A novel tetraarylpyrene host: Conformation-dependent inclusion of guest molecules in the crystal lattice

Article abstract of DOI:10.1007/s12039-010-0058-z

Tetrakis(2,6-dimethyl-4-acetoxyphenyl)pyrene H2 containing flexible acetate functionalities at the para positions of sterically-hindered and rigid aryl rings functions as an inclusion host system. Depending on the orientations of the acetate functionaliti

Full text of DOI:10.1007/s12039-010-0058-z

J. Chem. Sci., Vol. 122, No. 5, September 2010, pp. 697–706. © Indian Academy of Sciences.
A novel tetraarylpyrene host: Conformation-dependent inclusion of
guest molecules in the crystal lattice
1 2,
PALANI NATARAJAN , PALOTH VENUGOPALAN * and
1,
JARUGU NARASIMHA MOORTHY *
1
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016
2
Department of Chemistry, Panjab University, Chandigarh 160 014
e-mail: moorthy@iitk.ac.in; venugopalanp@yahoo.com
Abstract. Tetrakis(2,6-dimethyl-4-acetoxyphenyl)pyrene H2 containing flexible acetate functionalities
at the para positions of sterically-hindered and rigid aryl rings functions as an inclusion host system. De-
pending on the orientations of the acetate functionalities, a variety of conformers may indeed be
expected. A limited number of the crystal structures of the inclusions compounds of H2 reveal that one
indeed observes 2 different conformations for the host based on the orientations of the acetate functiona-
lities. The inclusion compound of H2 with benzene guest molecules is particularly appealing in terms of
how the latter are held in trough domains of the host by weak C−H⋅⋅⋅O and C−H⋅⋅⋅π hydrogen bonds.
More experimentation and analyses of crystal structures of such systems is expected to lead to better
insights toward realizing multicomponent molecular crystals in a rational manner.
Keywords. Inclusion compounds; X-ray diffraction; conformational analysis; self-assembly; supra-
molecular chemistry.
1. Introduction
rene H1 with D2h-symmetry as a host system with
three distinct domains, viz. trough, basin and con-
16,17
The investigation of solid state host–guest molecular
aggregates that represent a paradigm for multicom-
ponent molecular assemblies is important, and paves
way for understanding the link between chemistry
cave (chart 1), for guest inclusion.
In a compre-
hensive investigation, we have shown that diverse
guest molecules are included by the host H1 in solid
state in all the three domains. Indeed, the crystal
structure analysis of a large number of inclusion
compounds permitted the recognition of selective
binding of larger aromatics in trough regions and
aliphatic guests in concave regions. Based on this
differential binding of guests in two distinct
domains of the host H1, we demonstrated a rational
approach to the creation of ternary inclusion com-
1–5
and biology. The host–guest inclusion chemistry
is relevant in diverse applications that range from
optical resolution through polymerization in matrices
to the protection and stabilization of hazardous
chemicals and sensitive pharmaceuticals, respec-
6–11
tively.
Thus, there is an emphasis of interest in
new and novel inclusion host systems discovered
2
either by serendipity or by rational design. The latter
17
pounds with two dissimilar guests. In a logical
constitutes an endearing goal, which entails the de-
sign at a molecular level taking into consideration of
topological attributes of the overall skeleton and the
intermolecular interactions that the molecular sys-
extension of these studies, we sought to explore the
inclusion chemistry of tetraacetyl analog H2 of host
H1 (chart 1). The motivations for our interest in H2
are the following:
12–15
tems may lend themselves to.
There has been considerable interest in the crea-
tion of molecular receptors that selectively bind neu-
tral organic molecules, as they constitute models for
• The host H2 represents a homolog with an addi-
tional C=O group that may effectively serve
as a hydrogen bond acceptor for the included
guests.
1–5
substrate–receptor biochemistry. In our laborato-
ries, we recently designed sterically-hindered
1,3,6,8-tetrakis(2,6-dimethyl-4-methoxyphenyl) py-
• The acetoxy group may enhance the guest binding
owing to its flexibility, which might allow adop-
tion of conformations that enable better binding of
the guests.
