Spectroscopic investigation and direct comparison of the reactivities of iron pyridyl oxidation catalysts

The growing interest in green chemistry has fueled attention to the development and characterization of effective iron complex oxidation catalysts. A number of iron complexes are known to catalyze the oxidation of organic substrates utilizing peroxides as the oxidant. Their development is complicated by a lack of direct comparison of the reactivities of the iron complexes. To begin to correlate reactivity with structural elements, we compare the reactivities of a series of iron pyridyl complexes toward a single dye substrate, malachite green (MG), for which colorless oxidation products are established. Complexes with tetradentate, nitrogen-based ligands with cis open coordination sites were found to be the most reactive. While some complexes reflect sensitivity to different peroxides, others are similarly reactive with either H₂O₂ or tBuOOH, which suggests some mechanistic distinctions. [Fe(S,S-PDP)(CH₃CN)₂](SbF₆)₂ and [Fe(OTf)₂(tpa)] transition under the oxidative reaction conditions to a single intermediate at a rate that exceeds dye degradation (PDP=bis(pyridin-2-ylmethyl) bipyrrolidine; tpa=tris(2-pyridylmethyl)amine). For the less reactive [Fe(OTf)₂(dpa)] (dpa=dipicolylamine), this reaction occurs on a timescale similar to that of MG oxidation. Thus, the spectroscopic method presented herein provides information about the efficiency and mechanism of iron catalyzed oxidation reactions as well as about potential oxidative catalyst decomposition and chemical changes of the catalyst before or during the oxidation reaction.

An amphoteric reactivity of a mixed-valent bis(μ-oxo)dimanganese(III,IV) complex acting as an electrophile and a nucleophile

A mixed-valent bis(μ-oxo)dimanganese(III,IV) complex, [(dpaq)Mn(III)(O)2Mn(IV)(dpaq)](+) (1), was prepared by reacting a hydroxomanganese(III) complex, [(dpaq)Mn(III)(OH)](+), with hydrogen peroxide in the presence of triethylamine. The mixed-valent bis(μ-oxo)dimanganese(III,IV) complex (1) was well characterised by UV-vis, EPR and CSI-MS techniques. The electrophilic reactivity of 1 was investigated in the oxidation of 2,6-di-tert-butylphenol derivatives by 1, in which the relative rate afforded a good Hammett correlation with a ρ value of -1.0. The nucleophilic character of 1 was then investigated in aldehyde deformylation reactions, using 2-phenylpropionaldehyde (2-PPA) and benzaldehyde derivatives as substrates. In contrast to the case of the reaction of 1 with 2,6-di-tert-butylphenol derivatives, a positive ρ value of 0.89 was obtained in the Hammett plot, demonstrating that the bis(μ-oxo)-dimanganese(III,IV) complex is an active nucleophilic oxidant. Thus, 1 exhibited an amphoteric reactivity in both electrophilic and nucleophilic oxidative reactions.

Mononuclear nonheme iron(iii) complexes that show superoxide dismutase-like activity and antioxidant effects against menadione-mediated oxidative stress

Iron(iii) complexes with pentadentate monocarboxylamido ligands display excellent SOD-like antioxidant activity to reduce superoxide radicals in cultured cells.

Hemin-block copolymer micelle as an artificial peroxidase and its applications in chromogenic detection and biocatalysis

Following an inspiration from the fine structure of natural peroxidases, such as horseradish peroxidase (HRP), an artificial peroxidase was constructed through the self-assembly of diblock copolymers and hemin, which formed a functional micelle with peroxidase-like activity. The pyridine moiety in block copolymer poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG-b-P4VP) can coordinate with hemin, and thus hemin is present in a five-coordinate complex with an open site for binding substrates, which mimics the microenvironment of heme in natural peroxidases. The amphiphilic core-shell structure of the micelle and the coordination interaction of the polymer to the hemin inhibit the formation of hemin μ-oxo dimers, and thereby enhance the stability of hemin in the water phase. Hemin-micelles exhibited excellent catalytic performance in the oxidation of phenolic and azo compounds by H2O2. In comparison with natural peroxidases, hemin-micelles have higher catalytic activity and better stability over wide temperature and pH ranges. Hemin-micelles can be used as a detection system for H2O2 with chromogenic substrates, and they anticipate the possibility of constructing new biocatalysts tailored to specific functions.