Binding site-driven sensing properties of a quinazoline derivative with metal cations

Abstract

A compound, 6-furyl-5,6-dihydrobenzoimidazo[1,2-c]quinazoline (L) was synthesized and characterized. L showed weak fluorescence emission at 416 nm in CH₃OH when excited by 383 nm. Upon addition of Al³⁺, Cr³⁺ or Fe³⁺ ions, enhanced fluorescence emission and spectral shift (55 nm) could be observed. Job's plot, fluorescence titration, ¹H NMR and ESI-MS data for binding of L with Al³⁺, Cr³⁺ and Fe³⁺ ions showed 1 : 1 metal to L complexation with the transformation of the quinazoline compound (L) to the Schiff base ligand [2-(1H-benzoimidazol-2-yl)-phenyl]-furan-2-ylmethyleneamine (L¹). Cr³⁺ can be differentiated from Al³⁺ and Fe³⁺ by time-dependent fluorescence spectra, while Al³⁺ can be distinguished from Fe³⁺ by the emission at 430 nm when excited by 300 nm in the aqueous medium CH₃OH–H₂O (1/9, v/v). The detection limits were on the order of 10⁻⁶ M for Al³⁺, Cr³⁺ and Fe³⁺ ions. L is insensitive to Cd(II) ions. The crystal structures of Cd(II) complexes, [CdL₂(OAc)₂] (1) and [CdL₂Cl₂] (2) indicate the binding site of L with Cd(II) is one imidazole nitrogen atom. UV-vis spectra and fluorescence spectra of L, 1 and 2 in CH₃OH showed no significant difference. Also ¹H NMR signals of 1 were almost the same as those of L. These observations indicated that the coordination of L with Cd(II) has little effect on the spectroscopic properties of L. The response mechanism of L to Al³⁺, Cr³⁺ and Fe³⁺ ions was discussed.

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Introduction

Aluminum is the most abundant metal in earth's crust (approximately 8%) and is extensively used in modern life.¹ The general population is exposed to aluminum from its widespread use in water treatment, food additives, aluminum-based pharmaceuticals and cooking utensils. The WTO recommended tolerable weekly aluminum dietary intake in the human body to be 7 mg kg⁻¹ body weight.² But it is neurotoxic to humans and induces many health problems, such as Alzheimer's disease and Parkinson's disease.³ Chromium(III) is an essential trace component in human nutrition and has great impacts on the metabolism of carbohydrates, fats, proteins and nucleic acids. The deficiency of chromium would cause disturbances in the glucose levels and lipid metabolism, and lead to a variety of diseases including diabetes and cardiovascular disease.⁴ In addition, high levels of chromium(III) can bind to DNA, negatively affecting cellular structures and damaging the cellular components.⁵ Among the biologically important metals, iron(III) is an essential element in human body and plays an important role in the transport and storage of oxygen.⁶ A deficiency or excess of iron can cause various pathological disorders in humans.⁷ Thus, detection of Al³⁺, Cr³⁺ and Fe³⁺ is important to control the concentration level in the biosphere and minimize direct effect on human health. In recent decades, fluorescent chemosensors have attracted significant interest because of their potential applications in medical and environmental sciences. The development of chemosensors for the facile detection of Al³⁺, Cr³⁺ and Fe³⁺ is of great importance in environmental monitoring and biological assays. However, compared to divalent metal ions, such as Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺ and Cu²⁺, limited chemosensors for Al³⁺, Cr³⁺ and Fe³⁺ based on a single molecule have been reported.⁸

It has been shown that the quinazoline derivatives as fluorescent chemosensors are available for detection of M³⁺ ions.⁹ We have confirmed with a combination of experiments and theoretical calculations that in sensing process, the appropriate metal ion can assist the C–N bond breakage of the quinazoline ring, forming a metal Schiff-base complex with metal-containing six-membered ring.¹⁰ At the same time a large spectral shift as well as great fluorescence-enhancement were observed.¹¹ The chelation-enhanced fluorescence (CHEF) effect makes the quinazoline-based compounds to be excellent probes for metal ions. As is known, Schiff-base compounds are good platform for detection of Al³⁺, Cr³⁺ and Fe³⁺ ions.¹² Al³⁺, Cr³⁺ and Fe³⁺ possess smaller radius and larger positive charge, they as strong Lewis acids have high affinity for oxygen atom and tend to form M–O bond, resulting in the metal(iii) complexes when combined with oxygen-containing ligands like metal–furyl complexes.¹³ Herein, we report a quinazoline-based compound as chemosensor for Al³⁺, Cr³⁺ and Fe³⁺ ions obtained by coupling 2-(2-aminophenyl)-1H-benzimidazole with furfural. And the response mechanism was discussed.

