Highly strained phenylene bicyclophanes

Article abstract of DOI:10.1002/anie.201306299

Bending the rules: Strained bicyclophanes (see structure) with highly bent biphenylene units and a central aromatic moiety (yellow) forced into a perpendicular position were accessible in high yields by cyclization of the appropriate bromides by Yamamoto condensation. They were able to bind to graphite cutouts in solution and were adsorbed at the liquid/highly oriented pyrolytic graphite (HOPG) interface to form extended 2D structures. Copyright

Full text of DOI:10.1002/anie.201306299

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DOI: 10.1002/anie.201306299
Strained Aromatic Compounds
Highly Strained Phenylene Bicyclophanes**
Gabi Ohlendorf, Christian W. Mahler, Stefan-S. Jester, Gregor Schnakenburg, Stefan Grimme,*
and Sigurd Hçger*
Dedicated to Professor Karl Heinz Dçtz on the occasion of his 70th birthday
The self-assembly of rigid organic molecules at the liquid/
HOPG (highly oriented pyrolytic graphite) interface enables
the bottom-up synthesis of designed two-dimensional (2D)
crystalline superstructures with adjustable symmetry and
lattice parameters. Enhanced understanding of 2D supra-
molecular engineering has led to an enhanced understanding
of the essential key mechanisms of molecule–surface and
molecule–molecule interactions.[1] Usually, aromatic com-
Figure 1. a) Schematic representation of a large framework structure
pounds lie flat on the substrate.[2] Architectures with aromatic
with a central aromatic unit able to move into the plane of the
structure when brought into contact with a graphite surface.[2] b) Sche-
units aligned perpendicularly to the graphite surface are
rarely observed[3] but may become of interest when function-
matic representation of a large structure with a central aromatic unit
confined perpendicularly in a tight and rigid framework.
alization towards the volume phase is desired.
Our approach is the construction of a rigid molecular
cyclic framework with long alkyl chains that drive adsorption
to the graphite surface by van der Waals interactions. The
diameter and height of the central lumen cause a central
aromatic unit to be confined in an arrangement perpendicular
to the overall oblate-shaped molecule structure. The central
unit is neither able to rotate, nor able to move into the plane
of the structure (Figure 1b) and will therefore point into the
volume phase.[2] We assumed that small cyclic oligo(meta-/
para-phenylene)s with an intraannular central aromatic unit
would fulfill these criteria. However, the synthesis of such
cyclophanes is challenging owing to their high strain energy.[4]
Herein, we describe a straightforward synthetic approach
to the highly strained bicyclic oligophenylene structures 1a/b,
2a/b, and 3a/b, in which the central aromatic unit and the
para-substituted arylene rings of the bicyclic backbone are
aligned perpendicular to the plane of the molecule, as
determined by single-crystal X-ray structure analysis. Fur-
thermore, we describe the adsorption of 1b and 2b onto
HOPG as well as studies of the binding of 1b, 2b, and 3b to
appropriate model compounds. All experimental studies are
well-supported by state-of-the-art DFT calculations.
The synthesis is based on our previously reported
convenient route to phenyl-substituted all-para quinquephe-
nylenes from readily accessible pyrylium salts (Scheme 1).[5]
As the synthesis tolerates halogen substituents, bromo-
substituted intermediates can be prepared without the use
of protective groups. By the condensation of the triaryl
pyrylium salts 4a and 4c with sodium 1,4-phenylenediacetate
(5), tetrabromides 6a and 6c were obtained in acceptable
yields of approximately 20%. Compound 6c was transformed
into 6b, and tetrabromides 6a and 6b were treated with
bis(1,5-cyclooctadiene)nickel ([Ni(cod)2]) and 2,2’-bipyridine
in a mixture of THF and 1,5-cyclooctadiene (COD; Yama-
moto conditions)[6] in a microwave oven. When the cyclization
reactions were carried out under high-dilution conditions (c =
10ꢀ3–10ꢀ4 molLꢀ1), the formation of higher oligomers was
mostly avoided,[7] and the desired bicyclophanes 1a and 1b
were the main products, as judged by analytical gel perme-
ation chromatography (GPC; 1a was isolated in 3% yield
owing to its restricted solubility, and 1b was isolated in 57%
yield, in both cases after purification by recycling GPC).