*For correspondence
697

698
Palani Natarajan et al
Chart 1. Structures of H1 and H2 and cartoon drawing that exemplifies the structures of hosts H1 and
H2 with locations for guest binding.
Scheme 1. The synthetic route for the preparation of host H2.
• Depending on the orientations of the acetoxy phy (TLC) using aluminum sheets pre-coated with
groups in the crystal lattice, one may envisage silica gel. Column chromatography was conducted
1
conformation-dependent guest binding as well as with silica gel (60–120 μm mesh). H NMR spectra
18
conformational polymorphism.
were recorded on a 500 MHz spectrometer using
deuterated solvents. TGA measurements were car-
Here, we report that 1,3,6,8-tetrakis(2,6-dimethyl-4- ried out at a heating rate of 10°C/min under nitrogen
acetoxyaryl)pyrene H2 does indeed function as a gas atmosphere. Commercial chemicals were used
unique host system. The limited investigations of as received.
guest inclusion reveal that H2 behaves as a respon-
sive host system in that it appears to bind different
2.1 Synthesis of 1,3,6,8-tetrakis(2,6-dimethyl-4-
guest molecules by exploiting the conformational
acetoxyphenyl)pyrene H2
flexibility inherent to four acetoxy groups. In other
words, the host H2 lends itself to what may be
The synthesis of 1,3,6,8-tetrakis(2,6-dimethyl-4-
termed conformation-dependent guest binding.
methoxyphenyl)pyrene H1 from 4-fold Suzuki
coupling of 1,3,6,8-tetrabromopyrene with 2,6-
dimethyl-4-methoxyphenylboronic
acid
using
2. Experimental
Pd(PPh3)4 as a catalyst has been previously reported
19
Anhydrous tetrahydrofuran (THF) was freshly dis- by us. To a solution of H1 (1⋅0 g, 1⋅36 mmol) in
tilled over sodium prior to use. All other solvents 30 mL of dry CH2Cl2 at 0°C was added BBr3
were distilled prior to use. The progress of reactions (0⋅6 mL, 5⋅42 mmol) drop-wise under a N2 gas at-
was monitored by analytical thin layer chromatogra- mosphere. The reaction mixture was allowed to stir

A novel tetraarylpyrene host
2.3 X-ray crystal structure determination
699
overnight. Subsequently, it was quenched with 10%
HCl (ca. 10 mL), extracted with ethyl acetate, dried
over Na2SO4, treated with charcoal, filtered and
concentrated. Filtration over a short-pad of silica gel
using a mixture of ethyl acetate and pet. ether
(50 : 50) led to 1,3,6,8-tetrakis(2,6-dimethyl-4-
hydroxyphenyl)pyrene (scheme 1) as a pure colour-
less solid in a quantitative yield (>95%); IR (KBr)
A good quality crystal in each case was mounted
over a glass fibre, cooled to 100 K, and the intensity
data were collected on a Bruker Nonius SMART
APEX CCD detector system with Mo-sealed Sie-
mens ceramic diffraction tube (λ = 0⋅71073 Å) and a
highly oriented graphite monochromator operating
at 50 kV and 30 mA. The data were collected on a
hemisphere mode and processed with Bruker
SAINTPLUS. Empirical absorption correction was
made using Bruker SADABS. The structure was
solved in each case by Direct Methods using
SHELXTL package and refined by full matrix least-
–1 1
cm 2920, 3394(b); H NMR (500 MHz, DMSO-d6)
δ 1⋅77 (s, 24H), 6⋅61 (s, 8H), 7⋅40 (s, 2H), 7⋅43 (s,
4H), 9⋅27 (s, 4H); C NMR (125 MHz, DMSO-d6) δ
13
21⋅1, 114⋅8, 124⋅9, 126⋅8, 128⋅6, 128⋅8, 130⋅6,
136⋅6, 137⋅8, 151⋅0. Anal. Calcd. For C48H42O4: C
84⋅43, H 6⋅20. Found: C 84⋅42, H 6⋅18.
