Amphiphilic bistable rotaxanes

FULL PAPER

Amphiphilic Bistable Rotaxanes

Jan O. Jeppesen,*^{[a, b]}Kent A. Nielsen,^{[a, b]}Julie Perkins,^[a]Scott A. Vignon,^[a]

[c]

Alberto Di Fabio,^[c]Roberto Ballardini,^{[c, d]}M. Teresa Gandolfi,*^[c]Margherita Venturi,*

Vincenzo Balzani,^[c]Jan Becher,^[b]and J. Fraser Stoddart*^[a]

Abstract: Two

molecular

shuttles/

component and cyclobis(paraquat-p-

phenylene) as the ring component, has

been investigated. The dumbbell-shaped

components were constructed using con-

ventional synthetic methodologies to

assemble 1) the hydrophobic, tetraaryl-

methane stopper and 2) the hydrophilic,

dendritic stopper. Next, 3) the hydro-

phobic stopper was fused to the 1,5-

dioxynaphthalene moiety and/or the

monopyrrolotetrathiafulvalene unit by

appropriate alkylations, followed by 4)

attachment of the hydrophilic stopper,

once again by alkylation to give the

dumbbell-shaped compounds. Finally, 5)

the [2]rotaxanes were self-assembled by

using the dumbbells as templates for the

formation of the encircling cyclobis-

(paraquat-p-phenylene) tetracations. The

two [2]rotaxanes differ in their arrange-

ment of the p-electron-rich units, one in

which the SMe group of the monopyr-

rolotetrathiafulvalene unit points to-

ward the 1,5-dioxynaphthalene moiety

(2 ¥ 4PF₆) and another in which it points

away from the 1,5-dioxynaphthalene

moiety (3 ¥ 4PF₆). This seemingly small

difference in the orientation of the

conformations wherein the monopyrro-

lotetrathiafulvalene unit is involved in

an ™alongside∫ interaction with the tet-

racationic cyclophane. In the molecular

shuttle/switch 2 ¥ 4PF₆a ™knob∫, in the

shape of the SMe group, is situated

between the monopyrrolotetrathiafulva-

lene and the 1,5-dioxynaphthalene rec-

ognition sites, making it possible to

isolate both translational isomers

(2 ¥ 4PF₆¥ GREEN and 2 ¥ 4PF₆¥ RED)

and to investigate the kinetics of the

shuttling of the cyclobis(paraquat-p-

phenylene) tetracation between the

two recognition sites. The shuttling

processes, which are accompanied by

clearly detectable color changes, can be

followed by ¹H NMR and UV-visible

spectroscopy, allowing the rate constants

and energies of activation for the trans-

lation of the cyclobis(paraquat-p-phe-

nylene) tetracations between the two

recognition sites to be determined. In

the molecular shuttle/switch 3 ¥ 4PF₆,

there is no ™knob∫ situated between

the 1,5-dioxynaphthalene and the mo-

nopyrrolotetrathiafulvalene recognition

sites, resulting in a considerably faster

shuttling of the cyclobis(paraquat-p-

phenylene) tetracation between these

two sites, making the separation of the

two possible translational isomers of 3 ¥

4PF₆impractical. However, the shut-

tling of the cyclobis(paraquat-p-phenyl-

ene) tetracation can be followed by

switches–a slow one and a fast one–

in the shape of amphiphilic, bistable

[2]rotaxanes have been synthesized and

characterized. Both [2]rotaxanes con-

tain a hydrophobic, tetraarylmethane

and a hydrophilic, dendritic stopper.

They are comprised of two p-electron-

rich stations–a monopyrrolotetrathia-

fulvalene unit and a 1,5-dioxynaphtha-

lene moiety–which can act as recogni-

tion sites for the tetracationic cyclo-

phane, cyclobis(paraquat-p-phenylene),

to reside around. In addition, a model

[2]rotaxane, incorporating only a mono-

pyrrolotetrathiafulvalene unit in the

rod section of the amphiphilic dumbbell

[a] Prof. J. F. Stoddart, Dr. J. O. Jeppesen,

K. A. Nielsen, Dr. J. Perkins, S. A. Vignon

Department of Chemistry and Biochemistry

University of California, Los Angeles

405 Hilgard Avenue, Los Angeles,

CA 90095-1569 (USA)

Fax : (1)310-206-1843

E-mail: stoddart@chem.ucla.edu

joj@chem.sdu.dk

monopyrrolotetrathiafulvalene

unit

[b] Dr. J. O. Jeppesen, K. A. Nielsen,

Prof. J. Becher

leads to profound changes in the phys-

ical properties of these rotaxanes. The

bistable [2]rotaxanes were both isolated

as brown solids. ¹H NMR and UV-

visible spectroscopy, and electrochemi-

cal investigations, reveal the presence of

both possible translational isomers at

ambient temperature. As a consequence

of the existence of both possible trans-

lational isomers in these bistable [2]ro-

taxanes, they exhibit a complex electro-

chemical behavior, which is further

complicated by the presence of folded

Department of Chemistry

Odense University

(University of Southern Denmark)

Campusvej 55,

DK-5230, Odense M (Denmark)

Fax : (45)66-15-87-80

1

[c] Prof. M. T. Gandolfi, Prof. M. Venturi,

A. Di Fabio, Dr. R. Ballardini,

Prof. V. Balzani

dynamic H NMR spectroscopy. At low

temperatures, the major translational

isomer is 3 ¥ 4PF₆¥ RED, while 3 ¥ 4PF₆¥

GREEN is the major isomer at higher

temperature. In the bistable [2]rotax-

anes shuttling of the cyclobis(paraquat-

p-phenylene) tetracations can be driven

by electrochemical oxidation of the

monopyrrolotetrathiafulvalene unit. In

complexes in which one of the two

dumbbell stoppers is missing, electro-

chemical oxidation causes dethreading.

Dipartimento di Chimica ™G. Ciamician∫

¡

Universita di Bologna

Via Selmi 2, 40126 Bologna (Italy)

Fax : (39)051-209-9456

E-mail: vbalzani@ciam.unibo.it

[d] Dr. R. Ballardini

ISOF-CNR Institute

Via Gobetti 101, 40129 Bologna (Italy)

Keywords:

molecular devices

self-assembly ¥ tetrathiafulvalenes

electrochemistry

¥

Supporting information for this article is

available on the WWW under http://

www.chemeurj.org/ or from the author.

¥

rotaxanes

2982

¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI: 10.1002/chem.200204589

Chem. Eur. J. 2003, 9, 2982 3007

2982 3007

try^[14]–is an ideal redox-active unit in view of its excellent p-

Introduction

electron-donating properties. This led to its forming^[15]

a

A [2]rotaxane^[1]that contains two different recognition sites

(™stations∫) in its dumbell-shaped component can exist as two

different translational isomers, whose populations reflect

their relative free energies as determined primarily by the

strength of the two different sets of noncovalent bonding

interactions. In suitably designed [2]rotaxanes, the ring

component resides preferentially (in ideal cases only) on

one of the two recognition sites. In these systems, the

properties of the preferential recognition site can be rever-

sibly altered by external stimuli, such as light excitation, redox

processes (either heterogeneous or homogeneous), and acid/

base reactions. As a consequence, shuttling of the ring

component between the two stations can occur.^[2]Because

of the extended molecular rearrangement involved in these

shuttling processes, nondegenerate [2]rotaxanes are prime

candidates for the construction of artificial molecular ma-

chines^{[3, 4, 5]}and for the fabrication of molecular electronic

devices.^{[6, 7, 8]}A number of different protocols, based on self-

assembly^[9]have been developed^{[2, 10, 11]}for the template-

directed syntheses^[12]of rotaxanes. The desirable features for

the redox-controllable, amphiphilic [2]rotaxanes that have

been employed^[13]to fabricate single-molecule thick electro-

chemical junctions in electronic devices include 1) the siting of

redox-active units along the rod section of the dumbbell

component and 2) the presence of both hydrophobic and

hydrophilic groups as stoppers at the ends of the dumbbell

component.

strong green 1:1 complex (K_a 8000m^À1in MeCN)^[2d]with the

p-electron-accepting tetracationic cyclophane,^[16]cyclobis-

(paraquat-p-phenylene) (CBPQT⁴), making it ideal for

incorporation into redox-switchable [2]rotaxanes, along with

a 1,5-dioxynaphthalene (DNP) moiety which also interacts,

but somewhat more weakly^[17]with CBPQT⁴, affording a red

color in the process. Although a TTF unit and a DNP moiety

have been incorporated^[18]into the crown ether component of

a redox-switchable [2]catenane–a compound which has

already been employed in the fabrication of a solid-state

electronically-reconfigurable switch^[6]–no rotaxanes employ-

ing these two recognition sites for a CBPQT⁴component had

been described in the literature^[19]prior to the publication of a

preliminary communication^[20]describing a small part of the

research reported in this paper. In addition, rotaxanes

incorporating TTF units in dumbbell components comprising

two different stoppers had been unknown hitherto, most

likely because of the lack of an appropriate TTF building

block. Now that such a building block is available in the shape

of the pyrrolo[3,4-d]tetrathiafulvalene unit,^[21]we have de-

signed amphiphilic bistable [2]rotaxanes in which the ring

component is CBPQT⁴and the dumbbell component–

containing a monopyrrolo-TTF (MPTTF) unit and a DNP

moiety within its rod section–is terminated by a hydrophilic

dendritic stopper at one end and a hydrophobic tetraaryl-

methane stopper at the other end. Here, we describe the

template-directed syntheses of three amphiphilic [2]rotaxanes

(Figure 1), namely 1) a model compound 1 ¥ 4PF₆comprising

only an MPTTF unit in the dumbbell-shaped component, 2) a

In the context of such devices, the tetrathiafulvalene (TTF)

unit–which has found widespread use in materials chemis-

Figure 1. Molecular formulas of the single-station [2]rotaxane 1 ¥ 4PF₆, the slow two-station [2]rotaxane 2 ¥ 4PF₆, and the fast two-station [2]rotaxane 3 ¥ 4PF₆

(only one translational isomer is shown in each case).

Chem. Eur. J. 2003, 9, 2982 3007

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slow molecular shuttle/switch 2 ¥ 4PF₆with a ™knob∫ in the

shape of an SMe group situated between the MPTTF and

DNP recognition sites, hindering fast interconversion be-

tween the two translational isomers, and 3) a fast molecular

shuttle/switch 3 ¥ 4PF₆without a ™knob∫ between the two

recognition sites. Thereafter, we describe and discuss some

mass spectrometric, photophysical, electrochemical, and

¹H NMR spectroscopic investigations of the [2]rotaxanes 1 ¥

4PF₆, 2 ¥ 4PF₆, and 3 ¥ 4PF₆and their corresponding dumbbell

compounds, together with an extensive range of model

compounds and complexes. Next the separation of the two

possible translational isomers of the slow molecular shuttle/

switch 2 ¥ 4PF₆is described, and the kinetic and thermody-

namic processes involved in their slow interconversion are

discussed. Finally, it is shown that the relative populations of

the two stable translational isomers of the fast molecular

shuttle/switch 3 ¥ 4PF₆in solution are heavily temperature

dependent.

Synthesis: The syntheses of the hydrophobic stopper 11^{[13a, 22]}

and the hydrophilic stoppers 16 and 22^{[13a, 22]}have already

been reported. Here, we describe improved syntheses of the

MPTTF building block 6^{[21b, 22]}and the single-station [2]rotax-

ane 1 ¥ 4PF₆,^{[13a, 22]}before outlining the syntheses of the slow

[20]

molecular shuttle/switch 2 ¥ 4PF₆

shuttle/switch 3 ¥ 4PF₆.^[13b]

and the fast molecular

Our improved synthesis of the asymmetric MPTTF building

block 6 was carried out as outlined in Scheme 1. Cross-

coupling of 5-tosyl-(1,3)-dithiolo[4,5-c]pyrrole-2-one (4)^[21]

with two equivalents of 4-(2-cyanoethylthio)-5-methylthio-

1,3-dithiole-2-thione (5)^[23]in neat (EtO)₃P gave 6 (64%) in

gram quantities after column chromatography.^[24]

SMe

S

Ts N

O

+

S

CN

4

5

64%

SMe

S

(EtO)₃P

S

135 °C / 1.5 h

S

CN

Results and Discussion

6

Scheme 1. Synthesis of the asymmetric MPTTF building block 6.

Design and synthetic strategy: Retrosynthetic analyses of the

amphiphilic [2]rotaxanes 1 ¥ 4PF₆, 2 ¥ 4PF₆, and 3 ¥ 4PF₆(Fig-

ure 1) reveal a range of possible disconnections. In the

synthetic approaches that were eventually adopted, the

dumbbell-shaped components were constructed by using

conventional methodologies, followed by clipping of the

tetracationic cyclophane CBPQT⁴employing the dumbbell-

shaped components as templates. The syntheses (Scheme 1

6) of the amphiphilic dumbbells 7, 17, and 23, require up to

four different types of components: 1) a hydrophobic stopper,

2) a hydrophilic stopper, 3) an MPTTF unit, and 4) a DNP

moiety. In order to minimize the number of synthetic steps, it

was decided to prepare all the dumbbell-shaped components

from common intermediates. For the hydrophobic stopper, a

tetraarylmethane-based phenol 11 (Scheme 3),^{[13a, 22]}which

can be easily functionalized by using simple alkylation

reactions, was chosen. For the hydrophilic stopper, den-

drons 16 (Scheme 3) or 22

We have previously reported^{[13a, 22]}that the single-station

[2]rotaxane 1 ¥ 4PF₆can be self-assembled under ambient

conditions by using the dumbbell-shaped component 7 as the

template for the formation of the encircling CBPQT⁴

[10]

tetracation from the dicationic precursor 8 ¥ 2PF₆and the

dibromide 9. However, the yield was only 8% in this

template-directed reaction.^[25]By employing high-pressure

conditions,^[26]the supramolecular assistance to covalent syn-

thesis (Scheme 2) was considerably enhanced, and the single-

station [2]rotaxane 1 ¥ 4PF₆was isolated in 21% yield from a

mixture of the dumbbell 7, 8 ¥ 2PF₆, and 9 subjected to a

10 kbar pressure in DMF at room temperature for three days.

The dumbbell 17–the precursor to the slow molecular

shuttle/switch 2 ¥ 4PF₆–was synthesized as shown in

Scheme 3. Alkylation of the monotosylate 10^{[13a, 20]}with the

(Scheme 5),^{[13a, 22]}which both

contain glycol chains and can

be functionalized by nucleophilic

substitution reactions, were

chosen. The DNP moiety was

introduced as the monotosylate

10 (Scheme 3).^{[13a, 20]}For the TTF

unit, the asymmetric MPTTF

building block 6 (Scheme 1),^{[21b, 22]}

which can be functionalized re-

gioselectively by means of sim-

ple S-and Na- lkylations in high

yields, was chosen. With these

components, the syntheses of the

amphiphilic [2]rotaxanes 1¥4PF₆,

2 ¥ 4PF₆, and 3 ¥ 4PF₆were

achieved (Scheme 2 6) in

a

series of reactions involving

relatively few steps.

Scheme 2. Synthesis of the single-station[2]rotaxane 1 ¥ 4PF₆.

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2984

Chem. Eur. J. 2003, 9, 2982 3007

Amphiphilic Rotaxanes

2982 3007

Scheme 3. Synthesis of the dumbbell compound 17.

hydrophobic tetraarylmethane stopper 11^{[13a, 22]}in MeCN with

LiBr as catalyst gave the alcohol 12 in 80% yield. Subsequent

bromination of the free alcohol with CBr₄and Ph₃P afforded

the bromide 13 in good yield (94%). It could be coupled with

the MPTTF building block 6, following its in situ deprotection

with one equivalent of CsOH ¥ H₂O to give 14 in 74% yield.

The tosyl protecting group on the MPTTF unit was removed

in excellent yield (95%) by using NaOMe in a THF/MeOH

mixture. The resultant pyrrole nitrogen in 15 was alkylated

with the chloride 16 carrying the hydrophilic stopper and,

following purification by column chromatography, the dumb-

bell 17 was isolated in 83% yield. Formation of the

[2]rotaxane 2 ¥ 4PF₆was achieved (Scheme 4) in 23% yield^[25]

by using the dumbbell 17 as the template for the formation of

Scheme 4. Synthesis of the slow two-station [2]rotaxane 2 ¥ 4PF₆.

Chem. Eur. J. 2003, 9, 2982 3007

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Scheme 5. Synthesis of the dumbbell compound 23.

the encircling CBPQT⁴tetracation from the dicationic

precursor^[10]8 ¥ 2PF₆and the dibromide 9.

construct the dumbbell 23 (Scheme 5), the free hydroxyl

function in compound 18 was initially converted to a mesyl

group in 95% yield (18 ! 19), then to an iodide in 93% yield

(19 ! 20), and finally to a thiocyanate group in 81% yield

(20 ! 21). The thiocyanate 21 could also be obtained directly

from the mesylate 19 in 97% yield. The thiocyanate group was

reduced in situ with NaBH₄, and the resulting thiolate^[28]was

subsequently coupled with the hydrophilic chloride^{[13a, 22]}22 in

THF/EtOH to give the dumbbell 23 in 90% yield. The

The preparation of the fast molecular shuttle/switch 3 ¥ 4PF₆

is outlined in Schemes 5 and 6. The synthesis of the semi-

dumbbell-shaped compound 18 has been described previous-

ly.^[13a]Direct reaction between the alcohol 18 and the chloride

16 in either THF or DMF containing NaH at 608C gave, in

both cases, an inseparable mixture of unidentified products

containing none of the desired dumbbell.^[27]So, in order to

Scheme 6. Synthesis of the fast two-station [2]rotaxane 3 ¥ 4PF₆.

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Amphiphilic Rotaxanes

2982 3007

synthesis of the [2]rotaxane 3 ¥ 4PF₆was completed

(Scheme 6) by the introduction of CBPQT⁴using a clipping

procedure. The dumbbell 23, 8 ¥ 2PF₆, and the dibromide 9,

were dissolved in anhydrous DMF. The reaction mixture was

stirred at room temperature for ten days, and the pure

[2]rotaxane 3 ¥ 4PF₆was isolated in 15% yield, following

column chromatography. In addition, it was possible to carry

out the clipping procedure at high pressures in a reaction

whereby the dumbbell 23, 8 ¥ 2PF₆, and the dibromide 9 were

dissolved in anhydrous DMF in a teflon-tube and subjected to

10 kbar pressure at room temperature for three days. In this

case, the pure [2]rotaxane 3 ¥ 4PF₆was isolated in 47% yield,

indicating the advantage of carrying out this type of reaction

at ultra high pressures.

Figure 2. FAB-MS spectrum of the [2]rotaxane 2 ¥ 4PF₆.

ibrated MeCN, Me₂CO, or Me₂SO solutions at room temper-

ature. Four systems have been investigated (Schemes 2 8):

1) the semi-dumbbell components 18 and 24; 2) the

[2]pseudorotaxanes^[29]18 ꢀ CBPQT⁴and 24 ꢀ CBPQT⁴; 3)

the dumbbell compounds 7, 17, and 23; and 4) the [2]rotaxanes

1⁴, 2⁴, and 3⁴. We have also investigated the model

compounds 25,^[21]26,^[29d]27,^[30]and 28^[13a](Figure 3). These

compounds and complexes contain several chromophoric

units exhibiting strong absorption bands in the UV region.

Some of these units–namely, the DNP and the oxybenzene

ones–are also expected to show fluorescence.^{[11d, 31]}In the

case of the pseudorotaxanes and rotaxanes, broad and

relatively weak charge-transfer (CT) bands^{[10, 18, 19f,k,p, 31a, 32]}

are also expected to appear in the visible spectral region.

Such low-lying CT energy levels are usually responsible for

the quenching of the fluorescent excited states of the DNP

and oxybenzene units.^{[10, 18, 19p, 31a]}There are also some struc-

tural differences that are important in comparing the photo-

physical properties of the systems under examination: 1) in all

cases, except dumbbell 17, the MPTTF unit is closer to the

Mass spectrometric investigations: All the [2]rotaxanes re-

ported in this paper were characterized (Table 1) by fast atom

bombardment (FAB) mass spectrometry. The spectra ob-

tained gave peaks corresponding to the [M À PF₆] , [M À

2PF₆] , and [M À 3PF₆] ions, as well as some corresponding

to the doubly positively charged ions [M À 2PF₆]², [M À

3PF₆]², and [M À 4PF₆]². The FAB mass spectrum of the

[2]rotaxane 2 ¥ 4PF₆is illustrated in Figure 2. Furthermore, the

[2]rotaxane 3 ¥ 4PF₆was characterized by electrospray (ES)

mass spectrometry. Its ES mass spectrum revealed peaks

corresponding to the triply positively charged [M À 3PF₆]³

and quadruply positively charged [M À 4PF₆]⁴ions. A

comparison of the FAB mass spectra of the hydrophilic

stopper 16 and the TTF derivatives 7 and 17 revealed the

incipient radical cation character of the MPTTF unit. The

FAB mass spectrum of the hydrophilic stopper 16 showed

only peaks corresponding to fragmentations, and no molec-

ular ion was detected. Attachment of an MPTTF unit to the

hydrophilic stopper, as, for ex-

ample, in 17 changed this sit-

uation entirely. The FAB mass

spectrum of 17 showed a mo-

lecular ion as the major peak,

and almost no fragmentation

peaks were observed in the

spectrum. This observation

demonstrates

a fundamental

property of TTF, namely, that

the TTF unit easily forms a

radical cation.

Photophysical investigations:

The photophysical properties

have been studied in air-equil-

Figure 3. Molecular formulas of the model compounds of the p-electron donor stations (25 and 26) and the

stoppers (27 and 28) of the dumbbell-shaped components of the [2]rotaxanes shown in Figure 1.

Table 1. FAB-MS data^[a]for the [2]rotaxanes 1 ¥ 4PF₆, 2 ¥ 4PF₆, and 3 ¥ 4PF₆.

[2]Rotaxane

M^[b]

[M À PF₆]

[M À 2PF₆]

[M À 3PF₆]

[M À 2PF₆]²

[M À 3PF₆]²

[M À 4PF₆]²

1 ¥ 4PF₆

2 ¥ 4PF₆

3 ¥ 4PF₆

(2836)

(3066)

(3064)

2691

2921

2919

2546

2776

2774

2401

2631

2629

1273

1388

1387

1201

1316

1315

1128

1243

1242

[a] FAB spectra were obtained with a ZAB-SE mass spectrometer. The samples were dissolved in a small amount of 3-nitrobenzyl alcohol and the spectra

were recorded in the positive-ion mode. [b] The peaks corresponding to the molecular ions were not observed. The molecular weights are shown in

parentheses.

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hydrophobic stopper than it is to the hydrophilic one; 2)

dumbbells 17 and 23 differ from each other, not only because

of the relative positions of the two p-electron-donating units

on the rod section of the dumbbell, but also on account of

there being a flexible S-containing diethyleneglycol chain

adjacent to the hydrophilic stopper in 23; and 3) an SMe

™knob∫ on the MPTTF unit which slows down the shuttling of

the tetracationic cyclophane component between this unit and

the DNP one in the [2]rotaxane 2⁴, but not in the [2]rotaxane

3⁴, or in its related [2]pseudorotaxane 18 ꢀ CBPQT⁴. As a

result of these structural differences, difficulties are antici-

pated when comparing the properties derived from the CT

interactions between the p-electron-donating units, contained

in the semi-dumbbell and dumbbell components, and the p-

electron-accepting tetracationic cyclophane. Nevertheless,

interesting conclusions will likely be drawn by comparing

the results obtained for ™homogenous∫ couples, such as 1) the

two [2]pseudorotaxanes 18 ꢀ CBPQT⁴and 24 ꢀ CBPQT⁴,

2) the [2]pseudorotaxane 24 ꢀ CBPQT⁴and the [2]rotaxane

1⁴, 3) the [2]pseudorotaxane 18 ꢀ CBPQT⁴and the [2]ro-

taxane 3⁴, and 4) the three [2]rotaxanes 1⁴, 2⁴, and 3⁴.

Figure 5. Absorption spectra of the semi-dumbbell compounds 18 (

and 24 (––) recorded in MeCN at room temperature.

)

occurrence of energy-transfer processes from the emitting

excited states of the DNP moiety and the oxybenzene units in

the tetraarylmethane groups to the lower energy excited

states of the MPTTF unit.^{[18, 19p, 33]}

Mixing equimolar amounts (8.3 Â 10^À4m) of 24 and

CBPQT⁴in Me₂CO at 298 K leads to the formation of the

[2]pseudorotaxane 24 ꢀ CBPQT⁴(Scheme 7),^[34]as indicated

by the immediate appearance of a green solution; this is

Tetracationic cyclophane and model compounds: The absorp-

tion spectra of CBPQT⁴and of the model compounds 25 28

are shown in Figure 4. None of these compounds displays

absorption bands in the visible spectral region, apart from the

MPTTF derivative 25, which exhibits a weak tail. The DNP

derivative 26 and the hydrophobic stopper fragment 27 reveal

the expected fluorescence bands (Figure 4, inset), whereas the

hydrophilic stopper fragment 28 does not show any emission,

despite the fact that oxybenzene units are known to exhibit

fluorescence. No emission is observed in the case of the

MPTTF derivative 25.

Scheme 7. Complexation of 24 by CBPQT⁴

.

related to the presence of a broad CT absorption band

centered on 805 nm.^[35]This band is characteristic^{[15, 19p]}of

Figure 4. Absorption spectra of CBPQT⁴and of the model compounds

25 28 recorded in MeCN at room temperature. The fluorescence spectra

for the emitting species are shown in the inset.

superstructures containing

a TTF unit located inside

CBPQT⁴. By using the CT band as a probe, a dilution

experiment^{[15c, 35]}was carried out and a binding constant (K_a)

for the 1:1 complexation of CBPQT⁴with the semi-dumbbell

24 was obtained. The K_avalue of 1300 Æ 200m^À1(e

1310 Lmol^À1cm^À1) in Me₂CO at 298 K corresponds to a free

energy of complexation^[36](ÀDG8) of 4.2 kcalmol^À1.

Semi-dumbbells and their [2]pseudorotaxanes: The absorption

spectra of the semi-dumbbell compounds 18 and 24 (Figure 5)

are quite similar to the sum of the spectra of their model

compounds (Figure 4). These results indicate that in the semi-

dumbbell compounds 18 and 24 there are no significant

interactions among their chromophoric units in the ground

state. The characteristic fluorescence of the model compounds

26 and 27 (Figure 4, inset) is completely quenched in 18 and

24. The most likely explanation for this quenching is the

Mixing equimolar amounts of 18 and the tetracationic

cyclophane^[16]CBPQT⁴in Me₂CO leads to the formation of

the bistable [2]pseudorotaxane 18 ꢀ CBPQT⁴(Scheme 8), as

evidenced by the spontaneous formation of a brown solu-

tion.^[13a]Since the [2]pseudorotaxane contains two donor units

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Chem. Eur. J. 2003, 9, 2982 3007

Amphiphilic Rotaxanes

2982 3007

bands as probes.^{[15c, 35]}The for-

mation of 18 ꢀ CBPQT⁴and

24 ꢀ CBPQT⁴has also been fol-

lowed by titrations of solutions of

18 or 24 with CBPQT⁴in MeCN

(5.0 Â 10^À5m) up to a value of

seven for the [CBPQT⁴]/[18]

and [CBPQT⁴]/[24] ratios. As

previously observed for analo-

gous systems,^{[18, 19f,p]}formation of

the [2]pseudorotaxanes is accom-

panied by a decrease of intensity

in the UV region, and the ap-

pearance of weak and broad

bands in the visible region. The

titrations were carried out by

monitoring the growth of the

CT absorption bands at suitable

wavelengths. Figure 6 illustrates

the titration curves that have

been fitted, assuming 1:1 com-

plexation. Both the K_avalues and

the molar absorption coefficients

(Table 2) at the selected wave-

lengths were taken as adjustable

parameters, and

a

Newton

Raphson procedure was used^[37]

to minimize the squares of the

residuals. After addition of seven

equivalents of CBPQT⁴, 90 and

Scheme 8. Self-assembly of the two-station [2]pseudorotaxane 18 ꢀ CBPQT⁴

.

70% of the semi-dumbbell com-

(MPTTF and DNP), it can exist as a mixture of two

translational isomers–one (18 ꢀ CBPQT¥ GREEN⁴), in

which the CBPQT⁴resides around the MPTTF unit, and

the other (18 ꢀ CBPQT¥ RED⁴), in which the CBPQT⁴

resides around the DNP moiety. Indeed, a 1:1 mixture of

CBPQT⁴and the semi-dumbbell 18 exhibits CT bands in

Me₂CO centered around 745 nm (MPTTF/CBPQT⁴) and

545 nm (DNP/CBPQT⁴). These observations, taken togeth-

er, indicate that the [2]pseudorotaxane 18 ꢀ CBPQT⁴is

indeed a mixture of two translational isomers. The K_aand

derived ÀDG8 values^[36]for the 1:1 complexation between

CBPQT⁴and the semi-dumbbell 18 were obtained (Table 2)

in Me₂CO at 298 K by UV-visible dilution experiments, by

using both the MPTTF/CBPQT⁴and the DNP/CBPQT⁴CT

pounds 18 and 24, respectively, were complexed. The ÀDG8

values,^[36]derived from the K_avalues are also recorded in

Table 2. Complexation is stronger in MeCN than in Me₂CO, a

situation that has been observed previously for other do-

nors.^{[15c, 38]}After appropriate corrections, the absorption

spectra of the two [2]pseudorotaxanes 18 ꢀ CBPQT⁴and

24 ꢀ CBPQT⁴in MeCN were obtained as shown in Figure 7.

In the case of 24 ꢀ CBPQT⁴, the band with l_max 820 nm can

be attributed to the CT interaction between the MPTTF

unit^{[15, 19p]}and CBPQT⁴. A comparison of the absorption

spectrum of 24 ꢀ CBPQT⁴with the sum of the spectra of its

separated components also shows a small increase in the

absorption in the 350 450 nm region, which can be attributed

to a CT interaction^{[11d, 31]}between CBPQT⁴and the oxy-

benzene unit present in the

hydrophobic stopper. Such an

Table 2. Binding constants^[35](K_a) and free energy changes^[36](ÀDG8) for the complexation of CBPQT⁴with

the semi-dumbbell compounds 24 and 18 determined by UV-visible spectroscopy. Temperature: 298 K.

increase was also observed for

18 ꢀ CBPQT⁴. In the case of

18 ꢀ CBPQT⁴, the band with

l_max 780 nm, assigned to the

Com-

pound

Solvent

l_max

e

Data

points

Correlation

coefficient

K_a

[m^À1

À DG8

[a]

[nm]

[Lmol^À1cm^À1

]

[kcalmol^À1

]

24^[b]

24^[d]

18^[b]

18^[d]

Me₂CO

MeCN

Me₂CO

MeCN

805

1310

1500

760

590

1050

920

15^[c]

19

0.984

0.997

0.917

0.959

0.998

0.999

1300^[d,e]

8800^[d,e]

25000^[d,e]

36000^[d,e]

26000^[d,e]

4.2

5.4

6.0

6.2

6.0

CT

interaction

between

CBPQT⁴and the MPTTF unit,

is accompanied by broad

820

545^[e]

745

22^[c]

17

a

absorption in the 450 650 nm

region (Figure 7), which was

assigned previously to a CT

interaction between CBPQT⁴

and the DNP moiety,^{[29d, 31a, 39]}in

520^[e]

780

17

[a] Estimated error on K_a: Æ 15%. [b] Determined from dilutions experiments. [c] Measurements were carried

out from dilutions of two different stock solutions. [d] Determined from titration experiments. [e] Observed as a

shoulder.

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interactions can be extracted from the spectroscopic data. If it

is assumed that the CT interaction between the ™inside∫

MPTTF and CBPQT⁴is identical in 18 ꢀ CBPQT⁴and 24 ꢀ

CBPQT⁴, then the contribution of the CT interaction

between the ™inside∫ MPTTF and CBPQT⁴will be 50% of

the 820 nm band of 24 ꢀ CBPQT⁴. When this contribution is

subtracted from the experimental band then an absorption

band with l_max 780 nm and e ꢁ 450 Lmol^À1cm^À1appears

(Figure 7, inset), which can be assigned to the ™alongside∫

MPTTF/CBPQT⁴CT interactions. It is also worth noting that

this e value represents the ™average∫ contribution to the CT

interactions of all the possible conformers in which the

MPTTF unit is ™outside∫ the tetracationic cyclophane.

Dumbbells: The absorption spectra (Figure 8) of the dumb-

bell components 7, 17, and 23 are similar to the sums of the

spectra of their model compounds (Figure 4), indicating that

there are no significant electronic interactions among the

Figure 6. Titration curves for the formation of the [2]pseudorotaxanes

a) 24 ꢀ CBPQT⁴and b) 18 ꢀ CBPQT⁴in MeCN at room temperature.

For more details, see text.

agreement with the electrochemical findings (vide infra); this

suggests that 18 ꢀ CBPQT⁴exists (Scheme 8) as a 1:1

mixture of the two translational isomers 18 ꢀ CBPQT¥

GREEN⁴and 18 ꢀ CBPQT¥ RED⁴. A careful examination

of the spectral region around 800 nm reveals that the MPTTF/

CBPQT⁴CT bands in the two [2]pseudorotaxanes have

different shapes, with their maxima displaced by 40 nm, an

observation which is consistent with the existence in 18 ꢀ

CBPQT¥ RED⁴of folded conformations, wherein the

MPTTF unit is involved in an ™alongside∫ interaction with

CBPQT⁴, as confirmed by the electrochemical results (vide

infra). Although an in-depth analysis is made difficult because

of the broad and overlapping nature of the two CT bands,

information about the ™alongside∫ MPTTF/CBPQT⁴CT

Figure 8. Absorption spectra of the dumbbell compounds 7 (

(––), and 23 ( ¥¥ ) recorded in MeCN at room temperature.

), 17

chromophoric units in the ground state. Just as observed for

the semi-dumbbell compounds 18 and 24, the characteristic

fluorescence of the model compounds 26 and 27 is completely

quenched in the dumbbells. The most likely explanation for

this quenching is the occurrence of an energy-transfer process

from the emitting excited states of the DNP moiety and the

oxybenzene units to the lower energy excited states of the

MPTTF unit.^{[18, 19p, 33]}

Single-station [2]rotaxane: The [2]rotaxane 1 ¥ 4PF₆was iso-

lated as a green solid. A comparison of the absorption

spectrum (Figure 9) of 1⁴recorded in MeCN^[40]with the sum

of the spectra for CBPQT⁴and the dumbbell compound 7

reveals the expected decrease in the intensity in the UV

region, and the appearance of weak and broad absorption

bands in the 350 1100 nm region, typical of CT interac-

tions.^{[10, 18, 19f,l,p]}The band with l_max 830 nm (e

1500 Lmol^À1cm^À1) can be assigned to the interaction between

the MPTTF unit and CBPQT⁴surrounding it. This band is

similar to that exhibited by 24 ꢀ CBPQT⁴(Figure 9). The

absorption in the 350 520 nm region could arise from CT

interactions between the tetracationic cyclophane and oxy-

benzene units present in both stoppers, implying that folded

Figure 7. Absorption spectra of the [2]pseudorotaxanes 18 ꢀ CBPQT⁴

(––) and 24 ꢀ CBPQT⁴

(

) recorded in MeCN at room temperature.

The contribution of the CT band of the ™alongside∫ interacting MPTTF

unit in 18 ꢀ CBPQT¥ RED⁴is shown in the inset. For more details, see

text.

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Chem. Eur. J. 2003, 9, 2982 3007

Amphiphilic Rotaxanes

2982 3007

Figure 10. Comparison of the absorption spectra of the single-station

Figure 9. Comparison of the absorption spectra of the single-station

[2]rotaxane 1⁴

(

¥¥ ) and the two-station [2]rotaxanes 2⁴(––) and 3⁴

[2]rotaxane 1⁴(––) and the [2]pseudorotaxane 24 ꢀ CBPQT⁴

(

)

(

) recorded in MeCN at room temperature.

recorded in MeCN at room temperature.

conformations may be populated to some extent in solu-

tion.^{[11d, 31b,c]}

Slow and fast two-station [2]rotaxanes: Both the [2]rotaxanes

2 ¥ 4PF₆and 3 ¥ 4PF₆were isolated as brown solids. They differ

from the single-station [2]rotaxane 1⁴because of the

presence of a second p-electron-donating station, namely, a

DNP moiety that, in 2⁴, is inserted between the MPTTF unit

and the hydrophobic stopper, whereas in 3⁴the DNP moiety

is inserted between the MPTTF unit and the hydrophilic

stopper. It should also be noted that in 2⁴there is a bulky

SMe group located ™between∫ the two recognition sites. A

comparison of the absorption spectra of 2⁴and 3⁴(Fig-

ure 10)^[41]with the sums of the spectra for CBPQT⁴and the

dumbbell compounds 17 and 23, respectively, shows the

expected intensity decreases in the UV region and the

appearance of very broad CT absorption bands across the

350 1100 nm spectral region. The two-station [2]rotaxane 3⁴

is expected to behave like the [2]pseudorotaxane 18 ꢀ

CBPQT⁴. This is indeed the case, as it is evident from an

inspection of the spectra shown in Figure 11. The spectra of

both 3⁴and 18 ꢀ CBPQT⁴exhibit a relatively large absorp-

tion in the 480 600 nm region, which is not observed for the

[2]rotaxane 1⁴(Figure 10) and the [2]pseudorotaxane 24 ꢀ

CBPQT⁴(Figure 7). This absorption can clearly be assigned

to the CT interaction^[31a]be-

Figure 11. Comparison of the absorption spectra of the two-station

[2]rotaxane 3⁴

(

) and the [2]pseudorotaxane 18 ꢀ CBPQT⁴(––)

recorded in MeCN at room temperature.

it is displaced toward shorter wavelengths in comparison with

the spectra of 1⁴and 24 ꢀ CBPQT⁴(Table 3 and Figure 10).

As in the case (vide supra) of 18 ꢀ CBPQT⁴, we believe that

this shift arises from the contribution of ™alongside∫ CT

interactions between the MPTTF unit and CBPQT⁴in folded

conformations. The lower energy absorption band of 2⁴

(Figure 10) is similar to those observed for 1⁴and 24 ꢀ

CBPQT⁴(Figure 9). This observation is consistent with the

electrochemical and ¹H NMR spectroscopic results (vide

infra), which show that, in the case of 2⁴, the predominant

tween the DNP station and

CBPQT⁴. According to elec-

Table 3. Photophysical data at 298 K for the [2]pseudorotaxanes 24 ꢀ CBPQT⁴and 18 ꢀ CBPQT⁴and for the

trochemical and ¹H NMR spec-

troscopic data (vide infra), ap-

proximately half of 3⁴is pres-

ent in MeCN at room

temperature as the translation-

al isomer in which CBPQT⁴

encircles the DNP station.^[42]

The lower energy absorption

band (600 1100 nm), which

can be assigned to the CT

interactions of the MPTTF en-

circled by CBPQT⁴, is similar

to that observed for 18 ꢀ

CBPQT⁴(Figure 11), although

[2]rotaxanes 1⁴, 2⁴, and 3⁴

.

GREEN

e [Lmol^À1cm^À1

RED

e [Lmol^À1cm^À1

Compound/Complex

Solvent

l_max[nm]

]

l^[a][nm]

]

24 ꢀ CBPQT⁴

Me₂CO

MeCN

Me₂CO

MeCN

Me₂CO

MeCN

Me₂CO

MeCN

Me₂CO

MeCN

Me₂SO

805

820

745

780

810

830

805

820

785

800

765

1310

1500

590

24 ꢀ CBPQT⁴

18 ꢀ CBPQT⁴

545

520

760

1050

18 ꢀ CBPQT⁴

920

1⁴

2⁴

3⁴

1400

1500

860

1100

740

540

520

540

520

540

760

880

760

960

640

1300

1310

[a] Observed as a shoulder.

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FULL PAPER

(ꢁ75%) translational isomer is the one in which CBPQT⁴

exhibit some electron-donating power.^{[11d, 31b,c]}The results

obtained concerning the two stations are listed in Table 4,

along with the data for the MPTTF 25 and DNP 26 model

compounds (Figure 3). The semi-dumbbell 24 and the dumb-

bell 7 show two reversible, monoelectronic processes (Fig-

ure 13 and Table 4), assigned to the MPTTF unit and, at more

positive potentials, irreversible oxidation processes associated

with the stoppers. The observed processes take place at

practically the same potentials as those recorded for the

model compounds 25, 27, and 28 (MPTTF 25: 0.44, 0.74 V;

27: 1.55, 1.76 V; 28: 1.31, 1.63, 1.81 V vs SCE),

suggesting that there are no interactions among the electro-

encircles the MPTTF unit.

Figure 12 shows the absorption spectra of the [2]rotaxane

3⁴recorded in MeCN, Me₂CO, and Me₂SO. In all of these

solvents, a broad absorption band is observed in the region

600 1100 nm (Table 3); this can be assigned to the CT

interactions of the MPTTF unit encircled by CBPQT⁴. In

Table 4. Electrochemical data^[a]for the MPTTF and DNP model com-

pounds 25 and 26, the semi-dumbbell compounds 24 and 18, the dumbbell

compounds 7, 17, and 23, the [2]psudorotaxanes 24 ꢀ CBPQT⁴and 18 ꢀ

CBPQT⁴, and the [2]rotaxanes 1⁴, 2⁴, and 3⁴

.

MPTTF^[b]

E_ox[V]^[c]

DNP^[b]

E_ox[V]^[d]

Compound/Complex

25

26

0.44

0.74

1.12

1.24

24

18

7

17

0.44

0.41

0.44

0.34

0.39

0.47

0.48

0.77

0.50

0.54

0.74

0.72

0.74

0.70

0.72

0.74

0.74^[e]

0.81

0.74^[e]

0.76^[e]

Figure 12. Absorption spectra (298 K) of an equilibrium mixture of the

[2]rotaxanes 3 ¥ GREEN⁴and 3 ¥ RED⁴recorded in MeCN ( ¥¥ ),

23

Me₂CO (

), and Me₂SO (––).

24 ꢀ CBPQT⁴

18 ꢀ CBPQT⁴

1.24

1⁴

2⁴

3⁴

both MeCN and Me₂CO (brown solutions), a relatively large

absorption in the 480 600 nm region is observed and can be

assigned to a CT interaction between the DNP station and

CBPQT⁴. However, in Me₂SO (dark green solution), only a

tiny shoulder at 540 nm is observed, indicating that 3 ¥

GREEN⁴is the major translational isomer in this solvent, a

[a] Argon-purged MeCN, room temperature, tetraethylammonium hexa-

flourophosphate (TEAPF₆) as supporting electrolyte, glassy carbon as

working electrode, potential values in V versus SCE. [b] Units involved in

the observed processes. [c] Reversible and monoelectronic processes,

unless otherwise indicated. [d] Irreversible process. [e] Overlapping of

processes. For more details, see text.

1

conclusion that is in agreement with the H NMR spectro-

scopic findings (vide infra). They suggest that 3⁴exists as a

1:1 mixture in MeCN, a 1:3 mixture in Me₂CO, and a 2:1

mixture in Me₂SO of the two translational isomers, 3 ¥

GREEN⁴and 3 ¥ RED⁴.

Electrochemical investigations: The electrochemical studies

were carried out in argon-purged MeCN solutions at room

temperature by using cyclic voltammetry (CV) and differ-

ential pulse voltammetry (DPV). Since the compounds and

complexes that have been investigated contain numerous

electro-active units, their electrochemical behavior is rather

complex: for example, in the case of the [2]rotaxanes 2⁴and

3⁴, at least seven oxidation and four reduction processes were

observed. We have mainly focussed our studies on the

oxidation processes of the MPTTF and DNP electron-

donating units contained in the semi-dumbbell and dumbbell

compounds, and on the reduction processes of the CBPQT⁴

electron-accepting cyclophane.

Semi-dumbbells, dumbbells, and tetracationic cyclophanes:

The semi-dumbbells 18 and 24 and the dumbbells 7, 17, and 23

contain one (MPTTF) or two (MPTTF and DNP) electron-

donating units^{[18, 19p, 31a]}and either one or two stoppers,

containing oxybenzene units, which are also expected to

Figure 13. DPV peaks in MeCN solutions of a) the MPTTF model

compound 25, b) the semi-dumbbell compound 24, and c) the [2]pseudo-

rotaxane 24 ꢀ CBPQT⁴. The current intensity has been corrected to take

into account differences in diffusion coefficients and concentrations.

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Amphiphilic Rotaxanes

2982 3007

active units. Comparison of the results obtained for the semi-

dumbbell 18 and the dumbbells 17 and 23 (Figure 14 and

Table 4) with those of the MPTTF model compound 25 shows

that both the first and second oxidation processes associated

with the MPTTF unit take place at a potential that is less

24 ꢀ CBPQT⁴) are very large compared with the small

amount of uncomplexed semi-dumbbell compounds 18 and

24. It has been possible, therefore, to study the oxidation of

the electron-donating units engaged in these two [2]pseudo-

rotaxanes. The low solubilities of the semi-dumbbell com-

pounds 18 and 24, however, have prevented us from perform-

ing electrochemical studies under conditions in which the

complexed fraction of CBPQT⁴is large relative to the

uncomplexed one.

Since the semi-dumbbell 24 contains only an MPTTF

station for CBPQT⁴the [2]pseudorotaxane 24 ꢀ CBPQT⁴is

the simplest system to have been examined. The results

obtained for 24 and 24 ꢀ CBPQT⁴(Figure 13 and Table 4)

show that, in the [2]pseudorotaxane, the first oxidation of the

MPTTF unit is slightly shifted (30 mV) towards more positive

potentials with respect to that of the semi-dumbbell 24, while

the second oxidation takes place at the same potential. These

results can be explained^{[18, 19f]}as follows: 1) in the [2]pseudo-

rotaxane, the MPTTF unit is engaged in CT interactions with

CBPQT⁴; and 2) after the first oxidation of the MPTTF unit,

dethreading takes place as expected, because the CT inter-

action is being destroyed and a Coulombic repulsion arises

between the oxidized MPTTF unit and the tetracationic

cyclophane.

In the [2]pseudorotaxane 18 ꢀ CBPQT⁴, there are two

stations (one MPTTF and one DNP) on the semi-dumbbell

component for CBPQT⁴to encircle. The simplest hypothesis

is that CBPQT⁴encircles the MPTTF station, which accord-

ing to its oxidation potential, is the better electron donor. If

this is the case, the electrochemical behavior should be similar

to that found in the [2]pseudorotaxane 24 ꢀ CBPQT⁴. At

first sight, the observed potentials (0.48 and 0.74 V vs.

SCE, both in the range expected for MPTTF oxidation;

Table 4) appear to be consistent with the above hypothesis.

However, the CV shapes and current intensities, and the DPV

peak areas for the two oxidation processes are different

(Figure 14), indicating a more complex situation. The pres-

ence of two discrete translational isomers of 18 ꢀ CBPQT⁴

(Scheme 8) is also inconsistent with the results obtained,

because there is no evidence for a noninteracting MPTTF

station. These results suggest that the MPTTF station is

always engaged in donor acceptor interactions in at least two

different ways. The simplest hypothesis that accounts for these

results is that: 1) both the translational isomers are present

and 2) in the isomer in which CBPQT⁴encircles the DNP

moiety, the MPTTF unit is engaged in an ™alongside∫

interaction with CBPQT⁴in a folded superstructure. We

can thus assign the process at 0.48 V (shifted by 70 mV

toward more positive potentials with respect to the same

process in its semi-dumbbell 18) to the first oxidation of the

MPTTF unit interacting in an ™alongside∫ manner with

CBPQT⁴. The comparison with the current intensity of the

CV wave and the area of the DPV peak at 0.48 V with those

of 18 at 0.41 V (Figure 14) shows that approximately 50%

of the MPTTF units are involved in ™alongside∫ interactions.

This situation implies that, in the [2]pseudorotaxane 18 ꢀ

CBPQT⁴, there is also approximately 50% of the transla-

tional isomer in which the MPTTF unit is located ™inside∫ the

electron-accepting cyclophane. It should be noted that the

Figure 14. DPV peaks in MeCN solutions of a) the MPTTF model

compound 25, b) the semi-dumbbell compound 18, and c) the [2]pseudo-

rotaxane 18 ꢀ CBPQT⁴. The current intensity has been corrected to take

into account differences in diffusion coefficients and concentrations.

positive than these observed for the model compound 25. This

behavior can be attributed to the presence of interactions

.

arising between both the MPTTF and MPTTF²and the

electron-donating units, for example, the DNP moiety,

incorporated in the compounds.^[18]Besides the MPTTF-based

processes, the semi-dumbbell 18 and the dumbbells 17 and 23

show, at more positive potentials, irreversible processes that

can be attributed to the oxidation of the DNP moiety as well

as the oxybenzene units present in the two stoppers. For the

dumbbells 17 and 23, the first irreversible oxidation of the

hydrophilic dendritic stopper takes place at a potential similar

to that of the DNP moiety. The electron-accepting CBPQT⁴

cyclophane shows the well-known reversible and bielectronic

processes at À0.29 and À0.71 V versus SCE, which can be

assigned to the simultaneous first and second reductions of

the two bipyridinium units.^[10]

[2]Pseudorotaxanes: The binding constants of CBPQT⁴with

the semi-dumbbells 18 and 24 (33000 and 8800m^À1in MeCN

for 18 ꢀ CBPQT⁴and 24 ꢀ CBPQT⁴, respectively) are not

large enough to avoid the presence of substantial amounts of

free species starting from equimolar concentrations of the two

components. Therefore, the electrochemical characterization

of the electron-donating and electron-accepting units in these

[2]pseudorotaxanes has to be investigated in the presence of

an excess of CBPQT⁴or semi-dumbbell components, re-

spectively. In solutions of 18 (9.3 Â 10^À5m) or 24 (1.5 Â 10^À4m)

in MeCN (in both cases, close to the solubility limit) and

an excess (10 equivalents) of CBPQT⁴, the fractions of

[2]pseudorotaxanes (96% for 18 ꢀ CBPQT⁴and 92% for

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FULL PAPER

potential for the first oxidation of the MPTTF unit in the

translational isomer in which it is encircled by CBPQT⁴is

closer to that observed for the [2]rotaxane 1⁴(vide infra)

than for the [2]pseudorotaxane 24 ꢀ CBPQT⁴. This shows

that in the case of 24 ꢀ CBPQT⁴dethreading is faster than

for 18 ꢀ CBPQT⁴, as might be expected. Therefore, in

agreement with the behavior of the [2]rotaxane 1⁴, the first

oxidation of the ™inside∫ MPTTF unit has to be much more

positively shifted than that of the ™alongside∫ MPTTF unit, to

the extent that it is expected to overlap with the second

oxidation process. We can, therefore, assign the process

observed at 0.74 V to both the first and second oxidation of

the ™inside∫ MPTTF unit, as well as to the second oxidation of

the ™alongside∫ MPTTF unit. This assignment is consistent

with the current intensity of the CV wave and the area of the

DPV peak of the process observed at 0.74 V, with respect to

those of the process taking place at 0.48 V. The fact that the

second oxidation of the MPTTF unit in 18 ꢀ CBPQT⁴takes

place approximately at the same potential as that observed in

the semi-dumbbell 18, regardless of its ™alongside∫ and

™inside∫ position, shows that, after the first oxidation of the

MPTTF unit, CBPQT⁴leaves the MPTTF station. Since the

potential for the oxidation of the DNP moiety is the same in

18 ꢀ CBPQT⁴as that found in the semi-dumbbell 18, we can

conclude that after the second oxidation of the MPTTF unit

the cyclophane is no longer engaged with the semi-dumbbell

component. Whether dethreading had already occurred after

the first MPTTF oxidation, as could be expected if the

Coulombic repulsion between the monooxidized MPTTF unit

and the tetracationic cyclophane overcomes the CT inter-

action of CBPQT⁴with the DNP moiety, cannot be said from

the experimental results.

Figure 15. DPV peaks in MeCN solutions of a) the single-station [2]rotax-

ane 1⁴, b) the two-station [2]rotaxane 2⁴, c) the two-station [2]rotaxane

3⁴, and d) the [2]pseudorotaxane 18 ꢀ CBPQT⁴. The current intensity has

been corrected to take into account differences in diffusion coefficients and

concentrations.

A more complex situation arises in the case of the

[2]rotaxanes 2⁴and 3⁴, which can be expected to give two

translational isomers (Schemes 4 and 6). In the potential

range of MPTTF oxidation, two processes take place in both

2⁴and 3⁴: at 0.50 and 0.74 V for 2⁴, and at 0.54 and

0.76 V versus SCE for 3⁴(Figure 15 and Table 4). These

two processes exhibit different CV current intensities and

DPV peak areas. Following the manner in which we treated

the [2]pseudorotaxane 18 ꢀ CBPQT⁴and taking the single-

station [2]rotaxane 1⁴as an appropriate reference for an

MPTTF unit located ™inside∫ CBPQT⁴, we can attribute the

first process for 2⁴and 3⁴to the first oxidation of the

MPTTF unit interacting ™alongside∫ with CBPQT⁴. The

comparison of the CV current intensity and the DPV peak

area for this process in these two [2]rotaxanes and their

corresponding dumbbells allows us to estimate that the

percentage of the translational isomer with MPTTF interact-

ing ™alongside∫ is 20% for 2⁴and 60% for 3⁴; as a

consequence, the remaining 80% of 2⁴and 40% of 3⁴

correspond to the translational isomer in which CBPQT⁴

encircles the MPTTF unit. Therefore, we can attribute the

process occurring at 0.74 V for 2⁴and at 0.76 V for 3⁴to

the first and second oxidation processes of the ™inside∫

MPTTF unit and to the second oxidation of the ™alongside∫

MPTTF unit. These attributions are consistent with the CV

current intensities and also with the DPV peak areas for the

first and second oxidation processes observed in these two

[2]rotaxanes. We have noticed that some of the results seem to

indicate that the hydrophilic dendritic stopper plays an

important role as far as the distribution of CBPQT⁴between

the two stations is concerned. In the case of the [2]pseudo-

rotaxane 18 ꢀ CBPQT⁴, which does not contain the hydro-

[2]Rotaxanes: Since the single-station [2]rotaxane 1⁴con-

tains, in its dumbbell component, only an MPTTF station for

CBPQT⁴, it can be considered as a model for the electro-

chemical behavior of an MPTTF unit ™inside∫ the CBPQT⁴

.

On going from the dumbbell 7 to the [2]rotaxane 1⁴, the two

monoelectronic oxidation processes of the MPTTF unit, well

separated in the dumbbell, occurs at two very close potentials,

0.77 and 0.81 V versus SCE (Figure 15 and Table 4). We

assign the process at 0.77 V to the first oxidation of the

MPTTF unit located ™inside∫ the electron-accepting cyclo-

phane. The shift of 330 mV toward more positive potential for

the first oxidation process of the MPTTF unit in 1⁴is easily

explained on the basis of the electrostatic repulsion and the

strong CT interaction with the two-electron-accepting bipyr-

idinium units in CBPQT⁴. Since the electrochemical behav-

ior of the [2]pseudorotaxanes 18 ꢀ CBPQT⁴and 24 ꢀ

CBPQT⁴shows that CBPQT⁴leaves the MPTTF stations

after their first oxidations, we can assign the process at

0.81 V to the second oxidation of an MPTTF unit that is no

longer encircled by CBPQT⁴. The shift of 70 mV toward

more positive potential for this process, compared to the same

process in the dumbbell 7, can be accounted for by the

presence of the two stoppers and the shortness of the

dumbbell component, both features that force the tetracat-

ionic electron-accepting cyclophane to remain near to the

dioxidized MPTTF unit.

