Endo vs Exo Bowl: Complexation of Xanthone by ... - ACS Publications

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Endo vs Exo Bowl: Complexation of Xanthone by Pyrogallol[4]arenes Constance R. Pfeiffer, Drew A. Fowler, and Jerry L. Atwood* Department of Chemistry, University of Missouri, 125 Chemistry Building, 601 South College Avenue, Columbia, Missouri 65211, United States S Supporting Information *

ABSTRACT: Cocrystal systems containing pyrogallol[4]arene, xanthone, and solvent molecules are examined and discussed. Three types of pyrogallol[4]arenes are cocrystallized with xanthone in isopropanol, acetonitrile, dimethyl sulfoxide, and a mixture of isopropanol and methanol. It is found that the solvent controls complexation of the xanthone molecule, whereas pyrogallol[4]arene tail length affects cocrystal packing. With a singlesolvent system, the xanthone molecule is outside (exo) of the bowl of the pyrogallol[4]arene, but with a two-solvent system, the xanthone molecule is inside (endo) of the bowl of the pyrogallol[4]arene.

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INTRODUCTION Calixarenes are a class of macrocycles central to the field of supramolecular chemistry. These versatile compounds have the ability to act as hosts for a variety of types of guest molecules. One application of calixarenes that has been of interest is their use as host molecules in cocrystals with different guest molecules, potentially molecules with fluorescent properties. Pyrogallol[4]arenes are a class of calixarenes that are ideal for cocrystallization applications. They are bowl-shaped molecules containing 12 hydroxyl groups on the upper rim of the bowl and aliphatic tails of a predetermined length on the lower rim. Hydrogen bonding plays a crucial role in the formation of cocrystals containing pyrogallol[4]arenes. The hydroxyl groups allow for intermolecular hydrogen bonding throughout the crystal lattice. Adding guests or altering the aliphatic chain on the pyrogallol[4]arene often changes the overall crystal architecture.1 Along with their ability to participate in hydrogen bonding, pyrogallol[4]arenes are useful for cocrystallizations because of their capability to form various supramolecular structures such as nanocapsules and nanotube assemblies.2 Pyrogallol[4]arenes are also ideal for cocrystallization due to their capacity to act as hosts for small inorganic molecules, drug molecules, and ionic liquids.3−5 Fluorescent probes are advantageous to cocrystallization with pyrogallol[4]arenes due to the fact they can lead to insight into the assembly of supramolecular structures with regard to encapsulation and intermolecular interactions. Additionally, studies can be performed to determine whether fluorescent probe properties change upon cocrystallization from solution. It has been demonstrated that pyrogallol[4]arenes will cocrystallize with several fluorescent probe molecules. Specifically, Chexylpyrogallol[4]arene has been cocrystallized with ADMA and PBA fluorophores in acetonitrile.6 This study is focused on determining the trends that arise from varying the solvent and the pyrogallol[4]arene aliphatic tail group. Eight cocrystals are presented herein: xanthone and Cmethylpyrogallol[4]arene (PgC1) in isopropanol (cocrystal 1), © 2014 American Chemical Society

xanthone and PgC1 in a mixture of isopropanol and methanol (cocrystal 2), xanthone and C-ethylpyrogallol[4]arene (PgC2) in isopropanol (cocrystal 3), xanthone and PgC2 in a mixture of dimethyl sulfoxide and methanol (cocrystal 4), xanthone and PgC2 in a mixture of isopropanol and methanol (cocrystal 5), xanthone and C-propylpyrogallol[4]arene (PgC3) in acetonitrile (cocrystal 6), xanthone and PgC3 in isopropanol (cocrystal 7), and xanthone and PgC3 in a mixture of isopropanol and methanol (cocrystal 8) (Figure 1).

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EXPERIMENTAL SECTION

Reagents and solvents were obtained commercially and used without further purification. PgC1, PgC2, and PgC3 were synthesized using an adapted method previously described by Gerkensmeier et al. that was modified by using acetaldehyde, propionaldehyde, and butyraldehyde, respectively.7 Cocrystal 1 was prepared by mixing PgC1 and xanthone in a 1:1 molar ratio (0.1:0.0286 g) in 20 mL of isopropanol, followed by sonication for 30 min. The solution was permitted to crystallize by slow evaporation. Cocrystal 2 was crystallized in the same manner by mixing PgC1 and xanthone in a 1:3.4 molar ratio (0.05:0.0481 g) in a mixture of 5 mL of methanol and 10 mL of isopropanol. Cocrystal 3 was crystallized by mixing PgC2 and xanthone in a 1:3.9 molar ratio (0.5:0.056 g) in 15 mL of isopropanol. Cocrystals 4 and 5 were cocrystallized by mixing PgC2 and xanthone in a 1:3.8 molar ratio (0.025:0.028 g) in 10 mL of isopropanol and a few drops of dimethyl sulfoxide (cocrystal 4) and in a mixture of 5 mL of methanol and 10 mL of isopropanol (cocrystal 5). Cocrystal 7 was crystallized by mixing PgC3 and xanthone in a 1:4.45 molar ratio (0.1:0.121 g) in 20 mL of isopropanol. The mixture was sonicated for 30 min and allowed to slowly evaporate until crystallization of colorless prisms. Cocrystals 6 and 8 were crystallized with the same method except that the molar ratio was 1:1 (0.1:0.0241 g) and instead of isopropanol, the solvents used were 10 mL of acetonitrile (cocrystal 6) and a mixture of 5 mL of Received: May 30, 2014 Revised: July 14, 2014 Published: July 15, 2014 4205

