Capturing a [c2]daisy chain using the threading-followed-by-swelling approach

Article abstract of DOI:10.1039/b716331b

We have used the threading-followed-by-swelling approach to fix a daisy chain structure in solution, leading to the isolation of a captured [c2]daisy chain in 77% yield. The Royal Society of Chemistry.

Full text of DOI:10.1039/b716331b

Chemical Communications
www.rsc.org/chemcomm
Number 7 | 21 February 2008 | Pages 785–892
ISSN 1359-7345
COMMUNICATION
Sheng-Hsien Chiu et al.
Capturing a [c2]daisy chain using the threading-followed-by-swelling
approach

COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Capturing a [c2]daisy chain using the threading-followed-by-swelling
approachw
Shau-Hua Ueng,a Sheng-Yao Hsueh,a Chien-Chen Lai,b Yi-Hung Liu,a
Shie-Ming Penga and Sheng-Hsien Chiu*a
Received (in Cambridge, UK) 23rd October 2007, Accepted 22nd November 2007
First published as an Advance Article on the web 11th December 2007
DOI: 10.1039/b716331b
We have used the ‘‘threading-followed-by-swelling’’ approach to
fix a daisy chain structure in solution, leading to the isolation of
a captured [c2]daisy chain in 77% yield.
Interlocked molecules such as rotaxanes and catenanes have
potential applicability as molecular actuators and switches
within mesoscale molecular electronics devices.1 Interest in
extending their degree of mechanical interlocking and function
into macromolecules has led to the development of a range of
polyrotaxanes and polycatenanes, either through the polymer-
ization of interlocked monomers or the stoppering of poly-
pseudorotaxane complexes.2 In the past decade or so, attempts
have been made to construct linear and cyclic daisy chain-like
supramolecular structures in both interwoven and interlocked
forms because their unique molecular structures resemble
prototypical molecular actuators and muscles.3 Because of
Fig. 1 Cartoon representation of the generation of higher-order
entropic factors, however, [c2]daisy chains have usually pre-
cyclic daisy chains from a rigid linear monomer containing ortho-
dominated in solution.3e Intuitively, the propensity of a pair of
gonally complexing host and guest units.
hermaphroditic monomers to form a [c2]daisy chain will be
abated if the host and guest components are aligned ortho-
gonally in their complex and are linked together (i.e., within
each monomer) through an inflexible spacer. In such a con-
figuration, we might expect higher-order cyclic oligomers, such
as [c3]- and [c4]daisy chains, to become predominant species in
solution (Fig. 1). Herein, we report the preparation of an
diastereoisomeric daisy chain oligomers—we replaced the
central diethylene ether motif of the macrocycle 1 with a
pyridyl unit to allow substitution of a secondary ammonium
hermaphroditic daisy chain monomer comprising
macrocycle,
dibenzylammonium (DBA+) ion, and
a
a
a
dialkenylcyclopropane end group. Using the ‘‘threading-
followed-by-swelling’’ approach,4 we heated a solution of the
monomer to fix the daisy chain structure through efficient
swelling of its reactive terminal groups to form cyclohepta-
diene stoppers, leading to isolation of—disappointingly—the
captured [c2]daisy chain, albeit in a gratifyingly high yield
of 77%.
Previously, we reported that the oxygen-deficient macro-
cycle 1 complexes DBA+ ions with high affinity in low-
polarity solvents (Scheme 1).5 To obtain a symmetrical her-
maphroditic daisy chain monomer—one that would not form
a Department of Chemistry, National Taiwan University, Taipei,
Taiwan 10617, R. O. C.. E-mail: shchiu@ntu.edu.tw;
Fax: +886 2 33661677; Tel: +886 2 33661675
b Institute of Molecular Biology, National Chung Hsing University and
Department of Medical Genetics, China Medical University Hospital,
Taichung, Taiwan, R. O. C.
w Electronic supplementary information (ESI) available: Experimental
procedures for the preparation of all new compounds and their
characterization data. See DOI: 10.1039/b716331b
Scheme 1
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This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 817–819 | 817

Fig. 2 Ball-and-stick representation of the solid state structure of the
[2]pseudorotaxane-like complex [2*DBA]+. Hydrogen bonding geo-
metries, Xꢁ ꢁ ꢁN, Xꢁ ꢁ ꢁH [A], and N–Hꢁ ꢁ ꢁX [1]: (a) 2.95, 2.05, 171.0; (b)
3.07, 2.66, 107.7; (c) 3.07, 2.64, 113.0; (d) 2.89, 2.07, 172.1.
ion at the 4-position; macrocyclization of the dichloride 3 and
diol 4 gave the macrocycle 2 in 7% yield.
Scheme 2 Synthesis of the hermaphroditic daisy chain monomer
5-H ꢁ PF6.
The 1H NMR spectrum of an equimolar mixture of macro-
cycle 2 and DBA ꢁ PF6 in CD3CN (10 mM) displays three sets
of resonances: one set for the unthreaded macrocycle 2, one
for unthreaded DBA ꢁ PF6, and one for the 1 : 1 complex
formed between macrocycle 2 and the DBA+ ion (see the
ESIw). Because the shifts in the signals of the complex formed
between macrocycle 2 and DBA+ are similar to those ob-
served for the pseudorotaxane formed between 1 and DBA+,
we suspected that the [2]pseudorotaxane-like complex
[2*DBA]+ formed in solution (Scheme 1).5 Using a single-
point method,6 we determined the association constants (Ka)
contrast to our previously reported swelled-rotaxane synth-
esis,4 where we constructed the dialkenylcyclopropane motif in
the final step, here we found that the chiral amine 7 was
reasonably stable under ambient conditions so that we could
perform the condensation (with aldehyde 6), reduction, acid-
ification, and ion exchange steps to give the desired herma-
phroditic daisy chain monomer 5-H ꢁ PF6 (Scheme 2).
The 1H NMR spectrum of monomer 5-H ꢁ PF6 in CD3CN at
room temperature displayed relatively broad resonances,
which suggested the existence of intermolecular interactions
between the monomers, most likely the formation of daisy
chains. Heating this solution at 323 K and monitoring it using
1H NMR spectroscopy indicated the gradual consumption of
the signals of the putative supramolecular daisy chains and the
concomitant rise in the intensities of the signals of the possible
captured molecular daisy chains (Fig. 3). Subsequent ion
exchange and chromatography of the reaction products
provided the [c2]daisy chain 8-2H ꢁ 2PF6 in 77% yield
(Scheme 3).11
for this system in CD3CN and CD3NO2 to be 40 and 770 Mꢂ1
,
respectively. The binding affinity of macrocycle 2 toward the
DBA+ ion is substantially weaker than that of macrocycle 1,
possibly because of the reduced number of (or improperly
positioned) lone pairs of electrons in the pyridine unit of
2 relative to those in the more-flexible diethylene oxide
motif of 1.7
Following vapor diffusion of isopropyl ether into a CH3CN
solution containing a 1 : 1 mixture of macrocycle 2 and
DBA ꢁ PF6, we obtained single crystals that were suitable for
X-ray crystallographic analysis, which revealed the [2]pseudo-
rotaxane geometry of [2*DBA]+ in the solid state (Fig. 2).8 z
The presence of [N+–Hꢁ ꢁ ꢁO] and [N+–Hꢁ ꢁ ꢁN]9 hydrogen
We grew single crystals suitable for X-ray crystallography
by slowly evaporating a CD3CN solution of 8-2H ꢁ 2PF6.
X-Ray crystallographic analysis12 z of 8-2H ꢁ 2PF6 revealed
the [c2]daisy chain structure stabilized by [N+–Hꢁ ꢁ ꢁO] and
[N+–Hꢁ ꢁ ꢁN] hydrogen bonds (Fig. 4).13 Thus, the herma-
phroditic monomer 8-2H ꢁ 2PF6 was not sufficiently rigid to
+
bonds between the NH2 center and pyridyl nitrogen atom
and its adjacent oxygen atoms led us to hypothesize that an
hermaphroditic daisy chain monomer derived from 2 might
favor aggregation in a linear supramolecular structure because
the pyridyl unit of the macrocycle would prefer to position its
lone pair of electrons perpendicular to the orientation of the
avoid the formation of the [c2]daisy chain. We suspect
+
that the structural flexibility of the ArCH2NH2
and
OCH2(C6H3N)CH2O components of the hermaphroditic
monomer was sufficient to deviate its molecular structure from
linear upon complexation so that, once again, the entropically
favored [c2]daisy chain predominated.
We have demonstrated that macrocycle 2 recognizes DBA+
ions with reasonable affinity and is capable of being deriva-
tized into symmetrically monosubstituted compounds, in-
creasing its potential for use as an anchor for appending
such complexes onto the surfaces of solid supports or for
building complicated supermolecules. Although the applica-
tion of this threading-followed-by-swelling approach to the
+
thread-like NH2 unit to maintain these favorable hydrogen
bonds.10 Our hope was that the structural rigidity of the
hermaphroditic monomer would be sufficient to retard the
formation of the entropically favored [c2]daisy chain such that
higher-order daisy chains would be generated in solution.
Thus, we synthesized the hermaphroditic daisy chain mono-
mer 5-H ꢁ PF6.
To avoid unnecessary complications arising from stereo-
isomeric mixtures of daisy chains, we employed an optically
pure dialkenylcyclopropane as the terminal swellable unit. In
ꢀc
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818 | Chem. Commun., 2008, 817–819

