Concomitant polymorphism and supramolecular isomerism in 1,3,5-tris(4-cyanobenzoyl)benzene

Article abstract of DOI:10.1039/b307492g

The title compound, 1,3,5-tris(4-cyanobenzoyl)benzene, exhibits the phenomenon of concomitant polymorphism. Two distinct isomeric solid-state networks, honeycomb and ladder, were obtained upon crystallization from acetone-water solution. Conformational flexibility of the molecule appears to facilitate polymorph formation. Each network is maintained via formation of multiple C-H...N and C-H...O hydrogen bonds.

Full text of DOI:10.1039/b307492g

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Concomitant polymorphism and supramolecular isomerism in
,3,5-tris(4-cyanobenzoyl)benzene
1
V. S. Senthil Kumar, F. Christopher Pigge* and Nigam P. Rath
Department of Chemistry & Biochemistry, University of Missouri-St.Louis, 8001
Natural Bridge Road, St.Louis MO 63121-4499, USA. E-mail: piggec@jinx.umsl.edu
L e t t e r
Received (in London, UK) 30th June 2003, Accepted 1st August 2003
First published as an Advance Article on the web 22nd September 2003
The title compound, 1,3,5-tris(4-cyanobenzoyl)benzene, exhibits
the phenomenon of concomitant polymorphism. Two distinct
isomeric solid-state networks, honeycomb and ladder, were
obtained upon crystallization from acetone–water solution.
Conformational flexibility of the molecule appears to facilitate
polymorph formation. Each network is maintained via formation
of multiple C–Hꢀ ꢀ ꢀN and C–Hꢀ ꢀ ꢀO hydrogen bonds.
A molecule that adopts more than one distinct packing
1
arrangement in the solid state is said to be polymorphic.
The occurrence of polymorphs implies that the free energies
2
of the various crystalline forms are comparable. Polymor-
phism is important across a wide range of industries in which
the properties and/or performance of various products are a
function of structure. Reproducibly obtaining a specific poly-
morph of a given compound can be a critical issue for the
proper formulation of pharmaceuticals, dyes, explosives and
3
pigments. Polymorphism results when different (but energeti-
Scheme 1 Examples of three-connected networks: (a) honeycomb;
b) herringbone; (c) ladder; (d) brick wall.
cally similar) packing interactions are operative during crystal
growth. With rigid molecules, polymorphism is manifested
through different packing motifs. With flexible molecules,
different low-energy conformational arrangements can give
rise to polymorphic modifications (i.e., conformational poly-
morphs). Indeed, in conformationally flexible molecules, the
energies required to rotate about single bonds are comparable
in magnitude to the lattice energy differences observed between
(
1
2
compounds, but there are fewer examples of coordination
7
,13
polymers that exhibit this network topology.
hand, brick wall, ladder, herringbone, and bilayer networks
On the other
1
are rarely observed in organic crystals, but they are relatively
common in coordination polymers. Here we describe two
X-ray crystal structures of 1,3,5-tris(4-cyanobenzoyl)benzene
4
7
,15
4
polymorphs. Consequently, a greater tendency to exhibit
polymorphism might be expected for conformationally flexible
1
6
(
1), a material that exhibits concomitant polymorphism
1
7
5
molecules. For example, nineteen independent conformations
and conformational polymorphism in the same system. The
dimorph 1 also provides an example of supramolecular iso-
merism through assembly of discrete honeycomb and ladder
networks.
As a part of ongoing studies aimed at defining the inclusion
host properties of substituted 1,3,5-triaroylbenzenes and
determining the ability of functionalized triaroylbenzenes to
mediate the formation of metal-organic coordination poly-
of the same molecule were found in the tetramorphic cluster of
4
6
,4-diphenyl-2,5-cyclohexadienone. Materials that form crys-
1
8
talline polymer networks in which individual molecules are
connected via discrete intermolecular interactions also exhibit
polymorphism. In these instances, polymorphic forms result
in different network superstructures, and the terms supramole-
1
9
7
8
cular isomerism and architectural isomerism have been
coined to describe these systems.
2
0
mers, the title compound 1,3,5-tris(4-cyanobenzoyl)benzene
1) was prepared (Scheme 2). Recrystallization of 1 from 1 : 1
acetone–water solution afforded two concomitant polymorphs
forms A and B) as revealed by low temperature single-crystal
(
Visualization of organic crystal structures as networks with
molecules acting as nodes and supramolecular synthons as
node connectors facilitates retrosynthetic analysis in the con-
(
9
–11
X-ray diffraction. The two crystallographically independent
text of crystal engineering applications.
In particular, the
self-assembly of honeycomb, brick wall, ladder, bilayer and
herringbone networks from purely organic as well as metal-
organic building blocks is currently under active investiga-
7
,9b
tion.
to three-connected scaffolds in the retrosynthetic sense
Scheme 1). These networks are topologically identical and self-
The design of such network structures may be traced
(
assembly is controlled by supramolecular isomerism. Honey-
comb networks derived from three-connected building blocks
are commonly encountered in crystal structures of organic
Scheme 2
1
554
New J. Chem., 2003, 27, 1554–1556
DOI: 10.1039/b307492g
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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molecules observed in the dimorphs of 1 differ in the rotation
