Resonance Raman (RR) spectra are reported for a series of ferric complexes of meso-tetramesitylporphyrin XFeIII(TMP) (where X = F-, Cl-, m-chlorobenzoate (mCB-), CF3O2SO- (trif-) and ClO4 -) and their oxidized derivatives, OFeIV(TMP•+)(X). Mode assignments are made by RR studies of the β-pyrrole-deuterated and 18O-labeled analogues. The results demonstrate that the oxo−iron bond strength is sensitive to the nature of the trans-axial ligand (X), the ν(FeO) stretching mode appearing near 800 cm-1 for the F-, Cl-, and mCB- complexes and near 835 cm-1 for the trif- and ClO4 - complexes. The high-frequency marker modes and the 1H chemical shifts are consistent with stronger electron donation by the former three anions. An explanation for the apparent absence of a correlation between the ν(FeO) stretching frequencies and the previously reported rate constants for substrate epoxidation is also suggested.

During the catalytic action of heme-dependent enzymes such as catalases and peroxidases two oxoiron porphyrin intermediates are usually observed, commonly referred to as compound I and compound 1I.I Regarding synthetic iron porphyrins, both oxoiron(1V) porphyrins and oxoiron(1V) porphyrin cation radicals,2 which are considered synthetic analogs of compound I1 and compound I, respectively, were isolated and characterized by various spectroscopic method^.^ Most attention was given to oxoiron(1V) porphyrin cation radicals, since such species can mimic reactions mediated by P-450 monooxygenases and are considered to be the reactive intermediates in catalytic oxidation systems based on iron porphyrin^.'.^ One important difference among the various hemoproteins is the identity of their axial ligand. The unique spectroscopic and chemical features that distinguish P-450 from the other enzymes are due to its cysteinate ligand, whose role was elucidated for most steps in the catalytic cycle of P-450q5 The last step, which is the oxygenation of substrates by compougd I, was intensively studied by model porphyrins? but to our knowledge no systematic investigation was carried out for the exploration of an axial ligand effect for that reaction. Because of the extreme instability of P-450's compound I, such a study is a formidable task in natural systems. It also cannot be studied in synthetic systems under catalytic conditions, since after a few catalytic cycles the identity of the original axial ligand in the iron(II1) porphyrin catalyst is lost by reaction with the solvent, the substrate, or the oxidant.' (1) Watanabe, Y.; Groves, J. T. In The Enzymes; Sigman, D. S., Ed.;In the present study we have overcome these problems and found a very significant ligand effect on the reactivity of oxoiron(1V) porphyrin cation radicals toward styrene.Oxoiron(1V) tetramesitylporphyrin cation radical ( [ ( T M P ) -FeIV=O] +, 1) is by far the most extensively characterized complex of its type. We have prepared a series of iron(II1) tetramesitylporphyrins with different axial ligands ((TMP)Fe-X, 2-X), oxidized them by ozone* to the corresponding 1-X (X = -F, HOCH3, C1, -0Ac, -0SO2CF3, -0C103), and examined their EPR and lH NMR ~pectra.~ The typical S = 3/2 EPR spectra' (that of 1-OClO3, which is representative of the spectra of all other compounds, is presented in Figure 1) served to confirm the formation of oxoiron(1V) porphyrin cation radicals by this procedure and to eliminate the possibility of other reactive oxidative ~pecies.~b.c Inspection of the last column of Table 1 reveals that the presence of different axial ligands in 1-X had only a minor effect on the EPR spectra, but fortunately the NMR spectra were very indicative for that purpose. The effect of the axial ligands on the chemical shifts of the various protons in 1-X is illustrated in the NMR spectra of Figure 1 and presented in Table 1. This clearly shows that the axial ligands from 2-X remained intact in 1-X after ozonolysis. In addition, we have also confirmed that after th...

A series of oxoiron(IV) porphyrin cation radical complexes was investigated as compound I analogs of cytochrome P-450. Both the spectroscopic features and the reactivities of the complexes in oxygen atom transfer to olefins were examined as a function of only one variable, the axial ligand trans to the oxoiron-(IV) bond. The results disclosed two important kinetic steps -electron transfer from olefin to oxoiron(IV) and intramolecular electron transfer from metal to porphyrin radical -which are affected differently by the axial ligands. The large kinetic barrier of the latter step in the reaction of olefins with the perchlorato-bound oxoiron(IV) porphyrin cation radical complex enabled the trapping of a reaction intermediate in which the metal, but not the porphyrin radical, is reduced. The first electron transfer step is probably followed by s-bond formation, which readily accounts for formation of isomerized organic products at low temperatures. It is finally postulated that part of the enhanced oxygenation activities of cytochrome P-450 monooxygenases and chloroperoxidases is due to a lowering of the energy barrier for the second electron transfer step via participation of their redox-active cysteinate ligand.

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