Two novel 3D borates: porous-layer and layer-pillar frameworks

Abstract

Two new borates, NaK_1.5[B{B₅O₉(OH)}{B₅O₈(OH)₂}_1/2] (1) and Na_3.5[Al{B₃O₆(OH)}{B₅O₁₀}_1/2{BO(OH)}] (2), have been made by a solvothermal method. Structure 1 is made of two types of oxoboron clusters ([B₅O₉(OH)]⁴⁻ and [B₅O₈(OH)₂]³⁻) and BO₄ tetrahedra to form a 3D porous layer, in which the [B₅O₉(OH)]⁴⁻ clusters are bridged by BO₄ groups to produce a 2D B–O layer with 11-membered ring (MR) windows, while the [B₅O₈(OH)₂]³⁻ clusters as 2-connected pillars join the adjacent B–O layers to make a 3-D porous layer. Structure 2 is built from two types of oxoboron clusters ([B₃O₆(OH)]⁴⁻ and [B₅O₁₂]⁹⁻), AlO₄ units and BO₂(OH) triangles to produce 3D pillar-layered aluminoborate (ABO), in which the [B₅O₁₂]⁹⁻ clusters are linked together to form a 2-D B–O layer; the [B₃O₆(OH)]⁴⁻ clusters and AlO₄ groups join each other to form 1D tubular ABO; and the BO₂(OH) triangles act as linkers to connect the adjacent B–O layer and the ABO tubes, resulting in 3D pillar-layered ABO. Interestingly, the ABO tubes decorated with the BO₂(OH) triangles act as pillars to make a 3D layer-pillared structure. UV-vis diffuse spectra indicate that both have cut-off edges below 190 nm, and the calculated band gaps are 5.75 and 5.78 eV, respectively.