Síntese de compostos organometálicos de rutênio como catalisadores da desidrogenação do ácido fórmico

Abstract

The present work proposes the synthesis of new organometallic compounds to be applied as catalysts in the dehydrogenation reaction of formic acid. Formic acid was chosen because it contains 4.4% hydrogen, being a good source of molecular hydrogen (H2). The metal chosen was ruthenium and p-cymene and diimine compounds were used as ligands. Diimines were synthesized from condensation reactions between different types of anilines and glyoxal. The complexes have the general formula [RuCl(η6-arene)(N-N)](PF6), where η6-arene = para-cymene and N-N = α-diimines; (1) = 2,6-dimethylaniline, (2) = 2,4-dimethylaniline, (3) = 2,4,6-trimethylaniline, (4) = 2,6-diisopropylaniline, (5) = biphenylamine, (6 ) = 4-fluoroaniline, (7) = 4-chloroaniline and (8) = cyclohexylamine. Its characteristics were studied by various experimental techniques, such as electronic absorption spectroscopy in the UV/Vis region, in the infrared region, 1H and 13C nuclear magnetic resonance and elemental analysis (C, H, N). The next step was to carry out the syntheses of the ruthenium complexes to be used as catalysts, followed by the study of their properties. Using infrared, it was possible to observe bands referring to the stretching of the bond between ruthenium and nitrogen in regions close to 430 cm-1, indicating the coordination of the diimine ligands to the metal, also verifying the presence of the band referring to the PF6 counter-ion, which is located in the region of 800 cm-1. 1H NMR analyzes of the complexes showed signals around 8 ppm, referring to the hydrogen nuclei linked to the imine carbons, in addition to signals in the region between 4 and 6 ppm, characteristic of the hydrogens of the p-cymene ring. The 13C NMR spectra showed signals above 150 ppm, which are attributed to the carbons directly linked to the diimine nitrogen and between 78-100 ppm, related to the carbons of the p-cymene ring. Elemental analyzes are in agreement with the C, N and H contents for the suggested structures. TD-DFT calculations showed the possibility of forming conformers for the complexes obtained, due to the orientation of the p-cymene ring in relation to the other ligands. The catalytic study began by determining the catalyst/formic acid ratio, starting from the precursor complex as a reference, with the ratio 1:1200 being defined as the most ideal. Soon after, the synthesized complexes containing p-cymene and α-diimines as ligands were used as catalysts. The best result was for complex (1), which had an average conversion of 88.45 ± 1.18% and average TOF of 584.59 ± 21.02 h-1, with a second addition of formic acid reaching a conversion of 92.61% and TOF equal to 889 h-1. Analysis by gas chromatography using a thermal conductivity detector (TCD) confirmed that the products obtained in the reaction were H2 and CO2, without the presence of CO, effecting the reaction involved as the dehydrogenation of formic acid. Study of base variation indicated the need for a Brønsted-Lowry base, such as triethylamine (NEt3). The presence of Lewis bases promoted a decrease in catalytic activity. The study of temperature variation allowed, through the Arrhenius and Eyring equations, the determination of physical-chemical parameters of the reaction, such as: activation energy (Ea), variation of the Gibbs free energy of activation and variation of the entropy of activation of the reaction. These results indicated that the reaction is spontaneous (ΔG‡ = + 12.22 kJ mol-1), in a dissociative mechanism (ΔS‡ = 49.50 eu). From the results of kinetics, thermodynamics of the activated state, and DFT calculations, a mechanism for the dehydrogenation of formic acid was proposed, where the formation of a complex hydride occurs as a reaction intermediate and the Brønsted-Lowry base acts as a system activator. The synthesis of complex hydride, one of the intermediates of the proposed mechanism, was carried out. Infrared analysis of this compound showed a band at 2111 cm-1, characteristic of the stretching of the Ru-H bond. The last step was to monitor the reaction between the complex hydride and HNEt3+Cl- by UV/Vis, with the aim of regenerating the formation of the complex containing the chloride ligand (1).