Cl
Cl
Cl
6' 7'
8' 9'
10'
1'
N
e
4'
O
N
O
O
+
f
3'
5'
Et3N, CH2Cl2
4%
O
2'
NH2
H2N
g
O
O
h
O
i
j
O
O
8·Cl
O
O
O
6
Scheme 3 Synthesis of 2-methylimidazolium rotaxane 8·Cl via a condensation method.
70
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chromatography, anion exchange to the hexafluorophosphate salt
40 and trituration with methanol. ‘Stoppered’ axle 14·PF6 was also
isolated from the reaction mixture.
To aid purification, rotaxane synthesis was attempted in the
absence of the TBTA stabilising ligand. However, in addition to
the expected reduction in reaction rate (completion after three
45 days compared to 15 h with TBTA), a considerable loss in the
yield of rotaxane was observed (10% by NMR integration). It is
not evident why the presence of TBTA would favour interlocked
structure formation to such an extent.¶
Fig. 6 1H NMR spectrum (500 MHz) of rotaxane 8·Cl in CDCl3 at 293 K.
The synthesis of 8·Cl was repeated with ten equivalents of
bis-amine 6 and 5-nitroisophthaloyl dichloride in an attempt to
favour interlocked structure formation, however after removal of
the increased oligomeric by-products, 1H NMR integration of the
crude mixture revealed no significant enhancement in the
rotaxane yield.
5
O
11·Cl
+
4
+
Although 2-methylimidazolium rotaxane 8·Cl is the first
12
10 non-pyridinium interlocked structure prepared via this anion
templated condensation strategy, this is not a viable route for the
effective synthesis of anion host systems due to the very low
yield. It is unclear why this is the case, given that the
macrocyclisation yield was considerably enhanced in the
15 presence of thread 2·Cl.
DIPEA, Cu(MeCN)4PF6
TBTA, CH2Cl2
1.
2. NH4PF6
a
b
Rotaxane Synthesis By ‘Stoppering’
PF6
O
O
c
To exploit the ability of the 2-methylimidazolium motif to form
strong anion templated pseudorotaxanes with macrocycle 4 for
effective rotaxane synthesis, an alternative ‘threading-followed-
20 by-stoppering’ approach was employed. Mono-stoppered
2-methylimidazolium thread 11·Cl, incorporating a terminal azide
group, was synthesised as shown in Scheme 4. Imidazole
NH
d
HN
N
N
e
N
O
f
N
N
O
g
h
O
O
O
O
i
O
j
k
derivative
917
was
reacted
with
an
excess
of
13·PF6
4,4’-bis(chloromethyl)-1,1’-biphenyl
under
microwave
Crude yield ~ 25%
Isolated yield 15%
25 irradiation to give the desired alkylated product 10·Cl in 84%
yield. Conversion of the terminal chloro-functionality to the key
azide group was achieved using NaN3, and repeated washing with
aqueous NH4Cl afforded target thread 11·Cl.
+
9
1
2
3
4
5
12
6
8
13
For rotaxane formation via ‘stoppering’, a copper(I)-catalysed
30 alkyne-azide cycloaddition (CuAAC) reaction between azide
11·Cl and stoppered alkyne 1226 was undertaken in the presence
of macrocycle 4 (one equivalent compared to 11·Cl), using
DIPEA as the base, Cu(MeCN)4PF6 as the catalyst and TBTA as
a stabilising ligand (Scheme 5). Although 1H NMR integration of
35 the reaction mixture indicated a crude yield of approximately
25% for interlocked structure formation, difficulties in
purification resulted in target rotaxane 13·PF6 being isolated in
15% yield following preparative silica gel thin layer
14
20 21
22 23 24
O
N
N
15
7
16
N
O
N
10 11
N
PF6
19
18
17
14·PF6
50
Scheme 5 Synthesis of 2-methylimidazolium rotaxane 13·PF6 via a
‘stoppering’ method.
Scheme 4 Synthesis of mono-stoppered 2-methylimidazolium thread 11·Cl.
