Electrocatalytic and Hg2+ Fluorescence Identifiable Bifunctional

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Electrocatalytic and Hg2+ Fluorescence Identifiable Bifunctional Sensors for a Series of Keggin Compounds Ai-Xiang Tian,* Meng-Le Yang, Yu-Bo Fu, Jun Ying, and Xiu-Li Wang* Department of Chemistry, Bohai University, Jinzhou, 121013, P. R. China

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S Supporting Information *

ABSTRACT: By using a 2,2′-dimethyl-4,4′-bithiazole (dm4bt) ligand, Keggin polyanions, and different metal ions, nine Kegginbased compounds, namely, {[Ag(dm4bt) 2 ][Ag 2 (dm4bt) 3 ]} 2 (PW12O40)(H2PWV2WVI10O40) (1), [CuI(dm4bt)2][CuII(dm4bt)2(PW12O40)] (2), [CuI(dm4bt)2]4(SiW12O40) (3), {[Zn(dm4bt)2]2(SiW12O40)} (4), [Zn(dm4bt)2(H2O)]2(HPMoV2MoVI10O40)·2H2O (5), [Zn(dm4bt)2(Mo2O7)] (6), [Cd(dm4bt)3][Cd(dm4bt)2(H2O)(PMo12O40)]2·2H2O (7), [Cd(dm4bt)3](PMo12O40)·(Hdm4bt) (8), and [Cd(dm4bt)2(H2O)2]2(HPMoV2MoVI10O40)· 2H2O (9), have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analysis, IR spectroscopy, and elemental analyses. Compounds 1−9 are zero-dimensional structures except compound 6, which exhibits a onedimensional structure. In compound 1, there are three isolated subunits: Keggin anions, binuclear [Ag2(dm4bt)3]2+, and mononuclear [Ag(dm4bt)2]+ clusters. The binuclear [Ag2(dm4bt)3]2+ cluster has a Ag−Ag bond. In compound 2, a monosupporting anion {[CuII(dm4bt)2](PW12O40)}− and an isolated [Cu(dm4bt)2]+ cluster exist. By changing the transition metal ions, we obtained two different structures: a supramolecular 3 and a bisupporting anion in 4. By a one-pot method, we successfully obtained compounds 5 and 6, 7 and 8, respectively. In compound 5, the [Zn(dm4bt)2(H2O)]2+ subunit links adjacent PMo12 anions via S···O interactions to form a one-dimensional (1D) supramolecular chain. In compound 6, some PMo12 ions have transformed to [Mo2O7]n2n− chains. The [Zn(dm4bt)2]2+ clusters buckle up and down the chain. Compound 7 has a monosupporting anion and an isolated [Cd(dm4bt)3]2+ cluster. Compound 8 has isolated anions and [Cd(dm4bt)3]2+ clusters. By changing the pH of 7 and 8, a distinct supramolecular compound 9 was obtained. Additionally, the optical band gap, electrochemical, and photocatalytic properties of 1−9 have been investigated in detail. The carbon paste electrodes can be used as bifunctional amperometric and fluorescence sensors for recognition of Hg2+. The n-CPEs as electrochemical sensors can show accurate selectivity for NO2− ions in some common ions. In the fluorescence sensor experiment, fluorescence intensities decrease more than 80% when quenched by Hg2+.

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INTRODUCTION Polyoxometalates (POMs), as inorganic high nuclear metal oxides1 (mainly, Mo, W, V, Nb, and Ta), have been an increasingly important topic, owing to their intriguing architectures and numerous potential applications in catalysis,2 medicine,3 electronics,4 magnetism,5 and optics.6 Recently, the POMs act as the inorganic linkers to coordinate with transition metal complexes (TMCs) with various structures and good performance, inducing more complex POM−TMC structures and broader application prospects.7 Recently, many POM−TMC compounds have been obtained. Owing to the redox chemistry of POM polyoxoanions, the electrochemical properties of POM−TMCs have been extensively researched, using many test methods. Among them, the POM−TMCs modified carbon paste electrodes (CPEs) have become a popular and widely used test method especially for POM−TMCs synthesized under hydrothermal conditions. The CPEs have many advantages: cheap raw material, simple preparation, high stability and reproducibility, and so on. The POM−TMCs modified CPEs usually exhibit excellent electrocatalytic properties for reduction of NO2−, BrO3−, and H2O2, oxidation of L-ascorbic acid (AA), and so © XXXX American Chemical Society

on. However, these POM−TMC modified CPEs acting as amperometric sensors for detecting NO2−, BrO3−, H2O2 and AA are relatively rare.8 The study of POM-based amperometric sensors can expand the scope of POM applications. Thus, in this work, we used the title compounds acting as amperometric sensors for detecting NO2−, and the sensing effect of NO2− is obvious. Up to now, for POM−TMC systems, the usage of organic ligands containing N donors, such as pyridine,9 imidazole,10 pyrazole,11 triazole,12 tetrazolium,13 and their derivatives, is very extensive.13 In order to get distinct structures and properties, we tried to use an organic ligand containing other donor atoms, such as S atoms. So in this work, we selected the 2,2′-dimethyl-4,4′-bithiazole (dm4bt) as an organic moiety, which has unique merits: (i) two N donors in two thiazole groups have strong coordination ability with transition metal (TM) ions; (ii) the two N donors have a long N···N distance compared with bipyridine type ligands, which may induce metal−metal bonds with ease; (iii) it has two S atoms, which Received: November 22, 2018

A

DOI: 10.1021/acs.inorgchem.8b03248 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Fluorescence Residual Intensity Formula of n-CPE (n = 1− 9). The fluorescence residual intensity of n (n = 1−9) is obtained through the formula:

can combine with toxic metal ions (soft acid) according to Pearson’s hard and soft acids and bases theory. For example, the Hg2+ ion as a soft acid conduces to link S atoms (soft base). As everyone knows, among the many heavy metal ions, Hg2+ ions are the most harmful, and one of the most polluting metal ions in the world. Furthermore, Hg2+ ions are continuously enriched in humans and animals, which can lead to various diseases. Thus, the selection of dm4bt as an organic ligand to design and synthesize POM-based compounds for detecting Hg2+ ions has a certain significance in environmental protection issues and applications of POMbased compounds.14 However, up to now, there are fewer reports on POM− TMC compounds that can be used as an electrochemical sensor and as a Hg2+ ion sensor.15 So the design and preparation of new POM−TMCs that act as electrocatalytic and Hg2+ fluorescence identifiable bifunctional sensors make sense. Herein, in this work, by using a dm4bt ligand and Keggin-TCM system, we have obtained nine compounds, {[Ag(dm4bt)2][Ag2(dm4bt)3]}2(PW12O40)(H2PWV2W10O40) (1), [Cu I (dm4bt) 2 ][Cu I I (dm4bt) 2 (PW 1 2 O 4 0 )] (2), [CuI(dm4bt)2]4(SiW12O40) (3), {[Zn(dm4bt)2]2(SiW12O40)} (4), [Zn(dm4bt)2(H2O)]2(HPMoV2MoVI10O40)·2H2O (5), [Zn(dm4bt)2(Mo2O7)] (6), [Cd(dm4bt)3][Cd(dm4bt)2(H2O)(PMo12O40)]2·2H2O (7), [Cd(dm4bt)3](PMo12O40)· (Hdm4bt) (8), and [Cd(dm4bt)2(H2O)2]2(HPMoV2MoVI10O40)·2H2O (9). The electrocatalytic, photocatalytic, and electrochemical properties of compounds 1−4 and 6−7 were studied.

