Single-Crystal Time-of-Flight Neutron Diffraction and Magic-Angle

Aug 7, 2017 - The Apex 3 software package was used to process the collected diffraction data, including corrections for background, polarization, and ...
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Single-Crystal Time-of-Flight Neutron Diffraction and Magic-AngleSpinning NMR Spectroscopy Resolve the Structure and 1H and 7Li Dynamics of the Uranyl Peroxide Nanocluster U60 Travis A. Olds,† Mateusz Dembowski,‡ Xiaoping Wang,§ Christina Hoffman,§ Todd M. Alam,∥ Sarah Hickam,† Kristi L. Pellegrini,† Junhong He,⊥ and Peter C. Burns*,†,‡ †

Department of Chemistry and Biochemistry and ‡Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States § Chemical and Engineering Materials Division and ⊥Instrument and Source Division, Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee 37831, United States ∥ Department of Organic Material Science, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *

ABSTRACT: Single-crystal time-of-flight neutron diffraction has provided atomic resolution of H atoms of H2O molecules and hydroxyl groups, as well as Li cations in the uranyl peroxide nanocluster U60. Solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to confirm the dynamics of these constituents, revealing the transportation of Li atoms and H2O through cluster walls. H atoms of hydroxyl units that are located on the cluster surface are involved in the transfer of H2O and Li cations from inside to outside and vice versa. This exchange occurs as a concerted motion and happens rapidly even in the solid state. As a consequence of its large size and open hexagonal pores, U60 exchanges Li cations more rapidly compared to other uranyl nanoclusters.

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INTRODUCTION Uranyl peroxide cage clusters developed over the past decade provide model systems for studying the structure−size− property relationships.1 They self-assemble in water and contain as many as 124 uranyl polyhedra, with diameters extending to 4 nm.2 The anionic cages are built from linear (UO2)2+ uranyl ions that contain two multiply bonded “yl” O atoms in a trans configuration, and these O atoms stabilize the cage on both the inside and outside. With more than 60 reported varieties, uranyl peroxide cage clusters adopt a broad range of topologies, several of which are fullerenes such as U60, which is topologically identical with C60. Countercations and the solution pH are important factors in the hydrolysis and condensation reactions that occur between oligomeric uranyl hydroxide peroxide species in solution to form cages3 and impact the resultant topology by stabilizing specific ring structures.4−7 Countercations are located both inside and outside the assembled clusters, and some can pass through the pores in the cage walls.8 In solution, transfer of countercations and protonated species through pores in the clusters is important in determining the solubility and behavior in water,9 including aggregation into blackberries.8,10−12 The structural whereabouts of H atoms of H2O molecules and hydroxyl (OH−) groups is a significant factor impacting these properties, yet little reliable information is available. Because of © 2017 American Chemical Society

their high charge densities and formation under alkaline conditions, protonation of the cages of uranyl peroxide clusters is typical and most contain hydroxyl bridges. Despite ubiquitous protonation, H atoms of hydroxyl bridges in uranyl polyoxometalate clusters have not been located. The lack of information on the countercation and H-atom locations in crystals of uranyl peroxide cage clusters is a barrier to a deeper understanding of the role of these constituents in determining the structures and properties of the clusters. X-ray diffraction data for cluster crystals is dominated by electron-rich uranium, making resolution of the H and Li positions particularly problematic. Neutron scattering efficiencies for the atoms of interest are much more favorable, although singlecrystal neutron diffraction is experimentally more demanding than X-ray diffraction because it requires a much larger crystal, a neutron beam, and an array of neutron detectors. Recently, we have demonstrated that the combination of neutron diffraction and 31P NMR is a powerful probe for hydrogenated uranyl peroxide pyrophosphate nanoclusters.13 In situ 31P NMR allowed for a better understanding of the formation mechanisms of U24Pp12 clusters in solution; however, solution NMR studies are not well suited to determine the dynamics of Received: May 10, 2017 Published: August 7, 2017 9676

