Bioinspired phosphate homeostasis for atomically precise regulation of silver nanoclusters

ABSTRACT

The dynamic dissociation equilibrium of phosphate in living organisms plays a crucial role in maintaining the balance necessary for sustaining life. In the field of metal clusters, [H_3−xPO₄]^x− (x = 1–3) anions also serve as effective templates for constructing silver clusters, with their innate structural flexibility bringing tremendous promise for structural regulation. However, current understanding of the effects of phosphate balance on the dynamic assembly of high-nuclearity silver clusters (metal atom number > 100) remains limited. In this study, we first demonstrate that different forms of phosphates (orthophosphate, hydrogen phosphate and dihydrogen phosphate) can controllably provide tetrahedral PO₄³⁻ oxyanions in the basic environment, thereby directing the structural evolution of silver clusters. A multilayered, rosette-shaped 104-nuclei silver nanocluster (Ag104a) is successfully isolated by utilizing Na₃PO₄/Na₂HPO₄ as the PO₄³⁻ source. This unique structure features a silver-containing (PO₄)@Ag₄@(PO₄)₁₂ template layer, enveloped by an outer Ag₁₀₀ shell composed of an Ag₇₂ garland and two Ag₁₄ units. Notably, Ag104a represents the silver alkynyl cluster with the highest number of encapsulated tetrahedral anions to date. In contrast, using NaH₂PO₄ results in the formation of a different co-crystallized silver cluster: Ag104b·Ag108a. Time-dependent ³¹P nuclear magnetic resonance analysis on the reaction solution reflected the different release rates of PO₄³⁻ anions, which can affect the assembly of silver clusters. This work not only makes a significant advancement in the structural regulation of high-nuclearity silver clusters by phosphates, but also offers valuable insights into the intricate interplay between phosphate balance and the dynamic assembly of silver clusters.

This study demonstrates how different forms of phosphates regulate the assembly processes of high-nuclearity (metal atom number >100) silver clusters.

INTRODUCTION

The burgeoning trajectory of interest in the field of cluster chemistry is currently captivating the scientific community, expanding the structural diversity of metal nanoclusters for next-generation functional materials [1–10]. Over the past two decades, one of the most important members in the cluster family—silver cluster—has emerged as a focal point of significant interest, garnering attention owing to its diverse structure, properties and promising applications [11–18]. In particular, high-nuclearity silver clusters (metal atom number > 100) boast charming aesthetic structures, rendering them one of the pursuits of synthetic chemists in a challenging field [19–33]. Various ligands have been utilized for stabilizing large metallic skeletons, including phosphine [19,20], thiolate [21–26], alkynyl [27–29], halides [30–32], p-tert-butylthiacalix[4]arene [33] and so on. Anion templates, distinguished by their multiple negative charges and diverse geometries, play a vital role in mediating the size, structure and property of silver clusters from within [34]. The anion-template synthetic strategy is recognized as a popular route for the controlled assembly of these clusters [35]. Compared with the simple ions (Cl⁻, S²⁻) [36–38], oxyanion templates with rich geometric shapes (such as triangular NO₃⁻, CO₃²⁻; tetrahedral ⁹⁹TcO₄⁻, SO₄²⁻, CrO₄²⁻, MoO₄²⁻, PO₄³⁻, AsO₄³⁻, VO₄³⁻; octahedral TeO₆⁶⁻ and higher bulk polyoxometalates) have larger size and higher negative charge, providing more coordination sites for the silver atoms and thus beneficial for constructing high-nuclearity silver clusters [39–46]. Although diverse anion-templated silver clusters have been successfully isolated, the synthesis of silver clusters with a total metal count of >100 remains a rare achievement. Notable examples include: [Ag₁₀₂S₆(PO₄)₈(CyS)₃₀(H₂PO₄)₆(HPO₄)₆(CF₃COO)₁₈]·6MeOH·6H₂O [47], [Ag₁₂@Ag₂₀@(KPO₄)₁₀@Ag₇₀(^tBuPhS)₆₀(CF₃COO)₁₀(DMF)₂] [48] and (H₃O)₂[(SO₄)₃₆S₂₂@Ag₁₉₂(CyS)₆₆(NO₃)₁₂]·4CH₃OH [26], all of which contain tetrahedral anion templates by using NaH₂PO₄, KH₂PO₄ and VOSO₄·xH₂O, respectively.

