Competitive Inclusion of Carboxylic Acids with a Metastable Crystal Polymorph of p‑tert-Butylthiacalix[4]arene

Nanoporous molecular crystals (NMCs) are potential separation materials, but it is difficult to simultaneously achieve both high selectivity and wide applicability for adsorbates using NMCs, similar to other separation materials. Although the crystal of p-tert-butylthiacalix­[4]­arene (2α) selectively includes organic molecules, its applicability is limited. To achieve higher applicability while maintaining the potential selectivity of 2α, a metastable crystal polymorph (2β) was prepared, and its inclusion capability for carboxylic acids was investigated. Crystal 2β could be prepared based on the finding that inclusion crystal 2·MeOH, deposited during the crystallization of compound 2 from a mixture of MeOH/toluene, spontaneously desorbs the methanol upon leaving the crystal in the solvent. Crystal 2β selectively included formic acid and acetic acid from 1:1 mixtures of HCO2H/MeCO2H and MeCO2H/EtCO2H, respectively. Further, 2β included each acid with complete selectivity from a 1:1 mixture of HCO2H/EtCO2H by changing the temperature. This performance is superior to that of 2α, which exhibited notable selectivity only for propionic acid over formic acid. The guest selectivity of 2β can be rationalized based on (i) the inclusion rates of carboxylic acids, (ii) thermal stability of the resulting inclusion crystals, and (iii) aggregation states of carboxylic acids in acid mixtures.