Four-connected metal–organic frameworks constructed by tetracarboxylate acid-based ligands

Abstract

Three tetracarboxylate acid ligands, 5,5′-(1H-1,2,3-triazole-1,4-diyl)diisophthalic acid (H₄TADIP), 5-(bis(4-carboxybenzyl)amino)isophthalic acid (H₄BCBAIP), and 5-(3,5-dicarboxybenzyloxy)isophthalic acid (H₄DBIP) have been reacted with Mn(II), Co(II) and In(III) salts to construct four new metal–organic frameworks, [Mn₂(TADIP)(DMF)₃]·2DMF·3H₂O (1), [Co₂(TADIP)(DEF)₃]·0.5DEF (2), [Mn₂(BCBAIP)(DMF)₃]·7.5DMF (3), and (Me₂NH₂)[In(DBIP)]·3DMF·7H₂O (4) under solvothermal conditions. Due to the rotation ability of these ligands that exhibit a quadrangle or tetrahedral geometry, these compounds show diverse three-dimensional four-connected topologies. Compound 1 is composed of binuclear Mn(II) units and quadrangle type ligand H₄TADIP with pts topology, while compound 2 possesses a zeolitic abw topology constructed from the 4-connected tetrahedral type ligand H₄TADIP and binuclear Co(II) units. Compound 3 is a 3D structure possessing a zeolitic sod topology from the coordination of binuclear Mn(II) units with the tetrahedral type ligand H₄BCBAIP. Compound 4 is an anionic framework constructed from tetrahedral type ligand H₄DBIP and In(III) ions that has a topology of unc. Variable temperature magnetic susceptibility measurements show that compound 1 exhibits antiferromagnetic behavior. The selective dye-adsorption and luminescent properties of compound 4 have been studied in detail.

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Introduction

In recent years, considerable effort has been focused on metal–organic frameworks (MOFs) due to not only their aesthetic structures, but also their versatile applications such as gas storage and separation,¹ catalysis,² magnetism,³ luminescence,⁴ and so on. Some recent excellent reviews have deeply discussed all aspects of the development and progress in the last two decades.⁵ Among these studies, development of new MOFs with diverse structures has always drawn great attention. The common strategy of crystal engineering has parted the structures into inorganic secondary building units (I-SBUs) and organic secondary building units (O-SBUs). I-SBUs can be thought as “joint” and O-SBUs as “strut”.⁶ The rational combination of I-SBUs and O-SBUs has been an important strategy in the design, prediction and synthesis of diverse MOFs.

Setting aside I-SBUs, which are too complicated to describe,⁷ organic linkers that can be regarded as O-SBUs participate in the construction of frameworks. Through selection of organic linkers with different geometries (2-, 3-, and 4-connected ligands can be thought as linear, triangular, planar quadrangular or tetrahedral, respectively), a variety of MOFs with different topologies and properties have been synthesized. In the selection of organic linkers, 4-connected tetracarboxylate ligands with rigid plane quadrangle or tetrahedral geometry are widely employed.⁸ For example, in 2005 Chen et al. reported the synthesis of MOF-505 for the first time by utilization of 4-connected tetracarboxylate ligand H₄BPTC (H₄BPTC = 3,3′,5,5′-biphenyltetracarboxylic acid) to react with a copper salt.⁹ Such a MOF has a topology of nbo and exhibits good adsorption behavior to H₂. Schröder and coworkers developed a series of tetracarboxylate ligands with expansion of the length, and obtained a number of MOFs with different structures, which have good properties in gas adsorption.¹⁰ Some other groups also reported MOFs synthesized with 4-connected tetracarboxylate ligands based on isophthalic acid.¹¹ However, most works reported up to now have made use of 4-connected ligands to react with Cu, Zn or In salts to form MOFs with topologies of nbo, pts, fcu, etc. From the crystal engineering point of view, many more structures might be built by rational exploitation of 4-connected I-SBU and O-SBU, so there remain lots of MOFs to be constructed when considering the features of these SBUs.

On the other hand, zeolite-like MOFs (ZMOFs) with sod, rho, and mtn topologies have been explored these years and found their applications in lots of areas.¹² In order to build ZMOFs, research mainly focused on the utilization of imidazole, imidazole carboxylic acid and heterocyclic carboxylic acid by the judicious mimicking of the Si–O–Si bond angle of about 145° through the employment of L–M–L angles.¹³ Recently, Zheng et al. reported another synthetic strategy for the creation of ZMOFs using carboxylic acid.¹⁴ It is not surprising that ZMOFs can be constructed by the linkage of 4-connected I-SBU and O-SBU through direct coordination interaction. Feng and coworkers reported several ZMOFs with sod and rho topologies based on 4-connected boron imidazolate complexes and 4-connected Li⁺ or Cu⁺ ions.^12e,13c However, reports are still rare on the construction of ZMOFs with 4-connected tetracarboxylate acid (Scheme 1).

Scheme 1 Representation of the three ligands used in this work.

Recently, we have paid great attention to the design and synthesis of MOFs with novel ligands based on isophthalic acid.^15,16 Some of the MOFs formed exhibited sensing behaviors for the detection of metal ions and nitroaromatic explosives.^15b,c We have designed and synthesized a ZMOF with sod topology through the 4 + 4 synthetic strategy by cross-linking the designed 4-connected tetracarboxylate ligand with the 4-connected In³⁺ ion.¹⁶ Here, we use three tetracarboxylate acid ligands, 5,5′-(1H-1,2,3-triazole-1,4-diyl)diisophthalic acid (H₄TADIP), 5-(bis(4-carboxybenzyl)amino)isophthalic acid (H₄BCBAIP) and 5-(3,5-dicarboxybenzyloxy)isophthalic acid (H₄DBIP) to react with Mn(II), Co(II) and In(III) salts to construct diverse structures. As mentioned in previous work, for ligand H₄TADIP,^15b,17 the two isophthalic acids linked to the 1,2,3-triazole ring can rotate with the C–N and C–C single bonds in some content, which can make the ligand act as two different geometries, i.e. planar quadrangular and tetragonal, and these can potentially lead to a number of different structures with metal ions. In ligand H₄BCBAIP, the free rotation of the C–N single bond may introduce a ligand with tetrahedral configuration. As for ligand H₄DBIP, Cao and others have reported the usage of such a ligand for the preparation of MOFs with diverse topologies.^11j,k We present here four MOFs based on these three tetracarboxylate acid ligands, [Mn₂(TADIP)(DMF)₃]·DMF (1), [Co₂(TADIP)(DEF)₃]·DEF (2), [Mn₂(BCBAIP)(DMF)₃]·7.5DMF (3), and (Me₂NH₂)[In(DBIP)]·3DMF·7H₂O (4). Compound 1 has a pts topology, while 2 has a zeolitic abw topology. Compound 3 possesses a zeolitic sod topology, and compound 4 has an unc topology.

Experimental

Materials and physical measurements

All reagents were purchased commercially and used without further purification. H₄TADIP,^15bH₄BCBAIP¹⁶ and H₄DBIP^15e were synthesized according to the procedures reported by our group previously. Thermogravimetric analyses (TGA) were performed under air atmosphere from room temperature to 800 °C with a heating rate of 10 °C min⁻¹ on TGA Q500 thermogravimetric analyzer. Elemental analyses (C, H and N) were carried out with a vario MICRO elemental analyzer. The infrared (IR) spectra were recorded from 400 to 4000 cm⁻¹ on a Nicolet Impact 410 FTIR spectrometer using KBr pellets. Powder X-ray diffraction (PXRD) data were collected on a Rigaku D-Max 2550 diffractometer with Cu-Kα radiation (λ = 1.5418 Å) over the 2θ range of 4–40° at room temperature. Temperature dependent magnetic susceptibility data were recorded on a Quantum-Design MPMS-XL SQUID magnetometer under an applied field of 1 kOe over the temperature range of 2–300 K. Photoluminescent spectra were obtained with a RF-5301 PC spectrometer. The solid state UV-vis absorption spectra were taken on a HITACHI U-4100 spectrophotometer while the liquid spectra were taken on a SHIMADZU UV-2450 UV-visible spectrophotometer.

Single-crystal X-ray studies

Single-crystal X-ray diffraction measurements were carried out on a Rigaku R-AXIS RAPID diffractometer for compound 1 and a Bruker Apex II CCD diffractometer for compounds 2–4, with graphite monochromated Mo Kα (λ = 0.71073 Å) radiation at 293 or 296 K. Data processing was accomplished with the SAINT processing program.¹⁸ The structures of these compounds were solved by direct method and refined on F² by full-matrix least-squares with the SHELEX-97 program.¹⁹ For compound 1, all non-hydrogen atoms were refined with anisotropic displacement parameters except the C and N atoms of coordinated DMF molecules. Some restraints like dfix were used on the DMF molecules for a better geometry configuration. The hydrogen atoms on the aromatic rings were placed geometrically with isotropic thermal parameters 1.2 times those of the attached carbon atoms. As for compound 2, all non-hydrogen atoms were refined with anisotropic displacement parameters except the C and N atoms of coordinated DMF molecules. Some restraints like dfix were used for a better geometry configuration. For compound 3, the C25 was split to C25 and C25′. All non-hydrogen atoms on the backbone were refined with anisotropic displacement parameters except the terminal DMF molecules. The restraint dfix was used to the terminal DMF molecules for a better geometry. For compound 4, all non-hydrogen atoms were refined with anisotropic displacement parameters (except for C9, C9′, O9 and O9′ in compound 4). Some restraints like exyz, eadp and dfix were used to refine the frameworks. Attempts to locate and model the highly disordered solvent molecules in the pores were unsuccessful. Therefore, further refinement with the SQUEEZE routine of PLATON was used to remove the diffraction contribution from guests to produce a set of solvent-free diffraction intensities.²⁰ The summary of the crystallographic data and refinement parameters is given in Table 1. The selective bond lengths and angles are given in Tables S1–S4 (ESI†). The contribution of the solvent molecules has been incorporated in both the empirical formula and formula weight in Table 1. The formula was designated from the electrons resulted after the SQUEEZE routine and the experimental results (see CIF).

Table 1 Crystallographic data for 1–4

Compound	1	2	3	4
a R 1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑[w(Fo^2 − Fc^2)^2]/∑w(Fo^2)^2]^1/2.
Formula	C33H48Mn2N8O16	C35.5H45.5Co2N6.5O11.5	C111H153Mn4N23O37	C26H43InN3O23
fw	922.65	865.145	2621.29	880.44
Temp (K)	296(2)	293(2)	296(2)	296(2)
Wavelength (Å)	0.71073	0.71073	0.71073	0.71073
Cryst syst	Tetragonal, P4̄21c	Monoclinic, P21/n	Hexagonal, R3c	Tetragonal, P41212
a (Å)	19.1999(10)	10.213(2)	38.9881(10)	18.0570(3)
b (Å)	19.1999(10)	17.607(4)	38.9881(10)	18.0570(3)
c (Å)	25.815(3)	28.123(6)	22.2962(11)	13.4495(5)
V (Å^3)	9516.3(13)	5000.3(17)	29 351.2(18)	4385.28(19)
Z	8	4	16	4
F(000)	2791	1556	7164	900
θ range (°)	1.59–28.29	3.07–27.48	1.04–28.30	1.59–28.29
Reflns collected/unique	69 096/11 809	45 488/11 374	71 268/16 106	32 419/5452
R int	0.1481	0.1065	0.0698	0.0467
Data/restraints/params	11 809/9/359	11 374/8/408	16 106/2/391	5452/0/148
GOF on F^2	1.047	1.041	1.010	1.026
R 1 , wR2^b [I > 2σ(I)]	0.0896, 0.2219	0.0810, 0.2022	0.0445, 0.0680	0.0276, 0.0520
R 1 , wR2^b (all data)	0.1784, 0.2723	0.1462, 0.2298	0.0967, 0.0730	0.0344, 0.0530

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Synthesis of [Mn₂(TADIP)(DMF)₃]·2DMF·3H₂O (1). A mixture of MnCl₂·4H₂O (6 mg, 0.03 mmol), H₄TADIP (5 mg, 0.013 mmol) and 70 μL of AcOH in DMF (4 mL) was added to a 20 mL vial, then the solution was heated at 110 °C for 48 h to give colourless block-shaped crystals. Anal. Calcd for C₃₃H₄₈Mn₂N₈O₁₆: C, 42.96; H, 5.24; N, 12.14%; Found: C, 41.66; H, 5.01; N, 11.36%. IR spectrum (KBr, cm⁻¹): 3418 (br), 3088 (w), 2928 (w), 1660 (s), 1615 (s), 1551 (m), 1441 (m), 1390 (s), 1246 (m), 1103 (m), 1061 (m), 908 (m).

Synthesis of [Co₂(TADIP)(DEF)₃]·0.5DEF (2). A mixture of Co(NO₃)₂·6H₂O (10 mg, 0.034 mmol), H₄TADIP (5 mg, 0.013 mmol) and AcOH (50 μL) in DEF (1 mL) was added to a 20 mL vial, then the solution was heated at 110 °C for 48 h to give colourless block-shaped crystals. Anal. Calcd for C_35.5H_45.5Co₂N_6.5O_11.5: C, 49.28; H, 5.30; N, 10.52%. Found: C, 48.60; H, 5.759; N, 10.33%. IR spectrum (KBr, cm⁻¹): 3405 (br), 3094 (w), 2977 (m), 2938 (w), 2884 (w), 1631 (s), 1592 (m), 1452 (s), 1382 (s), 1304 (w), 1265 (m), 1211 (m), 1118 (w), 1109 (w), 1051(w), 1028 (w), 998 (w).

Synthesis of [Mn₂(BCBAIP)(DMF)₃]·7.5DMF (3). A mixture of Mn(NO₃)₂ aqueous (48 μL, 0.5 mmol) and H₄BCBAIP (22.5 mg, 0.05 mmol) was dissolved in DMF (5 mL) and HNO₃ (10 μL, 2.7 M in DMF), after stirring for 15 min, the mixture was sealed in a vial and kept at 95 °C for 72 h. After cooling to room temperature, the light brown needle-like crystals were collected by filtration, washed with DMF and air-dried. Anal. Calcd for C₁₁₁H₁₅₃Mn₄N₂₃O₃₇: C, 50.86; H, 5.88; N, 12.29%; found: C, 48.28; H, 5.75; N, 9.63%. IR (KBr, cm⁻¹): 3406 (br), 3063 (w), 2925 (w), 1663 (s), 1603 (s), 1551 (s), 1388 (s), 1242 (m), 1176 (w), 1151 (w), 1104 (m), 1064 (m), 1015 (m), 993 (w).

Synthesis of (Me₂NH₂)[In(DBIP)]·3DMF·7H₂O (4). In(NO₃)·xH₂O (20 mg) and H₄DBIP (10 mg, 0.03 mmol) were dissolved in a 20 mL vial with DMF (1 mL) and HNO₃ (0.5 mL, 2.7 M in DMF), the mixture was then stirred for 30 min and sealed. After being kept at 85 °C for 12 h, the reaction mixture was cooled to room temperature, the light brown block crystals were collected, washed with DMF, and air dried. Anal. Calcd for C₂₆H₄₃InN₃O₂₃: C, 35.47; H, 4.92; N, 4.77%; found: C, 34.67; H, 5.617; N, 4.93%. IR (KBr, cm⁻¹) 3445 (br), 2965 (w), 2931 (w), 2886 (w), 2792 (w), 2468 (br), 1654 (s), 1625 (m), 1569 (s), 1469 (s), 1380 (s), 1257 (s), 1133 (s), 1106 (s), 1033 (s).

The dye-adsorption experiments

Several batches of compound 4 (25 mg) were added to the ethanol solution (2 mL) containing 0.05 mg of different dyes including Methylene Blue (MB), Coumarin 6 (C6), Coumarin 343 (C343), Rhodamine B (RB), Methyl Orange (MO), respectively. The liquid state UV-vis adsorption spectra were taken every at fixed intervals over a period of 6 h. The products obtained were named MB@4, C6@4, C343@4, RB@4 and MO@4.

Results and discussion

Crystal structures

Structural description of compound 1. Compound 1 crystallizes in the tetragonal space group P4̄2₁c. The asymmetric unit consists of two crystallographically independent Mn(II) ions (Mn1, Mn2), one TADIP⁴⁻ anion and three terminal DMF molecules coordinated to the Mn2 ion. The Mn1 ion is six-coordinated with six oxygen atoms (O4–O6, O8–O10) from four TADIP⁴⁻ anions to form a distorted octahedral geometry. The Mn2 ion is six-coordinated with three oxygen atoms (O3, O7, O9) from three TADIP⁴⁻ anions and another three (O1, O2, O11) from the DMF molecules to form an octahedral geometry. The average Mn–O bond length ranges from 2.067(5) to 2.305(5) Å. In compound 1, the carboxylate groups of TADIP⁴⁻ display three kinds of coordination modes: the bridging bismonodentate mode (O3–C11–O5, O4–C18–O7), the chelating bidentate mode (O8–C5–O10), and the bridging-chelating mode (O6–C1–O9), where O9 acts as a μ₂ atom. In the crystal structure of compound 1, Mn1 and Mn2 act as a binuclear unit with a Mn⋯Mn distance of 3.556 Å. The binuclear unit is then linked by four organic TADIP⁴⁻ anions to form a 4-connected 3D framework (Fig. 1). When viewed along the c axis, it can be seen that there exist two different kinds of one-dimensional channels, which can be further parted into four due to their left & right orientation. The ellipsoid-type channel is blocked in the middle by the coordinated DMF molecules, while the square one has a diameter of about 5 Å (considering the van der Waals radii) (Fig. 1c). Topological analysis shows that 1 has a pts topology with a point symbol of (4²·8⁴) when considering the Mn₂ units and TADIP⁴⁻ as 4-connected nodes (Fig. 1e,f). The solvent accessible volume is 43.2% of the unit cell volume, calculated from the crystal structure using the PLATON program.

Fig. 1 (a) The coordination environment of binuclear Mn(II) cluster viewed as a 4-connected I-SBU; (b) the ligand H₄TADIP coordinated to four binuclear Mn(II) I-SBUs and viewed as a 4-connected O-SBU; (c) the 3D framework of 1; (d) space-filling model of channels along the [001] direction; (e) polyhedral view of the pts topology; (f) the pts topology.

Structural description of compound 2. Compound 2 crystallizes in the monoclinic space group P2₁/n. The asymmetric units consist of two crystallographically independent Co(II) ions (Co1, Co2), one TADIP⁴⁻ anion and three terminal DEF molecules coordinated to the Co2 ion. The Co1 ion is 4-coordinated with four oxygen atoms (O1, O3, O5, O11) from four TADIP⁴⁻ anions to form a tetrahedral geometry, while the Co2 ion is 6-coordinated with three oxygen atoms (O4, O6, O10) from three TADIP⁴⁻ anions and another three oxygen atoms (O7–O9) from the DEF molecules to form an octahedron. The average Co–O bond length ranges from 1.959(4) to 2.105(5) Å. In compound 2, the carboxylate groups of ligand TADIP⁴⁻ display two kinds of coordination modes: the bridging bismonodentate mode (O4–C1–O3, O5–C14–O6, O10–C15–O11), and the monodentate mode (O1–C10–O2). In the crystal structure of compound 2, the Co1 and Co2 ions act as a binuclear unit with a Co⋯Co distance of 3.648 Å. The binuclear unit is then linked by four organic TADIP⁴⁻ anions to form a 4-connected 3D framework. There exist three dimensional cross-typed channels in the framework, one is about 5 × 6 Å along the b axis, while the other is blocked by the coordinated DEF molecules and has a pore of about 3 × 6 Å along the a axis (Fig. 2c,d). The topological analysis reveals that compound 2 has a zeolite-type abw topology with a point symbol of (4²·6³·8) when considering the Co2 unit and the TADIP⁴⁻ ligand as 4-connected building blocks (Fig. 2e,f). The solvent accessible volume is 37.6% of the unit cell volume, calculated from the crystal structure using the PLATON program.

Fig. 2 (a) The coordination environment of binuclear Co(II) cluster viewed as a 4-connected I-SBU; (b) the ligand H₄TADIP coordinated to four binuclear Co(II) I-SBUs and viewed as a 4-connected O-SBU; (c) the 3D framework of 2; (d) space-filling model of channels along the [100] direction; (e) polyhedral view of the abw topology; (f) the abw topology.

Structural description of compound 3. Compound 3 crystallizes in the hexagonal R3c space group. The asymmetric unit contains two crystallographically independent Mn(II) ions (Mn1, Mn2), half of an organic BCBAIP⁴⁻ anion, and three terminal DMF molecules coordinated to the binuclear Mn cluster. The Mn1 ion is six-coordinated with six oxygen atoms from three carboxylate (O4, O6, O8) and three DMF molecules (O9, O10, O11) to form an octahedral geometry, while the Mn2 ion is six-coordinated with six oxygen atoms from four carboxylates (O1–O5, O7) to form a distorted octahedral geometry. The average Mn–O bond length ranges from 2.096(2) to 2.319(2) Å. In this compound, the organic ligand exhibits three kinds of coordination mode: the bridging bismonodentate mode (O5–C24–O6, O7–C1–O8), the chelating bidentate mode (O1–C22–O2), and the bridging-chelating mode (O3–C23–O4), where O4 acts as a μ₂ atom. In the compound, the Mn1 and Mn2 ion act as a Mn₂ binuclear unit with a Mn⋯Mn distance of 3.499 Å. The binuclear unit is then linked by four organic BCBAIP⁴⁻ anions to form a 4-connected 3D framework. When viewed along the c axis, one kind of one dimensional channel can be found. The pore dimension of the channel is approximately 11 Å. Fig. 3c illustrates the compound along the c axis with the hexagonal windows. As shown in Fig. S5,† the hexagonal window is constructed by three O-SBUs (BCBAIP⁴⁻) and three I-SBUs (Mn₂). Further connection of the hexagonal windows can form a SOD cage with twelve I-SBUs and twelve O-SBUs. This cage has a maximum space of about 15 Å, with a dimension of about 11 Å along the windows. When considering the Mn₂ cluster as a 4-connected inorganic building unit and BCBAIP⁴⁻ as a 4-connected organic linker, a topological analysis reveals that this compound has a zeolitic type sod topology (Fig. 3e,f). Such a topology has been realized in our previously reported work using the same ligand and In³⁺ ions.¹⁶ The void volume of compound 3 with the removal of guest solvent molecules is about 54.4% by using PLATON/SQUEEZE. However, although the framework of compound 3 exhibits porous features, the investigation of the N₂ adsorption–desorption properties failed.

Fig. 3 (a) The coordination environment of binuclear Mn(II) cluster viewed as a 4-connected I-SBU; (b) the ligand H₄BCBAIP coordinated to four binuclear Mn(II) I-SUBs and viewed as a 4-connected O-SBU; (c) the 3D framework of 3; (d) space-filling model of channels along the [001] direction; (e) polyhedral view of the sod topology; (f) the sod topology.

Structural description of compound 4. Compound 4 crystallizes in the tetragonal space group P4₁2₁2. The asymmetric unit consists of one independent In³⁺ ion, half of a DBIP⁴⁻ anion, and one Me₂NH₂⁺ cation, which was decomposed from the solvent DMF molecules to balance the negative charged framework. The In³⁺ ion is located on a crystallographically-imposed 2-fold axis. Each In³⁺ ion is four coordinated to four carboxylate groups from four DBIP⁴⁻ ligands, and serves as a tetrahedral 4-connected inorganic building unit. Each carboxylate group in an organic ligand adopts a bidentate chelating mode, and coordinates with one indium ion to form a 4-connected organic linker. Further linkage of the 4-connected {In(O₂C)₄} inorganic building unit with 4-connected organic linker forms a 3D framework (Fig. 4c,d). The structure of compound 4 features two types of one-dimensional channels along the c axis. The large one is about 14 Å, and the small one is 11 × 7 Å. Topological analysis of this network gives rise to a 4-connected net of unc topology with a point symbol of (6⁶) (Fig. 4e,f). The void volume of 4 calculated using PLATON/SQUEEZE is 68.8% of the unit cell volume upon removal of the Me₂NH₂⁺ cations and guest solvent molecules.

Fig. 4 (a) The coordination environment of In(III) unit viewed as a 4-connected I-SBU; (b) the ligand H₄DBIP coordinated to four binuclear In(III) I-SBUs and viewed as a 4-connected O-SBU; (c) the 3D framework of 4; (d) space-filling model of channels along the [001] direction; (e) polyhedral view of the unc topology; (f) the unc topology.

Summary of the coordination modes for these ligands and their usage as 4-connected linkers

A summary of the diverse coordination modes of ligands H₄TADIP and H₄BCBAIP is listed in Fig. 5. The coordination modes of H₄DBIP have been depicted in previous work reported by Cao and coworkers.^11k It can be seen that all the three ligands chosen here can rotate with different functional groups or bonds to adopt diverse geometries. For compound 1, the ligand H₄TADIP can be regarded as a rectangular geometry, while in compounds 2–4, these ligands can be regarded as tetrahedral building blocks. Combined with external factors such as temperature, pH, solvent, and metal salts, these ligands could adopt a tetrahedral or rectangular geometry to fit the need of a better coordination. Thus, the usage of such semi-rigid ligands to coordinate with the metal ions can lead to the formation of 4-connected MOFs with diverse topologies.

Fig. 5 Summary of diverse coordination modes for H₄TADIP and H₄BCBAIP.

Magnetic properties of compound 1

For compound 1, the Mn1 and Mn2 ions act as a binuclear unit with a Mn⋯Mn distance of 3.556 Å. The temperature-dependent magnetic susceptibility measurement of compound 1 has been performed on the powder sample in the ranges of 2–300 K under a field of 1 kOe. As shown in Fig. 6, the observed value of χ_MT per Mn unit is 4.58 cm³ K mol⁻¹ at room temperature, which is slightly higher than the value expected for a free Mn(II) ion (4.375 cm³ K mol⁻¹). Upon cooling, the χ_MT value decreases gradually, until a sharp decrease is observed below 50 K to a value of 0.37 cm³ K mol⁻¹ at 2 K, this feature is possibly the result of intramolecular antiferromagnetic interactions dominating the magnetic properties in compound 1. The plot of χ_M⁻¹vs. T over the temperature range 25–300 K obeys the Curie–Weiss law [χ = C/(T − θ)] with C = 4.88 cm³ K mol⁻¹ and θ = −18.40 K (Fig. S6†).

Fig. 6 Temperature dependence of χ_M (○) and χ_MT (□) vs. T plots for compound 1 under an applied field of 1 kOe from 2–300 K.

The luminescent properties of compound 4

The UV-vis spectra of the free ligand H₄DBIP and compound 4 in the solid state are shown in Fig. S9.† The ligand shows the absorption from 200 to 350 nm with the maximum at about 316 nm. The maximum absorption of compound 4 is slightly red-shifted (about 8 nm) compared to the ligand and has an absorption ranging from 200 to 300 nm. Their luminescent spectra are also investigated. When excited at 316 nm, the solid state luminescent spectrum of H₄DBIP exhibits a maximum emission at about 380 nm. The emission spectrum of 4 was obtained with an excitation wavelength of 324 nm, and shows a maximum emission at about 460 nm. Compound 4 shows a relatively superior red-shift compared to H₄DBIP which might be ascribed to the ligand-to-metal charge transfer (LMCT) (Fig. 7).²¹

Fig. 7 The luminescent spectra of compound 4 and H₄DBIP.

The selective dye-adsorption kinetics

To evaluate the porosity of compound 4, a nitrogen (at 77 K) adsorption experiment was performed. Unfortunately, no significant nitrogen uptake was observed. In order to demonstrate the pore accessibility and considering the existence of Me₂NH₂⁺ cation in the framework, dye-adsorption experiments were performed using compound 4 with differently charged dyes (cationic MB, RB; neutral C343, C6; anionic MO). When immersed into the ethanol solutions of Methylene Blue (MB), Coumarin 6 (C6), Coumarin 343 (C343), Rhodamine B (RB), and Methyl Orange (MO) for 6 h at room temperature, compound 4 presents different kinetic adsorption behavior toward these dyes as monitored using UV-vis absorption spectroscopy (Fig. S7†). Compound 4 exhibits an exchange rate of MB > C343 > RB > C6 > MO. The diverse exchange behaviors of compound 4 towards these dyes may be ascribed to the anionic nature of the framework and the resulting exchangeable moderate pore size. The Me₂NH₂⁺ cation in the channels may be exchanged rapidly by the cationic MB molecules, while RB is resisted due to its large molecule dimensions, and the neutral C343 and C6 are not the favored dyes for the exchange. The negatively charged MO is hard to exchange. To further demonstrate the selectivity of compound 4 for the cationic dyes, especially MB, and considering the easily distinguishable UV-vis spectra, the dyes of MB, MO and RB were mixed together to observe the unique selection behaviors of compound 4 to the cationic dyes. As shown in Fig. 8, compound 4 shows an obvious selection ability for the cationic MB molecules and absorbs the most amount of MB, about 85.1% estimated from the UV-vis absorption spectra.

Fig. 8 (a) The UV-vis absorption spectra for the mixture of MB, RB and MO in ethanol before (black line) and after (red line) the addition of compound 4. (b) Peak-height ratios of MB, RB and MO after the addition of compound 4 to the initial intensity.

Conclusions

Four novel MOFs were synthesized by the coordination of metal ions (Mn, Co, In) with rigid and semi-rigid tetracarboxylate acid ligands such as H₄TADIP, H₄BCBAIP, and H₄DBIP under solvothermal conditions. Due to the fact that these ligands can exhibit diverse geometries with free rotation through characteristic bonds or functional groups, these ligands can be regarded as 4-connected linkers with quadrangle or tetrahedral building blocks. All of these synthesized MOFs are 4-connected frameworks composed of the above mentioned 4-connected inorganic secondary building units (I-SBUs) and organic secondary building units (O-SBUs). Topological analyses of these compounds reveal the formation of four-connected pts and unc topologies for compounds 1 and 4, and zeolitic abw and sod topologies for compounds 2 and 3. Compound 1 shows an antiferromagnetic behavior. Compound 4 was used to selectively exchange the cationic dyes due to the anionic nature of the frameworks. The anionic nature of compound 4 can be further used in the selective exchange of the cationic MB dye from the mixture of MB, RB and MO in ethanol solution. This work may help to further develop 4-connected metal–organic frameworks with diverse topologies through a 4 + 4 synthetic strategy, and further extension of this work with other metal ions is in progress in our laboratory.

Acknowledgements

We thank the National Natural Science Foundation of China (grants: 91122029 and 21320102001) and the State Basic Research Project of China (grants: 2011CB808703 and 2014CB931802) for support of this work. Z.L. thanks the open project in Key Lab of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, P. R. China.

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Footnote

† Electronic supplementary information (ESI) available: Complete X-ray crystallographic data for 1–4 in CIF format. PXRD patterns, TGA and IR spectra for 1–4. UV-vis, luminescence and dyes adsorption for 4. Selected bond lengths and angles for compound 1–4. CCDC 993507–993510. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qi00047a

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