Pseudopolymorphism of a Highly Adaptable ... - ACS Publications

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Pseudopolymorphism of a Highly Adaptable Tetraarylpyrene Host that Exhibits Abundant Solid-State Guest Inclusion Published as part of the Crystal Growth & Design virtual special issue In Honor of Prof. G. R. Desiraju Palani Natarajan,† Alankriti Bajpai,† 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: Tetraarylpyrene host 1,3,6,8-tetrakis(2,6-dimethyl-4-methoxyphenyl)pyrene (TP) is found to crystallize with inclusion of diverse guests in two different modifications with simple variation of the crystallization conditions involving addition of a cosolvent. While the guest stoichiometry is found to vary in the pseudopolymorphs with some guests, polymorphic behavior of the host−guest compounds is observed with cyclohexenone and toluene as guests. The crystal structures of the polymorphic inclusion compounds constitute rather rare examples of supramolecular isomerism. The thermogravimetric analyses of the inclusion compounds show that the guest in one of the modifications is released in a relatively higher temperature range. The structural diversity exhibited by the host TP with various guest molecules and its ability to exhibit different forms with the same guest has been to attributed its unique structural features that inherently confer the system with guest inclusion behavior, conformational freedom associated with the methoxy groups and the near-orthogonal p-anisyl rings, and its potential to exploit weak C−H···O and C−H···π hydrogen bonds in response to the guest.

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INTRODUCTION The area of crystal engineering has advanced exponentially in the past few years. A better understanding of intermolecular interactions has led to a myriad of factors governing solid-state structure and topology to be unraveled. The crystal structure prediction1 for a given compound still remains a far-fetched dream, despite the little success achieved via terrific advancements in computations.2 Evolution of the field of crystal engineering has had a great bearing on the development of drugs and their formulations in pharmaceutical industry.3 Different crystal forms of the same chemical speciestermed polymorphs4exhibit differences in bioavailability, solubility, dissolution rate, chemical stability, physical stability, melting point, color, filterability, density, flow behavior, etc.5 Thus, different polymorphs of the same drug can have different physicochemical properties.6 Similarly, a chemical substance may crystallize in different solvated forms; the latter are called “pseudopolymorphs” or “variable solvates”.7,8 Understanding of the factors underlying the phenomenon of pseudopolymorphism is important from several perspectives, which include fundamental relevance to the area of molecular recognition, crystal engineering, design and development of new solid materials and manipulation of solid-state properties. In general, solvents interact with solute/host molecules via a number of interactions and modes, and are retained sometimes as solvents of crystallization to stabilize the lattice. Pseudopolymorphs differ with regard to the nature and stoichiometry of © 2012 American Chemical Society

the solvent molecules present in the crystal lattice. In certain instances, interactions of the solvent with host molecules in more than one way may lead to different crystal forms, in spite of the fact that the chemical components are identical. Accordingly, one may have polymorphstwo or more different crystal forms for a given compoundof the same host−guest compound and/or pseudopolymorphs with the same guest, but in different stoichiometries. In general, the phenomenon of pseudopolymorphism appears to be most frequently observed with polar hosts and polar solvents that are capable of forming at least two hydrogen bonds, i.e., DMF, DMSO, dioxane, etc.8,9 Herein, we report a rather unusual occurrence of two different crystal structures for each of the pseudopolymorphs of a highly adaptable and relatively less polar tetraarylpyrene host TP (Chart 1) with different solvents. Remarkably, polymorphs are observed for at least two host−guest compounds, while the stoichiometries of the guests are found to vary in the pseudopolymorphs of others. Polymorphs of the same host−guest complexes constitute rare instances of supramolecular isomerism10existence of two or more superstructures for the same building blocks. The steric rigidification of the p-anisyl moietiesvia installation of two methyl groups at the meta-positions of the p-anisyl ringsleading to near-orthogonal Received: August 31, 2012 Revised: October 28, 2012 Published: November 1, 2012 6134

dx.doi.org/10.1021/cg3012684 | Cryst. Growth Des. 2012, 12, 6134−6143

Crystal Growth & Design

Article

Chart 1. Molecular Structure of Host TP and a Schematic Drawing That Depicts the Various Domains Inherent to Its Structure

Table 1. The Details of Host:Guest Stoichiometry, Space Group, Guest Accessible Volume (V), Shape of the Crystal, Crystallization Solvent and Code for all Modifications inclusion compound TP·CHCl3 TP·cyclohexenone TP·cyclododecene TP·dicyclopentadiene TP·nitrobenzene TP·toluene

solvent of crystallization

shape

host/guest

code

space group

V (%)

CHCl3 + EtOAc CHCl3 cyclohexenone + CHCl3 cyclohexenone cyclododecene + CHCl3 cyclododecene dicyclopentadiene + CHCl3 dicyclopentadiene nitrobenzene + CHCl3 nitrobenzene toluene + CHCl3 toluene

needles columnar chunks needles columnar chunks plates blocks plates columnar chunks flakes cubes tabular blocks

