Hexaphenylbenzenes as potential acetylene sponges

Article abstract of DOI:10.1021/ol902723q

(Figure presented) Acetylene sponges can be created by taking advantage of the nonplanar geometry of hexaphenylbenzenes and the special capacity of the central aromatic ring to engage in C(sp)-H...π interactions reinforced by secondary C(sp²)-H

Full text of DOI:10.1021/ol902723q

ORGANIC
LETTERS
2010
Vol. 12, No. 2
380-383
Hexaphenylbenzenes as Potential
Acetylene Sponges
Eric Gagnon,†,‡ Alain Rochefort,§ Vale´rie Me´tivaud,† and James D. Wuest*,†
De´partement de Chimie, UniVersite´ de Montre´al, Montre´al, Que´bec H3C 3J7
Canada, and De´partement de ge´nie physique and Regroupement que´be´cois
´
sur les mate´riaux de pointe (RQMP), Ecole Polytechnique de Montre´al,
Montre´al, Que´bec H3C 3A7 Canada
james.d.wuest@umontreal.ca
Received November 25, 2009
ABSTRACT
Acetylene sponges can be created by taking advantage of the nonplanar geometry of hexaphenylbenzenes and the special capacity of the
central aromatic ring to engage in C(sp)-H···π interactions reinforced by secondary C(sp2)-H···π interactions, as revealed by X-ray
crystallographic studies and DFT calculations.
Nonplanar aromatic compounds with multiple contiguous aryl
substituents, represented by hexaphenylbenzene (1a ) HBP),
have special properties that make them widely useful in science
and technology.1 In particular, their complex topologies limit
conjugation and disfavor extensive intermolecular π-π and
C-H···π interactions.2,3 As a result, these compounds can
be expected to show higher HOMO-LUMO gaps, lower
degrees of self-association, less efficient packing, and higher
solubilities than planarized analogues such as hexa-peri-
† Universite´ de Montre´al.
‡ Fellow of the Natural Sciences and Engineering Research Council of
Canada (2003-2009).
§
´
Ecole Polytechnique. (Present address: CEA-Grenoble, INAC/SprAM,
17 rue des Martyrs, 38054 Grenoble Cedex 9, France).
(1) Zhi, L.; Mu¨llen, K. J. Mater. Chem. 2008, 18, 1472–1484. Bauer,
R. E.; Grimsdale, A. C.; Mu¨llen, K. Top. Curr. Chem. 2005, 245, 253–
286. Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV. 2001, 101,
1267–1300. Wiesler, U.-M.; Weil, T.; Mu¨llen, K. Top. Curr. Chem. 2001,
212, 1–40. Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. ReV. 1999,
99, 1747–1785.
hexabenzocoronene (2) or linear analogues such as p-
septiphenyl (3). These marked differences have been under-
scored by the results of a recent systematic crystallographic
analysis of the structures of HPB (1a) and its relatives.4 For
example, the ratio of H···H contacts to C···H contacts in
the structure of compound 1a (67:32) greatly exceeds the
ratio observed in the structure of linear isomer 3 (39:61).
(2) For recent reviews of the subject of C-H···π interactions, see:
Tsuzuki, S.; Fujii, A. Phys. Chem. Chem. Phys. 2008, 10, 2584–2594.
Nishio, M. CrystEngComm 2004, 6, 130–158
.
(3) A database related to C-H···π interactions, maintained by Professor
Motohiro Nishio, is available via the Internet at http://www.tim.hi-ho.ne.jp/
dionisio.
10.1021/ol902723q  2010 American Chemical Society
Published on Web 12/17/2009

