Synthesis, Crystal Structures, and Solid-State Luminescent Properties

Article pubs.acs.org/crystal

Synthesis, Crystal Structures, and Solid-State Luminescent Properties of Diverse Ln−Pyridine-3,5-Dicarboxylate Coordination Polymers Modulated by the Ancillary Ligand Xiao Wang,†,‡ Quan-Guo Zhai,*,† Shu-Ni Li,† Yu-Cheng Jiang,† and Man-Cheng Hu*,† †

Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi, 710062, People’s Republic of China ‡ Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan’an University, Yan’an, Shaanxi 716000, People’s Republic of China S Supporting Information *

ABSTRACT: Nine members of the novel Ln−pyridine-3,5-dicarboxylate coordination polymer family, namely, [Ln(PDC)(GA)]n (Ln = Gd (1), Tb (2), Dy (3), Er (4)), [Ln(PDC)(OAc)(H2O)]n·nH2O (Ln = Sm (5), Eu (6), Gd (7)), [Gd(PDC)(OAc)(H2O)2]n·nH2O (8), and [Tb(PDC)1.5(H2O)]n (9) (PDC = pyridine-3,5dicarboxylate, GA = glycolate, OAc = acetate), have been successfully obtained by carefully regulating the ancillary ligand or reaction temperatures. Complexes 1−4 are isomorphous two-dimensional networks generated by Ln−glycolate chains and bridging PDC ligands. When the HOAc was utilized instead of glycolic acid, isomorphic three-dimensional compounds 5−7 were isolated. The Ln3+ atoms are first bridged by acetate anions to give dinuclear clusters, which are extended by nearby six PDC ligands forming a 3D (3,6)-connected f lu-topological framework. Notably, the increase of reaction temperature from 160 to 180 °C during the synthesis of compound 7 led to compound 8, the other 3D (3,6)-connected structure on the base of dinuclear subunits with rtl topology. Furthermore, the absence of HOAc introduced the formation of compound 9, in which each binuclear cluster links adjacent eight PDC anions to give a 3D (3,8)-connected tfz-d topological structure. The elemental analyses, XRPD, FT-IR, and TGA were also investigated to characterize compounds 1−9. Furthermore, solid-state photoluminescence measurements show that these Ln−pyridine-3,5-dicarboxylate coordination polymers produce strong emissions at room temperature.

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INTRODUCTION

chromophoric antenna ligands, which are good sensitizers to stimulate lanthanide ion luminescence.7 Among the various aromatic multidentate ligands, pyridine dior multicarboxylates have been intensively employed to provide a great variety of topological architectures due to their remarkable versatile coordination modes.8 More recently, we have focused on pyridine-3,5-dicarboxylic acid (H2PDC), a triangular multidentate ligand to construct Ln-MOFs due to four inherent chemical features: first, the π-electric conjugated system of the pyridine group maybe acts as a chromophore. Second, the H2PDC ligand possesses multiple types of potential metal binding sites and shows flexible and various coordination modes, wherein the oxygen/nitrogen atoms could coordinate with the lanthanide ions in terminal monodentate or diverse bridging motifs, or chelate Ln3+. Third, carboxyl groups with strong coordination ability can effectively preclude the formation of Ln−H2O coordination bonds and, therefore, reduce the activation of nonradiative decay. Finally, the H2PDC ligand can

Coordination polymers (or called metal−organic frameworks, MOFs) have provoked great interests for not only the diverse structures but also their potential technological applications.1 Specially, the lanthanide (Ln)-MOFs showing unique photophysical properties are excellent candidates for optical devices2 due to the transitions within the partially filled 4f shells of Ln cations. In the past decade, plenty of Ln coordination polymers exhibiting fascinating structures and various properties, such as luminescence and atypical magnetism, have been synthesized.3 It is well-known that the lanthanide cations are luminescent over the entire spectrum. For example, the emission of Gd is in the ultraviolet region, but Sm, Eu, Tb, Dy, and Tm all exhibit visible emissions. In the near-infrared regions, Nd, Yb, and Er show interesting luminescent emissions.4 However, the Ln3+ usually gives weak luminescence because of the Laporte forbidden f−f transitions.5 To overcome this problem, one effective way is the selection of suitable organic ligands as antenna chromophores, which can improve the emission efficiency of Ln cations.6 Up to now, a lot of aromatic carboxylate or pyridine-carboxylate ligands have been demonstrated to be © 2013 American Chemical Society

Received: September 13, 2013 Revised: November 19, 2013 Published: November 26, 2013 177

dx.doi.org/10.1021/cg401365x | Cryst. Growth Des. 2014, 14, 177−188

Crystal Growth & Design

Article

Table 1. Crystal Data and Structure Refinement Summary for Compounds 1−9 1

2

3

T/K empirical formula formula weight crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Dc/Mg·m−3 F(000) reflns collected data/restraints/params Z final R R indices (all data) GOF on F2

293(2) H6C9NO7Gd 397.40 monoclinic P2(1)/c 7.6349(3) 17.1484(5) 8.4454(3) 90.00 104.922(4) 90.00 1068.44(6) 2.471 1032 4800 2186/0/163 4 0.0259 0.0574 0.963 4

293(2) H6C9NO7Tb 399.07 monoclinic P2(1)/c 7.6339(5) 17.0720(9) 8.3853(4) 90.00 104.517(6) 90.00 1057.93(10) 2.506 752 4441 2165/0/163 4 0.0332 0.0696 1.023 5

293(2) H6C9NO7Dy 402.65 monoclinic P2(1)/c 7.6299(3) 17.0477(7) 8.3505(3) 90.00 104.340(5) 90.00 1052.33(7) 2.541 756 4495 2155/0/163 4 0.0332 0.0674 0.992 6

T/K empirical formula formula weight crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Dc/Mg·m−3 F(000) reflns collected data/restraints/params Z final R R indices (all data) GOF on F2

