Polymorphism in Atomically Precise Cu₂₃ Nanocluster Incorporating Tetrahedral [Cu₄]⁰ Kernel

Because of the typical instability of copper nanoclusters, atom-precise structural elucidation of these nanoclusters has remained elusive. Herein, we report an air- and moisture-stable 23-copper nanocluster (SD/Cu23a or SD/Cu23b) isolated from the reaction of Cu(CF₃COO)₂, BuC≡CH, Cu powder, and Ph₂SiH₂ using a gradient reduction (Cu^II → Cu^I → Cu⁰) strategy (GRS), which is competent for controlling the kinetics of the reduction reaction, thus avoiding formation of pure Cu^I complexes or large Cu⁰ nanoparticles. The solid-state structure of the Cu₂₃ nanocluster shows a rare [Cu₄]⁰ tetrahedral kernel surrounded by an outer Cu₁₉ shell, which is protected by BuC≡C^- and CF₃COO^- ligands. The Cu₂₃nanocluster is a rare four-electron superatom with a 1S²1P² electronic shell closure and can be crystallized in two polymorphs (R3c and R3̅) depending on the solvent used. The crystallization of SD/Cu23a in the R3c space group is mainly governed by van der Waals forces and C-H···F interactions, whereas additional intermolecular C-H···Cl_chloroform interactions are responsible for the R3̅ space group of SD/Cu23b. This work not only shows the ingenuity of a gradient reduction strategy for the synthesis of copper nanoclusters but also provides a better fundamental understanding of how to produce the polymorphic copper nanoclusters in a precisely tunable fashion.