Structural flexibility in cationic metal–organic framework for boosting ReO₄⁻ capture

ReO₄⁻, as a typical anion, its capture is now receiving increasing attentions. However, there still lacks effective method to boost the uptake performance of materials, except for designing new materials. In this work, we found that the structural flexibility could be a good candidate to boost ReO₄⁻ uptake. The used flexible cationic MOF, [Co(H₂O)₂(TIB)][NO₃]₂ (named as Co-TIB-NO₃, TIB = 3,3′,5,5′-tetra(1H-imidazol-1-yl)-1,1′-biphenyl), showed a multiple response towards temperature, counterion, and solvents with the observation of three different structures, including crystalline OP (open pore) phase, crystalline CP (closed pore) phase, and amorphous CP phase. The origin of the structural flexibility is quite complicated, involving a cage-to-channel migration for counterion, interlayer compression, and deformation of organic ligands. The developed material, Co-TIB-NO₃-313, showing the amorphous CP phase, was found to perform outstanding ReO₄⁻ capture with a high uptake capacity, rapid adsorption rate, large distribution coefficient, excellent selectivity, and recyclability, suggesting its superior application in ReO₄⁻ capture.