Radical reaction of C70Cl10 with P(OEt)3: Isolation and characterization of C70[P(O)(OEt)2] nHn (n = 1, 2)

Article abstract of DOI:10.1016/j.mencom.2014.06.007

The Arbuzov-type reaction of chlorofullerene C₇₀Cl₁₀ with P(OEt)₃ leads to the formation of C₇₀[P(O)(OEt) ₂]H and C₇₀[P(O)(OEt)₂]₂H ₂, whose molecular struc

Full text of DOI:10.1016/j.mencom.2014.06.007

Mendeleev
Communications
Mendeleev Commun., 2014, 24, 211–213
Radical reaction of C70Cl10 with P(OEt)3: isolation
and characterization of C70[P(O)(OEt)2]nHn (n = 1, 2)
Ekaterina A. Khakina,a Anastasiya A. Yurkova,a Artem V. Novikov,a,b Nataliya P. Piven,c
Alexander V. Chernyak,a,d Alexander S. Peregudove and Pavel A. Troshin*a
a Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka,
Moscow Region, Russian Federation. Fax: +7 496 515 5420; e-mail: troshin2003@inbox.ru
b Department for Fundamental Physicochemical Engineering, M. V. Lomonosov Moscow State University,
119991 Moscow, Russian Federation
c Institute of Energy Problems of Chemical Physics (Branch), 142432 Chernogolovka, Moscow Region,
Russian Federation
d Joint Analytical Center, Scienific Center in Chernogolovka, Russian Academy of Sciences,
142432 Chernogolovka, Moscow Region, Russian Federation
e A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation
DOI: 10.1016/j.mencom.2014.06.007
The Arbuzov-type reaction of chlorofullerene C70Cl10 with P(OEt)3 leads to the formation of C70[P(O)(OEt)2]H and C70[P(O)(OEt)2]2H2,
whose molecular structures were determined using ESI mass spectrometry in combination with 1D and 2D NMR spectroscopy.
(EtO)2(O)P
H
Functionalized fullerene derivatives are promising materials for
organic electronics1–3 and biomedicine4,5 applications. Cycloaddi-
tion reactions are commonly used for the derivatization of the
carbon cages of C60 and C70 fullerenes.6–11 However, cycloaddi-
tion reactions suffer from poor regio- and stereoselectivity.12,13
The addition of two or more organic addends to the fullerene
cage produces hardly separable mixtures of many isomers.14,15
Chlorinated fullerenes are accessible precursors for the highly
selective synthesis of multifunctional fullerene derivatives. The
chlorination of C60 or C70 using a solution of ICl affords indi-
vidual isomers of C60Cl6 and C70Cl10, respectively.16–19 High-
temperature chlorination of C60 results in the quantitative forma-
tion of C60Cl24, C60Cl28 or C60Cl30 depending on the reaction
conditions.20–22 Fullerene C70 gives C70Cl28 under similar con-
ditions.23
P(OEt)3
toluene
+
C70
C70Cl10
1 (35%)
(EtO)2(O)P
H
P(O)(OEt)2
H
+
The chlorofullerenes C60Cl6 and C70Cl10 react with C- and
O-nucleophiles such as aromatic hydrocarbons in the presence
of Lewis acids, MeLi, H2C=CHCH2SiMe3 and alcohols.24–29
We reported the synthesis of water-soluble [70]fullerene deri-
vatives via the Friedel–Crafts arylation of C70Cl8 and C70Cl10.30
The first air-stable salts of the fullerene anion [C60(CN)5]– were
synthesized in the reaction of C60Cl6 with organic cyanides.31
A family of aminofullerenes was obtained using the reactions
of C60Cl6 with amines.32,33 A novel group of sulfur-containing
fullerene derivatives was designed using highly efficient reactions
of C60Cl6 with thiols.34
More recently, we reported the Arbuzov reaction of chloro-
fullerene C60Cl6 with P(OR)3 leading to the highly selective
formation of C60[P(O)(OR)2]5H.35 Here, we explored the appli-
cability of this reaction to other fullerene halides, in particular,
C70Cl10. Pristine chlorofullerene C70Cl10 (120 mg, 0.1 mmol)
prepared in accordance with a published procedure18 was dis-
solved in 100 ml of freshly distilled toluene with stirring at room
temperature to afford a lemon-yellow solution. A solution of
P(OEt)3 (1660 mg, 10 mmol) in 10 ml of toluene was added drop-
wise to the chlorofullerene solution with stirring. The reaction
mixture changed its color to dark red and then to reddish brown.
