Unusual fcc-structured Ag₁₀ kernels trapped in Ag₇₀ nanoclusters

Abstract

Controlled trapping atom-precise ultrasmall silver nanoparticles into silver nanoclusters is challenging; thus only limited progress has been made in this area. We are therefore inspired to isolate two novel silver nanoclusters, Ag₁₀@Ag₇₀ (SD/Ag80a and SD/Ag80b; SD = SunDi), where a novel fcc-structured Ag₁₀ kernel built from two single-edge opened Ag₆ octahedra by sharing one edge is trapped. The bioctahedral Ag₁₀ kernel is locked by a pair of Mo₇O₂₆¹⁰⁻ anions to form an inner Ag₁₀@(Mo₇O₂₆)₂ core which is further encapsulated by an outer Ag₇₀ shell to form three-shell Ag₁₀@(Mo₇O₂₆)₂@Ag₇₀ nanoclusters. Notably, the bioctahedral Ag₁₀ kernel has not been observed in silver nanoclusters ever before, thus representing a new embryo state of silver nanoparticles. SD/Ag80a emits in the near infrared (NIR) region (λ_em = 730 nm) at low temperature. This work will deepen our understanding on the atomic-level growth of silver nanoparticles and complicated three-shell self-assembly involving polyoxometalate (POM) and two different silver nanoclusters.

---

Introduction

Ultrasmall silver nanoparticles (e.g., few-atom clusters, <1 nm) represent the embryo states of larger silver nanoparticles (typically >2 nm) to some extent, which have defined molecular structures and compositions and thus can deepen the understanding on the size evolution of silver nanoparticles.¹ Given this, X-ray single crystal structures become a prerequisite to get atomic-level information including the surface ligands, inorganic–organic interfaces and silver atoms packed in silver nanoparticles.² While chasing large silver nanoclusters such as Ag₁₄, Ag₂₁, Ag₂₃, Ag₄₄, Ag₅₀, Ag₆₂, Ag₆₇, Ag₇₄, and Ag₁₄₁ and even the largest known Ag₃₇₄,³ chemists almost neglect the significance of the embryo states of silver nanoparticles that however are quite difficult to be captured due to their typical kinetics-controlled growth course.⁴ Therefore, controlling the reductive transformation from Ag(I) to Ag⁰ and then trapping the transient Ag aggregates into the thermodynamically stable crystalline product during the self-assembly is an urgent need and thus a major challenge.

Learning from the solvent-controlled synthesis of multiple-twin decahedral and icosahedral silver nanoparticles with special favourable [111] facets,⁵ we found that DMF (N,N-dimethylformamide), compared to widely used NaBH₄, is a much more mild reductive agent which facilitates the formation of Ag₆ octahedral kernels during the slow reduction process as seen in Ag₃₄ and Ag₆₂ nanoclusters.⁶ Such Ag₆ octahedra can be seen as the smallest fragment cut from the unit cell of face-centered cubic (fcc) bulk silver metal, whereas other silver nanoclusters smaller than the most common icosahedral Ag₁₃ are still not directly observed in any reported silver nanoclusters.⁷ Thus, the species in the early evolution from discrete Ag atoms to the metallic state are still largely vague and the exploration of a suitable synthesis strategy to trap them is scientifically desired.

With these considerations in mind, we used a DMF-containing mixed solvent system to isolate two novel silver nanoclusters [Ag₁₀@(Mo₇O₂₆)₂@Ag₇₀(MoO₄)₂(CyhS)₃₆(CF₃SO₃)₁₆(DMF)₆]·2DMF·4ⁿPrOH (SD/Ag80a; SD = SunDi; CyhSH = cyclohexanethiol) and [Ag₁₀@(Mo₇O₂₆)₂@Ag₇₀(MoO₄)₂(ⁱPrS)₃₆(CF₃SO₃)₁₆(DMF)₆] (SD/Ag80b). Two silver nanoclusters have the same metallic core but different organic coatings. In the innermost of cluster, an unusual fcc-structured Ag₁₀ nanocluster constructed from two single-edge opened Ag₆ octahedra by sharing one edge is locked by a pair of Mo₇O₂₆¹⁰⁻ anions to form an inner Ag₁₀@(Mo₇O₂₆)₂ core which acts as a template to support the outer Ag₇₀ nanocluster to form a final three-shell Ag₁₀@(Mo₇O₂₆)₂@Ag₇₀ nanocluster. This unprecedented bioctahedral Ag₁₀ nanocrystal can be deemed as a new nanofragment cut from fcc silver metal and represents a possible transient species in the growth of large silver nanoparticles.

Results and discussion

X-ray structures of SD/Ag80a and SD/Ag80b

SD/Ag80a and SD/Ag80b were synthesized through a facile one-pot solvothermal reaction of silver-thiolate polymeric precursors, CF₃SO₃Ag and molybdates in different DMF-containing mixed solvent systems (Scheme 1). In spite of several attempts, we still couldn't isolate SD/Ag80a and SD/Ag80b using the same Mo sources. Their samples were collected as brown-yellow and red crystals, respectively, after evaporation of solvents at room temperature for 1–2 weeks. Several synthetic parameters were optimized and are listed in Tables S1 and S2 (ESI)† for details. Details of the synthesis and some basic characterization are shown in the ESI.†

Scheme 1 Synthetic routes for SD/Ag80a and SD/Ag80b.

The molecular structures of SD/Ag80a and SD/Ag80b were revealed by single-crystal X-ray diffraction (SCXRD) analysis. They crystallize in monoclinic P2₁/n and triclinic P1̄ space groups, respectively. In each asymmetric unit only half of the corresponding clusters were resolved. Due to the structural similarities, only that of SD/Ag80a is described in detail here. The structural diagrams of SD/Ag80b are shown in Fig. S1.† Selected details of the data collection and structure refinements are listed in Table S3.†

SD/Ag80a is an elongated spheroid (1.0 × 1.4 × 2.1 nm) that sits on the crystallographic inversion center (i). The Ag₈₀ nanocluster is composed of a Ag₇₀ shell and a Ag₁₀ kernel. The Ag₇₀ shell is capped by 36 CyhS⁻, 16 CF₃SO₃⁻, 2 MoO₄²⁻ and 6 DMF (Fig. 1a and b). All cyclohexyl groups of 36 CyhS⁻ ligands show a unified chair configuration. Two different coordination modes (μ₃ and μ₄) are found in 36 CyhS⁻ ligands capped on the silver trigons or tetragons (Ag–S distances: 2.389(5)–2.722(5) Å). The 16 CF₃SO₃⁻ anions exhibit three different coordination fashions including μ₃-η¹:η²:η⁰, μ₃-η¹:η¹:η¹, and μ₂-η¹:η¹:η⁰. Two MoO₄²⁻ anions (yellow tetrahedra in Fig. 1) adopt a μ₈-η²:η³:η³ mode to bind in the equatorial region of the Ag₇₀ shell. Six DMF molecules as terminal ligands finished the organic ligand coverage on the surface of the Ag₇₀ shell. Three different O donor ligands (CF₃SO₃⁻, MoO₄²⁻, and DMF) interact with Ag atoms with the bonding distances in the ranges of 2.406(15)–2.789(17), 2.251(11)–2.568(11) and 2.390(13)–2.458(14) Å, respectively. The Ag₇₀ shell was further consolidated by the argentophilic interaction⁸ ranging from 2.833(2) to 3.4394(16) Å. The surface of the Ag₇₀ shell consists silver trigons, tetragons, pentagons and heptagons (Fig. 1c). The silver trigons, tetragons, and pentagons are capped by CyhS⁻ or CF₃SO₃⁻, whereas MoO₄²⁻ shapes the large silver heptagons (green rings in Fig. 1c).

Fig. 1 (a) and (b) The X-ray crystal structure of Ag₁₀@(Mo₇O₂₆)₂@Ag₇₀ nanoclusters viewed along two orthogonal directions. The inner silver atoms of the Ag₁₀ kernel are highlighted in black. Mo₇O₂₆¹⁰⁻ and MoO₄²⁻ are represented by green and yellow polyhedra, respectively. (c) The Ag₇₀S₃₆ shell with silver heptagons highlighted in green.

There are two crescent-like Mo₇O₂₆¹⁰⁻ anions under the Ag₇₀ shell (Fig. 2a). During the synthesis of SD/Ag80a and SD/Ag80b, although different Mo sources, Na₂MoO₄·2H₂O and [(ⁿBu₄N)₂Mo₆O₁₉], were used, respectively, the same Mo₇O₂₆¹⁰⁻ anion was trapped as the template in the final silver nanoclusters. Thus, the novel Mo₇O₂₆¹⁰⁻ anions should be in situ transformed from Na₂MoO₄·2H₂O or [(ⁿBu₄N)₂(Mo₆O₁₉)] in different solvent environments. We used Bond-Valence Sum (BVS) calculations for seven Mo atoms of Mo₇O₂₆¹⁰⁻, which confirmed that all of them are in the +6 oxidation state (Table S5†).⁹ The Mo₇O₂₆¹⁰⁻ is constructed from seven edge-shared MoO₆ octahedra. The total 26 O atoms are divided into four kinds based on their binding fashion to Ag atoms, 2 μ₀, 4 μ₁, 16 μ₂, and 4 μ₃. Such highly negative-charged Mo₇O₂₆¹⁰⁻ totally binds 35 silver atoms. Among them, 7 are from the inner Ag₁₀ kernel and the remaining 28 are from the Ag₇₀ shell (Fig. 2b). Notably, this novel molybdate has neither been observed in classic POM chemistry nor in silver nanoclusters. More importantly, this molybdate carries the second highest negative charges¹⁰ which effectively enhanced its template effect by binding more Ag atoms (Table S6†).

Fig. 2 (a) Two Mo₇O₂₆¹⁰⁻ anions in SD/Ag80a shown in polyhedral (left) and ball-and-stick modes (right). (b) Binding fashion of Mo₇O₂₆¹⁰⁻ toward 35 Ag atoms (black: Ag from the Ag₁₀ kernel; purple: Ag from the Ag₇₀ shell). (c) Animation showing the formation of a Ag₁₀ bioctahedron from two single-edge opened Ag₆ octahedra by fusing one Ag–Ag edge. (d) The Ag₁₀ bioctahedron locked by a pair of Mo₇O₂₆¹⁰⁻ anions. (e) The Ag₁₀ bioctahedron (claybank space-filling balls) residing in the Ag₇₀ shell.

The most interesting feature in SD/Ag80a is the unusual Ag₁₀ kernel underlying the equatorial region of the Ag₇₀ shell which is built from two single-edge opened Ag₆ octahedra by sharing one edge (Fig. 2c). The shared edge is the longest Ag⋯Ag edge (Ag38⋯Ag38ⁱ = 3.457(1) Å, symmetry code i: −x + 1, −y + 1, −z + 1) within the Ag₁₀ kernel, which is out of the normal Ag⋯Ag interaction range. All other eleven Ag⋯Ag edges are distributed in the range of 2.659(2)–2.980(1) Å (Fig. S2†) and the average Ag⋯Ag distance is 2.814 Å, which is 2.5% shorter than the Ag⋯Ag distance in metallic silver (2.886 Å),¹¹ indicating strong argentophilic interactions as in bulk silver metal. All exposed trigons of the Ag₁₀ bioctahedral kernel are [111] facets which are capped by Mo₇O₂₆¹⁰⁻ anions through Ag–O bonding (Ag–O distances: 2.284(10)–2.433(10) Å; Fig. 2d). As such, the Ag₁₀ bioctahedron is doubly clamped by a pair of Mo₇O₂₆¹⁰⁻ anions to form an inner Ag₁₀@(Mo₇O₂₆)₂ core, which was enwrapped by an outer Ag₇₀ shell to form a three-shell Ag₁₀@(Mo₇O₂₆)₂@Ag₇₀ nanocluster. The two polar sites of the Ag₁₀ bioctahedron are also linked with the outer Ag₇₀ shell through argentophilic interactions (Ag⋯Ag: 2.8523(19)–3.3621(18) Å; Fig. S3†).

