Formation of singly bonded PhCH2C60-C 60CH2Ph dimers from 1,2-(PhCH2)HC60 via electroreductive C60-H activation

Article abstract of DOI:10.1021/jo2006768

Singly bonded PhCH₂C₆₀-C₆₀CH₂Ph dimers are generated via controlled-potential bulk electroreduction and electrooxidation of 1,2-(PhCH₂)HC₆₀. The reaction mixture was purified by HPLC, and the isolated fraction was characterized with single-crystal X-ray diffractions, ¹H and ¹³NMR, MS, elemental analysis, and cyclic voltammetry. It was shown that the fraction consists of two HPLC-inseparable PhCH₂C₆₀-C ₆₀CH₂Ph regioisomers, which are assigned as the meso and racemic regioisomers. The bulk electrolysis processes for the formation of the dimers were followed by in situ cyclic voltammetry and were further corroborated with an in situ voltammetric titration of 1,2-(PhCH₂)HC₆₀ with tetra-n-butylammonium hydroxide (TBAOH), on the basis of which a reaction mechanism is proposed.

Full text of DOI:10.1021/jo2006768

ARTICLE
pubs.acs.org/joc
Formation of Singly Bonded PhCH2C60ÀC60CH2Ph Dimers from
1,2-(PhCH2)HC60 via Electroreductive C60ÀH Activation
Wei-Wei Yang, Zong-Jun Li, and Xiang Gao*
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
5625 Renmin Street, Changchun 130022, Jilin, China
S Supporting Information
b
ABSTRACT: Singly bonded PhCH2C60ÀC60CH2Ph dimers are generated
via controlled-potential bulk electroreduction and electrooxidation of
1,2-(PhCH2)HC60. The reaction mixture was purified by HPLC, and the
isolated fraction was characterized with single-crystal X-ray diffractions, 1H
and 13NMR, MS, elemental analysis, and cyclic voltammetry. It was shown
that the fraction consists of two HPLC-inseparable PhCH2C60ÀC60CH2Ph
regioisomers, which are assigned as the meso and racemic regioisomers. The
bulk electrolysis processes for the formation of the dimers were followed by
in situ cyclic voltammetry and were further corroborated with an in situ
voltammetric titration of 1,2-(PhCH2)HC60 with tetra-n-butylammonium
hydroxide (TBAOH), on the basis of which a reaction mechanism is
proposed.
’ INTRODUCTION
contact properties than thiols, demonstrating a promising po-
tential for the compounds to be used as molecular wires.16 Singly
bonded pristine C60 has been reported; however, such com-
pound is stable only at low temperature and can exist only as
the anionic form (C60À)2 due to the existence of unbonded
electrons.11 On the contrary, singly bonded C60 dimers of the
RC60ÀC60R type are much more stable and are formed involving
The functionalization of fullerenes, a family of nanosized
carbon molecules with three-dimensional geometry and convex
surface represented by C60, has attracted great interest due to
their unique geometric and electronic structures.1 Among var-
ious fullerene derivatives, organo(hydro) fullerenes2 are of
particular interest, since they are promising precursors for
fabrication of novel organofullerenes with the breakdown of
the CfullereneÀH bond.3,4 In fact, the CÀH activations for
construction of CÀC bonds have been the pivotal focus in
organic chemistry, which has led to a great number of organic
transformations.5 However, the activation of the C60ÀH bond is
still in its infancy and is achieved mainly with the utilization of
strong bases by taking advantage of the acidic property of the
fullerenyl H atom6 due to the strong electron deficiency of C60.7
Considering the diversity of the protocols that have been
developed for CÀH activations during the past several decades5
and the promising potential of organofullerenes for various
applications,8 it is of both interest and importance to explore
new methodologies for the activation of C60ÀH bond, which has
a relative low bond dissociation energy of about 70 kcal/mol.2d,9
C60 dimers have been a subject of intense interest of fullerene
study,10À12 because such structures are potential building blocks
for fullerene polymers.13 In addition, the C60 dimers have also
displayed interesting electronic14 and optically switchable15
properties due to their unique structures. For example, the two
C60 cages in the C60-TTF-C60 (TTF = tetrathiafulvalene) dumb-
bell dimer triads have exhibited a cooperative effect that doubles
the lifetime of the corresponding charge-separated state in
comparison to the analogous dyad.14 Work on molecular elec-
tronics has shown that C60 dimers have better metal-molecule
the RC60 .