DOI: 10.1021/acs.inorgchem.7b01174 Inorg. Chem. 2017, 56, 9676−9683

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Spallation Neutron Source (SNS), ORNL.21 A block-shaped hydrogenated crystal of U60, with dimensions of 0.65 × 1.9 × 2.0 mm, was mounted on the tip of a Kapton capillary using fluorinated grease and transferred to the TOPAZ goniometer. Data were collected at 100 K, using 13 crystal orientations optimized with the CrystalPlan software22 for an estimated 99% coverage of symmetry-equivalent reflections of the cubic cell. Each orientation was measured for approximately 8 h. Integrated raw Bragg intensities were obtained via 3D ellipsoidal Qspace integration by previously reported methods.23 Data were corrected for neutron TOF spectral Lorentz effects, and corrections for the detector efficiency were made with the ANVRED3 program.24 The reduced data were not merged and were saved in SHELX HKLF2 format in which the wavelength is recorded separately for each reflection. An initial structure was refined starting from the X-ray structure, which supplied the starting positions of the U and O atoms of the U60 cage, the K atoms within the shell, and some interstitial H2O (Ow) species. The remaining O and H atoms of the H2O molecules and hydroxyl groups and the Li atoms were located from difference Fourier maps calculated using the neutron data. The structure was refined using a mixture of anisotropic and isotropic treatments to convergence using the SHELXL-14/7 program.20,25 Most of the H2O molecules exhibit positional disorder and were refined with manually applied occupancy constraints that were set using free variables and distance constraints using the DFIX command with d = 0.96 and s = 0.03 for O- and H-atom pairs where appropriate. The single-crystal neutron structure of U60 is cubic, Fm3,̅ a = 37.8411(9) Å, V = 54186(2) Å3, Z = 4, and T = 100(2) K. Other information regarding the refinement and additional supplementary crystallographic information are found in the Supporting Information. 1 H and 7Li MAS NMR. Solid-state 1H and 7Li MAS NMR spectra were obtained on a Bruker Avance-III 600 MHz instrument using a 2.5 mm broad-band probe spinning between 20 and 25 kHz. 1H chemical shifts were referenced to the secondary external standard adamantane with respect to tetramethylsilane (δ = 0.0 ppm) and 1 M LiCl for the 7 Li chemical shift. The 1D 1H MAS NMR spectra were obtained using a rotor synchronized Hahn echo with a 4 s recycle delay, while the 7Li MAS NMR spectra were obtained using a single-pulse Bloch decay, a 1 s recycle delay, and a π/12 excitation pulse. The 2D 1H NOESY NMR homonuclear correlation experiments were obtained using a standard TPPI phase-sensitive pulse sequence with 256 t1 increments, 8 scan averages, and a 2 s recycle delay over a wide range of mixing times. The 2D 1H−7Li HETCOR NMR experiments were obtained using short 250−500 μs 1H−7Li cross-polarization (CP) contact times, 128−256 scan averages, and 64−128 t1 increments. All reported sample temperatures were corrected for frictional heating under spinning using the 207Pb chemical shift change of a secondary Pb(NO3)2 sample. Spectral deconvolutions were performed using the DMFIT software package.26

hydrogen in uranyl clusters because of exchange broadening.14 Several solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR) studies have highlighted the rapid exchange of encapsulated cations and H2O molecules inside and outside of the U2415,16 and U28 uranyl peroxide nanoclusters.16 Single-crystal X-ray diffraction could not resolve the positions of hydroxyl ligand H atoms in these studies, but their interactions with anions and H2O were inferred using multinuclear MAS NMR techniques. Here we report the single-crystal neutron diffraction structure of U60 and complementary studies using solid-state 1 H and 7Li MAS NMR. In addition to resolving aspects of the Li- and H-atom distribution and dynamics in the solid U60, we used 2D heteronuclear correlation (HETCOR) NMR experiments to demonstrate that hexagonal-shaped windows of U60 allow passage of Li cations and H2O molecules even in the solid state.

