Crystal Engineering with the Uranyl Cation III. Mixed Aliphatic

Oct 6, 2011 - Cahill , C. L. ; ; Borkowski , L. A. Structural Chemistry of Inorganic Actinide ..... Nicolas P. Martin , Clément Falaise , Christophe ...
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Crystal Engineering with the Uranyl Cation III. Mixed Aliphatic Dicarboxylate/Aromatic Dipyridyl Coordination Polymers: Synthesis, Structures, and Speciation Andrew T. Kerr and Christopher L. Cahill* Department of Chemistry, The George Washington University, 725 21st Street N.W., Washington, D.C. 20052, United States

bS Supporting Information ABSTRACT: Seven novel U(VI)-bearing compounds have been synthesized using hydrothermal conditions and characterized via single crystal X-ray diffraction. These materials are the result of systematically pairing aliphatic dicarboxylic acids and dipyridyl (1,2-bis(4-pyridyl)ethane and trans-1,2-bis(4pyridyl)ethylene) molecules with the uranyl cation and are a conclusion to parts I and II of this study. A common factor of this family of materials is that the carboxylate group preferentially coordinates to the uranyl center, whereas the dipyridyl plays one of two roles: space filling or direct coordination uranyl cation. The role of the dipyridyl appears to be directly related not only to the length of the aliphatic dicarboxylate with which it is paired but also to the rigidity of the dipyridyl species. Further, this family of materials shows a tendency toward specific primary and secondary building units formed by the oligomerization of the uranyl cation despite the diversity suggested by other studies. This tendency appears to be due to the affinity of the ligand(s) for smaller building units.

’ INTRODUCTION Hybrid materials containing hexavalent uranium (UO22+, also known as the “uranyl cation”) are of considerable interest owing to their structural diversity,15 photophysical properties,610 and potential relevance to the nuclear fuel cycle.11,12 Synthesis of such materials is indeed quite similar to the approaches employed for the production of the more common d- and f-metal coordination polymers and metalorganic frameworks (MOFs) that are currently being explored for gas separations/storage, drug delivery, and catalysis.1317 Our interests, however, are less focused on these potential applications but rather toward a fundamental exploration of interactions of the uranyl cation with small organic molecules. This area of inquiry is relevant to the fate of radionuclides in the environment, spent fuel reprocessing, and long-term stewardship of nuclear waste in general. Moreover, this highlights an important intellectual overlap with d- and f-metal materials synthesis: hydrothermal chemistry and the relationship between solution phase speciation and solid-state crystal chemistry. Several research groups have explored reactions of the uranyl cation (systematic or otherwise) with a wide range of organic linker species.1826 In general, hydrothermal (or solvothermal) reaction conditions are utilized to dissolve uranyl salts, wherein solution phase species are then assembled via multitopic linker molecules to form extended solid-state compounds. This has given rise to an exceptionally diverse family of materials, made even more intriguing when one considers uranyl hydrolysis: mUO2 2þ þ nH2 O T ðUO2 Þm ðOHÞn 2m  n þ nHþ r 2011 American Chemical Society

Hydrolysis within aqueous uranyl systems has been studied at length27 and is arguably responsible for the range of building units seen in U-bearing coordination polymers. Monomeric species, either square, pentagonal, or hexagonal bipyramids (i.e., primary building units (PBUs)) will yield to secondary building units (SBUs) such as dimers, tetramers, chains, and sheets as a function of pH and/or concentration. As such, subtle changes in reaction conditions can have rather marked influence on crystalline reaction products, perhaps more so than in systems less susceptible to such diversity among hydrolysis products. Presented herein is an extension (and in some ways a conclusion) of our previous studies of systematic pairings of the uranyl cation with either aliphatic dicarboxylate anions or combinations of dicarboxylates (oxalate to sebacate) and bipyridines (4,40 bipyridine and 1,2-bis(4-pyridyl)ethane).28,29 Chart 1 shows a snapshot of the pairings of aliphatic dicarboxylic acids and bipyridines prior to this study, and it highlights missing data points. As such, we report seven new crystal structures along with a comprehensive treatment of structural systematics within this and, indeed, the broader family of related materials.

