A facile, fast responsive and highly selective mercury(II) probe characterized by the fluorescence quenching of 2,9-dimethyl-1,10-phenanthroline and two new metal–organic frameworks

Abstract

A fast responsive and highly selective mercury(II) sensor was developed using the fluorescence quenching mechanism of 2,9-dimethyl-1,10-phenanthroline (2,9-DMP) towards mercury(II). Furthermore, two kinds of metal–organic frameworks (MOFs) of 2,9-DMP–Hg(II) have been synthesized and characterized by X-ray diffraction. The probe 2,9-DMP exhibited a fast and highly selective response to mercury(II) ions in aqueous solution, showing a good linear relationship over the concentration range of 0.050–2.0 μM for Hg(II) with R² = 0.9994. The established procedure was used to monitor the concentration of mercury(II) in drinking water and Zhujiang River water samples with satisfactory recoveries of 85.42–101.5% and 81.72–96.09%, respectively. Furthermore, the fluorescence quenching mechanism of 2,9-DMP by mercury(II) was investigated and clarified by the data of single crystal X-ray diffraction of MOFs and the Job's plot analyses of 2,9-DMP towards Hg(II).

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Introduction

As is well known, mercury ion(II) is hazardous to human health due to its high toxicity, mobility and ability to accumulate through food chains or the atmosphere in ecological systems. Since a high risk can be caused by very low concentrations of mercury, mercury contamination has attracted much attention as a global environmental problem. Mercury is found in many commodities in daily life, such as batteries, paints and electronic equipment. It exists in different forms (ionic, metallic, inorganic and organic salts, as well as complexes). No matter what form of mercury is consumed by humans, it can accumulate in the body. Even a very small amount of mercury ions could cause serious diseases such as Minamata disease, cardiovascular disease, serious cognitive and motion disorders as well as coronary heart disease. Therefore, the determination of mercury in biological and environmental samples is very important both for the monitoring of environmental pollution and the diagnosis of clinical disorders.

The conventional methods for the determination of Hg(II) ions include atomic absorption/emission spectroscopy (AAS/AES),^1,2 cold vapor atomic fluorescence spectrometry,³ inductively coupled plasma mass spectrometry (ICP-MS),^4–6 and anodic stripping voltammetry.^7,8 Although these methods allow sensitive and quantitative determination of Hg(II) ion levels, they require expensive and sophisticated instrumentation, along with tedious sample preparation procedures. So those methods are not suitable for real-time and in situ measurement in the field. Thus, there is a strong incentive to develop a fast, simple, selective, sensitive, reliable and convenient approach for real-time monitoring of Hg(II) in the environment.

In the past decade, much effort has been made to develop portable sensors, including fluorescent sensors^9–13 and colorimetric sensors.^14–18 Especially, the fluorescent Hg(II) probes or/and sensors have attracted a huge interest because of its simple operation, high sensitivity, and adaptability for Hg(II) determination in-field. Recently, a number of fluorometric probes or/and sensors using organic chromophores,^19,20 conjugated polymers,^21–25 quantum dots,^26–28 carbon nanodots^29,30 and metal nanoparticles³¹ have been developed to detect Hg(II) ions in environmental and biological samples. Although these approaches have made great contributions toward Hg(II) assay, there still existed some limitations such as insufficient water-solubility, irreversible Hg(II) complex, poor selectivity, long response-time and sophisticated synthesis of the probe or sensor materials.

Herein, we report a facile, fast responsive, highly selective probe for monitoring Hg(II) in water samples based on the fluorescence quenching of 2,9-dimethyl-1,10-phenanthroline (2,9-DMP) by Hg(II). Meanwhile, the binding model of complex 2,9-DMP–Hg(II) was explored and characterized by the Job's plot analyses and the single crystal X-ray diffraction for two new metal–organic frameworks (MOFs) of 2,9-DMP–Hg(II).

