English

• 0.   Introduction

  Self-assembled supramolecular coordination cages have attracted considerable interest because of their prominent applications in gas storage, catalysis, and electrochemical sensing^[1-6]. The coordination cages could be rationally assembled with directional bridging organic ligands and different types of metal ions^[7-9]. From this perspective, calix[4]resorcinarene-based derivatives, possessing large inner cavities and controllable cavity environments, have been extensively utilized as building blocks for supramolecular self- assembly^[10-13]. For instance, Pei et al. synthesized a giant coordination cage that was formed by the self- assembly of calix[4]resorcinarene, Zn(NO₃)₂·6H₂O and 3,3′, 5,5′-azobenzene tetracarboxylic acid^[14]. He et al. reported a huge cubic supramolecular nanocapsule bearing six calix[4]resorcinarene subunits^[15]. The functionalized calix[4]resorcinarene cavitand is an essential building block for the controllable assembly of giant coordination cages^[16-19].

  Polyoxometalates (POMs), one fascinating class of metal-oxo clusters, feature high-valent metal ions (e.g., V, Mo, W) and abundant redox active sites, which have been proven to be candidate anode materials used in lithium-ion batteries (LIBs)^[20-23]. Nonetheless, POMs are easily soluble in electrolytes, limiting their wide application^[24]. Therefore, the self-assembly of supramolecular coordination cages and POMs is a synthetic approach to take advantage of their structure features^[25]. The POMs could act as inorganic building blocks and guest counter ions in the construction of inorganic-organic hybrid complexes^[26-34]. Nevertheless, to our knowledge, POM-calixarene-based coordination cages have been rarely reported, since giant coordination cage self-assembly and the strategy of introducing POMs are still substantially challenging for crystal engineering in supramolecular chemistry^[35].

  Based on the above consideration, we designed a new POM-calixarene-based [Co₈] coordination cages, namely [Co₈(MTR4A)₆Cl₈](α-SiW₁₂O₄₀)₂·30DMF·74EtOH (cage-1), synthesized with a methyl imidazole functionalized resorcin^[4]arene-based ligand (MTR4A), H₄[SiO₄(W₃O₉)₄] and Co(Ⅱ) cations under solvothermal conditions (Scheme 1)^[36]. Notably, cage-1 is a unique example of hexameric [Co₈] cationic coordination cages surrounded by six MTR4A molecules with POMs counter anions. Importantly, cage-1 featured a satisfied lithium-ion storage capacity and stability. Moreover, cage-1 showed good electrochemical functions in reducing nitrite (NO₂^-) and oxidizing ascorbic acid (AA).

  Scheme 1

  

  Scheme 1.  Synthetic procedure for cage-1

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  1.   Experimental

  1.1   Materials and physical measurement

  All materials were provided by pharmaceutical companies. FTIR spectroscopy was characterized using a Nicolet Magna 560 Fourier transform IR spectrometer in 4 000-400 cm^-1. The VarioEL Ⅲ Elemental Analyzer was employed to collect elemental results on carbon, nitrogen, and hydrogen. The Rigaku SmartLab X-ray diffractometer was utilized to obtain powder X-ray diffraction (PXRD) patterns, under the graphite monochromatized Cu Kα radiation (λ=0.154 nm, U=45 kV, I=40 mA, 2θ=2°-30°). An electrochemical workstation CHI660E was employed to record electrochemical behaviors. The LAND test system (LANHE CT2001A) was adopted for measuring the galvanostatic charge- discharge curves at 100 mA·g^-1 and the potential of 0.01-3 V (vs Li/Li⁺).

  1.2   Synthesis of cage-1

  Purple crystals of cage-1 (24 mg, 80% based on MTR4A) were achieved from the solvothermal reaction of MTR4A (11 mg, 10 mmol), CoCl₂·6H₂O (10 mg, 40 mmol) and H₄[SiO₄(W₃O₉)₄] (14 mg, 5 mmol) in DMF/EtOH (8 mL, 1∶1, V/V) at 80 ℃ for 3 d. Anal. Calcd. for Co₈C₅₇₄H₉₉₀N₇₈O₂₃₂S₂₄Cl₈Si₂W₂₄(%): C, 36.85; H, 5.30; N, 5.84. Found(%): C, 37.22; H, 5.12; N, 5.78. IR data (KBr, cm^-1): 3 121 (w), 2 937 (w), 1 666 (s), 1 530 (w), 1 467 (m), 1 386 (m), 1 337 (w), 1 285 (w), 1 251 (m), 1 147 (m), 1 095 (m), 1 055 (w), 1 013 (m), 971 (s), 922 (s), 804 (s), 691 (w), 662 (w), 584 (w), 534 (w).

