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3D organic-inorganic hybrid framework built upon [β-Mo8O26]4− units and polymeric copper(II) complexes with magnetic properties and electrocatalytic activities for H2O2 reduction
Inorganic Chemistry Communications 104 (2019) 160–164
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
3D organic-inorganic hybrid framework built upon [β-Mo8O26]4− units and polymeric copper(II) complexes with magnetic properties and electrocatalytic activities for H2O2 reduction
T
Qiao Gaoa, , Lin Xub, Donghua Hua, Dehui Lia, Jing Yanga ⁎
a b
School of Pharmaceutical Sciences, Changchun University of Chinese Medicine, Changchun 130117, PR China Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China
GRAPHICAL ABSTRACT
A new 3D organic-inorganic hybrid complex bulit upon the [β-Mo8O26]4− units and 1D {Cu2(C2H6O2)4Cl}n chains connected by K+ cations has been synthesized to find its magnetic properties and electrocatalytic activities for the reduction of H2O2.
ARTICLE INFO
ABSTRACT
Keywords: Polyoxometalates Organic-inorganic hybrid Electrochemistry Magnetic properties
A new 3D organic-inorganic hybrid complex, H2K2[HN(CH2CH2OH)3]2[Cu2(C2H6O2)4Cl][Mo8O26]·17H2O (1) has been synthesized and structurally characterized. In compound 1, the [β-Mo8O26]4− units and 1D {Cu2(C2H6O2)4Cl}n chains are joined together via K+ ions, leading to the 3D porous framework. The magnetic measurements of 1 indicates the presence of antiferromagnetic interactions between copper(II) ions. Electrochemical experiments reveal that 1-CPE exhibits the characteristic multi-electron redox processes ascribed to MoVI centers, and shows good electrocatalytic activity for H2O2 reduction.
Polyoxometalates (POMs), as a fascinating class of metal‑oxygen cluster compounds with unique structural diversity, have been widely used in catalysis, magnetism, medicine and materials science [1–3]. An intriguing branch in POM chemistry is the modification of suitable POM building blocks with organic components or metal-organic coordination complexes for creating the novel organic-inorganic hybrids with useful functional properties [4–6]. Among the various types of POMs, the
⁎
octamolybdates is a class of ideal inorganic building blocks for the synthesis of organic-inorganic hybrid materials, owing to their various isomeric forms (α, β, γ, δ, ε, ζ, η, and θ isomers) and versatile coordination modes arised from the oxygen-enriched surfaces [7–9]. Meanwhile, the octamolybdates can be easily prepared from Na2MoO4·2H2O under conventional conditions [10]. In addition, the suitable organic ligands also play an important role
Corresponding author. E-mail address: [email protected] (Q. Gao).
https://doi.org/10.1016/j.inoche.2019.04.015 Received 14 March 2019; Received in revised form 7 April 2019; Accepted 9 April 2019 Available online 10 April 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 104 (2019) 160–164
Q. Gao, et al.
hydrogen bonded interactions (OeH⋯OPOM: 2.795(8)-2.978(11)Å) with the surface oxygen atoms of POM fragments (Fig. S11). Obviously, such classical hydrogen bonds play the vital role in stabilizing the supramolecular fabrication of 1 [26]. The phase purity of 1 was confirmed by the agreement between the experimental powder X-ray diffraction pattern and the simulated pattern based on the structure analysis (Fig. S1). The IR spectra of 1 exhibit five intense bands at 895, 792, 625, 516 and 451 cm−1, characterizing respectively the vibrations of ν(Mo-Ot), ν(Mo-Ob-Mo) and ν(Mo-Oc-Mo) in the [β-Mo8O26]4− fragments [27]. The bands at 1629-1090 cm−1 are attributed to characteristic vibrations of TEA and Ac groups [28]. The strong and broad bands at 3376–3197 cm−1 are ascribed to the stretching vibrations of υ(OeH) from the crystalline water molecules [29] (Fig. S2). The thermal gravimetric curve of 1 exhibits three weight loss steps in the temperature range 30–600 °C (Fig. S3). The first weight loss of 9.2% in the temperature range 30–112 °C corresponds to the release of seventeen crystalline water molecules. The second weight loss of 16.2% (17.1% calculated), which occurs from 112 to 415 °C, is attributed to the loss of two triethanolamine molecules and four Ac ligands. The last stage is the decomposition of the [β-Mo8O26]4− polyoxoanions, and the observed total weight loss of 29.2% can compare with the calculated value of 28.7%. The UV electronic spectra of 1 in the solution were also