Synthesis and Characterization of Constitutionally Isomeric Oriented Calix[6]arene-Based Rotaxanes

Article abstract of DOI:10.1002/ejoc.201501522

Oriented rotaxanes composed of tris(N-phenylureido)calix[6]arene wheel 1 and N,N′-dialkyl viologen-based axles were synthesized in which the span between the diphenylacetyl stoppers at the wheel upper and lower rim and the bis-pyridinium cation portion of

Full text of DOI:10.1002/ejoc.201501522

DOI: 10.1002/ejoc.201501522
Full Paper
Molecular Devices
Synthesis and Characterization of Constitutionally Isomeric
Oriented Calix[6]arene-Based Rotaxanes
Valeria Zanichelli,[a] Giulio Ragazzon,[b] Arturo Arduini,[a] Alberto Credi,[b,c] Paola Franchi,[b]
Guido Orlandini,[a] Margherita Venturi,[b,c] Marco Lucarini*[b] Andrea Secchi*[a] and
Serena Silvi*[b,c]
Abstract: Oriented rotaxanes composed of tris(N-phenyl- upper or lower rim of the wheel. The synthetic strategy adopted
ureido)calix[6]arene wheel 1 and N,N′-dialkyl viologen-based involves the upper rim threading of axles 2a–c to yield oriented
axles were synthesized in which the span between the diphen- pseudorotaxanes, which were then stoppered in situ. The struc-
ylacetyl stoppers at the wheel upper and lower rim and the bis- ture of the oriented rotaxanes was unravelled by detailed NMR
pyridinium cation portion of the axial component is different in experiments, and their spectroscopic, electrochemical and dy-
length. A sequential synthetic pathway was followed to achieve namic properties were investigated by UV/Vis, cyclic voltamme-
the selective formation of orientational isomers, wherein the try, and EPR measurements. The results were compared with
short (or long) chain of the axle is positioned either on the those of their symmetric counterparts.
Introduction
movement of the components and (2) establish the relative di-
rection of motion.
One of the challenges that scientists have to face in the field of
artificial molecular machines is the exploitation of movement
at the molecular level to carry out some specific task.[1] The first
requisite that needs to be fulfilled for this purpose is direction-
ality of motion; that is, the components of the machine must
move in a defined direction one with respect to the other.[2]
Beside the detailed design of the molecular components, a key
requirement for the accomplishment of this objective is the
possibility to monitor the direction of motion. Recently,[3] the
combined use of spin labelling and EPR has proven to be an
informative technique for the structural characterization of
rotaxane-based systems. In particular, the relative distance be-
tween molecular components can be evaluated in suitably la-
belled systems; moreover, EPR is complementary to NMR when
paramagnetic species are involved.
The investigated rotaxanes are composed of a calix[6]arene
wheel and a series of axles of different length comprising a
bipyridinium core. Three main features of these rotaxanes
(Scheme 1) make them suitable for this investigation. First, both
the axle and the wheel components are nonsymmetric: the
calixarene presents two different rims, the upper one, function-
alized with three phenyl urea moieties, and a lower one, func-
tionalized with long alkyl chains. The axles are characterized by
a bipyridinium core with alkyl substituents of different lengths.
Secondly, it is possible to obtain orientational isomers, wherein
the short (or long) chain of the axle is positioned either on the
upper or lower rim of the calixarene. Third, the bipyridinium
core can be reduced to the monocationic radical, which is an
EPR-active species.
We know from previous studies[4] that the calix[6]arene and
the bipyridinium axles interact strongly, mainly by virtue of
charge transfer and solvophobic effects. The direction of the
axle threading into the wheel in homologous pseudorotaxane
systems can be biased by using an appropriate solvent, thus
allowing the preparation of orientational isomers.[5] The interac-
tion between the components can be weakened upon reduc-
tion of the bipyridinium core, causing for example, the de-
threading of pseudorotaxane systems.[4] A number of questions
arise: Is this stimulus effective to cause a shuttling of the com-
ponents also in an interlocked system, in the absence of a sec-
ondary station for the calixarene? Is there a preferential direc-
tion of motion? We envisaged that it may be possible to ad-
dress these issues by combining spectroelectrochemical and
EPR techniques. The investigations on the prepared rotaxanes
should indeed reveal whether (1) the different length of the
axles affects their stimuli-triggered rearrangement, and (2) the
With this in mind, we planned to investigate a series of non-
symmetric rotaxanes with the aim of investigating whether, on
applying an appropriate stimulus, it is possible to (1) induce
[a] Dipartmento di Chimica, Università di Parma,
Parco Area delle Scienze 17/A, 43124 Parma, Italy
E-mail: andrea.secchi@unipr.it
[b] Dipartimento di Chimica “G. Ciamician”, Università di Bologna,
Via Selmi 2, 40126 Bologna, Italy
E-mail: serena.silvi@unibo.it
http://www.chimica.unibo.it/it/ricerca/aree-di-ricerca/
fotochimica-e-chimica-supramolecolare
[c] Interuniversity Center for the Chemical Conversion of Solar Energy,
Bologna Unit,
Via Selmi 2, 40126 Bologna, Italy
Supporting information and ORCID(s) from the author(s) for this article
are available on the WWW under http://dx.doi.org/10.1002/
ejoc.201501522.
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The diphenylacetic group was selected as stopper for both
the upper and the lower ends of the axles of the pseudorotax-
anes. To ensure that a clear difference between the two isomers
of each rotaxane could be detected, it was necessary to con-
struct axles bearing two alkyl chains with an appropriate differ-
ence in length. On this basis, we opted for isomers with one
propyl and one dodecyl chain and with one hexyl and one
hexadecyl chain. The monostoppered axles 2a–c were synthe-
sised according to Scheme 2. Tosylate 4a–c, obtained from the
corresponding diol 3a–c, was first reacted with diphenylacetyl
chloride to insert the first stopper. The resulting product 5a–c
was heated to reflux overnight with 4,4′-bipyridine in aceto-
nitrile to give salt 6a–c×TsO. Subsequent alkylation with the
OH-terminated tosylate 4b–d afforded the monostoppered axle
2a–c (see experimental for all characterization details).
Scheme 2. Reagents and conditions: (i) TsCl, NEt3, DMAP, CH2Cl2 room temp.,
3 h; (ii) Ph2CHCOCl, THF room temp., 12 h; (iii) 4,4′-bipyridine, CH3CN reflux,
12 h; (iv) CH3CN reflux, 7 d.
Scheme 1. Calix[6]arene-based “wheel” 1, bipyridinium-based “axles” 2a–
d×2TsO, and rotaxanes R(CxCy)×2TsO used in this study.
In the following discussion, pseudorotaxanes and rotaxanes
will be named P(CxCy) and R(CxCy), respectively. Subscripts x
and y indicate the length of the alkyl chains at the upper and
lower rim of the wheel, respectively.
relative motion of the components takes place with a preferen-
tial direction in the orientationally isomeric rotaxanes.
