Highly selective colorimetric and fluorescent sensors for the fluoride anion based on imidazo[4,5-f]-1,10-phenanthroline metal-complexes

Abstract

A new imidazo[4,5-f]-1,10-phenanthroline ligand and its Pt(II), Ru(II) and Re(I) derivatives have been synthesized. It is shown that these metal complexes can be utilised as colorimetric and fluorescence chemosensors for anions. The interaction of these complexes with different anions has been characterised by UV/Vis absorption spectra, fluorescence spectra and titration studies. The results indicated that two of the three complexes showed selective recognition for F⁻, of which [Re(CO)₃ClL₁] showed an immediate change from yellow to pink when the fluoride anion was added in DMSO. The phenomenon could be observed with the naked eye. On the other hand, the emission intensities of [Ru(bpy)₂L₁](PF₆)₂ and [PtCl₂L₁] in DMSO were strongly enhanced upon the addition of F⁻.

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Introduction

Over the last few years, the design and synthesis of chemosensors for binding anions selectively has attracted much attention because of the fundamental role of anions in a wide range of biological and chemical processes.¹ In particular, research into anion sensing using electrochemical change,² fluorescent change³ or colorimetric change⁴ has been actively studied. In colorimetric sensing, anion recognition requires changes in the absorption spectra along with corresponding colour changes which can be detected by the naked eye.⁵ Among various kinds of anions, the fluoride anion has attracted the interest of chemists because of its importance in many areas such as the human diet and dental care.^6,7 Therefore, it is of considerable importance to develop an effective sensor to detect the fluoride anion with the naked eye.⁸

A large number of different receptors, based on the introduction of electrostatic interactions, Lewis acid groups, hydrogen bond donor groups and hydrophobic interactions, have been designed that use metal complexes as synthetic receptors to recognize anions.^9,10 A single ligand metal complex may provide sufficient binding ability to obtain stable aggregates with high affinities for anions.¹¹ Metal complexes such as Pt(II), Ru(II), Re(I), Ir(III), Au(I), and Os(II) based complexes have been well developed as chemosensors, because of the advantageous photophysical properties of heavy-metal complexes, such as sensitivity of emission properties, significant single-photon excitation in the visible region and a relatively long lifetime compared with purely organic chemosensors.¹²

2,2′-Bipyridine and its derivatives, such as phenanthroline, have played an important role in coordination chemistry due to their excellent coordination ability with a range of transition metals. When hydrogen bond donors are introduced after coordination, metal complexes can sometimes serve as good candidates as anion receptors because of some of their novel properties.¹³ Usually, heterocyclic ring systems containing the NH group can be used to detect anions such as calixpyrroles,¹⁴ porphyrinoids,¹⁵ pyrrole derivatives,¹⁶ indoles,¹⁷ bisindoles,¹⁸ bisimidazoles, indolocarbazoles¹⁹ and benzimidazoles.²⁰ For instance, Molina and coworkers²¹ have synthesised imidazo[4,5-f]-1,10-phenanthroline as the ligand to form ruthenium(II) complexes. This new chemosensor could effectively recognize hydrogen pyrophosphate and the organic anions ADP and ATP. Wang et al. have reported a Eu(III) complex containing imidazo[4,5-f]-1,10-phenanthroline, in which the addition of F⁻, HSO₄⁻ and AcO⁻ causes significant changes in the UV/Vis absorption and emission spectra.²²

Following the above, we have designed and synthesized a new ligand containing the imidazo[4,5-f]-1,10-phenanthroline block (L₁), followed by reaction of the ligand L₁ with three metal salts (Pt(II), Ru(II) and Re(I) salts) to give appropriate complexes (Scheme 1). Their structures have been fully confirmed by NMR spectroscopy and mass spectrometry. In this case, the introduction of the long alkoxyl chain can improve the solubility. The anion binding was investigated by UV-Vis and fluorescence titration. These complexes showed colorimetric or fluorescent properties for the selective detection of fluoride ions. In this, [Re(CO)₃ClL₁] showed an immediate change from yellow to pink, which could be observed by the naked eye, with the addition of F⁻ in DMSO. On the other hand, the emission intensities of [Ru(bpy)₂L₁](PF₆)₂ and [PtCl₂L₁] in DMSO were strongly enhanced upon the addition of F⁻.

