A twin-axial hetero[7]rotaxane

Article abstract of DOI:10.1002/anie.201105375

Two in one: Two pseudorotaxanes can be combined to form a twin-axial hetero[7]rotaxane (see picture) by using the copper-catalyzed alkyne-azide "click" reaction. The synthetic route, in which twin-axial and single-axial rotaxanes are formed, combines self

Full text of DOI:10.1002/anie.201105375

Communications
DOI: 10.1002/anie.201105375
Rotaxanes
A Twin-Axial Hetero[7]rotaxane**
Zhi-Jun Zhang, Heng-Yi Zhang, Hui Wang, and Yu Liu*
Rotaxanes are challenging and interesting because of their
potential applications in nanotechnology and molecular
machines.[1] Template synthesis has been widely used to
realize more efficient synthesis of rotaxanes, while various
protocols have been reported such as the popular “threading
followed by stoppering”, “slippage”, and “clipping”
approaches.[2] However, the synthesis of high-order rotaxanes
still remains a challenge.[3,4] In this respect, it is very difficult
to obtain heterorotaxanes with two or more similar rings,[5] as
well as [n]rotaxanes with more than one axle molecule
threaded through the same ring.[6]
hetero[7]rotaxane is synthesized by using the highly efficient
CuAAC reaction among the preorganized building blocks.
The preparation of 1 and 2 involves the condensation of
the corresponding benzaldehyde with an alkyl or benzyl
amine, and then reduction, protonation, and anion-exchange
1
steps. We performed H NMR experiments to confirm the
formation of self-assembly systems in solution. It has been
reported that BPP34C10 can form a 1:2 complex with
secondary dibenzyl ammonium ions.[8b] From the character-
1
istic complexation-induced chemical shifts in the H NMR
spectra, the formation of [3]pseudorotaxane 22@BPP34C10
and [2]pseudorotaxane 1@B21C7 was confirmed, when the
axle molecules and corresponding ring components were
mixed (see Figures S28 and S38 in the Supporting Informa-
tion). The signals of He and Hf in 22@BPP34C10 as well as
those of Ha and Hc in 1@B21C7 can be identified from the
1H NMR spectrum of a 2:2:2:1 mixture of 1, 2, B21C7, and
BPP34C10 (Figure 2b). These observations indicate that the
two pseudorotaxanes still exist as the dominant species in the
four-component system. Moreover, the 1H NMR spectrum of
a 1:1:1:1 mixture of 1, 2, B21C7, and dibenzo-24-crown-8
(DB24C8) shows that two pseudorotaxanes 1@B21C7 and
2@DB24C8 are predominantly formed in this four-compo-
nent system (see Figure S30 in the Supporting Information).
The CuAAC reaction of 1@B21C7 and 22@BPP34C10 in
the presence of Cu(MeCN)4PF6 and 2,6-lutidine in dichloro-
methane at room temperature, and subsequent alkylation of
the 1,2,3-triazole[11a] resulted in the successful synthesis of
hetero[7]rotaxane 5, which was isolated in moderate yield
after purification by column chromatography. The HRMS
spectrum of 5 shows a signal at m/z 663.2944, which corre-
sponds to the product after loss of six PF6À ions; this finding is
consistent with the calculated mass isotope distribution
(Figure S52 in the Supporting Information). The methylation
of the 1,2,3-triazole was performed to enhance the polarity of
the final product for more efficient isolation of the rotaxane
from the reactants.
To further confirm the structure of hetero[7]rotaxane 5,
we synthesized hetero[4]rotaxane 4, which contains one
DB24C8 and two B21C7 rings, as well as [3]rotaxane 3 with
two B21C7 rings, by following a similar procedure. We
assigned all the resonances in 3 and 4 by analyzing their 1D
and 2D NMR spectra. The protons H1, H2, H3, and Hc on the
axle component in 3 are shifted upfield when compared with
free 6, while Ha and Hb adjacent to the outer ammonium site
are shifted downfield (Figure 3c) because of the complex-
ation of the B21C7 ring. It is notable that protons H4 in 3 can
be detected because of the stabilizing effect of the hydrogen
bonding interactions between the crown ether and the
ammonium hydrogen atoms. On the other hand, we noted
that not only the central ammonium hydrogen atoms H8 in 3
cannot be detected, but also that H6, H7, and Hf remain mostly
Macrocyclic polyethers have been applied widely in the
construction of various rotaxanes[7] because of the unique
complexation behavior of the macrocycles toward threadlike
guests such as secondary benzyl or alkyl ammonium ions.[8]
Herein, we propose a strategy to integrate two different
macrocyclic polyethers into one [n]rotaxane molecule by
simultaneous twin-axial and single-axial rotaxanation
(Figure 1). We firstly designed a four-component self-assem-
bly system, which involves two macrocyclic polyethers,
namely bis(p-phenylene-34-crown-10) (BPP34C10) and
benzo-21-crown-7 (B21C7) and two secondary ammonium
compounds 1 and 2. When all materials are mixed, the
[3]pseudorotaxane 22@BPP34C10[8b,9] and the [2]pseudoro-
taxane 1@B21C7[10] are formed exclusively. The two pseudo-
rotaxanes contain azide and alkyne groups as potential
reactive sites. With the pseudorotaxanes in hand, the cop-
per(I)-catalyzed Huisgen alkyne–azide 1,3-dipolar cycloaddi-
tion (CuAAC “click” reaction), which has often been used in
rotaxane synthesis, was performed.[11] In the obtained hetero-
[7]rotaxane, four B21C7 rings can be stoppered by the outer
phenyl groups,[10] while the central BPP34C10 is “cascade-
stoppered” by the B21C7 rings.[5a] The combination of self-
assembly and synthetic strategy ensures the correct position
for two kinds of rings in the final product. In sharp contrast,
the synthesis of higher-order rotaxanes often needs the
corresponding components to contain more binding sites,
which increases the synthetic difficulty; a high association
constant is needed to obtain the dynamic multicomponent
complex, which is required in the common “threading
followed by stoppering” protocol. Our method utilizes the
concept of modularization, which means that the high-order
[*] Z.-J. Zhang, Prof. Dr. H.-Y. Zhang, H. Wang, Prof. Dr. Y. Liu
Department of Chemistry
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin, 300071 (P.R. China)
E-mail: yuliu@nankai.edu.cn
[**] We thank the 973 Program (2011CB932500), and the NNSFC (Nos.
20932004 and 20972077) for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105375.
10834
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Angew. Chem. Int. Ed. 2011, 50, 10834 –10838

Figure 2. Partial 1H NMR spectra (400 MHz, 298 K, CDCl3/
CD3CN=7:1, [2]=[1]=[B21C7]=5.0 mm, [BPP34C10]=2.5 mm) of
a) 2:1 mixture of 2 and BPP34C10, b) 2:2:2:1 mixture of 1, 2, B21C7,
and BPP34C10, and c) equimolar mixture of 1 and B21C7. *=solvent
signal. See Figure 1 for atom labels.
Figure 3. Partial 1H NMR spectra (400 MHz, 298 K, CD3CN) of a) het-
ero[7]rotaxane 5, b) hetero[4]rotaxane 4, c) [3]rotaxane 3, and d) axle 6.
*=solvent signal.
in their original location, thus suggesting that the central
ammonium site on the axle component in 3 is not encircled by
the crown ether.
The proton signals of the two outer ammonium sites and
the resonances of B21C7 in the 1H NMR spectrum of
hetero[4]rotaxane 4 are at similar positions to those in the
spectrum of 3 (Figure 3b), thus indicating that the two B21C7
rings are still located on the outer ammonium sites of the axle
component. Moreover, the resonance of the central ammo-
nium hydrogen atoms H8 can be found in the spectrum. The
signals of hydrogen atoms H6 and H7 on the phenyl rings and
of He near the central ammonium site are shifted upfield, and
the signal of Hf is shifted downfield. In addition, the protons
HB of the catechol rings in DB24C8 and the ethylidene
protons also show their characteristic pattern. These com-
bined observations not only validate the formation of
Figure 1. Structures of compounds 1, 2, 6, B21C7, BPP34C10, DB24C8,
[3]rotaxane 3, and hetero[4]rotaxane 4, and formation of hetero[7]rotax-
ane 5 by the modularized four-component self-assembly strategy of 1,
2, B21C7, and BPP34C10.
Angew. Chem. Int. Ed. 2011, 50, 10834 –10838
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Communications
hetero[4]rotaxane 4, but are also useful for analyzing the
structure of hetero[7]rotaxane 5.
As can be seen in the 1H NMR spectrum of 5 (Figure 3a),
the chemical shifts of protons on the outer ammonium sites
and B21C7 rings of 5 are consistent with those of 3 and 4, thus
suggesting that the location of B21C7 remains unchanged.
