Conformational Supramolecular Isomerism in Two-Dimensional

Article pubs.acs.org/crystal

Conformational Supramolecular Isomerism in Two-Dimensional Fluorescent Coordination Polymers Based on Flexible Tetracarboxylate Ligand Da-Shu Chen,† Li-Bo Sun,‡ Zhi-Qiang Liang,‡ Kui-Zhan Shao,† Chun-Gang Wang,† Zhong-Min Su,*,† and Hong-Zhu Xing*,† †

Institute of Functional Material Chemistry, College of Chemistry, Northeast Normal University, Changchun, 130024 China State Key Lab of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, 130012 China

‡

S Supporting Information *

ABSTRACT: Six new coordination polymers, (Me2NH2)[In(L)]· DMF (1), (Me2NH2)2[In2(L)2]·(DMF)3·(H2O)2 (2), (Me2NH2)4[Cd2(L)2]·(DMF)2 (3), (Et2NH2)2[Cd(L)]·H2O (4), (Et 2 NH 2 ) 3 [Cd 2 (L) 2 ]·(DEF) 2 ·(H 2O) 4 (5), (Et 2 NH 2 ) 3 [Cd 2 (L) 2 ]· (EtNH2)2·(H2O)2 (6) have been prepared based on a newly designed flexible tetracarboxylate ligand of H4L {H4L = 5-[bis(3-carboxybenzyl)amino]isophthalic acid; DMF = N,N-dimethylformamide; DEF = N,Ndiethylformamide}. Compounds 1−6 possess very similar twodimensional framework structures with (4,4)-net topology. These compounds comprise two groups of conformational supramolecular isomers, owing to identical compositions of the frameworks but different conformations of ligands. It is worth noting that as much as seven pairs of stereoisomers of ligands have been found in these compounds, which demonstrated that the introduction of multiple single bonds has greatly expanded the flexibility of the ligand. The photoluminescent properties of compounds 1−6 have also been investigated where the emission of these supramolecular isomers are from blue to green.

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INTRODUCTION Coordination polymers (CPs) have received enormous attention in recent years in view of their topologically diverse structures1,2 and potential applications in fields such as gas storage,3 separations,4 catalysis,5 drug delivery,6 luminescence,7 and so on.8 Over the last few decades, a vast number of CPs have been synthesized and investigated. However, a targeted architecture cannot be obtained frequently, even though making a wise choice for “building blocks” because structural uncertainty of CPs is an inherent characteristic of a selfassembled system.9 This structural uncertainty can be mainly ascribed to the various crystallization conditions and the different arrangements of the same component in the solid compounds.10 In order to well-understand the self-assembled system of coordination polymers and reach the goal of target synthesis, a detailed study of supramolecular isomerism is necessary. Supramolecular isomerism has attracted much attention in material science because not only may supramolecular isomers display different physical and chemical properties but also controlling supramolecular isomerism truly reflects the core ideology of crystal engineering.11 For coordination polymers, supramolecular isomerism is used when the structures are different but the whole crystals or the coordination frameworks have the same chemical composition.12 The supramolecular © XXXX American Chemical Society

isomerism in coordination polymers can be categorized into four different sorts: structural,13 conformational,14 catenane,15 and optical16 isomerism. These categorizations reveal the structural origins of supramolecular isomers, which can also be regarded as the guiding principle for designing a supramolecular isomerism system.12 The application of flexible ligands has already been proven to be a successful approach to reach structural diversity of coordination polymers.17 However, it is still less popular for the study of supramolecular isomerism and the resulting properties. With the aforementioned consideration, we focused on the conformational supramolecular isomerism using a newly designed tetracarboxylate ligand H4L {H4L = 5-[bis(3carboxybenzyl)amino]isophthalic acid; Scheme 1}. Both C−N (Csp3−N) and C−C (Csp3−C′sp2) covalent bonds were introduced to expand the flexibility of the ligand where the C−N bond cooperated with the C−C bond in regulating the relative location of the p-carboxyphenyl group. Six new two-dimensional coordination polymers have been synthesized by solvothermal method, namely (Me2NH2)[In(L)]·DMF (1), (Me2NH2)2[In2(L)2]·(DMF)3·(H2O)2 (2), Received: June 17, 2013 Revised: July 26, 2013

