Synthesis, Crystal Structures, and Properties of Novel Heterometallic

Jan 21, 2010 - Blue crystals of 1 and 2 had been produced, which were collected by filtration and washed repeatedly with distilled water and diethyl e...
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DOI: 10.1021/cg900299v

Synthesis, Crystal Structures, and Properties of Novel Heterometallic La/Pr-Cu-K and Sm/Eu/Tb-Cu Coordination Polymers

2010, Vol. 10 1059–1067

Ya-guang Sun,*,† Xiao-fu Gu,† Fu Ding,‡ Philippe F. Smet,§ En-jun Gao,† Dirk Poelman,§ and Francis Verpoort‡ †

Laboratory of Coordination Chemistry, Shenyang Institute of Chemical Technology, Shenyang, 100142, China, ‡Centre for Ordered Materials, Organometallics and Catalysis, Department of Inorganic and Physical Chemistry, Ghent University, Ghent, 9000, Belgium, and §LumiLab, Department of Solid State Sciences, Ghent University, Ghent, 9000, Belgium Received March 15, 2009; Revised Manuscript Received December 21, 2009

ABSTRACT: Five new heterometallic coordination polymers, {[LnCu2K(pydc)4(H2O)9] 3 H2O}n [Ln = La (1), Ln = Pr (2), H2pydc = pyridine-2,5-dicarboxylic acid], {[Ln2Cu(pydc)4(H2O)8] 3 H2O}n [Ln = Sm (3), Ln = Eu (4), Ln = Tb (5)], have been synthesized under hydrothermal conditions and characterized by elemental analysis, IR, thermogravimetric analysis, and singlecrystal X-ray diffraction. X-ray structural analysis revealed that complexes 1 and 2 are the first Ln/Cu/K structures containing Cu(II) sandwiched in the interlayer regions of the two La/K planes alternately arranged along the b-axis. In complexes 3-5, two distinct types of building blocks, Ln(pydc)(H2O)4 and Cu(pydc)2, are linked together by pydc ligands to form a two-dimensional net structure. The magnetic properties of 1 and 2 and the luminescence properties of 3-5 have been investigated.

Introduction The design and construction of metal-organic frameworks (MOFs) continue to attract much attention owing to the fact that they might be developed into more advanced solid-state materials.1 In recent years, much work has been focused on MOFs containing transition or lanthanide metal ions.2 Multidimensional coordination polymers incorporating both lanthanide and transition metals have been relatively little reported, although in some research d-f or f-f heterometallic compounds have been found to exhibit excellent properties in magnetic applications, catalysis, sensors, or molecular recognition.3 The reasons for this are (a) lanthanide ions have high affinity for and a preference to bind with hard donor atoms, whereas many later d-block transition metal ions prefer to be coordinated by soft donor atoms;4 (b) the synthesis of heterometallic MOFs is frequently influenced by many subtle factors, such as the temperature, molar ratio of the raw materials, solvent, template, counterion, and pH of the medium; (c) in addition, the coordination number of the metal ion and the shape, flexibility, and symmetry of the organic ligands also influence the construction of heterometallic MOFs. Therefore, researchers in this field need to select suitable ligands to assemble lanthanide and transition metal ions under different reaction conditions.5 Symmetrical N-heterocycle carboxylate ligands are good candidates for use in the construction of heterometallic MOFs since they contain both N- and O-donor atoms.6 For example, zeolite-type 3d-4f heterometallic MOFs with a three-dimensional (3D) nanotubular structure based on pyridine-2,6-carboxylate and a series of 3D 4d-4f heterometallic MOFs with helical Ln-O-Ag subunits based on isonicotinic acid have been reported.7 Pyridine-2,5-dicarboxylic acid (H2pydc) is a very important unsymmetrical carboxylate derivative and has attracted much interest with regard to the construction of MOFs. The pydc ligand could potentially provide various coordination modes to form both discrete and consecutive metal complexes under appropriate synthesis conditions. Systematic *To whom correspondence should be addressed. r 2010 American Chemical Society

