Proton-Conductive Keggin-Type Clusters Decorated by the Complex

Jan 10, 2019 - Proton-Conductive Keggin-Type Clusters Decorated by the Complex Moieties of Cu(II) 2,2′-Bipyridine-4,4′-dicarboxylate/Diethyl Analo...
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Article Cite This: Inorg. Chem. 2019, 58, 1020−1029

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Proton-Conductive Keggin-Type Clusters Decorated by the Complex Moieties of Cu(II) 2,2′-Bipyridine-4,4′-dicarboxylate/Diethyl Analogues Hui Yang,†,§ Xian-Ying Duan,*,‡,§ Jia-Jia Lai,† and Mei-Lin Wei*,† †

School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, People’s Republic of China Institute of Chemistry, Henan Academy of Sciences, Zhengzhou 450002, People’s Republic of China

Inorg. Chem. 2019.58:1020-1029. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/25/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Three proton-conductive decorated Keggin-type clusters, {[Cu(debpdc)(H 2 O) 3 ][Cu(debpdc)(H 2 O)Cl][PMo12O40]}·2CH3OH·1.5CH3CN·3H2O (1), {[Cu(H2bpdc)(H 2 O) 2 Cl0.5 ]2 [PW 12O 40 ]}·10H2 O (2), and {[Cu(H 2 bpdc)(H2O)2.5]2[SiW12O40]}·10H2O (3) (where debpdc is diethyl 2,2′-bipyridine-4,4′-dicarboxylate and H2bpdc is 2,2′-bipyridine4,4′-dicarboxylic acid), were synthesized through electrostatic and coordination interactions between Keggin-type anions and Cu(II) H2bpdc/debpdc complex moieties. Interestingly, in the three complexes, both the H2bpdc/debpdc and the Keggin anion are covalently linked to the Cu2+ ions as polydentate organic and inorganic ligands, respectively. Notably, complexes 2 and 3 are the first examples of the functionalization of a Keggin-type cluster with Cu(II)-H2bpdc complex moieties, thereby providing a pathway to design and synthesize multifunctional hybrid materials with cluster structures based on two building units. In them, the free COOH groups of the H2bpdc ligand can act as both hydrogen bond acceptors and proton carriers. 1 has debpdc ligands with ethoxycarbonyl groups, while 2 and 3 have the H2bpdc ligands with free COOH groups; thus, the three complexes help us to understand the influence of the different substituents on the proton conductivity. The measurement results reveal that 2 and 3 have a high conductivity value of over 10−3 S cm−1 at 100 °C under 98% relative humidity, which is 2 orders of magnitude higher than that of 1 under the same conditions.

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INTRODUCTION

Carboxylate ligands are suitable for constructing protonconductive carboxylate-based metal−organic frameworks (MOFs)/coordination polymers (CPs). Carboxyl groups as the proton sources are covalently bonded to the organic linkers of the backbone of MOFs/CPs. This could ensure that the proton sources are immobilized in the conduction pathway under hydrated conditions.15−17 As types of carboxylate ligands, N-heterocyclic carboxylic ligands allow various acidity-dependent coordination modes and easily give MOFs/CPs as the organic linkers.24−29 So far, it has been hard to build organic−inorganic complexes in which both N-heterocyclic multicarboxylic acids and POMs as multifunctional ligands coordinate with metal ions.21,22 Therefore, the rational design of Keggin-type clusters decorated by metal N-heterocyclic multicarboxylic acid complex moieties with free COOH groups remains an arduous task. In this paper, we focused our attention on metal−organic moieties from 2,2′-bipyridyl-4,4′-dicarboxylic acid (H2bpdc). The H2bpdc ligand is an N-heterocyclic multicarboxylic acid. In contrast with its acid analogue, diethyl 2,2′-bipyridine-4,4′-

Due to transport dynamics and their applications in fuel cells, the study of proton-conductive crystalline materials has become necessary and interesting. Crystalline materials have become some of the ideal candidates for proton-conductive applications. Their crystalline nature could provide an opportunity to study the pathway and mechanism for proton conduction.1−17 Since two Keggin-type acids, H3PM12O40· 29H2O (M = Mo, W), as low-temperature proton-conductive crystalline materials were first reported by Nakamura in 1979,18 Keggin anions, a class of proton-conductive inorganic building blocks with a globe structure, have been found to be suitable for the construction of proton-conductive organic− inorganic hybrid materials.19−23 In comparison to simple Keggin-type polyoxometalates (POMs), organic−inorganic hybrid materials could show good stability and water retention, because the Keggin-type anions are not easy dissociated from the hybrids on the basis of electrostatic and/or coordinated interactions. Inserting Keggin-type anions into ordered cavities/layers/channels could not only strengthen the stability and hydrophilicity of organic−inorganic hybrids but also furnish a pathway for proton conduction. © 2019 American Chemical Society

Received: March 13, 2018 Published: January 10, 2019 1020

DOI: 10.1021/acs.inorgchem.8b00667 Inorg. Chem. 2019, 58, 1020−1029

Article

Inorganic Chemistry

the bottom of a test tube. Then, a methanol/acetonitrile/water (1/1/ 1, v/v/v) solution (8 mL) was used as a buffer layer. Finally, a methanol solution (5 mL) of debpdc (30 mg, 0.10 mmol) and Cu(NO3)2·6H2O (19 mg, 0.10 mmol) was carefully layered over the buffer layer. Finally, the test tube containing the solutions was stoppered and laid aside at room temperature. Blue-green crystals formed after 5−6 weeks and were collected and dried in air after quickly being washed with water. Yield: 70 mg, 70% based on polyoxometalates. Anal. Calcd for C37H60N5.5O57Cu2Mo 12PCl (2838.68): C, 15.65; H, 2.13; N, 2.71. Found: C, 15.56; H, 2.08; N, 2.79. IR (KBr): ν (cm−1) 810 ν(Mo−Oc), 873 ν(Mo−Ob), 961 ν(Mo−Ot), 1065 ν(P−Oa) (characteristic vibrations resulting from heteropolyanions with the Keggin structure); 3407, 3156, 1729, 1619, 1401, 1256 (vibrations resulting from the debpdc ligands). The Cl atom was proved by X-ray photoelectron spectroscopy in the energy region of Cl 2p (Figure S1a in the Supporting Information). Complex 2. The copper heteropolyacid salt CuHPW12O40·nH2O (30 mg, 0.01 mmol) and CuCl2·6H2O (1.7 mg, 0.01 mmol) were dissolved in 4 mL of water, which was placed in the bottom of a test tube. Then, a methanol/acetonitrile/water (1/1/1, v/v/v) solution (6 mL) was used as a buffer layer. Finally, an acetonitrile solution (10 mL) of [(HOCH2)3CNHCO]2(C5H3N)2 (9.0 mg, 0.02 mmol) was carefully layered over the buffer layer. Finally, the test tube containing the solutions was stoppered and laid aside at room temperature. Light blue crystals formed after 4−5 weeks and were collected and dried in air after quickly being washed with water. Yield: 20 mg, 67% based on polyoxometalates. Anal. Calcd for C 24 H 44 N 4 O 62 Cu 2 W 12 PCl (3780.33): C, 7.62; H, 1.17; N, 1.48. Found: C, 7.55; H, 1.11; N, 1.41. IR (KBr): ν (cm−1) 804 ν(W−Oc), 882 ν(W−Ob), 990 ν(W− Ot), 1079 ν(P−Oa) (characteristic vibrations resulting from heteropolyanions with the Keggin structure); 3454, 1720, 1619, 1407, 1228 (vibrations resulting from the H2bpdc ligands). The content of Cl was proved by X-ray photoelectron spectroscopy in the energy region of Cl 2p (Figure S1b in the Supporting Information). Complex 3. Except for Cu2SiW12O40·nH2O (30 mg, 0.01 mmol) replacing CuHPW12O40·nH2O (30 mg, 0.01 mmol), complex 3 was prepared in the same way as for complex 2. Light blue crystals formed after 4−5 weeks and were collected and dried in air after quickly being washed with water. Yield: 21 mg, 70% based on polyoxometalates. Anal. Calcd for C24H46N4O63Cu2W12Si (3760.02): C, 7.67; H, 1.23; N, 1.49. Found: C, 7.59; H, 1.28; N, 1.40. IR (KBr): ν (cm−1) 798 ν(W−Oc), 883 ν(W−Ob), 922 ν(W−Ot), 973 ν(Si−Oa) (characteristic vibrations resulting from heteropolyanions with the Keggin structure); 3452, 1721, 1619, 1408, 1229 (vibrations resulting from the H2bpdc ligands). Measurements. A PerkinElmer 240C analyzer was used for elemental analyses (C, H, and N). A Thermo Scientific ESCALAB 250Xi XPS instrument was used for X-ray photoelectron spectroscopy in the energy region of Cl 2p. X-ray powder diffractions were carried out with a Bruker D8 Avance Instrument (Cu Kα radiation and a fixed power source of 40 kV, 40 mA). A VECTOR 22 Bruker spectrophotometer was used for IR spectra with KBr pellets in the region from 400 to 4000 cm−1 at room temperature. Thermogravimetric analyses were recorded on a PerkinElmer thermal analyzer. Water absorption experiments were performed on an AQUALAB VSA instrument. A chi660d (Shanghai chenhua) electrochemical impedance analyzer was used for ac impedance spectroscopy measurements with copper electrodes.21,22 The resistances were obtained from the extrapolation to the real axis. The conductivities of solid samples were calculated on the basis of σ = (1/R)(h/S) (where R is the resistance, h is the thickness, and S is the area of the tablet). Single-Crystal X-ray Diffraction Analysis. Diffraction data for suitable single crystals of the three complexes were collected on a Bruker Apex CCD area detector diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 293(2) K. The program SAINT was used for processing data.41 The SADABS program was applied for absorption correction on the basis of symmetry equivalent reflections.42 Structures were solved by direct methods (SHELX97)43 and refined with the full-matrix least-squares technique using the SHELXL-2014 package.44 Non-hydrogen atoms, except for some disordered oxygen

dicarboxylate (debpdc) reduces synthesis complications resulting from the effect of the pH.30−33 Notably, H2bpdc and debpdc can provide two nitrogen atoms to coordinate in a cis conformation with transition-metal ions such as Ru3+/Pd2+/ Pt2+ and so on, resulting in metal−organic clusters with carboxyl/carboxylate ester groups being free.34,35 Moreover, ester groups of the debpdc ligand and COOH groups of H2bpdc ligands can act as hydrogen bond acceptors.36−39 debpdc can use its carboxylate ester groups to cause metal complexes to be attached on the surface of metal oxides with surface hydroxyl groups. Moreover, the carboxylate ester group of the debpdc ligand may be hydrolyzed into its carboxylate form in solution. Importantly, to the best of our knowledge, there have been no reports on Keggin-type clusters decorated by metal H2bpdc (or debpdc) complex moieties. Therefore, the functionalization of a Keggin-type cluster with metal H2bpdc (or debpdc) complex moieties becomes necessary and interesting. Here, a Cu2+ ion, debpdc/H2bpdc, and typical Keggin-type anions ([PMo12O40]3−, [PW12O40]3−, and [SiW12O40]4−) were chosen to construct new types of proton-conductive organic− inorganic hybrid materials. We synthesized three protonconductive decorated Keggin-type clusters, {[Cu(debpdc)(H 2 O) 3 ][Cu(debpdc)(H 2 O)Cl][PMo 12 O 40 ]}·2CH 3 OH· 1.5CH3CN·3H2O (1), {[Cu(H2bpdc)(H2O)2Cl0.5]2[PW12O40]}·10H2O (2), and {[Cu(H2bpdc)(H2O)2.5]2[SiW12O40]}·10H2O (3), at room temperature. In these three complexes, both debpdc and H2bpdc act as bidentate nitrogen donor ligands in a chelating fashion. In 1, the [PMo12O40]3− anion acts as a tridentate ligand coordinated to three Cu2+ ions, resulting in a zigzag chain formed by decorated Keggin-type clusters based on the Cu(II) debpdc complex. In 2 and 3, the [PW12O40]3− and [SiW12O40]4− anions act as bidentate ligands coordinated to two Cu2+ ions to give decorated Keggin-type clusters based on Cu(II) H2bpdc complex moieties for the first time. The debpdc ligand has ethoxycarbonyl groups, while the H 2 bpdc ligand has protonated carboxyl groups. Carboxyl groups covalently bonding to the organic linkers of 2 and 3 not only increases the number of protons within the frameworks but also ensures the proton sources are immobilized in the conduction pathway under hydrated conditions. Thus, these three complexes would provide models to test the influence of different substituents on the proton conductivity. The measurement results reveal that 2 and 3 give high conductivity values of over 10−3 S cm−1 at 100 °C under 98% relative humidity (RH), while 1 gives a low conductivity value of over 10−5 S cm−1 under the same conditions. Here, we report their syntheses, crystal structures, and proton conductivities as a function of temperature and RH.

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EXPERIMENTAL SECTION

Materials. The title complexes were synthesized using organic solvents and materials of reagent grade. CuHPMo12O40·nH2O, CuHPW12O40·nH2O, and Cu2SiW12O40·nH2O were prepared according to a literature method.21,23 Diethyl 2,2′-bipyridine-4,4′-dicarboxylate (debpdc) was obtained from Zhengzhou Alfachem Co. Ltd. The synthesis of the [(HOCH2)3CNHCO]2(C5H3N)2 ligand was achieved by using a reported procedure,40 involving the condensation of tris(hydroxymethyl)aminomethane with diethyl 2,2′-bipyridine4,4′-dicarboxylate in methanol at reflux temperature. Syntheses. Complex 1. The copper heteropolyacid salt CuHPMo12O40·nH2O (100 mg, 0.05 mmol) and CuCl2·6H2O (8.5 mg, 0.05 mmol) were dissolved in 5 mL of water, which was placed in 1021

DOI: 10.1021/acs.inorgchem.8b00667 Inorg. Chem. 2019, 58, 1020−1029

Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Details for 1−3 param

1

2

3

empirical formula formula wt cryst syst space group a/Å b/Å c/Å β/deg V/Å3 Z Dc/g cm−3 μ/mm−1 F(000) no. of measd rflns no. of indep rflns Rint no. of refinement params R1/wR2 (I ≥ 2σ(I)) R1/wR2 (all data) GOF

C37H60N5.5O57Cu2Mo12PCl 2838.68 monoclinic P21/c 20.5213(14) 15.5426(10) 26.9702(18) 105.963(1) 8270.6 (10) 4 2.280 2.412 5482 41398 10948 0.0408 1001 0.0550/0.1497 0.0778/0.1610 1.067

C24H44N4O62Cu2W12PCl 3780.33 orthorhombic Pmmn 16.067(8) 20.577(10) 13.506(7) 90 4465(4) 2 2.812 15.992 3380 24076 3600 0.0728 275 0.0506/0.1098 0.0704/0.1164 1.071

C24H46N4O63Cu2W12Si 3760.02 orthorhombic Pmmn 15.6913(11) 20.3771(14) 13.3712(9) 90 4275.4(5) 2 2.921 16.668 3364 20610 3498 0.0524 268 0.0843/0.2276 0.0922/0.2333 1.083

atoms from the polyanion, were refined anisotropically. Hydrogens of organic molecules were generated geometrically. Coordinated water H atoms were located from difference Fourier maps and refined with isotropic temperature factors. The solvent water H atoms were not treated. It is unavoidable that there appear to be close contacts among atoms such as solvent atoms in such complex systems as the three title complexes. In addition, it is unavoidable that there is a larger than usual U(eq) range a the specified element type such as C in the nonsolvent/anion part of the structure in such a very complex system as 1 and that the Cu1 atom in 2 has a low “MainMol” U(eq) value in comparison to its neighbors. The use of a DAMP instruction (1000.0 report) in the final refinement job of 3 was carried out in order to improve the precision of C−C bonds. In 3, there are no atoms within the hydrogen bond distance of the O16 atom, resulting in O16− H16A without an acceptor. The crystal parameters, data collection, and refinement results are summarized in Table 1. Selected hydrogen bond lengths (Å) for the three complexes are summarized in Table 2. CCDC 1821826 (for 1), 1821827 (for 2), and 1821828 (for 3) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Table 2. Selected Hydrogen Bond Lengths (Å) for 1−3 D···A O1W···O34 O1W···O42 O1W···O49 O2W···O44 O2W···O50 O3W···O25 O3W···O31 O4W···O5W O4W···O50 O49···O5W O50···O9W O1W···O4W O1W···O1W O1W···O6W O16···O12W

