Pd-Mediated Synthesis of Ag33 Chiral Nanocluster with CoreâShell

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Pd-mediated Synthesis of Ag Chiral Nanocluster with Core-shell Structure in T Point Group Fan Tian, and Rong Chen J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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Journal of the American Chemical Society

Pd-mediated Synthesis of Ag33 Chiral Nanocluster with Core-shell Structure in T Point Group Fan Tian, Rong Chen* School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Donghu New & High Technology Development Zone, Wuhan, 430205, PR China ABSTRACT: We report a Pd-mediated synthesis and crystal structure of a new chiral Ag33(SCH2CH2Ph)24(PPh3)4 nanocluster with open shell electronic structure. Single-crystal X-ray structure reveals that the kernel of the cluster comprises a keplerate Ag13 icosahedron core with one Ag atoms in the center and a shell framework of Ag20S24P4 block. The Ag20S24P4 shell framework is fully arranged by helical –S-Ag-S- staples along four Ag-P bonds involved C3 axis, which endows the kernel structure of nanocluster chirality and symmetry in T point group. The geometry and chirality of Ag33 nanocluster are further confirmed by nuclear magnetic resonance (NMR), electronic circular dichroism (ECD) spectra and time-dependent density functional theory calculation. Our results show that the formation of the new chiral Ag33 nanocluster is strongly dependent on the presence of Pd regent with the form of Pd(PPh3)4 functioning in the synthesis. This work not only presents a novel chiral structure of silver nanocluster, but also provides a new strategy for the development of novel nanocluster.

KEYWORDS: Sliver, nanocluster, chirality, X-ray crystallographic, palladium

1. INTRODUCTION Metallic nanoclusters with precisely atomic arrangement have been emerged increasing research interests due to its fundamental structure-related electronic/optical properties and potential applications in catalysis, life science and nanotechnology1-2. The past decades have witnessed prosperities of atomic precisely gold, silver and its intermetallics nanoclusters since the synthesis and fully crystallographic elucidation of Au102(P-MBAs)44 by single-crystal X-ray diffraction2-7. With employing thiol8-14, phosphines15-16, alkynyl6, 17-18 or selenolate ligand19, the nanoclusters family with fully determined structures realizes rapid progress. Although the family is expanding rapidly in recent years, exploring new members with unique features and unreported number of metallic atoms still remains challenging. In the reported Ag nanocluster family with fully-determined structure, most of the representative structures could be categorized into the structure with atoms arranged in face-center-cubic like pattern (Ag1420, Ag2321, Ag3822, Ag6322 and Ag6723 nanocluster) and the structure with keplerate Ag icosahedron core (Ag2024, Ag2125, Ag2526, Ag2927 and Ag4428-29 nanoclusters). Moreover, there are size-forcing30, metal exchange31 and ligands exchange32 strategy at present for exploring nanocluster with atomic precisely. However, those strategies are efficient for Au and Au based alloyed nanoclusters’ exploration, but less robust in the specific systems of Ag nanocluster. Therefore, exploring new strategy for synthesizing Ag based novel nanocluster is urgent. Chirality is ubiquitous property in nature and have pervades widely in material science33, chemistry13, catalysis34 and bio-related field35. Previous works reveal that the nanoclusters with special screw spindle or chiral environmental ligands would exhibit chiroptical properties, therefore chiral active4, 15, 21, 36-41. Those nanoclusters are believed to be capable of enantioselective adsorption

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for chiral compounds, thus having potential applications in enantiospecific catalysis, separation and sensing1-2, 4, 13, 15, 21, 34, 40, 42. However, the preparation of chiral nanoclusters with precisely atomic arrangement is still infant due to the lack of mature guiding theories and sufficient experimental explorations4, 21, especially for the nanoclusters with the chirality originated from intrinsically chiral inorganic kernel rather than chiral ligands. Among the reported nanoclusters with intrinsically chiral kernel so far, frameworks with low symmetries such as C and D point group are widely observed, while higher symmetries such as T or O point group are rarely reported3-4, 11-12, 15, 21, 36-38, 43-46. In the Ag nanoclusters family, only Ag23 and Ag20 with chiral active owing to its kernel arrangements have been reported, which shares the symmetries of C point group. Herein, we report an open shell chiral keplerate Ag33(SCH2CH2Ph)24(PPh3)4 (abbreviated as Ag33) nanocluster with kernel structure of four C3 axis and three C2 axis, which could be clarified as T point group symmetry. The synthesis of Ag33 nanocluster is found only be valid in the presence of small amount of Pd reagent. X-ray crystallographic analysis reveals that it comprises an Ag13 icosahedron core with one Ag atom in the center and a chiral shell framework of Ag20S24P4 motif. The charity and symmetry of the nanocluster are further verified by NMR and ECD spectra. To the best of our knowledge, it is the first report on the metallic nanocluster of 33 atoms with fully determined chiral structure, which might act as a prototype for the exploration of other new type nanoclusters with 33 metallic atoms or similar framework. Furthermore, the formation of the new Ag33 nanocluster involving of small amount of Pd reagent, which has never been reported in the synthesis for pure metallic nanocluster, might get insight for exploring other novel metallic nanoclusters.

