Single-crystal to single-crystal mechanical contraction of metal-organic frameworks through stereoselective post-synthetic bromination.

The properties of metal-organic frameworks (MOFs) can be tuned by post-synthetic 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 ex-tensively characterised, including the crystal structures of the post-synthetically 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.