Antiferromagnetic Interactions of

Article pubs.acs.org/crystal

Diverse Structures and Ferro-/Ferri-/Antiferromagnetic Interactions of Pyridyltetrazole Coordination Polymers with Polycarboxylate Auxiliary Ligands Ming-Xing Li,*,† Yan-Fei Zhang, Xiang He,† Xue-Min Shi,† Ya-Ping Wang,† Min Shao,‡ and Zhao-Xi Wang*,† †

Department of Chemistry, College of Sciences, Shanghai University, Shanghai 200444, People’s Republic of China Laboratory for Microstructures, Shanghai University, Shanghai 200444, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Metal salts and 3/4-pyridyltetrazoles (3/4-Hptz) react with oxalic acid (H2ox), 1,2,4,5-benzenetetracarboxylic acid (H4btec), isophthalic acid (H2ip), and 1,2,3,4-butanetetracarboxylic acid (H4btca) to afford six coordination polymers, namely, [Cu(4-ptz)(ox)0.5]n (1), [Cu(4-Hptz)(btec) 0.5 ] n (2), [Co 2 (3-ptz)(ip)(OH)(H 2 O)] n (3), {[Co 1.5 (3-ptz)(btca)0.5(H2O)]·H2O}n (4), {[Co3(4-ptz)2(btca)(H2O)3]·2H2O}n (5), and {[Cd2(4-ptz)2(btca)0.5(H2O)3]·DMF·4H2O}n (6). Their structures were characterized by EA, IR, PXRD, TGA, and single-crystal XRD. 3/4Pyridyltetrazoles display μ2-, μ3-, and μ5-coordination modes in them. Complex 1 is a 3D MOF constructed from a 2D (4,4) [Cu(4-ptz)]n grid pillared by an oxalate spacer. Complex 2 displays a 2D network containing a square planar Cu1 and elongated octahedral Cu2 centers. Complex 3 shows a 2D network containing μ5-ptz, μ3-ip, and μ3-OH. Complex 4 exhibits a 3D MOF assembled by a 2D [Co1.5(btca)0.5]n network pillared by a 3-ptz spacer. Complex 5 displays a 3D porous MOF with nanosized rectangular tunnels. Complex 6 features a 3D porous MOF built by 2D [Cd2(4-ptz)(btca)0.5]n network pillared by a 4-ptz spacer. Variable-temperature magnetic susceptibility and magneto-structural correlation studies indicate that five CuII/CoII complexes present remarkable magnetic coupling diversity from ferro-, to ferri-, to antiferromagnetic interactions. Complex 1 shows strong antiferromagnetic behavior originating from the oxalate-bridging CuII dimer (Jox = −94.7 cm−1) and the tetrazole-bridging CuII dimer (Jptz = −61.0 cm−1). Complex 2 presents a weak ferromagnetic interaction (J = 0.82 cm−1) originating from the magnetic orbital orthogonality between square planar CuII and octahedral CuII centers. Complex 3 displays ferrimagnetic coupling in the Co3(μ3-OH) triangle. Two btca CoII complexes show antiferromagnetic interactions originating from carboxyl-bridging trinuclear CoII clusters with the θ values of −17.08 K for 4 and −8.6 K for 5. The thermal behaviors of these nitrogen-rich tetrazole complexes are investigated. Anhydrous complexes 1−3 containing rigid polycarboxylates are thermally stable to 250−300 °C and then decompose explosively, whereas complexes 4−6 with flexible butanetetracarboxylate decompose smoothly.

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relatively low pKa values (in the range 3−5),27 which means that their deprotonation processes easily occur and give rise to tetrazolate ligands. To date, a number of pyridyltetrazole-based coordination polymers have been synthesized and structurally characterized.28−30 Pyridyltetrazoles possess five nitrogen donors, which can serve as valuable bridging ligands to propagate metal magnetic exchange interactions.31−33 Moreover, these nitrogen-rich tetrazole derivatives and their metal complexes also arouse much attention in the application as high energy materials.34−36 Among the many strategies to construct coordination polymers, the self-assembly of metal ions and N-heterocycles

INTRODUCTION In the past two decades, coordination polymers (CPs) and metal−organic frameworks (MOFs) have attracted great interest owing to their intriguing structural motifs and distinctive physical properties,1−5 such as magnetism,6−8 luminescence,9,10 chirality,11,12 and porous absorption.13−15 Much effort was focused on the rational design and controlled synthesis of functional coordination polymers using Nheterocyclic and polycarboxylate ligands.16−20 Most N-heterocyclic compounds, such as triazole, tetrazole, and pyridyl derivatives, exhibit varied coordination modes, which make them very appealing for constructing coordination polymers with various structures and properties.21−23 Tetrazole and its derivatives are good multidentate ligands due to their tunable backbones and versatile coordination structures.24−26 Similar to the pKa values of carboxylic acids, pyridyltetrazolates have © XXXX American Chemical Society