• 12 Komatsu and co-workers12d have shown a control-
lable approach that utilizes the stepwise nature of the reaction of
C602À with organic halides17 to produce the RC60À intermediate,
which affords the RC60ÀC60R dimer with subsequent I2 oxida-
tion. However, it is desirable that the R group is a sterically bulky
group in order to avoid the consumption of RC60À intermediate
via further SN2 addition of organic halides,17 which may restrict
the types of R suitable for the synthesis of singly bonded C60
dimers. Recently, Itami and co-workers have reported a palladium-
catalyzed activation of organo(hydro)fullerenes for the carbonÀ
carbon bond formation,4 which is capable of preparing singly
bonded RC60ÀC60R dimers but is also applicable only for bulky
R group. Notably, Meier and co-workers have shown that the
fullerenyl H can be cleaved from C60 cage for C60H2 upon
electroreduction;18 however, no further functionalization work
utilizing such C60ÀH activations has appeared to date. Inspired
by such results, we are exploring new methodologies for fullerene
functionalization based on electroreductive C60ÀH activations.
Herein, we report the formation of singly bonded PhCH2C60À
C60CH2Ph dimers via electroreduction and electrooxidation of
1,2-(PhCH2)HC60, where the C60ÀH bond is cleaved after
Received: March 31, 2011
Published: June 13, 2011
r
2011 American Chemical Society
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Figure 1. HPLC trace of the reaction mixture obtained by electroox-
idation of the dianionic 1,2-(PhCH2)HC60 solution. The mixture
was eluted with toluene over a Buckyprep column with a flow rate of
4.0 mL/min. The detector wavelength was set at 380 nm.
Figure 2. ORTEP diagram for the meso form of singly bonded
PhCH2C60ÀC60CH2Ph dimer molecule with 50% thermal ellipsoids.
Hydrogen atoms and the solvent molecules are omitted for clarity.
Scheme 1. Illustrated Structures of the Meso and Racemic
RC60ÀC60R Dimers
Only a very small amount of the starting material, 1,2-(PhCH2)
HC60, is shown in the chromatogram, demonstrating the high
reactivity of 1,2-(PhCH2)HC60 under the experimental condi-
tions. The major fraction is eluted out from the column with a
retention time of 10.2 min, which is identified as an inseparable
mixture of PhCH2C60ÀC60CH2Ph dimers, and the isolated yield
of the dimers is 36%. The remainder of the reaction mixture is
insoluble in toluene or CS2 and remains to be identified.
In principle, there are meso and racemic regioisomers for the
singly bonded C60 dimers4,12cÀ12f as shown in Scheme 1. Com-
putational calculations using the Gaussian 03 program21 at the
HF (Hatree-Fock)/3-21G level predict that the energies for the
meso and racemic PhCH2C60ÀC60CH2Ph isomers are À5053.4421
and À5053.4431 au (1 au = 627.5095 kcal/mol), respectively,
which corresponds to a difference of about 0.6 kcal/mol between
the two regioisomers, implying that both regioisomers can be
formed as a result of the small energy difference.
The coalescence of two C60 cages is confirmed by X-ray single-
crystal diffraction. Crystals suitable for X-ray diffractions were
obtained from a saturated benzene solution stored at about 5 °C.
Only crystals of the meso regioisomer were resolved, which is
similar to the previous cases.12d,f There are two orientations for
the connected C60 cages, with each C atom disordered over two
sites. Each cage has an occupancy factor of 0.5. The asymmetric
unit consists of half of the molecule, with the remaining parts
generated by an inversion center. For clarity, only one orienta-
tion of the molecule is shown in Figure 2.
As is shown in Figure 2, two C60 cages are connected by a
singly bond at C1 and C1* with a bond length of 1.64(5) Å, which
is longer than the typical CÀC bond length (1.541 Å)22 and
slightly longer than that for the singly bonded C60 dimer bearing
diethoxyphosphorylmethyl groups (1.576 Å)12d and the one with
phosphonate esters (1.582 Å).12f The centroid-to-centroid dis-
tance of the two connected C60 cages is only 9.265 Å, which is
significantly shorter than that between C60 cages interacted with
weak intermolecular bonding as observed in the crystal packing
of C60 derivatives.23 In addition, it shows that two benzyls are
bonded to C60 with a 1,4-addition pattern at C4 and C4*.
