Carboxylic acid stimulated silver shell isomerism in a triple core–shell Ag₈₄ nanocluster

Abstract

Isomerization is highly important in all aspects of science, yet it is rarely observed in nanoscience. Here, we synthesized a unique triple core–shell Ag₈₄ nanocluster displaying isomerism, which is controlled by different carboxylic acids and a one-way transformation (SD/Ag84a → SD/Ag84b). The innermost core is a rare Ag₁₀ nanocluster which comprises an Ag₆ octahedral unit as seen in face-centred cubic (fcc) silver metal and four capped Ag atoms. It templates two crescent-shaped polyoxometalate (W₇O₂₆)¹⁰⁻ shells which are then enclosed in a shell of silver shaped as rugby balls. The organic ligands (ⁱPrS⁻, ⁿPrCOO⁻ and PhCOO⁻) finally shield the metallic clusters. Due to slight differences in structure at two poles and the steric hindrance of ⁿPrCOO⁻ and PhCOO⁻, SD/Ag84a and SD/Ag84b adopt the shapes of flat-headed and cuspidal prolate spheres, respectively. Interestingly, PhCOOH is dominant over ⁿPrCOOH whereby crystals of SD/Ag84b were isolated if PhCOOH is added during the synthesis of SD/Ag84a. This demonstrates that PhCOOH not only alters the organic coats but also induces metal shell re-organization. This work reveals carboxylate-controlled skeletal isomerism in silver nanoclusters for the first time, thus deepening the understanding of silver nanocluster assembly, flexibility and reactivity.

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Introduction

Isomerization plays a vital role in many fundamental biological processes in nature such as ligand binding, enzymatic catalysis, protein folding and photosynthesis.¹ Diverse isomerizations in inorganic, organic, and supramolecular chemistry have also attracted attention since isomers—compounds with the same atoms arranged in different manners—may deliver substantially different physicochemical properties and chemical reactivities without changing their compositions;² for example, the sweetness of optically active sugars. For polyatomic systems, such as metal nanoclusters, isomerization is not a highly probable event because (i) structural variation usually involves the re-organization of several dozens of metal atoms and motion of several ligands and (ii) unravelling atomic-precision in nanometric structures or even macromolecules is still a major challenge. Thus, it is difficult to experimentally achieve atomic-level isomerization of specific sites/regions in a cluster without destroying the other parts of the main skeleton. An important breakthrough was made in 2015 by Jin and Wu who reported the first genuine gold nanocluster isomers, Au_38T and Au_38Q.³ Following this work, their group reported ligand-induced core isomerization in two thiolate-protected Au₂₈ nanoclusters.⁴ Tsukuda also discovered anion-packing induced isomerization between crown- and butterfly-Au₉.⁵ Although there has been sporadic advances in Au nanocluster isomerization recently,⁶ those for silver are yet to be achieved.

The development of silver nanoclusters has expedited the establishment of general synthetic methodologies such as anion-templation and geometric polyhedral principles.⁷ The gradual accumulation of knowledge about silver nanocluster synthesis is also a reminder for us to revisit the ligand strategy⁸ based on the classic hard-soft-acid-base (HSAB) theory,⁹ that is controlling the ‘hardness’ of carboxylic acids in competition with ‘soft’ thiolates, allowing flexibility of the overall silver nanoclusters. The abundant availability of commercial alkyl or aryl carboxylates provides ample options to tune the structure and flexibility of silver nanoclusters through steric hindrance or/and electronic effects. Flexible silver nanoclusters may produce isomers under specific stimuli including acid, base, ligand exchange and so on. Given this situation, using this soft/hard double-ligand strategy in the rational synthesis of isomeric silver nanoclusters is very attractive and challenging.

