clusters of actinides with oxide, peroxide, or hydroxide bridges

Clusters of Actinides with Oxide, Peroxide, or Hydroxide BridgesJie 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

CONTENTS

1. Introduction 10972. Clusters Containing An(IV) 1099

2.1. Clusters with Four An(IV) Cations 11002.2. Clusters with Six An(IV) Cations 11002.3. Clusters with Eight An(IV) Cations 11002.4. Clusters with 10 An(IV) Cations 11012.5. Clusters with 38 An(IV) Cations 1101

3. Clusters Containing An(IV) and An(V) 11023.1. Clusters with Four or Five An(IV) and An(V)

Cations 11023.2. Clusters with Six An(IV) and An(V) Cations 11023.3. Clusters with 12 An(IV) and An(V) Cations 11023.4. Clusters with 16 An(IV) and An(V) Cations 1103

4. Clusters Containing An(V) 11034.1. Clusters with Four An(V) Cations 11034.2. Clusters with Six An(V) Cations 1104

5. Clusters Containing An(V) and An(VI) 11046. Clusters Containing An(VI) 1104

6.1. Clusters with Four An(VI) Cations 11046.2. Clusters with Six An(VI) Cations 11056.3. Clusters with Eight An(VI) Cations 1106

7. Actinyl Peroxide Clusters 11067.1. Importance of the Peroxide Bridge 11067.2. Synthesis of Actinyl Peroxide Clusters 11077.3. Representation of Uranyl Peroxide Clusters 11107.4. Cluster Compositions 11107.5. Cluster Descriptions 1110

7.5.1. Open Clusters 11107.5.2. Cage Clusters: Fullerenes 11117.5.3. Cage Clusters Containing Topological

Squares 11127.5.4. Miscellaneous Cage Clusters 1113

7.6. Role of the Solution pH 11137.7. Stability and Electrochemistry of Uranyl

Peroxide Cage Clusters 11147.8. Isomer Selection in Uranyl Peroxide Cage

Clusters 1115

7.9. Factors Impacting the Size of UranylPeroxide Cage Clusters 1115

8. Transition-Metal-Based Actinide-Bearing Clusters 11169. Summary and Discussion 1116Author Information 1118

Corresponding Author 1118Notes 1118Biographies 1118

Acknowledgments 1118References 1118

1. INTRODUCTION

Metal oxide clusters, especially transition-metal polyoxometa-lates, have been studied for decades because they provide a rareopportunity to study nanoscale materials with well-definedstructures and unusual properties, with emerging diverseapplications.1−9 In contrast to those of transition metals,actinide oxide clusters are relatively unexplored, although therehave been many developments in this area over the past decade.No doubt studies of actinide (An) clusters have lagged behindthose of other metals in part due to the experimental difficultiesof working with actinides, all of which are radioactive, and tothe scarcity of facilities suitable for studies of transuraniumelements. Furthermore, the availability of transuraniumelements for basic research is limited, owing to the considerablecost of their production and their strategic importance.The industrial-scale manipulation of actinides during and

after the Manhattan project achieved the goals of production ofvast quantities of isotopically pure plutonium for weapons anddevelopment of fuels for commercial nuclear power plants thatremain essential to the energy supplies of many countries.However, the environmental costs of these projects have beenconsiderable.10 Perhaps controlling actinide materials on thescale of a few nanometers will provide more effective andenvironmentally sound methods for processing nuclearmaterials in a future advanced nuclear energy system. Suchapplications include in separations and materials fabrication. Ithas also been suggested that actinide clusters can be usefulmodels for understanding the behavior of actinides inenvironmental and geochemical systems, as well as in thedesign of catalysts and molecular magnets.11 It is even possiblethat nanoscale clusters of actinides are responsible for theirdispersal under some environmental conditions.12−14

If one were to review the actinide literature during the 1990swhile transition-metal polyoxometalates emerged as a major

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research theme, one might erroneously assume that f elementsare incapable of self-assembly into well-structured and complexnanomaterials. The first high-nuclearity cluster for uraniumappeared in 1953 as part of an extended framework structure,15

but essential no further progress occurred until the later part ofthe same century. The authors of a study of a uranium cluster in1996 observed, “The results presented here demonstrate thatthere is nothing inherently unstable about high nuclearityactinide complexes, and we expect to see the number of suchwell-characterized complexes grow rapidly because of theirpotential applications.” 16 This prediction has proven correct,with especially abundant and complex clusters realized foruranyl peroxides.The efforts of a relatively few researchers have produced a

myriad of complex and beautiful actinide oxide clusters over thepast few years that are beginning to rival the complexity oftransition-metal clusters. At this point, most studies havefocused on describing the synthesis and structures of novelactinide clusters. Exploration of the properties and applicationsof such clusters is mostly in its infancy, but holds considerablepromise for future research efforts.The coordination chemistry of actinides is complex and very

oxidation state dependent (Figure 1).17 Where the actinide

cation has a valence of IV or less, symmetrical distributions ofligands about the large cations are typical, and coordinationnumbers range from 6 to 12 or higher. Cations in the V and VIoxidation states usually bond to two “yl” O atoms, forminglinear dioxo actinyl cations.17 The importance of yl O atoms iswell recognized in transition-metal oxide clusters,8,18 but inthese cases the metal coordination includes only a single yl Oatom that truncates the outer edge of the cluster or two ylatoms in a cis configuration. The presence of two yl O atoms inhigh-valence actinide coordination polyhedra ensures that theiroxide clusters will present interesting new cluster types andtopologies. Linear trans-dioxo actinyl ions are related to theinteraction 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 uranylion, giving triple bonds. Six linear combinations of O p orbitals

exist in the uranyl ion, and the symmetry of a subset of theseonly matches with uranium f orbitals. The existence of f orbitalsthus favors the linear dioxo cation, in contrast to the bentconfiguration typical in transition-metal cases.Here we focus on reviewing the structures of clusters in

which actinide cations are bridged through an oxygen, aperoxide, or a hydroxyl. We note that other important actinideclusters with nonoxygen bridges have been reported,21−28 butthese are outside the scope of our review. We choose here torestrict our attention to clusters containing four or moreessential actinide cations. There are many important aqueousspecies, such as some actinyl carbonates, sulfates, and peroxides,which contain fewer than four actinide cations; the interestedreader is directed to several reviews.29−34 Here we restrict ourattention to clusters that have been shown to exist in solution,or at least plausibly could exist in a solution. Most of ourattention is focused on cluster structures that have beenestablished by X-ray diffraction studies of their crystallizedforms. However, we emphasize that it is the properties ofactinide clusters in solution that are expected to be the mostuseful. In the solid state such clusters are distinguished by theattribute that they usually are not linked into an extendedstructure (a chain, sheet, or framework of polyhedra) throughhigher valence cations (although there are some notableexceptions). In other words, the structural unit in the crystalstructure is the finite cluster. Our discussion extends fromchemically and topologically simple clusters containing a smallnumber of actinide cations to clusters containing as many as120 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 andactinide polyoxometalates, it is the yl O atoms that stabilize thesurface of the cluster and effectively prevent further growth. Forcation coordination polyhedra lacking yl O atoms, clusterstypically only form where the surface is passivated by organicligands, or in rare cases halogen anions. A detailed comparisonof transition metal polyoxometalates with the emerging familyof actinide polyoxometalates is presented elsewhere.35

In this review several different types of illustrations areprovided to aid in the understanding of the connectivity ofactinide-based clusters (Figures 1−10). The approach takendepends mostly on the number of atoms in the cluster, withball-and-stick representations being effective for the smallerclusters and polyhedral models for the larger clusters. We alsoprovide graphical representations of the clusters in which eachactinide cation corresponds to a vertex, and lines connectingvertexes designate bridges between the corresponding cations.Although each of the clusters selected for coverage herecontains one or more oxide, peroxide, or hydroxide bridge,various other chemical types of bridges are present in someclusters. With the exception of bridges through pyrophosphateor oxalate groups in Figure 8, bridges other than oxide,peroxide, or hydroxyl are excluded from the graphicalrepresentations of clusters.We have chosen to arrange our discussion of actinide clusters

according to the cation oxidation states. This approach iswarranted because the coordination chemistry of actinides isstrongly oxidation state dependent. For clusters containingAn(IV) cations, their surfaces are usually passivated by organicligands, whereas for An(VI) clusters, surface passivation isalmost exclusively through the yl oxygen atoms. No clusters ofactinides containing oxygen, peroxide, or hydroxide bridgeshave been reported for actinide oxidation states less than IV.

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

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2. CLUSTERS CONTAINING AN(IV)

Clusters containing Th(IV), U(IV), and Pu(IV) have beenreported with crystal structure determinations, and a Np(IV)-bearing cluster has been proposed on the basis of spectroscopicstudies. Several of the clusters built from An(IV) cations arerelated to the fluorite structure type of the An(IV)O2

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

Figure 2. Illustrations of clusters built from An(IV) cations. The vertexes in the graphs represent the locations of An(IV) cations, and lines designateoxide or hydroxyl bridges. (a, b) [U4(L

2)2(H2L2)2(py)2O][CF3SO3]2, H4L

2 = 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 withThIV(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′-b i s ( 3 - h y d r o x y s a l i c y l i d e n e ) - 1 , 2 - p h e n y l e n e d i am i n e . ( n , o ) [U 1 0O 8 (OH) 6 ( P hCO2 ) 1 4 I 4 (H 2O) 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.



2.1. Clusters with Four An(IV) Cations

The compound [U4(L2)2(H2L

2)2(py)2O][CF3SO3]2, H4L2 =

N,N′-bis(3-hydroxysalicylidene)-2,2-dimethyl-1,3-propanedi-amine, py = pyridine, was synthesized and has tetranuclearclusters (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-Oatom. 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 asolution of pyridine into which mononuclear metal complexesof Cu and U had been introduced. The compound is thought tohave incorporated adventitious oxygen, and attempts toproduce the compound in a controlled fashion wereunsuccessful.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 Itcontains four Th(IV) cations arranged at the vertexes of atetrahedron, with a central μ4-O atom bonded to each, similarto 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 toadventitious oxygen. Such μ4-O oxygen atoms are relatively rarein clusters of An(IV) despite their occurrence in the fluorite-type 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)-1,2-phenylenediamine (Figure 2l,m), and [Pu38O56Cl54(H2O)8]

14−

(Figure 2q,r) also contain μ4-O atoms.The cluster [Cp(CH3COO)5U2O]2, cp = η5-cydopentadien-

yl, was synthesized in low yield under a nitrogen atmosphere(Figure 2c,d).38 The cluster has four U(IV) cations arrangedabout an inversion center. There are two μ3-O atoms, and theremainder of the O atoms that coordinate the U(IV) cationsare part of acetate groups. The two distinct U(IV) cations inthe cluster exhibit markedly different coordination environ-ments. One is coordinated by an μ3-O atom and fivemonodentate acetate groups in a very one-sided arrangement.The other is bonded to two μ3-O atoms and is also coordinatedby 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 ahexanuclear An6O8 cage was in 1953 for U6O4(OH)4(SO4)6(Figure 2e,f),15 although the cage in this compound is linkedinto a framework structure through bridging sulfate tetrahedraand there is no evidence of preassembly of the cages in solutionprior to crystallization. The compound was synthesized byheating an aqueous solution of U(IV) in 0.5 M H2SO4 in asealed glass tube at 200 °C; crystals of the compound formedbefore the solution was cooled.Another example of an An6(O,OH)8 cage cluster was

reported for a uranium phosphate complex (Figure 2f) in1996.16 It was obtained by reacting [TpVCl2(dmf)], Tp =hydridotris(pyrazolyl)borate, with sodium diphenylphosphateand uranyl acetate in aqueous acetonitrile under an inertatmosphere.16 Although the uranyl ion was not present in thecompound and the uranium had been reduced, it is unclearfrom the study which oxidation state the uranium adopted.Following the early work on the U6(O,OH)8 cage

clusters,15,16 several groups synthesized clusters with similar

An6(O,OH)8 cages, with four containing Th(IV)39,40 and eightwith U(IV) (i.e., Figure 2g).40−43 The latter studies confirmedthat the actinides are present in this cage in the tetravalentoxidation state. However, one compound has been isolated inwhich the U cations in a U6O8 cluster appear to be a mixture ofvalences (see section 3.2). The cage consists of six actinidecations that are arranged at the vertexes of an octahedron(Figure 2f), analogous to their local arrangement in the fluoritestructure. The eight anions that bridge between the cations arelocated above the centers of the octahedral faces. As such, eachof these anions is bonded to three cations, and each cation isbonded to four of these anions. The coordination environmentsof the cations are completed by a rich variety of ligands,including formate40 and dibenzoylmethanate.43 We note thatsimilar cage clusters consisting of non-actinide tetravalentmetals have been reported, including for Zr and Ce.44−47

The An6(O,OH)8 cage has been synthesized in the formsAn6(μ3-O)8

41,42 and An6(μ3-O)4(μ3-OH)4.39,40,43 Three clus-

ters of the latter form were synthesized containing Th.39 TheTh(IV) compounds were synthesized under ambient con-ditions in air, whereas the U(IV) compound syntheses wereconducted in aqueous solutions under an inert atmosphere.Although the crystal structure determinations did not reveal theH atoms of the μ3-OH, Th−O bond lengths in one of thecompounds clearly indicate an ordered distribution of μ3-OH inthe cage cluster.39 The possibility of μ3-OH disorder exists inthe other two compounds, although density functional theory(DFT) calculations of model structures indicate that atetrahedral arrangement of the OH ions on the Th6(μ3-O)4(μ3-OH)4 cages is energetically favored.39

A combination of UV−vis and X-ray absorption spectrasupport 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 withRCOO, where R = H, CH3, or CH2SH. The study concludedthat the hexanuclear core cluster was the dominant form ofNp(IV) in solution above about pH 1.5 under the conditions ofthe room-temperature experiments. The authors argued that, inthe absence of terminating ligands (RCOO in this case),hydrolization of the An(IV) cation could lead to a colloidalmaterial.A computational study examined Th6O8 and U6O8 clusters,

as well as several larger variants, with density functionaltheory.49 The simulations indicated that Th6O8 and U6O8clusters are stable entities in the absence of bridging ligands,but the anionic ligands are necessary to prevent further growthof the clusters.

2.3. Clusters with Eight An(IV) Cations

A recent study reported the synthesis and characterization offour octanuclear clusters with Th8(IV)(μ3-O)4(μ2-OH)4 cores(Figure 2h,i).50 The compounds have the compositionsTh8(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·nH2Oand crystallize as extended structures in which selenatetetrahedra bridge the clusters. An amorphous thoriumprecipitate was dissolved into selenic acid and water for thesynthesis of these compounds. The first two compoundscrystallized following heating to boiling and subsequentcooling, whereas the latter two synthesis experiments weredone entirely at room temperature. The novel core is bondedto selenate tetrahedra that are integral to the cluster (Figure



2h). In all of these compounds each Th(IV) cation iscoordinated by one μ3-O atom and two μ2-OH groups. Eachcluster contains two selenate tetrahedra near the center of thecluster that link to four Th(IV) cations, as well as four morethat each link to three Th(IV) cations (Figure 2i). The clustersin these four compounds are distinguished by the number ofH2O groups and nonbridging monodentate selenate tetrahedrathat complete the coordination spheres of the Th(IV) cations.Note that these clusters cannot be regarded as fragments of thefluorite-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 toestimate the deprotonation reaction free energies in aqueoussolution for different related model clusters.50

Two octanuclear clusters with U(IV) cations have beenisolated, although they are very distinct from each other (Figure2j−m).51,52 The cluster U8Cl24O4(cp*py)2, cp*py = tetrameth-yl-5-(2-pyridyl)cyclopentadiene (Figure 2j,k), was synthesizedin a nominally oxygen-free atmosphere.51 The cluster wasunintentionally obtained while exploring the synthesis ofuranium complexes with cp*py. The presence of O in thecluster is ascribed to adventitious oxygen that entered thereaction flask during heating, and crystals of the clustercompound formed when the heat-treated solution was leftstanding at room temperature. No yield or purity informationwas provided in the structure report. Four O atoms are locatednear the center of the cluster, where two of these are μ4-O, withU(IV) cations geometrically distributed in a tetrahedron similarto the coordination of μ4-O atoms in UO2. Two μ3-O atoms arebonded to three U(IV) cations that are approximately coplanarwith bond lengths of ∼2.2 Å, consistent with these O atomsbeing unprotonated. Of the 24 Cl anions in the cluster, 16bridge between 2 U(IV) cations and the others are terminal. Atboth ends of the elongated cluster, the cp*py ligandcoordinates a U(IV) cation.Cluster [U8L4Cl10O4]

2−, H4L = N,N′-bis(3-hydroxysalicyli-dene)-1,2-phenylenediamine, is shown in Figure 2l,m.52 Thiscluster crystallized by incorporating adventitious O thatprobably entered the reaction vessel during prolonged heatingat 80 °C; attempts to synthesize the compound in a controlledfashion were unsuccessful.52 This highly complex clustercontains four μ4-O atoms, each of which are surrounded byfour U(IV) cations in a tetrahedral arrangement with bondlengths similar to those in UO2. Eight of the Cl anions arebonded to a single U(IV) cation, and the remaining twoprovide μ2 bridges between U(IV) cations. The U(IV) cationsare also linked through eight μ2-phenoxyl bridges.

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) isthe only U10O14 core reported to date. The U10O14 topologyconsists of two U6O8 cages that are fused through two shared Uand two shared O atoms (Figure 2o). As in the U6O8 topology,the O atoms are located above the faces of the octahedradefined by the An(IV) cations. There are twelve μ3-O bridges,whereas the two O atoms that belong to each U6O8 are μ4-Obridges. Six of the μ3-O bridges correspond to OH groups withan average U−O bond length of 2.43(6) Å, and the remainingμ3-O bridges are O anions with an average U−O bond length

of 2.23(5) Å. Eight bridging benzoate ligands and two bridgingiodine 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·4MeCNwere synthesized from an acetonitrile reaction mixture formedby the hydrolysis of [UI3(thf)4] in the presence of benzoate.11

These compounds were obtained reproducibly even whendifferent uranium to benzoate ratios were used, whichprompted the authors to suggest that these are the onlyclusters in the acetonitrile solution.11 Given their earliersynthesis of a cluster with a U6O8 core using pyridine, theauthors hypothesized that the choice of organic base could beused to tune the cluster topology.11 Using a stronger organicbase, tetramethylethylenediamine (TMEDA), a cluster con-taining a U16O24 core was obtained (see section 3.3), consistentwith the importance of the organic base in determining clustersize.11

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 ambientconditions (Figure 2q,r).53 However, the reactants used for thissynthesis had a complex history that included passing throughan ion exchange column and repeated heat cycles to drynessand reconstitution in HCl solutions. Subsequently, a morecontrolled synthesis approach was used to obtain clusters withthe same Pu38O56 core.

54 A solution of Pu(IV) in concentratedHCl was boiled, with the addition of LiOH during boiling thatcontinued until the volume was reduced by half. The solutionwas allowed to cool to room temperature, and crystals ofLi12[Pu38O56Cl54(H2O)8](H2O)n formed when the solutionwas almost completely evaporated.54 The authors of the studyreport that the yield was not quantitative and that a more directsynthetic route was under investigation.The Pu38O56 core is a fragment of the PuO2 fluorite

structure, in which Pu(IV) cations are coordinated by eight Oatoms and O atoms toward the center of the cluster are μ4-Osurrounded by four Pu(IV) cations. Toward the edges of thenanoscale fragment of the PuO2 structure, μ3-O anions arebonded to three Pu(IV) cations with considerably shorter Pu−O bond lengths than in the case of the μ4-O anions. ThePu38O56 core is stabilized by addition of Cl and H2O, as shownin Figure 2r. Hydroxyl is notably absent. A cluster with theanalogous core and composition [Pu38O56Cl42(H2O)20]

2− wasalso isolated and characterized.54

Crystals containing the [Pu38O56Cl54(H2O)8]14− cluster were

readily dissolved in an aqueous solution of 2 M LiCl to producea green solution that gave an optical spectrum typical of aPu(IV) polymer.53 High-energy X-ray scattering (HEXS)provides pair distribution functions capable of giving insightinto actinide speciation in solution.55 HEXS data collected for asolution created by dissolving crystals containing the[Pu38O56Cl54(H2O)8]

14− cluster demonstrated that the clustersremain intact in solution.53 This nanostructured PuO2 fluoritematerial is thought to correspond to the well-known Pu(IV)polymer (sometimes designated colloid) that is challenging toseparate from complex systems. It was later demonstrated thatthe surfaces of the Pu38O56 core can be manipulated, therebymaking it possible to develop a separation scheme that targetsthe nanoscale material, rather than mononuclear metal speciesthat are only obtained through harsh chemical conditions.54



3. CLUSTERS CONTAINING AN(IV) AND AN(V)

Several clusters have been synthesized that contain a mixture ofAn(IV) and An(V) cations. Where good-quality crystallo-graphic data are available, these oxidation states can bedistinguished because An(V) cations usually are present asactinyl ions. Np(V) is the only pentavalent actinide cation withsignificant stability in aqueous solution, but its linkage intoclusters is unexplored. The U(V) cation disproportionates toU(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 ionshas provided two polynuclear clusters containing U(IV) andU(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-

tBu2)]2[(U(salen)]2(μ-O)3(μ3-O)}, H2salen = N,N′-ethylenebis(salicylideneimine),H2salophen = N,N′-phenylenebis(salicylideneimine), areshown in Figure 3a−d. That of {[UO2(mesaldien)−U-(mesaldien)]2(μ-O)} was obtained in 76% yield, togetherwith a U(VI) complex, and contains four U cations in a lineararrangement that are bridged through μ2-O atoms (Figure 3a).Although each of the U cations are in pentagonal bipyramidalcoordination polyhedra, there are two symmetrically distinctsites, and bond lengths indicate one contains U(IV) and theother U(V).58 The valence states alternate along the lineararrangement, such that the oxo atoms of the U(V) uranyl ionare bridges to U(IV) cations along the chain length. Thecoordination environments about the U(IV) and U(V) cationsare completed by O and N atoms of the Schiff ligands.In {[UO2(salen)][U(salophen-

tBu2)]2[(U(salen)]2(μ-O)3(μ3-O)}, the cluster contains five uranium cations (Figure3c,d). Again, the cluster was found to contain both U(IV) andU(V), as well as two cation sites for which the oxidation statesare 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 thecluster bridge to two different U(IV) cations. The core of thecluster also contains one μ3-O, at the center, and five μ2-Oatoms between the U cations (Figure 3d). The remainders ofthe coordination spheres of the five cations are completed by Nand O atoms of the organic components.

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

The controlled hydrolysis of trivalent uranium in acetonitrilehas provided hexanuclear clusters of uranium, as well as onewith a dodecanuclear cluster of uranium (see section 3.3).59

The details of the clusters formed were dependent on both thereaction time and the ligand used. The approach provided adiscrete hexanuclear cluster, a two-dimensional array ofhexanuclear clusters, a three-dimensional zeolite-like topologybuilt from hexanuclear clusters, and a discrete dodecanuclearcluster.59

The discrete hexanuclear cluster [U6(μ3-O)8(μ2-OTf)12-(H2O)3]·23H2O with a U6(μ3-O)8 core contains uraniumwith an average valence of 4.66, and the high symmetry of thecluster suggests that the mixture of two U(IV) and four U(V)cations in the cluster is completely delocalized.59 The analogouscore in the two-dimensional structure is reported to containfour U(IV) and two U(V) cations, whereas that whichcondensed into a framework only contains U(IV).59

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

The compound [U12(μ3-OH)8(μ3-O)12I2(μ2-OTf)16-(CH3CN)8]·2CH3CN·2H2O contains a discrete dodecanuclearcluster with a U12O20 core (Figure 3e,f).

