Investigating the structural evolution of thiolate protected gold clusters from first-principles

Unlike bulk materials, the physicochemical properties of nano-sized metal clusters can be strongly dependent on their atomic structure and size. Over the past two decades, major progress has been made in both the synthesis and characterization of a special class of ligated metal nanoclusters, namely, the thiolate-protected gold clusters with size less than 2 nm. Nevertheless, the determination of the precise atomic structure of thiolate-protected gold clusters is still a grand challenge to both experimentalists and theorists. The lack of atomic structures for many thiolate-protected gold clusters has hampered our in-depth understanding of their physicochemical properties and size-dependent structural evolution. Recent breakthroughs in the determination of the atomic structure of two clusters, [Au-25(SCH2CH2Ph)(18)](q) (q = 1, 0) and Au-102(p-MBA)(44), from X-ray crystallography have uncovered many new characteristics regarding the gold-sulfur bonding as well as the atomic packing structure in gold thiolate nanoclusters. Knowledge obtained from the atomic structures of both thiolate-protected gold clusters allows researchers to examine a more general inherent structure rule underlying this special class of ligated gold nanoclusters. That is, a highly stable thiolate-protected gold cluster can be viewed as a combination of a highly symmetric Au core and several protecting gold-thiolate staple motifs, as illustrated by a general structural formula [Au](a+a')[Au(SR)(2)](b)[Au-2(SR)(3)](c)[Au-3(SR)(4)](d)[Au-4(SR)(5)](e) where a, a', b, c, d and e are integers that satisfy certain constraints. In this review article, we highlight recent progress in the theoretical exploration and prediction of the atomic structures of various thiolate-protected gold clusters based on the divide-and-protect concept in general and the inherent structure rule in particular. As two demonstration examples, we show that the theoretically predicted lowest-energy structures of Au-25(SR)(8)(-) and Au-38(SR)(24) (-R is the alkylthiolate group) have been fully confirmed by later experiments, lending credence to the inherent structure rule.