55
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The 1H NMR spectrum of 13·PF6, along with those of its
component macrocycle 4 and axle 14·PF6, is shown in Fig. 7. 40 the amide protons (d) were not able to be followed due to
macrocycle and axle imidazolium protons 10 and 11. However,
Importantly, the signal corresponding to macrocycle phenolic
protons g is shifted upfield in rotaxane 13·PF6 due to the donor–
acceptor interactions characteristic of interlocked structures of
this type ( = − 0.47 ppm). Shielding from the macrocycle
deuterium exchange. Interestingly, only moderate shifts in
imidazolium protons 10 and 11 were observed, possibly due to
opposing contributions from anion binding and donor–acceptor
interactions. Importantly, no significant shifts occurred in the
5
electron-rich aromatic groups are responsible for upfield shifts in 45 signals corresponding to macrocycle phenolic protons g, axle
axle protons 8, 9 and 12, all of which are - to the imidazolium
ring, whilst macrocycle protons e are split in the rotaxane,
10 reflecting the asymmetry of the axle component. Finally, amide
protons d appear at a significantly higher chemical shift in
interlocked host 13·PF6 than in macrocycle 4.
CH2 protons 9 and 12, and triazole proton 18, indicating no major
co-conformational changes occur upon anion binding, and that
the axle triazole group does not participate in any interactions
with the anion.
50
Fig. 8 Partial 1H NMR spectra (500 MHz) in 1:1 CDCl3/CD3OD at 293 K
of a) rotaxane 13·PF6 and b) rotaxane 13·PF6 plus 5.0 equivalents of
TBACl.
Para-isophthalamide proton c was monitored during the titration
55 experiments (Fig. S16) and 1:1 stoichiometric association
constants determined using WinEQNMR2 analysis of the data
(Table 2).
Table 2 Association constants, K (M-1), of rotaxanes 13·PF6 and 15·PF6
with anions in 1:1 CDCl3/CD3OD at 293 K. Obtained by monitoring para-
60 isophthalamide proton. Errors are given in parentheses.
Fig. 7 1H NMR spectra (500 MHz) in CDCl3 at 293 K of a) macrocycle 4,
15
b) rotaxane 13·PF6, and c) axle 14·PF6.
The interlocked nature of rotaxane 13·PF6 was confirmed by high
17
1
Anion
Cl−
Rotaxane 13·PF6
307 (21)
Rotaxane 15·PF6
360 (24)
resolution electrospray mass spectrometry and 2D H-1H NMR
ROESY spectroscopy, and the observed intercomponent ROESY
correlations are shown in Fig. S10. There is the potential for a
Br−
I−
397 (24)
446 (30)
20 number of possible interactions between the macrocycle and axle
components of rotaxane 13·PF6, especially between the
macrocycle amide groups and axle triazole unit formed in the
‘stoppering’ reaction.27 It was expected that even in the absence
of a coordinating anion, the macrocycle would reside primarily
25 over the axle imidazolium group due to the favourable donor–
acceptor and methyl-polyether hydrogen bonding interactions,
and the observed correlations indicate that this is indeed the case.
424 (37)
360 (27)
−
H2PO4
OAc−
128 (12)
153 (15)
171 (21)
108 (6)
Rotaxane host 13·PF6 exhibits significantly stronger binding for
the spherical halide guest species over the larger, more basic
oxoanions dihydrogen phosphate and acetate. This selectivity has
65 been achieved by the creation of a three dimensional binding
cavity within the interlocked structure, and the behaviour of
imidazolium methyl protons 9 during the titrations is evidence for
different binding modes for the halides and oxoanions, internal
and external to the rotaxane host cavity (Fig. S17). These protons
70 shift upfield upon addition of the halides ( = − 0.06–0.08 ppm
after 10.0 equivalents), slightly downfield with dihydrogen
phosphate (+ 0.03 ppm), and do not shift with acetate
Rotaxane Anion Binding Studies
In order to investigate the anion recognition properties of
30 2-methylimidazolium rotaxane 13·PF6, 1H NMR titration
experiments were carried out in the competitive 1:1
CDCl3/CD3OD solvent mixture using a range of TBA anion salts
– the halides chloride, bromide and iodide, and oxoanions
dihydrogen phosphate and acetate. Spectra from the titration of
35 13·PF6 with TBA chloride are shown in Fig. 8. Throughout the
titrations, downfield shifts were observed for the rotaxane cavity
protons involved in anion binding via C-H hydrogen bonding,
namely isophthalamide proton c and amide protons d of the
6
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(< 0.01 ppm). Hence, the contrasting binding modes displayed by
13·PF6 appear to alter the position of the imidazolium methyl
It is noteworthy that no significant changes in the binding
selectivity, either between the halides and oxoanions or within the
protons with respect to the hydroquinone ring currents in 40 halides themselves, were observed for 13·PF6 in comparison with
different ways.