59 As with some of thesmaller clusters discussed in the preceding section, this clusterwas produced by controlled hydrolysis of trivalent uranium inacetonitrile. The cluster, which was obtained in quantitativeyield, contains 16 μ3-O and 4 μ3-OH anions that bridge theuranium cations. The cations are further coordinated by O andN atoms of the organic molecules, as well as terminal iodineanions in two cases. The average valence of the uranium was

Figure 3. Illustrations of clusters containing An(IV) and An(V). Thevertexes 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′-(2-aminomethyl)diethylenebis(salicylimine). (c, d) {[UO2(salen)][U-(salophen-tBu2)]2[(U(salen)]2(μ-O)3(μ3-O)}, H2salen = N,N′-ethylenebis(salicylideneimine), H2salophen = N,N′-phenylenebis-(sa l icyl ideneimine). (e , f) [U12(μ3 -OH)8(μ3 -O)12I2(μ2 -O T f ) 1 6 ( C H 3 CN ) 8 ] · 2 CH 3 CN · 2 H 2 O . ( g , h ) { [ K -(MeCN)]2[U16O22(OH)2(C6H5COO)24]}·4MeCN. See Table 2 forthe legend of the figures.



reported to be 4.16, which was formally interpreted to be dueto a mixture of ten U(IV) and two U(V) cations. The bondlengths in the cluster suggested a delocalization of charge. Thepresence of U(IV) was confirmed by UV/vis spectroscopy, andmagnetic measurements were consistent with the presence ofU(V).59

3.4. Clusters with 16 An(IV) and An(V) Cations

Th e c ompo u n d { [K (MeCN) ] 2 [U 1 6O 2 2 (OH) 2 -(C6H5COO)24]}·4MeCN contains a U16O24 core (Figure3g,h) and was synthesized using the organic base TMEDA inhigh yield.11 The study demonstrated the importance of theorganic base in synthesizing clusters of U(IV), with smallerclusters obtained with other bases (see section 2.4). The core ofthe cluster contains four U6O8 units that are fused, in general asin [U10O8(OH)6(PhCO2)14I4(H2O)2(MeCN)2], through pairsof shared U cations and pairs of shared O atoms. Linkagesbetween U cations are through 16 μ3-O atoms, 2 μ3-OHgroups, and 6 μ4-O atoms that occur as caps on the octahedralfaces of the U6O8 cages. Bond-valence analyses indicate that theU cations are a mixture of valences, with 12 being U(IV) and 4U(V). The presence of U(V) is supported by the measuredmagnetism of the compound.11

4. CLUSTERS CONTAINING AN(V)Cation−cation interactions between U(V) uranyl ions areemerging as important features in oxo-bridged clusters. Inactinide chemistry, a cation−cation interaction refers to thesituation where an O atom of an actinyl cation coordinatesanother actinyl cation in the equatorial plane of itscorresponding coordination polyhedron.60−66 Although theactual bonding involves an An−O−An linkage, the term“cation−cation interaction” has been in regular usage since itwas introduced to the literature in 1961.67 Although a fewcompounds have been reported with cation−cation interactionsbetween U(VI) uranyl ions, they are much more common inthe case of An(V) compounds. For example, more than 40% ofthe reported Np(V) crystal structures contain cation−cationinteractions.68

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 exhibitsa tetranuclear cation−cation complex based on U(V) (Figure4a,b).69 The compound was obtained by reacting thecoordination polymer {[UO2Py5][KI2Py2]}n with dibenzoylme-thanate in an oxygen-free atmosphere. In the cluster four U(V)uranyl ions form the core, with each uranyl ion donating onecation−cation interaction, as well as accepting one donated byanother uranyl ion (Figure 4b). The remainders of thecoordination environments of the uranyl pentagonal bipyr-amids are completed by O atoms of the organic ligands. Thepresence of the cluster in a pyridine solution was verified byNMR. In contrast, when the cluster is dissolved in dimethylsulfoxide (DMSO), it fragments.69 It is rapidly oxidized in thepresence of trace amounts of oxygen and breaks down inpyridine solution.In a continuation of efforts to stabilize U(V) uranyl cation−

cation interactions, two tetranuclear clusters were reported witht h e s am e b a s i c c o r e s t r u c t u r e a s t h a t i n{[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 vertexesof the four uranyl pentagonal bipyramids that are not involvedin the cation−cation interactions are two N and two O atoms

from the organic ligand. Using 1H NMR this study showed thatthis complex remains intact in pyridine for one month. Cyclicvoltammetry of the complex dissolved in pyridine demon-strated that a reversible one-electron oxidation does not destroythe structure of the cluster, but a subsequent three-electronoxidation under higher potential results in the irreversiblebreakdown of the cluster.64 Very recently, the first Np(V)example of the same tetrameric unit was isolated in thecompound [{NpO2(salen)}4(μ8-K)2][K(18C6)Py]2.

70

The compounds {[UO2(dbm)2]2[μ-K(Py)2]2[μ8-K-(Py)]}2I2·Py2 and {[UO2(dbm)2]2[μ-K(MeCN)2][μ8-K]}2 arebased on clusters with the same basic core structure as{[UO2(dbm)2]4[K6Py10]}·I2·Py2.

65 In pyridine solution thelatter of these clusters disproportionates to [U(dbm)4] and[(UO2(dmb)2] species. The presence of the cation−cationinteractions in the tetranuclear cluster between U(V) uranylions was found to be important for the disproportionationreaction.65 In other words, disrupting the cation−cationinteractions also disrupted disproportionation of U(V) toU(IV) and U(VI).Four additional clusters have been reported that contain the

s a m e b a s i c u r a n i u m − o x y g e n c o r e a s i n{[UO2(dbm)2]4[K6Py10]}·I2·Py2: [UO2(acacen)]4[μ8-K]2[K-( 1 8C6 ) ( p y ) ] 2 , H 2 a c a c e n = N ,N ′ - e t h y l e n e b i s -(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]C-6)]2.

71 This study explored the importance of both the organicligands and alkali-metal counterions in determining the stabilityof the clusters in pyridine solution. The authors concluded thatthe tetranuclear core containing U(V) could be highly stablebut careful tuning of the ligand geometry and the presence ofcoordinating counterions is essential to the stability. Thereaction of compounds containing the tetranuclear cluster withH+ results in immediate decomposition and conversion ofU(V) to a mixture of U(IV) and U(VI).71

Figure 4. Illustrations of clusters containing An(V). The vertexes inthe graphs represent the locations of An(V) cations, and linesd e s i g n a t e o x i d e o r h y d r o x y l b r i d g e s . ( 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.



4.2. Clusters with Six An(V) Cations

The compound [Cp*4 (bpy)2][U6O13], Cp* = 1,2,4-tBu3C5H2,bpy = bipyridine, contains a novel U(V)6O13 core (Figure4c,d).72 The U(V) cations are arranged at the vertexes of anoctahedron, as in the An6(O,OH)8 clusters discussed above, butin this case μ2-O atoms are located along the edges of theoctahedron and a single μ4-O atom is located at the center ofthe core. The core cluster is analogous to the Lindquist-typemolybdenum and tungsten oxide clusters. Whereas the latterare stabilized by the presence of yl O atoms, the U(V)6O13 coreis truncated by Cp* in the case of four U(V) cations andbipyridine in the other two cases.

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

which contains only U(V), [{UO2(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.

6. CLUSTERS CONTAINING AN(VI)

All but one of the clusters reported with An(VI) contain theU(VI) uranyl ion. The vast majority of these are uranylperoxide 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 containtetrameric units built from U(VI) uranyl ions that contain Obridges. In each of these, the uranyl ions are present as uranylsquare, pentagonal, or hexagonal bipyramids and each of thefour 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 ofcombinations of μ2-O and μ3-O atoms, and the bipyramidsshare either equatorial edges or single equatorial vertexes withinthe cluster. Graphical representations of the observedconfigurations are in Figure 5. Note that these graphs areconstructed differently from those elsewhere in this review, asthe sharing of polyhedral edges and vertexes is distinguished. Inthe graphs, black circles represent a uranyl ion bipyramid andthe connections between the polyhedra are shown as single or

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



double lines for the sharing of a single vertex and the sharing ofan 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 ligandswere used to connect uranyl ions. These clusters differ in thenumber of μ2-O and μ3-O atoms, and their specific topologiescan be related to the role of the organic ligands in both directlycoordinating the uranyl ions and providing for charge balancein the crystal structures. In these compounds, the aliphaticdicarboxylates contained from 5 to 10 C atoms and thedipyridine species were either 4,4′-bipyridyl or 1,2-bis(4-pyridyl)ethane.73 The dicarboxylates were found to alwayscoordinate uranyl ions, whereas the dipyridyl species coordinatethe uranyl ions, provide charge balancing, or provide spacefilling.73

The compound [HNEt3]2[(UO2)4(3−8H)(OH)2(H2O)4]·-1 . 5 N E t 3 · 2 . 5 H 2 O · CH 3 OH , 3 − 8 H = p - t e r t -butyloctahomotetraoxacalix[8]arene, contains four U(VI) ur-anyl ions, each of which is present as a uranyl pentagonalbipyramid (Figure 5i).74 It was obtained in very low yield byreacting p-tert-butyloctahomotetraoxacalix[8]arene with uranylnitrate in a solution of CH3CN and NEt3. The core of thiscluster involves two types of linkages. There are six μ2-O atoms,but they assume two different structural roles. Two pairs ofthese O atoms bridge between pairs of uranyl ions, thusdefining shared equatorial edges of pentagonal bipyramids. Twoadditional μ2-O atoms bridge between uranyl ions such thatthey correspond to shared bipyramidal vertexes. The result is acluster core in which the four U(VI) cations are approximatelycoplanar, with the equatorial vertexes of the bipyramids inapproximately the same plane and the uranyl ions themselvesextending roughly perpendicular to the plane. Two additionalO atoms that coordinate each of the uranyl ions are providedby the organic ligands.The tetranuclear cluster shown in Figure 5o has been

reported in several compounds obtained by a relatively broadrange of synthesis conditions. It was found in the compound(UO2)4O2Cl4(C4H8O)2(H2O)4 synthesized in air by theoxidation and hydration of a solution of (1,4,7-t r ime t h y l i n d e n y l ) u r a n i um( IV) t r i c h l o r i d e b i s -(tetrahydrofuran).75 In this case the terminal ligands of thecluster correspond to O atoms of the organic ligand as well asμ2-Cl atoms. The cluster in which the terminal ligands are μ1-Nor μ2-N of cyanate ligands was obtained by synthesis under anitrogen atmosphere.76 It was also found with terminal μ1-Oand μ2-O atoms corresponding to DMSO and acetate in acompound synthesized via room-temperature evaporation in airof a solution of [UO2(O2CCH3)2(OH2)2] in DMSO pretreatedat 100 °C.77 The cluster with μ2-Cl atoms and μ1-N atoms ofacetonitrile coordinating uranyl was obtained from a solution ofacetonitrile in a sealed vessel at room temperature.78 It wasobtained with μ2-Cl atoms and terminal H2O groups in thecompounds MU2O5Cl4(H2O)2, M = Rb, Cs, by crystallizationat 30 °C.79 Finally, a tetrameric cluster in which N atoms of two5-pyrimidyltetrazolate (pmtz) ligands coordinate the uranylions was synthesized by hydrothermal reaction.80

The cluster shown in Figure 5q, consisting of two uranylpentagonal bipyramids and two uranyl hexagonal bipyramids,was also reported with only O atoms coordinating the uranylions in the form (UO2)4(μ3-O)2O12, which was cocrystallizedwith a dimer of uranyl ions and linked via phthalate groups.81

This compound was synthesized hydrothermally at 200 °C witha 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, wasreported in the compound [(UO2)2(C2O4)(OH)2(H2O)2]-(H2O) that was synthesized hydrothermally at 120 °C over fourdays from an aqueous solution of uranyl nitrate, oxalic acid, andpotassium nitrate.82 Uranyl ions are bridged by OH groups andare also coordinated by edge-on oxalate groups that bridgebetween different tetrameric clusters. The complex[(UO2)4O4(LH8)]·10CH3OH, which contains the same typeof tetrameric cluster, was crystallized by slow evaporation atroom 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 thetetrameric clusters shown in parts g and e, respectively, ofFigure 5.84 These compounds were obtained in good yield viathe hydrothermal treatment of an aqueous solution containingorganic ligands at 160 °C for three days, followed by slowcooling.Hydrothermal synthesis provided the compound

[Zn2(phen)4U4O10(OAc)2(NA)2-(QA)2] (phen = 1,10-phen-anthroline; HOAc = acetic acid; HNA = nicotinic acid; H2QA =quinolinic acid) in high yield at 160 °C.85 It contains thetetranuclear cluster shown in Figure 5e, in which there are twoμ3-O atoms and the remainder of the coordination polyhedraabout the uranyl ions are completed by the organic ligands.

