Clusters of Actinides with Oxide, Peroxide, or Hydroxide Bridges

Review pubs.acs.org/CR

Clusters of Actinides with Oxide, Peroxide, or Hydroxide Bridges Jie Qiu† and Peter C. Burns*,†,‡ †

Department of Civil and Environmental Engineering and Earth Sciences and ‡Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States 7.9. Factors Impacting the Size of Uranyl Peroxide Cage Clusters 8. Transition-Metal-Based Actinide-Bearing Clusters 9. Summary and Discussion Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Clusters Containing An(IV) 2.1. Clusters with Four An(IV) Cations 2.2. Clusters with Six An(IV) Cations 2.3. Clusters with Eight An(IV) Cations 2.4. Clusters with 10 An(IV) Cations 2.5. Clusters with 38 An(IV) Cations 3. Clusters Containing An(IV) and An(V) 3.1. Clusters with Four or Five An(IV) and An(V) Cations 3.2. Clusters with Six An(IV) and An(V) Cations 3.3. Clusters with 12 An(IV) and An(V) Cations 3.4. Clusters with 16 An(IV) and An(V) Cations 4. Clusters Containing An(V) 4.1. Clusters with Four An(V) Cations 4.2. Clusters with Six An(V) Cations 5. Clusters Containing An(V) and An(VI) 6. Clusters Containing An(VI) 6.1. Clusters with Four An(VI) Cations 6.2. Clusters with Six An(VI) Cations 6.3. Clusters with Eight An(VI) Cations 7. Actinyl Peroxide Clusters 7.1. Importance of the Peroxide Bridge 7.2. Synthesis of Actinyl Peroxide Clusters 7.3. Representation of Uranyl Peroxide Clusters 7.4. Cluster Compositions 7.5. Cluster Descriptions 7.5.1. Open Clusters 7.5.2. Cage Clusters: Fullerenes 7.5.3. Cage Clusters Containing Topological Squares 7.5.4. Miscellaneous Cage Clusters 7.6. Role of the Solution pH 7.7. Stability and Electrochemistry of Uranyl Peroxide Cage Clusters 7.8. Isomer Selection in Uranyl Peroxide Cage Clusters

© 2012 American Chemical Society

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1. INTRODUCTION Metal oxide clusters, especially transition-metal polyoxometalates, have been studied for decades because they provide a rare opportunity to study nanoscale materials with well-defined structures and unusual properties, with emerging diverse applications.1−9 In contrast to those of transition metals, actinide oxide clusters are relatively unexplored, although there have been many developments in this area over the past decade. No doubt studies of actinide (An) clusters have lagged behind those of other metals in part due to the experimental difficulties of working with actinides, all of which are radioactive, and to the scarcity of facilities suitable for studies of transuranium elements. Furthermore, the availability of transuranium elements for basic research is limited, owing to the considerable cost of their production and their strategic importance. The industrial-scale manipulation of actinides during and after the Manhattan project achieved the goals of production of vast quantities of isotopically pure plutonium for weapons and development of fuels for commercial nuclear power plants that remain essential to the energy supplies of many countries. However, the environmental costs of these projects have been considerable.10 Perhaps controlling actinide materials on the scale of a few nanometers will provide more effective and environmentally sound methods for processing nuclear materials in a future advanced nuclear energy system. Such applications include in separations and materials fabrication. It has also been suggested that actinide clusters can be useful models for understanding the behavior of actinides in environmental and geochemical systems, as well as in the design of catalysts and molecular magnets.11 It is even possible that nanoscale clusters of actinides are responsible for their dispersal under some environmental conditions.12−14 If one were to review the actinide literature during the 1990s while transition-metal polyoxometalates emerged as a major

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Special Issue: 2013 Nuclear Chemistry

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Received: April 16, 2012 Published: October 24, 2012 1097

dx.doi.org/10.1021/cr300159x | Chem. Rev. 2013, 113, 1097−1120

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exist in the uranyl ion, and the symmetry of a subset of these only matches with uranium f orbitals. The existence of f orbitals thus favors the linear dioxo cation, in contrast to the bent configuration typical in transition-metal cases. Here we focus on reviewing the structures of clusters in which actinide cations are bridged through an oxygen, a peroxide, or a hydroxyl. We note that other important actinide clusters with nonoxygen bridges have been reported,21−28 but these are outside the scope of our review. We choose here to restrict our attention to clusters containing four or more essential actinide cations. There are many important aqueous species, such as some actinyl carbonates, sulfates, and peroxides, which contain fewer than four actinide cations; the interested reader is directed to several reviews.29−34 Here we restrict our attention to clusters that have been shown to exist in solution, or at least plausibly could exist in a solution. Most of our attention is focused on cluster structures that have been established by X-ray diffraction studies of their crystallized forms. However, we emphasize that it is the properties of actinide clusters in solution that are expected to be the most useful. In the solid state such clusters are distinguished by the attribute that they usually are not linked into an extended structure (a chain, sheet, or framework of polyhedra) through higher valence cations (although there are some notable exceptions). In other words, the structural unit in the crystal structure is the finite cluster. Our discussion extends from chemically and topologically simple clusters containing a small number of actinide cations to clusters containing as many as 120 actinide cations in complex topological arrangements. In general, metal oxide clusters form in solution when the surface of the cluster is passivated. In transition-metal and actinide polyoxometalates, it is the yl O atoms that stabilize the surface of the cluster and effectively prevent further growth. For cation coordination polyhedra lacking yl O atoms, clusters typically only form where the surface is passivated by organic ligands, or in rare cases halogen anions. A detailed comparison of transition metal polyoxometalates with the emerging family of actinide polyoxometalates is presented elsewhere.35 In this review several different types of illustrations are provided to aid in the understanding of the connectivity of actinide-based clusters (Figures 1−10). The approach taken depends mostly on the number of atoms in the cluster, with ball-and-stick representations being effective for the smaller clusters and polyhedral models for the larger clusters. We also provide graphical representations of the clusters in which each actinide cation corresponds to a vertex, and lines connecting vertexes designate bridges between the corresponding cations. Although each of the clusters selected for coverage here contains one or more oxide, peroxide, or hydroxide bridge, various other chemical types of bridges are present in some clusters. With the exception of bridges through pyrophosphate or oxalate groups in Figure 8, bridges other than oxide, peroxide, or hydroxyl are excluded from the graphical representations of clusters. We have chosen to arrange our discussion of actinide clusters according to the cation oxidation states. This approach is warranted because the coordination chemistry of actinides is strongly oxidation state dependent. For clusters containing An(IV) cations, their surfaces are usually passivated by organic ligands, whereas for An(VI) clusters, surface passivation is almost exclusively through the yl oxygen atoms. No clusters of actinides containing oxygen, peroxide, or hydroxide bridges have been reported for actinide oxidation states less than IV.

research theme, one might erroneously assume that f elements are incapable of self-assembly into well-structured and complex nanomaterials. The first high-nuclearity cluster for uranium appeared in 1953 as part of an extended framework structure,15 but essential no further progress occurred until the later part of the same century. The authors of a study of a uranium cluster in 1996 observed, “The results presented here demonstrate that there is nothing inherently unstable about high nuclearity actinide complexes, and we expect to see the number of such well-characterized complexes grow rapidly because of their potential applications.” 16 This prediction has proven correct, with especially abundant and complex clusters realized for uranyl peroxides. The efforts of a relatively few researchers have produced a myriad of complex and beautiful actinide oxide clusters over the past few years that are beginning to rival the complexity of transition-metal clusters. At this point, most studies have focused on describing the synthesis and structures of novel actinide clusters. Exploration of the properties and applications of such clusters is mostly in its infancy, but holds considerable promise for future research efforts. The coordination chemistry of actinides is complex and very oxidation state dependent (Figure 1).17 Where the actinide

Figure 1. A selection of typical actinide−oxygen coordination polyhedra. In (a) and (b), the An(IV) cation is 8-coordinated and coordination numbers can range from 6 to 12. In (c)−(g), the presence of an actinyl ion is designated by triple bonds and these correspond to An(V) or An(VI) cations. In (f) and (g), the coordination polyhedra include bidentate peroxide ligands. See Table 2 for the legend of the figures.

cation has a valence of IV or less, symmetrical distributions of ligands about the large cations are typical, and coordination numbers range from 6 to 12 or higher. Cations in the V and VI oxidation states usually bond to two “yl” O atoms, forming linear dioxo actinyl cations.17 The importance of yl O atoms is well recognized in transition-metal oxide clusters,8,18 but in these cases the metal coordination includes only a single yl O atom that truncates the outer edge of the cluster or two yl atoms in a cis configuration. The presence of two yl O atoms in high-valence actinide coordination polyhedra ensures that their oxide clusters will present interesting new cluster types and topologies. Linear trans-dioxo actinyl ions are related to the interaction of atomic orbitals on the actinide and O atoms.19,20 In the U(VI)O22+ actinyl ion, the O atoms provide 12 p electrons that completely fill the bonding orbitals of the uranyl ion, giving triple bonds. Six linear combinations of O p orbitals 1098


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Figure 2. Illustrations of clusters built from An(IV) cations. The vertexes in the graphs represent the locations of An(IV) cations, and lines designate oxide or hydroxyl bridges. (a, b) [U4(L2)2(H2L2)2(py)2O][CF3SO3]2, H4L2 = N,N′-bis(3-hydroxysalicylidene)-2,2-dimethyl-1,3-propanediamine, py = pyradine. (c,d) [Cp(CH3COO)5U2O]2, cp = η5-cyclopentadienyl. (e,f) U6O4(OH)4(SO4)6. (h, i) Th8(μ3-O)4(μ2-OH)8(H2O)15(SeO4)8·7.5H2O, Th8(μ3-O)4(μ2-OH)8(H2O)17(SeO4)8·nH2O, Th9(μ3-O)4(μ2-OH)8(H2O)21(SeO4)10], and Th9(μ3-O)4(μ2-OH)8(H2O)21(SeO4)10·nH2O with ThIV(μ3-O)4(μ2-OH)4 cores. (j, k) U8Cl24O4(cp*py)2, cp*py = tetramethyl-5-(2-pyridyl)cyclopentadiene. (l, m) [U8L4Cl10O4]2−, H4L = N,N′bis(3-hydroxysalicylidene)-1,2-phenylenediamine. (n, o) [U10O8(OH)6(PhCO2)14I4(H2O)2(MeCN)2]. (p) [U10O8(OH)6(PhCO2)12.79I3.2(H2O)4(MeCN)4]2I·4MeCN. (q, r) [Pu38O56Cl54(H2O)8]14− cluster. See Table 2 for the legend of the figures.

2. CLUSTERS CONTAINING AN(IV)

compounds. In this structure, each An(IV) cation is coordinated by eight O atoms arranged at the vertexes of a cube and each O atom is coordinated by four An(IV) cations at the vertexes of a tetrahedron. The An(IV) cations are arranged at the vertexes of octahedra, whereas the O atoms are distributed at the vertexes of cubes.

Clusters containing Th(IV), U(IV), and Pu(IV) have been reported with crystal structure determinations, and a Np(IV)bearing cluster has been proposed on the basis of spectroscopic studies. Several of the clusters built from An(IV) cations are related to the fluorite structure type of the An(IV)O2 1099


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An6(O,OH)8 cages, with four containing Th(IV)39,40 and eight with U(IV) (i.e., Figure 2g).40−43 The latter studies confirmed that the actinides are present in this cage in the tetravalent oxidation state. However, one compound has been isolated in which the U cations in a U6O8 cluster appear to be a mixture of valences (see section 3.2). The cage consists of six actinide cations that are arranged at the vertexes of an octahedron (Figure 2f), analogous to their local arrangement in the fluorite structure. The eight anions that bridge between the cations are located above the centers of the octahedral faces. As such, each of these anions is bonded to three cations, and each cation is bonded to four of these anions. The coordination environments of the cations are completed by a rich variety of ligands, including formate40 and dibenzoylmethanate.43 We note that similar cage clusters consisting of non-actinide tetravalent metals have been reported, including for Zr and Ce.44−47 The An6(O,OH)8 cage has been synthesized in the forms An6(μ3-O)841,42 and An6(μ3-O)4(μ3-OH)4.39,40,43 Three clusters of the latter form were synthesized containing Th.39 The Th(IV) compounds were synthesized under ambient conditions in air, whereas the U(IV) compound syntheses were conducted in aqueous solutions under an inert atmosphere. Although the crystal structure determinations did not reveal the H atoms of the μ3-OH, Th−O bond lengths in one of the compounds clearly indicate an ordered distribution of μ3-OH in the cage cluster.39 The possibility of μ3-OH disorder exists in the other two compounds, although density functional theory (DFT) calculations of model structures indicate that a tetrahedral arrangement of the OH ions on the Th6(μ3O)4(μ3-OH)4 cages is energetically favored.39 A combination of UV−vis and X-ray absorption spectra support a structure model with a Np6(μ3-O)4(μ3-OH)4 core, although no crystallographic study is available.48 The Np(IV) was dissolved in acidic aqueous solutions and complexed with RCOO, where R = H, CH3, or CH2SH. The study concluded that the hexanuclear core cluster was the dominant form of Np(IV) in solution above about pH 1.5 under the conditions of the room-temperature experiments. The authors argued that, in the absence of terminating ligands (RCOO in this case), hydrolization of the An(IV) cation could lead to a colloidal material. A computational study examined Th6O8 and U6O8 clusters, as well as several larger variants, with density functional theory.49 The simulations indicated that Th6O8 and U6O8 clusters are stable entities in the absence of bridging ligands, but the anionic ligands are necessary to prevent further growth of the clusters.

