Sulfate-Incorporated Co(II) Coordination Frameworks with Bis

Article pubs.acs.org/crystal

Sulfate-Incorporated Co(II) Coordination Frameworks with Bis-imidazole Bridging Ligands Constructed by Covalent and Noncovalent Interactions Ji Hye Park,† Woo Ram Lee,† Dae Won Ryu,† Kwang Soo Lim,† Eun A Jeong,† Won Ju Phang,† Eui Kwan Koh,‡ and Chang Seop Hong*,† †

Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-713, Korea Nano-Bio System Research Team, Korea Basic Science Institute, Seoul 136-713, Korea

‡

S Supporting Information *

ABSTRACT: Three 3D supramolecular networks, [Co(H2L1)2]· (SO4)·2H2O (1), [Co(H2L1)(SO4)(H2O)(DMF)] (2), and [Co(H2L2)(SO4)(H2O)] (3), were prepared by reacting Co(II) sulfate with the rigid bis-imidazoles of 1,4-di(1H-imidazol-4yl)benzene (H2L1) and 1,3-di(1H-imidazol-4-yl)benzene (H2L2) in solvothermal conditions. Compounds 1 and 2 containing the H2L1 ligand were isolated under different solvent-pair ratios. The structure of 1 can be described as a 6-fold interpenetrating 3D dia net in which the sulfate anions are positioned in the void spaces to balance the overall charge of the framework. In comparison, complex 2 shows a rectangular 2D grid consisting of 1D sulfatebridged chains linked by H2L1. When H2L2 is used in the reaction, the complex 3 having a 3D interdigitaed network with helical chains is formed, which is the first example of an H2L2-connected coordination polymer. The sulfate ions essentially contribute to the entanglement of the structures through extensive hydrogen bonding. Magnetic measurements for 2 indicate that very weak ferromagnetic interactions are operative between anisotropic Co(II) centers via sulfate bridges.

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INTRODUCTION Metal−organic frameworks (MOFs) have been actively investigated because of not only their intriguing topological variety but also their potential applications such as gas storage,1 separation,2 and catalysis.3 In terms of the fabrication of MOFs from an aesthetic viewpoint, the resulting structural types are totally dependent on metal ions with various coordination abilities, organic spacers with different binding sites, lengths, and directions, and reaction conditions such as temperature, reaction time, pH, and solvent.4 In this regard, organic linkers with N- or O-donors have been often explored to produce a great number of MOFs with interesting structures and properties. Among them, one of the best building bricks that can be used to attain such MOFs is an imidazole-containing ligand. For instance, polytopic organic ligands with 1-imidazole moieties have been intensively engaged in the formation of MOFs associated with diverse structures ranging from one-dimensional (1D) tubes to threedimensional (3D) interpenetrating nets.5 Compared with these neutral ligands, 4-imidazole-based ligands can be used as a pillar or bridge to ligate metal nodes, while the respective deprotonated entities can possess coordination sites available for the construction of rigid zeolitic imidazolate frameworks (ZIFs). The polyimidazole ligands are exemplified in ditopic ligands of 1,4-di(1H-imidazol-4-yl)benzene (H2L1) and 1,3-di(1H-imidazol-4-yl)benzene (H2L2), and a tritopic ligand of 1,3,5-tris(1H© 2012 American Chemical Society

imidazol-4-yl)benzene (H3L3). A series of M (M = Mn, Co, Ni, Zn, Cu, Cd, Ag, and so on) coordination networks with H2L1 were synthesized frequently in combination with carboxylate coligands, giving rise to rich topologies such as helical chains, multifold interpenetration, and self-penetration, as well as intriguing physical characteristics.6 However, a microporous Cu(II)imidazolate framework with high chemical stability was obtained by using deprotonated L12− ligands, exhibiting temperaturedependent selectivity of gas adsorption properties.7 Moreover, the use of partially deprotonated HL32− in aqueous NH3 solution yielded two isomeric Co(II)-imidazolate frameworks, which show special sorption behaviors relying on the isomeric forms.8 We attempted to employ the ditopic organic ligands of H2L1 and H2L2 to develop exotic supramolecular frameworks in the absence of polycarboxylates as coligands. Herein, we report the syntheses, crystal structures, and properties of three 3D supramolecular complexes, [Co(H2L1)2]·(SO4)·2H2O (1), [Co(H2L1)(SO4)(H2O)(DMF)] (2), and [Co(H2L2)(SO4)(H2O)] (3), that were obtained by reacting Co(II) sulfate and bis-imidazoles in solvothermal conditions. To the best of our knowledge, compound 3 marks the first example of a coordination polymer containing the Received: March 1, 2012 Revised: April 10, 2012 Published: April 11, 2012 2691

