Substituted 2-Aminobenzothiazoles Salicylidenes Synthesis and Characterization as Cyanide Sensors in Aqueous Medium

(E)-2-((benzo[d]thiazol-2-ylimino)methyl)-4-nitrophenol 1 and (E)-2-(((6-methoxybenzo[d]thiazol-2-yl)imino)methyl)-4-nitrophenol 2 were synthesized efficiently under microwave conditions. The structures were confirmed using IR, 1H NMR, and 13C NMR. UV-vis. Fluorescence investigations demonstrated that 1 and 2 are sensitive and selective sensors for detection of cyanide over all other anions SCN−, AcO−, N3−, H2PO4−, H2AsO4−, F−, Cl−, Br−, and I− in aqueous media. Cyanide induces colorimetric change from pale yellow to dark yellow and from transparent to pale yellow for 1 and 2, respectively. It enhances the absorption at wavelengths 385 nm and 425 nm of 1 and 385 nm and 435 nm of 2. Acidic anions H2PO4− and H2AsO4− displayed significant interference with the interaction of cyanide and sensors 1 and 2. Sensor 1 has lower detection limit (LDL) 1 × 10−6 M, while 2 has LDL 1.35 × 10−6 M.


Introduction
Cyanide detection is very crucial since it might cause death if inhaled or digested. It causes intracellular hypoxia by binding to ferric ion in cytochrome oxidase a 3 within the mitochondria [1]. Sources of cyanide contamination are wide. Artificial sources of cyanide are numerous such as industrial waste from polyacrylonitrile manufacturing, use in metallurgy, gold mining, and cyanide fishing. Natural sources of cyanide contamination are forest fires and food products such as cassava, seeds of bitter almonds, apples, and apricots in the form of cyanogenic glycosides [2]. The allowed level of cyanide in drinking water according to World Health Organization (WHO) should not exceed 2 µg/L [3].
Anion's sensing has gained scientists interest because of its importance in a wide variety of applications. Recently, many reports published on the rational design of chemical sensors that are able to detect cyanide selectively at micromolar level based on nucleophilic addition [4]. A chemical sensor should have a binding site that is connected covalently to a signalling unit (chromophore) that responds to the interaction with the anion giving a signal. The signal may be due to the electronic changes in the transition in π → π * bonds giving rise to absorption and emission spectral changes. Electronic structure change induced signal transduction takes place upon binding of cyanide to the binding site which is part of conjugated system of the signalling unit [5].
The 1 H and 13 C NMR spectra were recorded on a Bruker AVANCE-400 spectrometer (Bruker BioSpin, Billerica, MA) operating at 400 and 101 MHz respectively using DMSO-d 6 as a solvent. IR spectra were obtained using PerkinElmer FT-IR spectrometer (PerkinElmer, Waltham, MA, USA). UV-vis spectra were carried out using Agilent 8453 machine (Agilent, Santa Clara, CA, USA) in 1.0 cm quartz cuvette. Fluorescence peaks were carried out on PerkinElmer LS 45 Fluorescence spectrometer using 1.0 cm quartz cuvette. Scan speed was 700 nm min −1 , excitation slit was 10 nm, and emission slit was 10 nm. The microwave reactions were carried out in a biotage initiator 8 instrument (Biotage, Uppsala, Sweden). The temperature and time were pre-set as required. The pressure was monitored and indicated.

Results and Discussion
Sensors 1 and 2 were synthesized efficiently under microwave conditions in 5 min compared to the long conventional reflux processes that took 1 h [4] or 2 h [36]. Their chemical structures were confirmed using IR, 1 H NMR, and 13 C NMR. The presence of the p-nitro group increases the aldehyde reactivity that will facilitate the condensation reaction. The imines were obtained in good yields.
Absorption behavior of 1 and 2 is greatly influenced by the solvent. The solvent effect was investigated by increasing the polarity of the solvent. In CH 3 CN, sensor 1 has an absorption peak at 360 nm, while sensor 2 has an absorption peak at 385 nm. Escalating water ratio from 0% to 90% displays that absorption of both sensors in acetonitrile is affected upon addition of water. Figure 1a,b show that increasing water ratio resulted in increased absorption of the new peak of 1 at wavelength 450 nm until it reaches the maximum absorption at 70% water. Increasing the water ratio beyond 70% resulted in absorption decrease. Such reduction in the absorption may be due to the reduction in the solubility of the imine in the mixed solvents. Figure 1c,d show that absorption change of the peak at 450 nm of 1 and 2. First, the peak is increased upon increasing water ratio to 50%, where dissociation increases with polar protic solvent increase. Further addition of water led to absorption decrease due to the decrease in the solubility.

