Fabrication of a family of atomically precise silver nanoclusters via dual-level kinetic control

Abstract

The controllable preparation of metal nanoclusters in high yield is an essential prerequisite for their fundamental research and extensive application. Here a synthetic approach termed “dual-level kinetic control” was developed to fabricate a family of new silver nanoclusters. The introduction of secondary ligands was first exploited to retard the reduction rate and accomplish the first-level kinetic control. And the cooling of the reaction was performed to further slow the reduction down and accomplish the second-level kinetic control. A family of atomically precise silver nanoclusters (including [Ag₂₅(SR)₁₈]⁻, [Ag₃₄(SR)₁₈(DPPP)₃Cl₄]²⁺, [Ag₃₆(SR)₂₆S₄]²⁺, [Ag₃₇(SR)₂₅Cl₁]⁺, and [Ag₅₂(SR)₂₈Cl₄]²⁺) were controllably prepared and structurally determined. The developed “dual-level kinetic control” hopefully acts as a powerful synthetic tool to manufacture more nanoclusters with unprecedented compositions, structures, and properties.

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Introduction

With the recent establishment of modern nanochemistry, capabilities toward dictating the sizes and structures of metal nanoparticles are flourishing.^1–3 Recent years have witnessed significant advances in the preparation of atomically precise nanoparticles in the quantum size regime, also known as nanoclusters.^4–6 Metal nanoclusters, bridging between organometallic complexes and plasmonic metal nanoparticles, are a large family of metallo-inorganic-organic hybrid nanomaterials having core@shell structures consisting of internal metal cores and peripheral ligand shells.^7–11 Owing to their discrete electronic energy levels and the quantum size effect, nanoclusters display molecule-like and structure-dependent chemical/physical properties, rendering them prominent nanomaterials being applied in optics, catalysis, sensing, biochemistry, and so on.^12–24 Among all research branches, the controllable preparation of metal nanoclusters in high yield (or with high purity) is an essential prerequisite for fundamental research (e.g., structure evolutions and property mechanisms) and extensive applications of these nanomaterials.

In the past few decades, several approaches have been proposed to controllably prepare new clusters with dictated structures and properties, or to efficiently increase the synthetic yields of preexisting clusters, including the “pre-adjusting in situ reduction”,^25–27 the “cluster from clusters”,^28–30 the “one- or two-phase ligand exchange”,^31–33 the “anti-galvanic reduction” or “metal exchange”,^34–36 the “paste-based reaction”,^37,38 the “cluster-assembled framework”,^39–42etc. Previous experimental and theoretical efforts have demonstrated that the formation of nanocluster entities resulted from both their thermodynamic and kinetic stabilities, which was closely relevant to the reaction environment, especially for the in situ synthetic procedure.^43,44 In 2008, our group developed a facile approach for the preparation of Au₂₅(SR)₁₈ in a high yield (40%) via controlling the reaction kinetics, in vivid contrast against the uncontrolled preparation of Au₂₅(SR)₁₈ with a low yield (8%).²⁵ One challenging question subsequently arises: how can we extend the “kinetic control” to fabricate more nanoclusters, and further improve their synthetic yields by amplifying the “kinetic control”? The in-depth application of such a control would yield more new clusters with novel structures and enhanced properties, significantly assisting the development of this unique class of nanomaterials in terms of both fundamental investigations and practical applications.

Herein, we report the controllable preparation of a family of silver nanoclusters (including [Ag₂₅(SR)₁₈]⁻, [Ag₃₄(SR)₁₈(DPPP)₃Cl₄]²⁺, [Ag₃₆(SR)₂₆S₄]²⁺, [Ag₃₇(SR)₂₅Cl₁]⁺, and [Ag₅₂(SR)₂₈Cl₄]²⁺) in high yield via a “dual-level kinetic control”. Specifically, the direct reduction of Ag_x(S-Adm)_y complexes produced polydisperse nanoparticles. In contrast, monodispersed nanoclusters could be fabricated via “dual-level kinetic control” – (i) the first-level kinetic control: the introduction of phosphine ligands to the reaction retarded the reduction rate, accomplished the kinetic control, and gave rise to different Ag nanoclusters correlating with the phosphine ligand type; (ii) the second-level kinetic control: the cooling of the reaction further reduced the reduction rate, advanced the kinetic control, and remarkably improved the synthetic yields of such Ag clusters. The two-stage braking of the reduction was recorded by tracking photography, and the participation of phosphine ligands in the reduction was verified by mass spectrometry. Together, the dual-level kinetic control enabled the formation of several new silver nanoclusters and further contributed to their high-yield preparation.

