An investigation of ligand effects on the visible light-induced formation of porphyrin–iron( iv )-oxo intermediates

High-valent porphyrin–iron-oxo intermediates are the central oxidizing species in heme-containing enzymes and synthetic oxidation catalysts. In this work, we investigated a new photochemical entry to porphyrin–iron( iv )-oxo derivatives in a variety of porphyrin ligands with different electronic and steric environments. In non-electron-deficient ligand systems, the visible light photolysis of porphyrin–iron( iii ) bromates gave porphyrin–iron( iv )-oxo radical cations (compound I analogues). In contrast, the photolysis of porphyrin–iron( iii ) bromates with electron-deficient and sterically encumbered ligands produced one-electron reduced iron( iv )-oxo porphyrins (compound II analogues). Formation of iron( iii ) μ-oxo dimers was also observed in the sterically unencumbered systems containing electron-deficient substituents. The photochemical generation of porphyrin–iron( iv )-oxo radical cations and iron( iv )-oxo porphyrins permits direct kinetic studies of their oxidations with organic reductants. As expected, more oxidized porphyrin–iron( iv )-oxo radical cations reacted 2–3 orders of magnitudes faster than the iron( iv )-oxo porphyrins, and the rate constants obtained in this work are in comparison to those of iron( iv )-oxo derivatives formed by chemical methods. A model including internal electron-transfer (ET) and comproportionation of the putative iron( v )-oxo species pathways is presented.