Ligand Configuration-Induced Manganese(II) Coordination Polymers

Article pubs.acs.org/IC

Ligand Configuration-Induced Manganese(II) Coordination Polymers: Syntheses, Crystal Structures, Sorption, and Magnetic Properties Wen-Juan Shi,†,‡ Li-Yun Du,† Hong-Yun Yang,† Kun Zhang,† Lei Hou,*,† 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 ‡ Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Shantou 515063, Guangdong, P. R. China S Supporting Information *

ABSTRACT: Three new coordination polymers, {[Mn3(pzbtz)2(Cl)2(H2O)2]·4H2O}n (1), {Mn2(pzbtz)(SO4)(H2O)3]· 3H2O}n (2), and {[Mn3(pzbtz)3(H2O)3]·1.5DMA·2H2O}n (3), have been solvothermally synthesized with MnCl2/MnSO4 and a bitriazole ligand, 5′-(pyrazin-2-yl)-2H,4′H-3,3′-bi(1,2,4-triazole) (H2pzbtz), in different solvent systems. H2pzbtz shows different geometrical configurations and coordination modes, leading to the diverse threedimensional (3D) frameworks of 1−3. Complex 1 contains the trinuclear Mn3(tr)4X2 (X = Cl or H2O) clusters and reveals an unobserved (3,4,8)connected sqc929 topological framework with two types of cages. Complex 2 is a new (3,4,6)-connected network based on dinuclear Mn2(tr)2 clusters, and 3 shows an 8-connected bcu topology with a novel tetranuclear Mn4(tr)6 cluster. Complexes 1 and 2 show antiferromagnetic properties, while 3 reveals spin-canting magnetic behavior with an uncommonly high Tc around 44 K. In addition, 1 also possesses good adsorption selectivity for CO2 over CH4 and N2 and an uncommon gate-opening phenomenon.

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INTRODUCTION An increasing number of investigations have been devoted to the design and construction of novel coordination polymers (CPs), owing to their intriguing topologies and great promise as materials.1 The fact is that some crucial factors, such as the coordination habits of metal ions and the individual characters of organic ligands as well as counteranions and reacting conditions influence the topological architectures and properties of CPs, so it is still very challenging to predict and obtain a CP with a predesigned topology.2 For example, during the formation of CPs, the solvents not only decide the solubility of reactants but also can behave as essential ligands, lattice molecules, or structure-directing agents, consequently altering the final structures of frameworks. More importantly, the character of organic linkers, such as the geometries, configurations, and bridging orientations, exerts a critical effect on the architectures of CPs.3 Therefore, the investigations on the structures and properties of different CPs with the same metal ions and ligands caused by the changes of coordination modes and/or the configuration of ligands as well as reaction conditions are a vital and interesting project for understanding the assembly rules of functional CPs. 1,2,4-Triazole (Htz), because of its advantage of a strong binding ability with various transition-metal ions, is widely used as an organic linker in the fabrication of CPs.4 Furthermore, the facile modifiability of Htz that is accessed by introducing varied substituents has hastened numerous Htz derivative ligands. Because of these unique properties, investigating the bi-1,2,4© 2017 American Chemical Society

triazole derivative would be an exciting project due to not only having more coordination modes but also having cis and trans configurations that depend on the relative orientations of −N− NH− groups in two triazolyl units (Scheme 1a,b). However, Scheme 1. Various Geometric Configurations of Bi-1,2,4triazole and H2pzbtz

the complexes based on the bi-1,2,4-triazole derivative ligands in which the ligands display multiform coordination modes with a variety of metal ions are scarcely reported.5 To assess the influences on the structures and properties arising from the coordination modes and/or configurations of organic ligands, a less-investigated pyrazinyl-substituted bi1,2,4-triazole ligand, 5′-(pyrazin-2-yl)-2H,4′H-3,3′-bi(1,2,4-triazole) (H2pzbtz), which possesses eight accessible coordinated N atoms, is examined. Uniquely, besides two triazolyl units, Received: June 28, 2017 Published: August 10, 2017 10090