dx.doi.org/10.1021/cg500794w | Cryst. Growth Des. 2014, 14, 4205−4213

Crystal Growth & Design

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Cocrystal 7. C62.07H83.30O17.35, M = 1107.11, colorless prism, a = 12.799(2) Å, b = 15.169(2) Å, c = 16.067(2) Å, α = 75.896(2)°, β = 73.037(2)°, γ = 87.394(2)°, space group P1̅, V = 2892.7(7) Å3, Z = 2, Dc = 1.271 g/cm3, F000 = 1189, Mo Kα radiation (λ = 0.71073 Å), T = 100 K, 9830 reflections collected. Final ooF = 1.038, R1 = 0.097, wR2 = 0.267; R indices are based on reflections with I (refinement on F2), 856 parameters, 65 restraints. Lp and absorption corrections were applied (μ = 0.092 mm−1) Cocrystal 8. C57H68O16, M = 1009.11, colorless plate, a = 12.7772(8) Å, b = 13.0828(9) Å, c = 15.464(1) Å, α = 97.204(1)°, β = 90.784(1)°, γ = 97.800(1)°, space group P1̅, V = 2539.6(3) Å3, Z = 2, Dc = 1.320 g/cm3, F000 = 1076, Mo Kα radiation (λ = 0.71073 Å), T = 100 K, 11 284 reflections collected. Final GoF = 1.039, R1 = 0.044, wR2 = 0.110; R indices are based on reflections with I (refinement on F2), 679 parameters, no restraints. Lp and absorption corrections were applied (μ = 0.096 mm−1).

Figure 1. Chemical structure of (a) xanthone and (b) schematic structure of pyrogallol[4]arene.

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methanol and 10 mL of isopropanol (cocrystal 8). All crystallizations resulted in colorless prism- or plate-shaped crystals. Single-crystal X-ray data for cocrystal 5 was collected at 173 K on a Bruker Apex II CCD diffractometer using a Cu Kα radiation source (λ = 1.54178 Å). All other cocrystals were collected at 173 K on a Bruker Apex II CCD diffractometer, using a Mo Kα radiation source (λ = 0.71073 Å). Crystallographic Data. Cocrystal 1. C54H64O17, M = 985.09, colorless prism, a = 14.121(6) Å, b = 12.638(5) Å, c = 27.71(1) Å, β = 95.022(5)°, space group P21/n, V = 4927(4) Å3, Z = 4, Dc = 1.328 g/ cm3, F000 = 2096, Mo Kα radiation (λ = 0.71073 Å), T = 100 K, 10 924 reflections collected. Final GoF = 1.068, R1 = 0.086, wR2 = 0.236; R indices are based on reflections with I (refinement on F2), 665 parameters, 0 restraints. Lp and absorption corrections were applied (μ = 0.099 mm−1). Cocrystal 2. C50H56O17, M = 928.95, colorless prism, a = 12.544(1) Å, b = 13.176(1) Å, c = 14.583(1) Å, α = 68.605(1)°, β = 87.655(1)°, γ = 82.533(1)°, space group P1̅, V = 2225.0(3) Å3, Z = 2, Dc = 1.387 g/cm3, F000 = 984, Mo Kα radiation (λ = 0.71073 Å), T = 100 K, 9775 reflections collected. Final GoF = 1.024, R1 = 0.045, wR2 = 0.115; R indices are based on reflections with I (refinement on F2), 642 parameters, no restraints. Lp and absorption corrections were applied (μ = 0.104 mm−1). Cocrystal 3. C54.5H76O17, M = 1003.15, colorless prism, a = 13.195(3) Å, b = 26.798(5) Å, c = 14.844(3) Å, β = 91.929(2)°, space group P21/n, V = 5245.9(18) Å3, Z = 4, Dc = 1.270 g/cm3, F000 = 2156, Mo Kα radiation (λ = 0.71073 Å), T = 100 K, 9955 reflections collected. Final GoF = 1.025, R1 = 0.066, wR2 = 0.172; R indices are based on reflections with I (refinement on F2), 692 parameters, 72 restraints. Lp and absorption corrections were applied (μ = 0.094 mm−1). Cocrystal 4. C52H58O16S, M = 971.04, colorless prism, a = 12.218(1) Å, b = 12.245(1) Å, c = 17.232(2) Å, α = 86.549(1)°, β = 83.574(1)°, γ = 64.197(1)°, space group P1̅, V = 2306.3(4) Å3, Z = 2, Dc = 1.398 g/cm3, F000 = 1028, Mo Kα radiation (λ = 0.71073 Å), T = 100 K, 10 433 reflections collected. Final GoF = 1.047, R1 = 0.039, wR2 = 0.103; R indices are based on reflections with I (refinement on F2), 657 parameters, 6 restraints. Lp and absorption corrections were applied (μ = 0.146 mm−1). Cocrystal 5. C53H60O16, M = 953.01, colorless prism, a = 12.7479(3) Å, b = 13.0603(3) Å, c = 14.1910(4) Å, α = 98.372(1)°, β = 90.522(1)°, γ = 98.042(1)°, space group P1̅, V = 2313.47(10) Å3, Z = 2, Dc = 1.368 g/cm3, F000 = 1012, Cu Kα radiation (λ = 1.54178 Å), T = 100 K, 8227 reflections collected. Final GoF = 1.039, R1 = 0.041, wR2 = 0.111; R indices are based on reflections with I (refinement on F2), 641 parameters, 0 restraints. Lp and absorption corrections were applied (μ = 0.836 mm−1) Cocrystal 6. C57H62O14N2, M = 999.09, colorless plate, a = 12.417(2) Å, b = 12.4314(2) Å, c = 19.473(3) Å, α = 72.312(2)°, β = 83.962(2)°, γ = 60.700(2)°, space group P1̅, V = 2493.9(7) Å3, Z = 2, Dc = 1.330 g/cm3, F000 = 1060, Mo Kα radiation (λ = 0.71073 Å), T = 100 K, 10 928 reflections collected. Final GoF = 1.054, R1 = 0.054, wR2 = 0.136; R indices are based on reflections with I (refinement on F2), 676 parameters, 0 restraints. Lp and absorption corrections were applied (μ = 0.095 mm−1)

