A new benzimidazole-based quinazoline derivative for highly selective sequential recognition of Cu2+ and CN−

Tetrahedron Letters 54 (2013) 536–540

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A new benzimidazole-based quinazoline derivative for highly selective sequential recognition of Cu2+ and CN Lijun Tang a,⇑, Nannan Wang a, Qiang Zhang a, Jiaojiao Guo a, Raju Nandhakumar b,⇑ a b

Department of Chemistry, Liaoning Key Laboratory for the Synthesis and Application of Functional Compounds, Bohai University, Jinzhou 121013, China Department of Chemistry, Karunya University, Karunya Nagar, Coimbatore 641114, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 17 August 2012 Revised 5 November 2012 Accepted 20 November 2012 Available online 29 November 2012

a b s t r a c t A new benzimidazole-based quinazoline derivative (1) has been designed and synthesized as a fluorescent probe. Probe 1 exhibits high selectivity and sensitivity to Cu2+ as fluorescence ‘on–off’ behavior in HEPES-buffered CH3OH/H2O (1:1, v/v, pH = 7.0) solution. The in situ formed 1-Cu2+ complex is further utilized to sense the cyanide ions with high selectivity and fluorescence enhancement performance. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Copper detection Cyanide recognition Fluorescence revival

Introduction N

The development of fluorescent artificial receptors for the recognition of biologically and environmentally important ions (including metal ions and anions) has received numerous attention due to their basic roles in a wide range of chemical, environmental, and biological processes.1 As the third adequate element in the human body, Cu2+ plays vital roles in many fundamental physiological processes in organisms. For instance, it serves as a catalytic cofactor for a variety of metalloenzymes including superoxide dismutase, cytochrome c oxidase, and tyrosinase.2 However, overloading of Cu2+ is associated with severe neurodegenerative diseases such as Alzheimer’s or Parkinson’s diseases.3 It is also suspected to cause infant liver damage in recent years.4 Thus, highly selective and sensitive recognition of Cu2+ by chemosensors is still imperative. Consequently, great efforts have been devoted to the development of probes for the recognition of Cu2+.5 Among the vast number of important anions, cyanide has attracted increasingly more attention due to its high toxicity. Cyanide ions can bind heme cofactors to inhibit the process of cellular respiration in mammals.6 Most environmental cyanides are released by industries which are involved in nylon and acrylic polymers synthesis, electroplating, and gold mining.7 Hence, great efforts have been devoted to the detection of cyanide by using highly selective, sensitive, and convenient chemical probes.8 During the past decades, a number of detecting strategies have been adopted in cyanide recognition, which include hydrogen bonding ⇑ Corresponding authors. Tel.: +86 416 3400302. E-mail address: [email protected] (L. Tang). 0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2012.11.078

CHO

H2N N

OH OH

N H

2

N reflux Ethanol

3

N H OH OH 1

Scheme 1. Synthesis of probe 1. 9

interactions, formation of cyanide complexes,10 nucleophilic attack of CN to activated carbonyl groups11 or C@C double bond12 as well as a boron center,13 and demetalation of preassembled complexed sensor.14 Recognition of cyanide by Cu2+ displacement approach has been proved to be a valuable method to built highly selective chemosensors for cyanide in aqueous media,15 and can also effectually avoid the interference from other anions such as fluoride and acetate. Herein we designed and synthesized a new benzimidazolebased quinazoline derivative (1) (Scheme 1). Compound 1 displays highly selective and sensitive fluorescence ‘on–off’ recognition to Cu2+. In addition, the in situ formed 1-Cu2+ complex serves as a highly selective cyanide probe through fluorescence ‘off–on’ performance. Results and discussion Compound 1 was easily synthesized by an elegant condensation of 2-(2-aminophenyl)-1H-benzimidazole (2) with (S)-2,20 -dihy-

