Supramolecular fullerene chemistry: A comprehensive study of cyclophane-type mono- and bis-crown ether conjugates of C70

Article abstract of DOI:10.1002/1522-2675(200207)85:7<2009::AID-HLCA2009>3.0.CO;2-G

The covalently templated bis-functionalization of C₇₀, employing bis-malonate 5 tethered by an anti-disubstituted dibenzo[18]crown-6 (DB18C6) ether, proceeds with complete regiospecificity and provides two diastereoisomeric pairs of enantiomeric C₇₀ crown ether conjugates, (±)-7a and (±)-7b, featuring a five o'clock bis-addition pattern that is disfavored in sequential transformations (Scheme 1). The identity of (±)-7a was revealed by X-ray crystal-structure analysis (Fig. 6). With bis-malonate 6 containing a syn-disubstituted DB18C6 tether, the regioselectivity of the macrocylization via double Bingel cyclopropanation changed completely, affording two constitutionally isomeric C₇₀ crown ether conjugates in a ca. 1:1 ratio featuring the twelve (16) and two o'clock ((±)-15) addition patterns, respectively (Scheme 3). The X-ray crystal-structure analysis of the twelve o'clock bis-adduct 16 revealed that a H₂O molecule was included in the crown ether cavity (Figs. 7 and 8). Two sequential Bingel macrocyclizations, first with anti-DB18C6-tethered (5) and subsequently with syn-DB18C6-tethered (6) bis-malonates, provided access to the first fullerene bis-crown ether conjugates. The two diastereoisomeric pairs of enantiomers (±)-28a and (±)-28b were formed in high yield and with complete regioselectivity (Scheme 9). The cation-binding properties of all C₇₀ crown-ether conjugates were determined with the help of ion-selective electrodes (ISEs). Mono-crown ether conjugates form stable 1:1 complexes with alkali-metal ions, whereas the tetrakis-adducts of C₇₀, featuring two covalently attached crown ethers, form stable 1:1 and 1:2 host-guest complexes (Table 2). Comparative studies showed that the conformation of the DB18C6 ionophore imposed by the macrocyclic bridging to the fullerene is not particularly favorable for strong association. Reference compound (±)-22 (Scheme 4), in which the DB18C6 moiety is attached to the C₇₀ sphere by a single bridge only and, therefore, possesses higher conformational flexibility, binds K⁺ and Na⁺ ions better by factors of 2 and 20, respectively. Electrochemical studies demonstrate that cation complexation at the crown ether site causes significant anodic shifts of the first reduction potential of the appended fullerene (Table 3). In case of the C₇₀ mono-crown ether conjugates featuring a five o'clock functionalization pattern, addition of 1 equiv. of KPF₆ caused an anodic shift of the first reduction wave in the cyclic voltammogram (CV) by 70 to 80 mV, which is the result of the electrostatic effect of the K⁺ ion bound closely to the fullerene core (Fig. 14). Addition of 2 equiv. of K⁺ ions to C₇₀ bis-crown ether conjugates resulted in the observation of only one redox couple, whose potential is anodically shifted by 170 mV with respect to the corresponding wave in the absence of the salt (Fig. 16). The synthesis and characterization of novel tris- and tetrakis-adducts of C₇₀ are reported (Schemes 5 and 6). Attempts to prepare even more highly functionalized derivatives resulted in the formation of novel pentakis- and hexakis-adducts and a single heptakis-adduct (Scheme 7), which were characterized by ¹H- and ¹³C-NMR spectroscopy (Fig. 10), as well as matrix-assisted laser-desorption-ionization mass spectrometry (MALDI-TOF-MS). Based on predictions from density-functional-theory (DFT) calculations (Figs. 12 and 13), structures are proposed for the tris-, tetrakis-, and pentakis-adducts.

Full text of DOI:10.1002/1522-2675(200207)85:7<2009::AID-HLCA2009>3.0.CO;2-G

Helvetica Chimica Acta Vol. 85 (2002)
2009
Supramolecular Fullerene Chemistry: A Comprehensive Study of
Cyclophane-Type Mono- and Bis-Crown Ether Conjugates of C70
by Maurice J. van Eis, Paul Seiler, Liya A. Muslinkina, Martin Badertscher, Ernˆ Pretsch*, and FranÁois
Diederich*
Laboratorium f¸r Organische Chemie, ETH-Hˆnggerberg, HCI, CH-8093Z¸rich
Robert J. Alvarado and Luis Echegoyen*
Department of Chemistry, Clemson University, Clemson, SC 29634, USA
and
Ignacio Pe¬rez Nunƒez
¬
ƒ
ƒ
Departamento de QuÌmica Fundamental, Universidade da Coruna, E-15071 A Coruna
Dedicated to Professor Friedrich Bickelhaupt on the occasion of his 70th birthday.
The covalently templated bis-functionalization of C70, employing bis-malonate 5 tethered by an anti-
disubstituted dibenzo[18]crown-6 (DB18C6) ether, proceeds with complete regiospecificity and provides two
diastereoisomeric pairs of enantiomeric C70 crown ether conjugates, (Æ)-7a and (Æ)-7b, featuring a five o×clock
bis-addition pattern that is disfavored in sequential transformations (Scheme 1). The identity of (Æ)-7a was
revealed by X-ray crystal-structure analysis (Fig. 6). With bis-malonate 6 containing a syn-disubstituted
DB18C6 tether, the regioselectivity of the macrocylization via double Bingel cyclopropanation changed
completely, affording two constitutionally isomeric C70 crown ether conjugates in a ca. 1:1 ratio featuring the
twelve (16) and two o×clock ((Æ)-15) addition patterns, respectively (Scheme 3). The X-ray crystal-structure
analysis of the twelve o×clock bis-adduct 16 revealed that a H2O molecule was included in the crown ether cavity
(Figs. 7 and 8). Two sequential Bingel macrocyclizations, first with anti-DB18C6-tethered (5) and subsequently
with syn-DB18C6-tethered (6) bis-malonates, provided access to the first fullerene bis-crown ether conjugates.
The two diastereoisomeric pairs of enantiomers (Æ)-28a and (Æ)-28b were formed in high yield and with
complete regioselectivity (Scheme 9). The cation-binding properties of all C70 crown-ether conjugates were
determined with the help of ion-selective electrodes (ISEs). Mono-crown ether conjugates form stable 1:1
complexes with alkali-metal ions, whereas the tetrakis-adducts of C70, featuring two covalently attached crown
ethers, form stable 1:1 and 1:2 host-guest complexes (Table 2). Comparative studies showed that the
conformation of the DB18C6 ionophore imposed by the macrocyclic bridging to the fullerene is not particularly
favorable for strong association. Reference compound (Æ)-22 (Scheme 4), in which the DB18C6 moiety is
attached to the C70 sphere by a single bridge only and, therefore, possesses higher conformational flexibility,
‡
‡
binds K and Na ions better by factors of 2 and 20, respectively. Electrochemical studies demonstrate that
cation complexation at the crown ether site causes significant anodic shifts of the first reduction potential of the
appended fullerene (Table 3). In case of the C70 mono-crown ether conjugates featuring a five o×clock
functionalization pattern, addition of 1 equiv. of KPF6 caused an anodic shift of the first reduction wave in the
‡
cyclic voltammogram (CV) by 70 to 80 mV, which is the result of the electrostatic effect of the K ion bound
‡
closely to the fullerene core (Fig. 14). Addition of 2 equiv. of K ions to C70 bis-crown ether conjugates resulted
in the observation of only one redox couple, whose potential is anodically shifted by 170 mV with respect to the
corresponding wave in the absence of the salt (Fig. 16). The synthesis and characterization of novel tris- and
tetrakis-adducts of C70 are reported (Schemes 5 and 6). Attempts to prepare even more highly functionalized
derivatives resulted in the formation of novel pentakis- and hexakis-adducts and a single heptakis-adduct
1
(Scheme 7), which were characterized by H- and 13C-NMR spectroscopy (Fig. 10), as well as matrix-assisted
laser-desorption-ionization mass spectrometry (MALDI-TOF-MS). Based on predictions from density-

2010
Helvetica Chimica Acta Vol. 85 (2002)
functional-theory (DFT) calculations (Figs. 12 and 13), structures are proposed for the tris-, tetrakis-, and
pentakis-adducts.
1. Introduction.
The first higher fullerene C70 [1] (Fig. 1) features, in many
respects, similar reactivity to that displayed by the more-abundant buckminsterfuller-
ene, C60 [2]. However, there are also marked differences. Whereas the more-symmetric
Ih-C60 possesses only one type of C-atom and two different bond types (shorter 6-6
bonds at the junction between two hexagons and longer 6-5 bonds within pentagons),
D5h-C70 with lower symmetry features five types of C-atom (a e in Fig. 1,a) and eight
different bonds (four 6-6 and four 6-5 bonds). The different 6-6 bonds in C70 have been
classified according to their local curvature as a-, b-, e-, and k-type bonds (Fig. 1,c),
with the degree of local curvature decreasing from a-type bonds located near the poles
(e.g., C(1)ÀC(2) (for the numbering, see [3])) to k-type bonds located at the equator of
the C-sphere (e.g., C(21)ÀC(22)) [4]. Increasing degree of local curvature correlates
with increasing reactivity in nucleophilic attacks and cycloadditions to C70: thus the
Bingel monocyclopropanation [5a] occurs regioselectively at the a-type 6-6 bond
C(1)ÀC(2) to give 1 (Fig. 2) [5b][6], whereas azomethine ylide 1,3-dipolar cyclo-
additions afford, as the major products, regioisomeric mono-adducts originating from
attacks at a- (C(1)ÀC(2)) and b-type (C(5)ÀC(6)) bonds [7][8]. In addition to local
curvature (the degree of pyramidalization of C-atoms can be quantified by the p-orbital
axis vector (POAV) angles introduced by Haddon; see [9 11]), high p-bond order is
another factor favoring reactivity (in C70, the C(5)ÀC(6) bond has the highest p-bond
order, followed by the C(1)ÀC(2) bond; see [3a][12][13]).
Double Bingel cyclopropanations of C70 with diethyl malonate occur on opposite
hemispheres at the most curved a-type bonds, yielding the three constitutional isomers
2, (Æ)-3, and (Æ)-4, two of which are pairs of enantiomers due to the chirality of the
addition pattern [5][6][14]. The isomeric ratio 2/(Æ)-3/(Æ)-4 is 2.8 :6.8 :1.0. In a
Newman-type projection looking down the C5 axis of the C70 core onto the two polar
pentagons, the two addends in 2 adopt a twelve o×clock, those in (Æ)-3 a two o×clock,
and those in (Æ)-4 a five o×clock geometrical relationship (Fig. 2). Similar addition
patterns were also obtained in other bis-functionalization reactions, with the two
o×clock geometric relationship being highly favored in each case [15 18]. This product
distribution strongly deviates from the statistical 1:2 :2 distribution. Since steric
interactions between the addends are absent, electronic factors must be involved.
Several theoretical investigations indeed support that the regioselectivity of multiple
cyclopropanations of both C60 [19] and C70 [14] is governed by the frontier molecular
orbitals of the fullerene core (see below).
The chemistry of even higher adducts of C70 is less well-developed [1][17][20].
Starting from twelve and five o×clock bis-adducts such as 2 and (Æ)-3, Diederich and co-
workers prepared defined tris-, tetrakis-, and hexakis-adducts by sequential Bingel
cyclopropanation [6][14]; in addition, they isolated pure heptakis- and octakis-adducts
of yet unknown constitution.
Although the regioselectivity in sequential multiple functionalizations of C70 with
6-6 bonds of different reactivity
generally is higher than in the corresponding
reactions of C60, isomeric mixtures are often obtained that are difficult to separate. In
1994, Diederich and co-workers introduced the tether-directed remote functionaliza-

