One-step synthesis of fullerene hydride C60H2 via hydrolysis of acylated fullerenes

Article abstract of DOI:10.1021/jo900934j

(Figure Presented) The hitherto unexplored class of acylated fullerene compounds has been shown to be excellent C₆₀H₂ precursors. Upon a simple treatment with basic Al₂O₃, they are hydrolyzed quantitatively into

Full text of DOI:10.1021/jo900934j

pubs.acs.org/joc
This utility arises from the high acidity of the fullerenyl C-H
One-step Synthesis of Fullerene Hydride C60H2 via
Hydrolysis of Acylated Fullerenes
bond (for C60H2, pKa1 = 4.7, pKa2 =16),6 which facilitates
deprotonation of hydrogenated fullerenes (with very mild bases)
to afford fullerene anions for further functionalization. C60H2 is
also of fundamental interest as a model compound for other
fullerene derivatives7,8 and has been the subject of several
experimental and theoretical studies.9 In addition, C60H2 has
been shown to be an effective hole transport material with
potential applications in organic field-effect transistors (OFETs)
and organic light-emitting devices (OLEDs).10 These diverse
applications of C60H2, the simplest possible hydride of C60, have
made any synthetic method leading to its preparation of con-
siderable practical interest.
Manolis D. Tzirakis,† Mariza N. Alberti,† Leanne C. Nye,‡
Thomas Drewello,‡,§ and Michael Orfanopoulos*,†
†Department of Chemistry, University of Crete, 71003 Voutes,
Heraklion, Crete, Greece, and ‡Department of Chemistry,
University of Warwick, Coventry CV4 7AL, U.K.
§Present address: Physical Chemistry I, University
Erlangen-Nuremberg, 91058 Erlangen, Germany
orfanop@chemistry.uoc.gr
Received May 7, 2009
Established routes toward the synthesis of the hydroge-
nated fullerene C60H2 include hydroboration,11,12 hydrozir-
conation,13 photoinduced electrontransfer,14 dissolvingmetal
reductions,15,16 ultrasonically irradiating solutions of C60 in
2-
decahydronaphthalene,17 electrochemical reduction to C60
followed by protonation,18 as well as chemical reduction with
diimide or chromous acetate19,12 and NaBH4.20,5b Since hy-
drogen reduction of C60 is of fundamental interest, catalytic
hydrogenation has been also utilized for the synthesis of
C60H2 in solution with rhodium(0) on alumina,21 palladium
or palladium on carbon,22 as well as in the solid phase.23,4
The hitherto unexplored class of acylated fullerene
compounds has been shown to be excellent C60H2 pre-
cursors. Upon a simple treatment with basic Al2O3, they
are hydrolyzed quantitatively into C60H2. This key
feature led to the development of a new, straightforward
protocol for the selective synthesis of the simplest [60]
fullerene hydride, C60H2. This protocol may offer an
advantageous alternative to previously known methods
for the synthesis of C60H2 allowing for a rapid access to
C60H2 in good yield and high purity without tedious
separating processes.
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Reduced fullerenes, also known as fullerene hydrides, are the
simplest derivatives of the fullerene family of carbon allotropes.1
Due to its unique molecular structure, fullerene is the only form
of carbon which potentially can be chemically hydrogenated and
dehydrogenated reversibly,2 thereby making such structures of
interest as high-capacity hydrogen-storage materials.3,4 Further-
more, hydrogenated fullerenes, and especially C60H2, have
attracted considerable interest due to their synthetic utility as a
versatile starting material for further fullerene derivatization.5
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5746 J. Org. Chem. 2009, 74, 5746–5749
Published on Web 07/02/2009
DOI: 10.1021/jo900934j
r
2009 American Chemical Society

Tzirakis et al.
JOCNote
Most of these methods afford a single regioisomer of C60H2,
the 1,2-addition product which has the lowest energy of the 23
possible all-exo isomers of C60H2,8,24 along with a mixture of
highly hydrogenated derivatives (C60Hn, n g 2). The yields of
the desired C60H2 are typically low, while the purification
procedure requires extensive use of preparative or semipre-
parative high-pressure liquid chromatography (HPLC);25
there are only a few cases where the C60H2 yields lie in the
range of 60-70%.13-15, 20 Thus, the mild and selective hydro-
genation of C60 remains a major goal in fullerene chemistry.