Phosphate salts, including Na₃PO₄, Na₂HPO₄ and NaH₂PO₄, play crucial roles in both health and the environment due to their active involvement in numerous industrial and biological processes (Scheme 1a). Corresponding phosphates, [H_3−xPO₄]^x− (x = 1−3), belong to the tetrahedral oxyanion family and are effective templates to construct high-nuclearity silver clusters [47–50]. Significant progress has been achieved in thiolate-protected silver clusters by using [H_3−xPO₄]^x− as templates, yet it remains unclear whether different protonated phosphate salts can markedly impact the structure, size and properties of high-nuclearity silver clusters. An essential consideration is that a complex acid–base dissociation balance can exist, such as PO₄³⁻ ⇆ HPO₄²⁻ ⇆ H₂PO₄⁻, resulting in multiple forms of phosphates with different charged states in solutions depending on the pH [51,52]. In our previous work, without the involvement of an acid or base, we isolated an Ag₁₀₂ cluster containing three forms of phosphates when using NaH₂PO₄ as the incipient template agent, while the utilization of Na₃PO₄, a solely PO₄³⁻ templated Ag₄₂ cluster, was obtained in a similar reaction system [50]. However, in the presence of a base, what direction does the dynamic dissociation balance of [H_3−xPO₄]^x− proceed toward during the synthesis of high-nuclearity silver clusters? Furthermore, what effects does this have on the formation of the ultimate silver clusters?

Scheme 1.

Synthetic conception. (a) Phosphate balance in nature. (b) Synthetic routes for Ag104a and Ag104b·Ag108a. Color legend: Ag, purple and green; P, orange; S, yellow; O, red; C, gray.

Herein, we attempt to answer these unsolved puzzles and present the effect of different sodium phosphate salts on the formation of high-nuclearity silver clusters. An unprecedented high-nuclearity silver cluster, Ag104a, exhibiting a special silver-containing (PO₄)@Ag₄@(PO₄)₁₂ template layer and an outer Ag₁₀₀ shell protected by cPrC≡C⁻ (cPr = cyclopropyl) and Ph₂PS₂⁻ ligands, was isolated by introducing Na₃PO₄ or Na₂HPO₄ as the PO₄³⁻ source in the presence of a base. To our knowledge, Ag104a represents the silver alkynyl cluster with the maximum count tetrahedral oxyanions to date. Notably, we obtained the other new co-crystallized silver cluster Ag104b·Ag108a with different structures from Ag104a by controlling the release rate of PO₄³⁻ using NaH₂PO₄. As revealed by the time-dependent ³¹P nuclear magnetic resonance (NMR), the release rates of PO₄³⁻ anions play a crucial role in regulating their structures. These findings underscore the effectiveness of various forms of phosphates in achieving precise control over the assembly processes of high-nuclearity silver clusters, thereby advancing our understanding of the structural dynamics of silver clusters.

RESULTS

The synthetic routes for Ag104a and Ag104b·Ag108a are summarized in Scheme 1b by using a facile one-pot solvothermal method. Briefly, Ag104a was synthesized by the reaction of [cPrC≡CAg]_n, Na₃PO₄·12H₂O, Ph₂PS₂·HEt₃N, CF₃SO₃Ag and N,N,N',N'-tetramethylethylenediamine (TMEDA) in CH₃OH at 70°C. Light-yellow block crystals of Ag104a were obtained by evaporation of the filtrate at room temperature for 2 d. Ag104a was first characterized by using single-crystal X-ray diffraction (SCXRD) and the SQUEEZE protocol in PLATON was employed to remove the electron contribution of highly disordered counter ions and solvents. A total of 1951 electrons were removed from the unit cell (Z = 2) (Fig. S1), which can be approximately assigned to five CF₃SO₃⁻, thirty-three CH₃OH and one H₂O per formula. Therefore, the total formula of Ag104a is {[Ag₁₀₄(PO₄)₁₃(cPrC≡C)₄₈(Ph₂PS₂)₁₂]·5CF₃SO₃·33CH₃OH·H₂O}, which is further verified by using thermogravimetric analysis (Fig. S2) and electrospray ionization mass spectrometry (ESI-MS) hereinafter. Remarkably, the use of equimolar Na₂HPO₄·12H₂O instead of Na₃PO₄·12H₂O also led to the isolation of Ag104a. However, by exclusively replacing Na₃PO₄·12H₂O with an equimolar amount of NaH₂PO₄·2H₂O while maintaining the other conditions, we found that colorless crystals were formed (Scheme 1b and Fig. S3). SCXRD analysis revealed that it was a co-crystal (Ag104b·Ag108a) composed of two silver clusters: [(PO₄)@Ag₄@(PO₄)₁₂@Ag₁₀₀S₄(cPrC≡C)₄₀(Ph₂PS₂)₁₆]⁺ (Ag104b) and [(PO₄)@Ag₈@(PO₄)₁₂@Ag₁₀₀S₄(cPrC≡C)₄₀(Ph₂PS₂)₁₆]⁵⁺ (Ag108a) at a ratio of 1:1 (Fig. S4) with partial site-occupancy disordered Ag₄ and Ag₈ cores, respectively. The co-crystallized silver nanoclusters have sporadically been observed in paired Ag₄₀/Ag₄₆ [53] and Ag₂₁₀/Ag₂₁₁ [20] with and without site-occupancy disordered features, respectively. The S²⁻ should have originated from the P–S bond cleavage of Ph₂PS₂⁻ ligands [54,55]. Using KH₂AsO₄·2H₂O, similar co-crystal (Ag104c·Ag108b) containing [(AsO₄)@Ag₄@(AsO₄)₁₂@Ag₁₀₀S₄(cPrC≡C)₄₀(Ph₂PS₂)₁₆]⁺ (Ag104c) and [(AsO₄)@Ag₈@(AsO₄)₁₂@Ag₁₀₀S₄(cPrC≡C)₄₀(Ph₂PS₂)₁₆]⁵⁺ (Ag108b) can also be obtained (Figs S3 and S5), indicating that the co-crystallization of large silver clusters in a single crystal is not accidental in this system. More detailed synthetic procedures and characterizations (Figs S6 and S7) are given in the Supporting Information.

