Three 3D coordination polymers based on [1,1′:4′,1′′-terphenyl]-2′,4,4′′,5′-tetracarboxylate demonstrating magnetic properties and selective sensing of Al³⁺/Fe³⁺over mixed ions

Abstract

Three three-dimensional (3D) coordination polymers with the formulae [Cd(TPTC)_0.5(H₂O)₂] ·DMA (1), [Co₂(TPTC)(H₂O)_1.5(CH₃OH)_0.5] (2), and [Mn₂(TPTC)(H₂O)(DMA)]·DMA (3) (TPTC = [1,1′:4′,1′′-terphenyl]-2′,4,4′′,5′-tetracarboxylate acid) were solvothermally synthesized. They were characterized by thermogravimetric analyses, IR spectroscopy, X-ray powder diffraction, and single crystal X-ray diffraction. 1 features a 3D porous coordination polymer formed from Cd-carboxylate chains and the TPTC bridges. In 2 and 3, the uniform metal–carboxylate chains with triple bridges, (μ-EO-H₂O)(μ-syn,syn-COO)₂ and (μ-EO-H₂O/DMA)(μ-syn,syn-COO)₂ (EO = end-on), were extended by the TPTC arms. Complex 1 displayed strong fluorescent emission in the visible region. Interestingly, the emission intensities of 1 were increased upon the addition of Al³⁺ and quenched upon the addition of Fe³⁺, even in mixtures of ions. Thus, 1 can act as a useful material for sensing Fe³⁺ and Al³⁺ ions. The magnetic studies of 2 and 3 show that antiferromagnetic interactions between the Co(II) and Mn(II) centers exist.

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Introduction

Metal–organic coordination polymers or metal–organic frameworks (MOFs) have become an increasingly promising area, for their esthetically pleasing crystalline structures as well as their enormous practical applications, such as gas adsorption and separation,¹ catalysis,² luminescence,³ electrical conductivity,⁴ and magnetism.⁵ A number of luminescent materials for sensing small molecules or metal ions are widely used in many areas, such as luminescent probes in biomedical assays and time-resolved microscopy, fluorescent lighting, and luminescent probes for chemical species,⁶ which have made MOF chemosensors a hot research topic.⁷ MOFs offer high photoluminescence efficiency, based on components, antennae effects, adsorbate-based emission and sensitization, and surface functionalization.⁸ Recently, studies of luminescent MOFs for sensing metal ions have developed significantly, and some successful fluorescent probes have been employed to determine the concentration of metal ions such as Ca²⁺, Cd²⁺ and Fe³⁺.⁹

The design and construction of functional porous MOFs with magnetic properties is a challenge because it is difficult to obtain large pore sizes and relatively strong magnetic interactions simultaneously.¹⁰ Magnetic superexchange requires relatively short exchange pathways between two adjacent metal centers. The most popular synthetic strategy uses magnetic chains or clusters connected by longer organic linkers to form a porous open framework.¹¹

Organic carboxylate ligands have been widely used in synthesizing MOFs with esthetic structures and unusual properties because of the controllable length of ligand and the various coordination modes of the carboxyl group.¹² Herein, we use a new quadrangular ligand, H₄TPTC, [1,1′:4′,1′′-terphenyl]-2′,4,4′′,5′-tetracarboxylic acid. We present a study of three 3D porous MOFs, [Cd(TPTC)_0.5(H₂O)₂]·DMA (1), [Co₂(TPTC)(H₂O)_1.5(CH₃OH)_0.5] (2), and [Mn₂(TPTC)(H₂O)(DMA)]·DMA (3), which were generated by changing only the metal salt. More interestingly, 1 exhibits Al³⁺/Fe³⁺-modulated fluorescence over mixed ions; 2 and 3 also show antiferromagnetic interactions between metal ions.

