Anion-templated assembly of a [2]catenane

Article abstract of DOI:10.1021/ja045080b

The first example of a [2]catenane structure to be synthesized using anion templation is described. The nature of the anion template is demonstrated to be crucial to the assembly process, with only chloride anion producing the [2]catenane in acceptable yi

Full text of DOI:10.1021/ja045080b

Published on Web 11/04/2004
Anion-Templated Assembly of a [2]Catenane
Mark R. Sambrook, Paul D. Beer,* James A. Wisner, Rowena L. Paul, and Andrew R. Cowley
Inorganic Chemistry Laboratory, Department of Chemistry, UniVersity of Oxford, South Parks Road,
Oxford, OX1 3QR, United Kingdom
Received August 16, 2004; E-mail: paul.beer@chem.ox.ac.uk
The assembly of mechanically bonded molecules such as
rotaxanes and catenanes is of great interest to the supramolecular
chemist,1 not only for the challenge of their synthesis but also for
the potential uses these systems may have such as molecular
switches, sensors, and machines.2 Access to the assembly of these
interpenetrated structures has been dominated by templated strate-
gies using π-π donor-acceptor interactions,3 hydrogen bonding,4
metal-ligand coordination,5 and hydrophobic effects6 between two
Figure 1. Strategy for assembly of [2]-catenanes via anion templation.
or more species that are neutral or cationic in nature. The challenge
of using anions to direct the assembly of interlocked supramolecular
architectures remains underexploited,7 which may be attributed to
their diffuse nature, pH dependence, and relative high solvation
energy as compared to cations.8 We have recently reported a general
method of using anions to template the formation of a wide range
of [2]pseudorotaxanes.9 The assembly process is based on coupling
anion recognition with ion-pairing where in noncompetitive solvent
media a coordinatively unsaturated chloride anion of a tight ion-
pair threading component facilitates the interpenetration of a
pyridinium, imidazolium, or guanidinium thread through the annulus
of an isophthalamide macrocycle. In a major development of this
anion templation methodology, herein, we describe the first example
of using anion templation to synthesize [2]-10 and [3]catenanes and
Figure 2. Anion-binding macrocycle, 1, and catenane precursor, 2.
Scheme 1. Catenane Synthesis
demonstrate after template removal that the [2]catenane exhibits
chloride anion-selective binding behavior. The anion templation
strategy employed for preparing a [2]catenane is shown in Figure
1 where a chloride anion templates the initial formation of a
pseudorotaxane assembly and a subsequent clipping reaction using
ring-closing metathesis (RCM) affords the catenane structure.11
Compound 1 (Figure 2) provides one macrocyclic unit of the
target catenane, incorporating an amide cleft for anion recognition.
The pyridinium chloride thread component, 2a, has been designed
to complement the binding sites of the macrocycle: a pyridinium
chloride tight ion-pair where the chloride anion’s coordination
sphere can be satisfied by the macrocycle’s amide cleft, the presence
of an electron-deficient aromatic ring capable of favorable second-
sphere π-π stacking to the hydroquinone moieties of the macro-
cycle and a methyl group suitable for hydrogen bonding to the
macrocyclic polyether chain. In addition, functionalization of the
thread with terminal allyl groups enables a RCM reaction to be
carried out.
the chloride ion template plays where threading of the pyridinium
cation 2+ is driven by recognition of the halide anion by the
macrocycle.
Pale yellow crystals were grown by slow diffusion of diisopropyl
ether into a chloroform solution of the [2]catenane and analyzed
by single-crystal X-ray diffraction. The crystal structure is shown
in Figure 3 and confirms the interlocked nature of the two
macrocyclic rings and the location of the chloride anion within the
amide cavity. Aromatic stacking and pyridinium-N+-CH3‚‚‚
polyether hydrogen bonds are also observed.
Chloride anion template removal was achieved by addition of
AgPF6 to produce the [2]catenane PF6- salt, 3b. Interestingly, only
small changes in the 1H NMR spectrum are observed upon chloride
removal, indicating that the catenane, 3b, retains a nearly identical
molecular structure. To determine the effect that catenane formation