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Amphiphilic Rotaxanes

2982 3007

1

Table 5. Selected H NMR spectroscopic data^[a](d and Dd values) for the

dumbbell compound 7 and the single-station [2]rotaxane 1⁴in CD₃COCD₃

at 298 K.

philic dendritic stopper, the two translational isomers are

present in a 1:1 ratio, whereas, in case of the [2]rotaxane 3⁴,

the station closer to the hydrophilic dendritic stopper (i.e.,

DNP) is preferred, as shown by the 3:2 ratio. The fact that the

station closer (i.e., the MPTTF one) to the hydrophilic

dendritic stopper is even more preferred (4:1 ratio) in the case

of the [2]rotaxane 2⁴can be related to the different structures

of the dumbbell components, including the presence of the

SMe ™knob∫ between the two electron-donating stations

in 2⁴.

Compound SCH₃

SCH₂CH₂O SCH₂CH₂O NCH₂Ar Pyr-H^[b]

7

1⁴

2.39^[c]

2.64^[c]

0.25^[d] 0.27^[d]

3.02^[c]

3.29^[c]

3.72^[c]

3.95^[c]

0.23^[d]

4.88^[c]

5.18^[c]

0.30^[d]

6.71^[c]

6.44^[c]

À 0.27^[d]

[a] ¹H NMR spectra were recorded at 400 MHz. [b] The resonances

associated with the two pyrrole protons are nonequivalent and AB systems

(J 2 Hz) are observed in the spectra. The values reported correspond to

the centroids of the AB systems. [c] d values. [d] Dd values in ppm.

As far as the translational isomer with the MPTTF unit

located ™inside∫ CBPQT⁴is concerned, it is impossible to

demonstrate the occurrence of the expected ring shuttling

after oxidation of the MPTTF unit, since both oxidation

processes for the free and encircled DNP moiety overlap with

the irreversible oxidation processes of the oxybenzene units

present in both of the stoppers. However, the fact that in the

[2]rotaxanes 2⁴and 3⁴the second oxidation of the MPTTF

unit, regardless of its ™alongside∫ or ™inside∫ location, is only

slightly shifted (40 mV) toward more positive potentials

compared (Table 4) with the same process in their corre-

sponding dumbbells, seems to indicate that CBPQT⁴shut-

tling does take place. This shift, observed in the [2]rotaxanes,

can be accounted for by 1) the presence of CBPQT⁴, which

destroys the interactions of the oxidized MPTTF with the

other electron-donating units believed to be present in the

case of the dumbbell compounds 17 and 23; and 2) a

Coulombic repulsion, which arises between the tetracationic

cyclophane and the oxidized MPTTF unit.

1

Slow two-station [2]rotaxane: H NMR spectroscopy indicat-

ed the presence of the two stable translational isomers present

in the isolated product. The ¹H NMR spectrum (400 MHz) of

2⁴recorded at 298 K in CD₃COCD₃showed two singlets at

d 2.64 and 2.47 ppm (Figure 16), which can be assigned to

the protons in the SMe groups, attached to the MPTTF units

in 2 ¥ GREEN⁴and 2 ¥ RED⁴, respectively. From the inte-

grals of the two different SMe resonances, the ratio of the two

translational isomers was estimated to be approximately 1:1.

For all the rotaxanes investigated, reduction processes

involving the tetracationic cyclophane were also expected. In

most of the pseudorotaxanes, rotaxanes, and catenanes,

previously studied, the redox processes of the cyclophane

were clean and very useful for interpreting the structures and

the dynamic behavior of the systems.^{[2e,g, 10, 18, 19f, 31a]}In the case

of the present rotaxanes, however, although several reduction

processes were observed, unfortunately all the reduction

processes were irreversible and, in some cases, affected by

adsorption phenomena. We suppose that such an unusual

electrochemical behavior, which prevented us from obtaining

useful pieces of information, is related to the extensive folding

around CBPQT⁴of portions of the dumbbell components in

these long and flexible compounds.

Figure 16. Partial ¹H NMR spectrum of an equilibrium mixture of the

[2]rotaxanes 2 ¥ GREEN⁴and 2 ¥ RED⁴recorded at 400 MHz in

CD₃COCD₃at 298 K.

1

[2]Pseudorotaxane 18 ꢀ CBPQT⁴: The H NMR spectrum

(500 MHz) of an equilibrated 1:1 mixture of the semi-dumb-

bell 18 and CBPQT⁴recorded in CD₃COCD₃at 300 K

revealed broad signals, because of fast exchange between

complexed (i.e., 18 ꢀ CBPQT⁴) and free species on the

¹H NMR timescale. The presence of both 18 ꢀ CBPQT¥

GREEN⁴and 18 ꢀ CBPQT¥ RED⁴in the complexed spe-

cies is confirmed by the identification of singlets resonating at

d 2.70 and 2.34 ppm, which can be assigned to the SMe

resonances, attached to the MPTTF unit in 18 ꢀ CBPQT¥

GREEN⁴and 18 ꢀ CBPQT¥ RED⁴, respectively. The ratio

of the two co-conformations is roughly^[43]estimated to be 3:1

in favor of 18 ꢀ CBPQT¥ RED⁴. Upon cooling the sample

¹H NMR investigations: These were carried out in

CD₃COCD₃, CD₃CN, or CD₃SOCD₃. Four systems were

investigated (Schemes 2, 4, 6, and 8): 1) the single-station

[2]rotaxane 1⁴, 2) the slow two-station [2]rotaxane 2⁴, 3) the

[2]pseudorotaxane 18 ꢀ CBPQT⁴, and 4) the fast two-station

[2]rotaxane 3⁴.

1

down to 245 K, all the signals sharpened and the H NMR

Single-station [2]rotaxane: A comparison of the ¹H NMR

spectra (400 MHz, CD₃COCD₃, 298 K) of the dumbbell

compound 7 and the [2]rotaxane 1⁴reveals significant

chemical shift differences for the resonances associated with

the protons located close to the MPTTF unit (Table 5),

indicating that the tetracationic cyclophane encircles the

MPTTF unit in the [2]rotaxane 1⁴.

spectrum (500 MHz; Figure 17) revealed that 18 ꢀ CBPQT¥

RED⁴is almost exclusively present^[44]in the CD₃COCD₃

solution at 245 K. The most diagnostic evidence, which

indicates that CBPQT⁴encircles the DNP moiety, is the very

high upfield shift of the resonances for the DNP-H-4/8

protons, which are observed as two doublets (J 8 Hz) at d

2.59 and 2.69 ppm. The d values for the DNP-H-4/8 protons in

Chem. Eur. J. 2003, 9, 2982 3007

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FULL PAPER

Figure 17. Partial ¹H NMR spectrum (500 MHz) of the 1:1 complexes

formed between the semi-dumbbell compound 18 and CBPQT⁴recorded

in CD₃COCD₃at 245 K.

Figure 18. Partial ¹H NMR spectrum of an equilibrium mixture of the

[2]rotaxanes 3 ¥ GREEN⁴and 3 ¥ RED⁴, recorded at 500 MHz in CD₃CN

at 300 K.

18 ꢀ CBPQT¥ RED⁴and the same protons in the semi-

dumbbell compound 18 are shifted upfield by 5.21 and

5.14 ppm, respectively. They are of a similar magnitude to

those previously reported.^[45]Moreover, one intense SMe

singlet (Figure 17) resonating at d 2.29 ppm is observed.

The limited amount of 18 ꢀ CBPQT¥ GREEN⁴present in

the CD₃COCD₃solution at 245 K is indicated by a very small

singlet at d 2.71 ppm arising from the SMe protons in 18 ꢀ

CBPQT¥ GREEN⁴.

4.89 ppm). From the integrals of the signals for the DNP H-2/6

protons in 3 ¥ RED⁴and the two pyrrole protons in 3 ¥

GREEN⁴, the ratio of the two translational isomers was

estimated to be approximately 1:1 at 300 K in CD₃CN.

The ¹H NMR spectrum (500 MHz) of the [2]rotaxane 3⁴in

CD₃COCD₃at 300 K revealed that the ratio of 3 ¥ RED⁴and

3 ¥ GREEN⁴in this solvent is 3:1 in favor of 3 ¥ RED⁴. On

cooling this sample down to 245 K, the color of the solution

1

Fast two-station [2]rotaxane: The ¹H NMR spectrum

(500 MHz) of the [2]rotaxane 3⁴recorded in CD₃CN at

300 K (Figure 18) showed two triplets (J 8 Hz) at d 5.79

and 5.91 ppm and two doublets (J 8 Hz) at d 6.12 and

6.24 ppm; these can be assigned to the DNP-H-3/7 and the

DNP-H-2/6 protons, respectively, in 3 ¥ RED⁴. The presence

of 3 ¥ GREEN⁴in the isolated

changed to red, and the H NMR spectrum revealed that 3 ¥

RED⁴is almost exclusively present in the CD₃COCD₃

solution at 245 K (Figure 19). The most diagnostic signals,

indicating that CBPQT⁴encircles the DNP moiety, are the

very high upfield shifted ones for the DNP-H-4/8 protons,

which are observed as two doublets (J 8 Hz) at d 2.44 and

mixture is confirmed by the

appearance of an AB system

(J 2 Hz) at d 6.30 and

6.32 ppm (Figure 18), which

can be assigned to the two

chemically nonequivalent pyr-

role protons in 3 ¥ GREEN⁴.

Furthermore, two singlets at

d 2.27 and 2.61 ppm are ob-

served, which can be assigned

to the SMe resonances attached

to the TTF unit in 3 ¥ RED⁴

and 3 ¥ GREEN⁴, respectively.

Finally, four singlets are ob-

served in the region d 4.6

5.0 ppm (Figure 18), in the form

of an ¹H NMR spectroscopic

signature; this results from the

two different sets of ArCH₂O

protons in the hydrophilic den-

dritic stopper of 3 ¥ RED⁴

Figure 19. Full ¹H NMR spectrum (500 MHz) of the fast two-station [2]rotaxane 3⁴recorded in CD₃COCD₃at

245 K.

(d 4.71 and 4.86 ppm) and

3 ¥ GREEN⁴(d 4.79 and

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Chem. Eur. J. 2003, 9, 2982 3007

Amphiphilic Rotaxanes

2982 3007

2.51 ppm. The values of Dd for the DNP-H-4/8 protons in 3 ¥

4PF₆¥ RED and the same protons in the dumbbell compound

23 are À5.35 and À5.36 ppm, respectively. They are of similar

magnitudes to those previously reported.^[45]Moreover, only

one intense SMe singlet, resonating at d 2.27 ppm, is

observed. The limited amount of 3 ¥ GREEN⁴present in

CD₃COCD₃at 245 K is, for example, evidenced by a very

small singlet at d 2.67 ppm arising from the SMe protons in

3 ¥ GREEN⁴.

dumbbell-shaped component is encircled exclusively by

CBPQT⁴at 400 K in CD₃SOCD₃.

Dynamic investigations: These were carried out in

CD₃COCD₃, CD₃CN, or CD₃SOCD₃. Three systems were

investigated: 1) the single-station [2]rotaxane 1⁴, 2) the slow

two-station [2]rotaxane 2⁴, and 3) the fast two-station

[2]rotaxane 3⁴.

1

The H NMR spectrum (500 MHz, CD₃SOCD₃) recorded

Single-station [2]rotaxane: The resonances associated with the

a-and b-bipyridinium protons on the cyclophane component

CBPQT⁴in the single-station [2]rotaxane 1⁴are observed to

undergo coalescence with increasing temperature. This is

most clearly seen with the a-bipyridinium protons (Fig-

ure 21b), which exist as two sets of signals at low temperature

and coalesce into one set of signals at high temperature

(presumably each of these sets consists of multiple over-

lapping signals). The process by which these protons are

interconverted is termed pyridinium rotation (Figure 21a,

process I), and involves rotation of one of the pyridinium

units about its axis of substitution. The barrier^[47](DG⁼) for

this process to occur was determined^[48](Table 6) and

observed to be 15.3 kcalmol^À1at 260 K.^[49]This can be

compared to values obtained for the same process in a series

of [2]catenanes,^[50]for which the barriers were observed to

range from 16 17 kcalmol^À1. The lower barrier in the present

case can be attributed to the much less constricted nature of

the tetracationic cyclophane in the [2]rotaxane relative to the

[2]catenanes.

on the [2]rotaxane 3⁴at 300 K indicated that 3 ¥ GREEN⁴is

the major translational isomer in CD₃SOCD₃. Although, all

resonances are broad at this temperature, most likely on

account of slow site-exchange processes in this solvent, the

presence of 3 ¥ GREEN⁴as the major isomer is evident. The

¹H NMR spectrum revealed a broad singlet at d 6.49 ppm,

which can be assigned to the two pyrrole protons in 3 ¥

GREEN⁴. The presence of 3 ¥ RED⁴in this solvent can be

identified from the presence of a broad multiplet at d 6.05

6.20 ppm, which can be assigned to the DNP-H-2/6 protons in

3 ¥ RED⁴. From the integrals of the two pyrrole protons in 3 ¥

GREEN⁴and the DNP-H-2/6 protons in 3 ¥ RED⁴, the ratio

of the two translational isomers was estimated to be

approximately^[46]2:1 in favor of 3 ¥ GREEN⁴at 300 K in

CD₃SOCD₃. Upon heating (400 K) of this sample, all the

signals sharpen and the existence of 3 ¥ GREEN⁴as the only

translational isomer at 400 K is easily discerned from the

¹H NMR spectrum (500 MHz). For example, only one SMe

singlet (Figure 20) resonating at d 2.68 ppm is observed.

Moreover, a singlet resonating at d 6.35 ppm and integrat-

ing for two protons, is also evident, and can be assigned to the

two pyrrole protons in 3 ¥ GREEN⁴. Finally, no signals are

observed in the region d 6.0 6.3 ppm that would result

from the DNP-H-2/6 protons being located inside the cyclo-

phane, supporting the conclusion that the MPTTF unit of the

In addition to rotation of the pyridinium units, evidence for

rotation of the p-xylyl units (Figure 21a, process II) of the

CBPQT⁴component can also be seen in the ¹H NMR

spectra. At low temperatures it is possible to observe two sets

of signals again (Figure 21c), which coalesce into one signal at

higher temperatures. The DG⁼value for this process can be

similarly determined, and was

found to be 11.7 kcalmol^À1at

193 K. Once again, a compari-

son can be made with the data

obtained for p-xylyl rotation in

the [2]catenanes (a range of

values from 11 13 kcalmol^À1).

In this case the value for the

[2]rotaxane is nearly the same

as that for the [2]catenanes,

suggesting that the environ-

ment of the p-xylyl units is

similar in both cases.

Slow two-station [2]rotaxane:

Thin-layer

chromatography

(TLC) of the [2]rotaxane 2 ¥

4PF₆showed green and red

spots with similar intensities,

indicating the existence of two

isolable translational isomers,

one in which CBPQT⁴encir-

cles the MPTTF unit (i.e., 2 ¥

GREEN⁴) and another in

Figure 20. Full ¹H NMR spectrum (500 MHz) of the fast two-station [2]rotaxane 3⁴recorded in CD₃SOCD₃at

400 K.

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Figure 22. Partial ¹H NMR spectrum of the isolated [2]rotaxane 2 ¥ RED⁴

recorded at 500 MHz in CD₃COCD₃at 225 K.

component, four doublets (J 6 7 Hz) for both the a-and b-

bipyridinium protons and four singlets for the protons on the

1

p-xylyl units are observed in the H NMR spectrum of the

[2]rotaxane 2 ¥ RED⁴at 225 K (Figure 22). The UV-visible

spectrum of 2 ¥ RED⁴reveals a CT absorption band in the

form of a shoulder at 540 nm (Figure 24). It results from the

Figure 21. a) A schematic representation of the pyridinium unit rotation

process (I) occurring in the bipyridinium units present in the tetracationic

cyclophane component of the single-station [2]rotaxane 1⁴and the p-xylyl

unit rotation process (II) occurring in the tetracationic cyclophane

component of 1⁴. Rotation of the pyridinium units around the axis of

the bipyridinium units brings about the exchange of H_awith H_a'and H_b

with H_b'. Rotation of the p-xylyl units around their substituted axis causes

exchange between H_aand H_b. Partial variable temperature ¹H NMR

spectra (500 MHz) of a CD₃COCD₃solution of the single-station [2]rotax-

ane 1⁴of b) the a-bipyridinium region and c) the p-xylyl region.

DNP moiety being located inside CBPQT^{4 [29d, 39]}

Further-

.

more, no absorption band is observed in the region 750

850 nm for a CT interaction that would result from the

MPTTF unit being located inside the cyclophane^[15]and so

support the conclusion that the DNP moiety in the dumbbell

component is encircled exclusively by CBPQT⁴. Leaving the

red solution of 2 ¥ RED⁴to stand for 24 h at room temper-

ature results in a return to the ™original∫ spectrum (Fig-

ure 24), as a consequence of the shuttling of CBPQT⁴from

the DNP recognition site in 2 ¥ RED⁴to the MPTTF

recognition site. The kinetics of the shuttling of CBPQT⁴

from the DNP recognition site in 2 ¥ RED⁴to the MPTTF

recognition site were investigated by using ¹H NMR spectros-

which CBPQT⁴encircles the DNP moiety (i.e., 2 ¥ RED⁴).

By employing preparative thin-layer chromatography

(PTLC), it was possible to separate the red and green

translational isomers, that is, 2 ¥ RED⁴and 2 ¥ GREEN⁴.^[35]

¹H NMR and UV-visible spectroscopy clearly suggest that

CBPQT⁴encircles the DNP moiety in the [2]rotaxane 2 ¥

1

RED⁴. After isolation of 2 ¥ RED⁴, a H NMR spectrum

(500 MHz) was recorded (Figures 22 and 23a) at 225 K in

CD₃COCD₃. As expected, it shows only one signal for the

SMe protons, as a singlet resonating (Figure 23a) at d

2.49 ppm. The resonances for the protons attached to the

DNP moiety are shifted significantly to higher field relative to

the resonances for the same protons in the dumbbell

compound 17. For example, the DNP-H-2/6 protons resonate

as two doublets (J 8 Hz) at d 6.33 and 6.29 ppm and one of

the DNP-H-3/7 protons resonates as a triplet (J 8 Hz) at d

6.14 ppm (Figure 22). The signal for the other DNP-H-3/7

proton is obscured by the multiplet for the NCH₂protons.

