Assembly Chemistry between Lanthanide Cations and Monovacant

DOI: 10.1021/cg900248j

Assembly Chemistry between Lanthanide Cations and Monovacant Keggin Polyoxotungstates: Two Types of Lanthanide Substituted Phosphotungstates [{(r-PW11O39H)Ln(H2O)3}2]6- and [{(r-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}2]10-

2009, Vol. 9 4362–4372

Jingyang Niu,† Kaihua Wang, Huanni Chen, Junwei Zhao, Pengtao Ma, Jingping Wang,* Mingxue Li, Yan Bai, and Dongbin Dang Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University Kaifeng, Henan 475004, P. R. China. †E-mail: [email protected] Received February 27, 2009; Revised Manuscript Received June 20, 2009

ABSTRACT: Two 2:2 types of monolanthanide substituted polyoxometalates [{(R-PW11O39H)Ln(H2O)3}2]6- (Ln=NdIII for 1 and GdIII for 2) and [{(R-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}2]10- (Ln=SmIII for 3, EuIII for 4, GdIII for 5, TbIII for 6, HoIII for 7 and ErIII for 8) have been synthesized in aqueous solution and characterized by elemental analyses, IR spectra, UV-vis-NIR spectra, thermogravimatric analyses and single-crystal X-ray diffraction. The common structural features are that they are constructed from monovacant Keggin-type polyoxoanions [R-PW11O39]7- and trivalent lanthanide cations. Both 1 and 2 are essentially isomorphous, and the molecular structure is built by two symmetrically related monolanthanide substituted Keggin units [R-PW11O39Ln(H2O)3]4- linked via two Ln-O-W bridges, representing the first monovacant Keggin polyoxotungstate dimers constituted by two [R-PW11O39]7- polyoxoanions and two lanthanide cations in polyoxometalate chemistry. 3-8 are also isostructural and display another dimeric structure constructed from two monolanthanide substituted units [(R-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO]5- bridged by two (η2,μ-1,1)-acetato ligands. The photoluminescence properties of 4 and 6 were investigated at room temperature. Magnetic susceptibility measurements of 1, 3, 4, 7 and 8 between 2 and 300 K exhibit that their magnetic behaviors mainly result from the spin-orbital coupling interactions as well as weak antiferromagnetic exchange interactions within magnetic centers. The electrochemical properties of 1, 3, 4, 5, 7 and 8 were studied by means of cyclic voltammetry in aqueous solution with 0.5 M Na2SO4 as supporting electrolyte.

Introduction Current continuous research activities of polyoxometalate (POM) chemistry are greatly driven not only by diverse structural and electronic characteristics but also realized and potential applications in fields as diverse as catalysis, medicine, analytical chemistry, materials science and molecular electronics, etc.1 POMs can be versatile inorganic building blocks for the construction of molecule-based materials and can also bind most lanthanide (Ln) cations, resulting in a family of lanthanide substituted polyoxometalates (LSPs) that exhibit interesting luminescence and magnetic properties.2 Recently, considerable attention has been focused on exploring the reaction of Ln cations with lacunary polyoxoanions because (1) Ln cations have stronger affinity for the basic oxygen atoms at the defect sites in lacunary polyoxoanions than the 3d transition-metal cations; (2) owing to their multiple coordination requirements and stronger oxophilicity, Ln cations can readily link different lacunary polyoxoanions together leading to some novel LSP oligomers or larger aggregates with unexpected structures and properties. For example, in 2001, Howell et al. reported two isostructural tetrameric LSPs [(PM2W10O38)4(W3O14)]30- (M=EuIII and YIII) by reaction of trivacant Keggin precursor Na9PW9O34 3 15H2O with Eu3þ or Y3þ ions in aqueous solution.2a In 2005, Fang and coworkers addressed two POM-supported YIII- and YbIII-hydroxo/ oxo dimeric clusters [{Y4( μ3-OH)4(H2O)8}(R-P2W15O56)2]16*Corresponding author. Fax: (þ86) 378 3886876. E-mail: jpwang@henu. edu.cn. pubs.acs.org/crystal

Published on Web 08/19/2009

and [{Y6( μ6-O)( μ3-OH)(H2O)6}(R-P2W15O56)2]14-.2b Hitherto, the largest cyclic LSP [Ln16As12W148O524(H2O)36]76- composed of twelve trilacunary [AsW9O34]9- units and four [W5O18]6- units linked by Ln cations was reported by Pope et al. in 1997.2c To date, however, the progress of this research field is rather laggardly and the system containing Ln cations and lacunary polyoxoanions remains largely unexplored, therefore, this context not only provides us great challenges and opportunities but also gives us a great impetus to exploit this domain. Since monovacant Keggin species ([PW11O39]7-, [SiW11O39]8- and [GeW11O39]8-) were reported by Tourne et al. in 1969,3 the system containing Ln cations and monovacant polyoxoanions began to be continuously exploited. In 1971, Peacock and Weakley first put forward that the combination of monovacant Keggin polyoxoanions [XW11 O39]n- (X = SiIV, PV) with Ln cations can form both 1:1-type and 1:2-type derivatives in solution.4 According to the known structure of [U(GeW11O39)]12- reported by Tourne and co-workers in 1980,5 Blasse et al. predicted that, in the structures of the 1:2-type [Ln(SiW11O39)2]13- series, an eight-coordinate Ln ion is sandwiched by two monovacant Keggin polyoxoanions.6 It was not until 2000 that Pope et al. employed a simple strategy to isolate and characterize first two infinite one-dimensional 1:1-type POM-based Ln derivatives, [Ln(RSiW11O39)(H2O)3]5- (Ln=LaIII and CeIII), which represent two types of polymeric chain structures in the solid state.7 Moreover, the obtainment of these two one-dimensional 1:1-type POM-based Ln derivatives proved the presence of the 1:1 structural type put forward by Peacock and Weakley.4 r 2009 American Chemical Society

