Superatom chemistry: promising properties of near-spherical noble metal clusters

Abstract

Properties of near-spherical metal clusters are best understood on the basis of the concept of conventional atoms. Their conduction electrons occupy cluster orbitals that remind of hydrogen-like orbitals since they have the same angular dependence. When populated with electrons, maxima in their ionization potentials and minima in electron affinities reveal the closing of shells in the same sense as for noble gases. This suggests that the periodic table of elements should be amended by a third dimension reflecting the number of atoms in a cluster of the element. In a bonded situation the symmetry of cluster atoms is broken, and the atomic orbital momentum is quenched to a large extent. However, if superatoms are axially symmetric, there are superatomic orbital angular moments that are locked along this symmetry axis. If their z-component is non-zero, this leads to large magnetic moments and to significant spin-orbit interactions, which greatly complicate spectroscopic observation. This magnetic interaction is anisotropic and may lead to hysteresis loops with corresponding blocking temperatures up to room temperature. The number of unpaired electrons in such a system is crucial, and it may be influenced by doping with different atoms or by chemical bonds to capping ligands. Stable superatom clusters with size-tuned, tailored band gaps and band edge energies may be attractive replacements for toxic or rare elements in photovoltaic cells or batteries; they form chemically inert and well-defined stoichiometric complexes with various ligands. This reminds of the established transition metal complexes and may lead to a novel branch of chemistry in which the central ion of organometallic complexes is replaced by a metal superatom.

Theoretical Analysis of the Mackay Icosahedral Cluster Pd55(PiPr3)12(μ3‐CO)20: An Open‐Shell 20‐Electron Superatom

“…In regular LCAO-MO DFT calculations on group 11 clusters, these electrons are identified as those occupying the Kohn-Sham orbitals of dominant valence s character. [10,[14][15][16][17][18]20] It turns out that most of the coinage metal clusters isolated so far obey the above-mentioned closed-shell superatomic rule. [14][15][16][17][18][19] It is also noteworthy that in the case of group 11 clusters, both the whole cluster and its metal superatomic kernel possess a HOMO-LUMO gap for the considered "magic" electron count.…”

“…In 2016, the first isolation of the crystalline icosahedral Mackay Pd 55 (PiPr 3 ) 12 (m 3 -CO) 20 (1) cluster was reported. [1] This fascinating compound that ideally conforms to I h symmetry (without isopropyl substituents) is somewhat unique.…”

“…[1] The 12 outer Pd atoms that are located on the C 5 axis are coordinated by a phosphine ligand, whereas 20 symmetry-equivalent triangular faces of the Pd 42 outer shell (metal atoms not attached to the 12 phosphine ligands) are capped by carbonyl ligands. The icosahedral two-shell Pd architecture of Pd 55 (PiPr 3 ) 12 (m 3 -CO) 20 is shown in Figure 1.…”

“…These rules generally apply well to hollow clusters made of metal atoms, the valence d orbitals of which participate in the bonding. This is not the case for group 11 metal clusters, for example, which have their (localized) valence d shell filled and then can be ignored 20 : a) "Matryoshka doll sequence" of the Pd@Pd 12 @Pd 42 metal kernel, and b) outer Pd 42 shell with surrounding ligands (isopropyl groups not shown for clarity). The purple, yellow, dark blue, orange, grey, and red spheres are central Pd (Pd'), inner Pd (Pd''), outer Pd (Pd'''), P, C, and O atoms, respectively.…”

“…They usually tend to aggregate in a compact (non-hollow) and often spherical way. To understand their structure and composition, the concept of superatom, [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] based on the spherical jellium model, [25][26][27] is used. This qualitative model considers electrons within a radial phenomenological potential supposed to describe the average electrostatic potential associated with the cluster atom nuclei.…”