Threading Process and Stoppering
Results and Discussion
Selective preparation of the oriented assemblies from the tri-
phenylureido calix[6]arene-based wheel 1 and each nonsym-
metric axle 2a–c was possible because of the presence in the
Design
To evaluate the dynamic and spectroscopic properties of non- latter compounds of a bulky stopper group on the termini of
symmetric calix[6]arene-based oriented rotaxanes, we designed one of its alkyl chains and because of the inherent complexing
two sets of nonsymmetric axles that were characterised by properties of 1.[7] Studies performed in low polarity solvents on
three main features: (1) a 4,4′-bipyridinium core capable of driv- the insertion of different kinds of bipyridinium-based stoppered
ing the formation of pseudorotaxane complexes with wheel dumbbells provided evidence[8] for the selective threading of
1,[6,4] (2) an alkyl substituent of the bipyridinium core bearing OH-terminated axles from the upper rim of the wheel. Indeed,
a terminal bulky group that cannot slip through the calix[6]- formation of hydrogen bonds between the ureido groups and
arene rims; (3) another alkyl chain ending with an OH moiety. the anions of the guest favour the insertion of the stoppered
This latter head group neither hampers nor alters the threading axle from the upper rim, whereas the methoxy groups, oriented
process and it is useful for the attachment of a second stopper inward the cavity in the NMR timescale, hamper the access of
upon insertion of the monocapped dumbbell in the wheel cav- the dumbbell from the lower rim. For instance, in axle 2a×2TsO,
ity.
the diphenylacetic group prevents the threading of the shortest
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chain C3, and the univocal insertion of chain C12 from the up-
per rim of the wheel[9] leads to the exclusive formation of pseu-
dorotaxane P(C3C12). Starting from this information, we decided
to exploit the results described above to obtain two sets of
nonsymmetric oriented rotaxanes, the formation of which was
achieved by following the threading-stoppering strategy[7] sum-
marised in Scheme 3. The same protocol was followed for all
the rotaxanes. Wheel 1 was dissolved in toluene and equili-
brated for two hours at room temperature with one equivalent
of stoppered axle 2×2TsO. Upon formation of the pseudorotax-
ane complexes P(C3C12), P(C6C16), and P(C16C6), indicated by the
complete dissolution of the otherwise insoluble axle and by the
appearance of a typical dark-red color of the resulting homoge-
neous solution, a slight excess of triethylamine and diphenyl-
acetyl chloride was added. The mixture was then stirred at
room temperature overnight (see Scheme 3). After chromato-
graphic separation, rotaxanes R(C3C12), R(C6C16), and R(C16C6)
were isolated in 57, 43, and 40 % yield, respectively. The oppo-
site orientational isomer of R(C3C12), namely the rotaxane
R(C12C3) (see Scheme 1), was obtained in very poor yield and
was contaminated by several by-products that prevented its full
characterization.
Scheme 4. Reagents and conditions: (i) 4,4′-bipyridine, CH3CN reflux,7 d;
(ii) a. 1, toluene, room temp., 3 h; b. Ph2CHCOCl, NEt3, room temp., 12 h.
Each synthesised rotaxane was analysed by 1D and 2D NMR
techniques to gain information on the structure and the relative
orientation of the components (Table 1). Significantly, in all the
rotaxanes, the wheel component undergoes a conformational
rearrangement upon threading of the axle and its signals expe-
rience a shift that is consistent with the presence of a rotaxane-
type assembly.[9,11] The methoxy groups that are no longer ori-
ented inward towards the cavity resonate in the 3.7–4 ppm
range typical for these type of protons,[12] and the NH protons
of the ureido groups, involved in hydrogen bonds, show a re-
markable downfield shift in comparison with the spectrum of
free 1. In the proton spectra of R(C3C12), R(C6C16) and R(C16C6),
the methoxy groups of the wheel resonate as a sharp singlet
at δ = 3.85, 3.86 and 3.80 ppm, respectively. The sharp singlet
shape of this signal for each rotaxane is diagnostic for the pres-
ence of single orientational isomers. In fact, the chemical shift
of methoxy groups is remarkably affected by the proximity of
the lower stopper of the axle, and consequently by the length
of the lower alkyl chain. In the presence of a mixture of the two
possible orientational isomers, this signal would be split into
two singlets. The length of each alkyl chain, and therefore the
distance of each stopper from the rims of the calixarene cavity,
also affects the chemical shift of several protons of the dumb-
bell component, especially the methyne protons α and α′ and
the O-CH2 protons 1 and 1′.
Scheme 3. Reagents and conditions: (i) toluene, room temp.,
(ii) Ph2CHCOCl, NEt3, room temp., 12 h.
3 h;
The properties of the nonsymmetric rotaxanes R(C3C12),
R(C6C16) and R(C16C6) were compared with those of the corre-
sponding symmetrical systems R(C3C3), R(C6C6), R(C12C12) and
Comparison of these signals with those found in the 1H NMR
R(C16C16) (see Scheme 1). R(C3C3), R(C6C6) and R(C12C12) are spectra of the symmetrical rotaxanes R(C3C3), R(C6C6), R(C12C12)
known compounds,[10] whereas R(C16C16) was synthesised by and R(C16C16) provided evidence for the relative orientation of
equilibrating wheel 1 and axle 7c×2TsO in toluene at room tem- the components. In R(C3C12), α and α′ protons resonate at δ =
perature and adding two equivalents of triethylamine and di- 5.30 and 5.07 ppm, respectively, in agreement with the pres-
phenylacetyl chloride (Scheme 4).
ence of an upward C3 chain and a downward C12 chain. The
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Table 1. Chemical shifts (δ, ppm) of some of the most significant signals of
around 460 nm, arises from a charge transfer interaction be-
tween the aromatic rings of the calixarene and the bipyridinium
unit contained in the rotaxane axles. Both the energy and the
absorption coefficient of this band are independent of the
length of the alkyl chain, suggesting that the donor-acceptor
electronic coupling is the same for all the rotaxanes. On the
other hand, the absorption coefficient of the band in the UV
region depends on the length of the alkyl chain.
synthesized symmetric and nonsymmetric rotaxanes.[a]
Rotaxane O-CH3
α
α′
1
1′
3.6–3.8 4.06
3.7–4 4.74
#
§
R(C3C12
R(C3C3)
)
3.85
3.7–3.9 5.34
5.30
5.07
5.56
5.09
5.07
5.10
5.17
5.07
3.60
3.50
3.60
3.6–3.7
3.90
R(C12C12
R(C6C16
R(C16C6)
R(C6C6)
R(C16C16
)
3.90
3.86
3.80
3.84
3.86
5.09
5.13
5.06
5.16
5.07
4.1–4.2 4.1–4.2 3.75
)
4.03
4.02
4.08
4.03
4.03
4.33
4.38
4.03
3.70
3.70
3.60
3.7–3.8
3.70
3.70
)
3.7–3.8 3.7–3.8
[a] 1H NMR spectra were recorded in C6D6 at room temperature with a
300 MHz spectrometer.
absence of signals at δ = 5.56 ppm excluded the presence of
the opposite orientational isomer with a C3 lower chain. As
further evidence, the signal of O-CH2 protons (1) relating to the
upper chain of R(C3C12) was detected at 3.7 ppm, confirming
the presence of a C3 upper chain. As shown in the symmetrical
rotaxane R(C12C12), the same signal would be shifted to
4.2 ppm in the opposite orientational isomer with a C12 upper
chain. In the same way, the signal of 1′ protons located at δ =
4.06 ppm was consistent with a C12 lower chain. In case of a
C3 lower chain, this signal would be shifted to δ = 4.74 ppm.
