Oxidation of fullerene C60 with the system aluminum tri-tert-butoxide-tert-butyl hydroperoxide

Article abstract of DOI:10.1007/s11172-008-0047-2

Oxidation of fullerene C₆₀ with the system aluminum tri-tert-butoxide-tert-butyl hydroperoxide, in which electron-excited dioxygen is generated, gave a complex mixture of fullerene oxides C₆₀O _x (x = 1-6). The pathways of their formation were proposed.

Full text of DOI:10.1007/s11172-008-0047-2

Russian Chemical Bulletin, International Edition, Vol. 57, No. 2, pp. 304—308, February, 2008
304
Oxidation of fullerene C60 with the system
aluminum triꢀtertꢀbutoxide—tertꢀbutyl hydroperoxide
b
b
a
E. A. Zaburdaeva,a M. A. Lopatin, T. I. Lopatina, and V. A. Dodonov
ꢀ
aN. I. Lobachevsky Nizhny Novgorod State University,
23 prosp. Gagarina, 603950 Nizhny Novgorod, Russian Federation.
Fax: +7 (831) 434 5056. Eꢀmail: zaea4@rambler.ru
bG. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
49 ul. Tropinina, 603600 Nizhny Novgorod, Russian Federation.
Fax: +7 (831) 466 1497. Eꢀmail: lopatin@imoc.sinn.ru
Oxidation of fullerene C60 with the system aluminum triꢀtertꢀbutoxide—tertꢀbutyl hydroꢀ
peroxide, in which electronꢀexcited dioxygen is generated, gave a complex mixture of fullerene
oxides C60Ox (x = 1—6). The pathways of their formation were proposed.
Key words: fullerene C60, fullerene oxides, oxidative systems, electronꢀexcited dioxygen,
tertꢀbutyl hydroperoxide.
The preparative synthesis1 of fullerene C60 in 1990
gave impetus to investigations of fullerenes and their funcꢀ
tional derivatives obtained, e.g., by oxidation. The major
and (in most cases2) sole oxidation product of fullerene
Thus, C60 can serve as an efficient sensitizer for geꢀ
neration of singletꢀexcited dioxygen. Note that groundꢀ
state fullerene C60 is not oxidized by singletꢀexcited
dioxygen.3,5
C
60 is its monoxide C60O. This oxide forms even when the
Deep oxidation of C60 under the action of dimethylꢀ
and methyl(trifluoromethyl)dioxiranes6,7 gives its monoꢀ,
diꢀ, and trioxides in ~67, 30, and 8% yields, respectively.
The oxide C60O is formed when mꢀchloroperoxybenzoic
acid is used as an oxidant.8 The maximum yield (35.4%)
of monoxide is achieved in a reaction of C60 with the
system methyltrioxorhenium—hydrogen peroxide.9 Specꢀ
troscopic studies have revealed that epoxidation always
occurs at the 6,6 double bond; this has been confirmed by
quantum chemical calculations.10
starting fullerene is exposed to atmospheric oxygen in the
light (Scheme 1); however, the rate of this reaction is low.
Scheme 1
It has been noted11 that fullerene cannot be regarded
as classic fully delocalized aromatic systems; they retain
the features of fused planar conjugated π systems with
nonequivalent C atoms and the least steric stress. The
electronic structure of C60 allows reversible acceptance of
up to six electrons because of the presence of the triply
degenerate LUMO (t1u), which are higher in energy by
1.5—2.0 eV than the fivefold degenerate HOMO (hu).4
As reported earlier,2 the major product of fullerene
oxidation is C60O; however, the pathways of its formation
for most cases have not yet described in the literature.
Systems based on aluminum, titanium, and vanadium
tertꢀbutoxides and tertꢀbutyl hydroperoxide are efficient
lowꢀtemperature oxidants.12—14 These systems in benzene,
chlorobenzene, and CCl4 at room temperature generate
electronꢀexcited dioxygen via the formation of intermeꢀ
diate trioxides (ozonides) of the above elements. Oxygen
Fullerene strongly absorbs in the UV and visible reꢀ
gions (longestꢀwavelength peak at λ = 620 nm); C60 passes
into a singletꢀexcited state via the hu→t1u electron transiꢀ
tion and then into lowꢀenergy tripletꢀexcited state by inꢀ
tersystem crossing (ISC). The tripletꢀexcited state generꢀ
ates in air singletꢀexcited dioxygen (1∆g) in the quantum
yield φ = 0.96 0.04 at 532 nm.3 The increase in the conꢀ
tent and the decomposition of the latter can be monitored
by observing phosphorescence in the nearꢀIR region at
1268 nm (1∆g→3Σg–). In degassed solvents, the lifetime of
the tripletꢀexcited state of C60 is limited (133 µs), ending
in a radiationless transition into the singlet ground state
via vibrational relaxation.3,4
.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 296—300, February, 2008.
1066ꢀ5285/08/5702ꢀ0304 © 2008 Springer Science+Business Media, Inc.