’ EXPERIMENTAL SECTION Synthesis. Caution! Whereas the uranyl nitrate hexahydrate [UO2(NO3)2] 3 6H2O used in these experiments contained depleted Received: September 9, 2011 Revised: October 5, 2011 Published: October 06, 2011 5634

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Chart 1. Representation of Known Dicarboxylate Dipyridyl Pairings with the Role of the Dipyridyla

a

Missing data points are circled for emphasis. Detailed references can be found in the Supporting Information.

Table 1. Synthesis Conditions for Compounds 17 1

2

3

4

5

6

7

empirical formula

C32H46N2O13U2

C27H52N2O39U6 C36H48N2O16U2

C48H60N4O26U5

C30H38N2O12U2

C22H28N2O10U2

C39H58N2O18U2

aliphatic

sebacic acid

glutaric acid

suberic acid

sebacic acid

azelaic acid

sebacic acid

azelaic acid

dipyridyl

BPE

BPE0

BPE0

BPE0

BPE0

BPE0

BPE0

pH (init./final)

4.95/4.91

3.55/4.27

4.83/3.85

4.86/4.83

2.14/4.02

7.09/5.67

3.63/3.46

topology

3D

2D sheets

2D anionic chains 3D

2D sheets

2D sheets

2D anionic chains

edge sharing

monomers

monomers

point sharing

monomers

dicarboxylate

secondary building monomers unit

chains no

monomers and tetramers

pure? yield (%)

yes

role of dipyridyl

direct coordination charge balancing charge balancing

no

uranium, standard precautions for handling radioactive material should be observed. All materials used in the synthesis are available commercially and were used as received. Compounds 16 were prepared by combining [UO2(NO3)2] 3 6H2O, aliphatic dicarboxylate, dipyridyl (1,2-bis(4-pyridyl)ethane, BPE, or trans-1,2-bis(4-pyridyl)ethylene, BPE0 ), and water (molar ratio 1:1:1:151). The initial pH values were then adjusted with concentrated ammonium hydroxide. These reagents were then sealed in a 23 mL Teflon-lined Parr bomb and heated statically at 120 °C for 3 days. Compound 7 was prepared by the same method as compounds 16, yet with a modification of the molar ratio to 1:3:2:151. This was part of a systematic effort to explore phase purity within this system, and 7 could be made reproducibly under these conditions. For all preparations, crystalline products were collected after decanting the mother-liquor and washing with water

no

chains no

no

no

direct coordination direct coordination direct coordination charge balancing

and ethanol. The reagents used, reaction conditions, purity, and role of the dipyridyl are summarized in Table 1. Compounds 27 were not obtained as pure phases, owing to formation of the doubly protonated BPE0 nitrate, (C12H12N2)(NO3)2.30 The impurity does not appear in the powder pattern collected from these samples (as sample preparation included grinding in acetone), yet crystals were observed in the bulk sample, which allowed for the determination of the unit cell parameters of the BPE0 nitrate. An excess amount of nitrogen and carbon observed in elemental analyses also suggests that the impurity is BPE0 nitrate. This impurity can be removed by washing the products with chloroform. Compound 2 crystallized with a second phase that appears to be related to the glutarate structure formed with 4,40 -dipyridyl,29 wherein BPE0 replaced the charge balancing 4,40 dipyridyl in between the uranyl glutarate “slabs”. This second phase 5635

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Table 2. Crystallographic Data for Compounds 17 compd

