A Fullerene-Based Molecular Torsion Balance for Investigating Noncovalent Interactions at the C60 Surface

Article abstract of DOI:10.1002/anie.202005888

To investigate the nature and strength of noncovalent interactions at the fullerene surface, molecular torsion balances consisting of C₆₀ and organic moieties connected through a biphenyl linkage were synthesized. NMR and computational studies

Full text of DOI:10.1002/anie.202005888

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Fullerene-Based Molecular Torsion Balance for Investigating
Noncovalent Interactions at C60 Surface
Authors: Michio Yamada, Haruna Narita, and Yutaka Maeda
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Fullerene-Based Molecular Torsion Balance for Investigating
Noncovalent Interactions at C60 Surface
[
a]
[a]
[a]
Michio Yamada,* Haruna Narita, and Yutaka Maeda
Dedicated to Professor François Diederich on the occasion of his retirement
[
a]
Prof. Dr. M. Yamada, H. Narita, Prof. Dr. Y. Maeda
Department of Chemistry,
Tokyo Gakugei University,
Nukuikitamachi 4-1-1, Koganei, Tokyo 184-8501 (Japan)
E-mail:
Supporting information for this article is given via a link at the end of the document.
Abstract: In order to investigate the nature and strength of
noncovalent interactions at the fullerene surface, molecular torsion
balances consisting of C60 and organic moieties connected through a
biphenyl linkage were designed, synthesized, and characterized.
NMR spectroscopy combined with computational studies showed that
the unimolecular system remains in equilibrium between well-defined
folded and unfolded conformers owing to restricted rotation around
the biphenyl C–C bond. The measured energy differences between
the two conformers depend on the substituents and can, in turn, be
ascribed to the differences in the intramolecular noncovalent
interactions between the organic moieties and the fullerene surface.
Notably, the results showed that fullerenes favor interacting with the
p-faces of benzenes bearing electron-donating substituents. The
correlation between the folding free energies and the corresponding
Hammett constants of the substituents in the arene-containing torsion
balances is reflective of the contributions of the electrostatic
interactions and dispersion force to the face-to-face arene–fullerene
interactions.
been reported that porous metal–organic cages are suitable for
fullerene binding.[7] Nevertheless, determining the nature and
strength of the noncovalent interactions between fullerenes and
the functional groups of the organic moiety in question remains
challenging because of the difficulty in observing such weak
interactions and distinguishing between the various factors
contributing to them.[8,9]
The distribution of the electrostatic potentials of C60 is different
from that of benzene. In the case of benzene, the positive
electrostatic potentials are associated with hydrogens while the
negative electrostatic potentials are the strongest above and
below the benzene ring. In contrast, in
60
C , the positive
electrostatic potentials are located above the pentagonal and
hexagonal rings while the negative electrostatic potentials are
located above the edges shared by the two adjacent hexagonal
rings, as demonstrated by Wang et al.[8d] His group also
performed symmetry-adapted perturbation theory (SAPT)[10]
calculations of the noncovalent interactions between benzene
and C60 in the gas phase and showed that the face-to-face
configurations of the benzene–C60 complex are more strongly
bound than the edge-to-face configurations.[8d]
Introduction
Meanwhile, noncovalent arene–arene interactions have been
studied extensively.[11–13]
A key feature of the aromatic
Fullerenes are unique molecules with spherical hydrophobic
structures and curved p-electron systems, and are expected to
find widespread use in a variety of applications in the materials
science and medicinal chemistry fields.[1] Therefore, quantitative
investigations of the noncovalent interactions at the fullerene
surface, which are often very weak, are essential for
understanding the assembled molecular systems of fullerene as
well as its molecular recognition events.[2] The fact that the
fullerene molecule has a unique surface suggests the possibility
of effective noncovalent interactions with aromatic and aliphatic
biomolecular moieties such as proteins, nucleic acid bases, and
antibiotics in biosystems. In addition, the organocatalytic activity
of fullerene conjugates featuring anion–p interactions between
the anionic transition states or the intermediates of the reactants
and the fullerene surface have been reported.[3] Moreover, recent
studies on the supramolecular chemistry of fullerenes have
shown that structurally well-defined molecular receptors exhibit
strong noncovalent interactions with fullerenes to form stable
complexes.[4] In many cases, such fullerene receptors are
