Temperature and Dipyridyl Ligands Induced Diverse

Article pubs.acs.org/crystal

Solvent/Temperature and Dipyridyl Ligands Induced Diverse Coordination Polymers Based on 3‑(2′,5′-Dicarboxylphenyl)pyridine Bo Liu, Lei Wei, Nan-nan Li, Wei-Ping Wu, Hui Miao, Yao-Yu Wang,* and Qi-Zhen Shi Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. China S Supporting Information *

ABSTRACT: Eleven new multidimensional transition coordination polymers, [Cd2(dcpy)2(H2O)] (1), [Cd(dcpy)]·H2O (2), [Cd(dcpy)(H2O)1.5]·2H2O (3), [Cd(dcpy)(bipy)0.5(H2O)]·5H2O (4), [Cd2(dcpy)2(bipy)(H2O)3]·2.5H2O (5), [Cd2(dcpy)2(bipy)]·4H2O (6), [Cd(dcpy)(bpa)0.5]·2H2O (7), [Ni(dcpy)(H2O)2] (8), [Ni(dcpy)(bpe)0.5(H2O)]·3H2O (9), [Ni(dcpy)(bpe)0.5(H2O)]· 0.5H2O (10), and [Ni(dcpy)(bpa)0.5(H2O)2]·H2O (11), have been synthesized by using a tritopic ligand 3-(2′,5′-dicarboxylphenyl)pyridine acid (H2dcpy) and three different N-donor ancillary ligands (bipy = 4,4′-bipyridine, bpe = 1,2-bi(4pyridyl)ethene, and bpa = 1,2-bi(4-pyridyl)ethane) in different solvents and temperatures. 1 is a four-connected three-dimensional (3D) framework based on two kinds of Cd−O−Cd and Cd−(COO)−Cd chains. 2 shows also a fourconnected 3D framework with one-dimensional (1D) channels formed by double left- and right-handed helical chains. 3 displays a 3D microporous framework composed of Cd2O3 as a secondary building unit (SBU) with a (4,6)connected net. 4 is a two-dimensional (2D) layer based on Cd2O2 SBU and further stacks to generate a 3D supramolecular framework with open channels. 5 is a rare 3D homochiral microporous framework with left-handed helical chains. 6 presents the first (3,4,6)-connected 3D self-penetrating framework based on 1D metal−organic nanotubes (MONTs). 7 is a (3,4)-connected 3D microporous pillar-layer framework with 1D square channels filled by left- or right-handed helical water clusters chains. 8 is a 2D (3,6)-connected net based on Ni2 dimers. 9 shows a wavelike 2D stacked layer framework possessing two types of open channels. 10 is a structural isomer of 9, showing an intriguing (3,4)-connected self-penetrating network, while 11 exhibits a 2D (3,4)-connected layer. The effects of solvents, temperatures, metal cations, and the lengths and rigidness/flexibility of the dipyridyl ligand on the crystal architectures are discussed. The solid-state photoluminescence for 1−7 and the magnetic properties for 8 were also investigated. Gas adsorption studies for 9 shows a high selective adsorption of CO2 over N2.

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INTRODUCTION Studies on the synthesis and design of coordination polymers (CPs) have been extensively developed in recent years,1 owing to their dynamic structures and functional properties in gas storage/separation,2 heterogeneous catalysis,3 molecular magnetism,4 luminescence,5 and drug delivery.6 Up to now, numerous CPs were assembled by various approaches, such as combining the same organic ligands with different metal ions, introducing different anions, changing the substituent groups or lengths of organic ligands, incorporating mixed ligands,7 and so on. However, it remains a considerable challenge to control the structures of CPs for many external factors, such as solvent systems, pH values, and temperatures impose crucial influences on the structures of compounds.8 A subtle change of one of the foregoing factors may cause a drastic change in the spacial dimensionality and topology, and even for a fixed chemical composition, structural diversity of coordination polymers is also a common phenomenon.9 Thus, a particular series of CPs species that can be systematically tuned by one factor and undergo structural transformations still remain largely unexplored thus far. © 2014 American Chemical Society

On the other hand, the reasonable selection of multidentate organic ligands is one of the most effective ways in the manipulation of the expected structures of CPs.10 Any changes in ligand geometry, ligand substituents, or synthetic conditions may impose a strong influence on the final structure. Among multitudinous organic ligands, multicarboxylic ligands have been proven to be good candidates because they can be regarded not only as hydrogen-bonding acceptors but also as hydrogen-bonding donors, depending upon the degree of deprotonation. In particular, aromatic polycarboxylate ligands have been used fruitfully for the preparation of rigid and highly connected porous or helical coordination frameworks due to their bent backbones, strong linking capability, and versatile bridging fashions.11 Apart from the multicarboxylate ligands, aromatic N-donor linkers are frequently used as ancillary bridging, chelating, and charge balance ligands based on their versatile conformation.12 Moreover, the ancillary N-donor Received: October 27, 2013 Revised: January 16, 2014 Published: January 21, 2014 1110

