Influence of Angular Dicarboxylate Ligand on the Structures of Single

Article pubs.acs.org/crystal

Influence of Angular Dicarboxylate Ligand on the Structures of Single and Double Pillared-Layer Coordination Polymers of Co(II) In-Hyeok Park,† Raghavender Medishetty,‡ Hyeong-Hwan Lee,† Tun Seng Herng,§ Jun Ding,*,§ Shim Sung Lee,*,† and Jagadese J. Vittal*,‡ †

Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, South Korea Department of Chemistry, National University of Singapore, 3 Science Drive 3, 11753, Singapore § Department of Materials Science and Engineering, National University of Singapore, 119260, Singapore ‡

S Supporting Information *

ABSTRACT: Two pillared-layer coordination polymeric compounds, namely, [Co2(bpeb)2(obc)2]·DMF·5H2O (1) and [Co2(bpeb)(obc)2]·2DMF·H2O (2), have been synthesized using a long dipyridyl spacer ligand (1,4-bis[2-(4pyridyl)ethenyl]benzene, bpeb), 4,4′-oxybisbenzoate (obc), and Co(NO3)2·6H2O under solvothermal conditions, using appropriate ratios of Co(II) to bpeb. In compound 1, the double pillared-layer structure has a dimeric repeating unit and exhibits rob topology, while single pillared-layer compound 2 has an unusual building block which is a linkage isomer of the well-known paddle-wheel structure and has a pcu topology. Twofold and threefold interpenetrations are observed in 1 and 2, respectively. The variable-temperature magnetic properties of 1 and 2 were also investigated. The double pillared-layer structure of 1 exhibits antiferromagnetic behavior while a relatively rare ferromagnetism has been observed for the single pillared-layer structure of 2.

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INTRODUCTION

Pillared-layer coordination polymers are a class of metal− organic frameworks (MOFs) or porous coordination polymers (PCPs) found to possess interesting properties and potential applications in gas storage and separation.1−9 These structures can be designed from dicarboxylates, linear dipyridyl based spacer ligands, and metal(II) ions. Commonly encountered pillared-layer structures are of two types: single pillared-layer is usually based on paddle-wheel building blocks while double pillared-layer has a slightly different metal(II)-carboxylate dimer core. Both these structures differ in the metal carboxylate to pyridyl spacer ligand ratio. However, when a bent dicarboxylate ligand is used instead of linear dicarboxylates, the topology and connectivity are somewhat unpredictable, but in many occasions similar pillared-layer structures have been retained.10−20 For the given bent carboxylate ligand and linear spacer ligand, it has not yet been demonstrated whether both single pillared-layer and double pillared-layer compounds could be formed to verify the design strategy in these PCPs. Here we have chosen the V-shaped 4,4′-oxybisbenzoate (obc) as the bent carboxylate and 1,4-bis[2-(4-pyridyl)ethenyl]benzene (bpeb, shown in Figure 1) as the linear spacer ligand in an attempt to construct both types of the pillared layer by varying the metal carboxylate and ligand ratio. Herein we report two PCPs, namely, [Co2(bpeb)2(obc)2]· DMF·5H2O (1) and [Co2(bpeb)(obc)2]·2DMF·H2O (2) by solvothermal reaction as depicted in Scheme 1. Pink platy © XXXX American Chemical Society

Figure 1. Structure of bpeb ligand.

