Trigonal rigid triphenols: Self-assembly and multicomponent lattice inclusion

Article abstract of DOI:10.1021/cg200074z

The sterics introduced via methyl groups impart rigidity and inclusion behavior to trigonal C₃-symmetric triphenol hosts H1-H4. Triphenol H1 is found to mimic the O-H...O hydrogen-bonded self-assembly of trimesic acid to yield porous honeycomb nets. It is found that 18-crown-6?H1 in turn binds guest molecules in the hexagonal voids to yield guest?guest?host multicomponent molecular crystals. The triphenol H2 and the homologous derivatives H3 and H4 are also found to crystallize with 18-crown-6 and other guests to yield multicomponent crystals, but in these cases the 18-crown-6 is found to serve as a spacer. While the structure of H2 is determined in its guest-free form, some of the inclusion compounds of triphenols lend themselves to crystal packings that are deciphered based on network topologies. The networks observed for H1-Tol and H3-C-B-Et are unique; in the latter, the crystal packing analysis reveals organization of molecules into a pattern that is reminiscent of borromean rings.

Full text of DOI:10.1021/cg200074z

ARTICLE
pubs.acs.org/crystal
Trigonal Rigid Triphenols: Self-Assembly and Multicomponent Lattice
Inclusion
Published as part of a virtual special issue on Structural Chemistry in India: Emerging Themes
Jarugu Narasimha Moorthy,*,† Palani Natarajan,† Alankriti Bajpai,† and Paloth Venugopalan*,§
†Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
§Department of Chemistry, Panjab University, Chandigarh 160014, India
S Supporting Information
b
ABSTRACT:
The sterics introduced via methyl groups impart rigidity and inclusion behavior to trigonal C3-symmetric triphenol hosts H1ÀH4.
Triphenol H1 is found to mimic the OÀH O hydrogen-bonded self-assembly of trimesic acid to yield porous honeycomb nets. It
3 3 3
is found that 18-crown-6⊂H1 in turn binds guest molecules in the hexagonal voids to yield guest⊂guest⊂host multicomponent
molecular crystals. The triphenol H2 and the homologous derivatives H3 and H4 are also found to crystallize with 18-crown-6 and
other guests to yield multicomponent crystals, but in these cases the 18-crown-6 is found to serve as a spacer. While the structure of
H2 is determined in its guest-free form, some of the inclusion compounds of triphenols lend themselves to crystal packings that are
deciphered based on network topologies. The networks observed for H1ÀTol and H3ÀCÀBÀEt are unique; in the latter, the
crystal packing analysis reveals organization of molecules into a pattern that is reminiscent of borromean rings.
’ INTRODUCTION
self-assembly into porous networks for guest inclusion.7 It was
shown that both H1 and H2 do indeed exhibit strong propensity
to self-assemble into porous networks in which the guest
molecules could be included to construct multicomponent
molecular crystals. A feature that is inherent to these host systems
is that the sterics enforced via methyl groups ensure that the
hydroxyphenyl/aryl rings lie orthogonally to the central aromatic
core. A result of this is higher thickness of the interior cavities
thus generated via self-assembly; we have shown that host H1
undergoes hydrogen-bonded self-assembly into honeycomb net-
works with hexagonal voids that include 18-crown-6 and other
neutral as well as ionic guest molecules to afford guest⊂-
guest⊂host multicomponent crystals in a manner akin to Russian
dolls.8 Further, in these systems, the self-assembly via phenolic
hydroxy groups over carboxyl functionalities was deemed ad-
vantageous from the point of view of flexibility in the adoption of
supramolecular synthons.9 The phenolic hydroxy groups hydro-
gen bond in a variety of modes/synthons10 as compared to the
The ability to organize molecules in a desirable manner is
pertinent to the development of materials with predefined
properties.1 Although crystal structures, which represent an
extreme of periodic organization of molecules in three dimen-
sion, cannot yet be predicted a priori, it is increasingly becoming
possible to control the molecular order to some degree based on
directed hydrogen bonding and metal-assisted self-assembly.2
One aspect of self-assembly that has been advantageously
exploited is in the construction of porous materials.1,3 Indeed,
development of porous metalÀorganic materials/frameworks
(MOMs/MOFs) for the binding of gases is an intensely pursued
topic of research at present.4 Insofar as hydrogen-bonded self-
assembly is concerned, a prototype example that yields a
predictable honeycomb porous network constitutes the case of
trimesic acid.5 It has been found to exhibit dimorphism with
hexagonal channels of diameter ca. 14 Å, which are interpene-
trated. The interpenetration has been shown to be destroyed by
crystallization of trimesic acid in the presence of guest molecules
such as coronene, [18]annulene, hexahelicene, and pyrene.6
Recently, we envisaged that the C3-symmetric triphenolsH1and
H2 (Chart 1) should be unique in undergoing hydrogen-bonded
Received: January 18, 2011
Revised:
May 12, 2011
Published: June 07, 2011
r
2011 American Chemical Society
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Chart 1. Molecular Structures of Hosts H1 and H2, and their Phenyl-Extended Homologues H3 and H4, respectively
carboxylic acids, which predominantly exhibit dimeric and cate-
meric motifs.11 The flexibility in synthon adoption, although
detrimental to predictable organization of molecules, may confer
the hosts with the liberty to explore packing modes that are
suitable to a given guest system. In other words, the synthon
flexibility may render the hosts to be responsive systems to include
any guest molecule type. In the present investigation, we have
expanded our initial findings with hosts H1 and H2 and designed
homologous systems H3 and H4 for guest inclusion. Given that
the phenyl rings are almost coplanar in the crystal structures
of simple biaryls,12 it was surmised that the self-assembly of
homologous hosts H3 and H4 may lead to networks with
considerably larger cavities.13 We have found that the solvent
of crystallization is crucial to obtain the inclusion compounds,
which alludes to the importance of solvent on synthon adoption.
