Systematic Ligand Modification Tunes the Stability and Reactivity of Copper Complexes for the Electrocatalytic...

Compared to first-row transition metal complexes, such as Mn, Fe, Co, and Ni, the development of molecular copper electrocatalysts for CO2 reduction has been plagued by the instability of low-valent Cu intermediates, which are prone to undergo demetallation or aggregation. A series of rational and systematic ligand modifications was employed to transform a CO2-fixing yet electrochemically unstable pyridine-2,6-dicarboxamide copper complex into a stable electrocatalytic system. Guided by the Hard-Soft Acid-Base principle, incorporation of a thioether donor into the nitrogen-based pincer framework stabilized Cu(I) intermediates, thereby promoting reversible electron transfer and mitigating the instability of the hard nitrogen-donor environment. Furthermore, the introduction of a macrocyclic structure not only enhances the overall stability of the complex but also minimizes structural reorganization to support the redox reversibility of the redox-active ligand. The resulting complex mediated the electroreduction of CO2 to CO through an alternating sequence of electron-transfer (E) and chemical (C) steps (ECEC mechanism). The mechanistic studies identified the key CO2-bound and protonated HCO2-bound copper intermediate species using a combination of spectroscopic tools, complementary in situ electrochemical methods, and isotopic labeling. In addition, comparative experimental and computational analyses of the copper complexes further revealed the factors governing catalyst stability. These results demonstrate that the interplay among the coordination sphere, redox-active ligand, and ligand rigidity dictates the electrocatalytic performance and stability of molecular copper complexes. This work provides design principles and mechanistic insights applicable to both homogeneous electrocatalysts and copper-complex-derived heterogeneous systems for CO2 reduction.