Syntheses, Crystal Structures, and Magnetic Properties of Metal

Article pubs.acs.org/crystal

Syntheses, Crystal Structures, and Magnetic Properties of Metal− Organic Hybrid Materials of Co(II) Using Flexible and Rigid NitrogenBased Ditopic Ligands as Spacers Musheer Ahmad,† Manish K. Sharma,† Raja Das,‡ Pankaj Poddar,‡,* and Parimal K. Bharadwaj*,† †

Department of Chemistry, Indian Institute of Technology Kanpur, 208016, India Physical & Materials Chemistry Division, National Chemical Laboratory, Pune - 411 008, India

‡

S Supporting Information *

ABSTRACT: A carboxylate-based flexible ligand, 5-(4-carboxybenzyloxy)isophthalic acid (p-cbiaH3), readily reacts with Co(II) salts in the presence of different pyridinebased coligands such as 4-bis(4-pyridinylmethyl)piperazine (bpmp) or 1,2-di(4pyridyl)ethylene (dpe) under hydrothermal conditions to afford two different threedimensional (3D) coordination polymers, {[Co 4 (OH) 2 (p-cbia) 2 (bpmp)(H2O)3]·2H2O}n (1) and {[Co4(OH)2(p-cbia)2(dpe)(H2O)4]·2H2O}n (2). Both compounds have been characterized by X-ray crystallography, IR spectroscopy, thermogravimetry, and elemental analysis. Single-crystal X-ray diffraction studies reveal that both compounds consist of a tetranuclear core [Co4(μ3-OH)2]6+. Compound 1 is a unprecedented 3D non-interpenetrated (3,4,9)-connected pkb3 net. On the other hand, compound 2 is a 2-fold interpenetrated (3,8)-connected tfz-d net. Variable temperature static and dynamic magnetic measurements show that both compounds exhibit ferromagnetic behavior associated with mixed hydroxyl/carboxylate-bridged tetranuclear Co(II) clusters as subunits.

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INTRODUCTION Recent years have witnessed intense research activity in the rational design and synthesis of coordination polymers not only due to their interesting topologies but also because of their potential applications as functional materials in fields such as molecular magnetism, catalysis, gas adsorption,1 and so on. Synthesis of coordination polymers with desired magnetic properties are extremely important with an ever growing number of useful applications. A significant number of coordination polymers exhibiting ferromagnetic,2 antiferromagnetic,3 spin canting,4metamagnetism,5 single chain magnets,6 spin glass behavior,7 etc. have been observed. To date, large numbers of coordination architectures with interesting compositions and properties have been made using a wide variety of aromatic polycarboxylate- and pyridine-based ligands. The key for designing such materials is to search for good bridging ligands that can effectively mediate the magnetic coupling. The ligand shown in Scheme 1 has been designed to have a flexible part flanked on either side by rigid aromatic carboxylates. This design provides more conformational freedom for the coordination environment around the transition metal ions. Use of a second ligand to extend the metal carboxylate systems to a higher dimensional network is a common method adopted.8 In most cases, different N-donor neutral ligands have been used for this purpose.9 A significant ferromagnetic behavior has been observed for both compounds reported. © 2012 American Chemical Society

EXPERIMENTAL SECTION

Materials. 4-Bromomethyl benzoic acid methyl ester, 5-hydroxyisophthalic acid, piperazine, 4-(chloromethyl)pyridine hydrochloride, 1,2-di(4-pyridyl)ethylene (dpe), cobalt chloride, and cobalt acetate were acquired from Aldrich and used as received. Acetonitrile (MeCN) and K2CO3 were procured from S. D. Fine Chemicals, India. Acetonitrile was purified prior to use. Physical Measurements. Infrared spectra were obtained (KBr disk, 400−4000 cm−1) on a Perkin-Elmer Model 1320 spectrometer; ESI mass spectra were recorded on a WATERS Q-TOF Premier mass spectrometer. 1H NMR and 13C NMR spectra were recorded on a JEOL-ECX 500 FT (500 and 125 MHz, respectively) instrument in CDCl3 and DMSO-d6 respectively with Me4Si as the internal standard. Thermogravimetric analyses (TGA) were recorded on a Mettler Toledo Star System (heating rate of 5 °C/min). Microanalyses for the compounds were obtained using a CE-440 elemental analyzer (Exeter Analytical Inc.). Magnetic Measurements. All magnetic measurements were carried out on polycrystalline samples using a Physical Property Measurement System (PPMS) from Quantum Design Inc., San Diego, USA, equipped with a 7 T superconducting magnet and a vibrating sample magnetometer. The magnetic signal from the sample holder was negligible to affect our data accuracy. Corrections are based on subtracting the sample-holder signal and diamagnetic corrections estimated from Pascal constants.10 DC magnetization vs T curves were taken at 50 Oe field in field cooled (FC) and zero field cooled (ZFC) modes with a heating/cooling rate of 2 K/min. Magnetizations vs field Received: December 7, 2011 Revised: January 12, 2012 Published: January 18, 2012 1571