This study is born out of the successes of the recent afore-mentioned synthetic strategy achieving two Ag₈₄ nanoclusters (SD/Ag84a and SD/Ag84b). Their impressive structures comprise a Ag₁₀ nanocluster core, a pair of novel crescent-shaped (W₇O₂₆)¹⁰⁻ shells and a 74-silver outer shell, thus establishing a novel common rugby-ball shaped three-shell [Ag₁₀@(W₇O₂₆)₂@Ag₇₄] motif that differs in skeletal organization and ligand coverage at the two poles. Their flat-headed and cuspidal prolate spherical structures, respectively, are likely driven by the different steric hindrances between ⁿPrCOO⁻ and PhCOO⁻. Although the organic shells are different, these two Ag₈₄ nanoclusters have identical elemental contents, and hence, they belong to pseudo-isomers. What’s more interesting is that we can isolate SD/Ag84b in the mother solution of SD/Ag84a by adding PhCOOH at the second-step reaction, which demonstrates that PhCOOH not only changes the organic coverage but also induces metal shell distortion or re-organization.

Results and discussion

Synthesis of SD/Ag84a and SD/Ag84b

The details of the synthesis of SD/Ag84a and SD/Ag84b are given in the ESI.† Briefly, (ⁱPrSAg)_n, silver salts, RCOOH, and Na₂WO₄ were mixed in MeOH/DMF (v/v = 4/1) and reacted under solvothermal conditions (Scheme 1). Red crystals were collected and characterized by single-crystal and powder X-ray diffraction (SCXRD, PXRD), IR, UV-Vis, elemental analyses, and luminescence.

Scheme 1 Schematic representation of the assembly and conversion of SD/Ag84a and SD/Ag84b.

X-ray structures of SD/Ag84a and SD/Ag84b

Both SD/Ag84a and SD/Ag84b crystallize in the triclinic P1̄ space group with half of the cluster existing as the asymmetric unit. Apart from the carboxylate group they have similar formulae of [Ag₁₀@(W₇O₂₆)₂@Ag₇₄S₂(ⁱPrS)₄₀(ⁿPrCOO)₁₈]·2CH₃OH (SD/Ag84a) and [Ag₁₀@(W₇O₂₆)₂@Ag₇₄S₂(ⁱPrS)₄₀(PhCOO)₁₈] (SD/Ag84b). Their overall appearance is a prolate sphere with similar dimensions of about 1.1 × 2.5 nm. Their coverage by organic ligands is also similar to 40 ⁱPrS⁻ and 18 RCOO⁻ ligands (Fig. 1a and b). Both Ag₈₄ clusters consist of an Ag₇₄ outer shell and two crescent-shaped (W₇O₂₆)¹⁰⁻ inner shells clamping a subnanometer Ag₁₀ kernel. The Ag₁₀ kernel is constructed from an octahedral Ag₆ core by adding four Ag caps at the triangular faces (Fig. 1c). Two (W₇O₂₆)¹⁰⁻ anions symmetrically wrap the Ag₁₀ kernel through Ag–O bonds (Fig. 1d). Each (W₇O₂₆)¹⁰⁻ anion is formed by seven edge-sharing WO₆ octahedra (Fig. 1e) and coordinates to 32 silver atoms in total (Fig. S1†). Such (W₇O₂₆)¹⁰⁻ anions are not only observed in the POM@Ag cluster family for the first time but also have never been documented in classical POM chemistry. Two in situ generated μ₆-S²⁻ ions were inserted into the equatorial waist section, consolidating the linkage between the inner Ag₁₀ kernel and the Ag₇₄ outer shell (Fig. S2†). We listed several important geometry parameters of SD/Ag84a and SD/Ag84b such as Ag⋯Ag distances in the Ag₁₀ kernel and the Ag₇₄ shell, Ag–S_ligand, Ag–S_sulfide, Ag–O_carboxylate, Ag–O_POM, and W–O bond lengths, and coordination modes of ligands and (W₇O₂₆)¹⁰⁻ for comparison (Table S1†).

