Calixarene-Based {Ni18} Coordination Wheel - ACS Publications

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Calixarene-Based {Ni18} Coordination Wheel: High Efficient Electrocatalyst for the Glucose Oxidation and Template for the Homogenous Cluster Fabrication Shentang Wang, Xiaohui Gao, Xinxin Hang, Xiaofei Zhu, Haitao Han, Xiaokun Li, Wuping Liao, and Wei Chen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13193 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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Journal of the American Chemical Society

Calixarene-Based {Ni18} Coordination Wheel: High Efficient Electrocatalyst for the Glucose Oxidation and Template for the Homogenous Cluster Fabrication Shentang Wang,†,§,¶ Xiaohui Gao,‡,§,¶ Xinxin Hang,†,§ Xiaofei Zhu,† Haitao Han,†,§ Xiaokun Li,‡ Wuping Liao*†and Wei Chen*‡ † State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China § University of Chinese Academy of Sciences, Beijing 100049, China Supporting Information ABSTRACT: Catalyst plays a very important role in the exploration of new energy. To obtain a high-efficient electrocatalyst for the glucose oxidation and tiny metal nanocluster catalysts, a calixarene-based {Ni18} coordination wheel with sulfur atoms on the cavity surface was designed, synthesized and used as the porous template. Contributing from the active sites of nickel cations, the as-synthesized coordination wheels can efficiently catalyze the electrochemical oxidation of glucose with the onset and peak potentials of 0.3 and 0.46 V in alkaline medium. And the catalysis does not depend on the atmosphere (N2, air or O2), which indicates that the coordination wheel will be a promising electrocatalyst candidate for the compartment-less glucose-air fuel cell. Meanwhile, benefiting from its confined cavity and inner sulfur surface, such coordination wheel can serve as a general template for the fabrication and encapsulation of tiny metal nanoclusters of Au, Pd, Ir, Ru, Rh, Pt and AuPd. In electrochemical examinations, the bimetallic AuPd clusters confined in the coordination wheel show higher current density than commercial Pt/C toward hydrogen evolution reaction (HER). The present study shows that the designed coordination wheel can be used as not only a type of novel catalyst themselves but also a class of template for metal cluster catalysts.

1. INTRODUCTION With the depletion of fossil fuels and increasingly serious environmental pollution, the exploration of the clean, ecofriendly and sustainable energy sources has become a focus topic.1 Glucose fuel cells provide an attractive solution due to their renewability, nontoxicity, low cost and free availability.2,3 However, the development of stable, efficient, economic catalysts for glucose oxidation is still a big challenge because of the disadvantages of traditional catalysts based on enzymes and noble metals such as easy suffering from the environmental change, high cost, and ambiguous reaction mechanisms.4-6 So far, non-noble metal catalysts have been studied for the glucose oxidation. For instance, a porous cuprous oxide microcube,7 a 2D nanocomposite of the Ni(OH)2 nanoplate and reduced graphene oxide, 8 a graphene/Ni-Fe layered double hydroxide nanocomposite9 have been synthesized for the glucose oxidation. Recently, discrete porous metal-organic coordination entities have drawn increasing attention as catalysts due to their fascinating and defined structures, abundant active sites and high atomic effectiveness.10-13 It was reported that some vanadium compounds can be employed as the catalysts for ethylene polymerization reaction14 and the oxovanadium complexes can act as active catalysts for the oxidation of alcohols and polymerization of olefins.15 It is expected that the porous coordination complexes of transition metals would be a class of good catalysts for the glucose oxidation. As reported, for discrete porous metal-organic coordination entities, calixarenes can act as a kind of effective multidentate ligands for the construction of porous coordination compounds especially

