A Macrocycle Based on a Heptagon-Containing Hexa-peri-hexabenzocoronene

Article abstract of DOI:10.1002/anie.202003785

A cyclophane is reported incorporating two units of a heptagon-containing extended polycyclic aromatic hydrocarbon (PAH) analogue of the hexa-peri-hexabenzocoronene (HBC) moiety (hept-HBC). This cyclophane represents a new class of macrocyclic structures that incorporate for the first time seven-membered rings within extended PAH frameworks. The saddle curvature of the hept-HBC macrocycle units induced by the presence of the nonhexagonal ring along with the flexible alkyl linkers generate a cavity with shape complementarity and appropriate size to enable π interactions with fullerenes. Therefore, the cyclophane forms host–guest complexes with C₆₀ and C₇₀ with estimated binding constants of K_a=420±2 m^?1 and K_a=(6.49±0.23)×10³ m^?1, respectively. As a result, the macrocycle can selectively bind C₇₀ in the presence of an excess of a mixture of C₆₀ and C₇₀.

Full text of DOI:10.1002/anie.202003785

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Accepted Article
Title: A Heptagon-Containing HBC-Based Macrocycle
Authors: Vicente G. Jiménez, Arthur H. G. David, Juan M. Cuerva,
Victor Blanco, and Araceli G. Campaña
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A Heptagon-Containing HBC-Based Macrocycle
Vicente G. Jiménez, Arthur H. G. David, Juan M. Cuerva, Victor Blanco* and Araceli G. Campaña*
Abstract: A cyclophane incorporating two units of a heptagon-
containing extended polycyclic aromatic hydrocarbon (PAH),
analogue of the hexa-peri-hexabenzocoronene (HBC) moiety (hept-
HBC), is reported. This cyclophane represents a new class of
macrocyclic structures that incorporate for the first time seven-
membered rings within extended PAH frameworks. The saddle
curvature of the hept-HBC macrocycle units induced by the
presence of the non-hexagonal ring along with the flexible alkyl
in combination with other structural defects such as helicenes.[9]
Moreover, relevant magnetic or electronic properties have also
been predicted for these structures.[10]
linkers generate
a cavity with shape complementarity and
appropriate size to enable  interactions with fullerenes. Therefore,
the cyclophane forms host-guest complexes with C60 and C70 with
estimated binding constants of Ka = 420 ± 2 M-1 and Ka = (6.49 ±
0.23) × 103 M-1 respectively. As a result, the macrocycle can
selectively bind C70 in the presence of an excess of a mixture of C60
and C70.
Many examples have been reported of organic macrocycles and
metallacycles with different size and shape that incorporate
polycyclic aromatic hydrocarbons (PAHs) in their structures.[1]
However, most of those examples, with the exception of some of
the recent carbon nanobelts,[2] are restricted to PAHs with a
small number of fused rings. Thus, the examples of macrocycles
that exhibit extended aromatic systems such as the hexa-peri-
hexabenzocoronene (HBC) unit are much scarcer. In fact, only a
few examples of HBC-containing cyclophanes or strained
macrocycles and cages derived from [n]cycloparaphenylenes
(CPP) have been reported. Among the cyclophanes, we can
highlight the seminal work of Watson, Rabe, Müllen and co-
workers describing a macrocycle with two HBC units linked by
aliphatic chains[3] and more recent similar structures differing in
the spacer units (Figure 1a) or the substitution pattern of the
HBC core.[4] As part of the increasing interest on the
development of strained nanohoops,[1g,h, 5] which started with the
synthesis of the first CPPs, a variety of derivatives of different
size incorporating up to eight HBC units have been prepared in
the last few years (Figure 1a).[6] Interestingly, the strained nature
of the structures formed resulted in the distortion of the planarity
of the HBC units giving rise to curved surfaces.
On the other hand, it has recently emerged a growing
interest in the development of extended PAHs incorporating
seven-membered rings, resulting in nonplanar structures with a
negative saddle curvature.[7,8] These compounds are relevant
due to the properties they exhibit as their usual higher solubility
in comparison with their purely hexagonal counterparts, which
enhances their processability and characterization, but also their
interesting (chiro)optical or non-linear optical properties, mainly
[*]
V. G. Jiménez, A. H. G. David, Prof. J. M. Cuerva, Dr. V. Blanco, Dr.
A. G. Campaña
Departamento de Química Orgánica, Unidad de Excelencia de
Química aplicada a Biomedicina y Medio Ambiente, Facultad de
Ciencias
Universidad de Granada
Avda. Fuente Nueva, s/n, 18071, Granada (Spain)
E-mail: victorblancos@ugr.es, araceligc@ugr.es
Figure 1. a) Previous HBC-based cyclophanes and strained nanohoops. b)
Macrocycle
incorporating
two
saddle-shaped
heptagon-containing
hexabenzocoronene analogues (hept-HBC) described in this work. (Mes = 4-
(1,3,5-trimethylphenyl)).
