Synthesis, structure, and properties of a 3D porous Zn(II) MOF constructed from a terpyridine-based ligand

Abstract

A new 3D porous metal–organic framework [Zn₂L₂]·5H₂O (H₂L = 4′-(furan-2-yl)-[2,2′:6′,2′′-terpyridine]-4,4′′-dicarboxylic acid) has been solvothermally synthesized and structurally characterized by TGA, PXRD, elemental analysis, IR spectroscopy and single-crystal X-ray diffraction. Complex 1 possesses a 3D porous framework with a sra topology. Gas adsorption measurements with N₂, H₂ and CO₂ confirm its permanent porosity. Importantly, complex 1 exhibits excellent fluorescence properties that have been exploited to sense organic nitro compounds and metal ions, and the results show that complex 1 bears rapid and selective sensing capabilities for 4-NPH and 4-NP through fluorescence quenching.

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1. Introduction

Over the past decade, metal–organic frameworks (MOFs), which are assembled from metal ions/clusters and organic ligands, have gathered considerable attention as a new frontier for materials research,^1,2 not only for their intriguing structural diversities and fascinating topology, but also for their potential applications in functional materials such as luminescence,³ catalysis,⁴ electronics,⁵ gas adsorption and separation,⁶ magnetism.⁷ It is worth mentioning that study on MOFs as chemosensors becomes one of the hot topics.⁸ Recently, the development of selective and sensitive detection of target species, such as nitroaromatic explosives and heavy-metal ions, has drawn great attention due to its importance in homeland security, battlefield protection, environmental and health-care issues.^9,10

Heavy metal ions induce many environmental issues, and thereby a lot of efforts have been focused on their monitor and detection, especially in plant, soil, and waste water.¹¹ Among them, Fe³⁺ ions are very important for the most organisms and play a crucial role in many biochemical processes at the cellular level, such as the storage and transport of oxygen to tissues and enzymatic reactions of the mitochondrial respiratory chain.^12,13 The deficiency and overload of Fe³⁺ ions can induce various disorders such as Alzheimer's, Huntington's, and Parkinson's disease. On the other hand, nitroaromatic compounds, such as (4-nitrophenyl)-hydrazine (4-NPH), 2,4-dinitrotoluene (2,4-DNT), 1,4-dinitrobenzene (1,4-DNB), 4-nitrophenol (4-NP), nitrobenzene (NB), 1-methyl-4-nitrobenzene (1-M-4-NB) are also notorious environmental pollutants,¹⁴ and those containing di- or tri-nitro groups are known as high explosives that could cause serious safety problems. Hence, the convenient and highly efficient detective technologies based on the Fe³⁺ ions and nitroaromatic compounds have received much attention of chemists. In response, new ways to detect explosives are being developed to improve security, including gas chromatography coupled with mass spectrometry,¹⁵ and chemical sensors.¹⁶ However, it is urgent to seek more convenient and quick sensing methods for these pollutants. With a high surface area, fast response time, sensitive detection and convenient preparation, MOFs are promising sensing materials for those detection purposes.

In 2009, Li et al. first reported a Zn MOF exhibiting fast and reversible detection of nitroaromatic explosives.¹⁷ Following that, a series of fluorescent MOFs were investigated for the rapid fluorescence detection of nitroaromatic explosives or heavy-metal ions.¹⁸ Very recently, Lang et al. reported a photochemically modifiable one-dimensional (1D) coordination polymer [Zn₂(L)₂(bpe)₂(H₂O)₂] that displayed highly selective sensing of Fe³⁺ ions.¹⁹ Moreover, there have also been comparatively few reports of fluorescent MOFs used to detect 4-NP and 4-NPH.²⁰

Moreover, a large number of functional MOFs with gas storage/separation and fluorescent sensing have been synthesized and reported based on the carboxylate and/or pyridine-based organic ligands. However, reports on functional MOFs constructed from terpyridine-based ligands are somewhat rare.²¹ Actually, terpyridine ligands (tpy) have gained huge interest due to their excellent complexing properties toward numerous main group, transition metal and lanthanide cations.²² In the case of transition metal complexes, the σ-donor/π-acceptor character of the M–N bond contributes additionally to the stability of the resulting edifices, which have been found to have widespread applications in biomedical sciences, for photovoltaic applications, as luminescent materials and catalysts.²³ Herein, we report a novel Zn MOF, [Zn₂L₂]·5H₂O (1), based on a terpyridine ligand, 4′-(furan-2-yl)-[2,2′:6′,2′′-terpyridine]-4,4′′-dicarboxylic acid. Complex 1 possesses a 3D porous framework exhibiting rapid and selective sensing properties for Fe³⁺ ion and nitroaromatic compounds, especially for (4-nitrophenyl)-hydrazine (4-NPH) and 4-nitrophenol (4-NP) through fluorescence quenching.