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]octaneprovided (UO2

2+)6(L7−)2(O

2−)2(HDABCO+)6·3CH3CN·-CHCl3·5CH3OH·3H2O, HDABCO = 1,4-diazabicyclo[2.2.2]-octane, crystals of which are unstable even in their mothersolution.86 The core of the cluster consists of six U(VI) uranylions (Figure 6a,b). Two and four of these are present as square

Figure 6. Illustrations of selected clusters containing An(VI). (a, b)(UO2

2+)6(L7−)2(O

2−)2(HDABCO+)6·3CH3CN·CHCl3·5CH3OH·-3H2O, H7L = p-benzylcal ix[7]arene, HDABCO = 1,4-d i a z a b i c y c l o [ 2 . 2 . 2 ] o c t a n e . ( c ) [ H N E -t3]8[(UO2)8(H2L)4(O2)8]·22H2O, H4L = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene. See Table 2 for thelegend of the figures.



and pentagonal bipyramids, respectively. There are two μ3-Oatoms that are shared between uranyl pentagonal bipyramids, aswell as six μ2-O atoms. Two of these are involved in unusualU(VI) uranyl ion cation−cation interactions that are donatedby uranyl ions in the pentagonal bipyramids and are acceptedby those in the square bipyramids. Whereas the central group offour edge-sharing uranyl pentagonal bipyramids is a commonconfiguration, as part of clusters discussed in the previoussection or of a larger structural unit in uranyl minerals andvarious synthetic compounds, the occurrence of cation−cationinteractions associated with it is highly unusual. This clusteralso represents a relatively rare example of surface passivationby both yl O atoms and organic ligands. Note that the uranylions that accept the cation−cation interactions are also eachcoordinated by three O atoms of the calixarene, resulting insquare bipyramids. The organic ligands undoubtedly play animportant role in directing the assembly of a cluster containingU(VI) uranyl cation−cation interactions.

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-etha-noanthracene, contains a cage cluster in which two four-membered rings of uranyl peroxide polyhedra are linkedthrough organic molecules into the structural unit (Figure6c).83 Four uranyl ions are bridged by peroxide ligands that arebidentate to two uranyl ions and consequently form equatorialedges of uranyl hexagonal bipyramids. Each uranyl ion iscoordinated by two peroxo groups, as well as two O atomsprovided by the organic ligands. Two years later, similar rings ofuranyl peroxide polyhedra were found to self-assemble intoinorganic cage clusters, as discussed in section 7.

7. ACTINYL PEROXIDE CLUSTERS

Beginning in 2005,87 an extensive family of nanoscale actinylperoxide cage clusters have been reported (Figures 7 and8).87−99 Currently, 38 uranyl-based clusters and 1 neptunyl-based cluster are in the literature. In all 39 clusters actinyl ionsare bridged by bidentate peroxide ligands, and other bridgesbetween 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 stringthat represents chemical species other than peroxide orhydroxyl, and m indicates the quantity of these species.Where the UnXm designations are nonunique, additionaldesignations 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

linkages of uranyl hexagonal bipyramids through sharedequatorial edges, usually to form four-, five-, or six-memberedrings that correspond to topological squares, pentagons, andhexagons, respectively. We first review computational studiesrelated to these clusters, as well as synthesis methods, prior todescribing 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 compoundin which actinyl ions were bridged through peroxide.100 Studtiteoccurs naturally in uranium deposits,101 where it forms byincorporating peroxide created by α-radiolysis of water.102 Italso forms where used nuclear fuel interacts with water103−106

and in one case where complex nuclear materials created by areactor core-melt incident reacted with water.107 It is easy tosynthesize in the laboratory by combining the uranyl ion andhydrogen peroxide in acidic aqueous solutions and has longbeen important in the nuclear industry. However, the structurewas only determined in 2003 using a natural crystal;100 noknown synthesis method produces crystals suitable for single-crystal diffraction, and all known crystals from Nature are thinand easily damaged needles.In the structure of studtite, uranyl ions are coordinated by

two peroxide groups that are bidentate to the uranyl ion in atrans arrangement, as well as two H2O groups in a transconfiguration. The result is a uranyl hexagonal bipyramid withtwo of its equatorial edges defined by peroxide groups. It isthese two trans edges that are shared between adjacent uranylions, giving a one-dimensional chain of polyhedra that extendsthrough the structure. Adjacent chains are linked through Hbonding only. Despite the pliable nature of the H bondingnetwork, the uranyl−peroxide−uranyl bridges have dihedralangles that depart significantly from 180°. The dihedral anglesare about 140°, giving the chains a corrugated appearance. In2010, it was argued that the uranyl−peroxide−uranylinteraction is inherently bent and that this provides thecurvature that fosters the formation of actinyl peroxide cageclusters.94 The authors concluded, on the basis of an analysis ofknown structures with uranyl−peroxide−uranyl bridges, thatthe dihedral angles were always bent and that this wasimportant for the formation of cage clusters.The first computational studies of the uranyl−peroxide−

uranyl bridge appeared late in 2010.108,109 These examinedmodel species representing fragments of the larger clusters.Whereas computational studies of entire clusters of uranylperoxo polyhedra could provide various useful insights, stericconstraints of the cages require that the uranyl−peroxide−uranyl dihedral angles be bent. The smaller fragments studiedare not part of larger structures that require a specific geometry

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



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 multiconfigurationalmethods (CASSCF/CASPT2).109 These models ranged from asimple dimer of uranyl polyhedra with a peroxide bridge to aring structure consisting of five uranyl hexagonal bipyramids,with all bridges between uranyl ions being bidentate peroxidegroups. Full geometry optimizations of these model clusterswere conducted, both in the absence of counterions and in thepresence of Li, Na, K, Rb, or Cs cations added to balance thecharge. When counterions were included in the simulations, allmodel clusters optimized to geometries that included stronglybent uranyl−peroxide−uranyl dihedral angles. The bentconfiguration was attributed to a partially covalent interactionbetween the U(VI) cation of the uranyl ion and the peroxideligands. The calculated uranyl−peroxide−uranyl dihedral anglewas found to depend on the ionic radius and electronegativityof the specific counterion, with the smallest angles obtained forthe smallest cations. The calculated dihedral angles ranged from140° in the case of Li to 164° for Cs. Geometry optimizationsdone for related clusters derived by replacing each peroxidebridge by two hydroxyl groups gave U−(OH)2−U dihedralangles close to 180°.109

In another computational study, four-, five-, and six-membered rings of uranyl peroxide polyhedra were consid-ered.108 In each case all of the bridges between the uranyl ionswere through bidentate peroxide ligands. The geometries of themodel clusters were fully optimized without symmetryconstraints using DFT methods. Using a hypothetical[(UO2)2(μ-η

2:η2-O2)(H2O)6]2+ model cluster, the researchers

determined that the bent configuration is only about 0.5kcal·mol−1 below a planar configuration (i.e., with a uranyl−peroxide−uranyl dihedral angle of 180°). However, thiscomputation was done in the absence of counterions andthus, importantly, cations that can bridge between the O atomsof uranyl ions. When they examined a ring of four uranylhexagonal bipyramids bridged through peroxide ligands,including counterions, the planar configuration was 16kcal·mol−1 above the one containing bent uranyl−peroxide−uranyl dihedral angles. The study also examined the complex-ation energies of Li, Na, K, Rb, and Cs with the model clusterscontaining four, five, or six uranyl polyhedra. The calculatedcomplexation energy was greatest for Na and a five-memberedring, 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 Thisstudy emphasized five-membered rings of uranyl peroxidepolyhedra and the U20 cluster, which is the smallest cage builtfrom uranyl ions reported to date. It adopts a fullerenetopology that consists of only 12 pentagons (see below). TheDFT-optimized geometries of the rings and cluster matchedthose from X-ray structure determinations within a few percenterror for bond distances and angles. The study evaluated thecomplexation energy for counterions and uranyl peroxide ringsor cages and further established the importance of counterionsin stabilizing different types of rings. In the case of U20, thelowest energy was obtained for Na as a counterion (Li, K, Rb,and Cs were also considered), consistent with the use of Na inthe actual synthesis of the cluster.In summary, the results of the computational studies of

fragments of the uranyl peroxide clusters indicate that theuranyl−peroxide−uranyl dihedral angle can be strongly

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

7.2. Synthesis of Actinyl Peroxide Clusters

Of the 39 actinyl peroxide clusters in the literature, all but 2were synthesized in aqueous solution under ambientconditions. The remaining two, U40 and U50, were crystallizedfrom solutions heated to 80 °C in sealed Teflon-lined vessels.99

Characterization of species in solution provides ampleevidence that uranyl peroxide clusters self-assemble in solutionprior to crystallization. Small-angle X-ray scattering dataindicate clusters are present in solution in as little as 1 h,which is the shortest time needed for sample preparation andinitialization of data collection.92,93 Early studies of uranylperoxide cluster formation suggested that cluster abundanceschanged in solution over the course of months, although laterstudies indicated clusters can self-assemble and crystallize in aslittle as 15 min.93 A very recent study provided time-resolvedsmall-angle X-ray scattering studies of a uranyl core−shellcluster and demonstrated that the core formed essentiallyinstantaneously and the shell required about two weeks.92

Uranyl peroxide cage clusters that contain only peroxide andhydroxyl bridges self-assemble in aqueous solution over the pHrange of about 6.7−13 under ambient conditions. Thepreparations are very elementary, with simple combinationsof uranyl nitrate, hydrogen peroxide, an alkali-metal or alkaline-earth-metal base, and water giving the desired results. Althoughno measurements of the solubility of uranyl peroxide cageclusters have been reported, it seems reasonable to expect thattheir solubilities are very similar, given the topological andchemical relationships of the family of clusters. However,relatively modest changes in the details of the synthesisconditions have been found to give crystals with differentclusters. Also, it is likely that the solubility of uranyl peroxidecage clusters will be impacted by counterions in solution.Although the kinetics of uranyl peroxide cage cluster

assembly are mostly unstudied, kinetic factors are likelyimportant in determining the identity of the cluster crystallizedin any given case. For example, almost all of the uranyl peroxidecage clusters reported in the literature adopt clusters with highsymmetry (see below).Uranyl peroxide polyhedra in cage clusters are linked to three

other uranyl polyhedra, almost always through the sharing ofequatorial edges. In many clusters of uranyl peroxide polyhedrahydroxyl groups are essential constituents. It is common foruranyl ions to be coordinated by two peroxide groups arrangedalong two equatorial edges of a hexagonal bipyramid, withanother edge defined by two hydroxyl groups. The bipyramidshares its two peroxide edges with two different bipyramids andits edge defined by the two hydroxyl groups with a thirdbipyramid. As the dihedral angle of the U−(OH)2−U bridge ispliable, incorporation of hydroxyl bridges has the effect ofchanging the average dihedral angle of the cluster. Clusters thatare larger than those that would be consistent with peroxidebridges can form with hydroxyl bridges.The primary lens through which self-assembly of uranyl

peroxide clusters in solutions has been studied is through thecrystals that they form. In other words, it is those clusters thatcrystallized that have been characterized, and the solutions mayhave been polydisperse. The relatively few studies of clusters inmother solutions suggest such polydispersity, but over thecourse of weeks and months, solutions move toward a moremonodisperse distribution of clusters.87 A reliance on crystals



