Solvent-Controlled Syntheses, Structure, and Magnetic Properties of

Article pubs.acs.org/crystal

Solvent-Controlled Syntheses, Structure, and Magnetic Properties of Trinuclear Mn(II)-Based Metal−Organic Frameworks Fei-Yan Yi and Zhong-Ming Sun* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China S Supporting Information *

ABSTRACT: Solvothermal reactions of manganese(II) salts with hexa[4(carboxyphenyl)oxamethyl]-3-oxapentane acid (H6L) afforded a family of porous metal−organic frameworks, namely, Mn3(L)(DMA)4·2DMA (1, C2/ c), Mn3(L)(H2O)2(DMF)2·8DMF (2, Cc), and Mn3(L)(H2O)2(DMF)·4DMF (3, P21/c). All compounds have been characterized by elemental analysis and thermogravimetric analysis and structurally confirmed by single-crystal X-ray diffractions. Their structures consist of three types of trinuclear MnII subunits, which are further bridged by the carboxylic ligand, resulting in two types of topological nets (pts and sra). All of the MnII3 subunits are terminally coordinated by solvent molecules. The structure of the MnII3 core in 1 is symmetric with an inversion center, whereas those in 2 and 3 display a symmetry-breaking phenomenon. Their magnetic behaviors exhibit interesting variations, in which the local net magnetization at low temperature increases gradually from 1 to 3. Such magnetic evolution behavior in trinuclear subunits has never been observed previously.

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based on flexible multidentate tetracarboxylate ligands {tetrakis[4-(carboxyphenyl)oxamethyl]methane acid}, some porous frameworks have been reported,7 which display great gas sorption capacities, ferroelectric properties, etc. Bearing this point in the mind, a flexible hexacarboxylate ligand, hexa[4(carboxyphenyl)oxamethyl]-3-oxapentane acid (H6L), is selected to build porous MOFs. In our previous studies, the combination of H6L and UO22+ gave the first examples of threedimensional (3D) uranium organic frameworks (UOFs) with 3fold interpenetrated networks.8 As an ongoing study, we extend our work toward the transition-metal ions, especially being interested in their magnetic properties. However, it remains a challenging task to design and construct the porous MOFs with excellent magnetism for the device-related perspectives, because it is difficult to obtain simultaneously large pore sizes and relatively strong magnetic interactions. The porosity usually depends on the use of relatively long bridging ligands. However, magnetic superexchange requires relatively short exchange pathways between two adjacent metal centers. To resolve this problem, the primary synthetic strategy is to form magnetic chains or clusters linked by organic ligands in porous MOFs. In large porous MOFs, metal chains or clusters are bridged by longer organic linkers, so the intercluster magnetic interaction is very weak. Intracluster magnetic behavior plays the most important role. Physically, a ferromagnetic material is the one in which the

INTRODUCTION Metal−organic frameworks (MOFs), also known as coordination polymers, were regarded to have been developed around 20 years ago.1 They have emerged as a new class of crystalline porous materials, which are constructed from metal ions and bridging organic linkers. Yaghi brought the notion of secondary building units (SBUs), where the atoms in a crystal are replaced by a cluster and the organic connections between them make the bonds.2 The porous MOFs have attracted tremendous attention not only because of their potential applications in ferroelectrics, nonlinear optics, porous materials, and catalysis in recent years, as can be seen from the rapidly increasing publications devoted to this field,3−5 but also because of their intriguring varieties of molecular architectures and topologies.6 Organic carboxylated ligands have been widely used in synthesizing porous MOFs with various coordination modes of the carboxyl group. However, most of these carboxylate ligands reported to date are rigid to obtain firm frameworks. The final rigid structural topology is rarely influenced by several factors, such as metal−ligand ratio, solvent, etc. In comparison with rigid ligands, porous MOFs based on flexible ligands are less documented. The major reason is that the starting entities are too flexible to maintain their structural geometries during the self-assembly process. Hence, it is difficult to predict the final topologies of MOFs using flexible ligands. Meanwhile, the resultant structures are more sensitive to many subtle factors. Therefore, the design and synthesis of such porous MOFs based on flexible ligands is still a challenging aspect of crystal engineering. However, the flexibility of ligands is essential to form some particular properties and structures. For example, © 2012 American Chemical Society