2. RESULTS AND DISCUSSION The chiral Ag33(SCH2CH2Ph)24(PPh3)4 nanocluster was prepared via the reduction of a mixture of Ag(NO3)3, phenylethanethiol and triphenylphosphine by NaBH4 aqueous solution in the presence of small amount of Pd(PPh3)2Cl2 at room temperature. After reaction for 24 hours, a purplish red solution is obtained, indicative of the formation of nanocluster (See details in Supporting Information). Black single-crystals for X-ray diffraction of the nanoclusters are grown in CH2Cl2/hexane at 4 oC (Figure S1, Supporting Information). The structure was determined by the single-crystal X-ray diffraction (See details in experimental section and Table S1 in Supporting Information). The yield of the cluster is estimated to be 14% according to the mass of crystals based on Ag element. Crystallographic structure analysis reveals that the Ag33 nanocluster and its enantiomer with occupations of 1 and 0.25 crystallized in a cubic centrosymmetric P43𝑛 space group. Accompanying by-product is also found, which could be assigned as disordered small molecule Pd(PPh3)4 (occu: 0.875) and Pd(PPh3)3Cl (occu: 0.125) (See Figure S2a and S2b, Supporting Information). X-ray photoelectron spectrum (XPS) of the crystal shows that C, S, P, Ag and Pd elements are presented in the sample (Figure S3a, Supporting Information). Both of Ag and Pd elements coexist with 0 and +1 valence state in the crystal (Figure S3b and S3c, Supporting Information). There is no counter ion such as NO3- or Na+ observed, indicating that the Ag33 nanocluster is electroneutral. The validation of the assignments for Ag and Pd atoms is verified by comparing the absolute amount of the two elements in the crystals with the theoretical mole ratio in the solved structure, using ICP-MS determination. It is found that the calculated Ag/Pd mole ratio of 41.16 from ICP-MS measurement is very close to the theoretical ratio of 41.25 (See Table S2, Supporting Information). The final anisotropic refinement converges at R1 = 7.81% for all the collected data. Different from the previously reported chiral nanoclusters crystals with racemic mixture of 1:1

3, 10, 12, 24, 40, 43, 46,

non-identified occupations of the

paired enantiomers are observed in the obtained crystals. Figure 1a shows the spacefilled structures of the paired Ag33 enantiomers with C and H atoms omitted. For each structure, three -Ag-S-Ag-S- staples are screwed counter-clockwise (Figure 1a, left) or clockwise (Figure 1a, right) along a C3 axis. The converged occupancies of the counter-clockwise and clockwise enantiomer are 0.25 and 1.00, respectively, indicative of an un-balanced probability of the two presented structures. It might be a cue for an un-identical production of the two enantiomers in this reaction. However, we have no idea why it happens so far. The keplerate Ag33 nanocluster could be dissected to an Ag13 icosahedron core with a centered Ag atom (AgC) surrounded by the other 12 Ag atoms in icosahedron (AgI) and a chiral framework of Ag20S24P4 block composed of -SR-Ag-SR- motifs and -Ag-P terminals


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(Figure 1b). The bond lengths of the Agc-AgI and AgI-AgI bonds are in accord with the correlated bonds in the icosahedrons of Ag25, Ag29, Ag44, Au25 and their derivatives27, 47-48 (Table S3, Supporting Information). The core-shell framework adopts in T point group with four C3 rotational axis through each -Ag-P terminals and three C2 rotational axis bisecting the connection of two arbitrary P atoms (Figure S4 and S5, Supporting Information). To the best of our knowledge, this symmetry is only found in previously reported Ag28Cu12(SR)24 nanocluster40, which is not adopted in any other chiral metallic nanoclusters.

Figure 1. Atomic packing of Ag33 paired enantiomers (a) and the skeletons structure illustration of the Ag33 nanoclusters (b). The atoms are colored as green (Ag), yellow (S) and orange (P). Some S and Ag atoms are colored in violet in (a) to show the screwing directions in the spacefill draws.

In the shell structure, the 24 sulfur atoms could be categorized as two types: -S*-Ag-P and -S*-Ag-S- (the asterisks denote the specific atoms). If we check the structure along the C3 axis, a pair of the specific sulfur atoms is found at the top and the bottom, respectively, as shown in Figure 2a. The first type of S atoms (S1) coordinates with adjacent three Ag atoms (AgI atom in Ag13 icosahedron, AgT atom in –Ag-P terminals and Ags atoms screwing along the C3 axis, Figure 2b). Considering the extra α-C connected, the sulfur atom is four coordinated to form unusual quadrihedron geometry with the coplanar α-C, AgT and AgI, and the non-coplanar Ags (Figure S6, Supporting Information). The bond lengths of the Ag-S1 vary from 2.40 to 2.60 Å in the order of S1-AgT > S1-AgI > S1-Ags (Table S4, Supporting Information). The second type of sulfur atoms (S2) bond with two Ag atoms, which is the Ag atom in the bottom of the C3 axis (AgB) and another equivalent Ag atom bridged two types of S atoms (AgS), respectively (Figure 2c). The average bond length of S2-AgS and S2-AgB is 2.54 and 2.37 Å, respectively. More interestingly, it is found that the ligands are in chiral arrangement if the carbon skeleton is visible, in which the phenyl rings in PPh3 screws counter-clockwise (Figure 2d) and phenyl ethyl groups in sulfur ligands presents clockwise (Figure 2e and 2f), owing to the C-H⋅⋅⋅π hyperconjugation between inter-ligands 13. The distances between the meta-H in PPh3 and the centroid of hexatomic rings in phenyl ethyl S (S2) groups are widely observed to be 2.93 ±0.2 Å, indicative of the presence of –H…π interactions (Figure S7, Supporting Information). These evidences demonstrate a rigidly chain arrangement of ligands on the surface of nanocluster.



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Figure 2. Shell framework of Ag33 nanoclusters: two type of S atoms (the yellow balls) along the C3 axis (a); geometries of the coordination for the first type (S1, b) and the second type (S2, C) of S atoms; screwing illustration for phenyl rings around P atoms (d), phenyl ethyl connecting to the first type (S1, e) and the second type (S2, f) of S atoms.

To verify the stereochemistry of Ag33 nanocluster as the single crystal structure depicted, we further examine the NMR spectroscopies for the collected crystals dissolved in chloroform-d3 with additional PPh3 to stabilize the nanocluster. To facilitate the assignments, 1H and

13C

NMR spectrum of PPh3 in chloroform-d3 are also recorded for comparison (Figure S8 and S9,

Supporting Information). Overlaps in the signals of 1H and 13C NMR spectrum between Ag33 nanocluster and the dissociate PPh3 or other impurity from the solvent are indexed by comparing the resonance intensities between spectrum of two different concentration of Ag33 nanocluster (Figure S10~S13, Supporting Information). Figure 3a shows the 1H NMR spectrum of the Ag33(SCH2CH2Ph)24(PPh3)4 in the presence of dissociate PPh3 in chloroform-d3. The peaks in the ranges of 4.5-2.0 and 7.5-6.0 ppm mainly correspond to the protons on the methylene groups and phenyl groups of the ligands, respectively. Seven peaks with integrals of 1:2:1:1:1:1:1 at 4.24, 3.48, 3.30, 3.16, 2.84, 2.64 and 2.43 ppm are observed (Figure S10 and Table S5, Supporting Information), respectively. Due to the presence of two types sulfur atoms (S1 and S2) in the cluster, there are two types of PhCH2CH2S- ligands on the surface of the nanocluster (denoted as type 1 and type 2, Figure 3). If all the protons of α-CH2 and β-CH2 groups on the two ligands (where α and β refers to the positions relative to the sulfur atom) are diastereotopic, it would produce eight peaks with integrals of 1:1:1:1:1:1:1:1. We tentatively assign the peak at 3.48 ppm as two non-equivalent protons occasionally overlapped, therefore the observations in the region between 4.5 to 2.0 ppm could be in accord with the SCXRD predication. The validation of the assignments is confirmed by the corresponding 1H-1H COSY and 1H-13C HSQC spectra (Figure S14 and S15, Supporting Information). Based on the germinal (2JHH, through two bonds) and vicinal (3JHH, through three bonds) J-coupling, the hydrogen atoms could be manifested by cross-correlation peaks off the diagonal in the 2D 1H-1H COSY spectrum49. Only two sets of cross peaks (connected with blue or green line, Figure S14, Supporting Information) are presented, indicating that there are two types of thiol ligand in the crystals. In the 1H-13C HSQC spectrum, the peak at 3.48 ppm in 1H dimension is found to cross-link with two types of carbon: the α-C in type 1 ligand (αC1) and the β-C in the type 2 ligand (βC2), illustrating that the peak is indeed overlapped by two non-equivalent protons (Figure S15, Supporting Information). We label the protons according to their relative positions (α, β for methylene; o, m, p for phenyl group) and the ligands type with the subscript (1 or 2 for thiol and p for


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PPh3 ligands), as shown in Figure 3a. A single quotation mark is used to distinguish two protons bonding with an identical carbon atom. Then all the peaks in the region of 4.5 to 2.0 ppm could be assigned as αH1 (4.24 ppm), αH2 (3.49 ppm), βH2 (3.47 ppm), βH1 (3.30 ppm), αH2’ (3.16 ppm), β H2’ (2.84 ppm), αH1’ (2.64 ppm), β H1’ (2.43 ppm). The results show that all the protons of the methylene groups on thiol ligands of the Ag33 nanocluster are chemical non-equivalent, which is another evidence for the chiral geometry of the keplerate structure49-50. The rigidly chain arrangement of ligands on the Ag33 nanocluster could be also evidenced by the 1H NMR spectrum in the region of 7.5 to 6.0 ppm. It is worth noting that the proton resonance signals of phenyl groups on triphenylphosphine or phenylethanethiol ligands often lies in a narrow region from 7.5 ppm to 7.0 ppm, as previous works reported

49, 51.

However, a much broad phenyl

proton signal distribution of Ag33 nanocluster ranging from 7.5 to 6.0 ppm is observed. Seven groups of intense peaks center at 7.32, 7.24, 7.06, 6.95, 6.75, 6.60 and 6.45 ppm (Figure 3) with integral of 29, 6, 3, 12, 6, 6 and 6 after deducting the contribution of solvent (Figure S10 and Table S5, Supporting Information).

According to the 1H-1H COSY and 1H-13C HSQC spectrum (Figure

S16 and S17, Supporting Information), it is concluded that the broad peak centered at 6.95 ppm with integral of 12 could be convoluted by three peaks with integral of 3, 6 and 3. The peak at 7.32 ppm with integral of 29 is complicated, which comes from the signal of the protons in dissociative PPh3, Pd(PPh3)4/Pd(PPh3)3Cl as well as Ag33 nanocluster (See

31P

NMR in Figure S18,

Supporting Information). A reasonable integral of 6 from Ag33 could be detached according to the 1H-13C HSQC spectrum (Figure S17, Supporting Information). Therefore, nine independent peaks from three group with integrals of 3, 6 and 6 could be assigned to oHP (7.32 ppm), oH1 (7.24 ppm), pH2 (7.06 ppm), pH1 (6.97 ppm), mH2 (6.94 ppm), pHp (6.89 ppm), mH1 (6.75 ppm), mHp (6.60 ppm) and oH2 (6.45 ppm), respectively. It is in accord with the SCXRD analysis of the three type ligands on the cluster, which is also evidenced by the cross-peaks ascribed to the different ligands (dark, green and blue lines) in the corresponding 1H-1H COSY spectrum (Figure S16, Supporting Information). The enlargement diastereotopicity of protons on the ligands is a significant cue for the chirality of the nanocluster. Previous works report that the chemical shift difference (Δδ) of the diastereotopic protons should be dependent on the distance between the group and the chiral center 49. In phenylethanethiol ligands of a chiral kernel cluster structure, therefore, the Δδ value of the protons on α-CH2 would be larger than that on β-CH2 of the same ligand49. In this work, the Δδ value of α-CH2 in ligand 1 (Δδα1, 1.6 ppm) is large than that of β-CH2 (Δδβ1, 0.87 ppm), which follows the rule. However, the Δδ value for the protons on type 2 ligand shows unexpected difference that the Δδ value of β -CH2 (Δδ β 2, 0.63 ppm) is larger than that of α-CH2 (Δδα2, 0.33 ppm). It could be explained as the results of competition between kernel and ligands chirality, because that the phenyl groups of type 2 ligands also arrange in a chiral patterns due to the H- π interactions between the phenyls and op-H in PPh3 ligands (Figure S7, Supporting Information).



