Article pubs.acs.org/crystal
Structural Features in Metal−Organic Nanotube Crystals That Influence Stability and Solvent Uptake Ashini S. Jayasinghe, Daniel K. Unruh, Andrew Kral, Anna Libo, and Tori Z. Forbes* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States S Supporting Information *
ABSTRACT: Porous hybrid materials such as metal−organic nanotubes are of interest due to synthetic tunability, possibility for 1-D flow, and confinement of solvent molecules. In the current study, the stability and solvent selectivity of two hybrid materials with nanotubular arrays, (pip)0.5[(UO2)(HIDA)(H2IDA)]·2H2O (UIDA; IDA = iminodiacetate) and [(UO2)(PDC)(H2O)] (UPDC; PDC = pyridine dicarboxylate) were analyzed using X-ray diffraction, gas chromatography/ mass spectrometry, thermogravimetric analysis, and infrared spectroscopy. The fine details of the structural characterization, such as the presence of solvent molecules, were found to be important to the overall properties of the materials as evidenced by the increased stability of the UIDA compound when formed from a solvent mixture containing acetone. Careful analysis of the UPDC compound indicated that the ligated solvent molecule can be exchanged, which may impact the hydration state of the material. Overall, the UIDA compound displays complete selectivity to water, but the UPDC compound adsorbs THF, methanol, ethanol, and cyclohexane.
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INTRODUCTION Porous hybrid materials have important applications in advanced separations because of the tunability of both the metal and ligand precursors that create compounds with a wide range of cavity shapes and sizes.1−9 One of the most wellstudied group of compounds are metal−organic frameworks (MOFs), which contain a 3D network of pores that can be structurally engineered with diameters in the nanoscale range. The largest pore volume reported for this class of materials is 4.3 cm3/g,10 but even the standard MOF-5 material possesses a surface area of 2200 m2/cm3 and offers tremendous capacity for gas uptake and storage.11−13 Tunability of the pore dimensions also results in size exclusion and molecular selectivity for some MOF materials.14−16 An example of this phenomenon is the aluminum 1,3,5-benzentricarboxylate compound, MIL-96, which shows preferential uptake of CO2 over CH417 and has also been reported to separate aliphatic C5-diolefins, monoolefins, and paraffins for use in crude oil separation processes.18 Other hybrid materials, such as metal−organic nanotubes (MONTs), are much less studied than MOFs, but these compounds may also possess enhanced selectivity and transport properties for advanced separation technologies. The 1-D flow that is inherent in the nanotubular design of MONT materials is of interest for novel membrane technologies, but there are difficulties in materials synthesis due to the need to truncate the extended 3D lattice or promote curvature of 2D sheets to form the tubular motif. Only 30 MONT compounds have been reported,19−26 which pales in comparison to the over 20 000 MOFs found in the Cambridge Structural Database.27 Successfully engineered MONTs utilized ligands with the proper sterics to induce curvature or through organized © XXXX American Chemical Society
stacking of macrocyclic units by covalent or supramolecular interactions.20,22−26,28−36 Of the reported MONTs, several have exhibited unique ordering of the confined solvent molecules located in the interior of the tube and spontaneous rehydration of the material following a heat cycle. For example, Dai et al. reported that the compound [Zn(ATIBDC)(bpy)]· 3H2O (ATIBDC = 5-amino-2,4,6-triiodoisophthalic acid) can reversibly trap clustered water molecules within the interior channels of the nanotube.28 In addition, [ZnLCl2]·8H2O (L = 1,1′-methylenebis(3-(4-carboxy-2-methylphenyl)-1H-imidazol3-ium) formed nanotubular arrays that could incorporate large molecules (Congo Red, Oil Red O, Sudan Black B, and Methylene Blue) within its 2 nm diameter channels.37 Unusual confinement effects on solvents and molecules located within the interior channels have been observed for single-walled carbon nanotubes, but these properties have yet to be thoroughly investigated for MONT materials.31,38−51 We have recently begun a detailed investigation of a MONT developed within the research group, (pip)0.5[(UO2)(HIDA)(H2IDA)]·2H2O (UIDA), that exhibits unique solvent ordering and uptake properties.31,51 The UIDA nanotube is composed of six U(VI) metal centers linked through iminodiacetate (IDA) ligands to form macrocyclic units that utilize supramolecular interactions to form nanotubular arrays (Figure 1a). Six water molecules arranged in an ordered chair conformation are observed inside the nanotube and engage in H-bonding between nearest neighbors to create a unique ice nanotube Received: May 15, 2015 Revised: June 29, 2015
