Uranyl-Peroxide Clusters Incorporating Iron Trimers and Bridging by

Mar 23, 2017 - A hybrid uranium–iron cage nanocluster, [(UO2)24(FeOH)24(O2)24(PO4)8(CH(COO)(PO3)2)24]96– (U24Fe24), was synthesized using ...
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Uranyl-Peroxide Clusters Incorporating Iron Trimers and Bridging by Bisphosphonate- and Carboxylate-Containing Ligands Jie Qiu,† Sining Dong,§ Jennifer E. S. Szymanowski,† Malgorzata Dobrowolska,§ and Peter C. Burns*,†,‡ †

Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States § Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

spectroscopic and magnetic properties, and solution behavior of this anionic cluster. U24Fe24 self-assembled in controlled one-pot reactions conducted by mixing aqueous solutions of UO2(NO3)2, H2O2, LiOH, C3P2, and FeCl3 at ambient temperature (see Supporting Information). Dark yellow cubic crystals (1) containing U24Fe24 formed from solutions with a pH ranging from 6.2 to 8.6, which were adjusted by slightly changing the ratios of the reactants in solution. Chemical and thermogravimetric analyses, in combination with the crystal structure analysis, demonstrated that 1 has composition Li96[(UO2)24(FeOH)24(O2)24(PO4)8(CH(COO)(PO3)2)24](H2O)283 (see Supporting Information). The crystal structure analysis using single-crystal X-ray diffraction revealed that crystals of 1 contain U24Fe24 clusters that are nearly spherical with a diameter of 29.8 Å (Figure 1a,b). U24Fe24 contains 24 (UO2)2+ uranyl and 24 Fe3+ ions, in

ABSTRACT: A hybrid uranium−iron cage nanocluster, [(UO2)24(FeOH)24(O2)24(PO4)8(CH(COO)(PO3)2)24]96− (U24Fe24), was synthesized using bridging ligands containing bisphosphonate and carboxylate groups. U24Fe24 contains six tetramers of uranyl hexagonal bipyramids and eight iron trimers, each of which consists of three corner-sharing Fe3+ octahedra and is stabilized by in situ formed phosphate and 2,2-bis(phosphonato)acetate (C2P2) groups. Tetramers and trimers are bridged by 24 C2P2 groups into a cage cluster. Crystals of U24Fe24 present a paramagnetic-like behavior. X-ray scattering showed that U 24 Fe 24 forms in the reactant solution prior to crystallization and is stable upon dissolution in water.

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igh nuclearity metal-oxide clusters have been the subject of many studies due to their diverse topological structures, rich electronic and physical properties, and applications in various fields including catalysis, medical science, and materials.1−4 Over the past decade, a family of nanoscale uranyl peroxide cage clusters, with potential applications in the nuclear fuel cycle and elsewhere, has been developed by taking advantage of the strong interaction of the uranyl ion and peroxide.5−8 The first reported clusters formed in alkaline aqueous solutions and have structures consisting of combinations of four-, five-, and six-membered rings of uranyl polyhedra.9−12 Subsequently, additional inorganic and organic ligands were introduced to yield hybrid clusters with diverse structures and properties.6,7,13,14 Among these, pyrophosphate and analogous ligands containing bisphosphonate groups effectively bridge uranyl polyhedra and have been used to synthesize uranyl cage clusters.13−17 We are interested in the synthesis of hybrid uranium−transition metal clusters with new functionalities and physical properties and reported two U/ Mo,18 three U/V,19,20 and five U/W18,21 clusters. 3,3Diphosphono-propanoic acid (C3P2) that contains both bisphosphonate and carboxylate groups facilitated synthesis of hybrid U/Fe clusters that are reported herein. High nuclearity iron clusters are mostly synthesized by employing various alkoxide- or carboxylate-based ligands to passivate iron polyhedra and stabilize discrete clusters.22−25 Extending this strategy, we synthesized the first hybrid U/Fe cluster, designated U24Fe24, and report the synthesis, structure, © XXXX American Chemical Society

Figure 1. Mixed polyhedral and ball-and-stick (a) and graphical (b) representations of U24Fe24 and mixed polyhedral and ball-and-stick representations of two units of the cluster (c, d). Legend: U, yellow; Fe, turquoise; P, purple; O, red; C, black; and Li, blue. Received: February 12, 2017

