An icosahedral Au₁₃ nanocluster with three adjacent chlorides on opposite poles catalyses hydroamination of phenylacetylene

Abstract

A novel anionic gold nanocluster, [Au₁₃(dppeF)₃Cl₆]⁻, bearing six chlorides, has been synthesized using a bulky diphosphine, dppeF (1,2-bis[bis(3,5-ditrifluoromethylphenyl)phosphino]ethane) as a protecting ligand. Single-crystal X-ray diffraction analysis revealed that this nanocluster has an icosahedral Au₁₃ core, which is protected by each of three Cl ligands on opposite poles and by three dppeF ligands in an equatorial plane. This nanocluster acted as a homogeneous catalyst for hydroamination between phenylacetylene and aniline, facilitated by the rapid exchange of the Cl⁻ ligands with phenyl acetylides.

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Introduction

In recent decades, the development of ligand-protected gold nanoclusters (AuNCs) has emerged as a significant area of research, due to their unique properties, which can be finely adjusted through the precise control of their size, shape, and surface modifications.^1–3 Phosphines are some of the most common types of ligands used to protect AuNCs.^4–9 Among numerous phosphine-protected AuNCs reported, icosahedral Au₁₃ NCs protected by alkyl-bridged diphosphines (L = Ph₂P-(CH₂)_n-PPh₂; n = 1–5) provide an ideal opportunity to demonstrate how the ligand structure can precisely adjust the geometric structure, stability, and physicochemical properties of AuNCs. The chemical formulas in Table 1 show that the L/Cl ratio in the Au₁₃ NC depends on the n value of L.^10–12 The observed trend suggests that one bulkier L is replaced by two less bulky Cl ligands. This exchange decreases the total charge by two, keeping the total number of valence electrons at eight. Based on the sequence of chemical formulas in Table 1, we can predict that a negatively charged NC, such as [Au₁₃L₃Cl₆]⁻, will form when L is bulkier than dpppe.

Table 1 Au₁₃ NCs protected by alkyl-bridged diphosphines

Formula	L	Ref.
(Ar^F = 3,5-bis(trifluoromethyl)phenyl).
[Au13L6]^5+	Ph2P-(CH2)1-PPh2 (dppm)	10
[Au13L5Cl2]^3+	Ph2P-(CH2)2-PPh2 (dppe)	11
[Au13L4Cl4]^+	Ph2P-(CH2)3-PPh2 (dppp)	12
	Ph2P-(CH2)4-PPh2 (dppb)
	Ph2P-(CH2)5-PPh2 (dpppe)
[Au13L3Cl6]^−	(Ar^F)2P-(CH2)2-P(Ar^F)2 (dppeF)	This study

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The structure of L affects not only the L/Cl ratio but also the structure and optical properties of the Au₁₃ core.^13–17 The Au₁₃ core of [Au₁₃(dppe)₅Cl₂]³⁺ (Au₁₃:dppe) is a nearly perfect icosahedron, whereas that of [Au₁₃(dppm)₆]⁵⁺ (Au₁₃:dppm) is significantly distorted into D₃ symmetry, probably due to the mechanical stress imposed on the two P sites of dppm. Au₁₃:dppe exhibits photoluminescence (PL) at 766 nm with a quantum yield (QY) of 8%.¹¹ The PLQY drops to 0.8% for [Au₁₃(dppe)₄Cl₄]⁺ (Au₁₃:dppp) and is below the detection limit for Au₁₃:dppm.

