Introducing negative charges into bis-p-phenylene crown ethers: A study of bipyridinium-based [2]pseudorotaxanes and [2]rotaxanes

Article abstract of DOI:10.1002/chem.200700387

This paper describes novel host-guest systems comprising viologen cations (guests) and the derivatives of bis-para-phenylene-34-crown-10 (hosts) with anionic groups COO^- or SO₃^-. The structure of the resulting chargecompensated host-guest complexes, their association constants and their electrochemical behaviour have been studied. In the solid state, the viologen cations thread the negatively charged crown ethers forming electroneutral zwitterion-like [2]pseudorotaxane salts; in solution this threaded geometry is preserved. The association constants of [2]pseudorotaxane salts incorporating the 1,1′-diethylviologen moiety in solution are significantly higher than those of previously reported analogues. The extrapolated association free energies in non-aqueous media exceed -40 kJ·mol^-1 at 25°C. This significant increase of the interaction free energy makes these compounds stable even in aqueous solutions. The association constants of [2]pseudorotaxane salts incorporating sterically more hindered 1,1′-diethyl-3,3′-dimethylviologen moieties are significantly lower. Structurally related [2]rotaxane salts, in which the oppositely charged ionic components are mechanically interlocked, have been prepared in good yields. It has been shown that [2]rotaxane salts incorporating anti-isomers of bisfunctionalised crown ethers are cycloenantiomeric. In both [2]pseudorotaxane and [2]rotaxane salts, the electrostatic interactions between the viologen moieties and the negatively charged crown ethers lead to very significant negative shifts of viologen reduction potentials up to 450 mV. The findings of the present study are valuable for the design of nanoscale molecular electronic devices.

Full text of DOI:10.1002/chem.200700387

FULL PAPER
DOI: 10.1002/chem.200700387
Introducing Negative Charges into Bis-p-phenylene Crown Ethers: A Study
of Bipyridinium-Based [2]Pseudorotaxanes and [2]Rotaxanes
Elena Lestini, Kirill Nikitin,* Helge Müller-Bunz, and Donald Fitzmaurice[a]
Abstract: This paper describes novel
host–guest systems comprising viologen
cations (guests) and the derivatives of
bis-para-phenylene-34-crown-10 (hosts)
tion are significantly higher than those
of previously reported analogues. The
extrapolated association free energies
are significantly lower. Structurally re-
lated [2]rotaxane salts, in which the op-
positely charged ionic components are
mechanically interlocked, have been
prepared in good yields. It has been
shown that [2]rotaxane salts incorpo-
rating anti-isomers of bisfunctionalised
crown ethers are cycloenantiomeric. In
both [2]pseudorotaxane and [2]ro-
taxane salts, the electrostatic interac-
tions between the viologen moieties
and the negatively charged crown
ethers lead to very significant negative
shifts of viologen reduction potentials
up to 450 mV. The findings of the pres-
ent study are valuable for the design of
nanoscale molecular electronic devices.
in
non-aqueous
media
exceed
À
with anionic groups COOÀ or SO3 .
À40 kJ·molÀ1 at 258C. This significant
increase of the interaction free energy
makes these compounds stable even in
aqueous solutions. The association con-
stants of [2]pseudorotaxane salts incor-
porating sterically more hindered 1,1’-
diethyl-3,3’-dimethylviologen moieties
The structure of the resulting charge-
compensated host–guest complexes,
their association constants and their
electrochemical behaviour have been
studied. In the solid state, the viologen
cations thread the negatively charged
crown ethers forming electroneutral
zwitterion-like [2]pseudorotaxane salts;
in solution this threaded geometry is
preserved. The association constants of
[2]pseudorotaxane salts incorporating
the 1,1’-diethylviologen moiety in solu-
Keywords: electrostatic
interac-
tions · molecular recognition · ro-
taxanes · supramolecular chemistry ·
zwitterions
Introduction
such as [2]rotaxanes. Recently, for example, electron trans-
fer processes at the surface have been studied in systems in-
corporating orientation-specific tripodal linkers connecting
electroactive moieties.[3] The topology of [2]rotaxanes
allows, in principle, the macrocyclic component of a [2]ro-
taxane to undergo a reversible internal translational motion
(RITM), thus working as a molecular shuttle or switch.[4]
Despite the rapid growth in this area and various ap-
proaches to study RITM in switchable [2]rotaxanes, little
was known until recently about their detailed conformation
and methods to elucidate and control their conformations.[5]
While in many cases the RITM can be achieved by applying
an appropriate stimulus, it is noted that the most remarkable
results were obtained both in solution and on surfaces when
strong electrostatic interactions were used to control the
RITM of macrocyclic components of the interlocked mole-
cules.[1,6]
Molecular electronics and artificial molecular machinery are
undergoing a breakthrough in recent years.[1] The progress
in these fields relies on the ability of the molecular compo-
nents to perform their function consistently. Supramolecular
chemistry, surface chemistry and nanoparticle chemistry
offer a range of applications in material science, biomedical
monitoring and analysis.[2] Therefore, intelligent and robust
components are required for nanoscale molecular and
supramolecular architectures to function as intended in solu-
tion or at the surface.