Scheme 1. Details of the Formation of Two Different Pillared-Layer Co(II) Coordination Polymers

crystals of 1 were obtained from the reaction of equimolar ratio of Co(NO3)2·6H2O, H2obc, and bpeb in a mixture of dimethyl Received: June 3, 2015 Revised: June 29, 2015

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DOI: 10.1021/acs.cgd.5b00766 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

block [Co(bpeb)(obc)] and related to the other half by crystallographic center of inversion as shown in Figure 2a. The

formamide (DMF), DMSO, and H2O. Under the same experimental conditions, when the ratio of Co(NO3)2·6H2O, H2obc, and bpeb was changed to 2:2:1 dark violet block single crystals of 2 were obtained. Interestingly, 1 and 2 differ distinctly in terms of not only color and shape, but also secondary building units (SBUs). While 1 is a double pillaredlayer PCP, 2 is a single pillared-layer PCP. Further we are able to control the stoichiometry of the product from appropriate ratio of the reactants used. We have also measured the variable temperature magnetic properties of these single and double pillared-layer structures. The details of our investigations are described below.

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EXPERIMENTAL SECTION

General. All chemicals were purchased from commercial sources and used as received. All solvents used were of reagent grade. The bpeb ligand was synthesized by the reported procedure.21 Elemental analyses were carried out on an Elemental Analysis Laboratory, CMMAC, Department of Chemistry, National University of Singapore. Thermogravimetric analyses were recorded in a TA Instruments TGA-Q50 thermogravimetric analyzer. Samples were heated at a constant rate of 5 °C min−1 from room temperature to 700 °C and in a continuous flow nitrogen atmosphere. The FT-IR spectra were recorded using FTS165 Bio-Rad FT-IR spectrometer with KBr pellets. Magnetic susceptibilities were measured in the temperature range 2−300 K on a Quantum Design VSM-SQUID. Preparation of [Co2(bpeb)2(obc)2]·DMF·5H2O (1). A mixture of bpeb (20.2 mg, 0.071 mmol), H2obc (18.3 mg, 0.071 mmol), and Co(NO3)2·6H2O (20.8 mg, 0.071 mmol) dissolved in DMF (3 mL), H2O (1 mL), and DMSO (0.5 mL) were placed in a 5 mL glass tube, and then 2−3 drops of 0.1 M NaOH solution was added. The tube was sealed and kept at 110 °C for 48 h, followed by cooling to room temperature over 8 h. Pink plate-shaped crystals 1 suitable for X-ray analysis were obtained. (37.3 mg, yield 76% based on Co(NO3)2· 6H2O). Anal. Calcd for [C71H65Co2N5O16]: C, 62.60; H, 4.81; N, 5.14. Found: C, 62.20; H, 4.54 N, 5.10%. IR (KBr pellet, cm−1) 3422, 3073, 2926, 1676, 1606, 1548, 1499, 1396, 1238, 1161, 1096, 1014, 1094, 968, 878, 834, 738, 708, 659, 609, and 557. Preparation of [Co2(bpeb)(obc)2]·2DMF·H2O (2). A mixture of bpeb (20.1 mg, 0.070 mmol), H2obc (36.5 mg, 0.141 mmol), and Co(NO3)2·6H2O (41.1 mg, 0.141 mmol) dissolved in DMF (3 mL), H2O (1 mL), and DMSO (0.5 mL) were placed in a 5 mL glass tube, and then 2−3 drops of 0.1 M NaOH solution was added. The tube was sealed and kept at 110 °C for 48 h, followed by cooling to room temperature over 8 h. Dark violet plate-shaped crystals 2 suitable for X-ray analysis were obtained. (47.2 mg, yield 62% based on Co(NO3)2·6H2O). Anal. Calcd for [C54H48Co2N4O13]: C, 60.12; H, 4.48; N, 5.19. Found: C, 60.20; H, 4.08; N, 4.90%. IR (KBr pellet, cm−1) 3429, 3075, 2925, 1664, 1609, 1566, 1501, 1400, 1303, 1236, 1162, 1099, 1063, 1013, 972, 879, 839, 780, 696, 617, and 557. Crystallographic Structure Determinations. Crystal data for 1 and 2 at 173 K were collected on a Bruker SMART APEX II ULTRA diffractometer equipped with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) generated by a rotating anode. The cell parameters for the compounds were obtained from a least-squares refinement of the spot (from 36 collected frames). Data collection, data reduction, and absorption correction were carried out using the software package of APEX2.22 All of the calculations for the structure determination were carried out using the SHELXTL package.23,24 CCDC 1402417 (1) and 1402418 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif.

Figure 2. Crystal structure of 1: (a) Connectivity and atom labeling for the building block. Symmetry code: (A) 1 − x, 1 − y, 1 − z. (b) A portion of the (4,4) rhombic grid formed by [Co2(obc)2]. (c) Parallel interpenetration between highly corrugated grids. (d) Side view of the two interpenetrating [Co2(obc)2] layers. The hydrogen atoms and the minor disordered atoms are omitted for clarity.

Co(II) center is six-coordinate, being bound to four carboxylate oxygen atoms in a distorted square-plane from three different obc ligands of which one is in a chelating fashion and the other two bridge a pair of Co(II). The octahedral coordination sphere is completed by two bpeb ligands via nitrogen atoms (Figure 2a). Such a M2(O2C-R)4 unit is well-known secondary building structure in many MOFs.14,15 The connectivity extends to form a (4,4) rhombic grid 2D layer in the bcplane (Figure 2b). The diagonal distances between the centers of Co2 pairs are 26.54 and 18.43 Å. As a result of large cavity, parallel interpenetration occurs between two such corrugated grids as shown in Figure 2c and d. Due to corrugation of the 2D layer arising from the bent obc ligand, the neighboring bpeb ligand pairs are crisscrossed with respect to each other by ca. 114°. The bridging by the bpeb ligands between the doubly interpenetrated [Co2(obc)2] layers resulted in a 3D structure. The doubly interpenetrated structure has a rob topology25,26 (Figure 3a and b). The total potential solvent area volume as calculated by PLATON27−29 is 1305.3 Å3 which is 31.3% of the unit cell volume of 4168.3(5) Å3. Since we were not able to locate the guest solvents, they were removed using the program SQUEEZE29 in the least-squares refinement cycles. Further highly disordered ligands arising from rotation of 6-membered rings (Figure S5 in the Supporting Information) were observed.

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RESULTS AND DISCUSSION Compound 1 crystallized in monoclinic space group P2/c with Z = 2. The asymmetric unit contains half of the dimer building B



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Figure 3. (a) Topological representation of the connectivity in 1. Color code: Yellow − Co(II) pairs; Blue − a pair of bpeb ligands; Pink − obc ligand. (b) Schematic representation showing the interpenetration in 1.

The bpeb ligand pairs bonded to the [Co2(obc)2] units are parallel and separated by Co···Co distance of 4.11 Å, but the atoms are slip-stacked such that N atom of one pyridyl ring close to the center of the neighboring pyridyl ring by 3.94 Å. Further, the bpeb ligands exhibit a trans−cis−trans conformation (Figure S8) and the adjacent CC bonds pairs are crisscrossed with a distance of 4.11 Å between the center of the olefin bonds (Figure S7). Pedal motion has been found to favor parallel orientations of the olefin bonds to satisfy the Schmidt’s criteria for [2 + 2] photochemical cycloaddition reactions.30−36 However, crystal 1 was found to be photoinactive. Well-aligned olefin bonds that satisfy the Schmidt’s criteria in many crystals containing transition metal ions have been found to be photoinactive.37,38 A Cambridge Structural Database (CSD)39 search reveals that there are at least six hits for the [M2(N−N)2(obc)2] SBU observed in 140 and similar SBU41,42 showing M2(obc)2 layer structures. The crystal structure of [Ni2(bpy)2(obc)2] reported by Natarajan and co-workers has similar topology with parallel double interpenetration of [Ni2(obc)2] layers.43 Compound 2 crystallized in monoclinic space group P21/n with Z = 4. The asymmetric unit contains the whole formula units as shown in Figure 4a. In 2, each Co(II) is chelated by a obc ligand and bridged by two obc ligands. Further, one of the oxygen atoms, O6, is bonded to Co2. The coordination geometry is completed by a nitrogen atom of the bpeb ligand. Overall, Co1 has distorted trigonal bipyramidal geometry with O1 and O7 occupying the axial positions and highly distorted octahedral geometry has been found at Co2. This is an isomer of paddle-wheel structure with two carboxylates chelating each metal center instead of bridging. The [Co2(O2C−C)4] units are further connected to four more units by the obc ligands to furnish a (4,4) square-grid structure in the ab-plane (Figure 4b). The bent obc ligand makes this highly corrugated Co2(obc)2 layer which looks like it has hexagonal cavities when viewed from the c-axis. These layers are further pillared by the bpeb ligands to yield a pillaredlayer 3D structure having a pcu topology (Figure 5).24−26 The dimension of the (4,4) grid is 14.57 × 13.73 Å (distance between the centers of the building blocks) while the Co-bpebCo distance is 19.86 Å. The packing efficiency is maximized by threefold interpenetration. Further, the disordered bpeb ligand has both trans−trans−trans and trans−cis−trans conformation (51:49 ratio). The total potential solvent area volume calculated by PLATON27−29 is 1321.0 Å3 which is 26.2% of the unit cell volume of 5037.6 Å3. Like 1, the voids in 2 are occupied by DMF and water molecules (Figure S6). Based on these two structures it may be concluded that the change in the metal−ligand ratio retains the 3D network