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Figure 1. Exchange properties and stability of porous UIDA31 (a) and UPDC54 (b) materials are the focus of the current study. U atoms are represented by the yellow polyhedra, and the C, O, N, and H are depicted as black, red, blue, and pink spheres, respectively. thoroughly combined using a vortex mixer. The vials were covered with perforated parafilm and allowed to slowly evaporate at room temperature. After approximately 2 weeks acicular crystals were obtained with lengths between 1 and 10 mm at yields of 74% based upon U. This is an improvement in crystallite size, quality, and yield compared with the previously reported methods. Structural and Chemical Characterization. Structural characterization of the materials was performed by isolating high quality single crystals from the mother liquor, coating in oil, and placing on a Nonius Kappa CCD single-crystal X-ray diffractometer equipped with Mo Kα radiation (λ = 0.7107 Å) and a low temperature cryostat. Data were collected at 100 K with the Nonius Collect software package,56 and peak intensities were corrected for Lorentz, polarization, and background effects using the Bruker APEX II software.57 An empirical absorption correction was applied using the program SADABS, and the structure solution was determined by direct methods and refined on the basis of F2 for all unique data using the SHELXTL version 5 series of programs.58 U atoms were located by direct methods, and the O, N, and C atom positions were identified in the difference Fourier maps calculated following refinement of the partial-structure models. Hydrogen atom positions associated with the organic linkers were fixed using a riding model. Additional crystallographic data for each compound and fractional coordinates and site occupancies of the water for the UPDC compound have been included in Tables S1 and S2, respectively, in the Supporting Information. To determine the purity of the bulk material, powder X-ray diffraction was performed from 5 to 60° 2θ with a step size of 0.05 and a count time of 1 s/step on a Bruker D-8 ADVANCE diffractometer equipped with Cu Kα radiation and a LynxEye solid state detector. The amount and identity of the solvent within the material was determined using a TAQ500 thermogravimetric analyzer interfaced with a Nicolet 4700 FTIR spectrometer via a Thermo Scientific evolved gas analyzer. To perform solvent analysis, approximately 15− 20 mg of each sample was loaded onto an aluminum pan, placed on the TGA instrument, and heated from 25 to 200 °C under nitrogen atmosphere at a ramp rate of 20 °C/min. The solvent within the a-UIDA sample was also investigated by gas chromatography/mass spectrometry. Approximately 0.3 g of a-UIDA crystals were heated in an oven at 80 °C for 16 h to remove all solvent from the crystalline lattice and subsequently exposed to ambient conditions for 6 h prior to analysis with the mass spectrometer to fully hydrate the material. The crystalline material was transferred to an amber bottle, purged with nitrogen, and then sealed with a crimp-top cap with rubber septum. The vial was heated to 75 °C in a water bath for 15 min to remove the solvent from the a-UIDA material, and a syringe was used to sample the gases present in the headspace. This sample was analyzed using an Agilent Technologies 7890 5975C GCMS gas chromatography mass spectrometry. After the initial experiment, the same crystals were subjected to two additional heat
within the interior channel. Previous thermochemical characterization of the material has determined that there are limited interactions between the confined water molecules and the interior walls of the nanotube.51 Initial uptake investigations utilizing hexane and DMSO to represent nonpolar and polar solvents found no evidence of uptake into the material,31 suggesting that the compound exhibits selectivity to water, but the mechanism for this selectivity is poorly understood. In the current study, we investigate stability and solvent exchange properties of the UIDA nanotubes crystallized with two different solvents (acetone (a-UIDA) and methanol (mUIDA)) and compare these properties to a previously reported nanoporous material, [(UO2)(PDC)(H2O)] (UPDC).52,53 The UPDC compound contains 1-D nanopores formed via linkages between the (U(VI)O2)2+ cation and 2,6-pyridine dicarboxylate (PDC) and has previously been reported by Jiang et al.54 to exhibit selectivity to water and methanol (Figure 1b). The study focuses on the importance of small structural changes in the crystalline lattice that impact the overall chemical and physical properties of the resulting material.