Received: August 15, 2012 Revised: September 12, 2012 Published: October 2, 2012 5693

dx.doi.org/10.1021/cg3011844 | Cryst. Growth Des. 2012, 12, 5693−5700

Crystal Growth & Design

Article

40 kV, 40 mA, increment = 0.02°). Magnetic susceptibility measurements were carried out on a Quantum Design MPMSXL SQUID magnetometer and PPMS-9T system. The raw data were corrected for the susceptibility of the container and the diamagnetic contributions of the sample using Pascal constants. Synthesis of Mn3(L)(DMA)4·2DMA (1). MnCl2·4H2O (0.12 mmol, 23.7 mg) and H6L (0.04 mmol, 39.0 mg) in a mixed dimethylacetamide (DMA, 6 mL)/distilled water (H2O, 1 mL) solvent were placed in a 20 mL vial, and then sealed. The sample was heated to 85 °C in 300 min, maintained at this temperature for 3 days, and cooled to room temperature slowly. After they were washed with distilled water, colorless crystals were obtained. Yield: 62.2 mg (85% based on Mn). Its purity was confirmed by X-ray power diffraction (XRD) (Figure S1; see the Supporting Information). Anal. Calcd (%) for 1 C84H112Mn3N8O27 (Mr = 1830.64): C, 55.11; H, 6.17; N, 6.12. Found: C, 55.67; H, 6.01; N, 6.04. Syntheses of the Mn3(L)(H2O)2(DMF)2·8DMF (2) and Mn3(L)(H2O)2(DMF)·4DMF (3). The syntheses of compounds 2 and 3 are similar to that of compound 1, except that the solvent is replaced for the mixed dimethyl formamide (DMF, 6 mL)/distilled water (H2O, 1 mL) solvent for 2, and dimethyl formamide (DMF, 6 mL) solvent for 3. The yields are 15.2 mg (20% based on Mn) for 2 and 48.1 mg (81% based on Mn) for 3. Their purities were also confirmed by X-ray power diffraction (XRD) (Figure S1, Supporting Information). Anal. Calcd (%) for 2 C82H112Mn3N10O31 (Mr = 1898.64): C, 51.87; H, 5.95; N, 7.38. Found: C, 51.95; H, 6.12; N, 7.27. Anal. Calcd (%) for 3 C66H68Mn3N5O26 (Mr = 1512.07): C, 52.42; H, 4.53; N, 4.63. Found: C, 52.36; H, 4.67; N, 4.40. X-ray Crystal Structure Determination. Suitable single crystals with dimensions of 0.40 × 0.25 × 0.25 mm3 for 1, 0.30 × 0.25 × 0.20 mm3 for 2, and 0.20 × 0.20 × 0.15 mm3 for 3 were selected for singlecrystal X-ray diffraction analyses. Crystallographic data were collected at 273 K on a Bruker Apex II CCD diffractometer with graphite monochromated Mo−Kα radiation (λ = 0.71073 Å). Data processing was accomplished with the SAINT program. The structure was solved by direct methods and refined on F2 by full-matrix least squares using SHELXTL-97.15 Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. All hydrogen atoms of organic molecules were placed by geometrical considerations and were added to the structure factor calculation. A summary of the crystallographic data for the three complexes is listed in Table 1. Selected bond distances and angles are given in Table 2 and Table S1 (see the Supporting Information), respectively. The CCDC reference numbers are 885386−885384 for 1−3. More details on the crystallographic studies are given in the Supporting Information.

