Volume 56
Number 62
11 August 2020
Pages 8713–8870

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Yoko Sakata, Shigehisa Akine et al.
Efficient formation of [3]pseudorotaxane based oncooperative complexation of dibenzo-24-crown-8 with
diphenylviologen axle

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Cite this: Chem. Commun., 2020,
56, 8735

Eﬃcient formation of [3]pseudorotaxane based oncooperative complexation of dibenzo-24-crown-8
with diphenylviologen axle†

Received 30th April 2020,
Accepted 9th June 2020

Yoko Sakata,*ab Takaya Oguraa and Shigehisa Akine*ab

DOI: 10.1039/d0cc03131crsc.li/chemcomm

A novel cooperative [3]pseudorotaxane system consisting of dibenzo24-crown-8 (DB24C8) and diphenylviologen axle has been developed.
The two-step formation of the [3]pseudorotaxane occurred in apositive-cooperative manner. The corresponding [3]rotaxane was
successfully obtained from just a stoichiometric mixture of eachcomponent by end-capping without dissociation.

Among the various kinds of mechanically-interlocked molecules,1
rotaxane derivatives have highly contributed to the developmentof new types of molecular machines, such as molecular shuttles,2a molecular elevator,3 and a molecular muscle.4 The dynamics ofthe [2]rotaxanes originates from rotating and shuttling motions
e.g., the change in the relative positions of the wheel and axle.
Oligorotaxanes, such as [3]rotaxanes, which have an increased
number of wheel components, can be good candidates to enable
more complex motions, because they are expected to undergo avariety of movements such as the changes in the relative positions
or orientations of the two or more wheel components.5
The general strategy to obtain [3]rotaxanes requires an axle
molecule that has two station units so that each station unit canbind to one wheel component. In fact, [3]rotaxanes are easily
available when the wheel-station interactions are strong enough,as seen in those based on metal-coordination.6 If the interaction
is not strong enough, however, the formation of [3]rotaxane
becomes less eﬃcient. The synthetic attempts often produce a
mixture of [2]- and [3]rotaxanes unless a large excess amount of
the wheel is used.7 This is because the second wheel threading isless favourable than the first one due to a steric/electrostatic
repulsion. This is interpreted as a negative-cooperative process,a

Graduate School of Natural Science and Technology, Kanazawa University,
Kakuma-machi, Kanazawa 920-1192, Japan.
E-mail: sakata@se.kanazawa-u.ac.jp, akine@se.kanazawa-u.ac.jpb
WPI Nano Life Science Institute (WPI-NanoLSI), Kanazawa University,
Kakuma-machi, Kanazawa 920-1192, Japan† Electronic supplementary information (ESI) available: Detailed synthetic procedures, crystallographic data, additional 1H NMR spectra and ESI-TOF MS forthe complexes. CCDC 1998063. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/d0cc03131c

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but if the two-step complexation occurs in a positive-cooperative
fashion,8 [3]rotaxanes would be more eﬃciently synthesized, thus
allowing us to use them as a convenient motif for dynamic
functional molecules. Actually, positive cooperativity was observed
for the formation of several [3]pseudorotaxanes or [3]rotaxanes
containing a cryptand,9 shape-persistent macrocycles, etc.10 as thewheel units. Since crown ethers have been key players in rotaxane
chemistry,1–3 it is desirable to develop cooperative [3]pseudorotaxane systems based on crown ethers, which could be a versatileintermediate for the synthesis of [3]rotaxanes.
Recently, the crystal structure of a [3]pseudorotaxane, MV(DB24C8)2, in which two crown ether wheels simultaneouslyinteract with one dimethylviologen (MV) molecule, has beenreported.11 This was somewhat surprising because, previously,
the MV motif had solely been used to obtain [2]pseudorotaxane
structures, that is, only one wheel molecule interacts with the
MV station of the rotaxane axle through p–p and/or CT interactions between the electron-rich alkoxybenzene unit and theelectron-deficient bipyridinium unit.2,3,12 In this unique[3]pseudorotaxane MV(DB24C8)2, however, the binding withthe second wheel was reported to be weak so that only [2]pseudorotaxane was observable in solution. If the formation efficiency is improved, this [3]pseudorotaxane motif could bemuch more attractive not only for its unique structural feature
but also for the redox behaviour of the viologen axles. In thisstudy, we explored the [3]pseudorotaxane formation behaviourof the two MV analogues, A1 and A2, having phenyl and benzyl
groups, respectively, which could enhance the MV-crown ether
interactions in solution (Fig. 1). We discovered that the combination of diphenylviologen A1 and DB24C8 exhibits an unexpectedly highly positive cooperativity during the [3]pseudorotaxane
formation. We successfully applied this positive-cooperative binding to the efficient synthesis of [3]rotaxane by end-capping of the
carboxy groups at the two terminals of the axle molecule.
We used two kinds of viologen axles, diphenylviologen A1
and dibenzylviologen A2 as TFPB salts (TFPB = tetrakis[3,5bis(trifluoromethyl)phenyl]borate),13 which were prepared fromthe corresponding halide salts14,15 (Fig. 1a). Two carboxy groups