Figure 4. (a) Asymmetric unit in 2 showing the coordination geometry and labeling of selected atoms. (b) View of the corrugated [Co2(obc)2] layer with [4,4] grid structure. (c) A portion of the pillared-layer structure of 2. Only selected atoms are shown for clarity.

Figure 5. (a) Schematic diagram showing the cubic topology in 2. Color codes: Pink − Co(II) building block; yellow − obc ligand; blue − bpeb ligand. (b) Schematic representation of the threefold interpenetration in 2.

structure but changes its topology as expected. Although the bent obc ligand does not favor necklace rings leading to the formation of 2D sheets, as found in another bent ligand, 4,4′sulfonyldibenzoate (sdb)44,45 and products 1 and 2 are dictated by the mole-ratio of the reactants used. Both these compounds 1 and 2 produce 2D layer structure of [Co2(obc)2] despite differences in the composition of metal:bpeb ratio and nature of the building blocks. A quick CSD search involving obc ligands and transition(II) metal ions with the same composition of 2 reveals that this M2(obc)2 layer structure was observed only in MOFs including bpeb out of at least 11 hits.39 Further, the SBU C



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results indicate a relatively large uncompensated magnetic moment at lower temperature. The χMT curves for 1 and 2 are shown in Figure 7. At higher temperature above 80 K, the The χMT increases for 1 but

observed in 2 seems to be very unique and observed at least in two PCPs.46,47 When a linear dicarboxylate such as 2,6-naphthalene dicarboxylate (ndc) was used along with bpeb ligand, a paddle-wheel repeating unit was favored in 2.48 In this reported structure, bpeb-Zn-Zn-bpeb undulating chains interweave over ndc ligand to form a fabric sheet. On the contrary, the undulating [Co2(obc)2] sheets from the bent obc ligands in 1 form parallel interpenetration and dominates the packing. Pillared-layer Co(II) structures have exhibited interesting magnetic properties and hence they were investigated for 1 and 2.49,50 Temperature-dependent magnetic susceptibility measurements of 1 and 2 were performed on the polycrystalline powder samples using a SQUID magnetometer. The magnetic susceptibilities, χM of 1 were measured in the temperature range 2−300 K. Figure 6 shows χM−1 versus T and χM(T + 80) versus

Figure 7. χMT versus T plots for 1 and 2.

slowly decreases for 2. This is expected as 1 is antiferromagnetic with a negative Curie temperature and 2 is a ferromagnetic with a low but positive Tc. Whereas the plots of χM(T + 80) in Figure 6 shows constant values in the temperature range of 200−300 K. The temperature dependence of the magnetic susceptibility of 2 in the form of χM−1 versus T is shown in Figure 8a. The

Figure 6. Temperature dependence of χM−1 of 1 at H = 1 kOe from 2 to 300 K (above) and χM(T + 80) versus T plot (below).

