Facile route to an all-organic, triply threaded, interlocked structure by templated dynamic clipping

Article abstract of DOI:10.1002/anie.201207048

[2]Rotaxanes are a class of mechanically interlocked molecules which are generally depicted by an "axle-in-wheel" model, in which a dumbbell-shaped linear component is threaded through a macrocyclic component (Scheme 1 a). Functionalization of the rotaxan

Full text of DOI:10.1002/anie.201207048

Angewandte
Chemie
DOI: 10.1002/anie.201207048
Self-Assembly
Facile Route to an All-Organic, Triply Threaded, Interlocked Structure
by Templated Dynamic Clipping**
Andrew Pun, David A. Hanifi, Gavin Kiel, Evan OꢀBrien, and Yi Liu*
Dedicated to Professor Sir Fraser Stoddart on the occasion of his 70th birthday
[2]Rotaxanes are a class of mechanically interlocked mole-
cules which are generally depicted by an “axle-in-wheel”
model, in which a dumbbell-shaped linear component is
threaded through a macrocyclic component (Scheme 1a).[1]
Functionalization of the rotaxanes on the linear dumbbell or
the macrocycle units provides access to a range of molec-
ular,[2] supramolecular,[3] and polymeric materials[4] with
unique architectures and functions, which have found many
applications in nanomechanical devices,[5] molecular memo-
ries,[6] and reconfigurable nanovalves.[7] Substituting the two-
terminal linear geometry with a three-terminal one, such as
a triply threaded [2]rotaxane involving a macrobicyclic cage
and a trifurcated component (Scheme 1b), produces a new
kind of interlocked structure with higher symmetry which can
be valuable for building extended arrays or networks at
higher dimensions. High-symmetry interlocked structures,
such as triply or quadruply interlocked homo[2]catenanes
have been constructed using metal–ligand coordination.[8]
Meanwhile, little has been done towards a facile and highly
efficient synthesis of triply threaded rotaxanes, primarily
because of the lack of a C3-symmetric host/guest system with
complementary geometry. Existing macro-bicycles, such as
cyclophane-based cryptands,[9] are viable C3-symmetric hosts
and have been an active component in the synthesis of linear
[2]rotaxanes and [2]catenanes.[10] The guests involved are,
however, by-and-large linear bipyridinium-based electron
acceptors. To obtain three-terminal interlocked structures,
trifurcated guests having complementary shape and strong
binding affinity to these macro-bicycles are necessary.
Among the common synthetic strategies for two-terminal
[2]rotaxanes, clipping of a macrocycle around a dumbbell-
shaped template is one of the most convenient methods[11] as
it minimizes the need for a preformed macrocyclic compo-
nent, which often demands lengthy synthesis and tedious
workup. Furthermore, the clipping reaction can be accom-
plished with higher yields and product specificity when
combined with molecular-recognition-based, noncovalent
templating, and reversible dynamic covalent chemistry.[12]
Indeed the versatile imine chemistry has been employed for
the assembly of novel structures, such as molecular cages,[13]
borromean rings,[14] suitanes,[15] catenanes,[16] and rotaxanes.[17]
The assembly of triply threaded [2]rotaxanes can be con-
ceived using a similar strategy of clipping a cagelike macro-
bicycle around a trifurcated guest. Herein, a series of C3-
symmetric trications were synthesized and tested for their
ability to template the formation of symmetry-matching
macro-bicycles. Triply threaded structures have been
obtained based on one of the tricationic species which
templates the (2+3) clipping reaction between simple alde-
hyde and amine precursors through sixfold imine bond
formation. The C3-symmetric, interlocked structure of the
Scheme 1. Illustrations of a) two-terminal and b) three-terminal
[2]rotaxanes synthesized by dynamic clipping.
1
product has been unambiguously characterized by H NMR
[*] A. Pun, D. A. Hanifi, G. Kiel, E. O’Brien, Dr. Y. Liu
The Molecular Foundry, Lawrence Berkeley National Laboratory
One Cyclotron Road, MS 67R6110 (USA)
spectroscopy and X-ray structural analysis.