Furthermore, the signals of # and § protons, as well as 2D NMR
correlations, confirmed the relative orientation of the compo-
nents [see comparison with symmetrical rotaxanes R(C3C3) and
R(C12C12) in Figure 1 and Table 1]. Similarly, the univocal orien-
tation of R(C6C16) and R(C16C6) was established (see Table 1).
Figure 2. Absorption spectra of the rotaxanes in acetonitrile: R(C12C12
(green); R(C3C3) (black) and R(C3C12) (cyan); R(C6C6) (red) and R(C6C16) (pink);
R(C16C16) (yellow, partially overlapped with the blue line) and R(C16C6) (blue).
)
These results would suggest that the extent of encapsulation
of the bipyridinium core in the calixarene cavity is similar for all
the rotaxanes, whereas the portion of alkyl chain that extrudes
from the rims depends on the length of the chain. In particular,
as the signal in the UV region is ascribed to the phenylureido
moieties, it is expected that the chain at the upper rim of the
calixarene cavity would affect their absorption. Apparently, the
absorption intensity is not directly related to the chain length;
nevertheless, it appears that rotaxanes with the same chain at
the upper rim show comparable absorption (Figure 2), thus sup-
porting our hypothesis.
It is well known that the reduction of the bipyridinium core
of the axle weakens the interaction between the components,
causing the decomplexation of parent pseudorotaxane sys-
tems.[4] In rotaxanes, the dethreading is prevented by the bulky
stoppers and, indeed, previous investigations[10] on rotaxanes
R(C3C3), R(C6C6) and R(C12C12) suggest that the charge-transfer
interaction is still present after monoreduction of the bi-
pyridinium, irrespective of the length of the alkyl chain and
of the polarity of the solvent. After a preliminary screening in
dichloromethane and acetonitrile, we restricted our investiga-
tion to the latter solvent, because electrochemical experiments
in dichloromethane proved to be less reproducible. Moreover,
beside the charge-transfer interaction, which appears to be un-
affected by the solvent, other interactions contribute to the
Figure 1. 1H NMR spectra (300 MHz, C6D6, 3–6 ppm region) of (a) R(C3C3);
(b) R(C3C12), and (c) R(C12C12). See drawing for labelling.
Spectroscopic and Electrochemical Investigation
In agreement with previous investigations,[10] all the rotaxanes overall stability of the complex, such as solvophobic effects and
exhibited two main absorption bands, irrespective of the sol- hydrogen bonds. These interactions are weaker in the more po-
vent (either dichloromethane or acetonitrile, Figure 2). The in- lar solvent, and we envisaged that this difference could be ex-
tense band in the UV region, below 300 nm, is ascribed to the ploited to maximize the destabilization due to the electrochem-
absorption of the phenylureido moieties and bipyridinium core, ical reduction, and cause some displacement of the compo-
whereas the weaker absorption band in the visible region, nents.
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Electrochemical investigations were performed by cyclic to the other rotaxanes for all the investigated scan rates. Such
voltammetry (CV) and differential pulse voltammetry (DPV) on a quasireversible character suggests that the heterogeneous
the rotaxanes and on 1,1′-dioctyl-4,4′-bipyridinium (or dioctyl- electron-transfer process is slowed down in R(C3C3), possibly
viologen, DOV), which was used as a model compound for the because of the constrained structure of this rotaxane and con-
bipyridinium unit of the axles. All the potential values are re- sequent effective encapsulation of the electroactive site.[14]
ported versus the standard calomel electrode (SCE). The model
compound shows two reversible monoelectronic reduction
processes at –0.42 and –0.86 V. In the presence of an equimolar
To gain more information on the consequences of electro-
chemical reduction, we performed spectroelectrochemical ex-
periments on the model compounds and on the rotaxanes. We
will restrict our discussion to the radical cation species, because
the second reduction process is affected by reproducibility is-
sues. The absorption spectra of the radical cation of DOV, both
in the absence and in the presence of calix[6]arene, match the
typical absorption spectrum of the bipyridinium radical cation
in acetonitrile (Figure 3).[15] Two structured and intense absorp-
tion bands are present with λmax = 398 and 602 nm. The energy,
shape and intensity of the bands are not affected by the pres-
ence of the calixarene, thus confirming the conclusion that the
monoreduced bipyridinium axle dethreads from the wheel. The
amount of calix[6]arene, a pseudorotaxane is formed with an
–1
association constant around 5 × 104
M
in CH3CN,[13] therefore
at the concentration of the electrochemical experiments, 25 %
of axles are not associated with the wheel. The first reduction
process shows two cathodic peaks, related to the free and com-
plexed axle, and one anodic peak, at the potential of the free
axle. This behaviour suggests that the radical cation is not asso-
ciated with the calix[6]arene. Indeed, the second reduction
process takes place at –0.86 V, confirming that the pseudorotax-
ane dissociates after the first reduction.
On the other hand, all the rotaxanes show two reversible spectra of the monoreduced rotaxanes are characterized by the
or quasireversible monoelectronic reduction processes at more same absorption features, but with some significant differences
negative potential values with respect to the free bipyridinium (Figure 3). The band at higher energy is less structured, whereas
derivative (see Table 2). The reversibility of the electrochemical the band at lower energy is ca. 10 nm bathochromically shifted,
waves suggests that either no conformational rearrangement with no substantial difference among the rotaxanes. These ob-
takes place, or the rearrangement is fast on the timescale of servations suggest that the bipyridinium radical cation in the
the electrochemical experiment. The shift toward negative rotaxanes experiences a different environment in comparison
potential values of the second reduction potential shows that with the free axle, regardless of the length of the alkyl chains
the reduced forms of the bipyridinium still interact with the and their position with respect to the wheel. In other words,
calix[6]arene. It should be noted that the presence of a residual our results seem to indicate that, after monoreduction of the
interaction does not exclude the possibility that minor rear- bipyridinium unit, the wheel is still located relatively close to it.
rangements can occur. An inspection of the molecular models
reveals that R(C3C3), bearing two short alkyl side chains on the
axle, is a rather constrained structure in which large-scale rela-
tive movements of the wheel and axle are prevented. On the
other hand, in the other rotaxanes, characterized by a larger
distance between the bipyridinium unit and the stopper(s), a
translation of the wheel along the axle could take place. Given
that the calixarene is nonsymmetric, the movement of the
wheel towards one side of the axle is chemically not equivalent
to that towards the other side, possibly leading to a preferential
direction of motion and generation of specific translational iso-
mers.
Table 2. Halfwave potential values of the investigated compounds, referred
to the SCE (CH3CN, room temperature).
Figure 3. Absorption spectra of the radical cation species of 1,1′-dioctyl-4,4′-
bipyridinium DOV (black) and R(C6C6) (gray). The spectra are normalized at
the maximum of the lower energy band.