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fluorescence residual intensity = 100% × [Io − I ]/Io where Io and I represent the fluorescence intensity before and after adding Hg2+. Synthesis of {[Ag(dm4bt) 2 ][Ag 2 (dm4bt) 3 ]} 2 (PW 1 2 O 4 0 )(H2PWV2WVI10O40)} (1). A mixture of H3[PW12O40]·12H2O (0.08 g, 0.026 mmol), AgNO3 (0.074 g, 0.44 mmol), dm4bt (0.02 g, 0.1 mmol), and H2O (10 mL) was stirred for 30 min in air at room temperature. The pH value was adjusted to about 3.3 with 1.0 M HNO3, and then the suspension was transferred to a 20 mL Teflon lined autoclave and kept at 160 °C for 5 days. After slow cooling to room temperature, yellow-green block crystals of 1 were obtained (yield 50% based on W), and the final pH was 2.4. Anal. Calcd for C80H82Ag6N20O80P2S20W24 (8366): C 11.49, H 0.99, N 3.35, Found: C:11.42, H 0.95, N 3.40%. IR cm−1 (KBr): 3728(s), 3092(m), 2908(w), 2344(w), 1502(m), 1414(m), 1378(w), 1290(w), 1202(m), 1158(w), 1070(s), 975(s), 895(s), 804(s), 594(m), 506(m). Synthesis of [CuI(dm4bt)2][CuII(dm4bt)2(PW12O40)] (2). The synthetic procedure for 2 was the same as for 1 except that CuCl2 (0.07 g, 0.5 mmol) was used instead of AgNO3 and the pH value of about 3.5. After slow cooling to room temperature, green block crystals of 2 were obtained (yield 50% based on W) and the final pH is 3.8. Anal. calcd. for C32H32Cu2N8O40PS8W12 (3789): C 11.49, H 0.99, N 3.35, Found: C 11.57, H 0.92, N 3.39%. IR cm−1 (KBr): 3773(s), 3428(m), 3106(w), 2930(w), 2344(w), 1641(m), 1524(m), 1429(m), 1378(w), 1283(w), 1209(w), 1143(w), 1072(s), 975(s), 888(s), 799(s), 594(w), 506(m). Synthesis of [CuI (dm4bt)2 ] 4 (SiW 12 O 40 ) (3). A mixture of H4[SiW12O40]·14H2O (0.08 g, 0.026 mmol), CuCl2 (0.08 g, 0.6 mmol), dm4bt (0.02 g, 0.1 mmol), and H2O (10 mL) was stirred for 30 min in air at room temperature. The pH value was adjusted to about 3.8 with 1.0 M HCl and then the suspension was transferred to a 20 mL Teflon lined autoclave and kept at 140 °C for 4 days. After slow cooling to room temperature, red block crystals of 3 were obtained (yield 40% based on W), and the final pH was 4.2. Anal. calcd. for C64H64Cu4N16O40S16SiW12 (4699): C 16.36, H 1.37, N 4.77, Found: C 16.41, H 1.32, N 4.82%. IR cm−1 (KBr): 3839(m), 3451(m), 3085(m), 2909(w), 2330(m), 1776(w), 1650(m), 1510(w), 1422(m), 1364(m), 1276(m), 1203(m), 1144(m), 1010(m), 971(s), 922(s), 876(w), 791(m), 689(w), 663(m), 524(m). Synthesis of {[Zn(dm4bt) 2]2(SiW12O 40)} (4). A mixture of H4[SiW12O40]·14H2O (0.08 g, 0.026 mmol), Zn(CH3COO)2 (0.08 g, 0.44 mmol), dm4bt (0.02 g, 0.1 mmol), and H2O (10 mL) was stirred for 30 min in air at room temperature. The pH value was adjusted to about 3.9 with 1.0 M HCl, and then the suspension was transferred to a 20 mL Teflon lined autoclave and kept at 140 °C for 4 days. After slow cooling to room temperature, yellow block crystals of 4 were obtained (yield 55% based on W) and the final pH is 3.2. Anal. calcd. for C32H32N8O40S8SiW12Zn2 (3790): C 10.14, H 0.85 N 2.96, Found: C 10.21, H 0.91, N 3.02%. IR cm−1 (KBr): 3751(s), 3465(s), 3114(m), 2938(w), 2345(w), 1708(w), 1627(w), 1525(s), 1422(s), 1378(m), 1298(s), 1210(s), 1159(s), 1015(m), 978(s), 921(s),876(m), 790(s), 689(w), 632(w), 572(w), 516(m). Synthesis of [Zn(dm4bt)2(H2O)]2(HPMoV2MoVI10O40)·2H2O (5) and [Zn(dm4bt)2(Mo2O7)] (6). A mixture of H3PMo12O40·12H2O (0.08 g, 0.039 mmol), Zn(CH3COO)2 (0.08 g, 0.44 mmol), dm4bt (0.02 g, 0.1 mmol), and H2O (10 mL) was stirred for 30 min in air at room temperature. The pH value was adjusted to about 4.0 with 1.0 M HCl, and then the suspension was transferred to a 20 mL Teflon lined autoclave and kept at 140 °C for 4 days. After slow cooling to room temperature, yellow block crystals for 5 and white block crystals for 6 were obtained (18% yield for 5 and 50% yield for 6 based on Mo), and the final pH was 3.4. Anal. calcd. for C32H41Mo12N8O44PS8Zn2 (2811, compound 5): C 13.67, N 3.99, H 1.47, Found: C 13.60, N 4.05 H 1.42%. IR cm−1 (KBr): 3853(w), 3436(m), 3092(w), 2353(w), 1657(w), 1562(w), 1518(w), 1400(w), 1371(w), 1291(w), 1210(m), 1151(w), 1063(s), 947(s), 866(s), 795(s),

