Redox-Switchable Calix[6]arene-Based Isomeric Rotaxanes

A Journal of

Accepted Article

Title: REDOX-SWITCHABLE CALIX[6]ARENE-BASED ISOMERIC

ROTAXANES

Authors: Valeria Zanichelli, Margherita Bazzoni, Arturo Arduini, Paola

Franchi, Marco Lucarini, Giulio Ragazzon, Andrea Secchi,

and Serena Silvi

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To be cited as: Chem. Eur. J. 10.1002/chem.201800496

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REDOX-SWITCHABLE CALIX[6]ARENE-BASED ISOMERIC

ROTAXANES

Valeria Zanichelli,^[a]Margherita Bazzoni,^[a]Arturo Arduini,^[a]Paola Franchi,^[b]Marco Lucarini,^[b]Giulio

Ragazzon,^[b,c],* Andrea Secchi^[a],* and Serena Silvi^[b],*

Abstract: Operating molecular machines are based on switchable

systems, whose components can be set in motion in a controllable

fashion. The presence of non-symmetric elements is a mandatory

capable of delivering both chemical and biological cargos in a

controlled manner.^[7–10]The shuttling motion can be triggered by

a plethora of different stimuli,^[4,11]such as thermal treatment,^[12]

requirement to obtain and demonstrate the unidirectionality of motion. ion coordination,^[13,14]electrochemical^[15]or photochemical^[16,17]

Calixarene-based macrocycles have proven very efficient hosts in

the design of oriented rotaxanes and of pseudorotaxanes with a

strict control on the direction of complexation. We have synthesized

and characterized a series of two-station rotaxanes based on

activation, changes of solvent or pH.^[18]In order to obtain work

from a molecular machine, and to move from switches to real

molecular motors, full control on the direction of motion must be

attained.^[19–22]This is one of the main challenges when designing

a molecular machine and it involves the exploitation of inherently

non-symmetrical components, arranged in univocally oriented

architectures. Indeed, asymmetry is a fundamental factor in the

majority of systems reported to date, not only for obtaining

unidirectionality of motion, but also for demonstrating it.^[23]

bypiridinium-ammonium axles. We have exploited

a recently

reported supramolecular-assisted strategy for the synthesis of

different orientational isomers and we identified the ammonium unit

as a proper secondary station for the calixarene. We were able to

trigger the displacement of the macrocycle upon electrochemical

reduction of the bipyridinium primary station and we demonstrated

that the shuttling is influenced both by the length of the chain of the

axle component and by the position of the secondary station with

respect to the calixarene rims.

In

a rotaxane-based architecture, asymmetry can be

included both in the axle and wheel components. The majority of

two-station molecular shuttles reported so far is based on non-

symmetric axles,^[18,24,25]but in principle the direction of motion in

a rotaxane could be dictated by the asymmetry of the wheel

component (Figure 1).^[22,26–31]In a minimalistic design like the

one reported in Figure 1a, upon weakening the interaction

between the axle and the ring the formation of two orientational

isomers could be envisaged. If the non-symmetric ring has a

preferential direction of motion, dictated by the different nature of

its two rims, then it would be possible to get selectively only one

orientational isomer. If both the axle and the ring are non-

symmetric (Figure 1b), then two orientational isomers can be

synthesized, and the movement of the ring can give rise to four

translational isomers (Figure 1b).

Introduction

In the field of molecular level machines, the possibility to

synthesize and operate with supramolecular systems capable of

performing specific movements under the action of a defined

external energy input constitutes a fascinating challenge.^[1–5]

One of the simplest classes of molecular machines is

represented by molecular shuttles based on rotaxane

architectures, which were ﬁrst characterized by Stoddart et al. in

solution in 1991.^[6]Since that report, many mechanically

interlocked molecules have been designed, synthesized and

shown to mimic the complex functions of macroscopic switches

with a wide range of applications, including molecular electronic

devices, sensors, nanomechanical systems and instruments

a

b

[a]

Dr. V. Zanichelli, Margherita Bazzoni, Prof. A. Arduini, Prof. A.

Secchi

Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità

Ambientale

Università di Parma

Parco Area delle Scienze 17/A, I-43124 Parma, Italy

E-mail: andrea.secchi@unipr.it

Dr. P. Franchi, Prof. M. Lucarini, Dr. G. Ragazzon, Dr. S. Silvi

Dipartimento di Chimica “G. Ciamician”,

Università di Bologna

Figure 1. Rotaxanes with a) a symmetric axle and non-symmetric ring and b)

both non-symmetric components (two orientational isomers): on weakening

the interaction between the axle and the ring, different translational isomers

can be obtained, depending on the direction of motion of the ring.

[b]

[c]

Via Selmi 2, I-40126, Italy

E-mail: serena.silvi@unibo.it

Dr. Giulio Ragazzon

Dipartimento di Scienze Chimiche

Università di Padova

via Marzolo 1, I-35131, Padova

E-mail: giulio.ragazzon@unipd.it

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similar pseudorotaxane assemblies,^[34]no detectable movement

of the macrocyclic component takes place when the axle is

mechanically interlocked in

a

rotaxane-type structure,^[32]

regardless of the nature and the orientation of the threaded

dumbbell.^[33]From these results, it emerged that the presence of

a further recognition element is mandatory to facilitate the

displacement and promote the shuttling. We thus focused our

attention on double-station rotaxanes, equipped with two distinct

recognition sites (stations) on their dumbbell-shaped component,

in which the macrocycle can move from one site to the other in a

controlled and reversible manner. To achieve this goal, it is

crucial that the two stations exhibit a different association

strength with the receptor. If this is the case, the rotaxane can

exist as two different equilibrating co-conformations, the

populations of which reflect their relative free energies,

determined by the extent of the two different sets of non-

covalent bonding interactions. To this aim, in this paper we

tackled the synthesis of a series of novel double station

rotaxanes, constituted by the calix[6]arene wheel Cx and

dialkylviologen-based axles endowed with an ammonium station

as second recognition unit on the dumbbell (see Figure 2).

Results and Discussion

Synthesis of rotaxanes

To describe the results of this study, we labeled the compounds

with descriptors showing (i) the position of the ammonium

station with respect to the rims of the calixarene (Up if the

station is oriented towards the upper rim, Down if it is oriented

towards the lower rim), (ii) the length of the N-containing side

chain connected to the pyridinium nitrogen atom (Long or

Short) (see Figure 2). Labels DB, P and R denote, respectively,

dumbbells, pseudorotaxanes and rotaxanes. To gain information

about a possible preferential shuttling direction, pivoted by the

inherent asymmetry of Cx, we designed two different series of

orientational rotaxane isomers in which the ammonium station

(N-station) faces the two opposite rims of the macrocycle. To

have an insight about the influence of the length of the spacers,

and to assure that the movement is not impeded by

neighbouring molecular components, each series was

synthesized with a long (C₁₂NC₁₁) and a short (C₆NC₆) side

chain (Figure 2). Taking inspiration from the previously followed

protocols,^[33]we designed the synthesis of axles endowed with a

bulky stopper at one end, and a terminal OH group appended at

the opposite side. For the preparation of the rotaxanes having

the longer N-containing side chain toward the calixarene upper

rim, the appropriate N-Boc protected axles 6a and 6b were

synthesized according to Scheme 1. N-Boc protected ω-

aminoalcohols 1a-b were converted in tosylates 2a-b. These

latter were first reacted with 3,5-di-tert-butylphenol in the

presence of K₂CO₃to insert the stopper (3a-b) and then with a

molar excess of either 1,6-dibromohexane or 1,12-

dibromododecane to complete the backbone of the N-containing

alkyl side chains. Reaction of the resulting bromides 4a-b with

the pyridyl pyridinium tosylate 5 finally afforded axles 6a and 6b

in 46% and 50% yield, respectively (see Experimental).

Figure 2. Single and double-station rotaxanes derived from calix[6]arene Cx.

We have exploited the complexation abilities of

calix[6]arene derivative Cx (Figure 2) toward 4,4’-bipyridinium-

based axles^[32,33]to synthesize non-symmetric rotaxanes. In

these systems, both the axle and the wheel components are

non-symmetric (Figure 1b):^[33]the calixarene presents two rims

which are functionalized with three N-phenylureido groups

(upper rim) and three octyl chains (lower rim), and differ for both

size and chemical properties, and the dumbbells are decorated

with two alkyl chains with an appropriate difference in length.

Moreover, in each molecule, the components are arranged in a

univocal and controlled relative orientation, giving rise to

constitutionally isomeric oriented rotaxanes. Since the

functionalized calixarene is non-palindrome, the movement of

the wheel towards one side of the axle would be chemically not

equivalent to the one towards the opposite side and could

therefore generate specific translational isomers (Figure 1b). We

first planned to induce the shuttling by exploiting the response of

these redox-active systems to electrochemical stimulation.^[33]

However, it emerged that the sole electrochemical reduction of

the bipyridinium core is not sufficient to induce any mechanical

rearrangement in these rotaxanes. Cyclic voltammetry and EPR

measurements showed that, contrarily to what observed for

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moiety and a suitable leaving group (Cl) for the alkylation of

bipyridine. These compounds were reacted in refluxing

acetonitrile with the pyridyl pyridinium salt 10, functionalized with

a stoppered C₆alkyl chain, to afford axles 11a and 11b in 65%

and 88 % yield, respectively (see Experimental).