These chemical shifts values are similar to those reported

previously for

a

DNP moiety being encircled by

CBPQT⁴.^{[45, 51]}On account of the asymmetry in the dumbbell

Table 6. p-Xylyl and bipyridinium rotation kinetic data and thermody-

namic data for the single-station [2]rotaxane 1⁴in CD₃COCD₃.

[b]

[c]

Process

T [K]^[a]

k_ex[s^À1

]

DG⁼[kcalmol^À1

]

p-xylyl

rotation

bipyridinium

rotation

193

203

260

273

0.23

0.65

0.83

2.65

11.7

11.9

15.3

15.4

Figure 23. Partial ¹H NMR spectra (500 MHz) of the isolated [2]rotaxane

2 ¥ RED⁴, recorded in CD₃COCD₃at a) 225 K/0 h, b) 300 K/1 h, c) 300 K/

3 h, d) 300 K/5 h, e) 300 K/8 h, and f) 300 K/21 h. The singlet at d

2.49 ppm corresponds to the SMe resonance when CBPQT⁴encircles

the DNP moiety and the singlet at d 2.65 ppm corresponds to the SMe

resonance when CBPQT⁴encircles the MPTTF unit.

[a] Calibrated using neat MeOH sample, see: ref. [49]. [b] Measured using

spin saturation transfer method, see ref. [48]. [c] Æ 0.1 kcalmol^À1

.

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Amphiphilic Rotaxanes

-

2982 3007

revealed that the ratio between the two translational isomers

is approximately 6:1 in favor of 2 ¥ GREEN⁴. The a-and b-

bipyridinium protons in 2 ¥ GREEN⁴appear as two sharp

doublets (J 6 Hz) and the p-xylyl protons appear as one

sharp singlet. The NCH₂protons in 2 ¥ GREEN⁴are

observed as an AB system (J 13 Hz), which is a direct

consequence of the asymmetry present in the dumbbell

component and the interlocked nature of 2⁴. This asymmetry

means that the pairs of NCH₂protons in CBPQT⁴cannot

become equivalent under any circumstances, that is, they are

inherently heterotopic. Upon heating to 425 K, the color of

the solution changed from brown to green. The solution was

subsequently quenched at 273 K (ice-bath) and the conver-

sion of 2 ¥ GREEN⁴to 2 ¥ RED⁴was followed at 300 K, using

the signals for the protons on the MPTTF unit in 2 ¥ GREEN⁴

and the DNP-H-2/6 protons in 2 ¥ RED⁴as probes. On this

occasion, a first-order rate constant of 3 Â 10^À4s^À1was

obtained for the slippage of CBPQT⁴over the SMe group,

in the direction from 2 ¥ GREEN⁴to 2 ¥ RED⁴. The associ-

ated DG⁼value for this isomerization is 22 kcalmol^À1. Thus,

the barrier for the shuttling of CBPQT⁴over the SMe

group^[54]in CD₃SOCD₃is 2 kcalmol^À1less than that recorded

in CD₃COCD₃. This observation is probably a direct conse-

quence of the decrease in all intramolecular noncovalent

bonding interactions, when CD₃SOCD₃is the solvent.

Figure 26 shows the absorption spectra of the isolated 1:1

mixture of the slow two-station [2]rotaxane 2⁴recorded in

MeCN at 298 K. The spectrum changes with time, such that

the absorption bands for the MPTTF/CBPQT⁴CT inter-

action (centered around 820 nm) increased in intensity and

the DNP/CBPQT⁴CT interaction (centered around 520 nm)

decreased in intensity; this clearly indicates that shuttling of

CBPQT⁴from the DNP recognition site in 2 ¥ RED⁴to the

Figure 24. Absorption spectra recorded at 298 K in a Me₂CO solution of

an equilibrium mixture of the [2]rotaxanes 2 ¥ GREEN⁴and 2 ¥ RED⁴

(––) and of the [2]rotaxane 2 ¥ RED⁴

(

¥¥ ) immediately after its

isolation. Allowing the red solution of 2 ¥ RED⁴to stand for 24 h at room

temperature regenerates the ™original∫ spectrum ( ).

copy. After isolation of 2 ¥ RED⁴and recording of an

¹H NMR spectrum (500 MHz) at 225 K in CD₃COCD₃

(Figure 23a), the sample was heated to 300 K and the shuttling

of CBPQT⁴from the DNP recognition site in 2 ¥ RED⁴to

the MPTTF recognition site was followed by using the SMe

resonances as probes (Figure 23b f). After 21 h at 300 K,

equilibration is complete and the 1:1 mixture of 2 ¥ RED⁴and

2 ¥ GREEN⁴is re-established (Figure 23f). As a consequence

of these spectroscopic variations, the color of the solution

changes from red to brown. By employing a first-order kinetic

treatment,^{[52, 53]}a rate constant (k 2 Â 10^À5s^À1) for the

slippage of CBPQT⁴over the SMe group, in the direction

from 2 ¥ RED⁴to 2 ¥ GREEN⁴, was obtained. The free

energy of activation^[47](DG⁼) for this isomerization is

24 kcalmol^À1. Despite the fact that 2 ¥ GREEN⁴is less polar

than 2 ¥ RED⁴, it was only possible to extract an extremely

small amount of 2 ¥ GREEN⁴from the silica on the PTLC

plate. The UV-visible spectrum^[35]recorded in Me₂CO at

298 K of this fraction shows, as expected, only a broad CT

absorption band centered on 800 nm. Although it was not

possible to isolate sufficient amounts of 2 ¥ GREEN⁴after

PTLC to follow its interconversion into 2 ¥ RED⁴by ¹H NMR

spectroscopy, it turned out to be possible to shift the

equilibrium between the two translational isomers from 1:1

to 9:1 in favor of 2 ¥ GREEN⁴by heating a CD₃SOCD₃

solution of the brown 1:1 mixture to 425 K. Figure 25a

e

shows the partial ¹H NMR spectra (500 MHz) of 2⁴recorded

in CD₃SOCD₃at 310, 350, 365, 395, and 425 K. The AB system

centered on d 6.21 ppm (at 395 K) corresponds to the two

pyrrole protons on the MPTTF unit in 2 ¥ GREEN⁴, whereas

the doublet centered on d 6.27 ppm at 395 K can be assigned

to the two DNP-H-2/6 protons in 2 ¥ RED⁴. It is evident from

Figure 25a e that the relative populations of the two trans-

lational isomers shift upon heating. Additionally, the signals

arising from the protons on CBPQT⁴are simplified as a

result of fast exchange of all the relevant sites in the

Figure 25. Partial variable temperature ¹H NMR spectra of an equilibrium

mixture of the [2]rotaxanes 2 ¥ GREEN⁴and 2 ¥ RED⁴recorded at

500 MHz in CD₃SOCD₃at a) 310 K, b) 350 K, c) 365 K, d) 395 K, and

e) 425 K. Spectra f) and g) were recorded at 300 K 15 min and 3 h,

respectively, after the sample had been heated to 425 K and quenched.

1

cyclophane component CBPQT⁴. The H NMR spectrum

(500 MHz) of 2⁴,^[35]recorded in CD₃SOCD₃at 410 K

Chem. Eur. J. 2003, 9, 2982 3007

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Figure 26. Absorption spectra recorded at 298 K on a MeCN solution

(4.1 Â 10^À4m) of the slow two-station [2]rotaxane 2⁴immediately after its

preparation (––), after 1 h (

), and after 20 h (

¥

).

MPTTF recognition site occurs. After 20 h, the system had

reached equilibrium and no perceptible changes were ob-

served in UV-visible spectra recorded subsequently. The

¹H NMR spectrum (500 MHz, 300 K), recorded on a solution

of the isolated slow two-station [2]rotaxane 2⁴in CD₃CN,

showed immediately, after its preparation, that the ratio^[55]

between 2 ¥ GREEN⁴and 2 ¥ RED⁴was approximately 1:1,

1

whereas the H NMR spectrum (500 MHz, 300 K) recorded

on an equilibrated (after 48 h) solution of the two [2]rotax-

anes 2 ¥ GREEN⁴and 2 ¥ RED⁴in CD₃CN revealed that the

ratio^[55]was 3:1 in favor of 2 ¥ GREEN⁴.

Fast two-station [2]rotaxane: Since the ¹H NMR investigations

of the [2]rotaxane 3⁴show that shuttling of CBPQT⁴is

temperature dependent, variable-temperature (VT) ¹H NMR

spectroscopy was carried out in CD₃CN, since this solvent

allows spectra to be recorded over the widest temperature

range. The VT ¹H NMR experiment (500 MHz) shows

(Figure 27a) that 3 ¥ RED⁴is the major translational isomer

at low temperature (red solution), whereas 3 ¥ GREEN⁴is

the major isomer at higher temperature (green solution). This

observation was supported by a temperature controlled UV-

visible experiment. At 298 K in MeCN, absorption bands for

both the MPTTF/CBPQT⁴CT interaction (centered around

800 nm) and the DNP/CBPQT⁴CT interaction (centered

around 520 nm) were clearly evident (Figures 10 and 12). On

increasing the temperature, the CT band centered around

800 nm increased in intensity and that around 520 nm

decreased in intensity. The reverse was true on decreasing

the temperature. From integration of the signals for the DNP-

H-2/6 protons in 3 ¥ RED⁴, and for the two pyrrole protons in

3 ¥ GREEN⁴, and also for the ArCH₂O protons in 3 ¥ RED⁴

and 3 ¥ GREEN⁴, the temperature-dependent variations

(Figure 27b) in the population of the two translational isomers

were obtained. In order to study the kinetics of the shuttling of

CBPQT⁴between the two recognition sites, a sample of 3⁴in

Figure 27. a) Partial variable temperature (VT) ¹H NMR spectra of an

equilibrium mixture of the [2]rotaxanes 3 ¥ GREEN⁴and 3 ¥ RED⁴

recorded at 500 MHz in CD₃CN. b) Temperature controlled variation in

the population of two translational isomers 3 ¥ RED⁴and 3 ¥ GREEN⁴in

CD₃CN.

was re-established after less than five minutes. Thus, the

shuttling of CBPQT⁴between the two recognition sites in 3⁴

is a much faster process than the shuttling of CBPQT⁴in the

slow bistable [2]rotaxane 2⁴. All attempts to separate the two

translational isomers of the fast [2]rotaxane 3⁴by employing

PTLC failed on account of the fast shuttling of CBPQT⁴

between the two recognition sites in 3⁴.

Conclusion

In conclusion, the syntheses and characterizations of two

amphiphilic bistable [2]rotaxanes, with two different recog-

nition sites–an MPTTF unit and a DNP moiety–for

CBPQT⁴have been reported. In both cases, the [2]rotaxanes

were isolated as mixtures of the two possible translational

isomers. Based on the redox properties of the dumbbell

compounds 17 and 23, one could have expected a strong

preference for CBPQT⁴to reside around the MPTTF station

in the [2]rotaxanes 2⁴and 3⁴. The results obtained, however,

confirm^{[10, 15c, 18, 19l,p, 31a, 56]}that the interaction of CBPQT⁴

1

CD₃CN was heated to 350 K and an H NMR spectrum was

recorded, revealing the presence of 3 ¥ GREEN⁴as the major

isomer, approximately 8:1 in favor of 3 ¥ GREEN⁴. Next, the

solution was cooled down to 300 K, followed by an immediate

recording of an ¹H NMR spectrum, an experiment which

showed that the 1:1 mixture of 3 ¥ RED⁴and 3 ¥ GREEN⁴

3000

¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Chem. Eur. J. 2003, 9, 2982 3007

Amphiphilic Rotaxanes

2982 3007

(Scheme 3), 4-[bis(4-tert-butylphenyl)(4-ethylphenyl)methyl]phenol (11)^[13a]

(Scheme 3), compound 16^[13a](Scheme 3), the semi-dumbbell compound

18^[13a](Scheme 5), compound 22^[13a](Scheme 5), the semi-dumbbell com-

pound 24^[13a](Scheme 7), and cyclobis(paraquat-p-phenylene) tetrakis-

(hexafluorophosphate) (CBPQT¥ 4PF₆)^[16](Schemes 7 and 8) were all

prepared according to literature procedures. Solvents were dried according

to literature procedures.^[57]All reactions were carried out under an

anhydrous argon or nitrogen atmosphere. High-pressure experiments were

carried out in a teflon tube on a Psika high-pressure apparatus. Thin-layer

chromatography (TLC) was carried out by using aluminium sheets pre-

coated with silica gel 60F (Merck 5554). The plates were inspected under

UV light and, if required, developed in I₂vapor. Column chromatography

was carried out by using silica gel 60F (Merck 9385, 0.040 0.063 mm),

while preparative thin-layer chromatography (PTLC) was performed on

UNIPLATE silica gel PTLC plates. Melting points were determined on an

Electrothermal 9100 apparatus or a B¸chi melting point apparatus and are

uncorrected. ¹H and ¹³C spectra were recorded (at room temperature

except where stated otherwise) on either a Bruker AC200 (200 and

50 MHz, respectively), Bruker ARX400 (400 and 100 MHz, respectively),

Bruker ARX500 or Bruker AMX500 (500 and 125 MHz, respectively)

spectrometers, with residual solvent as the internal standard. All chemical

shifts are quoted on a d scale, and all coupling constants (J) are expressed in

Hertz (Hz). Electron impact ionization mass spectrometry (EIMS) was

performed on a AUTO-SPEC instrument. Fast atom bombardment (FAB)

mass spectra were obtained using a ZAB-SE mass spectrometer, equipped

with krypton primary atom beam, utilizing a 3-nitrobenzyl alcohol matrix,

with electron-donating units also depends on other factors,

such as extension of p p stacking and the formation of

hydrogen bonds. The electrochemical and photophysical

studies show that the [2]pseudorotaxane 18 ꢀ CBPQT⁴and

the [2]rotaxanes 2⁴and 3⁴–all containing both an MPTTF

and a DNP station for encircling CBPQT⁴–exhibit a

complicated behavior that cannot be explained by the

presence of two simple translational isomers. The results

obtained suggest that–owing to the length and flexibility of

18 ꢀ CBPQT⁴, 2⁴, and 3⁴–in the translational isomer in

which CBPQT⁴encircles the DNP moiety, the MPTTF unit is

engaged ™alongside∫ with CBPQT⁴in a folded structure in

order to maximize the CT interactions. The relative orienta-

tion of the MPTTF unit and the DNP moiety with respect to

the hydrophilic dendritic stopper also seems to affect the

localization of CBPQT⁴on the (semi)dumbbell component,

À

most likely because of CT and C H ¥¥¥ O hydrogen-bonding

interactions between the hydrophilic stopper and CBPQT⁴

.

The steric hindrance exhibited from the SMe group situated

between the two recognition sites in the slow molecular

shuttle/switch 2⁴made it possible to isolate the translational

isomers 2 ¥ RED⁴and 2 ¥ GREEN⁴and to study the kinetics

of the shuttling of the tetracationic cyclophane between the

two recognition sites. The processes, which are accompanied

by clearly detectable color changes, can be followed by

¹H NMR and UV-visible spectroscopy; this allows us to

determine the rate constants and the associated energies of

activation for both the shuttling of CBPQT⁴from the DNP

recognition site in 2 ¥ RED⁴to the MPTTF recognition site in

2 ¥ GREEN⁴, as well as the shuttling of CBPQT⁴from the

MPTTF recognition site in 2 ¥ GREEN⁴to the DNP recog-

nition site in 2 ¥ RED⁴. In the fast molecular shuttle/switch

3⁴, the steric hindrance between the two recognition sites is

decreased by the insertion of a planar pyrrole moiety, instead

of a bulky SMe group as in 2⁴, between the DNP and MPTTF

recognition sites. This interchange results in considerably

faster shuttling of CBPQT⁴between the two recognition

sites, rendering the separation of 3 ¥ RED⁴and 3 ¥ GREEN⁴

impossible. The shuttling of CBPQT⁴in 3⁴is highly

temperature dependent and has been followed by variable

temperature ¹H NMR spectroscopy. At low temperature

(235 K), the major isomer is 3 ¥ RED⁴, whereas at higher

temperature (330 K) the major isomer is 3 ¥ GREEN⁴. In the

bistable [2]rotaxanes 2⁴and 3⁴shuttling of the macrocyclic

ring component CBPQT⁴can be driven by electrochemical

oxidation of the MPTTF unit. In the [2]pseudorotaxanes 18 ꢀ

CBPQT⁴and 24 ꢀ CBPQT⁴, in which one of the stoppers on

the dumbbell component is absent, electrochemical oxidation

causes dethreading.

while electrospray mass spectrometry (ESMS) was performed on

a

Finnigan MAT TSQ 700 triple quadrupole mass spectrometer, whereby

the rotaxanes were electrosprayed from MeCN solutions. Infrared (IR)

spectra were recorded on a Perkin Elmer 580 spectrophotometer. Micro-

analyses were performed by Quantitative Technologies, Inc.

2-[4-(2-Cyanoethylthio)-5-methylthio-1,3-dithiole-2-yliden]-5-tosyl-(1,3)-

dithiolo[4,5-c]pyrrole (6): Ketone 4 (1.48 g, 4.75 mmol) and thione 5

(1.26 g, 4.75 mmol) were suspended in distilled (EtO)₃P (35 mL) and

heated to 1358C (during heating the two solids dissolved leaving a red

solution and after 10 15 min a yellow orange precipitate was formed). Two

additional portions of 5 (each, 0.63 g, 2.37 mmol) were added after 15 and

30 min, respectively. The red reaction mixture was stirred for another hour

at 1358C, and cooled to room temperature; addition of MeOH (80 mL)

yielded an orange solid, which was filtered and washed with MeOH (3 Â

50 mL). The yellow solid was suspended in boiling CH₂Cl₂(70 mL), and the

hot mixture was filtered, whereupon the filter was washed with boiling

CH₂Cl₂(30 mL). The combined organic-phase filtrate was concentrated in

vacuo, and the resulting yellow solid was resuspended in CH₂Cl₂(50 mL)

and subjected to column chromatography (SiO₂, CH₂Cl₂). (Before the

column was eluted a yellow solid exclusively containing the symmetric

bis(pyrrolo)TTF was removed carefully.) The yellow band (R_f 0.4) was

collected, and the solvent evaporated to give a yellow solid, which was

dissolved in CH₂Cl₂/MeOH (1:1 v/v, 400 mL) and concentrated to

approximately half of its volume to precipitate the product. The yellow

crystals were collected by filtration, washed with MeOH (2 Â 50 mL) and

dried in vacuo to give compound 6 (1.60 g, 64%) as yellow needles. M.p.

192.5 1938C (lit.^[21b]186.5 1878C); ¹H NMR (CD₃SOCD₃, 200 MHz):

d 2.39 (s, 3H), 2.50 (s, 3H), 2.83 (t, J 6.6 Hz, 2H), 3.10 (t, J 6.6 Hz,

2H), 7.41 (s, 2H), 7.46 (d, J 8.4 Hz, 2H), 7.83 ppm (d, J 8.4 Hz, 2H);

¹³C NMR (CD₃SOCD₃, 50 MHz): d 18.4, 18.7, 21.3, 31.0, 112.4, 113.0,

118.1, 119.1, 120.9, 126.2, 127.0, 130.6, 132.7, 134.6, 146.1 ppm (2 signals are

missing/overlapping); MS (FAB): m/z (%): 528 (100) [M] ; elemental

analysis calcd (%) for C₁₉H₁₆N₂O₂S₇(528.8): C 43.15, H 3.05, N 5.30; found:

C 42.83, H 3.02, N 5.07.

Single-station [2]rotaxane 1 ¥ 4PF₆: A solution of the dumbbell 7 (0.41 g,

0.24 mmol), 8 ¥ 2PF₆(0.50 g, 0.71 mmol), and the dibromide 9 (0.19 g,

0.72 mmol) in anhydrous DMF (10 mL) was transferred to a teflon-tube

and subjected to 10 kbar of pressure at room temperature for 3 d. The

green suspension was directly subjected to column chromatography (SiO₂),

and unreacted dumbbell was eluted with Me₂CO, whereupon the eluent

was changed to Me₂CO/NH₄PF₆(1.0 g NH₄PF₆in 100 mL Me₂CO) and the

green band was collected. Most of the solvent was removed in vacuo (T<

308C) followed by addition of H₂O (80 mL). The resulting precipitate was

collected by filtration, washed with Et₂O (50 mL) and dried affording 1 ¥

4PF₆(0.14 g, 21%) as a green solid. M.p. 1358C (decomp); ¹H NMR

Experimental Section

General methods: Chemicals were purchased from Aldrich and were used

as received, unless indicated otherwise. The compounds 5-tosyl-(1,3)-

dithiolo[4,5-c]pyrrole-2-one (4)^[21](Scheme 1), 4-(2-cyanoethylthio)-5-

methylthio-1,3-dithiole-2-thione (5)^[23](Scheme 1), the dumbbell com-

pound 7,^[13a]1,1''-[1,4-phenylenebis(methylene)]bis(4,4'-bipyridin-1-ium)

bis(hexafluorophosphate) (8 ¥ 2PF₆)^[10](Schemes 2, 4 and 6), 2-{2-{5-[2-(2-

hydroxyethoxy)ethoxy]naphthalene-1-yloxy}ethoxy}ethane tosylate (10)^[13a]

Chem. Eur. J. 2003, 9, 2982 3007

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FULL PAPER

(400 MHz, CD₃COCD₃): d 1.19 (t, J 7.6 Hz, 3H), 1.28 (s, 18H), 2.60 (q,

J 7.6 Hz, 2H), 2.64 (s, 3H), 3.28 (s, 6H), 3.29 (s, 3H), 3.29 (t, J 6.4 Hz,

2H), 3.47 3.50 (m, 6H), 3.61 3.65 (m, 6H), 3.77 3.81 (m, 6H), 3.95 (t,

J 6.4 Hz, 2H), 3.98 4.01 (m, 2H), 4.08 4.14 (m, 6H), 4.23 4.25 (m,

2H), 4.69 (s, 2H), 4.80 (s, 4H), 4.97 (s, 2H), 5.18 (s, 2H), 5.98 6.08 (m,

8H), 6.43 and 6.45 (AB q, J 2.1 Hz, 2H), 6.78 (s, 2H), 6.83 6.85 (m, 4H),

6.94 (d, J 8.7 Hz, 4H), 7.07 7.12 (m, 10H), 7.18 (d, J 8.6 Hz, 2H), 7.26

7.31 (m, 10H), 7.38 (brs, 4H), 7.69 (d, J 8.7 Hz, 2H), 7.94 8.06 (m, 8H),

8.45 (brs, 4H), 9.16 (brs, 4H), 9.48 ppm (brs, 4H); MS (FAB): m/z (%):

2691 (4) [M À PF₆] , 2546 (14) [M À 2PF₆] , 2401 (16) [M À 3PF₆] , 1738

(7), 1273 (15) [M À 2PF₆]², 1200.5 (35) [M À 3PF₆]², 1128 (20) [M À

4PF₆]²; UV/Vis (Me₂CO, 298 K): l_max(e) 810 nm (1400 Lmol^À1cm^À1);

UV/Vis (Me₂CN, 298 K): l_max(e) 830 nm (1500 Lmol^À1cm^À1); elemental

analysis calcd (%) for C₁₃₄H₁₄₅F₂₄N₅O₁₅P₄S₆¥ 2H₂O (2837.9): C 56.00, H

5.23, N 2.44; found: C 56.07, H 5.06, N 2.28.

compound 14 (0.47 g, 74%) as a yellow foam. ¹H NMR (CD₃COCD₃,

200 MHz): d 1.19 (t, J 7.6 Hz, 3H), 1.28 (s, 18H), 2.37 (s, 3H), 2.39 (s,

3H), 2.60 (q, J 7.6 Hz, 2H), 3.07 (t, J 6.3 Hz, 2H), 3.83 (t, J 6.3 Hz,

2H), 3.94 4.06 (m, 6H), 4.14 4.19 (m, 2H), 4.26 4.34 (m, 4H), 6.84 (d,

J 8.9 Hz, 2H), 6.88 6.96 (m, 2H), 7.05 7.13 (m, 10H), 7.22 7.43 (m,

10H), 7.79 7.85 ppm (m, 4H); MS (FAB): m/z (%): 1252 (100) [M] ;

elemental analysis calcd (%) for C₆₉H₇₃NO₇S₇(1252.8): C 66.15, H 5.87, N

1.12; found: C 66.34, H 6.02, N 1.05.

Compound 15: Compound 14 (0.42 g, 0.34 mmol) was dissolved in

anhydrous THF/MeOH (1:1 v/v, 50 mL) and degassed (Ar, 10 min) before

NaOMe (25% solution in MeOH, 1.1 mL, 0.27 g, 5.0 mmol) was added in

one portion. The yellow solution was heated under reflux for 20 min before

being cooled to room temperature, whereupon the solvent was evaporated.

The yellow residue was dissolved in CH₂Cl₂(100 mL), washed with H₂O

(3 Â 100 mL), and dried (MgSO₄). Concentration gave a yellow foam,

which was subjected to column chromatography (SiO₂, CH₂Cl₂). The

yellow band (R_f 0.5) was collected and concentrated to provide com-

pound 15 (0.35 g, 95%) as a yellow foam. ¹H NMR (CD₃COCD₃,

200 MHz): d 1.20 (t, J 7.6 Hz, 3H), 1.29 (s, 18H), 2.42 (s, 3H), 2.60 (q,

J 7.6 Hz, 2H), 3.09 (t, J 6.4 Hz, 2H), 3.85 (t, J 6.4 Hz, 2H), 3.93 4.05

(m, 6H), 4.14 4.19 (m, 2H), 4.28 4.32 (m, 4H), 6.79 (s, 2H), 6.84 (d, J

9.0 Hz, 2H), 6.93 (d, J 7.7 Hz, 1H), 6.95 (d, J 7.7 Hz, 1H), 7.05 7.13 (m,

10H), 7.24 7.39 (m, 6H), 7.82 (d, J 8.3 Hz, 1H), 7.85 (d, J 8.4 Hz, 1H),

Compound 12: A solution of the monotosylate 10 (2.45 g, 4.99 mmol) and

11 (2.38 g, 4.99 mmol) in anhydrous MeCN (50 mL) containing K₂CO₃

(6.9 g, 50 mmol), LiBr (0.2 g, catalytic amount), and [18]crown-6 (ꢁ50 mg,

catalytic amount) was heated under reflux for 20 h. After cooling down to

room temperature, the reaction mixture was filtered and the residue

washed with MeCN (50 mL). The combined organic-phase filtrate was

concentrated in vacuo, and the brown oily residue was dissolved in CH₂Cl₂

(150 mL), washed with H₂O (2 Â 100 mL), and dried (MgSO₄). After

removal of the solvent the residue was purified by column chromatography

(SiO₂, CH₂Cl₂/MeOH 49:1). The colorless band (R_f 0.2) was collected,

and the solvent evaporated to give a colorless oil, which was redissolved in

CH₂Cl₂(20 mL) and concentrated providing compound 12 (3.20 g, 80%) as

a white foam. ¹H NMR (CDCl₃, 200 MHz): d 1.26 (t, J 7.6 Hz, 3H), 1.33

(s, 18H), 2.19 (s, 1H), 2.65 (q, J 7.6 Hz, 2H), 3.71 3.82 (m, 4H), 3.97

4.10 (m, 6H), 4.14 4.19 (m, 2H), 4.27 4.35 (m, 4H), 6.80 6.88 (m, 4H),

7.06 7.15 (m, 10H), 7.24 7.42 (m, 6H), 7.90 (d, J 8.5 Hz, 1H), 7.92 ppm

(d, J 8.4 Hz, 1H); ¹³C NMR (CDCl₃, 50 MHz): d 15.4, 28.3, 31.5, 34.4,

61.9, 63.2, 67.4, 68.0, 68.1, 69.8, 70.0, 70.1, 72.7, 105.9 (2 signals overlapping),

113.3, 114.6, 114.9, 124.2, 125.2, 125.3, 126.7, 126.8, 126.9, 130.8, 131.1, 132.3,

139.9, 141.4, 144.3, 144.7, 148.4, 154.3, 154.4, 156.6 ppm; MS (FAB): m/z

10.35 ppm (brs, 1H); MS (FAB): m/z (%): 1098 (100) [M] ; elemental

analysis calcd (%) for C₆₂H₆₇NO₅S₆(1098.6): C 67.78, H 6.15, N 1.27; found:

C 67.81, H 6.15, N 1.24.

Dumbbell 17: Compound 15 (0.23 g, 0.21 mmol) and the chloride 16 (0.21 g,

0.23 mmol) were dissolved in anhydrous DMF (10 mL) and degassed (Ar,

10 min) before NaH (0.021 g of

a 60% suspension in mineral oil,

0.53 mmol) was added. The reaction mixture was stirred for 45 min at

room temperature, causing the initially yellow solution to become more

orange. H₂O (40 mL) was added (dropwise until no more gas evolution was

observed), followed by addition of brine (40 mL). The yellow precipitate

was filtered and dried. The crude product was purified by column

chromatography (SiO₂, CH₂Cl₂/EtOAc 2:1). The yellow band (R_f 0.4)

was collected and the solvent evaporated affording a yellow oil, which was

repeatedly dissolved in CH₂Cl₂(3 Â 20 mL) and concentrated, providing

compound 17 (0.34 g, 83%) as a yellow foam. ¹H NMR (CD₃COCD₃,

400 MHz): d 1.17 (t, J 7.6 Hz, 3H), 1.26 (s, 18H), 2.38 (s, 3H), 2.57 (q,

J 7.6 Hz, 2H), 3.05 (t, J 6.4 Hz, 2H), 3.26 (s, 9H), 3.45 3.48 (m, 6H),

3.60 3.63 (m, 6H), 3.74 3.79 (m, 6H), 3.81 (t, J 6.4 Hz, 2H), 3.90 4.00

(m, 6H), 4.05 4.13 (m, 8H), 4.24 4.27 (m, 4H), 4.88 (s, 2H), 4.96 (s, 2H),

5.00 (s, 6H), 6.72 and 6.75 (AB q, J 2.1 Hz, 2H), 6.79 (d, J 8.8 Hz, 2H),

6.80 (d, J 9.1 Hz, 2H), 6.83 (s, 2H), 6.88 6.93 (m, 8H), 7.03 7.12 (m,

10H), 7.15 (d, J 8.7 Hz, 2H), 7.22 7.33 (m, 8H), 7.36 (d, J 8.8 Hz, 4H),

7.80 (d, J 8.5 Hz, 1H), 7.82 ppm (d, J 8.5 Hz, 1H); MS (FAB): m/z (%):

(%): 794 (63) [M] , 689 (15), 661 (29), 383 (100); elemental analysis calcd

(%) for C₅₃H₆₂O₆(795.1): C 80.07, H 7.86; found: C 79.85, H 7.88.

Compound 13: Ph₃P (0.70 g, 2.67 mmol) was added portionwise to a

solution of the alcohol 12 (1.75 g, 2.20 mmol) and CBr₄(0.88 g, 2.65 mmol)

in anhydrous CH₂Cl₂(15 mL) at room temperature. The reaction mixture

was stirred for 16 h, whereupon additional CBr₄(0.88 g, 2.65 mmol),

followed by Ph₃P (0.70 g, 2.67 mmol) was added and the reaction mixture

was stirred for another 24 h. After concentration, the residue was purified

by column chromatography (SiO₂, CH₂Cl₂/hexane 2:1). The colorless band

(R_f 0.3) was collected and the solvent evaporated, affording a colorless

oil, which was repeatedly dissolved in CH₂Cl₂(3 Â 50 mL) and concen-

trated to provide compound 13 (1.77 g, 94%) as a white foam. ¹H NMR

(CDCl₃, 200 MHz): d 1.26 (t, J 7.6 Hz, 3H), 1.33 (s, 18H), 2.65 (q, J

7.6 Hz, 2H), 3.54 (t, J 6.2 Hz, 2H), 3.94 4.10 (m, 8H), 4.14 4.19 (m,

2H), 4.28 4.36 (m, 4H), 6.80 6.89 (m, 4H), 7.06 7.15 (m, 10H), 7.24 7.42

(m, 6H), 7.90 (d, J 8.4 Hz, 1H), 7.91 ppm (d, J 8.4 Hz, 1H); ¹³C NMR

(CDCl₃, 50 MHz): d 15.4, 28.3, 30.5, 31.5, 34.4, 63.2 (C(Ar)₄), 67.4, 68.0,

69.8, 70.0, 70.2, 71.6 (6 out of 7 CH₂O signals, one overlapping), 105.8 (2

signals overlapping), 113.3, 114.7, 114.9, 124.2, 125.2, 125.3, 126.7, 126.8 (2

signals overlapping), 130.8, 131.1, 132.3, 139.9, 141.4, 144.3, 144.7, 148.3,

1966 (100) [M] , 1757 (21), 1548 (13), 1203 (20); elemental analysis calcd

(%) for C₁₁₂H₁₂₇NO₁₈S₆(1967.6): C 68.37, H 6.51, N 0.71; found: C 68.17, H

6.49, N 0.66.

Slow two-station [2]rotaxane 2 ¥ 4PF₆: A solution of 17 (0.28 g, 0.14 mmol),

8 ¥ 2PF₆(0.30 g, 0.42 mmol) and 9 (0.11 g, 0.42 mmol) in anhydrous DMF

(10 mL) was stirred for 10 d at room temperature (after approximately 1 d

the color changed to dark green and a white precipitate was formed). The

green suspension was directly subjected to column chromatography (SiO₂),

and unreacted 17 was eluted with Me₂CO, whereupon the eluent was

changed to Me₂CO/NH₄PF₆(1.0 g NH₄PF₆in 100 mL Me₂CO) and the

brown band containing 2 ¥ 4PF₆was collected. Most of the solvent was

removed under vacuum (T< 308C) followed by addition of H₂O (50 mL).

The resulting precipitate was collected by filtration, washed with Et₂O

(20 mL) and dried, affording 2 ¥ 4PF₆(0.10 g, 23%) as a brown solid. M.p.

1508C (decomp). The data given below are for the 1:1 mixture of the two

154.3, 154.4, 156.6 ppm; MS (FAB): m/z (%): 858 (30) [M2] , 856 (27)

[M] , 753 (8), 751 (8), 725 (15), 723 (14), 383 (100); elemental analysis calcd

(%) for C₅₃H₆₁BrO₅(858.0): C 74.20, H 7.17; found: C 74.36, H 7.20.

Compound 14: A solution of 6 (0.27 g, 0.51 mmol) in anhydrous THF

(35 mL) was degassed (Ar, 10 min) before a solution of CsOH ¥ H₂O

(0.090 g, 0.54 mmol) in anhydrous MeOH (3.5 mL) was added dropwise by

syringe over a period of 1 h at room temperature. The mixture was stirred

for 15 min, whereupon a solution of the bromide 13 (0.46 g, 0.54 mmol) in

anhydrous THF (5 mL) was added in one portion and the reaction mixture

was stirred for 24 h at room temperature. The solvent was evaporated, and

the resulting yellow residue was dissolved in CH₂Cl₂(100 mL), washed with

brine (100 mL), H₂O (2 Â 100 mL), and dried (MgSO₄). Removal of the

solvent gave a yellowish orange foam, which was purified by column

chromatography (SiO₂, CH₂Cl₂/hexane 4:1). The broad yellow band (R_f

0.3) was collected and concentrated affording a yellow foam, which was

repeatedly dissolved in CH₂Cl₂(2 Â 20 mL) and concentrated to give

translational isomers. MS (FAB): m/z (%): 2921 (4) [M À PF₆] , 2776 (9)

[M À 2PF₆] , 2631 (6) [M À 3PF₆] , 1966 (3), 1388 (10) [M À 2PF₆]²

,

1315.5 (13) [M À 3PF₆]², 1243 (6) [M À 4PF₆]²; UV/Vis (Me₂CO, 298 K):

l_max(e) 540 (760), 805 nm (860 Lmol^À1cm^À1); elemental analysis calcd

(%) for C₁₄₈H₁₅₉F₂₄N₅O₁₈P₄S₆¥ 2H₂O (3068.1): C 57.26, H 5.29, N 2.26;

found: C 56.86, H 5.19, N 2.11.

Separation of the translational isomers of 2 ¥ 4PF₆: The two translational

isomers were separated by using preparative thin-layer chromatography

(PTLC), which was performed at room temperature with Me₂CO/NH₄PF₆

(1.0 g NH₄PF₆in 100 mL Me₂CO) as eluent. Immediately after elution, the

3002

¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Chem. Eur. J. 2003, 9, 2982 3007

Amphiphilic Rotaxanes

2982 3007

red band (R_f 0.45) containing 2 ¥ 4PF₆¥ RED was extracted into Me₂CO.