dx.doi.org/10.1021/cg401599x | Cryst. Growth Des. 2014, 14, 1110−1127

Crystal Growth & Design

Article

Synthesis of [Cd2(dcpy)2(H2O)] (1). A mixture of Cd(NO3)2·4H2O (61.6 mg, 0.2 mmol) and H2dcpy (24.3 mg, 0.1 mmol) in H2O (10 mL) was sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 160 °C for 72 h and then gradually cooled to room temperature at a rate of 5 °C min−1. Colorless block crystals of 1 were collected by filtration and washed with H2O. The yield was ca. 16.8 mg (46.4%, based on dcpy). Anal. Calcd for C26H16Cd2N2O9: C, 43.06; H, 2.22; N, 3.86. Found: C, 43.10; H, 2.08; N, 3.89. IR (KBr, cm−1): 3454s, 2026w, 1623m, 1573m, 1401m, 1372s, 1108w, 835w, 770m, 706m. Synthesis of [Cd(dcpy)]·(H2O) (2). The procedure was similar to the preparation of 1, except that the solvent H2O (10 mL) was replaced by H2O (8 mL)−DMF (2 mL), and the temperature was reduced to 130 °C. Colorless prism crystals of 2 were collected by filtration and washed with H2O−DMF (4:1). The yield was ca. 15.7 mg (42.5%, based on dcpy). Anal. Calcd for C13H9CdNO5: C, 42.02; H, 2.44; N, 3.77. Found: C, 42.07; H, 2.38; N, 3.86. IR (KBr, cm−1): 3421s, 2969w, 2026w, 1581m, 1404m, 1271w, 1068m, 1016m, 979w, 836m, 775m. Synthesis of [Cd(dcpy)(H2O)1.5]·2(H2O) (3). The procedure was similar to the preparation of 2, except that the solvent ratio of H2O− DMF was changed to H2O (4 mL)−DMF (6 mL). Colorless block crystals of 3 were isolated by washing with H2O−DMF (3:2). The yield was ca. 16.3 mg (38.6%, based on dcpy). Anal. Calcd for C13H14CdN2O7.5: C, 36.94; H, 3.34; N, 6.63. Found: C, 36.98; H, 3.22; N, 6.73. IR (KBr, cm−1): 3419s, 2027w, 1643w, 1570s, 1416m, 1371s, 1275m, 1194m, 1109m, 899m, 772m. Synthesis of [Cd(dcpy)(bipy)0.5(H2O)]·5H2O (4). A mixture of Cd(NO3)2·4H2O (61.6 mg, 0.2 mmol), H2dcpy (24.3 mg, 0.1 mmol), and bipy (15.6 mg, 0.1 mmol) in H2O (10 mL) was sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 130 °C for 72 h and then gradually cooled to room temperature at a rate of 5 °C min−1. The block crystals of 4 were collected by filtration and washed with H2O. The yield was ca. 21.7 mg (40.2%, based on dcpy). Anal. Calcd for C18H23CdN2O10: C, 40.05; H, 4.29; N, 5.19. Found: C, 39.88; H, 4.60; N, 4.17. IR (KBr, cm−1): 3402s, 2026w, 1555m, 1371m, 1270w, 1222m, 1194w, 1070m, 916w, 847m, 770m. Synthesis of [Cd2(dcpy)2(bipy)(H2O)3]·2.5H2O (5). The same synthetic method as that for 4 was used except that H2O (10 mL) was replaced by H2O (2 mL)−DMF (8 mL) as the solvent. The colorless block crystals of 5 were isolated by washing with H2O and DMF (1:4). The yield was ca. 16.8 mg (35.3%, based on dcpy). Anal. Calcd for C36H33Cd2N4O13.5: C, 45.30; H, 3.49; N, 5.87. Found: C, 45.48; H, 3.37; N, 5.91. IR (KBr, cm−1): 3447s, 2427w, 2026w, 1763w, 1582m, 1386s, 1289m, 1223m, 1074m, 1009m, 810s, 707m. Synthesis of [Cd2(dcpy)2(bipy)]·4H2O (6). The same synthetic method as that for 4 was used except that the temperature is increased to 160 °C. The colorless block crystals of 6 were isolated by washing with H2O. The yield was ca. 28.7 mg (61.4%, based on dcpy). Anal. Calcd for C36H30Cd2N4O12: C, 46.22; H, 3.23; N, 5.99. Found: C, 46.30; H, 3.16; N, 6.03. IR (KBr, cm−1): 3627s, 3449s, 2026w, 1776w, 1575m, 1470m, 1396m, 1272w, 1201m, 1107m, 1036s, 942m, 888m, 830m, 770m, 709m. Synthesis of [Cd(dcpy)(bpa)0.5]·2H2O (7). The procedure was similar to the preparation of 6, except that bipy was replaced by bpa, and the H2O (10 mL) was replaced by H2O (4 mL)−DMF (6 mL) as the solvent. The colorless block crystals of 7 were isolated by washing with H2O−DMF (2:3). The yield was ca. 35.5 mg (73.6%, based on dcpy). Anal. Calcd for C19H17CdN2O6: C, 47.37; H, 3.56; N, 5.82. Found: C, 47.42; H, 3.51; N, 5.79. IR (KBr, cm−1): 3395s, 2026w, 1612m, 1573m, 1496m, 1410s, 1275w, 1072m, 935m, 849s, 790m, 773m, 702m. Synthesis of [Ni(dcpy)(H2O)2] (8). The same synthetic method as that for 1 was used except that Cd(NO3)2·4H2O was replaced by NiSO4·6H2O (52.6 mg, 0.1 mmol). Green block crystals of 8 were obtained and washed with H2O. The yield was ca. 23.0 mg (68.5%, based on dcpy). Anal. Calcd for C13H11NNiO6: C, 46.48; H, 3.30; N, 4.17. Found: C, 46.53; H, 3.26; N, 4.20. IR (KBr, cm−1): 3452s, 2026w, 1631s, 1403m, 1371m, 1275w, 1195w, 1106w, 1040w, 908w, 875m, 819m, 782m, 702w.

ligands also play an important role in adjusting the coordination mode of polycarboxylate blocks and the structure of the resulting network, which is rarely documented to date. In contrast to fruitful research in bridging carboxylate or pyridyl ligands, limited works have focused on the investigation of elongated pyridyl carboxylate ligands in CPs.13 Known examples are mainly related to nicotinate,14 isonicotinate,15 and pyridinedicarboxylate.16 Surprisingly, less attention has been focused on the investigation of the elongated unsymmetrical bridging pyridyl carboxylate ligands.17 And recent research works reveal that CPs based on unsymmetrical linkers could exhibit previously undiscovered novel frameworks and properties. Following the above consideration, an unsymmetrical biphenyl tritopic ligand, 3-(2′,5′-dicarboxylphenyl)pyridine acid (H2dcpy), was selected to construct coordination architecture. In this work, through adjusting the reaction solvents and temperatures, we herein present 11 new CPs: [Cd2(dcpy)2(H2O)] (1), [Cd(dcpy)]·H2O (2), [Cd(dcpy)(H2O)1.5]·2H2O (3), [Cd(dcpy)(bipy)0.5(H2O)]·5H2O (4), [Cd2(dcpy)2(bipy)(H2O)3]·2.5H2O (5), [Cd2(dcpy)2(bipy)]·4H2O (6), [Cd(dcpy)(bpa)0.5]·2H2O (7), [Ni(dcpy)(H2O)2] (8), [Ni(dcpy)(bpe)0.5(H2O)]·3H2O (9), [Ni(dcpy)(bpe)0.5(H2O)]·0.5H2O (10), and [Ni(dcpy)(bpa)0.5(H2O)2]·H2O (11), which show a systematic variation of architectures from two-dimensional (2D) layers to three-dimensional (3D) frameworks by the employment of H2dcpy and different dipyridyl ligands (bipy, bpe, and bpa) (Scheme 1). Structures, luminescence, gas sorption, and magnetic properties were investigated. Scheme 1. H2dcpy and N-Donor Ancillary Ligands Used in This Work

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EXPERIMENTAL SECTION

Materials and General Methods. All the reagents were purchased commercially and used as received without further purification. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 2400C elemental analyzer. The IR spectra were recorded using KBr pellets on a Nicolet Avatar 360 FTIR spectrometer. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8-ADVANCE using Cu Kα radiation (λ = 1.5418 Å). Thermogravimetric analyses (TGA) was measured under a nitrogen stream using with a heating rate of 5 °C/min on Netzsch TG209F3 equipment. All the gas sorption isotherms were measured by using a ASAP 2020M adsorption equipment with an automatic volumetric sorption apparatus. Luminescent spectra were performed on a PerkinElmer LS55 luminescence spectrometer. Magnetic measurements were carried out on a Quantum Design MPMS-XL-7 SQUID magnetometer. 1111



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Table 1. Crystallographic Data and Structural Refinements for 1−11 1

2

3

4

formula Mr crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm−3) F(000) Rint GOF on F2 R1a [I > 2σ(I)] wR2b (all data)

C26H16Cd2N2O9 725.21 monoclinic P21/n 14.9929(11) 8.4425(6) 19.9004(13) 90 109.3370(10) 90 2376.8(3) 4 2.027 1416 0.0226 1.050 0.0237 0.0562 5

C13H9CdNO5 371.60 orthorhombic Pbca 7.6105(6) 16.9070(12) 18.8834(14) 90 90 90 2429.7(3) 8 2.021 1440 0.0255 1.019 0.0238 0.0542 6