Particularly interesting are electronically addressable and
switchable molecules and interlocked molecular systems,
[a] E. Lestini, Dr. K. Nikitin, Dr. H. Müller-Bunz, Prof. D. Fitzmaurice
School of Chemistry and Chemical Biology
University College Dublin, Belfield, Dublin 4 (Ireland)
Fax : (+353)01-7162-127
It has recently been demonstrated that electrostatic repul-
sion forces can be modulated to control the shuttling and/or
circumrotation movements in interlocked molecular system-
s.[6a,b] However, electrostatic attraction forces have not been
used in this capacity. It is surprising as the electrostatic at-
E-mail: kirill.nikitin@ucd.ie
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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traction is the most common intercomponent interaction
found in salts, ion pairs and supramolecular and biomolecu-
lar systems.[6c–k] It was shown recently that electrostatic at-
traction can direct ionic self-assembly.[6l]
will be determined by the chemical structure, stability and
electrochemical behaviour of [2]rotaxanes and [2]pseudoro-
taxanes incorporating model viologens 5–8 and charged
crown ethers 3 and 4.
A switchable [2]rotaxane is represented in Scheme 1. This
interlocked molecular system comprises the crown ether C
threaded by the dumbbell-shaped component (“axle”), in-
In the present paper, we report the discovery of a novel
type of [2]pseudorotaxanes and related [2]rotaxanes in
which the molecular components are oppositely electrically
charged. Consequently, the mutual molecular recognition is
significantly enhanced by the resulting intercomponent elec-
trostatic attraction forces. In these compounds, termed here
[2]pseudorotaxane and [2]rotaxane salts, respectively, the
positive and negative charges are internally compensated.
The findings reported allow us to predict that electrostatic
attraction interactions of components will help control shut-
tling-type RITM in interlocked molecular systems. Further
work on the preparation and application of interlocked sys-
tems of even greater complexity is under way.
Results and Discussion
Preparation of anionic crown ethers: The prototype bis-p-
phenylene-34-crown ether 1, its carbalkoxy derivatives as
well as derivatives of other crown ethers have been previ-
ously prepared and their association with cations has been
studied.[7] It has been found that association with viologens
increases with the addition of the ester groups to the back-
bone of 1.[7b] However, no attempts have been made to pre-
pare anionic bis-p-phenylene crown ethers and to test their
complexation with viologens. It is worth noting that strong
binding of metal cations to anionic aliphatic crown ethers[7c,d]
as well as strong binding of lysine to an anionic cylinder-
shaped artificial molecular receptor have been described.[7f]
It was demonstrated that the addition of carboxylates to the
crown-ether backbone gives a mean logK increment 2.6 for
divalent cations.[7d]
In the present work, we studied novel anionic crown
ethers 3 and 4 and compared them to the neutral crown
ethers 1 and 2 (Scheme 1). It was expected that the nega-
tively charged 3 and 4 would interact strongly with the posi-
tively charged viologens 5–8 and the resulting electroneutral
[2]pseudorotaxane salts would include one doubly negative-
ly charged macrocycle and one doubly positively charged vi-
ologen moiety. It should be noted, that the model viologens
5 and 6 incorporate a more electron deficient and less steri-
cally hindered viologen moiety, V1, than the viologens 7 and
8 (V2).[3b,4b,c] As a consequence, the V1 moiety possesses a
higher affinity for crown ethers and can be reduced at less
negative potentials than V2.
The synthesis of the carboxylate crown ether 3 was car-
ried out from an aromatic building block, 9, and a glycol
chain building block, via the ester 2 and acid 11 (Scheme 2).
Firstly, 9 was alkylated at the more accessible 5-hydroxyl by
using an excess of the dibromide at 608C. This was possible
due to the intramolecular hydrogen bonding between the 2-
hydroxyl group and the ethoxycarbonyl group in the ester 9.
The cyclisation of the resulting bromide 10 at a slightly
Scheme 1. Bistable [2]rotaxane (top) incorporating a crown ether (e.g. 1–
4) and two viologen stations (e.g. 5–8).
corporating two positively charged and, therefore, electro-
chemically active viologen sites V1 and V2. Assuming that
V1 represents the “strong” viologen recognition site, an elec-
trically neutral crown ether, such as 1 or 2, which is initially
localised at V1, can be switched to the “weak” site V2 by
means of electrochemical or photochemical reduction of
V1.[3b,4b,c,5] Let us consider how a negative charge on the
crown ether may influence its RITM in [2]rotaxane. In this
case, a strong electrostatic attraction additionally stabilises
both translational isomers (co-conformations) of the [2]ro-
taxane when the crown ether, for example, 3, is localised at
viologen site V1 or V2. The electrostatics will create deeper
energy wells for 3 at both viologen sites in this interlocked
system. It is important to find out whether the electrochemi-
cally controlled switching is still feasible in this case. This
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[2]Pseudorotaxanes and [2]Rotaxanes
FULL PAPER
phonic acid (Scheme 3). The
product is a mixture of two iso-
meric disulphonic acids syn-12
and anti-12 in equimolar quan-
tities. As attempts to isolate in-
dividual isomers of 12 failed,
the two acids were converted
into a mixture of tetrabutylam-
monium salts 4. This material
was used for the measurements
and for subsequent synthetic
steps.
For the purposes of compara-
tive study of the anionic crown
ethers, a model aromatic car-
boxylate 14 was prepared as
shown in Scheme 3. This com-
pound incorporates a p-dialkox-
yaromatic ring and a negatively
charged carboxylate group thus
representing the aromatic unit
of macrocyclic 3.
Scheme 2. Preparation of the macrocyclic carboxylate 3. Reaction conditions (room temperature unless other-
wise stated): i) ethanol, H2SO4, reflux, 6 d, 93%; ii) Br-(CH2CH2O)3CH2CH2Br, K2CO3, acetonitrile, 608C, 3 d,
31%; iii) K2CO3, acetonitrile, 808C, 3 d, 53%; iv) a) NaOH, ethanol, 608C, 2 h; b) HCl, 100%; v) tetramethy-
lammomnium hydroxide, methanol, 100%.
higher temperature, 808C, furnished the crown ester 2 in
good yield. The ester groups of 2 were saponified and the
acid 11 was isolated as a colourless monoclinic crystalline
material. The tetramethylammonium salt 3 was prepared
and used in the present study as it has the advantage of
being soluble in water, methanol, acetonitrile and other sol-
vents.
