A Series of Lanthanide Secondary Building Units Based Metal

Nov 23, 2010 - Compound 1 presents a four-connected two-dimensional (2D) network ... 3D anionic framework with (42510728)(4254) topology, when the Ce(...
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DOI: 10.1021/cg101343k

A Series of Lanthanide Secondary Building Units Based Metal-Organic Frameworks Constructed by Organic Pyridine-2,6-Dicarboxylate and Inorganic Sulfate

2011, Vol. 11 337–346

Jing Xu, Weiping Su,* and Maochun Hong State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China Received October 12, 2010; Revised Manuscript Received November 10, 2010

ABSTRACT: A series of novel lanthanide coordination polymers constructed from organic pyridine-2,6-dicarboxylate (2,6-pydc) and inorganic sulfate, namely, [Eu3(2,6-pydc)3(2,6-Hpydc)(SO4)(H2O)3 3 (H2O)3] (1), [Ln2(2,6-pydc)2(SO4)(H2O)2 3 (H2O)2] (Ln = Ce (2); Ln = Pr (3); Ln = Nd (4); Ln = Sm (5)), and [Ce5(2,6-pydc)6(SO4)2(H2O)3 3 (Me2NH2)] (6), have been synthesized under hydrothermal conditions. X-ray crystal structure analyses reveal that these compounds have a rich structural chemistry. Compound 1 presents a four-connected two-dimensional (2D) network based on hexanuclear {Eu6} secondary building units (SBUs). The 2D layers are further linked into a three-dimensional (3D) supramolecular framework via strong π-π stacking interactions of the pyridyl rings. Compounds 2-5 are isostructural and adopt uninodal six-connected 3D framework of pcu topology constructed from square-planar tetranuclear {Ln4} SBUs. Of particular interest is that compound 6 exhibits a protonated (Me2NH2)þ templated binodal (4,6)-connected 3D anionic framework with (42510728)(4254) topology, when the Ce(3) monomer and the planar tetranuclear {Ce4} SBUs are regarded as four-connected and six-connected nodes, respectively. Notably, sulfate serves as an auxiliary supporting bridge to strengthen the {Ln4} or {Ln6} SBUs in these compounds. Compound 1 displays intense red luminescence and exhibits the characteristic transition of the Eu3þ ion. The magnetic properties of 1, 4, and 5 reveal the weak antiferromagnetic characters. Furthermore, infrared (IR), thermogravimetric analysis (TGA), elemental analyses (EA), and powder X-ray diffraction (PXRD) properties of these compounds are also studied.

*Corresponding author. E-mail: [email protected]; fax: (þ86) 5918377-1575.

built from mixed organic ligands, for example, carboxylateoxalate,14 isonicotinate-carboxylate, 15 have been well reported and shown more diverse structure types, while mixed inorganic-organic hybrid ligands in establishing LnMOFs are still underdeveloped. Remarkably, anions play an important role in the structure control for self-assembly.16 Although a variety of inorganic-organic frameworks involving anions such as silicates,17 phosphates,18 polymolybdate,19 and polytungstate20 have been well documented, the assembly of inorganic-organic architectures containing sulfate,3m-o,10e,21 especially LnMOFs based on Ln SBUs with sulfate as an auxiliary bridge,8g are relatively rare. Therefore, we chose sulfate/2,6-pydc as inorganic/organic ligands to construct new LnMOFs, based on the following considerations: (1) The sulfate ion, as a simple tetrahedral oxoanion, has versatile coordination modes,3n,22 which allows it to coordinate to Ln ions and serves as auxiliary supporting bridge in construction of the Ln SBUs. (2) 2,6-pydc, as a chelating ligand with multiple coordination sites involving one pyridine nitrogen atoms and four carboxylate oxygen atoms, has limited steric hindrance combined with weak stacking interactions, offering possibilities to form and propagate Ln SBUs through carboxylate oxygen atoms.10a Accordingly, our aim is to use sulfate as an auxiliary bridge to construct Ln SBUs, which is further linked by 2,6-pydc to generate Ln SBUs based LnMOFs. In this contribution, six novel LnMOFs, namely, [Eu3(2,6pydc)3(2,6-Hpydc)(SO4)(H2O)3 3 (H2O)3] (1), [Ln2(2,6-pydc)2(SO4)(H2O)2 3 (H2O)2] (Ln = Ce (2); Ln = Pr (3); Ln = Nd (4); Ln = Sm (5)), and [Ce 5 (2,6-pydc)6 (SO 4 )2 (H 2O)3 3 (Me2NH2)] (6), based on different polynuclear lanthanide

r 2010 American Chemical Society

Published on Web 11/23/2010

Introduction In recent years, the design and synthesis of metal-organic frameworks (MOFs) have attracted considerable attention due to their intriguing topological structures and potential applications in gas storage and separation, magnetism, luminescence, and catalysis.1 Consequently, a large number of functional MOFs of transition metal (TM) ions with interesting architectures and topologies have been well-established and synthesized successfully.2 In contrast, the analogous chemistry of lanthanide (Ln) remains less developed.3 Nevertheless, Ln ions, with high coordination number and flexible coordination environments4 together with the special luminescent5 and magnetic6 properties resulting from 4f electrons, are good candidates for providing various building blocks in constructing diverse lanthanide-based MOFs (LnMOFs). On the other hand, secondary building units (SBUs) generally possess larger sizes and more coordination sites but smaller steric hindrance for ligands,7 and polynuclear Ln SBUs have been employed in the construction of LnMOFs more recently.8 Therefore, it should be a rational way to introduce polynuclear Ln SBUs to establish LnMOFs. It is well-known that Ln ions have a large radius and high affinity for oxygen donor atoms; thus multidentate ligands with oxygen or hybrid oxygen-nitrogen atoms, such as pyridinecarboxylate,3p,q,9-11 imidazoledicarboxylate,3l-o,8g,12 and thiophenedicarboxylate13 have been employed in the construction of LnMOFs. At present, a number of LnMOFs