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RESULTS AND DISCUSSION Complexes 1−3 reported here were synthesized in methanol/ acetonitrile/water solution by the layering method. H2bpdc ligands are insoluble in conventional solvents such as water, methanol, acetonitrile, and so on. Moreover, H2bpdc ligands allow various acidity-dependent coordination modes and easily give MOFs/CPs as the organic linkers. Therefore, it is very hard to build organic−inorganic complexes in which both H2bpdc ligands and electronegative POMs coordinate with metal ions in the same complex. In contrast with H2bpdc, debpdc and [(HOCH2)3CNHCO]2(C5H3N)2 ligands not only could reduce synthesis complications resulting from the effect of the pH30−33 but also have some solubility in methanol and acetonitrile. Moreover, the carboxylate ester group of the debpdc ligand and the amide group of the [(HOCH2)3CNHCO]2(C5H3N)2 ligand may be hydrolyzed into their carboxylate form in solution. With the aim of synthesisis of organic−inorganic hybrids based on Keggin-type anions and metal H2bpdc complexes and testing the ability of

O1W···O15 O1W···O8W O2W···O5

d(D···A) Complex 1 3.073 2.872 2.615 2.818 2.659 2.973 2.969 2.703 2.816 2.893 2.782 Complex 2 2.837 2.891 2.599 2.861 Complex 3 2.880 2.904 2.954

symmetry

−x, y − 1/2, −z + 1/2 −x, y + 1/2, −z + 1/2 −x, y − 1/2, −z + 1/2 −x, y + 1/2, −z + 1/2 −x, y − 1/2, −z + 1/2 −x, y + 1/2, −z + 1/2 −x, −y + 1, −z

−x + 3/2, y, z x + 1/2, −y + 2, −z + 1

−x + 2, −y + 2, −z + 2 −x + 1, −y + 2, −z + 2

the different substituents to hydrolyze into their carboxylate forms in solution, Cu2+ ion, debpdc/ [(HOCH2)3CNHCO]2(C5H3N)2, and typical Keggin-type anions were chosen to construct new types of protonconductive organic−inorganic hybrid materials. First, we tried the debpdc ligand to synthesize complex 1, a decorated Keggin-type cluster based on the Cu(II) debpdc complex. In 1, it is not observed that the debpdc ligand changed to the H2 bpdc ligand on the basis of the hydrolyzation of ethoxycarbonyl groups in situ. Then, we further tried to use the [(HOCH2)3CNHCO]2(C5H3N)2 ligand to replace the debpdc ligand. We succeeded in constructing complexes 2 and 3, two decorated Keggin-type clusters based on the Cu(II) ion and H2bpdc molecule. They are formed by an autoassembly process of the Keggin-type anion and the in situ generated 1022

DOI: 10.1021/acs.inorgchem.8b00667 Inorg. Chem. 2019, 58, 1020−1029

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Inorganic Chemistry

Figure 1. (a) {[Cu(debpdc)(H2O)3][Cu(debpdc)(H2O)Cl][PMo12O40]} cluster in 1 with atom labeling and 30% probability thermal ellipsoids. (b) Conformations of the ester groups of debpdc ligands. (c) 1D zigzag chain (blue broken lines show the Cu−O coordination interactions as Cu1−O34 and Cu2−O37; purple broken lines show the Cu−O semicoordination interactions as Cu1−O13#1). (d) 2D layer showing the hydrogen-bonding interactions (yellow broken lines). (e) 3D packing diagram view along the c axis showing two types of channels.

supported by the terminal oxygen O34 of the Mo8 octahedron, and the [Cu(debpdc)(H2O)Cl]+ ion is supported by the terminal oxygen O37 of the Mo10 octahedron. As shown in Figure 2a, in the {[Cu(H2bpdc)(H2O)2Cl0.5]2[PW12O40]} cluster of 2, two [Cu(H2bpdc)(H2O)2Cl0.5]1.5+ ions are located on symmetric sides of the [PW12O40]3− anion. The halves of crystallographically identical Cl− ions are located symmetrically at opposite sites of the decorated Keggin-type cluster, and both connect with the Cu2+ ions, featuring a Cl1···Cl1A separation of 19.44 Å. As shown in Figure 2b, in the {[Cu(H2bpdc)(H2O)2.5]2[SiW12O40]} cluster of 3, two [Cu(H2bpdc)(H2O)2.5]2+ ions are located on symmetric sides of the [SiW12O40]4− anion. The halves of the crystallographically identical coordinated water molecule (O2W) are located symmetrically at opposite sites of the decorated Keggin-type cluster, and both connect with the Cu2+ ions, featuring an O2W···O2W separation of 19.42 Å. The [Cu(H2bpdc)(H2O)2Cl0.5]1.5+ ion in 2 (or the [Cu(H2bpdc)(H2O)2.5]2+ ion in 3) is supported by the terminal oxygen O10 of the W3 octahedron of the [PW12O40]3− anion ([SiW12O40]4− for 3). As a result, the [PMo12O40]3− anion in 1 should act as a tridentate ligand coordinated to three Cu2+ ions, resulting in a zigzag chain formed by decorated Keggin-type clusters by the Cu(II) debpdc complex (Figure 1c), and the [PW12O40]3− and [SiW12O40]4− anions in 2 and 3 act as bidentate ligands coordinated to two Cu2+ ions. The P−O (1.525(5)−1.535(5) Å) and Mo−O (1.656(6)−2.441(5) Å) distances for 1, the P− O (1.541(11)−1.553(12) Å) and W−O (1.716(9)−2.467(7)

