Crystal growth of novel 3D skeleton uranyl germanium complexes: influence of synthetic conditions on crystal structures†
Five centrosymmetric uranyl germanate compounds, K8BrF(UO2)3(Ge2O7)2, Rb6(UO2)3(Ge2O7)2·0.5H2O, Cs6(UO2)2Ge8O21 and A+2(UO2)3(GeO4)2 (A+ = Rb+, Cs+), were synthesized in this work. K8BrF(UO2)3(Ge2O7)2 and Rb6(UO2)3(Ge2O7)2·0.5H2O were obtained under mixed KF–KBr flux and hydrothermal conditions, respectively. Both structures crystallized in the triclinic P space group and have similar anionic frameworks featuring novel hexagon shaped 12-membered channels. The condensation of two different types of SBU [UGe4] pentamers (A) and (A2) results in the formation of K8BrF(UO2)3(Ge2O7)2 and Rb6(UO2)3(Ge2O7)2·0.5H2O frameworks. Cs6(UO2)2Ge8O21 was obtained from a CsF–CsCl high temperature flux, and it also crystallized in the centrosymmetric triclinic P space group. The structure of Cs6(UO2)2Ge8O21 has a novel oxo-germanate layer composed of germanate tetrahedra and trigonal bipyramids. Two new SBU types, (42·52-A2) and (54-A2) [UGe4] pentamers, were found in the structure of Cs6(UO2)2Ge8O21. A+2(UO2)3(GeO4)2 (A+ = Rb+, Cs+) were synthesized by a high temperature/high pressure (HT/HP) technique, and both structures with oval-shaped 12-membered channels crystallized in the centrosymmetric orthorhombic Pnma space group. The extreme conditions led to the formation of [U2Ge2] tetramers (E), which consist of 7-coordinated U and 5-coordinated Ge. Different synthetic methods of uranyl germanate compounds resulted in a distinct coordination environment of the uranyl cations and a variety of UO and U−O bond lengths, further affecting the dimensionality and types of uranyl units and SBUs. The Raman and IR spectra of the five new phases were collected and analyzed.