A series of inorganic–organic hybrid compounds constructed from bis(undecatungstophosphate) lanthanates and copper-organic units

Inorganica Chimica Acta 363 (2010) 3823–3831

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A series of inorganic–organic hybrid compounds constructed from bis(undecatungstophosphate) lanthanates and copper-organic units Dong-Ying Du, Jun-Sheng Qin, Shun-Li Li, Xin-Long Wang, Guang-Sheng Yang, Yang-Guang Li, Kui-Zhan Shao, Zhong-Min Su * Institute of Functional Material Chemistry, Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China

a r t i c l e

i n f o

Article history: Received 22 February 2010 Received in revised form 5 July 2010 Accepted 12 July 2010 Available online 16 August 2010 Keywords: Polyoxometalate Lanthanide Magnetic property Copper

a b s t r a c t A series of inorganic–organic hybrid compounds built from bis(undecatungstophosphate) lanthanates and copper-complexes, namely, H8[Cu(en)2H2O]4[Cu(en)2]{[Cu(en)2][La(PW11O39)2]}218H2O (1), H6[Na2(en)2 (H2O)5][Cu(en)2H2O]4[Cu(en)2]{[Cu(en)2][Ce(PW11O39)2]}216H2O (2), H6[Na2(en)2(H2O)5][Cu(en)2H2O]4 [Cu(en)2]{[Cu(en)2][Pr(PW11O39)2]}218H2O (3), H6[Na2(en)2(H2O)4][Cu(en)2H2O]4[Cu(en)2]{[Cu(en)2] [Nd(PW11O39)2]}214H2O (4), H6[Na2(en)2(H2O)5][Cu(en)2H2O]4[Cu(en)2]{[Cu(en)2][Sm(PW11O39)2]}2 20H2O (5), and H7[Cu(en)2]2[Sm(PW11O39)2]10H2O (6) (where en = 1,2-ethylenediamine), have been prepared. In these compounds, two lacunary [PW11O39]7 anions sandwich an eight-coordinated Ln(III) cation to yield [Ln(PW11O39)2]11 anion in a twisted square anti-prismatic geometry, which is further bridged by [Cu(en)2]2+ fragments to generate a 1D zigzag-like chain. In 1–6, the coordination bond interactions and weak interactions between adjacent 1D chains play an important role in the zigzagging distances and angles of different 1D chains. The magnetic studies indicate that antiferromagnetic interactions exist in compounds 1, 2 and 4. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Never have polyoxometalates (POMs) failed to attract considerable interest in constructing multi-dimensional frameworks owing to their controllable shape and size, their highly negative charges, and their oxo-enriched surfaces, which may possess a series of potential applications in catalysis, analytical chemistry, ion exchange and biological chemistry [1–15]. An ongoing interest in this field is to modulate the properties of fascinating POM building blocks by secondary metal coordination complexes in order to obtain inorganic–organic hybrid compounds, such as POM-supported metal complexes and metal-complex-bridged POM extended structures [16–19]. A simple and powerful approach for constructing these kinds of inorganic–organic hybrid materials is the combination of POMs and transition metal–organic units. Up to now, a series of hybrid compounds based on the typical heteropolyanions (such as Keggin- and Dawson-type) or the vanadium/molybdenum isopolyanions have been explored [20–28]. Whereas, reported hybrid compounds built from lanthanopolyoxometalates (LnPOMs) and transition metal–organic units are comparatively unexplored [29–36]. One type of LnPOMs is the 1:2-type compounds, bis(undecatungstophosphate) lanthanates [Ln(PM11O39)2]11 (abbre-

* Corresponding author. Tel.: +86 431 85099108; fax: +86 431 85684009. E-mail address: [email protected] (Z.-M. Su). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.07.028

viated as Ln(PM11)2, M = W or Mo, Scheme S1), which was first documented by Peacock and Weakley [37]. In the past two decades, several lacunary heteropolyoxomolybdate/tungstate complexes of this structural type have been prepared and crystallographically characterized [38–44], which are almost discrete molecules. The development of multi-dimensional compounds based on Ln(PM11)2 and the transition metal complexes is still in its infancy [45]. Taking these points into account, we have investigated the reactions among [PW12O40]3 polyanions, Ln(III) cations and transition metal complexes in order to construct multi-dimensional inorganic–organic hybrid compounds. Herein, we report a family of compounds, H8[Cu(en)2H2O]4[Cu(en)2]{[Cu(en)2][La(PW11O39)2]}2 18H2O (1), H6[Na2(en)2(H2O)5][Cu(en)2H2O]4[Cu(en)2]{[Cu(en)2] [Ce(PW11O39)2]}216H2O (2), H6[Na2(en)2(H2O)5][Cu(en)2H2O]4 [Cu(en)2]{[Cu(en)2][Pr(PW11O39)2]}218H2O (3), H6[Na2(en)2-(H2 O)4][Cu(en)2H2O]4[Cu(en)2]{[Cu(en)2][Nd(PW11O39)2]}214H2O (4), H6[Na2(en)2(H2O)5][Cu(en)2H2O]4[Cu(en)2]{[Cu(en)2][Sm(PW11O39)2]}2 20H2O (5), and H7[Cu(en)2]2[Sm(PW11O39)2]10H2O (6), which all consist of 1D step-type chains constructed by alternating bis(undecatungstophosphate) lanthanates [Ln(PW11O39)2]11 and [Cu(en)2]2+ groups. All six compounds are synthesized in moderate yield and characterized by a range of physical and spectroscopic techniques, i.e., the infrared spectrum, X-ray powder diffraction, elemental analysis and thermogravimetric analysis. In addition, the magnetic property has also been studied.

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C, 2.65; H, 1.30; N, 3.09; W, 63.80; Cu, 3.51; La, 2.19. Found: C, 2.63; H, 1.34; N, 3.07; W, 63.83; Cu, 3.53; La, 2.12%. IR (KBr, cm1, Fig. S1a): 3522 (w), 3250 (w), 1580 (m), 1458 (m), 1091 (m), 1042 (s), 948 (s), 889 (s), 778 (s).

2. Experimental 2.1. Materials and measurements The H3PW12O40xH2O precursor was synthesized according to the procedure described in the literature [46] and characterized by IR spectrum. All other chemicals were obtained from commercial sources, and were used without further purification. Elemental analyses (C, H, and N) were performed on a Perkin–Elmer 240C elemental analyzer, W, Cu, Na and Ln were determined by a PLASMA-SPEC(I) ICP atomic emission spectrometer. IR spectra were recorded in the range 4000–400 cm1 on an Alpha Centaurt FT/IR spectrophotometer using KBr pellets. Thermogravimetric analyses (TGA) of the samples were performed using a Perkin–Elmer TG-7 analyzer heated from 40 to 800 °C under nitrogen atmosphere at the heating rate of 5 °C min1. X-Ray powder diffraction measurements were performed on a Siemens D5005 diffractometer with Cu Ka (k = 1.5418 Å) radiation in the range of 3–50° at 293 K. The magnetic susceptibilities were measured over 300–2 K at the magnetic field of 1000 Oe for 1, 2 and 4 on a Quantum Design MPMS XL-5 SQUID magnetometer.

2.2.2. Syntheses of 2–5 Compounds 2–5 were prepared following the procedure described for 1, but Ce(NO3)36H2O, Pr(NO3)36H2O, Nd(NO3)36H2O, Sm(NO3)36H2O (0.22 g, 0.50 mmol) were used instead of La(NO3)36H2O. Purple block crystals were obtained. For 2: Yield: 58% (based on Cu). Elemental Anal. Calc. for C32H184N32O181P4W44Cu7Ce2Na2 (12898.39): C, 2.98; H, 1.44; N, 3.47; W, 62.71; Cu, 3.45; Ce, 2.17; Na, 0.36. Found: C, 2.99; H, 1.42; N, 3.44; W, 62.73; Cu, 3.48; Ce, 2.13; Na, 0.34%. IR (KBr, cm1, Fig. S1b): 3478 (m), 3253 (w), 1582 (m), 1458 (w), 1093 (s), 1043 (s), 948 (s), 887 (s), 773 (s). For 3: Yield: 35% (based on Cu). Elemental Anal. Calc. for C32H188N32O183P4W44Cu7Pr2Na2 (12936.01): C, 2.97; H, 1.46; N, 3.46; W, 62.53; Cu, 3.44; Pr, 2.16; Na, 0.35. Found: C, 2.98; H, 1.44; N, 3.43; W, 62.55; Cu, 3.47; Pr, 2.12; Na, 0.33%. IR (KBr, cm1, Fig. S2a): 3452 (m), 3254 (w), 1582 (m), 1458 (w), 1093 (s), 1043 (s), 949 (s), 888 (s), 774 (s). For 4: Yield: 65% (based on Cu). Elemental Anal. Calc. for C32H178N32O178P4W44Cu7Nd2Na2 (12852.59): C, 2.99; H, 1.39; N, 3.49; W, 62.94; Cu, 3.46; Nd, 2.24; Na, 0.36. Found: C, 2.96; H, 1.42; N, 3.45; W, 62.93; Cu, 3.47; Nd, 2.22; Na, 0.33%. IR (KBr, cm1, Fig. S2b): 3500 (w), 3252 (w), 1581 (m), 1458 (m), 1095 (m), 1043 (s), 949 (s), 888 (s), 774 (s). For 5: Yield: 59% (based on Cu). Elemental Anal. Calc. for C32H192N32O185P4W44Cu7Sm2Na2 (12990.92): C, 2.96; H, 1.49; N, 3.45; W, 62.26; Cu, 3.42; Sm, 2.31; Na, 0.35. Found: C, 2.95; H, 1.46; N, 3.43; W, 62.31; Cu, 3.46; Sm, 2.33; Na, 0.32%. IR (KBr, cm1, Fig. S3a): 3452 (m), 3255 (w), 1583 (m), 1458 (w), 1098 (m), 1044 (s), 949 (s), 888 (s), 774 (s).

2.2. Syntheses 2.2.1. Synthesis of 1 H3PW12O40xH2O (0.26 g, 0.09 mmol) was dissolved in H2O (5 mL) with stirring and the pH was adjusted up to approximately 5.0 by addition of Na2CO3, and then a mixture of La(NO3)36H2O (0.23 g, 0.50 mmol) and isophthalic acid (0.08 g, 0.48 mmol) was added directly to the solution (solution A). Next, a 3 mL aqueous solution of en (0.10 mL, 1.50 mmol) was added to a 7 mL aqueous solution of CuSO45H2O (0.24 g, 1.00 mmol), so solution B was obtained. Solution B was added drop-wise to solution A, and then blue precipitates appeared in the beaker. The mixture was stirred for 30 min, and then transferred and sealed in a 23 mL Teflonlined stainless steel container, which was heated at 150 °C for 72 h. After the autoclave was cooled to room temperature, purple block crystals suitable for X-ray crystallography were obtained, washed with distilled water, and air-dried to give 1 in 73% yield (based on Cu). Elemental Anal. Calc. for C28H164N28O178P4W44Cu7La2 (12677.75):

2.2.3. Synthesis of 6 Compound 6 was prepared following the procedure described for 5, but the reaction temperature was at 170 °C. Purple block crystals were obtained. Yield: 37% (based on Cu). Elemental Anal. Calc. for C8H59N8O88P2W22Cu2Sm (6059.70): C, 1.58; H, 0.98; N,

Table 1 Crystal data and structure refinement for compounds 1–6.

Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dc (mg m3) Absorption coefficient (mm1) Reflections collected Independent reflections h Range (°) Goodness-of-fit (GOF) on F2 Rint R1, wR2 [I > 2r]a R1, wR2 (all data)b a b

1

2

3

4

5

6

C28H164N28P4W44O178Cu7La2 12677.75 triclinic  P1

C32H184N32P4W44O181Cu7Na2Ce2 12898.39 triclinic  P1

C32H188N32P4W44O183Cu7Na2Pr2 12936.01 triclinic  P1

C32H178N32P4W44O178Cu7Na2Nd2 12852.59 triclinic  P1

C32H192N32P4W44O185Cu7Na2Sm2 12990.92 triclinic  P1

C8H59N8P2W22O88Cu2Sm 6059.70 triclinic  P1

15.433(1) 18.253(1) 21.444(2) 114.993(1) 91.561(1) 94.182(1) 5449.2(3) 1 3.863 24.294 27 874 18 981 1.05–25.00 1.022 0.0318 0.0467, 0.1076 0.0754, 0.1245

15.934(2) 18.041(2) 21.159(2) 113.860(1) 90.783(2) 94.323(2) 5540.3(1) 1 3.866 23.929 29 913 20 659 1.24–25.68 1.040 0.0521 0.0616, 0.1315 0.1187, 0.1627

15.991(2) 18.029(2) 21.1160(2) 113.872(2) 90.7770(1) 94.239(2) 5545.4(1) 1 3.874 23.937 28 297 19 296 1.06–25.00 1.063 0.0524 0.0602, 0.1272 0.1166, 0.1586

15.958(2) 18.062(2) 21.120(2) 113.929(1) 90.783(2) 94.244(2) 5542.6(1) 1 3.851 23.975 28 323 19 347 1.06–25.08 1.039 0.0489 0.0583, 0.1289 0.1134, 0.1587

15.974(2) 18.025(2) 21.021(2) 113.976(2) 90.729(2) 94.169(1) 5510.0(3) 1 3.915 24.183 27 990 19 149 1.06–25.00 1.061 0.0388 0.0571, 0.1441 0.0859, 0.1638

12.731(1) 20.611(2) 21.662(2) 71.166(1) 82.228(1) 75.590(1) 5201.0(3) 2 3.869 25.299 26 503 18 089 0.99–25.05 1.029 0.0377 0.0551, 0.1364 0.0975, 0.1602

R1 = R||Fo|  |Fc||/R|Fo|. P P wR2 ¼ j wðjF o j2  jF c j2 Þj= jwðF 2o Þ2 j1=2 .

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1.85; W, 66.74; Cu, 2.10; Sm, 2.48. Found: C, 1.59; H, 0.96; N, 1.83; W, 66.73; Cu, 2.12; Sm, 2.45%. IR (KBr, cm1, Fig. S3b): 3518 (w), 3251 (w), 1581 (w), 1456 (w), 1097 (m), 1044 (m), 948 (s), 886 (m), 771 (s). 2.3. X-ray crystallography Single crystal X-ray diffraction data for 1–6 were recorded on a Bruker APEXII CCD diffractometer with graphite-monochromated Mo Ka radiation (k = 0.71069 Å) at 293 K. Absorption corrections were applied using multi-scan technique. All the structures were solved by Direct Method of SHELXS-97 [47] and refined by full-matrix least-squares techniques using the SHELXL-97 program [48] within WINGX [49]. Hydrogen atoms on the en ligands were placed on calculated positions and included in the refinement riding on their respective parent atoms. Anisotropic thermal parameters were used to refine all non-hydrogen atoms except for part of oxygen, carbon and nitrogen atoms. Those hydrogen atoms attached to lattice water molecules were not located. The crystal data and structure refinement results of 1–6 are summarized in Table 1. Selected bond lengths and bond angles for 1–6 are provided in Tables S1–S6 in Supporting information. 3. Results and discussion 3.1. Synthesis discussion POM-based coordination chemistry is often dominated by a large number of equilibria among different species, all of them present in solution. When H3PW12O40xH2O was dissolved in aqueous solution (pH 1), [PW12O40]3 is the major species present, as identified by 31P NMR spectroscopy. Raising the pH value up to 5.0 affords [PW11O39]7 as the predominant species in solution, and the lacunary anion can be precipitated from solution as [NBu4]+ salts and crystallized from MeCN in high yield [50– 54]. In these experiments, compounds 1–6 construct from [LnIII(PW11O39)2]11 polyoxoanions, although the reaction was carried out by using saturated Keggin polyoxoanions as the starting materials. The saturated [PW12O40]3 polyoxoanions transformed to the monovacant [PW11O39]7 polyoxoanions and then combined with Ln(III) ions, resulting in the well-known bis(undecatungstophosphate) lanthanate structures, which are further bridged by

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copper-organic units to generate six new products with analogous polyoxoanion structure features. Meanwhile, compounds 1–6 can be synthesized from [a-PW9O34]9 precursors under similar hydrothermal conditions, due to partly decomposition and reassembly of [a-PW9O34]9 polyanions. It is worthy to be mentioned that isophthalic acid did not exist in the final structure of compounds 1–6. If isophthalic acid was not introduced into the reaction mixture, no crystal products were isolated under similar reaction conditions, so it may be presumed that isophthalic acid might be beneficial for obtaining the high quality crystals and enhancing the product yields of 1–6. The similar synthesis phenomena also exist in the syntheses of other POMs [55–58]. On the other hand, the addition of a big excess of Ln(NO3)36H2O to the solution is unfavorable to the yield of the final product, indicating that the amount of Ln(NO3)36H2O is a key factor for constructing the final structure of the title compounds. In addition, the control of reaction temperature can tune the formation of different transitionmetal substituted POMs phases [59]. 3.2. Crystal structure of compounds 1–6 3.2.1. Structure of compound 1 X-ray diffraction study reveals that compound 1 crystallizes in  which consists of [La(PW11O39)2]11 the triclinic space group P1 2+ polyanions, [Cu(en)2H2O] units, [Cu(en)2]2+ units, and water molecules. As shown in Scheme S1, the {La(PW11O39)2} moiety is constructed from an eight-coordinated La(III) cation sandwiched between two monolacunary {PW11O39} units, resulting in a wellknown sandwich-type bis(undecatungstophosphate) lanthanate structure, which has been isolated by mixing solutions of the saturated Keggin-type POMs with approximately 8/3 equiv. of Ln(NO3)3 and then recrystallized from aqueous ammonium chloride for the first time [37]. Fig. 1a depicts the building block of compound 1, which is composed of one bis(undecatungstophosphate) lanthanate [La(PW11O39)2]11 polyoxoanion, two [Cu(en)2]2+ bridging group (Cu3O39 = 2.453(2) Å; Cu4#1O45 = 2.518(3) Å, respectively. #1: x, 1 + y, z), and two [Cu(en)2H2O]2+ decorated fragments (Cu1#2O19 = 2.805(1) Å and Cu1#2O8w = 2.609(2) Å; Cu5O37 = 2.570(5) Å and Cu5O2w = 2.478(3) Å. #2: 1 + x, 1 + y, z). [La(PW11O39)2]11 polyoxoanion is connected by [Cu(en)2]2+ units to give rise to a 1D chain (Fig. 1b), which further extends to a 2D sheet through weak interactions. As shown in Fig. 1d, a 2D supramolecular sheet is finally constructed by linking

Fig. 1. (a) Combined polyhedral/ball-and-stick representation of the building block of 1. (b) Scheme view of the 1D chain-like of 1. (c) View of the weak interaction mode between {Cu(2)(en)2} fragment and {La(PW11O39)2} polyanions. (d) Combined polyhedral/ball-and-stick representation of the 2D sheet of 1. Hydrogen atoms were omitted for clarity. Symmetry codes: #1: x, 1 + y, z; #2: 1 + x, 1 + y, z.

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the 1D chains through Cu  O weak interactions between Cu2 atom of the {Cu(en)2} fragment and O28 atom originated from {La(PW11O39)2} polyoxoanions with the distances of 2.989(2) Å (Fig. 1c) and hydrogen-bonds interactions between the en molecules coordinated to Cu5 and O17 derived from [La(PW11O39)2]11 polyoxoanions of adjacent ladder-like chains with the distance of 3.016(1) Å. The hydrogen-bonding interactions have strengthened the stability of the supramolecular structure of 1. In 1, the WO bond distances are in the range 1.6842.526 Å (avg. 1.961 Å), and the PO distances range from 1.515 to 1.540 Å (avg. 1.530 Å). These bond distances are in agreement with those in the known polyoxotungstates [20,45]. If each grafted [La(PW11O39)2]11 polyoxoanion is considered as a 4-connected node and [Cu(en)2]2+ group as a linker, the structure of 1 exhibits a 63 grid as shown in Fig. 2. 3.2.2. Structures of compounds 25 When La(NO3)36H2O was replaced with Ce(NO3)36H2O, Pr(NO3)36H2O, Nd(NO3)36H2O and Sm(NO3)36H2O, compounds 25 containing similar polyoxoanions have been successfully prepared, respectively. Compounds 25 also crystallize in the  and are isostructural with only slight differtriclinic space group P 1 ences in bond lengths, bond angles and the number of lattice H2O

molecules; thus only compound 2 is described in detail below. Compound 2 is composed of [Ce(PW11O39)2]11 polyanions, [Cu(en)2H2O]2+ groups, [Cu(en)2]2+ groups, binuclear sodium clusters [Na2(en)2(H2O)5]2+ cations, and water molecules. As shown in Fig. 3a, the inorganic building block [Ce(PW11O39)2]11 grafted by one [Cu(en)2H2O]2+ group with the Cu4#1–O7w distances of 2.673(1) Å (#1: 2  x, 2  y,1  z), is connected to each other by [Cu(en)2]2+ groups with Cu2O28 distance of 2.511(2) Å and Cu3O41 2.584(2) Å, respectively, to generate a 1D chain (Fig. 3b). The [Na2(en)2(H2O)5]2+ cluster is made up of two sodium cations, two en and five H2O molecules, in which each six-coordinated sodium is defined in the equatorial plane with a nitrogen atom from one en ligand and three H2O molecules with Na–N and Na–O bond lengths of 1.993(2) and 1.998(4)–2.557(4) Å, and two terminal oxygen atoms from two [Ce(PW11O39)2]11 polyanions in polar positions with Na–O bond lengths of 2.151(4) and 2.867(3) Å, exhibiting a distorted octahedral coordination environment (Fig. 3c). Then the binuclear sodium clusters bridge two adjacent 1D chains, generating a sheet structure as illustrated in Fig. 3d. All the bond lengths and bond angles in the [Ce(PW11O39)2]11 polyanions are within the usual range and consistent with those described in the literatures [20,45]. If each modified [Ce(PW11O39)2]11 polyoxoanion is considered as a

Fig. 2. (a) Ball-and-stick and polyhedral representation of the four-connected polyanion cluster in 1. (b) and (c) Ball-and-stick representations of the two-connected copperorganic units in 1. (d) Schematic view of the framework of 1.

Fig. 3. (a) and (b) Combined polyhedral/ball-and-stick representation of the building block and the 1D chain of 2. (c) View of the [Na2(en)2(H2O)5]2+ cluster in 2. (d) Polyhedral and ball-and-stick representation of 2D sheet of 2. H atoms and isolated {Cu(4)(en)2H2O} units are omitted for clarity. Symmetry code: #1: 2  x, 2  y, 1  z.

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3-connected node, [Cu(en)2]2+ group and binuclear sodium cluster as linkers, the structure of 2 exhibits a 63 topology as shown in Fig. 4. 3.2.3. Structure of compound 6 X-ray diffraction analysis illustrates that compound 6 crystal which consists of 1D zigzag-like lizes in the triclinic space group P 1 chain built from [Sm(PW11O39)2]11 polyanions, [Cu(en)2]2+ groups, and water molecules located inside the framework between neighboring chains. As shown in Fig. 5a, [Sm(PW11O39)2]11 polyanion was supported by two [Cu(en)2]2+ units in compound 6 (the occupancy factor of Cu1 and Cu2 is 0.5 per unit and the occupancy factor of Cu3 is 1.0 per unit, respectively). Cu1 and Cu2 atoms are occupied by four nitrogen atoms from two en molecules with Cu–N bond lengths ranging from 1.970(2) to 2.030(2) Å in the equatorial plane, and two terminal oxygen atoms from two [Sm(PW11O39)2]11 polyoxoanions in the polar positions with Cu– O bond lengths of 2.775(1) and 2.741(2) Å, adopting a distorted octahedral coordination environment. The five-coordinated Cu3 atom is completed by four nitrogen atoms from two en molecules with Cu–N bond lengths ranging from 1.980(3) to 2.030(3) Å, and one oxygen atom from the polyanion with Cu–O bond length of 2.724(1) Å. [Sm(PW11O39)2]11 polyoxoanions were connected to