EXPERIMENTAL SECTION

Materials and General Methods. 2,2′-Dimethyl-4,4′-bithiazole was prepared according to the procedure described previously.16−18 The rest of the reagents were of analytical grade and were used as received from commercial sources without further purification. Elemental analyses (C, H, and N) were performed on a PerkinElmer 2400 CHN elemental analyzer. The IR spectra were obtained by a Magna FT-IR 560 spectrometer with KBr pellet in the 400−4000 cm−1 region. The thermal gravimetric analyses (TGA) were carried out in N2 on a PerkinElmer DTA 1700 differential thermal analyzer with a rate of 10.00 °C/min. Powder X-ray diffraction (PXRD) patterns were recorded on an Ultima IV with D/teX Ultra diffractometer at 40 kV, 40 mA with Cu Kα (λ = 1.5406 Å) radiation in the 2θ range of 5−50°. Fluorescence spectra were completed with a Fluoromax-4NIR fluorescence spectrometer. X-ray photoelectron spectrum (XPS) analyses were performed on a VG ESCALAB MK II spectrometer with an Mg−Kα (1253.6 eV) achromatic X-ray source. The solid-state diffuse-reflectance UV−vis spectra were recorded on powder samples with a PerkinElmer Lambda 750 UV− vis spectrometer equipped with an integrating sphere, using BaSO4 as a white standard. Electrochemical measurements were performed with a CHI 660 electrochemical workstation. A conventional threeelectrode system was used. A saturated calomel electrode (SCE) was used as a reference electrode, and a Pt wire was used as a counter electrode. Chemically bulk modified CPEs were used as the working electrodes. UV−vis absorption spectra were obtained using a UV1801 ultraviolet spectrophotometer. CAT Formula of n-CPE (n = 1−4 and 7). The CAT (electrocatalytic efficiency) of n-CPE (n = 1−4 and 7) is obtained through the formula: CAT = 100% × [Ip(C, substrate) − Ip(C)] /Ip(C) where Ip (C, substrate) is the current intensity for an electrocatalyst with nitrite (NO2−) and hydrogen peroxide (H2O2), and Ip(C) is the current intensity without NO2− and H2O2. B

DOI: 10.1021/acs.inorgchem.8b03248 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for Compounds 1−9 1

2

3

4

5

6

7

8

9

formula

C80H82 Ag6 N20O80P2S20 W24

C32H32 N8O40 S8Si W12 Zn2 3790 triclinic P1̅ 11.115 12.390 13.018 108.66 99.52 92.64 1665.8 1 3.778 21.692 1690 0.0578 0.1164 1.034

C8H8 Mo2N2 O7S2 Zn

8366 triclinic P1̅ 11.720 15.263 23.100 84.71 80.52 84.85 4046.7 1 3.433 18.049 3750 0.069 0.1753 1.017

C64H64 Cu4 N16O40 S16 SiW12 4699 monoclinic P21/n 14.021 24.020 16.592 90 100.81 90 5488.8 2 2.843 13.669 4308 0.0736 0.1740 1.083

C32H41 Mo12 N8O44P S8Zn2

fw crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g·cm−3) μ (mm−1) F(000) R1a [I > 2σ(I)] wR2b (all data) GOF on F2

C32H32 Cu2N8 O40P S8 W12 3789 triclinic P1̅ 13.266 13.420 22.573 94.95 91.45 117.43 3544.0 2 3.551 20.320 3378 0.0429 0.1086 0.962

2811 triclinic P1̅ 11.064 12.394 13.636 74.38 86.01 85.23 1792.4 1 2.604 3.033 1347 0.0543 0.1473 1.020

566 triclinic P1̅ 6.977 10.283 10.746 107.33 103.97 103.81 672.9 2 2.791 3.950 544 0.0301 0.0643 0.990

C56H64 Cd3 Mo24 N14O84 P2S14 5428 monoclinic C2/c 25.666 14.231 36.138 90 95.61 90 13137 4 2.744 3.030 10304 0.0591 0.1044 0.931

C32H33 Cd Mo12 N8O40P S8 2721 monoclinic P21/c 15.696 12.482 35.691 90 97.35 90 6935.0 4 2.606 2.751 5184 0.0364 0.0973 0.988

C32H45 Cd2 Mo12 N8O46P S8 2941 triclinic P1̅ 11.762 12.478 13.759 66.92 89.79 82.85 1841.0 1 2.652 2.883 1403 0.0518 0.1170 1.028

R1 = ∑||F0| − |Fc||/∑|F0|. bwR2 = {∑[w(F02 − Fc2)2]/∑[w(F02)2]}1/2.

a

X-ray Crystallographic Study. X-ray diffraction analysis data for compounds 1−9 were collected with a Bruker Smart Apex CCD diffractometer with MoKα (λ = 0.71073 Å) at 293 K. All crystal structures were solved by direct methods and refined on F2 by fullmatrix least-squares methods using the SHELXTL package.19,20 A summary of the crystallographic data and structural determination for nine compounds is given in Table 1. Selected bond lengths and angles are listed in Table S1 (Supporting Information). CCDC numbers are 1817821, 1817822, 1841985, 1841987, 1831390, 1832009, 1841986, 1877533, and 1877534 for compounds 1−9, respectively.

654(m), 573(w), 507(w). Anal. calcd. for C8H8Mo2N2O7S2Zn (565, compound 6): C 16.99, N 4.95, H 1.43, Found: C 17.06, N 4.91, H 1.49%. IR cm−1 (KBr): 3839(m), 3436(s), 3106(m), 2924(w), 2337(w), 1801(w), 1642(m), 1510(s), 1430(s), 1371(m), 1283(m), 1203(s), 1151(s), 939(s), 881(s), 844(s), 756(s), 698(s), 580(s), 492(w). Synthesis of [Cd(dm4bt)3][Cd(dm4bt)2(H2O)(PMo12O40)]2·2H2O (7), [Cd(dm4bt)3](PMo12O40)·Hdm4bt (8), and [Cd(dm4bt)2(H2O)2]2(HPMoV2MoVI10O40)·2H2O (9). The synthetic procedure for 7−9 was the same as for 5 except that Cd(NO3)2 (0.08 g, 0.34 mmol) were used instead of Zn(CH3COO)2, and the pH value was about 2.9 (for compounds 7 and 8) and 1.5 (for compound 9). After slow cooling to room temperature, red rod crystals of 7 and red block crystals of 8 were obtained in one pot (54% yield for 7, 28% yield for 8 based on Mo), and the final pH was 3.1. After tuning the pH to 1.5, black block crystals of 9 were obtained (23% yield based on Mo), and the final pH was 2.5. Anal. Calcd for C56H64Cd3Mo24N14O84P2S14 (5428, compound 7): C 12.39, N 3.61, H 1.19, Found: C 12.45, N 3.68, H 1.25%. IR cm−1 (KBr): 3832(m), 3548(w), 3412(w), 3097(w), 2919(w), 2363(w), 1649(w), 1509(m), 1414(m), 1382(w), 1300(s), 1207(s), 1159(s), 1067(s), 969(s), 871(s), 800(s), 632(m). C32H33CdMo12N8O40PS8 (2721, compound 8): C 14.13, N 4.12, H 1.22, Found: C 14.21, N 4.07, H 1.27%. IR cm−1 (KBr): 3839(w), 31414(m), 3077(w), 2916(w), 2345(w), 1642(m), 1503(m), 1415(w), 1378(w), 1291(w), 1210(s), 1151(w), 1063(s), 959(s), 873(s), 800(s), 492(m). C32H45Cd2Mo12N8O46PS8 (2941, compound 9): C13.07, N 3.81, H 1.54, Found: C 13.15, N 3.90 H 1.48%. IR cm−1 (KBr): 3839(w), 3553(w), 3421(w), 3099(m), 2916(w), 2345(w), 1620(w), 1518(s), 1415(m), 1283(m), 1203(m), 1159(m), 1071(s), 961(s), 866(s), 800(s), 492(m). Preparation of Bulk-Modified CPEs. A compound 1 modified CPE (1-CPE) was fabricated as follows: graphite powder (0.10 g) and compound 1 (0.01 g) were mixed and ground together in an agate mortar for approximately 40 min to achieve a uniform mixture. Nujol (0.15 mL) was then added, with stirring. The homogenized mixture was packed into a glass tube with a 1.5 mm inner diameter. The tube surface was wiped with weighing paper. The electrical contact was established with a copper rod through the back of the electrode. In a similar manner, 2- to 9-CPEs was made with compounds 2−9.

