The smallest superatom Au₄(PPh₃)₄I₂ with two free electrons: synthesis, structure analysis, and electrocatalytic...

Atomically precise metal nanoclusters (NCs) have emerged as a new class of ultrasmall nanoparticles with both free valence electrons and precise structures (from the metal core to the organic ligand shell) and provide great opportunities to understand the relationship between their structures and properties, such as electrocatalytic CO₂ reduction reaction (eCO₂RR) performance, at the atomic level. Herein, we report the synthesis and the overall structure of the phosphine and iodine co-protected Au₄(PPh₃)₄I₂ (Au₄) NC, which is the smallest multinuclear Au superatom with two free e^- reported so far. Single-crystal X-ray diffraction reveals a tetrahedral Au₄ core stabilized by four phosphines and two iodides. Interestingly, the Au₄ NC exhibits much higher catalytic selectivity for CO (FE_CO: > 60%) at more positive potentials (from -0.6 to -0.7 V vs. RHE) than Au₁₁(PPh₃)₇I₃ (FE_CO: < 60%), a larger 8 e^- superatom, and Au(i)PPh₃Cl complex; whereas the hydrogen evolution reaction (HER) dominates the electrocatalysis when the potential becomes more negative (FE_H of Au₄ = 85.8% at -1.2 V vs. RHE). Structural and electronic analyses reveal that the Au₄ tetrahedron becomes unstable at more negative reduction potentials, resulting in decomposition and aggregation, and consequently the decay in catalytic performance of Au based catalysts towards the eCO₂RR.