Effects of bromine-containing counterion salts in directing the structures of medium-sized silver nanoclusters

Abstract

The preparation and structural determination of silver nanoclusters (especially the medium-sized Ag clusters) remain more challenging relative to those of their gold counterparts because of the comparative instability of the former. In this work, three medium-sized Ag clusters were controllably synthesized and structurally determined, namely, [Ag₅₂(S-Adm)₃₀Br₄H₂₀]²⁻ (Ag₅₂ for short), Ag₅₄(S-Adm)₃₀Br₄H₂₀ (Ag₅₄ for short), and [Ag₅₈(S-Adm)₃₀Br₄(NO₃)₂H₂₂]²⁺ (Ag₅₈ for short) nanoclusters. Specifically, the introduction of PPh₄Br gave rise to the generation of Ag₅₂ and Ag₅₄ nanoclusters with homologous compositions and configurations, while the TOABr salt selected Ag₅₈ as the sole cluster product, whose geometric structure was completely different from those of Ag₅₂ and Ag₅₄ nanoclusters. In addition, the optical absorptions and emissions of the three medium-sized silver nanoclusters were compared. The findings in this work not only provide three uniquely medium-sized nanoclusters to enrich the silver cluster family but also point out a new approach (i.e., changing the counterion salt) for the preparation of new nanoclusters with novel structures.

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Xi Kang

Xi Kang obtained his PhD in Chemistry from Anhui University in 2020 under the supervision of Prof. Manzhou Zhu. He is currently a Professor of Chemistry at Anhui University. His research interests include atomically precise nanoclusters and cluster-based nanomaterials.

Introduction

As a special kind of metal nanomaterial, atomically precise metal nanoclusters, whose sizes are between those of metallic complexes and those of bulk metals, have attracted widespread attention in the past few decades.^1–3 Owing to the strong quantum size effect and discrete electronic energy levels, metal nanoclusters or cluster-based nanomaterials exhibit structure-dependent performances, such as optical,^4–9 electrical,^10–13 magnetic, etc.^14–17 Besides, the atomic precision characterization of metal nanoclusters makes them ideal nano-platforms for investigating the structural evolution mechanisms of metal-based nanostructures and the related structure–property correlations, which could, in turn, act as guidelines for the preparation of metal clusters with customized structures and tailored properties. In this context, how to control the size/structure of nanoclusters and regulate their chemical environment to obtain functional nanoclusters has aroused in-depth research interests of nanocluster scientists.^18–23

Compared with the rich family of gold nanoclusters, there are relatively few structures on the atomically precise silver counterparts. Indeed, the comparative instability of silver nanoclusters relative to gold ones makes their preparation and isolation more challenging. To date, several silver nanoclusters with different core sizes have been prepared and structurally determined by means of one-pot synthesis, ligand engineering, intercluster assembly, etc.^24–35 Several of these reported silver nanoclusters comprise small-sized metallic kernels (e.g., Ag_14–16,^36–38 Ag_20–23,^39–41 Ag₂₅,⁴² Ag₂₉,⁴³etc.) or large-sized metallic kernels (e.g., Ag₁₀₀,⁴⁴ Ag₁₃₆,⁴⁵ Ag₁₄₁,⁴⁶ Ag₁₄₆,⁴⁷ Ag₁₅₂,⁴⁸ Ag₂₀₆,⁴⁹ Ag_210–211,⁵⁰ Ag₃₀₇,⁵¹ Ag₃₇₄,⁵²etc.). In comparison, there are fewer reports about structurally determined silver nanoclusters with medium-sized metallic kernels (i.e., 50–100-atom silver kernels), which partly restrict the formation of a rich family of silver nanoclusters. Besides, ligand engineering has been developed as a versatile approach for the synthesis of silver nanoclusters with novel structures and functionalities, and the ligand effect has been evaluated in such preparations. However, most of these works focused on the thiol or phosphine ligand effects in directing the structures of metal nanoclusters. As the halide-containing salts have been widely exploited for the preparation of silver nanoclusters via forming Ag–halide interactions, more efforts are needed to evaluate the effect of halide-containing counterion salts in the corresponding preparations and structure formations.

In the current work, the effect of bromine-containing salts in directing the structures of silver nanoclusters was investigated, and three clusters with medium-sized metallic kernels were controllably synthesized and structurally determined. Specifically, the introduction of PPh₄Br gave rise to the generation of [Ag₅₂(S-Adm)₃₀Br₄H₂₀]²⁻ (Ag₅₂ for short) and Ag₅₄(S-Adm)₃₀Br₄H₂₀ (Ag₅₄ for short) nanoclusters, while TOABr selected the [Ag₅₈(S-Adm)₃₀Br₄(NO₃)₂H₂₂]²⁺ (Ag₅₈ for short) nanocluster, where S-Adm is 1-adamantane thiol and TOABr is tetraoctylammonium bromide. The atomically precise structures of the three silver nanoclusters were determined by single-crystal X-ray diffraction (SC-XRD) and further confirmed by electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) measurements. The Ag₅₂ and Ag₅₄ nanoclusters were structurally homologous in terms of the kernel structures and the overall configurations, while the Ag₅₈ nanocluster exhibited a completely different geometric structure. In addition, the optical performances, including optical absorptions and emissions, of the three silver nanoclusters were compared.

Results and discussion

The three medium-sized Ag nanoclusters (i.e., Ag₅₂, Ag₅₄, and Ag₅₈) were controllably synthesized via a one-pot mild-reduction method (see the ESI for more details†). The borane-tert-butylamine complex was exploited as the reducing agent in this work to accomplish the mild reduction of the nanoclusters. Specifically, the structurally homologous Ag₅₂ and Ag₅₄ nanoclusters were prepared simultaneously and distinguished in the SC-XRD process, while the Ag₅₈ nanocluster was prepared separately. The critical points for directing different nanoclusters (Ag₅₂ and Ag₅₄, or Ag₅₈) was the introduction of different bromine-containing counterion salts—PPh₄Br or TOABr (Fig. 1). The different cluster products might be attributed to the following two aspects: (i) the alkane chain in TOABr was flexible, while the benzene ring in PPh₄Br was rigid. The different steric hindrances of two counterion salts resulted in distinct restriction effects in the structural rearrangement among the cluster formation process; (ii) the different interactions between counter-cations (PPh₄⁺ or TOA⁺) and cluster molecules would remarkably affect the final cluster formation.

Fig. 1 Synthetic approaches of Ag₅₂, Ag₅₄, and Ag₅₈ nanoclusters and their overall structures. Color labels: light blue/green = Ag; yellow = S; brown = Br; blue = N; and red = O. All C and H atoms are omitted for clarity.

The structures of Ag₅₂ and Ag₅₄ clusters were distinguished via SC-XRD (Fig. S1†). As depicted in Fig. 2, the Ag₅₂ and Ag₅₄ nanoclusters were highly structurally homologous in terms of their overall geometric structures. Both Ag₅₂ and Ag₅₄ clusters contained an Ag₁₂ kernel consisting of three octahedral Ag₆ units assembled via an Ag₃-face-fusing pattern (Fig. 2A and B). The Ag₁₂ kernel was further stabilized by four bromine ligands and seven Ag₅(SR)₄ motifs via Ag–Br and Ag–S interactions, respectively (Fig. 2C and D). The obtained A₄₆(SR)₂₈Br₄ structure presented the C₂ symmetry (Fig. 2D). The only difference between the geometric structures between Ag₅₂ and Ag₅₄ nanoclusters was their bridging motifs on two ends: two Ag linkers and two Ag₂(SR)₁ motifs in the Ag₅₂ nanocluster, or two Ag linkers and two Ag₃(SR)₁ motifs in the Ag₅₂ nanocluster (Fig. 2E and F). In this context, the Ag₅₂ and Ag₅₄ nanoclusters were highly structurally homologous with subtle differences in their terminal structures. Such similar frameworks might be the root cause for the co-generation of the two nanoclusters and the high similarity of their crystals; accordingly, the Ag₅₂ and Ag₅₄ clusters could only be distinguished and separated via SC-XRD. However, the cocrystallization of the two structure-correlated nanoclusters was not formed, which was probably due to their distinct crystal lattice energies that forced them to be crystallized into their individual crystals.^53,54 Indeed, the two nanoclusters followed different crystalline systems and space groups.

Fig. 2 Structural anatomy of Ag₅₂ and Ag₅₄ nanoclusters. (A) The three Ag₆ units. (B) The Ag₁₂ kernel. (C) The Ag₁₂Br₄ kernel structure. (D) The Ag₄₆(SR)₂₈Br₄ structure. (E) The overall structure of Ag₅₂(SR)₃₀Br₄. (F) The overall structure of Ag₅₄(SR)₃₀Br₄. Color labels: gold/light blue/green = Ag; yellow = S; and brown = Br. All C and H atoms are omitted for clarity.

Due to their different terminal structures, Ag₅₂ and Ag₅₄ nanoclusters exhibited distinct bond lengths despite their homologous frameworks. Of note, the difference in peripheral structures of nanoclusters would also lead to corresponding changes in their kernel structures. For example, the octahedral Ag₆ nuclei underwent elongation with the increasing cluster size. Specifically, the average bond length between Ag atoms in Ag₆ nuclei of Ag₅₂ was 2.825 Å, which increased to 2.908 Å for Ag₅₄ (Fig. S2A and S3A†). Besides, the average lengths of the Ag(core)–Ag(motif) and Ag(core)–Br bonds were shortened from 2.992 and 2.762 Å for Ag₅₂ to 2.946 and 2.715 Å for Ag₅₄, respectively. In addition, both average lengths of Ag(motif)–Ag(motif) and Ag(motif)–S(motif) bonds were also reduced with the cluster size growth (Fig. S2 and S3†). The most significant structural difference between Ag₅₂ and Ag₅₄ was their terminal motif structures: Ag₂(SR)₁ in Ag₅₂ and Ag₃(SR)₁ in Ag₅₄. Compared to the average Ag–S bond length of 2.603 Å in the Ag₂(SR)₁ motif of the Ag₅₂ cluster, the corresponding bonds in Ag₅₄ were remarkably shortened to 2.548 Å (Fig. S4†). In this context, the surface structure of Ag₅₄ becomes more densely packed relative to Ag₅₂ due to the incorporation of two more Ag atoms. At the same time, the kernel structure is expanded with the cluster size growth, which might result from the pulling effect of the peripheral motif frameworks of nanoclusters on their innermost kernel structures.

The introduction of the TOABr counterion that contained flexible carbon tails gave rise to the generation of the Ag₅₈ nanocluster. The overall structure of Ag₅₈ contained an Ag₆ core with Ag(core)–Ag(core) bond lengths ranging from 2.743 to 2.785 Å (on average, 2.770 Å). At both ends of the Ag₆ core, two polar Ag atoms (marked in blue) were symmetrically anchored onto the corresponding Ag₃ face, forming two Ag₄ tetrahedra and constituting an Ag₈ kernel (Fig. 3A and S5–S7†). The Ag₈ kernel was fixed by a ring-shaped Ag₈Br₄ motif, and four Br atoms were connected to four Ag atoms at adjacent positions on the Ag₆ core, constituting an Ag₁₆Br₄ structure (Fig. 3B). In contrast to the exposed Br atoms in Ag₅₂ and Ag₅₄, the Br atoms in the Ag₅₈ nanocluster were shielded. The average bond length of the Ag(core)–Br bonds in Ag₅₈ was 2.609 Å, indicating a more compact kernel structure relative to those in Ag₅₂ and Ag₅₄ (Fig. S2–S4†). Furthermore, three Ag₅(SR)₄ motifs further stabilized the Ag₁₆Br₄ structure, forming an Ag₄₆(SR)₂₄Br₄ framework (Fig. 3C). Each three adjacent Ag₅(SR)₄ subunits were inter-connected via Ag–S interactions, forming the Ag₁₅(SR)₁₂ structure in a “hand in hand” manner (Fig. S6B†). Besides, each Ag₅(SR)₄ motif structure corresponded to the three exposed Ag₃ faces (blue faces in Fig. 3C), wherein the middle Ag atom in Ag₅(SR)₄ connected the polar Ag atoms at the ends of the Ag₈ kernel. Finally, two Ag₆(SR)₃(NO₃) motifs were attached at both ends of the Ag₄₆(SR)₂₄Br₄ structure, giving rise to the overall Ag₅₈(SR)₃₀Br₄(NO₃)₂ structure (Fig. 3D). The average length of the Ag(core)–Ag(core) bonds in the kernel of Ag₅₈ was measured at 2.770 Å, remarkably shorter than those in the Ag₅₂ and Ag₅₄ nanoclusters (Fig. S5A†), while the average Ag(core)–Ag(motif) bond length of Ag₅₈ was much longer than those of Ag₅₂ and Ag₅₄ (Fig. S5B;† 3.191 Å versus 2.992 and 2.946 Å). In this context, the Ag₅₈ nanocluster exhibited a more compact kernel structure compared to a more scattered kernel–motif shell interaction relative to the Ag₅₂ and Ag₅₄ nanoclusters.

Fig. 3 Structural anatomy of the Ag₅₈ nanocluster. (A) The Ag₈ kernel. (B) The Ag₁₆Br₄ kernel structure. (C) The Ag₄₆(SR)₂₄Br₄ structure. (D) The overall structure of Ag₅₈(SR)₃₀Br₄(NO₃)₂. Color labels: gold/light blue/green = Ag; yellow = S; brown = Br; blue = N; and red = O. All C and H atoms are omitted for clarity.

Structurally, all three silver nanoclusters contained 30 thiol ligands, and the coordination modes of S (i.e., the interaction modes between S and Ag) in the three nanoclusters were compared. The 30 thiol ligands in Ag₅₂ and Ag₅₄ exhibited three types of bonding patterns, μ₂-S, μ₃-S, and μ₄-S, by analyzing the number of Ag atoms connecting the S atom, while only μ₂-S and μ₃-S were observed in the Ag₅₈ nanocluster. As the cluster size increased from Ag₅₂ to Ag₅₄, the surface structure of the nanocluster became more complex with a transformation of some μ₂-S into μ₃-S (Fig. S8†). The average S–Ag coordination numbers of Ag₅₂ and Ag₅₄ nanoclusters were determined to be 2.324 and 2.378, respectively, demonstrating that more S–Ag interactions were generated on the Ag₅₄ nanocluster surface. Moreover, a comparison of crystalline arrangement patterns was conducted (Fig. S9–S12†). The lattice symmetry of Ag₅₂ (orthogonal system), Ag₅₈ (monoclinic system), and Ag₅₄ (triclinic system) was ordered from high to low. Accordingly, the different molecular structures of these nanoclusters could remarkably affect their supramolecular assemblies. Indeed, the supramolecular crystalline density was increased in direct proportion to the size of the nanocluster (Fig. S12†). In addition, the X-ray photoelectron spectroscopy (XPS) results of these nanoclusters demonstrated that there was no change for the Ag 3d, Br 3d, and S 2p binding energies (Fig. S13–15†), further confirming their homologous structural characterization.

