Two Isostructural Metal–Organic Frameworks Directed by the Different

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Two Isostructural Metal−Organic Frameworks Directed by the Different Center Metal Ions, Exhibiting the Ferrimagnetic Behavior and Slow Magnetic Relaxation Yun-Long Wu, Fu-Sheng Guo, Guo-Ping Yang,* Lu Wang, Jun-Cheng Jin, Xiang Zhou, Wen-Yan Zhang, and Yao-Yu Wang* Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710127, P. R. China S Supporting Information *

ABSTRACT: Two 3D isostructural metal−organic frameworks with 1D ferrimagnetic chains, formulated as [M3(L)(μ3-OH)2(H2O)4] [H4L = (1,1′:4′,1″-terphenyl)-2′,3,3″,5′-tetracarboxylic acid, where M = Mn for 1 and Co for 2], have been successfully synthesized by employing different center metal ions and a multicarboxylate ligand under identical reaction conditions in this work. The single-crystal X-ray diffraction data of 1 and 2 reveal that the complexes are two 3D isostructural frameworks based on 1D [M3(OH)2]n chains composed of triangular subunits as rod-shaped secondary building units, which are classified as binodal 4,6-connected fry nets with the point symbol (510· 63·78)(54·62). The magnetic properties revealed that complexes 1 and 2 exhibit ferrimagnetic behavior. Also, the alternating-current susceptibility of 2 displays slow magnetic relaxation, showing interesting magnetic behavior of a single-chain magnet with an effective energy barrier of 32 K.

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INTRODUCTION

interchain and the magnetic properties for a specific chainlike complex. In the latest reported works by us and other groups, it has been proven that a multidentate tetracarboxylic acid, i.e., (1,1′:4′,1″-terphenyl)-2′,3,3″,5′-tetracarboxylic acid (H4L), is an excellent molecular building block (MBB) to build metal− organic frameworks (MOFs) for different functional materials, such as optical materials in the detection of metal ions (Fe3+ and Cu2+),7a,b catalysts in synthetic chemistry, etc.7c Furthermore, from structural analysis of the H4L ligand, as shown in Scheme 1, adjacent carboxylate groups of H4L may adopt chelating and bridging models with metal ions to produce 1D chain systems, then giving rise to a 3D framework sustained by

The spin dynamical behavior on a chain of the ferromagnetically coupled Ising spins named single-chain magnets (SCMs) were first predicted by Glauber in 1963;1 however, the experimental evidence of the first SCM was reported by Gatteschi et al. until 2001.2 SCMs, as analogues to single-ion magnets (SIMs) and single-molecule magnets (SMMs),3 were discovered in recent years, displaying higher relaxation barriers, slower magnetic relaxation behavior, and magnetic hysteresis of the molecule, which can further improve conditions as molecular memory devices, recording media, information storage, quantum computing, etc., for future potential applications.4 According to Glauber’s theory and reported experimental works, there are two key factors in constructing SCMs as follows: (i) the magnetic anisotropic fundamental spin unit built by bridging functional ligands and anisotropic spin center metal ions (Co2+, Mn3+, Fe2+, Dy3+, etc.); (ii) the use of appropriate diamagnetic separators to make the chains magnetically well isolated in order to weaken the intra/ interchain exchange interaction.5 Therefore, the rational synthesis approach of new SCM systems requires basic units with measurable geometries, ideal coordination abilities, fantastic magnetic characteristics, bulky coligands, and long spacers.6 In the current stage, the synthetic challenge for chemists is how to effectively fine-tune the distances of the © XXXX American Chemical Society

Scheme 1. Molecular Structure of the H4L Ligand

Received: March 26, 2016

A

DOI: 10.1021/acs.inorgchem.6b00757 Inorg. Chem. XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Description of the Crystal Structures. Single-crystal Xray data analyses reveal that MOFs 1 and 2 are the isostructural frameworks; therefore, the structure of 1 is only presented and discussed for brevity. 1 crystallizes in the monoclinic space group P21/c with a 3D framework built from the 1D ferrimagnetic [Mn3(OH)2]n chains and H4L ligands. Each asymmetric unit includes three crystallographically independent MnII ions (2Mn1 and Mn2), one L4−, one μ3-OH group, and four H2O molecules. As shown in Figure 1, all of the MnII ions

terphenyl groups. On the other hand, these 1D chains can be separated by the terphenyl groups into independent units, which may display interesting magnetic properties. Thus, H4L should be a versatile representative MMB to build multifunctional MOFs based on different metal ions. In this work, H4L is used as a MBB to synthesize peculiar MOFs with MnII/CoII ions to design and obtain homometallic MOFs with fascinating SCM magnetic behavior. Fortunately, two isostructural complexes, [M3(L)(μ3-OH)2(H2O)4] (M = Mn for 1 and Co for 2), were obtained via an identical solvothermal reaction system. Structural analyses of the two MOFs revealed that 1D [M3(OH)2]n chains composed of triangular subunits as rod-shaped secondary building units (SBUs) were further sustained by L4− in a 3D dense-packing motif. The weak magnetic mediator terphenyl group of L4− isolated the 1D ferrimagnetic chains; enlarging the distances between chains ensured vanishingly small interchain interactions in the crystal, which led to 1D chains with ferrimagnetic behavior and slow magnetic relaxation, showing interesting behavior of a SCM with an effective energy barrier of 32 K.

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Article

EXPERIMENTAL SECTION

Materials and Methods. Commercially available starting materials and reagents of analytical grade were used as received. The H4L ligand was purchased from the Jinan Camolai Trading Company. Powder X-ray diffraction (PXRD) data were collected using a Bruker D8 ADVANCE powder X-ray diffractometer (Cu Kα, λ = 1.5418 Å) with 2θ = 5−50°. Elemental analyses (C, H, and N) were performed on a PerkinElmer 2400C elemental analyzer. IR spectra were recorded in the range of 400−4000 cm−1 on an EQUINOX55 Fourier transform infrared (FT-IR) spectrophotometer. Thermogravimetric analyses (TGA) were collected by a NETZSCH STA 449C microanalyzer at 25−800 °C under the protection of N2. Magnetic susceptibility data of powdered samples restrained in parafilm were measured on a Quantum Design MPMS-XL-7 SQUID magnetometer in the range of 1.8−300 K. The diamagnetic corrections for the complexes were estimated using Pascal’s constants. Syntheses of Complexes 1 and 2. A mixture of MnSO4·H2O (0.2 mmol, 33.8 mg), H4L (0.1 mmol, 40.6 mg), H2O (4 mL), and dimethylacetamide (4 mL) was mixed in a 15 mL Teflon-lined stainless steel vessel and then heated at 145 °C for 72 h. After that, the vessel was cooled to room temperature at a rate of 10 °C h−1, and at last colorless block crystals were obtained. Yield: 65% (based on H4L). Yield: 65% (based on H4L). Elem anal. Calcd for 1: C, 39.25; H, 2.99. Found: C, 39.41; H, 2.83. FT-IR (KBr, cm−1): 3582 (s), 2965 (m), 1578 (s), 1401 (s), 902 (w), 840 (m), 766 (m), 665 (m), 572 (m). Similar to that of 1, except using Co(NO3)2·6H2O (0.2 mmol, 58.2 mg) instead of MnSO4·H2O in the synthesis process, red needlelike crystals of 2 were obtained. Yield: 53% (based on H4L). Elem anal. Calcd for 2: C, 38.56; H, 2.94. Found: C, 38.72; H, 2.83. FT-IR (KBr, cm−1): 3565 (s), 2990 (m), 1570 (s), 1392 (s), 775 (m), 589 (m). Crystallographic Data Collection and Refinement. Singlecrystal X-ray diffraction measurements were performed on a Bruker SMART APEXII CCD diffractometer equipped with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) using a ϕ/ω scan technique at 296 K. The diffraction data were corrected for Lorentz and polarization effects as well as for empirical absorption based on a multiscan. The structures of two complexes were solved by direct methods and refined anisotropically on F2 by a full-matrix leastsquares refinement with the SHELXTL program.8a Reflection data were corrected using the program SADABS.8b Anisotropic thermal parameters were applied to non-hydrogen atoms, and all hydrogen atoms from the organic ligands were calculated and added at idealized positions. Other details of relevant crystallographic data are given in Table S1. Selected bond lengths and angles are listed in Table S2. CCDC 1436509 and 1436510 are for 1 and 2, respectively.