The dimeric fraction is further characterized with NMR,
MALDI-TOF MS, UVÀvis, and cyclic voltammetry, which con-
firm the formation of the PhCH2C60ÀC60CH2Ph regioisomers. It
is worthwhile to mention that the obtained compound is reactive
transferring two electrons to the organo(hydro)fullerene. Since
there is no size restriction for the R group of C60HR as shown
in the literature,2À4,19 the protocol is expected to be more
versatile and controllable for the production of singly bonded
RC60ÀC60R dimers.
’ RESULTS AND DISCUSSION
Preparation and Characterization of Singly Bonded
PhCH2C60ÀC60CH2Ph Dimers from 1,2-(PhCH2)HC60. 1,2-
(PhCH2)HC60, which was obtained following procedures de-
scribed previously,19 was reduced with controlled-potential bulk
electrolysis (CPE) at À1.10 V vs SCE in benzonitrile (PhCN)
solution containing 0.1 M tetra-n-butylammonium perchlorate
(TBAP). After the theoretical Coulomb number for two-electron
transfer was completed, the solution was oxidized back to neutral
with a potential of 0 V vs SCE. The solution color changed from
magenta (neutral compound) to green after the electroreduction
was over, which is the typical color for anionic monosubstitued
C60 intermediates,3d,20 suggesting that the fullerenyl H was
cleaved from the C60 sphere (in either neutral or ionic form).
The solution color changed to brown after the green anionic
solution was electrooxidized back to neutral. The workup for the
purification of the dimer products is similar to that for the
isolation of C60 derivatives from the reactions of anionic C60.19
The toluene-soluble part of the reaction mixture was subjected to
HPLC for purification eluted with toluene over a Buckyprep
column. Figure 1 shows the HPLC trace of the crude product.
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Figure 3. 1H NMR spectrum of the HPLC dimer fraction recorded in o-
DCB-d4. Resonances labeled with asterisks are due to the toluene
residue used for HPLC purification. The inset shows the expanded
region from 4.4 to 5.0 ppm.
Figure 4. Cyclic voltammogram of the singly bonded PhCH2C60À
C60CH2Ph dimers in benzonitrile containing 0.1 M TBAP with a scan
rate of 0.1 V/s.
toward CS2, which is the most common solvent used for
organofullerene NMR measurements. It changed to a green
color and precipitated when the compound was put into CS2.
The NMR of the compound is therefore measured in o-dichlor-
obenzene-d4 (o-DCB-d4), which has a much better solubility for
the product than toluene. Figure 3 shows the 1H NMR spectrum
of the isolated HPLC dimer fraction. It exhibits two sets of AB
quartets having different intensities centered at 4.70 and 4.66
ppm, with δAB of 150 and 106 Hz, respectively, corresponding to
the methylene protons of the benzyls. The intensity variance for
the two AB quartets unambiguously indicates the presence of two
different products in the HPLC fraction. In addition, the
appearance of AB quartet indicates that the methylene protons
are diastereotopic to each other, suggesting that the benzyls of
each product are both likely positioned with a 1,4-addition
pattern,24 in agreement with the X-ray single-crystal diffraction
results. Resonances corresponding to the phenyl rings of the
benzyls are also observed in the region from 7.31 to 7.77 ppm,
consistent with the addition of benzyls on the obtained com-
pounds. It is noteworthy that the resonances of the aromatic
protons for the two products are coincidentally superimposed
with each other, implying that the two regioisomers are structu-
rally very similar to each other. Additional resonances from 7.10
and 7.25 ppm are due to the toluene residue used for HPLC
purification, suggesting the existence of strong interactions
between the dimers and toluene, and the molar ratio between
the dimer and the toluene is 1:1 based on the integration area.