constructed through the assembly of p-extended molecules such
as porphyrins[5] and polyaromatic hydrocarbons.[6] It has also
interactions is the fact that arene surfaces can exhibit a variety of
geometries with similar energies. In addition, various
intermolecular forces contribute equally to the arene–arene
interactions. Therefore, noncovalent arene–arene interactions
have become an important topic of study in computational
chemistry. The pioneering work of Hunter and Sanders led to the
development of a qualitative model that states that the strength
and preferred orientation of the arene–arene interactions may be
understood based on the aromatic quadrupole moments and the
electrostatic effects of the substituents can be attributed to the
polarization of the electrons in the attached aromatic ring by the
substituents.[14] Later, Wheeler and Houk introduced the local,
direct-interaction model for the substituent effects in the case of
face-to-face arene–arene interactions, stating that the substituent
effects for these interactions are primarily attributable to the direct,
through-space interactions of the substituent with the proximal
vertex of the other ring.[13f,13j]
In the 1990s, based on experimental studies, Wilcox and
coworkers developed a unimolecular model system, called a
molecular torsion balance, to evaluate edge-to-face arene–arene
interactions using the rigid V-shaped structure of Tröger’s base
1
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Angewandte Chemie International Edition
10.1002/anie.202005888
RESEARCH ARTICLE
as the scaffold.[15] Wilcox’s torsion balance was designed to be in
equilibrium between the well-defined folded and unfolded
conformers based on restricted rotation around the biphenyl C–C
bond. The conformationally flexible ester sidearm allows for the
adjustment of the effective edge-to-face contacts. It has been
suggested that, in Wilcox’s systems, motion in the ester linkage is
not an important factor with respect to folding.[16] By analyzing the
folding energies for torsion balances with different substituents,
Wilcox et al. concluded that the edge-to-face arene–arene
interactions are dominated by dispersion rather than by
electrostatic forces.[15] Since then, the concept of the synthetic
molecular balances has been employed for investigating a range
of noncovalent interactions, given their excellent applicability.
Recent studies have shown that such balances allow for highly
Figure 1) on ring Y. A 1,5-diethoxy-1,5-dioxopentan-3-yl (DDP)
group[22] was introduced at the N-position of the pyrrolidine ring
for solubility reasons. Various substituents were embedded in the
model system in order to elucidate the substituent effects: 1a–1f
contain a benzene ring (labelled ring Z), which is substituted with
different functional groups at the para position (1a: -NMe
OMe, 1c: -H, 1d: -Cl, 1e: -CN, 1f: -NO ), while 1g contains a
cyclohexyl group and 1h contains a methyl group.
2
, 1b: -
2
Y
X
Y
O
O
Me
Me
O
O
R
R
X
EtO2C
N
EtO2C
N
accurate and sensitive measurements of noncovalent
interactions.[17–19] For instance, Shimizu et al. assessed the
K
additivity of the substituent effects for arene–arene interactions
using a torsion balance that could adopt an offset face-to-face
stacking geometry and found that the additive substituent effects
were consistent with the Wheeler–Houk model.[19i] These
successes motivated us to extend the idea of molecular balances
to fullerenes in order to investigate the noncovalent interactions
at the fullerene surface. Thus, in this study, we designed,
synthesized, and characterized the first model system of a
fullerene-based molecular balance to quantitative analyze the
noncovalent interactions at the C60 surface.
EtO2C
EtO2C
Folded 1
Unfolded 1
Figure 1. Fullerene-based molecular torsion balances (1a–1h) designed to
study noncovalent interactions at fullerene surface.
The process for synthesizing the molecular torsion balances,
1
a–1h, is shown in Scheme 1. In short, the esterification of 2-iodo-
3
-methylbenzoic acid with the corresponding phenol or alcohol
derivatives affords iodoarenes 2a–2h. Next, Suzuki coupling with
Results and Discussion
4-formylboronic acid in the presence of PdCl (PPh ) and Na CO
3
2
3 2
2
yields aldehydes 3a–3h. Meanwhile, the reductive amination of
diethyl 1,3-acetonedicarboxylate with glycine benzyl ester
3
Molecular Design and Synthesis
Figure 1 depicts the fullerene-based molecular torsion balances,
hydrochloride in the presence of NaBH CN yields 4, and the
1
a–1h, designed in this study. In its folded conformation, the
subsequent deprotection of the benzyl group results in glycine
derivative 5. Finally, the Prato reaction[23] of C60 with 3a–3h and 5
in toluene affords 1a–1h in yields of 23–44%. The
characterization of 1a–1h was performed using matrix-assisted
laser desorption/ionization-time of flight mass spectrometry and
absorption spectroscopy[23] as well as by NMR spectral analyses.