In separated experiments, 11a and 11b were then

suspended in toluene and mixed with an equimolar amount of

wheel Cx. In these two reactions, no hints of complexation were

observed:

the

heterogeneous

mixture

never

turned

homogeneous and the supernatant toluene solution remained

uncolored. Heating the two mixtures to reflux for several hours

did not change the result. The ESI-MS analysis of the reaction

mixtures (carried out in MeOH to provide the solubility of the

axle) revealed the presence of the sole unreacted reagents,

meaning that no side reaction or reagents degradation took

place: in fact, after chromatographic separation, Cx was

recovered almost quantitatively.

The impossibility to obtain any pseudorotaxane complex

through these reactions was mainly ascribed to the bulkiness of

the protected secondary amine in 11a and 11b that prevent the

threading of the axle bearing the carbamate moiety in proximity

to the lower rim of the wheel. Overall, the above “threading-and-

capping” sequential strategy has shown the remarkable

advantages of a complete selectivity in the synthesis of

orientational isomers presenting the N-station facing the upper

rim of the calix[6]arene: R-UpShort and R-UpLong (Figure 2).

On the other hand, its major drawback is the impossibility to

obtain assemblies presenting bulky moieties exposed downward

or arranged in non-linear conformations, because such

dumbbells cannot efficiently thread the wheel component.

Scheme 1. Synthesis of N-Boc protected axles 6a and 6b.

In separated experiments, these axles were then

equilibrated at room temperature in toluene with an equivalent of

wheel Cx. After stirring for three hours at room temperature, the

initial suspensions gradually turned homogeneous and red

coloured. This coloration is usually ascribed to the charge

transfer interaction occurring in a pseudorotaxane complex

between the viologen core of the axle and the π-rich aromatic

cavity of the calixarene. As shown in previous studies,^[35,36]the

presence on the axle of a terminal bulky stopper leads to the

unidirectional threading of the axle from the upper rim of Cx with

its ω-hydroxy hexyl chain and thus to the exclusive formation of

oriented pseudorotaxanes P-UpShort and P-UpLong as

depicted in Scheme 2. The subsequent axle stoppering reaction

with diphenylacetylchloride, carried out directly in the toluene

solution of each psuedorotaxane, followed by treatment with

trifluoroacetic acid of the dichloromethane solutions of the

resulting N-Boc protected rotaxanes, allowed us to eventually

isolate the desired rotaxanes R-UpShort and R-UpLong in 55%

and 27% of overall yield, respectively (see Scheme 2).

For the preparation of the orientational rotaxane isomers

having the N-station facing the lower rim of the calixarene wheel

(R-DownLong and R-DownShort in Figure 2), we designed the

synthesis of axles 11a and 11b, which are endowed with the

secondary ammonium function between the viologen unit and

the terminal OH group (see Scheme 3). In the first step, the

hydroxyl group of 1a-b was protected with tert-butyldimethylsilyl

chloride (TBSCl), then the Boc-protected NH group of the

resulting doubly protected derivatives 7a-b, was alkylated with

the appropriate α,ω-dibromoalkane to yield amines 8a-b. In the

following step, all attempts to free the OH group of these

derivatives through the cleavage of the silyl ether function with

tetrabutylammonium fluoride (TBAF) led to an undesired

fluoro/bromo exchange. Finally, deprotection with anhydrous

CeCl₃afforded the chlorides 9a-b presenting a terminal hydroxyl

Scheme 2. Synthesis of target rotaxanes R-UpShort and R-UpLong.

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Rotaxanes Characterization

The inclusion of bipyridinium-based dumbbell in

a

calix[6]arene wheel can be assessed through the analysis of the

proton chemical shift of some diagnostic signals.^{[32,33,36,39-41]}The

presence of a single sharp signal at ca. 3.8 ppm for the lower

rim methoxy groups is used to confirm the presence of a single

orientational isomer in solution (see Figure 3). Considering the

cone shape adopted by the calix[6]arene cavity upon threading

and its extension due to the three N-phenylureas at its upper rim,

it is known that all signals of the stoppered alkyl chain facing the

phenylureas are more upfield shifted than those belonging to the

opposite side chain.^{[32,33,35,36]}To assess the orientation of the

asymmetric dumbbell DB-Short and DB-Long with respect the

rims of the wheel, we took advantage of our previous NMR

studies carried out on a symmetric rotaxane bearing two C6-

alkyl chains at the opposite sides of the bipyridinium station, R-

C6C6 (see Figure 2),^[32]which shares with the two-station

rotaxanes so far synthesized the same alkylated bipyridinium

moiety with a diphenylacetic stopper. In this rotaxane, the

methylene group nearby the diphenylacetic stopper, indicated

either with the label 1 or 1' depending on the orientation of its

hexyl chain with respect to the asymmetric aromatic cavity

(upper and lower rim, respectively, see Figure 2), is greatly

affected by changes of the magnetic environment. It gives rise to

two well-separated NMR signals at δ = 4.38 and 4.08 ppm (see

entry 1, Table 1), with the more upfield-shifted one relative to the

methylene group (1) of the hexyl chain located toward the wheel

upper rim (see Figure 2).^[32]Through a simple comparison of the

chemical shifts of these signals, it was then possible to foresee

that the side chain containing the N-station in R-UpShort and R-

UpLong (see Figure 3a-b and Table 1) is oriented toward the

wheel upper rim. The same applies for rotaxanes R-DownShort

and R-DownLong where the N-station is oriented in the

opposite direction (see Figure 3c-d and Table 1). In the two

“short” rotaxane isomers R-UpShort and R-DownShort,

characterized by a N-containing side chain with short spacers,

also the signals of the two methylene groups nearby the

ammonium moiety are affected by the orientation of the

dumbbell inside Cx. In R-UpShort these methylene groups

resonate as two broad signals at δ = 2.9 and 2.7 ppm, labelled

respectively as # and § (see Figure 3a). In the opposite

Scheme 3. Synthesis of N-Boc protected axles 11a and 11b.

To overcome these limitations, we envisaged to investigate

whether the recent results we obtained in the supramolecularly-

assisted

synthesis

of

oriented

calix[6]arene-based

rotaxanes^[37,38]can be extended to the preparation of rotaxanes

that cannot be synthesized with traditional procedures. In this

recent work it was indeed shown that the alkylation of pyridyl-

pyridinium salts such as 5 and 10 in presence of an analogue of

Cx takes place preferentially inside the calix[6]arene cavity with

the non-alkylated pyridine ring facing the calixarene upper rim.

This alkylation gives thus rise to the formation of

pseudorotaxane structures. If a bulky terminal group is present

on the alkyl chain of the pyridyl-pyridinium guest, the following

assisted alkylation and stoppering reactions lead to the

formation of oriented rotaxanes, in which the alkyl chain initially

appended to the pyridinium, regardless of its length, is located

close to the lower rim of the wheel.^[37,38]

Starting from these results, we therefore designed the

synthesis of the stoppered pyridyl-pyridinium salts having the

Boc-protected N-station between the phenolic stopper and

pyridylpyridinium moiety and alkyl spacers between the amine

group and the two opposite endings of different length. In

separated experiments, 4a-b were first reacted with a large

excess of 4,4’-bipyridine in refluxing acetonitrile (see scheme 4)

and the resulting pyridylpyridinium salts 12a and 12b, easily

isolated by filtration, were then mixed in toluene with equimolar

amounts of Cx and 13. The latter compound is a C₆alkylating

agent mono-stoppered with a bulky diphenylacetic moiety.

These supramolecularly-assisted rotaxanation reactions went to

completeness after two days of refluxing (solutions turned

homogeneous and red-colored). After chromatographic

separation, the rotaxanes were immediately treated with TFA in

dichloromethane (DCM) to deprotect the N-station. After solvent

removal, the oriented rotaxanes R-DownShort and R-

DownLong were obtained in 58% and 69% of overall yield.

Model compounds dumbbells DB-Long and DB-Short were

synthesized as depicted in Scheme 4.

orientational isomer R-DownShort one of these two signals (§′)

is downfield-shifted at δ = 3.2 ppm (see Figure 3c). In the longer

dumbbell of the rotaxane isomers R-UpLong and R-DownLong

both methylene groups give rise to a unique broad signal

centered at ca. 2.6 ppm regardless of their position with respect

to the cavity. This is coherent with the increased distance of the

ammonium group (C₁₂spacer) from the cavity in these two

rotaxanes. Moreover, for each rotaxane, 2D NMR

measurements (COSY, TOCSY, NOESY, HSQC) were carried

out to facilitate the complete characterization of the products.

The formation of the two-station rotaxanes was also confirmed

by high resolution mass spectrometry (see experimental).

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Table 1 ¹H NMR chemical shifts (δ, ppm, C₆D₆, 400 MHz) of the most

diagnostic methylene signals of the dumbbell in the series of two-station

rotaxanes R-Down and R-Up. The chemical shift of the same signals for

symmetric rotaxane R-C6C6 were reported for comparison.^[a,b]

Rotaxane

Chemical shift (ppm)

1’

1

#’

#

§’

§

R-C6C6

4.38

4.08

/

R-UpShort

R-UpLong

R-DownShort

R-DownLong

DB-Long

4.31

/

2.9

/

2.7

4.32

/

2.6

/

2.6

/

4.03

4.04

2.9

2.6

/

3.2

2.6

/

4.17

4.16

3.0^[c]

DB-Short

[a] For the NMR signals assignment see Figure 3; [b] a primed label indicates

the orientation of the methylene group toward the wheel lower rim; [c] in the

non-complexed dumbbells these signals are overlapped.

