Cd 12 Ag 32 (SePh) 36 : Non-Noble Metal Doped Silver Nanoclusters

: While there are numerous recent reports on doping of a ligand-protected noble metal nanocluster (e.g., Au and Ag) with another noble metal, non-noble metal (e.g., Cd) doping remains challenging. Here, we design a phosphine-assisted synthetic strategy and synthesize a Cd doped Ag nanocluster, Cd 12 Ag 32 (SePh) 36 (SePh: selenophenolate), which exhibits characteristic UV-vis absorption features and rare near-infrared (NIR) photoluminescence at ~ 1020 nm. The X-ray single crystal structure reveals an asymmetric two-shell Ag 4 @Ag 24 metal kernel protected by four nonplanar Cd 3 Ag(SePh) 9 metal-ligand frameworks. Furthermore, the electronic structure analysis shows that the cluster is a 20-electron “superatom” and density functional theory predicts that its chiral optical response is comparable to the well-known Au 38 (SR) 24 cluster. Our synthetic approach will pave a new path for introducing other non-noble metals into noble metal nanoclusters for exploring their effect

of a ligand-protected noble metal nanocluster (e.g., Au and Ag) with another noble metal, non-noble metal (e.g., Cd) doping remains challenging.Here, we design a phosphine-assisted synthetic strategy and synthesize a Cd doped Ag nanocluster, Cd12Ag32(SePh)36 (SePh: selenophenolate), which exhibits characteristic UV-vis absorption features and rare near-infrared (NIR) photoluminescence at ~1020 nm.The X-ray single crystal structure reveals an asymmetric two-shell Ag4@Ag24 metal kernel protected by four nonplanar Cd3Ag(SePh)9 metal-ligand frameworks.Furthermore, the electronic structure analysis shows that the cluster is a 20-electron "superatom" and density functional theory predicts that its chiral optical response is comparable to the well-known Au38(SR)24 cluster.Our synthetic approach will pave a new path for introducing other non-noble metals into noble metal nanoclusters for exploring their effect on optical and chemical properties.
Ligand stabilized few-atom (tens to hundreds) nanoclusters (NCs) of metals such as Au, Ag and Cu have gained much attention in recent years for their intriguing properties, including photoluminescence (PL), optical activity, catalysis, and size-and structureconversion. [1][2][3][4] The NCs (typical size <3 nm) bridge the gap between the metal-ligand complexes and nanoparticles (NPs, size ˃3 nm).Unlike the classical NPs with large size and composition distributions, the NCs are truly single-sized with precise molecular formulae.]10 The size, structure and composition are shown to influence the cluster properties significantly.]11 The common ligands used to attain different clusters are thiols, selenols, alkynyls, halides, hydrides, phosphines or their combinations, 1,2,[12][13][14][15][16][17] in which selenols are rarely used for stabilizing Ag clusters. 18,19 other path to modulate the cluster properties is by modifying the metal composition through substitutional doping with heteroatoms. 20,21 he doped NCs exhibit enhanced properties compared to undoped ones owing to synergistic effects. 4,22,23 Te dopants incorporated into Au and Ag NCs are mostly the noble metals such as Pd and Pt due to their close atomic diameters. 1,20 n contrast, doping a noble metal NC with a non-noble metal such as Cd is challenging largely due to mismatch in their atomic sizes and standard reduction potentials.Recently, several Cd doped Au NCs were synthesized by post-synthetic modification processes, [24][25][26][27] while those of Ag remain unexplored.
Here, we report a novel synthetic method for the non-noble metal doped Ag NCs by a phosphine assisted process.Specifically, by using triphenylphosphine (PPh3) as an assisting ligand and Cd as dopant, we successfully synthesized a selenophenolate stabilized Ag NC, Cd12Ag32(SePh)36.The details of its synthesis (Figure S1) and purification procedures are provided in the Supporting Information.The single crystals of NCs were grown (Figure S2) by the vapor diffusion of n-pentane into a DCM solution of the cluster.
The single crystal X-ray diffraction (SCXRD) analysis reveals that the final product is a 44 metal atom cluster stabilized by 36 SePh ligands with a molecular formula, Cd12Ag32(SePh)36 (Figure 1).Clearly, the Cd atoms are located on the cluster surface.The cluster crystallizes in a triclinic system of P-1 space group (Table S1) and its unit cell comprises two NCs (Figure S3).No counterions were identified in the structure, indicating that the cluster is neutral.Ag6 facet with four triangles (Figure 2B).Such four Ag6 facets cap the four faces of Ag4 tetrahedron to form a structure shown in Fig- ure 2C.The interfacet interactions through Ag-Ag bonding produce a Ag24 layer and it completely encapsulates the Ag4 tetrahedron, forming a two-shell Ag4@Ag24 core (Figure 2D).Notably, the central Ag3 triangles of Ag6 facets (placed exactly on top of the triangular faces of inner Ag4 core) are significantly elongated from the Ag4 core compared to other Ag-Ag bonds (Table S2).In Cd3Ag(SePh)9 surface motif (Figure 2E), each Cd atom is tetrahedrally coordinated with four Se atoms of four SePh ligands.Totally, three Se atoms from three CdSe4 tetrahedrons bind with one Ag to form a AgSe3 cap-like structure, forming the Cd3Ag(SePh)9 motif.The capping of four Cd3Ag(SePh)9 motifs on four Ag6 facets of Ag28, through Se atoms that are unbound to motif Ag atoms, produces the total structure of Cd12Ag32(SePh)36 cluster (Figure 2F).Notably, 12 and 24 Se atoms appear in µ-3 and µ-2 bridging modes, respectively.All the four Cd3Ag(SePh)9 motifs are mounted over four triangular faces of inner Ag4 tetrahedron, and therefore, it imparts the tetrahedral shape to the final structure.A similar Ag28 core of our cluster is observed in [Cu12Ag28(SR)24] 4- cluster, 28 however with vividly different metal and ligand composition, electronic charge and the surface structure.
