A Needle in a Haystack: Transient Porosity in a Closed Pore Square Lattice Coordination Network

Guest transport through discrete voids (closed pores) in crystalline solids is poorly understood. Herein, we report the gas sorption properties of a nonporous coordination network, {[Co(bib)₂Cl₂] ⋅ 2MeOH}_n (sql-bib-Co-Cl-α), featuring square lattice (sql) topology and the bent linker 1,3-bis(1H-imidazol-1-yl)benzene (bib). The as-synthesized sql-bib-Co-Cl-α has 11.3 % (313 ų) of its unit cell volume in closed pores occupied by methanol (MeOH). Upon desolvation and air exposure, sql-bib-Co-Cl-α underwent a single-crystal to single-crystal (SC-SC) phase transformation to sql-bib-Co-Cl-β′, wherein MeOH was replaced by water. Activation (vacuum or N₂ flow) resulted in dehydration and retention of the closed pores, affording sql-bib-Co-Cl-β with 7.7 % (194 ų) guest-accessible space. sql-bib-Co-Cl-β was found to preferentially adsorb C₂H₂ (at 265 K) over CO₂ (at 195 K) through gate-opening mechanisms, at gate-opening pressures of 59.8 and 27.7 kPa, respectively, while other C2 gases were excluded. PXRD was used to monitor transformations between the three phases of sql-bib-Co-Cl, while in situ DSC, in situ SCXRD under CO₂ pressure, and computational studies provided insight into the guest transport mechanism, which we attribute to the angular, flexible nature of the bib ligand. Further, the preferential adsorption of C₂H₂ over CO₂ and other C2 gases suggests that transiently porous sorbents might have utility in separations.