Tampering with molecular cohesion in crystals of hexaphenylbenzenes

Article abstract of DOI:10.1021/jo902175u

(Chemical Equation Presented) Hexaphenylbenzene (HPB) and analogous compounds have properties of broad utility in science and technology, including conformationally well-defined molecular structures, high thermal stability, high HOMO-LUMO gaps, little self-association, inefficient packing, and high solubilities. Previous structural studies of HPB and its analogues have revealed persistent involvement of the central aromatic ring in strong C-H...π interactions. These interactions can be blocked by adding simple ortho alkyl substituents to the peripheral phenyl groups. Comparison of the structures of HPB and a series of ortho-substituted derivatives has shown systematic changes in molecular cohesion and packing, as measured by packing indices, densities, solubilities, temperatures of sublimation, melting points, and ratios of H...H, C...H, and C...C contacts. These results illustrate how crystal engineering can guide the search for improved materials by identifying small but telling molecular alterations that thwart established patterns of association.

Full text of DOI:10.1021/jo902175u

pubs.acs.org/joc
Tampering with Molecular Cohesion in Crystals of Hexaphenylbenzenes
‡
†
Eric Gagnon,1,† Shira D. Halperin, Valerie Metivaud, Kenneth E. Maly,2,‡ and
ꢀ
ꢀ
James D. Wuest*,†
†
‡
Departement de Chimie, Universite de Montreal, Montreal, Quebec H3C 3J7, Canada and Department of
ꢀ
ꢀ
Chemistry, Wilfrid Laurier University, Waterloo, Ontario N2L 3C5, Canada
ꢀ
ꢀ
ꢀ
james.d.wuest@umontreal.ca
Received October 13, 2009
Hexaphenylbenzene (HPB) and analogous compounds have properties of broad utility in science and
technology, including conformationally well-defined molecular structures, high thermal stability,
high HOMO-LUMO gaps, little self-association, inefficient packing, and high solubilities. Previous
structural studies of HPB and its analogues have revealed persistent involvement of the central
aromatic ring in strong C-H π interactions. These interactions can be blocked by adding simple
3 3 3
ortho alkyl substituents to the peripheral phenyl groups. Comparison of the structures of HPB and a
series of ortho-substituted derivatives has shown systematic changes in molecular cohesion and
packing, as measured by packing indices, densities, solubilities, temperatures of sublimation, melting
points, and ratios of H H, C H, and C C contacts. These results illustrate how crystal
3 3 3
3 3 3
3 3 3
engineering can guide the search for improved materials by identifying small but telling molecular
alterations that thwart established patterns of association.
Introduction
notable for high charge-carrier mobilities, and polynuclear
aromatics such as hexa-peri-hexabenzocoronene (HBC, 2),6
which feature large π-surfaces. Unfortunately, compounds 1
and 2 cannot easily be used in thin-film molecular devices
Specialized aromatic compounds have found a niche in
modern materials science and molecular electronics.3 Their
properties permit the fabrication of various devices, includ-
ing light-emitting diodes, photovoltaic cells, field-effect tran-
sistors, lasers, sensors, RFID tags, and photorefractive
systems. Among the most promising classes of aromatic
compounds are acenes such as pentacene (1),4,5 which are
(5) Sele, C. W.; Kjellander, B. K. C.; Niesen, B.; Thornton, M. J.; van der
Putten, J. B. P. H.; Myny, K.; Wondergem, H. J.; Moser, A.; Resel, R.; van
Breemen, A. J. J. M.; van Aerle, N.; Heremans, P.; Anthony, J. E.; Gelinck,
G. H. Adv. Mater. DOI: 10.1002/adma.200901548. Published Online: Sep
18, 2009. Lee, S. S.; Kim, C. S.; Gomez, E. D.; Purushothaman, B.; Toney,
M. F.; Wang, C.; Hexemer, A.; Anthony, J. E.; Loo, Y.-L. Adv. Mater. 2009,
21, 3605–3609. Hamilton, R.; Smith, J.; Ogier, S.; Heeney, M.; Anthony, J.
E.; McCulloch, I.; Veres, J.; Bradley, D. D. C.; Anthopoulos, T. D. Adv.
Mater. 2009, 21, 1166–1171. Lim, Y.-F.; Shu, Y.; Parkin, S. R.; Anthony, J.
E.; Malliaras, G. G. J. Mater. Chem. 2009, 19, 3049–3056. Chen, J.;
Subramanian, S.; Parkin, S. R.; Siegler, M.; Gallup, K.; Haughn, C.; Martin,
D. C.; Anthony, J. E. J. Mater. Chem. 2008, 18, 1961–1969.
(1) Fellow of the Natural Sciences and Engineering Research Council of
Canada (2003-2009).
(2) Fellow of the Natural Sciences and Engineering Research Council of
Canada (2003-2004).
(3) Roncali, J.; Leriche, P.; Cravino, A. Adv. Mater. 2007, 19, 2045–2060.
Forrest, S. R. Nature 2004, 428, 911–918.
€
(6) For recent reviews, see: Zhi, L.; Mullen, K. J. Mater. Chem. 2008, 18,
€
1472–1484. Wu, J.; Pisula, W.; Mullen, K. Chem. Rev. 2007, 107, 718–747.
(4) For recent reviews, see: Anthony, J. E. Angew. Chem., Int. Ed. 2008,
47, 452–483. Anthony, J. E. Chem. Rev. 2006, 106, 5028–5048.
DOI: 10.1021/jo902175u
r
Published on Web 12/09/2009
J. Org. Chem. 2010, 75, 399–406 399
2009 American Chemical Society

Gagnon et al.
