Mechanistic insights into the two-phase synthesis of heteroleptic Au nanoclusters.

A mechanistic study on the two-phase synthesis of heteroleptic Au nanoclusters (NCs) is reported here. First, the effects of binary ligands on controlling the size of Au NCs were examined: (1) the binary ligands could exhibit an eclectic effect on the size control of Au NCs if the binding affinities of such hetero-ligands with Au are comparable and (2) the binary ligands could exhibit a competitive effect on the size control of Au NCs, and the size of the Au NCs could be determined by the ligand with stronger binding affinity to Au. This finding is interesting and can shed some light on the design of new functional metal NCs. Secondly, the formation mechanism of the heteroleptic Au NCs that originated from the complex precursors was unprecedentedly studied. The complex precursors of the heteroleptic Au NCs were identified to be the predominant hybridized ligand#1-Au(i)-ligand#2 species, which is helpful for understanding the synthetic mechanisms in depth. Moreover, the growth processes of the heteroleptic Au NCs were also monitored, and some fundamental perceptions about the growth pathway and the structures of the Au NCs were obtained.

From understanding the roles of tetraoctylammonium bromide in the two-phase Brust-Schiffrin method to tuning the size of gold nanoclusters.

The two-phase Brust-Schiffrin (B-S) method has been widely used for synthesizing small-sized Au nanoparticles (NPs) of size 2-6 nm, as well as Au nanoclusters (NCs) of size <2 nm. However, size tuning of Au NCs at the atomic level by this method is challenging probably due to a lack of in-depth understanding of its mechanism. Herein, we report the identification of two roles of tetraoctylammonium bromide (TOAB) in the two-phase B-S method: TOAB not only transfers Au(iii) precursors but also transfers the reducing agent NaBH4 from the aqueous to the organic phase. On this basis, we developed a novel two-phase synthetic strategy by decoupling the roles of the TOAB: (1) using the hydrophobic selenolate ligand to transfer Au(iii) precursors from the aqueous to the organic phase via the formation of selenolate-Au(i) complexes and (2) deploying a small amount of TOAB as "shuttles" to transfer NaBH4 into the organic phase for controlled reduction of selenolate-Au(i) complexes in organic phase. Using this strategy, size tuning of Au NCs at the atomic level could be achieved by simply varying the amount of TOAB. The high yields of Au NCs (≥76%) together with the short synthetic time (≤3 h) and size-tuning capability further illustrate the attractiveness of this synthetic strategy. These advantages also present the classical B-S method with greater strength and flexibility towards NC synthesis.

Effect of subtle changes of isomeric ligands on the synthesis of atomically precise water-soluble gold nanoclusters.

The subtle structural change of hydrophilic ligands on the size control of metal nanoclusters (NCs) is unclear but critically important for fundamental understanding. Herein, we report our findings that subtle changes of isomeric ligands lead to a dramatic difference in the size of water-soluble Au NCs. By using isomeric para-mercaptobenzoic acid (p-MBA), m-MBA, and o-MBA as model ligands, it was found that both the steric hindrance and the electronic effect of isomeric ligands significantly influences the size of Au NCs, resulting in the formation of different sized Au44(p-MBA)26 NCs, Au25(m-MBA)18 NCs, and Au37/43(o-MBA)22/26 NCs. Besides this, by collocating any two of the isomeric MBAs as ligand pairs to compare their protecting capability for Au NCs, the protecting abilities of such ligands were found to follow the trend: m-MBA > o-MBA > p-MBA. In addition, the growth process of Au44(p-/o-MBA)26 NCs from Au(i)-MBA complexes in the NaBH4 reduction system was also monitored by real-time UV-vis absorption spectroscopy and ESI mass spectrometry, which complies with the 2e- hopping growth principle, indicating the universal applicability of this principle in the synthesis of thiolated metal NCs. This study provides a fundamental understanding of the effect of ligands' steric hindrance and electronic factors on the size control of water-soluble metal NCs and sheds light on the formation of metal NCs in the NaBH4 reduction system.