2.1. Clusters with Four An(IV) Cations

The compound [U4(L2)2(H2L2)2(py)2O][CF3SO3]2, H4L2 = N,N′-bis(3-hydroxysalicylidene)-2,2-dimethyl-1,3-propanediamine, py = pyridine, was synthesized and has tetranuclear clusters (Figure 2a,b).36 The cluster contains four U(IV) cations that are arranged at the vertexes of a tetrahedron (Figure 2b). At the center of this tetrahedron there is a μ4-O atom. The Schiff base ligands also coordinate the U(IV) cations, providing eight O atoms that each bridge two U(IV) cations. The compound was crystallized at 80 °C from a solution of pyridine into which mononuclear metal complexes of Cu and U had been introduced. The compound is thought to have incorporated adventitious oxygen, and attempts to produce the compound in a controlled fashion were unsuccessful.36 A cluster with composition [Th4Cl8(O)(EO42‑)3]·3CH3CN, EO42− = tetraethylene glycolate, was crystallized in high yield from a solution of ThCl4 in pentaethylene glycol and CH3CN/ CH3OH that was heated to 60 °C and cooled to 20 °C.37 It contains four Th(IV) cations arranged at the vertexes of a tetrahedron, with a central μ4-O atom bonded to each, similar to the cluster shown in Figure 2a,b. Perhaps most notable about the cluster shown in Figure 2a is the presence of the μ4-O at its center and its attribution to adventitious oxygen. Such μ4-O oxygen atoms are relatively rare in clusters of An(IV) despite their occurrence in the fluoritetype structures. The clusters U8Cl24O4(cp*py)2, cp*py = tetramethyl-5-(2-pyridyl)cyclopentadiene (Figure 2j,k), [U8L4Cl10O4]2−, H4L = N,N′-bis(3-hydroxysalicylidene)-1,2phenylenediamine (Figure 2l,m), and [Pu38O56Cl54(H2O)8]14− (Figure 2q,r) also contain μ4-O atoms. The cluster [Cp(CH3COO)5U2O]2, cp = η5-cydopentadienyl, was synthesized in low yield under a nitrogen atmosphere (Figure 2c,d).38 The cluster has four U(IV) cations arranged about an inversion center. There are two μ3-O atoms, and the remainder of the O atoms that coordinate the U(IV) cations are part of acetate groups. The two distinct U(IV) cations in the cluster exhibit markedly different coordination environments. One is coordinated by an μ3-O atom and five monodentate acetate groups in a very one-sided arrangement. The other is bonded to two μ3-O atoms and is also coordinated by one bidentate and four monodentate acetate groups. 2.2. Clusters with Six An(IV) Cations

The first report of the synthesis of what may be regarded as a hexanuclear An6O8 cage was in 1953 for U6O4(OH)4(SO4)6 (Figure 2e,f),15 although the cage in this compound is linked into a framework structure through bridging sulfate tetrahedra and there is no evidence of preassembly of the cages in solution prior to crystallization. The compound was synthesized by heating an aqueous solution of U(IV) in 0.5 M H2SO4 in a sealed glass tube at 200 °C; crystals of the compound formed before the solution was cooled. Another example of an An6(O,OH)8 cage cluster was reported for a uranium phosphate complex (Figure 2f) in 1996.16 It was obtained by reacting [TpVCl2(dmf)], Tp = hydridotris(pyrazolyl)borate, with sodium diphenylphosphate and uranyl acetate in aqueous acetonitrile under an inert atmosphere.16 Although the uranyl ion was not present in the compound and the uranium had been reduced, it is unclear from the study which oxidation state the uranium adopted. Following the early work on the U 6 (O,OH) 8 cage clusters,15,16 several groups synthesized clusters with similar

2.3. Clusters with Eight An(IV) Cations

A recent study reported the synthesis and characterization of four octanuclear clusters with Th8(IV)(μ3-O)4(μ2-OH)4 cores (Figure 2h,i).50 The compounds have the compositions Th8(μ3-O)4(μ2-OH)8(H2O)15(SeO4)8·7.5H2O, Th8(μ3-O)4(μ2OH)8(H2O)17(SeO4)8·nH2O, Th9(μ3-O)4(μ2-OH)8(H2O)21(SeO4)10], and Th9(μ3-O)4(μ2-OH)8(H2O)21(SeO4)10·nH2O and crystallize as extended structures in which selenate tetrahedra bridge the clusters. An amorphous thorium precipitate was dissolved into selenic acid and water for the synthesis of these compounds. The first two compounds crystallized following heating to boiling and subsequent cooling, whereas the latter two synthesis experiments were done entirely at room temperature. The novel core is bonded to selenate tetrahedra that are integral to the cluster (Figure 1100


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of 2.23(5) Å. Eight bridging benzoate ligands and two bridging iodine anions complete the cluster. Compounds [U10O8(OH)6(PhCO2)14I4(H2O)2(MeCN)2] and [U10O8(OH)6(PhCO2)12.79I3.2(H2O)4(MeCN)4]2I·4MeCN were synthesized from an acetonitrile reaction mixture formed by the hydrolysis of [UI3(thf)4] in the presence of benzoate.11 These compounds were obtained reproducibly even when different uranium to benzoate ratios were used, which prompted the authors to suggest that these are the only clusters in the acetonitrile solution.11 Given their earlier synthesis of a cluster with a U6O8 core using pyridine, the authors hypothesized that the choice of organic base could be used to tune the cluster topology.11 Using a stronger organic base, tetramethylethylenediamine (TMEDA), a cluster containing a U16O24 core was obtained (see section 3.3), consistent with the importance of the organic base in determining cluster size.11

2h). In all of these compounds each Th(IV) cation is coordinated by one μ3-O atom and two μ2-OH groups. Each cluster contains two selenate tetrahedra near the center of the cluster that link to four Th(IV) cations, as well as four more that each link to three Th(IV) cations (Figure 2i). The clusters in these four compounds are distinguished by the number of H2O groups and nonbridging monodentate selenate tetrahedra that complete the coordination spheres of the Th(IV) cations. Note that these clusters cannot be regarded as fragments of the fluorite-type structure. DFT computations were used to determine likely H atom positions on the Th(IV)8(μ3-O)4(μ2-OH)4 cores and to estimate the deprotonation reaction free energies in aqueous solution for different related model clusters.50 Two octanuclear clusters with U(IV) cations have been isolated, although they are very distinct from each other (Figure 2j−m).51,52 The cluster U8Cl24O4(cp*py)2, cp*py = tetramethyl-5-(2-pyridyl)cyclopentadiene (Figure 2j,k), was synthesized in a nominally oxygen-free atmosphere.51 The cluster was unintentionally obtained while exploring the synthesis of uranium complexes with cp*py. The presence of O in the cluster is ascribed to adventitious oxygen that entered the reaction flask during heating, and crystals of the cluster compound formed when the heat-treated solution was left standing at room temperature. No yield or purity information was provided in the structure report. Four O atoms are located near the center of the cluster, where two of these are μ4-O, with U(IV) cations geometrically distributed in a tetrahedron similar to the coordination of μ4-O atoms in UO2. Two μ3-O atoms are bonded to three U(IV) cations that are approximately coplanar with bond lengths of ∼2.2 Å, consistent with these O atoms being unprotonated. Of the 24 Cl anions in the cluster, 16 bridge between 2 U(IV) cations and the others are terminal. At both ends of the elongated cluster, the cp*py ligand coordinates a U(IV) cation. Cluster [U8L4Cl10O4]2−, H4L = N,N′-bis(3-hydroxysalicylidene)-1,2-phenylenediamine, is shown in Figure 2l,m.52 This cluster crystallized by incorporating adventitious O that probably entered the reaction vessel during prolonged heating at 80 °C; attempts to synthesize the compound in a controlled fashion were unsuccessful.52 This highly complex cluster contains four μ4-O atoms, each of which are surrounded by four U(IV) cations in a tetrahedral arrangement with bond lengths similar to those in UO2. Eight of the Cl anions are bonded to a single U(IV) cation, and the remaining two provide μ2 bridges between U(IV) cations. The U(IV) cations are also linked through eight μ2-phenoxyl bridges.

2.5. Clusters with 38 An(IV) Cations

A large [Pu38O56Cl54(H2O)8]14− cluster was crystallized with Li as a counterion from acidified aqueous solution under ambient conditions (Figure 2q,r).53 However, the reactants used for this synthesis had a complex history that included passing through an ion exchange column and repeated heat cycles to dryness and reconstitution in HCl solutions. Subsequently, a more controlled synthesis approach was used to obtain clusters with the same Pu38O56 core.54 A solution of Pu(IV) in concentrated HCl was boiled, with the addition of LiOH during boiling that continued until the volume was reduced by half. The solution was allowed to cool to room temperature, and crystals of Li12[Pu38O56Cl54(H2O)8](H2O)n formed when the solution was almost completely evaporated.54 The authors of the study report that the yield was not quantitative and that a more direct synthetic route was under investigation. The Pu38O56 core is a fragment of the PuO2 fluorite structure, in which Pu(IV) cations are coordinated by eight O atoms and O atoms toward the center of the cluster are μ4-O surrounded by four Pu(IV) cations. Toward the edges of the nanoscale fragment of the PuO2 structure, μ3-O anions are bonded to three Pu(IV) cations with considerably shorter Pu− O bond lengths than in the case of the μ4-O anions. The Pu38O56 core is stabilized by addition of Cl and H2O, as shown in Figure 2r. Hydroxyl is notably absent. A cluster with the analogous core and composition [Pu38O56Cl42(H2O)20]2− was also isolated and characterized.54 Crystals containing the [Pu38O56Cl54(H2O)8]14− cluster were readily dissolved in an aqueous solution of 2 M LiCl to produce a green solution that gave an optical spectrum typical of a Pu(IV) polymer.53 High-energy X-ray scattering (HEXS) provides pair distribution functions capable of giving insight into actinide speciation in solution.55 HEXS data collected for a solution created by dissolving crystals containing the [Pu38O56Cl54(H2O)8]14− cluster demonstrated that the clusters remain intact in solution.53 This nanostructured PuO2 fluorite material is thought to correspond to the well-known Pu(IV) polymer (sometimes designated colloid) that is challenging to separate from complex systems. It was later demonstrated that the surfaces of the Pu38O56 core can be manipulated, thereby making it possible to develop a separation scheme that targets the nanoscale material, rather than mononuclear metal species that are only obtained through harsh chemical conditions.54

2.4. Clusters with 10 An(IV) Cations

The decanuclear cluster [U10O8(OH)6(PhCO2)14I4(H2O)2(MeCN)2] is illustrated in Figure 2n,o.11 It contains U(IV) and together with the closely related cluster [U10O8(OH)6(PhCO2)12.79I3.2(H2O)4(MeCN)4]2I·4MeCN11 (Figure 2p) is the only U10O14 core reported to date. The U10O14 topology consists of two U6O8 cages that are fused through two shared U and two shared O atoms (Figure 2o). As in the U6O8 topology, the O atoms are located above the faces of the octahedra defined by the An(IV) cations. There are twelve μ3-O bridges, whereas the two O atoms that belong to each U6O8 are μ4-O bridges. Six of the μ3-O bridges correspond to OH groups with an average U−O bond length of 2.43(6) Å, and the remaining μ3-O bridges are O anions with an average U−O bond length 1101


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3. CLUSTERS CONTAINING AN(IV) AND AN(V) Several clusters have been synthesized that contain a mixture of An(IV) and An(V) cations. Where good-quality crystallographic data are available, these oxidation states can be distinguished because An(V) cations usually are present as actinyl ions. Np(V) is the only pentavalent actinide cation with significant stability in aqueous solution, but its linkage into clusters is unexplored. The U(V) cation disproportionates to U(IV) and U(VI) in aqueous solutions under most conditions, but nonaqueous synthesis methods can stabilize the U(V) oxidation state.56,57 3.1. Clusters with Four or Five An(IV) and An(V) Cations

The U(IV)-mediated disproportionation of U(V) uranyl ions has provided two polynuclear clusters containing U(IV) and U(V).58 The clusters of these compounds, designated {[UO2(mesaldien)−U(mesaldien)]2(μ-O)}, H2mesaldien = N,N′-(2-aminomethyl)diethylenebis(salicylimine), and {[UO2(salen)][U(salophen- t Bu2 )] 2 [(U(salen)] 2(μ-O) 3 (μ3 O)}, H 2 salen = N,N′-ethylenebis(salicylideneimine), H2salophen = N,N′-phenylenebis(salicylideneimine), are shown in Figure 3a−d. That of {[UO2(mesaldien)−U(mesaldien)]2(μ-O)} was obtained in 76% yield, together with a U(VI) complex, and contains four U cations in a linear arrangement that are bridged through μ2-O atoms (Figure 3a). Although each of the U cations are in pentagonal bipyramidal coordination polyhedra, there are two symmetrically distinct sites, and bond lengths indicate one contains U(IV) and the other U(V).58 The valence states alternate along the linear arrangement, such that the oxo atoms of the U(V) uranyl ion are bridges to U(IV) cations along the chain length. The coordination environments about the U(IV) and U(V) cations are completed by O and N atoms of the Schiff ligands. In {[UO 2 (salen)][U(salophen- t Bu 2 )] 2 [(U(salen)] 2 (μO)3(μ3-O)}, the cluster contains five uranium cations (Figure 3c,d). Again, the cluster was found to contain both U(IV) and U(V), as well as two cation sites for which the oxidation states are unclear, but either U(IV) or U(V) or a mixture thereof.58 The oxo atoms of the single obvious U(V) uranyl ion in the cluster bridge to two different U(IV) cations. The core of the cluster also contains one μ3-O, at the center, and five μ2-O atoms between the U cations (Figure 3d). The remainders of the coordination spheres of the five cations are completed by N and O atoms of the organic components.

Figure 3. Illustrations of clusters containing An(IV) and An(V). The vertexes in the graphs represent the locations of An(IV) and An(V) cations, and lines designate oxide or hydroxyl bridges. (a, b) {[UO2(mesaldien)−U(mesaldien)]2(μ-O)}, H2mesaldien = N,N′-(2aminomethyl)diethylenebis(salicylimine). (c, d) {[UO2(salen)][U(salophen- t Bu2 )]2[(U(salen)]2(μ-O)3 (μ3-O)}, H2 salen = N,N′ethylenebis(salicylideneimine), H2salophen = N,N′-phenylenebis(salicylideneimine). (e, f) [U 1 2 (μ 3 -OH) 8 (μ 3 -O) 1 2 I 2 (μ 2 OTf)16(CH3CN)8]·2CH3CN·2H2O. (g, h) {[K(MeCN)]2[U16O22(OH)2(C6H5COO)24]}·4MeCN. See Table 2 for the legend of the figures.