E1 (V)
E2 (V)
DOV
–0.42[a]
–0.64[b]
–0.62[a]
–0.62[a]
–0.62
–0.60
–0.62
–0.61
–0.86[a]
–1.15[b]
–1.14[a]
–1.18[a]
–1.24[b]
–1.16
R(C3C3)
R(C6C6)
R(C12C12
R(C16C16
R(C3C12
R(C6C16
)
)
EPR Spectra
)
)
Further evidence for the large effect of rotaxanation on the
properties of the bipyridinium radical cation was obtained by
EPR spectroscopy. Bipyridinium radical cations of free DOV and
rotaxanes were generated inside the EPR cavity by electrochem-
ical reduction in situ in deoxygenated acetonitrile at room tem-
–1.22[b]
–1.11
R(C16C6)
[a] From ref.[10] [b] Poorly reversible process; value determined from DPV
peak.
From a detailed investigation of the voltammograms, it can perature (see Exp. Sect.). For comparison, we initially recorded
be noted that the anodic to cathodic peak separation of the the EPR spectrum of the radical cation obtained by in situ elec-
first reduction process in the CV of R(C3C3) is larger with respect trochemical reduction of DOV (see Figure 4, a).
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symmetric distribution of the spin density in the two hetero-
cyclic rings.
The spectral patterns were subsequently simulated as de-
scribed above; the fitting parameters are summarized in Table 3.
In each case, the g-factor was found to be close to 2.0031.
Comparing these data shows that rotaxanation has a modest
effect on the spin density at the two aryl CH sites. More signifi-
cant effects are observed, however, at the two CH2 and nitrogen
sites, at which it appears that the inductive effect of the non-
symmetric calix[6]arene wheel serves to transfer spin density
from one ring to the second. It is not possible to exactly deter-
mine which of the two rings shows a decrease in the value of
hyperfine splitting constants. However, we can tentatively iden-
tify the portion unit of the radical cation ring that experiences
an increase of the positive charge – and thus a decrease of the
spin density – with the pyridinium ring closer to the aromatic
electron rich part of the calix[6]arene. The EPR spectra of non-
symmetric rotaxanes R(C3C12) and R(C6C16) were also recorded
(as an example, see Figure 4, c). Also in this case, the spectra
could be reproduced by assuming different spin density distri-
bution in the two heterocyclic rings (see Table 3). The EPR be-
haviour of the rotaxane R(C16C6) – exhibiting an inverted rela-
tive orientation of the two components with respect to R(C6C16)
– is particularly interesting. The spectroscopic parameters ob-
tained from the corresponding theoretical simulation were very
similar to those measured in the radical cation derived from the
orientational isomer R(C6C16) (Table 3). This observation implies
an inversion of the spin density distribution in the two hetero-
cyclic rings of R(C16C6) compared with rotaxane R(C6C16).
Figure 4. EPR spectra (black) of the radical cation derived from DOV (a),
R(C16C16) (b) and R(C3C12) (c). The corresponding theoretical simulations are
plotted in red.
Table 3. EPR hyperfine splitting constants (a, in Gauss) of radical cations ob-
tained after electrochemical reduction of the bipyridinium unit at room temp.
in CH3CN.
aN
aN
aCH2 aCH2 a(Ar)2Hꢀ a(Ar)2Hꢀ a(Ar)2Hα a(Ar)2Hα
DOV
4.11
4.34
4.34
4.41
4.36
4.37
4.42
4.11 4.07 4.07 1.59
3.85 4.02 2.76 1.83
4.04 4.02 2.72 1.77
3.92 4.05 2.73 1.83
4.02 4.02 2.72 1.80
3.82 3.93 2.70 1.99
3.90 3.96 2.69 1.80
4.00 4.04 2.66 1.83
1.59
1.83
1.74
1.77
1.76
1.81
1.80
1.75
1.10
1.19
1.10
1.10
1.10
1.03
1.14
1.18
1.10
1.19
1.10
1.06
1.10
0.96
1.14
1.09
In agreement with prior studies[16] on the monocation radi-
cal derived from the parent 4,4′-dimethylbipyridinium, the EPR
spectrum can be simulated[17] (see red line in Figure 4, a) on
the basis of the coupling of the unpaired electron with two
equivalent N atoms, with a hyperfine coupling constant of
4.11 G, and 12 H atoms. The latter can be divided into four
protons on the methylene groups of the two chains (aCH2
4.07 G) and two equivalent sets, each of four protons [a(Ar)Hα
and a(Ar)Hꢀ] on the aromatic nucleus. On the basis of earlier
works,[16] the smaller hyperfine coupling constant for these H
atoms can be assigned to the aromatic α-protons (1.10 G)
whereas the larger coupling can be assigned to the aromatic ꢀ-
protons (1.59 G). The g factor was found to be 2.0031.
EPR spectra for the radical cations derived from bipyridinium
units trapped in the symmetric rotaxanes in deoxygenated
acetonitrile were then recorded. With all the symmetric rotax-
anes, a significant change in the shape of the EPR spectrum
was clearly observed; as an example, the EPR spectrum ob-
tained after electrochemical reduction of the symmetric rotax-
ane R(C16C16) is reported in Figure 4 (b). In particular, the spec-
R(C3C3)
R(C6C6)
R(C12C12
R(C16C16
R(C3C12
R(C6C16
)
)
)
)
R(C16C6) 4.32
In conclusion, similar EPR results were obtained for all the
investigated rotaxanes, suggesting that the reduced forms of
the bipyridinium still interact strongly with the calix[6]arene,
irrespective of the length and position of the alkyl chains on
the axle. These results, together with the electrochemical data,
show that, in mechanically interlocked systems, the addition of
one electron on the bipyridinium recognition site does not in-
duce the displacement of the wheel away from it.
Conclusions
tra centres no longer correspond to a line as instead observed We have designed and synthesized a series of rotaxanes con-
with the radical cation of the free model compound DOV. This taining bipyridinium-based axles and a calix[6]arene wheel.
spectral feature can be reproduced only by considering a non- These rotaxanes are characterized by two nonsymmetric com-
Eur. J. Org. Chem. 2016, 1033–1042
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a pulse height of 75 mV, and a duration of 40 ms. For reversible
processes, the same halfwave potential values were obtained from
the DPV peaks and from an average of the cathodic and anodic
cyclic voltammetric peaks. The potential values for not fully reversi-
ble processes were estimated from the DPV peaks. The experimen-
tal error on the potential values for reversible and not fully reversi-
ble processes was estimated to be 10 and 20 mV, respectively.
Spectroelectrochemical experiments were performed using the
same experimental conditions (solvent, sample concentration, sup-
porting electrolyte and reference) as for cyclic and pulsed voltam-
ponents, the relative position of which can be controlled during
the synthesis and assigned by NMR spectroscopy. The system
was designed with the aim of investigating the relative direc-
tion of motion of the components under an appropriate stimu-
lus. The reduction of the bipyridinium unit in the axle compo-
nent should (1) reduce its interaction with the wheel (thus es-
tablishing the premises for the motion of the components) and
(2) give rise to an EPR-active species. Similar to NMR, EPR should
help in assigning the relative position of the components after
reduction, thus giving information on their relative direction of metry experiments. Prolonged and vigorous argon purging of the
solution was performed to minimize the oxygen content in the in-
vestigated solution. Experiments were performed in a custom built
optically transparent thin-layer electrochemical (OTTLE) cell, having
Pt minigrids (ca. 0.3 cm2) as the working and counter electrodes,
and a Ag wire as a pseudo-reference electrode, all melt-sealed into
a polyethylene spacer. The thickness of the layer, determined by
spectrophotometry, was ca. 180 μm. The electrolysis was controlled
by means of the above described Autolab 30 instrument, and the
absorption spectra were recorded with an Agilent Technologies
8543 diode array spectrophotometer. The electrolysis times were
determined on the basis of the spectral changes observed; that is,
the electrolysis was stopped when no further spectral variations
occurred.
motion. Moreover, electrochemical and spectroelectrochemical
techniques can be combined with EPR studies.
