The self-assembly of fullerene-containing [2]pseudorotaxanes: Formation of a supramolecular C60 dimer

Article abstract of DOI:10.1039/a903687c

With its unique chemical and physical properties. C₆₀ is an attractive molecule to be incorporated into molecular assemblies and supramolecular arrays. This paper reports the syntheses of a C₆₀ derivative with the macrocyclic polyether dibenzo-24-crown-8 (DB24C8) attached to the carbon sphere and of a C₆₀ adduct with a dibenzylammonium salt covalently bonded to the carbon allotrope. The C₆₀ DB24C8 conjugate forms a stable, pseudorotaxane-like 1 : 1 complex (ΔG^o = -16.6 kJ mol^-1. T = 298 K, CDCl₃-CD₃CN 1 : 1) with dibenzylammonium hexafluorophosphate. Evidence for this superstructure was provided by ¹H NMR spectroscopic studies in solution and by mass spectrometric investigations in the gas phase. Equally, the C₆₀ dibenzylammonium conjugate threads through the cavity of DB24C8 to form a 1 : 1 complex with a pseudorotaxane-like geometry (ΔG^o = -23.3 kJ mol^-1, CDCl₃). Furthermore, the C₆₀ DB24C8 adduct and the C₆₀ dibenzylammonium conjugate interact with each other by means of hydrogen bonding and ion-dipole interactions to form the first supramolecular C₆₀ dimer (ΔG^o = -17.0 kJ mol^-1, CDCl₃-CD₃CN 90 : 10). In all three cases, the dibenzylammonium component is threaded through the cavity of the crown ether macrocycle. When DB24C8 is complexed to the C₆₀ dibenzylammonium conjugate, the luminescence associated with the catechol rings of the crown ether is partially quenched upon complex formation. We have profited from this special feature to monitor reversible, acid-base induced dethreading/rethreading processes.

Full text of DOI:10.1039/a903687c

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The self-assembly of fullerene-containing [2]pseudorotaxanes:
formation of a supramolecular C60 dimer
François Diederich,*a Luis Echegoyen,*b Marcos Gómez-López,a Roland Kessingera and
J. Fraser Stoddart*c
a Laboratorium für Organische Chemie, ETH-Zentrum, Universitätstrasse 16, 8092 Zürich,
Switzerland
b Department of Chemistry, University of Miami, Coral Gables, FL 33124, USA
c Department of Chemistry and Biochemistry, University of California at Los Angeles,
405 Hilgard Avenue, Los Angeles, CA 90095, USA
Received (in Cambridge) 7th May 1999, Accepted 9th June 1999
With its unique chemical and physical properties, C60 is an attractive molecule to be incorporated into molecular
assemblies and supramolecular arrays. This paper reports the syntheses of a C60 derivative with the macrocyclic
polyether dibenzo-24-crown-8 (DB24C8) attached to the carbon sphere and of a C60 adduct with a dibenzyl-
ammonium salt covalently bonded to the carbon allotrope. The C60 DB24C8 conjugate forms a stable, pseudo-
rotaxane-like 1:1 complex (∆GЊ = Ϫ16.6 kJ molϪ1, T = 298 K, CDCl3–CD3CN 1:1) with dibenzylammonium
hexafluorophosphate. Evidence for this superstructure was provided by 1H NMR spectroscopic studies in solution
and by mass spectrometric investigations in the gas phase. Equally, the C60 dibenzylammonium conjugate threads
through the cavity of DB24C8 to form a 1:1 complex with a pseudorotaxane-like geometry (∆GЊ = Ϫ23.3 kJ molϪ1
,
CDCl3). Furthermore, the C60 DB24C8 adduct and the C60 dibenzylammonium conjugate interact with each other by
means of hydrogen bonding and ion–dipole interactions to form the first supramolecular C60 dimer (∆GЊ = Ϫ17.0 kJ
molϪ1, CDCl3–CD3CN 90:10). In all three cases, the dibenzylammonium component is threaded through the
cavity of the crown ether macrocycle. When DB24C8 is complexed to the C60 dibenzylammonium conjugate, the
luminescence associated with the catechol rings of the crown ether is partially quenched upon complex formation. We
have profited from this special feature to monitor reversible, acid–base induced dethreading/rethreading processes.