dx.doi.org/10.1021/cg201619x | Cryst. Growth Des. 2012, 12, 1571−1578

Crystal Growth & Design

Article

Scheme 1. Synthetic Scheme of 5-(4-Carboxybenzyloxy)isophthalic acid (p-cbiaH3) and Chemical Structure of the Two CoLigands

loops were taken in a field sweep from −50 kOe to +50 kOe at the rate of 50 Oe/s. Single-Crystal X-ray Studies. Single crystal X-ray data on complexes 1−2 were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite monochromated MoKα radiation (λ = 0.71073 Å). The linear absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were taken from the International Tables for X-ray Crystallography.11 The data integration and reduction were carried out with SAINT12 software. Empirical absorption correction was applied to the collected reflections with SADABS,13 and the space group was determined using XPREP.14 The structure was solved by the direct methods using SHELXTL-9715 and refined on F2 by full-matrix least-squares using the SHELXL-97 program16 package. All non-hydrogen atoms were refined anisotropically. The Ow4 atom was refined isotropically in 2. In compound 1, two large residual electron density peaks (2.53 and 1.93 e Å−3) are present that are 0.817 and 0.699 Å from the Co(1), respectively. This is likely due to inefficient absorption correction for the Co(II) ion. The hydrogen atoms attached to carbon atoms were positioned geometrically and treated as riding atoms using SHELXL default parameters. The hydrogen atoms of coordinated water molecules in 1 and 2 were located from difference Fourier maps. The hydrogen atoms of lattice water molecules could not be located in the difference Fourier maps in 1 and 2. Several DFIX and DANG commands were used to fix the bond distances of metal coordinated and lattice water molecules. The crystal and refinement data are collected in Table 1, while selective bond distances and angles are given in Table S1, Supporting Information. Synthesis of the Ligand. Synthesis of the ligand was achieved in several steps as described below. Diethyl 5-(4-Methoxycarbonylbenzyloxy)isophthalate (pdmcbi). 5-Hydroxyisophthalic acid diethyl ester17 (2 g, 8.4 mmol) and dry K2CO3 (1.7 g, 12.6 mmol) were mixed in a round-bottom flask under an inert atmosphere. Dry acetonitrile (10 mL) was added to it, and the mixture was stirred for 30 min at 80 °C. The mixture was treated with 4-bromomethyl benzoic acid methyl ester (1.9 g, 8.40 mmol), and the resulting solution was refluxed for 24 h. At the end of this period, it was allowed to cool to room temperature and poured in ice-cold water (75 mL) to obtain a white solid that was collected by filtration and dried in air. Yield: 2.8 g (86%). Melting point 82 °C; IR: sharp peak at 1725 cm−1 corresponding to νCO (str). 1H NMR (CDCl3), δ (ppm): 1.4 (t, J = 7 Hz, 6H); 3.92 (s, 3H); 4.39 (q, J = 7 Hz, 4H); 5.2 (s,1H); 7.52 (d, J = 8.25 Hz, 1H); 7.82 (s, 2H); 8.06 (d, J

Table 1. Crystal and Structure Refinement Data for 1 and 2. compound

1

2

empirical formula formula wt crystal system space group a, Å b, Å c, Å α (°) β (°) γ (°) U, Å3 Z ρcalc Mg/m3 μ, mm−1 temperature (K) θ max F(000) refln collected independent refln GOF final R indices [I > 2σ(I)] R indices (all data)

