Regulation of Surface Structure of [Au9Ag12(SAdm)4(Dppm)6Cl6](SbF6)3 Nanocluster via Alloying

Tailoring of specific sites on the nanocluster surface can tailor the properties of nanoclusters at the atomic level, for the in-depth understanding of structure and property relationship. In this work, we explore the regulation of surface structure of [Au9Ag12(SAdm)4(Dppm)6Cl6](SbF6)3 nanocluster via alloying. We successfully obtained the well-determined tri-metal [Au9Ag8@Cu4(SAdm)4(Dppm)6Cl6](SbF6)3 by the reaction of [Au9Ag12(SAdm)4(Dppm)6Cl6](SbF6)3 with the CuI(SAdm) complex precursor. X-ray crystallography identifies that the Cu dopants prioritily replace the position of the silver capped by Dppm ligand in the motif. The Cu doping has affected the optical properties of Au9Ag12 alloy nanocluster. DPV spectra, CD spectra and stability tests suggest that the regulation of surface structure via Cu alloying changes the electronic structure, thereby affecting the electrochemical properties, which provides insight into the regulation of surface structure of [Au9Ag12(SAdm)4(Dppm)6Cl6](SbF6)3 via alloying.


INTRODUCTION
Atomically precise core-shell nanoclusters have become a promising material in catalysis, biomedicine, and chemical sensing due to the unique quantum confinement effect resulting in optical properties Yao et al., 2018;Xu et al., 2019;Jin R. et al., 2021;Sun et al., 2021;Zheng et al., 2021). The studies on correlation between the properties and structures of cluster compounds based on the determined crystal structures show that the core and shell structures have different effects on the performance of the cluster compounds, and modifications on the core and shell structures may induce variations on clusters properties (AbdulHalim et al., 2014;Chakraborty and Pradeep, 2017;Khatun et al., 2018;Yan et al., 2018;. The Pt core-doped nanocluster PtAu 24 (SC 6 H 13 ) 18 exhibits higher hydrogen production than that of Au 25 (Kwak et al., 2017), and the dopant AuAg 24 shows stronger fluorescence performance (Bootharaju et al., 2016). Surface shell dopant Au 24 Cu 6 exhibited superior catalytic activity compared to other homometallic and Au-Cu alloy nanoclusters (Chai at al., 2019). Therefore, alloying could serve as an efficient approach to tailor the properties of nanoclusters for more applications Jin et al., 2018a;Wang et al., 2018;Dias and Leite, 2019).
Frontiers in Chemistry | www.frontiersin.org January 2022 | Volume 9 | Article 793339 2 2019b; Kang et al., 2020) When the third metal is doped into the bimetallic alloy clusters, what site will it occupy and what effect will it have on the overall performance?Recently, for the active metal Cu doping, several surface Cu-doped nanoclusters such as Au 13 Cu x (x 2, 4, 8) (Yang et al., 2013), Cu x Au 25-x (Yang et al., 2017), Cu 3 Au 34 (Yang et al., 2017), Ag 28 Cu 12 , Ag 30 Cu 14  and Cu-internal-doped nanoclusters like Ag 61 Cu 30 have been observed and well-determined by x-ray crystallography (Zou et al., 2020). Specifically, the outer Au shells always are partially alloyed by the incorporated Cu heteroatoms for Au-based nanoclusters, while core-shell alloy nanoclusters with a shell-by-shell configuration could be generated for Ag-based nanoclusters. However, for the Au-Ag alloy nanocluster, how will the copper atoms choose the sites?
Herein, we use position-determined alloy clusters [Au 9 Ag 12 (SAdm) 4 (Dppm) 6 Cl 6 ](SbF 6 ) 3 as templates for the doping of the third metal copper (Jin et al., 2018b). The crystallography analysis suggested that the four Cu atoms priority replace the position of the silver capped by Dppm ligand in the motif for [Au 9 Ag 8 Cu 4 (SAdm) 4 (Dppm) 6 Cl 6 ](SbF 6 ) 3 (Scheme 1). And the Cu doping affected the electronicstructure, resulting in the difference of optical properties in CD spectra, DPV spectra and so on. This provides a good observation method for understanding the doping position.

Synthesis of Cu I SR Complex Precursor
CuCl (0.05 g, 0.5 mmol) was dissolved in 5 ml CH 3 CN, and AdmSH (0.09 g, 0.55 mmol) was dissolved in 5 ml CH 3 CN and added drop-wise to the solution under vigorously stirred. The resulting solution mixture was then washed several times with hexane. Then the final product was used directly.

Characterization
All UV/Vis absorption spectra of nanoclusters are recorded on a Techcomp UV1000 spectrophotometer. Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) measurement was performed using a UPLC H-class/XEV0G2-XS QTOF highresolution mass spectrometer. The sample was directly infused into the chamber at 5 μL/min. Photoluminescence spectra were measured using an FL-7000 spectrofluorometer with the same FIGURE 3 | (A) the overall structure of [Au 9 Ag 12 (SAdm) 4 (Dppm) 6 Cl 6 ](SbF 6 ) 3 ; (B) the overall structure of [Au 9 Ag 8 Cu 4 (SAdm) 4 (Dppm) 6 Cl 6 ](SbF 6 ) 3 ; Packing models of (C) [Au 9 Ag 12 (SAdm) 4 (Dppm) 6 Cl 6 ] 3+ and (D) [Au 9 Ag 8 Cu 4 (SAdm) 4 (Dppm) 6 Cl 6 ] 3+ from default view a, b, c. Color labels: Golden Au; Sky blue Ag; red S; purple P; Gray C; light green Cl; Turquoise Copper). optical density (OD) of ∼0.2. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo ESCALAB 250 configured with a monochromated Al Kα (1486.8 eV) 150 W X-ray source, 0.5 m mm circular spot size, a flood gun to counter charging effects, and an analysis chamber base pressure lower than 1 × 10 -9 mbar, and the data were collected with FAT 20 eV. CD spectra are recorded with a BioLogic MOS-500 CDspectropolarimeter in a 0.1-cm path length quartz cell. The spectra are recorded in diluted solutions of dichloromethane and the signal of the blank solvent is subtracted. The enantiomers of chiral [Au 9 Ag 8 Cu 4 (SAdm) 4 (Dppm) 6 Cl 6 ](SbF 6 ) 3 were separated by HPLC on an Agilent 1260 system equipped with a Chiralcel OD-H column (5 µm, 4.6 mm ø × 250 mm). A diode array detector (DAD) in situ monitors the entire optical absorption spectrum (190-950 nm range) of the eluted solution, and the 427, 482 and 710 nm wavelength were used for the chromatogram. The nanoclusters were pre-dissolved in solvent which has the same composition of the mobile phase (methanol/ isopropanol 35/65). The flow rate was at 0.4 ml min −1 and the temperature set at 20°C.
Furthermore, in order to have a deep understanding of the regulation process, the time-dependent UV-Vis spectra and ESI mass spectra of [Au 9 Ag 12 (SAdm) 4 (Dppm) 6 Cl 6 ](SbF 6 ) 3 in CH 2 Cl 2 after adding Cu I (SAdm) complex precursor were performed. As shown in Figure 2A, with the increase of time of Cu I (SAdm) complex precursor. adding, the peak centered at 427 nm always maintained, and the peak centered at 480 only 2 nm redshifts. While the 670 nm peak gradually red shift to 710 nm, with a redshift value of 40 nm. ESI mass spectra suggested the copper atoms are gradually replacing silver atoms, which leads to red shift ( Figure 2B). The successful determination of [Au 9 Ag 8 Cu 4 (SAdm) 4 (Dppm) 6 Cl 6 ] 3+ structure allowed us to know the site of doping clearly.
