Synthetic Methods

Preparing more metal nanoclusters with diverse sizes and structures is very significant to promote the development of nano cluster science. Therefore, the synthesis of selenolate-protected metal nanoclusters plays a significant role in the development of this area.

As discussed in the previous chapters, recent research has made great breakthroughs in the synthesis of atomically precise metal nanoclusters protected by thiolate and isolation from a mixture, which offer a good basis for preparing selenolate-capped metal nanoclusters [4, 5]. However, due to the difference between Se and S atoms [41], the synthesis of metal nano clusters protected by selenolate possesses new challenges and becomes more difficult. Thus, the reaction conditions (e.g., the concentration of the metal precursor, the type and concentration of ligands and the reducing agent, the type of solvent used as the reaction medium, reaction temperature, stirring speed, the mixing of metal salt and ligand, and the addition speed of the reductant) need to be improved and optimized. In terms of synthetic methodologies, three typical routes for preparing selenolate-capped metal nanoclusters are usually adopted: direct synthesis, ligand exchange, and size focusing.

Direct Synthesis

Generally, in this one-pot synthetic method, a mixture of the metal precursor and ligands was directly reduced by the well-established two-phase procedure pioneered by Brust and co-workers [63]. For the selenolate-protected metal nanoclusters, a slight modification has been made in the reaction process. For example, the metal precursor is first reduced in the absence of ligands. After 1 to 2 s, the ligands are rapidly added to the solution. Using this strategy, Negishi et al. synthesized [Au25(SeC8H17)18]‘ nanoclusters [43]. Furthermore, a co-reduction synthetic method was also proposed by Song et al. in which ligands (PhSeH) and the reductant (NaBH4] are added to the solution of the metal precursor simultaneously to convert Au(III) into Au(I) or Au(0). With this, [Au25(SePh)18]‘ and [Au24(SePh]20] nanoclusters were prepared in high yield [44-46].

Ligand Exchange

In this strategy, as-prepared metal nanoclusters with precise formula are used as precursors to react with another type of ligands (selenolate). With this method, [Au25(SePh)18]‘ nanoclusters were first prepared [51]. Interestingly, if the concentration of the precursor ([Au25(SCH2CH2Ph)18]") is reduced, Au18(SePh)14 and Au20(SePh)16 nanoclusters will be obtained [52], which are separated by HPLC (high-performance liquid chromatography) and characterized by ESI (electrospray ionization) mass spectrometry, indicating the product is highly sensitive to the ratio of precursor and selenolate (e.g., PhSeH). Furthermore, with this method, [Au38(SeC8H17)24],

[Au36(SePh)24], [Ag20{Se2P(OEt)2}12], and [MAg20{Se2P(OEt)2}12]+

(M = Ag and Au) nanoclusters were also obtained [53-55]. However, selenolate-capped metal nanoclusters with other sizes have not been prepared with this method, even though thiolate-capped metal nanoclusters with different sizes have been widely reported with ligand exchange.

Size Focusing

In this method, there are two primary steps. In step I, polydisperse metal nanoclusters capped by phosphine are obtained through moderating the reaction conditions (i.e., the static and dynamic factors). In step II, as-prepared polydisperse metal nanoclusters are focused into the monodisperse product by etching or aging under a severe condition. When this condition is applied to the as-prepared polydisperse metal nanoclusters, only the metal nanoclusters with most robustness can survive, while the other products are decomposed or converted to the most stable size [64, 65]. The ‘survival of the most robust' principle somewhat resembles nature's law'survival of the fittest' [66]. Withthis method, [Au11(L5)4(SePh)2]+, [Au25(PPh3)10(SePh)5Cl2]+/2+, [Au60Se2(PPh3)10[SePh)15]+, and

[Au13Cu4(dppy)4(SePh)9] are obtained through controlling the reaction conditions [56-59].

Structure of Selenolate-Capped Metal Clusters

Revealing of the total structure of atomically precise metal nanoclusters can be viewed as a holy grail in nanoscience [4], which has helped us not only to understand the core structure (i.e., the arrangements of metal atoms) and the surface structure (i.e., the arrangements of ligands and the bonding between the ligands and the metal core), but also to further understand their novel physicochemical properties (e.g., electronic, optical, catalytic) at the atomic level by correlation with the precise structures. In past decades, thiolate-capped metal nanoclusters have made a great breakthrough due to their extraordinary robustness [4, 5]. In the previous chapter, the structural analysis of thiolate-metal system was discussed in detail. Compared to S atom, the Se atom possesses longer atomic radius and lower electronegativity [41], which endow selenolate ligands with unique coordination modes with metal atoms. In this chapter, we first provide a brief overview of some early works, in which some selenolate-capped metal nanoclusters were structurally determined. Based on the protecting ligands, they can be divided into two categories: (i) the metal nanocluster protected by full selenolate ligands and (ii) the metal nanocluster co-protected by selenolate and phosphine ligands. Furthermore, the comparison of selenolate-capped metal nanoclusters with their thiolate (or phosphine) counterparts is also summarized for further understanding the effect of coordination modes on the atom-packing mode.

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