Fig. 1 Structures of the clusters SD/Ag84a (a) and SD/Ag84b (b). (c) The polyhedral mode showing an Ag₆ octahedron (green) capped by four additional silver tetrahedra (yellow) to form the Ag₁₀ kernel; (d) two (W₇O₂₆)¹⁰⁻ anions wrapping an Ag₁₀ kernel and (e) polyhedral mode showing the structure of a (W₇O₂₆)¹⁰⁻ anion. Color labels: purple, Ag; cyan, W; yellow, S; gray, C; and red, O.

Notably, such an Ag₁₀ kernel is observed for the first time, although its innermost subvalent Ag₆⁴⁺ octahedron has been reported in silver nanoclusters¹⁰ and some inorganic compounds such as Ag₆Ge₁₀P₁₂, Ag₅GeO₄, Ag₅SiO₄ and Ag₆O₂.¹¹ The formation of such subvalent Ag nanoclusters is related to the reductive effect of DMF, which has been recognized as a key factor in the controlled synthesis of multiple-twin silver nanocrystals by reducing Ag⁺ → Ag⁰.¹² Based on the digested ¹³C NMR spectra of reaction mother solutions of SD/Ag84a and SD/Ag84b (Fig. S3 and S4†), we observed typical C_carboxyl resonances at δ = 163.16 and 163.04 ppm corresponding to the oxidization product of DMF, Me₂NCOOH.¹³ In the same region, the expected C_aldehyde of DMF and C_carboxyl of ⁿPrCOOH and PhCOOH were also observed at δ = 164.39, 176.47, and 168.95 ppm, respectively. These results provided experimental evidence that the redox reaction happened in such a complicated self-assembly process. As the smallest unit in fcc bulk silver metal, the subnanometer Ag₆ octahedron with eight exposed [111] facets can be seen as an embryonic state of any other bigger silver nanocrystals and thus is of particular interest in the field of nanoparticles.¹⁴ The subnanometer Ag₁₀ kernel trapped in SD/Ag84a and SD/Ag84b can be seen as the Ag₆ “nuclei” grown by adding four tetrahedra on its four [111] facets, manifesting the atomic-level silver nanocrystal growth route, that is, stepwise growth of tetrahedral caps on specific facets. Such a growth route is quite similar to that proposed for larger decahedral and icosahedral silver nanocrystals by the Tsuji group.¹⁵ This result thus sheds light on the atomic details of the growth of silver nanocrystals in the embryonic stage.

Upon further carefully checking and comparing the structural features such as ligand distributions and skeletons of these two silver nanoclusters, we surprisingly found that the distinct differences of silver skeletons between SD/Ag84a and SD/Ag84b are in the polar sections and have nothing to do with the inner Ag₁₀ kernel and (W₇O₂₆)¹⁰⁻, although the silver polygons on the other regions of surfaces are almost identical with slight dislocations and distortions. The Ag₇₄ shells in SD/Ag84a and SD/Ag84b can be described as flat-headed (red skeleton in Fig. 2a) and cuspidal (green skeleton in Fig. 2a) prolate spheres, respectively. The superposed Ag₇₄ shells, especially in the polar regions (Ag₁₅ caps, Fig. 2b), showed silver polygon migrations and severe distortions, which are caused by the different distributions of ⁱPrS⁻ and RCOO⁻ ligands on this region. There are in total six ⁱPrS⁻ and six RCOO⁻ ligands on the polar regions of SD/Ag84a and SD/Ag84b. On one pole of SD/Ag84a, six ⁿPrCOO⁻ ligands are equally distributed at the two sides (Fig. 2c), whereas four PhCOO⁻ ligands are located at one side and the other two at another side of one pole of SD/Ag84b (Fig. 2d). Moreover, two μ₂-κ¹:κ¹ PhCOO⁻ ligands are adjacent and simultaneously coordinate to the same Ag atom (Ag32), creating a single peak (Ag32) on the pole of SD/Ag84b. Although the organic coats are different in SD/Ag84a and SD/Ag84b, the same silver atom counts suggested that they are cage isomers. The driving force of the isomeric silver nanocluster should be most probably generated from the steric hindrance of different carboxylate groups. Thus, we successfully filled in the blanks in isomeric silver nanoclusters using a soft/hard double-ligand strategy.