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EXPERIMENTAL METHODS
Crystal Synthesis. All solutions were prepared using Millipore water (18.2 MΩ), and chemicals purchased were used directly without further purification. Caution: (UO2)(NO3)2·6H2O contains radioactive 238 U, which is an α emitter, and like all radioactive materials must be handled with care. These experiments were conducted by trained personnel in a licensed research facility with special precautions taken toward the handling, monitoring, and disposal of radioactive materials. The a- and m-UIDA crystals were synthesized according to a previously published method described by Unruh et al.31 Briefly, 3.0 mL of a 0.2 M iminodiacetic acid (IDA) (Alfa Aesar) solution and 3.0 mL of 0.2 M piperazine (Alfa Aesar) solution were mixed with 1.5 mL of 0.2 M uranyl nitrate hexahydrate (Flynn Scientific, Inc.) solution in a glass scintillation vial. To promote crystallization of the nanotube, 7.0 mL of either methanol (m-UIDA) or acetone (a-UIDA) was carefully added to the aqueous solution, and the vials were capped. After 3 days, large (100−1000 μm) columnar crystals formed on the bottom of the vial with yields of 95% based upon U. These crystals were filtered, washed with acetone, and dried in air. The UPDC material was optimized based upon the previously reported method by Jiang et al.,54 Degetto et al.,55 and Harrowfield et al.52 One milliliter of a 0.2 M uranyl nitrate hexahydrate solution was mixed with an equal volume of 0.2 M 2,6-pyridine dicarboxylate (Sigma Aldrich) and the pH was adjusted to 4.0 using piperazine. Three milliliters of THF was added, and the resulting solution was B
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cycles (80 °C for 16 h), equilibrated at room temperature, and reanalyzed using the same procedure. Stability Experiments. To determine the long-term stability of the materials in the presence of water vapor, the UIDA and UPDC samples were subjected to saturated conditions and analyzed using Xray diffraction. Approximately 100 mg of each compound was weighed into separate 20 mL glass scintillation vials, which were placed into a larger 250 mL glass beaker. A small amount of water was added to the bottom of the 250 mL beaker, and then the container was sealed with parafilm to create a saturated atmosphere. A subsample of material was removed at various time points, and crystallinity of the samples was determined using powder X-ray diffraction. Diffractograms were collected on the Bruker D-8 ADVANCE diffractometer from 5 to 60° 2θ with a step size of 0.05 and a count time of 1 s/step. Hydration state of the material was also analyzed by the TGA instrument using the methods outlined previously. Crystalline degradation products were also studied by single-crystal X-ray diffraction with a Nonius Kappa CCD single-crystal X-ray diffractometer using methods described in an earlier section. The degradation product UIDA-2 crystallized in the triclinic space group P1̅ with a = 6.522(1) Å, b = 12.771(2) Å, c = 13.063(2) Å, α = 110.96(5)°, β = 90.31(5)°, and γ = 98.601(5)°. U atoms were identified during the initial solution, and the O, N, and C atoms were found in subsequent least-squares refinements. Hydrogen atoms associated with organic components were constrained using a riding model, whereas the H atoms on the interstitial water molecules for UIDA-2 were determined from the difference Fourier maps following subsequent least-squares refinement of the partial structure models. Select crystallographic data and bond lengths for UIDA-2 are located in Tables S3 and S4, respectively, in the Supporting Information. Solvent Exchange Experiments. Selectivity of the UIDA and UPDC compounds were investigated using solvents with a wide range of polarities, chain lengths, and molecular shapes, including methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 2-propanol, acetone, THF, hexane, and cyclohexane. Thirty milligram samples of