Results and Discussion
Sensors 1 and 2 were synthesized efficiently under microwave conditions in 5 min compared to the long conventional reflux processes that took 1 h [4] or 2 h [36]. Their chemical structures were confirmed using IR, 1 H NMR, and 13 C NMR. The presence of the p-nitro group increases the aldehyde reactivity that will facilitate the condensation reaction. The imines were obtained in good yields.
Absorption behavior of 1 and 2 is greatly influenced by the solvent. The solvent effect was investigated by increasing the polarity of the solvent. In CH3CN, sensor 1 has an absorption peak at 360 nm, while sensor 2 has an absorption peak at 385 nm. Escalating water ratio from 0% to 90% displays that absorption of both sensors in acetonitrile is affected upon addition of water. Figure 1a,b show that increasing water ratio resulted in increased absorption of the new peak of 1 at wavelength 450 nm until it reaches the maximum absorption at 70% water. Increasing the water ratio beyond 70% resulted in absorption decrease. Such reduction in the absorption may be due to the reduction in the solubility of the imine in the mixed solvents. Figure 1c,d show that absorption change of the peak at 450 nm of 1 and 2. First, the peak is increased upon increasing water ratio to 50%, where dissociation increases with polar protic solvent increase. Further addition of water led to absorption decrease due to the decrease in the solubility.
Sensors 1 and 2 exist in equilibrium between protonated and deprotonated forms in polar solvents, Scheme 1b. As water ratio increases in the solvent, the solubility of the sensors is enhanced and equilibrium is directed towards the deprotonated form. Sensor 2 is less soluble in polar solvents than 1 due to the presence of methoxy group. As deprotonation increases upon escalating the ratio of water, the absorption of the new peak at 450 nm increases until it reaches the maximum absorption at 70% for 1 and 50% water for 2. Further increase of water ratio lowered the solubility of the sensors and directed the equilibrium towards the protonated form resulting in absorption decrease. Sensors 1 and 2 exist in equilibrium between protonated and deprotonated forms in polar solvents, Scheme 1b. As water ratio increases in the solvent, the solubility of the sensors is enhanced and equilibrium is directed towards the deprotonated form. Sensor 2 is less soluble in polar solvents than 1 due to the presence of methoxy group. As deprotonation increases upon escalating the ratio of water, Selectivity of the sensors towards different anions is based on the nature of the anions. Cyanide is more nucleophilic than other reported anions and has less hydrogen bonding ability [36], so that the imine carbon of 1 and 2 is more susceptible to cyanide addition. Other anions have larger solvation effect with protic solvents than cyanide [17] and form hydrogen bonds in H 2 O/CH 3  The proposed mechanism of the interaction of 1 and 2 with anions was previously reported [36]. Intramolecular proton transfer takes place resulting in the keto-amine form via two pathways; (i) addition of cyanide to the electron-deficient carbon atom of the imine group and (ii) deprotonation of phenolic hydrogen by basic anions, Scheme 2. Altering the solvent ratio from H2O/CH3CN from 90:10 to become 10:90 changed the previous results. Basic anions such as AcO − , N3 − enhanced absorption intensity at 388 nm and 445 nm of 1 and 455 nm of 2, Figure 3. These results indicated that sensors 1 and 2 lose their selectivity for cyanide in less polar solvents. Selectivity of the sensors towards different anions is based on the nature of the anions. Cyanide is more nucleophilic than other reported anions and has less hydrogen bonding ability [36], so that the imine carbon of 1 and 2 is more susceptible to cyanide addition. Other anions have larger solvation effect with protic solvents than cyanide [17] and form hydrogen bonds in H2O/CH3CN 90:10. Sensors 1 and 2 have the electron-withdrawing nitro group in p-position rendering the phenolic group very acidic and liable for deprotonation by basic anions such as AcO − and N3 − in less polar solvent H2O/CH3CN 10:90.
The proposed mechanism of the interaction of 1 and 2 with anions was previously reported [36]. Intramolecular proton transfer takes place resulting in the keto-amine form via two pathways; (i) addition of cyanide to the electron-deficient carbon atom of the imine group and (ii) deprotonation of phenolic hydrogen by basic anions, Scheme 2. Sensors 1 and 2 have λ max < 400 nm. The absorption at λ max > 400 nm is enhanced upon addition of cyanide. The λ max < 400 nm is attributed to the enol form and the λ max > 400 nm is attributed to the keto-amine form [37,38]. Scheme 2 displays the conversion of phenol-imine form into keto-amine form with intramolecular proton transfer.
Investigation   Figure 5a,c with cyanide concentration. Benesi-Hildebrand plots are presented for each sensor (Figure 5b,d), from which Ka values were calculated. Moreover, lower detection limits (LDL) and isosbestic points are shown in Table 1.
Job plots of 1 and 2 displayed in Figure 6 indicating 1:1 complex from the sensors with CN − . The calculated LDL of 1 is much lower than the allowed level of cyanide in drinking water stated by World Health Organization (WHO), which should not exceed 2 µg/L [3].  Figure 5a,c with cyanide concentration. Benesi-Hildebrand plots are presented for each sensor (Figure 5b,d), from which Ka values were calculated. Moreover, lower detection limits (LDL) and isosbestic points are shown in Table 1.
Job plots of 1 and 2 displayed in Figure 6 indicating 1:1 complex from the sensors with CN − . The calculated LDL of 1 is much lower than the allowed level of cyanide in drinking water stated by World Health Organization (WHO), which should not exceed 2 µg/L [3].    Interference of H2PO4 − and H2AsO4 − with sensors 1 and 2 reactivity towards CN − is shown in Figure 8a,b. Remarkably, H2PO4 − and H2AsO4 − quenched the sensors reactivity towards cyanide. Formation of the protonated form of the sensors inhibited the ICT, thus re-induced the fluorescence intensity. Bar graphs of the interference of reported anions with sensors 1 and 2 reactivity towards cyanide are well displayed in Figure 8c,d. Intramolecular charge transfer (ICT) plays a very important role in the fluorescence behaviors of 1 and 2 since reduced ICT results in fluorescence intensity enhancement, whereas efficient ICT quenches the fluorescence intensity [39]. Addition of cyanide to the electron deficient imine carbon of the sensors resulted in the formation of the keto form which induced the ICT throughout the molecules leading to fluorescence intensity reduction of 1 and 2. The presence of the electron-withdrawing, nitro group in 1 enhanced the acidity of the phenolic group enabling deprotonation by basic anions AcO − and N 3 − .

Conclusions
Sensors 1 and 2 were synthesized efficiently and evaluated for their interaction with a wide range of anions. They showed selectivity towards cyanide in H 2 O/CH 3