Figure 8. continued



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 cage composition figure

U16 [(UO2)16(O2)24(OH)8]24− 8a,b

U18Pp2PCP6 [(UO2)18(O2)18(OH)2(CH2P2O6)6(P2O7)2]34− 8c,d

U20 [(UO2)20(O2)30]20− 8e,f

U20Pp6b [(UO2)20(O2)24(P2O7)6]32− 8g

U20Pp6a [(UO2)20(O2)24(P2O7)6]32− 8h

U20Pp10 [(UO2)20(O2)20(P2O7)10]40− 8i

U20R [(UO2)20(OH)16(O2)28]32− 8j,k

U24 [(UO2)24(OH)24(O2)24]24− 8l,m

U24Pp12 [(UO2)24(O2)24(P2O7)12]48− 8n

U24PCP12 [(UO2)24(O2)24(CH2P2O6)12]48− 8n

U24R [(UO2)24(O2)36(OH)12]36− 8o,p

U26Pp11 [(UO2)26(O2)28(P2O7)11]48− 8q,r

U26Pp6 [(UO2)26(O2)33(P2O7)6]38− 8s,t

U28a [(UO2)28(O2)28(OH)28]28− 8u,v

U28 [(UO2)28(O2)42]28− 8w,x

U30 [(UO2)30(O2)36(OH)22]34− 8y,z

U30a [(UO2)30(O2)30(OH)30]30− 8aa,ab

U30Pp6 [(UO2)30(O2)39(P2O7)6]42− 8ac,ad

U30Pp10Ox5 [(UO2)30(O2)30(P2O7)10(C2O4)5]50− 8ae,af

U30Pp12P1 [(UO2)30(O2)30(P2O7)12(PO4)(H2O)5]51− 8ag,ah

U32 [(UO2)32(OH)32(O2)32]32− 8ai,aj

U32Pp16 [(UO2)32(O2)32(P2O7)16]64− 8ak,al

U32R [(UO2)32(O2)40(OH)24]40− 8am,an

U36a [(UO2)36(O2)36(OH)36]36− 8ao,ap

U36 [(UO2)36(O2)41(OH)26]36− 8aq,ar

U36Ox6 [(UO2)36(O2)48(C2O4)6]36− 8as

U38Pp10Nt4 [(UO2)38(O2)40(P2O7)10(NO3)4]56− 8at,au

U40 [(UO2)40(OH)40(O2)40]40− 8av,aw

U42 [(UO2)42(O2)42(OH)42]42− 8ax,ay

U42Pp3 [(UO2)42(O2)42(OH)36(P2O7)3]48− 8az

U44 [(UO2)44(O2)66]44− 8bc,bd

U44a [(UO2)44(O2)44(OH)44]44− 8ba,bb

U45Pp23 Na64Li26[(UO2)45(O2)44(P2O7)23](H2O)n 8be,bfU50 [(UO2)50(OH)50(O2)50]

50− 8bg,bhU50Ox20 [(UO2)50(O2)43(OH)4(C2O4)20]

30− 8biU60 [UO2(O2)(OH)]60

60− 8bj,bkU60Ox30 [(UO2)60(O2)60(C2O4)30]

60− 8blU28U40R (K,Na)44[(UO2)68(O2)74(OH)16(NO3)16(H2O)16](H2O)155 8bm,bnU120Ox90 K134Li46[(UO2)120(O2)120(C2O4)90](H2O)n 8bo,bp



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

In Figure 8 and Table 1 uranyl peroxide clusters are illustratedand listed in order of increasing number of uranyl polyhedra.

The smallest published structure contains only 16 uranylpolyhedra, and the largest to date contains 120 polyhedra. All ofthe clusters shown in Figure 8 were synthetically prepared, andthe structures were derived from single-crystal X-ray diffractiondata.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 theuranyl ions are part of hexagonal bipyramids, with theequatorial 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 andare not particularly helpful in their representation (Figure 7).Here we use mostly polyhedral representations of the clusters,with uranyl polyhedra consistently colored yellow in recog-nition 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). Thegraphical 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 betweenthe U(VI) cations. Shared edges between uranyl polyhedra aretherefore shown as a single connection between thecorresponding vertexes. Where U(VI) cations are bridged byunits such as pyrophosphate or oxalate, a single connector joinsthe corresponding vertexes. As such, the graph contains onlyinformation on the positions of the U(VI) cations and theconnections between them, with no designation of the chemicaltype of connection in any case. This approach is useful becauseit simplifies the topological representations and facilitatesidentification 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-

connected, meaning that each is linked to three other uranylpolyhedra. The result is a family of three-connected graphs.One subset of this family is the fullerene topology, whichconsists of exactly 12 pentagons as well as some hexagons.Made famous by C60,

111 fullerene topologies have been foundin 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. Forexample, the first graph in Figure 8 is designated 16:4164. Thisindicates there are 16 vertexes that correspond to 16 uranylpolyhedra, there is a single topological square, and there are 4topological hexagons. In the case of cage clusters with threeconnected vertexes, summing the number of vertexes in thedifferent topological elements gives 3 times the total number ofvertexes, because each vertex is included in three topologicalelements. For example, the designation 24:4668 corresponds toa cage cluster with six squares and eight hexagons, (4 × 6) + (6× 8) = 72, which is triple the number of vertexes (24).

7.4. Cluster Compositions

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

7.5. Cluster Descriptions

7.5.1. Open Clusters. Only four of the clusters of uranylperoxide polyhedra that have been reported to date are notcages. Three of these are ring structures, whereas the other isthe open-bowl-shaped U16 (Figure 8a,b).96 It is the smallestextended cluster reported that consists only of uranyl peroxidepolyhedra. Its graph, 16:4164 (Figure 8a), consists of a singlesquare that is surrounded by four hexagons. This arrangementof polygons occurs in some of the cage clusters, most notablyU24.Clusters U20R,

96 U24R,96 and U32R

93 each consist of crown-shaped ring topologies (Figure 8j,k,o,p,am,an). The topologiesof U20R and U24R consist solely of pentagons (U20R) orhexagons (U24R) and are unusual in the lack of topologicaldiversity; the U20 cage cluster is the only other one that is builtfrom a single type of polygon. Each contains both types ofuranyl polyhedra: U20R is built from 16 diperoxide and 4triperoxide hexagonal bipyramids, and U24R has 12 diperoxideand 12 triperoxide hexagonal bipyramids. Each also hascounterions inside the ring. U20R contains 12 K cations, 1 ofwhich is below each of the 5 pentagons in the topology. InU24R, 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, whereit is linked to the crown structure through the Na and Cscations.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

Table 2. Legend of the Figures

element color element color

Th atoms andpolyhedra

dark cyan Cl atoms bright green

U atoms and polyhedra yellow S atoms andpolyhedra

pink

Pu atoms slate blue P atoms andpolyhedra

purple

Mn atoms dark yellow F atoms dark greenW polyhedra dark blue O atoms redK atoms turquoise N atoms blueI atoms plum C atoms blackSe atoms rose



of the introduction of hydrogen peroxide into a solutioncontaining ammonium hydroxide and uranyl nitrate.93 Thecluster contains eight pentagons and four hexagons and can bebuilt from different combinations of uranyl hexagonalbipyramids: 8 triperoxide and 24 diperoxide polyhedra or 16diperoxide 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 12pentagons. The smallest of these contains 20 vertexes, withonly pentagons and graph 20:512, and has the lowest number ofvertexes possible for a fullerene topology. The correspondingU20 cluster (C5v) was isolated with Na as the counterion and alluranyl ions present as triperoxide polyhedra (Figure 8e,f).94

Three uranyl peroxide pyrophosphate clusters have beenreported that are topological derivatives of U20 and graph20:512.

89 These contain either 6 (U20Pp6a, U20Pp6b) or 10(U20Pp10) pyrophosphate groups. Each pyrophosphate groupcoordinates two uranyl ions with “side-on” configurations, suchthat two adjacent equatorial vertexes of a uranyl hexagonalbipyramid correspond to O atoms of the pyrophosphate group.Pyrophosphate groups bridge uranyl ions in a fashion that istopologically analogous to that of peroxide groups or sharededges defined by hydroxyl groups. The U20Pp6a (Figure 8h),U20Pp6b (Figure 8g), and U20Pp10 (Figure 8i) clusters aretopologically identical to U20, but relative to U20, peroxidebridges have been replaced. There are 30 peroxide bridges inU20. Ten of these are replaced by pyrophosphate in U20Pp10,and six are replaced by pyrophosphate in each of the U20Pp6aand U20Pp6b clusters.Each of the U20Pp6a, U20Pp6b, and U20Pp10 clusters was

synthesized from aqueous solutions containing identical initialuranyl nitrate and hydrogen peroxide concentrations. U20Pp6aand U20Pp6b crystallized from solutions with pH ranging from10.3 to 11.5 and from 9.2 to 9.5, respectively. U20Pp10crystallized from solutions over the pH range of 8.7−9.2,although the solution contained twice as much pyrophosphateas 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 14uranyl triperoxide hexagonal bipyramids and 12 uranyldiperoxide hexagonal bipyramids in which the uranyl ions arealso coordinated by side-on pyrophosphate groups. Thesepolyhedra are linked into eight five-membered rings ofpolyhedra, all of which share polyhedra with at least oneadjacent five-membered ring. Six pyrophosphate units bridgebetween uranyl ions of these five-membered rings, resulting in acage 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-vertexfullerene topology adopted has the highest possible idealsymmetry for this number of vertexes. U28 is a relatively rareexample of a cluster that is assembled entirely from uranyltriperoxide polyhedra. The initial report of this cluster includedK as the counterion, with a very low yield of only a few crystals.Subsequently, a reliable synthesis route was reported toproduce U28 that features different combinations of templatingcations (K, Rb, Cs) and anions (uranyl tiperoxide polyhedra,Nb(O2)4, and Ta(O2)4).

112 The study found that the keys toobtaining a high yield of this cluster were to maintain synthesisconditions that are not extreme or dynamic and to usetemplating cations that ideally match each other and thetopology of the capsule interior. Crystals containing these

clusters were soluble in tetramethylammonium, as well aslithium, sodium, and potassium salt or hydroxide solutions.Nuclear magnetic resonance (NMR) studies of Cs135indicatethat the stability of the cluster in solution is related to thecounterions present.112

Recently, a core−shell cluster has been reported, U28U40R, inwhich the core cluster is also the U28 fullerene topology cluster(Figure 8bm,bn).92 However, in this case the polyhedra thatform the cluster are a mixture of uranyl triperoxide anddiperoxide hexagonal bipyramids. The U28 fullerene topologycage cluster forms quickly in solution, within 1 h as shown bysmall-angle X-ray scattering data. The shell structure, whichcontains 40 uranyl hexagonal bipyramids as well as 16 nitrategroups, is templated by the U28 cluster, although its assembly isdelayed by about two weeks, as shown by time-resolved small-angle X-ray scattering (SAXS) data.92 The shell structureconsists of topological pentagons, and these are located directlyabove topological pentagons of the U28 core. Linkages betweenthe core and shell structure are through K cations. The studyalso demonstrated that when crystals of the core−shell clusterwere dissolved in water, the clusters remained intact.In cluster U30Pp6, uranyl ions are bridged by peroxide groups

or through pyrophosphate groups, resulting in a fullerenetopology with graph symbol 30:51265 (Figure 8ac,ad).