Frontiers in Chemistry | www.frontiersin.org January 2022 | Volume 9 | Article 793339 structures DppmAg 2 Cl 2 (SR) 2 , the Au 9 Ag 12 was obtained. By contrast, Au 5 Ag 8 @Au 4 @Cu 4 is obtained when four copper atoms doped the position of the silver of peripheral structures DppmAg 2 Cl 2 (SR) 2 . Meanwhile, the copper doping has little effect on the bond length and angle of the icosahedron metal core (Supplementary Figure S4). Based on the doping sites of copper atoms, we realize that the Au 4 @Ag 8 Au 5 will be a stable metal core. In the packing model, the difference of arrangement can be observed clearly, and it is worth mentioning that the doping can affect the arrangement of clusters in the unit cell ( Figures  3C,D) from a crystal engineering point of view. As reported, the chirality of metal clusters mainly come from chiral metalcore, the arrangement of chiral ligands and local chiral patterns on an achiral surface (Zeng and Jin, 2017). The chirality of [Au 9 Ag 12 (SAdm) 4 (Dppm) 6 Cl 6 ] 3+ comes from the chiral Au 4 @Ag 8 Au 5 metallic kernel. After doping, the cluster will have a different CD spectrum compared to the parent compound. Importantly, herein, the Cu dopants also have some impacts on the chiral properties. As shown in Figure 4, the CD spectra of [Au 9 Ag 12 (SAdm) 4 (Dppm) 6 Cl 6 ] 3+ reveal multiple CD-active peaks at 325, 363, 428 and 483 nm, respectively, and some weak peaks. While the CD spectra of [Au 9 Ag 8 Cu 4 (SAdm) 4 (Dppm) 6 Cl 6 ] 3+ shows peaks at 340, 373, 442, and 493 nm, respectively. The Au 38 cluster with Pd atoms leads to core-doped Pd 2 Au 36 (SC 2 H 4 Ph) 24 . Comparison between the CD spectra of Au 38 (SC 2 H 4 Ph) 24 and Pd 2 Au 36 (SC 2 H 4 Ph) 24 shows significant differences, revealing core-doping has strong impacts on the electronic structure of the cluster (Barrabés et al., 2014). The comparison between the CD spectra of [Au 9 Ag 12 (SAdm) 4 (Dppm) 6 Cl 6 ] 3+ and [Au 9 Ag 8 Cu 4 (SAdm) 4 (Dppm) 6 Cl 6 ] 3+ shows that all the peaks from [Au 9 Ag 8 Cu 4 (SAdm) 4 (Dppm) 6 Cl 6 ] 3+ have redshift, different from the differences between Au 38 (SC 2 H 4 Ph) 24 and Pd 2 Au 36 (SC 2 H 4 Ph) 24 . The doping location may have different impacts on the CD spectra.
The [Au 9 Ag 12 (SAdm) 4 (Dppm) 6 Cl 6 ](SbF 6 ) 3 and [Au 9 Ag 8 Cu 4 (SAdm) 4 (Dppm) 6 Cl 6 ](SbF 6 ) 3 show good stability in an ambient environment ( Figures 6A,D) and the stability tests (i.e., under oxidizing/reducing environments) for Au 9 Ag 12 and Au 9 Ag 8 Cu 4 are also performed to explore the effects of copper dopants on the stability of nanoclusters. Under the oxidizing environment (by mixing 200 μL of H 2 O 2 (50%) with 6 mg of cluster in 10 ml of CH 2 Cl 2 ), the Au 9 Ag 12 can stabilize for several hours ( Figures  6B,E), and the peaks of the UV-vis spectra are obvious. However, the Au 9 Ag 8 Cu 4 decompose quickly to form complexes within several mins ( Figures 6C,F). This difference may be because the peripheral copper atom is easier to be oxidized. Meanwhile, the copper doping has an impact on the properties of clusters on reducing environment (by mixing the 10 ml CH 2 Cl 2 solvent of 6 mg of cluster with 200 μL of EtOH solvent of 1 mg of NaBH 4 ). The UV-vis of Au 9 Ag 12 changes quickly until there are no obvious peaks within 30s. And the UV-vis of Au 9 Ag 8 Cu 4 also changes quickly, but still some peaks can be observed within 60 min. These indicate the regulation of surface structure affects the stability of nanoclusters.
Intercluster reactions between Au 9 Ag 12 and Au 9 Ag 8 Cu 4 (Abs. 671nm 0.3 for Au 9 Ag 12 and Abs. 712nm 0.3 for Au 9 Ag 8 Cu 4 , respectively) are performed (Zhang et al., 2016;Khatun et al., 2020;Neumaier at al., 2021). As shown in Figure 7, the reaction was completed quickly (1 min), similar to the UV-vis spectrum that prolongs the reaction for 3 h. As shown in the Figures 7A,B, intercluster reactions produce a spectrum with 428, 482 and 702 nm, respectively. Learned from the Figures 7C,D, the products are Au 9 Ag 8 Cu 4 , Au 9 Ag 9 Cu 3 , Au 9 Ag 10 Cu 2 , Au 9 Ag 11 Cu 1 ,respectively. Theoretical and experimental isotopic distributions of them matched perfectly as shown in Supplementary Figures S6, 7. This indicates the copper migration between Au 9 Ag 12 and Au 9 Ag 8 Cu 4 upon mixing in solution, similar to silver migration between Au 38 (SC 2 H 4 Ph) 24 and doped Ag x Au 38-x (SC 2 H 4 Ph) 24 nanoclusters (Zhang et al., 2016).