3.2. Clusters with Six An(IV) and An(V) Cations

The controlled hydrolysis of trivalent uranium in acetonitrile has provided hexanuclear clusters of uranium, as well as one with a dodecanuclear cluster of uranium (see section 3.3).59 The details of the clusters formed were dependent on both the reaction time and the ligand used. The approach provided a discrete hexanuclear cluster, a two-dimensional array of hexanuclear clusters, a three-dimensional zeolite-like topology built from hexanuclear clusters, and a discrete dodecanuclear cluster.59 The discrete hexanuclear cluster [U6(μ3-O)8(μ2-OTf)12(H2O)3]·23H2O with a U6(μ3-O)8 core contains uranium with an average valence of 4.66, and the high symmetry of the cluster suggests that the mixture of two U(IV) and four U(V) cations in the cluster is completely delocalized.59 The analogous core in the two-dimensional structure is reported to contain four U(IV) and two U(V) cations, whereas that which condensed into a framework only contains U(IV).59

3.3. Clusters with 12 An(IV) and An(V) Cations

The compound [U 1 2 (μ 3 -OH) 8 (μ 3 -O) 1 2 I 2 (μ 2 -OTf) 1 6 (CH3CN)8]·2CH3CN·2H2O contains a discrete dodecanuclear cluster with a U12O20 core (Figure 3e,f).59 As with some of the smaller clusters discussed in the preceding section, this cluster was produced by controlled hydrolysis of trivalent uranium in acetonitrile. The cluster, which was obtained in quantitative yield, contains 16 μ3-O and 4 μ3-OH anions that bridge the uranium cations. The cations are further coordinated by O and N atoms of the organic molecules, as well as terminal iodine anions in two cases. The average valence of the uranium was 1102


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reported to be 4.16, which was formally interpreted to be due to a mixture of ten U(IV) and two U(V) cations. The bond lengths in the cluster suggested a delocalization of charge. The presence of U(IV) was confirmed by UV/vis spectroscopy, and magnetic measurements were consistent with the presence of U(V).59 3.4. Clusters with 16 An(IV) and An(V) Cations

The compound {[K(MeCN)]2[U16O22(OH)2(C6H5COO)24]}·4MeCN contains a U16O24 core (Figure 3g,h) and was synthesized using the organic base TMEDA in high yield.11 The study demonstrated the importance of the organic base in synthesizing clusters of U(IV), with smaller clusters obtained with other bases (see section 2.4). The core of the cluster contains four U6O8 units that are fused, in general as in [U10O8(OH)6(PhCO2)14I4(H2O)2(MeCN)2], through pairs of shared U cations and pairs of shared O atoms. Linkages between U cations are through 16 μ3-O atoms, 2 μ3-OH groups, and 6 μ4-O atoms that occur as caps on the octahedral faces of the U6O8 cages. Bond-valence analyses indicate that the U cations are a mixture of valences, with 12 being U(IV) and 4 U(V). The presence of U(V) is supported by the measured magnetism of the compound.11

Figure 4. Illustrations of clusters containing An(V). The vertexes in the graphs represent the locations of An(V) cations, and lines designate oxide or hydroxyl bridges. (a, b) {[UO2(dbm)2]4[K6Py10]}·I2·Py2, dbm = dibenzoylmethanate, py = pyradine. (c, d) [Cp*4 (bpy)2][U6O13], Cp* = 1,2,4-tBu3C5H2, bpy = bipyridine. See Table 2 for the legend of the figures.

4. CLUSTERS CONTAINING AN(V) Cation−cation interactions between U(V) uranyl ions are emerging as important features in oxo-bridged clusters. In actinide chemistry, a cation−cation interaction refers to the situation where an O atom of an actinyl cation coordinates another actinyl cation in the equatorial plane of its corresponding coordination polyhedron.60−66 Although the actual bonding involves an An−O−An linkage, the term “cation−cation interaction” has been in regular usage since it was introduced to the literature in 1961.67 Although a few compounds have been reported with cation−cation interactions between U(VI) uranyl ions, they are much more common in the case of An(V) compounds. For example, more than 40% of the reported Np(V) crystal structures contain cation−cation interactions.68

from the organic ligand. Using 1H NMR this study showed that this complex remains intact in pyridine for one month. Cyclic voltammetry of the complex dissolved in pyridine demonstrated that a reversible one-electron oxidation does not destroy the structure of the cluster, but a subsequent three-electron oxidation under higher potential results in the irreversible breakdown of the cluster.64 Very recently, the first Np(V) example of the same tetrameric unit was isolated in the compound [{NpO2(salen)}4(μ8-K)2][K(18C6)Py]2.70 The compounds {[UO 2 (dbm) 2 ] 2 [μ-K(Py) 2 ] 2 [μ 8 -K(Py)]}2I2·Py2 and {[UO2(dbm)2]2[μ-K(MeCN)2][μ8-K]}2 are based on clusters with the same basic core structure as {[UO2(dbm)2]4[K6Py10]}·I2·Py2.65 In pyridine solution the latter of these clusters disproportionates to [U(dbm)4] and [(UO2(dmb)2] species. The presence of the cation−cation interactions in the tetranuclear cluster between U(V) uranyl ions was found to be important for the disproportionation reaction.65 In other words, disrupting the cation−cation interactions also disrupted disproportionation of U(V) to U(IV) and U(VI). Four additional clusters have been reported that contain the same basic uranium−oxygen core as in {[UO2(dbm)2]4[K6Py10]}·I2·Py2: [UO2(acacen)]4[μ8-K]2[K(18C6)(py)]2, H2acacen = N,N′-ethylenebis(acetylacetoneimine), {[UO2(acacen)]4[μ8-K]}·2[K([222])(py)], {[UO2(salophen)]4[μ8-K]2[μ5-KI]2[(K(18C6)]}·2[K([18]C-6)(thf)2]·2I, and [UO2(salen)4][μ8-Rb]2[Rb([18]C6)]2.71 This study explored the importance of both the organic ligands and alkali-metal counterions in determining the stability of the clusters in pyridine solution. The authors concluded that the tetranuclear core containing U(V) could be highly stable but careful tuning of the ligand geometry and the presence of coordinating counterions is essential to the stability. The reaction of compounds containing the tetranuclear cluster with H+ results in immediate decomposition and conversion of U(V) to a mixture of U(IV) and U(VI).71

4.1. Clusters with Four An(V) Cations

The compound {[UO2(dbm)2]4[K6Py10]}·I2·Py2, dbm = dibenzoylmethanate, was the first to be isolated that exhibits a tetranuclear cation−cation complex based on U(V) (Figure 4a,b).69 The compound was obtained by reacting the coordination polymer {[UO2Py5][KI2Py2]}n with dibenzoylmethanate in an oxygen-free atmosphere. In the cluster four U(V) uranyl ions form the core, with each uranyl ion donating one cation−cation interaction, as well as accepting one donated by another uranyl ion (Figure 4b). The remainders of the coordination environments of the uranyl pentagonal bipyramids are completed by O atoms of the organic ligands. The presence of the cluster in a pyridine solution was verified by NMR. In contrast, when the cluster is dissolved in dimethyl sulfoxide (DMSO), it fragments.69 It is rapidly oxidized in the presence of trace amounts of oxygen and breaks down in pyridine solution. In a continuation of efforts to stabilize U(V) uranyl cation− cation interactions, two tetranuclear clusters were reported with the same basic core structure as that in {[UO2(dbm)2]4[K6Py10]}·I2·Py2.64 In [{UO2(salen)}4(μ8-K)2][{K(18C6)Py)}2], 18C6 = 18-crown-6, the equatorial vertexes of the four uranyl pentagonal bipyramids that are not involved in the cation−cation interactions are two N and two O atoms 1103


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Figure 5. Illustrations of clusters containing four An(VI) cations. In the graphical representations, the nodes represent uranyl polyhedra and lines and double lines represent single and double O bridges, respectively. See Table 2 for the legend of the figures.

4.2. Clusters with Six An(V) Cations

which contains only U(V), [{UO 2 (salen)μ-K(18C6)}{UO2(salen)}3(μ8-K)2] is not stable when dissolved in pyridine. Instead, it rearranges to yield a mixture of compounds.

The compound [Cp*4 (bpy)2][U6O13], Cp* = 1,2,4-tBu3C5H2, bpy = bipyridine, contains a novel U(V)6O13 core (Figure 4c,d).72 The U(V) cations are arranged at the vertexes of an octahedron, as in the An6(O,OH)8 clusters discussed above, but in this case μ2-O atoms are located along the edges of the octahedron and a single μ4-O atom is located at the center of the core. The core cluster is analogous to the Lindquist-type molybdenum and tungsten oxide clusters. Whereas the latter are stabilized by the presence of yl O atoms, the U(V)6O13 core is truncated by Cp* in the case of four U(V) cations and bipyridine in the other two cases.

6. CLUSTERS CONTAINING AN(VI) All but one of the clusters reported with An(VI) contain the U(VI) uranyl ion. The vast majority of these are uranyl peroxide cage clusters, which are treated separately in section 7. 6.1. Clusters with Four An(VI) Cations

More than a dozen compounds have been reported that contain tetrameric units built from U(VI) uranyl ions that contain O bridges. In each of these, the uranyl ions are present as uranyl square, pentagonal, or hexagonal bipyramids and each of the four uranyl ions extend in approximately the same direction, roughly perpendicular to the larger dimensions of the clusters. The uranyl ions in these clusters are bridged by a variety of combinations of μ2-O and μ3-O atoms, and the bipyramids share either equatorial edges or single equatorial vertexes within the cluster. Graphical representations of the observed configurations are in Figure 5. Note that these graphs are constructed differently from those elsewhere in this review, as the sharing of polyhedral edges and vertexes is distinguished. In the graphs, black circles represent a uranyl ion bipyramid and the connections between the polyhedra are shown as single or

5. CLUSTERS CONTAINING AN(V) AND AN(VI) The cluster compound [{UO 2 (salen)μ-K(18C6)}{UO2(salen)}3(μ8-K)2] was isolated by reacting precursors containing U(V) in an oxygen-free atmosphere.64 The cluster has the same arrangement of U cations and O atoms as in [{UO2(salen)}4(μ8-K)2][{K(18C6)Py)}2], but in contrast the U cations correspond to a mixture of U(V) and U(VI) cations. As such, this cluster represents the first example of the existence of cation−cation interactions between U(V) and U(VI) uranyl ions. The crystallographic study of this cluster indicated that the valence is localized on the U cations, with three of the four being U(V). Unlike [{UO2(salen)}4(μ8-K)2][{K(18C6)Py)}2], 1104


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This compound was synthesized hydrothermally at 200 °C with a solution pH in the range of 1.5−3. The cluster shown in Figure 5m, in which uranyl polyhedra form a four-membered ring by sharing single vertexes, was reported in the compound [(UO2)2(C2O4)(OH)2(H2O)2](H2O) that was synthesized hydrothermally at 120 °C over four days from an aqueous solution of uranyl nitrate, oxalic acid, and potassium nitrate.82 Uranyl ions are bridged by OH groups and are also coordinated by edge-on oxalate groups that bridge between different tetrameric clusters. The complex [(UO2)4O4(LH8)]·10CH3OH, which contains the same type of tetrameric cluster, was crystallized by slow evaporation at room temperature.83 The compounds (UO2)6O(OH)(m-BTC)2(m-HBTC)2(H2O)2(H3O)·6H2O and (UO2)2O(m-BTC)[NH2(CH3)2]2·H2O, m-BTC = 1,2,4-benzenetricarboxylate, are based on the tetrameric clusters shown in parts g and e, respectively, of Figure 5.84 These compounds were obtained in good yield via the hydrothermal treatment of an aqueous solution containing organic ligands at 160 °C for three days, followed by slow cooling. Hydrothermal synthesis provided the compound [Zn2(phen)4U4O10(OAc)2(NA)2-(QA)2] (phen = 1,10-phenanthroline; HOAc = acetic acid; HNA = nicotinic acid; H2QA = quinolinic acid) in high yield at 160 °C.85 It contains the tetranuclear cluster shown in Figure 5e, in which there are two μ3-O atoms and the remainder of the coordination polyhedra about the uranyl ions are completed by the organic ligands.

double lines for the sharing of a single vertex and the sharing of an edge, respectively. Hydrothermal syntheses using multiple organic ligands, specifically aliphatic dicarboxylates and dipyridyl species, provided four tetramers of uranyl ions (Figure 5a,c,e,k).73 This was the first study in which two different organic ligands were used to connect uranyl ions. These clusters differ in the number of μ2-O and μ3-O atoms, and their specific topologies can be related to the role of the organic ligands in both directly coordinating the uranyl ions and providing for charge balance in the crystal structures. In these compounds, the aliphatic dicarboxylates contained from 5 to 10 C atoms and the dipyridine species were either 4,4′-bipyridyl or 1,2-bis(4pyridyl)ethane.73 The dicarboxylates were found to always coordinate uranyl ions, whereas the dipyridyl species coordinate the uranyl ions, provide charge balancing, or provide space filling.73 The compound [HNEt3]2[(UO2)4(3−8H)(OH)2(H2O)4]·1.5NEt3·2.5H2O·CH3OH, 3−8H = p-tertbutyloctahomotetraoxacalix[8]arene, contains four U(VI) uranyl ions, each of which is present as a uranyl pentagonal bipyramid (Figure 5i).74 It was obtained in very low yield by reacting p-tert-butyloctahomotetraoxacalix[8]arene with uranyl nitrate in a solution of CH3CN and NEt3. The core of this cluster involves two types of linkages. There are six μ2-O atoms, but they assume two different structural roles. Two pairs of these O atoms bridge between pairs of uranyl ions, thus defining shared equatorial edges of pentagonal bipyramids. Two additional μ2-O atoms bridge between uranyl ions such that they correspond to shared bipyramidal vertexes. The result is a cluster core in which the four U(VI) cations are approximately coplanar, with the equatorial vertexes of the bipyramids in approximately the same plane and the uranyl ions themselves extending roughly perpendicular to the plane. Two additional O atoms that coordinate each of the uranyl ions are provided by the organic ligands. The tetranuclear cluster shown in Figure 5o has been reported in several compounds obtained by a relatively broad range of synthesis conditions. It was found in the compound (UO2)4O2Cl4(C4H8O)2(H2O)4 synthesized in air by the oxidation and hydration of a solution of (1,4,7trimethylindenyl)uranium(IV) trichloride bis(tetrahydrofuran).75 In this case the terminal ligands of the cluster correspond to O atoms of the organic ligand as well as μ2-Cl atoms. The cluster in which the terminal ligands are μ1-N or μ2-N of cyanate ligands was obtained by synthesis under a nitrogen atmosphere.76 It was also found with terminal μ1-O and μ2-O atoms corresponding to DMSO and acetate in a compound synthesized via room-temperature evaporation in air of a solution of [UO2(O2CCH3)2(OH2)2] in DMSO pretreated at 100 °C.77 The cluster with μ2-Cl atoms and μ1-N atoms of acetonitrile coordinating uranyl was obtained from a solution of acetonitrile in a sealed vessel at room temperature.78 It was obtained with μ2-Cl atoms and terminal H2O groups in the compounds MU2O5Cl4(H2O)2, M = Rb, Cs, by crystallization at 30 °C.79 Finally, a tetrameric cluster in which N atoms of two 5-pyrimidyltetrazolate (pmtz) ligands coordinate the uranyl ions was synthesized by hydrothermal reaction.80 The cluster shown in Figure 5q, consisting of two uranyl pentagonal bipyramids and two uranyl hexagonal bipyramids, was also reported with only O atoms coordinating the uranyl ions in the form (UO2)4(μ3-O)2O12, which was cocrystallized with a dimer of uranyl ions and linked via phthalate groups.81

