Dawson-type Polyoxotungstate Clusters with

Article pubs.acs.org/crystal

Assembly of Keggin-/Dawson-type Polyoxotungstate Clusters with Different Metal Units and SeO32− Heteroanion Templates Wei-Chao Chen, Li-Kai Yan,* Cai-Xia Wu, Xin-Long Wang,* Kui-Zhan Shao, Zhong-Min Su,* and En-Bo Wang Department Institute of Functional Material Chemistry, Key Lab of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Changchun, 130024, P. R. China S Supporting Information *

ABSTRACT: Using a pH-dependent synthetic approach, the combination of different simple metal salts or metal coordination complexes with SeO32− heteroanion templates was employed to synthesize five distinct assemblies of Keggin-/Dawson-type tungstoselenites: (C2H8N)10KNa[(α-SeW9 O34){Zr(H2 O)}{WO(H2O)}(WO2)(SeO3){α-SeW8 O31 Zr(H2O)}]2·14H2O (1) at pH = 1.3; (C2H8N)10KNa5[(Se2W18O60)2(μ2O)4]·12H2O (2) at pH = 2.5; (C2H8N)4Na4[Se2W18O62(H2O)2]·13H2O (3) at pH = 3.6; (C2H8N)4K3Na10[(α-SeW9O33)2{Ce2(CH3COO)(H2O)3W3O6}(α-Se2W14O52)]·26H2O (4) at pH = 4.5; K10Na5[(αSeW9O33)2{Ce2(H2O)4W3O6}{α-Se2W14O51(OH)}]·24H2O (5) at pH = 4.5. All five compounds were characterized by single-crystal X-ray structure analysis, IR spectroscopy, thermogravimetric, UV/vis spectroscopy, and ESI-MS. Moreover, their electrochemical properties were investigated. Keggin-type polyoxoanion of 1 remains the first reported Zr-containing tungstoselenites based on {α-SeW9} building blocks. X-ray analysis revealed that the 4d metal Zr centers have seven- and eight-coordinated modes, and SeO32− acts as the templates as well as the linkers. With the increasing of the pH, Dawson-type polyoxoanions of 2 and 3 based on the first reported basic lacunary {α-Se2W14} building blocks are obtained by using 3d-4f metal coordination complexes. Polyoxoanions of 4 and 5 remain similar structures stabilized by the 4f metal Ce centers at pH = 4.5 and that contain the basic Keggin-type {α-SeW9} and Dawson-type {α-Se2W14} building blocks in 1−3 at the same time, presenting the mixed multiple lacunary building blocks being combined into the single polyoxoanion architecture. Furthermore, the density functional theory calculations have been performed on polyoxoanions of 1 and 5 as the representatives to investigate their electronic properties.

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INTRODUCTION Polyoxometalates (POMs) constitute a unique class of molecular metal−oxygen clusters (including W, Mo, V, or Nb as the metal centers), which exhibit a wide variety of structural diversity and potential applications in magnetism, multifunctional materials, nanotechnology, and so on.1 Their formation mechanism is considered as self-assembly processes that involve the condensations of {MOx} units (M = W, Mo, V, or Nb) directed by the immediate regulations of flexile experimental factors, such as pH value, temperature, the molar ratio of the reactants, reaction solvents, different kinds of metal-ions and organic ligands, so that the one-pot self-assembly synthesis remains an important synthetic strategy for polyoxotungstates (POTs) clusters.2 The ever-increasing interest in preparing novel POTs architectures results in different structural types being reported: for instance, the well-known Keggin-type {XW12O40} and the Wells−Dawson-type {X2W18O62} with two {XO4} tetrahedral heteroanions inside the central cavities respectively (X = S, P, Si, etc.), which still inspire an enormous amount of new research owing to their intriguing range of applications in catalysis and materials.3 At this stage, different © XXXX American Chemical Society

metal units (such as the 3d metals, 4d metals, 3d-4f metals, or 4f metals)4 possess powerful abilities to link or stabilize the lacunary Keggin- or Dawson-type building blocks (BBs). Usually, POTs are synthesized in aqueous solution with its reactions being researched by careful modifications of synthetic conditions. Among the factors, the feature of the pH is a key factor in the formations of several POTs, and thus the pHdependent synthetic approach constitutes an important research objective to allow for the rational design of tailored POT assemblies.5 In this context, the use of the different metal units with suitable reaction pH ranges also seems to be valuable. More importantly, the types of the heteroanions (such as PO43−, SiO44−, GeO44−, and AsO33−, etc.) may also seriously affect the final architectures of the high-nuclear POTs.4a,6 Recently, Cronin et al. found that redox-active nonconventional (nontetrahedral) heteroanions XO32− (X = S, Se, and Te) from Group VI played an important role in the self-assembly process Received: May 16, 2014 Revised: June 18, 2014

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dx.doi.org/10.1021/cg500719q | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

factors for the precise control of the self-assembly process (Scheme 1). The introductions of 4d metal salts or 3d-4f metal

of design and synthesis hetero-POTs clusters due to their lone pairs of electrons.7 Furthermore, our previous work of CeIIIstabilized Keggin-type POT nanoclusters constructed from the combination of 4f cerium centers with SeO 32−/TeO 32− heteroanion templates has provided a further step for understanding the effects of nonconventional heteroanion templates.8 POMs based on Keggin-type BBs with multiple metal centers have already been studied for many years; the early 4d transition-metal Zr-containing POMs have been one of the least reported types so far. 9a The Keggin-type dimer [Si2W18Zr3O71H3]11− reported by Finke et al. remains the first example of Zr-containing POMs,9b and then the reactivities of different lacunary Keggin-type BBs with the Zr linkers have been investigated by many groups, including Pope, Hill, Nomiya, Yang, Kortz, etc. The area of Zr-containing POMs has experienced a renaissance since numerous dimer or trimer Keggin-based Zr derivatives have been reported.9 However, such Zr-containing POMs based on the above-mentioned nonconventional heteroanion templates have not been discovered. The same as the Keggin-type POMs, the other particularly interesting family seems to be the very stable Dawson structural type, of which the nonconventional POT clusters incorporated pyramidal anions have been developed in recent years. Moreover, the nonconventional heteroanions {XO32−} (X = S, Se, and Te)10 templates have their own advantages in expanding the nonconventional Dawson-type hetero-POTs family, for example, the first two examples of POT clusters incorporating the sulfite anion: [W18O54(SO3)2]4−, and [W18O56(SO3)2(H2O)2]8−,10b the latter of which undergoes a unique electron-transfer reaction when heated. [H3W18O57(TeO3)]5− has been detected by cryospray mass spectrometry, which is the first example of the pyramidal TeO32− acting as a template embedded within the Dawson-type {W18O54} cage.10c Recently, a series of selenotungstates based on the Dawson-type BBs (such as {Se2W12}, and {γ-Se2W14}, etc.),10d−f involving the first reported polyoxoanions that the multiple lacunary building blocks derived from different parent species (Keggin- and Dawson-type BBs) have been combined into a single POM architecture bridged through the Pd centers.10e But the heteroanion SeO32− in templating the nonconventional Dawson-type cage (eg, {W18} cages) seems to be less developed.10g According to the above-mentioned studies, the extension of the family of POTs based on nonconventional heteroanions templates with different metal linkers at desirable reaction pH may represent potential applications in preparation of novel architectures. Thus, we chose the selenite anion (SeO32−) to build POTs, owing to its flexible coordination modes shown in POM clusters compared to the other two anions.7 Using a pHdependent synthetic approach, after introduce simple metal salts or metal coordination complexes, five new selenite-based polyoxoanions from Keggin- to Dawson-type motifs have been isolated: [(α-SeW9O34){Zr(H2O)}{WO(H2O)}(WO2)(SeO3){α-SeW8O31Zr(H2O)}]212− (1a) at pH = 1.3; [(Se2W18O60)2(μ2-O)4]16− (2a) at pH = 2.5; [Se2W18O62(H2O)2]8− (3a) at pH = 3.6; [(α-SeW9O33)2{Ce2(CH3COO)(H2O)3W3O6}(αSe 2 W 14 O 52 )] 17− (4a) at pH = 4.5; [(α-SeW 9 O 33 ) 2 {Ce2(H2O)4W3O6}{α-Se2W14O51(OH)}]15− (5a) at pH = 4.5. All compounds were prepared under one-pot synthesis conditions; besides the heteroanions templates effects, the pH and different metal units were proven to be the determining

Scheme 1. Schematic Assembly of 1a−5a with the Basic Building Blocks (Keggin-type, Dawson-type, or Keggin +Dawson-Type Motifs) by the Combination of Different Metal Units with SeO32− Heteroanion Template at Suitable Reaction pH Value

coordination complexes at different pH values achieve the assemblies from Keggin- to Dawson-type motifs at the same reactions. In particular, the Keggin- and Dawson-type motifs along with the 4f Ce centers at pH 4.5 have been “transferred” into a single POT architecture.

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EXPERIMENTAL SECTION

General Considerations. All chemicals and solvents were commercially purchased and used without further purification. Synthesis of (C2H8N)10KNa[(α-SeW9O34){Zr(H2O)}{WO(H2O)}(WO2)(SeO3){α-SeW8O31Zr(H2O)}]2·14H2O (1). K2WO4 (1.00 g, 3.07 mmol) and Na2SeO3·5H2O (0.09 g, 0.34 mmol) were dissolved in 40 mL of water. The pH value of the solution was adjusted to 4.8 by the 50% (1:1) acetic acid solution. After the solution was stirred for around 30 min, solid ZrCl4 (0.35 g, 1.51 mmol) and dimethylamine hydrochloride (0.80 g, 9.81 mmol) were successively added. The final pH was kept at 1.3 by 6 M HCl. This solution was stirred for another 10 min and then heated to 60 °C stirred for 1 h, cooled down to room temperature, filtered, and left to evaporate slowly. Colorless blockshaped crystals were obtained after 3 weeks, which were then collected by filtration and air-dried. Yield: 0.14 g (16% based on W). IR (KBr disk (Figure S20 of the Supporting Information), ν/cm−1): 3743(w), 3166(w), 2785(w), 1612(s), 1468(s), 1410(w), 1363(w), 977(s), 842(s), 755(w), 707(w). Elemental analysis calc. for C20H120KN10NaO160Se6W38Zr4 (%): W 63.81, Se 4.33, K 0.36, Na 0.21, Zr 3.33, C 2.19, N 1.28; Found: W 63.70, Se 4.30, K 0.3, Na 0.2, Zr 3.30, C 2.20, N 1.22. XRD and TGA of 1 and the related discussions were shown in the Figures S25 and S30 of the Supporting Information. Synthesis of (C2H8N)10KNa5[(Se2W18O60)2(μ2-O)4]·12H2O (2). The procedure was exactly the same as the preparation of 1 except that freshly prepared CeMn6O9(O2CCH3)9(NO3)(H2O)2 (0.30 g, 0.19 mmol) was used instead of ZrCl4 and before heating the final pH was kept at 2.5 by 6 M HCl. Colorless needle-like crystals were obtained after 3 weeks, which were then collected by filtration and air-dried. Yield: 0.10 g (12% based on W). IR (KBr disk (Figure S21 of the Supporting Information), ν/cm−1): 3444(w), 3106(w), 2777(w), 1606(s), 1527(s), 1463(s), 1407(s), 1024(s), 950(s), 808(w), 753(w). Elemental analysis calc. for C20H104KN10Na5O136Se4W36 B


a

C

C20H104KN10Na5O136Se4W36 9749.62 0.71073 Å 296(2) triclinic P1̅ 14.1071(12) 15.4752(13) 21.2067(17) 107.4710(10) 97.6310(10) 97.3190(10) 4307.9(6) 1 3.758 24.898 4256 1.95−25.00 22591 15031 15031/272/952 0.0381 0.944 0.0422 0.1020

C20H120KN10NaO160Se6W38Zr4 10948.29 0.71073 Å 296(2) triclinic P1̅ 14.1124(14) 15.5421(16) 20.970(2) 105.483(2) 95.186(2) 94.100(2) 4392.4(8) 1 4.139 26.368 4796 1.87−25.00 22955 15255 15255/447/1135 0.0577 0.987 0.0571 0.1355

R1 = Σ||Fo| − |Fc||/Σ|Fo|. wR2 = {Σ[wFo2 − Fc2)2]/Σ[w(Fo2)2]}1/2.

empirical formula M λ/Å T/K crystal system space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z Dc/Mg m−3 μ/mm−1 F(000) θ range/° measured reflections independent reflections data/restraints/parameters Rint after SQUEEZE goodness-of-fit on F2 R1 (I > 2σ(I))a wR2 (all data)a

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1

Table 1. Crystal Data and Structure Refinements for 1−5 C8H62N4Na4O77Se2W18 5005.62 0.71073 Å 296(2) monoclinic C2/c 28.706(5) 13.6800(18) 22.750(4) 90 114.522(4) 90 8128(2) 4 4.091 26.387 8788 1.94−25.00 20450 7141 7141/186/472 0.0699 1.107 0.0557 0.1538

3 C10H93Ce2K3N4Na10O155Se4W35 10127.91 0.71073 Å 296(2) triclinic P1̅ 19.4192(13) 19.9929(13) 26.9568(17) 70.2250(10) 70.3850(10) 63.6810(10) 8611.2(10) 2 3.906 24.834 8860 1.79−25.00 45224 30019 30019/409/1838 0.0520 0.934 0.0518 0.1234

