Co(II) Coordination Polymers with Co-Ligand Dependent Dinuclear to

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Co(II) Coordination Polymers with Co-Ligand Dependent Dinuclear to Tetranuclear Core: Spin-Canting, Weak Ferromagnetic, and Antiferromagnetic Behavior Prem Lama,† Jerzy Mrozinski,‡ and Parimal K. Bharadwaj*,† †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie Street, 50-383 Wrocław, Poland

‡

S Supporting Information *

ABSTRACT: Four new Co(II) containing coordination polymers have been synthesized using an ether bridged tricarboxylic acid ligand, ocpiaH3 (5-(2-carboxy-phenoxy)-isophthalic acid). This ligand readily reacts with CoCl2·6H2O in the presence of different nitrogen donor ligands such as 1,10-phenanthroline (phen), 4,4′-bipyridyl (bpy), 4,4′azopyridine (apy), and 1,4-bis(4-pyridinylmethyl)piperazine (bpmp) under hydrothermal conditions to afford three 3D and one 2D coordination polymers, {Co 3(o-cpia) 2(phen)(H 2O)·5H 2O} n (1), {Co1.5(o-cpia)(bpy)}n (2), {Co2(o-cpia)(OH)(apy)0.5}n (3), and {Co(o-cpiaH)(bpmp)0.5}n (4). Single crystal X-ray studies show that these coordination polymers contain homometallic clusters varying from dimeric to tetrameric depending upon the co-ligand used. In complexes 1 and 2, Co(II) ions form an angular and linear trimeric unit that extends along all directions to generate an overall 3D structure. In contrast, 3 forms a 3D coordination polymer containing a tetranuclear Co(II) unit. When the distance between the two donor N atoms of the co-ligand is further increased as in 4, a (4,4) net connected 2D coordination polymer results where the Co(II) ions form a dimeric paddle wheel unit. In addition to single-crystal X-ray crystallography, the complexes are also characterized by IR spectroscopy, thermogravimetry, and elemental analysis. Variable temperature magnetic susceptibility measurements on the complexes were carried out over the temperature range 1.72−300 K. Complex 1 exhibits ferromagnetic interactions due to uncompensated magnetic moments of the system leading to spin-canted antiferromagnetic behavior, while 2 and 3 show predominantly antiferromagnetic interactions. Complex 4 exhibits weak ferromagnetic behavior below 9 K.

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such as ferromagnetism,5 antiferromagnetism,6 spin-canting,7 metamagnetism,8 single-chain magnetism,9 and so on. Construction of coordination polymers with metal ions capable of showing magnetic anisotropy is particularly attractive.10 In the present work, we have chosen Co(II) as the paramagnetic metal ion, that can readily bind to the ligand, ocpiaH3 (5-(2-carboxy-phenoxy)-isophthalic acid) with multiple bridging capability (Scheme 1) to form multinuclear clusters of Co(II) capable of magnetic interactions. Several N-donor coligands were used to probe how the overall architecture of the complexes are influenced by these co-ligands. Synthesis and characterization of four new coordination polymers are described along with their interesting variable temperature magnetic properties.

INTRODUCTION Studies on coordination polymers have witnessed an upsurge in recent years due to their novel architecture as well as potential applications as functional materials.1 In particular, these materials can be engineered via ligand design to impart useful magnetic properties.2 It has been our interest to construct coordination polymers with interesting magnetic properties using carboxylate as well as mixed carboxylate and nitrogen donor linkers. Herein, we have used a tricarboxylate ligand with different N-donor co-ligands to construct coordination polymers with Co(II) metal nodes. Organic linkers bearing carboxylate as bridging ligands have been used frequently to synthesize magnetic materials because of their diverse binding capability and efficiency in transmiting magnetic coupling.3 In 1999, Li and co-workers reported4 one of the first examples of carboxylate donor ligands with N-donor co-ligands that led to a series of two-dimensional (2D) coordination polymers with the general formula, [M(ox)(bpy)] (where ox = oxalate and bpy = 4,4′-bipyridyl and M = Fe(II), Co(II), and Ni(II)). Coordination polymers of paramagnetic metal ions have led to an interesting range of systems exhibiting magnetic phenomena © 2012 American Chemical Society

