Lanthanide Contraction and Temperature-Dependent Structures of

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DOI: 10.1021/cg100196j

Lanthanide Contraction and Temperature-Dependent Structures of Lanthanide Coordination Polymers with Imidazole-4,5-Dicarboxylate and Oxalate

2010, Vol. 10 4310–4318

Wen-Guan Lu,†,‡ Long Jiang,† and Tong-Bu Lu*,† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, and School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China, and ‡Department of Chemistry, Shaoguan University, Shaoguan, 512005, China Received February 7, 2010; Revised Manuscript Received August 29, 2010

ABSTRACT: Three series of two-dimensional (2D) lanthanide coordination polymers of [Ln(HIDC)(OX)0.5(H2O)2]n 3 (H2O)n (1Ln) (Ln = Pr and Nd), [Ln(HIDC)(OX)0.5(H2O)2]n 3 (H2O)n (2Ln) (Ln = Eu and Tb), and [Ln(HIDC)(OX)0.5(H2O)]n (3Ln) (Ln = Pr, Nd, Eu, and Tb) have been prepared by reacting Ln(NO3)3 3 6H2O with imidazole-4,5-dicarboxylic acid (H3IDC) and oxalic acid (H2OX) at different temperatures under hydrothermal conditions. The results of structural analyses indicate that 1Ln and 2Ln are supramolecular isomers, in which 1Ln displays a 2D monolayer structure of [Ln(HIDC)(OX)0.5(H2O)2]n with a (4.52)(4.53.72) topology network, while 2Ln shows a different 2D monolayer structure containing alternating one-dimensional (1D) left-handed and right-handed helical chains of [Ln(HIDC)]¥ bridged by OX2-. 3Ln has a 2D double-layer structure consisting of oxalate pillared [Ln(HIDC)(H2O)]nnþ layers. The effect of lanthanide contraction and of temperature on the structures of 1Ln-3Ln was investigated. The structure related luminescent properties of 2Eu and 3Eu, as well as 2Tb and 3Tb, were also examined.

Introduction Recently, many lanthanide coordination polymers have been designed and constructed due to their superior functional properties and variety of potential applications.1-3 In comparison with transition metal ions, lanthanide ions have higher coordination numbers and more flexible coordination geometries, which make it even more difficult to control the structures. Thus, to be able to rationally design and construct lanthanide coordination polymers with predicted geometries is still a great challenge, as many factors can affect the overall structural formation. In addition to the choice of ligands and metal ions, reacting conditions such as solvents, reacting time, and temperature can also affect the final structure. Investigations on how these factors affect the structures will help us understand which reaction conditions afford lanthanide coordination polymers of expected structures. We are interested in the coordination chemistry of imidazole-4,5-dicarboxylic acid (H3IDC), a rigid planar ligand with multiple coordination sites, which has been proven to be useful in the construction of coordination polymers. This is because H3IDC can be partially or fully deprotonated allowing for the generation of coordination polymers with different structures, interesting topologies and properties.4-9 Recently, we used H3IDC and oxalic acid (H2OX) as building blocks to react with Ln(NO3)3 3 6H2O under basic conditions (pH = 8) to generate three fully deprotonated three-dimensional (3D) lanthanide coordination polymers of {K5[Ln5(IDC)4(OX)4]}n 3 20nH2O (Ln = Gd, Tb, and Dy) with one-dimensional (1D) channels containing Kþ ions, in which the luminescent properties can be modified through the exchange of the Kþ ions with *To whom correspondence should be addressed. Fax: þ86-20-84112921. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 09/13/2010

