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)Fe^III(OH) reacts with the hydroxylamine TEMPO–H (1-hydroxy-2,2,6,6-tetramethylpiperdine) to yield the ferrous porphyrin, (TMP)Fe^II, together with H₂O and TEMPO. This reaction has a second order rate constant k₁ = 76 ± 5 M⁻¹ s⁻¹ and likely occurs by concerted e⁻/H⁺ transfer. Hydrazines PhNHNHPh and PhNHNH₂ similarly yield (TMP)Fe^II. A subsequent reaction between TEMPO (2,2,6,6-tetramethylpiperdinyl radical) and (TMP)Fe^II is observed to reversibly form the TEMPO-ligated ferric porphyrin, (TMP)Fe^III(TEMPO). A combination of ¹H NMR and optical spectroscopies were used to determine the thermodynamic and kinetic 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⁻¹ K⁻¹, ΔG^‡₄(235 K) = 21.3 ± 0.5 kcal mol⁻¹, ΔG^‡₋₄(235 K) = 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)Fe^II to irreversibly make a similar Fe^III(OR) complex. Both (TMP)Fe^II and (TMP)Fe^III(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)Fe^II and 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 Fe^III/II couple that is often overshadowed by its higher-valent counterparts.