Oxidation of hydrocarbons with hydrogen peroxide catalysed by nonheme manganese complexes represents one of the most promising approach for selective and sustainable functionalization of non-activated C-H bonds. The catalytic cycle has been proposed to be the same operating in iron catalytic systems, but very few Mn-OOH and Mn=O intermediates have been isolated and characterized up to date due to the intrinsic difficulty to detect and investigate Mn intermediates by the conventional experimental techniques. In this research project we plan to follow the reactions of the manganese catalysts with oxidants, by means of the combined ultrafast time-resolved energy dispersive X-ray absorption spectroscopy (EDXAS) at the Mn K-edge and UV-Vis spectroscopy technique, with the aim of detecting and characterizing the missing intermediates. A wide series of nonheme Mn-complexes containing both tetradentate and pentadentate aminopyridine ligands will be synthetized and the combined EDXAS/UV-Vis technique will be used to detect the formation of high valent manganese(IV)-oxo or manganese(V)-oxo intermediates.
Once characterized the Mn-oxo species the XAS/UV-Vis combined procedure will be used to analyze oxidation reactions of organic substrates such as aryl methyl sulfides by the non-heme manganese-oxo complexes. This will allow us to determine the oxidation mechanism and to determine the kinetic constants of the reactions by following the relative Mn K edge energy shift in the time-resolved XAS spectra.
On the collection of XAS and UV-Vis time-resolved spectra we will to apply a multivariate data analysis to extract the principal components and we will carry out theoretical simulations of the X-ray absorption near edge structure (XANES) to extract accurate structural information on the geometry of the short-living intermediates that are formed during the chemical reactions.
Reaction of nonheme Mn complexes with suitable oxidants such as hydrogen peroxide, t-alkyl hydroperoxides, peroxyacids, iodosylbenzene etc. leading to the formation of activated Mn-peroxo and Mn-oxo complexes and oxidation processes of organic substrates promoted by the latter speciesare less investigated than the corresponding oxidation reactions with nonheme iron complexes. Some of the spectroscopic techniques available for the analysis of the reactions involving iron catalysts cannot be applied to manganese complexes.
For the study of the activation process of nonheme manganese complexes and the analysis of bimolecular reactions of the Mn-peroxo and Mn-oxo complexes we will develop a new strategy that will provide kinetic and structural information not obtainable by conventional techniques. In particular a new experimental approach combining XAS experiments carried out at large scale facilities and UV-Vis spectroscopy will be employed. The kinetic constants of the oxidation reactions will bedetermined from the analysis of the energy shift of the X-ray absorption edge of the Mn center. Moreover, the analysis of the XANES will allow us to determine the structure of the intermediate species formed during the reaction, with a picometric sensitivity.
In order to extract the spectra of the single species formed during the reactions we will apply an innovative multivariate analysis of both the XANES and UV-Vis data. To this aim we will write specific computational tools that will be part of the PyFitIT software in collaboration with research groups of the University of Turin (Italy) and Southern Federal University of Rostov (Russia). With this new software it will be possible to perform a combined multivariate analysis of both the XANES and UV-Vis data with a higher accuracy as compared with the standard methods.
Through the analysis of the XANES spectra we will be able to definitely disclose the nature of the active species involved in the oxidation reactions for systems that are difficult to be studied with other experimental techniques. We envisage that this new approach will provide unique insights into chemical reaction mechanisms involving transition metals for which the direct and unambiguous assignment of the metal oxidation state as well as the definition of its first coordination sphere is a crucial step to discriminate among different mechanistic hypotheses. Moreover, this combined technique represents a powerful tool to characterize intermediates that are silent to common spectroscopic techniques.