Crystal engineering: Lattice inclusion based on O H...O hydrogen-bonded self-assembly and guest-induced structural mimicry

Article abstract of DOI:10.1021/jo3010292

Pyrene-tetraphenol TP2 constitutes a molecular system with inherent features for inclusion of two or more guest molecules that are complementary in terms of size and shape. Hydrogen-bonded selfassembly of TP2 in the solid state is shown to lead to voids within which the guest molecules are incorporated. A large aromatic expanse extant to the pyrene core in TP2 permits inclusion of two different types of guest species interchangeably. The robust association manifests in packing equivalence in all of the inclusion compounds of TP2 with the exception of the compound formed with pyridine and odichlorobenzene guests; in the latter, pyridine terminates the otherwise 3-dimensional hydrogen-bonded organization. The half-component of TP2, i.e., 4,6-bis(4-hydroxyphenyl)-m-xylene (BX), deduced by simple structural reduction, is shown to exhibit guest inclusion, but with considerably less guest accessible volume. The limited yet meaningful set of guests allows mimicry of the two expected patterns of molecular organization based on hydrogen bonding for both TP2 and BX in the solid state.

Full text of DOI:10.1021/jo3010292

Article
pubs.acs.org/joc
Crystal Engineering: Lattice Inclusion Based on O−H···O Hydrogen-
Bonded Self-Assembly and Guest-Induced Structural Mimicry
Alankriti Bajpai,† Palani Natarajan,† Paloth Venugopalan,‡ and Jarugu Narasimha Moorthy*,†
†Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
‡Department of Chemistry, Panjab University, Chandigarh 160014, India
S
* Supporting Information
ABSTRACT: Pyrene−tetraphenol TP2 constitutes a molecular system
with inherent features for inclusion of two or more guest molecules that
are complementary in terms of size and shape. Hydrogen-bonded self-
assembly of TP2 in the solid state is shown to lead to voids within
which the guest molecules are incorporated. A large aromatic expanse
extant to the pyrene core in TP2 permits inclusion of two different
types of guest species interchangeably. The robust association manifests
in packing equivalence in all of the inclusion compounds of TP2 with
the exception of the compound formed with pyridine and o-
dichlorobenzene guests; in the latter, pyridine terminates the otherwise
3-dimensional hydrogen-bonded organization. The half-component of
TP2, i.e., 4,6-bis(4-hydroxyphenyl)-m-xylene (BX), deduced by simple
structural reduction, is shown to exhibit guest inclusion, but with
considerably less guest accessible volume. The limited yet meaningful set of guests allows mimicry of the two expected patterns
of molecular organization based on hydrogen bonding for both TP2 and BX in the solid state.
methoxy-2,6-dimethylphenyl)pyrene TP1a molecular system
INTRODUCTION
■
that is devoid of any strongly interacting functional groups and
Strong and directional hydrogen bonds1 are an invaluable glue
characterized by a planar base decked up at its four corners with
in the tool kit of a molecular engineer concerned with
aromatic panesexhibits three different domains, namely
trough, basin, and concave, for inclusion of guest species in
the solid state,10c,d,f,g Chart 1; the sterics incorporated through
the methyl groups at the ortho positions of the aryl rings ensure
that the latter lie almost orthogonally. The otherwise moderate
conformational flexibility inherent to the aryl rings was shown
to impart the molecular system with considerable adaptability
to bind diverse guest molecules, despite the fact that the host
molecules in the lattice are stabilized by weak interac-
tions.10c,d,f,g The abundant inclusion behavior exhibited by the
host TP1 spurred us to examine O−H···O hydrogen-bonded
aggregation of such host systems. Of course, the objectives were
to (i) control molecular organization in pursuit of creating
porous solids with well-defined voids for guest inclusion and
(ii) examine the possibility of formation of biporous organic
solids that may allow size- and/or shape-selective guest
inclusion of two or more guest species leading to multi-