As for the 13C NMR of the fraction, it shows resonances at
68.26, 61.86, 50.97, and 50.75 ppm in the sp3 region. Resonances
at 50.97 and 50.75 ppm are due to the benzylic carbon atoms
according to previous work.24 Due to the symmetry nature of
the meso and racemic isomers, each compound is expected to
show only one resonance for the two benzylic carbons. The
appearance of two closely positioned resonances for the benzylic
carbons is consistent with the formation of two structurally
similar regioisomers. As for the other two resonances, the one
at 61.86 ppm is ascribed to the sp3 C60 carbon atom bonded to
the benzyl group (60.2 ppm for the same carbon atom in 1,4-
dibenzyl C60),24 and the one at 68.26 ppm is attributed to the sp3
C60 carbon atom bonded to the other C60 cage, where the
exhibited downfield shift (68.26 vs 61.86 ppm) is likely due to the
electron-withdrawing inductive effect of the electron-deficient
C60 cage. For the meso and racemic isomers, the chemical shifts
for the respective sp3 C60 carbon atoms bonded to the benzyls are
coincidentally overlapped, as well as those bonded to the C60
cages, demonstrating once again the structural similarity between
the two regioisomers.
In the sp2 13C NMR region, a total of 72 resonances are shown
from 157.9 to 136.6 ppm, which are ascribed to the sp2 C60
carbon atoms.19,24 Notably, many of these resonances appear in
pairs, consistent with the presence of two regioisomers. Due to
the intense resonances from the solvent, resonances from the
phenyl groups of the dimer compounds and also possibly several
sp2 C60 carbons with the most upfield chemical shifts are masked
and cannot be clearly identified. However, the appearance of
the 72 resonances is sufficient to show there are two HPLC-
inseparable dimeric regioisomers formed from the reaction.
The MALDI-TOF MS of the HPLC fraction shows the
protonated molecular ion [M + H]+ at 1623.1, consistent with
the formation of the dimer product. Elemental analysis of the
product shows the composition is 98.38% C and 1.67% H, which
is consistent with the formation of M C6H5CH3 (C141H22; calcd
3
C, 98.71%; H, 1.29%) as evidenced by the 1H NMR spectrum.
The UVÀvis spectrum of the isolated fraction displays absorp-
tion bands at 362 and 450 nm, out of which the broad band
at 450 nm is characteristic for C60 1,4-adducts.25 The results
therefore confirm the formation of two HPLC-inseparable singly
bonded C60 dimers with 1,4-addition pattern by electroreduction
and electrooxidation of 1,2-H(PhCH2)C60.
Figure 4 shows the cyclic voltammogram of the singly bonded
C60 dimers in benzonitrile containing 0.1 M TBAP at a scan rate
of 0.1 V/s, which is in general consistent with the one obtained
previously for the C60 dimers bearing diethoxyphosphorylmethyl
groups.12d It exhibits two reversible redox processes with E1/2
at À0.94 and À1.36 V and an irreversible redox couple with the
Epc at À0.50 V and Epa at À0.06 V. The large peak-to-peak
separation for the first irreversible redox couple (∼440 mV)
indicates that the two species involved in this redox transforma-
tion are different as the case observed previously for a C60
heterocyclic derivative,26 suggesting that there is a chemical
reaction accompanying the electron-transfer process in either
direction.
As is shown in Figure 4, the first reduction wave is relatively
broad, which is likely due to the reduction potential difference
between the meso and racemic dimers and also the involvement
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Figure 5. Cyclic voltammograms of 1,2-(PhCH2)HC60 (a) in neutral state, (b) after transferring one electron to 1,2-(PhCH2)HC60 with CPE at À0.70
V, (c) after transferring two electrons to 1,2-(PhCH2)HC60 with CPE at À1.10 V, and (d) after oxidizing the anionic solution shown in panel c back to
neutral with a potential at 0 V. The arrows indicate the initial potential and the scan direction for the cyclic voltammetry measurement. The
measurements were carried out in benzonitrile containing 0.1 M TBAP at a scan rate of 0.1 V/s.