C26H28Cd2N2O15 833.33 monoclinic C2/c 14.4791(9) 12.5483(9) 15.9612(11) 90 93.1400(10) 90 2895.6(3) 4 1.879 1600 0.0174 1.061 0.0244 0.0713 7

C18H13CdN2O5 449.71 monoclinic C2/m 14.2014(19) 31.085(3) 12.2254(13) 90 120.053(2) 90 4671.4(9) 8 1.407 1984 0.0488 1.015 0.0402 0.0897 8

formula Mr crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm−3) F(000) Rint GOF on F2 R1a [I > 2σ(I)] wR2b (all data)

C36H28Cd2N4O11 917.45 monoclinic C2 16.6363(13) 11.7664(9) 11.7332(9) 90 96.0520(10) 90 2284.0(3) 2 1.325 900 0.0179 1.054 0.0277 0.0708

C36H22Cd2N4O8 863.37 triclinic P1̅ 8.7982(16) 11.695(2) 18.886(3) 103.833(3) 93.527(3) 105.995(3) 1796.9(6) 2 1.596 852 0.0213 1.066 0.0665 0.2463

formula Mr crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm−3) F(000) Rint GOF on F2 R1a [I > 2σ(I)] wR2b (all data) a

C19H17CdN2O6 481.76 monoclinic P21/c 16.2673(17) 7.2800(7) 16.2873(16) 90 106.001(2) 90 1854.1(3) 4 1.711 948 0.0353 1.057 0.0344 0.0910

9

10

C19H20N2NiO8 463.076 monoclinic P2/c 17.523(8) 17.315(9) 15.910(8) 90 108.410(9) 90 4580(4) 4 1.279 1808 0.0634 1.050 0.0785 0.3078

C38H30N4Ni2O11 836.08 orthorhombic Fdd2 13.0656(10) 20.0395(15) 25.521(2) 90 90 90 6682.0(10) 8 1.662 3440 0.0460 1.007 0.0399 0.0833

C13H11NNiO6 335.94 triclinic P1̅ 7.0670(6) 9.5235(9) 10.4625(9) 111.7300(10) 101.4860(10) 96.8740(10) 626.38(10) 2 1.781 344 0.0128 1.048 0.0308 0.0806 11 C19H19N2NiO7 446.06 orthorhombic C2221 10.5388(8) 19.9776(18) 19.0700(14) 90 90 90 4015.0(6) 8 1.469 1832 0.0358 1.022 0.0454 0.1202

R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2.

Synthesis of [Ni(dcpy)(bpe)0.5(H2O)]·3H2O (9). A mixture of NiSO4· 6H2O (52.6 mg, 0.1 mmol), H2dcpy (25.5 mg, 0.1 mmol) and bpe

(18.2 mg, 0.1 mmol) in H2O (4 mL)-DMF (6 mL) was sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 130 °C for 72 h, 1112



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Figure 1. (a) The coordination environments of Cd atoms in 1, (b) 1D zigzag chain formed by Cd1 atoms are bridged by carboxylate O atoms, (c) 1D helical chain formed by Cd2 atoms and carboxyl group, (d) 2D layer, (e) 3D structure, and (f) four-connected net of 1. and then gradually cooled to room temperature at a rate of 5 °C min−1. Green plate crystals of 9 were obtained with the yield was ca. 37.5 mg (81.0%, based on dcpy). Anal. Calc. for C19H20N2NiO8: C, 49.28; H, 3.35; N, 6.05. Found: C, 49.42; H, 3.29; N, 6.17. IR data (KBr, cm−1): 3449s, 2026w, 1614m, 1567m, 1485w, 1403m, 1373s, 1272w, 1201w, 1131w, 1025w, 978m, 903w, 825m, 780m, 701w. Synthesis of [Ni(dcpy)(bpe)0.5(H2O)]·0.5H2O (10). The same synthetic method as that for 9 was used except that the solvent H2O (4 mL)−DMF (6 mL) was replaced by H2O (10 mL), and the temperature was increased to 160 °C. Green block crystals of 10 were obtained with the yield was ca. 13.4 mg (32.8%, based on dcpy). Anal. Calc. for C19H15N2NiO5.5: C, 55.65; H, 3.69; N, 6.83. Found: C, 55.69; H, 3.58; N, 6.94. IR data (KBr, cm−1): 3573m, 2026w, 1616m, 1482w,

1403m, 1371m, 1272w, 1129w, 1070w, 1024w, 980w, 829m, 778m, 703w. Synthesis of [Ni(dcpy)(bpa)0.5(H2O)2]·H2O (11). The same synthetic method as that for 9 was used except that bpe was replaced by bpa (18.4 mg, 0.1 mmol). Green block crystals of 11 were obtained with the yield was ca. 19.9 mg (44.6%, based on dcpy). Anal. Calc. for C19H19N2NiO7: C, 51.16; H, 4.29; N, 6.28. Found: C, 51.32; H, 4.17; N, 6.34. IR data (KBr, cm−1): 3389s, 2652w, 2026w, 1927w, 1612m, 1554m, 1369m, 1277w, 1218w, 1148w, 1116m, 1065m, 1034m, 984m, 924w, 897m, 827m, 772m, 711w. X-ray Crystallography. The diffraction experiments for all complexes were conducted on a Bruker SMART APEX II CCD detector at 296(2) K using Mo Kα radiation (λ = 0.71073 Å) and ω 1113



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Figure 2. (a) The coordination environments of Cd atoms in 2, (b) 3D structure contains 1D narrow channels formed by double left- and righthanded helical chains, and 1D left- and right-handed helical chains formed by hydrogen bonds between O1w and O3, and (c) four-connected net of 2.

ions. 2 and 3 were prepared at 130 °C in the mixed DMF−H2O (1:4) and DMF−H2O (3:2) solvents, respectively. The different products should mainly result from the different solubilities of Cd(NO3)2 and H2dcpy in DMF and H2O solvents, and the same reason may be for the structural difference between 4 and 5. Under the same reaction solvent, 4 was obtained at 130 °C, while 6 was crystallized at a high temperature 160 °C. This result indicates that the formations of 4 and 6 are thermodynamically controlled. In addition, as described later in this paper, 1−11 possess different coordination architectures. Using the reaction solvents for preparing 1 (H2O), 2 (DMF−H2O/1:4), and 3 (DMF−H2O/ 3:2), 1 resulted in a nonporous structure, and 2 and 3 possess the effective free volume of 8.1% and 17.3%. This result further indicates the solvent effect for constructing a structure, and the same effect may explain the structural difference between 4 and 5, and 9 and 10. Crystal Structure of Cd2(dcpy)2(H2O) (1). 1 crystallizes in the monoclinic space group P21/n, and the asymmetric unit of 1 consists of two Cd2+ ions, two dcpy2−, and one water molecule. Cd1 is seven-coordinated by six O atoms from three dcpy2− and one water molecule, and one N atom of one dcpy2− [Cd1−O 2.248(2)−2.607(2) Å, Cd1−N 2.360(3) Å]. Cd2 is bonded to four O atoms and one N atom from four dcpy2− [Cd2−O 2.223(2)−2.362(2) Å, Cd1−N 2.238(2) Å] (Figure 1a). The dcpy2− in 1 displays two kinds of coordination modes;

rotation scans at a width of 0.3°. The structures were solved by the direct methods and refined by full-matrix least-squares refinements based on F2.18 Absorption corrections were applied by utilizing SADABS routine. Non-hydrogen atoms of the networks were refined with anisotropic temperature parameters. The hydrogen atoms of the linker molecules were placed in calculated positions and refined with a riding model. However, the hydrogen atoms of the guest solvates could not be located from the difference Fourier maps. Because of the low residual electron density and heavy disorder, it was impossible to locate the guest molecules in the crystal structures of 4, 5, and 6. Application of the SQEEZE routine in the PLATON19 software package produced a new intensity data set excluding the intensity contribution from disordered solvent molecules. Crystal data as well as details of data collection and refinements for complexes 1−11 are summarized in Table 1. CCDC reference numbers for 1−11: 967997− 968007.

