Tetraarylbiphenyl as a New Lattice Inclusion Host by Structure

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Tetraarylbiphenyl as a New Lattice Inclusion Host by Structure Reductionism: Shape and Size Complementarity Based on Torsional Flexibility Ishita Neogi, Alankriti Bajpai, Govardhan Savitha,* and Jarugu Narasimha Moorthy* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India S Supporting Information *

ABSTRACT: 3,3′,5,5′-Tetrakis(2,6-dimethyl-4-methoxyphenyl)biphenyl (TB) was designed as a lattice inclusion host based on a structure reductionistic approach. TB binds guests in two different domains based on the premise that the two phenyl rings of the biphenyl core remain coplanar in the solid state. The versatility of TB as a host has been demonstrated by its ability to include different guest molecules in the crystal lattice, as revealed by X-ray crystal structure analyses. One observes preference for guest location in the concave domain with the trough domain compromised. The host TB is found to exploit torsional freedom about the central σ-bond between the two phenyl rings of the biphenyl core to adopt discrete structures that are complementary in terms of shape and size to include a given guest. In other words, TB behaves like a “molecular chameleon” that undergoes structural adaptation in response to the guest via torsional twist about the central σ-bond. Quite remarkably, the inclusion compounds with different guests having similar torsional angles between the phenyl rings of the biphenyl core are found to be isostructural. The torsion-induced structural morphosis in response to the guest is found to completely offset binding in the trough region.

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INTRODUCTION Crystallization is a process of purification of a solid material. So, crystallization of two or more different components together in a single crystal as a binary or multicomponent molecular material is a daunting task, and is indeed fundamentally challenging.1,2 In this regard, lattice inclusion compounds,3−7 which are single phase solids characterized by well-defined binding with shape complementarity between the host and guest(s), offer unique insights into the phenomenon of molecular recognition.8,9 They are important within the context of “crystal engineering”.10−12 In general, the lattice inclusion compounds are held together by a variety of noncovalent intermolecular forces, viz., electrostatic interactions, hydrophobic interactions, hydrogen bonds, van der Waals interactions, dipole−dipole interactions, and so forth.13 A number of applications have been demonstrated for lattice inclusion compounds, which include separation and purification,14 chiral resolution,15−18 catalysis and asymmetric synthesis,19−23 and gas storage;24 Sozzani and co-workers have shown that guest inclusion can be reversed in inclusion compounds, a behavior that is commonly observed in inorganic zeolites for storage of gases.25 One of the ways by which two or more components can be brought together in a single solid with well-defined stoichiometry to create binary or multicomponent co-crystals or salts1,2,26−30 is by employing carboxylic acids and pyridine bases.31−35 This strategy continues to be exploited tremendously in the industry to develop pharmaceutical co-crystals.36 Another strategy constitutes the design of molecular modules with concave features as host systems for guest inclusion.37 We © XXXX American Chemical Society

have been concerned for some time now with the development of lattice inclusion host systems based on a de novo design.38−42 We have shown that hydrogen-bonded aggregation in the solid state of a twisted D2d-symmetric tetraarylbimesityl TAB leads to generation of voids within which guest molecules are entrapped (Figure 1).43 In continuation of these studies, we demonstrated that tetraarylpyrene TP that is decked up with rigid aromatic planes at the four corners serves as a versatile host system with inherent concave features, and that it includes guest molecules in three different domains, referred to as concave, trough and basin. Host TP was further shown to exhibit site-selective guest inclusion of two different molecules simultaneously thereby allowing access to ternary solids.38−42 It has been shown that the aromatic guests are bound selectively in the trough regions, while aliphatic and small guest molecules are included in the concave and basin regions preferentially.38 In a logical progression from our initial tetraarylbimesityl-based host TAB to tetrarylpyrene TP, we wondered if the concave region could further be enlarged by truncating the structure of the pyrene core to a simple biphenyl one as in TB (Figure 1). Given that the two phenyl rings of biphenyl are nearly coplanar in the solid state,44 the molecular system TB can be readily likened to the extensively investigated wheel-and-axle class of lattice inclusion hosts.8,45 We were, therefore, motivated to explore the inclusion behavior of such a structurally modified host system. Of course, it should be recognized that such a Received: November 24, 2014 Revised: March 19, 2015

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Figure 1. Molecular structures of the hosts TAB, TP, and TB.

