Solvent-Induced Manipulation of Supramolecular Organic

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Solvent induced manipulation of Supramolecular Organic Frameworks Rahul S Patil, Amanda M. Drachnik, Harshita Kumari, Charles L. Barnes, Carol A Deakyne, and Jerry L. Atwood Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00148 • Publication Date (Web): 18 Mar 2015 Downloaded from http://pubs.acs.org on March 26, 2015

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Crystal Growth & Design

Solvent induced manipulation of Supramolecular Organic Frameworks Rahul S. Patil,† Amanda M. Drachnik, † Harshita Kumari, †,‡ Charles L. Barnes, † Carol A. Deakyne, † Jerry L. Atwood. † †Department of Chemistry, 601 S. College Avenue, University of Missouri-Columbia, Columbia, MO 65211. ‡James L. Winkle College of Pharmacy, 3225 Eden Avenue, University of Cincinnati, Cincinnati, OH 45267. KEYWORDS: Resorcin[4]arene; bipyridine; crystal engineering; supramolecular chemistry; pyrogallol[4]arene

Abstract

Supramolecular organic frameworks (SOFs) based on C-alkylresorcin[4]arene (RsCn) and conformationally flexible 1,2-bis(4-pyridyl)ethane (bpea) spacer are discussed here. The conformational flexibility of bpea molecule is predetermined with the help of electronic structural calculations of bpea in the gas and solution phases. The architectures of these frameworks are majorly governed by O–H···O and O–H···N intermolecular hydrogen bonding interactions between the components of the frameworks. The unique arrangement of bpea spacer

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around RsCn yields continuous 1D wave-like and 1D lateral hydrogen bonded frameworks. The cocrystals of RsCn with bpea show conformational variation as a function of a change in the solvent of crystallization. In acetonitrile, the spacer bpea exists in trans form solvent whereas in ethanol it adopts both gauche and trans forms. RsCn, on the other hand, adopts a pinched cone conformation in both solvents. The extended frameworks of RsCn-bpea in ethanol enclose continuous channels filled with arrays of hydrogen bonded gauche bpea molecules.

Introduction The design and synthesis of supramolecular organic frameworks (SOFs) have gathered the attention of chemists due to the potential applications of SOFs in the areas of gas sorption and separation.1-4A SOF is an aggregate of two or more organic species held together by noncovalent interactions such as hydrogen bonding, halogen bonding, cation-π, π-π, or van der Waals interactions. The building blocks of SOFs play a vital role in the functioning of the SOFs,5-9 as well as in their structural morphology. Resorcin[4]arenes (RsC), a subclass of calix[4]arenes, have proven to be useful in the construction of SOFs because of their unique shape and functional group flexibility. A typical C-alkylresorcin[4]arene (RsCn) has a bowl shape with eight hydroxyl groups at the upper rim and four C-alkyl tails emerging from the lower rim of the bowl. Thus, the upper rim of RsCn is hydrophilic compared to the lower rim, which remains hydrophobic in nature. These unique structural features render RsCn favorable in the design and synthesis of SOFs, which has helped to further our understanding of host-guest interactions10,11 and molecular recognition.12,13 RsCn itself crystallizes in a typical bilayer arrangement as a result of O–H···O intermolecular