2. Experimental

2.1. Materials and methods

All chemicals and solvents used in the syntheses were of reagent quality and used as commercially obtained without further purification. IR spectra were collected on a Nicolet 330 FTIR Spectrometer within the 4000–400 cm⁻¹ region. Photoluminescence spectra were recorded with a Hitachi F-7000 Fluorescence Spectrophotometer. Gas sorption experiments were carried out on the surface area analyzer ASAP-2020. Thermo-gravimetric analysis (TGA) experiments were carried out on a PerkinElmer TGA 7 instrument under a static N₂ atmosphere with a heating rate 10 °C min⁻¹ at the range of 25–900 °C. Elemental analyses were carried out on a CE instruments EA 1110 elemental analyzer. The powder XRD diffractograms were obtained on a Panalytical X-Pert pro diffractometer with Cu Kα radiation. The 4′-(furan-2-yl)-[2,2′:6′,2′′-terpyridine]-4,4′′-dicarboxylic acid was prepared according to the reported procedures.²⁴

2.2. Synthesis of [Zn₂L₂]·5H₂O (1)

Zn(NO₃)₂·6H₂O (20 mg, 0.067 mmol), H₂L (2 mg, 0.006 mmol) and 1 mL dioxane–H₂O (v/v = 1 : 1) were sealed in a glass tube, and heated to 120 °C in 500 min, kept at 120 °C for 4000 min, then the reaction system was cooled to room temperature slowly at a rate of 5 °C h⁻¹. The colorless, block-shaped crystals were obtained and washed with dioxane, and then dried in the air (yield: 85% based on H₂L). Elemental analysis calcd (%) for complex 1: C 50.87; H 3.25; N 8.48. Found: C 49.83; H 3.04; N 8.61. IR (KBr, cm⁻¹): 3418 (s), 1617 (s), 1555 (m), 1403 (m), 1360 (s), 1243 (w), 1115 (w), 1004 (w), 869 (w), 784 (w), 692 (m).

2.3. X-ray crystallography

The single crystal of complex 1 with appropriate dimensions were chosen under an optical microscope and quickly coated with high vacuum grease before being mounted on a glass fiber for data collection. The data for 1 were collected with an Agilent Supernova diffractometer with Cu-Kα radiation source (λ = 1.5418 Å). The structure was solved by direct methods using SHELXS-97 and refined on F² by full-matrix least-squares procedures with SHELXL-97.²⁵ All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were set in calculated positions and refined with a riding model. For complex 1, there are large solvent accessible void volumes, where many disordered water molecules reside. However, no satisfactory disorder model could be achieved. Therefore, these electron densities are removed by PLATON/SQUEEZE.²⁶ All structures were examined using the Addsym subroutine of PLATON to assure that no additional symmetry could be applied to the models. The crystallographic data are presented in Table 1. Selected bond lengths and angles are collated in Table S1.† The topological analysis and some graphs were generated using the TOPOS program.²⁷

Table 1 Crystal data and structure refinements for 1

a R1 = ∑(||F0| − |Fc||)/∑|F0|.b wR2 = [∑w(|F0|^2 − |Fc|^2)^2/∑(F0^2)^2]^1/2.
Identification code	1
Empirical formula	C42H32N6O15Zn2
Formula weight	991.40
Temperature/K	298
Crystal system	Orthorhombic
Space group	Pccn
a/Å	16.7284(3)
b/Å	20.8912(5)
c/Å	28.9741(6)
α/°	90.00
β/°	90.00
γ/°	90.00
Volume/Å^3	10 125.7(4)
Z	8
ρcalc mg mm^−3	1.183
μ/mm^−1	1.000
F(000)	3648.0
2θ range for data collection	3.12 to 50°
Reflections collected	22 385
Independent reflections	8930 [Rint = 0.0191, Rsigma = 0.0215]
Data/restraints/parameters	8930/5/541
Goodness-of-fit on F^2	1.072
Final R indexes [I ≥ 2σ(I)]^a^,^b	R1 = 0.0752, wR2 = 0.2317
Final R indexes [all data]	R1 = 0.0911, wR2 = 0.2486
Largest diff. peak/hole/e Å^−3	1.01/−0.63

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3. Result and discussion

3.1. The structure descriptions of complex 1

Single crystal X-ray diffraction analysis revealed that complex 1 crystallized in an orthorhombic space group of Pccn, and possessed a 2-fold interpenetrated 3D framework with sra topology. As shown in Fig. 1b, the asymmetric unit consists of two crystallographically independent Zn(II) ions and two L²⁻ ligands. Zn1 exhibits a distorted octahedral geometry, which is completed by three nitrogen atoms ([Zn1–N1 = 2.175(4) Å, Zn1–N2 = 2.071(4) Å, Zn1–N3 = 2.159(4) Å]) from one L²⁻ ligand and three oxygen atoms ([Zn1–O = 1.955(4)–2.256(9) Å]) from two independent L²⁻ ligands. Zn2 is five-coordinated by three nitrogen atoms from one L²⁻ ligand and two oxygen atoms from different L²⁻ ligands. The Zn2–N bond lengths fall in the range of 2.082(4)–2.224(4) Å, and Zn2–O bond lengths are 1.945(3) Å (Zn2–O1) and 1.973(3) Å (Zn2–O3), which are in accordance with the previously reported complexes.²⁸ The ligand adopts two types of coordination modes (Fig. 1b). The first type of ligands coordinate to one Zn1 by the chelation of three nitrogen atoms and two Zn2 by the two sides of single coordinated carboxylates to form 1D zigzag chains (Fig. 1a); while the second type of ligands connect to three above chains (Fig. 1d) to form a 3D framework (Fig. 1c), in which one Zn2 of one chain is connected by chelation of three nitrogen atoms, and two Zn1 of another two chains are linked by the two sides of chelating carboxylate groups. From the topological view, there are four adjacent Zn2 ions around every Zn1 ions connected by three different ligands. Similarly, there are also four adjacent Zn1 ions around every Zn2 ions linked by three different ligands. Hence, both Zn1 and Zn2 can be seen as four-connected nodes. In this way, the structure could be simplified to a binodal 4-connected sra (Schläfli symbol: {42·63·8}) topology network (Fig. 1e).

Fig. 1 (a) An infinite 1D zigzag chains of complex 1 (b) types of coordination modes of ligand. (c) View of the 3D framework of complex 1 along the b axis. (d). Three chains connected by the ligand H₂L. (e) Topology simplification of 2-fold interpenetrating along the c axis.

3.2. Powder X-ray diffraction analyses, IR spectra, and thermal analyses

The PXRD pattern for 1 was presented in ESI Fig. S2.† The diffraction peaks of experimental pattern match well with that simulated by single crystal X-ray diffraction data, indicating the phase purity of complex 1. The IR spectrum of complex 1 shows the asymmetric stretching vibration of carboxyl at 1619 cm⁻¹, which has a red-shift of 14 cm⁻¹ compared to the related absorption peak of ligand at 1633 cm⁻¹. This result indicates the deprotonated nature of carboxyls in complex 1, which can be clearly observed in crystal structure.

The thermal behavior of complex 1 was investigated under a dry nitrogen atmosphere from 25 to 900 °C and the TG curve was presented in Fig. S4 (ESI†). The first weight loss of 9.10% happens below 100 °C, which corresponds to the removal of lattice water molecules (calcd 9.08%). Beyond 340 °C, a rapid weight loss is observed, indicating the decomposition of complex 1.