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SBUs with sulfate as an auxiliary supporting bridge, have been successfully synthesized. Compound 1 presents a fourconnected 2D network based on hexanuclear {Eu6} SBUs. Compounds 2-5 adopt uninodal six-connected 3D framework of pcu topology constructed from square-planar tetranuclear {Ln4} SBUs. Of particular interest is that compound 6 exhibits a protonated (Me2NH2)þ templated binodal (4,6)connected 3D anionic framework with (42510728)(4254) topology, when the Ce(3) monomer and the planar tetranuclear {Ce4} SBUs are regarded as four-connected and six-connected nodes, respectively. The successful isolation of these six compounds demonstrates that using inorganic sulfate as an auxiliary ligand is a feasible method to construct Ln SBUs based LnMOFs. Simultaneously, the structural related luminescent and magnetic properties are also described in detail. Experimental Section Materials and Methods. All chemicals were purchased commercially and used without further purification. 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 N 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 spectra were measured at room temperature on an Edinburgh FL-FS920 TCSPC system. The polycrystalline magnetic susceptibility data were 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 Eu3(2,6-pydc)3(2,6-Hpydc)(SO4)(H2O)3 3 (H2O)3 (1). A mixture of EuCl3 3 6H2O (0.5 mmol, 0.1832 g), CuSO4 3 5H2O (0.2 mmol, 0.0499 g), H2pydc (1.0 mmol, 0.1672 g), isonicotinic acid (2.0 mmol, 0.2462 g), and H2O (8 mL) was sealed in a 25 mL Teflonlined autoclave at 170 C for 7 days, then cooled to room temperature. White prismatic crystals of 1 were obtained (yield: 32% based on EuCl3 3 6H2O). Anal. calc. for C28H23Eu3N4O26S (1319.44): C, 25.49; H, 1.76; N, 4.25%. Found: C, 25.61; H, 1.73; N, 4.27%. IR spectrum (KBr pellet, ν/cm-1): 3440(vs), 1625(vs), 1472(w), 1440(s), 1402(s), 1279(m), 1198(m), 1141(m), 1070(m), 1036(w), 1018(w), 979(w), 932(vw), 856(vw), 832(vw), 765(m), 737(m), 694(m), 657(m), 604(w). Synthesis of Ce2(2,6-pydc)2(SO4)(H2O)2 3 (H2O)2 (2). A mixture of Ce(NO3)3 3 6H2O (0.5 mmol, 0.2171 g), CuSO4 3 5H2O (0.2 mmol, 0.0499 g), H2pydc (2.0 mmol, 0.3342 g), nicotinic acid (0.5 mmol, 0.0616 g), and H2O (6 mL) was sealed in a 25 mL Teflon-lined autoclave at 170 C for 5 days and then cooled to room temperature. Light-yellow prismatic crystals of 2 were obtained (yield: 36% based on Ce(NO3)3 3 6H2O). Anal. calc. for C14H14Ce2N2O16S (778.57): C, 21.60; H, 1.81; N, 3.60%. Found: C, 21.82; H, 1.67; N, 3.43%. IR spectrum (KBr pellet, ν/cm-1): 3411(vs), 1601(vs), 1473(vw), 1449(s), 1383(s), 1279(m), 1208(m), 1155(w), 1108(m), 1018(m), 970(m), 932(w), 840(w), 765(m), 737(m), 704(w), 656(m), 600(w). Synthesis of Pr2(2,6-pydc)2(SO4)(H2O)2 3 (H2O)2 (3). A mixture of Pr(NO3)3 3 6H2O (0.25 mmol, 0.1088 g), CuSO4 3 5H2O (0.1 mmol, 0.0250 g), H2pydc (0.5 mmol, 0.0836 g), and H2O (4 mL) was sealed in a 25 mL Teflon-lined autoclave at 170 C for 6 days and then cooled to room temperature. Light-green prismatic crystals of 3 were obtained (yield: 26% based on Pr(NO3)3 3 6H2O). Anal. calc. for C14H14Pr2N2O16S (780.15): C, 21.55; H, 1.81; N, 3.59%. Found: C, 21.67; H, 1.74; N, 3.50%. IR spectrum (KBr pellet, ν/cm-1): 3411(vs), 1601(vs), 1473(vw), 1449(s), 1383(s), 1279(m), 1208(m), 1155(w), 1108(m), 1018(m), 970(m), 932(w), 840(w), 765(m), 737(m), 704(w), 656(m), 600(w).

