A Three-Dimensional (42.84)-lvt Topology

May 27, 2009 - Tianjin 300071, P. R. China, and Liuzhou Medical College, Liuzhou, Guangxi 545006, P. R. China. ReceiVed NoVember 10, 2008; ReVised ...
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A Three-Dimensional (42.84)-lvt Topology and a Two-Dimensional Brick-Wall Network: Two Pillared Supramolecular Isomers Exploring the Use of L-Cysteic Acid to Engineer Porous Frameworks

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 3191–3196

Fu-Ping Huang,†,‡ Hai-Ye Li,†,§ Jin-Lei Tian,‡ Wen Gu,‡ Yi-Min Jiang,*,† Shi-Ping Yan,*,‡ and Dai-Zheng Liao† College of Chemistry and Chemical Engineering, Guangxi Normal UniVersity, Guilin, 541004, P. R. China, Department of Chemistry, Nankai UniVersity, Tianjin 300071, P. R. China, and Liuzhou Medical College, Liuzhou, Guangxi 545006, P. R. China ReceiVed NoVember 10, 2008; ReVised Manuscript ReceiVed May 6, 2009

ABSTRACT: The preparations Via variation of the reactive conditions of the same metal-to-ligand ratios of Co(NO3)2 · 6H2O with mixed L-cysteic acid (H2cys) and 4,4′-bipyridine (bpy) ligands gave rise to two interesting pillared supramolecular isomers: {[Co2(cys)2(bpy)2(H2O)2] · 3H2O}n (1) and {[Co2(cys)2(bpy)(H2O)4](bpy) · H2O}n (2), which have the same formula but distinct solid phases. In the two complexes, Co(II) centers are chelated and bridged by different µ2-cys bridges to form a one-dimensional (1D) 41 helical chain and a 1D zigzag chain, respectively. The two chains are further linked by bpy pillars into a novel three-dimensional (42.84)-lvt structrue (1) and a two-dimensional brick-wall structure (2), respectively. The magnetic behavior of compounds 1 and 2 was studied, and it indicated the existence of small antiferromagnetic coupling. Best fit parameters for 1: J ) -0.14 K with R ) 4.77 × 10-4; for 2, J ) -0.64 K with R ) 3.62 × 10-4. Introduction The designed synthesis and characterization of porous metal-organic frameworks (MOFs), containing channels and cavities with well-defined sizes, shapes, and chemical environments, have achieved considerable progress in supramolecular chemistry and material chemistry.1,2 The increasing interest in this field is justified not only because of their tremendous potential applications in gas storage, chemical separations, ion exchange, microelectronics, nonlinear optics, and heterogeneous catalysis, but also owing to their intriguing variety of architectures and topologies.3–6 Consequently, a series of open MOFs with various structural motifs, including honeycomb, brick-wall, bilayer, ladder, herringbone, diamondoid and rectangular grid, have been deliberately designed and discussed in comprehensive reviews by Yaghi, Kitagawa, Rao, Chen, and their co-workers.7 An effective strategy for obtaining such extended porous frameworks is to utilize metal complexes and suitable linkers as building blocks.2b,8 On the other hand, supramolecular isomerism refers to a crystalline material that adopts two or more architectures but have identical chemical compositions in terms of building blocks and molecular connectivity; that is, the compound manifests more than one topology.9 As such, the different topologies for the compounds provide an invaluable insight into the factors that govern the self-assembly and growth of crystals.9,10 In the architectural isomerism, the connectivities of the organic building blocks as well as the geometries of metal ions [or metalcluster secondary building units (SBUs)] represent two crucial factors that invariably determine the outcome of the topology of the final structure.10 Fortunately, in this context, we use the same metal-to-ligand ratios of Co(NO3)2 · 6H2O, L-cysteic acid (H2cys) and 4,4′* To whom correspondence should be addressed. E-mail: huangfp2006@ 163.com (Y.J.); [email protected] (S.Y.). † Guangxi Normal University. ‡ Nankai University. § Liuzhou Medical College.

