Rhenium-templated regioselective polyhydrogenation reaction of [60]fullerene

Article abstract of DOI:10.1002/anie.200351722

Functionalizing fullerenes: The first regioselective polyhydrogenation of [60]fullerene has been achieved. Upon treatment of [60]fullerene or its derivative with 9,10-dihydroanthracene in the presence of [Re ₂(CO)₁₀], transfer hydrometallation reaction takes place to give a polyhydrogenation product (see scheme). The reaction provided a pathway to a variety of metal η⁵ complexes of hydrofullerenes.

Full text of DOI:10.1002/anie.200351722

Communications
Hydrogenation of Fullerenes
Rhenium-Templated Regioselective
Polyhydrogenation Reaction of [60]Fullerene**
Motoki Toganoh, Yutaka Matsuo, and
Eiichi Nakamura*
With the progress of the science of fullerenes, regioselective
polyfunctionalization reactions have acquired
a unique
importance in the development of new research areas.[1]
Owing to the isotropic structure of [60]fullerene, regiocontrol
of polyfunctionalization is a fundamental problem, on which
considerable effort has been expended to provide useful
solutions to the synthesis of polyalkylated fullerene mole-
cules.[2] On the other hand, in spite of a significant number of
synthetic routes and suggested utilities in materials applica-
tions,[3] polyhydrogenated fullerenes can still only be pro-
duced as a mixture of many compounds owing to the lack of
control of both the regioselectivity and the number of
hydrogen atoms delivered to the fullerene molecule.[4]
Herein we report a new class of polyhydrogenation method,
a rhenium-templated transfer hydrogenation from 9,10-dihy-
droanthracene (DHA) to [60]fullerene or its derivatives. The
reaction of a [60]fullerene adduct 1 with DHA in the presence
of [Re2(CO)10] and water affords cyclopentadiene 2 [Eq. (1)],
5
À
Scheme 1. Preparation of metal h complexes by C H bond activation.
hydrofullerene complexes (4, 6, 8, and 9, Scheme 2). The new
hydrogenation/hydrometallation can also be described as a
transfer hydrogenation reaction (see below) and represents
the first method for completely regiocontrolled delivery of
hydrogen atoms to a fullerene molecule.[5]
The hydrogenation reaction is illustrated for the reaction
of the fullerene tri-adduct C60(H)(PhCH2)2(Ph)[6] (1) (a near
3:2 mixture of regioisomers as to the position of the hydrogen
atom as shown in Equation (1)) with DHA (11 equiv) in the
presence of [Re2(CO)10] (2.5 equiv) and H2O (100 equiv) in
PhCN at 1608C for 24 h [Eq. (1)]. The dihydrogenation
product C60(H)3(PhCH2)2(Ph) (2) was obtained in 61% yield
(obtained as a regio-mixture as to the position of the
which can be converted into the corresponding metal com-
plexes, such as an iron(ii) complex 3 and a rhenium(i) complex
4 (Scheme 1). The reaction with the rhenium cluster under
anhydrous conditions, on the other hand, results in hydro-
metallation reaction that affords a variety of the h5-Re
[*] Prof. Dr. E. Nakamura, Dr. M. Toganoh, Dr. Y. Matsuo
Department of Chemistry
The University of Tokyo
Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax: (+81)3-5800-6889
E-mail: nakamura@chem.s.u-tokyo.ac.jp
[**] This study was supported by a grant-in-aid for Scientific Research
(Specially Promoted Research) and by the 21st Century COE
Program for Frontiers in Fundamental Chemistry from the Ministry
of Education, Culture, Sports, Science and Technology. M.T. thanks
the Japan Society for Promotion of Science for a predoctoral
fellowship. Generous supply of [60]fullerene from Frontier Carbon
Corporation is gratefully acknowledged.
Supporting information (experimental details) for this article is
available on the WWW under http://www.angewandte.org or from
the author.
Scheme 2. Hydrometallation reaction affording rhenium-hydrofullerene
complexes.
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DOI: 10.1002/anie.200351722
Angew. Chem. Int. Ed. 2003, 42, 3530 –3532