293(2) H6C9NO7Er 407.41 monoclinic P2(1)/c 7.6082(3) 17.0721(4) 8.2971(3) 90.00 104.215(4) 90.00 1044.70(6) 2.590 764 4384 2143/0/163 4 0.0407 0.1002 1.036 7

296(2) C9H10NO8Sm 410.54 monoclinic C2/c 15.8757(10) 9.0998(5) 18.7296(12) 90.00 111.039(7) 90.00 2525.4(3) 2.144 1552 5314 2585/0/172 8 0.0357 0.0928 1.053 8

293(2) H10C9NO8Eu 412.14 monoclinic C2/c 15.890(2) 9.0934(14) 18.737(3) 90.00 111.075(2) 90.00 2526.4(7) 2.151 1560 6790 2575/0/172 8 0.0483 0.1992 1.068 9

T/K empirical formula formula weight crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Dc/Mg·m−3 F(000) reflns collected data/restraints/params Z final R

293(2) H10C9NO8Gd 417.43 monoclinic C2/c 15.9586(7) 9.0571(4) 18.9040(12) 90.00 111.473(6) 90.00 2542.7(2) 2.165 1568 5701 2598/0/177 8 0.0268

293(2) H12C9NO9Gd 435.45 monoclinic P2(1)/c 10.2899(3) 12.2282(4) 10.1888(7) 90.00 91.531(4) 90.00 1281.57(10) 2.153 788 3349 2248/0/181 4 0.0303 178

293(2) H13C21N3O14Tb2 849.20 monoclinic C2/c 14.1072(10) 11.1298(8) 15.0715(12) 90.00 92.040(7) 90.00 2364.9(3) 2.385 1608 4975 2418/0/182 4 0.0509



Article

Table 1. continued 7 R indices (all data) GOF on F2

8

0.0773 1.063

0.0785 0.990

deprotonate in different processes, leading to HPDC− or PDC2− anions, which are beneficial for the formation of stable supramolecular structures through the hydrogen bonds. To combine the interesting luminescence of lanthanide cations and the diverse properties of the pyridine-3,5dicarboxylic acid ligand mentioned above, we have focused on the novel Ln−pyridine-3,5-dicarboxylate coordination polymers to evaluate their applications as luminescence materials. Moreover, ancillary ligands with excellent chelating ability, such as acetic acid, have been widely used to prevent the coordination of solvent or water molecules with lanthanide centers. In this contribution, glycolic acid and acetic acid are selected as the ancillary bridging ligand to regulate the assemblies of Ln cations and H2PDC under hydrothermal conditions. As a result, nine members of the Ln−PDC coordination polymer family, namely, [Ln(PDC)(GA)]n (Ln = Gd (1), Tb (2), Dy (3), Er (4)), [Ln(PDC)(OAc)(H2O)]n·nH2O (Ln = Sm (5), Eu (6), Gd (7)), [Gd(PDC)(OAc)(H2O)2]n·nH2O (8), and [Tb(PDC)1.5(H2O)]n (9) (PDC = pyridine-3,5-dicarboxylate, GA = glycolate, OAc = acetate), are successfully isolated. Singlecrystal X-ray diffractions show that these nine compounds are of four types of structures varying from 2D layers (1−4), 3D (3,6)connected f lu frameworks (5−7), a 3D (3,6)-connected rtl framework (8), to a 3D (3,8)-connected tfz-d topological net (9). Moreover, solid-state photoluminescence measurements show that these Ln−pyridine-3,5-dicarboxylate coordination polymers produce strong emissions at room temperature.