C48H50N4O21Co4 1254.65 triclinic P1̅ 10.659(3) 14.708(5) 16.638(5) 83.681(5) 71.443(2) 87.964(5) 2457.7(14) 2 1.690 1.414 100 25 1276 12357 8469 1.037 R1 = 0.0805 wR2 = 0.2182 R1 = 0.1111 wR2 = 0.2499

C22H21NO11Co2 593.26 triclinic P1̅ 8.005(2) 10.744(5) 14.402(5) 97.640(3) 91.050(5) 109.285(2) 1156.2(8) 2 1.704 1.499 100 25 604 6018 3989 1.060 R1 = 0.0364 wR2 = 0.0917 R1 = 0.0419 wR2 = 0.0974

= 8.25, 2H); 8.29 (s, 1H); 13C NMR (DMSO-d6), δ (ppm): 14.4, 52.3, 61.6, 69.8, 120.1, 123.5, 127.2, 130.1, 132.3, 141.3, 152.8, 155.7, 158.5, 165.7, 166.9. Elemental analysis: Calcd. for C21H22O7 (386.140): C, 65.26; H, 5.69%; Found: C, 64.59; H, 6.04%. ESI-MS: m/z (100%) 387.138 [M + 1]+. 5-(4-Carboxybenzyloxy)isophthalic Acid (p-cbiaH3). Compound p-dmcbi obtained as above (2 g, 5.17 mmol) was hydrolyzed by refluxing it with 6(N) NaOH solution (20 mL) for 24 h. After cooling to 5 °C, the resulting solution was acidified with 6(N) HCl solution to obtain a white precipitate. It was collected by filtration, washed thoroughly with water, and dried in air. Yield: 1.3 g (80%). Melting point = 280 °C. IR: sharp peak at 1690 cm−1 corresponding to νCO (str). 1H NMR (DMSO-d6), δ (ppm): 5.1 (s, 2H); 7.4 (d, J = 8.7 1572



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Figure 1. Crystal structure of 1 (a) perspective view of metal cluster. (b) Two different coordination modes of p-cbia3− ligand.

Figure 2. Crystal structure of 1 showing the extension of the tetranuclear metal clusters into 3D structure, (a) view along the b axis, (b) view along the a axis (hydrogen atoms and guest molecules are removed for clarity). under autogenous pressure to 180 °C for 3 days and then allowed to cool to room temperature at the rate of 1 °C/min. Block-shaped pink crystals of 2 were collected in 65% yield. The crystals were repeatedly washed with water followed by acetone and air-dried. Anal. Calcd. for C22H21NO11Co2: C, 44.53; H, 3.56; N, 2.36%. Found: C, 45.02; H, 3.81; N, 2.22%. IR (cm−1): 3539(s), 1608(m), 1564(s), 1454(s), 1406(m), 1359(s), 1313(s), 1259(m), 1175(m), 1114(m), 1027(s), 1018(s), 994(m), 955(m), 871(s), 826(s), 771(s), 715(s), 556(m) (see Figure S10, Supporting Information).

Hz, 2H); 7.69 (s, 2H); 7.92 (d, J = 7.95, 2H); 8.17 (t, 1H). 13C NMR (DMSO-d6), δ (ppm): 69.7, 119.9, 123.8, 127.1, 130.0, 130.8, 132.8, 141.2, 158.4, 167.3, 168.0, 170.5. Elemental analysis: Calcd. for C16H12O7 (316.051): C, 60.75 ; H, 3.79% Found: C, 59.97; H, 3.75%. ESI-MS: m/z (100%) 315.051 [M − 1]+. The spectra related to this ligand and its precursors are given in the Supporting Information as Figures S1−S8. 1,4-Bis(4-pyridinylmethyl)piperazine (bpmp). This compound was synthesized using an earlier reported procedure.18 {[Co4(OH)2(p-cbia)2(bpmp)·(H2O)3]·2H2O}n (1). A mixture containing p-cbiaH3 (0.02 g, 0.06 mmol), Co(CH3COO)2·4H2O (0.07 g, 0.28 mmol), and bpmp (0.015 g, 0.06 mmol) in 3 mL of water and 0.1 mL of 1 M NaOH solution were sealed in a Teflon-lined autoclave and heated under autogenous pressure to 180 °C for three days and then allowed to cool to room temperature at the rate of 1 °C/min. Blockshaped pink crystals of 1 were collected in 40% yield. The crystals were repeatedly washed with water followed by acetone and air-dried. Anal. Calcd. for C48H50N4O21Co4: C, 45.95; H, 4.01; N, 4.47%. Found: C, 45.21; H, 3.91; N, 4.14%. IR (cm−1): 3638(m), 3411(s), 2939(s), 2855(s), 1603(s), 1653(m), 1451(m), 1401(m), 1370(m), 1320(s), 1296(s), 1265(m), 1159(s), 1131(s), 1056(m), 1019(s), 1010(s), 927(s), 883(m), 842(m), 829(s), 789(s) (see Figure S9, Supporting Information). {[Co4(OH)2(p-cbia)2(dpe)(H2O)4]·2H2O}n (2). A mixture containing p-cbiaH3 (0.02 g, 0.06 mmol), CoCl2·6H2O (0.10 g, 0.42 mmol), and dpe (0.01 g, 0.05 mmol) in 3 mL of water and 0.1 mL of 1 M NaOH solution were sealed in a Teflon-lined autoclave and heated

