Polyoxometalate-mediated syntheses of three structurally new silver clusters†

Jing Zhang , Yuanyuan Dong , Lan Deng , Manzhou Chi , Yeqin Feng , Mengyun Zhao , Hongjin Lv * and Guo-Yu Yang
MOE Key Laboratory of Cluster Science, Beijing Key Laboratory of Photoelectric/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488, P. R. China. E-mail: hlv@bit.edu.cn

First published on 27th May 2024

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1. Introduction

Atomically precise coinage metal (Cu, Ag, Au, etc.) nanoclusters have attracted increasing attention from the scientific community due to their novel aesthetic structures, unique physicochemical properties and widespread application,^1–6 including their promising applications in various fields like catalysis, photochromism, chirality, biomedicine, and chemical sensing, etc.^7–14 The synthesis of Ag clusters is inextricably linked to the involvement of surface-protecting organic ligands, mainly thiolate,^15–19 phosphine,^20–22 alkynyl,^23–25 and amide ligands²⁶ or appropriate combination of these ligands,²⁷ which can offer more possibilities for the structure and properties of Ag clusters. In addition to the adjustable ligand type, the introduction of structure-directing agents like anionic templates has also largely contributed to the construction of high-nuclearity Ag clusters, and the anion-template synthetic strategy has become a popular route for making novel Ag clusters.^28–30

Polyoxometalates (POMs), typically constructed using early transition-metal elements (Mo, W, V, Nb, Ta) in their highest oxidation states, are a class of large anionic metal-oxo clusters with rich bridging and terminal oxygen atoms, exhibiting diverse applications.^31–37 Compared with simple conventional inorganic anions (e.g. SO₄²⁻, CO₃²⁻, CrO₄²⁻, and MoO₄²⁻, etc.), POM-based anionic templates exhibit bigger sizes, more active oxygen sites, and unique redox properties, which could not only induce the aggregation of Ag cations into high-nuclearity Ag clusters with interesting architectures, but also adjust the physicochemical properties of the resulting Ag clusters. To date, a large number of Ag clusters directed by POM templates have been successfully prepared and reported.^38–47 For instance, in 2014, Wang's group synthesized a novel Ag₇₀ alkynyl cluster by adopting lacunary POM [A-α-PW₉O₃₄]⁹⁻ in a distinctive ionic liquid environment.⁴⁰ Subsequently, they prepared another unique Ag₄₁ cluster containing a di-lacunary [α-SiW₁₀O₃₇]¹⁰⁻ as an anionic template and a saturated [β-SiW₁₂O₄₀]⁴⁻ as the counter anion, in which both the internal [α-SiW₁₀O₃₇]¹⁰⁻ template and the external [β-SiW₁₂O₄₀]⁴⁻ counter-anion are in situ derived from the precursor [γ-SiW₁₀(H₂O)₂O₃₄].⁴¹ Alternatively, Zang and co-workers also synthesized an Ag₆₇ cluster consisting of two lacunary Keggin [PW₉O₃₄]⁹⁻ cores and a Ag₆₇S₃₆ shell that was further protected by a thiol p-F-PhS⁻ ligand.⁴²

In contrast to homometallic clusters, the doping of heterometals in a discrete molecular skeleton may generate novel heterometallic clusters with attractive properties. Generally, transition-metal ions have been utilized for the construction of structurally new heterometallic clusters with the goal of exploring new photochemical, catalytic, and photoluminescent properties. However, the doping of transition metals into POM-templated Ag clusters has rarely been reported. A representative example has been reported by Lu and co-workers in 2018. The authors have prepared two structurally new high-nuclearity heterometallic ethynide clusters, Ag₃₄Cu₆ and Ag₃₇Cu₆, using Cu-substituted lacunary polyoxotungstates, phosphonic acid, and silver(I) ethynide. In their work, both clusters exhibit a C₃-symmetric cluster shell composed of silver(I) and copper(II) cations encapsulating a [SiW₉O₃₄]¹⁰⁻ template anion.⁴⁸