Xu et al. Synthesis of Nd2(2,6-pydc)2(SO4)(H2O)2 3 (H2O)2 (4). A mixture of Nd2O3 (0.25 mmol, 0.0841 g), CuSO4 3 5H2O (0.5 mmol, 0.1248 g), H2pydc (1.0 mmol, 0.1672 g), and H2O (8 mL) was sealed in a 25 mL Teflon-lined autoclave at 170 C for 7 days and then cooled to room temperature. Purple block crystals of 4 were obtained (yield: 38% based on Nd2O3). Anal. calc. for C14H14Nd2N2O16S (786.81): C, 21.37; H, 1.79; N, 3.56%. Found: C, 21.45; H, 1.65; N, 3.33%. IR spectrum (KBr pellet, ν/cm-1): 3411(vs), 1601(vs), 1473(vw), 1449(s), 1383(s), 1279(m), 1208(m), 1155(w), 1108(m), 1018(m), 970(m), 932(w), 840(w), 765(m), 737(m), 704(w), 656(m), 600(w). Synthesis of Sm2(2,6-pydc)2(SO4)(H2O)2 3 (H2O)2 (5). Compound 5 was made by a procedure similar to that of 4, except Sm2O3 (0.25 mmol, 0.0872 g) replaced Nd2O3. Colorless prismatic crystals of 5 were obtained (yield: 40% based on Sm2O3). Anal. calc. for C14H14Sm2N2O16S (799.03): C, 21.04; H, 1.77; N, 3.51%. Found: C, 21.03; H, 1.56; N, 3.31%. IR spectrum (KBr pellet, ν/cm-1): 3411(vs), 1601(vs), 1473(vw), 1449(s), 1383(s), 1279(m), 1208(m), 1155(w), 1108(m), 1018(m), 970(m), 932(w), 840(w), 765(m), 737(m), 704(w), 656(m), 600(w). Synthesis of Ce5(2,6-pydc)6(SO4)2(H2O)3 3 (Me2NH2) (6). A mixture of Ce(NO3)3 3 6H2O (0.5 mmol, 0.2171 g), CuSO4 3 5H2O (0.2 mmol, 0.0499 g), H2pydc (2.0 mmol, 0.3342 g), nicotinic acid (0.5 mmol, 0.0616 g), H2O (6 mL), and DMF (0.5 mmol) was sealed in a 25 mL Teflon-lined autoclave at 170 C for 5 days and then cooled to room temperature. Yellow prismatic crystals of 6 were obtained (yield: 35% based on Ce(NO3)3 3 6H2O). Anal.calc. for C44H32Ce5N7O35S2 (1983.49): C, 26.64; H, 1.63; N, 4.94%. Found: C, 26.44; H, 1.71; N, 4.67%. IR spectrum (KBr pellet, ν/cm-1): 3440(vs), 1611(vs), 1431(m), 1383(s), 1278(w), 1222(w), 1108(m), 1070(w), 1027(m), 956(w), 832(vw), 761(w), 737(w), 695(w), 657(w), 590(w). Crystal Structural Determination. The crystal structure data of compounds 1-6 were determined on a SCXmini CCD diffractometer equipped with a graphite-monochromated MoKR (λ = 0.71073 A˚) radiation using an ω scan mode at 293 K. An empirical absorption correction was applied using the SADABS program.23 All structures were solved by direct methods and refined by fullmatrix least-squares on F2 using the SHELXS-97 program package.24 All non-hydrogen atoms were refined anisotropically except the oxygen atoms of the free water molecules in 1-5 and the nitrogen atom of the free dimethylamine molecule in 6. The hydrogen atoms belonging to the water molecules were placed by CACLOH of the WinGX program package except those residing on one coordinated water molecule in 1, which cannot be added. The other hydrogen atoms were located geometrically and treated as riding. O(6) in 1 is protonated for charge balance, as confirmed by the bond valence sum (BVS) for O(6) being 1.30. A summary of the crystallographic data of 1-6 is listed in Table 1. The selected bond lengths and hydrogen bond parameters of 1, 4, and 6 are listed in Tables 2, 3, 4, and 5, respectively. The selected bond lengths and hydrogen bond parameters of 2, 3, and 5 are shown in Tables S1 and S2 (Supporting Information), respectively. CCDC-769424 (1), CCDC-783899 (2), CCDC-778325 (3), CCDC-771572 (4), CCDC-773511 (5), and CCDC773878 (6) contain the crystallographic data in CIF format.

Results and Discussion Synthesis and Characterization. The high temperature and pressure during the course of hydrothermal reactions can dramatically enhance the ligand solubility and the reactivity of reactants.25 Hydrothermal synthesis has been proven to be an useful technique to obtain metal coordination polymers. And then, single crystals of 1-6 were obtained under hydrothermal conditions at 170 C. All these compounds are airstable and insoluble in common solvents such as chloroform, toluene, acetonitrile, DMF, methanol, and ethanol. X-ray crystal structure analyses show that isonicotinic acid (HIN) and nicotinic acid (HNA) are not incorporated into the resulting frameworks of 1 and 2, respectively. However, 1 and 2 could not be synthesized in the absence of HIN and HNA.

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

1

2

3

4

5

6

formula Mr crystal system space group a [A˚] b [A˚] c [A˚] R [o] β [o] γ [o] V [ A˚3] Z F [g cm-3] μ [mm-1] F(000) GOF (F 2) collected reflns unique reflns observed reflns parameters R1a [I>2σ(I)] wR2b [I>2σ(I)]

C28H23Eu3N4O26S 1319.44 triclinic P1 10.0495(2) 11.0245(4) 18.1435(4) 81.327(7) 78.600(5) 79.416(6) 1923.23(9) 2 2.278 4.991 1264 1.113 14831 8389 7239 555 0.0364 0.0920

C14H14Ce2N2O16S 778.57 monoclinic P2(1)/n 10.6886(5) 16.4787(7) 12.4214(6) 90 100.007(4) 90 2154.55(17) 4 2.400 4.356 1488 1.223 16828 4870 4623 312 0.0496 0.1391

C14H14Pr2N2O16S 780.15 monoclinic P2(1)/n 10.6611(9) 16.4266(11) 12.4018(8) 90 99.793(7) 90 2140.2(3) 4 2.421 4.684 1496 1.102 14689 4579 4115 312 0.0684 0.1671

C14H14Nd2N2O16S 786.81 monoclinic P2(1)/n 10.623(3) 16.369(5) 12.362(4) 90 99.655(6) 90 2119.1(11) 4 2.466 5.033 1504 1.281 16012 4855 4449 312 0.0324 0.0812

C14H14Sm2N2O16S 799.03 monoclinic P2(1)/n 10.577(5) 16.299(8) 12.333(6) 90 99.566(10) 90 2096.6(18) 4 2.531 5.735 1520 1.199 15931 4783 4351 312 0.0437 0.1146