The molecular structure of the acid 11 was confirmed by
X-ray diffractometry (Figure 1). Clearly, the two carboxylic
groups of 11 are attached to the bis-p-phenylene-34-crown-
10 backbone in the anti-position and point in opposite direc-
tions. The distances between the oxygen atoms of the two
different carboxylate groups (7.2–8.0 Š) are similar to the
typical distances between positively charged nitrogen atoms
in viologen guests (7.0 Š), providing a good geometric
fit.[5d,e]
The synthesis of the crown ether 4 was carried out by the
direct sulphonation of the crown ether 1 with chlorosul-
Scheme 3. Top: preparation of macrocyclic sulphonate 4; bottom: prepa-
ration of acyclic carboxylate 14. Reaction conditions (room temperature
unless otherwise stated): i) HSO3Cl, 08C, 1 h, 78%; ii) tetrabutylammoni-
um hydroxide, methanol, 100%; iii) tetramethylammonium hydroxide,
methanol, 100%.
Structure of [2]pseudorotaxane salts: The [2]pseudoro-
taxane salts 15 and 16 were prepared by ion-exchange reac-
Figure 1. X-ray crystal structure of the macrocyclic acid 11.
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1097

K. Nikitin et al.
tions between the viologen hexafluorophosphate 6 and the
macrocyclic salts 3 and 4 in aqueous alcohol (Scheme 4).
These reactions yield deeply coloured solutions of the highly
polar [2]pseudorotaxane salts, while the insoluble tetraalkyl-
ammonium salts precipitate and can be filtered off.
The crystal structures of hydrated [2]pseudorotaxane salts
15 and 16 are shown in Figure 2a and b, respectively. In both
structures, the viologen moiety threads the anionic macrocy-
cle. This important finding demonstrates that in the solid
state the threaded species, 15 and 16, are more favourable
than other possible ion-paired species. It should be noted
that the position, orientation and internal dihedral angle of
the viologen moiety in 15 and 16 are similar to those in
common [2]pseudorotaxanes containing the neutral crown
ether 1.[5d,e] However, there are a few surprising features.
For example, in the [2]pseudorotaxane salt 15 the distance
between the anionic carboxylate groups of the crown ether
moiety (around 10 Š) is considerably greater than in the
structure of the acid 11 (7.2 Š) due, possibly, to electrostatic
repulsion. As a result, in the solid [2]pseudorotaxane salt 15,
the cationic nitrogen of the viologen moiety and the anionic
carboxylates are rather far from each other (the closest con-
tact is 5.5 Š) and the carboxylates point away from the viol-
ogen moiety into layers of hydrating water molecules. The
hydration shell of water molecules (twelve molecules per
each [2]rotaxane salt unit) form layers separating adjacent
[2]pseudorotaxane salt units from each other. This type of
hydration prevents direct contacts between the carboxylate
groups and the positively charged nitrogen atoms. As all at-
tempts to fully dehydrate the [2]pseudorotaxane salt 15 led
to irreversible decomposition, the constitutional water is
likely to be an additional stabilisation factor of the [2]pseu-
dorotaxane salt in the solid state. It is also noted that as the
viologen moieties are commonly extremely sensitive to
bases, the presence of excess base or excess carboxylate
Figure 2. a) X-ray crystal structure of the [2]pseudorotaxane salt 15. b) X-
ray crystal structure of the [2]pseudorotaxane salt 16.
should be strictly avoided when preparing [2]pseudoro-
taxane salt 15. By comparison, in the solid [2]rotaxane salt
16, the distance between the anionic sulphonate groups of
the crown ether moiety (8.9–10 Š) and their closest contacts
with viologen nitrogens (5.2 Š) are slightly shorter than in
the structure 15. Additionally, the number of the hydrating
water molecules (only two per each [2]pseudorotaxane salt
Scheme 4. Formation of [2]pseudorotaxane salts in solution
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[2]Pseudorotaxanes and [2]Rotaxanes
FULL PAPER
unit) is substantially lower that in 15. As a result, in the
solid [2]pseudorotaxane salt 16, the cationic nitrogen of the
viologen moiety and the anionic sulphonates are not sepa-
rated from each other by layers of water molecules. It can
be concluded, that the sulphonate groups provide better
charge delocalisation than the carboxylate groups and im-
prove the stability of [2]pseudorotaxane salts.
While the key finding of the above crystallographic study
is that the positively charged viologen moieties form stable
threaded [2]pseudorotaxane salts with negatively charged
crown ethers in the solid state, the question which arises is
whether the threaded structure remains preserved in solu-
tion. To answer this question, the study of the [2]pseudoro-
taxane salts and related species in solution was carried out
by NMR spectroscopy.
The proton NMR spectra of the individual viologen bro-
mide 5, tetramethylammonium carboxylate 3 and their equi-
molar mixture were recorded in methanol at 258C (Fig-
ure 3a). Given the high value of association constant Ka
under the specified conditions (vide infra), the association
equilibrium is fully shifted towards the formation of [2]pseu-
dorotaxane salt 15 (Scheme 4: R=H, X=Br). The interac-
tions between the viologen moiety and the negatively
charged crown ether in 15 are accompanied by significant
upfield shifts of the resonances in the region of d=8.0–
9.5 ppm (H1 and H2 of the viologen moiety, Scheme 4) and
in the region of d=6.0–7.3 ppm. (H3–H5 of the crown
ether). These shifts in the case of the [2]pseudorotaxane salt
15 are attributed to mutual strong diamagnetic shielding due
to the p-stacking of viologen and crown ether aromatic sys-
tems.[8] Consequently, it can be concluded that in solution
the viologen moiety threads the negatively charged crown
ether 3 to form [2]pseudorotaxane salt 15. A NOESY study
of 15 demonstrates that when irradiating the H2 protons of
the viologen (d=8.1 ppm) strong interactions are observed
with the H1 protons of the viologen at d=9.1 ppm and with
all three aromatic protons H3, H4, H5 (region d=6.0–
6.6 ppm) of the crown ether. Similar experimental observa-
tions were made for the interaction of the bromide 5 with
the sulphonate 4 in methanol.