Scheme 1 Synthesis of complexes 1–4.

Results and discussion

General information

As shown in Scheme 1, the metal complexes (1–4) were obtained by the reaction of the ligands with the corresponding metal salts, the compounds were characterized by ¹H NMR spectroscopy and mass spectrometry. The photophysical properties of these metal complexes with several anions (F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, H₂PO₄⁻, CH₃COO⁻ and ClO₄⁻) using their tetrabutylammonium salts in DMSO were investigated by UV-Vis, fluorescence measurements and titration studies.

Colorimetric anion-sensing in DMSO solution

An initial test of complex 1 in DMSO with the addition of F⁻ showed an immediate change from yellow to pink (Fig. 1). However, no obvious changes in colour were observed with the addition of other anions (Cl⁻, Br⁻, I⁻, HSO₄⁻, H₂PO₄⁻, CH₃COO⁻ and ClO₄⁻). These results suggested that the change in colour is caused only by fluoride and not by other anions. To further study the colorimetric sensing behaviour of receptor 1, UV-Vis absorption experiments were performed. The changes in UV-Vis absorption of complex 1 (2 × 10⁻⁵ mol L⁻¹ in DMSO) upon addition of F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, H₂PO₄⁻, CH₃COO⁻ and ClO₄⁻ (10 equiv.) are shown in Fig. 2. The UV-Vis absorption spectrum of receptor 1 exhibits two strong absorption bands at λ = 284 nm (ε = 7.35 × 10⁴ M⁻¹ cm⁻¹) and 368 nm (ε = 7.07 × 10⁴ M⁻¹ cm⁻¹). The addition of the above anions (2 × 10⁻⁴ mol L⁻¹ in DMSO) demonstrates that only F⁻ promotes a significant response. According to Fig. 2, after addition of F⁻, the absorbance at 284 nm showed a 14 nm red shift while no obvious changes were observed for the 368 nm absorbance. In addition, a new low energy broad band appeared at around 512 nm, thereby inducing a promising color change. However, there were no significant changes in the absorption behaviour in the presence of Cl⁻, Br⁻, I⁻, HSO₄⁻, H₂PO₄⁻, CH₃COO⁻ and ClO₄⁻.

Fig. 1 Colour change of complex 1 (2 × 10⁻⁵ mol L⁻¹ in DMSO) after addition of 10 equiv. of different anions. From left to right: F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, H₂PO₄⁻, CH₃COO⁻ and ClO₄⁻.

Fig. 2 Changes in the absorption spectra of complex 1 (2 × 10⁻⁵ mol L⁻¹ in DMSO) upon addition of 10 equiv. of several anions (2 × 10⁻⁴ mol L⁻¹ in DMSO).

In order to clarify the interaction of complex 1 with F⁻, we synthesized ligand L₂ by methylation of the N–H on the imidazo-ring of L₁ with CH₃I in the presence of K₂CO₃. Subsequently, reaction of ligand L₂ with Re(CO)₅Cl gave the complex 2. However, complex 2 showed no change upon addition of F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, H₂PO₄⁻, CH₃COO⁻ and ClO₄⁻ (10 equiv.) in DMSO. This demonstrated that F⁻ interacts with the N–H group of the imidazole ring. The absorption peak at 260 nm shifted to 284 nm when a protic solvent such as methanol or water was added, which leads to the suggestion that the interaction of complex 1 with F⁻ depends on deprotonation or hydrogen bonding. As a result of the greater contribution of electron density for the conjugated system in the deprotonated complex 1, it is suggested that the interaction is deprotonation rather than hydrogen bonding because of the significant absorption shift.⁵ UV-Vis absorption titrations of complex 1 in the presence of different equivalents of F⁻ were performed. As shown in Fig. 3, the intensity of the absorption band at 368 nm increased with the appearance of a new absorption band at 512 nm. On increasing the concentration of F⁻ up to 20 equiv., there was an obvious red shift of the absorption band at 284 nm to 308 nm. Compounds 3 and 4 showed no obvious color change with the addition of different anions.

