Electroreduction of μ-Oxo Iron(III) Porphyrins Adsorbed on an Electrode Leading to a Cofacial Geometry for the Iron(II) Complex: Unexpected Active Site for the Catalytic Reduction of O2 to H2O

Abstract

Acidification of a solution of (μ-oxo)bis[(5,10,15,20-tetraphenylporphyrinato)iron(III)] ([{Fe(tpp)}2O], II) in CH2Cl2 produced equimolar amounts of a hydroxoiron(III) complex [(tpp)FeIII(OH)] (III) and an iron(III) complex [(tpp)FeIII(ClO4)] (IV). The complex IV was isolated as a perchlorate salt, which crystallyzed in the triclinic space group P1-(#2); a = 11.909(3), b = 19.603(4), c = 10.494(3) Å, α = 95.74(2)°, β = 107.91(2)°, γ = 89.14(2)°, V = 2319.1(9) Å3, Z = 2, Dcalc = 1.328 g cm-3, μ(Mo Kα) = 4.35 cm-1, final R = 0.055 and Rw = 0.050. The crystal structure of IV revealed that ClO4- is coordinated to the iron atom, which may be driven by the preference of iron(III) to be five coordinate rather than four coordinate. Reduction of the complex II in the presence of acid by electrolysis and/or by a reducing agent, such as sodium dithionite, under argon produced [FeII(tpp)]. The addition of O2 to a solution of [Fe(tpp)] in acidic CH2Cl2 in the presence of an equimolar amount of the reducing agent produced the complex III. When the complex II was adsorbed on an electrode surface and placed in aqueous acidic electrolyte solutions, electroreduction of the adsorbate proceeded according to the half-reaction: [{Fe(tpp)}2O]+2H++2e-→2[Fe(tpp)]+H2O, at 0.031—0.059 pH V (vs. SCE, pH>1.0). Based on these results, oxo-bridged iron(III) porphyrin dimers were used as electrocatalysts for the reduction of O2. The catalytic reduction of O2 proceeded at potentials in the vicinity of those for II. As a whole, the proportion of H2O as the product increased from 50% for adsorbed [(tpp)FeIIICl] to >90% for the adsorbed dimer. Thus, electroreduction of the dimer adsorbed on a carbon electrode immersed in aqueous acid produced two solid state, cofacially fixed iron(II) porphyrin molecules: [PFeIIIOfeIIIP]ad+2H++2e-→[PFeII FeIIP]ad+H2O (P = porphyrin dianion). Coordination of molecular oxygen to the adjacent two iron(II) centers under acidic conditions allowed formation of O2-bridged iron(III) porphyrin [PFeIII(O2)FeIIIP]ad at the electrode surface. Electroreduction of the adsorbate under acidic conditions produced H2O and allowed the reformation of [PFeII FeIIP]ad. The implication is that the electroreduction of the adsorbed oxo-bridged dimer gives a cofacial geometry for PFeII on the electrode, facilitating the coordination and subsequent splitting of O2.

Oxygen Reduction to Water by a Cofacial Dimer of Iron(III)–Porphyrin and Iron(III)–Phthalocyanine Linked through a Highly Flexible Fourfold Rotaxane

“…It has been reported that -oxo Fe(III)porphyrin dimers are split into two Fe(II)porphyrins in the presence of both acids and reductants, whereas they dissociate into Fe(III)porphyrin monomers in the absence of a reductant under acidic conditions. [21] Therefore, it is reasonable that the -oxo structure of 7 was cleaved, to produce the Fe(II)phthalocyanine, by the one-electron reduction of 7 in aqueous sulfuric acid. Furthermore, the onset potential for O2 reduction by 7 (0.78 V) is considered to be a reasonable value for the reduction potential of the iron center in Fe(III)phthalocyanine.…”

Effect of Axial Coordination of Iron Porphyrin on Their Nanostructures and Photocatalytic Performance

“…A weak peak of m + / z = 1353.9 in S3 may be from a little conversion of FeTPPOH·H 2 O to [FeTPP] 2 O, as displayed in reaction . , Figure S2 exhibits a color change of S1, S2, and S3 in dichloromethane (CH 2 Cl 2 ) from brown to green. The typical Soret band (B-band) and Q-band of metal porphyrin (MTPP) can be clearly observed from their UV–vis spectra in Figure b . The UV–vis spectrum of S1, including a Soret band at 416 nm and three obvious Q-bands at 509, 572, and 690 nm, is according to that of the FeTPPCl molecule.…”

“…All of them have two obvious bands at 1003 and 994 cm –1 derived from the vibration modes of the skeleton of the porphyrin ring . The moderate intensity bands at 874 and 891 cm –1 in S2, as well as a broader and weaker vibration band at 872 cm –1 in S3 are the stretching vibration of Fe–O–Fe . The vibration band at 3645 cm –1 in S3 may be assigned to the stretching vibration of O–H from the Fe–O–H, , while the vibration band at 3742 cm – 1 is assigned to the stretching vibration of O–H from the weak coordination of water in crystal.…”

Investigation of macromolecule–metal complexes as cathode catalyst in polymer electrolyte membrane fuel cell system

“…The mechanism still has, however, many points to be elucidated. In order to reproduce the catalytic function of cytochrome c oxidase by artificial system, various metalloporphyrin derivatives have been synthesized 9–16. Interestingly enough, four‐electron reduction of oxygen is difficult to achieve with one metalloporphyrin derivative, but it becomes feasible if a macromolecule–metal complex such as molecular assembly system or polynuclear complex system is used.…”