Received: March 9, 2012 Revised: April 24, 2012 Published: April 26, 2012 3158

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Yield: 2.9 g (82%). This compound did not melt up to 300 °C. IR: sharp peak at 1700 cm−1 corresponding to νCO (str); 1H NMR (DMSO-d6), δ (ppm): 7.16 (d, J = 8.3 Hz, 1H); 7.33 (t, J = 7.6 Hz, 1H); 7.48 (s, 2H); 7.58−7.61 (m, 1H); 7.85 (d, J = 7.7 Hz, 1H); 8.12 (s, 1H); 13C NMR (DMSO-d6), δ (ppm): 120.6, 122.7, 123.9, 125.4, 125.7, 131.8, 133.9, 153.6, 158.5,166.4, 166.5; Anal. Calcd. for C15H10O7 (302.04): C, 59.61; H, 3.33%; Found: C, 59.74; H, 3.21%. ESI-MS: m/z (100%) 301.0427 [M − 1]+. Synthesis of 1,4-Bis(4-pyridinylmethyl)piperazine (bpmp). This compound was synthesized using an earlier reported procedure.13 Synthesis of {Co3(o-cpia)2(phen)(H2O)·5H2O}n (1). A mixture containing o-cpiaH3 (0.04 g, 0.13 mmol), CoCl2·6H2O (0.15 g, 0.63 mmol), and phen (0.023 g, 0.13 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 3 days and then allowed to cool to room temperature at the rate of 1 °C per minute. Block-shaped pink crystals of 1 were collected in 56% yield. The crystals were washed with water followed by acetone and air-dried. Anal. Calcd. for C42H34N2O20Co3: C, 47.43; H, 3.22; N, 2.63%. Found: C, 47.59; H, 3.32; N, 2.58%. IR (cm−1): 3610(s), 3441(w,br), 3067(s), 1625(m), 1580(m), 1453(m), 1379(m), 1316(s), 1260(m), 1213(s), 1161(s), 1096(s), 1002(s), 978(m), 903(s), 847(s), 803(s), 778(m), 756(s), 727(m), 664(s). Synthesis of {Co1.5(o-cpia)(bpy)}n (2). A mixture containing ocpiaH3 (0.04 g, 0.13 mmol), CoCl2·6H2O (0.15 g, 0.63 mmol), and bpy (0.02 g, 0.13 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 3 days and then allowed to cool to room temperature at the rate of 1 °C per minute. Block-shaped pink crystals of 2 were collected in 50% yield. The crystals were washed with water followed by acetone and air-dried. Anal. Calcd. for C25H15N2O7Co1.5: C, 55.21; H, 2.78; N, 5.15%. Found: C, 52.29; H, 2.89; N, 5.09%. IR (cm−1): 3065(s), 1607(m), 1557(m), 1484(s), 1450(m), 1408(m), 1311(s), 1245(s), 1212(m), 1145(s), 1097(s), 1063(s), 1000(s), 969(s), 939(s), 921(s), 903(s), 865(s), 815(m), 801(m), 777(m), 741(s), 720(m), 660(s). Synthesis of {Co2(o-cpia)(OH)(apy)0.5}n (3). A mixture containing ocpiaH3 (0.04 g, 0.13 mmol), CoCl2·6H2O (0.15 g, 0.63 mmol), and apy (0.024 g, 0.13 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 3 days and then allowed to cool to room temperature at the rate of 1 °C per minute. Block-shaped pink crystals of 3 were collected in 46% yield. The crystals were washed with water followed by acetone and air-dried. Anal. Calcd. for C20H12N2O8Co2: C, 45.65; H, 2.29; N, 5.32%. Found: C, 45.74; H, 2.36; N, 5.25%. IR (cm−1): 3550(s), 3068(s), 2927(s), 1606(m), 1576(m), 1530(m), 1482(s), 1444(s), 1395(m), 1266(m), 1227(m), 1192(m), 1167(s), 1151(s), 1113(s), 1093(s), 1050(s), 1025(s), 979(s), 923(s), 903(s), 868(s), 847(s), 818(s), 796(s), 767(m), 714(s), 685(m), 666(s). Synthesis of {Co(o-cpiaH)(bpmp)0.5}n (4). A mixture containing ocpiaH3 (0.04 g, 0.13 mmol), CoCl2·6H2O (0.15 g, 0.63 mmol), and bpmp (0.034 g, 0.13 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 3 days and then allowed to cool to room temperature at the rate of 1 °C per minute. Block-shaped pink crystals of 4 were collected in 45% yield. The crystals were washed with water followed by acetone and air-dried. Anal. Calcd. for C23H18N2O7Co: C, 55.99; H, 3.67; N, 5.67%. Found: C, 55.84; H, 3.59; N, 5.74%. IR (cm−1): 3551(m), 3075(s), 2933(s), 2815(s), 1702(s), 1628(m), 1585(m), 1452(m), 1382(m), 1317(s), 1255(s), 1215(s), 1152(s), 1113(s), 1093(s), 1009(s), 976(s), 916(s), 775(m), 756(m), 713(s), 662(s). Single-Crystal X-ray Studies. Single crystal X-ray data on 1−4 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.14 Data integration and reduction were carried out with SAINT15 software. Empirical absorption

Scheme 1. Schematic Representation of Ligand o-cpiaH3

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EXPERIMENTAL SECTION

Materials and Methods. 2-Fluorobenzonitrile, 5-hydroxyisophthalic acid, piperazine, 4-(chloromethyl)pyridine hydrochloride, 4,4′bipyridyl (bpy), 1,10-phenanthroline (phen), 4,4′-azopyridine (apy), and CoCl2·6H2O salt were acquired from Aldrich and used as received. N,N′-Dimethylformamide (DMF) and K2CO3 were procured from S. D. Fine Chemicals, India. DMF was purified prior to use. 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 DMSO-d6 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.). Electron paramagnetic resonance (EPR) spectra were recorded at 4.5 K on polycrystalline samples using an X-band Radiopan SE/X2543 EPR spectrometer. Magnetic Measurements. Variable temperature magnetic measurements of polycrystalline samples were carried out with a Quantum Design SQUID magnetometer (MPMSXL-5-type) at a magnetic field of 0.5 T over the temperature range 1.72−300 K. Corrections were based on subtracting the sample-holder signal while diamagnetic corrections were estimated from Pascal constants.11 Synthesis. Synthesis of the ligand was achieved in two steps as detailed below. Synthesis of Diethyl 5-(2-cyanophenoxy)-isophthalate (o-dcpi). 5-Hydroxy-isophthalic acid diethyl ester12 (3.5 g, 14.7 mmol) was mixed with dry K2CO3 (3.1 g, 22.1 mmol) in a 50 mL round-bottom flask under dinitrogen atmosphere and then treated with 20 mL of dry DMF. The mixture was stirred for 15 min at 80 °C followed by addition of 2-fluoro-benzonitrile (1.8 mL, 16.6 mmol) and the resulting mixture was stirred for 24 h in an oil-bath at 80 °C. At the end of the period, the solution was allowed to cool to room temperature and then poured into ice-cold water (100 mL) with vigorous stirring that afforded a white precipitated which was collected by filtration, washed with water, and dried in air. Yield: 4.6 g (92%). Melting point 48 °C; IR: sharp peaks at 2229 and 1720 cm−1 corresponding to νCN and νCO (str) respectively; 1H NMR (DMSOd6), δ (ppm): 1.28 (t, J = 7.2 Hz, 6H); 4.3 (q, J = 7.2 Hz, 4H); 7.1 (d, J = 8.3 Hz, 1H); 7.34 (t, J = 7.6 Hz, 1H); 7.66−7.69 (m, 1H); 7.8 (s, 2H); 7.91 (d, J = 7.6 Hz, 1H); 8.25−8.26 (m, 1H); 13C NMR (DMSO-d6), δ (ppm): 14.1, 61.7, 103.6, 115.7, 118.6, 123.8, 124.9, 125.6, 132.7, 134.5, 135.6, 155.7, 157.6,164.1; Anal. Calcd. for C19H17NO5 (339.11): C, 67.25; H, 5.05; N, 4.13%. Found: C, 67.39; H, 5.14; N, 4.02%. ESI-MS: m/z (100%) 339.1104 [M]+. Synthesis of 5-(2-Carboxy-phenoxy)-isophthalic Acid (o-cpiaH3). Diethyl 5-(2-cyanophenoxy)-isophthalate (4 g, 11.8 mmol) was hydrolyzed by refluxing it with 6(N) NaOH solution (50 mL) for 12 h. Finally, the resulting solution was acidified carefully with 6(N) HCl to obtain a white precipitate. After keeping it overnight in the freezer, the white solid was collected by filtration and dried at 80 °C. 3159

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

1

2

3

4

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 + F(000) refln collected independent refln GOF final R indices [I > 2σ(I)] R indices (all data)

C42H34N2O20Co3 1063.50 monoclinic P21/n 10.220(2) 23.211(3) 17.646(5) 90.000 90.688(5) 90.000 4185.6(15) 4 1.672 1.263 2124 22202 7764 1.053 R1 = 0.0564, wR2 = 0.1337 R1 = 0.0833, wR2 = 0.1522

C25H15N2O7Co1.5 543.78 triclinic P1̅ 10.140(2) 10.148(4) 11.369(5) 92.076(5) 94.575(4) 115.448(6) 1049.8(7) 1 1.720 1.253 551 5601 3805 1.068 R1 = 0.0382, wR2 = 0.0974 R1 = 0.0429, wR2 = 0.1020

C20H12N2O8Co2 526.18 triclinic P1̅ 9.604(3) 10.852(4) 11.414(6) 64.684(3) 85.395(5) 65.539(6) 972.0(7) 2 1.798 1.761 528 5179 3518 1.034 R1 = 0.0508, wR2 = 0.1182 R1 = 0.0676, wR2 = 0.1322

C23H18N2O7Co 493.32 triclinic P1̅ 8.422(3) 8.874(5) 14.509(4) 102.467(3) 90.171(6) 101.097(5) 1037.9(7) 2 1.579 0.877 506 5218 3733 1.026 R1 = 0.0453, wR2 = 0.1119 R1 = 0.0601, wR2 = 0.1232

Figure 1. (a−e) Coordination modes of ligand o-cpia3− in 1−4.

Figure 2. (a) A view of angular trinuclear unit in 1. (b) Diagram showing hexameric water clusters having a pentameric in 1. correction was applied to the collected reflections with SADABS,16 and the space group was determined using XPREP.17 The structure was solved by the direct methods using SHELXTL-9718 and refined on F2 by full-matrix least-squares using the SHELXL-9719 program package. All non-hydrogen atoms were refined anisotropically. The H atoms were refined as follows: H atoms attached to carbon atoms were positioned geometrically and treated as riding atoms using SHELXL default parameters, while H atoms of coordinated water molecules in complex 1 were located from difference Fourier maps and refined freely keeping the O−H bond distances constrained to 0.85 Å with the DFIX command. The hydrogen atoms of the lattice water molecules (O2W, O3W, O4W, O5W, and O6W) could not be located in

complex 1, even though its contribution has been added in the molecular formula and formula weight. The crystal and refinement data are collected in Table 1, while selected bonding and H-bonding distances and angles are given in Tables S1 and S2 (Supporting Information), respectively.