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RESULTS AND DISCUSSION Synthesis. The formation and/or crystallization of CPs are significantly influenced by the reaction temperature, solvent, metal cations, and so on.8 Reaction of the same initial reactants in different solvents or at different temperatures can generate diverse structures, which may provide direct evidence of the structural influence from these external factors. 1 and 8 were synthesized under the same external experimental environment except for the metal salts; the structural differences may be related to the different coordination environments of metal 1114



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Figure 3. (a) The coordination environments of Cd atoms in 3, (b) Cd2O3 SBU, (c) 3D structure, and (d) (4,6)-connected net of 3.

one links to four Cd2+ ions, and the carboxyl groups adopt η1:η1-μ1 and η1:η2-μ2, while the other also connects to four Cd2+ ions but in which the carboxyl groups adopt η1:η1-μ1 and η1:η1μ2 (Scheme S1a, Supporting Information). The adjacent Cd1 atoms are bridged by the carboxylate O atom along the b-axis to form a one-dimensional (1D) zigzag Cd−O−Cd chain (Cd1··· Cd1 = 4.390 Å) (Figure 1b), while adjacent Cd2 atoms are linked by one carboxyl group along the b-axis to generate a 1D left-handed helical Cd−(COO)−Cd chain (Cd2···Cd2 = 5.449 Å) (Figure 1c). As shown in Figure 1d, such two kinds of 1D chains extended via the bridge of dcpy2− alternately to generate a 2D net and further connected along the c-axis to give a 3D framework (Figure 1e). From a topological viewpoint, the framework of 1 can be simplified as a four-connected topological net, upon considering dcpy2− and Cd2+ ion as four-connected nodes (Figure 1f). Crystal Structure of [Cd(dcpy)]·(H2O) (2). 2 crystallizes in the orthorhombic space group Pbca and shows a 3D microporous framework based on 1D Cd−O−Cd chains and dcpy2− linkers. The asymmetric unit consists of one Cd2+ ion and one dcpy2−. Each Cd2+ ion shows a distorted octahedral geometry by five carboxylate O atoms and one N atom from four dcpy2− [Cd1−O 2.210(2)−2.517(2) Å, Cd1−N 2.236(2) Å] (Figure 2a). All dcpy2− in 2 display one coordination mode in which the carboxyl groups adopt η1:η1-μ1 and η1:η2-μ2 (Scheme S1b, Supporting Information). The adjacent Cd2+ ions are bridged by the carboxylate O atoms (O2) along the aaxis to form a 1D serrated Cd−O−Cd chain (Cd1···Cd1 = 4.282 Å), and the neighboring 1D chains are further connected by dcpy2− to generate a 3D framework that contains 1D narrow channels occupied by guest water (O1w) molecules (Figure 2b). Interestingly, these channels are formed by double left- or

right-handed 21 helical chains, which are arranged alternately in the structure. More interestingly, the strong hydrogen bonding extends in the channels between O1w and O3 along the a-axis direction to form also 1D left- and right-handed helical chains (O1w···O3 = 2.986 and 3.006 Å). Similar to 1, in 2, either the dcpy2− or Cd2+ ion can be considered as a four-connected node; therefore, the structure of 2 can be defined as a fourconnected topological net with a point symbol of (4·63·82) (Figure 2c). Crystal Structure of [Cd(dcpy)(H2O)1.5]·2(H2O) (3). 3 crystallizes in the monoclinic space group C2/c, and the asymmetric unit consists of one Cd2+ ion, one dcpy2−, and three and a half water molecules. Each Cd2+ ion is bonded to six O atoms of four dcpy2− and two water molecules, and one N atom form one dcpy2− [Cd1−O 2.249(3)−2.536(2) Å, Cd1−N 2.316(2) Å] (Figure 3a). The same as 2, each dcpy2− in 3 links four Cd2+ ions, showing only one coordination mode in which the two carboxyl groups adopt η1:η1-μ1 and η0:η2-μ2 (Scheme S1c, Supporting Information). The adjacent two Cd2+ ions are bridged by two carboxylate O atoms (O1) and one water (O1w) molecule to generate a Cd2O3 SBU with a Cd1···Cd1 distance of 3.582 Å (Figure 3b). By the dcpy2− linkers, these SBUs are connected to generate a 3D microporous framework (Figure 3c) which contains 1D open channels with a diameter of ca. 4.6 Å, taking into account the van der Waals radii. The removal of water molecules which filled in the channels gives 17.3% of accessible void volume. According to the simplification principle, the framework of 3 can be considered as a (4,6)-connected net, in which the dcpy2− and Cd2O3 SBU act as four- and six-connected nodes, respectively (Figure 3d). Crystal Structure of [Cd(dcpy)(bipy)0.5(H2O)]·5H2O (4). 4 crystallizes in the monoclinic C2/m space group, showing a 3D 1115



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Figure 4. (a) The coordination environments of Cd atoms in 4, (b) Cd2O2 SBU, (c) 2D plane layer, (d) 3D supramolecular structure formed by hydrogen bonding interactions (blue dotted line) and face to face π−π interactions (purple dotted line), and (e) space-filling model of the 3D supramolecular structure.

porous framework built on 2D stacked layers through hydrogen bonding and π−π interactions. The asymmetric unit is composed of one Cd2+ ion, one dcpy2−, a half bipy, and one coordinated H2O molecule. Like 3, Cd2+ ion is bonded to six O atoms from three dcpy2− and one water molecule, and one N atom from one bipy [Cd1−O 2.327(3)−2.544(3) Å, Cd1−N 2.294(3) Å] (Figure 4a). Adjacent Cd2+ ions are bridged by two carboxylate O atoms (O2) to generate a Cd2O2 SBU with a Cd1···Cd1 distance of 3.827 Å (Figure 4b). Interestingly, the pyridine group of each dcpy2− ligand does not participate in the coordination. So each dcpy2− connects to three Cd2+ ions through two carboxyl groups which adopt η1:η1-μ1 and η1:η2-μ2 (Scheme S1d, Supporting Information). Two Cd2O2 SBUs are linked by two dcpy2− and expand along the a-axis to form a 1D wavelike chain, while the adjacent chains are linked by the bipy via Cd−N bonds to form a 2D plane layer (Figure 4c). These layers are further interconnected through the hydrogen bonding interactions between a coordinated O1w molecule and a carboxylate O1 atom (O1w···O1 = 2.729 Å) and the faceto-face π−π interactions (C11···C11 = 3.357 Å) and stack in an AAA fashion resulting into a 3D supramolecular architecture (Figure 4d), which possesses open channels with a dimension of approximately 11.654 × 9.922 Å2 along the [001] direction (Figure 4e). The removal of water molecules gives 33.7% of accessible void volume. Topologically, the dcpy2− and Cd2O2