Experimental methods

Materials

HS-Adm was prepared by the reported procedure.⁴⁵ All the following reagents were purchased from Sigma-Aldrich and used without further purification, including silver nitrate (AgNO₃, 99% metal basis), triphenylphosphine (PPh₃, TPP, 99%), bis(diphenylphosphino)methane (Ph₂P–CH₂–PPh₂, DPPM, 98%), 1,2-bis(diphenylphosphino)ethane (Ph₂P–C₂H₅–PPh₂, DPPE, 98%), 1,3-bis(diphenylphosphino)propane (Ph₂P–C₃H₇–PPh₂, DPPP, 98%), 1,4-bis(diphenylphosphino)butane (Ph₂P–C₄H₉–PPh₂, DPPB, 98%), 1,5-bis(diphenylphosphino)pentane (Ph₂P–C₅H₁₁–PPh₂, DPPPE, 98%), 1,6-bis(diphenylphosphino)hexane (Ph₂P–C₆H₁₃–PPh₂, DPPH, 98%), sodium borohydride (NaBH₄, 99%), methylene chloride (CH₂Cl₂, HPLC grade), methanol (CH₃OH, HPLC grade), and n-hexane (Hex, HPLC grade).

Reduction of Ag_x(S-Adm)_y complexes in the absence of phosphine ligands

AgNO₃ (30 mg) was dissolved in CH₃OH (1 mL) and CH₂Cl₂ (15 mL) by sonication. The solution was vigorously stirred (1200 rpm) with magnetic stirring for 10 min. Then, Adm-SH (0.1 g) was added and the reaction was vigorously stirred (1200 rpm) for another 30 min. After that, NaBH₄ (1 mL) aqueous solution (20 mg mL⁻¹) was added quickly to the above reaction. The reaction was allowed to proceed for 12 h under a N₂ atmosphere. Then, the precipitate was removed and the supernatant was analyzed by STEM.

Preparation of a family of silver nanoclusters in the presence of phosphine ligands

AgNO₃ (30 mg) was dissolved in CH₃OH (1 mL) and CH₂Cl₂ (15 mL) by sonication. The solution was vigorously stirred (1200 rpm) with magnetic stirring for 10 min. Then, Adm-SH (0.1 g) and the phosphine ligand (0.1 g) were added together and the reaction was vigorously stirred (1200 rpm) for another 30 min at room temperature or in an ice bath. After that, NaBH₄ (1 mL) aqueous solution (20 mg mL⁻¹) was added quickly to the above reaction. The reaction was allowed to proceed for 12 h under a N₂ atmosphere. After that, the aqueous layer was removed, and the mixture in the organic phase was rotavaporated under vacuum. Then, approximately 15 × 3 mL of CH₃OH was used to wash the synthesized nanoclusters. The precipitate was then dissolved in CH₂Cl₂ for crystallization and characterization. Different silver nanoclusters were synthesized correlating with the phosphine ligand type, including [Ag₅₂(S-Adm)₂₈Cl₄]²⁺ by using TPP or DPPPE, [Ag₃₆(S-Adm)₂₆S₄]²⁺ by using DPPM, [Ag₂₅(S-Adm)₁₈][Ag₁(DPPE)₂] by using DPPE, [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺ by using DPPP, [Ag₃₇(S-Adm)₂₅Cl₁]⁺ by using DPPB, and [Ag₂₅(S-Adm)₁₈][Ag₃(S-Adm)₂(DPPH)₂] by using DPPH. The synthetic yields were calculated on Ag basis by analyzing the qualities of Ag nanocluster crystals and the AgNO₃ salts. Such synthetic yields are presented in Fig. 2B.

Fig. 1 Schematic illustration of the first-level kinetic control via introducing phosphine ligands. Controllable preparation of a family of silver nanoclusters by retarding the reaction rate through the introduction of phosphine ligands.

Fig. 2 The dual-level kinetic control for the preparation of silver nanoclusters. (A) Illustration of the controllable preparation of Ag nanoclusters by retarding the reaction rate through introducing phosphine ligands (i.e., nanocluster preparation via the first-level kinetic control) and lowering the reaction temperature (i.e., yield improvement via the second-level kinetic control). (B) Synthetic yields of Ag nanoclusters under different conditions: in the absence of phosphine ligands at room temperature (red region), in the presence of phosphine ligands at room temperature (blue region), and in the presence of phosphine ligands in an ice bath (green region).

Crystallization of the obtained silver nanoclusters

Single crystals of the obtained silver nanoclusters were grown at room temperature in CH₂Cl₂/Hex. Of note, for accelerating the crystallization processes of Ag₃₆(S-Adm)₂₆S₄ and Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄ and improving their crystal quality, the original counterion was replaced by SbF₆⁻. After 7 days, black crystals of these clusters were collected and their structures were determined by X-ray crystallography.

