English

• 0.   Introduction

  Polyoxometalates (POMs), a series of transition-metal oxygen clusters, have garnered significant attention due to their aesthetical structures, high electronic density, and outstanding redox properties^[1-5]. Lacunary POMs serve as highly effective inorganic multidentate ligands, facilitating the clustering of multinuclear metal-oxygen fragments. The combination of lacunary polyoxotungstates with transition-metal (TM) clusters, particularly those of the 3d-metals cluster, expands the family of POMs and endows them with a broader range of chemical properties^[6-7]. The resulting TM‑added POMs (TMAPs) have become a flourished branch of POM research^[8].

  As a special subclass of POMs, the sandwich-type POMs were first isolated in 1973 by Weakley et al. (i.e., the Weakley sandwich-type)^[9]. Subsequently, other sandwich-type POMs were synthesized, such as Herve, Krebs, and Knoth sandwich-type ones^[10-12]. The utilization of lacunary polyoxotungstates has been demonstrated as an effective approach for producing sandwich-type POMs^[13]. These fascinating structures and extensive applications of this subject have gained great interest in recent times. The design and synthesis of novel functional sandwich-type TMAPs remain a great challenge.

  Some sandwich-type cobalt-added POMs (CoAPs) have also been synthesized with distinct configurations and practical applications^[14-15]. An interesting example was reported by Sun′s group in 2016^[16], which involved a hexa-Co ring encapsulated by two di-vacant PW₁₀O₃₇ fragments and showed excellent nitrite reduction performance. In the same year, another sandwich-type CoAP, [{Co₇^ⅡAs₆^ⅢO₉(OH)₆}(A-α-SiW₉O₃₄)₂]^12-, containing a fused double-quasi-cubane Co₇O₆ core, was prepared by Wang′s group^[17]. It could be applied in visible-light-driven water oxidation.

  Most sandwich-type CoAPs are composed of silicotungstates or phosphotungstates. The investigation of germanotungstates-based counterparts has been limited due to the reduced coordination ability of germanotungstates, which is a result of the large radius of the germanium cation. As a result, only a few studies have been conducted in this area^[18]. Therefore, we always focus on the exploration of germanotungstates-based CoAPs. Herein, a novel sandwich-type CoAP was successfully synthesized via hydrothermal reaction: [Co(dien)₂]{[Co(dien)(H₂O)]₂[Co(dien)]₂[Co₆(en)₂(μ₃-OH)₂(H₂O)₆(GeW₈O₃₁)₂]}·5.5H₂O (1), where dien=diethylenetriamine and en=ethylenediamine. The Co₆ core of compound 1 is enveloped by two β-GeW₈O₃₁ units originating from the transformation of A-α-GeW₉O₃₄ units, resulting in a novel sandwich‑type polyoxoanion (Scheme 1).

  Scheme 1

  

  Scheme 1.  Mechanism of formation of {GeW₈} in this case and the conversion from starting dien to en under hydrothermal conditions

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  1.   Experimental

  1.1   Synthesis of compound 1

  All reagents were of analytical grade and purchased commercially, except the precursor K₈Na₂[A-α-GeW₉O₃₄]·25H₂O prepared by the reported method^[19]. K₈Na₂[A-α-GeW₉O₃₄]·25H₂O (0.400 g, 0.130 mmol), CoCl₂·6H₂O (0.200 g, 0.841 mmol), (NH₄)₂B₁₀O₁₆·8H₂O (0.250 g, 0.459 mmol), H₃BO₃ (0.400 g, 6.469 mmol), and CsCl (0.120 g, 0.713 mmol) were mixed into distilled water (6 mL, 333.0 mmol). After vigorously stirring for 20 min, 2 mL NaOH solution (1.5 mol·L^-1) was added into the system, then 0.05 mL dien was added after another 10 min of stirring. Finally, the resulting emulsion was sealed into a Teflon-lined autoclave and kept at 100 ℃ for 3 d. The brown block crystals were obtained (Yield: 10.12%, based on K₈Na₂[A‑α‑GeW₉O₃₄]·25H₂O). The elemental analysis was conducted on a UNICUBE elemental analyzer (Elementar Corp.). Elemental analysis calculated for C₂₈H₁₂₃N₂₂Co₁₁O_77.5Ge₂W₁₆(%): C 5.86, H 2.14, N 5.37; Found(%): C 5.99, H 2.10, N 5.35.