dx.doi.org/10.1021/cg300302n | Cryst. Growth Des. 2012, 12, 2691−2698

Crystal Growth & Design

Article

oven at 100 °C. After 16 h, pink rod crystals were formed, which were washed with DMF/H2O (3:1) and dried in air. Yield: 40%. Anal. Calcd for C15H18CoN5O6S: C, 39.57; H, 3.98; N, 15.38; S, 7.04. Found: C, 39.16; H, 4.31; N, 15.77; S, 6.92. Although the reaction time was changed from 16 to 72 h, we obtained the same phase. [Co(H2L2)(SO4)(H2O)] (3). H2L2 (15 mg, 0.071 mmol) and CoSO4·6H2O (19 mg, 0.071 mmol) in a 10 mL vial were dissolved in a mixed solvent of DMF/H2O (3:1, 6 mL). The mixture was stirred for 30 min. The sealed vial was put in a preheated oven at 100 °C for 16 h, resulting in the formation of purple plate crystals. The collected crystals were washed with DMF/H2O (3:1) and dried in air. Yield: 45%. Anal. Calcd for C12H12CoN4O5S: C, 36.61; H, 3.16; N, 14.62; S, 8.37. Found: C, 36.59; H, 3.24; N, 14.23; S, 8.17. Physical Measurements. Elemental analyses for C, H, and N were performed at the Elemental Analysis Service Center of Sogang University. Infrared spectra were obtained from KBr pellets with a Bomen MB-104 spectrometer. Thermogravimetric analyses were carried out at a ramp rate of 10 °C/min in an N2 flow using a Scinco TGA N-1000 instrument. PXRD data were recorded using Cu Kα (λ = 1.5406 Å) on a Rigaku Ultima III diffractometer with a scan speed of 3 deg/min and a step size of 0.01°. Magnetic susceptibility data for 2 were obtained using a Quantum Design MPMS-7 SQUID susceptometer and a PPMS magnetometer. Diamagnetic corrections of 2 were estimated from Pascal’s Tables. Crystallographic Structure Determination. X-ray data for 1−3 were collected using a Bruker SMART APEXII diffractometer equipped with graphite monochromated MoKα radiation (λ = 0.71073 Å). A preliminary orientation matrix and cell parameters were determined from three sets of ω scans at different starting angles. Data frames were obtained at scan intervals of 0.5° with an exposure time of 10 s per frame. The reflection data were corrected for Lorentz and polarization factors. Absorption corrections were carried out using SADABS.10 The structures were solved by direct methods and refined by full-matrix least-squares analysis using anisotropic thermal parameters for non-hydrogen atoms with the SHELXTL program.11 In 1, the one oxygen atom (O2) of sulfate was disordered with an occupancy of 0.5. All hydrogen atoms except for water molecules were calculated at their idealized positions and refined using the riding models. Crystallographic data and the details of data collection are listed in Table 1. Selected bond lengths and angles are given in Table 2.

meta-positioned bis-imidazole ligand H2L2. The formation of the resultant structures is realized by the integration of covalent and noncovalent interactions over the lattice. The sulfate anions play a critical role in mingling covalently linked units and elaborating such complicated 3D architectures. Compound 2 displays a 1D coordination polymer bridged by sulfate anions, exhibiting a weak ferromagnetic interaction between anisotropic Co(II) centers.