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Fig. 1 (a) Chemical structures of axle and wheel molecules used in thisstudy (TFPB = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate). (b) Equilibria
for the formation of [3]pseudorotaxane and definition of the binding
constants K1, K2 and the cooperativity factor a.

are introduced at the two terminals as reaction sites for conversion to the corresponding [3]rotaxane by end-capping. As
the wheel units, four kinds of crown ether derivatives, 24C8,
DB24C8, DN24C8, and DB30C10, were used.
The pseudorotaxane formation behaviour of the viologen
axle A1 and 24C8 was investigated by 1H NMR titration studies
in CDCl3/CD3CN (4 : 1). The 1 : 1 complex, [2]pseudorotaxane
A1(24C8), was newly formed when 0.5 equiv. of 24C8 wasadded to the viologen axle A1, and the 1 : 2 complex, [3]pseudorotaxane A1(24C8)2, was also observed upon further addition (1.0–5.0 equiv.) (Fig. 2a). The stepwise formation of A1
(24C8) and A1(24C8)2 was also confirmed by ESI–TOF mass
spectrometry (Fig. S1, ESI†). The binding constants, K1 and K2,
for this two-step complexation (Fig. 1b) were determined by
non-linear curve fitting of the changes in the mole fractions(Fig. 2a and Table 1). The negative cooperativity was clearly
demonstrated (K1 = 1190 M 1, K2 = 132 M 1, and a = 4K2/K1 =0.44), which can be explained by steric/electrostatic repulsion
between the first and second wheels.16
In contrast, a positive cooperativity was observed when
DB24C8 was used instead of 24C8. Upon the addition of
DB24C8 to A1, two sets of signals successively appeared in
the 1H NMR spectra as observed in the A1-24C8 system(Fig. 2b), indicating the formation of the [2]- and [3]pseudorotaxanes (A1(DB24C8) and A1(DB24C8)2). The significant
upfield shift of the viologen signal Ha upon the complexation(8.63 - 5.41 ppm) implied the strong shielding eﬀect of thephenylene rings in DB24C8. In this case, 95% of A1 wasconverted into the [3]pseudorotaxane, A1(DB24C8)2 when
3 equiv. of DB24C8 were added. This indicated that the DB24C8
more eﬃciently formed the [3]pseudorotaxane than 24C8,
which was confirmed by the binding constants, K1 = 920 M 1and K2 = 4200 M 1 (Fig. 2b and Table 1). To our surprise, thisA1-DB24C8 system showed a highly positive cooperativity (a = 18),
which is significantly greater than that of the A1-24C8 system(a = 0.44). A similar high cooperativity (K1 = 900 M 1,8736 | Chem. Commun., 2020, 56, 8735--8738

Fig. 2 1H NMR spectral changes of A1 upon the addition of (a) 24C8 and(b) DB24C8 (400 MHz, 25 1C, CDCl3/CD3CN (4 : 1), [A1] = 5 mM). The plotsof the mole fractions of the three components versus the equivalents of
the wheel are also shown. See Fig. 1 for the signal assignments.

