Mixed-ligand strategy for synthesizing water-soluble chiral gold clusters with phosphine ligands.

Water-soluble chiral metal clusters have drawn much attention by virtue of their fascinating physicochemical properties and potential biomedical applications, but currently, phosphine-protected Au clusters with both chirality and water-solubility are still very limited. In this article, we demonstrate a mixed-ligand strategy for the facile synthesis of atomically precise, water-soluble chiral Au clusters protected by phosphine alone. The clusters are obtained by the reduction of aurate ions in the presence of a phosphine mixture consisting of highly hydrophilic monophosphine (, triphenylphosphine trisulfonate; TPPTS) and hydrophobic chiral diphosphine (, -Segphos or -BINAP), both of which are commercially available. The clusters are size/composition-separated gel electrophoresis, and notably, heptanuclear cluster Au(-Segphos)(TPPTS) exhibits a large chiroptical activity with the maximum anisotropy factor (-factor) of 4.7 × 10, one of the largest values in such Au clusters. Quantum chemical calculations for model Au cluster species suggest two important factors to obtain large chiroptical activity: (i) more than two axially-chiral diphosphine ligands, and (ii) the absence of configurational isomer averaging. Consequently, despite the experimental use of a mixture containing both chiral and achiral phosphines, a large chiroptical activity can be created in Au clusters with high water-solubility.