various cations.9c However, when the reaction was carried out under slightly acidic conditions (pH = 6), H3IDC is partially deprotonated, and three new 2D lanthanide coordination polymers of formulas [Ln(HIDC)(OX)0.5(H2O)2]n 3 (H2O)n (1Ln) (Ln=Pr and Nd), [Ln(HIDC)(OX)0.5(H2O)2]n 3 (H2O)n (2Ln) (Ln=Eu and Tb), and [Ln(HIDC)(OX)0.5(H2O)]n (3Ln) (Ln = Pr, Nd, Eu, and Tb) were obtained. The X-ray determined structures of 1Ln-3Ln, as well as the structural related luminescent properties, are detailed. Experimental Section H3IDC was prepared from benzoimidazole in 75% yield. All the other reagents of analytical grade were obtained from commercial sources and used without further purification. Elemental analyses were determined using an Elementar Vario EL elemental analyzer. The IR spectra were recorded in the 4000-400 cm-1 region as KBr pellets with a Bruker EQUINOX 55 spectrometer. Thermal gravimetric analysis (TGA) data were collected on a Netzsch TG-209 instrument in the temperature range of 20-700 C with a heating rate of 10 C/min. The X-ray powder diffraction (XPRD) measurements were recorded on a Bruker D8 ADVANCE powder X-ray diffractometer (Cu KR, 1.5418 A˚). Photoluminescent spectra were measured using an Edinburgh FLS920 fluorescence spectrometer for the solid powder samples under ambient temperature. [Pr(HIDC)(OX)0.5(H2O)2]n 3 (H2O)n (1Pr). A mixture of Pr(NO3)3 3 6H2O (0.218 g, 0.50 mmol), H3IDC (0.078 g, 0.50 mmol), oxalic acid (0.032 g, 0.25 mmol), and piperazine (0.194 g, 1.0 mmol) in 10 mL of water was stirred at room temperature for 20 min, and then was transferred to a 25 mL Teflon-lined stainless steel vessel. The reaction mixture was heated at 170 C for 3 days under autogenous pressure. After the reactant was cooled slowly to room temperature with a cooling rate of 2.5 C/h, pale green block-shaped crystals of 1Pr were obtained. Yield: 0.079 g, 40% based on Pr(NO3)3 3 6H2O. Anal. Calcd for C6H8N2O9Pr (1Pr): C, 18.33; H, 2.05; N, 7.13%. Found: C, 18.36; H, 2.11; N, 7.34%. IR (KBr, cm-1): 3535 (m), 3212 (m), 1583 (vs), 1564 (vs), 1493 (s), 1414 (m), 1388 (m), 1314 (w), 1238 (w), 1086 (w), 822 (m), 652 (m). r 2010 American Chemical Society

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Table 1. The Crystallographic Data for 1Ln-3Ln formula formula weigth T (K) crystal system space group a/A˚ b/A˚ c/A˚ β/ V/A˚3 Z Dc (g cm-3) μ (mm-1) data collected/unique (Rint) goodness-of-fit on F2 R1, wR2 [I > 2σ(I )] R1, wR2 (all data)

1Pr

1Nd

2Tb

3Pr

3Nd

3Eu

C6H8N2O9Pr 393.05 293(2) monoclinic P2(1)/c 7.4853(13) 17.211(3) 8.5448(15) 110.838(3) 1028.8(3) 4 2.538 4.784 8295/2231(0.0401) 1.165 0.0287, 0.0707 0.0411, 0.0756

C6H8N2O9Nd 396.38 293(2) monoclinic P2(1)/c 7.4725(11) 17.173(3) 8.5294(12) 110.873(2) 1022.7(3) 4 2.574 5.126 7816/2230(0.0410) 1.153 0.0274, 0.0563 0.0430, 0.0701

C6H8N2O9Tb 411.06 293(2) monoclinic C2/c 22.320(10) 7.131(3) 14.065(7) 110.119(8) 2102.0(17) 8 2.598 6.777 8082/2224(0.0405) 1.068 0.0277, 0.0637 0.0381, 0.0679

C6H4N2O7Pr 357.02 293(2) monoclinic C2/c 22.010(5) 6.7934(16) 13.362(3) 106.028(4) 1920.2(8) 8 2.470 5.099 5104/2088(0.0285) 1.097 0.0280, 0.0666 0.0412, 0.0715

C6H4N2O7Nd 720.70 173(2) monoclinic C2/c 21.816(7) 6.786(2) 13.297(4) 106.049(5) 1891.8(10) 8 2.530 5.514 5415/2014(0.0395) 1.022 0.0352, 0.0836 0.0432, 0.0881

C6H4N2O7Eu 736.14 173(2) monoclinic C2/c 21.911(12) 6.740(4) 13.188(7) 106.900(7) 1863.4(17) 8 2.624 6.757 5138/2002(0.0377) 1.149 0.0339, 0.0866 0.0570, 0.1330

