Understanding Doping Effects on Electronic Structures of Gold Superatoms: A Case Study of Diphosphine-Protected M@Au12 (M = Au, Pt, Ir)

Abstract

Dopants into ligand-protected Au superatoms have been hitherto limited to group X–XII elements (Pt, Pd, Ag, Cu, Hg, and Cd). To expand the scope of the dopants to the group IX elements, we synthesized unprecedented [IrAu₁₂(dppe)₅Cl₂]⁺ [IrAu₁₂; dppe = 1,2-bis(diphenylphosphino)ethane] and [PtAu₁₂(dppe)₅Cl₂]²⁺ (PtAu₁₂) and compared their electronic structures with that of [Au₁₃(dppe)₅Cl₂]³⁺ (Au₁₃). Single-crystal X-ray diffractometry, ³¹P{¹H} NMR, and Ir L₃-edge extended X-ray absorption fine structure analysis of IrAu₁₂ revealed that the single Ir atom is located at the center of the icosahedral IrAu₁₂ core. Electrochemical analysis demonstrated that the energy levels of the highest occupied molecular orbitals are upshifted in the order of Au₁₃ < PtAu₁₂ < IrAu₁₂. This trend was qualitatively explained in such a manner that the jellium core potential at the central position becomes shallower by replacing Au⁺ with Pt⁰ and further with Ir^–. IrAu₁₂ underwent reversible redox reactions between the charge states of 1+ and 2+. The gradual increase of the energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital in the order of Au₁₃ < PtAu₁₂ < IrAu₁₂ was observed by electrochemical measurement and optical spectroscopy. This study provides a simple guiding principle to tune the electronic structures of heterometal-doped superatoms.