Unusual Robust Luminescent Porous Frameworks Self-Assembled

V, Å3, 1890.0(7), 1884.5(7), 1868.2(4). Z, 2, 2, 2. T, K, 298(2), 293(2), 293(2) ... a Symmetry operators for 1: #1, −x + 2, −y + 1, −z + 1; #2...
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Unusual Robust Luminescent Porous Frameworks Self-Assembled from Lanthanide Ions and 2,2′-Bipyridine-4,4′-dicarboxylate Jing-Yun

Wu,†

Tzu-Tang

Yeh,†,‡

Yuh-Sheng

Wen,†

Jen

Twu,‡

and Kuang-Lieh

Lu*,†

Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, and Department of Chemistry, Chinese Culture UniVersity, Taipei 111, Taiwan

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 2 467-473

ReceiVed August 8, 2005; ReVised Manuscript ReceiVed NoVember 1, 2005

ABSTRACT: Three lanthanide(III) coordination polymers, [{Ln2(bpdc)3(H2O)3}‚H2O]n (Ln ) Sm, 1; Eu, 2; Tb, 3), were selfassembled from 2,2′-bipyridine-4,4′-dicarboxylic acid (H2bpdc) and corresponding lanthanide(III) salts under hydrothermal conditions. The compounds were found to be isomorphous and isostructural. Single-crystal X-ray diffraction studies show that compounds 1-3 present a three-dimensional network with one-dimensional hydrophilic microchannels that are occupied by guest water molecules. An edge-sharing dinuclear polyhedral [Ln2O14] was found to be the building unit of the network, in which two eight-coordinated LnO8 cores exhibit distinct coordination environments, that is, one is in a dicapped trigonal prism and the other is in a dodecahedron. Surprisingly, the bpdc ligand exhibits four different types of bonding characteristics in a framework, including the bis(monodentate), bis(syn,syn-bridging bidentate), bis(syn,anti-bridging bidentate), and bis(chelating-bridging bidentate) modes. The pyridyl nitrogen atoms of the 2,2′-bipyridyl unit in the bpdc ligand are uncoordinated in an anti conformation along the central C-C bond of the ligand, resulting from the high oxophilic nature of lanthanide(III) ions. A thermogravimetric analysis of 1 showed a high thermal stability (decomposing under N2 at T > 470 °C), indicating that the coordination habit of the metal ions with the bpdc ligand has a profound effect on the overall rigidity of the framework and the thermal stability of the compound. Photoluminescence measurements indicate that europium compound 2 and terbium compound 3 are strong red and green emitters, respectively. Introduction Investigations of the assembly of metal-organic frameworks (MOFs) based on the lanthanide family have attracted great interest due to not only their versatile architecture1-37 but also their potential utilization as luminescent and sensory materials.38,39 Many of these frameworks also show excellent photoluminescence40-43 and have interesting magnetic properties.6 Because of the high coordination nature, controlling the overall frameworks of lanthanide-based coordination polymers (CPs), undoubtedly, is a challenge. However, this provides an opportunity for the construction of various metal-organic frameworks with different topologies. For lanthanide compounds, it is well-known they exhibit excellent photophysical properties, which can attribute to f-f transitions,40 with an extremely narrow bandwidth. These transitions usually give weak intensities due to their spin- and parity-forbidden nature.39 However, the excitation of the metal ion could be achieved through a ligand-to-metal energy transfer (LMET) mechanism,39,41 a process termed an antenna effect.44 On the other hand, depending on the instinctive high oxophilicity (hard acidhard base interaction),42,43 lanthanide ions complex strongly to oxygen donor ligands, hence, a ligand incorporated with carboxylate groups such as 2,2′-bipyridine-4,4′-dicarboxylate (bpdc) would be expected to be a good organic ligand in constructing lanthanide-organic hybrid networks. The benefits derived from a carboxylate moiety are as follows: (a) the inherent negative charge of carboxylate groups compensate for the charge induced by metal cations and can mitigate the counterion effect; (b) the diverse coordination modes of carboxylate groups provide the potential for the formation of metal-carboxylate clusters or bridging units, which help enhance the robustness of the resulting network architecture.45 * To whom correspondence should be addressed. Fax: +886-227831237. E-mail: [email protected]. † Academia Sinica. ‡ Chinese Culture University.