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EXPERIMENTAL SECTION

Caution! Isotopically depleted uranium (238U) used during these experiments is toxic and emits α particles. All work was carried out in facilities appropriate for the use of radioactive isotopes. Exposure of 238U to thermal neutrons leads to the formation of 239Pu, which emits α particles. Materials. General Considerations. Uranyl nitrate hexahydrate (UO2(NO3)2·6H2O) was heated at 450 °C for several hours to obtain UO3, which was subsequently dissolved in 14N HNO3. The resulting solution was evaporated to dryness on a hot plate, and the solid was recrystallized from ultrapure water, yielding fresh uranyl nitrate hexahydrate. All other reagents were used as received from commercial sources. Uranyl peroxide clusters are abbreviated as Un, where n corresponds to the number of [UO2(O2)(OH)]− units. Synthesis of U60, Li44K16[(UO2)(O2)(OH)]60·255H2O. Crystals containing U60 were prepared using methods adapted from those described previously.17 The reagents used below were passed through EMD Millipore 0.22 μm PTFE syringe filters and placed on a vibration-dampened shelf to promote the growth of large crystals. To an aqueous solution of 1.0 mL of 0.5 M UO2(NO3)2 (International Bio-Analytical Industries) was added 1.0 mL of 30% H2O2 (EMD Millipore) in a 20 mL scintillation vial, leading to precipitation of a pale-yellow solid, studtite [(UO2)(O2)(H2O)2]·2H2O. During stirring, the addition of potassium chloride (0.40 M, 0.1 mL; Sigma-Aldrich) and aqueous lithium hydroxide (4 M, 0.4 mL; Sigma-Aldrich) produced a clear yellow solution with a final pH of 9. Equant golden-yellow crystals formed by evaporation of the solution after 8 days, yielding ∼15 μg of crystals per vial. Several vials containing suitably large crystals (∼2 mm) were capped to preserve them for neutron diffraction studies. The remaining crystals were harvested by vacuum filtration and rinsed with 18 MΩ ultrapure water. Elemental analyses (%) by inductively coupled plasma mass spectrometry (ICPOES): Li, 0.364(6); K, 0.41(4); U, 0.495(17). Ideal formula based on 60 U pfu: Li, 0.367; K, 0.133; U, 0.500. The reported H2O content is based upon structure determination. Single-crystal X-ray diffraction data were collected on an optically homogeneous block-shaped crystal of U60 using Mo Kα X-rays from a microfocus source and an Apex II CCD-based detector mounted to a Bruker Apex II Quazar three-circle diffractometer. Data frames were collected using 0.5° widths on ω at 30 s frame−1. The Apex 3 software package was used to process the collected diffraction data, including corrections for background, polarization, and Lorentz effects. A multiscan empirical absorption correction was done using SADABS,18 and an initial model was obtained by the charge-flipping method using SHELXT.19 Refinement proceeded by full-matrix least squares on F2 using SHELX-2014/7,20 and the structure was refined to an R1 of 8.8% for 770 reflections with Iobs > 4σ(I). The space group Fm3̅ was confirmed as the most well-converged refinement. Single-crystal neutron diffraction data were collected using the TOPAZ single-crystal time-of-flight (TOF) Laue diffractometer at the

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RESULTS AND DISCUSSION

Single-Crystal Neutron Diffraction. The structure of the U60 cage was reported earlier based on X-ray data and confirmed by the neutron diffraction data. In brief, 60 uranyl ions with U−O bond lengths of ∼1.79 Å are arranged at the vertices of a fullerene topology with Oh symmetry, and these are bridged through peroxide groups that are bidentate to the uranyl ions and pairs of hydroxyl groups (Figure 1). Each uranyl ion is coordinated by two peroxide and hydroxyl groups, and the uranyl O atoms (Oyl) terminate the cage on both the inside and outside. The topology of U60 consists of 20 hexagons and 12 pentagons, which correspond to six- and five-membered rings of uranyl ions, respectively. The outer diameter of the cage is 24.2 Å wide, as measured from the centers of external Oyl atoms, and surrounds a cavity measuring ∼17 Å wide (20580 Å3) from the centers of the bounding internal Oyl atoms, which is populated with H2O molecules and K and Li cations. 9677