building block as H2bpdc, which should come from the [(HOCH2)3CNHCO]2(C5H3N)2 ligand on the basis of hydrolysis. Complexes 2 and 3 realize for the first time both H2bpdc and POMs as multifunctional ligands to coordinate with metal ions in decorated Keggin-type clusters. Complex 1 was synthesized by the reaction of CuHPMo12O40·nH2O, CuCl2·6H2O, and debpdc at room temperature. The reactions of CuHPW12O40·nH2O (or Cu2SiW12O40· nH2O), CuCl2·6H2O, and [(HOCH2)3CNHCO]2(C5H3N)2 ligand at room temperature give complexes 2 and 3. The results of single-crystal X-ray diffraction analyses at room temperature reveal that 1 crystallized in the monoclinic space group P21/c and 2 and 3 crystallized in the orthorhombic space group Pmmn. 1 is a decorated Keggin-type cluster based on the Cu(II) debpdc complex, and 2 and 3 contain Keggin-type clusters decorated by Cu(II) H2bpdc complex moieties. Notably, 2 and 3 are the first examples of Cu(II) H2bpdc complex moieties supported on Keggin-type anions. In them, the H2bpdc ligand acts as a bidentate nitrogen donor ligand in a chelating fashion and its COOH groups are free. Moreover, its COOH groups can act as both hydrogen bond acceptors and proton carriers. As shown in Figure 1a, the {[Cu(debpdc)(H2O)3][Cu(debpdc)(H2O)Cl][PMo12O40]} cluster in 1 is a decorated Keggin-type cluster based on the Cu(II) debpdc complex. In the cluster, {[Cu(debpdc)(H2O)3]2+ and [Cu(debpdc)(H2O)Cl]+ ions are located on two asymmetric sides of the [PMo12O40]3− anion. The [Cu(debpdc)(H2O)3]2+ ion is 1023

DOI: 10.1021/acs.inorgchem.8b00667 Inorg. Chem. 2019, 58, 1020−1029

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Inorganic Chemistry

Figure 2. (a) {[Cu(H2bpdc)(H2O)2Cl0.5]2[PW12O40]} cluster in 2 with atom labeling and 30% probability thermal ellipsoids. (b) {[Cu(H2bpdc)(H2O)2.5]2[SiW12O40]} cluster in 3 with atom labeling and 30% probability thermal ellipsoids. (c−e) views of the 2D layer, the 3D supramolecular structure down the c axis, and the 3D structure down the a axis with the 2D water layer in 2, respectively.

[Cu(debpdc)(H2O)3]2+ ion is involved in the CuN2O4 core in an octahedral environment. Two debpdc nitrogen atoms (N1 and N2) and two water oxygen atoms (O1W and O2W) occupy the equatorial coordination positions with average Cu− O and Cu−N distances of 1.988 and 1.995 Å, respectively. The water oxygen atom, O3W, and the terminal oxygen atom, O34, occupy the axial coordination positions with Cu1−O3W and Cu1−O34 distances of 2.252(6) and 2.509(5) Å, respectivel. By comparison, the Cu2 center in the [Cu(debpdc)(H2O)Cl]+ ion is involved in the CuN2O3Cl core in a tetragonally elongated octahedral environment. Two debpdc nitrogen atoms (N3 and N4), a water oxygen atom (O4W), and a Cl− ion occupy the equatorial coordination positions with Cu− O, average Cu−N, and Cu−Cl distances of 1.988, 1.997, and 2.205 Å, respectively. Two terminal oxygen atoms, O37 and O13#1, occupy the axial coordination positions. In 2 and 3, the Cu−O distances (Cu1−O10 2.539(10) Å for 2 and Cu1−O10

Å) distances for 2, and the Si−O (1.600(13)−1.612(12) Å) and W−O (1.684(9)−2.365(9) Å) distances for 3 are respectively comparable to those in organic−inorganic hybrid materials with Keggin anions as guests or ligands.21−23,45,46 Thus, the [PMo12O40]3−, [PW12O40]3−, and [SiW12O40]4− units in the three complexes have normal Keggin structures. Selected bond lengths (Å) and Cu(II) coordination spheres in 1−3 are summarized in Table 3. In 1, the Cu−O distances (Cu1−O34 2.509(5) Å and Cu2−O37 2.543(7) Å) are close to the sum (2.5−2.6 Å) of the van der Waals radii.47 Notably, the Cu2−O37 and Cu2−O13#1 (#1 −x + 1, y − 1, −z) distances are significantly different: 2.543(7) and 2.902(5) Å, respectively. The Cu2−O13#1 distance is much longer than the sum (2.5−2.6 Å) of the van der Waals radii. Thus, the [Cu(debpdc)(H2O)Cl]+ ion may be considered as coordinated to the terminal oxygen atom, O37, and semicoordinated to the terminal oxygen atom, O13#1.45 The Cu1 center in the 1024

