Three Co(II) MetalâOrganic Frameworks with Diverse Architectures for

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Three Co(II) Metal−Organic Frameworks with Diverse Architectures for Selective Gas Sorption and Magnetic Studies Arun Pal,† Santanu Chand,† Joan Cano Boquera,‡ Francesc Lloret,‡ Jian-Bin Lin,§ Shyam Chand Pal,† and Madhab C. Das*,† †

Department of Chemistry, Indian Institute of Technology, Kharagpur, Kharagpur-721302 WB, India Departament de Química Inorgànica, Instituto de Ciencia Molecular (ICMol), Facultat de Química de la Universitat de València, C/Catedrático José Beltrán 2, 46980 Paterna, Spain § Department of Chemistry, University of Calgary, Calgary, Alberta T2N 1N4, Canada

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/18/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Three Co(II) metal−organic frameworks, namely, {[Co2(L)2(OBA)2(H2O)4]·xG}n (1), {[Co(L)0.5(OBA)]· xG}n (2), and {[Co2(L)2(OBA)2(H2O)]·DMA·xG}n (3) [where L = 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene, H2OBA = 4,4′oxybisbenzoic acid, DMF = dimethylformamide, DMA = dimethylacetamide, and G denotes disordered guest molecules], have been synthesized under diverse reaction conditions through self-assembly of a bent dicarboxylate and a linear spacer with a Co(II) ion. While 1 is crystallized at room temperature in DMF to form a 2D layer structure, 2 is formed by the assembly of similar components under solvothermal conditions with a 3D network structure. On the other hand, changing the solvent to DMA, 3 could be crystallized at room temperature with a 3D architecture. Out of the three, activated sample 2 was found to be permanently microporous in nature, with a BET surface area of 385 m2/g, and exhibited moderately high uptake capacity for C2H2 and CO2 while taking up much less CH4 and N2 at ambient conditions. As a result, high ideal adsorbed solution theory (IAST) separation selectivities are obtained for CO2/N2 (15:85), CO2/CH4 (50:50), and C2H2/CH4 (50:50) gas mixtures, making 2 a potential candidate for those important gas separations at ambient conditions. Moreover, the magnetic properties of 1−3 were studied. 1 and 2 show antiferromagnetic interaction between the Co(II) centers, whereas 3 displays ferromagnetic behavior arising from a counter-complementary effect between two types of links among Co(II) centers in 3.

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INTRODUCTION Crystal engineering of metal−organic frameworks (MOFs)1 attracts considerable attention from both academia and industry due to the various possible applications of MOFs as functional materials in the fields of gas separation,1,2 gas storage,1−3 magnetism,4 proton conduction,5 catalysis,6 sensing,7 photoluminescence,8 and many others.9 Their interesting architectures and desirable topologies also contribute to the wide interest they receive.10 To build new molecular architectures, many efforts have been made to improve the basic building blocks by using various solvents, metals, ligands, © XXXX American Chemical Society

reaction temperature, other experimental factors such as pH value, and templates.1−10 Among these factors, the choices of solvent and reaction temperature are the most important.10b,c Microporous MOFs are promising adsorbents for selective carbon capture due to their suitable pore size, easily tunable structure, and functionality.2 It is well established that the incorporation of basic functional groups, such as amine (−NH2), azo (−NN−), and amide (−CONH−), into Received: February 17, 2019