Scheme 2a lists the structures of the three types of
tricationic salts, including the trisimidazolium 3·3PF6, the
isostructural tris(m-pyridinium) 4·3PF6, and the tris(p-pyr-
idinium) 5a-c·3PF6.[18] The clipping reactions were carried out
by adding a CD3CN solution of one trication into a mixture of
1,3,5-benzenetrialdehyde (1) and 2,2’-(ethylenedioxy)diethyl-
E-mail: yliu@lbl.gov
Homepage: http://foundry.lbl.gov/liugroup/index.html
A. Pun, G. Kiel
Department of Chemistry, University of California, Berkeley
Berkeley, CA 94720 (USA)
[**] This work was performed at the Molecular Foundry, Lawrence
Berkeley National Laboratory, supported by the Office of Science,
Office of Basic Energy Sciences, Scientific User Facilities Division, of
the U.S. Department of Energy under Contract No. DE-AC02-
05CH11231.
1
amine (2) in a molar ratio of 1:2:3. H NMR spectroscopy
indicated that the reaction involving either 3·3PF6 or 4·3PF6
gave rise to a new species that corresponded to the triply
threaded pseudo[2]rotaxane (see Figures S1 and S2 in the
Supporting Information). However the yields were only
around 40% for 3·3PF6 and 29% for 4·3PF6 even when two
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207048.
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changes were observed after 2 hours, thus resulting in
a ratio of 1:0.41:0.22 for 6a3+/5a3+/fc (Figure 1d). A binding
constant of 4.2 ꢀ 103 mÀ1 can be obtained from single-point
calculations. Upon addition of 1.1 more equivalent of 5a3+
into the reaction mixture, the equilibrium was further shifted
towards nearly complete consumption of the free cage
(Figure 1e). Further analysis of the 1H NMR spectrum of
the interlocked species 6a·3PF6 indicated that the H1 and Hb
resonances, together with the Ha and Hb resonances showed
significant upfield shifts in comparison to these of unbound
5a·3PF6 and the free cage, thus consistent with a mutual
shielding effect between the aromatic units. On the contrary,
the Ha and H2 resonances of 6a·3PF6 shifted downfield
relative to those of unbound 5a·3PF6, thus indicating
a deshielding environment imposed by the surrounding
polyimine aromatic core. All of these observations are
consistent with the triply threaded structure of 6a·3PF6
having a staggered stacking geometry, in which the pyridi-
nium units stick out through the cavities of the polyetheric
macrocycles and partially overlap with the trisimine aromatic
cores. The integrity of 6a·3PF6 was further confirmed by high-
resolution electrospray ionization mass spectrometry (ESI-
MS). Intense molecular ions at m/z 684.8797 and 408.2649
were observed in the ESI-MS of 6a·3PF6, thus corresponding
Scheme 2. a) Structures of the three types of trications used for
templating. b) The clipping reaction for the formation of triply inter-
locked molecules 6a–c·3PF6.
À
equivalents of the trications were used. Remarkably, the
clipping efficiency was significantly improved when the N-
hexyl-substituted salt 5a·3PF6 was employed as the template.
