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.

金属有机框架（metal–organic frameworks, MOFs）的性能可通过合成后修饰（postsynthetic modification, PSM）进行调控，即在其合成完成后引入特定功能。通常，合成后修饰可通过悬挂官能团或金属/配体交换实现，且能够保留MOF的骨架结构。本文报道了一对Zr⁴+与Hf⁴+基MOF中固有炔基的溴化反应，该反应以单晶到单晶（single-crystal to single-crystal）的方式立体选择性地进行。研究团队对改性后MOF的化学与力学变化进行了全面表征，其中包括合成后溴化产物的晶体结构，结果显示该材料的体积发生最高达3.7%的力学收缩。这类兼具稳定性与化学反应活性的MOF，既有望通过化学转化调控其力学性能，也为孔道内部的功能化修饰开辟了全新路径。