Received: February 17, 2016 Revised: April 4, 2016

A

DOI: 10.1021/acs.cgd.6b00258 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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radiation. Variable-temperature (2−300 K) magnetic susceptibilities were measured on a Quantum Design MPMS-XL7 SQUID magnetometer in a field of 1000 Oe. Diamagnetic corrections were made with Pascal’s constants. Synthesis of [Cu(4-ptz)(ox)0.5]n (1). A mixture of Cu(NO3)2·3H2O (0.2 mmol), 4-Hptz (0.1 mmol), sodium oxalate (0.2 mmol), and H2O (8 mL) was sealed in a 15 mL Teflon-lined reactor and heated at 120 °C for 72 h. Dark blue cubic crystals were obtained in 71% yield based on 4-Hptz. Anal. Calcd for C7H4CuN5O2: C, 33.14; H, 1.59; N, 27.61. Found: C, 33.11; H, 1.57; N, 27.68. IR (cm−1): 3103w, 3045w, 1652s, 1627s, 1435m, 1062m, 843m, 722m. Synthesis of [Cu(4-Hptz)(btec)0.5]n (2). A mixture of Cu(NO3)2· 3H2O (0.2 mmol), 4-Hptz (0.1 mmol), H4btec (0.1 mmol), and H2O (8 mL) was sealed in a 15 mL Teflon-lined reactor and heated at 120 °C for 72 h. Light purple cubic crystals were obtained in 51% yield based on 4-Hptz. Anal. Calcd for C11H6CuN5O4: C, 39.35; H, 1.80; N, 20.85. Found: C, 39.41; H, 1.49; N, 20.88. IR (cm−1): 3100w, 3062w, 1644s, 1622s, 1582s, 1508m, 1481m, 1339s, 1232m, 822s, 748s, 607m, 525m. Synthesis of [Co2(3-ptz)(ip)(OH)(H2O)]n (3). A mixture of Co(OAc)2·4H2O (0.2 mmol), 3-Hptz (0.1 mmol), H2ip (0.1 mmol), and H2O (8 mL) was sealed in a 15 mL Teflon-lined reactor and heated at 140 °C for 72 h. Purple rod crystals were obtained in 43% yield based on 3-Hptz. Anal. Calcd for C14H11Co2N5O6: C, 36.30; H, 2.40; N, 15.12. Found: C, 36.19; H, 2.73; N, 15.93. IR (cm−1): 3615m, 3566m, 3076m, 3054m, 1602s, 1547s, 1431s, 1380s, 1274m, 1187m, 1027m, 861m, 813m, 768s, 701s, 641m, 431s. Synthesis of {[Co1.5(3-ptz)(btca)0.5(H2O)]·H2O}n (4). A mixture of Co(NO3)2·6H2O (0.2 mmol), 3-Hptz (0.1 mmol), H4btca (0.1 mmol), NaOH (0.2 mmol), and H2O (8 mL) was sealed in a 15 mL Teflon-lined reactor and heated at 160 °C for 72 h. Purple rod crystals were obtained in 28% yield based on 3-Hptz. Anal. Calcd for C10H11Co1.5N5O6: C, 31.15; H, 2.88; N, 18.16. Found: C, 31.47; H, 2.67; N, 18.44. IR (cm−1): 3329m, 3069w, 2911w, 1581s, 1428s, 1289s, 1190m, 1142m, 1012m, 893m, 815m, 755m, 695s. Synthesis of {[Co3(4-ptz)2(btca)(H2O)3]·2H2O}n (5). A mixture of Co(NO3)2·6H2O (0.2 mmol), 4-Hptz (0.1 mmol), H4btca (0.1 mmol), NaOH (0.4 mmol), and H2O (8 mL) was sealed in a 15 mL Teflon-lined reactor and heated at 140 °C for 72 h. Purple rod crystals were obtained in 84% yield based on 4-Hptz. Anal. Calcd for C20H24Co3N10O13: C, 30.43; H, 3.06; N, 17.74. Found: C, 29.91; H, 3.23; N, 17.51. IR (cm−1): 3265m, 3058w, 2978w, 2949w, 2910w, 1575s, 1515s, 1413s, 1391s, 1315s, 1276s, 856s, 726s, 672m, 524m. Synthesis of {[Cd2(4-ptz)2(btca)0.5(H2O)3]·DMF·4H2O}n (6). A 3 mL aqueous solution of Cd(NO3)2·4H2O (0.2 mmol) and H4btca (0.1 mmol) was added to a 5 mL DMF solution of 4-Hptz (0.1 mmol). The mixture was left undisturbed at room temperature for two months. Light yellow cubic crystals were obtained in 48% yield based on 4-Hptz. Anal. Calcd for C19H32Cd2N11O12: C, 27.45; H, 3.88; N, 18.53. Found: C, 27.44; H, 3.73; N, 18.56. IR (cm−1): 3448m, 3279m, 2961w, 2929w, 1668s, 1621s, 1576s, 1416s, 1391m, 1219m, 1010m, 861m, 711s, 560m. X-ray Crystallography. Single-crystal X-ray diffraction data for complexes 1−6 were collected on a Bruker Smart Apex-II CCD diffractometer with graphite monochromatic Mo Kα radiation (λ = 0.71073 Å) at room temperature. Determinations of the crystal system, orientation matrix, and cell dimensions were performed according to the established procedures. The structures were solved by the direct methods and refined on F2 by full-matrix least-squares techniques using the SHELXTL program.61 All non-hydrogen atoms were located from the difference Fourier maps and refined anisotropically. All hydrogen atoms were placed geometrically and refined using the riding model. Table 1 shows the crystallographic data and structure refinement parameters. Selected bond distances and angles are listed in Table S1 (Supporting Information).