Helvetica Chimica Acta Vol. 85 (2002)
2011
Fig. 1. Views on D5h-C70 a) perpendicular to (indicating the five different types of C-atoms) and b) along the C5-
symmetry axis, and Schlegel diagrams showing c) the numbering (clockwise) of the C-atoms and d) the four
different types a, b, e, and k of 6-6 bonds. The curvature and reactivity towards nucleophiles decreases from a- to
k-type bonds.
tion technique as a general method to control regio- and stereoselectivity in covalent
fullerene chemistry [21]. This strategy has proven to be a powerful tool in the multiple
derivatization of C60 [22 26], but had not previously been applied to C70 or other
higher fullerenes.
Here, we report the application of the tether-directed remote functionalization to
prepare C70 bis- and tetrakis-adducts (for preliminary communications of parts of this
work, see [27][28]). We show that crown ether tethered bis-malonates undergo highly
regioselective macrocyclizations with the higher C-sphere, providing access to a new

2012
Helvetica Chimica Acta Vol. 85 (2002)
O
O
O
O
1
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
2
(±)-3
(±)-4
twelve o'clock (18%)
five o'clock (13%)
two o'clock (69%)
Fig. 2. Mono- (1) and regioisomeric bis-adducts (2, (Æ)-3, and (Æ)-4) formed by Bingel cyclopropanation of C70
.
Also shown are Newman-type projections looking down the C5-symmetry axis of the C70 core onto the two polar
pentagons, which show the relative orientations of the addends.
series of mono- and bis-crown ether conjugates (for cyclophane-type crown ether
conjugates of C60, see [29][30]; for examples of other C60-appended crown ethers, see
[31]). Crown etherÀfullerene conjugates attract substantial interest as advanced
supramolecular materials [32] and have found application in sensing devices for cations
[33]. We describe the X-ray crystallographic characterization of the new C70 crown
ether derivatives and their ionophoric properties, as determined with ion-selective
electrodes (ISEs). We also demonstrate in electrochemical studies that cation
complexation strongly affects the redox potentials of the C70 core as a result of the
close proximity of the ionophore-bound cation to the fullerene surface. In addition, we
report exploratory studies on the formation of new higher adducts up to a heptakis-
adduct starting from five o×clock bis-adduct (Æ)-4, which, for the first time, was
obtained in larger amounts by tether-directed synthesis.
2. Results and Discussion.
2.1. Cyclophane-Type C70 Mono-Crown Ether
Conjugates 2.1.1. Tether Design and Computational Predictions. For the first
covalently templated functionalization of C70, we chose bis-malonate 5 with an anti-
substituted dibenzo[18]crown-6 (DB18C6) tether (Fig. 3). In previous work, Bingel
‡
macrocyclization of 5 with C60 in the presence of K ions (to rigidify the template) had
proceeded with complete regioselectivity, affording exclusively a trans-1 C60 crown
ether conjugate in which the template spans the entire fullerene sphere [29].

Helvetica Chimica Acta Vol. 85 (2002)
2013
Fig. 3. MM2 Calculations on the relative stabilities of three regioisomeric bis-adducts that can form in the
macrocyclization of C70 with bis-malonate 5 featuring an anti-substituted DB18C6 unit. Relative energies are
given in kcal molÀ1. The EtOOC side chains at the methano bridge C-atoms are replaced by H-atoms in the
calculations. All H-atoms have been omitted for clarity. Depicted are the lowest-energy conformers calculated
for each regioisomer.
MM2 Calculations [34] were performed to investigate the relative stability of the
regioisomeric bis-adducts expected to form in the bis-cyclopropanation of C70 with 5.
We had previously found that calculations of relative product stabilities provide a good
prediction of the regioisomers that are formed preferentially [29]. Since two sequential
Bingel additions to C70 occur exclusively at the a-type 6-6 bonds on opposite poles
(Fig. 2), we considered only this mode of functionalization in the computations. To
reduce the number of possible conformers, the EtOOC residues at the two methano
bridge C-atoms were replaced by H-atoms.
Molecular-dynamics [35] and Monte Carlo [36] simulations suggest that [18]crown-
6 (18C6) is highly flexible, spanning a large number of low-energy conformations. Free
DB18C6 is less flexible as a result of the double benzanellation, and four preferred
conformations were identified in the literature [37]. Among these, the conformation in
which all ÀOÀC(sp3)ÀC(sp3)ÀOÀ rotamers are alternate Æ gauche, leading to a C2v-
symmetric crown ether, is the most favorable one. In our calculations on the C70-
DB18C6 conjugates, we considered this conformation as well as a structure in which the
ÀOÀC(sp3)ÀC(sp3)ÀO- rotamers are alternating gauche and anti. The latter, in which
two O-atoms are pointing outward away from the crown ether cavity, is structurally
similar to the calculated lowest-energy (Ci-symmetric) conformation of 18C6. For all
three regioisomers (twelve, two, and five o×clock), it was found that the all-gauche
conformation of the crown ether moiety, which is the preferred conformation upon
cation complexation, is more stable than the gauche-anti conformation, in line with

2014
Helvetica Chimica Acta Vol. 85 (2002)
previous force-field calculations [37d]. Differences in energy between the two
conformations range from 0.3kcal/mol in the two o×clock bis-adduct to 2.7 kcal/mol
in the five o×clock bis-adduct. Furthermore, the calculations predicted the five o×clock
regioisomer to be more stable than the twelve and two o×clock regioisomers by 6.9 and
10.4 kcal/mol, respectively (Fig. 3). Based on these computational results, we expected
the macrocyclization of 5 with C70 to proceed with a high degree of selectivity.
Additionally, it was calculated that the macrocyclization of 5 with C70 to afford the five
o×clock adduct is more exothermic (by 1.6 kcal molÀ1) than the corresponding reaction
[29] with C60.
In principle, in-in, in-out, and out-out stereoisomerism, depending on how the two
EtOOC residues (or the two H-atoms in the calculations) at the two methano bridge C-
atoms are oriented, could generate a mixture of configurational diastereoisomers for
each bis-addition pattern [38]. However, the calculations predicted only the out-out
isomers to be feasible for each regioisomer, with the two other configurations being by
several kcal molÀ1 higher in energy. Finally, it should be noted that, as a result of the
planar chirality of the crown ether tether, the two and five o×clock bis-adducts may be
formed as pairs of diastereoisomers. The calculated energy difference is small (1.6 kcal/
mol) in case of the five o×clock bis-adducts.
The structures of the three possible regioisomeric C70 bis-adducts formed by
macrocyclization of C70 with bis-malonate 6, featuring a syn-disubstituted DB18C6
tether, were also calculated (Fig. 4). It was found that the change from anti to syn
substitution of DB18C6 dramatically alters the relative stability of the regioisomeric
bis-adducts. In all cases, the lowest-energy conformation of the macrocyclic bridge was
found to be the same (out-out configuration with all-gauche conformation of the crown-
ether moiety), but the twelve o×clock regioisomer was now calculated to be the most
stable, followed by the two o×clock (‡0.1 kcal/mol) and the five o×clock (‡ 5.0 kcal/
mol) bis-adduct.
2.1.2. Synthesis of C70 Mono-Crown Ether Conjugates Featuring a Five O×Clock
Addition Pattern. Macrocyclization of C70 with bis-malonate 5 under modified Bingel
conditions (I2, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), PhMe) in the presence of
KPF6 (10 equiv.; added to rigidify the tether by complexation) afforded, after
chromatographic separation (SiO2-H; PhMe/AcOEt 1:1), compounds (Æ)-7a and
(Æ)-7b in a 56 :44 ratio and an overall yield of 41% (Scheme 1). Their identity as
isomeric C70 bis-adducts was established by the matrix-assisted laser-desorption-
ionization time-of-flight (MALDI-TOF) mass spectra, which depicted for each of the
‡
two compounds the Na complex of the molecular ion as the parent peak at m/z
‡
1507.1950 ([M ‡ Na] , C102H36NaO14; calc. 1507.1997). Transesterification of pure (Æ)-
7a and (Æ)-7b, or a mixture of both (Cs2CO3, THF/EtOH 1:1) afforded, as single
product, the C2-symmetric five o×clock bis-adduct (Æ)-4, as revealed by spectral
comparison (1H- and 13C-NMR) with an authentic sample obtained by sequential
Bingel bis-cyclopropanation [5a][14]. As predicted by the computer modeling
(Sect. 2.1.1), macrocyclization of C70 with 5 proceeds with complete regioselectivity
to provide the regioisomer least favored in the kinetically controlled, sequential bis-
cyclopropanation (Fig. 2). Furthermore, the transesterification results show that
compounds (Æ)-7a and (Æ)-7b are two diastereoisomeric pairs of enantiomeric
fullerene crown ether conjugates featuring the same addition pattern and differing only