Scheme 4. Supramolecularly assisted synthesis of rotaxanes R-DownShort

and R-DownLong and synthesis of dumbbells DB-Short and DB-Long.

Figure 3. ¹H NMR stack plots (C₆D₆, 400 MHz) of the series of two-station rotaxanes (the most diagnostic signals are indicated in sketches on the left).

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Rotaxanes Electrochemical Behavior

rotaxanes with shorter chains R-UpShort and R-DownShort,

bearing the ammonium unit closer to the macrocycle, is slightly

more difficult (regardless of the calix[6]arene orientation) with

respect to the reduction of the “long” isomers: this result

suggests that a little but non-negligible interaction between the

calixarene and the monoreduced bipyridinium is still taking place

in the “short” isomers, in agreement with EPR results (vide infra).

It is known^[34]that parent pseudorotaxanes composed of a

bipyridinium-based axle and a trisphenylureido calix[6]arene can

be operated by means of electrochemical stimulation, i. e.

electrochemical reduction of the bipyridinium unit to its radical

cation causes the dethreading of the molecular components. On

the other hand, in rotaxanes lacking a second recognition unit

for the calixarene the reduction of the bipyridinium ring does not

displace the ring from the station.^[32,33]The response of the two-

station rotaxanes to electrochemical stimulation was steered

through Cyclic Voltammetry (CV) and Differential Pulse

Voltammetry (DPV) measurements in acetonitrile, and the

measured potentials were compared to the ones of non-

complexed dumbbells DB-Long and DB-Short (see Table 2).

For sake of comparison, also the reduction potentials of the 1,1’-

dioctyl-4,4-bipyridinium (DOV) axle and of symmetric rotaxane

R-C6C6 are reported. In line with previously analyzed systems,

all compounds show two reversible or quasi-reversible

monoelectronic reduction processes.^[32]The values of the

reduction potentials of the two dumbbells are similar to the ones

of DOV, thus demonstrating that the electrochemical properties

of the bipyridinium moiety are not influenced neither by the

stoppers nor by the ammonium units.

Table 2. Electrochemical potentials (vs SCE) of the investigated compounds.

Conditions:

argon-purged

acetonitrile,

tetraethylammonium

hexafluorophosphate 0.04 M, analyte 300-400 μM.

Species

Reduction potentials (V)

E₁

E₂

DOV

-0.42^[42]

-0.41

-0.87^[42]

-0.85

DB-Short

DB-Long

R-UpShort

R-UpLong

R-DownShort

R-DownLong

R-C6C6

-0.40

-0.85

-0.60

-0.94

-0.61

-0.87

-0.52

-0.93

As expected, the first reduction potential of each two-

station rotaxane isomer is shifted towards more negative values

with respect to the corresponding free axle in solution, and falls

at almost the same potential as in model compound R-C6C6

(Figure 4), as a consequence of the stabilization of the

bipyridinium unit induced by the calixarene cavity.^[32–34]On the

other hand, the second reduction potentials are comparable to

the ones of non-complexed dumbbells (Figure 4). These results,

together with the EPR data (vide infra), indicate that the

calixarene ring moves away from the monoreduced bipyridinium

unit. Nevertheless, the reversibility of the first reduction waves in

the cyclic voltammetry suggests that the shuttling equilibria in

the monoreduced state are fast with respect to the scanning

speed. The scan rates explored with our apparatus did not

evidence any dependence of the peak position and separation

on the scan rate, but digital simulations of the experiments and

fitting of the experimental data confirmed our hypothesis.

-0.60

-0.88

-0.64^[32,33]

-1.15^[32,33]

The voltammetric curves obtained on rotaxane R-UpShort

were fitted according to the scheme reported in Figure 5, by

fixing the reduction potentials for the encapsulated and free

species to the experimental values. The vertical processes are

the electrochemical reactions, whereas the horizontal processes

are the shuttling reactions. The values of the thermodynamic

constants for these equilibria are also reported. A closer

inspection of the reduction potentials (Figure 4) suggests that

there is dependence both on the length of the N-containing alkyl

chains (short or long) and on the position of the ammonium site

with respect to the calixarene rims (up or down). The analysis of

the second reduction potentials shows that the reduction of the

Figure 4. Genetic diagram of the measured reduction potentials of the studied

compounds: dumbbells DB-Short (empty circles) and DB-Long (empty

diamonds), two-stations rotaxanes R-UpLong (black diamonds), R-

DownLong (grey diamonds), R-UpShort (black circles), R-DownShort (grey

circles), and one-station rotaxane R-C6C6 (empty squares).

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3.0 × 10⁷s^-1

+

K(0) =9.4

3.2 × 10⁶s^-1

e^--1.18 V

e^-

-0.94 V

1.0 × 10⁵s^-1

1.0 × 10⁴s^-1

.

+

K(+•) =10

+

e^-

-0.41 V

-0.64 V

1.0 × 10^-3s^-1

0.77 s^-1

K(2+) =1.3 × 10^-3

+ +

+

Figure 5. Reaction scheme reporting the thermodynamic and kinetic data for the simulated voltammogram of R-UpShort. Reduction potentials partly taken from

refs. [32] and [33].

This finding would also suggest that when the ammonium is

close to the bipyridinium unit, it is possible for the calixarene

macrocycle to interact with both sites simultaneously, regardless

of the orientation of the axle. This observation is somehow

surprising, because the two rims of the calix[6]arene are

different from a chemical point of view and it is therefore not

easy to imagine different interaction modes that result in very

similar interaction energies. The analysis of the first reduction

Table 3. In all cases, the g-factors were found to be close to

2.0031. According to the literature,^[33]the smaller hyperfine

coupling constant for H atoms was assigned to the aromatic α-

protons, whereas the larger coupling was attributed to the

methylene groups of the two alkyl chains. Inspection of these

data clearly shows that: i) EPR parameters of the mono-reduced

double-station rotaxanes are similar to those measured for the

non-complexed mono-reduced dumbbells DB-Short(+•) and DB-

Long(+•) (Figure 6); ii) the values of the hyperfine coupling

constants are consistent with a symmetric distribution of the two

aromatic rings of the bipyridine. We have already shown^[33]that

trapping bpy(+•) in the asymmetric wheel of Cx induces a non-

symmetric distribution of the spin density in the two heterocyclic

potential enlightens also

a dependence on the relative

orientation of the axle and the ring. Rotaxane R-DownShort,

with a short N-containing chain close to the lower rim of the

calixarene, is easier to reduce with respect to all the other

rotaxanes. This would suggest that the calixarene is less

engaged in charge transfer interactions with the bipyridinium unit, rings. Thus, present results suggest that bpy(+•) does not

possibly because involved in the interaction with the ammonium

site already in the oxidized form, besides in the monoreduced

state as suggested by EPR data (vide infra). This hypothesis is

also supported by the reversed position of the NMR signals of R-

DownShort and R-UpShort (vide supra).

interact significantly with the calix[6]arene cavity in all derivatives,

irrespective of the length of the alkyl chains on the axle and the

relative position of the two components of the rotaxane.

Table 3. EPR hyperfine splitting constants (a, in Gauss) of radical cations

obtained after electrochemical reduction of the bipyridinium unit at room

temperature in CH₃CN.

Rotaxanes EPR Measurements

The hypothesis of a shuttling motion of Cx was also supported

by the EPR spectra of non-complexed dumbbells and rotaxanes

recorded after electrochemical reduction in deoxygenated

acetonitrile at room temperature. The EPR spectra of the

bipyridinium radical cations, bpy(+•), obtained by one-electron

reduction are reported in Figure 6. According to previous studies

on the radical cation of 1,1'-dialkyl-4,4'-bipyridinium

derivatives,^[33]all the spectra can be well reproduced by

assuming the coupling of the unpaired electron with two

equivalent N atoms and three groups of four equivalent protons:

one group is due to the methylene groups of the two chains and

the other two equivalent sets arise from the aromatic protons. By

using the above spectral pattern all the spectra can be nicely

reproduced with the hyperfine splitting constants reported in

a_2N

a_2CH

a_4H

α

2

β

DB-Short(+•)

4.10

4.13

4.16

4.17

4.13

4.14

4.06

4.05

3.98

4.11

4.01

4.04

1.61

1.58

1.66

1.47

1.62

1.61

1.11

1.12

1.20

1.30

1.17

1.15

DB-Long(+•)

R-UpShort(+•)

R-DownShort(+•)

R-UpLong(+•)

R-DownLong(+•)

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the second recognition unit faces the lower part of the wheel,

were obtained exploiting a supramolecularly assisted strategy

which was recently reported.^[37,38]Moreover, we have

demonstrated that the ammonium moiety can play the role of a

secondary station for the calixarene and the ability of these

systems to behave as molecular shuttles driven by

electrochemical

stimulation.