hydrogen-bonding

interactions

between

the

hydroxyl

groups

of

the


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macrocycle.14,15 Previous examples have shown that the presence of water molecules provides additional hydrogen bonding between adjacent RsCn macrocycles leading to 6(RsCn).8(H2O) hexameric nanocapsule.16 Thus, both bilayer and hexameric, arrangements of RsCn are possible when the macrocycle remains in a crown/bowl shape. Note that of the four different conformers of RsC1, boat (C2v symmetry), chair (C2h), diamond (Cs) and crown/cone (C4v), the cone is believed to be the thermodynamically most favorable conformer.17 The preference for a particular conformer, however, is dependent on the nature of the lower rim tail of RsCn and on the synthetic conditions.17 The cone conformer of RsCn is selectively formed for the longer Calkyl tail lengths. C-methylresorcin[4]arene (n = 1), on the other hand, exhibits multiple conformers.17 Moreover, for RsC1 the occurrence of a cone or boat conformer in crystalline frameworks depends primarily on the type of solvent used for crystallization.15 A boat conformer of RsC1 is usually observed with a protic polar solvent of crystallization, such as ethanol (EtOH), whereas a cone or pinched cone conformer of RsC1 is usually observed with an aprotic polar solvent, such as acetonitrile (MeCN).18-20 The extended crystal structure of the boat conformer of RsC1 is typically arranged in a continuous 1D beam-like arrangement, whereas the cone conformer of RsC1 self-assembles in a typical bilayer-type arrangement.18-20 Thus, cocrystallizing RsC1 with various organic molecules leads to the formation of unique and distinct SOFs with respect to the molecular arrangement and orientation of guest molecules around the macrocycle.14,15,18,19,21-23 In particular, co-crystallization of RsC1 with 4,4’-bipyridine (bpy) has resulted in the formation of a variety of frameworks. Examples include discrete capsules and 1D wave-like, 1D linear, 2D brick-wall-like, 2D step-like and 3D networks.19 Our interest lies in the synthesis and analysis of SOFs from RsCn macrocycles and bpytype spacer molecules because the extended frameworks enclose void spaces. The void spaces


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are in the form of either discrete cavities or continuous channels. Recently, we have reported a library of frameworks (discrete capsule, brick-wall-like sheet, and 1D wave-like arrangement) synthesized from RsCn (n=1, 3) and bpy-type spacer molecules: 1,2-bis(4-pyridyl)ethylene (bpe) and 1,2-bis(4-pyridyl)acetylene (bpa).20A functionally similar but conformationally flexible spacer molecule, 1,2-bis(4-pyridyl)ethane (bpea), is co-crystallized with RsC1 and RsC3 in two solvent systems in this article (Scheme 1). Similarly to butane, bpea can adopt a gauche or a trans conformation;24 however, only the trans conformation was observed in the crystal structure reported by Ide et al.25 To support our contention that both conformations could be observed in the RsCn co-crystrals, yielding diverse extended frameworks, quantum chemical calculations were performed to determine 1) the relative stabilities of trans- and gauche-bpea, and 2) the activation barrier between them. The thermochemical data were evaluated in the gas phase and in (implicit) EtOH and MeCN solvents. To our knowledge an equilibrium structure and vibrational frequencies have been calculated previously only for the trans rotamer of bpea, and solvent effects were not taken into account in that calculation.26

Scheme 1. Components of SOFs.


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Materials and Methods RsC1 and RsC3 (C-propylresorcin[4]arene) were synthesized from an acid catalyzed condensation reaction of resorcinol with acetaldehyde and butyraldehyde, respectively.27 The bpea and solvents (acetonitrile and ethanol) were purchased from commercial vendors and used without further purification. The crystalline complexes were synthesized by mixing equimolar solutions of RsCn (n=1 and 3) and bpea in the same solvent in a 1:2 (v/v) ratio. The clear solution obtained after mixing was subjected to slow evaporation, which led to the growth of crystals within 7-10 days. Single-crystal X-ray diffraction (sc-XRD) data was collected on a Bruker Apex II CCD diffractometer at a temperature of 173(2) K/100(2) K using MoKα/CuKα radiation (0.71073 Å). The structure was solved and refined using SHELX with X-Seed28 as the interface. The synthetic procedures are explained in more detail in the notes section and the crystal structure details and cif files are given in the electronic supporting information (ESI). The quantum chemical calculations were carried out with use of the Gaussian 09 suite of programs29 and the results were viewed with Gaussview5.30 The geometries of trans-bpea and gauche-bpea were fully optimized using the int = ultrafine and opt = tight criteria. The ωB97X-D method was implemented with the 6-31G(d,p) basis set for the geometry optimizations. This DFT method includes an empirical correction for the dispersion energy. Normal-mode vibrational frequencies were evaluated to confirm that the two rotamers correspond to minima on the potential energy surface (PES) and to convert calculated energies to enthalpies. For better comparison with the literature energetic results for butane,31[single-point energies (SPEs) were computed at the ωB97X-D/6-311+G(d,p)//ωB97X-D/6-31G(d,p) and MP2/6-31G(d,p)//ωB97X-


