Electroreduction of mu-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 O-2 to H2O

Acidification of a solution of (mu-oxo)bis[(5,10,15,20-tetraphenylporphyrin ato)iron(III)] ([{Fe(tpp)}(2)O], II) in CH2Cl2 produced equimolar amounts o f a hydroxoiron(III) complex [(tpp)Fe-III(OH)] (III) and an iron(III) compl ex [(tpp)Fe-III(ClO4)] (TV). The complex IV was isolated as a perchlorate s alt, which crystallyzed in the triclinic space group P (1) over bar (#2); a = 11.909(3), b = 19.603(4), c = 10.494(3) Angstrom, alpha = 95.74(2)degree s, beta = 107.91(2)degrees, gamma = 89.14(2)degrees, V = 2319.1(9) Angstrom (3), Z = 2, D-calc = 1.328 g cm(-3), mu(Mo K alpha) = 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 iro n(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 age nt, such as sodium dithionite, under argon produced [Fe-II(tpp)]. The addit ion of O-2 to a solution of [Fe(tpp)] in acidic CH2Cl2 in the presence of a n equimolar amount of the reducing agent produced the complex III. When the complex II was adsorbed on an electrode surface and placed in aqueous acid ic electrolyte solutions, electroreduction of the adsorbate proceeded accor ding to the half-reaction: [{Fe(tpp)}(2)O] +2H(+) +2e(-) --> 2[Fe(tpp)] + H2O, at 0.031-0.059 pH V (vs . SCE, pH > 1.0). Based on these results, ore-bridged iron(III) porphyrin d imers were used as electrocatalysts for the reduction of O-2. The catalytic reduction of O-2 proceeded at potentials in the vicinity of those for II. As a whole, the proportion of H2O as the product increased from 50% for ads orbed [(tpp)(FeCl)-Cl-III] to > 90% for the adsorbed dimer. Thus, electrore duction of the dimer adsorbed on a carbon electrode immersed in aqueous aci d produced two solid state, cofacially fixed iron(II) porphyrin molecules: [(PFeOFeP)-O-III-P-III](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 O-2-bridg ed iron(III) porphyrin [PFeIII(O-2) (FeP)-P-III](ad) at the electrode surfa ce. Electroreduction of the adsorbate under acidic conditions produced H2O and allowed the reformation of [PFeII (FeP)-P-II](ad). The implication is t hat the electroreduction of the adsorbed ore-bridged dimer gives a cofacial geometry for PFeII on the electrode, facilitating the coordination and sub sequent splitting of O-2.