Experimental section

Materials and methods

All chemicals were commercially available and used without further purification. The C, H, and N microanalyses were carried out with a PerkinElmer 240 elemental analyzer. The FT-IR spectra were recorded from KBr pellets in the 4000–400 cm⁻¹ range on a Nicolet 5DX spectrometer. Thermogravimetric analyses (TGA) were performed on a PerkinElmer Pyrisl (20–800 °C, 10 °C min⁻¹, N₂ gas flow). X-ray powder diffraction was recorded with a Bruker AXS D8 advanced automated diffractometer with Cu-Kα radiation. Luminescence spectra for the solid samples and liquid samples were taken with a Hitachi F-4500 fluorescence spectrophotometer and a Varian Cary Eclipse Fluorescence spectrophotometer, respectively. The magnetic data were collected on a Quantum Design MPMS SQUID-XL-5 magnetometer.

Solvothermal synthesis

[Cd(TPTC)_0.5(H₂O)₂] ·DMA (1). A mixture of Cd(NO₃)₂·4H₂O (0.049 mmol, 0.015 g), H₄TPTC (0.012 mmol, 0.005 g), N,N′-dimethylacetamide (DMA) (1.5 mL) and H₂O (0.5 mL) was stirred for 20 min in a 5 mL vial, which was heated in an oven at 75 °C for 72 h. White block crystals were obtained and washed with distilled water and dried in air (yield: ca. 54%). Elemental analysis calcd (%) for 1 C₁₅H₁₈ CdNO₇ (%): C: 41.25; H: 4.15; N: 3.21; found: C: 42.82; H: 4.49; N: 3.41. IR (KBr, cm⁻¹): 3306s, 2922w, 1590vs, 1385vs, 1258m, 1183m, 1016m, 871m, 783m.

[Co₂(TPTC)(H₂O)_1.5(CH₃OH)_0.5] (2). The synthesis of 2 was similar to that of 1, except Cd(NO₃)₂·4H₂O was used instead of Co(NO₃)₂·6H₂O (0.052 mmol, 0.015 g) (yield: ca. 65%). Elemental analysis calcd (%) for 2 C_22.5H₁₅Co₂O₁₀ (%): C: 47.98; H: 2.68; found: C: 48.15; H: 2.38. IR (KBr, cm⁻¹): 3563w, 2932w, 1590vs, 1412vs, 1347m, 1254m, 1152m, 868m, 781m.

[Mn₂(TPTC)(H₂O)(DMA)]·DMA (3). The synthesis of 3 was similar to that of 1, except Cd(NO₃)₂·4H₂O was used instead of Mn(NO₃)₂·6H₂O (0.052 mmol, 0.015 g) (yield: ca. 72%). Elemental analysis calcd (%) for 3C₃₀H₃₀Mn₂N₂O₁₁ (%): C: 51.15; H: 4.29; N: 3.98; found: C: 51.25; H: 4.36; N: 3.98. IR (KBr, cm⁻¹): 3548w, 2936w, 1591vs, 1414vs, 1347m, 1255m, 1152m, 867m, 782m.

X-ray crystallography

Crystallographic data of all the complexes were collected at 173 K with an Apex II diffractometer with Mo-Kα radiation (λ = 0.71073 Å) and graphite monochromator using the ω-scan mode. The structure was solved by direct methods and refined on F² by full-matrix least squares using SHELXTL.¹³ In 2, DMA molecules were highly disordered, and attempts to locate and refine the peaks were unsuccessful. The DMA molecules were removed from the data set using the SQUEEZE routine of PLATON¹⁴ and refined further using the data generated. Crystallographic data and experimental details for structural analyses are shown in Table 1. The CCDC reference number is 1013700–1013702 for 1–3.†

Table 1 Crystallographic data for 1–3

	1	2	3
a R1 = ∑||Fo|−|Fc||/∑|Fo|; wR2 = ∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]^1/2.
Empirical formula	C15H18CdNO7	C22.5H15Co2O10	C30H28Mn2N2O11
Formula weight	436.70	563.21	702.42
Wavelength/Å	0.71073	0.71073	0.71073
Crystal system	Monoclinic	Monoclinic	Monoclinic
Space group	P21/c	C2/m	P21/n
a/Å	8.7173(6)	18.7009(15)	12.6852(11)
b/Å	19.4288(17)	7.2352(5)	14.7613(15)
c/Å	9.6558(8)	12.5595(9)	16.0030(15)
β/°	97.170(3)	123.488(2)	98.210(3)
V/Å^−3	1622.6(2)	1417.27(18)	2965.9(5)
Z	4	2	4
Dc/mg m^−3	1.788	1.320	1.573
μ/mm^−1	1.383	1.215	0.917
F(000)	876	568	1440
Range for data collection/°	2.99–25.00	3.10–26.38	3.05–26.38
Reflections collected	2800	5869	17987
Max., min. transmission	0.7694, 0.7064	0.7775, 0.6633	0.8052, 0.7290
T/K	173(2)	173(2)	173(2)
Data/restraints/parameters	2800/24/225	1516/60/116	5467/18/417
Final R indices [I > 2σ(I)]^a	R1 = 0.0479, wR2 = 0.0782	R1 = 0.0849, wR2 = 0.2254	R1 = 0.0503, wR2 = 0.0886
R indices (all data)	R1 = 0.0767, wR2 = 0.0859	R1 = 0.1206, wR2 = 0.2481	R1 = 0.1394, wR2 = 0.1136
Largest diff. peak and hole/e Å^−3	0.891, −0.860	1.417, −0.951	0.546, −0.442

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Chart 1 Structure of H₄TPTC.

Results and discussion

Crystal structures

Single-crystal X-ray structure analysis reveals that 1 crystallizes in the monoclinic space group P2₁/c, showing a 3D non-interpenetrated framework. The asymmetric unit of 1 contains one crystallographically independent Cd(II) center, a half TPTC ligand, two aqua ligands, and a disordered DMA molecule. As shown in Fig. 1a and b, the TPTC ligand coordinates to six Cd(II) ions via its four carboxylate groups, two of which adopt chelating mode and the other two adopt monodentate coordination modes. The central Cd(II) ion is coordinated by four oxygen atoms of the TPTC ligands and two aqua ligands. Thus, the central Cd(II) ions are connected by TPTC ligands through their two η¹:η¹ chelating and two syn–anti-μ₂-η¹:η¹ carboxylate groups to generate a 2D layer (Fig. 1c). The layers are connected by the carboxylate groups of the TPTC ligands to form a 3D framework with channel dimensions of 8.212 × 7.124 Ų (including the van der Waals radii of the atoms), and neighboring metal ions are linked by the haploid syn–anti-COO group, which induces a 1D uniform Cd-carboxylate chain running along the a-axis, with a Cd–Cd separation of 4.889 Å (Fig. 1d).

Fig. 1 (a) Coordination environment of Cd(II) center in 1. Symmetry mode: (A) x, y, z. (B) 1 + x, 1.5 − x, 0.5 + z. (C) x, 1 + y, z. (b) The coordination modes of TPTC ligand in 1. (c) 2D layer structure constructed from Cd(II) and TPTC ligands along the a-axis in 1. (d) The 1D channel of 1 in the 3D framework along the c-axis, embedded with DMA molecules.

In contrast to 1, complex 2 crystallizes in the monoclinic space group C2/m. There is 0.5 of a Co(II) ion, 0.25 of a fully-deprotonated TPTC ligand, 0.375 of an μ₂-aqua ligand, and 0.125 of a CH₃OH molecule in the asymmetric unit of 2. The CH₃OH molecule may come from an impurity in the DMF solvent. As shown in Fig. 2a and b, each TPTC ligand connects eight Co(II) ions and each Co(II) links to four ligands. The coordination geometry around the Co(II) center can be described as a distorted octahedron with O–Co–O bond angles ranging from 84.9(3) to 180.0(2), and Co–O bond lengths ranging from 2.051(5) to 2.172(4) Å. The carboxylate groups of TPTC exhibit μ₂-η¹:η¹ bridging and a syn–syn coordination configuration, and neighboring metal ions are linked by the triple (μ-EO-H₂O)(μ-syn,syn-COO)₂ bridges, with Co–Co = 3.6176(2) Å, which induces a 1D uniform Co-carboxylate chain running along the c-axis (Fig. 2c). The four carboxylate groups in the TPTC ligand separate the metal ions from different chains by 9.350 Å and 10.751 Å.