On the basis of carboxylate linkers, it is noteworthy that a number of investigations have led to robust, thermally stable MOFs with large pores,3c,22,45-53 some of which even exhibited interesting catalytic activity54 and adsorption properties.3b,4a,55-58 As part of our ongoing efforts in the design and synthesis of functional crystalline materials, we previously reported on several robust, thermally stable porous frameworks.45,46 Herein we report on some new lanthanide networks, [{Ln2(bpdc)3(H2O)3}‚H2O]n (Ln ) Sm, Eu, Tb) assembled from 2,2′bipyridine-4,4′-dicarboxylate and the corresponding lanthanide salts. The unusual thermal stability, the simultaneous existence of four carboxylate coordination modes in a framework, the possible adjustment of the carboxylate bonding mode upon heating, and the strong luminescent behaviors are all features of these new lanthanide networks. Experimental Section Materials and Instruments. Sm(NO)3‚6H2O, EuCl3‚6H2O, TbCl3‚ 6H2O, KCl, KOH, and ethanol were purchased commercially and were used as received without further purification. The ligand 2,2′-bipyridine4,4′-dicarboxylic acid (H2bpdc) was prepared by published methods.59 Thermogravimetric (TG) analyses were performed under flowing nitrogen on a Perkin-Elmer TGA-7 analyzer at a heating rate of 3 °C/ min for all measurements. Elemental analyses were conducted on a Perkin-Elmer 2400 CHN elemental analyzer. The luminescence spectra were obtained on a Hitachi F-4500 fluorescence spectrophotometer under a nitrogen atmosphere. Synthesis of [{Ln2(bpdc)3(H2O)3}‚H2O]n (Ln ) Sm, 1; Eu, 2; Tb, 3). Compounds 1-3 were prepared under hydro(solvo)thermal conditions. Lanthanide salts (Sm(NO)3‚6H2O for 1, EuCl3‚6H2O for 2, TbCl3‚ 6H2O for 3, 0.05 mmol), KCl (0.05 mmol), H2bpdc (0.11 mmol), KOH (0.10 M, 2 mL), and H2O (1 mL) were conducted in an acid digestion bomb at 180 °C for 72 h to produce pale-yellow crystals in quantitative yield. In the absence of KCl, the reactions gave the products in low yield. The pH values in the solution are in the range 5-6 (e.g., for 1, pH ) 5.5 and 5.1 in the start and end of the reaction, respectively). The crystals were collected by filtration, washed with distilled water and ethanol, and dried at room temperature. [{Sm2(bpdc)3(H2O)3}‚H2O]n (1). Anal. Found: C, 39.31; H, 2.38; N, 7.64. Calcd for C36H26N6O16Sm2: C, 39.33; H, 2.38; N, 7.64.

10.1021/cg050393j CCC: $33.50 © 2006 American Chemical Society Published on Web 12/02/2005

468 Crystal Growth & Design, Vol. 6, No. 2, 2006

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Table l. Crystal Structure Refinement Data for Compounds 1-3

empirical formula formula weight crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z T, K λ, Å Fcalcd, g/cm3 µ, mm-1 F(000) GOF R1a [I > 2σ(I)] R1a (all data) wR2a [I > 2σ(I)] wRa (all data) a

1

2

3

C36H26N6O16Sm2 1099.33 triclinic P1h 11.368(2) 12.223(2) 14.362(3) 95.16(3) 103.38(3) 100.60(3) 1890.0(7) 2 298(2) 0.710 73 1.932 3.161 1072 1.021 0.0268 0.0419 0.0673 0.0738

C36H26Eu2N6O16 1102.55 triclinic P1h 11.378(2) 12.208(2) 14.342(3) 95.01(3) 103.48(3) 100.86(3) 1884.5(7) 2 293(2) 0.710 73 1.943 3.382 1076 1.044 0.0325 0.0437 0.0869 0.0933