T plots. The sample exhibited antiferromagnetic behavior with a Weiss temperature at −80 K. The fitting using the Curie− Weiss formula resulted in a magnetic moment 4.8 Nβ for Co(II) ion which is close to the spin-only Co(II) cation (for S = 3/2 magnetic moment = 3.75 Nβ). Interestingly, the χMT values reported for two coordination polymers with similar repeating units are 5.40 and 5.04 Nβ, which are larger than the expected value.51 The deviation from the linear relationship of χM versus T in Figure 6a is certainly associated with the relatively high ordering temperature. The magnetic ordering temperature (Neel temperature) is expected to be in the range 45−50 K. The M−H curve measured at 50 K is in good agreement for an antiferromagnetic Co(II) compound and relatively large magnetization was measured at 2 K. These

Figure 8. (a) Temperature dependence of χM−1 of 2 at H = 1 kOe from 2 to 300 K. (b) Field dependence of magnetization of 2 at T = 2 K. D



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magnetic susceptibility can be fitted with the Curie−Weiss equation showing ferromagnetism with Curie temperature at 3 K. The fitting of Curie constant resulted in a magnetic moment of 3.3 Nβ for Co(II), which is slightly lower than the value expected for the two independent Co(II) centers (3.75 Nβ for S = 3/2 assuming g = 2.0). At 2 K, the sample exhibited high magnetization of 2.2 Nβ (Figure 8b). However, in other single pillared-layer coordination polymeric materials with paddle-wheel building blocks, antiferromagnetic behavior were observed.52−54 A few cobalt(II) coordination polymers have been found to show ferromagnetic behavior.55−57

CONCLUSION In conclusion, a long dipyridyl spacer ligand (bpeb) and a Vshaped bent ligand have been employed to successfully synthesize single and double pillared-layer cobalt(II) MOFs using appropriate stoichiometry of the reactants. Interestingly, stoichiometry is not retained in most of the reported cases, and when it is retained, they do not have the pillared-layer structure.58−63 It appears that the long spacer ligand bpeb is responsible for the formation of these structures. The double pillared-layer MOF, 1 is very highly distorted arising from parallel interpenetration of the layers. Due to the highly corrugated metal-carboxylate layer, all the bpeb ligands are not parallel to each other, whereas the unusual building blocks in 2, a linkage isomer of paddlewheel structure, render the bpeb spacer ligands to form pillars parallel to each other. The single pillared-layer MOF, 2, has a triply-interpenetrated pcu topology. Despite the bent nature of the obc ligand and varying metal to bpeb ratio in the synthesis, a highly corrugated layer structure of [Co2(obc)2] is maintained in 1 and 2 with different SBUs presumably due to the presence of bpeb ligand. Both compounds show interesting magnetic properties. The double pillared-layer structure of 1 exhibits antiferromagnetic behavior as the other reported compounds. A relatively rare ferromagnetism has been observed for the single pillared-layer structure of 2. ASSOCIATED CONTENT

S Supporting Information *

XRPD patterns, TGA curves, and crystal structures. CCDC reference numbers 1402417 (1) and 1402418 (2). These materials are available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00766.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NRF (2012R1A4A1027750), S. Korea, and the Ministry of Education, Singapore through NUS FRC grant R-143-000-604-112 and National Research Foundation through NRF-CRP Grant No. 10-2012-03. We would like to thank Caroline Evania Mulijanto for exploring new properties of the PCPs. E



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Influence of Angular Dicarboxylate Ligand on the Structures of Single

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