pH-Dependent Binary Metal−Organic Compounds Assembled from

Feb 25, 2010 - †Institute of Functional Material Chemistry, Key Lab of Polyoxometalate ... ‡Biological Scientific and Technical College, Changchun...
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DOI: 10.1021/cg9012763

pH-Dependent Binary Metal-Organic Compounds Assembled from Different Helical Units: Structural Variation and Supramolecular Isomers

2010, Vol. 10 1699–1705

Shun-Li Li,† Ke Tan,‡ Ya-Qian Lan,† Jun-Sheng Qin,† Mei-Na Li,† Dong-Ying Du,† Hong-Ying Zang,† and Zhong-Min Su*,† † Institute of Functional Material Chemistry, Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun, 130024, China, and ‡ Biological Scientific and Technical College, Changchun University, Changchun, 130022, China

Received October 14, 2009; Revised Manuscript Received February 10, 2010

ABSTRACT: Six binary metal-organic compounds, namely, [Cd(L)2] (1), R-[Cd(L)2(H2O)] (2), β-[Cd(L)2(H2O)] (3a and 3b), γ-[Cd(L)2(H2O)] (4), and [Cd(L)2(H2O)2] (5), where HL=4-(pyridin-3-ylmethoxy)benzoic acid, have been synthesized under hydrothermal conditions. Their structures were determined by single-crystal X-ray diffraction analyses and further characterized by elemental analyses, IR spectra, and thermogravimetric (TG) analyses. The structure of 1 is a two-dimensional (2D) f 2D 3-fold parallel interpenetrating network with (4,4) grid. Compounds 2, 3, and 4 are supramolecular isomers formed by Cd(II) cations, L- ligand, and water molecule, showing a 2D f 2D 2-fold parallel interpenetrating (4,4) network for 2, single (4,4) network for 3a and 3b, and (66) topological net for 4 through using different reaction materials. Compounds 3a and 3b show chiral structures by spontaneous resolution using an achiral ligand. In compound 5, Cd(II) cations are linked by L- ligands to form an infinite chain. Comparing these structures, the number of coordination water molecules, the coordination fashion of organic ligands, and reaction conditions play fundamental roles in the formation of the final products. In addition, the luminescent properties of these compounds are discussed.

Introduction Coordination polymers are of current interest not only for their potential applications in microelectronics, nonlinear optics, zeolite-like materials for molecular selection, ion exchange, and catalysis but also for their intriguing variety of architectures and topologies.1 Up to now, large numbers of coordination polymers with one (1D)-, two (2D)-, and three (3D)-dimensional structures have been obtained by hydrothermal methods, which are optimal for crystal growth.2 The mechanism of the complicated reaction under hydrothermal method remains unclear, which depends directly on the interplay of starting materials, pH value, template, and reaction temperature. On the basis of above account, we selected a simple binary system and investigated the influence of the reaction conditions on this system under hydrothermal conditions. Meanwhile, helicity is a topological motif that is highly attractive not only because of its fascinating structure but also for its realistic and potential applications in many fields.3 Discrete helices or low-dimensional helical coordination polymers have been reported,4 while the rational synthesis of MOFs containing a multihelical-array is still a great challenge.5 According to the previous literature, the construction of metal-organic frameworks mainly depends on the nature of the organic ligands (spacers) and metal ions (nodes).6 The rational design of prospective structures with specific properties can be constructed on the basis of careful selection of the properties of the ligands, such as shape, functionality, flexibility, symmetry, length, and substituent group.7 According

to our previous research,8 4-(pyridin-3-ylmethoxy)benzoic acid (L-) as a nitrogen-containing asymmetric bridging carboxylic acid ligand is an excellent ligand and can freely twist around the -CH2- group to meet the requirements of the coordination geometries of metal atoms in the assembly process, which often leads to helical structures, and can more easily produce the new classes of compounds. As accurate prediction of the final structures is impossible; we have tried different synthetic conditions and performed many experiments under different conditions. Fortunately, six binary metal-organic compounds, namely, [Cd(L)2] (1), [Cd(L)2(H2O)] (2), R-[Cd(L)2(H2O)] (3a and 3b), β-[Cd(L)2(H2O)] (4), and γ-[Cd(L)2(H2O)2] (5) were successfully isolated by hydrothermal methods. In addition, the infrared spectra and thermogravimetric analyses have been investigated in detail for six compounds, and the luminescent properties have been also investigated. Experimental Section

*To whom correspondence should be addressed. Tel: þ86 431 85099108. E-mail: [email protected].