bipyridine (bpy) to synthesize two polymeric supramolecular isomers: {[Co2(cys)2(bpy)2(H2O)2] · 3H2O}n (1) and {[Co2(cys)2(bpy)(H2O)4](bpy) · H2O}n (2), respectively. Complex 1 is an interesting three-dimensional (3D) 2-fold interpenetrated network, and it contains a racemic motif which originated from five interwoven helices. From the topological point of view, the 3D net exhibits a four-connected lvt network with short and long Schla¨fli symbols (42.84) and (4.4.84.84.88.88), which is obviously different from other well-known four-connected diamond, paddlewheel, quartz dual, NbO, PtS, CdSO4, SrAl2 topologies.11 Complex 2 is a two-dimensional (2D) brick-wall structure based on one-dimensional (1D) zigzag Co-cys chain, which is further assembled Via interlay hydrogen bonds into a 3D pillared-layer supermolecular structure, hosting the uncoordinated bpy molecules in cavities. Experimental Section Materials. L-Cysteic acid was prepared according to the literature.12 All reagents were used as received without further purification. IR spectra were taken on a Perkin-Elmer spectrum One FT-IR spectrometer in the 4000-400 cm-1 region with KBr pellets. Elemental analyses for C, H, N, and S were carried out on a model 2400 II, Perkin-Elmer elemental analyzer. The magnetic susceptibility measurements of the polycrystalline samples were measured over the temperature range of 2-300 K with a Quantum Design MPMS-XL7 SQUID magnetometer using an applied magnetic field of 1000 Oe. Field dependences of magnetization were measured using a flux magnetometer in an applied field up to 50 kOe generated by a conventional pulsed technique. A diamagnetic correction to the observed susceptibilities was applied using Pascal’s constants. TG-DTA tests were performed on a Perkin-Elmer thermal analyzer from room temperature to 800 °C under N2 atmosphere at a heating rate of 5 °C/min. Synthesis of {[Co2(cys)2(bpy)2(H2O)2] · 3H2O}n (1). A mixture of L-cysteic acid (0.094 g, 0.5 mmol), Co(NO3)2 · 6H2O (0.146 g, 0.5 mmol), KOH (0.056 g, 1 mmol), bpy (0.078 g, 0.5 mmol), water (10 mL), and methanol (5 mL) was stirred for 30 min in air, then transferred and sealed in an 25 mL Teflon-lined autoclave, which was heated at 100 °C for 6 days. After slow cooling to the room temperature, red block-like crystals of 1 were obtained in 15% yield. Elemental analysis for C26H36Co2N6O15S2 (%), Calcd: C, 36.54; H, 4.25; N, 9.83; S, 7.50;

10.1021/cg801243a CCC: $40.75  2009 American Chemical Society Published on Web 05/27/2009

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Huang et al. graphite monochromatic Mo-KR radiation (λ ) 0.71073 Å). The data were collected at 291(2) K. Absorption effects were corrected by semiempirical methods. The structure was solved by direct methods with the program SHELXS-9713 and refined by full-matrix least-squares methods on all F2 data with SHELXL-97.14 The non-hydrogen atoms were refined anisotropically. Hydrogen atoms of water molecules were located in a difference Fourier map and refined isotropically in the final refinement cycles. Other hydrogen atoms were placed in calculated positions and refined by using a riding model. The final cycle of fullmatrix least-squares refinement was based on observed reflections and variable parameters. Crystallographic data for 1 and 2 are given in Table 1. Selected bond lengths and bond angles are given in Table 2. The crystallographic data of 1 and 2 in CIF format have been deposited in the Cambridge Crystallographic Data Center (CCDC reference numbers 704828 and 704829). These data can be obtained free of charge Via www.ccdc.cam.ac.uk/conts/retrieving.html; e-mail: [email protected].

Table 1. Crystal Data and Structure Refinement for Complexes 1 and 2 complex

1

2

empirical formula formula weight crystal system space group

C26H36Co2N6O15S2 854.59 orthorhombic Pbca

a (Å) b (Å) c (å) R (°) β (°) γ (°) volume (Å3) Z calculated density (Mg/m3) crystal size (mm3)) F(000) θ range for data collection reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data)

19.244 (3) 18.468 (3) 19.444 (3) 90.00 90.00 90.00 6910.5 (19) 8 1.643 0.17 × 0.10 × 0.07 3520 2.21 to 25.50 50689 6433 R(int) ) 0.108 6433/0/460 1.03 0.0690 0.2202

C26H36Co2N6O15S2 854.59 triclinic P1 (Flack parameter 0.002(9)) 7.5630(13) 10.0792(15) 13.163(2) 69.370(6) 81.226(7) 69.133(5) 877.0(2) 1 1.618 0.48 × 0.30 × 0.20 440 2.29 to 27.48 6653 5057 R(int) ) 0.011 5057/6/499 0.99 0.0234 0.0577

Results and Discussion Crystal Structure. {[Co2(cys)2(bpy)2(H2O)2] · 3H2O}n (1). Complex 1 crystallizes in space group Pbca with a [Co2(cys)2(bpy)2(H2O)2] coordinated unit and three lattice water molecules in an asymmetric unit, as shown in Figure 1. The Co1 and Co2 centers both exhibit identically distorted octahedral geometries, in which deprotoned cys chelates a Co(II) ion through an amino N atom and a carboxylic O atom. The remaining two sites in the basal plane are occupied by two N atoms from different bpy ligands, and the two axial positions are occupied by a sulfonate O atom and a coordinated water molecule. All the metal-ligand bonds are consistent with other Co(II) systems, except for the Co-O (sulfonate) bonds distances which are relatively longer, 2.250 and 2.282 Å. However, compared to similar derivatives of Co analogues having a sulfonate substituent,15 these bonds in 1 are still reasonable. The L-cysteic moiety acts as a multidentate ligand, linking two cobalt atoms to form a 1D 41 helical chain, which may come from the different coordination environments of Co1 and Co2. The helix is generated along the crystallographic c axis

found: C, 36.69; H, 4.72; N, 9.48; S, 7.11. IR (KBr): 3433s, 1654s, 1641s, 1427m, 1218m, 1193m, 1036s, 853w, 606w. Synthesis of {[Co2(cys)2(bpy)(H2O)4](bpy) · H2O}n (2). To an aqueous solution (10 mL) of L-cysteic acid (0.094 g, 0.5 mmol) and KOH (0.056 g, 1 mmol), Co(NO3)2 · 6H2O (0.146 g, 0.5 mmol) in methanol (5 mL) was added slowly. The reaction solution was stirred for 30 min in air, and then bpy (0.078 g, 0.5 mmol) in methanol (5 mL) was added. The mixture was further stirred for 6 h at room temperature and filtered. Red prism-like crystals of 2 were obtained from the filtrate after several days in 45% yield. Elemental analysis for C26H36Co2N6O15S2 (%),Calcd: C, 36.54; H, 4.25; N, 9.83; S, 7.50; found: C, 36.72; H, 4.27; N, 9.09; S, 7.84. IR (KBr): 3435s, 1607s, 1582s, 1418m, 1212s, 1197m, 1097m, 1046s, 816m, 574w. X-ray Structure Determination. All the data for complexes 1 and 2 were collected with a Bruker SMART CCD instrument by using

Table 2. Selected Bond Lengths (Å) and Angles (°) for 1 and 2 1a Co1sO6 Co1sN6 Co2sO1 Co2sN3 O6sCo1sN6 O6sCo1sN2 N6sCo1sN2 O6sCo1sO12 N6sCo1sO12 N2sCo1sO12 O6sCo1sN4 N6sCo1sN4 N2sCo1sN4 O12sCo1sN4

2.044 (5) 2.111 (6) 2.044 (6) 2.108 (6) 79.4 (2) 171.3 (2) 96.1 (2) 95.0 (2) 88.1 (2) 92.3 (2) 94.1 (2) 173.4 (2) 90.4 (2) 91.7 (2)

Co1sN2 Co1sO12 Co2sN1 Co2sO11 O6sCo1sO4A N6sCo1sO4A N2sCo1sO4A O12sCo1sO4A N4sCo1sO4A O1sCo2sN3 O1sCo2sN1 N3sCo2sN1 O1sCo2sO11 N3sCo2sO11

2.132 (6) 2.146 (5) 2.113 (6) 2.124 (7) 86.4 (2) 91.5 (2) 86.3 (2) 178.4 (2) 88.8 (2) 80.0 (2) 167.4 (3) 93.0 (2) 96.4 (3) 88.8 (2)

Co1sN4 Co1sO4A Co2sN5C Co2sO10B N1sCo2sO11 O1sCo2sN5C N3sCo2sN5C N1sCo2sN5C O11sCo2sN5C O1sCo2sO10B N3sCo2sO10B N1sCo2sO10B O11sCo2sO10B N5C-Co2sO10B

2.154 (6) 2.250 (5) 2.131 (7) 2.282 (6) 93.9 (3) 94.3 (2) 174.0 (2) 92.9 (2) 89.9 (3) 82.9 (2) 93.4 (2) 87.1 (2) 177.5 (2) 87.9 (2)

2.106 (2) 2.135 (3) 2.115 (2) 2.136 (2) 89.1 (1) 90.1 (1) 92.1 (9) 174.5 (1) 94.6 (1) 174.2 (1) 88.0 (1) 90.2 (1) 77.4 (1) 104.1 (1)

Co1sN2 Co1sN3 Co2sN4B Co2sO4W O3W-Co2sN1A O5AsCo2sN4B O10sCo2sN4B O3W-Co2sN4B N1AsCo2sN4B O5AsCo2sO4W O10sCo2sO4W O3W-Co2sO4W N1AsCo2sO4W N4B-Co2sO4W

2.145 (2) 2.162 (3) 2.147 (3) 2.154 (2) 165.1 (1) 93.6 (1) 91.9 (1) 88.6 (1) 95.1 (1) 86.6 (1) 87.8 (1) 86.7 (1) 89.6 (1) 175.3 (1)

2b Co1sO4 Co1sO1W Co2sO5A Co2sO10 O4sCo1sO1W O4sCo1sO9 O1W-Co1sO9 O4sCo1sO2W O1W-Co1sO2W O9sCo1sO2W O4sCo1sN2 O1W-Co1sN2 O9sCo1sN2 O2W-Co1sN2

2.051 (2) 2.090 (3) 2.097 (2) 2.104 (2) 91.6 (1) 178.5 (1) 89.5 (1) 85.8 (1) 87.9 (1) 93.1 (1) 100.5 (8) 167.2 (1) 78.4 (1) 88.5 (1)

Co1sO9 Co1sO2W Co2sO3W Co2sN1A O4sCo1sN3 O1W-Co1sN3 O9sCo1sN3 O2W-Co1sN3 N2sCo1sN3 O5AsCo2sO10 O5AsCo2sO3W O10sCo2sO3W O5AsCo2sN1A O10sCo2sN1A

1 Symmetry codes: A: -x, y + 1/2, -z + 3/2; B: x + 1/2, -y + 1/2, -z + 1; C: -x + 1/2, -y + 1, z - 1/2. b 2 Symmetry codes: A: x, y - 1, z; B: x, y, z - 1. a

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Figure 3. (a) Schematic view of the 2-fold parallel interpenetration in 1. (b) A schematic illustration of the racemic motif originated from five interwoven helices.

Figure 1. ORTEP drawing of 1 with thermal ellipsoids at 30% probability.

Figure 4. ORTEP diagram of the coordination environments for the Co atoms in 2.

Figure 2. (a) View of the 3D network of 1 in the ab plane. (b) Schematic diagram of the extended net with 4284 topology. (c and d) Schematic representation of the helical arrangement of 1.

with a pitch length of 19.444 Å. The helical chain as a 1D subunit are connected by bpy pillars to generate a novel 3D open framework featuring channels with dimensions of about 11 × 11 Å, in which six guest water molecules are hosted (Figure 2a). Here, the bpy molecules also link Co(II) centers to form a 41 helical chain. From the topological point of view, this 3D network can be simplified by considering the cobalt atoms as similar 4-connected nodes, cys and bpy as linkers. The resulting net, shown in Figure 2b, is a four-connected 3D uniform net. The succeeding topology analysis by topos40 program16 suggests the 4-connected net with the 42 84 topology symbol. Analysis of the vertex symbols and coordination sequence of 1 reveals that the metal nodes result in the vertex symbols (4.4.84.84.88.88), and their unique coordination sequence confirms the identity of the generated network topology as lvt. Compared with the well-known four-connected 66-dia, (65.8)-cds, (64.82)-nbo and (42.63.8)-sra nets, the (42 84)-lvt net is very rarely reported.17

Very intriguingly, the 3D polymer is alternatively assembled by the two types of helices:18 the Cocys chain and Cobpy chain which display an opposite helical orientation to the former helix are shown in Figure S5, Supporting Information. These two distinct helices are in an orderly arrangement with the cobalt atoms functioning as hinges to form a 3D highly ordered helical array. It is interesting to note that the void space in the single framework is so large that two identical 3D frameworks interpenetrate each other to form a novel 2-fold interpenetration of the architecture, leaving a small space for the inclusion of solvent molecules. Viewing along the c direction, there are four types of helical chains coexisting in the 3D interpenetrated network (two racemic Cocys chains and two racemic Cobpy chains). If we choose a Cobpy helical chain from a single network, then there are four equivalent helices appearing from the other net interweaving it in all directions. As presented in Figure 3b, the five independent homochiral helices are interlocked in a manner similar to that reported by Lin19 and Wang.20 {[Co2(cys)2(bpy)(H2O)4](bpy) · H2O}n (2). Interestingly, when the same metal-to-ligand ratios of Co(NO3)2 · 6H2O with the mixed L-cysteic acid and 4,4′-bipyridine ligands were used in a reaction at room temperature, complex 2, having same formula but a distinct solid phase21 compared with 1, is obtained. Complex 2, crystallized in space group P1, is a pillared layer complex based on Cocys zigzag metal-organic chain. There are two crystallographically independent Co(II) centers in the linear subunit (Figure 4). Both of them coordinated by two carboxylate oxygen atoms from two different cys ligands,

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Figure 5. (a) View of the 2D brick-wall structure of 2 in the bc plane (uncoordinated bpy and water molecules are omitted for clarity). (b) The 2D hydrogen bonded layered structure in the ac plane.

Figure 7. The reduced magnetization curve, M(H) for complex 2, saturating at MS ) 4.7Nβ, as expected (4-6Nβ),24,29 follow practically the Brillouin function, indicating that the magnetic coupling is very small. (The inset shows the plot of hysteresis loop at 2 K.)

Figure 6. Plots of the χM product (O and ∆) and χMT (b and 2) vs T for complexes 1 and 2, respectively.

two nitrogen atoms from a cys ligand and a bpy ligand, and two coordinated water molecules. Each cys is coordinated to two Co atoms through a syn-anti carboxylato bridge to form the catenulate subunit. Complex 2 is a 2D brick-wall structure (Figure 5a) based on the 1D zigzag Cocys subunits, which is further assembled Via interchain hydrogen bonds [O1W · · · O10i ) 2.926 (4) Å, O2W · · · O7i ) 2.798 (4) Å, O3W · · · O3i ) 2.876 (4) Å, symmetry codes: (i) x - 1, y - 1, z] among coordinated water, the coordinated carboxylic oxygen atom and the uncoordinated sulfonate oxygen atoms of cys, to form a 3D pillared-layer supermolecular structure,22,23 with 1D channels along the a axis. The void space of the crystal volume is occupied by uncoordinated bpy and guest water molecules. Magnetic Properties. The magnetic properties of the complexes 1 and 2 were investigated over the temperature range 2.0-300.0 K, and the resulting data are shown in Figure 6. For 1, the χMT value at 300 K is 6.65 cm3 K mol-1 (7.29 µB), which is much higher than the expected value (3.75 cm3 K mol-1, 5.477 µB) of two isolated spin-only Co(II) ions (S ) 3/2, g ) 2.0). As T is lowered, χMT decreases continuously to a value of 3.22 cm3 K mol-1 at 2 K. This behavior indicates a dominant antiferromagnetic interaction between the Co(II) ions in 1. Compared with 1, there is a little difference in the χMT values of 2. The χMT value of 2 at 300 K is 6.38 cm3 K mol-1 (7.15 µB), which is much higher than the expected value of two isolated spin-only Co(II) ions (S ) 3/2, g ) 2.0). Upon cooling, the χMT value continuously decreases and reaches a local minimum of 3.88 cm3 K mol-1 at about 3 K, accounting for a

overall antiferromagnetic interaction between the Co(II) ions, and then increases to a value 3.96 cm3 K mol-1 at 2 K. This behavior can be qualitatively explained if we bear in mind that high-spin octahedral Co(II) has an orbitally degenerate ground state 4T1. The decrease in χMT down to 3 K basically corresponds to a single-ion behavior, which accounts for the splitting of the 4 T1 term into six Kramers doublets as a consequence of the combined effect of spin-orbit coupling and distortion from ideal octahedral symmetry.24 At low temperature (T < 20 K) only the ground Kramer’s doublet is populated, and it can be regarded as an effective spin doublet, Seff ) 1/2, with a g value in the range 3.8-4.3. So, the value of the effective magnetic moment for this ground Kramer’s doublet is in the range 3.4-3.7 µB (Figure 7).24 Note that these values are smaller than the smallest value observed for 2 (minimum in the χM plot at 3 K, µeff per Co(II) ) 3.94 µB). So, the increase in the values of χMT below 3 K must be attributed to the presence of intramolecular weak ferromagnetism caused by small uncompensated AF spincanting, which may be attributed to both the strong single-ion anisotropy of Co(II) and the asymmetric syn-anti carboxylato bridge.25 To fit the susceptibility data of compounds 1 and 2, the two complexes can be both regarded as 1D uniform Cocys chains, which include only one kind of magnetically isolated Co(II) center. Considering the magnetic exchange coupling through the bpy bridges is expected to be very weak,26 the magnetic behavior of the two complexes was described equivalently in Figure 8. According to the Curie-Weiss law χ ) C/(T - θ) at temperatures in excess of 10 K. The value of the Curie constant, C ) 3.41 cm3 K mol-1 for 1, C ) 3.35 cm3 K mol-1 for 2, respectively, is consistent with the presence of hexacoordinated high-spin Co(II) ions (C ≈ 2.8-3.4 cm3 K mol-1).24b Considering the spin-orbit coupling due to the 4T1g ground state for octahedral Co(II) complexes,27 no exact analytical expression is available in the literature to describe the temperature dependence of χMT for chains of Co(II) ions.28 In the present case, we tried to fit the χMT by using a magnetic susceptibility equation derived by Miller27a and using the relationship given for Ising chains.24,28b However, these models turned out to be unsatisfactory for the two 1D system of Co(II). More recently, Rueff et al.28b,29 have proposed a phenomenological approach for some low dimensional Co(II) systems

3D (42.84)-lvt Topology and 2D Brick-Wall Network

Figure 8. Temperature variation of the magnetic susceptibility of compounds 1 and 2; the unbroken lines correspond to the best fit of the experimental data with the relation given in the text.