Scheme 1. Synthetic Route for Preparation of the Host TB

Chart 1. Molecular Structures of the Guests with Which the Inclusion Compounds Were Isolated with Host TB

structural truncation eliminates the “basin” region for guest inclusion in addition to imparting considerable torsional flexibility to the phenyl rings of the central biphenyl core. Herein, we report that the host tetraarylbiphenyl TB displays guest inclusion behavior with a preference for guest binding in the concave region, as revealed by X-ray crystal structure analyses. The preponderant guest inclusion in the concave region can be understood based on the torsional twist about the two central phenyl rings, which the host TB can exploit to adopt structural attributes that are complementary to the guests in terms of shape and size.

in chloroform without any additive led to isolation of the crystals of the host alone that is devoid of guest inclusion. In Chart 1 are shown all the guest molecules for which the inclusion compounds were obtained and analyzed. X-ray Crystal Structures of the Inclusion Compounds of Host TB with Various Guests. Single crystals of all the inclusion compounds of TB formed with diverse guests shown in Chart 1 were subjected to X-ray crystal structure determinations. Table 1 lists details of the codes employed for the inclusion compounds and their host:guest ratios. One observes that the crystals of the inclusion compounds of host TB with ten guest systems belong to three different space groups. Also given in Table 1 are solvent-accessible volumes as

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RESULTS AND DISCUSSION Synthesis of Host TB and Preparation of Lattice Inclusion Compounds. The host, 3,3′,5,5′-tetrakis(2,6dimethyl-4-methoxyphenyl)biphenyl TB, was synthesized by Pd(0)-catalyzed 4-fold Suzuki cross coupling reaction between 3,3′,5,5′-tetrabromobiphenyl and 2,6-dimethyl-4-methoxyphenylboronic acid (Scheme 1). The precursor 3,3′,5,5′tetrabromobiphenyl was synthesized starting from 1,3,5tribromobenzene by following the literature reported procedure.46 To obtain inclusion compounds of the host TB with various guest molecules, a saturated solution of the host TB (50 mg) and the guest (15 mg) in 5 mL of chloroform was allowed to evaporate slowly at rt over a period of 2−3 d. The crystals of inclusion compounds that formed were isolated and characterized by single crystal X-ray diffraction, thermogravimetric analysis, and 1H NMR spectroscopy, cf., Supporting Information (SI). Slow evaporation of a saturated solution of the host

Table 1. Codes Employed for the Inclusion Compounds of Tetraarylbiphenyl Host TB, and Details of Host-Guest Ratios and Guest-Accessible Volumes (V)

B

guest

code

host:guest

V (%)

none 1,4-dioxane cyclooctane decalin o-dicholorobenzene cyclohexenone salicylaldehyde cyclododecene cylohexanone trans-cinnamaldehyde p-methoxybenzaldehyde

TB-GF TB•G1 TB•G2 TB•G3 TB•G4 TB•G5 TB•G6 TB•G7 TB•G8 TB•G9 TB•G10

− 1:2 1:1 1:1 2:1 2:1 2:1 1:1 1:1 2:1 2:1

− 23.4 20.4 24.1 10.6 10.5 11.2 29.7 15.8 14.6 13.0

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Figure 2. (a,b) Crystal packing diagrams for TB−GF with and without space filling. (c) Crystal packing of TB•G1 showing the guests, which are located in the channels down the b-axis.