of a chemical reaction. As for the second and the third redox waves,
they are both quite narrow and reversible, suggesting the presence
of only one species in the solution. Itisthereforerational to assumÀe
that the dimers dissociate into anionic monomeric PhCH2C60
after they are reduced via receiving two electrons. The reversible
potentials are closely related to the HOMO and LUMO energies,
the results indicate that the HOMO of the PhCH2C60À should
be much lower than that of the 1,2-(PhCH2)HC60À•, which will
require more energy (more positive potential) to remove the
electron from PhCH2C60À, while the LUMO of the two species
should be comparable. Computational calculations with the
Gaussian 03 program21Àat B3LYP/6-31G level29 predict the
HOMO of PhÀC•H2C60 is À1.71 eV and the one for 1,2-
(PhCH2)HC60 is À1.06ÀeV, whereas for the LUMO, it
is À0.19 eV for PhCH2C60 and À0.07 eV for 1,2-(PhCH2)
HC60À•. The results imply a difference of 0.65 eV between the
HOMOs of the two species, but a difference of only 0.12 eV
between the LUMOs of two species, which is consistent with the
significant positive shift observed for the oxidation potential of
redox couples at À0.94 and À1.36 V consequently correspond to
2À•
the transformation of PhCH2C60À/PhCH2C60
and PhCH2-
C602À•/PhCH2C60 respectively. The oxidation wave with Epa
3À
À
of À0.06 V is likely due to the oxidation of PhCH2C60 into
PhCH2C60•, followed by the radical combination into the meso
and racemic dimers. The broad andflat nature of this wave suggests
it is likely an EC (electron-transfer chemical) process.27
Compared with the reduction potential of 1,4-(PhCH2)2C60
(Epc = À0.54 V vs SCE, see Supporting Information), the dimers
are easier to be reduced (Epc = À0.50 V vs SCE), consistent with
PhCH2C60À and the small variance of the reduction potentials
À•
between PhCH2C60À and 1,2-(PhCH2)HC60
.
the strong electron-withdrawing inductive effect of C60 cage. It is
In Situ Electrochemical Study of the Controlled-Potential
Bulk Electrolysis. To have a better understanding of the reaction
mechanism, the CPE was followed by in situ cyclic voltammetry.
Figure 5 shows the cyclic voltammogram of the neutral 1,2-
(PhCH2)HC60 (the starting material), the in situ cyclic voltam-
mograms recorded after transferring one and two electrons
to 1,2-(PhCH2)HC60 via CPE, and that after oxidation of the
dianionic solution back to neutral with a potential at 0 V vs SCE.
The neutral 1,2-(PhCH2)HC60 undergoes three reversible one-
electron transfer redox processes within the electrochemical
À
intriguing that the oxidation potential of PhCH2C60 (Epa
=
À0.06 V) is much positively shifted as compared with that of 1,
À•
2-(PhCH2)HC60 (Epa = À0.49ÀV, see below), whereas the
reduction potential of PhCH2C60 (E1/2 = À0.94 V) remains
almost identical to that of 1,2-(PhCH2)HC60À• (E1/2 = À0.94 V,
see below). Similar positive potential shift has been observed
during the electrochemical study of malonated fullerenes,
which has been proposed as evidence for the formation of singly
bonded fullerene dimers.28 Since the first oxidation and reduction
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window of the solvent as shown in Figure 5a. The three anionic
products of the reduction are stable on the cyclic voltammetric
time scale, with the E1/2 at À0.54, À0.94, and À1.50 V vs SCE,
which are positively shifted with respect to the E1/2 of 1,2-
(PhCH2)2C60 (À0.62, À1.04, and À1.57 V vs SCE),24b reflect-
ing the difference of electron donation ability between the H
atom and the benzyl group.
Figure 5b shows the in situ cyclic voltammogram of the
reduced 1,2-(PhCH2)HC60 species after bulk electrolytic reduc-
tion at À0.70 V, which is suitable for transferring one electron to
the molecule. The voltammetry scan was started at the bulk
electrolysis potential of À0.70 V, since this is supposed to be the
equilibrium potential (I = 0 μA) for the produced bulk anionic
species. In fact, it shows that the initial current is very close to
zero, indicating that the bulk anionic species in solution have one
formal negative charge as expected. It is noteworthy that the
current heights for both the first redox couple at À0.54 V and the
third one at À1.50 V are decreased; in the meanwhile, the redox
couple at À0.94 V remains almost unchanged. An extra reversible
redox couple labeled with asterisks appears at E1/2 of À1.36 V,
Figure 6. Cyclic voltammograms for the titration of 1,2-H(PhCH2)C60
with TBAOH (top to bottom, 1,2-(PhCH2)HC60/TBAOH molar
ratio = 0, 1.0, 1.5, 2.0, 4.0) in benzonitrile containing 0.1 M TBAP at
a scan rate of 0.1 V/s. The scans were started from 0 V toward negative
potential.