15·PF6.
5
There is little discrimination between the halides, although it
does appear that bromide and iodide are bound slightly more
strongly than chloride. Furthermore, both the anion binding
strength and selectivity are similar to that exhibited by the
analogous 2-methylimidazolium rotaxane 15·PF6 (Fig. 9 and
Conclusions
The effect of varying the size of the macrocycle on the formation
of chloride anion templated imidazolium pseudorotaxane
45 assemblies was investigated using two macrocycle components.
Compared with the previously reported systems containing
macrocycle 3, substantial increases in pseudorotaxane stabilities
were observed with smaller macrocycle 4, whilst the use of a
different smaller macrocycle, 5, resulted in no significant
50 differences. The structural arrangement of the isophthalamide,
electron-rich aromatic and polyether groups within the
macrocycle components is responsible for this difference, as 4
10 Table 2), which contains a larger macrocycle component.
displayed
a greater degree of complementarity with the
substituted imidazolium threading components due to secondary
55 stabilising hydrogen bonding and donor–acceptor interactions,
especially with the 2-methylimidazolium motif. This increase in
complementarity was supported by the solid-state pseudorotaxane
crystal structure of assembly 2·4·Cl.
A rotaxane structure incorporating a similar macrocycle
60 component and a 2-methylimidazolium axle was synthesised via
a chloride anion templated amide condensation strategy, however
in a very low yield. Successful synthesis of an analogous
rotaxane in a reasonable yield was achieved by employing a
‘stoppering’ method using the CuAAC reaction. This host
65 structure exhibited binding selectivity towards the halides over
more basic oxoanions due to the creation of a three dimensional
interlocked cavity. The anion recognition properties of this
17
Fig. 9 2-Methylimidazolium rotaxanes 13·PF6 and 15·PF6
.
system are similar to that of a previously reported 2-
methylimidazolium rotaxane containing a larger macrocycle
70 component, although a number of factors besides macrocycle size
are also thought to affect anion binding.
It is difficult to make a direct comparison between the anion
binding properties of 2-methylimidazolium rotaxanes 13·PF6 and
15 15·PF6, due to a number of structural differences arising from
their methods of construction, besides a reduction in the size of
the macrocycle. Firstly, host 13·PF6 contains the
Experimental
General
5-tert-butyl-isophthalamide
motif,
compared
to
the
5-nitro-isophthalamide group of rotaxane 15·PF6. The nitro
20 functionalisation increases the acidity of the isophthalamide
amide protons, and this has been shown to have a considerable
effect on the strength of rotaxane anion binding, for example in
one such system a four-fold increase in chloride binding affinity
was observed compared with an unsubstituted isophthalamide
25 rotaxane analogue.28
Commercially available chemicals were used without further purification unless
75 otherwise stated. Dry solvents were obtained by purging with N2(g) and then passing
through an MBraun MPSP-800 column. Water was de-ionised and microfiltered
using a Milli-Q Millipore machine. Et3N was distilled and stored over KOH. All
tetrabutylammonium salts, TBTA and Cu(MeCN)4PF6 were stored in a vacuum
desiccator prior to use.
80 NMR spectra were recorded on Varian Mercury 300, Varian Unity Plus 500 and
Bruker AVII 500 (with 13C Cryoprobe) spectrometers at 293 K. Low resolution
electrospray ionisation (ESI) mass spectrometry (MS) was performed using a Waters
LCT Premier spectrometer, and high resolution ESI MS using a Bruker TOF
spectrometer. Melting points were recorded on a Gallenkamp capillary melting point
85 apparatus and are uncorrected. Microwave reactions were undertaken using a
Biotage Initiator 2.0 microwave.
Secondly, although the 1H NMR studies indicate that the
macrocycle component of 13·PF6 lies predominately over the
imidazolium group of the axle, there are several possible
co-conformational equilibria arising from the extended
30 asymmetrical axle component. Both of these factors would
significantly weaken the anion binding ability of rotaxane 13·PF6
compared with 15·PF6 and, given that the halide binding strengths
of these two systems are approximately equal (~ 400 M−1), it is
postulated that reducing the size of the macrocycle component
35 and incorporating the five oxygen polyether chain may be
responsible for enhancing anion complexation if these
considerations are taken into account.