The crystals of these inclusion compounds were found to be isostructural with the space group P1̅ (Table 2). In all these cases, the guest is found to lie about the inversion center with an occupancy of 0.5 such that the host:guest stoichiometry is 1:0.5. Therefore, the guest is statistically disordered about the inversion center such that there is a 50% chance of being found in either orientation at any given equivalent site. The packing diagram reveals channels down the a-axis in which the guest molecules are found to be located in similar orientations (Figure 3). Insofar as the location of the guests in each case with respect to the host TB is concerned, it is immediately apparent that they lie in the concave regions. The trough regions in each case are found to be filled by the aryl rings of the neighbors (Figure 3). TB•G7 and TB•G8. These inclusion compounds contain cyclododecene (G7) and cyclohexanone (G8) in their crystals and belong to the C2/c space group with the asymmetric unit cell consisting of one host and one guest molecule such that the host:guest ratio is 1:1, Table 2. The crystal structure is made up of layers of host and guest molecules placed alternately in the ac-plane. The concave−concave and trough−trough associations by weak van der Waals interactions lead to voids as shown in Figure 4. The guests are found within the voids formed between the confluence of concave domains. While one of the trough regions of the host is filled by an aryl ring of the neighbor, the second one lies vacant. The offset of layers down the b-axis ensures that the voids thus formed are closed up by the aryl rings. Even with such an offset arrangement of layers down the b-axis, one observes channels with considerable volume in which the guest cyclododecene molecules are found to be located. The crystals of TB•G8 show packing equivalence to those of TB•G7 except that the host molecules are closely placed leading to better packing. In the process, the smaller guest molecules are found exceptionally in trough domain. TB•G9 and TB•G10. The crystals of the inclusion compounds of TB with trans-cinnamaldehyde (G9) and pmethoxybenzaldehyde (G10) belong to the space group C2/c (Table 2). The asymmetric unit consists of one host molecule and a guest molecule with an occupancy of 0.5. The latter, as in the case of TB•G4−6, lies about the inversion center such that it is disordered statistically, cf., Figure 4. Thus, the host:guest stoichiometry is 1:0.5. The crystal packing analysis shows that the host molecules are organized in the ac-layer with concave− concave association leading to voids within which the

obtained from PLATON calculations when the guests were excluded. In the following are described briefly the crystal packings of the inclusion compounds for each space group type. TB−GF. The host TB is found to crystallize in the space group Pbcn, cf., SI. The crystal structure analyses show that the host molecule in the lattice interacts with the neighbors by weak CH···O hydrogen bonds involving the methoxy oxygen; each molecule interacts with the diagonally placed neighbors that crisscross in the crystal lattice. The trough regions of neighboring molecules show close van der Waals interactions as they are juxtaposed against each other, cf., Figure 2. One observes no solvent-accessible void volume in the lattice. Of course, in the presence of solvent molecules, void spaces are created to accommodate guest molecules, leading to the formation of inclusion compounds, vide infra. TB•G1. The crystals with the dioxane guest exhibit highest symmetry (Cmca) in this set of co-crystals with Z′ = 1/4, which utilize the inherent D4h symmetry of the molecule. The host sits on the crystallographic bc-plane as well as the inversion center such that only 1/4 of the molecule is identified. The guest dioxane is found to lie on the C2 element down the a-axis. Thus, for 1 host, 2 guest dioxane molecules are found. The crystal packing diagram shown in Figure 2 shows that the guest dioxane molecules lie in the channels generated down b-axis between the hosts that are arranged in an inclined fashion in the ac-plane. The guest molecules are found to be disordered with an occupancy of 0.5 at each location. TB•G2 and TB•G3. These inclusion compounds of TB contain cyclooctane (G2) and trans-decalin (G3) as guest species. The crystal structures of TB•G2 and TB•G3 are similar with the space group C2/c, cf., Table 2. In the crystal packing, one observes that concave−concave alliance of host molecules forms strands along the b-axis with channels down the c-axis (Figure 3). These strands are found to be offset on both sides such that the trough domains are occupied by the anisyl rings. Of course, the guests are located in the channels generated between the concave domains. Nevertheless, cyclooctane is present exactly in the center of the concavity, while decalin is found to be offset and located below the concavity due to its larger size. Furthermore, cyclooctane is found to be disordered about the C2-axis. TB•G4, TB•G5, and TB•G6. These inclusion compounds of TB contain three guests, namely, o-dichlorobenzene (G4), cyclohexenone (G5), and salicylaldehyde (G6), respectively. C