Figure 1.
(a) View of the structure of crystals of HPB (1a) grown from CH2Cl2 or CH2Br2.5 (b) View of the structure of crystals of
analogue 1b grown from CH2Cl2. (c) View of molecules of analogue 1b enchained along the c axis by multiple C-H···π interactions. In
all views, carbon atoms are shown in light gray and hydrogen atoms in white. C-H···π interactions are represented by broken lines, with
key distances indicated. In (c), only the primary C(sp)-H···π interactions are shown.
Even though the characteristic nonplanar topology of HPB
(1a) appears to set severe limits on access to the faces of all
seven aromatic rings, intermolecular C-H···π interactions
are not entirely eliminated in the structure of its crystals.4,5
Surprisingly, the fully substituted central aromatic ring serves
as a double acceptor of C-H···π interactions, one on each
face (Figure 1a). This unexpected feature is also found in
the structures of many related compounds with multiple
contiguous phenyl groups.4 We now report the results of an
integrated experimental and theoretical study that reveals the
particularly strong tendency of hexaphenylbenzenes to form
C-H···π interactions with alkynes. Their association defines
a new supramolecular synthon that can be used predictably
to engineer molecular crystals.6
Å/140°).11 The other face engages in a C(sp2)-H···π
interaction with a second neighboring molecule (2.75 Å/164°).
A special feature in the structure is reinforcement of the
primary C(sp)-H···π interaction by secondary C(sp2)-H···π
interactions in which the ethynyl group acts as acceptor (2.96
Å/140° and 3.00 Å/153°).12 Overall, the resulting structure
consists of supramolecular chains along the c axis maintained
by multiple C-H···π interactions (Figure 1c).
In the structure of compound 1b, the acetylenic C-H bond
competes successfully with a total of 29 other C-H bonds
for access to the faces of the central aromatic ring. We
reasoned that supramolecular organization could be better
controlled in analogue 1c, in which two C(sp)-H donors
are available for both faces of the central ring. Compound
1c was synthesized as shown in Scheme 1,7 and crystals of
composition 1c ·1 toluene were grown from toluene/hex-
anes.8,13 As planned, the molecules are linked exclusively
by short C(sp)-H···π interactions (2.47 Å/135° and 2.46
Å/138°), creating chains along the c axis reinforced by
additional C(sp2)-H···π interactions in which the ethynyl
groups serve as acceptors (Figure 2).
By the route summarized in Scheme 1,7 we made
compound 1b, in which a hexaphenylbenzene core and an
ethynyl group are present within a single molecule. Com-
pound 1b was crystallized by slow evaporation of a solution
in CH2Cl2. Routine analysis of disorder in the ethynyl group
yielded a structure with noteworthy features.7,8 As expected,
the core adopts a nonplanar conformation similar to those
of other derivatives of HPB (1a), with large torsional angles
between the central and peripheral aromatic rings.4,9,10 As
shown in Figure 1b, one face of the central aromatic ring
serves as acceptor in an unusually short C(sp)-H···π
interaction involving the ethynyl group as donor (2.38
Surprisingly, reinforced C-H···π interactions of the type
observed in the structures of compounds 1b and 1c are strong
enough to force the cocrystallization of suitable hexaphe-
nylbenzenes and acetylenes.14 Cooling a hot solution of HPB
(1a) in PhCt CH produced crystals that proved to belong to
the monoclinic space group Cc and to have the composition
1a·0.5 PhCt CH.13 The resulting structure is maintained by
multiple C-H···π interactions (Figure 3), including the
expected reinforced C(sp)-H···π interaction involving
PhCt CH and the central aromatic ring of HPB (1a). In the
(4) Gagnon, E.; Maris, T.; Arseneault, P.-M.; Maly, K. E.; Wuest, J. D.
Cryst. Growth Des. 2009; DOI: 10.1021/cg9010746.
(5) Lutz, M.; Spek, A. L.; Bonnet, S.; Klein Gebbink, R. J. M.; van
Koten, G., as communicated in 2006 to the Cambridge Crystallographic
Data Centre (CCDC 609800, Refcode: HPHBNZ03).
(6) Nangia, A.; Desiraju, G. R. Top. Curr. Chem. 1998, 198, 57–95.
Desiraju, G. R. Angew. Chem., Int. Ed. 1995, 34, 2311–2327.
(7) See the Supporting Information for details.
(8) Only the structure of the major component of the disordered model
(11) The values correspond to the H···centroid distance and the
C-H···centroid angle, respectively.
is described in detail.7
(9) Maly, K. E.; Gagnon, E.; Maris, T.; Wuest, J. D. J. Am. Chem. Soc.
(12) The values correspond to (1) the distance from the hydrogen atom
to the midpoint of the triple bond and (2) the C-H···midpoint angle,
respectively.
2007, 129, 4306–4322
.
(10) Kobayashi, K.; Sato, A.; Sakamoto, S.; Yamaguchi, K. J. Am. Chem.
Soc. 2003, 125, 3035–3045. Kobayashi, K.; Shirasaka, T.; Horn, E.;
Furukawa, N. Tetrahedron Lett. 2000, 41, 89–93. Kobayashi, K.; Shirasaka,
T.; Sato, A.; Horn, E.; Furukawa, N. Angew. Chem., Int. Ed. 1999, 38,
(13) The composition was determined by X-ray diffraction.
(14) For a related crystallographic and theoretical study of the co-
crystallization of benzene and acetylene, see: Boese, R.; Clark, T.;
Gavezzotti, A. HelV. Chim. Acta 2003, 86, 1085–1100.
3483–3486
.
Org. Lett., Vol. 12, No. 2, 2010
381