proposed model system features an intramolecular noncovalent
interaction between the organic moiety (R) and the C60 surface,
which competes with solvation. However, this interaction is
absent in its unfolded conformation. It is expected that ring X is
positioned parallel to the fullerene surface in order to prevent
steric repulsion between the aromatic protons of the ring and the
fullerene surface. This hypothesis is in keeping with the results of
crystallographic analyses of relevant pyrrolidinofullerenes.[20]
Moreover, Nierengarten and coworkers reported that rotation
around the phenyl–pyrrolidine bond in phenyl-substituted
pyrrolidinofullerenes is restricted.[21] The neighboring ring, that is,
ring Y, is preferentially perpendicular to ring X, owing to the steric
repulsion between the aryl protons. Accordingly, it is expected
that the organic moiety, R, which is connected with ring Y through
an ester linkage, becomes close to the fullerene surface in the
folded conformation, in a manner similar to that for the folded
conformation of the Wilcox’s torsion balance. On the other hand,
it is likely that R would move away from the fullerene surface in
the unfolded conformation. It should be noted that, as per density
functional theory (DFT) calculations, which are described later,
the proposed models appear to be suited for face-to-face
O
I
O
I
O
O
a)
b)
Me
Me
R
Me
R
OH
O
O
2
a–h
3a–h
e)
1a–h
EtO
OEt
EtO
OEt
O
HN
O
O
O
O
O
O
HN
O
c)
d)
EtO
OEt
O
O
OH
Ph
4
5
a: R =
e: R =
NMe2
CN
b: R =
f: R =
OMe
NO2
c: R =
g: R =
H
d: R =
Cl
Z
Z
Z
Z
Z
Z
h: R =
Me
Scheme 1. Synthesis of 1a–1h. (a) R-OH, N,N-dimethyl-4-aminopyridine, N,N'-
dicyclohexylcarbodiimide, CH Cl . (b ) 4-Formylboronic acid, PdCl (PPh
Na CO , THF, H O, 80 °C. (c) Glycine benzyl ester hydrochloride, NaBH CN,
MeOH. (d) Pd/C, EtOAc, MeOH, H . (e) C60, toluene, 110 °C.
interactions instead of edge-to-face ones. When rotation around
the biphenyl C–C bond is slow on the 1H NMR timescale, the
2
2
2
3 2
) ,
2
3
2
3
population of the two conformers can be determined by
2
1
integrating the H signals of the methyl group (denoted by “Me” in
2
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
(a)
C63
(b)
H9’
H5’, H6’ H , H6
5
H9
HMe
O
O
1.2
1.1
1.0
H7’, H8’
7
H , H8
H
4
O
O
Y
H3
H2
9(B) or 9’(B)
X
N
1
H
r 2 = 0.98
O
C61
5(A,B), 6(A,B),
5’(A,B), 6’(A,B)
OMe(A) OMe(B)
O
9(A), 9’(A),
9’(B) or 9(B)
HOMe
7
7
(A,B), 8(A,B),
’(A,B), 8’(A,B)
0
0
.9
.8
Me(B)
(A)
3(B)
Me(A)
3
2(A,B),
aromatic protons
A : B = 47 : 53
4(A,B)
-3
3
.5
4.0
4.5 x10
–1
T
/ K
1
(A,B)
2
33 K
(c)
DDP
N
DDP
RO
2
C
H
3N
H
3
2
CO R
H
2
H
1
H
1
H2
A : B = 50 : 50
2
98 K
C61-endo
C61-exo
9
8
7
6
5
4
3
2
1
Chemical shift / ppm
Figure 2. (a) 400 MHz 1H NMR spectra of 1h in CDCl
at 233 K and 298 K. Partial structure of 1h as well as labelling system used are shown. Same labelling
3
system was used for 1a–1g throughout the manuscript, except for HOMe. (b) Plot of N
A 3
/N . (c) Schematic representation of possible
as a function of T–1 in CDCl
B
nitrogen inversion of pyrrolidine ring of folded 1.