DOI: 10.1021/acs.inorgchem.8b00667 Inorg. Chem. 2019, 58, 1020−1029

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Inorganic Chemistry

exo,exo conformation. In the [Cu(debpdc)(H2O)3]2+ ion, two ester groups for the Cu1 ion are in the endo,endo conformation (Figure 1b). Crystal-packing and hydrogenbonding interactions result in the different conformations. As shown in Figure 1d and Table 2, the assembly of the {[Cu(debpdc)(H2O)3][Cu(debpdc)(H2O)Cl][PMo12O40]} clusters forms a two-dimensional layer via hydrogen-bonding interactions between coordinated water molecules and the carbonyl groups belonging to the [Cu(debpdc)(H2O)3]2+ ion (O1W···O42#1 2.873 Å; O2W···O44#2 2.818 Å (#1 −x, y − 1/2, −z + 1/2; #2 −x, y + 1/2, −z + 1/2)). Moreover, there are weak hydrogen-bonding interactions between the [Cu(debpdc)(H2O)3]2+ ions and the [PMo12O40]3− anions (O1W···O34 3.073 Å; O3W···O25#1 2.973 Å; O3W···O31#2 2.969 Å (#1 −x, y − 1/2, −z + 1/2; #2 −x, y + 1/2, −z + 1/ 2)). In 1, each debpdc molecule has two ester groups sticking out of the two-dimensional (2D) composite layer. Finally, the packing of these layers gives two kinds of one-dimensional (1D) channels filled by solvent molecules (Figure 1e). As shown in Figure 2c, in 2, the assembly of the {[Cu(H2bpdc)(H2O)2Cl0.5]2[PW12O40]} clusters, based on the short Cl0.5···Cl0.5 separation of 1.137 Å between two adjacent decorated Keggin-type clusters, forms a 1D composite chain. Then, these 1D composite chains form a 2D layer via short interactions between the Cl atoms and the O(5) centers of the [PW12O40]3− units from two adjacent 1D composite chains (Cl1···O5#1 2.955 Å; #1 −x + 3/2, −y + 5/2, z + 2). As shown in Figure S2a in the Supporting Information, in 3, the assembly of the {[Cu(H2bpdc)(H2O)2.5]2[SiW12O40]} clusters, based on the short O0.5···O0.5 separation of 0.954 Å between two adjacent decorated Keggin-type clusters, forms a 1D composite chain. Then, these 1D composite chains form a 2D layer via short interactions between the O2W centers and the O5 centers of the [SiW12O40]4− units from two adjacent 1D composite chains (O2W···O5#1 2.991 Å; #1 −x + 3/2, −y + 5/2, z + 2). Finally, these 2D composite layers form 3D network structures depending on the hydrogen-bonding interactions among H2bpdc ligands, coordinated water molecules, solvent water molecules, and the oxygen atoms of polyanions (Figure 2d for 2; Figure S2b for 3; hydrogenbonding interactions in Table 2). Therefore, the whole packing arrangement of the {[Cu(H2bpdc)(H2O)2Cl0.5]2[PW12O40]} clusters for 2 (or the {[Cu(H2bpdc)(H2O)2.5]2[SiW12O40]} clusters for 3) and solvent water molecules results in the phase separation of hydrophilic/hydrophobic domains and the formation of the 2D hydrophilic layers (Figure 2e for 2 and Figure S2c for 3 in the Supporting Information). The up and down surfaces of each 2D hydrophilic layer contain the COOH groups of H2bpdc ligands, coordinated water molecules, and some terminal or bridging oxygen atoms from the polyanions. Complex 1 contains 1D hydrophilic channels. Complexes 2 and 3 contain a 2D hydrophilic layer. These facts suggest that debpdc and H2bpdc ligands are suitable for constructing POMbased organic−inorganic hybrid materials with a phase separation of hydrophilic/hydrophobic domains. In the three complexes, the spherical surfaces of Keggin-type anions give an opportunity for forming coordination bonds or hydrogen bonds with organic−inorganic hybrid moieties such as [Cu(debpdc)(H2O)Cl]+ and {[Cu(debpdc)(H2O)3]2+ ions for 1, the [Cu(H2bpdc)(H2O)2Cl0.5]1.5+ ion for 2, and the [Cu(H2bpdc)(H2O)2.5]2+ ion for 3. On a PerkinElmer thermal analyzer, thermogravimetric analyses of the powder of the crystalline sample of the title

Table 3. Selected Bond Lengths (Å) and Cu(II) Coordination Spheres in Complexes 1−3a

a

Symmetry transformations used to generate equivalent atoms: for 1: #1: -x+1, y-1, -z; for 2 and 3: #1: -x+3/2, y, z.

2.367(15) Å for 3) are close to the sum (2.5−2.6 Å) of the van der Waals radii, which is thus indicative of the Cu−O coordination interactions in the two complexes.44 Thus, in the [Cu(H2bpdc)(H2O)2Cl0.5]1.5+ ion for 2 or the [Cu(H2bpdc)(H2O)25]2+ ion for 3, the coordination number of the Cu1 ion is 6 (two N atoms from the H2bpdc ligand, two O atoms from two water molecules, half a Cl− ion (for 2) or half a water molecule (for 3), and one O atom from the [PW12O40]3− anion (for 2) or [SiW12O40]4− anion (for 3). This Cu1 ion shows an octahedral geometry (Figure 2a,b). In 1, the COO unit of each ester group and the attached pyridyl group are roughly coplanar. On the basis of the plane which divides two pyridyl groups of the debpdc ligand, the conformations of the ester groups are endo (directed toward the plane) or exo (away from the plane). In the [Cu(debpdc)(H2O)Cl]+ ion, two ester groups for the Cu2 ion are in the 1025

DOI: 10.1021/acs.inorgchem.8b00667 Inorg. Chem. 2019, 58, 1020−1029

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Figure 3. For the three title complexes (a−c) ac impedance plots under 98% RH at different temperatures, (d) log σ (S cm−1) versus temperature plots under 98% RH, (e) log σ (S cm−1) versus RH plots at 25 and 100 °C, and (f) Arrhenius plots of the proton conductivities.