General Procedures. Chemicals were purchased from commercial sources and used without further purification. 4-(Pyridin-3-ylmethoxy)benzoic acid was prepared according to the literature.8 Synthesis of [Cd(L)2] (1). A mixture of HL (0.25 g, 1.00 mmol), Cd(OAc)2 3 2H2O (0.27 g, 1.00 mmol), and H2O (10 mL) was adjusted to approximately pH ≈ 1 with H2SO4 (6 M) and stirred for 1 h and then transferred and sealed in a 25 mL Teflon-lined stainless steel container. The container was heated to 150 C and held at that temperature for 72 h, then cooled to room temperature at a rate of 10 C 3 h-1. The final pH value was approximately 3.5. Colorless crystals of 1 were collected in 50.7%. Elemental analyses calcd for C26H20CdN2O6 (568.84): C, 54.90; H, 3.54; N, 4.92. Found: C, 55.01; H, 3.56; N, 4.89%. IR (cm-1): 1600 (s), 1524 (m), 1384 (s), 1247 (s), 1168 (m), 1031 (m), 787 (s), 667 (w), 586 (s). Synthesis of [Cd(L)2(H2O)] (2). Compound 2 was prepared in a manner similar to that used to prepare compound 1 by pH ≈ 3-4

r 2010 American Chemical Society

Published on Web 02/25/2010

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Table 1. Crystal Data and Structure Refinements of Six Compounds 1

2

3a

formula fw crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalcd.[g cm-3] F(000) reflns collected/ unique R(int) GOF on F2 R1a [I > 2σ (I)] wR2b largest residuals [e A˚-3]

C26H20CdN2O6 568.84 monoclinic C2/c 17.4720(3) 7.2970(4) 17.6590(4) 90 100.7370(10) 90 2211.99(14) 4 1.708 1144 6562/2597

C26H22CdN2O7 586.86 monoclinic P2/n 11.8260(3) 6.7690(5) 15.8830(7) 90 111.6150(10) 90 1182.03(11) 2 1.649 592 7009/2787

C26H22CdN2O7 586.86 monoclinic C2 15.6120(4) 6.0220(5) 14.1470(9) 90 111.2100(10) 90 1239.94(13) 2 1.572 592 3755/2306

0.0190 1.072 0.0208 0.0522 0.446/-0.400

0.0192 1.063 0.0265 0.0664 0.538/-0.470

0.0180 1.066 0.0185 0.0453 0.638/-0.321

3b

4

5

formula fw crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalcd.[g cm-3] F(000) reflns collected/ unique R(int) GOF on F2 R1a [I > 2σ (I)] wR2b largest residuals [e A˚-3]

C26H22CdN2O7 586.86 monoclinic C2 15.5830(7) 6.0180(9) 14.1150(15) 90 111.026(2) 90 1235.5(2) 2 1.577 592 3001/2030

C26H22CdN2O7 586.86 monoclinic C2/c 47.6730(3) 6.7800(8) 16.1100(12) 90 106.8830(10) 90 4982.7(7) 8 1.565 2368 14821/6000

C26H24CdN2O8 604.87 monoclinic P21 5.8270(5) 15.5200(12) 13.5940(14) 90 97.322(2) 90 1219.35(19) 2 1.647 612 7445/4195

0.0209 1.052 0.0234 0.0604 0.590/-0.553

0.0245 1.176 0.0769 0.1845 1.814/-0.945

0.0241 1.025 0.0335 0.0793 1.077/-0.646

a

R1 = Σ Fo| - |Fc /Σ|Fo|. b wR2 = |Σw(|Fo|2 - |Fc|2)|/Σ|w(Fo2)2|1/2. )

