Anion-Templated Cu14 Nanoclusters Reveal Ligand-Controlled Stability and Catalytic Evolution During Electrochemical CO2 Reduction

Two atomically precise sulfide-templated Cu14 nanoclusters (NCs), [S@Cu14(4MBT)12(PPh3)6] (S@Cu14-4MBT) and [S@Cu14(3MBT)12(PPh3)6] (S@Cu14-3MBT), were synthesized and structurally characterized to investigate the influence of ligand engineering on the stability and electrocatalytic performance of Cu NCs during the electrochemical CO2 reduction reaction (CO2RR). Single-crystal X-ray diffraction analysis revealed that both NCs possess a highly similar S@Cu14 core architecture stabilized by twelve positionally different methyl-substituted thiolate and six triphenylphosphine ligands, allowing direct evaluation of ligand effects without changes in core structure. Despite their structural similarity, positional variation of the methyl substituent generated distinct electronic environments, intermolecular interactions, and crystal-packing arrangements. The relatively electron-deficient Cu environment in S@Cu14-3MBT resulted in superior CO2RR performance, achieving highly selective Faradaic efficiencies of 52% for formate and 18% for CO at −1.4 V vs. RHE with suppressed hydrogen evolution, whereas S@Cu14-4MBT exhibited lower selectivity and stability. Theoretical calculations, together with comparison with structurally analogous metal NCs, reveal that precise modulation of the electronic configuration plays a critical role in promoting formate selectivity. Post-catalytic analyses revealed degradation and ligand loss in S@Cu14-4MBT, whereas S@Cu14-3MBT remained structurally intact. These results highlight ligand engineering and anion templation as key strategies for tuning the stability and CO2RR performance of atomically precise metal NCs.