A scalable synthesis of methano[60]fullerene and congeners by the oxidative cyclopropanation reaction of silylmethylfullerene

Article abstract of DOI:10.1021/ja201267t

1,2-Dihydromethano[60]fullerene and its congeners have attracted much interest, but they have been synthesized only in very low yields because of several insurmountable problems. A new three-stage synthesis involving addition of a silylmethylmagnesium chloride to [60]- and [70]fullerene and oxidation of the anionic intermediate with CuCl₂ afforded the methano[60]- and methano[70]fullerenes in 90% and 70% overall yield, respectively. The reaction with 1,4-diorgano[60]fullerene also proceeded smoothly to give a diastereomerically pure 56-π-electron fullerene that has a higher LUMO level than the parent fullerene and gave a higher open-circuit voltage and better power conversion efficiency when fabricated into an organic photovoltaic device.

Full text of DOI:10.1021/ja201267t

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pubs.acs.org/JACS
A Scalable Synthesis of Methano[60]fullerene and Congeners by
the Oxidative Cyclopropanation Reaction of Silylmethylfullerene
Ying Zhang, Yutaka Matsuo,* Chang-Zhi Li, Hideyuki Tanaka, and Eiichi Nakamura*
Department of Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S Supporting Information
b
in the presence of a base and CuCl2. The installation of a
ABSTRACT: 1,2-Dihydromethano[60]fullerene and its
congeners have attracted much interest, but they have been
synthesized only in very low yields because of several
insurmountable problems. A new three-stage synthesis
involving addition of a silylmethylmagnesium chloride to
[60]- and [70]fullerene and oxidation of the anionic inter-
mediate with CuCl2 afforded the methano[60]- and methano-
[70]fullerenes in 90% and 70% overall yield, respectively. The
reaction with 1,4-diorgano[60]fullerene also proceeded
smoothly to give a diastereomerically pure 56-π-electron
fullerene that has a higher LUMO level than the parent
fullerene and gave a higher open-circuit voltage and better
power conversion efficiency when fabricated into an organic
photovoltaic device.
methano group raised the LUMO level of the parent fullerene,
caused no significant change in the crystal packing, rigidified the
crystal structure, and improved the power conversion efficiency
(PCE) of OPV devices.
We synthesized the 58-π-electron methanofullerene 1 via two
routes (Scheme 1). We first prepared silylmethylfullerene 2 in
89ꢀ93% yield by addition of iPrOMe2SiCH2MgCl in the presence
of N,N-dimethylformamide (DMF).12 This was converted to dark-
green potassium complex 3at 25 °C, which was then heated together
with CuCl2 (4 equiv) at 100 °C for 12 h to give the purple
compound 1 in 79% isolated yield after silica gel chromatography
(CS2 as eluent). Although a precise mechanism has not yet been
elucidated, we believe that the reaction involves the cyclization of a γ-
silyl cation generated by Cu(II) oxidation of anion 3. The reaction
may have a kinship to a Cu(II)-catalyzed multiple CꢀC bond
formation that may involve a related species.13 Because the chroma-
tographic behavior of the silylmethyl intermediate 2 was very
different from those of both the starting fullerene and the methano-
fullerene, the methanofullerene was easily obtained in a pure state.
We also synthesized 1 (Scheme 1, bottom) using a higher-yiel-
ding, two-step route via fullerene dimer 4, which was obtained in
100% yield as an inseparable diastereomeric mixture by oxidation
of 3 with N-chlorosuccinimide (NCS) or I2.14 Heating dimer 4 in
o-dichlorobenzene (ODCB) at 100 °C for 4 h with CuCl2 af-
forded 1 in quantitative yield. Filtration through silica gel afforded a
purple product that showed spectral properties identical with those
reported for 1 (mass, m/z 734.02; 1H NMR, 3.93 ppm).9
The latter method was readily amenable to scale-up. Thus, to
a mixture of dimethylisopropoxysilylmethyl[60]fullerene dimer 4
(1.00 g, 0.59 mmol) and CuCl2 (316 mg, 2.35 mmol) was added
ODCB (0.5 L). After the dark reaction mixture was stirred for 4 h
at 100 °C, MeOH (10 mL) was added, and the dark solution was
filtered through a silica gel pad eluted with CS2. The volatiles were
removed by rotary evaporation, and the residue was suspended in
methanol; after filtration, the methanofullerene was obtained as a
brown powder (0.82 g, 95% purity by HPLC analysis).
The synthesis was extended to the cyclopropanation of [70]fuller-
ene (Scheme 2). iPrOMe2SiCH2MgCl was added to the fullerene in
thepresenceofDMFat25°C to obtain a mixture of 1,2-isomer 5and
5,6-isomer 6 in an approximate ratio of 2.5:1. Treatment of the
mixture with tBuOK and then with CuCl2 at 100 °C for 12 h afforded
a 2.5:1 mixture of 1,2-dihydromethano[70]fullerene 7 and 5,6-di-
hydromethano[70]fullerene 810b in a combined yield of 93% (70%
from [70]fullerene).
ecause [60]fullerene is widely employed in the development
B
of organic photovoltaic (OPV) devices,1 the control of its
electronic state and morphology by addition of organic addends
to the fullerene core has become a very important issue.2,3 For
instance, an increasing number of addends can reduce the
π-conjugation length, raise the LUMO level,4 and hence increase
the open-circuit voltage (VOC) of the OPV device, which is beneficial
for device performance.5ꢀ7 However, the addends inevitably change
the crystal packing, typically reduce the fullereneꢀfullerene contact
distance, and may also reduce the carrier mobility. As the most stable
and the smallest 1,2-disubstitution available for fullerene modifica-
tion, cyclopropanation is ideal for reducing the conjugation length
with minimal structural perturbation.8 Thus, 1,2-dihydromethano-
[60]fullerene9 (C61H2, 1) and its congeners10 have been synthesized
since 1993, typically through 1,3-dipolar cycloaddition of diazome-
thane9,10b or by the use of diiodomethane10c,d as the one-carbon
source. Because of polyaddition of the methylene group,10c,d forma-
tion of fulleroid isomers,9,10b and incomplete reaction,10c,d the pro-
duct yields are very low without exception, making them unusable for
practical applications. We report herein a new three-stage route to
the 58-π-electron methanofullerene 1 (Scheme 1), which we carried
out on a gram scale in 85% overall yield. The best reported yield of 1
is only 30ꢀ40% (70% yield of for C71H2 as opposed a previously
reported 4% yield; see Scheme 2).10 The reaction is also applicable
to the regiocontrolled synthesis of 56-π-electron methanofullerenes
11a, 11b, and 14 (see Scheme 3). The key step is the regioselective
monoaddition of a silylmethyl Grignard reagent to [60]- or 1,4-
di(organo)[60]fullerenes under our previously reported condi-
tions11,12 followed by an oxidative CꢀC bond-formation reaction
Received: February 10, 2011
Published: May 02, 2011
r
2011 American Chemical Society
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|