JOCArticle
without significant structural modification. For example, the
use of pentacene (1) itself is hampered by its ease of
crystallization and by the existence of multiple poly-
morphic forms with different electronic properties.7 More-
over, molecular packing in these polymorphs is guided by
those patterns are favored, thereby providing a blue-
print for making subtle structural alterations that
change how crystallization occurs or even prevent it from
happening.13
A useful class of compounds related to acenes and poly-
nuclear aromatics is defined by analogues of hexaphenyl-
benzene (3a, HPB), in which multiple contiguous aryl
groups are attached to an aromatic core. Systematic struc-
tural studies of HPB (3a) and similar compounds have
established that they are forced to adopt characteristic
nonplanar conformations, with large torsional angles
between the central and peripheral aromatic rings.14 The
resulting topology is propeller-shaped and toroidal. Its
surface is rich in exposed C-H bonds, and access to the
π-faces of the constituent aromatic rings is severely
restricted. These conformational preferences have the
predictable effect of limiting conjugation and dis-
extensive C-H π interactions, and the preferred struc-
3 3 3
tures have herringbone arrangements with minimal π
π
overlap, which are considered unfavorable for conductiv-
3 3 3
ity. To increase orbital overlap in acenes, various strategies
have been devised to favor π π interactions at the
3 3 3
expense of C-H π interactions, such as by adding
3 3 3
substituents or by modifying the surfaces on which thin
films are grown.4,5 Similarly, substituents have been at-
tached to the periphery of HBC (2) to control the extent of
π-stacking, increase solubility, and produce new functional
materials.8,9
favoring extensive intermolecular π-π and C-H
π
3 3 3
interactions,14 leading to higher HOMO-LUMO gaps,
lower degrees of self-association, less efficient packing,
and higher solubilities than observed in planarized
analogues such as HBC (2). These characteristic properties
make derivatives of HPB (3a) increasingly useful in various
areas of science and technology, particularly when poor
molecular cohesion and inefficient packing are benefi-
cial.14
Analysis of the Hirshfeld surface derived from the struc-
ture of HPB (3a) suggests that cohesion is maintained
primarily by diffuse H H contacts, and well-defined direc-
3 3 3
tional forces such as C-H π and π π interactions are
3 3 3
3 3 3
less important.15-18 Specifically, the ratio of H H to
These examples show that many compounds with inher-
ently attractive molecular properties, such as pentacene (1)
and HBC (2), cannot achieve their full potential as compo-
nents of materials without modifications specifically intro-
duced to control packing and cohesion. Of particular value
in the cases of compounds 1 and 2 are structural changes
3 3 3
C
H contacts is 67:32, and C C contacts are essentially
3 3 3
3 3 3
absent. Close examination of C-H π interactions in the
3 3 3
structure of HPB (3a) reveals that none of the hindered
peripheral phenyl groups serves as an acceptor, but both
faces of the central aromatic ring are used in this way
(Figure 1a).14 This is not merely because potential pockets
for binding are created above and below the central ring
by the characteristic toroidal topology of HPB (3a). In fact, a
primary driving force for association appears to be the ability
designed to alter the importance of C H contacts
3 3 3
(resulting from C-H π interactions)10,11 relative to
3 3 3
C
C contacts (created by π-stacking). Identifying effective
3 3 3
structural alterations requires insights about molecular
packing and cohesion best drawn from the field of
crystal engineering,12 which is therefore poised to play a
growing role in guiding the search for new functional
materials. In particular, crystal engineering can reveal
how classes of compounds prefer to crystallize and why
of the central ring to act as an acceptor in C-H π inter-
3 3 3
actions, which can be reinforced by secondary interactions
involving the ortho hydrogen atoms of the peripheral phenyl
groups. This phenomenon is illustrated by the binding
of a terminal acetylene in structure 4 (for clarity, only two
of the six equivalent secondary C-H π interactions are
3 3 3
drawn).19 DFT calculations have established that reinforced
(7) Siegrist, T.; Besnard, C.; Haas, S.; Schiltz, M.; Pattison, P.; Chernyshov,
D.; Batlogg, B.; Kloc, C. Adv. Mater. 2007, 19, 2079–2082.
(8) For a recent example, see: Feng, X.; Marcon, V.; Pisula, W.; Hansen,
C-H π interactions of this type can be as strong as
3 3 3
€
M. R.; Kirkpatrick, J.; Grozema, F.; Andrienko, D.; Kremer, K.; Mullen, K.
Nat. Mater. 2009, 8, 421–426.
(9) For recent reviews, see: Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc.
Rev. 2007, 36, 1902–1929. Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.;
ꢁ
(13) Lebel, O.; Maris, T.; Perron, M.-E.; Demers, E.; Wuest, J. D. J. Am.
Chem. Soc. 2006, 128, 10372.
€
Hagele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.;
Tosoni, M. Angew. Chem., Int. Ed. 2007, 46, 4832–4887.
(14) Gagnon, E.; Maris, T.; Arseneault, P.-M.; Maly, K. E.; Wuest.
J. D. Cryst. Growth Des. DOI: 10.1021/cg9010746. Published Online: Oct
29, 2009.
(15) Bart, J. C. J. Acta Crystallogr. 1968, B24, 1277–1287. 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).
(16) For a recent review of the analysis of Hirshfeld surfaces, see:
Spackman, M. A.; Jayatilaka, D. CrystEngComm 2009, 11, 19–32.
(17) Analyses of Hirshfeld surfaces were carried out with the program
Crystal Explorer Version 2.1.18
(18) Wolff, S. K.; Grimwood, D. J.; McKinnon, J. J.; Jayatilaka, D.;
Spackman, M. A. Crystal Explorer 2.1; University of Western Australia:
Perth, Australia, 2007 (http://hirshfeldsurface.net/CrystalExplorer).
(10) For recent reviews of the subject of C-H π interactions, see:
3 3 3
Tsuzuki, S.; Fujii, A. Phys. Chem. Chem. Phys. 2008, 10, 2584–2594. Nishio,
M. CrystEngComm 2004, 6, 130–158.
(11) A database related to C-H π interactions, maintained by Pro-
3 3 3
fessor Motohiro Nishio, is available via the Internet at http://www.tim.hi-ho.
ne.jp/dionisio.
(12) Braga, D. Chem. Commun. 2003, 2751–2754. Biradha, K. CrystEng-
Comm 2003, 5, 374–384. Hollingsworth, M. D. Science 2002, 295, 2410–2413.
Crystal Engineering: From Molecules and Crystals to Materials; Braga, D.,
Grepioni, F., Orpen, A. G., Eds.; Kluwer: Dordrecht, The Netherlands,
1999. Desiraju, G. R. Crystal Engineering: The Design of Organic Solids;
Elsevier: Amsterdam, The Netherlands, 1989.