Voltammetric

and

EPR

measurements evidenced that, upon the first monoelectronic

reduction of the axle’s bipyridinium unit, a rearrangement of the

systems takes place. This indicates that, coherently with our

initial hypothesis, the electrochemical input induces the shuttling

of the calixarene towards the more favored second recognition

site and the complete reduction of the radical cation to its neutral

form takes place outside the calixarene cavity. Finally, we have

observed a dependence of the shuttling both on the length of the

chain of the axles and on the relative orientation of the molecular

components. On the basis of the collected data, in a symmetric

rotaxane bearing a central bipyridinium unit and two peripheral

ammonium stations connected by short spacers, reduction of the

electroactive unit is expected to induce the shuttling of the

calixarene preferentially towards the ammonium station located

at its lower rim, i.e. a directional motion would be dictated only

by the orientation of the wheel onto a symmetric axle. Three-

stations rotaxanes based on these architectures are currently

under investigation in our laboratories.

Experimental Section

Figure 6. EPR spectra (recorded in acetonitrile) of electrochemically reduced

non-complexed dumbbells and two-station rotaxanes.

Synthesis: Toluene, THF, acetonitrile and dichloromethane were dried by

following standard procedures, other reagents were of reagent grade

quality, obtained from commercial sources and used without further

purification. Chemical shifts are expressed in ppm using the residual

solvent signal as internal reference. Mass spectra were determined in

ESI mode. Compounds 1a,^[43]1b,^[44]2a,^[43]7a,^[45]13,^[35]5,^[43]10,^[46]14,^[33]

R-C6C6,^[32]axle DOV,^[34]and wheel Cx^[47]were synthesized according to

reported procedures.

The hyperfine splitting constants measured in the EPR

spectrum of the “short” rotaxanes R-UpShort(+•) and R-

DownShort(+•) which are slightly different with respect to those

measured with the non-complexed dumbbells and “long”

rotaxanes radical cations deserve a brief comment. Actually,

these small differences suggest that it is possible for the

aromatic or aliphatic tails of the calix[6]arene to maintain a little

but non-negligible interaction with the monoreduced bipyridinium

unit when the ammonium recognition site is close to the

bipyridinium unit. In conclusion, EPR results, in agreement with

the electrochemical data, confirm that the addition of one

electron on the bipyridinium site induces the displacement of the

wheel away from it.

Tert-butyl-N-{11-[(4-methylbenzenesulfonyl)oxy]undecyl}carbamate

(2b): to a solution of 1b (3.5 g, 12.2 mmol) in dry dichloromethane (50

mL), triethylamine (1.7 g, 17.9 mmol), tosyl chloride (3 g, 15.8 mmol) and

a catalytic amount of DMAP were added in this order. After stirring at

room temperature for 24h, the reaction was quenched with water (50 mL).

The separated organic phase was dried over anhydrous CaCl₂, filtered

and the solvent was evaporated to dryness under reduced pressure. The

sticky residue was purified by column chromatography (n-hexane/ethyl

acetate 85:15) to afford 2b as a white solid (75%); mp 57-59 °C; ¹H NMR

(400 MHz, CDCl₃, δ): 1.2−1.4 (m, 14H), 1.47 and 1.4−1.5 (s, m, 11H),

1.6−1.7 (m, 2H), 2.47 (s, 3H), 3.11 (br. s, 2H), 4.04 (t, J = 6.5 Hz, 2H),

4.5 (br. s, 1H), 7.36 (d, J = 7.6 Hz, 2H), 7.81 (d, J = 7.6 Hz, 2H); ¹³C

NMR (100 MHz, CDCl₃, δ): 21.6, 25.3, 26.8, 28.4, 28.8, 28.9, 29.2, 29.3

(2 res.), 29.4, 30.0, 40.6, 70.7, 78.8, 127.8, 129.8, 133.2, 144.6, 156.2

ppm; ESI-MS(+) m/z (%) [ion]: 464 (100) [M+Na]⁺. Anal. Calcd for

C₂₃H₃₉NO₅S: C, 62.55; H, 8.90; N, 3.17. Found: C, 62.61.43; H, 8.95; N,

2.99%.

Conclusions

In this paper, the design, the synthesis and the structural

characterization of new oriented double-station calix[6]arene-

based rotaxanes were presented. Several challenges in the field

of calixarene-based molecular machines have been tackled.

First, two different synthetic procedures have been followed for

the synthesis of the two orientational isomers. The up isomers,

bearing an ammonium station in proximity to the upper rim of the

wheel, were synthesized following a traditional sequential

threading-and-capping procedure; the down isomers, in which

General procedure for the synthesis of 3a-b: to a solution of 3,5-di-

tert-butylphenol (6.3 mmol) and K₂CO₃(2.6 g, 18.8 mmol) in dry DMF (50

mL), tosylate (2a or 2b, 7 mmol) was added. The resulting reaction

mixture was stirred at 80 °C for 12h. After cooling at room temperature,

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the reaction was quenched with water (50 mL) and extracted with ethyl

acetate (3 × 100 mL). The separated organic phase was dried over

Na₂SO₄and evaporated to dryness under reduced pressure.

2H), 2.0–2.2 (m, 4H), 2.39 (s, 3H), 3.24 (t, J = 6.8 Hz, 4H), 3.60 (t, J =

6.2 Hz, 2H), 4.00 (t, J = 6.2 Hz, 2H), 4.78 (t, J = 7.5 Hz, 4H), 6.77 (s, 2H),

7.05 (s, 1H), 7.25 (d, J = 8 Hz, 2H), 7.70 (d, J = 8 Hz, 2H), 8.71 (d, J =

6.5 Hz, 4H), 9.30 (d, J = 6.5 Hz, 4H); ¹³C NMR (75 MHz, CD₃OD, δ): 18.0,

23.8, 23.9, 24.1, 24.2, 24.4, 24.8, 26.0, 26.8, 27.2, 27.4, 27.7, 29.1, 29.6,

30.4, 32.9, 46.2, 59.8, 60.3, 65.8, 77.8, 107.1, 112.8, 124.1, 125.4, 127.1,

138.8, 140.9, 145.4, 148.3, 150.3, 154.5, 157.3 ppm; ESI-MS(+) m/z (%)

[ion]: 744.5 (100) [M-H]⁺; Anal. Calcd for C₅₄H₈₂BrN₃O₇S: C, 65.04; H,

8.29; N, 4.21. Found: C, 64.22.43; H, 7.99; N, 4.42%.

Tert-butyl N-[6-(3,5-di-tert-butylphenoxy)hexyl]carbamate (3a): the

residue was purified by column chromatography (n-hexane/acetone

85:15) to afford 3a as a colorless oil (53%); ¹H NMR (400 MHz, CDCl₃,

δ): 1.33 (s, 18H), 1.4−1.6 and 1.47 (2m, s, 15H), 1.8−1.9 (m, 2H), 3.14 (t,

J = 6.7 Hz, 2H), 3.98 (t, J = 6.4 Hz, 2H), 4.5 (br. s, 1H), 6.77 (s, 2H), 7.03

(s, 1H); ¹³C NMR (100 MHz, CDCl₃, δ): 25.9, 26.6, 28.5, 29.4, 30.1, 31.5,

35.0, 40.6, 67.5, 77.2, 108.8, 114.8, 152.1, 156.0, 158.6 ppm; ESI-MS(+)

m/z (%) [ion]: 428 (100) [M+Na]⁺.

Axle 6b: the yellow precipitate was collected by Buchner filtration and

washed with cold acetonitrile (50%); mp > 300 °C dec.; ¹H NMR (400

MHz, CD₃OD, δ): 1.2−1.4 (m, 44H), 1.4−1.6 (m, 21H), 1.7−1.9 (m, 4H),

2.0−2.2 (m, 4H), 2.38 (s, 3H), 3.18 (t, J = 7.2 Hz, 4H), 3.58 (t, J = 6.4 Hz,

2H), 3.97 (t, J = 6.4 Hz, 2H), 4.76 (m, 4H), 6.74 (s, 2H), 7.02 (s, 1H), 7.24

(d, J = 8 Hz, 2H), 7.69 (d, J = 8 Hz, 2H), 8.69 (d, J = 6.4 Hz, 4H), 9.29 (d,

J =6.4 Hz, 2H); ¹³C NMR (100 MHz, CD₃OD, δ): 20.0, 25.0, 25.6, 25.9 (2

res.), 26.5, 26.6, 27.5 (2 res.), 27.8 (2 res.), 28.3, 28.5, 28.8, 29.1, 29.2

(2 res.), 29.3, 29.4, 30.6, 31.1, 31.2, 31.8, 32.6, 33.1 (2 res.), 34.4, 46.7,

61.2, 61.8, 67.4, 79.2, 108.6, 114.3, 125.5, 126.9, 128.5, 140.3, 142.3,

145.7, 149.8, 151.7, 156.0, 158.8 ppm; ESI-MS(+) m/z (%) [ion]: 899.8

(100), 900.7 (65) [M-H]⁺; 1070.6 (25), 1071.8 (20) [M+TsO]⁺; Anal. Calcd

for C₆₅H₁₀₄BrN₃O₇S: C, 67.80; H, 9.10; N, 3.65. Found: C, 66.95; H, 8.65;

N, 4.01%.

