Use of Zwitterionic Ligands in Uranyl Hybrid Materials: Explorations on

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Use of zwitterionic ligands in uranyl hybrid materials: Explorations on the structural features that control water ordering and mobility Maurice K. Payne, Rebecca C. Laird, Margaret A. Schnell, Samantha R. Mackin, and Tori Z. Forbes Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01183 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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Crystal Growth & Design

For submission to Crystal Growth and Design

Use of zwitterionic ligands in uranyl hybrid materials: Explorations on the structural features that control water ordering and mobility Maurice K. Payne, Rebecca C. Laird, Margaret A. Schnell, Samantha R. Mackin, and Tori Z. Forbes* Department of Chemistry, University of Iowa, Iowa City, IA 52242 *corresponding author: [email protected]

Abstract Development of novel materials for water purification and storage is reliant on our understanding of the interaction of water within solid-state materials and inspired by the high selectivity present in certain biological systems. The present study investigates the influence of zwitterionic ligands on water properties within hybrid materials through the synthesis and characterization of four novel coordination polymers built from the uranyl unit and an imidazolium dicarboxylate (Imd) linker. One of the compounds (UIM-5), hosts an infinite water chain of edge sharing hexamers and octamers of hydrogen bonded water molecules. The other three compounds (UIM-2A, 2B, 2C) make up a polymorphic system in which the guest water molecules are found in varying degrees of isolation. We further explore the expansive literature on uranyl hybrid materials to identify common water nets and possible structural features that may control water structure and properties.

ACS Paragon Plus Environment


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Introduction Porous hybrid materials are a growing class of materials that can be engineered to create artificial water channels or membranes, but one of the significant issues with the design strategies is selectivity. Current materials used in the development of artificial water membranes are porous organic, inorganic, or hybrid materials that rely on different strategies to impart some level of water specificity. Organic peptide-appended pillar[5]arenes,1 dendritic dipeptides,2 and lipophilic ureidoimidazole supramolecular structures3 have all been previously reported to contain water channel systems and could potentially be embedded in a lipid bilayer system to create a selective channel.4 Ions can be excluded within these systems and the mechanism for this selectivity is thought to be based on hydrophobic or steric effects instead of hydrodynamic processes. Single walled carbon nanotubes are inorganic material that can spontaneously adsorb water when the overall diameter of the tubes falls between 0.8-2 nm

5-7

and exhibit high water

transport that is three orders of magnitude faster than predicted by Hagen-Poiseuille flow.8,9 Unfortunately, carbon nanotubes are not selective to water and are somewhat difficult to align for the fabrication of membranes with one-dimensional transport.10 Metal organic frameworks (MOFs) are a well-known class of hybrid materials that have demonstrated high water sorption and functional use in water reclamation strategies.11-13 The high porosity of these materials render them highly desirable for water capture, but a vast majority of MOFs are not water stable and only a handful display the desired specificity.12 In all cases, fundamental understanding of water under confinement within these materials is in its infancy and it is difficult to predict the important structural aspects that can lead to the desired properties. Aquaporin channels are resolutely selective to water and have chemical and physical properties that provide inspiration for advancements in water purification and storage materials.


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Aquaporins are proteins that are located within cellular lipid bilayers and form pores in the membranes of biological cells to transport water between the interior and exterior of the cell wall.14-16 These channels are highly selective to water and can prevent other solutes or ions, including protons, from crossing the membrane.17, 18 This selectivity can be traced back to an exquisitely designed hour-glass channel that contains 269 amino acid residues and induces the formation of a hydrogen bonded water wire.19,