Experimental section

Materials and apparatus

N,N-Dimethylformamide (DMF) was purchased from Tianjin Damao Reagent Co., Ltd (Tianjin, China). Fluorescent probe 2,9-dimethyl-1,10-phenanthroline (2,9-DMP) was purchased from Macklin Reagent Co., Ltd (Shanghai, China). Aqueous solutions of metal ions, including Ag(I), Al(III), Ba(II), Be(II), Ca(II), Cd(II), Co(II), Cr(III), Cu(II), Fe(II), Fe(III), Hg(II), K(I), Li(I), Mg(II), Mn(II), Ni(II), Pb(II) and Zn(II), were prepared from their nitrates or chlorides, which were purchased from Aladdin Reagent Co., Ltd (Shanghai, China). Buffer solutions were prepared with tris(hydroxymethyl)aminomethane nitrate (Tris-HNO₃), which was purchased from Sangon Biotech Bioengineering Co., Ltd. (Shanghai, China). Purified drinking water was purchased from Huarun Cestbon Beverage Co., Ltd. (Shenzhen, China). Unless otherwise specified, all the reagents used were of analytical grade. Deionized water was used throughout the experiment. All the reagents were purchased from commercial suppliers and utilized without further purification.

Fluorescence spectra were recorded with a Hitachi FL-2700 fluorescence spectrophotometer (Hitachi, Japan). The pH measurements were performed on a PHS-3C digital pH-meter (INESA, China). X-ray diffraction data of single crystals were collected using a Bruker APEX II diffractometer (Bruker, Switzerland).

Fluorescence measurements

In general, 2,9-DMP stock solution was prepared by dissolving 0.0208 g (0.1 mmol) of 2,9-DMP in 100 mL methanol to get a concentration of 1.0 mM for subsequent use. The stock solutions of metal ions, including Ag⁺, Al³⁺, Ba²⁺, Be²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, Li⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺, were prepared from their nitrates or chlorides (guarantee reagent). The work solutions were diluted to the desired concentrations with the stock solution using deionized water or a Tris-HNO₃ buffer solution at pH 2.0.

For selectivity and interference investigation, the solution of 2,9-DMP and 19 metal ions were mixed to obtain the final concentration of 10 μM for 2,9-DMP and 20 μM for 19 metal ions in the mixed solution, respectively. Meanwhile, in order to evaluate the interference of the mixture of 18 metal ions on the fluorescence intensity, 10 μM 2,9-DMP and 1 μM Hg²⁺ were mixed. Then 18 reference metal ions (the concentration of each one was 1 μM) were added to above mixture. The resulted solution was shaken well and analyzed immediately. Then all the fluorescence spectra studies were performed on a Hitachi FL-2700 fluorescence spectrophotometer. The excitation wavelength of fluorescence spectrometry was set at 275 nm. Excitation and emission slits were set at 10 nm and the voltage of the photomultiplier was set at 400 V in the fluorescence test except for the quantitative determination (700 V).

In order to investigate the possibility of the quantitative detection of Hg(II) with the 2,9-DMP fluorescent probe, a series standard concentrations of Hg(II) were added to 3 μM 2,9-DMP solution at pH 2.0 adjusted with Tris-HNO₃ buffer solution. The fluorescence emission spectra of the standard solution were recorded under the excitation wavelength of 275 nm.

To investigate the binding stoichiometry of 2,9-DMP–Hg(II) complex, firstly, 10⁻⁴ M of 2,9-DMP and 10⁻⁴ M of Hg(II) stock solution at pH 2.0 were prepared respectively. Then the stock solution of 2,9-DMP and Hg(II) were mixed with ratio from 9 : 1 (v/v) to 1 : 9 (v/v) in test tubes to keep a total volume of 1 mL. Finally, the mixtures were diluted to 10 mL with a Tris-HNO₃ buffer solution at pH 2.0 to get the test solution with a total concentration of [2,9-DMP] + [Hg(II)] = 10 μM.

To test the applicability of the proposed fluorescence probe for the analysis of practical samples, the Hg(II) in drinking water and Zhujiang River water was detected by the proposed method. To estimate the detection accuracy, the water samples were spiked with Hg(II) standard solution to the concentrations of 0.50, 1.0 and 2.0 μM. The recoveries of spiked Hg(II) standard solution in drinking water were calculated by the following formulate:

where R% presents the percent recoveries, c_D is the detected concentration of Hg(II) with the proposed method and c_A presents the added concentration of Hg(II). Since there are various of organic substances in Zhujiang River, the river water sample was digested with HNO₃ to eliminate the interferences from organic substances. To estimate the detection accuracy for Zhujiang River sample, the recovery test was performed as following: 100 mL Zhujiang River water was put into a 250 mL conical flask. Then 10 mL concentrated HNO₃ and different concentration of Hg²⁺ standard solutions were added respectively and a small funnel was inserted in the conical flask to avoid bumping during digestion. Finally, the residues were dissolved and transferred to 100 mL flask for analysis. The recoveries were calculated as drinking water.

Synthesis and characterization of 2,9-DMP–Hg(II) MOF 1 and MOF 2

To further confirm that the binding stoichiometry of 2,9-DMP–Hg(II) complex is based on a 1 : 1 binding model. MOF 1 and MOF 2 were synthesized and characterized by single crystal X-ray diffraction with a Bruker APEX II diffractometer (Bruker, Switzerland).

Synthesis of MOF 1. A reaction mixture of Hg(NO₃)₂·H₂O (0.0685 g, 0.2 mmol), 2,9-DMP (0.0208 g, 0.1 mmol), methanol (4 mL) and deionized water (4 mL) was mixed and loaded into a glass vial. Then the glass vial was sealed and heated to 333 K for 75 h. After cooling down to room temperature, a yellow block crystal was obtained after being filtered and washed with methanol and dried in air.

Synthesis of MOF 2. A reaction mixture of HgCl₂ (0.0272 g, 0.1 mmol), 2,9-DMP (0.0208 g, 0.1 mmol), N,N-dimethylformamide (4 mL) and deionized water (2 mL) was mixed and loaded into a glass vial. Then the glass vial was sealed and heated to 353 K for 75 h. After cooling down to room temperature, a yellow block crystal was obtained after being filtered and washed with DMF and dried in air.

The evaluation and confirmation of MOF 1 and MOF 2 were performed by single crystal X-ray diffraction with a Bruker APEX II diffractometer (Bruker, Switzerland).

Results and discussion

Evaluation of the selectivity and anti-interference performance of 2,9-DMP

The complexity of environmental sample puts forward a great challenge to accurately determination of metal ions not only with improved detection limit, but also high selectivity. Since the selectivity of a probe is a crucial parameter to evaluate the properties of fluorescent probes, the competition experiments were carried out by monitoring the fluorescence intensity of the probe 2,9-DMP in response to 19 metal ions including Ag(I), Al(III), Ba(II), Be(II), Ca(II), Cd(II), Co(II), Cr(III), Cu(II), Fe(II), Fe(III), Hg(II), K(I), Li(I), Mg(II), Mn(II), Ni(II), Pb(II) and Zn(II) with fluorescence spectrophotometer. The results indicated that the fluorescence of probe 2,9-DMP (10 μM) was almost quenched completely by Hg(II) (20 μM) while no evident fluorescence quenching of 2,9-DMP by the other metal ions (20 μM) was observed, which suggested that 2,9-DMP exhibited an excellently selective response to Hg(II) over other metal ions (Fig. 1A). The outstanding selectivity of 2,9-DMP towards Hg(II) has attracted our interest and prompted us to further investigate the possibility using 2,9-DMP as the fluorescence probe for detection of Hg(II) in water.

Fig. 1 The fluorescence characterization of 2,9-DMP [(A): the fluorescence response of 10 μM 2,9-DMP to 20 μM of different metal ions in pH 2.0 Tris-HNO₃ buffer solution; (B) fluorescence responses of probe 2,9-DMP to various reference metal ions (black bars: 10 μM 2,9-DMP + 20 μM reference metal ion; red bars: 10 μM 2,9-DMP + 20 μM reference metal ion + 20 μM Hg(II), λ_ex = 275 nm); (C) the influence of 18 mixed metal ions on the fluorescence of 2,9-DMP–Hg(II) complex (C_2,9-DMP = 10 μM, C_mix = 19 μM, λ_ex = 275 nm, 400 V)].