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EXPERIMENTAL SECTION

Reagent. 1,4-Di(1H-imidazol-4-yl)benzene (H2L1) and 1,3-di(1Hdimidazol-4-yl)benzene (H2L2) were prepared according to the literature procedure.9 All the other chemicals and solvents in the synthesis were of reagent grade and used as received. All manipulations were performed under aerobic conditions. [Co(H2L1)2]·(SO4)·2H2O (1). H2L1 (15 mg, 0.071 mmol) and CoSO4·6H2O (19 mg, 0.071 mmol) were put in a 10 mL vial and dissolved in a mixed solvent of DMF/H2O (1:12, 6 mL). The vial was sealed and located in a preheated oven at 100 °C and reacted for 72 h. Violet red crystals were precipitated, washed with DMF, and dried in air. Yield: 10%. Anal. Calcd for C24H24CoN8O6S: C, 47.14; H, 3.96; N, 18.32; S, 5.24. Found: C, 47.30; H, 3.64; N, 18.39; S, 5.26. [Co(H2L1)(SO4)(H2O)(DMF)] (2). H2L1 (15 mg, 0.071 mmol) and CoSO4·6H2O (19 mg, 0.071 mmol) in a 10 mL vial were dissolved in a mixed solvent of DMF/H2O (3:1, 6 mL). The resulting solution was stirred for 1 h. The reaction vessel was sealed and placed in a preheated

Table 1. Crystallographic Data for 1−3 formula formula weight crystal system space group temperature (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z dcalc (g cm−3) μ (mm−1) F(000) reflections collected unique reflections R(int) goodness-of-fit R1a [I > 2σ(I)] wR2b [I > 2σ(I)] a

1

2

3

C24H24CoN8O6S 611.50 tetragonal P4̅ 296(2) 14.3520(3) 14.3520(3) 6.2985(3) 90 90 90 1297.36(7) 2 1.565 0.799 630 12887 3127 0.0405 1.075 0.0444 0.1204

C15H18CoN5O6S 455.33 monoclinic P2(1)/n 293(2) 10.3774(13) 13.9064(17) 13.7479(16) 90 110.139(4) 90 1862.7(4) 4 1.624 1.078 936 11141 4702 0.0409 1.016 0.0479 0.1141

C12H12CoN4O5S 383.25 monoclinic C2/c 298(2) 28.835(3) 7.3637(7) 14.1679(13) 90 109.161(5) 90 2841.6(5) 8 1.792 1.387 1560 12632 3524 0.0520 1.099 0.0460 0.1191

R1 = Σ||FO| − |FC||/Σ|FC|. bwR2 = [Σw(FO2 − FC2)2/Σw(FO2)2]1/2. 2692



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Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1, 2, and 3a 1 Co(1)−N(1) N(1)#1-Co(1)−N(1) N(3)#3-Co(2)−N(3)

1.992(3) 107.75(9) 106.62(9)

Co(2)−N(3) N(1)−Co(1)−N(1)#2 N(3)−Co(2)−N(3)#4

1.996(3) 112.97(19) 115.34(18)

2 Co(1)−O(3) Co(1)−O(6) Co(2)−O(2)#5 O(3)−Co(1)−O(6)#6 O(3)−Co(1)−N(1) O(3)−Co(1)−O(6) N(3)−Co(2)−O(2) N(3)#5-Co(2)−O(1) O(2)#5-Co(2)−O(1) C(7)−N(3)−C(9)

2.0901(18) 2.154(2) 2.126(2) 86.09(8) 93.21(9) 93.91(8) 88.80(9) 90.59(9) 92.85(8) 105.4(3)

Co(1)−N(1) Co(2)−N(3) Co(2)−O(1) O(3)−Co(1)−N(1)#6 N(1)−Co(1)−O(6)#6 N(1)−Co(1)−O(6) N(3)−Co(2)−O(2)#5 N(3)−Co(2)−O(1) O(2)−Co(2)−O(1) C(7)−N(3)−Co(2)

2.104(2) 2.113(3) 2.136(2) 86.79(9) 94.36(10) 85.64(10) 91.20(9) 89.41(9) 87.15(8) 125.8(2)

3 Co(1)−O(1) Co(1)−N(3) O(1)−Co(1)−N(1) N(1)−Co(1)−N(3) N(1)−Co(1)−O(5)

1.964(3) 2.005(3) 117.82(12) 115.43(12) 103.96(11)

Co(1)−N(1) Co(1)−O(5) O(1)−Co(1)−N(3) O(1)−Co(1)−O(5) N(3)−Co(1)−O(5)

2.001(3) 2.012(2) 114.43(12) 101.69(10) 99.82(11)

a Symmetry transformations used to generate equivalent atoms: (#1) y, −x + 1, −z; (#2) −x + 1, −y + 1, z; (#3) y + 1, −x + 1, −z + 3; (#4) −x + 2, −y, z; (#5) −x + 2, −y, −z; (#6) −x + 1, −y, −z.