20

At the channel’s narrowest point (3 Å), the

interior walls are lined with hydrophobic functional groups that are thought to facilitate the exceptionally high water permeability and provide a high dielectric barrier for other ions.21 Even with the small pore size, the water transport through the channel is 3 x 109 water molecules per subunit per second because of the formation of a single-file water wire that spans 20 Å in length.16, 20 High specificity and fast water transport rates are highly desired for applications in water delivery, purification, and storage and aquaporins provide inspirational strategies for targeted materials engineering and design with similar capabilities. We have previously described a metal organic nanotube (UMON) that has the stability, water ordering, and specificity with similarities to aquaporins and desired for artificial water channels.22, 23 The structural building units for UMON include a uranyl (UO2)2+ metal synthon chelated by iminodiacetate in a tridentate fashion and further linked into a hexameric macrocycle through a second iminodiacetate with a doubly protonated secondary ammonium group. Similar to MOFs, this is a hybrid material that forms large single crystals with 1.2 nm wide onedimensional pores that run the length of the nanotubes. Unlike MOFs, the UMON displays a highly ordered ice-like array of water molecules located in the channel, reversible (37 °C) dehydration, stability in humid and higher temperature ( 2σΙ) R indices (all data) Largest peak/hole (Å-3)

9.887(5) 11.785(5) 12.143(5) 61.353(12) 71.009(19) 74.31(2) 1162.9(9) 2 2.122 7.064 713 2.20-26.14 26966/4614 Rint = 0.0531 1.054 R1 = 0.0216 wR2 = 0.0433 R1 = 0.0263 wR2 = 0.0444 1.487, -1.009

8.3970(9) 10.3546(12) 12.3239(13) 77.192(4) 84.487(4) 67.058(4) 962.17(19) 2 2.321 8.510 636 1.70-25.30 23860/3493 Rint = 0.0366 1.061 R1 = 0.0138 wR2 = 0.0332 R1 = 0.0149 wR2 = 0.0337 0.437, -0.475

17.0609(16) 11.5767(11) 9.7259(9) 90 91.170(3) 90 1920.6(3) 4 2.325 8.527 1272 1.19-25.33 41878/3514 Rint = 0.0274 1.138 R1 = 0.0189 wR2 = 0.0479 R1 = 0.0232 wR2 = 0.0510 0.762, -1.166

8.4997(10) 9.4104(10) 13.2398(18) 107.866(5) 102.133(5) 97.487(5) 963.6(2) 2 2.317 8.498 636 2.62-29.35 35972/5237 Rint = 0.0478 1.106 R1 = 0.0187 wR2 = 0.0386 R1 = 0.0215 wR2 = 0.0391 0.895, -1.418



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Additional chemical characterization Crystals were filtered from their mother liquor, ground into an acetone slurry with a mortar and pestle, and deposited onto a low-background silicon wafer. The polycrystalline sample was air dried and analyzed on a Bruker D8 Advance diffractometer equipped with a LYNXEYE solid state detector. Powder diffractograms of all compounds were collected using Cu Kα radiation (λ = 1.5418 Å) from 4-50° 2θ with a step size of 0.02° at 1.0 s/step (Fig. S7S10). To determine phase purity the experimental diffractograms were compared to calculated patterns that were generated from the single crystal data in the program Mercury.33 Elemental analysis was performed to confirm the purity of the bulk material. Approximately 1.50 mg of each sample weighed in triplicate using a mettle Toledo microbalance with 1 µg accuracy. The sample was placed in a tin capsule, crimped to form a small pellet, and then combustion analysis was initiated using an Exeter Analytical elemental analyzer. Results from elemental analysis can be found in Table S5. Vibrational spectra were collected on the crystals to investigate the position of the uranyl bands and variations in the water stretching and bending modes. Raman spectra for single crystals were acquired on a SnowyRange Instrument High-Resolution Sierra 2.0 Raman Spectrometer. The instrument was equipped with a 785 nm laser source and all scans were acquired from 200 to 2000 cm-1 at the full laser power of 15 mW for a total of 30 averaged scans at a varying integration time of 2-60 s/scan. Infrared spectra were collected on a Nicolet Nexus 760 FT-IR from 400 to 4000 cm-1. To collect the IR spectra, a small amount (mg quantity) of each sample was ground in a mortar and pestle with KBr and the entire mixture pressed into pellet prior to acquisition.