the discrete porous coordination entities,16-18 and a series of calixarene-based coordination cages with tunable sizes and apertures were obtained with deliberately chosen ancillary ligands.16d,17,18 Here we present a novel isolated nickelcalixarene coordination wheel and study its electrocatalytic property toward the glucose oxidation. On the other hand, metal nanoclusters have attracted more and more interests regarding their unique physical and chemical properties, even though there are many challenges in the synthesis and characterizations.19 In recent years, various metal/alloy nanoclusters have been obtained with different stabilizing ligands.20-25 Typically, Jin, Xie and Dass’s groups reported a number of thiol-protected gold nanoclusters such as Au25, Au38, Au133, Au144 and so on.26 Wang’s group synthesized several alkyne- and phosphine-coordinated gold nanoclusters.27 Zheng’s group synthesized some alloy metal nanoclusters protected by thiols or alkyne ligands.28 Nevertheless, the instinct and catalytic properties (activity and stability) of the metal nanoclusters are far from satisfactory because of the spontaneous aggregation and collapse.29 Therefore, the syntheses of metal/alloy nanoclusters in some confined spaces of the porous covalent/coordination compounds become a feasible way.30 The coordination cages with heteroatom donors can entrap various nanoclusters to prevent their aggregation and migration31 and improve the catalytic stability of the inner metal nanoclusters.32 In this work, a novel calixarene-based wheel-like entity, {Ni18Cl6(TC4A)6(MNA)6} (CIAC-123, H4TC4A = p-tert-

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butylthiacalix[4]arene, H2MNA = 2-mercaptonicotinic acid) is presented and characterized. Different from the reported coordination cages with small apertures, this coordination wheel has more active nickel sites and two open ends which ensure the efficient mass transfer of the reactants. Thereby, this {Ni18} coordination wheel was examined for electrochemically catalyzing the glucose oxidation. The coordination wheel has an onset potential of 0.3 V and a peak potential of 0.46 V, which is independent on the atmosphere. This compound would be an efficient electrocatalyst for the compartment-less glucoseair fuel cell. Moreover, due to its open structure, small inner cavity and the surface sulfur atoms, the coordination wheel was used as a general template for the fabrication and accommodation of metal nanoclusters of Au, Pd, Pt, Ir, Ru, Rh and bimetal AuPd without any stabilizing and reducing agents. As an example, AuPd@CIAC-123 was studied as the catalyst for the hydrogen evolution reaction, which gave better performance than the commercial Pt/C. 2. METHODS 2.1 Chemicals and Materials. p-tert-Butyltetrathiacalix [4]arene (H4TC4A) was synthesized by literature method.33 Chloroauric acid (HAuCl4·4H2O, 99.9%) was bought from Shanghai Chemical Reagent Co. Ltd. 2-Mercaptonicotinic acid (H2MNA, technical grade), tetraoctylammonium bromide (TOABr, 98%) and sodium tetrachloropalladate (II) (Na2PdCl4.3H2O, 99%) were both bought from Sigma-Aldrich. Chloroplatinic acid (H2PtCl6·6H2O, Beijing Chemical Works), Iridous chloride (IrCl3, Alfa Aesar), rhodium chloride (RhCl3, Alfa Aesar), ruthenium trichloride (RuCl3, Alfa Aesar). Sulfuric acid (H2SO4, A.R, 95-98%), potassium hydroxide (KOH, A.R, ~82%), glucose (C6H12O6, A.R), dichloridemethane (DCM, CH2Cl2, A.R) and ethanol (CH3CH2OH, A.R., ≥99.5%) were purchased from Beijing Chemical Works. E-TEK Pt/C (nominally 20 % by wt of 2-5 nm Pt nanoparticles on Vulcan XC-72R carbon support) was obtained from Alfa Aesar. The water with the resistivity of >18.2 MΩ·cm was used in the whole experiments. 2.2 Synthesis of CIAC-123. Yellow block crystals were obtained from reaction of the mixture of H4TC4A (0.072 g, 0.10 mmol), NiCl2·6H2O (0.100 g, 0.42 mmol), H2MNA (0.018 g, 0.15 mmol), MeOH (3.5 ml) and DMF (3.5 ml) in a 20 ml Teflon-lined autoclave which was kept at 130 °C for 3 days and then slowly cooled to 20 °C at about 4 °C/h. The crystals were isolated by filtration and then washed with 1:1 MeOH-DMF and dry in air. Elemental analysis: calculated (%) for C276H288Cl6N6Ni18O36S30, C 51.17, H 4.20, N 1.30; found (after dried in vacuum): C 51.18, H 4.18, N 1.29. FT-IR (cm-1): 3423(m), 2960(s), 2868 (w), 1680(s), 1584(m), 1468(s), 1398(m), 1259(s), 1137(m), 1089(m), 889(m), 838(m), 779(w), 658(w), 542(w), 502(w), 422(w). 2.3 Synthesis of Au@CIAC-123. The hybrid material was synthesized through a simple and sequent solution impregnation of chloroauric acid. Briefly, the ethanol solution containing 200 µl, 2 mg/ml HAuCl4 and 200 µl, 2 mg/ml TOABr was added into 5 ml ethanol solution with 5 mg CIAC-123. The reaction mixture was vigorously stirred for 20 h. The product was collected by centrifugation and washed three times with ethanol.