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Figure 3. Full-range 1H NMR spectrum of Ag33 nanoclusters in CDCl3:1, 2 and p in subscripts denotes the thiol ligands coordinating with three, two Ag atoms and PPh3 ligand coordinating with one Ag atom, respectively; a and b represent each diastereotopic proton in α- or β-CH2 in thiol ligands; The red ‘*’ in (a) denotes the specific carbon atom in phenyl group without H atoms.

The optical adsorption, electronic circular dichroism (ECD) and photoluminescence spectra of the Ag33 nanocluster are recorded in CH2Cl2 with 10 mg/mL PPh3. The experimental UV-vis spectrum (Figure 4a，black line) consists of a strong adsorption peak at ~540 nm, a weak shoulder peak at 460 nm and a weak tail band at 580-760 nm. In the ECD spectra (Figure 4a, blue line), distinct Cotton effects is observed in the range of 500-580 nm. As non-chiral nanocluster would exhibit un-distinguishable Cotton effect when it is irradiated in polarized light (Figure S19, Supporting Information), the distinct Cotton effect of the solution further verifies the chirality of the nanocluster. The photoluminescence spectra of the nanocluster shows double emissions centered at 765 and 810 nm, regardless of the excitations wavelengths of 530, 600 or 700 nm (Figure 4a, red line). Electron paramagnetic resonance (EPR) spectrum (at 100 K) displays a strong signal at g=1.998 with one local maxima and one local minima (Figure 4b), which is characteristic of a system with one unpaired electron (s = ½)21, indicative of the open shell electron configuration of the nanocluster molecular. This is in accord with the odd number of free valence electrons in Ag33 nanocluster (i.e., 33(Ag 5S1)-24(SR) = 9e), further verifying that the assignment of atoms in the crystal structure is valid.


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Figure 4. Optical adsorption (a, black line), electronic circular dichroism (a, blue line), photoluminescence (a, red line), electron paramagnetic resonance spectra (b), TD-DFT simulated adsorption (c), energy alignment of molecular orbitals (d) and density of states (e) of Ag33 nanoclusters.

The excitation states and corresponding orbits of the nanocluster are further revealed by time-dependent density functional theory (TD-DFT) using Gaussian16 package52. By employing unrestricted calculations, we reproduce the experimental adsorption peaks shape in the range of 500 to 900 nm via a simplified model of Ag33(SCH3)24(PC3H12)4 according to the crystal structure of the nanocluster (Figure 4c). The simulated adsorption peak at 580 nm is mainly ascribed to the excitations of α -HOMO-1, α -HOMO-2, α-HOMO-3 to α-LUMO, α-LUMO+1, α-LUMO+2 and the excitations of β-HOMO, β-HOMO-1, β-HOMO-2 to β-LUMO+1, β-LUMO+2, β-LUMO+3 (Figure 4d, blue arrows). The weak tail band at 650-800 nm in the simulated adsorption spectrum is dominated by the excitation from α-HOMO to α-LUMO+5 ~α-LUMO+9 and β-HOMO, β-HOMO-1, β-HOMO-2