Synthesis
Macrocycle 5,18 bis-amine 6,20 axle 7·Cl,17 imidazole derivative 917 and alkyne 1226
were prepared by literature procedures.
90 Macrocycle 4. Procedure A: 5-tert-Butylisophthalic acid (329 mg, 1.48 mmol) was
suspended in SOCl2 (5 mL) and heated at reflux under N2(g) for 15 h. The excess
SOCl2 was then removed by distillation, and the residue dried in vacuo. The crude
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dichloride (assumed quantitative conversion) was immediately dissolved in dry
CH2Cl2 (30 mL) and added dropwise to solution of bis-amine (600 mg,
-CH2-N3), 4.02 (2H, t, 3J = 5.6 Hz, O-CH2-CH2), 2.84 (3H, s, NC-CH3), 2.37–2.42
a
6
(2H, m, CH2-CH2-CH2), 1.30 (27H, s, -tBu). 13C NMR (75.5 MHz, CDCl3) (ppm)
155.7, 148.4, 144.1, 144.0, 141.3, 140.4, 139.8, 134.9, 132.4, 132.0, 130.6, 128.8,
75 128.7, 128.0, 127.5, 124.1, 122.3, 121.8, 112.8, 63.5, 63.0, 54.4, 52.1, 45.9, 34.3,
1.48 mmol) and Et3N (0.52 mL, 3.71 mmol) in dry CH2Cl2 (75 mL) which had been
stirred for 30 min. The reaction mixture was then stirred at room temperature under
5 N2(g) for 2 h, after which time the solution was washed with 10% HCl(aq) (2 x 50 mL)
and H2O (2 x 50 mL) and dried over MgSO4. The solvent was removed in vacuo and
purified by silica gel column chromatography using 97:3 CH2Cl2/MeOH followed
by 7:3 CHCl3/acetone to elute the product as a white solid (117 mg, 0.20 mmol,
13%). Procedure B: Same as A, but with 2-methylimidazolium thread 2·Cl (425 mg,
10 1.48 mmol) added at the same time as bis-amine 6, yielding the product as described
above (280 mg, 0.47 mmol, 32%). Mp > 250 °C. 1H NMR (300 MHz, CDCl3)
(ppm) 8.04 (2H, s, Hb), 7.82 (1H, s, Hc), 7.15 (4H, d, 3J = 8.2 Hz, Hf), 6.69 (4H, d,
3J = 8.2 Hz, Hg), 6.47 (2H, br s, Hd), 4.51 (4H, d, 3J = 5.3 Hz, He), 3.97–4.02 (4H, m,
Hh), 3.77–3.80 (4H, m, Hi), 3.67–3.69 (8H, m, Hj & Hk), 1.30 (9H, s, Ha). 13C NMR
15 (75.5 MHz, CDCl3)  (ppm) 166.8, 157.9, 152.6, 133.8, 130.3, 129.5, 128.4, 120.7,
114.5, 70.7, 69.6, 67.2, 43.8, 35.0, 31.1 (one peak missing – coincidental). MS-ESI
31.3, 29.2, 10.8 (one peak missing
– coincidental). MS-ESI m/z 848.5257
([M − Cl]+, C58H66N5O, calc. 848.5262), 848.5 ([M − Cl]+) – only observed peak in
low resolution mass spectrum.
Rotaxane 13·PF6. To a solution of macrocycle 4 (67 mg, 0.113 mmol), azide 11·Cl
80 (100 mg, 0.113 mmol) and alkyne 12 (74 mg, 0.136 mmol) in dry CH2Cl2 (25 mL)
was added TBTA (6 mg, 0.011 mmol) and DIPEA (39 l, 0.226 mmol) followed by
Cu(MeCN)4PF6 (8 mg, 0.023 mmol). The reaction mixture was stirred under N2(g)
for 72 h and then the solvent removed in vacuo. Purification was undertaken by
preparative silica gel thin layer chromatography using 94:6 CH2Cl2/MeOH. The
85 crude product was dissolved in CH2Cl2 (10 mL), washed with 0.1 M NH4PF6(aq)
(10 x 5 mL) and H2O (4 x 5 mL), and dried over MgSO4. The solvent was removed
was then removed in vacuo and the residue triturated with methanol (3 x 3 mL) to
yield the product as an off-white solid (36 mg, 0.017 mmol, 15%). 1H NMR
m/z 613.2896 ([M
+
Na]+, C34H42N2NaO7, calc. 613.2884), 613.3 (100%,
[M + Na]+), 1203.7 (15%, [2M + Na]+); 625.3 (50%, [M + Cl]−), 703.3 (100%,
[M + CF3COO]−), 625.3 (30%, [2M + Cl]−).
