Rearrangements dioxygen complexes

The stoichiometric reaction of S02 with coordinated 02 to form coordinated sulfate appears to be general for dioxygen complexes. lsO isotope substitution and IR spectroscopy have been employed to study the reaction betwen [Ir(X)(CO)(02)(PPh3)2] (X = Cl, Br, I) and S02 (reaction 91). IR spectral results are consistent with a bidentate sulfate, ligand. The mechanism proposed for sulfate formation involves a peroxosulfate intermediate, that is insertion of S02 into one of the Ir —O bonds and bond formation between S and the leaving O, with subsequent rearrangement of the peroxo-sulfite intermediate thus generated (see Scheme 15).330,331... 

The reaction of SO2 with rf dioxygen complexes gives a chelated sulphate complex. Isotopic labelling studies with 62 show that one of the terminal oxygen atoms of the sulphate originates from the dioxygen complex and the other from the SO2. This has been interpreted in terms of the rearrangement of a five membered cyclic intermediate . ... 

A free radical initiator azobisisobutyronitrile or an inhibitor, hydroquinone, have no effect on the rate of oxygen consumption. In any case a dissociative mechanism is precluded since autoxidation of uncoordinated phosphines is a radical process which gives mixed R P(OXOR)3 products. The mechanism suggested involves the formation and rearrangement of a cobalt dioxygen complex, equations (81) and (82), possibly via a dissociative oxygen insertion step of the type proposed by Halpem for platinum complexes. 

The experimental observations were interpreted by assuming that the redox cycle starts with the formation of a complex between the catalyst and the substrate. This species undergoes intramolecular two-electron transfer and produces vanadium(II) and the quinone form of adrenaline. The organic intermediate rearranges into leucoadrenochrome which is oxidized to the final product also in a two-electron redox step. The +2 oxidation state of vanadium is stabilized by complex formation with the substrate. Subsequent reactions include the autoxidation of the V(II) complex to the product as well as the formation of aVOV4+ intermediate which is reoxidized to V02+ by dioxygen. These reactions also produce H2O2. The model also takes into account the rapidly established equilibria between different vanadium-substrate complexes which react with 02 at different rates. The concentration and pH dependencies of the reaction rate provided evidence for the formation of a V(C-RH)3 complex in which the formal oxidation state of vanadium is +4. 

The third ligand was assumed to be coordinated to the metal center via the deprotonated 3-hydroxy and 4-carbonyl groups. This coordination mode allows delocalization of the electronic structure and intermolecu-lar electron transfer from the ligand to Cu(II). The Cu(I)-flavonoxy radical is in equilibrium with the precursor complex and formed at relatively low concentration levels. This species is attacked by dioxygen presumably at the C2 carbon atom of the flavonoxyl ligand. In principle, such an attack may also occur at the Cu(I) center, but because of the crowded coordination sphere of the metal ion it seems to be less favourable. The reaction is completed by the formation and fast rearrangement of a trioxametallocycle. 

An unusual sulfur-nitrogen donor, benzothiazole-2-thiolate (58), has been reacted with Vaska s complex to produce 59 in high yield no bidentate adducts of 58 are produced even in refluxing solvent (156). Upon reaction with dioxygen, the extremely sensitive and reactive complex 60 is produced. Addition of water to 60 caused rearrangement to the carboxylate complex 61, while the addition of sulfur dioxide to 60 produces 62 (see Scheme 11). A proposed mechanism for the reaction of water with 60, based on labeling experiments, was outlined and can be found in Scheme 12. 

Ligand oxidation and rearrangement together with loss of one metal cation is observed in the reaction of dioxygen with the trinuclear nickel(II) complex Ni3(dmot)2(OH)2... 

With very few exceptions, side-on peroxo bis-ji-oxo rearrangement occurs very rapidly, so 02 binding to copper(I) complexes affords the most stable isomer (or their mixture in cases of their similar stabilities). When two isomers are in a rapid equilibrium, they appear to form from Cu(I) precursors and 02 with equal rates. When dicopper(III) bis- i-oxo form is favored, it forms directly from two molecules of copper(I) complex and a molecule of 02 (or, in some cases, from one molecule of dicopper(I) complex with a dicompartmental ligand and a molecule of 02). As usual, oxygenation rates are fast, even at low temperatures, and activation barriers are low.40 The overall dioxygen binding process can be described by Equation 4.30 ... 

Scheme 18-5 is an alternative proposed by analogy to the mechanism of dioxygen activation mediated by dicopper complexes [58]. Rearrangement of a fi-, 2 peroxo... 

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