component molecular materials. We specifically chose phenolic
functionality as in 1,3,6,8-tetrakis(4-methoxy-2,6-
dimethylphenyl)pyrene TP2 to engineer O−H···O hydrogen-
bonded aggregation of molecules in view of the noted
propensity of the phenolic hydroxyl groups to exhibit varied
set of synthons,13 which were surmised to manifest in different
constructing functional supramolecular structures.2 As the
properties of macroscopic bulk solids depend on organization
of the constituent molecular modules, the functional properties
can in principle be fine-tuned via subtle changes at the
molecular level.2 The knowledge of supramolecular synthons3
has advanced significantly in the past few years to facilitate
targeted self-assembly. The molecules can now be programmed
to self-assemble in a specific fashion by incorporating certain
functional groups that manifest in the form of supramolecular
synthons−the aggregation motifs between functional groups
that are reproduced reliably in molecular crystals.3,4 The prime
goal of crystal engineering5 is to exploit the wealth of
intermolecular interactions to control organization of molecules
in pursuit of predetermined functions. One aspect of research
that continues to be actively pursued is the development of
porous organic solids based on hydrogen bonds,6 which are
being increasingly uncovered for a variety of applications such
as gas adsorption in a manner akin to metal−organic open
frameworks,7 separation,8 stereoselective reactions of the
included guest molecules,9 etc. Further, lattice inclusion of
guests permits access to binary or higher order multicomponent
molecular crystals,10 which are of immense importance in
pharmaceutical industry.11
Development of organic solids with voids for inclusion of
guest species is challenging and entails a diligent design at the
molecular level.6,12 We showed recently that 1,3,6,8-tetrakis(4-
Received: May 18, 2012
Published: August 23, 2012
© 2012 American Chemical Society
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Chart 1
Figure 1. Schematic representation of the two different modes by which the hosts TP2 and BX pack with inclusion of the guests.
̈
modes of the assembly of TP2. A simple and naive analysis of
Table 1. Inclusion Compounds of TP2 and BX, Host:Guest
Ratios, Their Codes, Space Groups, and Guest-Accessible
Volumes (V)
the assembly of host TP2 based on the most prevalent “chain”
synthon suggests two ways, i.e., types 1 and 2, cf. Figure 1, by
which the molecules might be glued together. We herein
demonstrate that these two modes of organizations can be
mimicked with different guests. Further, the topological
features of the host system allow simultaneous inclusion of
two distinct guests. It is also shown that the half-component BX
deduced by slicing the host into two halves, i.e., bis(4-
hydroxyphenyl)-m-xylene, likewise undergoes self-assembly in a
manner akin to TP2 (Figure 1), but that the lack of aromatic
expanse alleviates its inclusion behavior.
host/
guest
ratio
space
V
host
inclusion compound
code
group (%)
TP2 TP2·benzene
1:4
TP2-B
P21
57
42
59
34
63
TP2 TP2·nitrobenzene
TP2 TP2·nitrobenzene·benzene
TP2 TP2·benzonitrile
TP2 TP2·o-dichloro
benzene·pyridine
1:2
TP2-NB
P21/c
1:1:2
1:2
TP2-NB-B P21
TP2-BN
TP2-C-P
P21/c
1:4:2
P-1
BX
BX
BX
BX
BX
BX·benzene
1:1
1:1
1:1
1:1
1:1
BX-B
P21/c
P21/c
P21/c
P-1
29
34
31
33
35
BX·nitrobenzene
BX·benzonitrile
BX·o-dichlorobenzene
BX·pyridine
BX-NB
BX-BN
BX-C
BX-P
RESULTS AND DISCUSSION
■
Synthesis and Inclusion Behavior of the Hosts TP2
and BX. The syntheses of hosts TP2 and BX were
accomplished via BBr3-mediated demethylation of the known
precursor methoxy derivatives,10c Scheme 1. The inclusion
behavior of both TP2 and BX was examined by dissolving the
compounds in methanol and ethyl acetate, repectively, in the
presence of added guest. The crystals that grew with time were
analyzed by 1H NMR as well X-ray crystal structure
determinations. In Table 1 are consolidated the results of
guest inclusion for both TP2 and BX with benzene,
benzonitrile, nitrobenzene, o-dichlorobenzene, and pyridine.
