Effect of Lanthanide Contraction on Crystal Structures of Three

ARTICLE pubs.acs.org/crystal

Effect of Lanthanide Contraction on Crystal Structures of Three-Dimensional Lanthanide Based Metal
Organic Frameworks with Thiophene-2,5-Dicarboxylate and Oxalate Jing Xu,†,§ Jianwen Cheng,‡ Weiping Su,*,† and Maochun Hong† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, the Chinese Academy of Sciences, Fuzhou, Fujian, 350002, P. R. China ‡ Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, Zhejiang, 321004, P. R. China § Graduate School of the Chinese Academy of Sciences, Beijing, 100039, P. R. China

bS Supporting Information ABSTRACT: Six three-dimensional (3D) lanthanide coordination polymers with formula [Ln(TDC)(ox)0.5(H2O)2] 3 (H2O) (1Ln) (Ln = Pr 1, Nd 2, Sm 3, Gd 4) and [Ln(TDC)(ox)0.5(H2O)2] 3 (H2O) (2Ln) (Ln = Dy 5, Er 6) have been synthesized by hydrothermal reactions of thiophene-2,5-dicarboxylate (TDC) and oxalate (ox) with corresponding lanthanide nitrate. X-ray crystal structure analyses reveal that 1Ln and 2Ln are isomers that may be produced by lanthanide contraction. Compounds 1
4 are isostructural and possess binodal (4,5)-connected 3D framework of tcj/hc topology with triangular channels along the [101] direction. Isostructural compounds 5 and 6 present trinodal (4,4,6)-connected 3D framework of sqc254 topology with rectangular channels along the b axis. The magnetic properties reveal that compound 2 has antiferromagnetic behavior, while compound 4 has ferromagnetic behavior. The luminescent property shows that compound 5 displays intense yellow luminescence and exhibits the typical Dy3þ ion emission. Furthermore, infrared (IR), thermogravimetric analyses (TGA), elemental analyses (EA), powder X-ray diffraction (PXRD), magnetic and luminescent properties of these compounds are also investigated.

’ INTRODUCTION The design and construction of metal
organic frameworks (MOFs) have provoked great interest because of their fascinating network topologies1 and potential applications in magnetism,2 luminescence,3 gas storage and separation,4 catalysis,5 and nonlinear optics (NLO).6 However, most of this work has so far focused on the rational design of multidimensional d-block MOFs,7 and the analogous chemistry of lanthanide (Ln) remains less developed, although some interesting findings have been reported.8
10 This is ascribed to the high coordination number and flexible coordination geometry of lanthanides, which may cause difficulty in controlling the synthetic reactions.11 Nevertheless, lanthanides, with special luminescent3b and magnetic3a,12 properties resulting from 4f electrons, are good candidates to construct functional compounds with specific properties and desired features. In addition, the ionic radius of the Ln ions decrease with increasing atomic number, which may influence the coordination number and lead to diversity in crystal structure. Though it is interesting to investigate how the lanthanide contraction works in crystal-structure formation, less attention has been paid to the influence of Ln ions’ radius on the diversity of Ln-based MOFs (LnMOFs) to date.13 It is well-known that Ln ions have high affinity and prefer to bind to hard donor atoms, thus multidentate ligands with oxygen r 2011 American Chemical Society

or hybrid oxygen
nitrogen atoms, such as pyridinecarboxylate,14 imidazoledicarboxylate,8e,10b
10d,15 and benzenepolycarboxylate,9c,10a,16 are widely used in the construction of lanthanide-containing coordination polymers. Up to now, most of the LnMOFs are built from just one type of multicarboxylate ligand. If a coligand was to be introduced, the cooperativity of both ligands may lead to the formation of unforeseen LnMOFs.17 Taking account of the above, we chose two types of thiophene2,5-dicarboxylic acid (H2TDC)/oxalate as mixed ligands to construct LnMOFs, based on the following considerations: (1) H2TDC, as a multidentate ligand with two carboxylate groups with a “V-shaped” configuration, shows various coordination modes, which makes it a useful bridge in the construction of coordination polymers;18 (2) oxalate, as one of the simplest bisbidentate connectors, can facilitate the formation of extended structures by bridging metal centers.19 Accordingly, we are inspired to explore and synthesize new LnMOFs with mixed ligands and different lanthanide ions. In this contribution, we report six mixed dicarboxylate ligandsbased 3D Ln coordination polymers of the same formula, Received: December 31, 2010 Revised: March 21, 2011 Published: April 14, 2011 2294