[Nd(HIDC)(OX)0.5(H2O)2]n 3 (H2O)n (1Nd). This compound was prepared at 150 C by a procedure analogous to that which resulted in 1Pr, but with Nd(NO3)3 3 6H2O instead of Pr(NO3)3 3 6H2O. Yield: 0.072 g, 36% based on Nd(NO3)3 3 6H2O. Anal. Calcd for C6H8N2O9Nd (1Nd): C, 18.18; H, 2.03; N, 7.07%. Found: C, 18.24; H, 2.03; N, 7.01%. IR (KBr, cm-1): 3531 (m), 3215 (m), 1587 (vs), 1567 (s), 1493 (m), 1412 (m), 1388 (w), 1315 (w), 1236 (w), 1084 (w), 822 (m), 652 (m). [Ln(HIDC)(OX)0.5(H2O)2]n 3 (H2O)n (2Ln), (Ln = Eu, Tb). These two compounds were prepared at 120 C by a procedure analogous to 1Pr, except using Ln(NO3)3 3 6H2O. Yield: 0.152 g, 75% for 2Eu (based on Eu(NO3)3 3 6H2O), and 0.185 g, 90% for 2Tb (based on Tb(NO3)3 3 6H2O). Anal. Calcd for C6H8N2O9Eu (2Eu): C, 17.83; H, 2.00; N, 6.93%. Found: C, 17.94; H, 2.06; N, 7.00%. IR (KBr, cm-1): 3270 (s), 3117 (m), 1658 (vs), 1594 (vs), 1492 (s), 1385 (vs), 1308 (m), 1241 (w), 1139 (w), 1088 (w), 853 (w), 807 (s), 662 (m). Anal. Calcd for C6H8N2O9Tb (2Tb): C, 17.53; H, 1.96; N, 6.81%. Found: C, 17.40; H, 2.02; N, 6.70%. IR bands (KBr, cm-1): 3270 (s), 3117 (m), 1663 (vs), 1594 (vs), 1493 (s), 1386 (vs), 1309 (m), 1243 (w), 1139 (w), 1089 (w), 853 (w), 808 (s), 663 (m). [Ln(HIDC)(OX)0.5(H2O)]n (3Ln), (Ln = Pr, Nd, Eu, Tb). Compound 3Pr was prepared at 190 C, while compounds 3Nd, 3Eu, and 3Tb were prepared at 170 C by a procedure analogous to 1Pr. Yield: 0.104 g, 58% for 3Pr, 0.127 g, 70% for 3Nd, 0.118 g, 64% for 3Eu, and 0.113 g, 60% for 3Tb (based on Ln(NO3)3 3 6H2O). Anal. Calcd for C6H8N2O9Pr (3Pr 3 2H2O): C, 18.33; H, 2.05; N, 7.13%. Found: C, 18.49; H, 2.01; N, 7.28%. IR (KBr, cm-1): 3435 (m), 3122 (m), 1683 (vs), 1574 (vs), 1499 (s), 1454 (m), 1410 (m), 1174 (w), 1088 (w), 811 (m), 650 (m); Anal. Calcd for C6H4N2O7Nd (3Nd): C, 20.00; H, 1.12; N, 7.77%. Found: C, 19.75; H, 1.17; N, 7.66%. IR (KBr, cm-1): 3148 (m), 3118 (m), 1678 (vs), 1571 (vs), 1507 (s), 1453 (m), 1392 (m), 1174 (w), 1087 (w), 859 (m), 650 (m); Anal. Calcd for C6H4N2O7Eu (3Eu): C, 19.58; H, 1.10; N, 7.61%. Found: C, 19.51; H, 1.18; N, 7.42%. IR (KBr, cm-1): 3431 (s), 3122 (m), 1684 (vs), 1572 (vs), 1498 (s), 1454 (m), 1395 (m), 1174 (w), 1088 (w), 863 (m), 650 (m); Anal. Calcd for C6H6N2O8Tb (3Tb 3 H2O): C, 18.33; H, 1.54; N, 7.13%. Found: C, 18.11; H, 1.37; N, 7.18%. IR (KBr, cm-1): 3428 (s), 3118 (m), 1680 (vs), 1578 (vs), 1495 (s), 1460 (m), 1398 (m), 1170 (w), 1083 (w), 856 (w), 646 (m). Determination of Crystal Structures. Single-crystal data for 1Pr, 1Nd, 2Tb, 3Pr, 3Nd, and 3Eu were collected on a Bruker Smart 1000 CCD diffractometer with graphite monochromatic Mo KR radiation (λ = 0.71073 A˚), but due to poor crystal quality data for 2Eu and 3Tb were not collected. Empirical absorption corrections were applied using the SADABS program.10 Structures were solved using direct methods, and refined by full-matrix least-squares on F2. Anisotropic thermal parameters were applied to all non-hydrogen atoms. The hydrogen atoms of the ligand were placed in calculated positions with fixed isotropic thermal parameters and included in structure factor calculations in the final stage of full-matrix leastsquares refinement. All the hydrogen atoms of water molecules were located in difference Fourier maps and refined isotropically. All

calculations were performed using the SHELXTL-97 system of computer programs.11 Crystallographic data and structural refinements are summarized in Table 1. Selected bond lengths are listed in Table 2.

