A 66‐Nuclear All‐Alkynyl Protected Peanut‐Shaped Silver(I)/Copper(I) Heterometallic Nanocluster: Intermediate in Copper‐Catalyzed Alkyne‐Azide Cycloaddition

Abstract Ligand‐protected heterometallic nanoclusters in contrast to homo‐metal counterparts show more broad applications due to the synergistic effect of hetero‐metals but their controllable syntheses remain a challenge. Among heterometallic nanoclusters, monovalent Ag‐Cu compounds are rarely explored due to much difference of Ag(I) and Cu(I) such as atom radius, coordination habits, and redox potential. Encouraged by copper‐catalyzed alkyne‐azide cycloaddition (CuAAC) reaction, comproportionation reaction of Cu(II)X2 and Cu(0) in the presence of (PhC≡CAg)n complex and molybdate generated a core‐shell peanut‐shaped 66‐nuclear Ag(I)‐Cu(I) heterometallic nanocluster, [(Mo4O16)2@Cu12Ag54(PhC≡C)50] (referred to as Ag54Cu12 ). The structure and composition of Ag‐Cu heterometallic nanocluster are fully characterized. X‐ray single crystal diffraction reveals that Ag54Cu12 has a peanut‐shaped silver(I)/copper(I) heterometallic nanocage protected by fifty phenylacetylene ligands in µ 3–modes and encapsulated two mutually twisted tetramolybdates. Heterometallic nanocage contains a 54‐Ag‐atom outer ellipsoid silver cage decorated by 12 copper inside wall. Nanosized Ag54Cu12 is a n‐type narrow‐band‐gap semiconductor with a good photocurrent response. Preliminary experiments demonstrates that Ag54Cu12 itself and activated carbon supported Ag54Cu12/C are effective catalysts for 1,3‐dipole cycloaddition between alkynes and azides at ambient conditions. The work provides not only a new synthetic route toward Ag(I)‐Cu(I) nanoclusters but also an important heterometallic intermediate in CuAAC catalytic reaction.