stoichiometry of the complex is determined by the size of the
Introduction
crown ether and the number of ammonium centres (–NH2ϩ–)
present in the thread-like component.11 The main driving forces
for formation of these superstructures are [Nϩ–H ؒ ؒ ؒ O] and
[C–H ؒ ؒ ؒ O] hydrogen bonds as well as contributions from
ion-pairing and dispersive interactions.12 These molecular
recognition motifs have been exploited to harness numerous
aesthetically appealing interlocked and/or threaded architec-
tures such as [n]rotaxanes,13 chemically controllable molecular
shuttles,14 a quadruple-stranded [5]pseudorotaxane accom-
modating one hexafluorophosphate counterion in the second
coordination sphere of the crown ether component,15 an inter-
woven molecular cage,16 an artificial analogue of the photo-
synthetic special pair,17 and daisy chain-like oligomers.18
Whereas the covalent functionalisation of buckminster-
fullerene, C60, has seen a dramatic development,1 the supra-
molecular aspects of its chemistry have not been explored to
the same extent.2 Supramolecular chemistry exploits non-
covalent bonding interactions in order to construct large,
yet ordered, molecular assemblies and supramolecular arrays3
with the potential for a wide range of real-life applications.4
The unique physical and chemical properties of covalent
derivatives of C60—such as their electronic absorption bands
expanding through the whole UV/Vis spectral region,5 their
singlet oxygen sensitising ability6 or their pronounced electron
acceptor character7 make them attractive candidates to
furnish functional supramolecular architectures with novel
properties.2,8,9
The research group of one of us (J. F. S.) discovered recently
that the complexation of secondary ammonium salts
(RRЈNH2ϩXϪ) by suitably sized crown ethers—such as
dibenzo-24-crown-8 (DB24C8) and bis-p-phenylene-34-crown-
10 (BPP34C10)—gives rise to a new family of complexes with
pseudorotaxane-like10 geometries.11 The linear ammonium ion
threads through the cavity of the macrocyclic polyether; the
Here, we report the incorporation of fullerenes into both a
DB24C8 macrocyclic polyether derivative19 and a secondary
ammonium salt to give 1 and 2-HؒPF6, respectively. The
formation of pseudorotaxane-like complexes in solution and
the gas phase is described, including the first example—to
the best of our knowledge—of a supramolecular fullerene
dimer.20,21 Furthermore, we show that the threading of a
fullerene-containing diammonium salt into DB24C8 changes
the luminescence properties of the catechol rings in the
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O
OH
OH
EtO
O
O
O
O
O
O
O
O
O
OR
OR
O
O
O
O
R
5
ii
O
6 R = COOEt
7 R = CH2OH
3 R = H
4 R = Ts
iii
i
O
O
O
O
O
O
O
O
O
iv
v
OEt
O
1
O
8
Scheme 1 Synthesis of the fullerene DB24C8 conjugate 1. i, TsCl, Et3N, DMAP, CH2Cl2, 1 h, 0→20 ЊC, 82%; ii, K2CO3, LiBr (cat.), MeCN, 60 h, ∆,
56%; iii, LiAlH4, THF, 3 h, ∆, 91%; iv, EtO2CCH2COCl, pyridine, CH2Cl2, 3 h, 0→20 ЊC, 53%; v, C60, DBU, I2, 12 h, 20 ЊC, 46%.
O
O
i
OEt
OH
O
N
N
OBut
O
OBut
9
10
O
O
O
ii
iii
2-H•PF6
O
O
N
OBut
O
11
Scheme 2 Synthesis of the fullerene ammonium salt 2-HؒPF6. i, EtO2CCH2COCl, pyridine, CH2Cl2, 1 h, 0→20 ЊC, 48%; ii, C60, DBU, I2, 10 h,
20 ЊC, 43%; iii, 1) CF3COOH, CHCl3, 12 h, 20 ЊC; 2) Conc. HCl, CHCl3; 3) NH4PF6, H2O–acetone, 82%.
crown ether, which could be exploited in chemical sensing or
in information storage at the molecular level.
1H NMR complexation studies
In a first study, we investigated the ability of the fullerene crown
ether conjugate 1 to form a [2]pseudorotaxane-like complex
with dibenzylammonium hexafluorophosphate (12-HؒPF6).
This salt, despite being highly soluble in MeOH, Me2CO
and MeCN, is virtually insoluble in CHCl3 and CH2Cl2. Upon
addition of 1, it becomes soluble in CH2Cl2 and CHCl3 solu-
tions, which provides a first evidence for complex formation.11a
Results and discussion
Synthesis
For the synthesis of 1 (Scheme 1), diol 322 was converted to
the tosylate 4 using a standard procedure (toluene-p-sulfonyl
chloride (TsCl)–Et3N–4-(dimethylamino)pyridine (DMAP)).
The crown ether 6 was obtained in 56% yield by macrocyclis-
ation (K2CO3–MeCN) of 4 with ethyl 3,4-dihydroxybenzoate
(5) under semi-high dilution conditions. Reduction with LiAlH4
produced benzyl alcohol 7 which was reacted with ethyl
malonyl chloride in the presence of pyridine to provide the
malonate-appended crown 8. Modified Bingel reaction23,24 (1,8-
diazabicyclo[5.4.0]undec-7-ene (1,5-5) (DBU), I2) of C60 with 8
finally afforded methanofullerene 1 in 46% yield.
The preparation of the fullerene-containing ammonium
salt 2-HؒPF6 started with the reaction of the previously
described benzyl alcohol 925 with ethyl malonyl chloride to
provide malonate 10 (Scheme 2). Bingel reaction (DBU–I2)
of 10 with C60 afforded the methanofullerene 11 which was
deprotected (CF3COOH–CH2Cl2), protonated (HCl) and sub-
jected to ion exchange (NH4PF6–H2O–acetone) to yield 2-Hؒ
PF6 (Scheme 2).
The 1H NMR spectrum (300 MHz, 298 K) of a 1:1 mixture
of 1 and 12-HؒPF6 in CDCl3 showed very broad signals and
displayed multiple resonances compared with the spectra of
the free species. The complicated pattern seems to indicate that
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species in equilibrium affords the association constant Ka = 820
Ϫ1 and the binding free energy ∆GЊ = Ϫ16.6 kJ molϪ1
The multiplet at δ = 4.60 ppm corresponds to the CH2

.
protons (Hacom, Fig. 1) of the complexed ammonium salt and
is the most diagnostic signal26 for complex formation. The
pseudorotaxane geometry of [1ؒ12-H][PF6] was confirmed by
NOE spectroscopy.11a Irradiation of the signals of the crown
ether OCH2 protons—probably the β and βЈ protons (Fig. 1)—
enhances the intensities of the resonances assigned to the
aromatic protons Hbcom and the benzylic protons Hacom belong-
ing to complexed 12-HؒPF6. These results are indicative of
the threading of the latter component through the crown ether
cavity of 1, since examinations of space filling models show
that the protons Hbcom and Hacom and the crown ether OCH2
protons come in close spatial proximity in a pseudorotaxane-
like complex. For the complex between DB24C8 and 12-HؒPF6,
the presumed co-conformation of the components has been
supported by X-ray crystallographic studies.11a
Next, the complexation of the fullerene ammonium salt con-
jugate 2-HؒPF6 with DB24C8 was investigated. Again, the
ammonium salt is only slightly soluble in solvents such as
CH2Cl2 and CHCl3, but is readily solubilised upon addition
of DB24C8. The 1H NMR spectrum (500 MHz, CDCl3, 298 K)
of a 1:1 mixture of 2-HؒPF6 and DB24C8 (c = 3.8 m; Fig. 2)
shows three sets of resonances, two for the free components
and one for the complex, in agreement with slow host–guest
exchange. The association constant for the formed 1:1 complex
[DB24C8ؒ2-H][PF6] in CDCl3 was calculated to be Ka = 12500
Ϫ1 (∆GЊ = Ϫ23.3 kJ molϪ1) and is of similar magnitude to
that reported for the 1:1 complex formed between DB24C8
and 12-HؒPF6 (Ka = 27000 Ϫ1) in that solvent.11a The 1H
NMR spectrum resembles the one recorded for the 1:1 mixture
of 1 and 12-HؒPF6 (Fig. 1) and displays the diagnostic signal
for the CH2N protons of the complexed ammonium salt as a
multiplet at δ = 4.62 ppm.
