Design Clues for Silver-Based Superatomic Molecules Unveiled in Recent Study

Researchers from Japan have conducted a study on superatomic molecules containing noble metal elements such as silver and gold. Although such molecules have potential in the synthesis of superatomic materials, the understanding of silver-based superatomic molecules has been limited. To fill this knowledge gap, the researchers focused on two bimetallic superatomic molecules with silver as a primary constituent to identify the key factors responsible for their formation. The findings of this study are anticipated to contribute to the development of new materials in the future.

In recent decades, metal nanoclusters made up of noble metals like gold (Au) and silver (Ag) have been increasingly investigated as superatoms for synthesizing materials with distinct properties and potential applications. These artificial atoms, which consist of clusters of a few to several hundred atoms, have properties that differ significantly from their bulk, traditional counterparts. However, similar to natural atoms, the stability of these superatoms is determined by the establishment of a closed-shell electron structure.

The properties and functions of Ag-based superatoms, such as photoluminescence and selective catalytic activity, are known to be superior to those of Au-based superatoms. Despite this, research in this area has mainly concentrated on Au-based superatomic molecules.

To bridge the gap in research, a team of Japanese scientists investigated the formation of superatomic molecules consisting of silver and analyzed the contributing factors. Their study was recently published on March 28, 2023, in the journal Communications Chemistry.

Professor Negishi explains the rationale for investigating Ag-based superatoms, stating, “As of now, we have synthesized numerous useful materials using the elements found on Earth. But given the challenges posed by energy and environmental issues, it is imperative to create materials with novel properties and functions.”

The researchers synthesized two di-superatomic molecules with bromine (Br) serving as the bridging ligand, in order to further their investigation. These molecules were named [Ag23Pt2(PPh3)10Br7]0 and [Ag23Pd2(PPh3)10Br7]0 (PPh3 = Triphenylphosphine). The former was composed of two icosahedral Ag12Pt superatoms that were connected by vertex sharing, and had platinum atoms (Pt) occupying the central position in each superatom. The latter, on the other hand, had two icosahedral Ag12Pd structures with palladium (Pd) as the central atom.

The researchers analyzed the geometric and electronic structure, as well as the stability of the two nanoclusters with bromine as the bridging ligand. These nanoclusters were then compared to [Ag23Pt2(PPh3)10Cl7]0 (1) and [Ag23Pd2(PPh3)10Cl7]0 (2), which are two nanoclusters with a similar geometry to the synthesized nanoclusters, but with chlorine as the bridging atom.

Upon analyzing the geometric structures of the four nanoclusters, the researchers noted a twist between the two icosahedral structures that contained Br as the bridging ligand. According to the researchers, this twist shortens the distance between the two structures and stabilizes the nanocluster.

Furthermore, the researchers found that the larger size of the Br atom led to steric hindrance within the molecule, resulting in a greater distance between the PPh3 molecule and the long axis of the metal nanocluster, as well as changes in the bond length of the Ag-P and Ag-Ag bonds. These results suggest that while the type of bridging halogen may impact the geometric structures of the metal nanoclusters to some extent, it does not impede their formation.

According to Prof. Negishi, “As long as the bridging halogen is large enough to keep a moderate distance between the two Ag12M structures, the type of bridging halogen seems to have minimal impact on the formation of superatomic molecules.”

The number of bridging halogens attached to the nanocluster was found to have a significant impact on its stability. As with atoms, stable metallic nanoclusters require a filled valence shell. In this study, the prepared nanoclusters had a total of 16 valence electrons, and the researchers found that they could only attach a maximum of five bridging halogens to maintain the metal nanocluster in a stable neutral or cationic state.

The incorporation of Pt or Pd central atoms was attributed to the formation of metallic nanoclusters. By substituting the central atom of Ag13 with Pt or Pd, the average binding energy within the nanoclusters increased, making the formation of superatomic molecules more favorable.

In summary, the researchers determined three essential conditions for the synthesis and isolation of superatomic molecules composed of two Ag13−xMx structures linked by vertex sharing. These include the requirement of a bridging halogen that can sustain an optimal distance between the two structures, a combination of heteroatoms and bridging halogens that results in 16 valence electrons, and the formation of a more robust icosahedral core than Ag13.

According to Prof. Negishi, “These findings provide specific design principles for developing molecular devices with diverse properties and functions, which could help address urgent issues related to clean energy and the environment.”

Source: Tokyo University of Science