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, tBuCCH, Cu powder, and Ph2SiH2 using a gradient reduction (CuII → CuI → Cu0) strategy (GRS), which is competent for
controlling the kinetics of the reduction reaction, thus avoiding
formation of pure CuI 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 tBuC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···Clchloroform 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.