Results and Discussion Synthesis Chemistry. Block-shaped crystals of 1Ln-3Ln were obtained by reacting Ln(NO3)3 3 6H2O, H3IDC, H2OX, and piperazine in water at different reacting temperatures under hydrothermal conditions. The initial and final pH value of the reaction mixture was approximately 6. The results of XPRD measurements indicate that the peaks displayed in the measured XPRD patterns closely match those in the simulated patterns generated from single-crystal diffraction data (Figure S1, Supporting Information), indicating single phases of 1Ln, 2Ln, and 3Ln were formed. The XPRD patterns of 1Ln, 2Ln, and 3Ln are obviously different from each other, while the XPRD patterns within 1Ln, 2Ln, and 3Ln are identical, indicating that the compounds 1Ln, 2Ln, and 3Ln are isomorphous. It is noteworthy that the structures of 1Ln-3Ln are dependent on the reacting temperature. As shown in Scheme 1, 1Ln and 2Ln were obtained at temperatures 20-50 deg lower than the temperatures used to produce 3Ln. Results from the crystal structure studies as well as TGA indicate that the numbers of coordination water molecules of Ln(III) were reduced at the higher reacting temperature. This also resulted in a change in the structure from a 2D monolayer to 2D double-layer geometries. Crystal Structures of 1Pr and 1Nd. Single-crystal X-ray diffraction analyses reveal that 1Pr and 1Nd are isomorphous, and thus only 1Nd is discussed in detail herein. As shown in Figure 1a, the Nd(III) is nine-coordinated with four oxygen and one nitrogen atoms from three individual μ3-HIDC2- (Scheme 2a), one bis-chelating oxalate anion and two water molecules. The coordination polyhedron of Nd(III) is a distorted tricapped trigonal prism (Figure 1b). The Nd-O distances of 2.397-2.613 A˚ and Nd-N distance of 2.578 A˚ are in the normal range for Nd(III) ions,12 while these distances are shorter than the Pr-O (2.419-2.628 A˚) and Pr-N distances (2.602 A˚) due to the lanthanide contraction.13 In 1Nd, the Nd(III) ions are connected via μ3-HIDC2(Scheme 2a) and oxalate linkers to form a 2D sheet along the bc plane (Figure 1c). If each metal center is considered as a four-connecting node, each μ3-HIDC2- is regarded as a three-connecting node, and each oxalate is considered as a

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Table 2. Selected Distances (A˚) for 1Ln-3Lna 1Pr Pr(1)-O(3)#1 Pr(1)-O(2)#1 Pr(1)-O(2W)

2.419(4) 2.443(3) 2.492(4)

Pr(1)-O(7)#2 Pr(1)-O(6) Pr(1)-O(1)

Nd(1)-O(3)#1 Nd(1)-O(2)#1 Nd(1)-O(2W)

2.397(4) 2.437(4) 2.478(4)

Nd(1)-O(6) Nd(1)-O(7)#2 Nd(1)-O(1)

Tb(1)-O(3)#1 Tb(1)-O(2W) Tb(1)-O(1W)

2.307(4) 2.331(3) 2.338(4)

Tb(1)-O(1) Tb(1)-O(2)#1 Tb(1)-O(5)

Pr(1)-O(4)#1 Pr(1)-O(2)#2 Pr(1)-O(3)#2

2.392(4) 2.407(4) 2.433(4)

Pr(1)-O(1) Pr(1)-O(1W) Pr(1)-O(6)#3

Nd(1)-O(4)#1 Nd(1)-O(2)#2 Nd(1)-O(3)#2

2.380(4) 2.392(4) 2.422(4)

Nd(1)-O(1W) Nd(1)-O(1) Nd(1)-O(6)#3

Eu(1)-O(2)#1 Eu(1)-O(4)#2 Eu(1)-O(3)#1

2.341(7) 2.356(7) 2.380(8)

Eu(1)-O(1) Eu(1)-O(1W) Eu(1)-O(5)

2.522(3) 2.527(3) 2.531(3)

Pr(1)-O(1W) Pr(1)-N(1) Pr(1)-O(4)#3

2.551(3) 2.602(4) 2.628(4)

2.509(4) 2.517(4) 2.519(4)

Nd(1)-O(1W) Nd(1)-N(1) Nd(1)-O(4)#3

2.542(4) 2.578(4) 2.613(4)

2.347(4) 2.406(3) 2.435(4)

Tb(1)-O(6)#2 Tb(1)-N(1)

2.437(4) 2.500(4)

2.446(3) 2.451(4) 2.484(4)

Pr(1)-O(5) Pr(1)-N(1)

2.490(4) 2.662(4)

2.426(4) 2.431(4) 2.466(4)

Nd(1)-O(5) Nd(1)-N(1)

2.471(4) 2.654(4)

2.391(8) 2.402(8) 2.428(8)

Eu(1)-O(6)#3 Eu(1)-N(1)

2.457(8) 2.588(9)

1Nd

2Tb

3Pr

3Nd

3Eu

a Symmetry transformations used to generate equivalent atoms: #1 x, -y þ 3/2, z - 1/2, #2 -x, -y þ 1, -z þ 1, #3 -x, y - 1/2, -z þ 1/2 for 1Pr and 1Nd. #1 -x þ 1/2, y þ 1/2, -z þ 3/2, #2 -x þ 1, -y, -z þ 2 for 2Tb. #1 x, -y þ 1, z þ 1/2, #2 x, -y, z þ 1/2, #3 -x þ 1, -y, -z þ 2 for 3Pr, 3Nd, and 3Eu.

