Interdependence between nanoclusters AuAg24 and Au2Ag41

Whole series of nanoparticles have now been reported, but probing the competing or coexisting effects in their synthesis and growth remains challenging. Here, we report a bi-nanocluster system comprising two ultra-small, atomically precise nanoclusters, AuAg24(SR)18− and Au2Ag41(SR)26(Dppm)2+ (SR = cyclohexyl mercaptan, Dppm = bis(diphenylphosphino)-methane). The mechanism by which these two nanoclusters coexist is elucidated, and found to entail formation of the unstable AuAg24(SR)18−, followed by its partial conversion to Au2Ag41(SR)26(Dppm)2+ in the presence of di-phosphorus ligands, and an interdependent bi-nanocluster system is established, wherein the two oppositely charged nanoclusters protect each other from decomposition. AuAg24(SR)18 and Au2Ag41(SR)26(Dppm)2 are fully characterized by single crystal X-ray diffraction (SC-XRD) analysis – it is found that their co-crystallization results in single crystals comprising equimolar amounts of each. The findings highlight the interdependent relationship between two individual nanoclusters, which paves the way for new perspectives on nanocluster formation and stability. Despite recent progress in individual nanocluster synthesis, understanding the competing or coexisting effects between particles in solution remains challenging. Here, the authors present the synthesis of a bi-nanocluster system comprising two atomically precise nanoclusters, and map out the interdependent relationship between them.

The methodology for exploring intermolecular interactions at the initial stage helps to regulate the synthesis of nanoparticles [31][32][33][34] . For example, the size focusing method for synthesizing atomically precise nanoparticles (nanoclusters), wherein the influence of reaction conditions (reaction temperature, growth kinetics, ligand bulkiness, etching time, ligand/metal ratio) on product nanoparticle size distribution are mapped out and then adjusted such that a specific size of product nanoparticles are obtained, is quite similar to the concept of "survival of the fittest" 35,36 . High-yield syntheses of molecularly pure Au 38 (SR) 24 37 , Au 144 (SR) 60 38 , Au 64 (SR) 32 39 have all been accomplished with this size-focusing methodology. However, the size focusing method does not necessarily result in monodisperse nanoclusters, and is more likely to afford a mixture of several different monodisperse nanoclusters. These nanoclusters can be regarded as the "fittest" in the environment in which they were formed, but although they can be separated and fully characterized, such studies generally do not inform our understanding of the relationship between them. Important open questions include: when in solution, how do nanoparticles interact with each other, leading to the coexistence of two or more nanoparticles, which are mutually beneficial? Can nanoparticles exist in the presence of each other, but not separately? The discovery of such phenomena will lead us to a deeper understanding of the formation and growth mechanism of nanoparticles, and will provide effective guideline for the design of new nanoparticles, especially metal nanoparticles with precise structure.
Herein, we report the synthesis and characterization of a binanocluster system, AuAg 24 and Au 2 Ag 41 , and the interdependent relationship between these two nanoclusters are mapped out. By working with precisely structured nanoclusters, we obviated the problem of poly-dispersion, which plagues the study of nanoparticles. The relatively simple nature of this binanocluster system also ensures that the exterior synthetic environment is the same for each individual nanoparticle, and therefore the interactions between them are more tractable. For example, we were able to determine that the negatively charged nanocluster AuAg 24 (SR) 18 − (denoted as AuAg 24 ) is produced first, and then some of it is converted into a positively charged nanocluster Au 2 Ag 41 (SR) 26 (Dppm) 2 + (denoted as Au 2 Ag 41 ). The two types of nanoclusters protect each other from decomposition, revealing the interdependent relationship between them. Moreover, co-crystallization of AuAg 24 and Au 2 Ag 41 resulted in crystals consisting of each of them in a molar ratio of 1:1.

