Structure, spectra and biological significance of high-valent iron(IV,V) porphyrins

Abstract

Oxoferryl(IV) porphyrins such as OFe (TPP) were prepared for the first time by laser photolysis (406.7 nm) of the corresponding dioxygen adducts in O 2 matrices at 15 K. Their resonance Raman spectra show the v (FeO) vibrations at 852 cm∗u$1  l$1 and the porphyrin core vibrations which indicate the Fe atoms to be in the low-spin and +IV state. The π-cation radical OFe (TPP − d ·+ 8) exhibits the v(FeO) band at 815 cm −1. The structure of an intermediate formed during the epoxidation reaction of ethylene by OFe (TPP) was elucidated based on the v(FeO) (804 cm −1) band of OFe (TPP) − C 2H 4 in O 2 C 2 H 4 matrices. Oxoferryl-porphyrin, OFe (TMP), can be produced via the cleavage of the O  O bond of its peroxo dimer in toluene solution at low temperatures. The v(FeO) vibrations of six-coordinate complexes, OFe (por) L, are in the range from 820 to 750 cm −1 depending on the nature of the axial ligand (L). Complexes containing Fe(V) are rare. However, intridoiron(V) porphyrin, NFe (OEP), can be prepared via laser photolysis of the corresponding azido complex (thin film, 30 K). Resonance Raman spectra show the v(FeN) vibration at 876 cm −1 and the oxidation state marker band at 1384 cm −1 which is higher than those of Fe(IV) complexes. It is expected to be high spin because NFe (OEP) is isoelectronic with the well-known high-spin complex OMn (por). The v(FeN) band disappears and a set of new bands emerges when NFe (OEP) is irradiated by the 413.1 nm line. These bands are attributed to the nitrido dimer (OEP) Fe  N  Fe (OEP), which shows the symmetric v(Fe  N  Fe) vibration at 438 cm −1. The biological significance of these high-valant iron (IV,V) porphyrins is discussed in relation to cytochome P-450, horseradish peroxidase and cytochrome P-450-LM3,4.