DOI: 10.1021/acs.inorgchem.7b01657 Inorg. Chem. 2017, 56, 10090−10098

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Inorganic Chemistry Table 1. Crystallographic Data of Complexes 1−3

a

complexes

1

2

3

formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm−3) reflns collected/unique Rint GOF on F2 R1a [I > 2σ(I)/all data] wR2b (I > 2σ(I)/all data)

C16H12Cl2Mn3N16O2 696.14 monoclinic P2/m 11.788(3) 10.907(3) 12.073(3) 90 106.232(4) 90 1490.4(7) 2 1.551 6483/3036 0.0214 1.065 0.0295/0.0901 0.0335/0.0929

C8H16Mn2N8O10S 526.23 monoclinic P21/c 12.7747(11) 10.3065(9) 13.4651(12) 90 104.7080(10) 90 1714.8(3) 4 2.038 8989/3360 0.0509 1.045 0.0483/0.0675 0.1176/0.1286

C24H18Mn3N24O3 855.44 monoclinic P21/c 17.066(10) 15.375(9) 20.269(9) 90 125.71(3) 90 4318(4) 4 1.316 23049/8416 0.0928 0.847 0.0553/0.1081 0.1190/0.1386

R1 = Σ∥F0| − |Fc∥/Σ|F0|. bwR2 = [Σw(F02 − Fc2)2/Σw(F02)2]1/2. mixture was sealed and heated at 145 °C for 72 h and then cooled to room temperature at a rate of 0.1 °C/min. Yellow sheet crystals were obtained in a yield of 43% (based on H2pzbtz). Anal. Calcd for C8H16Mn2N8O10S: C, 18.26; H, 3.06; N, 21.29%. Found: C, 18.31; H, 3.14; N, 21.34%. IR (KBr, cm−1): 3124 w, 1607 w, 1436 m, 1384 w, 1337 w, 1296 m, 1132 vs, 1027 s, 852 w, 764 w, 676 w, 611 m. Synthesis of {Mn3(pzbtz)3(H2O)3]·1.5DMA·2H2O}n (3). Complex 3 was prepared in a similar method to 2, except that EtOH was replaced by DMA. Yellow sheet crystals were obtained in a yield of 67% (based on H2pzbtz). Anal. Calcd for C30H35.5Mn3N25.5O6.5: C, 35.25; H, 3.50; N, 34.95%. Found: C, 35.32; H, 3.41; N, 34.86%. IR (KBr, cm−1): 3096 w, 2928 w, 1624 s, 1428 s, 1293 vs, 1108 s, 1020 m, 733 w, 663 w, 590 w. X-ray Crystallography. A Bruker SMART APEX II CCD detector was employed to collect the crystal data of complexes 1−3 at 296(2) K using ω rotation scan as well as Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and refined by full-matrix least-squares refinements based on F2 with the SHELXTL program.6 All non-hydrogen atoms were refined anisotropically with the hydrogen atoms added to their geometrically ideal positions and also refined isotropically. The solvent molecules in 1 and 3 cannot be modeled, and the SQUEEZE routine of PLATON was used in refination.7 The formulae were determined by uniting crystal structures, elemental microanalyses, and TGA data. Relevant structure refinement results are listed in Table 1. Bond lengths and bond angles are given in Table S1.

H2pzbtz contains one pyrazinyl group, which provides two additional coordination sites and also leads to four different configurations, cis−cis, cis−trans, trans−cis, and trans−trans, based on the orientations of N atoms between two outer units and a central triazolyl unit.5a Meanwhile, the eight N atoms in one H2pzbtz can adopt monodentate bridging and bidentate chelating fashions to coordinate with metal ions. Therefore, only through changing synthesis conditions H2pzbtz could tune its configuration to satisfy the coordination requirement of the metal ions, even for the same metal ions, giving rise to diverse CPs. Inspired by these, we herein presented a remarkable case of a solvent-mediated Mn-pzbtz system: {[Mn3(pzbtz)2(Cl)2(H2O)2]·4H2O}n (1), {Mn2(pzbtz)(SO4)(H2O)3]·3H2O}n (2) and {Mn3(pzbtz)3(H2O)3]·1.5DMA· 2H2O}n (3), which show distinct topological networks due to various coordination fashions of H2pzbtz. Their structures, gas sorption, and magnetic properties were investigated as well.