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Thermogravimetric analysis was performed using a TA Instruments TGA Q500 to confirm the presence of lattice solvent molecules and elucidate overall thermal stabilities. Approximately 15 mg of each compound was loaded onto an aluminum pan and placed on the instrument. The samples were heated under N2 gas from room temperature to 580° C at a ramp rate of 2°/min.

Results Structural Characteristics All of the compounds contain the uranyl unit as common synthon that is further coordinated by the Imd ligand to form a pentagonal bipyramidal coordination geometry. Each uranium (VI) ion is strongly bound to two O atoms to create the uranyl cation (UO2)2+ with bond lengths ranging from 1.767(2) to 1.782(2) Å.34 All of the uranyl bond angles can be considered nearly linear with the furthest deviation from linearity occurring in UIM-2C where the O-U-O bonding angle is 176.8(1)°. UIM-5 possesses four carboxylate oxygen atoms coordinated in a monodentate mode about the equatorial plane with distances from 2.316(3) to 2.376(3) Å (Fig. 1a). One additional ligated water molecule bonds at a distance of 2.455(3) Å to complete the first coordination shell and form the pentagonal bipyramidal geometry about the U(VI) cation. To complete the inner coordination sphere in UIM-2A, 2B, and 2C five carboxylate oxygen atoms from the Imd ligand bond in the equatorial plane with distances ranging from 2.273(3) to 2.538(2) Å. For these compounds, there are four distinct carboxylate groups, one in bidentate coordination and three bound in a monodentate fashion to create the pentagonal bipyramidal geometry (Fig. 1b-d). The major difference within these compounds is the orientation of the flexible Imd ligand, which creates variety in the long range structural topologies.



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(a)

(b)

(c)

(d)

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Figure 1. Thermal ellipsoid (50%) representation of the asymmetric unit for UIM-5 (a), 2A (b), 2B (c), 2C (d). The U, O, C, and N atoms are represented by yellow, red, black, and light blue ellipsoids, respectively. The O atoms associated with the water molecules have been depicted as teal ellipsoids.


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We begin by describing UIM-5 as it contains the most elaborate water structure within this series of compounds. UIM-5 exhibits a 2-D sheet topology (2,4-c net with point group symbol (84.122)(8)2). In this case, each uranyl polyhedron links to four others via monodentate carboxylate coordination, while a ligated water molecule occupies the fifth and final equatorial site. This architecture creates porous non-interpenetrated sheets with pore diameters of 13 x 20 Å that are occupied by an additional five lattice water molecules (Fig. 2). The five lattice waters interact via hydrogen bonds between each other, with donor to acceptor (D…A) distances suggesting moderately strong (2.64(4) Å) to relatively weak (3.45(4) Å) interactions (Fig. 2b). Some of the water molecules also all exhibit hydrogen bonding with carboxylates and the ligated water molecule with D…A distances ranging from 2.71(8) to 3.42(8) Å. Furthermore, a few nonclassical hydrogen bonds may be observed between lattice water molecules and the aromatic entity of the Imd ligand (O4W--C15 (2.98(5) Å), O5W--C15 (3.02(2) Å), and O5W--N11 (3.21(2) Å). All of these interactions combine to create an extended H-bonding array of edge sharing hexameric and octameric rings down the [001] direction (Figs. 2 b-c). This elaborate Hbonding network connects the 2-D into the crystalline lattice and results in an overall molecular formula of [(UO2)(C7O4N2H7)2H2O](H2O)5.



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(a)

(b)

(c)

Figure 2. a) Ball and stick representation of the 2-D sheet topology for UIM-5. U represented by yellow polyhedra, whereas the O, C, and N atoms are depicted as red, black and light blue spheres. H atoms and lattice water molecules omitted for clarity. b) The lattice water molecules within UIM-5 are shown as teal spheres and the hydrogen bond network is displayed with


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relevant donor to acceptor distances. c) Water channels are located between the 2-D sheets within UIM-5.