  1.2   Characterization

  The IR data was collected by Smart Omni-Trans-mission Spectrometer (KBr pellet, ThermoFisher Scientific Inc.) with a range of 4 000-400 cm^-1. The powder X-ray diffraction (PXRD) data was collected by a D8 Advance XRD diffractometer (Bruker Corp.) equipped a Cu Kα radiation (λ=0.154 056 nm), the operating voltage and current were 40 kV and 30mA, respectively, and the data were recorded in a 2θ range of 5° to 50°. The thermogravimetric analysis (TGA) was conducted on a Mettler‑Toledo TGA/DSC 1000 (Mettler-Toledo Measurement Instrument) under N₂ flowing with the heating rate of 10 ℃·min^-1 from 25 to 1 000 ℃.

  1.3   X-ray crystallography of compound 1

  The single-crystal X-ray diffraction data of compound 1 were collected by a Gemini A Ultra diffractometer (Rigaku Corp.) at 293(2) K with Mo Kα graphite monochromate (λ=0.071 073 nm). The absorption correction of data was performed by the SADABS program automatically. The structure solving and refining, by intrinsic phasing method and a full‑matrix least‑squares methods (based on F ²) respectively, were performed on the ShelXT program package embedded in Olex2 software^[20-21]. The disordered solvent molecules, located on voids in structure and hardly identified from the difference Fourier map, were treated by the Solvent Mask program^[22] in Olex2. To balance the charge, protons were added. The H atoms on dien and en are added by AFIX restraints. The parameters of the crystal are listed in Table 1.

  Table 1

  

  Table 1.  Crystal data and structure refinements for compound 1

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  Parameter	1		Parameter	1
  Empirical formula	C28H123N22Co11Ge2O77.5W16		Dc / (g·cm-3)	3.296
  Formula weight	5 743.35		μ / mm-1	17.967
  Crystal system	I2/a		F(000)	10 440
  Space group	Monoclinic		Limiting indices	-33 ≤ h ≤ 31, -14 ≤ k ≤ 14, -42 ≤ l ≤ 40
  a / nm	2.777 7(4)		Reflection collected, unique	41 479, 10 156 (Rint=0.159 2)
  b / nm	1.261 17(10)		Data, restraint, number of parameters	10 156, 243, 658
  c / nm	3.59 12(5)		Goodness-of-fit on F 2	0.908
  β / (°)	113.087		Final R indices [I > 2σ(I)]a	R1=0.069 8, wR2=0.132 2
  V / nm3	11.573(3)		R indices (all data) b	R1=0.148 7, wR2=0.153 4
  Z	4		Largest diff. peak and hole / (e·nm-3)	1 930 and -1 710
  ${ }^{\mathrm{a}} R_1=\sum\left\|F_{\mathrm{o}}|-| F_{\mathrm{c}}\right\| / \sum\left|F_{\mathrm{o}}\right| ;{ }^{\mathrm{b}} w R_2=\left[\sum w\left(F^2{ }_{\mathrm{o}}-F^2{ }_{\mathrm{c}}\right)^2 / \sum w\left(F^2{ }_{\mathrm{o}}\right)^2\right]^{1 / 2} .$

  1.4   Experiments of electrochemical properties

  All electrochemical experiments were performed on a workstation DH7000 (Jiangsu Donghua Analytical Instrument) with a standard three-electrode system: Ag/AgCl reference electrode with saturated KCl solution, Pt foil (10 mm×10 mm×0.1 mm) counter electrode and carbon paste working electrode modified by complex 1. The 1-modified carbon paste was prepared by manually grinding in a mortar with mixing: 1, graphite powder, and paraffin oil (m₁∶m_graphite∶m_{paraffin oil}=1∶6∶1.5). After sufficient homogenization, the carbon paste mixture was filled into the pocket of carbon paste electrodes (diameter: 2 mm) and packed firmly by a copper rod. The newly made working electrode could be used until 12 h later^[23], and the surface of the electrode was polished before the experiment. The electrolyte was prepared by mixing two solutions: 0.1 mol·L^-1 H₂SO₄ and 0.5 mol·L^-1 Na₂SO₄ (1∶2, V/V). The cyclic voltammetry (CV) test was performed at the potential range from -0.9 to 0.1 V (vs Ag/AgCl) with scan rate: 40, 80, 100, 140, 200, 300, 400, 500 mV·s^-1. Also, the CV technique was applied to explore the electrocatalytic activity of reduction of nitrite in the above-mentioned electrolyte with a scan rate of 100 mV·s^-1.