Recently, we have developed an effective method for the
direct acylation of [60]fullerene by using commercially avail-
able alkyl or aryl aldehydes.26 Acylated fullerenes represent a
novel class of fullerene compounds, and their chemical
properties are of fundamental interest. Our present studies
on their reactivity revealed, in accordance with earlier studies
reported by Mattay et al.27 and Saigo et al.,28 that these
compounds could be converted into C60H2 under mild
conditions. This result prompted us to conduct further
investigations for addressing this reactivity and explore
potential applications. For this purpose, a series of acylated
fullerenes 1-13 (Figure 1) was prepared according to the
previously reported method.26 We found that these com-
pounds could be used as efficient reagents for the selective
synthesis of 1,2-C60H2 in quantitative yield and high purity
(Scheme 1). Several studies under different experimental
conditions have been also conducted to explore this functio-
nalization and gain a valuable mechanistic insight.
For the purposes of the present study, ketones 1 and 11 were
initially used as model compounds for alkyl- and aryl-substi-
tuted ketones, respectively, and treated under different reaction
conditions. These conditions included treatment with silica or
alumina gel, acidic or alkaline solutions of different concentra-
tion, and the presence or absence of molecular oxygen. All these
reactions have been carried out by using a toluene solution of
ketone 1 or 11 (8ꢀ10-4 M). Some representative results ob-
tained from these studies are summarized in Table 1.
Initially, treatment of 1 with SiO2 or p-toluenesulfonic acid
(PTSA) did not result in any reaction for several hours,
suggesting that this compound is stable under acidic conditions
(cf. entries 1 and 2 in Table 1). In contrast, 1 was smoothly
converted to C60 and C60H2 when treated with aqueous NaOH
(entries 3-10, Table 1). Indisputably, hydroxide anions are
responsible for this transformation, presumably via the con-
ventional nucleophilic attack on the carbonyl carbon and/or
abstraction of the acidic fullerenyl hydrogen (vide infra).
Further studies revealed that both NaOH and dissolved oxygen
affect the rate of this reaction. In particular, the reaction rate
was found to vary considerably when three identical samples of
1 were treated under argon, ambient, or oxygen atmosphere
(cf. entries 3-8 in Table 1), respectively. Although the reaction
of 1, under an argon atmosphere, was hardly observed, it
FIGURE 1. Acylated [60]fullerenes investigated in this study.
SCHEME 1. Indirect Approach for the Synthesis of C60H2
through a Sequential Acylation/Hydrolysis Process
proceeded faster under ambient atmosphere. The effect of
dissolved oxygen became more pronounced when a pure
oxygen atmosphere was maintained over the reaction mixture,
thus resulting in a much faster reaction. For example, the time
required for achieving a 15% conversion of 1 was 21 h under
inert conditions, while under oxygen atmosphere, only 90 min
was needed (cf. entries 4 and 7 in Table 1). Apparently, the
presence of oxygen is necessary for this process. In addition, a
substantial increase in the reaction rate was observed with
increase of NaOH concentration (cf. entries 5, 6 and 9, 10 in
Table 1). It should also be mentioned that prolonged reaction
time and/or increased NaOH concentration led to the con-
sumption of both C60H2 and C60 and the formation of a brown
sludge, partially soluble in water. This is probably due to the
polyhydroxylation of C60, which results in the formation of
water-soluble fullerenols; this process is known to occur upon
treatment of C60 with NaOH under O2 atmosphere.29
At this point, it is worth mentioning that the C60/C60H2
ratio increases with increasing reaction time (cf. entries 9 and
10 in Table 1; see also below). This observation implies that
C60H2 should be considered as an intermediate compound in
the conversion of 1 to C60. To probe further this possibility,
we prepared a pure sample of C60H2 which was then treated
under the same experimental conditions mentioned above
with aqueous NaOH. Indeed, under these alkaline condi-
tions, C60H2 was readily converted to C60. Moreover, the
exclusion of oxygen from the reaction mixture inhibited this
process. This result is substantially consistent with that of
Wang et al.20 and provides further experimental evidence for
the intermediacy of oxygen in the reaction mechanism. More
importantly, this result may also rationalize the previously
reported lability of C60H2, and especially its conversion back
to C60 in the presence of oxygen.16,21
(24) (a) Henderson, C. C.; Cahill, P. A. Chem. Phys. Lett. 1992, 198, 570–
576. (b) Matsuzawa, N.; Dixon, D. A.; Fukunaga, T. J. Phys. Chem. 1992, 96,
7594–7604. (c) Dixon, D. A.; Matsuzawa, N.; Fukunaga, T.; Tebbe, F. N.
