Ligand Effects on the Structures of [Au₂₃L₆(CCPh)₉]²⁺ (L = N‑Heterocyclic Carbene vs Phosphine) with Au₁₇ Superatomic Cores

A new gold cluster compound [Au₂₃(NHC^ptol)₆(CCPh)₉]²⁺ (NHC^ptol = 1,3-di­(para-methylbenzyl)­benzimidazolin-2-ylidene) (Au₂₃NHC^ptol) was synthesized, and its geometric and electronic structures were compared to those of a known phosphine-protected analogue [Au₂₃(PPh₃)₆(CCPh)₉]²⁺ (Au₂₃PPh₃). Single-crystal X-ray diffraction analysis revealed that Au₂₃NHC^ptol has a common structural motif with Au₂₃PPh₃: a Au₁₇ core capped by six NHC ligands and surrounded by three Au₂(CCPh)₃ oligomers. However, a detailed inspection of the geometric structures elucidated two noticeable differences: (1) the Au₁₇ core of Au₂₃NHC^ptol is slightly elongated and sharpened along the C₃ axis compared with that of Au₂₃PPh₃ and (2) all of the phenyl rings of the alkynyl ligands in Au₂₃NHC^ptol face nearly parallel to the equatorial plane of the Au₁₇ core, whereas randomly aligned phenyl rings are found in Au₂₃PPh₃. Both clusters showed comparable stability at 60 °C in chloroform. UV–vis absorption spectroscopy and differential pulse voltammetry indicated that both clusters exhibit similar electronic structures, but the highest occupied molecular orbital (HOMO) of Au₂₃NHC^ptol is higher in energy and the HOMO–lowest unoccupied molecular orbital (LUMO) gap of Au₂₃NHC^ptol is larger compared to Au₂₃PPh₃. Theoretical analysis of the electronic structures showed that the Au₁₇ core common to both clusters cannot be viewed as a dimer molecule composed of a prolate Au₁₀(6e) superatom, but corresponds to a nonspherical superatom with 12 electrons.