Guest Exchange in a Robust Hydrogen-Bonded Organic Framework

Article pubs.acs.org/crystal

Guest Exchange in a Robust Hydrogen-Bonded Organic Framework: Single-Crystal to Single-Crystal Exchange and Kinetic Studies Helene Wahl, Delia A. Haynes,* and Tanya le Roex Department of Chemistry and Polymer Science, University of Stellenbosch, P. Bag X1, Matieland, 7602, Republic of South Africa S Supporting Information *

ABSTRACT: The salt 3,4-lutidinium pamoate crystallizes as its hemihydrate, forming a hydrogen-bonded organic framework with tetrahydrofuran (THF) as a guest in channels in the structure (1·THF). Extensive investigation has shown this framework to be highly robust: the THF in the channels can be exchanged for 20 different compounds, with 13 of these exchanges occurring in a single-crystal to single-crystal manner. The THF can also be exchanged for the volatile solids pyrazine or iodine, both via single-crystal to single-crystal transformations. Stepwise exchange of solvents is also possible, with a sequence of five exchanges occurring before the crystals begin to deteriorate. Investigation of the kinetics of exchange in 1·THF revealed that exchange occurs according to a deceleratory kinetic model for contracting volume.

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INTRODUCTION Porous materials have been the center of industrious research for potential applications in separation, storage, sensing, and catalysis.1−5 Materials such as metal−organic frameworks (MOFs) have proven to be tunable and adaptable materials in terms of porosity.6−9 There are, however, limited examples of hydrogen-bonded organic frameworks (HOFs) that exhibit porosity, despite the benefits in terms of flexibility that these systems would offer.10−13 There are various examples of HOFs consisting of neutral organic molecules,14−17 but our interest lies in frameworks that are generated through the use of charge-assisted hydrogen bonds. There are few reported examples of frameworks of this nature.18−20 The guanidinium sulfonate series discovered by Ward and co-workers is one of the first examples of ionic hydrogen-bonded organic frameworks. In this system, chargeassisted hydrogen bonds were used to control properties such as pore size and nonlinear optical properties.21−24 Recently it has also been shown that porous frameworks can be constructed with the guanidinium sulfonate building blocks.25 Yamamoto and co-workers have identified a series of diamondoid porous organic salts26−29 consisting of triphenylmethylamine and various sulfonic acids. These frameworks incorporate a combination of charge-assisted hydrogen bonds and neutral hydrogen bonds or π−π stacking, thereby forming robust and highly stable diamondoid porous organic salts (dPOSs). We have described a number of hydrogen-bonded organic frameworks based on the pamoate anion.30,31 Pamoic acid (Scheme 1), a well-known salt-former in the pharmaceutical industry that is used to reduce the solubility of active pharmaceutical ingredients, has proven to be a versatile building block for framework formation. Recently we showed that one of these pamoate-based frameworks, the tetrahy© XXXX American Chemical Society

Scheme 1

drofuran (THF) solvate of 3,4-lutidinium pamoate (1·THF), is porous: the THF in the framework can be exchanged for acetone, dichloromethane, or diethyl ether in a single-crystal to single-crystal (SC−SC) transformation.30 Herein we report on extensive further studies regarding guest exchange in 1·THF. We have also investigated the kinetics of guest exchange in 1·THF. Various host complexes have been widely studied for their abilities to encapsulate or release various molecules.26,27,32−37 The kinetics of guest uptake or encapsulation into an empty framework or a host structure has been studied,38−46 and the inclusion and desolvation kinetics of host−guest compounds has also been investigated.47−51 There have been some studies on guest exchange in host−guest systems,52−57 but there are very few examples of investigation into the kinetics of guest exchange in framework systems, particularly hydrogen-bonded organic frameworks.58 If the mechanisms involved during guest exchange in such systems are better understood, we take a step toward being able to regulate these processes. This would be highly relevant, for example, in the pharmaceutical industry for the controlled released of drug molecules in the body.59−62 Understanding Received: May 15, 2017