1:2 1:1.5 1:2 1:2 1:2.5 1:2 1:1 1:3 2:1 1:2 1:2 1:2

TPCH1 TPCH2 TPHE1 TPHE2 TPCD1 TPCD2 TPDP1 TPDP2 TPNB1 TPNB2 TPTO1 TPTO2

P21/c C2/c P21/c C2/c P1̅ P21/c P1̅ P2/n C2/c P21/c Pac21 P21/c

22 21 25 22 52 41 21 49 8 29 29 34

the host TP (ca. 0.03 g) was dissolved in 5 mL of guest/solvent and 0.1−0.2 mL of CHCl3/EtOAc was introduced. The resultant solution was allowed to evaporate slowly over a period of 7−10 days. This procedure afforded the crystals of TPHE1, TPCD1, TPDP1, TPNB1 and TPTO1 in 60−80% yields. An analogous crystallization in the presence of 0.1−0.2 mL of EtOAc gave crystals of TPCH1. The crystals of the inclusion compounds were found to be stable enough at room temperature in most cases to permit host/guest ratios and stabilities of the inclusion compounds to be established from thermogravimetric analyses (TGA).13 Further, the crystals in all cases were found to be amenable for X-ray crystal structure determinations. In Table 1 are consolidated host/guest stoichiometry, space group, guest accessible volume, shape of the crystal, crystallization solvent and code for each of the inclusion compounds. The details of crystal data as obtained from X-ray structure determinations are given in Table 2. X-ray Crystal Structures of the Pseudopolymorphs. As mentioned earlier, the crystal structures were determined for all the modifications. The pseudopolymorphs of TP with chloroform, i.e., TPCH1 and TPCH2, were found to exhibit crystal packing equivalence with those of TP with cyclohexenone, i.e., TPHE1 and TPHE2, respectively; notice the space group equivalence in Table 1. Also, the crystals of TPCD1 and TPTO1 were found to be similar in packing to those of TPDP2 and TPTO2, respectively. In the following are discussed the crystal structures and differences between their packing modes followed by the origin of the observed phenomenon. TP−Chloroform Pseudopolymorphs. Crystallization of TP in neat chloroform yielded colorless crystals with the space group C2/c (TPCH2), while further addition of ethyl acetate (ca. 2−4%) gave crystals with the space group P21/c (TPCH1). The crystals from CHCl3 appeared like columnar chunks, while those from CHCl3-EtOAc appeared needle-shaped. In TPCH1, the host crystallizes with two distinct molecules ‘A’ and ‘B’ lying on the special positions in the asymmetric unit together with two

orientation of the aryl rings with respect to the central pyrene core uniquely confers the host TP with three different domains, viz., trough, concave and basin, for selective guest inclusion. These topological features in conjunction with four methoxy groups that are flexible seemingly impart the right attributes to the host framework to permit inclusion of guests with different host/guest ratios as well as crystal packing patterns.

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RESULTS AND DISCUSSION Synthesis of 1,3,6,8-Tetrakis(2,6-dimethyl-4methoxyphenyl)pyrene (TP) and Preparation of Inclusion Compounds. 1,3,6,8-Tetrakis(2,6-dimethyl-4-methoxyphenyl)pyrene (TP) was synthesized starting from 1,3,6,8tetrabromopyrene and 2,6-dimethyl-4-methoxyphenylboronic acid via 4-fold Suzuki coupling reaction using Pd(PPh3)4 as a catalyst; the detailed procedure has been reported previously by us in the context of its application in organic light emitting diodes (OLEDs).11 The pseudopolymorphic modifications of the host TP were obtained by changing the solvent system used for crystallization. From several crystallization experiments, the host TP was found to form two morphologically different types of crystals with each of the guests, viz., CHCl3 (CH), cyclohexenone (HE), dicyclopentadiene (DP), cyclododecene (CD), nitrobenzene (NB) and toluene (TO), cf. Table 1. In all cases, one of the modifications, i.e., TPCH2, TPHE2 (Refcode: 226978), TPCD2 (Refcode: 726980), TPDP2 (Refcode: 726981) TPNB2 (Refcode: 726019) and TPTO2 (Refcode: 726983), was obtained upon crystallization of TP in the neat solvent; with the exception of the structure of TPCH2, the structures of all others have previously been reported elsewhere.12 The crystals of the second modification with each solvent, i.e., TPCH1, TPHE1, TPCD1, TPDP1, TPNB1 and TPTO1, were obtained upon addition of a cosolventeither EtOAc or CHCl3to the solution of TP in the corresponding solvent. In a typical crystallization experiment, 6135