C36H26N6O16Tb2 1116.47 triclinic P1h 11.3645(13) 12.1930(8) 14.271(2) 95.031(11) 103.576(12) 101.060(8) 1868.2(4) 2 293(2) 0.710 73 1.985 3.840 1084 1.050 0.0342 0.0415 0.0900 0.0950

R1 ) ∑||Fo|| - |Fc|/∑|Fo|; wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2. Table 2. Selected Bond Distances (Å) for Compounds 1-3a

Ln(1)-O(9) Ln(1)-O(5) Ln(1)-O(1) Ln(1)-O(82) Ln(1)-O(10)#1 Ln(1)-O(81) Ln(1)-O(4)#2 Ln(1)-O(3)#2 Ln(2)-O(7)#3 Ln(2)-O(11) Ln(2)-O(6) Ln(2)-O(8)#4 Ln(2)-O(83) Ln(2)-O(3)#2 Ln(2)-O(2) Ln(2)-O(1)

1 (Ln ) Sm)

2 (Ln ) Eu)

3 (Ln ) Tb)

2.327(3) 2.350(4) 2.381(3) 2.395(5) 2.405(3) 2.462(4) 2.465(3) 2.570(3) 2.294(3) 2.343(3) 2.361(3) 2.377(3) 2.471(4) 2.478(3) 2.483(3) 2.764(3)

2.311(3) 2.345(4) 2.371(4) 2.379(5) 2.391(4) 2.456(4) 2.456(4) 2.558(3) 2.283(3) 2.337(4) 2.341(3) 2.354(3) 2.457(4) 2.461(3) 2.474(3) 2.752(4)

2.291(3) 2.319(4) 2.339(3) 2.349(5) 2.363(3) 2.432(4) 2.427(3) 2.549(3) 2.257(3) 2.312(4) 2.317(3) 2.335(3) 2.429(3) 2.434(3) 2.440(3) 2.751(4)

a Symmetry operators for 1: #1, -x + 2, -y + 1, -z + 1; #2, x, y 1, z; #3, -x + 2, -y + 1, -z; #4, x - 1, y - 1, z.

[{Eu2(bpdc)3(H2O)3}‚H2O]n (2). Anal. Found: C, 39.21; H, 2.38; N, 7.62. Calcd for C36H26Eu2N6O16: C, 39.22; H, 2.38; N, 7.62. [{Tb2(bpdc)3(H2O)3}‚H2O]n (3). Anal. Found: C, 38.19; H, 1.89; N, 7.25. Calcd for C36H26N6O16Tb2: C, 38.73; H, 2.35; N, 7.53. Crystallographic Determination. Data collections for 1-3 were carried out with an Enraf-Nonius CAD4 diffractometer equipped with Mo KR radiation. The crystal structures were solved by direct methods60 and refined61 on F2 by full-matrix least-squares calculations using anisotropic displacement parameters for all non-hydrogen atoms. The weighted R-factor (wR) and goodness of fit (S) are based on F2, while conventional R-factors (R) are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2σ(F2) is used only for calculating R-factors etc. and is not relevant to the choice of reflections for refinement. All non-hydrogen atoms and hydrogen atoms of the water molecules were located from difference Fourier maps. Hydrogen atoms of the pyridyl rings were assigned by geometrical calculation and refined as a riding model. Experimental details for X-ray data collections of 1-3 are given in Table 1. Selected bond lengths and angles for 1-3 are provided in Tables 2 and 3, respectively.

Results and Discussion Synthesis. Under hydrothermal conditions, compounds 1-3 were successfully synthesized as pale-yellow crystals by the reactions of 2,2′-bipyridine-4,4′-dicarboxylic acid (H2bpdc) and the corresponding lanthanide(III) salts in quantitative yields.

