Imparting gas selective and pressure dependent porosity into a non-porous solid via coordination flexibility
Using a simple hard–soft acid–base concept we have deliberately designed gas-specific and pressure dependent porosity into a non-porous solid via coordination flexibility. This creates distinct gate-openings wherein the CO2 molecule opens-up the framework pores by rotating the ligand about the weaker hard-soft bonds (hard–soft gate control). For this, we have studied the CO2 gating behaviour of M(4-PyC)2 (M = Mg, Mn and Cu), which represent metals of varying hardness. A combination of quantum chemical calculations, molecular dynamics and Grand canonical Monte Carlo simulations were performed to examine the gate opening of the isonicotinate ligands in Mg(4-PyC)2. The simulations show that interaction of the CO2 molecules with the isonicotinate ligands at different CO2 loadings can result in pressure-dependent gate opening. Furthermore, the simulated CO2 uptake values calculated using the partially gate-opened structures at different loadings showed good agreement with the experimental uptake values. This provides an effective strategy for designing highly-stable dynamic porous solids employing rigid frameworks.