Figure 1 shows the progressive 1H NMR spectroscopic
changes of the clipping reaction (Scheme 2b) involving
a CD3CN/CD2Cl2 (1.6 mL, v/v 1:0.6) solution of the trisalde-
hyde 1, diamine 2, and 5a·3PF6 in a 2:3:1 ratio. A new set of
resonances corresponding to the formation of desired C3-
symmetric pseudorotaxane was observed within 10 minutes
(Figure 1a), together with resonances from small but dis-
cernible amounts of unreacted precursors. Significant quanti-
ties of the unbound guest 5a3+ and free cage were also present
in the solution. As the reaction progressed (Figure 1b–d),
both the aldehyde and diamine precursors were consumed,
and was commensurate with an increased amount of the
interlocked species and decreased amount of the unbound
guest 5a3+ and the free cage. No further spectroscopic
to the loss of two and three PF6 anions and consistent with
the theoretical predictions. In comparison, the mixture of
1 and 2 in the absence of a guest only resulted in a nonspecific
mixture.[17b]
The coexistence of the triply threaded species, the free
cage and the free trispyridinium guest in solution suggests that
these compounds are under slow equilibrium. The host–guest
complexation could occur through a threading process or the
=
cleavage and reformation of the C N bonds in the cases of
6a,b·3PF6. Since threading can be interrupted by steric end
groups, it is interesting to see whether the trispyridinium
5c·3PF6 having bulky 3,5-di-tert-butyl benzoic ester end
groups (Scheme 2) would disturb the equilibrium and the
corresponding clipping efficiency. The clipping reaction
involving 5c·3PF6, 1, and 2 in a 1:2:3 ratio resulted in a similar
mixture as that obtained with the nonstoppered 5a·3PF6,
where the triply threaded [2]rotaxane 6c·3PF6 coexisted with
free 5c·3PF6 and the free cage (see Figure S3a in the
Supporting Information). Upon the addition of one more e-
quivalent of 5c·3PF6, the free cage decreased to less than 5%,
commensurate with the increasing fraction of the [2]rotaxane
6c·3PF6. Since threading can be excluded on account of the
bulky end groups, the decrease of free cage fraction and the
concomitant formation of [2]rotaxane must follow the path-
=
way of C N bond cleavage and reformation.
The formation of the triply threaded species was verified
by single-crystal X-ray structural analysis.[19] Suitable single
crystals in the form of long colorless needles were obtained by
diffusing diethyl ether vapor into the reaction mixture
containing the N-ethanol derivative 5b·3PF6. As expected,
the guest is sandwiched within the cavity of the macrobicyclic
cage, with the pyridinium arms threading through the three
orifices (Figure 2). The six imine groups in the cage are
coplanar with the conjugating benzene, thus forming two
extended aromatic ring systems that lie nearly parallel with
Figure 1. The time-evolved partial 1H NMR spectra (298 K, 500 MHz,
CD2Cl2/CD3CN 3:5) of a mixture of 1, 2, and 5a3+ in 2:3:1 ratio.
a) 10 min, b) 20 min, c) 30 min, d) 2 h, and e) after the addition of 1.1
more equivalent of 5a·3PF6. fc=free cage molecule.
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interlocked rotaxanes through sixfold imine bond formation
using a trispyridinium-based p template. As revealed by solid-
state structures, the triply interlocked structure is stabilized by
À
À
p–p interactions and multiple [C H···O] and [C H···N]
interactions. Based on this p-template protocol, novel
[2]rotaxanes can be prepared by a one-pot reaction using
stoppered trispyridinium. Such triply interlocked molecules
serve as a complementary entry to the well-received “axle-
and-wheel”-type two-terminal [2]rotaxanes. These structures
have great potential as useful synthons for further assembly or
crosslinking to give higher-order assemblies or polymeric
materials. It should be noted that this is a higher symmetry
host–guest system built from all organic components, that is,
without contributions from the widely used metal–ligand
coordination,[21] thus setting a unique path to the design and
assembly of mechanically interlocked systems from simple
starting materials.
Figure 2. a) Top and b) side view of X-ray structure of 6b·3PF6.
c) Elongated p stacking in the 1:2 host–guest complex in the solid
state. The cage and the guest pyridimium units are represented in
stick and space-filled models, respectively. Solvent molecules and
anions are omitted for clarity. The blue dashed lines in (a) indicate
Received: August 31, 2012
Revised: October 22, 2012
Published online: November 19, 2012
Keywords: host-guest systems · rotaxanes · self-assembly ·
À
.
[C H···O] interactions. See the Supporting Information for detailed
supramolecular chemistry · template synthesis
bond angles and distances of such interactions.
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(2+3) clipping method for the assembly of C3-symmetric
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[18] See the Supporting Information for the synthetic details of the
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[19] See the Supporting Information for crystallographic data and
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data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
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