Although the single Ag₆ octahedron has been observed in several inorganic compounds¹² and a few silver nanoclusters,¹³ its dimer, Ag₁₀ bioctahedron, has never been observed before in silver nanoclusters. Such a Ag₁₀ bioctahedron can be seen as a bigger nanofragment than a Ag₆ octahedron. An important driven force for its formation should be the suitable reducibility of DMF.⁶ The oxidation product of DMF in the assembly process is Me₂NCOOH¹⁴ which can be recognized from the ¹³C NMR (nuclear magnetic resonance) of HCl digested reaction mother solution (Fig. S4†). In the chemical shift scale corresponding to aldehydes and carboxylates (δ = 150–200 ppm), two peaks appeared at δ = 164.64 and 162.92 ppm, which are assigned to the carbon resonances of DMF and Me₂NCOOH, respectively. We didn't observe any peaks in ¹³C NMR corresponding to the oxidation product of ⁿPrOH, which clearly excluded the possible reductive effect of ⁿPrOH in this assembly system. These results clearly evidenced the redox reaction between Ag(I) and DMF occurred during the self-assembly process. The emergence of a fcc-structured Ag₁₀ nanocluster, on the other hand, answered an important question, which is how the common observed smaller Ag₆ kernel grew up to larger structures. Based on the above structural information, we can tentatively assign a new edge-fusion mode to its growth mechanism, although several other growth modes for noble metal nanoparticles have been proposed such as face-fusion, interpenetration, shell-by-shell, layer-by-layer, and tetrahedron-based vertex-sharing growth modes.² Based on the formulae and charge neutrality considerations, we can determine that the valence of the Ag₁₀ kernel is +6, which means such a kernel carries four free electrons, belonging to a 4e superatom network. We also performed DFT calculations at the B3LYP/SDD theoretical level to study the free electron distributions on the frontier orbitals of the Ag₁₀ kernel (see details in the ESI†). According to the results identified experimentally, the inner Ag₁₀ kernel features C_i symmetry with +6 valence and four free electrons. Thus, frontier molecular orbital analysis (Fig. S5†) reveals that four free electrons occupy two A_u-symmetry HOMO-1 and HOMO. HOMO-1 and HOMO exhibit different components. HOMO involves in the 5s orbitals of two ends of Ag₁₀, while HOMO-1 concentrates on the 4d orbitals of two ends of Ag₁₀. Moreover, HOMO-2 features A_g symmetry with similar components to HOMO-1, and LUMO consists of 5s orbitals in the centre of Ag₁₀.

Combining the structural analysis and DMF-involved reductive process, we tentatively proposed a total shell-by-shell formation mechanism for such new silver nanoclusters. Weakly reductive DMF firstly induced the formation of an inner Ag₁₀ kernel (1st shell), which exposes highly active [111] facets that are quickly passivated by the formation of Ag–O interaction with Mo₇O₂₆¹⁰⁻ (2nd shell). The inner [Ag₁₀@(Mo₇O₂₆)₂] core acts as the authentic template to support an outer Ag₇₀ shell, forming the final core–shell type silver nanoclusters. Such a formation route resembled the mechanism revealed in the [Ag₆@(MoO₄)₇@Ag₅₆] family by electrospray ionization mass spectrometry.^6b

We also noted that the Au₂₁(S-Adm)₁₅ nanocluster has been reported by the Zhu group,¹⁵ who firstly found the bioctahedral Au₁₀ kernel formed by edge-sharing of two single-edge opened Au₆ octahedra. However, the shared edge is not the longest one (opened edge) and the overall Au₆ octahedral framework is severely disordered. Anyhow, as a counterpart of this Au₁₀ kernel, the bioctahedral Ag₁₀ kernel has not been reported before in silver nanoclusters.

The optical properties of SD/Ag80a

The UV/Vis spectrum of SD/Ag80a was measured in the solid state using diffuse reflectance mode. As shown in Fig. 3, SD/Ag80a showed an absorption maximum at 344 nm and a shoulder peak in the visible region (∼490 nm), which should be ascribed to ligand-based absorption and the charge transfer transition from the S 3p to Ag 5s orbitals, respectively. Similar assignments were also made in a hypothetical silver sulfide monomer and molecular [Ag₆₂S₁₃(SBu^t)₃₂]⁴⁺ cluster.¹⁶ Based on the Kubelka–Munk function (Fig. S6†), the band gap of SD/Ag80a was estimated to be ∼1.06 eV, which indicates that SD/Ag80a is a potential narrow-band-gap semiconductor. In comparison, the optical energy gap of the precursor (CyhSAg)_n is ∼2.09 eV.