4

H57Ce2K10Na5O152Se4W35 10026.24 0.71073 Å 296(2) triclinic P1̅ 18.806(3) 21.967(3) 23.6398(13) 24.552(3) 93.628(2) 111.811(2) 113.559(2) 2 3.983 25.733 8720 1.88−25.00 43430 29086 29086/937/1696 0.0538 0.972 0.0698 0.1856

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Crystal Growth & Design Article



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Scheme 2. Synthetic Procedure for the Formation of 1−5

(%): W 67.89, Se 3.24, K 0.4, Na 1.18, C 2.46, N 1.44; Found: W 67.92, Se 3.25, K 0.38, Na 1.2, C 2.50, N 1.45. XRD and TGA of 2 and the related discussions were shown in the Figures S26 and S31 of the Supporting Information. Synthesis of (C2H8N)4Na4[Se2W18O62(H2O)2]·13H2O (3). The procedure was exactly the same as the preparation of 1 except that freshly prepared CeMn6O9(O2CCH3)9(NO3)(H2O)2 (0.30 g, 0.19 mmol) was used instead of ZrCl4 and before heating the final pH was kept at 3.6 by 6 M HCl. Colorless block-shaped crystals were obtained after 2 weeks, which were then collected by filtration and air-dried. Yield: 0.21 g (24% based on W). IR (KBr disk (Figure S22 of the Supporting Information), ν/cm−1): 3445(w), 3136(w), 2785(w), 1605(s), 1532(s), 1466(s), 1406(s), 1021(s), 954(s), 803(w), 755(w). Elemental analysis calc. for C8H62N4Na4O77Se2W18 (%): W 66.11, Se 3.16, Na 1.84, C 1.92, N 1.12; Found: W 66.10, Se 3.15, Na 1.8, C 1.90, N 1.1. XRD and TGA of 3 and the related discussions were shown in the Figures S27 and S32 of the Supporting Information. Synthesis of (C 2 H 8 N) 4 K 3 Na 10 [(α-SeW 9 O 33 ) 2 {Ce 2 (CH 3 COO)(H2O)3W3O6}(α-Se2W14O52)]·26H2O (4). The procedure was exactly the same as the preparation of 1 except that Ce(NO3)3·6H2O (0.30 g, 0.69 mmol) was used instead of ZrCl4 and before heating the final pH was kept at 4.5 by 6 M HCl. Yellow block-shaped crystals were obtained after 4 weeks, which were then collected by filtration and airdried. Yield: 0.13 g (15% based on W). IR (KBr disk (Figure S23 of the Supporting Information), ν/cm−1): 3444(w), 3131(w), 2777(w), 1624(s), 1466(s), 1412(w), 1341(w), 955(s), 861(s), 752(w), 647(w). Elemental analysis calc. for C10H93Ce2K3N4Na10O155Se4W35 (%): W 63.53, Se 3.12, K 1.16, Na 2.27, Ce 2.77, C 1.19, N 0.55; Found: W 63.51, Se 3.1, K 1.15, Na 2.25, Ce 2.75, C 1.17, N 0.57. XRD and TGA of 4 and the related discussions were shown in the Figures S28 and S33 of the Supporting Information. Synthesis of K 1 0 Na 5 [(α-SeW 9 O 3 3 ) 2 {Ce 2 (H 2 O) 4 W 3 O 6 }{αSe2W14O51(OH)}]·24H2O (5). The procedure was exactly the same as the preparation of 4 except that KCl (0.50 g, 6.72 mmol) was used instead of dimethylamine hydrochloride. Yellow block-shaped crystals were obtained after 2 weeks, which were then collected by filtration and air-dried. Yield: 0.13 g (15% based on W). IR (KBr disk (Figure S24 of the Supporting Information), ν/cm−1): 3439(w), 1627(s), 957(s), 855(w), 743(w), 647(s). Elemental analysis calc. for H57Ce2K10Na5O152Se4W35 (%): W 64.18, Se 3.15, K 3.9, Na 1.15, Ce 2.80; Found: W 64.14, Se 3.12, K 4.0, Na 1.17, Ce 2.78. XRD and TGA of 5 and the related discussions were shown in the Figures S29 and S34 of the Supporting Information. Characterization. Elemental analysis of K, Na, Se, W, Zr, and Ce were performed with a Leaman inductively coupled plasma (ICP) spectrometer; C and N were performed on a PerkinElmer 2400 CHN elemental analyzer. IR spectra were recorded on an Alpha Centaurt FT/IR spectrophotometer with pressed KBr pellets in the range 400− 4000 cm−1. Water contents were determined by TG analyses on a PerkinElmer TGA7 instrument in flowing N2 with a heating rate of 10 °C·min−1. UV−vis absorption spectra were obtained using a 752 PC UV−vis spectrophotometer. XRD studies were performed with a

Rigaku D/max-IIB X-ray diffractometer at a scanning rate of 1° per min using Cu−Kα radiation (λ = 0.71073 Å). Electrospray ionization mass spectrometry was carried out with a Bruker Micro TOF-QII instrument. Single-Crystal X-ray Diffraction. Single-crystal X-ray diffraction data for 1−5 were recorded on a Bruker Apex CCD II area-detector diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 296(2) K. Absorption corrections were applied using multiscan technique and performed by using the SADABS program. The structures of 1−5 were solved by direct methods and refined on F2 by full-matrix leastsquares methods by using the SHELXTL package. Anisotropic thermal parameters were used to refine all nonhydrogen atoms. Hydrogen atoms attached to lattice water molecules were not located. The numbers of lattice water molecules and countercations for 1−5 were estimated by the results of elemental analyses, TG curves, and calculations of electron count in the voids with SQUEEZE. The detailed crystallographic data and structure refinement parameters for 1−5 are summarized in Table 1. CCDC 979287 (1), 979288 (2), 979289 (3), 979290 (4), and 979291 (5) contain the supplementary crystallographic data for this paper. Electrochemical Experiments. The electrochemical measurement was carried out on a CHI 660 electrochemical workstation at room temperature. Thrice-distilled water was used throughout the experiments. All the solutions were deaerated by pure argon bubbling prior to the experiments and the electrochemical cell was kept under an argon atmosphere throughout the experiment. A conventional three-electrode system was used. The working electrode was a 1.5 mm glassy carbon, a Ag/AgCl was used as the reference electrode and platinum wire as a counter electrode. The glassy carbon working electrodes were polished with alumina on polishing pads, rinsed with distilled water, and sonicated in H2O before each experiment. The scan rate was 50−500 mV s−1. All potentials were measured and reported versus the Ag/AgCl. The various media of 1−5 were all in 0.5 M H2SO4/Na2SO4. A pHS-25B type pH meter was used for pH measurement. Computational Details. Density functional theory (DFT) calculations presented here were carried out with the ADF2012.01.17 Electron correlation was treated within the local density approximation (LDA) in the Vosko-Wilk-Nusair (VWN) parametrization.18 The nonlocal corrections of Becke19 and Perdew20,21 were added to the exchange and correlation energies, respectively. The single point energy calculations were carried out based on the X-ray structures synthesized in this work. Slater TZP quality basis set was used to describe all electrons of O, Se, and H atoms, as well as the valence electrons of transition metals. The frozen core approximation for metal atoms has been used to reduce the large computational effort. The core electrons of metals (W, 1s-4d; Ce, 1s-5p; Zr, 1-3d) were kept frozen and described by single Slater functions. We applied scalar relativistic corrections to them via the zeroth-order regular approximation with the core potentials generated using the DIRAC program.22 To define the cavity surrounding the anions, we applied the D