a-UIDA, mUIDA, and UPDC solids were placed in 20 mL glass scintillation vials and heated in a gravimetric oven at 80 °C for 24 h to remove solvent from the pore spaces. This dehydration temperature was chosen to remove the water within the channels of all compounds but retain the ligated water in the UPDC solid. After sufficient heating, samples were removed from the oven and immediately added to glass vials that contained 3 mL of a solvent. These crystalline samples were allowed to equilibrate in the solvents for 16 h before the solid material was filtered, quickly rinsed with acetone, and immediately placed on the TGA instrument. During the degassing process, the solid state samples were heated to 180 and 220 °C for the UIDA and UPDC nanotubes, respectively, with a ramp rate of 20 °C/min. Gases evolved during the heating process were flushed through a transfer line that was stabilized at 225 °C and introduced into the FTIR chamber through the Thermo Scientific evolved gas analyzer. A library of IR spectra was established for this specific instrument setup for all solvents utilized during the solvent exchange experiments to confirm the identity of the vapors present in the solid state materials. Prior to the analysis, the TGA instrument was also primed with an initial heat cycle (up to 200 °C) while flushing the system with nitrogen to remove water absorbed on the inner walls of the heater and transfer lines.
nation sphere about the metal center and joining six individual polyhedra into a macrocycle. Each macrocyclic unit is significantly corrugated, allowing the synthons to stack into nanotubular arrays through hydrogen bonding interactions. The interior diameter of the nanotubes is 1.2 nm and can be filled with an ordered array of water molecules when fully hydrated. Additional single-crystal X-ray diffraction studies have indicated that the crystallinity of the material and the diameter of the channel remain constant upon removal of the water molecules.31 With detailed investigations of the extended lattice, one small variation in the structures emerges between the a-UIDA and mUIDA compounds. The nanotubular arrays are negatively charged; thus, additional piperazinium cations are located in the interstitial regions. Smaller channels form upon packing the nanotubes and charge-balancing cations into the crystallographic lattice. These cavities are 0.63 nm in diameter, and when the compound is synthesized in a H2O/methanol mixture (m-UIDA), the smaller channel is completely vacant (Figure 2a). Changing the solvent from H2O/methanol to H2O/ acetone (a-UIDA) results in inclusion of a disordered acetone molecule within this interstitial region (Figure 2b). The carbon (C12) and oxygen (O11) atoms associated with the ketone are both located on special positions within the trigonal space
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RESULTS AND DISCUSSION Characteristics of the Solid-State Materials. Major structural components of the dehydrated a- and m-UIDA compounds are identical with uranyl (U(VI)O22+) cations linked through the iminodiacetate ligands in the equatorial plane to create the macrocylic building units. One iminodiacetate molecule chelates the metal center in a tridentate fashion via the two carboxylate functional groups and the central amine group. Two additional iminodiacetate linkers coordinate to the uranyl cation through an O atom on the carboxylate endmembers, completing the pentagonal bipyramidal coordi-
Figure 2. Small cavity on the exterior of the nanotube (circled in red) is vacant in the (a) m-UIDA material, but an acetone molecule can be observed in the (b) a-UIDA compound. The U atoms are represented by the yellow polyhedra, and the C, O, N, and H are depicted as black, red, blue, and pink spheres, respectively. C
