Structural diversity, photoluminescence and magnet properties of complexes based on a V-shaped polycarboxylate

Inorganica Chimica Acta 366 (2011) 53–61

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Structural diversity, photoluminescence and magnet properties of complexes based on a V-shaped polycarboxylate Rui-Ting Liu, Lei Hou, Bin Liu, Ya-Nan Zhang, Yao-Yu Wang ⇑, Qi-Zhen Shi Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, PR China

a r t i c l e

i n f o

Article history: Received 30 July 2010 Received in revised form 5 October 2010 Accepted 8 October 2010 Available online 14 October 2010 Keywords: Complex Clusters Magnetic Luminescent

a b s t r a c t The self-assembly of a V-shaped ligand 3,30 ,4,40 -diphenylsulfonetetracarboxylate (dstc) and metal salts in the presence of a series of N-donor ligands yielded four new complexes, namely, [Cu4(H2dstc)4(phen)4] 12H2O (1), {[Cu2(dstc)(bpe)(H2O)2]4H2O}n (2), [Cu3(dstc)(bipy)(l2-OH)2(H2O)2]n (3), {[Cd5(dstc)2(bipy)2(l3-OH)2(H2O)4]4H2O}n (4) (phen = 1,10-phenanthroline; bpe = 1,2-bis(4-pyridyl)ethene; bipy = 4, 40 -bipyridine). All the complexes were structurally determined by single-crystal X-ray diffraction and characterized by elemental analyses, IR spectra, X-ray powder diffraction and TG analyses. Complex 1 is a discrete tetranuclear unit, which further assembles into a 3D supramolecular framework by intermolecular hydrogen bonding interactions. Complex 2 is composed of 2D 44 grid-like layers based on dinuclear copper units. Complex 3 features a rare 3D (6,8)-connected topological net consisting of trimetallic clusters. 12-connected pentanuclear cadmium clusters are observed in complex 4 and the resulting structure shows an uncommon (4,12)-connected topology. The structural differences among 1–4 demonstrate that the nature of the N-donor assistant ligands and metal ions can play critical roles in the formation and structures of the resulting complexes. Magnetic studies showed antiferromagnetic interactions for 1 and 3. In addition, the luminescent property of 4 was also studied. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Synthesis and characterization of inorganic–organic composite coordination polymers are of great interest because of their intriguing structures and potential applications for gas storage, chemical separations, microelectronics, nonlinear optics, and heterogeneous catalysis [1]. The formation of such extended coordination framework greatly depends on the sophisticated selection and utilization of multitopic building blocks as well as tectonic interactions. Multidentate O-donor ligands, especially carboxylates, are widely employed as attractive building blocks in coordination networks because of their various coordination modes [2,3]. Hitherto, more attention has been paid to the possibility of synthesizing MOFs using flexible polycarboxylate ligands [3]. The flexible nature of spacers allows the ligands to bend or rotate when coordinating to metal centers, which often causes diverse and intriguing structures. For example, two unprecedented self-penetrating coordination networks based on two V-shaped flexible ligands 3, 30 ,4,40 -benzophenonetetracarboxylate (bptc) and 4,40 -sulfonyldibenzoate (sdba) have been obtained [3a]. Recently, a series of complexes incorporating 3,30 ,4,40 -diphenylsulfonetetracarboxylic acid

⇑ Corresponding author. Tel.: +86 29 88303097; fax: +86 29 88373398. E-mail address: [email protected] (Y.-Y. Wang). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.10.011

(H4dstc, Scheme 1) with transition metal ions have been constructed, with the structures ranging from 1D linear chains to 3D architectures [4]. Herein, the characteristics of the H4dstc ligand can be summarized as follows: (1) it can provide multiple coordination modes with metal centers through complete or partial deprotonation of its four carboxylate groups, and thus generate a variety of polynuclear complexes ranging from discrete entities to three-dimensional frameworks; (2) the flexibility around the sulfonyl group can allow the sway of two phenyl rings of the H4dstc ligand to meet the coordination requirement of metal ions and direct the final structures; (3) it can provide directional conformation of network structures via dative bonds and also noncovalent contacts, such as hydrogen bonding and aromatic stacking. On the other hand, the coordination geometries of the metal centers and the nature of the secondary ligands play important roles in defining the overall structures of the complexes. For instance, the N-donor co-ligands have been widely used in the construction of multicarboxylate ligands with different metal centers, which manipulate the structural topologies through coordination in either terminal or bridging fashions, leading to many kinds of structures with interesting properties [5–7]. To make further investigation on coordination behavior of the H4dstc ligand and corresponding properties of its complexes, we have synthesised four new complexes through controllable hydrothermal reactions of the M(II)/dstc (M = Cu/Cd) system in the presence of three

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Scheme 1. Structures of H4dstc ligand and three neutral ligands.