SBU can be considered as two- and three-connected nodes, respectively. Therefore, the 2D layer of 4 can be defined as a three-connected net (Figure 4c). Crystal Structure of [Cd2(dcpy)2(bipy)(H2O)3]·2.5H2O (5). 5 crystallizes in the monoclinic C2 space group and exhibits a 3D homochiral microporous framework. The asymmetric unit is composed of two Cd2+ ions, two dcpy2−, one bipy, and three coordinated H2O molecules. Cd1 is six-coordinated by four O atoms from two dcpy2− and two water molecules, and two N atoms from two bipy [Cd1−O 2.254(3) and 2.363(3) Å, Cd1− N 2.332(10) and 2.349(5) Å]. Cd2 is bonded to five O atoms from two dcpy2− and one water molecule, and two N atoms from two dcpy2− [Cd2−O 2.339(12)−2.497(3) Å, Cd2−N 2.331(3) Å] (Figure 5a). Each dcpy2− connects to three Cd2+ ions, showing also one coordination mode in which the two carboxyl groups adopt η0:η1-μ1 and η1:η1-μ1 (Scheme S1e, Supporting Information). It is interesting to find that the dcpy2− links Cd2 to give a left-handed 21 helical chain along the b-axis, as well as a channel along the [110]-direction with the effective sizes of 8.50 × 6.90 Å2. Adjacent chains are connected by sharing Cd2 atoms to form a 2D layer, and each layer is chiral (Figure 5b), while adjacent Cd1 atoms are linked by bipy through Cd1−N2/Cd1−N3 bonds along the b-axis to form a 1D chain. The neighboring 2D layers are lined together by Cd1 atoms into a microporous framework (Figure 5c), leaving 1116



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Figure 5. (a) The coordination environments of Cd atoms in 5, (b) 2D layer formed by adjacent left-handed chains which are connected by sharing Cd2 atoms, (c) 3D framework formed by neighboring 2D layers are lined together through Cd1 atoms, and (d) (3,4)-connected net of 5.

31.5% voids. From a topological viewpoint, the final structure can be regarded as a (3,4)-connected network with a point symbol of (73)2(72·82·92)(74·9·11), by denoting the Cd2+ ion as a four-connected node and dcpy2− simplified as a threeconnected node (Figure 5d).

Crystal Structure of [Cd 2(dcpy)2(bipy)]·4H2O (6). 6 crystallizes in the triclinic P1̅ space group, and the asymmetric unit is composed of two Cd2+ ions, two dcpy2−, one bipy, and four free water molecules. Cd1 takes on a distorted pentagonal bipyramid geometry definited by five O atoms from three 1117



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Figure 6. (a) The coordination environments of Cd atoms in 6, (b) independently 0D [Cd4(dcpy)4] square loop, (c) 1D tubular structure, (d) two 1D tubular connected by Cd1−O5 bonds, (e) self-penetrating motif formed by three tubular through 1 + 2 form, and (f) (3,4,6)-connected net of 6.

dcpy2−, and two N atoms from two bipy [Cd1−O 2.295(5)− 2.435(6) Å, Cd1−N 2.329(6) and 2.334(6) Å]. Cd2 shows a distorted octahedral coordination geometry and connects with

four O atoms and two N atoms from four dcpy2− [Cd2−O 2.279(9)−2.444(11) Å, Cd2−N 2.261(7) and 2.295(7) Å] (Figure 6a). In 6, the dcpy2− takes two coordination modes; 1118



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Figure 7. (a) The coordination environments of Cd atoms in 7, (b) 2D layer contains left- and right-handed chains, (c) 3D pillar-layer structure, (d) left- and right-handed helical water cluster chains, and (e) (3,4)-connected net of 7.

one links to three Cd2+ ions and the carboxyl groups adopt η1:η1-μ1, and the other also connects to four Cd2+ ions in which the carboxyl groups adopt η1:η1-μ1 and η1:η2-μ2 (Scheme S1f, Supporting Information). Four Cd2+ ions (2Cd1 + 2Cd2) are first connected by four dcpy2− to give a [Cd4(dcpy)4] square with dimensions of 11.113 × 11.178 Å2 (the distance between the Cd1···Cd2) (Figure 6b). Furthermore, each bipy as a bridge

connects with two Cd2+ ions; therefore, the [Cd4(dcpy)4] square is infinitely extended by bipy to form a 1D metal− organic nanotube (MONT) (Figure 6c). Two such 1D MONT nets are connected through Cd1−O5 bonds, as shown in Figure 6d. Interestingly, another 1D MONT lies in the center flat of two connected 1D MONT nets and across them (1 + 2 form), which connects to the two MONT through the Cd2− 1119



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Figure 8. (a) The coordination environments of Ni atoms in 8, (b) 2D plane layer and its (3,6)-connected net (c), and (d) 3D supramolecular structure of 8 formed by the interlayers hydrogen bonds (O1w···O4 = 2.698 Å and O2w···O4 = 2.646 Å).

possesses a 10.9% solvent-accessible void (Figure 7c). It is interesting that the free water molecules filled in these channels are arranged in left- or right-handed helical water cluster chains (Figure 7d). This framework can be defined as a (3,4)connected topology with the point symbol of (4·82)(4·82·10· 11·12), upon considering the dcpy2− and Cd2+ as three- and four-connected nodes, respectively (Figure 7e). Crystal Structure of [Ni(dcpy)(H2O)2] (8). 8 crystallizes in the triclinic P1̅ space group and shows a dinuclear Ni2-based 2D layer-type structure. There is one Ni2+ ion, one dcpy2−, and two coordinated water molecules in the asymmetric unit. Each Ni2+ ion with distorted octahedral geometry is coordinated by five O atoms from three dcpy2− and two water molecules, and one N atom of one dcpy2− [Ni1−O 2.0479(17)−2.1070(18) Å, Ni1−N 2.096(2) Å] (Figure 8a). Two Ni2+ ions are combined by two η1:η1-μ2 carboxylate groups (Scheme S1g, Supporting Information) from two dcpy2− to afford a [Ni2(O2C)2(H2O)4] dimer with the Ni···Ni separations of 4.416 Å. These Ni2 dimers are first linked by the carboxylate groups to give a 1D chain, and the adjacent chains are further joined through the N atoms from dcpy2− to form a 2D layer parallel to the bc crystal face (Figure 8b). The adjacent 2D layers connect with each other through hydrogen bonding interaction between the coordinated O1w and O2w molecules and a carboxylate O4 atom (O1w···O4 = 2.698 Å, and O2w···O4 = 2.646 Å) and further stacked in an AAA fashion resulting in a 3D supramolecular architecture (Figure 8d). Topologically, 8 can