cooperating with polycarboxylates is one of the most effective approaches. Polycarboxylates have been extensively used to construct coordination polymers, owing to their versatile coordination modes and high structural stability.37−40 Several well-known MOFs are prepared by rigid polycarboxylates, such as terephthalate (MOF-5) and 1,3,5-benzenetricarboxylate (HKUST-1, MIL-100).41−43 Similarly, isophthalic acid (H2ip) and 1,2,4,5-benzenetetracarboxylic acid (H4btec) act as bridging ligand in many coordination polymers.44−47 Oxalic acid (H2ox) is used in preparing coordination polymers with strong magnetic exchange interactions.48,49 1,2,3,4-Butanetetracarboxylic acid (H4btca) is a valuable flexible ligand due to the facts that its four carboxyl groups can be fully or partially deprotonated and that it displays varied coordination modes.50−52 The magnetism of carboxyl complexes has been extensively investigated. Most CuII/CoII coordination polymers exhibit antiferromagnet properties, with fewer being ferromagnetic and ferrimagnetic.53−55 Generally, the interactions mediated by single or double carboxyl groups are weak, but they could be strengthened by additional bridging groups such as hydroxide and triazole groups.56,57 Similarly, tetrazole is a versatile ligand for the construction of coordination polymers. Tetrazole has ten different coordination modes and can be used as a good organic linker to propagate magnetic coupling among magnetic metal ions.57−59 However, the magnetism of tetrazole complexes is still less studied. In this work, we utilized 3/4-pyridyltetrazoles as primary ligands and four poly(carboxylic acid)s as auxiliary ligands (Scheme 1), and we successfully prepared four porous 3D Scheme 1. Pyridyltetrazoles and Polycarboxylic Acids

MOFs and two 2D networks. The pyridyltetrazoles display varied coordination modes. Variable-temperature magnetic susceptibility and magneto-structural correlation studies reveal that five CuII/CoII complexes present remarkable magnetic coupling diversity from ferro-, to ferri-, to antiferromagnetic interactions. Their thermal behaviors are also investigated.

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EXPERIMENTAL SECTION

Materials and Measurements. 3/4-Pyridyltetrazoles were prepared according to the literature method.60 Other reagents were obtained commercially as reagent grade and used without further purification. Elemental analyses (C, H, and N) were carried out on a Vario EL III elemental analyzer. Infrared spectra were recorded on a Nicolet A370 FT-IR spectrometer in the 400−4000 cm−1 range with KBr pellets. Thermogravimetric analyses (TGA) were performed on a Netzsch STA 449C thermal analyzer at a heating rate of 10 °C min−1 in air. Powder X-ray diffraction (PXRD) data were collected on a Rigaku DL/Max-2550 diffractometer with Cu Kα (λ = 1.5418 Å)

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RESULTS AND DISCUSSION Cu /CoII complexes 1−5 were synthesized under hydrothermal conditions, whereas CdII complex 6 was prepared in a B

II



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Table 1. Crystallographic Data and Structure Refinement Parameters for 1−6 formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm−3) F(000) μ (mm−1) reflections/unique Rint Data/restraints/param R1, wR2 [I > 2σ(I)] R1, wR2 (all data) GOF on F2 Δρmax, Δρmin (e Å−3) formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm−3) F(000) μ (mm−1) reflections/unique Rint Data/restraints/param R1, wR2 [I > 2σ(I)] R1, wR2 (all data) GOF on F2 Δρmax, Δρmin (e Å−3)

1

2

3

C7H4CuN5O2 253.69 orthorhombic Pbca 7.646(1) 14.935(2) 15.763(2) 90 90 90 1800.0(4) 8 1.872 1008 2.412 8536/1592 0.0209 1592/0/136 0.0226, 0.0578 0.0265, 0.0598 1.060 0.258, −0.269 4

C11H6CuN5O4 335.75 triclinic Pi ̅ 7.119(2) 8.765(2) 9.552(2) 110.735(3) 95.917(3) 93.970(3) 550.8(2) 2 2.024 336 2.011 2806/1905 0.0189 1905/0/197 0.0344, 0.0853 0.0491, 0.0941 1.042 0.434, −0.784 5

C14H11Co2N5O6 463.14 monoclinic P21 10.367(6) 6.665(4) 11.298(6) 90 108.531(5) 90 740.2(7) 2 2.078 464 2.292 4436/2997 0.0143 2997/7/ 253 0.0241, 0.0653 0.0247, 0.0660 1.058 0.394, −0.474 6

C20H24Co3N10O13 789.28 monoclinic P21 7.267(2) 23.567(7) 8.128(2) 90 95.906(4) 90 1384.6(7) 2 1.862 798 1.862 8651/4240 0.0248 5464/1/ 416 0.0420, 0.1077 0.0467, 0.1112 1.069 1.729, −0.647

C19H32Cd2N11O12 831.35 monoclinic P21/c 15.316 (1) 12.618 (9) 15.566(1) 90 96.165 (1) 90 2991.0(4) 4 1.846 1660 1.498 15291/5265 0.0253 5265/0/399 0.0284, 0.0677 0.0379, 0.0722 1.041 0.902, −0.599

C10H11Co1.5N5O6 385.63 triclinic Pi ̅ 7.279(1) 8.105(1) 11.970(2) 91.471(2) 101.122(1) 96.540(2) 687.65(15) 2 1.862 389 1.870 4297/3032 0.0233 3032/0/205 0.0429, 0.0979 0.0622, 0.1083 1.035 0.842, −0.486

2.189(2) Å. The four coordination bonds on the basal plane are slightly short, from 1.976(2) to 2.010(2) Å. Oxalate is a tetradentate ligand and adopts a symmetrical bisbidentate chelating mode to combine two CuII ions. 4-Ptz as a tridentate ligand binds to three CuII ions via two 2,3-positional tetrazolyl nitrogens and a pyridyl group. Two tetrazolyl groups link two CuII ions to form a Cu2N4 dimer. Each 4-ptz ligand connects three CuII ions to construct a [Cu(4-ptz)]n 2D (4,4) grid, where the Cu2N4 dimer acts as a node (Figure 1b). These 2D [Cu(4-ptz)]n grids are parallel arranged and pillared by an oxalate spacer to generate a porous 3D MOF with a rectangular tunnel (Figure 1c). The approximate dimensions of the rectangular channel are 6.94 × 9.95 Å2. From the viewpoint of the network topology,62 4-ptz and Cu1 can be treated as 3-

solution reaction. The additive metal salts, 3/4-Hptz, and corresponding poly(carboxylic acid)s were kept in a 2:1:1 molar ratio, except sodium oxalate was used in a 2:1:2 ratio in the preparation of 1. NaOH was added into the reaction mixtures to prepare 4 and 5. All the complexes were characterized by elemental analysis, IR spectra, PXRD, and TGA. Their structures were confirmed by single-crystal X-ray diffraction analysis. Structure of [Cu(4-ptz)(ox)0.5]n (1). Complex 1 is a porous 3D MOF. As shown in Figure 1a, CuII adopts a squarepyramidal geometry, coordinated by two tetrazolyl nitrogens, two carboxyl oxygens, and pyridyl N1. The tetrazolyl N4 occupies the apical position with a Cu1−N4 bond distance of C



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being protonated, the free pyridyl group hinders the 2D network from further extending to a 3D framework. The protonated pyridyl group was also found in the reported pyridyltetrazole complexes.63 Structure of [Co2(3-ptz)(ip)(OH)(H2O)]n (3). Complex 3 is a 2D network containing two distinct CoII ions (Figure 3a).

Figure 1. (a) Asymmetric unit of 1. (b) 2D (4,4) grid [Cu(4-ptz)]n. (c) Porous 3D MOF. (d) 2-fold interpenetrated 2-nodal (3,4)connected topology net.

and 4-connected nodes, respectively. The 3D framework can be simplified as a 2-nodal (3,4)-connected topology net with point symbol (4,82)(4,86), which is 2-fold interpenetrated as depicted in Figure 1d. Structure of [Cu(4-Hptz)(btec)0.5]n (2). Complex 2 is a 2D network. The asymmetric unit contains a half Cu1 and a half Cu2 ion, a neutral 4-Hptz and a half btec ligand (Figure 2a). Cu1 locates at the inversion center of a square geometry

Figure 3. (a) Asymmetric unit of 3. (b) 2D [Co2(3-ptz)]n network. (c) 2D [Co2(ip)(OH)]n network. (d) 2D network of 3.

Co1 adopts a mer-octahedral geometry completed by two tetrazolyl nitrogens, pyridyl N1, carboxyl O1, hydroxyl O5B, and water O1W. Co2 adopts a trans-octahedral geometry, coordinated by two tetrazolyl nitrogens, two carboxyl oxygens, and two hydroxyl oxygens (O5, O5A). The axial positions are occupied by two nitrogen atoms with a N4−Co2−N3A angle of 176.8°. All the Co−O and Co−N bond distances fall in the normal range of 2.029(2)−2.272(3) Å. 3-Ptz ligand is pentadentate and coordinates to five CoII ions via four tetrazolyl nitrogens and a pyridyl group, yielding a 2D [Co2(3-ptz)]n network (Figure 3b). The ip anion is a tridentate ligand that combines with three CoII ions via a bidentatebridging carboxyl group and a monodentate carboxyl group. It is noted that complex 3 has a μ3-hydroxyl group (O5) that links Co2, Co2A, and Co1A to form a triangular Co3(μ3-OH) cluster. These Co3(μ3-OH) triangles are further linked together by sharing Co2/Co2A and by the connection of a bridging carboxyl group, forming a 1D [Co2(μ3-OH)(μ2-COO)]n chain. Such 1D chains are parallel arranged and pillared by ip space to form a 2D [Co2(ip)(OH)]n network (Figure 3c), which is further connected with [Co2(3-ptz)]n network to generate the final 2D coordination polymer (Figure 3d). Structure of {[Co1.5(3-ptz)(btca)0.5(H2O)]·H2O}n (4). Complex 4 is a 3D porous MOF containing Co2 and a half Co1 (Figure 4a). Co1 locates at the inversion center of a transoctahedron sphere surrounded by two tetrazolyl nitrogens and four carboxyl oxygens. Co2 sits at a cis-octahedron sphere completed by three carboxyl oxygens, tetrazolyl N3, pyridyl N1A, and water O1W. All the Cd−O and Cd−N bond distances fall in the 2.018(3)−2.219(3) Å range. H4btca is fully deprotonated, and the btca anion acts as an octadentate ligand to combine with two Co1 ions and four Co2 ions. As described in Figure 4b, two middle β-carboxyl groups adopt a chelating-bridging fashion to chelate Co2 and to bridge Co1. Two terminal α-carboxyl groups adopt a bidentatebridging fashion to link Co1 and Co2. Such a coordination

Figure 2. (a) Asymmetric unit of 2. (b) 2D [Cu(btec)0.5]n network. (c) 2D network of 2.

completed by two tetrazolyl nitrogens and two carboxyl oxygens. Cu2 sits at the inversion center of an elongated octahedron sphere completed by two tetrazolyl nitrogens and four carboxyl oxygens. O1 and O1A occupy the axial positions with a weak coordination bond (2.356(3) Å) that is obviously longer than Cu2−O3 (1.963(3) Å) and Cu2−N1 (1.986(3) Å). Four carboxyl groups of H4btec are fully deprotonated. Btec anion is tetradentate and coordinates to four Cu(II) ions via two monodentate-bridging (O1) carboxyl groups and two monodentate (O3) carboxyl groups. Carboxyl O1 bridges Cu1 and Cu2 to form a 1D copper−oxygen chain that is further connected by btec to afford a 2D [Cu(btec)0.5]n network (Figure 2b). 4-Hptz is bidentate and links Cu1 and Cu2 via two 2,3-positional tetrazolyl nitrogens. 4-Hptz binds to the 2D [Cu(btec)0.5]n network via a tetrazolyl group and affords the final 2D network of 2 (Figure 2c). Owing to the pyridyl N5 D