Fig. 1 1H NMR spectrum (500 MHz, 298 K, CDCl3–CD3CN 1:1) of
a 1:1 mixture of the crown ether fullerene conjugate 1 and the second-
ary ammonium salt 12-HؒPF6; c = 3.8 m.
Fig. 2 1H NMR spectrum (500 MHz, 298 K, CDCl3) of a 1:1 mixture
of DB24C8 and the fullerene ammonium salt conjugate 2-HؒPF6,
c = 3.8 mM.
Finally, the complexation between the two fullerene deriv-
atives 1 and 2-HؒPF6 was studied. Again, the fullerene-
conjugated ammonium salt 2-HؒPF6 becomes readily soluble in
solvents such as CH2Cl2 and CHCl3 upon addition of the crown
1
complexation is indeed taking place. Nevertheless, the broad-
ness of the resonances does not allow for precise analysis of the
process of complex formation. The broad nature of the signals
in the 1H NMR spectrum in CDCl3 could be attributed to some
kind of aggregation.
ether 1. The H NMR spectrum (300 MHz, 298 K) in CDCl3
showed a complicated pattern with broad resonances, indicative
of aggregation, and a precipitate was observed to form within
a few minutes. Upon addition of a more polar solvent such as
CD3COCD3 and/or CD3CN, the solubility increased and the
resonances sharpened. The 1H NMR spectrum (500 MHz,
CDCl3–CD3CN 90:10, 298 K) of a 1:1 mixture of 1 and 2-
HؒPF6 (c = 1.5 m; Fig. 3) displayed three sets of resonances
which could be readily assigned to the individual components
and the 1:1 complex [1ؒ2-H][PF6], respectively. The association
constant was determined as Ka = 970 Ϫ1 (∆GЊ = Ϫ17.0 kJ
molϪ1). The diagnostic, strongly downfield shifted resonance
for the CH2N protons of complexed 2-HؒPF6 appears as a
broad multiplet at δ = 4.58 ppm. The topology of this first
example of a supramolecular fullerene dimer was studied by
Upon addition of a more protic solvent, such as CD3CN or
CD3COCD3, a sharpening of the resonances is observed. The
1H NMR spectrum (500 MHz, 298 K) of a 1:1 mixture of 1
and 12-HؒPF6 (c = 3.8 mM) in CDCl3–CD3CN 1:1 displays
three sets of resonances which correspond to the two free
components and the 1:1 complex [1ؒ12-H][PF6], respectively
1
(Fig. 1). Apparently, host–guest exchange is slow on the H
NMR time scale at room temperature.11a Integration of the
signals for the complexed species proves the formation of a 1:1
complex and evaluation of the signal intensities of the three
J. Chem. Soc., Perkin Trans. 2, 1999, 1577–1586
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Fig.
4
Space-filling representation of the geometry-optimised
supramolecular fullerene dimer [1ؒ2-H][PF6]. In the molecular mech-
anics calculations, the ethoxycarbonyl groups have been substituted by
hydrogens for computational ease.
Fig. 3 1H NMR spectrum (500 MHz, 298 K, CDCl3–CD3CN 9:1) of
a 1:1 mixture of the fullerene conjugates 1 and 2-HؒPF6, c = 1.5 m.
Fig. 5 MALDI-TOF mass spectrum of a 1:1 mixture of the fullerene
crown ether conjugate 1 and 12-HؒPF6; matrix: 2,5-DHB.
molecular modelling27 which suggested a centre-to-centre
distance between the two carbon spheres of ca. 20 Å (Fig. 4).
Mass spectrometric studies
The formation of the 1:1 complexes [1ؒ12-H][PF6], [DB24C8ؒ2-
H][PF6] and [1ؒ2-H][PF6] was also evidenced in the gas phase
by matrix assisted laser desorption ionisation time-of-flight
(MALDI-TOF) mass spectrometry in the positive ion mode.
The matrix in each case was 2,5-dihydroxybenzoic acid
(2,5-DHB). The spectrum recorded from a 1:1 mixture of
the fullerene crown ether conjugate 1 and ammonium salt
12-HؒPF6 displayed an intense peak at m/z 1655 (Fig. 5)
which corresponds to the supramolecular ion [1ؒ12-HؒPF6]ϩ.
The base peak is detected at m/z 1511 which can be assigned to
the 1:1 complex after loss of the PF6Ϫ counterion.
Fig. 6 MALDI-TOF mass spectrum of a 1:1 mixture of DB24C8 and
the fullerene ammonium salt conjugate 2-HؒPF6; matrix: 2,5-DHB.
The detection of the two other supramolecular complexes by
MALDI-TOF mass spectrometry was similarly successful. In
the spectrum obtained from a 1:1 mixture of DB24C8 and the
fullerene ammonium salt conjugate 2-HؒPF6, the base peak is
observed at m/z 1511 which corresponds to the supramolecular
1062—can be assigned to the molecular ion of the fullerene
ammonium salt conjugate minus the PF6Ϫ counterion.
