Issue 1, 2014

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 constant k1 = 76 ± 5 M−1 s−1 and likely occurs by concerted e/H+ transfer. Hydrazines PhNHNHPh and PhNHNH2 similarly yield (TMP)FeII. A subsequent reaction between TEMPO (2,2,6,6-tetramethylpiperdinyl radical) and (TMP)FeII is observed to reversibly form the TEMPO-ligated ferric porphyrin, (TMP)FeIII(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, ΔG4(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)FeII to irreversibly make a similar FeIII(OR) complex. Both (TMP)FeII and (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)FeII 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 FeIII/II couple that is often overshadowed by its higher-valent counterparts.

Graphical abstract: Radical reactivity of the Fe(iii)/(ii) tetramesitylporphyrin couple: hydrogen atom transfer, oxyl radical dissociation, and catalytic disproportionation of a hydroxylamine

Supplementary files

Article information

Article type
Edge Article
Submitted
23 Jul 2013
Accepted
14 Oct 2013
First published
16 Oct 2013

Chem. Sci., 2014,5, 372-380

Radical reactivity of the Fe(III)/(II) tetramesitylporphyrin couple: hydrogen atom transfer, oxyl radical dissociation, and catalytic disproportionation of a hydroxylamine

T. R. Porter and J. M. Mayer, Chem. Sci., 2014, 5, 372 DOI: 10.1039/C3SC52055B

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements