The first molecular dumbbell consisting of an endohedral Sc3N@C80 and an empty C60-fullerene building block

Article abstract of DOI:10.1039/c7cc03012f

An unprecedented hybrid dumbbell consisting of a metallofullerene and an empty fullerene was afforded via simple click reaction of suitable precursor derivatives of Sc₃N@C₈₀ and a C₆₀ hexakisadduct.

Full text of DOI:10.1039/c7cc03012f

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ISSN 1359-7345
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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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DOI: 10.1039/C7CC03012F
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The first molecular dumbbell consisting of an endohedral
Sc N@C and an empty C -fullerene building block
3
80
60
Received 00th January 20xx,
Accepted 00th January 20xx
+a
+a
a
Tao Wei, M. Eugenia Pérez-Ojeda, and Andreas Hirsch*
DOI: 10.1039/x0xx00000x
www.rsc.org/
An unprecedented hybrid dumbbell consisiting of
metallofullerene and empty fullerene was afforded via simple
click reaction of suitable precursor derivatives of Sc N@C80 and a
60 hexakisadduct.
a
Fm
Fn
3
C
Dumbbell shaped fullerenes represent an interesting class of
compounds. Their molecular architectures have been
attracting great attention due to their distinct
Bridge
1
chemical/physical properties. Up to date, various series of
such dimers based on the introduction of different type of
fullerenes (e.g. C60, C70, heterofullerenes, endofullerenes) as
Fig. 1 Previously reported types of dumbbell shaped fullerene dimers: (A, B) =
well as various linkers serving as bridged have been reported (C60, C60), (C60, C70), (C70, C70), (C59N, C59N), (C59N, C60), (C69N, C69N), (N@C60
,
2
-11
(
see Fig. 1).
One striking characteristic of dimeric fullerene N@C60), (N@C60, C60), (He@C60, C60), (H
2
@C60, H
2
@C60),(m,n) = 0 – 5, F denotes
the functionalized group.
dumbbells are their tunable properties by the possibility of
modifying the chemical nature of fullerene building blocks or
the chemical linkage between them. So far however, only
either empty fullerenes, heterofullerenes (C59N) or the non
pentakisadducts of C60 with C2v symmetry as useful building
blocks for the synthesis of large and highly functional
architectures involving an octahedral [5:1] addition pattern.
1
5
2
metallic endohedral fullerenes such as He@C60, H @C60 and
2
For the targeted synthesis of the first dumbbell consisting of
an endohedral and an empty fullerene we decided to prepare
a [5:1]-adduct of C60 equipped with 10 chains which will
facilitate the dumbbell solubility (aliphatic ones in this case)
and one terminal alkyne group. This should serve as an anchor
unit for the “click” coupling with a monoadduct of the
N@C60 have been employed where corresponding molecular
architectures containing endohedral clusterfullerenes are
elusive. Among these examples and due its unique high spin
behavior, dimers based on N@C60 promise feasibility for
11
quantum computer applications. However, the incorporation
of endohedral clusterfullerene, especially metal nitride
clusterfullerenes (NCFs), is still lacking. In these endofullerene
derivatives the encapsulated cluster comprising of both metal
and non-metal ions gives rise to an electron transfer between
endohedral guest and the carbon cage. This feature which is
absent in empty fullerenes or N@C60 makes the synthesis of
dumbbells an interesting task since the properties of both
kinds of fullerenes can be combined in one molecule. The only
cases of dimeric endohedral metallofullerenes were reported
endohedral clusterfullerene-Sc
complementary azide functionality in its periphery.
Sc N@I -C80was chosen as endohedral fullerene building block
3 h
N@I -C80 containing the
3
h
1
6
since it is easily available in rather high yields. On the other
hand, its high cage symmetry bearing two available reaction
positions allows the formation of only two possible
regioisomeric monoadducts. Altogether the synthesis of the
targeted fullerene dumbbell consists of three steps: 1) the
synthesis of an azidomalonate monoadduct of Sc N@I -C ; 2)
1
2-14
3
h
80
to take place in the solid state after crystallization.
Our group has demonstrated previously the versatility of [5:0]
the selective synthesis of a [5:1] hexakisadduct of C60 provided
with an alkyne and 3) the covalent connection of these
subunits via “click” reaction.
In the first step, we aimed to synthesize an azidomalonate
monoadduct of Sc N@I -C80 using Bingel-Hirsch reaction
3 h
conditions (scheme 1 (I) and for further details see S2.1). The
subsequent purification afforded a pure major product (named
as
time-of-flight (MALDI-TOF) mass spectroscopic analysis
showing an ionic peak at m/z 1308 (Fig. S6).
3) which was confirmed by matrix-assisted laser desorption
=
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O
O
N
3
O
O
O
O
DBU / I
2
(
I)
N
3
O
O
oꢀDCB 60 ºC
O
O
O
O
O O
O
O
N
N
O
O
O
1
2
3
O
O
O
N
CuSO
sodium ascorbate
4
—5H
2
O
O
O
O
O
O
CH Cl /H
2
2
2
O
O
O
O
O
O
O
O
O
O
O O
O
O
O
O
O
O
O O
O
O
O
O
O
O
O
O
7
O
O
O
O
DBU / CBr4
O
O
O
O
O
O
O
(
II)
toluene, rt
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
4
5
6
Scheme 1 Schematic synthesis route of dumbbell-7.
1 13
Additionally, both H and C NMR were performed to 96h (scheme 1). Upon such a mild and long reaction treatment
1
unambiguously elucidate its structure. The H NMR spectrum the assumed 1,2,3-triazole-linked dumbbell comprising of
3
of exhibits one set of signals featuring one quartet, one and was formed. Surprisingly, despite the presence of Cu(I)
3
6
singlet and two triplets centered at 4.59, 4.11, 3.50 and 2.09 catalyst, the clicked product afforded was not the expected
ppm, respectively (Fig. S7). The singlet at 3.50 ppm sole 1,4-adduct but an isomeric mixture in a 3:1 ratio of 1,4
corresponds to the methoxy group and the other three and 1,5-adduct which would have been likely under thermal
resonances can be attributed to protons of three remaining conditions. This interesting observation is indicative of some
methylene groups. A potential [5,6]-adduct would give rise to
two sets of nonequivalent protons as a result of different
chemical environment stemming from 5,6-ring junction,
whereas the more symmetrical [6,6]-adduct gives only one set
of proton signals. The fact that just one symmetrical [6,6]-
adduct was formed is in good agreement with previous studies
on
reactions. The C NMR spectrum of
3 h
M N@I -C80 adducts synthesized via Bingel-Hirsch
1
7
13
3
shows 5 signals in the
ranges of 20-60 ppm and two additional signals around 163
ppm for the attached group, besides, another 80 resonances
distributed in 85-155 ppm region were observed for carbon
cage, consistent with C
caused by addition of asymmetric functionalized moiety is in
line with previously reported Sc N@C80-PCBM, likewise, for
which the C symmetry was clearly confirmed by both C NMR
and X-ray crystallographic study.
In the second step, the [5:1] hexakisadduct of C60 (named as
was facilely synthesized utilizing Bingel-Hirsch reaction
scheme 1 (II) and for further details see S2.2). Finally, in order
to construct the target dumbbell, a copper-catalyzed Huisgen
,3-dipolar cycloaddition between azides and alkynes “click”
reaction was conducted by using and 1.5-fold of in
modified Sharpless conditions at room temperature with
CuSO •5H O (0.3 equiv.) and sodium ascorbate (0.9 equiv.) in
biphasic system CH Cl /H O, under nitrogen atmosphere for
1 1
symmetry (Fig. S8). The C symmetry
3
1
3
1
1
8
6
)
(
1
3
6
Fig. 2 MALDI-TOF mass spectrum of dumbbell-
calculated isotope distributions.
7
. Inset shows the measured and
4
2
2
2
2
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not yet well understood effect of copper on Sc
COMMUNICATION
1
3
3
N@I
properties, charge transfer or redox activity. On account of NMR spectrum.
very similar polarity of this two isomers, the completely The electronic properties of dumbbell-
separation of them is quite challenging. Fortunately, partial UV-vis-NIR spectroscopy. The UV-vis-NIR spectrum of
pure major isomer (named as ) was successfully isolated on a dumbbell- dissolved in toluene is shown in Fig. 3, which
silica-gel chromatographic column thanks to the good includes also those of monomers and for comparison.
solubility provided by the linkage to the hexakisadduct of C60 Apparently, the overall electronic absorption spectrum of is
h
-C80 limited amount of dumbbell-
7
precluded acquisition of its
C
1
9
7
were investigated by
7
7
3
6
.
3
The yield of major isomer, namely dumbbell-7, was roughly featureless without sharp absorption peaks and exhibits two
estimated to be 45% regardless of its small quantity that could broad shoulder peaks at about 440 and 748 nm, which is less
not be isolated from the isomers mixture.
The purified product enables the characterization to confirm absorption peaks are observed at 316, 334nm together with a
its molecular structure as well as its electronic properties. broad peak at 385 nm. For dumbbell- , several characteristic
rich in characteristics than that of 6, for which two obvious
7
Given that mass spectrometry can provide the most intuitive absorption peaks, which are derived from the two building
information about composition of the molecule, it was blocks, at 319, 336, 417 and 737 nm are clearly observed.
immediately applied to
mass spectrum of dumbbell-
at m/z = 2972 corresponding to the clicked product of
The chemical identification of dumbbell- was further for the remaining two absorption peaks in visible region.
achieved by comparing the isotopic distribution analysis of Besides, the absorption peaks at 319 and 336 nm of dumbbell-
experimental measured result with the calculated one since seem to be red-shifted compared to , in contrast, a blue-
well agreement is observed. Consequently, for the first time, shift for the other two peaks, 417 and 737 nm, is detected in
an unprecedented dumbbell based on endohedral comparison with that of . Regardless of the slight red/blue
clusterfullerene via chemical modification was successfully shift, the overall absorption spectrum of dumbbell- is quite
achieved. similar to the superposition spectra of two monomers,
The formation of dumbbell was further confirmed by H NMR indicating that dumbbell- inherits both electronic properties
spectroscopy study as shown in Fig. S19. In the H NMR of two constituents. Therefore this methodology opens up the
spectrum of dumbbell- , the four resonances centered at 4.52, possibility to combine the electronic properties from two
.12, 3.57 and 2.29 ppm are obviously derived from monomer fullerenes with deeply different natures. By carefully selection
and the other three resonances centered at 4.34, 3.80, 1.31 of the appropriate parental counterparts, the dumbbell
. Interestingly, all formation might be a very versatile tool to tune the electronic
are either low field or high properties as demanded by the applications. It will allow the
7
. As shown in figure 2, the MALDI-TOF Specifically, two sharp absorption peaks, 319 and 336 nm, in
exhibited an evident ionic peak UV region are apparently originated from one of the
and individuals of , whereas the other individual of is accounting
7
3
6.
6
3
7
7
6
3
7
1
7
1
7
4
3
ppm are clearly originated from monomer
these resonances of dumbbell-
6
7
field shifted compared to two monomers, suggesting that synthesis of novel fullerene derivatives with attractive
linking these two building blocks for the dumbbell formation properties which may diversify the potential applications of
brings about the changes in the chemical environment. fullerene-based multifunctional molecules.
Noticeable, a singlet resonance centered at 7.71 ppm Besides, in order to elucidate which is the real influence of the
corresponding to the proton situated at 1,2,3-triazole as a endofullerene, dumbbell-
result of the click reaction between azido group within and dimer- , which was synthesized by substitution of azido
alkynyl group within , states clearly that the formation of monoadduct of Sc N@C80 with azido monoadduct of C60 (8)
7 was further compared to congener
3
9
6
3
dumbbell has been successfully proceeded. Unfortunately, the (for details see S3). Under the same experimental conditions,
only the 1,4-adduct formation was observed which clearly
indicates the influence of the endofullerene in the copper
catalyzed click mechanism as explained above. As expected,
the UV-vis-NIR spectrum of dimer-
overlap of absorption peaks from the corresponding
monomers and (Fig. S20).
Moreover, noting existing six charges within
9 bears resemblance to the
8
6
3
as a result of
charge transfer from encaged species to the carbon cage,
investigation on the probably charge transfer between this
two building blocks is convinced to be quite attractive, which
will be carried out in the future.
In summary, we have synthesized and isolated the hitherto
unknown dumbbell-
clusterfullerene monodadduct
The structure of synthetic dumbbell-
7
via bonding of the endohedral
with the C60 hexakisaddcut
was unambiguously
3
6.
7
1
characterized by MALDI-TOF mass spectrometry and H NMR
spectroscopy. The electronic structure was carefully studied by
UV-vis-NIR spectroscopy and compared to parental building
blocks and congener dimer-
9, the peculiar electronic
Fig. 3 UV-vis-NIR spectrum of dumbbell-
with those of and 6. Insets: enlarged spectra in 300-500 nm (I) and 350-950 nm
II).
7
dissolved in toluene in comparison properties achieved by combination of two different moieties
3
in the dumbbell is unveiled. Our success on the first
construction of endohedral clusterfullerene dumbbell opens
up a new perspective in fullerene chemistry.
(
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This work was
Journal Name
supported
by
the
Deutsche
Forschungsgemeinschaft (DFG) through the SFB 953, the
Alexander von Humboldt (AvH) Foundation (Tao Wei) and
Marie Sklodowska-Curie IF European Actions 747734 Hy-
solFullGraph to M.E. Pérez-Ojeda.
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9 During the course of the reaction it was observed that the typical
yellowish color from aqueous phase containing Cu(I) specie
protected against oxidation in the presence of huge excess of sodium
ascorbate and inert nitrogen atmosphere turned into blue as a proof
of some oxidative process which could be tentatively promoted by
the endofullerene moiety. Other authors have previously described
that the presence of Cu additive in the electric arc reactor affects
3 h
dramatically the yield of metallofullerene Sc N@I -C80, especially
under equimolar amounts of Cu and Sc. However they have not been
able yet to address the reason of this behaviour. S. Stevenson, M. A.
Mackey, M. C. Thompson, H. L. Coumbe, P. K. Madasu, C. E. Coumbe
and J. P. Phillips. Chem. Commun., 2007, 4263–4265.
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