taken as shown in Fig. S4. Two absorption bands at ca. 206 and 234 nm were observed. The higher energy spectral absorption should be attributed to the charge transfer transitions of the Od → Mo band, while the lower energy absorption can be assigned to those of the Ob,c → Mo band [30]. Moreover, in the visible range from 400 nm to 1000 nm, a broad absorption band at ca. 796 nm could be observed, which is characteristic of B1g(dx2-y2) → Eg(dxz,yz) transition from Cu(II) cluster [31]. In addition, the UV–Vis diffuse reflectance spectra of 1 was measured to reveal achieve their band gap (Eg) from the Kubelka-Munk function F against E [32]. As shown in Fig. S5, the corresponding well defined optical absorption associated with Eg can be assessed at ca. 2.75 eV for 1, which reveals that compound 1 may have the potential as a wide band gap semiconductor. The plots of χMT versus T and χM versus T of 1 measured from 2 to 300 K in an applied magnetic field of 1000 Oe are shown in Fig. 4. The experimental χMT value of 1 at room temperature is 0.71 cm3·K·mol−1, which is consistent with the expected value 0.75 cm3·K·mol−1 for two isolated Cu2+ ions (S = 1/2, g = 2.00). The χMT values gradually decrease with decreasing temperature, and reach a minimum value of 0.06 cm3·K·mol−1 at 2 K, indicating a typical antiferromagnetic interaction. The 1/χM versus T plot for 1 obeys the Curie-Weiss law [χM = C/(T - θ)] from 2 to 300 K with C = 0.96 cm3·K·mol−1 and θ = −12.08 K. The negative Weiss constant suggests that the antiferromagnetism exists in compoud 1 (Fig. S6). Cyclic voltammetry (CV) experiments were performed to examine the redox properties of compound 1 modified carbon paste electrode (1-
in the formation of inorganic-organic hybrid materials. The acetate (Ac) captures our attention based on the following reasons: (i) The Ac ligand possesses flexible coordination sites and strong coordination ability with transition metal (TM) ions [11–13]. (ii) Introducing the Ac into the reaction system, it can form the HAc/Ac buffer solution, which can stabilize the aqueous solution in a certain pH range. (iii) The Ac ligand can also act as bridging group to mediate the magnetic interaction between TM ions [14,15]. Furthermore, POM-based hybrids incorporating copper(II) ions often exhibit controllable structures and attractive catalytic properties, derived from the synergistic effect between the copper(II) coordination complexes and POMs [16–18]. Moreover, after consulting the previous literatures, we know that the organic-inorganic hybrids constructed from octamolybdate and copper (II)-Ac‑chlorine complexes still remains virtually unexplored. Therefore, inspired by those clues, we attempt to explore the octamolybdate/copper(II)/Ac system in aqueous solution by one-pot reaction. Fortunately, we successfully isolated a new organic-inorganic hybrid complex H2K2[HN(CH2CH2OH)3]2[Cu2(C2H6O2)4Cl][Mo8O26]·17H2O (1), by facile one-pot self-assembly reactions under mild conditions. Compound 1 appears in a 3D porous framework constructed from 1D {Cu2(C2H6O2)4Cl}n chains and [β-Mo8O26]4− units linked together via K+ bridges. The magnetic investigation for 1 reveals the occurrence of antiferromagnetic interactions among the CuII centers. In addition, the 1-CPE exhibits excellent electrocatalytic performances for the reduction of H2O2. Single-crystal X-ray diffraction investigation reveals that compound 1 crystallizes in the orthorhombic space group Imma and is composed of the [β-Mo8O26]4− polyanions, [Cu2(C2H6O2)4Cl]− units, [HN (CH2CH2OH)3]+ cations and lattice water molecules [19–23] (Fig. S10). The bond valence sum (BVS) calculations for 1 confirmed the oxidation states of the Mo and Cu atoms are +6 and + 2, respectively [24]. The [β-Mo8O26]4− polyanion is constructed by eight {MoO6} octahedra connected with each other via edge-sharing oxygen atoms (Fig. 1a). Each MoVI atom in the {Mo8O26} cluster has a distorted octahedral geometry with an MoeO bond length in the range of 1.699(2)2.312(3) Å for terminal and bridging oxygen atoms, which are in consistent with that of the previously reported {Mo8O26} cluster [25]. In the crystal structure of 1, the neighboring {Cu2(C2H6O2)4} clusters connect alternately via Cl− bridges to form one-dimensional (1D) chain structure (Fig. 2b). Each Cu2+ ion in {Cu2(C2H6O2)4Cl} unit is five-coordinated with one chloride atom (Cu-Cl: 2.495(7) Å) and four O atoms from Ac ligands (Cu-O: 1.971(9)-1.978(7) Å) (Fig. 2c). In addition, the [β-Mo8O26]4− units and 1D {Cu2(C2H6O2)4Cl}n chains are connected together via K+ cations to from a 3D open framework containing a 1D channel with a dimension of ca. 14.06 × 5.79 Å2 (Fig. 3). The K+ ion links to four terminal oxygen atoms from [β-Mo8O26]4− units (K-O: 2.727(3)-2.950(5) Å), and four O atoms from Ac groups (KO: 2.879(4)-3.163(7) Å) to finish its eight-coordinated environment (Fig. 1b). Furthermore, protonated triethanolamine (TEA) cations and crystalline water molecules are located in the channels and form multipoint