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Table 2. Crystal Data for All the InclusionCompounds of TP identification code empirical formula formula weight (amu) temperature (K) wavelength (λ, Å) 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) theta range (deg) scan type reflections collected independent reflections refinement method data/restraints/ parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole (e·Å−3)

TPCH1

TPCH2

TPHE1

C54H52Cl6O4 977.66 100(2) 0.71073 monoclinic P21/c (No. 14) 17.1906(13) 11.4994(9) 24.8540(19) 90.00 91.5550(10) 90.00 4911.4(7) 4 1.322

C107H103Cl9O8 1835.94 100(2) 0.71073 monoclinic C2/c (No. 15) 20.1999(13) 21.9548(13) 22.6618(13) 90.00 107.967(2) 90.00 9560.1(10) 4 1.276

C32H33O3 465.58 100(2) 0.71073 monoclinic P21/c (No. 14) 12.793(2) 16.993(3) 11.506(2) 90.00 97.648(3) 90.00 2478.9(8) 4 1.248

0.395

0.320

2040 2.29−27.69 2θ−θ 24381 8555 [R(int) = 0.0394] full-matrix leastsquareson F2 8555/2/597 1.023 R1 = 0.0581, wR2 = 0.1383 R1 = 0.0717, wR2 =0.1464 0.813 and −0.498

TPDP1

TPNB1

TPTO1

C82H105O8 1085.28 100(2) 0.71073 triclinic P1̅ (No. 1) 11.858(2) 16.972(3) 19.789(3) 95.462(4) 103.095(3) 110.062(3) 3578.0(10) 2 1.072

C31H31O2 435.56 100(2) 0.71073 monoclinic P2/n (No. 13) 6.801(3) 12.486(6) 14.756(7) 104.804(9) 93.331(9) 97.280(9) 1196.4(10) 2 1.209

C110H105NO10 1600.95 100(2) 0.71073 monoclinic C2/c (No. 15) 40.570(6) 15.752(2) 13.5431(17) 90.00 97.797(5) 90.00 8575.0(2) 4 1.240

C66H66O4 923.19 100(2) 0.71073 monoclinic Pca21 (No. 29) 14.787(3) 15.341(3) 22.729(5) 90.00 90.00 90.00 5156.1(18) 4 1.189

0.078

0.064

0.074

0.078

0.072

3848 2.61−22.62 2θ−θ 24532 8356[R(int) = 0.0574] full-matrix leastsquareson F2 8356/6/605

996 2.15−25.00 2θ−θ 12284 4284 [R(int) = 0.0500] full-matrix leastsquareson F2 4284/64/367

1258 2.28−25.00 2θ−θ 18720 12365 [R(int) = 0.0587] full-matrix leastsquares on F2 12365/144/938




1.013 R1 = 0.0584, wR2 = 0.1331 R1 = 0.0912, wR2 = 0.1488 0.346 and −0.250

1.061 R1 = 0.0918, wR2 = 0.2449 R1 = 0.1856, wR2 = 0.2714 0.604 and −0.476

1.040 R1 = 0.0882, wR2 = 0.1078 R1 = 0.2744, wR2 =0.1352 0.188 and −0.195

1.049 R1 = 0.0590, wR2 = 0.1430 R1 = 0.0808, wR2 = 0.1584 0.316 and −0.217

1.050 R1 = 0.0681, wR2 = 0.1369 R1 = 0.1367, wR2 = 0.1586 0.357 and −0.286

1.014 R1 = 0.0824, wR2 = 0.1265 R1 = 0.0646, wR2 = 0.1440 0.423 and −0.258

chloroform guest molecules. Each of the two host molecules ‘A’ and ‘B’ show the same conformation with respect to the orientation of the methyl groups of the methoxy moieties (Figure 1). The pyrene cores of hosts ‘A’ and ‘B’ run along the a-axis as strands and alternate along the c-axis. The central flat pyrene rings of ‘A’ and ‘B’ are inclined at an angle (ca. 107.5°) with respect to each other such that they enclose voids that run down the b-axis; the voids are filled by chloroform guests. In each of the host molecules, concave and trough domains are occupied by the neighboring host molecules, while the basin region is occupied by the chloroform guest that undergoes C−H···π interactions with the pyrene core. While one of the crystallographically dissimilar host molecules occupies the concave region of the neighboring host, the other host molecule occupies its trough region. As a result, the two crystallographically inequivalent hosts orient in a trough-to-concave fashion. The host TP molecules are found to be associated via C−H···O and C−H···π intermolecular hydrogen bonds in the lattice. While C−H···O hydrogen bonds are formed with the methoxy groups, the central pyrene moiety is found to exhibit C−H···π hydrogen bonds with the methyl group of the methoxy moiety (Table 3). As revealed by PLATON, approximately 26% of the lattice porosity is filled by the included guest molecules. In TPCH2, the asymmetric unit cell is found to contain two host molecules in special positions with one guest chloroform in the general position and the other exhibiting disorder about the inversion center such that the host/guest ratio is 1:1.5. The