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dx.doi.org/10.1021/cg400913f | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

from 3° to 40°. Solid-state fluorescence spectra were measured on FLSP920 Edinburgh Fluorescence Spectrometer at room temperature. Synthesis of (Me2NH2)[In(L)]·DMF (1). A mixture of In(NO3)3· 4H2O (1.6 mg, 4.3 μmol), H4L (4 mg, 8.9 μmol), HBF4 (0.1 mL), and DMF (1 mL) was placed in a capped vial (20 mL). The vial was heated at 110 °C for 5 days. Colorless leaf-shaped crystals were obtained. Yield: ca. 50% based on In. Anal. Calcd (%) for C29H30InN3O9: C, 51.81; H, 4.93; N, 6.04. Found (%): C, 51.62; H, 4.71; N, 6.18. IR (KBr, cm−1): 3430 (w), 3064 (w), 2927 (w), 1656 (s), 1571 (s), 1454 (m), 1392 (s), 1165 (w), 1097 (w), 1021 (w), 995 (w), 821 (w), 775 (m), 754 (s), 705 (w), 683 (w), 451 (w). Synthesis of (Me2NH2)2[In2(L)2]·(DMF)3·(H2O)2 (2). A mixture of In(NO3)3·4H2O (6.7 mg, 18 μmol), H4L (8 mg, 18 μmol), HNO3 (0.15 mL, 7 mol/L), H2O (0.5 mL) and DMF (1.2 mL) was placed in a capped vial (20 mL). The vial was heated at 85 °C for 5 days. Colorless rodlike crystals were obtained. Yield: ca. 60% based on In. Anal. Calcd (%) for C61H71In2N7O21: C, 49.91; H, 4.88; N, 6.68. Found (%): C, 49.79; H, 4.93; N, 6.82. IR (KBr, cm−1): 3434 (m), 3058 (w), 2926 (w), 1660 (s), 1563 (s), 1447 (m), 1394 (s), 1145 (w), 1093 (w), 1055 (w), 877 (w), 765 (s), 693 (w), 449 (m). Synthesis of (Me2NH2)4[Cd2(L)2]·(DMF)2 (3). A mixture of Cd(NO3)2 (1.4 mg, 4.5 μmol), H4L (4 mg, 8.9 μmol), H2O (0.2 mL) and DMF (1 mL) was placed in a capped vial (20 mL). The capped vial was heated at 110 °C for 3 days. Colorless leaf-shaped crystals were obtained. Yield: ca. 70% based on Cd. Anal. Calcd (%) for C62H76Cd2N8O18: C, 51.49; H, 5.30; N, 7.75. Found (%): C, 51.32; H, 5.33; N, 7.84. IR (KBr, cm−1): 3424 (m), 3034 (w), 2930 (w), 2768 (w), 2486 (w), 1661 (s), 1571 (s), 1443 (w), 1389 (s), 1021 (w), 991 (w), 963 (w), 925 (w), 777 (m), 671 (w), 411 (w). Synthesis of (Et2NH2)2[Cd(L)]·H2O (4). A mixture of Cd(NO3)2 (0.91 mg, 3.0 μmol), H4L (4 mg, 8.9 μmol), H2O (0.10 mL), DEF (1 mL) was placed in a Teflon autoclave (20 mL) and heated at 110 °C for 3 days. Colorless platelike crystals were obtained. Yield: ca. 40% based on Cd. Anal. Calcd (%) for C32H41CdN3O9: C, 53.08; H, 5.71; N, 5.80. Found (%): C, 52.89; H, 5.82; N, 5.94. IR (KBr, cm−1): 3388 (w), 2973 (m), 1639 (s), 1577 (s), 1440 (m), 1382 (s), 1152 (w), 1113 (w), 1049 (w), 872 (w), 765 (s), 427 (w). Synthesis of (Et2NH2)3[Cd2(L)2]·(DEF)2·(H2O)4 (5). Compound 5 was prepared by the same reaction of 4 when it was carried out at 140 °C for 3 days. Colorless stripe-shaped crystals were obtained. Yield: ca.

Scheme 1. Molecular Structure of Flexible Tetracarboxylate Ligand (H4L)

(Me2NH2)4[Cd2(L)2]·(DMF)2 (3), (Et2NH2)2[Cd(L)]·H2O (4), (Et 2 NH 2 ) 3 [Cd 2 (L) 2 ]·(DEF) 2 ·(H 2 O) 4 (5), and (Et2NH2)3[Cd2(L)2]·(EtNH2)2·(H2O)2 (6). The six compounds can be ascribed to two groups of conformational supramolecular isomers, owing to the same composition of the frameworks and different conformations of ligands. The photoluminescence properties of 1−6 have been studied in solid state at room temperature.

