Iron(III) Complex of a Crown Ether−Porphyrin Conjugate and Reversible Binding of Superoxide to Its Iron(II) Form

The synthesis and characterization of the Fe(III) complex of a novel crown ether−porphyrin
conjugate, 52-N-(4-aza-18-crown-6)methyl-54,104,154,204-tetra-tert-butyl-56-methyl-5,10,15,20-tetraphenylporphyrin (H2Porph), as well as the corresponding hydroxo, dimeric, Fe(II), and peroxo species are reported.
The crystal structure of [FeIII(Porph)Cl]·H3O+·FeCl4-·C6H6·EtOH is also reported. [FeIII(Porph)(DMSO)2]+
and K[FeIII(Porph)(O22-)] are high-spin species (Mössbauer data:  δ = 0.38 mm s-1, ΔEq = 0.83 mm s-1
and δ = 0.41 mm s-1, ΔEq = 0.51 mm s-1, respectively), whereas in a solution of reduced [FeIII(Porph)(DMSO)2]+ complex the low-spin [FeII(Porph)(DMSO)2] (δ = 0.44 mm s-1, ΔEq = 1.32 mm s-1) and high-spin [FeII(Porph)(DMSO)] (δ = 1.27 mm s-1, ΔEq = 3.13 mm s-1) iron(II) species are observed. The reaction
of [FeIII(Porph)(DMSO)2]+ with KO2 in DMSO has been investigated. The first reaction step, involving
reduction to [FeII(Porph)(DMSO)2], was not investigated in detail because of parallel formation of an Fe(III)−hydroxo species. The kinetics and thermodynamics of the second reaction step, reversible binding of
superoxide to the Fe(II) complex and formation of an Fe(III)−peroxo species, were studied in detail (by
stopped-flow time-resolved UV/vis measurements in DMSO at 25 °C), resulting in kon = 36 500 ± 500 M-1
s-1, koff = 0.21 ± 0.01 s-1 (direct measurements using an acid as a superoxide scavenger), and KO2- =
(1.7 ± 0.2) × 105 (superoxide binding constant kinetically obtained as kon/koff), (1.4 ± 0.1) × 105, and (9.0
± 0.1) × 104 M-1 (thermodynamically obtained in the absence and in the presence of 0.1 M NBu4PF6,
respectively). Temperature-dependent kinetic measurements for kon (−40 to 25 °C in 3:7 DMSO/CH3CN
mixture) yielded the activation parameters ΔH⧧ = 61.2 ± 0.9 kJ mol-1 and ΔS⧧ = +48 ± 3 J K-1 mol-1.
The observed reversible binding of superoxide to the metal center and the obtained kinetic and
thermodynamic parameters are unique. The finding that fine-tuning of the proton concentration can cause
the Fe(III)−peroxo species to release O2- and form an Fe(II) species is of biological interest, since this
process might occur under very specific physiological conditions.