2.4 Synthesis of AuPd@CIAC-123. The synthesis is similar to that for Au@CIAC-123. Typically, the ethanolic mixture of HAuCl4 (2 mg/ml, 100 µl) and TOABr (2 mg/ml, 100 µl) was first introduced into 5 ml ethanol containing 5 mg CIAC-123. Before the solid was collected, the reaction mixture was kept constant stirring for 12 h. The collected solid was washed twice by ethanol to remove the possible unreacted gold species and then dispersed in another 5 ml ethanol. Shortly afterwards, 0.2 mg Na2PdCl4 dissolved in 100 µl ethanol was added and the mixture was kept stirring for another 10 h. The final product was isolated by centrifugation and washed three times with ethanol. 2.5 Synthesis of Pd@CIAC-123, Pt@CIAC-123, Ir@CIAC-123, Rh@CIAC-123 and Ru@CIAC-123. The preparation of other metal M@CIAC-123 hybrids was processed by the following procedures. Typically, 200 µl ethanol solutions with 2 mg/ml metal salts (including Na2PdCl4, H2PtCl6, IrCl3, RuCl3 and RhCl3) were introduced into the 3 ml DCM containing 5 mg CIAC-123 under the constant stirring. The reactions were allowed to continue for 24 h. The products were collected by the rotator evaporation and washed three times with ethanol. 2.6 Material characterization. Elemental analysis for C, H, N was recorded on a VarioEL instrument. TGA measurement is performed on a NETZSCH STA 449F3. FT-IR spectra (KBr pellets) were taken on a Bruker Vertex 70 spectrometer. UV-Vis spectra were measured at a UV-3000PC spectrophotometer (Shanghai Mapada Instrument Co. Ltd). X-ray photoelectron spectroscopy (XPS) experiments were performed on an AVG Thermo ESCALAB 250 spectrometer (VG scientific) operated at 120 W. Inductively coupled plasma-mass spectrometer was used to determine the composition of the products (ICP-MS, X Series 2, Thermo Scientific USA). Highresolution transmission electron microscopy (HRTEM) measurements were completed by using a JEM-2010 (HR) microscope operated at 200 kV. 2.7 Crystallographic Analysis. The intensity data were recorded on a Bruker APEX-II CCD system with Mo-Kα radiation (λ = 0.71073 Å) for CIAC-123. The crystal structures were solved by means of Direct Methods and refined employing full-matrix least squares on F2 (SHELXTL-97).34 The high R1 and wR2 factor of compound CIAC-123 might be due to the weak high-angle diffractions and the disorder of p-tertbutyl atoms. The diffraction data were treated by the “SQUEEZE” method as implemented in PLATON.35 The selected crystallographic parameters are given in Table S1. The crystallographic information files (CIF, CCDC 1526738) and the IUCr CheckCIF reports (PDF format) can be found in the supplemental materials. 2.8 Electrochemical Measurements. The whole electrochemical measurements were completed by using the CHI 660D electrochemical workstation with a standard threeelectrode cell at room temperature. The catalyst inks were prepared by mixing a certain amount of catalysts (CIAC-123 or AuPd@CIAC-123 or Pt/C or Au@CIAC-123) with ethanol and Nafion (1/0.025). The rotating disk electrode or common glassy carbon electrode (geometric area 0.196 or 0.07 cm2) deposited with a certain volume of catalyst inks were used as the working electrode and an Ag/AgCl (KCl-saturated) and a Pt coil were used as reference and counter electrodes, respectively. For the glucose oxidation reaction, 0.1 M KOH