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DOI: 10.1021/acs.cgd.7b00684 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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diverse compounds without collapse of the framework. Particularly remarkable is the exchange of THF for the volatile solids, iodine or pyrazine, in a SC−SC manner. SC−SC guest exchange of volatile liquids is rare in hydrogen-bonded organic frameworks, and only a few examples are known to date. Guest exchange of volatile solids in HOFs, however, is even more rare.63 To the best of our knowledge, 1·Pyrazine is the first ionic organic framework to include pyrazine in the crystal structure via guest exchange. Similarly, iodine uptake in framework materials such as MOFs64,65 and organic framework materials66,67 has been reported, but to our knowledge has not yet been reported for an ionic organic framework. In the crystal structure of 1·Pyrazine one pyrazine molecule and one THF molecule were modeled in the channel; that is, only a partial exchange has occurred in the particular crystal chosen for analysis (Figure 2a). Bulk measurements show that complete exchange has taken place in the bulk material (see Supporting Information). When crystals of 1·THF are exposed to iodine vapor, a striking color change occurs as the iodine permeates through the crystals from the edges to the center (Figure 2c). In the crystal structure of 1·Iodine the disordered iodine molecules spiral down the b axis (Figure 2b) with no THF remaining in the channels. The robust nature of the framework in 1·THF led us to investigate whether stepwise exchange was possible. A sample of 1·THF was exposed to several different solvents sequentially. It was found that in each case, complete exchange had taken place and the framework was still intact. Guests were exchanged as follows:

exchange processes would also allow better understanding of the factors that stabilize a framework material in the absence of solvent.

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RESULTS AND DISCUSSION 1·THF crystallizes as a 1:1 3,4-lutidinium pamoate salt, with half a molecule of water and one molecule of THF in the asymmetric unit. The crystal structure of 1·THF has been reported previously.30 Crystals of 1·THF are grown by dissolving a 1:1 molar ratio of 3,4-lutidine and pamoic acid in a minimum amount of 1:1 molar ratio THF/H2O. The 1:1 salt crystallizes as the THF solvate of a hydrate in the monoclinic space group C2/c. Four singly deprotonated pamoate ions hydrogen bond to one water molecule, forming an extended two-dimensional hydrogen-bonded network. The 3,4-lutidinium ions hydrogen bond to the remaining carboxylate oxygen of the pamoate ions, essentially locking the hydrogen-bonded network in place, and forming channels filled with disordered THF molecules (Figure 1). Previous work showed that the

THF → acetone → benzene → chloroform → diethyl ether → dichloromethane

It was also shown that exchange, at least in some cases, is reversible: on exposure to chloroform vapor, 1·THF converts to 1·Chloroform. Exposure of the chloroform-containing material to THF resulted in regeneration of 1·THF. Kinetic Studies. Because 1·THF undergoes guest exchange so readily with minimal change to the framework, it seemed an ideal candidate for investigating the kinetics of desolvation and guest exchange processes. There is much debate about whether solution-based kinetic models can be used to accurately describe the interaction between a solid and a gas,68 and conducting studies of these processes will help to determine whether solution-based kinetic models can accurately describe a guest exchange process in porous solids. In homogeneous reactions the change in concentration of the reactants and products is measured over time. However, in heterogeneous reactions between a solid and a gas or vaporized guest molecules, the reaction is monitored by the extent of the reaction, alpha (α), over time. The extent of the reaction is measured by mass loss (or gain) over time and can be expressed as m − mt α= 0 m0 − m f

Figure 1. Packing in 1·THF. The hydrogen bonding between the components of the framework is shown above; a packing diagram viewed down the c axis is shown below (framework in spacefill, THF molecules in stick representation).