The central ammonium hydrogen atom H8 can still be found
in the spectrum of 5, but is more downfield-shifted than in the
spectrum of 4. H6 and He are also downfield-shifted. These
observations indicate that this site is bound to a ring
component through a different binding mode. One explan-
ation is that the shielding effect from the catechol rings of
DB24C8 influences hydrogen atoms H6 and He in 4, while the
location of the hydroquinone rings of BPP34C10 does not
result in a similar shielding effect in 5. Moreover, the signal of
Hf is shifted upfield compared with both the other rotaxanes
and the uncomplexed axle molecule; this observation is
consistent with the twin-axial rotaxanation of BPP34C10.[8b,12]
Significant changes in the chemical shifts of the protons in
B21C7 and BPP34C10 of 5 are also observed (Figure S56 in
the Supporting Information).
Figure 5. Energy-minimized structure of 5 obtained by a molecular
modeling study. The geometry was optimized by the molecular
mechanics method with dreiding forcefield. B21C7 is shown in red,
axle 6 in blue, and BPP34C10 in violet.
The NOESY spectrum of 5 in CD3CN (Figure 4) shows a
cross-peak (peak Q) between the protons Hf adjacent to the
the construction of more complicated interlocked molecules
with well-defined structures and functions.
Experimental Section
General synthetic procedure as exemplified by the synthesis of
hetero[7]rotaxane 5: Cu(MeCN)4PF6 (373 mg, 1.00 mmol) and 2,6-
lutidine (12 mL, 0.10 mmol) were added to a solution of 1 (246 mg,
0.73 mmol), 2 (150 mg, 0.332 mmol), B21C7[13] (475 mg, 1.33 mmol),
and BPP34C10[14] (107 mg, 0.20 mmol) in dichloromethane (1.00 mL).
The reaction mixture was stirred for 24 h at room temperature, and
CH3CN (5 mL) and CH3I (5 mL) were subsequently added. The
mixture was heated at 408C for 4 days after which the solvent was
removed under reduced pressure. The crude product was suspended
in acetone (40 mL), a saturated aqueous solution of NH4PF6 was
added, and the mixture stirred until the suspension became clear. The
solvent was removed and water (100 mL) was added to the residue.
The resulting mixture was then filtered, washed with water, and dried.
The residue was purified by column chromatography on silica gel
(eluent: 20:1 acetone/MeOH) to afford 5 as a white solid (338 mg,
42%). 1H NMR (400 MHz, CD3CN): d = 8.40 (s, 4H; H5), 7.79 (s,
12H; H4, H8), 7.39 (m, 28H; H1, H2, H3, H7), 7.11 (d, J = 8.5 Hz, 8H;
H6), 7.03 (s, 8H; HC), 6.97 (m, 16H; HA), 5.30 (s, 8H; He), 4.61 (t, J =
7.1 Hz, 8H; Hd), 4.37 (m, 8H; Ha), 4.30–4.20 (m, 28H; HD, Hg), 4.13 (s,
8H; HJ), 4.00 (s, 8H; Hf), 3.85 (m, 16H; HE), 3.80 (s, 8H; HK), 3.75–
3.64 (m, 24H; HF, Hb), 3.62–3.53 (m, 32H; HG, HH), 3.42 (m, 16H; HI),
3.34 (s, 8H; HL), 3.17 (s, 8H; HM), 2.29 ppm (m, 8H; Hc); 13C NMR
(100 MHz, CD3CN) d = 157.9, 152.2, 146.5, 139.3, 132.0, 131.5, 129.7,
129.5, 129.1, 128.6, 124.2, 121.1 115.5, 114.7, 111.8, 70.8, 70.7, 70.3,
Figure 4. Partial NOESY spectrum (400 MHz, 298 K, CD3CN) of 5.
central ammonium center and HC on the BPP34C10 ring, and
the cross-peaks (peaks R, S, T, and U) between the polyether
protons (HL and HM) and the phenyl protons (H6 and H7) on
the axle component, thus clearly indicating that BPP34C10 is
located near the central site on the axle. Furthermore, cross-
peaks (peaks V, W, X, and Y) between the polyether protons
(HG, HH, and HI) of B21C7 and the outer ammonium protons
H4 and phenyl protons (H1, H2, and H3) are also observed,
thus confirming the location of the B21C7 rings. The energy-
minimized structure of 5 obtained by molecular modeling is
also consistent with the proposed structure (Figure 5).
