The Cu7Sc cluster is a stable sigma-aromatic seven-membered ring

Area of Science:

• Quantum Chemistry
• Computational Materials Science
• Cluster Chemistry

Background:

• Understanding the structure and properties of small metallic clusters is crucial for developing new materials.
• Aromaticity in metallic clusters is a fascinating phenomenon that challenges traditional chemical bonding theories.
• The Cu(7)Sc system presents an interesting case for studying electronic structure and bonding in mixed metal clusters.

Purpose of the Study:

• To determine the global minimum structure of the Cu(7)Sc cluster.
• To investigate the aromatic character and electronic properties of the Cu(7)Sc cluster.
• To elucidate the bonding nature and electron distribution within the Cu(7)Sc cluster.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to explore the potential energy surface of Cu(7)Sc.
• Nucleus-independent chemical shift (NICS) calculations, including NICS(1)(zz) and NICS(2)(zz), were used to assess aromaticity.
• Canonical Molecular Orbital (MO) decomposition and Electron Localizability Indicator (ELI-D) analysis were performed to understand electron distribution and bonding.

Main Results:

• The global minimum structure was identified as a planar, seven-membered copper ring with a central scandium atom (D(7h) symmetry).
• NICS calculations indicated significant aromatic character, consistent with Hückel's rule for electron counting (4s Cu/Sc + 3d Sc electrons).
• MO decomposition revealed that 4s atomic orbitals are primarily responsible for the observed aromaticity, and ELI-D analysis suggested sigma-aromatic behavior.

Conclusions:

• The planar Cu(7)Sc cluster represents a novel sigma-aromatic system.
• The electronic structure and aromaticity are well-described by DFT and supported by the phenomenological shell model.
• This study provides insights into the unique bonding and electronic properties of small, mixed-metal clusters.