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9 0.1233 1.075

3.48. Found: H, 1.55; C, 26.89; N, 3.43. IR (KBr, cm−1): 3454(m), 3061(w), 1624(s), 1606(s), 1550(m), 1515(m), 1393(m), 821(w), 773(w), 715(w). Calcd for C9H6NO7Er: H, 1.48; C, 26.53; N, 3.44. Found: H, 1.52; C, 26.57; N, 3.47. IR (KBr, cm−1): 3450(m), 3108(w), 1633(s), 1550(s), 1408(m), 1383(m), 1325(m), 889(w), 820(w), 721(w). Syntheses of [Sm(PDC)(OAc)(H2O)]n·nH2O (5), [Eu(PDC)(OAc)(H2O)]n·nH2O (6), and [Gd(PDC)(OAc)(H2O)]n·nH2O (7). Sm2O3 (0.1 mmol, 0.0349 g), H2PDC (0.2 mmol, 0.0334 g), HOAc (0.25 mmol, 0.015 mL), and H2O (5 mL) were mixed and heated in a 25 mL Teflon-lined stainless steel autoclave under autogenous pressure at 160 °C for 5 days. The reaction mixture was then cooled with a rate of 4 °C/ h. Colorless needle-like crystals were filtrated, washed, and dried. Yield: 69% (based on Sm). Elemental analysis (%): Calcd for C9H10NO8Sm: H, 2.46; C, 26.33; N, 3.41. Found: H, 2.51; C, 26.32; N, 3.43. IR (KBr, cm−1): 3446(m), 3022(w), 1628(s), 1610(s), 1538(m), 1514(m), 1419(m), 820(w), 785(w), 689(w). The similar process was followed to synthesize 6 and 7, except Sm2O3 was replaced by Eu2O3 (0.1 mmol, 0.0352 g) and Gd2O3 (0.1 mmol, 0.0363 g), respectively. Colorless needle-like crystals of 6 and 7 were collected in 72% and 76% yields (based on Eu and Gd). Elemental analysis (%): Calcd for C9H10NO8Eu: H, 2.45; C, 26.23; N, 3.40. Found: H, 2.47; C, 26.21; N, 3.42. IR (KBr, cm−1): 3440(m), 3039(w), 1640(s), 1574(s), 1532(m), 1466(m), 1377(m), 860(w), 767(w), 670(w). Calcd for C9H10NO8Gd: H, 2.41; C, 25.90; N, 3.36. Found: H, 2.44; C, 25.87; N, 3.34. IR (KBr, cm−1): 3455(m), 3103(w), 1634(s), 1570(s), 1540(m), 1508(m), 1461(m), 831(w), 773(w), 731(w). Synthesis of [Gd(PDC)(OAc)(H2O)2]n·nH2O (8). Gd2O3 (0.2 mmol, 0.0725 g), H2PDC (0.6 mmol, 0.0557 g), HOAc (0.6 mmol, 0.036 mL), and H2O (5 mL) were mixed and heated in a 25 mL Teflonlined stainless steel autoclave under autogenous pressure at 180 °C for 5 days. The reaction mixture was then cooled with a rate of 5 °C/h. Colorless block crystals were filtrated, washed, and dried. Yield: 64% (based on Gd). Elemental analysis (%): Calcd for C9H12NO9Gd: H, 2.78; C, 24.82; N, 3.22. Found: H, 2.82; C, 24.86; N, 3.24. IR (KBr, cm−1): 3450(m), 3061(w), 1607(s), 1581(s), 1550(m), 1500(m), 1461(m), 878(w), 773(w), 679(w). Synthesis of [Tb(PDC)1.5(H2O)]n (9). Tb2O3 (0.3 mmol, 0.1087 g), H2PDC (0.6 mmol, 0.0557 g), and H2O (8 mL) were mixed and heated in a 25 mL Teflon-lined stainless steel autoclave under autogenous pressure at 180 °C for 5 days. The reaction mixture was then cooled with a rate of 5 °C/h. Colorless prism-shaped crystals were filtrated, washed, and dried. Yield: 78% (based on Tb). Elemental analysis (%): Calcd for C10.5H6.5N1.5O7Tb: H, 1.54; C, 29.70; N, 4.95. Found: H, 1.52; C, 29.76; N, 4.94. IR (KBr, cm−1): 3452(m), 3027(w), 1629(s), 1598(s), 1532(m), 1520(m), 1389(m), 864(w), 718(w), 663(w). X-ray Crystallographic Determination. Crystal data for the complexes 1−9 were collected using a Gemini E diffractometer with graphite monochromated Mo Kα (λ = 0.71075 Å) radiation. The single crystal structures were solved by direct methods using the SHELXS-97 program and refined by the SHELXL-97 program.9 Non-hydrogen atoms were refined anisotropically by the full-matrix least-squares method. The C−H and O−H hydrogen atoms were also found and refined. The detailed crystallographic data and the structure refinement parameters of compounds 1−9 are summarized in Table 1. Selected bond distances and angles are given in Table2.

EXPERIMENTAL SECTION

Materials and Methods. All the solvents and starting materials were ordered and used without further purification. Elemental analysis (C, H, and N) was determined using a PerkinElmer 2400 elemental analyzer. FT-IR spectra were recorded by the Bruker EQUINOX-55 spectrophotometer (400−4000 cm−1, KBr pellet). TGA was carried out on a NETZSCH STA 449C thermal analyzer. PXRD measurements were performed on a D/Max2550VB+/PC diffractometer with Cu Kα (λ = 1.5406 Å), and the X-ray tube was operated at 40 kV and 40 mA. Solid-state photoluminescence emissions were collected on an Edinburgh Instrument F920 fluorescence spectrometer at ambient temperature. Syntheses of [Gd(PDC)(GA)]n (1), [Tb(PDC)(GA)]n (2), [Dy(PDC)(GA)]n (3), and [Er(PDC)(GA)]n (4). Gd2O3 (0.1 mmol, 0.0363 g), H2PDC (0.3 mmol, 0.0501 g), and HGA (0.2 mmol, 0.015 g) were mixed and dissolved in 8 mL of water in a 25 mL Teflon-lined stainless steel autoclave. The mixture was stirred for about 1 h at room temperature and heated at 160 °C for 5 days. The reaction system was then cooled with a rate of 5 °C/h. Colorless sheet-shaped crystals were filtrated, washed, and dried. Yield: 61% (based on Gd). Elemental analysis (%): Calcd for C9H6NO7Gd: H, 1.52; C, 27.02; N, 3.52. Found: H, 1.57; C, 27.01; N, 3.49. IR (KBr, cm−1): 3444(m), 2935(w), 1629(s), 1560(s), 1542(m), 1471(m), 1461(m), 931(w), 879(w), 769(w). A similar procedure was utilized to prepare compounds 2, 3, and 4, except Gd2O3 was replaced by Tb2O3 (0.1 mmol, 0.0366 g), Dy2O3 (0.1 mmol, 0.0373 g), and Er2O3 (0.1 mmol, 0.0382 g), accordingly. Colorless sheet-shaped crystals of 2 and 3 and pink sheet-shaped crystals of 4 were collected in 59, 54, and 58% yields based on Tb, Dy, and Er, respectively. Elemental analysis (%): Calcd for C9H6NO7Tb: H, 1.53; C, 27.09; N, 3.51. Found: H, 1.48; C, 26.89; N, 3.56. IR (KBr,cm−1): 3443(m), 3107(w), 1639(s), 1601(s), 1555(m), 1507(m), 1389(m), 879(w), 810(w), 721(w). Calcd for C9H6NO7Dy: H, 1.50; C, 26.85; N,

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RESULTS AND DISCUSSION Synthesis. The hydro/solvothermal method has been used to synthesize a large amount of metal−organic complexes with fascinating architectures and functions. It gives a more convenient and effective route to synthesize high-quality single crystals of MOFs over other methods.10 However, the research 179



Article

Table 2. Selected Bond Lengths (Å) and Angles (deg) for the Compounds 1−9 compound 1 Gd1−O1 Gd1−O4 Gd1−O6c O2a−Gd1−O5c O1−Gd1−O3d symmetry codes: a: −x, −0.5

2.324(3) 2.689(4) 2.358(3) 85.76(13) 75.28(12) + y, 0.5 − z; b: x, 1.5

2.327(4) Gd1−O2a 2.348(3) Gd1−O5b Gd1−O7c 2.460(4) O5c−Gd1−O7c 74.67(12) O6−Gd1−O3d 137.59(12) − y, 0.5 + z; c: x, 1.5 − y, −0.5 + z; d: −x, 2 − y, 1 − compound 2

Gd−O3d Gd1−O6 O1−Gd1−O2a O6−Gd1−O7c O6−Gd1−O4 z.

