KINETIC AND THERMODYNAMIC STUDIES OF IRON(III) AND IRON(IV) SIGMA-BONDED PORPHYRINS - FORMATION AND REACTIVITY OF [(OEP)FE(R)](N-C6F3H2, 2,4,6-C6F3H2, C6F4H, OR C6F5(), WHERE OEP IS THE DIANION OF OCTAETHYLPORPHYRIN (N = 0, 1, 2, 3) AND R = C6H5, 3,4,5)

A series of high-and low-spin iron(III) phenyl and fluorophenyl octaet hylporphyrin complexes are characterized by their electrochemical and spectroscopic properties in nonaqueous media. The investigated compoun ds 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 i n that these isomers differ in the spin state of the iron(III). Electr ochemical 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 sigm a-bonded ligand does not occur an the time scale of the experiment. Th e first one-electron oxidation of each compound leads to an Fe(IV) por phyrin, and this is followed by a migration of the axial group from th e iron center to one of the four nitrogen atoms independent of the nat ure 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 c oordination 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. Ou r results also show that the spin state of the iron(III) octaethylporp hyrin is not a key factor which governs the migration of the axial lig and of the electrooxidized species. For the first time, an overall mec hanism is proposed to explain the migration reaction in the sigma-bond ed iron porphyrin complexes.