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RESULTS AND DISCUSSION

As a part of our ongoing research efforts on coordination polymers, we have prepared a bent semirigid tripodal ligand, ocpiaH3 (Scheme 1) for binding of Co(II) ions in the presence of different N donor co-ligands. All the coordination polymers 3160

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Schlafli symbol {4.62}2{42.610.83}, considering the trinuclear metal unit as a 6-connected node and ligand o-cpia3− as a three connected linker (Figure 4). Total potential solvent accessible void in 1 calculated by PLATON25 is found to be 18.4% of the crystal volume. The asymmetric unit of 2 consists of two crystallographically independent Co(II) ions (one having full occupancy and the other with a half occupancy), one o-cpia3−, and one bpy ligand forming a linear trimeric unit (Figure 5a). It crystallizes in the triclinic space group P1̅. Here, three carboxylate groups of the ligand o-cpia3− bind the Co(II) ions in syn-syn, chelating, and bridging modes (Figure 1c). The Co1 is coordinated to six O atoms from four different o-cpia3− ligands in chelating and bridging fashion (Co−O = 1.968(2)−2.277(2) Å). On the other hand, Co2 forms a CoO4N2 environment provided by four oxygen atoms in chelating and bridging modes from three different o-cpia3− ligands (Co−O = 2.070(2)−2.125(2) Å) and two N atoms from two bpy ligands (Co−N = 2.102(2)− 2.160(2) Å). The linear trimeric Co(II) cluster present in the structure is further extended by both the o-cpia3− and bpy ligands to give an overall 3D coordination polymer (Figure 5b). Topological analysis shows that the framework consists of an (8,3) connected net with Schlafli symbol {43}2{46.618.84} (Figure 6). Complex 3 crystallizes in the triclinic space group P1̅ with two independent Co(II) ions, one o-cpia3−, one hydroxyl group, and a half of apy molecule in the asymmetric unit. In this centrosymmetric tetramer (Figure 7a), Co1 shows distorted CoO5 coordination from three bridging carboxylates of o-cpia3− ligands (Co−O = 1.997(3)−2.031(3) Å) besides two μ3-OH groups (Co−O = 2.002(3)−2.117(3) Å). The Co2 center, on the other hand, adopts a CoNO4 penta-coordination provided by three O atoms from three bridging carboxylates (Co−O = 1.988(3)−2.008(3) Å), one N atom from apy ligand (Co−N = 2.135(4) Å) and one μ3-OH group (Co−O = 2.052(3) Å). Both the metal ions (Co1 and Co2) show distorted square pyramidal geometry (τ = 0.46−0.48). The tetranuclear core [Co4(μ3-OH)2]6+ propagate in all directions via bridging carboxylates of o-cpia3− and N atom of the apy ligands into a 3D network (Figure 7b). In this case, all the three carboxylate groups of the o-cpia3− ligand bind the metal ions in syn-syn fashion (Figure 1d). In this case, the whole network can be extended to a 3D (8,3)-connected framework with Schlafli symbol 36.418.53.6 (Figure 8). Complex 4 also crystallizes in the triclinic space group P1̅ with one cobalt ion, one o-cpiaH2−, and a half of bpmp ligand in the asymmetric unit. Crystallographically, it is observed that the ligand o-cpiaH2− binds the Co(II) ions in syn-syn mode giving rise to a dimeric paddle wheel unit shown in Figure 1e. The metal ion forms a penta-coordinated geometry provided by one N atom from bpmp (Co−N = 2.053(3) Å) and four O from four bridging o-cpiaH2− ligands (Co−O = 2.009(2)−2.083(2) Å). All the Co−O and Co−N bond distances are within the range as reported for cobalt based coordination polymers.7a The Co···Co distance in the dimeric unit is found to be 2.775(1) Å. The dimeric unit in complex 4 is extended by ocpiaH2− along the crystallographic bc plane to give a 2D sheet structure (Figure 9). Between the 2D layers, strong π···π interactions exist between the benzene moieties of the ligands to give an overall 3D architecture (Figure 10). Each o-cpiaH2− binds to four different Co(II) ions and each Co(II) ion is linked by four o-cpiaH2− ligands. Therefore, both the Co(II) ions and the o-cpiaH2− can be regarded as 4-connected nodes

once isolated are stable in air and insoluble in common organic solvents and water. The IR spectra of 1−4 show strong absorption bands in the region, 1408−1628 cm−1 that are diagnostic20 of coordinated carboxylate groups.21 The broad peak at 3441 cm−1 in the case of 1 indicates22 the presence of both coordinated and noncoordinated water molecules. The peak around 3550 cm−1 suggests the presence of a hydroxyl group in the case of 3 and 4.23 Complex 1 crystallizes in the monoclinic space group P21/n where the asymmetric unit consists of three crystallographically independent Co(II) ions, two o-cpia3−, one phen ligand, one coordinated, and five lattice water molecules. Here, the three carboxylate groups of the o-cpia3− ligand bind Co(II) ions in different ways viz., syn-syn, syn-syn-anti, chelating and bridging (Figure 1a,b). The structure shows an angular trimeric Co(II) subunit constructed from bridging carboxylates and a phen ligand (Figure 2a). Co1 shows penta-coordination with ligation from one chelating and three bridging oxygen atoms from four different o-cpia3− ligands (Co−O = 1.994(3)−2.288(3) Å). In contrast, Co2 is hexa-coordinated with six O atoms from five different o-cpia3− ligand units in bridging mode (Co−O = 2.038(3)−2.193(3) Å) and one aqua molecule (Co−OW1 = 2.040(4) Å). The CoO4N2 environment of Co3 is provided by the two N atoms from one phen ligand (Co−N = 2.123(4)− 2.125(4) Å) and four oxygen atoms from three different ocpia3− ligands (Co−O = 2.005(3)−2.212(3) Å). The water molecule OW1 coordinated to Co2 forms a strong hydrogen bonding interaction with two lattice water molecules, (OW1···OW2 = 2.820(7) Å and OW1···OW3 = 2.726(6) Å). Further, the additional three water molecules (OW4, OW5 and OW6) present in the cavity are stabilized by strong H-bonding interactions with OW2 and OW3 to give a hexameric water cluster having a pentameric core (Figure 2b). A variety of water clusters in the voids of coordination polymers are reported in the literature.24 There are other noncovalent interactions in 1 that are collected in Table S2, Supporting Information.21 Figure 3 shows a view of water molecules present in the lattice of 1. The 3D topological network of 1 is analyzed and the result shows that the framework consists of a (6,3) connected net having a