studies of one-, two-, and three-dimensional coordination complexes based on H2pydc have been undertaken in some laboratories,8 which confirmed the rich variety of coordination motifs (Scheme 1). We chose pyridine-2,5-carboxylate (pydc) as a potential linker to connect lanthanide and transition metal ions, and thereby isolated five novel heterometallic coordination polymers following the reaction under hydrothermal conditions, {[LnCu2K(pydc)4(H2O)9] 3 H2O}n [Ln=La (1), Ln=Pr (2)] and {[Ln2Cu(pydc)4(H2O)8] 3 H2O}n [Ln=Sm (3), Ln=Eu (4), Ln = Tb (5)]. To the best of our knowledge, 1 and 2 are the first examples of Ln/Cu/K heterometallic complexes. Complex 1 exhibits a weak antiferromagnetic interaction between the Cu(II) ions. Complexes 3-5 show the characteristic optical emissions of Sm3þ, Eu3þ, and Tb3þ, respectively. Experimental Section Materials. All chemicals purchased were of reagent grade and were used without further purification. All syntheses were carried out in 25 mL Teflon-lined autoclaves under autogenous pressure. The reaction vessels were filled to approximately 60% volume capacity. Water used in the reactions was distilled water. Physical Techniques. We used the hydrothermal reaction method to generate the present series of Ln-Cu coordination polymers. Elemental analyses (C, H, N) were carried out with a Perkin-Elmer elemental analyzer. IR spectra were recorded on a Nicolet IR-470 spectrometer from samples in KBr pellets. Powder X-ray diffraction (PXRD) patterns of the samples were recorded on an X-ray diffractometer (Bruker D8 Advance) using Cu-KR radiation. Thermogravimetric analysis (TGA) experiments were performed on a Netzsch TG 209 instrument at a heating rate of 5 C min-1. Luminescence spectra were obtained on an FS920 fluorescence spectrometer (Edinburgh Instruments). Luminescence decay measurements were obtained using a pulsed nitrogen laser (excitation wavelength 337 nm, pulse length 800 ps, repetition rate 1 Hz) and a 1024-channel intensified CCD (Andor Technology) attached to a 0.5 m Ebert monochromator. Variabletemperature magnetic susceptibilities were measured on a Quantum Design MPMS-7 SQUID magnetometer. Diamagnetic corrections were applied using Pascal’s constants for all constituent atoms. Syntheses of 1 and 2. A mixture of H2pydc (2.0 mmol, 0.334 g), La2O3 (0.2 mmol, 0.107 g), or Pr6O11 (0.2 mmol, 0.204 g), Cu(CH3COO)2 3 H2O (0.6 mmol, 0.120 g), KOH (2.0 mmol, 0.102 g), Published on Web 01/21/2010

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and H2O (10 mL) was placed in a Teflon-lined autoclave and heated to 160 C for three days. The autoclave was then allowed to cool naturally to room temperature. Blue crystals of 1 and 2 had been produced, which were collected by filtration and washed repeatedly with distilled water and diethyl ether. The yields of 1 and 2 were 67% and 56% (based on Ln2O3), respectively. Elemental analysis calcd (%) for 1: C 29.47, H 2.43, N 4.91; found: C 29.38, H 2.40, N 4.85. Elemental analysis calcd (%) for 2: C 29.50, H 2.44, N 4.91; found: C 29.44, H 2.41, N 4.86. Infrared absorption spectra were measured and the major absorption peaks were identified (peak intensities are classified in brackets as w(eak), m(edium), or s(trong)). IR (KBr, cm-1) for 1: 3383 (m), 1609 (s), 1396 (s), 1354 (s), 1288 (m), 1260 (m), 1173 (w), 1130 (w), 1041 (w), 763 (w), 690 (w), 517 (w). IR (KBr, cm-1) for 2: 3434 (m), 1621 (s), 1403 (s), 1278 (s), 1221 (m), 1111 (m), 721 (w), 501 (w). Synthesis of 3-5. A mixture of H2pydc (2.0 mmol, 0.334 g), Ln2O3 (0.2 mmol, Sm 0.070 g; Eu 0.071 g; Tb 0.035 g), Cu(CH3COO)2 3 H2O (0.6 mmol, 0.120 g), and H2O (10 mL) in a molar ratio of 10:1:3:2778 was placed in a Teflon-lined autoclave and heated at 160 C for three days. The autoclave was then allowed to

Scheme 1. Coordinating Modes of pydc Ligands in Heterometallic Coordination Polymers

cool naturally to room temperature. The blue solids formed were collected by filtration and washed repeatedly with distilled water and diethyl ether. Elemental analysis calcd (%) for 3: C 28.34, H 2.55, N 4.72; found: C 28.29, H 2.53, N 4.67. Elemental analysis calcd (%) for 4: C 28.26, H 2.54, N 4.71; found: C 28.24, H 2.51, N 4.68. Elemental analysis calcd (%) for 5: C 27.93, H 2.51, N 4.65; found: C 27.89, H 2.47, N 4.59. IR (KBr, cm-1) for 3: 3405 (m), 1629 (s), 1390 (s), 1351 (s), 1335 (s), 1278 (w), 1040 (w), 834 (w), 765 (w), 686 (w), 518 (w). IR (KBr, cm-1) for 4: 3417 (m), 1596 (s), 1571 (s), 1386 (s), 1100 (w), 833 (w), 777 (w), 526 (w). IR (KBr, cm-1) for 5: 3405 (m), 1623 (s), 1390 (s), 1351 (s), 1334 (s), 1041 (w), 835 (w), 764 (w), 518 (w). Crystallographic Analyses. Crystallographic data of 1-5 were collected at 293 K on a Bruker SMART 1000 CCD diffractometer using monochromated MoKR radiation (λ = 0.71073 A˚), applying the ω-j scan technique. An empirical absorption correction was applied. The structures were solved by direct methods and refined by full-matrix least-squares against F2 using the SHELXS-97 and SHELXL-97 programs.9,10 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The hydrogen atoms were placed in calculated positions and refined as riding atoms with fixed isotropic thermal parameters. Analytical expressions of neutral atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data and selected bond lengths and angles for 1-5 are listed in Tables 1 and 2, respectively.