A

DOI: 10.1021/acs.inorgchem.9b00471 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry MOFs makes it possible to increase the interaction between the framework and gas molecules via quadrupole−quadrupole, π−π interactions and van der Waals forces.11 Chen et al. have shown that adding a polar group like H2O onto the MOF pore surface substantially influences CO2 sorption and separation performance.2b Such types of MOFs are very important for selective adsorption of C2H2 from C2H2/CH4 mixtures to get highly pure acetylene for organic synthesis and for purification of natural gas where C2H2 is present as an contaminant. Besides, selective capture of CO2 from CO2/N2 (flue gas) and CO2/CH4 (landfill gas) mixtures represents very important industrial gas separations. On the other hand, the similar kinetic diameters and boiling points of C2H2, CH4, and CO2 make their separation practically challenging. Taking all those points in account, we thought to immobilize a basic azine functionality on a linear N,N-donor spacer onto the MOF backbone to enhance affinity toward quadrupole molecules (CO2 and C2H2) for better performance in CO2/N2, CO2/ CH4, and C2H2/CH4 separation. The easiest models for core research on magnetic relaxation dynamics from both experimental and theoretical perspectives are the single-ion magnets having a single slow-relaxing metal center in a molecule. This behavior is determined by the delicate and minor structural distortions on the coordination environment around the metal center which ultimately regulate the type and extent of local single-ion magnetic anisotropy. Single-ion magnets are also particularly appealing due to their small sizes and easy handling, which make these simple molecules potential candidates for new magnetic devices, e.g., for use in molecular electronics and spintronics.12 In this contribution, we report three Co(II)-MOFs, {[Co2(L)2(OBA)2(H2O)4]·xG}n (1), {[Co(L)0.5(OBA)]· xG}n (2), and {[Co2(L)2(OBA)2(H2O)]·DMA·xG}n (3) (G denotes disordered guest molecules), self-assembled through a bent dicarboxylate OBA2−, a linear N,N-donor Schiff base linker L bearing azine functionality, and a Co(II) ion by varying the solvent system and crystallization temperature as well, leading to different architectures of MOF materials. The adsorption behavior of activated sample 2 for C2H2, CO2, CH4, and N2 gases has been analyzed. Ideal adsorbed solution theory (IAST) calculations have been performed to investigate the gas separation selectivity. In addition, a study of their magnetic properties shows that first two compounds are antiferromagnetic and 3 is ferromagnetic in nature.

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Scheme 1. Synthesis Routes toward MOFs 1−3

(s), 1566.3 (m), 1496.5 (m), 1398.8 (s), 1294.2 (w), 1227.9 (s), 1161.6 (s), 1091.9 (s), 1053.5 (m), 1011.6 (m), 879.1 (s), 784.8 (s), 694.2 (s), 648.8 (s), 533.7 (m), 446.5 (m). Synthesis of {[Co2(L)2(OBA)2(H2O)]·DMA·xG}n, 3. Complex 3 was synthesized by a procedure similarly to that for 1, except that DMF was replaced with DMA (4 mL) (Scheme 1). Red crystals of 3 were obtained after about 35 days. Yield: 40%, based on ligand L. FTIR (cm−1): 3425.6 (b), 3069.8 (w), 2926.7 (w), 1625.6 (s), 1594.2 (w), 1569.8 (w), 1496.5 (m), 1475.6 (m), 1391.9 (s), 1304.7 (m), 1231.4 (s), 1193.0 (m), 1161.6 (s), 1091.9 (m), 1046.5 (w), 1029.1 (w), 1015.1 (m), 875.6 (s), 854.6 (w), 802.3 (m), 784.8 (m), 704.6 (s), 655.8 (s), 627.9 (w), 519.7 (w).

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RESULTS AND DISCUSSION Synthesis. The synthesis of three Co(II)-MOFs (1−3) is fully dependent on the reaction solvents and temperatures. Room-temperature reaction of H2OBA, L, and Co(NO3)2· 6H2O (molar ratio 1:1:1) in DMF (4 mL) produces 1 with 2D layer structure, while 2 is formed by the assembly of exactly the same components under solvothermal conditions (110 °C for 48 h) with a 3D structure. On the other hand, upon changing the solvent from DMF to DMA, 3 is crystallized at room temperature with a 3D architecture. Crystal Structure of 1. 1 crystallizes in the monoclinic centrosymmetric P2/c space group.14 Figure 1a describes the core view around Co(II) centers, where both Co(II) centers (Co1, Co2) adopt distorted octahedral geometry surrounded by four O-atoms (two from OBA2− ligands, two from water molecules) and two N-atoms from spacer L. The two carboxylate groups of the OBA2− ligand show a monodentate coordination mode (Figure 1a). The topological study shows a sql topology with point symbol {44·62} (Figure 1b).15 Structural elucidation reveals a 2D layer structure of 1. Figures 1b and S2 represent the 2D layers stacked over each other through AA stacking sequence. Two different π···π stacking interactions are present between the pyridyl groups of L and the phenyl groups of the OBA2− ligand (centroid-to-centroid gaps of 3.946 and 4.066 Å) in each 2D layer. The overall framework of 1 is also stabilized by a number of non-covalent interactions among the constituent elements (Table S3). The total guest accessible void space in 1 is 1569 Å3 per unit cell volume of 6593 Å3 (23.8%), as determined by PLATON analysis.16 Crystal Structure of 2. X-ray study reveals that 2 crystallizes in the monoclinic centrosymmetric P21/n space group.14 Figure 2a depicts the core view around the Co(II) center surrounded by four O-atoms from four OBA2− ligands in the equatorial plane and one N-atom from spacer L at the axial position. The topological study shows a mab topology

EXPERIMENTAL SECTION

Materials. Ligand 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene (L) was prepared by the reported procedure.13 All other reagents were used as received. Synthesis of {[Co2(L)2(OBA)2(H2O)4]·xG}n, 1. Co(NO3)2·6H2O (29 mg), H2OBA (26 mg), and L (24 mg) were dissolved in 4 mL of DMF and stirred for 1 h. The solution was kept at room temperature for slow evaporation (Scheme 1). Red needle-type crystals of 1 were obtained after about 20 days. Yield: 50%, based on ligand L. FT-IR (cm−1): 3422.1 (b), 3069.8 (w), 2926.7 (w), 1674.4 (s), 1594.2 (s), 1531.4 (s), 1496.5 (m), 1475.6 (m), 1384.9 (s), 1301.1 (m), 1238.3 (s), 1196.4 (m), 1161.6 (s), 1091.9 (s), 1046.5 (m), 1011.6 (m), 879.1 (s), 812.8 (s), 788.4 (s), 756.9 (m), 704.7 (s), 655.8 (s), 627.9 (w), 519.7 (m). Synthesis of {[Co(L)0.5(OBA)]·xG}n, 2. Co(NO3)2·6H2O (29 mg), H2OBA (26 mg), L (24 mg), and DMF (4 mL) were placed in a 15 mL glass vial, heated at 110 °C for 48 h, and then cooled to room temperature over 12 h (Scheme 1). Dark red crystals of 2 were obtained. Yield: 70%, based on ligand L. FT-IR (cm−1): 3432.6 (b), 3059.3 (w), 2923.3 (w), 2846.5 (w), 1670.9 (s), 1622.1 (s), 1594.2 B


Article

Inorganic Chemistry

Figure 1. (a) Core view around the Co(II) cations in 1. (b) A sketch showing the layers of sql topology and AA stacking in 1. H-atoms are omitted for clarity.

Figure 2. (a) Core view of metal center in 2. (b) Representation of the mab topology of 2. (c) Packing diagram showing the channels of ∼5.0 × 6.2 Å2 along the crystallographic b-axis in 2. H-atoms are omitted for clarity.

Figure 3. (a) Core view around the Co(II) cations in 3. (b) Drawing showing the bnn hexagonal BN topology of 3. H-atoms are omitted for clarity.

having point symbol {44·610·8} (Figure 2b).15 Four carboxylate groups from four OBA2− ligands form bridges between two Co(II) centers, thereby generating a paddle-wheel dimer {Co2(CO2)4} with a Co···Co distance of 2.749 Å. These dimers are connected by both ligands to form a 3D framework (Figure 2c) with rhombus-shaped channels of ∼5.0 × 6.2 Å2 along the crystallographic b-axis (considering van der Waals radii) which possesses a guest-accessible void space of 29.7% (695.2 Å3 out of 2338.4 Å3) per unit cell volume, as

determined by PLATON analysis.16 Several non-bonding contacts between OBA2− and spacer L are also responsible for stabilization of the overall framework (Table S3). It is worth mentioning that 1 and 2 are formed by the combination of the same components with similar molar ratios; however, varying the reaction temperature produces either the 2D layer structure of 1 or the 3D microporous framework of 2. Crystal Structure of 3. When the solvent system is changed from DMF to DMA, a completely new framework, 3, C


Article

Inorganic Chemistry

Figure 4. C2H2,CO2, CH4, and N2 adsorption isotherms on activated 2 at 273 (a) and 295 K (b) (solid symbols, adsorption; open symbols, desorption).

Table 1. Uptake Amounts and IAST Separation Selectivities of Binary Gas Mixtures at 273 and 295 K under 1 Bar by 2′ uptake

IAST separation selectivity

temp, K

C2H2

CO2

CH4

N2

C2H2/CH4(50:50)

CO2/CH4(50:50)

CO2/N2(15:85)

273

77 mL/g (3.44 mmol/g) 58 mL/g (2.6 mmol/g)




