Iridium-catalyzed reductive carbon-carbon bond cleavage reaction on a curved pyridylcorannulene skeleton

Article abstract of DOI:10.1002/anie.201500819

The cleavage of C-C bonds in π-conjugated systems is an important method for controlling their shape and coplanarity. An efficient way for the cleavage of an aromatic C-C bond in a typical buckybowl corannulene skeleton is reported. The reaction of 2-pyridylcorannulene with a catalytic amount of IrCl₃n H₂O in ethylene glycol at 250 °C resulted in a structural transformation from the curved corannulene skeleton to a strain-free flat benzo[ghi]fluoranthene skeleton through a site-selective C-C cleavage reaction. This cleavage reaction was found to be driven by both the coordination of the 2-pyridyl substituent to iridium and the relief of strain in the curved corannulene skeleton. This finding should facilitate the design of carbon nanomaterials based on C-C bond cleavage reactions.

Full text of DOI:10.1002/anie.201500819

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201500819
German Edition: DOI: 10.1002/ange.201500819
Corannulenes
Hot Paper
Iridium-Catalyzed Reductive Carbon–Carbon Bond Cleavage
Reaction on a Curved Pyridylcorannulene Skeleton**
Shohei Tashiro, Mihoko Yamada, and Mitsuhiko Shionoya*
À
Abstract: The cleavage of C C bonds in p-conjugated systems
is an important method for controlling their shape and
coplanarity. An efficient way for the cleavage of an aromatic
À
strained buckybowl skeletons are limited to only a few special
examples, such as the fragmentation of corannulene during
mass measurements, the cleavage of a strongly distorted
corannulene derivative during flash vacuum pyrolysis, the
ring opening of decapentynylcorannulene via a diradical
intermediate, and laser annealing or oxone oxidation of
sumanene derivatives.[7] Herein, we report an efficient and
C C bond in a typical buckybowl corannulene skeleton is
reported. The reaction of 2-pyridylcorannulene with a catalytic
amount of IrCl3·nH2O in ethylene glycol at 2508C resulted in
a structural transformation from the curved corannulene
skeleton to a strain-free flat benzo[ghi]fluoranthene skeleton
À
selective C C bond cleavage reaction of a curved corannu-
À
through a site-selective C C cleavage reaction. This cleavage
lene skeleton by using IrCl3·nH2O as a catalyst under
microwave heating at 2508C in ethylene glycol to obtain
a flat benzo[ghi]fluoranthene skeleton (Figure 1). It was
suggested that relief of the nonplanar strain in the corannu-
lene skeleton was an important factor contributing to the
reaction was found to be driven by both the coordination of the
2-pyridyl substituent to iridium and the relief of strain in the
curved corannulene skeleton. This finding should facilitate the
À
design of carbon nanomaterials based on C C bond cleavage
reactions.
C
arbon nanomaterials, such as fullerenes, carbon nanotubes,
graphene, carbon nanorings, and buckybowls, are organic
materials that have recently attracted increasing attention,[1]
and the chemical modification of their basic skeletons is
essential to expand the capability of such materials. The most
typical transformations entail structural extension by attach-
ing functionality.[1,2] On the other hand, cutting the carbon
À
Figure 1. C C bond cleavage and concomitant planarization of a cor-
annulene skeleton.
À
nanomaterials through C C bond cleavage has also been
developed as a unique and useful transformation to obtain
hard-to-find structures.[3] For example, fullerenes were holed
to encapsulate guest species.[3c] Carbon nanotubes have also
been cleaved to open the nanotube termini[3e] or to shorten
the nanotubes.[3f] Another remarkable example is the unzip-
ping of carbon nanotubes[3g] and graphene sheets[3h] through
À
À
cleavage through insertion of iridium into the C C bond. C C
bond activation of strained compounds with transition metals
is a hot topic in this field,[8] and for example, oxidative
addition reactions of metals to strongly distorted carbon
nanomaterials, such as deeper buckybowls and open-cage
fullerenes have been reported, but the inserted metals were
still present in the products.[9] However, to the best of our
knowledge, catalytic bond cleavage reactions of less distorted
corannulenes[10] with metals have not been reported. Indeed,
it was found that among the late transition metals commonly
employed, this reaction was specific to iridium.
