Anion-Induced Structural Transformation of a Cage-Based Metal–Organic Framework

Cage-based metal–organic frameworks (MOFs) have large pore spaces but small pore windows, endowing them with unique potential in the fields of gas adsorption, separation, and so on. Here, we successfully synthesized a series of isostructural caged-based MOFs with different counteranions, i.e., {[Cu₃(TPA)₄(BF₄)₆]_n·(solvent)_x} (Cu-TPA–BF₄), {[Cu₃(TPA)₄(ClO₄)₆]_n·(solvent)_x} (Cu-TPA-ClO₄), and {[Cu₃(TPA)₄(NO₃)₆]_n·(solvent)_x} (Cu-TPA-NO₃), which are formed by the coordination of tridentate nitrogen-containing ligands tri(pyridin-4-yl)amine (TPA) and copper(II) cations. Nevertheless, after placing Cu-TPA–BF₄ in a glass tube for a long period, a more stable violet crystal {[Cu₃(TPA)₄(SiF₆)₃]_n·(solvent)_x} (Cu-TPA-SiF₆) is generated, while no transformation is observed for Cu-TPA-ClO₄ and Cu-TPA-NO₃. Cu-TPA–BF₄ belongs to a tbo topology and possesses octahedral cages and two kinds of cuboctahedral cages, with the uncoordinated BF₄^– anions filling in the channels. In comparison, as a two-connected node, SiF₆^2– anions participate in the construction of the tetrahedral cages and icosahedral cages, resulting in conversion to Cu-TPA-SiF₆ with an ith-d topology. Further investigation indicates that the hydrolysis of BF₄^– anions and further reaction with SiO₂ will afford SiF₆^2– anions, which can be directly involved in the formation of Cu-TPA-SiF₆ to induce the transformation. Additionally, the stability and adsorption behaviors of Cu-TPA–BF₄ and Cu-TPA-SiF₆ are also investigated.