Tert-butyl N-[11-(3,5-di-tert-butylphenoxy)undecyl]carbamate (3b):

the residue was purified by column chromatography (n-hexane/THF

85:15) to afford 3b as a colorless oil (43%); (400 MHz, CDCl₃, δ): 1.2−1.4

and 1.33 (m, s, 30H), 1.4−1.5 (m, 13H), 1.8−1.9 (m, 2H), 3.12 (t, J = 6.4

Hz, 2H), 3.98 (t, J = 6.4 Hz, 2H), 4.51 (br. s, 1H), 6.78 (s, 2H), 7.03 (s,

1H); ¹³C NMR (75 MHz, CDCl₃, δ): 26.1, 26.8, 28.4, 29.3, 29.5 (3 res.),

29.6, 30.1, 30.3, 31.5, 35.0, 40.6, 72.9, 79.1, 108.8, 114.7, 152.1, 156.0,

158.7 ppm; ESI-MS(+) m/z (%) [ion]: 499 (100) [M+Na]⁺.

General procedure for the synthesis of 4a-b: to a solution of the

proper N-protected amine (3a or 3b, 2.5 mmol) in dry DMF (50 mL), kept

under inert atmosphere and cooled at 0 °C through an external ice bath,

NaH (0.2 g of a 60% dispersion in mineral oil, ca. 5 mmol) was slowly

added. The resulting reaction mixture was stirred at room temperature for

3 h, then cooled again at 0 °C and the properα,ω-dibromoalkane (7.5

mmol) was added dropwise. After stirring for 18 h at room temperature,

the reaction was carefully quenched with water (40 mL) and extracted

with ethyl acetate (3 × 50 mL). The organic phase was dried over Na₂SO₄

and evaporated under reduced pressure.

General procedure for the sequential synthesis of two-station

rotaxanes R-UpShort and R-UpLong: the proper mono-stoppered axle

(6a or 6b, 0.06 mmol) was suspended in a solution of wheel Cx (0.09 g,

0.06 mmol) in anhydrous toluene (15 mL). The suspension was stirred at

room temperature for 3 hours until the solution turned homogeneous and

dark-red colored. Triethylamine (0.01 g, 0.1 mmol) and diphenylacetyl

chloride (0.03 g, 0.1 mmol) were then added, and the resulting reaction

mixture was stirred overnight at room temperature. After removing the

solvent under reduced pressure, the solid residue was portioned between

dichloromethane and water. The organic phase was separated and

evaporated under reduced pressure, and crude product was purified by

column chromatography (dichloromethane/methanol 50:1). The resulting

protected (N-Boc) rotaxane was then dissolved in 10 mL of anhydrous

dichloromethane and 2 mL of trifluoroacetic acid were added dropwise.

The solution turned yellow. After stirring at room temperature for 2 hours,

the solvent was evaporated under reduced pressure to afford the desired

deprotected rotaxane.

Tert-butyl

N-(6-bromohexyl)-N-[6-(3,5-di-tert-butylphenoxy)hexyl]

carbamate (4a): the residue was purified by column chromatography (n-

hexane/THF 95:5) to afford 4a as a colorless oil (0.75 g, 57%); ¹H NMR

(400 MHz, CDCl₃, δ): 1.33 (s, 18H) 1.4−1.6 and 1.48 (m, s, 21H), 1.8−1.9

(m, 4H), 3.1−3.2 (m, 4H), 3.42 (t, J = 6.8 Hz, 2H), 3.98 (t, J = 6.4 Hz, 2H),

6.77 (d, J = 1.6 Hz, 2H), 7.03 (t, J = 1.6 Hz, 1H); ¹³C NMR (100 MHz,

CDCl₃, δ): 26.0, 26.1, 26.7, 28.0, 28.5, 30.0, 31.5, 32.8, 33.8, 35.0, 46.9,

47.0, 67.6, 79.0, 108.8, 114.8, 152.1, 155.6, 158.6 ppm; ESI-MS(+) m/z

(%) [ion]: 590 (95), 591 (25), 592 (100) [M+Na]⁺.

R-UpShort: the product was isolated as a red solid (55%); mp > 300 °C

dec.; ¹H NMR (400 MHz, C₆D₆, δ): 0.8 (br. s, 2H), 0.8−1.0 (m, 17H), 1.1

(br. s, 6H), 1.2−1.4 and 1.33 (br. s, s, 68H), 1.5−1.7 and 1.64 (br. s, s,

46H), 1.8 (br. s, 12H), 1.9 (br. s, 2H), 2.00 (s, 12H), 2.10 (s, 4H), 2.7 (br.

s, 2H), 2.9 (br. s, 2H), 3.2−3.4 (m, 8H), 3.5−3.7 (2 br. s, 12H), 3.7−3.9

and 3.77 (m, s, 12H), 4.31 (t, J = 6.0 Hz, 2H), 4.42 (d, J = 12.0 Hz, 6H),

5.09 (s, 1 H), 6.59 (d, J =5.2 Hz, 2H), 6.7 (br. s, 2H), 6.9 (br. s, 3H), 7.0

(br. s., 14H), 7.2 (br. s, 11H), 7.42 (d, J = 7.2 Hz, 4H), 7.54 (s, 6H), 7.70

(d, J = 5.2 Hz, 2H), 7.83 (d, J = 6.8 Hz, 6H), 8.2 (br. s, 2H), 8.23 (d, J =

8.0 Hz, 4H), 9.0-9.3 (m, 7H); ¹³C NMR (100 MHz, C₆D₆, δ): 13.2, 20.0,

21.9, 24.6, 25.0, 25.2, 25.6, 25.7, 26.2, 27.1, 27.2, 28.3, 28.7, 28.8, 28.9,

29.9, 30.7, 31.1, 33.8, 33.9, 47.9, 48.2, 56.4, 59.8, 59.9, 60.3, 61.1, 62.8,

64.0, 66.5, 72.3, 108.3, 113.6, 115.8, 117.3, 120.4, 124.0, 124.8, 125.1,

125.7, 126.4, 126.7, 127.0, 127.3, 127.7 (2 res), 127.8, 128.0, 128.5,

128.6, 131.2, 132.8, 136.5, 138.2, 138.6, 140.3, 142.1, 142.5, 143.5,

145.3, 147.5, 151.1, 152.0, 152.5, 158.6, 171.1 ppm; HRMS (m/z): [M]²⁺

calcd for C₁₅₈H₂₀₉N₉O₁₂, 1212.3005; found, 1212.3027.

Tert-butyl

N-(12-bromododecyl)-N-[11-(3,5-di-tert-

butylphenoxy)undecyl] carbamate (4b): the residue was purified by

column chromatography (n-hexane/THF 95:5) to afford 4b as a colorless

oil (1.35 g, 58%).¹H NMR (400 MHz, CDCl₃, δ): 1.2−1.4 and 1.34 (m, s,

44H), 1.4−1.5 (m, 17H), 1.8−1.9 (2m, 4H), 3.2 (br. s, 4H), 3.43 (t, J = 6.8

Hz, 2H), 3.98 (t, J = 6.5 Hz, 2H), 6.78 (d, J = 1.6 Hz, 2H), 7.03 (t, J = 1.6

Hz, 1H); ¹³C NMR (100 MHz, CDCl₃, δ): 26.2, 26.9, 28.2, 28.5, 28.8, 29.4

(3 res.), 29.5 (4 res.), 29.6 (5 res.), 29.6, 30.4, 31.5, 32.9, 34.1, 35.0,

47.1, 67.8, 78.9, 108.8, 114.8, 152.1, 155.7, 158.7 ppm; ESI-MS(+) m/z

(%) [ion]: 744 (90), 745 (40), 746 (98), 747 (45) [M+Na]⁺.

General procedure for the synthesis of axles 6a-6b: in a sealed glass

autoclave, a solution of salt 5 (0.14 g, 0.3 mmol) and of the proper

bromide (4a or 4b, 0.6 mmol) in 5 mL of dry acetonitrile was refluxed for

7 days. After cooling to room temperature, the reaction mixture was

refrigerated at 0 °C until precipitation of a yellow solid was observed.