2
squares method based on F using SHELX-97 pro-
To a mixture of 1,3,6,8-tetrakis(2,6-dimethyl-4-
hydroxyphenyl)pyrene (0⋅75 g, 1⋅1 mmol) and ace-
tic anhydride (0⋅9 mL, 8⋅8 mmol) was added a few
drops of concentrated H2SO4. The reaction mixture
was stirred for 12 h at 60°C and cooled to room
temperature, and then poured into 50 mL of ice-cold
water. The organic matter was extracted with diethyl
ether. The combined extracts were washed with wa-
ter and dried over Na2SO4. The solvent was removed
in vacuum and the residual viscous solid was chro-
matographed on silica gel to afford 0⋅86 g (92%)
20
gram. All the non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were included
in the ideal positions with fixed isotropic U values
and were riding with their respective non-hydrogen
atoms. The experimental details of crystal data,
intensity measurements, structure solution and
refinements are presented in table 1. CCDC-773342
(H2•BZ), CCDC-773343 (H2•CH) and CCDC-
773344 (H2•NAP) contain the supplementary crys-
tallographic data for this paper. Copies of these data
can be obtained free of charge from the Cambridge
Crystallographic Data Centre (www.ccdc.cam.ac.uk/
data_request/cif).
of
1,3,6,8-tetrakis(2,6-dimethyl-4-acetoxyphenyl)
–1
pyrene H2 as a colourless solid, IR (KBr) cm
1
3042, 1592, 1179; H NMR (500 MHz, CDCl3) δ
1⋅90 (s, 24H), 2⋅27 (s, 12H), 6⋅87 (s, 8H), 7⋅51 (s,
4H), 7⋅55 (s, 2H); C NMR (125 MHz, CDCl3) δ
13
3. Results and discussion
20⋅7, 21⋅2, 120⋅2, 125⋅0, 125⋅9, 128⋅6, 128⋅9, 135⋅7,
137⋅4, 138⋅5, 149⋅9, 169⋅5. Anal. Calcd. For
C56H50O8: C 79⋅04, H 5⋅92. Found: C 78⋅99, H 5⋅91.
The host H2 was conveniently synthesized by de-
methylation of compound H1 using BBr3 followed
by treatment of the resulting tetraphenol with acetic
anhydride, scheme 1. The synthesis of host H1 has
already been reported by us in the context of organic
2.2 Preparation of inclusion compounds
19
light emitting diodes (OLEDs). Crystallization of
The inclusion compounds of H2 with benzene
(H2•BZ) and chloroform (H2•CH) were prepared
by dissolving host H2 (20⋅0 mg) in 5 mL of respec-
tive solvents, i.e. benzene and chloroform, that were
found to occupy lattice voids and evaporating the
resultant solutions over a period of 7–10 days to
yield colourless crystals quantitatively. The crystals
host H2 was carried out from a variety of solvents
and also with various combinations of solvents. The
crystals were readily obtained from benzene, CHCl3
and
a
mixture of naphthalene and 1,2-
dimethoxyethane (DME). The X-ray crystal struc-
1
ture analysis in conjunction with H NMR analysis
of the crystals revealed the presence of guest inclu-
sion. In table 1 are summarized the details of crystal
data, host:guest stoichiometry and guest accessible
volume, as calculated by the program PLATON.
1
were characterized by H NMR and X-ray crystal-
lography. The crystals of the compound with naph-
thalene and dimethoxyethane (H2•NAP) were
obtained as follows: 30 mg of H2 with 4 equivalents
of naphthalene were dissolved in 4 mL of 1,2-
dimethoxyethane (DME), and the resultant homoge-
3.1 The inclusion compound with benzene, H2•BZ
neous solution was slowly evaporated over a period The crystals of H2•benzene were found to belong to
of 10 days at room temperature. the monoclinic crystal system with space group

700
Palani Natarajan et al
Table 1. The crystal data, host:guest stoichiometry and guest accessible volume for the inclusion compounds of H2.