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with or without 5 mM glucose was used as the electrolyte. For hydrogen evolution reaction, the electrochemical tests were performed in 0.5 M H2SO4. In this work, all the currents were reported without ohmic drop correction during the measurements. For the glucose oxidation reaction, the currents were normalized to the Ni oxidation peak area (labeled as “area” in the Figures). For the hydrogen evolution reaction, the currents were normalized to mass of noble metals for comparison. 3. RESULTS AND DISCUSSIONS

the [6 + 6] condensation of the Ni3-TC4A SBUs and MNA linkers (Figure 1 and S1). Different from the trigonal prismatic cavity found in the former work,18a this hexagonal prismatic pore has two open ends that will make the transport of metal cations into the pore more easily and provide larger contact area for small molecules and the confined metal nanoclusters. There are six sulfur atoms in the side faces of the pore to bond and stabilize the metal nanocluster. It should be noted that these six sulfur sites cannot be accurately determined with the current data due to the disorder of the MNA ligands. The dimensions of the nanocage are approximately 25.7 × 27.5 × 15.1 Å3 and the size of the hexagonal prismatic pore is about of 9.0 × 9.0 × 13.3 Å3. The discrete coordination wheels are stacked through molecular interactions into a 3D supramolecular structure with some channels (Figure S3). The structure of the bridging MNA ligand would play an important role in the formation of this coordination wheel.16d,18a Table 1. Comparison of the catalytic performances of the {Ni18} coordination cage and some reported materials toward glucose oxidation.

Figure 1. A shuttlecock-like Ni3-TC4A SBU capped by a Clanion (a) and crystal structure of a {Ni18} coordination wheel (b, side view with the TC4A molecules and Cl- anions omitted for clarity; c, top view; d, top-view with Cyan truncated cone and yellow cylinder representing Ni3-TC4A SBU and the hexagonal prismatic pore, respectively). In (b), it is obvious that each Ni3TC4A SBU is connected to four others by three MNA ligands which act as some three-connection linkers through all their nitrogen, sulfur and oxygen atoms.

3.1. Structure of CIAC-123. Compound CIAC-123 crystallizes in the triclinic system with the space group Pī. In CIAC-123, a TC4A molecule adopting a cone conformation bonds three nickel atoms by four phenolic oxygen atoms and three sulfur atoms to form a seldom reported shuttlecock-like Ni3-TC4A SBU. Six such SBUs are bridged by six MNA ligands into a wheel-like entity, which has a hexagonal prismatic pore. There are nine crystallographically independent nickel sites (Ni1-Ni9), all of which are six-coordinated by two phenoxy oxygen atoms, one µ3-Cl, one sulfur atom from TC4A, and two oxygen atoms or one sulfur and one nitrogen or one sulfur and one oxygen of a same MNA ligand (Figure 1a, S1 and S2). Analysis of the bond lengths and bond valence sum calculations suggest all nickel cations to be divalent. Three adjacent nickel atoms bond a calixarene molecule to form a shuttlecock-like Ni3-TC4A SBU, whose bottom is capped by a µ3-Cl anion. In the structure, each Ni3-TC4A SBU is linked by three MNA ligands and each MNA molecule is bonded by three Ni3-TC4A SBUs (Figure S1 and S2). And then a wheellike {Ni18} unit with hexagonal prismatic pore forms through

Materials

Onset potentials (V)*

Peak potentials (V)*

Ref.

