Porous Uranyl Borophosphates with Unique Three-Dimensional Open-Framework Structures

Abstract

Two novel alkali-metal uranyl borophosphates have been prepared and characterized for the first time, namely, K₅(UO₂)₂[B₂P₃O₁₂(OH)]₂(OH)(H₂O)₂ and K₂(UO₂)₁₂[B(H₂PO₄)₄](PO₄)₈(OH)(H₂O)₆ denoted as KUPB1 and KUPB2, respectively. KUPB1 was obtained hydrothermally at 220 °C and crystallizes in a monoclinic structure in the chiral space group P2₁. The unit cell parameters of KUPB1 are a = 6.7623(2) Å, b = 19.5584(7) Å, c = 11.0110(4) Å, α = γ = 90°, β = 95.579(3)°, and V = 1449.42(8) ų. It features a unique three-dimensional (3D) open-framework structure, composed of two corner-sharing linked one-dimensional (1D) anionic borophosphates (BP), [B₂P₃O₁₃]^5-, along the a axis and uranyl phosphate (UP), [(UO₂)(PO₄)₃]^7-, chains along the c axis, further bridged by PO₄ tetrahedra. Multi-intersectional channels can be observed within the structure, in which the largest 11-ring (11-R) tunnel size is ∼7.0 Å × 8.8 Å. Its simplified framework can be described as a new 4-nodal net topological type with a point symbol of {4.8⁴.10}{4².6}₂{4³.6².8³.10²}{8².10}. By modification of the synthetic conditions of KUPB1 through an increase in the amount of H₃BO₃ as flux 4-fold and a reduction of water as the reaction medium, the novel compound KUPB2 is generated. The unit cell parameters of KUPB2 are a = b = 21.8747(3) Å, c = 7.0652(2) Å, α = β = γ = 90°, and V = 3380.72(12) ų. KUPB2 crystallizes in a tetragonal structure in the polar space group I4̅2m, and its structure is based on a highly complex 3D framework, {(UO₂)₁₂[B(PO₄)₄](PO₄)₈}^9-, in which 1D 8-R UP [(UO₂)(PO₄)]^- tubes can be observed along the c axis. The [(UO₂)(PO₄)]^- tubes consist of three uranyl chains along the c axis, which are linked alternately by [PO₄]^3- tetrahedra. Those isolated 1D [(UO₂)(PO₄)]^- tubes are further bridged through [(UO₂)₄B(PO₄)₄]^- clusters, forming an exceptional 3D open-framework structure. Its simplified cation network is a new 5-nodal net topological type such as {3².4³.5.6².7.8}₈{3⁴.4⁵.5⁴.6²}₈{4.6².8³}₄{4².6}₄{4⁴.6²}. Their facile hydrothermal synthetic routes, porous structure topology, thermal stability, and Raman spectroscopy properties are reported and discussed.