1
The H NMR spectra of the sterically hindered 3,3’-dime-
thylviologen bromide 7, the tetramethylammonium salt 3
and their equimolar mixture were also recorded in methanol
at 258C (Figure 3b). Given the high value of Ka under the
specified conditions, the association equilibrium is shifted
towards formation of [2]pseudorotaxane salt 17 (Scheme 4:
R=Me, X=Br). The interactions between the viologen
moiety and the negatively charged crown ether 3 in this case
are accompanied by significant upfield shifts of the resonan-
ces in the region of d=8.0–9.2 ppm (H1 and H2 of the violo-
gen moiety) and less significant upfield shifts in the region
of d=6.5–7.0 ppm (H3–H5 of the crown ether). Similarly to
the [2]rotaxane salt 15, the upfield shifts of H1 and H2 of the
viologen moiety can be attributed to diamagnetic shielding
by the closely approaching aromatic systems of the crown
ether. The relatively small values of the observed upfield
shifts of the crown ether protons H3–H5 probably indicate
that the sterically hindered 3,3’-dimethylviologen moiety in
[2]pseudorotaxane salt 17 cannot adopt a fully planar con-
formation. This steric barrier prevents the formation of a
“sandwiched” three-decked [2]pseudorotaxane structure and
diminishes the diamagnetic shielding effect. A NOESY
study of the [2]pseudorotaxane salt 17 demonstrates that
when irradiating the H2 protons of the viologen, strong in-
teractions are observed with the H1 protons of the viologen
and with all three aromatic protons H3, H4, and H5 of the
crown ether. It is concluded, that in solution, the viologen
moieties of 5 and 7 thread the negatively charged crown
ether 3 to form [2]pseudorotaxane salts 15 and 17.
The interaction of viologen bromide 5 with the non-mac-
rocyclic carboxylate 14 was studied and compared to the
macrocyclic anions 3 and 4. Although the interaction of 5
with carboxylate 14 can be observed visually (orange colour
of a 1:2 mixture of 5 and 14 in methanol), the 1H NMR
spectrum of this mixture at 258C (Figure 4) shows no up-
field shifts of the resonances. This demonstrates that the ob-
served upfield NMR shifts for [2]pseudorotaxane salts 15
and 17 should be attributed to threading rather than forma-
tion of ion pairs or charge-transfer interactions.
Figure 3. a) Partial 1H NMR spectra of viologen 5, crown ether 3 and
their equimolar mixture in CD3OD at 258C; b) partial 1H NMR spectra
of viologen 7, crown ether 3 and their equimolar mixture in CD3OD at
258C (concentration 1.00”10À2 moldmÀ3).
Having established that the viologens 5 and 7 thread neg-
atively charged crown ethers in solution, it was interesting
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K. Nikitin et al.
this case increases rapidly with the addition of methanol or
acetonitrile due, possibly, to the decrease of ionising power
of the solvent media.[10,11] Consequently, the association con-
stant Ka cannot be directly measured in neat methanol or
acetonitrile due to poor linearity of the acquired data.
To estimate association constants Ka in neat organic sol-
vents, the respective measurements were carried out in
aqueous mixtures and the results were extrapolated. This
was done on the basis of an assumption that the ionizing
power Y of a solvent mixture can be approximated by a
linear function of the composition of this mixture (see the
Supporting Information, part 2).[10]
Clearly, the association constants of 1,1’-diethylviologens
5 and 6 with the negatively charged crown ethers 3 and 4
are generally 4–5 orders of magnitude higher than the asso-
ciation constants with neutral crown ethers 1 and 2 in the
same solvent (Table 1). The association constants of [2]pseu-
dorotaxane salts incorporating 3,3’-dimethyl-1,1’-diethylviol-
ogens 7 and 8 and negatively charged crown ethers 3 and 4
are, as expected, 1–2 orders of magnitude lower than the as-
Figure 4. Partial 1H NMR spectra of viologen 5, carboxylate 14 and their
mixture in CD3OD at 258C (concentration of 5 1.00”10À2 moldmÀ3).
to study the possibility of an interaction between a sterically
stoppered dumbbell-shaped “axle” containing a viologen
moiety and a negatively charged crown ether. Additional ex-
periments (see the Supporting Information, part 1) have
shown that the electrostatic interactions in this case lead to
the formation of ion pairs, possibly by wrapping of the
crown ether around the viologen moiety.
On the basis of the above comparative NMR spectroscop-
ic study of the interactions between viologens and carboxyl-
ate anions 3 and 14, it is concluded that in solution, as in the
solid state, the viologens thread the anionic crown ethers to
form [2]pseudorotaxane salts.
sociation constants of 1,1’-diethylviologens
5
and
6
(Table 1). Apparently, the steric hindrance within the 3,3’-di-
methyl-1,1’-diethylviologen moiety decreases its predilection
for complexation with crown ethers.[3b,4b] In short, the ob-
served affinity of the disubstituted crown ethers CY2 (cf
Scheme 1) towards viologens decreases in the following
À
À
À
À
À
À
order: SO3 > COO @ COOEt> H.