In order to explore the possibility to use 2,9-DMP as probe for sensing Hg(II) in real water sample, the influence of coexisting metal ions on the fluorescence of 2,9-DMP–Hg(II) was also characterized in detail (Fig. 1B). As can be seen from Fig. 1B, the fluorescence quenching of 2,9-DMP–Hg(II) is not affected by most metal ions except for several metal ions such as Fe(II), Cr(III), Mn(II), Ba(II), Ca(II) and Co(II). Nevertheless, the interference of above six metal ions on the fluorescence of 2,9-DMP–Hg(II) is slight. Amongst them, only the relative error produced by Fe(II) is over 10%. This is possibly ascribed to that 2,9-DMP has the similar skeleton structure to phenanthroline which can complex with Fe(II). In fact, there is almost no Fe(II) in air-exposed water due to the high reduction capacity of Fe(II). In order to simulate the water sample, the cooperative influence of the mixture of 18 reference metal ions on the fluorescence of 2,9-DMP–Hg(II) was also investigated. It can be seen from Fig. 1C that the fluorescence of complex 2,9-DMP–Hg(II) was slightly enhanced by 7.5% due to the addition of 18 reference metal ions (Fig. 1C, blue line), which demonstrated that the selectivity of the probe 2,9-DMP for detection of Hg(II) in complicated water sample is acceptable.

Response time and pH dependence of 2,9-DMP towards Hg(II)

In order to optimize the condition of using probe 2,9-DMP for detecting Hg(II) in real water, the response time and pH dependence of 2,9-DMP towards Hg(II) were also explored in detail.

First of all, the rapid response capacity is very important for an on-site or on-line monitoring method in field. Unfortunately, a lot of literature did not mention the responsive time of the probes for sensing related targets. In some other literatures, the reaction time between probe and target metal ions has been reported to be more than half or one hour^32,33 For this purpose, the response time of 2,9-DMP to Hg(II) was monitored on-line with a Hitachi FL-2700 fluorescence spectrophotometer. As shown in Fig. 2, the complex reaction of 2,9-DMP and Hg(II) is completed within about 20 s, which is much faster than that of most literature reported.^32–38 Hence the probe 2,9-DMP can be utilized to rapid monitor Hg(II) in-site or/and on-line in environmental water.

Fig. 2 The fluorescence response process of the reaction of 2,9-DMP (10 μM) and Hg(II) (10 μM) monitored by a fluorescence spectrophotometer on-line.

On the other hand, the pH value of the environment usually shows an effect on the performance of probe toward target metal ions due to the protonation or deprotonation reaction of the fluorophore and the hydrolysis reaction of the metal ions. Thus, it is necessary to investigate the effect of pH value of aqueous solution on the fluorescence of probe 2,9-DMP. Accordingly, a series of 2,9-DMP solutions with pH value ranging from 0.0 to 6.0 adjusted with Tris-HNO₃ were prepared, since precipitation shall produce when pH is above 7.0. The variation of the fluorescence intensity of the probe (10 μM) in the absence and presence of Hg(II) (10 μM) with the pH value was studied and is outlined in Fig. 3. As depicted in Fig. 3, the fluorescence intensity of the probe 2,9-DMP is slightly affected by pH value of solution in the range of 0.5–3.5 while the fluorescence intensity of 2,9-DMP is decreased rapidly when pH > 4.0. On the other hand, the fluorescence intensities of complex 2,9-DMP–Hg are almost not affected by the pH value in the range from 0.0 to 6.0, indicating that the complex 2,9-DMP–Hg(II) is very stable in pH < 6.0 media. In consideration of the stability of 2,9-DMP, the following experiments were all performed in pH 2.0 solution adjusted with Tris-HNO₃.

Fig. 3 Effect of pH value on the fluorescence intensity of the probe 2,9-DMP and 2,9-DMP–Hg(II).

Quantitative determination of Hg(II) and applicability study

In order to investigate the possibility of the quantitative detection of Hg(II) with the fluorescent probe, 3.0 μM of 2,9-DMP solution was titrated with series of concentrations of Hg(II) (0.0–2.0 μM). As shown in Fig. 4, the fluorescence intensity of 2,9-DMP was decreased gradually with the increase of Hg(II) concentration (Fig. 4, insert). The fluorescence of 2,9-DMP was almost quenched completely when the concentration of Hg(II) increased to 2.0 μM. The fluorescence intensity of 2,9-DMP was linearly proportional to the concentration of Hg(II) in the range of 0.050–2.0 μM (Fig. 4). A linear regression equation for Hg(II), F = 8383.0 − 4033.4c (R² = 0.9994), was obtained, where F refers to the fluorescence intensity and c refers to the concentration of Hg(II). The limit of detection (LOD) (S/N = 3, the concentration necessary to yield a net signal equal to three times the standard deviation of the background) was calculated to be 40 nM. These results reveal that the probe 2,9-DMP is an excellent probe of Hg(II) and can be used to monitor Hg(II) levels both qualitatively and quantitatively in water sample.