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RESULTS AND DISCUSSION Syntheses. In this work, we obtained nine POMs-based compounds by using the dm4bt ligand, different metal ions (Ag+, Cu2+, Zn2+, and Cd2+) and Keggin polyoxoanions. At the same temperature, by using different metal ions, we obtained different structures compounds 1 and 2. Furthermore, by using different metal ions (Zn2+ and Cu2+), we get compounds 3 and Scheme 1. Summary of the Synthetic Conditions of Compounds 1−9

C

DOI: 10.1021/acs.inorgchem.8b03248 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. 1D supramolecular chain of compound 7.

Figure 1. 1D supramolecular chain of compound 1.

Figure 6. 1D supramolecular chain of compound 9. Figure 2. 1D supramolecular chain of compound 2 through S···O interactions.

Figure 7. Column diagrams of CAT% versus concentration of KNO2 (a) and CAT% vs concentration of H2O2 (b).

2.8−3.0) by a one-pot method (Figure S7). For the PMo12Zn2+-dm4bt system, when the pH range was 3.9−4.1, both compounds 5 and 6 are present in this system, whereas when the pH is close to 3.9 or 4.1, the system only has compound 6 (Figure S8). When the pH is lower than 3.7 or higher than 4.2, we only get some small white particles. When the pH range of the PMo12-Cd2+-dm4bt system is tuned to 2.8−3.0, compounds 7 and 8 coexisted in one pot, whereas when the pH is close to 2.8 or 3.0, the system only has compound 7 (Figure S9). When the pH is 1.7−2.7 or higher than 3.1, we only get some red precipitate. Owing to the distinct color and shape, compounds 5 and 6, and 7 and 8 can be separated mechanically by using a microscope. When we used the same synthetic conditions with that of compounds 7 and 8 except tuning the pH to 1.5, we obtained the distinct compound 9. Thus, using an appropriate pH range seems to be rationale to obtain two or more types of crystals in one pot. This work may provide informative examples for efficient usage of the hydrothermal technique to construct multiple crystals (Scheme 1). Description of Crystal Structures. Crystal structure analysis reveals that compound 1 consists of 6 AgI ions, 10 dm4bt ligands, 1 [H2PWV2WVI10O40]3− (abbreviated to H2PW12), and 1 [PW12O40]3− (abbreviated to PW12) anion (Figure S10). The valence sum calculations and XPS spectra

Figure 3. 2D supramolecular layer of compound 4.

Figure 4. 2D supramolecular layer of compound 6.

4, which have different structures. Interestingly, for the PMo12Zn2+-dm4bt and PMo12-Cd2+-dm4bt systems respectively, we obtained compounds 5 and 6 (pH = 3.9−4.1), 7 and 8 (pH = D

DOI: 10.1021/acs.inorgchem.8b03248 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 2. Analytical Data for 1- to 4- and 6- to 7-CPEs as Amperometric Sensors response time (s) 1-CPE 2-CPE 3-CPE 4-CPE 6-CPE 7-CPE

2.3 3.4 2.4 1.7 2.9 1.9

sensitivity (μA mM−1)

concentration range (mM) 0.008−0.052 0.012−0.092 0.008−0.092 0.008−0.092 0.008−0.092 0.008−0.092

Potassium Nitrite 23.089 31.761 97.611 231.463 1.074 240.899

correlation coefficient 0.98656 0.9948 0.9988 0.96272 0.9986 0.9770

detection limit (M) 1.0 2.9 3.0 2.7 1.7 1.1

× × × × × ×

10−5 10−6 10−5 10−4 10−6 10−4

Figure 8. Amperometric response for the 1- to 4- and 6- to 7-CPEs on successive addition of 0.1 mM nitrite to 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution, respectively. The inset: the steady-state calibration curve for current versus nitrite concentration, respectively (applied potential: −400 mV for 1- and 2-CPE, −600 mV for 3- and 4-CPE, 200 mV for 6-CPE, 150 mV for 7-CPE).

Figure 10. Plot of irradiation time versus concentration for compounds 1−4, 6, 7, and no catalyst in the solution of MB. Figure 9. Amperometric current responses of 2-, 4-, 6-, and 7-CPEs in aqueous solution upon addition of various inorganic ions.

two dm4bt ligands. The Ag2 and Ag3 are coordinated by three N atoms (N5, N6, and N9 for Ag2, N7, N8, and N10 for Ag3) from two dm4bt ligands and one Ag atom. By calculating of the SHAPE2 software22 (Table S2), the Ag1 forms a seesaw coordination geometry with an approximate C2v symmetry. The Ag2 forms a tetrahedron coordination geometry with an approximate Td symmetry, while the Ag3 forms a seesaw coordination geometry with an approximate C2v symmetry. The Ag−N distances are in the range of 2.13(2)−2.56(2) Å. The Ag−Ag distance is 3.025(3) Å. The N−Ag−N angles are in the range of 73.6(10)−54.6(10)°, while the N−Ag−Ag angles are in the range of 77.9(6)−100.4(7)°. The dm4bt

show that two of the 12 W are in the +V oxidation state (in the [H2PWV2WVI10O40]3− anion), other W atoms are in the +VI oxidation state, and all the Ag atoms are in +I oxidation state.21 In order to balance the charge, two protons have been added in one H2PW12 anion in the formula of 1. In compound 1, there are three crystallographically independent Ag centers (Ag1, Ag2, and Ag3). There are two different coordination modes for the three Ag ions. The Ag1 is four-coordinated by four N atoms (N1, N2, N3, and N4) from E

DOI: 10.1021/acs.inorgchem.8b03248 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 11. Solid-state optical diffuse-reflection spectra of compounds 1−9 derived from diffuse reflectance data at room temperature.

