Selective Complexation in Three Component ... - ACS Publications

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Selective Complexation in Three Component Cocrystals Composed of Pyrogallol[4]arene and Fluorescent Probes Pyrene and 1‑(2Pyridylazo)-2-naphthol 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: Cocrystals composed of a pyrogallol[4]arene molecule (one of Cpropylpyrogallol[4]arene in ethanol and one of C-hexylpyrogallol[4]arene in isopropanol) and a pyrene molecule were crystallized and investigated with the aim to develop trends based on solvent and aliphatic tail length of the pyrogallol[4]arene. These new trends along with previously determined trends for the fluorescent probe 1-(2-pyridylazo)-2-naphthol (PAN), were then used to crystallize a cocrystal composed of C-hexylpyrogallol[4]arene and two fluorescent probes, pyrene and PAN. Within a two-component cocrystal of pyrene or PAN, the probe molecules are complexed in both the endo (inside the pyrogallol[4]arene bowl) and exo (outside the pyrogallol[4]arene bowl) positions. However, in the three component system, selective complexation of the probes is demonstrated whereby the PAN molecules prefer to be endo and the pyrene molecules occupy exo positions.

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INTRODUCTION Pyrogallol[4]arenes are versatile molecules that have a widerange of applications. They are compounds composed of four aromatic groups joined into a bowl-shaped macrocycle by −CHR− bridging groups. The upper-rim of the macrocycle is adorned with 12 hydroxyl groups and the bottom-rim consists of four hydrocarbon tails of a given length (see Figure 1). Due to the flexibility of the bowl and the capacity of the upper-rim hydroxyl groups to partake in hydrogen bonding, these molecules can exhibit a variety of supramolecular architectures including bilayers, dimers, hexamers, nanocages, and nanotubes.1 Metal ions, such as Cu2+, Zn2+, and Ni2+, can displace the upper-rim hydroxyl hydrogen atoms to form metal-seamed dimers and hexamers.2 Along with the ability to display diverse structural chemistry, pyrogallol[4]arenes are model candidates for cocrystallizations. This is due to the cavity, which is ideal for host−guest interactions.3 Numerous guests including small inorganic molecules, pharmaceutical molecules, and ionic liquids have been bound into the cavity.4−6 Fluorescent probes are important candidates for cocrystallization with pyrogallol[4]arenes. Such probes can provide information regarding guest orientation and movement within the pyrogallol[4]arene cavity.7 However, few fluorescent probe and pyrogallol[4]arene cocrystals have been produced.8 However, recently there have been attempts to study and map the effects of solvent, aliphatic tail length, and probe on properties of consequential cocrystals such as pyrogallol[4]arene bowl shape, crystal packing, crystal architecture, and probe orientation.9 We now add insight into the formation of © XXXX American Chemical Society

Figure 1. (A) Pyrogallol[4]arene, (b) pyrene, and (c) 1-(2pyridylazo)-2-naphthol schematic structures.

cocrystals that consists of the fluorescent probe pyrene. It has been demonstrated that small changes in molecular structure, Received: May 4, 2015 Revised: May 22, 2015

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DOI: 10.1021/acs.cgd.5b00609 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Table 1. Reagents and Solvents Used and Their Corresponding Experimental Amounts cocrystal

PgCx

solvent

probe

molar ratio PgCx/probe

amount (g) PgCx/probe

solvent (mL)

1 2 3

PgC3 PgC6 PgC6

ethanol isopropanol methanol

pyrene pyrene pyrene/12P2N

1:2 1:1 1:1.2:1

0.1:0.0562 0.1:0.0227 0.05:0.0138:0.0140

10 10 10

such as altering the aliphatic tail length, can change intermolecular interactions.10 Thus, the aim of these studies is to document the changes caused by solvent, probe, and aliphatic tail length. With a database detailing the effects of such variables, one can create custom cocrystals that will exhibit desired properties. Herein this goal is achieved through the use of information learned from the current studies for the construction of a three-component cocrystal composed of a pyrogallol[4]arene and two different fluorescent cocrystals (one probe that resides inside the bowl of the pyrogallol[4]arene, and one probe that resides outside the bowl of the pyrogallol[4]arene). Within this contribution are presented two cocrystals containing pyrene: cocrystal 1 with C-propylpyrogallol[4]arene (PgC3) crystallized from ethanol, and cocrystal 2 with Chexylpyrogallol[4]arene (PgC6) crystallized from isopropanol. Further, cocrystal 3 containing two fluorescent probes, pyrene and 1-(2-pyridylazo)-2-naphthol (PAN), with PgC6 is prepared in methanol (see Figure 1).