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DOI: 10.1021/acs.inorgchem.7b00389 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry hexagonal bipyramidal and octahedral coordination environments, respectively. Four uranyl bipyramids are linked by sharing edges corresponding to peroxide groups to form a tetrameric subunit (Figure 1c), which is a common building unit for uranyl peroxide clusters.5 Groups of three Fe3+centered octahedra are linked by sharing corners, forming a building unit that is similar to the iron trimers (Figure 1c) in an iron-substituted polyoxometalate (POM) based on an A-type trivacant Keggin cluster.26 The six uranyl tetramers and eight iron trimers in U24Fe24 are bridged by bisphosphonate groups of 24 2,2-bis(phosphonato)acetate (C2P2) into a cage cluster with Oh symmetry. C2P2 formed in situ from degradation of C3P2 that was oxidized by H2O2 under the catalysis of iron salt.27 Each C2P2 group is bidentate to a uranyl ion and tridentate to an Fe3+ ion in the cluster. Within each uranyl tetramer, one equatorial edge of each uranyl bipyramid corresponds to two bisphosphonate O atoms, and the others are bidentate peroxide groups. Each tetramer hosts a Li+ ion, which bonds to four uranyl ion O atoms and contributes to the stability of the tetramer.28 Within each trimer, the octahedral vertices about each Fe3+ cation correspond to two μ2-OH groups that are shared between Fe3+ cations, one carboxylate O atom and two bisphosphonate O atoms from a C2P2, and one phosphate O atom. The phosphate groups also resulted from oxidative degradation of C3P2, and each is arranged below an iron trimer. U24Fe24 contains eight phosphate groups, each of which is linked to three iron polyhedra of a single trimer on the cluster interior. Tetramers of uranyl polyhedra are common constituents of uranyl peroxide cages. In contrast, the trimer of corner-sharing Fe3+ octahedra stabilized by C2P2 and phosphate groups is a novel feature in polyoxometalates. Eight are homogeneously distributed in U24Fe24. In iron clusters, a trimer consisting of three edge-sharing octahedra is a typical building unit.24,29 Iron has also been incorporated into POMs through reactions of lacunary POMs with iron salts to prepare compounds with rich magnetic and catalytic behaviors. 3,26,27,30−35 In clusters containing mixed metal centers, iron polyhedra cap or bridge lacunary POM fragments and are usually located at one end or in the center of the cluster. In other words, lacunary POM fragments acting as multidentate inorganic ligands bind and stabilize iron polyhedra and direct formation of new clusters. Raman, infrared (IR), and UV−vis spectra (see Supporting Information) were collected for crystals of 1 and confirmed the presence of UO22+, peroxide, PO43− groups, and C2P2. The Raman spectrum bands located at 803 and 856 cm−1 are due to the symmetrical UO22+ and peroxide stretches, respectively.36, Multiple bands in the region of 930 to 1180 cm−1 are due to stretches of the bisphosphonate group of C2P237 and the PO43− group.14,38 The IR spectrum contains a band at 873 cm−1 due to the antisymmetric stretch of the UO22+ group.38 Bands at 990 and 1081 cm−1 are the symmetric (ν1) and antisymmetric (ν3) stretches of the PO43− group.38 The band located at 1585 cm−1 is assigned to the asymmetric stretch of carboxylate.39 The broad band in the region from 2500 to 3700 cm−1 is assigned to H bonds.38 The UV−vis spectrum contains a typical uranyl absorption band in the region of 300−500 nm.14 A large crystal of 1 was used for measurement of magnetic properties. Figure 2 shows the magnetization vs temperature curves of the sample in conditions of zero-field-cooling (ZFC) and field-cooling (FC) at 5000 Oe. The sample was cooled from 300 to 4 K with zero field for the ZFC process and with a +5000 Oe field for the FC process. There is no significant

Figure 2. Temperature dependencies of the zero-field-cooled (black) and field-cooled (red) magnetizations for a crystal of 1 at a field of 5000 Oe.

