Assembly of Six Polyoxometalate-Based Hybrid Compounds from a

Article pubs.acs.org/crystal

Assembly of Six Polyoxometalate-Based Hybrid Compounds from a Simple Supramolecule to a Complicated Pseudorotaxane Framework via Tuning the pH of the Reaction Systems Shaobin Li, Huiyuan Ma,* Haijun Pang,* and Li Zhang Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China S Supporting Information *

ABSTRACT: Three pairs of inorganic−organic hybrid compounds respectively based on [Mo6O19]2−/[Mo8O26]4−, [PW12]3− and [P2W18]6− anions were obtained at different pH values under hydrothermal conditions, namely, [H2bib][Mo6O19] (1), CuII(bib)1.5(H2O)(β-Mo8O26)0.5 (2), [Hbib]2[HPW12O40] (3), [bib0.5][CuII(bib)3CuI(bib)0.5(PW12O40)] (4), [H2bib][CuII(bib)2.5(H2O)(H2P2W18O62)]·2H2O (5), and [CuIbib][(CuIbib)2(H2KP2W18O62)]·4H2O (6), (bib = 1,4-bis(1-imidazolyl)-2,5-dimethylbenzene). Their structures have been determined by single-crystal X-ray diffraction analyses and characterized by infrared spectra (IR), elemental analyses, and powder X-ray diffraction (PXRD) patterns. Compounds 1 and 3 are zerodimensional (0D) monomers, in which the protonated bib ligands as countercations combine with [Mo6O19]2−/[PW12O40]3− anions by the intermolecular hydrogen bonding interactions forming a supramolecular layer. In 2, the (4, 4) two-dimensional (2D) layer is formed by β-[Mo8O26]4− anions as double-dentate inorganic ligands to link “rail-like” Cu-bib chains. Compound 4 shows a complicated three-dimensional (3D) framework with a (61·102)(44·62)(62·81) topology. Compound 5 is a onedimensional (1D) polypendant chain, and the adjacent chains form a 1D + 1D → 2D interdigitated architecture by hydrogen bonding interactions. Compound 6 is an interesting 3D architecture with a novel pseudorotaxane framework. After careful investigations of these compounds, we noticed that with the increasing pH of the reaction systems, the compounds with more complicated structures were obtained without reference to the kinds of polyoxometalates (POMs). The isolation of these six compounds is very informative for systematically understanding of the effect of the pH on assembly of POM-based hybrids. Additionally, the electrochemical properties of the compounds have been investigated in detail.

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especially in catalysis and electrochemistry.4,5 The POM anions as a well-defined library of inorganic building blocks can link variable transition-metal complexes (TMCs) for the construction of various hybrid compounds, so-called POM-based inorganic−organic hybrid materials. Such materials can combine the advantages of both POMs and common inorganic−organic hybrid materials. Thus, the POM-based inorganic−organic hybrid materials have gained the attention of chemists, and much effort has been devoted to their preparation and relevant applications over the past few

INTRODUCTION Crystal engineering is the rational design and assembly of solid state structures with desired structural topologies and properties via the manipulation of ionic or molecular building blocks toward a specific disposition in the solid state.1 As a new generation of solid-state materials,1f inorganic−organic hybrid compounds have various structures as well as wide-ranging applications from gas storage and catalysis to material science,2,3 and the syntheses of inorganic−organic hybrid compounds through crystal engineering sprung up rapidly, thanks to the quick development of characterization techniques. Polyoxometalates (POMs), an outstanding class of inorganic building blocks with unique structural characteristics and diversity structures, exhibit versatile applications in given fields, © 2014 American Chemical Society

Received: April 17, 2014 Revised: June 23, 2014 Published: July 25, 2014 4450

dx.doi.org/10.1021/cg5005352 | Cryst. Growth Des. 2014, 14, 4450−4460

Crystal Growth & Design

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Table 1. Crystal Data and Structure Refinements for 1−6 formula fw T (K) 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) refl measured refl unique Rint F(000) R1a, wR2b [I > 2σ(I)] GOF on F2 a

1

2

3

4

5

6

C14H16Mo6N4O19 1119.93 293(2) triclinic P1̅ 7.741(5) 9.604(5) 10.200(5) 113.291(5) 107.492(5) 92.450(5) 652.9(6) 1 2.843 2.890 3256 2298 0.0153 530.0 R1a = 0.0285 wR2b = 0.0892 1.095

C21H22CuMo4N6O14 1029.74 293(2) triclinic P1̅ 11.4391(5) 11.9757(5) 12.3265(6) 67.301(1) 87.752(1) 73.715(1) 1490.78.(12) 2 2.290 2.416 11498 7428 0.0380 994.0 R1a = 0.0295 wR2b = 0.0699 1.013

C28H31PW12N8O40 3356.63 293(2) triclinic P1̅ 9.9705(16) 11.4405(19) 13.258(2) 76.876(2) 78.266(3) 65.948(3) 1334.5(4) 1 4.137 25.883 6804 4688 0.0257 1475.0 R1a = 0.0593 wR2b = 0.1529 1.059

C56H55Cu2N16O40PW12 3956.31 293(2) monoclinic P21/n 15.117(5) 24.346(5) 24.353(5) 90 99.291(5) 90 8845(4) 4 2.971 16.111 44158 15568 0.0606 7136.0 R1a = 0.0835 wR2b = 0.2372 1.099

C49H59CuN14O65P2W18 5318.62 293(2) triclinic P1̅ 14.505(5) 15.007(5) 22.506(5) 88.425(5) 83.881(5) 81.521(5) 4818(3) 2 3.659 21.743 16974 24119 16974 4704.0 R1a = 0.0585 wR2b = 0.1671 1.005

C42H44Cu3KN12O62P2W18 5309.67 293(2) monoclinic P21/n 13.152(5) 25.208(5) 34.395(5) 90 98.430(5) 90 11280(5) 4 3.125 18.971 56805 19871 0.0685 1675.0 R1a = 0.0718 wR2b = 0.1975 0.926

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

decades.6 The hydrothermal synthesis method is a powerful technique for the preparation of the POM-based hybrid materials. Nevertheless, from a crystal engineering point of view, structural control of the resulting hybrid compounds via hydrothermal synthesis is still very challenging work, because hydrothermal reactions are commonly termed as a “black box” and the final structures of hybrid compounds are frequently modulated by various factors.7 To address the effect of these factors on the construction of the hybrids, many efforts need to be made by researchers.8 It is well-known that the pH of a reaction system represents one of the important factors in the self-assembly process, and by tuning the pH the structures of the final compounds could be controlled. The influence of pH on the construction of POM-based hybrids has been explored in some pioneering work.9 However, all of the previous work studied the correlation between pH of the reaction and structure of the produced compounds just according to the reaction systems with one type of POM anion. In order to make the research on correlation more systematical and substantial, reaction systems with one more type of POM should be taken as representatives. So we synchronously introduce one kind of isopolymolybdate ((NH4)6Mo7O24) and two kinds of heteropolytungstates ([PW12]3− and [P2W18]6− anions) into an identical copperbib reaction system, respectively. Herein, six new POM-based inorganic−organic hybrid compounds, namely, [H2bib][Mo 6 O 19 ] (1), Cu II (bib) 1.5 (H 2 O)(β-Mo 8 O 26 ) 0.5 (2), [Hbib] 2 [HPW 12 O 40 ] (3), [bib 0.5 ][Cu II (bib) 3 Cu I (bib) 0.5 (PW12O40)] (4), [H2bib][CuII(bib)2.5(H2O)(H2P2W18O62)]· 2H2O (5), and [CuIbib][(CuIbib)2(H2KP2W18O62)]·4H2O (6) (bib = 1,4-bis(1-imidazol-yl)-2,5-dimethylbenzene), were obtained under similar hydrothermal conditions except different pH values. The structural analyses of these compounds reveal that with the increasing pH of the reaction, the dimensionality of the obtained hybrids rose without the effect of the kinds of POMs. The isolation of these six compounds is very informative for systematically understanding of the effect

of the pH on the assembly of POM-based hybrids. In addition, the electrochemical properties of the title compounds have been investigated in detail.

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EXPERIMENTAL SECTION

Materials and Methods. α-K6P2W18O62·15H2O was prepared according to the literature method.10 All other reagents were purchased commercially and were used without further purification. Elemental analyses (C, H, and N) were performed on a PerkinElmer 2400 CHN Elemental analyzer, and that of Cu, W, and Mo was carried out with a Leaman inductively coupled plasma (ICP) spectrometer. The FT-IR spectra were recorded from KBr pellets in the range 4000− 400 cm−1 with a Nicolet AVATAR FT-IR360 spectrometer. A CHI660 electrochemical workstation was used for control of the electrochemical measurements and data collection. A conventional threeelectrode system was used, with a carbon paste electrode (CPE) as a working electrode, a commercial Ag/AgCl as reference electrode, and a twisted platinum wire as counter electrode. The CPEs were fabricated according to the method reported in previous literature.8c The powder X-ray diffraction (PXRD) data were collected on a Rigaku RINT2000 diffractometer at room temperature. Synthesis of [H2bib][Mo6O19] (1). A mixture of (NH4)6Mo7O24· 4H2O (0.37 g, 0.3 mmol), CuCl2·2H2O (0.16 g, 0.9 mmol), bib (0.06 g, 0.25 mmol), and water (10 mL) was stirred for 1 h. The resulting solution was transferred to a Teflon-lined autoclave and kept under autogenous pressure at 160 °C for 4 days with a pH = 1.5−2.5 adjusted by 3 M HCl. After slow cooling of the solution to room temperature, yellow block crystals of 1 were filtered, washed with distilled water, and dried at room temperature (yield, 49%, based on Mo). Anal. Calcd for C14H16Mo6N4O19: C 15.01, H 1.44, N 5.00, Mo 51.40%; Found: C 14.29, H 1.52, N 4.87, Mo 50.11%. Synthesis of CuII(bib)1.5(H2O)(β-Mo8O26)0.5 (2). Compound 2 was prepared similarly to 1, except that the pH of the reaction was adjusted to 3.2−3.9. Blue block crystals of 2 were filtered, washed with distilled water, and dried at room temperature (yield, 43%, based on Mo). Anal. Calcd for C21H22CuMo4N6O14: C 24.49, H 2.15, N 8.16, Cu 6.17, Mo 37.27%; Found: C 24.11, H 2.24, N 8.25, Cu 6.01, Mo 36.48%. Synthesis of [Hbib]2[HPW12O40] (3). A mixture of H3PW12O40· 13H2O (0.32 g, 0.17 mmol), CuCl2·2H2O (0.16 g, 0.9 mmol), bib 4451