P21/c
As shown in Table 1, crystallization with benzene and
nitrobenzene led to ternary crystals with well-defined
stoichiometry. The crystallization with pyridine in the presence
of various guests was attempted, but the crystals were obtained
only with o-dichlorobenzene. In a similar vein, crystallization of
BX led to binary inclusion compounds with each of the guests,
but none of the attempts led to ternary crystals suggesting
thereby no evidence for their formation. The host-guest ratios
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Figure 2. Crystal packing of TP2-NB-B, which depicts alternate orientations of the adjacent host molecules. Notice the relative dispositions of
benzene (purple) and nitrobenzene (blue) channels (a). A section has been magnified to show the zigzag O−H···O hydrogen bonds down the a-axis,
which connect the 2-D layers formed along the bc-plane down the a-axis (b). The various domains of the host molecule occupied by the guest
benzene (purple) and nitrobenzene (green) are shown (c). A section of the 2-D layers propagating along the a-axis is shown with the hydrogen
atoms omitted for clarity (d).
Scheme 1
1
for all of the inclusion compounds were established by H
suffices to illustrate the inclusion phenomenon of TP2 in TP2-
B, TP2-BN, and TP2-NB. The crystals of TP2-NB-B were
found to belong to the P21 space group with the asymmetric
unit cell containing one molecule each of the host and
nitrobenzene and two molecules of benzene. Analysis of the
crystal structure shows that the host molecules are stitched up
by O−H···O hydrogen bonds leading to molecular self-
assembly in all the three dimensions, cf. Figure 2. One observes
that the host molecules in the bc-plane are organized in a
particular readily recognizable manner. As shown in Figure 2,
the trough region formed between the two dimethylphenol
rings of one TP2 molecule is closed up by two dimethylphenol
rings of a symmetry-related TP2 molecule that afford concave
enclosure. In other words, two host TP2 molecules that are
orthogonally oriented in the bc-plane are linked up by O−
NMR as well as thermogravimetric analyses; the TGA profiles
1
and H NMR spectra are included as part of the Supporting
Information. In the following sections are discussed the
inclusion modes of various guests in TP2 and BX.
The Guest Inclusion by 1,3,6,8-Tetrakis(4-hydroxy-
2,6-dimethylphenyl)pyrene, TP2. The crystal structure of
TP2·nitrobenzene·benzene, i.e., TP2-NB-B, is representative of
the guest inclusion phenomenon exhibited by TP2 with
benzene (TP2-B), benzonitrile (TP2-BN), and nitrobenzene
(TP2-NB), while the crystals of TP2-B, TP2-BN, and TP2-
NB-B are isostructural and contain guest molecules in all three
domains, i.e., trough, concave, and basin. Those of TP2-NB
differ marginally in the sense that the basin regions are devoid
of guests, vide infra. Thus, the crystal structure of TP2-NB-B
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Figure 3. Crystal packing of TP2-C-P depicting the large empty spaces, which extend as channels down the a-axis. These channels are filled by o-
dichlorobenzene guest (purple). Each host molecule accommodates o-dichlorobenzene guests in trough and concave regions and pyridine (green) in
the basin region. Pyridine molecules (green) act as terminators of hydrogen-bonding resulting in the formation of 1-D linear strands that propagate
along the c-axis (a). In each host molecule, o-dichlorobenzenes occupy the trough and concave regions, while the pyridine occupies the basin region
(b).
Figure 4. Crystal packing diagram of BX-B, which reveals remarkable similarity with that of TP2-NB-B in Figure 2 (a). Hydrogen bond mediated
propagation (b) and relative disposition of the “paired up” host molecules along with guest benzenes (purple) (c). 2-D undulating layers formed in
the bc-plane are shown separately (d).