along with a new irreversible oxidation wave at À0.05 V, which
2À•
are likely to correspondÀto the reduction of PhCH2C60
and
oxidation of PhCH2C60 as compared with the cyclic voltam-
mogram in Figure 4, indicating that part of Àmonoanionic 1,2-
(PhCH2)HC60 decomposes into PhCH2C60 species. Such a
decomposition of the monoanionic1,2-(PhCH2)HC60 on the
bulk electrolysis time scale is verified by the HPLC of the crude
product, which was obtained by oxidizing the monoanionic
1,2-(PhCH2)HC60 back to neutral with a potential at 0 V,
where a small amount of dimers was obtained (see Supporting
Information).
equilibrium with the electrolysis potential. The results therefore
indicate that it is the neutral H atom that is lost from
1,2-(PhCH2)HC60 via the homolytic cleavage of C60ÀH bond
after the molecule receives two electrons, consistent with the
previously reported decomposition of C60H2 under reducing
conditions.18 The observed dehydrogenation under reducing
conditions reflects the super stability of RC60À, which is likely
due to the strong electron affinity C60.7
Figure 5c shows the cyclic voltammogram of the anionic
product obtained after 1,2-(PhCH2)HC60 was electrolyzed
at À1.10 V vs SCE, which is about 150 mV more negative than
the E1/2 of the second reduction and is suitable for transfer-
ring two electrons to 1,2-(PhCH2)HC60. As is shown in the
cyclic voltammogram, the redox couples corresponding to
1,2-(PhCH2)HC60 species (À0.54, À0.94, À1.50 V) are com-
pletely missing, while well-resolved reversible redox couples
appear at À0.93 and À1.36 V, which correspond to the redox
processes of anionic singly bonded PhCH2C60 species. The slight
potential variance with respect to that shown in Figure 4 is likely
due to the subtle difference of the anionic species presented in
the solution. A small reduction peak appears at À0.47 V, which
should correspond to the first reduction of the dimers formed via
the oxidation of PhCH2C60À at about À0.05 V. Cyclic voltam-
mometry with a fast scan rate of 10 V/s was also performed (see
the Supporting Information). The voltammogram shows basi-
cally the same redox processes as those recorded with a slow scan
rate (0.1 V/s), with two quasi-reversible redox couples at À0.95
and À1.39 V, an irreversible oxidation at 0.09 V, and an
irreversible reduction at À0.64 V. The observed shifts for the
peak potentials (Epa and Epc) with the variation of the scan rate
indicate that the electron-transfer between the fullerene species
and the electrode is a relatively slow process.30 However, it shows
unambiguously that the dimers are formed from RC60À and are
Electrochemical oxidation with a potential set at 0 V of the
above anionic solution (Figure 5c) affords the singly bonded
dimers of PhCH2C60ÀC60CH2Ph as shown in the HPLC
chromatogram of Figure 1. Figure 5d displays the in situ cyclic
voltammogram of the crude reaction mixture after the oxidation
is completed. Compared with Figures 5c, the most notable
difference is the appearance of an intense irreversible reduction
wave with Epc at À0.47 V due to the reduction of dimers,
indicating the presence of a large amount of the dimers in the
solution. Redox couples at À0.94 and À1.37 V are ascribed to the
electron-transfer processes involving anionic PhCH2C60 species,
which are the dissociated products from the dimers after they are
reduced. In addition, the broad irreversible oxidation peak
À
corresponding to the oxidation of PhCH2C60 and the forma-
tion of the dimers is also observed at À0.05 V. The results
demonstrate that the singly bonded dimers of PhCH2C60À
C60CH2Ph are formed via the two-electron reduction of
1,2-(PhCH2)HC60, followed by an oxidation of the consequent
anionic species back to neutral.
Voltammetric Titration of 1,2-(PhCH2)HC60 with TBAOH
and Mechanistic Consideration. A voltammetric titration of
1,2-(PhCH2)HC60 with TBAOH was performed in order to
confirm the loss of a hydrogen atom during the reductive
electrolysis, since it has been shown that the fullerenyl proton
can be readily removed with a base3c due to the acidic nature of
fullerenyl proton.6 Figure 6 shows the cyclic voltammograms of
1,2-(PhCH2)HC60 with the addition of TBAOH at a molar ratio
of 0, 1.0, 1.5, 2.0, 4.0, respectively.
