Cage Catalyst: Tandem Assembly and Temperature‐Regulated Symmetry Breaking of Endogenous Metal Cluster for Phase‐Controlled CO ₂ 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 ‐Ag 4 X 4 @Cage‐2 , X = Cl, Br, I) via coordination‐driven tandem assembly, monitored by time‐resolved mass spectrometry. Upon heating, α ‐Ag 4 X 4 @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 CO 2 electroreduction, β ‐Ag 4 X 4 @Cage‐2 reaches 98.7% FE CO 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 ‐Ag 4 X 4 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.