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

A series of AuxSi4- 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 AuxSi4- (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).