TPCD1

crystal structure is characterized by 2-D layers in the ac-plane in which the crystallographically independent molecules are organized along the c-axis via trough-to-trough assembly through C−H···O and C−H···π hydrogen bonds. These one-dimensional strands are further associated with the adjacent ones such that the p-anisyl moieties fall on the aromatic expanse of pyrene core. Thus, the concave and basin regions are occupied by chloroform guests. The guest molecules are found to be disordered owing presumably to a loose fit of the guests within the available cavities. TP−Cyclohexenone Pseudopolymorphs. The two modifications of the inclusion compounds of TP with cyclohexenone, i.e., TPHE1 and TPHE2, are found to exhibit crystal packings similar to those of TPCH1 and TPCH2, respectively; indeed, their space groups are identical. The typical crystal packings for the two modifications are shown in Figure 2. TP−Cyclododecene Pseudopolymorphs. Two morphologically distinct crystals were obtained when the host TP was crystallized from cyclododecene/CHCl3 solution (plates, TPCD1) and from neat cyclododecene (thick blocks, TPCD2). The X-ray structure determinations revealed that the plates and blocks were found to belong to triclinic (P1̅) and monoclinic (P21/c) crystal systems, respectively (Table 2). In TPCD1, the asymmetric unit cell was found to contain two halfcomponents of the host and three disordered guest molecules (cyclododecene) in the host/guest ratio 1:2.5. The two host molecules (A and B) and one cyclododecene guest were found to 6136



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Figure 1. The crystal packing diagram of TPCH1 showing 1-D columns running down the b-axis (a). The crystal packing of TPCH2 (b).

lie on the crystallographic inversion centers. Orientation of molecules in a trough-to-trough and concave-to-basin manner leads to cavities that are filled with the cyclododecene guests. In other words, the basin and concave regions are found to be occupied by cyclododecene molecules (Figure 3). Since the two crystallographically dissimilar host molecules are oriented orthogonal to each other, channels that enclose cyclododecene run along both a- and c-axes. The entire molecular assembly is stabilized by C−H···O and C−H···π hydrogen bonds between dimethyl-p-anisyl rings and pyrene moieties akin to the case of TPCH1 (Table 3). The crystals of the monoclinic modification, i.e., TPCD2, were found to contain one molecule each of host (lying on the special position) and cyclododecene (in the general position) in the asymmetric unit. The guest molecule is found to be located between the trough and concave domains of the glide symmetry related molecules as shown in Figure 3. This structure has been described already elsewhere.12 TP−Dicyclopentadiene Pseudopolymorphs. Crystallization of TP from a mixture of dicyclopentadiene and CHCl3 led to triclinic (P1̅) crystals (TPDP1) containing the dicyclopentadiene guest in a host/guest ratio of 1:1. In contrast, crystallization of TP in neat dicyclopentadiene led to crystals that contained three equivalents of dicyclopentadiene and belonged to the monoclinic (P2/n) crystal system (TPDP2). In the triclinic modification of TPDP1, both host as well as guest were found to occupy special positions such that only halfcomponents were found in the asymmetric unit. The crystal packing analysis shows that the concave regions are occupied by dicyclopentadiene guests, while the basin and trough regions are occupied by neighboring host molecules via C−H···O hydrogen bonds (Figure 4); one observes 2-D layers along the bc-plane down the a-axis. The host molecules that stack down the a-axis assemble in a concave-to-concave manner with the adjacent neighbors along the c-axis leading to enclosures for guest location down the a-axis as shown in Figure 4. The host molecules are found to be organized along the b-axis through C−H···O hydrogen bonds. The crystal structure of the monoclinc TPDP2 modification has previously been reported;12 the asymmetric unit cell contains one host molecule lying in the special position with overall three guest molecules such that the host/guest ratio is 1:3. In this modification, the symmetry-related orthogonally oriented molecules organize by interjecting their trough regions into each other. The concave-to-basin union of these strands leads to voids in which the guests occupy both basin and concave domains.