C44H32Ce5N7O35S2 1983.49 triclinic P1 10.701(7) 10.737(7) 27.233(17) 95.068(8) 101.078(5) 103.937(9) 2950(3) 2 2.233 3.956 1894 1.085 22731 12887 9357 828 0.0757 0.1974

a

R1 = Σ||Fo| - |Fc||/Σ|Fo|. b wR2 = {Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]}1/2. Table 2. Selected Bond Lengths (A˚) for 1a

Eu1-O1 Eu1-O5 Eu1-O12#1 Eu1-O17 Eu1-N2 Eu2-O8 Eu2-O10 Eu2-O16#4 Eu2-O19 Eu3-O10 Eu3-O13 Eu3-O20 Eu3-O21 Eu3-N4

2.474(4) 2.506(4) 2.359(4) 2.513(4) 2.561(4) 2.328(4) 2.565(4) 2.523(4) 2.494(4) 2.461(4) 2.405(4) 2.597(4) 2.547(4) 2.537(4)

Eu1-O3 Eu1-O7 Eu1-O14#2 Eu1-N1 Eu2-O4#3 Eu2-O9 Eu2-O15#4 Eu2-O18 Eu2-O22#4 Eu3-O11 Eu3-O15 Eu3-O20#4 Eu3-N3

Table 4. Selected Bond Lengths (A˚) for 6a 2.431(4) 2.407(4) 2.369(4) 2.536(4) 2.414(4) 2.574(4) 2.529(4) 2.398(4) 2.417(4) 2.410(4) 2.460(4) 2.401(3) 2.515(4)

a Symmetry codes: #1: x þ 1, y - 1, z; #2: x, y - 1, z; #3: x, y þ 1, z; #4: -x, -y þ 1, -z.

Table 3. Selected Bond Lengths (A˚) for 4a Nd1-O1 Nd1-O5 Nd1-O6#2 Nd1-O10#2 Nd1-N1 Nd2-O3 Nd2-O7 Nd2-O11 Nd2-O14

2.498(3) 2.523(3) 2.446(3) 2.598(3) 2.545(4) 2.584(3) 2.454(3) 2.425(3) 2.513(3)

Nd1-O3 Nd1-O6 Nd1-O9#2 Nd1-O12#1 Nd2-O2#3 Nd2-O4 Nd2-O9 Nd2-O13 Nd2-N2

2.465(3) 2.577(3) 2.554(3) 2.423(3) 2.419(3) 2.565(3) 2.541(3) 2.485(3) 2.609(4)

Symmetry codes: #1: x þ 1/2, -y þ 1/2, z þ 1/2; #2: -x þ 1, -y, -z þ 4; #3: x - 1, y, z. a

Additionally, alkalescent organic solvent such as DMF has been proven to be able to automatically control the base/acid balance of the solution and at the same time provides cation such as (Me2NH2)þ at high temperature26 to either neutralize the overall charge in the solid or serve as a template. Taking this consideration into account, we introduced DMF into the reaction system of 2, and 6 was successfully isolated, in which DMF underwent hydrolysis to give (Me2NH2)þ. Moreover, even in the cases where DMF was introduced to the reaction systems of 3-5, other isostructural complexes with Pr, Nd, or Sm could not be obtained, indicating that the ionic radius of the Ln ion is the predominant factor governing the obtained structures.

Ce1-O1 Ce1-O8#3 Ce1-O11#3 Ce1-O14#3 Ce1-O33 Ce2-O3 Ce2-O5#3 Ce2-O9 Ce2-N1 Ce3-O4 Ce3-O15 Ce3-O19 Ce3-O34 Ce3-N4 Ce4-O23 Ce4-O27 Ce4-O29#4 Ce4-N(5) Ce5-O18#6 Ce5-O23 Ce5-O26#5 Ce5-O28#4 Ce5-O35

2.652(9) 2.457(8) 2.663(8) 2.486(9) 2.599(8) 2.441(9) 2.419(8) 2.554(8) 2.577(10) 2.499(9) 2.562(10) 2.547(9) 2.719(12) 2.612(11) 2.535(8) 2.546(8) 2.437(8) 2.585(10) 2.434(8) 2.664(8) 2.431(9) 2.600(9) 2.597(8)

Ce1-O2 Ce1-O10#1 Ce1-O12#3 Ce1-O16#2 Ce2-O2 Ce2-O5 Ce2-O6 Ce2-O11 Ce2-N2 Ce3-O13 Ce3-O17 Ce3-O21 Ce3-N3 Ce4-O22 Ce4-O25 Ce4-O29 Ce4-O30 Ce4-N6 Ce5-O20#4 Ce5-O24 Ce5-O27#4 Ce5-O31#4

2.683(8) 2.435(9) 2.552(9) 2.452(9) 2.543(8) 2.617(8) 2.582(8) 2.531(8) 2.647(10) 2.514(9) 2.513(9) 2.505(9) 2.651(11) 2.459(9) 2.555(8) 2.616(8) 2.573(9) 2.638(10) 2.508(9) 2.654(9) 2.649(8) 2.443(8)

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

The IR spectra of 1-6 are similar. The strong and broad absorption bands in the range of 3000-3700 cm-1 in 1-6 are attributable to the O-H stretching vibrations of the free or bonded water molecules. The strong vibrations appearing around 1600 and 1450 cm-1 correspond to the asymmetric and symmetric stretching vibrations of the carboxylate group, respectively. The absence of strong absorption bands around 1700 cm-1 indicates that the ligands are deproponated. The middle and narrow bands in the range of 1000-1470 cm-1 are attributed to C-N and C-C vibrations. The δO-C-O vibration in plane occurs in middle intensity peaks in the range of 685.55-858.65 cm-1. The IR spectra of 1-6 are in accordance with the results of the X-ray diffraction analysis (Figures S19-S21, Supporting Information). Crystal Structure of Eu 3(2,6-pydc)3(2,6-Hpydc)(SO4)(H2O)3 3 (H2O)3 (1). Compound 1 crystallizes in the triclinic P1 space group and possesses a 2D metal-organic framework