400 J. Org. Chem. Vol. 75, No. 2, 2010

Gagnon et al.
JOCArticle
SCHEME 1
molecular cohesion, increased volatility, and higher solubility.
In principle, the C-H π interactions of HPB (3a) can be
3 3 3
blocked by adding suitable ortho substituents to the periph-
eral phenyl groups. In this paper, we describe the syntheses
and structures of ortho-substituted derivatives 3b-e. As
planned, these compounds show substantial increases in the
ratio of H H to C H contacts, as well as other properties
3 3 3
3 3 3
that suggest decreased molecular cohesion.
Results and Discussion
Synthesis of Ortho-Substituted Hexaphenylbenzenes 3b-e.
The targets were prepared by an established general method
for making hexaphenylbenzenes, using Diels-Alder reactions
of suitably substituted tetraphenylcyclopentadienones with
diphenylacetylenes, followed by extrusion of carbon mon-
oxide(eq1).20 Therequired substituted diphenylacetylenes 521
and 622 were prepared by reported methods, and known
compounds 723 and 824 were made by straightforward new
procedures (Scheme 1). Diphenylacetylene 7 was synthesized
in 71% yield by Sonogashira coupling of 1,3-diethyl-2-iodo-
benzene25 with phenylacetylene, using Pd2dba3/PPh3 and CuI
as catalysts.26 Compound 8 was prepared from 2,6-diisopro-
pylaniline by the following two steps. A one-pot diazotiza-
tion-iodination developed by Knochel and co-workers25
provided the known 2-iodo-1,3-diisopropylbenzene (9)24,27
in 57% yield, and then Sonogashira coupling26 of intermedi-
ate 9 with phenylacetylene gave diphenylacetylene 8 in 71%
yield. Diphenylacetylenes 5-8 were then allowed to react with
tetraphenylcyclopentadienone to generate the desired ortho-
substituted hexaphenylbenzenes 3b-e in yields ranging from
52% to 77%. It is noteworthy that the high-temperature
Diels-Alder reactions leading to hexaphenylbenzenes 3d
and 3e were efficient despite very significant steric congestion.
The success of these reactions demonstrates the broad scope
FIGURE 1. (a) View of the structure of crystals of HPB (3a)
grown from CH2Cl2 or CH2Br2.15 (b) View of the structure of crystals
of monomethyl derivative 3b grown from chlorobenzene. Both views
show a central molecule (dark gray) and its primary neighbors (light
gray). Carbon atoms are shown in gray, hydrogen atoms involved in
C-H π interactions appear in white, and the interactions are
3 3 3
represented by broken lines. C-H
indicated.
πcentroid distances and angles are
3 3 3
normal hydrogen bonds, so they can make an important
contribution to molecular cohesion.19
(20) For an early example of the use of Diels-Alder reactions of aryl-
substituted cyclopentadienones with acetylenes to form aryl-substituted
€
arenes, see: Dilthey, W.; Schommer, W.; Trosken, O. Ber. Dtsch. Chem.
Ges. 1933, 66, 1627–1628.
(21) Rubin, M.; Trofimov, A.; Gevorgyan, V. J. Am. Chem. Soc. 2005,
127, 10243–10249.
(22) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org. Lett.
2000, 2, 1729–1731.
(23) Motti, E.; Rossetti, M.; Bocelli, G.; Catellani, M. J. Organomet.
Chem. 2004, 689, 3741–3749.
Detailed understanding of cohesion and packing in the
structures of HPB (3a) and related compounds allows crystal
engineers to devise subtle yet potent structural alterations that
can obstruct established preferences. We reasoned that pre-
venting the central aromatic ring of HPB (3a) from engaging
€
(24) Li, Z.; Dong, Y.; Mi, B.; Tang, Y.; Haussler, M.; Tong, H.; Dong, Y.;
Lam, J. W. Y.; Ren, Y.; Sung, H. H. Y.; Wong, K. S.; Gao, P.; Williams, I. D.;
Kwok, H. S.; Tang, B. Z. J. Phys. Chem. B 2005, 109, 10061–10066.
(25) Krasnokutskaya, E. A.; Semenischeva, N. I.; Filimonov, V. D.;
Knochel, P. Synthesis 2007, 81–84.
(26) Li, G. R.; Wang, X. H.; Li, J.; Zhao, X. J.; Wang, F. S. Chin. Chem.
Lett. 2005, 16, 719–722.
in reinforced C-H π interactions would increase the ratio
3 3 3
of H H to C H contacts and possibly lead to reduced
3 3 3
3 3 3
ꢀ
(19) Gagnon, E.; Rochefort, A.; Metivaud, V.; Wuest. J. D. Org. Lett.
Submitted for publication.
(27) Quiroga Norambuena, V. F.; Heeres, A.; Heeres, H. J.; Meetsma, A.;
Teuben, J. H.; Hessen, B. Organometallics 2008, 27, 5672–5683.
J. Org. Chem. Vol. 75, No. 2, 2010 401

Gagnon et al.
JOCArticle
TABLE 1. Crystallographic Data for Ortho-Substituted Hexaphenylbenzenes 3b-e
3b
3c
3d
3e
formula
crystal system
space group
C43H32
orthorhombic
Pna21
12.3269(3)
11.6835(3)
20.8753(5)
90
C44H34
C46H38
monoclinic
P21/n
12.3918(2)
11.7268(2)
23.2394(5)
90
98.217(1)
90
3342.39(11)
4
1.174
C48H42
monoclinic
P21
12.8715(2)
10.8218(2)
13.1831(2)
90
102.061(1)
90
1795.78(5)
2
1.144
orthorhombic
Pna21
12.3809(4)
11.7357(3)
21.3061(5)
90
˚
a (A)
˚
b (A)
˚
c (A)
˚
R (A)
˚
β (A)
90
90
90
90
˚
γ (A)
3
V (A )
˚
3006.49(13)
4
1.212
150
0.517
3095.74(15)
4
1.207
150
0.514
Z
F
calcd (g cm-3
T (K)
)
150
0.498
150
0.484
μ (mm-1
)
R1, I > 2σ (%)
R1, all data (%)
4.85
4.99
6.42
6.49
5.29
5.54
3.22
3.23
ωR2, I > 2σ (%)
ωR2, all data (%)
14.57
14.75
17.20
17.32
14.60
14.85
8.45
8.46
no. of measured reflections
no. of independent reflections
no. of obsd reflections, I > 2σ(I)
46644
5275
5078
49197
5511
5426
55091
5529
5193
29219
5704
5685
˚
˚
of the methodology employed, as well as the inherently high
thermal stability of hexaphenylbenzenes.