  2.   Results and discussion

  2.1   Description of crystal structure

  The single-crystal X-ray structural analysis shows that compound 1 crystallizes in the space group I2/a, and it consists of a subunit 1a: {[Co(dien)(H₂O)]₂[Co(dien)]₂[Co₆(en)₂(μ₃‑OH)₂(H₂O)₆(GeW₈O₃₁)₂]}^2- (Fig. 1a), a [Co(dien)₂]²⁺ cation and 5.5 lattice water molecules. In 1a, a sandwich-type polyoxoanion, [Co₆(en)₂(μ₃-OH)₂(H₂O)₆(GeW₈O₃₁)₂]^8-, is present, characterized by a belt-like Co₆O₁₈(en)₂ cluster covered by two pentadentate GeW₈O₃₁ units on its surface (Fig. 1b). In addition, two types of mononuclear Co²⁺ complex are captured at the outside of polyoxoanion and are linked to the {GeW₈} through Co—O=W linkages: [Co4(dien)(H₂O)]²⁺ (Fig. 1c) and [Co5(dien)]²⁺ (Fig. 1d). Using {GeW₉} precursor, only a few tetra-lacunary Keggin-type {GeW₈} was observed, and it was commonly found in the use of di-lacunary {GeW₁₀} precursor^[24-26], or one-pot synthesis between Na₂WO₄ and GeO₂^[27]. In 2014, our group observed the transformation from A-α-GeW₉ to B-α-GeW₈ during a hydrothermal reaction^[28], and the largest Zr₂₄-cluster was trapped by the mixed units of {GeW₉}, {GeW₈}, and W₂O₁₀. In addition, the degradation from dien into en was also observed under hydrothermal conditions (Scheme 1)^[29-30].

  Figure 1

  

  Figure 1.  (a) Structure of subunit 1a; (b) Co₆O₁₈(en)₂ cluster, where two [CoO₄N₂] octahedra are grafted onto classic Co₄O₁₆ cluster; (c) Coordination geometry of outside Co complexes: [Co4(dien)(H₂O)]²⁺ ([Co4O₃N₂]) and (d) [Co5(dien)]²⁺ ([Co5O₃N₃]) in 1a

  H atoms are omitted for clarity; Symmetry code: A: 1-x, 1-y, 1-z.

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  As shown in Fig. 1b, the sandwiched Co₆O₁₈(en)₂ core exhibits a central symmetric belt-like arrangement, similar to the rhombic Co₄O₁₆ cluster, whose diagonal positions are attached with two [CoO₄N₂] octahedra via μ₃-O4 and two μ₄-O18/O34 bridging atoms^{[9, 31]}. The Co2/Co2A and Co3/Co3A cations are located at the vacant sites of GeW₈O₃₁. The protonation levels of oxygen atoms on Co₆O₁₈(en)₂ are confirmed by BVS calculations^[32]: μ₃‑O4/O4A are monoprotonated oxygen atoms and terminal O3/O3A/O5/O5A/O6/O6A are water molecules.

  In 2008, a sandwich-type CoAP, [{Co(2, 2′-bpy)}₂ Co₄(H₂O)₂(α-GeW₉O₃₄)₂]^8- (2, 2′-bpy=2, 2′-bipyridine), was reported by Niu′s group (Fig. 2b)^[33]. In [{Co(2, 2′-bpy)}₂Co₄(H₂O)₂(α-GeW₉O₃₄)₂]^8-, four Co²⁺ ions in the middle position are surrounded with the O atoms from two GeW₉O₃₄ units, and two terminal Co²⁺ ions are chelated by a 2, 2′-bpy group with penta-coordinated distorted square pyramidal geometry, in which the length of axial Co—O bond reached 0.251 5 nm. In contrast, only partially coordinated O atoms of Co₆O₁₈(en)₂ originated from GeW₈O₃₁ due to their absence of a WO₆ octahedron. Therefore, Co₆O₁₈(en)₂ has more open metal sites than Co₆O₁₆(2, 2′-bpy)₂. Additionally, the terminal Co²⁺ ions of Co₆O₁₈(en)₂ are also chelated by en ligands presenting a hexa‑coordinated octahedral configuration, and the lengths of Co—O bonds in Co₆O₁₈(en)₂ range from 0.199 4 to 0.217 1 nm.

  Figure 2

  

  Figure 2.  (a) Polyoxoanion of compound 1; (b) Polyoxoanion [{Co(2, 2′-bpy)}₂Co₄(H₂O)₂(α-GeW₉O₃₄)₂]^{8- [33]}

  H atoms are omitted for clarity.

