Homolytic Cleavage of Molecular Oxygen by Manganese Porphyrins Supported on Ag(111)

Oxygen binding and cleavage are important for both molecular recognition and catalysis. Mn-based porphyrins in particular are used as catalysts for the epoxidation of alkenes, and in this study the homolytic cleavage of O2 by a surface-supported monolayer of Mn porphyrins on Ag(111) is demonstrated by scanning tunneling microscopy, X-ray absorption, and X-ray photoemission. As deposited, {5,10,15,20-tetraphenylporphyrinato}Mn(III)Cl (MnClTPP) adopts a saddle conformation with the average plane of its macrocycle parallel to the substrate and the axial Cl ligand pointing upward, away from the substrate. The adsorption of MnClTPP on Ag(111) is accompanied by a reduction of the Mn oxidation state from Mn(III) to Mn(II) due to charge transfer between the substrate and the molecule. Annealing the Mn(II)ClTPP monolayer up to 510 K causes the chlorine ligands to desorb from the porphyrins while leaving the monolayer intact. The Mn(II)TPP is stabilized by the surface acting as an axial ligand for the metal center....