LAUSR

Due to 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 reac-tion of Cu(CF3COO)2, tBuC≡CH, Cu powder and Ph2SiH2 using a gradient reduction (CuII→CuI→Cu0) strategy (GRS) which is competent to control the kinetics of the reduction reaction, thus avoiding formation of pure CuI complexes or large Cu0 nano-particles. 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 tBuC≡C- and CF3COO- ligands. The Cu23 nanocluster is a rare four-electron superatom with a 1S21P2 electronic shell closure and can be crystallized in two polymorphs (R3c and R-3) depending on the solvent used. The crystallization of SD/Cu23a in the R-3c space group is mainly governed by van der Waals forces and C-H•••F interactions, whereas additional intermolecular C-H•••Clchloroform interactions are responsible for the R-3 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.