Axle 6a: the yellow precipitate was collected by Buchner filtration and

washed with cold acetonitrile (46%); mp = 197-198 °C; ¹H NMR (300

MHz, CD₃OD, δ): 1.33 (s, 18H), 1.4−1.7 and 1.48 (m, s, 25H), 1.7−1.9 (m,

R-UpLong: the product was isolated as a red solid (27%); mp > 300 °C

dec.; ¹H NMR (400 MHz, C₆D₆, δ): 0.6−1.0 (m, 9H), 1.1−1.3 (m, 57H),

1.4−1.6 (m, 12H), 1.69 (s, 18H), 1.8 (br. s, 4H), 1.99 (s, 6H), 2.59 (br. s,

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4H), 3.38 (d, J = 14.4 Hz, 6H), 3.5 (br. s, 4H), 3.7 (br. s, 6H), 3.79 (s, 9H),

3.88 (t, J = 7.0 Hz, 2H), 4.32 (t, J = 6.8 Hz, 2H), 4.46 (d, J =14.4 Hz, 6H),

5.11 (s, 1H), 6.9 (d, J = 5.2 Hz, 2H), 6.77 (t, J = 6.8 Hz, 3H), 6.83 (d, J =

5.2 Hz, 2H), 6.94 (d, J = 8.0 Hz, 4H), 7.0 (m, 4H), 7.15 (m, 7H), 7.43 (m,

4H), 7.59 (s, 6H), 7.82 (s, 6H), 7.88 (d, J = 5.2 Hz, 2H), 8.14 (d, J = 8.0

Hz, 4H), 9.2-9.4 (m, 7H); ¹³C NMR (100 MHz, C₆D₆, δ): 14.0, 20.8, 22.8,

25.7 (2 res.), 25.8 (2 res.), 26.1 (2 res.), 26.4, 26.5, 26.6 (2 res.), 26.7,

28.8, 29.1, 29.2, 29.5 (2 res.), 29.6, 29.7 (2 res.), 29.8 (2 res.), 30.7, 30.8,

31.3, 31.5, 31.6, 31.9, 32.0 (2 res.), 34.6, 34.7, 38.2, 47.4, 47.5, 57.2,

59.7, 60.8, 64.8, 67.5, 73.1, 109.2, 114.4, 116.6, 118.1, 121.2. 124.8,

125.6, 126.4, 127.2, 128.3, 128.4, 128.5, 128.6, 128.7, 128.8, 129.3,

131.7, 132.0, 133.6, 137.3, 139.0, 139.6, 141.2, 142.9, 143.1, 144.1,

148.2, 148.4, 151.9, 152.9, 153.3, 159.5 ppm; HRMS (m/z): [M]²⁺calcd

for C₁₆₉H₂₃₁N₉O₁₂, 1289.3866; found, 1289.3859.

added. The reaction mixture was refluxed for 3 days. The solution was

then cooled at room temperature and the solvent removed under reduced

pressure. The oily residue was taken up with ethyl acetate (100 mL) and

washed with water (3 × 50 mL). The separated organic phase was dried

over Na₂SO₄and evaporated under reduced pressure.

Tert-butyl N-(6-chlorohexyl)-N-(6-hydroxyhexyl) carbamate (9a): the

crude product was purified by column chromatography (n-hexane/THF

6:4) to afford chlorinated product 9a as a colorless oil (50%). ¹H NMR

(400 MHz, CDCl₃, δ): 1.3−1.4 (m, 4H), 1.47 (s, 9H), 1.4−1.6 (m, 8H), 1.6

(br. s, 2H), 1.8−1.9 (m, 2H), 3.2 (br. s, 4H), 3.55 (t, J = 6.4 Hz, 2H), 3.65

(t, J = 6 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃, δ): 26.2, 26.7, 28.5, 31.0,

32.6, 32.7, 45.0, 46.8, 62.6, 62.9, 79.1, 155.7 ppm; ESI-MS(+) m/z (%)

[ion]: 336 (100), 338 (30), 337 (20) [M+H]⁺; 358 (100), 360 (30) 359 (20)

[M+Na]⁺; 374 (100), 376 (30), 375 (20) [M+K]⁺.

Tert-butyl N-{11-[(tert-butyldimethylsilyl)oxy]undecyl} carbamate

(7b): Tert-butyl-chlorodimethylsilane (0.89 g, 5.9 mmol) was slowly

added to a solution of 1b (1.4 g, 5 mmol) and triethylamine (0.6 g, 6

mmol) in dry dichloromethane (100 mL). A catalytic amount of DMAP

was added. After stirring at room temperature for 2 hours, the reaction

was quenched with water (50 mL) and the organic phase was separated,

washed with brine, dried over anhydrous CaCl₂and filtered. The solvent

was removed under reduced pressure to afford 7b as a colorless oily

residue (92%). ¹H NMR (400 MHz, CDCl₃, δ): 0.05 (s, 6H), 0.90 (s, 9H),

1.3−1.5 (m, 14H), 1.4−1.5 (m, 13 H), 3.1 (br. s, 2H), 3.60 (t, J = 6.8 Hz,

2H), 4.5 (br. s, 1H); ¹³C NMR (100 MHz, CDCl₃, δ): −5.3, 14.1, 18.4, 22.6,

25.7, 25.8, 26.0, 26.8, 28.4, 29.3, 29.4, 29.5 (2 res.), 29.6, 30.1, 31.6,

32.9, 40.7, 63.3, 78.9, 156.0 ppm; ESI-MS(+) m/z (%) [ion]: 424 (100),

425 (30) [M+Na]⁺.

Tert-butyl N-(12-chlorododecyl)-N-(11-hydroxyundecyl) carbamate

(9b): the crude product was purified by column chromatography (n-

1

hexane/ethyl acetate 9:1) to afford 9b as a colorless oil (0.22 g, 60%). H

NMR (400 MHz, CDCl₃, δ) 1.2−1.3 (m, 28H), 1.45 (s, 9H), 1.4−1.6 (m,

8H), 1.7−1.8 (m, 2H), 3.13 (br. s, 4H), 3.53 (t, J = 6.8 Hz, 2H), 3.63 (t, J =

6.8 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃, δ): 25.7, 26.9, 28.5, 28.9, 29.4

(2 res.), 29.5 (3 res.), 29.6, 32.6, 32.8, 45.1, 47.0, 63.0, 78.9, 155.7 ppm;

ESI-MS(+) m/z (%) [ion]: 490 (100), 491 (30), 492 (30) [M+H⁺]; 512 (100),

513 (30), 514 (30) [M+Na⁺]; 528 (100), 529 (30), 530 (20) [M+K]⁺.

General procedure for the synthesis of axles 11a and 11b: 10 (0.092

g, 0.15 mmol) and the proper chloride 9a or 9b (0.3 mmol) were

dissolved in 5 mL of dry acetonitrile and placed in a glass autoclave.

After sealing the autoclave under inert atmosphere, the resulting reaction

mixture was refluxed under stirring for 7 days. After this period, the

reaction mixture was cooled to room temperature, diluted with 10 mL of

ethyl acetate and then placed in a refrigerator at 0 °C to promote the

precipitation of the desired axle.

General procedure for the synthesis of 8a-b: to a solution of the

proper N-protected amine (5a or 5b, 4.4 mmol) in dry DMF (50 mL), kept

under inert atmosphere and cooled at 0 °C through an external ice bath,

NaH (0.35 g of a 60% dispersion in mineral oil, ca. 9 mmol) was slowly

added. The resulting reaction mixture was stirred at room temperature for

3 h, and then cooled again at 0 °C and the proper α,ω-dibromoalkane

(17.4 mmol) was added dropwise. After stirring for 24 h at room

temperature, the reaction was carefully quenched with water (100 mL)

and extracted with ethyl acetate (3 × 50 mL). The organic phase was

dried over Na₂SO₄and evaporated under reduced pressure.

Axle 11a: a sticky white solid was collected by Buchner filtration and

washed with cold ethyl acetate (65%). ¹H NMR (400 MHz, CD₃OD, δ):

1.3−1.6 (m, 36H), 1.83 (t, J = 6.8 Hz, 2H), 2.14 (t, J = 7.2 Hz, 4H), 2.37 (s,

6H), 3.1−3.2 (m, 4H), 3.5−3.6 (m, 2H), 3.99 (t, J = 6.0 Hz, 2H), 4.76 (t, J =

7.2 Hz, 4H), 6.74 (s, 2H), 7.03 (s, 1H), 7.24 (d, J = 8 Hz, 4H), 7.70 (d, J

=8 Hz, 4H), 8.66 (d, J = 6.0 Hz, 4H), 9.27 (d, J = 6.0 Hz, 4H); ¹³C NMR

(100 MHz, CD₃OD, δ) 19.9, 25.3, 25.6, 26.3, 27.4, 28.9, 30.5, 31.1, 32.2,

34.4, 61.2, 61.4, 61.8, 67.0, 79.3, 108.5, 114.4, 125.6, 126.9, 128.5,

140.3, 145.7, 149.9, 151.9, 156.0, 158.7, 160.7 ppm; ESI-MS(+) m/z (%)

[ion]: 745 (100), 746 (50), 747 (20) [M-H]⁺; Anal. Calcd for

C₅₄H₈₂ClN₃O₇S: C, 68.07; H, 8.68; N, 4.41. Found: C, 67.91; H, 8.32; N,

4.05%.

Tert-butyl N-(6-bromohexyl)-N-{6-[(tert-butyldimethylsilyl)oxy]hexyl}

carbamate (8a): the residue was purified by column chromatography (n-

hexane/THF 9:1) to afford 8a as a colorless oil (1.19 g, 55%). ¹H NMR

(400 MHz, CDCl₃, δ): 0.009 (s, 6H), 0.86 (s, 9H), 1.2−1.3 (m, 4H), 1.42 (s,

9H), 1.4−1.5 (m, 10 H), 1.8−1.9 (m, 2H), 3.1 (br. s, 4H), 3.36 (t, J =7 Hz,

2H), 3.56 (t, J = 8 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃, δ): −5.26, 14.1,

18.4, 22.7, 25.6, 25.9, 26.1, 26.7, 28.0, 29.7,30.3, 32.8, 33.8, 46.8, 47.0,

63.2, 79.0, 155.6 ppm; ESI-MS(+) m/z (%) [ion]: 516 (100), 518 (97), 519

(30) [M+Na]⁺.

Axle 11b: a sticky yellow solid was collected by Buchner filtration (88%).

