Vibrational Circular Dichroism of Thiolate-Protected Au₂₅ Clusters: Accurate Prediction of Spectra and Chirality Transfer within the Mixed Ligand Shell

We have prepared Au₂₅(PET)_18-2x((R)-FBI-NAS)_x(PET = 2-phenylethylthiol, (R)-FBINAS = (R)-5,5',6,6',7,7',8,8'-octafluoro-[1,1'-binaphthalene]-2,2'-dithiol) mixed ligand shell clusters via ligand exchange. Two fractions with different composition of the ligand shell were separated using size-exclusion chromatography and studied by infrared (IR) spectroscopy and vibrational circular dichroism (VCD). Both IR and VCD spectra are dominated by FBINAS vibrations although PET is more abundant on the cluster. Calculated VCD spectra on a model cluster Au₂₅(SH)₁₆(FBINAS)₁ depend on the charge state of the cluster, and the anion is in excellent agreement with the experimental spectra of the Au₂₅(PET)_18-2x(FBINAS)_x samples. Calculations of Au₂₅(SH)₁₄(FBINAS)₂ furthermore show very similar spectra for different adsorption isomers with one exception, where the two ligands share a SR-Au-SR-Au-SR staple motif. Model calculations show that it is not necessary to consider the entire cluster for obtaining reliable VCD and IR spectra, but the staple motifs cannot be neglected. A band that is assigned to PET shows significant vibrational optical activity, and an anisotropy factor that depends on the composition of the ligand shell. This shows that the FBINAS molecules can transfer chirality to achiral PET within the ligand shell of the cluster.