Synthesis and crystal structure of a [2+4]-type organic cage based on calix[4]arene

Abstract

Porous organic cages (POCs) have recently attracted attention as a relatively new class of porous materials with discrete molecular structures. Various POC architectures have been reported, but only a few examples employ macrocyclic molecules as molecular scaffolds for POCs. We successfully synthesized a [2+4] type calixarene-based organic cage (COC) composed of two bowl-shaped calix[4]arene molecules bearing four propyl groups, formed through covalent imine bonds (C=N) via a Schiff base reaction between formyl calix[4]arenes and propane diamines. The crystal structure of the COC was determined by single-crystal X-ray diffraction analysis, revealing a large internal cavity. Furthermore, the COC exhibited electronic resonance stabilization through the aromatic ring, which promoted an intramolecular acid–base reaction and resulted in keto–enol tautomerization in both the solid and liquid states. ¹H NMR and FT-IR confirmed the formation of imine bonds in the COC. The assembly of the cage was further supported by relative molecular weight analysis using FT-ICR MS; the protonated molecular ion peak of the COC was observed at m/z = 1225.55490 (C₇₆H₇₂N₈O₈ calculated value: m/z 1225.55459). In addition, results suggested that the COC undergoes partial collapse of its crystal structure upon solvent removal. Consequently, only negligible N₂ uptake was detected, yielding a low BET surface area of 6.1607 m²·g^-1. Nevertheless, due to the abundant imine linkages within the COC molecule, CO₂ storage was achieved, with uptakes of 9.052 cm³·g^-1 at 298 K and 20.191 cm³·g^-1 at 236 K under 100 kPa.