RESULTS With regard to the structures that result from the abovementioned crystallizations, the xanthone molecule inside the bowl-shaped cavity of the pyrogallol[4]arene is referred to as endo, whereas that outside the bowl is referred to as exo. Furthermore, when the xanthone molecule is oriented with the carbonyl group of the xanthone molecule pointing in the same (or opposite) direction as the hydroxyl groups of the pyrogallol[4]arene, the xanthone molecule is referred to as horizontal. On the other hand, if the carbonyl group of the xanthone molecule is oriented perpendicular to the hydroxyl groups of the pyrogallol[4]arene, then the xanthone molecule is referred to as vertical (Figure 2). With regard to Figure 3, when the cross-sectional distances of the bowl are equal, the bowl is said to be conical (C4v symmetry). When these two distances differ by more than 0.75 Å, the bowl is said to be pinched (C2v symmetry). With regard to C−H···π interactions, reference is made to the centroid of the π system. All of the crystal structures involve hydrogenbonded bilayers with extensive C−H···π interactions.

Figure 2. Orientation of the xanthone molecule: (a) horizontal and (b) vertical orientations. Hydrogen atoms have been removed for clarity. 4206



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Figure 5. Packing of the PgC1 molecules. Hydrogen atoms and solvent molecules have been removed for clarity.

Figure 3. Cross-sectional distances of pyrogallol[4]arene (dashed blue lines).

Cocrystal 1. The asymmetric unit of cocrystal 1 contains one xanthone molecule, one PgC1 molecule, and three isopropanol molecules (Figure 4). The xanthone is exo to the PgC1 bowl, and the space inside the bowl is occupied by an isopropanol molecule. The cross-sectional distances are 7.06 and 9.35 Å, demonstrating that the bowl is in the pinched conformation. In the bilayer structure, the upper rims of the PgC1 bowls are oriented as shown in Figure 5. Four PgC1 molecules surround the xanthone molecule, which is oriented horizontally (Figure 6). Within the crystal structure, there is an extensive hydrogen-bonding network. In total, there are 20 hydrogen bonds in which the hydroxyl groups on the upper rim of the PgC1 molecule are participating (1.80−2.71 Å (O−H··· O), 99.0−168.3° (O−H···O)). Five of these hydrogen bonds are intramolecular hydrogen bonds between hydroxyl groups. The remaining hydrogen bonds involve six hydroxyl hydrogen atoms donating to oxygen atoms of hydroxyl groups of adjacent PgC1 molecules (with two groups of two hydrogen atoms donating to the same PgC1), six oxygen atoms accepting hydroxyl groups of adjacent hydrogen atoms (with two oxygen atoms accepting hydrogen bonds of the same PgC1 molecule), two hydrogen atoms donating to an isopropanol molecule, and one oxygen accepting from an isopropanol molecule. The isopropanol molecule then hydrogen bonds to an adjacent PgC1 molecule, creating a bridge between the two PgC1 molecules. In addition to O−H···O hydrogen bonding, there is also one C−H···π interaction. One of the endo isopropanol