Identification code
H2•BZ
H2•CH
H2•NAP
C70H68O10
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C74H68O8
1085⋅28
100(2)
C60H54Cl12O8
1328⋅43
100(2)
1069⋅24
100(2)
0⋅71073
Monoclinic
P21/n (no⋅ 14)
12⋅831(2)
8⋅112(8)
30⋅865(6)
90⋅00
0⋅71073
0⋅71073
Monoclinic
C2/c (no⋅ 15)
15⋅070(7)
18⋅528(7)
21⋅117(8)
90⋅00
Monoclinic
P21/c (no⋅ 14)
15⋅063(5)
20⋅041(7)
10⋅590(4)
90⋅00
b (Å)
c (Å)
α (deg)
β (deg)
99⋅35(3)
90⋅00
3169⋅5(1)
2
92⋅96(3)
90⋅00
3193⋅0(2)
2
108⋅33(3)
90⋅00
5597⋅0(4)
4
γ (deg)
3
Volume (Å )
Z
3
Calculated density (mg/m )
–1
Absorption coefficient (mm )
F(000)
1⋅137
0⋅073
1152
1⋅382
0⋅571
1364
1⋅269
0⋅084
2272
Theta range (deg)
Scan type
2⋅36 to 25⋅00
2θ–θ
2⋅51 to 25⋅00
2θ–θ
2⋅28 to 25⋅00
2θ–θ
Reflections collected
Independent reflections
Refinement method
15687
15944
14103
5524 [R(int) = 0⋅0370]
Full-matrix
2
5558 [R(int) = 0⋅0757]
Full-matrix
2
4838 [R(int) = 0⋅0604]
Full-matrix
2
least-squares on F
least-squares on F
least-squares on F
Data/restraints/parameters
2
Goodness-of-fit on F
5524/0/370
5558/102/450
4838/119/414
1⋅051
1⋅046
1⋅086
Final R indices [I > 2σ (I)]
R indices (all data)
R1 = 0⋅0644,
wR2 = 0⋅1643
R1 = 0⋅1066,
wR2 = 0⋅1850
R1 = 0⋅0824,
wR2 = 0⋅1814
R1 = 0⋅1746,
wR2 = 0⋅2109
0⋅260 and –0⋅262
1 : 4
R1 = 0⋅0937,
wR2 = 0⋅2258
R1 = 0⋅1856,
wR2 = 0⋅2671
0⋅437 and –0⋅240
1 : 1 : 1
–3
Largest diff. peak and hole (eÅ ) 0⋅381 and –0⋅211
Host : Guest 1 : 3
V (%) 35
36
46
P21/n. The asymmetric unit was found to contain held in the trough region of host via C−H⋅⋅⋅O and
only half of the host H2 (located on the crystallo- C–H⋅⋅⋅π hydrogen bonds, as shown separately in
graphic centers of inversion) with 1⋅5 benzene guest figure 2a. Clearly, the guest molecules are accom-
molecules. Thus, the host:guest stoichiometry is modated in the two distinct domains of the host sys-
1 : 3, which was also established by TGA analysis. tem. Two neighbouring strands along a-axis are
The latter reveals that the occluded benzene mole- displaced in a staggered manner to make up a corru-
cules escape from the lattice at around 150°C and gated layer along c-axis. The layer is stabilized via
that the compound begins to decompose at 430°C. C−H⋅⋅⋅π hydrogen bonds between the central pyrene
The four 4-acetoxy-2,6-dimethylphenyl rings are and –CH3 groups of COCH3 moiety. The entire host–
found to be approximately orthogonal to the pyrene guest assembly is stabilized by C−H⋅⋅⋅O and C−H⋅⋅⋅π
Details of intermolecular interac-
21,22
platform; the calculated angles between the planes hydrogen bonds.
for the aryl rings with the central pyrene ring are: tions are listed in table 2. As revealed by PLATON,
77⋅45° and 88⋅56°. The host H2 is found to adopt approximately 35% of the volume is occupied by the
the conformation ‘in-in-in-in’ (figure 1A).
included guest molecules.