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Figure 2. Cross-sectional distances of the pyrogallol[4]arene (dashed blue bonds).

EXPERIMENTAL SECTION

Cocrystals were crystallized using a method previously described by Atwood et al., and the reagent and solvent experimental amounts are found in Table 1.9c Crystallographic Data. Cocrystal 1: C124H154O31, M = 2140.47, colorless prism, a = 20.392(2) Å, b = 30.310(2) Å, c = 19.413(2) Å, β = 104.241(1)°, space group P21/c, V = 11630.2(16) Å3, Z = 4, Dc = 1.222 g/cm3, F000 = 4584.0, Mo Kα radiation, λ = 0.71073 Å, T = 173 K, 25718 reflections collected. Final GOF = 1.031, R1 = 0.071, wR2 = 0.229, R indices based on reflections with I > 2σ(I) (refinement on F2), 1540 parameters, 78 restraints. Lp and absorption corrections applied, μ = 0.087 mm−1. Cocrystal 2: C183H224O29, M = 2887.62, colorless prism, a = 19.858(2) Å, b = 20.880(3) Å, c = 22.297(3) Å, α = 90.840(2)°, β = 114.042(2)°, γ = 105.603(2)°, space group P1̅, V = 8050.5(16) Å3, Z = 2, Dc = 1.191 g/cm3, F000 = 3108.0, Mo Kα radiation, λ = 0.71073 Å, T = 173 K, 24044 reflections collected. Final GOF = 1.05, R1 = 0.096, wR2 = 0.316, R indices based on reflections with I > 2σ(I) (refinement on F2), 1863 parameters, 309 restraints. Lp and absorption corrections applied, μ = 0.079 mm−1. Cocrystal 3: C170H199O30N6, M = 2806.35, orange plate, a = 16.9687(7) Å, b = 20.0883(9) Å, c = 22.439(1) Å, α = 101.325(3)°, β = 90.098(3)°, γ = 95.339(3)°, space group P1̅, V = 7465.7(6) Å3, Z = 2, Dc = 1.248 g/cm3, F000 = 3002.0, Cu Kα radiation, λ = 1.5418 Å, T = 100 K, 26473 reflections collected. Final GOF = 1.32, R1 = 0.117, wR2 = 0.350, R indices based on reflections with with I > 2σ(I) (refinement on F2), 1854 parameters, 93 restraints. Lp and absorption corrections applied, μ = 0.68 mm−1.

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are the distances from opposing, middle carbon atoms on the upper-rim of the bowl of the pyrogallol[4]arene (see Figure 2). The shape of the bowl of the pyrogallol[4]arene is determined from the cross-sectional distances. If the difference between these two distances is less than 0.75 Å, then the bowl is said to be conical or symmetrical in shape (C4v symmetry). If the difference between these two distances is greater than 0.75 Å, then the bowl is said to be pinched (C2v symmetry). Next, hydrogen bonding is discussed. Only for the upper-rim hydroxyl groups is the hydrogen bonding examined. Finally, C−H···π interactions are discussed. Cocrystal 1. In the asymmetric unit of cocrystal 1, there are two PgC3 molecules, two pyrene molecules, six ethanol molecules, and one water molecule (see Figure 3). Four of the ethanol molecules and the water molecule are disordered over two positions and are modeled at 50% occupancy. One pyrene molecule is endo to the bowl of one PgC3 molecule. The second pyrene molecule rests between the aliphatic tails of adjacent PgC3 molecules (see Figure 4). The second PgC3 molecule has an endo ethanol molecule. The PgC3 molecule that contains the endo pyrene molecule has a pinched-shaped bowl with cross-sectional distances of 7.26 and 9.53 Å. Crosssectional distances for the second PgC3 molecule are 8.23 and 8.77 Å; thus the bowl is more symmetrical. For the first PgC3 molecule (endo pyrene molecule), there are a total of 18 hydrogen bonds in which the hydroxyl groups participate (1.74−2.11 Å (H···O), 151.5−168.9° (O−H···O); see Figure 5). There are four intramolecular hydrogen bonds between the upper-rim hydroxyl groups on the same PgC3 molecule, ten intermolecular hydrogen bonds between hydroxyl groups of adjacent PgC3 molecules, and four hydrogen bonds between the hydroxyl groups of one PgC3 molecule and four ethanol molecules. The second PgC3 molecule (endo ethanol molecule) has a total of 19 hydrogen bonds (1.82−2.11 Å