  1.3   X-ray crystallography

  The Bruker D8 VENTURE X-ray diffractometer was used to record crystallographic data under the Cu Kα radiation (λ=0.154 178 nm) for cage-1 at 173 K. The structure was improved onto F² using the full-matrix least-squares by adopting SHELXTL-2018/3 in WINGX^[37-39]. All hydrogen atoms were placed geometrically. The highly disordered solvents were removed by the SQUEEZE routine in PLATON^[40]. Table S1 and Table S2 (Supporting information) list structure refinement parameters and crystallographic results.

  2.   Results and discussion

  2.1   Crystal structure of cage-1

  The calix[4]resorcinarene-based [Co₈] cationic coordination cage was achieved by self-assembly of MTR4A, H₄[SiO₄(W₃O₉)₄] and CoCl₂·6H₂O under solvothermal conditions. Single-crystal X-ray diffraction reveals that cage-1 crystallizes in space group P1 within a triclinic system. As shown in Fig. 1a, cage-1 is composed of six calix[4]resorciarene molecules, eight Co(Ⅱ) cations, two α-SiW₁₂O₄₀^4- counter anions, and eight Cl^- anions. Each Co(Ⅱ) center adopts the four- coordinated mode by three nitrogen atoms from the three adjacent ligands, with Co—N distances of 0.197 5-0.205 5 nm, and one chlorine atom, with Co—Cl distances of 0.205 5-0.22 51 nm, featuring an elegant nanosized [Co₈] coordination cage. The two free α-SiW₁₂O₄₀^4- anions act as counter ions to balance the negative charge. Noticeably, the α-SiW₁₂O₄₀^4- anions reveal two different connected modes through weak C—H…O hydrogen-bonding interactions. The related hydrogen bond data are listed in Table 1. They both are surrounded by three MTR4A ligands, the difference is that one α-SiW₁₂O₄₀^4- anion is surrounded by three cages from one direction, while the other from different directions to produce a 3D supramolecular structure (Fig. 1b and 1c). As far as we know, cage-1 represents the initial instance where six calix[4]resorcinarene molecules are linked together via metal cations to form a charming [Co₈] cationic cage with two free α-SiW₁₂O₄₀^4- anions (Fig. 1d).

  Figure 1

  

  Figure 1.  (a) Structure of cage-1; (b) α-SiW₁₂O₄₀^4- cation with three cages from one direction by hydrogen bonding; (c) α-SiW₁₂O₄₀^4- cation with three cages from different directions by hydrogen bonding; (d) Hydrogen-bonding supramolecular structure of cage-1

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  Table 1

  

  Table 1.  Hydrogen-bonding parameters for cage-1

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  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠D—H…A / (°)
  C49—H49#3…O61	0.095	0.259	0.330(2)	132.4
  C105—H105#1…O37	0.095	0.225	0.310(3)	149.7
  C114—H114#2…O66	0.095	0.251	0.326(3)	136.2
  C115—H11B#2…O79	0.098	0.240	0.322(2)	141.6
  C162—H162#6…O68	0.095	0.230	0.317(2)	152.2
  C163—H16C#6…O47	0.098	0.256	0.330(17)	132.2
  C214—H21B#5…O33	0.098	0.251	0.323(3)	130.3
  C280—H28C#4…O39	0.098	0.259	0.321(3)	121.2
  C336—H33G#3…O45	0.098	0.246	0.301(3)	115.1
  Symmetry codes: #1: -x+1, -y+1, z; #2: -x+1, -y+1, -z+1; #3: -x+2, -y, -z+1; #4: x, y+1, z; #5: x+1, y+1, z; #6: x+1, y+1, z+2.