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EXPERIMENTAL SECTION

Materials and General Methods. All starting materials were purchased commercially. Infrared spectra (IR) were performed in KBr discs on a Nicolet Avatar 360 FTIR spectrometer. Elemental analyses of C, H, and N were conducted with a PerkinElmer 2400C Element Analyzer. Thermogravimetric analyses (TGA) were carried out in an N2 stream using Netzsch TG209F3 equipment at a heating rate of 10 °C/min. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, 1.5418 Å). Sorption isotherms were measured using an ASAP 2020 M instrument. Magnetic measurements were performed on a Quantum Design MPMS-XL-7 SQUID magnetometer. Synthesis of {[Mn3(pzbtz)2(Cl)2(H2O)2]·4H2O}n (1). H2pzbtz (0.05 mmol, 11 mg) and MnCl2·4H2O (0.1 mmol, 20 mg) were mixed in CH3CN (7 mL) and H2O (2 mL) in a 25 mL vessel. The mixture was sealed and heated at 145 °C for 72 h and then cooled to room temperature at a rate of 0.1 °C/min. Yellow rhombus crystals were isolated in a yield of 79% (based on H2pzbtz). Anal. Calcd for C16H20Cl2Mn3N16O6: C, 25.02; H, 2.62; N, 29.17%. Found: C, 25.10; H, 2.69; N, 29.11%. IR (KBr, cm−1): 3621 w, 3520 w, 1964 m, 1460 m, 1426 s, 1306 vs, 1034 s, 995 s, 866 s, 775 m, 443 w. Synthesis of {Mn2(pzbtz)(SO4)(H2O)3]·3H2O}n (2). H2pzbtz (0.05 mmol, 11 mg) and MnSO4·H2O (0.1 mmol, 17 mg) were mixed in EtOH (4 mL) and H2O (4 mL) in a 25 mL vessel. The

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RESULTS AND DISCUSSION Synthesis. Complexes 1−3 were prepared in mixed CH3CN/H2O (7:2), EtOH/H2O (4:4), and DMA/H2O (4:4) solvents, respectively. The different products are closely dependent on the solubility differences of MnCl2/MnSO4 and H2pzbtz in reacting solvents as well as the different polarities of solvents. With regard to the above three solvent mediums and owing to the better solubility of H2pztz in DMA than in CH3CN and EtOH, H2pzbtz is most soluble in DMA/H2O; therefore, there is a higher H2pztz/Mn molar ratio (1:1) in the formula of 3 than 1 (2:3) and 2 (1:2). Meanwhile, although MnSO4 was used in the syntheses of 2 and 3 and differing from the coordination of SO42− anion in 2, no SO42− exists in 3, which is attributed to the fact that H2pztz has a relatively high solubility in DMA and preferably coordinates with the Mn2+ 10091


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Inorganic Chemistry ions by replacing SO42−. In addition, the coordination of the Cl− anion in 1 and the SO42− anion in 2 also brings about the different frameworks. These results verify the directing role of the anion salts and reacting solvents for the formations of various CPs. Crystal Structure of 1. Complex 1 has a monoclinic space group P2/m and exhibits a 3D framework with 1D channels. The asymmetric unit contains four independent Mn2+ ions, one pzbtz, two coordinated H2O molecules, and two Cl− anionic ligands. As shown in Figure 1a, all Mn2+ ions reveal similar