It was stirred for additional 4 h and then poured on the top of
2 (31%)
Scheme 1
a silica gel column (Acros organics, 40–60 mm). Elution with
toluene provided non-functionalized C70 which was formed from
chlorofullerene C70Cl10 via elimination of addends from the
fullerene cage under the action of phosphite. The following elution
with toluene–methanol mixtures produced C70[P(O)(OEt)2]H 1
and C70[P(O)(OEt)2]2H2 2 in 35 and 31% yields, respectively
(Scheme 1).
The molecular compositions of compounds 1 and 2 were con-
firmed using ESI mass spectrometry, which revealed only the
peaks corresponding to the title products with m/z 977 due to
[1–H]– and m/z 1115 due to [2–H]– (Figures S1 and S10, Online
Supplementary Materials).
The 1H NMR spectrum of 1 (Figure S2) revealed the signals
corresponding to the ethoxy group protons (~1.65 and ~4.65 ppm)
along with the characteristic signal of the proton attached to the
fullerene cage (~5.2 ppm). The high field part of the 13C NMR
spectrum (Figure S4) demonstrated three signals corresponding
to the sp3 carbon atoms: two signals at 16.9 and 65.1 ppm for the
© 2014 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2014, 24, 211–213
1950
1900
2
1850
C70Cl10
1 [1,2]
2 [1,2, 41,58]
1800
1750
1700
1
or
or
324 326 328 330 332 334 336 338 340
Magnetic field/mT
2' [1,2,56,57]
2' [1,2,67,68]
2' [1,2,3,4]
Figure 1 Schlegel diagrams of C70Cl10, products 1, 2 and possible molecular
structures of minor component 2'. Filled circles denote H, opened circles
denote P(O)(OEt)2, and filled diamonds denote Cl.
Figure 2 ESR spectra of (1) P(OEt)3–DMPO (weak signal due to some
impurity is visible) and (2) the reaction mixture P(OEt)3 + DMPO + C70Cl10
in toluene.
ethoxy unit and one signal at 48.7 ppm for the carbon atom of the
fullerene cage bearing hydrogen. This assignment was proved by
H–C HMQC spectrum (Figure S8). A signal for the cage carbon
atom bearing the phosphonate group was not observed presumably
due to strong C–P splitting. The low field 13C NMR spectrum
(Figure S5) showed 38 peaks due to the cage sp2 carbons, which
evidenced Cs molecular symmetry of compound 1. A somewhat
larger number of peaks (38 instead of 35 expected) is related
to the splitting of some signals on the phosphorus attached to
the carbon cage. Two-dimensional H–P HMBC (Figure S6) and
H–H ROESY (Figure S7) indicated that the phosphonate group
and hydrogen atom are attached to the neighboring positions of
the fullerene cage. It is well known that the absorption spectra
of C70 derivatives are very characteristic, and they can be used to
reveal an exact addition pattern for many fullerene derivatives.13
The absorption spectrum of 1 (Figure S9) unambiguously proved
that the addition of phosphonate group and hydrogen occurs
across the 1–2 double bond according to the Taylor’s numbering
scheme as shown in Figure 1.
phonate groups are attached in the adjacent positions of a carbon
cage in 2 and 2'. Only one reasonable C2v-symmetrical structure
of major component 2 is shown in Figure 1 and Scheme 1, which
satisfies all obtained spectral data. Moreover, it is strongly sup-
ported by the absorption spectrum (Figure S18), which resembles
closely the absorption spectra of C2v-symmetrical C70 adducts
with a 1,2,41,58-addition pattern reported previously.37 The
molecular structure of minor component 2' can be ascribed to the
C2-symmetrical 1,2,56,57- or 1,2,67,68-isomers with slightly
different arrangements of two pairs of addends at opposite poles
of the C70 cage (Figure 1). Alternatively, it could be represented
by a Cs-symmetrical 1,2,3,4-isomer with all organic addends
attached to the same pole of the C70 cage. The last option seems
the most plausible since the signal of the protons attached to the
carbon cage in 2' is downfield shifted by ~0.15 ppm compared
to the signal of the same type of protons in 2. The observed
downfield shift can be related to the presence of two electron-
withdrawing phosphonate groups surrounding each proton in
1,2,3,4-2' compared to the only one phosphonate group in the
major 1,2,41,58-isomer 2.