3.3. Gas sorption properties

To examine the pore characteristic, the gas adsorption properties of complex 1 was investigated. The as-synthesized 1 was guest-exchanged three times with the anhydrous methanol and dichloromethane, respectively. Then, the sample was activated at 60 °C under high vacuum for 5 hours to get the activated 1. The adsorption equilibrium data of N₂, H₂, CH₄, and CO₂ were collected. The N₂ sorption isotherm of complex 1 at 77 K is shown in Fig. 2a, which displays typical type-I adsorption isotherm, suggesting the retention of the microporous structure after the removal of solvents from the crystalline sample. The Brunauer–Emmett–Teller (BET) surface area is 367 m² g⁻¹ by fitting the N₂ isotherm. Activated 1 exhibits a classical reversible type-I isotherms for H₂ and CO₂. Under the conditions of 1 bar and 77 K, activated sample has the H₂ uptake capacity of 116 cm³ g⁻¹ (1.02 wt%). The gas isosteric heat of adsorption (Q_st) can be calculated by fitting the gas adsorption isotherms at different temperature to a virial-type expression.²⁹ By this method, the Q_st for H₂ has the estimated value of 7.3 kJ mol⁻¹, which is obtained by fitting the H₂ adsorption isotherms at 77 and 87 K (Fig. 2d). The sorption isotherm for CO₂ reveals that 1 stores CO₂ up to 77.4 cm³ g⁻¹ (15.07 wt%) at 273 K and 1 atm, and 58.9 cm³ g⁻¹ (11.47 wt%) at 295 K and 1 atm. The CO₂ sorption capacity of 1 is higher than those of the favorable zeolites such as ZIF-81, ZIF-82, and ZIF-100 (ref. 30) The Q_st for CO₂ at low coverage can be calculated by fitting the gas adsorption isotherms at 273 and 295 K, and the value for CO₂ is 29.6 kJ mol⁻¹ (Fig. 2d).

Fig. 2 (a) The N₂ sorption isotherms at 77 K for 1. (b and c) The H₂ and CO₂ sorption isotherms for 1: H₂, red 77 K, black 87 K; CO₂, red 273 K, black 295 K. (d) The Q_st of 1 for H₂ and CO₂.

3.4. Luminescence behavior and sensing properties

As the potential fluorescent materials, MOFs including d¹⁰ ions have been widely investigated in the optical field.³¹ Photoluminescence experiments of complex 1, as a typical d¹⁰ transition-metal MOFs that exhibits photoluminescent property, were performed in solid state at room temperature. As shown in Fig. S6 (ESI†), complex 1 exhibits clear emission at λ_max = 483 nm upon excitation at 330 nm, which has a blue-shift of 42 nm compared to the one of free ligand of H₂L at λ_max = 525 nm. The emission band of 1 can be tentatively ascribed to the intraligand π*–π and π*–n transitions because of the d¹⁰ nature of Zn²⁺ ions. However, the emission may be influenced by the coordinated Zn²⁺ ions because it can change rigidity of ligand and the electron donor–acceptor properties of carboxyls and nitrogen atoms.

Considering that the uncoordinated carboxylate oxygens distributed in the channels may form weak interactions with metal ions and small organic molecules, the fluorescent sensing of metal ions and nitroaromatic compounds was performed for 1. In the measurements, the DMF solvent was selected to prepare the suspension solution as the framework of complex 1 was confirmed to be stable in DMF by the comparison of PXRD patterns before and after soaking it in the DMF solution for 24 h (Fig. S3†). The experiment procedures are as follows: 2 mg of freshly prepared 1 was dispersed in 3 mL DMF solution, forming a suspension by an ultrasound method, then 10 μL of DMF solution of M(NO₃)_x (10 mM; M = Fe³⁺, Al³⁺, Ca²⁺, Pb²⁺, Zn²⁺, Cd²⁺, Cu²⁺, Ag⁺, Li⁺, Na⁺) was slowly dropped into the above solutions to form a uniform DMF suspension at room temperature, and then their photoluminescence spectra were recorded with a fluorescence spectrophotometer. The results showed that complex 1 exhibits selective sensing to Fe³⁺ by fluorescent quenching effect. When adding 100 μL of Fe³⁺ (10 mM), the fluorescent intensity decreases to the original 25%. In contrast, no significant change of fluorescent intensity is observed when adding other metal ions (Fig. 3a). The selective sensing properties to Fe³⁺ ion were also further studied by using mixed ions in DMF solution. The results show that the fluorescent quenching effect of Fe³⁺ ion on 1 exhibits no significant change after introducing equivalent amount of other metal ions including Na⁺, Ca²⁺, Al³⁺, Li⁺, Pb²⁺, Ag⁺, Zn²⁺ and Cu²⁺ (Fig. 3b), which further confirm that complex 1 can selectively detect Fe³⁺ ions over other cations.