Fig. 2 (a) The superposed Ag₇₄ shells of SD/Ag84a (red) and SD/Ag84b (green). (b) The top views of silver polygons at the pole regions of SD/Ag84a (red) and SD/Ag84b (green). The top views of ligand distributions at the pole region of SD/Ag84a (c) and SD/Ag84b (d).

Inspired by the isomeric silver skeletons of SD/Ag84a and SD/Ag84b, we also tried to explore the possibility of conversion between them under the carboxylic acid stimulus. The isomerization experiments were performed in respective reaction mother liquors without removing crystals and by adding a portion of another kind of carboxylic acid. Interestingly, we found that SD/Ag84b can be isolated after the synthesis of SD/Ag84a by adding bulkier PhCOOH for the second-step reaction; however, the transformation from SD/Ag84b to SD/Ag84a failed by adding smaller ⁿPrCOOH into the system for synthesizing SD/Ag84b, which indicated that the bulkier the carboxylic acid, the stronger the inducing effect.

Considering the subvalent characteristics of the Ag₁₀ kernel, we deduced the charge of it to be +4 based on the formula determined by high-quality SCXRD data, which was also further confirmed by the DFT calculations (see details in ESI†). The electronic structure of SD/Ag84a (Fig. 3a) demonstrates that both 5s states of the outer Ag₇₄ shell and 3p states of S play a pivotal role in the valence band maximum (VBM), while only the 5s states of Ag (especially from the inner Ag₁₀ kernel) are dominant in the conduction band minimum (CBM). The resultant band gap of 1.12 eV is comparable with the observed value (E_g = 1.24 eV) from solid UV-Vis measurement results discussed below. Thus, the corresponding absorption peak can be attributed to electronic transition from the outer Ag₇₄ shell to the inner Ag₁₀ kernel. Moreover, frontier molecular orbitals (Fig. 3b–g) also indicate that the three highest occupied orbitals (HOMO, HOMO−1 and HOMO−2) are concentrated in the 5s orbitals and 4d orbitals of the outer Ag₇₄ shell, while the two lowest unoccupied orbitals (LUMO and LUMO+1) mainly consist of 5s orbitals (derived from octahedron-like Ag₆) and 4d orbitals (involving two wing-like Ag₂ units) of the inner Ag₁₀ kernel. Therefore, it is concluded that the inner Ag₁₀ core features an electron-deficient state, consistent with the +4 valence estimated crystallographically.

Fig. 3 Total DOS and partial DOS of SD/Ag84a (a). Frontier molecular orbitals: HOMO−2 (b), HOMO−1 (c), HOMO (d) and LUMO (e), LUMO+1 (f) and LUMO+2 (g).

The optical properties of SD/Ag84a

The solid state UV-Vis absorption spectrum of SD/Ag84a was recorded at room temperature in the wavelength range from 300 to 1000 nm. As depicted in Fig. 4, there is one intense absorption band centered at 361 nm and a shoulder peak at 500 nm, respectively. The high energy absorption centered at 361 nm originates from the n → π* transition of ⁱPrS⁻, as similarly observed in the absorption spectrum of the (ⁱPrSAg)_n precursor. The low energy absorption centered at 500 nm can be attributed to the charge transfer transition from the S 3p to Ag 5s orbitals. Using a transformed Kubelka–Munk plot (Fig. S5†), the HOMO–LUMO gap of SD/Ag84a was found to be 1.24 eV, indicating that SD/Ag84a is a potential narrow-band-gap semiconductor. By contrast, the optical gap of the (ⁱPrSAg)_n precursor was calculated to be 2.52 eV, which matches with the color of the samples: SD/Ag84a is red, whereas the (ⁱPrSAg)_n precursor is yellow (see the insets of Fig. 4).