Herein, we report the formation of novel multicomponent
inclusion compounds with hosts H1, H3, and H4. The crystal
structure of host H2 in its guest-free form is also described.
of H1 and H2 in the absence and presence of guests, crystal-
lization in several combinations of solvents containing either
DMSO or EtOAc as a partner was explored. We were successful
in obtaining the crystals of both H1 and H2 in DMSO-CHCl3-
toluene and EtOAc-CH2Cl2-pet. ether, respectively. The crystals
obtained in these solvents were found to be indeed different from
those obtained in neat solvent systems, cf. Table 1. The solvent/
guest-dependent molecular assembly of hosts H1 and H2
prompted us to explore further such a property in the presence
of 18-crown-6 guest, which was shown to be readily included
in the crystal lattices of both H1 and H2 in our previous
investigation.7 Thus, crystallization of H1 with 18-crown-6 and
DME-H2O led to multicomponent molecular crystals containing
the solvent and water. Indeed, the crystals were found to be
isostructural with those containing methanolÀwater and metha-
nol-nitromethane,7 cf. Table 1. Likewise, multicomponent crys-
tals were also obtained when ammonium acetate was employed
in conjunction with 18-crown-6. The host H2 was found to
crystallize in its guest-free form, while our attempts to obtain the
crystals of H2 in our previous investigations were unsuccessful.
This shows that slight variation in solvent/s of crystallization
influences the hostÀguest phenomenon. The crystallization
of hosts H3 and H4 was explored in various solvents and in
several combinations of solvents with and without macrocyclic
18-crown-6 guest. The host H3 yielded crystals in an ethanolÀ
acetonitrileÀbenzene mixture containing 18-crown-6, while
H4 afforded crystals in tolueneÀEtOAc with and without
inclusion of 18-crown-6. All the hostÀguest inclusion com-
’ RESULTS AND DISCUSSION
Synthesis of Triphenols H1ÀH4. The synthesis and char-
acterization of triphenol hosts H1 and H2 have already been
published by us elsewhere.7 The hosts H3 and H4 were readily
synthesized starting from H1 and H2; the latter were converted
to their triflates and subjected to Suzuki coupling with 4-meth-
oxyphenyl boronic acid under Pd(0)-catalyzed conditions. The
resultant methoxy derivatives were demethylated using BBr3 to
afford extended triphenol hosts H3 and H4 (Scheme 1).
1
pounds were characterized by X-ray studies as well as by H
NMR analysis of the crystals. The details of inclusion com-
pounds, space groups, and abbreviations adopted for the inclu-
sion compounds are given in Table 1, and the crystal data are
collected in Table 2. The metrics for important hydrogen bonds
that are responsible for the self-assembly are collated in Table 3.
In the following are described the modes of self-assembly and
Inclusion Behavior of the Triphenols H1ÀH4. In our
numerous crystallization experiments with hosts H1 and H2,
we found that they invariably form inclusion crystals in neat
DMSO and EtOAc solvents, respectively.7 In the absence of
these solvents, both H1 and H2 failed to crystallize. To examine
the extent to which the choice of solvent influences crystallization
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Scheme 1. Synthetic Routes for the Inclusion Hosts H1ÀH4
the ways in which the guest species are bound in the crystal
lattices.
for the H-bonded molecular assembly is 44 611. To the best of
our knowledge, the network of this vertex symbol is heretofore
unknown.
3
H1-Tol. Good quality crystals of triphenol host H1 were
obtained from a DMSOÀCHCl3Àtoluene mixture by slow
evaporation. The crystals were found to be monoclinic (C2/c)
with one molecule of H1 and two molecules of toluene in the
asymmetric unit cell; the hostÀguest stoichiometry was also
confirmed by TGA analysis, cf. Supporting Information. The
dimethylhydroxyphenyl rings of H1 in its structure are found to
be roughly orthogonal to the plane of the central benzene ring
with the angles between the benzene plane and the orthogonal
aryl rings being ca. 87À88ꢀ. The crystal packing reveals that the
H1-C-DME. As mentioned already, crystallization of triphenols
H1 and H2 in the presence of 18-crown-6 yielded multicompo-
nent molecular crystals in which the crown is filled with solvents,
that is, CH3OH/CH3NO2/H2O.7 Host H1 readily yielded the
crystals with dimethoxyethane in the presence of 18-crown-6.
The X-ray crystal structure determination revealed that the
crystals belong to monoclinic lattice system (space group: C2/c)
with one molecule each of H1, 18-crown-6 and half molecule of
dimethoxyethane and 1.5 water molecules in the asymmetric
unit. As shown in Figure 2, the hydrogen-bonded self-assembly of
triphenol H1 leads to infinite 2D chicken-wire networks with
hexagonal voids along the ab-plane down the c-axis. The guest
18-crown-6 molecules nicely fit into the hexagonal enclosures
resulting from the assembly of three H1 building blocks. The
vacant space in the crown is found to be filled by two disordered
water molecules. The chicken-wire layers in the ab-plane are margi-
nally shifted relative to each other with the solvent dimethoxyethane
phenolic OH groups are bonded via OÀH O hydrogen bonds
3 3 3
in a zigzag manner down the b-axis to lead to a three-dimensional
(3D) assembly (Figure 1). One observes channels down the
b-axis in which the guest toluene molecules are found to be
located. The guests seemingly enjoy translational motion to
account for the observed disorder.
The topology of the hydrogen-bonded assembly was examined
by TOPOS program.14 The Schl€afli notation that is determined
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Crystal Growth & Design
ARTICLE
Table 1. Details of Crystallization of Hosts H1ÀH4 with and without Guests and their Codes
molecules occupying the interlayer voids (Figure 2). All the guest
molecules, that is, 18-crown-6, water and dimethoxyethane, are
found to be severely disordered. While each of the H1 molecules
unit cell was found to contain only half of the host H2 with the
molecule lying on the mirror plane. One of the hydroxyphenyl
rings is found to be disordered. The crystal packing analysis
shows that the molecules that are related by 4-fold rotational
self-assembles with its three hydroxyphenyl rings via OÀH
O
3 3 3
hydrogen bonds, a closer analysis shows that one of the water
symmetry are OÀH O hydrogen-bonded down the c-axis.