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based on hexanuclear {Eu6} SBUs. As shown in Figure 1, the asymmetric unit of 1 comprises three independent Eu3þ ions, three pydc2- dianions, one Hpydc- anion, one sulfate, three coordinated and three guest water molecules. Both of the Eu(1) and Eu(2) atoms are nine-coordinated with distorted tricapped trigonal prism geometries: four carboxylate oxygen atoms (OCOO-) and one nitrogen atom from four pydc2ligands, two OCOO- and one nitrogen atom from one Hpydcligand, together with one coordinated water molecule for Eu(1) (Figure S1, Supporting Information); five OCOO- from three pydc2- ligands, one OCOO- from one Hpydc- ligand, and one oxygen atom from one SO42- together with two coordinated water molecules for Eu(2) (Figure S2, Supporting Information), while Eu(3) atom is nine-coordinated and has Table 5. Hydrogen Bond Lengths (A˚) and Bond Angles () in 1a, 4b, and 6 Compound 1 D;H 3 3 3 A d(D;H) d(H 3 3 3 A) d(D 3 3 3 A) O18;H18D 3 3 3 O23#6 0.85 1.85 2.698(5) O25;H25D 3 3 3 O11#4 0.85 2.09 2.940(7)

— (DHA) 172.6 175.1

Compound 4 D;H 3 3 3 A O13;H13A 3 3 3 O15 O13;H13B 3 3 3 O10#6 O14;H14B 3 3 3 O7#7 O14;H14B 3 3 3 O8#7 O15;H15A 3 3 3 O5#5 O16;H16A 3 3 3 O8

d(D;H) 0.85 0.85 0.85 0.85 0.85 0.85

d(H 3 3 3 A) 1.96 1.91 2.25 2.56 2.58 2.06

d(D 3 3 3 A) 2.774(7) 2.749(5) 3.020(4) 3.288(5) 3.189(8) 2.885(10)

— (DHA)

d(D 3 3 3 A) 2.902(13) 3.061(17) 2.868(13) 2.947(13) 2.870(12) 2.813(13)

— (DHA)

160.1 170.6 151.6 144.7 130.0 164.7

Compound 6 D;H 3 3 3 A O33;H33A 3 3 3 O9 O34;H34B 3 3 3 O22 O35;H35A 3 3 3 O25 O35;H35B 3 3 3 O24 N7;H7A 3 3 3 O28 N7;H7B 3 3 3 O19

d(D;H) 0.85 0.85 0.85 0.85 0.90 0.90

d(H 3 3 3 A) 2.10 2.27 2.02 2.20 1.98 1.94

156.6 155.1 171.6 146.7 168.9 164.3

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

Figure 1. The asymmetric unit of 1. The thermal ellipsoids are drawn at 30% probability. All hydrogen atoms are omitted for clarity. Symmetry codes for the generated atoms are the same as Table 2.

distorted monocapped square antiprism coordination environments: four OCOO- and two nitrogen atoms from two pydc2- ligands together with three oxygen atoms from two SO42- (Figure S3, Supporting Information). The Eu-O bond lengths vary from 2.328(4) to 2.574(4) A˚ and Eu-N range from 2.515(4) to 2.561(4) A˚ (Table 2). Two coordination modes of three pydc2- ligands are observed in the structure (Scheme 1a,c). In mode a, the pydc2ligand is coordinated to three Eu3þ ions in a μ3-η1, η1, η2, η1 coordination mode. In mode c, the pydc2- ligand is coordinated to two Eu3þ ions and adopts a μ2-η1, η1, η1, η0 coordination mode. The carboxylate groups in Hpydcligand are partially deprotonated, and the coordination mode of tricoordinating Hpydc- in 1 is shown in Scheme 1d. Eu(2) links two neighboring Eu(3) via O10 and O15 to generate a square-planar tetranuclear {Eu4} subunit with Eu(2) 3 3 3 Eu(3) distances being 4.721(12) and 4.772(17) A˚, respectively. SO42- anion adopting a μ3-η1, η1, η2, η0 coordination mode bridges three Eu3þ ions of the {Eu4} subunit, with the fourth one (O(23)) being a free terminal oxygen (Scheme 1e). Meanwhile, the free O(23) atom is involved in the formation of the hydrogen bonds. That is, SO42- anion serves as an auxiliary supporting bridge to stabilize the {Eu4} subunit. The {Eu4} subunit further connects two neighboring Eu(1) through two double O(3)C(7)-O(4) and O(13)-C(27)-O(14) bridges yielding a hexanuclear {Eu6} SBUs (Figure 2). In the {Eu6} SBUs, the distances of Eu(1) 3 3 3 Eu(2) and Eu(1) 3 3 3 Eu(3) are 6.080(17) and 6.267(14) A˚, respectively. Then each {Eu6} SBUs links four neighboring SBUs through double carboxylate bridges (O(7)-C(14)-O(8) and O(11)-C(21)-O(12)) to form a 2D layer network in the ab plane (Figure 3a). From the topological point of view, the pydc2- ligands can be viewed as connectors and the {Eu6} SBUs as nodes (Figure S4, Supporting Information). Thus, the 2D network of 1 can be described as a four-connected topological network with the Schl€ afli symbol and vertex symbol being (44) and 4 3 4 3 4 3 4 analyzed by OLEX program,27 respectively (Figure 3b).