Thus, using a combination of ESI mass spectrometry and
different NMR-spectroscopic techniques, we deduced the mole-
cular structures of compounds 1 and 2 and proposed possible
structures for minor component 2'. Figure 1 shows that the mole-
cular structures of 1 and 2 have nothing similar with the molecular
structure of the parent chlorofullerene C70Cl10. In contrast to the
Arbuzov-type reaction of C60Cl6, which ends up with the simple
replacement of chlorine atoms with five phosphonate groups and
hydrogen, a similar reaction of C70Cl10 obviously proceeds via
a more complicated route. First, a number of chlorine atoms are
eliminated from the fullerene cage under treatment with P(OEt)3
which is a strong reducing agent. At the same time, the formal
chlorine substitution occurring at some reaction stages leads to
the addition of phosphonate groups and hydrogen atoms to the
poles of the C70 cage, while all chlorine atoms were attached to
the equator in C70Cl10. In view of the fact that the test reaction
included multiple chlorine eliminations, substitutions and rearrange-
ments, the formation of just four isolable products (C70, 1, 2 and
2') denotes unusually high selectivity of these processes. More-
over, the formation of only two C70[P(O)(OEt)2]2H2 isomers (2 and
2') also evidences extraordinary high selectivity of the reaction.
The exact mechanism of the reaction of C70Cl10 with P(OEt)3
is unclear. However, it was possible to figure out what type of
the reaction occurs between the chlorofullerene and phosphite
using ESR spectroscopy with 5,5-dimethyl-1-pyrroline N-oxide
(DMPO) as a radical trap. The ESR spectrum of a P(OEt)3–
DMPO mixture (Figure 2) showed a weak signal of a radical
impurity, which was also detected in the solution of DMPO.
Thus, we found that compound 1 has the molecular structure
of 1,2-addition product, as depicted in Figure 1 and Scheme 1.
Note that similar C60 derivatives were obtained previously in
the nucleophilic addition reaction of parent [60]fullerene with
HP(O)(OEt)2.36
1
The H NMR spectrum of 2 (Figure S11) revealed that the
sample consists of two components (2 and 2') formed in a 3:1
ratio. We will focus on the discussion of the spectral properties
of major component 2 since the weaker features corresponding
to minor component 2' could not be confidently detected and
1
resolved in all cases. Thus, the H NMR spectrum showed that
both hydrogen atoms and phosphonate groups are symmetrically
equivalent in 2. This conclusion is also supported by the 31P NMR
spectrum (Figure S12). The high field 13C NMR spectrum con-
tained signals due to ethoxy group carbons at 16.9 and 65.0 ppm.
A sharp signal at 49.3 ppm corresponded to the cage carbon linked
with hydrogen. Two broader signals at 56.5 and 57.5 ppm could
be attributed to the cage carbon bearing the phosphonate group.
Note that minor component 2' exhibited weak signals at 22.7
(Me) and 48.4 ppm (cage CH). This assignment was proved
by the 2D H–C HMQC spectrum (Figure S17). In the low field
13C NMR spectrum 55 signals of different intensities due to the
sp2 carbons of 2 and 2' were observed. This number can be
obtained for a mixture of C2v symmetrical isomer 2 with C2 (or
Cs) symmetrical isomer 2'. In this case, the expected number of
signals should be in a range of 52–53 (19+33 or 19+34), though
C–P splitting and incident overlapping of some signals might alter
this number. Both H–P HMBC (Figure S15) and H–H ROESY
(Figure S16) spectra confirmed that hydrogen atoms and phos-
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Mendeleev Commun., 2014, 24, 211–213
12 P. A. Troshin, A. S. Peregudov, D. Muhlbacher and R. N. Lyubovskaya,
However, the addition of C70Cl10 to the P(OEt)3–DMPO mixture
resulted in the appearance of a strong multi-line signal, that can
be attributed to the formation of several different radical species,
which are supposed to be reaction intermediates trapped with
DMPO.
In conclusion, we studied the reaction of C70Cl10 with P(OEt)3
and revealed the formation of C70[P(O)(OEt)2]H (1) and C70[P(O)-
(OEt)2]2H2 (2 and 2') along with unfunctionalized C70. Molecular
structures of all isolated products were deduced from the obtained
spectroscopic data. Using ESR spectroscopy and DMPO as a
radical trap, we revealed the formation of radical species in the
C70Cl10 + P(OEt)3 reaction mixture. This observation allowed
us to assume that the investigated reaction between chlorofulle-
rene C70Cl10 and P(OEt)3 proceeds via a complicated radical
mechanism.
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This work was supported by the Russian Ministry for Educa-
tion and Science (contract no. 14.512.11.0127) and the Russian
President Science Foundation (grant no. MK-6177.2013.3). We
are grateful to Professors V. K. Koltover and A. V. Kulikov for
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Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2014.06.007.
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Received: 19th November 2013; Com. 13/4246
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