X-ray structures

SCXRD analysis reveals that Ag104a crystallized in a tetragonal P4₂/nmc space group with D_4d symmetry. Its asymmetric unit contains a quarter of the Ag₁₀₄ cluster and the entirety exhibits a multilayered rosette-shaped structure. As portrayed in Fig. S8, the approximate dimension of the whole Ag₁₀₄ cluster is 3.0 nm × 3.0 nm × 2.3 nm and its metallic skeleton dimension is 1.9 nm × 1.9 nm × 1.1 nm. The Ag₁₀₄ cluster comprises 104 silver atoms, 13 PO₄³⁻ and 60 organic ligands, including 48 cPrC≡C⁻ and 12 Ph₂PS₂⁻. The presence of the CF₃SO₃⁻ counterion, although unresolved crystallographically, has been validated by using ESI-MS (Fig. S9). The composition and purity of Ag104a were confirmed by using ¹H NMR and ³¹P NMR (Figs S10 and S11). The ¹H NMR displays two sets of peaks at 0–2 and 7–9 ppm, corresponding to the H atoms of cPrC≡C⁻ and Ph₂PS₂⁻, respectively. The peaks centered at 65–70 and 15–22 ppm in ³¹P NMR are assigned to the P atoms of Ph₂PS₂⁻ and PO₄³⁻, respectively, which indicates dynamic averaging among chemically equivalent P atoms, as shown in the solid-state structure of Ag104a.

A more detailed anatomy was performed to gain in-depth insight into its structure (Fig. 1). The entire silver skeleton of the Ag₁₀₄ cluster can be divided into three hierarchies. In the center of the cluster is a distorted Ag₄ square without argentophilic interaction, which is encircled by a garland-shaped Ag₇₂ shell (Fig. 1c and d). The Ag₇₂ garland is intricately woven by two Ag₂₀ rings sharing four intersections to form Ag₃₆ rings (Fig. 1e), further reinforced by two smaller Ag₁₈ rings up and down (Fig. 1f). Positioned both above and below Ag₇₂ is an Ag₁₄ unit housing a central Ag₄ tetrahedron within its confines (Fig. 1g and h, and Fig. S12). Notably, this Ag₄ tetrahedron was known as the basic building unit commonly appearing in the silver cluster [57–61], but an example of such an appearance on the surface of a cluster has been rarely found [60,61]. Alternatively, it also can be seen as a distorted Ag₄ square wrapped in an integrated Ag₁₀₀ shell (Fig. S13). The argentophilic interactions (Ag···Ag) fall in the range of 2.75–3.37 Å and contribute to the stability of the overall metallic framework. Within the Ag₁₀₄ cluster, there are a total of 13 PO₄³⁻ anions supporting the silver skeleton through Ag–O bonds and they are symmetrically distributed around the crystallographic 4-fold rotoinversion axis. The innermost PO₄³⁻ anion is fixed in the center of the distorted Ag₄ square in a μ₈-κ²:κ²:κ²:κ² mode (Fig. S14), which is represented as a centered four-pointed star [(PO₄)@Ag₄]. Each silver atom on the distorted Ag₄ square ligates with three or four additional PO₄³⁻ anions to form the secondary anionic unity to support the outmost silver shell (Fig. 2a and b). These 12 PO₄³⁻ anions adopt μ₁₁-κ²:κ³:κ³:κ³ and μ₁₂-κ²:κ³:κ³:κ⁴ modes (Ag–O: 2.13–2.80 Å) at a ratio of 2:1. Specifically, the PO₄³⁻ anion plays three roles in the construction of Ag104a: (i) passivating the Ag₄ core; (ii) supporting the Ag₁₀₀ shell; and (iii) connecting the core and shell. It is noticed that the outermost cPrC≡C⁻ and Ph₂PS₂⁻ ligands regioselectively cover the Ag₁₀₀ shell of the Ag₁₀₄ cluster. The 48 cPrC≡C⁻ can be divided into two layers: 10 cPrC≡C⁻ ligands adopt μ₄-η¹:η¹:η¹:η¹, μ₃-η¹:η¹:η¹ and μ₂-η¹:η¹ coordination patterns at a ratio of 3:1:1, capping at the opposite two poles of the silver skeleton, and the remaining 28 are distributed near to the equator via 12 μ₃-η¹:η¹:η¹, 12 μ₄-η¹:η¹:η¹:η¹ and 4 μ₄-η¹:η¹:η¹:η² coordination patterns (Fig. 2c and Fig. S15) (Ag–C bond lengths: 2.04–2.68 Å). A total of 12 bidentate Ph₂PS₂⁻ ligands, 6 in one group, are inserted between the cPrC≡C⁻ layers (Fig. 2d and Fig. S16). Each caps on the square face in a μ₄-η²:η² binding mode and the Ag–S bond lengths fall within the range of 2.53–2.59 Å.