Figure 6. Each xanthone molecule is surrounded by four PgC1 molecules. The xanthone molecules are in blue, one four-membered PgC1 unit is in pink, and a second four-membered PgC1 unit is in purple. Hydrogen atoms and solvent molecules have been removed for clarity.

molecule’s hydrogen atoms is C−H···π interacting with one of the PgC1 molecule’s centroids (2.87 Å (C−H···π), 144.4° (C− H···π)). Cocrystal 2. Contained in the asymmetric unit of cocrystal 2 is one xanthone molecule, one PgC1 molecule, one isopropanol molecule, and two methanol molecules (Figure 7). The isopropanol molecule is disordered over two positions and is modeled at 70 and 30%. The xanthone molecule is endo to the PgC1 molecule’s bowl, and the bowl is pinched, with cross-sectional distances of 6.68 and 9.59 Å. Overall, there are 18 hydrogen bonds (1.88−2.22 Å (O−H···O), 100.4−166.2° (O−H···O)) (Figure 8). Six of the hydrogen bonds are intramolecular hydrogen bonds between upper-rim hydroxyl groups, eight are intermolecular hydrogen bonds between adjacent PgC1 molecules’ hydroxyl groups, three hydrogen bonds to solvent molecules (two with methanol molecules and

Figure 4. Asymmetric unit of cocrystal 1 (C32H32O12·C13H8O2· 3C3H7OH). Hydrogen atoms and the isopropanol molecules exo to the bowl are not shown for clarity.

Figure 7. Asymmetric unit of cocrystal 2 (C32H32O12·C13H8O2· C3H7OH·2CH3OH). Hydrogen atoms and solvent molecules have been removed for clarity. 4207



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molecule, and the xanthone molecule is oriented vertically. Because one of the isopropanol molecules is endo, the xanthone molecule is exo. Throughout the crystal structure, there are 18 hydrogen bonds (1.78−2.14 Å (O−H···O), 138.8−175.5° (O−H···O)): four intramolecular hydrogen bonds among the upper-rim hydroxyl groups, nine intermolecular hydrogen bonds among the hydroxyl groups of adjacent PgC2 molecules, and five hydrogen bonds among the hydroxyl groups of the PgC2 molecule and isopropanol hydroxyl groups. Additionally, there are two C−H···π interactions. The PgC2 molecule is involved in two C−H···π interactions from two endo isopropanols’ hydrogen atoms donating to two PgC2 centroids (2.78 Å (C−H···π), 136.6° (C−H···π); 2.82 Å (C− H···π), 127.2° (C−H···π)). The xanthone molecule also has two C−H···π interactions. Two PgC2 molecule aliphatic tail groups’ hydrogen atoms (−CH2 group, one from each PgC2) are donating to the middle centroid of the xanthone molecule (2.47 Å (C−H···π), 172.1° (C−H···π)) (Figure 10). Cocrystal 4. Cocrystal 4’s asymmetric unit contains one xanthone molecule, one PgC2 molecule, one dimethyl sulfoxide (DMSO) molecule, and one methanol molecule (Figure 11). The DMSO molecule is disordered over two positions and is modeled at 86 and 14%. The xanthone molecule is exo to the PgC2 bowl, whereas the DMSO is endo to the PgC2 bowl, and the PgC2 bowl is relatively conical shaped, with cross-sectional distances of 8.12 and 8.98 Å. It has a similar bilayer packing arrangement as that of cocrystal 1 in that there are four PgC2 molecules surrounding each xanthone molecule and the carbonyl group of the xanthone molecule is oriented horizontally. There are a total of 15 hydrogen bonds in which the PgC2 molecule’s upper-rim hydroxyl groups are involved (1.82−2.16 Å (O−H···O), 122.9−167.6° (O−H··· O)). Four hydrogen bonds are intramolecular hydrogen bonds among the upper-rim hydroxyl groups, six are intermolecular hydrogen bonds between adjacent PgC2 molecules’ hydroxyl groups, three are between the hydroxyl groups and methanol molecules, and two are between the hydroxyl groups and DMSO molecules. The hydrogen bonding between the hydroxyl groups and the methanol allows the methanol to act as a bridge between two PgC2 molecules that otherwise would not hydrogen bond with each other. In addition to the O−H··· O hydrogen bonding, there are also C−H···π interactions in which both the PgC2 molecule and xanthone molecule are participating. The PgC2 molecule has two C−H···π interactions

Figure 8. Example of the types of hydrogen bonding found in cocrystal 2. There are intramolecular (dashed green bonds), intermolecular (dashed pink bonds), solvent (dashed blue bonds), and xanthone (dashed purple bonds) hydrogen bonds.