Journal of the American Chemical Society
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Scheme 1. Synthesis of 1,2-Dihydromethano[60]fullerene
(1) from Silylmethylfullerene and Its Dimer
Figure 1. Crystal structures of 9a CS2 and 11a CHCl3: (a) side view
3
3
of 9a; (b) crystal packing in 9a along the a axis; (c) side view of 11a; (d)
crystal packing in 11a along the a axis. Solvent molecules have been
omitted for clarity.
Scheme 2. Synthesis of 1,2- and 5,6-
Dihydromethano[70]fullerene
Table 1. Reduction Potentials and LUMO Levels for Full-
erene Derivativesa
(V vs Fc/Fcþ)
red
1/2
E
Ar
E1
E2
E3
LUMO level (eV)b
C60
1
ꢀ1.10
ꢀ1.21
ꢀ1.19
ꢀ1.33
ꢀ1.20
ꢀ1.33
ꢀ1.15
ꢀ1.29
ꢀ1.49
ꢀ1.62
ꢀ1.60
ꢀ1.68
ꢀ1.61
ꢀ1.68
ꢀ1.53
ꢀ1.64
ꢀ1.94
ꢀ2.13
ꢀ2.09
ꢀ
ꢀ2.10
ꢀ2.35
ꢀ2.01
ꢀ2.30
ꢀ3.70
ꢀ3.59
ꢀ3.61
ꢀ3.47
ꢀ3.60
ꢀ3.47
ꢀ3.65
ꢀ3.51
9a
11a
9b
11b
12
14
a Potentials in V vs a ferrocene/ferrocenium (Fc/Fcþ) couple were
measured by cyclic voltammetry in 1,2-Cl2C6H4 solution containing
Bu4NþPF6ꢀ (0.1 M) as a supporting electrolyte at 25 °C with a scan rate
of 0.05 V/s. Glassy carbon, platinum wire, and Ag/Agþ electrodes were used
Scheme 3. Synthesis of the 56-π-Electron Methanofullerene
Derivatives
as the working, counter, and reference electrodes, respectively. b Estimated
red1
using the following equation:17 LUMO level = ꢀ(4.8 þ E ) eV.
1/2
gram scale using inexpensive reagents (Scheme 3). First, 1,4-bisbenzyl
adducts 9a and 9b were obtained from fullerene dianion using
K/PhCH2Br (9a, 61% yield; 9b, 52% yield).15 The regioselectivity
of the cyclopropanation was secured by the high regioselectivity of the
monoaddition11,12 of iPrOMe2SiCH2MgCl to 9a and 9b. Finally, the
CuBr2 oxidation of the anions of 10 afforded methanofullerenes 11 in
∼60% yield. Similarly, methanofullerene 14 was also synthesized in
56% overall yield from 1,4-diarylfullerene adduct 12.16
X-ray crystallographic analyses of single crystals of 9a contain-
ing CS2 and 11a containing CHCl3 indicated that the added
methano group in the latter indeed did not much affect either the
conformation of the benzyl side chains or the crystal packing. Thus,
the side orientations in the 9a CS2 and 11a 2CHCl3 cocrystals
3
3
illustrated in Figure 1a,c are almost identical to each other. In addi-
tion, the crystal parameters of the 9a CS2 and 11a 2CHCl3 co-
3
3
crystals (including the minimum fullereneꢀfullerene center-to-
center distances) are similar to each other (see the Supporting
Information for details).
We next demonstrated the regioselective cyclopropanation of 1,
4-diorgano[60]fullerenes. Thus, the synthesis of 56-π-electron com-
pound 11 from C60 was achieved through a three-step operation on a
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COMMUNICATION
Table 2. Comparison of 9b and 11b in pꢀn and BHJ
increasing the PCE, as demonstrated for methanofullerene 11b.
It is very important to note that 11b is a regioisomerically pure
56-π-electron molecule, as opposed to the popularly used 56-
π-electron fullerenes (e.g., bis-PCBM)6 that are produced as a
mixture of structural isomers and cannot form a well-defined
packing in the solid state. Obviously, each isomer has a different
electronic state, and the density of states of the isomer mixture in
the solid state would be broadened. We therefore expect that the
methanofullerenes will provide the organoelectronic community
with a novel tool for increasing the performance of OPV and
related devices.
Photovoltaic Cells
JSC
device type