has upon anion recognition, both the pyridinium allyl component,
2d, and [2]catenane, 3b, were titrated with three different anions
Mixing components 1 and 2a in dichloromethane followed by
addition of 10 wt % Grubbs’ catalyst and stirring overnight allows
the metathesis reaction to proceed. Separation of the reaction
mixture on preparative TLC using dichloromethane-methanol (94:
6) afforded the isolation of the [2]catenane, 3a, in 45% yield in
addition to a small amount (<5%) of the [3]catenane, 4 (Scheme
1). Both catenane systems were characterized by electrospray mass
1
spectrometry, elemental analysis, and H NMR. It is noteworthy
that analogous RCM reactions of 1 with 2b gave the bromide [2]-
catenane 5 in only 6% yield and no catenanes were isolated from
RCM reactions of 1 with 2c or 2d. This highlights the crucial role
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10.1021/ja045080b CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. We thank EPSRC for a postdoctoral fel-
lowship (J.A.W.) and a studentship (M.R.S.). Financial support for
Dr. Wisner from the Natural Sciences and Engineering Research
Council of Canada is also acknowledged. We are grateful to
Professor Michael J. Hynes (University College Galway, Ireland)
for his assistance with association constant determination.
Supporting Information Available: Full experimental details and
tables giving the crystal data and structure refinement information, bond
lengths and angles, atomic and hydrogen coordinates, and isotropic and
anisotropic displacement coordinates for 3a (PDF, CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
Figure 3. Crystal structure of the [2]-catenane. The polyether chain has
been modeled as disordered over two positions, but one of these has been
omitted for clarity. Chloride is represented as a green CPK sphere.
References
Table 1. Association Constants, M-1, for Various Anions with 2d
and 3b at 298 K in 1:1 CD3OD-CD3Cla
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host molecule
Cl-
H2PO4
-OAc
2d
K11 ) 230
K11 ) 1360
K12 ) 370
K11 ) 480
K12 ) 520
K11 ) 1500
K12 ) 345
K11 ) 230
3b
K11 ) 730
a Errors less than 10%.
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in a 1:1 CD3OD-CD3Cl solvent mixture, monitoring the aromatic
protons by 1H NMR. WinEQNMR12 analysis of the respective
titration curves gave association constant values shown in Table 1.
The pyridinium allyl component, 2d, displays a strong affinity for
acetate and dihydrogen phosphate, with only weak chloride binding
being observed. It would appear from these results that the basicity
of the oxoanions is the dominant factor in dictating the strength of
anion association with 2d.
By contrast, titrations with the [2]catenane, 3b, give a reverse
binding trend: Cl- > H2PO4- > -OAc. The binding of chloride is
enhanced significantly, whereas the binding of the oxoanions is
much weaker. These dramatic changes in anion selectivity are
postulated to be the result of the creation of a unique, topologically
constrained catenane binding pocket formed by the two amide clefts
of 3b. Large anions such as dihydrogen phosphate and acetate must
either bind outside of the cavity or force a large, and unfavorable,
conformational change upon the catenane. In both cases, it is
unlikely that a full complement of hydrogen bond donors will be
available to complex the oxo anionic guest. Association stoichi-
ometries also appear to be affected, with only a 1:1 binding ratio
observed for acetate as opposed to a mixed 1:1 and 1:2 with the
pyridinium allyl precursor, 2d. The reason for the cooperative
-
binding of a second H2PO4 guest species by the catenane is still
undetermined.
In conclusion, we have described the first [2]- and [3]catenane
structures to be synthesized using anion templation. The choice of
anion has been demonstrated to be crucial to the assembly process,
with only chloride producing the [2]catenane in acceptable yield.
Anion binding by the pyridinium component is greatly influenced
by catenation, leading to dramatic changes in anion selectivity
trends. The use of rotaxane and catenane cavities as binding
domains remains underexploited,13 and we are currently pursuing
a greater range of mechanically interlocked species with desirable
anion binding properties in our laboratories.
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