Figure 3. 3D view of complex 3 along the a axis with embedded water molecules (hydrogen atoms are omitted for clarity). 3161

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Figure 4. Topological view of (6,3) connected net in 1.

Figure 5. (a) A view of a linear trimeric unit in complex 2. (b) 3D view of complex 2 along the bc plane.

Figure 6. Topological view of the (8,3) connected net in 2.

having a Schlafli symbol 44.62, and the whole network can be extended to a (4,4)-connected layer net as displayed in Figure 11.

Overall, it could be seen that depending upon the distance between the two N atoms of the pyridine based co-ligands (Scheme 2) different types of Co(II) clusters are formed. When 3162

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Figure 7. (a) Tetrameric metal cluster formed by ligand in complex 3. (b) Propagation of the tetranuclear clusters into a 3D structure viewed along the ab plane.

Figure 8. Topological view of the (8,3) connected net in 3.

further increase in the distance between the N donors as in the case of bpmp a dimeric core results. Magnetic Studies. Variable-temperature magnetic susceptibility of 1 was carried out over the temperature range 1.72− 300 K. At 300 K, χMT value of 1 is 3.06 cm3 mol−1 K per Co(II) ion, which is much higher than the expected spin only value for S = 3/2 (1.87 cm3 mol−1 K) which is attributed to the spin− orbital coupling of the Co(II) ions. The effective magnetic moment (μeff) is found to be 4.95 μB at 300 K. A relation of χM (○) and 1/χM (●) versus T plots is shown in Figure 12. At the lowest temperature value of T = 1.72 K, a χMT value of 1 is found to be 3.07 cm3 mol−1K. Figure 13a,b shows an interesting relation of the χMT versus T in 1. The χMT decreases upon cooling and reaches the minimum of 2.97 cm3 mol−1 K at 41 K and then increases to 3.75 cm3 mol−1 K at 5 K followed by a drop toward zero. The maximum at 5 K suggests uncompensated magnetic moments of the system arising from spin-canting behavior of the antiferromagnetically coupled Co(II) magnetic centers.26 The magnetization M versus the magnetic field H (●) for 1 at 1.72 K is shown in the inset of Figure 13c. As discussed earlier, the crystal structure of 1 shows a 3D architecture having three crystallographically distinct Co(II) ions Co1, Co2, and Co3. The three Co(II) ions are arranged in an angular fashion that are bridged by the oxygen donors (O4 and O7) from two carboxylate groups. The Co1− O7−Co2 and Co2−O4−Co3 angles are 102.55° and 109.66°, respectively, indicating antiferromagnetic interactions between metal centers. It has been reported in the literature27 that the

Figure 9. 2D sheet along a bc plane in 4 having a dimeric paddle wheel unit.

the distance is short (i.e., in the case of phen and bpy) then it leads to trimeric cores. With an increase in the length of the ligand (in apy), a tetrameric core is formed although with 3163

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Figure 10. A view having a strong π···π interaction between the pyridine ring and the benzene ring bearing carboxylate group in 4.

Figure 11. Topological view of a 2D layer having a (4,4) connected net in 4.

and field-cooling process at 10.2 K. The appearance of TB results from small-size effects of magnetic grains. At both applied fields, magnetization curve shows an upturn below 10.2 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 net magnetization. Magnetic interaction is further investigated by performing magnetic hysteresis loop (M−H) measurements (Figure 14b) at different temperatures. However, even at the low temperature of 2 K, hysteresis loop could not be obtained. Magnetic susceptibity data of 2 show the existence of a negative value of Weiss constant (−18.7 K) indicating predominantly antiferromagnetic interactions between metal centers with a Curie constant of 3.07 cm3 mol−1 K in the temperature range 100−300 K. The effective magnetic moment of the linear {Co3} in 2 at 300 K is 4.82 μB per Co(II) ion which is significantly larger than the spin-only value of 3.87 μB A χT vs T plot is shown in Figure 15a. At 300 K, χMT value is

syn-anti bridging mode of carboxylate group mediates ferromagnetic exchange between metal centers. In our case, four syn-syn bridging modes exist between the three Co(II) centers (Co1, Co2, and Co3) leading to antiferromagnetic interactions where as one of the carboxylate groups bridges the distorted square pyramidal Co1 and distorted octahedral Co3 metal ions in syn-anti fashion that could possibly give rise to weak ferromagnetic interaction between the Co(II) pairs. As a result, the combination of weak ferromagnetic and antiferromagnetic exchange couplings in the 3D structure of 1 leads to noncompensation of spins leading to canted antiferromagnetic behavior. To achieve better understanding about the magnetic behavior at low temperatures, zero-field cooled (ZFC) and field-cooled (FC) magnetic susceptibility measurements were carried out at 25 and 100 Oe applied fields, respectively (Figure 14a). The temperature dependence of magnetization includes a magnetic blocking point (TB) between the zero-field-cooling 3164