Our results show that, in contrast to the behaviour of the
corresponding pseudorotaxane system, the monoelectronic re-
duction of the bipyridinium recognition site does not cause the
shuttling of the wheel. It indeed seems that, rather than resid-
ing on an alkyl chain, the calixarene tends to stay on the bi-
pyridinium radical cation. Nevertheless, the combined use of
EPR and spectroelectrochemistry enabled us to gain detailed
information on the effect of the electrochemical stimulus, be-
cause both techniques can clearly distinguish between a free
and a complexed bipyridinium radical. These results inspire us
to improve the design of the rotaxanes, for example by insert-
ing secondary recognition sites in the axle that can offer a de-
gree of stabilization to the wheel when the bipyridinium core
has been reduced.
UV/Vis Spectroscopy: Absorption spectra were recorded with Var-
ian Cary 50Bio, Agilent Technologies Cary 300 and Perkin–Elmer
Lambda45 spectrophotometers, on air-equilibrated acetonitrile so-
lutions at room temperature (ca. 20 °C), with concentrations ranging
from 1 × 10–5 to 5 × 10–4
M. Solutions were examined in 1-cm spec-
trofluorimetric quartz cells. The experimental error on the wave-
length values was estimated to be 1 nm.
EPR Measurements: EPR spectra were recorded at room tempera-
ture using an ELEXYS E500 spectrometer equipped with a NMR
gaussmeter for the calibration of the magnetic field and a fre-
quency counter for the determination of g-factors that were cor-
rected against that of the perylene radical cation in concentrated
sulfuric acid (g = 2.002583). The electrochemical cell was home-
made and consisted of an EPR flat cell (Wilmad WG-810) equipped
with a 25 × 5 × 0.2 mm platinum gauze (cathode), and a platinum
wire (anode).[19] The current was supplied and controlled by an
AMEL 2051 general-purpose potentiostat. In a typical experiment,
the cell was filled with an acetonitrile solution of the appropriate
Experimental Section
General: Toluene, THF and dichloromethane were dried by follow-
ing standard procedures, other reagents were of reagent grade
quality, obtained from commercial sources and used without fur-
ther purification. Chemical shifts are expressed in ppm using the
residual solvent signal as internal reference. Mass spectra were de-
termined in ESI mode. Compounds 1, 4–6a, 4–6b, 4d, R(C3C3),
R(C6C6) and R(C12C12) were synthesised according to reported pro-
cedures.[7,8,10,18]
Electrochemical Experiments: Cyclic voltammetric (CV) experi-
ments were carried out at room temperature in argon-purged
acetonitrile (Romil-high dry or Merck-Uvasol) with an Autolab 30
multipurpose instrument interfaced to a PC. The working electrode
was a glassy carbon electrode (Amel; 0.07 cm2); its surface was rou-
tinely polished with a 0.3 μm alumina–water slurry on a felt surface.
The counter electrode was a Pt wire, separated from the solution
by a frit; a Ag wire was employed as a quasi-reference electrode,
and ferrocene (Fc) was present as an internal standard [E1/2(Fc+/
Fc) = +0.395 vs. SCE]. Ferrocene was added from a concentrated
substrate (ca. 1 m
M) containing tetrabutylammonium perchlorate
(ca. 0.1 ) as supporting electrolyte. After thoroughly purging the
M
solution with N2, spectra were recorded at different potential set-
tings in the range 0 to –1.0 V. An iterative least-squares fitting pro-
cedure based on the systematic application of the Monte Carlo
method was performed to obtain the experimental spectral param-
eters of the radical species.[20]
16-Hydroxyhexadecyl 4-Methylbenzenesulfonate (4c): A solu-
tion of tosyl chloride (0.23 g, 1.2 mmol) in anhydrous dichlorometh-
ane (30 mL) was slowly added to a solution of 1,16-hexadecanediol
(0.44 g, 1.7 mmol) in 50 mL of a 1:1 mixture of anhydrous dichloro-
methane and THF. Triethylamine (0.26 g, 2.55 mmol) and a catalytic
amount of DMAP were added. After stirring at room temperature
acetonitrile solution (typically 0.1
pounds examined was in the range 2 × 10–4 to 4 × 10–4
ammonium hexafluorophosphate 0.04 was added as supporting
electrolyte. Cyclic voltammograms were obtained at sweep rates
M
). The concentration of the com-
M; tetraetyl-
M
varying typically from 0.02 to 1 V s–1. The IR compensation imple- for 5 h, the reaction was quenched with water (50 mL) and the
mented within the Autolab 30 was used, and every effort was made organic phase was separated, dried with anhydrous CaCl2 and fil-
throughout the experiments to minimize the resistance of the solu- tered. The solvent was removed under reduced pressure and the
tion. In any instance, the full electrochemical reversibility of the
voltammetric wave of ferrocene was taken as an indicator of the
absence of uncompensated resistance effects. Differential pulse vol-
residue was purified by column chromatography (n-hexane/ethyl
acetate, 7:3) to afford 4c (0.26 g, 52 %) as a white solid and unre-
acted diol (0.2 g), m.p. 43–46 °C. 1H NMR (400 MHz, CDCl3): δ = 1.2–
1.4 (m, 24 H, aliphatic CH2), 1.4–1.7 (m, 4 H, HO-CH2CH2), 2.47 (s, 3
tammograms (DPV) were performed with a scan rate of 20 mV s–1
,
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3
3
H, Ar-CH3), 3.66 [t, JH,H = 8 Hz, 2 H, HO-CH2], 4.04 [t, JH,H = 8 Hz, collected by Buchner filtration and purified by further precipitation
2 H, Ar-O-CH2], 7.37 [d, JH,H = 8 Hz, 2 H, ArH], 7.81 [d, 3JH,H = 8 Hz, in acetonitrile at room temperature, to give axle 2b×2TsO (38 %) as
3
2 H, ArH] ppm. 13C NMR (100 MHz, CDCl3): δ = 21.5, 25.2, 25.8, 28.7, a pale-yellow solid, m.p. 122–124 °C. 1H NMR (300 MHz, MeOD): δ =
28.8, 29.3, 29.4, 29.5, 29.6 (2 res.), 32.7, 62.6, 70.7, 127.8, 129.8, 133.1, 1.2–1.5 (m, 28 H, aliphatic CH2), 1.5–1.6 (m, 2 H, HOCH2CH2), 1.6–
144.6 ppm. ESI-MS(+): m/z (%) = 435 (90) [M + Na+], 451 (100) [M
1.7 [m, 2 H, (C=O)OCH2CH2], 2.0–2.1 (m, 2 H, N+-CH2CH2), 2.38 (s, 3
H, Ar-CH3), 3.55 [t, 3JH,H = 6.6 Hz, 2 H, HOCH2], 4.17 [t, 3JH,H = 6.6 Hz,
2 H, (C=O)OCH2], 4.6–4.8 (m, 4 H, N+-CH2), 5.09 [s, 1 H, (Ph)2CH],
+ K+].