Table 3. Selected Bond Angles (deg) for Compounds 1-3a

O(9)-Ln(1)-O(5) O(9)-Ln(1)-O(1) O(5)-Ln(1)-O(1) O(9)-Ln(1)-O(82) O(5)-Ln(1)-O(82) O(1)-Ln(1)-O(82) O(9)-Ln(1)-O(10)#1 O(5)-Ln(1)-O(10)#1 O(1)-Ln(1)-O(10)#1 O(82)-Ln(1)-O(10)#1 O(9)-Ln(1)-O(4)#2 O(5)-Ln(1)-O(4)#2 O(1)-Ln(1)-O(4)#2 O(82)-Ln(1)-O(4)#2 O(10)#1-Ln(1)-O(4)#2 O(9)-Ln(1)-O(81) O(5)-Ln(1)-O(81) O(1)-Ln(1)-O(81) O(82)-Ln(1)-O(81) O(10)#1-Ln(1)-O(81) O(4)#2-Ln(1)-O(81) O(9)-Ln(1)-O(3)#2 O(5)-Ln(1)-O(3)#2 O(1)-Ln(1)-O(3)#2 O(82)-Ln(1)-O(3)#2 O(10)#1-Ln(1)-O(3)#2 O(4)#2-Ln(1)-O(3)#2 O(81)-Ln(1)-O(3)#2 O(7)#3-Ln(2)-O(11) O(7)#3-Ln(2)-O(6) O(11)-Ln(2)-O(6) O(7)#3-Ln(2)-O(8)#4 O(11)-Ln(2)-O(8)#4 O(6)-Ln(2)-O(8)#4 O(7)#3-Ln(2)-O(83) O(11)-Ln(2)-O(83) O(6)-Ln(2)-O(83) O(8)#4-Ln(2)-O(83) O(7)#3-Ln(2)-O(3)#2 O(11)-Ln(2)-O(3)#2 O(6)-Ln(2)-O(3)#2 O(8)#4-Ln(2)-O(3)#2 O(83)-Ln(2)-O(3)#2 O(7)#3-Ln(2)-O(2) O(11)-Ln(2)-O(2) O(6)-Ln(2)-O(2) O(8)#4-Ln(2)-O(2) O(83)-Ln(2)-O(2) O(3)#2-Ln(2)-O(2) O(7)#3-Ln(2)-O(1) O(11)-Ln(2)-O(1) O(6)-Ln(2)-O(1) O(8)#4-Ln(2)-O(1) O(83)-Ln(2)-O(1) O(3)#2-Ln(2)-O(1) O(2)-Ln(2)-O(1)

1 (Ln ) Sm)

2 (Ln ) Eu)

3 (Ln ) Tb)

83.46(14) 85.20(12) 73.53(13) 98.66(18) 143.21(16) 70.10(16) 83.28(12) 69.18(12) 141.93(12) 147.60(16) 152.30(12) 116.42(13) 117.81(12) 77.10(17) 86.21(13) 77.65(14) 140.79(14) 137.29(14) 74.22(16) 74.67(14) 74.86(14) 156.15(11) 79.76(12) 73.88(11) 85.09(15) 106.12(11) 51.53(10) 125.70(13) 97.33(13) 85.51(12) 147.74(12) 99.94(12) 71.07(12) 140.31(12) 77.65(12) 139.91(12) 72.18(12) 70.77(12) 159.34(12) 102.27(12) 81.42(12) 80.55(11) 83.13(12) 74.12(11) 75.17(13) 74.78(12) 144.67(12) 137.70(12) 117.16(11) 122.95(11) 75.82(12) 75.80(11) 128.34(11) 140.12(11) 69.02(10) 49.11(10)

83.34(15) 85.37(13) 73.70(14) 98.55(18) 143.21(15) 69.87(15) 83.30(13) 69.19(13) 142.16(13) 147.59(14) 152.19(12) 116.26(15) 117.96(12) 77.73(18) 85.73(13) 77.69(14) 140.78(14) 137.18(15) 74.15(16) 74.70(15) 74.78(13) 155.81(12) 79.54(13) 73.53(12) 85.40(16) 106.09(12) 51.99(11) 126.02(13) 97.03(14) 85.99(13) 147.85(13) 99.18(12) 71.28(13) 140.03(13) 77.45(13) 140.34(14) 71.63(13) 70.97(13) 159.56(13) 102.19(13) 81.45(13) 80.70(12) 83.29(12) 74.53(12) 75.13(13) 74.91(13) 144.78(13) 137.35(13) 117.06(11) 123.61(12) 75.60(13) 76.18(12) 128.29(12) 139.96(12) 68.77(11) 49.31(11)