2.364(4) 2.409(3) 101.65(13) 63.58(11) 79.54(11)

2.314(4) Tb1−O2b 2.693(5) Tb1−O5 2.441(4) O5c−Tb1−O7c 77.56(14) O3a−Tb1−O2b 79.62(13) O2b−Tb1−O7c 0.5 + z; d: 1 − x, −y, 2 − z.

2.323(4) 2.388(4) 63.95(13) 85.49(14) 74.29(14)

2.679(5) Dy1−O3c 2.341(4) Dy1−O6a 2.430(4) O3c−Dy1−O6a 79.70(13) O1−Dy1−O5d 78.02(14) O4−Dy1−O5d + z; d: −x, 1 − y, 1 − z.

2.313(4) 2.303(4) 84.89(14) 75.60(14) 74.71(14)

2.278(5) Er−O2 2.356(5) Er1−O5b 2.399(5) O3d−Er1−O6 137.42(18) O4c−Er1−O6c 77.58(17) O1−Er1−O4c −0.5 + z; d: 1 − x, 2 − y, 2 − z.

2.298(4) 2.681(6) 74.55(19) 64.87(19) 129.45(2)

Tb1−O1 2.306(4) Tb1−O3a d Tb1−O4 2.342(4) Tb1−O6 2.343(4) Tb1−O7c Tb1−O5c O1−Tb1−O5c 73.97(13) O3a−Tb1−O5c O3a−Tb1−O4d 80.98(15) O2b−Tb1−O5c symmetry codes: a: 1 − x, −0.5 + y, 2.5 − z; b: x, 0.5 − y, −0.5 − z; c: x, 0.5 − y, compound 3 Dy1−O1 Dy1−O4b Dy1−O4 O1−Dy1−O4d O2−Dy1−O7b symmetry codes: a: −x, 0.5

2.297(4) 2.329(4) 2.384(4) 74.07(13) 71.36(14) + y, 0.5 − z; b: x, 1.5 −

Dy1−O2 Dy1−O5d Dy1−O7b O3c−Dy1−O4 O4c−Dy1−O6a y, −0.5 + z; c: x, 1.5 − y, 0.5 compound 4

Er1−O1 2.268(4) Er1−O4c d Er1−O3 2.323(5) Er1−O6 2.316(5) Er1−O7a Er1−O6c O1−Er1−O3d 75.58(17) O6c−Er1−O3d O2−Er1−O7a 73.79(18) O6c−Er1−O7a symmetry codes: a: 1 − x, −0.5 + y, 1.5 − z; b: x, 1.5 − y, 0.5 + z; c: x, 1.5 − y, compound 5 Sm1−O1 Sm1−O5 Sm1−N1 O2a−Sm1−O1a O6c−Sm1−O4d symmetry codes: a: 0.5 − x,

2.463(4) 2.539(5) Sm1−O2a 2.470(4) Sm1−O6c 2.397(5) 2.585(5) Sm1−O4d 2.432(4) 51.98(15) O3b−Sm1−O2a 84.16(17) 79.03(16) O3b−Sm1−O6 71.78(15) −0.5 + y, 0.5 − z; b: −0.5 + x, −0.5 + y, z; c: 0.5 − x, 0.5 − y, 1 − z; d: 1 − compound 6

Sm1−O3b Sm1−O6 Sm1−O8 O4d−Sm1−N1 O6c−Sm1−O2a x, 1 − y, 1 − z.

2.343(5) 2.638(4) 2.468(5) 76.79(17) 85.20(16)

Eu1−O1 Eu1−O3b Eu1−N1d O3b−Eu1−O7c O2−Eu1−O1 symmetry codes: a: 0.5 + x,

2.449(9) Eu1−O6 2.455(10) Eu1−O7C 2.313(10) Eu1−O8 2.453(10) Eu1−O2 2.580(10) Eu1−O7 2.629(9) Eu1−O4a C a 72.1(3) O7 −Eu1−O4 78.6(3) O4a−Eu1−O2 52.6(3) O1−Eu1−O8 70.9(3) N1d−Eu1−O8 1.5 − y, 1 − z; b: −x, +y, −0.5 − z; c: 0.5 − x, 1.5 − y, 1 − z; d: 0.5 − x, 0.5 + y, 0.5 − z. compound 7

2.399(9) 2.539(9) 2.416(9) 74.6(3) 71.6(3)

Gd1−O1 2.447(4) Gd1−O7b 2.375(4) Gd1−O5 2.551(4) Gd1−O6a 2.406(3) Gd1−O7 2.611(3) Gd1−O2 2.433(4) O4c−Gd1−O7b 71.76(13) O7b−Gd1−O6a 78.51(12) O1−Gd1−O3 74.33(14) O3−Gd1−N1d 71.69(13) symmetry codes: a: 0.5 + x, 1.5 − y, 0.5 + z; b: 1.5 − x, 1.5 − y, 2 − z; c: 1 − x, y, 1.5 − z; d: 1.5 + x, 0.5 compound 8 Gd1−O2 Gd1−O5 Gd1−O8 O2−Gd1−O1c O5a−Gd1−O3b symmetry codes: a: 2 − x, 1 Tb1−O1d Tb1−O4c O1d−Tb1−O2d

2.309(4) Gd1−O1c 2.725(4) Gd1−O5a 2.410(5) Gd1−O4b 89.22(17) O2−Gd1−O8 76.36(14) O3b−Gd1−O6 − y, −z; b: 1 − x, 1 − y, −z; c: x, 0.5 − y, −0.5 + z. compound 9 2.577(7) 2.331(7) 51.4(2)

Tb1−O2d Tb1−O5 Tb1−O3c 180

Gd1−O3 Gd1−O4c Gd1−N1d O5−Gd1−O7b O5−Gd1−N1d + y, 1.5 − z.