1

2

3

4

5

6

7

empirical formula

C32H46N2O13U2

C36H48N2O16U2

C36H47N2O16U2

C48H60N4O26U5

C30H38N2O12U2

C22H28N2O10U2

C39H58N2O18U2

formula weight

2439.74

2439.74

1239.58

2299.45

1094.58

956.58

1142.58

crystal system

monoclinic

orthorhombic

monoclinic

triclinic

triclinic

monoclinic

monoclinic

space group

C2

C2221

P21/n

P1

P1

P21/n

C2/c

a (Å)

20.357

12.534

9.4706

9.6535

8.1535

8.8042

25.4837

b (Å)

20.317

16.139

8.3098

9.7736

9.4052

8.2617

15.9313

c (Å)

10.2151

25.294

16.6977

17.4401

12.3338

17.433

25.4808

α (deg) β (deg)

90.00 117.279

90.00 90.00

90.00 98.2390

87.842 75.944

108.60 96.0540

90.00 94.808

90.00 118.76

γ (deg)

90.00

90.00

90.00

70.217

100.7710

90.00

90.00

V (Å3)

3755.0

5116.6

1300.53

1500.2

866.67

1263.6

9068.4

temp (K)

298

298

298

298

100

100

100

Z

4

4

4

1

2

4

8

λ (Mo Ka)

0.71073

0.71073

0.71073

0.71073

0.71073

0.71073

0.71073

Dcalc (g cm3)

1.950

3.197

2.026

2.545

2.097

2.437

1.932

μ (mm1) Rint

8.372 0.0964

19.031 0.1327

8.025 0.0390

13.532 0.0215

9.392 0.0181

12.492 0.0484

4.208 0.0478

R1 [I > 2σ(I)]

0.0361

0.0522

0.0220

0.0191

0.0199

0.0174

0.0263

wR2 [I > 2σ(I)]

0.0871

0.1216

0.0438

0.0423

0.0414

0.0428

0.0556

yielded poor crystals that allowed for only a preliminary refinement and a tentative formula of (UO2)2(C5H6O4)3 3 (C12H12N2). Compounds 3 and 4 coformed, and crystals were separated manually for analysis. Single-Crystal X-ray Diffraction. Single crystals of each compound were isolated from bulk samples and mounted on a MicroMount needle (Mitegen). Reflections were collected from 0.5 j and ω scans on a Bruker SMART diffractometer with an APEXII CCD detector and a Mo Kα source. The APEX II software suite31 was used to integrate the data and to apply an absorption correction. Structures were solved by direct methods using SIR9232 or SHELXS-97.33 All structures were refined using SHELXL-9733 and the WINGX software suite.34 All figures were prepared with CrystalMaker.35 Data collection and refinement details can be found in Table 2. Refinement of compound 1 yielded disorder about one of the aliphatic carbon chains and the dipyridyl. The disorder in the carbon chain was modeled successfully via the PART and ISOR commands (in order to allow the carbon atoms to be modeled anisotropically), yet carbon atoms in the chain continue to have high Hirshfeld differences. The carbon chains were further modeled via the DFIX commands, allowing bond lengths and angles to refine as sp3 hybridized carbon atoms. The disordered dipyridyl could not be modeled successfully due to overwhelming disorder in the carbon chain leaving C30 with large thermal parameters. Compound 2 had disorder about the dipyridyl cation that was modeled successfully via the PART and ISOR commands, yet the disorder caused the carboncarbon double bond to be short and have a distorted angle. Hydrogen atoms on the solvent water molecules could not be modeled satisfactorily. Compounds 3 and 4 each had disorder about one of the aliphatic carbon chains. The disorder was modeled satisfactorily in both compounds via PART commands. Compound 6 had disorder about one of the aliphatic carbon chains that was modeled via PART and DFIX commands to restrain the carbon atoms at appropriate bond lengths. Compound 7 had disordered dipyridyl cations. Attempts to model the disorder about the carboncarbon double bond failed, leaving the angle close to 180° and two of the carbon atoms (C39 and C39_d) with a short bond length. Also, hydrogen atoms on the solvent water molecules could not be satisfactorily modeled. Checks for missing symmetry in all compounds were performed with PLATON,36 and all space groups appear to be correct.