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water O3W, and three carboxyl oxygens. Co3 adopts an octahedron geometry surrounded by tetrazolyl N2, two water molecules (O1W, O2W), and three carboxyl oxygens. All the Co−O and Co−N bond distances fall in the 2.016(6)− 2.278(5) Å range. Similar to the mode in 4, btca is an octadentate ligand and combines with two Co1 ions, two Co2 ions, and two Co3 ions via two chelating-bridging β-carboxyl groups and two bidentatebridging α-carboxyl groups. This connection affords a 2D [Co3(btca)]n network (Figure 5b). Two distinct 4-ptz ligands show different coordination modes. One 4-ptz is tridentate and connects Co1, Co3, and Co2B via two 2,3-positional tetrazolyl nitrogens and a pyridyl group. Another 4-ptz is bidentate and links Co1 and Co2 by two 2,3-positional tetrazolyl nitrogens. Its pyridyl group is free. Both 4-ptz ligands link CoII ions to form a 1D [Co3(4-ptz)2]n zigzag chain (Figure S2a, Supporting Information). 2D [Co3(btca)]n networks are parallel arranged and pillared by 4-ptz ligands to form a porous 3D MOF with a nanosized rectangular tunnel. The approximate dimensions of the rectangular channel are 7.3 × 10.2 Å2 (Figure 5c). There exist 12 kinds of hydrogen bonds which connect carboxyl groups, water, and 4-ptz to stabilize the 3D framework (Table S2, Supporting Information). 4-Ptz, Co1, Co2, Co3, and btca can be treated as 3-, 4-, 4-, 3-, and 6-connected nodes, respectively. 3D MOF has a 5-nodal (3,3,4,4,6)-connected topology net with point symbol {3·4·5·82·9}{3·4·73·8}{3·43·5·6· 73·84·9·10}{4·82}{42·6} (Figure S2b, Supporting Information). Comparing with the tridentate 3-ptz in 4, one 4-ptz in 5 is bidentate and has a free pyridyl group; therefore, Co3 is coordinated by five oxygen atoms and only a nitrogen atom. This results in the structural difference between 4 and 5. Structure of {[Cd2(4-ptz)2(btca)0.5(H2O)3]·DMF·4H2O}n (6). Complex 6 is a 3D porous MOF. The asymmetric unit contains two independent CdII ions, two 4-ptz, a half btca, and three aqua ligands together with a guest DMF and four water molecules (Figure 6a). Cd1 sits in a cis-octahedral sphere

Figure 4. (a) Asymmetric unit of 4. (b) Coordination mode of btca. (c) 2D [Co1.5(btca)0.5]n network. (d) 3D framework.

mode has not occurred in previous butanetetracarboxylate complexes.64 Each btca ligand connects six CoII ions to form a 2D [Co1.5(btca)0.5]n network (Figure 4c). The 3-Ptz ligand is tridentate and binds to Co1 and two Co2 ions via two 2,3positional tetrazolyl nitrogens and a pyridyl group. The 2D [Co1.5(btca)0.5]n networks are parallel arranged and pillared by a 3-ptz spacer to generate a 3D porous MOF (Figure 4d). If we consider 3-ptz, Co1, Co2, and btca, respectively, as 3-, 4-, 4-, and 6-connected nodes, the 3D framework can be simplified as a 4-nodal (3,4,4,6)-connected topology net with point symbol {42·6}2{42·83·10}{43·62·8}2{44·62·86·103} (Figure S1, Supporting Information). Structure of {[Co3(4-ptz)2(btca)(H2O)3]·2H2O}n (5). Complex 5 is a porous 3D MOF. Three independent CoII ions all adopt an octahedron geometry (Figure 5a); however, their coordination environments are obviously different. Co1 is in a trans-octahedron geometry coordinated by two tetrazolyl nitrogens and four carboxyl oxygens. Co2 locates in a cisoctahedron geometry completed by tetrazolyl N6, pyridyl N9A,

Figure 6. (a) Asymmetric unit of 6. (b) Coordination mode of btca ligand. (c) 2D [Cd2(btca)0.5]n network. (d) Porous 3D MOF with guest DMF (pink).

Figure 5. (a) Asymmetric unit of 5. (b) 2D [Co3(btca)]n network. (c) 3D porous MOF. E



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The first weight-loss process in 100−200 °C was assigned to the release of two lattice water molecules (obsd 5.0%, calcd 4.6%). Three aqua ligands were released at 200−260 °C (obsd 6.9%, calcd 6.8%). The 4-ptz and btca ligands decomposed successively at 260−520 °C to afford 42.4% CoCO3 (calcd 45.2%). A final Co2O3 residue was left at 800 °C (obsd 29.0%, calcd 31.5%). Complex 6 lost four lattice water molecules and a DMF molecule at 30−140 °C with an endothermic peak at 90 °C (obsd 16.9%, calcd 17.4%). Three aqua ligands were released at 140−220 °C (obsd 6.4%, calcd 6.5%). The 4-ptz and btca ligands decomposed successively at 220−460 °C to afford 42.1% CdCO3 (calcd 41.5%). A residue at 800 °C was CdO (obsd 24.6%, calcd 30.9%). CdO can sublime at higher temperature.71 TGA experiments indicate that complexes 4−6, containing flexible butanetetracarboxylate, exhibit relatively higher thermal stability and decomposed smoothly, whereas anhydrous complexes 1−3 with rigid polycarboxylates were stable to about 250−300 °C and then decomposed explosively. This originates from the unstability of the tetrazolyl group. Therefore, only a small amount of tetrazole complexes can be handled with care. In order to further investigate the thermal stability, a variedtemperature PXRD experiment was carried out to study the stability of cobalt complex 5 in detailed (Figure 7). Upon