1 reveals that there is a weight loss of about 6.45% in the temperature range 20−300 °C, corresponding to the loss of all solvent molecules (6.32%). The results of complexes 2 and 3 reveal that there is a weight loss of about 4.61% for 2 and 4.64% for 3 in the temperature range 20−200 °C, corresponding to the loss of all solvent molecules (4.76% for 2 and 4.79% for 3). Subsequent heating results in the complete decomposition of the cluster structures of the three complexes at temperatures above 300 °C for 1 and 200 °C for 2 and 3 with a clifflike weight loss in the curves. The X-ray powder diffraction patterns of the title complexes after heating in air at 750 °C for 1 and at 800 °C for 2 and 3 are shown in Figures S4d−f in the Supporting Information. For 1, the peaks are mainly ascribed to the MoO3 phase. For 2 and 3, the peaks are ascribed to the WO3 phase. The three complexes are decorated Keggin-type clusters based on the Cu(II) ion and 2,2′-bipyridine derivatives (debpdc for 1 and H2bpdc for 2 and 3). Key factors for proton-conducting materials include stability, water retention, water insolubility, hydrophilic channels or layers, hydrogenbonding networks, and proton carriers. In comparison to simple Keggin-type POMs, the three title complexes are

complexes were performed under nitrogen at a heating rate of 10 °C min−1 (Figure S3 in the Supporting Information). The result of complex 1 reveals that there is a weight loss of about 6.53% in the temperature range 20−300 °C, corresponding to the loss of all solvent molecules (6.32%). The results of complexes 2 and 3 reveal that there is a weight loss of about 4.41% for 2 and 4.58% for 3 in the temperature range 20−300 °C, corresponding to the loss of all solvent molecules (4.76% for 2 and 4.79% for 3). The cluster structures of the three complexes begin to decompose above 300 °C due to the loss of coordinated water molecules and debpdc or H2bpdc ligands, as well as the disruption of the structural skeletons of the Keggin-type anions. The weight losses are about 3.32% for 1, 1.30% for 2, and 0.76% for 3 at 100 °C, suggesting that the hydrophilic channels or layers in three complexes could retain some uncoordinated water molecules at 100 °C. The results indicate that three complexes have good water retention in the temperature range 20−100 °C. In addition, thermogravimetric analyses of the powder of the crystalline sample of the title complexes were also performed in an air atmosphere at a heating rate of 10 °C min−1 (Figure S4a−c in the Supporting Information). The result of complex 1026

DOI: 10.1021/acs.inorgchem.8b00667 Inorg. Chem. 2019, 58, 1020−1029

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Inorganic Chemistry

groups of the H2bpdc ligand in 2 and 3, three complexes show better proton conductivities (over 10−5 S cm−1 for 1 and 10−3 S cm−1 for 2 and 3) at 100 °C than at 25 °C in the RH range 35−98%. In comparison with representative Keggin-typecluster-based compounds with good proton conductivities (Table S1 in the Supporting Information), the title complexes showed similar or lower proton conductivities under the same conditions, and complexes 2 and 3 reveal some of the best reported conductivity values (over 10−3 S cm−1) at 100 °C under 98% RH. Under the same conditions, the conductivities of 1 are lower than those of 2 and 3. It is necessary to determine the reasons for the favorable proton conductivities of 2 and 3 and the crucial difference resulting in the conductivities of 1 being lower than those of 2 and 3. One of the main reasons is the RH and temperature for the measurements. The RH and temperature are two of the key factors that improve proton conductivity. High RH and temperature could accelerate proton transition within the hydrophilic channels and layers.19,21 Another of the main reasons is the inseparable relation between the decorated Keggin-type clusters and the different substituents of 2,2′bipyridine. The Cu(II) 2,2′-bipyridine derivatives complexes supported by the Keggin-type clusters would provide enough proton hopping sites to speed up proton transportation in the three title complexes. Complex 1 has debpdc ligands with ethoxycarbonyl groups, while complexes 2 and 3 have H2bpdc ligands with free COOH groups, whose protons could be dissociated for proton conductivity. Carboxyl groups being covalently bonded to the organic ligands of decorated Keggintype clusters could ensure that the proton sources are immobilized in the conduction pathway under hydrated conditions. The protonated H2bpdc ligands have the ability to form hydrogen bonds to give hydrogen-bonding networks, which act as the pathways for proton translocation in 2 and 3. In addition, 2 and 3 have hydrophilic layers, while 1 has hydrophilic channels. These structural differences result in 2 and 3 having higher proton conductivities in comparison to 1 under the same conditions. Arrhenius plots of the proton conductivities of the three complexes in the temperature range of 25−100 °C under 98% RH are shown in Figure 3f. On the basis of an Arrhenius relationship, the activation energy of the proton transfer in the title complexes could be extracted from q variable-temperature impedance study. The ln σT values of each of the three complexes increase almost linearly with temperature in the range from 25 to 100 °C, and the corresponding activation energy (Ea) of conductivity was estimated to be 0.28 eV for 1, 0.52 eV for 2, and 0.57 eV for 3. These Ea values are comparable to those of pure heteropolyacids (0.25−0.4 eV).18 In addition, these Ea values are also comparable with those of representative examples of organic/inorganic complexes based on Keggin-type clusters and 2,2′-bipyridine derivatives. In the temperature range of 25−100 °C under 98% RH, their activation energy values are comparable to that of {[Cu(dmbipy)(H2O)2Cl0.5]2[PW12O40]}·7H2O (0.48 eV)47 and lower than those of {H[Cu(Hbpdc)(H2O)2]2[PM12O40]· nH2O}n (M = Mo, W) and {H[Ni(Hbpdc)(H2O)2]2[PW12O40]·nH2O}n (1.02 and 1.01 eV).21,22 The results of the X-ray powder diffraction studies indicated that the powder samples after the proton-conductive measurements have the same networks as those of the three complexes, respectively (Figure S5 in the Supporting Information). Thus, even when they are held at 100 °C with a RH higher than 98%,