In the lattice, the host H2 is found to form
C−H⋅⋅⋅O hydrogen bonded strands along the a-axis
with concave regions of the translationally related
molecules enclosing a square-shaped cavity down b-
axis, within which one benzene is trapped as a guest
molecule (figure 2c). The second guest molecule is
3.2 The inclusion compound with chloroform,
H2•CH
Crystallization of H2 in CHCl3 led to rectangular
crystals in a quantitative yield. The crystals of

A novel tetraarylpyrene host
701
Figure 1. Various conformations possible for the acetoxy host H2.
H2•CH were found to belong to the monoclinic
In the crystal lattice, the host H2 forms 2D layers
crystal system with the space group P21/c. The down c-axis with channels for guest accommoda-
asymmetric unit contains only half of the host skele- tion. The porous assemblies are stabilized by weak
ton with 2 guest CHCl3 molecules, which are disor- C−H⋅⋅⋅O hydrogen bonds between carbonyl oxygen
dered over two positions. The refinement for the and the hydrogens of methyl groups (figure 3b). The
latter was achieved by applying partial occupancies. channels down the c-axis are relatively much bigger
The calculated angles between the planes for the (approximate diagonal dimensions, 11⋅2 × 10⋅2 Å)
orthogonally oriented dimethylaryl rings and the than the overall size of the chloroform guest mole-
pyrene platform are: 70⋅54° and 81⋅84°, which are cules, and as a consequence a large number of solvent
ca. 10° lesser than those observed for the crystals of molecule (Host : Guest = 1 : 4) are accommodated in
H2•BZ. The conformation adopted by the host is the lattice. Another noteworthy feature is that the
in-out-in-out (figure 1C).
channels of the host lack any functional groups that

702
Palani Natarajan et al
Figure 2. The molecular structure of H2 with guest benzene (a and b), typical host–guest
assembly of H2•BZ in the crystal lattice down b-axis (c) and the arrangement of host H2
molecules down a-axis (d).
may bind the guest chloroform molecules strongly, cules in the unit cell. The calculated angles between
which renders the guests to adopt multiple orienta- the planes of the orthogonally oriented dimethylaryl
tions that lead to disorder.
rings and the central pyrene core are: 73⋅22° and
As shown in figure 3c, two neighbouring porous 82⋅12°, which are ca. 7° lesser than those observed
layers are displaced in a staggered manner to make in the crystals of H2•BZ. The conformation adopted
up the crystals. The layer displacement is stabilized by the host is in-out-in-out (figure 1C), which is
via C−H⋅⋅⋅π hydrogen bonds between the central similar to the one found for H2•CH.
pyrene and CH3 groups of COCH3 moiety. The
The crystal packing of H2•NAP with guest mole-
entire host–guest assembly is stabilized by C−H⋅⋅⋅O cules down c-axis is shown in figure 4c. The
and C−H⋅⋅⋅π hydrogen bonds and halogen-halogen C−H⋅⋅⋅O hydrogen bonding between the dimethy-
interactions. Details of intermolecular interactions laryl rings of the translationally-related molecules
are listed in table 2.
along the b-axis leads to voids in which the guests
are entrapped. Each of the host molecules is found
to involve in four C−H⋅⋅⋅O hydrogen bonds with the
neighbouring host molecules. Methyl groups of the
COCH3 are found to involve in C−H⋅⋅⋅π hydrogen
3.3 The inclusion compound with
naphthalene-DME, H2•NAP
Crystallization of H2 in DME containing 4 equiva- bonds with those of the neighbouring glide-related
lents of naphthalene led to block-shaped crystals. host molecules, such that one obtains a honeycomb
The crystals of H2•NAP were found to belong to structure with voids for guest inclusion (figure 4c).
monoclinic (space group C2/c) with four molecules A careful inspection of the molecular association
of host H2, four naphthalene and four DME mole- shows that one of the dimethylaryl rings of the

A novel tetraarylpyrene host
703
Table 2. Weak intermolecular hydrogen bonds observed in the inclusion
compounds of host H2 with different guest molecules.
