Di- and trinuclear [70]fullerene complexes: Syntheses and metal-metal electronic interactions

Full text of DOI:10.1002/anie.200902185

Angewandte
Chemie
DOI: 10.1002/anie.200902185
Fullerenes
Di- and Trinuclear [70]Fullerene Complexes: Syntheses and Metal–
Metal Electronic Interactions**
Yutaka Matsuo,* Kazukuni Tahara, Takeshi Fujita, and Eiichi Nakamura*
Transition-metal complexes of fullerenes are the molecules of
interest in a variety of fields of chemistry.[1] Their multinuclear
variants[2] are particularly interesting because of their rather
unusual extended d–p–d conjugated system, in which the
metal atoms interact with each other[3] through the semi-
conducting p system.[4] In this context, electronic interactions
between multiple metal atoms across a p bridge is an
interesting subject of study related to the development of
molecular electronics.[5] With such interests in mind, we
recently synthesized several stable mono- and dinuclear
complexes of [60]fullerene[6,7] and reported on the unique
photophysics of these molecules[8] as well as on their self-
assembled monolayers on electrode surfaces.[9] We report
herein the unique chemistry of di- and trinuclear [70]fullerene
complexes 2 and 4 (Scheme 1), including regioselective
sixfold addition of an arylcopper reagent to [70]fullerene to
form 1, regioselective addition of a seventh aryl group to the
dinuclear complex 2 to form 3, and the electronic communi-
cation among the metal atoms by way of the [70]fullerene
conjugated p system.
obtained as corresponding racemates. We determined unam-
biguously the positions of the addends by X-ray crystallo-
graphic analysis of the diprotio compound 1a and its diiron
and diruthenium complexes 2a and 2b (Figure 1a,b; the
The first step of the synthesis is the copper-mediated[10]
sixfold addition of arylmagnesium bromide to [70]fullerene to
produce hexaaryl adducts C70Ar6H2 (1a–d; Scheme 1) in the
presence of pyridine,[11] without which the reaction stops after
threefold addition.[12] The formation of six C C bonds took
À
place in one step in 40–71% yield of isolated product on a 1 g
scale. Other side products were unidentified insoluble prod-
ucts and adducts bearing more than six aryl groups. The
mechanism of the high regioselectivity is unknown at this
time. Repeated deprotonation of 1 (KOtBu) and treatment
with a cationic cyclopentadienyl complex (e.g., [FeCp*-
(MeCN)3]PF6; Cp* = C5Me5) produced the dinuclear com-
plexes [{M(C5R5)}2{C70Ar6}] (2a–c). They are chiral and were
[*] Prof. Y. Matsuo, Dr. K. Tahara,[+] T. Fujita, Prof. E. Nakamura
Department of Chemistry, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 (Japan)
Fax: (+81)3-5800-6889
E-mail: matsuo@chem.s.u-tokyo.ac.jp
nakamura@chem.s.u-tokyo.ac.jp
Prof. Y. Matsuo, Prof. E. Nakamura
Nakamura Functional Carbon Cluster Project
ERATO (Japan) Science and Technology Agency
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 (Japan)
[+] Present Address: Division of Frontier Materials Science, Graduate
School of Engineering Science, Osaka University (Japan)
[**] This work was partially supported by KAKENHI (no. 18105004) and
the Global COE Program for Chemistry Innovation of the MEXT
(Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902185.
Scheme 1. Syntheses and Schlegel diagrams of di- and trinuclear
[70]fullerene complexes.
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Figure 2. Electrochemistry of the diruthenium complex 2b. a) Cyclic
voltammogram of the electrochemical quasi-reversible oxidation of 2b.
The difference in the intensity of the anodic and cathodic scans
suggests that the process is chemically irreversible. Half-wave oxida-
tion potentials can be estimated to be 540 and 880 mV (vs. Fc/Fc+).