Figure 1.

Structure dissection of silver skeleton in Ag104a. (a) Top view of the overall Ag₁₀₄ core. (b) Front view of the overall Ag₁₀₄ core in a space-filling mode. (c) Top and (d) front views of Ag₇₆ consisting of a distorted Ag₄ square encircled by an Ag₇₂ garland. (e) Ag₃₆ shell interlaced together by two Ag₂₀ rings sharing four intersections highlighted by black dashed circles. (f) Two smaller Ag₁₈ rings. (g) Two Ag₁₄ units containing Ag₄ tetrahedron at the poles of the whole silver skeleton (h). Dashed lines indicate Ag···Ag distances of >3.44 Å corresponding to the absence of argentophilic interaction [56]. All atoms with different colors are silver atoms.

Figure 2.

(a) Top and (b) front views of the silver-containing (PO₄)@Ag₄@(PO₄)₁₂ template layer in Ag104a. Dashed lines indicate Ag···Ag distances of >3.44 Å. PO₄³⁻ anions are highlighted as pale yellow tetrahedrons. (c) Distribution of cPrC≡C⁻ ligands at the equator (brown) and poles (dark blue) on the Ag₁₀₀ shell of Ag104a. (d) Distribution of 12 Ph₂PS₂⁻ ligands on the Ag₁₀₀ shell of Ag104a. Color legend: Ag, green and pink; P, orange; S, yellow; O, red; C, dark blue, brown and gray.

Ag104b·Ag108a, a mixture of Ag₁₀₄ and Ag₁₀₈ at a 1:1 ratio, are crystalized in a tetragonal I space group. As in Ag104a, the Ag₁₀₄ cluster in Ag104b·Ag108a also has an Ag₄ square, connected with 13 PO₄³⁻, forming a silver-containing (PO₄)@Ag₄@(PO₄)₁₂ template layer (Figs S17 and S18). In contrast, the Ag₁₀₈ cluster displays a folded Ag₈ octagon with a uniform Ag···Ag distance of 2.66 Å, indicating the presence of nontrivial argentophilic interactions. Housing in the Ag₈ unit is one PO₄³⁻ tetrahedron in a μ₁₂-κ³:κ³:κ³:κ³ coordination mode, represented as [(PO₄)@Ag₈] (Figs S17–S19), which is further linked with 12 PO₄³⁻, forming a silver-containing (PO₄)@Ag₈@(PO₄)₁₂ template layer. Both silver-containing template layers are encapsulated by the same Ag₁₀₀ shell (Fig. 3a and b), which can also be viewed as an Ag₇₂ garland capped by two Ag₁₄ units (Fig. S18). Moreover, Ag₁₀₄ and Ag₁₀₈ clusters have identical ligand layers composed of 40 cPrC≡C⁻, 16 Ph₂PS₂⁻ and 4 S²⁻, with the same total count of ligands as Ag104a (48 cPrC≡C⁻ and 12 Ph₂PS₂⁻). Notably, unlike in Ag104a, both Ag₁₀₄ and Ag₁₀₈ clusters feature 8 cPrC≡C⁻ and 2 μ₃-bridging S²⁻ ligands at the opposite two poles of the silver skeleton, of which two S²⁻ ligands occupy the two cPrC≡C⁻ positions in Ag104a (Ag–S bond lengths: 2.33–2.62 Å), which leads to lower S₄ symmetry, leaving 24 cPrC≡C⁻ ligands located at the equator (Figs 2c, 3c and Fig. S20). The Ag–C distances are in the range of 1.99–2.68 Å. Except for the 12 μ₄-mode Ph₂PS₂⁻ inserted between the cPrC≡C⁻ layers (Fig. 3d and Fig. S21), there are still 4 μ₄-mode Ph₂PS₂⁻ cooperating with the equatorial 24 cPrC≡C⁻ to stabilize the silver shell (Ag–S bond lengths: 2.49–2.62 Å). Furthermore, the composition of Ag104b·Ag108a was confirmed by using ¹H NMR and ³¹P NMR (Figs S10 and S11). The ³¹P NMR displays a set of peaks centered at ∼20 ppm, evidencing the presence of only PO₄³⁻ anions in Ag104b·Ag108a, without HPO₄²⁻ and H₂PO₄⁻ anions. Additionally, four peaks at 67–71 ppm at a ratio of 1:1:1:1 are assigned to the P atoms of Ph₂PS₂⁻ in four different chemical environments, as shown in the structure of the ligand layer of both Ag₁₀₄ and Ag₁₀₈ clusters in Ag104b·Ag108a.