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located inside the cluster and below the hexagons of the U60 topology (Figures 3 and 4). The measured bond lengths are comparable to the distances we predicted earlier using molecular dynamics simulations for U60 (average 2.10 Å).4,9 For comparison, the Li−O distances from 796 structures deposited in the Cambridge Structural Database (2016) range from 1.78 to 2.40 Å, averaging 1.96(5) Å. The Li1 and Li2 atoms define the vertices of a regular dodecahedron, measuring 13.3 Å across (Figure 2c). The fourth H2O molecule bound to Li1 and Li2 (Ow1 and Ow2, respectively) is located beneath the hexagonal pore of the uranyl cage and can adopt several orientations (Figure 3). Two additional Li sites (Li3 and Li4), which coordinate to H2O, lie beneath the dodecahedron formed by the Li1 and Li2 atoms and provide further charge balance to the negatively charged cage (Figures S3 and S4). The two sites are positioned below several H2O molecules (Ow3, Ow4, and Ow5) that form hydrogen bonds with Oyl atoms. These hydrogen bonds are strong, with an average donor−acceptor distance of 2.60 Å. A single Li-atom position (Li5) is located outside the U60 cage based on the neutron data (Figure 5). It is coordinated to three disordered H2O molecules in a trigonal-planar arrangement (the average Li−O distance is 1.98 Å). Summing the refined site occupancies for the Li cations included in the neutron structure refinement indicates that at least 20 additional Li+ cations are not accounted for, presumably because of extensive disorder in the spaces between the clusters. In other uranyl peroxide nanoclusters, the most common binding mode of external cations is through side-on bonding with peroxide bridges.27,28 It is likely that scattering from the overlap of Li atoms with O atoms of H2O molecules acts to cancel their measured scattering intensity in the neutron structure. Such disorder is difficult to resolve; however, Li cations are likely positioned near the cage, within ∼4 Å from outstretched Oyl atoms. We were able to address this issue and understand the dynamic behavior of disordered Li atoms and H2O molecules using MAS NMR as shown below. The neutron study provided numerous H positions, which overall are most ordered near the cage wall. Most notably, H atoms of bridging hydroxyl units present in the cage of U60 are disordered over two sites and point both inside and outside of the hexagonal pore (Figure 3). These H atoms are locally ordered in a way that is consistent with the occupancy of the Li1 and Li2 sites and the hydrogen bonds emanating from their coordinating H2O groups. Where the Li1 or Li2 sites are

Figure 1. Polyhedral (a) and ball-and-stick (b) representations of the topology of U60. The cluster is built from 60 [(UO2)(O2)2(OH)2] subunits (c), which coordinate through bidentate peroxide (d) and hydroxyl units. Color scheme: U, yellow; K, purple;O, red; H, ivory. Note: hydroxyl H positions are split over two sites, pointing inside and outside.

The uranyl peroxide and hydroxyl cage of the U60 cluster has a net charge of 60−, which is balanced by K and Li cations located in close proximity to the cage walls. Cations within the U60 cage have variable coordination environments (Figure 2). The 12 K cations contained within U60 define the vertices of a regular icosahedron, measuring 16.7 Å across (Figure 2b). A K cation is associated with each topological pentagon inside the cluster and forms nine bonds with five Oyl atoms, three O atoms of peroxide groups of the pentagon, and a single disordered H2O molecule inside the cluster (Figure S3). According to chemical analyses, four additional K cations are present in the crystal structure, but these were not definitively located in the X-ray or neutron structures. The positions of the Li cations and H2O groups are disordered in many cases, complicating interpretation of the structure. However, it was possible to resolve several aspects of their distribution and bonding. Additionally, the neutron scattering amplitude from both the H and Li atoms is negative, providing enhanced discriminatory power. Two Li sites (Li1 and Li2) are tetrahedrally coordinated by four H2O groups at distances ranging from 1.84 to 2.03 Å, with the tetrahedra

Figure 2. Representation of encapsulated K (b; purple icosahedron) and Li (c; green dodecahedron) cation positions within the U60 cage (a). 9678

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Figure 3. Li atom that sits beneath each hexagonal pore coordinated to four H2O molecules, which form hydrogen bonds to μ2-OH bridges and Oyl atoms of the ring. On the right, a ball-and-stick representation highlights the protonation state of the hexagonal ring. Note: H atoms of the hydroxyl bridges can point inside or outside. Color scheme: U, yellow; O, red; Li, green; H, ivory.

