Synthesis of 5-dialkyl(aryl)aminomethyl-8-hydroxyquinoline dansylates as selective fluorescent sensors for Fe 3

A series of 5-dialkyl(aryl)aminomethyl-8-hydroxyquinoline dansylates were synthesized and their fluoroionophoric properties toward representative alkali ions, alkaline earth ions and transition metal ions were investigated. Among the selected ions, Fe3+ caused considerable quenching of the fluorescence, while Cr3+ caused quenching to some extent. The absence of any significant fluorescence quenching effects of the other ions examined, especially Fe2+, renders these compounds highly useful Fe3+-selective fluorescent sensors.


Introduction
A variety of metal ions are known to play vital roles in the structural, catalytic and regulatory aspects of biological systems [1a], therefore, sensitive and selective detection of these metal ions is of great importance.A large number of fluorescent sensors have been reported in recent years [1b-f].
Iron is one of the most important trace elements in the human body [2a].Increased iron availability in serum or tissues is associated with an increased risk of several tumors and may promote carcinogenesis [2b].Moreover, hereditary hemochromatosis is characterized by excess iron that causes tissue damage and fibrosis with irreversible damage to various organs [2c].Iron homeostasis is an important factor involved in neuroinflammation and progression of Alzheimer's disease [2d].
Compared to the number of fluorescent sensors for other transition metal ions, such as Hg 2+ [3] and Zn 2+ [4], Fe 3+ -specific fluorescent sensors are comparatively rare.
Regarding the Fe 3+ sensors reported up to date (Figure 1), their detection of Fe 3+ is always accompanied by interference from Cu 2+ and Cr 3+ , which affects their selectivity.For example, 4-amino-1,8-dicyanonaphthalene derivatives exhibit considerable fluorescence enhancement by both Fe 3+ and Cr 3+ , with a Fe 3+ :Cr 3+ amplification ratio of 1:0.84 [5a].Another naphthalene-derived fluorescent sensor showed a Fe 3+ :Cr 3+ fluorescence-quenching ratio of 1:0.83 [2a].A calix [4]arene-derived fluorescent sensor could selectively detect Fe 3+ and Cu 2+ [5b] with a Fe 3+ :Cu 2+ fluorescence-quenching ratio of 1:1.17, depending on the pH.The best sensor reported so far is a derivative of 1-oxa-4,10-dithia-7-aza-cyclododecane, which displayed significant Fe 3+ -amplified fluorescence with slight interference from Cu 2+ [5c].However, the test for its interaction with Cr 3+ was not undertaken.With an aim of developing more selective fluorescent sensors for Fe 3+ , we noticed that most fluorescent sensors are composed of metal-binding sites and a proximal fluorophore, and function via a photo-induced electron transfer (PET) mechanism [2a,3a,3j].Herein, we report the investigation of the 8-hydroxyquinoline dansylate structure as a fluorescent sensor.The dansyl group is a commonly used fluorophore that emits strong fluorescence [6], while 8-hydroxyquinoline is an excellent chelator of metal ions and has many significant applications [7].The sulfonic group in the dansylate group and the nitrogen in the quinoline could cooperatively chelate metal ions.In the predicted chelation process, concerted changes in the distances and angles between the two ligands are required.The structural limitations of such conformational changes could lead to ion selection.Therefore, a series of 8-hydroxyquinoline dansylate derivatives were synthesized and found to be highly sensitive and more selective for Fe 3+ .

Results and Discussion
The synthesis of the title compounds is shown in Scheme 1. First, 8-hydroxyquinoline (1) was chloromethylated to give crude 5-chloromethyl-8-hydroxyquinoline (2).High purity products were obtained by washing with concentrated hydrochloric acid (37%) instead of acetone [8].The residual 8-hydroxyquinoline was not removed completely by washing with acetone and a substitution reaction occurred during the crystallization in ethanol.Next, 2 was reacted with different secondary amines in dichloromethane at room temperature to give 3a-3d in high yields.Triethylamine was used as the base.The use of inorganic bases, such as sodium carbonate, sodium hydrogen carbonate and sodium hydroxide, led to either a very slow reaction rates or many unexpected byproducts.The final products, 4a-4d, were prepared by reaction of 3a-3d with dansyl chloride in tetrahydrofuran in good yield.4e was prepared by direct dansylation of 1.The structures were identified by 1 H-NMR, 13 C-NMR, elemental analysis and ESI-MS.

