Polyoxometalates DOI: 10.1002/anie.200805870 Symmetry versus Minimal Pentagonal Adjacencies in Uranium-Based Polyoxometalate Fullerene Topologies** Ginger E. Sigmon, Daniel K. Unruh, Jie Ling, Brittany Weaver, Matthew Ward, Laura Pressprich, Antonio Simonetti, and Peter C. Burns* Polyoxometalates are centuries old, [1] and Keggin established the first structure of a heteropolyoxometalate compound in 1933. [2] Actinides have recently been incorporated into polyoxometalates of transition metals, [3–7] but only three studies report actinide-based polyoxometalate clusters. [8–10] Yet actinides are ideal candidates for self-assembly into clusters that may possess interesting properties associated with the complexity of the f electrons, and such clusters could be useful in the nuclear fuel cycle. Duval et al. [9] reported a cluster with a polyoxometalate core containing six uranium atoms, and we recently described clusters of uranyl peroxide hydroxide isopolyhedra, consisting of 24, 28, 32, [8] 40, and 50 [10] polyhedra. Uranyl peroxide clusters self-assemble in alkaline solutions under ambient conditions and provide a glimpse into a potentially large and complex family of polyoxometalates. Clusters with fullerene topologies such as C 60 exhibit striking symmetric beauty and important properties. [11] Ful- lerenes consist of 12 topological pentagons and at least two hexagons. Topological pentagons placed in a sheet of hexagons create curvature, with twelve such units required to close the topology into a cluster. [12] Over the past decade, analogues of C 60 fullerene have been postulated, such as fullerene-like molecules of silicon atoms and mixtures of Group 13 and Group 15 elements, although confirmation by experiment is lacking. [13–15] Several inorganic fullerene-like cages have been synthesized using solid-state techniques. Zintl phases have produced a C 60 -like moiety in In 48 Na 12 , [16] and transition-metal phases with spherical fullerene-like molecules have been synthesized using iron and copper. [17] Superfullerene species have been reported that contain 132 molybdenum atoms. [18] Curvature in a fullerene structure relates to the distribu- tion of pentagons in its topology. [12] Adjacent pentagons increase local curvature. For carbon, increased curvature results in strain by decreasing orbital overlap. [12] No fullerene topology exists without adjacent pentagons and with less than 60 vertices. Only one fullerene structure with 60 vertices and no adjacent pentagons is possible, and it is adopted by C 60 because it results in the most favorable orbital overlap. In carbon fullerenes with less than 60 vertices, those with the minimum number of adjacent pentagons are expected to be the most stable. [19] The emerging structural complexities of uranyl peroxide clusters suggest the possibility of creating large fullerene- topology clusters with high stability. This prospect provides the impetus for a combinatorial approach to exploring the uranyl peroxide system. Subtle changes in growth conditions are known to result in different structures in this system. [8, 10] Clusters of composition [UO 2 (O 2 )(OH)] 60 60 (U60) formed when uranyl nitrate, hydrogen peroxide, potassium chloride, and lithium hydroxide were combined in aqueous solution at pH 9.0. Well-faceted millimeter-sized crystals with approximate composition Li 48+m K 12 (OH) m [UO 2 (O 2 )(OH)] 60 - (H 2 O) n (m  20 and n  310, see the Supporting Information) containing these clusters formed within seven days. Single- crystal X-ray diffraction gave the cubic space group Fm3 ¯ and resolved the atomic positions of the uranyl peroxide poly- hedra and potassium cations. The cluster is nearly spherical, with an outer diameter of 24.3 , measured from the centers of oxygen atoms on either side (Figure 1 a). U60 consists of sixty compositionally identical uranyl peroxide hydroxide polyhedra (Figure 1 a–c). Each has an Figure 1. Representations of the uranyl peroxide components of the structures of U60 (a–c), U36 (d–f), and U44 (g–i). Ball-and-stick (left) and polyhedral (center) representations are complemented by graphs showing the cluster topologies (right). [*] G. E. Sigmon, D. K. Unruh, J. Ling, B. Weaver, M. Ward, L. Pressprich, Prof. A. Simonetti, Prof. P. C. Burns Department of Civil Engineering and Geological Sciences University of Notre Dame, Notre Dame, IN 46556 (USA) E-mail: pburns@nd.edu Homepage: http://petercburns.com Prof. P. C. Burns Department of Chemistry and Biochemistry University of Notre Dame, Notre Dame, IN 46556 (USA) [**] This research was supported by the Chemical Sciences, Geo- sciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Grant No. DE-FG02- 07ER15880. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200805870. Angewandte Chemie 1 Angew. Chem. Int. Ed. 2009, 48,1–5  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü