title: Radical Reactivity of the Fe(III)/(II) Tetramesitylporphyrin Couple: Hydrogen Atom Transfer, Oxyl Radical Dissociation, and Catalytic Disproportionation of a Hydroxylamine.
abstract: The chemistry of low-valent iron porphyrin complexes with oxyl radical reagents has been explored. (Meso-tetramesityl porphyrinato) iron(III) hydroxide, (TMP)FeIII(OH) reacts with the hydroxylamine TEMPO-H (1-hydroxy-2,2,6,6-tetramethylpiperdine) to yield the ferrous porphyrin, (TMP)FeII, together with H2O and TEMPO. This reaction has a second order rate constantk1= 76 ± 5 M-11 s-1and likely occurs by concertede-/H+transfer. Hydrazines PhNHNHPh and PhNHNH2similarly yield (TMP)FeII. A subsequent reaction between TEMPO (2,2,6,6-tetramethylpiperdinyl radical) and (TMP)FeIIis observed to reversibly form the TEMPO-ligated ferric porphyrin, (TMP)FeIII(TEMPO). A combination of1H NMR and optical spectroscopies were used to determine the thermodynamic parameters for TEMPO binding:K4(25°C) = 535 ± 20 M-1, ΔH°4= -7.0 ± 1.5 kcal mol-1, ΔS°4= -11 ± 5 cal mol-1K-1, ΔG‡4(235K) = 21.3 ± 0.5 kcal mol-1, ΔG‡-4(235K) = 16.9 ± 0.5 kcal mol-1. The Fe-O bond is remarkably weak. The stable phenoxyl radical 2,4,6-tBu3C6H2O•(ArO•) forms a stronger bond to (TMP)FeIIto irreversibly make a similar FeIII(OR) complex. Both (TMP)FeIIand (TMP)FeIII(OH) are catalysts for the disproportionation of excess TEMPO-H to TEMPO and TEMP-H (2,2,6,6-tetramethylpiperdine). The lack of reactivity between (TMP)FeIIand the alkylated TEMPO-H analogue, TEMPO-CH3, suggests that the disproportionation involves a hydrogen atom transfer step. These results highlight the importance and versatility of the heme FeIII/IIcouple that is often overshadowed by its higher-valent counterparts.
authors.affiliation: Department of Chemistry, University of Washington, Seattle, WA 98195-1700.
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Porter
authors.affiliation: Department of Chemistry, University of Washington, Seattle, WA 98195-1700.
authors.forename: James M
authors.initials: JM
authors.lastname: Mayer
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