Tandem mechanochemical engineering yields highly crystalline metal-organic frameworks

Abstract

The formation of highly crystalline metal-organic frameworks (MOFs) relies on reversible metal–ligand (M–L) bond formation under conditions that enable defect annealing. While solvothermal synthesis remains the most common method for producing crystalline MOFs, mechanochemical synthesis is emerging as a greener alternative. However, the solid-state nature of mechanochemical reactions—even when assisted by catalytic amounts of liquid additives—limits molecular mobility, thereby impeding defect annealing and crystallization. This work introduces a tandem mechanochemical engineering strategy to achieve highly crystalline MOFs by incorporating a second class of reversible bond formation—imine condensation—alongside traditional M–L coordination. The critical role of cooperative dynamics between M–L and imine reversible bonds is highlighted by systematic investigations using similar high-connectivity ligand analogues featuring irreversible covalent link-ages (e.g., ether, amide, or alkyne), which fail to produce quality crystalline MOF phases under mechanochemical conditions. The synergistic effect of dual reversible bonds addresses sluggish reaction kinetics inherent to solid-state processes, enhances crystallization kinetics, and enables the efficient mechanochemical synthesis of MOFs with improved crystallinity under ambient conditions, particularly for frameworks constructed from high-connectivity ligands.