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

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.