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

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 <b>KUPB1</b> and <b>KUPB2</b>, respectively. <b>KUPB1</b> was obtained hydrothermally at 220 °C and crystallizes in a monoclinic structure in the chiral space group <i>P</i>2₁. The unit cell parameters of <b>KUPB1</b> are <i>a</i> = 6.7623(2) Å, <i>b</i> = 19.5584(7) Å, <i>c</i> = 11.0110(4) Å, α = γ = 90°, β = 95.579(3)°, and <i>V</i> = 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 <i>a</i> axis and uranyl phosphate (UP), [(UO₂)­(PO₄)₃]^7–, chains along the <i>c</i> 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 <b>KUPB1</b> through an increase in the amount of H₃BO₃ as flux 4-fold and a reduction of water as the reaction medium, the novel compound <b>KUPB2</b> is generated. The unit cell parameters of <b>KUPB2</b> are <i>a</i> = <i>b</i> = 21.8747(3) Å, <i>c</i> = 7.0652(2) Å, α = β = γ = 90°, and <i>V</i> = 3380.72(12) ų. <b>KUPB2</b> crystallizes in a tetragonal structure in the polar space group <i>I</i>4̅2<i>m</i>, 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 <i>c</i> axis. The [(UO₂)­(PO₄)]⁻ tubes consist of three uranyl chains along the <i>c</i> 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