16-(Tosyloxy)hexadecyl 2,2-Diphenylacetate (5c): To a solution
of 4c (0.57 g, 1.4 mmol) in anhydrous THF (100 mL), diphenylacetyl
chloride (0.43 g, 1.85 mmol) was added. After stirring at room tem-
perature for one day, the reaction was quenched with water (50 mL)
and extracted with ethyl acetate (2 × 100 mL). The separated or-
ganic phase was dried with Na2SO4, evaporated under reduced
pressure and the residue was purified by column chromatography
(n-hexane/ethyl acetate, 8:2) to afford 5c (0.75 g, 88 %) as a white
solid, m.p. 58–60 °C. H NMR (400 MHz, CDCl3): δ = 1.2–1.3 (m, 24
H, aliphatic CH2), 1.6–1.7 [m, 4 H, Ts-CH2CH2 + (C=O)OCH2CH2], 2.47
(s, 3 H, Ar-CH3), 4.04 [t, JH,H = 6.6 Hz, 2 H, Ts-CH2], 4.17 [t, JH,H
6.6 Hz, 2 H, (C=O)OCH2], 5.04 [s, 1 H, (Ph)2CH], 7.2–7.4 (m, 12 H,
ArH), 7.82 [d, 3JH,H = 8 Hz, 2 H, ArH] ppm. 13C NMR (100 MHz, CDCl3):
δ = 21.6, 25.3, 25.8, 28.5, 28.8, 28.9, 29.2, 29.4, 29.5 (3 res.), 29.6 (2
res.), 29.7, 57.2, 65.3, 70.7, 127.2, 127.9, 128.5, 128.6, 129.8 (2 res.),
138.8, 144.6, 172.5 ppm. ESI-MS(+): m/z (%) = 629 (100) [M + Na+],
645 (90) [M + K+].
3
7.2–7.4 (m, 14 H, ArH), 7.69 [d, JH,H = 8.1 Hz, 4 H, ArH], 8.65 [d,
3JH,H = 6.6 Hz, 4 H, ArH], 9.24 (m, 4 H, ArH) ppm. 13C NMR (100 MHz,
MeOD): δ = 20.0, 24.8, 25.2, 25.6, 25.9, 27.9, 28.8, 29.1, 29.2, 29.3,
29.4 (2 res.), 30.9, 31.2, 32.3, 56.9, 61.6, 61.7, 61.9, 64.5, 125.6, 126.9,
128.2, 128.3, 128.5, 138.9, 140.3, 142.3, 145.6, 149.8, 149.9,
172.9 ppm. ESI-MS(+): m/z (%) = 864 (100), 865 (60) [M2+ + Ts].
1-[16-(2,2-Diphenylacetoxy)hexadecyl]-1′-(6-hydroxyhexyl)-
[4,4′-bipyridine]-1,1′-diium Ditosylate (2c×2TsO): The solid was
collected by Buchner filtration and purified by further precipitation
in acetonitrile at room temperature, to give axle 2c×2TsO (42 %) as
1
3
3
=
1
a sticky white solid. H NMR (400 MHz, MeOD): δ = 1.2–1.5 (m, 30
H, aliphatic CH2), 1.5–1.7 [m, 4 H, HOCH2CH2 + (C=O)OCH2CH2], 2.0–
2.1 (m, 4 H, N+-CH2CH2), 2.37 (s, 3 H, Ar-CH3), 3.33 (m, 2 H, HOCH2),
3
4.16 [t, JH,H = 6.6 Hz, 2 H, (C=O)OCH2], 4.6–4.7 (m, 4 H, N+-CH2),
3
5.09 [s, 1 H, (Ph)2CH], 7.2–7.4 (m, 14 H, ArH), 7.70 [d, JH,H = 8.1 Hz,
3
4 H, ArH], 8.63 [d, JH,H = 6.6 Hz, 4 H, ArH], 9.22 (m, 4 H, ArH) ppm.
13C NMR (100 MHz, MeOD): δ = 20.0, 25.0, 25.4, 25.5, 25.6, 25.8,
28.2, 28.8, 29.1, 29.2, 29.3 (3 res.), 29.4, 31.2, 31.8, 48.1, 57.0, 61.2,
64.9, 100.0, 125.6, 127.9 (2 res.), 128.2, 128.3, 128.5, 138.9, 140.3,
145.6, 172.9 ppm. ESI-MS(+): m/z = 692 (100), 693 (50) [M2+ – H].
1-[16-(2,2-Diphenylacetoxy)hexadecyl]-[4,4′-bipyridin]-1-ium
Tosylate (6c): Compound 5c (0.75 g, 1.23 mmol) and 4,4′-bipyridine
(0.58 g, 3.7 mmol) were dissolved in anhydrous acetonitrile (50 mL)
and heated to reflux overnight. The solvent was then removed un-
der reduced pressure to afford a crude solid residue that was tri
turated with ethyl acetate (4 × 25 mL) to give 6c (0.47 g, 50 %) as
General Procedure for the Synthesis of Nonsymmetric Rotax-
anes R(C3C12), R(C6C16), R(C16C6): Axle 2a–c×2TsO (0.065 equiv.)
was suspended in a solution of 1 (0.065 equiv.) in anhydrous tolu-
ene (20 mL). The suspension was stirred at room temperature over-
night until the solution turned dark-red and the axle was com-
pletely dissolved. Triethylamine (0.13 equiv.) and diphenylacetyl
chloride (0.13 equiv.) were then added and the solution was stirred
at room temperature for one day. After removing the solvent under
reduced pressure, the residue was taken up with dichloromethane
and the organic phase was washed with water. The organic phase
was separated and evaporated under reduced pressure, and the
crude product was purified by column chromatography (dichloro-
methane/methanol, 50:1). The isolated rotaxane was redissolved in
1
a sticky white solid. H NMR (400 MHz, MeOD): δ = 1.2–1.4 (m, 22
H, aliphatic CH2), 1.4–1.5 [m, 2 H, (C=O)OCH2CH2CH2], 1.61 (m, 2 H,
N+-CH2CH2), 2.08 [m, 2 H, (C=O)OCH2CH2], 2.38 (s, 3 H, Ar-CH3), 4.17
[t, JH,H = 6.4 Hz, 2 H, (C=O)OCH2], 4.68 [t, JH,H = 7.6 Hz, 2 H, N+-
3
3
3
CH2], 5.09 [s, 1 H, (Ph)2CH], 7.2–7.4 (m, 12 H, ArH), 7.72 [d, JH,H
=
8 Hz, 2 H, ArH], 7.99 [d, 3JH,H = 8 Hz, 2 H, ArH], 8.52 [d, 3JH,H = 6.8 Hz,
3
3
2 H, ArH], 8.84 [d, JH,H = 8 Hz, 2 H, ArH], 9.12 [d, JH,H = 8 Hz, 2 H,
ArH] ppm. 13C NMR (151 MHz, MeOD): δ = 25.4, 25.7, 28.2, 28.7,
29.1, 29.3, 31.0, 56.7, 61.3, 64.8, 122.0, 122.1, 125.4, 125.7, 126.8,
128.2, 128.3, 138.9, 142.2, 145.0, 145.1, 150.3, 150.4, 153.7,
172.9 ppm. ESI-MS(+): m/z (%) = 590 (100) [M+ – H + H+].
dichloromethane (20 mL) and washed with 0.1
M silver p-toluene-
General Procedure for the Synthesis of Nonsymmetric Axles 2a–
c×2TsO: Salt 6a–c (0.34 equiv.) and 16-hydroxydodecyl tosylate 4b–
d (1 equiv.) were dissolved in anhydrous acetonitrile (20 mL) and
heated at 100 °C for 7 d. After cooling at 0 °C, precipitation of the
product was observed.
sulfonate solution in water (50 mL). The organic phase was sepa-
rated and the solvent was removed under reduced pressure to af-
ford pure rotaxane as the tosylate salt.
