Synthesis and Characterization of Manganese complexes as Photooxidation Catalysts

Abstract

This project has consisted in the synthesis of five manganese complexes containing the N-bidentate ligand 2,9-dimethyl-1,10-phenanthroline, dmp, together with two monodentate ligands, -Cl and -CF3SO3, the study of their spectroscopic, structural and electrochemical properties, and their evaluation as catalysts for the photooxidation of alcohols into aldehydes and ketones. The first three synthesized complexes were obtained by a process of mixing the reagents, stirring, filtrating the medium, and washing it. These three catalysts were [MnCl2(dmp)2] 1, [Mn2Cl4(dmp)2] 2 and [Mn(CF3SO3)2(dmp)2] 4. From the crystallization of compound 2’s mother liquor compound [MnCl2(dmp)(H2O)] 3 was obtained. On the other hand, recrystallization of complex 4 in an ethyl acetate solution produced crystals of [Mn(OAc)(dmp)(OH2)2](dpmH)(CF3SO3)2 5. All five compounds were subsequently characteried through IR spectroscopy, cyclic voltammetry, ESI-MS, elemental analysis and X-ray diffraction. After the characterization of the five catalysts, compounds 1, 2 and 4 and other three additional ones, 1a, 2a and 3a, synthesized during a previous project, were tested as photocatalysts in the oxidation of alcohols. Firstly, a study was made to determine the optimal reaction time, which was found to be 12 h. All the compounds showed moderate yield with high selectivity values for the photooxidation of 1-phenylethanol. The complexes containing the pyridine pyrazole ligand (1a-3a) presented better performance than 1-3 complexes, which contained the dmp ligand. In general, the yield values displayed by chloride catalysts were higher than those presented by the triflate complexes. Finally, the two chloride complexes, 1 and 2, were used to catalyze the photooxidations of five different alcohols in neutral and acidic media. The last phase of the project was dedicated to demonstrate the formation of high valent manganese species in water when Mn(II) is oxidized by a compound with a higher redox potential such as [RuIII(bpy)3]3+. This was visualized by UV-Vis and ESI-MS spectroscopy and accomplished by both photooxidation and chemical oxidation