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D/6-31G(d,p) levels of theory.The effect of ethanol and acetonitrile on the thermochemistry of bpea was studied using implicit solvent calculations. In particular, the above SPE calculations were repeated for both solvents using the SMD formalism of the polarizable continuum model.32 To ensure that the gauche and trans conformers are the only minima on the bpea potential energy surface (PES) and to locate the transition structure connecting these two conformers, a gas-phase relaxed PES scan was performed in which the molecule was rotated about the central C–C bond in 10° increments from 0° to 180°. MP2/6-31G(d,p) SPEs were obtained for each ωB97X-D/631G(d,p) structure along the scan. Results and discussion The relaxed PES scan varying the dihedral angle around the central C–C bond of bpea demonstrates that only the trans and gauche rotamers lie at minima on the PES (Figure 1). Similarly to butane, the enthalpy difference between the two rotamers is small, at 5 - 7 kJ/mol in the gas phase, ethanol solvent, and acetonitrile solvent, regardless of the level of calculation. At similar levels of calculation, the difference for butane ranges from 2 - 4 kJ/mol.31 Unlike butane, however, the gauche conformer is the more stable of the two. The gauche conformer was also found to be thermodynamically preferred for another 1,2-disubstituted ethane, 1-fluoro-2isocyanato-ethane, although only by about 2 kJ/mol.33As one might expect, the transition structure connecting gauche- and trans-bpea has a dihedral angle of 120°. The activation barrier for the gauche-to-trans transition in this conformationally flexible molecule is calculated to be about 22 kJ/mol, somewhat higher than the value of 14 kJ/mol reported for butane33,34 and for 1fluoro-2-isocyanato-ethane.33 In sum, the computational results suggest that the less thermodynamically favorable rotamer was observed in the reported crystal structure for bpea25


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and that it should be possible to observe gauche-bpea in the solid state, either independently or in combination with trans-bpea.

35 30 Relative Energy (kJ/mol)

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25 20 15 10 5 0 0

50

100

150

200

Dihedral Angle (○)

Figure 1. MP2/6-31G(d,p)//ωB97X-D/6-31G(d,p) relative energies of bpea as a function of rotation about the central C–C bond. Gauche-and trans-bpea are depicted next to their relative energies. The crystalline framework of 1a [(RsC1).(bpea).(MeCN)] consists of a rccc pinched cone RsC1 macrocycle. The pinching in the cone shape of a macrocycle is measured through comparison of the centroid-to-centroid distances separating the oppositely faced resorcinol units of the macrocycle. In 1a, these distances are 6.49Å and 7.13Å, thus making the RsC1 macrocycle slightly pinched with C2v symmetry instead of a perfect cone with C4v symmetry. Four O–H···O (O···O: 2.65Å - 2.79Å) intramolecular hydrogen bonds hold the macrocycle in the pinched cone geometry. A given RsC1 macrocycle hydrogen bonds with two adjacent RsC1 macrocycles (O···O: 2.63Å). The hydroxyl groups involved in these hydrogen-bonding