Fig. 3 (a) The luminescent intensity of 1 in DMF upon addition of 10 μL of various metal ions. (b) The luminescent intensity of 1 upon addition of 10 μL of various mixed metal ions.

Nitroaromatic compounds are widespread in refinery operations, plastic processing, and fuel operations, which are potentially neurotoxic and carcinogenic in nature. To investigate the fluorescent sensing effect of 1 on the nitrobenzene derivatives, the standard detecting suspension was prepared by adding 2 mg freshly synthesized 1 to 3 mL of DMF, to which was added the different analytes, including 1,3-dinitrobenzene (1,3-DNB), 2,4-dinitrotoluene (2,4-DNT), 1,4-dinitrobenzene (1,4-DNB), 4-nitrophenol (4-NP), nitrobenzene (NB), 1-methyl-4-nitrobenzene (1-M-4-NB) and (4-nitrophenyl)-hydrazine (4-NPH) in DMF (10 mM). As shown in Fig. 4, all nitrobenzene derivatives bear fluorescent quenching effect for 1. The order of quenching efficiency for the selected nitroaromatics is 4-NPH > 4-NP > 1,4-DNB > NB > 2,4-DNT > 1-M-4-NB > 1,3-DNB. However, 4-NPH and 4-NP clearly exhibit the higher quenching effect with the quenching percentage of 92.2% and 85.6%, respectively. This should be closely correlative with their structure characteristic. Because 4-NPH and 4-NP not only can produce weak π–π interactions with the conjugated groups of framework, but also can form hydrogen bond interactions with the uncoordinated carboxylic oxygen atoms in the channel of 1 by their amido, imido or hydroxyl.

Fig. 4 (a) Photoluminescence intensity of 1 dispersed in DMF. (b) Percentage of fluorescence quenching obtained for introducing different nitrobenzene derivatives into the DMF-suspension of 1. (c and d) Photoluminescence intensity of 1 dispersed in DMF upon incremental addition of a 4-NPH DMF solution (10 mM) and 4-NP DMF solution (10 mM). Insert: Stern–Volmer plot of I₀/I − 1 versus the 4-NPH (c) and 4-NP (d) concentration in DMF.

In order to further understand the sensing efficiency of 1 for 4-NPH and 4-NP, the fluorescence quenching efficiency was analyzed using the Stern–Volmer (SV) equation, (I₀/I) = K_sv[A] + 1, where I₀ is the initial fluorescence intensity before the addition of analytes, I is the fluorescence intensity in the presence of analyte, [A] is the molar concentration of analytes, and K_sv is the quenching constant (M⁻¹). The Stern–Volmer plots for 4-NPH and 4-NP are typically linear at low concentrations (Fig. 5), and the value of K_sv for 4-NPH and 4-NP is estimated to be 1.43 × 10⁴ M⁻¹ and 1.07 × 10⁴ M⁻¹, respectively.

Fig. 5 Corresponding Stern–Volmer plots of analytes.

4. Conclusions

In summary, a zinc–organic framework (1) based on a terpyridine ligand was solvothermally synthesized and characterized. Complex 1 bears a two-fold interpenetrating 3D porous structure with a sra topology, and displays excellent fluorescent properties. Fluorescent measurements indicate that complex 1 can not only sense Fe³⁺ ions in a moderate selectivity among 10 kinds of metal ions through fluorescence quenching, but also rapidly and selectively detect nitroaromatic compounds, especially for 4-NPH and 4-NP, which make it a promising luminescent probe for nitroaromatic compounds.

Acknowledgements

This work was supported by the NSFC (Grant No. 21271117, 21371179, 21571187), NCET-11-0309, the Shandong Natural Science Fund for Distinguished Young Scholars (JQ201003), and the Fundamental Research Funds for the Central Universities (13CX05010A, 14CX02158A).

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Footnote

† Electronic supplementary information (ESI) available: The crystallographic data in CIF format, IR, PXRD patterns, TG and other additional figures for 1. CCDC 1431152. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra27368d

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