Divalent Zinc, Cobalt, and Cadmium Coordination Polymers of a New

Article pubs.acs.org/crystal

Divalent Zinc, Cobalt, and Cadmium Coordination Polymers of a New Flexible Trifunctional Ligand: Syntheses, Crystal Structures, and Properties Shuang-Quan Zang,*,† Li-Hui Cao,† Ran Liang,† Hong-Wei Hou,† and Thomas C. W. Mak†,‡ †

Department of Chemistry, Zhengzhou University, Zhengzhou 450001, P. R. China Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, P. R. China

‡

S Supporting Information *

ABSTRACT: Three metal(II) coordination polymers [ZnL(H2O)]n (1), [Co2L2(H2O)]n (2), [Cd2L2(H2O)]n (3) (H2L = 1-(pyridin-4-ylthio)benzene-2,4-dioic acid) have been synthesized under hydro(solvo)thermal conditions and characterized by single-crystal X-ray diffraction, IR spectral, and thermogravimetric analyses. Though all three complexes have the same Schläfli symbol (4·62)2(42·610·83), under the inducement effect of the metal ions, 1 and 2 exhibit a distorted rtl topology, whereas 3 exhibits a sit net. For classification of the interpenetration network, complexes 1−3 belong to Class Ia, Class IIa, and Class IIa of 2-fold interpenetration, respectively. Furthermore, the solid luminescent properties of 1 and 3 at room temperature, and variable temperature magnetic susceptibilities of complex 2 were also studied.

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INTRODUCTION Recently, the design and synthesis of metal−organic frameworks (MOFs) have become the main challenges in the development of multifunctional materials.1 In particular, metal−organic frameworks which have novel topologies and special applications attract the most attention,2 and much research work has focused on the optimum design of ligands and rational choice of metals.3 Thus far, most networks reported to possess desirable properties have been constructed by a building-block methodology, which was based on transition metals of high coordination numbers and organic ligands with multidentate carboxylate functionalities.1b,4 Multitudinous interpenetrated nets with nodes of mixed connectivities subsist in nature. In this category, one of the dominant structures is the (3,6)-connected network.5 The (3,6)-connected topologies have been observed in rutile [Schläfli symbol (4·62)2(42·610·83)], pyrite (63)2(612·83), and anatase (42·6)2(44·62·88·10). The rutile (rtl)-like net has an uncommon sit net variation with the same Schläfli symbol. To date, there is no example to explore two such similar topologies in one system. In contrast to plentiful research on bridging carboxylic acid or pyridyl ligands, the investigation of polycarboxylic acids with additional pyridyl functional binding sites has remained largely unexplored in the assembly of supramolecular coordination polymers. Flexible ligands have been used widely to construct MOFs with fascinating topologies and useful properties because such ligands can adopt a variety of conformations according to the restrictions imposed by the coordination requirement of the metal. In this paper, we report the synthesis of a new flexible © 2012 American Chemical Society

trifunctional ligand, namely, 1-(pyridin-4-ylthio)benzene-2,4dioic acid (H2L, Scheme 1), and the construction of new Scheme 1. Structural Formula of 1-(Pyridin-4ylthio)benzene-2,4-dioic Acid (H2L)

coordination polymers that may exhibit interesting topologies and useful physical properties. As a flexible trifunctional ligand, the bis(bidentate) carboxylate groups of H2L can easily chelate two metal ions and lock their positions into a dinuclear M−O− C−O−M(M′) secondary building unit (SBU).6 At the same time, the pyridyl group can further coordinate to another metal ion to yield a higher-dimensional coordination polymer,7 and the flexibility around the etheric sulfur atom can adjust its modes to satisfy the coordination environment of metal ions. To explore various factors that influence the construction of coordination networks based on H2L and different metal(II) ions, we undertook synthetic and structural studies on three complexes of structural formulas [ZnL(H 2 O)] n (1), [Co2L2(H2O)]n (2), and [Cd2L2(H2O)]n (3). Received: October 14, 2011 Revised: February 12, 2012 Published: February 21, 2012 1830

dx.doi.org/10.1021/cg2013733 | Cryst. Growth Des. 2012, 12, 1830−1837

Crystal Growth & Design

Article

Table 1. Crystal Data and Structure Refinement Details for for 1−3

a

compound

1

2

3

formula Fw crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Dc (g·cm−3) F(000) reflns collected independent reflns R(int) GOF on F2 R1a (I > 2σ(I)) wR2b (I > 2σ(I)) max/min (e Å−3)