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EXPERIMENTAL SECTION

Materials and Measurements. Reagents were of analytical grade and used as purchased without further purification. Elemental analyses (C, H, N) were performed on a Perkin-Elmer 240C elemental analyzer. The Fourier transform infrared (FT-IR) spectra were recorded based on KBr pellets in the range of 4000−400 cm−1 on a Mattson Alpha-Centauri spectrometer. Thermogravimetric analyses (TGA) were performed on a Perkin-Elmer TG-7 analyzer heating from room temperature to 600 °C under atmosphere at a rate of 10 °C min−1. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku D-MAX 2550 radiation (λ = 0.15417 nm) with 2θ ranging

Table 1. Crystallographic Data and Structure Refinement Details for Compounds 1−6

a

compound

1

2

3

4

5

6

formula formula weight crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalc (g cm−3) μ (Mo Kα) (mm−1) F(000) total reflections unique reflections R(int) R1 [I > 2σ(I)]a wR2 [I > 2σ(I)]b R1 (all data) wR2 (all data) GOF on F2

C29H30InN3O9 679.38 monoclinic P21/c 9.8392(12) 21.748(3) 15.1973(16) 111.277(6) 3030.3 4 1.489 0.836 1384 15225 5336 0.0848 0.0482 0.0899 0.1098 0.1095 0.949

C61H71In2N7O21 1467.89 monoclinic P21 9.9183(16) 22.930(4) 15.082(3) 103.957(3) 3328.8(10) 2 1.464 0.771 1504 16401 11222 0.0566 0.0722 0.1734 0.1360 0.2104 1.009

C62H76Cd2N8O18 1446.13 monoclinic P21/c 15.972(2) 21.984(3) 20.090(3) 113.194(2) 6484.0(15) 4 1.481 0.732 2976 39154 15170 0.0869 0.0643 0.1419 0.1730 0.1891 0.994

C32H41CdN3O9 724.09 monoclinic P21/c 10.0732(6) 26.6408(15) 14.5918(7) 110.730(3) 3662.3 4 1.313 0.647 1496 14891 4667 0.0453 0.0466 0.1234 0.0692 0.1347 1.060



R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = [∑w(F2o − F2c)2/∑w(F2o)2]1/2 B



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Scheme 2. Synthesis of Compounds 1−6

Figure 1. (a) The coordination environment of the In3+ cation in 1 with the ellipsoids drawn at the 30% probability level. (b) A view of the structure of 1 where guest molecules are omitted for clarity. (c) View of a single two-dimensional (2D) layer of 1. (d) Ligand and In3+ ions can be regarded as 4-connected nodes and schematic representation of the (4,4)-net topology of 1. solved by direct methods and refined by full-matrix least-squares on F2 with anisotropic displacement using SHELXTL.19 The hydrogen atoms were added theoretically, riding on the concerned atoms. The relevant crystallographic data are listed in Table 1.

70% based on Cd. Anal. Calcd (%) for C70H96Cd2N7O22: C, 56.06; H, 6.45; N, 6.54. Found (%): C, 55.82; H, 6.52; N, 6.72. IR (KBr, cm−1): 3384 (w), 2976 (m), 1638 (s), 1574 (s), 1439 (m), 1379 (s), 1148 (w), 1112 (w), 1051 (w), 795 (w), 762 (s), 427 (w). Synthesis of (Et2NH2)3[Cd2(L)2]·(EtNH2)2·(H2O)2 (6). Laurelgreen platelike crystals of 6 were obtained (ca. 10%) as a byproduct of compound 5 when the reaction of 5 was carried out at 150 °C. The sample of 6 for fluorescent characterization was picked out from the mixture judging by the different crystal morphology. X-ray Crystallography. Single-crystal X-ray diffraction data collection of the compounds was performed on a Bruker Smart Apex II CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. Data processing was accomplished with SAINT.18 Absorption corrections were applied by using the multiscan program SADABS. All six crystal structures were

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RESULTS AND DISCUSSION Synthesis of the Compounds. Scheme 2 displays the synthesis of compounds 1−6. Compound 1 was prepared in DMF with the In3+ ion as an inorganic node, and fluoroboric acid (HBF4) was used to adjust the pH-dependent crystallization (pH ≈ 2). Synthesis of new coordination polymers in mixed solvent has been demonstrated to be very efficient. With the use of a mixed solvent of DMF/H2O, compound 2 was obtained with nitric acid as an additive (pH ≈ 1). In C



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Figure 2. (a−f) Different sterioisomers of ligands existed in compounds 1−6.