which allow an estimate of the strength of the antiferromagnetic exchange interactions. They postulate the phenomenological equation: χMT ) A exp(-E1/kT) + B exp(-E2/kT). In the equation, A + B equals the Curie constant (C ≈ 2.8-3.4 cm3 K mol-1 for octahedral cobalt(II) ions), and E1, E2 represent the “activation energies” corresponding to the spin-orbit coupling and the antiferromagnetic exchange interaction. This equation adequately describes the spin-orbit coupling, which results in a splitting between discrete levels, and the exponential low temperature divergence of the susceptibility [χT ∝ exp(RJ/ 2kT)]. Very good results have been reported in 1D Co(II) complexes.25c,28b Moreover, it is in excellent agreement with the experimental data obtained in the present work (Figure 8). The experimental data of 1 and 2 was analyzed30 using this model over the temperature range about 14-300 K and the obtained values of A + B ) 3.41 cm3 K mol-1 for complex 1 and 3.35 cm3 K mol-1 for complex 2, which perfectly agree with those given in the literature for the Curie constant (C ≈ 2.8-3.4 cm3 K mol-1). E1/k ) 50.01 K (for 1) and E1/k ) 39.92 K (for 2) are of the same magnitude than those reported 1D and 2D Co(II) complexes.25c,29 The value found for the antiferromagnetic exchange interaction, E2/k, is very weak for both 1 and 2: E2/k ) 0.07 K, corresponding to J ) -0.14 K for complex 1 and E2/k ) 0.32 K, corresponding to J ) -0.64 K for complex 2. Thermal Stability of the Supramolecular Isomers. Thermogravimetric experiments were conducted to study the thermal stability of 1 and 2, which is an important parameter for porous MOF materials. As shown in Figure S7, the TG-DTA curve of 1 suggests that there are several mass loss steps, although without a clear plateau. The first observed weight loss of 11.22% in the region of 40-130 °C (peak at 114 °C) corresponds to the dehydration process (calculated 10.63%). The residual framework starts to decompose owing to the expulsion of the lattice water and the coordinated water molecules beyond 135 °C with a series of complicated weight losses (peaks at 205, 307, and 413 °C) and does not stop until heating ends at 800 °C. For the TG-DTA curve of 2 (Figure S8, Supporting Information), a weight loss of 2.60% in the 46-124 °C (peak at 113 °C) corresponds to the release of the lattice water guest (calculated 2.02%). Then the decomposition of the residual components starts beyond 245 °C with two steps of weight

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Figure 9. XPRD patterns for 2 recorded (black) at room temperature, (red) after removal of the guest water molecules, and (blue) after reintroduction of the guest molecules.

losses (peaks at 267 and 318 °C) and does not end until heating stops at 800 °C. Removal and Reintroduction of Guest Solvates. Metalorganic coordination polymers with specific channels or cavities have attracted considerable interest because of their potential applications in catalysis, adsorption, and ion exchange. Complex 2 represents a fascinating 3D supramolecular array exhibiting 1D open cavities filled with bpy and water molecules, as described above. The TGA results suggest that the water guests could be excluded from the host framework upon heating. To further investigate the desorption/absorption properties, a freshly ground sample of 2 (112.6 mg) was placed inside a vacuum oven at 160 °C for 12 h. The sample experienced a weight loss of 2.78 mg, consistent with the removal of all included lattice water solvates per formular unit (calculated, 2.37 mg). The X-ray powder diffraction (XRPD) pattern recorded at this point (Figure 9) shows significant shifts of the sharp diffraction peaks compared with that of the original product. This indicates that the structure of the desolvated solid of 2 is changed. Interestingly, the desolvated sample can regain the guest molecules and partly revert to the original structure by soaking in water for 24 h. This is confirmed by XRPD, the weight gain of the material (3.18 mg). Therefore, the framework integrity of 2 can be partly maintained after a desorption/adsorption cycle, and the desolvated solid of 2 may be useful as a potential adsorbent material for small guest molecules. Conclusions In summary, we have prepared and characterized two interesting pillared supramolecular isomers Via using the same metal-to-ligand ratios of Co(NO3)2 · 6H2O with the mixed L-cysteic acid (H2cys) and 4,4′-bipyridine (bpy) ligands. When exploring the use of chains of amino acid to construct porous solids, we discovered that L-cysteic gave rise to two completely different architectures based on 1D coordination polymeric chains: a 1D helical chain, which is further connected by bpy pillars to generate a novel 3D (42.84)-lvt porous structure, and a 1D zigzag chain, which is further linked by bpy pillars to form a 2D brick-wall structure. Weak antiferromagnetic interactions were observed between the Co(II) atoms in 1 and 2. Acknowledgment. This work was supported by the Natural Science Foundation of Guangxi Province (No. 0731053). Supporting Information Available: Crystallographic data in CIF format, additional figures, IR, XRD and more magnetic data (PDF)

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for compound 1 and 2. This information is available free of charge Via the Internet at http://pubs.acs.org.

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