The ferrocene/ferrocenium couple is the internal standard. The mea-
surement was performed in CH2Cl2 containing equimolar amounts of
[FeCp2] and [nBu4N][B{C6H3-(CF3)2-3,5}4] (0.1m) as an electrolyte,
which was crucial for the successful measurements.[17] The sweep
started from À230 mV and moved towards the positive direction.
b) Cyclic voltammogram of the reversible reduction of 2b in THF
containing nBu4NClO4.
Figure 1. X-ray crystal structures of hexa- and heptaaryl [70]fullerenes
and their di- and trinuclear complexes. Coloring corresponds to that in
Scheme 1. a) Hexaphenyl adduct 1a. b) Diruthenium complex 2b.
c) Hepta-aryladduct 3b. The seventh aryl group is shown in light blue.
d) Triruthenium complex 4c.
determined by X-ray crystallographic analysis (Figure 1c).
Further conversion of 3 to the corresponding trinuclear
compounds 4 was achieved in a manner similar to the
structure of 2a is in the Supporting Information). The
ferrocene and ruthenocene moieties show the bonding
characteristics of an h5-indenyl metal complex (Scheme 1,
top).[10] The two h5-indenyl metal units are part of a biphenyl
motif on the fullerene surface (colored red in the Schlegel
diagram in Scheme 1, bottom).
synthesis of 2. A mixed diiron ruthenium compound
[(Cp*Fe)2(CpRu)(C70Ar7)] (4a; Cp = C5H5) was thus
obtained in 50% yield (Scheme 1), in which the third
ruthenium atom is coordinated by a fluorenyl motif (red in
Scheme 1, bottom).
Triruthenium complexes 4b–d were also synthesized from
3b–d in a similar fashion. The X-ray structure of 4c is shown
in Figure 1d. As indicated by the Schlegel diagram in
Scheme 1, the three metallocene groups (red) are conjuga-
tively connected through a cyclic poly(p-phenylene) array
(orange), as required for a three-way junction. The cyclic
voltammogram of the trinuclear complex 4d in CH2Cl2
exhibited a one-electron quasi-reversible or irreversible
oxidation wave for each metal center (Epa = 0.39, 0.65, and
0.85 V vs. Fc/Fc+; the E1/2 values (0.33, 0.55, and 0.82 V) are
less certain owing to irreversible processes, Figure 3a). The
first and the second oxidation events most likely occurred at
the two indenyl moieties, because the fluorenyl unit is the
most influenced by the strong electron-withdrawing nature of
[70]fullerene, thus giving a higher oxidation potential. The
stepwise oxidation with the large separations between the
waves demonstrates the metal–metal interaction through the
conjugated p-electron system of [70]fullerene. Reversible
reduction of 4b took place at À1.77 and À2.19 V versus Fc/
Fc+ in THF (Figure 3b), indicating that the trinuclear
complex is still highly electron-accepting.
In summary, we have developed a strategy for selective
synthesis of new classes of [70]fullerene derivatives 1 to 4
possessing many organic and metallic substituents in structur-
ally defined positions. The synthesis has been achieved with a
high level of regiocontrol and with minimal synthetic effort,
and the multinuclear compounds are thermally stable in air.