100.414°) and N···Csp3−C′sp2···C″sp2 (16.420° and 23.646°) (Table 2).

consideration that Cd2+ ions own the same electronic configuration and similar ionic radius as compared to In3+ ions, it would be an appropriate candidate replacing In3+ ions to construct isoreticular structures. Compound 3 with Cd2+ ion as an inorganic metal node was successfully synthesized using a mixed solvent of DMF/H2O. Compound 3 can be prepared under wide temperature from 85 to 120 °C. In addition, the volume ratio of DMF to H2O and the molar ratio of metal to ligand for the synthesis of 3 could vary from 10:1 to 1:1 and 1:1 to 1:4, respectively. When mixed solvent of DEF/H2O was used for the reaction of Cd2+ ions and ligand, compounds 4−6 can be prepared upon different reaction temperature. Pure phase of compound 4 was prepared at 110 °C, while stripeshaped crystals of compound 5 were obtained at 140 °C. When the temperature further increased, laurel-green platelike crystals of 6 can be found as a byproduct (yield: ca. 10%) of compound 5 when the reaction of 5 was carried out at 150 °C. However, it is unfortunate that attempts to prepare pure compounds of 6 with higher temperatures (150−170 °C) were not successful. Structural Descriptions of (Me2NH2)[In(L)]·DMF (1). Compound 1 crystallizes in the monoclinic space group P21/c. There is one In3+ ion, one ligand anion, one DMF molecule, and one Me2NH2+ cation in the crystallography asymmetric unit (Figure 1a). The In3+ ion, showing a distorted triangulated dodecahedral coordination geometry,20 binds eight oxygen atoms from four ligand anions with a typical In−O bond length ranging from 2.206(4) Å to 2.401(4) Å. The ligand coordinated to four In3+ ions through chelating bidentate modes of carboxylate groups constructing infinite anionic double-layered framework of compound 1 (Figure 1, panels b and c). There are pseudochannels along the [100] direction within the layers. The adjacent layers are in an ABAB packing model. Topologically, the double-layerd framework is a waved (4,4)net, where both the ligand anions and the In3+ cations in the structure can be simplified as 4-connected nodes (Figure 1d). Both the solvent molecules of DMF and protonated dimethylamine cations of Me2NH2+ exist in the pseudochannels of the structure. The charge balancing Me2NH2+ cations should result from the decomposition of DMF molecules.21 It is worth noting that ligands in compound 1 are propellerlike conformational isomers. The isomers are labeled by “Δ” and “Λ” which are usually used to describe propellerlike stereoisomers (Figure 2a).22 The torsion angles of Csp2···N− Csp3···C′sp2 and N···Csp3−C′sp2···C″sp2 were used to quantify a conformational change of ligands (Scheme 1). It is interesting that 1−Δ and 1−Λ are enantiomers, hence, they possess the same torsion angles of Csp2···N−Csp3···C′sp2 (94.465° and

Table 2. Torsion Angles of Different Sterioisomers of Ligand conformation

Csp2···N−Csp3···C′sp2

N···Csp3−C′sp2···C″sp2

1−Δ and 1−Λ 2−Δ 2−Λ 3−Δ and 3−Λ 3−Δ′ and 3−Λ′ 4−Δ and 4−Λ 5−Δ and 5−Λ 6−Δ and 6−Λ

94.465° and 100.414° 94.651° and 131.808° 103.349° and 128.779° 81.781° and 105.347° 109.445° and 113.147° 94.348° and 104.396° 92.543° and 146.645° 102.468° and 106.008°

16.420° and 23.646° 53.321° and 60.097° 52.914° and 59.031° 13.722° and 25.045° 36.222° and 55.439° 2.724° and 27.578° 52.360° and 55.603° 50.778° and 51.786°