C13H9NO5SZn 326.64 monoclinic P21/c 10.9338(10) 12.3885(12) 9.6317(9) 90 94.873(2) 90 1299.9(2) 4 1.822 720 6947 2290 0.0690 1.035 0.035 0.0863 0.759/−0.375

C26H16N2O9S2Co2 682.39 monoclinic P21/c 16.5392(10) 15.5258(9) 10.0965(3) 90 91.596(4) 90 2591.6(2) 4 1.749 1376 11920 4484 0.0300 1.073 0.0996 0.2833 1.058/−1.499

C26H16N2O9S2Cd2 789.33 monoclinic C/2c 19.436(6) 14.183(3) 10.148(2) 90 110.10(3) 90 2627.0(11) 4 1.996 1544 13036 2310 0.0491 1.030 0.0425 0.1158 0.611/−0.432

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2.

Table 2. Selected Bond Lengths (Å) for 1−3 Compound 1a Zn(1)−O(2)#1 Zn(1)−O(4)#3

1.963(2) 2.054(2)

Zn(1)−O(1) Zn(1)−O(1W) Compound 2b

1.972(2) 2.188(2)

Zn(1)−N(1)#2

2.044(2)

Co(1)−O(2) Co(1)−O(3)#3 Co(2)−O(1) Co(2)−O(1W)

2.010(6) 2.139(6) 1.889(18) 2.012(11)

Co(1)−O(5)#1 Co(1)−N(2)#1 Co(2)−O(8)#4

2.103(7) 2.147(7) 1.985(8)

Co(1)−N(1)#2 Co(1)−O(4)#3 Co(2)−O(6)#1

2.137(8) 2.189(6) 1.991(8)

2.253(4) 2.416(3)

Cd(1)−N(1)#3 Cd(1)−O(1)

2.307(5) 2.475(4)

Compound 3c Cd(1)−O(4)#1 Cd(1)−O(2)

2.206(4) 2.323(4)

Cd(1)−O(3)#2 Cd(1)−O(1W)

a Symmetry code: a #1, − x, − y, − z; #2, x − 1, − y + 1/2, z − 1/2; #3, − x, − y, − z + 1. bSymmetry code: b #1, − x, y − 1/2, − z − 1/2 ; #2, − x + 1, y − 1/2, − z + 1/2; #3, x, y, z−1; #4, −x, y −1/2, − z + 1/2. cSymmetry code: c #1, x, y, z − 1; #2, −x + 1, y, − z + 1/2; #3, −x + 3/2, y + 1/2, −z + 1/2.

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dimethyl 1-(pyridin-4-ylthio)benzene-2,4-dioate was obtained after removal of the solvent. Yield 0.85 g (56%). A mixture of dimethyl 1-(pyridin-4-ylthio)benzene-2,4-dioate (1.52 g, 5 mmol) and NaOH (0.40 g, 10 mmol) in distilled water (30 mL) was refluxed until the solution turned clear. After the pH value of the filtrate was adjusted to about 3−4 with HCl (3.0 mol/L), the filtrate was kept undisturbed overnight at room temperature. The yellowish solid of H2L was collected by filtration with a yield of 1.24 g (90%). IR/cm−1 (KBr): 3494 (s), 3066 (s), 1720 (s), 1698 (s), 1425 (m), 1210 (s). 1H NMR (300 MHz, DMSO): δ: 7.53 (d 2H), 7.86 (d 2H), 8.335 (d, 1H), 8.496 (d 1H), 8.59 (d 1H). Syntheses of Complexes 1−3. [ZnL(H2O)]n (1). A mixture of H2L (0.0138 g, 0.05 mmol), Zn(NO3)2·6H2O (0.0149 g, 0.05 mmol), and NaOH (0.0040 g, 0.1 mmol) in 6 mL of mixed H2O−MeCN (1:1 v/v) was placed in a Teflon-lined stainless steel vessel, heated to 120 °C for 3 days, and then cooled to room temperature. Colorless block-like crystals of 1 were obtained in 80% yield based on zinc. Anal. Calcd for C13H9NO5SZn (356.64): C, 43.74; H, 2.53; N, 3.93. Found: C, 43.70 ; H, 2.57; N, 3.98. IR/cm−1 (KBr): 3423(s), 1604(s), 1424(s), 1375(s), 1356(s).