magnetic moments of the atoms on different sublattices are opposed; as for antiferromagnetism, however, the opposing moments are unequal and a spontaneous magnetization remains. The sum of the sublattices displays the total ferromagnetic behavior. Therefore, the most effective strategy is to maintain the ferromagnetic behavior of sublattices, and obtain local net magnetization. Especially for the homometallic system, because only one kind of spin is present, the noncompensation of the individual spin moments is difficult to achieve in these systems. The noncompensation of the individual spin moments can only be achieved through geometric or symmetry restrictions.9,10 Natarajan and his colleague reported a 3D homometallic MnII-MOF, in which ferromagnetic behavior is mainly based upon two inequivalent MnII ions with different coordinated geometries.10 Zeng and co-workers also obtained a 3D MnII-MOF based on a benzotriazole-5-carboxylate ligand, in which metal centers with asymmetric coordinated geometries are bridged into magnetic chains.10 One mutually shared characteristic that is found is that there are asymmetric metal centers, but most of them are random. In contrast, the controllable symmetry breaking to obtain local net magnetization in the subunit is a synthetic challenge for homometallic systems. Bearing the aforementioned ideas in mind, a semirigid ligand, hexa[4-(carboxyphenyl)oxamethyl]-3-oxapentane acid (H6L, Scheme 1), was selected as a building block to construct new Scheme 1. Structure of Ligand H6L and Illustration of Crystallization Processes in the Synthesis of Compounds 1− 3

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RESULTS AND DISCUSSION Solvothermal reactions of MnCl2·4H2O and H6L ligand in the presence of different solvents, DMA/H2O, DMF/H2O, or DMF, led to three novel porous MnII-MOFs, with the following formulas: Mn3(L)(DMA)4·2DMA (1), Mn3(L)(H2O)2(DMF)2·8DMF (2), and Mn3(L)(H2O)2(DMF)·4DMF (3). All compounds were characterized by single-crystal X-ray diffraction, elemental analysis, and TGA. The carboxylate group has been widely used to build magnetic units with transition-metal ions. The carboxylate group can adopt syn−syn, syn−anti, and anti−anti coordination conformations, and corresponding compounds often show ferro- or antiferromagnetic behaviors. So far, a large number of manganese carboxylates have been reported with magnetic properties,9,16−18 but a family of porous MnII-MOFs with a unique structure−magnetic property relationship have been not documented. Single-crystal X-ray diffraction results reveal that MOFs 1 and 2 crystallize in monoclinic centrosymmetric space group C2/c and noncentrosymmetric space group Cc, respectively. Their structures are built by the linear trinuclear MnII3 building

structures. Solvothermal reactions of H6L and Mn2+ in different mixed solvent systems, DMA/H2O, DMF/H2O, and DMF, at 85 °C resulted in three novel porous compounds, 1 (C2/c), 2 (Cc), and 3 (P21/c), respectively. We describe in detail their syntheses, crystal structures, and interesting magnetic behavior.

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EXPERIMENTAL SECTION

Materials and Synthesis. All chemicals were purchased commercially and used without further purification. H6L was synthesized by a modified procedure previously documented.8,11−14 Elemental analyses of C, H, and N in the solid samples were performed with a VarioEL analyzer. Energy disperse thermogravimetric and differential thermal analysis (TG-DTA) data were recorded on a Thermal Analysis Instrument (SDT 2960, TA Instruments, New Castle, DE) from room temperature to 600 °C with a heating rate of 10 °C/min under an air atmosphere. Powder X-ray power diffraction (XRD) patterns were performed on a D8 Focus (Bruker) diffractometer with Cu Kα radiation (λ = 0.15405 nm, continuous, 5694



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Table 1. Summary of Crystal Data and Structure Results for 1−3

a

compounds

1

2

3

chemical formula structural formula fw temp (K) a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z space group 2θ max (deg) μ(Mo−Kα) mm−1 D, g/cm3 F(000) reflns collected R1a [I > 2σ(I)] wR2b [I > 2σ(I)] GOF

C84H112N8O27Mn3 Mn3(L)(DMA)4·2DMA 1830.64 273(2) 24.000(3) 11.077(1) 34.991(5) 90 99.660(2) 90 9170(2) 4 C2/c 46.12 0.467 1.074 3084 19 177 0.0631 0.1942 1.095

C82H112N10O31Mn3 Mn3(L)(H2O)2(DMF)2·8DMF 1898.64 273(2) 26.418(1) 10.3733(6) 34.280(2) 90.00 103.138(1) 90.00 9148.4(9) 4 Cc 52.10 0.493 1.379 3988 24 529 0.0536 0.1351 0.970