The presence of Cd, Ag and Se in the cluster was supported by energy dispersive X-ray spectroscopy (Figure S4).The matrix-assisted laser desorption ionization mass spectrometry shows only a single high-mass peak at ~9.6kDa, indicating that the cluster is single-sized.This peak with lower in mass by ~0.7kDa compared to 10.3kDa for Cd12Ag32(SePh)36 may be due to the fragmentation often observed in the Ag clusters 11 (Figure S5).The formula of the cluster is further validated by the elemental analysis (Figure S6).The thermogravimetric analysis shows a total weight loss of 54.5%, very close to a theoretical ligand weight of the cluster 54.0% (Fig- ure S7). 1 H nuclear magnetic resonance (NMR) spectrum of the cluster (Figure S8) shows proton signals spread between 6-8 ppm for SePh protection, 12 which is also in agreement with the infrared analysis (Figure S9A).The absence of H and P signals for PPh3 ligands, which were used during the cluster synthesis, in 1 H and 31 P NMR spectra is consistent with the SCXRD results.
The "superatom" theory 29 calculates a free metal electron count of 20 for Cd12Ag32(SePh)36 cluster i.e., [(12x2)+(32x1)-(36x1)] by considering the s-valence of two for Cd and one for Ag as well as one-electron withdrawing character of the SePh ligand.This electron count predicts that the HOMO state and the molecular orbitals just below HOMO should have 1D-2S symmetries present in a spherical angular momentum analysis, and the LUMO state should have 1F character.This is indeed what we observe by performing symmetry analysis and inspecting visually the frontier orbitals (Figures S10, S11), although the HOMO has a mixed 2S-1F character probably due to the non-spherical symmetry of the metal core.Furthermore, atomic charge analysis (Table S3) shows that the Ag atoms in the Ag4Ag24 shells are close to neutral, whereas the Ag and Cd atoms in the ligand motifs are clearly positively charged by 0.20 e and 0.66 e, respectively.The SePh ligands are shown as electron-withdrawing with a clear negative total charge by -0.28 e per ligand.
The optical properties and electronic structure of Cd12Ag32(SePh)36 cluster were further studied both experimentally by UV-vis spectroscopy and theoretically by the linear response formulation of the time-dependent density functional theory (LR-TDDFT, see SI for details).The experimental absorption spectrum (Figure 3) displays clear peaks at 451, 534 and 640 nm along with shoulder peaks at 245, 288, 335, and 493 nm.The absorption onset (Figure S9B) at ~900 nm (1.37 eV) is very close to the calculated HOMO-LUMO gap of 1.35 eV, with a slight underestimation of the gap being a typical feature of the used DFT PBE xc-functional.The calculated absorption spectrum for both the experimental (Figure 3) and PBE-relaxed cluster structure show a good agreement with the experiment, with at least six peaks/shoulders identifiable in the computed spectrum (labels a-f).
The absorption features a-f were analyzed by creating the dipole transition contribution maps and by breaking down the contributions by different parts of the cluster to a given transition (Figure S12).The lower energy peaks (a-c) have all similar characters, namely they are superatom-to-superatom transitions concentrated in the Ag core (intraband Ag(sp)→Ag(sp) transitions).It is interesting to note that the energy difference between a and b peaks seems to be a direct measure to the energy splitting of the superatom 1F manifold by the overall tetrahedral symmetry of the cluster (Figure S10).The high-energy features, d-f have increasing contributions also from the metal-ligand interface (mainly from Se(p)) and weakening contributions from the superatom states.We note that the Ag d-band is not actively participating to optical transitions in the analyzed and measured energy range, nor are the Cd atoms (since they are in the formal Cd(II) state).Interestingly, the Cd12Ag32(SePh)36 cluster shows a broad NIR PL (quantum yield: ~0.28%) peak at ~1020 nm (Figure 3), which is different from the visible PL of Ag NCs. 2 The similar PL excitation and UV-vis spectra, and the same emission at 1020 nm for different excitations together (Figure S13) indicate that the PL is originated from the cluster.The PL of Cd doped Ag cluster at 1020 nm is clearly different from that of a Cu doped cluster at 900 nm, 28 suggesting the successful tuning of NIR PL by the non-noble metal doping (Figure S14).Theoretically estimated PL emissions (see SI for details) at 1134 nm and 1150 nm are in qualitative agreement with the experiment (1020 nm), strongly suggesting that the origin of the high-energy end of broad PL is the de-excitation over the HOMO-LUMO energy gap. 30The low-energy end of PL is likely resulted from the relaxation through an intrinsic Cd3Ag(SePh)9 surface state.
The SCXRD shows that the two clusters in the unit cell are enantiomers (Figure S15) due to asymmetric Ag28 core.Since the solution of Cd12Ag32(SePh)36 is racemic, we cannot experimentally measure the chiral optical response, however, it can be straightforwardly calculated for one of the enantiomers.Our calculations predict that the strength of the chiral response in the UV-vis region (Figure 4) is comparable to, e.g., the well-known thiolated chiral Au38(SR)24 cluster. 31,32  note the absence of PPh3 in Cd12Ag32(SePh)36 cluster (Figure 1), which could be synthesized only in the presence of PPh3 and tetraoctylammonium bromide (Figure S16) in our experimental conditions.The absorption spectra (Figure S17) suggest that the formation of Cd12Ag32(SePh)36 proceeds through the intermediate clusters formed after NaBH4 reduction of metal-ligand complexes.The 31 P and 1 H NMR spectra confirm the binding of PPh3 in the metal-ligand complexes (Figures S18, S19).After NaBH4 addition, the PPh3 in the complex transfers the metals to form intermediates and subsequently released into solution, confirming PPh3-mediated synthesis of the Cd12Ag32(SePh)36 cluster.This cluster shows moderate and excellent ambient stability in the solution-and solid-state, respectively (Figure S20).
In summary, we designed a phosphine-assisted synthetic strategy to dope an atomically precise Ag nanocluster with a non-noble metal, Cd.By this method, we synthesized a single-sized neutral Cd12Ag32(SePh)36 cluster and elucidated its total structure using Xray crystallography.The Cd dopant is found to prefer the cluster surface to its core.Our cluster exhibits NIR PL and a chiral core, resulting optical activity.The absorption spectrum and electronic structure of this cluster are well compared with those from the DFT.Our ligand-assisted synthesis of nanocluster may become a general method to introduce other active metals into noble metal clusters.