Scheme 1. Synthetic Strategy of 1Ln, 2Ln, and 3Ln

connection; thus, the 2D sheet can be regarded as a unique (3,4)-connected topology network with the Schl€ afli topological symbol of (4.52)(4.53.72) (Figure 1d). The coordinated water molecules (O(2W)) point out of the 2D monolayer, and link to O(4) atom of μ3-HIDC2- and O(7) atom of oxalate from adjacent 2D monolayers via interlayer hydrogen bonding interactions (Table 3) resulting in a 3D supramolecular framework (Figure 1e). Crystal Structure of 2Tb. The results of XPRD measurements indicate that 2Eu and 2Tb are isomorphous (Figure S1b, Supporting Information), and the structure of 2Eu was not investigated due to poor crystal quality. Compound 2Tb can be regarded as a supramolecular isomer of 1Pr and 1Nd, as they all possess similar compositions but different supramolecular structures. As shown in Figure 2a, the Tb(III) ion coordinates to three oxygen and one nitrogen atoms from two individual μ2-HIDC2- (Scheme 2b), one bis-chelating oxalate anion, and two water molecules. In contrast to 1Pr and 1Nd, the coordination polyhedron of Tb(III) in 2Tb is a distorted bicapped trigonal prism (Figure 2b). The Tb-O distances (2.307-2.437 A˚) are slightly shorter than the Tb-N distance (2.500 A˚), and these values are comparable

to the reported values of Tb(III) complexes.9c,14-16 In 2Tb, the μ2-HIDC2- anions alternately bridge the Tb(III) ions to form a 1D right-handed helical chain of [Tb(HIDC)]n around the crystallographic 21 axis, with the pitch of 7.131 A˚. The oxalate anions coordinate to the remaining coordination sites of Tb(III) within the chain in a parallel way. Therefore, the neighboring chains of [Tb(HIDC)]n must be constructed in a left-handed helix in order to meet the request of the coordination of oxalate in the originally formed righthanded chain. Thus, the right-handed helicity of the original formed chain is transferred oppositely to the neighboring helical chains through the coordination of oxalate bridges,7d leading to the formation of an achiral 2D monolayer, in which the alternately arranged right- and left-handed chains are bridged by oxalate (Figure 2c). The achiral monolayers are further linked by interlayer hydrogen bonding interactions between the coordinated water molecules (O(2W)) and the oxygen atoms (O(4) and O(5)) of μ2-HIDC2- and μ2-OX2- (Table 3) to generate a 3D framework (Figure 2d). Crystal Structures of 3Pr, 3Nd, and 3Eu. The results of single-crystal X-ray diffraction analyses reveal that 3Pr, 3Nd, and 3Eu are isomorphous, and thus only the structure

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Figure 1. (a) The coordination environment of Nd(III) and bridging mode of μ3-HIDC2- in 1Nd. (b) The coordination polyhedron of Nd(III). (c) The 2D sheet of [Nd(HIDC)(OX)0.5]n along the bc plane (the coordinated water molecules were omitted for clarity). (d) The topological network of 2D sheet (the green and blue nodes in the network represent Nd(III) ions and the center of μ3-HIDC2- ligands). (e) The 3D framework constructed via the interlayer hydrogen bonding interactions.

of 3Nd is selected and described in detail herein. Similar to 1Nd, the Nd(III) ion in 3Nd coordinates to four oxygen and one nitrogen atoms from three individual μ3-HIDC2(Scheme 2c), and one bis-chelating oxalate anion (see Figures 1a and 3a). While there is only one water molecule coordinated to Nd(III) in 3Nd, the coordination numbers are reduced from nine in 1Pr and 1Nd to eight in 3Nd, and the

coordination polyhedron of Nd(III) in 3Nd is a distorted bicapped trigonal prism (Figure 3b). In 3Ln, the Ln-O distances are shorter than the corresponding Ln-N distances, and both Ln-O and Ln-N distances decrease along with increasing lanthanide atomic numbers (Table 2), due to the effect of lanthanide contraction.13 In 3Nd, the Nd(III) ions are connected via μ3-HIDC2- bridges to form a 2D

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Scheme 2. The Various Coordination Modes of HIDC2- Observed in 1Ln-3Ln