Results
Synthesis and characterization. The synthesis of the title nanoclusters was performed in a one-pot method. First, the stable precursor Ag-Au-Dppm complex was synthesized as a solution in toluene. Introduction of the thiol ligand in the presence of NaBH 4 resulted in the gradual formation of the nanoclusters. By adding 5 mL of n-hexane to the saturated toluene solution, single crystals could be obtained after 7 days. The resulting black single crystal was subjected to X-ray single crystal structure analysis (Supplementary Fig. 1), which revealed the co-crystallization of both AuAg 24 and Au 2 Ag 41 in a 1:1 molar ratio, and their hierarchical assembly in the triclinic space group. UV-Vis absorption spectrum of the (Au 2 Ag 41 )■(AuAg 24 ) co-crystal showed intense peaks at~443, 472, and 570 nm and a weaker peak at~710 nm ( Fig. 1), corresponding to excitation energies of 2.80, 2.63, 2.17, and 1.75 eV, respectively.
Atomic structure. X-ray crystallographic analysis revealed that AuAg 24 adopts an icosahedral M 13 kernel structure, with six, onedimensional Ag 2 S 3 motifs surrounding 12 Ag atoms surrounding a central Au atom (Fig. 2ai-ii). The Au 2 Ag 41 structure comprises a rod-shaped M 25 kernel composed of two M 13 structural units (Fig. 2aiii), wrapped by a "cage" like frame composed of the outer Ag-S-P layer (Fig. 2aiv). Detailed structural analyses of both nanoclusters are shown in Supplementary Fig. 2. The bond lengths of AuAg 24 and Au 2 Ag 41 were similar-for example, the average Au kernel -Ag kernel and Ag kernel -Ag kernel bond lengths were 2.91 Å for Au 2 Ag 41 , slightly longer than in AuAg 24 (2.88 Å). The Au 2 Ag 41 adopted the same M 25 kernel as the reported [(p-Tol 3 -P) 10 Au 13 Ag 12 Br 8 ] +40 , and the average bond lengths of Au 2 Ag 41 (2.91 Å) were almost identical with that of [(p-Tol 3 -P) 10 Au 13 Ag 12 Br 8 ] + (2.92 Å) ( Supplementary Fig. 3). AuAg 24 and Au 2 Ag 41 can be regarded as a structural unit in a stacked threedimensional structure (Fig. 2b-d). The occupancy of each nanocluster in the unit cell was 50%, and the interlayer distance of lamellar co-crystallization was 24.72 Å (calculated from the gap between each Au plane). The molar ratio of Au 2 Ag 41 and AuAg 24 was further confirmed by 1 H NMR spectroscopy ( Supplementary  Fig. 4); the integral area ratio of the benzene ring region (-C 6 H 5 ) to the partial methylene (-CH 2 ) region was 1:6.7, consistent with the theoretically calculated result of 1:6.6. Additionally, X-ray photoelectron spectroscopy (XPS) confirmed the elemental composition of the two nanoclusters (Supplementary Figs. 5-8). Dynamic growth process of the bi-nanocluster. In order to elucidate the nanocluster formation process, we undertook a thinlayer chromatography study based on the time-dependent absorption peak variation in the UV-Vis spectra (Fig. 3a). "Point 1" appeared on the thin-layer chromatography plate within ten minutes of reaction initiation. As the reaction progressed, the second "point 2" gradually appeared, indicating the formation of a new component. At the same time, we obtained the UV-Vis spectra of two key points plotted on the photon energy scale (Fig. 3ai-ii). Two absorption peaks centered at~2.17, and 2.68 eV were observed on the spectrum of "point 1", while four absorption peaks were observed on the spectrum of "point 2" centered at~1.79, 2.17, 2.68, and 2.90 eV, respectively. To identify the corresponding clusters, we crystallized component 1 ("point 1") and component 2 ("point 2"), respectively. The stabilities of components 1 and 2 were tracked by UV-Vis spectra (Fig. 3b, c) Fig. 9). The structure analysis showed it to be the negatively charged, eight-electron structure [AuAg 24 (SR) 18  Time-dependent UV-Vis spectra were recorded, to monitor the reaction (Fig. 3a). Characteristic peaks at 2.17 eV and 2.68 eV corresponding to AuAg 24 appeared within the first 10 min. Later, new peaks centered at 1.79 eV and 2.90 eV appeared, the intensity of which gradually increased. These reflect formation of another component, Au 2 Ag 41 and the formation of a stable bi-nanocluster system. 1 H NMR spectra were also acquired, to further track the reaction process and confirm the relationship between the two nanoclusters. At t = 10 min of the reaction, no signals corresponding to the benzene ring region could be observed in the 1 H NMR spectrum, indicating that the Au 2 Ag 41 stabilized by both thiol and phosphine ligands was not formed. As the reaction continued, the peaks centered at 7.0-8.0 ppm gradually increased in intensity compared with the peak centered at 3.0 ppm ascribed to S-CH of cyclohexanethiol ligand (-CH), indicating the formation of Au 2 Ag 41 ( Supplementary Fig. 12).
Mechanism of effect between the bi-nanocluster. The above results suggested that AuAg 24 was produced before Au 2 Ag 41 , but To map out the relationship between AuAg 24 and Au 2 Ag 41 , we designed a conversion reaction. A sample of AuAg 24 was dissolved in a mixture of dichloromethane and methanol. Then, phosphine and mercaptan ligands were added. The UV-Vis spectra of the mixture were recorded every hour from 0 min to 4 hr (Fig. 4). At 0 min, the UV-Vis spectra exhibited the characteristic absorption peaks of AuAg 24 at 463 nm and 570 nm. Gradually, new absorption peaks appeared at 695 nm and 425 nm. The spectrum of final product (after 4 h) was identical to that of the bi-nanocluster. These results indicated that the partial conversion of AuAg 24 50 1.75 2.00 2.25 2.50 2.75 3.00 3.25   Based on the above results, a mechanism of the transformation and interaction between AuAg 24 and Au 2 Ag 41 is proposed (Fig. 5). The reaction is divided into two steps: (I) The negatively charged AuAg 24 was formed at the beginning of the reaction; (II) Part of the AuAg 24 nanoclusters were converted to oppositely charged Au 2 Ag 41 due to the instability of AuAg 24 in the absence of suitable counterions. Briefly, AuAg 24 is susceptible to decomposition due to the absence of a suitable counter ion salt protection, causing part of it to be converted into a larger cluster with opposite valence as counter ion. These two types of nanoclusters stabilized each other to form a stable interdependent bi-nanocluster system (Supplementary Fig. 13). The behavior of the conversion from unstable nanoclusters to stable co-crystal system exhibit the synergy between different nanoclusters, which sheds light on the further preparation of new types of nanoparticles in the future.