In this context, we report herein the facile solvothermal preparation of three structurally new POM-templated silver clusters, homometallic [(SiW₉O₃₄)@Ag₂₄(ⁱPrS)₁₁(DPPP)₆Cl]₂(SiW₁₂O₄₀) (Ag₂₄), heterometallic [(SiW₉O₃₄)@Ag₂₂Cu(ⁱPrS)₁₁(DPPP)₆Cl](SbF₆)₂ (Ag₂₂Cu) and {Ag₁₆(ⁱPrS)₆(DPPP)₈(CH₃COO)₄[Co₄(OH)₃(H₂O)SiW₉O₃₃]₂}·(CH₃CN)₄ (Ag₁₆Co₈) (ⁱPrS⁻ = isopropanethiolate, DPPP = 1,3-bis(diphenylphosphino)propane, SbF₆⁻ = hexafluoroantimonate). The resulting three clusters all exhibited interesting temperature-dependent emission and photothermal conversion properties.

2. Results and discussion

Three atomically precise silver clusters Ag₂₄, Ag₂₂Cu, and Ag₁₆Co₈ have been synthesized by a facile solvothermal approach, using the lacunary POM ligand as anionic template. Orange block Ag₂₄ was synthesized through a reaction of TBA₄H₆[SiW₉O₃₄] (TBA-SiW₉), CH₃COOAg, ⁱPrSAg, and DPPP in mixed CH₃CN/H₂O/DMF solvent at 60 °C for 48 h. The synthetic procedures of Ag₂₂Cu and Ag₁₆Co₈ are similar to that of Ag₂₄, except for the extra addition of copper acetylacetonate and SbF₆⁻ to obtain orange block Ag₂₂Cu crystals, and cobalt acetylacetonate for the red block Ag₁₆Co₈ crystals (Scheme 1, see the ESI† for the detailed procedure). In particular, we obtained products with larger sizes and higher yields when the raw material silver acetate was changed to silver benzoate in these syntheses. Clear digital photographs of Ag₂₄, Ag₁₆Co₈ and Ag₂₂Cu crystals are shown in Fig. S1.† All three clusters were systematically characterized by single-crystal X-ray crystallography and various spectroscopic techniques.

Single-crystal X-ray diffraction analyses show that Ag₂₄ crystallizes in the triclinic P space group (Table S1†), which consists of 2 [Ag₂₄(ⁱPrS)₁₁(DPPP)₆(SiW₉O₃₄)Cl]²⁺ cations and 1 peripheral [SiW₁₂O₄₀]⁴⁻ counter anion. The lacunary POM ligand {SiW₉O₃₄} is encapsulated inside the Ag shell as an anionic structure-directing template, and {SiW₁₂O₄₀} is located outside the Ag shell as an anti-anion; both of them are in situ derived from the precursor TBA₄H₆[SiW₉O₃₄]. The Ag₂₄ shell can be divided into three parts: an {Ag₄} tetrahedral unit, an {Ag₅S₅} fragment, and three I-shaped {Ag₅S₂} fragments (Fig. 1a and S2†). The {Ag₄} tetrahedron connects with a lacunary [SiW₉O₃₄]¹⁰⁻ ligand via three Ag atoms, Ag18, Ag50, and Ag44, with Ag–μ₃-O bonds 2.410(2)–2.590(2) Å, and the Ag–Ag bond distances in the {Ag₄} tetrahedron are in the range of 2.683(6)–2.824(3) Å (Fig. S2 and Table S2†). The fourth atom Ag4 in the {Ag₄} tetrahedron as the central atom participates in the pentagonal {Ag₅S₅} motif (Fig. S2†), completed by an upper Cl⁻ (Ag–Cl = 2.412(7) Å) ion to meet hexa-coordination. All sulphur atoms in {Ag₅S₅} motifs originate from the ⁱPrS⁻ ligand. Three I-shaped {Ag₅S₂} fragments combine the above structures with Ag–Ag and Ag–S bonds and the outermost six DPPP ligands encapsulate the entire structure through Ag–O, Ag–S, and Ag–Ag bonds, which greatly improve the structural stability of the Ag₂₄ cluster. Structural analyses revealed that 11 ⁱPrS⁻ ligands coordinated to the Ag atoms with the same μ₃–η₁:η₁:η₁ modes in the Ag₂₄ cluster (Fig. S3†), with Ag–S bond distances in the range of 2.162(2)–2.96(2) Å (Table S2†). The bond valence sum (BVS) calculations in Table S3† give the valences of 5.832, 6.133, 6.111, 6.222, 6.039, 6.161, 5.874, 6.059, and 6.106 for W and 3.939 for Si, confirming that all W and Si atoms are in +6 and +4 oxidation states in Ag₂₄, respectively.

The Ag₂₂Cu cluster was obtained according to the synthetic approach of the Ag₂₄ cluster by doping the heterometallic copper and a little SbF₆⁻, which crystallized in tje monoclinic P2₁/n space group (Table S1†). Herein, it is noted that the addition of NaSbF₆ was beneficial for obtaining a high-quality Ag₂₂Cu crystal, and the SbF₆⁻ anion worked as the counterion to balance the positive charge of the structure. In this structure, the Ag1, Ag2, Ag3 and Ag4 atoms in the {Ag₅S₅} fragment of the Ag₂₄ cluster adopted 50% occupancy, leading to two fewer Ag atoms in the Ag₂₂Cu cluster; additionally, one Ag site was fully occupied by a Cu atom (Fig. S4†). The rest of the structure was similar to that of the Ag₂₄ cluster (Fig. 1b). Moreover, the bond distance of Ag–Cu was 2.925(9) Å (Fig. S4 and Table S4†) and BVS calculations confirmed the +6 and +5 oxidation states of W and Si atoms in the Ag₂₂Cu cluster, respectively (Table S5†).

X-ray crystallographic structural analyses show that the intact Ag₁₆Co₈ cluster is built with 2-fold symmetry of the [Ag₈(ⁱPrS)₃(DPPP)₄(CH₃COO)₂(Co₄(OH)₃(H₂O)SiW₉O₃₃)](CH₃CN)₂ fragment which consists of one central [Co₄SiW₉O₃₃] unit, two CH₃COO⁻, three ⁱPrS⁻ ligands, four DPPP, three OH⁻, eight Ag⁺ and one free H₂O as well as two peripheral acetonitrile molecules, as shown in Fig. 2. The [Co₄SiW₉O₃₃] unit consists of two intrinsic [SiW₉O₃₄]¹⁰⁻ and two {Co₄O₃} motifs, with the two quaternary-Co-substituted {SiW₉O₃₄} connected by sharing the two terminal oxygens of the {WO₆} unit, where the distances of the Co–O bonds are in the range of 2.011(6)–2.359(6) Å (Table S6†). The calculated BVS values for these O atoms in the two {Co₄O₃} units are 1.117, 1.093, 1.069, 1.117, 1.093, and 1.069, respectively (Table S7†), suggesting that these 6 oxygens are protonated, which causes the valence state of [Co₈(OH)₆(SiW₉O₃₃)₂] unit to be −6. The sixteen silver atoms in the outer shell are connected by six ⁱPrS⁻ ligands to form four {Ag₃S} and two {Ag₂S} fragments covering the hybrid POM surface, and these units are connected to each other by long-chain DPPP ligands. These six ⁱPrS⁻ ligands adopt two different geometrical coordination modes, including μ₃–η₁:η₁:η₁ (four) and μ₂–η₁:η₁ (two) in Fig. S5.† In addition, two CH₃COO⁻ ligands connect with each [Co₄SiW₉O₃₃] unit through Co–O bonds, and the overall structure is further surrounded by 8 DPPP ligands to form an overturned “S” shape in Fig. 2.