À
successive C C bond cleavage reactions.
Corannulene[4a] is a typical buckybowl,[4] and its character-
istic properties, which result from the curved structure, such
as bowl-to-bowl inversion,[5a,b] molecular recognition,[5c,d]
metal complexation,[5e,f] and self-association,[5g,h] have
attracted much attention. As with other carbon nanomate-
rials, there are many reports of the structural extension of the
corannulene skeleton through modifications of its perip-
We have previously reported that 2-pyridylcorannulene
(1) reacted with a PdII salt to form the cyclopalladated
complex [Pd(1ÀH)(CH3CN)2]BF4 under mild conditions
(608C).[11] In the current study, it was found that the
corannulene skeleton of 1 was readily cleaved by a catalytic
amount of IrCl3·nH2O (0.1 equiv) under microwave heating
(2508C) for 30 minutes (Figure 2a). The resulting yellow
suspension that contained some metallic solid was initially
purified by extraction with CH2Cl2 to obtain a crude product.
1H NMR spectroscopy of the crude product indicated that the
starting material 1 had been completely consumed, and that
a new species had been generated instead (Figure 2b,c).
hery.[1e, 5c,g,h,6] In contrast, C C bond cleavage reactions of less
À
[*] Dr. S. Tashiro, Dr. M. Yamada, Prof. Dr. M. Shionoya
Department of Chemistry, Graduate School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: shionoya@chem.s.u-tokyo.ac.jp
[**] This study was supported by the JSPS (KAKENHI Grant 26248016 to
M.S.) and the “Nanotechnology Platform” (12024046) of MEXT
(Japan). M.Y. thanks the JSPS for a JSPS Research Fellowship for
Young Scientists.
À
After further purification, the product of C C bond cleavage,
2-(6-ethylbenzo[ghi]fluoranthen-4-yl)pyridine (2), was iso-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500819.
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Angewandte
Communications
fragment compounds of 1 (Figure 4).
For example, unsubstituted corannu-
lene (3) did not react at all with
IrCl3·nH2O at 2508C in ethylene
glycol under microwave irradiation.
This indicates that the pyridine
moiety is essential for this reaction.
Furthermore, the simple addition of
pyridine as a reagent to a reaction
mixture with 3 did not facilitate the
carbon–carbon bond scission reac-
tion at all, suggesting that the 2-
pyridyl group was required to be
a substituent on the substrate.
We also examined a series of
other fragment compounds possess-
ing a smaller p-plane than 1, namely
2-(naphthalen-1-yl)pyridine (4), 2-
(phenanthren-9-yl)pyridine (5), and
Figure 2. a) Reaction conditions of the iridium-catalyzed conversion of 1 into 2. b,c) 1H NMR
spectra (500 MHz, CDCl3, 300 K) of 1 (b) and 2 (c).
2-(fluoranthen-3-yl)pyridine
Figure 4). In spite of the presence of
(6;
lated in 87% yield as a yellow solid. The 1H NMR spectrum of
2 in CDCl3 gave two signals characteristic of an ethyl moiety
in the aliphatic region at 3.42 and 1.57 ppm. The number of
1H NMR signals in the aromatic region also decreased to
À
twelve protons. These results suggested that one of the C C
bonds of the corannulene skeleton had been cleaved, and that
the resulting vinyl group had been hydrogenated to form an
ethyl group (Figure 2c). This presumption was also supported
by electrospray ionization time-of-flight (ESI-TOF) mass
spectrometry (m/z = 332.14 for [2 + H]+).
The molecular structure of 2 was determined by single-
crystal X-ray diffraction (XRD). As expected, the corannu-
lene skeleton had been cleaved, and a benzo[ghi]fluoranthene
skeleton bearing one ethyl group at the 6-position had been
Figure 4. Fragment compounds of 1. None of them gave any products
generated instead (Figure 3a). The resulting benzo-
[ghi]fluoranthene skeleton[12] is flat, which is in contrast to
the curved structure of the corannulene skeleton of 1 with
a bowl depth of 0.89 Š.[11] The flat moieties of 2 form
a column through slipped p–p stacking of the benzo-
[ghi]fluoranthene skeletons in the crystal structure (Fig-
ure 3b). This is in sharp contrast with the crystal packing of
1, where no p–p stacking of the curved corannulene skeletons
was observed.[11]
À
that arise from a C C bond cleavage reaction.