Results and Discussion Crystal Structures of 1 and 2. Since 1 and 2 are isostructural, the structure of 1 is described representatively. As shown in Figure 1, the unit cell of 1 contains one La(III) ion, one Cu(II) ion, one K(I) ion, and two crystallographically unique pydc ligands. Each La(III) ion is coordinated by nine oxygen atoms from four pydc ligands and five water molecules, resulting in a distorted monocapped square-antiprism geometry. The La-O bond lengths range from 2.495 to 2.658 A˚. The O-La-O angles range from 61.5 to 143. Each K(I) ion is coordinated by eight oxygen atoms from four pydc ligands and four water molecules. The average K-O bond length is 2.845 A˚. The O-K-O angles range from 57.57 to 152.47. In 1, the coordinated water O11 is disordered over two sites with occupancy factors of 0.8 and 0.2; the lattice water O14 has been given a 0.5 occupancy factor in order to retain an acceptable displacement parameter. As far as the Cu(II) ion is concerned, it exhibits a square-pyramidal geometry, being coordinated by two nitrogen atoms from two pydc ligands (Cu1-N1, 2.002 A˚; Cu1-N2, 1.980 A˚) and three oxygen atoms from three carboxylate groups of pydc ligands Table 1. Crystal Data and Structure Refinement for 1-5 data

1

2

3

4

5

empirical formula crystal system space group a [A˚] b [A˚] c [A˚] R [] β [] γ [] V (A˚3) Z T (K) λ (A˚) Dcalcd (g/cm3) μ (mm-1) GOF on F2 R1 [I > 2σ(I)] wR2 [I > 2σ(I)]

C14H13.75Cu K0.50La0.50N2O13 monoclinic P2/c 7.9928(9) 10.9734(11) 20.816(2) 90 93.657(2) 90 1822.03(3) 4 293(2) 0.71073 2.080 2.533 1.078 0.0271 0.0713

C14H13.80Cu K0.50Pr0.50N2O12.90 monoclinic P2/c 7.9977(14) 10.9472(19) 20.722(4) 90 93.796(3) 90 1810.3(5) 4 293(2) 0.71073 2.091 2.714 1.052 0.0345 0.0791

C28H30Cu Sm2N4O25 triclinic P1 7.7642(12) 9.4384(14) 13.219(2) 75.781(2) 76.272(2) 78.024(2) 900.7(2) 1 294(2) 0.71073 2.188 3.909 1.038 0.0223 0.0593

C28H30Cu Eu2N4O25 triclinic P1 7.7216(11) 9.3835(13) 13.1827(19) 75.752(2) 76.215(2) 78.194(2) 888.2(2) 1 294(2) 0.71073 2.225 4.189 1.033 0.0215 0.0549

C28H30Cu Tb2N4O25 triclinic P1 7.7483(12) 9.3676(14) 13.239(2) 75.677(2) 76.062(2) 78.296(2) 893.1(2) 1 294(2) 0.71073 2.239 2.239 1.014 0.0265 0.0663

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Table 2. Selected Bond Lengths (A˚) and Angle () for Compounds 1-5 Compound 1a La(1)-O(10) La(1)-O(8)#2 La(1)-O(9)#3 La(1)-O(1)#3 Cu(1)-O(4)#4 Cu(1)-N(2) K(1)-O(6)#5 K(1)-O(3)#7 K(1)-O(12)#5 O(10)-La(1)-O(8)#1 O(10)-La(1)-O(9) O(10)-La(1)-O(9)#3 O(10)-La(1)-O(1) O(1)-La(1)-O(1)#3 O(10)-La(1)-O(110 )#3 O(8)#2-La(1)-O(110 )#3 O(8)#1-La(1)-O(110 ) O(4)#4-Cu(1)-O(5) O(5)-Cu(1)-N(1)#4 O(6)-K(1)-O(3)#6

2.495(4) 2.526(2) 2.565(3) 2.570(2) 1.935(2) 1.981(3) 2.697(3) 2.702(3) 3.091(4) 137.07(5) 113.81(7) 113.81(7) 71.11(6) 142.22(12) 61.5(11) 143.0(11) 143.0(11) 170.85(12) 97.36(11) 89.28(8)

La(1)-O(8)#1 La(1)-O(9) La(1)-O(1) La(1)-O(11) Cu(1)-O(5) Cu(1)-O(2) K(1)-O(3)#6 K(1)-O(13) K(1)-O(12) O(8)#1-La(1)-O(8)#2 O(8)#1-La(1)-O(9) O(9)-La(1)-O(9)#3 O(9)-La(1)-O(1) O(10)-La(1)-O(11) O(8)#1-La(1)-O(110 )#3 O(9)-La(1)-O(110 )#3 O(8)#2-La(1)-O(110 ) O(4)#4-Cu(1)-N(2) N(1)#4-Cu(1)-O(2) O(6)-K(1)-O(3)#7

2.526(2) 2.565(3) 2.570(2) 2.658(7) 1.940(3) 2.400(3) 2.702(3) 2.891(3) 3.091(4) 85.86(11) 71.35(9) 132.38(14) 68.65(8) 67.3(2) 84.3(11) 68.8(11) 84.3(11) 93.60(11) 90.08(10) 71.54(11)

O(5)-Cu(1)-N(2) O(4)#4-Cu(1)-N(1)#4 O(5)-Cu(1)-N(1)#4 N(2)-Cu(1)-N(1)#4 O(3)#5-K(1)-O(6)#7 O(3)#6-K(1)-O(6)#7 O(6)-K(1)-O(6)#7 O(3)#5-K(1)-O(13)#7 O(3)#6-K(1)-O(13)#7 O(6)-K(1)-O(13)#7 O(6)#7-K(1)-O(13)#7 O(3)#5-K(1)-O(13) O(3)#6-K(1)-O(13) O(6)-K(1)-O(13) O(6)#7-K(1)-O(13) O(13)#7-K(1)-O(13) O(3)#5-K(1)-O(12)#7 O(3)#6-K(1)-O(12)#7 O(6)-K(1)-O(12)#7 O(6)#7-K(1)-O(12)#7 O(13)#7-K(1)-O(12)#7 O(13)-K(1)-O(12)#7 O(3)#5-K(1)-O(12) O(3)#6-K(1)-O(12)

83.94(15) 83.56(15) 97.41(15) 169.92(16) 139.19(11) 89.98(11) 124.34(16) 94.90(12) 138.60(13) 77.35(13) 74.79(13) 138.59(13) 94.90(12) 74.79(13) 77.35(13) 117.93(19) 75.89(12) 81.24(12) 130.34(12) 65.22(11) 57.37(13) 142.30(12) 81.24(13) 75.89(12)

Compound 2b Pr(1)-O(10) Pr(1)-O(8)#2 Pr(1)-O(9)#1 Pr(1)-O(1) Pr(1)-O(11) Cu(1)-O(4)#4 Cu(1)-O(5) Cu(1)-N(2) Cu(1)-N(1)#4 Cu(1)-O(2) K(1)-O(3)#5 K(1)-O(3)#6 K(1)-O(6) K(1)-O(13) K(1)-O(12)#7 O(10)-Pr(1)-O(8)#2 O(8)#2-Pr(1)-O(8)#3 O(10)-Pr(1)-O(9)#1 O(8)#3-Pr(1)-O(9)#1 O(10)-Pr(1)-O(1) O(9)-Pr(1)-O(1) O(1)-Pr(1)-O(1)#1 O(10)-Pr(1)-O(11) O(4)#4-Cu(1)-N(2)

2.460(6) 2.474(3) 2.519(4) 2.529(3) 2.625(5) 1.928(3) 1.937(3) 1.978(4) 1.997(4) 2.411(4) 2.683(4) 2.683(4) 2.687(4) 2.878(4) 3.125(5) 136.73(8) 86.54(15) 113.10(10) 71.48(12) 70.49(8) 69.18(12) 140.99(15) 66.79(11) 93.58(15) Compound 3c

Sm(1)-O(4)#1 Sm(1)-O(11) Sm(1)-O(10) Sm(1)-O(9) Cu(1)-O(8)#2 Cu(1)-N(2) O(4)#1-Sm(1)-O(1) O(1)-Sm(1)-O(11) O(1)-Sm(1)-O(12) O(12)-Sm(1)-N(1) N(2)-Cu(1)-N(2)#2

2.368(2) 2.392(2) 2.443(2) 2.455(2) 1.946(2) 1.985(3) 85.63(8) 77.91(9) 153.42(9) 139.91(8) 180.0