Figure 3.

(a) Overall Ag₁₀₄ framework in Ag104b. (b) Overall Ag₁₀₈ framework in Ag108a. (c) Distribution of cPrC≡C⁻ ligands at the equator (brown) and poles (dark blue) on the Ag₁₀₀ shell of both Ag₁₀₄ and Ag₁₀₈ clusters in Ag104a·Ag108a. (d) Distribution of 16 Ph₂PS₂⁻ ligands on the Ag₁₀₀ shell of Ag104a·Ag108a. Dashed lines indicate Ag···Ag distances of >3.44 Å. Color legend: Ag, pink, blue, apricot, buff and green; P, orange; S, yellow; C, dark blue, brown and gray.

Phosphate balance-regulating assembly of silver clusters

As observed in Ag₁₀₄ and Ag₁₀₈ clusters from Ag104b·Ag108a, only trivalent tetrahedral PO₄³⁻ anion was ultimately found as a template, despite the H₂PO₄⁻ being used in the synthesis of the silver clusters. One potential factor contributing to the structural difference in Ag104a and Ag104b·Ag108a may be attributed to variances in the release rate of PO₄³⁻ in solution. Under a weakly alkaline environment created by TMEDA, Na₃PO₄ directly released PO₄³⁻ whereas the H₂PO₄⁻ of the NaH₂PO₄ slowly transformed into PO₄³⁻. TMEDA can consume H⁺ ions as in the following equation: (CH₃)₂N(CH₂)₂N(CH₃)₂ + 2H⁺ → [(CH₃)₂NH(CH₂)₂NH(CH₃)₂]²⁺ [62], which promotes the equilibrium of H₂PO₄⁻ toward the release of PO₄³⁻ anions (H₂PO₄⁻ ⇆ H⁺ + HPO₄²⁻, HPO₄²⁻ ⇆ H⁺ + PO₄³⁻) [63]. In a previous example, we observed three phosphate species (H₂PO₄⁻, HPO₄²⁻ and PO₄³⁻) coexisting in a thiolate-protected Ag₁₀₂ cluster when H₂PO₄⁻ ion was utilized without a base in the reaction system [47]. This also emphasizes the essential role of TMEDA in the formation of silver clusters in this system, as it provides a weak base environment to control the slow release of PO₄³⁻ from H₂PO₄⁻ (Table S1).

³¹P NMR has proven to be an effective method for distinguishing various [H_3−xPO₄]^x− species that are present in the reaction mixture during cluster synthesis [47]. Thus, to further verify the above deduction, time-dependent ³¹P NMR was used to track the synthesis of Ag104a and Ag104b·Ag108a. As anticipated, the ³¹P NMR analysis of Ag104a revealed a characteristic signal of PO₄³⁻ that appeared at 18.43 ppm, even at the initial reaction stage, concurrently with HPO₄²⁻ (13.8 ppm, from the partial hydrolysis of PO₄³⁻) and a broad Ph₂PS₂⁻ envelope (65–70 ppm, centered at 66 ppm, reflecting the formation of Ag–S bonds) (Fig. 4a). Notably, these chemical shifts show significant downfield displacements compared with reference sodium phosphate salt [47], indicating the occurrence of phosphate-templated nucleation. With progression, the HPO₄²⁻ signal diminished entirely, leaving the single signal of PO₄³⁻. The ³¹P NMR of the reaction solution after 30 h exhibits nearly identical signals to those of Ag104a dissolved in CD₂Cl₂, indicating the formation of Ag104a. In contrast, in the ³¹P NMR of Ag104b·Ag108a before solvothermal treatment, only signals for H₂PO₄⁻ and HPO₄²⁻ were clearly detected at 0.64 and 15.87 ppm, respectively (Fig. 4b). Critical spectral transitions occurred at 10 h: emergent PO₄³⁻ anions (64–71 ppm) mark template deprotonation and core fixation, while the Ph₂PS₂⁻ envelope evolves from a single peak (67.33 ppm) to split into multiple peaks (67–71 ppm), indicating coordination symmetry at silver cluster interfaces. Furthermore, the ³¹P NMR of the reaction solution after reacting for 30 h exhibits almost identical signals to those of Ag104b·Ag108a dissolved in CD₂Cl₂, suggesting their formation in the solution. These results indicate that the release rate of PO₄³⁻ in Ag104b·Ag108a is slower than that in Ag104a. This is because H₂PO₄⁻ undergoes two chemical reactions involving deprotonation to release PO₄³⁻ in a weakly alkaline environment (Fig. 4c and d) whereas Na₃PO₄ directly releases PO₄³⁻.

Figure 4.

Time-dependent ³¹P NMR of reaction solution for (a) Ag104a and (b) Ag104b·Ag108a. Schematic representation of the dissociation equilibrium of [H_3−xPO₄]^x− in the reaction solution of (c) Ag104a and (d) Ag104b·Ag108a. Stars indicate starting materials. Positive-ion mode ESI-MS of (e) Ag104a and (f) Ag104b·Ag108a. Inset: Zoom-in mass spectra of experimental (blue line) and simulated (pink line) isotope patterns for +5 labeled species.