Table 3. Important C−H···O and C−H···π Hydrogen Bonds Observed between the Host Molecules in the Pseudopolymorphs of TP with Various Guestsa compound TPCH1

TPCH2

TPHE1

TPCD1

TPDP1

TPNB1

TPTO1

hydrogen bonds

d/Å

θ/°

C52−H···O2 Me C17−H···O3 Me C43−H···π Me C26−H···π Ar C22−H···O3 Me C43−H···O3 Me C43−H···O5 Me C52−H···O4 Ar C5−H···π Me C15−H···π Ar C20−H···π Me C25−H···π Ar C2−H···O1 Me C26−H···O1 Ar C11−H···π Me C25−H···O2 Me C52−H···O3 Ar C4−H···π Ar C11−H···O1 Me C17−H···O2 Me C17−H···π Ar C24−H···π Me C25−H···π Me C17−H···O2 Me C26−H···O4 Me C25−H···π Me C25−H···O2 Me C25−H···O4 Me C34−H···O1 Me C43−H···O4 Me C52−H···O3 Ar C2−H···π Me C34−H···π Me C52−H···π

2.78 2.63 2.81−2.92 2.78−3.00 2.50 2.80 2.77 2.69 2.87 2.89−2.92 3.01 2.79−2.89 2.51 2.80 2.94 2.56 2.67 2.89 2.59 2.71 2.99−2.99 2.81 2.85−2.87 2.51 2.60 2.85−2.87 2.71 2.67 2.65 2.67 2.58 2.01 2.84−2.86 2.88−2.91

120.78 109.67

Me

168.31 133.91 136.29 144.03

170.74 133.27 160.47 109.41 166.21 101.31

141.89 127.85 115.57 106.60 134.68 113.94 133.80

a

The interactions between host and guest are not included due to disorder of the guests.

TP−Nitrobenzene Pseudopolymorphs. As mentioned at the outset, the crystals of the two different modifications were 6137



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Figure 2. The crystal packings of TPHE1 (P21/c) (a) and TPHE2 (C2/c) (b) modifications.

Figure 3. The crystal packing diagrams of TPCD1 (a) and TPCD2 (b). It should be noted that the guest cyclododecene molecules in the former are disordered.

Figure 4. The crystal packing diagrams of triclinic TPDP1 (a) and monoclinic TPDP2 (b) modifications.

columns of host molecules ‘A’ and ‘B’ propagates in the ac-plane, while the molecules of kind ‘A’ are connected along the b-axis by two C−H···O hydrogen bonds. The disordered nitrobenzene guests are found to be located in the basin regions of the host with their pyrene platforms in the ac-plane. Approximately 8% of the volume in the crystals is found to be occupied by the guests that are disordered; the refinement was accomplished by associating partial occupancies to the disorder atoms. The crystal structure analysis of the modification TPNB2 (cubic blocks) grown from neat nitrobenzene (space group: P21/c) revealed the host/guest ratio to be 1:2; this structure has previously been reported and the typical packing is shown in Figure 5. As can

obtained when the host TP was crystallized from neat nitrobenzene and from CHCl3−nitrobenzene mixture. The crystals of the modification obtained from CHCl3−nitrobenzene, i.e., TPNB1 (thin flakes), were found to belong to the monoclinic crystal system with the space group C2/c. In this modification, the asymmetric unit cell was found to contain two half-components of host molecules ‘A’ and ‘B’ and one nitrobenzene guest molecule with half occupancy about the inversion center such that the stoichiometry between host/guest is 2:1. One finds in the crystal lattice that the molecules along the a-axis are organized in a manner that the troughs of host ‘A’ interject into those of host ‘B’ molecules as shown in Figure 5. The alternate arrangement of the 6138



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Figure 5. The crystal packing diagrams for the modifications of TP·nitrobenzene, TPNB1 (a) and TPNB2 (b).

Figure 6. The crystal packing diagrams of TPTO1 (a) and TPTO2 (b).

be seen, the host molecules propagate along the b-axis as strands. These strands are alternately displaced along the c-axis by half of the b-axis dimension such that one observes C−H···π interactions between the methyl hydrogens of methoxy group and the central pyrene rings of the adjacent molecules. Within each strand, the neighboring molecules are connected via weak C−H···O hydrogen bonds (Table 3). A careful inspection reveals a rather novel association of the four symmetry-generated molecules in which the methyl hydrogens of methoxy group of the host fall on the central pyrene rings of the adjacent ones leading to an enclosure down a-axis as shown in Figure 5. The enclosure thus is made up of both concave and trough regions of the hosts, and the guest nitrobenzene molecules within these cavities are found to be disordered. TP−Toluene Pseudopolymorphs. Two morphologically distinct crystals were obtained when the host TP was crystallized from its toluene solution containing CHCl3 (TPTO1) and from neat toluene (TPTO2). While the crystals were found to be tabular in shape when grown from chloroform as an additive, those grown from neat toluene were found to be blocks. The X-ray crystallography revealed that the tabular crystals and blocks belong to orthorhombic (Pca21, TPTO1) and monoclinic (P21/c, TPTO2) crystal systems, respectively. In TPTO1 modification, the asymmetric unit cell was found to contain one host TP and two guest toluenes in their general positions with a host/ guest ratio of 1:2. The asymmetric unit in the crystals of the orthorhombic modification TPTO2 was found to contain two half-components of host TP molecules ‘A’ and ‘B’ with two toluene guests such that the stoichiometry between host/guest is 1:2 as in the crystals of TPTO1 (Table 1).