Fig. 1. (a) Ball-and-stick representation of the [βMo8O26]4− unit in 1. (b) The coordination modes of the [β-Mo8O26]4− with K+ and [HN(CH2CH2OH)3]+ cations, together with {Cu2(C2H6O2)4Cl} units. Colour scheme: Mo-blue, Cu-green, K-pink, C-black, N-violet, O-red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. (a) Mixed polyhedral and ball-and-stick representation of the 2D layered framework in 1. The protonated TEA cations and crystalline water molecules have been omitted for clarity. (b) Ball-and-stick representation of 1D {Cu2(C2H6O2)4Cl}n chains. (c) Polyhedral representation of the coordination geometry of the {Cu2(C2H6O2)4Cl} unit.
CPE) in 1 mol/L H2SO4 solution at a scan rate of 30 mV s−1. As depicted in Fig. S7, two pairs of reversible redox peaks (I-I′ and II-II′) are detected with E1/2 = (Epa + Epc)/2 of −0.43 V (I-I′) and − 0.24 V (II-II′), which can be ascribed to two consecutive two-electron redox processes of Mo(VI) centers in compound 1 [33,34]. The two reduction waves (III’) and their oxidation counterpart (III), which are observed at −0.05, −0.15 and + 0.04 V, can be ascribed to the redox processes of the Cu2+ centers. This behavior is in agreement with the observations in other Cu(II)-containing POMs [35]. In addition, the stability of 1-CPE in 1 mol/L H2SO4 solution is also assessed by monitoring CV curves. As shown in Fig. S8, the peak currents remain almost unchanged after 24 h, which indicated the long-term stability of 1-CPE. Furthermore, the XRD data of 1 before and after electrocatalytic reaction have little change, indicating no structural change of 1 in the redox process (Fig. S9). Fig. 5a exhibits the cyclic voltammograms of 1-CPE at different scan rates from 30 to 130 mV·s−1. With the increase of scan rates, the peak currents increases gradually and the potentials of cathodic and anodic peaks shifted slightly. As shown in Fig. 5b, the peak currents of the I-I′ shows a linear dependence on the scan rates ranged from 30 to 130 mV·s−1, indicating that the redox process of 1-CPE is surface-controlled.
Fig. 4. χMT versus T curves of 1.
Fig. 3. (a) Mixed polyhedral and wires-stick of the 3D framework of 1 along the a axis. (b) Wires-stick diagram of the 3D structure of 1. The protonated TEA cations and crystalline water molecules in the channels are omitted for clarity. Colour codes: Moblue, Cu-green, K-pink, C-black, N-violet, O-red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. (a) Cyclic voltammograms of the 1-CPE in 1 mol/L H2SO4 solution under scan rates from inner to outer: (scan rates: 30, 50, 70, 90, 110, 130 mV·s−1). (b) The dependence of the I-I′ peak currents on the scan rates of 1-CPE.
using a one-pot self-assembly method. Compound 1 exhibits a 3D framework constructed from the [β-Mo8O26]4− units and 1D {Cu2(C2H6O2)4Cl}n chains joined together via K+ ions. Magnetic studies indicate the presence of antiferromagnetic interactions related to the Cu(II) cluster, Furthermore, The electrocatalytic experiments indicate that 1-CPE exhibits the characteristic multi-electron redox processes arised from MoVI centers, and has good electrocatalytic activities for the reduction of H2O2. These results indicate that the titile complex may have the promising applications in electrochemical sensing of H2O2. In further work, our interest will be focus on exploring the photocatalytic properties of 1. Acknowledgment This work was supported by the Thirteen Five-Year Science and Technology Research Project of the Education Department of Jilin Province (Grant number JJKH20181234KJ). Appendix A. Supplementary material
Fig. 6. Reduction of H2O2 at the 1-CPE in 1 mol/L H2SO4 solution containing H2O2 in various concentrations: (a) 0 mmol/L, (b) 0.1 mmol/L, (c) 0.2 mmol/L, (d) 0.3 mmol/L, (e) 0.4 mmol/L, Scan rate: 50 mV s−1.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.04.015.
