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 center dot(solvent)x} (Cu-TPA-BF4), {[Cu3(TPA)4(ClO4)6]n center dot(solvent)x} (Cu-TPA-ClO4), and {[Cu3(TPA)4(NO3)6]n center dot(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 center dot (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, SiF62- 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 SiF62- 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.