Sm(1)-O(1) Sm(1)-O(5) Sm(1)-O(12) Sm(1)-N(1) Cu(1)-O(8) Cu(1)-N(2)#2 O(4)#1-Sm(1)-O(11) O(11)-Sm(1)-O(10) O(11)-Sm(1)-O(9) O(8)#2-Cu(1)-O(8) O(13)-O(13)#4

2.383(2) 2.410(2) 2.454(2) 2.600(2) 1.946(2) 1.985(3) 80.72(9) 145.39(8) 145.64(8) 179.998(1) 0.780(13)

Eu(1)-O(11) Eu(1)-O(1) Eu(1)-O(12) Eu(1)-N(1) Cu(1)-N(2) O(11)-Eu(1)-O(5) O(5)-Eu(1)-O(1) O(9)-Eu(1)-N(1) N(2)#2-Cu(1)-N(2) O(13)-O(13)#4

2.369(3) 2.393(2) 2.425(3) 2.577(3) 1.980(3) 77.72(10) 115.56(9) 77.59(9) 180.0 0.847(14)

Compound 4d Eu(1)-O(7)#1 Eu(1)-O(5) Eu(1)-O(10) Eu(1)-O(9) Cu(1)-O(3)#2 O(7)#1-Eu(1)-O(11) O(7)#1-Eu(1)-O(1) O(1)-Eu(1)-O(10) O(3)#2-Cu(1)-O(3) C(7)-O(7)-Eu(1)#3

2.350(2) 2.370(2) 2.421(3) 2.430(2) 1.943(2) 80.82(10) 141.05(9) 140.27(8) 180.0 141.9(2)

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Tb(1)-O(11) Tb(1)-O(1) Tb(1)-O(9) Tb(1)-O(10) Cu(1)-O(7)#2 Cu(1)-N(2)#2 O(4)-Tb(1)#3 O(11)-Tb(1)-O(4)#1 O(11)-Tb(1)-O(9) O(9)-Tb(1)-N(1) N(2)#2-Cu(1)-N(2)

2.340(3) 2.355(3) 2.400(3) 2.412(3) 1.949(3) 1.986(3) 2.343(3) 80.89(12) 82.53(13) 140.25(11) 180.0

Tb(1)-O(4)#1 Tb(1)-O(5) Tb(1)-O(12) Tb(1)-N(1) Cu(1)-O(7) Cu(1)-N(2) O(13)-O(13)#4 O(11)-Tb(1)-O(5) O(4)#1-Tb(1)-O(12) O(7)#2-Cu(1)-O(7) (7)#2-Cu(1)-N(2)#2

2.343(3) 2.377(3) 2.403(3) 2.555(3) 1.949(3) 1.986(3) 0.869(17) 73.03(10) 73.60(11) 179.999(2) 83.38(13)

a Symmetry transformations used to generate equivalent atoms: (#1) -x þ 1, y, -z þ 1/2; (#2) x þ 1, y, z; (#3) -x þ 2, y, -z þ 1/2; (#4) -x þ 2, -y þ 1, -z þ 1; (#5) -x þ 1, y, -z þ 3/2; (#6) x - 1, y, z; (#7) -x þ 2, y, -z þ 3/2. b Symmetry transformations used to generate equivalent atoms: (#1) -x þ 2, y, -z þ 1/2; (#2) -x þ 1, y, -z þ 1/2; (#3) x þ 1, y, z; (#4) -x þ 2, -y þ 1, -z þ 1; (#5) x - 1, y, z; (#6) -x þ 2, y, -z þ 3/2; (#7) -x þ 1, y, -z þ 3/2. c Symmetry transformations used to generate equivalent atoms: (#1) x, y þ 1, z; (#2) -x þ 2, -y, -z þ 1; (#3)x, y - 1, z; (#4) -x þ 1, -y þ 1, -z þ 1; d Symmetry transformations used to generate equivalent atoms: (#1) x, y þ 1, z; (#2) -x þ 3, -y, -z; (#3) x, y - 1, z; (#4) -x þ 2, -y þ 1, -z; e Symmetry transformations used to generate equivalent atoms: (#1) x, y - 1, z; (#2) -x, -y þ 2, -z þ 2; (#3) x, yþ 1, z; (#4) -x þ 1, -y þ 1, -z.

Figure 1. The coordination environments of Cu(II), La(III), and K(I) in 1.