C66H68N5O26Mn3 Mn3(L)(H2O)2(DMF)·4DMF 1512.07 273(2) 24.233(2) 11.3031(9) 36.079(3) 90 98.213(2) 90 9781.3(13) 4 P21/c 52.28 0.428 0.844 2556 53 067 0.0433 0.0824 0.912

R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = {∑w[(Fo)2 − (Fc)2]2/∑w[(Fo)2]2}1/2.

coordinated modes of terminal metal centers are totally different. Compound 1 consists of two unique MnII ions, a half L6− ligand, and two coordinated DMA molecules in its asymmetric unit (Figure S3a, Supporting Information). Mn(2) is located at the inversion site and is coordinated by six trans-related carboxylate oxygen atoms from four different L6− anions in an octahedral geometry. The Mn(1) atom is five-coordinated in a square-pyramid geometry by five oxygen atoms from three different carboxylic ligands and two DMA molecules with Mn− O distances of 2.070(3) Å (Mn(1)−O(7)) and 2.393(9) Å (Mn(1)−O(14)), respectively (Figure 2). One L 6− is coordinated to 10 MnII ions through each unidentate carboxylic oxygen atom (Figure 1). The polyhedra of three metal atoms are bridged by carboxylate groups and give rise to centrosymmetric trinuclear manganese(II) subunits (MnII3). The Mn···Mn distance within the trinuclear metal unit is 3.755(1) Å, slightly longer than those in other carboxylatebridged trinuclear MnII complexes.9 The shortest distance of inter-MnII3 units is 11.077(1) Å. The asymmetric unit of compound 2 consists of three crystallographically independent Mn2+ ions, one L6− anion, two aqua ligands, and two coordinated DMF molecules (Figure S3b, Supporting Information). In Figure 2, Mn(1) has an octahedral coordination environment surrounded by six oxygen atoms from four different L6− ligands. Terminal Mn(2) and Mn(3) atoms are five-coordinated in distorted square pyramid geometries. Three oxygen atoms are from L6− ligands, one from the DMF molecule [O(20) and O(21)], and the last one in the apical position from aqua ligands [O(1W) and O(2W)]. Bond lengths are as follows: Mn(2)−O(21) 2.194(4) Å, Mn(2)− O(1W) 2.184(3) Å, Mn(3)−O(20) 2.179(4) Å, Mn(3)− O(2W) 2.179(3) Å. Each oxygen atom in the unique L6− ligand is unidentate and bridges 1 Mn2+ ion, and the L6− anion links 10 Mn2+ ions. Three Mn atoms are bridged by syn−syn L6− ligands, forming linear noncentrosymmetric MnII3 subunits with Mn···Mn distances of 3.627(1) and 3.641(1) Å.

Table 2. Selected Bond Lengths for 1−3 compound 1 Mn(1)−O(6) Mn(1)−O(14) Mn(1)−O(9)#2 Mn(2)−O(12)#1 Mn(2)−O(5)#3 Mn(2)−O(12)#5 Mn(1)−O(5) Mn(1)−O(10)#1 Mn(1)−O(18)#2 Mn(2)−O(15) Mn(2)−O(1W) Mn(2)−O(19)#2 Mn(3)−O(9)#1 Mn(3)−O(20) Mn(1)−O(5) Mn(1)−O(14)#1 Mn(1)−O(10)#3 Mn(2)−O(5) Mn(2)−O(1W) Mn(2)−O(16)#2 Mn(3)−O(18) Mn(3)−O(2W) Mn(3)−O(7)#4

2.026(3) Mn(1)−O(7) 2.393(9) Mn(1)−O(11)#1 2.099(3) Mn(2)−O(5) 2.08(2) Mn(2)−O(13)#2 2.14(4) Mn(2)−O(13)#4 2.35(2) compound 2 2.155(3) Mn(1)−O(14) 2.134(3) Mn(1)−O(16)#1 2.183(4) Mn(1)−O(7)#3 2.108(4) Mn(2)−O(21) 2.184(3) Mn(2)−O(17)#1 2.140(4) Mn(3)−O(6) 2.099(4) Mn(3)−O(8)#3 2.179(4) Mn(3)−O(2W) compound 3 2.185(1) Mn(1)−O(18) 2.095(2) Mn(1)−O(17)#2 2.114(2) Mn(1)−O(8)#4 2.268(2) Mn(2)−O(6) 2.154(2) Mn(2)−O(20) 2.118(2) Mn(2)−O(9)#3 2.365(2) Mn(3)−O(19) 2.090(2) Mn(3)−O(15)#1 2.323(2) Mn(3)−O(8)#4