2.85 A in HPB (3a) to 2.88 and 2.89 A in substituted
derivative 3b.31,32
Structure of Crystals of Dimethyl Derivative 3c of HPB
(3a). Cooling a hot saturated solution of compound 3c in
toluene gave colorless crystals belonging to the ortho-
rhombic space group Pna21, as previously observed for
both HPB (3a) and monomethyl derivative 3b. Additional
crystallographic data are presented in Table 1, and the
structure is shown in Figure 2a. The unit cell parameters
are very similar to those of analogues 3a and 3b, with a
slight increase in the lengths of the axes to accommodate
the two methyl groups. In addition, the packing index
(67.5%) and density (1.207 g cm-3) are also close to those
observed for compounds 3a and 3b. Molecular packing in
the structure of dimethyl derivative 3c (Figure 2a) resembles
that of less-substituted analogues 3a and 3b (Figure 1).
However, addition of a second methyl group allows the
Structure of Crystals of Monomethyl Derivative 3b of HPB
(3a). After substantial effort, we found that monomethyl
derivative 3b of HPB (3a) could be crystallized by slow
evaporation of a solution in chlorobenzene. The resulting
crystals proved to belong to the orthorhombic space group
Pna21, and other crystallographic data are summarized in
Table 1. The observed structure is essentially identical with
that of HPB (3a), which has the same space group and closely
similar unit cell parameters.15,28 Comparison of parts a and b
of Figure 1 reveals the striking overall similarity of the two
structures. The failure of an added methyl group to alter the
structure radically may result in part from inefficiencies in
the packing of HPB (3a) itself. Its Kitaigorodskii packing
index (67.1%)29,30 and density (1.208 g cm-3) are slightly
lower than those of monomethyl derivative 3b (67.3% and
1.212 g cm-3), suggesting that the substituent simply fits into
previously unoccupied voids, without disrupting the original
lattice appreciably. Although the added methyl group does
central aromatic ring to accept only one C-H π inter-
3 3 3
action, instead of one on each face. In addition, the
C-H πcentroid distance in the surviving interaction rises
3 3 3
˚
to 2.98 A, whereas the average values in HPB (3a) and
˚
monomethyl derivative 3b are 2.82 and 2.88 A, respectively.
These results (1) confirm that ortho substituents can alter the
association of hexaphenylbenzenes by interfering with nor-
mal C-H π interactions and (2) suggest that cohesion can
3 3 3
be reduced to even lower levels by introducing larger ortho
substituents.
(31) The existence of significant intermolecular C-H π aromatic
interactions can be assessed in various ways. In the case of the structure of
3 3 3
HPB (3a) itself (Refcode HPHBNZ03),15 for example, the central aromatic
ring engages in an intermolecular C-H π interaction that can be char-
3 3 3
acterized by various geometric parameters, including (1) the distance of the
hydrogen atom to the mean-square plane of the central aromatic ring (2.76
˚
A), (2) the distance of the same hydrogen atom to the nearest carbon atom of
not eliminate C-H π interactions, it makes them weaker,
3 3 3
˚
the central aromatic ring (2.91 A), and (3) the corresponding
and the two C-H πcentroid distances rise from 2.80 and
3 3 3
˚
C-H πcentroid distance (3.00 A). To avoid ambiguity, we consider that
3 3 3
an intermolecular C-H π aromatic interaction is present only when the
3 3 3
˚
πcentroid distance is e3.05 A, which is 1.05 times greater than the sum
(28) To facilitate comparison of the structures of hexaphenylbenzenes
3a-e, all crystallographic data were collected at 150 K.
(29) Kitaigorodskii, A. I. Organic Chemical Crystallography; Consultants
Bureau: New York, 1961.
C-H
of the accepted van der Waals radii of hydrogen (1.20 A) and carbon (1.70 A).
3 3 3
˚
˚
Similar criteria have been used previously in other studies of C-H
π
3 3 3
interactions.32
(30) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
Utrecht University: Utrecht, The Netherlands, 2003. Spek, A. L. J. Appl.
Crystallogr. 2003, 36, 7–13.
(32) Umezawa, Y.; Tsuboyama, S.; Takahashi, H.; Uzawa, J.; Nishio, M.
Tetrahedron 1999, 55, 10047–10056. Umezawa, Y.; Tsuboyama, S.; Honda,
K.; Uzawa, J.; Nishio, M. Bull. Chem. Soc. Jpn. 1998, 71, 1207–1213.
402 J. Org. Chem. Vol. 75, No. 2, 2010

Gagnon et al.
JOCArticle
FIGURE 2. (a) View of the structure of crystals of dimethyl derivative 3c grown from toluene. (b) View of the structure of crystals of diethyl
derivative 3d grown from toluene/hexanes. Both views show a central molecule (dark gray) and its primary neighbors (light gray). Carbon
atoms are shown in gray, hydrogen atoms involved in C-H π interactions appear in white, and the interactions are represented by broken
3 3 3
lines. C-H
πcentroid distances and angles are indicated.
3 3 3
Structure of Crystals of Diethyl Derivative 3d of HPB (3a).
Hexaphenylbenzene 3d, with two added ethyl groups, was
significantly more soluble in typical organic solvents than
less-substituted analogues 3a-c. Crystals grown by cooling a
hot saturated solution in toluene/hexanes were found to
belong to the monoclinic space group P21/n, and additional
crystallographic data are provided in Table 1. The introduc-
tion of two ethyl groups had the desired effect of lowering
both the packing index (66.3%) and density (1.174 g cm-3
)
relative to the values observed for analogues 3a-c. Despite
these differences, molecular organization in the structure
remains largely unaltered (Figure 2b). The space group is
different from those of the structures of less-substituted
analogues 3a-c, but the unit cell dimensions are still very
similar, and the β angle remains close to 90° (Table 1).