EXPERIMENTAL SECTION

Materials and Physical Measurements. All reagent and solvents employed in the present work were of analytical grade as obtained from commercial sources without further purification. Elemental analysis for C, H, and N were performed on a Perkin-Elmer 240 elemental analyzer. The FT-IR spectra were recorded from KBr pellets in the range from 4000 to 400 cm−1 on a Bruker VECTOR 22 spectrometer. Thermal analysis was performed on a SDT 2960 thermal analyzer from room temperature to 800 °C with a heating rate of 20 °C/min under nitrogen flow. Luminescence spectra for the solid samples were recorded on a Hitachi 850 fluorescence spectrophotometer. Synthetic Procedures. Syntheses of H2L. The ligand 1(pyridin-4-ylthio)benzene-2,4-dioic acid (H2L) was synthesized according to the following procedure. To a solution of pyridine4-thiol (0.69 g, 6.25 mmol) and NaOH (0.25 g, 6.25 mmol) in anhydrous ethanol (25 mL) stirred for 2 h, dimethyl 1bromobenzene-2,4-dioate (1.36 g, 5 mmol) was added. The mixture was heated to reflux for 48 h and then filtered after cooling to room temperature. Next, the filtrate was evaporated under reduced pressure, and the residue was chromatographed on silica gel with hexane-ethyl acetate (1/2, v/v). The product 1831



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Figure 1. (a) Metal coordination and atom labeling in complex 1 (thermal ellipsoids at 50% probability level) and the hydrogen atoms are omitted for clarity. (b) Schematic view of the helical chains. (c) Three dimensional metal−organic framework for 1 (the different helices channels denoted by different colors). (d) Schematic presentations of a single 3D net of 1: Six-connecting nodes are represented by blue balls, and three-connecting nodes are represented by red balls. (e) Schematic view of the 2-fold interpenetration structure, the translationally related two “colored” rtl nets in 1 with the arrows showing the full interpenetration vector applied one time.

different L2− ligands, a nitrogen atom (Zn1−N1 = 2.044(3)) from another L2− ligands and an aqua ligand to furnish a distorted square-pyramidal geometry with an Addison trigonality factor τ = 0.46911 (Figure 1a). The coordination mode of the ligand in this complex is illustrated in Scheme 2a. Two

[Co2L2(H2O)]n (2). The procedure for the synthesis of 1 was repeated except that Co(NO3)2·6H2O (0.0146 g, 0.05 mmol) was used instead of Zn(NO3)2·6H2O. Yield: 86% based on cobalt. Anal. Calcd for C26H16N2O9S2Co2 (682.39): C, 45.76; H, 2.36; N, 4.11. Found: C, 45.81; H, 2.33; N, 4.14. IR/cm−1 (KBr): 2942(v), 1599(s), 1554(s), 1420(s), 1359(s), 813(m), 779(m), 739(m). [Cd2L2(H2O)]n (3). The above procedure was repeated using Cd(NO3)2·4H2O (0.0154 g, 0.05 mmol) as the metal source. Yield: 75% based on cadmium. Anal. Calcd for C26H16N2O9S2Cd2 (789.33): C, 39.56; H, 2.04; N, 3.55. Found: C, 39.58; H, 2.09; N, 3.60. IR/cm−1 (KBr): 2924(w), 1599(s), 1554(s), 1420(s), 1359(s), 815(m), 778(m), 720(m). X-ray Crystallography. Single-crystal X-ray diffraction data for complexes 1−3 were collected on a Bruker SMART APEX CCD diffractometer8 using graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) at room temperature using the ω-scan technique. Lorentz polarization and absorption corrections were applied. All the structures were solved by direct methods with SHELXS-979 and refined with the full-matrix least-squares technique using the SHELXL9710 program. All non-hydrogen atoms were refined anisotropically. In complex 2, the O1 atom of the ligand which was disordered was assigned 0.6 and 0.4 occupancies. The hydrogen atoms of the coordination water molecules and ligands were included in the structure factor calculation at idealized positions by using a riding model and refined isotropically. Analytical expressions of neutral atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data and selected bond lengths for 1−3 are listed in Tables 1 and 2.

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Scheme 2. Coordination Modes of the L2− Anions in 1 (a), 2 (a, b), and 3 (b), respectively

symmetrically related Zn atoms are bridged by two carboxylate groups to form a dinuclear unit with a Zn···Zn separation of 3.295 Å. Such units are further linked to eight equivalent neighbors through six L2− ligands to give a three-dimensional (3D) metal−organic network. In addition, the aqua ligand is involved in hydrogen bonding (O1W−H1WA···O3#5 2.865(3) Å and O1W−H1WA···O3#6 2.850(3) Å; symmetry codes: #5, x, y, z − 1; #6, x, −y + 1/2, z − 1/2) with O3 atoms from

RESULTS AND DISCUSSION

Structural Description of [ZnL(H2O)]n (1). Single crystal X-ray analysis of complex 1 reveals that each Zn(II) atom is coordinated by three oxygen atoms (Zn1−O1 = 1.972(2), Zn1−O2 = 1.963(3), and Zn1−O4 = 2.054(3) Å) from three 1832



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Figure 2. (a) Metal coordination and atom labeling in complex 2 (thermal ellipsoids at 50% probability level) and the hydrogen atoms are omitted for clarity. (b) Two different helical chains in the complex 2. (c) and (d) two substructures with opposite chirality. (e) Schematic view of the singlenet of the overall framework. (f) Schematic view of the 2-fold interpenetration structure.

cross-linking complexes chains is highlighted, in which the three-connecting centers lie in the sides of the channels and the six-connecting centers occupy the corners, and the adjacent chains are mutually inclined at angle of ca. 56.7°, which is different in that they are perpendicular in the perfect rutile net (TiO2) (Figure S2, Supporting Information). The overall structure of the complex 1 belongs to a Class Ia 2-fold interpenetration, and the translational degree of interpenetration (Zt) of the structure is 2 based on the novel method for the investigation of interpenetrated architectures.5a,12 According to the translation equivalence, the 2-fold parallel interpenetration network structure can be categorized into two topologically equivalent 3D subsets. Each subset is related by the translation vector [1, 0, 0] (10.93 Å). This means that the overall array is generated by application of one translational operation, and viewed along the vector [1, 0, 0] direction showing the eclipsing of the two nets (Figure S3, Supporting Information). Structural Description of [Co2L2(H2O)]n (2). Complex 2 has a monoclinic space group P2 1 /c and exhibits a homogeneously 2-fold interpenetrated (3,6)-connected structure. The asymmetric unit of the complex 2 contains two crystallographically independent cobalt ions, two L2− (L and L′) anions and a coordinated water molecule. As shown in Figure 2a, the Co1 ion is six coordinated by a 2-carboxylate from one L2− (L) ligand with chelating mode (Co1−O3 = 2.139(6) Å and Co1−O4 = 2.189(6) Å), one oxygen atom from 4-

another substructure to stabilize this interpenetration framework. Viewed along the b axis, the dinuclear Zn units are first linked by nitrogen atoms and the two oxygen atoms of the 4carboxyl group to form (R) right- and (L) left-handed helical channels with a pitch distance of 12.389 Å (Figure 1b), the helical axis corresponds to the b axis. These helical chains with opposite chirality are united alternately together through dinuclear Zn units to exhibit a 2D metal−organic sheet (Figure S1, Supporting Information). Such layers are further linked by the 2-carboxyl of L2− ligands to give a 3D metal−organic framework (Figure 1c). The dinuclear zinc unit and L2− can be considered as sixconnected and three-connected nodes, respectively. According to the simplification principle, the structure of 1 is binodal with six-connected (Zn units) and three-connected (ligand) nodes and exhibits a fascinating 3D 2-fold parallel interpenetration structure consisting of two equivalent interwoven nets of rutile topology (Figure 1d,e). As a single net of the 2-fold interpenetration structure, Figure 1d exhibits the rutile (rtl) net with (4.62)2(42.610.83) topology, in which the six-connected dinuclear zinc clusters with the Schläfli symbol (42.610.83) and three-connected ligands with the Schläfli symbol (4.62) correspond to the Ti and O atoms in the rutile net, respectively (Figure 1e and Figure S3, Supporting Information). The single net of the complex 1 is a distortion version of the ideal rtl net, as shown in Figure 1d, a rhombus channel constructed by 1833