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  The pendant Co complexes all exhibit unique configurations. The Co4 of [Co4(dien)(H₂O)]²⁺ is ligated by a dien and water molecules, bonding to {GeW₈} through μ₃-O28 (Fig. 1c). The coordination geometry of [Co4O₃N₂] is a penta-coordinated trigonal bipyramidal, with the axial sites occupied by O24 water and N6, and equatorial plane by N7/N8 and O28. The Co5 of [Co5(dien)]²⁺ is connected to the {GeW₈} through three bridging μ₃-O atoms (O13/O29/O36) and stabilized by a dien molecule (Fig. 1c). The [Co5O₃N₃] exhibits a slightly distorted octahedral geometry, with O—Co—O ranging from 74.56° to 84.13° and N—Co—N angles ranging from 80.59° to 86.73°.

  2.2   IR spectrum

  The IR spectrum of compound 1 is shown in Fig.S1 (Supporting information). The peaks located in 950-600 cm^-1 are evidence of {GeW₈} in 1: 930 cm^-1 (W=O_d, terminal O atoms), 830 cm^-1 (Ge—O), 771 cm^-1 (W=O_b, terminal O atoms), 737 and 685 cm^-1 (W—O_c—W, bridging O atoms)^[34]. The peaks around 3 400 and 1 610 cm^-1 are ascribed to the stretching and bending vibrations of O—H. The presence of dien and en can be confirmed by those signals: the peaks at 2 920 and 1 456 cm^-1 indicate the presence of —CH₂— groups, and the peaks at 3 243 and 1 044 cm^-1 can be ascribed to the stretching vibrations of N—H and C—N single bonds^[35-36].

  2.3   PXRD patterns

  The simulated pattern of compound 1 was obtained from the calculation of the crystal data. As shown in Fig. 3, the experimental pattern was consistent with the simulated one. The slight difference between them may be due to the change in the preferred orientation of the powder sample during the acquisition of the experimental PXRD pattern.

  Figure 3

  

  Figure 3.  Experimental and simulated PXRD patterns of compound 1

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  2.4   TGA

  The TG curve of compound 1 (Fig. 4) showed a weight loss of 17.506% (Calcd. 17.420%) from 25-800 ℃, corresponding to the loss of five and a half lattice water molecules, eight coordinated water molecules, two hydroxy groups (The weight loss is equal to one water molecule.), six dien ligands, and two en ligands.

  Figure 4

  

  Figure 4.  TG curve of compound 1

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  2.5   Electrochemical properties

  The CV technique was used for investigating the electrochemical behavior of compound 1. As depicted in Fig. 5, two pairs of reversible redox waves and one single reduction peak were observed at the potential range from -0.9 to 0.0 V (vs Ag/AgCl), which could be ascribed to the redox of W^Ⅵ centers in β-GeW₈O₃₁ units^{[18, 25, 37-38]}. The anodic (E_pa), cathodic (E_pc) peak potentials, and the half-wave potentials (E_1/2) of Ⅰ/Ⅰ′ and Ⅱ/Ⅱ′ are listed in Table 2. The electrochemical process is diffusion-controlled, identified by the peak currents depending linearly on the square root of the scan rate in a range from 40 to 500 mV·s^-1[25].

  Figure 5

  

  Figure 5.  CV curves of compound 1

  Inset: the fitted curve of peak currents with the square root of the scan rate; The peak currents were extracted from cathodic peak Ⅲ.

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  Table 2

  

  Table 2.  E_pa, E_pc, and E_1/2 (vs Ag/AgCl) of Ⅰ/Ⅰ′ and Ⅱ/Ⅱ′ of compound 1* V

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  Peak	Epa	Epc	E1/2
  Ⅰ/Ⅰ′	-0.349	-0.404	-0.377
  Ⅱ/Ⅱ′	-0.452	-0.505	-0.479
  Ⅲ		-0.737
  * Scan rate: 100 mV•s-1.

  In addition, the application of compound 1 in electrocatalytic reduction of nitrite has been investigated. The cathodic peak currents surged by the increased NO₂^- concentration, in contrast to the anodic peak current, indicating that 1 presented a terrific performance in nitrite reduction (Fig. 6). Further, the catalytic efficiency (CAT) values of 1 were calculated via the equation: CAT=(-I_POM)/I_POM×100%^[38-39], where I_POM + NO₂^- and I_POM are the cathodic currents in the presence and absence of NO₂^-, respectively. The CAT values at different NO₂^- concentrations of 2, 4, 8, and 12 mmol·L^-1 are plotted in Fig. 7.