89

U30Pp10Ox5 is a rare example of a cluster that contains bothpyrophosphate and oxalate bridges (Figure 8ae,af).113 Itscrystallization was found to be strongly dependent on thesolution pH, with a superior yield obtained for pH 5.1.Solutions with lower pH favor formation of uranyl oxalateclusters, and those with higher pH give uranyl pyrophosphateclusters. In U30Pp10Ox5, uranyl ions are bridged by peroxide,pyrophosphate, and oxalate groups. The result is a fullerenetopology 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 hexagonsshare edges with other hexagons, forming a ring of hexagons. Inthe U30Pp6 topology, the hexagons are distributed over twodifferent segments that are separated by pentagons (Figure8ac).Cluster U30Pp12P1 exhibits several novel features (Figure

8ag,ah). The two most uncommon aspects of the cluster arethat it contains a single phosphate tetrahedron, along with the12 pyrophosphate groups, and 4 of the equatorial ligands ofuranyl hexagonal bipyramids are nonbridging and are assumedto correspond to H2O groups. Ten of the pyrophosphategroups bridge uranyl ions with the typical side-on configuration,but two bridge between three uranyl ions, one with a side-onlinkage and the other two by sharing single vertexes with theuranyl bipyramids. The graph of U30Pp12P1 contains 12pentagons and 6 hexagons, but it is not the same fullerenetopology 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 groupsor pairs of hydroxyl groups. The resulting topology is afullerene with eight hexagons and graph symbol 36:51268.Cluster U36Ox6 also exhibits this topology (Figure 8as), with sixof the uranyl bridges that correspond to two hydroxyl groupsbeing replaced by oxalate.91 The oxalate groups in U36 arearranged 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 contains12 hexagons with graph symbol 40:512612. In the case of



fullerene topologies with 44 vertexes, there are several clustersthat have higher symmetry than that with the least adjacentpentagons. U44 adopts one of these higher symmetry isomers,which was taken as evidence that symmetry is an importantconsideration in isomer selection for uranyl peroxideclusters.95,97,98

Cluster U50 (D5h) is built from uranyl diperoxide hexagonalbipyramids, such that uranyl ions are bridged by either peroxidegroups or pairs of hydroxyl groups (Figure 8bg,bh).99 Itstopology is a fullerene that contains 15 hexagons, with graphsymbol 50:512615. Of the possible fullerene isomers with 50vertexes, U50 selects the isomer that has both the highestsymmetry and the least number of pentagonal adjacencies. TheCl-stabilized C50Cl10 cluster adopts an identical topology.114

U50 is one of only two uranyl peroxide cage clusters thatcrystallized at temperatures other than ambient, in this case at80 °C. Cluster U50Ox20 forms with the same topology as U50,but it contains three different types of bridges between uranylions: peroxide, hydroxide, and oxalate groups (Figure 8bl).Cluster U60 (Oh) consists only of uranyl diperoxide

hexagonal bipyramids (Figure 8bj,bk).9595 Its topology is afullerene that contains 20 hexagons and no adjacent pentagons,with graph symbol 60:512620. The cluster is topologicallyidentical to C60, but the cluster is much larger, more massive,and more chemically complex than C60. It has only beencrystallized 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 havebeen replaced by oxalate bridges (Figure 8bl).91 This results ina 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−shellstructure in which the core is identical to the U60Ox30 cluster.The shell consists of 12 five-membered rings of uranyldiperoxide hexagonal bipyramids that share vertexes, and eachof the uranyl ions within the ring are coordinated by an oxalategroup. The oxalate groups in the shell structure are terminal;they each coordinate only one uranyl ion. One such ring islocated above each of the 12 topological pentagons of theU60Ox30 core. SAXS data indicate that upon dissolution inwater the 60 uranyl polyhedra that form the shell structureprobably detach from the cluster but the U60Ox30 cage remainsintact.7.5.3. Cage Clusters Containing Topological Squares.

Four-membered rings of uranyl hexagonal bipyramids areessential components of 13 cage clusters reported to date. Thisring always has four bidentate peroxide groups that bridgebetween four uranyl ions. As such, each uranyl ion containedwithin the four-membered ring is coordinated by two peroxidegroups. The remaining two equatorial vertexes correspondeither 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 13cage clusters containing four-membered rings of hexagonalbipyramids correspond to 11 different graphs (Figure 8). Inthese graphs there are no examples of square−square orsquare−pentagon adjacencies. Rather, in all cases each square issurrounded by hexagons. It has been noted that, in the case oftopological squares, geometric constraints mandate that theedges shared between the four uranyl polyhedra be peroxide.Assuming that the cluster is built from uranyl diperoxidepolyhedra, square−square adjacencies are prohibited, althoughnot in the case where the cluster is built from uranyl triperoxide

polyhedra. It is currently unclear why square−pentagonadjacencies are absent in these cluster graphs. In any case,the combination of topological squares, pentagons, andhexagons to form cage clusters of uranyl polyhedra results inan almost unlimited variety of potential cage clusters, muchgreater than the huge number of potential fullerene topologies.Cluster U18Pp2PCP6 is unusual because it contains peroxide,

hydroxyl, pyrophosphate, and methylenediphosphonate bridgesbetween uranyl ions (Figure 8c,d).89 There are two structuralunits that are built from uranyl polyhedra, one with eighthexagonal bipyramids that are linked into two five-memberedrings and the other with ten hexagonal bipyramids linked intoan open ring through shared peroxide equatorial edges.Pyrophosphate groups are located inside this ten-memberedring, and six methylenediphosphonate groups link the two unitsof 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 Itwas also one of the first three that were reported in the initialmanuscript in 2005. It is built from uranyl diperoxidepolyhedra, such that uranyl ions are bridged through eitherperoxide groups or pairs of hydroxyl groups. The topologyconsists of six squares and eight hexagons, 24:4668, arrangedsuch that each square shares edges with four different hexagonsand each hexagon shares three edges with other hexagons andthree 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 wassynthesized from an aqueous solution under ambientconditions with Li provided as a counterion. Addition ofNp(V) and peroxide to the aqueous solution resulted in blackcrystals of Np24 in good yield. The structural analysis indicatedmost of the Np in the cluster is hexavalent, but Np−O bondlengths suggested Np(V) is also present. A mixture of oxidationstates could help to explain the unusually dark color of thecrystals.U24Pp12 and its methylenediphosphonate (PCP) analogue

are topologically identical to U24 (Figure 8n).89 These clusters

contain pyrophosphate or methylenediphosphonate bridgesbetween uranyl ions in place of the bridges through twohydroxyl groups that are in U24. There are six four-memberedrings of uranyl polyhedra in the cage cluster. Four of these areconcave toward the center, and the other two are concavetoward the outside. This lowers the overall symmetry relative tothat of U24.Cluster U26Pp11 (Figure 8q,r)

89 has two distinct units that arebuilt from uranyl hexagonal bipyramids, with these two unitslinked through pyrophosphate groups that bridge betweenuranyl ions. The uranyl polyhedra are both triperoxide anddiperoxide hexagonal bipyramids; where the uranyl ion is onlycoordinated by two peroxide groups, it is also coordinated by aside-on pyrophosphate group. Its topological graph has thesymbol 26:4551062.U28a (C3v) is built from 24 uranyl diperoxide and 4

triperoxide hexagonal bipyramids (Figure 8u,v). Its topologydeparts significantly from others in the family in that there is nocenter of symmetry in the cluster.97 It is egg-shaped, with thebottom consisting of hexagons and squares in a configurationsimilar to that in U24 and the top consisting only of pentagons.It is also an unusual cluster because a SO4

2− tetrahedron isencapsulated 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



combined into four-, five-, and six-membered rings (Figure8y,z).97 The cluster is unusual in that it is one of only threereported that lack a center of symmetry. Its topological graph,30:425867, contains a base that consists of a single hexagon thatis surrounded by six pentagons. The upper portions of thecluster consist of two squares and two hexagons, and the topand bottom are fused into the cage through four hexagons andtwo pentagons.Cluster U30a is built from uranyl diperoxide hexagonal

bipyramids (Figure 8aa,ab).98 It is the only such uranylperoxide cage cluster reported that contains a 5-fold rotationalsymmetry axis. Its topology, with graph symbol 30:4552610, isdominated by hexagons that surround isolated pentagons onthe 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 (Figure8ai,aj).87

Cluster U32Pp16 is a rare example of a uranyl peroxidepyrophosphate cage cluster that is not a topological derivativeof a known cluster built only from uranyl polyhedra (Figure8ak,al).89 Four-membered rings of uranyl diperoxide hexagonalbipyramids dominate its structure, and these are linked by six-membered rings of polyhedra. Its graph contains topologicalsquares, hexagons, and two octagons: 32:486882. The cluster isunusual in that it is rather flattened, with its outer long andshort dimensions being 28.2 and 18.0 Å, respectively. The cagealso has pores of highly differing sizes. The largest of these,which correspond to the topological octagons, has a freeopening 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 inFigure 8ap, is identical to the top of U30. The bottom half ofU36a is the mirror image of the top, but rotated 90° so that thepentagons 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 uranyldiperoxide polyhedra, and bridges between uranyl ions arethrough either peroxide or hydroxyl groups. The correspondingtopological graph contains 2 squares, 8 pentagons, and 12hexagons: 40:4258612. Two ends of the graph correspond to thatof the U16 open cluster described above. These two ends areseparated through hexagons and pairs of pentagons.Cluster U42 (D3h) is built from uranyl diperoxide hexagonal

bipyramids that are arranged into four-, five-, and six-memberedrings (Figure 8ax,ay).98 The graph has 3 squares, 6 pentagons,and 14 hexagons, with graph symbol 42:4356614. K and Licounterions are located within the cage, with K below thetopological pentagons and Li below the topological squares.Cluster U42Py3 is a topological derivative of U42, in which threeof the shared hydroxyl−hydroxyl edges of U42 have beenreplaced by pyrophosphate units that bridge uranyl ions (Figure8az).Cluster U44a (D2v) is built from uranyl diperoxide polyhedra

that form four-, five-, and six-membered rings (Figure8ba,bb).97 The elongated cluster has a maximum dimensionof 31.3 Å, as measured from the outer edges of boundingoxygen 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 ofpolyhedra, and these are linked through distorted six- and eight-

membered rings of polyhedra. The corresponding graphsymbol is 44:4861482.

7.5.4. Miscellaneous Cage Clusters. Cluster U38Pp10Nt4has two distinct but symmetrically identical uranyl peroxidepyrophosphate “lobes” that are linked through nitrate groups(Figure 8au).113 Uranyl ions are bridged by nitrate, hydroxyl,peroxide, and pyrophosphate groups. It was created bycombining two solutions that had already crystallized otheruranyl pyrophosphate clusters. This combination of solutionsresulted in one with an unusually high nitrate concentrationand potentially also in the commingling of cluster fragmentsthat had already formed in the earlier solutions. The uranylpyrophosphate lobes that occur in U38Pp10Nt4 are similar tocluster U20Pp6b, except that some of the peroxide bridges inU20Pp6b correspond to hydroxyl bridges in U38Pp10Nt4. Theauthors of the study hypothesized that this difference preventedthe lobes in U38Pp10Nt4 from growing into complete cages andthat they were instead bridged through nitrate groups.113

Cluster U45Pp23 is highly unusual in several respects (Figure8be,bf).90 The most striking of these is that this cluster both hasan odd number of uranyl polyhedra and lacks any symmetry,both attributes being unique. In addition, although most of theuranyl ions contained within the cluster are in hexagonalbipyramids, the cluster contains the first and only occurrence ofa uranyl ion in a pentagonal bipyramidal polyhedron in which asingle equatorial edge corresponds to a peroxide group. Most ofthe pyrophosphate groups bridge two uranyl ions in the typicalside-on fashion, but some bridge three uranyl ions by beingbidentate to one and monodentate to two.Cluster U45Pp23 crystallizes from solution within 24 h of

mixing uranyl nitrate and hydrogen peroxide, and if the systemis left undisturbed, crystals of U32Pp16 form after about a weekand coexist with those of U45Pp23.