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Table 1. Crystal Data and Structure Refinements for 1
6 compound

2

3

4

5

6 C7H8ErO9S

formula

C7H8PrO9S

C7H8NdO9S

C7H8SmO9S

C7H8GdO9S

C7H8DyO9S

Mr

409.10

412.43

418.54

425.44

430.69

435.45

crystal system

monoclinic

monoclinic

monoclinic

monoclinic

monoclinic

monoclinic C2/c

space group

P21/c

P21/c

P21/c

P21/c

C2/c

a [Å]

8.619(2)

8.591(2)

8.554(2)

8.531(2)

19.2794(14)

19.25(2)

b [Å]

17.959(3)

17.914(3)

17.844(5)

17.790(4)

9.8604(5)

9.796(9)

c [Å]

8.135(2)

8.1002(18)

8.026(2)

7.979(2)

12.4549(10)

12.388(13)

β [deg] V [Å3]

110.828(14) 1177.0(5)

110.854(14) 1165.0(4)

110.587(3) 1146.9(5)

110.334(4) 1135.4(5)

106.871(4) 2265.8(3)

106.392(17) 2241(4)

Z

4

4

4

4

8

8

F [g cm
3]

2.309

2.352

2.424

2.489

2.525

2.582

μ [mm
1]

4.353

4.673

5.340

6.063

6.818

7.716

F(000)

788

792

800

808

1632

1648

GOF (F2)

1.217

1.152

1.095

1.151

1.117

1.049

collected reflns

9181

9063

8908

8884

8724

8439

unique reflns Rint

2705 0.0447

2668 0.0344

2626 0.0569

2577 0.0380

2555 0.0331

2538 0.0584

observed reflns

2584

2555

2421

2479

2263

1777

parameters

164

164

164

164

159

159

R1a [I > 2σ(I)]

0.0459

0.0358

0.0412

0.0383

0.0368

0.0419

R2b [I > 2σ(I)]

0.1206

0.1013

0.1060

0.1074

0.0923

0.1251

)

R1 = Σ Fo|
|Fc /Σ|Fo|. b R2 = {Σ[w(Fo2
Fc2)2]/Σ[w(Fo2)2]}1/2. )

a

1

[Ln(TDC)(ox)0.5(H2O)2] 3 (H2O) (1Ln Ln = Pr 1, Nd 2, Sm 3, Gd 4; 2Ln Dy 5, Er 6; TDC = thiophene-2,5-dicarboxylate, ox = oxalate), with two distinct structure types. 1Ln and 2Ln are isomers, in which 1Ln possesses binodal (4,5)-connected tcj/hc net, while 2Ln adopts trinodal (4,4,6)-connected sqc254 net. These compounds represent good examples of tuning crystal structures by the Ln contraction effect.

’ EXPERIMENTAL SECTION Materials and Methods. All chemicals except Dy(NO3)3 3 6H2O were purchased commercially and used without further purification. Dy(NO3)3 3 6H2O was prepared by dissolving Dy2O3 with (1:1 v/v) HNO3, and then evaporating at 100
C until the crystal film formed. The hydrothermal reaction was performed in a 25 mL Teflon-lined stainless steel autoclave under autogenous pressure. Infrared spectra were recorded on a Magna 750 FT-IR spectrometer using KBr pellets. C, H, and S microanalyses were measured with an elemental Vairo EL analyzer. Powered X-ray diffraction (PXRD) patterns of the samples were recorded in the 2θ = 5
50
range by a desktop X-ray diffractometer (RIGAKU-Miniflex II) with Cu KR radiation. The generator voltage is 30 kV and the tube current is 15 mA. Thermogravimetric analyses (TGA) were performed with a heating rate of 15
C/min in N2 atmosphere using a NETZSCH STA 449C simultaneous TG-DSC instrument. Fluorescence spectrum was measured at room temperature on an Edinburgh FL-FS920 TCSPC system. The polycrystalline magnetic susceptibility data was collected on a Quantum Design PPMS model 6000 magnetometer in the temperature range from 2 to 300 K at an external magnetic field of 1 KOe. The diamagnetic corrections were estimated with Pascal’s constants for all the complexes. Synthesis of [Pr(TDC)(ox)0.5(H2O)2] 3 (H2O) (1). A mixture of Pr(NO3)3 3 6H2O (0.25 mmol, 0.1088 g), Na2C2O4 (0.25 mmol, 0.0335 g), H2TDC (0.25 mmol, 0.0425 g), and H2O (5 mL) was sealed in a 25 mL Teflon-lined autoclave at 140
C for 4 days and then cooled to room temperature. Light-green prismatic crystals of 1 were obtained

(yield: 68% based on Pr(NO3)3 3 6H2O). Anal. Calcd for C7H8PrO9S (409.10): C, 20.55; H, 1.97; S, 7.84. Found: C, 20.50; H, 1.78; S, 7.96. IR spectrum (KBr pellet, ν/cm
1): 3598(m), 3515(w), 3307(vs), 3123(w), 1655(s), 1573(s), 1544(s), 1505(s), 1379(s), 1316(w), 1118(w), 1040(w), 861(w), 803(m), 779(m), 711(w), 676(m), 604(w), 537(w), 498(m), 464(m). Synthesis of [Nd(TDC)(ox)0.5(H2O)2] 3 (H2O) (2). Compound 2 was synthesized by a procedure similar to that of 1, except Nd(NO3)3 3 5H2O (0.25 mmol, 0.0826 g) replaced Pr(NO3)3 3 6H2O. Light-purple prismatic crystals of 2 were obtained (yield: 55% based on Nd(NO3)3 3 5H2O). Anal. Calcd for C7H8NdO9S (412.43): C, 20.38; H, 1.96; S, 7.77. Found: C, 20.41; H, 1.72; S, 7.94. IR spectrum (KBr pellet, ν/cm
1): 3597(m), 3514(w), 3315(vs), 3121(w), 1654(s), 1575(s), 1540(s), 1508(s), 1372(s), 1314(w), 1115(w), 1041(w), 862(w), 804(m), 770(m), 711(w), 677(m), 605(w), 537(w), 496(m), 465(m). Synthesis of [Sm(TDC)(ox)0.5(H2O)2] 3 (H2O) (3). Compound 3 was synthesized by a procedure similar to that of 1, except Sm(NO3)3 3 6H2O (0.25 mmol, 0.1111 g) replaced Pr(NO3)3 3 6H2O. Light-yellow prismatic crystals of 3 were obtained (yield: 47.8% based on Sm(NO3)3 3 6H2O). Anal. Calc. for C7H8SmO9S (418.54): C, 20.09; H, 1.93; S, 7.66. Found: C, 20.12; H, 1.76; S, 7.83. IR spectrum (KBr pellet, ν/cm
1): 3600(m), 3525(w), 3323(vs), 3130(w), 1660(s), 1576(s), 1542(s), 1509(s), 1374(s), 1315(w), 1115(w), 1038(w), 862(w), 803(m), 769(m), 711(w), 677(m), 601(w), 542(w), 493(m), 467(m). Synthesis of [Gd(TDC)(ox)0.5(H2O)2] 3 (H2O) (4). Compound 4 was synthesized by a procedure similar to that of 1, except Gd(NO3)3 3 6H2O (0.25 mmol, 0.1128 g) replaced Pr(NO3)3 3 6H2O. Colorless prismatic crystals of 4 were obtained (yield: 47% based on Gd(NO3)3 3 6H2O). Anal. Calcd for C7H8GdO9S (425.44): C, 19.76; H, 1.90; S, 7.54. Found: C, 19.71; H, 1.55; S, 7.71. IR spectrum (KBr pellet, ν/cm
1): 3597(m), 3514(w), 3330(vs), 3203(s), 3123(w), 1654(s), 1575(s), 1546(s), 1507(s), 1381(s), 1314(w), 1117(w), 1041(w), 862(w), 804(m), 773(m), 711(w), 677(m), 605(w), 536(w), 493(m), 464(m). Synthesis of [Dy(TDC)(ox)0.5(H2O)2] 3 (H2O) (5). Compound 5 was synthesized by a procedure similar to that of 1, except 2295

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Table 2. Selected Bond Lengths (Å) for 1a and 5b

Table 3. Hydrogen Bond Lengths (Å) and Bond Angles (deg) in 1a and 5b

compound 1 Pr
O(1)#3

2.448(4)

Pr
O(2)

compound 1

2.498(4)

Pr
O(3)#2

2.388(5)

Pr
O(5)

2.534(4)

Pr
O(4)#1

2.371(5)

Pr
O(7)

2.533(4)

Pr
O(6)#4

2.509(5)

Pr
O(8)

2.619(5)

D—H 3 3 3 A

compound 5 Dy(1)
O(1)

2.281(4)

Dy(1)
O(1)#1

2.281(4)

Dy(1)
O(5)

2.432(4)

Dy(1)
O(4)#2

2.362(4)

Dy(1)
O(5)#1 Dy(1)
O(4)#3

2.432(4) 2.362(4)

Dy(1)
O(6)#5 Dy(1)
O(6)#4

2.403(4) 2.403(4)

Dy(2)
O(3)

2.292(4)

Dy(2)
O(2)#7

2.326(4)

Dy(2)
O(2)#3

2.326(4)

Dy(2)
O(3)#6

2.292(4)

Dy(2)
O(7)

2.472(4)

Dy(2)
O(7)#6

2.472(4)

Dy(2)
O(8)

2.400(4)

Dy(2)
O(8)#6

2.400(4)

Symmetry codes: 1: #1
x, y
1/2,
z þ 3/2; #2 x þ 1,
y þ 3/2, z þ 1/2; #3
x,
y þ 1,
z þ 1; #4
x,
y þ 1,
z þ 2. b Symmetry codes: 5: #1
x, y,
z þ 3/2; #2 x
1/2, y
1/2, z; #3
x þ 1/2, y
1/2,
z þ 3/2; #4
x,
y,
z þ 1; #5 x,
y, z þ 1/2; #6
x þ 1, y,
z þ 3/2; #7 x þ 1/2, y
1/2, z. a

Dy(NO3)3 3 6H2O (0.25 mmol, 0.1141 g) replaced Pr(NO3)3 3 6H2O. Colorless prismatic crystals of 5 were obtained (yield: 51% based on Dy(NO3)3 3 6H2O). Anal. Calcd for C7H8DyO9S (430.69): C, 19.52; H, 1.87; S, 7.44. Found: C, 19.48; H, 1.56; S, 7.57. IR spectrum (KBr pellet, ν/cm
1): 3399(vs), 2934(w), 1631(s), 1554(s), 1520(s), 1379(s), 1316(m), 1123(w), 1035(w), 842(w), 793(s), 769(s), 677(w), 546(vw), 498(m), 464(m). Synthesis of [Er(TDC)(ox)0.5(H2O)2] 3 (H2O) (6). Compound 6 was synthesized by a procedure similar to that of 1, except Er(NO3)3 3 5H2O (0.25 mmol, 0.1108 g) replaced Pr(NO3)3 3 6H2O. Pink prismatic crystals of 6 were obtained (yield: 55% based on Er(NO3)3 3 5H2O). Anal. Calc. for C7H8ErO9S (435.45): C, 19.31; H, 1.85; S, 7.36. Found: C, 19.07; H, 1.54; S, 7.48. IR spectrum (KBr pellet, ν/cm
1): 3435(vs), 1611(vs), 3398(vs), 2932(w), 1630(s), 1556(s), 1522(s), 1381(s), 1314(m), 1124(w), 1033(w), 843(w), 794(s), 769(s), 677(w), 546(vw), 495(m), 464(m). Crystal Structural Determination. X-ray data of compounds 1
6 were collected on a SCXmini CCD diffractometer equipped with a graphite-monochromated MoKR (λ = 0.71073 Å) radiation using an ω scan mode at 293 K. An empirical absorption correction was applied using the SADABS program.20 All structures were solved by direct methods and refined by full-matrix least-squares on F2 using the SHELXS-97 program package.21 All non-hydrogen atoms were refined anisotropically except the oxygen atoms of the free water molecules in 5 and 6. All the hydrogen atoms belonging to the water molecules were placed by CACL-OH of the WinGX program package. The other hydrogen atoms were located geometrically and treated as riding. A summary of the crystallographic data of 1
6 is listed in Table 1. The selected bond lengths and hydrogen bond parameters of 1 and 5 are given in Tables 2 and 3. The selected bond lengths and hydrogen bond parameters of 2, 3, 4, and 6 are listed in Tables S1 and S2 (Supporting Information), respectively. CCDC-790201 (1), CCDC-791995 (2), CCDC-812828 (3), CCDC-791994 (4), CCDC-793683 (5), and CCDC-793682 (6) contain the crystallographic data in CIF format.