Figure 2. View of the hexanuclear {Eu6} SBUs in 1. Hydrogen atoms are omitted for clarity.

Scheme 1. Coordination Modes of the pydc2- Ligand in 1 (a, c), 4 (a), and 6 (a, b); the Hpydc- Ligand in 1 (d); and SO42- Ion in 1, 4, and 6

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Figure 4. 3D supramolecular framework of 1 generated via strong π-π stacking interactions of the pyridyl rings. Hydrogen atoms and guest water molecules are omitted for clarity. The π-π stacking interactions are shown as yellow dashed lines.

Figure 3. (a) Polyhedral representation of the 2D layer network based on {Eu6} SBUs for 1 in the ab plane. Hydrogen atoms and guest water molecules are omitted for clarity. (b) Schematic representation of four-connected 2D 44-network for 1 based on {Eu6} node (purple sphere).

In addition, there are two types of π-π stacking interactions in 1: the interlayer one that exists between the parallel pyridyl rings (N(1)-C(1)-C(5)) (Cg(1)) with a centroidto-centroid distance of 3.687(6) A˚ and further connects the 2D layer network to give a 3D supramolecular framework, while the intralayer one that appears between the parallel pyridyl rings (N(4)-C(22)-C(26)) (Cg(2)) is less weak with a centroid-to-centroid distance of 3.815(13) A˚ and plays a role in stabilizing the 2D layer (Figure 4). The lattice water molecules are present at the interlamellar region through strong hydrogen bonds interacting with a layer network (Table 5, Figure S5, Supporting Information). Crystal Structure of Ln2(2,6-pydc)2(SO4)(H2O)2 3 (H2O)2 (Ln = Ce (2); Ln = Pr (3); Ln = Nd (4); Ln = Sm (5)). X-ray structure analyses reveal that 2-5 are isostructural and crystallize in the monoclinic P2(1)/n space group. Therefore, only the structure of 4 is described in detail. Compound 4 contains 3D metal-organic frameworks based upon squareplanar tetranuclear {Nd4} SBUs. As shown in Figure 5, there are two independent Nd3þ ions, two pydc2- dianions, one sulfate, two coordinated and two lattice water molecules in the asymmetric unit of 4. The Nd(1) atom is nine-coordinated and has monocapped square antiprism coordination

Figure 5. The asymmetric unit of 4. The thermal ellipsoids are drawn at 30% probability. All hydrogen atoms are omitted for clarity. Symmetry codes for the generated atoms are the same as Table 3.

environments: five OCOO- and one nitrogen atom from three pydc2- ligands together with three oxygen atoms from two SO42- (Figure S6, Supporting Information), while Nd(2) atom is nine-coordinated with a distorted tricapped trigonal prism geometry: five OCOO- and one nitrogen atom from three pydc2- ligands, and one oxygen atom from one SO42together with two coordinated water molecules (Figure S7, Supporting Information). The Nd-O bond lengths vary from 2.419(3) to 2.598(3) A˚ and Nd-N are 2.545(4)-2.609(4) A˚ (Table 3). Owing to the effect of lanthanide constriction, the Ce-O and Ce-N bonds in 2 and Pr-O and Pr-N bonds in 3 are slightly longer than the corresponding Nd-O and Nd-N bonds in 4, while the Sm-O and Sm-N bonds in 5 are slightly shorter than the corresponding Nd-O and Nd-N bonds in 4 (Table 3, Table S1, Supporting Information).

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Figure 6. View of the tetranuclear {Nd4} SBUs in 4. Hydrogen atoms are omitted for clarity.

Both pydc2- ligands adopt the same coordination mode as shown in Scheme 1a. Each of them connects three Nd3þ ions and adopts a μ3-η1, η1, η2, η1 coordination mode. Nd(1) links two neighboring Nd(2) through O(3) and O(9) to give a square-planar tetranuclear {Nd4} SBUs (Figure 6) with Nd(1) 3 3 3 Nd(2) distances being 4.777(5) and 4.801(14) A˚, respectively. SO42- anion in 4 adopts the same coordination mode as described in 1 with the fourth free O(8) atom involved in the formation of hydrogen bonds, and also behaves as an auxiliary supporting bridge to strengthen the {Nd4} SBUs. These {Nd4} SBUs are double-bridged by two carboxylate groups (O(1)-C(6)-O(2)) to form a 1D chain along the a axis (Figure 7a, Figure S8, Supporting Information). Then each chain is further linked to four adjacent and parallel chains by a single carboxylate (O(11)C(14)-O(12)) bridge to give rise to a 3D framework in the bc plane (Figure 7b). The guest water molecules occupy the void of the interspace region and interact with the framework through hydrogen bonds (Table 5, Figure S9, Supporting Information). PLATON calculated no residual solventaccessible void by excluding the guest water molecules. According to the simplification principle adopted in 1, the pydc2- ligands in 4 can be viewed as connectors and the {Nd4} SBUs in 4 are six-connected nodes (Figure S10, Supporting Information). Therefore, the 3D framework of 4 can be abstracted as an uninodal six-connected topological network with the Schl€ afli symbol of (41263) (Figure 7c). The network can be specified by the vertex symbol of 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 * 3 * 3 * analyzed by the OLEX program,27 which is a typical pcu topological network (a nomenclature proposed by O’Keeffe et al).28 Crystal Structure of [Ce 5 (2,6-pydc)6(SO4 )2 (H2O)3 3 (Me2NH2)] (6). Compound 6 crystallizes in the triclinic P1 space group and forms a 3D porous anionic metal-organic framework, which is constructed from square-planar tetranuclear {Ce4} SBUs and Ce(3) monomer, and the charge compensating (Me2NH2)þ cations occupy the void of the channels. As shown in Figure 8, the asymmetric unit of 6 contains five independent Ce3þ ions, six pydc2- dianions, two sulfates, three coordinated water molecules and one (Me2NH2)þ cation. All the Ce3þ ions are nine-coordinated with distorted tricapped trigonal prism geometries, but with

Figure 7. View of the 1D chain along the a axis (a) and polyhedral representation of the 3D framework in the bc plane (b) based on {Nd4} SBUs in 4. Hydrogen atoms and guest water molecules are omitted for clarity. (c) Schematic representation of six-connected 3D framework with pcu topology for 4 based on {Nd4} node (purple sphere).

