One-, Two-, and Three-Dimensional Coordination Polymers of

Dec 2, 2008 - Under similar synthetic conditions, the assembly of trans-stilbene-4,4′-dicarboxylate with different metal ions in the presence of pyr...
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CRYSTAL GROWTH & DESIGN

One-, Two-, and Three-Dimensional Coordination Polymers of Stilbenedicarboxylate with Different Metal Ions Yu Ma,† Ai-Ling Cheng,† Jian-Yong Zhang,† Qi Yue,† and En-Qing Gao*,†,‡ Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal UniVersity, Shanghai 200062, China, and State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 35000, China

2009 VOL. 9, NO. 2 867–873

ReceiVed May 15, 2008; ReVised Manuscript ReceiVed October 9, 2008

ABSTRACT: Using trans-stilbene-4,4′-dicarboxylic acid (H2STDC) as bridging ligand, we have prepared four new coordination polymers with different transition metal ions, [Cu(STDC)(H2O)(py)2] · 2py (1), [Cd(STDC)(py)2] · 0.5py (2), [Co(STDC)(py)2] · 0.5py (3), and [Ni2(STDC)2(py)4(H2O)] · py · 0.5H2O (4), by hydrothermal reactions in the presence of pyridine (py). In compound 1, single metal ions are linked into quasi-linear coordination chains by the organic ligand, and the chains are further assembled into 3D supermolecular architectures through strong O-H (coordinate water) · · · O (carboxylate) and weak C-H (py) · · · π (benzene) hydrogen bonds. In compounds 2 and 3, which are isomorphous, binuclear motifs with double carboxylate bridges are linked into twodimensional (4,4) coordination layers exhibiting 2-fold inclined interpenetration. The last compound exhibits an abnormal 5-fold interpenetration of diamond networks, in which the tetrahedral building unit is defined by a binuclear motif with triple (two carboxylates and a water) bridges. Although it is impossible to rationalize the formation of these distinct structures, the differences can be related to the different disposition of the coordinated py (and water in 1) molecules around metal ions. All these coordination polymers contain voids occupied by guest molecules. The stability and guest inclusion properties of 2, 3, and 4 have been investigated. While 3 and 4 undergo permanent framework collapse upon evacuation by heating, 2 exhibits reversible framework transformation upon evacuation/absorption of the guest molecules. Introduction Metal-organic coordination polymers are currently of great interest, not only for their potential applications such as nonlinear optics, magnetism, porosity, chemical sensing, and catalysis, but also for the fundamental scientific issues such as their intriguing topological variety and the physical laws underlying the assembling processes.1-3 The final structures of coordination polymers are dependent upon multiple factors, among which the most important ones are the geometrical and electronic properties of the metal ions and ligands. The crystal engineering of such materials has attained a relatively mature level, and some coordination networks with specific topologies can be “designed” by the judicious selection of metal ions and organic ligands with definite coordination preferences.4 For example, the so-called “reticular synthesis” approach that requires the use of secondary building units to direct the assembly has been proposed for the design of new materials with predetermined topologies.5 The most successful examples of this approach are the three-dimensional (3D) metal-organic frameworks derived from aromatic di- or multicarboxylate ligands with specific geometry. A different approach for the design of coordination polymers is to incorporate terminal ligands that can regulate the connectivity and geometry by blocking some coordination sites of metal ions. This “ligandregulation” approach often leads to lower-dimensional coordination polymers, as exemplified by the large number of onedimensional (1D) coordination chains derived from dicarboxylate ligands and bidentate terminal ligands, such as 2,2′-bipyridine and 1,10-phenanthroline.6,7 Despite these successes, the rational structural prediction and design of coordination polymers still remain a long-term challenge in most systems, due to the * To whom correspondence should be addressed. E-mail: eqgao@ chem.ecnu.edu.cn. † East China Normal University. ‡ Chinese Academy of Sciences.

coordination flexibility and versatility of metal ions and ligands, the subtle influences of weak interactions (e.g., hydrogen bonds), and other factors including synthetic conditions. Aromatic dicarboxylate ligands, such as benzenedicarboxylates, biphenylcarboxylates, have been widely used to construct coordination polymers. In this context, a relatively long ligand of this type, trans-stilbene-4,4′-dicarboxylic acid (H2STDC), has been explored by us and others recently.7,8 In a previous paper, we described a series of Zn(II)-STDC compounds with systematically varied terminal ligands (pyridine, 2,2′-bipyridine, 1,10-phenanthroline, and 2,2′-bisquinoline).7 It is noted that although monodentate, two pyridine (py) molecules play the same role as those of the bidentate ligands to block two cis coordination sites, leading to zigzag coordination chains. To further explore the effect of py coordination on the structure, we present here the syntheses and characterizations of a series of M(II)-STDC-py coordination polymers with different metal ions: [Cu(STDC)(H2O)(py)2] · 2py (1), [Cd(STDC)(py)2] · 0.5py (2), [Co(STDC)(py)2] · 0.5py (3), and [Ni2(STDC)2(py)4(H2O)] · py · 0.5H2O (4). The coordination motifs of these compounds are all distinct from the Zn(II) compound, although the stoichiometry is similar. Compound 1 consists of quasilinear chains, 2 and 3 contain 2D networks with 2-fold inclined interpenetration, and 4 exhibits 5-fold interpenetration of 3D diamond networks. The thermal stability and the guest absorption/desorption properties of 2, 3, and 4 have also been investigated. Experimental Section Physical Measurements. Elemental analyses were determined on an Elementar Vario ELIII analyzer. The FT-IR spectra were recorded in the range 500-4000 cm-1 using KBr pellets on a Nicolet NEXUS 670 spectrophotometer. Thermogravimetric analyses (TGA) were performed on a Mettler Toledo TGA/SDTA851 instrument under flowing N2 atmosphere at a heating rate of 10 °C min-1. Powder X-ray

10.1021/cg800506g CCC: $40.75  2009 American Chemical Society Published on Web 12/02/2008

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Table 1. Crystal Data and Structure Refinement for Compounds 1-4

empirical formula fw cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z D (g m-3) µ (mm-1) F(000) no. of reflns collected no. of unique reflns GOF on F2 Rint R1 [I > 2σ(I)] wR2 (all data)

1

2

3

4

C36H32N4O5Cu 664.20 monoclinic P2/c 13.868(5) 5.817(2) 20.578(8) 91.174(5) 1659.6(11) 2 1.329 0.706 690 9914 3772 0.997 0.0637 0.0579 0.1567

C28.50H22.5N2.5O4Cd 576.39 monoclinic C2/c 31.638(3) 8.5009(8) 20.568(2) 108.2260(10) 5254.3(9) 8 1.457 0.868 2328 15940 6017 1.034 0.0335 0.0397 0.0583

C28.50H22.5N2.5O4Co 522.92 monoclinic C2/c 31.384(6) 8.2869(17) 21.465(4) 111.61(3) 5190.2(18) 8 1.338 0.698 2160 9670 5104 0.870 0.0637 0.0491 0.1291

C57H48N5O9.5Ni2 1072.42 tetragonal P43 15.8465(14) 15.8465(14) 21.775(3) 90 5468.0(10) 4 1.303 0.749 2228 35500 9776 1.019 0.1164 0.0811 0.2328