J. Phys. Chem. 1992, 96, 6107–6110.
ꢀ
(25) Bucsi, I.; Szabo, P.; Aniszfeld, R.; Prakash, G. K. S.; Olah, G. A.
Chromatographia 1998, 48, 59–64.
(26) Tzirakis, M. D.; Orfanopoulos, M. J. Am. Chem. Soc. 2009, 131,
4063–4069.
(27) Siedschlag, C.; Luftmann, H.; Wolff, C.; Mattay, J. Tetrahedron
1999, 55, 7805–7818.
(28) Tada, T.; Ishida, Y.; Saigo, K. Org. Lett. 2007, 9, 2083–2086.
(29) (a) Li, J.; Takeuchi, A.; Ozawa, M.; Li, X.; Saigo, K.; Kitazawa, K.
J. Chem. Soc., Chem. Commun. 1993, 1784–1785. (b) Husebo, L. O.;
Sitharaman, B.; Furukawa, K.; Kato, T.; Wilson, L. J. J. Am. Chem. Soc.
2004, 126, 12055–12064. (c) Alves, G. C.; Ladeira, L. O.; Righi, A.;
Krambrock, K.; Calado, H. D.; Gil, R. P. F.; Pinheiro, M. V. B. J. Braz.
Chem. Soc. 2006, 17, 1186–1190.
J. Org. Chem. Vol. 74, No. 15, 2009 5747

Tzirakis et al.
JOCNote
TABLE 1. Conversion of Ketones 1 and 11 to C60H2 and/or C60 under Various Conditionsa
relative yieldb (%)
entry
1c
substrate/conditions
time (h)
conversionb (%)
C60H2
C60
1/SiO2
1/PTSA
0
0
3
15
10
54
15
58
26
83
>99
0
0
20
92
2d
15
5
21
6
22
1.5
16
0.25
2
3e
1/NaOH 1 M, Ar
1/NaOH 1 M, Ar
1/NaOH 1 M, air
1/NaOH 1 M, air
1/NaOH 1 M, O2
1/NaOH 1 M, O2
1/NaOH 10 M, air
1/NaOH 10 M, air
1/basic Al2O3
33
27
34
28
38
33
40
15
67
73
66
72
62
67
60
85
4e
5 f
6 f
7g
8g
9h
10h
11i
12c
13d
14e
15 f
16 f
17g
18g
19i
>99
11/SiO2
11/PTSA
15
2
2
27
2
22
11/NaOH 1 M, Ar
11/NaOH 1 M, air
11/NaOH 1 M, air
11/NaOH 1 M, O2
11/NaOH 1 M, O2
11/basic Al2O3
53
71
10
44
6
>99
47
22
90
56
94
100
95
100
>99
aAll reactions were carried out by using 8ꢀ10-4 M 1or 11 in toluene (3 mL) at ambient temperature. Further details can be found in the Supporting
Information. bDetermined by HPLC analysis. These values are typically consistent with the obtained isolated yields. cThe reaction mixture was passed
through a column packed with silica gel. dThree milliliters of H2O containing ca. 80 mg of PTSA was added into the reaction mixture. eThree milliliters of
aqueous NaOH solution (1 M) was added. The reaction was carried out by using degassed solvents under argon atmosphere. fNaOH (1 M, 3 mL) was
added, and the reaction mixture was kept under ambient atmosphere. gAs above but under oxygen atmosphere (760 Torr). hNaOH (10 M, 3 mL) was
added under ambient atmosphere. iThe reaction mixture was passed through a column packed with Al2O3 (basic).
Finally, a toluene solution of 1 was treated with basic
Al2O3, under essentially the same conditions mentioned
above in the case of SiO2. To our delight and in contrast to
what was observed so far, ketone 1 was readily converted
to C60H2 in a quantitative yield without further conver-
sion to C60 (entry 11, Table 1).28 In this case, alumina
surface groups (i.e., negatively charged hydroxyl groups)
are most likely responsible for mediating this reaction.
We surmised that this solid material provides mild alka-
line conditions for the selective hydrolysis of 1 to C60H2;
subsequent conversion of C60H2 to C60 should be rela-
tively slow, thereby facilitating the selective formation of
C60H2 in essentially quantitative yield. To substantiate
this assumption, a pure sample of C60H2 was treated
similarly with basic Al2O3. Under these conditions,
C60H2 was indeed converted to C60, albeit at a much lower
rate (i.e., several hours, see the Supporting Information
for details).