H···O hydrogen bonds such that the union of concave and
trough regions of the neighboring host molecules gives rise to
pentagonal voids that run down the a-axis. Within the
pentagonal voids are found guest benzene and nitrobenzene
molecules arranged in the bc-plane in a checkerboard fashion
(type 1, Figure 1). Additional benzene guests are found to be
sandwiched between basin regions of the TP2 host molecules
stacked down the a-axis. As mentioned previously, the
molecular packing of TP2-NB is similar to TP2-B/TP2-NB-
B/TP2-BN in the bc-plane but is devoid of guest molecules in
the basin regions of the host TP2 down a-axis, cf. Supporting
Information; otherwise, the host is found to include two guest
molecules in the concave and trough regions. Presumably, size
of the guest is sufficiently large as to be sandwiched between
the basin regions of the host TP2 to give rise to the crystal
packing that is similar to others discussed above. The crystal
lattice, in the absence of guests in the basin region, is seemingly
stabilized by sliding of layers such that the aryl rings of the host
molecules in the neighboring layer occupy the basin regions.
Crystallization of TP2 in the presence of pyridine was carried
out in an attempt to explore how the latter may intercept O−
H···O hydrogen-bonded self-assembly of the host tetraphenols;
it should be noted that the phenols interact strongly with
azaaromatics, and this has been exploited tremendously to
engineer stereoselective [2 + 2] cycloaddition reactions in the
solid state.5 From several experiments, crystals suitable for X-
ray studies were obtained when TP2 was crystallized with
pyridine in the presence of o-dichlorobenzene. The crystals
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Figure 5. Crystal packing of BX-P showing the hydrogen-bonding pattern that leads to closely stacked 1-D strands. The void spaces extending as
channels down the c-axis are filled with pyridine molecules (green) (a). Notice the relative orientations of host molecules belonging to the two
adjacent layers (black and orange). The pyridine guests terminate the O−H···O hydrogen bonding, thereby limiting the propagation of the host BX
into 1-D strands (b).
were found to belong to the space group P1 with one host, four
region. Indeed, all of its inclusion compounds with benzene,
nitrobenzene, and benzonitrile crystallize in P21/c as that of
TP2-NB. Further, the crystal packing of BX-C is similar to
those of BX-B, BX-BN, and BX-NB except that it crystallizes in
̅
o-dichlorobenzene and two pyridine molecules in the
asymmetric unit cell. The crystal structure analysis shows that
o-dichlorobenzene molecules occupy trough and concave
regions, while pyridine guests are located in basin region of
the host TP2 (Figure 3). In this instance, the host molecules
are O−H···O hydrogen bonded in a manner that the concave
regions of the adjacent host TP2 molecules are joined to give
rise to 1-dimensional linear strands. The pyridine guests are
hooked up to these strands as pendants, thereby restricting the
formation of an interconnected 3-D network structure. In other
words, pyridine guests dissect the 3-D network into 1-D linear
strands (Figure 3) that enclose large void spaces for inclusion of
o-dichlorobenzene guests (Type 2, Figure 1). Indeed, the voids
containing o-dichlorobenzene guests extend as channels down
a-axis. The pyridine molecules that lie as pendants of 1-
dimensional strands protrude into the basin regions of the hosts
in the adjacent strand leading to a compact lattice.
Guest Inclusion Behavior of 2,4-Bis(4-hydroxy-2,6-
dimethylphenyl)-1,5-dimethylbenzene (Bis-
(hydroxyaryl)-m-xylene, BX). As in the case of TP2
discussed above, the half component BX also exhibits guest
inclusion with various guests. Indeed, the inclusion compounds
of BX with benzene (BX-B), benzonitrile (BX-BN), nitro-
benzene (BX-NB) and o-dichlorobenzene (BX-C) exhibit
packing equivalence despite the fact that the crystals of BX-C
the P1 space group, cf. the Supporting Information. The crystal
̅
packing analysis shows that the trough and the seemingly
concave regions of neighboring molecules are joined together
by O−H···O hydrogen bonds to yield pentagonal voids that are
filled by the guest molecules (Type 3, Figure 1). Due to the
slight offset of such “paired-up” host molecules, the 2-D layers
undulate in the bc-plane.