diffraction data were collected on a Bruker D8-ADVANCE diffractometer equipped with Cu KR at a scan speed of 1° min-1. Synthesis. The reagents were obtained from commercial sources and used without further purification. The ligand trans-stilbene-4,4′dicarboxylic acid (H2STDC) was prepared according to the literature.9 [Cu(STDC)(H2O)(py)2] · 2py (1). A mixture of Cu(OAc)2 · H2O (0.2 mmol, 0.04 g), H2STDC (0.2 mmol, 0.054 g), and py (3 mL) in water (9 mL) was stirred for 30 min at room temperature and then heated in a 23 mL Teflon-lined autoclave at 130 °C for 4 days. After being cooled to room temperature slowly, green prism crystals of 1 were obtained. The product was contaminated by uncharacterized microcrystals that are difficult to remove. Our attempts to obtain the pure sample of 1 were unsuccessful, which prevented us from carrying out measurements on the bulk sample. A few crystals were sorted out manually for IR and single-crystal X-ray analyses. IR (KBr, cm-1): 3400(br), 1600(s), 1543(m), 1447(w), 1378(vs), 1287(w), 1217(w), 1177(w), 1069(w), 856(w), 787(w), 691(w). [Cd(STDC)(py)2] · 0.5py (2). A procedure similar to that for 1 was followed to prepare 2, except Cu(OAc)2 · H2O was replaced by Cd(OAc)2 · 2H2O (0.2 mmol, 0.053 g). Yield: 60% based on Cd. Anal. Found: C, 59.3; H, 4.22; N, 6.15%. Calcd for C28.5H22.5N2.5O4Cd: C, 59.39; H, 3.93; N, 6.07%. IR (KBr, cm-1): 1587(s), 1538(s), 1484(w), 1444(m), 1396(vs), 1219(w), 1176(w), 1069(w), 858(w), 789(w), 703(m). [Co(STDC)(py)2] · 0.5py (3). A procedure similar to that for 1 was followed to prepare 3 except Cu(OAc)2 · H2O was replaced by Co(OAc)2 · 4H2O (0.2 mmol, 0.043 g). Yield: 45% based on Co. Anal. Found: C, 65.29; H, 4.74; N, 6.85%. Calcd for C28.5H22.5N2.5O4Co: C, 65.46; H, 4.34; N, 6.70%. IR (KBr, cm-1): 1605(s), 1549(m), 1533(s), 1485(w), 1444(m, sh), 1401(vs), 1219(w), 1178(w), 1071(w), 1038(w), 859(w), 788(w), 702(m). [Ni2(STDC)2(H2O)(py)4] · py · 0.5H2O (4). A procedure similar to that for 1 was followed to prepare 4 except Cu(OAc)2 · H2O was replaced by Ni(OAc)2 · 4H2O (0.2 mmol, 0.043 g). Yield: 45% based on Ni. Anal. Found: C, 63.72; H, 4.88; N, 6.41%. Calcd for C57H48N5O9.5Ni2: H, 4.51; C, 63.84; N, 6.53%. IR (KBr, cm-1): 3425(br), 1630(s), 1605(s), 1555(w), 1522(w), 1480(w), 1447(w), 1388(vs), 1213(w), 1171(w), 1038(w), 846(w), 769(m), 696(m). X-ray Crystallographic Measurements. Diffraction data for 1, 2, 3, and 4 were collected at 293 K on a Bruker Apex II CCD area detector equipped with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). Empirical absorption corrections were applied using the SADABS program.10 The structures were solved by the direct method and refined by the full-matrix least-squares method on F2, with all non-hydrogen atoms refined with anisotropic thermal parameters.11 All the hydrogen atoms attached to carbon atoms were placed in calculated positions and refined using the riding model, and the hydrogen atoms of the coordinated water molecules in 1 and 4 were located from the difference maps. The uncoordinated guest molecules in 4 were not located because of the limited quality of the data set. According to elemental analytic data and TGA analyses, the guest molecules were proposed to be py and H2O. All calculations were carried out with the SHELXTL

crystallographic software. A summary of the crystallographic data, data collection, and refinement parameters for complexes 1-4 are provided in Table 1.

Results and Discussion Crystal Structures. Compound 1. Single-crystal X-ray analyses revealed that complex 1 contains 1D linear polymeric chains. As shown in Figure 1, the Cu center resides on the crystallographic 2-fold axis and is five-coordinated by two equivalent carboxylate oxygen atoms from two different STDC ligands, two py molecules, and a water molecule. The coordination geometry is axially elongated square pyramidal with the water oxygen (O3) at the apex (see Table 2 for the relevant structural parameters). The four basal donor atoms are essentially coplanar, and the Cu atom is displaced out of the basal plane toward the apex by only 0.113(2) Å. The STDC ligand is centrosymmetric and serves as a bridge linking two neighboring Cu ions (Cu · · · Cu ) 17.44(4) Å), with the carboxylate groups in the monodentate coordination mode. Consequently, an infinite chain is formed along the (20-1) direction. The chain is quasilinear due to the trans coordination of the quasi-linear bridges around metal ions. All the chains are aligned in parallel and stacked into a 3D architecture through strong and weak hydrogen bonds (Figure 2). Each coordinated water molecule (O3) from one chain forms

Figure 1. Local coordination environments of the Cu center and the STDC ligands in compound 1 (hydrogen molecules were omitted for clarity). Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compound 1 Cu1-O1 Cu1-O3 O1-Cu1- O1#1 O1-Cu1-O3 O1-Cu1-N1

1.955(2) 2.201(4) 173.32(13) 93.34(6) 88.91(10)

Cu1-N1 O1-Cu1-N1#1 O3-Cu1-N1 N1-Cu1-N1#1

Symmetry operations in 1: #1 -x + 1, y, -z + 3/2.

2.030(3) 90.73(10) 93.15(7) 173.71(14)

Coordination Polymers of STDC with Different Metal Ions

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Figure 3. (a) Binuclear building unit in compound 2. (b) 2D (4,4) network based on the binuclear units. Table 3. Selected Bond Lengths (Å) and Angles (deg) for Compounds 2 and 3

Figure 2. (a) 2D network of 1 connected by hydrogen bonds; (b) 3D packing of the layers through C-H · · · π interaction.

two equivalent O-H · · · O interchain hydrogen bonds with two uncoordinated oxygen atoms (O2) from another chain, where the two O2 atoms arise from the carboxylate groups ligating the same Cu (II) ion. The relevant parameters are O-H · · · O ) 173(5)°, H · · · O ) 1.94(4) Å, O · · · O ) 2.675(3) Å. As a result, an interesting hydrogen bonding cyclic motif with graph set R22(10) containing two carboxylate groups, a Cu atom and a water molecule is formed between the chains (Figure 2a). Such hydrogen bonding motifs lead to a relatively short interchain Cu · · · Cu distance of 5.817(2) Å, and interlink the chains into a supramolecular layer extending along the (102) plane. On the other hand, the coordinated py molecules stretch out from above and below the layer, and the C11-H groups point toward the benzene rings of adjacent layers, with H · · · M ) 2.73 Å and C-H · · · M ) 144.7° (M is the center of the benzene ring) (Figure 2b). This indicates the occurrence of weak C-H · · · π hydrogen bonding. The C-H · · · π interactions require that the chains in adjacent layers be slipped with respect to each other, and generate a 3D network featuring 1D rectangle-shaped channels along the b direction. Uncoordinated py molecules are enclosed within the channels. Compounds 2 and 3. These two compounds are isomorphous and exhibit 2-fold interpenetrated 2D layer networks. The metal ion assumes a pseudo-octahedral coordination geometry completed by four carboxylate oxygen atoms (O1, O2, O3 and O4) from three different STDC ligands, and two py nitrogen atoms (N1 and N2) in trans positions (Figure 3a). The distortion of the geometry mainly arises from the chelating coordination of a carboxylate group, which has a very small bite angle (54.34(9)° for 2, 60.51(1)° for 3) and dissymmetric M-O bonds (2.309(2) Å and 2.506(3) Å for 2, 2.140(2) Å and 2.223(3) Å for 3) (Table 3). The STDC ligands, all having inversion centers at the

M1-O1 M1-O2 M1-O3 M1-O4#1 M1-N1 M1-N2 O1-M1-O2 O1-M1-O3 O1-M1-O4#1 O1-M1-N1 O1-M1-N2 O2-M1-O3 O2-M1-O4#1 O2-M1-N1 O2-M1-N2 O3-M1-O4#1 O3-M1-N1 O3-M1-N2 N1-M1-O4#1 N2-M1-O4#1 N1-M1-N2

2 (M ) Cd)

3 (M ) Co)

2.506(3) 2.309(2) 2.310(2) 2.229(2) 2.342(3) 2.344(3) 54.34(9) 104.15(9) 141.71(10) 85.23(10) 80.32(10) 158.49(9) 89.80(9) 91.73(10) 92.48(10) 111.00(9) 85.76(10) 82.68(9) 111.56(11) 89.37(11) 158.66(11)

2.223(3) 2.140(2) 2.005(2) 2.039(2) 2.190(3) 2.166(3) 60.49(9) 103.98(9) 151.13(10) 87.70(11) 83.86(10) 164.39(10) 91.33(10) 93.12(10) 90.78(10) 103.88(10) 87.49(10) 85.57(10) 100.96(11) 90.86(11) 167.45(11)

Symmetry operations: #1 -x + 1/2, -y + 1/2, -z.

midpoint of the central CdC bonds, exhibit two distinct bridging coordination modes: µ2-bis(chelating) and µ4-tetradentate. Each carboxylate group in the latter mode serves as a syn-anti bridge between metal ions. Two such carboxylate groups from different ligands bridge two equivalent M ions to give a centrosymmetic binuclear motif, with the M · · · M distances being 4.291(3) Å for (2) and 4.478(9) Å for (3). Each binuclear motif is linked to four identical motifs through four STDC ligands (two µ2 and two µ4), leading to a 2D coordination layer (Figure 3b). Taking the binuclear motifs as 4-connecting nodes and the ligands as linkers, the layer can be considered to be a (4,4) net. The dimensions of the rhombic windows of the layer are about 17.83 Å × 16.38 Å for 2 and 17.38 Å × 16.23 Å for 3 (the shortest M · · · M distances spanned by the two independent STDC ligands). To fill the large windows, interpenetration occurs between two sets of layers extending parallel to different planes, (111j)

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Figure 4. 2-Fold interpenetration of the layers in 2. (a) View showing the weak hydrogen bonds between two interpenetrating layers. (b) Space-filling diagram of the interpenetration.

and (1j11) (Figure 4). This inclined interpenetration is of the diagonal/diagonal type,4b with the binuclear nodes of one set residing at the centers of the windows of the other set. Between the interpenetrating layers, weak hydrogen bonds are operative, which involve the py C23-H group from one layer and the carboxylate O2 atom from the other layer (Figure 4a). The relevant parameters are O-H · · · O ) 162(5)°, H · · · O ) 2.56(4) Å, and O · · · O ) 3.458(3) Å for 2, and the corresponding data are 168(3)°, 2.573 Å and 3.489(4) Å for 3. The interpenetration leaves small open channels along the b direction, which are filled by guest py molecules. PLATON calculations12 show that the effective free voids comprise 15.7% for (2) and 16.0% for (3) of the crystal volume. Compound 4. This compound crystallizes in the chiral space group P43 and exhibits 5-fold interpenetrated three-dimensional diamond frameworks, with binuclear motifs as tetrahedral building units. The asymmetric unit of the framework contains two Ni(II) ions, two STDC dianions, four py molecules and a water molecule. As shown in Figure 5a, each metal ion exhibits a highly distorted octahedral coordination geometry completed by three carboxylate oxygen atoms (O1, O3, and O8 for Ni1; O2, O5, and O7 for Ni2) from three different STDC ligands, a water oxygen atom (O9) and two cis py nitrogen atoms (N1 and N2 for Ni1; N3 and N4 for Ni2). The Ni-O bond distances fall in the narrow range of 2.029(5)-2.088(5) Å, and the Ni-N ones are slightly longer (2.080(8)-2.128(8) Å) (Table 4). The two independent STDC ligands assume similar µ3-tridentate coordination modes: each ligand uses a carboxylate group to bridge Ni1 and Ni2 in the bidentate syn-syn mode, with the other carboxylate group binding a Ni1 or Ni2 atom in the monodentate mode. The water molecule serves as an additional monatomic bridge between Ni1 and Ni2, with Ni-O-Ni ) 117.8°. Thus, Ni1 and Ni2 are triply bridged by a water molecule and two syn-syn carboxylate groups to give a binuclear motif

Figure 5. (a) Binuclear tetrahedral building unit in compound 4. (b) Adamantane unit of the diamond network. (c) 5-Fold interpenetration of the diamond networks. Note that the translation of the red net along the a or b direction can generate the nets in other colors. Table 4. Selected Bond Lengths (Å) and Angles (deg) for Compound 4 Ni1-O1 Ni1-O3#2 Ni1-O8#1 Ni1-O9 Ni1-N1 Ni1-N2 O1-Ni1-O3#2 O1-Ni1-O8#1 O1-Ni1-O9 O1-Ni1-N1 O1-Ni1-N2 O3#2-Ni1-O8#1 O3#2-Ni1-O9 O3#2-Ni1-N1 O3#2-Ni1-N2 O8#1-Ni1-O9 O8#1-Ni1-N1 O8#1-Ni1-N2 O9-Ni1-N1 O9-Ni1-N2 N1-Ni1-N2