Fig. 4 The linear relationship between the fluorescence intensity and the concentration of Hg(II) (insert: the fluorescence response of 2,9-DMP to the various concentrations of Hg(II), 0.0–2.0 μM, 700 V).

To test the applicability of the proposed fluorescence probe for the analysis of practical samples, the Hg(II) in drinking water and digested Zhujiang River water samples were detected by the proposed method. Since the concentrations of Hg(II) in drinking water and Zhujiang River water samples are too low to detect, the drinking water samples were spiked directly with Hg(II) standard solution to concentrations of 0.50, 1.0 and 2.0 μM. And the Zhujiang River water samples were spiked with Hg(II) standard solution to concentrations of 0.50, 1.0 and 2.0 μM and then digested with concentrated HNO₃. Then the residues were dissolved in deionized water and detected using the proposed method. The recoveries were listed in Table S1.† As can be seen from Table S1,† the satisfactory recoveries of addition Hg²⁺ standard solution were obtained at 85.42–101.5% and 81.72–96.09% for drinking water and Zhujiang River water, respectively. In summary, the fluorescence probe 2,9-DMP provides a very highly selective and accurate monitoring method for determination of Hg(II) in drinking water and Zhujiang River water samples within a practical concentration range.

The binding mode investigation and confirmation of complex 2,9-DMP–Hg(II)

Normally, the binding mode of probe with target ion is investigated by the mean of Job's plot analysis in most literature. In addition, we have also synthesized successfully two new MOFs of 2,9-DMP–Hg(II). In this study, hence, the binding mode and mechanism of 2,9-DMP with Hg(II) were also investigated in detail by the single crystal X-ray diffraction data of the two new MOFs of 2,9-DMP–Hg(II). These single crystal X-ray diffraction results would display a more intuitive binding mode to reader.

Firstly, the Job's plot analysis was used for determination the stoichiometry between 2,9-DMP and Hg(II). All the analyses were performed with the total concentration of [2,9-DMP] and [Hg(II)] set at 10 μM. As shown in Fig. 5, the related fluorescence intensity (F₀ − F) of 2,9-DMP is increased firstly and then is decreased gradually with the increase of the ratio of Hg(II) to [2,9-DMP] + [Hg(II)]. The fluorescence intensity reaches the maximum value when the ratio of Hg(II) to [2,9-DMP] + [Hg(II)] is about 0.5, which suggests a 1 : 1 complex between 2,9-DMP and Hg(II) has been formed.

Fig. 5 Job's plot for determining the stoichiometry of the probe and Hg(II) ions (the total concentration of 2,9-DMP and Hg(II) ions was 10 μM, λ_ex = 275 nm).

On the other hand, the 1 : 1 complexation mode of 2,9-DMP with Hg(II) has been further confirmed by the single crystal X-ray diffraction results of the two new 2,9-DMP–Hg(II) MOFs which were synthesized by hydro–solvothermal synthesis in the existence of Cl⁻ and NO₃⁻ media, respectively. The two similar yellow block crystals were obtained and the appearance of MOF 1 was depicted in Fig. S1.†

The two new MOFs were further characterized by single-crystal X-ray diffraction measurement with a Bruker APEX II diffractometer at 298 K using graphite monochromatic Mo-Kα radiation (λ = 0.71073 A). The structures were solved by direct methods and refined by full-matrix least squares on F2 with SHELXS-97 and SHELXL-97 program. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were included in calculated positions and refined with isotropic thermal parameters riding on the parent atoms. The single-crystal X-ray diffraction studies reveal that the Hg–MOF 1 synthesized in NO₃⁻ media is a 1-D MOF. Hg–MOF 1 crystallized in the monoclinic space group P2₁/c with an asymmetric unit containing two Hg(II) ions, one 2,9-DMP and two nitrate ions. As shown in Fig. 6A, Hg1 is tetra-coordinated to two pyridine nitrogen atom (N1, N2), one nitrate oxygen atoms (O6a), exhibiting a distorted tetrahedral coordination geometry. Another Hg2 ion is hexa-coordinated with five nitrate oxygen atoms (O2, O3, O3a, O5, O6), showing distorted octahedral coordination geometry. The bond lengths of Hg–O are in the range from 2.239(12) Å to 3.032(12) Å while that of Hg–N are in the range from 2.263(13) Å to 2.369(13) Å. Every neighboring N center is linked together with an Hg⋯Hg bond with the distance of 2.5205(10) Å to generate a MOF unit which is further connected by the oxygen atoms of the nitrate, giving rise to a 1-D Hg-chain along with a axis (Fig. 6B). The more detailed crystal data and structure refinement for Hg–MOF 1 are shown in Table S2.† The selected bond lengths and angles for Hg–MOF 1 are listed in Table S3.†