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In 2003, Mialane et al. expanded the Pope’s work and discovered another two structural types: one is the onedimensional linear arrangement shown by [Yb(R-SiW11O39)(H2O)2]5-, and the other is the two-dimensional layer arrangement exhibited by [Nd2(R-SiW11O39)(H2O)11]2-; furthermore, they investigated the influence of the nature of Ln ions and the stoichiometric Ln/POM ratios on the structural arrangements.8 Subsequently, Mialane et al. isolated two 2:2-type dimers [{(R-SiW11O39)Ln(COOCH3)(H2O)}2]12(Ln=GdIII and YbIII) and quantitatively analyzed the antiferromagnetic behavior of [{(R-SiW11O39)Gd(COOCH3)(H2O)}2]12-.9 In 2004, Nogueira et al. reported a novel discrete CeIV phosphotungstate [Ce2(PW10O28)(PW11O39)2]17-, containing a monovacant [PW11O39]7- unit and an unusual 1,4-bilacunary [PW10O28]11- unit linked by two CeIV ions.10 Later, he and co-workers communicated a one-dimensional zigzag lanthanopolytungstoborate [Ce2(BW11O39)(H2O)6]12-.11 Recently, Kortz et al. prepared a family of monolanthanide substituted β2-Keggin silicotungstates [Ln(β2-SiW11O39)2]13(Ln = LaIII, CeIII, SmIII, EuIII, GdIII, TbIII, YbIII, LuIII) by reaction of Ln ions with the precursors [β2-SiW11O39]8- in a 1:2 molar ratio in 1 M KCl medium at pH=5.12 Since 2004, we have been exploiting the reactions of Ln cations with lacunary polyoxoanion precursors in the conventional aqueous solution conditions. We first reported two novel one-dimensional chainlike derivatives [(Pr(H2O)4SiW11O39)(NaPr2(H2O)12)(Pr(H2O)4 SiW11O39)]3-,13 and {[Sm(H2O)7]2Na[R-SiW11O39Sm(H2O)4]2}3-,14 which are constructed from alternating Ln centers, sodium centers and monovacant polyoxoanion units via terminal and bridging oxygen atoms from polyoxoanion units. With our continuous efforts, we extended our research from the Ln/lacunary silicotungstate system to the Ln/lacunary germanotungstate system; furthermore, some small organic molecular ligands (DMF = N,N-dimethylformamide, DMSO = dimethyl sulfoxide) were also introduced to our research system, thus, a class of novel organic-inorganic hybrid lanthanide derivatives of monovacant Keggin silicotungstates and germanotungstates were obtained,15 and these derivatives exhibit the rich structural diversity. For example, [Sm2(GeW11O39)(DMSO)3(H2O)6]2- displays a novel double-parallel chainlike structure constructed by two linear wires linked by samarium coordination cations.15a As far as we know, to date, the investigations on the reactions of Ln cations with lacunary phosphotungstate precursors ([R-PW11O39]7and [R-PW9O34]9-) are almost laggardly. Therefore, recently, we concentrated our attention on this branch. A large number of exploratory experiments have been performed; eventually, we isolated two kinds of 2:2-type dimeric lanthanophosphotungstates: [(CH3)4N]6[{(R-PW11O39H)Nd(H2O)3}2] 3 8H2O (1), [(CH3)4N]6[{(R-PW11O39H)Gd(H2O)3}2] 3 6H2O (2), and their general formula can be described as [{(R-PW11O39H)Ln(H2O)3}2]6- (Ln = NdIII for 1 and GdIII for 2); [(CH3)4N]10[{(R-PW11O39)Sm(H2O)(η2,μ-1,1)-CH3COO}2] 3 6H2O (3), [(CH3)4N]10[{(R-PW11 O39)Eu(H2O)(η2,μ-1,1)-CH3COO}2] 3 8H2O (4), [(CH3)4N]10[{(R-PW11O39)Gd(H2O)(η2,μ-1,1)-CH3COO}2] 3 8H2O (5), [(CH3)4N]10[{(R-PW11O39)Tb(H2O)(η2, μ-1,1)-CH3COO}2] 3 7H2O (6), [(CH3)4N]10[{(R-PW11O39)Ho(H2O)(η2,μ-1,1)-CH3COO}2] 3 8H2O (7) and [(CH3)4N]10[{(R-PW11O39)Er(H2O)(η2,μ-1,1)-CH3COO}2] 3 8H2O (8), and their general formula can be described as [{(R-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}2]10- [Ln=SmIII for 3, EuIII for 4, GdIII for 5, TbIII for 6, HoIII for 7 and ErIII for 8]. The common structural feature is that they are constructed from monovacant Keggin-type polyoxoanions [R-PW11O39]7- and trivalent

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lanthanide cations. Compunds 1-8 exhibit two different 2:2 structural types of dimeric structures. Both 1 and 2 are essentially isomorphous, and their molecular structure is built by two mono-Ln substituted Keggin fragments [R-PW11O39Ln(H2O)3]4- linked through two Ln-O-W bridges, representing the first 1:1-type monovacant Keggin phosphotungstate dimers constituted by two [R-PW11O39]7- polyoxoanions and two Ln cations in POM chemistry. 3-8 are also isostructural and display another dimeric structure constructed from two mono-Ln substituted fragments [{(R-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}]5- bridged by the two (η2,μ-1,1)-acetato ligands, and this structural type was first observed by Mialane et al. in 2004 during the course of studying the lanthanide/monovacant silicotungstate system.9 The influences of the sizes of Ln ions and the pH values of the synthetic media on the structures of products were discussed. The photoluminescence properties of 4 and 6 were investigated at room temperature. Magnetic susceptibility data of 1, 3, 4, 7 and 8 were measured between 2 and 300 K, and their magnetic behaviors mainly result from spin-orbital coupling interactions as well as weak antiferromagnetic exchange interactions within magnetic centers. The electrochemical properties of 1, 3, 4, 5, 7 and 8 were studied by means of cyclic voltammetry in aqueous solution with 0.5 M Na2SO4 as supporting electrolyte. Experimental Section General Methods and Materials. The Na7[R-PW11O39] 3 nH2O and Na9[R-PW9O34] 3 16H2O precursors were prepared according to the literature16,17 and confirmed by IR spectra. Other chemicals were purchased commercially and used without further purification. Elemental analyses (C, H and N) were performed on a PerkinElmer 2400 CHN elemental analyzer (see Supporting Information). Inductively coupled plasma (ICP) analysis was performed on a Jobin Yvon ultima 2 spectrometer (see Supporting Information). IR spectra were recorded on a Nicolet AV ATAR 360 FTIR spectrophotometer using KBr pellets in the range of 4000-400 cm-1 (see Supporting Information). Thermogravimetric analyses were performed on an Exstar 6000 analyzer under nitrogen flow with a heating rate of 10 °C/min. UV-vis-NIR spectra were obtained on a Unican UV-500 spectrometer (distilled water as solvent) in the range of 900-190 nm at room temperature. Photoluminescence measurements were performed on F-7000 fluorescence spectrometer. Magnetic measurements on the polycrystalline samples were carried out on a quantum design MPMS7 SQUID magnetometer in the temperature region of 2-300 K. Electrochemical measurements were performed on a LK98 microcomputer-based electrochemical system (LANLIKE, Tianjin, China). A three-electrode system was employed for cyclic voltammetry. A 4 mm diameter glassy carbon disk electrode (GCE) was used as a working electrode, a platinum wire served as the counter electrode and an Ag/AgCl electrode as the reference electrode. Synthesis of [(CH3)4N]6[{(r-PW11O39H)Nd(H2O)3}2] 3 8 H2O (1). 2.10 g (ca. 0.73 mmol) of Na7PW11O39 3 nH2O was dissolved in 15 mL of water at 80 °C, followed by addition of 0.33 g (1.30 mmol) of NdCl3 in 15 mL of water dropwise. The resulting solution was adjusted to the desired pH = 5.4 using 4 mol L-1 NaOH under stirring. After 1 h, the solution was cooled to room temperature and filtered. Then 0.20 g (1.30 mmol) of tetramethylammonium bromide was added under stirring. After 0.5 h, the resulting solution was filtered and left to evaporate at room temperature. Purple rhombic crystals of 1 were obtained after two weeks. Yield: ca. 22% (based on Na7PW11O39 3 nH2O). Anal. Calcd (%) for 1: C 4.55, H 1.62, N 1.33, P 0.98, Nd 4.55, W 63.77. Found: C 4.49, H 1.78, N 1.27, P 0.92, Nd 4.57, W 63.48. Synthesis of [(CH3)4N]6[{(r-PW11O39H)Gd(H2O)3}2] 3 6 H2O (2). The synthetic procedure was identical to that for 1, but used 0.34 g (1.30 mmol) of GdCl3 as the Ln reagent. Colorless rhombic crystals of 2 were obtained after two weeks. Yield: ca. 15% (based on Na7PW11O39 3 nH2O). Anal. Calcd (%) for 2: C 4.55, H 1.56, N 1.33,

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P 0.98, Gd 4.97, W 63.87. Found: C 4.51, H 1.77, N 1.22, P 0.90, Gd 4.69, W 63.99. Synthesis of [(CH3)4N]10[{(r-PW11O39)Sm(H2O)(η2,μ-1,1)-CH3COO}2] 3 6H2O (3). Method A. 0.36 g (1.40 mmol) SmCl3 was first dissolved in 30 mL of CH3COOH-CH3COONa buffer solution (0.5 mol L-1, pH=4.75) under stirring. Then 1.98 g (0.73 mmol) of Na9PW9O34 3 16H2O was added. After the solution was kept at 90 °C for 1 h, the solution was cooled to room temperature and filtered. Then 0.40 g (2.60 mmol) of tetramethylammonium bromide was added under stirring. After 0.5 h, the resulting solution was filtered and left to evaporate at room temperature. Light yellow prismatic crystals of 3 were obtained after two weeks. Yield: ca. 34% (based on Na9PW9O34 3 16H2O). Method B. 2.10 g (ca. 0.73 mmol) of Na7PW11O39 3 nH2O was dissolved in 15 mL of CH3COOH-CH3COONa buffer solution (0.5 mol L-1, pH=5.5) at 80 °C, followed by addition of 0.33 g (1.30 mmol) of SmCl3 in 15 mL of CH3COOH-CH3COONa buffer solution (0.5 mol L-1, pH=5.5) dropwise under stirring. After 1 h, the solution was cooled to room temperature and filtered. Then 0.25 g (1.62 mmol) of tetramethylammonium bromide was added with stirring. After 30 min, the resulting solution was filtered and left to evaporate at room temperature. Light yellow prismatic crystals of 3 were obtained after two weeks. Yield: ca. 26% (based on Na7PW11O39 3 nH2O). Anal. Calcd (%) for 3: C 7.94, H 2.15, N 2.10, P 0.93, Sm 4.52, W 60.74. Found: C 7.31, H 2.25, N 1.84, P 1.01, Sm 4.41, W 60.47. Synthesis of [(CH3)4N]10[{(r-PW11O39)Eu(H2O)(η2,μ-1,1)-CH3COO}2] 3 8H2O (4). 4 was prepared according to method A described for 3, but used 0.36 g (1.40 mmol) of EuCl3 as the Ln reagent. Colorless prismatic crystals of 3 were obtained after two weeks. Yield: ca. 30% (based on Na9PW9O34 3 16H2O). Anal. Calcd (%) for 4: C 7.89, H 2.20, N 2.09, P 0.92, Eu 4.54, W 60.38. Found: C 7.92, H 2.13, N 2.08, P 0.88, Eu 4.44, W. 60.91. Synthesis of [(CH3)4N]10[{(r-PW11O39)Gd(H2O)(η2,μ-1,1)-CH3COO}2] 3 8H2O (5). 5 was prepared according to method A described for 3, but used 0.37 g (1.40 mmol) of GdCl3 as the Ln reagent. Colorless prismatic crystals of 5 were obtained after two weeks. Yield: ca. 37% (based on Na9PW9O34 3 16H2O). Anal. Calcd (%) for 5: C 7.88, H 2.19, N 2.09, P 0.92, Gd 4.69, W 60.29. Found: C 7.88, H 2.13, N 2.06, P 0.87, Gd 4.78, W 60.58. Synthesis of [(CH3)4N]10[{PW11O39Tb(H2O)(η2,μ-1,1)-CH3COO}2] 3 6H2O (6). 6 was prepared according to method A described for 3, but used 0.37 g (1.40 mmol) of TbCl3 as the Ln reagent. Colorless prismatic crystals of 6 were obtained after two weeks. Yield: ca. 27% (based on Na9PW9O34 3 16H2O). Anal. Calcd (%) for 6: C 7.92, H 2.14, N 2.10, P 0.93, Tb 4.46, W 60.58. Found: C 7.61, H 2.37, N 1.92, P 0.98, Tb 4.55, W 60.19. Synthesis of [(CH3)4N]10[{(r-PW11O39)Ho(H2O)(η2,μ-1,1)-CH3COO}2] 3 8H2O (7). 7 was prepared according to method A described for 3, but used 0.38 g (1.40 mmol) of HoCl3 as the Ln reagent. Light yellow prismatic crystals of 7 were obtained after two weeks. Yield: ca. 35% (based on Na9PW9O34 3 16H2O). Anal. Calcd (%) for 7: C 7.88, H 2.19, N 2.09, P 0.92, Ho 4.92, W 60.29. Found: C 7.91, H 2.42, N 2.09, P 0.84, Ho 5.10, W 61.04. Synthesis of [(CH3)4N]10[{(r-PW11O39)Er(H2O)(η2,μ-1,1)-CH3COO}2] 3 8H2O (8). 8 was prepared according to method A described for 3, but used 0.38 g (1.40 mmol) of ErCl3 as the Ln reagent. Pink prismatic crystals of 8 were obtained after two weeks. Yield: ca. 40% (based on Na9PW9O34 3 16H2O). Anal. Calcd (%) for 8: C 7.85, H 2.19, N 2.08, P 0.92, Er 5.10, W 60.11. Found: C 7.71, H 2.36, N 1.96, P 0.98, Er 5.01, W 60.40. X-ray Crystallography. Intensity data of 1-8 were collected on a Bruker Apex-2 CCD detector using graphite monochromatized Mo KR radiation (λ=0.71073 A˚) at room temperature. The structures were solved by direct methods and refined using full-matrix leastsquares on F2. All calculations were performed using the SHELXL97 program package.18 Intensity data were corrected for Lorentz and polarization effect as well as for multiscan absorption. All of the non-hydrogen atoms were refined anisotropically. Hydrogen atoms were not included in the refinements. In addition, the solventaccessible volumes of 1 and 2 calculated using PLATON19 are 12.8% and 13.8% respectively. Such phenomenon is very common in POM chemistry,20a-c the main reason of which may be that the packing of the large volume of the polyoxoanions leads to the

Niu et al. presence of large solvent accessible voids in the packing arrangements.20d-f A summary of crystal data and structure refinements for compounds 1-8 was listed in Table 1. CCDC 694167694172 and 694174-694175 contain the supplementary crystallographic data for this paper for 1-8, respectively. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Results and Discussion Synthesis. Based on our previous findings on lacunary silicotungstate and germanotungstate Ln derivatives,13-15 we further developed the synthetic strategy of the combination of lacunary phosphotungstates with Ln cations. We chose lacunary phosphotungstate precursors Na7[R-PW11O39] 3 nH2O16 and Na9[R-PW9O34] 3 16H2O17 as the original materials to react with Ln cations under the different conditions. The 2:2-type dimers 1 and 2 were first obtained by reaction of the [R-PW11O39]7- polyoxoanion with NdIII or GdIII cations by controlling the pH value at 5.4. When other Ln cations were introduced to this system, similar species could be isolated; however, crystals of these compounds effloresced easily when they were moved out of the parent solutions; as a result, it is very difficult to collect intensity data on the X-ray single-crystal diffractometer. So, the product phases have to be confirmed by IR spectra (Figure S1 in the Supporting Information). Interestingly, when Ce(NO3)3 3 6H2O reacts with Na7[R-PW11O39] 3 nH2O under the condition of pH = 4.8 in the participation of (CH3)4NBr, a one-dimensional zigzag chainlike phosphotungstate cerium derivative [(CH3)4N]4[(R-PW11O39)Ce(H2O)2] 3 2H2O was isolated, which possesses a similar architectural motif to [Ce(R-SiW11O39)(H2O)3]5- reported by Pope et al. in 2000.7 The exploration of such a one-dimensional chainlike lanthanophosphotungstate is in progress. Thus, encouraged by the aforementioned results, we chose the CH3COOH-CH3COONa buffer solution to control the pH value of the reaction system. At the CH3COOH-CH3COONa buffer solution (pH=5.5), when SmIII, EuIII, GdIII, TbIII, HoIII and ErIII cations were used to react with the [R-PW11O39]7- precursor, respectively, another structural type of the 2:2-type dimers 3-8 was successively isolated. The isostructural species of LaIII, CeIII, PrIII and NdIII including acetate groups were also obtained and identified by IR spectra (Figure S2 in the Supporting Information). Unexpectedly, when LaIII, CeIII and PrIII were used as the Ln reagents in the CH3COOH-CH3COONa buffer solution (pH 5.5), the 2:2-type dimers [{(R-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}2]10- (Ln=LaIII, CeIII and PrIII) and 1:2-type dimeric compounds [Ln(R-PW11O39)2]11(Ln=LaIII, CeIII and PrIII), were obtained at the same time, which suggests that there exists an equilibrium between 2:2type and 1:2-type species in the solution. A similar phenomenon has been noticed by Mialane et al.9 The further systematic investigation on this equilibrium is in progress. The aforementioned facts indicate that the smaller atomic radii of Ln atoms are favorable to the formation of 2:2-type dimeric compound [{(R-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}2]10-, which is in good agreement with the conclusions deduced from the Ln/lacunary silicotungstate system.9 To exploit the reactions of the [R-PW9O34]9- precursor with Ln cations, interestingly, when the pH of the CH3COOH-CH3COONa buffer solution was set at 4.75, 3-8 could also be successfully obtained. In such a case, the transformation of [R-PW9O34]9- f [R-PW11O39]7- must have happened in the formation of 3-8. It is well-known that the [R-PW9O34]9- polyoxoanion

