Systematic Ligand Modification Tunes the Stability and Reactivity of Copper Complexes for Electrocatalytic Reduction of Carbon Dioxide

ABSTRACT: Compared to the 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 is prone to undergo demetallation or aggregation. A series of rational and systematic ligand modifications were employed to transform a CO2-fixing yet electrochemically unstable pyridine-2,6-dicarboxamide copper complex into a robust system with enhanced stability and electrocatalytic activity. 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 hard nitrogen donor environment. Furthermore, the introduction of a macrocyclic structure not only enhances the overall complex stability but also minimizes structural reorganization to support redox reversibility of the redox-active ligand. The resulting com-plex mediated the electroreduction of CO2 to CO through an ECEC-type mechanism, an alternating sequence of electron transfer (E) and chemical (C) steps. Further mechanistic studies reveal the key CO2-bound copper intermediate species and the ligand-centered redox behavior that promotes C-O bond cleavage using the combination of spectroscopic tools, complementary electro-chemical methods and isotopic labelling. These results highlight the cooperative interplay among the primary coordination sphere, redox-active ligand and ligand rigidity. Collectively, the lessons demonstrated here provide new directions for improving the elec-trocatalytic performance of molecular copper complexes and mechanistic insights applicable to both homogeneous electrocatalysts and copper-complex-derived heterogeneous systems for CO2 reduction.