insoluble in water and show good stability and water retention. In terms of the stability of solid proton-conducting materials, water-insoluble compounds are more promising than watersoluble compounds. On the basis of coordinated and electrostatic interactions, the Keggin-type anions are not easily dissociated from the hybrid clusters. The pronounced hydrophobic/hydrophilic separation in the three complexes could induce water segregation into hydrophilic channels for 1 and hydrophilic layers for 2 and 3. Keggin-type anions being inlaid into the ordered channels (or layers) could provide more hopping sites in the channels (or layers) for proton conduction. Furthermore, protons as possible carriers should be one of the necessary conditions for proton conductors. The types of proton carriers mainly include nonvolatile guest molecules (H2SO4, H3PO4, imidazole, triazole, NH4+, and (Me2NH2)+), organic ligands (−COOH, −NH2, −OH), coordinated water molecules, and free protons for charge balance.2 In the three complexes, open coordination sites of Cu2+ ions are bound by water molecules, which could act as proton donors. In addition, the COOH groups of the H2bpdc ligand in 2 and 3 could also act as proton carriers. Their proton conductivity should originate from these protons. In a word, the three complexes may be good solid proton-conducting materials and suitable models to study the effect of different substituents of 2,2′-bipyridine on their proton conductivities. Their water vapor adsorption isotherms have been measured at room temperature and normal pressure (Figure S6 in the Supporting Information). Three complexes show good capacity for the inclusion of water molecules. Complexes 2 and 3 adsorb water molecules to a similar degrees. Conductivity measurements help to give some insight into the dependence of the proton mobility in their hydrophilic channels or layers on the temperature and RH. Alternating current (ac) impedance spectroscopy measurements using compacted pellets of the powdered crystalline samples were performed on an electrochemical impedance analyzer with copper electrodes. The agreement between the observed and simulated X-ray powder diffraction patterns confirmed the phase purities of the three bulk materials (Figure S5 in the Supporting Information). The ac impedance method was used to evaluate their proton conductivities when the temperature and RH were varied (such as at 25 or 100 °C in the RH range 35−98% and at 98% RH in temperature range from 25 to 100 °C). Nyquist plots of the impedance spectra of pelleted samples of 1−3 under 98% RH in the temperature range from 25 to 100 °C are shown in Figure 3a−c. As seen from Figure 3, proton conductivities of three complexes depend closely on the temperature and humidity. It can be seen from Figure 3e that proton conductivities of complex 1 show almost no change with the RH (10−6 S cm−1 at 25 °C and 10−5 S cm−1 at 100 °C), while conductivities of complexes 2 and 3 increase gradually with the RH (10−6−10−5 S cm−1 at 25 °C and 10−4− 10−3 S cm−1 at 100 °C). It can be seen from Figure 3d that the proton conductivities of three complexes under 98% RH increase gradually with the temperature. At 98% RH, the conductivities of 1 increase from 6.44 × 10−6 S cm−1 at 25 °C to 4.71 × 10−5 S cm−1 at 100 °C, those of 2 rise to 1.40 × 10−3 S cm−1 at 100 °C from 3.07 × 10−5 S cm−1 at 25 °C, and those of 3 add up to 1.77 × 10−3 S cm−1 at 100 °C from 2.36 × 10−5 S cm−1 at 25 °C. Due to a high carrier concentration based on the dissociating processes of protons from coordination water molecules and the COOH 1027

DOI: 10.1021/acs.inorgchem.8b00667 Inorg. Chem. 2019, 58, 1020−1029

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the three complexes should not hydrolyze. In the three complexes, the hydrophilic channels for 1 and the hydrophilic layers for 2 and 3 could provide favorable pathways for proton conduction.The dependence of the proton conductivities of the three complexes on the RH demonstrates that water-rich conditions are important in the conduction pathways. Moreover, the dependence of proton conductivities on the temperature indicates that a high temperature is more important for obtaining a high carrier concentration on the basis of the dissociating processes of protons from coordination water molecules and the COOH groups of the H2bpdc ligand in 2 and 3. Thermal analyses of the three complexes indicated that a part of the solvent molecules is lost below 100 °C. These facts suggest that the proton conduction for the three complexes is based on the direct diffusion of additional protons with water molecules: that is, the vehicle mechanism.48

§

H.Y. and X.-Y.D. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21501047 and 21171050).

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CONCLUSION In summary, we designed and synthesized three decorated Keggin-type clusters based on Cu(II) debpdc/H2bpdc complexes with proton conductivity in the temperature range of 25−100 °C under 35−98% RH. H2bpdc as a protonated ligand could supply acidic protons in 2 and 3, resulting in good proton conductivity as high as 1.40 × 10−3 S cm−1 for 2 and 1.77 × 10−3 S cm−1 for 3 at 100 °C under 98% RH. In comparison with representative Keggin-type-cluster-based compounds with good proton conductivities, 2 and 3 are two of the best Keggin-type-cluster-based materials with one of the highest conductivity values. The successful syntheses of the three complexes will provide a method for the preparation of organic−inorganic hybrid based on Keggin-type anions and metal−organic complexes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00667. X-ray photoelectron spectroscopy in the energy region of Cl 2p in complexes 1 and 2, additional structural figures, additional characterization data such as TGA curves and PXRD patterns, water vapor absorption isotherms for three complexes, and representative Keggin-type-cluster-based compounds with good proton conductivities (PDF) Accession Codes

CCDC 1821826−1821828 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

AUTHOR INFORMATION

Corresponding Authors

*E-mail for X.-Y.D.: [email protected]. *E-mail for M.-L.W.: [email protected]. ORCID

Xian-Ying Duan: 0000-0001-8535-6329 Mei-Lin Wei: 0000-0003-3592-3545 1028

DOI: 10.1021/acs.inorgchem.8b00667 Inorg. Chem. 2019, 58, 1020−1029

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DOI: 10.1021/acs.inorgchem.8b00667 Inorg. Chem. 2019, 58, 1020−1029