Supporting information for this article is given via a link at the
end of the document.
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Nevertheless, despite this increasing interest in the design
Glaser alkyne homocoupling of 3 in dilute conditions afforded
macrocycle 4 which underwent selective hydrogenation of the
butadiyne groups using PtO2 as catalyst to yield the target
cyclophane 1 (Scheme 1).
and development of
a
variety of extended PAHs or
nanographenes incorporating heptagons, their inclusion in
macrocyclic structures and the study of the resulting properties,
especially the possible supramolecular interactions with curved
guests, has remained, to the best of our knowledge, completely
unexplored. This is somewhat surprising, taking into account
that the curvature induced by the presence of the seven
membered ring could facilitate the formation of less strained
macrocyclic structures or increase the shape complementarity
with curved  systems in comparison with their planar purely
hexagonal counterparts.
Here we report the synthesis, characterization and
supramolecular behaviour as host for fullerenes of a novel
macrocycle constituted by two units of heptagon-containing HBC
analogues (hept-HBC) (1) (Figure 1b).
Scheme 1. Synthesis of hept-HBC-based macrocycle
1 and model 5.
Reagents and conditions: a) 3-[(trimethylsilyl)ethynyl]phenylboronic acid
pinacol ester, Pd(PPh3)4, K2CO3, toluene/H2O, reflux, 16 h, 38%; b)
tetrabutylammonium fluoride (TBAF), THF/H2O, RT, 2 h, 99%; c) CuCl, CuCl2,
pyridine, 27 h, RT, 28%; d) H2, PtO2, THF/MeOH, RT, 18 h, 72%; e)
phenylboronic acid, Pd(PPh3)4, K2CO3, toluene/H2O/EtOH, reflux, 20 h, 70%.
Figure 2. a) Partial 1H NMR (400 MHz, [D4]-o-DCB, 25 ºC) spectra of 1. b)
MALDI-TOF mass spectra of
1 (left) and experimental (top right) and
calculated (bottom right) HR-MS isotopic distribution for the [M]+ ion. c) Side
(left) and top (right) views of the DFT (B97XD/def2SVP in o-DCB) optimized
structure of syn-1 (top) and anti-1 (bottom). Colour coding: C, grey; H, white; O,
red.
The key intermediate for the synthesis of cyclophane 1 is the
saddle hept-HBC analogue 2,[11] functionalized with two Br
atoms in the adequate positions to enable the introduction of the
linker moieties and the formation of a macrocycle (Scheme 1).
We prepared this dibromide derivative following our reported
synthetic strategy that gives access to distorted heptagon-
containing HBC analogues with different functionalization
patterns.[12] This strategy involved subsequent Sonogashira
The good solubility of hept-HBC-based macrocycle 1 in
chlorinated organic solvents such as dichloromethane,
tetrachloroethane or o-dichlorobenzene (o-DCB) allowed its
characterization by NMR spectroscopy. In principle, two main
possible conformers could be considered depending on whether
the ketone moieties have the same (syn) or opposite (anti)
orientation. Thus, broad resonances could be observed in the 1H
NMR spectrum in C2D2Cl4 at room temperature (Figure S26a),
which resolved into well-defined signals at 358 K (Figure
couplings starting from 2,2’-dibromobenzophenone,
a Co-
catalyzed alkyne cyclotrimerization and a final Scholl-type
oxidative cyclodehidrogenation (see the Supporting Information
for further details). Suzuki coupling between
2 and 3-
[(trimethylsilyl)ethynyl]phenylboronic acid pinacol ester followed
by cleavage of the trimethylsilyl protecting group yielded diyne 3.
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S24).[13] This could be explained by a coalescence situation that
results in fast exchange in the NMR timescale upon heating. On
supramolecular aggregates with curved  systems. The
extended  surface together with the curvature induced by the
presence of the seven-membered ring, which favours the
geometrical complementarity are factors that can account for
this molecular recognition.