Fig. 2 (a) Coordination environment of Co(II) center in 2. Symmetry mode: (A) x, y, z. (B) x, −y, z. (C) 0.5 − x, 0.5 + y, 1 − z. (D) 0.5 − x, 0.5 − y, 1 − z. (E) −0.5 + x, 0.5 + y, z. (F) 1 − x, −y, 1 − z. (G) 0.5 − x, 0.5 − y, 1 − z. (b) Coordination modes of TPTC ligand in 2. (c) 1D co–carboxylate chain along the b-axis in 2.

The single-crystal X-ray diffraction analysis reveals that complex 3 crystallizes in a monoclinic P2₁/n space group. The asymmetric unit of 3 is composed of three crystallographically independent Mn(II) centers, one fully-deprotonated TPTC ligand, one μ₂-aqua ligand, one μ₂-DMA molecule, and one free DMA molecule. As shown in Fig. 3a and b, the three types of Mn are all six-coordinated with distorted octahedral geometry, with four carboxylate O atoms from four different TPTC ligands, and also with two O atoms from one aqua and one terminally coordinated DMA ligand for Mn1, two DMA ligands for Mn2, and two aqua ligands for Mn3, respectively. Similar to 2, the carboxylate groups of the TPTC adopt a syn–syn coordination configuration and neighboring metal ions are linked by the triple (μ-EO-H₂O/DMA)(μ-syn,syn-COO)₂ bridges. Thus, 3 is built on a 1D Mn carboxylate chains and TPTC bridges (Fig. 3c) and is connected further by the TPTC ligands, which induces a large 1D irregular quadrangular channel with dimensions of approximately 9.944 × 6.286 Ų (including the van der Waals radii of the atoms) (Fig. 3d).

Fig. 3 (a) Coordination environment of Mn(II) center in 3. Symmetry mode: (A) 1 − x, 1 − y, −z. (B) −1 + x, y, z. (C) 2 − x, 1 − y, −z. (D) 1.5 − x, −0.5 + y, 0.5 − z. (E) 2.5 − x, −0.5 + y, 0.5 − z. (F) −1.5 + x, 0.5 − y, −0.5 + z. (G) −0.5 + x, 0.5 − y, −0.5 + z. (H) 1 − x, −y, −z. (b) Coordination modes of TPTC ligand in 3. (c) 1D Mn-carboxylate chain along the b-axis in 3. (d) 1D channel of 3 in the framework along the a-axis, embedded with DMA molecules.

PXRD patterns and thermogravimetric analysis

The simulated and experimental XRD patterns of the three compounds are shown in Fig. S1–S3.† Their peak positions are in good agreement with each other, confirming the purity of the synthesized bulk materials. The TGA curve (Fig. S4†) of 1 shows the first weight loss of 12.1% at ca. 275 °C, corresponding to the release of free DMA molecules (calcd 19.9%). For complex 2 (Fig. S5†), the first weight decrease of 27.0% appears in the range of 163 to 235 °C, which is consistent with the loss of coordinated water molecules and two lattice DMA molecules (calcd 26.9%). The second loss of 52.3% between 360 and 390 °C implies sudden decomposition, which corresponds to the collapse of the framework. The TGA curve (Fig. S6†) of 3 displays a decrease of 26.16% from 170 to 240 °C, coinciding with the loss of the aqua ligands and DMA molecules (calcd 27.3%), and then the framework collapse upon further heating.

Photoluminescence properties

Previous studies have shown that d¹⁰ coordination polymers containing zinc may exhibit photoluminescence properties.¹⁵ The luminescence properties of 1 were investigated. As expected, 1 exhibits extraordinary photoluminescence behavior in the solid state (Fig. S7†) and in a methanol suspension. In the solid state, a strong fluorescent emission band at 387 nm was observed at room temperature, excited at 274 nm, and in methanol solution, a fluorescent emission band at 390 nm was observed at room temperature, excited at 250 nm. 1 shows a weaker emission band blue-shifted to different extents compared with the free H₄TPTC ligand (Fig. S8†), which probably originates from the ligand-to-metal charge transfer (LMCT).¹⁶

Based on the XRD patterns (Fig. S9†), complex 1 retains its framework after immersion in methanol solutions containing different metal ions. The effects of a variety of relevant cations K⁺, Na⁺, Ag⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Hg²⁺, Ni²⁺, Zn²⁺, Al³⁺, Cr³⁺, Gd³⁺, In³⁺ and Fe³⁺ on the fluorescent intensities of 1 were investigated, and the luminescence properties are shown in Fig. 4. The results indicate that the emission intensity of 1 increased significantly upon addition of Al³⁺ and the emission intensity of 1 was almost quenched upon addition of Fe³⁺. The highest peak at 390 nm is at least twice as intense as the corresponding band in the solution without Al³⁺. However, the introduction of other metal ions either did not affect or weakened the intensity.