(Cu1-O2, 2.400 A˚; Cu1-O4, 1.935 A˚; Cu1-O5, 1.940 A˚). The coordination modes of pydc in previously structurally characterized heterometallic coordination polymers are summarized in Scheme 1. In 1, the pydc ligands exhibit two distinct coordination modes, as depicted in Scheme 1a,b, namely, the μ4-bridging coordination mode and the μ3-bridging coordination mode. On the basis of these coordination modes of the pydc ligands, three different building blocks of La(pydc)(H2O)5, K(pydc)(H2O)4, and Cu(pydc)2 are constructed along the aaxis, along which they are alternately arranged to form a onedimensional (1D) chain. All La(III) and K(I) in the chain are perfectly coplanar, and the distance between adjacent La(III) and K(I) centers is 10.913 A˚. The chains are connected by pydc and further extend, resulting in the assembly of a two-dimensional (2D) framework in the ac plane. It is interesting to note that Cu(II) is sandwiched in the interlayer regions of the two La/ K planes alternately arranged along the b-axis (Figure 2). To the best of our knowledge, 1 and 2 are the first examples of Ln(III)-Cu(II)-K(I) heterotrimetallic complexes. There are abundant hydrogen-bonding interactions ( — O11-H11A 3 3 3 O4 = 143.12, O11-O4 = 3.075 A˚; — O13-H13B 3 3 3 O7 = 115.33, O13-O7 = 2.827 A˚; — O12-H12B 3 3 3 O5 = 94.36, O12-O9 = 2.927 A˚; — O10-H10B 3 3 3 O12 = 137.84, O10O12 = 2.749 A˚) as well as π-π stacking interactions in an offset fashion with edge-to-edge distances of 3.606 A˚ between

adjacent nets, which extend the 2D frameworks into 3D supramolecular architectures (Figure 3). The crystal structures of 1 and 2 differ from the 1D Ln(III)-Cu(II) heterometallic coordination polymers constructed from pyridine-2,6-carboxylate.11 This may be due to the differences in steric hindrance of the two carboxylate groups on the respective pyridine rings. The topological approach is a good way to understand the nature of the framework involved because it can lead to reduced multidimensional structures. On the basis of the simplification principle, La(III), K(I), and Cu(II) can be defined as a 4-, 4-, and 3-connected nodes, respectively. The two types of coordination mode of the pydc ligands can be simplified as 4- and 3-connected nodes, respectively. On the basis of this simplification, the structure of 1 can be described as a (482)2(428310)2(4284) topological network; the topological mode is shown in Figure 4. Crystal Structures of 3-5. Since 3-5 are isostructural, the structure of 4 is described representatively. An ORTEP view of 4 is shown in Figure 5. Eu(III) is coordinated by one nitrogen atom and three oxygen atoms from pydc ligands, and the remaining four sites are occupied by four oxygen atoms of the coordinated water molecules. Thus, the coordination geometry about Eu(III) may be viewed as distorted square antiprismatic. The Eu-O distance varies from 2.350 to 2.430 A˚, and the average Eu-O and Eu-N bond lengths are 2.394 and 2.577 A˚, respectively. In 4, O2 of the carboxylate group is disordered over two sites, with occupancy factors of 0.8 and 0.2; the lattice water O13 is also disordered over two sites, with occupancy factors of 0.5 each. The O-Eu-O bond angles fall in the range 68.43-153.04 and are comparable to those found in the previously reported Ln(III)-Cu(II) complexes.8e The Cu(II) is coordinated by four oxygen atoms and two nitrogen atoms from pydc ligands, and these centers complete an elongated octahedral geometry due to the Jahn-Teller effect. In 4, the pydc ligands exhibit two distinct coordination modes, as shown in Scheme 1c,d, namely, the μ3-bridging coordination mode, the same as that in 1, and the μ2-bridging coordination mode. A similar motif was found in the isostructural heterometallic coordination polymer Pr-Zn reported by Yan et al.12 Thus, two distinct types of building blocks, Eu(pydc)(H2O)4 and Cu(pydc)2, are linked together by pydc ligands to form a 2D network structure (Figure 6). In the crystal structure of 3, on the basis of the topological approach of the simplification principle, Eu(III) can be defined as a 3-connected node and Cu(II) can be defined as a 4-connected node, and the

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Figure 2. The Cu(II) layer is sandwiched in the interlayer regions of the two La-K-layers alternately along the b-axis.

Figure 3. The three-dimensional supramolecular architectures of 1.

Figure 4. Projection of the 3D topological structure of 1. Cu, cyan, K, purple, Ln, green, pydc, 3-connect, blue, pydc, 4-connect, yellow.

pydc ligands can be simplified as 3-connected nodes and 2-connected nodes (lines), so that the topological mode of 4 can be described as a (6284)(83)2 topological network, as shown in Figure 7. Hong et al. reported a series of Ln-Cu coordination polymers containing pydc ligands,8e,8f but the structures of 3-5 are different from these, which may be because Hong et al. added 2,20 -bipyridine ligands to their reaction system, resulting in different crystal growth conditions even though 2,20 -bipyridine did not coordinate to the Ln(III) or Cu(II) ions. In the complexes 1-5, the average Ln-O bond length decreased from 2.578 to 2.555 A˚ with increasing atomic number, and the Ln-N bond length decreased from 2.660 to 2.555 A˚ with increasing atomic number among complexes 3-5. These trends may be ascribed to the lanthanide contraction effect. IR, Thermogravimetric Analysis, and PXRD. IR spectra were obtained from powdered samples of complexes 1-5.