The crystal structure data on [2]pseudorotaxane salts can
be used to estimate the electrostatic energy contribution to
their stability in solution. For example, considering the asso-
ciation free energy, DG8, of the viologen 6 with the neutral
crown ether 1 (À11.8 kJmolÀ1 in acetonitrile, Table 1) and
with the charged crown ether 3 (À40.2 kJmolÀ1 in acetoni-
trile) one can conclude that the estimated electrostatic con-
Association constants of [2]pseudorotaxane salts: It is ex-
pected that the thermodynamic stability of the [2]pseudoro-
taxane salts incorporating negatively charged crown ethers 3
and 4 and viologens would be substantially higher than that
of the [2]pseudorotaxanes incorporating neutral crown
ethers 1 and 2. The equilibrium association constants Ka be-
tween viologens 5–8 and crown ethers 1–4 (Scheme 4) were
measured by using a dilution method.[9] The equilibrium was
monitored by UV/visible spectroscopy at 258C in water/
methanol mixtures for bro-
tribution to the DG8 value in the case of 3 is DACHTREUNG(DG8)=
À28.4 kJmolÀ1. Assuming that the viologen 6 threads the
charged crown ether 3 in solution forming [2]rotaxane salt
15 (Scheme 4), the electrostatic energy contribution to this
process is equal to the electrostatic interaction energy, Ee,
between the viologen dication and the crown ether dianion
in solution. Ee can be estimated (see the Supporting Infor-
mides 5 and 7 and in water/
Table 1. Association constants of [2]pseudorotaxanes and [2]pseudorotaxane salts.[a]
acetonitrile mixtures for hexa-
fluorophosphates 6 and 8. The
results are given in Table 1.
Viologen
Host
Complex
Solvent
Ka [dm3 molÀ1
(5.1Æ0.2)”102
(1.2Æ0.1)”102
(7.3Æ0.4)”102
(6.0Æ0.3)”102
(7.8Æ3)”105
(3.6Æ1.6)”106
(1.2Æ0.5)”107
(4Æ2)”106
(4Æ2)”107
(3.4Æ0.8)”104
(6.8Æ1)”104
(1.5Æ0.2)”102
]
DG8 [kJmolÀ1
]
5
6
5
5
5
5
6
5
6
7
8
5
1
1
2
3
3
3
3
4
4
3
3
14
5·1
6·1
5·2
15
15
15
15
16
16
17
17
--
MeOH
MeCN
MeOH
H2O
G
À15.5Æ0.1
À11.8Æ0.4
À16.3Æ0.1
À15.9Æ0.1
À33.6Æ2.0
À37.4Æ1.6
À40.2Æ2.0
À37.7Æ2.0
À43.0Æ2.0
À25.9Æ0.7
À27.6Æ0.6
À12.3Æ0.3
AHCTREUNG
The association of the violo-
gen 5 with the neutral crown
ethers 1 and 2 is not observed
in water (due to sparing solu-
bility of the crown ether) and
is moderately strong in metha-
nol (Table 1). On the contrary,
the association of 5 with the
anionic crown ether 3 is suffi-
ciently strong to be measured
ACHTREUNG
AHCTREUNG
MeOH[b]
MeCN[b]
MeCN[b]
MeOH[b]
MeOH[b]
MeOH[b]
MeCN[b]
MeOH
ACHTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
[a] Dilution method monitored by UV spectroscopy at 258C. [b]Extrapolated value (see Supporting Informa-
even in neat water. The Ka in tion, part 2 for details).
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[2]Pseudorotaxanes and [2]Rotaxanes
FULL PAPER
mation, part 3) on the basis of the atom charges, interatomic
distances and the dielectric constant of the solvent. The esti-
mated value Ee =À28.7 kJmolÀ1 is in good agreement with
clic component C to V2. Earlier, it has been demonstrated
that the complexation of crown ethers with viologens shifts
the peak reduction potentials Ep of the viologen to more
negative values.[3b,4b,7b] It can be expected that due to strong
electrostatic interactions, the negative electrochemical shifts
in [2]pseudorotaxane salts will be far more pronounced than
in [2]pseudorotaxanes incorporating neutral crown ethers.
The cyclic voltammograms of the viologen hexafluorphos-
phate 6, in the absence and in the presence of carboxylate
crown ether 3, were recorded in acetonitrile containing
Et4NPF6 as supporting electrolyte (Figure 5). As expected,
the peak reduction potentials of the viologen moiety in the
resulting [2]pseudorotaxane salt 15 are significantly shifted
towards more negative values both for the first and second
reduction stages (Table 2). It can be noted, that while the
negative shifts observed for [2]pseudorotaxane salts reach
exceptional values, up to À235 mV, no shift is observed for
the first stage of the reverse oxidation process (Table 2).
the observed DACHTREUNG(DG8). It also is in a reasonable agreement
with the recent findings on the electrostatic interaction in a
molecular system comprising a quadruply positively charged
ring component and a doubly positively charged axle com-
ponent.[12] The electrostatic repulsion interaction energy in
that case was +37.7 kJmolÀ1.
Association of the viologen 5 with the non-macrocyclic
carboxylate 14 (Table 1) was also studied in neat methanol.
As expected, the association (Ka =1.5”102 dm3 molÀ1) is sig-
nificantly weaker than that of the preorganised macrocyclic
doubly-charged crown ethers 3 and 4. It is concluded, there-
fore, that multiple electrostatic interactions as well as com-
plementary dimensions of anionic crown ethers and violo-
gens lead to the strong association of viologens with anionic
crown ethers.