Helvetica Chimica Acta Vol. 85 (2002)
2015
Fig. 4. MM2 Calculations on the relative stabilities of three regioisomeric bis-adducts that can form in the
macrocyclization of C70 with bis-malonate 6 featuring a syn-substituted DB18C6 unit. Relative energies are given
in kcal molÀ1. The EtOOC side chains at the methano bridge C-atoms are replaced by H-atoms in the
calculations. All H-atoms have been omitted for clarity. Depicted are the lowest-energy conformers calculated
for each regioisomer.
in the orientation of the DB18C6 tether (planar chirality). Although KPF6 was added
in the first Bingel macrocyclizations, it was later found that the absence of salt did not
affect the regioselectivity at all; a result in contrast to observations made in the addition
of 5 to C60 [29a,b]. However, the diastereoisomeric ratio changed slightly in the
absence of salt, providing (Æ)-7a and (Æ)-7b in a ratio of 37:63 and a substantially
enhanced yield of 68%. The increase in yield probably results from more-efficient
‡
extraction and purification in the absence of coordinating K ions.
1
The H- and 13C-NMR spectra (CDCl3) support the C2 symmetry of the two
conjugates (Æ)-7a and (Æ)-7b. For example, the 13C-NMR spectrum of (Æ)-7a displayed
two CˆO resonances (163.29 and 162.00 ppm), 33 signals for fullerene C(sp2)-atoms
(156.17 111.64 ppm), two for the fullerene C(sp3)-atoms (66.90 and 66.56 ppm), and
1
one for the methano bridge C-atoms (36.68 ppm). Variable-temperature H-NMR
(VT-NMR) indicated that a considerable barrier exists for the rotation of the crown
ether moiety around the arms linking it to the C-sphere. The spectrum of (Æ)-7a in
(CDCl2)2 did not change significantly upon heating from 293to 393K. In particular, the
AB system of the benzylic CH2 protons (d 5.77 and 4.88 ppm) did not show any signs of
broadening due to exchange. Even after prolonged heating (10 h) of a sample of (Æ)-7a
in (CDCl2)2 at 393 K, the spectrum did not indicate the presence of the typical AB

2016
Helvetica Chimica Acta Vol. 85 (2002)
Scheme 1. Synthesis of (Æ)-7a and (Æ)-7b
5
a) or b)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
+
O
O
O
O
O
O
c)
(±)-7a
(±)-7b
(±)-4
a) C70, I2, DBU, KPF6 (10 equiv.), PhMe/MeCN, r.t.; 23% ((Æ)-7a), 18% ((Æ)-7b). b) C70, I2, DBU, PhMe/
MeCN, r.t.; 25% ((Æ)-7a), 43% ((Æ)-7b). c) Cs2CO3, THF/EtOH 1:1, KPF6, r.t.; 41%.
system (d 5.44 and 5.38 ppm) for the benzylic CH2 protons of (Æ)-7b. Hence, the two
diastereoisomers do not interconvert.
Mono- and bis-cyclopropanations of C70 are accompanied by a significant color
change from red (C70) to red-brown (for the mono- and bis-adducts). However, the
fullerene p-chromophore is only slightly perturbed, as evidenced by the close similarity
between the UV/VIS spectra of (Æ)-7a and C70 [39]. Also, the differences between the
UV/VIS spectra of (Æ)-7a and (Æ)-7b are negligibly small (Fig. 5). Both spectra show
weak longest-wavelength absorption maxima around 691 and 689 nm, respectively,
followed by characteristic bands around lmax ˆ 470 464, 444 443, 403 402, and 370
368 nm. In the UV region, strong absorptions with molar extinction coefficients e
approaching 100000 mÀ1 cmÀ1 are observed around 297 and 275 nm.
2.1.3. X-Ray Crystal Structure of (Æ)-7a. The presence of the inherently chiral five
o×clock addition pattern and the out-out orientation of the EtOOC groups in (Æ)-7a
were unequivocally established by X-ray crystallography (Fig. 6). The fullerene core in
this first crystal structure of a cyclopropanated C70 derivative has a spheroidal shape
with a distance of 8.03ä along the principal axis. As a consequence of cyclo-
propanation, the C-atoms C(1), C(2), C(67), and C(68) are pulled out of the C70-
surface leading to a slightly longer distance along the main axis than observed for the
complex [(h2-C70)Ir(CO)Cl(PPh3)2] (7.90 ä) [40]. Distances from the C-atoms at the
equator to the center of the sphere range from 3.52 3.56 ä with a mean value of
3.54 ä. The bond lengths between the bridgehead atoms C(1)ÀC(2) and C(67)ÀC(68)
of 1.605(9) and 1.596(10) ä, respectively, are strongly elongated compared to the
analogous bonds reported for several h2-transition metal C70-complexes (1.512
1.523ä) [16][17][41]. However, they are similar to the lengths of the corresponding
bonds reported for the trans-1 bis-cyclopropanated C60 crown ether conjugate (1.606
1.610 ä) mentioned above [29b] and a Diels-Alder mono-adduct of C70 (1.603ä) [42].

Helvetica Chimica Acta Vol. 85 (2002)
2017
Fig. 5. Comparison of the UV/VIS spectra (CH2Cl2) of (Æ)-7a, (Æ)-7b, (Æ)-15, and 16. a) Entire spectrum; b)
expansion of the region between 400 and 700 nm.

2018
Helvetica Chimica Acta Vol. 85 (2002)
Fig. 6. Crystal structure of (Æ)-7a. Atomic displacement parameters obtained at 233 K are drawn at the 30%
probability level.
Most geometrical features of (Æ)-7a are comparable to those of the latter Diels-Alder
adduct. However, a significant difference is observed for the 6-5 bonds connected to the
bridgehead atoms. In the case of (Æ)-7a, the lengths of these bonds are significantly
shortened (1.480 1.500 ä vs. 1.523 1.531 ä in the Diels-Alder adduct). Note that the
data of the Diels-Alder adduct, obtained at 193K, are more accurate than the present
data obtained at 233 K.
The crown ether moiety exists in a different conformation compared with those
reported so far in the literature for uncomplexed DB18C6 [37]. To bridge the fullerene
poles, it adopts an −umbrella-shaped× geometry with a distorted C2 symmetry. As a
result, the two aromatic rings are slightly twisted against each other, as can be seen from
the dihedral angles C(84)ÀC(85)ÀC(94)ÀC(95), C(83)ÀC(86)ÀC(106)ÀC(103),
and C(81)ÀC(82)ÀC(104)ÀC(105), which are ca. 128. The angle between the two
mean aromatic ring planes is ca. 1248. This is comparable to the value reported for
DB18C6 complexes (128 1028) [29b][37c,d], but considerably larger than for the
uncomplexed C2v-symmetric DB18C6 (98.88) [37d]. Several close contacts between the
DB18C6 tether and the fullerene surface are observed. One of the aromatic rings
(C(94) to C(106)) is almost parallel to a fullerene hexagon with an interplanar distance
of 3.97 ä. For the other aromatic ring, the closest contacts with the fullerene surface
range from 3.38 3.97 ä, with a mean value of 3.61 ä.
2.1.4. Synthesis of C70 Mono-Crown Ether Conjugates Featuring Twelve and Two
O×Clock Addition Patterns. Bis-malonate 6 with a syn-substituted DB18C6 tether was
previously obtained only as an unseparable 1:1 mixture together with anti-isomer 5
[29a,b][43]. Synthesis of pure 6 started from 3,4-dihydroxybenzaldehyde 8, which was
regioselectively benzyl-protected to give 9 (Scheme 2) [43]. Minor amounts of the

Helvetica Chimica Acta Vol. 85 (2002)
2019
other protected regioisomer and dibenzylated side product were removed by column
chromatography. Alkylation of 9 with (ClCH2CH2)2O readily afforded 10 but
subsequent cleavage of the benzyl ether moieties led to unforeseeable difficulties.
Hydrogenolysis (H2, Pd/C (10%)) in CH2Cl2 did not provide dicarbaldehyde 11, but
rather led in 93% yield to diphenol 12, in which the aldehyde groups are reduced to Me
groups. Similar observations were made by Stoddart and co-workers for the reduction
of a closely related system [44]. Fortunately, the undesired aldehyde reduction could be
suppressed by decreasing the solvent polarity, and, when the hydrogenolysis of 10 was
performed in toluene, no 12 could be detected, and 11 was isolated in 77% yield.
Subsequent Williamson macrocyclization with (BrCH2CH2)2O in the presence of KI
afforded the DB18C6-dicarbaldehyde 13 in remarkable 92% yield (for a synthesis of an
inseparable mixture of 13 and the corresponding anti-dicarbaldehyde, see [29a,b][45]).
Reduction to 14 and esterification proceeded smoothly to provide the desired bis-
malonate 6.
Scheme 2. Synthesis of syn-DB18C6-Tethered Bis-Malonate 6
O
O
O
O
O
a)
b)
OH
O
OH
OH
O
O
O
O
H
H
H
H
8
9
10
d)
c)
O
O
O
O
O
O
O
O
O
O
O
e)
O
O
O
O
H
H
H
H
OH HO
OH HO
O
12
13
11
f)
O
g)
O
O
O
O
HO
OH
6
O
14
a) PhCH2Br, K2CO3, DMF, 608; 74%. b) (ClCH2CH2)2O, K2CO3, DMF, 808; 82%. c) H2 (1 bar), Pd/C (10%),
CH2Cl2, r.t.; 93%. d) H2 (1 bar), Pd/C (10%), PhMe, r.t.; 77%. e) (BrCH2CH2)2O, K2CO3, KI, DMF, 908; 92%.
f) NaBH4, MeOH, r.t.; 98%. g) ClOCCH2CO2Et, pyridine, CH2Cl2, r.t.; 81%.
Bingel macrocyclization of 6 with C70 afforded the two regioisomeric bis-adducts
(Æ)-15 and 16 in yields of 26 and 32%, respectively, after chromatographic separation
(Scheme 3). As in the case of (Æ)-7a/b, the identity of these compounds was
1
unequivocally established by transesterification. The H-NMR spectra obtained after
transesterification were identical to those of the two o×clock ((Æ)-3) and twelve o×clock
(2) bis-adducts obtained by sequential Bingel cyclopropanation [5a][14].
1
The structure assignments of (Æ)-15 and 16 were also supported by the H- and
1
13C-NMR spectra. In the H-NMR spectrum (CDCl3) of (Æ)-15, the four benzylic