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(0.06 g, 0.25 mmol), and water (10 mL) was stirred for 1 h. The resulting solution was transferred to a Teflon lined autoclave and kept under autogenous pressure at 160 °C for 4 days with a pH = 2.5−3.5 adjusted by 3 M HCl. After slow cooling of the sample to room temperature, brown block crystals of 3 were filtered, washed with distilled water, and dried at room temperature (yield, 47%, based on W). Anal. Calcd for C28H31PW12N8O40: C 10.02, H 0.93, N 3.34, W 65.72%; Found: C 10.17, H 1.04, N 3.21, W 63.49%. Synthesis of [bib0.5][CuII(bib)3CuI(bib)0.5(PW12O40)] (4). Compound 4 was prepared similarly to 3, except that the pH of the reaction was adjusted to 4.0−4.6. Green block crystals of 4 were filtered, washed with distilled water, and dried at room temperature (yield, 41%, based on W). Anal. Calcd for C56H55Cu2N16O40PW12: C 17.00, H 1.40, N 5.66, Cu 3.21, W 55.76%; Found: C 16.88, H 1.50, N 5.79, Cu 3.03, W 54.91%. Synthesis of [H2bib][CuII(bib)2.5(H2O)(H2P2W18O62)]·2H2O (5). A mixture of α-K6P2W18O62·15H2O (0.56 g, 0.12 mmol), CuCl2·2H2O (0.16 g, 0.9 mmol), bib (0.06 g, 0.25 mmol), and water (10 mL) was stirred for 1 h. The resulting solution was transferred to a Teflon lined autoclave and kept under autogenous pressure at 160 °C for 4 days with a pH = 2.0−3.0 adjusted by 3 M HCl. After slow cooling of the sample to room temperature, green block crystals of 5 were filtered, washed with distilled water, and dried at room temperature (yield, 45%, based on W). Anal. Calcd for C49H56CuP2W18N14O65: C 11.07, H 1.06, N 3.69, Cu 1.20, W 62.25%; Found: C 11.26, H 1.15, N 3.88, Cu 1.32, W 63.41%. Synthesis of [CuIbib][(CuIbib)2(H2KP2W18O62)]·4H2O (6). Compound 6 was prepared similarly to 5, except that the pH of the reaction was adjusted to pH = 3.4−3.8. Red sheet crystals of 6 were filtered, washed with distilled water, and dried at room temperature (yield, 49%, based on W). Anal. Calcd for C42H44Cu3KN12O62P2W18: C 9.50, H 0.84, N 3.17, Cu 3.59, W 62.32%; Found: C 9.38, H 0.95, N 3.29, Cu 3.41, W 60.53%. X-ray Crystallographic Study. Single-crystal X-ray diffraction data collections of compounds 1−6 were performed using a Bruker Smart Apex CCD diffractometer with Mo−Kα radiation (λ = 0.71073 Å) at 293 K. Multiscan absorption corrections were applied.11 The structure was solved by Direct Methods and refined by full-matrix least-squares on F2 using the SHELXTL 97 crystallographic software package.12 The organic hydrogen atoms were generated geometrically. All H atoms on C and N atoms were fixed at the calculated positions, while H atoms on water molecules cannot be found from the residual peaks and were directly included in the final molecular formula. The reported refinement of 6 is of the guest-free structures using the *.hkp file produced by using the SQUEEZE routine. There are at least four solvent water molecules in the crystal structure of 6. These water molecules were directly included in the final molecular formula based on the elemental analysis and SQUEEZE analyses. A summary of the crystal data, data collections, and refinement parameters for 1−6 is listed in Table 1. Crystallographic data for the structures reported in this paper have been deposited in the Cambridge Crystallographic Data Center with CCDC Nos. 997156−997161 for 1−6.