Fig. 3 Optical absorption spectra of SD/Ag80a and the silver-thiolate precursor. Insets are photographs of solid samples of SD/Ag80a (brown microcrystals) and the polymeric precursor (CyhSAg)_n (pale yellow powder).

The luminescence properties of SD/Ag80a were studied in the solid state. As shown in the insets of Fig. 4, we can observe that SD/Ag80a isn't emissive under the UV light irradiation (λ_ex = 365 nm) at room temperature; however, it emits red luminescence at 77 K. The varied-temperature emission spectra of SD/Ag80a in the solid state were recorded from 293 to 83 K with 30 K as an interval, showing luminescence thermochromic behavior. When gradually cooled to 83 K, the intensity of emission shows an 18-fold enhancement, which should be assigned to the low-temperature induced increase of radiative decay. The emission maximum was blue-shifted from 754 to 730 nm (λ_ex = 469 nm) in the temperature range of 173–83 K (Fig. 4), which may be related to the enhanced molecular rigidity at lower temperature.¹⁷ This near-infrared (NIR) emission should be assigned to ligand-to-metal-charge-transfer (LMCT) transition from S 3p to Ag 5s orbitals.¹⁸ The emission lifetime of SD/Ag80a, falling on the microsecond scale at 83 K (Fig. S7†), suggests the triplet phosphorescence origin.

Fig. 4 Varied-temperature luminescence spectra of SD/Ag80a from 293–83 K in the solid state. Insets show the photographs of the sample SD/Ag80a under a hand-held UV lamp (365 nm) at 298 and 77 K.

Conclusions

In conclusion, we developed a DMF-controlled strategy to successfully capture an atom-precise ultrasmall Ag₁₀ kernel into a gigantic silver nanocluster. The DMF with mild reductive ability plays a key role in the formation of such a novel cluster-in-cluster silver nanocluster. The fcc-structured Ag₁₀ kernel is built from two single-edge opened Ag₆ octahedra by sharing one edge and further locked by a pair of Mo₇O₂₆¹⁰⁻ anions to form an inner Ag₁₀@(Mo₇O₂₆)₂ core which is finally encapsulated by an outer Ag₇₀ shell to form three-shell Ag₁₀@(Mo₇O₂₆)₂@Ag₇₀ nanoclusters. Notably, both the bioctahedral Ag₁₀ kernel and crescent-like Mo₇O₂₆¹⁰⁻ have not been observed in silver nanocluster and POM chemistry ever before, respectively. The bioctahedral Ag₁₀ core can be deemed as a brand-new embryo state of silver nanoparticles; moreover, it also provides a new edge-fusion growth route for silver nanoparticles from the smallest Ag₆ nanofragment of metallic silver. We hope that this work can popularize the new controllable synthetic method to expand the scope of silver nanoclusters with higher complexity.

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, and 21671172), the Natural Science Foundation of Shandong Province (No. JQ201803 and ZR2017MB061), the Qilu Youth Scholar Funding of Shandong University and the Fundamental Research Funds of Shandong University (104.205.2.5).