6.2. Clusters with Six An(VI) Cations

The reaction of (UO2)(NO3)2(H2O)6 with p-benzylcalix[7]arene (H7L) in the presence of 1,4-diazabicyclo[2.2.2]octane provided (UO2 2+ ) 6 (L 7− ) 2 (O 2− ) 2 (HDABCO + ) 6 ·3CH 3 CN·CHCl3·5CH3OH·3H2O, HDABCO = 1,4-diazabicyclo[2.2.2]octane, crystals of which are unstable even in their mother solution.86 The core of the cluster consists of six U(VI) uranyl ions (Figure 6a,b). Two and four of these are present as square

Figure 6. Illustrations of selected clusters containing An(VI). (a, b) (UO22+)6 (L7−) 2(O2−) 2(HDABCO+)6·3CH3CN·CHCl3·5CH3OH·3H 2 O, H 7 L = p-benzylcalix[7]arene, HDABCO = 1,4diazabicyclo[2.2.2]octane. (c) [HNEt3]8[(UO2)8(H2L)4(O2)8]·22H2O, H4L = 2,3,6,7-tetrahydroxy-9,10dimethyl-9,10-dihydro-9,10-ethanoanthracene. See Table 2 for the legend of the figures. 1105


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Figure 7. U60 cluster of uranyl peroxide hydroxide hexagonal bipyramids in different representations: (a) traditional ball-and-stick view, (b) spacefilling view, (c) polyhedral representation, (d) graphical representation. Each vertex in the graph represents a U cation. Segments in the graph that connect vertexes designate shared edges between the corresponding U polyhedra. See Table 2 for the legend of the figures.

and pentagonal bipyramids, respectively. There are two μ3-O atoms that are shared between uranyl pentagonal bipyramids, as well as six μ2-O atoms. Two of these are involved in unusual U(VI) uranyl ion cation−cation interactions that are donated by uranyl ions in the pentagonal bipyramids and are accepted by those in the square bipyramids. Whereas the central group of four edge-sharing uranyl pentagonal bipyramids is a common configuration, as part of clusters discussed in the previous section or of a larger structural unit in uranyl minerals and various synthetic compounds, the occurrence of cation−cation interactions associated with it is highly unusual. This cluster also represents a relatively rare example of surface passivation by both yl O atoms and organic ligands. Note that the uranyl ions that accept the cation−cation interactions are also each coordinated by three O atoms of the calixarene, resulting in square bipyramids. The organic ligands undoubtedly play an important role in directing the assembly of a cluster containing U(VI) uranyl cation−cation interactions.

linkages of uranyl hexagonal bipyramids through shared equatorial edges, usually to form four-, five-, or six-membered rings that correspond to topological squares, pentagons, and hexagons, respectively. We first review computational studies related to these clusters, as well as synthesis methods, prior to describing each cluster. 7.1. Importance of the Peroxide Bridge

The structure of the mineral studtite, [(UO2)(O2)(H2O)2](H2O)2, presented the first example of an inorganic compound in which actinyl ions were bridged through peroxide.100 Studtite occurs naturally in uranium deposits,101 where it forms by incorporating peroxide created by α-radiolysis of water.102 It also forms where used nuclear fuel interacts with water103−106 and in one case where complex nuclear materials created by a reactor core-melt incident reacted with water.107 It is easy to synthesize in the laboratory by combining the uranyl ion and hydrogen peroxide in acidic aqueous solutions and has long been important in the nuclear industry. However, the structure was only determined in 2003 using a natural crystal;100 no known synthesis method produces crystals suitable for singlecrystal diffraction, and all known crystals from Nature are thin and easily damaged needles. In the structure of studtite, uranyl ions are coordinated by two peroxide groups that are bidentate to the uranyl ion in a trans arrangement, as well as two H2O groups in a trans configuration. The result is a uranyl hexagonal bipyramid with two of its equatorial edges defined by peroxide groups. It is these two trans edges that are shared between adjacent uranyl ions, giving a one-dimensional chain of polyhedra that extends through the structure. Adjacent chains are linked through H bonding only. Despite the pliable nature of the H bonding network, the uranyl−peroxide−uranyl bridges have dihedral angles that depart significantly from 180°. The dihedral angles are about 140°, giving the chains a corrugated appearance. In 2010, it was argued that the uranyl−peroxide−uranyl interaction is inherently bent and that this provides the curvature that fosters the formation of actinyl peroxide cage clusters.94 The authors concluded, on the basis of an analysis of known structures with uranyl−peroxide−uranyl bridges, that the dihedral angles were always bent and that this was important for the formation of cage clusters. The first computational studies of the uranyl−peroxide− uranyl bridge appeared late in 2010.108,109 These examined model species representing fragments of the larger clusters. Whereas computational studies of entire clusters of uranyl peroxo polyhedra could provide various useful insights, steric constraints of the cages require that the uranyl−peroxide− uranyl dihedral angles be bent. The smaller fragments studied are not part of larger structures that require a specific geometry

6.3. Clusters with Eight An(VI) Cations

The compound [HNEt3]8[(UO2)8(H2L)4(O2)8]·22H2O, H4L = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene, contains a cage cluster in which two fourmembered rings of uranyl peroxide polyhedra are linked through organic molecules into the structural unit (Figure 6c).83 Four uranyl ions are bridged by peroxide ligands that are bidentate to two uranyl ions and consequently form equatorial edges of uranyl hexagonal bipyramids. Each uranyl ion is coordinated by two peroxo groups, as well as two O atoms provided by the organic ligands. Two years later, similar rings of uranyl peroxide polyhedra were found to self-assemble into inorganic cage clusters, as discussed in section 7.

7. ACTINYL PEROXIDE CLUSTERS Beginning in 2005,87 an extensive family of nanoscale actinyl peroxide cage clusters have been reported (Figures 7 and 8).87−99 Currently, 38 uranyl-based clusters and 1 neptunylbased cluster are in the literature. In all 39 clusters actinyl ions are bridged by bidentate peroxide ligands, and other bridges between actinyl ions are usually present in the clusters as well. These include bridges through two hydroxyl groups, pyrophosphate, or oxalate. All but four are closed-cage clusters, with the others being three open rings and a cup-shaped cluster. The many uranyl-based clusters are designated as UnXm. n is the number of uranyl ions in the cluster. X is a character string that represents chemical species other than peroxide or hydroxyl, and m indicates the quantity of these species. Where the UnXm designations are nonunique, additional designations are added (i.e., a, b, c) to differentiate the clusters. Various representations of U60 are shown in Figure 7. In general, each of the uranyl peroxide clusters is dominated by 1106


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influenced by the counterions present and that certain counterions favor topological squares, pentagons, or hexagons.

and thus provide good insight into the impact of the uranyl− peroxide−uranyl interaction on cluster formation. In one computational study, several model clusters of uranyl polyhedra were examined by DFT and multiconfigurational methods (CASSCF/CASPT2).109 These models ranged from a simple dimer of uranyl polyhedra with a peroxide bridge to a ring structure consisting of five uranyl hexagonal bipyramids, with all bridges between uranyl ions being bidentate peroxide groups. Full geometry optimizations of these model clusters were conducted, both in the absence of counterions and in the presence of Li, Na, K, Rb, or Cs cations added to balance the charge. When counterions were included in the simulations, all model clusters optimized to geometries that included strongly bent uranyl−peroxide−uranyl dihedral angles. The bent configuration was attributed to a partially covalent interaction between the U(VI) cation of the uranyl ion and the peroxide ligands. The calculated uranyl−peroxide−uranyl dihedral angle was found to depend on the ionic radius and electronegativity of the specific counterion, with the smallest angles obtained for the smallest cations. The calculated dihedral angles ranged from 140° in the case of Li to 164° for Cs. Geometry optimizations done for related clusters derived by replacing each peroxide bridge by two hydroxyl groups gave U−(OH)2−U dihedral angles close to 180°.109 In another computational study, four-, five-, and sixmembered rings of uranyl peroxide polyhedra were considered.108 In each case all of the bridges between the uranyl ions were through bidentate peroxide ligands. The geometries of the model clusters were fully optimized without symmetry constraints using DFT methods. Using a hypothetical [(UO2)2(μ-η2:η2-O2)(H2O)6]2+ model cluster, the researchers determined that the bent configuration is only about 0.5 kcal·mol−1 below a planar configuration (i.e., with a uranyl− peroxide−uranyl dihedral angle of 180°). However, this computation was done in the absence of counterions and thus, importantly, cations that can bridge between the O atoms of uranyl ions. When they examined a ring of four uranyl hexagonal bipyramids bridged through peroxide ligands, including counterions, the planar configuration was 16 kcal·mol−1 above the one containing bent uranyl−peroxide− uranyl dihedral angles. The study also examined the complexation energies of Li, Na, K, Rb, and Cs with the model clusters containing four, five, or six uranyl polyhedra. The calculated complexation energy was greatest for Na and a five-membered ring, followed closely by Na and a four-membered ring. A recent computational study has presented the first simulation of a complete uranyl peroxide cage cluster.110 This study emphasized five-membered rings of uranyl peroxide polyhedra and the U20 cluster, which is the smallest cage built from uranyl ions reported to date. It adopts a fullerene topology that consists of only 12 pentagons (see below). The DFT-optimized geometries of the rings and cluster matched those from X-ray structure determinations within a few percent error for bond distances and angles. The study evaluated the complexation energy for counterions and uranyl peroxide rings or cages and further established the importance of counterions in stabilizing different types of rings. In the case of U20, the lowest energy was obtained for Na as a counterion (Li, K, Rb, and Cs were also considered), consistent with the use of Na in the actual synthesis of the cluster. In summary, the results of the computational studies of fragments of the uranyl peroxide clusters indicate that the uranyl−peroxide−uranyl dihedral angle can be strongly

7.2. Synthesis of Actinyl Peroxide Clusters

Of the 39 actinyl peroxide clusters in the literature, all but 2 were synthesized in aqueous solution under ambient conditions. The remaining two, U40 and U50, were crystallized from solutions heated to 80 °C in sealed Teflon-lined vessels.99 Characterization of species in solution provides ample evidence that uranyl peroxide clusters self-assemble in solution prior to crystallization. Small-angle X-ray scattering data indicate clusters are present in solution in as little as 1 h, which is the shortest time needed for sample preparation and initialization of data collection.92,93 Early studies of uranyl peroxide cluster formation suggested that cluster abundances changed in solution over the course of months, although later studies indicated clusters can self-assemble and crystallize in as little as 15 min.93 A very recent study provided time-resolved small-angle X-ray scattering studies of a uranyl core−shell cluster and demonstrated that the core formed essentially instantaneously and the shell required about two weeks.92 Uranyl peroxide cage clusters that contain only peroxide and hydroxyl bridges self-assemble in aqueous solution over the pH range of about 6.7−13 under ambient conditions. The preparations are very elementary, with simple combinations of uranyl nitrate, hydrogen peroxide, an alkali-metal or alkalineearth-metal base, and water giving the desired results. Although no measurements of the solubility of uranyl peroxide cage clusters have been reported, it seems reasonable to expect that their solubilities are very similar, given the topological and chemical relationships of the family of clusters. However, relatively modest changes in the details of the synthesis conditions have been found to give crystals with different clusters. Also, it is likely that the solubility of uranyl peroxide cage clusters will be impacted by counterions in solution. Although the kinetics of uranyl peroxide cage cluster assembly are mostly unstudied, kinetic factors are likely important in determining the identity of the cluster crystallized in any given case. For example, almost all of the uranyl peroxide cage clusters reported in the literature adopt clusters with high symmetry (see below). Uranyl peroxide polyhedra in cage clusters are linked to three other uranyl polyhedra, almost always through the sharing of equatorial edges. In many clusters of uranyl peroxide polyhedra hydroxyl groups are essential constituents. It is common for uranyl ions to be coordinated by two peroxide groups arranged along two equatorial edges of a hexagonal bipyramid, with another edge defined by two hydroxyl groups. The bipyramid shares its two peroxide edges with two different bipyramids and its edge defined by the two hydroxyl groups with a third bipyramid. As the dihedral angle of the U−(OH)2−U bridge is pliable, incorporation of hydroxyl bridges has the effect of changing the average dihedral angle of the cluster. Clusters that are larger than those that would be consistent with peroxide bridges can form with hydroxyl bridges. The primary lens through which self-assembly of uranyl peroxide clusters in solutions has been studied is through the crystals that they form. In other words, it is those clusters that crystallized that have been characterized, and the solutions may have been polydisperse. The relatively few studies of clusters in mother solutions suggest such polydispersity, but over the course of weeks and months, solutions move toward a more monodisperse distribution of clusters.87 A reliance on crystals 1107


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Figure 8. continued

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Figure 8. Polyhedral and graphical representations of uranyl peroxide clusters. See Table 2 for the legend of the figures. Pp = pyrophosphate, Ox = oxalate, Nt = nitrate, P = phosphate, and PCP = methylenediphosphonate.