Scheme 2. The coordination modes of the H4dstc ligand in complexes 1–5.

N-containing auxiliary ligands, 1,10-phenanthroline (phen), 1,2bis(4-pyridyl)ethene (bpe) and 4,40 -bipyridine (bipy) (Scheme 1). These complexes are formulated as [Cu4(H2dstc)4(phen)4]12H2O (1), {[Cu2(dstc)(bpe)(H2O)2]4H2O}n (2), [Cu3(dstc)(bipy)(l2-OH)2-

(H2O)2]n (3), {[Cd5(dstc)2(bipy)2(l3-OH)2(H2O)4]4H2O}n (4), respectively. As expected, the H4dstc ligand adopts various coordination modes with metal centers (Scheme 2a–e), which lead to diverse structures from 0D discrete unit to 3D frameworks.

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2. Experimental

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1094(s), 1069(s), 891(w), 832(s), 806(s), 784(s), 755(s), 669(s), 648(s), 624(s), 552(m), 490(s).

2.1. Reagents and physical measurements All reagents and solvents employed were commercially available and used as received without further purification. Elemental analyses were performed on a Perkin–Elmer 240C analyzer. Infrared spectra on KBr pellets were recorded on Nicolet 170SX FT-IR spectrophotometer in the range of 4000–400 cm1. Thermal analyses were performed on a NETZSCH STA 449C microanalyzer with a heating rate of 10 °C min1 under N2 atmosphere. The X-ray powder diffraction patterns were recorded with a pigaku D/Max 3III diffractometer. Luminescence spectra for the solid samples were registered on a Hitachi F-4500 fluorescence spectrophotometer at room temperature. The magnetic susceptibility of microcrystalline sample restrained in parafilm was measured on a Quantum Design MPMS-XL7 SQUID magnetometer with an applied field of 1 kOe. Diamagnetic correction was estimated from Pascal’s constants. 2.2. Synthesis of the complexes 2.2.1. Synthesis of [Cu4(H2dstc)4(phen)4]12H2O (1) A mixture of Cu(NO3)23H2O (0.10 mmol, 24.1 mg), 3,30 ,4,40 diphenylsulfonetetracarboxylic dianhydride (DSPA) (0.05 mmol, 17.9 mg), phen (0.05 mmol, 9.9 mg) and water (15 ml) was stirred for 20 min in air, then transferred and sealed in a 25 mL Teflon reactor, which was heated at 160 °C for 72 h. Well-shaped blue block crystals were obtained when the solution was cooled to room temperature at a rate of 5 °C h1. Yield: 65%. Anal. Calc. for C112H88Cu4N8O52S4 (2760.37): C, 48.73; H, 3.21; N, 4.06. Found: C, 48.52; H, 2.97; N, 4.01%. IR (KBr, cm1): 3423(s), 3068(w), 2925(w), 2614(w), 1713(s), 1643(s), 1569(s), 1519(w), 1478(w), 1430(s), 1379(s), 1323(s), 1252(m), 1170(w), 1145(m), 1120(w), 1091(m), 1065(m), 895(w), 848(s), 794(s), 720(s), 667(w), 643(w), 562(m). 2.2.2. Synthesis of {[Cu2(dstc)(bpe)(H2O)2]4H2O}n (2) A mixture of Cu(NO3)23H2O (0.10 mmol, 24.1 mg), DSPA (0.05 mmol, 17.9 mg), bpe (0.05 mmol, 9.1 mg), NaOH (0.2 mmol, 8.0 mg) and water (15 ml) was stirred for 20 min in air, then transferred and sealed in a 25 mL Teflon reactor, which was heated at 160 °C for 72 h. Blue block crystals were obtained when the solution was cooled to room temperature at a rate of 5 °C h1. Yield: 36%. Anal. Calc. for C28H28Cu2N2O16S (807.68): C, 41.64; H, 3.49; N, 3.47. Found: C, 41.55; H, 3.76; N, 3.70%. IR (KBr, cm1): 3427(m), 2904(w), 1613(s), 1479(m), 1404(m), 1606(m), 1321(m), 1149(m), 1121(w), 1092(m), 1068(m), 1032(w), 978(w), 906(w), 836(m), 783(s), 740(w), 672(w), 606(w), 552(w), 480(w) 2.2.3. Synthesis of [Cu3(dstc)(bipy)(l2-OH)2(H2O)2]n (3) Complex 3 was prepared as 2 by using bipy (0.05 mmol, 7.8 mg) instead of bpe. Yield: 40%. Anal. Calc. for C26H20Cu3N2O14S (807.15): C, 38.69; H, 2.50; N, 3.47. Found: C, 38.76; H, 2.66; N, 3.67%. IR (KBr, cm1): 3404(m), 2361(s), 1611(s), 1478(m), 1384(s), 1323(m), 1225(m), 1175(m), 1123(m), 1098(m), 1072(s), 902(w), 847(m), 824(s), 785(s), 760(s), 726(w), 668(s), 640(m), 555(s), 513(s), 490(m). 2.2.4. Synthesis of {[Cd5(dstc)2(bipy)2(l3-OH)2(H2O)4]4H2O}n (4) Complex 4 was synthesised by a method similar to that of 3, using Cd(CH3COO)22H2O (0.1 mmol, 26.6 mg) instead of Cu(NO3)23H2O. Colorless crystals were obtained. Yield: 43%. Anal. Calc. for C52H46Cd5N4O30S2 (1833.12): C, 34.07; H, 2.53; N, 3.06. Found: C, 34.19; H, 2.24; N, 3.17%. IR (KBr, cm1): 3434(m), 2361(s), 1603(m), 1489(m), 1389(s), 1318(s), 1174(s), 1117(s),

2.2.5. Synthesis of {[Zn2(dstc)(bipy)2]8H2O}n (5) Complex 5 was obtained with a method similar to that of 3 by using Zn(CH3COO)22H2O (0.1 mmol, 21.9 mg) instead of Cu(NO3)23H2O. However, 5 was also obtained using different method in the previous literature [4c]. 2.3. X-ray crystallography Single-crystal X-ray diffraction datum of 1–4 were collected on a Bruker SMART APEX CCD diffractometer with graphite-monochromated Mo Ka radiation (k = 0.71073 Å) at room temperature. The structures were solved by using direct methods and successive Fourier difference synthesis (SHELXS-97) [8], and refined using the full-matrix least-squares method on F2 with anisotropic thermal parameters for all non-hydrogen atoms (SHELXL-97) [9] except for disordered atoms. The hydrogen atoms for pyridyl rings, benzene rings and coordinated water molecules were generated by a riding model on idealized geometries. The hydrogen atoms for lattice water molecules in 1 and 2 were located from difference Fourier maps and refined with isotropic thermal parameters 1.5 times those of their carrier atoms. The hydrogen atoms were not added to the lattice water molecules in 4. The crystallographic data and selected bond lengths and angles for 1–4 are listed in Table 1 and Tables S1 and S2. 3. Results and discussion 3.1. Synthesis The 3,30 ,4,40 -diphenylsulfonetetracarboxylic dianhydride (DSPA) was selected instead of the carboxylate H4dstc in the reaction systems, and all the complexes are based on the products of the hydrolysis of DSPA. The direct reaction of Cu(II) salt, DSPA and phen ligand gave complex 1 possessing a 0D discrete tetranuclear unit. While by the introduction of NaOH to control the hydrolysis of DSPA and the replacement of the second ligand, complexes 2 and 3 were obtained, with their structures showing 2D and 3D networks, respectively. In order to investigate the effects of the metal ions on the framework structures, Cd(II) salts were selected instead of Cu(II) salts, thus complex 4 was synthesised, which indicates different structure with that of 3. Additionally, in order to complete our systematic study, we take the complex {[Zn2(dstc)(bipy)2]8H2O}n (5) into account, which was also obtained by using different method with that in the previous literature [4c]. 3.2. Description of crystal structures 3.2.1. [Cu4(H2dstc)4(phen)4]12H2O (1) Complex 1 is a discrete tetranuclear [Cu4(H2dstc)4(phen)4] 12H2O unit based on two symmetry-related dinuclear copper(II) units (Fig. 1a). Each dinuclear unit contains two independent Cu(II) ions: Cu1 is six-coordinated by four O atoms from three different dstc ligands and two N atoms from one chelating phen ligand, whereas Cu2 is five-coordinated by three O atoms from three dstc ligands and two N atoms from one phen ligand, showing a distorted square pyramid geometry. The Cu1–O2 (2.708 Å), Cu1– O17A (2.400 Å) and Cu2–O1 (2.356 Å) bonds are remarkably weaker, but are not negligible [10]. In 1, the two Cu(phen) units are bridged together by two carboxylic l2-O atoms and one COO group to form a dinuclear unit with the Cu  Cu distance of 3.177 Å. There are two types of partially deprotonated H2dstc2

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Table 1 Crystal data and structure refinement for complexes 1–4.

a

Complex no.

1

2

3

4

Formula Formula mass Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) F (0 0 0) h (°) Data/restraints/parameters Final Ra indices [I > 2r(I)]

C112H88Cu4N8O52S4 2760.37 orthorhombic Pbca 9.7620(9) 24.535(2) 46.925(4) 90 90 90 11239.1(18) 4 1.631 0.927 5648 2.11–25.2 10 126/0/813 R1 = 0.0631 wR2 = 0.1552

C28H28Cu2N2O16S 807.68 triclinic  P1

C26H20Cu3N2O14S 807.15 monoclinic C2/c 10.4736(10) 9.3446(9) 26.918(3) 90 97.0130(10) 90 2614.8(4) 4 2.050 2.582 1620 1.52–25.10 2341/4/222 R1 = 0.0320 wR2 = 0.0982

C52H46Cd5N4O30S2 1833.12 monoclinic P2(1)/n 7.6233(6) 29.191(2) 13.0266(9) 90 102.9980(10) 90 2824.5(4) 2 2.146 2.028 1780 1.75–25.10 5006/0/425 R1 = 0.0273 wR2 = 0.0660

R1 =

P

8.1370(7) 13.4714(12) 15.5541(14) 71.3210 82.4700(10) 73.3800 1546.2(2) 2 1.735 1.525 824 2.61–25.10 5404/15/472 R1 = 0.0400 wR2 = 0.0952

P P P ||F0|  |Fc||/ |F0|, wR2 = { [w(F 20  F 2c )2]/ (F 20 )2}1/2.

Fig. 1. (a) Tetranuclear unit in 1, hydrogen atoms of the aromatic rings and lattice water molecules are omitted for clarity. (A: 1  x, 1  y, z) (b) 3D structure formed by intermolecular hydrogen-bonding interactions in 1. All H atoms are omitted. (c) Schematic illustration of the 3D structure of 1 (blue parallelogram: schematic illustration of the tetranuclear unit, green dotted line: intermolecular H-bonding interactions). (For interpretation of references to colors in this figure legend, the reader is referred to see the web version of this article.)

ligands: one acts as a terminal ligand adopting a l2-g2:g1-bridging fashion, the other functions as a bridging ligand with two carboxylate groups exhibiting l2-g2:g0-bridging and syn, syn-l2-g1:g1bridging modes, respectively (Scheme 2a and b). The tetranuclear unit is stabilized by the intramolecular p–p stacking interactions between the aromatic rings of the phen ligand and the dstc ligand (centroid-to-centroid distances of 3.748 and 3.933 Å). In addition, the intermolecular hydrogen bonding interactions between the lattice water molecules, the carboxylic groups and the sulfonyl groups of the H2dstc2 ligand, with the average O  O distance of 2.756 Å [11], assemble the discrete tetranuclear units into a 3D supramolecular architecture (Fig. 1b and c). 3.2.2. {[Cu2(dstc)(bpe)(H2O)2]4H2O}n (2) The structure of 2 consists of a dinuclear unit (Fig. 2a). Cu1 atom exhibits a distorted octahedral geometry by coordinating to four

carboxylic O atoms from two different dstc ligands, one N atom of bpe ligand and one water molecule, while the coordination polyhedron of Cu2 exhibits a distorted trigonal bipyramidal geometry, with the equatorial plane occupied by two carboxylic O atoms and one O atom from the coordinated water molecule and the apical positions defined by one carboxylic O atom of dstc ligand and one N atom of bpe ligand. Each dstc ligand links four Cu atoms with three of the carboxylate groups in l1-g1:g1-chelating modes and the additional one in a l1-g0:g1-monodentate way (Scheme 2c), which results in the generation of 1D chains (Fig. S1a). The further extension of these chains by the bpe linkers lead to a 2D 44 gridlike layer (Fig. 2b) with the node defined by a Cu(II) dimer (Cu  Cu 5.484 Å). Though with such a big grid size (18.47  13.60 Å), there are no interpenetration in the framework, instead, the adjacent 2D 44 nets pack on top of each other in a staggered fashion, and further form a bilayer through interlayer hydrogen bonding interactions

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Fig. 2. (a) Coordination environments around Cu(II) centers in 2, H atoms of the aromatic rings and guest water molecules are omitted for clarity. (A: 1 + x, 1 + y, z; B: 1 + x, y, 1 + z) (b) The 2D 44 grid-like layer. (c) View of the bilayer motif (purple dotted line: H-bonding interactions). (d) View of lattice water molecules among adjacent bilayers. (For interpretation of references to colors in this figure legend, the reader is referred to see the web version of this article.)

[O(11)–H  O(5) 2.763 Å] (Fig. 2c). Lattice water molecules are located in the channels of the adjacent bilayers and unite the bilayers to generate a 3D supramolecular architecture by H-bonding interactions between lattice water molecules [O(12), O(13), O(14), O(15)], coordinated water molecules [O(11), O(16)] and COO [O(4), O(6), O(8)] groups of the dstc ligand (Fig. 2d and Fig. S1b). 3.2.3. [Cu3(dstc)(bipy)(l2-OH)2(H2O)2]n (3) The structure of complex 3 is a 3D metal–organic framework containing linear trinuclear Cu(II) cluster units (Fig. 3a). There are two crystallographically independent Cu(II) centers, both of which are six-coordinated and adopt slightly distorted octahedral geometries. Cu1 is ligated by two O atoms from two hydroxyl groups and four O atoms from four dstc ligands, while Cu2 is coordinated by one hydroxyl group, three carboxylic O atoms, one N atom of bipy ligand and one water molecule. The three Cu(II) ions are connected together through two l2-OH groups, two carboxylic l2-O atoms and two carboxylate groups to form a linear trimeric unit with the Cu1  Cu2 distances of 3.159 Å. The carboxylate

groups of dstc ligand exhibits two types of unusual coordination modes (Scheme 2d) to connect ten Cu(II) atoms. One type set adopts a l2-g1:g1-bridging mode while the other set displays a l3-g2:g1-bridging fashion. On the basis of this connection mode, the trinuclear Cu clusters are connected together by dstc ligands to furnish a 3D structure (Fig. S2a). Additionally, bipy ligands further join two Cu2 atoms to complete the coordination sphere of the Cu atoms and generate a more complicated 3D structure (Fig. 3b). Topologically, each trinuclear cluster can be defined as an 8-connected node (connecting six dstc ligands and two bipy ligands, Fig. S2b), while each dstc ligand can be regarded as a 6-connected node (connecting six trimeric units, Fig. S2c), thus the whole framework of 3 will be defined as a 3D (6,8)-connected binodal net (Fig. 3c). 3.2.4. [Cd5(dstc)2(bipy)2(l3-OH)2(H2O)4]4H2O}n (4) X-ray structural analysis reveals that complex 4 features a 3D coordination polymer with the pentanuclear cadmium cluster as the secondary building unit (Fig. 4b). The crystal structure of 4 con-

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The Cd2 atom is linked to other four Cd atoms through two l3OH groups and two l3-O (dstc) atoms to give a [Cd5(l3-OH)2(l3– O)2(H2O)2] core, with the Cd  Cd separation of 3.3421–6.1961 Å. Each dstc ligand connects eight Cd atoms giving l1-g0:g1-monodentate, syn, syn, anti-l3-g2:g1 mode and l2-g1:g1 modes with syn, syn conformations, respectively (Scheme 2e). The dstc ligands integrate the pentanuclear clusters into a 2D layer (Fig. S3a). In the 2D layer, each pentanuclear cluster is surrounded by eight dstc ligands and can be regarded as an 8-connected node, in turn, each dstc ligand links four pentanuclear clusters and serve as a 4-connecting node (Fig. S3b), thus, this 2D layer can be simplified as a (4,8)-connected net, as shown in Fig. 4c. The 2D layers are further united together by bipy ligands into a 3D network by the coordination interaction between Cd3 ions and N atoms (Fig. S3c), and the resulting structure can be further defined as a uniform (4,12)connected binodal network (Fig. 4d), in which the pentanuclear clusters feature 12-connected nodes (surrounded by eight dstc ligands and four bipy ligands, Fig. S3d). Noticeably, similar pentanuclear cluster [Cd5(l3-OH)2(l3-O)2] has been reported in the 2D complex [Cd5(l3-OH)2(sdba)4(2,20 bipy)2(H2O)2]2H2O [13] (sdba = 4,40 -sulfonyldibenzoato, 2,20 bipy = 2,20 -bipyridine), in which 12 organic ligands (ten sdba ligands and two 2,20 -bipy ligands) surround each pentanuclear cluster. The final structure shows a 2D (3,10)-connected network. Obviously, the chelating effect of 2,20 -bipyridine in that prevents further extension of the framework. While in 4, the bridging coordination of 4,40 -bipyridine makes it possible for the formation of the higher-connected 3D (4,12)-connected network. The difference between the two complexes also demonstrates the key role of the second ligand in modulating the resulting frameworks. To the best of our knowledge, MOFs containing 12-connected pentanuclear cadmium units are rare [14], and the (4,12)-connected network based on pentanuclear cadmium clusters has never been reported. 3.2.5. {[Zn2(dstc)(bipy)2]8H2O}n (5) To further investigate the effects of metal ions on the framework structures, Zn(II) salts were also selected instead of Cu(II) salts, and complex 5 was synthesised, with the same structure as that in the previous report [4c]. 3.3. Discussion

Fig. 3. (a) The trinuclear copper second building unit in 3. (A: 1.5  x, 1.5  y, 2  z; B: 0.5 + x, 0.5 + y, z; C: 2  x, 1  y, 2  z; D: 1 + x, y, z; E: 2.5  x, 1.5  y, 2  z; F: 1  x, y, 1.5  z; G: 2  x, y, 1.5  z) (b) The 3-D metal–organic framework of complex 3. (c) Schematic representation of the 3D binodal (6,8)-connected net of 3 (yellow nodes: dstc ligands, turquoise nodes: trinuclear Cu clusters, purple lines: bipy ligands). (For interpretation of references to colors in this figure legend, the reader is referred to see the web version of this article.)

tains three crystallographically independent Cd atoms that are all octahedrally coordinated (Fig. 4a). The Cd2 atom, lying on the center of the symmetric unit, is coordinated by two hydroxyl groups and four carboxylic O atoms from four different dstc ligands, Cd1 is coordinated by one hydroxyl group, three carboxylic O atoms and two water molecules, whereas Cd3 is coordinated by three O atoms of three different dstc ligands, two N atoms from two bipy ligands and one l3-OH group. The bond lengths of Cd–O (2.224– 2.404 Å) and Cd–N (2.362–2.374 Å) are consistent with values reported for carboxylate and bipy-containing Cd(II) complexes [12].

From the structural analysis of complexes 1–5 described above, five coordination modes of the dstc ligands can be observed (Scheme 1). In 1, the H2dstc2 ligands are partially deprotonated and adopt two kinds of coordination modes. The chelating phen ligands, together with the partially deprotonated H2dstc2 ligands, lead to the low dimensionality of 1. In 2, the dstc ligand adopts a l4-bridging mode to link the adjacent Cu(II) dimers to form 1D chains. Moreover, as bpe possesses moderate flexibility, it can adjust itself to the dstc ligand and acts as a bridging ligand to further extend the 1D chains into a 2D layer. For 3, the dstc ligand adopts a complicated mode to link ten Cu(II) centers, which makes it possible for the formation of the Cu trimers and the resulting 3D structure by interlinking between the dstc ligands and trinuclear clusters. The 3D structure is further stabilized by the rigid bipy ligands. By comparison of 1–3, it can be concluded that different architectures and dimensionalities of structures can be rationalized by the changes of the secondary ligand and the linking modes of the dstc ligand. On the other hand, the metal centers can also play an important role in the presence of the same ligands, which can be reflected by comparison of 3, 4 and 5. The distinction may be attributed to the disparate radii of the metal ions, and the dstc ligand changed its coordination modes in order to satisfy the different geometric needs of the metal ions. In 3, dstc ligands link the trinuclear Cu clusters into a 3D structure in l10-bridging

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Fig. 4. (a) Coordination environments around Cd(II) centers in 4. (A: 1  x, 2  y, 2  z; B: 1 + x, y, z; C: 1 + x, y, 1 + z; D: 0.5 + x, 1.5  y, 0.5 + z; E: 1  x, 2  y, 1  z; F: x, 2  y, 1  z; G: x, y, 1 + z) (b) View of the pentanuclear cadmium cluster. (c) Schematic representation of the 2D (4,8)-connected net of 4. (d) Schematic illustration of the 3D binodal (4,12)-connected net of 4 (yellow nodes: dstc ligands, purple nodes: penternuclear Cd clusters, blue lines: bipy ligands). (For interpretation of references to colors in this figure legend, the reader is referred to see the web version of this article.)

fashions. In 4, each dstc ligand adopts a l8-bridging mode to bridge the pentanuclear Cd clusters into a 2D net. While in the case of 5, the ligand links four Zn centers in a simple tetra(monodentate) fashion, leading to the formation of 1D chains. The bipy ligands in 4 and 5 act as pillars to extend the 1D chains and 2D layers into 3D structures. Moreover, the change in coordination geometry of the metal centers leads to the distinctness of the linking fashion of the subunits and finally results in the different connectivity of the ultimate net. In 3, trinuclear Cu clusters are 8-connected to give a complicated (6,8)-connected framework. In 4, pentanuclear Cd clusters serve as 12-connected nodes, resulting an unusual (4,12)-connected framework. In 5, the Zn centers exist in a mononuclear fashion and act as 4-connected nodes, giving a 4-connected network. Thus it can be demonstrated that the choice of metal ions also play critical roles in the formation of novel dimensional networks. Additionally, the reaction pH also influence the final structures. Since all the above-mentioned factors work together to affect the structures of complexes, it is difficult to control and separate them. 3.4. XRPD measurements and thermal analysis In order to confirm the phase purity of the complexes, X-ray powder diffraction (XRPD) experiments were performed at room temperature. The experimental patterns are match well with those simulated from the single-crystal X-ray data of the complexes, although there are minor differences in the positions, intensities, and widths of some peaks (Fig. S4). Thermogravimetric analysis (TGA) of the polymers were performed to study the stability of 1–4 (Fig. S5). 1 loses its lattice water molecules (obsd 7.4%, calcd 7.8%) before 142 °C, and the remaining framework can be stable to 241 °C. For 2, the first

weight loss (11.8%) from 25 to 143 °C corresponds to evacuation of six coordinated and lattice water molecules (calcd 13.4%), after which, the net remains stable up to 243 °C. Heated to 130 °C, 3 begins to lose its two coordinated water molecules and two hydroxyl groups (obsd 8.1%, calcd 8.7%), further heating to 278 °C gives a rapid weight loss. The residues of 1, 2 and 3 are CuO (obsd 11.7%, 20.4%, 30.7%, calcd 11.5%, 19.7%, 29.6%). The TGA curve of 4 exhibits two weight-loss stages: the first loss (obsd 9.4%, calcd 9.7%) is within the range of 25–190 °C corresponds to the release of coordinated waters and hydroxyl groups, the second can be detected from 269 to 490 °C, indicating the decomposition of dstc and bipy ligands. The residue is CdO (obsd 34.6%, calcd 35.0%). 3.5. Magnetic properties Magnetic studies have been performed on powdered samples for 1 and 3 in the range 2–300 K. The vMT and vM versus T plots of 1 are shown in Fig. 5. The value of vMT at room temperature is 0.87 cm3 mol1 K, which is higher than the theoretical value of 0.75 cm3 mol1 K expected for two noninteracting Cu(II) ions. The vM shows a maximum at 9 K. These behaviors suggest antiferromagnetic exchange interactions between Cu1 and Cu2, connected to each other through two l2-O and one O–C–O bridges. From the magnetic point of view, complex 1 is a simple Cu(II) dimer with very weak interdimer interactions, the data can be analyzed by the following equation [15]:

v ¼ ð1  qÞ

2Ng 2 b2 1 Ng 2 b2 þ q  2J=kT 3þe kT 2kT

where q is the paramagnetic impurity. The best fit obtained with g = 2.16, J = 5.4 cm1 and q = 1.8%.

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Fig. 5. vMT and vM versus T plots of 1.