33.8

4.5

31.7

24.2

3.8

18

295

until 300 °C, and decomposition of the structure occurred upon further heating. 3 was stable up to 100 °C, and after that, the loss of solvent molecules (both coordinated and lattice) as well as decomposition of the framework occurred gradually. Gas Adsorption Measurements. Gas adsorption measurements were performed with the desolvated samples 1−3 to test their permanent porosity. However, activated samples 1 and 3 do not adsorb any gases, because after activation the frameworks of 1 and 3 are transformed to amorphous phase, as confirmed by PXRD measurements (Figures S5 and S9). The activated sample of 2 (hereafter 2′) shows permanent microporosity and moderately high uptake capacity for C2H2 and CO2, while it takes up much less CH4 and N2 at ambient conditions. Prior to gas adsorption measurements, the fresh crystals were dipped in dry CHCl3 for 2 days to exchange the guest molecules with CHCl3, followed by activation at 373 K for 10 h under high vacuum. The retention of crystallinity of the CHCl3 exchanged, the activated sample 2′, and postsorption measurements were confirmed by PXRD (Figure S7) measurements. TGA analysis for the desolvated material ensured the complete activation process of sample (Figure S11). The permanent microporosity was established through the N2 sorption isotherm at 77 K (Figure S12), exhibiting a BET surface area of 385 m2/g with pore volume of 0.23 cm3/g. The BET surface area was also calculated from the CO2 isotherm at 195 K/1 bar (Figure S13) and was similar (386 m2/g) to the value obtained from the N2 sorption isotherm at 77 K. This sample adsorbs 103 cm3/g of hydrogen at 77 K and 1 bar, corresponding to an uptake amount of 0.92 wt% (Figure S14). Single-component C2H2, CO2, CH4, and N2 gases are used to assess the uptake capacity of 2′ for its plausible application in selective gas separations at ambient conditions. As depicted in Figure 4, 2′ shows moderately high uptake capacity for C2H2 and CO2, while it takes up much less CH4 and N2 at

is obtained crystallized with monoclinic P21/n space group.14 Figure 3a represents the core view around the Co(II) centers, where both Co1 and Co2 adopt highly distorted octahedral geometry. Co1 and Co2 centers are bridged by a coordinated water molecule, and two carboxylate groups of two OBA2− ligands form a {Co2(CO2)2(H2O)} unit with a Co1···Co2 separation distance of 3.656 Å. Two OBA2− ligands act as two different bridging ligands, one a μ4-bridging ligand and the other a μ2-bridging ligand. The {Co2(CO2)2(H2O)} units are bridged by both ligands to form a 3D framework with channels of ∼4.3 × 5.3 Å2 along the crystallographic a-axis (considering van der Waals radii) (Figure S3). The total guest-accessible void space is 1319 Å3 per unit cell volume of 6560 Å3 (20.1%), as determined by PLATON analysis.16 Careful inspection of the structure of 3 reveals that two different π···π stacking interactions are present between the pyridyl groups of L and the phenyl groups of the OBA2− ligand, with centroid-tocentroid gaps of 3.881 and 4.110 Å. The topological analysis reveals a 5-c uninodal net with bnn hexagonal BN topology having point symbol {46·64} (Figure 3b).15 The overall 3D framework (Figure S2) is also stabilized by one hydrogenbonding interaction and several C−H···O and C−H···N noncovalent interactions with spacer L, OBA2− linker, and DMA (Table S3). PXRD Patterns and Thermal Analyses. The PXRD patterns recorded for complexes 1−3 to confirm the bulk phase purity of these samples are similar to their corresponding simulated patterns, indicating phase purity of each sample (Figures S4, S6, and S8). To check their thermal stability, thermogravimetric analysis (TGA) was performed with a heating rate of 5 °C per min under a nitrogen atmosphere (Figure S10). For 1, with increasing temperature, the loss of coordinated and lattice water molecules followed by gradual degradation of the framework could be observed. For 2, the first weight loss occurred in the temperature range 100−200 °C, which corresponds to lattice solvents. The weight was then stable D


Article

Inorganic Chemistry

Figure 5. Separation selectivities at 273 (a) and 295 K (b) by 2′ as projected through IAST for CO2/N2 (15/85), CO2/CH4 (50/50), and C2H2/ CH4 (50/50).