  2.2   Electrochemical performance

  Given the potential application of POM-based materials, the electrochemical properties of cage-1 as anode materials for LIBs were studied^[41-43]. The storage performance of LIBs was analyzed through galvanostatic current charge-discharge experiments and cyclic voltammetry (CV). Fig. 2a presents CV curves for cage-1 from 0.01 to 3 V at a scan rate of 0.1 mV·s^-1. One wide reduction peak could be observed near 1.48 V during the first anodic scan, which disappeared in subsequent cycles and was associated with solid electrolyte interface (SEI) generation. And another peak at 0.8 V is attributed to the Li⁺ ions insertion. In the first cathodic scan, the peak detected near 1.2 V could be caused by Li⁺ ions deintercalation^{[22, 44-46]}. The electrochemical redox process mainly occurred on the W species in POMs and the probable reaction mechanism may be expressed as follows^[47]:

  $ \begin{align} & \left[ \text{C}{{\text{o}}_{8}}{{(\text{MTR}4\text{A})}_{6}}\text{C}{{\text{l}}_{8}} \right]{{\left( \alpha -\text{Si}{{\text{W}}^{\text{VI}}}_{12}{{\text{O}}_{40}} \right)}_{2}}+24\text{L}{{\text{i}}^{+}}+24{{\text{e}}^{-}}\rightleftharpoons \\ & \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \left[ \text{C}{{\text{o}}_{8}}\text{L}{{\text{i}}_{24}}{{(\text{MTR}4\text{A})}_{6}}\text{C}{{\text{l}}_{8}} \right]{{\left[ \alpha -\text{Si}{{\text{W}}^{\text{V}}}_{12}{{\text{O}}_{40}} \right]}_{2}} \\ \end{align} $

  Figure 2

  

  Figure 2.  Electrochemical performance of cage-1: (a) CV curves at 0.1 mV·s^-1; (b) galvanostatic discharge-charge profiles; (c) cycling performance for 1 000 cycles; (d) rate performance under diverse current densities from 0.1 to 5 A·g^-1

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  This similar reaction mechanism has been widely observed in the reported POM-calixarene-based complexes^{[22, 43]}. In subsequent scans, the CV curves matched well, indicating that cage-1 has good electrochemical stability and reversibility.

  To explore the charge-discharge capability, galvanostatic current charge-discharge experiments with the current density of 100 mA·g^-1 were studied. According to Fig. 2b, cage-1 had the original charge and discharge capacities of 394 and 1 081 mAh·g^-1, respectively. Relative to commercial graphite, cage-1 exhibited a higher capacity. Discharge capacity in the second cycle was reduced to 428 mAh·g^-1, mainly attributed to the irreversible consumption of Li during SEI film generation. Obviously, the storage capacity of cage-1 was higher than that of commercial graphite, which may be the contribution of the synergistic effect between POMs and the MTR4A ligand.

  Cycling stability is also a critical factor for evaluating the electrochemical performance of cage-1 as the LIBs anode material. Comprehensive long-term cycling performance at 100 mA·g^-1 was further investigated. As shown in Fig. 2c, the original discharge capacity can be ignored due to SEI formation in the initial discharge process. The anodes delivered a 428 mAh·g^-1 capacity in the second cycle, which remained 83% (356 mAh·g^-1) following 100 cycles. Further, the cycling performance was detected under various current rates (Fig.S1), where the discharge capacity remained good. The relatively stable electrochemical performance suggests that cage-1 is an effective anode material for LIB.

  Normally, the rate performance of LIB is the important assessment criterion for commercialized applications. The rate capability of cage-1 was assessed under diverse current densities. According to Fig. 2d, the electrode showed capacities of 321, 281, 244, 203, 192, 149, 108, 92, 325, and 333 mAh·g^-1 at 0.1, 0.2, 0.4, 0.8, 1, 2, 4, 5, and 0.1 A·g^-1, respectively. The 333 mAh·g^-1 reversible capacity was resumed when the current density was lowered back to 0.1 A·g^-1, demonstrating that cage-1 has good reversibility as the LIBs anode material. Moreover, the electrochemical performance comparison of cage-1 with those reported complexes-based anodes for LIBs is given in Table S3. Notably, the discharge capacity was comparable to the reported complexes. The Li⁺ ions storage mechanism of cage-1 may attribute to the synergistic effects of POMs and the MTR4A ligand. On the one hand, POMs in cage-1 contain rich O sites, which can bind with Li⁺. In addition, the valence of metal W can be changed from +6 to +5 to promote the storage of Li^+[38]. On the other hand, the MTR4A ligand possesses uncoordinated N and S atoms, which can capture Li⁺ ions and participate in the insertion of Li, resulting in better electrochemical performance^{[43, 48]}.

