Au₃Si₄^- and Au₄Si₄: Electronically Equivalent but Different Polarity Superatoms

A series of AuSi₄^- cluster anions (x = 1-4) were generated most abundantly by laser ablation of a Au₄Si alloy target. Photoelectron spectroscopy and density functional theory (DFT) calculation of AuSi₄^- (x = 1-4) revealed that Au₃Si₄^- can be viewed as an electronically closed superatom and is composed of a Si₄ unit whose three adjacent edges of a single facet are bridged by three Au atoms. Such phase-segregated structure is facilitated by aurophilic interaction between the three Au atoms and results in a large permanent dipole moment (4.43 D). DFT calculations on an electronically equivalent superatom Au₄Si₄ predicted a new structure in which the uncoordinated Si atom of Au₃Si₄^- is bonded by Au⁺. This Au₄Si₄ is much more stable than a cubic structure previously reported and has a large HOMO-LUMO gap (1.68 eV) and a small permanent dipole moment (0.41 D).