Furthermore, the MALDI-TOF mass spectrum of a 1:1
mixture of the two fullerene conjugates 1 and 2-HؒPF6 displays
a strong peak at m/z = 2373 which corresponds to the supra-
Ϫ
Ϫ
ion of the 1:1 complex minus the PF6 counterion, i.e.
molecular ion minus the PF6 counterion (Fig. 7). The base
[DB24C8ؒ2-H]ϩ (Fig. 6). A peak of less intensity—at m/z
peak is observed at m/z = 1335 and can be assigned to the
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λ = 310 nm
λ = 277 nm
hν
hν
λ = 277 nm
O
O
O
O
O
O
O
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
+
O
N
H
O
N
H2
CF3COOH
2
O
–
PF6
[DB24C8·2-H][PF6]
DB24C8
Fluorescence OFF
Fluorescence ON
Scheme 3 A reversible acid–base controlled de-/rethreading process. The two states can be monitored by the on/off-switching of the fluorescence of
the crown ether component.
nium salt conjugate 2-HؒPF6 was added to a CH2Cl2 solution
of DB24C8 (Fig. 8). Since 2-HؒPF6 features a strong absorption
at λ = 277 nm, this decrease can be attributed, at least partially,
to the fact that the fullerene component is absorbing some of
the photons previously employed to excite the catechol units in
DB24C8.
Subsequent experimentation suggested that some of the
decrease in luminescence intensity also originates from either
energy or electron transfer from the photoexcited DB24C8
donor to the fullerene acceptor in the supramolecular complex
[DB24C8ؒ2-H][PF6].8f,9d,29 The 1:1 complex in CH2Cl2 was
treated with the non-nucleophilic base quinuclidine with the
expectation that it would deprotonate the –NH2ϩ– centre,
thereby disrupting the noncovalent bonding interactions that
initially brought the components together and, hence, pro-
moting (Scheme 3) the dethreading of the linear component.
When 1.5 equivalents of quinuclidine were added, around
one fourth of the initial fluorescence intensity was recovered
(Fig. 8). This result seems to indicate that the [2]pseudo-
rotaxane [DB24C8ؒ2-H][PF6] undergoes a disassembly process
after the deprotonation of the –NH2ϩ– centre and, hence, the
luminescence of the catechol rings is partially restored. The
total recovery of the DB24C8 luminescence is not possible since
after dethreading, the C60 unit in amine 2 is still absorbing
strongly at λexc = 277 nm. Addition of 1.75 equivalents of
trifluoroacetic acid results again in the complete disappearance
of the fluorescence band at λmax = 310 nm. This observation
suggests that the –NH2ϩ– centre is regenerated and rethreading
(Scheme 3) of the linear component through the cavity of
DB24C8 is taking place, thus rendering the assembly/
disassembly process reversible.
Fig. 7 MALDI-TOF mass spectrum of a 1:1 mixture of the fullerene
conjugates 1 and 2-HؒPF6; matrix: 2,5-DHB.
Fig. 8 Fluorescence spectra (λexc = 277 nm, CH2Cl2) of (a) a solution
of pristine DB24C8; (b) a solution of DB24C8 after the addition of
1 equiv. 2-HؒPF6; (c) solution (b) after adding 1.5 equiv. of quinuclidine;
(d) solution (c) after adding 1.75 equiv. of CF3COOH.
Although the difference in signal intensity between the two
states—“on” and “off ”—is relatively small,30 the observation
that the photophysical properties of the 1:1 complex differ
significantly from those of the individual species—and from
those of the corresponding supramolecular systems lacking
the C60 moiety—is of significance and further investigations
on the nature of the luminescence quenching in this supra-
molecular switching system are undertaken.
sodium complex of 1. A smaller peak at m/z = 1062 corre-
sponds to the molecular ion of the fullerene ammonium
conjugate after loss of the PF6Ϫ counterion.
Fluorescence studies
We made a series of interesting observations when the com-
plexation between DB24C8 and the fullerene-containing
ammonium salt 2-HؒPF6 was investigated by luminescence
studies. Preliminary results are reported here.
It is known that the catechol rings in DB24C8 exhibit an
intense short-lived fluorescence at λmax = 310 nm when
irradiated at λexc = 277 nm.28 A very large decrease in intensity
of this luminescence was observed when the fullerene ammo-
Electrochemical studies
The redox properties of the fullerene conjugates and their
complexes were studied by cyclovoltammetry (CV) at room
temperature in CH2Cl2–MeCN 1:1 or in pure CH2Cl2 (ϩ0.1 
Bu4NPF6) and potentials were measured against the ferrocene/
ferricinium (Fc/Fcϩ) couple as internal reference (Table 1).7a
J. Chem. Soc., Perkin Trans. 2, 1999, 1577–1586
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Table 1 Redox parameters of the fullerene adducts and their complexesa
Compound
Solvent
E11/2 (red.)
E21/2 (red.)
E31/2 (red.)
1
CH2Cl2–MeCN 1:1
CH2Cl2–MeCN 1:1
CH2Cl2
CH2Cl2
CH2Cl2
Ϫ1.0 (80)
Ϫ1.0 (82)
Ϫ1.07 (78)
Ϫ1.06 (91)
Ϫ1.03 (68)
Ϫ1.07 (140)
Ϫ1.41 (90)
Ϫ1.41 (90)
Ϫ1.46 (58)
Ϫ1.43 (103)b
Ϫ1.43 (58)b
Ϫ1.48 (150)
[1ؒ12-H][PF6]
11
Ϫ1.92 (83)
Ϫ1.92 (75)
Ϫ1.90 (81)
Ϫ1.91
2-HؒPF6
[DB24C8ؒ2-H][PF6]
[1ؒ2-H][PF6]
CH2Cl2
a The values are given in V vs. Fc/Fcϩ; ∆E values in mV are in parentheses; solvent contains 0.1  Bu4NPF6. b The presence of a shoulder at Ϫ1.58 V
makes it difficult to exactly determine the value of E2
.