The resulting three clusters have been further characterized using different spectroscopic techniques. Thermal gravimetric analyses (TGA) were performed to examine the thermal stabilities of Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ clusters in a dry air atmosphere from 45 to 800 °C (Fig. S14†). The first stages of weight losses of 24.81% and 30.47% occurred at 176.5 and 181.3 °C for Ag₂₄ and Ag₁₆Co₈, respectively, while Ag₂₂Cu had a higher degradation temperature at 213.25 °C with a weight loss of about 2.77%. Fourier transform infrared (FT-IR) spectra demonstrated the characteristic vibrational peaks of functional groups in the crystal structures of Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ clusters (Fig. S6†). The ultraviolet–visible (UV-vis) spectra of Ag₂₄ (DMF), Ag₂₂Cu (DMF) and Ag₁₆Co₈ (DMSO) in Fig. S7† in solution were detected in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), respectively, revealing that Ag₂₄ has a distinct absorption peak near 420 nm and Ag₂₂Cu is similar to Ag₂₄ with a strong absorption peak at 430 nm, whereas Ag₁₆Co₈ has two weak and broad bands in the range of 450–530 nm and a sharp absorption peak at 570 nm. The solid-state UV-vis diffuse reflection spectra of Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ clusters were measured at room temperature as shown in Fig. S15.†Ag₂₄ shows a broad absorption band in the range of 350–800 nm, and Ag₂₂Cu and Ag₁₆Co₈ show a band from 200–800 nm. The strong and broad absorption bands in the UV region from 200 to 350 nm can be attributed to the n → π* transition of the ⁱPrS⁻ ligands, whereas the peaks higher than 350 nm can be classified as ligand-to-metal charge transfer (LMCT). The powder X-ray diffraction (PXRD) spectra (Fig. S8†) are in good agreement with the simulation results, confirming the phase purities of all three clusters. X-ray photoelectron spectroscopy (XPS) is used to determine the elemental composition and chemical oxidation states of samples. As shown in Fig. S9–S11,† high resolution XPS spectra give the oxidation states of Ag, Cl, and S in Ag₂₄ as +1, −1, and −2, respectively, while the oxidation states of Ag, Cu, W, and S in the Ag₂₂Cu cluster are +1, +2, +6 and −2, and the oxidation states of Ag, Co, W, and S in Ag₁₆Co₈ cluster are +1, +2, +6 and −2, respectively, which is consistent with previous BVS calculations. Scanning electron microscopy with energy dispersive spectroscopy (SEM/EDX) revealed the microscopic morphology of Ag₂₄ and Ag₁₆Co₈ clusters and confirmed the element types (Fig. S12 and S13†), which is in line with the X-ray crystallographic structural analyses and XPS spectroscopies.

We further investigated the solid-state photoluminescence (PL) properties of the Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ powders. At ambient temperature, solid- and solution-stated clusters showed lightlessness under hand-held ultraviolet light irradiation (λ_ex = 365 nm). However, Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ clusters emitted red, orange, and yellow light at a cryogenic temperature, respectively. Therefore, the variable temperature (83–293 K, with a temperature interval of 30 K) solid-state emission spectra of Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ were further measured upon excitation at 468, 450 and 358 nm, respectively. As shown in Fig. 3a–c, Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ displayed maximum emission at 632, 606 and 517 nm at 293 K, respectively. As the temperature decreased from 293 to 83 K, these three clusters exhibited different emission properties. The maximum emission peak of Ag₂₄ was gradually blue-shifted from 632 to 603 nm, while the Ag₁₆Co₈ cluster was gradually red-shifted from 517 to 564 nm. Compared to Ag₂₄ and Ag₁₆Co₈, there was no significant shift of the maximum emission peak of Cu-doped Ag₂₂Cu accompanied by a decrease in temperature. At a cryogenic temperature, the emission intensities of Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ were significantly enhanced by 9, 51, and 30 times compared to those under ambient conditions, respectively. The movement of the externally coordinated DPPP ligands is greatly restricted at low temperatures, which might cause the blue-shift phenomenon of Ag₂₄.^49,50 For Ag₁₆Co₈, the obvious red shift can be attributed to the reduction of the Ag–Ag bond distances at low temperatures, which could be accompanied by a decrease in the energy gap of the metal-centered d¹⁰ → d⁹s¹ crystal transition.⁵¹ The normalized photoluminescence decay kinetics of Ag₂₄, Ag₂₂Cu, and Ag₁₆Co₈ clusters are shown in Fig. 3d–f and the nanosecond lifetimes of Ag₂₄ and Ag₂₂Cu under 455 nm laser excitation are 10.494 and 5.017 μs at 83 K, whereas the microsecond lifetime of Ag₁₆Co₈ with a microsecond lamp is 86.804 μs. In addition, plots of maximum emission intensity (I_max) versus temperature for Ag₂₄ and Ag₁₆Co₈ clusters show perfect single exponential behaviour in the range of 113–293 K (Fig. S16†), demonstrating that these clusters may be promising for potential applications as molecular luminescent thermometers under specific circumstances.

