Cage Catalyst: Tandem Assembly and Temperature-Regulated Symmetry Breaking of Endogenous Metal Cluster for Phase-Controlled CO2 Electroreduction.

Encapsulation of metal clusters in porous organic cages (POCs) is a promising strategy for metalloenzyme-mimetic catalysts. However, constructing POCs with endogenous metal clusters and achieving atomic-level control over their formation and structural evolution remains a key challenge. Here, we realize the in situ growth of a cubic tetranuclear Ag-halide cluster in an imine-based [4+6] POC (α-Ag4X4@Cage-2, X = Cl, Br, I) via coordination-driven tandem assembly, monitored by time-resolved mass spectrometry. Upon heating, α-Ag4X4@Cage-2 (X = Cl, Br) undergoes a single-crystal-to-single-crystal transformation, with symmetry breaking via Ag-X bond cleavage and structural transition from cubane to distorted hexahedron (β phase). This is a rare atomic-level observation of thermally induced structural change of endogenous metal clusters in molecular cages. For CO2 electroreduction, β-Ag4X4@Cage-2 reaches 98.7% FECO at -1.05 V versus RHE with 100 h stability (outperforming its α-phase counterpart), a TOF of 100 060 h-1 at 500 mA cm-2, ranking among top molecular materials. Mechanistic studies reveal that the β-Ag4X4 structural distortion disrupts charge symmetry of the four Ag atoms and localizes electrons at the bond-cleaved Ag site, lowering the energy barrier for key *COOH intermediate formation. This work offers insights into the dynamic evolution of confined metal clusters via precise host-guest structural engineering.