82.86(14) 85.16(12) 73.72(14) 98.73(17) 143.69(15) 70.31(15) 83.33(13) 69.39(13) 142.39(13) 146.92(14) 152.20(12) 116.41(14) 118.37(12) 77.77(17) 85.24(13) 77.46(14) 140.48(14) 136.91(14) 73.78(15) 74.51(14) 75.09(14) 155.48(12) 79.75(12) 73.48(11) 85.50(15) 106.25(11) 52.32(11) 126.58(13) 96.65(14) 86.60(12) 147.68(13) 98.45(12) 71.24(13) 140.26(12) 77.62(13) 140.61(13) 71.55(12) 71.19(12) 160.14(13) 101.85(13) 81.66(12) 80.92(11) 83.47(12 74.70(12) 75.11(13) 74.81(13) 144.61(13) 116.98(11) 137.20(12) 123.90(11) 75.60(12) 76.00(12) 128.46(11) 68.55(10) 139.68(11) 49.39(11)

a Symmetry operators for 1: #1, -x + 2, -y + 1, -z + 1; #2, x, y 1, z; #3, -x + 2, -y + 1, -z; #4, x - 1, y - 1, z.

Compounds 1-3 were characterized as having the formula [{Ln2(bpdc)3(H2O)3}‚H2O]n (Ln ) Sm, 1; Eu, 2; Tb, 3) based on elemental analysis results and confirmed by a single-crystal X-ray diffraction analysis. Structural Description. Compounds 1-3 are isomorphous and isostructural; hence, only the structure of 1 is described. A single-crystal X-ray diffraction analysis shows that 1 adopts a three-dimensional framework with one-dimensional hydrophilic channels along the (010) direction (Figure 1). The coordination environments around samarium ions and the numbering scheme of 1 are illustrated in Figure 2. There are two distinct samarium ions with different coordination spheres in the structure. The coordination polyhedron around Sm1 is an eight-coordinated

Unusual Robust Luminescent Porous Frameworks

Figure 1. Perspective view of the microporous framework of 1 along the (010) direction, showing hydrophilic channels where guest water molecules reside: blue sphere, guest water molecules; yellow edgesharing dinuclear polyhedra, [Sm2O14].

dicapped trigonal prism consisting of six oxygen atoms from five carboxylate groups and two water molecules, while the local geometry around Sm2 is an eight-coordinated dodecahedron made up of seven oxygen atoms from six carboxylate groups and one water molecule (Figure 2b). The Sm-O bond lengths are in the range of 2.294(3)-2.764(3) Å, compared with that in 2 (Eu-O 2.283(3)-2.752(4) Å) and in 3 (Tb-O 2.257(3)2.751(4) Å), showing a significant lanthanide contraction that reduces the atomic radius following the increase in the atomic number of the lanthanide elements.62,63 The Sm(1)O8 and Sm(2)O8 polyhedra are edge-shared through O1 and O3, leading

Crystal Growth & Design, Vol. 6, No. 2, 2006 469

to the formation of one-dimensional infinite metallic chains along the (111) direction as illustrated in Figure 3. The metal centers are arrayed in the order Sm1*‚‚‚Sm1‚‚‚Sm2‚‚‚Sm2* with three consecutive Sm‚‚‚Sm distances of 5.366, 4.096, and 5.119 Å, respectively. The chains are further cross-linked by the bipyridyl spacer of the bpdc anions and could be viewed as secondary building blocks in the formation of 1. Four types of crystallographically independent bpdc ligands with different carboxylate coordination modes are present in the framework of 1, bis(monodentate) (bpdcI), bis(syn,synbridging bidentate) (bpdcII), bis(syn,anti-bridging bidentate) (bpdcIII), and bis(chelating-bridging bidentate) (bpdcIV) as shown in Figure 4. Two of these modes, bpdcI and bpdcIII, are situated at inversion centers. These ligands bridge two (bpdcI) or four (bpdcII, bpdcIII, and bpdcIV) metal centers. The uncoordinated bipyridine portions of these bpdc ligands are present in an anti conformation along the central C-C bond with dihedral angles of 0°, 16.54°, 0°, and 10.24° for bpdcI, bpdcII, bpdcIII, and bpdcIV, respectively. Furthermore, the carboxylate groups and the corresponding linked pyridyl rings are not coplanar in the four crystallographically distinct bpdc ligands. They show ca. 4.72°, 15.66° (the other one 2.18°), 39.56°, and 17.29° (the other one 21.83°) twists from the pyridine linker in bpdcI, bpdcII, bpdcIII, and bpdcIV, respectively. One bpdcIII and two bpdcIV ligands are observed that are stacked almost parallel in one set of Sm2*‚‚‚Sm1*‚‚‚Sm1‚‚‚Sm2 arrays with π-π interactions (dihedral angle of 6.88° and centroid‚‚‚centroid distance of 3.688 Å, Figure 3). The hydrogen-bonding interactions from the coordinated water molecules to the carboxylate oxygen atoms and from coordinated/guest water molecules to guest water molecules as well as pyridyl nitrogen atoms are observed (Table 4). In addition, C-H‚‚‚O (carboxylate/water) interactions are also observed in 1, despite their weakness.