Table 3. Hydrogen Distances (A˚) and Angle (deg) for 1Ln-3Ln d(D 3 3 3 A) — (DHA) 1Pr N(2)-H(2) 3 3 3 O(1)#6 2.982(5) 154.7 O(1W)-H(1B) 3 3 3 O(3W)#7 2.756(5) 165.2 O(2W)-H(2A) 3 3 3 O(7)#8 2.984(5) 117.2 O(3W)-H(3A) 3 3 3 O(7)#2 2.818(5) 177.2 O(2W)-H(2B) 3 3 3 O(4)#9 2.707(5) 173.1 O(1W)-H(1A) 3 3 3 O(6)#10 2.994(5) 140.2 2Tb N(2)-H(2) 3 3 3 O(1)#4 2.905(5) 125.9 O(1W)-H(1A) 3 3 3 O(3W)#5 2.617(7) 165.3 O(1W)-H(1B) 3 3 3 O(4)#6 2.798(6) 160.1 O(2W)-H(2A) 3 3 3 O(5)#7 2.723(5) 172.2 O(2W)-H(2B) 3 3 3 O(4)#8 2.733(5) 162.2 O(3W)-H(3A) 3 3 3 O(4)#9 2.840(7) 153.7 3Nd N(2)-H(2) 3 3 3 O(6)#6 2.864(6) 160.9 O(1W)-H(1A) 3 3 3 O(1)#7 2.689(5) 154(5) O(1W)-H(1B) 3 3 3 O(3)#8 2.871(6) 140(4) D-H 3 3 3 A

symmetry operation x - 1, -y þ 3/2, z - 1/2 x, y, z - 1 x þ 1, y, z -x, -y þ 1, -z þ 1 x þ 1, -y þ 3/2, z þ 1/2 -x, -y þ 1, -z x, - y, z - 1/2 x, y - 1, z -x þ 1/2, -y - 1/2, -z þ 1 -x þ 1, y, -z þ 3/2 -x þ 1/2, -y þ 1/2, -z þ 1 x þ 1/2, -y þ 1/2, z þ 1/2

d(D 3 3 3 A) — (DHA) symmetry operation 1Nd O(1W)-H(1A) 3 3 3 O(6)#6 3.003(6) 140.6 -x, -y þ 1, -z O(2W)-H(2B) 3 3 3 O(4)#7 2.718(6) 173.6 x þ 1, -y þ 3/2, z þ 1/2 O(3W)-H(3A) 3 3 3 O(7)#2 2.815(6) 165.8 -x, -y þ 1, -z þ 1 O(2W)-H(2A) 3 3 3 O(7)#8 2.965(6) 124.2 x þ 1, y, z O(1W)-H(1B) 3 3 3 O(3W)#9 2.762(6) 166.6 x, y, z - 1 N(2)-H(2) 3 3 3 O(1)#10 2.967(6) 154.6 x - 1, -y þ 3/2, z - 1/2 3Pr N(2)-H(2) 3 3 3 O(6)#6 2.873(6) 160.5 -x þ 1, y þ 1, -z þ 3/2 O(1W)-H(1A) 3 3 3 O(1)#7 2.704(5) 141(4) -x þ 1/2, y þ 1/2, -z þ 3/2 O(1W)-H(1B) 3 3 3 O(3)#8 2.888(5) 148(5) -x þ 1/2, -y þ 1/2, -z þ 1 D-H 3 3 3 A

-x þ 1, y þ 1, -z þ 3/2 N(2)-H(2) 3 3 3 O(6)#6 -x þ 1/2, y þ 1/2, -z þ 3/2 O(1W)-H(1A) 3 3 3 O(1)#7 -x þ 1/2, -y þ 1/2, -z þ 1 O(1W)-H(1B) 3 3 3 O(3)#8

3Eu 2.846(12) 158.6 2.692(10) 123.9 2.904(11) 142.1

-x þ 1, y þ 1, -z þ 3/2 -x þ 1/2, y þ 1/2, -z þ 3/2 -x þ 1/2, -y þ 1/2, -z þ 1

Figure 2. (a) The coordination environment of Tb(III) and bridging mode of μ2-HIDC2- in 2Tb. (b) The polyhedron of the Tb(III). (c) The achiral 2D sheet containing the alternate right-handed and left-handed helical chains bridged by the parallel oxalate anions. (d) The 3D supramolecular framework constructed via the interlayer hydrogen bonding interactions.

monolayer of [Nd(HIDC)(H2O)]nnþ along the bc plane (Figure 3c), with a (6,3) topological network (Figure 3d).

Two 2D monolayers are further pillared by the oxalate to result in a 2D double-layer (Figure S2, Supporting

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Figure 3. (a) The coordination environment of Nd(III) and bridging mode of μ3-HIDC2- in 3Nd. (b) The coordination polyhedron of Nd(III). (c) The 2D layer of [Nd(HIDC)(H2O)]nnþ along the bc plane (the coordinated water molecules were omitted for clarity). (d) The (6,3) topological network of 2D layer (the green and yellow nodes in the network represent Nd(III) and the center of μ3-HIDC2- ligands, respectively). (e) The 3D framework constructed via the interlayer hydrogen bonding interactions between the 2D double-layer.