Considering the high strain of the molecules, as suggested by
molecular models (see the Supporting Information), the high
reaction yield is quite astonishing. The driving force of this
exceptional reaction is founded in the preorganization of the
phenyl units in the tetrabromo-substituted precursors.[8] After
insertion of the transition metal into one of the aryl–bromide
bonds, the transmetalation can readily occur prior to the
competing dehalogenation.[9] To prove the generality of our
synthetic approach, we tested further aromatic diacetates as
building blocks for the central unit of the bicyclophanes:
[*] G. Ohlendorf, Dr. C. W. Mahler, Dr. S.-S. Jester, Prof. Dr. S. Hçger
Kekulꢀ-Institut fꢁr Organische Chemie und Biochemie
Rheinische Friedrich-Wilhelms-Universitꢂt Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
E-mail: hoeger@uni-bonn.de
Dr. G. Schnakenburg
Institut fꢁr Anorganische Chemie
Rheinische Friedrich-Wilhelms-Universitꢂt Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Prof. Dr. S. Grimme
Mulliken Center for Theoretical Chemistry
Institut fꢁr Physikalische und Theoretische Chemie
Rheinische Friedrich-Wilhelms-Universitꢂt Bonn
Beringstrasse 4, 53115 Bonn (Germany)
E-mail: grimme@thch.uni-bonn.de
[**] Financial support by the DFG (SFB 624) and the FCI is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306299.
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Scheme 1. a) Benzoic anhydride, 1508C, 3 h; b) 1. BBr3, CH2Cl2, ꢀ788C!RT, 16 h; 2. 3,4,5-trihexadecyloxybenzyl chloride, Cs2CO3, DMF, 1108C,
48 h; c) [Ni(cod)2], 2,2’-bipyridine, THF/COD, microwave irradiation, 1208C, 12 min. DMF=N,N-dimethylformamide.
Sodium 1,4-naphthylenediacetate (7)[10] and 9,10-anthracene-
diacetate (9)[11] were condensed with the pyrylium salts 4a
and 4c. The reaction gave the desired products, which were
subsequently converted into 2a/b (2a: 58%; 2b: 80%) and
3a/b (3a: 22%; 3b: 81%) by analogous procedures to those
used for the synthesis of 1a and 1b. The isolation of 3a was
again hindered by low solubility. Owing to its higher solubility
in organic solvents, bicylophane 2a with a central naphthy-
lene moiety was isolated in considerably higher yield.
All six bicyclophanes were white, thermally stable solids.
The tert-butyl-substituted compounds 1a, 2a, and 3a did not
melt below 3008C, whereas the hexadecyloxy-substituted
analogues had lower melting points of 140–1458C (1b), 1038C
(2b), and 1368C (3b). NMR and mass spectra together with
the analytical GPC data unambiguously proved the structure
and purity of the compounds. Furthermore, single crystals of
2a suitable for X-ray diffraction studies were obtained, and
the experimental data (Figure 2) agreed well with dispersion-
corrected density functional theory (DFT-D3) calculations
(see the Supporting Information); that is, the four para-
phenylene units in the bicyclophane backbone as well as the
central naphthalene unit are oriented almost perpendicularly
to the bicyclophane plane. However, a slight twist of the
orthogonal phenylene rings D and E was observed. This
torsion eliminates H–H contacts that would be too close and
leads to a dihedral angle of 12.78 (calculated value: 17.58).
As the space-filling model shows very impressively (Fig-
ure 2b), the central naphthalene ring is clamped between the
outer biphenylene bridges and thus is not able to rotate, also
in agreement with DFT calculations. For 1a, the calculated
energy difference between the molecular conformations with
the central phenylene ring in the orthogonal orientation or in
the bicyclophane plane is already 47.3 kcalmolꢀ1, which
cannot be overcome under normal conditions. As suggested
by the structural formula, the surrounding biphenylene units
are not linear but highly curved. The imaginary line through
the bridging carbon atoms of phenyl ring D and the
neighboring atom in phenyl ring E (Figure 2) encloses an
angle ]1 of 141.38 (calculated value: 142.58). The correspond-
ing angle ]2 between the bridging carbon atoms of E and the
neighboring atom in D is 144.48 (calculated value: 142.58). To
the best of our knowledge, this deviation from linearity is the
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expected from the structure of the bicyclophane. Owing to the
molecular symmetry of 1b and tBu6-HBC, we expected the
formation of infinite alternating stacks, whereas sterical
shielding of one side of 2b should lead to a sandwichlike
2:1 complex in which two bicyclophanes encapsulate one
tBu6-HBC unit. As the exchange between the complexes and
free hosts and guests is fast on the NMR time scale, the Job
method of continuous variation[17] was used to determine the
complex stoichiometries (see the Supporting Information).