Oxidation of fullerene C60
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 2, February, 2008
305
The consumption of oxygen was checked as follows. Freshly
distilled benzaldehyde was placed in one leg of two Hꢀshaped
tubes; the second leg of one tube contained a mixture 1—2—C60
(2 : 4 : 1) in chlorobenzene and the second leg of the other tube
contained an analogous mixture without C60. Both the tubes
were evacuated, sealed, and left at room temperature. Crystals
of benzoic acid in the leg containing benzaldehyde appeared
after three days. The tubes were kept for seven days; then the
parts containing benzoic acid were opened and analyzed.
in such a state substantially differs in reactivity from
O2(1∆g). Being in the coordination sphere of the metal
atom, this oxygen oxidizes under mild conditions (20 °C)
the C—H bonds of hydrocarbons12—14 and the N—H bonds
of aromatic amines15 and virtually completely convert sulꢀ
fides into sulfones.16 This allowed some sulfides to be
proposed as test reagents for electronꢀexcited forms of
dioxygen in the liquid phase. The total yield of oxygen is
~80%.12 Under these conditions, the yields of singletꢀ
excited dioxygen determined from reactions with anꢀ
thracene and 9,10ꢀdimethylanthracene are 40 and 50% in
benzene and CCl4, respectively.
Results and Discussion
Oxidation of fullerene C60 with the system 1—2 was
carried out at room temperature in benzene, CCl4, chloꢀ
robenzene, and ethylbenzene. These solvents, except for
ethylbenzene, are inert to dioxygen generated in the
system, while ethylbenzene serves as a test compound in
the study of lowꢀtemperature oxidation of the C—H bonds.
We used predominantly chlorobenzene because fullerene
is best soluble in just this solvent among those mentioned
above. The molar ratio 1 : 2 : C60 was varied from 1 : 2 : 1
to 10 : 20 : 1; the exposure time of the reaction mixture
was varied from 1 to 11 days. The lightꢀprotected reaction
mixture turned claretꢀred (the characteristic color
of fullerene solutions is bright violet), which indicates
a chemical oxidation of C60 into oxygenꢀcontaining
products.
Special experiments showed that neither hydroperoxꢀ
ide 2 nor diꢀtertꢀbutoxyꢀtertꢀbutylperoxyaluminum 3
formed in the system oxidizes fullerene C60 under analoꢀ
gous conditions.
Apparently, fullerene is oxidized by electronꢀexcited
dioxygen in the coordination sphere of aluminum, which
is generated in the system 1—2 (Scheme 2).
The goal of the present work was to study a reaction of
fullerene C60 with the system aluminum triꢀtertꢀbutoxide
(1)—tertꢀbutyl hydroperoxide (2) that generates electronꢀ
excited dioxygen.
Experimental
Commercial solvents (analytical and reagent grades) were
used. Benzene and ethylbenzene were purified by distillation
over P2O5 and stored over metallic sodium. Chlorobenzene and
CCl4 were purified by distillation and stored over Na2SO4.
Fullerene C60 was prepared by the electric arc technique
and purified according to a known procedure.17 Aluminum
triꢀtertꢀbutoxide was prepared from tertꢀbutanol and aluminum18
and purified by sublimation (160—170 °C/1 Torr). Found (%):
Al, 11.12. C12H27AlO3. Calculated (%): Al, 10.98. All manipuꢀ
lations with aluminum triꢀtertꢀbutoxide were carried out in an
atmosphere of dry oxygenꢀfree argon. The concentration of
tertꢀbutyl hydroperoxide was no less than 99.5—99.8%.
Electronic absorption spectra were recorded on a Perkin—
Elmer Lambdaꢀ25 instrument (UV/VIS Spectrometer) in a
2ꢀmm quartz cell. HPLC analysis was performed on a KNAUER
highꢀresolution liquid chromatograph fitted with a UVꢀspectroꢀ
photometric detector (λ = 324 nm; stainless steel column
150×3 mm, sorbent Diasfer 110 C18, toluene as an eluent,
elution rate 1 mL min–1. Calculations were carried out by interꢀ
nal normalization with the MultiChrom for Windows
program. MALDIꢀTOF mass spectra were recorded on a Bruker
Scheme 2
AutoFlex II MALDIꢀTOF mass spectrometer with
a
nitrogen laser (λ = 337 nm). transꢀ2ꢀ[3ꢀ(4ꢀtertꢀButylphenyl)ꢀ
2ꢀmethylpropylidene]malononitrile (DCTB) (99%, Fluka)
was employed as a matrix.
Reaction of fullerene C60 with the system aluminum triꢀtertꢀ
butoxide (1)—tertꢀbutyl hydroperoxide (2) (1 : 2 : 4). In a roundꢀ
bottom flask filled with argon, compound 1 (0.04 g, 0.16 mmol),
hydroperoxide 2 (0.03 g, 0.33 mmol), and C60 (0.06 g,
0.08 mmol) in chlorobenzene (5 mL) were mixed. The resulting
bright violet reaction mixture was kept in the dark at room
temperature for 3 days. Then the mixture was hydrolyzed with
10% H2SO4 and the product was extracted with chlorobenzene.
The organic layer was dried with Na2SO4; the solvent and volaꢀ
tile products were removed under reduced pressure, collecting
them in a trap cooled by liquid nitrogen. The dark powdery
residue was analyzed by UV spectroscopy, liquid chromatograꢀ
phy, and mass spectrometry. The amounts of C60O and C60Ox
(x = 2—6) with respect to the nonconsumed fullerene C60 were
11.58 and 9.60%, respectively.
The reaction is catalytic. The energy of formation of
the electronꢀexcited coordinated dioxygen 5 calculated
from the binding energies of ozonide 4 substantially exꢀ
ceeds the energy of formation of O2(1∆g).16
In chlorobenzene at room temperature, the yield of
oxygen was 85%, while in the presence of fullerene under
analogous conditions, the yield of dioxygen was 59%.