Introduction
High-nuclearity ligand-protected heterometallic nanoclusters are more attractive than homo-metal nanoclusters for potential applications such as catalysis, photoluminescence, electrochemistry, and other fields, which is due to the synergistic effect of hetero-metal nanoclusters on physicochemical properties. [1]The key to tuning physicochemical behavior of heterometallic clusters is the preparation of nanoclusters with controllable composition/doping sites. [2]Generally, heteroatoms that can be dope into silver nanoclusters are noble metals with similar radius in the periodic table. [3]Conventional doping methods are based on co-reduction, metal exchange, metal deposition of atoms/ions [(also known as anti-galvanic reaction (AGR)], or a combination of these strategies. [4]hese strategies are less effective in controlling reduction of double metal salts and formation of heteroatomic nanoclusters.A search reveals that the availability of heteronuclear Ag(I)-Cu(I) alkynyl clusters via Cu doping are rare. [5]As far as dopants of Cu element are concerned, the standard potential of Cu 2+ /Cu + couple is as low as 159 mV, which indicates that the +1 oxidation state of copper is susceptible to be oxidized to much more stable +2 state.Furthermore, the aggregative nature of Cu(I) alkynyl causes a larger challenge in isolation of discrete Ag(I)-Cu(I) alkynyl nanoclusters.To solve this puzzle, the comproportionation reaction based coppercatalyzed alkyne-azide cycloaddition (CuAAC) reaction could provide a route toward stable homogeneous high-nuclearity monovalent copper alkynyl clusters. [6]The origin of Cu(I) intermediate is generated by Cu(II) with various Cu(0) sources such as wire, turnings, powder and nanoparticles in CuAAC reaction.This reaction could overcome potential difference to produce monovalent copper, [7] which in combination with a variety of alkynyl precursors give metal nanoclusters with tunable composition, structure, and properties. [8]In spite of success in homogeneous copper, the implementation of the experiments remains a challenge in heterogeneous Cu/Ag nanoclusters(.On the other hand, Cu(I) alkynyl complexes have been extensively studied as key intermediates in copper-catalyzed transformations of alkynes as well as in click chemistry synthesis of 1,2,3-triazole. [9]The widely utilized CuAAC involves generation of Cu I -alkynyl species of various nuclearities. [10]Reported examples include Cu 33 and Cu 62 , [11] and Cu 20 [12] nanocluster. [13]Apart from these homogeneous intermediates, there are only sporadic reports on heterometallic intermediates involving the click reaction.However, understanding of interaction of Cu(I) and hetero-metals in heterogeneous CuAAC catalytic systems is not well clear.Recently, Zhu et al captured three crucial Au 4 Cu 4 −alkyne intermediates and discussed an abnormal mechanism in CuAAC reaction, which is different from comproportionation reaction to dehydrogenate. [14]Therefore, the revealing of Cu(I) and Ag(I) interaction in heterometallic Cu/Ag clusters is of significance in click chemistry of CuAAC reaction.It is still an open question whether heterometallic intermediates can be achieved by comproportionation reaction and alkyne precursor.Inspired by the widely used methodology for Cu-ethyne nanoclusters, we would like to explore the similar story in Ag-ethyne cousins.
In order to generate new heterometallic nanoclusters and intermediates in CuAAC reaction, a series of Ag-Cu-alkyne products in the system need to be obtained by alkyne ligands.Our strategy is to creatively use silver-phenylacetylene precursor instead of alkyne or copper-alkyne ligand during CuAAC reaction without azides.Silver-phenylacetylene precursor provides alkyne source for CuAAC intermediate because alkynyl as a -acid ligand can bind to d 10 ions of Au, Ag, Cu via - modes and form various metal-carbon interactions. [15]As such, the pronounced interactions of alkynyl in a variety of coordination modes may affect the physicochemical properties of metal clusters due to metal atom kernel and metal-alkyne interactions. [16]This allows the atomic-level understanding of structure-property correlations, which in turn favours the targeted preparation of metal nanoclusters. [17]In addition, the slow release of Ag + from silverphenylacetylene in a weakly reducing DMF can bind Cu + in situ. [18]As in Scheme 1, we envisage that the polymolydate anion template will target Ag and Cu metal centres to aggregate in a core-shell structure, resulting in the formation of Ag-Cu clusters.
Compared to reported metal nanoclusters encapsulating multiple polyoxometallates (POMs), the twisted configuration of two unconnected [Mo 4 O 16 ] 8-units is unique (reports listing these high nuclear clusters are in Table S3, Supporting Information).The unconnected arrangement of two POMs allows more oxygen sites to be exposed, which is beneficial for enhancing the template effect.The composition of hetero-metal cluster was supported by PXRD and FI-IR spectra (Figures S4 and S5, Supporting Information, Experimental Section).XPS spectra shown the presence of Ag, Cu, Mo, C, and O elements, which was consistent with the results of EDS-mapping (Figures S6 and S7a, Supporting Information).High-resolution spectra clearly illustrated the valences of metal ions of Ag 54 Cu 12 .As shown in Figure S7b (Supporting Information), XPS data of the Ag 3d 5/2 and Ag 3d 3/2 binding energies are 368.26 and 374.24 eV, respectively, confirming that Ag atoms in the cluster are positively charged. [23]The Mo binding energy at 232.9 and 235.2 eV in Figure S7c (Supporting Information) can be attributed to Mo 6+ 3d 5/2 and 3d 3/2 spin-splitting slits. [24]Figure S7d (Supporting Information) shows XPS peaks of monovalent Cu(I).The Cu LMM Auger chemical shift also showed monovalent Cu I state (Figure S9, Supporting Information).
The solid-state UV absorption spectra of Ag 54 Cu 12 show a broadband absorption in the wavelength range 300-400 nm, which is attributed to the →* transition due to appearance of similar band in (PhC≡CAg) n precursor.The optical bandgap of Ag 54 Cu 12 was determined by Tauc equation to be 1.98 eV, which is narrower than that of 2.79 eV in precursor (PhC≡CAg) n (Figure 3c).18a,25] The photoelectrochemical behavior of Ag 54 Cu 12 was tested in a typical three-electrode system in a 0.2 m Na 2 SO 4 aqueous solution.Compared with (PhC≡CAg) n , an obvious photocurrent response was detected upon on-off cycling irradiation, indicating a better electron and hole separation efficiency of Ag 54 Cu 12 .The photocurrent density could reach up to 0.11 μA cm −2 , which remained nearly constant with increased test times, indicating high photophysical stability of Ag 54 Cu 12 .Considering the board absorption semiconductor property of Ag 54 Cu 12 , Mott-Schottky (M-S) measurements were performed at frequencies of 300, 500 and 1000 Hz in darkness (Figure 3d).The positive slope of the C −2 -E plot confirms that Ag 54 Cu 12 is an n-type semiconductor. [26]The flat band potential (EFB) was determined by the intersection to be ≈−1.3V versus Ag/AgCl, corresponding to a potential of −0.68 V versus NHE.It is expected that Based on the previous reports, the conduction band edge of semiconductor should be ≈0.10V more negative than the EFB.Therefore, it can be estimated that the conduction band (LUMO) of Ag 54 Cu 12 is approximately −1.2 V versus NHE.Copper-catalyzed [3+2] Huisgen cycloadditions of terminal alkynes and organic azides (CuAAC) were the cornerstone of Meldal and Sharpless research, most of which involved homometallic Cu-based materials. [27,28]To investigate and develop heterometallic 1,3-dipole cycloaddition reaction catalysis, Ag 54 Cu 12 was implemented by using phenylacetylene and benzyl azide at 40 °C as the model reaction.This reaction exhibits completely regioselectivity and is a powerful method for the rapid assembly of 1,4-disubstituted-1,2,3-triazoles.The solution of Ag 54 Cu 12 cluster was gradually added to a suspension of activated carbon/titanium dioxide in ethanol, and resulted mixture was stirred and centrifuged to give C/TiO 2 supported nanocluster catalysts.Transmission electron microscopy (TEM) illustrated that all particles of as-synthesized products were less than 2 nm   First, the influence of different solvents on the reaction was investigated.A relatively higher yield was realized in CH 3 (99%), compared to CH 3 OH solvent (Table 1, entries 1-5).In addition, the effect of supports and loading capacity was also investigated under the same reaction conditions, and a high isolated yield of 99% was realized (Table 1, entry 1-2 and 4-5).The unsupported Ag 54 Cu 12 NCs gave a lower isolated yield of 88% (Table 1, entry 6).Importantly, the control experiments showed that Ag 54 Cu 12 significantly contributes to the cycloaddition process (Table 1, entries 7-11).
Based on optimized standard conditions, the generality of cycloaddition was explored for various aryl-terminated acetylenes and the corresponding results were summarized in Table 2. Good yields (90%-94%) were obtained for substituted aryl alkynes with both electron-donating (-OCH 3 and -CH 3 ) and electronwithdrawing (-F, -Cl, and -NO 2 ) substituents (Table 2, entries 1-6).These results demonstrated the important interaction between Ag 54 Cu 12 and activated carbon substrate.
Interestingly, some 1,2,3-triazole products were crystallized from reaction environment (Tables 1, entry 6 and 2, entries 7-8).The single crystal data of products were collected and solved (Figure S12a and Tables S5 and S6, Supporting Information), directly confirming their corresponding identity.Both Ag 54 Cu 12 and Ag 54 Cu 12 /C as catalysts exhibited excellent cycle stability after six experiments (Figure S12b, Supporting Information).These results demonstrated that Ag 54 Cu 12 NCs could be a highperformance molecular catalysts for CuAAC.

