An Efficient Novel Acetate Anion Receptor based on Isatin

Um receptor para reconhecimento de ânions derivado da tiouréia foi sintetizado combinandose indolina-2,3-diona e 1,3-diaminotiouréia. A interação pode ser facilmente monitorada acompanhando-se as mudanças no espectro de absorção na região do UV-vis induzidas pela complexação de um dado ânion. Por exemplo, verificou-se que o fluoreto liga-se ao receptor na proporção de 1:2, mas esta muda para 1:1 no caso dos ânions di-hidrogenofosfato e acetato. Este último apresenta a maior constante de afinidade dentre eles indicando um melhor reconhecimento molecular pelo novo receptor. Finalmente, a natureza da interação entre o novo receptor e o ânion acetato foram explorados utilizando-se espectroscopia de H-RMN.


Introduction
Anions play significant roles in life processes and in the environment, such that the development of new anion receptors is of great interest and significance in host-guest chemistry. 1 Anion artificial receptor has represented an unique application prospect in anion sensor, 2 membrane transmit carrier 3 and simulation, enzyme catalyst synthesis, etc. 4,5 Also their effects as environmental pollutants have only recently been realized and several types of synthetic chemosensors have been developed to date. For the molecular design of the chemosensors, how to achieve the specific recognition of a certain anion and how to convert the recognition event into a signal are the key points. 6 Among the various important anionic analytes, biologically important acetate anion is one of the most significant due to its specific biochemical functions in the enzymes and antibodies. 7 Acetate has unique chemical properties and can form the strongest hydrogen-bond interaction with hydrogen-bond donors because of the trigonal geometry and the high basicity. Many examples are available on selective receptor molecules for acetate anion. [8][9][10] However, there is paucity of reports that describe the change in color in the visible region of the spectrum, and thereby allowing the visual detection of the acetate anion using a receptor. Accordingly, we designed and synthesized the new receptor 1 (Scheme 1) containing indoline-2,3-dione and 1,3-diaminothiourea subunits. UV-Vis spectral and 1 H NMR titration experiments were employed to investigate the interaction between receptor 1 and several anions.

Reagents
Unless otherwise specified, all reagents used in the synthesis were obtained commercially and were used without further purification. All anions used in the titration experiments were added in the form of tetra-n-butylammonium (TBA) salts, which were purchased from Sigma-Aldrich Chemical Co., stored under vacuum in a dessiccator containing self-indicating silica-gel and fully dried before use. DMSO (dimethyl sulfoxide) was dried over CaH 2 and then distilled at reduced pressure.
Apparatus 1 H NMR spectra were obtained in a Varian UNITY Plus-400 MHz Spectrometer using tetramethylsilane (TMS) as an internal reference. SI-MS experiments were performed in a MARINER apparatus. C, H, N elemental analyses were carried out in an Elementar Vario EL. UV-vis spectra were recorded in a Shimadzu UV-2450 Spectrophotometer using a quartz cuvette (path length = 1 cm).

General method
All experiments were carried out at 298 K, unless otherwise mentioned. A 2.0 × 10 -4 mol L -1 solution of the compound 1 in DMSO was prepared and stored in dry atmosphere. This freshly prepared stock solution was used in all spectroscopic studies after appropriate dilution. 1.0 × 10 -2 mol L -1 solutions of tetra-n-butylammonium (TBA) salts of the respective anions were prepared using dried and distilled DMSO and were appropriately stored in dry atmosphere. 1 H NMR titration experiments were carried out in DMSO-d 6 solution (TMS as internal reference). Receptor 1 (0.91 mg) was dissolved in DMSO-d 6 to obtain a 5 × 10 -3 mol L -1 solution. Then, increasing amounts of acetate anion (in DMSO-d 6 ) were added into that solution, and the host-guest interaction was monitored by 1 H NMR spectroscopy.

