Kinetic and Thermodynamic Studies of Iron(III) and Iron(IV)σ -Bonded Porphyrins. Formation and Reactivity of [(OEP)Fe(R)]ⁿ⁺, Where OEP Is the Dianion of Octaethylporphyrin (n = 0, 1, 2, 3) and R = C₆H₅, 3,4,5-C₆F₃H₂, 2,4,6-C₆F₃₂, C₆F₄H, or C₆F₅

A series of high- and low-spin iron(III) phenyl and fluorophenyl octaethylporphyrin complexes are characterized by their electrochemical and spectroscopic properties in nonaqueous media. The investigated compounds are represented as (OEP)Fe(R), where R = C₆H₅, 3,4,5-C₆F₃H₂, 2,4,6-C₆F₃H₂, C₆F₄H, or C₆F₅ and OEP is the dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin. The two C₆F₃H₂ complexes are of special interest in that these isomers differ in the spin state of the iron(III). Electrochemical studies indicate that three one-electron oxidations are seen for all of the (OEP)Fe(R) derivatives which were investigated both at room and low temperature under conditions where migration of the σ-bonded ligand does not occur on the time scale of the experiment. The first one-electron oxidation of each compound leads to an Fe(IV) porphyrin, and this is followed by a migration of the axial group from the iron center to one of the four nitrogen atoms independent of the nature of the axial group or the iron(III) spin state. The kinetics were examined to evaluate the migration rate constants in the presence and absence of pyridine as a sixth axial ligand. The results of this study show that the stronger the electron donor ability of the R group, the faster the migration rate in the case of the five-coordinate species. However, an increase in charge density at the metal center by axial coordination of pyridine retards the migration rate and this result is interpreted in terms of a rate determining electron transfer step from R to Fe(IV) of the singly oxidized species prior to the migration. Our results also show that the spin state of the iron(TII) octaethylporphyrin is not a key factor which governs the migration of the axial ligand of the electrooxidized species. For the first time, an overall mechanism is proposed to explain the migration reaction in the σ-bonded iron porphyrin complexes.