Preparation, Characterization, and Reaction of an Oxo-Fe(V)-Porphyrin Complex

Abstract

Oxidation of Fe^IIITDCPP (m-chlorobenzoate) (1-mCB, 2.2 × 10^-5 M, TDCPP: 5,10,15,20-tetrakis-2,6-dichlorophenylporphyrin) with 1.8 equiv amount of p-nitroperoxybenzoic acid in dry methylene chloride at -90°C produced an oxo-ferryl porphyrin cation radical (2). The addition of 4 equiv of methanol to the solution afforded a red species (3) with the loss of the characteristic broad band (600–750 nm) for porphyrin cation radicals. The spectrum of 3 is rather similar to that of oxo-ferryl (O=Fe^IV) porphyrin species (4) but very different from those of ferric porphyrin dications and ferric porphyrin N-oxides. Whereas 4 is stable at -5°C, the absorption spectrum of 3 changed to that of ferric high-spin porphyrin (1) even at -70°C in several hours. The addition of norbornylene to a methylene chloride solution of 3 caused the absorption spectrum change of 3 to that of 1 at -90°C, while 4 does not react with the olefin under similar condition. Titration of 3 with iodide ion indicates the oxidation state of 3 to be two-electron oxidized from the ferric state. That ²H-NMR spectra of 3 gave pyrrole deuterium and meta-phenyl deuterium signals at -35.1 and 8 ppm at -95°C, respectively, is consistent with the oxo-perferryl (O=Fe(V)) porphyrin formulation. ESR spectrum (g = 4.33, 3.69, and 1.99 at 4.2K) and solution magnetic susceptibility (µgff = 4.0 ± 0.2 µg) of 3 indicate that 3 is in a high-spin state (S = 3/2). While 3 catalyzes the epoxidation of olefins at -90°C, 3 is about 10 times less reactive than 2.