UIM-2A, UIM-2B, and UIM-2C are polymorphs, as the flexibility of the Imd linker results in different arrangements of the mono- and bidentate coordination modes to create unique 1-D chain structures (Fig. 3). The topology of UIM-2A contains a U(VI) monomer linked through two Imd ligands to form a chain in the [2ത01] direction (Figure 3a) with a 2,4-c net (point symbol for net (42)(4)2). Each Imd ligand has one end-member engaging in a monodentate coordination and the other in a bidentate mode, which links the individual polyhedra into the chain topology. The extended structure in UIM-2B resembles a 1-D double stepped ladder (2,2,4-c net; 4.84.12(4)(8) point symbol for net) where two Imd molecules comprise the horizontal rungs connecting two uranyl polyhedra and two additional ligands create the vertical rails that extend the chain (Fig. 3b). While Imd molecules that form the rungs link the uranyl cations through monodentate bonding, the Imd ligand that creates the rails alternates its denticity between mono- and bidentate modes. A similar 1-D ladder-like chain with a 2,2,4-c net is formed in the case of UIM-2C, but the intra-ligand alternation between mono- and bidenticity occurs among the rungs of the system while the rails remain consistently monodentate (Fig. 3c).



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(a)

(b)

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(c)

Figure 3. Extended structure view for UIM-2A (a), 2B (b), 2C (c). Hexavalent uranium is depicted as yellow polyhedra whereas the O, N, and C atoms are shown as red, light blue, and black spheres. Water molecules are illustrated as teal spheres and H atoms have been removed for clarity.


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Despite similar ladder topology, the unit cell volume of UIM-2B is twice as large as that of UIM-2C, which can be explained by the packing efficiencies within the crystalline lattice. In both cases, a parallelogram can be drawn with each of the U atoms comprising a corner (Fig. S1S3). For UIM-2B, the U-U distance across the horizontal rung is 11.77 Å and the distance from one U to another along the vertical rail is 11.58 Å, this creates an overall area of 136 Å2. Parameters of the analogous parallelogram in UIM-2C are significantly smaller. The parallelogram of UIM-2C (101 Å2) encompasses an area 75% of that in UIM-2B with rung distances of 10.80 Å and rail distances of 9.41 Å. All three 1-D chain structures contain additional supramolecular interactions, namely π-π interactions that result in the formation of a crystalline lattice. These π-π interactions occur across two aryl centers of adjacent chains, stitching together the structure in the dimension perpendicular to chain propagation (Figs. S4-S6). In UIM-2A, the interplanar distance of the neighboring imidazolium rings is 3.54 Å and the centroid-centroid distance between the rings is 3.66 Å. This interaction corresponds to a displacement angle of 15.2° (angle between ring normal and centroid vector), which is common among hybrid materials constructed with aromatic nitrogen containing ligands.35 UIM-2B and 2C contain π-π interactions that are occurring through neighboring imidazolium rings of the ligands comprising the ladder rails, with interplanar distances of the π-π interaction 3.47 Å and 3.16 Å, respectively. The centroidcentroid distances for UIM-2B and UIM-2C are 4.06 Å and 3.82Å, respectively. All three compounds of the polymorphic system also contain two lattice water molecules to complete the overall molecular formula [(UO2)(C7O4N2H7)2](H2O)2. In UIM-2A, one of the interstitial water molecules donates hydrogen bonds (D…A = 2.785(3) and 2.835(3) Å) to equatorially coordinating carboxylate oxygen atoms of different chains, while the second water