Fig. 3 Changes in the absorption spectra complex 1 (2 × 10⁻⁵ mol L⁻¹ in DMSO) upon addition of F⁻ (equiv. = 0, 0.2, 0.4, 0.6, 0.8, 1, 5, 10, 15, 20.).

Fluorescence anion-sensing in DMSO solution

The Ru(bpy)₃²⁺ moiety has been widely used in the development of fluorescence anion sensors bearing metal-based fluorescent units. The photochemistry of the Ru(bpy)₃²⁺ group is very well known.²³ The recognition ability of the ruthenium(II) complex for F⁻ was investigated by emission spectrophotometric studies. The emission intensity of the complex in DMSO was strongly enhanced by the addition of F⁻ with a small red shift of emission maxima possibly due to deprotonation of NH in the presence of the fluoride anion, which was in agreement with a previous report.²¹

As is shown in Fig. 4, complex 3 exhibits a very weak fluorescence in DMSO (2 × 10⁻⁵ mol L⁻¹). The emission spectrum showed a structureless band at 489 nm typical of the imidazo-phenanthroline ring³ with a low quantum yield of 0.022. Upon addition of other anions as their tetrabutylammonium salts, no significant changes in the emission spectrum could be observed for Cl⁻, Br⁻, I⁻, HSO₄⁻, H₂PO₄⁻, CH₃COO⁻ and ClO₄⁻. However, the emission band showed a strong enhanced fluorescence in the presence of 10 equiv. of F⁻ with a quantum yield Φ = 0.070.

Fig. 4 Fluorescence spectra of complex 3 (2 × 10⁻⁵ mol L⁻¹ in DMSO) upon addition of 10 equiv. of several different anions (2 × 10⁻⁴ mol L⁻¹ in DMSO) excited at 364 nm.

Quantitative investigations of the binding affinity of complex 3 with F⁻ were carried out in DMSO by fluorescence titration (Fig. 5). The emission intensity of 3 was gradually enhanced with increasing F⁻ concentration, and reached saturation when 10 equiv. of F⁻ was added. The Job’s plot for the binding between complex 3 and F⁻ showed a 1 : 1 stoichiometry (Fig. 6). From the fluorescence titration, the apparent association constant was calculated to be 1.2 × 10⁵ M⁻¹.

Fig. 5 Changes in the fluorescence emission of complex 3 (2 × 10⁻⁵ mol L⁻¹ in DMSO) upon addition of F⁻ (equiv. = 0, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60) excited at 364 nm.

Fig. 6 Job's plot for the reaction of [Ru(bpy)₂L₁](PF₆)₂ with F⁻ in DMSO.

Subsequently, the recognition ability of the platinum(II) complex 4 for anions was also investigated by emission spectrophotometric studies, and analogous fluorescence titrations of complex 4 with different anions were performed (Fig. S2†). Similar results were obtained for the solution of the platinum(II) complex 4 in DMSO, since the emission intensity of complex 4 was strongly enhanced upon addition of F⁻ and reached saturation when 10 equiv. of F⁻ was added. The Job's plot for the complexation of complex 4 with F⁻ also showed that the overall anion-to-receptor binding stoichiometry is 1 : 1, the same as the ruthenium(II) complex 3. In addition, the apparent association constant was observed to be 1.4 × 10⁴ M⁻¹ (Fig. S4†). Such strong enhancement of the emission intensities for [Ru(bpy)₂L₁](PF₆)₂ and [PtCl₂L₁] could possibly be explained by the high sensitivity of N–H in the imidazo-phenanthroline in these complexes. Finally, complex 1 exhibited a very weak fluorescence in DMSO, and there was no significant change with the addition of different anions.

