Stabilization of Thiolate-Protected Gold Clusters Against Thermal Inversion: Diastereomeric Au₃₈(SCH₂CH₂Ph)_24–2x(R-BINAS)_x

Abstract

Intrinsically chiral thiolate-protected gold clusters were recently separated into their enantiomers, and their circular dichroism (CD) spectra were measured. Introduction of the chiral R-1,1′-binaphthyl-2,2′-dithiol (BINAS) into the ligand layer of rac-Au38(2-PET)24 clusters (2-PET: 2-phenylethylthiolate, SCH2CH2Ph) was shown to be diastereoselective. In this contribution, we isolated and characterized the diastereomeric reaction products of the first exchange step, A-Au38(2-PET)22(R-BINAS)1 and C-Au38(2-PET)22(R-BINAS)1 (A/C, anticlockwise/clockwise) and the second exchange product, A-Au38(2-PET)20(R-BINAS)2. The absorption spectra show minor, but significant influence of the BINAS ligand. Overall, the spectra are less defined as compared to Au38(2-PET)24, which is ascribed to symmetry breaking. The CD spectra are similar to those of the parent Au38(2-PET)24 enantiomers, readily allowing the assignment of handedness of the ligand layer. Nevertheless, some characteristic differences are found between the diastereomers. The anisotropy factors are slightly lower after ligand exchange. The second exchange step seems to confirm the trend. Inversion experiments were performed and compared to the racemization of Au38(2-PET)24. It was found that the introduction of the BINAS ligand effectively stabilizes the cluster against inversion, which involves a rearrangement of the thiolates on the cluster surface. It therefore seems that introduction of the dithiol reduces the flexibility of the gold–sulfur interface.

Amplification of enantiomeric excess by dynamic inversion of enantiomers in deracemization of Au38 clusters

“…In addition to the first two peaks, which belong to the enantiomers of the Au 38 (2-PET) 24 cluster, several other peaks were observed in HPLC. Based on previous MALDI 34 , 36 (composition) and CD analysis (absolute configuration of the metal cluster) 31 , those peaks in the chromatogram were assigned to different species containing R-BINAS in their ligand shell. We will use the following nomenclature to describe these species: The anticlockwise cluster with one R-BINAS and 22 2-PET ligands in its ligand shell will be called (A-38, 22, R-1).…”

“…In this model no Au–S bonds are broken and the racemization proceeds via a rotational reconstruction of the metal core. Interestingly, after introducing a rigid di-thiolate, 1,1′-binaphthyl-2,2′-dithiol (BINAS), into the ligand shell of the cluster the racemization drastically slows down 34 . For example at 70 °C the racemization of Au 38 (2-PET) 22 (BINAS) 1 is about 27 times slower compared to the parent cluster.…”

Nonlinear Optical Properties of Thiolate-Protected Gold Clusters: A Theoretical Survey of the First Hyperpolarizabilities

“…Therefore, we chose the axially chiral 1,1'-binaphthyl-2,2'-dithiol (BINAS), which was used in several ligand exchange studies. [44][45][46][47] Computational Methods…”

Ligand-protected gold clusters: the structure, synthesis and applications

“…The problem can be solved by introducing one or two 1,1 Hbinaphthyl-2,2 H -dithiol substituents into the cluster. 203 In this case, the energy barrier to interconversion of enantiomers increases and the optical properties of the cluster are retained in a wide temperature range.…”