306
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 2, February, 2008
Zaburdaeva et al.
A
in a day were ~13.84 (C60O2/C60) and ~8.24% (C60O/C60)
and those in three days were ~9.60 (C60O2/C60) and
~11.58% (C60O/C60).
3.0
2.4
1.8
1.2
0.6
Unfortunately, the absence of chromatographic stanꢀ
dards for comparative analysis of the individual comꢀ
pounds C60Ox (x = 1—6) precluded strict assignment of
these peaks. Nevertheless, our conclusions were confirmed
by comparing the HPLC profiles of our reaction mixture
and the C60—dimethyldioxirane system6 under virtually
the same RP HPLC conditions (Inertisil ODSꢀ2 5U,
methanol—toluene, 55 : 45), as well as by NMR spectroꢀ
scopic data. By analyzing the retention times and comꢀ
paring our HPLC profiles with the literature data,6 we
assigned the peak with a retention time of 8.94 min to the
fullerene monoxide C60O and the peaks with lower retenꢀ
tion times to the oxides C60Ox (x = 2—6).
1
2
400
460
520
580
640
λ/nm
Fig. 1. UV spectra of the reaction mixture C60—(ButO)3Al—
ButOOH (1 : 2 : 4) (ethylbenzene, 20 °C, two days) (1) and
unoxidized C60 (2).
These results were confirmed by the MALDIꢀTOF
mass spectrum of oxidation products (1 : 2 : C60 = 2 : 4 : 1,
one day) with a DCTB matrix (Fig. 3). The spectrum
contains a molecular ion peak with m/z 720 (starting
fullerene C60), a peak with m/z 736 (C60O), and a number
of peaks spaced at m/z 16: 752 (C60O2), 768 (C60O3), 784
(C60O4), 800 (C60O5), and 816 (C60O6) (Fig. 3). A number
of peaks with m/z 970 and above are due to complexes of
the starting fullerene and its oxides with the matrix.
We assume that electronꢀexcited dioxygen 5 first oxiꢀ
dizes fullerene at the bond 6,6 into endoperoxide C60O2
(6) (Scheme 3). It is not improbable that trioxide 4 can
also act as an oxidant.
Therefore, ~26% of the oxygen generated in the system is
consumed in the reaction with fullerene.
The system 1—2 oxidizes the C—H bonds of the
methylene group of ethylbenzene to give acetophenone in
20% yield.19 The latter was identified by GLC (up to 30%
yield) in the oxidation of fullerene C60 with this system
(1 : 2 : C60 = 2 : 4 : 1) in ethylbenzene. However, the UV
spectra suggest competitive oxidation of C60 (Fig. 1). It
can be seen in Fig. 1 that the spectrum does not contain
the valley at 430 nm characteristic of unoxidized fullerene.
However, the presence of a wide absorption band at
424—600 nm relating to the starting fullerene suggests its
incomplete conversion.
The sixꢀmembered fullerene ring exhibits the properꢀ
ties of fused unsaturated hydrocarbons.11 With cyclic
cyclohexaꢀ1,3ꢀdiene endoperoxides (ascaridole, cycloꢀ
hexadiene derivatives, 1,3,5,5ꢀtetramethylcyclohexadiene,
and methyl Lꢀpyruvate) as examples, it has been demonꢀ
Oxidation products were identified by reverseꢀphase
(RP) HPLC (Fig. 2). The intense peak with a retention
time of 10.71 min relates to the nonconsumed C60.
However, the HPLC profile shows a series of peaks with
lower retention times. All of them (except the peak with
tr = 2.22 min due to the solvent) were assigned to fullerene
oxides containing different numbers of the oxygen atoms.
At the molar ratio 1 : 2 : C60 = 2 : 4 : 1, the relative yields
I•107 (rel. unit)
8
6
4
2
10.71
3.45
2.22
8.94
4.30
4.99
6.21
3
6
9
12
15
tr/min
800
1000
1200
m/z
Fig. 2. HPLC profile (Diasfer 110, toluene—methanol, 1 : 1) of
the solid residue upon hydrolysis and removal of the solvent
from the reaction mixture C60—(ButO)3Al—ButOOH (1 : 2 : 4)
(chlorobenzene, 20 °C, one day).
Fig. 3. MALDI mass spectrum of the solid residue upon
hydrolysis and removal of the solvent from the reaction
mixture C60—(ButO)3Al—ButOOH (1 : 2 : 4) (chlorobenzene,
20 °C, one day).