2.065(6) 2.088(5) 2.029(5) 2.075(6) 2.125(8) 2.128(8) 93.3(2) 90.9(3) 89.4(3) 86.3(3) 176.8(3) 174.2(3) 88.7(2) 91.0(3) 86.1(3) 95.2(2) 85.4(3) 89.4(3) 175.7(3) 93.8(3) 90.5(3)

Ni2-O2 Ni2-O5 Ni2-O7#1 Ni2-O9 Ni2-N4 Ni2-N3 O7#1-Ni2-O5 O2-Ni2-O7#1 O7#1-Ni2-O9 O7#1-Ni2-N4 O7#1-Ni2-N3 O2-Ni2-O5 O9-Ni2-O5 O5-Ni2-N4 O5-Ni2-N3 O2-Ni2-O9 O2-Ni2-N4 O2-Ni2-N3 O9-Ni2-N4 O9-Ni2-N3 N4-Ni2-N3

2.055(5) 2.070(5) 2.044(6) 2.066(6) 2.080(8) 2.121(7) 90.4(2) 92.8(2) 91.9(3) 86.5(3) 177.0(3) 176.1(3) 89.2(2) 89.5(3) 89.3(3) 93.0(2) 88.5(3) 87.5(3) 177.9(3) 91.1(3) 90.5(3)

Symmetry operations in 4: #1 -y + 1, x + 1, z - 1/4. #2 -y + 2, x - 1, z - 1/4.

[Ni · · · Ni ) 3.545(1) Å], which is further reinforced by the strong hydrogen bonds involving the bridging water molecule and the uncoordinated oxygen atoms from two monodentate carboxylate groups (O9-H · · · O4 ) 172(7)°, H · · · O4 ) 1.69(2) Å,O9 · · · O4 ) 2.547(8) Å and O9-H · · · O6 ) 168(8)°, H · · · O6

Coordination Polymers of STDC with Different Metal Ions

) 1.70(3) Å,O9 · · · O6 ) 2.543(8)Å). Similar motifs with water and carboxylate bridges and intramolecular hydrogen bonds have been identified in some binuclear Ni(II) compounds of general formula {[Ni(OOCR)2(A)2]2(µ2-OOCR)2(µ2-OH2)} [A ) monodentate ligands such as py and THF, or (A)2 ) bipyridine], which have been investigated as model compounds for metalloenzymes.13 The Ni-O, Ni-O-Ni and Ni · · · Ni parameters of 4 are comparable to those reported for the binuclear complexes (2.02-2.14 Å for Ni · · · O, 110-118° for Ni-O-Ni, and 3.28-3.70 Å for Ni · · · Ni).13 In 4, the binuclear motifs serve as the secondary building units of a 3D network. Each motif is furnished by four quasilinear STDC ligands radiating in four noncolinear directions, thus defining the motif as a tetrahedral 4-connecting node. The connection of the tetrahedral nodes in the lattice leads to a diamond network. An adamantane cage of the network is shown in Figure 5b. The center-to-center X · · · X distances (X is the center of a node, defined at the midpoint between Ni1 and Ni2) between adjacent nodes are 18.51 Å and 18.56 Å, whereas the X · · · X · · · X angles are in the range of 93.06(0)-145.76(0), which represents a significant distortion from the ideal tetrahedral angle of 109.5°. The long spacer between the nodes results in a very large cavity within the adamantane unit, with maximum dimensions of 50.111(4) × 50.111(4) × 21.775(3) Å3 (the diagonal distances between nodes across the unit along the (31j0), (130), and (001) directions, corresponding to 10a, 10a, and c, respectively). It has been well-known that diamond networks tend to interpenetrate to fill the voids within a single net. In 4, the large cavities are filled via 5-fold interpenetration. In a “normal” n-fold interpenetrated diamond net, the interpenetrating nets are related by the translation of 1/n of the diagonal distance across the adamantane unit along a shared 2-fold axis.14 The five interpenetrating nets in 4 are also translationally equivalent, but the translation is along the a and b directions, which are not the axes of the adamantane unit. The unit translation vectors are equal to the unit cell dimensions (Figure 5c). In other words, the unit cell translations of a net along a and b directions generate the four nets interpenetrating with the first net. This “abnormal” 5-fold interpenetration is similar to that found in a few hydrogen-bonded networks and coordination polymers.15,16 Despite the 5-fold interpenetration, PLATON calculations12 suggest that there are large solvent accessible cavities in the structure, the void volume comprising 23.2% of the crystal volume (1266.6 Å3 per unit cell). A close inspection into the structure indicates that the cavities are not open but enclosed in the framework. Because of the limited quality of the diffraction data, we were unable to locate the guest molecules in the structure, but elemental and thermogravimetric analyses suggest that water and py molecules are enclosed in the cavities. Structural Discussion. Up to now, we have synthesized five coordination polymers with STDC as bridging ligand and py as terminal ligand, including a Zn(II) compound ([Zn(STDC)(py)2] · py, 5) we described earlier.7b All these compounds, with different metal centers (Cu(II), Zn(II), Cd(II), Co(II), and Ni(II)), were synthesized under similar conditions, but they exhibit distinct coordination networks. It is worthwhile to make comparative analyses on these structures to gain some information, although the exact correlation between the structures obtained and the metal ions used is impossible at the current stage. In all these compounds, each metal ion is decorated by two cis- or trans-coordinated py molecules, which play an important role in determining the networks. The Cu(II) compound is unique among these compounds because Cu(II)

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Figure 6. View of the zigzag chain in the previous Zn(II) compound (5)7b for comparison with compound 1.

prefers the square pyramidal five-coordination. The coordination of two trans py molecules at basal positions and a water molecule at the apical position leaves only two basal trans positions for the STDC ligand, which assumes the bis(monodentate) bridging mode to give rise to a quasi-linear coordination chain. For Zn(II), the py ligands occupy two cis positions, and thus the STDC ligand can take the bis(chelating) mode to fulfill the six-coordination and to link the metal ions into a zigzag chain. It is worth noting that the carboxylate group in 5 adopts a highly asymmetric bidentate mode with the Zn-O distances being 1.94 and 2.76 Å.7b Neglecting the very weak interactions, the coordination geometry becomes tetrahedral (Figure 6). This may be a reflection of the preference of Zn(II) to tetrahedral coordination, which accounts for the cis arrangement of the two py molecules. In the other three compounds, the six-coordination of the metal ions is completed by forming binuclear motifs, which are four-connecting in favor of high-dimensional networks. In 2 and 3, the axial positions of the coordination geometry are occupied by the py ligands, leaving four equatorial positions for the bridging and chelating carboxylates. This feature, combining with the quasi-linearity of the STDC ligand, defines the binuclear motif as a planar building unit, which leads to 2D (4,4) layers. On the other hand, the cis arrangement of the py ligands and the incorporation of a water bridge in 4 dictate that the STDC ligands of the binuclear motif radiate in four noncolinear directions, and thus define the binuclear motif as a tetrahedral building unit, which leads to a 3D diamond network. Both the 2D and 3D networks exhibit interpenetration due to the long STDC ligands. IR Spectra, Thermal Stability, and Guest Inclusion. The IR spectra of 1 and 4 display very broad absorption bands around 3400 and 3425 cm-1, respectively, attributable to the ν(O-H) vibration of the hydrogen-bonded water molecules in the compounds. The broad shape of the band makes it difficult to distinguish the coordinated water molecules from the lattice ones in 4. All compounds exhibit characteristic asymmetric (νas) and symmetric (νs) absorptions of the carboxylate groups. The difference between νas and νs (∆ ) νas - νs) has been widely used as a diagnosis of the coordination mode of the carboxylate group. Generally, the monodentate carboxylate exhibit a much larger ∆ value than the chelating one, and the value for the bridging mode is intermediate.17 In 1, the νas(COO) and νs(COO) vibrations appear as strong bands at 1600 and 1378 cm-1, respectively. The large ∆ value (νas - νs ) 222 cm-1) is consistent with the monodentate coordination of the group, as revealed by structural analysis. In 2 and 3, there are two types of carboxylate groups in different coordination modes. The strong bands at about 1600 and 1535 cm-1 are assignable to the asymmetric vibrations of the carboxylate groups in bridging and chelating modes, respectively. The very strong band at about 1400 cm-1 should be due to the symmetric vibrations, and the broad shape of this band may suggest that the absorptions of the two different coordination modes are enveloped. The ∆

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Ma et al.