one with an isopropanol molecule), and one hydrogen bond from a hydroxyl hydrogen atom donating to the xanthone molecule’s carbonyl oxygen atom. The hydrogen bonding with the solvent causes the solvent to act as a bridge between adjacent PgC1 molecules as in cocrystal 1. Also as in cocrystal 1, there are C−H···π interactions. Two hydrogen atoms on the endo xanthone molecule are interacting with two PgC1 molecule’s centroids (2.42 Å (C−H···π), 177.8° (C−H···π), 3.02 Å (C−H···π), 103.7° (C−H···π)). Additionally, one exo PgC1 molecule’s tail hydrogen atom and one exo isopropanol molecule’s hydrogen atom are C−H···π interacting with two PgC1 molecule’s centroids (2.49 Å (C−H···π), 149.6° (C− H···π), 2.91 Å (C−H···π), 164.9° (C−H···π), respectively). Cocrystal 3. Included within the asymmetric unit of cocrystal 3 is one xanthone molecule, one PgC2 molecule, and four isopropanol molecules (Figure 9). The xanthone molecule is modeled over two positions, each at 50% occupancy. One of the isopropanol molecules is disordered over two positions and is modeled at 81 and 19% occupancy. The bowl of the PgC2 molecule is conical shaped, with crosssectional distances of 8.10 and 8.82 Å. In the packing motif, there are four PgC2 molecules surrounding one xanthone

Figure 10. C−H···π interactions (dashed orange bonds) between the PgC2 aliphatic tail groups and the xanthone molecule. C−H···π interactions to the endo isopropanol molecule are not shown for clarity. Hydrogen atoms and solvent molecules have been removed for clarity.

Figure 9. Asymmetric unit of cocrystal 3 (C36H40O12·C13H8O2· 4C3H7OH). Hydrogen atoms and the other isopropanol molecules have been removed for clarity. 4208



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Figure 11. Asymmetric unit of cocrystal 4 (C36H40O12·C13H8O2· (CH3)2SO·CH3OH). Hydrogen atoms and the methanol molecule have been removed for clarity.

from the endo DMSO molecule’s methyl groups to the centroids of the PgC2 molecule (2.55 Å (C−H···π), 131.8° (C−H···π); 2.65 Å (C−H···π), 158.9° (C−H···π)). The xanthone molecule has a total of four C−H···π interactions. The two end aromatic groups each have two C−H···π interactions: one from a PgC2 molecule tail group’s −CH2 group on each side of the centroid (2.70−2.82 Å (C−H···π), 138.6−153.4° (C−H···π)) (Figure 12). Cocrystal 5. Within the asymmetric unit of cocrystal 5 there is one xanthone molecule, one PgC2 molecule, one methanol molecule, and one isopropanol molecule (Figure 13). In this cocrystal, the xanthone molecule is endo to the PgC2 bowl. Its bowl is pinched, with cross-sectional distances of 6.74 and 9.56 Å. There are a total of 19 hydrogen bonds (1.74−2.39 Å (O− H···O), 107.7−174.9° (O−H···O)). Eight hydrogen bonds are intramolecular hydrogen bonds between hydroxyl groups, eight are intermolecular bonds between hydroxyl groups of adjacent PgC2 molecules, two are hydrogen bonds to methanol molecules, and one is a hydrogen bond donating to an adjacent xanthone molecule’s carbonyl oxygen atom. Furthermore, there is also C−H···π interactions throughout the cocrystal structure. Three hydrogen atoms on the endo xanthone molecule are C− H···π interacting with three PgC2 centroids (2.41−3.08 Å (C− H···π), 98.2−175.3° (C−H···π)) (Figure 14). Additionally, an adjacent PgC2 molecule’s tertiary hydrogen atom is C−H···π interacting with xanthone molecule’s centroid farthest out of the PgC2 molecule (2.76 Å (C−H···π), 141.9° (C−H···π)). Cocrystal 6. Contained in the asymmetric unit of cocrystal 6 is one xanthone molecule, one PgC3 molecule, and two acetonitrile molecules (Figure 15). One of the acetonitrile molecules is endo to the PgC3 molecule, whereas the other

Figure 13. Asymmetric unit of cocrystal 5 (C36H40O12·C13H8O2· C3H7OH·CH3OH). Hydrogen atoms and solvent molecules have been removed for clarity.

Figure 14. Hydrogen bonding (dashed red bond) and C−H···π interactions (dashed orange bonds) involving the xanthone molecule and PgC2 molecule. Hydrogen atoms and solvent molecules have been removed for clarity.

molecule is found in the center of the tail groups of the PgC3 molecule. Thus, because the acetonitrile is endo to the bowl, the xanthone molecule is exo to the PgC3 molecule’s bowl. The bowl of the pyrogallol[4]arene is conical, with cross-sectional distances of 8.18 and 9.08 Å. Once again, the PgC3 molecules form a bilayer structure. However, they are not cupped within each other as they are in cocrystals 1 and 3. Instead, the rims of the bowls are directly on top of each other (Figure 16). Furthermore, in the packing motif, four PgC3 molecules surround two xanthone molecules that are oriented horizontally. Again, there is an extensive hydrogen-bonding network.