POMs is a promising kind of electrocatalysts, which can undergo a series of continuous and reversible multi-electron redox processes without changing their initial structures [36]. Herein, we further studied the electrocatalytic properties of 1-CPE towards the reduction of H2O2. As given in Fig. 6, with the addition of a modest amount of H2O2, the reduction peak currents of I-I′ and II-II′ increase and the corresponding oxidation peak currents decrease, while the redox peak current of III-III′ remains almost unchanged. These results suggest that H2O2 was reduced by the two- and four-electron reduced species of 1. The electrocatalytic efficiency (CAT) of 1-CPE for H2O2 reduction can be calculated by using the following CAT formula [37]:
CAT = 100% × [Ip (POM, substrate)
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Ip (POM)]/ Ip (POM)
where Ip(POM) and Ip(POM, substrate) are the peak currents of 1-CPE in the absence and presence of the H2O2 substrate, respectively. According to the current intensities of I′ and II′ waves in the presence of 0.1 mM H2O2, the CAT values are ca. 482% and 40%. The results demonstrated the catalytic activity was enhanced with the reduced degree of polyoxoanions, and the four-electron reduced species exhibited the best electrocatalytic activity. Based on the results from the previous literatures [38–40], the relevant mechanism of H2O2 reduction at 1-CPE could be expressed (see the Eqs. (1)–(4) in ESI). In conclusion, a new 3D organic-inorganic hybrid H2K2[HN (CH2CH2OH)3]2[Cu2(C2H6O2)4Cl][Mo8O26]·17H2O was obtained by
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Inorganic Chemistry Communications 104 (2019) 160–164
Q. Gao, et al. 276 mmol) and Na2MoO4.2H2O (0.6g, 2.48mmol) were successively dissolved in 0. 4 mol/L triethanolamine solution (10 mL), and then the pH was adjusted to 1.0 by 37% hydrochloric acid. The resulting solution was heated at 60°C for 2h. Then solid CuCl2·2H2O (0.094g, 0.552mmol) was added to the hot solution and the pH was adjusted to 3.5 by adding CH3COOK solution (3mL glacial acetic acid and 15g CH3COOK were dissolved in 15mL H2O). The mixture was filtered and allowed to cool to ambient temperature. The blue crystals were isolated after two weeks (Yield 0.127 g, 10% based on Mo). Elemental analysis (%) calc (found): C 7.62 (7.18), N 0. 89 (0.76), H 2.94 (2.82), Cl 1.12(1.02), K 1.24(1.04), Mo 24.33 (23.86), Cu 4.03 (3. 91). IR (2% KBr pellet, ν/cm−1): 895(s), 792(s), 625(s), 516(m) and 451(m). [20] Crystal data for 1: H92N2C20KClMo8Cu2O57, Mr = 2281.18, Orthorhombic, Imma, a = 14.617(5)Å, b = 120.662(5)Å, c = 21.639(5)Å, α = 90°, β = 90°, γ = 90°, V = 6535(3)Å3, Z = 1, Dc = 2.318g/cm3, F(000) = 4524.0, R1[I > 2σ(I)] = 0.0628, wR2[all data] = 0.1985, GOF = 1.070. Crystal data of 1 were measured on a Bruker Apex CCD diffractometer using Mo-Kα radiation (λ = 0.71073 Å) at 293(2) K [21]. The crystal structure of 1 was solved by the direct method and refined by the fullmatrix least squares on F2 using the SHELXTL-2014 crystallographic software package [22]. Empirical absorption corrections were applied [23]. Non-hydrogen atoms were easily found from the Fourier difference maps and refined anisotropically. The hydrogen atoms attached to the triethanolamine and Ac ligands were not located and were included in the final molecular formula. The selected bond lengths and angles for 1 are given in Tables S2. CCDC-1900777 for 1. [21] CrysAlisCCD and CrysAlisRED, Oxford Diffraction Ltd, Abingdon, UK, 2010. [22] G.M. Sheldrick, SHELXTL, a Software for Empirical Absorption Correction, Bruker
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
3D organic-inorganic hybrid framework built upon [β-Mo8O26]4− units and polymeric copper(II) complexes with magnetic properties and electrocatalytic activities for H2O2 reduction
T
Qiao Gaoa, , Lin Xub, Donghua Hua, Dehui Lia, Jing Yanga ⁎
a b
School of Pharmaceutical Sciences, Changchun University of Chinese Medicine, Changchun 130117, PR China Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China
GRAPHICAL ABSTRACT
A new 3D organic-inorganic hybrid complex bulit upon the [β-Mo8O26]4− units and 1D {Cu2(C2H6O2)4Cl}n chains connected by K+ cations has been synthesized to find its magnetic properties and electrocatalytic activities for the reduction of H2O2.