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Table 1. Crystallographic Data and Structure Refinements for 1-8 formula Mr (g mol-1) T (K) space group crystal system a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dc (g cm-3) μ (mm-1) Rint reflns collected indep reflns params GOF on F 2 R1, wR2 [I > 2σ(I )] R1, wR2 [all data]

1

2

3

4

[(CH3)4N]6[PW11O39HNd(H2O)3]2 3 8H2O 6342.24 293(2) P2(1)/c monoclinic 13.028(2) 22.133(4) 20.933(4) 90.00 100.696(3) 90.00 5931.2(18) 2 3.551 22.217 0.1175 29883 10385 676 1.009 0.0588, 0.1210 0.0950, 0.1313

[(CH3)4N]6[PW11O39HGd(H2O)3]2 3 6H2O 6332.22 296(2) P2(1)/c monoclinic 13.0146(8) 22.2158(13) 21.0096(12) 90.00 101.0260(10) 90.00 5962.4(6) 2 3.527 22.340 0.0898 29628 10211 621 0.901 0.0506, 0.0817 0.0896, 0.0889

[(CH3)4N]10[PW11O39Sm(COOCH3)(H2O)]2 3 6H2O 6659.02 293(2) P2(1)/c monoclinic 25.229(3) 12.7407(15) 22.394(3) 90.00 115.473(2) 90.00 6498.5(14) 2 3.403 20.391 0.0990 32430 11401 749 1.005 0.0495, 0.1034 0.0809, 0.1106

[(CH3)4N]10[PW11O39Eu(COOCH3)(H2O)]2 3 8H2O 6698.27 296(2) P2(1)/c monoclinic 25.177(3) 12.7367(18) 22.305(3) 90.00 115.199(2) 90.00 6471.9(16) 2 3.437 20.538 0.0786 33782 11360 757 0.995 0.0441, 0.0694 0.0814, 0.0763

5

6

7

[(CH3)4N]10[PW11O39[(CH3)4N]10[PW11O39[(CH3)4N]10[PW11O39Gd(COOCH3)(H2O)]2 3 8H2O Tb(COOCH3)(H2O)]2 3 6H2O Ho(COOCH3)(H2O)]2 3 8H2O Mr (g mol-1) 6708.85 6676.16 6708.21 T (K) 296(2) 296(2) 296(2) space group P2(1)/c P2(1)/c P2(1)/c crystal system monoclinic monoclinic monoclinic a (A˚) 24.881(16) 25.0043(17) 25.070(5) b (A˚) 12.573(8) 12.7529(9) 12.744(3) c (A˚) 22.116(14) 22.2642(16) 22.335(5) R (deg) 90.00 90.00 90.00 β (deg) 115.462(9) 114.6940(10) 115.135(4) γ (deg) 90.00 90.00 90.00 6246(7) 6450.3(8) 6460(2) V (A˚3) Z 2 2 2 3.567 3.437 3.449 Dc (gcm-3) 21.337 20.729 20.829 μ (mm-1) 0.0708 0.0666 0.0725 Rint reflns collected 22406 32378 41176 indep reflns 14068 11330 15684 params 705 695 702 2 1.000 1.008 1.009 GOF on F 0.0402, 0.0842 0.0482, 0.1065 R1, wR2 [I > 2σ(I )] 0.0579, 0.1192 0.1276, 0.1454 0.0695, 0.0917 0.0898, 0.1296 R1, wR2 [all data] formula

is not stable in acidic aqueous solution and can easily rearrange slowly to the [R-PW11O39]7- anion and, almost certainly, this transformation is accelerated under the reaction conditions (CH3COOH-CH3COONa: pH 4.75 and 90 °C).22 However, to date, we have not found suitable conditions to prepare the 2:2-type dimers 1 and 2 by reaction of the [R-PW9O34]9precursor with Ln cations. The effects of pH value, counterions and stoichiometry were well investigated. In all cases, the pH value between 4.5 and 6.0 was found to be suitable to the synthesis process. The CH3COOH-CH3COONa buffer solutions with different pH values were adopted. In the preparations of 3-8, the CH3COO- anion acts not only as a reaction medium but also as a chelating ligand coordinated to Ln centers. The counterions play an important role in the synthesis of 1-8, when Naþ (or Kþ) was used to replace [(CH3)4N]þ as the countercation; however, good quality crystals could not be obtained. It can be supposed that the large organic counterions [(CH3)4N]þ could easily interact with the [R-PW11O39]7polyoxoanion generating the ion pairs, which is favorable

8 [(CH3)4N]10[PW11O39Er(COOCH3)(H2O)]2 3 8H2O 6728.87 296(2) P2(1)/c monoclinic 25.2010(15) 12.7295(7) 22.3888(13) 90.00 115.3560(10) 90.00 6490.3(6) 2 3.443 20.806 0.0519 32879 11437 709 1.007 0.0383, 0.0934 0.0641, 0.1054

to the formation of the 2:2-type dimers 1-8 and can enhance their chemical stability.23 The structures of products were obviously affected by the stoichiometry of Ln3þ/[R-PW11O39]7-. When the Ln3þ/[R-PW11O39]7- ratio increased from 2:1 to 3:1, although the solution became more turbid, 1-8 could be obtained. In contrast, when the ratio decreased to 1:1, the formation of the 1:2-type [Ln(PW11O39)2]11- series was favorable. Structural Description. The phase purity of 3-8 was characterized by the powder X-ray diffraction patterns of the bulk products (Figures S3-S6 in the Supporting Information). Both 1 and 2 are essentially isomorphous, and their structures consist of six discrete [(CH3)4N]þ cations, one [{(R-PW11O39H)Ln(H2O)3}2]6- (Ln=NdIII 1 and GdIII 2) dimeric core, and eight (or six) water molecules of crystallization. Because 1 and 2 are essentially isomorphous, only the structure of 1 is described here in detail. The dimeric core of [{(R-PW11O39H)Nd(H2O)3}2]6- in 1 possesses a centric head-to-head architecture, and can be regarded as the combination of two identical symmetrically related moieties [(R-PW11O39H)Nd(H2O)3]3-

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through two Nd-O-W bridges (Figure 1a,b). The Nd1 cation, incorporated to the vacant site of the [R-PW11O39]7- subunit in the “cap” region, is eight-coordinate, adopting a distorted square antiprismatic coordination geometry (pseudo-D4d) (Figure 1c) bonding to four oxygen atoms from the defect site of the [R-PW11O39]7- framework [Nd-O: 2.351(7)-2.413(7) A˚], to three water molecules [Nd-O: 2.504(9)-2.540(9) A˚], and to a terminal oxygen atom from another [(R-PW11O39H)Nd(H2O)3]3- moiety [Nd-O: 2.577(8) A˚]. The Nd1 cation is displaced outward and away from the normal twelfth position in the R-Keggin framework. In the coordination polyhedron around the Nd1 cation in 1, the O12, O26, O17 and O16 group and the O1W, O2W, O3W and O2A group constitute two bottom planes of the square antiprism, and their average deviations from their least-squares planes are 0.0242 and 0.2175 A˚, respectively. The dihedral angle for the two bottom surfaces is 26.5°. The distances between the Nd1 cation and the two bottom planes are 1.1558 and 1.0643 A˚, respectively, and the O-Nd-O bond angles are in the range of 68.7(3)143.6(3)°. The above-mentioned data indicate that the square antiprism is severely distorted, which may be related to the coordination environments of different coordination atoms. In the dimeric core of [{(R-PW11O39H)Nd(H2O)3}2]6-, the P atoms reside in the center of PO4 tetrahedra, which have been somewhat distorted resulting from the removal of one [WdOt]4þ group and the incorporation of a Ln cation into the monovacant POM framework as compared to the saturated Keggin structure. The P-O distances in the PO4 polyhedra vary from 1.515(8) to 1.552(7) A˚, and the O-P-O bond angles are in the range of 108.7(4)-110.8(4)°. Similarly, all the WO6 octahedra are distorted to some extent. Additionally, the distance between two NdIII cations is 6.6081(13) A˚ in the [{(R-PW11O39H)Nd(H2O)3}2]6- dimeric core. The structure of the dimeric Keggin core [{(R-PW11O39H)Nd(H2O)3}2]6is somewhat similar with those of two dimeric Dawson units [Ce2(R1-P2W17O61)2(H2O)8]14- and [Eu2(R2-P2W17O61)2(H2O)6]14- previously reported by Pope et al.24 and Luo et al.,25 respectively. In these two Dawson-based dimer units, the Ce 3 3 3 Ce and Eu 3 3 3 Eu distances are 6.0803(4) A˚ and 6.600(1) A˚, respectively. To the best our knowledge, 1 and 2 represent the first 2:2-type monovacant Keggin polyoxotungstate dimers constituted by two [R-PW11O39]7- polyoxoanions and two lanthanide cations in POM chemistry. In addition, the molecules of 1 are stacked along the c-axis, forming circular channels with dimensions of ca. 11.0 7.1 A˚, in which the discrete [(CH3)4N]þ cations and solvent water molecules are filled (Figure 1d). 3-8 all crystallize in the monoclinic P21/c space group, and their molecular structures are composed of the discrete [(CH3)4N]þ cations, the hybrid dimeric units [{(R-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}2]10- [Ln = SmIII for 3, EuIII for 4, GdIII for 5, TbIII for 6, HoIII for 7 and ErIII for 8] and water molecules of crystallization. 3-8 are isostructural and display another dimeric structure consisting of two monosubstituted units [{(R-PW11O39)Ln(H2O)(η2,μ-1,1)CH3COO}]5- bridged by the two (η2,μ-1,1)-acetato ligands, and this structural type was first observed by Mialane et al. in [{(R-SiW11O39)Ln(COOCH3)(H2O)}2]12- (Ln=GdIII and YbIII) during the course of investigating the lanthanide/monovacant silicotungstate system.9 It should be noted that the synthetic approaches of 3-8 and [{(R-SiW11O39)Ln(COOCH3)(H2O)}2]12- (Ln =GdIII and YbIII) are different. 3-8 were synthesized by direct reaction of the Na7[R-PW11O39] 3 nH2O or Na9[R-PW9O34] 3 16H2O precursor with Ln

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Figure 1. (a) Ball-and-stick representation of the molecular structure of 1. (b) Combined ball-and-stick/polyhedral representation of the molecular structure of 1. Atoms with “A” in their labels are symmetrically generated (A: 1 - x, 1 - y, 2 - z). (c) The square antiprismatic geometry of the Nd1 cation in 1. (d) The packing arrangement of 1 along the c axis. The [(CH3)4N]þ cations and water molecules of crystallization are omitted for clarity. Color code: W, blue; Nd, green; P, purple; O, red; WO6 octahedra, orange; PO4 tetrahedra, purple.

cations in the CH3COOH-CH3COONa buffer solution whereas [{(R-SiW11O39)Ln(COOCH3)(H2O)}2]12- (Ln = GdIII and YbIII) were obtained based on K5[Yb(SiW11O39)(H2O)2] 3 24H2O or K5[Gd(SiW11O39)] 3 25H2O as the starting materials in the CH3COOH-CH3COOK buffer solution. In the hybrid dimeric units of 3-8 [{(R-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}2]10-, the LnIII cations are located in the defect sites of the [R-PW11O39]7- fragments and adapt the eight-coordinate square antiprism arrangement, where four of eight oxygen atoms are provided by monovacant Keggin POM fragments, three are from acetate groups and one is the terminal water molecule. In the following description, only the structure of 3 is discussed here in detail. The molecular structure of 3 consists of one inorganic-organic hybrid dimeric core [{(R-PW11 O39)Sm(H2O)(η2,μ-1,1)-CH3COO}2]10-, ten discrete [(CH3)4N]þ cations and six water molecules of crystallization (Figure 2a,b). The centric head-to-head hybrid dimeric core [{(RPW11O39)Sm(H2O)(η2,μ-1,1)-CH3COO}2]10- is built by two identical symmetrically related monosubstituted moieties [{(R-PW11O39)Sm(H2O)(η2,μ-1,1)-CH3COO}]5- bridged by two (η2,μ-1,1)-acetato ligands. In 3, the Sm1 cation is in a distorted square antiprismatic geometry (pseudo-D4d) (Figure 2c), defined by four oxygen atoms from the defect site of the tetradentate [R-PW11O39]7- ligand [Sm-O: 2.365(5)-2.405 (6) A˚], a terminal water molecule [Sm-O: 2.473(6) A˚] and three carboxyl oxygen atoms from a bidentate acetate ligand and another acetate ligand on the other [{(R-PW11O39)Sm(H2O)(η2,μ-1,1)-CH3COO}]5- subunit [Sm-O: 2.501(6)2.524(6) A˚]. In the coordination configuration of the Sm1 cation, the O32, O33, O34 and O35 group and the O40, O40A, O41A and O1W group constitute the two bottom planes of the square antiprism, and their average deviations are 0.0047 and 0.1172 A˚, respectively. The dihedral angle for the two bottom surfaces is 3.9°. The distances between the Sm1 cation and the two bottom planes are 1.1786 and 1.5632 A˚, respectively. The two SmIII cations are therefore doubly bridged by two (η2,μ-1,1)-acetate ligands, separated by a Sm 3 3 3 Sm distance of 4.154(2) A˚. It is interesting that the

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Figure 2. (a) Ball-and-stick representation of the molecular structure of 3. (b) Combined ball-and-stick/polyhedral representation of the molecular structure of 3. Atoms with “A” in their labels are symmetrically generated (A: 1 - x, 1 - y, 1 - z). (c) The square antiprismatic geometry of the Sm1 cation in 3. (d) The packing arrangement of 3 in a unit cell. The [(CH3)4N]þ cations and water molecules of crystallization are omitted for clarity. Color code: W, blue; Sm, green; P, purple; O, red; C, black; WO6 octahedra, orange; PO4 tetrahedra, purple.

acetate ligands are combined with two samarium ions in an especially interesting motif. Each carboxylate group contributes one oxygen atom as a μ3-O bridge coordinated to two Sm atoms and one C atom while the second oxygen atom serves as a μ2-O bridge linking one Sm atom and one C atom. This connection motif has been previously observed for two ytterbium dimers where each Ln center is coordinated to two cyclopentadienyl ligands with Yb 3 3 3 Yb distances of Yb 3 3 3 Yb: 3.905 and 3.930 A˚,26 and an ytterbium substituted silicotungstate [{(R-SiW11O39)Yb(CH3COO)(H2O)}2]12- with the Yb 3 3 3 Yb distance of 4.085(18) A˚.9 In addition, in 2003, Kortz addressed a lanthanum Dawson-based POM [{La(R2P2W17O61)(H2O)2}2( μ-CH3COO)2]16- synthesized by rearrangement of the hexavacant [R-H2P2W12O48]12- precursor in the presence of LaIII ions in acetate buffer.27 In this Dawson-based POM, the LaIII ion is nine-coordinate with a monocapped square antiprismatic geometry and two monolanthanum substituted units [{La(R2-P2W17O61)(H2O)2}( μCH3COO)]8- are also connected through two (η2,μ-1,1)acetate ligands with the Yb 3 3 3 Yb distance of 4.36(2) A˚. Mialane et al. also investigated the reactivity of the [Yb(RSiW11O39)(H2O)4]5- and [Yb(R2-P2W17O61)(H2O)4]7- precursors with oxalate ligands, resulting in two tetrameric [{Yb(POM)}4(C2O4)3(H2O)4]n- (POM=[R-SiW11O39]8-, n= 26; [R2-P2W17O61]10-, n=34) clusters.2g In addition, in the ac plane, hybrid dimeric polyoxoanions [{(R-PW11O39)Sm(H2O)(η2,μ-1,1)-CH3COO}2]10- are arranged in parallel with each other viewed down the [001] direction, and discrete [(CH3)4N]þ cations distribute the interspaces surrounded by dimeric polyoxoanions (Figure 2d). IR Spectra. The IR spectra of 1-8 display four characteristic νas(P-Oa), terminal νas(W-Ot), corner-sharing νas(W-Ob) and edge-sharing νas(W-Oc) asymmetrical vibration peaks for the Keggin-type polyoxoanion frameworks (Figures S1, S2 in the Supporting Information).28 In comparison with the IR spectrum of the R-Na7PW11O39 3 nH2O precursor, the νas(W-Ot) vibration peaks for 1-8 have different red-shifts of 4-10 cm-1, the possible reason for which may be that charge compensation cations have stronger interactions to terminal oxygen atoms of polyoxoanions, impairing the W-Ot