Figure 8. The asymmetric unit of 6. The thermal ellipsoids are drawn at 30% probability. All hydrogen atoms are omitted for clarity. Symmetry codes for the generated atoms are the same as Table 4.

different coordination environments: seven OCOO- from five pydc2- ligands, and one oxygen atom from one SO 42together with one coordinated water molecule for Ce(1) (Figure S11, Supporting Information) and Ce(5) (Figure S15,

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Figure 9. View of the tetranuclear {Ce4} SBUs of Ce(1) and Ce(2) (a) and {Ce4} SBUs of Ce(4) and Ce(5) (b) in 6. Hydrogen atoms are omitted for clarity.

Supporting Information); four OCOO- and two nitrogen atoms from two pydc2- ligands and three oxygen atoms from two SO42- for Ce(2) (Figure S12, Supporting Information) and Ce(4) (Figure S14); six OCOO- from four pydc2- ligands and two nitrogen atoms from two pydc2ligands together with one coordinated water molecule for Ce(3) (Figure S13, Supporting Information). The Ce-O bond lengths vary from 2.419(8) to 2.719(12) A˚ and Ce-N range from 2.577(10) to 2.651(11) A˚ (Table 4). Six pydc2- ligands in the asymmetric unit adopt two different coordination modes as present in Scheme 1a,b. In mode a, the pydc2- ligand adopts the same coordination mode as depicted in 1 and 4. In mode b, the pydc2- ligand adopts a μ3-η1, η1, η1, η1 coordination mode by coupling with three Ce3þ ions. Ce(1) links two neighboring Ce(2) by O(2) and O(11) to form a square-planar tetranuclear {Ce4} SBUs (Figure 9a) with Ce(1) 3 3 3 Ce(2) distances of 4.884(9) and 4.917(16) A˚, respectively. Meanwhile, Ce(4) connects two neighboring Ce(5) through O(23) and O(27) to generate another square-planar tetranuclear {Ce4} SBUs (Figure 9b) with Ce(4) 3 3 3 Ce(5) distances being 4.874(25) and 4.910(16) A˚, respectively. Both SO42- anions in the two {Ce4} SBUs adopt the same coordination mode as shown in Scheme 1e. Each SO42- anion connects three Ce3þ ions of the {Ce4} SBUs with the fourth oxygen atom (O(7) or O(32)) free. Therefore, both of the two {Ce4} SBUs are consolidated by two SO42- anions acting as auxiliary supporting bridges. The linkage between the two {Ce4} SBUs and nine-coordinated Ce(3) monomers by pydc2- ligands give a novel 3D anionic framework with the same 1D channels along the a (Figure 10a) and b (Figure S16, Supporting Information) axes. The guest (Me2NH2)þ cations are filled in the channels along the b axis and interact with the anionic framework through hydrogen

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Figure 10. (a) Polyhedral representation of the 3D anionic framework for 6 based on {Ce4} SBUs and Ce(3) monomer viewed in the bc plane, showing 1D channels along the a axis. Hydrogen atoms are omitted for clarity. Color code: Ce(1) and Ce(2), purple; Ce(3), cyan; Ce(4) and Ce(5), yellow. (b) Schematic representation of the equivalent 4,6-connected 3D anionic framework with sqc422 topology for 6 based on {Ce4} SBUs (purple and yellow sphere) and Ce(3) monomers (cyan sphere).

bonds (Figure S16, Supporting Information). In addition, there are strong hydrogen bonds between the coordinated water molecules and the oxygen atoms of the carboxylate groups (Figure S17, Supporting Information). PLATON calculated a suggested solvent-accessible volume of about 165.5 A˚3 by excluding the guest (Me2NH2)þ cations. As shown in Figure 10b, each Ce(3) monomer is linked to four {Ce4} SBUs, among which two are of Ce(1) and Ce(2) and the other two of Ce(4) and Ce(5) through two pydc2ligands (Figure S18a, Supporting Information), while both {Ce4} SBUs are linked to two nearest {Ce4} SBUs and four Ce(3) monomers through 10 pydc2- ligands (Figure S18b,c, Supporting Information). Therefore, the connection between Ce(3) monomers and {Ce4} SBUs can be abstracted as a (4,6,6)-connected net by viewing the Ce(3) monomer as a four-connected node and the two {Ce4} SBUs as two sixconnected nodes. Thus, the overall anionic framework topology for 6 can be described with the Schl€ afli symbol of (42510728)(42510728)(4254). When further analyzed by OLEX program,27 the vertex symbols of the two Ce4 nodes are both 4.4.5.5.5.5.5.5.5.5.5(2).5(2).7(4).7(4).8(16), so the anionic framework of 6 can be simplified as a binodal (4,6)-connected net with the Schl€ afli symbol of (42510728)(4254), which is a typical sqc422 topological structure. Powder X-ray Diffraction. The synthesized products of 1-6 have been characterized by powder X-ray diffraction (PXRD) (Figures S22 and S23, Supporting Information). The experimental PXRD patterns correspond well with the

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Figure 12. Temperature dependence of χMT and χM for 1 at 1 KOe. Figure 11. Solid-state emission and excitation (inset) spectra of 1 at room temperature.