The next set of experiments assessed the reactivity of
ketone 11, which was expected to be different from that of 1
(Figure 1), due to the presence of an aryl instead of an alkyl
substituent. Indeed, benzyl ketone 11 was found to be much
more reactive than aliphatic ketone 1 as evidenced by the
shortening of the time required for completion of the
reaction (compare entries 3-8 with 14-18 in Table 1).
For example, a 58% conversion of 1 was observed over 16
h, whereas the time required for an almost quantitative
conversion of 11 was only 2 h (cf. entries 8 and 17). This
result clearly indicates that the susceptibility of the C60-
attached carbonyl group toward a nucleophilic attack by a
hydroxide anion is directly determined by the R-carbonyl
substituent. The lability of the aryl ketone moiety in 11, by
virtue of the resonance stability in the newly formed benzoic
acid derivative (the major hydrolysis product, see below),
may account for the observed reactivity. In this case, the
rapid hydrolysis of 11 to C60H2 combined with the relatively
slow conversion of C60H2 to C60 results in the accumulation
of significant amounts of C60H2 at short reaction times. On
the other hand, as seen in Table 1 (cf. entries 15-18), the
C60/C60H2 ratio increases over time and C60 can be almost
quantitatively obtained as the final product at prolonged
reaction times in the presence of molecular oxygen. This
result corroborates the aforementioned conclusion that
molecular oxygen plays an important role in this reaction
and that C60 is primarily, if not exclusively, derived from
C60H2. Moreover, in line with our previous findings, 11 was
quantitatively converted into C60H2 upon simple treatment
with basic Al2O3 (entry 19, Table 1). The practical utility of
this latter process, which is largely based on its simplicity,
was then established by studying a series of structurally
diverse ketones 2-10, 12, and 13 (Figure 1). An almost
quantitative conversion of these substrates to C60H2 was
again achieved, indicating the great synthetic potential of
this approach in the selective synthesis of the otherwise
5748 J. Org. Chem. Vol. 74, No. 15, 2009

Tzirakis et al.
JOCNote
SCHEME 2. Proposed Mechanism for the Conversion of Acy-
lated Fullerenes to C60 and C60H2
affords fullerene C60. This latter pathway, namely the conver-
sion of C60H2 to C60 is much slower in basic Al2O3, thus giving
selective access to the formation of C60H2 in high yield.
In conclusion, we have shown that acylated [60]fullerenes
can be either converted quantitatively to C60H2 or regenerate
C60 under certain reaction conditions. Regeneration of C60 is
of fundamental interest as a measure of the chemical stability
of several C60 compounds, while the synthesis of the simplest
fullerene hydride, C60H2, is of considerable practical interest.
Thus, the decatungstate-catalyzed acylation of C60 followed
by hydrolysis represents a new, straightforward, route to the
selective synthesis of C60H2. This facile methodology obvi-
ates the need for HPLC separation, thus allowing for rapid
access to C60H2 through a simplified purification procedure,
in high yield and purity.
Experimental Section
Representative experimental procedures are described here.
Full experimental details for all reactions can be found within
the Supporting Information.
hardly accessible C60H2 (eq 1).
Al2O3
General Procedure for the Synthesis of Acylated Fullerene
Adducts 1-13. A solution of C60 (20 mg, 0.028 mmol) in a
mixture of chlorobenzene/acetonitrile 85:15 (80 mL) was added
in a 150-mL glass flask containing a magnetic stirring bar. This
solution was degassed by performing three freeze-pump-thaw
cycles under argon, and then TBADT (18.5 mg, 0.0056 mmol)
and the corresponding aldehyde (2.78 mmol) were added. This
solution was subsequently irradiated at ca. 5 °C under argon
atmosphere. The progress of all reactions was monitored by
HPLC. Then, the solvent and aldehyde were distilled from the
reaction mixture under reduced pressure, and the remaining
crude product was washed with acetonitrile. The isolated pro-
duct was further purified by flash column chromatography
(hexane/toluene 4:1 v/v) to afford the fullerene adducts 1-13
(30-50% isolated yield).
