The four-electron reduction of dioxygen by decamethylferrocene (Fc*) to water is efficiently catalyzed by a binuclear copper(II) complex (1) and a mononuclear copper(II) complex (2) in the presence of trifluoroacetic acid in acetone at 298 K. Fast electron transfer from Fc* to 1 and 2 affords the corresponding CuI complexes, which react at low temperature (193 K) with dioxygen to afford the η2:η2-peroxo dicopper(II) (3) and bis-μ-oxo dicopper(III) (4) intermediates, respectively. The rate constants for electron transfer from Fc* and octamethylferrocene (Me8Fc) to 1 as well as electron transfer from Fc* and Me8Fc to 3 were determined at various temperatures, leading to activation enthalpies and entropies. The activation entropies of electron transfer from Fc* and Me8Fc to 1 were determined to be close to zero, as expected for outer-sphere electron-transfer reactions without formation of any intermediates. For electron transfer from Fc* and Me8Fc to 3, the activation entropies were also found to be close to zero. Such agreement indicates that the η2:η2-peroxo complex (3) is directly reduced by Fc* rather than via the conversion to the corresponding bis-μ-oxo complex, followed by the electron-transfer reduction by Fc* leading to the four-electron reduction of dioxygen to water. The bis-μ-oxo species (4) is reduced by Fc* with a much faster rate than the η2:η2-peroxo complex (3), but this also leads to the four-electron reduction of dioxygen to water.

The selective two-electron reduction of O2 by one-electron reductants such as decamethylferrocene (Fc*) and octamethylferrocene (Me8Fc) is efficiently catalyzed by a binuclear Cu(II) complex ([CuII2(LO)(OH)]2+ (D1) {LO is a binucleating ligand with copper-bridging phenolate moiety} in the presence of trifluoroacetic acid (HOTF) in acetone. The protonation of the hydroxide group of [CuII2(LO)(OH)]2+ with HOTF to produce [CuII2(LO)(OTF)]2+ (D1-OTF) makes it possible for this to be reduced by two equiv of Fc* via a two-step electron transfer sequence. Reactions of the fully reduced complex [CuI2(LO)]+ (D3) with O2 in the presence of HOTF led to the low-temperature detection of the absorption spectra due to the peroxo complex ([CuII2(LO)(OO)]) (D) and the protonated hydroperoxo complex ([CuII2(LO)(OOH)]2+ (D4). No further Fc* reduction of D4 occurs, and it is instead further protonated by HOTF to yield H2O2 accompanied by regeneration of [CuII2(LO)(OTF)]2+ (D1-OTF) thus completing the catalytic cycle for the two-electron reduction of O2 by Fc*. Kinetic studies on the formation of Fc*+ under catalytic conditions as well as for separate examination of the electron transfer from Fc* to D1-OTF reveal there are two important reaction pathways operating. One is a rate-determining second reduction of D1-OTF, thus electron transfer from Fc* to a mixed-valent intermediate [CuIICuI(LO)]2+ (D2) which leads to [CuI2(LO)]+ which is coupled with O2 binding to produce [CuII2(LO)(OO)]+ (D). The other involves direct reaction of O2 with the mixed-valent compound D2 followed by rapid Fc* reduction of a putative superoxo-dicopper(II) species thus formed, producing D.

The cytochromes P450 are versatile enzymes involved in various catalytic oxidation reactions, such as hydroxylation, epoxidation and dehydrogenation. In this work, we present combined experimental and theoretical studies on the change of regioselectivity in cyclohexadiene oxidation (i.e., epoxidation vs dehydrogenation) by oxoiron(IV) porphyrin complexes bearing different porphyrin ligands. Our experimental results show that meso-substitution of the porphyrin ring with electron-withdrawing substituents leads to a regioselectivity switch from dehydrogenation to epoxidation, affording the formation of epoxide as a major product. In contrast, electron-rich iron porphyrins are shown to produce benzene resulting from the dehydrogenation of cyclohexadiene. Density functional theory (DFT) calculations on the regioselectivity switch of epoxidation vs dehydrogenation have been performed using three oxoiron(IV) porphyrin oxidants with hydrogen atoms, phenyl groups, and pentachlorophenyl (ArCl 5 ) groups on the meso-position. The DFT studies show that the epoxidation reaction by the latter catalyst is stabilized because of favorable interactions of the substrate with halogen atoms of the meso-ligand as well as with pyrrole nitrogen atoms of the porphyrin macrocycle. Hydrogen abstraction transition states, in contrast, have a substrate-binding orientation further away from the porphyrin pyrrole nitrogens, and they are much less stabilized. Finally, the regioselectivity of dehydrogenation versus hydroxylation is rationalized using thermodynamic cycles.