Y@Cu₁₅: a novel spherical aromatic 18-ce bare superatomic molecular cluster

Finding prototypical species that expand the structural and electronic features of molecular clusters is of interest for expanding the understanding of well-defined structures with particular stability. Here, we investigate the Y@Cu₁₅ binary cluster using a modified basin-hopping (MBH) structure search method, revealing a stable configuration with a yttrium atom encapsulated in a low-symmetry Cu₁₅ cage exhibiting C_2v geometry, displaying different surface faces. The interaction energy ensuring Y-Cu₁₅ encapsulation amounts to -220.4 kcal mol^-1, mainly driven from electrostatic and orbital contributions, with orbital interactions dominated by charge transfer from 5s-Y→Cu₁₅ and "back-donation" from 4d-Y←Cu₁₅ orbitals, resulting in a sizable net charge transfer of -6.0|e| to yttrium. This charge distribution creates electron-deficient Cu sites analogous to the σ-hole regions, suggesting catalytic potential. The electronic structure of the cluster follows the 1S²1P⁶1D¹⁰ superatomic shell model, fulfilling Hirsch's 2(N + 1)² rule for spherical aromaticity. Through-space nuclear independent chemical shift (NICS) analysis confirmed the presence of a shielding cone characteristic of aromatic species, supported by electron density of delocalized bond (EDDB_G) analysis, which showed extensive electron delocalization consistent with spherical aromaticity. These findings establish Y@Cu₁₅ as a novel, medium-sized spherical aromatic superatomic cluster with promising reactive sites for catalysis, thereby expanding the understanding of superatomic cluster chemistry and encouraging further exploration of its reactivity and applications.