Estudo sobre a síntese de derivados de bases de Schiff, com estruturas baseadas em fluoresceína, com potencial aplicação como sensores químicos de metais de transição

Abstract

There is a growing need for the development of chemosensors capable of detecting metal ions, due to concerns about the presence of these metals in uncontrolled amounts in the environment, which can cause harmful effects in various systems. In this context, Schiff bases (BS) is considered a promising class of substances, according to its applications in several areas, such as in coordination chemistry, in which they act as versatile ligands capable of coordinating with a variety of metallic cations, leading to the formation of complexes with different geometries and oxidation states. Fluorescein, on the other hand, is a substance with recognized photochemical properties, having applications as dyes, markers and chemosensors. Therefore, in this study, syntheses of Schiff bases with structures based on fluorescein (BSF) were performed, aiming to obtain efficient colorimetric chemosensors in the detection of transition metals. First, the reaction between fluorescein and hydrazine hydrate was carried out, to obtain fluorescein hydrazide, where the applied methodology proved to be efficient, by obtaining a significant yield of 68% and high product purity. In sequence, the catalytic methods with and without application of niobium catalysts in the synthesis of BSF from the reaction between fluorescein hydrazide and benzaldehyde were evaluated, in which it was found that the best applied methodology corresponded to the use of 25 equivalent of NbCl5, obtaining a yield of 83% in just 10 minutes of reaction. After evaluating the catalytic methods, several BSF were prepared using NbCl5 as catalyst. BSF were obtained with yields of 70 - 84%, in 10 minutes of reaction. The products obtained were analyzed by 1H NMR, which indicated the possible formation of the molecules of interest. The synthesized BSF proved to be promising chemosensors for the detection of the Co2+ ion, since a sensitivity of the four ligands (BSF) for this cation was found, with the acquisition of UV-Vis spectra of their mixtures, in which close absorptions were verified. at 500 nm for all BSF. It was noted that the presence of the NO2 group in BSF provided a sensitivity to other metallic cations such as Cu2+, Cr3+ and Ni2+. It was also verified a possible preferential metal-ligand interaction via imine group when comparing the UV-Vis spectra of different BSF synthesized in the presence of Co2+.