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.