X-ray crystallography

The data collection for single-crystal X-ray diffraction (SC-XRD) of all nanocluster crystal samples was carried out on a Stoe Stadivari diffractometer under a nitrogen flow, using graphite-monochromatized Cu Kα radiation (λ = 1.54186 Å). Data reductions and absorption corrections were performed using the SAINT and SADABS programs, respectively. The structure was solved by direct methods and refined with full-matrix least squares on F² using the SHELXTL software package. All non-hydrogen atoms were refined anisotropically, and all the hydrogen atoms were set in geometrically calculated positions and refined isotropically using a riding model. All crystal structures were treated with PLATON SQUEEZE, and the diffuse electron densities from these residual solvent molecules were removed. The CCDC number of the Ag₅₂(S-Adm)₂₈Cl₄ nanocluster (prepared in the presence of TPP) is 2094270. The CCDC number of Ag₃₆(S-Adm)₂₈S₄ is 2094271. The CCDC number of [Ag₂₅(S-Adm)₁₈][Ag₁(DPPE)₂] is 2094272. The CCDC number of Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄ is 2094273. The CCDC number of Ag₃₇(S-Adm)₂₅Cl₁ is 2094275. The CCDC number of the Ag₅₂(S-Adm)₂₈Cl₄ nanocluster (prepared in the presence of DPPPE) is 2094276. The CCDC number of [Ag₂₅(S-Adm)₁₈][Ag₃(S-Adm)₂(DPPH)₂] is 2094482.

Measurements

All UV-vis absorption spectra of the nanoclusters dissolved in CH₂Cl₂ were recorded using an Agilent 8453 diode array spectrometer.

The dynamic light scattering (DLS) of each metal complex sample was recorded using a Malvern Zetasizer Nano ZS instrument.

Electrospray ionization mass spectrometry (ESI-MS) measurements were performed by using a Waters XEVO G2-XS QTof mass spectrometer. The sample was directly infused into the chamber at 5 μL min⁻¹. For preparing the ESI samples, nanoclusters were dissolved in CH₂Cl₂ (1 mg mL⁻¹) and diluted (v/v = 1 : 1) with CH₃OH.

Thermogravimetric analysis (TGA) was carried out on a thermogravimetric analyzer (DTG-60H, Shimadzu Instruments, Inc.) with 10 mg of the sample in a SiO₂ pan at a heating rate of 10 K min⁻¹ from room temperature to 1073 K.

The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) technique was performed by using a FEI Themis Z microscope. The electron beam energy was 200 kV. The HAADF-STEM image was obtained using Thermo Scientific Velox software using 1024 × 1024 pixels and the dwell time was set to 10 μs.

Results and discussion

The first-level kinetic control

We first directly reduced the Ag_x(S-Adm)_y complexes using NaBH₄ for the sake of preparing new silver nanoclusters (see Methods for more details). However, the reaction was ultra-violent after the introduction of the reductant, and the reaction turned black within three seconds, accompanied by the formation of massive precipitation (Fig. S1†). Previous studies have demonstrated the time-dependent size aggregation of nanoclusters by the synthetic procedure,^46–48 while the resultant products of this reaction remained polydisperse nanoclusters (or nanoparticles) in the range from 1 to 10 nm, even after 12 hours of the reaction (Fig. S2A†). The TGA result demonstrated that the Ag-to-SAdm ratio of these nanoparticles was 16.67% (Fig. S2B†). Through observation of the reaction phenomena, we suspected that the unsuccessful attempt at the monodispersed nanocluster preparation resulted from the over-quick reduction that generated excessively heterogeneous metallic nuclei. Besides, such an ultrafast reduction may also yield large-sized metallic nuclei, which were responsible for the formation of massive precipitation in the reaction. Accordingly, the core issue herein for the preparation of monodispersed nanoclusters lies in retarding the reduction rate, or in other words the kinetic control.