with H2SO4 (6 M) instead of pH ≈ 1. The final pH value was approximately 5-5.5. Colorless crystals of 2 were collected in 76.1%. Elemental analyses calcd for C26H22CdN2O7 (586.86): C, 53.21; H, 3.77; N, 4.77. Found: C, 53.41; H, 3.56; N, 4.81%. IR (cm-1): 1597 (s), 1539 (m), 1383 (s), 1247 (s), 1171 (m), 1012 (m), 787 (s), 669 (w), 592 (s). Synthesis of [Cd(L)2(H2O)] (3). Compound 3 was prepared in a manner similar to that used to prepare compound 2 by using CdCl2 (0.18 g, 1.00 mmol) and with HCl (6 M) instead of Cd(OAc)2 3 2H2O (0.27 g, 1.00 mmol) and with H2SO4 (6 M). The final pH value was approximately 5-5.5. Colorless crystals of 3 were collected in 61.9%. Elemental analyses calcd for C26H22CdN2O7 (586.86): C, 53.21; H, 3.77; N, 4.77. Found: C, 53.35; H, 3.46; N, 4.86%. IR (cm-1): 1596 (s), 1533 (m), 1363 (s), 1244 (s), 1169 (m), 1027 (m), 784 (s), 667 (w), 586 (s). Synthesis of [Cd(L)2(H2O)] (4). Compound 4 was prepared in a manner similar to that used to prepare compound 2 by using Cd(NO3)2 3 2H2O (0.31 g, 1.00 mmol) and with HNO3 (6 M) instead of Cd(OAc)2 3 2H2O (0.27 g, 1.00 mmol) and with H2SO4 (6 M). The final pH value was approximately 5-5.5. Colorless crystals of 4 were collected in 52.6%. Elemental analyses calcd for C26H22CdN2O7 (586.86): C, 53.21; H, 3.77; N, 4.77. Found: C, 53.43; H, 3.40; N, 4.73%. IR (cm-1): 1597 (s), 1548 (m), 13849 (s), 1247 (s), 1171 (m), 1012 (m), 783 (s), 674 (w), 592 (s). Synthesis of [Cd(L)2(H2O)2] (5). Compound 5 was prepared in a manner similar to that used to prepare compound 1 by pH ≈ 6 instead of pH ≈ 1 with H2SO4 (6 M) and NaOH (1 M). The final pH value was approximately 7. Colorless crystals of 5 were collected in 53.1%. Elemental analyses calcd for C26H24CdN2O8 (604.87): C, 51.63; H, 4.00; N, 4.63. Found: C, 51.59; H, 3.96; N, 4.59%. IR (cm-1): 1593 (s), 1536 (m), 1364 (s), 1245 (s), 1169 (m), 1017 (m), 784 (s), 668 (w), 587 (s). Physical Measurements. The C, H, and N elemental analysis was conducted on a Perkin-Elmer 240C elemental analyzer. The FT-IR spectra were recorded from KBr pellets in the range 4000-400 cm-1 on a Mattson Alpha-Centauri spectrometer. TGA was performed on a Perkin-Elmer TG-7 analyzer heated from 35 to 600 C under nitrogen. The emission/excitation spectra were recorded on a Varian Cary Eclipse spectrometer. X-ray Crystallography. Single-crystal X-ray diffraction data for compounds 1-5 were recorded on a Bruker Apex CCD diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) at 293 K. Absorption corrections were applied using the multiscan technique. All of the structures were solved by Direct Method of SHELXS-979 and refined by full-matrix least-squares techniques using the SHELXL-97 program10 within WINGX.11 Non-hydrogen atoms were refined with anisotropic temperature parameters. The hydrogen atoms of the organic ligands were refined as rigid groups. H atoms of water molecules were located from difference Fourier maps. The detailed crystallographic data and structure refinement parameters for 1-5 are summarized in Table 1. Structure Description of 1. Selected bond distances and angles for compounds 1-5 are listed in Table S1 (Supporting Information). Compound 1 shows a rare 2D f 2D example with parallel interpenetration of (4,4) layers. As shown in Figure 1a, the structure of 1 contains one kind of unique Cd(II) atom and one kind of unique L- ligand. The Cd(II) center is coordinated by four oxygen atoms and two nitrogen atoms (Cd(1)-O(2)=2.2483(13), Cd(1)-N(1)#2= 2.2928(14), and Cd(1)-O(3) = 2.5275(14) A˚) from four L- ligands to give the {CdO4N2} octahedral geometry. The Cd-O/N bond lengths are within the normal range.12 Adjacent Cd(II) atoms are linked by L- anions in a mono and bidentate fashion with a dihedral angle between the pyridine ring and phenyl ring of 126.4 (Chart S1a and Figure S1a, Supporting Information) to generate a simple puckered sheet with (44,62) topological structure, showing large square windows with dimensions of 14.004 A˚  14.004 A˚ for each side and 17.472 A˚  21.891 A˚ for each diagonal (Figures 1b and S1b (Supporting Information)), in which there exists a right-handed helical chain with a pitch of 21.891 A˚ along the b axis. The fascinating feature of the overall structure of 1 is the nature of the sheets, where the large square windows may offer a good chance for an interpenetration network. Three such 2D networks are interpenetrating each other as illustrated in Figure 1c, in which there are 3-fold helical chains (Figure 1d) along the b axis. Namely,

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three independent 2D sheets interpenetrate each other in a parallel fashion, still making an infinite 2D polycatenated network sheet (Figures 1e and S1c (Supporting Information)). Within the layer, all helical chains display the same chirality, and the adjacent polycatenated network sheets display opposite chirality (Figure 1f). Compound 1 is an achiral structure. To our knowledge, some products with the interesting interpenetrating nets have been researched.13,14 Among them, such 2D f 2D 3-fold parallel interpenetrating networks are scarce. Compounds [Pb(timpt)(NO3)2] 3 (Et2O)0.417 3 (H2O)0.167 (timpt=2,4,6-tris[4-(imidazol1-ylmethyl)phenyl]-1,3,5-triazine)14a with the (4 3 82) topology and bis{3-[2-(4-pyridyl)ethenyl]benzoato}cadmium14b with the (4, 4) topology have been reported. The structure of 1 shows another example of the 2D f 2D 3-fold parallel interpenetrating network, which could help us deeply understand the nature of coordination polymer frameworks and better design functional materials. Structure Description of 2. In complex 2, the Cd(II) ion adopts a pentagonal bipyramidal geometry coordinated by two N atoms, four carboxylic oxygen atoms, and one water molecule (Cd(1)N(1) = 2.3072(18), Cd(1)-O(3)#2 = 2.3924(18), Cd(1)-O(2)#2 = 2.4739(18) and Cd(1)-O(1W) = 2.513(3) A˚ (Figure 2a)). Each Lanion shows a mono and bidentate fashion with a dihedral angle between the pyridine ring and phenyl ring of 106.1 (Chart S1a and Figure S2a, Supporting Information) and links Cd(II) ions to

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Table 2. Selected Hydrogen-Bonding Geometry for 2, 3, 4, and 5 (in A˚ and deg)a D-H...A

Figure 1. (a) Coordination environment of the CdII atom in 1 with the ellipsoids drawn at the 30% probability level; all hydrogen atoms were omitted for clarity. (b) View of 2D (4,4) layers in 1 with a right-handed helical chain along the b axis. (c) Ball-and-stick and schematic representation of the parallel interpenetrating (4,4) nets in the structure of 1. (d) Ball-and-stick representation of threefolded helical chain in 1. (e) Schematic representation of the parallel interpenetrating (4,4) nets in the structure of 1. (f) The arrangement of adjacent layers in 1.

Figure 2. (a) Coordination environment of the CdII atom in 2 with the ellipsoids drawn at the 30% probability level; hydrogen atoms were omitted for clarity. (b) View of 2D (4,4) layers in 2 with a righthanded helical chain. (c) Ball-and-stick and schematic representation of the parallel interpenetrating (4,4) nets in the structure of 2. (d) Ball-and-stick representation of two-folded helical chain in 2. (e) Schematic representation of the parallel interpenetrating (4,4) nets in the structure of 2. (f) The 3D supramolecular structure of 2. generate a simple sheet with (44,62) topological structure, which contains windows with approximate dimensions of 13.626 A˚  13.626 A˚ for each side and 13.538 A˚  23.652 A˚ for each diagonal (Figures 2b and S2b (Supporting Information)). It is interesting that there exists a right-handed helical chain with a pitch of 13.538 A˚ along the b axis. When we compared the dimensions of the sheet for 2, it showed a smaller grid than that in 1. Therefore, two independent 2D sheets interpenetrate each other in a parallel fashion, showing a 2-fold helical chain (Figure 2d) along the b axis and make an infinite 2D polycatenated network sheet (Figure 2e). Some fascinating structures showing 2D-2D characteristics have been observed.13d-g Within the polycatenated network sheet in 2, the water molecules from one independent sheet donate hydrogen bonds to the carboxylic oxygen atoms from the other one, which stabilize the 2D

d(D-H)

d(H...A)

d(D...A)

— (D-H...A)

compound 2 O(1W)-H(1A)...O(3)#5 0.85(4) 1.994(19) 2.778(3)

154(3)

compound 3a O(1W)-H(1A)...O(2)#5 0.85(4) 1.88(5)

2.680(3)

158(5)

compound 3b O(1W)-H(1A)...O(2)#5 0.86(6) 1.86(3)

2.678(5)

161(8)

compound 4 O(1W)-H(1B)...O(1)#5 0.85(7) 1.89(7) O(1W)-H(1A)...O(5)#5 0.85(8) 2.32(9)

2.739(8) 172(11) 2.884(14) 124(8)

O(1W)-H(1B)...O(2) O(1W)-H(1A)...O(1)#3 O(2W)-H(2A)...O(4)#4 O(2W)-H(2B)...O(5)#1

compound 5 0.86(5) 1.83(3) 0.85(5) 1.97(3) 0.85(4) 1.98(4) 0.85(5) 1.82(3)