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. Thermal Stability Analyses. To study the thermal stabilities of compounds 1-6, thermogravimetric analyses (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 (Figure S24, Supporting Information). These six compounds all undergo two steps of weight loss. TG curve of 1 shows that three guest water and three coordinated water molecules were gradually lost in the temperature range of 70-310 C (calcd/found: 8.19/8.26%). Above this temperature, the whole framework collapses with the release of all organic carboxlylate ligands and one sulfate. Compounds 2-5 with the same structure show a similar thermal behavior. TG curves of 2-5 show that from 60 to 260 C two guest water and two coordinated water molecules were gradually lost for 2 (calcd/found: 9.26/ 9.27%), for 3 (calcd/found: 9.02/9.10%), for 4 (calcd/found: 9.16/9.12%), for 5 (calcd/found: 9.02/8.95%), respectively. Above 450 C, the frameworks began to collapse accompanying the decomposition of the pydc2- ligands and the removal of one sulfate. TG curve of 6 shows that (Me2NH2)þ cation and three coordinated water molecules were gradually lost in the temperature range of 80-290 C (calcd/found: 5.05/5.00%). Then the framework collapses with the decomposition of the pydc2- ligands and the departure of two sulfates. Luminescent Property. The solid-state luminescent property of 1 was investigated at room temperature. The excitation wavelength (394 nm) was selected as the maximum of the solid-state excitation spectrum (Figure 11, inset). Compound 1 displays intense red luminescence (Figure 11) and exhibits the characteristic transition of the Eu3þ ion with a decay lifetime of 601.1 μs; the peaks at 579, 592, 614, 651, and 696 nm are attributed to the 5D0 f 7FJ (J = 0-4) transitions, respectively. The appearance of the symmetry-forbidden emission 5D0 f 7F0 at 579 nm indicates that Eu3þ ions in 1 occupy sites with low symmetry and have no inversion center,5 which is further confirmed by the intensity ratio

Figure 13. Temperature dependence of χMT and χM for 5 at 1 KOe.

of about 5.2 for I(5D0 f 7F2)/I(5D0 f 7F1).29 This is in accordance with the result of the single-crystal X-ray analysis. Magnetic Properties. The temperature-dependent magnetic susceptibility measurements of compounds 1, 4, and 5 have been performed on the polycrystalline samples in the temperature range 2-300 K at a 1 KOe external field 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 vs T for compounds are given in Figures 12-14. Both of the compounds 1 and 5 show a typical antiferromagnetic character. As shown in Figures 12 and 13, at 300 K, the experimental χMT values for 1 and 5 are 4.11 cm3 3 K 3 mol-1 and 0.79 cm3 3 K 3 mol-1, which are close to the expected value (theoretical: 4.5 cm3 3 K 3 mol-1 and 0.64 cm3 3 K 3 mol-1) for the excited states of three isolated Eu3þ and two isolated Sm3þ ions, respectively. As the temperature cools down, the χMT values monotonously decrease, which are obviously attributed to the thermal depopulation of the stark components of lanthanide ions at low temperature.29,6c,30 At 2 K, χMT for 1 and 5 are 0.039 and 0.084 cm3 3 K 3 mol-1, both of which are also close to the expected values of their ground state (calculated: 0 cm3 3 K 3 mol-1 for 7F0 of Eu3þ and 0.090 cm3 3 K 3 mol-1 for 6 H5/2 of Sm3þ ions).30 As shown in Figure 14, at 300 K, χMT of 4 is equal to 3.11 cm3 3 K 3 mol-1, as expected for two isolated Nd3þ ions (3.28 cm3 3 K 3 mol-1) with a 4I9/2 ground

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Figure 14. Temperature dependence of χMT, χM, and χM-1 (inset) for 4 at 1 KOe.

state.6c As the temperature is lowered, χMT decreases to a minimum at about 4 K and then increases rapidly to reach 1.54 cm3 3 K 3 mol-1 at 2 K. The plot of χM-1 vs T over the temperature range 30-300 K obeys the Curie-Weiss law [χM = C/(T - θ)] with C = 3.22 cm3 3 K 3 mol-1 and θ = -33.34 K. The decrease of χMT and the negative value of θ may be due to the populations of the Stark levels and/or possible antiferromagnetic interactions between the two Nd3þ ions,31 and the increase at low temperature reveals dominant ferromagnetic interaction between Nd3þ ions in the carboxylate-bridged chain.

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Conclusions In summary, using inorganic sulfate as an auxiliary ligand, we have successfully synthesized a series of novel Ln SBUs based LnMOFs under hydrothermal conditions. Compound 1 presents a four-connected 2D network based on hexanuclear {Eu6} SBUs. Compounds 2-5 adopt uninodal six-connected 3D framework of pcu topology constructed from squareplanar tetranuclear {Ln4} SBUs. Compound 6 exhibits a protonated (Me2NH2)þ templated binodal (4,6)-connected 3D anionic framework with (42510728)(4254) topology, when the Ce(3) monomer and the planar tetranuclear {Ce4} SBUs are regarded as four-connected and six-connected nodes, respectively. Therefore, the introduction of inorganic sulfate ligand into LnMOFs not only provides a rational route to construct Ln SBUs based LnMOFs but also provides a good way to make new materials.

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Acknowledgment. Financial support from the 973 Program (2009CB939803), NSFC (20821061, 20925102), “The Distinguished Oversea Scholar Project”, “One Hundred Talent Project”, and Key Project from CAS is greatly appreciated. Supporting Information Available: CIF of compounds 1-6, Figures S1-S24, selected bond lengths and hydrogen bond tables for 2, 3, and 5. This information is available free of charge via the Internet at http://pubs.acs.org/.

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