2.439(4) 2.344(3) 2.576(4) 75.56(13) 73.77(13)

2.327(4) 2.416(4) 2.514(4) 78.71(18) 80.32(13)

Gd1−O3b Gd1−O6 Gd1−O7 O7−Gd1−O1c O6−Gd1−O7

2.492(4) 2.458(4) 2.473(4) 71.15(15) 71.13(15)

2.466(6) 2.335(8) 2.456(7)

Tb1−O2a Tb1−O6b Tb1−O7

2.381(7) 2.244(7) 2.308(7)



Article

Table 2. continued compound 9 70.6(2) O3c−Tb1−O6b 79.1(3) O2a−Tb1−O3c c 79.2(3) O4c−Tb1−O2d 68.8(2) O7−Tb1−O4 symmetry codes: a: −0.5 + x, 0.5 + y, z; b: −x, 1 − y, −z; c: −0.5 + x, 0.5 − y, −z; d: −x, −y, −z.

O4c−Tb1−O1d O2a−Tb1−O6b

82.8(2) 84.3(3)

Crystal Structures of [Ln(PDC)(OAc)(H2O)]n·nH2O (Ln = Sm (5), Eu (6), Gd (7)). Under the similar hydrothermal conditions as those of compounds 1−4, the utilizations of HOAc instead of glycolic acid led to compounds 5−7, which are isomorphic 3D frameworks in the monoclinic C2/c space group. For convenience, compound 7 is selected herein to be discussed in detail. Each asymmetric unit of 7 contains one unique Gd ion, one PDC anion ligand, one OAc ligand, one coordinated H2O, and two lattice H2O molecules (Figure 2a). The Gd1 atom is nine-coordinated by one water molecule (O3), four carboxylate oxygens (O2, O5, O4c, O6a) of three PDC ligands, three carboxylate oxygens (O1, O7, O7b) of two OAc anion ligands, and a nitrogen atom (N1D) from a PDC ligand to generate a distorted monocapped square antiprism coordination polyhedron with the [NO8] donors set. The coordination Gd−O/N bonds, varying from 2.344(3) to 2.608(3) Å, are within the reported results.12 The OAc ligands adopt a μ2-kO:kO,O coordination mode (Scheme 1f) linking two Gd3+ centers to form a [Gd2(OAc)2] binuclear cluster with the Gd···Gd separation of 3.9848(3) Å (Figure 2b). Each binuclear cluster is coordinated by six PDC ligands through the bidentate bridging, bidentate chelating, and monodentate coordination modes. On the other hand, each PDC ligand adopts a μ4kO:kO:kO,O:kN coordination mode (Scheme 1b) to bridge three binuclear clusters. Thus, the binuclear clusters and PDC ligands led to the novel 3D structure of compound 7 (Figure 2c). On the other hand, the framework 7 can also be described in a step-by-step method. As shown in Figure 2d, adjacent binuclear clusters are joined by two PDC ligands through bridging bidentate and chelating bidentate modes to generate a 1D double chain. Adjacent double chains link each other through the PDC ligands via the pyridine N atoms to give a 2D network lying in the bc plane. Finally, the 2D networks are extended to the 3D framework of 7 via the residue N donors along the a-axis direction. To analyze the net, we can define PDC ligands and the binuclear clusters as three-connected and six-connected nodes. These two nonequivalent nodes lead to a (3,6)-connected network of 7 (Figure 2e) with the Schläfli symbol of (42.6)2(44.62.87.102), which corresponds to the f lu topology. Crystal Structure of [Gd(PDC)(OAc)(H2O)2]n·nH2O (8). Compound 8 crystallizes in the monoclinic P2(1)/c space group. The unique unit contains one unique Gd3+, one PDC2− ligand, one OAc− anion, two coordinated H2O water molecules, and one free H2O molecule (Figure 3a). The nine-coordinated Gd3+ is a distorted monocapped square antiprism coordination polyhedron, which is completed by four carboxylic O atoms (O2, O4b, O5b, O1c) from three PDC2− liands, two water oxygen atoms (O7 and O8), and three O atoms (O5, O6, O5a) of two OAc ligands. The Gd−O distances are in the range of 2.310(4)− 2.723(4) Å for Gd−Ocarboxylate and 2.406(5)−2.470(4) Å for Gd−Owater. The O−Gd−O bond angles are in the range of 49.02(12)−147.26(16)°. Just like compound 7, the OAc anions in 8 also adopt a μ2-kO:kO,O coordination mode linking two Gd3+ centers to form a [Gd2(OAc)2] binuclear cluster (Figure 3b). However, the Gd···Gd separation increases to 4.3876(3) Å because there does not exist a bidentate bridging carboxylate