Comparison of the unit cell parameters observed for the
structures of hexaphenylbenzenes 3b-d (Table 1) shows
nearly monotonic evolution in response to the increasing
size of the ortho substituents. Despite evidence of poorer
cohesion in the structure of diethyl derivative 3d, both faces
FIGURE 3. View of the structure of crystals of diisopropyl deriva-
tive 3e grown from toluene/hexanes, showing a central molecule
(dark gray) and its principal neighbors (light gray). Carbon atoms
of the central aromatic ring engage in C-H π interac-
3 3 3
˚
centroid distance of 2.94 A.
are shown in gray, hydrogen atoms involved in C-H π interac-
tions appear in white, and the interactions are represented by
tions, with an average C-H
The average distance is significantly longer than those
π
3 3 3
3 3 3
broken lines. C-H
π
centroid distances and angles are indicated.
˚
πcentroid distance is 2.76 A, the
3 3 3
observed in the crystal structures of HPB (3a) and mono-
˚
methyl derivative 3b (2.82 and 2.88 A, respectively), but
slightly shorter than that found in dimethyl derivative 3c
interaction, and the C-H
shortest observed in the series of compounds 3a-e. The
3 3 3
˚
(2.98 A).
unexpected observation of the shortest C-H πcentroid
3 3 3
Structure of Crystals of Diisopropyl Derivative 3e of HPB
(3a). Like diethyl analogue 3d, diisopropyl derivative 3e
showed higher solubility than less-substituted hexaphenyl-
benzenes 3a-c. Crystals grown from toluene/hexanes
proved to belong to the monoclinic space group P21, and
additional crystallographic data are presented in Table 1.
As expected, further decreases are observed in the packing
index (65.1%) and density (1.144 g cm-3). Moreover, the
bulky substituents induce a major change in packing, and
the consistent pattern observed in compounds 3a-d is no
longer favored by analogue 3e (Figure 3). Only one face
distance in the most hindered derivative (3e) of HPB under-
scores the special difficulty of engineering predictable mo-
lecular crystals when no strong directional interactions are
present. However, it is important to note that the unusually
short contact appears to come at the cost of foregoing any
C-H π interaction involving the other face of the central
3 3 3
ring. Indeed, no C-H π contact involving the second face is
3 3 3
˚
shorter than 5.06 A, and an empty pocket with a volume of
3
approximately 13 A lies directly above the face.
33
˚
(33) The value was obtained by using the command VOID in the program
PLATON.30
of the central aromatic ring participates in a C-H
π
3 3 3
J. Org. Chem. Vol. 75, No. 2, 2010 403

Gagnon et al.
JOCArticle
FIGURE 4. Bar graph showing the nature of intermolecular con-
tacts in crystals of hexaphenylbenzenes 3a-e, as determined by
analyses of Hirshfeld surfaces.
FIGURE 5. Thermogravimetric analyses of crystalline samples of
hexaphenylbenzenes 3a-e, carried out under N2 at a rate of heating
of 10 deg/min.
Comparison of Structural Features Exhibited by Hexa-
phenylbenzenes 3a-e. Analysis of the structures of com-
pounds 3a-e confirms that the central aromatic ring of
hexaphenylbenzenes is strongly predisposed to engage in
sublimation, as confirmed visually. It is noteworthy that the
temperatures at which sublimation begins decrease as the
size of the ortho substituents increases. This observation is
consistent with the notion that the substituents decrease
intermolecular C-H π interactions. However, the number
overall molecular cohesion by increasing diffuse H
contacts at the expense of directional C H contacts. In
H
3 3 3
3 3 3
and strength of these interactions can be controlled rationally
by the simple expedient of placing alkyl groups at the ortho
positions of a single peripheral phenyl group. As planned,
adding substituents of increasing size reduces the efficiency of
packing, as demonstrated by progressively lower packing
3 3 3
the case of diisopropyl derivative 3e, subsequent loss of
weight is less rapid than in the cases of analogues 3a-d,
possibly because their packing is markedly different.
Hexaphenylbenzenes 3a-e also show systematic diffe-
rences in their melting points, which tend to decrease as
the ortho substituents increase in size. HPB (3a) is reported
to melt at 454-456 °C,34 and monomethyl derivative 3b
also melts above 400 °C; however, dimethyl, diethyl, and
diisopropyl derivatives 3c-e melt at 364-365, 282-283, and
285-287 °C, respectively. Enthalpies of sublimation and
lattice energies are directly related,35 but melting points do
not provide a reliable measure of molecular cohesion; never-
theless, the observed values are consistent with the notion
that ortho substituents reduce cohesion as they increase
in size.
indices, lower densities, and longer average C-H πcentroid
3 3 3
distances. When the substituents reach a critical size, the
characteristic packing observed in HPB (3a) and simple
derivatives can no longer be sustained.
The series of compounds 3a-e is designed to show in-
creasing resistance to the formation of intermolecular
C-H π interactions, so the ratio of H H to C
3 3 3
H
3 3 3
3 3 3
contacts should increase systematically. To test this hypoth-
esis, we used standard analyses of Hirshfeld surfaces to
decipher and quantify intermolecular contacts in the struc-
tures of hexaphenylbenzenes 3a-e (Figure 4).16-18 These
analyses confirm that progressively larger ortho substituents
cause a monotonic increase in H H contacts at the expense
of C H and C C contacts. HPB (3a) itself has an
Conclusions
3 3 3
3 3 3
3 3 3
A traditional pursuit of crystal engineers is the systematic
structural analysis of a series of related compounds to
discern characteristic patterns of crystallization, which then
allow the behavior of other compounds to be predicted. An
exciting new thrust in crystal engineering is to use accumu-
lated structural knowledge in a contrary way, by identifying
small but telling molecular alterations that can foil estab-
lished patterns of association, thereby preventing crystal-
lization or forcing it to occur in different ways. We
have tested the power of this strategy by examining the
representative case of hexaphenylbenzenes, which have
properties of broad utility in science and technology, includ-
ing conformationally well-defined molecular structures, high
thermal stability, high HOMO-LUMO gaps, low degrees of
self-association, inefficient packing, and high solubilities.
impressively high ratio of H H to C H contacts
3 3 3
3 3 3
(67:32), but the value reaches an even higher level in diiso-
propyl derivative 3e (83:17).
