Solvent-switchable continuous-breathing behaviour in a diamondoid metal–organic framework and its influence on CO2 versus CH4 selectivity

Understanding the behaviour of flexible metal–organic frameworks (MOFs)—porous crystalline materials that undergo a structural change upon exposure to an external stimulus—underpins their design as responsive materials for specific applications, such as gas separation, molecular sensing, catalysis and drug delivery. Reversible transformations of a MOF between open- and closed-pore forms—a behaviour known as ‘breathing’—typically occur through well-defined crystallographic transitions. By contrast, continuous breathing is rare, and detailed characterization has remained very limited. Here we report a continuous-breathing mechanism that was studied by single-crystal diffraction in a MOF with a diamondoid network, (Me2NH2)[In(ABDC)2] (ABDC, 2-aminobenzene-1,4-dicarboxylate). Desolvation of the MOF in two different solvents leads to two polymorphic activated forms with very different pore openings, markedly different gas-adsorption capacities and different CO2 versus CH4 selectivities. Partial desolvation introduces a gating pressure associated with CO2 adsorption, which shows that the framework can also undergo a combination of stepped and continuous breathing. Breathing metal–organic frameworks (MOFs) are functional materials whose molecular-scale pores can reversibly open and close. In contrast to typical defined structural transitions, continuous breathing has now been observed for a diamondoid MOF. Removal of two different solvents leads to two desolvated MOF polymorphs with dramatically different porosities and gas uptake properties, including CO2/CH4 selectivities. Partial desolvation introduces pressure-gated CO2 adsorption.