Cobalt cyclen-based supramolecular framework for efficient photocatalytic CO₂ reduction

Precise regulation of the electronic structure of molecular metal centers remains a key challenge in the rational design of photocatalysts for CO₂ reduction. Herein, three structurally defined cobalt-cyclen complexes, denoted Co(cyclen)-1, Co(cyclen)-2, and Co(cyclen)-3, were constructed to investigate how peripheral ligand substituents tune cobalt center electron deficiency and catalytic performance. Single-crystal structural analysis revealed that the peripheral substituents and nitrate species collectively regulate the coordination environment and supramolecular organization of the complexes. In a Ru(bpy)₃Cl₂-sensitized photocatalytic system, Co(cyclen)-1 delivered the highest CO evolution rate of 25.6 mmol g^-1 h^-1 with a CO selectivity of 98%, outperforming Co(cyclen)-2 and Co(cyclen)-3. Combined experimental and theoretical analysis indicate that the superior activity of Co(cyclen)-1 originates from an optimized electron-deficient Co center, which promotes charge transfer, facilitates *COOH formation, and enables favorable CO desorption. By contrast, Co(cyclen)-2 and Co(cyclen)-3 suffer from kinetic limitations associated with CO desorption and CO₂ adsorption, respectively. This work identifies the natural population analysis (NPA)-derived cobalt center electron deficiency as a useful electronic descriptor linking molecular structure with photocatalytic activity, thereby providing a molecular-level design strategy for cobalt-based photocatalysts.