À
the 2-pyridyl substituent, no C C bond cleavage reactions
occurred in all cases, and the starting materials were
recovered. Therefore, the non-planar structure of the cor-
annulene skeleton should be another key factor facilitating
À
the C C cleavage reaction. In other words, this reaction is
specific to 2-pyridylcorannulene (1), which comprises both
the 2-pyridyl substituent and the curved corannulene skel-
eton.
We next focused on the mechanism of this reaction, which
À
entails the cleavage of an aromatic C C bond and subsequent
reduction of the resulting vinyl group. To evaluate the
mechanism, we conducted several control experiments using
Based on this finding, we propose a preliminary mecha-
nism for this reaction (Figure 5). First, the starting iridium
salt, IrCl3·nH2O, is reduced to a lower-valent iridium species,
0
I
À
Ir or Ir , which then inserts into the aromatic C C bond of the
corannulene skeleton of 1 to form intermediate I, which
features five- and seven-membered metallacycles with an IrII
or IrIII atom in association with its binding to the pendant 2-
pyridyl group. After the oxidative addition, reduction of the
resulting vinyl moiety and protonation afford product 2 and
an IrII or IrIII species. Although other intermediates, such as
cyclometalated IrIII complexes or olefin complexes, may be
involved before or after the formation of I, we focus only on
the key steps of the proposed mechanism to simply explain
Figure 3. Crystal structure of 2.[18] a) ORTEP view with the thermal
ellipsoids set at 50% probability. b) The slipped p–p stacking arrange-
ment of 2.
À
the site-selective C C cleavage reaction of 1.
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with a seven-membered metallacycle. Therefore, the relief of
the nonplanar strain of the curved corannulene skeleton
should be another key driving force for the formation of
intermediate I. In the same way, more curved compounds
(with a deeper cavity), such as a half fullerene (C30H12) and
À
open-cage fullerenes, are known to undergo C C bond
activation through oxidative addition,[9] and their nonplanar
strain energy is much higher than that of corannulene
according to a recent calculation.[10] Our observation of
À
Figure 5. Plausible mechanism of the iridium-catalyzed reductive C C
bond cleavage reaction of 1. X=solvents, ClÀ, and/or hydrides, y=1–
À
3.
aromatic C C bond cleavage in a carbon nanomaterial with
a shallower curvature, namely corannulene, thus constitutes
a rare example.[7]
The first step of this reaction, the reduction of IrCl3·nH2O,
was experimentally confirmed by X-ray photoelectron spec-
troscopy (XPS). When we analyzed black solid samples
obtained by microwave heating of IrCl3·nH2O in ethylene
glycol at 25088C for five minutes, two distinct peaks corre-
sponding to Ir 4f7/2 and Ir 4f5/2 were observed at 61.4 and
64.4 eV, respectively. These binding energies were signifi-
cantly lower than those of IrIIICl3 (62.7 and 65.6 eV),
suggesting that the IrIIICl3 salt was reduced to a lower-valent
iridium species (Supporting Information, Figure S19). The
disappearance of the peaks that are due to the Cl atoms also
supports the reduction of IrIIICl3. The binding energies of the
reduced Ir components are close to those of metallic iridium,
Ir0 (61.35 and 64.35 eV),[13] indicating that the resulting
iridium species should be based on Ir0 or IrI. Indeed, these
reaction conditions are very similar to those of a known
method, the so-called polyol process, where metallic nano-
particles are produced from a transition-metal salt by heating
in ethylene glycol at high temperature.[14] According to the
literature, ethylene glycol reacts with a metal salt to afford
elemental metal, 2,3-butanedione, and HCl.[14d] As iridium
nanoparticles are also synthesized by a similar process,[14e] the
formation of Ir0 species is likely to occur under these
conditions.
