A High-Nuclearity Copper Sulfide Nanocluster [S-Cu50] Featuring a Double-Shell Structure Configuration with Cu(II)/Cu(I) Valences

This work represents an important step in the quest for creating atomically precise binary semiconductor nanoclusters (BS-NCs). Compared with coinage metal NCs, the preparation of BS-NCs requires strict control of the reaction kinetics to guarantee the formation of an atomically precise single phase under mild conditions, which otherwise could lead to the generation of multiple phases. Herein, we developed an acid-assisted thiolate dissociation approach that employs suitable acid to induce cleavage of the S-C bonds in the Cu-S-R (R = alkyl) precursor, spontaneously fostering the formation of the [Cu-S-Cu] skeleton upon the addition of extra Cu sources. Through this method, a high-nuclearity copper sulfide nanocluster, Cu₅₀S₁₂(SC(CH₃)₃)₂₀(CF₃COO)₁₂ (abbreviated as [S-Cu₅₀] hereafter), has been successfully prepared in high yield, and its atomic structure was accurately modeled through single-crystal X-ray diffraction. It was revealed that [S-Cu₅₀] exhibits a unique double-shell structural configuration of [Cu₁₄S₁₂]@[Cu₃₆S₂₀], and the innermost [Cu₁₄] moiety displays a rhombic dodecahedron geometry, which has never been observed in previously synthesized Cu metal, hydride, or chalcogenide NCs. Importantly, [S-Cu₅₀] represents the first example incorporating mixed Cu(II)/Cu(I) valences in reported atomically precise copper sulfide NCs, which was unambiguously confirmed by XPS, EPR, and XANES. In addition, the electronic structure of [S-Cu₅₀] was established by a variety of optical investigations, including absorption, photoluminescence, and ultrafast transient absorption spectroscopies, as well as theoretical calculations. Moreover, [S-Cu₅₀] is air-stable and demonstrates electrocatalytic activity in ORR with a four-electron pathway.