A new active intermediate in monooxygenations catalyzed by iron porphyrin complexes

A new type of high-valent oxoiron porphyrin (3b) has been prepared by the reaction of Fe-III(tdcpp) (1b) [tdcpp: 5, 10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin] with an oxidant such as p-nitroperbenzoic acid and pentafluoroiodosylbenzene at -90 degrees C in CH2Cl2 in the presence of a small amount of methanol. The UV-vis spectrum of 3b (lambda(max) at 418 and around 550 nm) is similar to that of O=Fe-IV(tdcpp) (4b, lambda(max) 419, 543 nm). However, titration of 3b by iodide ion indicates that the oxidation state of 3b is two-electron oxidized from the iron(III) state. Further, 3b catalyzes oxygenation of olefins such as styrene even at -90 degrees C. Possible formulation of 3b as a pi-cation radical is readily ruled out by deuterium NMR observation. The solution magnetic susceptibility (mu(eff) = 4.0+/-0.2 mu(B)) of 3b indicates that 3b has three unpaired electrons. These results indicate the formal description of 3b as being a high spin complex of either an O=Fe(V) porphyrin or . O-Fe(IV) porphyrin. The same oxidation of Ib in the absence of methanol gave O=Fe-IV(tdcpp) pi-cation radical (2b). We think the ligation of methanol causes the destabilization of iron d orbitals and eventually turns upside down the energy levels between the iron d(xz), d(yz) orbitals and the porphyrin HOMO orbital (a(2u)) since the a(2u) orbital is stabilized by the introduction of electron-withdrawing groups on the porphyrin ring.