Single-Crystal to Single-Crystal Mechanical Contraction of Metal–Organic Frameworks through Stereoselective Postsynthetic Bromination

The
properties of metal–organic frameworks (MOFs) can be
tuned by postsynthetic modification (PSM) to introduce specific functionalities
after their synthesis. Typically, PSM is carried out on pendant functional
groups or through metal/ligand exchange, preserving the structure
of the MOF. We report herein the bromination of integral alkyne units
in a pair of Zr4+ and Hf4+ MOFs, which proceeds
stereoselectively in a single-crystal to single-crystal manner. The
chemical and mechanical changes in the MOFs are extensively characterized,
including the crystal structures of the postsynthetically brominated
materials, which show a mechanical contraction of up to 3.7% in volume.
The combination of stability and chemical reactivity in these MOFs
leads to the possibility of tuning mechanical properties by chemical
transformation while also opening up new routes to internal pore functionalization.