Fig. 6 The single-crystal X-ray diffraction data of Hg–MOF 1 ((A) the coordination environment of Hg(II) ion, all the H atoms are omitted for clarity. Asymmetry code: (a) x, 0.5 − y, 0.5 + z; (B) 1-D chain structure, all the H atoms are omitted for clarity).

Meanwhile, another yellow block Hg–MOF 2 was also obtained in the Cl⁻ media and crystallized in monoclinic, C2/c space group. The X-ray diffraction data of Hg–MOF 2 (Fig. 7A) suggests that its asymmetric unit contain three Hg(II) ions, two 2,9-DMPs and four chlorides. Hg1 is tetra-coordinated to four chlorides (Cl1, Cl2, one μ2-bridged and the other terminal coordinated), exhibiting a distorted tetrahedral coordination geometry. Hg2 ion is tetra-coordinated with one μ2-bridged chlorides (Cl1a), two nitrogen atoms (N1, N2) on 2,9-DMP, and another Hg2 ion, showing distorted tetrahedral coordination geometry. The bond lengths of Hg–N are in the range from 2.348(12) Å and 2.351(12) Å while that of Hg–Cl are in the range from 2.391(13) Å to 2.619(13) Å. Hg–MOF 2 exhibits a 1-D chain with an Hg⋯Hg distance of 2.5523(8) Å (Fig. 7B). The detailed crystal data and structure refinement for Hg–MOF 2 are shown in Table S4† and the selected bond lengths and angles for this MOF are listed in Table S5.†

Fig. 7 The single-crystal X-ray diffraction data of Hg–MOF 2 ((A) the coordination environment of Hg(II) ion, all the H atoms are omitted for clarity. Asymmetry code: (a) 1 − x, y, 0.5 − z. (B) 1-D chain structure of Hg–MOF 2, all the H atoms are omitted for clarity).

In a word, both the Job's plot analyses and the results of single crystal X-ray diffraction of two new Hg–2,9-DMP MOFs demonstrate that the complexation mode of 2,9-DMP and Hg(II) is 1 : 1 type. And the data of single crystal X-ray diffraction indicate that 1-D chain structures of two new Hg–2,9-DMP MOFs are connected by the bridges of Hg(NO₃)₃⁻ or HgCl₄²⁻ in NO₃⁻ or Cl⁻ media, respectively.

Conclusion

In summary, a facile, fast responsive and highly selective mercury(II) probe has been found and characterized by the fluorescence quenching of 2,9-dimethyl-1,10-phenanthroline (2,9-DMP) towards mercury(II) and the single crystal X-ray diffraction of two kinds of new metal–organic frameworks (MOFs) of 2,9-DMP–Hg(II). When used for sensing mercury(II) ions in drinking water and Zhujiang River water samples, the probe 2,9-DMP exhibited a fast and highly selective response to mercury(II) ions with the satisfactory recoveries of 85.42–101.5% and 81.72–96.09%, respectively. Meanwhile, the 1 : 1 binding mode of 2,9-DMP towards Hg(II) was demonstrated and clarified by the Job's plot analyses and the method of single crystal X-ray diffraction of two new Hg–2,9-DMP MOFs. The further applications of probe 2,9-DMP for sensing Hg(II) for chemical and biological use are underway in our research group.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21477026, 41373118), Science and Technology Key Project of Ministry of Education (212129), Natural Science Foundation of Guangdong Province (2014A030313525). The authors also sincerely appreciate Mr Hu Lei for single-crystal X-ray diffraction service.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1479671 and 1479677. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra13514e

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