For fulfilling the kinetic control of the reaction, we introduced phosphine ligands to the reaction for two reasons – (i) the sizes of Ag–(S-Adm)–PR complexes before the reduction were prone to be more concentrated, while those of Ag–(S-Adm) complexes were much more dispersed (Fig. S3†). The more size-focused Ag–(S-Adm)–PR complexes were more accessible to be reduced uniformly, and thus produced monodispersed nanoclusters. Besides, the sizes of such Ag–(S-Adm)–PR complexes were, generally, in inverse proportion to the sizes of phosphine ligands. (ii) The coexistence of different ligands in metal complex precursors (i.e., thiol and phosphine co-protected complexes, as shown in Fig. S4 and S5†) might prevent the rapid formation of large-sized metallic nuclei due to their complex stabilizing patterns, holding the potential to retard the reduction rate and accomplish the kinetic control.⁴⁹ Experimentally, the reduction rate was remarkably reduced in the presence of phosphine ligands (Fig. S6–S12†). For instance, the reaction solution turned black after three minutes when TPP or DPPM ligands were involved (Fig. S6 and S7†), which was in stark contrast to the three-second mutation in the absence of phosphine ligands. Such a kinetic control resulted in the mild reaction environment and gave rise to different silver nanoclusters correlating with the phosphine ligand type, including [Ag₂₅(SR)₁₈]⁻, [Ag₃₄(SR)₁₈(DPPP)₃Cl₄]²⁺, [Ag₃₆(SR)₂₆S₄]²⁺, [Ag₃₇(SR)₂₅Cl₁]⁺, and [Ag₅₂(SR)₂₈Cl₄]²⁺. The formation of different Ag nanoclusters in the presence of different phosphine ligands resulted from the ligand selection effect, corresponding to the “survival of the fittest” in the thermodynamically selective synthesis. Collectively, the kinetic control was accomplished via introducing phosphine ligands to the reaction (termed “first-level kinetic control” in this work), resulting in the control of nanocluster sizes and the formation of a family of monodispersed silver nanoclusters (Fig. 1).

The second-level kinetic control

Although the first-level kinetic control (i.e., introducing phosphine ligands) resulted in monodispersed nanoclusters rather than polydisperse nanoparticles, the synthetic yields of these nanoclusters were relatively low (∼10% yield for each Ag cluster; see Fig. 2). Based on the above understanding that the slowing down of the reduction rate induced the formation of nanoclusters, we perceive a good opportunity to further advance the kinetic control (or to implement the second-level kinetic control) for increasing the synthetic yield of these Ag nanoclusters – lowering the temperature of the reaction via an ice bath.

As shown in Fig. S6–S12,† cooling the reaction could remarkably slow down the reaction rate. For example, the time required for turning the solution black for the preparation of Ag₂₅(S-Adm)₁₈ even doubled with the ice bath (Fig. S12†). After analyzing the synthetic yield of each case for the nanocluster preparation, we concluded that the proposed second-level kinetic control (i.e., cooling the reaction) was capable of increasing the yield of each silver nanocluster (Fig. 2B). For example, the synthetic yields of Ag₅₂ in the presence of TPP or DPPPE were enhanced from ∼10% to ∼25% or from ∼15% to ∼27%, respectively. The synthetic yields of other Ag nanoclusters also exhibited different degrees of enhancement with the second-level kinetic control (Fig. 2B). In this context, a combination of dual-level kinetic controls was exploited to fabricate a family of monodispersed silver nanoclusters and further improve their synthetic yields (Fig. 2A).

Structural anatomy of the [Ag₅₂(S-Adm)₂₈Cl₄]²⁺ nanocluster

The presence of monodentate TPP or bidentate DPPPE ligands in the reaction gave rise to the formation of the [Ag₅₂(S-Adm)₂₈Cl₄]²⁺ nanocluster (Fig. 2B and S13†). The [Ag₅₂(S-Adm)₂₈Cl₄]²⁺ cluster entities are crystallized in an orthorhombic crystal system with a Pccn space group. Structurally, [Ag₅₂(S-Adm)₂₈Cl₄]²⁺ contains a tetrahedral Ag₄ kernel that is enwrapped by an Ag₂₄ shell (Fig. 3A–C). Such a two-shell Ag₄@Ag₂₄ configuration has been previously observed in Ag₂₈Cu₁₂(SPhCl₂)₂₄ and Cd₁₂Ag₃₂(SePh)₃₆ nanoclusters.^50–52 Then, this Ag₂₈ core is encircled by four same Ag₆(S-Adm)₆ ring-like surface motif structures to form an Ag₂₈(core)@Ag₂₄(S-Adm)₂₄(shell) structure (Fig. 3D and E). Of note, the Ag₂₄(S-Adm)₂₄ shell structure in Ag₅₂(S-Adm)₂₈Cl₄, composed of hexameric Ag₆(S-Adm)₆ surface motifs, is relatively loose compared to 4 × [Cu₃(SPhCl₂)₆] or 4 × [Cd₃Ag₁(SePh)₉] surface structures in Ag₂₈Cu₁₂(SPhCl₂)₂₄ and Cd₁₂Ag₃₂(SePh)₃₆ nanoclusters, respectively. In this context, three Cl and three S-Adm ligands, acting as bridges, fill up surface spaces (i.e., the exposed Ag₃ triangles on the Ag₄@Ag₂₄ core) on the Ag₂₈@Ag₂₄(S-Adm)₂₄ structure to make up the final [Ag₅₂(S-Adm)₂₈Cl₄]²⁺ framework (Fig. 3F and G). The Cl ligands are proposed to originate from the CH₂Cl₂ solvent, which has been discovered in previously determined nanoclusters,^53,54 and Ag₃₄ and Ag₃₇ nanoclusters in this work. The overall structure of [Ag₅₂(S-Adm)₂₈Cl₄]²⁺ is highly symmetrical with four C₃ symmetry axes, and each symmetry axis passes through the center of the innermost Ag₄ kernel and polar S/Cl atoms at contrapositions (Fig. 3H and I).