Interestingly, the association constant between the violo-
gen bromide 5 and the carboxylate crown ether 3 is 4.6
times
higher
in
acetonitrile
(Table 1,
Ka =3.6”
106 dm3 molÀ1) than in methanol (Ka =7.8”105 dm3 molÀ1).
This difference can reasonably be explained by a higher
degree of solvation for the carboxylate anions in methanol
when compared to acetonitrile.[11] The association constant
in neat acetonitrile is 3.3 times higher when using hexafluor-
ophosphate 6 (Table 1, Ka =1.2”107 dm3 molÀ1) compared to
the bromide 5. As shown in the Supporting Information
(part 4), this effect can be explained by a possible difference
in the ionic strength created by the coformed tetramethyl-
ammonium salt Alk4NX (cf. Scheme 4).
It can be concluded that the viologen cations as guests
thread anionic crown ethers and form stable [2]pseudoro-
taxane salts. The association constants of [2]pseudorotaxane
salts in non-aqueous solvents exceed 107 dm3 molÀ1. The as-
sociation of the highly polar [2]pseudorotaxane salts de-
creases in polar media and in the presence of added ions.
Electrochemistry of [2]pseudorotaxane salts: As discussed
above, the RITM function of a switchable [2]rotaxane
(Scheme 1) involves the electrochemical reduction of the vi-
ologen moiety V1 followed by the shuttling of the macrocy-
Figure 5. Cyclic voltammograms of viologen 6 (2.0”10À3 moldmÀ3) and
[2]pseudorotaxane salt 15 (2.0”10À3 moldmÀ3) recorded at 100 mVsÀ1 in
acetonitrile containing Et4NPF6 (0.010 moldmÀ3) at 258C.
Table 2. Peak reduction and oxidation potentials[a] of [2]rotaxane and [2]pseudorotaxane salts.[b]
+
2+
V2+/V+ [V]
V+ /V [V]
V /V [V]
V+ /V [V]
C
C
CC
CC
C
C
Viologen
Host
Complex
6
6
6
6
8
8
20
20
20
20
20
–
14
3
4
–
3
–
11
–
–
À0.839
À0.853
À1.073
À1.122
À1.252
À1.443
À0.676
À0.846
À1.128
À0.894
À0.979
À1.305
À1.347
À1.484
À1.466
À1.510
À1.686
À1.008
À1.208
À1.440
À1.339
À1.324
À1.126
À1.098
À1.124
À1.123
À1.266
À1.261
À0.910
À0.913
À1.058
À1.010
À1.097
À0.667
À0.634
À0.770
À0.735
À1.049
À1.168
À0.550
À0.656
À0.721
À0.797
À0.782
15
16
–
17
–
21
22
24
23
3
syn-4
anti-4
[a] Values have been extracted numerically; accuracy not poorer than 10 mV. [b]Concentration 2.0”10À3 moldmÀ3 in acetonitrile containing supporting
electrolyte Et4NPF6 (0.010 moldmÀ3); scan rate 100 mVsÀ1; 258C.
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1101

K. Nikitin et al.
The above findings can be interpreted in terms of reduc-
tion and dissociation of the [2]pseudorotaxane salt 15 as
shown in Scheme 5. The large negative shifts of reduction
(Scheme 1), incorporating two viologen stations V1 and V2,
the anionic crown ether will be localised predominantly at
the “strong” viologen station V1.
[2]Rotaxane salts: The [2]ro-
taxane salts were prepared by
using an efficient three-compo-
nent self-assembly reaction
under high concentration condi-
tions as shown in Scheme 6.[3a]
Typically, the monocation pre-
Scheme 5. Electrochemical reduction and dissociation of [2]pseudorotaxane salt 15.
potentials Ered1 and Ered2 clearly demonstrate strong electro-
static interaction of the components in the [2]pseudoro-
taxane salt 15 in its parent oxidised state and in the first re-
duced state 15À. However, full reduction of the viologen
moiety leads to dissociation of the electroneutral thread 5(0)
and the carboxylate 3. Consequently, the first oxidation po-
tential of the system Eox1 is not shifted.
The cyclic voltammograms of 3,3’-dimethylviologen 8 in
the absence and in the presence of the carboxylate 3 were
also recorded under the above conditions. Similarly to the
viologen 6 and the corresponding [2]pseudorotaxane salt 15,
the peak reduction potentials of the viologen moiety of
[2]pseudorotaxane salt 17 are significantly shifted towards
more negative values both for the first and second reduction
stages (Table 2). It is noted, however, that due to the steric
hindrance, the electrostatic interaction in 17 is weaker and
the negative shifts observed (À190 and À175 mV upon first
and second reduction, respectively) for 17 are smaller than
those observed for the [2]pseudorotaxane salt 15.
By comparison, cyclic voltammograms of the viologen 6
in the absence and in the presence of the carboxylate 14
were recorded under the above conditions. It was found that
the first and second peak reduction potentials of the violo-
gen moiety were shifted towards more negative values by 14
and 43 mV (Table 2) and the respective peak oxidation po-
tentials were shifted towards more positive values by 30 mV.
As the resulting average potentials were virtually unaffect-
ed, it was reasoned that the electrostatic interaction of the
viologen moiety with the acyclic carboxylate 4 was insuffi-
cient to maintain stable molecular associates.
Finally, it can be concluded that viologens and negatively
charged p-phenylene crown ethers form very stable [2]pseu-
dorotaxane salts in solution and in the solid state. These
complexes retain the threaded geometry of the [2]pseudoro-
taxanes formed by neutral crown ethers. Electrostatic non-
covalent interaction in these complexes is one of the stron-
gest (up to 40 kJmolÀ1) among known intermolecular inter-
actions in solution. The stability of the [2]pseudorotaxane
salts is upheld by the cooperative electrostatic attraction
and preorganised geometry of the host and guest.