Crystal structure analysis reveals that compound 2 consists of two Cu ions (CuI and CuII), four dm4bt ligands, and one PW12 anion (Figure S11). The valence sum calculations and XPS spectra show that all the W atoms are in the +VI oxidation state, and the Cu atoms are in +I and +II oxidation states, respectively.21 In compound 2, there are two crystallographically independent Cu centers (Cu1, Cu2). The Cu1 is fivecoordinated by four N atoms (N3, N4, N5, and N6) from two dm4bt ligands and one O8 atom from one PW12, while the Cu2 shows a tetrahedron geometry, four-coordinated by four N atoms (N1, N2, N7, and N8) from two dm4bt ligands. By calculating of the SHAPE2 software22 (Table S2), the Cu1 forms a spherical square pyramid coordination geometry with an approximate C4v symmetry, while the Cu2 forms a tetrahedron coordination geometry with an approximate Td symmetry. The Cu−N distances are in the range of 1.963(12)−2.070(13) Å and Cu−O distances is 2.251(8) Å. The bond angles are of 80.9(5)−136.3(5)° for N−Cu−N, 80.9(5)−136.3(5)° for N−Cu−O. In compound 2, two dm4bt ligands link one Cu2 to form a discrete [Cu(2)(dm4bt)2]+ subunit. A similar [Cu(1)(dm4bt)2]2+ cluster links a PW12 anion through a Cu1−O8 bond to generate a monosupporting Keggin structure. The adjacent monosupporting anions further connect each other through S···O interactions (S1···O6 = 3.216 Å) to form a 1D supramolecular chain (Figure 2), and the [(Cu(2)(dm4bt)2]+ subunit is hanging on the 1D chain by S···O interactions (S2··· O25 = 3.301 Å). Crystal structure analysis reveals that compound 3 consists of four CuI ions, eight dm4bt ligands, and one [SiW12O40]4− (abbreviated to SiW12) anion. The valence sum calculations

Figure 12. Fluorescence intensity of 1 suspension with the addition of different metal ions (excited at 365 nm).

ligands in compound 1 show two kinds of coordination modes. The a-type dm4bt offers two N donors to link one Ag atom, exhibiting a chelate coordination mode. The b-type one provides two N atoms to connect two Ag atoms, which may be caused by the long distance between N···N atoms. In the compound 1, there are two kinds of Ag clusters: (i) two a-type dm4bt ligands offer four N atoms to link one Ag1 atom, forming a cross shape [Ag(dm4bt)2]+ cluster; (ii) the Ag2 and Ag3 atoms are linked by two a-type and one b-type dm4bt ligands to form a [Ag2(dm4bt)3]2+ cluster, containing a Ag−Ag bond. Adjacent PW12 anions form a 1D supramolecular POM-based chain by hydrogen bonding interactions (O4···O4 = 2.848 Å). Two kinds of clusters ([Ag(dm4bt)2]+ and [Ag2(dm4bt)3]2+) are suspended on the 1D chain by S···O interactions (O23···S7 = 3.313 Å for Ag1, S1···O6 = 3.305 Å for Ag2 and Ag3), as shown in Figure 1.

Figure 13. (Left) Fluorescence intensity of 4 with gradually increased Hg2+, (right) histogram reflects fluorescence residual intensity’s change (inset: plot showing a fluorescence change after adding Hg2+). F

DOI: 10.1021/acs.inorgchem.8b03248 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Fluorescence Quenching Rate of Hg2+ by Different Compounds rate (%)

1

2

3

4

5

6

7

8

9

88.68

85.61

85.31

82.27

95.07

93.50

89.92

88.46

86.39

Figure 14. Fluorescence intensity of compound 7 at about 440 nm in suspension at room temperature upon the addition of Hg2+ or Hg2+ + Mn+ ions (excited at 365 nm).

lattice water molecules, and one [PMo12O40]3− (abbreviated to PMo12) anion (Figure S18). The valence sum calculations and XPS spectra show that two of the 12 Mo are in the +V oxidation state, other Mo atoms are in the +VI oxidation state, and all the Zn atoms are in +II oxidation state.21 In order to balance the charge, one proton has been added in one PMo12 anion in the formula of 5. In compound 5, there is one crystallographically independent Zn center, five-coordinated by four N donors (N1, N2, N3, and N4) and one coordinated water molecule (O1W). By calculating of the SHAPE2 software22 (Table S2), the Zn1 forms an octahedron coordination geometry with an approximate Oh symmetry. The Zn−N distances are in the range of 2.067(9)−2.183(9) Å. The Zn−O distance is 2.363(17) Å. The bond angles are of 79.3(3)−174.1(4)° for the N−Zn−N, 87.4(4)−162.2(5)° for the N−Zn−O. In compound 5, two dm4bt ligands and one coordinated water molecule coordinate with one ZnII ion to form a [Zn(dm4bt)2(H2O)]2+ subunit, which links adjacent PMo12 anions via S···O interactions (S4···O11 = 3.296 Å, S1···O18 = 3.224 Å) to construct a 1D supramolecular chain (Figure S19). Crystal structure analysis reveals that compound 6 consists of one Zn II ion, one dm4bt ligand, one [Mo 2 O 7 ] 2− (abbreviated to Mo2O7) anion (Figure S20). The valence sum calculations show that all the Mo atoms are in the + VI oxidation state and the all Zn atoms are in + II oxidation state.21 In compound 6, there is only one crystallographically independent Zn center. The Zn atom shows a distorted octahedral coordination geometry, six-coordinated by four O atoms (O2, O3, O5, and O6) and two N atoms (N1 and N2). By calculating of the SHAPE2 software22 (Table S2), the Zn1 forms an octahedron coordination geometry with an approximate Oh symmetry. The Zn−N distances are 2.087(3)−2.111(3) Å. The Zn−O distance is 2.109(10) Å. The bond angles are 79.3(4)−174.2(4)° for N−Zn−N, 87.5(4)−138.5(4)° for N−Zn−O. In compound 6, there are two kinds of subunits. One is the [Zn(dm4bt)2]2+ cluster with one Zn2+ linked by two dm4bt ligands. The other is the [Mo2O7]n2n− chain transformed from reactant PMo12. The [Zn(dm4bt)2]2+ clusters buckle up and down the chain through Zn−O bonds (Figure S21, Figure