ESI-MS of Ag104a and Ag104b·Ag108a

The ESI-MS technique was further employed to verify the compositions of these silver clusters. As shown in Fig. 4e, the ESI-MS of Ag104a dissolved in CH₂Cl₂ in the positive-ion mode shows three major groups of peaks, each corresponding to different charge states of species: +6 species (m/z = 3000–3170), +5 species (m/z = 3630–3850) and +4 species (m/z = 4580–4820). The most prominent set of peaks is a group of signals consisting of 14 +5 species (5a–5n), of which 5f at the m/z = 3713.92 is assigned to [Ag₁₀₄(PO₄)₁₃(cPrC≡C)₄₈(Ph₂PS₂)₁₂]⁵⁺ (calcd. m/z = 3713.87), which can be regarded as the molecular ion of Ag104a. 5g–5n contain an intact Ag₁₀₄ cluster appended by solvents or ions, whereas 5a–5e are formed by ligand exchange in the Ag₁₀₄ cluster (Fig. S22). In the other two envelopes, all of the +4 (4a–4 m) and +6 (6a–6l) species also contain a complete 104-nuclei silver framework, as evidenced by the assigned formula by matching experimental and simulated isotopic distributions (Figs S23 and S24). From the small-angle X-ray scattering results, Ag104a dispersed in solution is relatively stable without aggregation (Fig. S25) and it can be modeled as a sphere (Fig. S26) with dimensions that are close to those for the nanocluster obtained from crystal data [64–66].

Moreover, ESI-MS provides powerful evidence to verify the coexistence of two different silver clusters in Ag104b·Ag108a [53]. To guarantee the purity of the sample, the single crystals were selected to perform measurement in CH₂Cl₂–CH₃OH mixed solvents. Similarly to Ag104a, there are mainly also three grouped peaks with +6, +5 and +4 species, but the groups of +5 and +4 species show two relatively separated peaks rather than the consecutive peaks that were observed in Ag104a (Fig. 4f). Among them, 5b' at m/z = 3949.27 is assigned to [Ag₁₀₄(PO₄)₁₃S₄(cPrC≡C)₃₇(Ph₂PS₂)₁₅(H₂O)]⁵⁺ (calcd. m/z = 3749.36), which can be attributed to the molecular ion of the Ag₁₀₄ cluster in Ag104b·Ag108a by removing three cPrC≡C⁻ ligands and one Ph₂PS₂⁻ ligand but adding one H₂O molecule (Fig. S27). Of note, 5c' centered at m/z = 3921.02 corresponds to [Ag₁₀₈(PO₄)₁₃S₄(cPrC≡C)₄₀(Ph₂PS₂)₁₆]⁵⁺ (calcd. m/z = 3921.10), which can be attributed to the molecular ion of the Ag₁₀₈ cluster in Ag104b·Ag108a (Fig. S27). Similarly, the compositions of both the Ag₁₀₄ cluster and the Ag₁₀₈ cluster can also be detected in the set of +4 species (Fig. S28). Comparison of the experimental (pink line) and simulated (blue line) isotopic distributions confirms the formula assignments. By contrast, only the species of the Ag₁₀₈ cluster can be identified in the set of peaks with +6 valence (Fig. S29). The above observations prove the real presence of two clusters in a single crystal of Ag104b·Ag108a.

Theoretical analysis of Ag104a

In order to gain further insights into the electronic structure of the ligand-protected silver phosphate cluster, computational calculations on Ag104a were carried out. The relaxed geometries of the metal backbone are in good agreement with the experimentally characterized results, as denoted by the root-mean-square deviation (RMSD) of 0.099 Å for Ag104a in comparison with the X-ray structures. For the ligand-protecting layer provided by external organic ligands, the RMSD increases to 0.475 Å, denoting a deviation from the experiment and theory, owing to the fact that such a layer is more affected by packing effects in the crystal structure. The density of states is able to expose the contribution of the electronic shells in terms of the constitutive protecting-layer-, phosphate- and 4d/5s-Ag-based orbitals (Fig. 5a). The lower occupied levels are dominated by 4d-Ag/PO₄/ligand contributions, turning to a main PO₄³⁻ character at the high-lying occupied orbitals, denoting a ligand character for the low-lying unoccupied orbitals, with contributions of the 5s-Ag levels (Fig. 5b), resulting in a calculated highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap of 1.00 eV for the overall Ag₁₀₄ cluster. The appearance of 5s-Ag levels as part of the unoccupied molecular orbitals manifold denotes that the metal centers are formally Ag(I) ions, as supported by the Hirshfeld charge analysis averaging to +0.65 ē per Ag atom within the Ag₁₀₄ core backbone. The obtained ultraviolet-visible (UV-vis) spectrum for Ag104a exhibits a distinctive peak at ∼420 nm. The simulated UV–vis pattern shows a shoulder at 435 nm (Fig. 5c). The ∼420-nm peak in Ag104a is of the main PO₄/4d-Ag-to-ligand charge transfer, centered at the aryl rings at Ph₂PS₂⁻ moieties, thus ascribed as a core-to-ligand character [67].