Conclusion
In conclusion, we have isolated an all-alkyne pro-

Figure 1 .
Figure 1.a) Full structure of Ag 54 Cu 12 nanocluster.b) The core-shell peanut shaped of Ag 54 Cu 12 .c) The Cu 12 Ag 54 ellipsoid cage as peanut shell.d) The two tetrabolydates serve as seeds for the nanocluster.e) Ag 54 shell.f) Cu 12 motif.
Careful check revealed that two [Mo 4 O 16 ] 8− units are bonded to 12 Cu atoms through 6 μ 2 -O and 24 terminal oxygen atoms via weak Cu•••O bonds, and to 34 Ag atoms via 20 terminal oxygen atoms.The 20 Ag atoms uncoordinated to POMs were found to be embedded into Ag 54 shell by Ag─Cu, Ag─C,

Figure 4 .
Figure 4. a) Total and partial DOS of Ag 54 Cu 12 cluster.b) Frontier molecular orbitals: HOMO and LUMO.
tected silver(I)/copper(I) heterometallic nanocluster, [(Mo 4 O 16 ) 2 @Cu 12 Ag 54 (PhC≡C) 50 ] (Ag 54 Cu 12 ), which was synthesized by the reduction of Cu (II) salt and copper powder in presence with (PhC≡CAg) n and Na 2 MoO 4 under solvothermal method.Two mutually twisted [Mo 4 O 16 ] 8− anions were encapsulated by Cu 12 Ag 54 cage to form a core-shell peanut-shaped (Mo 4 O 16 ) 2 @Cu 12 Ag 54 nanocluster.The surface PhC≡C − ligands are ligated to Ag and Cu in - modes to stabilize the nanocluster.XRD, XPS, and ESI-TOF-MS certify the structure and composition of Ag 54 Cu 12 .The n-type narrow-band-gap material was confirmed by solid state UV-vis spectra and DFT calculation.Furthermore, Ag 54 Cu 12 itself and Ag 54 Cu 12 /C are good catalysts for 1,3-dipole cycloaddition between alkynes and azides at ambient conditions.Different from conventional methods for heterometallic clusters, the comproportionation reaction in the work could provide an effective method in controlling reduction of double metal salts and formation of heteroatomic high-nuclearity monovalent copper alkynyl clusters.
b)Catalyst loading defined as mmol% Cu-based Ag 54 Cu 12 cluster.c) Isolated yield.

Table 2 .
Scope of benzyl azide with different alkynes.