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The spectra of 1 and 2 feature broad peaks in the range 3500-3300 cm-1 due to the presence of hydrogen bonding between water molecules. The asymmetric stretching vibrations of the carboxylate groups in 1 and 2 were observed at 1609 and 1621 cm-1 as broad bands, while the symmetric stretching vibration bands were observed at 1396 and 1403 cm-1, respectively. For 3-5, the asymmetric stretching vibrations of the carboxylate groups were observed at 1629, 1596, and 1623 cm-1, and the symmetric stretching vibrations were observed at 1390, 1386, and 1390 cm-1, respectively. The absence of the characteristic band at around 1700 cm-1 in the spectra of 1-5 indicates complete deprotonation of the carboxylate groups of pydc upon reaction with the metal ions. The analyses of the IR spectra of 1-5 are in good agreement with the findings from single-crystal X-ray diffraction. Because complexes 1 and 2 and 3-5 are isomorphic, compounds 1 and 4 were selected to examine the thermal stability. Crystalline samples of compounds 1 and 4 were subjected to TGA in the temperature range 20-800 C (Figure 8).

Figure 5. The coordination environments of Cu(II) and Eu(III) in 4.

Sun et al.

For 1, the first weight loss of 14.19% in the range 25-320 C corresponds to the loss of 10 water molecules (calculated weight loss 15.67%, including one uncoordinated and nine coordinated water molecules). The second weight loss above 320 C corresponds to complete collapse of the coordination network with decomposition of the pydc ligands. The final weight, 33.24% (calculated residual weight 32.42%), suggests that the residue was probably largely composed of La2O3, CuO, and K2O. The TGA curve of compound 4 shows the first weight loss of 13.65% in the range 25-280 C, which corresponds to the loss of one uncoordinated and eight coordinated water molecules (calculated weight loss 13.61%). The weight loss above 280 C corresponds to complete collapse of the coordination network with decomposition of the pydc ligands. The final weight, 35.87% (calculated residual weight 36.30%), suggests that the residue was probably largely composed of Eu2O3 and CuO. The simulated and experimental PXRD patterns of 1 and 4 are shown in Figure 9, while those of 2, 3, and 5 are shown in Figures S1-S3, Supporting Information. The simulated and experimental patterns are in good agreement, confirming the phase purity of the as-synthesized products. Magnetic Properties. The temperature dependences of the magnetic susceptibilities of complexes 1 and 2 were measured under an applied field of 1000 Oe in the temperature range

Figure 7. Projection of the 3D topological structure of 3. Cu, 4connect, purple, Eu, 3-connect, green, pydc, 3- connect, cyan, pydc, 2-connect, line.

Figure 6. Two distinct types of building blocks, Eu(pydc)(H2O)4 and Cu(pydc)2 were linked together by pydc ligands to form a two dimensional nets structure.

Article

Figure 8. TGA curve of 1 and 4.

Figure 9. The simulated and experimental PXRD patterns of 1 and 4.

Figure 10. Plots of χMT versus T for 1 (O) and 2 (Δ).

2-300 K. The plots of χMT versus temperature for complexes 1 and 2 are shown in Figure 10. For 1, the observed χMT value of 0.378 cm3 K mol-1 at room temperature is slightly larger than the theoretical value of 0.375 cm3 K mol-1 for a noninteracting [La(III)Cu(II)2] system. The χMT value increases steadily on decreasing the temperature from 300 to 12 K and reaches a maximum of 0.589 cm3 K mol-1 at 12 K, which implies ferromagnetic coupling between adjacent Cu2þ centers due to the diamagnetic La3þ ions. On lowering the

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Figure 11. Excitation (thin line) and emission (thick line) for 4: Eu3þ. The excitation spectrum was monitored at 615 nm. The emission spectrum was obtained at 395 nm.

