Charge Neutralization Strategy: A Novel Synthetic Approach to Fully Cationized Thiolate‐Protected Au₂₅(SR⁺)₁₈ Clusters with Atomic Precision

Abstract

Many thiolate‐protected molecular Au clusters, of different cluster sizes and with varying neutral or anionic thiolate ligands, have been reported over the past two decades. Despite this abundance of information, to date there has been no report of atomically pure cationized molecular Au clusters that are protected with cationic thiolates. We herein present a novel, facile and high‐yield synthetic approach towards producing a new series of thiolate‐protected molecular Au clusters: a fully cationized Au25(SR+)18 cluster with atomic precision. The simple “charge neutralization strategy” presented enables treatment of the cationic thiols as neutral thiols, thus the well‐established method for preparing Au25 clusters with neutral thiolates can be adopted for cationic Au25 clusters at a very high synthetic yield. We believe that this report will widen the spectrum of gold cluster chemistry and also help in the further development of cationized molecular clusters with various metals, core sizes, and ligand structures.

Kinetics of Cationic-Ligand-Exchange Reactions in Au25 Nanoclusters

“…In the second case (ii), cationic groups densely populated on the surface of a Au 25 cluster experience strong Coulombic repulsions. A higher number of cationic ligands strongly decreases the thermal stability of the Au 25 structure, consistent with our previous thermogravimetry-differential thermal analysis data, which indicate that a fully cationized Au 25 (SR + ) 18 cluster exhibits a lower decomposition temperature . Similar ligand-induced instability was also reported for anionic Au 25 (S­(CH 2 ) n COOH) 18 nanoclusters …”

“…This observation is different from the ligand-exchange reactions reported for Au 25 nanoclusters with neutral thiolate ligands, where absorbances were maintained for long times in high-temperature reactions. − On the basis of this observation, the use of a cationic thiol (SR + ) for the incoming ligand strongly affects Au 25 nanocluster stability, as discussed in our previous report on the synthesis of the fully cationic Au 25 (SR + ) 18 cluster using the same cationic thiol . The main reason for the observed decomposition involves Coulombic repulsions between the densely populated charged functional groups on the cluster surfaces, as reported by us and others. ,, …”

“…Nowadays, the ideal method for the synthesis of atomically precise Au nanoclusters involves a “size-focusing” step, which leads to the formation of thermally metastable structures that are typically referred to as “magic-number” nanoclusters. , In the case of cationic-ligand-protected Au cluster synthesis, the size-focusing methodology has two limitations: (i) Coulombic attractions between the anionic AuCl 4 – and the cationic thiol (SR + ), which dramatically affect the solubilities of the Au­(I)–SR + complexes in solution and further restrict homogeneous reduction and Au-nanocluster growth, and (ii) Coulombic repulsions induced by the densely populated cationic ligands on the small Au cluster surface, which promote thermal nanocluster decomposition. In our recent reports, , size-focusing at optimized reduction and etching rates resulted in the first successful synthesis of a pure Au 25 (SR + ) 18 cluster in 47% yield (Au basis).…”

Super Polycationic Molecular Compounds: Au144(SR+)60 Clusters

“…19 Recently, we have succeeded in the first synthesis of an atomically pure, per-cationized Au 25 (SR + ) 18 cluster compound using a modified Brust method. 14,15 Furthermore, we recently reported the kinetics of the cationic-ligand-exchange processes of neutral ligands on Au 25 clusters by cationic ligands, which is different from that of the neutral-thiol-to-neutral-thiol ligand exchange and is mainly controlled by the cationic-ligandinduced interactions between SR + ligands. 16 In this paper, we report a new series of cationized Au cluster compounds, among which Au 144 (SR + ) 60 is the largest compound reported with the most stable and studied molecular Au cluster (Au 144 ).…”

Sputter Deposition toward Short Cationic Thiolated Fluorescent Gold Nanoclusters: Investigation of Their Unique Structural and Photophysical Characteristics Using High-Performance Liquid Chromatography

“…In this paper, we report the synthesis of fluorescent Au nanoclusters stabilized using TC ligands via the sputtering method. Information about cationic thiolate-protected nanoclusters − is scarce because the electrostatic repulsion between cationic ligands on the surface of the nanoclusters hinders the formation of small nanoclusters. Our sputtering technique, however, enables the synthesis of this material owing to a unique mechanism via sputtering deposition, where small cluster species grow in both gas and liquid phases, in contrast to the common chemical means .…”