2.070(3) 2.072(4) 2.18(3) 2.07(4) 2.16(3)

2.137(4) 2.154(4) 2.184(3) 2.194(4) 2.128(4) 2.120(4) 2.124(3) 2.179(3) 2.185(2) 2.109(2) 2.251(2) 2.414(2) 2.190(2) 2.124(2) 2.177(2) 2.077(2) 2.244(2)

Symmetry transformations used to generate equivalent atoms: For 1: #1 x − 1/2, −y + 1/2, z − 1/2; #2 x, −y, z − 1/2; #3 −x + 1/2, −y + 1/2, −z; #4 −x + 1/2, y + 1/2, −z + 1/2; #5 −x + 1, y, −z + 1/2. For 2: #1 x + 1/2, y − 1/2, z; #2 x + 1/2, −y + 1/2, z + 1/2; #3 x, −y + 1, z − 1/2. For 3: #1 x, −y + 3/2, z + units and possess the same topology. The MnII3 subunits are symmetric in 1, but asymmetric in 2. Their corresponding 5695



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Figure 2. Coordination environments around the Mn(II) ions in the trinuclear clusters of 1, 2, and 3 with Mn···Mn distances.

Compound 3 crystallizes in the space group P21/c with three unique Mn(II) ions, one L6− anion, two aqua ligands, and one coordinated DMF molecule in its asymmetric unit (Figure S3c, Supporting Information). Each Mn(II) ion is 6-coordinated in an octahedral geometry. The Mn(1) atom coordinates to six oxygen atoms from four different L6− anions. Two coordinated nodes of the terminal Mn(2) atom are from the aqua ligand [O(1W)] and DMF molecule [O(20)]; the others are from carboxyl oxygen atoms. An axial site of the terminal Mn(3) atom is coordinated by an aqua ligand [O(2W)]; the other oxygen atoms are from H6L ligands. The Mn−O distances range from 2.077(2) to 2.414(2) Å. The six carboxyl groups in the L6− anion are bidentate or tridentate and bridge 10 Mn2+ ions. The Mn(1)O6 octahedron connects to terminal Mn(2) and Mn(3) octahedra via corner and edge sharing to form trinuclear MnII3 subunits. There are two types of bridges (syn− syn μ2-carboxylate bridge and oxygen bridges from L6− ligands) in the trinuclear unit, and it shows obvious symmetric breaking with Mn···Mn distances of 3.308(1) and 3.495(1) Å and Mn− O−Mn angles of Mn(1)−O(5)−Mn(2) 103.7(1)°, Mn(1)− O(8)−Mn(3) 94.8(2)°, and Mn(1)−O(18)−Mn(3) 92.9(2)°. The distances and angles between metal centers are within a range of F coupling reported.9 They are shorter and smaller than distances and angles between metal centers of similar complexes in the literature, respectively.9,16−18 As seen in Figure 2, such trimetallic building units (SBUs) are linked by L6− ligands into a 3D porous framework along the (010) direction with four types of 1D channels (11.97 × 8.35 Å, 8.89 × 7.92 Å, 8.78 × 8.05 Å, and 8.03 × 7.80 Å). All pore sizes in this work are calculated through taking into account the van der Waals radius. The solvent-accessible volume is approximately

Figure 1. Coordination modes of H6L ligands in the asymmetric units of 1, 2, and 3. C atoms are drawn as gray balls. The hydrogen atoms are omitted for clarity.