Porous Cu2O cubes

0.0 V

+0.6 V

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Ni(OH)2 nanoplates on rGO

/

+0.54 V

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+0.6 - 0.7 V

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NiCu alloy

+0.4 V

+0.5 - 0.6 V

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PtNi nanowires

+0.337 V

+0.487 V

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{Ni18}coordination cages

+0.3 V

+0.46 V

this work

*The potential value is relative to Ag/AgCl (saturated KCl). 3.2. Electrocatalytic activity of Ni18 for the glucose oxidation. Due to two unique open ends and accessible metal nickel active sites, the present coordination wheel Ni18 was evaluated for the electrochemical oxidation of glucose in alkaline solution, which is an important anodic reaction in glucose fuel cells. As displayed in Figure 2a, in the pure 0.1 M KOH electrolyte, the cyclic voltammetry curve from CIAC-123 shows an oxidative peak at +0.43 V and a reductive peak at +0.35 V in the reversed scan, which corresponds to the redox couple of Ni(II) and Ni(III). In the electrolyte containing 5 mM glucose, the current density obviously increases, indicating the electrochemical oxidation of glucose. Herein, the onset potential is found to be about +0.3 V and the anodic peak current density is doubled at the peak potential of +0.46 V compared to that from the electrolyte without glucose. As shown in Figure 2b, linear sweep voltammetry measurements were further performed. As expected, the polarization curve presents a nickel oxidation peak at 0.47 V in blank 0.1 M KOH. In the electrolyte containing 5 mM glucose, the polarization curve gives the maximum current density of 55 mA·area-2 at the potential of 0.5 V, which is almost five times higher than that from the blank electrolyte. The onset potential from linear sweep voltammetric curve is determined to be 0.31 V, in con-



sistent with the result of cyclic voltammetry. For the electrooxidation of glucose, the onset potential and peak potential are the two key parameters to evaluate the catalytic activity of an electrocatalyst. Table 1 compares the onset and peak potentials of the previously reported catalysts and the present {Ni18} coordination cage for glucose oxidation. It is obvious that CIAC-123 would be a good catalyst for the glucose oxidation with a low overpotential, much superior to the pure nickel powders (Figure S7) and noble metal-free materials and comparable to some noble metal-based catalysts.7-9,36,37 It should be noted that the overlap between the nickel oxidation and glucose oxidation peaks implies that the possible catalytic active sites is the metal Ni centers. The catalytic stability of CIAC-123 was examined through the accelerated durability by multiple cycles in 0.1 M KOH. From the LSV curves shown in Figure 2c, after 5000 cycles, the peak current density is almost unchanged with the maintenance of the onset potential. With the further increase of the test cycles, the current density tends to be enhanced, i.e, J20000th > J15000th > J10000th ≈ J5000th, which can be due to the electrochemical activation of CIAC-123. As shown in Figure S8, during the durability test, the peak current density of nickel oxidation from CIAC-123 also increases in the blank electrolyte, similar to the situation in the electrolyte containing 5 mM glucose, indicating high stability of the CIAC-123 compound. Note that the maximum current density is dependent on the concentration of glucose. The little increase of current density after durability tests may be ascribed to the adsorption of some species on the surface of CIAC-123. During the cyclic voltammetric cycles, the species can be removed gradually and more active sites are available. 20

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In addition, the influence of gas atmosphere on the glucose oxidation in this system was investigated, which is of importance for the construction of compartment-less glucose fuel cells.38 As showed in Figure 2d, in the presence of N2 or air or O2, the cyclic voltammograms do not show any obvious changes in the electrolyte with 5 mM glucose, suggesting the high catalytic selectivity of CIAC-123 to the glucose oxidation and its inertness to the oxygen, different from the Rhbased noble catalysts.39,40 Thus, the CIAC-123 can be a promising noble-metal free catalyst for compartment-less glucoseair fuel cells. In order to demonstrate the electrocatalytically active centers of above mentioned CIAC-123, the calixarene and the mixture of calixarene and nickel dichloride modified glassy carbon electrodes were respectively allowed to catalyze the glucose oxidation. For the calixarene alone (Figure S9), the cyclic voltammograms obtained from the electrolyte with or without glucose show small currents without obvious change, indicating its negligible catalytic activity to the glucose oxidation. However, for the mixture, enhanced oxidation current density was obtained in the electrolyte with 5 mM glucose compared to that from the electrolyte without glucose. Such results clearly illustrate the vital role of metal nickel in CIAC123 for glucose oxidation. As previously reported,36b Ni metal or Ni(II) can be oxidized to Ni(III) in the positive scan. The formed Ni(III) has the ability to oxidize glucose and itself is reduced back to Ni(II). These reaction processes indicate that the nickel-based PCCs can be promising catalyst candidates for glucose oxidation. Therefore, the high catalytic activity of CIAC-123 can be attributed to the enough catalytic active centers of Ni and the open ends which are beneficial for the transfer of reactants, intermediates and products.