Fig. 3 Structural anatomy of [Ag₅₂(S-Adm)₂₈Cl₄]²⁺. (A) The tetrahedral Ag₄ kernel + (B) the Ag₂₄ shell = (C) the Ag₂₈ core. (D) The Ag₆(SR)₆ ring-like motif. (E) The Ag₅₂(SR)₂₄ structure. (F) Surface Cl or SR bridges. (G) The Ag₅₂(SR)₂₈Cl₄ framework. (H) and (I) The overall structure of [Ag₅₂(S-Adm)₂₈Cl₄]²⁺ exhibits four C₃ symmetry axes. Color legends: blue/light blue/orange sphere, Ag; red/pink sphere, S; green sphere, Cl; grey sphere, C; light grey sphere, H.

Structural anatomy of the [Ag₃₆(S-Adm)₂₆S₄]²⁺ nanocluster

The [Ag₃₆(S-Adm)₂₆S₄]²⁺ nanocluster was prepared when DPPM was introduced to the reaction (Fig. 2B and S14†). The [Ag₃₆(S-Adm)₂₆S₄]²⁺ cluster entities are crystallized in a monoclinic crystal system with a P2₁/n space group. The structure of Ag₃₆(S-Adm)₂₆S₄ comprises an anti-z-shaped Ag₈S₄ core that is covered by two Ag₁₀(S-Adm)₁₀ motif structures via both Ag(core)–S(motif) and Ag(motif)–S(core) interactions, making up an Ag₂₈(S-Adm)₂₀S₄ structure (Fig. 4A–C). The sulfurs without carbon tails should stem from the thiol (i.e., S-Adm) in the formation of the nanoclusters, which has been detected in previously reported nanoclusters, such as Au₃₈S₂(SR)₂₀, Au₆₀S₆(SR)₃₆, Ag₄₆S₇(SR)₂₄, etc.^55–57 Then, the side of the Ag₂₈(S-Adm)₂₀S₄ structure is stabilized by two Ag₃(SR)₂ units (Fig. 4D and E), and the two symmetrical Ag₁₀(S-Adm)₁₀ motifs are held together via two SR and two Ag bridges, constituting the Ag₃₆(S-Adm)₂₆S₄ framework with a flattened configuration (Fig. 4F and G). No symmetrical element except for a center of symmetry is observed for the overall structure of the [Ag₃₆(S-Adm)₂₆S₄]²⁺ nanocluster, which is located at the center of the Ag₆S₄ core (Fig. 4H and I).

Fig. 4 Structural anatomy of [Ag₃₆(S-Adm)₂₆S₄]²⁺. (A) The Ag₆S₄ core + (B) two Ag₁₀(SR)₁₀ surface motifs = (C) the Ag₂₈(SR)₂₀S₄ structure. (D) The Ag₃(SR)₂ side motif structures. (E) The Ag₃₄(SR)₂₄S₄ structure. (F) The bridging SR and Ag units. (G) The Ag₃₆(SR)₂₆S₄ framework. (H) and (I) The overall structure of [Ag₃₆(S-Adm)₂₆S₄]²⁺ exhibits a center of symmetry. Color legends: blue/light blue/orange sphere, Ag; red/yellow sphere, S; grey sphere, C; light grey sphere, H.

Structural anatomy of the [Ag₂₅(S-Adm)₁₈]⁻ nanocluster

The presence of DPPE or DPPH ligands yielded [Ag₂₅(S-Adm)₁₈]⁻ nanoclusters (Fig. 2B, S15 and S16†), while the counterions were different – [Ag₁(DPPE)₂]⁺ for the DPPE-associate [Ag₂₅(S-Adm)₁₈]⁻, or [Ag₃(DPPH)₂(S-Adm)₂]⁺ for the DPPH-associate [Ag₂₅(S-Adm)₁₈]⁻ (Fig. 5, S15 and S16†). Structurally, the [Ag₁(DPPE)₂]⁺ cation follows a configuration of DPPE(side)–Ag(center)–DPPE(side) (Fig. 5E); by comparison, the [Ag₃(DPPH)₂(S-Adm)₂]⁺ cation contains a linear Ag₃ structure wherein the two side Ag atoms are bonded with DPPH, and the side and central Ag atoms are anchored by S-Adm (Fig. 5F). The [Ag₂₅(S-Adm)₁₈][Ag₁(DPPE)₂] cluster entities are crystallized in a triclinic crystal system with a P1̄ space group, whereas the [Ag₂₅(S-Adm)₁₈][Ag₃(DPPH)₂(S-Adm)₂] cluster entities are crystallized in a monoclinic crystal system with a P2₁/c space group.