on the complex mechanism of the reaction process in the reaction vessel is still in the primary exploration phase at present, and many possible mechanisms could only be deduced by the intuitive results. For complexes 1−9, the ancillary anionic ligands and reaction temperatures are two key factors. As mentioned above, ancillary ligands with excellent chelating ability can effectively prevent the coordination of solvent or H2O molecules with the lanthanide centers. In our experiments, the utilization of glycolic acid led to four 2D isomorphous networks of 1−4. The addition of HOAc instead of glycolic acid produced three isomorphic 3D frameworks of 5−7. During the synthesis of compound 7, the increase of reaction temperature from 160 to 180 °C led to 8, another 3D compound. Furthermore, the absence of HOAc introduced the formation of compound 9. An isostructure of complex 9 with a Dy ion were also synthesized through the reaction of Dy(NO3)3·6H2O with H2PDC at 165 °C under hydrothermal conditions.11 However, our repeated experiments showed that the usage of Ln2O3 under hydrothermal conditions slowly makes H2PDC ligands deprotonate without hydrolysis, which helps to improve the purity and productivity of 1−9. Crystal Structures of [Ln(PDC)(GA)]n (Ln = Gd (1), Tb (2), Dy (3), Er (4)). Complexes 1−4 are isomorphic, and 1 is selected herein as an example. X-ray crystallography reveals that 1 is a 2D network crystallizing in the monoclinic P2(1)/c space group. The asymmetric unit of 1 contains one independent Gd3+ ion, one PDC2− ligand, and one GA− anion (Figure 1a). The Gd3+ ion is of a distorted bicapped triangle prism geometry surrounding by an [O8] donor set, generated by four carboxylate O atoms of four PDC ligands, three carboxylate oxygen atoms of two GA anions, and one OH group from the GA anion. The Gd−O bond lengths vary from 2.324(3) to 2.689(3) Å, and the O−Gd−O bond angle varies from 50.23(12) to 157.03(13)°. These bond lengths and angles all are comparable with those reported in the Gd3+carboxylate compounds.12 It is noted that each GA anion in complex 1 exhibits a unique double chelating mode between adjacent Gd3+ ions forming the [GdOCO] four-membered and [GdOCCO] five-membered chelating rings (Scheme 1e). Thus, Gd3+ are linked via GA anions to give a 1D chain with the Gd··· Gd separation of 4.3690(4) Å (Figure 1b). On the other hand, the H2PDC ligand is completely deprotonated and adopts a μ4kO:kO:kO:kO coordination mode (Scheme 1a) linking four Gd3+ centers from two adjacent 1D chains to generate a 2D wavelike layered structure in the ac plane (Figure 1b). The strong π···π interactions (centroid-to-centroid distance of 3.6877 Å) between pyridine rings of PDC2− ligands help to stabilize the 2D network of 1. To analyze the network topology, the PDC ligands and Gd3+ center can be simplified as 4- and 5-connected nodes, and the GA anions are regarded as lines. On the other hand, the 1D chain can further be simplified as a rodlike subunit, which led to a simple rod-packing network of 1 (Figure 1c). It is worth mentioning that the strong interlayer hydrogen bonds between the OH group of glycolic acid and the pyridine N atom (O6−H··· N1#1 = 2.641 Å, #1 = 1 − x, − 0.5 − y, 1.5 − z) finally pack the adjacent layers into a 3D supramolecular architecture. 181



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Scheme 1. Coordination Environments of PDC2−, GA−, and OAc− Anions

group as 7. Also, each binuclear cluster is surrounded by six PDC ligands through the bidentate chelating and monodentate modes. The PDC2− ligands adopt a μ3-kO:kO:kO,O coordination mode (Scheme 1c) to link three adjacent binuclear clusters. Thus, the 6-connected binuclear clusters and 3-connected PDC ligands link each other to form the 3D (3,6)-connected framework of compound 8 (Figure 3c). It is different to 7; these two nonequivalent nodes lead to a familiar (3,6)-connected rutile topological net with the Schläfli symbol of (42.6)2(42.610.83). Crystal Structure of [Tb2(PDC)3(H2O)2]n (9). X-ray crystallography reveals that complex 9 is of the monoclinic C2/c space group. The asymmetric unit of 9 (Figure 4a) contains one independent Tb3+ ion, one and a half PDC2− ligands, and one coordinated H2O. Each Tb3+ ion coordinates to seven carboxylate oxygen atoms (O4, O5, O1a, O2a, O2b, O3c, O6d) and one terminal H2O (O7) to give a distorted square antiprism. The Tb−O bond lengths (2.244(7)−2.577(7) Å) are comparable to those reported Ln−carboxylate coordination polymers.2a,11 As shown in Figure 4b, Tb1 atoms are joined by two carboxylate groups with μ2-kO:kO,O fashion and two carboxylate groups with μ2-kO:kO fashion from four PDC ligands to give a [Tb2(CO2)4] binuclear motif with the Tb···Tb separation of 3.790 Å, which is slightly longer than the Dy···Dy distance in the isomorphic compound.11 Furthermore, the other four PDC ligands also connected to this binuclear unit via the monodentate fashion. Thus, each binuclear cluster links eight adjacent PDC ligands. On the other hand, two independent PDC anion ligands adopt the μ4-kO:kO:kO:kO (type-A, Scheme 1a) and μ4kO:kO:kO:kO,O (type-B, Scheme 1d) coordination modes, respectively. It should be noted that each type-A ligand links two adjacent binuclear clusters, but the type-B ligand connects three binuclear subunits. The [Tb2(CO2)4] binuclear motifs are first jointed by the type-B PDC ligands to produce a 2D layer (Figure 4c), which is further connected by the type-A PDC ligands to give the 3D framework (Figure 4d) of compound 9. To analyze the net, we can define the type-A PDC ligands as a linker, and type-B PDC ligands and [Tb2(CO2)4] binuclear clusters as threeconnected and eight-connected nodes, respectively. These two nonequivalent nodes lead to a (3,8)-connected network (Figure 4e) with the Schläfli symbol of (43)2(46.618.84), which is ascribed to the tfz-d topology.

Figure 1. (a) View of the basic coordination environments of compound 1. (b) The 2D framework of 1 in the bc plane. (c) The topology of compound 1. Symmetry codes: a: −x, −0.5 + y, 0.5 − z; b: x, 1.5 − y, 0.5 + z; c: x, 1.5 − y, −0.5 + z; d: −x, 2 − y, 1 − z. 182



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Figure 2. (a) View of the basic coordination environments of compound 7. (b) Structure of the binuclear cluster unit in 7. (c) 3D framework of complex 7. (d) 2D network of 7 in the bc plane. (e) The (3,6)-connected topological net for 7. Symmetry codes: a: 0.5 + x, 1.5 − y, 0.5 + z; b: 1.5 − x, 1.5 − y, 2 − z; c: 1 − x, y, 1.5 − z; d: 1.5 + x, 0.5 + y, 1.5 − z.