H2O molecules for Mn3. Mn2 is surrounded by four N atoms of four pzbtz, one H2O molecule, and one μ2-Cl− ion. Mn4 is ligated by two pairs of chelating N atoms of two pzbtz, one μ2H2O molecule, and one Cl− ion. Each pzbtz connects five Mn2+ ions with a trans−cis configuration (Scheme 1), producing two similar linear trinuclear clusters as secondary building units (SBUs). SBU1 contains one Mn1, two Mn2, two μ2-Cl− ions, four pyrazinyl, and four outer triazoyl units of eight pzbtz (Figure 1b). SBU2 consists of one Mn3, two Mn4, two μ2-H2O ligands, and four chelating pyrazinyl-triazoyl units of four pzbtz (Figure 1c). Both SBU1 and SBU2 are centrosymmetric with intracluster Mn···Mn distances of 3.598 and 3.619 Å. Notably, no similar metal−azolate−X (X = OH−, halide anions, H2O, and so on) cluster in 1 was previously observed according to the latest CCDC research. Each SBU1 and SBU2 are extended by eight and four pzbtz, respectively, and each pzbtz connects two SBU1 and one SBU2, affording a 3D framework (Figure 2a). Interestingly, two types of cages are formed in 1; one contains four pzbtz and two Mn2+ ions (Figure 2b), and the other is composed of four pzbtz and eight Mn2+ ions (Figure 2c). Adjacent cages are interconnected along the 101 direction to yield 1D square channels with voids of 30.4%7 and effective opening sizes of about 4.5 × 4.5 Å2 (Figure 2a). Topologically, pzbtz, SBU1, and two SBU2 can be designated as 3-, 8-, and 4connected nodes, respectively, thus 1 forms an uncommon trinodal (3,4,8)-connected sqc929 topology with the point symbol (4·82)4(86)(412·812·104) (Figure 2d).8 In comparison to those commonly reported uninodal and binodal topologies, the multiple nodal nets are not well explored because it requires different numbers of mixed nodes. So far, some trinodal nets, such as (3,4,5)-, (3,4,6)-, (3,4,7)-, (3,4,8)-, (3,4,9)-, and (3,4,10)-connected nets have been encountered in recent coordination frameworks,9 but they are still scarce. In particular, the (3,4,8)-connected topology was rarely reported in several

Figure 1. (a) Coordination environment of Mn2+ ions in 1, symmetry codes: #1:1−x, y, 2−z; #2: 1−x, −y, 2−z; #3: x, −y, z; #4: −x, y, 1−z; #5: −x, −y, 1−z; #6:1−x, y, 1−z; #7:1−x, 1−y, 1−z; #8: x, 1−y, z; (b) SBU1 and (c) SBU2.

distorted octahedral coordination geometries but different environments. Both Mn1 and Mn3 are coordinated by four equatorial triazoyl and pyrazinyl N atoms from four pzbtz and are coordinated also by two axial μ2-Cl− ions for Mn1 and μ2-

Figure 2. (a) 3D framework with the 1D channels along the 101 direction, (b,c) two different cages, and (d) the (3,4,8)-connected topology in 1 (pzbtz: blue balls, SBU1: cyan balls, SBU2: red balls). 10092


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Figure 3. (a) Coordination environment of Mn2+ ions in 2, symmetry codes: #1: 1−x, 1−y, −z; #2: 1−x, 0.5+y, 0.5−z; #3: x, 1.5−y, 0.5+z; #4: −x, y−0.5, 0.5−z, (b) the layer parallel to the bc plane with the opposite charities, (c) 3D structures viewed along the b axis (the coordinated H2O molecules and selected SO42− were omitted for clarity), and (d) the (3,4,6)-connected topology in 2(pzbtz: blue balls, Mn2 atoms: red balls, two Mn1 dimers: cyan balls).