RESULTS

Cocrystal characteristics are discussed. The first examined is the positioning of the probe molecule. All pyrogallol[4]arenes are arranged in a bilayer arrangement unless otherwise noted. If a portion of the probe is positioned inside the bowl of the pyrogallol[4]arene, then the probe is said to be endo. If a portion of the probe is found outside of the bowl of the pyrogallol[4]arene, then the probe is said to be exo. The second quality analyzed is the cross-sectional distances. These B



Article

Figure 3. Asymmetric unit of cocrystal 1, 2C40H48O12·2C16H10· 6CH3CH2OH·H2O.

(H···O), 149.6−174.3° (O−H···O)). There are two intramolecular hydrogen bonds, 12 intermolecular hydrogen bonds, and five hydrogen bonds with ethanol molecules. Both the PgC3 molecules and the pyrene molecules have C−H···π interactions. The first PgC3 molecule has two C−H···π interactions with the endo pyrene molecule (2.78 Å (C− H···π), 148.5° (C−H···π); 2.89 Å (C−H···π), 139.9° (C− H···π)). The second PgC3 molecule has three C−H···π interactions with the endo ethanol molecule (2.97−3.02 Å (C−H···π), 139.8−155.0° (C−H···π)). The endo pyrene molecule has two C−H···π interactions with an adjacent ethanol molecule (2.97 Å (C−H···π), 138.9° (C−H···π); 3.01 Å (C−H···π), 137.1° (C−H···π)). The second pyrene molecule has four C−H···π interactions. All four aromatic centroids have C−H···π interactions with an aliphatic tail of a PgC3 molecule (2.76−3.15 Å (C−H···π), 123.0−170.2° (C−H···π)). Cocrystal 2. The asymmetric unit of cocrystal 2 has two PgC6 molecules, four pyrene molecules, and five isopropanol molecules (see Figure 6). Two aliphatic tails of one PgC6 molecule are disordered over two positions and are modeled at 60% and 40% and at 50%. Three of the pyrene molecules are disordered over two positions and are all modeled at 50%. Four of the isopropanol molecules are disordered over two positions, and two are modeled at 50%, one at 60% and 40%, and one at 70% and 30%. In the bowl of one of the PgC6 molecules is an endo isopropanol molecule. The second PgC6 molecule contains an endo pyrene molecule. Two of the pyrene molecules are located among the aliphatic tail groups of adjacent PgC6 molecules, and one pyrene molecule is under the bowl of the PgC6 molecule, surrounded by all four of the aliphatic tail groups (see Figure 7). The first PgC6 molecule (endo isopropanol molecule) has a conical bowl with cross-sectional distances of 8.23 and 8.89 Å, while the second PgC6 molecule (endo pyrene molecule) has a pinched bowl with cross-sectional distances of 7.60 and 9.66 Å. With regard to hydrogen bonding, the first PgC6 molecule (endo isopropanol molecule) has 17 hydrogen bonds (1.88− 2.13 Å (H···O), 125.0−175.6° (O−H···O)): four intramolecular hydrogen bonds, nine intermolecular hydrogen

Figure 4. Packing arrangement of cocrystal 1 (a) along the a-axis and (b) along the c-axis. Pyrene molecules are represented in green.