Previous studies have shown that the photothermal effect of molecules is related to radiative emission (fluorescence), and fluorescence quenching of photothermal materials improves the efficiency of photothermal energy conversion.^52,53 The Ag₂₄ cluster exhibited the highest fluorescence intensity among these three clusters under ambient conditions, so Ag₂₂Cu and Ag₁₆Co₈ were theoretically considered to have a better photothermal conversion efficiency.^54,55 The photothermal conversion properties of Ag₂₄, Ag₂₂Cu, and Ag₁₆Co₈ clusters under the same green laser irradiation using a 532 nm laser are shown in Fig. 4a–c. Upon turning on the laser switch, the surface temperatures of the Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ increased rapidly within 15 seconds; correspondingly, the surface temperatures of these samples gradually decreased to room temperature within 30 seconds after the laser switch was turned off, which indicated that these three types of clusters have photothermal conversion ability. Moreover, the temperature increase was positively correlated with the gradual increase in power density from 0.1 to 1.0 W cm⁻². At 1.0 W cm⁻², the temperatures of Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ reached maximum values of 133, 183.9 and 150.7 °C (Fig. 4a–c), respectively. Compared to the previously reported POM-templated silver clusters, Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ exhibited decent photothermal conversion performance (Table S8†). And the temperature of Ag₂₂Cu was higher than those of Ag₂₄ and Ag₁₆Co₈, which is consistent with the above description. All these three clusters could be re-used for at least 20 successive photothermal cycles without a decline in performance, indicating good stability and photothermal recyclability (Fig. 4d–f). More importantly, the maximum temperatures of Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ remained at around 96.6, 136.9, and 121.4 °C without obvious reduction during multiple cycles of heating and cooling processes. Furthermore, the good photothermal stability of the three clusters was confirmed by comparing the FT-IR spectra of Ag₂₄, Ag₂₂Cu and Ag₁₆Co₈ before and after irradiation (Fig. S6†). These results confirmed the excellent photothermal conversion capabilities of Ag₂₄, Ag₂₂Cu, and Ag₁₆Co₈ clusters, implying potential applications in photothermal therapy, seawater desalination, and catalysis, etc.

3. Conclusions

In summary, three structurally new silver clusters Ag₂₄, Ag₂₂Cu, and Ag₁₆Co₈ were successfully prepared by adopting a classical lacunary [SiW₉O₃₄]¹⁰⁻ anion template under solvothermal conditions. The resulting clusters have been systematically characterized using various spectroscopic techniques and single-crystal X-ray crystallography. The Ag₂₄, Ag₂₂Cu, and Ag₁₆Co₈ clusters emit red, orange, and yellow light under a liquid nitrogen environment, respectively, exhibiting different temperature-dependent photoluminescence behaviours. In addition, all three clusters also show good photothermal conversion properties with promising potential applications. The present work contributes to enriching the family of POM-templated silver clusters, and more research on transition-metal-containing heterometallic silver clusters is ongoing in our lab.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 21871025, 21831001), the Beijing Natural Science Foundation (2242017), the Recruitment Program of Global Experts (Young Talents), and the Beijing Institute of Technology (BIT) Excellent Young Scholars Research Fund. The instrumental support from the Analysis and Testing Center of Beijing Institute of Technology is also highly appreciated.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2312087, 2336489 and 2343569. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nr02016b

This journal is © The Royal Society of Chemistry 2024