Figure 5.

(a) Density of states for Ag104a, denoting the contribution from 4d-Ag, 5s-Ag, PO₄³⁻ and ligand-based levels and the respective Ag-PO₄³⁻ overlap denoting bonding (positive) and anti-bonding (negative) contributions. (b) Frontier orbitals denoting the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) and the orbitals involved in the main UV–vis absorbance are given for Ag104a. (c) Experimental and calculated UV–vis absorbance profiles for Ag104a.

Furthermore, in order to evaluate the role of the encapsulated PO₄³⁻ anions, an energy decomposition analysis (EDA) [68] of the interaction energy between PO₄³⁻ and [Ag₄@(PO₄)₁₂Ag₁₀₀(cPrC≡C)₄₈(Ph₂PS₂)₁₂]⁸⁺ was conducted for Ag104a. The calculated interaction energy (ΔE_int) amounts to −1705.1 kcal/mol, denoting that the encapsulation of PO₄³⁻ anions is largely favored. The EDA analysis is able to decompose the interaction energy into chemically meaningful terms accounting for the electrostatic (ΔE_elstat), orbital (ΔE_orb) and London dispersion (ΔE_disp), which are stabilizing contributions accounting for the efficient PO₄³⁻ encapsulation. Such values denote a stabilizing contribution of 55.2% from the electrostatic term, which amounts to −1304.4 kcal/mol for the formation of Ag104a, and of 43.7% from the orbital term, amounting to −1033.2 kcal/mol for the cluster. In addition, a small contribution from London dispersion is given, accounting for 1.1% of the stabilizing interactions, amounting to −25.6 kcal/mol. Thus, efficient encapsulation of PO₄³⁻ is given mainly by both the electrostatic and the orbital character of the interaction.

Moreover, we computationally explore both the structural and the electronic changes upon removal of the central PO₄³⁻ anion in order to evaluate its role in the resulting characteristics of the overall Ag₁₀₄ cluster. Geometry relaxation of the hypothetical hollow [Ag₄@(PO₄)₁₂Ag₁₀₀(cPrC≡C)₄₈(Ph₂PS₂)₁₂]⁸⁺ counterpart shows an increased distortion of the metallic backbone of the Ag₁₀₄ cluster, as given by the RMSD values of 0.269 Å in comparison with the parent cluster. Hence, the PO₄³⁻ ion is relevant to retain the Ag₁₀₄ core architecture within the cluster. The removal of the central PO₄³⁻ ion leads to a decrease in the HOMO–LUMO gap to 0.64 eV, which leads to the strong variation in the UV–vis absorption profile (Figs S30 and S31). The charge transfer is estimated via the Hirshfeld charge analysis leading to a charge of −0.06 e for the central PO₄³⁻ unit, denoting that the orbital interaction term from the EDA is given by the sizable PO₄³⁻→[Ag₄@(PO₄)₁₂Ag₁₀₀(cPrC≡C)₄₈(Ph₂PS₂)₁₂]⁸⁺ charge transfer of 2.94 e. Thus, the central PO₄³⁻ unit serving as a structural template and modifier of the both frontier orbital and UV–vis features for the overall Ag104a suggests the term ‘non-innocent templates’, which was coined to denote embedded ions or molecular motifs that were able to tune the inherent cluster characteristics.

Optical properties of Ag104a

The solid-state UV–vis absorption spectrum of Ag104a was measured at room temperature. The cluster exhibits broad absorption across the UV and visible regions, with absorption in the range of 330–495 nm (Fig. S32a). The band gap of Ag104a was determined as 1.26 eV by using the Kubelka–Munk and Tauc functions, which indicates the narrow gap semiconductor nature (Fig. S32b). Ag104a exhibits a weak photoluminescence (PL) band near the edge of the visible region with a peak centered at 616 nm under an excitation of 480 nm, producing a large Stocks shift (i.e. 1239.83/480 = 2.58 eV to 1239.83/616 = 2.01 eV). The PL dynamics were studied by using the time-correlated single-photon counting technique; two microsecond components at 83 K (τ₁ = 0.97 μs, B₁ = 44.29%, τ₂ = 12.18 μs, B₂ = 55.71%) are required to fit the decay for Ag104a, giving rise to an average lifetime of 11.54 μs. In the same way, two lifetime components for Ag104a at 293 K (τ₁ = 0.58 μs, B₁ = 6.25%, τ₂ = 11.58 μs, B₂ = 93.75%) are required, with an average lifetime of 11.51 μs. The fitting results suggest that the PL may originate from a triplet excited state with two relaxation channels [69]. To verify the triplet characteristic, electron paramagnetic resonance spectra were measured to confirm the singlet oxygen (¹O₂) generation by using 2,2,6,6-tetramethylpiperidine (TEMP) as the trapping reagent. Ag104a shows a significant ¹O₂ signal under xenon-lamp irradiation (Fig. S33) in the solid state, suggesting its triplet spin-multiplicity of the excited state.