In conclusion, we have described a methodology for
preparing high-order hetero[n]rotaxanes through the combi-
nation of self-assembly and covalent synthetic “click”
chemistry. This strategy allows precise positional control in
the final [n]rotaxane product and will thus be beneficial for
70.1, 70.0, 69.9, 69.4, 68.0, 67.5, 57.6, 51.3, 51.2, 50.8, 43.5, 38.3,
6+
25.6 ppm; HRMS (ESI): m/z: calcd for C184H264F24N18O42P4
:
663.2945 [(M-6PF6)6+]; found: 663.2944.
1
Hetero[4]rotaxane 4: Yield 72%. H NMR (400 MHz, CD3CN)
d = 8.38 (s, 2H; H5), 7.78 (s, 4H; H4), 7.49 (s, 2H; H8), 7.40 (s, 6H; H1,
H2), 7.36 (m, 8H; H3, H7), 6.98 (m, 8H; HA), 6.81 (m, 12H; HB, H6),
5.15 (s, 4H; He), 4.62 (m, 8H; Hf, Hd), 4.35 (m, 4H; Ha), 4.28 (m, 4H;
HD), 4.20 (m, 10H; HD’, Hg), 4.07 (s, 8H; HN), 3.82 (m, 16H; HE, HO),
3.67 (m, 20H; HF, HP, Hb), 3.61–3.51 (m, 16H; HG, HH), 3.46–3.39 (m,
8H; HI), 2.26 ppm (m, 4H; Hc); 13C NMR (100 MHz, CD3CN) d =
157.1, 147.1, 146.5, 139.4, 132.0, 130.9, 129.7, 129.4, 129.1, 128.6, 125.6,
121.1, 120.8, 114.2, 112.1, 111.9, 70.8, 70.7, 70.4, 70.3, 70.1, 69.9, 69.4,
10836 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 10834 –10838

68.0, 67.5, 57.4, 51.3, 50.8, 43.5, 38.4, 25.6 ppm; HRMS (ESI): m/z:
calcd for C102H144F6N9O24P4+: 506.2494 [(M-4PF6)4+]; found: 506.2491.
[3]Rotaxane 3: Yield 76%. 1H NMR (400 MHz, CD3CN) d = 8.41
(s, 2H; H5), 7.77 (s, 4H; H4), 7.52 (d, J = 8.4 Hz, 4H; H7), 7.38 (m,
10H; H1, H2, H3), 7.30–7.26 (m, 2H; H8), 7.11 (d, J = 8.4 Hz, 4H; H6),
6.98 (d, J = 9.3 Hz, 8H; HA), 5.32 (s, 4H; He), 4.61 (t, J = 6.8 Hz, 4H;
Hd), 4.38–4.32 (m, 4H; Ha), 4.31–4.16 (m, 18H; Hg, Hf, HD), 3.84 (m,
8H; HE), 3.67 (m, 12H; HF, Hb), 3.55 (m, 16H; HG, HH), 3.45–3.36 (m,
8H; HI), 2.32–2.22 ppm (m, 4H; Hc); 13C NMR (100 MHz, CD3CN)
d = 157.8, 146.5, 139.3, 132.0, 131.9, 129.7, 129.6, 129.1, 128.6, 124.0,
121.1, 114.7, 111.9, 70.8, 70.7, 70.3, 70.1, 69.4, 68.0, 57.5, 51.3, 50.8,
50.4, 43.5, 38.4, 25.6 ppm; HRMS (ESI): m/z: calcd for
C78H110N9O163+: 476.2685 [(M-5PF6-2H)3+]; found: 476.2692.
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Axle 6: See the Supporting Information for detailed synthetic
procedure. Yield 91%. 1H NMR (400 MHz, CD3CN) d = 8.45 (s, 2H;
H5), 7.49 (m, 14H; H1, H2, H3, H7), 7.14 (d, J = 8.4 Hz, 4H; H6), 5.34
(s, 4H; He), 4.63 (t, J = 6.9 Hz, 4H; Hd), 4.28 (s, 6H; Hg), 4.21 (m, 8H;
Ha, Hf), 3.16 (m, 4H; Hb), 2.36 ppm (m, 4H; Hc); 13C NMR (100 MHz,
CD3CN) d = 157.9, 139.4, 131.9, 129.9, 129.8, 129.6, 128.9, 123.8, 114.8,
57.8, 51.5, 50.5, 50.4, 44.1, 38.3, 24.8 ppm; HRMS (ESI): m/z: calcd for
C42H53N9O2 [(M-5PF6-3H)2+]: 357.7156; found: 357.7173.
2+
Received: July 30, 2011
Revised: August 18, 2011
Published online: September 20, 2011
Keywords: click chemistry · host–guest systems · rotaxanes ·
.
self-assembly · supramolecular chemistry
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