3 3 3
molecules that is bonded to the crown forms a strong OÀH
O
The typical crystal packing diagram is shown in Figure 4. The
tetrameric association between the molecules that are related by
C4-rotational symmetry is one of the common modes of the
assembly of hydroxy compounds.10 We note that the triphenol
H2 does not readily crystallize in its guest-free form unless
crystallized under the conditions described above. A perspective
view of the crystal packing down the c-axis shows that the
hydrogen-bonding motifs in the close packed layers reminisce
the “piedfort” motif.16
3 3 3
hydrogen bond with one of the hydroxy group of H1 in the
adjacent layer approximately down the c-axis, cf. Figure 2b.
H1-C-Aac. As mentioned already, host H1 forms honeycomb
networks in all crystals containing 18-crown-6 as one of the
guests. Recently, Nangia and co-workers have demonstrated the
crucial role of cation in the self-assembly of cis,cis-1,3,5-cyclohex-
anetricarboxylic acid.15 With the latter, they have shown that one
obtains networks of β-arsenic topology with Na+⊂18-crown-6
+
and black phosphorus network with NH4 ⊂18-crown-6. We
H2-C-W. We noted earlier that the methyl groups of the central
mesitylene ring in H2 may fill the voids partially to account
for the absence of honeycomb organization for inclusion of
18-crown-6.7 The guest crown molecules were found to mediate
the assembly by functioning as spacers. It was shown that crystal-
lization of H2 in EtOAc containing ethanol and benzene leads to
multicomponent crystals in which the ethanol guests expand the
dimensions of hexagonal voids to allow inclusion of 18-crown-6
that also binds two water molecules.7 The crystallization in
dioxane and EtOAc mixture containing 18-crown-6 with exclu-
sion of ethanol led to rhombus-shaped crystals, whose structure
determination revealed the presence of 18-crown-6 and adven-
titious water in the lattice. The crystals were found to belong
to triclinic (space group: P1) with one molecule each of H2 and
18-crown-6 and two molecules of water in the asymmetric unit
cell. The typical crystal packing of H2-C-W is shown in Figure 5.
As can be seen, both H2 and 18-crown-6 are found to form
columnsthatalternatealongthec-axis. The crownisfoundto serve
as a guest as well as a spacer in the hydrogen-bonded assembly of
triphenols into distorted honeycomb networks (Figure 5).
H3-C-B-Et. The phenyl extended triphenol H3 is expected to
exhibit the self-assembly and guest inclusion behavior similar to
its simple analogue H1 with more porosity. The crystallization of
H3 was tried out with and without the addition of 18-crown-6.
wished to explore if the presence of a cation such as NH4+ can
make any difference. The crystallization of host H1 in the
presence of 18-crown-6 and CH3COONH4 yielded crystals,
whose structure determination revealed the presence of one host
molecule H1, one 18-crown-6, one CH3COONH4, and an
adventitious water molecule in the asymmetric unit cell, that is,
H1 18-crown-6 CH3COONH4 H2O. The crystal packing re-
3
3
3
veals the absence of anticipated honeycomb network. One
observes that the host triphenol molecules are interconnected
by water as well as the acetate counteranion to lead to a two-
dimensional (2D) layered structure along the ac plane as shown
in Figure 3. The layers in the ac plane are offset with respect to
each other to make up the crystal. The guest crown molecules are
found to be located in the voids thus formed. Of course, the
ammonium cation is found to be trapped by the crown via
NÀH O hydrogen bonds with the disordered oxygens, which
3 3 3
is also supported by cationÀdipole interactions.
H2. The crystals of host H2 in its guest-free form were not
realized from several crystallization experiments.7 As mentioned
earlier, the crystallization of H2 in a EtOAcÀCH2Cl2Àpet. ether
solvent mixture led to needle-shaped crystals. The crystal struc-
ture determination revealed that the crystals belong to the
tetragonal lattice system with I4cm space group. The asymmetric
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Table 2. The Crystal Data for Multicomponent Molecular Compounds of Hosts H1ÀH4 with Various Guests
H1-Tol
H1-C-DME
H1-C-Aac
H2
molecular formula
formula weight
crystal system
space group
C
81H84O6
C88H124O23
C44H63NO12
C27H24O3
396.46
1153.48
1549.87
797.95
monoclinic
monoclinic
monoclinic
tetragonal
C2/c (No. 15)
C2/c (No. 15)
P21/n (No. 14)
I4cm (No. 108)
23.059(2)
23.059(2)
7.478(3)
90.00
a (Å)
23.376(7)
23.562(5)
13.932(3)
b (Å)
13.341(3)
13.960(5)
11.104(2)
c (Å)
23.042(5)
28.168(7)
28.633(6)
R (deg)
90.00
90.00
90.00
β (deg)
105.77(7)
113.15(9)
100.75(6)
90.00
γ (deg)
90.00
90.00
90.00
90.00
volume (Å3)
6916.0(3)
8519.0(4)
4351.9(5)
3975.8(6)
8
Z
4
4
4
calculated density (mg/m3)
absorption coefficient (mmÀ1
F(000)
1.108
1.208
1.218
1.325
)
0.068
0.086
0.088
0.085
2472
3344
1720
1680
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
1.050
1.086
1.034
1.046
R1 = 0.0852, wR2 = 0.1936
R1 = 0.1621, wR2 = 0.2261
R1 = 0.1033, wR2 = 0.2736
R1 = 0.1695, wR2 = 0.3162
R1 = 0.0656, wR2 = 0.1221
R1 = 0.1278, wR2 = 0.1398
R1 = 0.0401, wR2 = 0.0996
R1 = 0.0447, wR2 = 0.1027
H2-C-W
H3-C-B-Et
H4-Tol
H4-C-Tol
molecular formula
formula weight
crystal system
space group
C
39H48O11
C138H164N1O36
C52H44O3
C85H91O12
692.77
2419.23
716.87
1304.58
triclinic
trigonal
tetragonal
orthorhombic
P1 (No. 2)
R3 (No. 148)
I4 (No. 82)
P212121 (No. 19)
a (Å)
10.446(5)
21.037(3)
34.693(5)
9.393(1)
b (Å)
13.758(7)
21.037(3)
34.693(5)
22.498(1)
c (Å)
16.201(8)
53.661(1)
7.198(3)
37.809(9)
R (deg)
108.82(8)
90.00
90.00
90.00
β (deg)
93.86(1)
90.00
90.00
90.00
γ (deg)
108.65(1)
120.00
90.00
90.00
volume (Å3)
2050.1(8)
20566.0(3)
8664.0(4)
7990.0(2)
Z
2
3
8
4
calculated density (mg/m3)
absorption coefficient (mmÀ1
F(000)
1.122
1.184
1.099
1.090
)
0.082
0.085
0.067
0.071
740
7784
3040
2812
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
1.092
1.134
1.062
1.061
R1 = 0.1315, wR2 = 0.3219
R1 = 0.1735, wR2 = 0.3736
R1 = 0.1180, wR2 = 0.3001
R1 = 0.1981, wR2 = 0.3498
R1 = 0.0934, wR2 = 0.2188
R1 = 0.1527, wR2 = 0.2513
R1 = 0.1046, wR2 = 0.2318
R1 = 0.1927, wR2 = 0.2699
Good quality crystals of host H3 were readily obtained when the
crystallization was performed in the presence of 18-crown-6
using an EtOHÀCH3CNÀC6H6 solvent mixture. The crystals
were found to belong to the trigonal lattice system with R3 space
group. The asymmetric unit cell was found to contain two host
H3 molecules sitting on the special position with one-third of
occupancy along with one 18-crown-6, half CH3CN, one-sixth of
benzene, and four water molecules. The crystal packing down the
b-axis reveals two independent host molecules in the asymmetric
unit cell, that is, A and B, which self-assemble into layers that are
connected by crowns as bridges (Figure 6). Each of the layers in
the ab-plane shows that the hydroxyphenyl rings of the triphenol
water molecules that are accommodated as guests in 18-crown-6.