2020
Helvetica Chimica Acta Vol. 85 (2002)
Scheme 3. Synthesis of (Æ)-15 and 16
6
a)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
+
(±)-15
16
2
b)
b)
(±)-3
a) C70, I2, DBU, KPF6, PhMe/MeCN, r.t.; 26% ((Æ)-15), 32% (16). b) Cs2CO3, THF/EtOH (1:1), KPF6, r.t.,
2 h.
protons appear as two AB systems, indicating C1 symmetry. This symmetry is also
inferred from the 13C-NMR spectrum in which sixty-six fullerene C(sp2)-atom
resonances are observed. In contrast, the benzylic protons of 16 appear as a single
AB system, indicating a higher degree of symmetry, either C2 or Cs. The Cs symmetry of
16 could be deduced from the 13C-NMR spectrum, which displayed thirty-eight
fullerene C(sp2)-atom resonances, ten of which exhibit half the intensity; in case of C2
symmetry, one would expect to observe thirty-three signals of similar intensity.
The UV/VIS spectra of the two C70 crown ether conjugates are similar to those
previously reported for C70 bis-adducts featuring two and twelve o×clock addition
patterns (Fig. 5) [6][14]. Both (Æ)-15 and 16 have their onset of absorption around
700 nm, and the lowest-energy absorption bands appear as shoulders centered at ca. 660
and 655 nm, respectively. Compared with (Æ)-7a, this corresponds to a small
hypsochromic shift of ca. 30 nm. Overall, the spectra of (Æ)-7a, (Æ)-15, and 16 are
very similar; however, there are some subtle differences that are characteristic for the
different addition patterns. In the spectrum of (Æ)-7a, a band is present at lmax 444 nm.
This band is shifted to lmax 434 nm in the case of (Æ)-15 and to lmax 423nm for 16. A
unique characteristic of 16 is the short-wavelength band at lmax 270 nm (Fig. 5,a).
2.1.5. X-Ray Crystal Structure of 16. Evidence for the twelve o×clock addition
pattern of 16 was provided by its X-ray crystal structure (Fig. 7). It clearly shows the
tangential orientation of the ionophore moiety with the out-out geometry and linkage
to the fullerene sphere through the methano bridges at C(1)ÀC(2) and C(41)ÀC(58).
The structure is slightly distorted from C2 symmetry as demonstrated by the different
orientation of the EtOOC side chains.
An interesting feature of 16 is the inclusion of a H2O molecule (O(200)); see Fig. 8)
in the DB18C6 cavity. It is located close to the center and somewhat outside the crown

Helvetica Chimica Acta Vol. 85 (2002)
2021
Fig. 7. Crystal structure of 16. Atomic displacement parameters obtained at 208 K are drawn at the 30%
probability level.
ether cavity. The distances between O(200) and the six crown ether O-atoms range
from 3.08 to 3.25 ä, and the distance between O(200) and the mean plane of the
corresponding O-atoms is ca. 1.5 ä. As a consequence of binding a H2O molecule, all
ÀOÀCH2ÀCH2ÀOÀ rotamers are in a pseudo-gauche conformation with angles
ranging from 74 768 and the ÀCH2ÀCH2ÀOÀCH2À rotamers adopt an anti
conformation with angles between 172 1788. The crown ether moiety adopts an
−umbrella-shaped× structure, with an angle between the mean aromatic ring-planes of
ca. 1198. A search in the Cambridge Crystallographic Database (CCD) for inclusion
complexes of H2O with crown ethers revealed that, in the majority of cases, the H2O
molecule residing in the cavity void is stabilized by additional H-bonds to neighboring
molecules [46]. In some cases, this leads to large H-bonded networks [47]. Inclusion
complexes in which the H2O molecule is bound by a crown ether moiety only are rare
[48]. The H2O molecule typically forms two H-bonds and as a consequence,
O(H2O)ÀO(ether) bond distances fall into two categories, ranging from 2.8 3.1 ä
for the H-bonded O(ether)-atoms and 3.4 4.0 ä for the nonbonded ones [46 48]. All
these cases concern 18C6 complexes in which the crown ether adopts a pseudo-D3d
symmetry. In the present X-ray structure, O(200) shows one CÀH ¥¥¥ O contact to a
CH2Cl2 molecule and one contact to C(83') in the DB18C6 moiety of a neighboring
fullerene conjugate, the C ¥¥¥ O distances being 3.53 and 3.55 ä, respectively (Fig. 8).
Both benzene rings of the tether are in close stacking contact with the fullerene
surface. The benzene ring C(81) to C(86) is nearly parallel to the neighboring fullerene
pentagon C(5)ÀC(4)ÀC(17)ÀC(18)ÀC(19), the interplanar distance being 3.4 ä. The
midpoint of a second benzene ring formed by C(94) to C(106) is positioned on top of
C(56) with an atom-to-plane distance of 3.20 ä (Fig. 8).

2022
Helvetica Chimica Acta Vol. 85 (2002)
Fig. 8. Two molecules of 16 (related by an inversion center) showing intermolecular contacts to the enclosed
H2O O-atom (O(200)). The C ¥¥¥ O distances in the contacts of O(200) to CH2Cl2 and to C(83') in the DB18C6
moiety of a neighboring fullerene conjugate are 3.53 and 3.55 ä, respectively.
2.1.6. Synthesis of a C70 Mono-Adduct with a DB18C6 Moiety Connected by a Single
Flexible Linker. In the cyclophane-type C70-DB18C6 conjugates discussed so far, the
flexibility of the crown ether moiety is substantially restrained. Furthermore, the close
proximity of the fullerene sphere to the DB18C6 moiety inhibits complexation on the
concave side of the crown ether. Since both factors may affect cation-binding affinities
of the ionophoric site, it was desirable to compare the complexation properties of these
compounds with those of a suitable reference conjugate containing a flexible crown
ether (vide infra). For this purpose, we synthesized the C70 mono-adduct (Æ)-22, in
which the crown ether moiety is covalently attached to the C-sphere by one arm only,
thus retaining its full conformational flexibility (Scheme 4). Double alkylation of
catechol (ˆ benzene-1,2-diol, 17) with (ClCH2CH2)2O afforded dichloride 18, which
was transformed into crown ether 19 [45][49] by macrocyclization with 3,4-dihydrox-
ybenzaldehyde (8) [50]. Reduction ( ! 20), esterification ( ! 21), and Bingel addition
provided the desired C1-symmetric conjugate (Æ)-22, which was fully characterized.
2.2. Formation of Higher Adducts of C70. 2.2.1. Synthesis of Tris- and Tetrakis-
Adducts Starting from Five O×Clock Bis-Adducts (Æ)-4 and (Æ)-7a. Transesterification
of a mixture of (Æ)-7a and (Æ)-7b (vide supra) provided, for the first time, good access
to the five o×clock bis-adduct (Æ)-4 (Scheme 1), which was formed only in low yield by
sequential double Bingel addition (Fig. 2) [5b][6][14]. This improved availability of
(Æ)-4 enabled us to investigate its reactivity in additional cyclopropanation reactions.
When (Æ)-4 was reacted with 1 equiv. of diethyl malonate (modified Bingel conditions:
I2, DBU, PhMe/Me2SO 4 :1), a single isolable new compound ((Æ)-23) was obtained in

Helvetica Chimica Acta Vol. 85 (2002)
Scheme 4. Synthesis of (Æ)-22
2023
O
O
O
O
O
a)
b)
c)
HO
HO
Cl
Cl
O
O
O
O
O
O
O
O
O
O
H
HO
O
O
18
19
17
20
O
O
O
O
O
O
d)
O
O
O
O
O
O
e)
O
O
O
O
O
O
O
O
21
(±)-22
a) (ClCH2CH2)2O, K2CO3, DMF, 858; 35%. b) 8, K2CO3, KI, DMF, 808; 34%. c) NaBH4, MeOH, r.t.; 95%. d)
ClOCCH2CO2Et, pyridine, CH2Cl2, r.t.; 50%. e) C70 (0.8 equiv.), I2 (0.8 equiv.), DBU (2.4 equiv.), PhMe, r.t.;
81%.
36% yield, together with unreacted starting material (30%; Scheme 5). The new C70
derivative was identified by MALDI-Fourier-transform-MS (MALDI-FT-MS) as a
‡
tris-adduct (m/z 1337.1623 ([M ‡ Na] , C91H30NaO12; calc. 1337.1629)).
Reaction of (Æ)-4 with 2 equiv. of diethyl malonate afforded, besides (Æ)-23 (18%),
a single isolable product ((Æ)-24) in 54% yield, which was characterized by MALDI-
TOF-MS as a C70 tetrakis-adduct. The most intense peak in the mass spectrum
‡
corresponds to the Na complex of the molecular ion at m/z 1495.2206 ([M ‡ Na] ,
C98H40NaO16; calc. 1495.2209). Reaction of (Æ)-4 with 3equiv. of diethyl malonate
afforded tetrakis-adduct (Æ)-24 as the sole product in 75% yield. The formation of even
higher adducts was not observed under these conditions. No change in product
distribution was observed when (Æ)-4 was reacted with diethyl 2-bromomalonate under
the original Bingel conditions.
Similarly, reaction of (Æ)-7a with 2 equiv. of diethyl malonate under modified
Bingel conditions afforded a single new compound ((Æ)-25; 50%), which was fully
characterized as a tetrakis-adduct featuring the same addition pattern as shown for (Æ)-
24 (Scheme 6). Apparently, the presence of the crown ether tether does not affect the
regioselectivity for attachment of the third and fourth addends.
Tris- and tetrakis-cyclopropanation of (Æ)-4 (Scheme 5) and the corresponding
reaction of (Æ)-7a (Scheme 6) occur with remarkable degree of regioselectivity. In
theory, addition to the different 6-6 bonds of (Æ)-4 could result in 24 possible tris-
adducts, whereas ten different C2-symmetric tetrakis-adducts are possible starting from
(Æ)-23, assuming that the additions occur only at 6-6 bonds. Nevertheless, only one
unique tris-adduct and one unique tetrakis-adduct are formed.
1
Although the H- and 13C-NMR spectra are in full agreement with the proposed
geometries of (Æ)-23 (C1 symmetry), (Æ)-24 (C2 symmetry), and (Æ)-25 (C1
symmetry), they do not allow an unambiguous structure assignment. The addition