Notes and references
z CCDC 664527 and 664528 for 8-2H ꢁ 2PF6 and [2*DBA] ꢁ PF6
respectively. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b716331b
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1
Fig. 3 Partial H NMR spectra [400 MHz, CDCl3–CD3CN (10 : 1),
323 K] displaying (a–d) the formation of the [c2]daisy chain 8-
2H ꢁ 2PF6 from the monomer 5-H ꢁ PF6 over time—(a) 0, (b) 45, (c)
85, and (d) 110 h—and (e) the purified 8-2H ꢁ 2PF6.
4 C.-W. Chiu, C.-C. Lai and S.-H. Chiu, J. Am. Chem. Soc., 2007,
129, 3500–3501.
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2006, 71, 2373–2383.
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Chrystal, P. T. Glink, S. Menzer, C. Schiavo, N. Spencer, J. F.
Stoddart, P. A. Tasker, A. J. P. White and D. J. Williams,
Chem.–Eur. J., 1996, 2, 709–728.
7 The association constants for the interactions between macrocycle
1 and DBA+ ions are 200 and 15 000 Mꢂ1 in CD3CN and
CD3NO2, respectively. See: K. W. Cheng, C.-C. Lai, P.-T. Chiang
and S.-H. Chiu, Chem. Commun., 2006, 2854–2856 and ref. 4.
Scheme 3 Fixing the [c2]daisy chain using the swelling approach.
8 Crystal data for [2*DBA] ꢁ PF6: [C43H43O4N2][PF6]; Mr
=
796.76; orthorhombic; space group P212121; a = 12.50690(10); b
= 17.0522(2); c = 18.6392(2) A; V = 3975.19(7) A3; rcalcd = 1.331
g cmꢂ3; m(MoKa) = 0.142 mmꢂ1; T = 295(2) K; colorless plates;
9055 independent measured reflections; F2 refinement; R1
0.0546; wR2 = 0.1510.
=
9 In this case, the [N+–Hꢁ ꢁ ꢁN] hydrogen bonds are relatively weak,
as indicated by their longer bond lengths and smaller bond angles.
10 The two phenolic rings sandwich the highlighted methylene pro-
+
tons adjacent to the NH2 center at distances (Cꢁ ꢁ ꢁphenolic ring
centroid) of 3.53 and 3.66 A, respectively, which agrees well with
our previous computational finding (see ref. 5b) that such com-
plexes might be stabilized partially by [N+C–Hꢁ ꢁ ꢁp] interactions.
This arrangement explains the large upfield shifts of these protons
in the NMR spectra.
Fig. 4 Ball-and-stick representation of the solid state structure of the
[c2]daisy chain [8-2H]2+
.
11 No higher-order cyclic daisy chains were observed in the crude
product analyzed using electrospray mass spectrometry.
12 Crystal data for 8-2H ꢁ 2PF6: [C16H49O4N2 ꢁ 4CHCl3][PF6]; Mr =
successful preparation of higher-order daisy chains will require
the design of even-more-rigid hermaphroditic monomers, our
efficient synthesis of this captured [c2]daisy chain suggests that
this swelling protocol will be a useful means of fixing poly-
meric daisy chains.
ꢀ
1315.31; triclinic; space group P1; a = 12.8645(4); b = 13.4307(3);
c = 18.2973(5) A; V = 3010.00(14) A3; rcalcd = 1.451 g cmꢂ3
;
m(MoKa) = 0.640 mmꢂ1; T = 295(2) K; colorless cubes; 13743
independent measured reflections; F2 refinement; R1 = 0.1094;
wR2 = 0.2561.
13 The phenyl and pyridyl rings in the solid state structure of
[8-2H]2+ were elegantly p-stacked with a centroid-to-centroid
distance of 3.6 A.
This study was supported by the National Science Council,
Taiwan (NSC-95-2113-M-002-016-MY3).
ꢀc
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Chem. Commun., 2008, 817–819 | 819