Figure 1. Asymmetric unit of compound 1 (all of the hydrogen atoms are omitted for clarity).

is a supramolecular structure, and the intermolecular hydrogen bondings existing between the adjacent Mo6 anion and biprotonated bib ligands as countercations stabilize a twodimensional (2D) supramolecular structure (the typical hydrogen bondings included in the 2D layer: C1−H1···O5 = 2.866 Å, C2−H2···O8 = 2.999 Å) (Figure 2). Structure Description of Compound 2. Compound 2 was prepared similarly to 1, except that the pH of the reaction was adjusted to 3.2−3.9. X-ray crystal structure analysis reveals that compound 2 consists of one Cu(II) ion, one and a half bib ligands, and half a [β-Mo8O26]4− anion (abbreviated as β-Mo8) (Figure 3). The β-Mo8 anion acts as a double-dentate inorganic ligand to link two Cu ions. Cu1(II) is five-coordinated in tetragonal pyramidal coordination geometry,6c coordinated by three nitrogen atoms (N1, N3, N5) from three bib ligands and two oxygen atoms (O9, O1w) from one β-Mo8 anion and one crystalline water molecule, respectively. The Cu−N bond lengths are in the range of 1.988(3)−2.002(3) Å, and the Cu− O bond lengths are in the range of 1.980(2)−2.234(3) Å. All of these bond lengths are within the normal ranges observed in other Cu(II)-containing complexes.14 The structure of compound 2 shows a 2D grid layer and achieved by β-Mo8 anions as double-dentate inorganic ligands linking to “rail-like” Cu-bib chain (Figure 4a,b). There are two kinds of windows in the layer (Figure S1, Supporting Information), in which the windows I is comprised by four Cu atoms and four bib ligands, and the size of the window is ca. 13.469 × 13.275 Å. The windows II is comprised by four Cu atoms, two bib ligands, and two β-Mo8 anions, and the size of the window ca. 13.469 × 11.903 Å. Furthermore, these adjacent layers are fused together via hydrogen bonding interactions to form a porous 3D framework (Figure 4c). Structure Description of Compound 3. Compound 3 was obtained at the pH = 2.5−3.5. Single-crystal X-ray diffraction analysis reveals that 3 consists of one protonated Keggin-type [HPW12O40]2− anion (abbreviated as PW12) and two monoprotonated bib ligands (Figure 5). In addition, the central four μ4-O atoms of PW12 polyanions are observed to be disordered over eight positions with each oxygen site halfoccupied, which is a usual phenomenon for Keggin clusters.15 Compound 3 is a supramolecular structure, and the intermolecular hydrogen bondings existing between the adjacent PW12 anion and monoprotonated [Hbib]+ countercations stabilize the 2D supramolecular layer (the hydrogen bondings included in the 2D layer: C10−H10···O11 = 2.609 Å, C11−H11A···O5 = 2.458 Å) (Figure 6). Structure Description of Compound 4. Compound 4 was prepared similarly to 3, except that the pH of the reaction was adjusted to 4.0−4.6. The asymmetric unit of 4 consists of

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RESULTS AND DISCUSSION Structure Description of Compound 1. Compound 1 was obtained at the pH = 1.5−2.5. Single-crystal X-ray diffraction analysis reveals that compound 1 consists of one [Mo6O19]2− anion (abbreviated as Mo6) and one biprotonated bib ligand (Figure 1). The structure of Mo6 anion displays the well-known Lindqvist configuration that consists of six MoO6 octahedras with three distinct types of Mo−O bond lengths. According to their different coordination environments in the polyanion, the oxygen atoms can be divided into three groups: terminal oxygen atoms (Ot); bridging oxygen atoms (Ob); and central oxygen atoms (Oc). The average distances are 1.685 Å, 1.924, and 2.318 Å for Mo−Ot, Mo−Ob, and Mo−Oc, respectively, which are consistent with the previous reports.13 Compound 1 4452



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Figure 2. 2D supramolecular structure formed by hydrogen bondings.

Figure 5. Asymmetric unit of compound 3. (All of the hydrogen atoms are omitted for clarity.)

and O8) from the PW12 anions and two nitrogen atoms (N1 and N3) from two bib ligands. The Cu−N bond lengths are in the range of 1.90(3)−2.01(2) Å, and the Cu−O bond lengths are in the range of 2.46(2)−2.68(7) Å. One structural feature of 4 is a complicated 3D structure constructed by 2D Cu-bib undulated layers and 2D PW12−Cubib polypendant layers, which can be described as follows: (i) On the one hand, Cu1 atoms connect μ2-mode bib ligands leading to a (4, 4)-connected 2D Cu1-bib layer with quadrangular windows (The dimension of each window is ca. 13.254 × 13.342 Å, Figure S2a, Supporting Information.) When the 2D Cu1-bib layer is rotated clockwise for ∼90° along the given direction, an undulated layer can be observed (Figure 8a). On the other hand, the PW12 anions, Cu2 atoms, and μ1- and μ2-mode bib ligands are fused together via Cu−O and Cu−N

