A combination of variable-temperature and variable-pressure single-crystal and powder X-ray diffraction is used to study the thermo- and piezo-mechanical properties of the metal–organic framework (MOF) silver(I) 2-methylimidazolate, Ag(mim). We find the material to exhibit a number of anomalous mechanical properties: negative thermal expansion, colossal positive thermal expansion and the most extreme negative linear compressibility yet observed for a MOF. By considering the mechanical response of individual supramolecular motifs we are able to rationalise the varied and unconventional behaviour of the bulk material. A general inverse correspondence between strength of supramolecular interaction and magnitude of mechanical response is identified. We propose that the consideration of MOF structures in terms of their underlying mechanical building units provides a straightforward qualitative method of directing framework design in order to maximise anomalous mechanical response.

Thermal expansion in ABX₃formates is correlated to the size and molecular anisotropy of the A- and B-site cations.

Metal–organic frameworks (MOFs) are a broad and interesting class of materials known for their mechanical flexibility. As such, their response to pressure is usually extreme and often counterintuitive. This tutorial review surveys the structural response of MOFs to pressure as observed experimentally. It describes the experimental tools exploited in high-pressure crystallographic measurements and highlights some of the experiment design choices that influence the actual physics probed in these measurements. The main focus of the review is a description of the key pressure-driven structural responses exhibited by MOFs: isosymmetric compression, including negative compressibility; symmetry-lowering transitions; changes in connectivity; amorphization; and inclusion of the pressure-transmitting medium within the MOF pores. The review concludes both by highlighting some functional implications of these responses and by flagging some future directions for the field.