1
the contrary, the H NMR spectrum in [D4]-o-DCB showed only
one set of well-defined signals at room temperature (Figure 2a
and Figure S17 and S26c), which could be assigned with the aid
of 2D NMR experiments, confirming the proposed structure.[14]
Moreover, the identity of the compound was further confirmed by
MALDI-TOF mass spectrometry. The spectrum showed a single
signal corresponding to the [M]+ ion with a HRMS exact mass
and isotopic distribution in excellent agreement with the
calculated ones (Figure 2b). The UV-Vis spectrum of 1 in o-DCB
shows an absorption band between 320 and 420 nm (max = 365
nm,  = 1.4 × 106 L mol-1) with a shoulder at 395 nm ( = 4.9 ×
105 L mol-1). Upon irradiation at 365 nm, the macrocycle shows
luminescence, with an emission band in the 450-570 region, with
its maximum at 490 nm (Figure S35).
Having demonstrated that simple hept-HBC derivatives can
participate in supramolecular interactions, we studied the host-
guest chemistry of cyclophane 1. Again, we chose C60 and C70
as potential guests for our macrocycle as the curvature on the
aromatic surface would enhance the shape complementarity
with curved aromatic guests.[1h,21] In addition, the C60 van der
Waals diameter (ca. 10.5 Å) reasonably fits the size of the
macrocycle cavity, which could also slightly adapt due to the
presence of alkyl chains linking the two hept-HBC units.
The resonances of both 1H and 13C NMR spectra of mixtures
of cyclophane 1 and C60 or C70 were shifted in comparison with
those of the free host and guests, thus indicating the
establishment of  interactions between them and the formation
of host-guest complexes (Figures 3 and S49-S50, S52-S53 and
S55-S56).[22] In particular, the signals of He and Hf, located in
the equatorial region of the receptor, are shifted to lower
The structure of the two different conformers of 1 was also
investigated by means of DFT calculations at the
B97XD/def2SVP level. The optimized structure of syn-1
displays a basket or calix-like shape defined by the saddled
curvature induced by the seven-membered rings of the hept-
HBC units. The distance between the central part of the hept-
HBC units ranges ca. 12.8-18.0 Å, while the distance between
the side aliphatic chains is ca. 15.6 Å (Figures 2c and S58),
giving a cavity effective size of ca. 9.4−14.6 Å × 12.2 Å (see the
Supporting Information for further details).[15] Anti-1 also exhibits
a basket-like structure, although more distorted than the syn
conformer (Figures 2c and S59).
1
frequencies due to the shielding by the curved guest. H NMR
titration of receptor 1 with C60 or C70 in [D4]-o-DCB allowed us to
determine the binding constants as Ka = 420 ± 2 M-1 with C60 and
Ka = (6.49 ± 0.23) × 103 M-1 with C70 (Figure 3 and the Supporting
Information).
The presence of an internal cavity demarcated by the curved
hept-HBC units could enable macrocycle to act as
a
supramolecular receptor of aromatic compounds by means of 
interactions between the distorted -extended PAH derivatives
and the guest. Although it has been suggested that heptagon-
containing extended PAHs can interact with fullerene,[16] the
supramolecular behaviour of these saddle systems has
remained essentially unexplored. For this reason, we decided to
study their ability to form supramolecular entities with fullerenes.
We first investigated the self-association of hept-HBC model
5,[17] prepared from dibromide 2 by Suzuki coupling with
phenylboronic acid (Scheme 1). The 1H NMR spectra of 5 in the
1-100 mM range showed important changes in the chemical shift
of the aromatic signals (Figures S40-S41). The fitting of the 
values at the different concentrations gave us the value of the
monomer-dimer equilibrium constant (Kd = 24 ± 4 M-1) (Figure
Figure 3. Partial 1H NMR (400 MHz, [D4]-o-DCB) spectra for the titration of 1
with increasing amounts of C70. Inset: Fitting of δ for the signal initially at 9.04
ppm (labelled in orange in the NMR spectra) with [C70].