fullerene (mA/cm2) VOC (V) FF PCE (%)
small-molecule pꢀna
9bb
11bb
9b
3.2
4.3
4.8
7.1
0.59
0.77
0.69
0.82
0.53
0.57
0.49
0.58
1.0
1.9
1.6
3.4
polymer BHJc
11b
a The general device structure was ITO/PEDOT:PSS/BP/fullerene/
NBphen/Al. PCEs were derived from the equation PCE
=
(JSC VOC FF)/P0, where P0 is the incident light intensity (=100
3
3
mW/cm2). b For the n layer, 0.7 wt % 9b in toluene or 0.7 wt % 11b
’ ASSOCIATED CONTENT
c
in 1:1 CS2/chlorobenzene was spin-coated onto the BP p layer. The
general device structure was ITO/PEDOT:PSS/P3HT:fullerene (1:1)/
Ca (20 nm)/Al (130 nm).
S
Supporting Information. Synthetic procedures and
b
characterization for methanofullerene derivatives, their electro-
chemical and thermotropic properties, and crystallographic data
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
A comparison of the thermotropic properties of 9 vs 11 as
studied by differential scanning calorimetry measurements
showed that the methanofullerene compounds 11a and 11b in
the solid state exhibit thermal behavior similar to that of 9a and
9b, but the phase transition temperatures are shifted consistently
by ∼45 °C. We therefore consider that the methanofullerene
molecules 11a and 11b in crystals are less mobile than 9a and 9b
(see the Supporting Information for details).
’ AUTHOR INFORMATION
Corresponding Author
matsuo@chem.s.u-tokyo.ac.jp; nakamura@chem.s.u-tokyo.ac.jp
The LUMOs of the 56-π-electron methanofullerene com-
pounds 11a, 11b, and 14 (as estimated by cyclic voltammetry
measurements) were found to be consistently higher than those
of the correponding 58-π-electron 1,4-diorganofullerenes by as
much 0.14 V, as opposed to 0.11 V found for the parent
compound 1 (Table 1). Methanofullerenes 1, 11b, and 14
underwent reversible three-electron reduction, as does pristine
C60, while 11a underwent two-electron reduction.
In agreement with the higher LUMO, the VOC values of OPV
devices using a methanofullerene were also found to be higher
than that of the device using the parent fullerene (Table 2). First,
we examined the performance of a pꢀn heterojunction, small-
molecule OPV device using tetrabenzoporphyrin (BP) as an
electron donor and a fullerene as an electron acceptor.2 A device
using 9b as the acceptor performed with a 1.0% PCE, while the
one using methanofullerene 11b performed much better (1.9%).
The short-circuit current density (JSC) increased by 1.1 mA/cm2
and the VOC value by as much as 0.18 V, while the fill factor (FF)
remained essentially the same. We may surmise that the higher
LUMO level of 11b contributes to the improvement of VOC. We
also fabricated a polymer fullerene bulk heterojunction (BHJ)
OPV device18 by using poly(3-hexylthiophene) (P3HT) as the
donor. The device using 11b as the acceptor showed a PCE of
3.4% with a VOC of 0.82 V, while the one using 9b showed a PCE
of 1.6% with a VOC of 0.69 V. The VOC of 0.82 V is substantially
higher than the value obtained for a BHJ device using phenyl-
C61-butyric acid methyl ester (PCBM) (0.58 V).19
’ ACKNOWLEDGMENT
This study was partially supported by KAKENHI (22000008),
the Global COE Program “Chemistry Innovation through Co-
operation of Science and Engineering” from MEXT, Japan, and
Strategic Promotion of Innovative Research and Development
from the Japan Science and Technology Agency (JST). Y.M.
thanks the Funding Program for Next Generation World-Lead-
ing Researchers.
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