Angewandte
Chemie
cyclopentadienyl hydrogen atom; all yields reported herein
are based on the fullerene compound). The assigned structure
rests on the 1H NMR and mass spectra, and was subsequently
confirmed by the single-crystal X-ray analysis of its rhenium
derivative 4 (see below).
rhenium complex 4 in 65% yield (Scheme 2a).[13] Similarly,
the reaction of a dialkyl fullerene C60(PhCH2)2 (5)[14] with
DHA and [Re2(CO)10] afforded a trihydrogenation product
[Re(CO)3{C60(H)3(PhCH2)2}] (6) in 55% yield (Scheme 2b).
The molecular weight determined from the mass spectrum
(APCI, [MÀ] 1176) is consistent with the assigned structure.
The IR (2020 and 1924 cmÀ1) and NMR spectra are similar to
those of 4. For instance, the protons Ha and Hb are coupled to
the neighboring sp3 carbon atoms with 1JCH = 144 Hz and
145 Hz, respectively. We therefore assigned the cyclopenta-
dienyl rhenium structure 6 to the product. In the same
The cyclopentadiene 2 was readily converted into the
5
5
À
corresponding h -Fe and h -Re complexes through C H bond
activation (Scheme 1).[7] Upon heating of 2 with [{(h-
C5H5)Fe(CO)2}2] (4.0 equiv) in PhCN at 1608C for 24 h, 2
afforded [(h-C5H5)Fe{C60(H)2(PhCH2)2(Ph)}] (3) in 28%
yield. Similarly, treatment of 2 with [Re2(CO)10] (2.5 equiv)
gave [Re(CO)3{C60(H)2(PhCH2)2(Ph)}] (4) in 74% yield. The
rhenium h5-complex 4 showed IR (2024 and 1939 cmÀ1), mass
atmospheric pressure chemical ionization (APCI) sepctra
([M+] 1252), and NMR spectra consistent with the assigned
structure. The two protons attached to the fullerene skeleton
appear as doublet signals at d = 5.32 and 5.51 ppm, which are
manner, the reaction of
a
monoalkyl fullerene
C60(H)(PhCH2)[15] (7) under the same conditions afforded a
tetrahydrogenation product [Re(CO)3{C60(H)4(PhCH2)}] (8)
in 42% yield (Scheme 2c). The 13C NMR signals indicated the
presence of only three kinds of sp3 fullerene carbon atoms
(Ca = 44.19 ppm, Cb = 44.56 ppm, and Cc = 55.77 ppm), which
indicates Cs symmetry on the NMR time scale (supported also
coupled with each other with JHH = 2.8 Hz. In the 13C NMR
5
1
spectrum the signals of the two carbon atoms attached to
by the H NMR spectrum). Carbon atoms Ca and Cb have
1
these protons (both at d = 44.35 ppm) show the same JCH
1JCH = 145 Hz. Mass (APCI, [MÀ] 1086) and IR spectra (2020,
1922 cmÀ1) are also consistent with its structure.
value of 145 Hz, which indicates 29% s character[8] (that is
sp2.4) of these “sp3” carbon atoms.[9]
Finally, the reaction of [60]fullerene with DHA and
[Re2(CO)10] afforded the hydrometallated product 9 albeit in
low yield (Scheme 2d). The compound 9 had mass (APCI
[MÀ] 996), IR (2017, 1921, and 1901 cmÀ1) and NMR spectra
consistent with the assigned structure. The 1H NMR spectrum
has only one singlet signal at d = 5.23 ppm, and the 13C NMR
spectrum has only nine signals (indicating C5v symmetry on
the NMR time scale) namely, those of an sp3 (d = 44.73 ppm,
1JCH = 145 Hz), an sp2 cyclopentadienide (d = 102.97 ppm), a
carbonyl carbon atom (d = 190.64 ppm), and six sp2 carbon
atoms assigned to the remaining carbon atoms of the fullerene
core.
Unambiguous structure determination of 4 was by X-ray
crystallographic analysis of
a single crystal (Figure 1),
obtained by slow diffusion of hexane to a CS2 solution of
All the reactions described above proceeded cleanly and
gave exclusively one regioisomer of the desired hydrogena-
tion product, accompanied only by Diels–Alder adducts
between the starting fullerene compound and anthracene
(dehydrogenation product of DHA),[16] as well as recovered
starting fullerene compound.[17] The solvent PhCN was
uniquely effective among the variety of aromatic and other
high-boiling-point solvents used. Simple polyhydrogenation
products other than 2 have not been obtained to date in
synthetically significant yields, this is due to the instability of
such polyhydrogenated products towards oxygen and light.[5]