Scheme 1. Synthesis of hexaphenylbenzenes 1b and 1c
structure of the cocrystals of HPB (1a) and PhCt CH, as
well as in the crystals of hexaphenylbenzenes 1b and 1c,
the thermal parameters of the -Ct C-H group are reduced,7
presumably because the group is bound in a pocket and
engages in strong C(sp)-H···π interactions.15
The pronounced tendency of hexaphenylbenzenes to
engage in reinforced C(sp)-H···π interactions whenever
possible suggests that such compounds can serve as sponges
for acetylenes. To test this notion, we used DFT calculations
to evaluate the association of acetylene with benzene, HPB
(1a), and 1,3,5-triphenylbenzene held in three distinct
conformations. The geometries were fully optimized at the
B3LYP/6-31G* level, with corrections for van der Waals
interactions and basis set superposition error.7 The results
are summarized in Figure 4. The complex of acetylene and
benzene is calculated to favor an orthogonal geometry, with
a short C(sp)-H···π interaction (2.31 Å), and the dissociation
energy is estimated to be 3.5 kcal/mol, in good agreement
with the conclusions of previous studies.14,16 The addition
of three phenyl groups does not increase this energy when
they are forced to remain in the plane of the central ring;
however, when they are orthogonal, the energy increases to
5.0 kcal/mol. Finally, the complex of acetylene with HPB
(1a) is estimated to be stabilized by 6.4 kcal/mol, and the
C(sp)-H···π interaction is linear and very short (2.18
Å/180°).
Together, these results suggest that the central C(sp)-
H···π interaction contributes 3.5 kcal/mol, and each cofacial
ortho C-H bond of an orthogonal phenyl group adds a
reinforcing C(sp2)-H···π interaction worth 0.5 kcal/mol.
Figure 2. View of the structure of crystals of compound 1c grown
from toluene/hexanes, with guest molecules of toluene omitted for
clarity. Carbon atoms are shown in light gray and hydrogen atoms
in white. C-H···π interactions are represented by broken lines,
with key distances indicated.
Figure 3. View of the structure of 2:1 cocrystals of HPB (1a) and
PhCt CH. Carbon atoms are shown in light gray and hydrogen
atoms in white. C-H···π interactions are represented by broken
lines, with key distances indicated.
382
Org. Lett., Vol. 12, No. 2, 2010

Figure 4. Structures and dissociation energies of complexes formed when acetylene is bound by benzene, 1,3,5-triphenylbenzene (in three
conformations), and HPB (1a), as estimated by DFT calculations.
Simple C(sp)-H···π interactions are well-known, but they
account for only about half of the overall stabilization of
the acetylene-HPB (1a) complex. Moreover, the reinforcing
secondary interactions help draw the bound acetylene closer
to the π-acceptor. For both reasons, it is clear that acetylene-
hexaphenylbenzene complexes have unique features not
previously noted. Careful examination of electron densities
in the calculated acetylene-HPB (1a) complex shows reduc-
tions on the cofacial ortho hydrogens,7 thereby confirming
the importance of the secondary reinforcing interactions.
Furthermore, these interactions appear to be strong enough
to slightly distort the geometry of HPB (1a) in a way that
brings the ortho hydrogens closer to the bound acetylene.7
Although theory suggests that the reinforced C(sp)-H···π
interactions of hexaphenylbenzenes should be linear, com-
peting intermolecular interactions and geometric constraints
in crystals can induce significant deformations, as observed
in the structures formed by compounds 1a-c.
that are ideal for binding acetylenes by C(sp)-H···π
interactions reinforced by secondary C(sp2)-H···π interac-
tions. The importance of this new associative motif is
revealed by its persistent presence in the structures described
above. Moreover, calculations show that its strength is nearly
twice that of normal C(sp)-H···π interactions, making it
similar to hydrogen bonds in its ability to direct molecular
association with predictable directionality. If so, it should
be possible to observe the binding of acetylenes by hexaphe-
nylbenzenes in solution; however, compounds 1a-c are too
poorly soluble in suitable solvents to permit such tests, and
we are currently preparing derivatives with higher solubility.
We believe that such compounds can be engineered for new
applications in supramolecular chemistry, such as the creation
of sponges for the selective sorption of acetylene itself.17
Acknowledgment. We are grateful to the Natural Sciences
and Engineering Research Council of Canada, the Ministe`re
´
The characteristic nonplanar topology of hexaphenylben-
zenes creates sites on both faces of the central aromatic ring
de l’Education du Que´bec, the Canada Foundation for
Innovation, the Canada Research Chairs Program, and
Universite´ de Montre´al for financial support. We also thank
Dr. Thierry Maris of the Universite´ de Montre´al for his help
in analyzing crystallographic data.
(15) Steiner, T. Chem. Commun. 1994, 101–102.
(16) Singh, N. J.; Min, S. K.; Kim, D. Y.; Kim, K. S. J. Chem. Theory
Comput. 2009, 5, 515–529. Shibasaki, K.; Fujii, A.; Mikami, N.; Tsuzuki,
S. J. Phys. Chem. A 2007, 111, 753–758. Tekin, A.; Jansen, G. Phys. Chem.
Chem. Phys. 2007, 9, 1680–1687. Sundararajan, K.; Viswanathan, K. S.;
Kulkarni, A. D.; Gadre, S. R. J. Mol. Struct. 2002, 613, 209–222. Takahashi,
O.; Kohno, Y.; Iwasaki, S.; Saito, K.; Iwaoka, M.; Tomoda, S.; Umezawa,
Y.; Tsuboyama, S.; Nishio, M. Bull. Chem. Soc. Jpn. 2001, 74, 2421–2430.
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Supporting Information Available: Detailed experimen-
tal procedures, spectroscopic data for all new compounds,
and additional crystallographic and computational details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(17) For recent related work, see: Xiang, S.; Zhou, W.; Gallegos, J. M.;
Liu, Y.; Chen, B. J. Am. Chem. Soc. 2009, 131, 12415–12419.
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