Experimental section

Materials and method

All solvents and reagents were used as received (analytical grade and spectroscopic grade). The solutions of metal ions were prepared from LiCl, NaCl, CaCl₂, KNO₃, MgCl₂·6H₂O, CrCl₃·6H₂O, Mn(ClO₄)₂·6H₂O, Fe(NO₃)₃·9H₂O, Co(ClO₄)₂·6H₂O, Ni(ClO₄)₂·6H₂O, Cu(ClO₄)₂·6H₂O, Cd(ClO₄)₂·6H₂O, Zn(ClO₄)₂·6H₂O, Al(ClO₄)₃·9H₂O and Pb(ClO₄)₂·3H₂O, respectively. Fluorescence spectra were recorded on a Cary Eclipse fluorescence spectrophotometer and UV-vis absorption spectra were recorded by a spectrophotometer UV-2450, with a quartz cuvette (path length = 1 cm). ¹H NMR was obtained using a Bruker Avance III 400 MHz spectrometer. Mass spectra (ESI-MS) were obtained on Quattro microtriple quadrupole mass spectrometer. PH was measured on PHS-3C PH meter.

X-ray crystallography

Single-crystal data were collected on a Bruker APEX IICCD diffractometer with graphite monochromated Mo-Kα radiation (λ) at 293 K. The structure was solved by the direct method and refined by full matrix least squares based on F² using the SHELX 97 program.¹⁴ All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions. Crystallographic data for Cd(II) complexes (1 and 2) are summarized in Table 1. Selected bond distances and angles of two structures are summarized in Table S1 (ESI).†

Table 1 Crystal data and structure refinement parameters of 1 and 2^a

Compounds	1	2
a R1 = ∑(|Fo| − |Fc|)/|Fo|; wR2 = {∑[(w|Fo^2| − |Fc^2|)^2/∑w(Fo^2)^2]}^1/2.
Empirical formula	C40H32CdN6O6	C36H26CdCl2N6O2
Formula weight	805.11	757.94
Crystal system	Monoclinic	Monoclinic
Space group	C2/c	C2/c
a (Å)	18.697(3)	18.732(7)
b (Å)	10.5490(18)	9.935(4)
c (Å)	18.017(3)	17.851(7)
α (°)	90	90
β (°)	110.149(3)	103.003(7)
γ (°)	90	90
V (Å^3)	3336.2(10)	3237(2)
Z	4	4
Dc (g cm^−3)	1.603	1.555
μ (mm^−1)	0.716	0.883
F (000)	1640	1528
θ range (°)	2.25 to 27.52	2.33 to 27.60
Rint	0.0318	0.0617
R1, wR2[I > 2σ(I)]	0.0448, 0.1234	0.0465, 0.1098
R1, wR2 (all data)	0.0512, 0.1272	0.0805, 0.1399

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Preparation of L

A mixture of furfural (44 μL, 0.4 mmol) and 2-(2-aminophenyl)benzimidazole (0.0837 g, 0.4 mmol) in 2 mL isopropanol was sealed in 25 mL Teflon-lined autoclave and heated at 80 °C for 1 day and then the resulting solution was kept at room temperature for 4–5 days. The reddish brown block-shaped crystals were obtained and filtered, washed with isopropanol several times and dried at room temperature. Brown block-shaped crystals of L were collected in a yield of 43.7%. C₁₈H₁₃N₃O, anal. found (%): C, 75.40; H, 4.46; N, 14.72; calc. (%): C, 75.25; H, 4.56; N, 14.63; IR (KBr pellet, cm⁻¹): 3261 (s), 1616 (s), 1585 (s), 1512 (s), 1474 (s), 1450 (s), 1396 (s), 1332 (s), 1263 (s), 1148 (s), 758 (s), 735 (s).