À
decomposed into RC60 after being reduced, and such a
transformation between the dimers and RC60À is rather fast. It
is noteworthy that the initial current of the in situ cyclic
voltammogram is very close to zero when the voltammetry is
started at À1.10 V, indicating that the anionic species in the
solution should bear two formal negative charges, which are in
As is shown in Figure 6, the voltammetric changes of
1,2-(PhCH2)HC60 under titration with TBAOH are exactly
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Scheme 2. Proposed Mechanism for the Formation of Singly Bonded RC60ÀC60R Dimers via Electroreductive Activation of
1,2-(PhCH2)HC60
the same as those when 1,2-(PhCH2)HC60 is under reductive
electrolysis. The reversible redox couples at À0.54 and À1.50 V
associated with 1,2-(PhCH2)HC60 gradually decreased along
with the addition of TBAOH and completely disappeared when
4.0 equiv of TBAOH was added; in the meantime, a new redox
couple appeared at À1.36 V, implying that this is associated with
study of the bulk electrolysis process and the 1,2-(PhCH2)
HC60/TBAOH titration has revealed the reaction mechanism,
where the fullerenyl hydrogen is homolytically cleaved from
1,2-(PhCH2)HC60 via two-electron reduction. The work de-
monstrates the preparation of singly bonded C60 dimer from
HRC60 via electroreductive C60ÀH bond activation, which may
extend the scope of fullerene functionalization utilizing
organo(hydro)fullerenes.
À
the dehydrogenated PhCH2C60 intermediate. However, the
current height for the redox couple at À0.94 V, which corre-
2À
sponds to 1,2-(PhCH2)HC60À•/1,2-(PhCH2)HC60
, re-
mained almost unchanged during the addition of TBAOH,
’ EXPERIMENTAL SECTION
except a slight variation of the Epc (À1.00 vs À0.98 V), indicating
2À•
that the reduction potential of PhCH2C60À/PhCH2C60
is
/
General Methods. Electrochemical grade TBAP was recrystallized
from absolute ethanol and dried in a vacuum at 298 K prior to use. PhCN
was distilled over P2O5 under vacuum at 305 K prior to use. 1,2-
H(PhCH2)C60 was prepared as report previously.19 All other chemicals
were commercially available and used as received.
À•
coincidentally superimposed with that of 1,2-(PhCH2)HC60
1,2-(PhCH2)HC602À. As for the waves at À0.04 and À0.49 V,
they are associated with the oxidative formation of the dimers
from PhCH2C60À and reductive dissociation of the dimers back
to PhCH2C60À. Notably, isocurrent points are shown in the
cyclic voltammogram, indicating that only dehydrogenated reac-
tion occurs for 1,2-(PhCH2)HC60 with the addition of TBAOH,
confirming the C60ÀH activation for 1,2-(PhCH2)HC60 under
the electroreductive conditions. A reaction mechanism is pro-
posed as shown in Scheme 2. It shows that the fullerenyl
hydrogen atom is cleaved from 1,2-(PhCH2)HC602À, which
leads to the formation of PhCH2C602À•. With the subsequent
Synthesis of the Singly Bonded PhCH2C60ÀC60CH2Ph Dimers.
Typically, 50 mg (61.6 μmol) 1,2-(PhCH2)HC60 was electrolyzed
at À1.10 V vs SCE in 50 mL of freshly distilled PhCN solution containing
0.1 M TBAP under an N2 atmosphere. The reductive electrolysis was
stopped after achieving the theoretical number of coulombs required for
two-electron transfer, and then the anionic solution was oxidized back to
neutral with the potential set at 0 V vs SCE. The reaction mixture was
dried with a rotary evaporator under reduced pressure, and the residue was
washed with methanol to remove TBAP. The obtained crude mixture
was put into toluene and sonicated for a few minutes, and the soluble
part was further purified by HPLC over a semipreparative Buckyprep
column using toluene as the eluent; 18.2 mg product was obtained, with
an isolated yield of 36%.
•
removal of two electrons from PhCH2C602À•, chiral PhCH2C60
are then produced as a result of the presence of two identical
hexagons containing the mono addend, leading to the formation
of both the meso and racemic dimers by the radical couplings of
•
different PhCH2C60
.