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Figure 1. Polyhedral and ball-and-stick representation of 1a and its basic building blocks. The {SeW8} or {SeW9} units are shown with green polyhedra and cyan balls. The Zr centers are shown as orange balls. The Se atoms are shown as cyan balls. The extra W units are shown as green balls. solvent-excluding method with a fine tesserae.23 The ionic radii for the POM atoms, which actually define the cavity in the conductor-like screening model (COSMO), are 1.26, 0.79, 2.35, 1.16, 1.52, and 0.32 Å for W6+, Zr4+, Ce3+, Se4+, O2−, and H1+, respectively.

turbid (pH = 4.8) to clarification (pH = 1.3). Colorless blockshaped crystals of 1 were obtained from the clarification after 3 weeks. No crystals formed when comparing the pH value from 2.0 to 4.5. In the presence of 3d-4f metal coordination c o m p l e x e s , C e I V M n I V 6 O 9 ( O 2 CC H 3 ) 9 ( N O 3 ) ( H 2 O ) 2 (CeMn6),11 2 and 3 were obtained. It is worth noting that CeMn6 did not present in the final architectures, but no crystals were found without this material. In view of this context, other metal raw materials were introduced to the same one-pot reaction system, such as 3d metal cluster Mn12-acetate containing MnIV, 4f metal ion CeIV, and both of them; unfortunately, they all failed. This phenomenon also occurred in other reports.11d When the carboxylic complexes were added in, the pH value decreases from about 4.8 to 4.5. With the pH value as the single variable, the pH was then adjusted from about 4.5 to 1.5 by the 6 M HCl solution, and the brown solution changes from clarification (pH = 4.5) to turbid (pH = 1.5). Colorless block-shaped crystals of 3 were obtained at a pH value of 3.6 and colorless needle-like crystals of 2 at the immediate pH of 2.5. Unlike the simple Zr4+ ion in building Keggin-type 1, the coordination complexe, CeMn6, guides the formation of the Dawson-type dimer 2 and its monomer 3. Besides the pH, the organo-ammonium cations (dimethylamine hydrochloride)1c,12 were employed to restrict the aggregation of more highly symmetrical clusters during the assembly process, resulting in the successful synthesis of the Dawsontype 2 and 3. The sodium−potassium-organic amine salt (C 2 H 8 N) 4 K 3 Na 10 4a·26H 2 O and sodium−potassium salt K10Na55a·24H2O were isolated after the introduction of the 4f ions Ce3+ at pH = 4.5, and the vacancy Keggin-type or Dawson-type BBs formed easily and remained stable at this pH. 4 and 5 possess similar structures except for the coordinated carboxylate and countercations. It is worth pointing that different countercations lead to different crystallization times: K+ (about 2 weeks) and [C2H8N]+ (4 weeks). Some efficient observations during the syntheses of 1−5 should be mentioned: (1) The combination of different metal uints with SeO32− heteroanion templates seems to be a potential strategy to build novel POTs, which may be due to the inducing effect of the lone pair of electrons in SeO32−; (2) pH-dependent synthetic approach is crucial for the formation of Keggin- or Dawson-type BBs and activating the reactivity of the linkers; (3) the correct choice of different countercations which may play an important role in both the crystallization time and the final architectures.

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RESULTS AND DISCUSSION Synthesis. The precise control of the one-pot reaction conditions of combining different metal units with the SeO32− anion templates at proper pH was employed for the assembly of 1−5. The procedures followed for the synthesis of 1−5 are summarized in Scheme 2. First, we chose K2WO4 and Na2SeO3·5H2O as the W- & Sesources according to the previous studies,7,8 which could provide K+ and Na+ counterions simultaneously. The acidification of K2WO4 and Na2SeO3·5H2O (W/Se molar ratio 9:1) by acetic acid was necessary. Acetic acid has already been proven to be a suitable reagent for acidifying Se-based POTs clusters,7a,8 and furthermore, the molar ratio also plays an important role that is in accordance with the final structures: such as the basic trilacunary Keggin-type {α-SeW9} fragment (W:Se molar ratio 9:1), the nonconventional Dawson-type fragment {α-Se2W18} (W:Se molar ratio 9:1) and the Cestabilized Keggin- and Dawson-type motif {Se4W35} (W/Se molar ratio 9:1). Subsequently, simple metal salts or metal coordination complexes were introduced to the acid solution. Since the SeO32− anion template has a lone pair of electrons and a series of novel coordination modes, a combination of different metal units and SeO32− anion templates seems to be powerful in construction of 1−5. After ZrCl4 was introduced into the reaction system, the mixed sodium−potassium-organic amine salts (C2H8N)10KNa1a·14H2O were isolated. The Zr-containing POTs were seldom obtained under the one-pot syntheses, and conversely, they tend to prepare generally by stepwise synthetic assembly approach by reaction Zr centers with various Keggin- or Dawson-type BBs.9 In this stage, other than the Zr centers, we find the impact of the pH should be considered during the one-pot syntheses. From the previously reported studies, the pH value is a known crucial parameter in POT chemistry: a series of available building units toward the formation of the final structures are driven by it and it also provides the chance to obtain Keggin-/Dawson-type motifs though a pH-dependent synthetic approach. Thus, we analyze the formation of 1 with the pH value as the single variable: the pH was then adjusted from about 4.8 to 1.3 by the dropwise addition of a 6 M HCl solution, and the solution changes from E



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by two SeO32− anions through Zr−O−Se and W−O−Se bonds (Figure 2d), which keep two parts to be nearly planar alignment at the same time, resulting in the polyoxoanion crystallized 1 in space group P1̅. Moreover, the SeO32− anions here act as the templates as well as the linkers. Bond lengths relevant to the coordination mode of the Se atom are given in Table S7 of the Supporting Information. With the increase of the pH and the introduction of the essential 3d-4f CeMn6 metal units, two Dawson-type anions [W18O54(SeO3)2}2(μ2-O)4]16− (Figure 3; Figures S2 and S7 of

Structural Analysis. The Keggin-type 1a has a tetrameric structure that contains 42 metal centers, including 38 W atoms and 4 Zr ions. X-ray structural analysis (Table 1) reveals that 1a can be divided into two same subunits linking via two pyramidal SeO32− anions (Figure 1; Figures S1 and S6 of the Supporting Information). Each part remains a dimer sandwich structure as shown in Figure 1: it consists of a well-known trivacant Keggin-type {α-SeW9O34} BBs (Figure 2a), a novel

Figure 2. Ball-and-stick representation of the basic building blocks (a) {SeW9}, (b) {SeW8Zr}, (c) the “V”-shaped unit {W2O3Zr(H2O)2}, and (d) the two SeO32− linkers in 1a (W: green, O: gray, Zr: orange, Se: cyan).

