Structural control of dynamic covalent cages: kinetic vs. thermodynamic assembly and PFAS removal from water

Abstract

Dynamic covalent chemistry is a powerful tool to synthesise complex structures from simple building blocks. However, even minor variations in the numerous parameters governing self-assembly can drastically influence the size and structure of the resulting assemblies. Herein, we report the selective formation of three cages belonging to the low-symmetry Tri22Tri2 cage topology for the first time, using highly symmetric tritopic building blocks, confirmed by single-crystal X-ray (SC-XRD) analysis. Fluorinated and non-fluorinated aldehydes were combined with two amines differing in their degree of structural flexibility. Applying either kinetic or thermodynamic control through solvent selection allowed for the selective synthesis of either the low-symmetry Tri22Tri2 or the larger, highly symmetric Tri4Tri4 assemblies. While the fluorinated linker strongly preferred the formation of the Tri22Tri2 cage topology under thermodynamic control, the non-fluorinated linker selectively formed the Tri4Tri4 species. Kinetic control, using methanol as a poor solvent, allowed for the selective precipitation of the Tri22Tri2 intermediate. Reduction of the Janus-like fluorinated Tri22Tri2 cages yielded the cages Et2F2red and TREN2F2red, which showed high potential for removing perfluorooctanoic acid (PFOA) from water, with Et2F2red exhibiting structural rearrangements in organic solvents to accommodate PFOA, as observed by 1H and 19F NMR titrations in combination with 19F DOSY measurements.