1. IntroductionBecause of structural flexibility and their application for switching materials and so on, Schiff base compounds are one of the most extensively used ligands in the field of coordination chemistry [1]. During the past decades, Schiff bases have been intensively studied due to their strong coordination capability as well as their diverse biological activities, such as antibacterial, antitumor, etc. [2]. Schiff bases have received much attention because of their potential applications with some of these compounds exhibiting various pharmacological activities, as noted by their anticancer [3], anti-HIV [4], antibacterial and antifungal properties. In addition, some of them may be used as analytical reagents for the determination of trace elements [5], Schiff-bases can readily form stable complexes with most transition metals, in which some may exhibit interesting properties, we have determined the crystal structure of the title compound.
In previous work, we have reported the synthesis and developed their function as selective fluorescent chemsensors for Cu2+ [6]. In continuing our research to develop new fluorescent chemsensors based on Schiff base, we have designed a new molecule of 1 which have big p-systems that can be used as the signaling subunit in their two sidewalls and two oxygen atoms of ring and two C=N atoms in their structure be used as the potential binding sites. We now report their synthesis and a binding study with different metal ions.
2. Experimental DetailsPreparation of 1 [6-8]: To a solution of 2-oxo-2H-chromene-3-carbohydrazide (0.3 g, 1.5 mmol) in ethanol (20 ml) was added 2-hydroxybenzaldehyde (0.3 ml, 3mmol). The reaction mixture was stirred for about 5 h. Subsequently, it was cooled to room temperature. The resultant orange solution was filtered. Yellow precipitate obtained was dissolved in ethanol (1.5 g, 31.8%). The synthesis of the shciff base molecules 1 is re presented in the following equation :
Yellow crystals suitable for XRD formed after a few days of slow evaporation of the solvent at room temperature over several days. Yellow single crystals of the title compound is shown in Figure 1. The crystal structures of 1 clearly revealed that it has well-defined geometry due to the rigidity that the fused rings confer on the molecule.
CharacterizationAll reagents obtained from commercial sources were of analyzed grade. Melting points were determined with XT4A micromelting point apparatus and were uncorrected. The 1H NMR was recorded on a Mercury Plus-400
spectrometer with TMS as internal reference and CDCl3 as solvent. IR were recorded on a Perkin-Elmer PE-983 IR spectrometer as KBr pellets with absorption in cm−1. MS were obtained with Finnigan Trace MS instrument using EI method. Elemental analyses were carried out on a Vario EL III instrument. Fluorescence spectra were determined on a Hitachi F-4500.
3. Results and Discussion3.1. Single Crystal X-Ray Diffraction AnalysisSingle crystal X-ray diffraction studies were carried out on the grown crystals. The X-ray date were collected using X-ray diffractometer (Model: Bruker Smart APEXCCD). The observed results indicate that the crystal belongs to triclinic crystal system and the determined unit cell parameters are a = 17.505 (5) Å, b = 6.3135 (18) Åc = 16.578 (5) Å, α = 90˚, β = 97.255 (5)˚, γ = 90˚ and V = 1817.5 (9) Å3.
3.2. The Structure CharacterizedCompound of 1. M.p. 145.2˚C - 147.8˚C (dec.). (KBr, cm−1): 3620, 1645, 1600, 1503, 1445. 1H NMR d: 5.2 (s, 2H, OH), 6.8 (d, 4H, PhH), 7.2 (q, 2H, PhH), 7.6 (d, 2H, PhH), 8.8 (s, 2H, CO2CH2CH3), 7.26 - 7.45 (m, 4H, ArH). MS (EI): m/z = 241 [M + H] + Anal. Calcd for C14H12N2O2 (240.26): C, 69.99; H, 5.03; N, 11.66; O, 13.32, Found: C, 69.67; H, 5.33; N, 11.57; O, 13.53.
3.3. Fluorescent SpectroscopyThe binding properties of the Schiff base with various metal ions were investigated by fluorescent spectroscopy titration experiments. Changes of the fluorescence properties of Schiff base (2 × 10−5 M in acetonitrile) solution caused by 15 equiv. of various metal ions (Co2+, Cr3+, Sn4+, Cu2+ , Sr2+ , Ag+, Ni2+, Pb2+, Fe3+) were measured until their emission intensity were constant.
The result showed that Cu2+ produced significant quenching in their fluorescent emission. The other metal ions that were tested only show a relatively insignificant change (Figure 2). It can be concluded that 1 has a higher selectivity for the recognition of Cu2+.
The sensitivity of the fluorescence emission response of 1 towards Cu2+ was also examined under the same conditions with various Cu2+ concentrations (Figure 3). The fluorescence intensity of 1 decreased continually upon addition of Cu2+. When the concentration of Cu2+ increased to 15 equiv, the fluorescence intensity of 1 was
reduced to 80% of the initial value. From a Stern-Volmer plot (Figure 3), the quenching constants were estimated 1.75 × 104 M−1.