and XPS spectra show that all the W atoms are in the +VI oxidation state and all the Cu atoms are in +I oxidation state, Figure S12.21 In compound 3, there are two crystallographically independent Cu centers. The Cu ions have the same coordination mode, four-coordinated by four N donors (N1, N2, N3, and N4 for Cu1, N5, N6, N7, and N8 for Cu2). By calculating of the SHAPE2 software22 (Table S2), Cu1 and Cu2 all form a seesaw coordination geometry with an approximate C2v symmetry. The Cu−N distances are in the range of 1.92(2)−2.18(2) Å. The N−Cu−N angles are in the range of 80.6(9)−149.3(13)°. In compound 3, two dm4bt ligands are linked by one CuI ion to form [Cu(dm4bt)2]+ subunits, which surround the SiW12 anion. Adjacent SiW12 anions form a 1D supramolecular chain via the S···O interactions (S4···O18 = 3.157 Å) (Figure S13). Crystal structure analysis reveals that compound 4 consists of two ZnII ions, four dm4bt ligands, one SiW12 anion (Figure S14). The valence sum calculations show that all the W atoms are in the +VI oxidation state, and all the Zn atoms are in +II oxidation state.21 In compound 4, there is only one crystallographically independent Zn center, which is five-coordinated by four N donors (N1, N2, N3, and N4) and one O20 atom. By calculating of the SHAPE2 software22 (Table S2), the Zn1 forms a vacant octahedron coordination geometry with an approximate C4v symmetry. The Zn−N distances are in the range of 2.069(14)−2.131(13) Å. The Zn−O distance is 2.113(11) Å. The bond angles are 80.0(6)−177.9(7)° for N− Zn−N and 86.6(5)−156.4(5)° for N−Zn−O. In compound 4, two dm4bt ligands are linked by one Zn ion to form a [Zn(dm4bt)2]2+ cluster, which is like a butterfly (Figure S15). Furthermore, two [Zn(dm4bt)2]2+ clusters connect the SiW12 anion via O20 to form a bisupporting anion. Adjacent bisupporting anions built a 1D supramolecular chain via S···O interactions (S3···O3 = 3.081 Å) (Figure S16). The adjacent 1D chains construct a 2D supramolecular layer through hydrogen bonding interactions (N2···O4 = 2.946 Å) (Figure 3, Figure S17). Crystal structure analysis reveals that compound 5 consists of two ZnII ions, four dm4bt ligands, two coordinated and two G

DOI: 10.1021/acs.inorgchem.8b03248 Inorg. Chem. XXXX, XXX, XXX−XXX

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hydrogen bonding interactions (O17···O2W = 1.992 Å) connect these chains to construct a supramolecular 2D layer (Figure S27). XPS Spectra. In order to confirm the oxidation states of W, Cu, and Mo, XPS measurements of compounds 1−3, 5, and 9 are further studied (Figure S28). The XPS spectra exhibit two peaks at 35.3 and 36.05 eV in compound 1, which are ascribed to WV(4f5/2) and WVI(4f5/2), respectively.23a,b In compound 2, the peaks at 935.5, 954.2, 944.1, and 963.5 eV are attributed to CuII ions.23c The peaks at 932.1 and 951.1 eV for compound 2, 932.2 and 952.2 eV for compound 3, are attributed to CuI ions.23d The four overlapped peaks at 231.6 and 232.6, 234.7, and 235.7 eV for compound 5, 231.6 and 232.7, 234.7, and 235.8 eV for compound 9, are ascribed to MoV(3d5/2), MoVI(3d5/2), MoV(3d3/2), and MoVI(3d5/2) ions, respectively.8 All these results further confirm the valence sum calculations and the structural analyses. Voltammetric Behavior and Electrocatalytic Activity. The electrochemical properties of compounds 1−9 were investigated in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution. Owing to their similar electrochemical behaviors, we used compounds 1−4 and 7 as examples (Figure S29). In the potential range from +600 to −850 mV for 1- and 2-CPEs, there exist three reversible redox peaks II−II′, III−III′, and IV−IV′ for 1-CPE and 2-CPE with mean peak potentials E1/2 = (Epa + Epc)/2 at −123, −421, and −665 mV for 1-CPE, -247, -457, and −675 mV for 2-CPE (scan rate: 250 mV·s−1), corresponding to two consecutive one-electron and one twoelectron processes of the PW12 anion.24 There is also one irreversible anodic peak I′ with a potential of +100 mV for 1CPE, +210 mV for 2-CPE (scan rate: 250 mV·s−1), which are assigned to the oxidation of the Ag25 and Cu centers, respectively. For 3-CPEs, in the range of 0 to −800 mV, there exist three reversible redox peaks I−I′, II−II′, and III− III′ with the half-wave potentials at −395, −553, and −687 mV mV (scan rate: 250 mV·s−1), corresponding to two consecutive one-electron and one two-electron processes of SiW12 anion.26 For 4-CPEs, in the range of 0 to −850 mV, there exist two reversible redox peaks I−I′ and II−II′ with the half-wave potentials at −477 and −655 mV (scan rate: 250 mV·s−1), corresponding to two consecutive two-electron redox processes of SiW12 anion.24 For 7-CPE, in the range of −300 to +700 mV, there are three reversible redox peaks I−I′, II−II′, and III−III′, with half-wave potentials at +292, +136, and −105 mV (scan rate: 250 mV·s−1). These three pairs of redox peaks belong to three consecutive, two-electron processes of the PMo12 anion.24 We studied the electrocatalytic reduction of KNO2, H2O2, and oxidation of AA for 1- to 4- and 7-CPEs. Figure S30 shows cyclic voltammograms for the electrocatalytic reduction of hydrogen peroxide at 1- to 4- and 7-CPEs. Upon addition of hydrogen peroxide, the second and third reduction peak currents of 1-, 2-, and 7-CPEs, the first and second reduction peak currents of 3- and 4-CPEs increase gradually, indicating that the four- and six-electron reduced species of PW12 in 1and 2-CPEs, PMo12 in 7-CPE, and the two- and four-electron reduced species of SiW12 in 3- and 4-CPEs show electrocatalytic activities for the reduction of H2O2. As shown in Figure S31, with the addition of nitrite, all the reduction peak currents increase gradually and the corresponding oxidation peak currents decrease, suggesting that all the reductive species of 1- to 4- and 7-CPEs all possess electrocatalytic activities for the reduction of NO2−. As shown in the Figure S32, with