Fig. 5 Structural anatomy of [Ag₂₅(S-Adm)₁₈]⁻. (A) The icosahedral Ag₁₃ kernel + (B) six Ag₂(SR)₃ dimeric motifs = (C) the Ag₂₅(SR)₁₈ framework. (D) The overall structure of Ag₂₅(S-Adm)₁₈. (E) The overall structure of [Ag₂₅(SR)₁₈]⁻[Ag₁(DPPE)₂]⁺. (F) The overall structure of [Ag₂₅(SR)₁₈]⁻[Ag₃(DPPH)₂(SR)₂]⁺. Color legends: green/light blue/orange sphere, Ag; red sphere, S; magenta sphere, P; grey sphere, C; light grey sphere, H.

The [Ag₂₅(S-Adm)₁₈]⁻ nanocluster is composed of an icosahedral Ag₁₃ kernel and six peripheral Ag₂(S-Adm)₃ dimeric motifs (Fig. 5A–D), which is reminiscent of the previously reported [Ag₂₅(S-PhMe₂)₁₈]⁻ nanocluster with the same metal–ligand composition and configuration, but with different thiol ligand types.⁵⁸ Here, the corresponding bond lengths in different Ag₂₅(SR)₁₈ nanoclusters were compared to figure out the ligand effect on the geometric structure of this nanocluster. As depicted in Table S8,† both the kernel Ag–icosahedral Ag and the icosahedral Ag–icosahedral Ag bonds in [Ag₂₅(S-Adm)₁₈]⁻ are much longer than those in [Ag₂₅(S-PhMe₂)₁₈]⁻, while both icosahedral Ag–motif S and motif Ag–motif S bonds in [Ag₂₅(S-Adm)₁₈]⁻ are much shorter. Accordingly, compared with [Ag₂₅(S-PhMe₂)₁₈]⁻, [Ag₂₅(S-Adm)₁₈]⁻ displays a more expansive kernel structure while a tighter kernel–surface interaction; in other words, the bulkier S-Adm ligand endows the Ag₂₅(SR)₁₈ nanocluster with a loose inside@tight outside intracluster environment.

Structural anatomy of the [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺ nanocluster

The presence of DPPP in the reaction gave rise to the formation of the [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺ nanocluster (Fig. 2B and S17†). The [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺ cluster entities are crystallized in a trigonal crystal system with a R3̄ space group. Of note, as for all silver nanoclusters in this work, only the [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺ nanocluster contains a phosphine ligand within the structure. In contrast, in other Ag nanoclusters, the phosphine ligands can not only slow down the reduction rate, but also act as a “dam” to temporarily store Ag and then release it to generate the nanoclusters.⁴⁹

Structurally, the [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺ nanocluster contains a twisted icosahedral Ag₁₃Cl₁ core (Fig. 6A). The three Ag atoms connecting the Cl ligand are unbound among each other, rendering the Ag₁₃ icosahedron twisted. Then, two types of ring-like motif structures, Ag₉(S-Adm)₆ and Ag₉(S-Adm)₆(DPPP)₃Cl₃, enwrap the Ag₁₃Cl₁ core from opposite faces to constitute an Ag₃₁(S-Adm)₁₂(DPPP)₃Cl₄ structure (Fig. 6B–E). Three monomeric Ag₁(S-Adm)₂ bridges are further introduced to fix the two ring-like motifs and fully protect the Ag₁₃Cl₁ core, making up the final Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄ framework (Fig. 6F and G). The overall configuration of [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺ is triple axisymmetric, and the C₃ symmetry axis crosses through the Cl atom and the innermost Ag atom in the Ag₁₃Cl₁ core (Fig. 6H and I).

Fig. 6 Structural anatomy of [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺. (A) The twisted icosahedral Ag₁₃Cl₁ core + (B) the Ag₉(SR)₆ ring-like motif = (C) The Ag₂₂(SR)₆Cl₁ structure. (D) The Ag₉(SR)₆(DPPP)₃Cl₃ ring-like motif. (E) The Ag₃₁(SR)₁₂(DPPP)₃Cl₄ structure. (F) Three bridging Ag₁(SR)₂ units. (G) The Ag₃₄(SR)₁₈(DPPP)₃Cl₄ framework. (H) and (I) The overall structure of [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺ exhibits a C₃ symmetry axis. Color legends: light blue/blue/orange/green sphere, Ag; red/pink sphere, S; purple sphere, Cl; magenta sphere, P; grey sphere, C; light grey sphere, H.