Oxidation of fullerene C60
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 2, February, 2008
307
Scheme 3
A
2.0
1
1.5
2
1.0
3
0.5
4
300
400
500
600
λ/nm
Fig. 4. UV spectra of the reaction mixture C60—(ButO)3Al—
ButOOH (1 : 2 : 4) (ethylbenzene, 20 °C) after 1 (1), 2 (2), 6 (3),
and 11 days (4).
mass spectrum contains no molecular ion peak with m/z
1456 (see Fig. 3).
strated20 that the latter easily transform into the correꢀ
sponding bisepoxides under irradiation or thermolysis conꢀ
ditions. Stabilization of intermediate 6 can be similarly
accompanied by homolysis of the O—O bond leading to
diepoxide 7 (see Scheme 3).
The fullerene molecule contains lowꢀenergy degenerꢀ
ate vacant orbitals; under the reaction conditions, C60 is
physically excited via its reaction with electronꢀexcited
dioxygen 5 (Scheme 4). The excited (most likely, tripletꢀ
excited)3,4 fullerene further reacts with endoperoxide 6
present in the system to give the monoxide C60O.
We are grateful to V. Yu. Markov for his assistance in
mass spectrometric studies.
This work was financially supported by the Council on
Grants of the President of the Russian Federation (Program for
State Support of Leading Scientific Schools of the Rusꢀ
sian Federation, Grant NShꢀ8017.2006.3) and the Presidium
of the Russian Academy of Sciences (Basic Research Proꢀ
gram “Development of Methods for the Synthesis of Comꢀ
pounds and the Creation of Novel Materials”).
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Scheme 4
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Received November 1, 2006;
in revised form November 5, 2007