The solvent was removed in vacuo (T< 108C) and the red residue dissolved

in CD₃COCD₃, giving a red solution, which was cooled to À788C in a

Me₂CO/dry ice bath for storage.

6.95 (d, J 8.0 Hz, 1H), 7.00 (d, J 8.0 Hz, 1H), 7.07 7.15 (m, 10H), 7.32

(d, J 8.6 Hz, 4H), 7.36 (t, J 8.0 Hz, 1H), 7.43 (t, J 8.0 Hz, 1H), 7.83 (d,

J 8.0 Hz, 1H), 7.86 ppm (d, J 8.0 Hz, 1H); MS (FAB): m/z (%): 1263

(100) [M] ; elemental analysis calcd (%) for C₆₇H₇₇NO₉S₇(1264.8): C 63.62,

H 6.14, N 1.11; found: C 63.55, H 5.85, N 1.24.

Data for 2 ¥ 4PF₆¥ RED: ¹H NMR (CD₃COCD₃, 500 MHz, 225 K): d 1.10

(t, J 7.6 Hz, 3H), 1.18 (s, 18H), 2.49 (s, 3H), 2.50 2.60 (m, 4H), 3.20 (s,

3H), 3.23 (s, 6H), 3.72 3.76 (m, 8H), 4.00 4.05 (m, 8H), 4.12 4.16 (m,

2H), 4.26 4.31 (m, 2H), 4.34 4.45 (m, 10H), 4.78 (s, 2H), 4.87 (s, 2H),

4.95 (s, 4H), 5.00 (s, 2H), 5.91 6.05 (m, 9H), 6.14 (t, J 8.2 Hz, 1H), 6.29

(d, J 8.2 Hz, 1H), 6.33 (d, J 8.2 Hz, 1H), 6.76 6.80 (m, 6H), 6.86 6.94

(m, 8H), 7.01 7.06 (m, 10H), 7.22 7.28 (m, 8H), 7.38 (d, J 8.4 Hz, 4H),

7.54 (d, J 5.8 Hz, 2H), 7.59 (d, J 5.8 Hz, 2H), 7.70 (d, J 6.3 Hz, 2H),

7.72 (d, J 6.6 Hz, 2H), 8.06 (s, 2H), 8.16 (s, 2H), 8.29 (s, 2H,), 8.36 (s, 2H),

9.07 (d, J 6.6 Hz, 2H), 9.09 (d, J 6.9 Hz, 2H), 9.19 (d, J 6.4 Hz, 2H),

9.34 ppm (d, J 6.5 Hz, 2H); the signals from 5 Â CH₂O (10H) are

obscured under the intense H₂O signal, which appears at 3.26 3.69; UV/

Vis (CD₃COCD₃, 298 K): l_max 540 nm.

Although 2 ¥ 4PF₆¥ GREEN appears to be less polar than 2 ¥ 4PF₆¥ RED, it

was only possible to extract an extremely small amount of 2 ¥ 4PF₆¥

GREEN (R_f 0.5) from the silica on the PTLC plate. The UV/Vis

spectrum recorded in CD₃COCD₃at 298 K of this fraction shows, as

expected, only a broad CT absorption band centered on 800 nm. As a

consequence of the extremely limited amount of 2 ¥ 4PF₆¥ GREEN,

isolated from the PTLC experiment it was not possible to record an

¹H NMR spectrum. Alternatively, it was possible to shift the equilibrium of

the two translational isomers from 1:1 to 9:1 in favor of 2 ¥ 4PF₆¥ GREEN

by heating a CD₃SOCD₃solution of the brown 1:1 mixture to 425 K. The

data given below are for the major isomer at 410 K.

Compound 20: Compound 19 (0.50 g, 0.40 mmol) was dissolved in

anhydrous Me₂CO (50 mL), and NaI (0.59 g, 3.94 mmol) was added in

one portion. The reaction mixture was heated under reflux for 14 h, before

being cooled to room temperature and the solvent removed in vacuo. The

yellow residue was dissolved in CH₂Cl₂(100 mL), washed with H₂O (2 Â

70 mL), and dried (MgSO₄). Concentration in vacuo gave a yellow foam,

which was purified by column chromatography (SiO₂: CH₂Cl₂). The yellow

band (R_f 0.6) was collected and concentrated to provide compound 20

(0.48 g, 93%) as a yellow foam. ¹H NMR (CD₃COCD₃, 500 MHz): d 1.21

(t, J 7.6 Hz, 3H), 1.31 (s, 18H), 2.43 (s, 3H), 2.60 (q, J 7.6 Hz, 2H), 3.06

(t, J 6.4 Hz, 2H), 3.39 (t, J 6.2 Hz, 2H), 3.75 (t, J 6.4 Hz, 2H), 3.79

3.81 (m, 2H), 3.83 3.85 (m, 2H), 3.89 (t, J 6.2 Hz, 2H), 3.90 3.93 (m,

2H), 4.00 4.02 (m, 2H), 4.08 4.10 (m, 4H), 4.26 4.28 (m, 2H), 4.32 4.33

(m, 2H), 6.75 and 6.77 (AB q, J 2.1 Hz, 2H), 6.81 (d, J 8.8 Hz, 2H), 6.92

(d, J 8.0 Hz, 1H), 6.98 (d, J 8.0 Hz, 1H), 7.09 7.16 (m, 10H), 7.32 (d,

J 8.5 Hz, 4H), 7.36 (t, J 8.0 Hz, 1H), 7.42 (t, J 8.0 Hz, 1H), 7.83 (d, J

8.0 Hz, 1H), 7.89 ppm (d, J 8.0 Hz, 1H); MS (FAB): m/z (%): 1295 (100)

[M] ; elemental analysis calcd (%) for C₆₆H₇₄INO₆S₆(1296.6): C 61.14, H

5.75, N 1.08; found: C 61.28, H 5.46, N 0.99.

Compound 21

Method A: Compound 20 (0.48 g, 0.37 mmol) was dissolved in anhydrous

Me₂CO (40 mL), and KSCN (0.36 g, 3.70 mmol) was added in one portion.

The reaction mixture was heated under reflux for 5 h. After being cooled to

room temperature, the solvent was removed in vacuo. The yellow residue

was dissolved in CH₂Cl₂(100 mL), washed with H₂O (2 Â 100 mL), and

dried (MgSO₄). Concentration in vacuo gave a yellow foam, which was

subjected to column chromatography (SiO₂, CH₂Cl₂/hexane 9:1). The

yellow band (R_f 0.3) was collected and concentrated to a yellow oil, which

was repeatedly redissolved in CH₂Cl₂(2 Â 20 mL) and concentrated to give

compound 21 (0.37 g, 81%) as a yellow foam.

Data for 2 ¥ 4PF₆¥ GREEN: ¹H NMR (CD₃SOCD₃, 500 MHz, 410 K): d

1.23 (t, J 7.6 Hz, 3H), 1.31 (s, 18H), 2.62 (q, J 7.6 Hz, 2H), 2.68 (s, 3H),

3.30 (s, 9H), 3.33 (t, J 6.2 Hz, 2H), 3.49 3.54 (m, 6H), 3.62 3.66 (m,

6H), 3.77 3.83 (m, 6H), 3.89 3.92 (m, 2H), 3.90 4.00 (m, 2H), 4.06 (t,

J 6.2 Hz, 2H), 4.11 4.23 (m, 10H), 4.32 4.36 (m, 2H), 4.45 4.47 (m,

2H), 4.90 (s, 2H), 5.05 (s, 6H), 5.11 (s, 2H), 5.86 and 5.90 (AB q, J

13.1 Hz, 8H), 6.16 and 6.17 (AB q, J 2.0 Hz, 2H), 6.83 6.89 (m, 6H),

6.95 6.99 (m, 6H), 7.03 7.11 (m, 12H), 7.20 7.41 (m, 14H), 7.83 (d, J

8.0 Hz, 2H), 7.87 (s, 8H), 8.08 (d, J 6.3 Hz, 8H), 9.29 ppm (d, J 6.3 Hz,

8H); UV/Vis (CD₃COCD₃, 298 K): l_max 800 nm.

Method B: Compound 19 (0.54 g, 0.43 mmol) was dissolved in anhydrous

Me₂CO (50 mL), and KSCN (1.24 g, 12.8 mmol) was added in one portion.

The yellow reaction mixture was heated under reflux for 1 d, whereupon

additional KSCN (0.83 g, 8.54 mmol) was added. The reaction mixture was

heated under reflux for a further 1 d and cooled to room temperature.

After removal of the solvent, the yellow residue was dissolved in CH₂Cl₂

(150 mL), washed with H₂O (3 Â 100 mL), and dried (MgSO₄). Concen-

tration in vacuo gave compound 21 (0.51 g, 97%) as a yellow foam.

¹H NMR (CD₃COCD₃, 500 MHz): d 1.21 (t, J 7.6 Hz, 3H), 1.31 (s,

18H), 2.45 (s, 3H), 2.62 (q, J 7.6 Hz, 2H), 3.08 (t, J 6.4 Hz, 2H), 3.38 (t,

J 5.8 Hz, 2H), 3.78 (t, J 6.4 Hz, 2H), 3.82 3.84 (m, 2H), 3.86 3.88 (m,

2H), 3.93 3.95 (m, 2H), 4.01 (t, J 5.8 Hz, 2H), 4.05 4.07 (m, 2H), 4.10

4.13 (m, 4H), 4.29 4.31 (m, 2H), 4.36 4.38 (m, 2H), 6.77 and 6.78 (AB q,

J 2.1 Hz, 2H), 6.84 (d, J 8.8 Hz, 2H), 6.95 (d, J 8.0 Hz, 1H), 7.01 (d,

J 8.0 Hz, 1H), 7.08 7.16 (m, 10H), 7.32 (d, J 8.5 Hz, 4H), 7.38 (t, J

8.0 Hz, 1H), 7.43 (t, J 8.0 Hz, 1H), 7.83 (d, J 8.0 Hz, 1H), 7.88 ppm (d,

[2]Pseudorotaxane 18 ꢀ CBPQT¥ 4PF₆: Dissolving a 1:1 mixture of the

semi-dumbbell compound 18 and CBPQT¥ 4PF₆in CD₃COCD₃at 298 K

produced a brown solution (c 2.12 Â 10^À3m), and a ¹H NMR spectrum

(500 MHz) was recorded at 300 K. The signals were extremely broad,

because the exchange between the complexed and free species occurs

rapidly on the ¹H NMR timescale. Upon cooling the sample down to 245 K,

1

all the signals sharpened, and the H NMR spectrum (500 MHz) revealed

(Figure 17) that 18 ꢀ CBPQT¥ RED⁴is almost exclusively present in the

CD₃COCD₃solution at 245 K. The data given below are for 18 ꢀ CBPQT¥

RED⁴in CD₃COCD₃at 245 K. ¹H NMR (CD₃COCD₃, 500 MHz, 245 K):

d 1.19 (t, J 7.6 Hz, 3H), 1.29 (s, 18H), 2.29 (s, 3H), 2.59 (d, J 8.0 Hz,

1H), 2.60 (q, J 7.6 Hz, 2H), 2.69 (d, J 8.0 Hz, 1H), 2.89 (t, J 6.2 Hz,

2H), 3.65 (t, J 6.2 Hz, 2H), 3.78 3.92, 4.02 4.10, and 4.30 4.58 (m,

20H), 5.10 (unresolved t, 1H), 5.95 6.05 (m, 2H), 6.15 6.25 (m, 8H), 6.29

(d, J 8.0 Hz, 1H), 6.47 (d, J 8.0 Hz, 1H), 6.86 (d, J 8.9 Hz, 2H), 7.05

7.35 (m, 18H), 7.75 (d, J 6.6 Hz, 2H), 7.87 (d, J 6.6 Hz, 2H), 7.89 (d, J

6.6 Hz, 2H), 8.20 (s, 2H), 8.27 (s, 2H), 8.31 (s, 2H), 8.49 (s, 2H), 8.69 (d, J

6.6 Hz, 2H), 9.16 (d, J 6.6 Hz, 4H), 9.62 ppm (d, J 6.6 Hz, 2H).

J 8.0 Hz, 1H); IR (KBr): nÄ 2154 cm^À1(S C N); MS (FAB): m/z (%):

À ꢂ

1226 (100) [M] ; elemental analysis calcd (%) for C₆₇H₇₄N₂O₆S₇(1227.8): C

65.54, H 6.08, N 2.28; found: C 65.49, H 6.02, N 2.13.

Dumbbell 23: Compound 21 (0.25 g, 0.20 mmol) and the chloride 22

(0.19 g, 0.24 mmol) were dissolved in anhydrous THF/EtOH (2:1 v/v,

50 mL), after which powdered NaBH₄(0.077 g, 2.04 mmol) was added in

one portion. The reaction mixture was stirred for 1 d at room temperature,

whereupon it was poured into ice containing saturated aqueous NH₄Cl

solution (50 mL) and extracted with CH₂Cl₂(3 Â 50 mL). The combined

organic extracts were washed with H₂O (2 Â 50 mL) and dried (MgSO₄).

Concentration in vacuo gave a yellow oil, which was purified by column

chromatography (SiO₂, CH₂Cl₂/EtOAc 3:2). The yellow band (R_f 0.4)

was collected, and the solvent evaporated affording a yellow oil, which was

repeatedly dissolved in CH₂Cl₂(3 Â 20 mL) and concentrated to give

compound 23 (0.35 g, 90%) as a yellow foam. ¹H NMR (CD₃COCD₃,

500 MHz): d 1.20 (t, J 7.6 Hz, 3H), 1.29 (s, 18H), 2.42 (s, 3H), 2.60 (q,

J 7.6 Hz, 2H), 2.61 (t, J 6.5 Hz, 2H), 3.04 (t, J 6.4 Hz, 2H), 3.29 (s,

9H), 3.48 3.50 (m, 6H), 3.62 3.65 (m, 6H), 3.73 3.81 (m, 14H), 3.82

3.85 (m, 2H), 3.88 3.91 (m, 2H), 3.93 3.95 (m, 2H), 4.07 4.13 (m, 10H),

Compound 19: MsCl (0.05 mL, 0.078 g, 0.68 mmol) was added dropwise to

an ice-cooled solution of the alcohol 18 (0.50 g, 0.42 mol) and Et₃N

(0.21 mL, 0.15 g, 1.48 mmol) in anhydrous CH₂Cl₂(40 mL). The yellow

reaction mixture was stirred for 1 h at 08C, whereupon the ice-bath was

removed. The reaction mixture was diluted with CH₂Cl₂(50 mL), washed

with H₂O (3 Â 50 mL), and dried (MgSO₄). Concentration in vacuo gave a

yellow oil, which was subjected to column chromatography (SiO₂: CH₂Cl₂/

MeOH 99:1). The yellow band (R_f 0.4) was collected and concentrated to

give compound 19 (0.50 g, 95%) as a yellow foam. ¹H NMR (CD₃COCD₃,

500 MHz): d 1.22 (t, J 7.6 Hz, 3H), 1.31 (s, 18H), 2.45 (s, 3H), 2.62 (q,

J 7.6 Hz, 2H), 3.08 (t, J 6.4 Hz, 2H), 3.10 (s, 3H), 3.78 (t, J 6.4 Hz,

2H), 3.82 3.84 (m, 2H), 3.87 3.89 (m, 2H), 3.93 3.95 (m, 4H), 4.04 4.06

(m, 2H), 4.11 4.13 (m, 4H), 4.29 4.31 (m, 2H), 4.35 4.37 (m, 2H), 4.43

4.45 (m, 2H), 6.77 and 6.78 (AB q, J 2.0 Hz, 2H), 6.84 (d, J 8.6 Hz, 2H),

Chem. Eur. J. 2003, 9, 2982 3007

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¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

3003

J. F. Stoddart, J. O. Jeppesen, M. T. Gandolfi, M. Venturi et al.

FULL PAPER

4.22 4.25 (m, 2H), 4.31 4.33 (m, 2H), 4.87 (s, 2H), 4.96 (s, 4H), 6.73 6.75

(m, 4H), 6.80 (d, J 8.4 Hz, 2H), 6.81 (d, J 8.9 Hz, 2H), 6.89 (d, J

8.1 Hz, 1H), 6.93 (d, J 8.3 Hz, 4H), 6.99 (d, J 8.1 Hz, 1H), 7.05 7.13 (m,

10H), 7.28 7.32 (m, 7H), 7.35 (d, J 9.0 Hz, 4H), 7.39 (t, J 8.1 Hz, 1H),

7.79 (d, J 8.1 Hz, 1H), 7.87 ppm (d, J 8.1 Hz, 1H); MS (FAB): m/z (%):

cationic compounds. UV/Vis absorption spectra were recorded with a

Perkin Elmer Lambda 40 spectrophotometer, a Cary 100 Bio spectro-

photometer, a Shimadzu UV-160 instrument, or a Shimadzu UV-1601PC

instrument. Uncorrected luminescence spectra were obtained with a

Perkin Elmer LS-50 spectrofluorimeter, equipped with a Hamamatsu

R928 phototube. The estimated experimental errors are 2 nm on band

maxima, Æ5% on the molar absorption coefficients and fluorescence

intensity.

1965 (100) [M] , 1756 (20), 1546 (22), 1202 (24); elemental analysis calcd

(%) for C₁₀₉H₁₂₉NO₁₈S₇(1965.7); C 66.60, H 6.61, N 0.71; found: C 66.67, H

6.54, N 0.61.