Structural anatomy of the [Ag₃₇(S-Adm)₂₅Cl₁]⁺ nanocluster

The [Ag₃₇(S-Adm)₂₅Cl₁]⁺ nanocluster was obtained when the DPPB ligand was introduced to the reaction (Fig. 2B and S18†). The [Ag₃₇(S-Adm)₂₅Cl₁]⁺ cluster entities are crystallized in a monoclinic crystal system with a P2₁/c space group. The [Ag₃₇(S-Adm)₂₅Cl₁]⁺ nanocluster comprises a planar Ag₁₈Cl₁ core that is capped by a ring-like Ag₆(S-Adm)₆ motif and a S-Adm on the same face (Fig. 7A–D). Then, three dimeric Ag₂(S-Adm)₃ motif structures wrapped the side of the planar Ag₁₈Cl₁ core, giving rise to an Ag₃₀(S-Adm)₁₆Cl₁ structure (Fig. 7E and F). Finally, another three dimeric Ag₂(S-Adm)₃ motifs that are anchored by an Ag atom stabilize another face of the planar Ag₁₈Cl₁ core (i.e., the opposite face with the Ag₆(S-Adm)₆ motif), forming the Ag₃₇(SR)₂₅Cl₁ framework (Fig. 7G–I). Similar to [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺, the overall structure of [Ag₃₇(S-Adm)₂₅Cl₁]⁺ nanocluster is triple axisymmetric with a C₃ symmetry axis passing through the Cl atom on the Ag₁₈Cl₁ core and the anchoring Ag atom on the surface (Fig. 7G and K).

Fig. 7 Structural anatomy of Ag₃₇(S-Adm)₂₅Cl₁. (A) The planar Ag₁₈Cl₁ core + (B) the Ag₆(SR)₆ ring-like motif + (C) the capping SR = (D) The Ag₂₄(SR)₇Cl₁ structure. (E) Three Ag₂(SR)₃ dimeric motifs. (F) The Ag₃₀(SR)₁₆Cl₁ structure. (G) Three Ag₂(SR)₃ dimeric motifs. (H) The bridging Ag. (I) The Ag₃₇(SR)₂₅Cl₁ framework. (J) and (K) The overall structure of Ag₃₇(S-Adm)₂₅Cl₁ exhibits a C₃ symmetry axis. Color legends: light blue/blue/orange/green sphere, Ag; pink/orange/purple sphere, S; light green sphere, Cl; grey sphere, C; light grey sphere, H.

Similar to the structure of [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺, the kernel of [Ag₃₇(S-Adm)₂₅Cl₁]⁺ also contains a surface Cl capping the core structure, making up an Ag₃–Cl tetrahedral subunit. In the Ag₁₃Cl₁ core of [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺, no Ag–Ag interaction was observed within the Ag₃–Cl subunit, and the average distances of Ag–Ag and Ag–Cl are 3.375 and 2.693 Å, respectively (Fig. 6). By comparison, the Ag₃–Cl tetrahedral subunit in the Ag₁₈Cl₁ core of [Ag₃₇(S-Adm)₂₅Cl₁]⁺ is more compact with average Ag–Ag and Ag–Cl distances of 2.904 and 2.540 Å, respectively (Fig. 7).

The Ag–Cl and Ag–S interactions in these obtained Ag nanoclusters were then compared. As shown in Table S9,† the bond lengths of surface Ag–Cl (or surface Ag–S) are much longer than those of kernel Ag–Cl (or kernel Ag–S). Besides, the bond lengths of Ag–S are much shorter than those of Ag–Cl, which resulted from the different interactions between Ag–S and Ag–Cl.

Characterization studies and optical absorptions

The ESI-MS measurement was performed to confirm the compositions and determine the valence states of the obtained silver nanoclusters. As shown in Fig. S19–S25,† the mass peaks at 5217.72, 4180.39, 5708.24, 4028.85, and 8208.69 Da confirmed the compositions of the crystal structures of these silver nanoclusters, and demonstrated their valence states to be [Ag₅₂(S-Adm)₂₈Cl₄]²⁺, [Ag₃₆(S-Adm)₂₆S₄]²⁺, [Ag₂₅(S-Adm)₁₈]⁻, [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺, and [Ag₃₇(S-Adm)₂₅Cl₁]⁺, respectively. The compositions of [Ag₁(DPPE)₂]⁺ and [Ag₃(S-Adm)₂(DPPH)₂]⁺ counterions of [Ag₂₅(S-Adm)₁₈]⁻ were also verified (Fig. S22 and S25†). Besides, the presence of “SbF₆⁻” counterions of [Ag₃₆(S-Adm)₂₆S₄]²⁺ and [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺ nanoclusters was also confirmed by ESI-MS (Fig. S21 and S23†).