2.624(5) 2.793(5) 2.814(5) 2.621(5)

153(7) 160(6) 167(6) 157(6)

a Symmetry transformations used to generate equivalent atoms for 2: #5 x þ 1, y, z. For 3a: #5 x, y þ 1, z. For 3b: #5 x, y - 1, z. For 4: #5 x, y þ 1, z. For 5: #1 x þ 1, y, z þ 1; #3 x þ 1, y, z; #4 x, y, z þ 1.

network (Table 2 and Figure S2c (Supporting Information)). All helical chains display the same chirality in the same layer, and the adjacent polycatenated network sheets display opposite chirality (Figure 2f). Compound 2 is an achiral structure. Interestingly, the π 3 3 3 π stacking interactions15 show between pyridine rings from adjacent layers, with a plane to plane distance of 3.312 A˚ and 3.264 A˚, respectively, and a centroid-centroid distance of 3.756 A˚ and 3.738 A˚, respectively, which link different polycatenated network sheets to finally form a 3D supramolecular framework. Structure Description of 3a and b. Different from compounds 1 and 2, compounds 3a and b are chiral structures and crystallize in the C2 space group. There is one kind of crystallographically independent Cd(II) ion, one kind of L- anion, and one coordination water molecule (Figure 3a). Each Cd(II) center shows a distorted pentagonal bipyramidal geometry, which is surrounded by four carboxylic oxygen atoms, two nitrogen atoms from four L- anions, and one water molecule (Cd(1)-O(1W) = 2.250(4), Cd(1)-N(1)#1 = 2.3536(16), Cd(1)-O(2) = 2.406(2), and Cd(1)-O(3) = 2.422(2) A˚ for 3a, and Cd(1)-O(1W) = 2.255(5), Cd(1)-N(1)#1 = 2.349(2), Cd(1)-O(2) = 2.406(3), and Cd(1)-O(3) = 2.422(4) A˚ for 3b). Each L- anion adopts a mono and bidentate fashion (Chart S1a and Figure S3a Supporting Information) with a dihedral angles between pyridine ring and phenyl ring of 108.6 for 3a (109.1 for 3b) and links Cd(II) ions to generate a simple sheet with (44,62) topological structure, which contains windows with approximate dimensions of 13.792 A˚  13.792 A˚ (3a) and 13.787 A˚  13.787 A˚ (3b) for each side, and 6.002 A˚  26.920 A˚ (3a) and 6.018 A˚  26.909 A˚ (3b) for each diagonal (Figures 3b, S3b for 3a (Supporting Information), and S3c (Supporting Information) for 3b). Although the distance of each side is similar to those in 1 and 2, and one of the distances of the diagonal is smaller than those in 1 and 2, which prevents the independent 2D sheets interpenetrating each other. Therefore, compound 3a only shows a 2D layer-like structure. In addition, each water molecule donates two hydrogen bonds to the adjacent carboxylic oxygen atoms in the same sheet, which stabilizes the sheet (Figure 3b). The most outstanding structural feature is that the left-handed helical chain with a pitch of 6.002 A˚ (3a) and the right-handed helical chain with a pitch of 6.018 A˚ (3b) exhibit in the layer (Figure 3c). Adjacent sheets are of the same chirality. Compounds 3a and b show chiral (44,62) topological structures (Figure 3d) by spontaneous resolution using an achiral ligand. Spontaneous resolution, known as the segregation of enantiomers upon crystallization, was discovered as early as 1846 by Louis Pasteur,16 and it is still a rare phenomenon that cannot be predicted a priori because the laws of physics determining the processes are not yet fully understood.17 It is an important route to construct chiral MOFs by using achiral ligands, which yields a conglomerate.18 Statistically,