Surprisingly, the maxima in both Job plots were, within the
experimental error, at c = 0.5, which indicated the formation
of 1:1 (n:n) complexes in both cases. However, the presence
of a small amount of a 2:1 complex cannot be excluded. At
present, we ascribe the more or less exclusive formation of the
1:1 complex in both cases to the presence of the long flexible
alkoxy chains, which may prevent interaction with a second
bicyclophane. Binding constants for the host–guest complex-
ation in THF were found by the double-reciprocal (Benesi–
Hildebrand) method to be approximately 70 Lmolꢀ1, which
corresponds to a free binding energy of ꢀ2.5 kcalmolꢀ1
(Table 1).[18]
Figure 2. Molecular geometry of 2a, as determined by single-crystal
X-ray diffraction: a) ball-and-stick model with the angles ]1 and ]2,
which are measures of the high strain of the bicyclophane system;
b) space-filling model with the encapsuled naphthalene ring high-
lighted in yellow.
Table 1: Results of Benesi–Hildebrand experiments for 1b and 2b with
tBu6-HBC.
largest observed for para-substituted biphenylenes (by single-
crystal X-ray analysis; see the Supporting Information).
Further evidence of high ring torsion is the aberration of
the phenylene rings D and E from planarity: The bridging
carbon atoms deviate from the best planes of D and E
(defined by their H-substituted carbon atoms) by 17.8 pm
(calculated value: 15.8 pm) on average. Comparable values
are observed for small para-connected cyclophenylenes, for
example, [6]cyclo-para-phenylene ([6]-CPP).[12] The calcu-
lated strain energy of 2a is 75.2 kcalmolꢀ1.[13] Each of the four
bent phenylene units of the surrounding biphenyl bridges
contributes 16.6 kcalmolꢀ1 to this strain: a contribution
similar to the corresponding strain per phenylene unit in
[6]-CPP (16.0 kcalmolꢀ1).[14] As the central and four bridging
phenylene moieties are oriented perpendicular to the bicyclic
plane of the molecule, ten aryl hydrogen atoms point to the
top of the molecular plane and a further ten hydrogen atoms
point to the bottom of the molecular plane. These hydrogen
atoms can in principle bind to polycyclic aromatic hydro-
carbons through aryl CH···p interactions.[15]
Host:guest
Dd [ppm]
K [Lꢀ1 molꢀ1
]
DG [kcalmolꢀ1
]
1b:tBu6-HBC
tBu6-HBC:1b
2b:tBu6-HBC
tBu6-HBC:2b
0.241
0.005
0.219
0.006
73ꢁ10[a]
72ꢁ10[a]
70ꢁ10[a]
66ꢁ30[a]
ꢀ2.6ꢁ0.1
ꢀ2.6ꢁ0.1
ꢀ2.5ꢁ0.1
ꢀ2.5ꢁ0.2
[a] Errors were determined by graphical data analysis (see the Supporting
Information).
We performed quantum-chemical computations of bicy-
clophane–tBu6-HBC complexes at the dispersion-corrected
TPSS-D3 level by using def2-TZVP AO basis sets and
including corrections derived from the COSMO-RS model
for the free energies (298 K) in THF (see the Supporting
Information for details).[19] The results reveal significant
stacking interactions between 1a’ (a model compound of 1a
in which the tBu substituents are neglected) and tBu6-HBC in
the gas phase (DE) as well as in solution (DG). For 1a’@tBu6-
HBC (Figure 3a), the computed DE value (estimated accu-
racy ꢁ 10%) is ꢀ43.3 kcalmolꢀ1, which is a typical value for
two unsaturated hydrocarbons of this size.[20] The binding in
the perpendicular conformation (Figure 3c) is significantly
less strong (ꢀ23.4 kcalmolꢀ1), which excludes such arrange-
ments under normal conditions. As the complexes of 1a’ with
tBu6-HBC (in particular, the sandwich complex; Figure 3b)
are extremely large for sophisticated DFT computations, we
could not afford the largest atomic-orbital basis sets and
hence expect a slight systematic overbinding (as a result of the
basis set superposition error). For the 1:1 association process
leading to 1a’@tBu6-HBC, we obtained a value for DG in
solution of ꢀ7 kcalmolꢀ1, which compares reasonably well
with the experimental values of ꢀ2.5 kcalmolꢀ1. We also
studied the 2:1 complex and found practically additive
interactions; that is, the second association free energy of
ꢀ7.6 kcalmolꢀ1 was very similar to that for the formation of
To confirm this supposed attractive interaction of the
bicyclophanes with extended planar p systems, binding with
a hexa-peri-hexabenzocoronene (HBC) derivative was inves-
tigated in solution. HBC can be viewed as a soluble cutout of
the honeycomb structure of graphite. Compounds 1b, 2b, and
3b were investigated as complexation partners for hexa-tert-
1
butyl-substituted HBC (tBu6-HBC)[16] by means of H NMR
spectroscopy. When tBu6-HBC was added to solutions of 1b
1
or 2b in [D8]THF at room temperature, the H NMR signals
corresponding to the bicyclic units of the cyclophanes were
shifted upfield. For example, the singlet in 2b at d = 6.07 ppm
moved steadily upfield up to d = 5.85 ppm as the ratio of tBu6-
HBC to 2b was increased; this shift indicated close proximity
between the flat side of the bicyclophane and the p plane of
the HBC derivative. In contrast, the NMR signals of 3b were
not influenced by the presence of the HBC derivative, as
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Figure 4. Scanning tunneling microscopy image and molecular model
of a self-assembled monolayer of 1b at the PHO/HOPG interface.
