Herein, we report the EPR spectroscopic study of the bioinspired catalyst systems for selective hydrocarbon oxofunctionalizations based on dinuclear ferric complexes with TPA* and PDP* aminopyridine ligands, hydrogen peroxide, and acetic acid (TPA* = tris(3,5-dimethyl-4-methoxypyridyl-2methyl)amine, PDP* = bis(3,5-dimethyl-4-methoxypyridyl-2methyl)-(S,S)-2,2′-bipyrrolidine). Using very low temperatures, −75 to −85 °C, the extremely unstable and reactive iron−oxygen intermediates, directly reacting with olefins even at −85 °C, have been detected for the first time. Their EPR parameters (g 1 = 2.070−2.071, g 2 = 2.005−2.008, g 3 = 1.956− 1.960) were very similar to those of the known oxoiron(V) complex [(TMC)Fe V O(NC(O)CH 3 )] + (g 1 = 2.053, g 2 = 2.010, g 3 = 1.971, TMC = 1,4,8,4,8,. On the basis of EPR and reactivity data, the detected intermediates were assigned to the Fe V O active oxidizing species of the catalyst systems studied.

The electronic structure of the iron–oxygen intermediates responsible for catalytic transformations in the biomimetic catalyst systems [((S,S)-PDP)­FeII(OTf)2]/H2O2/RCOOH has been found to be strongly dependent on the structure of the carboxylic acid RCOOH. For carboxylic acids with primary and secondary α-carbon atoms (acetic acid, butyric acid, caproic acid), the active species exhibit electron paramagnetic resonance (EPR) spectra with large g-factor anisotropy (g 1 = 2.7, g 2 = 2.4, g 3 = 1.7), whereas for those with tertiary α-carbon atoms (2-ethylhexanoic acid, valproic acid, 2-ethylbutyric acid), the active species display EPR spectra with small g-factor anisotropy (g 1 = 2.07, g 2 = 2.01, g 3 = 1.96). The EPR spectra of the latter intermediates are very similar to those of the intermediates previously assigned to oxoiron­(V) species. The systems featuring intermediates of the second type ensure higher enantioselection in the epoxidation of electron-deficient olefins.

The catalytic activity of a series of iron complexes of the PDP family (PDP = N,N'-bis(2-pyridylmethyl)-2,2'-bipyrrolidine) in the oxidation of aromatic substrates with H 2 O 2 has been studied. In the presence of acetic acid, these complexes efficiently catalyze the oxidation of benzene and alkylbenzenes with high selectivity for oxygen incorporation into the aromatic ring (up to 93 %), performing up to 84 catalytic turnovers. The parent complex, [(PDP)(OTf) 2 ], has demonstrated the highest catalytic efficiency and aromatic oxidation selectivity. The yield of products of oxidation of different substrates increases in line with increasing number of electron-donating alkyl groups of the substrates: halogenbenzenes < benzene < monoalkylbenzenes < dialkylbenzenes, which, together with the results of competitive oxidation experiments and the observation of inverse primary k H /k D (0.90-0.92), agrees with the S E Ar oxidation mechanism. Low-temperature EPR studies have witnessed the presence of low-spin (g 1 = 2.071, g 2 = 2.008, g 3 = 1.960) perferryl intermediates, demonstrating direct reactivity toward benzene. The oxidation of m-xylene in the presence of H 2 18 O has shown the same probability of 18 O incorporation into the aromatic ring and the aliphatic moieties, which is indicative that both the aromatic and aliphatic oxidations are conducted by the same active species.[a] N.