Periodic Trends in Hexanuclear Actinide Clusters

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.