The open network structure thus formed is found to be 4-fold
interpenetrated as shown in Figure 6. Thus, the voids still left
after interpenetration are found to be filled by benzene, acetoni-
trile, and ethanol as guest molecules. A further inspection of the
crystal packing shows that the oxygen atom of one hydroxyphe-
nyl ring of each triphenol molecule involves in a CÀH
O
3 3 3
= 3.42 Å, and
O
hydrogen-bond (dH
= 2.49 Å, DC
3 3 3
O
3 3 3
θCÀH
= 166.0ꢀ) to yield a cyclic array of hydroxyphenyl
O
rings i3n3 a3 chair geometry that is overall reminiscent of a rosette at
the center of which lies benzene guest, cf. Figure 6. A comparison
of the crystal packing of H3 with H1 is quite instructive: while H1
assembles into a 2D honeycomb network in the presence of
crowns, the expanded host H3 assembles into a hexagonal (6,3)
net that is bridged by crowns functioning as spacers. The (6,3)
net is 4-fold interpenetrated. The point to note here is that the
are found to be interconnected through OÀH O hydrogen
3 3 3
bonds with one water molecule, which is in turn OÀH
O
3 3 3
hydrogen-bonded to one of the oxygen atoms of 18-crown-6. In
other words, the host H3 self-assembles by the mediation of
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Crystal Growth & Design
ARTICLE
observed molecular assembly is exactly similar to that of the
H1-DMSO, which is 8-fold interpenetrated.7 These two struc-
tures illustrate the crucial dependence of the host size (in terms
of the lengths of the aryl groups) on the self-assembly to afford
voids that encapsulate crowns. It is also relevant to point out the
fact that the 4-fold penetration closely resembles 3-fold inter-
penetrated Borromean networks that are based on hydrogen-
bonded self-assembly reported recently.17
H4-Tol. Although the host H4 was found to include a variety
of guest molecules and yield crystals that were seemingly
good, their isolation from the mother liquor for structural
determinations proved very difficult; the crystals developed
cracks upon removal from the mother liquor to preclude
structure determinations successfully. Similar to the guest-free
self-assembly of H2, the phenyl-extended homologue H4 also
exhibited tetragonal crystal packing with the space group I4.
The asymmetric unit cell was found to contain one host and one
guest toluene molecule. One of the hydroxyphenyl rings and
the toluene guest molecule were found to be disordered. One
Table 3. The Geometrical Parameters for Important
OÀH O Hydrogen Bonds Observed in the Inclusion
3 3 3
Compounds of H1ÀH4
H
O
O
O
—OÀH
O
3 3 3
(Å)
3 3 3
(Å)
3 3 3
(deg)
compound
atomsa
H1-Tolb
O1ÀH1 O3
2.72(1)
2.72(1)
2.76(1)
2.76(1)
2.63(1)
2.65(1)
2.68(1)
2.63(1)
2.56(1)
2.79(1)
2.60(1)
2.75(1)
2.69(1)
2.75(1)
2.56(1)
2.64(1)
2.65(1)
2.65(1)
2.67(1)
3.02(1)
2.67(2)
2.69(1)
3 3 3
O2ÀH2 O1
3 3 3
observes that the OÀH O hydrogen-bonded molecules are
O2ÀH2 O2
3 3 3
3 3 3
related by C4-rotational symmetry down the c-axis. The adjacent
layers are offset in a manner that one observes again a “piedfort”
motif16 for the hydrogen-bonded motif down the c-axis
(Figure 7).