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interactions are from oppositely facing resorcinol units of the macrocycle, resulting in a lateral 1D hydrogen-bonded arrangement of the RsC1 macrocycles. (Figure 2A) The parent RsC1 molecule also forms three O–H···N (O···N: 2.58Å-2.64Å) hydrogen bonds with three trans-bpea molecules (Figure 2).The trans and gauche conformations of bpea were identified by evaluating the C3-C6-C7-C10 dihedral angle of the macrocycle (Figure 3C). The dihedral angles in the bpea molecules in 1a range between 176.2° and 180.0° (Figure 3C and Table 1),consistent with the reported crystal structure of bpea.25 Each of these bpea molecules is slightly tilted over the RsC1. The three ligands connect the RsC1 to two inverted macrocycles via O–H···N (2.60Å - 2.62Å) hydrogen bonds with the second nitrogen. These interactions yield a continuous 1D wave-like hydrogen-bonding pattern between the RsC1 macrocycles and bpea molecules (Figure 2B). Similar wave-like structures were observed for the relatively shorter bpy and bpe spacer ligands; however, those structures arise from hydrogen bonding between four bpy/bpe molecules and one RsC1 molecule.20 An acetonitrile molecule is situated in the center of the RsC1 bowl of 1a, and the hostguest complex is stabilized by CH-π interactions (3.43Å – 4.06Å). This acetonitrile molecule occupies the space between two head-to-tail arranged macrocycles in the extended framework.


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Figure 2. [A] Intermolecular hydrogen bonds formed in 1a. [B] Wave-like hydrogen bonding pattern in 1a. Color codes: Gray-carbon, Red-oxygen, Blue-nitrogen. To observe the effect of a more polar, protic solvent on the resultant extended framework, ethanol as a solvent of crystallization was employed in a similar experimental set up with RsC1 and bpea molecules. The distinctive asymmetric unit of 1b [(RsC1).(bpea).(ethanol)] has one trans-bpea molecule, one gauche-bpea molecule and one RsC1 macrocycle, a possibility predicted by the quantum chemical results. The pinched cone macrocycle has centroid-tocentroid distances of 6.54Å and 7.20Å. The two hydroxyl groups of one resorcinol unit form an O–H···N hydrogen bond with either a gauche- (2.63Å) or a trans- (2.67Å) bpea molecule. Similarly, the two hydroxyl groups of an oppositely facing resorcinol form O-H···N hydrogen bonds with another set of gauche (2.66Å) and trans (2.74Å) rotamers (Figure 3A).The C3-C6C7-C10 dihedral angles in the trans- and gauche-bpea are 168.8° and 58.5°, respectively (Table 1). Thus, one RsC1 macrocycle forms a hydrogen bond with four different bpea molecules (two


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gauche and two trans), all positioned horizontally with respect to the RsC1 bowl. In addition, these four bpea molecules link the parent macrocyle to four separate macrocycles via O–H···N (2.63Å - 2.80Å) hydrogen bonds. That is, both the trans and gauche conformers of bpea work as connectors, constructing a network between the parent RsC1 and four other RsC1 macrocycles. The RsC1 macrocycle connected to the other end of the trans-bpea is slightly twisted with respect to the parent RsC1. In contrast, the RsC1 linked to the other end of the gauche-bpea is oriented parallel to the parent RsC1. This unique arrangement of 1b differs from those observed for the bpy and bpe analogues in two ways. First, in the analogues the RsC1 exhibits a rccc boat conformation. Second, the extended frameworks of the analogues exhibit either a brick-sheet (bpy) or 1D beam-column-like (bpe) arrangement.20 A gauche-bpea molecule is situated on top of the bowl of the parent RsC1 in 1b as a result of CH-π interactions (3.66Å - 3.86Å) between the ethyl group of the bpea and the πelectron cloud of RsC1. Crystallographic expansion along the [001] axis reveals the formation of gauche-bpea occupied channels. The walls of these channels are supported by the array of transbpea and RsC1 molecules (Figure 3B).