Rotaxane R(C3C12): Rotaxane was obtained as a red solid (57 %).
1H NMR (400 MHz, C6D6): δ = 0.93 (m, 12 H), 1.10–1.20 (m, 8 H),
1.25–1.35 (br. s, 26 H), 1.35–1.45 (br. s, 14 H), 1.50–1.60 (m, 12 H),
1-[3-(2,2-Diphenylacetoxy)propyl]-1′-(12-hydroxydodecyl)-[4,4′-
bipyridine]-1,1′-diium (2a×2TsO): Pure axle 2a×2TsO was col-
2
1.71 (br. s, 20 H), 1.83 (m, 8 H), 1.96 (s, 6 H), 3.36 [d, JH,H = 16 Hz,
1
3
lected by Buchner filtration as a sticky white solid (47 %). H NMR
6 H], 3.7–3.8 (m, 12 H), 3.86 (s, 9 H), 4.06 [t, JH,H = 6 Hz, 2 H], 4.47
2
(300 MHz, MeOD): δ = 1.3–1.5 (m, 18 H, aliphatic CH2), 2.09 (m, 2
H, aliphatic CH2), 2.37 (s, 3 H, Ar-CH3), 2.48 (m, 2 H, aliphatic CH2),
[d, JH,H = 16 Hz, 6 H], 5.07 (s, 1 H), 5.30 (s, 1 H), 6.57 (br. s, 4 H),
3
6.75 (br. s, 4 H), 6.92 [d, JH,H = 8 Hz, 4 H], 6.95 (br. s, 4 H), 7.00–
3
3
3
3.55 [t, JH,H = 9 Hz, 2 H, HOCH2], 4.33 [t, JH,H = 6 Hz, 2 H, (C=
7.10 (m, 4 H), 7.10–7.20 (m, 14 H), 7.40 [d, JH,H = 8 Hz, 4 H], 7.52
O)OCH2], 4.75 (m, 4 H, N+-CH2), 5.09 [s, 1 H, (Ph)2CH], 7.2–7.3 (m, 14
H, ArH), 7.69 [d, JH,H = 9 Hz, 4 H, ArH], 8.54 [d, JH,H = 6 Hz, 2 H,
3
[d, JH,H = 8 Hz, 4 H], 7.59 (s, 6 H), 7.75–8.00 (m, 10 H), 8.19 (br. s, 4
3
3
H), 9.41 (br. s, 6 H) ppm. 13C NMR (100 MHz, C6D6): δ = 14.0, 20.8,
22.8, 25.9, 26.6, 28.6, 29.6, 29.9, 30.8, 31.5, 32.0, 34.6, 56.9, 57.3, 58.5,
60.7, 61.2, 64.7, 73.0, 116.6, 118.0, 121.3, 124.8, 125.7, 126.6, 127.1,
128.5, 128.6, 128.7, 128.8, 129.3, 132.0, 133.7, 137.4, 139.2, 139.5,
141.0, 142.9, 143.3, 144.7, 148.3, 152.8, 153.2, 171.7, 171.9 ppm. ESI-
MS(+): m/z (%) = 1187 (100) [M2+].
3
3
ArH], 8.63 [d, JH,H = 6 Hz, 2 H, ArH], 9.9 [d, JH,H = 6 Hz, 2 H, ArH],
3
9.26 [d, JH,H = 6 Hz, 2 H, ArH] ppm. 13C NMR (75 MHz, MeOD): δ =
20.0, 25.5, 25.8, 28.7, 29.1, 29.2 (2 res.), 29.3, 29.6, 31.2, 32.2, 56.4,
59.3, 61.5, 61.6, 62.0, 125.5, 126.8, 127.1, 128.3 (2 res.), 128.5, 138.7,
140.4, 145.8, 149.6, 149.9, 172.3 ppm. ESI-MS(+): m/z (%) = 593 (100),
594 (60) [M – H]+.
Rotaxane R(C6C16): Rotaxane was obtained as a red solid (40 %).
1H NMR (300 MHz, C6D6): δ = 0.72 (br. s, 8 H), 0.96 (br. t, 9 H + 4
H), 1.13 (m, 8 H), 1.2–1.4 (m, 40 H), 1.56–1.63 (m, 16 H), 1.71 (br. s,
1-[6-(2,2-Diphenylacetoxy)hexyl]-1′-(16-hydroxyhexadecyl)-
[4,4′-bipyridine]-1,1′-diium Ditosylate (2b×2TsO): The solid was
Eur. J. Org. Chem. 2016, 1033–1042
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2
20 H), 1.83 (m, 8 H), 1.95 (s, 6 H), 3.36 [d, JH,H = 14 Hz, 6 H], 3.7– 2 H), 7.03 (br. d, 4 H), 7.05 (br. d, 4 H), 7.10 (br. s, 4 H), 7.12 (br. s, 4
3
2
3.8 (m, 10 H), 3.86 (s, 9 H), 4.03 [t, JH,H = 6 Hz, 4 H], 4.47 [d, JH,H
=
H), 7.39 [d, 3JH,H = 9 Hz, 4 H], 7.58 (br. s, 6 H), 7.83 (m, 6 H), 7.95 (m,
3
14 Hz, 6 H], 5.07 (s, 1 H), 5.13 (s, 1 H), 6.65 (m, 2 H), 6.74 (m, 2 H),
6.91 [d, JH,H = 8 Hz, 4 H], 6.95–7.10 (m, 10 H), 7.1–7.2 (m, 16 H),
2 H), 8.20 [d, JH,H = 9 Hz, 4 H], 9.42 (br. s, 6 H) ppm. 13C NMR
3
(100 MHz, C6D6): δ = 14.0, 20.8, 22.8, 25.8, 26.6, 28.6, 28.9, 29.2, 29.6
(3 res.), 29.7 (2 res.), 29.8, 29.9 (4 res.), 30.1, 30.2, 30.8, 31.6, 32.0,
34.6, 57.3, 60.8, 64.8, 64.9, 73.0, 116.6, 117.9, 121.0 (2 res.), 124.8 (2
7.35–7.45 (m, 12 H), 7.58 (br. s, 6 H), 7.75–7.90 (m, 4 H), 7.12 (m, 2
H), 8.18 [d, 3JH,H = 8 Hz, 4 H], 9.39 (br. s, 6 H) ppm. 13C NMR (75 MHz,
C6D6): δ = 14.0, 20.8, 22.8, 25.8, 26.1, 26.5, 28.6, 28.9, 29.2, 29.5, 29.6, res.), 126.6, 127.0, 127.1, 127.6, 127.8, 128.5 (2 res.), 128.7 (2 res.),
29.7 (3 res.), 29.9 (3 res.), 30.7, 31.5, 31.9, 34.6, 57.3, 60.8, 64.9, 73.1, 129.4, 139.3, 141.1, 142.9, 144.3, 148.3, 152.9, 153.3, 171.9 ppm. ESI-
116.6, 118.0, 121.0, 124.9, 125.5, 126.5, 127.0, 128.5 (2 res.), 128.6, MS(+): m/z (%) = 1307 (100) [M2+].