surrounded by two tetrazolyl nitrogens, three water molecules, and a carboxyl O1. Cd2 adopts a pentagon-bipyramidal geometry completed by four carboxyl oxygens, tetrazolyl N8, and pyridyl N5 and N10. Carboxyl O2 and O3 occupy the apical positions with an O2−Cd2−O3 bond angle of 163.1°. All the Cd−O bond distances fall in the 2.245(3)−2.400(3) Å normal range, except two weak coordination bonds of Cd2−O3 (2.654(3) Å) and Cd2−O1 (2.715(3) Å). H4btca are fully deprotonated. The btca anion is octadentate and binds to two Cd1 and four Cd2 ions (Figure 6b). Two βcarboxyl groups adopt a chelating-bridging fashion, chelating Cd2 and bridging Cd1, whereas two α-carboxyl groups chelate Cd2. Such a coordination mode has not occurred in reported btca complexes.65,66 Two 4-ptz ligands display different coordination modes. One is tridentate and combines to Cd1 and two Cd2 ions via two 1,3-positional tetrazolyl nitrogens and a pyridyl group. Another one is bidentate and links Cd1 and Cd2 via tetrazolyl and pyridyl groups. Both 4-ptz ligands link CdII to form a 2D [Cd2(4-ptz)2]n double-layered network (Figure S3a, Supporting Information). Cd1 and Cd2 are connected by btca to construct a 2D porous [Cd2(btca)0.5]n network (Figure 6c). The tridentate 4ptz is coplanar with the 2D network and locates in the square pore (Figure S3b, Supporting Information). Such identical [Cd2(4-ptz)(btca)0.5]n networks are parallel arranged and pillared by a bidentate 4-ptz spacer to generate a beautiful 3D porous MOF (Figure 6d). Viewed along the b-axis, the 3D MOF exhibits a nanosized rectangular tunnel with the dimensions of 6.6 × 11.7 Å2. Guest DMF and water molecules perch in the tunnels. Upon removing the guest molecules, the void volume calculated by PLATON is 30.6%.62,67 There exist 15 kinds of hydrogen bonds to connect carboxyl groups, DMF, and water to further stabilize the 3D framework (Table S3, Supporting Information). If we consider tridentate 4-ptz, Co1, Co2, and btca, respectively, as 3-, 3-, 5-, and 6-connected nodes, the 3D framework is a 4-nodal (3,3,5,6)-connected topology net with point symbol {4·62}2{42·610·82·10}{42·67·8}2{63}2 (Figure S3c, Supporting Information). Thermogravimetric Analyses and PXRD Patterns. Generally, nitrogen-rich tetrazole complexes often exhibit controlled thermal decomposition and can be regarded as potential high-energy materials.68,69 To examine the stability of pyridyltetrazole complexes 1−6, their thermal behaviors were investigated by TGA (Figure S5, Supporting Information). Oxalate complex 1 was stable to 280 °C, and then decomposed explosively to afford CuCO3 at 320 °C (obsd 50.1%, calcd 48.7%).70 A final residue at 800 °C was CuO (obsd 34.8%, calcd 31.4%). Similarly, benzenetetracarboxylate complex 2 was stable to 300 °C, and then decomposed explosively. The intermediate at 320 °C is mainly CuCO3 (obsd 44.7%, calcd 36.8%). A final CuO residue was left at 800 °C (obsd 24.8%, calcd 23.7%). Isophthalate complex 3 was stable to 250 °C, and then decomposed continuously to afford 49.0% CoCO3 at 476 °C (calcd 51.4%). A final Co2O3 residue was left at 800 °C (obsd 35.0%, calcd 35.8%). Butanetetracarboxylate complex 4 lost a lattice water molecule at 68−110 °C with an endothermic peak at 101 °C (obsd 4.8%, calcd 4.7%). An aqua ligand released in 150−240 °C with an endothermic peak at 238 °C (obsd 4.9%, calcd 4.7%). The 3-ptz ligand decomposed rapidly in 350−410 °C (obsd 37.1%, calcd 37.9%). [Co1.5(btca)0.5]n intermediate further decomposed to afford a Co2O3 residue at 800 °C (obsd 27.9%, calcd 32.2%). Complex 5 was stable to 100 °C.

Figure 7. Varied-temperature PXRD of 5.

heating the sample to 200 °C, the peak position and intensity of the PXRD patterns were almost unchanged. This indicated that the porous 3D architecture remained well after two lattice water molecules were lost. Accompanying three coordinated water molecules released at 200−260 °C, as indicated by TGA experiment, the PXRD peak intensities weaken gradually. Over 280 °C, the diffraction peaks almost disappear, indicating the 3D architecture collapsed. The PXRD experimental data is in good agreement with the TGA result of 5. Room-temperature PXRD experiments were carried out to check the phase purity of the products 1−6. The measured PXRD patterns are in good agreement with those simulated on the basis of the single-crystal X-ray diffraction data (Figure S6, Supporting Information), which indicates that the crystal structures are truly representative of the bulk as-synthesized products. Magnetic Properties. Carboxyl and tetrazolyl groups provide effective pathways for the superexchange interactions. The variable-temperature magnetic susceptibilities of CuII/CoII complexes 1−5 were measured in the temperature range 2− F



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Figure 8. Plots of χMT and χM−1 vs T for (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5. Red solid lines represent the best fits.

300 K. The magnetic data as plots of χMT and χM−1 versus T are depicted in Figure 8. Complex 1 contains oxalate-bridging CuII dimer and tetrazole-bridging CuII dimer. Its χMT value at 300 K is 0.38 cm3 K mol−1, which is close to the expected only spin value of CuII ion (0.38 emu K mol−1, S = 1/2, g = 2.0). Upon cooling, the χMT value decreases very quickly down to 0.06 emu K mol−1 at 100 K, and then remains at this near zero value to 2 K. This experimental fact indicates a very strong antiferromagnetic coupling interaction existing in the CuII dimers. Considering the magnetic interaction originates from both oxalate-bridging (Jox) and tetrazole-bridging (Jptz) CuII ions, we fit the χMT versus T plot using an alternating chain model (eq 3) above 100 K.72,73

χd =

χd =

Ng 2β 2 6 3kT 3 + exp( −2Jptz /kT )

(1)

Ng 2β 2 Sd(Sd + 1) 3kT

(2)

χchain =

Ng 2β 2 1 + u Sd(Sd + 1) 3kT 1 − u

(3)

where u = coth(JoxSd(Sd + 1)/kT) − kT/JoxSd(Sd + 1). This gives the parameters Jox = −94.7 cm−1, Jptz = −61.0 cm−1, and g = 2.41. As a result, complex 1 exhibits very strong antiferromagnetic couplings in oxalate-bridging and tetrazolebridging dimers. G



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orbital overlap produces antiferromagnetic coupling, while magnetic orbital orthogonality gives rise to ferromagnetic coupling.83−85 However, no final conclusion has yet been reached on this mechanism due to some doubt.86 The above CuII complexes 1 and 2 are two good examples to support the magnetic orbital overlap/orthogonality mechanism. For CoII complex 3, the χMT value is 5.52 emu K mol−1 at 300 K, which is obviously higher than the value expected for two isolated high-spin CoII ions (3.75 emu K mol−1, S = 3/2, g = 2.0).87,88 This is due to the large orbital contribution of CoII ion, which is well-known to be significant for a CoII ion located in an octahedral geometry with the 4T1g ground state, and it is difficult to apply an exact theoretical model for fitting the magnetic susceptibility data.89 Upon cooling, the χMT value decreases gradually to a minimum of 2.84 emu K mol−1 at 30 K, and then increases sharply to 22.12 emu K mol−1 at 4 K. This phenomenon is a typical ferrimagnetic behavior.90,91 The data above 40 K can be fitted to the Curie−Weiss law, χM = C/(T − θ), yielding C = 6.29 emu K mol−1 and θ = −40.15 K. The ferrimagnetic behavior can be suggested to be arising from the competitive antiferromagnetic (AF) and ferromagnetic (FM) couplings in complex 3.92 The structural feature of 2D complex 3 possesses an isosceles triangular Co3(μ3−OH) cluster (Figure 9). The Co1···Co2,

Magnetostructural correlation study indicates that the unpaired electron centered on the CuII ion is in a dx2−y2 orbital mainly delocalized over the Cu1 and the four coordinated atoms on the basal plane (O1, O2, N1, and N3). The N4 atom occupies the apical position of a squarepyramidal geometry, as described in the crystal structure of 1. The dx2−y2 magnetic orbitals of Cu1 and Cu1A are coplanar and overlap each other by the close connection of the oxalate bridge. This results in strong antiferromagnetic coupling with a large Jox value. Oxalate is a short connector which can combine metal ions tightly. In fact, oxalate has been demonstrated to be one of the most versatile ligands used in the search for molecule-based magnets.74,75 Oxalate−CuII complexes often exhibit strong antiferromagnetic coupling, especially when oxalate adopts the symmetrical bis-bidentate chelating mode, such as for our complex 1. Several oxalate−CuII complexes exhibit strong magnetic interactions with large negative J values, such as [Cu2(ox)2(SCS)]·3H2O (−215 cm−1), [Cu2(ox)(Pz2CPh2)2Cl2] (−364 cm−1), and [Cu 6 (ox) 3 (dipyatriz) 2 (H 2 O) 9 (NO 3 ) 3 ](NO 3 ) 3 (−390 cm−1).76−78 Compared to the extensive study of carboxyl complexes, a limited number of tetrazole-based complexes have been magnetically characterized.59,79 For tetrazole-bridging CuII dimer in complex 1, the dx2−y2 orbitals of Cu1 and Cu1B are parallel with a Cu1···Cu1B distance of 3.86 Å. The apical N4 atoms of two tetrazolyl groups closely connect the two magnetic orbitals to give rise to strong antiferromagnetic coupling with a large negative Jptz value. Conjugate tetrazolate linkers propagate the magnetic exchange interaction. Complex 2 is a 2D network containing square planar Cu1 and octahedral Cu2 centers. The χMT value at 300 K is 0.43 emu K mol−1, which is slightly higher than the only spin value of CuII ion (0.38 emu K mol−1). Upon cooling, the χMT value increases gradually and reaches 0.54 emu K mol−1 at 2 K. This indicates a ferromagnetic interaction. The data is fitted by a uniform chain model (eq 4) above 6.8 K.80−82 χchain =

Ng 2β 2 1 + u S(S + 1) 3kT 1 − u

Figure 9. Co3(μ3-OH) cluster in the structure of 3.