(500 MHz, CDCl3)  (ppm) 8.22 (2H, d, 3J = 1.5 Hz, Hb), 7.53–7.63 (7H, m, Hc
&
90 Hd & H14 & H15), 7.34 (2H, d, 3J = 7.8 Hz, H16), 7.20–7.26 (15H, m, H2 & H13 & H18
& H23), 7.13–7.18 (6H, m, Hf & H4), 7.06–7.11 (15H, m, H3 & H11 & H21 & H22),
6.95 (1H, d, 3J = 2.0 Hz, H10), 6.82 (2H, d, 3J = 8.8 Hz, H20), 6.73 (2H, d,
3J = 8.8 Hz, H5), 6.22 (4H, d, 3J = 8.3 Hz, Hg), 5.49 (2H, s, H17), 5.02 (2H, s, H19),
4.94 (2H, s, H12), 4.60 (2H, d, 2J = 13.2 Hz, He’), 4.38 (2H, d, 2J = 13.2 Hz, He”),
95 3.81–3.85 (2H, m, H6), 3.72–3.77 (2H, m, H8), 3.57–3.67 (12H, m, Hi & Hj & Hk),
3.50–3.55 (2H, m, Hh’), 3.44–3.48 (2H, m, Hh”), 1.99–2.02 (3H, m, H9), 1.87–1.92
(2H, m, H7), 1.31 (9H, s, Ha), 1.26 (27H, s, H1 or H24), 1.26 (27H, s, H1 or H24).
19F NMR (282.5 MHz, CDCl3) (ppm) − 71.8 (d, 1J = 714 Hz, PF6). 13C NMR
(125.8 MHz, CDCl3) (ppm) 167.6, 156.8, 156.1, 156.0, 152.2, 148.4, 148.3, 144.0,
20 Rotaxane 8·Cl. 5-Nitroisophthalic acid (14 mg, 0.066 mmol) was suspended in
SOCl2 (1 mL) and heated at reflux under N2(g) for 15 h. The excess SOCl2 was then
removed by distillation, and the residue dried in vacuo. The crude dichloride
(assumed quantitative conversion) was immediately dissolved in dry CH2Cl2 (5 mL)
and added dropwise to a solution of bis-amine 6 (27 mg, 0.066 mmol), axle 7·Cl
25 (80 mg, 0.066 mmol) and Et3N (23 L, 0.166 mmol) in dry CH2Cl2 (13 mL) which
had been stirred for 30 min. The reaction mixture was then stirred at room
temperature under N2(g) for 2 h, after which time the solution was washed with 10%
HCl(aq) (2 x 10 mL) and H2O (2 x 10 mL) and dried over MgSO4. The solvent was
removed in vacuo and the residue purified by preparative silica gel thin layer 100 142.9, 141.4, 140.5, 140.4, 140.2, 134.2, 134.1, 133.9, 132.4, 132.3, 131.7, 130.7,
30 chromatography (1:1 CH2Cl2/MeCN followed by 96:4 CH2Cl2/MeOH) to yield the
product as an off-white solid (5 mg, 0.003 mmol, 4%). Mp 183–185 °C (dec.).
1H NMR (500 MHz, CDCl3) (ppm) 9.78 (2H, br s, Hc), 9.23 (1H, br s, Hb), 9.01
(2H, s, Ha), 7.38 (2H, s, H1), 7.27–7.30 (4H, m, He), 7.23–7.26 (12H, m, H9), 7.17
130.6, 130.3, 129.9, 128.8, 128.5, 128.0, 127.9, 124.1, 124.0, 122.7, 122.1, 120.5,
113.2, 113.1, 112.9, 71.2, 70.7, 70.0, 66.8, 64.1, 63.1, 62.0, 53.9, 51.4, 45.4, 42.0,
35.0, 34.3, 31.4, 31.2, 29.7, 29.1, 8.9 (five peaks missing – coincidental). MS-ESI
m/z 1002.5831 ([M − PF6 + Na]2+, C132H154N7NaO9, calc. 1002.5864), 1002.6 (85%,
(4H, d, 3J = 8.8 Hz, H7), 7.09–7.12 (12H, m, H8), 6.79 (4H, d, 3J = 8.8 Hz, H6), 6.36 105 [M − PF6 + Na]2+), 1981.9 (100%, [M − PF6]+).