S22). The adjacent 1D chains are still further linked by hydrogen bonding interactions (O3···C6 = 2.255 Å) to construct a 2D supramolecular layer (Figure 4). Crystal structure analysis reveals that compound 7 consists of three CdII ions, seven dm4bt ligands, two coordinated and two lattice water molecules, and one PMo12 anion (Figure S23). The valence sum calculations show that all the Mo atoms are in the +VI oxidation state, and all the Cd atoms are in +II oxidation state.21 In compound 7, there are two crystallographically independent Cd centers. Each CdII ion is six-coordinated and possesses a distorted octahedral coordination geometry. The Cd1 is coordinated by four N atoms (N1, N2, N3, and N4), a water molecule, and one O2 atom from one PMo12 anion. The Cd2 is coordinated by six N atoms (N5 to N7) from three dm4bt. The Cd−N distances are 2.275(14)− 2.379(16) Å. The Cd−O distances are 2.323(11)−2.402(10) Å. The bond angles are 72.5(6)−172.0(5)° for N−Cd−N, 83.0(4)−172.8(5)o for N−Cd−O, 94.1(4)° for O−Cd−O. In compound 7, the [Cd(dm4bt)2(H2O)]2+ cluster connects the PMo12 through a Cd−O bond to form a monosupporting structure. These adjacent monosupporting anions build a 1D supramolecular chain via hydrogen bonding interactions (O2W···O35 = 2.747 Å). The discrete clusters [Cd(dm4bt)3]2+ connect the 1D supramolecular chain through a lattice water molecule (O1W···C21 = 2.516 Å) and a coordinated water molecule (O2W···O1W = 1.985 Å) (Figure 5). Crystal structure analysis reveals that compound 8 consists of one CdII ion, three coordinated and one discrete dm4bt ligands, and one PMo12 anion (Figure S24). The discrete dm4bt ligand is protonated, showing +I oxidation states as (Hdm4bt)+. The valence sum calculations show that all the Mo atoms are in the +VI oxidation state, and all the Cd atoms are in +II oxidation state.21 In compound 8, there is only one crystallographically independent Cd center. The Cd1 shows a six-coordinated mode, connected six N atoms (N1 to N6) from three dm4bt ligands. The Cd−N distances are 2.317(5)−2.389(5) Å. The bond angles are 72.87(18)−172.62(17)° for N−Cd−N. The adjacent PMo12 anions connect [Cd(dm4bt)3]2+ subunits via S···O interactions (S4···O38 = 3.266 Å, S4···O39 = 3.217 Å) to construct a 1D supramolecular POM-based chain, and the discrete dm4bt ligands are suspended above the POM by S···O interactions (S7···O3 = 2.965 Å) (Figure S25). Crystal structure analysis reveals that compound 9 consists of two CdII ions, four dm4bt ligands, four coordinated and two lattice water molecules, and one PMo12 anion (Figure S26). The valence sum calculations and XPS spectra show that 2 of 12 Mo are in +V oxidation state, and all Zn atoms are in +II oxidation state.21 In order to balance the charge, one proton has been added in one PMo12 anion in the formula of 9. In compound 9, there is one crystallographically independent Cd center. The Cd1 is coordinated by four N atoms (N1 to N4) from two dm4bt ligands and two coordinated water molecules. The distances are 2.297(6)−2.403(8) Å for Cd−N, 2.285(7)−2.415(6) for Cd−O. The bond angles are 73.5(3)− 170.6(2)° for N−Cd−N, 85.0(2)−174.2(2)° N−Cd−O. In compound 9, two dm4bt ligands and two water molecules are fused by one CdII ion to form a [Cd(dm4bt)2(H2O)2]2+ cluster, connecting adjacent anions to build a 1D supramolecular chain (S1···O3 = 3.223 Å, O17···O2W = 1.992 Å) (Figure 6). Adjacent chains are parallel to each other. The H

DOI: 10.1021/acs.inorgchem.8b03248 Inorg. Chem. XXXX, XXX, XXX−XXX

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which measured the absorbance of the solution to estimate the photocatalytic performance of 1−4, 6, and 7 to MB solution. As shown in Figure S34, with the increase of reaction time, the absorption peaks of MB were significantly decreased after 150 min. The maximum percent conversions of MB increase from 14.7% (no catalyst) to 70.3%, 69.6%, 69.3%, 69.1%, 58.6%, and 74.5% for compounds 1−4, 6, and 7, respectively (Figure 10). The results show that compounds 1−4, 6, and 7 have a good catalytic efficiency under a UV lamp for degradation of MB dye. In addition, the stabilities of compounds 1−4, 6, and 7 in the photocatalytic process were further investigated. Figure S35 shows the PXRD patterns of compounds 1−4, 6, and 7 before (simulated and experimental data) and after photocatalysis for MB. The simulated and experimental diffraction peaks match well in positions, showing that the phase purities of compounds 1−4, 6, and 7 are good. Furthermore, the diffraction peaks of experimental and recycled patterns match well in positions. Thus, during the photocatalytic process 1−4, 6, and 7 can maintain their stability. Solid-State Diffuse-Reflection Study. In order to obtain the energy gap between the nine materials, we measured the optical diffuse reflection spectra of crystalline solid samples of 1−9 at room temperature. The absorption data were calculated from the reflection using the Kubelka−Munk function.30 The energy band gaps (Eg) obtained by extrapolation of the linear portion of the absorption edges were estimated as 2.19 eV for (1), 2.45 eV for (2), 2.47 eV for (3), 3.06 eV for (4), 3.09 eV for (5), 3.25 eV for (6), 2.24 eV for (7), 1.93 eV for (8), and 2.09 eV for (9) (Figure 11 and Figure S36), indicating their semiconductor properties. Detection of Metal Ions. The sensing ability of 1−9 on the metal ions was investigated. Herein, the chloride salts of different metals were dispersed in compound 1 or 2−9 (1 mg) dimethyl sulfoxide (DMSO) solutions (10 mL). The emission spectra of 1−9 suspension at room temperature excited at 365 nm show an intense emission band at 440 nm. Compound 1 as an example, as shown in Figure 12, the emission intensities for all suspensions exhibit the differences. When the Co2+/Zn2+/ Cd2+/Ni2+/Fe3+/Cu2+ was added to the solutions respectively, the emission intensities for compound 1 show a reduction with a different degree. However, when the Hg2+ was added to the solution, the emission for compound 1 is relatively large quenched, which means compound 1 can act as a chemosensor to selectively sense the Hg2+ ion. The rest of compounds also show corresponding experimental phenomena. A sensitivity test of 1−9 on the Hg2+ ion was further carried out. At this time, different moles of Hg2+ were added into these compounds’ DMSO solutions. As shown in Figure 13 and Figure S37, with the increase of the Hg2+ concentration, the emission intensity of 1 gradually decreases. When the Hg2+ concentration is 15 mM, the fluorescence intensity for 4 declines by ca. 50%, whereas when the Hg2+ concentration increases up to 30 mM, the fluorescence intensity for 4 decreases by ca. 82.27%, 88.5% for 1, 85.36% for 2, 85.31% for 3, 95.07% for 5, 93.51% for 6, 89.92% for 7, 88.46% for 8, 86.39% for 9 (the reduction compared to blank sample in intensity) in Table 3. The possible mechanism for the significant quenching effect of Hg2+ is that according to Lewis acid−base theory, Hg2+ ions (Lewis acid) have a high complexation affinity to S atoms (Lewis base). This result may decrease the degree of delocalization in these compounds, minimizing the energy-transfer efficiency and resulting in