about the C–C bonds between the cyanobenzoyl groups and
the central 1,3,5-substituted benzene ring. These conforma-
tions may be defined according to the three torsion angles
t
1
, t
2
3
and t (Table 1). Conformational changes from one
structure to the other leads to the formation of two different
polymorphs; hence, these concomitant polymorphs also may
be described as conformational polymorphs.
1
7
Each polymorph exhibits a distinct network structure
mediated by weak solid-state hydrogen bonding interactions.
In form A, each roughly Y-shaped molecule of 1 has three
pairs of donor/acceptor sites that participate in intermolecular
2
1
hydrogen bonding. Two of the three sites are adjacent and
interact with a complementary array of sites on a neighboring
molecule, thereby resulting in four contiguous hydrogen
bonds. Two approximately co-planar carbonyl oxygens and
two phenyl hydrogens form a centrosymmetric four point
recognition pattern through two unique C–Hꢀ ꢀ ꢀO hydrogen
Fig. 1 Crystal structure of form A. Note each molecule is connected
to three inversion related neighbors to produce the honeycomb
network. Hydrogen bonds are shown as dashed lines.
˚
ꢁ
˚
ꢁ
bonds (d, y: 2.43 A, 169.8 ; 2.61 A, 138.8 ). The other two
nodes of the three-connected molecules extend through C–
Hꢀ ꢀ ꢀN centrosymmetric dimers involving phenyl hydrogens
˚
ꢁ
˚
ꢁ
and nitrile acceptors (2.77 A, 123.7 ; 2.78 A, 125.5 ). Thus
the three-connected molecule produces a honeycomb network
as shown in Fig. 1. The central benzene ring and two cyano-
benzoyl rings participate in network formation. The third cya-
nobenzoyl ring fills the hexagonal void formed in the network
to avoid an open structure. Additional hydrogen bonding
between this nitrile group and a phenyl hydrogen within the
Fig. 2 Ladder network observed in form B.
the lower energy modification. This assertion is based on the
observation that almost all of the hydrogen bonding acceptor
sites (carbonyl oxygens and nitrile nitrogens) are engaged in
intermolecular interactions in the honeycomb structure
shown in Fig. 1. In contrast, fewer solid-state hydrogen bonds
are present in form B. Consequently, form A exhibits closer
crystal packing than form B, thus resulting in a smaller crystal
˚
ꢁ
void (2.50 A, 156.3 ) further stabilize the structure.
Form B has similar cell dimensions but differs slightly in
volume as compared to form A. The solid-state network in
form B consists of centrosymmetric C–Hꢀ ꢀ ꢀN hydrogen bond
˚
ꢁ
dimers (2.56 A, 137.4 ) similar to those encountered in form
A but involving a different aryl hydrogen bond donor
(
Fig. 2). The other two nodes of the three-connected T-shaped
˚
volume. Additionally, the calculated density of form A
,
building blocks extend via C–Hꢀ ꢀ ꢀN interactions (2.32 A,
ꢂ3
is greater than that of form B (1.384 vs. 1.352 g cm
respectively).
ꢁ
1
63.7 ) involving translation related molecules to furnish a lad-
der network. The inversion related ladders are connected via
C–Hꢀ ꢀ ꢀO hydrogen bond dimers (not shown in Fig. 2). The
conformation of 1 is slightly different in the two polymorphs,
and this presumably accounts for the dramatic change in
network architecture. While supramolecular isomerism has
In summary, two concomitant conformational polymorphs
of cyano-substituted 1,3,5-triaroylbenzene derivative 1 have
been structurally characterized. The conformational flexibility
of 1 coupled with the large number of potential hydrogen bond
donor and acceptor sites undoubtedly contribute greatly to the
observed polymorphism. In both crystalline forms, solid-state
C–Hꢀ ꢀ ꢀN and C–Hꢀ ꢀ ꢀO hydrogen bonds propagate isomeric
supramolecular networks. Triaroylbenzenes have previously
7
,14c–d,18
been observed in other crystalline networks,
of concomitant supramolecular isomerism appear to be less
examples
2
2
common.
The relative stability of the two polymorphs has not been
determined. Because these are concomitant polymorphs of
similar crystalline morphology (i.e., visually indistinguishable),
separation of the two forms was not possible. The solid-state
structures, however, would appear to indicate that form A is
19
been shown to act as crystalline inclusion hosts and to parti-
cipate in the assembly of metal-organic coordination poly-
20
mers. The results of the present study indicate that this
readily accessible molecular framework may also prove useful
in further exploring the phenomenon of polymorphism. Work
along these lines continues and results will be reported in due
course.
Table 1 Torsion angles observed in the dimorphs of 1
Experimental
Compound 1 was synthesized from p-cyanobenzaldehyde as
0
2
previously described. X-ray quality crystals of the concomi-
tant polymorphs were obtained by slow evaporation of a
1
: 1 acetone–water solution at ambient temperature. Both
crystalline modifications were found to be present as evidenced
by X-ray diffractometry (Figs. 1 and 2). Random selection
of additional single crystals and determination of unit cell
constants at 165 K also revealed the presence of both forms.
t
1
t
2
t
3
(
C1–C2–C7–O1) (C3–C4–C8–O2) (C5–C6–C9–O3)
[deg]
Crystal data
Polymorph [deg]
[deg]
Preliminary examination and data collection was performed
using a Bruker SMART CCD area detector system single
crystal X-ray diffractometer. The SHELXTL-PLUS software
Form A
Form B
39.2(2)
27.7(3)
146.1(2)
164.9(2)
36.8(3)
17.3(2)
New J. Chem., 2003, 27, 1554–1556
1555