Figure 2. (a) Numbering scheme and coordination environment around the two crystallographically independent Sm(III) centers in 1. Hydrogen atoms and guest water molecule are omitted for clarity. (b) Highlight of the coordination polyhedra for the two crystallographically independent Sm ions.

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Figure 3. Linking of Sm ions into a metallic chain via the bridged carboxylates along the (111) direction. Asterisks denote symmetry-equivalent atoms. Adjacent pyridyl rings that are engaged in π-π stacking are highlighted in dark-gray and light-gray plates.

Figure 4. Four coordination modes of 2,2′-bipyridine-4,4′-dicarboxylate (bpdc) in compounds 1-3: mode I, bis(monodentate); mode II, bis(syn,syn-bridging bidentate); mode III, bis(syn,anti-bridging bidentate); mode IV, bis(chelating-bridging bidentate). Table 4. Possible Hydrogen Bonding Interactions in 1a donor-H‚‚‚acceptor

D-H (Å)

H‚‚‚A (Å)

D‚‚‚A (Å)

D-H‚‚‚A (deg)

O(81)-H(81A)‚‚‚O(10)#1 O(81)-H(81B)‚‚‚N(12)#1 O(82)-H(82A)‚‚‚O(12) O(83)-H(83A)‚‚‚N(31)#2 O(83)-H(83B)‚‚‚N(1)#3 O(91)-H(91A)‚‚‚N(2) C(5)-H(5)‚‚‚O(11)#4 C(8)-H(8)‚‚‚O(83)#5 C(8)-H(8)‚‚‚N(1) C(14)-H(14)‚‚‚N(12) C(20)-H(20)‚‚‚O(7) C(23)-H(23)‚‚‚O(4)#6 C(26)-H(26)‚‚‚N(21)#6 C(32)-H(32)‚‚‚N(31)#7

0.77(5) 0.74(8) 0.70(9) 0.88(6) 0.78(7) 0.72(10) 0.99(4) 0.94(5) 0.94(5) 0.87(5) 0.85(6) 1.02(7) 0.68(6) 0.87(6)

2.20(5) 2.25(8) 2.01(10) 1.89(6) 2.34(7) 2.14(10) 2.52(4) 2.39(5) 2.46(5) 2.44(5) 2.52(5) 2.42(7) 2.49(5) 2.53(6)

2.870(6) 2.964(6) 2.704(7) 2.767(6) 3.051(5) 2.808(8) 3.477(6) 3.325(6) 2.782(6) 2.807(7) 2.839(6) 3.398(7) 2.795(7) 2.846(7)

145(4) 163(9) 174(10) 170(3) 152(7) 156(10) 164(4) 171(4) 100(3) 106(4) 104(4) 159(5) 109(5) 102(4)

a Symmetry operators: #1, 2 - x, 1 - y, 1 - z; #2, 1 + x, y, z; #3, x, -1 + y, z; #4, 1 - x, 1 - y, -z; #5, x, 1 + y, z; #6, 2 - x, 2 - y, 1 - z; #7, -x, -y, -z.

