Steering CO₂ Electroreduction to Methane via Secondary-Sphere Noncovalent Interactions in NHC-Protected Copper...

Copper-based catalysts are promising for CO₂ reduction to hydrocarbons; however, selective CH₄ generation remains challenging because of the presence of competing pathways and an insufficient understanding of active site control. Herein, we develop a ligand-microengineering strategy to synthesize a series of fully N-heterocyclic carbene (NHC)-protected Cu₄O clusters, [Cu₄(μ₄-O)]L₂(PF₆)₂ (L = 1a-1d, denoted as 2a-2d), via a green and efficient ball-milling approach. Of the four evaluated preparations, cluster 2c exhibits the optimal CH₄ Faradaic efficiency ( ${\mathrm{FE}}_{{\mathrm{CH}}_4}$ ), with a value of 67.5% ± 2.1% at -1.4 V versus RHE. Integrated in situ spectroscopy and density functional theory (DFT) calculations reveal that unique secondary-sphere noncovalent interactions (hydrogen-bonding and C-H···π) present in 2c provide additional stabilization for the key *CHO intermediate, maximizing CH₄ selectivity. This study not only demonstrates the unique advantages of NHC ligands in constructing stable and efficient copper cluster catalysts but also establishes a new paradigm for precisely steering CO₂ electroreduction selectivity through the rational design of noncovalent interactions within the secondary coordination sphere.