cluster. The average O−H···O distance for the six structurally unique peroxide O atoms is 2.67 Å. Individual clusters are bridged to one another through disordered H2O molecules and Li+ cations. The nearest approach between two clusters occurs between Oyl atoms (O14−O14) at 3.77 Å. These Oyl atoms form weak hydrogen bonds to H2O molecules bound to Li5 (Figure 5). An additional hydrogen-bonded linkage (O−H···O 2.64 Å) is formed between protruding peroxide bridges and the H2O molecule Ow29 (Figure 5). The remaining cluster cage interactions with external species are unclear and are comprised of disordered H2O and cations centered about the H2O molecule Ow13 (Figure S5). MAS NMR. The neutron structure determination revealed partially occupied Li and H2O sites inside U60 and significant disorder of Li and K cations and H2O molecules in the regions of the crystal structure between the clusters. Neither the neutron or X-ray structures distinguish between the dynamic or static disorder of these constituents as they probe the longrange structure. Here we turn to NMR spectra to gain better insight into local configurations of these constituents. Variable-temperature 1H MAS NMR for powdered U60 crystals (Figure 6) reveals a strong temperature dependence on the dynamics between the different H sites in this material. The resonances at δ = +11.8 (12%) and +9.3 (2%) ppm (315 K) show small shifts with cooling related to increasing hydrogen bonding; however, there are no exchange dynamics occurring between these two species. On the basis of previous 1 H MAS NMR studies in uranyl complexes,14,15 the δ = +11.8 ppm resonance was assigned to a μ2-OH strongly hydrogenbonded to H2O. On the basis of correlations developed between the 1H NMR chemical shift and the hydrogen bond length in uranyl complexes,14 this chemical shift predicts a 1.0 Å UO---H or a ∼2.88 Å O---O hydrogen-bonding distance. This is in relatively good agreement with the average μ2-OH hydrogen-bonding distances in the crystal structure of 2.74 Å. The observed 12% relative intensity of this 1H environment is in excellent agreement with the 10.7% relative fraction for the expected OH/H2O ratio of 60/510. The assignment for the δ = +9.3 ppm resonance is a bit more ambiguous. It could result from μ2-OH in a weaker hydrogen-bonded environment but, more likely, is an encapsulated and strongly hydrogen-bonded H2O molecule, including H2O groups hydrogen bonded to the Oyl atoms. The variable-temperature behavior of these hydrogen-bonded environments is in stark contrast to the behavior of the strong H2O resonance at δ = ∼4.1 (86%) ppm, which broadens upon cooling and resolves into multiple signals at 290 K, finally giving rise to a δ = −1.6 ppm resonance and an exchanged broadened resonance at δ = +6 ppm. At 277 K, both of these resonances are assigned to H2O species that are in

Figure 4. Disordered H2O molecule shared by the Li1 and Li2 atoms also forming hydrogen bonds (dashed lines) with internal Oyl atoms. Color scheme: U, yellow; Li, green; O, red; K, purple; H, ivory.

Figure 5. Intramolecular hydrogen bonds (dashed lines) and Li cations (Li5) that hold clusters together through interactions between Oyl and peroxide O atoms of the pentagonal ring. The bonds are easily broken upon dissolution, which leads to high aqueous solubility. Color scheme: U, yellow; K, purple; O, red; Li, green; H, ivory.

occupied, two of the three H atoms of the μ2-OH units must be directed outward (outside the cluster), and the μ2-OH O atom also accepts a hydrogen bond from H2O coordinating Li1. There are 60 Oyl atoms that extend outward from the cage, and each accepts at least one hydrogen bond from interstitial H2O molecules. Each of the 60 Oyl atoms pointing inward is bonded to K+ and also accepts hydrogen bonds from H2O groups that bridge Li1 and Li2 (Figure 4). Peroxide units that bridge uranyl ions are unprotonated, but each does accept at least one hydrogen bond from H2O located outside of the 9679

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Figure 7. 2D 1H MAS NMR NOESY exchange spectra for U60 (323 K, 500 μs mixing time), with off-diagonal cross peaks showing spatial contact between H2O molecules and several strongly hydrogenbonded H atoms of the bridging hydroxyl units (μ2-OH). Note: +9.3 and +11.8 ppm resonances do not show a cross peak (highlighted in blue) and therefore must be spatially removed from each other.

(>5 Å) from each other or that the 1H are not readily exchanging between these different environments. Variable-temperature 7Li MAS NMR spectra (Figure 8) reveal a single Li-atom environment at room temperature due

Figure 6. Variable-temperature 1H MAS NMR spectra (isotropic chemical shift region) revealing several different H-atom environments, including bridging hydroxyl units (μ2-OH), lattice H2O [H2O(L)], encapsulated H2O [H2O(E)].