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group, whereas the methyl groups (C11) are observed in partial occupancy around the central carbon. Weak intramolecular forces hold the acetone in the observed position, allowing the methyl carbon atoms to occupy several sites within the cavity. TGA analysis of the hydrated m- and a-UIDA materials indicates that dehydration takes place at 37 °C, and analysis of the evolved gas by FTIR did not find evidence of removal of the acetone from the crystalline material up to 100 °C (Figures S2 and S3 in the Supporting Information). Additional characterization of the gases using GC-MS indicated the presence of acetone as evidenced by the molecular ion peak at 58 m/z and the base peak at 43 m/z (Figure S4 in Supporting Information). After multiple heat cycles, the peak was still observed, suggesting that the release of acetone from the a-UIDA material is a relatively slow process. The UPDC compound was initially characterized by Immirzi et al.53 and further refined by both Harrowfield et al.52 and Jiang et al.54 The compound contains one crystallographically unique U(VI)O22+ cation coordinated through the equatorial plane by 2,6-pyridine dicarboxylate and bonded to the metal center in a tridentate fashion. Further linkages occur through the free O atoms on the carboxylate functional group and an additional ligated solvent molecule completes the pentagonal bipryamidal coordination sphere. Overall, the material is linked into a porous framework through the PDC ligands, creating open nanotubular cavities that are 0.6 nm in diameter. Water within the cavity was not identified by Immirzi et al.53 or Harrowfield et al.,52 but Jiang et al.54 suggested the presence of disordered water within the channels based upon TGA experiments.52−54 The current structural characterization provides evidence of water within the channel and the atomic positions of the O atoms were refined. One partially occupied water molecule was located in the central cavity of the UPDC compound (UPDC(1)) where the water could be modeled as a split site (OWa and OWb) with total occupancy of 0.3333, leading to an overall formula of [(UO2)(PDC)(H2O)]·0.3 H2O (Figure 3a). Water molecules present in the interior channel engage in H-bonding interactions with the ligated water molecules bound to the U(VI) metal center. Donor to acceptor distances of 2.85 and 3.02 Å (Figure 3b) are observed between the H atoms on the bound solvent and the interstitial water molecule, suggesting relatively strong intermolecular forces. This is in stark contrast to the water molecules present in the UIDA compounds, where there are limited interactions between the confined water molecules and the interior walls of the nanotube, but strong H-bonding networks between water molecules to form an ice-like conformation (Figure S5, Supporting Information).31 Single-crystal X-ray diffraction experiments on the UPDC compound suggested variations in hydration state and the identity of the ligated solvent molecule on the uranyl polyhedra. The interstitial water molecule was located again in the structural characterization of a different UPDC crystal, and the occupancy was allowed to free refine to a factor of 0.2014, indicating there may be some variability to the hydration state of the material. Within this second data set, C atoms were located within the difference Fourier map following refinement of the structural model that were approximately 1.5 Å from the bound O atom and refined with an occupancy factor of 0.25. Additional C atoms needed to complete the molecule could not be located in this manner, most likely due to positional disorder of the ligand. This suggests that a THF molecule can substitute for the bound water molecule within the uranyl pentagonal
Figure 3. (a) Water molecules (depicted by teal spheres) are observed within the channels of the UPDC compound and (b) engage in H bonding interactions with the ligated water molecules located on the interior walls. The U atoms are represented by the yellow polyhedra, and the C, O, N, and H are depicted as black, red, blue, and pink spheres, respectively.