2024
Helvetica Chimica Acta Vol. 85 (2002)
Scheme 5. Synthesis of Tris-Adduct (Æ)-23 and Tetrakis-Adduct (Æ)-24
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
(±)-23
a), b), or c)
O
O
+
O
O
O
O
O
O
O
O
(±)-4
O
O
O
O
O
O
O
O
O
O
(±)-24
a) (EtO2C)2CH2 (1 equiv.), I2, DBU, PhMe/Me2SO 4 :1, r.t.; 30% ((Æ)-4), 36% ((Æ)-23). b) (EtO2C)2CH2
(2 equiv.), I2, DBU, PhMe/Me2SO 4 :1, r.t.; 18% ((Æ)-23), 54% ((Æ)-24). c) (EtO2C)2CH2 (3equiv.), I 2, DBU,
PhMe/Me2SO 4 :1, r.t.; 75% ((Æ)-24).
Scheme 6. Synthesis of Tetrakis-Adduct (Æ)-25
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
a)
O
O
O
O
O
O
O
O
O
O
(±)-7a
(±)-25
O
a) (EtO2C)2CH2 (2.5 equiv.), DBU, PhMe/Me2SO 4 :1, r.t.; 50%.
patterns proposed are, however, strongly supported by earlier results from the
cyclopropanation of 2 and (Æ)-3 (Fig. 2) leading to tris- and tetrakis-adducts [6][14].
In these studies, it was found that the preferred pattern for bis-cyclopropanation within
the same hemisphere of C70 is similar to that established for C60. In the case of C60, the
second addition preferably occurs at a so-called equatorial (e) position (for positional
notation, see: [51]). In the C70 bis-adduct (Æ)-4, a similar (−e-type×) geometric

Helvetica Chimica Acta Vol. 85 (2002)
2025
relationship exists between the bridged a-type bond C(1)ÀC(2) and the b-type bond
C(31)ÀC(32) (and, correspondingly, between the bridged a-type bond C(67)ÀC(68)
and the b-type bond C(42)ÀC(43) in the opposite hemisphere). For steric and
electronic reasons (high LUMO coefficients [14]), the third and fourth kinetically
controlled Bingel additions to give (Æ)-23 and (Æ)-24, respectively, occur at these b-
type bonds.
The UV/VIS spectrum of tris-adduct (Æ)-23 exhibits a series of poorly resolved
absorption bands with an end-absorption near 700 nm (Fig. 9). The only characteristic
feature is an absorption maximum at lmax 406 nm. It should also be noted that the molar
extinction coefficients are strongly reduced when compared to those of the bis- (Fig. 5)
and tetrakis-adducts. For the latter ((Æ)-24 and (Æ)-25), the onset of absorption occurs
near 660 nm. The first absorption band, which corresponds to the HOMO-LUMO
transition, is visible as a shoulder at ca. 625 nm. A further characteristic feature of the
tetrakis-adducts is the absorption maximum at lmax 425 430 nm. The molar extinction
coefficients are similar to those of the bis-adducts.
Fig. 9. Comparison of the UV/VIS spectra (CH2Cl2) of (Æ)-23, (Æ)-24, and (Æ)-25. Shown is only the VIS region
between 400 and 700 nm; for the UV absorptions, see the Exper. Part.
2.2.2. Synthesis of C70 Pentakis- to Heptakis-Adducts Starting From Five O×Clock
Bis-Adduct (Æ)-4. Previous investigations on the Bingel cyclopropanation of the twelve
o×clock (2) and two o×clock ((Æ)-3) bis-adducts had shown that even higher adducts
(beyond tetrakis-adducts) could be obtained, if more polar solvents such as pure
Me2SO [14] were used. In this environment, the nucleophilic addition reaction is
greatly accelerated [52], whereas the selectivity is reduced. Similar findings were made
in this study, starting from five o×clock bis-adduct (Æ)-4.

2026
Helvetica Chimica Acta Vol. 85 (2002)
When (Æ)-4 was reacted with diethyl 2-bromomalonate (6 equiv.) and DBU
(9 equiv.) for 2 h in Me2SO, four main product Fractions I IV were obtained besides
traces of tris-adduct (Æ)-23 and tetrakis-adduct (Æ)-24 (Scheme 7). The four fractions,
which displayed distinct differences in color, were isolated by preparative TLC (SiO2;
CH2Cl2/AcOEt 98 :2) and identified by MALDI-FT-MS as a pentakis-adduct
‡
Fraction I (m/z 1653.2779 ([M ‡ Na] , C105H50NaO20; calc. 1653.2788)), two hexakis-
‡
adducts Fractions II and III (m/z 1811.3363 ([M ‡ Na] , C112H60NaO24; calc.
‡
1811.3367)), and
C119H70NaO28; calc. 1969.3951)).
a
heptakis-adduct Fraction IV (m/z 1969.3949 ([M ‡ Na] ,
Scheme 7. Synthesis of C70 Pentakis- to Heptakis-Adducts Starting from (Æ)-4
O
O
O
O
O
O
a)
O
O
O
O
O
n
O
Product
Fraction
(±)-4
n
I
5
6
6
7
II
III
IV
a) Diethyl 2-bromomalonate (6 equiv.), DBU (9 equiv.), Me2SO, r.t.; 9.8% (pentakis-adduct Fraction I),
18.8% (hexakis-adduct Fraction II), 7.4% (hexakis-adduct Fraction III), 15.6% (heptakis-adduct Fraction IV).
In the absence of X-ray crystal-structure analyses, a conclusive structure assignment
could not be made; however, from the NMR and UV/VIS data, some important
conclusions could be drawn. The 13C-NMR spectrum revealed that the dark-brown
pentakis-adduct Fraction I contained two isomers in a ratio of ca. 1:1. The appearance
of 120 resonances for the fullerene C(sp2)-atoms is consistent with two C1-symmetric
pentakis-adducts (Fig. 10,a). Accordingly, the spectrum displays ten signals between 43
and 36 ppm, the region where the methano bridge C-atoms in cyclopropanated
fullerenes resonate (Fig. 10,b). Thus, we can conclude that no methanofullerene
substructures with bridged 6-5-open (or possibly 6-6-open) bonds had formed, in which
the bridging C-atom resonance is shifted substantially downfield into the region at ca.
60 ppm [14]. The number of possible structures for the two compounds in Fraction I
could be narrowed down further with the aid of density-functional-theory (DFT)
calculations (vide infra).
The two bright-red and purple-red hexakis-adducts Fractions II and III, respec-
tively, were characterized only by HR (high resolution)-MALDI-FT-MS and UV/VIS,
since low quantities of material and the presence of impurities (including several minor
isomers) largely prevented the recording of NMR spectra with a good signal-to-noise
ratio. Meaningful 13C-NMR spectral data were obtained only for Fraction II, indicating
the presence of at least two hexakis-adducts in a ratio of ca. 4.5 :1. All fullerene C(sp2)-

Helvetica Chimica Acta Vol. 85 (2002)
2027
Fig. 10. 13C-NMR Spectra (125.8 MHz, CDCl3) of pentakis-adduct Fraction I (a, b) and heptakis-adduct
Fraction IV (c). Spectrum a shows the fullerene C(sp2)-atom region and spectrum b the methano-C-atom
region in the spectrum of Fraction I, which contains two C1-symmetric pentakis-adducts. The inset in spectrum c
shows two resonances at unusual chemical shift of the pure C1-symmetric heptakis-adduct present in
Fraction IV.
atoms were found to resonate between 150 126 ppm, which suggests that all additions
led to the formation of cyclopropanated 6-6 bonds rather than methano-bridged 6-6-
open or 6-5-open substructures. A methano-bridged 6-5-open substructure had
previously been reported for a C70 hexakis-adduct prepared by Bingel additions to
the two o×clock bis-adduct (Æ)-3 [14] (for other examples of 6-5-open methanofuller-

2028
Helvetica Chimica Acta Vol. 85 (2002)
enes, see [53]). The two major hexakis-adducts IIa and IIb in Fraction II could be
separated by preparative TLC (SiO2; CH2Cl2/AcOEt 39 :1).
The orange-yellow heptakis-adduct fraction consists of one pure C1-symmetric
isomer according to 1H- and 13C-NMR spectroscopy. The 13C-NMR spectrum (Fig. 10,c)
comprises 56 resonances between 150 and 126 ppm corresponding to the fullerene
C(sp2)-atoms. Six (out of seven expected for an all-cyclopropanated derivative) signals
are observed for the methano-bridge C-atoms in the region between 45 and 39 ppm,
whereas 13(out of 14 expected) resonances of the fullerene C(sp 3)-atoms appear in the
region between 74 and 53ppm. However, two additional signals with an anomalous
shift appear at 97 and 91 ppm (Fig. 10,c). All these features are consistent with a
structure in which one of the (EtO2C)2C addends bridges a 6-6-open or a 6-5-open
bond.
A comparison between the UV/VIS spectra (CH2Cl2) of Fractions I IV also
revealed some interesting details (Fig. 11). Whereas the onset of absorption of bis-
adduct (Æ)-4, tris-adduct (Æ)-23, tetrakis-adduct (Æ)-24, pentakis-adduct Fraction I,
and hexakis-adduct Fraction II occurs at ꢀ 660 nm, a change to ꢁ 600 nm is observed
for the second hexakis- (III) and the heptakis- (IV) adduct fractions. This shift suggests
a significant reduction of the remaining fullerene p-chromophore.
Fig. 11. Comparison of the UV/VIS spectra (CH2Cl2) of higher-adduct Fractions I IV. Shown is only the VIS
region between 400 and 700 nm; for the UV absorptions, see the Exper. Part.
2.2.3. Regioselectivity of Multiple Cyclopropanations: A Density-Functional-Theory
(DFT) Study. Successful theoretical rationalizations of the experimentally observed
regioselectivity in kinetically controlled nucleophilic Bingel cylopropanations of C60
[19] and C70 [14] are based on calculations of the coefficients of the lowest unoccupied
molecular orbital (LUMO) according to semi-empirical or ab initio methods.

Helvetica Chimica Acta Vol. 85 (2002)
2029
Additions occur preferentially at 6-6 bonds with large LUMO coefficients, providing
the best interaction with the highest occupied molecular orbital (HOMO) of the
nucleophile.
Here, we apply density-functional-theory (DFT) calculations [54] to optimize the
structures of the fullerene adducts and calculate their frontier molecular orbitals. In the
first step, the accuracy of the method was evaluated by comparing the results for C70,
mono-adduct 1 and the twelve (2) and two o×clock ((Æ)-3) bis-adducts with those
obtained in previous ab initio studies [14]. In all calculations, the (EtO2C)2C addends
were replaced by CH2 bridges (H2C).
The LUMO and the LUMO ‡ 1 of C70 are degenerate (Fig. 12,a), and both orbitals
are mainly located at the poles, with large coefficients at the polar a-type bonds. The
LUMO ‡ 2, which is nearly isoenergetic (DE ˆ 0.046 eV), bears no coefficients at the
pole. These findings are in agreement with the experimental results: mono-addition
occurs exclusively at the a-type bonds as a result of favorable interactions between the
HOMO of the nucleophile and the LUMO and LUMO ‡ 1 of C70. They differ,
however, from a recent study at the RHF-PM3(restricted Hartree-Fock-parametric
method 3) level of theory, in which was reported a different order for the lowest
unoccupied orbitals [8].
The DFT calculations revealed that the LUMO in mono-adduct 1* (* indicates in
the following that all (EtO2C)2C are replaced by CH2 in the calculations) has large
orbital coefficients at the a-type bonds C(56)ÀC(57) and C(59)ÀC(60) on the
opposite pole (Fig. 12,b). Addition to these bonds leads to formation of two o×clock
bis-adduct (Æ)-3*, which is, indeed, the main product in the Bingel bis-cyclopropana-
tion of C70 (Fig. 2). Two other a-type bonds, C(67)ÀC(68) and C(69)ÀC(70), bear
small LUMO coefficients. Addition to these bonds leads to five o×clock bis-adduct (Æ)-
4*, which is the least favored regioisomer in the sequential Bingel bis-cyclopropanation.
The formation of the twelve o×clock adduct 2* can be rationalized by the interaction of
the HOMO of the nucleophile with the LUMO ‡ 2 of the monoadduct. The LUMO ‡ 2
is only by 0.156 eV higher in energy and bears very large coefficients at the a-type bond
C(41)ÀC(58) at which attack occurs. These results are in good agreement with the
previous ab initio RHF-SCF (self-consistent field) calculations with the 3-21G basis set
[14].
The LUMO of the five o×clock bis-adduct (Æ)-4* has large coefficients at the b-type
bonds C(16)ÀC(17), C(28)ÀC(29), C(31)ÀC(32), C(39)ÀC(40), C(42)ÀC(43), and
C(61)ÀC(62) (Fig. 12,c). Additionally, the LUMO ‡ 1, which is nearly isoenergetic
(DE ˆ 0.01 eV), has substantial coefficients at the b-type bonds C(31)ÀC(32) and
C(42)ÀC(43). Experimentally, it is observed that the third addition preferentially
occurs at the two latter bonds. In the resulting C1-symmetric tris-adduct (Æ)-23*, the
LUMO coefficients at C(42)ÀC(43) become even more pronounced (Fig. 12,d) and
attack at that bond leads to the formation of C2-symmetric tetrakis-adduct (Æ)-24*.
The calculations also allow a tentative prediction of the structures for the two C1-
symmetric compounds in pentakis-adduct Fraction I (Sect. 2.2.3). The computed
LUMO of tetrakis-adduct (Æ)-24* has large coefficients at the hexagon
C(18)ÀC(19)ÀC(20)ÀC(38)ÀC(37)ÀC(36) through which the C2-axis passes
(Fig. 13,a). In particular, the coefficients at the 6-5 bonds C(18)ÀC(19) and
C(37)ÀC(38) are large. Addition to these bonds, followed by valence isomerization

2030
Helvetica Chimica Acta Vol. 85 (2002)
Fig. 12. Calculated LUMO, LUMO ‡ 1, and LUMO ‡ 2 of a) C70, b) C70 mono-adduct 1*, c) C70 bis-adduct (Æ)-
4*, and d) C70 tris-adduct (Æ)-23*. The star (*) indicates that the (EtO2C)2C addends in the experimentally
prepared products are replaced by CH2 addends in the computational study. The DFT calculations were
performed with the pBP/DN (perturbational Becke-Perdew/numerical split-valence basis set) model. Visual-
ization was done with the SPARTAN (SGI V. 5.1.3) graphics module [54].

Helvetica Chimica Acta Vol. 85 (2002)
2031
[14][53a], would lead to the formation of pentakis-adduct ((Æ)-26a*, Fig. 13,b), in
which the 6-5 bond is opened. Indeed, a geometry optimization of this regioisomer led
to the 6-5 open structure, although the input structure possessed a 6-5 bond. The NMR
data (Fig. 10), however, do not support formation of a 6-5-open methanofullerene.
Smaller coefficients are present at the b-type bonds C(5)ÀC(6), C(54)ÀC(55),
C(16)ÀC(17), and C(39)ÀC(40). Addition to the latter two bonds leads to the
formation of pentakis-adduct (Æ)-26b*.
Fig. 13. a) Calculated LUMO, LUMO ‡ 1, and LUMO ‡ 2 of C70 tetrakis-adduct (Æ)-24* and b) possible
structures for the two products in pentakis-adduct Fraction I. The star (*) indicates that the (EtO2C)2C addends
in the experimentally prepared products are replaced by CH2 addends in the computational study. The DFT
calculations were performed with the pBP/DB model and visualizations with the SPARTAN (SGI V. 5.1.3)
graphics module [54].

2032
Helvetica Chimica Acta Vol. 85 (2002)
The LUMO ‡ 1 is mainly located at the opposite face, centered around the k-type
bond C(46)ÀC(47), through which the C2-symmetry axis passes. Large coefficients are
present at the 6-5 bonds C(27)ÀC(28) and C(62)ÀC(63), the b-type 6-6 bonds
C(28)ÀC(29) and C(61)ÀC(62), the e-type 6-6 bonds C(27)ÀC(46) and C(47)ÀC(63),
and the k-type 6-6-bond C(46)ÀC(47). Addition to these bonds may lead to the
formation of pentakis-adducts (Æ)-26c*, (Æ)-26d*, (Æ)-26e*, and (Æ)-26f*, respectively
(Fig. 13,b).
For a tentative structure proposal, we introduce, in addition to the experimental
evidence for the absence of addition to 6-5 bonds, the following boundary conditions
derived from the previous experience with nucleophilic multiple substitutions of C70: i)
additions do not occur at 6-6 bonds of a hexagon that already bears an addend, and ii)
additions do not occur at 6-6 bonds on the equator. On this basis, only structures (Æ)-
26b*, (Æ)-26d*, and (Æ)-26e* are feasible. Of these regioisomers, (Æ)-26b* and (Æ)-
26d* are nearly isoenergetic and lower in energy than (Æ)-26e*. Therefore, we propose
structures (Æ)-26b*/(Æ)-26d* for the two compounds present in approximately equal
amounts in pentakis-adduct Fraction I.
2.3. Synthesis of C70 Bis-Crown Ether Conjugates. A model of (Æ)-7a/b showed that
the distance between the two reactive b-type 6-6 bonds (which, in C60, would be classified
as e-type; see above) is similar to that between the two bonds already functionalized.
Thus, it should be possible to bridge the C-sphere with a second crown ether by Bingel
macrocyclization, allowing, for the first time, the study of the electrochemistry of a
fullerene sandwiched between two crown-ether bound cations. MM2 Calculations [34]
predicted that bis-malonate 5 with an anti-DB18C6 tether would have the right length
to bridge the two b-type bonds in an almost strain-free fashion.
Reaction of (Æ)-7a with 5 under modified Bingel conditions afforded a mixture of
four isomeric products (27, Scheme 8) in 69% yield. The formation of C70 bis-crown
‡
ether adducts was confirmed by HR-MALDI-FT-MS (m/z 2151.4120 ([M ‡ Na] ,
C134H72NaO28; calc. 2151.4102)). We had anticipated the formation of two diaster-
Scheme 8. Macrocyclization of (Æ)-7a with Bis-Malonate 5 Afforded a Mixture of Four Major Isomers (27)
Crown ether
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
a)
O
O
O
O
O
O
O
O
O
O
O
(±)-7a
Crown ether
isomeric mixture 27
O
O
O
O
O
O
Crown ether
=
O
O
a) 5 (1 equiv.), I2 (2 equiv.), DBU (6 equiv.), PhMe/Me2SO 4 :1, r.t.; 69%.

