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

Abstract

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 (<b>SD/Cu23a</b> or <b>SD/Cu23b</b>) isolated from the reaction of Cu(CF₃COO)₂, ^<i>t</i>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 ^<i>t</i>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 (<i>R</i>3<i>c</i> and <i>R</i>3̅) depending on the solvent used. The crystallization of <b>SD/Cu23a</b> in the <i>R</i>3<i>c</i> 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 <i>R</i>3̅ space group of <b>SD/Cu23b</b>. 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.