Quasi-Closed-Loop Crystal-Amorphous Solid-Wet Gel Transformation in Copper MOFs for Exceptional Cycling Stability in Cascade Reactions.

Quasi-closed-loop transformations between crystals, amorphous solids, and wet gels are rarely explored in metal-organic frameworks (MOFs). We report a stimuli-responsive Cu(I)-MOF (Cu-cMOF) constructed from relatively stable 1D Cu(I) secondary building units (s-SBUs) and unstable trigonal Cu(I) SBUs (us-SBUs). This unique architecture enables quasi-closed-loop crystal-amorphous solid-wet gel transformations through the following steps: i) The crystalline framework first undergoes oxidative amorphization via Cu(I)-to-Cu(II) conversion at us-SBUs, forming amorphous Cu-aMOF; ii) Cu-aMOF subsequently reorganizes into wet gel (Cu-MOG) in preheated DMAc via oxygen- and dimethylamine-assisted framework fragmentation and colloidal assembly; and iii) Cu-MOG transforms back to Cu-cMOF via a solvent-mediated phase transition. The relative structural persistence of the s-SBUs-in contrast to the labile us-SBUs-confers a degree of reversibility to the system. Remarkably, mutual regulation between reaction conditions and catalyst structure is achieved in the azidation-click cascade reaction. The azidation step drives the transformation of Cu-aMOF into Cu-cMOF, while the click reaction reverses this process. Each catalyst phase exhibits step-specific catalytic selectivity, forming a self-sustaining structural transformation cycle that maintains over 98% efficiency after at least 20 cycles. Additionally, used catalysts can be repurposed as Cu-MOGs for large-sized molecule adsorption. This work establishes a cooperative switchable system with adaptive multifunctionality.