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Periodic Trends in Hexanuclear Actinide Clusters

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posted on 02.04.2012, 00:00 by Juan Diwu, Shuao Wang, Thomas E. Albrecht-Schmitt
Four new Th­(IV), U­(IV), and Np­(IV) hexanuclear clusters with 1,2-phenylenediphosphonate as the bridging ligand have been prepared by self-assembly at room temperature. The structures of Th6Tl3[C6H4(PO3)­(PO3H)]6(NO3)7(H2O)6·(NO3)2·4H2O (Th6-3), (NH4)8.11­Np12Rb3.89[C6H4(PO3)­(PO3H)]12(NO3)24·15H2O (Np6-1), (NH4)4U12Cs8[C6H4(PO3)­(PO3H)]12(NO3)24·18H2O (U6-1), and (NH4)4­U12Cs2­[C6H4(PO3)­(PO3H)]12(NO3)18·40H2O (U6-2) are described and compared with other clusters of containing An­(IV) or Ce­(IV). All of the clusters share the common formula M6(H2O)m[C6H3(PO3)­(PO3H)]6(NO3)n(6–n) (M = Ce, Th, U, Np, Pu). The metal centers are normally nine-coordinate, with five oxygen atoms from the ligand and an additional four either occupied by NO3 or H2O. It was found that the Ce, U, and Pu clusters favor both C3i and Ci point groups, while Th only yields in Ci, and Np only C3i. In the C3i clusters, there are two NO3 anions bonded to the metal centers. In the Ci clusters, the number of NO3 anions varies from 0 to 2. The change in the ionic radius of the actinide ions tunes the cavity size of the clusters. The thorium clusters were found to accept larger ions including Cs+ and Tl+, whereas with uranium and later elements, only NH4+ and/or Rb+ reside in the center of the clusters.