Powder X-ray Diffraction. Powder X-ray diffraction data was collected on a Rigaku Miniflex diffractometer (Cu Kα, 360°) and analyzed with the Jade software package.37 Powder XRD data were used to verify reproducibility as well as to explore the purity of the washed samples.

’ RESULTS AND DISCUSSION Structural Descriptions. Compound 1, [(UO2)2(C10H16O4)2(C12N2H12)H2O], consists of three crystallographically unique uranyl cations (UO22+) that form monomeric pentagonal bipyramidal building units as seen in Figure 1. U1 binds to two unique sebacate molecules in a bridging bidentate fashion (with U3) via O11 and O14, as well as a single water molecule (OW1). U2 is bound to a bridging bidentate sebacate anion (via O22a) and a water molecule (OW2) and is further coordinated to two symmetry equivalent BPE molecules via N1. U3 binds to four unique sebacate anions, three of which are in a bridging bidentate (with U1 and U2) via O12, O13, and O21a whereas the fourth is bidentate through O23a and O24a. Distances for the axial uranyloxygen bonds range from 1.670 to 1.709 Å and are within the typical values. The UO and UN bonds observed in the equatorial positions are also typical lengths.29,3840 This coordination geometry gives rise to chains of pentagonal bipyramidal building units that propagate along the [101] direction. These chains are cross-linked by sebacate groups to form a threedimensional coordination polymer wherein dipyridyl molecules “decorate” the chains and protrude into the channels (Figure 2). Compound 2, [(UO2)6(C5H6O4)3(OH)4O2] 3 (C12N2H12)(H2O)9, is composed of three crystallographically unique uranyl cations in the form of pentagonal bipyramidal building units (Figure 3). U1 is bound to two bridging bidentate glutarate anions via O15 and O14 (with U2 and U3), as well as three μ3 oxygen atoms (O7, O8, and O9 with U2 and U3). U2 shares (with U3) one monodentate glutarate anion through O10 and one bridging bidentate glutarate through O13. Finally, U3 is bound to one bridging bidentate glutarate through O12. The μ3 5636

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Figure 3. Local structure of 2. Carbon atoms are not labeled, and water and BPE0 molecules have been omitted for clarity.

Figure 1. Local structure of 1. Yellow spheres are uranium atoms, blue are nitrogen atoms, red are oxygen atoms, whereas black lines represent carbon chains. Individual carbons are not labeled for clarity. Fully labeled ORTEP representations are in the Supporting Information.

Figure 4. Polyhedral representation of 2 in the ac plane. Water and dipyridyl molecules have been omitted for clarity.

Figure 2. Polyhedral representation of compound 1 shown down [101]. The color scheme is the same as that in Figure 1.

oxygen atoms were verified as hydroxides (O7 and O8) or oxides (O9) via bond valence summations (all bond valence summations can be found in the Supporting Information).41,42 The Uμ3O bond distances (lengths between 2.185 and 2.248 Å) are shorter than the other equatorial UO bonds. These distances, however, are typical values for oxo groups bound by the urnayl cation.36 The pentagonal bipyramidal units edge share to form chains (along the [100] direction) that are further connected

by glutarate anions to form anionic sheets as seen in Figure 4. These sheets are charge balanced by diprotonated BPE0 molecules that occupy the interlayer. Compound 3, [(UO2)2(C8H12O4)3] 3 (C12N2H12), consists of one crystallographically unique uranyl cation in the form of a monomeric hexagonal bipyramidal building unit as seen in Figure 5. These monomers are connected via suberate anions and propagate along [101] forming anionic chains that are charge balanced with diprotonated BPE0 molecules as seen in Figure 6. Looking at the local structure, U1 is bound to three suberate anions in a bidentate fashion: the first via O3 and O4, the second via O5 and O6, and the third via O7a and O8a (Figure 5). As in the previous compounds, bond angles and distances are within typical values. Compound 4, [(UO2)5(O)2(C8H12O4)3(C12H10N2)2(H2O)2], consists of three crystallographically unique uranyl cations. 5637

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Figure 5. Local structure of compound 3. Carbon and nitrogen atoms are not labeled for clarity.