Discussion
In summary, we have successfully synthesized a bi-nanocluster system and mapped out the relationship between its two constituent nanoclusters. Owing to the lack of suitable counter ion, the negatively charged AuAg 24 nanocluster partially converts to the Au 2 Ag 41 nanocluster, which bears the opposite charge. Thus, the two nanoclusters act as counter ions of each other, establishing a stable and interdependent system. The interdependent effect revealed in this work further advances the understanding of inter-nanocluster correlations.
Synthesis of the (Au 2 Ag 41 )■(AuAg 24 ) co-crystal. AgNO 3 (60 mg) and bis-(diphenylphosphino)methane (Dppm, 40 mg) were added to a 15 mL methanol solution, and the Ag-Dppm complex was formed after vigorous stirring. Gold salt (HAuCl 4 • 3H 2 O, 4 mg) was injected into the reaction solution. After continuing to stir for 10 min, cyclohexyl mercaptan (C 6 H 12 S, 100 mg) was added, and the solution changed from white and turbid to yellow and clear. The stirring continued for 20 min until the color of the reaction no longer changed. Then, drop-wise addition of 2 mL of NaBH 4 ethanol solution (25 mg) to the reaction, the color of the reaction mixture changed to yellow and then to dark. This solution was incubated for 12 h at room temperature. The solution was centrifuged to give the black crude product, which was washed by methanol, then dissolved in toluene to prepare a saturated solution. A certain amount of n-hexane was added thereto and the solution was placed in a refrigerator, and rod-shaped black crystals were obtained in about 7 days.
Synthesis of AuAg 24 nanocluster. Typically, we obtained a yellow clear solution according to the above co-crystallization method. Immediately, the drop-wise addition of ice-cold Ethanol NaBH 4 (20 mg in 2 mL Ethanol). The mixed solution was continuously stirred for 7 h in an ice bath, after which the brown compound was isolated by purification and dissolved in CH 2 Cl 2 . After that, the methanolic PPh 4 Br (30 mg in 2 mL methanol) was added to the solution and which was crystallized in CH 2 Cl 2 /hexane for about 14 days in a refrigerator to obtain black hexagonal crystals. Characterization. Ultraviolet−visible (UV−Vis) absorption spectra were recorded on an UV-6000PC spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on Thermo ESCALAB 250 configured with a monochromated Al Kα (1486.8 eV) 150 W X-ray source, 0.5 mm circular spot size, a flood gun to counter charging effects, and the analysis chamber base pressure lower than 1 × 10 −9 mbar; data were collected with FAT = 20 eV. Nuclear magnetic resonance (NMR) analysis was performed on a Bruker Avance spectrometer operating at 400 MHz for CD 2 Cl 2 was used as the solvent to dissolve ∼5 mg clusters; the residual solvent peak (i.e., 1 H at 5.32 ppm) was used as reference. The data collection for single crystal X-ray diffraction was carried out on Stoe Stadivari diffractometer under liquid nitrogen flow at 170 K, using graphitemonochromatized Cu Kα radiation (λ = 1.54186 Å). Data reductions and absorption corrections were performed using the SAINT and SADABS programs, respectively.

Data availability
The X-ray crystallographic coordinates for structures reported in this work have been deposited at the Cambridge Crystallographic Data Center (CCDC), under deposition numbers CCDC-2007161 and 2007612. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif, which has been mentioned in the article.