H1-C-DME
H1-C-Aac
O1ÀH1 O3
1.93(1)
1.81(1)
1.82(1)
1.85(1)
1.82(1)
1.74(1)
1.81(1)
1.59(1)
1.95(1)
1.90(1)
178.7(2)
165.1(3)
170.9(2)
169.7(1)
161.0(2)
165.1(1)
156.4(1)
173.7 (1)
158.9(1)
160.9(3)
3 3 3
O2ÀH2 O(W)
3 3 3
O3ÀH3 O2
3 3 3
O3ÀH3 O2
3 3 3
H4-C-Tol. The crystallization of host H4 with 18-crown-6
using equimolar mixture of EtOAc and toluene furnished thick
needle-shaped crystals, which were found to belong to orthor-
hombic lattice system with the space group P212121. The
asymmetric unit cell was found to contain one molecule each
of host H4 and 18-crown-6 along with four toluene and three
water molecules. As in H2-C-W and H3-C-B-Et, the crown guest
molecules are found to function as spacers in linking up triphenol
H4 molecules. The two water molecules bonded to the crown
O1ÀH1 O(ac)
3 3 3
O2ÀH2 O(W2)
3 3 3
O(W1)ÀH O3
3 3 3
O(W3)ÀH O(ac)
3 3 3
H2
O1ÀH1 O1
3 3 3
H2-C-W
O1ÀH1 O3
3 3 3
O(W1)ÀH O2
3 3 3
O3ÀH3 O(W2)
1.77(1)
1.85(1)
1.81(1)
1.90(1)
1.91(1)
2.22(1)
1.83(1)
1.86(1)
163.4(1)
156.0(3)
174.5(3)
148.4(3)
149.3(3)
159.0(5)
173.0(7)
170.8(3)
3 3 3
H3-C-B-Et
H4-Tol
O1ÀH1 O(W1)
3 3 3
oxygens are also found to form OÀH O hydrogen bonds with
3 3 3
O2ÀH2 O(W2)
the hydroxyphenyl rings of H4. In the process, each triphenol
engages two hydroxyphenyl rings. The resultant 2D hydrogen-
bonded assembly propagates to develop a pseudohoneycomb
structure as shown in Figure 8. In other words, each triphenol
unit is found to self-assemble with the aid of 18-crown-6 guest
molecules into a pseudo-honeycomb structure to furnish corru-
gated 2D sheets.10 Two such adjacent sheets interpenetrate to
make up the crystal. As the host H4 is an extended version of H2,
3 3 3
O1ÀH1 O1
3 3 3
O2ÀH2 O2
3 3 3
H4-C-Tol
O3ÀH3 O1
3 3 3
O1ÀH1 O(W1)
3 3 3
O2ÀH2 O(W2)
3 3 3
a “W” and “ac” refer to water molecules and acetate anion. b Hydrogen
atoms are disordered.
Figure 1. The crystal packing of H1-Tol inclusion compound (a), and the pattern of OÀH O hydrogen-bonded assembly (b). Schematic
3 3 3
representation of the host assembly upon reduction of the host structure into a 3-connecting module (c).
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Figure 2. (a) The OÀH O hydrogen bond-mediated self-assembly of H1 into honeycomb layers. The water included in the crown hydrogen bonds
3 3 3
with hydroxyl oxygen of another H1 in the adjacent layer (b). The crystal packing down c-axis without the guests is shown at far right (c).
Figure 3. The crystal packing of H1-C-Aac (a) and the molecular organization in one of the layers to exemplify the self-assembly that leads to voids for
guest inclusion (b). A segment of hydrogen-bonded assembly involving acetate anion is also shown (c).
Figure 4. The crystal packing of H2 (a). The OÀH O hydrogen bond-mediated tetrameric association down the c-axis is shown separately (b).
3 3 3
Notice that the hydrogen-bonding motif down the c-axis appears like a piedfort motif (c). The hydrogen atoms have been removed for clarity.
larger cavities of the former are filled by interpenetration, and the
guest toluene molecules are found to be accommodated in the
void spaces.
materials in general and inclusion host compounds in particular
should be appealing from the point of synthon flexibility;1,3,9 the
latter may facilitate guest-dependent crystal packing or adoption
of synthons. A cursory perusal of the literature reveals a variety of
hosts that are based on phenolic functionality. These include
hydroquinone,18 Dianin’s compound,18 hexa-host,19 anthracene
bisresorcinols,20 bifluorene-tetraphenols,13a,b,21 bimesityl-based
tetraphenols reported from our laboratory,22 binaphthols,23
H-shaped tetraphenols,24 T-shaped bisphenols,25 calixarenes,26
resorcinarenes,26 etc.27 In the backdrop of this noted propensity
Generalizations Concerning Hydrogen-Bonded Self-As-
sembly of H1ÀH4 and Formation of Multicomponent Mo-
lecular Crystals. Phenols are acidic and hence can involve in
hydrogen bonding by functioning both as strong hydrogen bond
donors and acceptors.9 Indeed, one may observe a range of
synthons for the association of phenols, which include dimers,
trimers, linear chains, helices, etc.10 Thus, the self-assembly based
on phenols as applied to rational development of porous organic
of phenols to assemble via OÀH O hydrogen bonds, we were
3 3 3
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Figure 5. The molecular packing in the crystals of H2-C-W (a). The guest crown molecules mediate the assembly with the aid of water molecules that
are in turn bound via OÀH O hydrogen bonds (b). Notice that the crystal packing is akin to H1-C-DME, except that the trimeric assembly down the
3 3 3
a-axis is mediated by crown guest molecules (c).
Figure 6. The crystal packing of H3-C-B-Et down the b-axis (a); each of the layers in the ab-plane is shown separately on the right (b). The basic motif
of the (6,3) net (c), which undergoes 4-fold interpenetration (d). The water embedded in crown-mediated linkage of the triphenols is shown (e) along
with the CÀH O hydrogen-bonded motif that augments the self-assembly of the host H3 in the ab-plane (f).
3 3 3
intrigued by the reported observation that the trigonal 1,3,5-
tris(4-hydroxyphenyl)benzene does not undergo self-assembly
into an expected (6,3) honeycomb porous 2D net.28 We
envisaged that rigidification of the hydroxyphenyl rings by
exploiting sterics introduced through methyl groups as in H1
and H2 might allow porous honeycomb networks to be observed
in the presence of suitable guests.7 The motivation for rigidifica-
tion was indeed derived from remarkable inclusion behavior
uncovered for tetraarylpyrenes characterized by perpendicular
planes that are devoid of functional groups that exhibit strong
intermolecular hydrogen bonding.29
Although inclusion compounds were obtained with H1 and
H2 previously, the change in crystallization solvents evidently
leads to new inclusion compounds with H1 and H2. Indeed, H2
is found to crystallize in the guest-free form under the changed
solvent conditions. While the host H1 is found to undergo
self-assembly either into a highly interpenetrated 2D network
(in H1-DMSO) or a 3D unique net (in H1-Tol),7 one observes
the formation of expected porous 2D honeycomb networks with
18-crown-6 as a guest; indeed the crowns fit into the hexagonal
voids and in turn include solvent water molecules. It should be
noted that the multicomponent inclusion compounds with 18-
crown-6, DME, and water is isostructural with H1-C-MeW and
H1-C-MeNM (Table 1) reported earlier. This suggests the fact
that the self-assembly remains unaltered with subtle variation of
solvents and is robust to replacement of one kind of solvent with
another. InthecaseofH1-C-Aac, water, and acetate anions are found
to expand the hydrogen-bonded assembly to yield corrugated layers
+
with pseudohexagonal voids in which NH4 ⊂crown complexes
are bound. From the present as well as our previous investiga-
tions, host H2 is found not to yield such hydrogen-bonded 2D
honeycomb networks with hexagonal voids in the presence of
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Figure 7. The crystal packing of H3-Tol (a); notice that one of the hydroxyphenyl rings is disordered. The OÀH O hydrogen bonded tetrameric
3 3 3
assembly (b). Notice that the layers down the c-axis are offset such that the hydrogen-bonded motif appears like a piedfort (c).