¹H NMR titration experiment

To further investigate the anion binding properties of fluoride and the metal-complexes 1, 3 and 4, NMR titration experiments were carried out in the absence and presence of different equiv. of fluoride anion in d⁶-DMSO. Fig. S5† shows the changes in chemical shift when the fluoride anion was added to compound 1. As can be seen from the ¹H NMR spectra, for the addition of TBAF, the resonance signals of protons on the unsymmetrical phenanthroline ring exhibited little change when the fluoride anion at 0.5 equiv. was added in d⁶-DMSO solution at room temperature. An obvious downfield shift was observed when the fluoride anion at 1.0 equiv. was added under the same conditions. Further additions of the fluoride anion showed almost no new shifts. At the same time, the hydrogen donor NH signal disappears along with the addition of the fluoride anion due to the formation of [FHF]⁻, a result in good agreement with our previous reports.²⁴ When fluoride anions were added the NH signals completely disappeared. However, a new signal at around 16.00 ppm was observed when the fluoride anion reached 5.0 equiv. of complex 1 in concentration due to the formation of [FHF]⁻ and a broad peak at 16.0 ppm was observed, a result in good agreement with previous reports.²⁴ Similar results were observed in the titration experiments of complexes 2 and 3 (see ESI†, Fig. S5–7).

Conclusions

We have synthesized and characterized a new imidazo[4,5-f]-1,10-phenanthroline ligand and three metal complexes, which can be utilised as colorimetric and fluorescent chemo-dosimeters for anions. The results showed that [Re(CO)₃ClL₁] revealed an immediate change in colour from yellow to pink, which could be observed by the naked eye, with the addition of F⁻, and the emission intensities of [Ru(bpy)₂L₁](PF₆)₂ and [PtCl₂L₁] in DMSO were strongly enhanced with the addition of F⁻. Furthermore, these metal complexes could be used as colorimetric and fluorescent chemo-dosimeters for F⁻.

Experimental

General information

The compounds cis-[Ru(bpy)₂Cl₂]·2H₂O, [PtCl₂(NCPh)₂] and 1,10-phenanthroline-5,6-dione were synthesized according to literature methods.²¹ All other starting materials were obtained commercially at analytical-grade and were used without further purification. All anions, in the form of tetrabutylammonium salts, were purchased from Alfa Aesar (USA), and they were used as received without further purification. Elemental analyses (C, H, N) were performed by Microanalytical Services, College of Chemistry, CCNU. ¹H and ¹³C NMR spectra were collected on a American Varian Mercury Plus 400 spectrometer (400 MHz). ¹H and ¹³C NMR chemical shifts are relative to TMS. UV-Vis spectra were obtained on a U-3310 UV spectrophotometer. Fluorescence spectra were taken on a Fluoromax-P luminescence spectrometer (Horiba Jobin Yvon inc.).

Synthesis

Rhenium(I) complex [Re(CO)₃ClL₁] (1). A mixture of Re(CO)₅Cl (0.1 g, 0.28 mmol) and L₁ (0.2 g, 0.30 mmol) in toluene (30 mL) was heated and refluxed at 115 °C for 6 h. After the solution was cooled to room temperature, most of the solvent was removed under reduced pressure. The solid precipitate was filtered and washed with diethyl ether and dried under vacuum to give an orange solid. Yield 84%. ¹H NMR (400 MHz, CDCl₃): δ = 0.96 (6H, CH₃), 1.40 (8H, CH₂), 1.53 (4H, CH₂) 1.85 (4H, CH₂), 4.02 (4H, OCH₂), 6.98 (7H, Ar–H), 7.21 (4H, Ar–H), 7.40 (1H, Ar–H), 7.72 (2H, pyridine-H), 8.27 (1H, pyridine-H), 8.63 (1H, pyridine-H), 8.83 (1H, pyridine-H), 8.15 (1H, pyridine-H), 11.33 (1H, NH). MS (m/z): 969.27 [M]⁺. Anal. calcd for C₄₆H₄₅ClN₅O₅Re: C, 56.98; H, 4.68; N, 7.22. Found: C, 56.71; H, 4.50; N, 7.23.