’ RESULTS AND DISCUSSION Synthesis and Characterization. 1Ln and 2Ln were obtained in the same conditions by the hydrothermal reactions of lanthanide

d(D—H) d(H 3 3 3 A) d(D 3 3 3 A) — (DHA)

O(7)—H(7B) 3 3 3 O(6)#5 O(8)—H(8A) 3 3 3 O(5)#6

0.85 0.85

1.90 2.01

2.740(6) 2.855(7)

167.7 170.7

O(9)—H(9A) 3 3 3 O(7)

0.85

2.22

2.799(8)

125.7

compound 5 D—H 3 3 3 A O(7)—H(7B) 3 3 3 O(6)#8 O(8)—H(8A) 3 3 3 O(5)#7 O(9)—H(9A) 3 3 3 S#3 O(9)—H(9B) 3 3 3 S

d(D—H) d(H 3 3 3 A) d(D 3 3 3 A) — (DHA) 0.85

2.08

2.911(6)

164.3

0.85

2.02

2.847(5)

163.4

0.85

2.81

3.403(12)

128.7

0.85

2.78

3.574(12)

155.8

Symmetry codes: 1: #5 x, y, z
1; #6
x þ 1,
y þ 1,
z þ 2. b Symmetry codes: 2: #3
x þ 1/2, y
1/2,
z þ 3/2; #7 x þ 1/2, y
1/2, z; #8
x þ 1/2,
y þ 1/2,
z þ 1. a

Figure 1. Local coordination environment of 1 with 30% thermal ellipsoids. All hydrogen atoms are omitted for clarity. Symmetry codes for the generated atoms are the same as Table 2.

nitrates with thiophene-2,5-dicarboxylate and oxalate ligands in water at 140
C. X-ray crystallographic studies revealed that 1Ln and 2Ln are isomers with two distinct structure types with decreasing lanthanide radius. 1Ln and 2Ln are air-stable and insoluble in water and common organic solvents, such as chloroform, toluene, acetonitrile, DMF, methanol, and ethanol. The IR spectra of 1
6 are similar. The strong and broad absorption bands in the range of 3000
3700 cm
1 in compounds 1
6 are assigned as characteristic peaks of O
H vibration. The strong vibrations appearing around 1600 and 1400 cm
1 correspond to the asymmetric and symmetric stretching vibrations of the carboxylate group, respectively. The absence of strong absorption bands ranging from 1690 to 1730 cm
1 indicates TDC and oxalate ligands are deproponated. The middle and narrow bands in the range of 1000
1470 cm
1 are attributed to C
C vibrations. The δO
C
O vibration in plane occurs in middle intensity peaks in the range of 675
865 cm
1. The IR spectra of 1
6 are in accordance with the results of the X-ray diffraction analysis (Figures S1
S3, Supporting Information). Crystal Structure of [Ln(TDC)(ox)0.5(H2O)2] 3 (H2O) (1Ln) (Ln = Pr 1, Nd 2, Sm 3, Gd 4). X-ray structural analyses reveal that compounds 1
4 are isostructural and crystallize in the monoclinic P21/c space group. As a representative example, the crystal structure of 1 is described in detail. The asymmetric unit of 1 contains one unique Pr3þ ion, one TDC ligand, one-half 2296

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Scheme 1. Coordination Modes of the TDC and Oxalate Ligands Observed in 1
6

Figure 3. (a) TDC ligand regarded as an organic four-connected node linked with four Pr3þ ions (Pr nodes, green; TDC nodes, orange). (b) Inorganic five-connected Pr3þ node coordinated with four TDC ligands and one [Pr(ox)]þ unit. (c) Schematic representation of (4,5)-connected net with tcj/hc topology along the [101] direction for 1.

Figure 2. (a) 1D double zigzag chain along the c axis. (b) View of the 3D framework of 1 along the [101] direction.

oxalate ligand, and two coordinated and one guest water molecules. As shown in Figure 1, the Pr atom is eight-coordinated and has distorted bicapped trigonal prism coordination environments: four carboxylate oxygen atoms (OCOO
) from four TDC ligands, two OCOO
from one oxalate ligand, together with two coordinated water molecules (Figure S4, Supporting Information). The Pr
O bond lengths vary from 2.371(5) to 2.619(5) Å (Table 2) and the Pr
O
Pr bond angles are in the range of 64.11(14)80.79(16)
. Owing to the effect of lanthanide contraction, the Nd
O bonds in 2, the Sm
O bonds in 3, and Gd
O bonds in 4 (Table S1, Supporting Information) are slightly shorter than the corresponding Pr
O bonds in 1. Although the TDC and oxalate ligands have more than eight18 and seventeen19 coordination modes, respectively, the TDC ligand adopts only a single μ4-(η1, η1)-(η1, η1) bis-bidentate coordination mode by connecting four Pr3þ ions and the oxalate ligand links two Pr3þ ions adopting a single tetradentate μ2-η1, η1, η1, η1 coordination mode (Scheme 1) in 1, which indicates the high cooperativity between the TDC and oxalate ligands in the formation of 1. The Pr centers are double-bridged by bidentate carboxylate groups from TDC ligands to generate a one-dimensional (1D) double infinite zigzag chain with the Pr 3 3 3 Pr distances being 4.470(1) and 5.233(1) Å, respectively (Figure 2a). These zigzag chains are further bridged through