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Crystal Growth & Design Table 2. Crystal Data for Inclusion Compounds of Host TB with Different Guest Molecules molecular formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z calculated density (mg/m3) absorption coefficient (mm−1) F(000) goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data)

molecular formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z calculated density (mg/m3) absorption coefficient (mm−1) F(000) goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data)

TB•G1

TB•G2

TB•G3

TB•G4

TB•G5

C48H50O4•2C4H8O2 867.09 orthorhombic Cmca 32.353(8) 5.8839(14) 24.670(6) 90.00 90.00 90.00 4696(2) 4 1.226 0.081

C48H50O4•C8H16 803.09 monoclinic C2/c 26.616(3) 14.999(11) 11.5435(9) 90.00 102.60(2) 90.00 4497.2 4 1.186 0.073

C24H25O2•C5H9 414.56 monoclinic C2/c 24.471(12) 15.774(12) 12.252(7) 90.00 92.56(16) 90.00 4725(5) 8 1.166 0.071

C48H50O4•0.5C6H4Cl2 764.38 triclinic P1̅ 11.7393(6) 13.7204(7) 14.1348(7) 96.80(10) 98.73(10) 108.61(10) 2098.34(18) 2 1.210 0.136

C48H50O4•0.5C6 H8O 738.94 triclinic P1̅ 11.118(5) 13.770(6) 14.464(6) 92.82(9) 96.74(8) 107.27(8) 2091.6(16) 2 1.173 0.073

1864 1.074 R1 = 0.0471, wR2 = 0.1151 R1 = 0.0680, wR2 = 0.1265 TB•G6

1736 1.080 R1 = 0.0802, wR2 = 0.2117 R1 = 0.1246, wR2 = 0.2769 TB•G7

1792 1.022 R1 = 0.0921, wR2 = 0.2228 R1 = 0.1733, wR2 = 0.2914 TB•G8

814 1.010 R1 = 0.0530, wR2 = 0.1038 R1 = 0.0991, wR2 = 0.1201 TB•G9

792 1.115 R1 = 0.1049, wR2 = 0.2617 R1 = 0.1706, wR2 = 0.3711 TB•G10

C48H50O4•0.5C7H6O2 751.94 triclinic P1̅ 11.554(6) 13.665(7) 14.373(8) 91.05(11) 99.12(10) 107.76(12) 2128(2) 2 1.173 0.074

C48H50O4•C12H24 859.19 monoclinic C2/c 28.348(6) 11.778(2) 30.986(5) 90.00 101.909(6) 90.00 10123 8 1.128 0.072

C54H60O5 789.02 monoclinic C2/c 30.156(13) 12.688(5) 25.591(12) 90.00 115.696(18) 90.00 8823(6) 8 1.188 0.074

C48H50O4•0.5C9 H8 O 755.95 monoclinic C2/c 28.915(12) 11.849(4) 24.852(10) 90.00 91.715(14) 90.00 8511(6) 4 1.180 0.074

C48H50O4•0.5C8H8O2 758.95 monoclinic C2/c 29.044(7) 11.731(2) 24.658(5) 90.00 92.399(5) 90.00 8394(3) 8 1.201 0.076