  Additionally, POMs have been widely used in the field of electrocatalysis due to their good electron transfer and storage ability^[49-53]. Some inorganic reagents and small biological molecules, such as nitrite (NO₂^-), bromine ion (BrO₃^-), H₂O₂, and ascorbic acid (AA), can be usually chosen to be the redox probes for electro-catalytic redox. Herein, NO₂^- and AA were selected to explore the electro-catalytic behaviors of cage-1 modified carbon paste electrode (cage-1/CPE). Firstly, CV curves for cage-1/CPE were measured at 50 mV·s^-1 in an H₂SO₄-Na₂SO₄ solution. As illustrated in Fig. 3a, the midpoint potentials (E_m) for Ⅰ-Ⅰ′, Ⅱ-Ⅱ′ and Ⅲ-Ⅲ′ redox couples have been calculated using the tangential method for cage-1/CPE. The observed values of 0.03, -0.35, and -0.63 V can be attributed to the electron redox process involving central metal W in α-SiW₁₂O₄₀^4-[54]. Moreover, the E_m of cage-1/CPE at 0.44 V (Ⅳ-Ⅳ′) suggests the occurrence of an electron redox process associated with the Co(Ⅱ)/Co centers within the cage-1. Furthermore, the CV curves at different scan rates were determined in the same electrolyte. And with the increase of scan rates, the oxidation peak gradually moved towards the direction of higher potential, while the reduction peak moved in the opposite direction. Further, the scan rates were found to increase proportionally upon increasing the redox peak currents, which proves that cage-1/CPE adopts a surface-confined route (Fig. 3b)^[55].

  Figure 3

  

  Figure 3.  CV curves of cage-1/CPE in H₂SO₄-Na₂SO₄ solutions: (a) at the 50 mV·s^-1 scan rate; (b) at diverse scan rates of 50-500 mV·s^-1 (Inset: linear relationships of the anode and the cathode peak current of Ⅲ-Ⅲ' vs scan rate); (c) with different AA concentrations (Scan rate: 50 mV·s^-1; Inset: linear relationship of I_p vs c_AA); (d) with different NO₂^- concentrations (Scan rate: 50 mV·s^-1; Inset: linear relationship of I_p vs c_NO2^-)

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  The CV responses for cage-1/CPE were analyzed with NO₂^- and AA at a current density of 50 mA·g^-1 within the H₂SO₄-Na₂SO₄ aqueous solutions. As shown in Fig. 3c, oxidation peak currents for cage-1/CPE changed as AA concentration elevated (from 0 to 20 mmol·L^-1). The AA oxidation is catalyzed by electron transfer via redox couple of Co(Ⅱ)/Co. Furthermore, as shown in Fig. 3d, the reduction peak currents continued to increase with adding NaNO₂ (from 0 to 20 mmol·L^-1), demonstrating the occurrence of the reduction of NO₂^-. And the proposed redox mechanism is defined as shown in equations^[56]:

  $ \begin{array}{l} {\rm{Si}}{{\rm{W}}^{\rm{V}}}_{12 - n}{{\rm{W}}^{\rm{V}}}_n{{\rm{O}}_{40}}^{(4 + n) - } + {\rm{NO}}_2^ - + x{{\rm{H}}^ + } \to \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;{\rm{Si}}{{\rm{W}}^{{\rm{VI}}}}_{12}{{\rm{O}}_{40}}^{4 - } + {\rm{products }} \end{array} $

  $ \begin{array}{l} {\rm{Si}}{{\rm{W}}^{{\rm{VI}}}}_{12 - n}{{\rm{W}}^{\rm{V}}}_n{{\rm{O}}_{40}}^{(4 + n) - } + {\rm{NO}} + y{{\rm{H}}^ + } \to \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;{\rm{Si}}{{\rm{W}}^{{\rm{VI}}}}_{12}{{\rm{O}}_{40}}^{4 - } + {\rm{products }} \end{array} $

  Co²⁺ + C₆H₈O₆ → Co + C₆H₆O₆

  The electrocatalytic efficiency (η_CAT) of cage-1/CPE was calculated to be 77.68%, 134.04%, 209.70%, and 285.66% for AA, and η_CAT of NO₂^- was calculated to be 63.03%, 110.33%, 163.79%, and 187.58% (Fig. 4 and Table S4)^[57-58]. According to the results, cage-1/CPE shows better performance for the oxidation of AA than the reduction of NO₂^-.