Preparation of [CdL₂(OAc)₂] (1)

A mixture of cadmium acetate dihydrate (0.0533 g, 0.2 mmol), furfural (44 μL, 0.4 mmol) and 2-(2-aminophenyl)benzimidazole (0.0837 g, 0.4 mmol) in 5 mL methanol and isopropanol (v(methanol)/v(isopropanol) = 4 : 1) solution, was sealed in 25 mL Teflon-lined autoclave and heated at 80 °C for 1 day and then the resulting solution was evaporated at room temperature for 4–5 days. The brown block-shaped crystals were obtained and filtered, washed with methanol several times and dried at room temperature in a yield of 26.1%. C₄₀H₃₂CdN₆O₆, anal. found (%): C, 59.52; H, 4.18; N, 10.43; calc. (%): C, 59.67; H, 4.00; N, 10.44; IR (KBr pellet, cm⁻¹): 3264 (s), 1622 (s), 1551 (s), 1514 (s), 1458 (s), 1412 (s), 1383 (w), 1267 (s), 1152 (s), 1013 (s), 735 (s), 669 (s).

Preparation of 1′

A mixture of cadmium acetate dihydrate (0.0533 g, 0.2 mmol) and L (0.1148 g, 0.4 mmol) in 5 mL methanol and isopropanol (v(methanol)/v(isopropanol) = 4 : 1) solution, was sealed in 25 mL Teflon-lined autoclave and heated at 80 °C for 1 day and then the resulting solution was evaporated at room temperature for 4–5 days. The brown block-shaped crystals were obtained and filtered, washed with methanol several times and dried at room temperature in a yield of 28.7%. C₄₀H₃₂CdN₆O₆, anal. found (%): C, 59.31; H, 4.08; N, 10.43; calc. (%): C, 59.67; H, 4.00; N, 10.44; IR (KBr pellet, cm⁻¹): 3264 (s), 1622 (s), 1553 (s), 1516 (s), 1456 (s), 1414 (s), 1383 (s), 1267 (s), 1152 (s), 1013 (s), 735 (s), 669 (s).

Preparation of [CdL₂Cl₂] (2)

A mixture of cadmium chloride hemipentahydrate (0.0456 g, 0.2 mmol), furfural (44 μL, 0.4 mmol) and 2-(2-aminophenyl) benzimidazole (0.0837 g, 0.4 mmol) in 5 mL methanol and isopropanol (v(methanol)/v(isopropanol) = 4 : 1) solution, was sealed in 25 mL Teflon-lined autoclave and heated at 80 °C for 1 day and then the resulting solution was evaporated at room temperature for 4–5 days. The brown block-shaped crystals were obtained and filtered, washed with methanol several times and dried at room temperature in a yield of 38.8%. C₃₆H₂₆CdCl₂N₆O₂, anal. found (%): C, 57.07; H, 3.48; N, 11.08; calc. (%): C, 57.05; H, 3.46; N, 11.09; IR (KBr pellet, cm⁻¹): 3325 (s), 1618 (s), 1585 (s), 1530 (s), 1504 (s), 1479 (s), 1456 (s), 1412 (s), 1315 (s), 1263 (s), 1229 (s), 1146 (s), 1013 (s), 762 (s), 748 (s), 733 (s).

Results and discussion

Crystal structure of L

The quinazoline derivative, L was prepared by condensation of furfural and 2-(2-aminophenyl)-1H-benzimidazole under solvothermal condition. The crystal structure (Fig. S1†) is the same as reported.¹⁵ The selected bond distances and angles were tabulated in Table S1.†

Photophysical property of L

Since L is insoluble in H₂O while soluble in CH₃OH, its fluorescence property in CH₃OH was observed. An emission band at 416 nm for L in CH₃OH could be seen (Fig. S2†). It is attributed to π–π* transition of L. Fig. S2† also showed emission changes of L upon addition of HCl in CH₃OH. A double band at 416 nm and 462 nm appeared when c_HCl : c_L = 2 : 1. Further addition of HCl to CH₃OH solution of L made the emission band at 416 nm for L completely disappear. This could be ascribed to formation of a new compound. But the emission band of L remained unchanged when NaOH was added to L in CH₃OH.

Fluorescence responses of L to metal ions in CH₃OH

The fluorescence responses of L to metal cations were examined in CH₃OH. The addition of 1.0 equiv. of Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Fe³⁺, Co²⁺, Cr³⁺, Cd²⁺, Cu²⁺, Pb²⁺, Ni²⁺, Zn²⁺ and Al³⁺ ions to L in CH₃OH was used to measure the selectivity of L for metal ions. As shown in Fig. 1, when excited by 383 nm, L showed weak fluorescence emission at 416 nm in CH₃OH. While upon the addition of Al³⁺, Cr³⁺ or Fe³⁺ ions, an intensive fluorescence emission at 471 nm with a larger peak shift (55 nm) can be observed from the fluorescence spectra. For other metal ions except Al³⁺, Cr³⁺ and Fe³⁺, no obvious fluorescence change could be observed upon their addition to the methanol solution of L. The shifting of emission peak for the response system makes L more sensitive to the metal ions.