THF in the channels could be exchanged for dichloromethane (DCM), diethyl ether, and acetone as a single-crystal to singlecrystal transformation when exposed to the vapor of these solvents.30 Investigation of Guest Exchange. In order to determine whether other solvents could be included in framework 1, crystals of 1·THF were exposed to vapors of 28 different compounds. Table 1 contains a list of the compounds tested. Of the 28 compounds investigated, 20 exchanged with the THF in 1·THF. Of these, 13 compounds underwent exchange in a SC−SC fashion. Those compounds that did not exhibit SC− SC exchange often only showed partial exchange and required slightly elevated temperatures for exchange to take place at all. Framework 1 is clearly remarkably robust. The compounds tested for exchange vary in shape, size, boiling point, polarity, and density, yet many are able to exchange with the THF in framework 1, and a large number via SC−SC transformations. The charge-assisted hydrogen bonds in framework 1 lend it strength and stability, allowing for exchange with so many

where m0 is the initial mass, mt is the mass at time t, and mf is the final mass. A graph of α versus time at fixed temperatures can then be used to identify a kinetic model that best describes the data. Values of α are substituted into rate equations that describe particular kinetic models (listed in Table S2 in the Supporting Information). The activation energy can then be determined by using the Arrhenius equation. There is some B



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Table 1. Vapor Exchange Experiments with 1·THFa successful exchanges

compound

stoichiometry of productb

molecules of solvent per asymmetric unit (NMR)

unsuccessful exchanges

molecules of solvent per asymmetric unit (XRD model)

molecules of solvent per asymmetric unit (TGA best model)

acetone30 benzene chloroform dichloromethane30 diethyl ether30 1,4-dioxane

1:1:0.5:1 1:1:0.5:1 1:1:0.5:0.7 1:1:0.5:0.75 1:1:0.5:0.7 1:1:0.5:1

1 0.88 0.72 0.75 0.57 1

1 1 0.84 0.87 not modeled 1

0.8 2.8c 0.6 0.75 0.7 0.5

ethanol (EtOH) methanol (MeOH) 1-propanol (PrOH) iodomethane toluene

1:1:0.5:1 1:1:0.5:2 1:1:0.5:1 1:1:.0.5:1 1:1:0.5:0.9 (trace THF) 1:1:0.5:2 1:1:0.5:0.8 (trace THF) 1:1:.0.5:1 1:1:.0.5:0.25:0.5 1:1:.0.5:0.4:0.1 1:1:.0.5:0.7:0.3 1:1:.0.5:0.9:0.2

1.54 1.55 0.89 1 0.79 (trace THF)

1.05 1.87 1 0.93 0.87

0.9 0 0.9 0.96 0.9

0.78 (trace THF)

0.71 0.5:0.5 THF

decomposition 0.9:0.3 THF

1.52 0.25:0.56 THF 0.39:0.14 THF 0.7:0.3 THF 0.9:0.2 THF

not not not not not

0.6 0.25:0.45 THF 0.4:0.1 THF

1:1:.0.5:0.5:0.3 1:1:.0.5:0.5

0.5:0.3 THF 0.5

not SC−SC not SC−SC

iodine pyrazine acetonitrile benzonitrile naphthalene 1-ethyl-pyrrolidone N-methylpyrrolidone (NMP) p-xylene m-xylene

SC−SC SC−SC SC−SC SC−SC SC−SC

compound cyclohexane cyclohexanone 2-pyrrolidone aniline morpholine dimethylformamide (DMF) pyridine o-xylene

product unchanged 1·THF unchanged 1·THF unchanged 1·THF unknown product unknown product mixture of SIQCIFd and TAPDARe TABMAKf unchanged 1·THF