Figure 3. Polyhedral and ball-and-stick representation of 2a (top) with its four μ-oxo bridges connection modes (down). (W: green, O: gray, Se: cyan).

Zr-containting Keggin-type {α-SeW8O31Zr(H2O)} fragment (Figure 2b), and a “V”-shaped planar-like {W2O3Zr(H2O)2} fragment between them. As in the previous reports, Zrcontaining POMs were prepared generally by a stepwise synthetic assembly approach with various BBs, and thus it is inclined to form the dimer sandwich strutures with Zr-cluster cation units between two lacunary units.9 At this stage, the sandwich dimer structure in 1a still possesses some interesting features: the two Zr centers here are independent, and they exist as seven- and eight-coordination modes, respectively, the former of which links two W centers by two μ2-oxo bridges (Zr−O46−W and Zr−O50−W) (Figure 2c), forming the “V”shaped planar-like unit with an angle of 73.764(45)° via the Zr−O bond lengths from 2.045(16) to 2.276(16) Å, the latter of which fills the monolacunary site of the {α-SeW8} unit with the Zr−O bond lengths from 2.089(15) to 2.739(15) Å. Moreover, the central “V”-shaped planar-like unit (Figure 2c) appears to be doubly protonated with the protons located at the terminal oxygen (O69) upon of the tungsten centers (W− OH2: 2.205 Å (17)) according to the bond valence sum (BVS) calculations13 (the results of BVS calculations for all the oxygen atoms in 1a are listed in Table S1 of the Supporting Information). Additionally, the residual coordination sites of the Zr atoms are each occupied by one water molecule. Bond lengths relevant to the two coordinated modes of the Zr atoms are given in Table S6 of the Supporting Information. It is worth noting that the eight-coordinated Zr center linked via a oxo bridge to the {SeO3} hetero group with a Zr−O bond length of 2.739(15) Å remains the longest so far4b,9 (Figure 2b). With the formation of the dimer sandwich structure, further connection appears because of the existence of the hereoanion SeO32−. Two dimer sandwich structures are bridged

the Supporting Information) and [W18O56(SeO3)2(H2O)2]8− (Figure 4; Figures S3 and S8 of the Supporting Information)

Figure 4. (a) Polyhedral and ball-and-stick representation of 3a. (b) The basic building block {α-Se2W14} unit shown in polyhedra with four neighboring equatorial tungsten centers shown in ball-and-stick modes (W: green, O: gray, Se: cyan).

were isolated from the solutions. 3a has a similar structure with the sulfite-based Dawson-type POTs [W18O56(SO3)2(H2O)2]8− isolated as potassium or organo-ammonium cation salts. It can be viewed as one BBs {α-Se2W14} unit with four neighboring equatorial tungsten centers (Figure 4b) which are uncoordinated to the templating selenium groups and thereby reduce the overall molecular to C2v. Moreover, each of the four tungsten units (Figure 4b) has two terminal ligands, and these extra terminal ligand positions are occupied by water or oxo ligands. The different orientations of the selenium ions lead to the Se···Se distance of 3.417(3) Å, while the average Se−O distance is 1.700 Å, which is a little shorter than the S···S distance in [W18O56(SO3)2(H2O)2]8−.10a,b At a suitable pH value of 2.5, its dimer of 2a involving four μ2-oxo bridges W3− F



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O5−W5, W1−O12−W2, W2−O12−W1, and W5−O5−W3 (bond lengths: W3−O5 2.189, W1−O12 2.211, W2−O12 1.736, W5−O5 1.771 Å) was obtained (Figure 3). The results of BVS calculations for all the oxygen atoms in 2 and 3 are listed in Tables S2 and S3 of the Supporting Information, which indicate that all the oxygen atoms in 2 are without any protonations. Moreover, each of the two {W18O54(SeO3)2} subunits has one four-neighboring equatorial tungsten unit as shown in 3a, which does not combine to each other through the two centers, giving a mirror symmetry configuration. Instead, two of the W centers in the four-neighboring equatorial tungsten unit link the other W centers at the equator position in another {W18O54(SeO3)2} subunit, presenting a central symmetry configuration, which provides the opportunity for monomer (Se2W18), dimer (Se4W36), or tetramer (Se8W72) “chainshaped” hetero-POTs architectures; they are similar to the reported “layered” molecular assembly based on the TeO32− heteroanion template.7c Herein, it can be concluded that the selenium-based novel POTs clusters are an extensive field to be explored. The unprecedented Keggin-/Dawson-type 4a (Figure 5; Figures S4 and S9 of the Supporting Information) and 5a Figure 6. Polyhedral and ball-and-stick representation of 5a. The {SeW9} or {Se2W14} units are shown with green polyhedra and cyan balls. The {Ce2(H2O)4W3O6} core is shown in ball-and-stick mode (W: green, O: gray, Se: cyan, Ce, yellow).

Figure 5. Comparison of the difference between 4a (a) and 5a (b) (W: green, O: gray, C, blue, H, light blue, Se: cyan, Ce, yellow).

(Figure 5; Figures S5 and S10 of the Supporting Information) were isolated at pH = 4.5, which possess similar constructions except for the 9- and 10-coordinated modes of the 4f Ce centers in 4a, ascribed to the coordination of carboxylate (Figure 5). BVS calculations for all the oxygen atoms (Tables S4 and S5 of the Supporting Information) show that there is only one single protonated oxygen atom located at one polar position of {α-Se2W14O51(OH)} BBs in 5a, while there is no protonated oxygen atoms in 4a, and thus two Dawson-type lacunary BBs possess different charges. 5a is selected as a representative to discuss their structures. This polyanion contains two types of BBs: two trilacunary Keggin-type {αSeW9O33} units and one tetra-lacunary Dawson-type {αSe2W14O51(OH)} unit (Figure 6), which have already been discussed in 1a−3a. Moreover, the {Ce2(H2O)4W3O6} core stabilizes two types of BBs simultaneously, resulting in 5a crystallized in space group P1̅. The “V”-shaped planar-like {CeO2W2} fragment fills the trivacant {α-SeW9O33} unit, giving the stable and saturated {M12} Keggin-like motif (Figure 7a). As well as the “V”-shaped planar-like fragment mentioned in 1a, the nine-coordinated Ce center links the two W centers with a smaller angle of 66.644(31)° by two μ2-oxo bridges (Ce1−O62− W13 and Ce1−O64−W2) (Figure 7c) via the Ce−O bond lengths Ce1−O62 2.472(18) and Ce1−O64 2.498(15) Å, which are longer than the Zr−O bonds. Thereby this {MW2} metal motif remains powerful energy in building novel POTs.6,8

Figure 7. Polyhedral and ball-and-stick representation of the “saturated” Keggin or Dawson units in 5a: [{Ce2 O4 W2}{αSe2W14O51(OH)}] unit (a), [{CeO2W2}{α-SeW9O33}] unit (b), {CeO2W2} unit (c), and {Ce2O4W2} unit (d) (W: green, O: gray, Se: cyan, Ce, yellow).

Furthermore, the tetravacant {α-Se2W14O51(OH)} unit is filled with a square-planar-like {Ce2O4W2} fragment with an angle of 82.807(32)°, showing the stable and saturated {M18} Dawson-like motif (Figure 7b). It is worth noting that the Se··· Se distance (3.7038(30) Å) is longer than that in 2a and 3a, which is mainly due to the different {M4} units at the equatorial position filling in the vacancy. The square-planar-like {Ce2O4W2} fragment here includes four μ2-oxo bridges (Ce1−O62−W13, Ce2−O53−W13, Ce2−O52−W21, and Ce1− O3−W21) (Figure 7d) with average Ce−O bond lengths of 2.522 Å, such long bond lengths “brace” the square-planar-like fragment, further to the tetravacant {α-Se2W14O51(OH)} unit “elongating” the Se···Se distance. Three factors are keys to the G



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{E1/2 = (Epa + Epc)/2} (vs Ag/AgCl), which corresponds to the redox processes of WVI centers (Figure 9). No prominent

formation of this unprecedented Keggin- & Dawson-type cluster: (1) the SeO32− anion template, which can be viewed as “inorganic ligands”, has been easily employed to form unsaturated Keggin-type or Dawson-type units due to the existence of lone pair electrons. (2) pH value of 4.5 is crucial for the formations of lacunary Keggin- and Dawson-type units at the same time. (3) the 4f ions Ce3+ stabilize the lacunary BBs with its high coordination number, large radius, and reconcilable long bond lengths (2.447−2.609 Å). In addition, BVS calculations indicate that the oxidation states of all W, Se, Zr, and Ce centers are +6, +4, +4, and +3, respectively. Electrospray Ionization Mass spectroscopy (ESI-MS) Studies. The stability of the five clusters in water was investigated by UV/vis spectroscopy and ESI-MS. During the several hours monitoring process, the UV/vis spectra of the five polyoxoanions in solution remained unchanged (Figure S11 of the Supporting Information). The mass spectra of 1−5 in deionized water shows that the whole clusters are present, and all the main peaks can be assigned to its different charge/cation states (Figure 8; Figures S12−S15 of the Supporting

Figure 9. Cyclic voltammograms of 1 in 0.5 M H2SO4/Na2SO4 solution (pH = 1.3). The scan rate was 50 mV s−1. The working electrode was glassy carbon, and the reference electrode was Ag/AgCl.

electrochemical signature for the Zr unit is found, which is consistent with the previous reported Zr-containing POMs.9 Similarly, in a sulfate pH 2.5 medium at a scan rate of 100 mV s−1, the CV of 2 possesses three pairs of redox processes of WVI centers: one strong redox pair (III/III′) at E1/2 = −0.288 V and two relative weak redox pairs (II/II′) at E1/2 = −0.687 V and (I/I′) E1/2 = −0.808 V (vs Ag/AgCl) (Figure 10a, Table S13 of

Figure 8. ESI-MS of 3 in H2O.

Information). For 1−2, we found the clusters associated with [C2H8N]+, K+, Na+, and H+ ions dominated the solution of pure, redissolved 1−2 (Tables S8 and S9 of the Supporting Information). For 3 as shown in Figure 8, the strongest ones are centered at m/z 1164.2 and 1576.4, which have charges of −4 and −3, respectively. The other peak, which has an m/z value of 2400.2 with lower intensity compared to the former two, has a charge of −2. Accordingly, we found that the Dawson-type {Se2W18} cluster associated with [C2H8N]+, Na+, and H+ ions dominated the solution of redissolved pure 3 (Table S10 of the Supporting Information). Moreover, the ESIMS of 4 and 5 are very similar, two peaks with the same charges, the m/z values of 4 (28.5 or 35.0 mass units) are bigger than that of 5, which is mainly due to their key differences with different species and numbers of cations, coordinated carboxylate to the Ce centers, and crystallized water molecules. These two clusters associated with [C2H8N]+, K+, Na+, and H+ ions dominated the solution of redissolved pure 4−5 (Tables S11 and S12 of the Supporting Information). Electrochemistry. The cyclic voltammetry (CV) experiments were performed to examine the redox properties of 1−5 in 0.5 M H2SO4/Na2SO4 solution. These compounds display various behaviors, which may result from the different coordination environments of metal centers in the structures of 1−5.14 The CV of 1 examined in 0.5 M H2SO4/Na2SO4 solution (pH = 1.3) shows one broad reduction wave located at Epc = −0.481 V (II′) and one redox pair (I/I′) at E1/2 = −0.586 V

Figure 10. Cyclic voltammograms of 2 (a) and 3 (b) in 0.5 M H2SO4/ Na2SO4 solution (pH = 2.5). The scan rate was 100 mV s−1. The working electrode was glassy carbon, and the reference electrode was Ag/AgCl.

the Supporting Information), respectively. As for its monomer, 3, at the aforementioned scan rate and scanning toward the negative region of potential values, the reduction of W centers also occurs through three quasi-reversible redox couples with the corresponding E1/2 peak potentials (Table 2) located at −0.194, −0.451, and −0.628 V (vs Ag/AgCl), respectively, as shown in Figure 10b. To remove any possible redox process from dimethylamine hydrochloride molecules, we repeated the CV under the same conditions without the POM molecule where no redox process has been observed.10a As was expected, the observed redox peak potentials (E1/2) of 2 is shifted toward H



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Table 2. Redox Peak Potentials for all the Tungsten Waves Determined by Cyclic Voltammetry in 0.5 M H2SO4/Na2SO4 solution (pH = 2.5) for 3 Epa (V)

Epc (V)

E1/2 (V)

ΔEp (mV)

−0.145 −0.417 −0.600

−0.243 −0.484 −0.655

−0.194 −0.451 −0.628

98 67 55

more negative potential values compared to 3 since the dimer is harder to reduce as well as maybe a higher formal negative charge under such conditions.14c Besides, the observed voltammogram pattern of 3 is similar to the corresponding one for the [W18O56(SO3)2(H2O)2]8− cage. As previously reported, the [W18O56(SO3)2(H2O)2]8− undergoes a unique electron-transfer reaction when heated, in which a structural rearrangement allows the two embedded pyramidal sulfite anions to release up to four electrons to the surface of the cluster generating the sulfate-based, deep blue, mixed-valence cluster [W18O54(SO4)2]8−,10b while the 3a has no electrontransfer reactions here. This is not unexpected since the encapsulated sulfite anions act as embedded reducing agents and are more easily to be oxidized. Consequently, even though the two cages represent almost the same Dawson structural motifs, especially the “correct” orientation for the encapsulated anions, no oxidation processes are observed for the selenite anions present in 3. Furthermore, the CVs of both compounds at different scan rates are measured (Figures S16−S17 of the Supporting Information). The peak currents were proportional to the square root of the scan rate under these conditions, confirming ideal diffusion-controlled behavior in the 100−500 mV s−1 region of the scan rate. Also, we find the Keggin- or Dawson-type units possess similar WVI-waves depending on their own pH, and this is a well-known electrochemical behavior for the majority of the POMs.14 Figure 11 shows the main characteristic peaks associated with W-centered redox couples of 4 and 5 in the region −1.100 to +1.100 V of potential values versus Ag/AgCl at a scan rate of 50 mV s−1. At the pH value of 4.5 in sulfate medium, CV curves of 4 and 5 resemble each other (4 taken as the discussion case). Reduction of the WVI centers is expected to occur in the negative potential domain as mentioned above. The Wcentered waves of 4 are located at E1/2 = −0.472 V (III/III′), E1/2 = −0.677 V (II/II′), and another peak at Epc = −0.834 V (I) (vs Ag/AgCl). In the positive region of potentials, we observed an oxidation peak located at Epc = +0.821 V (vs Ag/ AgCl), while the ΔEp value of 113 mV for the oxidation process of interest suggests a quasi-reversible one-electron process (the theoretical value of ΔEp for a reversible electron transfer is about 59 mV),14d which is attributed to the cerium(IV/III) redox processes, exactly as Ce-containing POTs are want to do.8,14d−f Theoretical Analysis. In order to rationalize the electronic properties of the synthesized compounds, 1a and 5a were selected to investigate by density functional theory (DFT) calculations. The molecular orbital diagram for 1a is shown in Figure 12. For 1a, the highest occupied molecular orbital (HOMO) is localized over the bridging oxygens, which bond to Se and Zr/W, and the lowest unoccupied molecular orbital (LUMO) is localized over the W atoms and adjacent oxygens. It suggests that the W centers will be reduced in the twoelectron redox process, which is in well agreement with the experimental result. The HOMO−LUMO (H-L) gap of 1a is

Figure 11. Cyclic voltammograms of 4 (a) and 5 (b) in 0.5 M H2SO4/ Na2SO4 solution (pH = 4.5). The scan rate was 50 mV s−1. The working electrode was glassy carbon, and the reference electrode was Ag/AgCl.

Figure 12. Frontier molecular orbital diagram for 1a.

2.54 eV, which is in the range of H-L gaps of most POMs (2−3 eV).15 For 5a, the state with multiplicity (2S + 1 = 3) is more stable. The atomic electron spin densities for two Ce atoms are 0.868e and 0.846e, indicating that two unpaired electrons are effectively localized over the Ce atoms. The molecular orbital diagram for 5a is shown in Figure 13. The energy level of αHOMO is quite lower than that of β-HOMO. It is interesting to find that the α-HOMO and α-LUMO are 4f orbitals of Ce atoms. For beta components, the HOMO localizes on oxo ligands, while the LUMO is mainly contributed by d orbitals of W atoms. Furthermore, the oxygen basicity of 1a is analyzed. In order to clearly visualize the molecular electrostatic potential I



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Figure 13. Frontier molecular orbital diagram for 5a.

O−W and W−O−Se oxygens are most basic, while the terminal oxygens OW are the least basic. Although bridging W−O−Se oxygens are most basic, they are not the oxygen site accessible for protonation due to the internal oxygens.

(MEP) of 1a by plotting the electrostatic potential (EP) over an electron density isosurface is presented in Figure 14. Herein,

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CONCLUSIONS The versatile pH-dependent one-pot synthetic strategy was used to prepare five Keggin-/Dawson-type tungstoselenites by the combination of different metal uints with SeO 3 2− heteroanion templates. All five compounds were characterized by single-crystal X-ray structure analysis, IR spectroscopy, thermogravimetric analysis, UV/vis spectroscopy, ESI-MS, and DFT calculations. Moreover, their electrochemical properties were investigated. These five clusters possess three “features” as follows: (1) the first reported tetrameric Zr-containing tungstoselenites (1) with the 4d metal Zr centers in sevenand eight-coordinated modes; (2) the second reported nonconventional Dawson-type cage {Se2W18} (3) and its dimer {Se4W36} (2) templating by the SeO32− heteroanion based on the first reported {α-Se2W14} building blocks; (3) the second reported Keggin- ({α-SeW9}) and Dawson-type ({αSe2W14}) building blocks appeared at the same time into the single polyoxoanion architectures 4 and 5. The present work realizes the important roles of the SeO32− heteroanion template, pH, different metal units, and effects of the counter cations in guiding the one-pot synthetic strategy that reveals a fascinating potential for further exploration toward the design and synthesis of novel POM-based materials. In the future, we will continue to investigate the assembly and the functionality of different synthesis metal units with the heteroanion templates in more detail. Moreover, the heteroanions (such as SbO32−, TeO32−, and AsO33−, etc.) may also be good

Figure 14. MEP distribution for 1a.

the red-yellow stands for nucleophilic regions and green-blue for electrophilic regions. In POMs, there are three structural types of oxygen sites: bridging W−O−W, W−O−Se, internal W−O−Se and terminal OW sites. The MEP analysis predicts that the bridging W−O−W oxygens in 1a should be the most basic in the whole cluster and that W−O−Se oxygens are less nucleophilic than bridging W−O−W ones. Terminal oxygens OW are the least basic as usual in POMs.15a,16 As for polyanion 5 (Figure S19 of the Supporting Information), the bridging W− J