Interaction
d/Å
θ/deg
Range of C–H⋅⋅⋅π d/Å
H2•BZ
C28–H⋅⋅⋅O4
C18–H⋅⋅⋅O2
C23–H⋅⋅⋅O3
C13–H⋅⋅⋅O1
C18–H⋅⋅⋅O2
C29–H(g)⋅⋅⋅O4
C30–H(g)⋅⋅⋅O2
C35–H(g)⋅⋅⋅O1
2⋅918
2⋅762
2⋅679
2⋅656
2⋅548
2⋅549
2⋅874
2⋅712
102⋅21
135⋅39
164⋅25
169⋅38
119⋅61
144⋅72
166⋅68
143⋅98
2⋅84–2⋅97 Å
H2•CH
C25–H⋅⋅⋅O2
C18–H⋅⋅⋅O1
C28–H⋅⋅⋅O2
C16–H⋅⋅⋅O2
C18–H⋅⋅⋅O1
2⋅864
2⋅819
2⋅650
2⋅610
2⋅536
118⋅59
155⋅77
138⋅74
156⋅28
144⋅63
2⋅92–2⋅99 Å
H2•NAP
C21–H⋅⋅⋅O4
C18–H⋅⋅⋅O4
C25–H⋅⋅⋅O4
C16–H⋅⋅⋅O1
C18–H⋅⋅⋅O2
2⋅796
2⋅754
2⋅662
2⋅636
2⋅412
153⋅41
132⋅17
162⋅13
117⋅24
149⋅73
2⋅91–2⋅99 Å
Figure 3. The molecular structure of H2 with guest CHCl3 (a), 2D C−H⋅⋅⋅O hydrogen
bonded sheet arrangement of host H2 (b) and typical host–guest assembly of H2•CH in the
crystal lattice down c-axis (c).

704
Palani Natarajan et al
Figure 4. The molecular structure of H2 with guest naphthalene and DME (a) and the
C−H⋅⋅⋅O hydrogen bond-mediated strands of host H2 along b-axis. Notice that the aryl rings
of the host interject into the trough regions of the neighbouring molecules (b). The honey-
comb kind of host–guest assembly down c-axis (c) and the arrangement of host H2 as corru-
gated sheets down a-axis (d).
adjacent molecules interjects into the trough of the lead to as many as 7 conformational isomers shown
neighbouring host molecule via two C−H⋅⋅⋅O and in figure 1. Accordingly, based on the orientations
two C−H⋅⋅⋅π hydrogen bonds that leads to columns of the acetoxy groups inward/outward of the trough,
down b-axis (figure 4b). As shown in Figure 4d, the the conformational isomers are denoted by descrip-
molecules of H2 are arranged in corrugated sheets, tions such as in-in-in-in, in-out-in-out, in-out-out-in,
and their packing is stabilized by stacking interactions etc. Such a conformational flexibility is a priori
between the dimethylaryl rings of adjacent molecules. expected to lend considerable liberty to the host sys-
The guest naphthalene forms C−H⋅⋅⋅O and C−H⋅⋅⋅π tems to adopt geometries complimentary to varying
interactions with host H2 in the lattice. Details of sizes and shapes of guest molecules to allow excel-
intermolecular interactions are listed in table 2.
lent guest inclusion behaviour. A cursory glance at
First of all, our strategy that the molecules charac- the various conformers in figure 1 shows that the
terized by a flat aromatic base decked up with rigid geometries A–C are centrosymmetric and D–E are
aromatic panels can exhibit the phenomenon of mirror symmetric, while F and G are unsymmetrical.
guest inclusion is demonstrated by guest-binding in- Given the tendency of symmetric molecules to ex-
16,17
clusion behaviour of the host H2.
Because of the ploit crystallographic symmetry, one should expect
sterics, the dimethylaryl rings in H2 are expected to guest inclusion with various conformational isomers
be orthogonal and rigid. Thus, the acetoxy groups on leading to conformation-dependent guest inclusion.
the rigid aryl rings were meant as handles to abet the Viewed differently, the host may be said to explore
binding of guests in concave/trough domains of the conformational changes in response to the shape,
host via additional C−H⋅⋅⋅O/C−H⋅⋅⋅π hydrogen size and electronic complementarity towards binding
21,22
bonds.