Figure 8. The crystal packing of H4-C-Tol (a). Water molecules bound in the crowns are found to expand the hydrogen-bonded assembly of H4 (b).
Notice the 2-fold interpenetration of corrugated 2D nets constructed by the mediation of crown-water complexes (c).
18-crown-6, which underscores the importance of (i) thickness
of the interior pores that result and (ii) location of methyl groups
that protrude into the hexagonal voids, if the self-assembly
analogous to H1 were to occur. Otherwise, it should be noted
that H2 does crystallize with inclusion of 18-crown-6 in the
lattice. In the crystal structure of H2-C-W, water and the crown
that bind the former are found to mediate the overall self-
assembly as shown in Figure 5. In other words, the crown is
found to function as a spacer in all of the inclusions compounds
of H2. Presumably, the assembly of the triphenol does not lead to
pores with sufficient thickness to include 18-crown-6. The
extended triphenol H3 is found to crystallize in the presence of
18-crown-6 to yield multicomponent molecular crystals by
including benzene and acetonitrile also as guest species. The
crystal packing reveals the absence of a honeycomb structure. A
careful analysis shows that the crowns act as spacers to yield a
hexameric assembly that is highly porous to undergo self-inter-
penetration. In fact, the packing akin to H1-DMSO in which the
(6,3) net undergoes 8-fold interpenetration.7 In this instance, the
crown-mediated hexameric honeycomb network is found to be
4-fold interpenetrated (Figure 6). In the crystal structure, the
hydroxyphenyl ring and the dimethyl phenyl ring are found to be
almost coplanar with the angles between the planes to be ca. 32ꢀ;
in fact, this is approximately the magnitude that is found in a
majority of the crystal structures of biphenyls deposited in
CSD.30 This means that the biphenyl ring can be considered to
be rigid enough to permit assembly of the host into the basic
trimeric motif with large hexagonal pores. The fact that one does
not observe honeycomb network could also be a consequence of
the dimensions of the employed guest crown being too small
compared to the void dimensions that would otherwise be
created. There is no reason to believe that the porous 2D network
cannot be observed with a suitable guest. Insofar as 18-crown-6 as
the guest is concerned, H4 self-assembles with the former as a
spacer to afford pseudohexagonal (6,3) net that is self-interpe-
netrated. A scenario that is similar to H2 is observed in the
crystals of H4-C-Tol; the crown is found to serve as a spacer to
yield a pseudohoneycomb assembly that is doubly interpenetrated.
’ CONCLUSIONS
All the triphenol hosts H1ÀH4 exhibit notable guest inclu-
sion, which attests to the importance of orthogonal aromatic
planes—a feature ensured via sterics. In other words, the results
illustrate the fact that the desired crystal packing with porosity for
guest inclusion can be engineered via rational molecular design
that exploits sterics.29,31 The host H1 reliably yields perfect
honeycomb structure with hexagonal voids in which the 18-
crown-6 nicely fits; of course, the latter further binds guests such
as methanol,7 nitromethane,7 ammonium cation, water, etc. to
furnish guest⊂guest⊂host multicomponent inclusion compounds.8
That the length of the rigid aryl moiety is crucial for such an
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assembly is demonstrated by the crystal structure of H3-C-B-Et,
which yields crown-spaced assembly. The host triphenols H2
and H4 do assemble with inclusion of crown, but the latter serves
as a spacer. In the absence of crown, the triphenols include
solvent molecules and self-assemble into nets that are 2- and 3D
(for H1-Tol and H4-Tol). The host H2 is found to crystallize in
General Procedure for the Suzuki Coupling with Aryl
Triflates. Triflate (1.0 mmol), arylboronic acid (4.5 mmol), Pd(PPh3)4
(10 mol %, with respect to the triflate), and K2CO3 (10 mmol) were
introduced into an initially oven-dried and cooled (under N2) pressure
tube. To this mixture, dry toluene (15 mL) was added. The reaction
mixture was heated slowly at 70À100 ꢀC and maintained at this
temperature for 36 h with constant stirring. After this period, the
reaction mixture was cooled to 20 ꢀC, and a saturated aqueous solution
of NH4Cl was added. The organic matter was extracted with CHCl3 and
the combined extract was dried over Na2SO4 and filtered. The filtrate
was concentrated in vacuo to yield a crude product, which was subjected
to column chromatography to isolate the methyl aryl ether as a colorless
solid material.
1,3,5-Tris(2,6-dimethyl-4-(p-anisyl)phenyl)benzene. Yield
57%. Colorless powder; mp 136À140 ꢀC; IR (KBr) cmÀ1 2924 (s),
2854 (m), 1607 (s), 1512 (s), 1467 (s); 1H NMR (400 MHz, CDCl3) δ
2.23 (18H, s), 3.85 (9H, s), 6.97 (6H, d, J = 8.20 Hz), 7.00 (3H, s), 7.31
(6H, s), 7.56 (6H, d, J = 8.20 Hz); 13C NMR (125 MHz, CDCl3) δ 21.1,
55.3, 114.1, 125.7, 128.1, 128.4, 133.6, 136.3, 139.4, 140.2, 141.2, 159.0;
ESI-MS+ m/z Calcd for C51H48O3 708.93 [M + H], found 709.3680.
