The discovery of carbon fullerene cages and their solids opened a new avenue to build materials from stable cage clusters as “artificial atoms” or “superatoms” instead of atoms. However, cage clusters of other elements are generally not stable. In 2001, ab initio calculations showed that endohedral doping of Zr and Ti atoms leads to highly stable Zr@Si16 fullerene and Ti@Si16 Frank–Kasper polyhedral clusters with large HOMO–LUMO gaps. In 2002, Zr@Ge16 was shown to form a Frank–Kasper polyhedron, suggesting the possibility of designing novel clusters by tuning endohedral and cage atoms. These results were subsequently confirmed from experiments. In the past nearly two decades, many experimental and theoretical studies have been carried out on different clusters, and many very stable cage clusters with possibly high abundance have been found by endohedral doping. Indeed in 2017, Ta@Si16 and Ti@Si16 cage clusters have been synthesized in bulk quantity of about 100 mg using a dry-chemistry method, giving rise to a new hope of developing cluster-based materials in macroscopic quantity besides the well-known C60 fullerene solid. Also, wet-chemistry methods have been used to synthesize endohedrally doped clusters as well as ligated clusters and their solids, which auger well for the development of novel nanostructured materials using atomically precise clusters with unique properties. In this comprehensive review, we present results of many such developments in this fast-growing field including (i) endohedrally doped Al, Ga, and In clusters, (ii) small endohedral carbon fullerene cages with ≤ 28 carbon atoms, (iii) metal doped boron cages, (iv) endohedrally doped cages of group 14 elements (Si, Ge, Sn, and Pb), (v) coinage metal (Cu, Ag, Au) cages doped with a transition metal atom as well as their ligated clusters and crystals, (vi) endohedrally doped cages of compound semiconductors, and (vii) multilayer Matryoshka cages and core–shell structures. In a large number of cases, we have performed ab initio calculations to present updated results of the most stable atomic structures and fundamental electronic properties of the endohedrally doped cage clusters. We discuss electronic, magnetic, optical, and catalytic properties in order to shed light on their potential applications. The stability of the doped cage clusters has been correlated to the concept of filling the electronic shells for superatoms such as within a spherical potential model and also using various electron counting rules including Wade–Mingos rules, systems with 18 and 32 electrons, and the spherical aromaticity rule. We also discuss cluster–cluster interaction in cluster dimers and assemblies of some of the promising doped cage clusters in different dimensions. Finally, we give a perspective of this field with a bright future.

Gas-phase metal clusters have been a subject of research interest for allowing reliable strategies to explore the stability and reactivity of materials at reduced sizes with atomic precision. Here we have prepared well-resolved copper cluster anions Cu n − (n = 7−37) and systematically studied their reactivity with O 2 , NO, and CO. We found remarkable stability of an open-shell cluster Cu 18 − , which is comparable with the closed-shell clusters Cu 17 − and Cu 19 − within the picture of an electronic shell model. Even without having a magic number of valence electrons, intriguingly, the unpaired electron on the singly occupied molecular orbital of Cu 18 −

Understanding the stability and reactivity of silver clusters toward oxygen provides insights to design new materials of coinage metals with atomic precision. Herein, we report a systematic study on anionic silver clusters, Ag n − (n = 10-34), by reacting them with O 2 under multiple-collision conditions. Mass spectrometry observation presents the odd-even alternation effect on the reaction rates of these Ag n − clusters. A few chosen clusters such as Ag 13 − and Ag 17-19 − hold up in the presence of excessive oxygen gas reactants. First-principles calculation results reveal that the chemical stability of D 4d Ag 17 − is associated with its symmetric ellipsoidal structure and the electronic shell closure of superatomic orbitals (1S 2 | 1P 4 |1P 2 |1D 4 |1D 6 ||2S 0 ). This results in 17c-2e multicenter bonding and a large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, the highest electron detachment energy and incremental binding energy among all the studied Ag n − clusters, as well as the smallest O 2 -binding energy and least charge transfer from Ag to O 2 . We fully demonstrate the superatomic signature of these silver clusters and emphasize the unique Ag 17 with both geometric and electronic shell closure, shedding light on the 18e stability for the coinage of metal clusters. The superatomic characteristics are also disclosed for Ag 16 − , Ag 18 − , and Ag 32 − clusters.

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