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)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.