Spectral Characterization of the Singly Bonded PhCH2C60
1
ÀC60CH2Ph Dimers. H NMR (600 MHz, o-DCB-d4) δ 7.75 (d,
2H), 7.42 (t, 2H), 7.31(t, 1H), 4.70 (ABq, ΔνAB = 150 Hz, JAB = 13.2
Hz), 4.66 (ABq, ΔνAB = 106 Hz, JAB = 13.2 Hz); 13C NMR (150 MHz,
o-DCB-d4) δ 157.9, 157.8, 155.3, 155.0, 154.3, 154.2, 150.8, 150.6,
150.5, 150.2, 149.9, 149.7, 149.6, 149.5, 149.4, 148.6, 148.3, 148.1, 148.0,
147.8, 147.8, 147.7, 147.7, 146.5, 146.5, 146.4, 146.2, 145.8, 145.6, 145.5,
145.4, 145.4, 145.4, 145.3, 145.3, 145.2, 145.2, 145.1, 145.1, 145.0, 144.8,
’ CONCLUSION
The formation of singly bonded PhCH2C60ÀC60CH2Ph
dimers, which consist of the meso and racemic regioisomers, is
achieved via controlled-potential bulk electroreduction and elec-
trooxidation of 1,2-(PhCH2)HC60. In situ cyclic voltammetric
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The Journal of Organic Chemistry
ARTICLE
144.8, 144.7, 144.3, 144.2, 144.1, 144.0, 144.0, 144.0, 143.9, 143.9, 143.9,
143.6, 143.5, 143.4, 143.3, 143.0, 143.0, 143.0, 142.9, 142.8, 141.8, 141.8,
140.6, 140.5, 140.4, 139.9, 139.9, 139.0, 139.0, 138.4, 136.6, 68.3, 61.9,
51.0, 50.8. Positive MALDI-TOF MS: C134H14, m/z [M + H]+ calcd
M. Fullerenes: Chemistry and Reactions; Wiley-VCH: Weinheim, Germany,
2005. (f) Martin, N. Chem. Commun. 2006, 2093–2104. (g) Thilgen, C.;
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3
C 98.71, H 1.29; found C 98.38, H 1.67. UVÀvis (toluene) λmax/nm 362
and 450.
X-ray Crystallographic Data Collection and Structure Re-
finement. Black lamellar crystals of the singly bonded PhCH2C60À
C60CH2Ph dimer suitable for an X-ray analysis were obtained by slow
evaporation of a benzene solution at a low temperature (around 5 °C).
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14.6106(8) Å; R = 100.4(1)°, β = 91.5(1)°, γ = 115.59(1)°, and V =
2402.1(2) Å3, z = 1, Dcalc = 1.500 Mg mÀ3, μ = 0.086 mmÀ1, T = 143(2)
K, crystal size 0.25 Â 0.12 Â 0.10 mm; reflections collected 17255,
independent reflections 8439; 5272 with I > 2σ(I); R1 = 0.0974 [I >
2σ(I)], wR2 = 0.2508 [I > 2σ(I)]; R1 = 0.1449 (all data), wR2 = 0.2851
(all data), GOF (on F2) = 1.051.
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’ ASSOCIATED CONTENT
S
Supporting Information. Single-crystal X-ray CIF file of
b
the singly bonded C60 dimer, 1H and 13C NMR, UVÀvis, MS of
the dimer fraction, HPLC of the crude product obtained by
oxidation of the monoanionic 1,2-(PhCH2)HC60 back to neu-
tral, cyclic voltammogram of 1,4-(PhCH2)2C60, cyclic voltam-
mogram of the anionic species after transferring two electrons to
1,2-(PhCH2)HC60 with CPE at À1.10 V with a scan rate of
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: xgao@ciac.jl.cn.
’ ACKNOWLEDGMENT
ꢀ
1997, 209–210. (d) Segura, J. L.; Martin, N. Chem. Soc. Rev. 2000,
We are grateful to Prof. Ning-Hai Hu and Ms. Zhen-Zhen Mo
for single-crystal X-ray diffraction measurements. The work was
supported by the National Natural Science Foundation of China
(Grant No. 20972150) and the Solar Energy Initiative of the
Chinese Academy of Sciences (Grant No. KGCX2-YW-399 + 9)
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