Progressive gas adsorption squeezing through the narrow channel of a soft porous crystal of [Co2(4,4′-bipyridine)3(NO3)4]

Abstract

Reactions of the ternary components of Co2+ ion, 4,4′-bipyridine, and NO3− give several coordination polymers, which are often obtained in mixed phases. Herein, we explore the condition for the selective formation of Co-1D chain and Co-tongue-and-groove coordination polymers and find reversible interconversion pathways between them. The crystal structures of Co-tongue-and-groove in desolvated and two different CO2-adsorbed states show a one-dimensional corrugated channel with small windows through which CO2 is unlikely to pass. Nevertheless, a sufficient amount of CO2 is adsorbed at 195 K. The CO2 molecules are accommodated in the swollen cavity, forcing their way through the seemingly impermeable window of the channel, which we have named squeezing adsorption. The local motion of the ligand of the window frame plays an essential role in the guest permeation, which proves that the tongue-and-groove coordination polymers are essentially locally flexible porous frameworks. Mixed-phase coordination polymers are often formed when using ternary components. Here, conditions for the selective formation of [Co2(4,4′-bpy)3(NO3)4] are deduced, which shows unique gas adsorption squeezing through seemingly impassable narrow windows due to local structural flexibility.