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RESULTS AND DISCUSSION Synthesis and Thermal Stabilities. Three sulfate-containing Co(II) complexes were prepared in solvothermal conditions. Interestingly, although the same reactants (CoSO4·6H2O and H2L1) in equimolar amounts were reacted in each case, different products, denoted 1 and 2, were precipitated. The generation of these dissimilar products depends on the ratio of solvents used; 1 was obtained in 1:12 DMF/H2O solvent pairs, while 2 was isolated in 3:1 DMF/H2O solvent pairs. This observation demonstrates that the reaction solvent-pair ratio plays an important role in influencing the final product. During the synthesis of 3, H2L2 was employed instead of H2L1 with identical reaction conditions as those for 2. The nature of the ligand naturally affects the consequent structure, as observed in 2 and 3. The purity of the bulk samples of 1−3 was checked by PXRD, from which we found that the profiles are consistent with the simulated patterns (Figures S1−S3, Supporting Information). Thermogravimetric analysis (TGA) was carried out to investigate the thermal stability of the samples (Figures S4−S6, Supporting Information). The TGA diagram of 1 indicates a weight loss of 6.0% in the temperature range 140−340 °C, which corresponds to the decomposition of two H2O molecules (5.9%). The framework began to decompose at about 450 °C. In the case of 2, a weight loss of 4.3% in the temperature range of 90−240 °C is equal to the removal of one H2O molecule, and that of 15.9% in the range 240− 410 °C is equal to one DMF molecule (16.1%). The decomposition of the framework occurred at about 430 °C. For 3, one bound H2O and one lattice H2O (9.0%) were completely removed at about 365 °C (8.6%), and then, the framework was eventually disorganized at about 405 °C. Crystal Structure of 1. Complex 1 is illustrated in Figure 1a with a selected atom-labeling scheme. The Co center adopts a tetrahedral geometry surrounded by four N atoms from H2L1, and the Co−N bond lengths are 1.992(3) Å for Co1−N1 and 1.996(3) Å for Co2−N3. The long spacer ligand H2L1 acts as a bridge to connect two neighboring Co atoms with the Co−Co

distance of 13.865 Å. The extended complicated network structure is shown in Figure 1b in the ab plane where L1 is simplified as a stick and Co lies at the joint. One layer extracted from Figure 1b shows that the Co center serves as a node and the ligand as a linker, leading to a 4-connected uninodal net with a point symbol of (66) (Figure 1c). This diamond (dia) structure with large hexagonal channels is constructed from the extension of the adamantanoid cages (Figure 1d).12 Interestingly, the adamantanoid-based 2D layers are mutually interpenetrated to generate a 6-fold structure, as shown in Figure 1e. This network belongs to Class Ia.13 The void space of the interpenetrated cationic net is filled by water molecules and sulfate anion. The water molecules participate in hydrogen bonding with the free N−H groups and sulfate O atoms, while the sulfate O atoms additionally form hydrogen bonds with the other N−H moieties. The anions are also required in the structure to maintain the charge balance. The existence of the multiple hydrogen bonds of the sulfate anions likely accounts for the stabilization of the dia structure. It is compared with the 4-fold interpenetrating dia net [Ni(H2L1)(SO4)] in which the sulfate anion is bound to the Ni center in a bidentate mode.6e In addition, a 5-fold 3D dia net was found in [Zn(H2L1)(1,4benzendicarboxylate)], where both H2L1 and dicarboxylate are considered to act as linkers to build up the dia framework.6d A 6-fold interpenetrating dia net was also observed in the partially deprotonated [Co(HL1)2].6d Crystal Structure of 2. Complex 2 is shown in Figure 2a with a selected atom-labeling scheme. Unlike 1, the Co geometry can be described as a distorted octahedron; Co1 is coordinated by two N atoms from H2L1, two O atoms from bound DMF molecules, and two O atoms from the bridging sulfate anion, whereas Co2 is encircled by two N atoms from H2L1, two O atoms from water molecules, and two O atoms from the anion. The average Co−N(O) bond lengths are similar, 2.12(3) Å for Co1 and 2.13(1) Å for Co2. The sulfate anions link adjacent Co atoms, leading to the formation of a 1D linear coordination polymer. Note that the coordinated water molecules have intrachain 2693