Figure 8. Polyhedral representation of compound 4 shown down [100].

Figure 6. Polyhedral representation of compound 3 shown down [001].

Figure 9. Local structure of compound 5. Carbon atoms are not labeled for clarity.

Figure 7. Local structure of compound 4. Carbon atoms are not labeled for clarity.

The first is a UO2(O)6 hexagonal bipyramidal building unit, the second is a UO2(O)4N pentagonal bipyramidal unit, and the third is a UO2(O)4N2 hexagonal bipyramidal monomer as seen in Figure 7. U1 is bound to both a bidentate suberate anion, via oxygen atoms O6 and O7, as well as a BPE0 molecule via N1. U2 is bound by two crystallographically unique suberate anions via O9 and O11 (sharing with U3), as well as one BPE0 molecule (N2) and one unique μ3 oxo group (O12 with U3). U3 is bound by two suberate anions via O8, O9, O10, and O11, as well as one water molecule (OW1). The water molecule and the μ3 oxo

group were verified via bond valence summations. Again, the Uμ3O bond lengths are short, with distances of 2.184, 2.232, and 2.264 Å, as expected with an oxo group. Also, the UO2(O)6 hexagonal bipyramid has long bond lengths of 2.639 and 2.609 Å. These long distances are treated as bonds due to the more relaxed bond angles of the hexagonal (65.95°) versus the square bipyramid (115.50°). All other UO and UN bond lengths are reasonable. U2 and U3 edge share and are reproduced through an inversion center to form tetramers as seen in Figure 7. The building units are connected to one another via suberate anions and BPE0 molecules. This coordination geometry provides a threedimensional structure with channels down the [100] direction (Figure 8). Compound 5, [(UO2)2(C9H14O4)2(C12N2H10)], consists of one crystallographically unique uranyl cation in the form of a monomeric, UO2(O)4N, pentagonal bipyramidal building unit (Figure 9). U1 is bound to a bidentate azelate anion via oxygens O3 and O4, two bridging bidentate azelate anions via oxygens O5 and O6, and one BPE0 molecule via N1. All angles and lengths are within reasonable values for UO and UN bonds. The monomers are connected to one another through bridging azelate anions forming “dimers” that are further connected via BPE0 molecules forming neutral sheets that propagate in the [011] as seen in Figure 10. Compound 6, [(UO2)2(OH)2(C10H16O4)(C12N2H10)], consists of one crystallographically unique UO2(O)4N pentagonal 5638

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Figure 10. Polyhedral representation of compound 5 shown down [110].

Figure 13. Local structure of compound 7. Carbon atoms are not labeled, and water molecules have been omitted for clarity.

Figure 11. Local structure of compound 6. Carbon atoms are not labeled for clarity.

Figure 12. Polyhedral representation of compound 6 shown down [101].

bipyramidal building unit (Figure 11). U1 is bound to two sebacate anions in a bridging bidentate fashion via O3 and O4, one BPE0 via N1, and one bridging hydroxide (O5), the latter of which has been verified using bond valence summations. All bond lengths and angles are as expected. U1 point shares though O5 to form chains that propagate in the [010] that are further connected by sebacate anions and BPE0 molecules to form neutral sheets as seen in Figure 12. Compound 7, [(UO2)2(C9H14O4)3] 3 (C12N2H12)(H2O)2, consists of two crystallographically unique uranyl cations, both

of which are hexagonal bipyramidal building units. U1 binds to three azelate anions in a bidentate fashion via O3, O4, O5, O6, O7, and O8, as seen in Figure 13. U2 is also bound to three azelate anions via O11, O12, O13, O14, O15, and O16. The uranyl monomers, along with the azelate anions, form linear chains approximately along [110]. The chains are then tethered to one another via a second (unique) azelate anion to form an unusual “double” uranyl azelate chain (Figure 14). Such chains (and those generated by symmetry that run approximately along [110]) pack in a crossed fashion and are charge balanced by cationic BPE0 and solvent water molecules in the voids. Structural Systematics. The dipyridyl species found in the materials not only in this study but also in the entire family of dicarboxylatebipyridine pairings have exhibited two different roles: space filling (the molecule occupies the empty space formed by the coordination polymer and can balance charge)43 or direct coordination to the uranyl cation. In the cases where the dicarboxylate is approximately 3 Å or more shorter than the dipyridyl, the latter will not be incorporated in any manner, and instead, reactions thereof result in a uranyl dicarboxylate. In cases where the length of the dicarboxylate is equal to or no more than approximately 3 Å shorter than the dipyridyl, the latter may be incorporated as a space filling species. Finally, in the cases where the dicarboxylate is longer than the dipyridyl, the latter may coordinate directly to the uranyl cation. These trends can be observed graphically in Chart 2 and the lengths of the organics (as obtained from the Cambridge Structural Database44,45 (CSD) and measured using Mercury44) can be found in Table 3. In cases where the dipyridyl is flexible, such as BPE, it has a tendency to coordinate directly to the uranyl cation when paired with a dicarboxylate of the appropriate length. If the dipyridyl is rigid, such as BPE0 , it may not coordinate directly to the uranyl cation as readily as its flexible counterpart (BPE), as observed in the formation of compounds 3, 4, 5, and 7. One may therefore speculate that the tendency of the rigid dipyridyl is to space fill. Owing to this tendency, both space filling and direct coordination by the dipyridyl can be observed when the aliphatic is of appropriate length for direct coordination and slightly beyond. An example of this is observed when BPE0 is paired with suberate or azelate anions (the two acids that are of the appropriate length). The dipyridyl is charge balancing in compounds 3 and 7 5639

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Chart 2. Representation of Full Series Including New Compounds

Table 3. Lengths of Organic Species Used in This Studya dicarboxylate

a

length (Å)

dipyridyl 0

length (Å)

oxalic malonic

2.66 4.42

4,4 -bipy BPE

6.95 9.16

succinic

5.03

BPE0

9.27

glutaric

6.49

adipic

7.52

pimelic

9.04

suberic

10.11

azelaic

11.64

sebacic

12.59

CSD codes and references can be found in the Supporting Information.

(see Figure 14 for a representation of compound 7), while the dipyridyl coordinates directly to the uranyl cation in compounds 4 and 5. This same rigidity argument may be made for 4,40 -bipy, where a similar trend is observed when paired with pimelic acid.29 Speciation. Uranyl hydrolysis typically gives rise to a large diversity of secondary building units at the concentrations and pH values used in this study.27 Interestingly, primarily monomers and a few examples of tetramers or chains have been observed in the family of dicarboxylate dipyridyl pairings despite the diversity of other SBUs suggested by other studies.46,47 This observation can be explained by monomeric and lower order oligomerization species persisting at higher pH values.27 Also, the ligands explored herein may have an affinity for smaller secondary building units due to the bite angle, chelation effects, and length of the dicarboxylates, as observed in previous studies.46,47 Moreover, we note the presence of tetrameric SBUs when the carboxylate and dipyridyl molecules are of comparable length.

Figure 14. Polyhedral representation of compound 7 shown down [101]. Water molecules have been omitted for clarity.