Fig. 1 Fluorescence spectra of L (30 μM) with 1.0 equiv. of Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Li⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺ and Zn²⁺ in CH₃OH.

To further test the selectivity of L as a fluorescent chemosensor for Al³⁺, Cr³⁺ or Fe³⁺ ions, the competitive experiments to measure the fluorescence intensity of L at 471 nm with Al³⁺ (Fig. 2), Cr³⁺ (Fig. S4(a)†) or Fe³⁺ (Fig. S4(b)†) ions in the presence of various metal ions in CH₃OH were carried out. As a result, there is no or little interference for the fluorescence detection of Al³⁺, Cr³⁺ or Fe³⁺ ions in presence of other metal ions, so the binding of the sensor with Al³⁺, Cr³⁺ or Fe³⁺ ions is not affected by concomitant ions. Therefore, L was shown to be a promising selective fluorescent sensor for Al³⁺, Cr³⁺ and Fe³⁺ ions in the presence of the competing metal ions stated.

Fig. 2 Fluorescence intensity of L with Al³⁺ in the presence of various metal ions in CH₃OH. λ_ex = 383 nm and λ_em = 471 nm. Red bars: L (30 μM) with 1.0 equiv. of Al³⁺ and 1.0 equiv. of other metal ions stated. Yellow bars: L (30 μM) with 1.0 equiv. of other metal ions stated.

The Job's plot for Al³⁺ system was obtained as shown in Fig. 3 (Fig. S5† for Cr³⁺ and Fe³⁺). 1 : 1 stoichiometric complexation of L with Al³⁺, Cr³⁺ and Fe³⁺ in CH₃OH was confirmed, respectively. In the fluorescence titration profiles (Fig. 4 and S6†), an increase of fluorescence intensity at 471 nm could be observed with increasing Al³⁺, Cr³⁺ and Fe³⁺ concentration until 1.0 equiv. while the emission intensity tends to be the same with further increasing Al³⁺, Cr³⁺ and Fe³⁺ concentration, respectively. The saturation behaviors of the fluorescence intensity after 1.0 equiv. of Al³⁺, Cr³⁺ or Fe³⁺ also reveal the 1 : 1 stoichiometry.

Fig. 3 Job's plot for the determination of the stoichiometry of L and Al³⁺ in the complexation.

Fig. 4 (a) Fluorescence emission spectra (λ_ex = 383 nm) of L (33 μM) in the presence of increasing amounts of Al³⁺ (0.30, 0.40, 0.50, 0.55… 0.80, 0.85, 0.90, 0.95, 1.00, 1.20, 1.40, 1.60, 1.80 equiv.) in CH₃OH. (b) Spectrofluorimetric titration curve at λ_em = 471 nm.

Based on the fluorescence titration data, the association constant K of L for Al³⁺, Cr³⁺ or Fe³⁺ was calculated by the Benesi–Hildebrand expression. The association constants were determined to be 1.02 × 10⁴, 1.44 × 10⁴ and 3.04 × 10⁴ for the complex of L with Al³⁺, Cr³⁺ and Fe³⁺ in CH₃OH, respectively as shown in Fig. 5(a), S7(a) and (c).† The detection limits of L as a fluorescent chemosensor for the analysis of Al³⁺, Cr³⁺ and Fe³⁺ were calculated and these were found to be 12, 15 and 11 μM, respectively (Fig. 5(b), S7(b) and (d)†).

Fig. 5 (a) Benesi–Hildebrand plot of L with Al³⁺ in CH₃OH (λ_em = 471 nm). (b) Normalized response of emission signal at 471 nm with changing Al³⁺ concentrations.

Considering Al³⁺, Cr³⁺ and Fe³⁺ have the same fluorescence response to L in methanol when excited with 383 nm, some strategies were adopted to distinguish them by L. As shown in Fig. 6, the response of L to Cr³⁺ is time-dependent in 20 minutes upon the addition of Cr³⁺. While the response of L to Al³⁺ and Fe³⁺ was quick (Fig. S8†). This may be attributed to slow hydrolysis rate for Cr(III) ion, forming Cr(III) species containing L¹ and OH⁻. Therefore, Cr³⁺ could be differentiated from Al³⁺ and Fe³⁺ by time-dependent emission spectra. As to Al³⁺ and Fe³⁺, it is found that their emission spectra are different when excited by 300 nm in the aqueous media CH₃OH–H₂O (1/9, v/v). As is shown in Fig. 7, the Al³⁺ system emits at 430 nm, while Fe³⁺ system emits at 471 nm. So they can be distinguished from each other. The blue-shifted fluorescence for Al³⁺ could be ascribed to formation of [AlL¹(OH)₃], which is confirmed by ESI-MS (Fig. S9†). In a word, the results showed that our work is comparable with the reported (Table S2†).