References

1. (a) T. U. B. Rao and T. Pradeep, Angew. Chem., Int. Ed., 2010, 49, 3925–3929; (b) T. U. B. Rao, B. Nataraju and T. Pradeep, J. Am. Chem. Soc., 2010, 132, 16304–16307; (c) H. Xiang, S.-H. Wei and X. Gong, J. Am. Chem. Soc., 2010, 132, 7355–7360.
2. (a) R. C. Jin, C. J. Zeng, M. Zhou and Y. X. Chen, Chem. Rev., 2016, 116, 10346–10413; (b) S. L. Zhuang, L. W. Liao, Y. Zhao, J. Y. Yuan, C. H. Yao, X. Liu, J. Li, H. T. Deng, J. L. Yang and Z. K. Wu, Chem. Sci., 2018, 9, 2437–2442.
3. (a) H. Yang, J. Lei, B. Wu, Y. Wang, M. Zhou, A. Xia, L. Zheng and N. Zheng, Chem. Commun., 2013, 49, 300–302; (b) R. S. Dhayal, J.-H. Liao, Y.-C. Liu, M.-H. Chiang, S. Kahlal, J.-Y. Saillard and C. W. Liu, Angew. Chem., Int. Ed., 2015, 54, 3702–3706; (c) C. Liu, T. Li, H. Abroshan, Z. M. Li, C. Zhang, H. J. Kim, G. Li and R. C. Jin, Nat. Commun., 2018, 9, 744; (d) A. Desireddy, B. E. Conn, J. Guo, B. Yoon, R. N. Barnett, B. M. Monahan, K. Kirschbaum, W. P. Griffith, R. L. Whetten, U. Landman and T. P. Bigioni, Nature, 2013, 501, 399–402; (e) W. J. Du, S. Jin, L. Xiong, M. Chen, J. Zhang, X. J. Zou, Y. Pei, S. X. Wang and M. Z. Zhu, J. Am. Chem. Soc., 2017, 139, 1618–1624; (f) S. Jin, S. X. Wang, Y. B. Song, M. Zhou, J. Zhong, J. Zhang, A. D. Xia, Y. Pei, M. Chen, P. Li and M. Z. Zhu, J. Am. Chem. Soc., 2014, 136, 15559–15565; (g) M. J. Alhilaly, M. S. Bootharaju, C. P. Joshi, T. M. Besong, A.-H. Emwas, R. Juarez-Mosqueda, S. Kaappa, S. Malola, K. Adil, A. Shkurenko, H. Hakkinen, M. Eddaoudi and O. M. Bakr, J. Am. Chem. Soc., 2016, 138, 14727–14732; (h) M. Qu, H. Li, L. H. Xie, S. T. Yan, J. R. Li, J. H. Wang, C. Y. Wei, Y. W. Wu and X. M. Zhang, J. Am. Chem. Soc., 2017, 139, 12346–12349; (i) L. Ren, P. Yuan, H. Su, S. Malola, S. Lin, Z. Tang, B. K. Teo, H. Hakkinen, L. Zheng and N. Zheng, J. Am. Chem. Soc., 2017, 139, 13288–13291; (j) H. Y. Yang, Y. Wang, X. Chen, X. J. Zhao, L. Gu, H. Q. Huang, J. Z. Yan, C. F. Xu, G. Li, J. C. Wu, A. J. Edwards, B. Dittrich, Z. C. Tang, D. D. Wang, L. Lehtovaara, H. Hakkinen and N. F. Zheng, Nat. Commun., 2016, 7, 12809.
4. (a) Y. Wang, Y. Q. Zheng, C. Z. Huang and Y. N. Xia, J. Am. Chem. Soc., 2013, 135, 1941–1951; (b) M. Zhu, P. Wang, N. Yan, X. Q. Chai, L. Z. He, Y. Zhao, N. Xia, C. H. Yao, J. Li, H. T. Deng, Y. Zhu, Y. Pei and Z. K. Wu, Angew. Chem., Int. Ed., 2018, 57, 4500–4504; (c) L. Z. He, J. Y. Yuan, N. Xia, L. W. Liao, X. Liu, Z. B. Gan, C. M. Wang, J. L. Yang and Z. K. Wu, J. Am. Chem. Soc., 2018, 140, 3487–3490.
5. (a) A. Tao, P. Sinsermsuksakul and P. D. Yang, Angew. Chem., Int. Ed., 2006, 45, 4597–4601; (b) M. Tsuji, Y. Maeda, S. Hikino, H. Kumagae, M. Matsunaga, X. L. Tang, R. Matsuo, M. Ogino and P. Jiang, Cryst. Growth Des., 2009, 9, 4700–4705; (c) M. Tsuji, M. Ogino, R. Matsuo, H. Kumagae, S. Hikino, T. Kim and S. H. Yoon, Cryst. Growth Des., 2010, 10, 296–301; (d) M. Tsuji, X. L. Tang, M. Matsunaga, Y. Maeda and M. Watanabe, Cryst. Growth Des., 2010, 10, 5238–5243.