TDC and oxalate ligands forming a 3D framework with 1D triangular channels along the [101] direction (Figure 2b). The guest water molecules occupy the void interspace region and interact with the coordinated water molecules of frameworks (Table 3, Figure S5 in Supporting Information). PLATON calculated suggested a solvent-accessible volume of 96.2 Å3 (approximately 8.2% of unit cell) by excluding the guest water molecules. In a N2 sorption experiment, no N2 sorption was observed at 77 K, thus indicating the solvent-accessible channel is too small. Network topological approach has been proven to be an important and essential tool for design and analysis of MOFs.22 By reducing complicated MOFs to simple node-and-connector reference nets, topological approach has been extensively used in the analyses of structures in recent years.23 A better insight into the nature of this intricate 3D framework can be achieved by the application of topological approach. In this case, the oxalate ligands can be viewed as connectors; the TDC ligands act as fourconnected nodes (Figure 3a) and the Pr atoms function as fiveconnected nodes (Figure 3b). Thus, the 3D framework can be reduced to a binodal (4,5)-connected net (Figure 3c). Further analyzed by TOPOS program,24 the vertex symbols of TDC/Pr nodes are 4 3 4 3 5 3 5 3 7 3 7 and 4 3 4 3 5 3 5 3 5 3 7 3 7 3 72 3 82, respectively, and the 3D topological network of 1 has the Schl€afli symbol of (42 3 52 3 72)(42 3 53 3 75), which is a typical tcj/hc topological net. Structures of [Ln(TDC)(ox)0.5(H2O)2] 3 (H2O) (2Ln) (Ln = Dy 5, Er 6). X-ray structural analyses reveal that compounds 5 and 6 are isostructural and crystallize in the monoclinic C2/c space group. Therefore, only the structure of 5 is described in detail. There are two half Dy3þ ions, one TDC ligand, one-half oxalate ligand, two coordinated and one guest water molecules in the asymmetric unit of 5. As shown in Figure 4, Dy(1) and Dy(2) atoms are 2297

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Figure 4. Local coordination environment of 5 with 30% thermal ellipsoids. All hydrogen atoms are omitted for clarity. Symmetry codes for the generated atoms are the same as Table 2.

Figure 5. (a) 1D double zigzag chain along the b axis. (b) View of the 3D framework of 5 in the ac plane. Color codes: Dy1, green; Dy2, cyan.

both eight-coordinated with distorted bicapped trigonal prism geometries: four OCOO
from four TDC ligands and four OCOO
from two oxalate ligands for Dy(1) (Figure S6, Supporting Information); four OCOO
from four TDC ligands and four coordinated water molecules for Dy(2) (Figure S7, Supporting Information). The Dy
O bond lengths are in the range of 2.281(4)
2.472(4) Å (Table 2) and the Dy
O
Dy bond angles vary from 67.44(12) to 90.62(14)
. Because of the effect of lanthanide contraction, the Er
O bonds in 6 (Table S1, Supporting Information) are slightly shorter than the corresponding Dy
O bonds in 5. Because of the high cooperativity between TDC and oxalate ligands, both TDC and oxalate ligands in 5 adopt the same coordination mode as described in 1. Each of TDC ligands connects two Dy(1)3þ and two Dy(2)3þ ions and adopts a μ4-(η1, η1)-(η1, η1) bis-bidentate coordination mode. The oxalate ligand acts as tetradentate chelating ligand linking two Dy(1)3þ ions. The Dy(1) and Dy(2) centers are double bridged by bidentate carboxylate groups from TDC ligands also to produce a 1D double infinite zigzag chain (Figure 5a), which differs from the 1D chain in 1. The Dy(1) 3 3 3 Dy(2) distances separated by bridging TDC ligands are 4.922 and 4.939 Å, respectively. Then these chains are further connected by TDC and oxalate ligands to give rise to a 3D framework with rectangular channels along the b axis (Figure 5b). The guest water

