Ligand Effects on the Structures of [Au<sub>23</sub>L<sub>6</sub>(C≡CPh)<sub>9</sub>]<sup>2+</sup> (L = N-Heterocyclic Carbene vs Phosphine) with Au<sub>17</sub> Superatomic Cores
K. Hirano, Shinjiro Takano, Tatsuya Tsukuda
Abstract
A new gold cluster compound [Au23(NHCptol)6(C≡CPh)9]2+ (NHCptol = 1,3-di(para-methylbenzyl)benzimidazolin-2-ylidene) (Au23NHCptol) 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 Au23NHCptol 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 Au23NHCptol 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 Au23NHCptol 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 Au23NHCptol 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 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.