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bond, reducing the W-Ot bond force constant and leading to decreasing of the W-Ot vibration frequency.15 The νas(P-Oa) and νas(W-Oc) vibration frequencies for 1-8 have different blue-shifts of 4-21 and 10-24 cm-1, respectively, the possible reason for which may be that the symmetry of 1-8 increases as compared to that of R-Na7PW11O39 3 nH2O. Furthermore, with the decreasing of the sizes of the Ln ions, the extent of blue-shifts of νas(P-Oa) and νas(W-Oc) in 3-8 gradually increases due to the lanthanide contraction effects. The resonances at 1557-1561 and 1340-1344 cm-1 in 3-8 are assigned to the νas(CdO) and νas(C-O) stretching vibrations of the CH3COO- ligands, respectively. Comparing with those of the free CH3COOH,29 the vibration peaks of the νas(CdO) and νas(C-O) red-shift from 1563 cm-1 to 1557-1561 cm-1 and from 1416 cm-1 to 1340-1344 cm-1, respectively. These results suggest that the CH3COO- ligands are coordinated to the Ln cations by means of the carboxyl oxygen atoms. The IR spectral results are in good agreement with the X-ray single-crystal structural analyses. UV-Vis-NIR Spectra. The UV-vis-NIR spectra of 1-8 were performed in aqueous solution in the 900-190 nm range. The UV spectra of 2, 5 and 6 in the range of 300-190 nm all reveal two strong absorption bands centered at 194-200 and 252 nm (Figure S7 and Table S1 in the Supporting Information). The former higher energy absorption band can be assigned to the pπ-dπ charge-transfer transitions of the Ot f W bond whereas the latter lower energy absorption band is attributed to the pπ-dπ charge-transfer transitions of the Ob(c) f W bonds.30 The UV spectra of 1, 3, 4, 7 and 8 in the range of 300-190 nm show only the lower energy absorption band centered at 250-252 nm assigned to the pπ-dπ chargetransfer transitions of the Ob(c) f W bonds (Figure S7 and Table S1 in the Supporting Information), however, their higher energy absorption bands assigned to the pπ-dπ charge-transfer transitions of the Ot f W bond are blue-shifted to the near UV region that is lower than 190 nm, which may be related to the influence of different Ln cations on the structure of the [RPW11O39]7- ligand. Such a phenomenon has been already encountered in our previous studies on the reactions of Ln cations with monovacant Keggin silicotungstate or germanotungstate precursors.13-15 In addition to the above-mentioned strong absorption bands (Ot f W or Ob(c) f W), five absorption bands centered at 524, 584, 749, 801, and 869 nm for 1 can be ascribed to the transitions of 4I9/2 f 4F5/2, 4I9/2 f 4F3/2, 4 I9/2 f 4I15/2, 4I9/2 f 4I13/2 and 4I9/2 f 4I11/2 of the NdIII ion, respectively (Figure S8 in the Supporting Information);31 and four absorption bands centered at 378, 488, 521, and 653 nm for 8 correspond to the transitions of 4I15/2 f 4F11/2, 4I15/2 f 4F7/2, 4 I15/2 f 2H11/2 and 4I15/2 f 4F9/2 of the ErIII cation, respectively (Figure S9 in the Supporting Information).32 Photoluminescence Properties. In order to investigate the photoluminescence behavior of 1-8, the photoluminescence spectra of solid samples of 1-8 were measured at room temperature upon photoexcitation. The results of photoluminescence measurements reveal that only 4 and 6 show obvious photoluminescence phenomena (Figure 3). When the europium compound 4 was measured under excitation at 273 nm, 4 exhibits intense red photoluminescence. The emission spectrum of 4 displays five characteristic emission bands of the EuIII ion at 581, 592, 618, 652, and 694 nm, ascribed to the 5D0 f 7F0, 5D0 f 7F1, 5D0 f 7F2, 5D0 f 7F3 and 5D0 f 7F4 transitions, respectively. These results are in good accordance with previous data.8,33 The 5D0 f 7F1,3 transitions are magnetic-dipolar transitions and insensitive

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Figure 3. Photoluminescence spectra of 4 and 6 at room temperature. Exciting light wavenumbers are 273 and 254 nm for 4 and 6, respectively.

to their local environments, and the 5D0 f 7F0,2,4 transitions are electric-dipolar transitions and sensitive to their local environments.33a When the interactions of the Ln cation with its local chemical environment are stronger, the complex becomes more nonsymmetrical and the intensity of the electric-dipolar transitions becomes more intense.33a As we know, the 5D0 f 7F0 transition is strictly forbidden in a field of symmetry. The symmetric forbidden transition 5D0 f 7F0 at 581 nm can be hardly found in 4, suggesting that 4 employs comparatively higher symmetry. The 5D0 f 7F1 transition at 592 nm is a magnetic-dipole transition, and its intensity varies with the ligand field strength acting on the EuIII ion. The 5D0 f 7F2 transition at 618 nm is an electric-dipole transition and is extremely sensitive to chemical bonds in the vicinity of the EuIII ion. The intensity of the 5D0 f 7F2 transition increases as the site symmetry of the EuIII ion decreases. So, the intensity ratio of the 5D0 f 7F2/5D0 f 7F1 transition is widely used as a measure of the coordination state and site symmetry of the rare earth ions.33a,34 For 4, the strongest emission is in the 5D0 f 7F1 transition region and the 5D0 f 7F1 peak splits into two levels at 590 and 594 nm. This small splitting of the highest 7F1 Stark component indicates the presence of high local symmetry for the europium local site.8 The intensity ratio of the 5D0 f 7F2/5D0 f 7 F1 transition is small, which is related to the centric structure of 4. The terbium compound 6 emits green photoluminescence when excited at 254 nm. The emission spectrum of 6 exhibits four characteristic peaks, which are assigned to the 5D4 f 7FJ (J=3, 4, 5, 6), 5D4 f 7F6 (489 nm), 5D4 f 7F5 (545 nm), 5D4 f 7F4 (583 nm) and 5D4 f 7F3 (622 nm) transitions, being in good accordance to the previous results.35 The mechanism of energy transfer from the ligand to the metal has been widely discussed to interpret the luminescence of lanthanide complexes.36 When the triplet-state energy of the ligand is greater than or equal to the energy gap (ΔE) between the excited state and ground state of the metal ion, efficient luminescence could be obtained.37 From the results discussed above, we could presume that the energy gap (ΔE) of the EuIII ion may be smaller than that of the TbIII ion. It means that the ligand-to-metal charge transfer (LMCT) of the EuIII ion is more effective than that of the TbIII ion.

Niu et al.