It is remarkable that the cohesion of compounds with
largely aromatic surfaces can be so thoroughly dominated by
diffuse H H contacts. The growing utility of hexaphenyl-
3 3 3
benzenes in science and technology is based in part on
properties associated with poor molecular cohesion, such
as inefficient packing and high solubility. Our results suggest
that these valuable properties can be enhanced by astute
structural alterations, without simultaneously changing
other useful features of hexaphenylbenzenes, such as their
well-defined topologies and high HOMO-LUMO gaps.
Thermal Analyses of Hexaphenylbenzenes 3a-e. Thermo-
gravimetric analyses of crystalline samples of hexaphenyl-
benzenes 3a-e demonstrated that all five compounds have
high thermal stability (Figure 5). Heating the samples
above 250 °C led to a 100% weight loss due to complete
(34) Fieser, L. F. Org. Synth. 1966, 46, 44–48. Fieser, L. F. Organic
Syntheses; Wiley: New York, 1973; Collect. Vol. V, pp 604-608.
(35) Ouvrard, C.; Mitchell, J. B. O. Acta Crystallogr. 2003, B59, 676–685.
404 J. Org. Chem. Vol. 75, No. 2, 2010

Gagnon et al.
JOCArticle
1
Previous structural studies of hexaphenylbenzene (3a) and
related compounds have revealed persistent involvement of
2.7 mmol, 71%) as a colorless liquid: H NMR (CDCl3, 400
MHz) δ 7.57-7.53 (m, 2H), 7.41-7.31 (m, 3H), 7.23 (AK2
system,40 1H, 3J = 7.6 Hz), 7.11 (AK2 system,40 2H, 3J = 7.6
the central aromatic ring in strong C-H π interactions.
3 3 3
Hz), 2.92 (q, 4H, J = 7.6 Hz), 1.33 (t, 6H, J = 7.6 Hz); 13C
NMR (CDCl3, 100 MHz) δ 146.7, 131.5, 128.5, 128.4, 128.2,
125.5, 124.1, 121.6, 97.0, 86.8, 28.3, 15.0; HRMS (APCI-TOF)
calcd for [C18H18 þ H]þ m/e 235.14812, found 235.14768.
2-Iodo-1,3-diisopropylbenzene (9).24,27 2,6-Diisopropylani-
line (15.1 mL, 14.2 g, 80.1 mmol) was added to a suspension
3
3
14,19 In principle, these key cohesive forces can be blocked by
adding simple ortho substituents to the peripheral phenyl
groups. Comparison of the structures of HPB (3a) and
ortho-substituted derivatives 3b-e has shown systematic
changes in molecular cohesion and packing, as measured
by packing indices, densities, solubilities, temperatures of
sublimation, melting points, and ratios of H H, C H,
of p-TsOH H2O (68.5 g, 360 mmol) in a mixture of t-BuOH
3
(480 mL) and water (20 mL), and the mixture was cooled to 10 °C.
A solution of sodium nitrite (16.6 g, 240 mmol) and potassium
iodide (49.8 g, 300 mmol) in water (70 mL) was then added
dropwise during 2.5 h, without allowing the temperature of the
mixture to rise above 10-15 °C. The temperature was then
allowed to rise to 25 °C, and the mixture was stirred for an
additional 1.5 h. Solid NaHCO3 (∼30 g) was then added to bring
the mixture to pH 9-10, followed by solid Na2S2O3 (79 g). The
resulting mixture was stirred vigorously for 30 min, giving rise to
an orange solution that was poured into water (2.0 L). The mixture
was extracted with Et2O (4 ꢀ 100 mL), and the combined extracts
were washed with water and brine, dried with anhydrous MgSO4,
and filtered. Removal of volatiles by evaporation under reduced
pressure left a residue of dark red liquid, which was purified by
distillation to give a slightly pink fraction boiling at 100-105 °C/
1.5 mm Hg. The pink color was removed by passing the product
through a plug of silica gel, using hexanes as eluent. Removal of
solvent from the eluent by evaporation under reduced pressure
gave 2-iodo-1,3-diisopropylbenzene (9; 13.1 g, 45.5 mmol, 57%) as
a colorless liquid: 1H NMR (CDCl3, 400 MHz) δ 7.25 (t, 1H, 3J =
7.7 Hz), 7.08 (d, 2H, 3J = 7.7 Hz), 3.41 (septet, 2H, 3J = 6.8 Hz),
1.24 (d, 12H, 3J = 6.8 Hz); 13C NMR (CDCl3, 100 MHz) δ 151.2,
128.5, 123.9, 109.3, 39.5, 23.5; HRMS (ESI-TOF) calcd for
[C12H17I þ H]þ m/e 289.04477, found 289.04513. Anal. Calcd
for C12H17I: C, 50.02; H, 5.95. Found: C, 49.85; H, 5.90.
3 3 3
3 3 3
and C C contacts. As planned, adding ortho substi-
3 3 3
tuents to hexaphenylbenzene (3a) has the beneficial effect
of altering molecular cohesion without necessarily changing
other properties of value.
Our work has focused on the particular case of hexaphenyl-
benzenes, but it illustrates a potentially general strategy for
using principles of crystal engineering to guide the search for
improved molecular materials. At the core of this strategy is the
ability to control association by making astute molecular altera-
tions based rationally on systematic crystallographic analyses.
Experimental Section
2,6-Diisopropylaniline was purified by distillation from
CaH2 prior to use. 2-Methyl-(1-phenylethynyl)benzene (5),21
1,3-dimethyl-2-(phenylethynyl)benzene (6),22 tetraphenylcyclo-
pentadienone,36 and 1,3-diethyl-2-iodobenzene25 were prepared
according to reported procedures. Anhydrous and oxygen-free
solvents were obtained by passage through columns packed with
activated alumina and supported copper catalyst (Glass Con-
tour, Irvine, CA). All other reagents and solvents were pur-
chased from commercial sources and used without further
purification unless otherwise indicated.
