Cis-Trans 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.