This work aimed at developing atomically precise AuNC catalysts with a Cl-rich surface. The idea comes from a report that the hydrochlorination of phenylacetylene (PA-H) was catalysed by a halide-rich AuNC: [Au₁₃(NHC)₆Br₆]⁻ (NHC = N-heterocyclic carbene).¹⁴ We attempted to synthesize a Cl-rich Au₁₃ NC using (Ar^F)₂PC₂H₄P(Ar^F)₂ (Ar^F = 3,5-bis(trifluoromethyl)phenyl; dppeF) as a ligand, which is bulkier than dppe due to the presence of multiple CF₃ groups within an otherwise identical molecular framework. We also expect that the resulting Au₁₃ core will be stabilized electronically by the CF₃ groups via an inductive mechanism.¹⁸ We successfully obtained [Au₁₃(dppeF)₃Cl₆]⁻ (Au₁₃:dppeF) (Table 1) and determined its structure using single-crystal X-ray diffraction (SCXRD) analysis. Au₁₃:dppeF has an icosahedral Au₁₃ core that is surrounded by three dppeF ligands at the equator and six Cl ligands at the axial Au₃ triangles on opposite poles. The superatomic nature of the Au₁₃ core was confirmed by density functional theory (DFT) calculations. Au₁₃:dppeF exhibited higher catalytic activity than Au₁₃:dppe for the hydroamination of PA-H using aniline as the nucleophile.¹⁹ The reaction pathways were probed using in situ electrospray ionization mass spectrometry (ESI-MS) by taking advantage of homogeneous catalytic conditions.^20–27 Based on the reaction intermediates detected by in situ ESI-MS, we propose that the reaction starts with the activation of PA-H via ligand exchange with Cl, followed by attack by a nucleophile (aniline) to produce the corresponding enamine, which releases imine as a final product.

Results and discussion

Synthesis and structural characterization of Au₁₃:dppeF

A tetraphenylphosphonium (PPh₄⁺) salt of Au₁₃:dppeF was synthesized using a method analogous to the one used to prepare Au₁₃:dppe.¹¹ The synthetic details are provided in the Supporting Information (SI). Briefly, the (AuCl)₂dppeF complex in tetrahydrofuran was reduced by NaBH₄ at 253 K. The resultant polydispersed mixture of dppeF-protected AuNCs was subjected to an HCl etching process to focus the AuNC sizes. Au₁₃:dppeF was recrystallized from dichloromethane and hexane after alumina column chromatography and repeated reprecipitation, yielding black crystals. The yield was 35% based on Au.

Fig. 1a presents a typical negative-mode ESI mass spectrum of (PPh₄⁺)·Au₁₃:dppeF. The intense mass peak was assigned to the formula of [Au₁₃(dppeF)₃Cl₆]⁻, as confirmed by comparing the experimental and the calculated isotope patterns. To the best of our knowledge, this is the first example of a negatively charged Au₁₃ NC protected by phosphines. Fig. 1b shows the ultraviolet-visible (UV-Vis) absorption spectra of acetonitrile solutions of Au₁₃:dppeF and Au₁₃:dppe, which were prepared by dissolving crystals of (PPh₄⁺)·Au₁₃:dppeF and Au₁₃:dppe·(PF₆⁻)₃, respectively. Both spectra exhibit characteristic peaks that reflect the quantized electronic structures. The spectral profiles of Au₁₃:dppeF and Au₁₃:dppe are similar, but do not match due to their structural differences. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of Au₁₃:dppeF is estimated to be 2.0 eV from a spectral onset of the UV-Vis absorption spectrum. This value is comparable to that of Au₁₃:dppe (∼1.9 eV).^8,11,16 By combining with the nuclear magnetic resonance (NMR) data of (PPh₄⁺)·Au₁₃:dppeF (Fig. S1), we concluded that the target Au₁₃ NC with six chlorides was successfully obtained.

Fig. 1 (a) Negative-mode ESI mass spectrum of (PPh₄⁺)·Au₁₃:dppeF. The inset compares the isotope patterns experimentally observed (red) and calculated (black) for Au₁₃:dppeF. (b) UV-Vis absorption spectra of (PPh₄⁺)·Au₁₃:dppeF (red) and Au₁₃:dppe·(PF₆⁻)₃ (black) in acetonitrile.