1/2
of a different fullerene-containing species—by means of an
irreversible chemical process occurring after reduction.
Furthermore, it was observed that after the first one-electron
reduction a film starts to precipitate on the electrode making
the monitoring of subsequent redox processes more difficult.
The CV of the 1:1 mixture of the DB24C8 and 2-HؒPF6 gave
basically the same results.
The two first reduction waves in the CV of the solution of
the supramolecular dimer [1ؒ2-H][PF6] are slightly broadened
(Fig. 10), which seems to indicate that each wave corresponds
to a two-electron transfer process involving reduction of
both fullerene moieties in the complex. No evidence for possible
electronic interactions between the two fullerenes within the
supramolecular dimer was obtained, which can be explained by
the large spatial distance between the two carbon spheres in the
complex, as disclosed by the molecular modelling studies. This
result is in agreement with electrochemical studies on covalent
fullerene dimers.21b,31f
Conclusions
The new methanofullerenes 1 and 2-HؒPF6 feature molecular
recognition motifs suitable for the construction of pseudo-
rotaxane-like supramolecular architectures. In aprotic solvents,
fullerene crown ether conjugate 1 forms a 1:1 complex with the
dibenzylammonium salt 12-HؒPF6 and the fullerene ammo-
nium salt conjugate 2-HؒPF6 complexes with DB24C8. Finally,
1 and 2-HؒPF6 undergo association to form the first supra-
molecular C60 dimer. The pseudorotaxane-like architectures of
these complexes and their association strengths were derived
from 1H NMR spectroscopic measurements; furthermore,
MALDI-TOF mass spectra revealed the high stability of these
supramolecular species in the gas phase. Threading of the
fullerene ammonium salt conjugate 2-HؒPF6 into DB24C8
changes the luminescence properties of the catechol moieties
in the crown ether and this property can be used to monitor
the pH-dependent, reversible formation (on/off-switching) of
the pseudorotaxane-like complex. By describing the construc-
tion of supramolecular fullerene materials whose properties can
be tuned by external stimuli, this paper provides yet another
example for the promising technological potential32 of the
rapidly emerging field of supramolecular fullerene chemistry.
Fig. 9 Cyclovoltammograms in CH2Cl2 (ϩ0.1  Bu4NPF6) of (a) 2-
HؒPF6 and (b) [DB24C8ؒ2-H][PF6].
Fig. 10 Cyclovoltammogram of the supramolecular dimer [1ؒ2-H]-
[PF6] in CH2Cl2 (ϩ0.1 M Bu4NPF6).
For both free and complexed fullerene derivatives, two or three,
mostly reversible C60-centred reduction waves were observed
at potentials similar to those measured previously for other
methano[60]fullerenes.21b,31 The comparison between the redox
properties of free and complexed fullerene conjugates revealed
that complexation does not significantly influence the reduction
potentials.
The CV of 2-HؒPF6 (Fig. 9) shows a quasi-reversible first
one-electron reduction wave at E1/2 = Ϫ1.06 V. This wave
exhibits a shoulder which could be indicative of some irre-
versible chemical process taking place after one-electron
reduction. The second reduction wave becomes chemically
irreversible and it clearly shows an even larger shoulder at
E = Ϫ1.58 V. This shoulder might arise from the formation
Experimental
General
Reagents and solvents were purchased reagent-grade and
used without further purification, except for THF which was
distilled in the presence of Na. Fullerene soot extract was
purchased from MER Corporation, Tucson, Arizona (AZ)
85706, USA. All reactions were performed in standard glass-
ware. Evaporation and concentration in vacuo were done at
water aspirator pressure, and compounds were dried at 10Ϫ2
Torr. Column chromatography (CC): SiO2 60 (230–400 mesh,
0.040–0.063 mm) from E. Merck. TLC glass plates coated with
SiO2 60 F254 from E. Merck; visualisation by UV light. UV/Vis
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spectra: Varian Cary-5 spectrophotometer. Melting points:
Büchi Melting Point B-540, uncorrected. IR spectra: Perkin-
Elmer; 1600-FTIR. NMR Spectra: Bruker AM 500 and Varian
Gemini 300 or 200 spectrometers at 298 K, with solvent peaks
as reference. FABMS: VG ZAB 2SEQ instrument, 3-nitro-
benzyl alcohol as matrix; MALDI-TOF-MS: measured with
reflectron detection in the positive or negative ion mode,
MeOH 99:1, then CH2Cl2–MeOH 98:2) afforded 6 (2.62 g,
56%) as a white solid: mp 79.2–79.5 ЊC [Found: C, 62.11; H,
6.96. Calc. for C27H36O10 (520.58): C, 62.30; H, 6.97%];
νmax(KBr)/cmϪ1 3461, 1923, 1707, 1592, 1500, 1269, 1246, 1200,
1123, 731; δH(200 MHz, CDCl3) 1.37 (3 H, t, J 7.0), 3.79–3.86
(8 H, m), 3.90–3.98 (8 H, m), 4.13–4.24 (8 H, m), 4.34 (2 H, q,
J 7.0), 6.85 (1 H, d, J 8.3), 6.86–6.90 (4 H, m), 7.54 (1 H, d,
J 2.0), 7.65 (1 H, dd, J 2.0, 8.3); δC(50 MHz, CDCl3) 11.87,
51.04, 58.34, 66.88, 67.01, 67.10, 67.23, 67.39, 67.54, 68.88,
68.94, 69.04, 109.70, 111.73, 112.05, 119.09, 120.90, 121.51,
126.01, 126.04, 145.95, 146.64, 150.55, 164.07; m/z (FABMS)
475 ([M Ϫ OEt]ϩ, 100), 520 (Mϩ, 77), 543 ([M ϩ Na]ϩ, 11%).
acceleration voltage 15–20 kV, on
a Bruker REFLEX
spectrometer; matrices are 2,5-dihydroxybenzoic acid (DBH,
0.1  in MeCN–EtOH–H2O 50:45:5), α-cyano-4-hydroxy-
cinnamic acid (CCA, 0.1  in MeCN–EtOH–H2O 50:45:5)
or anthracene-1,8,9-triol (dithranol, 0.05  in CHCl3–MeOH
1:1). Elemental analyses were performed by the Mikrolabor
at the Laboratorium für Organische Chemie, ETH-Zürich.