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Figure 3.[A] Parent RsC1 hydrogen bonded to gauche- and trans-bpea in 1b.[B] Channels occupied by gauche-bpea in 1b. [C] Trans-and gauche-bpea. Color codes: Red-RsC1, Greentrans-bpea, CPK- gauche-bpea. As a second component of this study, co-crystallization of bpea and a RsC macrocycle with a longer propyl tail was investigated. The RsC3 macrocycles in the resultant crystalline framework 2a [(RsC3).(bpea).(MeCN)] have a pinched cone geometry,with centroid-to-centroid distances of 5.88Å and 7.61Å. Four hydroxyl groups of a given RsC3 hydrogen bond with four trans-bpea molecules through O–H···N (2.60Å-2.75Å) intermolecular interactions.The C3-C6C7-C10 dihedral angles of the trans-bpea molecules are 174.9° and 179.7° (Table 1). Note that again only the trans rotamer is observed in the co-crystals synthesized in MeCN. The four bpea molecules are horizontally oriented with respect to the parent RsC3 and are paired on opposite sides of the macrocycle. Each pairs of lateral trans-bpea molecules bridges between the parent


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RsC3 and another RsC3 macrocycle, hydrogen bonding through the second nitrogen atoms to the same resorcinol unit of the second macrocycle (Figure4A). Also, one hydroxyl group of each RsC3 forms an O–H···O (2.76Å) intermolecular interaction with a hydroxyl group of an inverted RsC3. The O–H···N intermolecular hydrogen bonding induces an extended 1D beam-like arrangement of the RsC1 and bpea molecules. The O–H···O interactions connect the beam-like arrays and are responsible for a typical AB bilayer-type arrangement of the RsC3 macrocycles (Figure 4B). The intermolecular interactions in the bpy and bpe analogues of 2a lead to a typical 1D wave-like arrangement and a discrete extended capsule, respectively. Thus, those frameworks enclose either continuous channels (bpy) or discrete void spaces (bpe).20


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Figure 4. [A] Horizontal arrangement of the components in 2a. Color codes: Gray-carbon, Redoxygen, Blue-nitrogen. [B] O–H···O intermolecular hydrogen bonds in 2a. Color codes: CPKRsC3, Green -trans-bpea. The crystalline framework of 2b [(RsC3).(bpea).(ethanol)] has RsC3 macrocycles with a pinched cone conformation with centroid-to-centroid distances of 6.47Å and 7.18Å. Two diagonally opposed hydroxyl groups of a RsC3 form O–H···N (2.60Å-2.67Å) hydrogen bonds with gauche-bpea spacer molecules (Figure 5A). A third gauche-bpea molecule is situated on top of the bowl-shaped cavity of the RsC3, resulting in CH-π interactions between the ethyl group of the bpea molecule and the π-electron cloud of the bowl. The C3-C6-C7-C10 dihedral angles of the gauche-bpea range between 65.4° and 66.1° (Table 1). The other end of each spacer bpea exhibits O–H···N (2.60Å-2.67Å) interactions with a hydroxyl group of an adjacently positioned RsC3 macrocyle. The RsC3 macrocycles also hydrogen bond to four ethanol molecules through O–H···O (2.64Å-2.98Å) interactions. Crystallographic expansion of 2b along the [010] axis reveals the presence of channels filled with gauche-bpea molecules (Figure 5B). The bpy and bpe analogues of 2bhave a 1D lateral arrangement of the RsC3 and spacer molecules,an arrangement that also creates discrete cavities within the extended frameworks.21 Similarly to 2a, a trans rotamer (C3-C6-C7-C10 dihedral angle = 180.0°, Table 1 and Figure 5B) is situated in the channels of 2b, providing another example of a mixed rotamer system. However, this trans-bpea molecule is not hydrogen bonded to RsC3.Instead it forms O– H···N (2.65Å) bonds with two residual ethanol molecules.


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Figure 5. [A] Intermolecular hydrogen bonding pattern in 2b. Color codes: Gray-carbon, Redoxygen, Blue-nitrogen. [B] Channels filled with gauche bpea in 2b. Color codes: Red-RsC3, Green – trans-bpea, CPK – gauche-bpea Table 1.Summary of RsCn and bpea conformations within the crystallographic frameworks.