Next, we examined the geometric structure of Au₁₃:dppeF using SCXRD analysis. (PPh₄⁺)·Au₁₃:dppeF crystallized in the triclinic crystal system with the P1̄ space group (Table S1). Fig. 2a shows the determined structure of Au₁₃:dppeF and reveals that it has an icosahedral Au₁₃ core. The continuous symmetry measure (CSM),²⁸ which quantifies deformation from an ideal icosahedron (CSM = 0), was calculated to be 0.029 for the Au₁₃ core in Au₁₃:dppeF. This value is closer to 0 than that of Au₁₃:dppe (CSM = 0.036)⁸ and indicates that the Au₁₃ core of Au₁₃:dppeF is a nearly perfect icosahedron. Fig. 2b shows the coordination geometry of the ligands around the Au₁₃ core. Three dppeF ligands coordinate with the Au₆ ring in a chair configuration on the equator of the Au₁₃ core. The remaining axial Au₃ triangles at the top and bottom are fully capped by three chlorides each, giving pseudo-C₃ symmetry along the axial direction. Halide-accumulated facets were also observed in [Au₁₃(NHC)₆Br₆]⁻, but they were divided into asymmetric sites of Br₄ and Br₂.¹⁴ These results imply that halides tend to segregate on the AuNC surface. However, the limited examples of polyhalogenated Au₁₃ NCs make it difficult to conclude whether the segregated behavior of halides is a general feature.

Fig. 2 (a) The entire structure of Au₁₃:dppeF, as determined by SCXRD analysis. (b) Top and side views of Au₁₃:dppeF. Hydrogen atoms, Ar^F moieties, a solvent molecule, and the PPh₄⁺ ion are omitted for clarity. Color code: Au, yellow; P, orange; Cl, green; C, gray; F, yellow-green.

The electronic structure of Au₁₃:dppeF was investigated using DFT calculations with a full ligand model. Structural optimization was conducted using the crystal structure of Au₁₃:dppeF as the initial structure. The optimized structure, shown in Fig. S2, reproduced the crystal structure shown in Fig. 2. Fig. 3 shows the energy diagrams and the corresponding Kohn–Sham (KS) orbitals of Au₁₃:dppeF. As expected, given the eight valence electrons calculated for Au₁₃:dppeF, the HOMO, HOMO−1, and HOMO−2 are assigned to three 1P superatomic orbitals. This confirms the electron configuration of (1S)²(1P)⁶. Despite the nearly perfect icosahedral Au₁₃ core, the 1P orbitals are split into two subgroups, 1P_z and 1P_x/1P_y. The 1P_z orbital is more stable than the 1P_x/1P_y orbitals due to electrostatic stabilization by the six electron-withdrawing chlorides located along the z-axis. This also explains why the 1D_z² orbital distributed along the z axis corresponds to the LUMO. The HOMO–LUMO gap was calculated to be 2.6 eV, which is larger than 2.0 eV determined by the optical method. A similar overestimation was observed in the case of Au₁₃:dppe.¹³

Fig. 3 Energy diagrams and KS orbitals of Au₁₃:dppeF (isovalue = 0.02). The top-right figure shows the orientation of the Au₁₃:dppeF NC without the Ar^F moieties and H atoms, which have been removed for clarity.

Catalytic application of Au₁₃:dppeF to hydroamination reaction

Fig. 4 compares the space-filling models of Au₁₃:dppeF and Au₁₃:dppe. The Au₁₃ core surface of Au₁₃:dppeF is clearly visible through the gap between the three Cl ligands. In contrast, the Au₁₃ core of Au₁₃:dppe is barely visible. This remarkable difference in the steric environment suggests that reactants can readily access the Au₁₃ core of Au₁₃:dppeF than that of Au₁₃:dppe. The catalytic application of well-defined AuNCs can provide useful information for establishing a correlation between structure and catalysis.^29–32 Inspired by the hydrochlorination of PA-H by the halide-rich [Au₁₃(NHC)₆Br₆]⁻,¹⁴ we compared the catalytic activity of Au₁₃:dppeF and Au₁₃:dppe for the hydroamination of terminal alkynes by amines under homogeneous conditions. PA-H and aniline were selected as the model terminal alkyne and amine, respectively.¹⁹ As shown in Table 2, Au₁₃:dppeF catalysed the reaction (entry 1): the conversion after 24 h was 29% (Fig. S3), increasing up to ∼95% after two weeks. Conversion after 24 h increased with the amount of Au₁₃:dppeF loading and reached up to 82% at 0.040 mol% (entries 2–4). The gradual decrease in TON values with increasing catalyst loading (entries 2–4) is due to the decreasing substrate concentration. In contrast, the conversions after 2 h were nearly proportional to the catalyst concentration, and TON values remained constant (Table S2). These results suggest that the active sites of the catalyst are uniform, despite the presence of multiple halide sites. The blank test (entry 10) and the poor conversion by the (AuCl)₂-dppeF complex (entry 8) indicate that the higher conversion is associated with Au₁₃:dppeF. Conversion by Au₁₃:dppeF was higher than that by Au₁₃:dppe (entry 9). This result supports our hypothesis that the less crowded chlorinated area provides the active site.