Figure 4. Temperature dependence of χmT for 1, 3, 4, 7 and 8 in the temperature range of 2-300 K.

Magnetic Properties. In contrast to the effect of the crystal field, spin-orbital coupling interactions in general play a more important role in the magnetic properties of Ln compounds, which can partly remove the degeneracy of the 2Sþ1L group term of lanthanide ions, generating the 2Sþ1LJ states. The 2Sþ1LJ states further split into Stark levels by the crystal field perturbation.38 For most of the trivalent Ln ions, the 2Sþ1 LJ free-ion ground state is well separated in energy from the first excited state so that only the ground state is thermally populated at room and low temperature, 37a,b but this situation is possibly exceptional for the SmIII and EuIII ions in which the first excited state may be thermally populated because of the small energy separation [1000 cm-1 for SmIII and 400 cm-1 for EuIII]. Therefore, in the following discussion, the crystal field effect and the possible thermal population of the higher energy state should be taken into account for SmIII- and EuIII-containing POMs. The variable-temperature magnetic susceptibilities of 1, 3, 4, 7 and 8 were measured in the temperature range 2-300 K. The temperature dependence of the χmT and χm-1 are shown in Figure 4 and Figures S10-S14 in the Supporting Information, respectively. For the neodymium complex 1, the value of χmT is 3.86 cm3 K mol-1 at 300 K, which is consistent with the value expected for two isolated NdIII ions (3.28 cm3 K mol-1), and decreases slowly to 1.12 cm3 K mol-1 at 2 K. This behavior is mainly due to the splitting of the 10-fold degenerate 4I9/2 ground state affected by the crystal field perturbation and the progressive depopulation of the higher energy state upon cooling.38 Thus, the nature of the interactions between the two NdIII ions with an orbital momentum cannot be unambiguously deduced only from the shape of the χmT vs T curve.37a,39 The whole profile of χmT versus T is very similar to those previous reported mononuclear and homodinuclear NdIII compounds.38-40 Therefore, even if there are magnetic interactions, they should be very weak. The relationship of 1/χm versus T in 80-300 K obeys the Curie-Weiss law; however, as the temperature decreases from 80 to 2 K, the relation of 1/χm versus T does not follow the Curie-Weiss law (Figure S10 in the Supporting Information). It is well-known that the depopulation of the NdIII Stark levels as the temperature decreases is a distinct magnetic phenomenon. The 4I9/2 ground state for the free NdIII ion in the crystal field is split into five Kramers doublets.40 At room temperature

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those doublets are equally populated; as the temperature decreases, the Kramers doublets of higher energy are successively depopulated and the magnetic behavior significantly deviates from the Curie-Weiss law predicated by the free-ion approximation.41 For the samarium compound 3, the χmT value decreases from 1.63 cm3 K mol-1 at 300 K to 0.059 cm3 K mol-1 at 2 K when the temperature is lowered. The curve of 1/χm versus T in 100-300 K can be described using the Curie-Weiss law (Figure S11 in the Supporting Information), but the relation of 1/χm versus T between 100 and 2 K does not follow the Curie-Weiss law chiefly because the Kramers doublets of higher energy are successively depopulated as the temperature decreases. The 6H5/2 ground state for the free SmIII ion in the crystal field is split into six states by spin-orbit coupling, and the spin-orbit coupling parameter is 1200 cm-1, so the crystal field effect and the possible thermal population of the high energy states should be considered for 3.40 The value of 0.059 cm3 K mol-1 at 2 K is evidently smaller than that for two noninteractiong SmIII ions (0.178 cm3 K mol-1), revealing the occurrence of weak antiferromagnetic interactions within two SmIII ions mediated by carboxylic oxygen atoms from CH3COO- as well as the spin-orbital coupling and the crystal field effect. This observation has been encountered in a dinuclear SmIII complex [Sm2(4-cba)6(phen)2(H2O)2].38 For the europium compound 4, the 7F ground term is split by the spin-orbit coupling into seven states, 7FJ, with J taking integer values from 0 to 6. Due to the small energy separation between the ground state and the first excited state, the first excited state may be thermally populated at room temperature and above.40 The χmT value decreases from 15.32 cm3 K mol-1 at 300 K to 5.78 cm3 K mol-1 at 2 K when the temperature is lowered, which should be attributed to the depopulation of the Stark levels for Eu(III) ions.42 The curve of 1/χm versus T in 2-300 K can follow the Curie-Weiss law (Figure S12 in the Supporting Information) mainly because of the spin-orbital coupling and the presence of thermally populated excited states.42 For the holmium compound 7 and erbium compound 8, their magnetic behaviors are very similar. Their room temperature χmT values are 22.21 and 22.76 cm3 K mol-1, respectively. Upon cooling the samples, the χmT values decrease continuously from 22.21 and 22.76 cm3 K mol-1 to 5.37 and 12.85 cm3 K mol-1, respectively. For 7, the value of χmT (5.37 cm3 K mol-1) at 2 K is obviously smaller than that the value expected for two free HoIII ions (28.14 cm3 K mol-1),43 exhibiting stronger antiferromagnetic interactions within HoIII centers, which is further proved by the Weiss constant θ = -8.20 K for 7 (Figure S13 in the Supporting Information). For 8, the value of χmT (12.85 cm3 K mol-1) at 2 K is obviously lower than the value expected for two free ErIII ions (22.96 cm3 K mol-1),32,44 which reveals the presence of the spin-orbital coupling interactions and antiferromagnetic interactions between ErIII centers. Furthermore, the curve of 1/χm versus T in 2-300 K follows the Curie-Weiss law with θ = -6.53 K for 8 (Figure S14 in the Supporting Information), and this behavior also indicates the occurrence of weak antiferromagnetic interactions at lower temperature. Elecrochemical Properties. In order to explore the electrochemical behaviors of the as-synthesized compounds, we carried out cyclic voltammetric (CV) measurements of 1, 3, 4, 5, 7 and 8 in aqueous solution in the presence of 0.5 M Na2SO4 as supporting electrolyte (Figures 5 and 6 and


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Figure 5. Cyclic voltammograms of 0.5 mM compound 1 with the pH e 4.0 (a) and pH > 4.0 (b) regions in 0.5 M Na2SO4 aqueous solution; scan rate: 50 mV/s. The pH value was adjusted with 0.1 M H2SO4 and 0.1 M NaOH aqueous solution, respectively. Curves: a, pH = 4.0; b, pH =3.68; c, pH =3.26; d, pH= 3.03; e, pH =5.00; f, pH = 6.00; g, pH = 6.95.

Figures S15-S16 in the Supporting Information). The study results indicate that these compounds show similar cyclic voltammetric behaviors because of the presence of monovacant Keggin phosphotungstate fragments [R-PW11O39]7in their structures (Figure S17 in the Supporting Information). The a curve in Figure 5a shows the evolution of the cyclic voltammetry for 0.5 mM compound 1, where the monovacant Keggin phosphotungstate fragment [R-PW11O39]7- exhibits a kinetically stable and reproducible CV pattern, at least in the pH 4.00 media studied here. At pH 4.00, three couples of redox waves are observed at (Epc1 = -0.811 V, Epa1=-0.903 V), (Epc2=-0.973 V, Epa2=-1.050 V) and (Epc3 = -1.211 V, Epa3 = -1.316 V), respectively, and their midpoint peak potentials (Ep1/2) of -0.857 V, -0.1.012 and -1.264 V, respectively, where Ep1/2=(Epc þ Epa)/2, Epc and Epa are cathodic and anodic peak potentials. The peak potential separations of three couples of redox waves are 92 mV, 77 mV and 105 mV, respectively, indicating that they are the one-electron charge-transfer processes. These waves are assigned to the redox processes of WVI centers.45 To highlight the influence of pH value on the peak potentials in the cyclic voltammograms for 1, the experiments were carried out in the same above-mentioned media in the wide pH region (Figure 5a:b-d and Figure 5b:f,g). In the acidic direction, three peak potentials in the pH value from 4.00 to 3.03 (Figure 5a.b-d) become more positive in comparison with those in the pH value of 4.00. Moreover, the peak current intensities become stronger. This phenomenon exhibits that redox processes are gradually becoming irreversible with the decreasing of the solution pH values, which may be due to the reaction of hydrogen evolution or the framework change of the polyoxoanion fragment [R-PW11O39]7-.46 On the contrary, with the increasing of solution pH value (Figure 5b:e-g), the three redox peaks are turning weaker step by step. This change can be explained by the degeneration of the polyoxoanion fragment [R-PW11O39]7with the increasing of the solution alkalinity. From the above analysis, the solution pH value has a marked influence on the polyoxoanion fragment [R-PW11O39]7- of 1. The cyclic voltammograms of 3, 4, 5, 7 and 8 display a similar CV pattern to that of 1 (Figure 6 and Figures S15-S16 in the Supporting Information), which also suggests that the acetate groups do not evidently affect the electrochemical behaviors of polyoxoanion fragments [R-PW11O39]7- in 3-8. Therefore, we do not discuss them redundantly here. Thermogravimetric (TG) Analyses. TG analyses were examined on pure samples of 1-8 under flowing nitrogen