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Figure 1. (a) Crystal structure of 1 displaying coordination geometry around Co1 with the selected atom-labeling scheme. H atoms were omitted for clarity. The color codes are cyan for Co, blue for N, and gray for C atoms. (b) Three-dimensional extended framework in the ab plane. (c) Single layer of the dia net in the ab plane. (d) Hexagonal channels of the single layer showing a diagonal distance of 24.581 Å. (e) Representation of the 6-fold interpenetrating diamond net. (f) Three-dimensional network formed by hydrogen bonds among sulfate anions, water molecules, and imidazole N−H groups. The dotted lines stand for the hydrogen bonding. The color codes are cyan for Co, blue for N, gray for C, red for O, and yellow for S atoms.

The 1D chains are extended toward a 2D sheet by virtue of the coordination of H2L1 to neighboring chains, as depicted in Figure 2b. In the simplification routine, Co can be treated as a

hydrogen bonding to free sulfate O atoms, with a distance of 2.8158(2) Å for O1a−O3 (a = 2 − x, −y, −z). The shortest Co− Co distance within a chain is 5.1887(7) Å for Co1−Co2. 2694



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Figure 2. (a) One-dimensional chain linked by sulfate bridges and intrachain hydrogen bonds of 2 with the selected atom-labeling scheme. The dotted lines show hydrogen bonds. Symmetry transformation used to generate equivalent atoms: a = 2 − x, −y, −z. The color codes are cyan for Co, blue for N, gray for C, red for O, and yellow for S atoms. (b) Two-dimensional grid lattice. (c) Three-dimensional network architecture fabricated by multiple hydrogen bonds between 2D layers. The dotted lines denote hydrogen bonds. (d) Enlargement of the circled area of panel c illustrating detailed hydrogen bonding interactions.

node, and sulfate and H2L1 as linkers, resulting in a 4-connected uninodal sql sheet with a point symbol of (44.62). The sql lattice has a window size of 5.1887 Å × 14.0874 Å. The rectangular lattices are interlinked together via multiple hydrogen bonds built between free sulfate O atoms and nonbound N−H groups of nearby layers in the bonding interaction range of 2.720−2.741 Å, giving rise to the construction of a 3D network structure (Figure 2c,d). Hence, the sulfate ions crucially contribute to the stabilization of the 1D chain as well as creating such an entangled 3D architecture. Crystal Structure of 3. Figure 3a shows the structure of 3 with a selected atom-labeling scheme. The central environment around Co can be viewed as a distorted tetrahedron with two N atoms from H2L2, one O atom from the water molecule, and one O atom from the sulfate anion, as found in 1. The Co− N(O) length spans from 1.964 to 2.012 Å. Notably, there is a very weak bonding interaction between Co and the other

sulfate O atom (O4) with a distance of 2.5990(2) Å for Co1−O4. The H2L2 spacer joins two Co atoms and the Co−Co distance is 10.274(1) Å, much shorter than the separation by L1 in 1 and 2. The linkage through H2L2 allows for the formation of a 1D coordination chain that is reinforced by the presence of intrachain hydrogen bonding between the bound water molecules and noncoordinate sulfate O atoms (O3c− O5b = 2.679(1) Å; b = 0.5 − x, −0.5 + y, 1.5 − z and c = 0.5 − x, 0.5 + y, 1.5 − z). The intermetallic Co−Co distance through the hydrogen bonding path is 7.3637(7) Å, which is even smaller than that through the coordination bond by H2L2. Two coordination chains are incorporated via noncovalent forces such as hydrogen bonding and face-to-face π−π contacts, as illustrated in Figure 3b. The hydrogen bonding takes place among sulfate anions, water molecules, and free N−H groups in the range of 2.679−2.893 Å, affording the interconnection of the chains and a consequent 3D network (Figure 3c,d). 2695



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Figure 3. (a) Molecular diagram of 3 with the selected atom-labeling scheme. The thick and thin dotted lines present a weak covalent bond between Co and O atoms of sulfate and hydrogen bonding interactions between bound water molecules and free sulfate O atoms, respectively. Symmetry transformations used to generate equivalent atoms: b = 0.5 − x, −0.5 + y, 1.5 − z and c = 0.5 − x, 0.5 + y, 1.5 − z. The color codes are cyan for Co, blue for N, gray for C, red for O, and yellow for S atoms. (b) Entangled two helical chains between which hydrogen bonding (left) and π−π stacking (right) play a role. The dotted lines stand for the noncovalent bonding interactions. (c) Three-dimensional framework structure constructed by hydrogen bonds among sulfate, water molecules, and imidazole N−H groups. (d) Enlargement of bonding situation around sulfate anion of the circled region of panel c. Hydrogen bonding takes place between sulfate anion and neighboring hydrogen donors. The fragments with darker colored C atoms belong to the identical chain. (e) Top view of helical chains in the crystal packing diagram. Left-handed (green) and right-handed (pink) helical chains are arranged in an alternating way. (f) Side view of helical chains running along the b axis.