804 0.966 R1 = 0.1162, wR2 = 0.2725 R1 = 0.2461, wR2 = 0.3955

3728 1.253 R1 = 0.1189, wR2 = 0.3251 R1 = 0.1951, wR2 = 0.3945

3392 0.936 R1 = 0.0913, wR2 = 0.2029 R1 = 0.2273, wR2 = 0.2896

3232 1.034 R1 = 0.1039, wR2 = 0.2649 R1 = 0.1905, wR2 = 0.3384

3248 1.101 R1 = 0.0652, wR2 = 0.1732 R1 = 0.0964, wR2 = 0.2367

disordered guests are found. While the molecules within the layer are found to be associated purely by van der Waals interactions, the layers down the b-axis are interconnected by CH···O hydrogen bonds formed with oxygen atoms of the methoxy groups. Rational Design by Structure Reductionism. As mentioned at the outset, design of the host TB was based on a logical progression from bimesityl and pyrene-based molecular systems (TAB and TP), whose guest inclusion behavior has been sufficiently demonstrated.38−43 It was a priori surmised that removal of the two central double bonds in the TP host would result in considerable increase of the space in the concave region at the expense of basin domain that is extant to TP. Yet, the molecular system was anticipated to exhibit two different domains for guest inclusion. Of course, how the loss of rigidity in conjunction with torsional flexibility would manifest in the anticipated guest inclusion behavior was our motivation.

As is evident from the structural investigations of the inclusion compounds of TB with diverse guest systems, it is amply evident that the TB does indeed function as a lattice inclusion host system. The following emerges from the crystal structure analyses of the inclusion compounds: (i) the basin domain vanishes, (ii) out of the ten guest systems, only one guest is found to be located in the trough region, (iii) guest inclusion occurs predominantly via binding of the guests in the concave domain, and (iv) one observes different packing modes as exemplified pictorially in Figure 5. Although, the structural reduction from pyrene to biphenyl conserves guest inclusion behavior, the abundant lattice inclusion observed with TB is attenuated in the present case. The host system with truncated binding domains limits guest inclusion largely to the concave region. Otherwise, one observes a remarkable guest-dependent structural adaptation achieved through twisting of the central phenyl rings of the biphenyl core, vide infra. D

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Figure 3. (a) Crystal packing of the inclusion compound TB•G3 showing the strands of host molecules along the b-axis with guests filled in the channels down the c-axis. (b) Crystal packing diagram of TB•G5 with channels running down the a-axis.

Figure 4. (a) Crystal packing of the inclusion compound TB•G7 showing offset of two layers in the ac-plane with respect to each other with guests located in the channels down the b-axis. (b) Crystal packing diagram of the inclusion compound TB•G9 with the guests located in the channels down the b-axis (see text).

Guest-Dependent Structural Adaptation by the Host. We have shown previously that the rigid TP host displays guest inclusion by accommodating guests of different sizes and shapes in one or more inherently available domains.38−43 One observes a paradigm change in the way the guests are bound in the present instance. The very fact that one does not observe guest inclusion in the trough region except in the case of cyclohexanone, but that the guest accommodation is observed almost exclusively in the concave region, points to torsional flexibility that the host system is seemingly able to exploit. A careful analysis of the angles between the two phenyl rings in the biphenyl coretermed twist angle (ϕ)in all the structure of the inclusion compounds of TB reveals that the angle ϕ varies from ca. 0° to 41.0°. Thus, the host may, in principle, explore infinite number of conformations and associated mutually exclusive structures for varying twist angles. As may be seen in Figure 6, ϕ is close to 0° in the case of TB•G1, ca. 22.0° for TB•G2, and as high as ca. 40.0° for TB•G9 and TB•G10 with that for the compounds of other guests falling within these limits. Further, for every twist, the change in the void region available for guest inclusion in the concave domain may be ascertained by the distance (d) between the two anisyl rings that create the concave domain; we have calculated