Figure 6. (a) TDC ligand regarded as an organic four-connected node linked with two Dy(1)3þ and two Dy(2)3þ ions (Dy(1) nodes, green; Dy(2) nodes, cyan; TDC nodes, orange). (b) Inorganic four-connected Dy(2)3þ node coordinated with four TDC ligands. (c) Inorganic sixconnected Dy(1)3þ node coupled with four TDC ligands and two [Dy(1)(ox)]þ units. (d) Schematic representation of (4,4,6)-connected net with sqc254 topology along the b axis for 5.

molecules occupy the void of the interspace region and interact with the S atoms of TDC ligands (Table 3, Figure S8 in Supporting Information). PLATON calculated suggested a solventaccessible volume of 232.0 Å3 (approximately 10.2% of unit cell) by excluding the guest water molecules. The N2 sorption isotherm of 5 did not indicated any appreciable amount of adsorption at 77 K, presumably because of the limited size of 5. To better understand the structure of 5, topological analysis has also been used as the simplification principle in 1. From the topological point of view, the oxalate ligands in 5 act as connectors; the TDC ligands can be viewed as four-connected nodes (Figure 6a) and Dy(2)/Dy(1) atoms as four/six-connected nodes (Figure 6b, c), respectively. Therefore, the 3D framework of 5 can be abstracted as a trinodal (4,4,6)-connected topological net (Figure 6d). Further analyzed by TOPOS program,24 the vertex symbols of TDC/Dy(2)/Dy(1) nodes are 4 3 4 3 4 3 4 3 62 3 64, 4 3 4 3 4 3 4 3 62 3 62, and 4 3 4 3 4 3 4 3 6 3 6 3 6 3 6 3 6 3 6 3 6 3 6 3 62 3 62 3 84, respectively. Thus, the resulting net of 5 has the (44 3 62)2(44 3 62)(44 3 610 3 8) Schl€afli symbol, which is a typical sqc254 topological net. Lanthanide Contraction and Structural Diversity. As 1Ln and 2Ln were synthesized from the same starting materials and same reaction conditions, two isomers of 1Ln and 2Ln obtained provide a fair assessment of the critical influence of lanthanide contraction. In these compounds, both TDC and oxalate ligands adopt the same coordination mode and the Ln ions in compounds 2298

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Figure 7. Variable-temperature PXRD patterns for 1 (a) and 5 (b).

1
6 are all trivalent and eight-coordinated. However, the Ln ions adopt different coordination environment in these compounds, in 1Ln, Ln ion is coordinated with two coordinated water molecules, four TDC and one oxalate ligands; in 2Ln, Ln1 ion is coordinated with four TDC and two oxalate ligands, while Ln2 ion is coordinated with four TDC ligands and four coordinated water molecules, it may be the lanthanide contraction effect that leads to the different crystal structures. Moreover, we have also compared the average Ln-O bond lengths among the compounds 1-6. The average lengths between lanthanide and O atoms are decreasing continuously from 2.488 (1), 2.471 (2), 2.440 (3), 2.413 (4), 2.371(5) to 2.351 Å (6). PXRD Patterns and Thermal Properties. The synthesized products of compounds 1
6 have been characterized by PXRD (Figure S9, Supporting Information). The experimental PXRD patterns’ peak positions correspond well with the results simulated from the single crystal data, indicating the high purity of the synthesized samples. The difference in reflection intensities between the simulated and experimental patterns was due to the variation in preferred orientation of the powder samples during the collection of the experimental PXRD data. To study the thermal stabilities of compounds 1
6, TGA in N2 atmosphere with a heating rate of 15
C/min were performed on polycrystalline samples to determine their thermal stabilities from 30 to 1000
C (Figures S10
S12, Supporting Information). Compounds 1
6 show simple thermal behavior and merely undergo two steps of weight loss. Owing to the similar structures of 1
4 and the similarity of 5 and 6, compounds 1 and 5 were selected to examine the experimental variable-temperature PXRD after calcination at the temperature range of RT
150
C. As

ARTICLE

Figure 8. (a) Temperature dependence of χMT and χM
1 for 2 at 1 KOe between 2 and 300 K. (b) Temperature dependence of χMT and χM
1 for 4 at 1 KOe between 2 and 300 K.