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Figure 4. The TGA curves of 1Nd and 3Nd, as well as 2Eu and 3Eu.

Information). All the coordinated water molecules point out of the 2D double-layer (Figure S2, Supporting Information), and link to the O(1) and O(3) atoms of μ3-HIDC2- from adjacent 2D double-layers via interlayer hydrogen bonding interactions (Table 3) to generate a 3D supramolecular framework (Figure 3e). Lanthanide Contraction and Temperature-Dependent Structures. From the above description of crystal structures, it can be found that 1Ln, 2Ln, and 3Ln exhibit different structures. The structural difference between 1Ln and 2Ln may be due to lanthanide contraction rather than the temperature of the reaction, as the nine-coordinated 1Pr and 1Nd were prepared at higher temperatures (170 and 150 C), while the eightcoordinated 2Eu and 2Tb were obtained at lower temperature (120 C). Were it not for lanthanide contraction, the coordination numbers of 1Pr and 1Nd should be eight rather than nine, as the higher reacting temperature should reduce the numbers of coordination water molecules of Ln(III). The lanthanide contraction refers to the decrease of ionic radius and coordination numbers from Pr to Tb. The larger Pr(III) and Nd(III) ions can coordinate to three HIDC2- anions, while the smaller Eu(III) and Tb(III) ions can only coordinate to two HIDC2- anions due to steric effects (see Figures 1a and 2a). Obviously, a ninecoordinated environment in 1Ln is too crowded for the smaller Eu(III) and Tb(III) ions, and the structure will become unstable if Eu(III) or Tb(III) is incorporated into a 1Ln type structure; thus, only eight coordinated 2Eu and 2Tb were formed even at lower temperature (120 C). The lanthanide contraction from Pr(III) to Tb(III) generates two series of supramolecular isomers of 1Ln and 2Ln with the same composition and different supramolecular structures. To our knowledge, supramolecular isomerism as a result of lanthanide contraction has not been reported so far. When the same reactions were carried out at higher temperatures,17 the numbers of coordination water molecules are reduced from two in 1Ln and 2Ln to one in 3Ln, resulting in

Figure 5. The emission spectra of (a) 2Eu and 3Eu, and (b) 2Tb and 3Tb in the solid state at room temperature.

changes in the structural motifs from 2D monolayers in 1Ln and 2Ln to 2D double-layers for 3Ln, in which the coordination numbers of Ln(III) reduce from nine in 1Pr and 1Nd to eight for 3Ln. Thermal Analyses and XPRD Patterns. The TGA measurements for 1Ln-3Ln indicate that all the compounds within 1Ln, 2Ln and 3Ln show similar thermal behavior owing to their isomorphous structures; thus, only the thermal stabilities of 1Nd and 3Nd are discussed here in detail. As shown in Figure 4, the TGA curve of 1Nd shows that there is no weight loss before 120 C due to strong hydrogen bonding interactions of lattice water molecule with the host framework (see Table 3), and then an initial weight loss of 9.14%, corresponding to the loss of one lattice and one coordination water molecules (calcd 9.08%). The second coordination water molecule was lost in the temperature range of 165-262 C, with the found and calculated weight losses of 4.28 and 4.54%, respectively. The thermal behavior of 1Nd clearly demonstrates that if the reaction is carried out at 170 C, only one coordinated water molecule would be stabilized, leading to the formation of 3Nd rather than 1Nd (see Scheme 1). The TGA curve of 3Nd shows there is no weight loss before 190 C, and that the coordinated water molecule is lost in the 190-270 C temperature range; the observed weight loss of 5.0% is consistent with that calculated (5.0%). The above thermal behavior was also confirmed by the results of variable temperature XPRD measurements of 1Nd and 3Nd. As shown in Figure S3a, Supporting Information, the initial phase in 1Nd changed to a second phase when the sample was heated to 120 C. This second phase was stable in the 120-240 C temperature range and changed to a third phase upon further heating. The initial phase in 3Nd changed