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to β-LUMO (Figure 4d, red line). Projected density of state (PDOS) plots for different types of atoms show that the most closest occupied states below the α -HOMO are composed of orbits from Ag atoms both in Ag13 icosahedron core and Ag20S24P4 shell. However, the closest unoccupied states above the α -HOMO state are mainly composed of orbits from Ag13 icosahedron core (Figure 4e and Figure S20 in Supporting Information ), indicating that the adsorption peak at 580 nm are induced by excitation from the kernel Ag atoms of Ag33 nanocluster to Ag13 icosahedron core. The α-HOMO state is found to be composed solely from orbits of Ag atoms in Ag13 icosahedron, and therefore the associated excitation of electron from α -HOMO to α -LUMO+5 ~ α -LUMO+9 are originated from electron transfer in Ag13 icosahedron core, which partially contributes to the tail band at 650-800 nm. The calculations predict that the excitations by visible light mainly originate from electrons in core-shell structure of the nanocluster, which is in good agreement with the previous reports on other metallic nanoclusters28-29. All above results show that a novel chiral Ag33 nanocluster is definitely successful synthesized under a conventional thiol and triphenylphosphine ligands environment. It need to be noted that the presence of Pd(PPh3)2Cl2 is vital important for the synthesis. If the Pd(PPh3)2Cl2 is absence, the obtained product is previously reported Ag23 nanocluster (Figure S21, Supporting Information). Changing the Pd(PPh3)2Cl2 to PdCl2 or Pd(PPh3)4 obtains the same results (Figure S22, Supporting Information), indicating Pd atoms in the solution is responsible for the formation of Ag33 nanocluster. Substituting Au or Pt for Pd mainly results in the formation of Ag23 nanoclusters, demonstrating the unique role of Pd in the synthesis (Figure S22, Supporting Information). It is unexpected that the Pd atoms co-crystallized with the Ag33 nanocluster in the form of Pd-P complexes rather than doped into the nanocluster. In the previous reports, co-exists of two or more kinds of metallic atoms in a synthesis system often results doped or alloyed nanoclusters. For Ag and Pd co-existing system with thiol ligand present, only PdAg24(SR)182- forms either in one-pot reaction under NaBH4 reduction or metal exchanging used Ag25 nanocluster as precursor48, 53-54. The Pd atom is centrally doping into the Ag13 icosahedron core of Ag25 nanocluster by replacing the center Ag atom, resulting in an Ag12Pd icosahedron. It is interesting that our synthesized Ag33 nanocluster showing a semblable Ag13 icosahedron core without being exchanged by Pd atom, as depicted in the results. It is because the PPh3 presents in the system, which could easily coordinated with zero valence Pd atoms to form Pd(PPh3)4. Considering the synthetic procedure and the presence of Pd(PPh3)4/Pd(PPh3)3Cl in the crystals, we propose that Pd(PPh3)4 might be an key intermediate in the formation of Ag33 chiral nanocluster. In this synthesis, the presence of Pd reagent is firstly reduced by NaBH4 to produce Pd0, and then coordinated with PPh3 to form Pd(PPh3)4. Pd(PPh3)4 can react with Ag+ to produce Ag0 and Pd(PPh3)3+. The followed self-assembly of Ag33 nanocluster might proceed by using Ag0 and NaBH4 reduced Ag+ as precursors, as illustrated in Scheme 1. However, our current understanding of the synthesis is superficial. The detailed formation mechanism of Ag33 involved of Pd regent is still in process.

Scheme 1. Pd cycle in the synthesis of Ag33 nanocluster.


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3. CONCLUSIONS In summary, we have reported a Pd mediated synthesis and crystal structure of the first 33 atoms metallic nanocluster formulated as Ag33(SCH2CH2Ph)24(PPh3)4. The nanocluster comprises an Ag13 icosahedron core and a chiral shell framework of Ag20S24P4 motif, which adopts in T symmetries and divides the ligands into two kinds of thiol and one kind of PPh3. The validation of atom assignments, geometry and chirality of Ag33 nanocluster are further confirmed by nuclear magnetic resonance (NMR), electronic circular dichroism (ECD) spectra and time-dependent density functional theory calculation. The presence of Pd in the synthesis is functioning as a catalyst in the form of Pd(PPh3)4, which plays a key role for the self-assembly of Ag33 nanocluster.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The detail experiment sections, X-ray crystallography, supplementary figures and tables, and NMR spectrum (PDF) Ag33(SCH2CH2Ph)24(PPh3)4 (CIF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Tel.: (+86)13659815698; Fax: (+86)2787195680.

ORCID Fan Tian: 0000-0003-0534-9749 Rong Chen: 0000-0003-1455-5093

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21671153) and Department of Education of Hubei Province under the Project of Science and Technology Innovation Team of Outstanding Young and Middle-aged Scientists (T201606). We thanks to Dr. Yongqing Xu (HUST), Dr. Yuncheng Cai (HUST), Dr. Yunlei Peng (NKU) and Dr. Ye Yuan (WUST) for their assistance in SCXRD and NMR characterizations.

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