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molecule donates hydrogen bonds to an uncoordinated carboxylate oxygen and a uranyl oxo of a chain adjacent (D…A = 2.796(3) and 3.096(3) Å). Within UIM-2B and UIM-2C, the two lattice water molecules donate hydrogen bonds (D…A = 2.440(4)-3.29(11) Å) to a ladder above and below, with the strongest interactions occurring in the case of UIM-2C. Also notable in UIM-2C is the removal of the water molecules from between the horizontal rungs and the addition of water molecules between the 1-D chains. This is likely due to the more efficient packing within UIM-2C, which prohibits the incorporation of the water between the rungs. UIM-0 was previously reported by Martin et al. and contains a double layer network arranged in a densely packed manner.31 Notably there are no waters of solvation within this compound and it is held together through strong interactions between the carboxylate O atoms and the U(VI) metal center. As is common in this system, there are π-π stacking of nearby aryl groups (planar distance = 3.47 Å; centroid-centroid distance = 3.90 Å) and some additional weak CH-π hydrogen bond interactions. One of the methylene hydrogens of the ligand is oriented directly towards an adjacent aryl center with parameters archetypal for this type of interaction (H---Aromatic = 2.96Å, C—H—Aromatic = 141.3°).36 Analysis with the ToposPro software confirmed that the structure is a complex interpentrating 2-D sheet (2,2,4-c net; point symbol (12)(4.125)(4)). The 2D sheet runs along the (100) plane and has a width of 8.15 Å, indicating that it spans the entire length of the unit cell. The room-temperature synthesis described vide supra resulted in higher yields (>90%) compared to 19% for the previously reported methodology that utilized synthetic temperatures of 120 °C.


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Chemical Characterization All compounds exhibit phase purity per the powder X-ray diffraction and elemental analysis (SI) and were further characterized by vibrational spectroscopy (Fig. 4 and Table S6). Raman spectra for all compounds are similar, with minor variations in the uranyl symmetric stretching band (ν1) located between 830 and 835 cm-1. UIM-2A contains two separate ν1 vibrations within this window at 832 and 835 cm-1, whereas all other compounds contain a single band within that region (Fig. 4a). In the infrared spectra, the uranyl asymmetric stretch (ν3) was observed in a similarly narrow energy window from 906-912 cm-1 for this series of compounds (Fig. 4b). For the compounds containing lattice water molecules the characteristic broad O-H stretching bands are observed in the region 3200-3600 cm-1 with this feature being unusually sharp for UIM-2A at 3617, 3523, and 3488 cm-1. The expected group stretches are found for the Csp3-H, Csp2-H, C=O, C-N and aromatic C-N and C-C bonds of the ligand.



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(a)

(b)

Figure 4. Raman (a) and infrared (b) spectroscopy of UIM-5, UIM-2A, UIM-2B, UIM-2C and UIM-0 (see Martin et al.31 for IR of UIM-0).


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Examination of the TGA data show good agreement with the hydration state predicted by single-crystal X-ray diffraction (Fig. S11-14). An initial 14% mass loss for UIM-5 occurs at temperatures less than 65 °C that can be attributed to the loss of the five lattice and one ligated water molecule (theoretical = 14.5%). The polymorphs all show an initial mass loss of approximately 5% that can be attributed to the loss of the two lattice water molecules (theoretical = 5.35%). The temperature at which this transition ends varies among those three compounds. Dehydration occurs slowest for UIM-2A, as the mass loss continues until 130 °C suggesting stronger water-framework interactions. UIM-2B shows this water loss complete by 90 °C and UIM-2C has the fastest dehydration, with complete removal of the water molecules at 55 °C. All of the compounds remain thermally robust following any water loss, with thermal stabilities until 260-290 °C. At this temperature regime the Imd ligand undergoes a degradation process and the loss of organic components leads to the transition to a U(VI) oxide material.

Discussion Structural features The topological features of UIM-5, 2A, 2B, and 2C have some similarities to previously reported compounds, but differ significantly from the uranyl Imd compounds recently characterized by Martin et al.31 The topological features of UIM-2A can also be observed in two uranyl acteylendicarboxylates37 and a related chain is observed in [UO2(DCT)DMF]· H2O (DCT = 4,4”-dicarboyxl-(1,1’, 3’1”)-terphenyl)38 and [UO2(BDC)H2O)] (BDC = benzophenone-4,4’dicarboxylate).39 A ladder-like array has only been reported in the case of uranyl thiodiglycolate compounds that exhibit varying degrees of curvature based upon interactions with the interstitial



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pyridine molecules.40 Comparing to the compounds reported by Martin et al.,31 similar linkages for the ligand are observed in (UO2)2(Imd)2(ox)3H2O, but the higher temperatures and pressures during the synthesis resulted in the in-situ formation of oxalate. Furthermore, increasing the pH leads to hydrolysis of the uranyl cation and favors the formation of oligomeric building units and extended

hydroxo-bridged

arrays

observed

in

(UO2)3O2(H2O)(Imd)·H2O

and

(UO2)3O(OH)3(Imd)·2 H2O.31 In both of those cases, the Imd ligand decorates the outside of a hexamer or 1-D chain structure.