In TPTO1, one observes a layered structure with the layers in the ab-plane stacked along the c-axis as shown in Figure 6. As in the case of TPNB2, association of four symmetry-generated molecules in which the methyl hydrogens of methoxy groups fall on the central pyrene rings of the adjacent host molecules leads to an enclosure down the c-axis (Figure 6). Further, the neighboring molecules are found to be connected via weak C−H···O hydrogen bonds within each layer. The enclosure thus is made up of both concave and trough regions of the hosts. The crystal packing of TPTO2 is found to be similar to that of TPTO1 (Figure 6) in spite of the fact that the space groups are different. This structure has been reported already elsewhere.12 Thermal Properties of the Pseudopolymorphs of Host TP. Investigations on thermal decomposition behavior of clathrates have been found to be useful for the detection of new polymorphic and pseudopolymorphic phases of certain hosts. Such investigations shed light on the mechanisms of clathrate formation.14 We have performed TGA analyses for all pseudopolymorphs of the host TP with different guests under nitrogen gas atmosphere. The TGA profiles for all the new compounds described herein, i.e., TPCH1, TPCH2, TPHE1, TPCD1, TPDP1, TPNB1 and TPTO1, are shown in Figure 7. The mass losses in the TGA profiles of all the compounds correspond to the guest molecules, and are in reasonable agreement with the host/guest ratios observed from X-ray structure determinations.13 We have examined the temperatures at which the included guests are released (desolvation temperatures) in all modifications from thermogravimmetric analysis, cf. Table 4. The temperature range within which the guests are desorbed from crystals (Te − Tb, Table 4) reflects the influence of host matrix on the guest binding and release. With the exception of toluene as the guest, 6139



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Figure 7. The TGA plots for the inclusion compounds of TP with various guest molecules.

strate formation of ternary inclusion compounds with both aliphatic and aromatic guests.15 In general, compounds containing functional groups that effectively involve directional N/O−H···N/O hydrogen bonds have been found to be sensitive to crystallization conditions.16 The crystallization of host molecules bearing polar protic functional group/s under different conditions (temperature, solvent, pressure, etc.) yields pseudopolymorphic compounds either with same guest in different ratios or with more than one guest. In contrast, for a variety of hydrophobic hosts 1−7 shown in Chart 2, pseudopolymorphic behavior with the same guest in different stoichiometric ratios is scarce. For example, the well-known 9,9′bianthryl host (7) yields inclusion crystals with cyclohexane and pyridine guest in 1:1 stoichiometry invariably in spite of changing the crystallization conditions drastically.17 In this regard, the inclusion behavior observed in the present investigation for the host TP that does not possess polar protic functional groups is remarkable. Notably, the crystallization of TP in neat solvent/ guest leads to a higher-symmetry modification with aliphatic guests (TPCH2, TPHE2, TPCD2 and TPDP2) and a lowersymmetry modification with aromatic guests (TPNB2 and TPTO2). Addition of a cosolvent CHCl3/EtOAc (2−5%) results in the second modification for all the pseudopolymorphs. In particular, one observes two polymorphs for each of the inclusion compounds of TP with cyclohexenone and toluene.18 Two or more crystal structures for the same host−guest compounda supramoleculeconstitute isomeric superstructures such that the observed phenomenon may be termed ‘supramolecular isomerism’.10 What causes a given host−guest system to explore two or more proximate lattice energies in the process of crystallization? Why does the same host system crystallize with the same guest in different stoichiometries? A more basic and logical question still: how and when does the host exhibit a tendency to bind one or two guest species? Answers to these questions appear to be traceable to structural attributes of the host TP, which is characterized by orthogonal aromatic planes; it is noteworthy that Weber and Bishop have extensively investigated the inclusion behavior of a large number of host systems typified by aromatic planes.19 Insofar as the intermolecular interactions are concerned, the host TP is devoid of any strongly interacting functional groups that may decisively control the crystal packing. A careful crystal packing analysis suggests that all the inclusion compounds are stabilized by plenty of

Table 4. The Guest Desolvation Temperatures (from TGA Analysis) for all Pseudopolymorphic Modifications of Host TP with Different Guest Molecules desolvation temperature (°C) compound TPCH1 TPCH2 TPHE1 TPHE2 TPCD1 TPCD2 TPDP1 TPDP2 TPNB1 TPNB2 TPTO1 TPTO2

guest

bp (°C)

CHCl3

61−63

cyclohexenone

171−173

cyclododecene

232−245

dicyclopentadiene

169−171

nitrobenzene

210−211

toluene

110−111

begin (Tb)

end (Te)

Te−bp (°C)