The ESI-MS measurement was performed to confirm the compositions and determine the valence states of these silver nanoclusters. The mass peaks at 5484.72 and 5808.24 Da confirmed the compositions of the Ag₅₂ and Ag₅₈ nanoclusters and demonstrated their valence states as [Ag₅₂(S-Adm)₃₀Br₄H₂₀]²⁻ and [Ag₅₈(S-Adm)₃₀Br₄H₂₂]²⁺, respectively (Fig. S16†). It should be noted that the two NO₃ ligands were dissociated in the ESI-MS measurement. According to the valence states of these nanoclusters, their nominal electron counts were determined as 52(Ag) − 30(SR) − 4(Br) −20(H) + 2(charge) = 0e for [Ag₅₂(SR)₃₀Br₄H₂₀]²⁻ and 58(Ag) − 30(SR) − 4(Br) − 22(H) − 2(charge) = 0e for [Ag₅₈(SR)₃₀Br₄H₂₂]²⁺, demonstrating the absence of the free electron in such cluster frameworks. However, no mass signal was detected for the Ag₅₄ cluster (Fig. S17†). Considering that the Ag₅₂ and Ag₅₄ nanoclusters shared a homologous structure, we proposed that the molecular formula for Ag₅₄ should be Ag₅₄(SR)₃₀Br₄H₂₀. In this context, the Ag₅₄ nanocluster should be electrically neutral and was difficult to ionize in the mass detection. All three silver clusters contained a large number of hydride ligands, which was rare in previously reported metal nanoclusters. However, the precise locations of such hydrides were hard to determine using an SC-XRD diffractometer due to their low electron densities. To address this issue, we attempted to replace the “H” ligands with “D” to further verify their numbers in nanoclusters by ESI-MS. However, no deuterated borane-tert-butylamine complex (the reducing agent in this work) was commercially available. We have tried several times to replace the reducing agent with NaBH₄, for which deuterated NaBD₄ was available; however, no cluster product was obtained since all products were decomposed after the preparation. In this context, we just employed ¹H NMR spectroscopy to confirm the presence of hydrides in the nanocluster. For the Ag₅₂ nanocluster, three prominent NMR peaks at 4.67, 5.29, and 8.07 ppm were detected for the nanocluster with an area ratio of 1 : 2 : 1, while no corresponding signal was observed for the HS-Adm ligand. Consequently, it is reasonable that there are three types of hydrides within the framework of the Ag₅₂ nanocluster.

The structure of clusters is the decisive factor of their physical–chemical properties. The differences in molecular structures of the three nanoclusters resulted in their distinct optical absorptions and emissions (Fig. 4). Specifically, the Ag₅₂ nanocluster exhibited two distinctive absorptions at 350 and 475 nm, while Ag₅₄ displayed two absorptions at 350 and 460 nm, and a shoulder band at 530 nm. In comparison, the Ag₅₈ nanocluster presented two absorptions at 350 and 460 nm. Besides, the emission wavelengths of the Ag₅₂, Ag₅₄, and Ag₅₈ nanoclusters were 613, 617, and 602 nm, respectively. In addition, compared to the Ag₅₄ and Ag₅₂ nanoclusters with homologous structures, the Ag₅₈ nanocluster exhibited a remarkably reduced photoluminescence intensity. Besides, we found that the Ag₅₄ nanocluster exhibited an enhanced PL intensity relative to its structure-related Ag₅₂ nanocluster. Such an enhancement was rational by considering that the Ag₅₄ nanocluster showed a more compact molecular structure, which decreased the non-radiative transition and enhanced the radiative transition (i.e., the PL).^55,56

Fig. 4 Comparison of optical absorptions (solid lines) and emissions (dot lines) of Ag₅₂ (black lines), Ag₅₄ (red lines), and Ag₅₈ (blue lines) nanoclusters (cluster molecules were dissolved in CH₂Cl₂).

Conclusion

In summary, three medium-sized silver nanoclusters, namely, Ag₅₂, Ag₅₄, and Ag₅₈, were controllably synthesized by adjusting the bromine-containing counterion salts in the synthetic process. Accordingly, the effects of Br-based counterion salts in directing the structures of silver nanoclusters were evaluated. Among the three nanoclusters, Ag₅₂ and Ag₅₄ were structurally homologous in terms of their kernel structures and overall configurations, while the configuration of the Ag₅₈ nanocluster was completely different. The structure-dependent optical properties (i.e., optical absorption and emission) of the three nanoclusters were evaluated. The findings in this work not only expand the structural diversity of the silver nanocluster family but also demonstrate the counterion effect in the preparation of new nanoclusters with novel structures.

Data availability

CCDC 2301180, 2301182 and 2301183 contain the supplementary crystallographic data for this paper.†

Author contributions

H. L. and X. W. performed the experiments. H. L., X. W. and X. K. wrote the manuscript. X. K. and M. Z. supervised the project. All authors commented on and agreed on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support by the NSFC (21631001, 21871001, 22371003, and 22101001) and the Ministry of Education, and the University Synergy Innovation Program of Anhui Province (GXXT-2020-053).

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S18 and Tables S1–S3 for the structure comparison, and ESI-MS, XPS, and NMR results of nanoclusters. CCDC 2301180, 2301182 and 2301183. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3nr05464k

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