Ordered water channels within solid materials The major purpose of this study is to investigate the structural features that promote the formation of water channels in hybrid materials, with the major focus on the impact of the zwitterionic ligand. The compounds presented herein contain well-ordered water molecules that provide some insight into the chemical parameters that may influence this feature. In this series of compounds, water is stabilized by the ligand and interacts stronger with the negatively charged carboxylate group than with the positively charged imidazolium. In addition, the water molecules are relatively isolated from each other in the polymorph series (UIM-2A, 2B, 2C), but they create an extended network in UIM-5. Only UIM-5 contains a pore size within the extended lattice that is greater than one nm, whereas the polymorphic compounds can be considered more densely packed networks. To gain some insight into the ligand itself, we turn to imidazolium-based ionic liquids (ILs) because they are a well-studied system that contain positively charged ligands typically paired with a halide or PF6- anion.41, 42 Classical molecular dynamics experiments have indicated that there are strong water-anion H-bonding interactions, but little to no interactions with the


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imidazolium cation as indicated by no change in the dipole moment of the molecule in the presence of water.43 In our structures, we also observe limited interactions with the imidazolium functional group and stronger interactions with free carboxylate O atoms. In addition, the imidazolium

groups of the ILs typically showed aggregation amongst themselves through

intermolecular interactions that increased with increasing alkyl chain length.43 In response to this aggregation, water networks can form that increase in size with increasing water concentration, suggesting the formation of nano-scale segregation or micelles. Isolation of water molecules in the ILs occurs with low water content in the structure, whereas the water network-like structure increases as the hydration state increases.43 We acknowledge that water ordering within ionic liquids is likely very different from solid-state materials; thus, we next turned our attention to the vast literature on porous hybrid materials and the ordering of water within solid-state compounds. For this initial analysis, we chose to limit our investigations to uranyl hybrid materials that were deposited into the Cambridge Structural Database.44 The search terms within the Conquest software45 included anything with uranium and at least three water molecules located 2.5-3.5Å (O-O distance) from each other and intermolecular H-bonding angles between 120° and 175°. Combining these search terms yielded 71 unique structures. We further limited our analysis to compounds with widely available Crystallographic Information Files (from the Cambridge Structural Database website), R1 values of less than 5%, and structures that contained displacement parameters for solvent water molecules that are less than 0.2. Structures that contained water chains with no direct solvent water connections (e.g. chains with three water molecules where the middle linking water molecule was ligated to a metal center) were not included in the analysis. This led to a total of 15 structures we chose to analyze in further detail,



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which are summarized in Table 3. The water networks within these compounds were analyzed for strong H-bonding only, which was identified as water-water interactions with donor to acceptor distances between 2.5 to 3.0 Å.


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Table 3. A subset of uranyl hybrid materials culled from the Cambridge Structural Database58 that contain water networks or channels. Pore volume was calculated assuming the van der Waals radii using the Crystalmaker X software.59 CSD code 246085 275318 709255 712239 774760 832118 832122 839613

Compound Formula

Water-water interactions

Ref

1-D 1-D 3-D 0-D 0-D 1-D 2-D 1-D

Pore volume(Å3), % Unit Cell 827.74, 21.65% 167.55, 22.85% 237.42, 28.31% 779.39, 28.86% 224.92, 24.88% 298.35, 34.42% 1140.65, 26.65% 595.49, 29.09%

Hexamer, Square Square 1-D wire (4 H2O) 1-D wire (3 H2O) 1-D wire (4 H2O) Square Square Hexamer