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to a second phase when the sample was heated to 180 C, and this second phase was stable up to 380 C (Figure S3b, Supporting Information). The thermal behavior of 2Eu is different from that of 1Nd. As shown in Figure 4b, compound 2Eu lost one lattice water molecule in the 20-150 C temperature range, with the found and calculated weight losses of 4.73 and 4.54%, respectively. The first coordinated water molecule was lost in the 150-230 C temperature range, with found and calculated weight losses of 6.96 and 4.54%, respectively. The compound decomposed along with the loss of the second coordinated water molecule upon further heating. This behavior was also observed in the TGA curve of 3Eu, in that the compound began to decompose along with the loss of the coordinated water molecule. There was no obvious plateau in the TGA curves of 2Eu and 3Eu after the loss of coordinated water molecules. The result of XPRD measurements indicates that the initial phase in 2Eu remained until 210 C, indicating that the framework of 2Eu did not change after the loss of water molecules; the compound became amorphous upon further heating (Figure S3c, Supporting Information). The initial phase in 3Eu changed to a second phase when the sample was heated to 240 C, and this second phase was stable up to 380 C (Figure S3d, Supporting Information). Luminescent Properties. The luminescent properties of 2Eu, 3Eu, 2Tb, and 3Tb were investigated at room temperature. As shown in Figure 5, 2Eu and 3Eu, as well as 2Tb and 3Tb, show very similar emission positions except for the difference in the emission intensities, and they all exhibit good luminescent properties with narrow, sharp, and wellseparated emission bands. 2Eu and 3Eu display strong red luminescence and characteristic transition spectra of Eu(III) ion under excitation at 276 nm. As shown in Figure 5a, the transitions from 5D0 to different 7FJ (J = 0-4) states were observed in the emission spectra, that is, 5D0 f 7F0 at 579 nm, 5D0 f 7F1 at 592 nm, 5D0 f 7F2 at 613 nm, 5D0 f 7F3 at 650 nm, and 5D0 f 7F4 at 701 nm,14a,16,18 respectively. Under excitation at 276 nm, 2Tb and 3Tb show the characteristic emission bands of Tb(III) at 488, 544, 582, and 621 nm, respectively, which are assigned to 5D4 f 7FJ (J = 6, 5, 4, and 3) transitions of Tb(III) (Figure 5b).9c,14-16,19 Furthermore, the fluorescence lifetimes of 2Eu and 3Eu at 613 nm, and 2Tb and 3Tb at 544 nm are 96.84, 118.54, 356.01, and 374.04 μs, respectively (Figure S4, Supporting Information). It is interesting to note that 3Eu and 3Tb exhibit stronger emission intensities and longer fluorescence lifetimes than 2Eu and 2Tb, which can be attributed to their different structural motifs. The luminescent intensity of Ln(III) relies on the efficiency of the energy transfer from the ligand to Ln(III).9c,15,16,18-20 It has been demonstrated that the presence of lattice and coordination water molecules decrease the luminescent emission intensity of lanthanide coordination polymers, as the thermal oscillation of water molecules will consume some excitation energy absorbed by “antenna” ligands.12a,14c,15 There are one lattice and two coordination water molecules in 2Eu and 2Tb, while there is only one coordination water molecule in 3Eu and 3Tb; thus the emission intensities of 3Eu and 3Tb are much stronger than those of 2Eu and 2Tb due to less deactivation in 3Eu and 3Tb. In addition, the HIDC2- anion adopts a μ3-HIDC2coordination mode (Scheme 2c) in 3Eu and 3Tb, while it adopts a μ2-HIDC2- coordination mode (Scheme 2b) in 2Eu and 2Tb, the μ3-HIDC2- coordination mode in 3Eu and 3Tb makes HIDC2- more rigid; this reduces vibration-induced

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deactivation of μ3-HIDC2- and increases the efficiency of the energy from μ3-HIDC2- to Ln(III).9c,14c,19,20 Therefore, it is reasonable that 3Eu and 3Tb exhibit stronger emission intensities and longer fluorescence lifetimes than 2Eu and 2Tb due to the existence of the more rigidly coordinated μ3-HIDC2- ligand and the lesser number of water molecules in 3Eu and 3Tb. Conclusions In summary, eight lanthanide coordination polymers of 1Ln-3Ln with three different structural motifs have been synthesized at different reacting temperatures under hydrothermal conditions. 1Ln and 2Ln are supramolecular isomers produced by lanthanide contraction. In addition, a higher reacting temperature reduces one coordination water molecule of Ln(III) from 1Ln and 2Ln to 3Ln, resulting in a structural change from 2D monolayers in 1Ln and 2Ln to 2D double-layers in 3Ln. The results of luminescent measurements for 2Eu/2Tb and 3Eu/3Tb indicate that the emission intensities of 3Eu/3Tb are stronger than those of 2Eu/2Tb, which are attributed to the existence of more rigid μ3-HIDC2and lesser numbers of water molecules in 3Eu/3Tb. Acknowledgment. This work was supported by NSFC (20625103, 20831005, 20821001) and 973 Program of China (2007CB815305). We thank Rudy L. Luck at Michigan Technological University for assistance with corrections. Supporting Information Available: X-ray crystallographic file in CIF format, the XPRD patterns of 1Ln-3Ln. This material is available free of charge via the Internet at http://pubs.acs.org.

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