Fig. 4 Space-filling models of Au₁₃:dppeF (left) and Au₁₃:dppe (right) around the Cl coordination sites. Color code: Au, yellow; P, orange; Cl, green; C, gray; F, yellow-green.

Table 2 Catalytic activity of Au₁₃ NCs in the hydroamination of PA-H with aniline^a

Entry	Catalyst	mol%^b	Conversion (%)	TON^c
a General conditions: PA-H (1.0 M), aniline (5.0 M), 500 rpm, 24 h. Conversion was estimated by ^1H NMR analysis using 1,3,5-trimethoxybenzene as an internal standard. b Molecular base. c TON = Conversion (%)/mol%. d Not determined.
1	Au13:dppeF	0.010	29	2.9 × 10^3
2	Au13:dppeF	0.015	41	2.7 × 10^3
3	Au13:dppeF	0.027	67	2.4 × 10^3
4	Au13:dppeF	0.040	82	2.1 × 10^3
5	Au13:dppeF/Br	0.0098	6.3	6.3 × 10^2
6	Au13:dppeF/I	0.010	<1	n.d.^d
7	Au13:dppeF/PA	0.011	32	3.2 × 10^3
8	(AuCl)2-dppeF	0.065	9.1	1.4 × 10^2
9	Au13:dppe	0.010	4.8	4.8 × 10^2
10	blank	—	<1	n.d.^d

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It has been reported that the Cl ligands in Au₁₃:dppe can be replaced by PA⁻.¹³ To confirm that the same ligand exchange occurs in Au₁₃:dppeF, we measured the ESI mass spectra of a mixed solution of Au₁₃:dppeF and 50 equivalents of PA-H in the presence of the Brønsted base triethylamine. As expected, the dominant species detected was the fully PA-exchanged AuNCs [Au₁₃(dppeF)₃(PA)₆]⁻ (Au₁₃:dppeF/PA) (Fig. S4). This observation led us to hypothesize that Au₁₃:dppeF undergoes ligand exchange from Cl to PA, and that the resultant PA-functionalized Au₁₃ NCs are involved in the catalytic reactions. To investigate this possibility, we characterized the AuNCs after a 24-hour catalytic reaction of Au₁₃:dppeF using ESI-MS. As shown in Fig. S5, the main species detected was the fully PA-exchanged AuNCs Au₁₃:dppeF/PA. We also studied the catalytic activity of the pre-synthesized Au₁₃:dppeF/PA. As shown in entry 7 of Table 2, the catalytic activity of Au₁₃:dppeF/PA was slightly higher than that of Au₁₃:dppeF (entry 1, Table 2). These results suggest that the PA ligands introduced onto the Au₁₃ NCs are key to hydroamination by aniline.