Figure 3. Asymmetric unit of compound 2 and the coordination environments around the Cu atom. (All of the hydrogen atoms are omitted for clarity.)

one Cu(II) ion, one Cu(I) ion, three and a half bib ligands, one PW12 anion, and free half a bib ligand (Figure 7). There are two crystallographically independent Cu ions with two kinds of coordination modes. Cu1(II) is six-coordinate in distorted octahedral geometry,7e and achieved by two oxygen atom (O2, O19) from the PW12 anion and four nitrogen atoms (N7, N9, N11, N14) from four bib ligands. Cu2(I) is four-coordinate in a “seesaw” geometry,6d and achieved by two oxygen atoms (O1

Figure 4. (a) β-Mo8 anions as double-dentate inorganic ligands linking to “rail-like” Cu-bib chain, (b) 2D layers formed by β-Mo8 anions and “raillike” Cu-bib chain, (c) the porous 3D framework in 2 (symmetry code: #1, −x, 1 − y, −z). 4453



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as P2W18), one coordination water molecules, two free water molecules, and free biprotonated half a bib ligand (Figure 10). The P2W18 anion acts as a bidentate inorganic ligand linking two symmetrical Cu1 ions. The Cu1(II) is six-coordination in distorted octahedral geometries,7e coordinated by three nitrogen atoms (N1, N5, N9) from three bib ligands and three oxygen atoms (O29, O42, O1w) from two P2W18 anion and one crystalline water molecule (Figure 10). The Cu−N bond lengths range from 1.974(2) to 2.008(3) Å, and the Cu−O lengths are in the range of 2.043(2)−2.611(3) Å. All of these bond lengths are within the normal ranges observed in other Cu(II)-containing complexes.14 One of the structural features for 5 is the unusual 1D + 1D → 2D interdigitated architecture that can be described as follows: first, the P2W18 anions as bidentate ligands link Cu atoms to form an inorganic chain, and these adjacent inorganic chains are further connected by μ2-mode bib ligands to form an inorganic−organic chain with many honeycomb-like windows (the dimension of each window is ca. 7.55 × 13.30 Å, Figure S3, Supporting Information) (Scheme S1). When the inorganic−organic chain is rotated for ∼90° along a given direction, it is also a polypendant chain, in which the μ1-mode bib molecules as pendants are appended to the two sides of the chain (Figure 11a,b). Interestingly, the windows of each chain provide sufficient voids for the μ1-mode bib pendants in adjacent chains to interdigitate each other, forming a 1D + 1D → 2D interdigitated architecture (Figure 11c,d). Complementary intermolecular hydrogen bondings existing between the two adjacent layers stabilize this structure (typical hydrogen bondings: C12−H12···O12 = 3.39 Å, C2−H2···O61 = 3.14 Å, and C3−H3···O28 = 3.18 Å). Structure Description of Compound 6. Compound 6 was prepared similarly to 5, except that the pH of the reaction was adjusted to 3.4−3.8. Single-crystal X-ray diffraction analysis reveals that the asymmetric unit of 6 consists of three Cu(I) caions, three bib ligands, one K+ cation, one P2W18 anion, and four H2O molecules (Figure 12). There are three crystallographically independent Cu ions with two kinds of coordination modes. Both Cu1(I) and Cu2(I) are four-coordinate in a “seesaw” geometry6d and achieved by two oxygen atom (O25, O56 and O53, O59) from the P2W18 anions and two nitrogen atoms (N1, N3 and N6, N8) from bib ligands, respectively. The Cu3(I) is twocoordination in line coordination geometry9e and achieved by two nitrogen atoms (N10 and N11) from two bib ligands (Figure 12). The Cu−N bond lengths range from 1.86(2) to 1.91(3) Å, and the Cu−O lengths are in the range of 2.67(1)− 2.74(2) Å. The most fascinating structural feature for 6 is the complicated (1D + 3D) metal−organic pseudorotaxane framework (MOPRF) that can be described as follows: first, the P2W18 anions link Cu1 and Cu2 cations to generate a 2D inorganic layer, which possess two kinds of windows. The dimension of window A is ca. 16.77 × 10.59 Å, and B is ca. 8.27 × 11.66 Å (Figure S4, Supporting Information). The μ2-mode bib ligands as pillars link adjacent inorganic layers to generate a highly opened 3D framework with two kinds of channels (channel A and B) (Figure S5a). From the topological view, if each P2W18 anion, Cu1 and Cu2 is considered as a 4-connected node, the structure of the 3D framework in 6 is a novel (4, 4, 4)-connected framework with (84·62)(64·82)(63·83) topology (Figure S5b). Meanwhile, the Cu3 cations link other μ2-mode bib ligands to form an infinite inorganic−organic chain (Figure

Figure 6. 2D supramolecular layer formed by hydrogen bonding interactions.