bipyramid and its presence has been confirmed using chemical characterization techniques that will be described in greater detail below. Because THF was not utilized in the previously reported synthetic techniques,52−54 this ligand substitution is unique to the UPDC compound. Stability of Material in Humid Environments. Powdered m-UIDA and a-UIDA nanotubes that were placed in a saturated environment displayed differences in the structural integrity of the material as evidenced by powder X-ray diffraction (Figure 4a,b). Degradation of the a-UIDA sample was not observed over the course of 2 months as evidenced by no alteration of the experimental and predicted diffractograms. In contrast, the diffractograms of the m-UIDA powders begin to change after exposure to water vapor for 1 week, with a decrease in the intensity of the peaks associated with the nanotube and an ingrowth of new peaks at 9.99°, 14.38°, 16.25°, and 18.06° 2θ, corresponding to the formation of a degradation product. After 2 weeks of exposure to a saturated environment, the degradation phase comprises a significant component of the solid state material and after 30 days, the peaks associated with the original compound are completely absent from the diffractogram. During the degradation process, single crystals associated with a secondary phase were isolated at day 25, and the structure was characterized by single-crystal X-ray diffraction. The trigonal symmetry of the m-UIDA material was reduced to the triclinic P1̅ space group for the degradation phase, UIDA-2. Within the degradation product, the original inner coordination sphere of the (UO2)2+ moiety is maintained, but the sixD
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Figure 4. Powder X-ray diffractograms of the (a) a-UIDA and (b) m-UIDA nanotubes during exposure to water vapor for 60 days.
mole ratio increased from 1:2 for m-UIDA to 1:4 for the UIDA-2 degradation product. Comparing the predicted powder pattern obtained from the single crystal data for UIDA-2 to the diffractogram of the degraded sample did not lead to a complete identification of the final product (Supporting Information, Figure S6). Peaks at 7.5°, 14.5°, 16.7°, and 21.7° 2θ likely correspond to the UIDA2 compound and appear in the diffractogram after 7 and 14 days of exposure to a saturated environment. Additional peaks are observed starting at day 7 that suggest the presence of a multiphase system, and we suspect that the UIDA-2 dimeric phase is likely a transition phase that occurs before complete degradation of the sample. Additional small crystallites were observed in the final product, but were not suitable for analysis with single-crystal X-ray diffraction. We postulate that additional hydration of the compound will further disrupt the linkers, leading to the formation of a phase containing isolated uranyl IDA monomers. Overall, these experiments indicate that the presence of acetone in the a-UIDA compound provides additional stability for the solid-state material in humid environments. This stability is likely due to the inability of additional water molecules to occupy the smaller channels and disrupt the H-
membered macrocycle originally observed in the m-UIDA compound has been replaced by a dimeric species (Figure 5a). Hydrogen bonding interactions occur between the singly protonated amines on the IDA chelators and the carboxylate functional group on the neighboring linker molecules, with donor to acceptor distances of 3.16 Å. Overall the molecular formula for the dimer is [(UO2)2(HIDA)2(H2IDA)2]2−, and additional piperazinium cations are located in the crystalline lattice to account for charge balancing for the solid state material (Figure 5b). As a reminder, the water molecules in mUIDA were arranged into hexagonal rings within the interior cavity of the nanotube and a majority of the hydrogen bonding interactions occurs between neighboring water molecules. Channels of water molecules still exist in UIDA-2, but the hydrogen bonding between the water molecules has been disrupted so that additional interactions occur with other structural components, including the piperazinium cations and IDA ligands. Hydrogen bonding between the interstitial water molecules and other donors/acceptors within the lattice occur at distances ranging from 2.7 to 2.8 Å. As expected, the hydration state of the resulting material increases upon exposure to humid environments because the final U/H2O E
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Figure 5. (a) UIDA-2 degradation product contains isolated U(VI) dimers linked through iminodiacetate ligands. (b) Additional H-bonding interactions are observed in the crystalline lattice, and water molecules within the structure form an extended network within the interstitial regions. The U atoms are represented by the yellow polyhedra, and the C, O, N, and H are depicted as black, red, blue, and pink spheres, respectively.