Table 1. Summary of Symbolic Notations and Cluster Compositions for Uranyl Peroxide Clusters symbol U16 U18Pp2PCP6 U20 U20Pp6b U20Pp6a U20Pp10 U20R U24 U24Pp12 U24PCP12 U24R U26Pp11 U26Pp6 U28a U28 U30 U30a U30Pp6 U30Pp10Ox5 U30Pp12P1 U32 U32Pp16 U32R U36a U36 U36Ox6 U38Pp10Nt4 U40 U42 U42Pp3 U44 U44a U45Pp23 U50 U50Ox20 U60 U60Ox30 U28U40R U120Ox90

figure

cage composition 24−

[(UO2)16(O2)24(OH)8] [(UO2)18(O2)18(OH)2(CH2P2O6)6(P2O7)2]34− [(UO2)20(O2)30]20− [(UO2)20(O2)24(P2O7)6]32− [(UO2)20(O2)24(P2O7)6]32− [(UO2)20(O2)20(P2O7)10]40− [(UO2)20(OH)16(O2)28]32− [(UO2)24(OH)24(O2)24]24− [(UO2)24(O2)24(P2O7)12]48− [(UO2)24(O2)24(CH2P2O6)12]48− [(UO2)24(O2)36(OH)12]36− [(UO2)26(O2)28(P2O7)11]48− [(UO2)26(O2)33(P2O7)6]38− [(UO2)28(O2)28(OH)28]28− [(UO2)28(O2)42]28− [(UO2)30(O2)36(OH)22]34− [(UO2)30(O2)30(OH)30]30− [(UO2)30(O2)39(P2O7)6]42− [(UO2)30(O2)30(P2O7)10(C2O4)5]50− [(UO2)30(O2)30(P2O7)12(PO4)(H2O)5]51− [(UO2)32(OH)32(O2)32]32− [(UO2)32(O2)32(P2O7)16]64− [(UO2)32(O2)40(OH)24]40− [(UO2)36(O2)36(OH)36]36− [(UO2)36(O2)41(OH)26]36− [(UO2)36(O2)48(C2O4)6]36− [(UO2)38(O2)40(P2O7)10(NO3)4]56− [(UO2)40(OH)40(O2)40]40− [(UO2)42(O2)42(OH)42]42− [(UO2)42(O2)42(OH)36(P2O7)3]48− [(UO2)44(O2)66]44− [(UO2)44(O2)44(OH)44]44− Na64Li26[(UO2)45(O2)44(P2O7)23](H2O)n [(UO2)50(OH)50(O2)50]50− [(UO2)50(O2)43(OH)4(C2O4)20]30− [UO2(O2)(OH)]6060− [(UO2)60(O2)60(C2O4)30]60− (K,Na)44[(UO2)68(O2)74(OH)16(NO3)16(H2O)16](H2O)155 K134Li46[(UO2)120(O2)120(C2O4)90](H2O)n 1109

8a,b 8c,d 8e,f 8g 8h 8i 8j,k 8l,m 8n 8n 8o,p 8q,r 8s,t 8u,v 8w,x 8y,z 8aa,ab 8ac,ad 8ae,af 8ag,ah 8ai,aj 8ak,al 8am,an 8ao,ap 8aq,ar 8as 8at,au 8av,aw 8ax,ay 8az 8bc,bd 8ba,bb 8be,bf 8bg,bh 8bi 8bj,bk 8bl 8bm,bn 8bo,bp


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connected, meaning that each is linked to three other uranyl polyhedra. The result is a family of three-connected graphs. One subset of this family is the fullerene topology, which consists of exactly 12 pentagons as well as some hexagons. Made famous by C60,111 fullerene topologies have been found in a range of cage clusters over the past two decades. Each cluster is designated by an alphanumeric descriptor (see above) and each topological graph by a numeric descriptor. For example, the first graph in Figure 8 is designated 16:4164. This indicates there are 16 vertexes that correspond to 16 uranyl polyhedra, there is a single topological square, and there are 4 topological hexagons. In the case of cage clusters with three connected vertexes, summing the number of vertexes in the different topological elements gives 3 times the total number of vertexes, because each vertex is included in three topological elements. For example, the designation 24:4668 corresponds to a cage cluster with six squares and eight hexagons, (4 × 6) + (6 × 8) = 72, which is triple the number of vertexes (24).

for insight into the clusters that may have been present in solution will favor those clusters that reach their solubility limit in solution, either because they form in large quantities or because they have a lower solubility than other clusters present. As yet unknown kinetic factors may favor formation of specific clusters and their accumulation to sufficient concentration to trigger crystallization, even though the specific cluster may or may not be the most thermodynamically stable. 7.3. Representation of Uranyl Peroxide Clusters

In Figure 8 and Table 1 uranyl peroxide clusters are illustrated and listed in order of increasing number of uranyl polyhedra. Table 2. Legend of the Figures element

color

element

color

Th atoms and polyhedra U atoms and polyhedra

dark cyan

Cl atoms

bright green

yellow

pink

Pu atoms

slate blue

Mn atoms W polyhedra K atoms I atoms Se atoms

dark yellow dark blue turquoise plum rose

S atoms and polyhedra P atoms and polyhedra F atoms O atoms N atoms C atoms

7.4. Cluster Compositions

purple

Restricting discussion to clusters that consist only of uranyl peroxide polyhedra, two distinct environments occur about the uranyl ions. A few clusters are built exclusively by uranyl ions that are each coordinated by three peroxide groups (triperoxide hexagonal bipyramids, Figure 1g). More contain uranyl ions that are coordinated by two peroxide groups that delineate two equatorial edges of the corresponding hexagonal bipyramid in a cis arrangement (diperoxide hexagonal bipyramids, Figure 1f). The remaining two equatorial vertexes correspond to hydroxyl groups. To date, all cage clusters consisting of only uranyl polyhedra are built from one or both of these types of uranyl polyhedra; none contain two peroxide groups in a trans arrangement about the uranyl ion. Cluster compositions are given in Table 1.

dark green red blue black

The smallest published structure contains only 16 uranyl polyhedra, and the largest to date contains 120 polyhedra. All of the clusters shown in Figure 8 were synthetically prepared, and the structures were derived from single-crystal X-ray diffraction data. All of the uranium in the clusters shown in Figure 8 and listed in Table 1 is in the hexavalent oxidation state. In all cases, the U(VI) cation is strongly bonded to two O atoms, forming (approximately) linear uranyl ions. In almost all cases the uranyl ions are part of hexagonal bipyramids, with the equatorial ligands consisting of O atoms of peroxide, hydroxyl, pyrophosphate, or oxalate. Because of their size and complexity, ball-and-stick representations of the clusters are awkward and are not particularly helpful in their representation (Figure 7). Here we use mostly polyhedral representations of the clusters, with uranyl polyhedra consistently colored yellow in recognition of the color of the uranyl ion in solution and many solids (Figure 7c). Graphical representations of clusters of uranyl polyhedra are very useful for examining their topologies (Figure 7d). The graphical representation is shown for most clusters in Figure 8. In all cases each vertex in the graph corresponds to a U(VI) cation. Lines in the graphs correspond to connections between the U(VI) cations. Shared edges between uranyl polyhedra are therefore shown as a single connection between the corresponding vertexes. Where U(VI) cations are bridged by units such as pyrophosphate or oxalate, a single connector joins the corresponding vertexes. As such, the graph contains only information on the positions of the U(VI) cations and the connections between them, with no designation of the chemical type of connection in any case. This approach is useful because it simplifies the topological representations and facilitates identification of relationships. The graphs of clusters of uranyl peroxide polyhedra usually have some combination of squares, pentagons, and hexagons. In the cage clusters, the uranium polyhedra are three-

7.5. Cluster Descriptions

7.5.1. Open Clusters. Only four of the clusters of uranyl peroxide polyhedra that have been reported to date are not cages. Three of these are ring structures, whereas the other is the open-bowl-shaped U16 (Figure 8a,b).96 It is the smallest extended cluster reported that consists only of uranyl peroxide polyhedra. Its graph, 16:4164 (Figure 8a), consists of a single square that is surrounded by four hexagons. This arrangement of polygons occurs in some of the cage clusters, most notably U24. Clusters U20R,96 U24R,96 and U32R93 each consist of crownshaped ring topologies (Figure 8j,k,o,p,am,an). The topologies of U20R and U24R consist solely of pentagons (U20R) or hexagons (U24R) and are unusual in the lack of topological diversity; the U20 cage cluster is the only other one that is built from a single type of polygon. Each contains both types of uranyl polyhedra: U20R is built from 16 diperoxide and 4 triperoxide hexagonal bipyramids, and U24R has 12 diperoxide and 12 triperoxide hexagonal bipyramids. Each also has counterions inside the ring. U20R contains 12 K cations, 1 of which is below each of the 5 pentagons in the topology. In U24R, there is a Cs cation below each hexagon in the topology, as well as eight Na cations located toward the lips of the crown, and a dimer of uranyl diperoxide polyhedra at the center, where it is linked to the crown structure through the Na and Cs cations. Cluster U32R, with graph 32:5864, is the largest crown-shaped structure that has been reported (Figure 8am,an). Remarkably, it self-assembles in solution and crystallizes within about 15 min 1110


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clusters were soluble in tetramethylammonium, as well as lithium, sodium, and potassium salt or hydroxide solutions. Nuclear magnetic resonance (NMR) studies of Cs135indicate that the stability of the cluster in solution is related to the counterions present.112 Recently, a core−shell cluster has been reported, U28U40R, in which the core cluster is also the U28 fullerene topology cluster (Figure 8bm,bn).92 However, in this case the polyhedra that form the cluster are a mixture of uranyl triperoxide and diperoxide hexagonal bipyramids. The U28 fullerene topology cage cluster forms quickly in solution, within 1 h as shown by small-angle X-ray scattering data. The shell structure, which contains 40 uranyl hexagonal bipyramids as well as 16 nitrate groups, is templated by the U28 cluster, although its assembly is delayed by about two weeks, as shown by time-resolved smallangle X-ray scattering (SAXS) data.92 The shell structure consists of topological pentagons, and these are located directly above topological pentagons of the U28 core. Linkages between the core and shell structure are through K cations. The study also demonstrated that when crystals of the core−shell cluster were dissolved in water, the clusters remained intact. In cluster U30Pp6, uranyl ions are bridged by peroxide groups or through pyrophosphate groups, resulting in a fullerene topology with graph symbol 30:51265 (Figure 8ac,ad).89 U30Pp10Ox5 is a rare example of a cluster that contains both pyrophosphate and oxalate bridges (Figure 8ae,af).113 Its crystallization was found to be strongly dependent on the solution pH, with a superior yield obtained for pH 5.1. Solutions with lower pH favor formation of uranyl oxalate clusters, and those with higher pH give uranyl pyrophosphate clusters. In U30Pp10Ox5, uranyl ions are bridged by peroxide, pyrophosphate, and oxalate groups. The result is a fullerene topology with five hexagons and graph symbol 30:51265, although this is not the same topology as found for U30Pp6. Specifically, in the U30Pp10Ox5 topology, all five of the hexagons share edges with other hexagons, forming a ring of hexagons. In the U30Pp6 topology, the hexagons are distributed over two different segments that are separated by pentagons (Figure 8ac). Cluster U30Pp12P1 exhibits several novel features (Figure 8ag,ah). The two most uncommon aspects of the cluster are that it contains a single phosphate tetrahedron, along with the 12 pyrophosphate groups, and 4 of the equatorial ligands of uranyl hexagonal bipyramids are nonbridging and are assumed to correspond to H2O groups. Ten of the pyrophosphate groups bridge uranyl ions with the typical side-on configuration, but two bridge between three uranyl ions, one with a side-on linkage and the other two by sharing single vertexes with the uranyl bipyramids. The graph of U30Pp12P1 contains 12 pentagons and 6 hexagons, but it is not the same fullerene topology that is adopted by cluster U30Py6. Cluster U36 (D6h) consists of 10 uranyl triperoxide and 26 uranyl diperoxide hexagonal bipyramids (Figure 8aq,ar).97 Uranyl ions are thus bridged through either peroxide groups or pairs of hydroxyl groups. The resulting topology is a fullerene with eight hexagons and graph symbol 36:51268. Cluster U36Ox6 also exhibits this topology (Figure 8as), with six of the uranyl bridges that correspond to two hydroxyl groups being replaced by oxalate.91 The oxalate groups in U36 are arranged about the equatorial plane of an elongated spheroid. Cluster U44 (D3d) is built only from uranyl triperoxide hexagonal bipyramids (Figure 8bc,bd).97 Its topology contains 12 hexagons with graph symbol 40:512612. In the case of

of the introduction of hydrogen peroxide into a solution containing ammonium hydroxide and uranyl nitrate.93 The cluster contains eight pentagons and four hexagons and can be built from different combinations of uranyl hexagonal bipyramids: 8 triperoxide and 24 diperoxide polyhedra or 16 diperoxide and 16 triperoxide polyhedra. 7.5.2. Cage Clusters: Fullerenes. Of the 34 cage clusters presented in Figure 8, there are 26 unique topological graphs, and 11 of these are fullerene topologies containing 12 pentagons. The smallest of these contains 20 vertexes, with only pentagons and graph 20:512, and has the lowest number of vertexes possible for a fullerene topology. The corresponding U20 cluster (C5v) was isolated with Na as the counterion and all uranyl ions present as triperoxide polyhedra (Figure 8e,f).94 Three uranyl peroxide pyrophosphate clusters have been reported that are topological derivatives of U20 and graph 20:512.89 These contain either 6 (U20Pp6a, U20Pp6b) or 10 (U20Pp10) pyrophosphate groups. Each pyrophosphate group coordinates two uranyl ions with “side-on” configurations, such that two adjacent equatorial vertexes of a uranyl hexagonal bipyramid correspond to O atoms of the pyrophosphate group. Pyrophosphate groups bridge uranyl ions in a fashion that is topologically analogous to that of peroxide groups or shared edges defined by hydroxyl groups. The U20Pp6a (Figure 8h), U20Pp6b (Figure 8g), and U20Pp10 (Figure 8i) clusters are topologically identical to U20, but relative to U20, peroxide bridges have been replaced. There are 30 peroxide bridges in U20. Ten of these are replaced by pyrophosphate in U20Pp10, and six are replaced by pyrophosphate in each of the U20Pp6a and U20Pp6b clusters. Each of the U20Pp6a, U20Pp6b, and U20Pp10 clusters was synthesized from aqueous solutions containing identical initial uranyl nitrate and hydrogen peroxide concentrations. U20Pp6a and U20Pp6b crystallized from solutions with pH ranging from 10.3 to 11.5 and from 9.2 to 9.5, respectively. U20Pp10 crystallized from solutions over the pH range of 8.7−9.2, although the solution contained twice as much pyrophosphate as that for U20Pp6a and U20Pp6b. Cluster U26Pp6 crystallized from aqueous solutions with pH ranging from 10.2 to 10.8 (Figure 8s,t). The cluster contains 14 uranyl triperoxide hexagonal bipyramids and 12 uranyl diperoxide hexagonal bipyramids in which the uranyl ions are also coordinated by side-on pyrophosphate groups. These polyhedra are linked into eight five-membered rings of polyhedra, all of which share polyhedra with at least one adjacent five-membered ring. Six pyrophosphate units bridge between uranyl ions of these five-membered rings, resulting in a cage cluster with topological graph symbol 26:51263 (Figure 8s). Cluster U28 (Td) has a fullerene topology with four hexagons and graph symbol 28:51264 (Figure 8w,x).87 The 28-vertex fullerene topology adopted has the highest possible ideal symmetry for this number of vertexes. U28 is a relatively rare example of a cluster that is assembled entirely from uranyl triperoxide polyhedra. The initial report of this cluster included K as the counterion, with a very low yield of only a few crystals. Subsequently, a reliable synthesis route was reported to produce U28 that features different combinations of templating cations (K, Rb, Cs) and anions (uranyl tiperoxide polyhedra, Nb(O2)4, and Ta(O2)4).112 The study found that the keys to obtaining a high yield of this cluster were to maintain synthesis conditions that are not extreme or dynamic and to use templating cations that ideally match each other and the topology of the capsule interior. Crystals containing these 1111