Synthesis of C60H2. Ketone 1 (10 mg, 0.012 mmol) was passed
through a long column packed with activated alumina (basic,
Brockmann I) to afford C60H2 quantitatively. This simple proce-
dure was similarly applied to ketones 2-13. Again, a quantitative
conversion of 2-7 and 9-13 to C60H2 was readily achieved.
Hydrolysis of ketones 7 and 9 to C60H2 was found to be relatively
slow. Thus, 7 and 9 were effectively converted to C60H2 after three
consecutive passages through a short column of basic alumina. On
the other hand, ketone 8 was hardly converted to C60H2 even after
consecutive passing of 8 through the alumina column three times
(ca. 10% conversion). The isolated yield for this reaction sequence
(acylation/hydrolysis) was routinely 30-50% with respect to the
starting C60 material, being depended on the ketone used. For
example, a 50% of C60H2 (0.014 mmol) was isolated via acylation of
1-13
f
1 -7, 9 -13; > 99%
8; 10%
C60H2
ð1Þ
Another important result obtained from these latter stu-
dies concerns the steric effect on the rate of this reaction. This
was evidenced by the longer time required for the hydrolysis
of ketones 7-9 which have secondary or tertiary substitution
at the R-carbon, respectively (see Supporting Information).
Accordingly, we conclude that this reaction is sensitive to
steric, in addition to electronic factors.
Scheme 2 depicts a mechanistic approach for the transforma-
tions of acylated fullerene adducts under alkaline conditions,
consistent with the results described in the preceding discussion,
as well as with previous findings.20,28 This process should be
initiated by nucleophilic attack of a hydroxide anion at the
carbonyl carbon, leading to the formation of HC60- anion and
carboxylic acid RCOOH. The fullerene anion acts as a good
leaving group, due to the strong electron-withdrawing ability of
the fullerene moiety.28,30 This mechanistic pathway has been
experimentally verified by studying the hydrolysis of ketone 12
with 1 M NaOH; in this case, p-methoxybenzoic acid, along with
a mixture of C60/C60H2, has been isolated as the reaction
product (see the Supporting Information for details). On the
other hand, the accumulation of C60H2 molecules is giving rise to
a second equilibrium process, which is also induced under basic
conditions. In this process, hydroxide anions abstract a proton
from C60H2 to form C60H- anion which is then converted to
fullerene hydroperoxide HC60OOH via two distinct pathways,
both of which involve the intervention of molecular oxygen.20
In one route, direct electron transfer from C60H- to O2 results in
two radical intermediates, namely superoxide radical anion
(O2•-) and fullerene radical (HC60•). Coupling of these radical
intermediates and subsequent protonation affords the full-
erene hydroperoxide HC60OOH. Alternatively, direct nucleo-
philic addition of fullerene anion (HC60-) to oxygen followed
by protonation results in the same fullerene hydroperoxide
HC60OOH. Subsequently, the elimination of a H2O2 molecule
C60 (20 mg, 0.028 mmol) with valeraldehyde followed by hydrolysis.
C60H2: IR (KBr) ν (cm-1)=2914, 1426, 1182, 577, 526; UV-vis
(CHCl3) λmax (nm)=257, 328, 405, 433; MS (MALDI, positive,
DCTB) m/z 722; 1H NMR (500 MHz, CDCl3/CS2) δ 7.00 (s, 1H);
1H NMR (500 MHz, C6D6): δ 5.91 (s, 1H); 1H NMR (300 MHz,
ODCB-d4) δ 6.85 (s, 1H); 13C NMR (ODCB-d4) δ 152.49, 147.79,
147.40, 146.39, 146.33, 146.11, 145.52, 145.44, 144.73, 143.31,
142.55, 141.98, 141.96, 141.61, 140.38, 136.36, 53.32.
Acknowledgment. The Greek National Scholarships
Foundation (IKY) is acknowledged for providing a three-
year fellowship to M.D.T.
Supporting Information Available: Detailed experimental
procedures, spectral data, 1H and 13C NMR, FT-IR, and UV-
vis spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
(30) For similar transformations, see: (a) Timmerman, P.; Anderson,
H. L.; Faust, R.; Nierengarten, J.-F.; Habicher, T.; Seiler, P.; Diederich, F.
Tetrahedron 1996, 52, 4925–4947. (b) Murata, Y.; Motoyama, K.; Komatsu,
K.; Wan, T. S. M. Tetrahedron 1996, 52, 5077–5090.
J. Org. Chem. Vol. 74, No. 15, 2009 5749