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

Abstract

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, SiSO 3 − ( 4), and both, SiSO 3 −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/O axial at 2.26 Å for 3; four Mn–N at 2.03 Å and two Mn–O axial at 2.27 Å for 4, and four Mn–N at 2.02 Å and two Mn–N/O axial 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.