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, K(UO)BPO(OH)(HO) and K(UO)B(HPO)(OH)(HO) denoted as KUPB1 and KUPB2, respectively. KUPB1 was obtained hydrothermally at 220 °C and crystallizes in a monoclinic structure in the chiral space group P2. 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) Å. It features a unique three-dimensional (3D) open-framework structure, composed of two corner-sharing linked one-dimensional (1D) anionic borophosphates (BP), [BPO], along the a axis and uranyl phosphate (UP), [(UO)(PO)], chains along the c axis, further bridged by PO 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 {4.8.10}{4.6}{4.6.8.10}{8.10}. By modification of the synthetic conditions of KUPB1 through an increase in the amount of HBO 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) Å. KUPB2 crystallizes in a tetragonal structure in the polar space group I4̅2m, and its structure is based on a highly complex 3D framework, {(UO)B(PO)}, in which 1D 8-R UP [(UO)(PO)] tubes can be observed along the c axis. The [(UO)(PO)] tubes consist of three uranyl chains along the c axis, which are linked alternately by [PO] tetrahedra. Those isolated 1D [(UO)(PO)] tubes are further bridged through [(UO)B(PO)] clusters, forming an exceptional 3D open-framework structure. Its simplified cation network is a new 5-nodal net topological type such as {3.4.5.6.7.8}{3.4.5.6}{4.6.8}{4.6}{4.6}. Their facile hydrothermal synthetic routes, porous structure topology, thermal stability, and Raman spectroscopy properties are reported and discussed.