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are in distorted octahedral geometry and the Co1−O−Co2 angle is found to be 113.36°(8). It has been generally observed that this type of larger angle gives rise to a negative value of magnetic exchange interaction.28 The negative θ value indicates dominating antiferromagnetic interactions between the Co(II) ions via the Co/O−C−O/Co and Co−O−Co pathways. The magnetization M versus the magnetic field H (●) at 1.72 K for 2 is shown in Figure 15b. Susceptibility studies of 3 also indicate a negative value of Weiss constants (−15.2 K) indicating a predominant antiferromagnetic interaction with a Curie constant of 1.59 cm3 mol−1K in the temperature range 100−300 K. The effective magnetic moment is 3.49 μB at 300 K per Co(II) ion. In this case, the indicated χMT value equals 1.52 cm3 mol−1 K that is lower than the spin-only value expected for S = 3/2 (1.87 cm3 mol−1 K) indicating a strong antiferromagnetic exchange interactions between the Co(II) centers. A χT vs T plot for this complex is shown in Figure 16a. The χMT value at the lowest temperature (1.72 K) is found to be 0.109 cm3 mol−1 K with an effective magnetic moment of 0.94 μB. As discussed earlier, the complex has a tetranuclear unit where both the crystallographically different metal ions Co1 and Co2 are in distorted square pyramidal geometry. Here, the Co−OH−Co angle is found in the range of 100.51° to 122.92° suggesting an antiferromagnetic interaction between the Co(II) ions. The magnetization M versus the magnetic field H (●) for 3 at 1.72 K is shown in Figure 16b. Variable-temperature magnetic susceptibility data on 4 again indicate a negative value of Weiss constants (−12.7 K) suggesting an antiferromagnetic interaction between the Co(II) centers in the temperature range 100−300 K with a Curie constant of 3.34 cm3 mol−1 K. At 300 K, the effective magnetic moment is found to be 5.09 μB. For this complex, χM (○) and 1/χM(●) versus T plots are shown in Figure 17. The room temperature χMT value is much higher than the spin-only value expected for S = 3/2 (1.87 cm3 mol−1 K) which is attributed to the spin−orbital coupling of the Co(II) ions. At 1.72 K, the χMT value is found to be 2.26 cm3 mol−1 K with an effective magnetic moment of 4.25 μB. The interesting relationship between the χMT versus T is illustrated in Figure 18a,b. The χMT versus T plot shows that with the decrease in temperature, the χMT value decreases due to Boltzmann population of the lower spin-state and reaches effectively a minimum of 2.86 cm3 mol−1 K at 44 K. The value then increases to a maximum of 3.39 cm3 mol−1 K at 9 K followed by a drop to zero. The maximum existing at 9 K suggests uncompensated magnetic moments of the system.26 The magnetization M versus the magnetic field H (●) for 4 at 1.72 K is shown in the inset of Figure 18c. As discussed before, the structure of 4 contains a dimeric paddle wheel cluster of Co(II) ions with four bridging carboxylates and each Co(II) ion is further axially coordinated by pyridyl nitrogen of the bpmp ligand. When such four O−C− O bridge is present in the dimeric paddle wheel unit, the exchange interaction is usually large giving rise to predominantly antiferromagnetic interactions between the metal pairs. In addition, a weak metal−metal interaction exists within the dimeric unit (Co···Co distance = 2.776(8) Å).29 The maximum magnetic moment reported30 for the high spin Co(II) complex having square pyramidal geometry is found to be 5.07 BM, which is almost comparable with the value 5.09 μB obtained for this complex at room temperature. From the crystal structure of 4, it is observed that the paddle wheel unit is somewhat distorted from the ideal one with the O−Co−O angle in the

Scheme 2. Scheme Showing the Distance Observed between the Two Coordinating Nitrogen Atoms of Pyridyl Group in Different Co-Ligands

Figure 12. Experimental magnetic data plotted as χM (○) and 1/χM (●) versus T for 1.

Figure 13. (a) χMT (○) versus T for complex 1. (b) The inset showing χMT (○) versus T up to 80 K. (c) Variation of the magnetization M versus the magnetic field H (●) for 1 at 1.72 K.

much higher than expected for the spin-only value for S = 3/2 (1.87 cm3 mol−1 K) due to orbital contribution of the high-spin Co(II) ion. Values of χMT at the lowest temperature (1.72 K) are found to be 0.668 cm3 mol−1 K with a effective magnetic moment value of 2.31 μB. In this complex, the three Co(II) ions are placed linearly in the trinuclear unit, where the terminal Co2 and the central Co1 are linked by one carboxylate bridge and one bridging oxygen atom (O6) from another carboxylate group (Co1 lies on the inversion center). Both the Co(II) ions 3165

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Figure 14. (a) Temperature dependence of magnetization for complex 1 in the 2−20 K range and a field of 25 and 100 Oe, respectively: ○ represent ZFC (zero-field cooling), ● represent FC (field-cooling). (b) Magnetization M versus the magnetic field H for 1 at different low temperatures.