The final step in the proposed mechanism entails reduc-
tion of the vinyl moiety and protonation to give product 2. It
was experimentally confirmed that our reaction conditions
allowed for the hydrogenation of styrene to form ethyl-
benzene, even though the conversion was only approximately
50% (Table S1). Accordingly, the possibility of direct hydride
insertion from the iridium center to the metalated vinyl group
cannot be excluded. The following protonation is likely to
take place because HCl should be generated according to the
mechanism of the polyol process. Consequently, the released
iridium species with a higher oxidation state, IrII or IrIII,
should again be reduced by the polyol process to enter the
catalytic cycle once again. Another possible pathway, the
reductive elimination of IrII or IrIII from a hydride complex
and subsequent dissociation of Ir0 or IrI, might also be
involved.
Finally, we investigated the metal specificity of this
reaction by conducting screening experiments with various
transition-metal chlorides (FeCl3, CoCl2·6H2O, NiCl2·6H2O,
CuCl2·2H2O, RuCl3, RhCl3·3H2O, PdCl2, AgCl, IrCl3·nH2O,
PtCl2, and Na[AuCl4]·2H2O) under almost identical condi-
tions (with ca. 1 equiv or an excess amount of the metal salts).
Interestingly, it was found that only IrCl3·nH2O provided full
conversion of 1 into 2. Other metal salts (except for RuCl3,
which afforded a rather complicated mixture including 1 and
2) did not react with 1 at all, and the starting material was fully
recovered after the reaction (Figures S23–S33). These results
As electron-rich transition metals in low oxidation states
À
generally prefer to insert into C C bonds through oxidative
addition,[8a] we propose that the resulting low-valent iridium
species, Ir0 or IrI,[15] inserted into the aromatic C C bond of
strongly indicate that the aromatic C C bond cleavage
À
À
the corannulene skeleton of 1 to form intermediate I with an
IrII or IrIII center (Figure 5). This mechanism is well supported
by a recent theoretical study, in which metal insertion into
reaction of 1 is specific to IrCl3·nH2O among the late
transition metal chlorides that we tested, which is consistent
À
À
with recent progress in C C and C H bond activation with
Group 9 metals (Rh, Ir). The superiority of Ir over Rh has
À
a C C bond at the edge of a carbon nanotube was proposed as
À
the first step of the transition-metal-catalyzed unzipping of
carbon nanotubes.[3j] Furthermore, the proposed structure of
intermediate I also meets several conditions that need to be
been reported for some C C activation processes and was
[8c]
À
explained in terms of the stability of the M C bonds.
In summary, we have demonstrated the efficient and site-
À
À
fulfilled for C C bond activation to occur, such as relieving
selective C C cleavage reaction of the corannulene skeleton
strain energy, inducing aromatic stabilization, forming stable
metallacyclic complexes, and chelation-assisted activation.[8a]
For instance, intermediate I comprises two metallacycles, one
of which is stabilized by chelation with the 2-pyridyl
substituent. In fact, the 2-pyridyl moieties often serve as
of 1 by using IrCl3·nH2O and ethylene glycol as catalyst and
solvent, respectively, with the aid of microwave heating at
2508C.[17] Several control experiments suggest that an Ir
species obtained by in situ reduction reacted with 1 to form
intermediate I in a process that is driven by both 2-pyridyl
chelation and the relief of the nonplanar strain in the
À
chelating auxiliaries in transition-metal-mediated C H and
C C bond activation reactions.
By the formation of intermediate I, the curved p-system of
the corannulene skeleton of 1 is likely to be flattened to some
extent through the iridium insertion because the planar
benzo[ghi]fluoranthene skeleton of I is only loosely tethered
[16]
À
À
À
corannulene. Although several C H and C C bond activa-
tion reactions have been developed to date by means of
elaborate iridium catalysts, the commercially available iri-
dium salt IrCl3·nH2O used here is one of the simplest iridium
catalysts. We envision that a further expansion of this reaction
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À
Keywords: annulenes · C C activation · cleavage reactions ·
iridium · reaction mechanisms
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[17] This reaction was very slow at 2008C; see the Supporting
Information.
[18] CCDC 1042770 (2) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
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Received: January 28, 2015
Published online: March 10, 2015
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