Fluorescence. Fluorescence data were collected for the compounds in which pure samples were obtained, either by washing or manual separation. Compound 1 (containing BPE) exhibits weak uranyl fluorescence and the spectrum, as seen in the Supporting Information, does not show the fine structure typical of strong emission. Compounds 2, 3, 4, 5, 6, and 7 (containing BPE0 ) do not exhibit any significant uranyl fluorescence. The more conjugated BPE0 may allow energy transfer from one uranyl species to the next in the solid state causing selfquenching of the fluorescence.48,49 This explanation is admittedly speculative at this point, yet the observations are in fact consistent between studies, and a more thorough examination of these and related materials is underway.

’ CONCLUSIONS The effects of pairing different aliphatic dicarboxylates with dipyridyl molecules and the uranyl cation have been investigated. 5640

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Crystal Growth & Design The compounds studied herein have exhibited direct dicarboxylate coordination to the uranyl cation whereas the dipyridyl plays different roles (space filling and direct coordination) as the two ligands explored were varied in size. Chart 2 shows the completed series including the new compounds that have been prepared in this study. The ligands may tend to prefer coordinating to secondary building units that have not undergone significant oligomerization. Due to this preferential binding, monomers and other “smaller” secondary units are primarily observed.

’ ASSOCIATED CONTENT

bS

Supporting Information. ORTEP figures of all compounds, charts detailing references and building units, bond valence summations of oxygen atoms of interest, and X-ray crystallographic files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. CIFs have also been deposited at the Cambridge Crystallographic Data Centre and may be obtained from http://www.ccdc.cam.ac.uk by citing reference codes 843713843719 for compounds 17, respectively.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: (202) 994 6959. Fax: (202) 994 5873. E-mail: cahill@ gwu.edu.

’ ACKNOWLEDGMENT This material is based upon work supported as part of the Materials Science of Actinides, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001089. We are grateful to Dr. Raymond Butcher (Howard University) for assistance with modeling the disordered structures. ’ REFERENCES (1) Cahill, C. L.; de Lill, D. T.; Frisch, M. CrystEngComm 2007, 1, 15–26. (2) Ephritikhine, M. Dalton Trans. 2006, 21, 2501–2516. (3) Giesting, P. A.; Burns, P. C. Crystallogr. Rev. 2006, 3, 205–255. (4) Wang, K.; Chen, J. Acc. Chem. Res. 2011, 7, 531–540. (5) Cahill, C. L.; Borkowski, L. A. Structural Chemistry of Inorganic Actinide Compounds; Krivovichev, S. V., Burns, P. C., Tananaev, I. G., Eds.; Elsevier: 2007; pp 409442 (6) Burrows, H. D.; Kemp, T. J. Chem. Soc. Rev. 1974, 2, 139–165. (7) Harrowfield, J. M.; Lugan, N.; Shahverdizadeh, G. H.; Soudi, A. A.; Thuery, P. Eur. J. Inorg. Chem. 2006, 2, 389–396. (8) Chen, W.; Yuan, H.; Wang, J.; Liu, Z.; Xu, J.; Yang, M.; Chen, J. J. Am. Chem. Soc. 2003, 125 (31), 9266–9267. (9) Liao, Z.; Li, G.; Bi, M.; Chen, J. Inorg. Chem. 2008, 11, 4844–4853. (10) Yu, Z.; Liao, Z.; Jiang, Y.; Li, G.; Chen, J. Chem.—Eur. J. 2005, 9, 2642–2650. (11) Choppin, G. J. Radioanal. Nucl. 2007, 3, 695–703. (12) Ewing, R. C.; Runde, W.; Albrecht-Schmitt, T. E. MRS Bull. 2010, 859–866. (13) Ferey, G. Dalton Trans. 2009, 23, 4400–4415. (14) Wang, Z.; Tanabe, K. K.; Cohen, S. M. Chem.—Eur. J. 2010, 1, 212–217. (15) Allendorf, M. D.; Bauer, C. A.; Bhakta, R. K.; Houk, R. J. T. Chem. Soc. Rev. 2009, 5, 1330–1352. (16) James, S. L. Chem. Soc. Rev. 2003, 5, 276–288.

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