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>i.e.</i>, {[Cu₃(TPA)₄(BF₄)₆]<i>_n</i>·(solvent)<i>_x</i>} (<b>Cu-TPA–BF</b>_<b>4</b>), {[Cu₃(TPA)₄(ClO₄)₆]<i>_n</i>·(solvent)<i>_x</i>} (<b>Cu-TPA-ClO</b>_<b>4</b>), and {[Cu₃(TPA)₄(NO₃)₆]<i>_n</i>·(solvent)<i>_x</i>} (<b>Cu-TPA-NO</b>_<b>3</b>), which are formed by the coordination of tridentate nitrogen-containing ligands tri(pyridin-4-yl)amine (TPA) and copper(II) cations. Nevertheless, after placing <b>Cu-TPA–BF</b>_<b>4</b> in a glass tube for a long period, a more stable violet crystal {[Cu₃(TPA)₄(SiF₆)₃]<i>_n</i>·(solvent)<i>_x</i>} (<b>Cu-TPA-SiF</b>_<b>6</b>) is generated, while no transformation is observed for <b>Cu-TPA-ClO</b>_<b>4</b> and <b>Cu-TPA-NO</b>_<b>3</b>. <b>Cu-TPA–BF</b>_<b>4</b> belongs to a <i>tbo</i> 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 <b>Cu-TPA-SiF</b>_<b>6</b> with an <i>ith-d</i> 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 <b>Cu-TPA-SiF</b>_<b>6</b> to induce the transformation. Additionally, the stability and adsorption behaviors of <b>Cu-TPA–BF</b>_<b>4</b> and <b>Cu-TPA-SiF</b>_<b>6</b> are also investigated.