Electrochemical experiments: Cyclic voltammetric (CV) and differential

pulse voltammetric (DPV) experiments were carried out in argon-purged

MeCN at room temperature with an Autolab 30 multipurpose instrument

interfaced to a personal computer. The working electrode was a glassy

carbon electrode (0.08 cm², Amel), whose surface was routinely polished

with a 0.05 mm alumina water slurry on a felt surface immediately prior to

use. In all cases, the counter electrode was a Pt spiral, separated from the

bulk solution with a fine glass frit, and an Ag wire was used as a quasi-

reference electrode. 1,1-Dimethylferrocene (0.31 V vs SCE)^[58]was

present as an internal standard. The concentrations of the compounds

examined were of the order of 5 Â 10^À4m, unless otherwise noted. The

experiments were carried out in the presence of tetraethylammonium

hexafluorophosphate (5 Â 10^À2m) as supporting electrolyte. Cyclic voltam-

mograms were obtained with sweep rates in the range 0.05 1.0 Vs^À1. DPV

experiments were performed with a scan rate of 20 or 4 mVs^À1, a pulse

height of 75 or 10 mV, and a duration of 40 ms. The reversibility of the

observed processes was established by using the criteria of 1) separation of

60 mV between cathodic and anodic peaks, 2) the close to unity ratio of the

intensities of the cathodic and anodic currents, and 3) the constancy of the

peak potential on changing sweep rate in the cyclic voltammograms. The

same halfwave potentials were obtained from the DPV peaks and from an

average of the cathodic and anodic CV peaks, as expected for reversible

processes. For irreversible processes, the potentials were estimated from

the DPV peaks. The experimental errors on the potentials were estimated

to be Æ10 mV. For the [2]rotaxane 1⁴the potential values for the two

oxidations associated with the MPTTF unit were obtained by deconvolu-

tion of the DPV profile by employing the equations proposed by Parry and

Osteryoung.^[59]The number of exchanged electrons for reversible processes

involving the MPTTF unit was measured by comparing the current

intensity of the CV waves and the area of the DPV peaks with those found

for the two reversible and monoelectronic oxidation processes of the

MPTTF model compound 25, after correction for differences in concen-

trations and diffusion coefficients.^[60]

Fast two-station [2]rotaxane 3 ¥ 4PF₆

Method A:

A solution of 23 (0.35 g, 0.18 mmol), 8 ¥ 2PF₆(0.38 g,

0.54 mmol), and 9 (0.14 g, 0.53 mmol) in anhydrous DMF (10 mL) was

stirred for 10 d at room temperature (after approximately 1 d the color

changed to dark green and a white precipitate was formed). The dark green

suspension was directly subjected to column chromatography (SiO₂), and

unreacted 23 was eluted with Me₂CO, whereupon the eluent was changed

to Me₂CO/NH₄PF₆(1.0 g NH₄PF₆in 100 mL Me₂CO) and the brown band

containing 3 ¥ 4PF₆was collected. Most of the solvent was removed under

vacuum (T< 308C) followed by addition of H₂O (50 mL). The resulting

precipitate was collected by filtration, washed with Et₂O (20 mL) and

dried, affording 3 ¥ 4PF₆(0.084 g, 15%) as a brown solid. M.p. 2208C

(decomp).

Method B:

A solution of 23 (0.40 g, 0.20 mmol), 8 ¥ 2PF₆(0.43 g,

0.61 mmol), and 9 (0.16 g, 0.61 mmol) in anhydrous DMF (12 mL) was

transferred to a teflon tube and subjected to 10 kbar pressure at room

temperature for 3 d. The dark green solution was directly subjected to

column chromatography (SiO₂), and unreacted 23 was eluted with Me₂CO,

whereupon the eluent was changed to Me₂CO/NH₄PF₆(1.0 g NH₄PF₆in

100 mL Me₂CO) and the brown band containing 3 ¥ 4PF₆was collected.

Most of the solvent was removed in vacuo (T< 308C) followed by addition

of H₂O (200 mL). The resulting precipitate was collected by filtration,

washed with H₂O (30 mL) and Et₂O (40 mL), and dried affording 2 ¥ 4PF₆

(0.29 g, 47%) as a brown solid. M.p. 2158C (decomp). The data given below

are for the mixture of the two translational isomers; MS (FAB): m/z (%):

2919 (3) [M À PF₆] , 2774 (8) [M À 2PF₆] , 2629 (9) [M À 3PF₆] , 1964 (6),

1387 (11) [M À 2PF₆]², 1314.5 (24) [M À 3PF₆]², 1242 (12) [M À 4PF₆]²

;

UV/Vis (MeCN, 298 K): l_max(e) 520 (960), 800 nm (1300 Lmol^À1cm^À1);

UV/Vis (Me₂CO, 298 K): l_max(e) 540 (760), 785 nm (740 Lmol^À1cm^À1);

UV/Vis (Me₂SO, 298 K): l_max(e) 540 (640), 765 nm (1310 Lmol^À1cm^À1);

elemental analysis calcd (%) for C₁₄₅H₁₆₁F₂₄N₅O₁₈P₄S₇(3066.2): C 56.80, H

5.29, N 2.28; found: C 56.43, H 5.20, N 2.21.

All attempts to separate the two translational isomers by employing PTLC

failed on account of the fast shuttling of CBPQT⁴between the two

recognition sites in 3 ¥ 4PF₆. Instead, 3 ¥ 4PF₆¥ RED and 3 ¥ 4PF₆¥ GREEN

were characterized as a consequence of the fact in CD₃COCD₃at 245 K, 3 ¥

4PF₆exists almost exclusively as 3 ¥ 4PF₆¥ RED, whereas in CD₃SOCD₃at

400 K, it exists almost exclusively as 3 ¥ 4PF₆¥ GREEN.

Acknowledgement

This research was funded by the Defense Advanced Research Projects

Agency (DARPA) in the United States, the Carlsbergfondet in Denmark,

and EU (HPRN-CT-2000-00029) in Italy. Some of the compound/complex

characterization were supported by the National Science Foundation under

equipment grant no CHE-9974928.

Data for 3 ¥ 4PF₆¥ RED: ¹H NMR (CD₃COCD₃, 500 MHz, 245 K): d 1.19

(t, J 7.5 Hz, 3H), 1.28 (s, 18H), 2.27 (s, 3H), 2.44 (d, J 8.0 Hz, 1H), 2.51

(d, J 8.0 Hz, 1H), 2.59 (q, J 7.5 Hz, 2H), 2.87 (t, J 6.1 Hz, 2H), 3.15

(unresolved t, 2H), 3.24 (s, 6H), 3.28 (s, 3H), 3.49 3.53 (m, 6H), 3.60 (t,

J 6.1 Hz, 2H), 3.63 3.68 (m, 6H), 3.73 3.77 (m, 2H), 3.79 3.83 (m,

8H), 4.03 4.08 (m, 10H), 4.18 4.22 (m, 2H), 4.23 4.27 (m, 4H), 4.30

4.34 (m, 2H), 4.38 4.42 (m, 2H), 4.48 4.52 (m, 2H), 4.67 (s, 2H), 4.88 (s,

4H), 5.88 6.15 (m, 10H), 6.22 (d, J 8.0 Hz, 1H), 6.34 (d, J 8.0 Hz, 1H),

6.75 (s, 2H), 6.78 6.86 (m, 8H), 7.07 7.18 (m, 12H), 7.21 7.26 (m, 8H),

7.33 (d, J 8.4 Hz, 4H), 7.57 (d, J 6.0 Hz, 2H), 7.61 (d, J 6.0 Hz, 2H),

7.85 (d, J 6.0 Hz, 2H), 8.14 (s, 2H), 8.22 (s, 2H), 8.26 (s, 2H), 8.27 (s, 2H),

8.65 (d, J 6.0 Hz, 2H), 8.97 (d, J 6.0 Hz, 2H), 9.09 (d, J 6.0 Hz, 2H),

9.27 ppm (d, J 6.0 Hz, 2H).

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[35] See Supporting Information for further details.

[36] The DG8 values were calculated using the relationship DG8

ÀRTlnK_a, in which R is the gas constant and T is the absolute

temperature.

[20] J. O. Jeppesen, J. Perkins, J. Becher, J. F. Stoddart, Angew. Chem. 2001,

113, 1256 1261; Angew. Chem. Int. Ed. 2001, 40, 1216 1221.

[21] Recently some of us developed an efficient synthesis of the parent

pyrrolo[3,4-d]tetrathiafulvalene unit using a simple and nonclassical

pyrrole synthesis, see: a) J. O. Jeppesen, K. Takimiya, F. Jensen, J.

Becher, Org. Lett. 1999, 1, 1291 1294; b) J. O. Jeppesen, K. Takimiya,

F. Jensen, T. Brimert, K. Nielsen, N. Thorup, J. Becher, J. Org. Chem.

2000, 65, 5794 5805.

[37] D. J. Leggett, Computational Methods for Determination of Formation

Constants, Plenum, London and New York, 1985, Chapter 1.

[38] A. Mirzoian, A. E. Kaifer, J. Org. Chem. 1995, 60, 8093 8095.

¬

[39] a) R. Wolf, M. Asakawa, P. R. Ashton, M. Gomez-Lopez, C. Hamers,

S. Menzer, I. W. Parsons, N. Spencer, J. F. Stoddart, M. S. Tolley, D. J.

Williams, Angew. Chem. 1998, 110, 1018 1022; Angew. Chem. Int. Ed.

¬

1998, 37, 975 979; b) V. Balzani, A. Ceroni, A. Credi, M. Gomez-

¬

Lopez, C. Hamers, J. F. Stoddart, R. Wolf, New J. Chem. 2001, 25, 25

[22] J. O. Jeppesen, J. Perkins, J. Becher, J. F. Stoddart, Org. Lett. 2000, 2,

3547 3550.

[23] K. B. Simonsen, N. Svenstrup, J. Lau, O. Simonsen, P. M˘rk, G. J.

Kristensen, J. Becher, Synthesis 1996, 407 418.

31.

[40] The absorption spectrum of the [2]rotaxane 1⁴recorded in Me₂CO at

298 K, shows a broad CT absorption band centered on 810 nm (e

1400 Lmol^À1cm^À1).

[24] Although the preparation of 6 has been described before in the

literature (see: ref. [21b]), it was synthesized by a different route and

only on a very small scale.

[41] The absorption spectra were recorded on equilibrium mixtures of 2⁴

and 3⁴. In the fast [2]rotaxane 3⁴equilibration occurs immediately.

However, in the case of the slow [2]rotaxane 2⁴equilibration takes

24 h (see dynamic investigations).

[25] Unreacted dumbbell can be recovered and reused.

[26] It is well established that organic reactions with a negative volume of

activation (DV⁼) are favored by high pressure, see: F.-G. Kl‰rner, F.

Wurche, J. Prakt. Chem. 2000, 342, 609 636.

[27] The FAB mass spectrum of the isolated mixture revealed only peaks

with a substantially lower mass than the molecular weight of the

desired dumbbell.

[42] However, it cannot be certain as to whether the observed absorption

in the 480 600 nm region of the spectrum arises only from this

translational isomer. It is possible that it receives some contribution

from folded conformations of the other translational isomer in which

the DNP moiety enters into ™alongside∫ fleeting interactions with

CBPQT⁴

.

[28] Reductive cleavage of thiocyanate groups with NaBH₄as a way of

accessing thiolates has only been introduced recently into the growing

field of TTF chemistry, see: a) J. O. Jeppesen, K. Takimiya, N.

Thorup J. Becher, Synthesis 1999, 803 810; b) J. O. Jeppesen, K.

Takimiya, J. Becher, Org. Lett. 2000, 2, 2471 2473.

[43] The word ™roughly∫ is used because an exact integration of the two

SMe resonances proved to be elusive on account of broad signals in

the ¹H NMR spectrum and the appearance of an intense H₂O

resonance (d 2.72 2.85 ppm) close to the singlet resonating at d

2.70 ppm.

[29] A [2]rotaxane is a molecule composed of a ring and dumbbell-shaped

component. The ring encircles the linear portion of the dumbbell-

shaped component and is trapped mechanically around it by two bulky

stoppers. By contrast, in a [2]pseudorotaxane, at least one of the

stoppers on the dumbbell-shaped component is absent, with the

consequence that dissociation into its two components can occur

spontaneously, that is, it behaves like a 1:1 complex. The inclusion of

TTF derivatives inside the cavity of the tetracationic cyclophane

CBPQT⁴is well documented (see: refs. [15, 18, 19]) and leads to the

formation of pseudorotaxanes. Pseudorotaxanes are obtained under

thermodynamic control upon mixing of their acyclic and cyclic

components in solution, and the occurrence of the threading process

is evidenced by the ¹H NMR and absorption spectra, see: a) ref. [10];

b) ref. [19f]; c) ref. [19k]; d) P. R. Ashton, R. Ballardini, V. Balzani,

[44] In the case of the [2]pseudorotaxane 18 ꢀ CBPQT⁴, only resonances

for complexed CBPQT⁴were observed in the ¹H NMR spectrum,

recorded on a 1:1 mixture of the semi-dumbbell compound 18 and

CBPQT⁴at low temperature (245 K), indicating that the complex-

ation of 18 by CBPQT⁴is very strong at this temperature.

[45] When a DNP moiety is located inside CBPQT⁴the DNP-H-4/8

protons resonate at very high field (d 2.0 3.0 ppm), see: a) M.

Asakawa, P. R. Ashton, V. Balzani, C. L. Brown, A. Credi, O. A.

Matthews, S. P. Newton, F. M. Raymo, A. N. Shipway, N. Spencer, A.

Quick, J. F. Stoddart, A. J. P. White, D. J. Williams, Chem. Eur. J. 1999,

5, 860 875; b) ref. [29d].

[46] On account of the broad signals in the ¹H NMR spectrum, an exact

integration of the resonances proved to be elusive and therefore the

word ™approximately∫ is used.

S. E. Boyd, A. Credi, M. T. Gandolfi, M. Gomez-Lopez, S. Iqbal, D.

Philp, J. A. Preece, L. Prodi, H. G. Ricketts, J. F. Stoddart, M. S. Tolley,

M. Venturi, A. J. P. White, D. J. Williams, Chem. Eur. J. 1997, 3, 152

170.

[47] The DG⁼values were calculated using the relationship DG⁼

ÀRTln(kh/k_BT), in which R is the gas constant, T is the absolute

temperature, k is the rate constant, h is the Planck constant, and k_Bis

the Boltzmann constant.

¬

[30] The synthesis of compound 27 will be reported elsewhere.

[31] a) P. R. Ashton, R. Ballardini, V. Balzani, A. Credi, M. T. Gandolfi,

[48] The exchange rate constants (k_ex) were measured using the spin

saturation transfer method, see: a) B. E. Mann, J. Magn. Reson. 1977,

25, 91 94; b) M. Feigel, H. Kessler, D. Leibfritz, J. Am. Chem. Soc.

1979, 101, 1943 1950.

¬

D. J.-F. Marquis, L. Perez-Garcia, L. Prodi, J. F. Stoddart, M. Venturi,

A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1995, 117, 11171

3006

¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Chem. Eur. J. 2003, 9, 2982 3007

Amphiphilic Rotaxanes

2982 3007

[49] Temperatures were calibrated using a neat MeOH sample. See: A. L.

Van Geet, Anal. Chem. 1970, 42, 679 680.

and this isomer was isolated by preparative thin-layer chromatog-

raphy (PTLC). After isolation of 29 ¥ RED⁴, various attempts to

observe the shuttling of CBPQT⁴from the DNP station to the

MPTTF station were made by ¹H NMR spectroscopy. However, no

interconversion was observed, even at elevated temperatures (425 K,

CD₃SOCD₃). This observation is consistent with our hypothesis that it

is the SMe group in 2⁴which constitutes the steric barrier to the

shuttling of CBPQT⁴in the [2]rotaxane.

[50] H.-R. Tseng, S. A. Vignon, P. C. Celestre, J. F. Stoddart, A. J. P. White,

D. J. Williams, Chem. Eur. J. 2003, 9, 543 556.

1

[51] In the case of 2 ¥ RED⁴, in which the H NMR spectrum (500 MHz)

was recorded at 225 K, the resonances for the DNP-H-4/8 protons are

observed as part of a multiplet at d 2.50 2.60 ppm, together with

the resonances for the CH₂CH₃protons. In the ¹H NMR spectrum

(400 MHz) of the 1:1 mixture of 2 ¥ RED⁴and 2 ¥ GREEN⁴recorded

at 298 K in CD₃COCD₃a doublet (J 8 Hz) resonating at d

2.72 ppm is observed, which can be assigned to the resonance of one

of the DNP-H-4/8 protons in 2 ¥ RED⁴. The other DNP-H-4/8 proton

resonance is obscured by the intense H₂O signal, which appears at d

2.76 2.80 ppm.

[55] The ratio between 2 ¥ GREEN⁴and 2 ¥ RED⁴was determined from

the integrals of the signals for the pyrrole protons in 2 ¥ GREEN⁴and

2 ¥ RED⁴, respectively.

[56] a) M. Asakawa, C. L. Brown, S. Menzer, F. M. Raymo, J. F. Stoddart,

D. J. Williams, J. Am. Chem. Soc. 1997, 119, 2614 2627; b) K. N.

Houk, S. Menzer, S. P. Newton, F. M. Raymo, J. F. Stoddart, D. J.

Williams, J. Am. Chem. Soc. 1999, 121, 1479 1487; c) F. M. Raymo,

M. D. Bartberger, K. N. Houk, J. F. Stoddart, J. Am. Chem. Soc. 2001,

123, 9264 9267.

[52] K. J. Laidler, Chemical Kinetics, Harper Collins, New York, 1987.

[53] The rate constant k was obtained from the slope of a plot of

lnI(SMe, 2 ¥ RED⁴) against t, in which I(SMe, 2 ¥ RED⁴) denotes the

integral of the SMe resonance for the red translational isomer, 2 ¥

RED⁴and t the time, see: ref. [52].

[57] D. D. Perrin, W. L. F. Armarego, Purification of Laboratory Chem-

icals, Pergamon, New York, 1988.

[54] In order to test our hypothesis that it is indeed the SMe group that

poses the barrier to the shuttling of CBPQT⁴, we synthesized a

derivative (i.e., 29⁴, see Supporting Information) of the [2]rotaxane

2⁴in which the SMe group was replaced by the more bulky SEt group.

As in 2⁴, the [2]rotaxane 29⁴was isolated as a mixture of transla-

tional isomers, 29 ¥ GREEN⁴(in which CBPQT⁴resides around the

MPTTF unit) and 29 ¥ RED⁴(in which CBPQT⁴resides around the

[58] S. F. Nelsen, L.-J. Chen, M. T. Ramm, G. T. Voy, D. R. Powell, M. A.

Aceola, T. R. Seehafer, J. J. Sabelko, J. R. Pladziewicz, J. Org. Chem.

1996, 61, 1405 1412.

[59] E. P. Parry, R. A. Osteryoung, Anal. Chem. 1965, 37, 1634 1637.

[60] J. B. Flanagan, S. Margel, A. J. Bard, F. C. Anson, J. Am.Chem. Soc.

1978, 100, 4248 4253.

DNP moiety). Perhaps surprisingly, the main isomer was 29 ¥ RED⁴

,

Received: November 19, 2002 [F4589]

Chem. Eur. J. 2003, 9, 2982 3007

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¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

3007

Article Doi

Amphiphilic bistable rotaxanes

DOI: 10.1002/chem.200204589

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