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polyhedra. It is currently unclear why square−pentagon adjacencies are absent in these cluster graphs. In any case, the combination of topological squares, pentagons, and hexagons to form cage clusters of uranyl polyhedra results in an almost unlimited variety of potential cage clusters, much greater than the huge number of potential fullerene topologies. Cluster U18Pp2PCP6 is unusual because it contains peroxide, hydroxyl, pyrophosphate, and methylenediphosphonate bridges between uranyl ions (Figure 8c,d).89 There are two structural units that are built from uranyl polyhedra, one with eight hexagonal bipyramids that are linked into two five-membered rings and the other with ten hexagonal bipyramids linked into an open ring through shared peroxide equatorial edges. Pyrophosphate groups are located inside this ten-membered ring, and six methylenediphosphonate groups link the two units of uranyl polyhedra into a cage cluster. U24 (Td) is the smallest cluster built solely from uranyl polyhedra that contains topological squares (Figure 8l,m).87 It was also one of the first three that were reported in the initial manuscript in 2005. It is built from uranyl diperoxide polyhedra, such that uranyl ions are bridged through either peroxide groups or pairs of hydroxyl groups. The topology consists of six squares and eight hexagons, 24:4668, arranged such that each square shares edges with four different hexagons and each hexagon shares three edges with other hexagons and three with squares. This is the well-known sodalite topology. Np24 is the only neptunyl peroxide cluster repored to date.87 It is topologically identical to U24 (Figure 8l,m). It was synthesized from an aqueous solution under ambient conditions with Li provided as a counterion. Addition of Np(V) and peroxide to the aqueous solution resulted in black crystals of Np24 in good yield. The structural analysis indicated most of the Np in the cluster is hexavalent, but Np−O bond lengths suggested Np(V) is also present. A mixture of oxidation states could help to explain the unusually dark color of the crystals. U24Pp12 and its methylenediphosphonate (PCP) analogue are topologically identical to U24 (Figure 8n).89 These clusters contain pyrophosphate or methylenediphosphonate bridges between uranyl ions in place of the bridges through two hydroxyl groups that are in U24. There are six four-membered rings of uranyl polyhedra in the cage cluster. Four of these are concave toward the center, and the other two are concave toward the outside. This lowers the overall symmetry relative to that of U24. Cluster U26Pp11 (Figure 8q,r)89 has two distinct units that are built from uranyl hexagonal bipyramids, with these two units linked through pyrophosphate groups that bridge between uranyl ions. The uranyl polyhedra are both triperoxide and diperoxide hexagonal bipyramids; where the uranyl ion is only coordinated by two peroxide groups, it is also coordinated by a side-on pyrophosphate group. Its topological graph has the symbol 26:4551062. U28a (C3v) is built from 24 uranyl diperoxide and 4 triperoxide hexagonal bipyramids (Figure 8u,v). Its topology departs significantly from others in the family in that there is no center of symmetry in the cluster.97 It is egg-shaped, with the bottom consisting of hexagons and squares in a configuration similar to that in U24 and the top consisting only of pentagons. It is also an unusual cluster because a SO42− tetrahedron is encapsulated within it, as well as Na cations. Cluster U30 (C2v) is built from a mixture of 14 uranyl diperoxide and 16 triperoxide hexagonal bipyramids that are

fullerene topologies with 44 vertexes, there are several clusters that have higher symmetry than that with the least adjacent pentagons. U44 adopts one of these higher symmetry isomers, which was taken as evidence that symmetry is an important consideration in isomer selection for uranyl peroxide clusters.95,97,98 Cluster U50 (D5h) is built from uranyl diperoxide hexagonal bipyramids, such that uranyl ions are bridged by either peroxide groups or pairs of hydroxyl groups (Figure 8bg,bh).99 Its topology is a fullerene that contains 15 hexagons, with graph symbol 50:512615. Of the possible fullerene isomers with 50 vertexes, U50 selects the isomer that has both the highest symmetry and the least number of pentagonal adjacencies. The Cl-stabilized C50Cl10 cluster adopts an identical topology.114 U50 is one of only two uranyl peroxide cage clusters that crystallized at temperatures other than ambient, in this case at 80 °C. Cluster U50Ox20 forms with the same topology as U50, but it contains three different types of bridges between uranyl ions: peroxide, hydroxide, and oxalate groups (Figure 8bl). Cluster U60 (Oh) consists only of uranyl diperoxide hexagonal bipyramids (Figure 8bj,bk).9595 Its topology is a fullerene that contains 20 hexagons and no adjacent pentagons, with graph symbol 60:512620. The cluster is topologically identical to C60, but the cluster is much larger, more massive, and more chemically complex than C60. It has only been crystallized using both Li and K counterions. U60Ox30 forms with the same fullerene topology as U60, but all of the hydroxyl bridges between uranyl ions in U60 have been replaced by oxalate bridges (Figure 8bl).91 This results in a larger cluster with larger pores in the cage wall. U120Ox90 is the largest cluster of uranyl peroxide polyhedra reported to date (Figure 8bo,bp).88 It consists of a core−shell structure in which the core is identical to the U60Ox30 cluster. The shell consists of 12 five-membered rings of uranyl diperoxide hexagonal bipyramids that share vertexes, and each of the uranyl ions within the ring are coordinated by an oxalate group. The oxalate groups in the shell structure are terminal; they each coordinate only one uranyl ion. One such ring is located above each of the 12 topological pentagons of the U60Ox30 core. SAXS data indicate that upon dissolution in water the 60 uranyl polyhedra that form the shell structure probably detach from the cluster but the U60Ox30 cage remains intact. 7.5.3. Cage Clusters Containing Topological Squares. Four-membered rings of uranyl hexagonal bipyramids are essential components of 13 cage clusters reported to date. This ring always has four bidentate peroxide groups that bridge between four uranyl ions. As such, each uranyl ion contained within the four-membered ring is coordinated by two peroxide groups. The remaining two equatorial vertexes correspond either to hydroxyl groups or to pyrophosphate groups. The four-membered rings of uranyl bipyramids in the clusters correspond to squares in the cluster graphs. The 13 cage clusters containing four-membered rings of hexagonal bipyramids correspond to 11 different graphs (Figure 8). In these graphs there are no examples of square−square or square−pentagon adjacencies. Rather, in all cases each square is surrounded by hexagons. It has been noted that, in the case of topological squares, geometric constraints mandate that the edges shared between the four uranyl polyhedra be peroxide. Assuming that the cluster is built from uranyl diperoxide polyhedra, square−square adjacencies are prohibited, although not in the case where the cluster is built from uranyl triperoxide 1112


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combined into four-, five-, and six-membered rings (Figure 8y,z).97 The cluster is unusual in that it is one of only three reported that lack a center of symmetry. Its topological graph, 30:425867, contains a base that consists of a single hexagon that is surrounded by six pentagons. The upper portions of the cluster consist of two squares and two hexagons, and the top and bottom are fused into the cage through four hexagons and two pentagons. Cluster U30a is built from uranyl diperoxide hexagonal bipyramids (Figure 8aa,ab).98 It is the only such uranyl peroxide cage cluster reported that contains a 5-fold rotational symmetry axis. Its topology, with graph symbol 30:4552610, is dominated by hexagons that surround isolated pentagons on the top and bottom of the cluster. U32 (C4v) is built from diperoxide hexagonal bipyramids that are arranged to give two topological squares, eight pentagons, and eight hexagons, with graph symbol 32:425868 (Figure 8ai,aj).87 Cluster U32Pp16 is a rare example of a uranyl peroxide pyrophosphate cage cluster that is not a topological derivative of a known cluster built only from uranyl polyhedra (Figure 8ak,al).89 Four-membered rings of uranyl diperoxide hexagonal bipyramids dominate its structure, and these are linked by sixmembered rings of polyhedra. Its graph contains topological squares, hexagons, and two octagons: 32:486882. The cluster is unusual in that it is rather flattened, with its outer long and short dimensions being 28.2 and 18.0 Å, respectively. The cage also has pores of highly differing sizes. The largest of these, which correspond to the topological octagons, has a free opening of about 6.3 Å. Cluster U36a (D2d) is built from uranyl diperoxide polyhedra arranged in four-, five-, and six-membered rings of polyhedra (Figure 8ao,ap).97 Its graph contains 4 squares, 4 pentagons, and 12 hexagons: 36:4454612. The right half of U36a, as drawn in Figure 8ap, is identical to the top of U30. The bottom half of U36a is the mirror image of the top, but rotated 90° so that the pentagons share edges with the hexagons. Cluster U40 (D4v) crystallized from a solution heated to 80 °C (Figure 8av,aw).99 The cage cluster is built from uranyl diperoxide polyhedra, and bridges between uranyl ions are through either peroxide or hydroxyl groups. The corresponding topological graph contains 2 squares, 8 pentagons, and 12 hexagons: 40:4258612. Two ends of the graph correspond to that of the U16 open cluster described above. These two ends are separated through hexagons and pairs of pentagons. Cluster U42 (D3h) is built from uranyl diperoxide hexagonal bipyramids that are arranged into four-, five-, and six-membered rings (Figure 8ax,ay).98 The graph has 3 squares, 6 pentagons, and 14 hexagons, with graph symbol 42:4356614. K and Li counterions are located within the cage, with K below the topological pentagons and Li below the topological squares. Cluster U42Py3 is a topological derivative of U42, in which three of the shared hydroxyl−hydroxyl edges of U42 have been replaced by pyrophosphate units that bridge uranyl ions (Figure 8az). Cluster U44a (D2v) is built from uranyl diperoxide polyhedra that form four-, five-, and six-membered rings (Figure 8ba,bb).97 The elongated cluster has a maximum dimension of 31.3 Å, as measured from the outer edges of bounding oxygen atoms. At its narrowest, the cluster is only 12.1 Å wide. The ends of the cluster are very similar to a fragment of U24, each with four four-membered and six-membered rings of polyhedra, and these are linked through distorted six- and eight-

membered rings of polyhedra. The corresponding graph symbol is 44:4861482. 7.5.4. Miscellaneous Cage Clusters. Cluster U38Pp10Nt4 has two distinct but symmetrically identical uranyl peroxide pyrophosphate “lobes” that are linked through nitrate groups (Figure 8au).113 Uranyl ions are bridged by nitrate, hydroxyl, peroxide, and pyrophosphate groups. It was created by combining two solutions that had already crystallized other uranyl pyrophosphate clusters. This combination of solutions resulted in one with an unusually high nitrate concentration and potentially also in the commingling of cluster fragments that had already formed in the earlier solutions. The uranyl pyrophosphate lobes that occur in U38Pp10Nt4 are similar to cluster U20Pp6b, except that some of the peroxide bridges in U20Pp6b correspond to hydroxyl bridges in U38Pp10Nt4. The authors of the study hypothesized that this difference prevented the lobes in U38Pp10Nt4 from growing into complete cages and that they were instead bridged through nitrate groups.113 Cluster U45Pp23 is highly unusual in several respects (Figure 8be,bf).90 The most striking of these is that this cluster both has an odd number of uranyl polyhedra and lacks any symmetry, both attributes being unique. In addition, although most of the uranyl ions contained within the cluster are in hexagonal bipyramids, the cluster contains the first and only occurrence of a uranyl ion in a pentagonal bipyramidal polyhedron in which a single equatorial edge corresponds to a peroxide group. Most of the pyrophosphate groups bridge two uranyl ions in the typical side-on fashion, but some bridge three uranyl ions by being bidentate to one and monodentate to two. Cluster U45Pp23 crystallizes from solution within 24 h of mixing uranyl nitrate and hydrogen peroxide, and if the system is left undisturbed, crystals of U32Pp16 form after about a week and coexist with those of U45Pp23.90 The authors of the study concluded that either formation of U45Pp23 was kinetically favored, such that its concentration rose to the point of crystallization, or it has an unusually low aqueous solubility, which caused it to crystallize even if it was not the dominant species in solution.90 7.6. Role of the Solution pH

Many of the open and cage clusters built from uranyl peroxide polyhedra, and including bridges such as pyrophosphate and oxalate, are gathered in Figure 9, where they are arranged according to the solution pH they crystallized from and the total number of uranium polyhedra in the cluster. The pKa values for oxalate are 1.12 and 4.19, for pyrophosphate they are 0.85, 1.49, 5.77, and 8.22, and for hydrogen peroxide the value is 11.62. In the published structures all of these species were reported to be completely deprotonated. However, resolution of H atom positions in such structures using X-ray diffraction is impossible because of the dominance of U in scattering the Xrays and the overall complexity of the structures. Whereas bond-valence arguments clearly indicate that the peroxide and oxalate in these clusters are deprotonated, there is uncertainty in the case of the pyrophosphate bridges, each of which have two terminal O atoms that could in some cases be protonated. The synthesis details for the various clusters shown in Figure 8 often contain more than one counterion, and various acids and bases were used to adjust the pH. Furthermore, the synthesis conditions were initially dynamic, with the decomposition of peroxide and bubbling of oxygen from solution. Evaporation in air was used to encourage crystallization. Insight into the speciation in solution is provided by the crystallized cluster, but 1113


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and U60, because the dihedral angles for U−(OH)2−U bridges are pliable and ideally ∼180°, as shown by the DFT simulations.109 Figure 9 also shows that cage clusters of uranyl ions with pyrophosphate bridges form from solutions with a large range of solution pH, from 4 to 10.8. The highest number of uranyl ions in a pyrophosphate-bearing cluster is 42, although this cluster contains only 3 pyrophosphate groups. The U32Pp16 cage cluster has the most uranyl ions of any containing at least six pyrophosphate groups. Owing to the lack of clusters containing pyrophosphate and more than 42 uranyl ions, and to the broad range of solution pH in which such clusters form, the left side of Figure 9 is dominated by uranyl pyrophosphate cage clusters. In five clusters uranyl ions are bridged through oxalate groups. These all form from solutions prepared using oxalic acid, with a maximum pH of 6.5. In contrast to the uranyl pyrophosphate clusters, which contained relatively few uranyl ions, uranyl oxalate clusters contain at least 30 uranyl ions and 3 contain more than 50. As such, these clusters dominate across the bottom of Figure 9.