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Figure 3. (a) Coordination environment of the CdII atom in 3a with the ellipsoids drawn at the 30% probability level; hydrogen atoms were omitted for clarity. (b) Ball-and-stick and schematic representation of the (4,4) nets in the structure of 3a. (The yellow dashed lines are interlayer hydrogen bonds.) (c) Ball-and-stick representation of the helical chain in 3a. (d) Schematic representation of the (4,4) nets in the structure of 3a. between 5 and 10% of all racemates form conglomerate crystals,19 indicating that heterochiral interactions are prevalent and more facile than homochiral interactions in the formation of crystalline racemates.20 Therefore, the construction of chiral MOFs is one of the most challenging issues in synthetic chemistry and material science. Structure Description of 4. In compound 4, there is one kind of Cd(II) cation, two kinds of L- anions, and one coordination water molecule (Figure 4a). The pentagonal bipyramidal arrangement {CdN2O5} around the Cd(II) cation is completed by four carboxylate oxygen atoms, two nitrogen atoms from four L- anions, and one water molecule (N(1)-Cd(1) = 2.311(6), N(2)-Cd(1) = 2.328(5), O(1)-Cd(1) = 2.405(5), O(2)-Cd(1) = 2.457(5), O(4)Cd(1) = 2.276(14), O(5)-Cd(1) = 2.649(12) and Cd(1)-O(1W) = 2.427(6) A˚). All L- anions link Cd(II) cations to generate a sheet. Two types of L- anions exist in the sheet: one assumes a mono and bidentate fashion with a dihedral angle between pyridine ring and phenyl ring of 128.8 (L1-), and the other adopts the similar conformation with a corresponding angle of 101.9 (L2-) (Chart S1a, Supporting Information). Within the layer, all L1- anions connect all Cd(II) ions to show a single helical chain with a pitch of 6.972 A˚, and all L2- anions connect all Cd(II) ions to show a double helical chain with a pitch of 13.584 A˚. Two kinds of helical chains with homochirality form a sheet by sharing Cd(II) ions (Figure 4b). If each Cd(II) ion is considered as a four-connected node, the structure of 4 is a (66) topological net (Figure 4c). In addition, each coordination water molecule donates two hydrogen bonds to two carboxylate oxygen atoms in the same layer, which make the layer-like structure more stable (Figure 4d). Similar to compounds 1 and 2, adjacent sheets show opposite chirality; therefore, compound 4 is also an achiral structure. A striking structural feature of 4 is the alternating assembly of the single helix and double helix with homochiralities and different pitches along the crystallographic b axis. Hong’s group has reported a tubular coordination sheet [Zn(spcp)(OH)] (spcp = 4-sulfanylmethyl-40 -phenylcarboxylate pyridine) which shows two distinct homochiral single helices in an orderly arrangement with the zinc atoms functioning as hinges.21 Obviously, the structure of 4 is different from the above-mentioned compounds and shows the first example of the alternating assembly of the single helix and double helix with homochiralities. In addition, the supramolecular isomerism defined by Moulton and Zaworotko22 is the existence of more than one type of network

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Figure 4. (a) Coordination environment of the CdII atom in 4 with the ellipsoids drawn at the 30% probability level; hydrogen atoms were omitted for clarity. (b) Ball-and-stick representation of two kinds of helical chains in 4 and ball-and-stick and schematic representation of the (66) nets in the structure of 4. (c) Schematic representation of the (66) nets in the structure of 4. (d) The 3D supramolecular structure of 4. superstructure for the same molecular building blocks and is therefore related to structural isomerism at the molecular level. However, most reported examples in coordination networks have different solvent molecules in the structures,23 and only a few are true supramolecular isomers with a fixed stoichiometry for all components.24 A comparison of structures 2, 3, and 4 show the same compositions of [Cd(L)2(H2O)], in which L- anions link Cd(II) ions coordinated by one water molecule to generate three different 2D structures. Therefore, compounds 2, 3, and 4 are true supramolecular isomers. To date, three very different supramolecular isomers are scarce. In the reported literature, Fujita and coworkers have reported three different supramolecular isomers depending upon the nature of guests or templates: 1D chains, 2D sheets, and 3D diamondoid networks.25 Chen and Masciocchi et al. have synthesized three new supramolecular isomers of Cu(im)2 as polycrystals which have a 3D sodalite (SOD), a 3D moganite, and 2D four-connected net topologies, respectively.26 Recently, our group reported a series of supramolecular isomers with polythreaded topology based on different octamolybdate isomers.27 Different from them, compounds 2, 3, and 4 are supramolecular isomers with various 2D structures. Structure Description of 5. In omparison to the structures of 1-4, the structure of 5 displays an infinite chain-like structure. There is one kind of Cd(II) ion, two kinds of L- anions, and two coordination water molecules in the independent crystallographical unit (Figure 5a). Each Cd(II) ion shows an octahedral coordination geometry which is completed by two carboxylate oxygen atoms and two nitrogen atoms from four L- anions and two coordination water molecules (Cd(1)-O(1)=2.320(3), Cd(1)-O(2W)=2.327(4), Cd(1)N(2)#1=2.327(4), Cd(1)-N(1)=2.338(4), Cd(1)-O(1W)= 2.347(4), and Cd(1)-O(4)#1 = 2.349(3) A˚). Two kinds of L- anions exhibit the similar bis-monodentate coordination modes with dihedral angles between the pyridine ring and phenyl ring of 80.6 and 73.3, respectively (Figure S5a Supporting Information). Two Lanions coordinate to two Cd(II) ions to form a 24-membered ring, which is linked to generate an infinite chain-like structure by sharing Cd(II) ions (Figure 5b and Figure S5b (Supporting Information)). It is interesting that each water molecule donates one hydrogen bond to one carboxylate oxygen atom in the same chain and the other one to a carboxylate oxygen atom from the adjacent chain. Therefore, the structure of 5 shows a 2D supramolecular structure (Figure 5c). Structure Description of 6. The structure of [Cd2L4(H2O)] 6 has been reported in our previous article.8 A simple description will be given in this section for the convenience of comparison with other