dynamic exchange with one another. At 277 K, the chemical shift of the encapsulated H2O at δ = −1.6 ppm is similar to that previously observed for Li−U24 and corresponds to an isolated H2O molecule in an isolated weak hydrogen-bonding environment. The distinct encapsulated (OH)−1 H NMR resonance seen at δ = −10 ppm in LiK−U24 and Li−U24 is not observed in U60, possibly suggesting a rapid internal exchange between the encapsulated H2O and (OH)− environments at these temperatures, and may contribute to the δ = 9.3 ppm resonance. However, charge balance considerations imposed by ICP-OES chemical analyses do not support the presence of encapsulated (OH)− in U60, such that a distinct OH resonance would not be expected. Details about the proximity of the H2O groups and μ2-OH H-atom environments were examined using 2D 1H MAS NMR NOESY homonuclear correlation experiments (Figure 7). At relatively short mixing time (10−500 μs), strong spatial correlations between bulk H2O [δ(1H) = +4.1 ppm] and μ2OH [δ(1H) = +11.8 ppm] environments were observed. This is consistent with H2O being directly hydrogen-bonded to μ2OH. It is also possible that these NOESY correlations could arise from exchange between H2O and the μ2-OH H atoms. Similarly, weakly hydrogen-bonded μ2-OH or strongly hydrogen-bonded H2O at δ = +9.3 ppm also revealed strong correlations with the H2O environment. More importantly, the δ(1H) = +9.3 and +11.8 ppm resonances did not show a NOESY cross peak and therefore must be spatially removed

Figure 8. Variable-temperature 7Li MAS NMR for U60. As the temperature is lowered, two different Li-atom environments are revealed: lattice Li at δ = +1 ppm and encapsulated Li at −5.2 ppm.

to dynamic averaging, and upon cooling, a distinct shielded encapsulated Li-atom environment appears at δ = −5.2 (29%) ppm and a lattice environment at δ ∼ +0.97 (61%) ppm. The assignment of encapsulated Li to the δ = −5.2 (29%) ppm resonance is consistent with the ratio of the lattice (29.6 apfu) to encapsulated (14.4 apfu) Li atoms from the neutron structure refinement of 37.2%. In addition, a lattice Li atom in the structure can essentially be compared to a hydrated Li 9680

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Figure 9. Left: 2D 1H−7Li HETCOR NMR spectra for U60 at room temperature (left) and reduced temperature (right). At 276 K, both the 1D 7Li MAS NMR spectrum (upper) and the 1D 7Li projection are shown.

Assuming a two-site exchange (A → B) between unevenly populated environments (pA − pB = Δp = 0.1) for two 7Li resonances separated by ∼1550 Hz, the correlation time, τc = (kA + kB)−1 can be estimated to be ∼175 μs at a coalescence temperature of 290 K.29 The Li exchange rates between the lattice and encapsulated environments for these different uranyl peroxide materials are as follows: U60 ∼ Li−U24 > LiK−U24. Because K cations bind to and presumably block transfer through pentagonal pores in U60 and hexagonal pores in U24, Li cations can migrate through the hexagonal pores of U60 more easily than square pores in LiK−U24.15 [At coalescence, the correlation time for the dynamics can be related to the population difference between the two environments]:

solution, similar to the 1 M LiCl chemical shift standard (δ = 0.0 ppm). The 2D 1H−7Li HETCOR NMR spectra are shown in Figure 9. At room temperature (299 K), a single Li-atom and H2O environment is observed as a result of dynamic averaging between the different Li- and H-atom environments, with the HETCOR spectra showing correlations between this average Li species at both the H2O and strongly hydrogen-bonded μ2-OH H-atom environments. At reduced temperatures, the 2D 1 H−7Li HETCOR NMR now shows a strong correlation between the lattice Li-atom environment [δ(Li) = +1 ppm] and both the lattice H2O [δ(H) = +5 ppm] and μ2-OH [δ(H) = +11.8 ppm] environments and weaker correlations (see projections in Figure 9) between the encapsulated Li-atom environment [δ(Li) = −5.2 ppm] and both the μ2-OH and broadened H2O resonance. No clear 1H−7Li correlation was observed between Li and δ(H) = +9 ppm of the H2O species, suggesting that this H2O is spatially removed from Li, or dynamics significantly averaging the 1H−7Li heteronuclear dipolar interactions are present. Similarly, no correlation was observed between the encapsulated Li-atom environment [δ(Li) = −5.2 ppm] and the encapsulated H2O environment at δ(H) = −1.6 ppm. Because the encapsulated Li cation is shown to correlate to both μ2-OH and H2O based on the 2D HETCOR NMR (Figure 9) and was observed under the hexagonal pore in the neutron structure (Figure 3), this lack of correlation to the encapsulated H2O may reflect poor 1H−7Li CP efficiency because of dynamics still being present for the various encapsulated species at the analysis temperatures. This is in contrast to the lattice Li species, which appear to have dynamics that are significantly reduced at the temperatures probed in the MAS NMR experiments (∼276 K). Lower temperature studies would be required to fully freeze out the dynamics of U60-encapsulated H and Li. Previous studies of the U24 cluster demonstrate that Li-cation exchange is significantly slowed in the presence of K cations.15 Li-cation exchange in U60 occurs at a lower temperature and faster exchange rate than that observed for the LiK−U24 capsules but has dynamics similar to that observed in Li−U24.

