Chiral Inversion of Thiolate-Protected Gold Nanoclusters via Core Reconstruction without Breaking a Au-S Bond.

Sami Malola,Hannu Häkkinen

doi:10.1021/jacs.9b01204

Abstract

On the basis of density functional theory computations of the well-known chiral Au38(SR)24 nanocluster and its Pd- and Ag-doped derivatives, we propose here a mechanism for chiral inversion that does not require the breaking of a metal–sulfur bond at the metal–ligand interface but features a collective rotation of the gold core. The calculated energy barriers for this mechanism for Au38 and Pd-doped Au38 are in the range of 1–1.5 eV, significantly lower than barriers involving the breakage of Au–S bonds (2.5 eV). For Ag-doped Au38, barriers for both mechanisms are similar (1.3–1.5 eV). Inversion barriers for a larger chiral Au144(SR)60 are much higher (2.5−2.8 eV). Our computed barriers are in good agreement with racemization barriers estimated from existing experiments for bare and doped Au38. These results highlight the sensitivity of chiral inversion to the size, structure, and metal composition of the metal core and sensitivity to the detailed structure of the metal–thiolate interface. Our work also predicts that enantiopure Au144(SR)60 clusters would be promising materials for applications requiring high resistance to chiral inversion.

Highlights

Chirality is instrumental to many functions and processes of biomolecules, surface reactions, and organic catalysis, to name a few
The first mechanism requires simultaneous Au−S bond breaking and formation, whereas the second is about reconstructing the metal core without Au−S bond breaking
The results show that the chiral inversion of the cluster can occur energy optimally without any Au−S bond breaking through a rotational reconstruction of the metal core

Summary

Introduction

Chirality is instrumental to many functions and processes of biomolecules, surface reactions, and organic catalysis, to name a few. Experimental work on chiral MPCs has successfully surveyed the separation of chiral enantiomers during and after synthesis,[6−10] properties affecting chiral activity, and thermal stability against chiral inversion.[6,11−16] One of the most extensively studied clusters in this perspective is the prolate, bi-icosahedral Au38(SR)[24] and its derivatives afforded by doping or ligand exchange For this cluster, relatively low activation barriers (0.8−1.3 eV) have been reported experimentally for the racemization that doping and ligand exchange affect.[11−14] Surprisingly, the reported barriers are much too low for reactions including breaking Au−S bonds on the cluster surface,1b which would be the most obvious way to rearrange the Au−S interface structure into the opposite chirality

Methods

Results

Conclusion