In compounds 1 and 2, all MnII3 subunits are further linked by the L6− ligands into similar types of porous 3D frameworks (Figure 3a). Each MnII3 subunit is connected by four L6− ligands and thus can be considered as a four-connected node. Each L6− site is also four-connected despite that it has six carboxyl groups. Because two couples of carboxyl groups chelate two Mn3 subunits, each L6− site is bonded to four MnII3 trimeric units. TOPOS analysis reveals that their framework can be simplified as a 4-coordinated binodal pts net with a point symbol of 42.84. Calculations using PLATON indicate that 32% and 45.5% of the total volumes are occupied by solvent molecules in 1 and 2, respectively.19 5696



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Figure 3. 3D porous frameworks for 1 and 2 (a) and 3 (b), their space filling along the b axis, and the simplified pts net for 1 and 2 and sra net for 3. Mn and O atoms are drawn as green and red circles, respectively. C and N atoms are shown as blue sticks.

5330.5 Å3 per unit cell volume, and the pore volume ratio is calculated to be 54.5% using the PLATON program.19 Each L6− site is also a four-coonected node with two couples of carboxyl groups chelating two Mn3II subunits, similar to compounds 1 and 2. TOPOS analysis reveals that this framework can be simplified as a 4-coordinated uninodal sra net with a point symbol of 42.63.8.

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Figure 4. Temperature dependence of χmT and χm−1 for 1−3 at H = 1 kOe from 2 to 300 K. The solid line is the best fit according to the corresponding classical model and the Curie−Weiss law. Inset: the field-dependent isothermal magnetization for 1−3 at 2 K.

MAGNETIC PROPERTY MEASUREMENTS By simply changing reaction solvents, the current synthetic method provides an effective way to build different topological 3D frameworks. In addition, the solvents also induce symmetry breaking within the MnII3 subunits, thus directly leading to various magnetic behaviors of compounds 1−3 at low temperature. As shown in Figure 4, the magnetic properties of 1−3 are displayed in the form of χm−1 and χmT versus T plots from 2 to 300 K. The χmT versus T plot exhibits values of 13.49, 13.46, and 12.16 cm3 K mol−1 at 300 K for 1−3, respectively. The inverse susceptibility plot as a function of temperature is linear above 15 K, following the Curie−Weiss law with a Weiss constant of θ = −12.72 K for 1, −17.32 K for 2, and −22.70 K for 3. Curie constants are 14.11, 14.13, and 13.11 cm3 mol−1 K for 1−3, respectively, close to the spin-only value as expected for three isolated Mn2+ ions (13.125 cm3 mol−1 K with S = 5/2) with g = 2.0 each. For 1, the χmT versus T plot decreases slightly in most of the temperature range on cooling and decreases more rapidly under 60 K to a value of 4.86 cm3 K mol−1 at 2 K. The negative value

of θ and the total decrease of χmT should be attributed to the overall antiferromagnetic coupling between metal centers within the trimer. For 2 and 3, their χmT curves show similar diversification at high temperature. They decrease continuously with decreasing temperature and reach a minimum of 5.10 cm3 mol−1 K at 6.5 K and 4.53 cm3 mol−1 K near 8 K, respectively, indicating an antiferromagnetic coupling among the Mn2+ ions, which also indicates that the dominant interaction between spin carriers is antiferromagnetic at high temperature. Below this minimum, the χmT rises quickly to 6.94 cm3 mol−1 K for 2 and 8.38 cm3 mol−1 K for 3 at 2 K; that may be the signature of the ferrimagnetic behavior or spin-canting response. In contrast to the above measurement for compound 1, the low-temperature magnetization for 2 and 3 is clearly different from each other. When the structure data of three compounds are combined, three Mn atoms are bridged by L6− ligands, forming linear MnII3 subunits. Assuming that the exchange coupling within the 5697



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cluster is J12 = J23 = J1, J13 = J2 (where J12 and J23 are the exchange interactions between central MnII and terminal MnII ions; J2 is the exchange interaction between terminal MnII ions and intratrinuclear MnII3), zJ′ is referring to the intercluster coupling constants in 3D MOFs. The variable-temperature magnetic susceptibility data of MnII trimers were fitted according to the analytical equation derived from the Hamiltonian20 with S1 = S2 = S2 = 5/2. n