Table 1 Binding constants for the complexation between the viologen
axles and crown ethers

Axle

Crown ether

K1 a

K2a

a

A1
A1
A1
A1
A2
A2

24C8
DB24C8
DN24C8
DB30C10
DB24C8
DN24C81190  70
920  80
900  500
1190  190
36  371  6132  44200  200
4300  1200
—b
—b
—b

0.44
18
B20
B0
B0
B0

a
The binding constants were determined by 1H NMR spectroscopy in
CDCl3/CD3CN (4 : 1) at 25 1C. b Negligibly small.

K2 = 4300 M 1, and a E 20) was observed for the combinationof A1 and DN24C8, which contains two naphthylene moieties(Fig. S5, ESI† and Table 1). Based on these results, we foundthat the introduction of aromatic rings into the 24C8 scaﬀold isessential to induce the positive cooperativity during the 1 : 2complexation with A1.
In order to obtain further insight into the positive cooperativity, we investigated the complexation behaviour of A1 with
the DB30C10 wheel, a larger macrocycle having two phenylene
moieties. Upon the addition of DB30C10, the Ha signal in theviologen axle A1 was gradually shifted upfield, indicating
the formation of the pseudorotaxane complex (Fig. S6, ESI†).
The chemical shift changes were nicely fitted to the 1 : 1
binding isotherm with K1 = 1190 M 1 (Table 1). Thus, in thiscase, [2]pseudorotaxane A1(DB30C10) was mainly formed
and the formation of the [3]pseudorotaxane A1(DB30C10)2
was negligible.17

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We also investigated the complexation behaviour of thedibenzylviologen axle A2 in a similar manner. When DB24C8 or
DN24C8 was added to A2, a new set of signals appeared in the1
H NMR spectra (Fig. S7 and S8, ESI†). The quantitative analysis
yielded smaller binding constants for the 1 : 1 complex, K1 = 36
and 71 M 1 for DB24C8 and DN24C8, respectively, without
forming the 1 : 2 complex (Table 1). Thus, the 1 : 2 complexes(e.g., [3]pseudorotaxanes) were formed only when the combination of diphenylviologen A1 and DB- or DN24C8 was used.
This observation is consistent with previous reports describingthat dialkyl- and dibenzylviologen axles form [2]pseudorotaxanes with DB24C8.11,12c
The formation of the [3]pseudorotaxane A1(DB24C8)2 wassignificantly influenced by the solvent. The [3]pseudorotaxane
formation became less eﬃcient in more polar media containing
less CDCl3 (Fig. S9–S13, ESI† and Table 2). Notably, however,higher cooperativity factors were observed in the polar solvents(a = 63 in CD3CN; a = 41 in CD3OD). Although the origin of thecooperativity is unclear at this point, the solvophobic effect orsome other factors might contribute to the positive cooperativity.
Single crystals of [3]pseudorotaxane A1(DB24C8)2 wereobtained by slow diﬀusion of pentane into a methanol/chloroform solution. As we expected, the A1 axle was threaded into
the two DB24C8 molecules in the crystal structure (Fig. 3). The
two DB24C8 molecules were arranged in an interdigitatedfashion so as to wrap the two pyridinium rings, but no p–p
stacking interaction was observed in the DB24C8–DB24C8 or
DB24C8–A1 combinations (Fig. 3a and b). A similar interdigitated arrangement was found in the crystal structure of MV
(DB24C8)2, although the formation of the [3]pseudorotaxane
was negligible in solution probably due to the lack of phenylgroups in the viologen axle.11 Each of the phenylpyridinium
moieties was surrounded by eight oxygen atoms of the DB24C8
so that they can form eight intramolecular C–H  O hydrogenbonds ranging from 2.11 Å to 2.92 Å (Fig. 3c and Table S3, ESI†).
This multiple hydrogen bonding, in which the phenyl C–H
groups in A1 also participate, might account for the enhanced
interaction between the A1 axle and the DB24C8 wheel. In
addition, C–H  p interactions between the pyridinium C–H
groups (Ha in Fig. 1a) in A1 and the phenylene rings in DB24C8
were also observed (from 2.66 Å to 2.93 Å, Fig. 3b). This
interdigitated structure of [3]pseudorotaxane in solution was
supported by the NOE correlation between Ha viologen protonand phenylene protons of DB24C8 (Fig. S14, ESI†).

