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) 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.