Compounds 3 and 9 were synthesised according to refs. 22 and
25, respectively.
6,7,9,10,12,13,20,21,23,24,26,27-Dodecahydrodibenzo[b,n]-
[1,4,7,10,13,16,19,22]octaoxacyclotetracosin-2-ylmethanol (7)
LiAlH4 (1  in THF) (12.1 ml) was added under N2 to a
solution of 6 (1.26 g, 2.4 mmol) in THF (200 ml). The mixture
was stirred for 1 h at 20 ЊC, then heated to reflux for 2 h. After
cooling, H2O was added dropwise (very carefully!) to destroy
the excess of LiAlH4. The suspension was washed with CH2Cl2
(3 × 100 ml), the combined organic phases were washed by
sat. aq. NaHCO3 solution (100 ml) and sat. aq. NaCl solution
(100 ml). Drying (MgSO4) and evaporation in vacuo yielded 7
(1.05 g, 91%) as a white powder: mp 90.9–91.2 ЊC; νmax(KBr)/
cmϪ1 3431, 2923, 2854, 1584, 1511, 1454, 1427, 1254, 1127,
1100, 942, 727; δH(200 MHz, CDCl3) 3.83 (8 H, s), 3.85–3.93
(8 H, m), 4.12–4.17 (8 H, m), 4.57 (2 H, s), 6.83–6.90 (7 H,
m); δC(50 MHz, CDCl3) 23.11, 62.56, 66.97, 67.13, 67.48,
68.46, 110.65, 111.60, 111.79, 117.54, 119.10, 126.01, 132.14,
146.68; m/z (FABMS) 461 ([M Ϫ OH]ϩ, 87), 478 (Mϩ, 100),
501 ([M ϩ Na]ϩ, 37%); m/z (HR-FABMS) 478.2204 (Mϩ,
[C25H34O9]ϩ; calc. 478.2203).
Electrochemical studies
A 3 mm glassy carbon electrode was used as the working
electrode, while a Pt wire served as the counter electrode. An
aq. Ag/AgCl electrode, separated by a Vycor tip, was used as the
reference. In every run, ferrocene (Fc) was added as an internal
potential reference, and all the values reported in Table 1 are
relative to the Fc/Fcϩ couple. Typical scan rates were 100 mV
s
Ϫ1, and the potentiostat used was an EG&G PAR model 263A.
Fluorescence studies
Fluorescence studies were performed with solutions (~10Ϫ5 )
of the fluorescent component DB24C8 to which was added
sequentially 1 equiv. of 2-HؒPF6, 1.5 equiv. of quinuclidine and
1.75 equiv. of CF3COOH. Excitation occurred at λexc = 277 nm
in a fluorescence spectrometer (Spex 1860) equipped with a
0.22 m double-grating monochromator and a 450 W xenon
lamp with a band pass of 3.4 nm.
1-(6,7,9,10,12,13,20,21,23,24,26,27-Dodecahydrodibenzo[b,n]-
[1,4,7,10,13,16,19,22]octaoxacyclotetracosin-2-ylmethyl)
3-ethyl propane-1,3-dioate (8)
1,2-Phenylenebis(oxyethylenoxyethylenoxyethylene) ditosylate
(4)
To 7 (915 mg, 2 mmol) and C5H5N (0.72 ml, 2 mmol) in CH2Cl2
(50 ml) was added at 0 ЊC under N2 ethyl malonyl chloride
(430 mg, 3 mmol) and the solution was stirred for 1 h at 0 ЊC
and for 2 h at 20 ЊC. The mixture was washed with sat. aq.
NH4Cl solution (2 × 100 ml), H2O (100 ml) and sat. aq. NaCl
solution (2 × 50 ml). Drying (MgSO4) and evaporation in
vacuo, followed by column chromatography (SiO2, CH2Cl2–
MeOH 99:1) afforded 8 (620 mg, 53%) as a white powder: mp
68.8–69.3 ЊC [Found: C, 61.01; H, 6.77. Calc. for C30H40O12
(592.64): C, 60.80; H, 6.80%]; νmax(KBr)/cmϪ1 2946, 2261, 1731,
1592, 1515, 1446, 1369, 1325, 1253, 1130, 731; δH(200 MHz,
CDCl3) 1.26 (3 H, t, J 7.3), 3.41 (2 H, s), 3.78 (8 H, s), 3.85–
3.96 (8 H, m), 4.15–4.22 (10 H, m), 5.11 (2 H, s), 6.87–6.90 (7 H,
m); δC(50 MHz, CDCl3) 13.61, 21.93, 25.17, 41.20, 61.11, 66.72,
68.98, 69.07, 69.42, 69.52, 113.23, 113.71, 114.02, 121.04,
121.42, 124.63, 127.93, 148.50, 148.56, 148.76, 161.49, 166.09;
m/z (FABMS) 461 ([M Ϫ OCOCH2CO2Et]ϩ, 100), 592 (Mϩ,
99), 615 ([M ϩ Na]ϩ, 47%).
To 3 (9.0 g, 24 mmol), Et3N (10.1 g, 100 mmol) and a catalytic
amount of DMAP in CH2Cl2 (100 ml) at 0 ЊC under N2
was added dropwise TsCl (11.5 g, 60 mmol) in CH2Cl2 (200 ml)
and the solution was stirred at 0 ЊC for 1 h, then warmed to
20 ЊC. Filtration and concentration of the filtrate in vacuo left
an oil which was taken up in CH2Cl2 and washed with 0.1 
HCl (2 × 200 ml) and sat. aq. NaCl solution (2 × 200 ml).
