Chirality in Au9 clusters protected by chiral/achiral mixed bidentate phosphine ligands: influence of the metal core and ligand array

Abstract

In this article, the chiroptical responses of Au₉ clusters protected by chiral/achiral mixed bidentate phosphine ligands are reported. The mixed phosphine we use is (S)-BINAP/Xantphos in the molar ratio of 1/0 (= pure (S)-BINAP), 3/1, 1/1, or 0/1 (= pure Xantphos), where BINAP and Xantphos represent 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl and 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, respectively. Electronic absorption spectra of the clusters are similar between the samples with different molar diphosphine ratios, but the chiroptical activity or g-factor decreases nonlinearly with an increase in the fraction of Xantphos. Quantum chemical calculations and geometrical quantifications based on the Hausdorff chirality measure (HCM) for model Au₉ cluster species suggest that (i) two types of metal core structures with pseudo-P- and M-chirality are found, and their appropriate contributions would cancel out the chiroptical response in the low-energy region; (ii) the origin of optical activity in pure (S)-BINAP-protected Au₉ clusters can mainly be attributed to the metal core chirality, whereas that of other mixed-ligand protected clusters would be due to the chiral ligand arrangement. This work demonstrates that the modulation of chiroptical activity in Au₉ clusters by chiral/achiral mixed-diphosphine ligation is controlled by the difference in the degree of chirality existing in the cluster core and/or the ligand array.