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

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)Fe(III)(OH)] (III) and an iron(III) complex [(tpp)Fe(III)(ClO₄)] (IV). The complex IV was isolated as a perchlorate salt, which crystallized in the triclinic space group P1̄ (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) ų, Z = 2, D(calc)= 1.328 g cm^-3, μ(Mo Kα) = 4.35 cm^-1, final R = 0.055 and R(w) = 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 [Fe(II)(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)Fe(III)Cl] 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: [PFe(III)OFe(III)P](ad)+2H⁺+2e^-→[PFe(II) Fe(II)P](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 [PFe(III)(O₂) Fe(III)P](ad) at the electrode surface. Electroreduction of the adsorbate under acidic conditions produced H₂O and allowed the reformation of [PFe(II) Fe(II)P](ad). The implication is that the electroreduction of the adsorbed oxo-bridged dimer gives a cofacial geometry for PFe(II) on the electrode, facilitating the coordination and subsequent splitting of O₂.