temperature further, the χMT value decreases dramatically to 0.558 cm3 K mol-1 at 2 K due to zero-field splitting. The complexes 1 and 2 are isostructural. However, their magnetic properties differ significantly, mainly due to the contribution of the two f electrons and the orbital angular momentum of the Pr3þ ion. The χMT value for 2 at room temperature is 1.104 cm3 K mol-1, which is slightly lower than the expected value (1.175 cm3 K mol-1). In the temperature range from 300 to 150 K, the value of χMT is almost constant, and this behavior is interpreted in terms of the compensation of the ferromagnetic coupling between adjacent Cu2þ ions, which causes the value of χMT to change only slowly over a certain temperature range. On further decreasing the temperature, χMT decreases to a minimum value of 0.440 cm3 K mol-1 at 2 K. In principle, the Stark levels arising from the 3H4 ground state of the Pr3þ ion are thermally populated, and a progressive depopulation of these levels occurs with decreasing temperature. As a result, χMLnT (denoting the molar magnetic susceptibility of Ln3þ ions) decreases on cooling. Consequently, the nature of the interactions between the Pr3þ and Cu2þ cannot be unequivocally deduced purely from the smooth decrease in χMT values on cooling.3b Luminescent Properties. The luminescent properties of 3-5 were investigated in the solid state. The PL emission from 4 (Eu3þ) originates from internal transitions in the 4f6 configuration of Eu3þ (Figure 11). The emissive transitions mainly originate from the 5D0 excited state. The spectrum is dominated by the electric-dipole transition to the 7F2 state, indicative of a lack of inversion symmetry at the Eu3þ site, which is consistent with the crystal structure. Apart from the weak 5D1-7F1 transition, hardly any transitions from the higher 5DJ states are observed, indicating a fast relaxation to the 5D0 excited state. The excitation spectrum consists of several sharp transitions (mainly to the 5L6 and 5D2 levels) and a broadband below 300 nm due to charge transfer. The luminescence decay profile (as shown in Figure 14) is characterized by an exponential decay with a decay constant of 440 μs. The emission spectrum of 5 (Tb3þ) consists of the transitions from 5D4 to 7FJ, with J = 0, 1, ..., 6 (Figure 12). No emission peaks originating from the higher excited states 5DJ, expected to appear in the wavelength region from 400 to 460 nm, were observed. This indicates that a rapid relaxation occurs toward the 5D4 ground level of the excited state. The excitation spectrum is characterized by two broad bands, which are related to the transition from the

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Figure 12. Excitation (thin line) and emission (thick line) for 5: Tb3þ. The excitation spectrum was monitored at 544 nm. The emission spectrum was obtained at 315 nm.

Sun et al.

Figure 15. Decay profile for 5 (Tb3þ). Three exponential decay profiles (dotted lines) were fit to the experimental data (open circles).

materials.14 No decay constants could be determined for the Sm-doped compound, because of the low overall luminescence intensity and the mismatch between the laser wavelength of 337 nm and the excitation spectrum of the Sm-doped compound. Conclusion

Figure 13. Excitation (thin line) and emission (thick line) for 3: Sm3þ. The excitation spectrum was monitored at 643 nm. The emission spectrum was obtained at 403 nm.

In summary, we have synthesized five lanthanide-transition metal coordination polymers by hydrothermal reactions. The complexes 1 and 2 are the first Ln/Cu/K structures containing Cu(II) sandwiched in the interlayer regions of two La/K planes alternately arranged along the b-axis. Moreover, two distinct types of building blocks, Ln(pydc)(H2O)4 and Cu(pydc)2, are linked together by pydc ligands to form a 2D network in the complexes 3-5. Furthermore, complex 1 exhibits a weak ferromagnetic interaction between Cu2þ ions. The complexes 3 (Sm3þ), 4 (Eu3þ), and 5 (Tb3þ) exhibit characteristic lanthanide-centered luminescence. Acknowledgment. This work was conducted in the framework of a project sponsored by the Natural Science Foundation of China (No.20671064), SRF for ROCS, SEM, Liaoning BaiQianWan Talents Program and the Doctor Foundation of Liaoning Province (No. 20071016); P.F.S. is Postdoctoral Fellow of the Research Foundation - Flanders (FWO). Supporting Information Available: The simulated and experimental PXRD patterns of 2, 3, and 5 are shown in Figures S1-S3. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 14. Decay profile for 4 (Eu3þ).

4f8 ground-state configuration to the 4f75d1 excited state. From the luminescence decay profile (Figure 15), at least three components can be discerned, with lifetimes of 0.85, 3.0, and 32 μs. Although the emission of 3 (Sm3þ) is weak compared to those of 4 and 5, several transitions are observed in the green to red region of the visible spectrum (Figure 13). Transitions are observed from the 4G5/2 excited state to the 6HJ level, with J = 5/2, 7/2, 9/2, and 11/2. The excitation spectrum consists of several sharp lines due to internal transitions in the 4f5 configuration. As strong mixing occurs between the multiplet terms,13 no straightforward assignments can be made, although the excitation spectrum is very similar to those of other Sm3þ-doped

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