Figure 9. Relationship among the solution pH, composition, and number of uranyl ions in various uranyl peroxide clusters. Blue, black, and yellow balls represent uranyl peroxide pyrophosphate clusters, uranyl peroxide oxalate clusters, and clusters that are built only from uranyl peroxide polyhedra, respectively. See Table 2 for the legend of the figures. Pp = pyrophosphate, Ox = oxalate, Nt = nitrate, P = phosphate, and PCP = methylenediphosphonate.

7.7. Stability and Electrochemistry of Uranyl Peroxide Cage Clusters

A variety of studies have shown that cage clusters built from uranyl peroxide polyhedra can be harvested from solutions after months or even years of aging.97,98 Although such observations clearly demonstrate persistence of these cage clusters, the subject of cluster stability, from a thermochemical perspective, is unstudied. A recent study included an examination of the fate of U60 dissolved in ultrapure water.116 In this, crystals of U60 were harvested from their mother solution, followed by washing and dissolution of the crystals in ultrapure water. ESI-MS data collected for solutions extracted and diluted from this bulk solution showed peaks that are characteristic of the cluster. These persisted through 290 days, at which time the study ended. This result is surprising because the water contained no free peroxide at the onset of crystal dissolution, and the ESI-MS data indicated little change occurred over the course of almost a year. The only thermochemical study of a uranyl peroxide cage cluster reported to date is for U60.116 Crystals of the cluster were grown and harvested. Drop-solution calorimetry for the resulting compound, together with a series of thermochemical cycles, permitted determination of the heat of formation of the compound under standard conditions and evaluation of various hypothetical reactions. The study concluded that U60 is thermodynamically stable and kinetically persistent in the absence of free peroxide. On the basis of this study, it was noted that such clusters could form and persist in solutions in contact with damaged nuclear fuel after a reactor accident.117 The electrochemical behavior of the U28 cluster has been examined.115 This is one of the few clusters in which each uranyl ion is coordinated by three bidentate peroxide groups and no hydroxide groups. Initially yellow solutions of U28 were irreversibly reduced, corresponding to about 81 electrons per cluster. Upon reduction the solution became colorless. The authors attributed the electrochemical behavior of U28 to either a two-electron reduction of each of the peroxide ligands or a combination of reduction of U(VI) to U(V) and reduction of 28 of the 42 peroxide ligands contained in the cluster. The loss

other clusters were also likely present, as shown by SAXS and electrospray ionization mass spectrometry (ESI-MS) in some cases.87,89 Given that the potential applications of such clusters reside in the solution realm, the complexity of the systems cannot be ignored. It is, however, currently prudent to discuss the importance of pH in the formation and crystallization of specific clusters only in a general sense. The notable exception to the uncertainties due to dynamic synthesis conditions described in the preceding paragraph is the work of Nyman,111,115 who focused on different synthetic routes to the U28 cluster. In this study, reactants are combined in aqueous solution maintained at 5 °C in an ice bath and a yellow precipitate that forms is collected and dissolved in water that is subsequently maintained at 8 °C until crystals containing U28 with various encapsulated cations are recovered. This approach provided yields in the range of 52−74% based on uranium. Clusters that are built only from uranyl ions that are bridged through peroxide or hydroxyl groups form in general under alkaline conditions. Cluster U44 formed from a solution with pH 6.7, U42 formed from a solution with pH 7.9, and all the other clusters assembled in solutions with pH 9 or higher. Inspection of Figure 9 also reveals that the smaller cage clusters, built from 30 or less uranyl ions, formed from solutions with a pH of at least 10.5. In contrast, the largest cluster with 60 uranyl ions formed at pH 9. Most of the cage clusters that are built only from uranyl polyhedra contain both peroxide and hydroxyl bridges, consistent with the higher pH conditions of their formation. According to DFT simulations, the dihedral angle of the U− (O2)−U bridge is ideally ∼140°,109 and this is reflected in the geometries of the bridge in the many cage clusters reviewed here. As such, if all uranyl ions are bridged only through peroxide groups to form cage clusters, the angle limits the size of the cluster because of the curvature required. Incorporation of bridges between uranyl ions that are two shared hydroxyl groups should foster formation of larger clusters, such as U50 1114


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number of pentagons and hexagons.98 Thirty-nine of these include three squares and have no square−square or square− pentagon adjacencies. The graph adopted by U42 and U42Pp3 has the highest symmetry of these 39 graphs. In the case of graphs containing 28 vertexes and at least 1 square, there are 151 topologies. Fifty of these have three squares. Cluster U28a adopts the highest symmetry isomer of these 50, and it is also the highest symmetry isomer of the 151 that contain squares.97 There are 198 three-connected graphs that contain 36 vertexes and 4 squares. Cluster U36a corresponds to the highest symmetry graph of these 198 graphs.97 In this discussion of the role of symmetry in topology selection, one should not ignore U45Pp23, which has no symmetry.90 It is discussed above, where it is suggested that this unusual cluster is kinetically favored during the synthesis experiment. In summary, for a given number of vertexes, cage clusters of uranyl peroxide polyhedra exhibit a strong tendency to avoid square−square and square−pentagon adjacencies and to adopt high-symmetry topologies. The number of polyhedra in the cluster depends on other factors, as discussed below.

of color is most consistent with the latter scenario, and it was concluded that the clusters fragmented during the experiment. The study of U28115 is the only electrochemical experiment that has been published for a uranyl peroxide cluster. There are many opportunities for additional electrochemical studies of uranyl peroxide clusters that could probe the behavior of a wide range of cluster topologies that differ in their counterions and uranyl bridges. It will be particularly interesting to see if the electrochemical behavior of clusters containing less peroxide, specifically those incorporating hydroxide, oxalate, or pyrophosphate bridges, will be similarly succesptible to fragmentation. 7.8. Isomer Selection in Uranyl Peroxide Cage Clusters

As discussed above and shown in Figure 8, with only a couple of exceptions, cage clusters built from uranyl peroxide polyhedra can be represented by graphs in which each vertex is three-connected. The number of possible three-connected graphs for a given number of vertexes grows rapidly with the number of vertexes. For example, considering only fullerene topologies with 60 vertexes, there are 1812 possible graphs. Three studies have discussed topology selection in the case of uranyl cage clusters.95,97,98 These studies argue that high symmetry is the most important factor in topology selection. The high-symmetry preference is not particularly striking in the case of clusters where the number of vertexes, and correspondingly the number of topologies, is small. It becomes compelling by the point where the total vertexes is 60, and the U60 cluster adopts the highest symmetry fullerene topology of the possible 1812 choices.95 This is the same fullerene topology that is adopted by C60, but for different reasons. The C60 topology is favored because there are no adjacent pentagons in the topology, which are unfavorable because they produce excessive curvature locally and reduce bonding orbital overlaps. Where uranyl polyhedra are linked into a cage cluster with 60 vertexes, high symmetry is favored. There is only one fullerene topology with 60 vertexes that has no pentagonal adjacencies, and it also has the highest symmetry in the pool of 1812 topologies. In contrast to the case of U60, where the fullerene topology has 44 vertexes, adjacent pentagons are a topological requirement. The topology with the lowest number of adjacent pentagons does not have the highest symmetry of the 44-vertex pool, and the U44 cluster with a fullerene topology has higher symmetry, rather than fewer topological pentagonal adjacencies.95 Inclusion of combinations of topological squares, pentagons, and hexagons in graphs increases the number of possible topologies dramatically relative to that of fullerene topologies. The large number of possible topologies complicates evaluation of selection criteria. In the case of a 30-vertex graph there are 227 graphs that contain at least 1 square and any number of pentagons and hexagons.98 Two clusters of uranyl peroxide polyhedra have been reported that have 30 vertexes, and these correspond to 2 different graphs. The avoidance of square− square and square−pentagon adjacencies appears to be a factor in selection, and only 5 of the 227 graphs lack these features. Two of these correspond to the U30 and U30a clusters.98 Clusters U30Pp6 and U30Pp12P1 have fullerene topologies. There are only 3 fullerene topologies with 30 vertexes, and these 2 clusters exhibit 2 of these topologies. Considering next the U42 and U42Pp3 clusters, there are 2373 graphs with 42 vertexes and at least 1 square, as well as any

7.9. Factors Impacting the Size of Uranyl Peroxide Cage Clusters

Restricting the discussion to cage clusters built solely of uranyl peroxide polyhedra with a single shell, clusters have been synthesized with from 20 to 60 uranyl polyhedra. The smallest, U20, has a diameter of 18.0 Å,94 as measured from the outer edges of the bounding O atoms. The corresponding diameter of U60 is 27.0 Å.95 In the case of U20, all of the uranyl ions are bridged through peroxide groups, and the average dihedral angle of the bridge is 140.4°.94 In U60 uranyl ions are bridged by both peroxide groups and pairs of hydroxyl groups. The average dihedral angle of the bridges is 154.8°.95 The average dihedral angle of the bridges between uranyl ions is an essential factor related to the curvature of the cage wall and therefore also the number of uranyl ions contained in the cluster. The larger dihedral angles arise from an averaging over peroxide and hydroxyl bridges. DFT simulations have given considerable insight into the factors that determine cluster size.108−110 The role of the counterion included in the synthesis reaction is important in determining the dihedral angle over the uranyl ion bridges,109 and the complexation energies for different cations with four-, five-, and six-membered rings of bipyramids differ considerably.108 Specifically, Li as a counterion strongly favors fourmembered rings of uranyl polyhedra (topological squares), Na, K, and Rb favor five-membered rings (pentagons), and Cs favors six-membered rings (hexagons).108,110 However, the degree to which the complexation energies differ for a given cation and the different macrocycles varies considerably. In the case of Li, complexation with the four-membered ring gives −19.2 kcal·mol−1, with five-membered rings gives −10.4 kcal·mol−1, and with six-membered rings gives −8.9 kcal·mol−1, assuming that the cation is located at the center of the ring corresponding to the O atoms of the uranyl ions.108 In contrast, the complexation energies for K and the different macrocycles considered only range from −14.9 to −18.8 kcal·mol−1.108 1115


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8. TRANSITION-METAL-BASED ACTINIDE-BEARING CLUSTERS Actinide cations, including transuranium elements, have been introduced as addenda metals in transition-metal polyoxometalates. Interest in these clusters derives from their potential applications in separation cycles, as well as the unique possibilities they provide for probing the role of 5f electrons in clusters. Several studies reported clusters containing a single actinide cation that bridges between two transition-metal polyoxometalate clusters or fragments.118−122 Additional studies have examined transition-metal polyoxometalate clusters bridged through two123−129 or three130−134 actinide cations. A large mixed transition-metal/actinide cluster was reported in the compound [Th6Mn10O22(OH)2(O2CPh)16(NO3)2(H2O)8]·10MeCN (Figure 10a,b).135,136 This compound was

The crystallization and characterization of a cluster that combines a P6W36 polyoxometalate with two four-membered rings of uranyl peroxide polyhedra provides an important intermediate between traditional transition-metal polyoxometalates and uranyl peroxide clusters (Figure 10c).137 The cluster contains eight uranyl ions that are in hexagonal bipyramidal coordination and was synthesized at pH 4. Each is coordinated by two peroxide groups located along cis equatorial edges of the bipyramids and two additional O atoms that are also bonded to W cations. Uranyl ions are bridged through peroxide groups to form four-membered rings in a fashion analogous to that of the numerous uranyl peroxide cage clusters discussed above. Two such four-membered rings of uranyl polyhedra are enclosed in the curved transition-metal polyoxometalate. As such, this cluster represents a novel hybrid in which it is surfacepassivated by yl O atoms on both transition metals and actinides. The compound Na32[(UO2)12(μ3-O)4(μ2-H2O)12(P2W15O56)4]·77H2O contains the cluster [(UO2)12(μ3-O)4(μ2H2O)12(P2W15O56)4]32− shown in Figure 10d.138 It consists of four P2W15O56 lobes that are connected through four unusual trimers of uranyl pentagonal bipyramids. In these trimers three uranyl ions share a μ3-O and pairs of uranyl ions are bridged through three μ2-H2O groups. The remainder of the equatorial ligands of the pentagonal bipyramids are provided by the P2W15O56 units. A combined experimental and computational study provided a novel wheel-shaped mixed tungsten uranium polyoxometalate, {[W5O21]3[(UO2)2(μ2-O)]3}30− (Figure 10e).139 This cluster contains two distinct building units. The first consists of pairs of uranyl ions that are bridged by bidentate peroxide groups and that are coordinated by additional O atoms to produce a dimer of uranyl hexagonal bipyramids. There are three such dimers in the cluster, and they are arranged about the equatorial plane of the overall wheel-shaped cluster. The second building unit consists of five Mo(VI) cations, each of which is coordinated by either six O atoms in an octahedral arrangement or five in a square pyramidal arrangement. Four octahedra and one square bipyramid are linked to produce the building unit with composition [W5O21]12−, and there are three such units in the cluster. The tungstate and uranyl building units are linked into the larger cluster through the sharing of polyhedral edges and vertexes. DFT simulations reproduced the geometry of the cluster and provided a classical polyoxometalate electronic structure containing metal and oxo bands. The calculations also provided the sites for likely protonation of O atoms of the cluster, as the X-ray study did not produce H atom positions.