Figure 7. Asymmetric unit of compound 4 and the coordination environments around the Cu atoms. (All of the hydrogen atoms are omitted for clarity.)

bonds to form a (6, 3)-connected 2D PW12-Cu-bib layer with an approximate rectangular window ca. 13.542 × 24.346 Å. When the 2D PW12-Cu-bib layer is rotated clockwise for ∼90° along a given direction, it is a polypendant layer, in which the μ1-mode bib molecules as pendants are appended to the two sides of the layer (Figure 8b, Figure S2b). (ii) The Cu-bib undulated layer and adjacent polypendant layer are connected together via Cu1−O8 bonds to achieve a 3D framework (Figure 8c). From the topological view, if each the PW12 anion, Cu1 and Cu2 cation is considered as a 3-, 4-, and 3-connected node, respectively, the structure of 4 is a novel (3, 4, 3)connected framework with (61·102)(44·62)(62·81) topology (Figure 9). Structure Description of Compound 5. Compound 5 was obtained at the pH = 2.0−3.0. The asymmetric unit of 5 consists of one Cu(II) ion, two and a half bib ligands, and one protonated Dawson-type [H2P2W18O62]3− anion (abbreviated 4454



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Figure 8. (a) View of the 2D undulated layers, (b) view of the 2D polypendant layer, (c) illustration of the 3D framework in 4.

comprehensive range and classifies the POM-based MOPRFs into four categories: (i) 3D + 1D, (ii) 2D + 1D, (iii) 1D + 1D, and (iv) 0D + 1D, according to the combination pattern of their single motifs in these compounds. Among them, there are only four compounds that show the 3D + 1D MOPRF structures up to now,9e,17 and these four compounds are constructed as either octamolybdate or decavanadate. Therefore, compound 1 represents the first exceptional case assembled from Wells-Dawson clusters. In addition, it is worth mentioning that in 6 each of the molecular “‘loop A’” encircles two molecular “strings” (Figure 13c). This entangled fashion is also rare in the previous reports on rotaxane and pseudorotaxane structures.15b,18 Influence of the pH on the Structures of Compounds 1−6. To explore the influence of pH on the construction of POM-based hybrid compounds, we respectively introduce one kind of isopolymolybdate ((NH4)6Mo7O24) and two kinds of heteropolytungstates ([PW12]3− and [P2W18]6− anions) into an identical copper-bib reaction system except different pH values. And three pairs of inorganic−organic hybrid compounds respectively based on Mo6/Mo8, PW12, and P2W18 POMs were obtained by us (Scheme 1). After careful investigations, we notice that the pH mainly influences the protonated extent of the organic ligands in these compounds. And the protonated extent of organic ligands can further affect the final structures of the hybrid compounds.9g,h Namely, when the pH was adjusted to 1.5−2.5, compound 1 was obtained. In the structure of 1, all of the bib ligands were biprotonated; meanwhile, the Mo6 anions were formed by [Mo7O24]6− anions in situ. Such biprotonated bib ligands generally show weak coordination ability but possess high potential (N−H) donor sites for hydrogen bonding toward oxygen atoms of POMs forming N− H···O interactions. Thus, the biprotonated bib ligands and adjacent Mo6 anions via hydrogen bonding interactions form the simple supramolecular structure (Scheme 1). However, with the pH increase, it is difficult to be protonated for the bib ligands, and such unprotonated bib ligands with “exposed” nitrogen coordination sites usually show a high coordinated ability toward the metal fragments to form complicated architectures. Therefore, when we employed the identical experiment conditions with 1 except that the pH of the reaction was increased to 3.2−3.9, the 2 with the 2D layer that is more complicated than the supramolecular structure of 1 was obtained (Scheme 1). The pH also show similar influences

Figure 9. Topology of 3D framework in 4. Color codes: blue, connected nodes of Cu cations, yellow, connected nodes of PW12 anions.

Figure 10. Asymmetric unit of compound 5 and the coordination environments around the Cu atoms. (Two free water molecules and all of the hydrogen atoms are omitted for clarity.)