Powder X-ray Diffraction and Thermal Stabilities. XRPD patterns for the bulk samples of 1−9 were also done at room temperature and are depicted in Figure S1 (Supporting Information). The well-matched patterns between the simulated and experimental results indicate the phase purity of the

products. It should be pointed out that the intensity differences maybe caused by the preferred orientation of the powder samples. The thermogravimetric analyses for crystal samples of 1−9 were performed under a nitrogen atmosphere in the temperature 183



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to the easy elimination of water molecules (calcd 8.5% for 7, 12.4% for 8). The Ln−PDC frameworks can stabilize to about 350 °C; then complex 7 starts to decompose. A further sharp weight loss was observed from 350 to 680 °C, implying that the PDC2− and OAc− ligands decompose; the final residual should be Gd2O3, supported by the calculated values of 43.9% for 7, 42.3% for 8 (calcd 43.4% for 7, 41.6% for 8). An initial weight loss of approximately 8.1% over a temperature range of 90−260 °C was observed for compound 9. This should be attributed to the loss of H2O water molecules (calcd 8.2%). A further sharp weight loss from 310 to 670 °C implies the decomposition of PDC ligands. The final residual (42.6%) corresponds to Tb2O3 (calcd 43.1%). Overall, the final products after thermal decomposition of the compounds 1−9 all were rare earth oxide. Solid-State Luminescent Properties. Compounds 1−9 are all Ln-MOFs, which maybe have interesting luminescent properties since the trivalent lanthanide ions are well-known for their line-like and color-pure emissions. The Tb3+, Eu3+, Dy3+, Sm3+, Er3+, and Gd3+ coordination complexes exhibit excellent luminescence properties. In addition, we speculate that the H2PDC ligand is an ideal chromophore, which may help to sensitize the lanthanide ion luminescence. Figure 5 showed the solid-state emission spectra of complexes 1−9 at room temperature. The detailed emission information is summarized in Table 3. The luminescence images of complexes 1−9 irradiated under a UV lamp with a wavelength of 254 nm are also given in Figure 6. Figure 5a presents the emission and excitation spectra of Tb3+ complexes 2 and 9 in the solid state. Upon excitation at 290 nm for 2 and 300 nm for 9, the emission spectra of 2 and 9 in the solid state show the strongest emission with four characteristic emission bands at 488 nm (5D4 → 7F6), 543 nm (5D4 → 7F5), 583 nm (5D4 → 7F4), and 621 nm (5D4 → 7F3). In addition, the green emission of the 543 nm (5D4 →7F5) transition is the most intense. A sharp characteristic f−f transition of the Tb3+ at 548 nm around the 543 nm emission band is observed, which is assigned to the 7F5 → 5G6 transition.13 The ligand-based emissions are not observed for 2 and 9. In our opinion, the single intense 4f absorption of Tb3+ indicates the effective sensitization from the H 2 PDC ligands to the Tb 3+ centers under luminescence.14 As depicted in Figure 5b, the excitation of 6 at 330 nm produced five characteristic peaks in the visible region at 579, 593, 617, 652, and 698 nm, which are attributed to the 5D0 → 7F0, 5 D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 transitions, respectively. It should be noted that the 5D0 → 7F0 transition could only be observed in the Cnv, Cn, or Cs symmetry groups according to the selection rules for electric dipole transitions. Thus, the 5D0 → 7F0 transition (579 nm) is very weak since compound 6 is in the monoclinic system. The 5D0 → 7F1 transition is independent of the ligand field effects showing one peak at 593 nm. However, the 5D0 → 7F2 transition is sensitive to the crystal field symmetry and produces an intense band at 618 nm. The intensity of the 5D0 → 7F2 transition is much higher than that of the 5D0 → 7F1 transition, indicating that the Eu3+ ion is in a highly polarizable chemical environment,15 which is in accordance with the single crystal structure. Two broad peaks with less intensity are observed for the 5D0 → 7F3 (652 nm) and 5D0 → 7F4 (698 nm) transitions. Furthermore, the intraligand broad emission band is absent. This result also indicates that PDC ligands can effectively transfer the absorbed energy to the Eu3+ center.14 On the other hand, the excitation spectrum of 6 is also investigated by monitoring the strongest

Figure 3. (a) The asymmetric unit of compound 8. (b) Structure of the binuclear cluster unit in 8. (c) The (3,6)-connected topological net of 8. Symmetry codes: a: 2 − x, 1 − y, −z; b: 1 − x, 1 − y, −z; c: x, 0.5 − y, −0.5 + z.

range from 30 to 800 °C with a heating rate of 10 °C min−1 (Figure S2 and Table S1, Supporting Information). TG analyses indicate that the compounds 1−4 and 5−7 exhibit similar thermal behavior because they are isostructural structures. Herein, the TGA results of 1, 7, 8, and 9 were selected as examples to be discussed in detail. The TGA curves of 1 are stable up to 330 °C. The PDC2− and GA− organic ligands started to decompose gradually at 330 °C and completed at 700 °C. The weight loss of 53.7% for 1 is in good agreement with the calculated values (54.4%), which indicates that the final residual should be Gd2O3. Complexes 7 and 8 show a similar decomposition process. In the temperature range of 60−260 °C, the weight loss (found 8.6% for 7, 11.9% for 8) was attributed 184



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Figure 4. (a) The asymmetric unit of compound 9. (b) The binuclear cluster unit in 9. (c) 2D network of 9 in the bc plane. (d) 3D network of 9. (e) The (3,8)-connected topological framework of 9. Symmetry codes: a: 0.5 − x, 1.5 − y,1 − z; b: −x, 1 − y, 1 − z; c: x, 1 + y, z.