variation of the distance between the two methoxy oxygen atoms of the anisyl rings for all inclusion compounds of the host TB. This distance is found to vary from 10.6 to 12.5 Å. Figure 6 is an exemplar of a correlation between various twist angles observed for the host TB in the structures of its inclusion compounds investigated herein with the distance d; as should be expected, the value of d should increase with an increase in the twist angle ϕ. One is immediately tempted to pose the question: do inclusion compounds of TB with different guests with similar twist angles (ϕs) of the host exhibit similar crystal packing? Indeed, we readily identified the fact that the compounds TB•G4, TB•G5, and TB•G6 with similar twist angles (32−34°) are isostructural. Likewise, TB•G9 and TB•G10 with ϕ ca. 40° are found to exhibit packing equivalence with the same space group. In the same manner, TB•G7 and TB•G8 with approximate twist angles were found to exhibit packing equivalence. It therefore emerges that the host can explore innumerable structures via torsion about the σ bond between the two phenyl rings of the biphenyl core. Clearly, the conformation that is observed in the crystal structures is a consequence of size and structural complementarity with respect to the guest. E

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Figure 5. Schematic representation showing the packing patterns of host TB with different guest molecules G1−G10; the guest are shown with a filled blue circle.

Figure 6. Plot of the average span (d) of the concave region vs the twist angle (ϕ) about the biphenyl rings in TB.

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CONCLUSIONS TB was designed as a simple host system based on our earlier investigations with bimesityl and pyrene-based molecular systems. Structural truncation from tetraarylpyrene (TP) to tetraarylbiphenyl (TB) was anticipated to allow increased concave region in the latter with the basin domain compromised, provided that the two phenyl rings of the biphenyl core remain coplanar. Guest binding in the solid state was investigated through single crystal X-ray structure determinations of the crystals obtained in the presence of different guests. The structural analyses show that the host TB

indeed includes a variety of guest molecules in the lattice. TB displays a common mode of guest binding, largely in the concave region. The crystal structures reveal a remarkable structural adaptation of the host in response to size and shape attributes of the guests. Accordingly, one observes that the two phenyl rings of the central biphenyl twist with respect to each other into geometries appropriate for the given guest. This propensity seemingly offsets the binding of guests in trough regions completely, as evident from the fact that the latter is observed for only one guest, namely, cyclohexanone. Thus, the host TB may be likened to a molecular chameleon, which F

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X-ray Crystal Structure Determinations. The crystal data for the inclusion compounds of TB were collected at 100 K on a Bruker Nonius SMART APEX CCD detector system provided with Mo-sealed Siemens ceramic diffraction tube (λ = 0.71073 Å) and highly oriented graphite monochromator. The instrument was operated at 50 kV and 30 mA. The lattice parameters and standard deviations were obtained by a leastsquares fit using 25 frames with 20 s/frame exposures with the Bruker APEX247 software. The data were collected for a hemisphere by φ and ω scans with 2θ = 40° and 10 s/frame exposures. Data processing and reduction were done using Bruker SAINT,47 and empirical absorption correction was made using Bruker SADABS.18 The structures were solved and refined using the WINGX48 and SHELXLe-201449 programs. The hydrogen atoms were included in the ideal positions with fixed isotropic U values and were riding with their respective non-hydrogen atoms and refined. The VOID50 program from PLATON software was used for calculating guest-accessible volumes after omitting the guest molecule in the input files.

adapts itself into suitable conformations necessary for binding a given guest in the concave region through twisting about the central σ bond between the phenyl rings of the biphenyl core. It is noteworthy that the twisting that is observed is as much as ca. 40° for guests such as 4-methoxybenzaldehyde and transcinnamaldehyde, while it is ca. 0° for dioxane and ca. 22° for cyclooctane. The inclusion compounds of cyclohexanone and cyclohexenone are very instructive from the point of view of how even subtle changes in the structures of the guests alter the mode of crystal packing; the difference between the two guest molecules is seemingly minor in that only two of the carbons in the ring are either sp2 or sp3 hybridized, yet the inclusion compound of TB with cyclohexenone corresponds to a lower symmetry space group (P1̅), while that of cyclohexanone belongs to the C2/c space group. It is noteworthy that the inclusion compounds with different guests having similar torsional angles between the phenyl rings of the biphenyl core are found to be isostructural. Development of a new lattice inclusion host based on a structure reductionistic approach has thus been compellingly demonstrated. Application of such molecular systems with torsional flexibility in the central core and rigidity at the periphery should be appealing for development of metal−organic materials/frameworks (MOMs/MOFs).46 We are presently exploring metal-assisted self-assembly of tetraarylbiphenyls.