S4). Finally, a pair of infinite inorganic−organic chain penetrates into channel A of the 3D framework to form the (1D + 3D) MOPRF structure (Figure 13a,b). Among the rapidly increased POM-based coordination polymers, those that show MOPRF structures have been seldom observed so far. Very recently, the progress of POMbased MOPRFs was well-documented in a Coord. Chem. Rev. paper by Su, Lan and their co-workers,16 which spans a 4455



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Figure 11. (a) View of the inorganic−organic chain, (b) the polypendant chain, (c) the 1D + 1D → 2D interdigitated architecture, (d) schematic illustration of the interdigitated architecture.

on other two pairs of compounds: 3, 4 and 5, 6 (Scheme 1). Meanwhile, since POM anion is acting as an inorganic ligand with both terminal and bridging oxygen atoms, protonation and nucleophilicity of the oxygen atoms are also playing an important role in increasing the complexity of the hybrid structures.9a,b Compound 5 was obtained at the pH = 2.0−3.0, which is a 1D polypendant chain. When the pH of the reaction was adjusted to 3.4−3.8, compound 6 was obtained, which exhibits a complicated (1D + 3D) metal−organic pseudorotaxane framework. The pH also shows similar influences on compounds 3 and 4. This result reveals that with the increase in pH, more complicated structures of these hybrid compounds may be obtained without reference to the kinds of POMs. The informative structures of these six compounds contribute to systematically understanding of the effect of the pH on the assembly of POM-based hybrids. In addition, we have tried other pH values by doing many parallel experiments. However, we could not obtain any good quality crystals when the pH was adjusted to lower or higher values rather than the ones that we used in preparations of 1−6, which further reveals that, to some

Figure 12. Asymmetric unit of compound 6 and the coordination environments around the Cu atoms. (All of the hydrogen atoms, and H2O molecules are omitted for clarity.)

Figure 13. (a) View of the (1D + 3D) MOPRF structure, (b) the topology of the (1D + 3D) MOPRF structure, (c) detailed view of the pseudorotaxane fragment in 6. 4456



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Scheme 1. Summary of the Influence of the Different pH on the Structures of 1−6

Figure 14. CVs for CPEs in 1 M H2SO4 aqueous solution at different scan rates (from inner to outer: 25, 50, 75, 100, and 125 mV s−1) for (a) 2CPE and (b) 4-CPE (from inner to outer: 125, 150, 175, 200, and 225 mV s−1) for (c) 6-CPE.

Ot), νas(W−Ob−W), and νas(W−Oc−W) from PW12 and P2W18 anions,24 respectively. Additionally, the bands in the region of 1625−1127 cm−1 could be ascribed to the character peaks of bib ligand in 1−6.25 As shown in Figure S7, the X-ray powder diffraction patterns measured for the as-synthesized samples of 1−6 are all in good agreement with the PXRD patterns simulated from the respective single-crystal X-ray data, proving the purity of the bulk phases. Electrochemical and Electrocatalytic Properties. The POM-modified carbon paste electrode (CPE) often show the abilities to undergo reversible mono- and/or multielectron redox processes, which endows them attractive electrochemical and electrocatalytic properties.26 Herein, the 2-, 4-, and 6-CPEs have been taken as examples to study their electrochemical and electrocatalytic properties. The CPEs (2-, 4-, and 6-CPE) were prepared according to the previous method.7e (The prepared process was shown in Supporting Information.) The cyclic voltammograms (CVs) for the 2-, 4- and 6-CPEs in the 1 M H2SO4 aqueous solution at different scan rates are presented in Figure 14. It can be seen clearly that two reversible redox peaks (I−I′, II−II′) appear in the potential range from +0.45 to −0.1 V for the 2-CPE (Figure 14a). The mean peak potentials E1/2 = (Epa + Epc)/2 are +0.27 V (I−I′) and 0.032 V (II−II′) (scan rate: 100 mV·s−1), which can be ascribed to redox processes of MoVI/MoV.27 The redox peak of the Cu center for 2-CPE was not observed, perhaps because of the overlap of the MoVI/MoV redox peak, which is consist with the previous work.28 In the potential range of +0.45 to −0.65 V for 4-CPE (Figure 14b), it can be seen that there are three pairs of reversible redox peaks

extent, the pH is also crucial for the formation of crystals with high quality. Analyses of BVS, IR Spectra, and PXRD. In 1−6, all Mo and W atoms are in the +VI oxidation state obtained by bond valence sum (BVS) calculations.19 As shown in Tables S1 and S2, Supporting Information, the copper atoms are in the +II oxidation state for 2 and 5, in both +I and +II oxidation states for 4, in +I in oxidation state for 6 confirmed by their coordination environments, BVS calculations, and color of the crystals. The Cu(I) cations in 4 and 6 were from the reduction of starting material Cu(II) cations. Such phenomena have often been observed in reactions of N-containing ligands in the presence of Cu(II) cations under hydrothermal conditions.20 In addition, for charge balance, since title compounds were isolated from acidic aqueous solution, one proton is attached to the PW12 cluster in 3, as well as two protons are respectively attached to the P2W18 cluster in 5 and 6, which are similar to the cases of [Ag2(3atrz)2]2[(HPMoVI10MoV2O40)]21 and [Ag7(btp)5(HP2WVI16WV2O62)]·H2O.22 The IR spectra and stretching frequencies of 1−6 are shown in Figure S6 and Table S3, Supporting Information, respectively. The IR spectra exhibit characteristic peaks at 966, 913, 800, and 709 cm−1 in 1 as well as 956, 903, 801, and 693 cm−1 in 2, which are attributed to ν(MoOt) and νas(Mo−Oc−Mo) from Mo6 and β-Mo8 anions,23 respectively. The IR spectra exhibit the characteristic peaks at 1094, 951, 913, and 792 cm−1 in 3, 1086, 953, 913, and 791 cm−1 in 4, 1089, 958, 913, and 778 cm−1 in 5 as well as 1088, 951, 906, and 792 cm−1 in 6, which are attributed to ν(P−O), ν(W 4457