To further support the above suggestions, we examined the impact of halogens on activity. To this end, we synthesized the halide-exchanged analogues [Au₁₃(dppeF)₃Br₆]⁻ (Au₁₃:dppeF/Br) and [Au₁₃(dppeF)₃I₆]⁻ (Au₁₃:dppeF/I) by mixing Au₁₃:dppeF with KBr and KI, respectively.¹⁵ The synthetic details and characterization results are provided in the SI (Fig. S6–S9). SCXRD analysis of the halide-exchanged products revealed the same coordination geometry (Fig. S10 and Table S1), indicating that these NCs can provide direct insight into the effect of halides on the catalytic activity. As shown in entries 1, 5, and 6 of Table 2, the reactivity decreases in the following order: Au₁₃:dppeF > Au₁₃:dppeF/Br ≫ Au₁₃:dppeF/I. This trend reveals that the nature of the halides significantly affects activity. To elucidate halide-dependent catalytic activity, we compared the ligand exchange activity of Au₁₃:dppeF, Au₁₃:dppeF/Br, and Au₁₃:dppeF/I with the PA⁻ ligands using ESI-MS. We added 10 equivalents of PA-H to solutions of Au₁₃:dppeF, Au₁₃:dppeF/Br, and Au₁₃:dppeF/I, in the presence of 10 equivalents of triethylamine, to promote the deprotonation of the terminal alkynyl proton. Fig. S11 shows the SI mass spectra of the mixed solutions after 24 h. Partially or fully PA-exchanged AuNCs [Au₁₃(dppeF)₃(PA)_nX_6−n]⁻ (X: Cl, Br, or I) were detected similarly. The average number of PA ligands introduced (n) was ∼1.6, ∼0.3, and <0.1 for Au₁₃:dppeF, Au₁₃:dppeF/Br, and Au₁₃:dppeF/I, respectively. These numbers increased to 5.9, 4.1, and 0.3, respectively, after 24 h of the catalytic reaction, where an excess of PA-H is present (Fig. S12). These results indicate that reactivity toward PA exchange decreased in the following order: Au₁₃:dppeF > Au₁₃:dppeF/Br ≫ Au₁₃:dppeF/I. This is consistent with the activity order shown in entries 1, 5 and 6 of Table 2. Fig. S13 shows the time-course results for the catalysts of Au₁₃:dppeF, Au₁₃:dppeF/Br, Au₁₃:dppeF/I and Au₁₃:dppeF/PA (entries 1 and 5–7, Table 2). These results suggest that activating the PA ligands on Au₁₃via ligand exchange is a critical step in the reaction.

The catalytic hydroamination reaction has been studied using an Au complex,³³ an Au surface,³⁴ Au nanoparticles (AuNPs),^35–37 and AuNCs.¹⁹ All of these Au catalysts, except for the AuNCs, activate PA-H via a π-bond on an Au atom while leaving the terminal hydrogen atoms intact, as shown in Fig. 5a. In the case of [Au₁₆(bi-NHC)₅(PA)₃Br₂]³⁺ (bi-NHC = bidentate NHC ligand),¹⁹ although PA forms novel surface units of (bi-NHC)–Au(I)–PA on the icosahedral Au₁₃ core, it is π-bonded to the Au₁₃ core. Contrary to these examples, we propose σ-bonding of PA to the Au₁₃ core of Au₁₃:dppeF (Fig. 5b), based on the SCXRD result of a similar system, [Au₁₃(dppe)₅(PA)₂]³⁺.¹³ To further support this hypothesis, DFT structural optimization was conducted for Au₁₃:dppeF/PA by replacing six Cl ligands of Au₁₃:dppeF with σ-ligated PA ligands. The optimized structure, shown in Fig. S14, supports σ-bonding of PA to the Au₁₃ NC. The difference in the PA bonding mode is likely due to the difference in the steric environment at the adsorption site: σ-bonding is preferred at sterically crowded sites, while π-bonding is possible only at sterically open sites. The σ-bonding of PA to the Au₁₃ NC alters the charge distribution of the PA molecule via electronic coupling between the 1P superatomic orbital of the Au₁₃ core and the π orbital of the PA molecule, as theoretically demonstrated by Konishi.¹³ According to the natural bond orbital (NBO) analysis of our DFT-optimized structures of Au₁₃:dppeF/PA and free PA⁻, the electronic charge transfers from PA to the Au₁₃ NC. The NBO charges of the terminal C (C_α) and the adjacent C (C_β) atoms of the PA ligand suggest that C_α becomes electron-rich and C_β becomes electron-deficient upon ligation (Table S3).

Fig. 5 Schematic representation of (a) π-ligation of PA-H to an Au surface and (b) σ-bonding of PA to Au NC. (c) Proposed reaction mechanism of hydroamination of PA-H on Au₁₃ NCs.