0.1 M KOH 0.1 M KOH + 5 mM glucose

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Figure 2. (a) Cyclic voltammograms of CIAC-123 in 0.1 M KOH without or with 5 mM glucose, potential scan rate: 100 mV/s; (b) Linear sweep voltammograms of CIAC-123 in 0.1 M KOH without or with 5 mM glucose, scan rate : 10 mV/s. (c) Linear sweep voltammetric curves from the CIAC-123 in 0.1 M KOH with 5 mM glucose after 5000, 10000, 15000 and 20000 cycles tests, scan rate: 10 mV/s; (d) Cyclic voltammetric curves from the CIAC-123 in 0.1 M KOH with 5 mM glucose in the atmospheres of N2, air and O2, scan rate: 100 mV/s. Note that (a-c) and (d) were obtained from the rotatory glassy carbon disk electrode and common glassy carbon electrode, respectively.


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Figure 3. HRTEM images of (a) Au@CIAC-123, (b) Pd@CIAC-123, (c) Pt@CIAC-123, (d) Ir@CIAC-123, (e) Ru@CIAC-123 and (f) Rh@CIAC-123. The scale bars: 2 nm. 3.3. Template for the homogenous clusters formation and the electrocatalytic performance of AuPd@CIAC-123 for HER. Because of the open structure, the small cavity and sulfur donors on the framework, the present coordination wheel of CIAC-123 can be also used to synthesize and confine various metal nanoclusters, including Au, Pd, Pt, Ru, Rh, Ir and even bimetal AuPd nanoclusters, following the processes described in experimental section. Here, high resolution transmission electron microscopy is first used to examine the size and morphology of the formed products. As shown in Figure 3a-f and Figure 4b, tiny metal clusters with size of ~1 nm (marked by the red cycles) can be observed with good dispersion within CIAC-123 (thereafter, donated as M@CIAC-123), manifesting the successful formation of mono- and bi-metallic clusters with maintenance of the pristine crystal structure of the coordination wheel. Meanwhile, the results from inductively coupled plasma mass spectrometry (ICP-MS) indicated that the metal loadings are 14.5%, 2.08%, 23%, 0.76%, 2.32%, 2.16%, and 32% for the Au@CIAC-123, Pd@CIAC-123, Pt@CIAC-123, Ru@CIAC-123, Rh@CIAC-123, Ir@CIAC-123 and AuPd@CIAC-123, respectively. And for the AuPd@CIAC123, the atomic ratio of Au to Pd was calculated to be 7: 16. Therefore, the CIAC-123 is an effective template for synthesizing and accommodating various metal nanoclusters with limited size, similar to the previously reported PAMAM denrimer.41

Figure 4. (a) UV-Vis absorption spectra of CIAC-123 and AuPd@CIAC-123.; (b) HRTEM image of the AuPd@CIAC123, the scale bar is 2 nm; (c) Far infrared spectra of AuPd@CIAC-123 (black line) and CIAC-123 (red line).; (d) Polarization curves of HER on the AuPd@CIAC-123, Au@CIAC-123 and the commercial Pt/C in 0.5 M H2SO4. To further demonstrate the effectiveness of the CIAC-123 as a general template, bimetallic AuPd@CIAC-123 hybrid is taken as an example for detail investigations. The UV-Vis absorption spectrum and far infrared red (FIR) spectra were measured. As shown in Figure 4a, the UV-Vis absorption