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Figure 3. (a) Metal coordination and atom labeling in complex 3 (thermal ellipsoids at 50% probability level) and the hydrogen atoms are omitted for clarity. (b) Two different helical chains in the complex 3. (c) and (d) Two substructures in 3. (e) Schematic view of the single-net of the overall framework. (f) Schematic view of the 2-fold interpenetration structure, which is highlighted used the two different colors.

Noteworthy of mention here is that the coordinated water molecules also play an important role in stabilizing the 2-fold interpenetration net through the formation of O1W− H1WA···O7#4 and O1W−H1WB···O7#9 (symmetry codes: #4, −x, y − 1/2, −z + 1/2; #9, − x, −y + 1, −z; D···A: 2.708(13) and 2.875(13) Å) hydrogen bonds between two adjacent substructures. As shown in Figure 2b, along the b axis, the dinuclear Co units are connected by the 4-COO− group (or 2-COO− group) and nitrogen atoms of the different type ligands (L and L′) to form two similar rhombic helical channels (L, L′ or R, R′). The winding axis corresponds to the b axis and the pitch to the length of 15.526 Å. These different helical chains with the same rotation direction connected each other to form chiral helical layers with right-handed (R helix) or left-handed screws (L helix) (Figures S4 and S5). The Co atoms which locate the corners of the helical channels in the helical layers connected the rest carboxyl of the ligands to give a single chiral 3D substructure (Figure 2c,d). Because the two substructures exhibit just opposite chirality, complex 2 is a mesomeric network in general. Similar to complex 1, the dinuclear cobalt units can be considered as six-connected nodes and the L2− ligands can be simplified as three-connected nodes. As discussed above, the multidimensional structures can be simplified as a (3,6)connected 2-fold interpenetration net which consist of two

carboxylate of the other symmetry-related ligand with monodentate mode (Co1−O2 = 2.010(6) Å), two nitrogen atoms (Co1−N1 = 2.137(8) Å and Co1−N2 = 2.147(7) Å) from two symmetry-unrelated ligands, and one oxygen atom from the 4-carboxylate of another ligand (L′) with monodentate mode (Co1−O5 = 2.103(7) Å). The Co2 ion is four coordinated by three oxygen atoms from three different ligands with monodentate mode (Co2−O1 = 1.889(18) Å, Co2−O6 = 1.991(8) Å, and Co2−O8 = 1.985(8) Å) and one oxygen atom from the coordinated water molecule. The Co1 atom exhibits a distorted octahedron geometry; however, the Co2 atom adopts a distorted tetrahedron geometry. Two coordination modes of the ligand in this complex are illustrated in Scheme 2. The crystallographically different ligands adopt two different coordination modes. The 4-COO− group of the first ligand (L) is bidentate and the 2-COO− group is chelate. In contrast, the 4-COO− group of the other ligand (L′) is bidentate and the O8 atom of 2-COO− group adopts monodentate fashion and the O7 is free. As to the two crystallographically independent L2− anions, the dihedral angle between the phenyl ring and pyridine ring of each ligand is 59.9° and 68.3°, respectively. One pair of symmetry-unrelated carboxylate bridges two Co atoms to give dinuclear units with the Co···Co distance of 3.728(2) Å. Each dinuclear Co unit connects six L2− anions, and each L2− anion links three dinuclear Co units to form a (3, 6)-connected 3D 2-fold equivalent interpenetration structure. 1834



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structure are just opposite, so complex 3 does not show chirality. As the same as in 1 and 2, the dinuclear cadmium atoms can be considered as six-connected nodes and the L2− ligands can be simplified as three-connected nodes in 3. The single subnet of the overall structure exhibits the (3,6)-connected sit net. The Schläfli symbol for 3 is (4.62)2(42.610.83) (Figure 3e). Finally, the 3D framework can be simplified as a (3,6)-connected 2-fold interpenetration net that consists of two equivalent interwoven nets with sit topology (Figure 3f). The whole structure of complex 3 exhibits a Class IIa 2-fold interpenetration related by inversion operation. Structure Differences and Evolution between Complexes 1−3. With further structure analysis, we noticed the change of the topologies and classes of interpenetration nets from the complex 1 to 3. Although the three complexes have the same Schläfli symbol, the topology of the complexes 1−3 exhibit a little distortional rtl net, a serious distortional rtl net, and a sit net, respectively. As shown in Figure 1e, two sixconnected metal units share two three-connected nodes to form a parallelogram, in which the four nodes locate the same plane. However, such nodes of the complexes 2 and 3 (Figures 2e and 3e) locate two different planes, and the dihedral angles between the two planes are 31.1° in 2 and 10.5° in 3, respectively. For the interpenetration structures, there is also interesting change from complex 1 to 3. As discussed above, the complex 1 belongs to Class Ia and the full interpenetration vectors (FIV) of the complex 1 is [1, 0, 0] (10.93 Å). It implies that we can obtain the overall entanglement through translating either single interpenetration net 10.93 Å along the vector [1, 0, 0] one time. Opposite to that of 1, the framework of 2 also exhibits a (3,6)-connected rtl interpenetration network; however, the overall structure belongs to Class IIa related by inversion operation, which means that the overall structure can be generated by a inversion operation. Like complex 2, the framework of 3 exhibits a (3,6)-connected net and has the same Schläfli symbol with complex 2, but the topology of complex 3 belongs to sit net. And the overall structure of 3 belongs to Class IIa related by inversion operation. Thermogravimetric analysis. Thermogravimetric (TG) analysis for complexes 1−3 was studied to reveal their thermal stability. TG curves for complexes 1−3 are shown in Figure S9, Supporting Information. For complex 1, a gradual weight loss from room temperature to 160 °C is attributed to the release of coordinated water molecules (observed: 5.01%; calculated: 5.05%), and the decomposition temperature of the residual composition spans the range of 300−520 °C. For complex 2, weight loss corresponding to the release of the coordinated water molecules occurred from room temperature to 155 °C (observed, 2.51%; calculated, 2.43%), and then the host framework started to decompose after 300 °C. For complex 3, 2.33% weight loss shows that the coordinated water molecules (calculated, 2.28%) were lost from 80 to 160 °C, and from 320 °C the host framework started to decompose. Photoluminescent Properties. As complexes 1 and 3 are hybrid inorganic−organic coordination polymers with d10 metal centers,13 their emission spectra in the solid state are investigated at room temperature. As shown in Figure 4, complex 1 displays two emission maxima at ca. 462 and 491 nm, and complex 3 shows two emission maxima at ca. 397 and 514 nm. The ligand displays an intense luminescent emission maximum at ca. 460 nm in the solid state at ambient

equivalent interwoven nets with rutile (rtl) topology (Figure 2e). The single subnet of the overall structure discloses the (3,6)-connected rtl net [Schläfli symbol (4·62)2(42.610.83)] (the first symbol for L2− ligand, the second one for the dinuclear Co units) (Figure 2f). The subnet of the overall structure is a serious distortion version of the ideal rtl net. An analysis of the topology of the interpenetration nets to a classification indicates that the overall structure of complex 2 belongs to infrequent Class IIa related by inversion operation. To the best of our knowledge, the (3,6)-connected rtl interpenetration net mostly belongs to the Ia classification.5a Structural Description of [Cd2L2(H2O)]n (3). X-ray single crystal diffraction reveals that complex 3 crystallizes in the monoclinic system, space group C2/c. The asymmetric unit in complex 3 contains a cadmium ion, L2− anion, and a half of a coordinated water molecule. The single crystal X-ray analysis of complex 3 shows that the local coordination geometry for sixcoordination CdII center is close to an octahedral geometry. Each CdII atom binds six atoms, two oxygen atoms from a chelating carboxylate group of a distinct L2− ligand (Cd1−O1 = 2.473(5) and Cd1−O2 = 2.322(4) Å), two oxygen atoms from monodentate carboxylate groups (Cd1−O3 = 2.252(5) and Cd1−O4 = 2.205(4)Å) of two different ligands, a nitrogen atom (Cd1−N1 = 2.310(5) Å) from the other L2− ligand, and an oxygen atom from the coordinated water molecule which bridges two Cd(II) atoms (Figure 3a). The whole L2− anion bond to four CdII cations with two carboxylate and one pyridine ring in the μ4-η2:η1:η1 coordination mode (Scheme 2b). Different from complexes 1 and 2, the two oxygen atoms of the 4-carboxyl of ligands chelated one Cd center in complex 3, whereas they bridged two Zn (Co) centers in complexes 1 and 2. The dihedral angle between the aromatic rings and pyridine ring is about 73.1°, and the two carboxylate groups make dihedral angles of 19.9 and 26.7° with the corresponding linked benzene rings, respectively. The two crystallographically identical Cd atoms are bridged by 2-carboxylate groups and the coordinated water molecule to form a dinuclear units with a Cd···Cd distance of 3.596 Å that is further linked to eight equivalent neighbors through six L2− ligands into a 3D 2-fold interpenetration network. This interpenetration structure was further stabilized through the formation of hydrogen bonds (O1W−H1WA···O(2)#7, symmetry codes: #7, x, −y, z − 1/2. 2.681(5) Å) between the coordinated water molecules and O2 atoms from adjacent subnet and the face-to-face π···π interactions (intercentroid distances: 3.747 Å) between two phenyl rings from adjacent substructures (Figure S6, Supporting Information). Viewed along the a axis, the dinuclear Cd units are linked by 2-carboxyl and the nitrogen atoms of the ligands to form two different type helical channels with triangular section because of the dihedral angle between the phenyl ring and the pyridine ring achieved 73.1° (Figure 3b). The helical axis corresponds to the a axis and the pitch to the length of 19.436 Å. The Cd centers of the helical channels are further cross-linked by the 4carboxyl of the ligands along the c direction into a chiral double-layer helical sheet with right- or left-hand screw (Figures S7 and S8). The double-layer helical sheets which have the same helix direction are connected by the 2-carboxyl and the nitrogen atoms along the b direction to form a 3D substructure (Figure 3c,d). The single subnet of the whole 2fold interpenetration structure exhibits a chiral structure with right-handed screw (R helix) or left-handed (L helix), but the rotation directions of two single structures of the whole 1835



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coupling, which results in a splitting between discrete levels. It is in excellent agreement with the experimental data obtained in this work (Figure 5). The best fit gave the parameters A = 1.60

Figure 4. Fluorescent emission spectra of complexes 1 and 3. Figure 5. Temperature dependence of XMT and XM for 2. Open points are the experimental data.

temperature. The 462 nm band, one of the dual emission bands of complex 1 in a range similar to that of the ligand, can be assigned to the π−π* intraligand fluorescence of the ligands,14 whereas the emissions observed at 491 nm for 1 might be assigned to a combination of the charge transfer (CT) process and intraligand emission states.15 In comparison with the free ligand, the emission band of 514 nm is red-shifted of about 54 nm, that may be tentatively assigned to a ligand-to-metal charge transfer (LMCT) process, and the emission spectrum with a strong peak (397 nm) is obviously different from that of the ligand, which may be due to its unique 3D 2-fold interpenetrating framework with π−π stacking interactions between the phenyl rings of the different substructure.16 Compounds 1 and 3 may be suitable as excellent candidates for the exploration of blue-fluorescent materials, since they are highly thermally stable and insoluble in common solvents. Magnetic Properties. For complex 2, similar to the magnetic behavior of other reported Co(II) complexes, the χmT value of 2 at 300 K is 4.98 cm3 K mol−1, which is larger than the expected value of 3.64 cm3 K mol−1 for two noninteracting S = 3/2 Co(II) centers with a g value of 2. This is a common phenomenon for Co(II) ions because of its strong spin−orbit coupling interaction.17 Upon cooling of the sample, the χmT value smoothly decreases to 4.26 cm3 K mol−1 around 50 K, which is a typical manner of spin−orbit coupling and is mainly due to the single-ion behavior of Co(II). And along cooling of the sample to 1.8 K, the χmT value sharply decreases to 1.68 cm3 K mol−1, which is close to the expected one for a magnetically isolated Co(II) ion. Between 50 K and 300 K, the χm−1 versus T data can be well fit by the Curie−Weiss law with C = 5.08 cm3 K mol−1 and θ = −7.11 K. The negative θ value implies the occurrence of a very weak antiferromagnetic interaction between the cobalt(II) ions. From 1.8 K to 300 K, the temperature dependence of the χmT value was roughly fitted with the simple the phenomenological equation18 which is described by eq 1: χmT = Ae−E1/ kT + Be−E2 / kT