N2 bonds to give a novel self-penetrating motif (Figure 6e). It is worth pointing out that 6 presents the first 3D architecture showing a self-penetrating structure based on 1D MONT. From the view of topology, adjacent Cd1 atoms are bridged by two carboxylate O atoms (O5) to form a Cd2O2 SBU, which is considered as a six-connected node, while the Cd2 and dcpy2− can be considered as four- and three-connected nodes, respectively, so 6 can be defined as a (3,4,6)-connected net (Figure 6f). Crystal Structure of [Cd(dcpy)(bpa)0.5]·2H2O (7). 7 crystallizes in the monoclinic space group P21/c and shows a 3D microporous pillar-layer framework. There is one Cd2+ ion, one dcpy2−, a half bpa, and two free water molecules in the asymmetric unit. As shown in Figure 7a, each Cd2+ ion shows distorted octahedral geometry via coordinating to four carboxylate O atoms of two dcpy2−, and two N atoms form one dcpy2− and one bpa [Cd1−O 2.289(3)−2.491(3) Å, Cd1− N 2.273(3) and 2.282(3) Å]. Like 5, each dcpy2− links to three Cd2+ ions with two carboxyl groups adopting the same η1:η1-μ1 coordination mode (Scheme S1b, Supporting Information). The interlinkage between Cd2+ ions and dcpy2− generates a 2D double layer, in which two kinds of 21 helical chains are formed along the b-axis and are alternately arranged in left- and righthanded sequences (Figure 7b). Each layer is achiral, and it further links the adjacent layers through a bpa pillar along the aaxis, leading to a 3D pillar-layer network. The network contains 1D square channels with a diameter of ca. 6.45 × 7.69 Å2, which 1120



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Figure 9. (a) The coordination environments of Ni atoms in 9, (b) 2D layer of 9 and its simplified net, (c) 2D stacked layers along the (101) directions, and (d) space-filling model of 2D stacked layers.

direction. It should be pointed out that 9 is isoreticular with the complex [Ni(dcpy)(bipy)0.5(H2O)] recently reported by our group.20 Comparing the two structures, the most interesting observation is that when we use a longer bpe linker to replace the bipy (Figure S1, Supporting Information) the result is that both the pore size and the accessible void volume for 9 are apparent improved, while the framework type does not change. The void volume of 9 is 39.0% after removal of water molecules, which is much higher than [Ni(dcpy)(bipy)0.5(H2O)] (27.8%). From a topological view, 9 is a (3,4)connected topology with the point symbol of (42·6)(42·6·83) (Figure 9e). Crystal Structure of [Ni(dcpy)(bpe)0.5(H2O)]·0.5H2O (10). 10 is a structural isomer of 9 due to the same framework components except different amounts of water guest molecules.21 10 crystallizes in the orthorhombic space group Fdd2 and shows an unusual 3D self-penetrating framework. There is one Ni2+ ion, one dcpy2−, a half bpe, and one and a half water molecules in the asymmetric unit. Each Ni2+ ion is six-coordinated by four O atoms of two dcpy2− and one water molecule, and two N atoms of one dcpy2− and one bpe [Ni1− O 2.041(2)−2.179(3) Å, Ni1−N 2.062(3) and 2.073(3) Å] (Figure 10a). The dcpy2− adopt the same coordination mode in which two carboxyl groups adopt η1:η1-μ1 and η1:η1-μ1 (Scheme S1i, Supporting Information). Each dcpy2− connects to three Ni2+ ions and extends into a 3D microporous structure (Figure

be described as a (3,6)-connected sheet with the point symbol of (4·62)2(42·62·82). Crystal Structure of [Ni(dcpy)(bpe)0.5(H2O)]·3H2O (9). 9 crystallizes in the monoclinic C2/c space group and displays a 3D porous framework built on 2D stacked layers through hydrogen bonding interactions. There are two Ni2+ ions in the asymmetric unit, while both Ni1 and Ni2 have the same distorted octahedral geometry, and each one is coordinated to four O atoms from two dcpy2− and one water molecule, and two N atoms from one dcpy2− and one bpe (Figure 9a). Each dcpy2− bonds to three Ni2+ ions to induce a 1D zigzag chain, in which two carboxyl groups adopt η 0:η1-μ1 and η 1:η1-μ1 coordination mode (Scheme S1h, Supporting Information), and adjacent chains are connected by the bipy linkers via Ni−N bonds to form a 2D wavelike layer (Figure 9b). Furthermore, these 2D layers are interconnected by the hydrogen bonding interactions between the coordinated water molecule (O10) and a carboxylate atom O7 (O10···O7 = 2.764 Å), the guest water molecules (O1w and O3w) and the coordinated water molecule (O9) (O1w···O9 = 2.807 Å, O1w···O1w = 2.743 Å, O3w···O9 = 2.848 Å, and O3w···O3w = 2.636 Å), and the guest water molecule (O2w) with a carboxylate atom O8 (O2w···O8 = 2.951 Å and O2w···O2w = 2.654 Å) and stack in an ABAB fashion into a 3D supramolecular framework (Figure 9c,d), which has two kinds of open channels with dimensions of approximately 5.17 × 5.17 and 7.0 × 7.0 Å2 along the [001] 1121



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Figure 10. (a) The coordination environments of Ni atoms in 10, (b) 3D framework formed by only Ni2+ and dcpy2−, (c) 3D framework of 10, (d) (3,4)-connected net, and (e) schematic representation of the shortest catenated seven-membered rings.

10b). However, the bpe bridges two Ni2+ ions filling in the pores to allow the structure of 10 to become a nonporous framework (Figure 10c). Topology analysis indicates that the structure can be regarded as a (3,4)-connected network, by denoting the Ni2+ ions to four-connected nodes and dcpy2−

simplified as three-connected nodes, with the point symbol of (4·122)(4·125) (Figure 10d). Remarkably, this 3D network also shows an intriguing self-penetrating entangled fashion. As shown in Figure 10e, the catenated 7-rings are the shortest topological rings, and two such 7-rings interlocked with another 1122



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Figure 11. (a) The coordination environments of Ni atoms in 11, 2D layer of 11 (b) and its simplified topological net (c), 3D supramolecular framework formed by interlayers hydrogen bonds (O2w···O2 = 2.754 Å) which is stacked in an ABAB fashion (d).

between 6 and 7, and 9 and 11 are related to the lengths and rigidness/flexibility of the dipyridyl ligands for the bipy and bpe are rigid ligands. While the bpa is a flexible ligand, bpa and bpe are longer than bipy. The decisive effect of ancillary N-donor ligands employed in 4−7 and 9−11 leads to different products revealing that the N-donor ligands play a significant role in adjusting the final structures. Furthermore, through comparison of these structures mentioned above and the ones previously reported based on H2dcpy and the N-donor ancillary ligands,8d,9b we could find that introducing the rigid auxiliary N-donor ligands makes it easy to form 2D layer-type porous structures, while interpenetration usually occurred in the 3D structures. So both the pore width and void volume of the 3D structures are much smaller than these 2D frameworks. On the other hand, although the H2dcpy displays various coordination modes in these structures, most of the H2dcpy served as a three-connected node to form a 2-nodal (3,4)-connected network. PXRD and TGA. The experimental PXRD patterns of 1−11 demonstrate good agreement with the simulated ones (Figure S2, Supporting Information), indicating the phase purity. The TGA was performed in N2 atmosphere on polycrystalline samples of 1−11 (Figure S3, Supporting Information). For 1, the preliminary weight loss in the range 30−135 °C corresponds to the release of coordinated water molecules (found. 2.41%; calcd. 2.48%) and then no further weight loss