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RESULTS AND DISCUSSION Compounds 1 and 2 once isolated are stable in air and insoluble in common organic solvents and water. The IR spectra of all the compounds show strong absorption bands between 1608 and 1454 cm−1 that are diagnostic19 of coordinated carboxylate groups,20 and broad bands at region ∼3638−3411 cm−1 indicates21 the presence of coordinated, noncoordinated water molecules and hydroxyl groups22 (Figures S9−S10, Supporting Information). X-ray Structural Studies. Compound 1 crystallizes in the triclinic space group P1̅, and the asymmetric unit consists of four crystallographically independent Co(II) ions, two cbia3− ligands, two μ3-OH groups, three metal coordinated water molecules, two crystallographically independent half bpmp ligands, and two lattice water molecules. The four cobalt ions 1573



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form a tetranuclear unit [Co4(μ3-OH)2]6+ through bonding from two bridging μ3-OH and carboxylate groups of cbia3− ligands, and this core is further coordinated by the ligand bpmp (Figure 1a). In the tetramer core, each Co(II) ion exhibits distorted octahedral geometry: Co(1) and Co(2) are coordinated to four bridging carboxylates of p-cbia3−, one μ3OH (Co−O = 2.027(5)−2.187(7) Å) besides pyridyl group of bpmp (Co−N = 2.116(7) Å); Co(3) is coordinated to three bridging carboxylates of p-cbia3−, two μ3-OH and one water molecule (Co−O = 2.038(5)−2.162(5) Å); Co(4) is coordinated to two bridging carboxylates, two μ3-OH groups, and two aqua molecules (Co−O = 2.030(5)−2.204(6) Å). The coordination modes of p-cbia3− ligand are shown in Figure 1a. These tetrameric cores extend to 3D framework via bridging carboxylate group of p-cbia3− ligands and N atoms of bpmp ligands (Figure 2). The framework can be described as a novel (3,4,9) connected pkb3 net considering p-cbia3− ligands as 3and 4-connected nodes while metal clusters as 9-connected nodes23 (Figure 3). Figure 4. Crystal structure of 2 showing, (a) perspective view of metal cluster. (b) Coordination modes p-cbia3− ligand. (c) Three-dimensional packing (H atoms are omitted for clarity).

Figure 3. Structure showing the topology of 1, (a) 3-connected pcbia3− node. (b) 4-connected p-cbia3− node. (c) 9-connected metal cluster node. (d) (3,4,9)-connected pkb3 net.

Figure 5. Structure showing the topology of 2, (a) 3-connected pcbia3− node, (b) 8-connected metal cluster node, (c) (3,8)-connected tfz-d net. (d) View of 2-fold interpenetrated framework.

