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 UO_x (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.