7-ring to form a self-penetrating motif, which is a rare selfpenetrating motif. Crystal Structure of [Ni(dcpy)(bpa)0.5(H2O)2]·H2O (11). 11 crystallizes in the monoclinic space group of C2221, and the asymmetric unit consists of one Ni2+ ion, one dcpy2−, a half bpa, and three H2O molecules. Each Ni2+ ion with a distorted octahedral geometry is coordinated by four O atoms from two dcpy2− and two water molecules, and two N atoms from one dcpy2− and one bpa [Ni−O 2.082(2)−2.110(2) Å, Ni1−N 2.075(3) and 2.091(3) Å] (Figure 11a). The two carboxylate groups of dcpy2− display the same η0:η1:μ1 coordination mode (Scheme S1j, Supporting Information) and bonds to three Ni2+ ions to induce a 2D layer (Figure 11b), in which the adjacent Ni2+ ions are further bridged by one bpa. The neighboring 2D layers are interconnected through hydrogen bonds and stacked in an ABAB fashion to result in a 3D supramolecular architecture (Figure 11d). Topology analysis indicates that both the dcpy2− and Ni2+ ions can be considered as threeconnected nodes, so 11 is a three-connected net sheet with the point symbol of (32·4·92·10)(3·92). As described above, 1−11 possess 11 different polydimensional frameworks. Besides the effects of the reaction solvents, temperatures and metal cations have been discussed in the synthesis. The significant structural differences between 1 relative to 6 and 7, 8 relative to 10 should be ascribed to the introduction of the auxiliary N-donor ligands. The differences 1123



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Figure 12. Gas sorption isotherms of 9 for N2, H2, and CO2: (a) N2, 77 K; H2, 77 K; CO2, 195 K, (b) N2 and CO2, 273 and 298 K.

before the decomposition at 355 °C. For 2, a slow loss between 30 and 205 °C is attributed to the release of free H2O guests (found. 4.83%; calcd. 4.84%). No obvious weight loss is observed until the decomposition of the framework occurs at 385 °C. For 3, the TGA curve exhibits a weight loss about 14.7% within the temperature range 30−300 °C, which is ascribed to the release of both the coordinated and lattice water molecules (calcd. 15.1%), and then the residual framework starts to decompose. For 4, the preliminary weight loss in the range 30−130 °C is ascribed to the losses of lattice and coordinated water molecules (found. 20.7%; calcd. 20.0%) and then no further weight loss before the decomposition at 310 °C. The TGA curve of 5 reveals a steady weight loss between 30−165 °C, corresponding to the loss of the lattice and coordinated water molecules (found. 10.3%; calcd. 10.2%). The structure began to decompose from approximately 320 °C. For 6, first, the structure is stable up to 105 °C, with a total weight loss of a 7.7% in the temperature range of 105−175 °C, corresponding to 4H2O per formula unit (calcd. 7.7%). Then the framework followed by a plateau of stability until 355 °C. For 7, the weight loss of 7.8% in the range 30−160 °C corresponds to the removal of 2H2O per formula unit (calcd. 7.5%). Above 350 °C, a further heating induces an abrupt weight loss owing to the decomposition of 7. Similar to 6, the structure of 8 is stable up to 145 °C, and then, with a total weight loss of a 10.6% in the temperature range 145−175 °C, corresponding to coordinated water molecules per formula unit (calcd. 10.7%). For 9, a total weight loss of 16.3% below 250 °C corresponds to the release of lattice and coordinated water molecules (calcd. 15.6%) and no further weight loss before the decomposition at 310 °C. For 10, its weight loss of 7.2% below 155 °C corresponds to the release of water guest and coordinated water molecules (calcd. 6.5%), and no weight loss before the decomposition at 350 °C. For 11, the preliminary weight loss of 12.3% beginning at 30−185 °C corresponds to the removal of the lattice and coordinated water molecules (calcd. 12.1%). The framework began to decompose slowly from 150 °C and decomposed rapidly above 320 °C. Sorption Properties. Porous CPs (PCPs) are currently attracting great attention toward their application as CO2 separation and capture materials from flue gas owing to their well-defined chemical structures and facile porous modification.22 While these materials are intensively focused on 3D PCPs, limited works have focused on the investigation of 1D and 2D networks for gas separation. Recent reports demonstrated that 2D PCPs are significant motifs for selective

gas adsorption because of the possible movement of the 2D nets in addition to local bond lengths and angles shift or change during guest removal or accommodation.23 On the basis of the excellent sorption properties of [Ni(dcpy)(bipy)0.5(H2O)],20 gas sorption studies were also conducted for 9, which has a larger pore size and volume. The desolvated samples were prepared by heating 9 at 150 °C for 4 h and subsequent 180 °C for 2 h under a vacuum, which is verified by TGA and PXRD analysis. As shown in Figure 12a, the desolvated 9 shows reversible type-I sorption isotherms for N2 at 77 K and CO2 at 195 K but displays obvious hysteresis curves for both N2 and CO2 desorption. The maximum N2 and CO2 uptakes at 1 atm are 93.9 and 172.4 cm3 g−1, respectively. From the N2 sorption isotherm at 77 K, the BET and Langmuir surface areas were calculated to be 225 and 301 m2 g−1, respectively, and a median pore width of 5.7 Å was determined by applying the Horvath− Kawazoe method. In addition, the desolvated 9 takes H2 of 84.0 cm3 g−1 or 0.75 wt % at 77 K and 1 atm, which is comparable to the best zeolite ZSM-5 (0.7 wt %).24 Interestingly, the H2 sorption isotherm also displays remarkable hysteresis, although the pore size is much bigger than the kinetic diameter of H2, indicating strong interaction between H2 and host framework. It should be noted that both the adsorption and desorption process for N2, CO2, and H2 are almost the same with [Ni(dcpy)(bipy)0.5(H2O)], but the adsorption amounts are significantly increased (Figure S4a−4c, Supporting Information), and the measured value of pore width for 9 is also larger than [Ni(dcpy)(bipy)0.5(H2O)] (Figure S4d). These results imply that by using the longer bpe linker to replace the bipy not only can the architecture of the 2D layer-type porous be retained but also the pore width and volume increase, to further improve the adsorptivity. Interestingly, at 273 and 298 K, the desolvated 9 is almost nonadsorptive for N2 (6.7 and 1.5 cm3 g−1); however, it exhibits high CO2 uptakes of 73.4 and 52.8 cm3 g−1, respectively (Figure 12b). This CO2 sorption amount (10.4 wt %) at 298 K, which is moderately comparable or even superior to that of many famous porous materials with large pores,25 implies that the small pore may be more attractive for CO2 under low pressure. The initial slopes of the CO2 and N2 adsorption isotherms were calculated, and the ratios of these slopes were used to estimate the adsorption selectivity for CO2 over N2 (Figure S5, Supporting Information). From these data, the calculated CO2/N2 selectivity is 51.3 at 273 K and 120.7 at 298 K. Such high CO2 selectivity over N2 is rarely reported and among the best excellent CO2 selectivity values to date in PCPs.26 1124



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solvents. These results indicate its potential application in gas separation, which contains a certain amount of moisture. In addition, gas adsorption measurements were also carried out for 4 and 5, which possess a solvent accessible void of 33.7% and 31.5%, respectively. Unfortunately, no obvious N2, H2, and CO2 adsorption is observed (Figure S8, Supporting Information), which is probably caused by structural distortion or the framework part disintegration after desolvation. The desolvated samples of 4 and 5 are proven by the following PXRD experiments (Figure S7, Supporting Information). Photoluminescence Properties. CPs have been investigated for fluorescence properties owing to their application in the field of luminescent materials, especially for the compounds with d10 metal centers, such as Zn2+ and Cd2+ ions.33 In this work, the solid-state photoluminescence of 1−7 (Figure 14)