90 The authors of the studyconcluded that either formation of U45Pp23 was kineticallyfavored, such that its concentration rose to the point ofcrystallization, or it has an unusually low aqueous solubility,which caused it to crystallize even if it was not the dominantspecies in solution.90

7.6. Role of the Solution pH

Many of the open and cage clusters built from uranyl peroxidepolyhedra, and including bridges such as pyrophosphate andoxalate, are gathered in Figure 9, where they are arrangedaccording to the solution pH they crystallized from and thetotal number of uranium polyhedra in the cluster. The pKavalues for oxalate are 1.12 and 4.19, for pyrophosphate they are0.85, 1.49, 5.77, and 8.22, and for hydrogen peroxide the valueis 11.62. In the published structures all of these species werereported to be completely deprotonated. However, resolutionof H atom positions in such structures using X-ray diffraction isimpossible because of the dominance of U in scattering the X-rays and the overall complexity of the structures. Whereasbond-valence arguments clearly indicate that the peroxide andoxalate in these clusters are deprotonated, there is uncertaintyin the case of the pyrophosphate bridges, each of which havetwo terminal O atoms that could in some cases be protonated.The synthesis details for the various clusters shown in Figure 8often contain more than one counterion, and various acids andbases were used to adjust the pH. Furthermore, the synthesisconditions were initially dynamic, with the decomposition ofperoxide and bubbling of oxygen from solution. Evaporation inair was used to encourage crystallization. Insight into thespeciation in solution is provided by the crystallized cluster, but



other clusters were also likely present, as shown by SAXS andelectrospray ionization mass spectrometry (ESI-MS) in somecases.87,89 Given that the potential applications of such clustersreside in the solution realm, the complexity of the systemscannot be ignored. It is, however, currently prudent to discussthe importance of pH in the formation and crystallization ofspecific clusters only in a general sense.The notable exception to the uncertainties due to dynamic

synthesis conditions described in the preceding paragraph is thework of Nyman,111,115 who focused on different syntheticroutes to the U28 cluster. In this study, reactants are combinedin aqueous solution maintained at 5 °C in an ice bath and ayellow precipitate that forms is collected and dissolved in waterthat is subsequently maintained at 8 °C until crystals containingU28 with various encapsulated cations are recovered. Thisapproach provided yields in the range of 52−74% based onuranium.Clusters that are built only from uranyl ions that are bridged

through peroxide or hydroxyl groups form in general underalkaline conditions. Cluster U44 formed from a solution withpH 6.7, U42 formed from a solution with pH 7.9, and all theother 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 apH of at least 10.5. In contrast, the largest cluster with 60uranyl 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 thegeometries of the bridge in the many cage clusters reviewedhere. As such, if all uranyl ions are bridged only throughperoxide groups to form cage clusters, the angle limits the sizeof the cluster because of the curvature required. Incorporationof bridges between uranyl ions that are two shared hydroxylgroups should foster formation of larger clusters, such as U50

and U60, because the dihedral angles for U−(OH)2−U bridgesare pliable and ideally ∼180°, as shown by the DFTsimulations.109

Figure 9 also shows that cage clusters of uranyl ions withpyrophosphate bridges form from solutions with a large rangeof solution pH, from 4 to 10.8. The highest number of uranylions in a pyrophosphate-bearing cluster is 42, although thiscluster contains only 3 pyrophosphate groups. The U32Pp16cage cluster has the most uranyl ions of any containing at leastsix pyrophosphate groups. Owing to the lack of clusterscontaining pyrophosphate and more than 42 uranyl ions, and tothe broad range of solution pH in which such clusters form, theleft side of Figure 9 is dominated by uranyl pyrophosphate cageclusters.In five clusters uranyl ions are bridged through oxalate

groups. These all form from solutions prepared using oxalicacid, with a maximum pH of 6.5. In contrast to the uranylpyrophosphate clusters, which contained relatively few uranylions, uranyl oxalate clusters contain at least 30 uranyl ions and 3contain more than 50. As such, these clusters dominate acrossthe bottom of Figure 9.

7.7. Stability and Electrochemistry of Uranyl Peroxide CageClusters

A variety of studies have shown that cage clusters built fromuranyl peroxide polyhedra can be harvested from solutions aftermonths or even years of aging.97,98 Although such observationsclearly demonstrate persistence of these cage clusters, thesubject 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 wereharvested from their mother solution, followed by washing anddissolution of the crystals in ultrapure water. ESI-MS datacollected for solutions extracted and diluted from this bulksolution showed peaks that are characteristic of the cluster.These persisted through 290 days, at which time the studyended. This result is surprising because the water contained nofree peroxide at the onset of crystal dissolution, and the ESI-MSdata indicated little change occurred over the course of almost ayear.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 theresulting compound, together with a series of thermochemicalcycles, permitted determination of the heat of formation of thecompound under standard conditions and evaluation of varioushypothetical reactions. The study concluded that U60 isthermodynamically stable and kinetically persistent in theabsence of free peroxide. On the basis of this study, it wasnoted that such clusters could form and persist in solutions incontact with damaged nuclear fuel after a reactor accident.117

The electrochemical behavior of the U28 cluster has beenexamined.115 This is one of the few clusters in which eachuranyl ion is coordinated by three bidentate peroxide groupsand no hydroxide groups. Initially yellow solutions of U28 wereirreversibly reduced, corresponding to about 81 electrons percluster. Upon reduction the solution became colorless. Theauthors attributed the electrochemical behavior of U28 to eithera two-electron reduction of each of the peroxide ligands or acombination of reduction of U(VI) to U(V) and reduction of28 of the 42 peroxide ligands contained in the cluster. The loss

Figure 9. Relationship among the solution pH, composition, andnumber 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 fromuranyl peroxide polyhedra, respectively. See Table 2 for the legend ofthe figures. Pp = pyrophosphate, Ox = oxalate, Nt = nitrate, P =phosphate, and PCP = methylenediphosphonate.



of color is most consistent with the latter scenario, and it wasconcluded that the clusters fragmented during the experiment.The study of U28

115 is the only electrochemical experimentthat has been published for a uranyl peroxide cluster. There aremany opportunities for additional electrochemical studies ofuranyl peroxide clusters that could probe the behavior of a widerange of cluster topologies that differ in their counterions anduranyl bridges. It will be particularly interesting to see if theelectrochemical behavior of clusters containing less peroxide,specifically those incorporating hydroxide, oxalate, or pyro-phosphate bridges, will be similarly succesptible to fragmenta-tion.

7.8. Isomer Selection in Uranyl Peroxide Cage Clusters

As discussed above and shown in Figure 8, with only a coupleof exceptions, cage clusters built from uranyl peroxidepolyhedra can be represented by graphs in which each vertexis three-connected. The number of possible three-connectedgraphs for a given number of vertexes grows rapidly with thenumber of vertexes. For example, considering only fullerenetopologies with 60 vertexes, there are 1812 possible graphs.Three studies have discussed topology selection in the case ofuranyl cage clusters.95,97,98 These studies argue that highsymmetry 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, andcorrespondingly the number of topologies, is small. It becomescompelling by the point where the total vertexes is 60, and theU60 cluster adopts the highest symmetry fullerene topology ofthe possible 1812 choices.95 This is the same fullerene topologythat is adopted by C60, but for different reasons. The C60topology is favored because there are no adjacent pentagons inthe topology, which are unfavorable because they produceexcessive curvature locally and reduce bonding orbital overlaps.Where uranyl polyhedra are linked into a cage cluster with 60vertexes, high symmetry is favored. There is only one fullerenetopology with 60 vertexes that has no pentagonal adjacencies,and it also has the highest symmetry in the pool of 1812topologies.In contrast to the case of U60, where the fullerene topology

has 44 vertexes, adjacent pentagons are a topological require-ment. The topology with the lowest number of adjacentpentagons does not have the highest symmetry of the 44-vertexpool, and the U44 cluster with a fullerene topology has highersymmetry, rather than fewer topological pentagonal adjacen-cies.95

Inclusion of combinations of topological squares, pentagons,and hexagons in graphs increases the number of possibletopologies dramatically relative to that of fullerene topologies.The large number of possible topologies complicates evaluationof selection criteria. In the case of a 30-vertex graph there are227 graphs that contain at least 1 square and any number ofpentagons and hexagons.98 Two clusters of uranyl peroxidepolyhedra have been reported that have 30 vertexes, and thesecorrespond to 2 different graphs. The avoidance of square−square and square−pentagon adjacencies appears to be a factorin 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, andthese 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

number of pentagons and hexagons.98 Thirty-nine of theseinclude three squares and have no square−square or square−pentagon adjacencies. The graph adopted by U42 and U42Pp3has 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 threesquares. Cluster U28a adopts the highest symmetry isomer ofthese 50, and it is also the highest symmetry isomer of the 151that contain squares.97

There are 198 three-connected graphs that contain 36vertexes and 4 squares. Cluster U36a corresponds to the highestsymmetry graph of these 198 graphs.97

In this discussion of the role of symmetry in topologyselection, one should not ignore U45Pp23, which has nosymmetry.90 It is discussed above, where it is suggested that thisunusual cluster is kinetically favored during the synthesisexperiment.In summary, for a given number of vertexes, cage clusters of

uranyl peroxide polyhedra exhibit a strong tendency to avoidsquare−square and square−pentagon adjacencies and to adopthigh-symmetry topologies. The number of polyhedra in thecluster depends on other factors, as discussed below.

7.9. Factors Impacting the Size of Uranyl Peroxide CageClusters

Restricting the discussion to cage clusters built solely of uranylperoxide polyhedra with a single shell, clusters have beensynthesized with from 20 to 60 uranyl polyhedra. The smallest,U20, has a diameter of 18.0 Å,94 as measured from the outeredges of the bounding O atoms. The corresponding diameter ofU60 is 27.0 Å.95 In the case of U20, all of the uranyl ions arebridged through peroxide groups, and the average dihedralangle of the bridge is 140.4°.94 In U60 uranyl ions are bridgedby both peroxide groups and pairs of hydroxyl groups. Theaverage dihedral angle of the bridges is 154.8°.95 The averagedihedral angle of the bridges between uranyl ions is an essentialfactor related to the curvature of the cage wall and thereforealso the number of uranyl ions contained in the cluster. Thelarger dihedral angles arise from an averaging over peroxide andhydroxyl bridges.DFT simulations have given considerable insight into the

factors that determine cluster size.108−110 The role of thecounterion included in the synthesis reaction is important indetermining 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 consid-erably.108 Specifically, Li as a counterion strongly favors four-membered rings of uranyl polyhedra (topological squares), Na,K, and Rb favor five-membered rings (pentagons), and Csfavors six-membered rings (hexagons).108,110 However, thedegree to which the complexation energies differ for a givencation and the different macrocycles varies considerably. In thecase of Li, complexation with the four-membered ring gives−19.2 kcal·mol−1, with five-membered rings gives −10.4kcal·mol−1, and with six-membered rings gives −8.9 kcal·mol−1,assuming that the cation is located at the center of the ringcorresponding to the O atoms of the uranyl ions.108 In contrast,the complexation energies for K and the different macrocyclesconsidered only range from −14.9 to −18.8 kcal·mol−1.108



8. TRANSITION-METAL-BASED ACTINIDE-BEARINGCLUSTERS

Actinide cations, including transuranium elements, have beenintroduced as addenda metals in transition-metal polyoxome-talates. Interest in these clusters derives from their potentialapplications in separation cycles, as well as the uniquepossibilities they provide for probing the role of 5f electronsin clusters. Several studies reported clusters containing a singleactinide cation that bridges between two transition-metalpolyoxometalate clusters or fragments.118−122 Additionalstudies have examined transition-metal polyoxometalateclusters bridged through two123−129 or three130−134 actinidecations.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

s y n t h e s i z e d b y t h e r e a c t i o n o f ( N n B u 4 ) -[Mn4O2(O2CPh)9(H2O)] with Th(NO3)4(H2O)3 in MeCN/MeOH under aerobic conditions. The core of this cluster hasthe composition [Th6Mn10O22(OH)2]

18 (Figure 10b). In totalthere are 6 Th(IV) and 10 Mn(IV) cations, and these are linkedthrough 4 μ4-O, 16 μ3-O, 2 μ2-O, and 2 μ2-OH bridges. Two ofthe μ4-O atoms bridge between three Th(IV) cations and oneMn(IV) cation , whereas the other two bridge two Th(IV) andtwo Mn(IV) cations. All Th(IV) cations are eight-coordinated,and Mn(IV) cations are octahedrally coordinated. Thecoordination spheres of the cations in the core of the clusterare completed by O atoms of the organic ligand, a nitrategroup, and H2O groups, which passivate the surfaces of thecluster.