ARTICLE INFO
ABSTRACT
Keywords: Polyoxometalates Organic-inorganic hybrid Electrochemistry Magnetic properties
A new 3D organic-inorganic hybrid complex, H2K2[HN(CH2CH2OH)3]2[Cu2(C2H6O2)4Cl][Mo8O26]·17H2O (1) has been synthesized and structurally characterized. In compound 1, the [β-Mo8O26]4− units and 1D {Cu2(C2H6O2)4Cl}n chains are joined together via K+ ions, leading to the 3D porous framework. The magnetic measurements of 1 indicates the presence of antiferromagnetic interactions between copper(II) ions. Electrochemical experiments reveal that 1-CPE exhibits the characteristic multi-electron redox processes ascribed to MoVI centers, and shows good electrocatalytic activity for H2O2 reduction.
Polyoxometalates (POMs), as a fascinating class of metal‑oxygen cluster compounds with unique structural diversity, have been widely used in catalysis, magnetism, medicine and materials science [1–3]. An intriguing branch in POM chemistry is the modification of suitable POM building blocks with organic components or metal-organic coordination complexes for creating the novel organic-inorganic hybrids with useful functional properties [4–6]. Among the various types of POMs, the
⁎
octamolybdates is a class of ideal inorganic building blocks for the synthesis of organic-inorganic hybrid materials, owing to their various isomeric forms (α, β, γ, δ, ε, ζ, η, and θ isomers) and versatile coordination modes arised from the oxygen-enriched surfaces [7–9]. Meanwhile, the octamolybdates can be easily prepared from Na2MoO4·2H2O under conventional conditions [10]. In addition, the suitable organic ligands also play an important role
Corresponding author. E-mail address: [email protected] (Q. Gao).
https://doi.org/10.1016/j.inoche.2019.04.015 Received 14 March 2019; Received in revised form 7 April 2019; Accepted 9 April 2019 Available online 10 April 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 104 (2019) 160–164
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hydrogen bonded interactions (OeH⋯OPOM: 2.795(8)-2.978(11)Å) with the surface oxygen atoms of POM fragments (Fig. S11). Obviously, such classical hydrogen bonds play the vital role in stabilizing the supramolecular fabrication of 1 [26]. The phase purity of 1 was confirmed by the agreement between the experimental powder X-ray diffraction pattern and the simulated pattern based on the structure analysis (Fig. S1). The IR spectra of 1 exhibit five intense bands at 895, 792, 625, 516 and 451 cm−1, characterizing respectively the vibrations of ν(Mo-Ot), ν(Mo-Ob-Mo) and ν(Mo-Oc-Mo) in the [β-Mo8O26]4− fragments [27]. The bands at 1629-1090 cm−1 are attributed to characteristic vibrations of TEA and Ac groups [28]. The strong and broad bands at 3376–3197 cm−1 are ascribed to the stretching vibrations of υ(OeH) from the crystalline water molecules [29] (Fig. S2). The thermal gravimetric curve of 1 exhibits three weight loss steps in the temperature range 30–600 °C (Fig. S3). The first weight loss of 9.2% in the temperature range 30–112 °C corresponds to the release of seventeen crystalline water molecules. The second weight loss of 16.2% (17.1% calculated), which occurs from 112 to 415 °C, is attributed to the loss of two triethanolamine molecules and four Ac ligands. The last stage is the decomposition of the [β-Mo8O26]4− polyoxoanions, and the observed total weight loss of 29.2% can compare with the calculated value of 28.7%. The UV electronic spectra of 1 in the solution were also taken as shown in Fig. S4. Two absorption bands at ca. 206 and 234 nm were observed. The higher energy spectral absorption should be attributed to the charge transfer transitions of the Od → Mo band, while the lower energy absorption can be assigned to those of the Ob,c → Mo band [30]. Moreover, in the visible range from 400 nm to 1000 nm, a broad absorption band at ca. 796 nm could be observed, which is characteristic of B1g(dx2-y2) → Eg(dxz,yz) transition from Cu(II) cluster [31]. In addition, the UV–Vis diffuse reflectance spectra of 1 was measured to reveal achieve their band gap (Eg) from the Kubelka-Munk function F against E [32]. As