Especially, at 0.15 atm, a typical partial pressure of CO2 in industrial flue gas, 9 still holds 14.3 cm3 g−1 (28.1 mg g−1) CO2, but no adsorption for N2. The significant selectivity for CO2 is mainly related to the different electrostatic interactions between the porous surface and CO2 molecules.20,23,27 In 9, the porous surface is decorated with the exposed Ni2+ sites, stemming from the release of coordinated H2O, giving rise to an electric field interacting with quadrupole molecules, which induces strong interactions between framework and CO2 because of its much larger quadrupole moment (−1.4 × 10−39 C m2) than N2 (−4.7 × 10−40 C m2). In addition, the small kinetic diameter of CO2 (3.30 Å) compared to N2 (3.64 Å) enables it to enter easily into the pore. To further check the affinity of 9 for CO2, the CO2 adsorption enthalpies (Qst) were calculated according to the virial equation from the sorption isotherms at 273 and 298 K (Figure S6, Supporting Information). At initial coverage, Qst exhibits maxima of 34.6 kJ mol−1 (Figure 13), and this value is

Figure 14. Solid-state emission spectra of 1−7 at room temperature. Figure 13. The CO2 adsorption heat calculated according to the virial equation.

and the free ligand H 2 dcpy (Figure S9, Supporting Information) were investigated at room temperature. The emission spectra have broad peaks with maxima at 442 nm (λex = 340 nm) for 1, 521 nm (λex = 320 nm) for 2, 451 nm (λex = 340 nm) for 3, 465 nm (λex = 360 nm) for 4, 445 nm (λex = 340 nm) for 5, 396 nm (λex = 320 nm) for 6, and 359 nm (λex = 280 nm) for 7, respectively. As we all known, Cd2+ ion takes on the d10 configuration, which is proven to be hard to oxidize or reduce; the emissions of 1−7 should neither be metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT). According to the close emission energy, they are tentatively attributed to π* → n and π* → π transitions of the intraligands. In addition, the emission spectra of the H2dcpy ligand show a very weak peak at 400 and 470 nm (λex = 290 nm) in the range 300−600 nm. The free bipy and bpa ligands, which show a weak peak at 415 and 345 nm, respectively, may also influence the emission for 4−7. So the red- or blue-shift of the emission maximum can probably be ascribed to the metal− ligand coordinative interactions. Comparing the differences of luminescences between 1−3 and 4−6, the result suggests that the differences of the ligands, the structures, and coordination environments of the central metal ions may also participate in the process of energy transfer involved in the luminescence. Magnetic Property. Solid-state magnetic susceptibility measurements for 8 was performed in the range 1.8−300 K under a field of 1000 Oe. As shown in Figure 15, The magnitude of χMT at 300 K is 1.25 cm3 K mol−1, which is close to the expected value of 1.21 cm3 K mol−1 for one spin-only Ni2+ (S = 1) ion with g = 2.2. Upon cooling, the χMT value declines monotonously and reaches 0.04 cm3 K mol−1 at 1.8 K,

comparable with that of some previous PCPs containing amine groups or open unsaturated metal sites that have high Qst values for CO2 (32−47 kJ mol−1),28 and the flexible PCPs with “breathing effect” MIL-53 (32−35 kJ mol−1),29 while it slightly surpasses the value of MOF-5 [Zn4O(bdc)3] (34 kJ mol−1, bdc = 1,4-benzenedicarboxylate).30 The Qst slowly decreases with the increase of pressure, remaining up to 33.0 kJ mol−1 at maximum loading. This value even exceeds the zero-loading Qst for CO2 in some PCPs31 and siliceous zeolites (ca. 27 kJ mol−1).32 In the overall adsorption region, the mean Qst is 33.8 kJ mol−1. The high Qst for CO2 in 9 is attributed to the following crucial factors: first, the exposed Lewis acidic Ni2+ sites in the porous surface induce strong electrostatic interactions with quadrupole CO2 molecules. Second, the numerous uncoordinated carboxyl O donors and pyridine groups on the pore walls to form strong affinity for CO2 molecules. Third, the small pore promotes the overlap of potential fields from multiple sides of the pore walls, which strengthens framework−CO2 interactions. Meanwhile, CO2 molecules are forced to be close together in small pores, inducing CO2−CO2 interactions and providing moderate contributions for Qst. On the other hand, the stability toward water is one of the main challenges in applying PCPs as CO2 adsorbents.22b The hydrothermal stability of 9 demonstrates that the framework integrity could be retained in water at room temperature for one week (Figure S2b, Supporting Information). Moreover, 9 can also be stable in common organic 1125



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material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the State Key Program of National Natural Science of China (Grant No. 20931005), Key Research Planning Program of National Natural Science Foundation of China (Grant No. 91022004), and National Natural Science Foundation of China (Grant Nos. 21001088 and 21201139).

Figure 15. Temperature dependence of magnetic susceptibilities in the form of χM (black) and χMT (blue) vs T plot.

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suggesting a dominant antiferromagnetic exchange behavior in Ni2+ ions. In the Ni2 unit of 8, the magnetic coupling between two Ni2+ centers is transmitted through two μ1,3-carboxylate bridges with a long Ni···Ni distance of 4.416 Å, which is responsible for the antiferromagnetic interaction in 8. Data fitting of the χM−1−T curve using the Curie−Weiss Law gives the Curie constant (C = 1.26 cm3 K mol−1) and a negative Weiss constant (θ = −11.42 K) above 50 K. The negative θ further confirms a dominant antiferromagnetic interaction between the Ni2+ ions.

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CONCLUSION In summary, we have constructed 11 CPs based on an unsymmetrical pyridyldicarboxylatic acid. 1−11 display appealing structural features from 2D layers to 3D frameworks, such as a rarely reported 4-connected 3D framework with double left- and right-handed helical chains of 2, (3,4)-connected 3D homochiral microporous framework with left-handed helical chains of 5, the first 3D (3,4,6)-connected network which shows a self-penetrating structure based on 1D MONT of 6, 3D microporous pillar-layer framework containing 1D left- and right-handed helical water clusters chains of 7, wavelike 2D stacked layer pore framework possessing large open channels of 9, and a 3D (3,4)-connected self-penetrating feature of 10. The results indicate that the variations of reaction solvents and temperatures are critical to the assemblies of CPs in some particular systems. Furthermore, the flexibility and length of the dipyridyl ligands play significant roles in adjusting the coordination modes of pyridyl carboxylic acids and coordination architecture. 1−7 exhibit solid-state luminescence with different emission energies, and 8 displays antiferromagnetic interaction. More importantly, 9 exhibits a high water stability and highly 2D polar pore material with excellent selective adsorption of CO2 over N2. This work will further enrich the synthesis and design of CPs based on elongated pyridyl carboxylic acid ligands, and the extendable work will construct stabilized and functionalized materials through employing a variety of analogous ligands.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in CIF format, additional structural figures, TGA, PXRD patterns of 1−11, and detailed calculations on the adsorption heat and selectivity in 9. This 1126



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