Scheme 6. Synthesis of [2]rotaxane 21 and its conversion into [2]rotaxane
salt 22. Reaction conditions (room temperature): i) PhCl/PhCN, 10 d,
yield of 21: 50%; ii) sodium bicarbonate, water/nitromethane, 1 min,
83%.
cursor 18, the crown ether and the stopper component 19
were dissolved in a minimum amount of an aromatic solvent
(benzonitrile, chlorobenzene) and were left at ambient con-
ditions until the reaction is complete. Usually, the viologen
axle component, 20, was formed as a side product.[5d]
The above findings are valuable for the design of novel
[2]ionorotaxanes. For example, on the basis of [2]rotaxane
salt stability constants for the viologens 5 and 7 it can be
reasonably expected that in a switchable [2]rotaxane salt
1102
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Chem. Eur. J. 2008, 14, 1095 – 1106

[2]Pseudorotaxanes and [2]Rotaxanes
FULL PAPER
The carboxylic diacid [2]rotaxane 21 was prepared from
the macrocyclic acid 11 in a mixture of benzonitrile/chloro-
benzene as the solvent (Scheme 6). The [2]rotaxane 21 was
obtained in reasonably good yield (50%) which was limited
by the solubility of the acid 11 in the reaction media.
In the proton NMR spectrum of 21 (Figure 6), the reso-
nances of the methylene protons Ha and Hb (Scheme 6) are
represented by two doublets at d=5.96 and 6.12 ppm, re-
Figure 7. Cyclic voltammograms of 20 (2”10À3 moldmÀ3), [2]rotaxane 21
(2”10À3 moldmÀ3) and [2]rotaxane salt 22 (2”10À3 moldmÀ3) recorded at
100 mVsÀ1 in dry acetonitrile containing TBAP(0.010 moldm À3) at 258C.
1
Figure 6. Partial H NMR spectra of [2]rotaxanes 21 and 22 in acetone at
258C.
The peak reduction potentials of the [2]rotaxane 21, in
comparison to the axle component 20, are shifted to more
negative values by 170–200 mV. In the [2]rotaxane salt 22,
the intercomponent interaction leads to a very significant
peak shifts up to À450 mV. The latter figure is considerably
greater than the expected electrochemical shift estimated on
the basis of the electrostatic interaction in analogous
[2]pseudorotaxane salt 15 (see the Supporting Information,
part 5). This discrepancy is, possibly, caused by a different
spatial position of the carboxylate groups in 22, permitting
closer contacts with the positively charged nitrogen atoms of
the viologen moiety.
spectively, with a proton–proton coupling constant of J=
14 Hz. This interesting behaviour can be explained by the
cycloenantiomerism[13] of the [2]rotaxane 21. Indeed, al-
though the individual components of 21 (the dumbbell-
shaped 20 and the crown ether 11) are not chiral, no symme-
try element is present when 20 and 11 are interlocked form-
ing chiral [2]rotaxane 21. The [2]rotaxane 21 can be depro-
tonated by sodium bicarbonate and the neutral [2]ionoro-
taxane 22 can be extracted into nitromethane. Individual 22
is a deep-red glassy solid which, when dissolved, can be
easily converted back into 21 by treatment with HPF6.[14]
The proton NMR spectra reveal striking differences be-
tween acidic the form of 21 and its conjugated base [2]ro-
taxane salt 22 (Figure 6). In the proton NMR spectrum of
22 all signals of the viologen moiety, the crown ether aro-
matic rings and the benzylic protons are significantly broad-
ened. However, the original spectrum of acid 21 can be fully
restored simply by addition of HPF6.
The electrochemical behaviour of [2]rotaxanes 21 and 22
provides important insights in relation to the nature and
strength of the intercomponent interactions in these inter-
locked molecules. The cyclic voltammograms of axle compo-
nent 20, [2]rotaxanes 21 and [2]rotaxane salt 22 in acetoni-
trile containing tetraACHTRE(UGN n-butyl)ammonium perchlorate
(TBAP) are shown in Figure 7; the electrochemical peak po-
tential data are given in Table 2.
Neutral sulphonate [2]rotaxane salts 23 and 24 were pre-
pared (Scheme 7) by using a mixture of syn- and anti- iso-
mers of the crown ether 4. Each of the isomeric [2]rotaxane
salts is formed in 50% yield due to the presence of equal
amounts of the syn- and the anti- isomers of the salt 4 in the
mixture. Unexpectedly, unlike the starting 4, the [2]rotaxane
salts 23 and 24 can be separated by column chromatography.
Any protonation effects have not been noticed for isomeric
sulphonate [2]rotaxane salts 23 and 24. Consequently, the
application of sulphonate anionic crown ethers and sulpho-
nate [2]rotaxane salts is considered to be more promising in
a wider range of solvents and under a wider range of condi-
tions.
1
Detailed examination of the H NMR spectra of the two
diastereomeric [2]rotaxane salts 23 and 24 (Figure 8) reveals
Chem. Eur. J. 2008, 14, 1095 – 1106
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1103

K. Nikitin et al.
column chromatography is that the [2]rotaxane salt 23 is the
one which can be eluted with neat acetone while [2]rotaxane
salt 24 (its symmetrical configuration is analogous to a
meso-form) behaves as a very polar compound and cannot
be eluted with neat acetone. Apparently, anti-orientation of
the sulphonate groups in 23 allows a better electrostatic
shielding of the positively charged viologen moiety of 23. It
is believed that the [2]rotaxane salt 24, unlike 23, cannot
adopt a conformation in which both positive charges of the
viologen moiety are shielded by anionic groups and, conse-
quently, it behaves like a more-polar compound.