Fig. 6 Fluorescence spectra variation of L (67 μM) upon addition of 1.0 equiv. of Cr³⁺ in CH₃OH after 1, 2, 3, 4, 5, 10, 15, and 20 min, λ_ex = 383 nm.

Fig. 7 Fluorescence spectra of L (100 μM) upon addition of 1.0 equiv. of Al³⁺ and Fe³⁺ in the aqueous media CH₃OH–H₂O (1/9, v/v), λ_ex = 300 nm.

Binding modes of L with Al³⁺, Cr³⁺ or Fe³⁺ and possible fluorescent species formed in CH₃OH

As shown in Fig. 8(a), proton signals of L were assigned similarly as the reported.¹⁵ The binding modes of L with Al³⁺ was studied by ¹H NMR titration, shown in Fig. 8(a–d). Upon complexation of L with Al³⁺ (1 : 1), the proton signal (H₉) of L downfield shifted from 7.03 to 7.51 (Δδ = 0.48) due to HC (the quinazolie ring carbon)–N (the imidazole ring nitrogen) bond cleavage upon coordination of the two nitrogen atoms with Al³⁺ ion. Accordingly, benzimidazole protons and phenyl protons except H₅ shifted to low magnetic field because of the reduction of electron density upon coordination to the Al³⁺. Also, furyl proton signals (H₁₀–H₁₂) shifted downfield after binding of furyl oxygen atom from L with Al³⁺, Δδ = 0.15–0.31 ppm. Similarly, upon complexation of L with Cr³⁺ in CH₃OH-d₆, the proton peak (H₉) downfield shifted from 7.03 to 7.46 ppm (Fig. S10†). The ¹H NMR signals of the quinazoline ring protons, benzimidazole ring protons and furyl ring protons of L showed changes for the both systems. The results suggest that Al³⁺ and Cr³⁺ are coordinated with the aldimine nitrogen atom, imidazole nitrogen atom and the furyl oxygen atom of L, resulting from C–N bond cleavage of the quinazole ring to form a Schiff base [2-(1H-benzoimidazol-2-yl)-phenyl]-furan-2-ylmethyleneamine (L¹) and M³⁺–L¹ complexes were obtained.^10,11c In addition, there were no significant changes for the proton signals upon the addition of 1.5 equiv. of Al³⁺ (Fig. 8(c) and (d)) and Cr³⁺ (Fig. S10(c) and (d)†) to L. This also confirmed the 1 : 1 stoichiometry for L to Al³⁺ and Cr³⁺, respectively. We tried to observe ¹H NMR spectrum of L with Fe³⁺, we failed to get satisfactory result because of its paramagnetic nature.

Fig. 8 ¹H NMR spectra in CH₃OH-d₆: (a) L only; (b) L and 0.5 equiv. of Al³⁺; (c) L and 1.0 equiv. of Al³⁺; (d) L and 1.5 equiv. of Al³⁺.

To further study the coordination of L with Al³⁺, Cr³⁺ and Fe³⁺, their ESI-MS spectra were recorded. As shown in Fig. 9, the mass spectrum of L upon addition of 1.0 equiv. of Al³⁺ exhibited an intense peak at m/z 366.1, corresponding to the ion [AlL¹(OH)₂(H₂O)]⁺ (calcd m/z 366.3). This could be attributed to the fact that –OH and H₂O can supply a hard-base environment for the hard-acid Al³⁺, which makes it easier to form the Al³⁺ complex.¹⁶ Also, in the literature [Al(OH)₂H₂O]⁺ could be found.¹⁷ In Fig. S11(a),† the peak at m/z 397.9 corresponds to the ion [CrL¹(OH)₃]Li⁺ (calcd m/z 397.3) and the peak at m/z 416.8 corresponds to the ion [FeL¹(OH)₃]Na⁺ (calcd m/z 417.2) (Fig. S8(b)†). From Job plot, ¹H NMR data and ESI-MS results, it can be concluded that the Al³⁺, Cr³⁺ and Fe³⁺ may form [AlL¹(OH)₂(H₂O)]⁺, CrL¹(OH)₃ and FeL¹(OH)₃ in CH₃OH, respectively.