27.6/298 K),21a ZJNU-83 (35/278 K, 26.5/298 K),21b UTSA36a (16.1/273 K, 13.8/296 K),21c ZJU-16a (38/273 K, 13/298 K),21d ZnSDB (18.6/273 K, 16.9/298 K),21e and QMOF-1a (15.2/273 K, 13.5/298 K).19 The high selectivity for separation of acetylene over methane indicates the potential of this material for selective adsorption of C2H2 from a C2H2/ CH4 mixture. The CO2/N2 and CO2/CH4 selectivity values are also comparable to or higher than those of many reported MOFs, ZIFs, zeolites, and activated carbon under the similar conditions (Table S5).2b,11b,18,22 This result also indicates its potential to act an absorbent for flue and landfill gas separation at ambient conditions. Magnetic Properties. The magnetic properties of 1 in the form of χMT against T are shown in Figure 6, where χM

ambient conditions. The uptake amounts of these four gases at two different temperatures are given in Table 1. To investigate the affinity of the 2′ framework toward gas molecules, the Qst values of C2H2, CO2, CH4, and N2 were derived using their single-component isotherms (273 and 295 K) through the Clausius−Clapeyron equation (Figure S15). The obtained values at near zero coverage are 25, 30.4, 15.5, and 13.6 kJ mol−1 for C2H2, CO2, CH4, and N2, respectively. The high Qst values for C2H2 and CO2 may be due to the higher polarizabilities of C2H2 (39.3 × 10−25 cm−3) and CO2 (29.1 × 10−25 cm−3) than of CH4 (25.9 × 10−25 cm−3) and N2 (17.4 × 10−25 cm−3). On the other hand, the larger quadrupole moment of CO2 is responsible for enhancing its interaction with the framework containing basic azine sites immobilized into the pore walls. The Qst of C2H2 is comparable with those of MOF-505 (24.7 kJ mol−1),17a ZJU-25 (25.4 kJ mol−1),17b UTSA-68a (25.87 kJ mol−1),17c UTSA-100a (22 kJ mol−1),17c M′MOF-3a (25 kJ mol−1),17c and M′MOF-6a (28 kJ mol−1).17d The Qst of CO2 is comparable with those of UTSA-15a (28.6 kJ mol−1),2b UTSA-16 (34.6 kJ mol−1),2b UTSA-20a (32.4 kJ mol−1),2b UTSA-33a (30 kJ mol−1),2b SIFSIX-2-Cu-i (31.9 kJ mol−1),18a PCN-88 (27 kJ mol−1).18b It may be noted that, although Qst of C2H2 is lower than that of CO2 at near zero coverage, a steadily increasing value could be observed with higher loadings. The reason is still unclear; however, a similar phenomenon has also been observed by others.19 To evaluate the separation ability of the activated framework 2′ for C2H2 and CO2, the gas selectivities of C2H2/CH4 (50:50), CO2/N2 (15:85, flue gas composition), and CO2/ CH4 (50:50, landfill gas composition) at different temperatures were calculated via the popular IAST method.20 Experimental single-component sorption isotherms of C2H2, CO2, CH4, and N2 were fitted based on the dual-site Langmuir−Freundlich (DSLF) model to get the adsorption parameters (Figures S16−S19), which were subsequently used to calculate multicomponent adsorption with IAST (Table S4). The outcomes of the calculations reveal that, at 1 bar, the selectivity for the CO2/N2 mixture is 31.7 at 273 K and 18 at 295 K, the selectivity for the CO2/CH4 mixture is 4.5 at 273 K and 3.8 at 295 K, and the selectivity for the C2H2/CH4 mixture is 33.8 at 273 K and 24.2 at 295 K (Figure 5a,b, Table 1). C2H2/CH4 selectivity values are comparable to or higher than those of ZJNU-70 (37.8/278 K, 28/298 K),21a ZJNU-71 (32.4/278 K,

Figure 6. Plot of χMT vs T for 1, where the solid line symbolizes the best theoretical fit as defined by the Hamiltonian in eq 1. The inset shows the plot of M vs H/T over the temperature range 2.0−10.0 K.

represents the magnetic susceptibility for a single Co(II) ion. At room temperature, a larger value of 2.90 cm3 mol−1 K for χMT than the spin-only value (1.874 cm3 mol −1 K for a spin quartet, S = 3/2 and g = 2.0) could be obtained, suggesting a substantial magnetic orbital contribution.23 Upon cooling, χMT continuously decreases to 1.64 cm3 mol −1 K at 2.0 K. The decrease of χM T in 1 might be because of the intermolecular antiferromagnetic interactions between the Co(II) centers, the depopulation of the higher energy Kramer’s doublets of the hexa-coordinated Co(II) ions, or both. Because of the large E


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Inorganic Chemistry value for the shortest intermolecular Co(II)−Co(II) distance (ca. 6.6 Å), an insignificant magnetic interaction between the local spin quartets is anticipated. Therefore, the data were evaluated using the Hamiltonian shown in eq 1.23 ÄÅ É ÅÅ 2 L(L + 1) ÑÑÑÑ Å ̂ ̂ ̂ ̂ H = −αλLS + ΔÅÅLz − ÑÑ + βH( −αL̂ + 2S)̂ ÅÅÇ ÑÑÖ 3 (1)