behavior at low temperature will be ascribed to the local net magnetization, and the value increases as the symmetric breaking increases in the trinuclear unit. Considering their structral information shown in Figure 2, in compound 1, the MnII3 subunit is linear and centrosymmetric with a Mn···Mn distance of Mn(1)···Mn(2) 3.755(1) Å. Compound 2 is linear and noncentrosymmetric with one type of carboxylate bridge [Mn···Mn distances of 3.627(1), 3.641(1), and 7.267(1) Å]. In compound 3, Mn(1) atoms are linked to Mn(2) by one μ2-O bridge from the carboxylate group and two carboxylate bridges; Mn(1) is linked to Mn(3) by two μ2-O bridges from carboxylate groups and one carboxylate bridge. Therefore, there are two sets of magnetic exchange pathways within the trinuclear: carboxylate bridges and μ2-O bridges. The different coordinated molecules of terminal metal centers cause symmetry breaking in trinuclear units. The bent angle of Mn(2)−Mn(1)−Mn(3) is 155.02(3)°, for compound 3, as shown in Figure 2. From centrosymmetric trinuclear subunits (1) to obvious symmetry breaking (2 and 3), the consequent magnetization data at low temperature increases sharply: 4.86 cm3 mol−1 K for 1, 6.94 and 8.38 cm3 mol−1 K for 2 and 3, respectively. The fitting data (Weiss constant θ and intrasubunit coupling constants J1) also show similar increasing variation. J1 (intrasubunit coupling constants) values are 1 < 2 < 3 (increasing symmetry breaking). This is in view of developing the structure−magnetic property relationships, and to provide a platform for further development of interesting porous magnetic materials.

n

Ĥ = −2 ∑ ∑ Jij Si⃗ · Sj⃗ i=1 j>i

Ĥ = −2J12 S1⃗ · S⃗2 − 2J23 S⃗2 · S⃗3 − 2J13 S1⃗ · S3⃗ χt =

∑ ST(ST + 1)(2ST + 1)e−E(ST)/ kT Nβ 2g 2 × S 3kT ∑ (2ST + 1)e−E(ST)/ kT S

χm =

χt 1 − (2zJ ′/Ng 2β 2)χt

Fitting of magnetic data using the linear trinuclear Mn2+ model gives satisfactory results with the superexchange parameters: for 1, J1/kB = −1.58 K, J2/kB = 0.52 K, zJ′/kB = 0.05 K, and g = 2.01; for 2, J1/kB = −1.95 K, J2/kB = −0.04 K, zJ′/kB = 0.11 K, and g = 1.98; for 3, J1/kB = −2.66 K, J2/kB = 0.87 K, zJ′/kB = 0.14 K, and g = 1.93. The agreement factor defined by R = Σ(χmTexp − χmTcal)2/Σ(χmTexp)2 is equal to 2.78 × 10−3. These values confirm the presence of antiferromagnetic interaction between the Mn2+ ions within a trinuclear subunit. The intercluster magnetic interaction (zJ′) is rather small, indicating that the exchange interactions between two magnetic clusters are very weak, which is probably due to the large separations [11.077 (1), 10.373(1), and 11.303(1) Å for 1−3, respectively] between neighboring magnetic subunits (Figure 3). The magnetization curves at 2 K for 1−3, which are shown in the inset of Figure 3, sharply increase to reach a value of ca. 4.62, 4.24, and 2.73 Nβ at a field of 20 kOe, then slowly increases to a saturation value of 5.95, 5.37, and 4.21 Nβ at 50 kOe. The saturation value for compound 3 is far from the value anticipated for a net spin value of S = 5/2 with g = 2.0 (5.00 Nβ) in the local ferrimagnetic [5/2 + 5/2 − 5/2], further confirming the predominant antiferromagnetic coupling between the MnII ions. Its magnetization curve exhibits a hysteresis loop with a critical field of 500 Oe and a remnant magnetization of 0.05 Nβ, which is in agreement with the weak ferromagnetic response of this compound at low temperature. No hysteresis loop was observed at the temperature for 2. To assess whether the local ferrimagnetic behavior or spin canting in compounds 2 and 3 displays long-range ordering, the dynamics of the magnetization were performed from the alternating current susceptibility measurements (T < 30 K). As depicted in Figure S2 (Supporting Information), there is no divergence in the field-cooled and zero-field-cooled magnetization below 30 K under a small field, so the magnetic phase transition is not verified. The long-range ordering was also not verified, because frequency-dependent peaks did not appear in the out-of-phase magnetic susceptibility while operated in a 3.0 Oe ac field and a zero dc field. On the basis of the above results, the transition temperature may be lower than 2 K. The intertrinuclear subunit interactions within the 3D porous framework are very weak for 2 and 3. Therefore, the magnetic