46

1-D

1207.86, 29.03%

1-D wire (3 H2O)

52

0-D 0-D 3-D 1-D 0-D 3-D

977.40, 34.22% 272.61, 29.49% 347.77, 27.91% 875.73, 30.91% 305.96, 30.52% 13880.65, 41.96%

Extended hexameric Ice Extended 1D wire Hexamer, Square Hexamer Extended 1-D wire Square connected into 2D net

53

Topology

K2Mg2U2(C2O4)7(H2O)2·9H2O [N4C6H22][(UO2)(H2O)(SO4)2]2 ·6 H2O [(UO2)2(C6O8)(H2O)2] ·2 H2O [CB6][(UO2)(H2O)2(NO3)2] ·7 H2O [(UO2)(CrO4)(H2O)(C6NH5O2)2] ·2 H2O [(UO2)(C12N2O4H6)(H2O)]·3 H2O [(UO2)3(C12H8O6)2(H2O)6]·10 H2O (NH(CH3)2)2(CB6)[(UO2)4O2(OH)2(C8H12O4)] ·8 H2O 839615 (NH(CH3)2)2(CB6)[(UO2)4O2(OH)2(C9H14O4)] ·8 H2O 915893 (C4N2H8)0.5[(UO2)(C4NH6O4)(C4NH5O4)] ·2 H2O 927979 (C4N2H12)[(UO2)(C2O4)2] ·5H2O 978940 [(UO2)3(C9H9(PO3H)3)2(H2O)2] ·14 H2O 1033538 [(UO2)(C7H7SO3)2(C7H8SO3)] ·4H2O 1400496 (C4N2H8)0.5[(UO2)(C4NH6O4)(C4NH5O4)] ·4 H2O 1404926 [Ni(C10N2H10)3][(UO2)2(C14H8O4)3] ·6 H2O


47 48 49 50 51 51 52

54 55 56 23 57


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The structural compositions of the final set of compounds are quite diverse and display either hydrophilic or hydrophobic interactions with the solvent water molecules.

Most

compounds contain water molecules that directly interact with hydrogen bond donors (amines) and acceptors (carboxylates, phosphates, and sulfates) associated with the ligands. In the case of (NH(CH3)2)2(CB6)[(UO2)4O2(OH)2(C8H12O4)]

·8

H2O

and

(NH(CH3)2)2(CB6)[(UO2)4O2(OH)2(C9H14O4)] ·8 H2O,52 water molecules are confined within hydrophobic regions surrounded by long chain alkyl groups and cucurbituril molecules. The notable exceptions to these two classifications are UIM-5 and the uranyl nanotube (UMON), (C4N2H8)0.5[(UO2)(C4NH6O4)(C4NH5O4)] ·2 H2O, which both fall between these two extremes. In the UMON compound, the central amine within the iminodiacetate linker is doubly protonated, leading to charge separation within the molecule. The H atoms of these doubly protonated functional groups are located on the exterior wall of the nanotube, leaving the relatively hydrophobic methylene groups and unreactive uranyl oxo groups to interact weakly with the solvent water molecules. For UIM-5 the water engages in far fewer interactions with the interior walls of the pore space, with the vast majority of the interactions taking place between the free solvent water. In addition, the water weakly interacts with the positive charge on the imidazolium group of the Imd ligand through electrostatics, rather than strong H-bonding interactions. Both cases lead to situations that are similar to the mechanisms observed in aquaporin, where electrostatics are thought to influence the formation of the H-bonding water wire and the selectivity of the channel.21, 60 This preliminary structural analysis of uranyl hybrid materials suggests that zwitterionic ligands are not necessary for the formation of water clusters and channels; however, they likely influence the behavior of water within the material in addition to aiding in the selectivity for water (over other guest species).


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Despite the diversity in the composition of the compounds analyzed, the structural features of the water channels typically display isolated clusters or extended wire networks (Fig. 5). In many cases, the pores extend in one direction and contain isolated water clusters with three, four, or six waters per cluster. The commonly observed isolated motifs include truncated chains (3 water molecules long), square nets with extensions (four water molecules plus additional water molecules extending into available pore spaces) and hexamers (six water molecules).

