Isolated Square-Planar Copper Center in Boron Imidazolate Nanocages for Photocatalytic Reduction of CO₂ to CO

Photocatalytic reduction of CO₂ to value-added fuel has been considered to be a promising strategy to reduce global warming and shortage of energy. Rational design and synthesis of catalysts to maximumly expose the active sites is the key to activate CO₂ molecules and determine the reaction selectivity. Herein, we synthesize a well-defined copper-based boron imidazolate cage (BIF-29) with six exposed mononuclear copper centers for the photocatalytic reduction of CO₂ . Theoretical calculations show a single Cu site including weak coordinated water delivers a new state in the conduction band near the Fermi level and stabilizes the *COOH intermediate. Steady-state and time-resolved fluorescence spectra show these Cu sites promote the separation of electron-hole pairs and electron transfer. As a result, the cage achieves solar-driven reduction of CO₂ to CO with an evolution rate of 3334 μmol g^-1 h^-1 and a high selectivity of 82.6 %.