CPs,9d,10 while the sqc929 topology presented in 1 has only been predicted theoretically.11 Crystal Structure of 2. Complex 2 crystallizes in the monoclinic space group P21/c and consists of two independent Mn2+ ions, one pzbtz, one SO42− anion, three H2O ligands, and three lattice H2O molecules in the asymmetric unit. As shown in Figure 3a, all Mn2+ ions reveal the distorted octahedral geometries: Mn1 is surrounded by three N atoms of two pzbtz, two O atoms of two SO42−, and one coordinated H2O; Mn2 is coordinated by four N atoms of three pzbtz and two H2O molecules. One SO42− with two O atoms bridges two Mn1 atoms. One pzbtz adopting a cis−cis geometric configuration connects five Mn2+ ions. Two Mn1 are bridged by two triazolates of two pzbtz to produce a centrosymmetric Mn2(tr)2 cluster, while one Mn1 and one Mn2 are bridged by one triazolate. Mn1 and Mn2 atoms are connected alternatively by triazolates to form 21 helical chains along the b axis (Figure 3b). The neighboring helices with the opposite chirality are jointed together through sharing the clusters to generate an achiral 2D layer parallel to the bc plane. The adjacent layers along the a axis are interlinked by the coordination of pyrazinyl N atom of pzbtz to give rise to a 3D framework (Figure 3c). The framework contains two different channels along the b axis, which are occupied by SO42− and H2O ligands, leaving small voids of 9.1% for lattice H2O residing.7 In 2, one Mn2(tr)2 cluster links four clusters through four SO42− and is further coordinated to two pzbtz and thus can be topologically simplified as a 6-connected node. One Mn2 atom connected by three pzbtz can be regarded as a 3-connected node. One pzbtz links one cluster and three Mn2 atoms and can be designated as a 4-connected node. Therefore, 2 forms an unusual trinodal (3,4,6)-connected net with the point symbol (4·5·7)2(4·52·7·82)2(44·54·62·74·10) (Figure 3d).8 The (3,4,6)connected topology is only reported in sporadic CPs;9b,12 the topology of 2 is unprecedented and differs from the various (3,4,6)-connected topologies observed in those CPs.

Crystal Structure of 3. Complex 3 with the monoclinic space group P21/c displays a 3D framework based on tetranuclear Mn4(tr)6 clusters. The asymmetric unit contains three independent Mn2+ ions, three pzbtz, and three H2O ligands. All Mn2+ ions possess the distorted octahedral geometries (Figure 4): Mn1 is coordinated by four N atoms

Figure 4. Coordination environment of Mn2+ ions in 3, symmetry codes: #1: x−1, 1/2−y, −0.5+z; #2: x, 1/2−y, 1/2+z; #3: −x, 1/2+y, 1/2−z.

from two chelating pyrazinyl-triazoyl units of two pzbtz and two H2O molecules; Mn2 is ligated by five N atoms from three pzbtz and one water O atom; Mn3 is surrounded by six triazoyl N atoms from four pzbtz. In 3, all pzbtz reveal the same cis− trans configuration with a trans coordination mode for the two chelating sites. Two Mn2 and two Mn3 are bridged by six outer triazoyls of six pzbtz to form a tetranuclear Mn4(tr)6 cluster (Mn···Mn = 4.317 and 4.478 Å) (Figure 5a). To our knowledge, the similar tetranuclear cluster in 3 was undocumented in metal−azolate systems. In 3, one Mn4(tr)6 cluster connects other four clusters through four pzbtz to form a 2D layer parallel to the bc plane (Figure 5b). The neighboring 10093


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Figure 5. (a) Tetranuclear Mn4(tr)6 cluster in 3, (b) 2D layer parallel to the bc plane, (c) 3D framework by Mn1 atom (purple polyhedra) linking neighboring layers, and (d) the 8-connected topology.

layers along the a axis are further aggregated by the coordination of interlayer pzbtz with Mn1 atoms, affording a 3D framework (Figure 5c). Each cluster extended by eight pzbtz can be viewed as an 8-connected node, so 3 forms an 8connected body-centered cubic (42464)-bcu topology (Figure 5d).8 Coordination Feature of H2pzbtz. In complexes 1−3, all of the pzbtz display the same −2 charges and similar planarity but different coordination features. In 1, pzbtz reveals the tran− cis geometric configuration and contains one chelating coordination site (Scheme 2a), while pzbtz in 2 shows the