NMR spectroscopic characterization and conformational
not equivalent and that the rotation around the N–C63 bond is
slower than the NMR timescale, owing to steric congestion.
As for the pyrrolidine ring of 1h, the resonances of H3 were
observed as two singlets, at 5.46 ppm (H3(A)) and 5.45 ppm (H3(B)),
at 233 K. Because the relative integral ratio of H3(A) to H3(B) was
similar to that of HMe(A) to HMe(B), it is reasonable to conclude that
the splitting of the resonances of H3 into two at 233 K is
attributable to the conformational isomerism between conformer-
A and conformer-B. The two singlets of the H3 resonances
merged together as the temperature was increased and appeared
as one singlet at 283–298 K.
analysis
1
Figure 2a shows the H NMR spectra of 1h in CDCl
3
at 298 K and
2
33K as a typical example of the synthesized torsion balance
systems. At 298 K, several signals appeared as broad
resonances, and these signals became sharper with a decrease
in the temperature. Notably, two sets of resonances for the methyl
(
HMe(A) and HMe(B)) and methoxy (HOMe(A) and HOMe(B)) protons on
ring Y as well as two sets for the CH proton on the pyrrolidine ring
H3(A) and H3(B)) were observed clearly in the spectra; this
(
indicated the presence of two conformers, conformer-A and
conformer-B. The relative integral ratio of conformer-A to
conformer-B was 47:53 at 233 K. The large differences in the
chemical shifts between HMe(A) and HMe(B) (Dd = 0.44 ppm at 233
K) as well as those between HOMe(A) and HOMe(B) (Dd = 0.52 ppm
at 233 K) suggest that the methyl and methoxy groups on ring Y
are located in significantly different environments in the two
2
The resonance of methylene proton H could be distinguished
1
from that of the other methylene proton, H , based on the ROESY
spectrum of 1h at 233 K, wherein a positive cross-peak, that is,
3
2
an ROE correlation, was observed between H and H . On the
other hand, there was no correlation between H3 and H . The
1
1
2
resonance of H also appeared as two doublets, at 5.12 ppm ( J
conformers. The two sets of the resonances (HMe(A) and HMe(B)
,
= 8.7 Hz) and 5.11 ppm ( J = 8.7 Hz), at 233 K. Further, these two
2
and HOMe(A) and HOMe(B)) do not coalesce in the 233–298 K
range.[24] In the rotating frame Overhauser enhancement
spectroscopy (ROESY) spectrum of 1h at 233 K, negative cross-
peaks were observed between HMe(A) and HMe(B) as well as
between HOMe(A) and HOMe(B), indicating that a chemical exchange
had occurred between conformer-A and conformer-B over the
NMR timescale. The relative populations of conformer-A and
doublets merged as the temperature was increased, appearing as
2
a doublet ( J = 8.7 Hz) at 283–298 K. Similar trends were also
2
observed for the resonance of H , though, in this case, the signals
overlapped with the other proton signals (at 233 K, the resonance
2
of H appeared as a triplet because of overlapping with the other
resonances).
Notably, the chemical shifts for H1 and H2 were markedly
different in the range of 233–298 K (Dd = 0.60–0.64 ppm)
regardless of the temperature, indicating that H1 and H were
positioned in very different environments. In addition, the
pyrrolidine protons would exhibit four sets of resonances if the
nitrogen inversion of the pyrrolidine ring were to yield two
observable conformers (see Figure 2c),[25] because both
conformer-A and conformer-B would split into two. However, this
was not the case. Therefore, we surmised that the relative
difference in the energies of one pyrrolidine conformer and the
conformer-B (i.e., N
A
and N
B
, respectively; N
A
+ N
B
1
= 1) were then
2
determined by the integration of the Lorenz-fitted H NMR signals
of HMe(A) and HMe(B) at different temperatures. As shown in Figure
B A
b, N /N
of 1h was linearly dependent on T–1 in the range of 233–
2
2
2
98 K (r = 0.98).
Meanwhile, the four sets of triplets (three of which overlapped
1
with each other) in the H NMR spectrum of 1h at 233 K could be
9
assigned to the methyl protons of the ethoxy moieties, labeled H
9’
and H , indicating that the two ester units of the DDP group are
3
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Angewandte Chemie International Edition
10.1002/anie.202005888
RESEARCH ARTICLE
other nitrogen-inverted conformer is large enough such that only
one side of the pyrrolidine conformers is responsible for the
observed NMR resonances. This conclusion is further supported
by the results of theoretical calculations, as described later. Thus,
it can be concluded that the conformational isomerism between
conformer-A and conformer-B comes from the restricted rotation
around the biphenyl C–C bond between rings X and Y.