The term αλLŜ ̂ takes into accounts the spin−orbit coupling with α = Aκ, where A is a crystal field parameter with values of 1.5 and 1 for the weak and strong field limits, respectively, κ stands for orbital reduction factor, and λ denotes spin−orbit coupling constant. The middle term in the equation represents the one-center operator accountable for the axial distortion on the hexa-coordinated metal center where Δ denotes the energy difference concerning singlet 4A2 and doublet 4E levels resulting from orbital splitting of triplet ground-state 4T1g under such axial distortion. Finally, the last term denotes the Zeeman interaction. For the sake of calculation, benefit of the isomorphism between the orbital triplet T1 (originating from 4 F term) and the triplet L = 1 from a P term was taken which eventually suggests that ||T1|| = −α||P||.23 In order to simulate the experimental data, numerical matrix diagonalization methods with the VPMAG suite were employed because of the non-availability of analytical expressions of the magnetic susceptibility as a function of parameters A, λ , κ, and Δ.24 The best fitted parameters obtained for 1 were α = 1.30(1), Δ = 675(9) cm−1, and λ = −167(2) cm−1. The theoretical plot and the experimental data match quite well over the entire temperature range, as depicted in Figure 6. The values of the obtained parameters are within the normal range observed for high-spin hexa-coordinated Co(II) complexes.25 The magnetization curve at 2.0 K tends to 2.15 μB (inset of Figure 6), which is the expected value for one hexa-coordinated Co(II) ion with α = 1.3 (eq 2). In addition, the reduced magnetization curves undoubtedly indicates that the low-lying levels in 1 are similar in energy, as the magnetization isotherms eventually overlay. M = (10 + 2α)/3

Figure 7. Dependency of frequency for the alternating current out-ofphase molar magnetic susceptibility of 1 with an applied static field of Hdc = 2500 G over the temperature range 2.0−10.0 K.

temperature could be fitted to the exponential Arrhenius equation, τ = τ0 exp(E#/kT), as anticipated for a thermally activated Orbach process (insets of Figures 8, S24, and S25)

(2) Figure 8. Spin−lattice relaxation rates for 1 and its temperature dependence under static dc fields of 2500 G. The inset shows the Arrhenius plot for 1 as ln(τ) against 1/T.

Considering that the octahedral distortion is adequate for total quenching of the orbital momentum, we have used the spin Hamiltonian in eq 3 to analyze the experimental magnetic data of 1. ÄÅ É S(S + 1) ÑÑÑÑ ÅÅÅ ̂ 2 ̂ H = DÅÅSz − ÑÑ + βH[g||Sẑ + g⊥(Sx̂ + Sŷ )] ÅÅÇ ÑÑÖ 3 (3)

affording effective energy barriers of the magnetization reversal, E#, and pre-exponential factors, τ0 (Table 2). From the relationship E# = D(S2 − 1/4), an approximate estimate for |D| ≈ 3.7 cm−1 was obtained for 1, a value that has nothing to do with that obtained from eq 2. The lower temperatures data indicate the occurrence of a faster relaxation process and thus possibly correlating a tunneling effect. A satisfactory match of the experimental

In this equation, the first term relates to the zero-field splitting being 2D the energy gap between the doublets of Ms = ±1/2 and Ms = ±3/2 resulting from the second-order spin−orbit coupling of the spin quartet ground state, and the second term denotes the Zeeman effect. The best-fit parameters obtained were g = 2.48(1) and |D| = 84(1) cm−1 (Figure S21). However, this value is too large (2|D| = 168 cm−1) and may not have a real physical meaning. No out-of-phase signals (χM″) could be detected in absence of outer static field (H = 0) while alternating current (ac) magnetic susceptibility measurements were performed by furnishing a dc magnetic field (H = 0.0−2.5 kG) for 1. The use of outer dc fields of 500, 1000, and 2500 G (Figures 7, S22, and S23) undoubtedly produced signals for both components, χM′ and χM″, as a function of frequency. The data at high-

Table 2. Best-Fit Parameters from Eq 4 in All the Temperature Range and from Arrhenius Equation τ = τ0 exp(E#/kT) in the High-Temperature Region (Figures 8, S24, and S25)

F

H (G)

E# (cm−1)

τ0 (s)

A (K−1 s−1)