Figure 7. TGA curves for compounds (a) 2, (b) 3, and (c) 4.

values for the bridging and chelating modes are about 200 and 135 cm-1, respectively, in good agreement with those for similar compounds.17 In 4, there are also two types of coordinated carboxylate groups. One is monodentate and should be responsible for the absorption at 1630 cm-1 (νas), while the other acts as bridge, for which the νas absorption is observed at 1605 cm-1. The νs(COO) absorption in 4 also appears as a very strong envelope band, which appears at about 1390 cm-1. The ∆ values of about 215 and 240 cm-1 are consistent with the bridging and monodentate modes, respectively. Thermogravimetric analyses (TGA) were performed on compounds 2, 3, and 4 (We were unable to obtain pure sample of 1, although great efforts have been made). As shown in Figure 7, complex 2 exhibits a large weight loss of 33.5% at 100-300 °C, corresponding to the loss of all the py molecules in the structure (calcd, 34.8%). The weight loss beginning at 380 °C corresponds to decomposition of the framework. The thermal stability of 3 is similar, with the weight loss of 35.7% in the range of 100-300 °C corresponding to the loss of py molecules (calcd, 37.8%). TGA of 4 suggests that the release of the guest molecules begins at a higher temperature (about 150 °C), perhaps due to the nonopenness of the cavities. The total weight loss (37.2%) of 4 up to 380 °C corresponds to the loss of all the guest and coordinated molecules (water and py, calcd. 37.7%), in agreement with the analytic data. The powder X-ray diffraction (PXRD) pattern for the evacuated sample of 2 obtained by heating 2 at 160 °C under a vacuum for 24 h is different from that of the as-synthesized sample (Figure 8), suggesting the occurrence of certain structural transformation upon evacuation. However, the original structure can be recovered by reabsorbing the guests: after the evacuated solids being immersed in a mixture of py and H2O (1/3 v/v) at room temperature for 10 h, the PXRD pattern of the resulting sample is coincident with that for the as-synthesized sample. This suggests that 2 is a dynamic microporous material that can undergo reversible structural transformation upon the removal/inclusion of guest molecules. PXRD measurements suggest that compound 3 become amorphous upon evacuation (160 °C under a vacuum for 24 h). The crystallinity cannot be recovered by immersing the evacuated solids in py/H2O. This indicates that the structure of 3 undergoes permanent collapse upon evacuation. Similar studies suggest that compound 4 also become amorphous upon evacuation (160 °C under a vacuum for 24 h)

Figure 8. PXRD patterns of 2: (a) simulated from the single-crystal data, (b) for the as-synthesized sample, (c) for the evacuated sample, and (d) for the resolvated sample.

and that the original phase cannot be recovered by immersing the evacuated solids in py/H2O. Conclusions We have synthesized and described a new series of coordination polymers of different divalent metal ions (Cu(II), Cd(II), Co(II), and Ni(II)) with STDC and py as bridging and terminal ligands, respectively. The structures of these compounds, except for the isomorphous Cd(II) and Co(II) species, are distinct from one another and from the previous Zn(II) compound,7b in which single Zn(II) ions are linked into zigzag chains. In compound 1, single Cu(II) ions are linked into quasi-linear coordination chains, which are assembled through hydrogen bonds. The Cd(II) and Co(II) compounds (2 and 3) contain 2-fold interpenetrated (4,4) layers with doubly carboxylate-bridged binuclear motifs as planar nodes. Compound 4 exhibits an abnormal 5-fold interpenetration of diamond networks, with triply (carboxylate and water) bridged binuclear motifs as tetrahedral building units. This series of coordination polymers of different metal ions with the same bridging and terminal ligands highlights the great challenges faced by crystal engineering. Although it is impossible to rationalize the formation of these distinct structures, phenomenologically, the different disposition of the coordinated py (and water in 1) molecules around metal ions is important in generating the structures. According to thermogravimetric and PXRD studies, compounds 3 and 4 undergo permanent framework collapse upon evacuation, whereas 2 exhibits reversible framework transformation upon evacuation/absorption of the guest molecules. Acknowledgment. The authors thank NSFC (20571026, 20771038, and 20490210), MOE (NCET-05-0425), Shanghai Leading Academic Discipline Project (B409), and STCSM (06SR07101) for financial support. Supporting Information Available: X-ray crystallographic information files (CIF) are available for compounds 1-4. This material is available free of charge via the Internet at http://pubs.acs.org.

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