to satisfy the coordination requirement of metal centers under varied reaction conditions. PXRD and Thermal Analyses. The phase purity of complexes 1−3 was confirmed by the coincidence of the experimental PXRD patterns with those simulated from the single-crystal structures (Figure S1). TGA of 1 displays a weight loss of 15.1% from 30−285 °C, corresponding to the removal of all H2O molecules (calcd 14.1%), and then is followed by a plateau ending at 440 °C (Figure S2). This thermostable temperature is significantly high compared to those (below 400 °C) reported in major CPs. Complex 2 shows two-step weight losses in the ranges of 30−200 °C (12.0%) and 290−430 °C (6.0%), arising from the departure of all H2O molecules (calcd 20.5%), and then decomposes at a higher temperature. Complex 3 exhibits a rapid weight loss of 17.1% from 30 to 220 °C, which is attributed to the release of one and a half DMA and two H2O solvent molecules per formula unit (calcd 16.3%). Then, a slow weight loss of 5.9% from 220−350 °C suggests the removal of coordinated H2O molecules (calcd 5.3%), followed by an abrupt weight loss due to framework decomposition. Magnetic Properties. The magnetic-susceptibility measurements of complexes 1−3 were performed under an external field of 1000 Oe in the temperature range of 2−300 K. As shown in Figure 6a, the χmT product per Mn3 unit of 1 at 300 K is 13.25 cm3 K mol−1, which is close to the spin-only value (13.125 cm3 K mol−1) expected for three magnetically isolated high-spin Mn2+ ions (S = 5/2 and g = 2.00). The temperature cooling leads to the continuous decrease of the χmT value, reaching the minimum of 3.67 cm3 K mol−1 at 2.0 K, while the χm value increases monotonically. The χm data above 25 K follows the Curie−Weiss law with C = 14.70 cm3 K mol−1 and θ = −32.34 K (Figure S3a). The negative θ value suggests antiferromagnetic couplings between the neighboring Mn2+ centers. The χmT value per Mn2 unit of 2 at 300 K is 10.22 cm3 K mol−1, moderately higher than the spin-only value for two isolated Mn2+ ions (8.75 cm3 K mol−1). Upon cooling, χmT and χm exhibit a monotonic decrease and increase, respectively

Scheme 2. Three Different Coordination Features of H2pzbtz in 1 (a), 2 (b), and 3 (c)

cis−cis configuration and has two chelating coordination sites (Scheme 2b). Meanwhile, although each pzbtz in 1 and 2 connects the same number of Mn2+ ions, it involves six- and seven-coordinated N atoms, respectively. Consequently, pzbtz serves as the 3- and 4-connected topological nodes in 1 and 2, respectively. Differing from 1 and 2, pzbtz in 3 as a 2-connected node shows a cis−trans configuration with two chelating coordination sites (Scheme 2c). Moreover, the two chelating sites of pzbtz in 2 and 3 disclose the trans and cis modes, respectively. The various coordination features of pzbtz contribute to the diverse structures of 1−3. These findings also suggest that pzbtz possesses multiform structural freedoms 10094


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Figure 6. χmT and χm vs T plots for 1 (a) and 2 (b), respectively.

(Figure 6b). The χm data above 25 K follows the Curie−Weiss law with θ = −8.25 K and C = 10.28 cm3 K mol−1 (Figure S3b), indicating an overall antiferromagnetic behavior. The magnetic behavior of 3 is different from those of 1 and 2. As shown in Figure 7, the χmT value per Mn3 unit of 3 at 300

value expected for three Mn2+ ions, suggests the occurrence of ferromagnetic-like ordering of the uncompensated spins in the framework. In addition, the magnetic susceptibility decreases as the measuring field increases, also confirming the spin-canted antiferromagnetism of 3 (Figure 7, inset). Also, long-range magnetic order is evident from both low-field magnetization and ac susceptibility measurements. The divergence of the zero field-cooled magnetization and field-cooled magnetization (ZFC/FC) data below Tc = 44 K indicates irreversible behavior arising from the formation of a magnetically ordered state (Figure 8). The ac susceptibility shows a maximum at 41 K in

Figure 7. χmT vs T curve measured at 1000 Oe for 3. The inset depicts curves at varying applied fields.