Figure 5. Types of hydrogen bonding found in cocrystal 1.

bonds among hydroxyl groups of adjacent PgC6 molecules, and four hydrogen bonds with isopropanol molecules. The second PgC6 molecule (endo pyrene molecule) has 18 hydrogen bonds (1.80−2.07 Å (H···O), 141.4−171.6° (O− H···O)): four intramolecular hydrogen bonds, 12 intermolecC



Article

Figure 6. Asymmetric unit of cocrystal 2, 2C52H56O12·4C16H10· 5C3H7OH.

ular hydrogen bonds between hydroxyl groups of adjacent PgC6 molecules, and two hydrogen bonds with isopropanol molecules. Both PgC6 molecules and three of the pyrene molecules have C−H···π interactions. The first PgC6 molecule (endo isopropanol molecule) has five C−H···π interactions involving the aromatic centroids (2.70−3.09 Å (C−H···π), 102.4−148.3° (C−H···π)): two with the endo isopropanol molecule, two with two different exo isopropanol molecules, and one with an exo pyrene molecule (see Figure 8). The second PgC6 molecule has three C−H···π interactions involving the aromatic centroids (2.59−2.73 Å (C−H···π), 114.0−179.2° (C−H···π)): two with the endo pyrene molecule and one with an exo pyrene molecule (see Figure 8). One centroid of the endo pyrene molecule is participating in two C−H···π interactions with two different isopropanol molecules (on opposite sides of the centroid) (2.86 Å (C−H···π), 134.7° (C− H···π); 2.87 Å (C−H···π), 139.0° (C−H···π); see Figure 9). Two centroids of a second pyrene molecule have C−H···π interactions with an aliphatic tail of one PgC6 molecule, and a third centroid has C−H···π interactions with a second aliphatic tail of a second PgC6 molecule (2.79−2.89 Å (C−H···π), 135.2−163.0° (C−H···π); see Figure 9). Three centroids of a third pyrene molecule have C−H···π interactions with an aliphatic tail group of a PgC6 molecule, and two of those centroids also have C−H···π interactions with a second aliphatic tail group of a second PgC6 molecule (2.85−3.04 Å (C−H···π), 135.8−155.3° (C−H···π)) (see Figure 9). The fourth pyrene molecule does not have C−H···π interactions. Cocrystal 3. Contained in the asymmetric unit of cocrystal 3 are two PgC6 molecules, two pyrene molecules, two PAN molecules, and four methanol molecules (see Figure 10). Three methanol molecules are disordered over two positions: two are modeled at 50%, and one is modeled at 60% and 40%. One of the pyrene molecules is also disordered over two positions and is modeled at 58% and 42%. Each of the PAN molecules is

Figure 7. Cocrystal 2 packing motif depicted (a) in ball-and-stick representation, (b) with PgC6 molecules in ball-and-stick representation and pyrene molecules in space-filling representation, and (c) with PgC6 and pyrene molecules in space-filling representation. Isopropanol molecules are represented in orange, endo pyrene molecules are represented in purple, exo pyrene molecules along the aliphatic tail groups are represented in green, and exo pyrene molecules within the aliphatic tail groups are represented in blue.

endo to a bowl of one of the PgC6 molecules. Both of the pyrene molecules rest between the aliphatic tail groups of adjacent PgC6 molecules (see Figure 11). The bowls of the PgC6 molecules are pinched with cross sectional distances of 7.14 and 9.40 Å and 7.17 and 9.41 Å. The upper-rim hydroxyl groups of both PgC6 molecules participate in 12 hydrogen bonds (1.88−2.16 Å (H···O), 123.3−171.2° (O−H···O)). One PgC6 molecule has five intramolecular hydrogen bonds, five intermolecular hydrogen bonds between hydroxyl groups of adjacent PgC6 molecules, one hydrogen bond with a methanol D



Article

Figure 8. C−H···π interactions of cocrystal 2 involving the PgC6 molecules. Figure 10. Asymmetric unit of cocrystal 3, 2C52H56O12·2C16H10· 2C15H11ON3·4CH3OH.

Figure 11. Asymmetric unit of cocrystal 3 (a) with PgC6 molecules in ball-and-stick representation and PAN and pyrene molecules in spacefilling representation and (b) with PgC6, PAN, and pyrene molecules in space-filling representation. PAN molecules are represented in blue, and pyrene molecules are represented in green.

Figure 9. C−H···π interactions of cocrystal 2 involving the pyrene molecules.

molecule, and one hydrogen bond with a nitrogen atom of a PAN molecule (not endo). The second PgC6 molecule has four intramolecular hydrogen bonds, six intermolecular hydrogen bonds with hydroxyl groups of adjacent PgC6 molecules, and two hydrogen bonds with methanol molecules. Centroids of both PgC6 molecules and both pyrene molecules have C−H···π interactions. Each PgC 6 molecules has two C−H···π interactions with the endo PAN molecule (2.53−2.71 Å (C− H···π), 130.7−146.4° (C−H···π); see Figure 12a). One pyrene molecule has one C−H···π interaction with an aliphatic tail group of a PgC6 molecule and the second pyrene has two C− H···π interactions with two aliphatic tail groups of a PgC6 molecule (2.92−3.07 Å (C−H···π), 137.2−148.5° (C−H···π); see Figure 12b).