Compound 2 also crystallizes in the triclinic space group P1.̅ The asymmetric unit of 2 consists of the two crystallographically independent cobalt ions, one p-cbia3− ligand, a half dpe ligand, one μ3-OH group, two coordinated and one lattice water molecules. In the tetrameric unit, Co(1) shows distorted octahedral CoO6 coordination from three carboxylate O (Co(1)−O, 2.028(2)−2.174(2) Å) from three p-cbia3− ligands, two O (Co(1)−O, 2.152(3)−2.186(2) Å) from two coordinated water molecules and one O atom (Co(1)−O, 2.030(2) of μ3-OH (Figure 4a). Here, the three carboxylate groups of the pcbia3− ligand bind in μ6-η2:η0:η1:η1:η1:η1 modes. The Co(2) has a distorted octahedral CoNO5 coordination by three O (Co(2)−O, 2.084(2)−2.124(2) Å) from three bridging carboxylates, one N (Co(2)−N, 2.139(3)Å) from a dpe ligand and two O (Co(2)−O, 2.076(2)−2.135(2) Å) from two μ3OH groups. These tetrameric units propagate into a 3D framework via N atoms of the rigid dpe ligands (Figure 4c). On considering cbia3− ligands as 3-connected nodes and metal clusters as 8-connected metal nodes, framework of 2 can be described as (3,8) connected tfz-d net (Figure 5) with a Schläfli symbol (43)2(46.618.84}.23,24 Because of mutual interpenetration

of identical 3D frameworks, a 2-fold interpenetrating architecture is generated. The thermogravimetric analysis20 of both complexes (Figure S11, Supporting Information) were performed in N 2 atmosphere. The TGA of 1 exhibit a gradual weight loss of ∼7.2% corresponding to the loss of three coordinated and two noncoordinated water molecules. The compound shows gradual decomposition beyond 350 °C. Compound 2 shows a sharp weight loss of ∼9.1% in the temperature range of 80− 105 °C that corresponds to loss of two coordinated and one lattice water molecules. Decomposition of the framework is observed above 410 °C. Magnetic Studies. {[Co 4 (OH) 2 (p-cbia) 2 (bpmp)(H2O)3]·2H2O}n (1). The four metal centers are bridged by two triply bridging hydroxides forming a core similar to the known “butterfly” core. Although this is a common structural motif for the trivalent 3d metals such as Mn, Fe, V, and Cr, it is not often found for divalent metals. In the common butterfly complexes, the triply bridging ligand is an oxide, while in 1 it is a hydroxide. The Co−O(H)−Co angles range between 122.0° and 96.7°, resulting in competing antiferromagnetic and ferromagnetic interactions. The temperature dependence of 1574



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Figure 6. (a) The temperature dependence of the magnetic susceptibility χ and χT for 1 at 50 Oe applied field. (b) MH loops in low field region taken at temperatures 3, 8, 12, and 25 K for 1.

Figure 7. Temperature dependence of (a) real part (b) imaginary part of a.c. susceptibility for 1.

spin glass or a superparamagnetic state.27 Below 18 K, the FC magnetization curve shows a gradual increase and changes its slope further at 9.5 K and shows no saturation effect until the lowest measured temperature. Similar to the FC curve, the ZFC magnetization also shows upright behavior, until it reaches a maximum at 12 K, then decreases until 2 K. The maxima at 12 K in the ZFC magnetization is a signature of spin-canted weak ferromagnetic interaction between the Co(II) ions. The remnant magnetization (FC-ZFC) shows a sharp upturn below 25 K, where the bifurcation of the ZFC and FC curve was observed. This shows the presence of an uncompensated canted spin below this temperature which contributes to this net magnetization. To further investigate the magnetic interactions, we performed isothermal magnetic hysteresis loop (M−H) measurements (Figure 6b) at different temperatures around which magnetic transition was observed in the magnetic susceptibility study. The M−H curve does not saturate up to 5 T for all the measured temperatures. At 12 K, the magnetization shows opening up of the hysteresis loop along with the nonlinear behavior indicating the onset of ferromagnetic exchange interaction. On the other hand, at 25 K, the magnetization shows complete linear behavior, indicating a paramagnetic behavior. The value of coercive field (HC) increases immensely from 22 Oe at 12 K, to 862 Oe at 3 K, with an intermediate value of 126 Oe at 8 K. Large coercive fields (HC) and large departure from saturation of the M−H curve indicates a spin-canted weak ferromagnetic state. Similarly, the remnant magnetization (Hr) also increases