0.65

a Successful exchange of the THF in 1·THF for the first 13 solvents/compounds, given in bold in the table, has been confirmed by single-crystal Xray diffraction. For the remaining seven compounds, it was not possible to collect single-crystal X-ray diffraction data as the crystals were either too small or did not diffract sufficiently to yield a full dataset. Other methods were therefore used to confirm that exchange had taken place. Details are given in the Supporting Information. bAll stoichiometries are given as pamoate/lutidinium/water/solvent. If THF is also present, stoichiometries are given as pamoate/lutidinium/water/solvent/THF. The stoichiometries given here are “best estimates”, with the NMR and TGA results (as bulk measurements) being given more significance. It seems likely that in most cases, one THF per asymmetric unit is completely replaced with one molecule of the exchanged solvent. This is not always reflected in the bulk measurements, but this could be due to solvent loss during analysis. The exception to this is MeOH, where two solvent molecules are included per ASU. It is probable that when single crystals were chosen, those in which complete exchange had not yet occurred were of better quality, and thus in some cases THF could still be modeled in the crystal structure. cWe are unable to explain this anomalous result. dPamoic acid DMF solvate. e3,4-Lutidinium pamoate DMF solvate. fPyridinium pamoate salt.

temperature dependence, do have practical importance and give quantitative information regarding the temperature-dependence of solid-state reactions. Thermogravimetric analysis (TGA) of 1·THF shows that decomposition of this material has a clear two-step mass loss profile, with the first step corresponding to loss of THF (see Supporting Information). An attempt was therefore made to measure the desolvation kinetics of this material using isothermal TGA. It was clear that desolvation occurs faster at higher temperatures, but the mass loss never reaches a plateau, even once complete loss of THF has occurred. The two mass loss steps (loss of THF followed by loss of lutidine leading to collapse of the framework) are not completely isolated from one another, and the kinetics of loss of THF could not be resolved. 1·Ether, 1·Dichloromethane, and 1·MeI gave similar problems. Efforts therefore turned to investigating the kinetics of solvent exchange. Exchange of THF for iodomethane (MeI) was investigated. Because of the large mass difference between THF and MeI, change in mass on exchange can be measured using a sensitive balance. Kinetic studies of this guest exchange process were performed on a vapor balance developed in-house.70 Exchanges were carried out in duplicate at five temperatures (17, 22, 25,

Figure 2. Packing diagrams of 1·Pyrazine (a) and 1·Iodine (b) viewed down the c axis. The framework is shown in space-filling, while guests are drawn in stick mode. In 1·Pyrazine one molecule of pyrazine and one molecule of THF have been modeled on special positions. In 1·Iodine the disordered iodine molecules spiral along the b axis. (c) The color change that occurs when crystals of 1·THF are exposed to iodine vapor. Photographs were taken at 0 min (left), 5 min (middle), and 50 min (right) after initial exposure.

controversy regarding the use of the Arrhenius equation to describe heterogeneous reactions, but Brown and Galway69 argue that the parameters Ea and A, and particularly their C



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30, and 35 °C). From the mass gain over time, α-time curves at the different temperatures were plotted (Figure 3).

rate constant for this process increases with increasing temperature. This suggests that removal of the initial guest, which would occur faster with increasing temperature, is a significant rate-determining step in the process of exchange. The Arrhenius equation and the natural logarithm thereof were used to determine the activation energy Ea and the preexponential factor A (Figure 5). The activation energy was

Figure 3. α-Time curves of the exchange of the THF in the channels of 1·THF for iodomethane, at different temperatures. Figure 5. Plot of ln k versus 1/T for the exchange of THF for MeI in 1·THF.

The α-time plots resemble both the deceleratory and sigmoidal kinetic models, although the deceleratory model better describes the exchange reaction: the initial reaction is fast, but as the reaction nears completion the reaction rate decreases. Various rate laws (Table S1) were fitted to the αtime curves, and it was found that the deceleratory model R3 for contracting volume best describes the data. This is illustrated in Figure 4, where the plots for the geometrical

