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

Abstract

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: delta = 0.38 mm s(-1), DeltaEq = 0.83 mm s(-1) and delta = 0.41 mm s(-1), DeltaEq = 0.51 mm s(-1), respectively), whereas in a solution of reduced [FeIII(Porph)(DMSO)2]+ complex the low-spin [FeII(Porph)(DMSO)2] (delta = 0.44 mm s(-1), DeltaEq = 1.32 mm s(-1)) and high-spin [FeII(Porph)(DMSO)] (delta = 1.27 mm s(-1), DeltaEq = 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 degrees 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) x 10(5) (superoxide binding constant kinetically obtained as kon/koff), (1.4 +/- 0.1) x 10(5), and (9.0 +/- 0.1) x 10(4) 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 degrees C in 3:7 DMSO/CH3CN mixture) yielded the activation parameters DeltaH = 61.2 +/- 0.9 kJ mol(-1) and DeltaS = +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.

Transition Metal Complexes and the Activation of Dioxygen

“…Further characterization of the O-O and Fe-O bonding by IR, rR, and EXAFS spectroscopy supported a symmetrically bound peroxo ligand [39,41]. These data have served as benchmarks for the characterization of other model compounds (14b-d, 16) ( Figure 6), which all show similar spectroscopic features [42][43][44]. In a more recent report, compound 16 was found to be in equilibrium with an Fe(II)-O [42].…”

“…These data have served as benchmarks for the characterization of other model compounds (14b-d, 16) ( Figure 6), which all show similar spectroscopic features [42][43][44]. In a more recent report, compound 16 was found to be in equilibrium with an Fe(II)-O [42]. Kinetic and equilibrium studies showed the reaction between the Fe(II) precursor and O À 2 to be fast, with the equilibrium strongly in favor of the bound species.…”

Reactions of Superoxide with Iron Porphyrins in the Bulk and the Near-Surface Region of Ionic Liquids

“…These reactions are usually very fast, even when they proceed in a stoichiometric manner in aprotic media, and for their detailed investigations usually low temperatures are needed. 4,23 Since the majority of reactions of metal complexes are significantly slowed down in ionic liquids, 24,25 this can be valuable for their kinetic/mechanistic studies, and may enable their examination under room temperature conditions.…”

“…To pursue the above-mentioned concept of utilizing ILs as a medium for monitoring reactions between superoxide and metal centers, we have studied the reactions of highly charged Fe(P4+), Fe(P8+), and Fe(P8−) iron porphyrins ( 4 ], by using time-resolved UV/vis stopped-flow measurements, mass spectrometry, EPR, and XPS measurements. These complexes are known to be water-soluble, 36,37 and we found that they are soluble in ionic liquids as well.…”

Synthesis, structure, and cation complexation of a novel crown ether porphyrin

“…17 Since the first example of crown ether-appended porphyrins reported by Thanabal and Krishnan, 18 a number of porphyrins bearing crown ether moieties have been synthesized, and the complexation of the crown ether units with a variety of cations to form porphyrin dimers or donor-acceptor dyads has been studied. [19][20][21][22][23][24] We report here the synthesis and cation-induced dimerization of zinc 5,10,15,20-tetrakis[(benzo-18-crown-6)-4 0 -ylethynyl]-porphyrin (1), in which the crown ether and porphyrin ring are bridged by an ethynyl linker as shown in Figure 1. The crown ether units in 1 are more flexible for rotation than those in the porphyrin system with benzo-crown ethers appended directly at the mesopositions such as zinc 5,10,15,20-tetrakis[(benzo-18-crown-6)-4 0 -yl]porphyrin (2) 18 and zinc 5,10,15,20-tetrakis[(benzo-15-crown-5)-4 0 -yl]porphyrin (3).…”