DFT study of the active intermediate in the Fenton reaction

Density functional theory has been used to investigate the nature of the ox idizing agent in the Fenton reaction. Starting from the primary intermediat e [Fe-II(H2O)(5)H2O2](2+), we show that the oxygen - oxygen bond breaking m echanism has a small activation energy and could therefore demonstrate the catalytic effect of the metal complex. The O-O bond cleavage of the coordin ated H2O2, however, does not lead to a free hydroxyl radical. Instead, the leaving hydroxyl radical abstracts a hydrogen from an adjacent coordinated water leading to the formation of a second Fe-OH bond and of a water molecu le. Along this reaction path the primary intermediate transforms into the [ Fe-IV(H2O)(4)(OH)(2)](2+) complex and in a second step into a more stable h igh valent ferryl-oxo complex [Fe-IV(H2O)(5)O](2+). We show that the energy profile along the reaction path is strongly affected by the presence of an extra water molecule located near the iron complex. The alternative interm ediate [Fe-II(H2O)(4)(OOH-)(H3O+)](2+) suggested in the literature has been also investigated, but it is found to be unstable against the primary inte rmediate. Our results support a picture in which an Fe-IV-oxo complex is th e most likely candidate as the active intermediate in the Fenton reaction, as indeed first proposed by Bray and Gorin already in 1932.