Based on the above discussions, we propose the following catalytic mechanism for Au₁₃:dppeF, as depicted schematically in Fig. 5c. The reaction begins with ligand exchange with PA-H in the presence of excess aniline, which acts as a Brønsted base (step i). Next, the σ-bonded PA is attacked by the nucleophile aniline. The nucleophilic attack by aniline occurs at the electron-deficient C_β, to produce an enamine according to the Markovnikov rule (step ii). This nucleophilic attack of aniline on the σ-bonded PA substrate has also been proposed as a key step in the catalysis of Au₁₆ NC.¹⁹ Finally, enamine tautomerizes to the imine (step iii), and then protodeauration and ligand exchange with a new PA occur (step iv).

Finally, we evaluated the stability of Au₁₃:dppeF during the catalytic reaction. After 24 h of the reaction, we collected the catalysts by evaporating the solvent, followed by washing with a toluene–hexane mixture (1 : 3) to remove the remaining reactants and the products. The precipitate exhibited a similar UV-Vis absorption profile to that of (PPh₄⁺)·Au₁₃:dppeF/PA (Fig. 6), which is consistent with the ESI-MS results. The catalysts after the 1st and 2nd usage exhibited similar spectral profiles (Fig. 6). These results suggested that the NC retained its Au₁₃:dppeF/PA structure, indicating the stability of Au NCs during the catalysis. We also compared catalytic activity during recycled usage. The conversion by 0.01 mol% catalyst decreased from 32% to 19% and 11%, respectively, in the 2nd and 3rd cycles (Table 3). The gradual reduction in conversion with reuse is due to the reduced recovery of Au₁₃:dppeF/PA NCs by washing with a toluene–hexane mixture. The weight of the recovered catalyst was reduced to ∼60% of the original value by washing (Table 3). Assuming the recovered precipitate was pure Au₁₃:dppeF/PA NCs, the TON at the 2nd and 3rd cycles was calculated to be 3.1 × 10³ and 3.0 × 10³, respectively (Table 3). These values closely match the TON for Au₁₃:dppeF/PA (3.2 × 10³, entry 7, Table 2), which reinforces the high durability of this NC.

Fig. 6 UV-Vis absorption spectra of initial Au₁₃:dppeF (black) and Au₁₃:dppeF after cycles 1 (dark brown) and 2 (brown), along with the spectrum of Au₁₃:dppeF/PA (red) as a reference.

Table 3 Recycling test of catalytic activity of Au₁₃:dppeF^a

Cycle	Weight (mg)^b	mol%^c	Conversion (%)	TON^d
a General conditions: PA-H (1.0 M), aniline (5.0 M), 500 rpm, 24 h. Conversion was estimated by ^1H NMR analysis using 1,3,5-trimethoxybenzene as an internal standard. b Weight of the loading catalyst in each run. c Molecular base. d TON = Conversion (%)/mol%. e Calculated as Au13:dppeF/PA.
1	6.0	0.010	29	2.9 × 10^3
2	3.9	0.0061^e	19	3.1 × 10^3
3	2.3	0.0036^e	11	3.0 × 10^3

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Conclusions

We successfully synthesized an anionic Au₁₃ NC with six Cl ligands by using bulky diphosphines as protecting ligands. Single-crystal X-ray diffraction analysis revealed that the six chlorides coordinate with the two Au₃ facets located at opposite poles of the icosahedral Au₁₃ core. The Au₁₃ NC was then used to catalyse the hydroamination reaction between PA-H and aniline to form an enamine. Based on the reaction intermediates detected by in situ ESI-MS, we propose that the reaction begins with the dissociative adsorption of PA-H, while releasing a Cl ligand. Then, a nucleophile aniline attacks at the electron-deficient C_β of the adsorbed PA, producing the corresponding enamine. The efficient introduction of PA was attributed to the less crowded environment of the Cl-accumulated Au₃ facets.

Author contributions

S. Wu: investigation, visualization, writing – original draft, writing – review & editing. S. Takano: conceptualization, investigation, funding acquisition, supervision, validation, writing – original draft, writing – review & editing. T. Tsukuda: conceptualization, funding acquisition, resources, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5cp03444b.

CCDC 2481066, 2484473 and 2484474 contain the supplementary crystallographic data for this paper.^38a–c

Acknowledgements

This work was financially supported by JST CREST (grant number: JPMJCR20B2) and JSPS KAKENHI grants (no. JP23H00284 and JP23K26610).

Notes and references

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