Photocatalytic activation of oxygen by iron(III) porphyrins

Photochemical charge transfer excitation of tetraphenyl(porphyrinato)iron(III) complexes yields tetraphenyl(porphyrinato)iron(II) which is able to coordinate molecular oxygen under formation of oxo-[tetraphenyl(porphyrinato)]iron(IV). Based on this photochemical reaction pathway photocatalytic oxygenation of alpha-pinene and other alkenes can be initiated. Iron(III) complexes of tetramesitylpolphyrin, tetrakis-(pentafluorophenyl)porphyrin, octa-beta-bromo-tetrakis-(pentafluorophenyl)porphyrin, and octa-beta-chloro- tetrakis-(pentafluorophenyl)porphyrin were investigated photochemically with the aim to improve the low photochemical efficiency of tetraphenyl(porphyrinato)iron(III). The influence of substituents on the porphyrin ligand on the photochemical behavior of the corresponding iron(III) complexes is measured mainly by means of temperature dependent UV/Vis spectroscopy. Both, the yield of oxygenation products formed photocatalytically with alpha-pinene and the product distribution (allylic alcohols versus epoxide) depend on the design of the porphyrin ligands coordinated with iron(III).