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ASSOCIATED CONTENT

S Supporting Information *

The 1H and 13C NMR spectral reproductions of the host TB, 1 H NMR spectra for the inclusion compounds of TB dissolved in CDCl3, crystal data for TB-GF, and TGA profiles for the inclusion compounds of TB with different guest molecules. This material is available free of charge via the Internet at http://pubs.acs.org.

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EXPERIMENTAL SECTION General Aspects. Column chromatography was conducted with silica gel of 100−200 μ particle size. 1H and 13C NMR spectra were recorded on JEOL (400 and 500 MHz) spectrometers in CDCl3 as a solvent. Chemical shifts are reported in δ scale. The ESI mass spectra were recorded on Waters QTOF machine. IR spectra were recorded on a Bruker Vector 22 FT-IR spectrophotometer. Melting points were determined with a Perfit melting point apparatus. The TGA measurements were carried out using Mettler-Toledo instrument at a heating rate of 10 °C/min under nitrogen atmosphere. The crystal data were collected at 100 K on a Bruker Nonius SMART APEX CCD detector system. Synthesis of Host TB. The precursor 3,3′,5,5′-tetrabromobiphenyl was synthesized from 1,3,5-tribromobenzene by following the literature reported procedure.46 It was later subjected to Pd(0)-catalyzed Suzuki coupling with 2,6dimethyl-4-methoxyphenylboronic acid. For the coupling, an oven-dried two-necked round-bottom flask was charged with tetrabromobiphenyl (0.6 g, 1.3 mmol), 2,6-dimethyl-4-methoxyphenylboronic acid (1.83 g, 10.2 mmol), Pd(PPh3)4 (0.30 g, 20 mol %), and NaOH (0.61 g, 15.3 mmol) under N2 atmosphere. The contents were dissolved in 18 mL of dioxane-EtOH-H2O mixture (3:2:1) and heated at reflux for 48 h. After completion of the reaction, as monitored by thinlayer chromatography, the contents were evaporated to dryness in vacuo and the residue extracted with CHCl3. The organic phase was dried over anhyd. Na2SO4 and concentrated in vacuo to obtain the crude product. Silica gel column chromatography was performed using CHCl3/pet. ether (10%) as an eluent to obtain pure product as a colorless solid, yield (0.66 g, 75%); mp 180−182 °C; IR (film) cm−1 2951, 2918, 2836, 2248, 1606, 1486, 1315, 1153; 1H NMR (CDCl3, 500 MHz) δ 2.10 (s, 24H), 3.81 (s, 12H), 6.67 (s, 8H), 6.89 (s, J = 2.0 Hz, 2H), 7.38 (d, J = 2.0 Hz, 4H); 13C NMR (CDCl3, 125 MHz) δ 21.3, 55.1, 112.5, 126.3, 129.9, 134.0, 137.3, 140.8, 141.4, 158.2; EI-MS+ m/z Calcd for C48H51O4 691.3787 [M + H]+, found 691.3788.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.N.M. is thankful to SERB for generous research funding. G.S. is grateful to IITK, Kanpur for Institute Postdoctoral fellowship. I.N. and A.B. gratefully acknowledge CSIR for senior research fellowships.

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REFERENCES

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DOI: 10.1021/cg501711t Cryst. Growth Des. XXXX, XXX, XXX−XXX

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DOI: 10.1021/cg501711t Cryst. Growth Des. XXXX, XXX, XXX−XXX