¹H NMR (400 MHz, CD₃OD, δ): 1.3−1.6 (m, 67H), 1.8−1.9 (m, 2H),

2.1−2.2 (m, 4H), 2.39 (s, 3 H), 3.19 (t, J = 7.6 Hz), 3.55 (t, J = 6.8 Hz, 2H),

4.00 (t, J = 6.4 Hz, 2H), 4.8 (m, 4H), 6.75 (s, 2H), 7.02 (s, 1H), 7.25 (d, J

= 8 Hz, 2H), 7.72 (d, J =8 Hz, 2H), 8.71 (d, J = 6.4 Hz, 4H), 9.3 (m, 4H);

¹³C NMR (100 MHz, CD₃OD, δ): 25.6, 25.9, 26.5 (2 res.), 27.4, 28.8, 28.9,

29.1, 29.2, 29.3, 30.5, 31.1, 31.2, 32.3, 34.4, 61.6, 61.8, 61.9, 67.1, 79.2,

108.6, 125.6, 127.1, 128.5, 140.3, 142.1, 145.7, 149.8, 151.8, 156.1,

158.8 ppm; ESI-MS(+) m/z (%) [ion]: 900 (100) [M-H]⁺; Anal. Calcd for

C₆₅H₁₀₄ClN₃O₇S: C, 70.52; H, 9.47; N, 3.80. Found: C, 69.77; H, 9.12; N,

3.15 %.

Tert-butyl

N-(12-bromododecyl)-N-{11-[(tert-

butyldimethylsilyl)oxy]undecyl} carbamate (8b): the residue was

purified by column chromatography (n-hexane/THF 98:2) to afford 8b as

a colorless oil (2.52 g, 54%). ¹H NMR (300 MHz, CDCl₃, δ): 0.06 (s, 6H),

0.91 (s, 9H), 1.2−1.3 (m, 28H), 1.46 (s, 9H), 1.4−1.6 (m, 8H), 1.8−1.9 (m,

2H), 3.15 (t, J = 7 Hz, 4H), 3.42 (t, J = 6.8 Hz, 2H), 3.61 (t, J = 6.6 Hz,

2H); ¹³C NMR (100 MHz, CDCl₃, δ): −5.3, 25.8, 26.0, 26.9, 28.2, 28.5,

28.8, 29.4, 29.5, 29.6, 32.8, 32.9, 34.0, 47.0, 63.3, 78.8, 155.6 ppm; ESI-

MS(+) m/z (%) [ion]: 648 (95), 650 (100), 651 (40) [M+H]⁺.

General procedure for the synthesis of 12a and 12b: bromides 4a or

4b (0.3 mmol) and 4,4’-bipiridyne (1.0 mmol) were dissolved in dry

acetonitrile (50 mL) and refluxed overnight. The solvent was then

General procedure for the synthesis of 9a-b: to a solution of the

proper bromide 8a or 8b (1.6 mmol) in a mixture of dry acetonitrile (80

mL) and DMF (10 mL), anhydrous cerium(III) chloride (3.1 mmol) was

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removed under reduced pressure to afford a crude solid residue that was

2H), 3.9−4.0 (m, 11H), 4.04 (t, J = 6.8 Hz, 2H), 4.47 (d, J = 14.8 Hz, 6H),

5.13 (s, 1H), 6.55 (d, J = 5.2 Hz, 2H), 6.68 (t, J = 7.2 Hz, 3H), 6.85 (d, J

=5.2 Hz, 2H), 6.93 (d, J = 8.4 Hz, 4H), 7.0−7.1 (m, 8H), 7.12 (s, 1H),

7.1−7.2 and 7.19 (m, s, 8H), 7.44 (d, J = 7.2 Hz, 6H), 7.58 (s, 6H), 7.72

(d, J = 5.2 Hz, 2H), 7.8 (br. s, 6H), 7.9 (br. s, 4H), 8.14 (d, J = 8.4 Hz, 4H),

9.3-9.6 (m, 7H); ¹³C NMR (100 MHz, C₆D₆, δ): 14.1, 20.8, 22.8, 24.9,

25.5, 25.8, 26.4, 26.7 (2 res.), 26.9, 28.0, 28.3, 28.6, 29.1, 29.2, 29.5,

29.6, 29.7 (3 res.), 29.8 (2 res.), 29.9, 30.0, 30.4, 30.6, 30.8, 31.2, 31.3,

31.5, 31.7, 32.0, 34.6, 34.8, 47.2 (2 res.), 57.4, 60.5, 60.8, 61.1, 64.6,

67.4, 73.0,109.1, 114.6, 116.7, 118.0, 121.3, 124.7, 125.6, 125.4, 127.2,

127.6, 128.6, 128.7 (2 res.), 128.9, 129.3, 132.0, 133.7, 137.3, 139.2,

139.6, 141.0, 142.9, 143.1, 144.1, 147.9, 148.3, 152.0, 152.8, 153.2,

triturated with ethyl acetate and hexane (4 × 25 mL).

1-(3-{[(tert-butoxy)carbonyl]

[6-(3,5-di-tert-butylphenoxy)

hexyl]amino}hexyl–[4,4’-bipyridin]-1-ium bromide (12a): after suction

filtration 12a was obtained as a sticky yellow solid (52%). ¹H NMR (300

MHz, CD₃OD, δ): 1.30 (s, 18H), 1.30−1.6 (m, 21H), 1.78 (t, J =7.5 Hz,

2H), 2.19 (t, J =6.8 Hz, 2H), 3.22 (t, J = 7.3 Hz, 4H), 3.97 (t, J = 7.5 Hz,

2H), 4.75 (t, J = 6.8 Hz, 4H), 6.73 (s, 2H), 7.01 (s, 1H), 8.04 (d, J = 8 Hz,

2H), 8.56 (d, J = 8 Hz, 2H), 8.84 (d, J = 8 Hz, 2H), 9.19 (d, J = 8 Hz, 2H);

¹³C NMR (75 MHz, CD₃OD, δ): 25.6 (2 res.), 26.3, 27.5, 28.3, 29.1, 30.6,

31.0, 31.1, 34.4, 46.5, 61.9, 67.4, 79.3, 108.6, 114.3, 122.3, 125.8, 142.2,

145.2, 150.4, 151.8, 153.5, 156.0, 158.8 ppm; ESI-MS(+) m/z (%) [ion]:

645 (100), 646 (95), 647 (80) [M]⁺.

159.4, 172.0 ppm; HRMS (m/z): [M]²⁺calcd for C₁₆₉H₂₃₁N₉O₁₂

,

1289.3866; found, 1289.3857.

General procedure for the synthesis of dumbbells DB-Short and DB-

Long: 14 (1 eq.) and the appropriate alkylating agent (4a for DB-Short

or 4b for DB-Long, 2 eq.) were dissolved in 20 mL of dry acetonitrile and

placed in a glass autoclave. After sealing the autoclave under inert

atmosphere, the resulting reaction mixture was refluxed under stirring for

7 days. After this period, the reaction mixture was cooled to room

temperature and the solvent was evaporated to dryness under reduced

pressure. The crude residue was taken up with ethyl acetate (10 mL) and

placed in a refrigerator. The solid that precipitated upon standing in the

refrigerator was collected by Buchner filtration and then dissolved in

dichloromethane (50 mL). The resulting solution was treated with 5 mL of

trifluoroacetic acid and stirred at room temperature for 3 hours. After this

period, the solvent was evaporated to dryness under reduced pressure to

afford the dumbbells DB-Short and DB-Long.

1-(3-{[(tert-butoxy)carbonyl]

[11-(3,5-di-tert-butylphenoxy)

undecyl]amino}dodecyl–[4,4’-bipyridin]-1-ium bromide (12b): after

suction filtration 12b was obtained as a sticky yellow solid (60%). ¹H

NMR (400 MHz, CDCl₃, δ): 1.2–1.4 (m, 52H), 1.77 (t, J = 8 Hz, 2H), 2.08

(t, J =7 Hz, 2H), 3.18 (t, J = 7.4 Hz, 4H), 3.97 (t, J = 8 Hz, 2H), 4.73 (t, J

= 7 Hz, 2H), 6.73 (s, 2H), 7.02 (s, 1H), 8.03 (d, J = 8 Hz, 2H), 8.55 (d, J =

8 Hz, 2H), 8.85 (d, J = 8 Hz, 2H), 9.17 (d, J = 8 Hz, 2H); ¹³C NMR (100

MHz, CDCl_3,δ) 25.8, 26.5 (2 res.), 27.5, 28.8, 29.1, 29.2 (2 res.), 29.3,

29.4, 30.6, 31.1, 34.4, 61.3, 67.5, 79.4, 108.6, 114.5, 122.4, 126.1, 142.2,

145.5, 150.5, 152.0, 153.7, 156.2, 159.1 ppm; ESI-MS(+) m/z (%) [ion]:

799 (100) 800 (80) 801 (40) [M]⁺.

General procedure for the supramolecularly assisted synthesis of

down rotaxanes: wheel Cx (0.04 mmol), salt 12a or 12b (0.06 mmol)

and bromide 13 (0.08 mmol) were suspended in dry toluene (15 mL), and

the mixture was stirred for two days at 110°C. After few hours, the

mixture turned red and homogeneous. The solvent was then removed

under reduced pressure and the residue was portioned between

dichloromethane and water. The separated organic phase was

evaporated under reduced pressure, and crude product was purified by

column chromatography (dichloromethane/methanol 50:1). Isolated Boc-

protected rotaxane was then dissolved in 5 mL of dry dichloromethane

and 5 mL of trifluoroacetic acid were added dropwise. The red solution

turned yellow. After stirring at room temperature for 2 hours, the solvent

was evaporated under reduced pressure to give the deprotected

rotaxane.

