Estudo da reatividade de ligantes diimínicos com Ni2+ e Pd2+ e do potencial catalítico dos seus complexos em reações de Heck e Sonogashira

Abstract

The search for new methodologies capable of promoting the formation of bonds between carbon atoms is a constant process and of great interest. Therefore, several important works have been developed with the objective of accomplishing more selective and effective syntheses, mainly involving transition metal complexes, and the investigation of new catalytic cycles. Thus, the present work sought to investigate the reactivity of diimine ligands with the Ni2+ and Pd2+ metal ions as well as the potential use of the products as catalysts in carbon-carbon coupling reactions. Thus, compounds derived from two subclasses of different diimine ligands were synthesized and properly characterized: I) bis(thiosemicarbazones), so called H2L1R (R = Me or Et) and II) α-diimine ligands, referred to here as L2x (x = H or alkyl). The PdII complexes obtained from these ligands, [Pd(L1Me)] and [PdCl2(L2x )], were tested in Heck and Sonogashira reactions. Regarding the catalytic tests for Heck-type reactions, using styrene and iodobenzene, it was possible to achieve promising results, where the complexes promoted high rates of conversion and selectivity, especially for the complexes [PdCl2(L2x )] which showed conversions even at low concentrations of the catalysts. The tests for Sonogashira reactions were made only for [Pd(L1Me)] and further investigation is still necessary, however, it was possible to observe a structural relationship through a comparison with [PdCl2(PPh3)2]. In addition, the reactivity of the H2L1R ligands was evaluated with different NiII precursors and reaction conditions, with the formation of products with different colors being evidenced, whose spectroscopic analyzes confirmed the acquisition of distinct compounds. Surprisingly, it was possible to obtain crystals for both products, and it was possible to confirm by single-crystal X-ray diffraction that we were dealing with linkage isomers. Considering the difficult to obtain such type of isomers with a tetradentate ligand, we decided to investigate more deeply the kinetic effects involved in their formation as well as to perform an electrochemical study. Finally, theoretical calculations by DFT were accomplished in order to explain the electronic spectra of the two isomer species. In general, this work promoted the acquisition of promising and unprecedented results both in relation to catalysis and coordination chemistry, bringing novelty to understand the behavior of diimine ligands front of the ions of Pd2+ and Ni2+.