Divergent Structural Chemistry of Uranyl Borates Obtained from Solid State and Hydrothermal Conditions

A series of novel uranyl borates, K4Sr4[(UO2)13(B2O5)2(BO3)2O12], A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs), and Rb3[(UO2)3(BO3)2O(OH)](H2O), were synthesized using conventional conditions. Among them, K4Sr4[(UO2)13(B2O5)2(BO3)2O12] and A6[(UO2)12(BO3)8O3](H2O)6 were obtained through a high-temperature solid-state reaction method, whereas Rb3[(UO2)3(BO3)2O(OH)](H2O) was synthesized via a hydrothermal reaction. All compounds adopt novel two-dimensional (2D) layered structures in which basic building units (BBUs) consist of corner- or edge-sharing UOx (x = 6, 7 and 8) polyhedra linked with planar BO3 triangles or B2O5 dimers. K4Sr4[(UO2)13(B2O5)2(BO3)2O12] is the first mixed alkali–alkaline earth metal uranyl borate. This compound has the most complex 2D anion topology observed thus far in 2D uranyl borates. The fundamental building block (FBB) in this structure, [(UO2)13(B2O5)2(BO3)2O12]12–, consists of 3 UO8 hexagonal bipyramids and 10 UO7 pentagonal bipyramids connected with 2 BO3 triangles and 2 B2O5 dimers. The FBB of Rb6[(UO2)12(BO3)8O3](H2O)6 is [(UO2)6(BO3)4O3]6–, comprised of five edge-sharing UO7 pentagonal bipyramids, one UO6 tetragonal bipyramid, and four BO3 triangles. The simplest FBB, [(UO2)3(BO3)2O2]4–, occurs in Rb3[(UO2)3(BO3)2O(OH)](H2O), where three UO7 pentagonal bipyramids are linked with two BO3 triangles via edge sharing. The aforementioned FBBs are further polymerized into the corresponding infinite uranyl borates layers. The synthetic methods, novel topologies, thermal stability, and spectroscopic properties of these compounds are reported herein