Helvetica Chimica Acta Vol. 85 (2002)
2033
eoisomeric pairs of enantiomers, in analogy to the formation of (Æ)-7a and (Æ)-7b, and
the isolation of additional isomers, which could result from configurational (in-out)
isomerism at the methano-bridge C-atoms, was quite unexpected. Although partial
product separation was achieved by TLC, we did not succeed in preparative separations
by column chromatography. Also, attempted separation by HPLC proved to be
unsatisfactory, since it was tedious and resulted in extensive decomposition of the
products.
We subsequently changed to bis-malonate 6 with the syn-DB18C6 tether to achieve
the second bridging, hoping that the reduced tether length would make in-out
isomerism less likely. Furthermore, the syn-tether does not introduce additional planar
chirality into the product. When (Æ)-7a was reacted with 6 under modified Bingel
conditions, a single bis-crown ether conjugate, (Æ)-28a, was indeed formed regio- and
stereoselectively in 31% yield (Scheme 9). Similarly, the reaction of (Æ)-7b with 6 led
to tetrakis-adduct (Æ)-28b in 26% yield. The addition pattern in (Æ)-28a/b is C2-
symmetric and identical to that in (Æ)-24 and (Æ)-25, but the overall symmetry is C1, as
inferred from the 1H- and 13C-NMR spectra, due to the two different types of DB18C6
tethers. Thus, the 1H-NMR spectrum shows in the region between 5.9 and 4.6 ppm two
sets of benzylic CH2 resonances, belonging to the anti- and syn-substituted tether,
respectively. The protons of each benzylic CH2 group appear as one AB system
resulting in a total of four well-resolved AB systems. The UV/VIS spectra of (Æ)-24,
(Æ)-25, and (Æ)-28a/b expectedly are virtually identical, providing additional support
for the correct structural assignment of the bis-crown ether conjugates.
2.4. Ionophoric Properties. The cation binding properties of all C70 crown ether
conjugates were studied in the organic membrane phase of ion-selective electrodes
(ISEs). The compound under study was dissolved in the ISE membrane bis(2-
ethylhexyl) sebacate (ˆdecanedioate) (DOS)/poly(vinyl chloride) (PVC) together
with the cation exchanger sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
(NaTFPB) that determines the total concentration of cations in the organic phase
[55][56]. Close to Nernstian responses of 54 to 59 mV/decade proved the existence of a
local equilibrium at the membrane/sample phase boundary in all cases. Therefore, the
electromotive-force (EMF) values could be used for the determination of thermody-
namic parameters [57].
The ionophoric properties of the crown ether conjugates are indicated by a
‡
significantly improved K -ion selectivity as compared to membranes containing only
the ion exchanger (NaTFPB) but no ligand (Table 1, last entry). The selectivities of the
‡
five o×clock and twelve o×clock bis-adducts (Æ)-7a/b and 16 with respect to Na and
‡
NH4 ions do not differ significantly. The selectivity of the two o×clock derivative (Æ)-15
‡
for Na ions, however, is by ca. one order of magnitude higher than that of the two
other C70-ionophore conjugates with regioisomeric addition patterns. Since the C70 bis-
crown ether conjugates may also form 1:2 (ligand/ion) complexes, they were
additionally investigated in membranes having a molar excess of ion exchanger.
Membranes with an excess of C70 bis-crown ether conjugate (Æ)-28a or (Æ)-28b show a
‡
‡
somewhat lower selectivity for Na and a higher one for NH4 ions than the C70 mono-
crown ether conjugates. With a molar excess of ion exchanger, all selectivities are closer
to those of the ionophore-free membrane. The membrane with reference compound
(Æ)-22, in which the DB18C6 moiety has a higher conformational flexibility than in the

2034
Helvetica Chimica Acta Vol. 85 (2002)
Scheme 9. Synthesis of C70 Bis-Crown Ether Conjugates (Æ)-28a/b
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
(±)-7a
(±)-7b
a)
O
O
O
O
O
O
O
O
O
a)
O
O
O
O
O
6
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
(±)-28b
(±)-28a
O
O
a), I2, DBU, PhMe/Me2SO 4 :1, r.t.; 31% ((Æ)-28a), 26% ((Æ)-28b).
‡
other crown ether conjugates shows a significantly weaker discrimination of Na ions
than the membranes with the other ionophores.
eff
Effective complex-formation constants, b (I: alkali metal ion, L: ligand; see
IL
‡
Table 2), with K ions were determined by the sandwich-membrane method [58]. The
potential of a double membrane with the same composition of both of its layers except
that one contains an ionophore and the other not is a direct measure of the decrease of
free-ion activities through complexation. Since the potential is defined by the activities
of free ions, the formal complex-formation constants include effects of ion-pair
‡
‡
‡
formation [58]. The complex-formation constants with NH4 , Na , and H ions were

Helvetica Chimica Acta Vol. 85 (2002)
2035
pot
KJ
Table 1. Potentiometric Selectrivity Coefficients, log K , of ISEs Based on C70 Crown Ether Conjugates and
Concentrations of Investigated Ligand and Ion Exchenger NaTFPBa) in the ISE Membranes
pot
KJ
Ligand
log K
Ligand
NaTPFPB
[mmol/l]
‡
‡
‡
Na
NH4
H
[mmol/l]
(Æ)-7a
(Æ)-7b
(Æ)-15
16
(Æ)-22
(Æ)-25
(Æ)-28a
À 2.11 Æ 0.01
À 2.29 Æ 0.01
À 3.64 Æ 0.01
À 2.28 Æ 0.01
À 1.19 Æ 0.03
À 2.38 Æ 0.04
À 1.61 Æ 0.01
À 1.53 Æ 0.01
À 1.50 Æ 0.01
À 1.45 Æ 0.01
À 1.47 Æ 0.01
À 1.63 Æ 0.03
À 2.06 Æ 0.04
À 1.75 Æ 0.01
À 2.68 Æ 0.02
À 1.84 Æ 0.02
À 1.84 Æ 0.02
À 1.85 Æ 0.02
À 2.97 Æ70.032.3
À 1.39 Æ 0.01
À 2.88 Æ 0.030.92
4.95
4.94
4.81
5.01
1.15
2.52
2.53
2.54
2.54
2.53
1.40
0.25
1.65
0.44
(27.34 mol-%)
(180 mol-%)
(Æ)-28a
(Æ)-28b
(Æ)-28b
No ligand
À 0.54 Æ 0.02
À 2.09 Æ 0.01
À 0.84 Æ 0.03
À 0.48 Æ 0.03
À 0.31 Æ 0.01
À 1.71 Æ 0.02
À 0.50 Æ 0.01
À 0.18 Æ 0.02
À 0.32 Æ 0.030.92
À 2.38 Æ 0.031.11
À 0.71 Æ 0.02
(39.50 mol-%)
1.91
(174.40 mol-%)
1.11
0.14 Æ 0.04
a) Sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. Mol-% are given with respect to the ligand
concentration.
Table 2. Effective Complex Formation Constants of C70 Crown Ether Conjugates, log beILff, Obtained for the
‡
Organic Membrane Phase with the Sandwich Membrane Method (for K ions) and from the Potentiometric
Selectivity Coefficients (for the other ions)a)
‡ c
c
‡
‡
‡c
Ligand
Stoichiometryb)
K
Na
4.28 Æ 0.05
)
Na4
)
H )
(Æ)-7a
(Æ)-7b
(Æ)-15
16
(Æ)-22
(Æ)-25
(Æ)-28a
(Æ)-28a
(Æ)-28b
(Æ)-28b
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 2
1 : 1
1 : 2
5.90 Æ 0.05
5.90 Æ 0.034.10
6.03 Æ 0.06
6.06 Æ 0.04
6.34 Æ 0.04
6.04 Æ 0.01
6.24 Æ 0.05d)
4.57 Æ 0.05
.1Æ70.033
3.12 Æ 0.06
Æ 0.04
Æ 0.034.60
2.88 Æ 0.06
4.28 Æ 0.04
5.63 Æ 0.05
4.25 Æ 0.05
4.82 Æ 0.04
4.79 Æ 0.06
4.79 Æ 0.04
4.91 Æ 0.05
4.82 Æ 0.06
4.39 Æ 0.04
4.14 Æ 0.07
4.16 Æ 0.05
3.03 Æ 0.06
3.86 Æ 0.06
2.94 Æ 0.06
11.51 Æ 0.07d,e
)
6.37 Æ 0.07d)
4.45 Æ 0.09
4.53 Æ 0.10
3.59 Æ 0.10
11.48 Æ 0.09d,e
)
a) Error bounds: standard deviations of five measurements. b) Assumed host-guest stoichiometries. The values
for (Æ)-28a and (Æ)-28b were obtained from two measurements, with a molar excess of the ligand and the ion
exchanger, respectively. c) The ISE membrane conditions do not allow a correct determination of the formation
constant for the second complexation step. d) Calculated by iteration. e) log beIff
.
2L
subsequently calculated from the ratio of the potentiometric selectivity coefficients
‡
obtained with membranes with and without ionophore, with K as the reference ion
[59] (see Table 1 and Exper. Part).
‡
‡
As indicated by Table 2, the complex-formation constants with K and NH4 ions
(0.8 À 2.3 Â 106 and 2.5 À 8.1 Â 104 l molÀ1, resp.) are rather indifferent from the nature
of the C70 mono- and bis-crown ether conjugates. On the other hand, more significant
‡
differences between the various ligands are observed in the complexation of Na and