Figure 15. (a) Experimental magnetic data plotted as χMT (○) versus T for complex 2. (b) The inset showing variation of the magnetization M versus the magnetic field H (●) for 2 at 1.72 K.

Figure 17. Experimental magnetic data plotted as χM (○) and 1/χM (●) versus T for 4.

Figure 16. (a) Experimental magnetic data plotted as χMT (○) versus T for complex 3. (b) The inset showing variation of the magnetization M versus the magnetic field H (●) for 3 at 1.72 K.

Figure 18. (a) χMT (○) versus T for complex 4. (b) The inset showing χMT (○) versus T up to 80 K. (c) Variation of the magnetization M versus the magnetic field H (●) for 4 at 1.72 K.

range 87.30−93.34°. This kind of distortion from the ideal paddle wheel structure could contribute in minimizing the exchange interaction between the metal pairs. Moreover, due to the semirigid nature of the ligand, it gets twisted in such a way that the dihedral plane passing through the two paddle wheel dimeric units is found to be 87.45° that is close to being orthogonal. Such an angle makes magnetic exchange difficult between the metal centers. 21 This kind of accidental orthogonality exchange may lead to uncompensated magnetic

moments of the system. The sudden rise of the magnetization value below 11 K is also accompanied by a strong divergence of the zero-field-cool/field-cool measurements at 100 Oe indicating the onset of a ferromagnetic state (Figure 19). The temperature dependence of magnetization includes a magnetic blocking point (TB) between zero-field-cooling and fieldcooling processes at 10.5 K. The appearance of TB results from a small-size effect of magnetic grains. The weak ferromagnetic nature of this complex is further confirmed by the hysteresis loops seen at different low temperature ranges 3166

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coupling between Co(II) centers in the dimeric unit is found to be antiferromagnetic. Thermal stabilities of all four complexes were examined.21 Complex 1 exhibits a weight loss of ∼10.1% in the temperature range of 80−170 °C that corresponds to loss of one coordinated and five lattice water molecules. Decomposition of 1 takes place only beyond 400 °C. Complex 2 and 4 show no weight loss up to ∼400 °C indicating high thermal stability of these complexes. Decomposition of these complexes takes place only above 400 °C. Complex 3 also shows a high thermal stability up to 400 °C. Complete decomposition is observed beyond 400 °C. The X-band EPR spectra of all the complexes have been measured for the polycrystalline samples at liquid helium temperature giving g values ≥ 3.3 indicative of high-spin complexes.21 Complex 1 shows a single line at 4.5 K with geff = 3.3 and δHpp = 900 Gs, while the EPR spectrum of 2 at 4.5 K exhibits two single lines at geff = 5.4 and δHpp = 200 Gs and geff = 4.4, δHpp = 140 Gs. Like 1, complex 3 also shows a single line at 4.5 K with spectroscopic splitting factor equal geff = 4.7 and δHpp = 670 Gs. However, complex 4 does not show any EPR spectrum which could be attributed to very fast relaxation time for this complex.

Figure 19. Temperature dependence of magnetization for complex 4 in the 2−20 K range and a field of 25 and 100 Oe, respectively: ○ represent ZFC (zero-field-cooling), ● represent FC (field-cooling).

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(Figure 20). The development of the hysteresis loop upon heating shows a lowering of the area of the hysteresis loop with increasing the temperature and going to zero at about 8 K. Thus, the magnetic structures of the weak ferromagnetic 4 can be described as ferromagnetically coupled between the two different dimeric {Co2} units that are almost perpendicular to each other due to twisting of the tricarboxylate ligand. The

CONCLUSION In summary, we have synthesized four different coordination polymers of Co(II) ions having different polynuclear clusters using a bent tripodal ether-bridged tricarboxylate ligand with different N donor co-ligands under hydrothermal conditions.

Figure 20. Magnetization M versus the magnetic field H for 4 at different low temperatures. 3167