Structural Descriptions of (Me2NH2)2[In2(L)2]·(DMF)3· (H2O)2 (2). Compound 2 crystallizes in the chiral space group P21. An attempt to find higher symmetry using Platon was not successful, indicating the chiral nature of the structure.23 There are two In3+ cations, two ligand anions, two water molecules, two Me2NH2+ cations, and three DMF molecules in the crystallography asymmetric unit of 2 (Figure 3a). The two crystallographically independent eight-coordinated In3+ cations are both coordinated by eight oxygen atoms from carboxylic groups. The In−O bond lengths range from 2.133(11) Å to 2.563(12) Å. The coordination modes of carboxylate groups belonging to ligand anions are all of chelating modes. The ligand anions and In3+ cations connected to each other form a double-layered structure, which is similar to 1 (Figure 3b). The layered structure of 2 can be simplified as the (4,4)-net and the adjacent layers are of ABAB packing. However, additional H2O and DMF molecules occurred in 2 as compared to 1. The guest molecules in 2 not only existed in the intralayer pseudochannels but also appeared in the interlayer void space. It is worth noting that ligands in 2 constitute a new pair of stereoisomers, and the location of the p-carboxyphenyl groups in ligands seems more “upright” as compared to that in compound 1 (Figure 2b). Different from the ligands in the racemic compound 1 (space group P21/c) are pairs of enantiomers, the sterioisomers of ligand in 2 are not enantiomorphs because of the chiral nature of the structure (space group P21), hence 2−Δ and 2−Λ possess independent Csp2···N−Csp3···C′sp2 and N···Csp3−C′sp2···C″sp2 torsion angles. The enatiomer of 2−Δ possesses Csp2···N−Csp3···C′sp2 torsion angles of 94.651° and 131.808° and N···Csp3−C′sp2···C″sp2 torsion angles of 53.321° and 60.097°, while enatiomers of 2−Λ have Csp2···N−Csp3···C′sp2 torsion angles of 103.349° and D



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Figure 3. (a) Coordination environment of the In3+ cation in 2 with the ellipsoids drawn at the 30% probability level. (b) Presentation of the layered structure of 2, where guest molecules are omitted for clarity.

Figure 4. (a) The coordination environment of Cd2+ cation in 3 with the ellipsoids drawn at the 30% probability level. (b) Presentation of the layered structure of 3 where guest molecules are omitted for clarity. (c) A view showing the decussated ligand in 3. (d) A view showing the π−π interactions in 3.

128.779° and N···Csp3−C′sp2···C″sp2 torsion angles of 52.914° and 59.031° (Table 2). It is interesting that the structures of both compounds 1 and 2 possess identical components but different structures. Hence, they present supramolecular isomerism where 1 and 2 are supramolecular isomers. The analyses of crystal structures indicate that the difference between 1 and 2 results from the conformational change of the ligand. Therefore, compounds 1 and 2 are conformational supramolecular isomers originating from the flexibility of the ligand.

In order to study conformational supramolecular isomerism of coordination polymers using this ligand, we replaced In3+ ions by Cd2+ ions in further experiments. Compounds 3−6 were prepared by reaction of Cd2+ ions with H4L in mixed solvents of DMF/H2O (for 3) and DEF/H2O (for 4−6). Single-crystal structures revealed that 3−6 possess similar double-layered structures and the same (4,4)-net topology as 1 and 2. Structural analyses indicate that compounds 3−6 are also conformational supramolecular isomers. E



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Figure 5. The layered structures of compounds (a) 4, (b) 5, and (c) 6.

