The nature of the bonding between metalloporphyrins [Fe(II), Co(II), Mn(II)] and oxygen species (·O·, H₂⁺O·/HO·, HOO·, ·O·)

The interaction of reduced transition-metal porphyrins [e.g. PMn(II), PFe(II), PCo(II)] with oxygen species (·O·, H₂⁺O·/HO·, HOO·, ·O·) involves radical/radical coupling to form covalent bonds. Coordinately unsaturated iron(II) porphyrins form two sigma bonds with ·O·(−ΔG_BF = 1–6 kcal mol⁻¹) via two of the four unpaired d-electrons of the iron center. The hydroxyl radical couples with an unpaired electron of the PM(II) porphyrins to form a single covalent bond . The bond energies (−ΔG_BF) for and are 65 ± 4 and 58 kcal mol⁻¹, respectively. Under neutral conditions the uncharged species [(Cl₈TPP)Mn^III–OH, (Cl₈TPP)Fe^III–OH, (Cl₈TPP)Co^III–OH] have bond-formation energies (−ΔG_BF) of 70 ± 4, 63, and 61 kcal mol⁻¹, respectively. In acetonitrile solutions atomic oxygen (·O·; derived from ozone, O₃) couples with (Cl₈TPP)Mn^III[ClO₄], (Cl₈TPP)Mn^II, (Cl₈TPP)Fe^III[ClO₄], and (Cl₈TPP)Fe^II to form (Cl₈TPP⁺)Mn^VO (−ΔG_BF = 59 ± 4 kcal mol⁻¹), (Cl₈TPP)Mn^IVO (85 kcal mol⁻¹), (Cl₈TPP⁺·)Fe^IVO (68 kcal mol⁻¹; model for compound I of horseradish peroxidase), and (Cl₈TPP)Fe^IVO (78 kcal mol⁻¹; model for compound II of horseradish peroxidase). The weakly bonded species [(Cl₈TPP⁺)Mn^VO and (Cl₈TPP⁺·)Fe^IVO] epoxidize olefins with near-stoichiometric efficiency; the other two species are essentially unreactive with olefins. Copyright © 2001 John Wiley & Sons, Ltd.