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the loss of 8 crystal water molecules and 2 coordination water molecules (calcd 2.69%). The other weight loss of 11.98% between 185 and 600 °C is approximately attributed to the decomposition of 10 tetramethylammonium groups and 2 acetato ligands (calcd 12.83%). For 5, the first weight loss is 3.27% between 26 and 167 °C, attributable to the removal of 8 crystal water molecules and 2 coordination water molecules (calcd 2.69%). The second weight loss of 10.73% between 167 and 600 °C is followed by the decomposition of 10 tetramethylammonium groups and 2 acetato ligands (calcd 12.81%). For 6, the weight loss of 2.51% between 30 and 181 °C corresponds to the loss of 6 crystal water molecules and 2 coordination water molecules (calcd 2.16%). After 181 °C until 600 °C, the weight loss of 13.54% is observed and approximately assigned to the decomposition of 10 tetramethyl-ammonium groups and 2 acetato ligands (calcd 12.87%). For 7, the first one is a dehydration process below 199 °C [found (calcd) for 8 crystal water molecules and 2 coordination water molecules: 2.80% (2.69%)], followed by the release of 10 tetramethylammonium groups and 2 acetato ligands [13.30% (12.81%)] between 199 and 600 °C. For 8, the first weight loss of 3.31% (calcd 2.68%) ranging from 26 to 182 °C is attributed to the removal of 8 crystal water molecules and 2 coordination water molecules. The second weight loss of 10.79% (calcd 12.77%) is assigned to the loss of 10 tetramethylammonium groups and 2 acetato ligands. Figure 6. (a, b) Cyclic voltammograms of 0.5 mM compound 3 with the pH e 4.65 and pH>4.65 regions in 0.5 M Na2SO4 aqueous solution; curves: a, pH = 4.65; b, pH=3.95; c, pH = 3.15; d, pH= 5.00; e, pH = 5.48; f, pH = 6.32. (c, d) Cyclic voltammograms of 0.5 mM compound 5 with the pH e 4.60 and pH > 4.60 regions in 0.5 M Na2SO4 aqueous solution; curves: a, pH=4.60; b, pH=4.00; c, pH=3.68; d, pH=3.20; e, pH=5.20; f, pH=5.58; g, pH=5.72; h, pH = 6.09. (e, f) Cyclic voltammograms of 0.5 mM compound 8 with the pH e 4.60 and pH>4.60 regions in 0.5 M Na2SO4 aqueous solution; curves: a, pH = 4.60; b, pH = 3.92; c, pH = 3.42; d, pH = 3.05; e, pH = 5.22; f, pH = 5.56; g, pH = 6.29; h, pH = 6.56. Scan rate: 50 mV/s. The pH value was adjusted with 0.1 M H2SO4 and 0.1 M NaOH aqueous solution, respectively.

atmosphere in the range of 25-600 °C (Figure S18 in the Supporting Information). The TG curve of 1 shows two steps of weight loss, being associated with the loss of lattice water and tetramethylammonium cations with a total loss of 11.75% (calcd 11.28%) in the range of 25-600 °C. The first weight loss of 4.38% between 25 and 138 °C corresponds to the loss of 8 crystal water molecules and 6 coordination water molecules (calcd 3.98%). The second weight loss of 7.37% until 490 °C is assigned to the decomposition of 6 tetramethylammonium groups and the dehydration of 2 protons (calcd 7.30%). Similarly, the TG curve of 2 also exhibits two steps of weight loss between 25 and 600 °C. The first weight loss is 3.85% between 25 and 108 °C due to the removal of 6 crystal water molecules and 6 coordination water molecules (calcd 3.41%). The second weight loss is 7.87% in the range of 108-600 °C because of the decomposition of 6 tetramethylammonium groups and the dehydration of 2 protons (calcd 7.31%). For 3, the first weight loss of 1.95% between 40 and 174 °C can be attributed to the removal of 6 crystal water molecules and 2 coordination water molecules (calcd 2.71%). The second weight loss of 13.27% from 174 to 600 °C is assigned to the decomposition of 10 tetramethylammonium groups and 2 acetato ligands (calcd 13.18%). For 4, one weight loss of 3.17% between 30 and 185 °C corresponds to

Conclusions In this paper, two 2:2 types of monolanthanide substituted POMs [{(R-PW11O39H)Ln(H2O)3}2]6- (Ln=NdIII for 1 and GdIII for 2) and [{(R-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}2]10- [Ln = SmIII for 3, EuIII for 4, GdIII for 5, TbIII for 6, HoIII for 7 and ErIII for 8] have been synthesized in aqueous solution and characterized by elemental analyses, IR spectra, UV-vis-NIR spectra, TG analyses and single-crystal X-ray diffraction. The common structural features are that they are constructed from monovacant Keggin-type polyoxoanion fragments [R-PW11O39]7- and trivalent Ln cations. Both 1 and 2 are essentially isomorphous and their molecular structure is built by two monolanthanide substituted Keggin units [R-PW11O39Ln(H2O)3]4- linked through two Ln-O-W bridges, representing the first monovacant Keggin polyoxotungstate dimers constituted by two [R-PW11O39]7- polyoxoanions and two Ln cations in POM chemistry. 3-8 are also isostructural and display another dimeric structure constructed from two monolanthanide substituted units [{(RPW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}]5- bridged by the two (η2,μ-1,1)-acetato ligands. Photoluminescence measurements show that 4 and 6 exhibit obvious luminescence properties. Magnetic susceptibilitiy data of 1, 3, 4, 7 and 8 were measured between 2 and 300 K, and their magnetic behaviors mainly result from spin-orbital coupling interactions as well as weak antiferromagnetic exchange interactions within magnetic centers. The electrochemical properties of 1, 3-5, 7 and 8 were studied by means of cyclic voltammetry in aqueous solution with 0.5 M Na2SO4 as supporting electrolyte. In conclusion, the extended research on the structure and properties of a series of compounds built by lacunary Keggin POMs and Ln cations will be useful for the design and exploration of novel materials. In the following work, we will introduce some magnetic transition-metal cations to the present system to exploit the novel magnetic materials simultaneously containing

Article


lanthanide and transition-metal cations. Meanwhile, some organic polycarboxylic ligands will be used as bridging connectors to construct multidimensional organic-inorganic hybrid structures and open-framework materials. Acknowledgment. This work was supported by the National Natural Science Foundation of China, Program for New Century Excellent Talents in University of Henan Province, the Foundation of Education Department of Henan Province and Natural Science Foundation of Henan Province. Supporting Information Available: Representations of IR spectra, XRD patterns, UV-vis spectra, the temperature evolution of the inverse magnetic susceptibility χm-1, thermogravimetric analyses and cyclic voltammograms. This material is available free of charge via the Internet at http://pubs.acs.org.

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Assembly Chemistry between Lanthanide Cations and Monovacant

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