der Waals radii of the relevant nuclei (~2.9 Å). In a related work,
Nishio et al. reported that the C–H···fullerene distance was
determined to be 2.9 Å or lower (as low as 2.5 Å), as per a survey
by the Cambridge Crystallographic Database.[29] The DFT
calculations also suggested that the optimized structure of the
folded conformation of 1a has the face-to-face configuration, as
shown in Figure 4. The smallest C···C distance between ring Z
and the fullerene surface was calculated to be 3.0 Å. The nitrogen
atom of the N,N-dimethylamino group on ring Z was also located
close to the fullerene carbon atom, and the interatomic distance
was 3.2 Å. These results suggest that the rigid framework of the
proposed model system ensures intramolecular face-to-face
arene–fullerene interactions, in keeping with expectations.
To gain further insights into the conformational behaviors, DFT
calculations were performed at the M06-2X[26] level using the 6-
3
1G(d)[27] basis sets. The effects of chloroform as the solvent were
Figure 4. Two orthogonal views of optimized structure of folded conformation
of 1a determined using M06-2X[26]/6-31G(d)[27] level of theory. Effects of
chloroform as solvent were introduced through self-consistent reaction field
theory calculations performed using IEFPCM method.[28]
1
The H NMR chemical shifts were calculated for structures I and
III at the GIAO[30]-B3LYP[31]/6-31G(d) level of theory using the
M06-2X[26]/6-31G(d)[27] optimized geometries.[32] We focused on
the HMe protons as the indicators of the conformers, because the
methyl group is the one least affected by the local ring currents of
the C60 core[ and the flexible DDP moieties. The calculated
33]
1
Me
average H chemical shift of the H protons in structure I appears
at a lower magnetic field (2.06 ppm) than that in structure III (1.91
ppm). Therefore, it is suggested that conformer-A and conformer-
B are the unfolded and folded conformers, respectively. Based on
this assignment, the folding free energies (DGfold) can be obtained
from Equation (1).
Figure 3. Optimized structures and relative energies (in kJ mol–1) of four
possible conformers (structures I, II, III, and IV) of 1h determined using M06-
X[26]/6-31G(d)[27] level of theory. Effects of chloroform as solvent were
2
introduced through self-consistent reaction field theory calculations performed
using IEFPCM method.[28]
introduced based on self-consistent reaction field theory
calculations performed using the IEFPCM method.[28] Figure 3
shows four possible conformers of 1h as representative molecular
torsion balances as well as their relative energies.
DGfold = – RT ln (N
B
/N
A
)
(1)
The DGfold value of 1h (0.06 kJ mol–1 in CDCl
3
) is close to zero,
indicating that the relative energies of conformer-A and
conformer-B are almost identical. Thus, it is reasonable to
conclude that, because of the negligibly weak interaction between
R and the fullerene surface, conformer-A and conformer-B are
almost isoenergetic in 1h and that the very small DGfold value may
be attributable to the solvent effects and long–range polar
interactions.