The electrochemical characterisation of the two isomeric
[2]rotaxane salts 23 and 24 provided additional insights into
the nature and strength of the intercomponent interactions
in these interlocked systems. The cyclic voltammograms of
axle dumbbell-shaped component 20 and the two isomeric
[2]rotaxane salts 23 and 24 in acetonitrile containing TBAP
appear as Figure 9. The respective electrochemical peak po-
Scheme 7. Synthesis of [2]rotaxane salts 23 and 24. Reaction conditions:
i) room temperature, benzonitrile, 6 d.
Figure 8. Partial 1H NMR spectra of the [2]rotaxane salts 23 and 24 in
acetone.
that only in one of them, the N-benzylic methylene protons
Ha and Hb are diastereotopic due to cycloenantiomerism
(proton–proton coupling constant JH-H =14 Hz). This is in
full agreement with the structure of the anti-isomer 23
(Scheme 7) in which the protons Ha and Hb are non-equiva-
Figure 9. Cyclic voltammograms of 20, 23 and 24 (each 2.0”
10À3 moldmÀ3) recorded at 100 mVsÀ1 in acetonitrile containing TBAP
(0.010 moldmÀ3) at 258C.
1
lent.[13] As the H NMR spectrum of the [2]rotaxane salt 24
demonstrates no non-equivalence of the benzylic proton res-
onances, it is concluded that in 24 the sulphonate groups are
syn- to each other.
The different symmetry characteristics of the two isomeric
[2]rotaxane salts 23 and 24 control their chemical behaviour.
For example, the reason that 23 and 24 can be separated by
tentials are given in Table 2. The key observation is that
both [2]rotaxane salts 23 and 24 demonstrate large negative
shifts of the first and second reduction processes (Table 2).
This observation is in full agreement with strong electrostat-
ic intercomponent interaction in [2]ionorotaxanes. As dis-
cussed above, the less-polar anti-isomer 23 has a more effec-
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Chem. Eur. J. 2008, 14, 1095 – 1106

[2]Pseudorotaxanes and [2]Rotaxanes
FULL PAPER
tive shielding of the positively charged viologen dication.
Consequently, the first electrochemical reduction peak of
the viologen moiety in this rotaxane is shifted by À303 mV.
In the more-polar 24, the same viologen dication is shielded
by the negatively charged syn-isomer of 4 to a lesser extent.
Consequently, the first electrochemical reduction peak in
the [2]rotaxane salt 24 is shifted by only À218 mV.
4) [2]Rotaxane salts with anti-orientation of the functional
groups at the crown ether moiety are cycloenantiomeric
unlike their individual interlocked components. It was
found that electrochemical behaviour and polarity of the
diastereomeric [2]rotaxane salts depend significantly on
the orientation of the functional groups.
Apart from [2]pseudorotaxane salts (vide supra), in which
the first oxidation potentials were not shifted due to dissoci-
ation of the reduced species (Scheme 5), the first oxidation
potential shifts were observed for both [2]rotaxane salts 23
and 24 (Table 2). As the charged crown ether is secured at
the viologen moiety by the bulky stopper groups, the disso-
ciation of the reduced [2]rotaxane salt is precluded. There-
fore, the viologen moiety of an [2]rotaxane salt (for exam-
ple, 23) experiences the electrostatic effect of the negatively
charged ring component even in the fully reduced state,
232À; consequently, even the first oxidation potential of this
viologen is negatively shifted. A similar, although weaker,
effect has been previously observed for a common [2]ro-
taxane containing the neutral crown ether 1.[5d]
The use of controlled intramolecular electrostatic interac-
tions already helps to narrow the gap between actual and
desirable parameters of functional molecular entities.[1,6] Al-
though the synthesis of interlocked molecular systems has
become a routine among modern chemists, little has been
done to redesign the existing [2]rotaxanes and modify them
with ionic groups to fully realise their development potential
for molecular electronics and nanoscience. The findings of
the present study of prototype rotaxane salts indicates that
the enhanced stability and electrochemical behaviour of
these systems can be useful for a range of exciting potential
applications.
It can be concluded that novel [2]rotaxane salts represent
an important new class of artificial molecular systems in
which the components are not only mechanically interlocked
but are also strongly electrostatically attached. Intramolecu-
lar electrostatic interactions in [2]rotaxane salts can be con-
trolled by applying external electrical or chemical stimuli.
This opens possibilities for the design of controlled reversi-
ble molecular switching systems driven by electrostatic at-
traction forces.
Experimental Section
Details on the experimental procedures, calculations, measurements and
characterization of the compounds are given in the Supporting Informa-
tion, parts 1–7. CCDC-633834 and -633836 contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif
Acknowledgement
Conclusion
This research was funded by the Science Foundation Ireland. Authors
thank Prof. R.A. More OꢁFerrall for valuable discussions.
Electrically neutral [2]pseudorotaxane and [2]rotaxane salts
have been prepared and characterised for the first time. In
these systems, doubly positively charged viologen cations
thread doubly negatively charged crown ethers. Due to the
strong electrostatic attraction between the host and guest
components there are several aspects in which the behaviour
of [2]pseudorotaxane salts and [2]rotaxane salts differs from
previously known [2]rotaxane systems:
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[14] The synthesis of the [2]rotaxane salt 22 should be feasible by a
direct assembly of the monocation precursor 18, the anionic crown
ether 3 and the stopper component 19. However, attempts to pre-
pare 22 in this way were unsuccessful, due to fast competitive alky-
lation of the carboxylate anionic groups of 3 with the stopper com-
ponent 19.
Received: March 8, 2007
Revised: October 16, 2007
Published online: December 4, 2007
1106
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