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Scheme 1. Schematic View of Six Compounds in This Work

Figure 5. (a) Coordination environment of the CdII atom in 5 with the ellipsoids drawn at the 30% probability level; hydrogen atoms were omitted for clarity. (b) Schematic view of the 1D chain of 5. (c) Ball and stick representation of the 2D supramolecular structure of 5.

Figure 6. A schematic representation of the 3D network of 6 based on the double chains (left) and a schematic diagram (OLEX) showing the (412,612,84)(46)2 network (green and pink balls represent four- and eight-connected nodes, respectively) (right). compounds (Figure 6). Different from 1-5, there exist two CdII ions, four L- ligands, and one water molecule in the crystallographically independent unit in 6. Four kinds of L- ligands link all Cd(II) cations to form a 3D framework with the flu topology network. Syntheses of the Compounds. Hydrothermal synthesis is a relatively complex process, and the final products under a given set of conditions are often unpredictable. Many coordination polymers with diverse structural architectures have been synthesized using hydrothermal methods28 because this versatile method can cause a reaction to shift from the kinetic to the thermodynamic domain and avoid thermal decomposition of the organic component while providing conditions which overcome the differential solubilities of the organic and inorganic components and which are optimal for crystal growth. The architecture of the final product depends directly on the interplay of starting materials, pH value, template, and reaction temperature. In this case, the different reaction pH values have an influence on the final products.9 When the same raw starting materials are selected, different compounds are isolated at pH ≈ 1 for 1, 3-4 for 2-4, 6 for 5, and 9 for 6. In addition, starting materials (different anions) can induce various structures for 2, 3, and 4, which are obtained by using Cd(OAc)2, CdCl2, and Cd(NO3)2 3 2H2O. Although these anions (OAc-, Cl-, and NO3-) are not present in the ultimate structures, they may influence the formation of 2-4 during crystal growth. All compounds are stable in air and are insoluble in common solvents. Comparisons with the Structures of 1-6: (Scheme 1). According to previous research, the coordination ability of the nitrogen atoms and carboxylate groups from organic ligands is stronger than that of water. In compounds 1-6, each Cd(II) cation is first coordinated by two nitrogen atoms and two carboxylate groups from four L- ligands to form various [CdL2] units. At lower pH

