Configurational Entropy Driven High‐Pressure Behaviour of a Flexible Metal–Organic Framework (MOF)

Tuning the High‐Pressure Phase Behaviour of Highly Compressible Zeolitic Imidazolate Frameworks: From Discontinuous to Continuous Pore Closure by Linker Substitution

The high‐pressure behaviour of flexible zeolitic imidazolate frameworks (ZIFs) of the ZIF‐62 family with the chemical composition M(im)_2−x(bim)_x is presented (M²⁺=Zn²⁺, Co²⁺; im⁻=imidazolate; bim⁻=benzimidazolate, 0.02≤x≤0.37). High‐pressure powder X‐ray diffraction shows that the materials contract reversibly from an open pore (op) to a closed pore (cp) phase under a hydrostatic pressure of up to 4000 bar. Sequentially increasing the bim⁻ fraction (x) reinforces the framework, leading to an increased threshold pressure for the op‐to‐cp phase transition, while the total volume contraction across the transition decreases. Most importantly, the typical discontinuous op‐to‐cp transition (first order) changes to an unusual continuous transition (second order) for x≥0.35. This allows finetuning of the void volume and the pore size of the material continuously by adjusting the pressure, thus opening new possibilities for MOFs in pressure‐switchable devices, membranes, and actuators.

Solvent-controlled elongation and mechanochemical strain in a metal-organic framework

Under high pressure, crystals of [Zn(m-btcp)₂(bpdc)₂]·2DMF·H₂O, referred to as DMOF are particularly sensitive to the type of pressure-transmitting media (PTM) employed: large PTM molecules seal the pores and DMOF is compressed as a closed system, whereas small PTM molecules are pushed into the pores, thereby altering the stoichiometry of DMOF. Compression in glycerol and Daphne 7474 leads to negative linear compressibility (NLC), while a mixture of methanol : ethanol : water 'hyperfills' the pores of the chiral framework, adjusting its 3-dimensional strain and resulting in pressure-induced amorphization around 1.2 GPa. The uptake of the small-molecule PTM strongly increases the dimensions of DMOF in the direction perpendicular to that of the NLC of the crystal.