ASSOCIATED CONTENT Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.Synthesis, characterization and computational details of the Cd12Ag32(SePh)36 cluster (PDF).Crystallographic data for the Cd12Ag32(SePh)36 cluster (CIF).

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Figure 1 .
Figure 1.The total structure of Cd12Ag32(SePh)36 cluster.H atoms of SePh ligands are omitted for clarity.The detailed structural analysis (Figure2) shows that the NC consists of a Ag28 core stabilized by four Cd3Ag(SePh)9 motifs.Further anatomy of the Ag28 core unveils a Ag4 tetrahedron (Figure2A) at the center.The average Ag-Ag bond distance of 2.84 Å is close to that of the bulk Ag, indicating the strong Ag-Ag interactions in the NC.Six Ag atoms arrange nearly coplanarly to form a

Figure 2 .
Figure 2. Construction of the structure of Cd12Ag32(SePh)36.(A) Ag4 inner core and (B) Ag6 facet.Capping of Ag4 core with Ag6 facets and interfacet interactions (purple arrows) result in C and D, respectively.Mounting of Cd3Ag(SePh)9 motifs (E) on the Ag28 core (D) gives the total structure of the cluster (F).The phenyl rings of ligands are omitted for clarity.

Figure 4 .
Figure 4. Computed circular dichroism (CD) spectra of one of the enantiomers of Cd12Ag32(SePh)36 (red curve) and Au38(SCH2CH2Ph)24 (black curve, offset vertically for better visualization) clusters.Inset: the DFT optimized structure of the Cd12Ag32(SePh)36 enantiomer.We note the absence of PPh3 in Cd12Ag32(SePh)36 cluster (Figure1), which could be synthesized only in the presence of PPh3 and tetraoctylammonium bromide (FigureS16) in our experimental conditions.The absorption spectra (FigureS17) suggest that the formation of Cd12Ag32(SePh)36 proceeds through the intermediate clusters formed after NaBH4 reduction of metal-ligand complexes.The 31 P and 1 H NMR spectra confirm the binding of PPh3 in the metal-ligand complexes (FiguresS18, S19).After NaBH4 addition, the PPh3 in the complex transfers the metals to form intermediates and subsequently released into solution, confirming PPh3-mediated synthesis of the Cd12Ag32(SePh)36 cluster.This cluster shows moderate and excellent ambient stability in the solution-and solid-state, respectively (FigureS20).In summary, we designed a phosphine-assisted synthetic strategy to dope an atomically precise Ag nanocluster with a non-noble metal, Cd.By this method, we synthesized a single-sized neutral Cd12Ag32(SePh)36 cluster and elucidated its total structure using Xray crystallography.The Cd dopant is found to prefer the cluster surface to its core.Our cluster exhibits NIR PL and a chiral core,