42 48 109 112 165 167 117 124 154 151 47 48

82 94 201 214 299 313 186 203 237 223 138 140

20 32 29 42 44 58 16 33 26 12 27 29

one observes that the guest in one of the two modifications of the inclusion compounds of TP is released in a higher temperature range. For example, the cyclohexenone guest is released in a higher temperature range (ca. 12 °C) in TPHE2 modification than in TPHE1. It also emerges that both modifications of TP with toluene exhibit similar desorptions in agreement with their crystal packing equivalence. It should be noted that the second weight loss beyond 400 °C corresponds to the guest-free apohost decomposition. Plausible Basis of Pseudopolymorphism. Overall, the host TP shows two types of structural organizations with each guest/solvent molecule used for crystallization. In Figure 8 are consolidated the observed crystal packing patterns schematically. In all crystal lattices, 2-D strands of molecules are observed with the crystal packing stabilized by weak C−H···O and C−H···π hydrogen bonds. Depending on the size and nature of the aliphatic guest, the concave and basin regions of host TP are found to be engaged. In contrast, aromatic guests are found to occupy trough regions selectively via C−H···π hydrogen bonds. It is worth mentioning that this noted preference for inclusion of the aromatic guests in the trough regions has earlier been exploited to demon6140



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Figure 8. A schematic diagram showing different modes of packing observed in the polymorphs and pseudopolymorphs of host TP.

weak C−H···O and C−H···π hydrogen bonds between host−host and host−guest pairs (Table 3). In all these cases, the stacking between central flat pyrene rings is precluded due to steric hindrance provided by the four orthogonally oriented dimethylaryl rings. We believe that the observed structural diversity of host TP with guests is a consequence of (i) slight yet meaningful conformational freedom of the dimethylaryl rings installed orthogonally to the central pyrene unit (the angles between the p-anisyl rings and the central pyrene core vary in the range of 72−90°; see Table 5 and ref 12), (ii) multiple weak intermolecular interactions such as C−H···O and C−H···π hydrogen bonds that are involved in the lattice stabilization, (iii) molecular topology that permits three different domains for guest accommodation and (iv) freedom associated with methoxy functionalities. As mentioned earlier, experimental variables such as temperature,20 crystallization conditions involving slow (thermodynamic) and fast (kinetic) evaporation of solutions,21 pressure,22 additives and cosolvent23 have been reported to lead to the formation of pseudopolymorphs. In the present investigation, addition of a cosolvent brings about alternative pseudopolymorph/polymorph. Clearly, variation of the stoichiometry of the included guest with respect to the host depending on the solvent

employed and alternative crystal packing in the case of cyclohexenone and toluene guests emphasize the importance of differences in weak solute−solvent interactions in nucleation and crystal growth. While solvent-induced protein denaturation,24 solvent-dependent growth of MOFs,25 solvent-mediated supramolecular organizations,26 etc. are well-known, the inclusion phenomenon of a sensitive host such as TP, which responds to subtle variations in solvents, may offer invaluable insights concerning the influence of solvent forces on the development of nucleithe embryos of crystal structures27at the onset of crystallization.

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CONCLUSIONS The host TP has been found to crystallize with different solvents in two different modifications. It is shown that simple variation of the crystallization conditions involving addition of a cosolvent leads to alternative modification of the host−guest compound. While the guest stoichiometry is found to vary in the pseudopolymorphs with some guests, very rare polymorphic behavior of the host−guest compounds is observed with cyclohexenone and toluene guests. The crystal structures of these inclusion compounds constitute rather rare examples of supramolecular isomerism. The TGA analyses show that the guest in one of the 6141



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Chart 2. Structures of Well-Known Hydrophobic Host Molecules

correction was made using Bruker SADABS. The structure was solved in each case by Direct Methods using SHELXL package and refined by full matrix least-squares method based on F2 using SHELX-97 program. The experimental details of crystal data, intensity measurements, structure solution and refinements are presented in Table 2. Refinement of Disordered Guest Molecules. The evidence for disordered guest molecules was quite apparent from difference Fourier maps. The residual peaks which corresponded to guest molecules in different orientations were included in the refinement with partial occupancies. To determine the occupancy factors, the isotropic thermal parameters were kept at fixed values while an overall occupancy factor was refined for each partial molecule. When the R index improved significantly, the model was accepted. Otherwise, the same procedure was repeated until a satisfactory solution was obtained. After convergence, the occupancy factors were kept constant, while the atomic thermal parameters were allowed to vary individually.

