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

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 Tri² ₂Tri² 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 Tri² ₂Tri² or the larger, highly symmetric Tri⁴Tri⁴ assemblies. While the fluorinated linker strongly preferred the formation of the Tri² ₂Tri² 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 Tri² ₂Tri² intermediate. Reduction of the Janus-like fluorinated Tri² ₂Tri² cages yielded the cages Et²F² _red and TREN²F² _red, which showed high potential for removing perfluorooctanoic acid (PFOA) from water, with Et²F² _red exhibiting structural rearrangements in organic solvents to accommodate PFOA, as observed by ¹H and ¹⁹F NMR titrations in combination with ¹⁹F DOSY measurements.