difference between the ZFC and FC curves, and there is no indication of an impurity. The continuous decrease of magnetization with increasing temperature demonstrates that U24Fe24 presents paramagnetic-like behavior, which is also confirmed by the M−H loop measured at 4 K (see Supporting Information). The closest Fe···Fe distances in U24Fe24 are 3.546 Å, whereas the shortest distance between Fe atoms from different trimers is 10.788 Å. Therefore, the magnetic interaction within the trimer should dominate the magnetic response. Thermal effects may strongly influence the magnetic order of the trimers,40 resulting in paramagnetic (or superparamagnetic41)-like properties. Small angle X-ray scattering (SAXS) was used to explore the behavior of U24Fe24 in aqueous solution. Crystals of 1 were dissolved in ultrapure water, and the SAXS plot for the resulting solution shows typical scattering features of a hollow spherical nanocluster (Figure 3).42 The data were fitted using a sphereshell model with a radius of 14.5 Å, which is consistent with the cluster size from the crystal structure. This indicates that U24Fe24 persists as a macroion after dissolution in pure water.

Figure 3. Experimental (black) and fitted (red) SAXS plot of the solution prepared by dissolving crystals of 1 in ultrapure water. B

DOI: 10.1021/acs.inorgchem.7b00389 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(6) Qiu, J.; Nguyen, K.; Jouffret, L.; Szymanowski, J. E. S.; Burns, P. C. Time-resolved assembly of chiral uranyl peroxo cage clusters containing belts of polyhedra. Inorg. Chem. 2013, 52, 337−345. (7) Qiu, J.; Ling, J.; Jouffret, L.; Thomas, R.; Szymanowski, J. E. S.; Burns, P. C. Water-soluble multi-cage super tetrahedral uranyl peroxide phosphate clusters. Chem. Sci. 2014, 5, 303−310. (8) Qiu, J.; Ling, J.; Sui, A.; Szymanowski, J. E. S.; Simonetti, A.; Burns, P. C. Time-resolved self-assembly of a fullerene-topology coreshell cluster containing 68 uranyl polyhedra. J. Am. Chem. Soc. 2012, 134, 1810−1816. (9) Burns, P. C.; Kubatko, K. A.; Sigmon, G.; Fryer, B. J.; Gagnon, J. E.; Antonio, M. R.; Soderholm, L. Actinyl peroxide nanospheres. Angew. Chem., Int. Ed. 2005, 44, 2135−2139. (10) Sigmon, G. E.; Unruh, D. K.; Ling, J.; Weaver, B.; Ward, M.; Pressprich, L.; Simonetti, A.; Burns, P. C. Symmetry versus minimal pentagonal adjacencies in uranium-based polyoxometalate fullerene topologies. Angew. Chem., Int. Ed. 2009, 48, 2737−2740. (11) Forbes, T. Z.; McAlpin, J. G.; Murphy, R.; Burns, P. C.Metaloxygen isopolyhedra assembled into fullerene topologies. Angew. Chem., Int. Ed. 2008, 47, 2824−2827. (12) Unruh, D. K.; Burtner, A.; Pressprich, L.; Sigmon, G. E.; Burns, P. C. Uranyl peroxide closed clusters containing topological squares. Dalton Trans. 2010, 39, 5807−5813. (13) Ling, J.; Qiu, J.; Sigmon, G. E.; Ward, M.; Szymanowski, J. E. S.; Burns, P. C. Uranium pyrophosphate/methylenediphosphonate polyoxometalate cage clusters. J. Am. Chem. Soc. 2010, 132, 13395− 13402. (14) Liao, Z.; Ling, J.; Reinke, L. R.; Szymanowski, J. E. S.; Sigmon, G. E.; Burns, P. C. Cage clusters built from uranyl ions bridged through peroxo and 1-hydroxyethane-1,1-diphosphonic acid ligands. Dalton Trans. 2013, 42, 6793−6802. (15) Ling, J.; Qiu, J.; Szymanowski, J. E. S.; Burns, P. C. Lowsymmetry uranyl pyrophosphate cage clusters. Chem. - Eur. J. 2011, 17, 2571−2574. (16) Ling, J.; Ozga, M.; Stoffer, M.; Burns, P. C. Uranyl peroxide pyrophosphate cage clusters with oxalate and nitrate bridges. Dalton Trans. 2012, 41, 7278−7284. (17) Unruh, D. K.; Ling, J.; Qiu, J.; Pressprich, L.; Baranay, M.; Ward, M.; Burns, P. C. Complex nanoscale cage clusters built from uranyl polyhedra and phosphate tetrahedra. Inorg. Chem. 2011, 50, 5509− 5516. (18) Ling, J.; Hobbs, F.; Prendergast, S.; Adelani, P. O.; Babo, J.-M.; Qiu, J.; Weng, Z.; Burns, P. C. Hybrid uranium-transition-metal oxide cage clusters. Inorg. Chem. 2014, 53, 12877−12884. (19) Senchyk, G. A.; Wylie, E. M.; Prizio, S.; Szymanowski, J. E. S.; Sigmon, G. E.; Burns, P. C. Hybrid uranyl-vanadium nano-wheels. Chem. Commun. 2015, 51, 10134−10137. (20) Qiu, J.; Dembowski, M.; Szymanowski, J. E. S.; Toh, W. C.; Burns, P. C. Time-resolved X-ray scattering and Raman spectroscopic studies of formation of a uranium-vanadium-phosphorus-peroxide cage cluster. Inorg. Chem. 2016, 55, 7061−7067. (21) Miro, P.; Ling, J.; Qiu, J.; Burns, P. C.; Gagliardi, L.; Cramer, C. J. Experimental and computational study of a new wheel-shaped {[W5O21]3[(UVIO2)2(μ-O2)]3}30‑ polyoxometalate. Inorg. Chem. 2012, 51, 8784−8790. (22) Liu, T.; Zhang, Y.-J.; Wang, Z.-M.; Gao, S. A 64-nuclear cubic cage incorporating propeller-like FeIII8 apices and HCOO− edges. J. Am. Chem. Soc. 2008, 130, 10500−10501. (23) Zhang, Z.-M.; Yao, S.; Li, Y.-G.; Clérac, R.; Lu, Y.; Su, Z.-M.; Wang, E.-B. Protein-sized chiral Fe168 cages with NbO-type topology. J. Am. Chem. Soc. 2009, 131, 14600−14601. (24) Sadeghi, O.; Zakharov, L. N.; Nyman, M. Aqueous formation and manipulation of the iron-oxo Keggin ion. Science 2015, 347, 1359−1362. (25) Nachtigall, O.; Kusserow, M.; Clerac, R.; Wernsdorfer, W.; Menzel, M.; Renz, F.; Mrozinski, J.; Spandl, J. [Fe19] ″super-Lindqvist″ aggregate and large 3D interpenetrating coordination polymer from solvothermal reactions of [Fe2(OtBu)6] with ethanol. Angew. Chem., Int. Ed. 2015, 54, 10361−10364.