Table 3. Structure Differences among Compounds 1−6 compound

1

2

3

ligand isomers

In 8 Me2NH2+ DMF 1−Δ,1−Λ

In 8 Me2NH2+ DMF, H2O 2−Δ,2−Λ

COO− coordination modes

bidentate

bidentate

Cd 7 Me2NH2+ DMF 3−Δ,3−Λ 3−Δ′,3−Λ′ mono, bidentate

metal coordination number guest molecules

4

5

6

Cd 7 Et2NH2+ H2O 4−Δ,4−Λ

Cd 6 Et2NH2+ DEF, H2O 5−Δ,5−Λ

Cd 7 Et2NH2+ EtNH2, H2O 6−Δ,6−Λ

mono, bidentate

mono, bidentate

mono, bidentate

Csp2···N−Csp3···C′sp2 torsion angles of 94.348° and 104.396° and N···Csp3−C′sp2···C″sp2 torsion angles of 2.724° and 27.578° (Figure 2d and Table 2). Structural Descriptions of (Et2NH2)3[Cd2(L)2]·(DEF)2· (H2O)4 (5). Compound 5 crystallizes in the same space group P21/c as 4. The asymmetric unit is composed of one Cd2+ cation, one ligand anion, one DEF molecule, two water molecules, and one and a half Et2NH2+ cations. The octahedral Cd2+ ion is six coordinated with two monodentate carboxylate groups and two chelating carboxylate groups. The Cd−O bond distances are in the range of 2.218(5)−2.575(6) Å. A new pair of enantiomers 5−Δ and 5−Λ exists in the structure (Figure 2e). As compared to compound 4, the (4,4)-net double-layerd framework is obviously distorted (Figure 5b) and the conformation of ligand enantiomers changes seriously (Figure 2 and Table 2). Structural Descriptions of (Et2NH2)3[Cd2(L)2]·(EtNH2)2· (H2O)2 (6). Compound 6 crystallizes also in the space group P21/c. The symmetric unit consists of one Cd2+ cation, one ligand anion, one water molecule, one and a half Et2NH2+ cations, and one EtNH2 molecule. The distorted pentagonal biyramid Cd2+ ion is seven-coordinated with four carboxylate groups belonging to four ligands. The Cd−O bond distances are in the range of 2.234(8)−2.665(13) Å. The 2D structure of 6 is also constructed from the 4,4-connection of ligands and Cd2+ ions (Figure 5c). As shown in Figure 2f, a new pair of ligand enantiomers 6−Δ and 6−Λ appears as expected. The conformation of the ligand in 6 is very close to that in 5 (Figure 2, panels e and f), with the primary difference resulting from the change of the Csp2···N−Csp3···C′sp2 torsion angle (Table 2). It is worth noting that compounds 4−6 were prepared from the same reaction where the only difference existed in the reaction temperature. As shown in Figure 5, the adjacent layers in these compounds seem to pack denser correspondingly when the temperature increased. The interlayer distances are approximately 12.6, 12.0, and 10.6 Å for compounds 4−6 by measuring the distances between adjacent planes in the (4,4)net double layers, where the planes are defined by inversion centers between ligand enantiomers (Figure S12 of the Supporting Information). In addition, the calculated density of the structures is also increased from 4 to 6 (Table 1). It is well-known for hydro/solvothermal synthesis that the selfgenerated pressure in autoclaves increases while the temper-

Structural Descriptions of (Me2NH2)4[Cd2(L)2]·(DMF)2 (3). Compound 3 crystallizes in the monoclinic P21/c space group, and there are two Cd2+ cations, two ligand anions, two DMF molecules, and four Me2NH2+ cations in the crystallography asymmetric unit. As illustrated in Figure 4a, both Cd(1) and Cd(2) are seven-coordinated by oxygen atoms from four adjacent ligands to generate a distorted pentagonal biyramid coordination geometry. The Cd−O distances range from 2.259(5) to 2.644(6) Å. The Cd2+ ions make the framework bearing more negative charges as compared to the In3+ ions, so more cations should exist in the void space of the structure to keep the charge balance. Dimethylamine cations of Me2NH2+ resulting from the decomposition of DMF molecules exist in the intralayer channel and the interlayer void space to balance the framework charge. As shown in Figure 4b, compound 3 has 2D double-layered (4,4)-net structures and adjacent layers are of ABAB packing. Although ligands with new conformations could be forecasted here, it is still unexpected that two pairs of new ligand enantiomers with different conformations coexist in compound 3. As shown in Figure 2c, the two pairs of enantiomers are obviously different. The enatiomers of 3−Δ and 3−Λ possess Csp2···N−Csp3···C′sp2 torsion angles of 81.781° and 105.347° and N···Csp3−C′sp2···C″sp2 torsion angles of 13.722° and 25.045°, while enatiomers of 3−Δ′, 3−Λ′ have Csp2···N−Csp3···C′sp2 torsion angles of 109.445° and 113.147° and N···Csp3−C′sp2···C″sp2 torsion angles of 36.222° and 55.439° (Table 2). The four sterioisomers of ligand are decussated in the layer (Figure 4c). Two kinds of interlayer π−π interactions between benzene rings of p-carboxyphenyl groups can be found in the structure (Figure 4d), and the faceto-face distances of π−π interactions are 3.683 Å and 3.469 Å. Structural Descriptions of (Et2NH2)2[Cd(L)]·H2O (4). Compound 4 crystallizes in the space group P21/c, and the asymmetric unit consists of one Cd2+ cation, one ligand anion, one water molecule, and two Et2NH2+ cations. The distorted pentagonal biyramid Cd2+ ion is seven-coordinated with seven oxygen atoms from four symmetry related ligands. The Cd−O bond distances fall in the range of 2.283(5)−2.615(4) Å. The carboxylate groups of ligand anions adopt three bidentate and one monodentate coordination mode to coordinate Cd2+ ions forming the (4,4)-net framework (Figure 5a). A new pair of propellerlike enantiomers 4−Δ and 4−Λ can be found with F