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droxy-1,10 -binapthlaene-3-carbaldehyde (3)16 in absolute ethanol (Scheme 1). The structure of 1 was confirmed by 1H NMR, high resolution mass spectroscopy (HRMS), and single-crystal X-ray diffraction methods. The crystal suitable for X-ray analysis was obtained by slow evaporation of its ethanol solution and the ORTEP structure of 1 was shown in Figure 1. The X-ray crystallography structural investigation of 1 clearly revealed the formation of quinazoline skeleton. A summary of the crystallographic data and structure refinements for 1 are listed in Table S1 (Supplementary data). To explore the photophysical properties, the fluorescence spectrum of solution 1 (10 lM) in CH3OH/H2O (1:1, v/v) at different pH values was examined first (Fig. S1). The results showed that probe 1 exhibits strong fluorescence emission at neutral pH condition. The fluorescence intensity decrease of probe 1 at low pH values could be attributed to protonation of benzimidazole moiety which led to proton-induced fluorescence quenching.17 Under high pH conditions, the base induced deprotonation of phenolic OH might lead to photoinduced electron transfer (PET) from the electron-rich binaphthol moiety to benzimidazole fluorophore, which may be responsible for the fluorescence quenching of probe 1.18 Thus, pH of 7 was selected as working moiety throughout the following spectroscopic experiments. The fluorescence response of solution 1 (10 lM) toward various metal ions was explored to examine its metal ion selectivity. As shown in Figure 2, only the introduction of Cu2+ (1.0 equiv) into solution 1 could induce a dramatic fluorescence quenching. Whereas, addition of other metal ions such as Hg2+, Ag+, Pb2+, Sr2+, Ba2+, Cd2+, Ni2+, Co2+, Fe2+, Mn2+, Fe3+, Zn2+, Al3+, Cr3+, Mg2+, Cu+, Ca2+, K+, and Na+ (1.0 equiv of each) did not cause significant

fluorescence spectra changes. These observations indicated that probe 1 has an excellent selectivity to Cu2+ ion. To get further insight into the sensing behavior of probe 1 to Cu2+, fluorescence titration experiment was performed (Fig. 3). Upon incremental addition of Cu2+ (0–2.0 equiv) to solution 1, a gradual decrease of fluorescence intensity was observed. The fluorescence emission was almost completely quenched (>95%)19 when 1.0 equiv of Cu2+ was employed. Non-linear least-squares fitting of the titration profiles based on the 1:1 binding mode strongly support the 1:1 stoichiometry of 1 and Cu2+, and the binding constant was estimated to be 7.38  106 M1. The 1:1 binding stoichiometry of Cu2+ and 1 was also supported by Job’s plot method. The fluorescence intensity of the tested solution exhibited a turning point when the molar fraction of Cu2+ was 0.5 (Fig. S2), which also indicates the 1:1 binding stoichiometry of 1 and Cu2+. In addition, a solid evidence for proving the 1:1 binding of 1 and Cu2+ come from the TOF-ES high resolution mass spectrum. The most prominent peak at m/z = 567.1050 is assignable to [1+Cu2+H+]+ (Fig. S3). This result credibly supports the 1:1 interaction of 1 and Cu2+. To get further insight into the binding mode of 1-Cu2+, a several of quantum chemical calculations with the DFT/B3LYP method have been carried out to get the possible configurations of 1Cu2+. The 6-31G basis set was adopted for H, N, and O atoms, for Cu atom, the LANL2DZ effective core potential was used. The initial configures of 1-Cu2+ for the optimizations were obtained by randomly placing the copper ion nearby the possible coordinated O and N atoms, all of them can converge to the same stable configuration, as depicted in Figure 4. The calculated energy-minimized structure reveals that the Cu2+ ion binds to 1 very well through 0

three coordination sites. The Cu–O(1) bond length is 1.82 Å A, and 0

0

A (Cu– the Cu–N bond lengths are 2.01 Å A (Cu–N(3)) and 2.16 Å N(2)). These results provided valuable information for understanding the bind mode of 1 and Cu2+. All the calculations were performed with GAUSSIAN 03 package. Subsequently, fluorescence competition experiments were conducted to demonstrate the high selectivity of 1 to Cu2+ (Fig. 5). The fluorescence changes of 1 in CH3OH/H2O (1:1, v/v, HEPES 20 mM, pH = 7.0) were measured by the treatment of 1.0 equiv of Cu2+ ion in the presence of equimolecular ratio of other potential competitive metal ions. The results show that the tested background metal ions, such as Hg2+, Ag+, Pb2+, Sr2+, Ba2+, Cd2+, Ni2+, Co2+, Fe2+, Fe3+,Mn2+, Zn2+, Al3+, Cr3+, Mg2+, Cu+, Ca2+, K+ and Na+, displayed scarce interference on the detection of Cu2+ ion. Figure 1. Crystal structure of 1, all hydrogen atoms were omitted for clarity.

800 2+

2+

2+

600

400

200

800

0

2+

600

Cu

Fluorescence Intensity (a.u.)

2+

1 a nd 1 + Ni , Hg , Ba , Mg , Fe , 3+ 2+ 2+ + 2+ 2+ + Al , Mn , P b , Na , Sr , Cu , K 2+ 2+ 2+ 3+ 3+ + 2+ Co , Zn , Cd , Cr , Fe , Cu , Ca

Fluorescence Intensity (a.u.)