Fig. 3 Crystal structure of A1(DB24C8)2. (a) Space-filling model, and(b and c) capped stick models. Hydrogen atoms (b and c), solvent molecules
and TFPB anions are omitted for clarity. Hydrogen atoms participating in the
C–H  p and C–H  O interactions are shown in (b) and (c), respectively(C–H  p distances, H7  PhA, 2.66 Å; H8  PhB, 2.93 Å; H10  PhC, 2.74 Å;
H13  PhD, 2.83 Å). The green dotted lines indicate the C–H  O hydrogenbonds (for the C–H  O distances, see Table S3, ESI†).

Finally, we applied this positive-cooperative binding to theeﬃcient synthesis of [3]rotaxane by end-capping of the carboxy
groups in A1 via amide formation (Fig. 4a).18 We prepared asolution containing A1 (20 mM) and DB24C8 (40 mM) in CDCl3/
CD3CN (4 : 1), in which 89% of A1 is converted to [3]psedorotaxane A1(DB24C8)2 as estimated from the binding constants.
The end-capping was achieved without dissociation of the wheel

Table 2 Binding constants for the complexation between the viologen
axle A1 and DB24C8 in various kinds of solvents

Solvent

K1a

K2 a

a

CDCl3/CD3CN (4 : 1)
CD3CN
CDCl3/CD3OD (4 : 1)
CD3OD
CDCl3/DMSO-d6 (4 : 1)
DMSO-d6

920  8048  81500  200
201  13420  100—b4200  200
760  904800  400
2070  70
1500  200
—b

18
63
13
41
15
—b

a

The binding constants were obtained by 1H NMR spectroscopy at
25 1C. b Negligibly small.

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Fig. 4 (a) Reaction scheme of [3]rotaxane R1(DB24C8)2 formation by
amide formation, and (b) 1H NMR spectrum of the reaction mixture(400 MHz, CDCl3/CD3CN (4 : 1)).

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components by amide formation with 3,5-di-tert-butylaniline under
the conditions using 1-hydroxybenzotriazole (HOBt) and 1-ethyl-3(3-dimethylamino)propylcarbodiimide (EDC). The almost exclusive
formation of [3]rotaxane R1(DB24C8)2 was confirmed by the1
H NMR spectrum (Fig. 4b) and the ESI-TOF mass spectrum(m/z = 834.9 for [R1(DB24C8)2 2TFPB]2+, Fig. S15, ESI†). Notably,
the signals for the [2]rotaxane R1(DB24C8) were scarcely observedin the 1H NMR spectrum even though only a stoichiometricamount of the DB24C8 wheel was used. Thus, the [3]rotaxane R1
(DB24C8)2 was eﬃciently synthesized without consuming an excess
amount of the wheel component based on the newly discovered[3]pseudorotaxane system exhibiting a positive-cooperativecomplexation behaviour.
In conclusion, we have demonstrated a new positivecooperative [3]pseudorotaxane system consisting of DB24C8
and the diphenylviologen axle A1. The maximum cooperativity
factor of 63 was achieved in the CD3CN solution. The X-ray
crystallographic analysis revealed that both the multiple C–H  O
and C–H  p interactions between the A1 and DB24C8 components contributed to the stabilization of the [3]pseudorotaxane.
With these results in hand, [3]rotaxane R1(DB24C8)2 was efficiently obtained via the amide formation from a 1 : 2 stoichiometric mixture of A1 and DB24C8. We believe that these results
open the way to the development of not only new [3]rotaxanes
based on various kinds of DB24C8 derivatives, but also oligo- andpolyrotaxanes that have more than two DB24C8 wheels forpotential application to more complex molecular machines. The
investigation of the electrochemical responses of the [3]rotaxane
based on the redox active viologen unit is currently underway.
This work was supported by JSPS KAKENHI Grant Number
JP20H04667, JP18H04511 (Soft Crystals), and JP16H06510(Coordination Asymmetry), Inoue Science Research Award,
the Asahi Glass Foundation, the Iwatani Naoji Foundation,and the World Premier International Research Center Initiative(WPI), MEXT, Japan.

Conflicts of interest
There are no conflicts to declare.

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