Structures I and II correspond to the folded conformers while
structures III and IV correspond to the unfolded conformers with
respect to the restricted rotation around the C–C bond between
rings X and Y. In addition, structures I and III exhibit the C61-endo
61
form whereas structures II and IV exhibit the C -exo form with
respect to the nitrogen atom of the pyrrolidine ring. The results
6
1
–1
indicated that the C -exo conformers are 37–46 kJ mol less
1
The H NMR spectra of 1a–1h at 233 K exhibited very similar
61
stable than the C -endo conformers. Thus, it is reasonable to
1H chemical shifts and spin–spin coupling patterns for the protons
61
1
conclude that the contribution of the C -exo conformers to the H
common to 1a–1h. This suggested similar conformational
isomerism, that is, the existence of only two NMR-detectable
conformers (conformer-A and conformer-B), in 1a–1h (see Figure
S111). Thus, we assigned the respective conformers-A and -B of
61
NMR spectra is negligible and that the C -endo forms are the
dominant ones. In addition, structures I and III of 1h have similar
1
stabilities. These results are in good agreement with those of H
NMR spectral studies, which showed that the differences in the
free energies of 1h are small. The separation between the methyl
proton and the nearest carbon of the fullerene cage in structure I
was calculated to be 2.8 Å, which is similar to the sum of the van
1
a–1h based on the similarities in the chemical shifts as well as
the patterns in their 1H NMR spectra. The folding free energy,
DGfold, values at 298 K in CDCl and C , obtained from Eq. 1,
3
6 6
D
are listed in Table 1. The corresponding Hammett constants
4
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Angewandte Chemie International Edition
10.1002/anie.202005888
RESEARCH ARTICLE
(
s
para),[34] and experimental polarizability (a) values[35,36] of the
p
The high linearity of the correlation of DGfold with s in the aromatic
2
2
corresponding R-H molecules for 1a–1h are also shown in the
table. Finally, Figure 5 shows the relationship between the
individual DGfold values and the Hammett constant for the
substituent R.
3 6 6
systems both in CDCl (r = 0.83) and in C D ( r = 0.95) can be
explained based on electrostatic interactions. These interactions
direct the molecular torsion balances to the folded conformers and
are counterbalanced by the Pauli repulsion terms. The results
also suggest that the noncovalent interactions between the arene
moieties and the fullerene surface become more attraction-based
as the p-electron density of R increases. This trend is in contrast
to that observed in the case of the face-to-face arene–arene
interactions, wherein electron-withdrawing substituents lead to
the stabilization of the interactions while electron-donating
substituents destabilize the interactions. The magnitude of the
p
Table 1. Folding free energy (DGfold) values of 1a–1h; Hammett constant, s , for
substituents in ring Z; and polarizability (a) values of corresponding R-H
molecules
DGfold[a] in
DGfold[a]
[b]
Compd.
in
s
p
a
CDCl
3
6 6
C D
–1
positive slope (0.76 kJ mol ) is smaller than those measured by
1
1
1
1
1
1
1
1
a (R = C
b (R = C
c (R = C
d (R = C
e (R = C
f (R = C
6
H
4
NMe
2
)
–0.20
0.18
0.78
0.43
1.14
0.91
1.02[c]
0.06
–0.46
–0.07
0.71
– [d]
–0.83
–0.27
0.00
16.3[f]
13.1[f]
10.4[f]
–1
2
[19a]
Gung et al. (–3.24 kJ mol ; r = 0.94),
and Shimizu et al. (–
–1
2
[19i]
0
.80 kJ mol ; r = 0.41 for the phenanthrene system)
for flat
6
H
4
OMe)
aromatic surfaces that face-to-face stack in CDCl
3
. Diederich et
6
H
5
)
al. also observed a linear correlation between the association
constants and the s values in a bimolecular host–guest model
p
12.4[f]
6
H
4
Cl)
0.23
system consisting of a Rebek-imide-type receptor; this correlation
was indicative of the electrostatic effects of the substituent on the
aromatic platform of the receptor with respect to the parallel-
displaced face-to-face arene–arene interactions, as predicted by
the Hunter–Sanders model.[37] The opposite correlations of the
12.6[f]
6
H
4
CN)
1.68
1.46
– [d]
0.66
13[f]
6
H
4
NO
2
)
0.78
g (R = cyclohexyl)
h (R = methyl)
N/A[e]
N/A[e]
11.0[f]
2.6[g]
folding energies with s
p
values between the arene–arene
0.22
interactions and arene–fullerene interactions could be related to
the unique electrostatic potentials and the strong electron-
accepting ability of C60. In the case of 1a, DGfold is negative,
indicating that the attractive electrostatic interactions overcome
the repulsive interactions to direct the folded conformer more
[
a] All data are average values in kJ mol–1 obtained from multiple
–1
3
measurements. Uncertainty is ±0.12 kJ mol . [b] Ref. 34. [c] Integral ratio of H
signals instead of those of HMe were used for determining thermodynamic
parameters because of overlapping of HMe and proton signals of cyclohexyl
group. [d] Data could not be obtained because solubility was too low. [e] Not
preferably in CDCl
3
. The substituent effect is more pronounced in
3
3
applicable. [f] Values in Å ; Ref. 35. [g] Values in Å ; Ref. 36.