Fluorescence Intensity (a.u.)

800

2+

2.0 eq.

600

400

200

0 0 .0

5.0x10 -6

400

1.0x10-5

1.5x10-5

2.0x10-5

2+

[ Cu ]

200

2+

1+Cu

0 400

0 40 0

450 500 Wavelength (nm)

550

600

Figure 2. Fluorescence spectra of 1 (1.0  105 M) upon addition of a specific metal ions (1.0 equiv of each) in CH3OH/H2O (1:1, v/v, HEPES 20 mM, pH = 7.0).

450

500

550

600

Wavelength (nm) Figure 3. Fluorescence spectra changes of 1 (1.0  105 M) upon successive addition of Cu2+ (0–2 equiv) in CH3OH/H2O (1:1, v/v, HEPES 20 mM, pH = 7.0). Inset: non-linear fit plots monitored by the fluorescence quench at 431 nm.

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and maximum intensities. Plotting of (IminI)/(IminImax) versus log[Cu2+] afforded a good linear relationship (R = 0.9918) (Fig. 6). The point at which this line crossed the ordinate axis is regarded as the detection limit and found to be 3.8  107 M, which is much lower than the limit of copper in drinking water (20 lM) and the typical concentration of blood copper (15.7–23.6 lM) in normal individuals defined by the U.S. Environmental Protection Agency.5c These results indicate that our probe 1 is sensitive enough to monitor Cu2+ concentration in water. Considering the high stability constants of cyanide and copper,15b 1-Cu2+ complex was expected to act as a chemosensing ensemble for cyanide recognition. Thus, the probe 1-Cu2+ was prepared in situ by addition of 1.0 equiv of Cu2+ to solution 1. The stability of the in situ prepared probe 1-Cu2+ complex was evaluated by monitoring the fluorescence intensity changes against time (0– 60 min). The fluorescence intensity of the solution is almost constant after 5 min of Cu2+ addition (Fig. S4), which indicates that the 1-Cu2+ complex has a good stability and suitable for anion recognition. Subsequently, the responses of probe 1-Cu2+ (10 lM) to a variety of anions were investigated (Fig. 7). Upon addition of 150 equiv of CN (K+ salt) to solution 1-Cu2+, the fluorescence intensity was greatly enhanced. However, addition of other anions including F, Cl, Br, I, AcO, HSO4, H2PO4, HPO42, PO43, SCN, NO3, CO32, HCO3, and ClO4 (all used as Na+ salt, 150 equiv for each) induced negligible fluorescence enhancement.

Figure 4. Calculated energy-minimized structure of 1-Cu2+.

1+other metal ion 2+ 1+other metal ion+Cu

600

400 500

200

Na + N i 2+ Pb 2+ S r 2+ Z n 2+ Cu + C a 2+

K+ M g 2+ M n 2+

Ag + A l 3+ Ba 2+ C d 2+ C o 2+ C r 3+ F e 2+ Fe 3+ Hg 2+

1 C u 2+

0

Metal ions 5

Figure 5. Fluorescence intensity of 1 (1.0  10 M) at 431 nm to various metal ions. The black bars represent the fluorescence of 1 in the presence of 1 equiv of miscellaneous metal ions, the red bars represent the fluorescence of the above solution upon further addition 1.0 equiv of Cu2+.

Fluorescence Intensity (a.u.)

Fluorescence Intensity (a.u.)

800

2+

1-Cu + CN

-

400

300

200

2+

-

-

-

-

-

3-

1-C u + F , Cl , Br , I , SCN , PO 4 , -

H2PO 4 ,

2-

-

HPO4 , NO 2 , NO 3 , AcO

2-

-

2-

SO4 , HSO4 , CO3 ,

100

-

-

-

HC O3 , ClO4

-

0 400

450

500

550

600

Wave length (nm)

In addition, to check its practical utility, the fluorescent detection limit of 1 for Cu2+ was evaluated by a reported method.20 The intensity at 431 nm was normalized between the minimum

Figure 7. Fluorescence spectra of 1-Cu2+ (1.0  105 M) upon addition of various anions (150 equiv of each) in CH3OH/H2O (1:1, v/v, HEPES 20 mM, pH = 7.0).

500 0.4

150 eq. Fluorescence Intensity (a.u.)

Y = 2.74787+0.42797*X R = 0.99175

(I min -I)/(Imin -Imax )

0.3

0.2

400 CN

-

300 0 200

100

0.1

0 400 -6.4

-6.3

-6 .2

-6.1

-6.0

-5.9

-5.8

-5.7

-5.6

-5 .5

450 500 Wavelength (nm)

550

600

2+

Log[Cu ] Figure 6. Normalized response of the fluorescence signal to log[Cu2+].