calculated to be 55.1 (±4.0) kJ/mol, and A was calculated to be 1.81 × 107 min−1 (from ln A = 16.7 ± 1.6). Thus, for iodomethane to exchange for the THF in the channels of 1·THF an energy barrier of 55.1 (±4.0) kJ/mol must be overcome. Single Crystal Exchange and TGA. To further investigate the exchange process, attempts were made to follow the exchange of THF for MeI in single crystals of 1·THF. Four separate single crystals of 1·THF were exposed to MeI vapor at 25 °C for 1, 2, 4, or 6 h, respectively. Data collected after 1 and 2 h of exposure yielded satisfactory solutions and stable refined models, but after exposure for 4 or 6 h the crystals had cracked, and data from the resulting distinct domains could not be deconvoluted. These results are summarized in Table 3. The unit cell parameters of 1·THF do not change significantly on exchange, although there is a slight decrease in the β angle. No satisfactory model could be obtained for the channel contents after exposure to MeI for 1 or 2 h, but analysis of the SQUEEZE71,72 results is interesting. The electron count of 44 electrons for 1·THF correlates well with the 40 electrons expected for a single THF molecule. Similarly, the electron count of 69 in 1·MeI is close to the expected count of 62 for one molecule of iodomethane. This shows that SQUEEZE results in this system give a reasonable indication of what is occurring in the crystal structure during exchange. After exposure to iodomethane for an hour, an electron count of 55 is obtained, which correlates to half a THF and half an iodomethane. The presence of iodomethane is also implied by the existence of a large residual electron density peak in the difference map. After 2 h of exposure to iodomethane, an electron count of 80 is obtained. This indicates that the structure has more than one solvent molecule per asymmetric unit, possibly because the exchange process has not yet reached equilibrium. These results also suggest that inflow of iodomethane and outflow of THF occur simultaneously and not via an intermediate empty host structure, since both molecules are present in the crystal structure during the initial stages of exchange. TGA further supports this conclusion. Analysis of samples that were exposed to iodomethane for between 5 min and 48 h show a general increase in percentage mass loss as the exposure

Figure 4. Plots of the deceleratory model R3 rate law with linear regression lines and correlation coefficients.

model R3 are shown with linear regression lines. The equation of each linear regression line is of the form y = mx + c where m, the slope of the curve, is the rate constant k. This model implies that the rate of exchange is dependent on the volume of the particle exposed to the vapor of iodomethane. Rate constants for each temperature are given in Table 2. It is clear that the Table 2. Rate Constants for Exchange of THF in 1·THF for Iodomethanea temperature/°C

k/min−1

standard deviation

17 22 25 30 35

0.00189 0.00326 0.00523 0.00616 0.00719

±0.0002 ±0.00002 ±0.0005 ±0.0011 ±0.0010

a

Rate constants were determined based on the R3 model of contracting volume. D



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Table 3. Selected crystallographic data for single crystals of 1·THF exposed to iodomethane for 1 h (1·MeI_1h), 2 h (1· MeI_2h), 4 h (1·MeI_4h), and 6 h (1·MeI_6h)a 1·THF

1·MeI_1h

1·MeI_2h

space group a/Å b/Å

C2/c 22.117(1) 19.857(1)

C2/c 22.0503(8) 19.8214(7)

C2/c 21.996(6) 19.843(4)

c/Å β/° volume/Å3 temperature/K crystal mosaicity/° crystal dimensions/mm

14.6154(8) 115.240(1) 5806.0(6) 100(2) 0.49 0.144 × 0.163 × 0.386 44

14.5772(7) 115.065(1) 5771.2(4) 100(2) 0.55 0.164 × 0.232 × 0.459 55

14.607(3) 114.570(3) 5798(2) 100(2) 0.69 0.106 × 0.186 × 0.254 80

SQUEEZE e‑count (per asymmetric unit) a

1·MeI_4h

could not be solved

100(2) 0.74 0.112 × 0.146 × 0.262

1·MeI_6h

could not be solved

100(2) 0.62 0.089 × 0.091 × 0.289

1·MeI C2/c 22.040(2) 19.803(2) 14.597(2) 114.718(2) 5787.2(11) 100(2) 0.52 0.258 × 0.294 × 0.490 69

1·MeI is the structure obtained after exposure of crystals of 1·THF to iodomethane for 5 days; i.e., full exchange has taken place.