C
6
D
6
than that in CDCl
3
. The fact that the substituent effect is
stronger in the apolar solvent than in the polar one can be
explained based on the solute–solvent interactions.[18b,38] This is
in keeping with the distinct contribution of the electrostatic
interactions between the organic moieties and the fullerene
surface. In fact, the DGfold value of 1b becomes negative when the
Substituent effects
It is worth noting that the DGfold values varied with the choice of R
in 1a–1h. Based on the similarity of the structures of 1a–1h, we
propose that the noncovalent interactions between R and the
fullerene surface are responsible for the differences in the DGfold
values. The observed positive DGfold values indicate that the
primary contributors are the Pauli repulsion terms, which direct
the molecular torsion balances to the unfolded conformers, in the
solvent is changed from CDCl
3 6 6
to C D . This trend contrasts with
that observed for the complexation between
C
60 and
calix[4]arene-linked bis-porphyrins reported by Boyd and
coworkers. In their system, the binding constant and fullerene
solubility were found to be inversely correlated, indicating that
desolvation of the fullerene was a major factor determining the
magnitude of the binding constant.[5e] The DGfold value is larger
when ring Z is unsubstituted (i.e., 1c) as compared with that for
the chloro-substituted system, 1d. This deviation from the
case of 1b–1g in CDCl
3
. As shown in Figure 5, the DGfold values
were correlated not with the a values of the corresponding R-H
molecules but with their s
p
values in the aromatic systems.
2
1
1
0
0
.0
.5
.0
.5
.0
CN
p
correlation between DGfold and s could be due to the difference
NO2
in the dispersion interactions and/or the absence of direct
interactions between the substituent of the arene ring and the
fullerene surface. In a related study, Wheeler and Houk showed
that, in the case of the correlation between the computed gas-
phase interaction energies and the Hammett constants for
sandwich dimers of benzenes, there exists an outlier, namely, the
unsubstituted benzene dimer.[13f] They stated that this deviation
was attributable to the dispersion interactions, which supposedly
stabilize all types of substituted benzene dimers, in contrast to the
case for the unsubstituted benzene dimer. In fact, the R-H for 1c
exhibits smaller polarizability values than those for 1a, 1b, 1d, 1e,
and 1f. The DFT-calculated folding free energies for 1a–1f using
the M06-2X[26]/6-31G(d)[27] level of theory and the IEFPCM
method[28] are essentially reproduce the experimental trends, as
shown in Figure S120. This observation suggests that substituted
H
Cl
OMe
-0.5
NMe2
-
1.0
-0.5
0.0
σpara
0.5
1.0
3 6 6
Figure 5. Experimental DGfold values in CDCl (open circles) and C D (solid
circles) plotted against Hammett constant spara of respective substituents in ring
Z.
5
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Angewandte Chemie International Edition
10.1002/anie.202005888
RESEARCH ARTICLE
arenes having negative and positive s
p
values favor folded and
Keywords: fullerenes • conformational analysis • pi interactions
unfolded conformers, respectively, while unsubstituted arenes
highly disfavor folded conformers. The large DGfold value of 1g can
also be explained based on the low polarizability of the cyclohexyl
moiety, which contributed to the dispersion term. It is likely that
the possible edge-to-face aliphatic CH–fullerene interaction is
overwhelmed by the repulsive terms. On the whole, the
substituent effects observed in these experiments suggest that
the electrostatic interactions have a significant influence on the
arene–fullerene interactions.
•
electrostatic interactions • London dispersion force
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Conclusion
In summary, we attempted to quantify the noncovalent
interactions between organic moieties and the fullerene surface
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Nierengarten et al. performed SAPT calculations on metalloporphyrin–
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Entry for the Table of Contents
Measurement of
Arene–Fullerene
Interactions
Fullerene-based molecular torsion balance is constructed for investigating noncovalent arene–fullerene interactions for the
first time. Results show that the interactions are weak but measurable. The linear correlation between the difference in the
free energies of two well-defined conformers and the Hammett constants of the substituents on the arene moieties indicate
that the electrostatic interactions contribute significantly to the overall interactions.
9
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