B (K−n s−1)

n

500 1000 2500

7.71(5) 7.79(5) 6.80(4)

1.81(3) 1.73(2) 2.01(2)

1800(9) 2020(10) 2890(12)

150(2) 90(1) 69(1)

3.60(2) 3.72(2) 3.79(2)


Article

Inorganic Chemistry data within the temperature range of 2.0−5.0 K could be obtained while an effort was made to fit the relaxation times of 1 to combine the two processes (one-phonon direct prevailing at very low temperatures and two-phonon Raman processes accountable for the relaxation time at high temperatures) through eq 4 as presented in Figures 8, S24, and S25. The best fitting parameters are given in Table 2. τ −1 = Adirect T + BRaman T n

(4) 26

In general, a value of n = 9 is expected for Kramer’s ions, and thus, a very small value of n (∼3.7) may not be suggestive of a two-phonon Raman process. The Cole−Cole plots as depicted in Figures S26−S28 display semicircular profiles, as estimated for Orbach processes. Similar behaviors were also observed in other Co(II) complexes.25a,b The magnetic measurements of 2 were performed in the temperature range of 2−300 K. A plot for χM and χMT against T (per two Co(II) ions) is shown in Figure 9. A maximum

Figure 10. Pot of χMT against T for 3. The best theoretical fit is symbolized by the solid line as defined by the Hamiltonian eq 6. Inset represents the plots of M versus H/T (2.0−10.0 K).

mol−1 K, g = 2.0) indicating an involvement of the orbital angular momentum, characteristic for a 4T1g ground term.28 Upon cooling, the magnetic moment is decreased as anticipated for the depopulation of the higher energy spin− orbit levels. A plateau developing at ca. 8 K with χMT value at about 3.6 cm3 mol−1 K falls off until 3.37 cm3 mol−1 K at 2 K. The ground Kramer’s doublet is populous only at lower temperature region (T < 20 K) which can be considered as an effective spin doublet (Seff = 1/2) with allied g value within the range of 3.9−4.3. Therefore, the χMT value for this ground Kramer’s doublet spans in the range of 1.4−1.7 cm3 mol−1 K (2.8−3.4 cm3 mol−1 K for two Co atoms).23 It is noteworthy to mention that these values are lower than that realized for the incipient plateau at ca. 8 K (ca. 3.6 cm3 mol−1 K) and near to the minimum value found at 2 K (3.4 cm3 mol−1 K). These phenomena indicate the existence of a feeble intramolecular ferromagnetic communication (producing a lowest in the χMT curve with subsequent rise at low temperature)29 in addition with a weak intermolecular antiferromagnetic communication (responsible for the devoid of the rise of χMT at lower temperatures). Equation 6 represents the full Hamiltonian to define the magnetic properties of 3 where the first term takes into considerations of magnetic exchange communication between the two spin quartets (S = 3/2) from each Co(II) ion. The other terms have been described above in the Hamiltonian eq 1. 2 2 ÄÅ É Å 2 2 ÑÑ Ĥ = −JS1̂ S2̂ − ∑ αλLî Sî + ∑ ΔÅÅÅÅLzî − ÑÑÑÑ ÅÇ 3 ÑÑÖ i=1 i=1 Å

Figure 9. Thermal dependence of χMT and χM for 2. The best theoretical fit is symbolized by the solid line as defined by eq 5.

value of 4.6 cm3 mol−1 K for χMT could be obtained at 300 K, which subsequently drops upon freezing and essentially disappears at low temperature. On the other hand, the χM arc shows a maximum at 75 K representing a typical behavior for an antiferromagnetically coupled system. Due to the relatively large antiferromagnetic interaction, which dominates the low temperature range, we can neglect the orbital contribution and use an isotropic spin-only formalism H = −J S1·S2 (S1 = S2 = 3/2).27 So, the experimental data were fitted through eq 5, derived from the above isotropic Hamiltonian, where x = J/kT.28 2Nβ 2g 2 l o o e x + 5e 3x + 14e 6x | o o χM = m } o x 3x 6x o o kT n 1 + 3e + 5e + 7e o (5) ~ The best least-squares fitting of the experimental data is acquired with values of g = 2.49(1) and J = −32.6(2) cm−1, where the negative value of J is the indication for antiferromagnetically coupled system between the two Co(II) ions. The magnetic measurements of powdered sample of 3 were studied in the temperature range 2−300 K and a plot of χMT against T is depicted in Figure 10. The χMT value obtained at room temperature per two Co (II) ions is 5.65 cm3 mol−1 K, which subsequently falls off to 3.4 cm3 mol−1 K at 2 K. The room-temperature χMT value is larger than the spin-only value for a hexa-coordinated high-spin Co(II) complex (3.0 cm3