Fig. 4 Room-temperature luminescent intensity of 1 at 390 nm in methanol suspension upon addition of various metal ions (excited at 250 nm).

The luminescent intensity of Mⁿ⁺ relies on the efficiency of the energy transfer from the ligand to the Mⁿ⁺ center.¹⁷ Sun et al. prepared [H₂N(CH₃)₂][Eu(H₂O)₂(BTMIPA)]·2H₂O, with [H₂N(CH₃)₂]⁺ located in the channels, which can be replaced by Fe³⁺ and Al³⁺, which has a significant effect on the luminescence emissions.^18a Similar to the reported metal-sensitive luminescent metal probes, the change of luminescent intensity in our investigation may result from the more effective intramolecular energy transfer process from the TPTC ligand to the Cd(II) with the addition of Al³⁺, whereas with the additional Fe³⁺, this energy transfer process is less effective.

To explore the fluorescence quenching and enhancement further, detailed studies of the luminescence properties of 1 in the presence of Fe³⁺ and Al³⁺ ions were carried out. Concentration-dependent luminescence measurements were also carried out (Fig. S10a and b†). However, the PL intensities for Fe³⁺- and Al³⁺-loaded samples changed dramatically when the concentration was increased. For the Fe³⁺-loaded sample, the PL intensity decreased gradually, and when the concentration reached 10⁻³ mol L⁻¹, the luminescence was quenched completely. For the Al³⁺-loaded sample, the PL intensity increased gradually and reached a maximum at a concentration of 9 × 10⁻⁴ mol L⁻¹, and after that, the intensity decreased gradually. Based on these results, we propose the following mechanism. First, it is possible for the Fe³⁺ and Al³⁺ ions to bind to the inner surface of the channels. In the structure of 1, the carboxylic oxygen atoms inside the channel, the coordinated water molecules located on Cd²⁺, and the water molecules in the channel can create a favorable coordination environment for metal ions. Second, the exchange between Fe³⁺/Al³⁺ and Cd²⁺ may collapse the framework of 1.¹⁸

To confirm the high selectivity for Al³⁺ and Fe³⁺ ions over other metal ions, we studied the luminescence of 1 in a methanol solution containing mixed metal ions (Na⁺, Hg²⁺, Mg²⁺, Ni²⁺, Zn²⁺). The emission spectrum of the mixed-ion-loaded methanol solution decreased significantly, compared with the original one, as shown in Fig. 5a. However, when 1 was immersed in a methanol solution containing mixed metal ions including Fe³⁺ ions, the luminescence of 1 was quenched, indicating that the presence of other metal ions did not interfere with selectivity for Fe³⁺ ions. The detection of Al³⁺ ions over other metal ions was also examined and the emission spectrum shows that the intensity decreased significantly when Al³⁺ ions were absent, and the luminescence increased when Al³⁺ ions were present, as shown in Fig. 5b. At low Al³⁺/Fe³⁺ concentrations in mixed metal ion solutions, 1 is also highly sensitive to Al³⁺/Fe³⁺ ions (Fig. S10†). Thus, 1 is a good material for the selective sensing of Fe³⁺ and Al³⁺ ions. The high sensitivity of 1 for Al³⁺/Fe³⁺ ions indicates the promise of this type of luminescent material for sensing ions in biological systems.¹⁹

Fig. 5 Comparison of the photoluminescence intensity of 1 in methanol suspension with the introduction of other Mⁿ⁺ ions (Na⁺, Hg²⁺, Mg²⁺, Ni²⁺, Zn²⁺) in the absence and presence of 10 equiv. Fe³⁺ (a) and 9 equiv. Al³⁺ (b).

Magnetic properties

Magnetic measurements were performed for crystalline samples of complexes 2 and 3 in the temperature range of 2–300 K at 1 kOe, and the χ_M and χ_MT versus T curves are shown in Fig. 6. The two compounds are actually 3D porous frameworks, although in terms of magnetism, they can be considered as linear Co-(μ₂-O)-Co and Mn-(μ₂-O)-Mn chains, linked by the long carboxylate bridging ligands.

Fig. 6 Temperature dependence of χ_MT and χ_M under an applied field of 1 kOe for 2 (a) and 3 (b). The inset shows the χ_M⁻¹ vs. T plot.

For complex 2, the χ_M increased with decreasing temperature, reaching a maximum of 0.04 cm³ mol⁻¹ at around 2.19 K. The χ_MT value per two Co(II) units at 300 K was 2.58 cm³ K mol⁻¹, which is lower than the spin-only value of 3.75 cm³ K mol⁻¹ expected for two magnetically isolated Co(II) ions (S = 3/2, g = 2.0).²⁰ Decreasing the temperature decreased χ_MT gradually by a small amount from 2.58 cm³ K mol⁻¹ at 300 to ca. 80 K, then a little more steeply, reaching 0.76 cm³ K mol⁻¹ at 2.0 K, suggesting a dominant antiferromagnetic interaction between Co(II) centres.²¹ The inverse susceptibility plot as a function of temperature was linear above 100 K, following the Curie–Weiss law with a Weiss constant of θ = −42.22 K, and a Curie constant of C = 0.74 cm³ K mol⁻¹. The negative Weiss constant indicates that there is a predominantly antiferromagnetic interaction between two adjacent Co(II) centres. However, there was a jump in the χ_MT vs. T plot at ca. 250 K that cannot be explained by the current model. This cannot be caused by a structural transition because the crystal of 2 remained unchanged down to 100 K. Other effects beyond structural transformations have not yet been found.²²

For complex 3, χ_M increased with decreasing temperature, reaching a maximum of 0.02 cm³ mol⁻¹ at around 2.09 K. The χ_MT value per Mn(II) unit at 300 K was 6.72 cm³ K mol⁻¹, which is much lower than the spin-only value of 13.12 cm³ K mol⁻¹ expected for three magnetically isolated Mn(II) ions (S = 5/2, g = 2.0). Decreasing the temperature decreased the χ_MT gradually by a small amount from 6.72 cm³ K mol⁻¹ at 300 to ca. 80 K, then a little more steeply, reaching 0.43 cm³ K mol⁻¹ at 2.0 K, suggesting a diamagnetic ground state between Mn(II) centres.²³ The inverse susceptibility plot as a function of temperature was linear above 100 K, following the Curie–Weiss law with a Weiss constant of θ = −7.32 K, and a Curie constant of C = 0.23 cm³ K mol⁻¹. The negative Weiss constant indicates that there is a predominantly antiferromagnetic interaction between two adjacent Mn(II) centres.

Conclusions

In summary, three 3D coordination polymers were constructed from rigid quadrangular ligands and different metal ions under the same reaction conditions. Polymers 1–3 exhibit different structures based on metal–carboxylate chains incorporating the TPTC ligand, and mean that the materials have excellent properties. 1 exhibits strong luminescence emissions in the solid state and in a methanol suspension at room temperature. The luminescence of 1 displayed high selectivity for Al³⁺ and Fe³⁺ ions, suggesting that it may be used as a luminescent probe for Al³⁺ or Fe³⁺. Magnetic studies of 2 and 3 showed that there are antiferromagnetic interactions between the metal centers.

Acknowledgements

This work was granted financial support from National Natural Science Foundation of China (Grant 20871063, 21271096), Liaoning BaiQian Wan Talents (2010921066), the Program for Liaoning Excellent Talents in University (LR2011001), the Liaoning Natural Science Foundation, China (201102079), the Innovative Team Project of Department of Education of Liaoning Province, China (LT2011001), and the Liaoning University 211-Projects of the third period.

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Footnote

† Electronic supplementary information (ESI) available: The TGA curves, simulated, experimental X-ray powder diffraction patterns and X-ray crystallographic file (CIF) for 1–3. CCDC 1013700–1013702. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra10124c

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