  Figure 6

  

  Figure 6.  Cathodic current change for compound 1 caused by increased NO₂^- concentration

  Scan rate: 100 mV•s^-1.

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  Figure 7

  

  Figure 7.  CAT values of compound 1 under different NO₂^- concentrations

  The data were calculated from the cathodic currents at-0.9 V (vs Ag/AgCl).

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  3.   Conclusions

  In summary, a novel sandwich-type CoAP (1) was synthesized under hydrothermal conditions, featuring a unique belt-like Co₆ cluster encapsulated by two β-GeW₈O₃₁ units derived from the degradation of starting precursor A-α-GeW₉O₃₄. The CV analysis of 1 was conducted, showing the diffusion-controlled redox process. Moreover, 1 has a terrific electrocatalytic nitrite reduction performance. The research focused on the synthesis and properties of novel CoAPs is still ongoing in our lab.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Scheme 1  Mechanism of formation of {GeW₈} in this case and the conversion from starting dien to en under hydrothermal conditions

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  Figure 1  (a) Structure of subunit 1a; (b) Co₆O₁₈(en)₂ cluster, where two [CoO₄N₂] octahedra are grafted onto classic Co₄O₁₆ cluster; (c) Coordination geometry of outside Co complexes: [Co4(dien)(H₂O)]²⁺ ([Co4O₃N₂]) and (d) [Co5(dien)]²⁺ ([Co5O₃N₃]) in 1a

  H atoms are omitted for clarity; Symmetry code: A: 1-x, 1-y, 1-z.

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  Figure 2  (a) Polyoxoanion of compound 1; (b) Polyoxoanion [{Co(2, 2′-bpy)}₂Co₄(H₂O)₂(α-GeW₉O₃₄)₂]^{8- [33]}

  H atoms are omitted for clarity.

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  Figure 3  Experimental and simulated PXRD patterns of compound 1

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  Figure 4  TG curve of compound 1

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  Figure 5  CV curves of compound 1

  Inset: the fitted curve of peak currents with the square root of the scan rate; The peak currents were extracted from cathodic peak Ⅲ.

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  Figure 6  Cathodic current change for compound 1 caused by increased NO₂^- concentration

  Scan rate: 100 mV•s^-1.

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  Figure 7  CAT values of compound 1 under different NO₂^- concentrations

  The data were calculated from the cathodic currents at-0.9 V (vs Ag/AgCl).

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  Table 1.  Crystal data and structure refinements for compound 1

  Parameter	1		Parameter	1
  Empirical formula	C28H123N22Co11Ge2O77.5W16		Dc / (g·cm-3)	3.296
  Formula weight	5 743.35		μ / mm-1	17.967
  Crystal system	I2/a		F(000)	10 440
  Space group	Monoclinic		Limiting indices	-33 ≤ h ≤ 31, -14 ≤ k ≤ 14, -42 ≤ l ≤ 40
  a / nm	2.777 7(4)		Reflection collected, unique	41 479, 10 156 (Rint=0.159 2)
  b / nm	1.261 17(10)		Data, restraint, number of parameters	10 156, 243, 658
  c / nm	3.59 12(5)		Goodness-of-fit on F 2	0.908
  β / (°)	113.087		Final R indices [I > 2σ(I)]a	R1=0.069 8, wR2=0.132 2
  V / nm3	11.573(3)		R indices (all data) b	R1=0.148 7, wR2=0.153 4
  Z	4		Largest diff. peak and hole / (e·nm-3)	1 930 and -1 710
  ${ }^{\mathrm{a}} R_1=\sum\left\|F_{\mathrm{o}}|-| F_{\mathrm{c}}\right\| / \sum\left|F_{\mathrm{o}}\right| ;{ }^{\mathrm{b}} w R_2=\left[\sum w\left(F^2{ }_{\mathrm{o}}-F^2{ }_{\mathrm{c}}\right)^2 / \sum w\left(F^2{ }_{\mathrm{o}}\right)^2\right]^{1 / 2} .$

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  Table 2.  E_pa, E_pc, and E_1/2 (vs Ag/AgCl) of Ⅰ/Ⅰ′ and Ⅱ/Ⅱ′ of compound 1* V

  Peak	Epa	Epc	E1/2
  Ⅰ/Ⅰ′	-0.349	-0.404	-0.377
  Ⅱ/Ⅱ′	-0.452	-0.505	-0.479
  Ⅲ		-0.737
  * Scan rate: 100 mV•s-1.

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