The latter π−π stackings occur among imidazole and benzene rings. The centroid distance between imidazole rings is 3.615 Å, while that between imidazole and benzene rings is slightly longer at 3.768 Å. The overall extended structure of 3 in the ac plane is shown in Figure 3e. It is manifest that each chain embraces the helicity running along the b axis (Figure 3f). The right-handed (P) and lefthanded (M) chains are stacked in an alternating fashion along the c direction. It appears that each helical chain becomes stabilized because H2L2 can bind the Co centers to give such a spiral pattern for the chain with the assistance of noncovalent interactions. Specially, it is worth mentioning that the sulfate anions play a pivotal role in all the hydrogen bonds, resulting in the realization of an aesthetic 3D supramolecular array with heterohelicity.14 Structural Comparison. Solvent-controlled different structures of 1 and 2 are stabilized. When water is more than DMF, the structure of 1 is preferentially precipitated, whereas more DMF than water generates the structure of 2. Therefore, the relative solvent ratios play a critical role in determining the stabilization of the final structures.15 All three complexes consist of a Co ion as a node and bis-imidazole ligands as linkers. Importantly, the sulfate ions are incorporated to facilitate coordination bonds and/or supramolecular interactions, as well as to maintain charge balance in the structure. For 1, the basic 3D backbone is composed of Co2+ and the ligand H2L1, engendering a 6-fold interpenetrating dia net where the sulfate ions reside in the void spaces of the net in order to strengthen hydrogen bonding in the framework. Comparatively, the sulfate anions serve as bridges to give a 1D

coordination chain in 2, while they behave as monodentate ligands to complete the coordination sphere around Co in 3. As found in 1, the anions in 2 and 3 are O donors to the N−H groups of the imidazole ligands, which are responsible for extensive hydrogen bonding and the subsequent supramolecular entities. It is noteworthy that the free N−H groups are also essential ingredients for hydrogen bonding to O donor atoms, eventually promoting the interdigitation of the resultant network structures. Magnetic Properties. Magnetic data were collected only for 2 because the paramagnetic Co(II) centers are close to each other in 2, while they lie far away from each other in the other complexes. The temperature dependence of the molar magnetic susceptibility of 2 was measured in the temperature range of 2−300 K at H = 1000 G (Figure 4a). The room-temperature χmT value of 3.76 cm3 K mol−1 is higher than the spin-only value (1.875 cm3 K mol−1) expected for an independent Co(II) ion with S = 3/2, which is due to the significant orbital contribution of the metal ion.16 The χmT product decreases slowly until Tmin = 9 K as the temperature is lowered. Below Tmin, χmT undergoes an obvious upturn. The initial decay of χmT in the temperature range of 9−300 K is a consequence of spin−orbit coupling for an octahedral Co(II) ion with a 4T1g ground state and a degree of magnetic couplings between spin centers. The spin−orbit coupling effect is estimated using the expression for a six-coordinate Co(II) ion under a molecular field perturbation at T > 25 K (Figure S7, Supporting Information).17 A best fit provides parameters of λ (spin−orbit coupling constant) = −73 cm−1, belonging to the usual scope of Co(II) systems,18 2696