35 (4H, d, 3J = 8.8 Hz, Hf), 4.61–4.65 (4H, m, Hd), 3.98–4.00 (4H, m, H5), 3.90–3.93
(4H, m, H3), 3.57–3.65 (12H, m, Hh & Hi & Hj), 3.48–3.51 (4H, m, Hg), 2.04–2.09
Axle 14·PF6. Isolated from the reaction mixture for the preparation of rotaxane
(4H, m, H4), 1.66 (3H, s, H2), 1.27 (54H, s, H10). 13C NMR (125.8 MHz, CDCl3)
(ppm) 164.9, 157.1, 156.0, 149.0, 148.4, 144.0, 143.4, 140.1, 136.4, 133.6, 132.4,
130.6, 130.4, 126.2, 124.1, 121.5, 113.1, 112.9, 71.1, 70.5, 67.8, 66.7, 64.3, 63.1,
40 53.4, 45.8, 42.3, 34.3, 31.4, 8.0 (one peak missing – coincidental). MS-ESI m/z
1752.0318 ([M − Cl]+, C114H136N5O11, calc. 1752.0264), 1751.9 ([M − Cl]+) – only
observed peak in low resolution mass spectrum.
13·PF6 as a white solid. 1H NMR (500 MHz, CDCl3) (ppm) 7.56–7.59 (3H, m, H14
& H18), 7.53 (2H, d, 3J = 8.1 Hz, H15), 7.34 (2H, d, 3J = 8.1 Hz, H16), 7.30 (2H, d,
3J = 8.1 Hz, H13), 7.21–7.24 (13H, m, H2 & H11 & H23), 7.19 (1H, s, H10), 7.03–7.11
110 (16H, m, H3 & H4 & H21 & H22), 6.83 (2H, d, 3J = 8.8 Hz, H20), 6.68 (2H, d,
3J = 9.3 Hz, H5), 5.56 (2H, s, H17), 5.24 (2H, s, H12), 5.15 (2H, s, H19), 4.33 (2H, t,
3J = 6.6 Hz, H8), 3.96 (2H, t, 3J = 5.4 Hz, H6), 2.62 (3H, s, H9), 2.24–2.29 (2H, m,
H7), 1.26 (27H, s, H1 or H24), 1.25 (27H, s, H1 or H24). 19F NMR (282.5 MHz,
CDCl3)  (ppm) − 73.3 (d, 1J = 713 Hz, PF6). 13C NMR (75.5 MHz, CDCl3)  (ppm)
Thread 10·Cl. Imidazole derivative
9
(300 mg, 0.48 mmol) and
4,4’-bis(chloromethyl)-1,1’-biphenyl (600 mg, 0.68 mmol) were dissolved in 115 156.1, 155.7, 148.4, 148.3, 144.8, 144.2, 144.0, 143.9, 141.1, 140.4, 140.4, 140.2,
45 acetone (30 mL) and heated under microwave irradiation at 150 °C for 2 h. The
solvent was then removed in vacuo and the residue purified by silica gel gradient
column chromatography using 95:5 to 85:15 CH2Cl2/MeOH to elute the product as a
134.0, 132.4, 132.3, 132.0, 130.7, 130.6, 128.8, 128.7, 128.1, 127.8, 124.1, 124.0,
122.6, 122.1, 121.7, 113.1, 112.8, 63.5, 63.0, 62.0, 53.8, 52.1, 45.9, 34.3, 31.4, 29.2,
10.9 (three peaks missing – coincidental). MS-ESI m/z 1391.8887 ([M − PF6]+,
C98H112N5O2, calc. 1391.8843), 1391.8 (100%, [M − PF6]+) – only observed peak in
white solid (353 mg, 0.40 mmol, 84%). Mp
>
250 °C. 1H NMR (300 MHz,
CDCl3) (ppm) 7.62 (1H, d, 3J = 2.4 Hz, NC-H), 7.57 (2H, d, 3J = 8.0 Hz, Ar-H), 120 low resolution mass spectrum.