Figure 10. Transition-metal-based clusters containing actinides. (a, b) [Th6Mn10O22(OH)2(O2CPh)16(NO3)2(H2O)8]·10MeCN (Figure 8a,b). (c) K 6 Li 19 [Li(H 2 O)K 4 (H 2 O) 3 {(UO 2 ) 4 (O 2 ) 4 (H 2 O) 2 } 2 (PO3OH)2P6W36O136]·74H2O. (d) Na32[(UO2)12(μ3-O)4(μ2-H2O)12 (P2W15O56)4]·77H2O. See Table 2 for the legend of the figures.

9. SUMMARY AND DISCUSSION Stabilization of finite clusters of metal cations and oxygen requires surface passivation. This can be achieved either by truncating the cluster with appropriate organic ligands, or in a few cases simple inorganic ions, or by incorporation of cations with yl oxygens that terminate the clusters. Both of these approaches have been applied in the case of transition metals, although truncation via yl O atoms is the most developed and corresponds to the extensive family of polyoxometalates. Both approaches are also evident for actinides, in a few cases in combination in a single cluster. Where clusters are built from An(IV) cations, there are no yl O atoms to truncate the clusters. Most of the clusters shown in Figure 2 contain actinide(IV) oxide cores that are truncated

synthesized by the reaction of (NnBu4)[Mn4O2(O2CPh)9(H2O)] with Th(NO3)4(H2O)3 in MeCN/ MeOH under aerobic conditions. The core of this cluster has the composition [Th6Mn10O22(OH)2]18 (Figure 10b). In total there are 6 Th(IV) and 10 Mn(IV) cations, and these are linked through 4 μ4-O, 16 μ3-O, 2 μ2-O, and 2 μ2-OH bridges. Two of the μ4-O atoms bridge between three Th(IV) cations and one Mn(IV) cation , whereas the other two bridge two Th(IV) and two Mn(IV) cations. All Th(IV) cations are eight-coordinated, and Mn(IV) cations are octahedrally coordinated. The coordination spheres of the cations in the core of the cluster are completed by O atoms of the organic ligand, a nitrate group, and H2O groups, which passivate the surfaces of the cluster. 1116


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into finite clusters by capping organic ligands. The principle exceptions are Th(IV) clusters that are bridged by selenite tetrahedra (Figure 2h) and Pu(IV) clusters that are terminated by H2O groups and Cl anions (Figure 2r). Many of these An(IV) clusters have An−O connectivities similar to those of the fluorite structure, and these may be important species in An(IV) hydrolysis reactions. It is only the An(V) and An(VI) cations that form actinyl ions. Clusters built from actinyl polyhedra that are truncated by yl oxygens are polyoxometalates and have some similarities to those of the transition metals. However, unlike transition metals, actinyl ions have two yl O atoms in a trans configuration. As such, actinyl polyhedra tend to assemble into cage clusters in which both the inner and outer surfaces are truncated by yl oxygen atoms. Assembly of such cage clusters requires curvature of the walls, and to date they have only been found for actinyl peroxide polyhedra. It is the peroxide bridge between the actinyl ions that fosters the curvature needed to form cage clusters. In the absence of peroxide, actinyl polyhedra tend not to form clusters, but rather crystallize into extended structures that are dominated by sheets of polyhedra. The most notable exception is the series of U(V)-based tetrameric clusters in which cation−cation interactions bridge U(V) and the clusters are passivated by various organic molecules (Figure 4a,b). A few clusters are surface-passivated by both the yl O atoms of actinyl ions and by capping organic ligands (i.e., Figure 5). With the single exception of the Pu38O56 core, all of the clusters that contain 18 or more actinide cations are members of the uranyl peroxide family. Many of the clusters containing An(IV) or An(V) cations were synthesized in nominally oxygen-free atmospheres, whereas all of the uranyl peroxide clusters self-assemble in aqueous solution in the presence of air. Given the ease of their synthesis, and their stability under ambient conditions, the uranyl peroxide clusters may have the most promise for applications in an advanced nuclear fuel cycle. One of the more promising applications of actinide oxide clusters appears to be in separations related to a nuclear fuel cycle. The uranyl peroxide clusters spontaneously self-assemble under ambient conditions in alkaline water and hydrogen peroxide. Several studies have emphasized the feasibility of an alkaline-based system for used nuclear fuel reprocessing, in contrast to plutonium−uranium extraction (PUREX), which is an acidic process.140−145 Used uranium dioxide fuel dissolves in alkaline water in the presence of peroxide. Although detailed studies of the speciation of uranium in solution following dissolution of used fuel, or a surrogate, are currently lacking, the conditions are appropriate for the rapid self-assembly of soluble uranyl peroxide cage clusters. Realizing this, it should be possible to develop an approach to separating the uranium from the bulk solution on the basis of the size, mass, or unique chemical properties of the cage clusters. Such an approach may also be appropriate for purification of uranium at the front end of the fuel cycle. A second potential application of actinide oxide clusters in the nuclear fuel cycle is in the fabrication of nanocomposite materials. Although this area is entirely undeveloped, we imagine that the controlled deposition of actinide oxide clusters onto various substrates could contribute to development of novel fuel types. A very recent study has documented the deposition of plutonium(IV) oxide clusters onto the surface of mica.146

With a few notable exceptions, studies of actinide oxide clusters have emphasized their synthesis and structural characterization more so than the details of their properties and potential applications. These studies have established a family of clusters that form the basis for future studies of properties and applications. Taken together, the family of actinide oxide clusters reported to date demonstrates that their composition and size can be tailored by controlled synthesis techniques. For example, in the case of clusters built from the uranyl ion, although peroxide is an essential bridge to foster curvature, other bridges between uranyl ions can be incorporated and the size of the cluster is impacted by the counterion used. There are many unexplored aspects of the unique family of uranyl peroxide cage clusters that have been synthesized over the past several years. It now seems likely that the peroxide bridge between uranyl ions is essential for the formation of these clusters. Symmetry is important in topology selection, and counterions are important for determining the size of the cluster. However, little is known about the mechanisms of selfassembly of the clusters, the diversity of clusters that may form under specific conditions, or the relative stabilities and aqueous solubilities of the many clusters. The electrochemical behavior of uranyl peroxide clusters is mostly unstudied, as is the importance of ion pairing in solution. The aggregation behavior of uranyl peroxide clusters in solution has not been addressed, nor has the cation-exchange properties in solution. Computations are emerging as important components of the studies of actinide oxide clusters. Recent examples include studies of uranyl peroxide cage clusters108−110 and thorium oxide clusters.39 We expect the role of computations to continue to grow in studies of actinide clusters, with simulations beginning to guide experimental approaches to create tailored clusters with specific properties.147 The contribution of computations will be particularly important in studies of transuranium systems, where the expense of experiments is very high. Actinide oxide clusters provide an abundance of future research directions. At the most fundamental level, they can be expected to provide unique insights into the complex behavior of elements in which the 5f orbitals are important contributors to their chemistry. Largely unexplored areas for future studies include their expected complex redox behavior, their relative thermodynamic stabilities, the role of actinide oxide clusters in transport in the environment, aggregation behavior in solution, and application development. The vast majority of actinide oxide clusters that have been reported are based on uranium or thorium, with a small subset of the total corresponding to neptunium or plutonium. The few clusters that are built from neptunium for which there is data suggest they are similar in structure to those containing uranium, but the dominance of the pentavalent oxidation state of Np in aqueous solution is likely to result in some divergence from U(VI). Specifically, the bonds within the Np(V) neptunyl ion are weaker than those in the U(VI) uranyl ion; thus, the yl O atoms of the Np(V) neptunyl ion are more reactive, and cation−cation interactions are much more common than for U(VI).148,149 The propensity of cation−cation interactions in Np(V) systems could even prevent the formation of cage clusters analogous to the uranyl peroxide system because the yl O atoms may participate in additional bonding, rather than passivating the surface of the cluster. The single neptunyl peroxide cluster that has been reported to date, Np24, appears 1117


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to be built from mostly Np(VI) but also may contain some Np(V). It is topologically identical to U24, which only contains U(VI). As Pu is reduced by peroxide in aqueous systems, it is unlikely that plutonyl peroxide clusters will form under conditions similar to those used to develop the family of uranyl peroxide cage clusters. It is possible that clusters of Np(IV) and Pu(IV) will follow the generalities revealed for Th(IV) and U(IV), but the single Pu cluster (Pu38O56) reported to date is much larger than those of Th(IV) or U(IV), although clusters of each of these tetravalent actinides are known that are based upon the fluorite structure type. No clusters have been reported for Pu(V) or Pu(VI). There are also no reported clusters of An(III) cations bridged through oxygen, although lanthanide(III) clusters are known150−153 and may provide some insight into possibilities for the actinides.

postdoctoral appointments at the University of Cambridge and the University of New Mexico and one year on the faculty at the University of Illinois, he joined the faculty of the University of Notre Dame in Indiana in 1997. He is currently Massman Professor of Civil and Environmental Engineering and Earth Sciences and Concurrent Professor of Chemistry and Biochemistry. He is also Director of the U.S. Department of Energy’s Energy Frontier Research Center Materials Science of Actinides, which was created in 2009.

ACKNOWLEDGMENTS This material is based upon work supported as part of the Materials Science of Actinides Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001089.

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REFERENCES

Corresponding Author

(1) Hasenknopf, B.; Micoine, K.; Lacote, E.; Thorimbert, S.; Malacria, M.; Thouvenot, R. Eur. J. Inorg. Chem. 2008, 5001. (2) Hill, C. L. J. Mol. Catal. A: Chem. 2007, 262, 2. (3) Long, D. L.; Burkholder, E.; Cronin, L. Chem. Soc. Rev. 2007, 36, 105. (4) Long, D. L.; Tsunashima, R.; Cronin, L. Angew. Chem., Int. Ed. 2010, 49, 1736. (5) Mal, S. S.; Dickman, M. H.; Kortz, U.; Todea, A. M.; Merca, A.; Bogge, H.; Glaser, T.; Muller, A.; Nellutla, S.; Kaur, N.; van Tol, J.; Dalal, N. S.; Keita, B.; Nadjo, L. Chem.Eur. J. 2008, 14, 1186. (6) Mitchell, S. G.; Streb, C.; Miras, H. N.; Boyd, T.; Long, D. L.; Cronin, L. Nat. Chem. 2010, 2, 308. (7) Muller, A.; Peters, F.; Pope, M. T.; Gatteschi, D. Chem. Rev. 1998, 98, 239. (8) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (9) Pradeep, C. P.; Long, D. L.; Cronin, L. Dalton Trans. 2010, 39, 9443. (10) Crowley, K. D.; Ahearne, J. F. Am. Sci. 2003, 91, 5. (11) Biswas, B.; Mougel, V.; Pecaut, J.; Mazzanti, M. Angew. Chem., Int. Ed. 2011, 50, 5744. (12) Kersting, A. B.; Efurd, D. W.; Finnegan, D. L.; Rokop, D. J.; Smith, D. K.; Thompson, J. L. Nature 1999, 397, 56. (13) Novikov, A. P.; Kalmykov, S. N.; Utsunomiya, S.; Ewing, R. C.; Horreard, F.; Merkulov, A.; Clark, S. B.; Tkachev, V. V.; Myasoedov, B. F. Science 2006, 314, 638. (14) Powell, B. A.; Dai, Z. R.; Zavarin, M.; Zhao, P. H.; Kersting, A. B. Environ. Sci. Technol. 2011, 45, 2698. (15) Lundgren, G. Ark. Kemi 1953, 5, 349. (16) Mokry, L. M.; Dean, N. S.; Carrano, C. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1497. (17) Morss, L. R.; Edelstein, N. M.; Fuger, J.; Katz, J. J. The Chemistry of the Actinide and Transactinide Elements; Springer: Dordrecht, The Netherlands, 2006. (18) Coronado, E.; Gomez-Garcia, C. J. Chem. Rev. 1998, 98, 273. (19) Denning, R. G. Struct. Bonding (Berlin, Ger.) 1992, 79, 215. (20) Denning, R. G. J. Phys. Chem. A 2007, 111, 4125. (21) Arliguie, T.; Blug, M.; Le Floch, P.; Mezailles, N.; Thuery, P.; Ephritikhine, M. Organometallics 2008, 27, 4158. (22) Arliguie, T.; Thuery, P.; Le Floch, P.; Mezailles, N.; Ephritikhine, M. Polyhedron 2009, 28, 1578. (23) Berthet, J. C.; Thuery, P.; Ephritikhine, M. Eur. J. Inorg. Chem. 2008, 5455. (24) Berthet, J. C.; Thuery, P.; Ephritikhine, M. Angew. Chem., Int. Ed. 2008, 47, 5586. (25) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Science 2005, 309, 1835. (26) Kiplinger, J. L.; Pool, J. A.; Schelter, E. J.; Thompson, J. D.; Scott, B. L.; Morris, D. E. Angew. Chem., Int. Ed. 2006, 45, 2036. (27) Monreal, M. J.; Khan, S. I.; Kiplinger, J. L.; Diaconescu, P. L. Chem. Commun. 2011, 47, 9119.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Jie Qiu received her B.S. in Chemistry from Shandong Normal University in 2004 and Ph.D. degree in Polymer Chemistry and Physics from the Institute of Chemistry, Chinese Academy of Sciences, in 2009, both in China. She has been a postdoctoral researcher in Prof. Peter C. Burns’ group at the University of Notre Dame since 2009. Her research interest is focused on the synthesis and growth mechanism of uranyl peroxide nanoclusters.

Peter C. Burns received his B.S., M.S., and Ph.D. degrees from the University of New Brunswick, University of Western Ontario, and University of Manitoba, respectively, all in Canada. Following 1118


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