Figure 12. C−H···π interactions (dashed orange bonds) involving the xanthone molecule and PgC2 molecules. Other C−H···π interactions are not shown for clarity, and hydrogen atoms and solvent molecules have been removed for clarity. 4209



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Figure 15. Asymmetric unit of cocrystal 6 (C40H48O12·C13H8O2· CH3CN). Hydrogen atoms have been removed for clarity.

Figure 17. Asymmetric unit of cocrystal 7 (C40H48O12·2C13H8O2· 4C3H7OH). Hydrogen atoms and solvent molecules have been removed for clarity.

are arranged in a bilayer structure where the rims are cupped within each other. There are two xanthone molecule surrounded by four PgC3 molecules in the packing scheme, and the xanthone molecules are oriented vertically (Figure 18). In cocrystal 7, there are 16 hydrogen bonds (1.74−2.39 Å (O− H···O), 107.7−174.9° (O−H···O)). Four of these are intramolecular hydrogen bonds between the upper-rim hydroxyl groups, and eight are intermolecular hydrogen bonds between hydroxyl groups on adjacent pyrogallol[4]arenes. The remaining four hydrogen bonds are between the PgC3’s hydroxyl groups and an isopropanol molecules’ hydroxyl group. There is also C−H···π interactions throughout the crystal network involving both the PgC3 molecule and xanthone molecules. First, three of the endo isopropanol’s hydrogen atoms are donating to three of the PgC3 molecule’s centroids (2.80−2.85 Å (C−H···π), 133.9−167.9° (C−H···π)). One xanthone centroid (the one nearest to PgC3 molecule’s bowl) is C−H···π interacting with one PgC3 molecule’s tail hydrogen atom and one adjacent PgC3 molecule’s tail hydrogen atom (2.89 Å (C−H···π), 144.1° (C−H···π)). The centroid furthest from the PgC3 molecule’s bowl is C−H···π interacting with a different adjacent PgC3 molecule’s tail hydrogen atom (2.89 Å (C−H···π), 144.1° (C−H···π)). The second xanthone has the same C−H···π scheme for the xanthone centroid closest to the PgC3 molecule except that the centroid is interacting with a

Figure 16. Packing of the PgC3 molecules. Hydrogen atoms, xanthone molecules, and solvent molecules have been removed for clarity.

There are 18 hydrogen bonds in which the hydroxyl groups of the PgC3 molecule are involved (1.87−2.14 Å (O−H···O), 148.4−173.5° (O−H···O)). Five of these are intramolecular hydrogen bonds between the hydroxyl groups, whereas six oxygen atoms are hydrogen-bond accepting from adjacent PgC3 molecules’ hydroxyl groups (with two accepting to the same PgC3 molecule). Seven hydrogen atoms are donating hydrogen bonds to adjacent PgC3 molecules’ hydroxyl groups (with three pairs of hydrogen atoms donating to the same PgC3 molecule). Additional hydrogen bonding is occurring through C−H···π interactions from both the PgC3 molecule’s and xanthone’s aromatic groups. One of the PgC3 aromatic groups is accepting a C−H···π hydrogen bond from the endo acetonitrile (2.88 Å (C−H···π), 144.0° (C−H···π)). The xanthone molecule is also participating in C−H···π bonding. Each of the xanthone molecule’s end aromatic groups is accepting a C−H···π interaction from −CH2 groups closest to the bowl of the PgC3 molecule (2.87 Å (C−H···π), 145.3° (C−H···π); 2.99 Å (C−H···π), 145.3° (C−H···π)) and from −CH2 groups closest to the bowl of an adjacent PgC3 molecule (3.00 Å (C−H···π), 123.1° (C−H···π); 2.85 Å (C−H···π), 156.1° (C−H···π)). Cocrystal 7. In the asymmetric unit of cocrystal 7 there are two xanthone molecules, one PgC3 molecule, and four isopropanol molecules (Figure 17). Both of the xanthone guest molecules are disordered over two positions. One xanthone molecule is modeled with each position at 50% occupancy. The second is disordered with an isopropanol molecule where the xanthone is modeled at 37.5% occupancy and the isopropanol at 62.5% occupancy. The rest of the isopropanol molecules are disordered over two positions, two modeled at 60 and 40% occupancies and the third at 70 and 30% occupancies. The bowl of the pyrogallol[4]arene is relatively conical (8.14 Å, 8.83 Å), and one of the isopropanol molecules is endo to the PgC3 bowl. Once again, the xanthone molecule is exo to the PgC3 molecule, and the PgC3 molecules

Figure 18. Packing of the PgC3 molecules and xanthone molecules. Hydrogen atoms and solvent molecules have been removed for clarity. 4210