The crystallization and characterization of a cluster thatcombines a P6W36 polyoxometalate with two four-memberedrings of uranyl peroxide polyhedra provides an importantintermediate between traditional transition-metal polyoxome-talates and uranyl peroxide clusters (Figure 10c).137 The clustercontains eight uranyl ions that are in hexagonal bipyramidalcoordination and was synthesized at pH 4. Each is coordinatedby two peroxide groups located along cis equatorial edges of thebipyramids and two additional O atoms that are also bonded toW cations. Uranyl ions are bridged through peroxide groups toform four-membered rings in a fashion analogous to that of thenumerous uranyl peroxide cage clusters discussed above. Twosuch four-membered rings of uranyl polyhedra are enclosed inthe curved transition-metal polyoxometalate. As such, thiscluster represents a novel hybrid in which it is surface-passivated by yl O atoms on both transition metals andactinides.The compound Na32[(UO2)12(μ3-O)4(μ2-H2O)12(P2W15-

O56)4]·77H2O contains the cluster [(UO2)12(μ3-O)4(μ2-H2O)12(P2W15O56)4]

32− shown in Figure 10d.138 It consistsof four P2W15O56 lobes that are connected through fourunusual trimers of uranyl pentagonal bipyramids. In thesetrimers three uranyl ions share a μ3-O and pairs of uranyl ionsare bridged through three μ2-H2O groups. The remainder ofthe equatorial ligands of the pentagonal bipyramids areprovided by the P2W15O56 units.A combined experimental and computational study provided

a novel wheel-shaped mixed tungsten uranium polyoxometa-late, {[W5O21]3[(UO2)2(μ2-O)]3}

30− (Figure 10e).139 Thiscluster contains two distinct building units. The first consistsof pairs of uranyl ions that are bridged by bidentate peroxidegroups and that are coordinated by additional O atoms toproduce a dimer of uranyl hexagonal bipyramids. There arethree such dimers in the cluster, and they are arranged aboutthe equatorial plane of the overall wheel-shaped cluster. Thesecond building unit consists of five Mo(VI) cations, each ofwhich is coordinated by either six O atoms in an octahedralarrangement or five in a square pyramidal arrangement. Fouroctahedra and one square bipyramid are linked to produce thebuilding unit with composition [W5O21]

12−, and there are threesuch units in the cluster. The tungstate and uranyl buildingunits are linked into the larger cluster through the sharing ofpolyhedral edges and vertexes. DFT simulations reproduced thegeometry of the cluster and provided a classical polyoxometa-late electronic structure containing metal and oxo bands. Thecalculations also provided the sites for likely protonation of Oatoms of the cluster, as the X-ray study did not produce H atompositions.

9. SUMMARY AND DISCUSSIONStabilization of finite clusters of metal cations and oxygenrequires surface passivation. This can be achieved either bytruncating the cluster with appropriate organic ligands, or in afew cases simple inorganic ions, or by incorporation of cationswith yl oxygens that terminate the clusters. Both of theseapproaches have been applied in the case of transition metals,although truncation via yl O atoms is the most developed andcorresponds to the extensive family of polyoxometalates. Bothapproaches are also evident for actinides, in a few cases incombination 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 inFigure 2 contain actinide(IV) oxide cores that are truncated

Figure 10. Transition-metal-based clusters containing actinides. (a, b)[Th6Mn10O22(OH)2(O2CPh)16(NO3)2(H2O)8]·10MeCN (Figure8a,b). (c) K6Li19[Li(H2O)K4(H2O)3{(UO2)4(O2)4(H2O)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.



into finite clusters by capping organic ligands. The principleexceptions are Th(IV) clusters that are bridged by selenitetetrahedra (Figure 2h) and Pu(IV) clusters that are terminatedby H2O groups and Cl anions (Figure 2r). Many of theseAn(IV) clusters have An−O connectivities similar to those ofthe fluorite structure, and these may be important species inAn(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 byyl oxygens are polyoxometalates and have some similarities tothose of the transition metals. However, unlike transitionmetals, actinyl ions have two yl O atoms in a transconfiguration. As such, actinyl polyhedra tend to assembleinto cage clusters in which both the inner and outer surfaces aretruncated by yl oxygen atoms. Assembly of such cage clustersrequires curvature of the walls, and to date they have only beenfound for actinyl peroxide polyhedra. It is the peroxide bridgebetween the actinyl ions that fosters the curvature needed toform cage clusters. In the absence of peroxide, actinyl polyhedratend not to form clusters, but rather crystallize into extendedstructures that are dominated by sheets of polyhedra. The mostnotable exception is the series of U(V)-based tetramericclusters in which cation−cation interactions bridge U(V) andthe clusters are passivated by various organic molecules (Figure4a,b). A few clusters are surface-passivated by both the yl Oatoms 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 membersof the uranyl peroxide family. Many of the clusters containingAn(IV) or An(V) cations were synthesized in nominallyoxygen-free atmospheres, whereas all of the uranyl peroxideclusters self-assemble in aqueous solution in the presence of air.Given the ease of their synthesis, and their stability underambient conditions, the uranyl peroxide clusters may have themost 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 fuelcycle. The uranyl peroxide clusters spontaneously self-assembleunder ambient conditions in alkaline water and hydrogenperoxide. Several studies have emphasized the feasibility of analkaline-based system for used nuclear fuel reprocessing, incontrast to plutonium−uranium extraction (PUREX), which isan acidic process.140−145 Used uranium dioxide fuel dissolves inalkaline water in the presence of peroxide. Although detailedstudies of the speciation of uranium in solution followingdissolution of used fuel, or a surrogate, are currently lacking, theconditions are appropriate for the rapid self-assembly of solubleuranyl peroxide cage clusters. Realizing this, it should bepossible to develop an approach to separating the uranium fromthe bulk solution on the basis of the size, mass, or uniquechemical properties of the cage clusters. Such an approach mayalso be appropriate for purification of uranium at the front endof the fuel cycle.A second potential application of actinide oxide clusters in

the nuclear fuel cycle is in the fabrication of nanocompositematerials. Although this area is entirely undeveloped, weimagine that the controlled deposition of actinide oxide clustersonto various substrates could contribute to development ofnovel fuel types. A very recent study has documented thedeposition of plutonium(IV) oxide clusters onto the surface ofmica.146

With a few notable exceptions, studies of actinide oxideclusters have emphasized their synthesis and structuralcharacterization more so than the details of their propertiesand potential applications. These studies have established afamily of clusters that form the basis for future studies ofproperties and applications. Taken together, the family ofactinide oxide clusters reported to date demonstrates that theircomposition and size can be tailored by controlled synthesistechniques. For example, in the case of clusters built from theuranyl ion, although peroxide is an essential bridge to fostercurvature, other bridges between uranyl ions can beincorporated and the size of the cluster is impacted by thecounterion used.There are many unexplored aspects of the unique family of

uranyl peroxide cage clusters that have been synthesized overthe past several years. It now seems likely that the peroxidebridge between uranyl ions is essential for the formation ofthese clusters. Symmetry is important in topology selection,and counterions are important for determining the size of thecluster. However, little is known about the mechanisms of self-assembly of the clusters, the diversity of clusters that may formunder specific conditions, or the relative stabilities and aqueoussolubilities of the many clusters. The electrochemical behaviorof uranyl peroxide clusters is mostly unstudied, as is theimportance of ion pairing in solution. The aggregation behaviorof 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 includestudies of uranyl peroxide cage clusters108−110 and thoriumoxide clusters.39 We expect the role of computations tocontinue to grow in studies of actinide clusters, withsimulations beginning to guide experimental approaches tocreate tailored clusters with specific properties.147 Thecontribution of computations will be particularly important instudies of transuranium systems, where the expense ofexperiments is very high.Actinide oxide clusters provide an abundance of future

research directions. At the most fundamental level, they can beexpected to provide unique insights into the complex behaviorof elements in which the 5f orbitals are important contributorsto their chemistry. Largely unexplored areas for future studiesinclude their expected complex redox behavior, their relativethermodynamic stabilities, the role of actinide oxide clusters intransport 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 subsetof the total corresponding to neptunium or plutonium. The fewclusters that are built from neptunium for which there is datasuggest they are similar in structure to those containinguranium, but the dominance of the pentavalent oxidation stateof Np in aqueous solution is likely to result in some divergencefrom U(VI). Specifically, the bonds within the Np(V) neptunylion are weaker than those in the U(VI) uranyl ion; thus, the ylO atoms of the Np(V) neptunyl ion are more reactive, andcation−cation interactions are much more common than forU(VI).148,149 The propensity of cation−cation interactions inNp(V) systems could even prevent the formation of cageclusters analogous to the uranyl peroxide system because the ylO atoms may participate in additional bonding, rather thanpassivating the surface of the cluster. The single neptunylperoxide cluster that has been reported to date, Np24, appears



to be built from mostly Np(VI) but also may contain someNp(V). It is topologically identical to U24, which only containsU(VI). As Pu is reduced by peroxide in aqueous systems, it isunlikely that plutonyl peroxide clusters will form underconditions similar to those used to develop the family ofuranyl peroxide cage clusters. It is possible that clusters ofNp(IV) and Pu(IV) will follow the generalities revealed forTh(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 areknown that are based upon the fluorite structure type. Noclusters have been reported for Pu(V) or Pu(VI). There arealso no reported clusters of An(III) cations bridged throughoxygen, although lanthanide(III) clusters are known150−153 andmay provide some insight into possibilities for the actinides.

AUTHOR INFORMATIONCorresponding Author

*E-mail: [email protected]

The authors declare no competing financial interest.

Biographies

Jie Qiu received her B.S. in Chemistry from Shandong NormalUniversity in 2004 and Ph.D. degree in Polymer Chemistry andPhysics 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 growthmechanism of uranyl peroxide nanoclusters.

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

postdoctoral appointments at the University of Cambridge and theUniversity of New Mexico and one year on the faculty at theUniversity of Illinois, he joined the faculty of the University of NotreDame in Indiana in 1997. He is currently Massman Professor of Civiland Environmental Engineering and Earth Sciences and ConcurrentProfessor of Chemistry and Biochemistry. He is also Director of theU.S. Department of Energy’s Energy Frontier Research CenterMaterials Science of Actinides, which was created in 2009.

ACKNOWLEDGMENTSThis material is based upon work supported as part of theMaterials Science of Actinides Center, an Energy FrontierResearch Center funded by the U.S. Department of Energy,Office of Science, Office of Basic Energy Sciences, under AwardNo. DE-SC0001089.

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clusters of actinides with oxide, peroxide, or hydroxide bridges

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