Synthesis of N2-Type Superatomic Molecules

Abstract When atoms form covalent bonds, the atomic orbitals combine linearly to produce molecular orbitals. A similar bonding scheme exists for metallic nanoclusters. Spherical gold nanoclusters called “superatoms” are stabilized when valence electrons occupy the atomic orbital-like electronic shell called superatomic orbital¹⁻⁴. Nonspherical gold nanoclusters can be viewed as “superatomic molecules” composed of fused superatoms sharing several gold atoms, and the superatomic orbitals are combined to form superatomic molecular orbitals⁵⁻¹². Artificial superatomic molecules have been isolated, and their bonding scheme with superatomic molecular orbitals has been theoretically predicted⁷,⁸. However, isolated examples of natural molecule-like superatomic molecules are limited to the F₂-type⁹⁻¹²; thus, the bonding theory and electron transitions of superatomic molecules remain poorly understood. Herein, we report the first synthesis of two N₂-type superatomic molecules, M₂Au₁₇(depp)₁₀Cl₇ (M = Pd, Pt, depp = diethylphenylphosphine) nanoclusters bearing triple bonds of superatomic molecular orbitals. Irradiating visible light on MAu₁₂ superatoms triggered a novel degradative fusion reaction to afford M₂Au₁₇ superatomic molecules. Single-crystal X-ray diffraction analysis and density functional theory (DFT) calculation revealed that the 10 valence electrons are accommodated in the superatomic molecular orbitals with the electron configuration (1Σs)²(1Σs*)²(1Πpx, py)⁴(1Σpz)², similar to that of N₂ molecule. Furthermore, the UV-Vis absorption spectrum and time-dependent DFT calculations clarified that an electron transition occurs between superatomic molecular orbitals with the same symmetry, similar to that between molecular orbitals. Our results demonstrate a facile synthetic approach to superatomic molecules and their molecule-like optoelectronic behaviour. This synthetic approach could expand the library of superatomic molecules. Furthermore, a better understanding of the link between bonding theory and optoelectronic nature for both conventional and superatomic molecules will facilitate the development of functional metallic nanomaterials.