Characterization of Mononuclear Non-heme Iron(III)-Superoxo Complex with a Five-Azole Ligand Set

Enhancing Tris(pyrazolyl)borate-Based Models of Cysteine/Cysteamine Dioxygenases through Steric Effects: Increased Reactivities, Full Product Characterization and Hints to Initial Superoxide Formation

The design of biomimetic model complexes for the cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO) is reported, where the 3-His coordination of the iron ion is simulated by three pyrazole donors of a trispyrazolyl borate ligand (Tp) and protected cysteine and cysteamine represent substrate ligands. It is found that the replacement of phenyl groups-attached at the 3-positions of the pyrazole units in a previous model-by mesityl residues has massive consequences, as the latter arrange to a more spacious reaction pocket. Thus, the reaction with O₂ proceeds much faster and afterwards the first structural characterization of an iron(II) η² -O,O-sulfinate product became possible. If one of the three Tp-mesityl groups is placed in the 5-position, an even larger reaction pocket results, which leads to yet faster rates and accumulation of a reaction intermediate at low temperatures, as shown by UV/Vis and Mössbauer spectroscopy. After comparison with the results of investigations on the cobalt analogues this intermediate is tentatively assigned to an iron(III) superoxide species.

A Chromium(III)-Superoxo Complex as a Three-Electron Oxidant with a Large Tunneling Effect in Multi-Electron Oxidation of NADH Analogues

Metal-superoxo species are involved in a variety of enzymatic oxidation reactions, and multi-electron oxidation of substrates is frequently observed in those enzymatic reactions. A Cr^III -superoxo complex, [Cr^III (O₂ )(TMC)(Cl)]⁺ (1; TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), is described that acts as a novel three-electron oxidant in the oxidation of dihydronicotinamide adenine dinucleotide (NADH) analogues. In the reactions of 1 with NADH analogues, a Cr^IV -oxo complex, [Cr^IV (O)(TMC)(Cl)]⁺ (2), is formed by a heterolytic O-O bond cleavage of a putative Cr^II -hydroperoxo complex, [Cr^II (OOH)(TMC)(Cl)], which is generated by hydride transfer from NADH analogues to 1. The comparison of the reactivity of NADH analogues with 1 and p-chloranil (Cl₄ Q) indicates that oxidation of NADH analogues by 1 proceeds by proton-coupled electron transfer with a very large tunneling effect (for example, with a kinetic isotope effect of 470 at 233 K), followed by rapid electron transfer.

A structural and functional model for the 1-aminocyclopropane-1-carboxylic acid oxidase

The hitherto most realistic low-molecular-weight analogue for the 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO) is reported. The ACCOs 2-His-1-carboxylate iron(II) active site was mimicked by a TpFe moiety, to which the natural substrate ACC could be bound. The resulting complex [Tp(Me,Ph) FeACC] (1), according to X-ray diffraction analysis performed for the nickel analogue, represents an excellent structural model, featuring ACC coordinated in a bidentate fashion-as proposed for the enzymatic substrate complex-as well as a vacant coordination site that forms the basis for the first successful replication also of the ACCO function: 1 is the first known ACC complex that reacts with O2 to produce ethylene. As a FeOOH species had been suggested as intermediate in the catalytic cycle, H2 O2 was tested as the oxidant, too, and indeed evolution of ethylene proceeded even more rapidly to give 65 % yield.