1,3-Diisopropyl-2-(phenylethynyl)benzene (8).24 Pd2dba3
(0.636 g, 0.695 mmol), CuI (0.132 g, 0.693 mmol), and PPh3
(0.728 g, 2.78 mmol) were combined under N2. Triethylamine (20
mL) was added, followed by 2-iodo-1,3-diisopropylbenzene (9;
2.00 g, 6.94 mmol) and phenylacetylene (1.52 mL, 1.41 g, 13.8
mmol), and a stream of N2 was bubbled through the mixture for 10
min. The mixture was then heated at reflux for 24 h under N2 and
allowed to cool to 25 °C. Volatiles were removed by evaporation
under reduced pressure, and the residue was partitioned between
CH2Cl2 (50 mL) and 1 N aqueous HCl (50 mL). The aqueous
phase was discarded, and the organic phase was further washed
with 1 N aqueous HCl, saturated aqueous NaHCO3, and brine.
The solution was then dried with anhydrous MgSO4 and filtered.
Removal of volatiles by evaporation under reduced pressure left a
residue that was purified by flash chromatography (silica gel,
hexanes) to afford 1,3-diisopropyl-2-(phenylethynyl)benzene (8;
1.29 g, 4.92 mmol, 71%) as a colorless liquid: 1H NMR (CDCl3,
400 MHz) δ 7.57-7.53 (m, 2H), 7.41-7.32 (m, 3H), 7.29 (t,
1H, J = 7.7 Hz), 7.16 (d, 2H, J = 7.7 Hz), 3.62 (septet, 2H,
3J = 6.9 Hz), 1.32 (d, 12H, 3J = 6.9 Hz); 13C NMR (CDCl3, 100
MHz) δ 150.9, 131.4, 128.6, 128.5, 128.2, 124.2, 122.3, 121.0, 97.5,
86.8, 32.0, 23.4; HRMS (ESI-TOF) calcd for [C20H22 þ H]þ m/e
263.17943, found 263.17871.
1-(2-Methylphenyl)-2,3,4,5,6-pentaphenylbenzene (3b). Tetra-
phenylcyclopentadienone (2.00 g, 5.20 mmol) and 2-methyl-(1-
phenylethynyl)benzene (5; 1.00 g, 5.20 mmol) were combined in
diphenyl ether (4 mL), and the mixture was heated at reflux for 4
days. The mixture was allowed to cool to 25 °C, and the resulting
yellow crystalline precipitate was separated by filtration
and washed with EtOH and hexanes. Recrystallization from
In studies of single crystals by X-ray diffraction, data were
collected at 150 K with a Bruker Microstar diffractometer with
Cu KR radiation. The structures were solved by direct methods
with SHELXS-97 and refined with SHELXL-97.37,38 Non-
hydrogen atoms were refined anisotropically, whereas hydrogen
atoms were placed in ideal positions and refined as riding atoms.
In all structural studies, calculated X-ray powder diffraction
patterns closely matched those obtained experimentally by
analysis of bulk crystalline samples.39
1,3-Diethyl-2-(phenylethynyl)benzene (7).23 Pd2dba3 (0.352 g,
0.384 mmol), CuI (0.073 g, 0.38 mmol), and PPh3 (0.403 g, 1.54
mmol) were combined under N2. Triethylamine (10 mL) was
added, followed by 1,3-diethyl-2-iodobenzene (1.00 g, 3.84
mmol) and phenylacetylene (0.84 mL, 0.78 g, 7.6 mmol), and a
stream of N2 was bubbled through the mixture for 10 min. The
mixture was heated at reflux for 18 h under N2 and then allowed
to cool to 25 °C. Removal of volatiles by evaporation under
reduced pressure left a dark residue, which was dissolved in
CH2Cl2 (75 mL). The solution was washed with 1 N aqueous
HCl, saturated aqueous NaHCO3, and brine. The organic phase
was dried with anhydrous MgSO4 and filtered through a small
pad of silica gel, using CH2Cl2 as eluent. Removal of volatiles
from the eluent by evaporation under reduced pressure left a
residue, which was purified by flash chromatography (silica gel,
hexanes) to give 1,3-diethyl-2-(phenylethynyl)benzene (7; 0.64 g,
3
3
(36) Johnson, J. R.; Grummitt, O. Org. Synth. 1943, 23, 92–93. Johnson,
J. R.; Grummitt, O. Organic Syntheses; Wiley: New York, 1955; Collect. Vol.
III, pp 806-807.
(37) Sheldrick, G. M. SHELXS-97, Program for the Solution of Crystal
Structures and SHELXL-97, Program for the Refinement of Crystal Struc-
€
€
€
tures; Universitat Gottingen: Gottingen, Germany, 1997.
(38) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(39) See the Supporting Information for details.
(40) Friebolin, H. Basic One- and Two-Dimensional NMR Spectroscopy;
Wiley-VCH: Weinheim, Germany, 1998.
J. Org. Chem. Vol. 75, No. 2, 2010 405

Gagnon et al.
JOCArticle
o-xylene yielded 1-(2-methylphenyl)-2,3,4,5,6-pentaphenylben-
zene (3b; 1.49 g, 2.72 mmol, 52%) as a colorless solid. A sample
for thermal analysis was prepared by crystallization from diox-
ane: mp (sealed capillary) >400 °C; 1H NMR (400 MHz,
CDCl3) δ 6.95-6.70 (m, 29H), 2.03 (s, 3H); 13C NMR (175
MHz, CDCl3) δ 140.79, 140.52, 140.43, 140.39, 140.35, 140.18,
139.84, 135.85, 132.15, 131.71, 131.62, 131.59, 131.50, 130.49,
128.95, 126.71, 126.65, 126.55, 126.43, 126.27, 125.42, 125.27,
124.08, 20.87;41 HRMS (ESI-TOF) calcd for [C43H32 þ H]þ m/e
549.25768, found 549.25640. Anal. Calcd for C43H32: C, 94.12;
H, 5.88. Found: C, 94.01; H, 5.72.
solid. A sample for thermal analysis was prepared by crystal-
lization from toluene: mp (sealed capillary) 282-283 °C; 1H
NMR (CDCl3, 400 MHz) δ 6.92 (t, 1H, 3J = 7.6 Hz), 6.88-6.74
(m, 27H), 2.46 (q, 4H, 3J = 7.5 Hz), 1.06 (t, 6H, 3J = 7.5 Hz);
13C NMR (CDCl3, 175 MHz) δ 140.99, 140.90, 140.84, 140.73,
140.40, 140.34, 140.28, 139.14, 138.26, 131.76, 131.73, 130.74,
126.86, 126.66, 126.65, 126.33, 125.57, 125.22, 125.20, 123.33,
26.13, 13.36; HRMS (ESI-TOF) calcd for [C46H38 þ H]þ m/e
591.30463, found 591.30381. Anal. Calcd for C46H38: C, 93.52;
H, 6.48. Found: C, 93.46; H, 6.35.