6. (a) D. Sun, G.-G. Luo, N. Zhang, R.-B. Huang and L.-S. Zheng, Chem. Commun., 2011, 47, 1461–1463; (b) Z. Wang, H. F. Su, M. Kurmoo, C. H. Tung, D. Sun and L.-S. Zheng, Nat. Commun., 2018, 9, 2094.
7. R. S. Dhayal, Y.-R. Lin, J.-H. Liao, Y.-J. Chen, Y.-C. Liu, M.-H. Chiang, S. Kahlal, J.-Y. Saillard and C. W. Liu, Chem.–Eur. J., 2016, 22, 9943–9947.
8. (a) H. Schmidbaur and A. Schier, Angew. Chem., Int. Ed., 2015, 54, 746–784; (b) H. Liu, C.-Y. Song, R.-W. Huang, Y. Zhang, H. Xu, M.-J. Li, S.-Q. Zang and G.-G. Gao, Angew. Chem., Int. Ed., 2016, 55, 3699–3703; (c) S. Li, X.-S. Du, B. Li, J.-Y. Wang, G.-P. Li, G.-G. Gao and S.-Q. Zang, J. Am. Chem. Soc., 2018, 140, 594–597; (d) R.-W. Huang, Y.-S. Wei, X.-Y. Dong, X.-H. Wu, C.-X. Du, S.-Q. Zang and T. C. W. Mak, Nat. Chem., 2017, 9, 689–697.
9. W. Liu and H. H. Thorp, Inorg. Chem., 1992, 31, 1585–1588.
10. R.-W. Huang, Q.-Q. Xu, H.-L. Lu, X.-K. Guo, S.-Q. Zang, G.-G. Gao, M.-S. Tang and T. C. W. Mak, Nanoscale, 2015, 7, 7151–7154.
11. P. Pyykkö, Chem. Rev., 1997, 97, 597–636.
12. (a) H. G. V. Schnering and K. G. Hausler, Rev. Chim. Miner., 1976, 13, 71–81; (b) W. Beesk, P. G. Jones, H. Rumpel, E. Schwarzmann and G. M. Sheldrick, J. Chem. Soc., Chem. Commun., 1981, 664–665; (c) C. Linke and M. Jansen, Inorg. Chem., 1994, 33, 2614–2616; (d) M. Jansen and C. Linke, Angew. Chem., Int. Ed. Engl., 1992, 31, 653–654.
13. (a) Y. Kikukawa, Y. Kuroda, K. Suzuki, M. Hibino, K. Yamaguchi and N. Mizuno, Chem. Commun., 2013, 49, 376–378; (b) Z.-Y. Wang, M.-Q. Wang, Y.-L. Li, P. Luo, T.-T. Jia, R.-W. Huang, S.-Q. Zang and T. C. W. Mak, J. Am. Chem. Soc., 2018, 140, 1069–1076.
14. I. Pastoriza-Santos and L. M. Liz-Marzan, Pure Appl. Chem., 2000, 72, 83–90.
15. S. Chen, L. Xiong, S. X. Wang, Z. Y. Ma, S. Jin, H. T. Sheng, Y. Pei and M. Z. Zhu, J. Am. Chem. Soc., 2016, 136, 10754–10757.
16. G. Li, Z. Lei and Q. M. Wang, J. Am. Chem. Soc., 2010, 132, 17678–17679.
17. (a) C. Yang, O. Elbjeirami, C. S. P. Gamage, H. V. R. Dias and M. A. Omary, Chem. Commun., 2011, 47, 7434–7436; (b) C.-M. Che, M.-C. Tse, M. C. W. Chan, K.-K. Cheung, D. L. Phillips and K.-H. Leung, J. Am. Chem. Soc., 2000, 122, 2464–2468; (c) D. Sun, L. L. Zhang, H. F. Lu, S. Y. Feng and D. F. Sun, Dalton Trans., 2013, 42, 3528–3532; (d) J. Jin, W.-Y. Wang, Y.-H. Liu, H.-W. Hou and Y.-T. Fan, Chem. Commun., 2011, 47, 7461–7463.
18. V. W.-W. Yam, V. K.-M. Au and S. Y.-L. Leung, Chem. Rev., 2015, 115, 7589–7728.

---

Footnotes

† Electronic supplementary information (ESI) available: IR, ¹³C NMR, CV, UV, EDS, PXRD and luminescence decay curve, and details of the data collection and structure refinements, and crystal data. CCDC 1850394 and 1850395 for SD/Ag80a and SD/Ag80b. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8sc03396j

‡ These authors contributed equally.

---