shown in Fig. S5, the corresponding well defined optical absorption associated with Eg can be assessed at ca. 2.75 eV for 1, which reveals that compound 1 may have the potential as a wide band gap semiconductor. The plots of χMT versus T and χM versus T of 1 measured from 2 to 300 K in an applied magnetic field of 1000 Oe are shown in Fig. 4. The experimental χMT value of 1 at room temperature is 0.71 cm3·K·mol−1, which is consistent with the expected value 0.75 cm3·K·mol−1 for two isolated Cu2+ ions (S = 1/2, g = 2.00). The χMT values gradually decrease with decreasing temperature, and reach a minimum value of 0.06 cm3·K·mol−1 at 2 K, indicating a typical antiferromagnetic interaction. The 1/χM versus T plot for 1 obeys the Curie-Weiss law [χM = C/(T - θ)] from 2 to 300 K with C = 0.96 cm3·K·mol−1 and θ = −12.08 K. The negative Weiss constant suggests that the antiferromagnetism exists in compoud 1 (Fig. S6). Cyclic voltammetry (CV) experiments were performed to examine the redox properties of compound 1 modified carbon paste electrode (1-
in the formation of inorganic-organic hybrid materials. The acetate (Ac) captures our attention based on the following reasons: (i) The Ac ligand possesses flexible coordination sites and strong coordination ability with transition metal (TM) ions [11–13]. (ii) Introducing the Ac into the reaction system, it can form the HAc/Ac buffer solution, which can stabilize the aqueous solution in a certain pH range. (iii) The Ac ligand can also act as bridging group to mediate the magnetic interaction between TM ions [14,15]. Furthermore, POM-based hybrids incorporating copper(II) ions often exhibit controllable structures and attractive catalytic properties, derived from the synergistic effect between the copper(II) coordination complexes and POMs [16–18]. Moreover, after consulting the previous literatures, we know that the organic-inorganic hybrids constructed from octamolybdate and copper (II)-Ac‑chlorine complexes still remains virtually unexplored. Therefore, inspired by those clues, we attempt to explore the octamolybdate/copper(II)/Ac system in aqueous solution by one-pot reaction. Fortunately, we successfully isolated a new organic-inorganic hybrid complex H2K2[HN(CH2CH2OH)3]2[Cu2(C2H6O2)4Cl][Mo8O26]·17H2O (1), by facile one-pot self-assembly reactions under mild conditions. Compound 1 appears in a 3D porous framework constructed from 1D {Cu2(C2H6O2)4Cl}n chains and [β-Mo8O26]4− units linked together via K+ bridges. The magnetic investigation for 1 reveals the occurrence of antiferromagnetic interactions among the CuII centers. In addition, the 1-CPE exhibits excellent electrocatalytic performances for the reduction of H2O2. Single-crystal X-ray diffraction investigation reveals that compound 1 crystallizes in the orthorhombic space group Imma and is composed of the [β-Mo8O26]4− polyanions, [Cu2(C2H6O2)4Cl]− units, [HN (CH2CH2OH)3]+ cations and lattice water molecules [19–23] (Fig. S10). The bond valence sum (BVS) calculations for 1 confirmed the oxidation states of the Mo and Cu atoms are +6 and + 2, respectively [24]. The [β-Mo8O26]4− polyanion is constructed by eight {MoO6} octahedra connected with each other via edge-sharing oxygen atoms (Fig. 1a). Each MoVI atom in the {Mo8O26} cluster has a distorted octahedral geometry with an MoeO bond length in the range of 1.699(2)2.312(3) Å for terminal and bridging oxygen atoms, which are in consistent with that of the previously reported {Mo8O26} cluster [25]. In the crystal structure of 1, the neighboring {Cu2(C2H6O2)4} clusters connect alternately via Cl− bridges to form one-dimensional (1D) chain structure (Fig. 2b). Each Cu2+ ion in {Cu2(C2H6O2)4Cl} unit is five-coordinated with one chloride atom (Cu-Cl: 2.495(7) Å) and four O atoms from Ac ligands (Cu-O: 1.971(9)-1.978(7) Å) (Fig. 2c). In addition, the [β-Mo8O26]4− units and 1D {Cu2(C2H6O2)4Cl}n chains are connected together via K+ cations to from a 3D open framework containing a 1D channel with a dimension of ca. 14.06 × 5.79 Å2 (Fig. 3). The K+ ion links to four terminal oxygen atoms from [β-Mo8O26]4− units (K-O: 2.727(3)-2.950(5) Å), and four O atoms from Ac groups (KO: 2.879(4)-3.163(7) Å) to finish its eight-coordinated environment (Fig. 1b). Furthermore, protonated triethanolamine (TEA) cations and crystalline water molecules are located in the channels and form multipoint