Framework

Conformation of RsCn

Conformation of bpea

1a

Pinched cone

trans

1b

Pinched cone

trans and gauche

2a

Pinched cone

trans

2b

Pinched cone

trans and gauche

Dihedral angle C3-C6-C7-C10 of bpea 176.2°- 180.0° 168.8° and 58.5° 174.9°- 179.7° 180.0° and 65.4°- 66.1°


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Conclusion Novel SOFs of RsC1 and RsC3 with 1,2-bis(4-pyridyl)ethane were successfully synthesized in acetonitrile and ethanol. The intermolecular hydrogen-bonding interactions between the components result in the formation of extended lateral 1D to wave-like 2D frameworks. Use of the protic polar solvent ethanol brought about two types of change in the resultant co-crystals. First, unlike the SOFs formed with the 1,2-bis(4-pyridyl)ethylene and 1,2bis(4-pyridyl)acetylene analogues in EtOH,20 the conformation of the RsC1/RsC3 macrocyle is a pinched cone rather than a boat. Second, only the less stable trans-bpea (by 5-7 kJ/mol) is observed in the co-crystals synthesized in MeCN, but a mixture of trans- and gauche-bpea is observed in the co-crystals synthesized in EtOH. With these changes in conformation, continuous 1D channels filled with gauche-bpea molecules are enclosed in the extended frameworks. Overall, as one would expect on the basis of the computational results, it appears that the bpea molecule is sufficiently conformationally flexible (barrier height ~22 kJ/mol) to adopt the conformation required to maximize the hydrogen-bonding interactions between the RsCn macrocycles. ASSOCIATED CONTENT Supporting Information. Crystallographic information file for all the crystals structure is available with checkcif report. This material is available free of charge via the internate at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Authors *Dr. Jerry L.Atwood, Dr. Carol A. Deakyne, Dr. Harshita Kumari *Email: [email protected]; [email protected]; [email protected] Present Addresses †Department of Chemistry, 601 S. College Avenue, University of Missouri-Columbia, Columbia, MO 65211. ‡James L. Winkle College of Pharmacy, 3225 Eden Avenue, University of Cincinnati, Cincinnati, OH 45267. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources National Science Foundation Notes General crystal growing/synthesis procedure: In a 20 mL glass vial, 1mL 0.01M solution of RsCn in acetonitrile or ethanol was mixed with 2mL 0.01 M solution (in same solvent) of bpea molecule. The clear mixture was then sonicated for 5 minutes and allowed to stand for slow solvent evaporation. Crystals suitable for single-


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crystal X-ray diffraction were obtained over 7-10 days. Similar procedures were followed to grow the crystal for all the frameworks described in the article. ABBREVIATIONS RsCn: C-alkylresorcin[4]arene RsC1: C-methylresorcin[4]arene RsC3: C-propylresorcin[4]arene bpea: 1,2-bis(4-pyridyl)ethane bpy: 4,4’-bipyridine bpe: trans-1,2-bis(4-pyridyl)ethylene bpa: trans-1,2-bis(4-pyridyl)acetylene

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Synopsis: Synthesis and architectures of four novel Supramolecular Organic Frameworks (SOF) of Calkylresorcin[4]arenes with 1,2-Bis(4-pyridyl)ethane (bpea) are reported in this article. The flexibility in the spacer molecule (bpea) and the effect of solvent (protic vs. aprotic) resulted in the conformational changes of the components and formation of 2D wave-like and 1D lateral hydrogen bonded frameworks. Combined theoretical and experimental results suggest that the spacer molecule is sufficiently conformationally flexible to adopt the conformation required to maximize the hydrogen-bonding interactions between the RsCn macrocycles.


Solvent-Induced Manipulation of Supramolecular Organic

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