1,3,5-Tris(4-(p-anisyl)phenyl)mesitylene. Yield 48%. Color-
less powder; mp 160À164 ꢀC; IR (KBr) cmÀ1 2924 (m), 2836 (m),
1607(s), 1495(s), 1458 (s); 1H NMR (400 MHz, CDCl3) δ 1.76 (9H, s),
3.78 (9H, s), 6.91 (6H, d, J = 8.80 Hz), 7.22 (6H, d, J = 8.20 Hz), 7.53
(6H, d, J = 8.80 Hz), 7.56 (6H, d, J = 8.20 Hz); 13C NMR (125 MHz,
CDCl3) δ 19.6, 55.3, 114.2, 126.7, 128.0, 129.8, 133.4, 133.5, 138.7, 139.6,
140.4, 159.0; ESI-MS+ m/z Calcd for C48H42O3 666.85 [M + NH4],
found 684.3474.
guest-free form via OÀH O hydrogen-bonded tetrameric
3 3 3
assembly that propagates.
A variety of C3-symmetric inclusion compounds are reported
in the literature. This symmetry is so much apparent in several
important host systems such as tri-ortho-thymotide (TOT),32
perhydrotriphenylene,32 triphenyl methane,32 tris(o-phenylene-
dioxy)cyclotriphosphazene,4e,f,33 etc.34 The prototype example
of a C3-symmetric host that undergoes hydrogen-bonded self-
assembly into a porous hexagonal network constitutes 1,3,5-
benzenetricarboxylic acid (trimesic acid).5,6 The results with H1
show that honeycomb networks with hexagonal voids can like-
wise be constructed with triphenol H1. Given the synthon
flexibility that is inherent to phenols, we believe that the rigid
triphenols H1ÀH4 reported herein should be an invaluable
addition to the class of C3-symmetric inclusion hosts.
’ EXPERIMENTAL SECTION
Anhydrous tetrahydrofuran (THF) was freshly distilled over sodium
prior to use. All other solvents were distilled prior to use. The progress of
reactions was monitored by analytical thin layer chromatography (TLC)
using aluminum sheets precoated with silica gel. Column chromatogra-
Synthesis of Extended Triphenols H3 and H4
1
phy was conducted with silica gel (60À120 mesh). H and 13C NMR
General Procedure for Demethylation of Methyl Aryl
Ethers. To a solution of methyl aryl ether (0.5 mmol) in 15 mL of
dry CH2Cl2 at 0 ꢀC was added dropwise BBr3 solution (0.6 mmol)
under a N2 gas atmosphere. The reaction mixture was allowed to stir
overnight. Subsequently, it was quenched with 10% HCl, extracted with
ethyl acetate, dried over 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
pet. ether (50:50), followed by recrystallization from ethyl acetate and
pet. ether.
spectra were recorded on 400 and 500 MHz spectrometers using
deuterated solvents. The TGA and DSC measurements were carried
out on a TGA-DSC1 with a heating rate of 10 ꢀC/min under N2
atmosphere. Commercial chemicals were used as received.
Synthesis of Triphenol Hosts H1ÀH4. A general procedure for
the synthesis of triphenols H1ÀH4 involved 3-fold Suzuki coupling of
tribromobenzenes/triaryl triflates with suitably functionalized boronic
acids using Pd(PPh3)4 as a catalyst (Scheme 1). The preparation of
triphenol hosts H1 and H2, that is, 1,3,5-tris(2,6-dimethyl-4-hydroxy-
phenyl)benzene and 1,3,5-tris(4-hydroxyphenyl)mesitylene, has pre-
viously been published by us.7
General Procedure for the Esterification of Phenols with
Triflic Anhydride. To a solution of triphenol (1.0 mmol) in anhy-
drous CH2Cl2 (30 mL) were added Et3N (4.5 mmol) and triflic
anhydride (3.1 mmol). The reaction mixture was stirred at 0À5 ꢀC
for 2 h. At the end of this period, the reaction mixture was quenched by
pouring the contents into 50 mL of ice-cold water. The organic material
was extracted with DCM, washed with water, dried over anhyd Na2SO4,
and concentrated in vacuo. Filtration over a short-pad of silica gel gave
the pure triaryl triflate as a colorless solid.
1,3,5-Tris(2,6-dimethyl-4-trifluoromethylsufonylphenyl)-
benzene. Yield 78%. Colorless powder; mp 164À168 ꢀC; IR
(KBr) cmÀ1 2962 (m), 1602 (s), 1427 (s); 1H NMR (500 MHz,
CDCl3) δ 2.14 (18H, s), 6.94 (6H, s), 7.03 (3H, s); 13C NMR (125
MHz, CDCl3) δ 21.1, 118.8 (q, J = 38.75 Hz), 119.9, 128.4, 138.6, 140.8,
141.2, 148.4; ESI-MS+ m/z Calcd for C33H27O9F9S3 834.75 [M + Cl]À,
found 869.0362.
1,3,5-Tris(4-trifluoromethylsulfonylphenyl)mesitylene.
Yield 30%; mp 128À132 ꢀC; IR (KBr) cmÀ1 2930 (m), 1914 (w), 1504
(s), 1432 (s), 1219 (s); 1H NMR (400 MHz, CDCl3) δ 1.59 (9H, s),
7.21 (6H, d, J = 8.20 Hz), 7.30 (6H, d, J = 8.20 Hz); 13C NMR
(125 MHz, CDCl3) δ 19.4, 118.7 (q, J = 39.0 Hz), 121.6, 131.1, 133.6,
138.3, 141.9, 148.4; ESI-MS+ m/z Calcd for C30H21O9F9S3 792.67
[M + Cl], found 826.9898.
1,3,5-Tris(2,6-dimethyl-4-(p-hydroxyphenyl)phenyl)benzene
(H3). Yield 72%. mp 188À192 ꢀC colorless powder; IR (KBr) cmÀ1
1
3404 (s, br), 1608 (m), 1514 (m), 1466 (m); H NMR (400 MHz,
DMSO-d6) δ 2.15 (18H, s), 6.82 (6H, d, J = 8.6 Hz), 6.90 (3H, s), 7.33
(6H, s), 7.49 (6H, d, J = 8.6 Hz), 9.47 (3H, s); 13C NMR (100 MHz,
DMSO-d6) δ 20.6, 115.6, 124.9, 127.5, 135.6, 139.4, 177.8; ESI-MS+
m/z Calcd for C48H42O3 666.85 [M + Cl]À, found 701.2825.