DB-Short: (60%). ¹H NMR (400 MHz, CD₃OD, δ): 1.31 (s, 18H), 1.4−1.6

(m, 14H), 1.6 (br. s, 2H), 1.7 (br. s, 4H), 2.0 (br. s, 2H), 2.1 (br. s, 2H),

2.33 (s, 3H), 3.0 (br. s, 4H), 3.97 (t, J = 6 Hz, 2H), 4.16 (t, J = 6 Hz, 2H),

4.67 (t, J = 7.6 Hz, 2H), 4.77 (t, J = 7.6 Hz, 2H), 5.08 (s, 1H), 6.75 (s, 2H),

7.03 (s, 1H), 7.2−7.3 (m, 12H), 7.69 (d,J = 8 Hz, 2H), 8.64 (d, J = 6 Hz,

4H), 9.22 (d, J = 6 Hz, 2H), 9.28 (d, J = 6 Hz, 2H); ¹³C NMR (100 MHz,

CD₃OD, δ): 20.0, 24.8, 25.1, 25.2, 25.4 (2 res.), 25.8, 25.9, 27.9, 28.9,

30.5, 30.7, 30.9, 34.4, 56.9, 61.6, 61.7, 64.5, 67.2, 108.6, 114.4, 125.5,

126.9, 128.2, 128.3, 128.6, 138.9, 140.4, 142.3, 145.6, 145.7, 149.8,

149.9, 151.9, 158.8, 172.9 ppm; ESI-MS(+) m/z (%) [ion]: 839 (100) [M-

H]⁺.

DB-Long: (55%). ¹H NMR (400 MHz, CD₃OD, δ): 1.31 (s, 18H), 1.4−1.5

(m, 36H), 1.7 (br. s, 4H), 1.8 (br. s, 2H), 2.0 (br. s, 2H), 2.1 (br. s, 2H),

2.36 (s, 3H), 2.97 (t, J = 8 Hz, 4H), 3.97 (t, J =6.4 Hz, 2H), 4.17 (t, J = 6.4

Hz, 2H), 4.68 (t, J = 7.2 Hz, 2H), 4.74 (t, J = 7.2 Hz, 2H), 5.08 (s, 1H),

6.74 (s, 2H), 7.02 (s, 1H), 7.2−7.4 (m, 12H), 7.69 (d, J = 8 Hz, 2H), 8.65

(d, J = 6 Hz, 4H), 9.22 (d, J = 6 Hz, 2H), 9.26 (d, J = 6 Hz, 2H); ¹³C NMR

(100 MHz, CD₃OD, δ): 20.0, 24.8, 25.1, 25.8, 25.9, 26.2, 27.8, 28.8 (3

res.), 29.0, 29.1, 29.2 (2 res.), 30.5, 30.9, 31.2, 34.4, 56.9, 61.7, 61.9,

64.5, 67.5, 108.6, 114.3, 125.5, 126.9, 128.2, 128.3, 128.5, 138.9, 140.4,

145.7, 149.9 (2 res.), 151.8, 158.8, 172.9 ppm; ESI-MS(+) m/z (%) [ion]:

993 (100) [M-H]⁺.

R-DownShort: the rotaxane was isolated as a red solid (0.06 g, 58%).

mp > 300 °C dec.; ¹H NMR (400 MHz, C₆D₆, δ): 0.76 (br. s, 2H), 0.9−1.0

(m, 9H), 1.1−1.4 and 1.35 (m, s, 61H), 1.6 (m, 2H), 1.63 (s, 18H), 1.8 (br.

s, 2H), 1.9 (br. s, 8H), 2.00 (s, 6H), 2.2 (br. s, 4H), 2.9 (br. s, 2H), 3.2 (br.

s, 2H), 3.4−3.5 (m, 8H), 3.8 (br. s, 6H), 3.8 (br. s, 2H), 3.9 (br. s, 2H),

3.93 (s, 9H), 4.03 (t, J = 7.2 Hz, 2H), 4.50 (t, J = 15 Hz, 6H), 5.12 (s, 1H),

6.6−6.7 (m, 5H), 6.82 (d, J = 5.2 Hz, 2H), 7.0 (br. s, 11H), 7.2 (br. s, 10H),

7.4−7.5 (m, 8H), 7.58 (s, 6H), 7.80 (d, J = 5.2 Hz, 2H), 7.9 (br. s, 6H),

8.03 (d, J = 5.2 Hz, 2H), 8.14 (d, J = 8 Hz, 4H), 9.2−9.6 (m, 7H); ¹³C

NMR (100 MHz, C₆D₆, δ): 14.1, 20.8, 22.8, 24.9, 25.5, 25.6, 25.9, 26.4,

26.7, 28.3, 29.3, 29.6, 29.8, 30.7, 31.3, 31.5, 32.0, 34.6, 34.8, 38.2, 46.8,

57.4, 60.5, 61.1, 64.7, 67.1, 73.3, 109.0, 114.8, 116.8, 118.1, 121.3,

124.8, 125.7, 126.4, 127.2, 128.6, 128.7 (2 res.), 128.8129.3, 132.0,

133.6, 137.4, 139.2, 139.7, 141.1, 142.9, 143.2, 144.3, 146.3, 148.3,

148.4, 152.1, 152.8, 153.3, 159.2, 172.0 ppm; HRMS (m/z): [M]²⁺calcd

for C₁₅₈H₂₀₉N₉O₁₂, 1212.3005; found, 1212.3013

Electrochemical measurements and simulation. Cyclic voltammetric

(CV) experiments were carried out at room temperature in argon-purged

acetonitrile or acetone with an Autolab 30 multipurpose instrument

interfaced to a PC using a glassy carbon as the working electrode, a Pt

wire as the counter electrode, and an Ag wire as a quasi-reference

electrode. The oxidation wave of ferrocene [E_1/2(Fc⁺/Fc) = +0.395 vs

SCE], added as a standard, was used to calibrate the potential scale and

assess electrochemical reversibility. The concentration of the compounds

R-DownLong: product was isolated as a red solid (0.08 g, 69%). mp >

300 °C dec.; ¹H NMR (400 MHz, C₆D₆, δ): 0.7 (br. s, 2H), 0.9 (br .s, 2H),

0.99 (t, J = 6 Hz, 9H), 1.1−1.5 and 1.33 (m, s, 69H), 1.5−1.7 and 1.69 (m,

s, 44H), 1.8 (br. s, 4H), 1.9 (br. s, 4H), 1.98 (s, 6H), 2.2 (br. s, 2H), 2.6 (br.

s, 4H), 3.37 (d, J = 14.8 Hz, 6H), 3.6 (br. s, 2H), 3.7 (br. s, 6H), 3.8 (br. s,

examined was in the range 2×10⁻⁴

M

−

tetraetylammonium hexafluorophosphate (TEAPF₆) 0.04 M was used as

4×10⁻⁴M, and

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the supporting electrolyte. Scan rates varying typically from 50 to 1000

mV s^–1were utilized. Differential pulse voltammograms (DPV) were

performed with a scan rate of 20 mV s^–1, 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 CV peaks. The potential values for not fully

reversible processes were estimated from the DPV peaks. The

experimental error on the potential values was estimated to be ±10 and

±20 mV, respectively. Digital fitting of the experimental CV for the R-

UpShort system was obtained on the basis of the square scheme

mechanism illustrated in Figure 5 by using the software package DigiSim

3.05.^[48]The redox potentials were fixed in the simulation and the

shuttling equilibrium constant in the monoreduced form was fixed to 10;

shuttling rate constants were optimized in the simulation. A value of 1 cm

s^-1, representative of a reversible process under our conditions was

employed for the electron transfer rates.^[49]The charge-transfer

coefficients were taken as 0.5 in all cases.

[8]

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appropriate substrate (ca.

1 mM) containing tetrabutylammonium

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The authors thank Centro Interdipartimentale di Misure of the

University of Parma for NMR and MS measurements. This work

was supported by the Italian MIUR (PRIN 2010CX2TLM) and

the Universities of Parma and Bologna (FARB SLaMM project).

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Keywords: calixarenes • molecular machines • switches •

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2009, 0, 5515.

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Valeria Zanichelli, Margherita Bazzoni,

Arturo Arduini, Paola Franchi, Marco

Lucarini, Giulio Ragazzon,* Andrea

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Page No. – Page No.

Redox-Switchable Calix[6]Arene-

based Isomeric Rotaxanes

One-way rotaxanes: a series of isomeric and redox-switchable calix[6]arene-based

rotaxanes have been synthesized with a reagent-less strategy. It was demonstrated

that the orientation of the calix[6]arene wheel can give rise to a thermodynamic

preference for shuttling among orientational isomers.

Article Doi

Redox-Switchable Calix[6]arene-Based Isomeric Rotaxanes

DOI: 10.1002/chem.201800496

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Article abstract of DOI:10.1002/chem.201800496

Full text of DOI:10.1002/chem.201800496

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