The host molecule in BX-P was found to readily yield chunky
cubic crystals in ethyl acetate with pyridine and o-
dichlorobenzene as additives. The inclusion compound so
obtained was found to contain only pyridine as the guest. The
crystals were found to belong to the space group P21/c with
one molecule each of the host BX and the pyridine guest in the
asymmetric unit. The crystal packing analysis reveals that the
host molecules are interconnected by O−H···O hydrogen
bonds with voids that lead to channels down the c-axis (Figure
5a). The pyridine molecules seemingly compel the host
molecules to be confined to 1-D linear strands by terminating
the propagation of O−H···O hydrogen bonds via O−H···N
hydrogen bonding (dO−H···N = 1.81 Å, DO···N = 2.64 Å, θO−H···N
= 166.9°). Because of the angular geometry of the host BX, the
adjacent strands fit tightly into the neighboring strands, leaving
virtually no space for further guest inclusion (type 4, Figure 1).
In other words, the host BX undergoes self-inclusion in the
trough region of the adjacent host molecule such that the 2-D
layers in the ab-plane are compactly packed (Figure 5b).
Overall Rationalizations. In comparison to the aggrega-
tion of carboxylic acids, phenolic compounds are known to
exhibit considerable flexibility in terms of the synthon
adoption.12 The synthons that are exhibited by hydroxyl
groups include (a) dimers, (b) chains, and (c) rings (Figure
6).12 Therefore, the host systems based on phenols may be
expected to display guest-dependent adaptability. Indeed, there
are diverse host systems known that are based on the self-
assembly of hydroxyl groups.12 By assuming the prevalence of
“chain” synthon in the crystals of the hosts TP2 and BX, which
is typically observed in rigid 2-/3-dimensional molecular
modules, one may envisage two types of assembly for the
tetraphenol TP2 and bis-phenol BX as shown schematically at
correspond to P1, while those of all others correspond to P2 /c.
̅
1
The crystal structure of BX-B described below serves to
illustrate the packing features of all. In contrast, the inclusion
compound of BX with pyridine, i.e., BX-P, corresponds to an
altogether different pattern as should be expected.
Figure 4 shows the crystal packing of BX-B down a-axis,
which is strikingly similar to the TP2-NB-B structure (Figure
2a). Like tetraphenol TP2, its half component bisphenol BX
also undergoes O−H···O hydrogen-bonded self-assembly as
though two of them are joined together to yield a 4-connecting
TP2-like module that self-assembles. As the half-component
BX modules contain methyl groups at the 1,3 positions, they
cannot remain in plane and are displaced vertically. This
expectedly should destroy the aromatic expanse that is inherent
to TP2 to preclude guest inclusion in the basin region. Thus,
the self-assembly should correspond to the case of TP2-
nitrobenzene (TP2-NB) that is devoid of guests in the basin
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analyses of the solutions of their crystals. TGA analyses of the
crystals further corroborate the host-guest ratios, cf. the
Supporting Information. More incisive analysis of the
thermogravimetric analysis reveals incremental release of the
guests in the case of TP2-NB-B, cf. the Supporting
Information; this is particularly appealing from the point of
view of correlating the binding of the guests with their
exclusion from the crystals. In this instance, benzene guests are
found to be located in basin and trough regions, while
nitrobenzene can be said to be found in the concave domain.
Accordingly, one observes three steps for release of the guests
in the TGA, cf. the Supporting Information. Insofar as the
binding of the guest species in all of the inclusion compounds is
concerned, the crystal packing analyses show that the guests
occupying the voids are held by weak C−H···O and C−H···π
hydrogen bonds. It is the size and shape complementarity that
appears to drive the inclusion of guests in the solid state of the
host systems that seemingly behave like tweezers with some
conformational flexibility for the aryl arms. The crystal structure
analyses show that there exists a small window within which the
aryl rings lie near orthogonally; the angle between the aryl rings
and the planar pyrene core is found to vary in the range of 73−
90°.