128.7, 132.0, 133.6, 137.3, 139.0, 139.3, 139.4, 141.2, 143.3, 148.3,
152.9, 153.4, 171.9 ppm. ESI-MS(+): m/z (%) = 1237 (100) [M2+].
Acknowledgments
Rotaxane R(C16C6): Rotaxane was obtained as a red solid (43 %).
This work was supported by the Italian Ministero dell'Università
e della Ricerca (MIUR) (PRIN 2010CX2TLM “InfoChem” and
“MULTINANOITA”), the University of Bologna (Finanziamenti di
Ateneo alla Ricerca di Base, SLaMM Project), University of Parma
and Centro Interdipartimentale di Misure “G. Casnati” for NMR
and Mass measurements.
1H NMR (300 MHz, C6D6): δ = 0.85–1.00 (br. s, 14 H), 1.10 (br. s, 6
H), 1.2–1.4 (m, 48 H), 1.51 (br. s, 4 H), 1.58 (m, 8 H), 1.70 (br. s, 22
H), 1.82 (m, 8 H), 1.95 (s, 6 H), 3.37 [d, 2JH,H = 14 Hz, 6 H], 3.45–3.65
3
(br. s, 6 H), 3.70 (m, 4 H), 3.80 (s, 9 H), 4.02 [t, JH,H = 6 Hz, 2 H],
3
2
4.33 [t, JH,H = 6 Hz, 2 H], 4.48 [d, JH,H = 14 Hz, 6 H], 5.06 (s, 1 H),
3
5.10 (s, 1 H), 6.61 (m, 2 H), 6.73 (br. t, 2 H), 6.89 [d, JH,H = 8 Hz, 4
H], 6.93 (br. s, 2 H), 7.00–7.10 (m, 10 H), 7.10–7.20 (m, 12 H), 7.35–
3
7.50 (m, 12 H), 7.59 (br. s, 6 H), 7.80–7.90 (m, 10 H), 8.15 [d, JH,H
=
Keywords: Supramolecular chemistry · Molecular devices ·
Rotaxanes · EPR spectroscopy · Electrochemistry
8 Hz, 4 H], 9.39 (br. s, 6 H) ppm. 13C NMR (100 MHz, C6D6): δ = 14.1,
20.8, 22.8, 25.0, 25.5, 25.8, 26.6, 28.3, 28.6, 29.2, 29.6, 29.7, 29.8 (2
res.), 30.0, 30.1, 30.2, 30.4 (2 res.), 30.8, 31.1, 31.5, 32.0, 34.6, 57.3,
57.4, 60.4, 60.7, 64.6, 64.8, 73.0, 116.6, 118.0, 121.1, 124.8, 125.5,
126.6, 127.1, 127.2, 127.6, 127.8, 128.5, 128.6, 128.7 (2 res.), 128.8,
129.3, 132.1, 133.6, 139.2 (2 res.), 141.1, 142.9, 144.3, 148.3, 152.8,
153.3, 171.9, 172.0 ppm. ESI-MS(+): m/z (%) = 1237 (100) [M2+].
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tosylate (7c×2TsO): In a sealed glass reactor, 4c (0.22 g, 0.54 mmol)
and 4,4′-bipyridine (0.02 g, 0.13 mmol) were dissolved in anhydrous
acetonitrile (5 mL) and heated at 110 °C for 7 d. After cooling at
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solid was collected by Buchner filtration and washed with cold
acetonitrile. Axle 7c×2TsO (60 mg, 48 %) was obtained as a white
1
solid, m.p. 165–170 °C. H NMR (300 MHz, MeOD): δ = 1.2–1.6 (m,
3
56 H, aliphatic CH2), 2.39 (s, 3 H, Ar-CH3), 3.55 [t, JH,H = 6.6 Hz, 4
H, HO-CH2], 4.74 [t, 3JH,H = 7.5 Hz, 4 H, N+-CH2], 7.24 [d, 3JH,H = 8 Hz,
3
3
4 H, ArH], 7.69 [d, JH,H = 8 Hz, 4 H, ArH], 8.67 [d, JH,H = 6.9 Hz, 4
H, ArH], 9.27 [d, JH,H = 6.9 Hz, 2 H, ArH] ppm. 13C NMR (100 MHz,
3
MeOD): δ = 20.0, 25.5, 25.8, 28.8, 29.1, 29.2 (2 res.), 29.3 (2 res.),
31.2, 32.2, 61.6, 62.0, 125.5, 126.9, 128.5, 140.3, 142.1, 145.6,
149.9 ppm. ESI-MS(+): m/z (%) = 678 (100), 639 (40) [M2+ – H].
Rotaxane R(C16C16): Axle 7c×2TsO (0.05 g, 0.05 mmol) was sus-
pended in a solution of 1 (0.08 g, 0.05 mmol) in anhydrous toluene
(20 mL). The suspension was stirred at 50 °C for 3 h until the solu-
tion turned dark-red and the axle was completely dissolved. Trieth-
ylamine (15 μL, 0.1 mmol) and diphenylacetyl chloride (25 mg,
0.1 mmol) were then added and the solution was stirred at room
temperature for one day. After removing the solvent, the residue
was partioned between dichloromethane and water. The organic
phase was separated and evaporated under reduced pressure, and
crude product was purified by column chromatography (dichloro-
methane/methanol, 50:1). The isolated rotaxane was redissolved in
dichloromethane (20 mL) and washed with 0.1
M silver p-toluenes-
ulfonate solution in water (50 mL). The organic phase was separated
and the solvent was removed under reduced pressure, to afford
rotaxane (70 mg, 46 %) as a red solid. 1H NMR (300 MHz, C6D6): δ =
0.90 (br. s, 4 H), 0.96 (br. t, 9 H + 4 H), 1.12 (m, 14 H), 1.2–1.4 (m,
64 H), 1.5–1.6 (m, 16 H), 1.73 (br. s, 24 H), 1.84 (m, 8 H), 1.96 (s, 6
2
H), 3.36 [d, JH,H = 12 Hz, 6 H], 3.61 (m, 6 H), 3.72 (m, 4 H), 3.86 (s,
3
2
9 H), 4.03 [t, JH,H = 6 Hz, 4 H], 4.48 [d, JH,H = 12 Hz, 6 H], 5.07 (s,
2 H), 6.69 (m, 6 H), 6.83 (m, 2 H), 6.91 [d, 3JH,H = 9 Hz, 4 H], 7.01 (m,
Eur. J. Org. Chem. 2016, 1033–1042
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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Received: December 3, 2015
Published Online: January 20, 2016
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Eur. J. Org. Chem. 2016, 1033–1042
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim