A series of multidimensional MOFs incorporating a new N-heterocyclic building block: 5,5′-di(pyridin-4-yl)-3,3′-bi(1,2,4-triazole)

Abstract

By using a new N-heterocyclic building block, 5,5′-di(pyridin-4-yl)-3,3′-bi(1,2,4-triazole) (4,4′-H₂dbpt), six novel coordination polymers with diversiform connectivity from one- to three-dimensional were successfully constructed. By regulating the different auxiliary ligands, central metal ions, and some other synthetic conditions, 4,4′-H₂dbpt adopted various coordination modes. Consequently, 1 adopts a 2D (3,6)-topology, with the (4³)₃(4⁶.6⁶.8³)₂ Schläfli symbol. 2 shows a 3D 8-connected topology with a (3⁶.4¹⁸.5³.6) Schläfli symbol. 3 and 4, which are isostructural, both have a 2D 4-connected topology, with a (4⁴.6²) Schläfli symbol. 5 has a complex 3D porous architecture with a 1D solvent-filled channel. 6 reveals a 1D helical chain extended along a 4-fold screw axis. These results indicate that 4,4′-H₂dbpt is an excellent multi-connection linker from which we can construct MOFs with interesting structures and properties.

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Introduction

The design and synthesis of coordination polymers, especially metal–organic frameworks (MOFs) has attracted considerable interest in the realm of supramolecular chemistry and crystal engineering, owing not only to their appealing structural and topological novelty but also to their tremendous potential applications in gas storage and separation,¹ electrical conduction,² and as luminescence materials,³ molecular magnets,⁴ and heterogeneous catalysts.⁵ Thus, a series of studies in this field have mainly focused on the design, preparation, and structure–property relationships.⁶ The structures of coordination polymers are determined by several factors, including the coordination geometry of the central metal ion, solvents,⁷ ligand structure, metal-ligand ratio,⁸ counterions,⁹ pH,¹⁰ and temperature.¹¹ Among the reported studies, much effort has been focused on the rational design and controlled synthesis of coordination polymers using multidentate ligands such as polycarboxylate and N-heterocyclic ligands. The N-heterocyclic ligands are good molecular building blocks and co-ligands for constructing MOFs with interesting structures and properties, which have been widely reported by Yaghi,¹² Snurr,¹³ Chen,¹⁴ Du,¹⁵ Tong,¹⁶ and Zhang.¹⁷

Our groups have systematically explored the coordination assemblies on the basis of some polycarboxylate ligands that cooperate with some angular N-heterocyclic-like ligands: 1H-3,5-bis(4-pyridyl)-1,2,4-triazole (4,4′-bpt), 1H-3-(3-pyridyl)-5-(4-pyridyl)-1,2,4-triazole (3,4′-bpt), 1H-3,5-bis(3-pyridyl)-1,2,4-triazole (3,3′-bpt), 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole (4,4′-abpt), and 1,4-bis(5-(4-pyridyl)-1H-1,2,4-triazol-3-yl)benzene (bptb).¹⁸ In most cases, the angular N-heterocyclic-like ligands act as 2-connected linkers with different distortion angles and directions. Relying on the versatile coordination modes of the polycarboxylate ligands, a variety of structures were obtained. Recently, various compounds with complex topologies based on some ditriazole–pyridine ligands, have been reported by Hou,¹⁹ Li,²⁰ Sha,²¹ and Zhou.²² These ligands revised various conformations, highlighting the fact that they are another class of excellent multi-connection linkers in addition to polycarboxylate ligands.²³ Thus, these considerations inspired us to explore new coordination frameworks with this kind of ligand.

In this paper, we introduce a new ditriazole–pyridine ligand: 5,5′-di(pyridin-4-yl)-3,3′-bi(1,2,4-triazole) (4,4′-H₂dbpt) which is different to our previous N-heterocyclic-like ligands and introduces a chelating effect between the adjacent triazole rings. Thus, it could adopt various conformations that may lead to unpredictable and interesting structures. By regulating the rotation angles of the four aromatic rings with respect to one another, the deprotonation effort (4,4′-H₂dpbt, 4,4′-Hdpbt⁻, 4,4′-dpbt²⁻) and the flexing of the angles can be investigated. To the best of our knowledge, MOFs based on 5,5′-di(pyridin-4-yl)-3,3′-bi(1,2,4-triazole) (4,4′-H₂dbpt) have not been documented to date.

A series of coordination polymers based on 4,4′-H₂dbpt, namely, {[Co₅(4,4′-dbpt)₂Cl₈]·2(C₂H₅)₃NH}_n (1), {[Co₃(p-BDC)₂(4,4′-dbpt)·0.5CH₃OH]}_n (2), {[Co(4,4′-Hdbpt)₂]·H₂O}_n (3), {[Ni(4,4′-Hdbpt)₂]·H₂O}_n (4), {[Cu₂(4,4′-dbpt)]·H₂O}_n (5), {[Mn(4,4′-dbpt)(H₂O)₂]·2H₂O}_n (6) were successfully constructed. By regulating the different auxiliary ligands, central metal ion, 4,4′-H₂dbpt displayed five different coordination modes (Scheme 1) based on alteration of the rotation angles, valence state and the flexing angles. As a result, 1–6 adopted five different architectures (3 and 4 are isostructural) with diversiform connectivity from one- to three-dimensional.

Scheme 1 The versatile coordination modes of 4,4′-H₂dbpt used in this work.

Experimental section

Materials and physical measurements

With the exception of the ligand 4,4′-H₂dbpt that was prepared according to the literature procedure,²⁴ all reagents and solvents for synthesis and analysis were commercially available and used as received. IR spectra were taken on a Perkin-Elmer spectrum One FT-IR spectrometer in the 4000–400 cm⁻¹ region with KBr pellets. Elemental analyses for C, H and N were carried out on a Perkin-Elmer Model 2400 II elemental analyzer. Magnetic susceptibility measurements of the polycrystalline samples were obtained over the temperature range of 2–300 K with a Quantum Design MPMS-XL7 SQUID magnetometer using an applied magnetic field of 1000 Oe. A diamagnetic correction to the observed susceptibilities was applied using Pascal's constants. X-ray powder diffraction (XRPD) intensities were measured on a Rigaku D/max-IIIA diffractometer (Cu-Kα, λ = 1.54056 Å). The crystalline powder samples were prepared by crushing the crystals and spectra scanned from 3 to 60° with a step of 0.1° s⁻¹. Calculated patterns of 1–6 were generated with the PowderCell software.

Syntheses of complexes 1–6

{[Co₅(4,4′-dbpt)₂Cl₈]·2(C₂H₅)₃NH}_n (1). A mixture containing 4,4′-H₂dbpt (87 mg, 0.3 mmol), CoCl₂·6H₂O (238 mg, 1 mmol), ethanol (15 mL) and triethylamine (0.5 mL) was sealed in a Teflon-lined stainless steel vessel (25 mL), which was heated at 160 °C for 3 days and then cooled to room temperature at a rate of 10 °C h⁻¹. Glaucous needle-like crystals of 1 were obtained, picked out, washed with ethanol and dried in air. Yield: 47% (based on Co(II)). Elemental analysis for C₄₀H₄₆Cl₈Co₅N₁₈ (%) calcd: C, 35.40; H, 3.42; N, 18.58. Found: C, 35.47; H, 3.49; N, 18.52. IR (KBr, cm⁻¹): 3410s, 1620s, 1437s, 1354m, 1307s, 1217w, 1160w, 1066m, 1019s, 839s, 713s, 616w, 576s, 526, 436.

{[Co₃(p-BDC)₂(4,4′-dbpt)·0.5CH₃OH]}_n (2). A mixture containing 4,4′-H₂dbpt (87 mg, 0.3 mmol), Co(NO₃)₂·6H₂O (291 mg, 1 mmol), p-H₂BDC (83 mg, 0.5 mmol), water (10 mL), methanol (5 mL) and triethylamine (0.5 mL) was sealed in a Teflon-lined stainless steel vessel (25 mL), which was heated at 160 °C for 3 days and then cooled to room temperature at a rate of 10 °C h⁻¹. Purple block crystals of 2 were obtained, picked out, washed with distilled water and dried in air. Yield: 30% (based on Co(II)). Anal. calcd for (C_30.5H₁₈Co₃N₈O_8.5): C, 45.26; H, 2.24; N, 13.85. Found: C, 45.39; H, 2.17; N, 13.92. IR (KBr, cm⁻¹): 3446s, 1717m, 1599s, 1498w, 1426m, 1365s, 1300m, 1271m, 1217w, 1156w, 1102w, 1016m, 882w, 839m, 814s, 742s, 717m, 587m, 515w, 450w.

{[Co(4,4′-Hdbpt)₂]·2H₂O}_n (3). A mixture containing 4,4′-H₂dbpt (87 mg, 0.3 mmol), Co(NO₃)₂·6H₂O (291 mg, 1 mmol), water (15 mL), and triethylamine (0.5 mL) was sealed in a Teflon-lined stainless steel vessel (25 mL), which was heated at 160 °C for 3 days and then cooled to room temperature at a rate of 10 °C h⁻¹. Orange block, X-ray-quality crystals of 3 in 32% yield (based on Co(II)) were obtained. Anal. calcd for (C₂₈H₂₂CoN₁₆O₂): C, 49.93; H, 3.29; N, 33.27. Found: C, 49.81; H, 3.35; N, 33.18. IR (KBr, cm⁻¹): 3428s, 3028m, 2668m, 1620s, 1574m, 1559w, 1426s, 1390m, 1329w, 1304s, 1210m, 1178s, 1113m, 1052m, 1008s, 990s, 890m, 961w, 839s, 753w, 724s, 699m, 656w, 533m, 515s, 461w, 418w.

{[Ni(4,4′-Hdbpt)₂]·2H₂O}_n (4). The same synthetic procedure as that for 3 was used except that Co(NO₃)₂·6H₂O was replaced by Ni(NO₃)₂·6H₂O giving green, needle-like, X-ray-quality crystals of 4 in 39% yield (based on Ni(II)). Anal. calcd for (C₂₈H₂₂NiN₁₆O₂): C, 49.95; H, 3.29; N, 33.29. Found: C, 49.86; H, 3.31; N, 33.15. IR (KBr, cm⁻¹): 3428s, 3028m, 2668m, 1620s, 1574m, 1559w, 1307s, 1214m, 1174s, 1113m, 1052m, 1012m, 990s, 980m, 890w, 861m, 839s, 756s, 724s, 699m, 652w, 533m, 515s, 461w, 422w.

{[Cu₂(4,4′-dbpt)]·H₂O}_n (5). The same synthetic procedure as that for 3 was used except that Co(NO₃)₂·6H₂O and water were replaced by Cu(NO₃)₂·6H₂O and DMF, respectively, thus giving red, needle-like, X-ray-quality crystals of 5 in 45% yield (based on Cu(I)). Anal. calcd for (C₁₄H₁₀Cu₂N₈O): C, 38.80; H, 2.33; N, 25.86. Found: C, 38.87; H, 2.29; N, 25.93. IR (KBr, cm⁻¹): 3435s, 3107w, 3050w, 2456w, 1944w, 1660w, 1610s, 1516m, 1430s, 1383m, 1318s, 1214w, 1152w, 1098w, 1030w, 1005m, 987m, 839s, 746w, 710s, 526w, 497w.

{[Mn(4,4′-dbpt) (H₂O)₂]·2H₂O}_n (6). The same synthetic procedure as that for 3 was used except that Co(NO₃)₂·6H₂O was replaced by Mn(NO₃)₂·6H₂O giving colourless, needle-like, X-ray-quality crystals of 6 in 44% yield (based on Mn(II)). Anal. calcd for (C₁₄H₁₆MnN₈O₄): C, 40.49; H, 3.88; N, 26.98. Found: C, 40.57; H, 3.72; N, 27.07. IR (KBr, cm⁻¹): 3415s, 3046w, 2948w, 1932w, 1612s, 1526w, 1499m, 1455w, 1434s, 1403m, 1385w, 1351w, 1317s, 1292m, 1234w, 1215w, 1157w, 1120w, 1092w, 1065w, 1006s, 834m, 745w, 723m, 708s, 671w, 640w, 523w, 489w.

X-ray crystallographic determination

All reflection data were collected on an Agilent Supernova diffractometer (Mo, λ = 0.71073 Å) at room temperature. A semi-empirical absorption correction was applied using the SADABS program and the raw data frame integration was performed with SAINT.²⁵ The crystal structures were solved by direct methods using the program SHELXS-97 (ref. 26) and refined by the full-matrix least-squares method on F² with anisotropic thermal parameters for all non-hydrogen atoms using SHELXL-97.²⁷ All hydrogen atoms were placed in calculated positions and refined isotropically, except the hydrogen atoms of water molecules, which were located in a difference Fourier map and refined isotropically. The details of the crystal data are summarized in Table 1 and selected bond lengths and angles are listed in Table S1 for compounds 1–6 (ESI†).

Table 1 Crystal data and structure refinement for 1–6

Complex	1	2	3	4	5	6
Empirical formula	C40H48Cl8Co5N18	C30.5H18Co3N8O8.5	C28H22CoN16O2	C28H22NiN16O2	C14H10Cu2N8O	C14H16MnN8O4
Formula weight	1359.20	809.32	673.55	673.33	433.36	415.29
Crystal system	Triclinic	Triclinic	Monoclinic	Monoclinic	Orthorhombic	Tetragonal
Space group	P1̄	P1̄	P21/n	P21/n	Pbcn	I41/acd
a (Å)	11.5936 (6)	9.2529 (12)	9.2567 (10)	9.1787 (10)	14.6978 (14)	19.1314 (3)
b (Å)	11.9201 (7)	10.6355 (11)	15.269 (4)	15.2181 (11)	19.3933 (17)	19.1314 (3)
c (Å)	12.1223 (6)	11.0493 (13)	10.2578 (3)	10.3355 (8)	10.3560 (9)	20.6667 (3)
α (°)	63.593 (5)	63.622 (11)	90	90	90	90
β (°)	88.025 (4)	86.489 (10)	96.720 (5)	96.657 (8)	90	90
γ (°)	67.331 (5)	74.907 (10)	90	90	90	90
Volume (Å^3)	1365.40 (12)	938.68 (19)	1439.9 (4)	1433.9 (2)	2951.9 (5)	7564.2 (2)
Z	1	1	2	2	8	16
Calculated density (mg m^−3)	1.651	1.432	1.554	1.559	1.941	1.459
Goodness-of-fit on F^2	1.040	1.171	1.040	1.029	1.033	1.079
Independent reflections	5578	3840	2938	2922	3018	1933
Rint	0.0712	0.0249	0.0247	0.0841	0.0521	0.0313
R1[I > 2σ(I)]	0.0652	0.0596	0.0391	0.0530	0.0538	0.0397
wR2 (all data)	0.1433	0.1933	0.0963	0.1073	0.1390	0.1180

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XRPD results

To confirm whether the crystal structures are truly representative of the bulk materials, X-ray powder diffraction (XRPD) experiments have also been carried out for 1–6. The XRPD experimental and calculated patterns of the corresponding complexes are shown in ESI, Fig. S2.† Although the experimental patterns have a few unindexed diffraction lines and some are slightly broadened in comparison with those calculated from the single crystal models, it can still be considered favorably that the bulk synthesized materials and the crystals as grown are homogeneous for 1–6.

Results and discussion

Description of the crystal structures

{[Co₅(4,4′-dbpt)₂Cl₈]·2(C₂H₅)₃NH}_n (1). Single crystal X-ray diffraction analysis reveals that the asymmetric unit of 1 contains three crystallographically independent Co atoms, one 4,4′-dbpt²⁻ anion, four Cl⁻ anions, and one lattice protonated triethylamine ion. As illustrated in Fig. 1a, Co1 is coordinated by two N atoms (N6, N3A, symmetry codes: A: x, y, z + 1) from two 4,4′-dbpt ligands, and two Cl atoms (Cl1, Cl2), to yield distorted tetrahedron geometry. Co3 also adopts a distorted tetrahedron geometry coordinated by two N atoms (N1C, N8D, C: −x + 1, −y + 1, −z + 1; D: x, y, z − 1) from two 4,4′-dbpt ligands, one μ₂-Cl (Cl3) atom and one terminal Cl atom (Cl4). While Co2, lying on a symmetry center, adopts a distorted octahedral geometry coordinated by four N atoms (N4, N5, N4A, N5A) from two 4,4′-dbpt ligands in the equatorial plane and two μ₂-Cl atoms (Cl3, Cl3A) in the axial positions, the middle Co2 ion is linked to terminal ions, Co1 and Co3, with two triazoles from two 4,4′-dbpt ligands (Co1⋯Co2 = 3.788 (2) Å) and one μ₂-Cl⁻ (Co2⋯Co3 = 4.493 (2) Å), respectively, to form a Co₅ unit (Fig. 1b). The adjacent Co₅ units share vertices (Co3) and connect to each other to form a 2D ladder structure (Fig. 1c). The lattice protonated triethylamine ions fill the gaps of the neighboring 2D layers that arrange in a manner parallel to each other (Fig. 1d). A topological analysis reveals that the Co₅ unit and the shared vertices, at Co3, serve as 6-connected node and 3-connected nodes, respectively, to link each other. According to Wells' topology definition,²⁸ an interesting topology with the short Schläfli symbol of (4³)₃(4⁶.6⁶.8³)₂ is formed (Fig. 1e).

Fig. 1 (a) The coordination environments of the Co(II) atoms (symmetry codes: A: x, y, z + 1; B: x + 1, y, z; C: −x + 1, −y + 1, −z + 1; D: x, y, z − 1); (b) the Co5 unit; (c) view of the 2D ladder network; (d) the 3D extended structure; (e) the schematic description for the 2D architecture with (4³)₃(4⁶.6⁶.8³)₂ symbol.

{[Co₃(p-BDC)₂(4,4′-dbpt)·0.5CH₃OH]}_n (2). The asymmetric unit of 2 contains two crystallographically independent Co atoms: half a 4,4′-dbpt²⁻ anion, one p-BDC anion, and one quarter of a lattice methanol molecule. As illustrated in Fig. 2, 2 reveals a novel 3D coordination polymer with a linear trinuclear Co(II) unit. In the trinuclear unit, the middle Co1 ion, lying on a symmetry center, exhibits a distorted octahedral geometry coordinated by four O atoms (O1, O3, O1A, O3A, A: −x + 1, −y + 1, −z + 1) from four p-BDC ligands in the equatorial plane, and two N atoms (N3B, N3C, B: −x, −y + 1, −z + 1; C: x + 1, y, z) from two dbpt ligands in the axial positions. The terminal Co2 ions exhibit a distorted trigonal-bipyramidal geometry, which is provided by two N atoms (N2C, N4D, D: −x, −y, −z + 1) from two 4,4′-dbpt ligands in the axial positions, two O atoms (O2A, O4A) from two p-BDC ligands and one N atom (N4D, D: −x, −y, −z + 1) from a 4,4′-dbpt ligand in the equatorial plane. The middle Co1 ion is linked to terminal Co2 ions with two syn–syn carboxylates from two p-BDC ligands and a triazole from 4,4′-dbpt [Co1⋯Co2 = 4.008 (5) Å] to form the Co₃ unit. The adjacent Co₃ units are connected by 4,4′-dbpt²⁻ using connection mode 2 (Scheme 1) to form a 2D layer (Fig. 2b). The layer is further connected by p-BDC pillars to generate a 3D open framework with a 1D solvent-filled channel of diameter about 9 Å (Fig. 2d). Better insight into this framework can be achieved by topology analysis. The Co₃ unit serves as an 8-connected node, and both p-BDC and 4,4′-dbpt serve as linkers. In this way, this framework can be simplified to be an 8-connected 3D architecture with the short Schläfli symbol of (3⁶.4¹⁸.5³.6) (Fig. 2e).

Fig. 2 (a) The coordination environments of the Co(II) atoms (symmetry codes: A: −x + 1, −y + 1, −z + 1; B: −x, −y + 1, −z + 1; C: x + 1, y, z; D: −x, −y, −z + 1; E: x − 1, y, z; F: −x + 1, −y, −z + 2); (b) view of the 2D network connected by the 4,4′-dbpt ligands; (c) view of the 3D network along the a axis; (d) the 3D network with parallel channels viewed along the b axis; (e) the schematic description for the 3D architecture with (3⁶.4¹⁸.5³.6) symbol.

{[M(4,4′-dbpt)₂]·2H₂O}_n, M = Co (3), M = Ni (4). Compounds 3 and 4 are isostructural having an identical space group and closely similar cell dimensions. As shown in Fig. 3a, there is only one independent M atom in the asymmetric unit. The M ions lie on symmetry centers and exhibit a distorted octahedron geometry coordinated by four triazole N atoms (N4, N5, N4B, N5B, B: x + 1/2, −y − 1/2, z − 1/2) from two 4,4′-Hdbpt⁻ ions in the equatorial plane, and two pyridine N atoms (N8A, N8C, A: −x + 1, −y, −z + 2; C: −x + 1/2, y + 1/2, −z + 5/2) from two 4,4′-Hdbpt⁻ ions in the axial positions. The adjacent M ions are connected through 4,4′-Hdbpt⁻ anions with connection mode 3 (Scheme 1) and with a separation of 10.56 (1) Å for 3 and 10.54 (1) Å for 4, respectively, to form a 2D layer (Fig. 2b). The neighboring 2D layers lie parallel to each other (Fig. 3c). The distance of the nearest M atoms in neighboring 2D layers is 9.25 (1) Å for 3 and 9.18 (1) Å for 4. The M ion serves as a 4-connected node and 4,4′-Hdbpt⁻ serves as a linker. As a result, this framework can be simplified to be a 4-connected 2D architecture with the short Schläfli symbol of (4⁴.6²) (Fig. 3d).

Fig. 3 (a) The coordination environment of the M(II) atom (symmetry codes: A: −x + 1, −y, −z + 2; B: x + 1/2, −y − 1/2, z − 1/2; C: −x + 1/2, y + 1/2, −z + 5/2); (b) view of the 2D network connected by the 4,4′-Hdbpt ligand; (c) the 3D extended structure; (d) the schematic description for the 3D architecture with (4⁴.6²) symbol.

{[Cu₂(4,4′-dbpt)]·H₂O}_n (5). The asymmetric unit of 5 contains three crystallographically independent Cu atoms, one 4,4′-dbpt²⁻ anion, and a lattice water molecule. As illustrated in Fig. 4a, Cu1 is coordinated by four N atoms (N1, N1A, N7B, N6C, symmetry codes: A: −x, y, −z + 1/2; B: −x + 1/2, y − 1/2, z; C: x − 1/2, y − 1/2, −z + 1/2) from four 4,4′-dbpt ligands in a distorted tetrahedral geometry. Cu2, lying on a symmetry site, is two-coordinated by two triazole N donors (N4, N4D, D: −x + 1, y, −z + 3/2; (v) x, −y + 1, z + 1/2) from two 4,4′-dbpt ligands. The distance of Cu2–N5 is 2.913 (2) Å, indicating a weak coordination interaction between them. Cu3 adopts a planar triangular configuration coordinated by three N atoms (N3, N6E, N8F, E: x, −y + 1, z + 1/2; F: x − 1/2, y − 1/2, −z + 3/2) from three 4,4′-dbpt ligands. The 4,4′-dbpt ligands adopt mode 4 (Scheme 1) connecting Cu(I) atoms to form a 3D reticular structure with a 1D solvent-filled channel of diameter about 7 Å (Fig. 4b and c).

Fig. 4 (a) The coordination environments of the Cu(I) atoms (symmetry codes: A: −x, y, −z + 1/2; B: −x + 1/2, y − 1/2, z; C: x − 1/2, y − 1/2, −z + 1/2; D: −x + 1, y, −z + 3/2; E: x, −y + 1, z + 1/2; F: x − 1/2, y − 1/2, −z + 3/2); (b) the space-filling plot of the 3D network connected by the 4,4′-dbpt ligands; (c) view of the 3D network connected by the 4,4′-dbpt ligands.

{[Mn(4,4′-dbpt) (H₂O)₂]·2H₂O}_n (6). Compound 6 crystallizes in the tetragonal space group I4₁/acd. As shown in Fig. 5a, there is only one independent Mn atom in the asymmetric unit. The Mn atoms exhibit a distorted octahedral geometry coordinated by four triazole N atoms (N1, N1A, N3B, N3C, symmetry codes: A: −x + 1/2, y, −z; B: −y + 1/4, −x + 1/4, −z + 1/4; C: y + 1/4, −x + 1/4, z − 1/4) from two 4,4′-dbpt ligands, and two water molecules (O1, O1A). The adjacent Mn atoms are connected by 4,4′-dbpt²⁻ with mode 5 (Scheme 1) with the separation of 5.9493 (9) Å to form a quadruple helical chain extended along the c axis (Fig. 5b). The neighboring helical chains arrange in a parallel manner with the opposite spiral direction and are connected to each other through H-bonds around another 4-fold screw axis to form a 3D open framework with a 1D water-filled channel (Fig. 5c and d). The water molecules in the channel are arranged with S₄ symmetry (Fig. 5e). The distance of the nearest Mn atoms between neighboring helical chains is 9.327 (1) Å.

Fig. 5 (a) The coordination environment of the Mn(II) atoms (symmetry codes: A: −x + 1/2, y, −z; B: −y + 1/4, −x + 1/4, −z + 1/4; C: y + 1/4, −x + 1/4, z − 1/4); (b) view of the helical chain extended along the c axis; (c) view of the 3D extended structure along the c axis; (d) the connection type of the H-bond between the adjacent helical chains; (e) arrangement of lattice water molecules filling the gaps between neighboring chains.

Structural diversity of 1–6

Six new MOFs, from one to three dimensions, based on the 4,4′-H₂dbpt ligand were presented. Among them, 1, 3, and 4 form 2D layer architectures, 2 and 5 form 3D network architectures, and 6 forms a 1D chain architecture with diversiform connectivity. Clearly, the phenomenon of structural diversification in 1–6 may arise from the three sources as follow. First, the 4,4′-H₂dbpt ligand plays a dominating effect on constructing the polymer structures. Owing to the different rotation angles of the four aromatic rings with respect to each other, deprotonation effort (4,4′-H₂dpbt, 4,4′-Hdpbt⁻, 4,4′-dpbt²⁻) and the flexing angles, the ligand could adopt various conformations, which may lead to unpredictable and interesting structures. In this paper, the 4,4′-H₂dbpt ligand adopts five different conformations (Scheme 1) according to the geometric requirements of metal ions and/or the introduction of auxiliary ligands. Secondly, the introduction of auxiliary ligands may play a significant role in the formation of different topological structures. Compound 3, with no auxiliary ligand, reveals a 2D, 4-connected topology with a (4⁴.6²) Schläfli symbol. For 1, Cl⁻ is involved in coordination and, as a result, a more complex 2D ladder structure based on a Co₅ unit with the short Schläfli symbol of (4³)₃(4⁶.6⁶.8³)₂ is formed. For 2, with the employment of H₂ p-BDC, an 8-connected, 3D pillared layered architecture with the short Schläfli symbol of (3⁶.4¹⁸.5³.6) is formed. Thirdly, different metal ions may have different charges, electron configurations, and ionic radii, and hence exhibit different coordination geometries. Therefore, the connectivity of polymeric frameworks is also strongly related to the metal centers. In 5, Cu(I) exhibits the coexistence of two-, three- and four-coordination patterns, which is different from the others and a complex 3D architecture is formed.

Magnetic properties

Magnetic susceptibility measurements were carried out on polycrystalline samples of 1–4 and 6 in the temperature range of 2.0–300.0 K at 1000 Oe (Fig. 6a and 7).

Fig. 6 (a) Plots of χ_MT vs. T and χ_M⁻¹ vs. T (inset) for 1; (b) FC magnetization of 1 in different fields; (c) plots of zero-field cooled magnetization (ZFC) and field-cooled magnetization (FCM) and in a field of 10 Oe for 1 using a SQUID; (d) Arrhenius plot for 1 fitted by Arrhenius law τ = τ₀ exp(−ΔE/k_BT), inset: the zero-field ac magnetic susceptibilities for 1.

Fig. 7 Plots of χ_MT vs. T and χ_M⁻¹ vs. T (inset) for 2–4 and 6.

For 1, the data above 50 K follow the Curie–Weiss law with C = 10.81 cm³ K mol⁻¹ and θ = −22.35 K. The χ_MT value at 300 K is 10.12 cm³ K mol⁻¹, which is larger than the spin-only value 9.38 emu K mol⁻¹ for five magnetically active Co(II) ions (S = 3/2, g = 2.0), as expected for Co(II) systems with a significant contribution from the effects of spin–orbital coupling. As the temperature is lowered, the χ_MT values decrease continuously and reach a local minimum of 5.98 cm³ K mol⁻¹ at about 12 K, indicative of a strong single-ion behavior admixture with a weak antiferromagnetic interaction,²⁹ and then increase slightly to reach a maximum at ca. 8.0 K, and then rapidly decrease to a minimum of 2.29 cm³ K mol⁻¹ at 2 K. The observed increase in χ_MT below this temperature is no longer coming from the single-ion behaviour but rather from a ferromagnetic or canted antiferromagnetic Co(II)–Co(II) exchange interaction, and the final decrease in χ_MT below 10 K indicates a magnetic phase transition. The dependence of χ_MT vs. T curves of 1 at different fields is pronounced at a low temperature, the larger increase of χ_MT values being at small fields (Fig. 6b). This is an important feature of spin canting behavior.³⁰

The zero-field ac susceptibility data of 1 shows obvious peaks and slight frequency dependence (Fig. 6d, inset) below 8 K, which confirms the magnetic phase transition. The shift of peak temperature (T_p) of χ′′_M for 1 was measured by a parameter ø = ΔT_p/[T_pΔ(log f)] ≈ 0.06, which is in the range of spin glass systems.³¹ The relaxation time τ₀ was obtained from the Arrhenius law: the best set of parameters is τ₀ = 2.53 × 10⁻¹⁸ s and ΔE/k_B = 201.55 K (Fig. 6d), where ΔE is the energy barrier and k_B is the Boltzmann constant, suggesting a thermally activated mechanism.

For 2, the data above 2 K follow the Curie–Weiss law with C = 7.48 cm³ K mol⁻¹ and θ = −18.35 K. The χ_MT value at 300 K is 7.05 cm³ K mol⁻¹, which is larger than the spin-only value of 5.63 cm³ K mol⁻¹ for three magnetically active Co(II) ions (S = 3/2, g = 2.0), as expected for Co(II) systems with a significant contribution from the effects of spin–orbital coupling. As the temperature is lowered, the χ_MT values decrease continuously and reach a local minimum of 1.01 cm³ K mol⁻¹ at about 2 K, indicative of a strong single-ion behavior admixture with a weak antiferromagnetic interaction.

For 3, the data above 2 K follow the Curie–Weiss law with C = 3.01 cm³ K mol⁻¹ and θ = −12.47 K. The χ_MT value at 300 K is 2.89 cm³ K mol⁻¹, which is much larger than the spin-only value of 1.88 cm³ K mol⁻¹ for a magnetically active Co(II) ion (S = 3/2, g = 2.0), as expected for Co(II) systems with a significant contribution from the effects of spin–orbital coupling. As the temperature is lowered, the χ_MT values decrease continuously and reach a local minimum of 0.61 cm³ K mol⁻¹ at about 2 K, indicative of a strong single-ion behavior. The distance of the nearest Co atoms in 3 is 9.25 (1) Å, indicating that the weak interactions between them could be ignored. The in- and out-of-phase ac susceptibilities have no dependence on frequency between 2 and 10 K for 3, indicating that there is no single-ion magnet (SIM) behavior in 3 (Fig. S3†).

For 4, the data above 2 K follow the Curie–Weiss law with C = 1.33 cm³ K mol⁻¹ and θ = −1.94 K. The χ_MT value at 300 K is 1.32 cm³ K mol⁻¹, which is slightly larger than the spin-only value of 1.21 cm³ K mol⁻¹ for a magnetically active Ni(II) ion (S = 1, g = 2.2), and is relatively constant down to approximately 40 K. Below this temperature, it collapses, indicating significant ZFS in the S = 1 ground state.³² Similar to 3, the distance of the nearest Ni atoms in 4 is 9.18 (1) Å, indicating that the weak interactions between them could be ignored. The in- and out-of-phase ac susceptibilities have no dependence on frequency between 2 and 10 K for 4, indicating that there is no SIM behavior in 4 (Fig. S3†).

For 6, the data above 2 K follow the Curie–Weiss law with C = 4.73 cm³ K mol⁻¹ and θ = −4.70 K. The χ_MT value at 300 K is 4.65 cm³ K mol⁻¹, which is slightly larger than the spin-only value of 4.38 cm³ K mol⁻¹ for a magnetically active Mn(II) ion (S = 5/2, g = 2.2) and is relatively constant down to approximately 30 K. Below this temperature, the χ_MT values decrease continuously and reach a local minimum of 1.27 cm³ K mol⁻¹ at about 2 K, indicative of a weak antiferromagnetic Mn(II)–Mn(II) exchange interaction. The in- and out-of-phase ac susceptibilities have no dependence on frequency between 2 and 10 K (Fig. S3†).

Thermal analyses

The thermogravimetric (TG) analysis was performed in a N₂ atmosphere on polycrystalline samples of complexes 1–6, and the TG curves are shown in Fig. 8.

Fig. 8 TG curves for complexes 1–6.

For 1, the first weight loss of 15.08% between 30 and 112 °C corresponds to the release of the lattice triethylamine molecules (calculated, 14.89%). The residual framework decomposed beyond 360 °C in a series of complicated weight losses until 920 °C. For 2, the first weight loss of 1.82% between 30 and 85 °C corresponds to the release of the lattice methyl alcohol molecules (calculated, 1.98%). The residual framework decomposed beyond 316 °C in a series of complicated weight losses until 885 °C. For 3, the two steps of the weight loss of 5.14% between 30 and 268 °C correspond to the release of the two lattice water molecules (calculated, 5.34%). The residual framework decomposed beyond 450 °C in a series of complicated weight losses until 844 °C. For 4, similar to 3, the two steps of the weight loss of 5.46% between 30 and 280 °C correspond to the release of the two lattice water molecules (calculated, 5.35%). The residual framework decomposed beyond 428 °C in a series of complicated weight losses until 880 °C. For 5, the first weight loss of 4.16% between 30 and 77 °C corresponds to the release of the lattice water molecules (calculated, 4.13%). The residual framework decomposed beyond 502 °C in a series of complicated weight losses and was still continuing when heating ended at 1000 °C. For 6, the first weight loss of 82.21% between 80 and 184 °C corresponds to the release of the lattice and the coordinated water molecules (calculated, 82.66%). The residual framework decomposed beyond 409 °C in a series of complicated weight losses until 834 °C.

Conclusions

In this paper, we have presented the synthesis and crystal structures of six new MOFs from one to three dimensions based on the 4,4′-H₂dbpt ligand. The structural diversities indicate that the 4,4′-H₂dbpt ligand could adopt different conformations according to the geometric requirements of metal ions and/or the introduction of auxiliary ligands owing to the different rotation angles of the four aromatic rings with respect to each other, deprotonation effort (4,4′-H₂dpbt, 4,4′-Hdpbt⁻, 4,4′-dpbt²⁻) and the flexing angles. As a result, five diverse and interesting architectures were obtained. 1 has a 2D (3,6)-topology with (4³)₃(4⁶.6⁶.8³)₂ Schläfli symbol. 2 has a 3D 8-connected topology with (3⁶.4¹⁸.5³.6) Schläfli symbol. 3 and 4 which are isostructural, have a 2D 4-connected topology with (4⁴.6²) Schläfli symbol. 5 has a complex 3D porous architecture with a 1D solvent-filled channel. 6 reveals a 1D helical chain extended along a 4-fold screw axis. Accordingly, our present findings will further enrich crystal engineering strategy and offer the possibility of controlling the formation of desired network structures.

Acknowledgements

We gratefully acknowledge the National Nature Science Foundation of China (nos 21101035, 21361003 and 21461003), the Guangxi Natural Science Foundation of China (2012GXNSFBA053017, 2012GXNSFAA053035, and 2014GXNSFBA118056) and the Foundation of Key Laboratory for Chemistry and Molecular Engineering of Medicinal Resources.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1033515–1033520. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra06273j

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