Ligand Effects on the Structures of [Au 23 L 6 (CCPh) 9 ] 2+ (L = N‑Heterocyclic Carbene vs Phosphine) with Au 17 Superatomic Cores

A new gold cluster compound [Au 23 (NHC p tol ) 6 (CCPh) 9 ] 2+ (NHC p tol = 1,3-di­( para -methylbenzyl)­benzimidazolin-2-ylidene) ( Au 23 NHC p tol ) was synthesized, and its geometric and electronic structures were compared to those of a known phosphine-protected analogue [Au 23 (PPh 3 ) 6 (CCPh) 9 ] 2+ ( Au 23 PPh 3 ). Single-crystal X-ray diffraction analysis revealed that Au 23 NHC p tol has a common structural motif with Au 23 PPh 3 : a Au 17 core capped by six NHC ligands and surrounded by three Au 2 (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 p tol is slightly elongated and sharpened along the C 3 axis compared with that of Au 23 PPh 3 and (2) all of the phenyl rings of the alkynyl ligands in Au 23 NHC p tol face nearly parallel to the equatorial plane of the Au 17 core, whereas randomly aligned phenyl rings are found in Au 23 PPh 3 . 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 Au 23 NHC p tol is higher in energy and the HOMO–lowest unoccupied molecular orbital (LUMO) gap of Au 23 NHC p tol is larger compared to Au 23 PPh 3 . 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.