Table 5. Calculated Angles (°) between the Aryl Rings and the Central Pyrene Ring in the Structures of TP with all Guest Molecules compound TPCH1 TPCH2 TPHE1 TPHE2 TPCD1 TPCD2

angle (deg) 86.12 68.45 84.89 84.91 76.61 79.80 80.03 80.06 80.77 89.89

82.80 89.86 81.05 86.78 71.54 84.48 80.88 82.29 86.96 89.98

compound

angle (deg)

TPDP1

87.18

72.33

TPDP2

88.78

88.67

TPNB1 TPNB2

86.57 89.44

88.47 86.63

TPTO1

82.96 79.07 78.74 83.55

76.25 88.91 77.11 87.63

TPTO2

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S Supporting Information *

modifications is released in a higher temperature range. From the crystal packing analyses, the structural diversity exhibited by the host TP with guest molecules has been attributed to (i) its unique structural features that inherently confer the system with guest inclusion behavior, (ii) conformational freedom associated with the methoxy groups and the significant flexibility of the near-orthogonal p-anisyl rings, and (iii) its potential to exploit weak C−H···O and C−H···π hydrogen bonds in response to the guest.

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ASSOCIATED CONTENT

Crystallographic information file. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.V.); [email protected] ( J.N.M.).

EXPERIMENTAL SECTION

Notes

The authors declare no competing financial interest.

X-ray Crystal Structures of the Pseudopolymorphs. In each case, a good quality crystal was mounted over a glass fiber, 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 = 0.7107 Å) and a highly oriented graphite monochromator operating at 50 kV and 30 mA. The data were collected in a hemisphere mode and processed with Bruker SAINTPLUS. Empirical absorption

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ACKNOWLEDGMENTS J.N.M. is thankful to DST (SERB, Science and Engineering Research Board), New Delhi for financial support. A.B. is grateful to CSIR for a senior research fellowship. 6142



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Article

Su, H. J. Phys. Org. Chem. 2000, 13, 368 and references therein. (d) Ibragimov, B. T., Talipov, S. A. In Encyclopedia of Supramolecular Chemistry; Atwood, J. L., Steed, J. W., Eds.; Marcel Dekker: New York, 2004; p 606. (21) Sumarna, O.; Seidel, J.; Weber, E.; Seichter, W.; Ibragimov, B. T.; Beketov, K. M. Cryst. Growth Des. 2003, 3, 541. (22) For influence of pressure on pseudopolymorphs formation, see: (a) Fabbiani, F. P. A.; Allan, D. R.; Dawson, A. D.; David, W. I. F.; McGregor, P. A.; Oswald, I. D. H.; Parsons, S. Chem. Commun. 2003, 24, 3004. (b) Boldyreva, E. Cryst. Growth Des. 2007, 7, 1662. (c) Oswald, I. D. H.; Pulham, C. R. CrystEngComm 2008, 10, 1114. (23) Pseudopolymorphs formation with additives/co-solvent, see: Nakano, K.; Sada, K.; Miyata, M. J. Chem. Soc., Chem. Commun. 1996, 989. (24) (a) Akdogan, Y.; Hinderberger, D. J. Phys. Chem. B 2011, 115, 15422 and references therein. (b) Li, W.; Qin, M.; Tie, Z.; Wang, W. Phys. Rev. E 2011, 84, 041933. (25) (a) Tynan, E.; Jensen, P.; Kruger, P. E.; Lees, A. C. Chem. Commun. 2004, 776. (b) Du, M.; Zhao, X. J.; Guo, J. H.; Batten, S. R. Chem. Commun. 2005, 4836. (c) Hill, D. J.; Moore, J. S. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 5053. (d) Baxter, P. N. W.; Khoury, R. G.; Lehn, J.M.; Baum, G.; Fenske, D. Chem.−Eur. J. 2000, 6, 4140. (e) Khatua, S.; Harada, T.; Kuroda, R.; Bhattacharjee, M. Chem. Commun. 2007, 3927. (f) Mamula, O.; Lama, M.; Stoeckli-Evans, H.; Shova, S. Angew. Chem., Int. Ed. 2006, 45, 4940. (26) (a) Huang, X. C.; Zhang, J. P.; Lin, Y. Y.; Chen, X. M. Chem. Commun. 2005, 2232. (b) Gale, P. A.; Light, M. E.; Quesada, R. Chem. Commun. 2005, 5864. (c) Cockroft, S. L.; Hunter, C. A. Chem. Commun. 2006, 3806. (d) Cook, J. L.; Hunter, C. A.; Low, C. M. R.; Perez-Velasco, A.; Vinter, J. G. Angew. Chem., Int. Ed. 2007, 46, 3706. (e) Schnatwinkel, B.; Stoll, I.; Mix, A.; Rekharsky, M. V.; Borovkov, V. V.; Inoue, Y.; Mattay, J. Chem. Commun. 2008, 3873. (f) Das, D.; Barbour, L. J. Chem. Commun. 2008, 5110. (27) (a) Anthony, A.; Desiraju, G. R. Supramol. Chem. 2001, 13, 11. (b) Jetti, R. K. R.; Boese, R.; Thallapally, P. K.; Desiraju, G. R. Cryst. Growth Des. 2003, 3, 1033.

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