On the other hand, the metal complexes 3, 4, 6, 8, and 9 are
very stable.
The mechanism of the present reaction remains unclear.
There are nonetheless pieces of experimental information
that give some insight into the reaction pathway. First,
consumption of [60]fullerene by reaction with DHA in the
presence of [Re2(CO)10] is much faster than in its absence,
and, under the latter conditions, the reaction produced a
mixture of various hydrogenated products, of which C60(H)36
was predominant.[4] Second, the reaction of [Re2(CO)10]
(2.8 equiv) with C60(H)36 (prepared by the reaction with
DHA and which is thus contaminated with various other
hydrogenated products) at 1608C in PhCN did not give 9 but
only produced [60]fullerene. Therefore, hydrofullerenes such
as C60(H)36 are unlikely to be involved in the reaction
Figure 1. Molecular structure of the hydrofullerene rhenium complex 4.
A) ORTEP drawing with thermal ellipsoids at 30% probability. The CS2
molecule found in the unit cell was omitted for clarity. B) Space-filling
models.
4.[10] The X-ray data clearly indicates that cyclopentadienyl
rhenium moiety is surrounded by five sp3 carbon centers
(rather than sp2 centers) as in the penta-alkylated [60]full-
erene metal h5-complexes reported previously.[7,11] The geom-
etry around the rhenium center is similar to that of [(h-
C5H5)Re(CO)3],[12] as the three phenyl rings bound to the
fullerene core in 4 point away from the metal center to
minimize steric interaction with the carbonyl ligands.
The rhenium-templated polyhydrogenation under anhy-
drous conditions provides a direct entry to a variety of
rhenium h5-complexes of hydrofullerenes (Scheme 2). Treat-
ment of 1 with DHA (11 equiv) and [Re2(CO)10] (2.5 equiv)
at 1608C for 24 h in anhydrous PhCN directly afforded the
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Communications
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(C60(D)(PhCH2)2(Ph), > 99% D) with DHA (10 equiv) and
[Re2(CO)10] (2.5 equiv) at 1608C in PhCN for 24 h afforded a
deuterated product [D]4 ([Re(CO)3{C60(H)(D)(PhCH2)2-
(Ph)}) in 17% yield and its diprotio compound 4 in 43%
À
yield. It is thus likely that the cyclopentadienyl C D bond
cleavage occurred first to generate a {Re-D} intermediate
(transfer deuteration), which delivers the deuterium atom
back to the periphery of the cylopentadienyl moiety. Similar
À
C H bond activation may also be involved in the reaction of
7. Finally, the rhenium complex 4 could not be converted into
the metal-free compound 2 under the wet conditions where 2
was obtained, which suggests that 2 was not formed via 4 in
Equation (1).
In summary, we have developed a new transfer hydro-
genation reaction to introduce hydrogen atoms regioselec-
tively to [60]fullerene and its derivatives. The reaction under
anhydrous conditions provides a flexible method for the
synthesis of the h5-metal complexes of a variety of hydro-
fullerenes, while the reaction under wet conditions has proven
to be effective for the synthesis of the trihydro compound 2.
In contrast to the known polyhydrogenation reactions that
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both the regioselectivity and the numbers of hydrogen atoms
is remarkably high. The h5-structures of the metal com-
plexes[7,11] reported herein are the first examples of such
hydrofullerenes. The similarity of the regioselectivity of the
formation of 4, 6, 8, and 9 to that of the regioselective
polyalkylation reported previously[2b,19] suggests certain
mechanistic kinship among these reactions, for example, an
electron-transfer mechanism. We anticipate that the hydro-
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Received: April 23, 2003 [Z51722]
Published online: July 11, 2003
Keywords: cyclopentadienyl ligands · fullerenes ·
.
hydrogenation · hydrometallation · rhenium
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Angew. Chem. Int. Ed. 2003, 42, 3530 –3532