magnetic susceptibility for complex 1 is shown in Figure 6a. The χT value for four Co(II) at 300 K is 8.04 emu K mol−1 Oe1−, which is higher than the spin-only value of 7.5 emu K mol−1 Oe1− for four Co (II) ions with spin value 3/2 and Landé g value of 2. This could be due to a possible orbital contribution.25 Upon cooling, the χT value remains almost constant down to ∼120 K. Below 120 K, the χT value decreases gradually to reach a minimum of 5.42 emu K mol−1 Oe1− (at ∼26 K) and then increases to a peak of 38.8 emu K mol−1 Oe1− at 12 K followed by a drop to 9.07 emu K mol−1 Oe1− at 2 K. The first decrease of χT until 26 K could be due to the antiferromagnetic interaction of the Co (II) ions, and sharp increase between 26 and 12 K could be due to the presence of ferri/ferromagnetic interaction between Co(II) ions through oxygen atom of the p-cbia3−.26 This type of χT behavior is characteristic of a spin-canted system or reminiscent of a ferrimagnet. Fitting of data above 80 K with the Curie−Weiss law gives the Curie constant C = 0.12 emu K mol−1 Oe1− and Weiss constant θ = −67 K. The large negative Weiss constant value indicates the presence of antiferromagnetic coupling between the Co(II) ions, although the spin−orbit coupling and anisotropy can also contribute to this value. To gain more insight into the magnetic behavior at low temperatures, zero field cooled (ZFC) and field cooled (FC) magnetic susceptibility measurements (Figure 6a) were performed from 2−300 K under 50 Oe applied field. The FC and ZFC magnetization show irreversibility below the critical temperature of 25 K, indicating a phase transition from a paramagnetic state to either a long-range antiferromagnetic, 1575



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Figure 8. (a) The temperature dependence of the magnetic susceptibility χ and χT for 2 at 50 Oe applied field. (b) MH loops in the low field region taken at temperatures 2.7, 5, 12, and 30 K for 2.

Figure 9. Temperature dependence of (a) real part (b) imaginary part of a.c. susceptibility for 2.

continuously from 0.07 emu g−1 at 12 K to 0.7 emu g−1 at 3 K. The initial magnetization at 3 K reveals an S-shaped curve, indicating occurrence of a magnetic transition from a nonmagnetic ground state to spin-ordered state. While analyzing the field dependence of initial magnetization at 8 and 12 K, we observed that the magnetization increases very sharply at low field followed by continuous increase at higher field. The sharpness of initial magnetization at low field is higher at 12 K than at 8 K. This behavior also indicates a weak canted ferromagnetism. The presence of magnetic ordering was further confirmed by ac magnetic susceptibility data (Figure 7). A peak in the in-phase component of the susceptibility χ′ observed at 12 K, in agreement with the DC magnetic data, confirms the presence of phase transition. The imaginary part of susceptibility (χ″) exhibits peak ∼12.5 K. In both cases, we did not observe any frequency dependence in the measured frequency range (101−9999 Hz), confirming the absence of any spin-glass and superparamagnetic behavior.28 {[Co4(OH)2(p-cbia)2(dpe)(H2O)4]·2H2O}n (2). This compound also has a tetrameric core like compound 1. The asymmetric unit gives only two Co(II) ions and the remaining two are related by center of symmetry to give the tetrameric unit. The Co−O(H)−Co angles range between 122.3° and 96.4°, resulting in competing antiferromagnetic and ferromagnetic interactions. The temperature dependence of magnetic susceptibility for 2 is shown in Figure 8a. The χT value at 300 K is 4.08 emu K mol−1 Oe1− which is higher than the spin-only value of 3.75 emu K mol−1 Oe1−with spin value 3/2 and Landé g value of 2. This could be due to a possible orbital contribution.25 Upon cooling, the χT value decreases sharply and changes its slope near 170 K below which the slope became

