The nature of the bonding between metalloporphyrins [Fe(II), Co(II), Mn(II)] and oxygen species (·O2·, H2+O·/HO·, HOO·, ·O·)

Abstract

The interaction of reduced transition-metal porphyrins [e.g. PMn (II), PFe (II), PCo (II)] with oxygen species (· O 2·, H 2+ O ·/ HO ·, HOO ·, · O ·) involves radical/radical coupling to form covalent bonds. Coordinately unsaturated iron(II) porphyrins form two sigma bonds with · O 2·(-Δ G BF = 1–6 kcal mol -1) via two of the four unpaired d-electrons of the iron center. The hydroxyl radical [Formula: see text] couples with an unpaired electron of the PM (II) porphyrins to form a single covalent bond [[Formula: see text], PM III– OH ]. The bond energies (-Δ G BF ) for [Formula: see text] and [Formula: see text] are 65 ± 4 and 58 kcal mol-1, respectively. Under neutral conditions the uncharged species [( Cl 8 TPP ) Mn III– OH , ( Cl 8 TPP ) Fe III– OH , (Cl8TPP)CoIII- OH ] have bond-formation energies (-Δ G BF ) of 70 ± 4, 63, and 61 kcal mol-1, respectively. In acetonitrile solutions atomic oxygen (· O ·; derived from ozone, O 3) couples with ( Cl 8 TPP ) Mn III[ ClO 4], ( Cl 8 TPP ) Mn II, ( Cl 8 TPP ) Fe III[ ClO 4], and ( Cl 8 TPP ) Fe II to form ( Cl 8 TPP +) Mn V O (-ΔG BF = 59 ± 4 kcal mol -1), ( Cl 8 TPP ) Mn IV= O (85 kcal mol -1), ( Cl 8 TPP +·) Fe IV= O (68 kcal mol -1; model for compound I of horseradish peroxidase), and ( Cl 8 TPP ) Fe IV= O (78 kcal mol -1; model for compound II of horseradish peroxidase). The weakly bonded species [( Cl 8 TPP +) Mn V= O and ( Cl 8 TPP +·) Fe IV= O ] epoxidize olefins with near-stoichiometric efficiency; the other two species are essentially unreactive with olefins.