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different PgC3 tail group than the first xanthone and it does not interact with the adjacent molecule (2.89 Å (C−H···π), 139.3° (C−H···π)) (Figure 19). Cocrystal 8. There is one xanthone molecule, one PgC3 molecule, one methanol molecule, and one isopropanol molecule found in the asymmetric unit of cocrystal 8 (Figure 20). As in cocrystal 5, the xanthone molecule is endo to the PgC3 molecule. The bowl of the PgC3 molecule is pinched, with cross-sectional distances of 6.77 and 9.57 Å. It forms a bilayer structure like that of cocrystal 5. As with the previous three cocrystals, there is an extensive hydrogen-bonding network found in cocrystal 8. The hydroxyl groups on the pyrogallol[4]arene’s upper rim are participating in 17 hydrogen bonds (1.93−2.22 Å (O−H···O), 114.6−170.6° (O−H···O)). Between hydroxyl groups of the pyrogallol[4]arene, three are six intramolecular hydrogen bonds. Four hydroxyl hydrogen atoms are donating hydrogen bonds to hydroxyl oxygen atoms on adjacent pyrogallol[4]arenes, and three hydroxyl oxygen atoms are accepting hydrogen bonds from adjacent pyrogallol[4]arenes. Three hydrogen bonds are from hydroxyl hydrogen atoms donating to solvent oxygen atoms. The one remaining hydrogen bonds is donated from the PgC 3 molecule’s hydroxyl hydrogen to an adjacent xanthone’s carbonyl oxygen (1.91 Å (O−H···O), 146.4° (O−H···O)). There are C−H···π interactions throughout the cocrystal. A hydrogen atom on one end of the xanthone molecule is donating C−H···π interactions to three opposite facing aryl groups on the PgC3 molecule (2.41−3.09 Å (C−H···π), 99.0− 175.5° (C−H···π)). There is also a C−H···π interaction between a hydrogen atom of the xanthone and another aryl group of the PgC3 molecule (2.80 Å (C−H···π), 140.2° (C− H···π)).

Figure 20. (a) Asymmetric unit of cocrystal 8 (C40H48O12·C13H8O2· C3H7OH·CH3OH), (b) spaced-filled representation of the xanthone molecule, and (c) space-filled representation of both the xanthone molecule and PgC3 molecule.

Solvent is seen to have an effect on the structure of the cocrystal. When in just isopropanol or acetonitrile, or in the mixture of DMSO/methanol (cocrystals 1, 3, 4, 6, and 7), the xanthone molecule is exo to the pyrogallol[4]arene, but when in the two-solvent system of isopropanol and methanol (cocrystals 2, 5, and 8), the xanthone molecule becomes endo. The positioning of the xanthone in the packing arrangement alters with solvent. In isopropanol, for one carbon atom (cocrystal 1), the xanthone molecule is oriented horizontally. However, with two and three carbon atoms (cocrystals 3 and 7), the xanthone molecule is oriented vertically. This suggests that with more than one carbon atom there is more room and that in order to fill more space the xanthone rotates. When the tail length increases to three carbon atoms, there is even more room, and in order to fill the space, another xanthone molecule is added. Changing the solvent to acetonitrile (cocrystal 6) has a different effect on the orientation of the xanthone molecule. Unlike PgC3 in isopropanol, the xanthone molecule in cocrystal 6 is oriented horizontally. There is more space because of the longer aliphatic tails (and thus there is two xanthone molecules surrounded by four PgC3 molecules), but the one xanthone molecule is C−H···π interacting with two tail groups (of one macrocycle), so it is forced into the horizontal arrangement. In isopropanol (cocrystal 7), each xanthone molecule is C−H···π interacting with one tail group (of one macrocycle) (Figure 21). Furthermore, with changes in solvent, there are changes in the hydrogen-bonding networks of the host−guest cocrystals. Cocrystal 1 has more hydrogen bonds to solvent molecules than in any of the other cocrystals. Cocrystals 1, 3, 6, and 7 have twice the amount of intermolecular hydrogen bonding as intramolecular hydrogen bonding (Table 1). They are all cocrystals that do not have a mixed solvent system. Mixed solvent systems, cocrystals 2, 4, 5, and 8, have a more equal distribution of intramolecular and intermolecular hydrogen bonds. Additionally, with a mixed solvent system of isopropanol and methanol, cocrystals 2, 5, and 8 are the only cocrystals in which the xanthone molecule hydrogen bonds with the pyrogallol[4]arene molecule. Solvent plays an important role in the resulting architecture of the hydrogen-bonding network. In cocrystals 1, 3, and 7, the pyrogallol[4]arenes are cupped within each other, whereas in cocrystal 6, they are stacked. Additionally, cocrystals 1, 3, 4, 5, 7, and 8 have solvents containing hydroxyl groups. These solvents participate in hydrogen bonding, acting as bridges between adjacent pyrogallol[4]arenes. Acetonitrile in cocrystal 5 does not take part in hydrogen bonding, leaving the hydroxyl groups of

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DISCUSSION There are similarities between the cocrystals formed, such as the formation of a bilayer structure and a complex hydrogenbonding network. Nevertheless, solvent and aliphatic tail lengths appear to have considerable influence on the properties of the cocrystal produced. First, tail length appears to play a part in the ratio of pyrogallol[4]arene to xanthone in the final cocrystal structure. When the tail length is one carbon (PgC1) and the solvent is isopropanol (cocrystal 1), the ratio of pyrogallol[4]arene to probe is 1:1. However, when maintaining the same solvent but increasing the tail length to three carbons (PgC3) (cocrystal 7), the ratio changes to 1:2.

