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

A new gold cluster compound [Au-23(NHCptol)(6)(C CPh)(9)](2+) ( NHCptol = 1,3-di(paramethylbenzyl)benzimidazolin-2-ylidene) (Au23NHCptol) was syn- thesized, and its geometric and electronic structures were compared to those of a known phosphine-protected analogue Au-23(PPh3)(6)(C CPh)(9)](2)(+) (Au23PPh3). Single-crystal X-ray diffraction analysis revealed that Au(23)NHC(ptol )has a common structural motif with Au23PPh3: a Au-17 core capped by six NHC ligands and surrounded by three Au-23(C CPh)(3) oligomers. However, a detailed inspection of the geometric structures elucidated two noticeable differences: (1) the Au-17 core of Au(23)NHC(ptol )is slightly elongated and sharpened along the C-3 axis compared with that of Au23PPh3 and (2) all of the phenyl rings of the alkynyl ligands in Au23NHCptol face nearly parallel to the equatorial plane of the Au-17 core, whereas randomly aligned phenyl rings are found in Au23PPh3. Both dusters showed comparable stability at 60 degrees 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 Au(23)NHC(ptol )is higher in energy and the HOMO-lowest unoccupied molecular orbital (LUMO) gap of Au23NHCptol is larger compared to Au23PPh3. Theoretical analysis of the electronic structures showed that the Au-17 core common to both clusters cannot be viewed as a dimer molecule composed of a prolate Au-10(6e) superatom, but corresponds to a nonspherical superatom with 12 electrons.

Automated summary

A new gold cluster compound Au23NHCptol was synthesized and compared with a known phosphine-protected analogue Au23PPh3. Both clusters have a similar structural motif with some noticeable geometric differences, and also exhibit slight differences in their electronic structures.