Small one-dimensional channels with a compressed rhombicshaped are observed along the (010) direction in 1. The guest water molecules reside in the channels and form O-H‚‚‚N hydrogen-bonding interactions (O(91)-H(91A) 0.72 Å, H(91A)‚‚‚ N(2) 2.14 Å, O(91)‚‚‚N(2) 2.81 Å, O(91)-H(91A)‚‚‚N(2) 156°) with the uncoordinated pyridyl nitrogen atoms. On the other hand, as shown in Figure 5, within the one-dimensional channels, in addition to guest water molecules, a bpdcI ligand also occupies the channels along the (111) direction, compensating the charge from the cationic three-dimensional host network in 1 that is sustained by three different connectors (bpdcII, bpdcIII, and bpdcIV). A PLATON64 analysis indicates that the

Figure 5. (a) Perspective view of the 3-D porous framework of 1 along the (111) direction, showing the bpdcI ligands and guest water molecules situated within the channels. All hydrogen atoms are omitted for clarity. (b) Space-filled model of 1 showing the 1-D channels; the bpdcI ligands and guest water molecules are omitted for clarity.

host framework is quite dense (4.3% of the unit cell volume is solvent-accessible). It is noteworthy that the coordination modes of the bpdc ligand, which contains a 2,2′-bipyridyl chelating site and two carboxylate coordinating sites in a triangular fashion, has been efficiently adjusted depending on the nature of the metal centers. For first- and second-row late transition metals, the carboxylate groups show only a few bonding modes, such as bis(syn,synbridging bidentate), bis(monodentate), or chelating-µ3-bridging bidentate bonding modes, in the M(bpdc)-based (M ) Mn,65a,66 Co,65b and Cd45) CPs. On the other hand, the bpdc ligand in the [Ln2(bpdc)3]-based polymeric frameworks present in 1-3 exhibits versatile coordination characteristics due to the high

Unusual Robust Luminescent Porous Frameworks

Crystal Growth & Design, Vol. 6, No. 2, 2006 471

Figure 6. Thermogravimetric diagram of compound 1.

oxophilic and high coordination properties of lanthanide(III) ions. The 2,2′-bipyridine unit of the bpdc ligand in the firstand second-row transition metal CPs is chelated to the metal centers. However, this is not observed in lanthanide(III) compounds 1-3 due to the high affinity of lanthanide(III) ions for oxygen and the flexibility of the bpdc ligand along the central C-C bond. Thermal Stability Analysis of 1. As shown in Figure 6, compound 1 was found to be exceptionally thermally stable. A thermogravimetric (TG) trace of 1 shows that the guest water molecule is removed initially with increasing temperature, and the coordinated water molecules then follow when the temperature reaches 230 °C. The total weight loss for one guest and three coordinated water molecules is 6.50% (calcd 6.55%). In the temperature range of 230-470 °C, about a 3.30% weight loss is observed, which may correspond to the release of 1 mol of CO2 per formula unit (calcd 4.00%). This suggests that the decarboxylate reaction of the bpdcI ligand takes place due to the comparatively weak bonding to Sm ions. Finally, the decomposition of 1 started at 470 °C and ended at above 540 °C. Sm2O3 is assumed to be the final residual product (31.66% weight containing), supported by the expected value of 31.72%. Compound 1 represents an unusual thermally stable phase in comparison with a robust Cd(II)-bpdc CP (Tdecomp > 440 °C)45 and gives a temperature difference ca. 160 °C higher than Mn(II)-bpdc CPs.65a This result indicates that the high coordination number and the coordination habit of lanthanide ions with the bpdc ligand have a profound effect on overall framework rigidity and thermal stability.4b,45,67 The π-π interactions between the pyridyl rings of the coordinated bpdc moiety also play an important role in stabilizing the network. In addition, when the coordinated water molecules are removed upon heating, the bpdcIII ligand (O9 and O10) may adjust its carboxylate group and transform to type IV with a bis(chelatingbridging bidentate) mode (Figures 2 and 4). The transformation is likely to take place when O10 further binds to the neighboring Sm1 with the concomitant adjustment of the Sm1-Sm1* distance. The structural integrity of the framework is therefore maintained, and as a result, the thermal stability is strengthened. As a consequence, 1 exhibits a high thermal stability. Employing oxophilic metals such as lanthanides provides potential for the construction of frameworks with high thermal stabilities.67 Examples for coordination polymers that show a high thermal stability are also observed in compounds [Er(TMA)] (TMA ) benzene-1,3,5-tricarboxylate),67 [Er4(bdc)6‚6H2O] (bdc ) benzene1,4-dicarboxylate),4b [Ln(SIP)(H2O)4]n (Ln ) Eu, Gd, or Ce and SIP ) 5-sulfoisophthalate),24c [{Yb2(NDC)3(H2O)}‚H2O]n (NDC

