New Insight into the Conjugation Effect of Tetranuclear Copper(I) Cluster Catalysts for Efficient Electrocatalytic Reduction of CO₂ into CH₄

The electrochemical CO2 reduction reaction (eCO2RR) is a sustainable approach for converting CO2 into high-value-added products to promote carbon neutrality but is limited by low reaction selectivity and activity. Multinuclear Cu(I) cluster complexes are considered to be one of the most promising catalysts due to abundant copper sites, high atom utilization, and excellent stability. Herein, we synthesized two tetranuclear Cu(I) complexes [{Cu2(μ-dppm)2}2(μ3-η2(N,N),η1(N),η1(N)-pytz)2](ClO4)2 (1) and [{Cu2(μ-dppm)2}2(μ3-η2(N,N),η1(N),η1(N)-mpytz)2](ClO4)2 (2) and investigated their performance for eCO2RR. X-ray structural analysis revealed that 1 and 2 were two Cu(I) clusters with similar planar Cu4N8 units, but 2 showed worse planarity than 1 due to the steric hindrance of the methyl into the 3-position on the pyridyl ring. Complex 1 achieved an optimal CH4 Faradaic efficiency (FECH4) of 43% with a partial current density (jCH4) of 70.85 mA·cm–2 at −1.1 V, which was superior to that of methylated derivative 2. Mechanistic investigations demonstrated that stronger π-conjugation in complex 1 upshifted the d-band center, enhancing the adsorption and activation of the Cu site to the key reaction intermediate. And the highest occupied molecular orbital–lowest-unoccupied molecular orbital (HOMO–LUMO) gap was decreased, which facilitated electron transfer between active sites and CO2. Moreover, π-conjugation enhanced the electropositive properties of the Cu site, thereby forming an acidic local microenvironment to promote the hydrogenation of intermediates toward CH4. This study provides new insights into the design of efficient multinuclear Cu(I) catalysts for the electrocatalytic reduction of CO2 to CH4 by modulating conjugation effects.