⎛ X2 − pA − pB = Δp = ⎜ ⎝ 3

2⎞ ⎟ ⎠

3/2

1 X

X = 2πτcδν τc = (kA + kB)−1 = pA /kB = pB /kA

The MAS NMR results demonstrate rapid exchange between U60 internal and external Li-cation environments in the solid state. Strongly bound K cations associated with the pentagonal rings effectively block transfer of Li cations and H2O molecules, whereas hexagonal pores can more readily accommodate transfer. Because only a weak 1H−7Li HETCOR NMR signal was observed between encapsulated Li and μ2-OH H atoms (Figure 9), movement of the Li cations and H atoms of μ2-OH units may occur in a concerted, but opposing, motion. That is, an encapsulated Li cation beneath the hexagonal pore is associated with dominantly outward pointing hydroxyl units (2 of 3). Likewise, interstitial Li cations are associated with outward-facing μ2-OH’s but may enter the cage when μ2-OH H atoms predominantly point inward, allowing the hydration shell of the Li cations to approach the H atom closer to the pore. In solution, the movement of cations and H2O molecules across 9681

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performed on the ORNL SNS’s TOPAZ single-crystal diffractometer was supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC05-00OR22725 with UTBattelle, LLC. The NMR portion of the work was performed at Sandia National Laboratories, which is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract DE-NA0003525.

the shell may be associated with a cluster shell breathing motion,30 involving expanding pores and H atoms of μ2-OH bridges, which may act as a diaphragm for transfer. Additional experimental verification of this concerted process is still required.

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CONCLUSIONS The solid-state techniques used herein partially resolved the positions and dynamics of the H and Li atoms in U60 and provided unprecedented resolution of their interactions with the uranyl cage and other encapsulated species. For the first time, we have resolved H atoms of bridging hydroxyl units in a uranyl peroxide cluster. H atoms of these units are involved in the transport of cations and H2O molecules across the cage boundary of uranyl peroxides, and using MAS NMR, we describe the dynamics of that transfer. The MAS NMR results revealed that the chemical shift dynamics of Li and protonated species are fast on the NMR time scale in the solid state, and multinuclear NMR experiments inform us of how the Li and H motions correlate. Ultimately, this allows us to extend our knowledge of how other porous clusters exchange H2O molecules and cations in the solid state.