Figure 7. Photoluminescence spectra of 2 (λex ) 339 nm) and 3 (λex ) 343 nm).

) 2,6-naphthalenedicarboxylate),11 and [{Eu2(1,4-BDS)(4SB)2}‚3H2O] (1,4-BDS ) benzene-1,4-disulfonate and 4-SB ) 4-sulfobenzoate)33 and so on, for which the robustness may be attributed to the multidentate functionality of the organic ligands and the rigidity of the extended frameworks.24c Other thermally robust inorganic-organic hybrid species such as metal pyrimidin-2-olate derivatives [M(pymo)2]n (M ) Co, Zn)68 and metallosilicate-organic materials [Zn8(SiO4)(C8H4O4)6]n,69 which are exceptionally stable at temperatures over 560-570 °C, are clearly in good agreement with the nature of the noninterpenetrated diamondoid framework68 and the combination of more metals at a network vertex joined by more organic linkers through coordination bonds.69 Photophysical Studies. We examined the ligand-assisted photoluminescence properties of powder samples of the lanthanide compounds 1-3 at room temperature. When excited at 339 nm, 2 emits red light, while 3 shows green light emission upon excitation at 343 nm. The photoluminescence spectra are shown in Figure 7. The characteristic emission bands of 2 at 578, 590, 615, 650, and 696 nm are assigned to 5D0 f 7FJ (J ) 0, 1, 2, 3, and 4) transitions, while the emission peaks of 3 at 493, 548, 587, and 621 nm can be attributed to 5D4 f 7FJ (J ) 6, 5, 4, and 3) transitions, respectively. The additional peaks at 676 nm in 2 and at 694 nm in 3 are concluded to be overtones of excited wavelengths. However, these characteristic emission bands indicate that the ligand-to-metal energy transfer is moderately efficient under the experimental conditions used.39 Note that the forbidden transition, 5D0 f 7F0, was also observed in the europium species 2. Moreover, the hypersensitive transition, 5D0 f 7F2, the preferred transition for europiumcontaining luminescent materials, is much more intense than the 5D0 f 7F1 transition in 2. These results can be attributed to the noncentrosymmetric coordination environment around the Eu centers.21,42 Upon excitation at 305 nm, the very weak emission bands of 1 appeared at 563, 600, and 646 nm, assigned to 4G5/2 f 6H5/2, 4G5/2 f 6H7/2, and 4G5/2 f 6H9/2 transition, respectively.70 Conclusion The reaction of lanthanide (Sm, Eu, Tb) salts with 2,2′bipyridine-4,4′-dicarboxylate under hydro(solvo)thermal conditions leads to the formation of unusual thermally stable three-

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dimensional frameworks. The bpdc ligand exhibits four coordination characteristics simultaneously in a network, providing a good example for studies of the metal-carboxylate bonding mode. The high thermal stability (decomposition under N2 at T > 470 °C) of the frameworks can be attributed to the high coordination number of lanthanide ions, the strong metalcarboxylate bonds, the π-π interactions between the pyridyl rings of the coordinated bpdc ligands, and the possible adjustment of the carboxylate bonding mode. Photoluminescence properties are illustrated by europium compound 2 and terbium compound 3, which are strong red and green emitters, respectively. Acknowledgment. We thank Academia Sinica and the National Science Council of Taiwan for financial support. Supporting Information Available: The crystallographic data of 1-3 in CIF format. This material is available free of charge via the Internet at http: //pubs.acs.org.

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