Rhenium(I) complex [Re(CO)₃ClL₂] (2). A mixture of Re(CO)₅Cl (0.1 g, 0.28 mmol) and L₂ (0.21 g, 0.30 mmol) in toluene (30 mL) was heated and refluxed at 115 °C for 6 h. After the solution was cooled to room temperature, most of the solvent was removed under reduced pressure. The solid precipitate was filtered and washed with diethyl ether and dried under vacuum to give an orange solid. Yield 47%. ¹H NMR (400 MHz, CDCl₃): δ = 0.92 (6H, CH₃), 1.36 (8H, CH₂), 1.50 (4H, CH₂), 1.82 (4H, CH₂), 3.96 (4H, OCH₂), 4.31 (3H, CH₃), 6.90 (4H, Ar–H), 7.13 (6H, Ar–H), 7.56 (2H, Ar–H), 7.79 (2H, pyridine-H), 8.95 (1H, pyridine-H), 9.24 (3H, pyridine-H). MS (m/z): 983.28 [M]⁺. Anal. calcd for C₄₇H₄₇ClN₅O₅Re: C, 57.39; H, 4.82; N, 7.12. Found: C, 57.51; H, 4.72; N, 7.03.

Ruthenium(II) complex [Ru(bpy)₂L₁](PF₆)₂ (3). To a solution of cis-[Ru(bpy)₂Cl₂] (0.11 g, 0.22 mmol) in ethanol (20 mL) was added L₁ (0.15 g, 0.22 mmol), and the reaction mixture was refluxed for 7 h. After cooling to room temperature, glacial acetic acid (three drops) and a solution of NH₄PF₆ (0.30 g, 1.84 mmol) in water (10 mL) were added. The solution was boiled, partially concentrated, and cooled overnight in the fridge. The resulting precipitate was collected, washed with ether (10 mL), and dried under vacuum. The product obtained was dissolved in acetone (10 mL), then hexane (10 mL) was added to induce the precipitation of the complex which was purified by crystallization from acetonitrile. Yield 82%. ¹H NMR (400 MHz, CDCl₃): δ = 0.91 (6H, CH₃), 1.28 (8H, CH₂), 1.49 (4H, CH₂), 1.78 (4H, CH₂), 4.02 (4H, OCH₂), 6.87 (8H, Ar–H), 7.15 (4H, Ar–H), 7.42 (2H, Ar–H), 7.64 (2H, pyridine-H), 7.84 (2H, Ar–H), 7.96 (2H, Ar–H), 8.06 (2H, Ar–H), 8.16 (6H, Ar–H), 8.26 (2H, Ar–H), 8.83 (3H, pyridine-H), 9.15 (1H, pyridine-H). MS (m/z): 1077.40 [M]⁺. Anal. calcd for C₆₃H₆₁N₉O₂Ru: C, 70.24; H, 5.71; N, 11.70. Found: C, 70.28; H, 5.65; N, 11.89.

Platinum(II) complex [PtCl₂L₁] (4). A solution of [PtCl₂(NCPh)₂] (0.066 g, 0.14 mmol) in chloroform was added to a solution of ligand L₁ (0.107 mmol, 0.14 g) in chloroform, and the mixture was refluxed for 5 h. The solvent was then removed under reduced pressure, and diethyl ether was added. Excess [PtCl₂(NCPh)₂] was removed by filtration, and ethanol was added until precipitation occurred. The orange precipitate was filtered off and dried in a vacuum oven at 50 °C. Yield 67%. ¹H NMR (400 MHz, CDCl₃): δ = 0.87 (6H, CH₃), 1.31 (8H, CH₂), 1.42 (4H, CH₂), 1.74 (4H, CH₂), 3.91 (4H, OCH₂), 6.79 (6H, Ar–H), 7.39 (2H, Ar–H), 7.83 (4H, Ar–H), 8.37 (2H, pyridine-H), 9.26 (4H, pyridine-H). MS (m/z): 928.26 [M]⁺. Anal. calcd for C₄₃H₄₅Cl₂N₅O₂Pt: C, 55.54; H, 4.88; N, 7.53. Found: C, 55.59; H, 4.78; N, 7.39.

Acknowledgements

The authors acknowledge financial support from the National Natural Science Foundation of China (No. 20931006, 21072070) and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0953).

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Footnote

† Electronic supplementary information (ESI) available: NMR and mass spectra. See DOI: 10.1039/c2ra20331f/

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