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

Abstract

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., { [ C u 3 ( T P A ) 4 ( B F 4 ) 6 ] n • ( s o l v e n t ) x } ( C u -T P A − B F 4 ) , {[Cu 3 (TPA) 4 (ClO 4 ) 6 ] n •(solvent) x } (Cu-TPA-ClO 4 ), and {[Cu 3 (TPA) 4 (NO 3 ) 6 ] n •(solvent) x } (Cu-TPA-NO 3 ), 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−BF 4 in a glass tube for a long period, a more stable violet crystal {[Cu 3 (TPA) 4 (SiF 6 ) 3 ] n • (solvent) x } (Cu-TPA-SiF 6 ) is generated, while no transformation is observed for Cu-TPA-ClO 4 and Cu-TPA-NO 3 . Cu-TPA−BF 4 belongs to a tbo topology and possesses octahedral cages and two kinds of cuboctahedral cages, with the uncoordinated BF 4 − anions filling in the channels. In comparison, as a two-connected node, SiF 6 2− anions participate in the construction of the tetrahedral cages and icosahedral cages, resulting in conversion to Cu-TPA-SiF 6 with an ith-d topology. Further investigation indicates that the hydrolysis of BF 4 − anions and further reaction with SiO 2 will afford SiF 6 2− anions, which can be directly involved in the formation of Cu-TPA-SiF 6 to induce the transformation. Additionally, the stability and adsorption behaviors of Cu-TPA−BF 4 and Cu-TPA-SiF 6 are also investigated.

Self-similar chiral organic molecular cages

“…Despite the significant strides made in constructing superstructural OMCs, the current approach primarily focuses on building 3D superstructures with intricate geometric shapes using lower-dimensional organic building blocks. Moreover, there is still a need to enhance the properties (such as porosity, catalysis, and recognition) of existing OMCs, which currently lag behind polymers like COFs and MOFs 34 . Furthermore, ongoing studies are investigating the construction of supramolecular architectures by employing 3D structural OMCs as monomers to create OMC-based polymers with improved properties 18 , 35 , 36 .…”