spectrum shows that the AuPd@CIAC-123 has an obvious absorption peak at 325 nm and a broad shoulder from 350 to 365 nm. Compared to the parent CIAC-123, the red-shift of broad absorption peak from ca. 314 nm to 328 nm and the less prominence of the shoulder peak suggest that the electronic structure of CIAC-123 coordination wheel was modified by the encapsulated metal species. Such changes can be ascribed to the chemical interaction between the AuPd NCs and the sulfur groups in CIAC-123. In Figure 4c, compared to that of CIAC-123, the FIR spectrum of AuPd@CIAC-123 has additional peak at 329 cm-1 which can be ascribed to the metal-S bonds (Au-S or Pd-S).42 This result demonstrates clearly again that the encapsulation of AuPd clusters in the coordination wheel. On account of the prominent properties of bimetallic clusters and the accessible active sites from the open structure, the potential application of CIAC-123 and AuPd@CIAC-123 for hydrogen evolution reaction was explored. As shown in Figure 4d, the AuPd@CIAC-123 shows a comparable onset potential of -0.28 V vs Ag/AgCl to commercial Pt/C for HER in 0.5 M H2SO4. In particular, the current density of HER from AuPd@CIAC-123 runs up to two and three times of the commercial Pt/C at the potentials of -0.6 and -0.7 V, respectively. Meanwhile, the AuPd@CIAC-123 shows significantly higher HER catalytic activity than Au@CIAC-123. Such results indicate the excellent electrocatalytic activity of the synthesized AuPd@CIAC-123 for HER. Moreover, as shown in Figure S12 and S13, the AuPd@CIAC-123 shows high catalytic stability for HER (See the discussion in Supporting Information). Such catalytic properties can be revealed by the XPS results (Figure S15-19). For the studied CIAC-123, Au@CIAC-123 and AuPd@CIAC-123 samples, the AuPd@CIAC-123 shows the most positive Ni 2p binding energy, indicating the higher electron density on the bimetallic AuPd clusters. Meanwhile, compared to the Au@CIAC-123, the negative shift of Au 4f binding energy from AuPd@CIAC-123 suggests that Pd metal serves as electron donor in AuPd clusters. Therefore, it can be proposed that the catalytic activity sites in AuPd@CIAC-123 are probably from Pd species while gold and nickel can effectively regulate the electronic structure of Pd, thus promoting the catalytic activity for HER. Combining the above electrochemical results (glucose oxidation and hydrogen evolution reactions), one can see that CIAC-123 and AuPd@CIAC-123 show different catalytic performances for the two studied electrochemical reactions (The performance of AuPd@CIAC-123 for glucose oxidation reaction is discussed in Figure S10). The displayed catalytic properties can be attributed to the selectivity of the open structure of CIAC-123. As a type of large molecule, glucose (diameter: ~1 nm) cannot go through the pore (diameter: 0.9 nm) and contact with the confined AuPd clusters; therefore the catalytic glucose oxidation happens on the outside surface and metal nickel sites are the active sites. On the other hand, hydrogen ions with small size can go through easily the inner cavity and be absorbed on the clusters. Therefore, hydrogen evolution reaction occurs on the Pd sites with the synergetic effects from Au and Ni components. Therefore, based on the size of reaction molecules and pore size of the designed {Ni18}, the selective catalysis can be realized.



4. CONCLUSIONS In summary, we designed and synthesized a novel calixarenebased coordination wheel with two open ends and sulfur atoms on the cavity surface. Benefited from the open structure of coordination wheels, the CIAC-123 exhibited promising catalytic performance for the glucose electrochemical oxidation, which is superior to the noble-metal free materials and comparable to some noble-metal catalysts. This application has not yet been reported for polyhedron coordination cages. On the other hand, by using the sulfur atoms in the framework as immobilizing sites, tiny but stable metal nanoclusters can be efficiently formed in the cavities. Taking the bimetallic one as an example, the synthesized AuPd@CIAC-123 showed excellent electrocatalytic activity for the hydrogen evolution reaction due to the electron regulating effect. We hope this work can open a new page for the development of calixarene-based discrete porous coordination entities, and provide a new strategy for synthesizing highly active and stable metal nanoclusters to speed up their practical catalytic applications in energy conversion and storage. Moreover, by taking advantage of the pore size of the coordination cages, selective catalytic applications can be achieved.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.xx Single-crystal structure determination, MALDI mass spectra, electrocatalytic activities, XPS, TEM, other figures and detailed analyses (PDF). Crystallographic data in CIF format (CIF).

AUTHOR INFORMATION Corresponding Author

*[email protected] *[email protected] Author Contributions ¶

S.-T.W., and X.-H.G. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Nos. 21571172, 21633008, 21521092, and 21575134) and National Key Research and Development Plan (2016YFA0203200).

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