2036
Helvetica Chimica Acta Vol. 85 (2002)
‡
‡
H ions. The two o×clock conjugate (Æ)-15 forms ca. 20 times weaker complexes with
Na ions than the other crown ether conjugates. Reference compound (Æ)-22, in which
the crown ether is linked to C70 by one arm only, forms significantly stronger complexes
‡
‡
with K and Na ions than the cyclophane-type crown ether conjugates. This
demonstrates that the double bridging in the latter reduces the ability of the
DB18C6 moiety to adopt the most favorable binding conformation. As a consequence,
‡
‡
the binding affinity of the cyclophane-type conjugates toward K and Na ions
eff
decreases by ca. 0.5 and 1.2 logarithmic units in log b , respectively. The stronger
decrease of the binding constant for Na ions and the concomitant improvement of the
IL
‡
‡
K
ion selectivity of the cyclophane-type C70 mono-crown ether conjugates nicely
reflects the fact that the cavity size of the doubly fixed crown ether nearly matches the
‡
‡
diameter of a K ion, while it is too large for a Na ion [37c,d].
The C70 bis-crown ether conjugates may form complexes with both 1:1 and 1:2
‡
host-guest stoichiometry. The respective complex formation constants with K ions
were obtained from two sandwich membrane measurements, one with an excess of
ligand (30 40 mol-% cation exchanger) and one with an excess of cations (170 À
180 mol-% ion exchanger). As indicated by the values of beff and bIefLf , the two binding
IL
2
‡
sites of the bis-crown ether conjugates of C70 have similar affinity for K ions.
Unfortunately, the determination of 1:2 complex formation constants of interfering
ions with C70 bis-crown ether conjugates was not possible, because their complexes are
so weak that, at submillimolar concentrations, the 1:2 complexes exist at very low
concentrations only. Under these conditions, the method is not applicable.
2.5. Electrochemistry. 2.5.1. C70 Mono-Crown Ether Conjugates. To determine the
effects of cation complexation on the redox properties of the crown ether conjugates,
cyclic voltammetric (CV) studies were performed in PhMe/MeCN 4 :1 in the absence
and presence of KPF6, NaBF4, Ba(CF3SO3)2, and NH4PF6 (Fig. 14 and Table 3).
Based on the DEpp values (separation between the potentials of the conjugated
oxidation and reduction peaks), the first fullerene-centered reduction waves of these
conjugates appear to be either reversible or quasi-reversible. However, with the
exception of (Æ)-16, the second reductions are chemically irreversible as judged by the
appearance of additional oxidation waves on the reverse scan. Consequently, cation-
binding effects were monitored for only the first reductions.
Based on the DEpp values and the cathodic/anodic current ratio, the first-reduction
waves of (Æ)-7a and (Æ)-7b are both electrochemically and chemically reversible. It
should be noted that the first-reduction potentials of (Æ)-7a and (Æ)-7b are slightly
cathodically shifted as compared to those of the five o×clock bis-adduct (Æ)-4 bearing
diethyl malonate addends (À1.11 V) [60]. This indicates some electron donation from
the benzocrown ether moiety to the C-sphere, presumably through p-p stacking of the
DB18C6 benzene rings with the fullerene surface. This effect is even more pronounced
for a fullerene with two appended crown ethers, as demonstrated by the differences in
the first-reduction potentials (170 mV) of (Æ)-28a and (Æ)-24 (Table 4). Similar to the
behavior of the previously reported cyclophane-type C60-DB18C6 conjugate [29a,b],
‡
addition of substoichiometric amounts of Na ions to a solution of (Æ)-7a causes the
appearance of a new, poorly resolved, yet clearly discernible redox wave, which is
anodically shifted by 70 mV relative to that for the free compound. This new redox
couple, assigned to the cation complex, grows at the expense of the one for the

Helvetica Chimica Acta Vol. 85 (2002)
2037
Fig. 14. CV Response of (Æ)-7a in the absence (––) and presence ( ¥¥¥¥¥¥ ) of 1 equiv. of NaBF4 at a scan rate of
100 mV sÀ1
‡
Table 3. Redox Potentials of C70 Crown Ether Conjugates (vs. Ferrocene/ferricinium (Fc/Fc ) couple) in the
Absence or Presence of Alkali Metal Cations. Values in parenthesis are the DEpp in mVa).
‡ KPF6
E1/V
‡ NaBF4
E1/V
‡ Ba(CF3SO3)2
E1/V
‡ NH4PF6
E1/V
E1/V
E2/V
(Æ)-7a
(Æ)-7b
(Æ)-15
16
(Æ)-22
(Æ)-28a
(Æ)-28b
(Æ)-25
À 1.13(67)
À 1.14 (68)
À 1.17 (75)
À 1.09 (69)
À 1.04 (63)
À 1.47b)
À 1.57b)
À 1.58b)
À 1.56b)
À 1.46 (73)
À 1.45b)
À 1.72b)
À 1.74b)
À 1.71b)
À 1.05 (65)c)
À 1.07 (68)c)
À 1.09 (68)c)
À 1.07 (66)c)
À 1.03(68)
À 1.06 (68)d)
À 1.06 (67)d)
À 1.09 (68)d)
À 1.04 (63)d)
À 1.05 (83)e)
À 1.06 (82)e)
À 1.06 (99)e)
À 0.98 (84)e)
À 1.08 (68)f)
À 1.09 (68)f)
À 1.10 (86)f)
À 1.09 (67)f)
À 1.31 b)g)
À 1.30 b)h)
À 1.27 (87)h)
À 1.34 b)d)
À 1.44 (83)
À 1.27 (76)g)
À 1.43b)
À 1.33 b)c)
a) Measurements were performed under Ar in PhMe/MeCN 4 : 1 containing 0.1m Bu4NPF6 as the supporting
electrolyte. Concentration of ionophores: 0.2 0.3m m. Salts were dissolved in MeCN and added in ml amounts.
‡
Glassy carbon working electrode. Nonaqueous Ag/Ag reference electrode. Pt wire counter electrode. Scan rate
100 mV sÀ1
.
b) Cathodic peak potential. The reversibility of the first reduction of (Æ)-28a and (Æ)-25 was
difficult to ascertain due to the presence of a small, yet significant redox couple resulting from a minor fullerene
impurity. c) ‡ 1.0 Equiv. of KPF6. d) ‡ 1.0 Equiv. of NaBF4. e) ‡ 1.0 Equiv. of Ba(CF3SO3)2. ) ‡ 1.0 Equiv. of
f
NH4PF6. g) ‡ 2.0 Equiv. of KPF6. h) ‡ 2.0 Equiv. of NaBF4.

2038
Helvetica Chimica Acta Vol. 85 (2002)
‡
uncomplexed species as the concentration of Na ions increases. This was firmly
established by the observation of isopotential points on both the forward and reverse
‡
scans. Addition of 1 equiv. of Na ions causes the redox wave of the free compound to
disappear completely, and further addition causes no further observable changes in the
voltammetry. This behavior implies strong complexation between the crown ether
‡
conjugate and one Na ion [61]. The anodic shift must be attributed to the electrostatic
‡
effect of the Na ion that is bound to the DB18C6 ionophore in close proximity to the
‡
fullerene surface. The behavior with K ions was very similar (Table 3), and the same
was observed for (Æ)-7b and (Æ)-15 with either cation. Similar electrostatic effects have
been observed for fulleropyrrolidinium salts that are reduced at a potential which is
60 mV less negative with respect to C60 [62].
‡
Table 4. Redox Potentials of Higher C70 Adducts with Malonate Addends (vs. Fc/Fc ). Values in parenthesis are
the DEpp in mVa).
E1/V
0.97 (60)
E2/V
E3/V
C70
29
À 1.37 (61)
À 1.44 (67)b)
À 1.47b)
À 1.82 (61)
À 1.84 (59)b)
À 1.88b)
À 1.05 (62)b)
À 1.11 (66)b)
À 1.16 (69)
À 1.30(78)
À 1.40c)
(Æ)-30
(Æ)-23
À 1.61c)
À 1.97c)
(Æ)-24
À 1.60c)
Pentakis-adduct Fraction I
Hexakis-adduct Fraction II
Heptakis-adduct Fraction IV
À 1.66c)
À 1.49c)
À 1.67c)
À 1.50c)
a) Measurements were performed under Ar in PhMe/MeCN 4 : 1 containing 0.1m Bu4NPF6 as the supporting
‡
electrolyte. Concentration of ionophores: 0.2 0.3m m. Glassy carbon working electrode. Nonaqueous Ag/Ag
reference electrode. Pt wire counter electrode. Scan rate 100 mV sÀ1 b) Taken from [60]. c) Cathodic peak
.
potential.
Similar cation complexation-induced effects were also obtained with the twelve
o×clock conjugate 16, but with a much smaller anodic shift of the first reduction
‡
potential in the presence of K ions (20 mV). The smaller shift can be attributed to a
larger average distance between the cation and the fullerene core. This increase in
‡
distance results in a weaker electrostatic interaction. Na -Ion complexation by 16
induces an anodic shift of 50 mV (Table 3).
‡
The addition of NH4 ions results in comparable anodic shifts of the first reduction
potential of compounds (Æ)-7a, (Æ)-7b, and (Æ)-15. On the other hand, the same ions
cause no observable changes in the first reduction of 16. Addition of Ba2‡ ions induces a
shift of 80 mV for compounds (Æ)-7a and (Æ)-7b and 110 mV for (Æ)-15. Interestingly,
Ba2‡-ion complexation also shifts the first reduction of 16 by 110 mV. A shift of this
magnitude was not anticipated given the behavior of 16 in the presence of monovalent
cations. An argument could be made that the increased charge of a divalent cation
induces larger shifts. However, the behavior of (Æ)-7a and (Æ)-7b as well as that
‡
previously reported for the C60-DB18C6 conjugate [29b] indicates that K and Ba2‡
ions cause similar shifts despite the difference in charge density of the two cations. In
the case of the C60-DB18C6 conjugate, this behavior was attributed to the stronger
solvation of the divalent cations, which inhibits their interaction with the crown ether