Understanding the High-Pressure Behavior of Honeycomb-like Metal-Organic Frameworks with Open Metal Sites: Influence of Metal Identity, Linker Extension and Diamine Functionalization

MOF-74 (M2(dobdc); dobdc4– = 2,5-dioxido-1,4-benzenedicarboxylate) and its isoreticular derivatives represent a chemically versatile class of metal-organic frameworks (MOFs) with high densities of open metal sites and one-dimensional channels. These features underpin their utility in gas storage, separation, and catalysis. However, their mechanical behavior under hydrostatic pressure, critical for shaping, membrane integration, and high-pressure separations, remains insufficiently understood. Here, we report a comparative high-pressure powder X-ray diffraction study of six M2(dobdc) materials (M2+ = Mg2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+) and the expanded analogue Mg2(dobpdc) (dobpdc4– = 4,4’-dioxido-[1,1’-biphenyl]-3,3’-dicarboxylate), both in its activated and diamine-loaded forms, under hydrostatic pressure up to 4000 bar. Two distinct compression mechanisms are identified: M2(dobdc) frameworks compress primarily along the crystallographic c-axis through distortion of the square-pyramidal [MO5] coordination units. The bulk modulus varies from 10 to 24 GPa and correlates strongly with metal ion hardness as well as bond length and angular distortion parameters. In contrast, Mg2(dobpdc) undergoes compression via bending of the extended dobpdc4– linker in the ab-plane, resulting in an exceptionally low bulk modulus of ~3 GPa. Notably, while guest-free Mg2(dobpdc) amorphizes irreversibly, the diamine-loaded variant transitions reversibly to a severely contracted, non-periodically distorted high-pressure phase and fully recovers its crystallinity upon decompression. Together, these findings establish how metal identity, linker architecture and guest loading govern pressure response in MOF-74-type materials, offering guiding principles for their integration in pressure-intensive industrial applications.