cm3 K mol−1, B = 3.55 cm3 K mol−1, E1 = 30.67 cm−1, E2 = 1.49 cm−1, and the Curie constant C = 5.15 cm3 K mol−1, which agrees with that obtained from the Curie−Weiss law in the high temperature range. The obtained values of A + B = 5.15 cm3 K mol−1 and E1/k (E1/k = 43.37 K, using a least-squares fitting method) are consistent with those given in the literature for the effect of spin−orbital coupling and site distortion (E1/k on the order of 100 K). The small E2 value indicates the presence of weak antiferromagnetic interaction.19,20

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CONCLUSION

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ASSOCIATED CONTENT

In this paper, we present the result of our investigation on a series of three complexes 1−3 exhibiting similar topologies and 2-fold interpenetration structures constructed from deprotonated forms of a new flexible trifunctional ligand H2L with both aromatic carboxylic acid and pyridine groups and divalent transition metals (Zn, Co, and Cd). Although having the same (3,6)-connected topology symbol [(4.62)2(42.610.83)], three complexes may lead to delicate geometric diversification of the resulting coordination polymers under the inducement effect of the metal ions. Complex 2 can be regarded as a topological transition from complex 1 to complex 3. Furthermore, the present study has demonstrated that H2L, and its positional isomers too, can serve as versatile flexible ligands in the construction of new MOFs in combination with appropriately chosen metal centers.

S Supporting Information *

X-ray crystallographic files (CIF), additional crystal packing diagrams, thermogravimetric analysis and PXRD. This material is available free of charge via the Internet at http://pubs.acs.org.

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(1)

AUTHOR INFORMATION

Corresponding Author

where A + B equals the Curie constant, and E1 and E2 represent the activation energies corresponding to the spin−orbital coupling and the magnetic exchange interactions, respectively.19 This equation adequately describes the spin−orbit

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1836



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Article

1617−1624. (d) Sun, H.-L.; Wang, Z.-M.; Gao, S. Inorg. Chem. 2005, 44, 2169−2176. (18) (a) Magnetism: Molecules to Materials; Miller, J. S., Drillon, M., Eds.; Wiley-VCH: Weinheim, Germany, 2005; Vol. 5, Chapter 10, p 347. (b) Drillon, M.; Panissod, P.; Rabu, P.; Souletie, J.; Ksenofontov, V.; G€utlich, P. Phys. Rev. B: Condens. Matter. Mater. Phys. 2002, 65, 104404/1−104404/8. (19) (a) Rabu, P.; Rueff, J. M.; Huang, Z.-L.; Angelov, S.; Souletie, J.; Drillon, M. Polyhedron 2001, 20, 1677−1685. (b) Rueff, J. M.; Masciocchi, N.; Rabu, P.; Sironi, A.; Skoulios, A. Eur. J. Inorg. Chem. 2001, 2843−2848. (20) (a) Liang, L.-L.; Ren, S.-B.; Wang, J.; Zhang, J.; Li, Y.-Z.; Du, H.B.; You, X.-Z. CrystEngComm 2010, 12, 2669−2671. (b) Chen, Q.; Lin, J.-B.; Xue, W.; Zeng, M.-H.; Chen, X.-M. Inorg. Chem. 2011, 50, 2321−2328.

ACKNOWLEDGMENTS We gratefully acknowledge financial support by the National Natural Science Foundation of China (No. 20901070) and Zhengzhou University (P. R. China).

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REFERENCES

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