K is 13.50 cm3 K mol−1, slightly higher than the value (13.125 cm3 K mol−1) expected for three isolated Mn2+ ions. Upon cooling, χmT decreases smoothly to a rounded minimum of 12.79 cm3 K mol−1 at about 45 K, then rises rapidly to a sharp maximum of 23.52 cm3 K mol−1 at 35 K, and finally drops rapidly upon further cooling to 2.0 K. The magnetic behavior above 45 K indicates an overall antiferromagnetic interaction, which is further confirmed by the fit of χm data with the Curie− Weiss equation (C = 14.1 cm3 K mol−1 and θ = −7.2 K) (Figure S3c). The abrupt increases in χmT below 45 K and the very large χmT maximum suggest that there is a mechanism of generating uncompensated spin moments and that an ferromagnetic-like correlation between the uncompensated spins is operative and develops into long-range ferromagnetic ordering. The descent of χmT below 35 K may be due to saturation effects and/or antiferromagnetic interactions.13 In particular, the uncompensated spin moment and ferromagnetic correlations at low temperature can be attributed to spin canting in the framework. The occurrence of spin canting is consistent with the structural characteristics of 3, the lack of an inversion center between neighboring Mn2+ ions, and the systematic alternation of the relative orientation of the coordination polyhedrons throughout the framework. Meanwhile, the very high value of the χmT maximum, far above the

Figure 8. ZFC and FC versus T curves measured at 20 Oe for 3.

the in-phase (χ′) region (Figure S4), and a peak is also detectable in the out-of-phase (χ″) region at the same temperature. The nonzero signal in the imaginary component indicates the presence of a weak ferromagnetism ordering as a result of the spin canting in this magnetic system. Notably, the critical temperature of 41 K in 3 is much higher, which is rarely reported in complexes.13a,14 The magnetization of 3 increases almost linearly until 70 kOe with the effective moment of 14.17 Nβ (Figure S5a), which is far from the expected saturated value Ms (15 Nβ) for three Mn2+ ions, supporting the existence of spin-canted antiferromagnetic interactions. A small hysteresis loop is observed at 1.8 K with a coercive field of 150 Oe and a remanent magnetization Mr of 0.04 Nβ (Figure S5b). The canting angle α is evaluated by the equation sin(α) = Mr/Ms to be 0.15°, which is comparable to other Mn2+-containing complexes.15 Sorption Properties. The good thermolstability of 1 encourages us to evaluate its porosity. Complex 1 was activated at 180 °C for 5 h under vacuum to remove solvent molecules. 10095


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Figure 9. (a) Gas sorption isotherms of 1 for CO2 (195 K), CH4 (195 K), and N2 (77 K) (inset: the enlarged adsorption isotherm of CO2 at 195 K). (b) Gas sorption isotherms of 1 for CO2 and CH4 at 273 and 298 K.

polar pores which generate the strong contacts with CO2 because of the large quadrupole moment (CO2, 4.30 × 10−26 esu m2; CH4, 0) and the high polarizability (CO2, 29.1× 10−25 cm−3; CH4, 25.9 × 10−25 cm−3) of CO2 relative to CH419 as well as the smaller kinetic diameter of CO2 (3.30 Å) than that of CH4 (3.80 Å).