1-(2,6-Diisopropylphenyl)-2,3,4,5,6-pentaphenylbenzene (3e).
1,3-Diisopropyl-2-(phenylethynyl)benzene (8; 0.750 g, 2.86
mmol) and tetraphenylcyclopentadienone (1.10 g, 2.86 mmol)
were combined in diphenyl ether (2 mL), and the mixture was
heated at reflux for 24 h under N2. The mixture was allowed to
cool to 25 °C and diluted with MeOH (30 mL). The resulting
precipitate was collected by filtration, washed with MeOH
(20 mL), and dried to give 1-(2,6-diisopropylphenyl)-2,3,4,5,6-
pentaphenylbenzene (3e; 1.37 g, 2.21 mmol, 77%) as a colorless
solid. A sample for thermal analysis was prepared by crystal-
lization from toluene/hexanes: mp (sealed capillary) 285-
1-(2,6-Dimethylphenyl)-2,3,4,5,6-pentaphenylbenzene (3c). 1,
3-Dimethyl-2-(phenylethynyl)benzene (6; 0.568 g, 2.75 mmol)
and tetraphenylcyclopentadienone (1.06 g, 2.76 mmol) were
combined in diphenyl ether (2 mL), and the mixture was
heated at reflux for 44 h. The mixture was allowed to cool to
25 °C and diluted with MeOH (5 mL). The resulting precipitate
was separated by filtration and dried to give 1-(2,6-dimethyl-
phenyl)-2,3,4,5,6-pentaphenylbenzene (3c; 1.19 g, 2.11 mmol,
77%) as an off-white solid. A sample for thermal analysis was
prepared by crystallization from toluene: mp (sealed capillary)
364-365 °C; 1H NMR (CDCl33, 400 MHz) δ 6.89-6.78 (m,
25H), 6.75 (AK2 system,40 1H, J = 7.6 Hz), 6.64 (AK2 sys-
tem,40 2H, 3J = 7.6 Hz), 2.10 (6H, s); 13C NMR (benzene-d6, 175
MHz) δ 141.58, 141.35, 141.33, 141.27, 140.87, 140.77, 140.36,
139.34, 136.10, 132.12, 132.06, 130.85, 127.28, 127.26, 127.23,
127.09, 126.90, 126.22, 125.80, 125.78, 21.79; HRMS (ESI-
1
3
287 °C; H NMR (CDCl3, 400 MHz) δ 7.09 (t, 1H, J = 7.7
Hz), 6.89-6.73 (m, 27H), 2.83 (septet, 2H, 3J = 6.7 Hz), 0.86 (d,
12H, 3J = 6.7 Hz); 13C NMR (benzene-d6, 175 MHz) δ 146.89,
141.96, 141.84, 141.65, 141.55, 141.30, 140.44, 138.16, 137.26,
132.77, 132.43, 132.01, 128.72, 127.27, 127.22, 126.94, 126.16,
125.74, 125.69, 123.44, 30.76, 25.13; HRMS (ESI-TOF) calcd
for [C48H42 þ H]þ m/e 619.33593, found 619.33432. Anal. Calcd
for C48H42: C, 93.16; H, 6.84. Found: C, 93.16; H, 6.70.
TOF) calcd for [C44H34
563.27245. Anal. Calcd for C44H34: C, 93.91; H, 6.09. Found:
C, 93.81; H, 5.97.
þ
H]þ m/e 563.27333, found
Acknowledgment. We are grateful to the Natural Sciences
1-(2,6-Diethylphenyl)-2,3,4,5,6-pentaphenylbenzene (3d). 1,3-
Diethyl-2-(phenylethynyl)benzene (8; 0.500 g, 2.13 mmol), tet-
raphenylcyclopentadienone (0.820 g, 2.13 mmol), and diphenyl
ether (2 mL) were combined, and the mixture was heated at
reflux for 48 h under N2. The mixture was allowed to cool to 25
°C and diluted with MeOH (30 mL). The resulting precipitate
was collected by filtration, washed with a small amount of
MeOH, and dried to yield 1-(2,6-diethylphenyl)-2,3,4,5,6-pen-
taphenylbenzene (3d; 0.828 g, 1.40 mmol, 66%) as a colorless
ꢁ
and Engineering Research Council of Canada, the Ministere
de l’Education du Quebec, the Canada Foundation for
ꢀ
ꢀ
Innovation, the Canada Research Chairs Program, and
ꢀ
ꢀ
Universite de Montreal for financial support. In addition,
we thank Dr. Thierry Maris for his help in analyzing crystal-
lographic data and Sylvain Essiembre for his assistance in
carrying out thermal analyses.
Supporting Information Available: 1H and 13C NMR spec-
tra of compounds 3b-e and 7-9 and additional crystallo-
graphic details, including ORTEP views, X-ray powder
diffraction patterns, and tables of structural data in CIF format.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(41) In hexaphenylbenzene 3b, rotation of the o-tolyl group about the
biphenyl bond is expected to be slow at 25 °C.42 As a result, compound 3b
possesses 28 aromatic carbon atoms that are unique but spectroscopically
very similar. We could not entirely resolve these signals by 13C NMR, even at
175 MHz using a cryoprobe.
(42) Gust, D. J. Am. Chem. Soc. 1977, 99, 6980–6982.
406 J. Org. Chem. Vol. 75, No. 2, 2010