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Figure 6. (a) Emisson spectra of ligand and supramolecular isomers 1 and 2. (b) Emisson spectra of ligand and supramolecular isomers 3−6. The insets showed the photographs of crystals 1−6 under UV excitation (λex = 365 nm).

ature rises.12 Therefore, it is rational to deduce that such structural change primarily results from the temperatureinduced pressure. Other structural differences among these compounds including coordination number of metals, guest molecules, ligand conformation, and coordination modes have been shown in Table 3. Totally, six coordination polymers 1−6 with very similar layered structures have been prepared by ligand H4L. The asprepared compounds can be ascribed to two groups of conformational supramolecular isomers based on different metal ions of In3+ (1 and 2) and Cd2+ (3−6). Interestingly, as much as seven pairs of ligand steroisomers with different conformations were observed in 1−6. As shown in Table 2, the ligand showed great diversification of conformation where the torsion angles of Csp2···N−Csp3···C′sp2 change from 81.781° to 146.645°, and the torsion angle of N···Csp3−C′sp2···C″sp are from 2.724° to 60.097°, respectively. These data indicate that the introduction of multiple single bonds indeed expanded the flexibility of the ligand, thereby facilitating the formation of conformational supramolecular isomers of coordination polymers. Luminescent Properties. The photoluminescence spectra of the free ligand H4L in DMF and compounds 1−6 in the solid state have been investigated at room temperature. The DMF solution of ligand displays fluorescent emission centered at 424 nm (λex = 390 nm), which could be ascribed to the π−π* or n−π* transition.24 Compounds 1 and 2 exhibit very similar blue emission at ca. 430 nm upon excitation at 306 nm, which are ascribed to ligand-based emission (Figure 6a).25 However, the photoluminescence of compounds based on cadmium ions is more interesting. When excited at 306 nm, the emission at 424 nm of the free ligand was blue-shifted to 412 and 402 nm for compounds 3 and 4, respectively (Figure 6b). Unexpectedly, the strongest emission of compound 5 is located at 493 nm under excitation at 306 nm, which is significantly red-shifted compared to ligand emission at 424 nm. Interestingly, a further red-shifted emission at 508 nm upon excitation at 306 nm was observed for compound 6, which shows a weak shoulder emission at 405 nm. It is noteworthy that compounds 4−6 displayed fluorescence emission from

blue (402 nm) to green (508 nm), which is among the maximum emission shifts for study of supramolecular isomers in coordination polymers.26 In consideration of the aforementioned structural analyses, such a large emission shift should arise from differences in solid-state structures.

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CONCLUSIONS By introducing multiple covalent single bonds to design a flexible luminescent multicarboxylic ligand, we have synthesized six 2D coordination polymers 1−6. They can be ascribed to two groups of conformational supramolecular isomers. The ligand showed great diversification of conformation that as much as seven distinct pairs of ligand stereoisomers with different conformations were observed in the as-prepared coordination polymers. The fluorescence of these supramolecular isomers displayed a large emission shift from blue to green. This study demonstrated that the combination of multiple single bonds to enrich the ligand conformations is a feasible strategy to construct conformational supramolecular isomerism for coordination polymers.

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ASSOCIATED CONTENT

S Supporting Information *

Coordination modes of ligand, PXRD patterns, TGA, IR, and X-ray crystallographic CIF files (CCDC 943574−943579). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Z.-M.S.: e-mail, [email protected]. H.-Z.X.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the China Natural Science Foundation (Grant 21101023), the Fundamental Research Funds for the Central Universities (Grant 10QNJJ010) and the G



Article

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Jilin Provincial Science and Technology Development Foundation (Grant 201101007).

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