Figure 12. C−H···π interactions found in cocrystal 3 involving (a) the PgC6 molecules and (b) the pyrene molecules. Both PgC6 molecules have the same C−H···π interactions; thus only one is showed for clarity. Aliphatic tails have been shortened for clarity. All unnecessary hydrogen atoms and solvent molecules are omitted for clarity.

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DISCUSSION Both cocrystals 1 and 2 have similar interactions. With regard to hydrogen bonding and C−H···π interactions, both cocrystals exhibit comparable amounts. The bowl of the pyrogallol[4]arene with the endo pyrene molecule is more pinched than that

of the pyrogallol[4]arene with the endo solvent molecule. Additionally, both have an endo pyrene molecule and an endo solvent molecule. However, cocrystal 1 has two pyrene E



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molecules, and cocrystal 2 has four pyrene molecules. This could be a result of the aliphatic tail length. Increasing the aliphatic tail length from three carbon atoms to six carbon atoms increases the space between adjacent pyrogallol[4]arenes, thus allowing for the accommodation of more pyrene molecules. Additionally, when the number of pyrene molecules is increased, the added pyrene molecules remain exo to the bowl of the pyrogallol[4]arene, even though there is one remaining pyrogallol[4]arene bowl that does not contain a pyrene molecule. These properties are valuable in the aim to form a cocrystal with two fluorescent probe molecules and a pyrogallol[4]arene molecule. If pyrene molecules were to be the first probe molecule, then from the trends obtained it is known that a second probe molecule would need to favor the endo position in order to fill the empty pyrogallol[4]arene bowl that remains in the pyrene cocrystals. Additionally, a longer aliphatic tail would be most suitable since it would provide the most space for probes to be incorporated. Cocrystals composed of pyrogallol[4]arenes and 1-(2-pyridylazo)-2-naphthol (PAN) molecules were previously published.9c It was found that the PAN molecules preferred the endo position. Unlike in the pyrene cocrystals, if there were two pyrogallol[4]arene molecules, both bowls would be occupied by PAN molecules. Furthermore, any additional PAN molecules would occupy positions near the endo PAN molecules, rather than by the aliphatic tails like the pyrene molecules. Thus, using this information, it was attempted to crystallize a cocrystal with pyrene, PAN, and PgC6 molecules, and the result was cocrystal 3. The PAN molecules prefer the endo position, filling the endo position in both bowls of the two PgC6 molecules, while the pyrene molecules favor an exo position, located near the aliphatic tails.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.L.A. thanks the NSF for funding of this work. ABBREVIATIONS PgC3, C-propylpyrogallol[4]arenel; PgC6, C-hexylpyrogallol[4]arene; MeOH, methanol; EtOH, ethanol; IPA, isopropanol; PAN, 1-(2-pyridylazo)-2-naphthol

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REFERENCES

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CONCLUSION The information gathered from the trends developed in previous sections was used to construct a multicomponent cocrystal containing two different fluorescent probes and a host macrocycle. The probes display selective complexation for specific regions of the macrocycle structure, with PAN molecules preferring endo positions and pyrene molecules preferring exo positions. In terms of the two pyrene cocrystals, increasing the aliphatic tail length from three carbon atoms to six carbon atoms increases the probe to pyrogallol[4]arene ratio. Solvent appears to affect the bowl-shape of the pyrogallol[4]arene molecule. It has been demonstrated that the trends occurring from solvent and aliphatic tail length changes provide useful information for the creation of tailored cocrystals with specific features. Further studies, including fluorescence spectroscopy and molecular modeling, will aid to further elucidate the causes for the observed trends.

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ASSOCIATED CONTENT

S Supporting Information *

Single crystal X-ray crystallographic information files (CIF) are available for all cocrystals. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00609. Crystallographic information files are also available from the Cambridge Crystallographic Data Center (CCDC) upon request (http://www.ccdc.cam.ac. uk, CCDC deposition numbers (1063101−1063103). F



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