Characterization of Mn(III)porphyrin immobilized on modified silica surfaces by EXAFS spectroscopy: A promising tool for analysis of supported metalloporphyrin catalysts

Herein we present the first Mn–K edge EXAFS spectra recorded for manganese(III)porphyrin catalysts containing [Mn{T(4-N-MePy)P}(L)n]5+ (where L = oxygenated or nitrogenated axial ligands, n = 1 or 2) grafted onto modified silica-surfaces with propylimidazole, IPG (3), sulfonatophenyl, SiSO3− (4), and both, SiSO3−IPG (5), where T(4-N-MePy)P is the ligand 5,10,15,20-tetra(4-N-methylpyridyl)porphyrinate. From the data analysis and the refinement results, we have obtained the following structural information concerning to the coordination environment around the Mn(III) ion: four Mn–N at 2.00 Å and two Mn–N/Oaxial at 2.26 Å for 3; four Mn–N at 2.03 Å and two Mn–Oaxial at 2.27 Å for 4, and four Mn–N at 2.02 Å and two Mn–N/Oaxial at 2.28 Å for 5. Correlations of EXAFS data with UV–vis spectra pattern and other properties such as colors of the materials allowed distinguishing between materials 3, 4 and 5. Materials 4 and 5 belong to the same category, which involves ionic complex–support interaction. Furthermore, there are evidences for the coordination of the imidazolic ligand to the manganese(III) ion in 3 and 5 from qualitative analysis of XANES data. In the case of the catalyst 5, bearing both sulfonate and imidazole as functional groups, we have suggested that the nature of the [Mn{T(4-N-MePy)P}(L)n]5+–support interaction is predominantly of ionic character, even though the existence of Mn–imidazole bonds can be detected.

Graphical abstractThe first Mn–K edge EXAFS spectra recorded for catalytic materials containing manganese(III)porphyrin [Mn{T(4-N-MePy)P}(L)2]5+(L = oxygenated or nitrogenated ligands)-grafted onto functionalized silica surfaces, which have been used in hydrocarbon oxygenations, gave structural evidences about the coordination environment of the active sites and their interaction nature with the silica matrix.Figure optionsDownload full-size imageDownload as PowerPoint slide