Molecular Barrel by a Hooping Strategy: Synthesis, Structure, and Selective CO₂ Adsorption Facilitated by Lone Pair-π...

A sophisticated molecular barrel 5 was efficiently constructed by hooping a 63-membered loop around a D_3h-symmetric, shape-persistent bis(tetraoxacalix[2]arene[2]triazine) core. The hooping strategy involved 3-fold ring-closing metathesis (RCM) reactions of six branched olefin arms which were preanchored on the inner core. Through hooping, the loop tightens the cage structure and significantly enhances its stability toward nucleophilic decomposition. The X-ray crystal structure showed the molecular barrel bears three enclosed fan-shaped cavities as divided by the triazine rings and each of the cavities can hold a solvent CHCl₃ or CH₂Cl₂ molecule. With the intrinsic porosity, the amorphous solids of 5 exhibit considerable CO₂ uptake with an exceptionally large isosteric enthalpy. Lone pair (lp)-π interactions between the electron-deficient triazine rings and CO₂ could contribute to the strong adsorption as supported by IR studies and DFT modeling.