Figure 19. C−H···π interactions (dashed orange bonds) involving the xanthone molecule and PgC3 molecules. Other C−H···π interactions are not shown for clarity, and hydrogen atoms and solvent molecules have been removed for clarity. 4211



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the bowl of the pyrogallol[4]arene is more symmetrical than it is for pyrogallol[4]arenes without endo solvent molecules. However, cocrystal 1 also has an endo isopropanol molecule, but its pyrogallol[4]arenes’ bowls are pinched. Thus, tail length could also play a role in the bowl shape of the pyrogallol[4]arenes. As the tail length is increased from PgC1 to PgC3, the pyrogallol[4]arenes’ bowls become more symmetrical with both isopropanol and the mixture of isopropanol and methanol (Table 2). For instances, looking at isopropanol systems (cocrystals 1, 3, and 7), as the tail length is increased from one carbon atom to three carbon atoms the difference between cross-sectional distances decreases from 2.910 to 0.711 to 0.694 Å (the bowl becomes more symmetric).

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CONCLUSIONS In summary, it has been demonstrated that cocrystals consisting of pyrogallol[4]arene and xanthone molecules form a variety of complexes in different solvent media. Notably, it was shown that the positioning of the xanthone molecule, exo or endo to the pyrogallol[4]arene bowl, can be controlled by the choice of solvent. In a single-solvent system, the xanthone molecule was exo to the bowl of the pyrogallol[4]arenes. However, in the two-solvent system of methanol and isopropanol, the xanthone was endo to the bowls of the pyrogallol[4]arenes. The solvent not only controls the exo/endo positioning of the xanthone molecules but also affects, as was demonstrated, the horizontal and vertical orientations of the probe, the hydrogen-bonding scheme, and the bowl shape of the pyrogallol[4]arenes. The aliphatic tail length of the pyrogallol[4]arenes also appears to dictate the final ratio of xanthone to pyrogallol[4]arene. The next step is to examine the fluorescent properties of these cocrystals to better understand the chemical environment and to study whether the solvent or aliphatic tail affects the fluorescent properties of the xanthone.

Figure 21. C−H···π interactions in cocrystals (a) 6 and (b) 7.

Table 1. Types and Number of Hydrogen-Bonds in the Cocrystals in Which the Pyrogallol[4]arene Molecule’s Hydroxyl Groups Are Involved

cocrystal

number of intramolecular hydrogenbonds

number of intermolecular hydrogenbonds

number of solvent hydrogenbonds

number of xanthone hydrogenbonds

1 2 3 4 5 6 7 8

5 6 4 4 8 5 4 6

12 8 9 6 8 13 8 7

3 3 5 5 2 0 4 3

0 1 0 0 1 0 0 1

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ASSOCIATED CONTENT

S Supporting Information *

Single-crystal X-ray crystallographic information files (CIF) for all cocrystals. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic information files are also available from the Cambridge Crystallographic Data Center (CCDC) upon request (http://www.ccdc.cam.ac. uk; CCDC deposition nos. 1002169−1002176).

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adjacent pyrogallol[4]arenes to bond with each other and thus leading to the cupped formation. Finally, solvent effects the shape of the pyrogallol[4]arenes’ bowl. When the solvent is endo, as in cocrystals 3, 4, 6, and 7,

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Table 2. Cross-Sectionals Distances for the Cocrystals cocrystal

PgCx/solvent

first cross-sectional distance (Å)

second cross-sectional distance (Å)

difference between distances (Å)

conformation

1 2 3 4 5 6 7 8

PgC1/IPA PgC1/IPA/MeOH PgC2/IPA PgC2/DMSO/MeOH PgC2/IPA/MeOH PgC3/MeCN PgC3/IPA PgC3/IPA/MeOH

7.06 6.68 8.11 8.12 6.74 8.18 8.14 6.77

9.35 9.59 8.82 8.98 9.56 9.08 8.83 9.57

2.29 2.91 0.71 0.86 2.82 0.90 0.69 2.80

pinched pinched conical conical pinched pinched conical pinched

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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

J.L.A. thanks the NSF for funding this work. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS PgC1, C-methylpyrogallol[4]arene; PgC2, C-ethylpyrogallol[4]arene; PgC3, C-propylpyrogallol[4]arenel; MeOH, methanol; IPA, isopropanol; MeCN, acetonitrile

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REFERENCES

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