1,3,5-Tris(4-(p-hydroxyphenyl)phenyl)mesitylene (H4).
Yield 73%. mp 182À186 ꢀC Colorless powder; IR (KBr) cmÀ1 3369
1
(s, br), 2926 (s), 1609 (m), 1498 (s), 1448 (m), 1258 (s); H NMR
(400 MHz, DMSO-d6) δ 2.45 (9H, s), 6.84 (6H, d, J = 8.65 Hz), 7.40
(6H, d, J = 8.25 Hz), 7.54 (6H, d, J = 8.65 Hz), 7.65 (6H, d, J = 8.25 Hz),
9.56 (3H, s); 13C NMR (100 MHz, DMSO-d6) δ 19.3, 115.7, 126.0,
127.6, 129.6, 130.5, 132.5, 138.3, 139.0, 139.5, 157.0; ESI-MS+ m/z
Calcd for C45H36O3 624.77 [M + Cl]À, found 659.2356.
Crystallization of Triphenol Hosts H1ÀH4 with and with-
out Guest Molecules. Crystallization of H1-Tol and H2. A 1:1
mixture of CHCl3 and toluene was layered over a solution of host H1
(50.0 mg, 0.114 mmol) in DMSO (2.0 mL). Slow evaporation of the
resultant clear solution over a period of a week led to square-shaped
1
crystals in a quantitative yield. The crystals were characterized by H
NMR, TGA, and X-ray crystallography. A similar procedure was used for
the crystallization of H2. In this instance, a CH2Cl2Àpet. ether mixture
was layered over H2 in ethyl acetate solution. The crystals thus obtained
were found to be devoid of any guest inclusion.
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Crystallization of H1-C-DME and H1-C-Aac. H1 (50.0 mg, 0.114
mmol) and 18-crown-6 (45.0 mg, 0.171 mmol) were dissolved in
dimethoxyethane (5.0 mL). Slow evaporation of the resultant clear
solution over a period of 4À5 days led to rectangular-shaped crystals.
Filtration followed by washing with a CHCl3Àpet. ether (50:50)
mixture led to isolation of the compound in 70À75% yield based on
host H1. The crystals were characterized by TGA, DSC, and X-ray
crystallography.
Berlin, 1998: Vol. 198. (c) Crystal Engineering: From Molecules and
Crystals to Materials; Braga, D., Orpan, A. G., Ed.; NATO ASI Series;
Kluwer: Dordecht, The Netherlands, 1999. (d) Steed, J. W., Atwood, J. L.,
Eds.; Supramolecular Chemistry; Wiley: New York, 2000. (e) Encyclopedia of
Supramolecular Chemistry; Atwood, J. L., Steed, J. W., Eds.; Marcel Decker:
New York, 2004; Vols. I and II. (f) Bradshaw, D.; Claridge, J. B.; Cussen,
E. J.; Prior, T. J.; Rosseinsky, M. J. Acc. Chem. Res. 2005, 38, 273.
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Chem., Int. Ed. 2002, 41, 48. (f) Laliberte, D.; Maris, T.; Wuest, J. D.
J. Org. Chem. 2004, 69, 1776. (g) Hosseini, M. W. Acc. Chem. Res. 2005,
38, 313. (h) Dalgarno, S. J.; Power, N. P.; Atwood, J. L. Coord. Chem. Rev.
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A similar procedure was used for the synthesis of H1-C-Aac. In this
case, ammonium acetate in water was employed; yield of the inclusion
compound was 80À85% based on host H1.
Preparation of H2-C-W, H3-C-B-Et, and H4-C-Tol. 18-Crown-
6 (45.0 mg, 0.171 mmol) dissolved in EtOAc (2.0 mL) was slowly added
to a solution of H2 (50.0 mg, 0.114 mmol) in dioxane (3.0 mL). Slow
evaporation of the resultant clear solution over a period of 5À7 days led
to square-shaped crystals in a quantitative yield.
A similar procedure was used for the preparation of H3-C-B-Et and
H4-C-Tol. In all cases, the appropriate guest and the solvent were used
for crystallization.
X-ray Crystal Structure Determinations. A good quality crystal
in each case was mounted in a glass capillary and cooled to 100 K, and
the intensity data were collected on a Bruker Nonius SMART APEX
CCD detector system with Mo-sealed Siemens ceramic diffraction tube
(λ = 0.71073) 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-squares method based on F2 using SHELX97 program. All the non-
hydrogen atoms were refined anisotropically. The hydrogen atoms were
included in their ideal positions with fixed isotropic U values and were
allowed to ride with their respective non-hydrogen atoms. The experi-
mental details of crystal data, intensity measurements, structure solution,
and refinement are presented in Table 2.
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R.; Sozzani, P. Chem. Commun. 2007, 350. For gas absorption by
metalÀorganic materials, see:(g) Cussen, E. J.; Claridge, J. B.; Rosseinsky,
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’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic infor-
b
1
mation files (CIF), details of refinement, H NMR spectral
reproductions and TGA profiles for the multicomponent crystals
are available. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: moorthy@iitk.ac.in (J.N.M.); venugopalanp@yahoo.com
(P.V.).
’ ACKNOWLEDGMENT
(5) (a) Herbstein, F. H. In Comprehensive Supramolecular Chemistry;
Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo€gtle, F., Eds.;
Pergamon: Oxford, 1996; Vol. 6, p 61. (b) Ermer, O.; Neud€orfl, J. Helv.
Chim. Acta 2001, 84, 1268.
J.N.M. is thankful to the Department of Science and Technol-
ogy (DST), India, for generous funding. P.N. and A.B. thank
UGC and CSIR, respectively, for their research fellowships.
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’ DEDICATION
This paper is dedicated to Prof. K. Venkatesan on the occasion of
his 80th birthday.
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