SAXS data measured for the evolving reactant solution of U24Fe24 indicate that smaller nanoclusters form within 1 day of combination of the reactants, with a cluster having similar dimensions as U24Fe24, and assumed to be U24Fe24, appearing after about one month (see the Supporting Information). U24Fe24 therefore assembles in solution prior to crystallization. In summary, isolation of the first hybrid U−Fe cluster demonstrates that employing bisphosphonate- and carboxylatecontaining ligands is an effective strategy to introduce Fe3+ cations into uranyl peroxide clusters. Furthermore, introduction of Fe3+ cations resulted in a novel structure that is stable in water. Future work will examine U24Fe24 redox properties and potential applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00389. Experimental methods; crystallographic data and bondvalence analysis; TGA data; PXRD pattern; IR, Raman, and UV−vis spectra; magnetic data; small-angle X-ray scattering data for reactant solutions (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jie Qiu: 0000-0001-7131-4881 Peter C. Burns: 0000-0002-2319-9628 Funding

This research was funded by the Office of Basic Energy Sciences of the U.S. Department of Energy as part of the Materials Science of Actinides Energy Frontier Research Center (DE-SC0001089). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Chemical analyses were done in the Center for Environmental Science and Technology at the University of Notre Dame. Spectra and diffraction data were collected in the Materials Characterization Facility of the Center for Sustainable Energy at the University of Notre Dame.

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DOI: 10.1021/acs.inorgchem.7b00389 Inorg. Chem. XXXX, XXX, XXX−XXX