S42).[18] We then studied the recognition of C60 and C70 by 5. 13
C
NMR experiments showed that addition of model derivative 5 to
a solution of C60 or C70 resulted in changes in the chemical shift
of the fullerene signals, indicating the presence of
supramolecular interactions between the hept-HBC unit and the
fullerene molecules (Figures S43, S45 and S47). 13C NMR
titrations allowed us to estimate the binding constants as Ka =
18.2 ± 0.1 M-1 (in C2D2Cl4) or 20.0 ± 0.1 M-1 (in [D4]-o-DCB) with
C60 and Ka = 3.75 ± 0.02 M-1 (in [D4]-o-DCB) with C70 using a 1:1
fitting model (see the Supporting Information for details).[19,20]
These are relevant results as they evidence that simple hept-
HBC derivatives can play an interesting role in supramolecular
chemistry, demonstrating that, although weakly, they can form
Remarkably, the association between heptagon-containing
macrocycle 1 and C70 is one order of magnitude stronger than
that with C60, which could enable the selective binding of one of
the fullerenes, a characteristic that has attracted interest over
the last years.[23] As a result, when the host was added to an
excess of a 1:1 mixture of both fullerenes, the macrocycle
selectively bound to C70. This was confirmed by comparison of
the 13C NMR signals of the fullerenes before and after the host
addition, which revealed a shift toward lower frequencies of the
C70 signals ( = −0.17−0.24 ppm) while the 13C signal of C60
remains essentially unaltered ( = −0.02 ppm) (Figure S57).
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The host-guest complexes were also modelled by DFT
calculations at the B97XD/def2SVP level in o-DCB (Figure 4).
The optimized structures of the syn-1 complexes show the
inclusion of the fullerene guests inside its cavity. The calculation
predicts C60 to be located in the center of the cavity, being the
closest distance to both hept-HBC units quite similar (ca. 3.4 Å).
On the contrary, C70 adopts, due to its prolate-like shape, a tilted
orientation to maximize contact with the hept-HBC  surface.
The computational methods also predict that the anti-1
conformer can be a receptor for fullerene guests (see the
Supporting Information).
programmes from the “Plan Propio de Investigación”) for
financial support. We thank the CSIRC-Alhambra, Universidad
de Granada, for providing supercomputing facilities and time.
Keywords: Macrocycles • nanographenes • nanostructures •
host-guest systems • fullerenes
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an important role, yet to be explored, in the design of new
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Acknowledgements
We acknowledge FEDER/Junta de Andalucía-Consejería de
Economía y Conocimiento (A-FQM-339-UGR18), the European
Research Council (ERC) under the European Union's Horizon
2020 research and innovation program (ERC‐2015‐STG‐
677023), Ministerio de Ciencia, Innovación y Universidades
(MICIU/FEDER/AEI, Spain; PGC2018‐101181‐B‐I00) and
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[13] The possibility of the existence of the syn and anti conformers is also
supported by the two sets of signals in a slow exchange regime
observed in the 1H NMR spectrum of 4 recorded at 256 K (Figures S13-
S14).
[22] The addition of fullerenes to 1 also resulted in the quenching of its
fluorescence. However, an inner filter effect due to the strong fullerene
absorbance at the excitation wavelength avoids any reliable estimation
of the binding constant.
[14] The reason behind the behaviour in different solvents is not completely
clear. Assuming that the interconversion barrier between conformers is
not significantly affected by the solvent, one single set of signals should
imply that the system is biased towards a major isomer. Possible
explanations for this are the higher dipole moment of o-DCB, which
would stabilize the most polar isomer, or the different supramolecular
interactions of the solvent molecules with each conformer, which could
estabilize one of them preferentially. However, we cannot
unambiguously give a reason behind this behaviour.
[23] For selected examples of organic receptors able to selectively bind to
different fullerenes, see, for example, references [4b], [6c,d] and: a) E.
Huerta, G. A. Metselaar, A. Fragoso, E. Santos, C. Bo, J. de Mendoza,
Angew. Chem. Int. Ed. 2007, 46, 202-205; Angew. Chem. 2007, 119,
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[15] The effective size was calculated by subtracting twice the van der
Waals radius of C (1.7 Å) to the measured distances.
[16] Miao and co-workers reported the cocristallization of an extended
heptagon-containing PAH with C60, suggesting the establishment of
supramolecular interactions between both components. However, as
pointed by the authors, the poor quality of the data precluded the full
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10.1002/anie.202003785
Angewandte Chemie International Edition
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Carbon nanobasket: a new
cyclophane bearing two units of
heptagon-containing hexa-peri-
hexabenzocoronene is presented. The
resulted macrocyclic offers an unusual
aromatic calix-shape cavity able to
interact with fullerenes. Selective
binding of C70 over C60 is
Vicente G. Jiménez, Arthur H. G. David,
Juan M. Cuerva, Victor Blanco,* Araceli
G. Campaña*
Page No. – Page No.
A Heptagon-Containing HBC-Based
Macrocycle
demonstrated.
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