Drying (MgSO4) and evaporation in vacuo, followed by column
chromatography (SiO2, CH2Cl2, then CH2Cl2–MeOH 99:1)
yielded 4 (13.46 g, 82%) as a colourless oil [Found: C, 56.42;
H, 6.22. Calc. for C32H42O12S2 (682.81): C, 56.29; H, 6.20%];
νmax(KBr)/cmϪ1 2923, 2869, 1592, 1600, 1446, 1358, 1169, 1123,
915, 761, 653, 546; δH(200 MHz, CDCl3) 2.42 (6 H, s), 3.59–3.61
(4 H, m), 3.65–3.79 (8 H, m), 3.81–3.84 (4 H, m), 4.11–4.18
(8 H, m), 6.91 (4 H, s), 7.33 (4 H, “d”, J 8.1), 7.79 (4 H, “d”,
J 8.1); δC (50 MHz, CDCl3) 19.11, 51.10, 66.28, 66.43, 66.91,
67.42, 68.37, 112.59, 119.35, 125.63, 127.54, 130.68, 142.55,
146.71; m/z (FABMS) 682 (Mϩ, 100%).
1-(6,7,9,10,12,13,20,21,23,24,26,27-Dodecahydrodibenzo[b,n]-
[1,4,7,10,13,16,19,22]octaoxacyclotetracosin-2-ylmethyl)
3-ethyl 1,2-methano[60]fullerene-61,61-dicarboxylate (1)
Ethyl 6,7,9,10,12,13,20,21,23,24,26,27-dodecahydrodibenzo-
[b,n][1,4,7,10,13,16,19,22]octaoxacyclotetracosine-2-carb-
oxylate (6)
DBU (402 mg, 2.7 mmol) was added to a solution of 8 (314 mg,
0.53 mmol), C60 (382 mg, 0.53 mmol) and I2 (140 mg,
0.53 mmol) in PhMe (500 ml) and the mixture was stirred
for 12 h under N2. Filtration through a plug (SiO2) eluting
first with PhMe until all the unreacted C60 had been collected
and then with CH2Cl2–MeOH 97:3, followed by column
chromatography (CH2Cl2–MeOH 99:1) gave 1 (319 mg, 46%)
as a glassy purple solid: mp >250 ЊC; νmax(KBr)/cmϪ1 3455,
1739, 1683, 1455, 1405, 1161, 1100, 694, 517; λmax(CH2Cl2)/nm
(ε/dm3 molϪ1 cmϪ1) 256 (110000), 326 (42000), 408 (sh, 3500),
A solution of 5 (1.62 g, 9.0 mmol), K2CO3 (5.04 g, 36 mmol)
and a catalytic amount of LiBr in MeCN (1 l) was heated to
reflux under N2 for 1 h. A solution of 4 (6.14 g, 9.0 mmol) in
MeCN (500 ml) was added dropwise over 12 h and the mixture
was heated to reflux for 48 h. Filtration and concentration of
the filtrate in vacuo left an oil which was taken up in CH2Cl2
(200 ml) and washed with 0.1  HCl (2 × 200 ml) and sat. aq.
NaCl solution (200 ml). Drying (MgSO4) and evaporation in
vacuo followed by column chromatography (SiO2, CH2Cl2–
J. Chem. Soc., Perkin Trans. 2, 1999, 1577–1586
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View Article Online
426 (3400), 477 (1600), 686 (320); δH(200 MHz, CDCl3) 1.40
(3 H, t, J 7.0), 3.84 (8 H, br s), 3.85–3.93 (8 H, m), 4.10–4.20 (8
H, m), 4.52 (2 H, q, J 7.0), 5.45 (2 H, s), 6.84–7.09 (7 H, m);
δC(125 MHz, CDCl3) 14.10, 22.63, 31.57, 52.10, 63.45, 68.96,
69.37, 69.41, 69.48, 69.52, 69.81, 69.96, 71.31, 71.37, 71.54,
113.43, 114.05, 114.95, 121.45, 122.77, 125.28, 127.68, 128.21,
129.02, 140.85, 140.93, 141.81, 141.85, 142.15, 142.18, 142.92,
142.98, 143.04, 143.81, 143.86, 144.46, 144.58, 144.63, 144.64,
144.66, 144.84, 145.01, 145.03, 145.12, 145.13, 145.20, 145.23,
145.28, 148.87, 148.92, 149.49, 163.46, 163.50; m/z (FABMS)
720 (C60ϩ, 100), 1311 (Mϩ, 49), 1333 ([M ϩ Na]ϩ, 37%); m/z
(HR-FABMS) 1310.2371 (Mϩ, [C90H38O12]ϩ; calc. 1310.2363).
solvent was removed in vacuo and the resultant solid suspended
in Me2CO (250 ml). Sat. aq. NH4PF6 solution was added to the
suspension until a clear solution formed. Evaporation of the
solvent Me2CO in vacuo, filtration of the resulting suspension,
and washing with copious amounts of H2O yielded 2-HؒPF6
(170 mg, 82%) as a brown solid: mp >250 ЊC; νmax(KBr)/cmϪ1
3522, 3222, 2944, 1739, 1600, 1456, 1428, 1233, 839, 556, 522;
λmax(CH2Cl2)/nm (ε/dm3 molϪ1 cmϪ1) 258 (102500), 326 (34000),
404 (sh, 2900), 414 (sh, 2400), 426 (2300), 478 (1600), 687 (430);
δH(200 MHz, CDCl3–CD3CN 1:1) 1.12–1.27 (3 H, br m), 3.97
(4 H, br m), 4.41 (2 H, br m), 5.42 (2 H, br s), 7.29–7.50 (9 H,
br m); δC(125 MHz, CDCl3–CD3CN 1:1) 12.80, 23.08, 50.42,
50.90, 58.15, 63.10, 67.61, 70.97, 128.69, 128.99, 129.16, 129.37,
129.76, 135.86, 138.23, 138.63, 140.38, 140.45, 141.21, 141.33,
141.61, 141.67, 142.39, 142.48, 142.50, 142.51, 142.55, 142.57,
143.28, 143.34, 143.90, 144.02, 144.09, 144.11, 144.17, 144.35,
144.52, 144.57, 144.60, 144.66, 144.69, 144.75, 162.71, 162.73;
m/z (MALDI-TOFMS) 1060, [M Ϫ PF6]ϩ.