Not only their molecular structures are striking but their
electronic properties are unique in that the metal atoms
electronically communicate with each other. The new syn-
thetic strategy has made possible the installation of organic
Electrochemical measurements further support the struc-
tural data for the diruthenium complex 2b. Cyclic voltam-
metry (CV) and differential pulse voltammetry (DPV) of 2b
in CH2Cl2 reveal two quasi-reversible one-electron oxidation
waves arising from the two metal atoms at E1/2 = 0.54 and
0.88 V versus ferrocene/ferrocenium (Fc/Fc+; Figure 2a). The
potential difference[13] DE = 340 mV is very large, because the
two metallocene moieties are directly conjugated with each
other through the [70]fullerene p system (Scheme 1 and
Figure 1; cf. HOMO conjugation[14] for the dinuclear [60]full-
erene system).[7,15] In the cathodic scan of 2b in THF, two
reversible one-electron reduction waves corresponding to
stepwise two-electron reduction of the [70]fullerene core
were observed (E1/2 = À1.50 and À2.06 V vs. Fc/Fc+; Fig-
ure 2b). These values are comparable to those of the second
and third one-electron reduction of [70]fullerene (À0.87,
À1.44, and À1.93 V vs. Fc/Fc+ in THF), thus indicating that
the dinuclear hexaaryl [70]fullerene still possesses a high
electron-accepting ability and stability to reduction—proper-
ties necessary for the fullerene to act as a viable device.[16]
The next challenge was the synthesis of triply substituted
compounds, which we achieved in good yield and with high
regioselectivity (Scheme 1). Introduction of the seventh aryl
group to the diiron complex 2a by addition of {C6H4-(tert-
C4H9)-p}MgBr in the presence of CuBr·SMe2 and pyridine
took place selectively and afforded a single product, the
heptaaryl compound [(Cp*Fe)2(C70{C6H4-(tert-C4H9)-p}7H)]
(3a) in 66% yield of isolated product. Analogous reactions
afforded the ruthenium compounds 3b–d regioselectively in
63–86% yield. The position of the Ar’ group in 3c was
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Figure 3. Electrochemistry of the triruthenium complex 4d. a) Cyclic
voltammogram of the three-electron oxidation of 4d. The measure-
ment was performed in CH2Cl2 containing [nBu4N][B{C6H3-(CF3)2-3,5}4]
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Experimental Section
1b: [C6H4-(tert-C4H9)-4]MgBr in THF (0.88m, 20.2 mL, 17.9 mmol)
was added to CuBr·SMe2 (3.64 g, 17.9 mmol) in pyridine (200 mL) at
288C. After stirring for 10 min at 408C, a degassed solution of C70
(1.00 g, 1.19 mmol) in o-dichlorobenzene (200 mL) was added. The
multiple addition reaction was complete within 6 h. The reaction
mixture was quenched with HCl(aq). Pyridine and THF were
removed under vacuum. The crude mixture was dissolved in toluene
and filtered through a pad of silica gel (toluene). The solution was
concentrated to a small volume, and the products were precipitated
by addition of MeOH to obtain a dark red solid. The crystals were
washed thoroughly with MeOH and dried under vacuum. Analyti-
cally pure C70[C6H4-(tert-C4H9)-4]6H2 (1.39 g, 71% yield) was
obtained after purification with HPLC (RPFullerene, 250 mm,
toluene/acetonitrile = 1:1).
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2a: KOtBu (1.0m in THF, 80 mL, 80 mmol) was added to 1b
(120 mg, 73.0 mmol) in THF (12 mL) at 288C. [FeCp*(CH3CN)3]PF6
(0.10m in CH3CN, 0.85 mL, 85 mmol) was added to the reaction
mixture. After stirring for 5 min at 288C, additional KOtBu (1.0m in
THF, 80 mL, 80 mmol) and [FeCp*(CH3CN)3]PF6 (0.10m in CH3CN,
0.85 mL, 85 mmol) was added to the mixture. After further stirring for
5 min, solvents were removed under vacuum. The black residue was
dissolved in hexane and purified by the silica gel column chromatog-
raphy (hexane/toluene = 1:0; ca. 9:1 as eluent). The solvents were
evaporated. Recrystallization from toluene/MeOH gave black plate
crystals of 2a (57.8 mg, 39% yield).
[13] The potential difference DE is often taken as a measure of
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[15] This difference is much larger than those of the diiron
decamethyl[60]fullerene complexes D5d [Fe2(C60Me10)Cp2]
(DE = 110 mV) and C2v [Fe2(C60Me10)Cp2] (DE = 135 mV), in
which there is no direct conjugation between the two ferrocene
moieties.
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Experimental details for compounds 1a,c,d, 2b,c, 3a,d, and 4a–d
and X-ray data are supplied in the Supporting Information.
Received: April 23, 2009
Revised: May 25, 2009
Published online: July 23, 2009
Keywords: electronic interactions · fullerenes · metallocenes ·
.
molecular electronics
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