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THERMAL STABILITIES Thermogravimetric (TG) analysis of 1−3 was carried out on a Thermal Analysis Instrument (SDT 2960, TA Instruments, New Castle, DE) with air flow from room temperature to 600 °C with a heating rate of 10 °C/min−1. As seen in Figure 5, the TGA curve of compound 1 shows two main steps of weight losses. The first step (129−242 °C)

Figure 5. TGA curves of compounds 1−3.

corresponds to the release of two noncoordinated DMA molecules and four coordinated DMA molecules; the observed weight loss of 14.2% is very close to the calculated value (13.9%). The second step covering a temperature range of 350−477 °C corresponds to the combustion of carboxylate ligands. The total weight loss at 477 °C is 85.8%. 5698



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The TGA result of 2 reveals the first weight loss of 23.9 wt % in the temperature range of 70−202 °C corresponding to the release of solvent molecules inserting into the pores (8 equiv of DMF per formula unit), coordinated DMF, and aqua ligands. The second weight loss occurred in the temperature range of 280−486 °C, which corresponds to the loss of coordinated DMF and aqua ligands, and combustion of carboxylate ligands and a total weight loss of ca. 82.9% at 486 °C. The TGA curve of compound 3 also exhibits two main steps of weight losses (Figure 5). The first step (48−178 °C) exhibits two continuous weight losses, which correspond to the release of five DMF molecules, coordinated DMF molecules, and two aqua ligands. The observed weight loss of 33.43% is larger than the calculated values (26.55%), which may be the water molecules of surface adsorption. The second step (316−423 °C) corresponds to the combustion of carboxylate ligands. The total observed weight loss is 84.9% at 423 °C for 3.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the support of this work by the National Nature Science Foundation of China (Nos. 21171662 and 21201162), SRF for ROCS (State Education Ministry), and CIAC startup fund. F.Y. thanks the China Postdoctoral Science Foundation (No. 20110491329) for support.

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CONCLUSIONS A family of porous MnII-MOFs based on MnII3 subunits and H6L ligands have been successfully synthesized. Herein, we describe their syntheses, crystal structures, and magnetic properties. For porous frameworks, the factors that affect structural types are rather complicated and difficult to be summarized, such as metal sources, solvent types, and configuration of ligands. It is one of the most simple and effective strategies to control topological classification of porous MOFs through varying solvents. This method also causes symmetry breaking within SBUs. Based on different coordinated small molecules of terminal metal centers, MnII3 units display different symmetry breaking. The symmetry breaking is apt to cause local non-offset of spins intradimers, such as the spin-canting phenomenon. It is reflected perfectly that the local net mganetization increases regularly at low temperature from centrosymmetric compound 1 to noncentrosymmetric compounds 2 and 3. In the homometalllic MnII system, such a phenomenon has never been reported. It is a pity that the longrange ordering was not observed. Maybe there is a long distance between trinuclear subunits in these 3D porous frameworks, so the local net magnetization is not extended well, or the temperature of long-range ordering is even lower than 2 K. Magnetic interaction between the nearest-neighbor subunit carriers is derived from different long structural modes that result in different signs and values. The magnetic strength decreases with the number of intervening bonded atoms between the subunits; when the connection is through fouratom bridges, long-range magnetic ordering was not observed above 2 K. The structure-directed symmetry breaking provides a good way to tune magnetism in a porous homometallic system. Taking into account both large porous and good magnetic exchange bridges, multinuclear subunits may be formed with a topological magnetic lattice. Future work will be focused on the explorative synthesis of ferromagnetic materials by changing reaction conditions in this system.

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Article

ASSOCIATED CONTENT

* Supporting Information S

X-ray crystallographic files in CIF format, simulated and measured XRD patterns, ORTEP representation of the asymmetric units, and table of selected bond angles for all compounds. This material is available free of charge via the Internet at http://pubs.acs.org. 5699



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