time to iodomethane increases (see Supporting Information). In other words, more iodomethane is present in the crystals after longer exposure to iodomethane vapor. The mass loss obtained in the TGA experiments correlates roughly with the electron counts from single crystals. These results confirm that influx of iodomethane and outflux of THF occur concurrently, with no evidence for an intermediate apohost phase. The simultaneous occurrence of these two processes also complicates interpretation of the kinetic results, as the two processes could well have different rate laws.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Full experimental details are provided in the Supporting Information. Exchange Experiments. Crystals of 1·THF were taken from the mother liquor and dried on filter paper. The crystals were placed in a glass vial, which was then placed inside a larger glass jar containing approximately 1 mL of the particular solvent. The lid of the jar was sealed with parafilm. Crystals were analyzed for exchange after approximately 1 week. In all cases the products after exchange with the relative compound were analyzed by powder X-ray diffraction to determine whether the framework structure was retained. 1H NMR, differential scanning calorimetry, and thermogravimetric analysis were used to determine or confirm the ratio of host/guest in the exchanged material. Details are given in the Supporting Information. Stepwise exchanges were carried out in the same way, except that approximately 20 mg of the crystals was removed for powder X-ray diffraction (PXRD) and 1H NMR analysis after exposure to each solvent. Crystals were exposed to each solvent for 5 days. Kinetics. Crystals of 1·THF were removed from the mother liquor, dried, and placed in a glass vial, which was placed in a larger jar that contained approximately 1 mL of iodomethane. Full exchange of THF for iodomethane occurred within a day, without a change in the framework structure. This was confirmed by single-crystal diffraction, PXRD, and 1H NMR analysis. Kinetic measurements were carried out at five temperatures on an in-house vapor balance.70 Data were analyzed using Alphatime, a program written by Prof. L. J. Barbour.73 Full details are given in the Supporting Information. Photographs of crystals were taken on a Leica Microsystems microscope camera (DFC295).

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CONCLUSIONS A detailed investigation of the porous salt, 3,4-lutidinium pamoate hemihydrate as its THF solvate, has been carried out. THF in the framework can be exchanged for numerous other solvents, often in a SC−SC process. The framework also takes up volatile solids (iodine and pyrazine), also in a SC−SC fashion. There is no apparent common property linking molecules that can be taken up by the framework, and it seems that this material will take up whatever is present in the atmosphere in sufficient quantities. The framework can also undergo stepwise exchange, with at least five sequential exchanges before the crystals start to deteriorate. The incorporation of charge-assisted hydrogen bonds clearly lends great stability to this framework. The kinetics of exchange in 1·THF were also investigated. Although it is difficult to unambiguously assign a kinetic model for the exchange, due to the simultaneous uptake and outflow of guests, some insight into the mechanism of guest exchange was obtained. It was shown that the rate of exchange is dependent on the volume of the particle exposed to the vapor (kinetic model of contracting volume). This model is in agreement with what is observed when monitoring this process using single crystals, as it can be seen that the framework does not undergo significant structural changes during the exchange process. Crystal size and not framework deformation is therefore the rate limiting factor. It was also shown that the rate constant for the exchange process increases with increasing temperature, indicating that removal of the initial guest may be an important rate-determining step. This study highlights the complexity of studying the kinetics of solid−gas interactions. It is clear that charged organic frameworks such as 1·THF can be extremely robust porous materials, and warrant further study and development.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00684. General experimental details; PXRD, NMR, TGA for exchanges; details of kinetic experiments (PDF) Accession Codes

CCDC 1549109−1549118 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. E



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Delia A. Haynes: 0000-0002-8390-5432 Notes

This material is based upon work supported financially by the National Research Foundation. Any opinion, findings and conclusions or recommendations expressed in this material are those of the author(s) and therefore the NRF does not accept any liability in regard thereto. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Research Foundation of South Africa and Stellenbosch University for funding. NMR data were collected on an instrument managed by the Central Analytical Facility at Stellenbosch University.

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