To study temperature-dependent emission behaviors, the emission spectra (λ_ex = 480 nm) of Ag104a in the solid state were collected from 83 to 283 K, with 20 K as an interval. As presented in Fig. 6a, the PL intensity exhibits a negative correlation with temperatures from 283 to 83 K, in which the emission intensity at 83 K showcases a nearly 9.5-fold increase compared with that at 283 K. Moreover, the PL position gradually shifted from 636 to 616 nm during the cooling process, which corresponds to an increase in the PL energy level. Meanwhile, the PL lifetimes of Ag104a at 283 and 83 K were almost unchanged, with both being ∼11.5 μs (Fig. 6b), reflecting that the relaxation channels and lifetimes of excited electrons are temperature-independent. As is widely reported, the temperature-dependent emissions should be in connection with the variations in molecule rigidity and argentophilic interactions [70–72]. Therefore, we tentatively deduced that the blue shift in the temperature-dependent PL of Ag104a should be attributed to the triplet core-to-ligand charge transfer (³MLCT) from the Ag atoms to the cPrC≡C⁻ and Ph₂PS₂⁻ligands and is more susceptible to temperature than the cluster-center (³CC)-dominated PL. As shown in Fig. 6c, with increasing temperature, the relative emission intensity of Ag104a (Δ = I/I₂₈₃, I₂₈₃ denotes the integrated intensity of the band at 283 K) decreases. The temperature-dependent emission intensity can be well fitted via Equation (1):

Figure 6.

(a) Temperature-dependent PL spectra of Ag104a in the solid state from 83 to 283 K with 20 K as an interval. (b) PL decay traces of Ag104a at 83 and 293 K. (c) Normalized integrated peak intensities and fitting by using Equation (1) (data from (a)). (d) Relative thermal sensitivity S_r as a function of temperature.

(1)

with a correlation coefficient R² of 0.989 for Ag104, which indicates that the cluster can be used as a luminescent thermometer between 83 and 283 K. The relative thermal sensitivity S_r (S_r = |Δ/T|/Δ) is used to characterize the performances of the thermometers. The S_r of Ag104a increases linearly with temperature, with a maximum value of 2.06% K⁻¹ at 283 K (Fig. 6d). These good thermometric characteristics and a wide temperature measurement range for Ag104a make it potentially valuable for optical thermometer applications [73,74].

CONCLUSION

In summary, we explore the release rate of PO₄³⁻ in weakly alkaline environments by utilizing different forms of phosphates (orthophosphate, hydrogen phosphate and dihydrogen phosphate) as a means to regulate the assembly of high-nuclearity silver clusters. Direct employment of Na₃PO₄ led to a multilayered rosette-shaped silver cluster Ag104a, which possesses a unique silver-containing (PO₄)@Ag₄@(PO₄)₁₂ template layer. Conversely, when utilizing NaH₂PO₄ to indirectly release PO₄³⁻, we obtained the other new co-crystallized silver cluster Ag104b·Ag108a. Furthermore, time-dependent ³¹P NMR was used to track the synthesis of Ag104a and Ag104b·Ag108a and the results revealed that different release rates of the PO₄³⁻ anions can affect the assembly of silver clusters. This work not only demonstrates silver alkynyl clusters containing the maximum count of tetrahedral oxyanions, but also provides crucial insights into regulating the assembly of high-nuclearity silver clusters by phosphate dissociation balance and opportunities for researchers to understand the structural dynamics of silver clusters.

DATA AVAILABILITY

The data that support the findings of this study are available within the article and its supplementary information files. Other relevant data are available from the corresponding author upon request. The X-ray crystallographic coordinates for structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 2410600, 2410601 and 2410602 for Ag104a, Ag104b·Ag108a and Ag104c·Ag108b. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

FUNDING

This work was supported by the National Natural Science Foundation of China (22201159 to Z.W., 22171164, 22325105, 52261135637 and 92361301 to D.S) and the Natural Science Foundation of Shandong Province (ZR2022QB008). A.M.-C. gives thanks for support from Agencia Nacional de Investigación y Desarrollo (ANID) FONDECYT Regular 1221676.

AUTHOR CONTRIBUTIONS

D.S. conceived and designed the experiments; W.D.S. conducted the synthesis and characterization; W.D.S., L.Y.X., Z.W. and D.S. performed the research and analysed the data; W.D.S., L.Y.X., A.M.C., C.Z., B.L.H., J.L.Z., Z.W., C.H.T. and D.S. contributed to scientific discussion; W.D.S., Z.W. and D.S. wrote the paper. All authors discussed the results and commented on the manuscript.

Conflict of interest statement. None declared.