Previous investigations by Vaitheeswaran et al. utilized molecular dynamic

simulations to investigate structural features of water clusters confined in non-polar cavities of fullerene.61 Their results indicated that these cavities could also host cyclic trimers, square tetramers, and hexamers when the cavity was greater than 1 nm. In addition to these structural motifs, both pentamers and octamers were observed within fullerenes, but were not observed in the current set of compounds. Vaitheeswaran et al.61 also found linear trimeric units to be less energetically favored than the cyclic form due to a smaller number of H-bonds formed within the linear array. The second classification of water within uranyl hybrid materials is the extended water chain. These topologies often had resemblance to water wires, but in some cases had structural features that mimicked portions of the Ice-I structure. Water wires and extended water networks have also been observed in aquaporin channels16 and are greatly influenced by the chemical interactions that take place with the interior walls of the confined channels. Thus, a wider range of water clusters and extended structures may be observed as we extend our structural investigations to a wider group of hybrid materials.



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Figure 5. Common water network topologies observed in uranyl hybrid materials.

Formation of water networks and channels require some level of free pore space to allow the water molecules to interact with each other. The pore volume for the group of compounds selected for this initial study varies significantly, but the percentage of pore volume per unit cell remains more consistent. The lowest pore volume calculated for this set of compounds was 167.55 Å3 and contained an isolated square tetramer. The highest pore volume was 13,880.65 Å3 and the structure exhibited a more expansive extended 2D water net. Calculating the pore volume per unit cell resulted in more consistent values ranging between 21 to 42%. The largest pore volume currently recorded in the literature (5,201,096 Å3) is a uranyl MOF material, but it does not contain well-defined solvent molecules.62 This is true in many MOF compounds because the larger pore volumes do not stabilize the solvent molecules within the crystalline lattice, which leads to diffuse electron density within the interstitial regions and disorder of these


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molecules.12 Our initial data suggests that there may be an optimal pore volume range to support well-ordered water molecules within hybrid materials.

However, there was no notable

relationship between pore volume and the formation of water clusters versus extended networks within this small sub-set of compounds.

Water removal and mobility Subtle differences in the H-bonding networks may influence other physical phenomena, such as dehydration temperature and infrared spectral features. The compounds of the current study show varied facility for water removal, with UIM-5 and UIM-2C possessing the lowest dehydration temperatures (55-65 °C).

This phenomenon of easily released water suggests

greater interactions between the solvent molecules themselves than they have with the uranyl Imd framework. Having established the H-bonding network present in UIM-5 it comes as no surprise that we observe features in the TGA and vibrational spectrum that confirm these limited interactions with the 2-D uranyl Imd sheet. The presence of extended H-bonding networks tend to broaden the signal for the O-H symmetric stretch of water molecules, in addition to shifting it to lower frequencies.63 UIM-5 exhibits a broad band in the O-H stretching region that is also red shifted relative to UIM-2A, 2B, and 2C. This, along with the very low dehydration temperature removes any doubt about the existence of an extended H-bonding network. Further investigation of the hydrogen bonding interactions in the polymorphs show that UIM-2C has the most and the strongest hydrogen bonding between water molecules, which may explain the low dehydration temperature observed for this compound. In contrast, UIM-2A displays no hydrogen bonding between the water molecules, while UIM-2B has only one weak hydrogen bond between water molecules (D…A = 3.35Å). The infrared spectrum of UIM-2A



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also supports a claim of strong water-ligand interactions because we observe sharp peaks in the O-H stretching region that are blue shifted relative to the rest of the compounds. Many of the compounds listed in Table 3 were not further characterized by vibrational spectroscopy and/or thermogravimetric analysis, but of the ones that were, an important trend is observed. Namely, low temperatures of dehydration (

Use of Zwitterionic Ligands in Uranyl Hybrid Materials: Explorations on

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