Figure 8. Fluorescence spectra changes of 1-Cu2+ (1.0  105 M) upon successive addition of CN in CH3OH/H2O (1:1, v/v, HEPES 20 mM, pH = 7.0).

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Figure 9. Proposed mechanism for Cu2+ and CN recognition.

2+

1-Cu +anions 2+ 1-Cu +anions+CN

Fluorescence Intensity (a.u.)

400

300

0.28

0.24 (I min -I)/(Imin -Imax )

The observed cyanide induced fluorescence enhancement of 1-Cu2+ indicates that the speculated Cu2+ displacement approach played a role. These results demonstrate that the complex probe 1-Cu2+ behaves high selectivity to CN, which makes 1-Cu2+ a good candidate for highly selective recognition of cyanide in aqueous media. The CN sensing behavior of 1-Cu2+ was then examined by fluorescence titration experiment (Fig. 8). The fluorescence intensity of 1-Cu2+ solution increased gradually upon stepwise increasing of CN concentration, and the emission reached saturation when 150 equiv of CN (relative to 1) was introduced. It is noteworthy that even at the saturation state, the fluorescence intensity of the solution could not regain its initial emission state of 1. This phenomenon might be attributed to the tight binding between 1 and Cu2+, which resulted in incomplete displacement of Cu2+ from the complex. Based on the abovementioned results, the fluorescence quenching and revival of 1 induced by Cu2+ and CN, respectively, the sensing mechanism for the two ions is proposed and illustrated in Figure 9. Furthermore, fluorescence competition experiments were conducted to evaluate the tolerance of 1-Cu2+ to other anions. As shown in Figure 10, the coexistence of equal amount of other anion did not induce any significant interference on the CN recognition. These results demonstrate that the CN recognition by probe 1Cu2+ has an excellent anti-jamming ability. In addition, the detection limit of 1-Cu2+ for CN was also evaluated following the above mentioned method and was calculated to be 1.86  105 M (Fig. 11), which is greater than the upper limit

Y=1.80874+0.38243X R=0.99946

0.20

0.16

0.12 -4.4

-4.3

-4.2

-4.1

-4 .0

-

Lo g[CN ] Figure 11. Normalized response of the fluorescence signal to log[CN].

(1.9 lM) for cyanide in drinking water set by the World Health Organization.21 This result indicates that complex probe 1-Cu2+ has a limitation in detection of cyanide in drinking water, but it still has the potential utility to detect cyanide concentration in highly cyanide polluted water. Conclusions In summary, we have developed a new benzimidazole-based quinazoline derivative (1) as a fluorescent probe for sequential recognition of Cu2+ and CN in HEPES-buffered CH3OH/H2O (1:1, v/v, pH = 7.0) solution. Probe 1 shows highly selective and sensitive fluorescence ‘on–off’ behavior toward Cu2+ over other heavy and transition metal ions. The in situ prepared 1-Cu2+ complex exhibits high selectivity to cyanide over other anions through fluorescence enhancement. This sequential recognition behavior makes probe 1 as a potential probe to detect Cu2+ and CN in the environmental monitoring works. Acknowledgments

200

100

4

-

SC4 NSO 2 -

-

NO2 PO 3 3

I-

4

NO

3

F2 PO HC 4 O HP 3 O 2HS 4 O H

Cl ClO CO 4 2

1-C

u 2+ CN Ac O Br -

0

We are grateful to the NSFC (21176029), the Natural Science Foundation of Liaoning Province (20102004) and the Program for Liaoning Excellent Talents in University (LJQ2012096) for financial support. This work was also sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. Supplementary data

Figure 10. Fluorescence intensity of 1-Cu2+ (1.0  105 M) at 431 nm to various anions. The black bars represent the fluorescence intensity of 1-Cu2+ in the presence of 150 equiv of miscellaneous anions, the red bars represent the fluorescence intensity of the prepared copper conjugate and miscellaneous anions on further addition of 150 equiv of CN.

Supplementary data (instrumental details, synthesis and characterization of 1, pH effect on fluorescence intensity of 1, Job’s plot and Crystal data and structure refinements for 1. Crystallographic

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data (excluding structure factors) for the structures in this Letter have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-892906. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK. Fax: +44 (0)1223 336033 or e-mail: [email protected]) associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.tetlet.2012.11.078.

6. 7. 8. 9. 10. 11.

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