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Giménez-Marqués, M.; Galán-MascarÓ s, J. R.; Vitoria, P.; GutiérrezZorrilla, J. M. Angew. Chem., Int. Ed. 2010, 49, 8384−8388. (c) Kortz, U.; Savelieff, M.; Bassil, B.; Dickman, M. Angew. Chem., Int. Ed. 2001, 40, 3384. (7) (a) Yan, J.; Long, D. L.; Cronin, L. Angew. Chem., Int. Ed. 2010, 49, 4117−4120. (b) Gao, J.; Yan, J.; Mitchell, S. G.; Miras, H. N.; Boulay, A. G.; Long, D. L.; Cronin, L. Chem. Sci. 2011, 2, 1502−1508. (c) Gao, J.; Yan, J.; Beeg, S.; Long, D. L.; Cronin, L. Angew. Chem., Int. Ed. 2012, 51, 3373−3376. (d) Yan, J.; Gao, J.; Long, D. L.; Miras, H. N.; Cronin, L. J. Am. Chem. Soc. 2010, 132, 11410−11411. (e) Gao, J.; Yan, J.; Beeg, S.; Long, D. L.; Cronin, L. J. Am. Chem. Soc. 2013, 135, 1796−1805. (8) Chen, W. C.; Li, H. L.; Wang, X. L.; Shao, K. Z.; Su, Z. M.; Wang, E. B. Chem.Eur. J. 2013, 19, 11007−11015. (9) (a) Nomiya, K.; Sakai, Y.; Matsunaga, S. Eur. J. Inorg. Chem. 2011, 179−196. (b) Finke, R. G.; Rapko, B.; Weakley, T. J. R. Inorg. Chem. 1989, 28, 1573. (c) Gaunt, A. J.; May, I.; Collison, D.; Holman, K. T.; Pope, M. T. J. Mol. Struct. 2003, 656, 101. (d) Fang, X. K.; Anderson, T.; Hill, C. L. Angew. Chem., Int. Ed. 2005, 44, 3540−3544. (e) Huang, L.; Wang, S. S.; Zhao, J. W.; Cheng, L.; Yang, G. Y. J. Am. Chem. Soc. 2014, DOI: 10.1021/ja413134w. (f) Bassil, B. S.; Mal, S. S.; Dickman, M. H.; Kortz, U.; Oelrich, H.; Walder, L. J. Am. Chem. Soc. 2008, 130, 6696−6697. (10) (a) Yan, J.; Long, D. L.; Miras, H. N.; Cronin, L. Inorg. Chem. 2010, 49, 1819−1825. (b) Long, D. L.; Abbas, H.; Kögerler, P.; Cronin, L. Angew. Chem., Int. Ed. 2005, 44, 3415. (c) Yan, J.; Long, D. L.; Wilson, E. F.; Cronin, L. Angew. Chem., Int. Ed. 2009, 48, 4376. (d) Cameron, J. M.; Gao, J.; Vilà-Nadal, L.; Long, D. L.; Cronin, L. Chem. Commun. 2014, 2155−2157. (e) Cameron, J. M.; Gao, J.; Long, D. L.; Cronin, L. Inorg. Chem. Front. 2014, 1, 178−185. (f) Kalinina, I. V.; Peresypkina, E. V.; Izarova, N. V.; Nkala, F. M.; Kortz, U.; Kompankov, N. B.; Moroz, N. K.; Sokolov, M. N. Inorg. Chem. 2014, 53, 2076−2082. (g) Wang, L. J.; Li, W. J.; Wu, L. Z.; Dong, X. B.; Hu, H. M.; Xue, G. L. Inorg. Chem. Commun. 2013, 35, 122−125. (11) (a) Tasiopoulos, A. J.; O’Brien, T. A.; Abboud, K. A.; Christou, G. Angew. Chem., Int. Ed. 2004, 43, 345. (b) Tasiopoulos, A. J.; Milligan, P. L., Jr; Abboud, K. A.; O’Brien, T. A.; Christou, G. Inorg. Chem. 2007, 46, 9678. (c) Fang, X. K.; Kögerler, P. Chem. Commun. 2008, 3396−3398. (d) Fang, X. K.; McCallum, K.; Pratt, H. D., III; Anderson, T. M.; Dennis, K.; Luban, M. Dalton Trans. 2012, 41, 9867−9870. (12) Long, D. L.; Kögerler, P.; Farrugia, L. J.; Cronin, L. Angew. Chem., Int. Ed. 2003, 42, 4180−4183. (13) (a) Brown, I. D.; Altermatt, D. Acta Crystallogr. Sect. B 1985, 41, 244. (b) Trzesowska, A.; Kruszynski, R.; Bartczak, T. J. Acta Crystallogr. Sect. B 2006, 62, 745. (14) (a) Bassil, B. S.; Kortz, U.; Tigan, A. S.; Clemente-Juan, J. M.; Keita, B.; Oliveira, P.; Nadjo, L. Inorg. Chem. 2005, 44, 9360. (b) Mbomekalle, I. M.; Keita, B.; Nierlich, M.; Kortz, U.; Berthet, P.; Nadjo, L. Inorg. Chem. 2003, 42, 5143. (c) Korenev, V. S.; Floquet, S.; Marrot, J.; Haouas, M.; Mbomekalle, I.; Taulelle, F.; Sokolov, M. N.; Fedin, V. P.; Cadot, E. Inorg. Chem. 2012, 51, 2349−2358. (d) Haraguchi, N.; Okaue, Y.; Isobe, T.; Matsuda, Y. Inorg. Chem. 1994, 33, 1015−1020. (e) Suzuki, K.; Tang, F.; Kikukawa, Y.; Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2014, 53, 5356− 5360. (f) Sadakane, M.; Dickman, M. H.; Pope, M. T. Inorg. Chem. 2001, 40, 2715−2719. (15) (a) López, X.; Bo, C.; Poblet, J. M. J. Am. Chem. Soc. 2002, 124, 12574−12582. (b) Bagno, A.; Bini, R. Angew. Chem., Int. Ed. 2010, 49, 1083−1086. (16) (a) Kempf, J. Y.; Rohmer, M. M.; Poblet, J. M.; Bo, C.; Benard, M. J. Am. Chem. Soc. 1992, 114, 1136−1146. (b) Fernández, J. A.; López, X.; Bo, C.; de Graaf, C.; Baerends, E. J.; Poblet, J. M. J. Am. Chem. Soc. 2007, 129, 12244−12253. (17) ADF2012, SCM, Theoretical Chemistry; Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com. (18) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200− 1211. (19) Becke, A. D. Phys. Rev. A 1988, 38, 3098−3100.

candidates for the construction of new novel architectures with the 3d metals, 3d−4f metals, 4d metals, or 4f metals centers to link, stabilize, and enrich the family of nonconventional POM clusters. Finding more unprecedented Keggin-type and Dawson-type clusters (such as 4 or 5) also provides us the chance to make a major step forward in POMs chemistry. These works will be reported in due course.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Details of additional structural figures, the tables of the BVS calculation results of all the oxygen atoms in 1−5, the tables of bond lengths for Zr4+ ions and selenite atoms, the IR, UV/vis, TGA, XRD, ESI-MS characterizations, electrochemistry, and the tables of the assignment of peaks of ESI-MS. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Authors

*(L.-K.Y.) E-mail: [email protected]. *(X,.-L.W.) E-mail: [email protected]. *(Z.-M.S.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the NNSF of China (Nos. 21001022, 21171033, 21131001), the NGFR 973 Program of China (No. 2010CB635114), NCET in Chinese University (No. NCET-10-0282), PhD Station Foundation of Ministry of Education (No.20100043110003), the FANEDD of the P. R. China (No. 201022), the STDP of Jilin Province (Nos. 201001169, 20111803), and the FRFCU (Nos. 09ZDQD003, 10CXTD001).

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