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2
3
package was used for structure solution and refinement.
Hydrogens were fixed at idealized geometries and treated
12 (a) Comprehensive Supramolecular Chemistry, vol. 6, eds.
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1
5
996; (b) A. Nangia, Curr. Opin. Solid State Mater. Sci., 2001,
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isotropically as riding groups.
¯
Form A: Triclinic, P1, a ¼ 5.9762(2), b ¼ 13.9047(4), c ¼
1
3
4
(a) H. J. Choi and M. P. Suh, J. Am. Chem. Soc., 1998,
20, 106 22; (b) N. R. Brooks, A. J. Blake, N. R. Champness,
˚
ꢁ
1
4.2152(4) A, a ¼ 71.790(2), b ¼ 84.290(2), g ¼ 87.967(2) ,
1
˚
3
ꢂ3
V ¼ 1116.51(6) A , Z ¼ 2, D
c
¼ 1.384 g cm , T ¼ 200 K,
J. W. Cunningham, P. Hubberstey, S. J. Teat, C. Wilson and
M. Schr o¨ der, J. Chem. Soc., Dalton Trans., 2001, 2530; (c)
S. R. Batten and R. Robson, Angew. Chem. Int. Ed., 1998,
˚
F(000) ¼ 480 l ¼ 0.71073 A, m ¼ 0.091, R1 ¼ 0.0530
for 3708 Fo > 2s(Fo), wR
2
¼ 0.1361 for 5486 independent
3
7, 1461.
(a) Y. Zhang, C. D. Kim and P. Coppens, Chem. Commun., 2000,
299; (b) K. T. Holman, A. M. Pivovar, J. A. Swift and M. D.
reflections. CCDC reference number 215270.y
1
¯
Form B: Triclinic, P1, a ¼ 7.7072(2), b ¼ 11.3057(2), c ¼
2
˚
ꢁ
1
3.7913(2) A, a ¼ 73.284(1), b ¼ 88.422(1), g ¼ 83.420(1) ,
Ward, Acc. Chem. Res., 2001, 34, 107; (c) L. R. MacGillivray,
J. L. Reid and J. A. Ripmeester, Chem. Commun., 2001, 1034;
(d ) B.-Q. Ma, Y. Zhang and P. Coppens, Cryst. Growth Des.,
˚
3
ꢂ3
V ¼ 1143.33(4) A , Z ¼ 2, D
c
¼ 1.352 g cm , T ¼ 165 K,
˚
F(000) ¼ 480 l ¼ 0.71073 A, m ¼ 0.089, R1 ¼ 0.0445 fo
2
002, 2, 7; (e) P. Vishweshwar, A. Nangia and V. M. Lynch,
Fo > 2s(Fo), wR
2
¼ 0.1084 for 4900 independent reflections.
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Acknowledgements
1
5
Acknowledgment is made to the donors of the Petroleum
Research Fund, administered by the American Chemical
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New J. Chem., 2003, 27, 1554–1556