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each other through copper-organic units to generate a 1D zigzag chain (Fig. 5b and c). In addition, Cu3 atoms have weak interactions with adjacent 1D chains (Cu3#1  O77#2 = 3.120 Å; #1: 1  x, y, 1  z; #2: x, 1 + y, z) and nitrogen atoms of the en ligand coordinated to Cu3 atoms donate hydrogen-bond interactions to polyanions of neighboring 1D chains (N5#1  O75#2 = 2.902 Å), thus giving rise to a two-dimensional supramolecular structure as depicted in Fig. 5d. All the bond lengths and bond angles in the [Sm(PW11O39)2]11 are within the normal range and consistent with those reported in the literatures [20,45]. The bond-valence calculations [60] indicate that all W atoms, Cu atoms and La atoms are in the +6, +2 and +3 oxidation state in compound 1, respectively. Since 1 was isolated from acidic aqueous solution, the negative charges ought to be compensated by the protonation of the lattice water molecules, which is similar to the case of K2H7[{Ln(PW11O39)2}{Cu2(bpy)2(l-ox)}]xH2O [45], then 1 should be formulated as H8[Cu(en)2H2O]4[Cu(en)2]{[Cu(en)2][La(PW11O39)2]}218H2O, which is also confirmed by elemental analysis. Similar results have also been obtained for compounds 2–6. In a comparative analysis on structures of compounds 16, they all construct from the secondary building blocks {Ln(PW11O39)2}, a Ln(III) cation sandwiched between two monolacunary [PW11O39]7

Fig. 4. (a) Ball-and-stick and polyhedral representations of the three-connected {Ce(PW11O39)2} polyanion cluster in 2. (b) and (c) Ball-and-stick representation of the twoconnected copper-organic unit and sodium cluster in 2, respectively. (d) Schematic view of the framework of 2.

Fig. 5. (a), (b) and (d) Combined polyhedral/ball-and-stick representation of the building block, step-like chain and 2D sheet of 6. (c) Scheme view of the 1D chain-like of 6. All hydrogen atoms were omitted for clarity. Symmetry code: #1: 1  x, y, 1  z.

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anions in which the trivalent lanthanide has a coordination number of eight and occupies square anti-prismatic geometry, and each lacunary [PW11O39]7 anion provides four unsaturated oxygen atoms to coordinate to the Ln(III) metal centre (Scheme S1). Subsequently, the secondary building blocks are bridged through copper-organic units to extend to 1D polyoxoanion chains. The successful syntheses of compounds 16 illustrate that the appropriate combination of materials, ratio of reactants and hydrothermal synthesis conditions can lead to the formation of POM-based inorganic–organic hybrid complexes with different geometrical and structural characteristics. The coordination bond and weak interactions (such as hydrogen-bonding and metal–O interactions) between adjacent 1D chains perhaps result in the 1D chains with different zigzagging distances and angles in compounds 1–6. 3.3. The lengths of Ln–O bond As expected, the Ln–O bond distances (Table 2, Tables S1–S6) decrease across the early lanthanide series because of the stronger interaction between the lanthanide and the unsaturated oxygen atoms as the ionic radius of the Ln(III) ion decreases, which is in accordance with the effect of the lanthanide contraction (Fig. S9) [61–63]. The origin of the different Ln–O bond lengths in each compound is rationalized by consideration of the parent Keggin structure, [XW12O40]3 (where X = a range of heteroatoms), in which four types of O atoms can be distinguished, i.e., (i) the l4-O atoms bound to a heteroatom (Oa), (ii) the terminal oxygen atoms combined with only one W atom (Ot), (iii) O atoms bonded to two W atoms through edge-shared WO6 octahedra (‘long’ W–O bonds, Oe), (iv) O atoms coordinated to two W atoms through cornershared WO6 octahedra (‘short’ W–O bonds, Oc). Removal of one terminal {W–O} unit from a saturated Keggin anion gives the monovacant species with four oxygen atoms available for bonding to the lanthanide ion. This difference is translated into the Ln–O

Table 2 Comparison of Ln–O bond lengths in compounds 1–5. Compounds 1 2 3 4 5

(La3+) (Ce3+) (Pr3+) (Nd3+) (Sm3+)

The range of Ln–O bond lengths (Å)

Average Ln–O bond lengths (Å)

2.458(1)–2.559(1) 2.433(2)–2.518(2) 2.431(2)–2.501(2) 2.430(2)–2.497(2) 2.401(2)–2.485(2)

2.500(4) 2.481(4) 2.459(8) 2.457(1) 2.424(5)

bond distances in the sandwich [Ln(PW11O39)2]11 complexes with the two longer Ln–O bonds owning oxygen atoms derived from edge-shared WO6 octahedra and the two shorter Ln–O bonds sharing oxygen atoms originating from corner-shared WO6 octahedra. 3.4. The one-dimensional chains with different zigzagging distances and angles According to previous research, the chelating coordination ability of the nitrogen atoms from en is stronger than the coordination ability of oxygen atoms from polyanions and water molecules [20,64], therefore copper cations are first coordinated by nitrogen atoms from en to generate metal–organic units, which are further linked by polyanions to form the final products. In these cases, the [Ln(PW11O39)2]11 anions with different modified modes are connected by [Cu(en)2]2+ units to show multifarious frameworks. In compound 1, each [La(PW11O39)2]11 anion was supported by two [Cu(en)2H2O]2+ groups, but in compounds 2–5, each [Ln(PW11O39)2]11 ion was grafted by only one [Cu(en)2H2O]2+ unit. While in compound 6, [Sm(PW11O39)2]11 was modified by [Cu(en)2]2+ which has weak interaction with adjacent one-dimensional chains. From Fig. 6, we can clearly see that 1D zigzag chains own distinct distances and angles in compounds 1–6. Based on the above description, the connectivity mode between POMs and metal–organic units plays an important role in the formation of the structures, meanwhile, the interactions modes and directions between POM-based 1D chains also play vital roles in the formation of the ultimate frameworks. In compounds 2–5, 1D step-like chains are connected by binuclear sodium clusters [Na2(en)2(H2O)n]2+ (n = 5, compounds 2, 3 and 5; n = 4, compound 4) to give rise to a 2D sheet through coordination bond interactions (Fig. 3d). Notice that the zigzagging distances and angles of the 1D chains are nearly unchanged in compounds 2–5, presenting that the Ln(III) cations do not have any influence on the final structures. In compound 1, 1D chains built from LnPOMs are linked through the supramolecular interactions to give rise to a 2D framework (Fig. 1d). It is worth mentioning that the coordination bond interactions between 1D chains and sodium cations in compounds 2–5 are in the same direction with the supramolecular interactions between adjacent 1D chains in compound 1, resulting in that the 2D sheet frameworks are in the same plane as shown in Fig. 1d and 3d. Comparing the structures of compounds 1–5, the 1D chain in 1 owning a bigger zigzagging angle maybe result from that the supramolecular interaction is weaker than the coordination bond interaction between adjacent 1D chains. While in compound 6, the similar inorganic–organic sheet as in compound 1 is formed

Fig. 6. Selected Ln  Ln distances (Å) and zigzagging angle (°) in the chains of polymeric compounds 2–5 (a), 1 (b) and 6 (c).

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through similar interactions but nearly vertical to the direction of that in compounds 1–5 (Fig. 5d). Comparing Fig. 5d with Fig. 1d, we presume that the different directions of the interactions between adjacent 1D chains lead to the 1D chain with different zigzagging angles and distances. The different modified types of building blocks, interactions between one-dimensional chains and the directions of the interactions may be the direct or/and indirect reasons to result in the pronounced increase in the hybrid chain zigzagging angle when going from compounds 2–5, 1 to 6 (Fig. 6). 3.5. XRPD, IR and TG analyses The XRPD patterns for compounds 1–6 are presented in Figs. S4–S6. The diffraction peaks of both simulated and experimental patterns match well, indicating that the phase purities of 1–6. The differences in intensity may be due to the preferred orientation of the powder samples. The FT-IR spectra were recorded as KBr pellets in the range of 4000–400 cm1 for 1–6 (Figs. S1–S3). The characteristic W–Ot band is observed in the range 948–950 cm1 for all the compounds, bands between 896 and 887 cm1 can be assigned to W–Oc–W stretching modes and bands covering from 773 to 775 cm1 can be attributed to W–Oe–W stretching modes, respectively [65]. It should be noted that there is a clear split (Fig. S10) of the P–O stretching band between 1100 and 1000 cm1 [66], due to the

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effective local C3v symmetry of the ‘PO4’ unit in the monovacant anion (the parent Keggin anion, [PW12O40]3, has Td symmetry and hence only one band at 1080 cm1). The size of the split increases from 45.0 cm1 in the uncomplexed lacunary anion [PW11O39]7 [67], to 48.8 cm1 in compound 1. The size of the split shows an increase trend across the Ln(III) complexes in general, from 48.8 cm1 (La, 1), to 50.0 cm1 (Ce, 2), 50.2 cm1 (Pr, 3), 52.1 cm1 (Nd, 4), and finally 54.0 cm1 (Sm, 5), perhaps due to the increased interaction between [PW11O39]7 and early lanthanide ions across the series. There is no evidence for the Ln–O stretching vibration in the far-infrared region, probably owing to the predominantly ionic character of the interaction between the vacant anion and the Ln(III) centre [68]. As the P–O bond lengths vary a little with Ln(III) centre then it would appear that, in this sense, infrared spectroscopy is more sensitive to change in coordination of Ln(III) ion to the [PW11O39]7 anion than X-ray crystallography. To examine the thermal stabilities of these compounds, the thermogravimetric analyses (TGA) was performed with a N2 atmosphere for compounds 1, 2, 4 and 5 from 40 to 800 °C (Supporting information, Figs. S7 and S8). For 1, the first step weight loss in the region 40–262 °C accounts for approximately 2.60% and can be attributed to the loss of lattice water molecules, which is higher than the calculated value of 2.56% due to the moisture of product. The second step between 262 and 568 °C accounts for 7.24% of the sample weight loss and can be assigned to the loss of coordinated

Fig. 7. vT vs. T and v1 vs. T curves of compounds (a) 1, (b) 2 and (c) 4.

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en and water molecules (calc. 7.21%). For compounds 2, 4 and 5, they show a total weight loss of 10.99–11.39% in the range of 40–563 °C, which agrees with the calculated value of 10.94– 11.41%. The weight loss of 3.88–4.42% at 40–281 °C corresponds to the loss of all lattice water molecules, and water and en molecules which coordinated to sodium ions (calc. 3.86%, 3.46% and 4.39%, respectively). Furthermore, the weight loss of 7.11–7.09% at 281–563 °C results from the loss of water and en molecules coordinated to copper ions (calc. 7.08%, 7.11% and 7.03%, respectively). Further weight loss for them is probably due to further reductive decomposition.

3.6. Magnetic properties The solid-state magnetic susceptibility data of compounds 1, 2 and 4 were measured in the temperature range of 2300 K in a 1000 Oe magnetic field and plotted as vT and v1 versus T, as shown in Fig. 7. For compound 1, the observed value of vT is 3.03 cm3 K mol1 at 300 K, which is slightly higher than the value expected of 2.62 cm3 K mol1 for seven non-interacting Cu(II) ions (vT ¼ l2eff =8, g = 2.0, S = 1/2). Upon cooling, the vT value decreases slowly with decreasing temperature until 76 K, and then the value of vT increases up to a maximum of 2.91 cm3 K mol1 at approximate 49 K, then abruptly decreases to reach 2.55 cm3 K mol1 at 2 K (as shown in Fig. 7a). The v1 versus T plot is fitted to the Curie–Weiss law [v = C/(T  H)] in the whole temperature range with C = 3.03 cm3 K mol1 and H = 2.46 K. The H value indicates that antiferromagnetic interaction exist in compound 1 and the magnetic interactions between Cu(II) ions are very weak. The vT versus T plots of compounds 2 and 4 exhibit different curves from that of compound 1 due to the intrinsic nature of the lanthanide (Fig. 7b and 7c). The vT value of 4.69 cm3 K mol1 at 300 K is a little higher than the calculated value of 4.23 cm3 K mol1 for seven uncoupled Cu(II) ions (g = 2.0, S = 1/2) and two uncoupled Ce(III) ions (g = 6/7, J = 5/2) [69] in compound 2. The vT product of 5.85 cm3 K mol1 at 300 K is very close to the calculated value of 5.89 cm3 K mol1 for seven uncoupled Cu(II) ions (g = 2.0, S = 1/2) and two uncoupled Nd(III) ions (g = 8/11, J = 9/2) [69] in compound 4. The v1 versus T plots agree with the Curie–Weiss law in the temperature range from 50 to 300 K with C = 5.09 cm3 K mol1 and H = 29.90 K for 2 and C = 6.29 cm3 K mol1 and H = 23.44 K for 4, respectively. Such magnetic behavior is characteristic of an antiferromagnetic coupling interaction.

and electrochemically active metal centers. Such kind of research is currently underway in our lab. Acknowledgments The authors gratefully acknowledge the financial support from the Program for Changjiang Scholars and Innovative Research Team in University, the National Natural Science Foundation of China (Nos. 20573016, 20901014 and 20703008), the Science Foundation for Young of Jilin Scientific Development Project (Nos. 20090125 and 20090129), the Science Foundation for Young Teachers of NENU (No. 20090407), the Training Fund of NENU’s Scientific Innovation Project (NENU-STC08019), Ph.D. Station Foundation of Ministry of Education for New Teachers (No. 20090043120004), the Postdoctoral Foundation of Northeast Normal University, and the Postdoctoral Foundation of China (No. 20090461029). Appendix A. Supplementary material CCDC 743390, 743391, 743392, 743393, 743394 and 743395 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_ request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2010.07.028. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

4. Conclusions In conclusion, a series of inorganic–organic hybrid materials based on monolacunary Keggin-type POM building blocks with sandwiched trivalent lanthanide ions and copper-organic bridging fragments have been prepared by hydrothermal reaction. In all the six compounds, [Ln(PW11O39)2]11 polyoxoanions served as an inorganic ligand to coordinate to Cu(II)-organic units and finally gave rise to a 1D zigzag-like chain. The zigzag-like chains with different distances and angles maybe result from the interaction modes between adjacent 1D chains. There is a systematic decrease in Ln–O distances and increase in splitting of the mP–O modes across early lanthanide series which can be attributed to the increased interaction between the Ln(III) cations and the lacunary phosphotungstate anions. The magnetic studies of compounds 1, 2 and 4 indicate that antiferromagnetic interactions exist in these three compounds. Further research will focus on the synthesis of inorganic–organic hybrid materials based on other LnPOM polyanions so as to obtain the high-dimensional materials with magnetically

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