Figure 1. Coordination environment of MnII ion in 1. Symmetry codes: #1, 1 − x, −y, −z; #2, x, −0.5 − y, −0.5 + z; #3, x, −1 + y, z; #4, x, 0.5 − y, −0.5 + z; #5, 1 − x, −0.5 + y, 0.5 − z; #6, 1 + x, y, z.

adopt distorted octahedral [MnO6] geometries. Mn1 is coordinated by two oxygen atoms from two different L4− ligands [Mn1−O 2.152(4)−2.259(4) Å], two μ3-OH group oxygen atoms [Mn1−O 2.159(4)−2.172(3) Å], and two H2O molecules [Mn1−O 2.212(4)−2.298(4) Å]. Mn2 is ligated by four oxygen atoms coming from four independent L4− ligands and two μ3-OH groups [Mn2−O 2.136(4)−2.208(4) Å]. The MnII ions are bridged by μ3-OH groups to form the repeating trimeric unit 1D [Mn3(OH)2]n cluster with a metal sequence of ···Mn2···μ3-OH···2Mn1··· (Mn1−Mn1/Mn2−Mn1 = 3.29/ 3.68 Å, Mn1−OH−Mn2 = 117.54°/116.18°, and Mn1−OH− Mn1 = 99.07°; Figures 2a,c,d and S2b), the carboxylate groups of L4− adopt one bridging model (η2μ2χ2) with the center metal ions (Figure S1). Then these neighboring 1D chains are further linked by the L4− ligands in the μ8 model to generate the 3D motif (Figures 2b and S4b). It is worth noting that the shortest distances of Mn···Mn between chains are greater than 6.40 Å, and there exists no significant interchain hydrogen-bonding or π−π-stacking interactions (Figure S3). Topologically, the carboxylate carbon atoms in the metal carboxylate chain are commonly viewed as the nodes of extension. When the 1D [Mn3(OH)2]n chains are simplified as rod-shaped SBUs, and the L4− ligands, acting as decussating linkers, are 4-connected nodes (Figure S4a), then the framework of 1 gives a binodal 4,6-connected fry topological net with the point symbol (510·63· 78)(54·62) (Figure S4b). Magnetic Behavior. Temperature-dependent magnetic susceptibility measurements for two MOFs were well performed in the range of 1.8−300 K at 1 kOe. The χmT versus T plot for the trimeric unit [Mn3] in 1 is shown in Figure 3a. The value of χmT is 11.513 cm3 mol−1 K (3 × 3.83), which is close to the theoretical value 13.13 cm3 mol−1 K (3 × 4.375) for the three spin-only MnII ions. The value gradually decreases to 7.5 cm3 mol−1 K at 50 K. The data were fitted by B

DOI: 10.1021/acs.inorgchem.6b00757 Inorg. Chem. XXXX, XXX, XXX−XXX

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reveals the antiferromagnetic (AF) couplings between MnII ions. As the temperature decreases by ∼4.5 K, χmT increases sharply to a maximum value of 37.38 cm3 mol−1 K; this increasing behavior in the low temperature range shows the characteristic of a ferrimagnet. Below 4.5 K, χmT declines quickly to a value of 20.9 cm3 mol−1 K at 1.8 K, which may be caused by the effect of saturation.9 As shown in Figure 3b, the field dependence of magnetization (M−H) of 1 was measured at 2, 3, and 5 K, showing a sudden jump at lower external fields. Also, the plots increase with slower growth to a saturation value (5 Nβ) for the formula 3Mn unit at 50 kOe consistent with the theoretical spin-only value for one uncompensated MnII ion (g = 2.0) in a triangular [Mn3] unit, supporting the AF coupling between the MnII ions. The temperature dependence of magnetization shows alternating-current (ac) susceptibility signals for 1 (Figure S8), which indicates AF order in 1. As for 2, the varying temperature magnetic susceptibility is similar to that of 1 (Figure 4a). The χmT value (∼8.9 cm3 mol−1

Figure 2. (a) 1D [Mn3(OH)2]n cluster with a metal sequence of ··· Mn2···2Mn1···. (b) 3D dense structure of 1. (c) Distances between the adjacent MnII ions. (d) Distances between the adjacent CoII ions.

Figure 4. (a) χmT versus T plots for 2. (b) M versus H plots for 2. Inset: χm−1 versus T plots fit by the Curie−Weiss law.

K) per formula is higher than the spin-only value (5.63 cm3 mol−1 K) for three S = 3/2 spins, which is caused by the unquenched orbital momentum contribution of the distorted octahedral CoII ions.10 The curve of χmT versus T reveals typical ferrimagnetic behavior. As the temperature is decreased, χmT slowly decreases to a minimum of ∼4.78 cm3 mol−1 K at 28 K and then increases dramatically to a sharp maximum of ∼12 cm3 mol−1 K at 7.6 K, suggesting ferrimagnetic character. After that, it rapidly falls to ∼2.1 cm3 mol−1 K at 1.8 K, causing the additional effect of a weak AF interchain and zero-field splitting.11 The data above 50 K were fit to the Curie−Weiss law [χm = C/(T − θ)], giving C = 11.1 cm3 mol−1 and θ = −58.3 K (inset of Figure 4b). The negative θ indicates dominant AF coupling between the neighboring CoII ions. The field dependence of magnetization of 2 was measured at 2, 3,

Figure 3. (a) χmT versus T plots for 1. (b) M versus H plots for 1. Inset: χm−1 versus T plots fit by the Curie−Weiss law.

