CO2-induced single-crystal to single-crystal transformations of an interpenetrated flexible MOF explained by in situ crystallographic analysis and molecular modeling

Abstract

A molecular-level investigation is reported on breathing behaviour of a metal–organic framework (1) in response to CO₂ gas pressure. High-pressure gas adsorption shows a pronounced step corresponding to a gate-opening phase transformation from a closed (1_cp) to a large-pore (1_lp) form. A plateau is observed upon desorption corresponding to narrow-pore intermediate form 1_np which does not occur during adsorption. These events are corroborated by pressure-gradient differential scanning calorimetry and in situ single-crystal X-ray diffraction analysis under controlled CO₂ gas pressure. Complete crystallographic characterisation facilitated a rationalisation of each phase transformation in the series 1_cp → 1_lp → 1_np → 1_cp during adsorption and subsequent desorption. Metropolis grand-canonical Monte Carlo simulations and DFT-PBE-D3 interaction energy calculations strongly underpin this first detailed structural investigation of an intermediate phase encountered upon desorption.