Article

Figure 15. Chart of the CAT vs concentration of the (a) IO3− and (b) NO2−.

with the mean peak potentials E1/2 = (Epa + Epc)/2 of −0.074 (II−II′), −0.351 (III−III′), and −0.588 V (IV−IV′), which can be ascribed to redox processes of WVI/WV.29 One irreversible anodic peak (I) exists with the mean peak potentials +0.26 V for 4-CPE, which is attributed to the redox processes of the Cu centers.7e,30 In the potential range of +0.4 to −0.7 V for 6-CPE (Figure 14c), it can be seen that there are three pairs of reversible redox peaks with the mean peak potentials E1/2 = (Epa + Epc)/2 of −0.089 (I−I′), −0.314 (II−II′), and −0.573 V (III−III′) (scan rate: 150 mV·s−1), which can be ascribed to redox processes of WVI/WV.24b One irreversible anodic peak (I) exists with the mean peak potentials +0.21 V for 6-CPE, which is attributed to the redox processes of the Cu centers. In addition, in acidic medium, the peaks below 0 E/V, particularly below −0.1 E/V in 4-CPE may correspond to evolution of hydrogen.31 The electrocatalytic properties of 2-, 4-, and 6-CPEs have also been investigated toward reduction iodate (IO3−) and nitrite (NO2−) in 1 M H2SO4 solution. The results show that the 2- and 4-CPEs have no obvious electrocatalytic activities, and the 6-CPE has good electrocatalytic activities for iodate. As shown in Figure S8, with addition of iodate, the reduction peak currents for II, IV increase gradually, while the corresponding oxidation peak currents decrease. The nearly equal current steps for each addition of iodate demonstrate stable and efficient electrocatalytic activity of 2-, 4-, and 6-CPEs (see the inset figures of Figure S8a−c). For nitrite, the 4- and 6-CPEs also have good electrocatalytic activities; however, the 2-CPE almost has no electrocatalytic activities (Figure S9a). As shown in Figure S9, with addition of NO2−, the reduction peak currents for IV of the 4- and 6-CPEs increase gradually, while the corresponding oxidation peak currents decrease. The nearly equal current steps for each addition of NO2− demonstrate stable and efficient electrocatalytic activity of 4- and 6-CPEs (see the insets of Figure S9b,c). To compare the electrocatalytic activity of 2-, 4-, and 6-CPEs for iodate and nitrite, the CAT (catalytic efficiency) of 2-, 4-, and 6-CPEs can be calculated by using the CAT formula.32 As shown in Table S4, the CAT of 2-, 4-, and 6-CPEs (catalyst 9 mg) toward 60 mM IO3− were calculated to be 34%, 53%, and 118%, respectively. And the CAT of 2-, 4-, and 6-CPEs toward 60 mM NO2− was calculated to be 4%, 79%, and 62%, respectively. As shown in Figure 15a, from the chart of the CAT, we found that the 4-CPE has a good electrocatalytic effect toward the reduction of nitrite, and 6-CPE has a good electrocatalytic effect toward the reduction of iodate, which

suggests that compounds 4 and 6 may have potential applications in the detection of nitrite and iodate.

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CONCLUSIONS In summary, by introducing one kind of isopolymolybdate anion ([Mo7O24]6−) and two kinds of heteropolytungstate anions ([PW12]3− and [P2W18]6−) into an identical copper-bib reaction system except for different pH values, six POMs-based hybrid compounds ranging from 0D monomer, 1D chain, 2D network, to 3D pseudorotaxane framework have been successfully prepared. The structural diversities of the compounds not only reveal that the pH plays a key role in the assembly of POM-based hybrid compounds without reference to the kinds of POMs but also confirm that tuning the pH of the reaction is an effective strategy in crystal engineering for preparing new POM-based hybrid materials.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

CIF files, Tables S1−S4, Scheme S1, and Figures S1−S9. This material is available free of charge via the Internet at http:// pubs.acs.org . Corresponding Authors

*(H.Y.M.) E-mail: [email protected]. *(H.J.P.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSF of China (No.21371041 and 21101045), the NSF of Heilongjiang Province (No.B201103) and innovative research team of green chemical technology in university of Heilongjiang Province, China.

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REFERENCES

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Assembly of Six Polyoxometalate-Based Hybrid Compounds from a

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