The results clearly demonstrate how the molecular design
with more than one location for guest binding in conjunction
with the knowledge of supramolecular synthons may be
exploited to develop cocrystals with packing types that may
be a priori expected. Quite remarkable is how structural
reduction based on the results of tetraaryl phenol TP2 has paved
the way for the inclusion phenomenon to be demonstrated for
diarylbenzene BX that incorporates all of the attributes of TP2,
except the aromatic expanse. It is compellingly evident that the
aromatic expanse in TP2 plays a crucial role for the observed
guest inclusion behavior (Table 1).
Figure 6. Typical synthons observed for the self-assembly of OH
groups: (a) dimer, (b) ring, and (c) chain.13
the outset in Figure 1. The first type in which the union of
concave and trough regions of the host leads to pentagonal
voids is observed in all four inclusion compounds of TP2, i.e.,
TP2-B, TP2-NB-B, TP2-NB, and TP2-BN. Based on packing
equivalence exhibited by TP2-B and TP2-NB, we surmised that
both benzene and nitrobenzene could be interchangeably
incorporated simultaneously in the crystal lattice. The fact that
one does indeed obtain ternary inclusion compound with
benzene and nitrobenzene is reflected by the crystal structure of
TP2-NB-B. What is noticeable in these cases is that benzene
gets accommodated in the basin region as well, while
nitrobenzene and a second molecule of benzene are located
in the pentagonal voids formed between the enclosure between
concave and trough domains. Our attempts with pyridine that
may terminate the hydrogen-bonded aggregation led to crystals
containing o-dichlorobenzene. As the crystal packing in Figure
3 suggests, it corresponds to the second type 2 in Figure 1. As
can be seen in Figure 3, TP2 host molecules form 1-
dimensional strands via linkage of the concave regions, which
close up to yield further voids in the trough regions within
which o-dichlorobenzene guests get accommodated. Clearly,
the mimicry of two packing types 1 and 2 that are conceived a
priori for TP2 is demonstrated via variation of the guests.
Based on the molecular packings observed in the crystals for
TP2-B, TP2-NB, TP2-BN, and TP2-NB-B, we wondered as to
how the half-component deduced by slicing TP2 host into two
halves may undergo hydrogen-bonded self-assembly. Of course,
the importance of aromatic expanse inherent to pyrene core in
TP2 host molecules was thus expected to be revealed in the
lattice inclusion of various guests. As in the case of TP2, the
methyl groups at 2,6 positions of the phenolic moieties in the
half-component BX ensure orthogonality. Thus, one may
expect only the trough domain to become accessible for guest
inclusion such that the region for guest inclusion is substantially
attenuated. In line with these expectations, the host BX was
indeed found to exhibit guest inclusion. A remarkable
equivalence is observed in the crystal packings of the crystals
BX-B with those of TP2-B/TP2-BN-B. The loss of basin region
is reflected in the reduction of volume accessible for guests,
which is 29% for BX-B as compared to 57% for TP2-B. Indeed,
BX is likewise found to include other guests such as
nitrobenzene, benzonitrile, and o-dichlorobenzene, which are
found to be included in the trough region in the inclusion
crystals of TP2. The packing in all of these cases (type 3, Figure
1) is synonymous with the type 1 observed for inclusion
compounds of TP2. The pyridine guests serve to truncating the
otherwise hydrogen-bonded propagation. It turns out that in
the crystals of BX-P, the pyridine guests interdigitate into the
adjacent 1-dimensional strands that are formed by the
aggregation of phenolic moieties (type 4, Figure 1).
CONCLUSIONS
■
Tetraarylpyrene host TP2 decked up with rigid 2,6-dimethyl-4-
hydroxyphenyl rings as panes at the four corners is shown to
undergo O−H···O hydrogen-bonded self-assembly adopting
the “chain” synthon into two expected patterns. The union of
the concave and trough regions of the neighboring host permits
pentagonal voids within which aromatic guests are readily
incorporated. That the assembly is robust is reflected from the
isostructurality exhibited by the host TP2 with different guests.
It is shown that benzene and nitrobenzene can be
simultaneously included in the lattice of the host TP2. Guest-
induced switchover allows the realization of alternative packing
mode; in the presence of pyridine, which truncates the
hydrogen bond-mediated propagation, the host TP2 crystallizes
by including o-dichlorobenzene. That the half-component
BXobtained by slicing host TP2should expectedly exhibit
guest inclusion likewise is demonstrated by X-ray structural
characterization of its inclusion compounds with analogous
guests. The structural analyses of the inclusion compounds of
TP2 and BX reveal remarkable similarities in terms of the
crystal packings that are mediated in both cases by O−H···O
hydrogen bonds. As in the case of TP2, the guest-induced
packing permits mimicry of the two possible modes of crystal
packings based on the chain synthon of the phenolic hydroxyl
groups of BX, Figure 1.
The host-guest stoichiometries as determined from the
inclusion compounds were also established by 1H NMR
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compounds; crystals of the latter were not easily obtained when
other solvents were employed, which suggests that the solvent plays an
important role in the formation of the inclusion compounds of BX.
In a typical experiment, the host TP2 (50.0 mg, 0.07 mmol) was
dissolved in 20 mL of ethyl acetate/methanol, and a few drops of
guests was introduced without causing precipitation. The resultant
solution was allowed to evaporate over a period of 7−10 days to yield
colorless crystals quantitatively. The crystals were characterized by 1H
NMR, TGA, and X-ray crystallography. A similar procedure was used
for the synthesis of inclusion compounds of BX.
EXPERIMENTAL SECTION
■
The starting materials, viz., 1,5-dibromo-2,4-dimethylbenzene and 2,6-
dimethyl-4-methoxyphenylboronic acid, were synthesized according to
the previously reported procedures.10c
Synthesis of Tetrakis(hydroxyaryl)pyrene (TP2) and Bis-
(hydroxyaryl)-m-xylene (BX). The preparation of the precursor
1,3,6,8-tetrakis(2,6-dimethyl-4-methoxyphenyl)pyrene has previously
been reported.10c The precursor of BX, namely 2,4-bis(4-methoxy-2,6-
dimethylphenyl)-1,5-dimethylbenzene, was prepared starting from 1,5-
dibromo-2,4-dimethylbenzene; the latter was subjected to Suzuki
coupling with 2,6-dimethyl-4-methoxyphenylboronic acid under Pd(0)
catalysis. Demethylation of these precursor compounds using BBr3/
CH2Cl2 led to TP2 and BX.
ASSOCIATED CONTENT
* Supporting Information
■
S
Preparation of 2,4-Bis(4-methoxy-2,6-dimethylphenyl)-1,5-
dimethylbenzene. In a two-necked round-bottom flask were taken
1,5-dibromo-2,4-dimethylbenzene (0.1 g, 0.38 mmol), 2,6-dimethyl-4-
methoxyphenylboronic acid (0.24 g, 1.3 mmol), Pd(PPh3)4 (0.024 g,
0.021 mmol), and NaOH (0.06 g, 1.50 mmol). To the resultant
mixture was added 15 mL of 1,4-dioxane−water mixture (2:1), and the
contents were heated at reflux for 12 h. After completion of the
reaction as monitored by thin-layer chromatography, the reaction was
quenched. The solvent, i.e., dioxane, was removed in vacuo, and the
solid residue was extracted with CHCl3. The combined extract was
washed with brine and dried over anhyd Na2SO4 and solvent removed
in vacuo to obtain the crude product. Silica gel column
chromatography (25% CHCl3/petroleum ether) yielded the required
product as a colorless solid: yield 91% (0.13 g, 0.35 mmol); mp 133−
NMR spectral reproductions, crystal data, X -ray crystallo-
graphic information files (CIF), and TGA profiles. This
information is available free of charge via the Internet at
http://pubs.acs.org/.
AUTHOR INFORMATION
Corresponding Author
*E-mail: moorthy@iitk.ac.in.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
A.B. is grateful to CSIR for a senior research fellowship. J.N.M.
is thankful to the Department of Science and Technology
(DST), India, for generous financial support.
137 °C; H NMR (500 MHz, acetone-d6) δ 1.96 (s, 12H), 1.97 (s,
6H), 3.78 (s, 6H), 6.57 (s, 1H), 6.70 (s, 4H), 7.25 (s, 1H); 13C NMR
(125 MHz, CDCl3) δ 20.8, 21.7, 55.6, 95.5, 112.4, 121.1, 130.4, 133.6,
138.2, 156.7, 158.1; HRMS m/z calcd for C26H31O2 375.2324, found
375.2328 [M+H]+.
REFERENCES
■
General Procedure for Demethylation of Aryl Methyl Ether.
To a solution of aryl methyl ether (2 mmol) in 20 mL of dry CH2Cl2
at 0 °C was added dropwise BBr3 solution (3 mmol) under N2
atmosphere. The reaction mixture was allowed to stir overnight.
Subsequently, it was quenched with 10% HCl and extracted with ethyl
acetate. The combined extract was dried over anhyd Na2SO4, treated
with charcoal, filtered, and concentrated. The pure product was
obtained as a colorless solid after filtration over a short pad of silica gel
using a mixture of ethyl acetate and petroleum ether (50:50) followed
by recrystallization from ethyl acetate and petroleum ether.
1,3,6,8-Tetrakis(2,6-dimethyl-4-hydroxyphenyl)pyrene
(TP2): colorless solid; yield 96% (0.97 g, 1.42 mmol); mp >300 °C; IR
(KBr) cm−1 3320, 3050, 2925, 1591, 1476, 1429, 1298, 1190; 1H
NMR (500 MHz, DMSO-d6) δ 1.77 (s, 24H), 6.60 (s, 8H), 7.40 (s,
2H), 7.44 (s, 4H); 13C NMR (125 MHz, DMSO-d6) δ 21.1, 114.8,
124.9, 125.8, 128.6, 128.8, 130.6, 136.6, 137.7, 157.0; HRMS m/z
calcd for C48H43O4 683.3161, found 683.3163 [M + H]+.
2,4-Bis(4-hydroxy-2,6-dimethylphenyl)-1,5-dimethylben-
zene (BX): colorless solid; yield 96% (0.11 g, 0.32 mmol); mp 163−
167 °C; IR (KBr) cm−1 3270, 3024, 2922, 1606, 1496; 1H NMR (500
MHz, CD3OD) δ 1.86 (s, 12H), 1.92 (s, 6H), 6.49 (s, 1H), 6.52 (s,
4H), 7.17 (s, 1H); 13C NMR (125 MHz, CD3OD) δ 19.3, 20.7, 114.9,
131.9, 132.6, 133.8, 135.9, 138.0, 139.6, 156.9; HRMS m/z calcd for
C24H27O2 347.2011, found 347.2013 [M + H]+.
Preparation of Inclusion Compounds. The inclusion com-
pounds were prepared by slow evaporation of the solution of host
TP2/BX in a mixed solvent system comprising of one in which the
host dissolves and the second that functions as a guest. Both TP2 and
BX were found to be readily soluble in polar solvents such as
methanol, ethyl acetate, acetone, ethanol, and 2-propanol. From the
screening of a range of solvents, methanol and ethyl acetate were
reckoned to be the most suitable solvents for obtaining good-quality
single crystals of the inclusion compounds of TP2 and BX,
respectively. Indeed, change of solvent from methanol to ethyl acetate
in the case of TP2 was found not to influence the formation of the
inclusion compound with the added guest/second solvent. However,
the host BX preferably underwent crystallization in ethyl acetate in the
presence of added guest to yield the corresponding inclusion
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