Co1···Co2A, and Co2···Co2A side distances are 3.586(2), 3.602(2), and 3.339(2) Å, respectively. The trinuclear cobalt clusters are linked together by sharing Co2/Co2A corners and by the connection of a bridging carboxyl group to form a [Co2(μ3-OH)(μ2-COO)]n chain. Previously, the magnetic behavior of M3(μ3-OH) and M3(μ3-OH)2 cores (M = CoII, CuII, et al.) has been extensively studied.93,94 A number of complexes containing a μ3-OH bridging trinuclear CoII unit exhibit ferrimagnetic behavior.95−97 The hydroxyl group is the major magnetic bridge because it usually transmits strong magnetic interaction. A smaller Co−O−Co bond angle often brings about FM coupling, while a larger Co−O−Co angle often gives rise to AF coupling.98−100 In complex 3, Co2 and Co2A locate at the short side of the Co3(μ3-OH) triangle with a smaller Co2−O5−Co2A bond angle of 107.9°, which indicates a FM coupling may exist between Co2 and Co2A. Comparatively, the Co1−O5−Co2 (120.3°) and Co1−O5− Co2A (119.8°) angles are larger, which indicate two AF couplings between Co1 and Co2/Co2A. Therefore, three CoII ions in the Co3(μ3-OH) triangle adopt an up−up−down spin configuration.99 The ferrimagnetic behavior of 3 is a competitive result of AF/AF/FM interactions.90,101 For CoII complex 4, the χMT value at 300 K is 4.58 emu K mol−1, which is obviously higher than the isolated high-spin value of one and a half CoII ions (2.81 emu K mol−1).102,103

(4)

where u = coth(JS(S + 1)/kT) − kT/JS(S + 1). The best fit parameters are J = 0.82 cm−1 and g = 2.15, which indicates the presence of a weak ferromagnetic interaction between Cu1 and Cu2 via the connection of the carboxyl O1 atom and the bidentate tetrazolyl bridge. Interestingly, complex 2 has a square planar Cu1 center and an elongated octahedral Cu2 center, and the magnetic orbitals of both CuII ions are orthogonal to each other. The unpaired electron of Cu1 occupies the dx2−y2 orbital lying on the coordination plane (O1, O1A, N2, and N2A). Owing to the evident Jahn−Teller distortion, Cu2 adopts an elongated octahedral geometry with a long axis along the O1−Cu2− O1A direction. The Cu−O1, Cu2−O3, and Cu2−N1 distances are 2.356(3), 1.963(3), and 1.986(3) Å, respectively. Therefore, the unpaired electron is expected to be in the dx2−y2 orbital, mainly delocalized over the Cu2 and the four atoms on the equatorial plane (O3, O3A, N1, and N1A). Both dx2−y2 magnetic orbitals of Cu1 and Cu2 are almost orthogonal to each other, with a dihedral angle of 72.9° (Figure S7, Supporting Information), which results in weak ferromagnetic coupling. It is important to study the magnetic exchange mechanism. For a long period, it is suggested that magnetic H



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This is also due to the large orbital contribution of CoII ion. Upon cooling, the χMT value decreases in a continuous fashion and finally reaches 1.06 emu K mol−1 at 2 K. The inverse magnetic susceptibility curve shows a linear behavior in the 2− 300 K range and obeys the Curie−Weiss law with C = 9.76 emu K mol−1 and θ = −17.08 K. The negative θ value suggests an antiferromagnetic coupling between Co1 and Co2/Co2A ions. For CoII complex 5, the χMT value at 300 K is 8.83 emu K mol−1, obviously higher than the value expected for three isolated high-spin CoII ions (5.62 emu K mol−1).104,105 Upon cooling, the χMT value decreases gradually and reaches 1.86 emu K mol−1 at 2 K. The data is fitted to the Curie−Weiss law, yielding C = 9.25 emu K mol−1 and θ = −8.6 K, which indicates an antiferromagnetic coupling in the trinuclear CoII cluster.

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (21171115, 21203117), and Natural Science Foundation of Shanghai (16ZR1411400).

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CONCLUSIONS Four 3D MOFs and two 2D coordination networks were successfully prepared based on 3/4-pyridyltetrazoles and four poly(carboxylic acid)s. The pyridyltetrazoles display varied coordination modes in them (bi-, tri-, and pentadentate). Flexible butanetetracarboxylate is a valuable multidentate ligand which constructs three porous MOFs with nanosized rectangular tunnels. Three complexes containing rigid polycarboxylate auxiliary ligands were stable to 250−300 °C and then decomposed explosively, while three 3D MOFs with flexible butanetetracarboxylate ligand decomposed smoothly. Five CuII/CoII complexes present remarkable magnetic coupling diversity from ferro-, ferri-, to antiferromagnetic interactions. Complex 1 shows strong antiferromagnetic behavior originating from oxalate-bridging CuII dimer and tetrazole-bridging CuII dimer. Complex 2 presents a weak ferromagnetic interaction originating from the magnetic orbital orthogonality between square planar CuII and octahedral CuII centers. CoII complex 3 displays ferrimagnetic coupling interaction with competitive AF/AF/FM interactions. Magnetostructural correlation studies reveal that the magnetic orbital overlap and orthogonality play important roles in the magnetic coupling interactions. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00258. Bond distances and angles, structural figures, IR spectra, PXRD patterns, and thermal analyses (PDF) Accession Codes

CCDC 1431396−1431401 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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AUTHOR INFORMATION

Corresponding Authors

*(M.-X.L.) E-mail: [email protected]. *(Z.-X.W.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. I



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