UV-vis spectroscopic measurement
Firstly, to evaluate the binding ability, UV-vis titration experiments were carried out using a solution of the receptor 1 in dry DMSO tetrabutylammonium salts of AcO -, F -, H 2 PO 4 -, Cl -, Brand I -, at 298.2 ± 0.1K. The set of UV-vis spectra of 1 (1.0 × 10 -5 mol L -1 ) recorded upon the addition of increasing concentration of AcOis shown in Figure 1a. Two bands, one at 380 and another at 490 nm can be easily identified in the spectrum of the pure solution of 1. Upon addition of AcO -, there is an decrease of the 380 nm band intensity, while the lower energy band intensity increased gradually as a function of acetate concentration, generating an isosbestic point at 404 nm. As a consequence, the color of the solution changed from yellow to red, which could provide a convenient method for naked-eye detection of AcO -, as it was coordinated to 1 and the negative charge was dispersed to the π-conjugated system through intramolecular charge transfer (ICT) effect. A similar spectral change was observed upon the addition of H 2 PO 4 but the change was smaller. However, as Cl -, Brand Iwere titrated into 1, the spectra hardly changed even after an excess of the anions were added. In the case of F -, spectra changed without clear isosbestic points (see Figure  1b) implying formation of higher order complexes such as 1/ F -= 1/2, 12 in analogy to the formation of [HF 2 ]-species. [13][14] This should be the result of the too small size of Fanion.
Continuous variation method was used to determine the stoichiometric ratios of the receptors to the anion guests. In Figure 2 Job plot 15,16 of receptor 1 and AcOin DMSO shows the maximum at a mole fraction of 0.5. This result indicates that the receptor 1 binds AcOguest with a 1:1 ratio. Moreover, similar results can also be obtained for H 2 PO 4 -. On the other hand, the Job plot reveals that 1 coordinated Fwith a 1:2 ratio, showing the maximum at a molar fraction of 0.67. For a complex of 1:1 stoichiometry, the relation in Equation (1) could be derived easily, where X is the absorption intensity, and C H or C G is the concentration of the host or the anion guest correspondingly. 17 For a complex of 1:2 stoichiometry, the relation in Equation (2) could be derived easily, where X is the absorption intensity, and C H or C G is the concentration of the host or the anion guest correspondingly. 18 (2) The affinity constants of receptor 1 for anionic species were calculated and listed in Table 1 below.
Obviously, the recognition function of 1 for AcOis more remarkable than H 2 PO 4 -. Because AcOis a trigonal planar species and the angle of O-C-O is about 120º while the angle of O-P-O is about 108º, 19 the distance between two oxygen atoms in AcOmight fit better to the recognition sites forming stronger hydrogen bonds. Furthermore, AcOis a stronger base than H 2 PO 4 and F -. The affinity constants of Cl -, Brand Ifor 1 were so small probably due to the much lower basicity of those species.
As a validation to the titration experiments, selective recognition experiments were performed by UV-vis spectral investigation. One equivalent of the AcO -, H 2 PO 4 -, F -,  Cl -, Brand Ianions, were added to the solution of 1 (1.0 × 10 -5 mol L -1 ), respectively. The UV-vis spectra were recorded and the results presented that 1 could recognize AcOwell ( Figure 3).

H NMR spectroscopic analyses
Further supporting for the notion that hydrogen bond formed between 1 and anions came from 1 H NMR spectroscopic analyses. They were carried out in DMSO-d 6 under normal conditions of so-called NMR titration where the spectra of the receptor 1 were recorded in the presence of increasing concentrations of anions ( Figure 4). Upon the addition of AcOto the solution of 1, the original peak of H a (marked in Scheme 1) at 14.95 ppm disappeared, but that at 11.34 ppm assigned to H b (marked in Scheme 1) shifted downfield. When 1.5 equivalents of AcOwere added to 1, the peaks corresponding to H b broadened and moved downfield to 12.64 ppm (Δd = 1.30 ppm), which suggests that AcOwas being combined with the two H b of receptor 1 by hydrogen bonding. However, on the one hand, as to the signals of H a , they were still not observed in the whole titration process of AcOanions. On the other hand, the signals of the phenyl group shifted upfield, which indicated the increase of the electron density on the phenyl ring owing to the deprotonation taking place. As a function of the titration experiments and spectra analysis, the proposed binding mode of receptor 1 and AcOis given in Scheme 2. 12

Conclusion
In conclusion, 1 has been developed as a novel colorimetric receptor for AcOanion. It has been demonstrated that the receptor can bind anions in a 1:1 stoichiometry (except for F -) with obvious colorimetric changes, where the complex had been formed between 1 and AcOby hydrogen bonding as proved by 1 H NMR titration.