Curie−Weiss above 50 K, which gives C = 14.3 cm3 mol−1 K and θ = −59.6 K (inset of Figure 3b). The negative value of θ C

DOI: 10.1021/acs.inorgchem.6b00757 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and 5 K to gain further exploration into the underlying magnetic properties. The results indicate that saturation magnetization is not reached even at the highest field of 50 kOe because of the strong magnetic anisotropy of the CoII metal ions in the 1D ferrimagnetic chains (Figure 4b).12 The straight line is the plot of ln(χmT) versus 1/T (Figure S9) from 13 to 20 K with Δξ/kB = 17 K and Ceff = 2.26 cm3 mol−1 K, where Δξ is the energy to create a domain wall and Ceff is the Curie constant, which demonstrate the 1D chain of the magnetic properties of 2. To gain the further insight into the magnetic properties of 2, the ac susceptibility was determined under a 2 Oe direct-field (dc) field at frequencies between 1 and 1000 Hz. The temperature and frequency dependencies (Figure 5) of the

Figure 6. Variable-frequency ac magnetic susceptibility data for real (χ′) and imaginary (χ″) components for 2 measured in a zero dc field.

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CONCLUSIONS In summary, two 3D homometallic MOFs with 1D chains based on MnII and CoII ions have been obtained by controlling the length of the organic ligand and center metal ions. These 1D chains containing the trinuclear [M3(OH)2]n ligand in two MOFs are well isolated by the bulky terphenyl of the L4− ligands, preventing efficient interchain magnetic interaction. Most importantly, the magnetic properties of two MOFs reveal that 1 and 2 display 1D ferrimagnetic chains, while 2 shows significant magnetic anisotropy and slow magnetic relaxation because of the nature of the center metal ion, displaying interesting magnetic behavior of the SCM with an effective energy barrier of 32 K. MOFs based on the CoII or other anisotropic magnetic center ions like lanthanides or other transition-metal ions may produce a series of MOFs with interesting magnetic properties, which will be act as potential functional materials in the area of molecular memory devices, recording media, information storage, and so on. Therefore, methods on how to design and synthesize new molecular magnets with higher effective energy barriers and block temperature are on the way in our group.

Figure 5. Temperature dependence below 8 K of the real (χ′) and imaginary (χ″) components of the ac susceptibility for 2 in the frequency range of 1−1000 Hz.

magnetization relaxation time τ were deduced from the maximum of the χ″ versus ν data (Figure S11a). Both the real (χ′) and imaginary (χ″) components of the ac susceptibility (Figure 6) are strongly frequency-dependent from 1.8 to 5.4 K, showing the characteristics of spin glasses, superparamagnets, or SCMs.13 However, the ac susceptibility data of the frequency dependence are analyzed by the parameter φ = (ΔTf/Tf)/ Δ(log ν) = 0.16, where Tf is the temperature at which χ″ reaches a maximum and ν is the frequency. This is the typical value for SCMs (0.1 < φ < 0.3).14 In addition, its magnetic behavior was further identified by the fitting of the Arrhenius law τ = τ0 exp(ΔUeff/T) with the best-fit parameters τ0 = 5 × 10−8 s and Ueff = 32 K, where τ0 is a preexponential factor and ΔUeff is an energy barrier quantifying the magnetization reversal; herein, the obtained values are physically meaningful and well consistent with most of the previously reported values for SCMs.15 Furthermore, the resulting Cole−Cole χ″ versus χ′ plots below 5.4 K using a generalized Debye model (Figure S10) can be fitted with the Arrhenius law (Figure S11b) with α in the range of 0.41−0.55 (Table S3), showing a moderate distribution of the relaxation time of SCMs.16,17

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00757. Additional figures, table of selected bond lengths and angles, PXRD, TGA, IR spectra, and crystallographic data of 1 and 2 (PDF) Crystallographic information files for compounds 1 and 2 (CCDC 1436509 and 1436510, respectively) (CIF) D

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support from the NSFC (Grants 21201139, 21371142, and 21531007), the NSF of Shaanxi Province (Grant 2013JQ2016), and the Open Foundation of Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education (Grant 338080049).

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DOI: 10.1021/acs.inorgchem.6b00757 Inorg. Chem. XXXX, XXX, XXX−XXX