Image parameters: 20.1ꢃ20.1 nm2, c=10ꢀ5 m, VS =ꢀ0.8 V, It =7 pA;
unit-cell dimensions: a=(5.9ꢁ0.2) nm, b=(3.9ꢁ0.1) nm,
g(a,b)=(73ꢁ2)8. Red and white lines indicate the unit cell and the
HOPG main axis directions, respectively.
Figure 3. a,b) Computed structures of van der Waals complexes of the
1:1 (a) and 2:1 complex (b) between 1a’ and tBu6-HBC. The
conformation of 1a’ (except for a bending along the long molecular
axis) in the complex is similar to that in the free state. c) The
perpendicular conformation is considerably less stable and can be
excluded.
The central and four of the six surrounding arylene groups are
oriented perpendicular to the plane of the bicyclophane, as
verified by single-crystal X-ray structure analysis. The biphe-
nylene moieties are highly bent and show the largest deviation
from linearity (141, 1448) measured so far (calculated strain
energy of 2a: 75.2 kcalmolꢀ1). In solution, the bicyclophanes
bind through CH···p noncovalent interactions to HBC
derivatives. STM investigations showed that the molecules
adsorb to HOPG in such a way that the central (intraannular)
and four of the circumventing phenylene units are oriented
perpendicular to the graphite surface. The intraannular unit
can be variably substituted at only one side without affecting
the graphite adsorption, as shown exemplarily with the
naphthylene-centered bicyclophane 2b. Therefore, this
molecular platform may enable volume-phase surface func-
tionalization.
the 1:1 complex and hence should be observable under the
experimental conditions. At present we can only speculate
that the reason for the apparent disagreement between theory
and experiment (the 2:1 complex was not observed) is rooted
in the presence of the long alkoxy chains of 1b/2b; such
chains could not be treated adequately by our ab initio
computations (MD simulations would be required). Never-
theless, our computations clearly established substantial
noncovalent stacking interactions between 1a’ and tBu6-
HBC that are not quenched in solution and hence can be
considered to be form-specific.
These results prompted us to investigate the 2D self-
assembly of the bicyclophanes 1b and 2b at the liquid/solid
interface of 1-phenyloctane (PHO) and HOPG by scanning
tunneling microscopy (STM). Both molecules formed crys-
talline monolayers with domains of lateral extension of more
than 1002 nm2 (for 2b, see the Supporting Information). The
aromatic backbone and hexadecyloxy side chains were
imaged with low and high tunneling resistivity and visualized
in bright and dark colors, respectively.[21] Throughout all
observed adlayer patterns, regions covered with pairwise-
assembled backbones (polymorph A; Figure 4) or continuous
lines of densely packed backbones (polymorph B; see the
Supporting Information) were separated by lamellar arrange-
ments of hexadecyloxy side chains, which were aligned along
an HOPG main axis direction.[22]
Received: July 19, 2013
Published online: && &&, &&&&
Keywords: CH···p interactions · cyclophanes ·
.
scanning tunneling microscopy · strained molecules ·
supramolecular 2D networks
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Communications
Strained Aromatic Compounds
Bending the rules: Strained bicyclo-
phanes (see structure) with highly bent
biphenylene units and a central aromatic
moiety (yellow) forced into a perpendicu-
lar position were accessible in high yields
by cyclization of the appropriate bro-
mides by Yamamoto condensation. They
were able to bind to graphite cutouts in
solution and were adsorbed at the liquid/
highly oriented pyrolytic graphite
G. Ohlendorf, C. W. Mahler, S.-S. Jester,
G. Schnakenburg, S. Grimme,*
S. Hçger*
&&&& — &&&&
Highly Strained Phenylene Bicyclophanes
(HOPG) interface to form extended 2D
structures.
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