shown in Figure 7, at 130
C, the experimental PXRD patterns of 1 and 5 are still in good agreement with the simulated PXRD patterns of 1 and 5, respectively, indicating that the crystal lattice of 1 and 5 remain intact at 130
C. In other words, the departure of the guest water and coordinated water molecules does not lead to an obvious phase transformation. When the samples of 1 and 5 are heated at 150
C, the structures of 1 and 5 begin to change and form amorphous phase. The guest water and two coordinated water molecules were completely lost at 265
C for 1 (calcd/found = 13.2/12.61%), for 2 (calcd/found = 13.1/13.69%), for 3 (calcd/ found = 12.9/12.95%), for 4 (calcd/found = 12.7/12.78%), for 5 (calcd/found = 12.54/13.26%), for 6 (calcd/found = 12.4/13.9%), respectively. Above this temperature, the weight loss is due to the decomposition of the TDC and oxalate ligands and the collapse of the whole frameworks. Magnetic Properties. The temperature-dependent magnetic susceptibility measurements of compounds 2 and 4 have been performed on the polycrystalline samples in the temperature range 2
300 K at an external field of 1 KOe with a Quantum Design PPMS model 6000 magnetometer. The experimental susceptibilities were corrected for Pascal’s constants. The temperature dependencies of the magnetic susceptibilities in the form of χMT and χM
1 versus T for compounds 2 and 4 are given in Figure 8. For 2, as shown in Figure 8a, at 300 K, the experimental value of χMT is 1.74 cm3 3 K 3 mol
1, as expected for one isolated Nd3þ ions (1.64 cm3 3 K 3 mol
1) with a 4I9/2 ground state.2a As the temperature is lowered, χMT decreases to a minimum at about 6 K and then increases rapidly to reach 0.79 cm3 3 K 3 mol
1 at 2 K. The continuous decrease of χMT with higher temperature region may be due to the depopulation of the 2299

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ARTICLE

’ ASSOCIATED CONTENT

bS

Supporting Information. CIF of compounds 1
6, Figures S1
S12, and selected bond lengths and hydrogen bond tables for 2, 3, 4, and 6. This information is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: (þ86) 591-8377-1575.

Figure 9. Solid-state emission spectrum of 5 at room temperature.

Stark levels for a single Nd3þ ion or possible antiferromagnetic interactions between Nd3þ ions,25 while the increase at lower temperature may be attributed to ferromagnetic interactions between Nd3þ ions in the carboxylate-bridged chain. The plot of χM
1 versus T obeys the Curie
Weiss law [χM = C/(T
θ)] with C = 1.95 cm3 3 K 3 mol
1 and θ =
46.67 K. The negative value of θ also gives the evidence of antiferromagnetic interactions existing in 2. For 4, as shown in Figure 8b, at 300 K, the experimental value of χMT is 7.88 cm3 3 K 3 mol
1, equal to the expected value of 7.88 cm3 3 K 3 mol
1 for one isolated Gd3þ ion with an 8S7/2 ground state.2a As the temperature decreased to ∼13 K, χMT increased smoothly with a following decrease. Upon further cooling, the χMT values continuously decrease and reach 7.55 cm3 3 K 3 mol
1 at 2 K. The plot of χM
1 versus T over the temperature range 30
300 K obeys the Curie
Weiss law [χM = C/(T
θ)] with C = 7.86 cm3 3 K 3 mol
1 and θ = 1.06 K. The increase of χMT with the higher temperature range coupled with the positive small θ value indicates the very weak ferromagnetic interactions between Gd3þ ions.26 Luminescent Property. The lanthanide compounds have shown good luminescent properties for their high color purity with high quantum efficiency,27 so the solid-state luminescent property of 5 was investigated at room temperature. The excitation wavelength was selected as the maximum of the solid-state excitation spectrum. Under excitation of 310 nm, compound 5 displays intense yellow luminescence (Figure 9) and exhibits the typical Dy3þ ion emission. The emission at 483, 574, and 665 nm is attributed to the characteristic emission of 4F9/2 f 6HJ (J = 15/ 2, 13/2, and 11/2) transitions of Dy3þ ion.3b It is obvious that the intensity of the yellow emission, corresponding to the 4F9/2 f 6 H13/2 transition, is much stronger than the blue emission of 4 F9/2 f 6H15/2. This indicates that the H2TDC ligand is suitable for the sensitization of yellow luminescence for Dy3þ ion.

’ CONCLUSIONS In summary, six 3D lanthanide coordination polymers constructed from mixed TDC and oxalate ligands have been successfully synthesized under hydrothermal conditions. 1Ln and 2Ln are isomers that may be produced by lanthanide contraction. 1Ln displays binodal (4,5)-connected 3D MOFs of tcj/hc topology, while 2Ln shows 3D MOFs of trinodal (4,4,6)-connected sqc254 topology. The successful isolation of these compounds provides a good example of tuning crystal structure by lanthanide contraction effect.

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