50 7.54 (1H, d, 3J = 2.4 Hz, NC-H), 7.51 (2H, d, 3J = 8.0 Hz, Ar-H), 7.44 (2H, d,
3J = 8.0 Hz, Ar-H), 7.40 (2H, d, 3J = 8.0 Hz, Ar-H), 7.20–7.26 (6H, m, Ar-H), 7.03–
Acknowledgements
7.11 (8H, m, Ar-H), 6.67 (2H, d, 3J = 8.8 Hz, OAr-H), 5.57 (2H, s, N-CH2-Ar), 4.63
(2H, s, -CH2-Cl), 4.51 (2H, t, 3J = 6.5 Hz, N-CH2-CH2), 4.02 (2H, t, 3J = 5.6 Hz,
O-CH2-CH2), 2.84 (3H, s, NC-CH3), 2.37–2.41 (2H, m, CH2-CH2-CH2), 1.30 (27H,
55 s, -tBu). 13C NMR (75.5 MHz, CDCl3)  (ppm) 155.7, 148.3, 144.1, 143.9, 141.2,
140.4, 139.9, 137.0, 132.4, 132.1, 130.6, 129.1, 128.8, 128.0, 127.3, 124.0, 122.3,
121.8, 112.8, 63.6, 63.0, 52.0, 50.6, 45.8, 34.2, 31.3, 29.2, 10.8 (one peak missing –
coincidental). MS-ESI m/z 841.4869 ([M − Cl]+, C58H66ClN2O, calc. 841.4858),
841.4 ([M − Cl]+) – only observed peak in low resolution mass spectrum.
G.T.S. thanks the EPSRC for a studentship (EP/F011504).
N.G.W. thanks the Clarendon Fund and Trinity College for a
studentship, and the Oxford University Crystallography Service
125 for instrument use. We are grateful to Diamond Light Source for
an award of beamtime on Beamline I19.
60 Thread 11·Cl. Imidazolium 10·Cl (350 mg, 0.40 mmol) and NaN3 (78 mg,
1.20 mmol) were dissolved in DMF (15 mL) and heated at 80 °C under N2(g) for
15 h. After this time, H2O (40 mL) was added and the crude product extracted with
CH2Cl2 (3 x 50 mL). The combined organic fractions were washed with H2O
(1 x 20 mL), dried over MgSO4 and the solvent removed in vacuo. The resulting
65 residue was dissolved in CH2Cl2 (15 mL), washed with 1.0 M NH4Cl(aq) (5 x 10 mL),
H2O (2 x 10 mL) and brine (1 x 10 mL), and dried over MgSO4. The solvent was
then removed in vacuo to yield the product as a white solid (306 mg, 0.35 mmol,
87%). Mp > 250 °C. 1H NMR (300 MHz, CDCl3)  (ppm) 7.64 (1H, d, 3J = 2.3 Hz,
NC-H), 7.51–7.60 (5H, m, 2 x Ar-H & NC-H), 7.35–7.42 (4H, m, 2 x Ar-H), 7.20–
70 7.26 (6H, m, Ar-H), 7.03–7.11 (8H, m, Ar-H), 6.68 (2H, d, 3J = 8.8 Hz, OAr-H),
5.58 (2H, s, N-CH2-Ar), 4.51 (2H, t, 3J = 6.8 Hz, N-CH2-CH2), 4.39 (2H, s,
Notes and references
a Chemistry Research Laboratory, Department of Chemistry, University
of Oxford, Mansfield Road, Oxford OX1 3TA, UK. Fax: +44 (0) 1865
130 272609; Tel: +44 (0) 1865 285142; E-mail: paul.beer@chem.ox.ac.uk
† Electronic Supplementary Information (ESI) available: spectral
characterisation of novel compounds, 2D 1H-1H ROESY spectra, 1H
NMR titration protocols and data, discussion of pseudorotaxane
equilibria, and crystallographic information. See DOI: 10.1039/b000000x/
135 ‡ With the same objective of optimising the properties of functional
interlocked structures through strategic design modifications, the effect of
8
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