Fig. 9 Positive-ion electrospray ionization mass spectra of L upon addition of 1.0 equiv. Al³⁺ in CH₃OH.

To investigate the structure of the species formed in the response system, we tried to cultivate the crystals of the Al³⁺, Cr³⁺ and Fe³⁺ complexes with L, but failed. However, two Cd complexes were obtained by one-pot reaction of furfural, 2-(2-aminophenyl)benzimidazole with cadmium acetate (1) and chloride (2), respectively. The crystal structures of the Cd(II) complexes 1 and 2 are shown in Fig. 10. In 1, Cd(II) ion is six-coordinated by four acetate oxygen atoms and two imidazole nitrogen atoms from two L ligands. The polycrystals sample of 1′ was obtained by the solvothermal reaction of cadmium acetate with L, which is confirmed to be the same phase as 1 by elemental analysis, IR and powder X-ray diffraction (Fig. S12†). In 2, Cd(II) ion is four-coordinated by two chloride anions and two imidazole nitrogen atoms from two L ligands. L in 1 and 2 is monodentate and the binding site of L with Cd(II) is not located at the quinazoline ring of L. The UV-vis spectra, fluorescence spectra and ¹H NMR of L and Cd(II)-species are shown in Fig. S13–15.† No significant difference between L and its Cd(II) species in the corresponding spectra was found. This shows that the coordination of L with Cd(II) has a little effect on the intramolecular π to π* transition and electric delocalization, and thus L for Cd(II) is insensitive.

Fig. 10 The crystal structures of the Cd(II) complexes. (a) [CdL₂(OAc)₂]; (b) [CdL₂Cl₂].

As is known, the ionic radii of Al³⁺, Cr³⁺, Fe³⁺ and Cd²⁺ are 67.5, 75, 78.5 and 109 pm, respectively with a coordination number of 6. For Cd²⁺ it is even 92 pm when coordination number falls to 4,¹⁸ which is much larger than that of Al³⁺, Cr³⁺ and Fe³⁺. According to the hard–soft acid–base theory, Cd²⁺ is soft acid, while Al³⁺, Cr³⁺, Fe³⁺ are hard acids. Thus Cd²⁺ as a soft acid has high affinity to nitrogen atom. Considering the steric effect and its large ionic radius, Cd²⁺ is apt to coordinate with the nitrogen atom at 3-position of imidazole section. While the Al³⁺, Cr³⁺, Fe³⁺ ions as hard acids have high affinity to oxygen atom. Their small radii make the furan oxygen atom and aldimine nitrogen atom available for them. Consequently the coordination of L with Al³⁺, Cr³⁺ and Fe³⁺ leads to the breakage of C–N bond, forming a Schiff base complex.

Conclusions

We synthesized a quinazoline derivative (L) and two Cd(II) complexes (1 and 2). Their structures have been determined by single-crystal X-ray diffraction. The interaction site of L with Cd(II) is one imidazole nitrogen atom from L and thus L is monodentate in 1 and 2. Spectroscopic determination showed that L for Cd(II) is insensitive. But L is employed as a highly sensitive and selective sensor for Al³⁺, Cr³⁺ and Fe³⁺ ions over a number of metal ions in CH₃OH. The Al³⁺, Cr³⁺ and Fe³⁺ detections have been demonstrated by absorption, fluorescence, ¹H NMR and ESI-MS spectral studies. The 1 : 1 binding stoichiometry of Al³⁺, Cr³⁺ and Fe³⁺ with L was confirmed by Job's plot, fluorescence titration and ESI-MS data, respectively. The interaction sites of L for Al³⁺ and Cr³⁺ have been suggested by spectral changes and the transformation from L to L¹ in the presence of Al³⁺, Cr³⁺ and Fe³⁺ ions was explored. The CHEF effect may be attributed to the formation of M³⁺-containing five- and six-membered rings via two nitrogen atoms and one oxygen atom from L¹ by increase of the donor–acceptor electric delocalization after complexation. It shows that nitrogen- and oxygen-containing quinazoline-base compounds can be served as a platform to explore chemosensors for multi-metal ions.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (20971015) and the Fundamental Research Funds for the Central Universities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Information on X-ray crystallographic data in CIF format, Fig. S1–S12, Table S1. CCDC 1041496 and 1041497. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra04841a

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