Introduction of such flexible groups may of the guest.

A novel tetraarylpyrene host
705
Figure 5. The structures of Rebek’s diacid and its inclusion compound with the guest
pyrazine (right).
The limited crystal structures of host H2 with References
benzene, naphthalene-DME and chloroform show
1. Atwood J L, Davies J E D and Macnicol D D 1984
In Inclusion compounds (London: Academic Press
Inc)
that the host crystallizes with two different confor-
mations. That is, the conformation adopted in the in-
clusion compounds of H2•BZ is in-in-in-in (A,
figure 1), while it is in-out-in-out (C, figure 1) in
H2•NAP and H2•CH. Thus, the guest-dependent
adoption of the conformation by H2 is clearly
2. Weber E 1987 and 1988 In Molecular inclusion and
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vols 140 and 149
23
evident. The results may also be described as con-
24
formation-dependent guest inclusion. Of the three
3. Desiraju G R 1989 Crystal engineering: The design
of organic solids (Amsterdam, Netherlands: Elsevier)
4. Atwood J L, Davies J E D and Macnicol D D 1991 In
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crystal structures of the inclusion compounds of H2
described, particularly appealing is the structure of
that with benzene guest. The guest benzenes in
H2•BZ are bound in ‘trough’ and ‘concave’
domains such that they are held by C−H⋅⋅⋅O and
C−H⋅⋅⋅π hydrogen bonds. The mode of binding of
benzene reminisces molecular clefts, which have re-
5. Atwood J L, Davies J E D, MacNicol D D, Vogtle F
and Lehn J-M 1996 In Comprehensive supramolecu-
lar chemistry (Oxford: Pergamon)
6. Ramamurthy
V
1991 Photochemistry in con-
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25
ceived much attention. In particular, the mode of
7. Desiraju G R 1997 Curr. Opin. Solid. State. Mater.
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benzene in ‘trough’ via weak hydrogen bonds is
akin to Rebek’s Kemp’s triacid based imide that was
demonstrated to bind pyrazine guest with 2 N−H⋅⋅⋅O
8. Toda F and Bishop R 2004 Separation and reactions
in organic supramolecular chemistry (New York:
Wiley)
26,27
hydrogen bonds as shown in figure 5.
9. Miyata M 2004 Inclusion reactions and polymeriza-
tion: Encyclopedia of supramolecular chemistry
(New York: Marcel Decker) p. 705
4. Conclusions
10. Sozzani P, Bracco S, Comotti A, Ferretti L and
Simonutti R 2005 Angew. Chem., Int. Ed. 44 1816
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Zaworotko M J 2006 J. Pharm. Sci. 95 499
12. Macnicol D D, Toda F and Bishop R 1996
Comprehensive supramolecular chemistry, solid state
chemistry: Crystal engineering (Oxford: Pergamon
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We have shown that tetraarylpyrene host H2 con-
taining flexible acetate functionalities at the para
positions of the sterically hindered and rigid aryl
rings functions as an inclusion host system. A lim-
ited number of crystal structures of the inclusion
compounds of H2 reveal that one observes 2 differ-
ent conformations for the host based on the orienta-
tions of the acetate functionalities. The inclusion
13. Malek N, Maris T, Perron M-E and Wuest J D 2005
Angew. Chem., Int. Ed. 44 4021
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J. Am. Chem. Soc. 129 4306
is particularly appealing in terms how the guest
15. Holman K T, Pivovar A M, Swift J A and Ward M D
is bound in the trough domains of the host by weak
2001 Acc. Chem. Res. 34 107
C−H⋅⋅⋅O and C−H⋅⋅⋅π hydrogen bonds.
16. Moorthy J N, Natarajan P and Venugopalan P 2009
J. Org. Chem. 74 8566
Acknowledgements
17. Moorthy J N, Natarajan P and Venugopalan P 2010
Chem. Commun. 46 3574
J N M is grateful to the Department of Science and
Technology (DST), Government of India for gener-
ous financial support.
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