<p>Single-atom catalyst has garnered widespread attention to mimic mature enzymes due to its well-defined atomic structure and coordination environments. However, since the carbon-carbon (C–C) coupling reactions require synergistic catalysis of multiple sites, single-atom catalysts suffer from insufficient active sites and unclear reaction mechanisms. Controlling the reaction intermediates in a precisely targeted pocket through careful metal-organic cage design is therefore crucial. Here, we prepare a tetrahedral [Cu₆L₄]-type boron–imidazolate cage integrating highly active Cu sites and optimized cavity, which exhibits enzyme like specific catalytic performance in electrochemical CO₂ reduction reaction (CO₂RR) to enhance the selectivity of C₂H₄. Electrochemical analyses and computational calculations suggest that the single Cu site together with neighboring boron-imidazolate ligands provides suitably synergistic effects that enable the energetically favorable formation of an *COCHO intermediate, a key step determining selectivity. As a result, the [Cu₆L₄]-type cage of BIC-145 achieves a Faradaic efficiency of 28% for C₂H₄ maintaining an average current density of −3.54 mA cm⁻² over a 5-hour electrolysis period. This work represents the first example for studying single-metal site catalysts with ultra-low coordination numbers through the rational design of metal-organic cages.</p>