2

+ βH ∑ ( −αLî + 2Sî ) i=1

(6)

The analytical expressions for the magnetic susceptibility as a function of parameters J, α, λ, and Δ could not be obtained; however, the matrix-diagonalization technique24 was employed to get those values. Additionally, the corrective term, Θ, in its form (T − Θ) was used in consideration with intermolecular interactions. The best-fitted parameters are those with the values of J = +1.06(1) cm−1, α = 1.295(3), λ = −165(1) cm−1, Δ = 615(2) cm−1, and Θ = −0.392(2) K. The values of the atomic G


Article

Inorganic Chemistry parameters are comparable to those observed for 1 and other hexa-coordinated Co(II) systems.25 The calculated curve equals quite well with the experimental data over the entire temperature range (Figure 10), and it unequivocally indicates the presence of an intramolecular ferromagnetic communication between the Co(II) ions. The magnetization curve at 2.0 K tends to 4.2 μB (inset of Figure 10), which it is the expected value for two magnetically isolated hexa-coordinated Co(II) ions with α = 1.3 (eq 2). In addition, the reduced magnetization curves evidently display that the low-lying levels in 3 are too close in energy as the magnetization isotherms essentially superimpose. To understand the magnetic-exchange mechanism, it is essential to note that compound 3 has one monatomic bridge (μ-oxo from a water molecule) and two μ-carboxylato (syn-syn conformation) bridges. It is well-acclaimed that the existence of last bridge produces antiferromagnetic coupling,28 such as occurs in compound 2. Depending upon the value of the M− O−M angle (M stands for metal) at the bridgehead position of the μ-oxo bridge, ferro- or antiferromagnetic coupling is perceived. A greater antiferromagnetic coupling is expected with the larger value of the angle.30 Given that this bond angle in 3 is quite large (112.2°) an antiferromagnetic coupling is anticipated through this bridge. A possible explanation for this disagreement (the observed ferromagnetic interaction versus the expected antiferromagnetic one) might be the well-known counter-complementary effect between these two types of bridges. When the bridging ligands are dissimilar, the two bridges may either enhance (complementarity) or counterbalance (counter complementarity) their effects.28 It has been observed that the μcarboxylato (syn-syn conformation) and the μ-oxo (mono atomic) bridges are counter complementary and the antiferromagnetic contributions of each bridge neutralize each other; therefore, the ferromagnetic term governs.31

Physical measurements, crystallographic data, PXRD, TGA diagram, calculation details, related plots and tabulated data of IAST selectivity, and related plots of magnetic measurements, including Tables S1−S5 and Figures S1−S28 (PDF) Accession Codes

CCDC 1896306−1896308 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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*E-mail: [email protected]. ORCID

Arun Pal: 0000-0002-0665-3446 Madhab C. Das: 0000-0002-6571-8705 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.P. acknowledges UGC for a SRF fellowship. M.C.D. gratefully acknowledges financial support received from SERB, New Delhi, as Early Carrier Research Award (ECR/ 2015/000041). F.L. acknowledges the support received from Ministerio Español de Economı ́a y Competitividad (MINECO Project CTQ2016-75068-P and Unidad de Excelencia MDM2015-0538).

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REFERENCES

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SUMMARY In summary, we have successfully synthesized three Co(III)MOFs with different architectures by using bent dicarboxylate OBA2−, linear N,N-donor Schiff base linker L with azine functionality, and Co(II) ions under diverse reaction conditions. By changing the reaction temperature and the solvent system, MOFs with 2D layer structure or 3D networks with varying topology have been realized. Gas sorption studies of activated 2 show preferential uptake for C2H2 and CO2 over CH4 and N2 with high IAST separation selectivity at ambient conditions, thus making this MOF a potential candidate for C2H2 purification and CO2 separation from flue gas and landfill gas mixtures under ambient conditions. Magnetic studies show that the first two MOFs are antiferromagnetic and 3 is ferromagnetic in nature. Given the vast library of organic linkers and linear spacers and comparatively lower number of MOFs showing microporosity based on Co(II) ions than their Zn/Cd (II) counterparts, this study may be useful for further exploration toward the design and construction of new Co(II)based MOFs with permanent microporosity for potential usage in gas separation and interesting magnetic properties.

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AUTHOR INFORMATION

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00471. H


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