  Figure 4

  

  Figure 4.  Histogram graph of ηCAT vs concentration of AA (red) and NO₂^- (blue) for cage-1/CPE

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  3.   Conclusions

  To conclude, we present a polyoxometalate- directing calix[4]resorcinarene-based giant [Co₈] coordination cage (cage-1). The cage is determined to be a nanoscale coordination cage constructed from six bowl-shaped calix[4]resorcinarene molecules, eight Co(Ⅱ) cations, two α-SiW₁₂O₄₀^4- counter anions, and eight Cl^- anions. The successful assembly of cage-1 demonstrates that the introduction of POM is conducive to constructing multifunctional coordination cages. Markedly, cage-1 features active POMs and Co(Ⅰ) sites, showing good lithium-ion storage capacity. And cage-1 could reduce NO₂^- and oxidize AA, making it a bifunctional catalyst with high activity. This work provides a feasible way to synthesize and design functionalized POM-calixarene-based coordination cages.

  
  Acknowledgments: The current work was supported by the Natural Science Foundation of Shanxi Province (Grant No.202103021223002), the Scientific and Technological Innovation Programs of the Higher Education Institutions of Shanxi (Grant No.2021L578), and the Doctoral Starting Research Foundation of Taiyuan University. Conflicts of interest: All authors claimed no competing interest.
  Supporting information is available at http://www.wjhxxb.cn
  
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  Scheme 1  Synthetic procedure for cage-1

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  Figure 1  (a) Structure of cage-1; (b) α-SiW₁₂O₄₀^4- cation with three cages from one direction by hydrogen bonding; (c) α-SiW₁₂O₄₀^4- cation with three cages from different directions by hydrogen bonding; (d) Hydrogen-bonding supramolecular structure of cage-1

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  Figure 2  Electrochemical performance of cage-1: (a) CV curves at 0.1 mV·s^-1; (b) galvanostatic discharge-charge profiles; (c) cycling performance for 1 000 cycles; (d) rate performance under diverse current densities from 0.1 to 5 A·g^-1

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  Figure 3  CV curves of cage-1/CPE in H₂SO₄-Na₂SO₄ solutions: (a) at the 50 mV·s^-1 scan rate; (b) at diverse scan rates of 50-500 mV·s^-1 (Inset: linear relationships of the anode and the cathode peak current of Ⅲ-Ⅲ' vs scan rate); (c) with different AA concentrations (Scan rate: 50 mV·s^-1; Inset: linear relationship of I_p vs c_AA); (d) with different NO₂^- concentrations (Scan rate: 50 mV·s^-1; Inset: linear relationship of I_p vs c_NO2^-)

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  Figure 4  Histogram graph of ηCAT vs concentration of AA (red) and NO₂^- (blue) for cage-1/CPE

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  Table 1.  Hydrogen-bonding parameters for cage-1

  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠D—H…A / (°)
  C49—H49#3…O61	0.095	0.259	0.330(2)	132.4
  C105—H105#1…O37	0.095	0.225	0.310(3)	149.7
  C114—H114#2…O66	0.095	0.251	0.326(3)	136.2
  C115—H11B#2…O79	0.098	0.240	0.322(2)	141.6
  C162—H162#6…O68	0.095	0.230	0.317(2)	152.2
  C163—H16C#6…O47	0.098	0.256	0.330(17)	132.2
  C214—H21B#5…O33	0.098	0.251	0.323(3)	130.3
  C280—H28C#4…O39	0.098	0.259	0.321(3)	121.2
  C336—H33G#3…O45	0.098	0.246	0.301(3)	115.1
  Symmetry codes: #1: -x+1, -y+1, z; #2: -x+1, -y+1, -z+1; #3: -x+2, -y, -z+1; #4: x, y+1, z; #5: x+1, y+1, z; #6: x+1, y+1, z+2.

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