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(1) Nyman, M.; Burns, P. C. Chem. Soc. Rev. 2012, 41, 7354−7367. (2) Qiu, J.; Burns, P. C. Chem. Rev. 2013, 113, 1097−1120. (3) Knope, K. E.; Soderholm, L. Chem. Rev. 2013, 113, 944−994. (4) Miró, P.; Pierrefixe, S.; Gicquel, M.; Gil, A.; Bo, C. J. Am. Chem. Soc. 2010, 132, 17787−17794. (5) Miro, P.; Vlaisavljevich, B.; Gil, A.; Burns, P. C.; Nyman, M.; Bo, C. Chem. - Eur. J. 2016, 22, 8571−8578. (6) Falaise, C.; Nyman, M. Chem. - Eur. J. 2016, 22, 14678−14687. (7) Vlaisavljevich, B.; Gagliardi, L.; Burns, P. C. J. Am. Chem. Soc. 2010, 132, 14503−14508. (8) Gao, Y. Y.; Szymanowski, J. E. S.; Sun, X. Y.; Burns, P. C.; Liu, T. B. Angew. Chem., Int. Ed. 2016, 55, 6887−6891. (9) Peruski, K. M.; Bernales, V.; Dembowski, M.; Lobeck, H. L.; Pellegrini, K. L.; Sigmon, G. E.; Hickam, S.; Wallace, C. M.; Szymanowski, J. E. S.; Balboni, E.; Gagliardi, L.; Burns, P. C. Inorg. Chem. 2017, 56, 1333−1339. (10) Gao, Y. Y.; Haso, F.; Szymanowski, J. E. S.; Zhou, J.; Hu, L.; Burns, P. C.; Liu, T. B. Chem. - Eur. J. 2015, 21, 18785−18790. (11) Flynn, S. L.; Szymanowski, J. E. S.; Gao, Y. Y.; Liu, T. B.; Burns, P. C.; Fein, J. B. Geochim. Cosmochim. Acta 2015, 156, 94−105. (12) Li, D.; Simotwo, S.; Nyman, M.; Liu, T. B. Chem. - Eur. J. 2014, 20, 1683−1690. (13) Dembowski, M.; Olds, T. A.; Pellegrini, K. L.; Hoffmann, C.; Wang, X.; Hickam, S.; He, J.; Oliver, A. G.; Burns, P. C. J. Am. Chem. Soc. 2016, 138, 8547−8553. (14) Alam, T. M.; Liao, Z. L.; Nyman, M.; Yates, J. J. Phys. Chem. C 2016, 120, 10675−10685. (15) Alam, T. M.; Liao, Z.; Zakharov, L. N.; Nyman, M. Chem. - Eur. J. 2014, 20, 8302−8307. (16) Nyman, M.; Alam, T. M. J. Am. Chem. Soc. 2012, 134, 20131− 20138. (17) Sigmon, G. E.; Unruh, D. K.; Ling, J.; Weaver, B.; Ward, M.; Pressprich, L.; Simonetti, A.; Burns, P. C. Angew. Chem., Int. Ed. 2009, 48, 2737−2740. (18) Sheldrick, G. M. Bruker/Siemens Area Detector Absorption Correction Program; Bruker AXS: Madison, WI, 1998. (19) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71, 3−8. (20) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (21) Jogl, G.; Wang, X.; Mason, S. A.; Kovalevsky, A.; Mustyakimov, M.; Fisher, Z.; Hoffman, C.; Kratky, C.; Langan, P. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2011, 67, 584−591. (22) Zikovsky, J.; Peterson, P. F.; Wang, X. P.; Frost, M.; Hoffmann, C. J. Appl. Crystallogr. 2011, 44, 418−423. (23) Schultz, A. J.; Jorgensen, M. R. V.; Wang, X.; Mikkelson, R. L.; Mikkelson, D. J.; Lynch, V. E.; Peterson, P. F.; Green, M. L.; Hoffmann, C. M. J. Appl. Crystallogr. 2014, 47, 915−921. (24) Schultz, A. J.; Srinivasan, K.; Teller, R. G.; Williams, J. M.; Lukehart, C. M. J. Am. Chem. Soc. 1984, 106, 999−1003. (25) Gruene, T.; Hahn, H. W.; Luebben, A. V.; Meilleur, F.; Sheldrick, G. M. J. Appl. Crystallogr. 2014, 47, 462−466. (26) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z. H.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70−76.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01174. Raman spectrum of U60 with peak assignments (Figure S1), concentration of cations for the empirical formula determined using ICP-OES (Table S1), the H2O content as confirmed by thermogravimetric analysis (Figure S2), and supplementary crystallographic information (PDF) Accession Codes

CCDC 1551577 contains 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 data_ [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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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID

Travis A. Olds: 0000-0001-5333-0802 Peter C. Burns: 0000-0002-2319-9628 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001089, as a part of the Materials Science of Actinides Center, an Energy Frontier Research Center. Electrospray ionization mass spectra were collected at the Mass Spectrometry and Proteomics Facility, University of Notre Dame. Raman spectroscopy and thermogravimetric analysis measurements were collected at the Materials Characterization Facility of the Center for Sustainable Energy, University of Notre Dame. Work 9682

DOI: 10.1021/acs.inorgchem.7b01174 Inorg. Chem. 2017, 56, 9676−9683

Article

Inorganic Chemistry (27) Ohtomo, N.; Arakawa, K. Bull. Chem. Soc. Jpn. 1979, 52, 2755− 2759. (28) Nyman, M.; Rodriguez, M. A.; Alam, T. M. Eur. J. Inorg. Chem. 2011, 2011, 2197−2205. (29) Sandström, J. Dynamic NMR Spectroscopy; Academic Press Inc.: London, 1982. (30) Müller, A.; Zhou, Y.; Bögge, H.; Schmidtmann, M.; Mitra, T.; Haupt, E. T. K.; Berkle, A. Angew. Chem., Int. Ed. 2006, 45, 460−465.

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DOI: 10.1021/acs.inorgchem.7b01174 Inorg. Chem. 2017, 56, 9676−9683