emission at 574 nm belongs to the 4F9/2 → 6H13/2 transition of the Dy3+ ion. Compound 5 exhibits the typical emission bands of the Sm3+ ion sensitized by the H2PDC ligand. Under excitation at 334 nm, three characteristic sharp bands of Sm3+ are observed at 560, 590, and 642 nm, which can be assigned to the 4G5/2 → 6HJ (J = 5/2, 7/2, 9/2) transitions18 (Figure 5c). Among these bands, the most intense one is the emission at 642 nm (4G5/2 → 6H9/2). Different to the compounds discussed above, the intraligand

emission band (617 nm). Four sharp lines at 383, 392, 413, and 462 nm are also found, which are characteristic of the Eu3+ ion energy level and attributed to the transitions from the 7F1 → 5G2, 7 F0 → 5L6, 7F1 → 5D3, and 7F1 → 5D2.16 Yellow luminescence was observed for complex 3.17 Excited at 305 nm, compound 3 shows characteristic narrow emission bands at 486 and 574 nm corresponding to the 4F9/2 → 6HJ (J = 15/2, 13/2) transitions (Figure 5c). The more intensive 185



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Figure 5. Solid-state emission spectra of complexes 1−9 and H2PDC.

fluorescence appears in the emission spectra of complexes 3 and 5 at about 390 nm. The red shift in the spectra of complex 5 compared to the free H2PDC ligand in 390−550 nm attributed to the ligand to Sm3+ charge transfer. The emission spectra of 1, 7, and 8 are illustrated in Figure 5d at the excitation wavelengths of 296, 326, and 329 nm, respectively. The emission spectra of complexes 1 and 7 exhibited a broad band in the range of 350−550 nm and 8 in the range of 350−400 nm. The emissions with λmax = 373 nm for 7 and 8 and λmax = 394 nm for 1 corresponded to the intraligand charge transfer. The emission spectra of the broad fluorescent emission band in 400−500 nm with λmax = 443 nm of complexes 1 and 7 are assigned to the ligand to Gd3+ fluorescence. The emission spectrum of compound 4 excited at 290 nm is illustrated in Figure 5e. The emission spectrum of complex 4

exhibited in the 360−440 nm region with the maximum wavelength of 415 nm. The free H2PDC ligand shows an emission at 390 nm (λex = 330 nm), which may be assigned to the π → π* or π → n transitions (Figure 5f). Apparently, the emission spectrum of complex 4 is similar to that of the free H2PDC ligand, indicating that the fluorescence of 4 is a ligandbased emission.19 The red shifts maybe caused by the existence of Ln−PDC coordination bonds. Overall, complexes 2, 6, and 9 exhibited the characteristic bands of the corresponding Ln ions and the intraligand emissions are absent. The emission spectra of complexes 3 and 5 exhibit not only the characteristic bands of luminescent lanthanide ions but also the emissions arising from the free ligand. The emissions of compounds 1, 7, and 8 all are due to the intraligand charge transfer; however, complexes 1 and 7 also show ligand to Ln3+ 186



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Table 3. Summary of the Photoluminescence Properties of 1−9 compd

Ln3+

λex (nm)

λem (nm)

mechanism

1

Gd3+

296

2 3

Tb3+ Dy3+

290 305

4 5

Er3+ Sm3+

290 334

6 7

Eu3+ Gd3+

330 326

8 9

Gd3+ Tb3+

329 300

394 400−450 488, 543, 583, 621 486, 574 390 413 560, 590, 642 390 390−550 579, 593, 617, 652, 698 373 400−450 373 488, 543, 583, 621

intraligand L → Ln f → f (5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4, 5D4 → 7F3) f → f (4F9/2 → 6H15/2, F9/2 → 6H13/2) intraligand intraligand f → f (4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2) intraligand L → Ln f → f (5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, 5D0 → 7F4) intraligand L → Ln intraligand f → f (5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4,5 D4 → 7F3)

Figure 6. Luminescence images of complexes 1−9 under a UV lamp at 254 nm.

fluorescence. Finally, the fluorescence of complex 4 only shows the ligand-based emission.

luminescence, whereas complex 5 emits bright red luminescence in the solid state.

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CONCLUSIONS In this paper, a series of lanthanide frameworks involving pyridine-3,5-dicarboxylic acid and glycolate or acetic acid ligands (1−9) have been hydrothermally synthesized and structurally characterized. The auxiliary ligands effectively modulate the assembly of these structures. The μ3-GA small ligands led to the formation of 1D Ln−organic chains, which further formed the isomorphous 2D networks of complexes 1−4. The μ2-OAc anions only generated a binuclear cluster in complexes 5−7, which is further extended by six PDC2− ligands to give a 3D (3,6)-connected f lu framework. The increase of reaction temperature reduced one coordination water molecule and changed the orientation of OAc in the binuclear motifs, which finally led to complex 8 showing 3D (3,6)-connected rtl topology. Furthermore, the absence of an anionic ancillary ligand produced compound 9, a 3D (3,8)-connected structure with tfz-d topology on the base of similar binuclear Ln clusters. The solid-state photoluminescence measurements of compounds 1−9 show that all of these Ln−pyridine-3,5-dicarboxylate MOFs produce remarkable emissions at room temperature. In particular, complexes 3 and 9 exhibit bright green

ASSOCIATED CONTENT

S Supporting Information *

The powder XRD patterns, TG curves and detailed assignments, and FT-IR spectra of compounds 1−9, as well as X-ray crystallographic files in CIF format for compounds 1−9 are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-29-81530767. Fax: +86-29-81530727. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21271123), and New Century Excellent Talents in University (NCET-12-0897). X.W. thanks 187



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the Innovation Funds of Graduate Programs of Shaanxi Normal University (SNU.2013CXB025).

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Synthesis, Crystal Structures, and Solid-State Luminescent Properties

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