<i>Cis</i>–<i>Trans</i> Selectivity of Haloalkene Isomers in Nonporous Adaptive Pillararene Crystals

The separation of haloalkene cis–trans isomers is difficult to achieve, yet highly desired in the chemical industry. Here, we report an energy-efficient adsorptive separation of 1,4-dichloro-2-butene (DCB) cis–trans isomers using nonporous adaptive crystals of perethylated pillararenes. Adaptive perethylated pillar[6]­arene (EtP6) crystals separate the trans-DCB isomer from its cis isomer with high selectivity while perethylated pillar[5]­arene (EtP5) crystals adsorb cis–trans DCB isomers without selectivity. The selectivity of EtP6 derives from the difference in the thermodynamic stabilities of guest-loaded EtP6 crystal structures upon capture of cis–trans DCB isomers, while the structural similarity of guest-loaded EtP5 leads to the loss of selectivity. EtP6 is highly recyclable due to the reversible transformations between guest-free and guest-loaded structures.