bonding network that holds the macrocycles into the extended 1D nanotubes. Weak H-bonding interactions that stabilize the acetone in the crystallographic lattice prevent diffusion of the molecule out of the solid state material even at relatively high temperatures. Thus, the existence of a small amount of secondary molecules within the UIDA compound provides additional stability, suggesting that the type of solvent utilized during the initial synthesis is an important consideration with the design of similar materials. The UPDC material is quite insoluble in aqueous solutions and maintains structural integrity in humid environments. Powder X-ray diffractograms of the UPDC samples remain constant over the course of the experiment and match the predicted pattern (see Supporting Information, Figure S7). Increased humidity does not result in the degradation of the UPDC pattern, which can be attributed to the presence of
strong bonding interactions between the structural components to create the extended lattice. Hydrogen bonds do exist within the UPDC material, but these interactions are not the basis for the structural integrity. Solvent Exchange and Water Selectivity. Thermogravimetric analysis combined with FTIR of the evolved gases was utilized to investigate the uptake of solvents into dehydrated mUIDA and a-UIDA samples. Water is removed from the nanotubes at 37 °C, and the solvents used in this study are expected to be desorb from the compound between room temperature and 180 °C. The weight loss in this region for all samples was less than 1%, which is within instrumental error and indicates that there was no solvent uptake for the UIDA material (Table 1). Evolved gases were analyzed by FTIR, and in all cases the only solvent observed was water, which was identified by vibrational modes from 1300 to 2000 and 3500− F
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Table 1. Observed Weight Loss between 25 and 180 °C for the UIDA and UPDC Compounds after the Exchange Reactiona
rehydration, the removal of solvent occurs much faster and at a lower temperature (115 °C) than observed for the initial sample (see Supporting Information, Figure S8). The final weight loss was 3.97%, which matches well the removal of the ligated water for the compound [(UO2)(PDC)(H2O)] with a predicted weight loss of 3.96%, and also for the rehydrated sample, no THF peak was observed in FTIR spectra. This suggests that both THF and the water in the interior channel are removed upon heating and rehydration of the channel does not occur upon exposure to ambient conditions. Comparison between the as-synthesized and rehydrated sample allows the quantitative determination of THF in this sample to be [(UO2)(PDC)(H2O)0.9(THF)0.1](H2O)0.3. Jiang et al. also reported a larger than expected mass loss of 8.8%, compared with the calculated weight loss of 4.31% (associated with the bound water) and attributed this weight loss to an additional disordered H2O molecule present in the interior channels.54 This material was synthesized using hydrothermal methods and did not include THF as a solvent, so the larger experimental value could not be due to the identity of the ligated molecule. Harrowfield et al.52 also crystallize the UPDC compound using hydrothermal conditions, and structural characterization did not find evidence of a water molecule within the pore space. Jiang et al. did find that dehydration of the sample led to a shift in the (100) reflection in the powder diffractogram from 6.88° to 7.60° 2θ, suggesting a decrease in the lattice spacing, and that rehydration was favorable under ambient conditions.54 This evidence would also support the removal of a bound water molecule in the crystalline lattice because it could result in strain on the structural integrity of the material. Rehydration of the UPDC compound by placing the crystalline material in liquid water did increase the hydration state to the values observed by Jiang et al.54 TGA analysis of the rehydrated sample indicated a two-step weight loss of 8% between room temperature and 100 °C corresponding to one weakly bound interstitial and one ligated water. Structural characterization (UPDC(2)) of the rehydrated sample indicated three unique water positions with occupancies refined to 0.3333. Additional water molecules located in the channel also bond to the interior walls and form trimeric clusters along the length of the pore space (Supporting Information, Figure S5). Interactions between the confined water molecules are still relatively weak, with bond distances between 2.8 to 3.4 Å. Ethanol, cyclohexane, acetone, and methanol were observed in the FTIR evolved gas analysis of the UPDC sample after the exchange reaction. The exact amount of solvent absorbed into the channels cannot be discerned due to the variation in bound H2O/THF molecule that was also removed during the heat cycle. Presence of several different solvents in the UPDC sample indicates that the same level of selectivity found in the UIDA nanotubes is not observed for this material. Jiang et al. also observed uptake by methanol but did not find ethanol being adsorbed into the nanotubes and mechanism for this selectivity was suggested to be a physical separation based upon size.54 The current data supports this idea with the uptake of smaller solvent molecules (