Fig. 7. The solid-state photoluminescent spectra of 4 at room temperature.

vchain ¼

Ng 2 b2 1 þ u 1 þ e2J=kT þ 10e3J=kT 4kT 1  u 1 þ e2J=kT þ 2e3J=kT

The temperature independent paramagnetism (TIP) was included, then the expression becomes:

v¼

Ng 2 b2 1 þ u 1 þ e2J=kT þ 10e3J=kT þ TIP 4kT 1  u 1 þ e2J=kT þ 2e3J=kT

where u ¼ cothðJc St ðSt þ 1Þ=kTÞ  kT=Jc St ðSt þ 1Þ The best fitting, shown as the solid line in Fig. S6, gives parameters g = 2.28, J = 17.9, Jc = 33.9 and TIP = 4  104. Fig. 6. vMT and vM versus T plots of 3.

3.6. Photoluminescence properties For complex 3, the vMT and vM versus T is shown in Fig. 6. The value of vMT at room temperature is 1.10 cm3 mol1 K, which is close to that of three uncoupled Cu(II) ions (1.125 cm3 mol1 K for g = 2.0). The vM value increases continuously on cooling from room temperature until approaching a maximum of 0.0086 cm3 mol1 at 50 K. Then it decreases, reaching a minimum of 0.0041 cm3 mol1 at 14 K, below which vM increases again. The maximum appearing in the vM curve indicates a significant antiferromagnetic interaction between Cu(II) ions. The upturn of the vM value below 14 K could be attributed to paramagnetic impurities. From the magnetic point of view, it is a 3D framework containing a 1D metal chain assembled from trinuclear clusters. On the basis of an approximate model of trinuclear Cu(II) clusters, the susceptibility data of 3 can be fitted by the following expression deduced from spin Hamiltonian H ¼ 2JðS1 S2 þ S1 S3 Þ, where the Weiss constant h accounts for the magnetic interactions between the trinuclear Cu(II) clusters.

vCu3 ¼

Ng 2 b2 1 þ e2J=kT þ 10e3J=kT  4kðT  hÞ 1 þ e2J=kT þ 2e3J=kT

The best fit of the experimental data gives J = 15.0 cm1 g = 2.14, and h = 60.6 K, large h value confirming the strong antiferromagnetic interaction between the Cu3 trimer which are linked by carboxylate groups in anti-anti coordination mode for complex 3 (Fig. S2d). To analyze the magnetic data of 3, we tried to use the second approximate approach for 1-D chain which can be treated as alternating uniform Cu3 trimers with the intratrimeric exchange (Jc) [16].

Inorganic–organic coordination polymers, especially those with d10 metal centers, have been investigated for their fluorescent properties and potential applications as fluorescent-emitting materials, such as light-emitting diodes (LEDs) [17]. Thus, the luminescence of complex 4 was investigated in the solid state at room temperature. As shown in Fig. 7, intense emission bands are observed at 390 nm for 4 (kex = 300 nm), the emission peak of free H4dstc ligand is at 356 nm according to the literature [4c]. The red shift of emission bands of 4 would be attributed to the metal-to-ligand charge transfer (MLCT) [18].

4. Conclusions In summary, we have successfully synthesised four new copper and cadmium coordination polymers with a V-shaped DSPA ligand in the presence of a series of N-donor ligands with different size, conformation and degrees of freedom. The results indicate that the dstc ligand with various coordination modes plays a key role in constructing polynuclear (i.e. bi-, tri-, tetra- and pentanuclear) complexes ranging from discrete entities to three-dimensional frameworks. Additionally, different N-heterocyclic ligands and metal ions also have impressive influence on the resulting structures. It is noteworthy that complexes 3 and 4 show intriguing and complicated (6,8)-connected and (4,12)-connected topologies, respectively. From the viewpoint of crystal engineering, this work encourages us to make further efforts on changing neutral ligands and metal ions to afford other novel metal-dstc crystalline networks.

R.-T. Liu et al. / Inorganica Chimica Acta 366 (2011) 53–61

Acknowledgments We gratefully acknowledge financial support of this work by the National Natural Science Foundation of China (Grant No. 20771090), State Key Program of National Natural Science of China (Grant No. 20931005), Natural Science Foundation of Shaanxi Province (Grant No. 2009JZ001), and Specialized Research Found for the Doctoral Program of Higher Education (Grant No. 20096101110005).

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Appendix A. Supplementary material CCDC 772602–772605 contain the supplementary crystallographic data for 1–4. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc. cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.ica.2010.10.011.

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