addition of AA, the oxidation peak currents of 1- to 4- and 7CPEs gradually increase, which suggests that 1- to 4- and 7CPEs present excellent electrocatalytic activities for the oxidation of AA. In a word, the 1- to 4- and 7-CPEs possess excellent multifunctional electrocatalytic activities toward not only reduction of KNO2, H2O2 but also oxidation of AA. To compare the electrocatalytic activity of 1- to 4- and 7CPEs for NO2− and H2O2, the CAT (catalytic efficiency) of 1to 4- and 7-CPEs can be calculated by using the CAT formula.27 The CATs of 2-CPE toward 2.0, 4.0, 6.0, 8.0, and 10.0 mM NO2− were calculated as 124.9, 258.2, 381.4, 460.8 and 536.7%, respectively, which shows that 2-CPE for the reduction of NO2− is superior to 1-, 3-, 4-, and 7-CPEs. Similarly, the CATs of 4-CPE toward 2.0, 4.0, 6.0, 8.0, and 10.0 mM H2O2 were calculated to be 317.4, 147.8, 608.7, 787.0, and 1178.1%, which shows that 4-CPE for reduction of H2O2 is superior to 1-, 2-, 3-, and 7-CPEs. Column diagrams of CAT% for 1- to 4- and 7-CPEs versus concentrations of NO2− and H2O2 are shown in Figure 7 for visual comparisons of the electrocatalytic behaviors (Tables S3−S4). In compounds 1−4 and 7 the different POM anions and different modified TMCs may exert a synergetic effect to induce distinct electrocatalytic activity of reduction of H2O2 or NO2−. 1- to 4- and 6- to 7-CPEs for Amperometric Sensors. We used the 1- to 4- and 6- to 7-CPEs as the nitrite amperometric sensors, respectively.28 We added nitrite successively to continuously stirred 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution. The 1-CPE response time was 2.3 s. The electrode response was linear for nitrite within the concentration range 8 × 10−6 to 5.2 × 10−5 M, and the sensitivity was 23.089 μA mM−1 (correlation coefficient of 0.98656). The detection limit was 1.0 × 10−5 M (signal-tonoise ratio: 3). Furthermore, we summarized the information concerning the amperometric sensors for the 1- to 4- and 6- to 7-CPEs including response time, concentration range, sensitivity, correlation coefficient, and detection limit in Table 2 and Figure 8. After a few weeks, we retested the stability of n-CPEs as amperometric sensor and found that they were stable (Table S5 and Figure S33). Selectivity plays an important role in electrochemical sensors.28 We choose 2-, 4-, 6-, and 7-CPEs as examples and choose various inorganic ions, such as CO32−, HCO3−, CH3COO−, SO42−, Cl−, and Br−, as possible interferences in the detection of nitrite ions. It can be observed in Figure 9 that when 0.1 M KNO2 was added to 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution, a significant increase occurred in the amperometric current responses. However, with addition of Na2CO3, NaHCO3, CH3COONa, KCl, Na2SO4, NaCl, and KBr at the same concentration of NO2− to 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution at regular intervals, these interfering substances had almost no influence on the amperometric current responses, suggesting that 2-, 4-, 6-, and 7-CPEs electrode possesses selectivity toward nitrite. Photodegradation Properties of Compounds 1−4, 6, and 7. In this work, we studied the photocatalytic activities of compounds 1−4, 6, and 7 in methylene blue (MB) solution under UV light irradiation.29 First, 150 mg of compound 1−4, 6, or 7 was dispersed in a 0.02 mmol·L−1 MB solution (90 mL) and magnetically stirred for about 15 min to make sure the equilibrium in the dark. Then, a UV Hg lamp irradiates the mixed solution with stirring continuously. At every interval 30 min for 1−4, 6 and 7, 5.0 mL samples were taken out for analysis by using a UV-1801 ultraviolet spectrophotometer I

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fluorescence quenching.14,31 Furthermore, in order to verify whether other metal ions affect the recognition of Hg2+ by these compounds, we added different metal ions (Co2+/Zn2+/ Cd2+/Ni2+/Fe3+/Cu2+) with a constant concentration Figure 14 and Figure S38. When we added these metal ions into the compound DMSO solutions, these metal ions had almost no influence on the emission intensities. However, when the Hg2+ ion was further added, the emission intensity dramatic declined, suggesting that these compounds can act as sensors to identify Hg2+ ions.

CONCLUSION In the work, by using a Keggin-TMs-dm4bt system, nine compounds have been synthesized under hydrothermal conditions. By tuning the reactant type and pH, we have obtained compounds 1−9. In particular, compounds 5 and 6, 7 and 8 were obtained in one pot. Compounds 1−9 exhibit good electrocatalytic properties for reduction of KNO2, H2O2, and oxidation of AA. The 1- to 4- and 6- to 7-CPEs can act as nitrite amperometric sensors. According to a fluorescence quenching experiment, the emission of compounds 1−9 could be quenched efficiently by adding Hg2+ ions, almost with no interference of other metal ions, suggesting that compounds 1−9 can serve as potential fluorescence sensors for the detection of Hg2+. More POMs-TM-dm4bt systems will be explored both as amperometric sensors and Hg2+ sensors. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03248. Tables of selected bond lengths and angles for compounds 1−9, structural figures of 1−9, the PXRD patterns, the XPS data, the IR spectra, the TGA curves of 1−9, and the figures of electrocatalytic activity, amperometric sensors and the fluorescence (PDF) Accession Codes

CCDC 1817821−1817822, 1831390, 1832009, 1841985− 1841987, and 1877533−1877534 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*(A.-X.T.) E-mail: [email protected]. *(X.-L.W.) E-mail: [email protected]. ORCID

Ai-Xiang Tian: 0000-0002-4186-9795 Xiu-Li Wang: 0000-0002-0308-1403 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Nos. 21571023, 21471021, and 21101015) and Talent-supporting Program Foundation of Education Office of Liaoning Province (LJQ2012097). J

DOI: 10.1021/acs.inorgchem.8b03248 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.8b03248 Inorg. Chem. XXXX, XXX, XXX−XXX