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shows an evident increase of χmT at low temperatures. An infinite chain model with the spin Hamiltonian H = −JΣiSiSi+1 was applied to fit (χmT)mag.20 The fitted parameters are g = 2.38 and J = 0.05 cm−1. This result definitely supports the operation of very weak intrachain ferromagnetic interactions between Co(II) centers mediated by sulfate anions. On the basis of a simple orbital scheme, the total exchange coupling constant (J) is the combination of the ferromagnetic (JF) and antiferromagnetic factors (JAF).21 The latter term is proportional to the overlap integral. The observed ferromagnetic interaction may be understood by the fact that the JF component is dominant over JAF under the geometry of the sulfate bridge weakening the overlap of relevant magnetic orbitals. The magnetization data were collected at T = 2 K as a function of field (Figure 4b). The magnetization value reaches 2.97 Nβ at H = 7 T, which is in the anticipated region (2−3 Nβ) for an octahedral high-spin Co(II).22 Co(II) can be regarded as an effective spin of S′ = 1/2 at low temperatures below 30 K. The magnetization at T = 2 K and H = 7 T can be matched with the Brillouin curve derived from S′ = 1/2 and g = 5.8. The calculated g value is reasonable because the g values for high-spin Co(II) in an octahedral environment are normally larger than 4.23 To check the spin dynamics of the magnetic system, we collected ac magnetic susceptibility data, pointing out that there is neither slow magnetic relaxation nor longrange magnetic order in this compound (Figure S8, Supporting Information).

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Figure 4. (a) Plot of χmT versus T for 2. The open squares show raw experimental data, which were fitted using the Rueff expression (solid line). The dotted line stands for the spin−orbit coupling effect contributed from A exp(−E1/kBT). The subtraction of the raw data from the dotted line results in the open circles. The solid line is the fitted result with the Fisher model. (b) Plot of M versus H at 2 K of 2. The solid line represents the theoretical curve derived from the Brillouin equation with an effective spin S′ = 1/2 for Co (II) in an octahedral environment.

CONCLUSIONS We have prepared three 3D supramolecular network compounds [Co(H2L1)2]·(SO4)·2H2O (1), [Co(H2L1)(SO4)(H2O)(DMF)] (2), and [Co(H2L2)(SO4)(H2O)] (3) by treating Co(II) sulfate and the rigid bis-imidazole ligands of H2L1 and H2L2 in solvothermal conditions. The corresponding crystal structures are varied under subtle changes (solvent pair ratios) in the reaction conditions and the ligands used. In particular, the sulfate anions play a crucial role in constructing 3D supramolecular frameworks because they donate O atoms in order to accommodate extensive hydrogen bonding as well as maintain the charge balance of the system. The results unveil that it is possible to achieve 3D network architectures without the assistance of coligands such as polycarboxylates. The magnetic measurements reveal that the sulfate bridge can mediate ferromagnetic couplings, albeit tiny ones, between magnetic centers.

and A (crystal field parameter, 1.5 for weak-field limit and 1.0 for strong-field limit) = 1.4. The term (k = λ/λ′), where k is a reduction factor for spin−orbit coupling affected by covalency and λ′ is the spin−orbit coupling parameter for the free ion (λ′ = −170 cm −1), is deduced to be 0.43. High-temperature magnetic data at T > 40 K were taken to calculate relevant parameters using the Curie−Weiss equation (χm = C/(T − θ)), and the linear fit gives C = 3.93 cm3 K mol−1 and θ = −14.5 K. The Curie constant is in the common range for octahedral high-spin Co(II) ions.19 The Weiss constant should reflect both spin−orbit and magnetic coupling effects. To separately inspect spin−orbit coupling and magnetic interaction in 2, we employed the simple phenomenological formula, χmT = A exp(−E1/kBT) + B exp(−E2/kBT), where A + B is the Curie constant, and E1 and E2 are the activation energies corresponding to the spin−orbit coupling and the magnetic interaction, respectively.19 The best fit affords A + B = 3.9 cm3 K mol−1, consistent with the Curie constant, as well as E1 = 40.7 cm−1 and E2 = −0.13 cm−1. The E1 value is similar to those reported for Co(II) systems.19 The negative E2 term suggests the presence of ferromagnetic interactions on the basis of the Ising chain model χmT ∝ exp(+βJ/kBT). Because the experimental data are a combination of spin−orbit coupling and magnetic exchange interaction, we subtracted the spin− orbit coupling effect (A exp(−E1/kBT)) to extract the magnetic contribution only ((χmT)mag). The resultant curve obtained from the relationship (χmT)mag = (χmT)exp − A exp(−E1/kBT)

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format and additional experimental data for the complexes. This material 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 This work was supported by grants from the Korea Research Foundation (KRF-2009-220-C00021) and by the Priority Research Centers Program through the National Research 2697



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Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (NRF20110018396).

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