Understanding the High-Pressure Behavior of Honeycomb-Like Metal-Organic Frameworks with Open Metal Sites: Influence...

MOF-74 (M₂(dobdc); dobdc^4- = 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) (dobpdc^4- = 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 dobpdc^4- linker in the ab-plane, resulting in an exceptionally low bulk modulus of ∼3 GPa. Notably, while guest-free Mg₂(dobpdc) undergoes irreversible amorphization at elevated pressure, the diamine-loaded variant transitions reversibly to a severely contracted, nonperiodically 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.