Fig. 1. (a) Ball-and-stick representation of the [βMo8O26]4− unit in 1. (b) The coordination modes of the [β-Mo8O26]4− with K+ and [HN(CH2CH2OH)3]+ cations, together with {Cu2(C2H6O2)4Cl} units. Colour scheme: Mo-blue, Cu-green, K-pink, C-black, N-violet, O-red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. (a) Mixed polyhedral and ball-and-stick representation of the 2D layered framework in 1. The protonated TEA cations and crystalline water molecules have been omitted for clarity. (b) Ball-and-stick representation of 1D {Cu2(C2H6O2)4Cl}n chains. (c) Polyhedral representation of the coordination geometry of the {Cu2(C2H6O2)4Cl} unit.
CPE) in 1 mol/L H2SO4 solution at a scan rate of 30 mV s−1. As depicted in Fig. S7, two pairs of reversible redox peaks (I-I′ and II-II′) are detected with E1/2 = (Epa + Epc)/2 of −0.43 V (I-I′) and − 0.24 V (II-II′), which can be ascribed to two consecutive two-electron redox processes of Mo(VI) centers in compound 1 [33,34]. The two reduction waves (III’) and their oxidation counterpart (III), which are observed at −0.05, −0.15 and + 0.04 V, can be ascribed to the redox processes of the Cu2+ centers. This behavior is in agreement with the observations in other Cu(II)-containing POMs [35]. In addition, the stability of 1-CPE in 1 mol/L H2SO4 solution is also assessed by monitoring CV curves. As shown in Fig. S8, the peak currents remain almost unchanged after 24 h, which indicated the long-term stability of 1-CPE. Furthermore, the XRD data of 1 before and after electrocatalytic reaction have little change, indicating no structural change of 1 in the redox process (Fig. S9). Fig. 5a exhibits the cyclic voltammograms of 1-CPE at different scan rates from 30 to 130 mV·s−1. With the increase of scan rates, the peak currents increases gradually and the potentials of cathodic and anodic peaks shifted slightly. As shown in Fig. 5b, the peak currents of the I-I′ shows a linear dependence on the scan rates ranged from 30 to 130 mV·s−1, indicating that the redox process of 1-CPE is surface-controlled.
Fig. 4. χMT versus T curves of 1.
Fig. 3. (a) Mixed polyhedral and wires-stick of the 3D framework of 1 along the a axis. (b) Wires-stick diagram of the 3D structure of 1. The protonated TEA cations and crystalline water molecules in the channels are omitted for clarity. Colour codes: Moblue, Cu-green, K-pink, C-black, N-violet, O-red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. (a) Cyclic voltammograms of the 1-CPE in 1 mol/L H2SO4 solution under scan rates from inner to outer: (scan rates: 30, 50, 70, 90, 110, 130 mV·s−1). (b) The dependence of the I-I′ peak currents on the scan rates of 1-CPE.
using a one-pot self-assembly method. Compound 1 exhibits a 3D framework constructed from the [β-Mo8O26]4− units and 1D {Cu2(C2H6O2)4Cl}n chains joined together via K+ ions. Magnetic studies indicate the presence of antiferromagnetic interactions related to the Cu(II) cluster, Furthermore, The electrocatalytic experiments indicate that 1-CPE exhibits the characteristic multi-electron redox processes arised from MoVI centers, and has good electrocatalytic activities for the reduction of H2O2. These results indicate that the titile complex may have the promising applications in electrochemical sensing of H2O2. In further work, our interest will be focus on exploring the photocatalytic properties of 1. Acknowledgment This work was supported by the Thirteen Five-Year Science and Technology Research Project of the Education Department of Jilin Province (Grant number JJKH20181234KJ). Appendix A. Supplementary material
Fig. 6. Reduction of H2O2 at the 1-CPE in 1 mol/L H2SO4 solution containing H2O2 in various concentrations: (a) 0 mmol/L, (b) 0.1 mmol/L, (c) 0.2 mmol/L, (d) 0.3 mmol/L, (e) 0.4 mmol/L, Scan rate: 50 mV s−1.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.04.015.