Scheme 1.
The absorption spectra of representative compounds 4c and 4e are shown in Figure 2. When excited at 360 nm, 4a-4e showed similar emission patterns.The maximum emission wavelengths were 466, 468, 465, 466 and 463 nm, respectively, for 4a-4e.Their fluoroionophoric properties toward representative alkali ions (K + ), alkaline earth ions (Mg 2+ , Ca 2+ , Ba 2+ ) and transition metal ions (Fe 3+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Ag + , Zn 2+ , Cd 2+ Pb 2+ , Mn 2+ , Cr 3+ ) were investigated in methanol solutions.The results (Table 1) revealed that only Fe 3+ and Cr 3+ caused considerable fluorescence quenching, similar to levels previously described in the literature [2a,5b].However, there were differences in the sensitivities of 4a-4e.The best result was obtained with 4e, which showed a quenching percentage (calculated by [(I 0 -I)/I 0 ] × 100%) of 91.2% for Fe 3+ .Compound 4d behaved in the same way as 4e, while 4a-4c behaved similarly, but showed slightly poorer selectivity.The fluorescence quenching data for sensors 4c and 4e are shown in Figure 3. Fe 3+ caused the highest level of fluorescence quenching, while Cr 3+ caused quenching to some extent.The other ions Wavelength(nm) Absorbance 1 eq.Fe 3+ showed little fluorescence quenching.However, the selectivity between Fe 3+ and Cr 3+ was improved comparative with previously reported Fe 3+ sensors.For example, the best quenching ratio was 1:0.58 (Fe 3+ :Cr 3+ ) for 4e, which is better than the previously reported ratio of 1:0.83 for a naphthalene-derived fluorescent sensor [5b].Furthermore, the absence of any significant fluorescence quenching effects by Cu 2+ , Mn 2+ , Co 2+ , Ni 2+ and especially Fe 2+ renders these derivatives highly useful Fe 3+ -selective sensors.In order to gain further insights into the analytical possibilities, fluorescence titration of 4e by Fe 3+ was performed and the obtained curves are shown in Figure 4.The quenching was very effective and a quenching efficiency of 91.2% was obtained at a ratio of 1:1.The break around 1.0 equiv of Fe 3+ suggested a 1:1 stoichiometry for the 4e-Fe 3+ complex system [3j].By working on the Stern-Volmer equation [9,3a], the association constant was calculated to be about 1.1×10 5 M -1 .Next, a competition experiment was carried out in the presence of 4.0 equiv of background metal ions in order to confirm the practical applicability of 4e as a Fe 3+ sensor (Figure 5).Most of the metal ions, including Cr 3+ , did not show any obvious interference with the detection of Fe 3+ , while Cu 2+ , Co 2+ , Zn 2+ and Mn 2+ showed only moderate interference.Therefore, 4e is a potential Fe 3+ -selective fluorescent chemosensor.In order to gain further insights into the interaction between Fe 3+ and designed sensors, related conformations were simulated using the MM2 method as implemented in Chem3D.The optimized conformation of 4e is shown in Figure 4e and the 4e-Fe 3+ complex in Figure 6.Among the changes, the most important would be the naphthyl conformation.It actually changed from a plane form to a twisted one.Therefore the conjugation system of dansyl moiety has been severely damaged and this may be the actual cause of the observed fluorescence quenching.

General
All solvents and reagents were purchased from Aldrich and used without further purification. 1H-NMR (300 MHz) and 13 C-NMR (75 MHz) spectra were recorded on a JOEL JNM-ECA300 spectrometer.Chemical shifts (δ) are reported in parts per million (ppm) relative to tetramethylsilane (TMS) as an internal standard, and coupling constants (J) are given in hertz (Hz).Elemental analyses were carried out on an Elementar Vario EL CHN element analysis instrument.All ESI-MS experiments were undertaken on a Bruker ESQUIRE-LC.Fluorescence intensity was measured using a HORABA Fluoromax 3.

General procedure for the preparation of 5-chloromethyl-8-hydroxyquinoline hydrogen chloride 2
A stream of hydrogen chloride gas was blown through a solution of 8-hydroxyquinoline 1 (0.1 mol) and formaldehyde (20 mL, 37%) in 37% hydrochloric acid (50 mL) for 8 hours at 50°C.After filtration, the product was washed with 37% hydrochloric acid and dried to afford 2 in 85% yield.

General procedure for the preparation of 5-dialkyl(aryl)aminomethyl-8-hydroxyquinoline 3a-3d
To a solution of 2 (10 mmol) and triethylamine (30 mmol) in dichloromethane (50 mL) was added dropwise a solution of the appropriate dialkyl(aryl)amine (10 mmol) in dichloromethane (50 mL) at room temperature over 30 minutes.After a further 15 minute of reaction, the solution was washed thoroughly with brine, dried over magnesium sulfate and concentrated to afford 3a-3d without further purification (yields are listed in Scheme 1).

General procedure for the preparation of 5-dialkylaminomethyl-8-hydroxyquinoline dansylate 4a-4e
To a solution of 3a-3d or 1 (1 mmol) in THF (10 mL) was added sodium hydroxide (5 mmol), followed by stirring for 5 minutes.Next, a solution of dansyl chloride (1 mmol) in THF (10 mL) was added dropwise at room temperature over 5 minutes.The reaction was kept at room temperature for a further 10 minutes and then filtered.The filtrate was evaporated under vacuum and the crude product was purified by silica gel column chromatography (EtOAc-Petroleum Ether).