According to the valence states of these nanoclusters, their nominal electron counts were determined:⁵⁹ 52(Ag) − 28(SR) − 4(Cl) − 2(charge) = 18e for [Ag₅₂(S-Adm)₂₈Cl₄]²⁺, 36(Ag) − 26(SR) − 4 × 2(S) − 2(charge) = 0e for [Ag₃₆(S-Adm)₂₆S₄]²⁺, 25(Ag) − 18(SR) + 1(charge) = 8e for [Ag₂₅(S-Adm)₁₈]⁻, 34(Ag) − 18(SR) − 4(Cl) − 2(charge) = 10e for [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺, and 37(Ag) − 25(SR) − 1(Cl) − 1(charge) = 10e for [Ag₃₇(S-Adm)₂₅Cl₁]⁺. Compared with other silver nanoclusters, the [Ag₃₆(S-Adm)₂₆S₄]²⁺ nanocluster had no nominal electron counts, and could be recognized as a nanocluster complex. Such a conclusion, i.e., [Ag₃₆(S-Adm)₂₆S₄]²⁺ was a nanocluster complex, corresponded to that the Ag₈S₄ kernel in this nanocluster was not a pure-metal kernel.

The optical absorptions of the obtained silver nanoclusters (dissolved in CH₂Cl₂) were then measured (Fig. S26†). [Ag₅₂(S-Adm)₂₈Cl₄]²⁺ displayed two intense absorptions at 500 and 620 nm, and two shoulder bands at 375 and 670 nm (Fig. S26A†). [Ag₃₆(S-Adm)₂₆S₄]²⁺ exhibited several shoulder bands at 500, 625, and 690 nm (Fig. S26B†). [Ag₃₄(S-Adm)₁₈(DPPP)₃Cl₄]²⁺ displayed two obvious absorptions at 420 and 575 nm, and two shoulder bands at 380 and 460 nm (Fig. S26D†). [Ag₃₇(S-Adm)₂₅Cl₁]⁺ showed a series of absorptions at 345, 435, 565, 730, and 895 nm (Fig. S26E†). [Ag₂₅(S-Adm)₁₈]⁻ displayed two intense bands at 500 and 690 nm, showing ∼10 nm red-shift relative to the corresponding bands of [Ag₂₅(S-PhMe₂)₁₈]⁻ (Fig. S26C and F†).⁵⁸ Such a red-shift resulted from the ligand effect on nanocluster electronic structures, since the alternation of ligands of nanoclusters would affect their molecular orbital energy levels, embodied by their optical absorptions.^33,60–62

Conclusions

In summary, a “dual-level kinetic control” was exploited for synthesizing atomically precise silver nanoclusters and contributing to their high-yield preparation. Specifically, the introduction of phosphine ligands to the reaction retarded the reduction rate and accomplished the first-level kinetic control. Then, the cooling of the reaction further retarded the reduction rate and fulfilled the second-level kinetic control. A family of monodispersed Ag nanoclusters (including [Ag₂₅(SR)₁₈]⁻, [Ag₃₄(SR)₁₈(DPPP)₃Cl₄]²⁺, [Ag₃₆(SR)₂₆S₄]²⁺, [Ag₃₇(SR)₂₅Cl₁]⁺, and [Ag₅₂(SR)₂₈Cl₄]²⁺) were controllably synthesized and structurally determined. The developed “dual-level kinetic control” in this work might potentially act as a powerful tool for the preparation of more nanoclusters with unprecedented atomic structures.

Data availability

All the data supporting this article have been included in the main text and the ESI.†

Author contributions

X. W. and C. X. carried out experiments and analyzed the data. H. L. assisted with the analysis. X. K. and M. Z. designed the project, analyzed the data, and wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support by NSFC (21631001, 21871001, and 22101001), the Ministry of Education, and the University Synergy Innovation Program of Anhui Province (GXXT-2020-053).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Fig. S1–S26 and Tables S1–S9 for the HAADF-STEM image, structure anatomy, structure comparison, ESI-MS results, DLS results, UV-vis spectra, and crystal data of nanoclusters. CCDC 2094270–2094273, 2094275, 2094276 and 2094482. For ESI and crystallographic data in CIF or other electronic format see https://doi.org/10.1039/d2sc01016j

‡ These authors contributed equally to this work.

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