POMs is a promising kind of electrocatalysts, which can undergo a series of continuous and reversible multi-electron redox processes without changing their initial structures [36]. Herein, we further studied the electrocatalytic properties of 1-CPE towards the reduction of H2O2. As given in Fig. 6, with the addition of a modest amount of H2O2, the reduction peak currents of I-I′ and II-II′ increase and the corresponding oxidation peak currents decrease, while the redox peak current of III-III′ remains almost unchanged. These results suggest that H2O2 was reduced by the two- and four-electron reduced species of 1. The electrocatalytic efficiency (CAT) of 1-CPE for H2O2 reduction can be calculated by using the following CAT formula [37]:
CAT = 100% × [Ip (POM, substrate)
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Ip (POM)]/ Ip (POM)
where Ip(POM) and Ip(POM, substrate) are the peak currents of 1-CPE in the absence and presence of the H2O2 substrate, respectively. According to the current intensities of I′ and II′ waves in the presence of 0.1 mM H2O2, the CAT values are ca. 482% and 40%. The results demonstrated the catalytic activity was enhanced with the reduced degree of polyoxoanions, and the four-electron reduced species exhibited the best electrocatalytic activity. Based on the results from the previous literatures [38–40], the relevant mechanism of H2O2 reduction at 1-CPE could be expressed (see the Eqs. (1)–(4) in ESI). In conclusion, a new 3D organic-inorganic hybrid H2K2[HN (CH2CH2OH)3]2[Cu2(C2H6O2)4Cl][Mo8O26]·17H2O was obtained by
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Q. Gao, et al. 276 mmol) and Na2MoO4.2H2O (0.6g, 2.48mmol) were successively dissolved in 0. 4 mol/L triethanolamine solution (10 mL), and then the pH was adjusted to 1.0 by 37% hydrochloric acid. The resulting solution was heated at 60°C for 2h. Then solid CuCl2·2H2O (0.094g, 0.552mmol) was added to the hot solution and the pH was adjusted to 3.5 by adding CH3COOK solution (3mL glacial acetic acid and 15g CH3COOK were dissolved in 15mL H2O). The mixture was filtered and allowed to cool to ambient temperature. The blue crystals were isolated after two weeks (Yield 0.127 g, 10% based on Mo). Elemental analysis (%) calc (found): C 7.62 (7.18), N 0. 89 (0.76), H 2.94 (2.82), Cl 1.12(1.02), K 1.24(1.04), Mo 24.33 (23.86), Cu 4.03 (3. 91). IR (2% KBr pellet, ν/cm−1): 895(s), 792(s), 625(s), 516(m) and 451(m). [20] Crystal data for 1: H92N2C20KClMo8Cu2O57, Mr = 2281.18, Orthorhombic, Imma, a = 14.617(5)Å, b = 120.662(5)Å, c = 21.639(5)Å, α = 90°, β = 90°, γ = 90°, V = 6535(3)Å3, Z = 1, Dc = 2.318g/cm3, F(000) = 4524.0, R1[I > 2σ(I)] = 0.0628, wR2[all data] = 0.1985, GOF = 1.070. Crystal data of 1 were measured on a Bruker Apex CCD diffractometer using Mo-Kα radiation (λ = 0.71073 Å) at 293(2) K [21]. The crystal structure of 1 was solved by the direct method and refined by the fullmatrix least squares on F2 using the SHELXTL-2014 crystallographic software package [22]. Empirical absorption corrections were applied [23]. Non-hydrogen atoms were easily found from the Fourier difference maps and refined anisotropically. The hydrogen atoms attached to the triethanolamine and Ac ligands were not located and were included in the final molecular formula. The selected bond lengths and angles for 1 are given in Tables S2. CCDC-1900777 for 1. [21] CrysAlisCCD and CrysAlisRED, Oxford Diffraction Ltd, Abingdon, UK, 2010. [22] G.M. Sheldrick, SHELXTL, a Software for Empirical Absorption Correction, Bruker
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