The Raman spectra of a series of monolayer-protected gold clusters were investigated with special emphasis on the Au–S modes below 400 cm^–1. These clusters contain monomeric (SR-Au-SR) and dimeric (SR-Au-SR-Au-SR) gold–thiolate staples in their surface. In particular, the Raman spectra of [Au₂₅(2-PET)₁₈]^0/–, Au₃₈(2-PET)₂₄, Au₄₀(2-PET)₂₄, and Au₁₄₄(2-PET)₆₀ (2-PET = 2-phenylethylthiol) were measured in order to study the influence of the cluster size and therefore the composition with respect to the monomeric and dimeric staples. Additionally, spectra of Au₂₅(2-PET)_18–2x(S-/rac-BINAS)_x (BINAS = 1,1′-binaphthyl-2,2′-dithiol), Au₂₅(CamS)₁₈ (CamS = 1R,4S-camphorthiol), and Au_nBINAS_m were measured to identify the influence of the thiolate ligand on the Au–S vibrations. The vibrational spectrum of Au₃₈(SCH₃)₂₄ was calculated which allows the assignment of bands to vibrational modes of the different staple motifs. The spectra are sensitive to the size of the cluster and the nature of the ligand. Au–S–C bending around 200 cm^–1 shifts to slightly higher wavenumbers for the dimeric as compared to the monomeric staples. Radial Au–S modes (250–325 cm^–1) seem to be sensitive toward the staple composition and the bulkiness of the ligand, having higher intensities for long staples and shifting to higher wavenumbers for sterically more demanding ligands. The introduction of only one BINAS dithiol has a dramatic influence on the Au–S vibrations because the molecule bridges two staples which changes their vibrational properties completely.

The ligand exchange reaction between racemic Au₃₈(2-PET)₂₄ (2-PET: 2-phenylethylthiolate) clusters and enantiopure 1,1’-binapththyl-2,2’-dithiol (BINAS) was monitored in situ using a chiral HPLC approach. In the first exchange step, a clear preference of R-BINAS towards the left-handed enantiomer of Au₃₈(2-PET)₂₄ is observed (about four times faster than reaction with the right-handed enantiomer). The second exchange step is drastically slowed compared to the first step. BINAS substitution deactivates the cluster for further exchange, which is attributed to (stereo)electronic effects. The results constitute the first example of a ligand exchange reaction in a thiolate-protected gold cluster with directed enrichment of a defined species in the product mixture. This may open new possibilities for the design of nanomaterials with tailored properties.

Chirality unveiled: Thiolate-protected Au₄₀(SR)₂₄ clusters were enantioenriched using an HPLC approach. CD spectra show strong mirror-image responses, indicating the intrinsic chirality of a cluster of unknown structure protected with achiral ligands.

Ligand exchange reactions on size-selected Au₃₈(2-PET)₂₄ and Au₄₀(2-PET)₂₄ clusters (2-PET: 2-phenylethylthiol) with mono- and bidentate chiral thiols was performed. The reactions were monitored with MALDI mass spectrometry and the arising chiroptical properties were compared to the number of incorporated chiral ligands. Only a small fraction of chiral ligands is needed to induce significant optical activity to the clusters. The use of monodentate camphor-10-thiol (CamSH) leads to comparably fast exchange on both clusters. The arising optical activity is weak. In contrast, the use of bidentate 1,1’-binaphthyl-2,2’-dithiol (BINAS) is slow, but the optical activity measured is strong. Moreover, a non-linear behaviour between optical activity and number of chiral ligands is found in the BINAS case for both Au₃₈ and Au₄₀, which may indicate different exchange rates of enantiopure BINAS with the enantiomers of inherently chiral clusters. This is ascribed to effects arising from the bidentate nature of BINAS. This is the first study where chiroptical effects are directly correlated with the composition of the ligand shell.

Bestowing chirality to metals is central in fields such as heterogeneous catalysis and modern optics. Although the bulk phase of metals is symmetric, their surfaces can become chiral through adsorption of molecules. Interestingly, even achiral molecules can lead to locally chiral, though globally racemic, surfaces. A similar situation can be obtained for metal particles or clusters. Here we report the first separation of the enantiomers of a gold cluster protected by achiral thiolates, Au₃₈(SCH₂CH₂Ph)₂₄, achieved by chiral high-performance liquid chromatography. The chirality of the nanocluster arises from the chiral arrangement of the thiolates on its surface, forming 'staple motifs'. The enantiomers show mirror-image circular dichroism responses and large anisotropy factors of up to 4×10^−3. Comparison with reported circular dichroism spectra of other Au₃₈ clusters reveals that the influence of the ligand on the chiroptical properties is minor.

Chiral gold clusters stabilised by enantiopure thiolates were prepared, size-selected and characterised by Circular Dichroism and mass spectrometry. The product distribution is found to be ligand dependent. Au₂₅ clusters protected with camphorthiol show clear resemblance of their chiroptical properties with their glutathionate analogue.

Size Exclusion Chromatography (SEC) on a semi-preparative scale (10 mg and more) was used to size-select ultrasmall gold nanoclusters (< 2 nm) from polydisperse mixtures. In particular, the ubiquitous byproducts of the etching process towards Au₃₈(SR)₂₄ (SR: thiolate) clusters were separated and gained in high monodispersity (based on mass spectrometry). The isolated fractions were characterized by UV/Vis spectroscopy, MALDI mass spectrometry and electron microscopy. Most notably, the separation of Au₃₈(SR)₂₄ and Au₄₀(SR)₂₄ clusters is demonstrated.

The thiolate-for-thiolate ligand exchange reaction between the stable Au₃₈(2-PET)₂₄ and Au₄₀(2-PET)₂₄ (2-PET: 2-phenylethanethiol) clusters and enantiopure BINAS (BINAS: 1,1′-binaphthyl-2,2′-dithiol) was investigated by circular dichroism (CD) spectroscopy in the UV/vis and MALDI mass spectrometry (MS). The ligand exchange reaction is incomplete, although a strong optical activity is induced to the resulting clusters. The clusters are found to be relatively stable, in contrast to similar reactions on [Au₂₅(2-PET)₁₈]^− clusters. Maximum anisotropy factors of 6.6 × 10^−4 are found after 150 h of reaction time. During the reaction, a varying ratio between Au₃₈ and Au₄₀ clusters is found, which significantly differs from the starting material. As compared to Au₃₈, Au₄₀ is more favorable to incorporate BINAS into its ligand shell. After 150 h of reaction time, an average of 1.5 and 4.5 BINAS ligands is found for Au₃₈ and Au₄₀ clusters, respectively. This corresponds to exchange of 3 and 9 monodentate 2-PET ligands. To show that the limited exchange with BINAS is due to the bidentate nature of the ligand, exchange with thiophenol was performed. The monodentate thiophenol exchange was found to be faster, and more ligands were exchanged when compared to BINAS.