The activated 1 reveals the slight shifts of some diffraction peaks in PXRD patterns (Figure S1), but remaining are the narrow and strong peaks, suggesting the flexibility of the framework to a degree. The N2, CO2, and CH4 sorption isotherms were measured at 77 and 195 K on the desolvated sample, respectively. The desolvated 1 reveals little uptake for CH4 and N2, with the corresponding adsorption amounts of 8.8 and 4.3 cm3 (STP) g−1 at 100 kPa. Interestingly, the CO2 sorption isotherm of 1 at 195 K shows a gate-opening phenomenon, wherein 1 shows the low CO2 uptake of 9.3 cm3 (STP) g−1 at 2.2 kPa (Figure 9a, inset). Complex 1 displays a type-I sorption isotherm for CO2 at 195 K, with the adsorption amounts increased abruptly to 87.8 cm3 (STP) g−1 at 9.8 kPa and 113.9 cm3 (STP) g−1 at 100 kPa. The Brunauer−Emmett− Teller (BET) and Langmuir surface areas are 351 and 476 cm2 g−1, respectively. Meanwhile, 1 also displays moderately hysteretic desorption, which is possibly ascribed to the hampered release of CO2 in the channels of 1 due to the close sizes of CO2 (3.4 × 3.3 × 5.4 Å3) and channel (4.5 × 4.5 Å2). Furthermore, the gate-opening phenomenon is clearer at 273 and 298 K, with corresponding triggered pressure points of 50 and 65 kPa, respectively (Figure 9b). This unique characteristic of sorption isotherms was not frequently observed in metal− organic frameworks (MOFs) or porous CPs,16 in which the pores are closed or opened at a moderate pressure because of the shrinkage-swelling of the flexible frameworks. The existence of gate-opening pressure in 1 is possibly associated with on one hand, the flexibility of the framework as reflected by PXRD with the slight shift of some peaks (Figure S1), leading to a breathing effect at the imposed pressures;16c,e−g on the other hand, the intrinsic polar porous surface decorated by the electron-rich N-heterocyclic units of pzbtz and Cl− ions, which could form strong interactions with CO2 even at low pressure, blocks the entrance of CO2 molecules into the pores until at a moderate pressure.17 The ascent of the gate-opening pressure with the measured temperatures in 1 has been similarly noted in other flexible MOFs.16a,d,18 This phenomenon results from the fact that at higher temperatures, the framework−CO2 interactions become weak due to the relative large kinetic energy of the CO2 molecules. Therefore, to make CO2 molecules be adsorbed in the framework, the density of CO2 should be enhanced at a higher temperature, which incurs a higher gate-opening pressure.18 At 273 K and 100 kPa, 1 shows significant adsorption of CO2 (40.1 cm3 (STP) g−1) but only a trace of uptake for CH4 (1.9 cm3 (STP) g−1) (Figure 9b). The remarkable adsorption selectivity for CO2 over CH4 is mainly attributed to the highly

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CONCLUSIONS In summary, three new CPs have been assembled by the reaction of Mn2+ ions with a pyrazinyl-bitriazole H2pzbtz ligand through tuning reaction conditions. H2pzbtz, due to the N-rich feature, reveals different geometrical configurations of tran−cis, cis−cis, and cis−trans, leading to diverse 3D cluster-based frameworks of complexes 1−3: 1 contains the new trinuclear Mn3(tr)4X2 (X = Cl or H2O) clusters and shows an unprecedented (3,4,8)-connected sqc929 topology; 2 exhibits an unobserved (3,4,6)-connected framework based on dinuclear Mn2(tr)2 clusters; 3 shows an 8-connected bcu net based on novel tetranuclear Mn4(tr)6 clusters. Complexes 1 and 2 show antiferromagnetic exchanges between the neighboring Mn2+ centers; however, 3 reveals spin-canting magnetic behavior and has an uncommonly high Tc around 44 K as well. In addition, 1 with 1D channels based on two types of cages shows not only good adsorption selectivity for CO2 over CH4 and N2 but also a rare gate-opening phenomenon for CO2 gas adsorption. In this presentation, the finding of new topologies with trinodal nodes in 1 and 2 enriches the database of mixed node-based CPs; meanwhile, the adoption of Nmultiple bitriazole systems affords an unusual assembly method for the crystal engineering of CPs.

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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.7b01657. Additional figures: TGA, PXRD, magnetic data, and selected bond length/angle table (PDF) Accession Codes

CCDC 1557493−1557495 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. 10096


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Lei Hou: 0000-0002-2874-9326 Yao-Yu Wang: 0000-0002-0800-7093 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (21471124 and 21531007), NSF of Shannxi province (15JK1731), Open Foundation of Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education (338050067), and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province (KLPAOSM201504).

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