U(VI) Uranyl Cation−Cation Interactions in Framework Germanates

The isomorphous compounds NH₄[(UO₆)₂(UO₂)₉(GeO₄)(GeO₃(OH))] (1), K[(UO₆)₂(UO₂)₉(GeO₄)(GeO₃(OH))] (2), Li₃O[(UO₆)₂(UO₂)₉(GeO₄)(GeO₃(OH))] (3), and Ba[(UO₆)₂(UO₂)₉(GeO₄)₂] (4) were synthesized by hydrothermal reaction at 220 °C. The structures were determined using single crystal X-ray diffraction and refined to R₁ = 0.0349 (1), 0.0232 (2), 0.0236 (3), 0.0267 (4). Each are trigonal, P3̅1c. 1: a = 10.2525(5), c = 17.3972(13), V = 1583.69(16) ų, Z = 2; 2: a = 10.226(4), c = 17.150(9), V = 1553.1(12) ų, Z = 2; 3: a = 10.2668(5), c = 17.0558(11), V = 1556.94(15) ų, Z = 2; 4: a = 10.2012(5), c = 17.1570(12), V = 1546.23(15) ų, Z = 2. There are three symmetrically independent U sites in each structure, two of which correspond to typical (UO₂)²⁺ uranyl ions and the other of which is octahedrally coordinated by six O atoms. One of the uranyl ions donates a cation−cation interaction, and accepts a different cation−cation interaction. The linkages between the U-centered polyhedra result in a relatively dense three-dimensional framework. Ge and low-valence sites are located within cavities in the framework of U-polyhedra. Chemical, thermal, and spectroscopic characterizations are provided.