Inclusion adducts of 4,4-bis(4'-biphenylyl)cyclohexa-2,5-dienone: a clay mimic organic host

Article abstract of DOI:10.1039/b106598j

In the lamellar architecture of the title crystal structure mediated by C-H...O hydrogen bonds, 1,2-dimethoxyethane and n-hexane are included in the hydrophobic galleries; isostructurality of these clathrates is a unique case of guest mimicry.

Full text of DOI:10.1039/b106598j

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Inclusion adducts of 4,4-bis(4A-biphenylyl)cyclohexa-2,5-dienone: a clay
mimic organic host
V. S. Senthil Kumar and Ashwini Nangia*
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India. E-mail: ansc@uohyd.ernet.in
Received (in Columbia, USA) 1st July 2001, Accepted 11th October 2001
First published as an Advance Article on the web 24th October 2001
In the lamellar architecture of the title crystal structure
mediated by C–H…O hydrogen bonds, 1,2-dimethoxyethane
and n-hexane are included in the hydrophobic galleries;
isostructurality of these clathrates is a unique case of guest
mimicry.
been obtained (Fig. 1). The biphenyl groups do not engage in the
usual herringbone T-motif or p–p stacking interactions,10
possibilities that would have resulted in a self-inclusion crystal
through interdigitation. Instead, inversion-related biphenyl
groups form parallelepiped channels of 8 3 5 Å down the c-axis
which are occupied by ordered DME guest species (Fig. 2).
Host–host and host–guest (Ph)C–H…O hydrogen bonds (2.71,
137.2; 2.57 Å, 159.5) further stabilise the ordered supramo-
lecular structure of 1·DME. The advantage of building host
networks with weak interactions was highlighted in a recent
paper on 14 inclusion compounds of tetrakis(4-nitrophenyl)-
methane.11 Open architectures assembled with C–H…O hydro-
gen bonds have a degree of both stability and flexibility, a
combination that results in adaptability and versatility towards
guest inclusion.
Host–guest inclusion compounds provide opportunities for the
design of novel open architectures for diverse applications such
as chemical separation, reactions and catalysis in a microcavity,
and for electrooptic, nonlinear and magnetic materials.1 Crystal
engineering of 2D and 3D robust metal–organic and soft
molecular open frameworks offer the promise of tailoring
specific structures, functions and properties through function-
alised organic molecules.2 For example, 4-tritylbenzoic acid
crystallises via a carboxylic acid dimer (O–H…O hydrogen
bond) with wheel–axle topology, 4-halophenoxy triazines self-
assemble through triangulo halogen synthons (Cl…Cl, Br…Br
interaction) as hexagonal channel hosts, and ligands such as
1,3,5-benzenetricarboxylic acid, 4-aminopyridine and 4,4A-
bipyridine continue to be exploited in coordination polymer
networks.3 We report herein the construction of a lamellar, C–
H…O hydrogen bonded host lattice which includes guest
species in the interlayer region, thereby expanding the family of
newly emerging neutral, organic clay mimics.4
Continuing further, 1 was recrystallised from n-hexane, a
solvent isosteric with DME but devoid of hydrogen bonding
groups. The crystal structure of 1·n-hexane (2+1) is iso-
5
Molecule 1† (Scheme 1) was designed as a tecton6 with
hydrogen bonding donor–acceptor groups in the quinone ring
and biphenyl spacers oriented in the third dimension. It was
expected that tecton 1 will self-assemble in a layer motif via C–
H…O hydrogen bonded tape 2 and the biphenyl groups will
constitute the pillars in the resulting 3D open framework. While
implementing this molecule ? crystal synthetic strategy for a
potential host, we are aware that interpenetration7 competes
with guest inclusion during crystallisation and that the latter
phenomenon (the goal of this study) occurs in only ca. 15%
cases.8 Recrystallisation of 1 from various solvents afforded
diffraction quality crystals from 1,2-dimethoxyethane (DME).
The X-ray structure of 1·DME (2+1 stoichiometry, C2/c space
group)‡ contains a C–H…O hydrogen bond chain along [001]
(H…O 2.66 Å, C–H…O 160.6°; C–H distance neutron-
normalised to 1.083 Å). The second C–H…O bond of the tape
is long (3.05 Å, 175.6°) but at the accepted distance limit
advocated by Desiraju and Steiner.9 What is significant from a
crystal engineering objective is that the target synthon 2 has
Fig. 1 Crystal structure of 1·DME (2+1) in the bc-plane to show the tape
synthon 2. Guest molecules are not shown for clarity. Notice that the
biphenyl groups project in the third dimension.
Fig. 2 Crystal structure of 1·DME (2+1) down the c-axis. The network of
host–host and host–guest C–H…O hydrogen bonds stabilise the ordered
superstructure.
Scheme 1
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Chem. Commun., 2001, 2392–2393
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106598j

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fractometer facility is acknowledged. VSSK thanks the CSIR
for fellowship.
Notes and references
†
1 was synthesised from 4,4A-diphenylbenzophenone using standard
reagents and reaction conditions. FT-IR: 1668, 763 cm21. 1H NMR: d 6.45
(d, J 10 Hz, 2 H), 7.20–7.50 (m, 12 H), 7.50–7.70 (m, 8 H). For synthesis
of 3, see ref. 5.
‡ Crystal data: (1)·(CH3OCH2CH2OCH3)0.5: (C30H22O)·(C4H10O2)0.5, C2/
c, a = 25.464(5), b = 19.751(4), c = 9.770(2) Å, b = 97.72(3)°, Z = 8,
V = 4868.9(17) Å3, 5886 unique reflections (3 < 2q < 57°), final R =
0.0522 for 1352 observed reflections [I > 2s(I)] for 308 parameters. Data
collected on Enraf-Nonius MACH-3 diffractometer at 293(2) K using Mo-
Ka X-rays (l 0.71073 Å) in the w scan mode. CCDC 172238. (1)·(n-
Fig. 3 Displacement ellipsoids at 50% probability of n-hexane in 1·n-hexane
(2+1). (a) Conformation with 45% occupancy of C34, (b) conformation with
55% occupancy of C33, (c) superposition of both conformers to show the
mimicry with 1,4-dimethylcyclohexane (sof of atoms).
hexane)0.5: (C30H22O)·(C6H14)0.5, C2/c, a = 25.052(5), b = 20.185(4), c =
9.722(2) Å, b = 98.27(3)°, Z = 8, V = 4865.4(17) Å3, 4736 unique
reflections (5 < 2q < 53°), final R = 0.0675 for 2282 observed reflections
[I > 2s(I)] for 317 parameters. Data collected on a Bruker SMART
diffractometer at 168(2) K using MoKa X-rays (l 0.71073 Å) in the w-2q
scan mode. For both structures, H-atoms were fixed and structure solution
and refinement were done with SHELX-97. CCDC 172239. See http://
www.rsc.org/suppdata/cc/b1/b106598j/ for crystallographic files in .cif or
other electronic format.
§ Aldrich Chemical Co. USA, mixture of isomers $71.20 per 25g, pure cis
$227.70 per 25g.
¶ PC Spartan Pro 1.0, Wave function Inc. USA.
∑ (1)·(Br(CH2)4Br)0.5: C2/c, a = 25.21, b = 20.21, c = 9.84 Å, b = 97.8°,
T = 293 K, R = 0.12. The heavy guest disorder in interlayer regions gives
a high diffraction R value typical of clays.4 The presence of 1,4-di-
bromobutane in crystals was confirmed by NMR (d 2.0, 3.4).
morphous‡ with identical arrangement of host atoms in the
crystal. The metrics of quinone tape C–H…O bonds are 2.67,
161.9 and 2.99 Å, 175.1°; the (Ph)C–H…O is 2.71 Å, 137.2°. In
the channel, n-hexane is disordered over two orientations with
55% and 45% occupancy (Fig. 3). The slightly elongated
displacement ellipsoid of the C33 atom in a low temperature
crystal structure implies that there are other minor conforma-
tions that contribute to guest disorder, but it is difficult to assign
their exact position because of very low electron density. Using
our refinement model, the structure converges (R factor 0.0675)
with site occupancy factor (sof) of 0.55 and 0.45 for C33 and
C34 guest atoms. Notably, neither of the conformations of n-
hexane trapped in the microcavity correspond to the one in the
crystal structure of pure solvent.12 Superposition of the two
conformers (Fig. 3c) suggests that host 1 may be able to separate
a mixture of cis- and trans-1,4-dimethylcyclohexane.§ The
guest is present in folded conformations because dimensions of
the host cavity (8 3 5 Å) force it to adopt a compact shape.13
The end-to-end C…C distance in the folded conformations of n-
hexane is 5.5 Å (5.3 Å in DME) while it is 6.4 Å in the extended
zigzag conformation.12 Energy of the three conformations of n-
hexane (RHF-631G*)¶ is 2235.2718 au (55% population),
2235.2933 au (45% population) and 2235.3678 au (zigzag). A
survey of the Cambridge Structural Database14 (version 5.21,
April 2001 update) shows that this is the first example of
structural mimicry between DME and n-hexane in a host
channel. The host cavity of 1, which also includes 1,4-di-
bromobutane,∑ is highly selective towards these guest species.
Co-solvents such as EtOAc, Et2O, CHCl3, CH2Cl2 are not
included, even when present in large excess.
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Finally, we note that while polymorphs of phenyl benzoqui-
none 3 were found to have divergent, even abstruse, crystal
¯
packing in P1 (Z = 8) and P21 (Z = 2) space groups,15
pseudopolymorphs of biphenyl analogue 1 are isostructural.
Therefore replacement of geminal phenyl group with biphenyl
steers crystal packing towards a recurring (robust) supramo-
lecular synthon16 and yields a novel, organic host material with
clay-like properties. Metric engineering2 of the lamellar–pillar
molecular framework of 1 is a current goal in our studies.
We thank Professor W. T. Robinson and Dr R. Kadirvelraj
(University of Canterbury, New Zealand) for X-ray data
collection on a Bruker SMART diffractometer. Financial
assistance from the DST in the form of a research project (SP/
S1/G29/98) and for supporting the Enraf-Nonius X-ray dif-
10 C. A. Hunter, Chem. Soc. Rev., 1994, 23, 101.
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Desiraju, J. Am. Chem. Soc., 2001, 123, 4432.
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14 F. H. Allen and O. Kennard, Chem. Des. Autom. News, 1993, 8, 31.
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Madhavi, A. Nangia, R. Thaimattam and V. R. Thalladi, Cryst. Eng.,
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Chem. Commun., 2001, 2392–2393
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