1-({4-[(N-{[(tert-Butyl)oxy]carbonyl}[(phenyl)methyl]amino)-
methyl]phenyl}methyl) 3-ethyl propane-1,3-dioate (10)
Ethyl malonyl chloride (1.5 g, 10 mmol) was added to a solution
of 9 (1.63 g, 5 mmol) and C5H5N (0.79 g, 10 mmol) in CH2Cl2
(100 ml) at 0 ЊC and the mixture was stirred under N2 for
1 h. The solution was washed with sat. aq. NH4Cl solution
(2 × 200 ml) and H2O (2 × 200 ml), the organic phase was dried
(MgSO4) and evaporated in vacuo. Column chromatography
(SiO2, hexane–EtOAc 1:1) provided 10 (1.05 g, 48%) as a
colourless oil [Found: C, 68.03; H, 7.25; N, 3.14. Calc. for
C25H31NO6 (441.52): C, 68.01; H, 7.08; N, 3.17%]; νmax(KBr)/
cmϪ1 2978, 1744, 1689, 1455, 1405, 1361, 1156, 1033, 883, 700;
δH(200 MHz, CDCl3) 1.27 (3 H, t, J 7.0), 1.51 (9 H, s), 3.44
(2 H, s), 4.20 (2 H, q, J 7.0), 4.27–4.42 (4 H, br m), 5.20 (2 H, s),
7.25–7.37 (9 H, m); δC(50 MHz, CDCl3) 11.55, 25.96, 39.17,
46.53, 59.13, 64.50, 77.76, 124.97, 125.66, 126.04, 126.24,
132.01, 135.60, 136.11, 153.69, 164.13; m/z (FABMS) 340
([M Ϫ CO2But]ϩ, 35), 386 ([M Ϫ But]ϩ, 23), 440 (Mϩ, 5%).
Acknowledgements
This work was supported by the Swiss National Science
Foundation and the US National Science Foundation
(CHE-9313018). We thank Dr Monica Sebova for NMR
measurements and Ms Brigitte Brandenberg for NOE difference
spectra. We would also like to thank Dr Carlo Thilgen for
assistance with the nomenclature.
References
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DBU (0.7 mg, 4.6 mmol) was added to a solution of 10 (204
mg, 0.46 mmol), C60 (500 mg, 0.47 mmol) and I2 (162 mg,
0.46 mmol) in PhMe (500 ml) and the mixture was stirred for
10 h at 20 ЊC under N2. Filtration through a plug (SiO2) using
first PhMe as the eluent to isolate unreacted C60 and then
CH2Cl2–MeOH 90:10, followed by column chromatography
(SiO2, CH2Cl2) yielded 11 (232 mg, 43%) as a glassy solid:
mp >250 ЊC; νmax(KBr)/cmϪ1 2978, 1739, 1689, 1450, 1400,
1355, 1228, 1156, 694, 522; λmax(CH2Cl2)/nm (ε/dm3 molϪ1
cmϪ1) 258 (255900), 326 (79000), 403 (sh, 4600), 413 (sh, 3600),
426 (3500), 484 (2070), 624 (sh, 400), 688 (230); δH(200 MHz,
CDCl3) 1.40 (3 H, t, J 7.0), 1.52 (9 H, s), 4.37–4.42 (4 H, br m),
4.48 (2 H, q, J 7.0), 5.54 (2 H, s), 7.26–7.37 (7 H, m), 7.49 (2 H,
“d”, J 7.9); δC(125 MHz, CDCl3) 14.11, 28.42, 63.42, 68.58,
71.46, 80.17, 80.32, 127.28, 127.66, 127.96, 128.18, 128.25,
128.54, 129.30, 133.62, 137.85, 138.75, 139.26, 140.86, 140.90,
141.80, 142.14, 142.16, 142.31, 142.91, 142.96, 142.97, 143.03,
143.05, 143.82, 143.84, 144.47, 144.51, 144.57, 144.63, 144.76,
144.83, 145.03, 145.05, 145.12, 145.14, 145.19, 145.21, 155.87,
163.37, 163.61; m/z (MALDI-TOFMS) 1159 (Mϩ), 1103
[M Ϫ But]ϩ.
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3-ethyl 1,2-methano[60]fullerene-61,61-dicarboxylate
hexafluorophosphate (2-HؒPF6)
A solution of 11 (200 mg, 0.17 mmol) and CF3COOH (0.4 ml,
3.4 mmol) in CHCl3 (30 ml) was stirred under N2 at 20 ЊC for 12
h. After evaporation in vacuo, the residue was taken up into
CH2Cl2 (200 ml) and washed with 6  NaOH (200 ml). The aq.
phase was washed with CH2Cl2 (2 × 100 ml) and the combined
organic layers were dried (MgSO4) and evaporated in vacuo to
yield the corresponding amine which was dissolved in CHCl3
(20 ml). Conc aq. HCl solution was added to reach pH < 2, the
1584
J. Chem. Soc., Perkin Trans. 2, 1999, 1577–1586

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,
Fig. 1) appears as a second-order multiplet as a result of coupling to
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27 The geometry optimisation of the complex [1ؒ2-H][PF6] was carried
out by molecular mechanics calculations using the force field
SYBYL as implemented in the program package SPARTAN 4.0,
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