steeper and reaches a minimum of 1.12 emu K mol−1 Oe1− at ∼22 K. Below this temperature, this value increases to a peak value of 2.09 emu K mol−1 Oe1− at 12.5 K followed by a drop, which again shows a kink at 2.9 K (0.8 emu K mol−1 Oe1−) and showed a minimum value of 0.52 emu K mol−1 Oe1− at 2 K. The first decrease of χT until 22 K could be due to weak antiferromagnetic interaction of the Co(II) ions and the sharp increase between 22 and 12.5 K could be due to the presence of canted ferromagnetic interaction between Co(II) ions through oxygen atom of the ligand. This type of χT behavior is characteristic of a spin canted system or reminiscent of a ferrimagnet. The second drop of χT value below 12.5 K shows the presence of an antiferromagnetic interaction and the kink near 2.9 K shows the presence of a ferri or ferromagnetic interaction. In the measured temperature range, inverse χ does not follow Curie−Weiss law. This shows the presence of interaction between the cobalt centers in the measured temperature range. It can be noted that the χT value at 300 K was higher in the measured temperature range. The FC and ZFC magnetization curves (Figure 8a) show irreversibility in the measured temperature range. Below 300 K, the FC and ZFC magnetization curve shows a gradual increase and changes its slope further at 28 K and shows no saturation effect until the lowest measured temperature. Similar to the FC curve, the ZFC magnetization also shows an increasing trend, until it reaches a maximum at 12 K, then decreases until 4.7 K; below this temperature the magnetization value in ZFC again increases and shows a maximum at 2.7 K and further drops until the lowest measured temperature. Like ZFC, FC magnetization also shows similar maxima at 2.7 and 9.5 K. The maximum at 12 K in the ZFC magnetization is a signature 1576



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ACKNOWLEDGMENTS We gratefully acknowledge the financial support received from the Department of Science and Technology, New Delhi, India (to P.K.B.) and SRFs from the CSIR to M.A. and M.K.S. NCL authors acknowledge the financial help from the CSIR Young Scientist Award grant.

of ferromagnetic interaction between the Co(II) ions. The shifting of this maximum to 9.5 K in FC magnetization may be due to the perturbation of the magnetic field. The maximum at 2.7 K in ZFC and FC magnetization may be due to the presence of a second type of ferromagnetic interaction. The remnant magnetization (FC-ZFC) shows a sharp upturn below 20 K; in the measured temperature range it shows a nonzero value. This shows the presence of an uncompensated cantedspin below which it contributes to this net magnetization. The M−H curve (Figure 8b) does not saturate up to 5 T for all the measured temperatures. Below 12 K, the magnetization shows opening up of the hysteresis loop and nonlinear behavior due to the onset of ferromagnetic coupling. On the other hand, at 30 K, the magnetization shows complete linear behavior. The value of coercive field (HC) increases from 181 Oe at 2.7 K, to 230 Oe at 5 K. Large coercive fields (HC) and a large departure from saturation of the M−H curve indicates a canted ferromagnetic state. Unlike the coercive field (HC), the remnant magnetization (Hr) also decreases continuously from 0.08 emu g−1 at 2.7 K to 0.5 emu g−1 at 5 K. The initial magnetization at 2.7 K reveals an S-shaped curve, indicating the occurrence of a magnetic transition from a nonmagnetic ground state to a spin ordered state, whereas 5 and 12 K initial magnetization showed the behavior of ferro/ferrimagnetic nature. The sharpness of initial magnetization at low field is higher at 12 K than 5 K. The in-phase component of the susceptibility χ′ (Figure 9) measured at zero-dc field and a 5 Oe oscillating field in the frequency range of 11−10000 Hz shows two distinct peaks at 13.8 and 2.7 K, in agreement with the static magnetic data, confirming the presence of phase transition. It can be noted that unlike the transition at 12 K, the transition observed at 2.7 K does not shift in all measurements. This confirms the robustness of 2.7 K transition. The imaginary part of susceptibility (χ″) exhibits a peak at ∼13.5 K and showed upturn behavior at low temperature. In both transitions, no frequency (9999−101 Hz) dependency was observed, confirming the absence of any spin-glass or superparamagnetic behavior.

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CONCLUSIONS In conclusion, we have demonstrated the construction of two novel cobalt(II)-organic frameworks using a flexible and a rigid nitrogen donor coligands with a flexible tripodal ether bridge carboxylate ligand, p-cbiaH3 under hydrothermal conditions. Currently, the ortho-isomer (5-(2-carboxy-benzyloxy)-isophthalic acid) of this ligand with different transition metal ions is under investigation in our laboratory. Variable temperature static and dynamic magnetic measurements show that both complexes exhibit ferromagnetic behavior. ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in CIF format, table for selected bonds and distances for 1 and 2, and complete data for IR, TGA analysis, ESI-MS, and NMR. This material is available free of charge via the Internet at http://pubs.acs.org/.

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*E-mail: [email protected] (P.P.), [email protected] (P.K.B.). Notes

The authors declare no competing financial interest. 1577



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Syntheses, Crystal Structures, and Magnetic Properties of Metal

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