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Yamashita, M.; Clerac, R. Inorg. Chem. 2009, 48, 3420. (c) Brockman, J. T.; Stamatatos, T. C.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Inorg. Chem. 2007, 46, 9160. (d) Oullette, W.; Prosvirin, A. V.; Whitenack, K.; Dunbar, K. R.; Zubieta, J. Angew. Chem., Int. Ed. 2009, 48, 2140. (10) (a) Yang, C.-I.; Chuang, P.-H.; Lu, K.-L. Chem.Commun. 2011, 4445. (b) Rodríguez-Diéguez, A.; Palacios, M. A.; Sironi, A.; Colacio, E. Dalton Trans. 2008, 2887. (c) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, Heidelberg, 1986. (d) Meyer, A.; Gleizes, A.; Girerd, J.- J.; Verdaguer, M.; Kahn, O. Inorg. Chem. 1982, 21, 1729. (11) König, E. Magnetic Properties of Coordination and Organometallic Transition Metal Compounds; Springer: Berlin, 1966. (12) Collman, J. P.; Brauman, J. I.; Fitzgerald, J. P.; Hampton, P. D.; Naruta, Y.; Sparapany, J. W.; Ibers, J. A. J. Am. Chem. Soc. 1988, 110, 3477. (13) Niu, Y.; Hou, H.; Wei, Y.; Fan, Y.; Zhu, Y.; Du, C.; Xin, X. Inorg. Chem. Commun. 2001, 4, 358. (14) International Tables for X-Ray Crystallography; Kynoch Press: Birmingham, England, 1952; Vol. III. (15) SAINT, version 6.02; Bruker AXS: Madison, WI, 1999. (16) Sheldrick, G. M. SADABS: Empirical Absorption Correction Program; University of Göttingen: Göttingen, Germany, 1997. (17) XPREP, version 5.1; Siemens Industrial Automation Inc.: Madison, WI, 1995. (18) Sheldrick, G. M. SHELXTL Reference Manual, version 5.1; Bruker AXS: Madison, WI, 1997. (19) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refinement; University of Göttingen: Göttingen, Germany, 1997. (20) (a) Lisnard, L.; Mialane, P.; Dolbecq, A.; Marrot, J.; ClementeJuan, J. M.; Coronado, E.; Keita, B.; de Oliveira, P.; Nadjo, L.; Secheresse, F. Chem.Eur. J. 2007, 13, 3525. (b) Li, X.-J.; Wang, X.Y.; Gao, S.; Cao, R. Inorg. Chem. 2006, 45, 1508. (21) See Supporting Information. (22) (a) Tang, Y.-Z; Wang, X.-S.; Zhou, T.; Xiong, R.-G. Cryst. Growth Des. 2006, 6, 11. (b) Dobrzynska, D; Jerzykiewicz, L. B.; Jezierska, J.; Duczmal, M. Cryst. Growth Des. 2005, 5, 1945. (23) (a) Luo, J.; Zhao, Y.; Xu, H.; Kinnibrugh, T. L.; Yang, D.; Timofeeva, T. V.; Daemen, L. L.; Zhang, J.; Bao, W.; Thompson, J. D.; Currier, R. P. Inorg. Chem. 2007, 46, 9021. (b) Jia, H.-P.; Li, W.; Ju, Z.F.; Zhang, J. Inorg. Chem. Commun. 2007, 265. (24) (a) Das, M. C.; Maity, S. B.; Bharadwaj, P. K. Curr. Opin. Solid State Mater. Sci. 2009, 13, 76. (b) Ghosh, S. K.; Bharadwaj, P. K. Angew. Chem., Int. Ed. 2004, 43, 3577. (25) Spek, A. L. PLATON; The University of Utrecht: Utrecht, The Netherlands, 1999. (26) (a) Lines, M. E. J. Phys. Chem. Solids 1970, 31, 101. (b) Darriet, J.; Haddad, M. S.; Duesler, E. N.; Hendrickson, D. N. Inorg. Chem. 1979, 18, 2679. (c) Yang, B.-P.; Provirin, A. V.; Guo, Y.-Q.; Mao, J.-G. Inorg. Chem. 2008, 47, 1453. (27) (a) Sakiyama, H.; Adams, H.; Fenton, D. E.; Cummings, L. R.; McHugh, P. E.; Okawa, H. Open J. Inorg. Chem. 2011, 1, 33. (b) Cheng, X.-N.; Xue, W.; Chen, X.-M. Eur. J. Inorg. Chem. 2010, 3850. (c) Wang, X.-Y.; Sevov, S. C. Inorg. Chem. 2008, 47, 1037. (28) (a) Goodenough, J. B. Magnetism and the Chemical Bond; John Wiley and Sons: New York, 1963. (b) Kanamori, J. J. Phys. Chem. Solids 1959, 10, 87. (c) Kanamori, J. In Magnetism; Rado, G. T.; Suhl, H., Eds.; Academic Press: New York, 1963; Vol. I, Chapter 4, p 127. (29) Benbellat, N.; Gavrilenko, K. S.; Gal, Y. L.; Cador, O.; Golhen, S.; Gouasmia, A.; Fabre, J.-M.; Ouahab, L. Inorg. Chem. 2006, 45, 10440. (30) (a) Thuiry, P.; Zarembowitch, J. Inorg. Chem. 1986, 25, 2001. (b) Banci, L.; Bencini, A.; Benelli, C.; Gatteschi, D.; Zanchini, C. Struct. Bonding (Berlin) 1982, 52, 37 and references therein.

The overall structures of the polymers depend on the length of N-donor co-ligand used. Magnetic studies reveal that complex 1 exhibits spin-canted antiferromagnetic behavior, whereas complexes 2 and 3 display predominantly antiferromagnetic interactions. On the other hand, complex 4 shows a weak ferromagnetic behavior at low temperature.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data for 1−4 in CIF format, table for selected bonds and distances for 1−4, IR, TGA analysis, ESI-MS, and NMR. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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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 P.L. Prof. J. M. acknowledges the financial support received from the National Science Centre (Poland) under Grant No 2011/01/B/ST5/ 01624 for this research.

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REFERENCES

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