values (pH ≈ 1), the water molecule cannot coordinate to the Cd(II) center, and compound 1 shows a 2D three-folded interpenetrating framework. When one water molecule coordinates to the Cd(II) center, compounds 2-4 show a 2D 2-fold interpenetrating net for 2 or simple 2D structures for 3 and 4. This may be due to the production of a steric hindrance from coordinate water molecules, which may force the higher interpenetrating structure. In the similar structures of 1-3 with the (4,4) net, the dimensions of the large square windows are 14.004 A˚  14.004 A˚ for each side and 17.472 A˚  21.891 A˚ for each diagonal in 1, 13.626 A˚  13.626 A˚ for each side and 13.538 A˚  23.652 A˚ for each diagonal in 2, and 13.792 A˚  13.792 A˚ (3a) and 13.787 A˚  13.787 A˚ (3b) for each side, and 6.002 A˚  26.920 A˚ (3a) and 6.018 A˚  26.909 A˚ (3b) for each diagonal in 3. Therefore, the corresponding areas of each square window are 382.5 A˚2 for 1, 320.2 A˚2 for 2, 160.6 A˚2 for 3a, and 161.9 A˚2 for 3b, respectively. Therefore, the structure of 1 shows a larger window than those in 2 and 3, which can benefit the formation of the higher degrees of the interpenetrating network. In compounds 1-4, the L- ligand shows a mono and bidentate coordinate fashion and acts as a mono and monodentate ligand in 5. When two water molecules coordinate to the Cd(II) center, compound 5 has been obtained, showing a chain-like structure. Different from compounds 1-5, there are dinuclear cadmium clusters, a mononuclear cadmium atom, and four kinds of Lligands (showing three types of coordination fashions) in 6, in which the dinuclear cadmium clusters as eight-connected nodes, and mononuclear cadmium atoms as four-connected nodes are linked by L- ligands to give a (4,8)-connected framework. As mentioned above, many factors, such as the number of coordination water molecules, the coordination fashion of organic ligands, and reaction conditions play fundamental roles in the formation of the final products. These factors work together and have a significant effect on the structures of the coordination polymers. It is a feasible method to introduce ligands to construct coordination polymers with different structural types. Luminescent Properties. Luminescent compounds are of great current interest because of their various applications in chemical sensors, photochemistry, and electroluminescent display.29 The luminescent properties of cadmium carboxylate compounds have been investigated.30 The main emission peaks of HL are at 510 nm and a shoulder peak at 467 nm (λex = 415 nm),8 which may be assigned to the π*/n transition. Compounds 1-5 exhibit the emission maxima at 465 and 518 (λex = 310) nm for 1, 465 and 495 (λex = 310) nm for 2, 467 and 499 (λex = 310) nm for 3, 466 and 498 (λex = 310) nm for 4, and 509 (λex = 310) nm for 5 (Figure 7). The emission maximum for compounds 1-5 maybe assigned to the intraligand fluorescent emission of the HL ligand and/or charge transfer transitions between the coordinated ligands and the metal center. The shift relative to HL may be caused by the deprotonation and the coordination to the metal center, which effectively increases the rigidity of the ligand and reduces the loss of energy by nonradiative decay.31 Thermal Analysis. In order to characterize the compounds more fully in terms of thermal stability, their thermal behaviors were studied by TGA. The experiments were performed on samples consisting of numerous single crystals of 1-5 under N2 atmosphere

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Li et al. Supporting Information Available: Selected bond distances and angles for compounds 1-5; two coordination modes of the Lanion in 1-5; coordination modes of the L- anion in 1, 2, 3a and b, 4, 5; and TGA of compounds 1-5. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 7. Solid-state photoluminescent spectra of 1-5 at room temperature. with a heating rate of 10 C/min, as shown in Figure S6 in the Supporting Information. For compound 1, the weight losses in the range of 158-456 C correspond to the removal of the corresponding organic components, and the remaining weight corresponds to the formation of CdO (obsd, 23.3%; calcd, 22.6%). For compounds 2, 3, and 4, the weight loss in the range of 17-84 C for 2, 21-112 C for 3, and 27-133 C for 4 corresponds to the departure of one water molecule (obsd, 2.9%; calcd, 3.1% for 2; obsd, 3.0%; calcd, 3.1% for 3; and obsd, 3.2%; calcd, 3.1% for 4), and the weight losses in the range of 126-409 C for 2, 151-435 C for 3, and 175-477 C for 4 correspond to the removal of the corresponding organic components, and the remaining weight corresponds to the formation of CdO (obsd, 21.6%; calcd, 21.9%) for 2, (obsd, 22.1%; calcd 21.9%) 3, and (obsd, 22.5%; calcd, 21.9%) 4. The TGA curve of 5 shows that it loses two water molecules from room temperature to 130 C (obsd, 6.3%; calcd, 6.0%), and then the anhydrous compound begins to decompose, leading to the formation of CdO as the residue (obsd, 22.0%; calcd, 21.2%).

Conclusions In summary, six binary metal-organic compounds with diverse structural types have been synthesized under hydrothermal conditions. The number of coordination water molecules, the coordination fashion of organic ligands, and reaction conditions play fundamental roles in the formation of the final products. Compounds 2, 3, and 4 are supramolecular isomers. Compounds 3a and b are enantiomers. The successful isolation of these species not only produces intriguing examples of enantiomerically pure architectures and supramolecular isomer architectures but also may provide a rational strategy for the synthesis of these compounds by selecting ligands. Acknowledgment. We are thankful for financial support from the Program for Changjiang Scholars and Innovative Research Team in University, the National Natural Science Foundation of China (No. 20573016, 20901014and 20703008), the Science Foundation for Young of Jilin Scientific Development Project (No. 20090125 and 20090129), the Science Foundation for Young Teachers of NENU (No. 20090407), the Training Fund of NENU’s Scientific Innovation Project (NENU-STC08019), the Ph.D. Station Foundation of Ministry of Education for New Teachers (No. 20090043120004), the Postdoctoral Foundation of Northeast Normal University, and the Postdoctoral Foundation of China (No. 20090461029).

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