Fig. 4 The UV-Vis absorption spectra of SD/Ag84a and the (ⁱPrSAg)_n precursor.

SD/Ag84a does not emit at room temperature in the solid state under a 365 nm hand-held UV light; however, it emits bright-red light at 77 K, which is detectable by the naked eye (see the insets of Fig. 5). The temperature dependent fluorescence spectra were recorded under 468 nm excitation in the temperature range of 293–83 K. From Fig. 5a, we can see that SD/Ag84a is almost non-emissive from 293 to 203 K. Upon cooling from 203 to 83 K, SD/Ag84a starts emitting with the maximum emission band centered at 689 nm, which is gradually blue-shifted to 685 nm at 83 K along with the increase of luminous intensity. This low-temperature emission should originate from the ligand-to-metal charge transfer (LMCT, charge transfer from S 3p to Ag 5s) perturbed by Ag⋯Ag interactions.¹⁶ The temperature dependence behavior should be relevant with variable molecular rigidity and Ag⋯Ag interactions at different temperatures. The emission intensity was found to be sensitive to temperature and showed good linearity with the corresponding temperature ranging from 83 to 203 K (Fig. 5b), with the linear equation I_max = 1511090 − 7364.75T. The linear equation correlation coefficient is 0.992, which is suitable for temperature detection in the temperature range of 83–203 K. The fluorescence lifetime of SD/Ag84a was measured at 83 K (Fig. 5c), with the lifetime value falling on the scale of microseconds (τ₁ = 86.30 μs and τ₂ = 227.23 μs), suggesting a triplet state emission.

Fig. 5 (a) The temperature-dependent emission spectra of SD/Ag84a under 468 nm excitation. Insets: the photographs of SD/Ag84a irradiated with 365 nm UV light at 298 and 77 K. (b) The plot of temperature vs. maximum emission intensity (red line is the linear fitting in the range of 83–203 K). (c) Luminescence lifetime of SD/Ag84a recorded at 83 K (red line is the fitting curve).

Conclusions

In conclusion, we successfully achieved the assembly of isomeric Ag₈₄ nanoclusters for the first time by the combination of anion template and soft/hard double-ligand strategies. The comparative structural analysis indicated that the isomerism mainly occurred on the polar regions of the prolate spheres. The differences are found in both the silver skeleton and ⁱPrS⁻/RCOO⁻ ligand distributions. The driving force for such an isomerism is dominated by the steric hindrance of carboxylates. What’s more interesting is that the Ag₁₀ kernel as a grown nanocrystal from the smallest octahedral Ag₆ unit in face-centred cubic (fcc) silver metal was first identified in the innermost region of the Ag₈₄ nanocluster. The results obtained in this study are not only the pioneering results for isomeric silver nanoclusters but also enrich the library of multi-shell silver nanoclusters containing both novel POM templates and ultrasmall reductive silver nanocrystals.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21822107, 21571115, 21671172), the Natural Science Foundation of Shandong Province (No. JQ201803 and ZR2017MB061), the Taishan Scholar Project of Shandong Province of China, the Qilu Youth Scholar Funding of Shandong University and the Fundamental Research Funds of Shandong University (104.205.2.5). Dr Zhuang was financially supported by the Zhejiang Provincial Natural Science Foundation (No. LR19B010001).

References

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Footnote

† Electronic supplementary information (ESI) available: IR, ¹³C NMR, UV-Vis, EDS and PXRD data, and details of the data collection and structure refinements, and crystal data. CCDC 1878911 and 1878912 for SD/Ag84a and SD/Ag84b. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8sc05666h

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