Complexes of molecular oxygen

The most common pathway for catalysis of autoxidations by transition metal complexes involves the decomposition of alkyl hydroperoxides. Another route that may be possible for chain initiation involves direct oxygen activation, whereby the complexation of molecular oxygen by a transition metal would lower the energy of activation for direct reaction with the substrate [reaction (9)]. For example, oxygen coordinated to a metal might be expected to possess properties similar to alkylperoxy radicals and undergo hydrogen transfer with a hydrocarbon ... 

From a practical point of view the study of the chemistry of dioxygen complexes has considerable interest. Complexation of molecular oxygen by a transition metal has been widely adopted by biological systems as a means of reducing the considerable kinetic barrier to the reduction of O2. Quite apart from the inherent interest of the biological systems, the transition metal complexes offer the possibility of efficient catalysis of autooxidation reactions, and have recently attracted interest as possible catalysts for the reduction of O2 in fuel cells ... 

The counterpart of 50 (Y = OH, OOH) in an aprotic medium is the zwitterion 77 or its cyclic tautomer 78. Precisely how such species are formed from the weak, brownish charge-transfer complex of molecular oxygen with 1 is a matter for debate. Spin-forbidden as it... 

Charge transfer 23, 34 Charge-transfer complex, of molecular oxygen and tetraaminoethylenes 730... 

Inner-sphere incomplete charge transfer occurs, in particular, in complexes of molecular oxygen with the coiqrer dipyridyl complex [dipy2Cu02 in which the oxidation state of copper is intermediate between 1 and n. 

Many complexes of molecular oxygen with transition metal ions are known. In all cases, the election doisity shi from the metal to O2. According to this fact, die ccanplexes of the )>eioxide complexes are distinguished... 

The ff-oxidation of carbonyl compounds may be performed by addition of molecular oxygen to enolate anions and subsequent reduction of the hydroperoxy group, e.g. with triethyl phosphite (E.J. Bailey, 1962 J.N. Gardner, 1968 A,B). If the initially formed a-hydroperoxide possesses another enolizable a-proton, dehydration to the 1,2-dione occurs spontaneously, and further oxidation to complex product mitctures is usually observed. 

Write a balanced equation for the reduction of molecular oxygen by reduced cytochrome e as carried out by complex IV (cytochrome oxidase) of the electron transport pathway. 

Activation of molecular oxygen on interaction with transition metal complexes. A. V. Savitskii and V. I. Nelyubin, Russ. Chem. Rev. (Engl. Transl.), 1975,44,110-121 (124). 

In the field of nonheme iron complexes, Miinck, Collins, and Kinoshita reported the oxidation of benzylic alcohols via stable p-oxo-bridged diiron(IV) TAME complexes, which are formed by the reaction of iron-28 complexes with molecular oxygen (Scheme 23) [142]. 

An additional study on the reaction of molecular oxygen with an NHC complex is depicted in Scheme 10.3 [13]. The tri-NHC cobalt complex 9 is obtained by reaction of T1MEN > 8 (TIMEN = tris[2-(3-alkylimidazol-2-ylidene)ethyl]amine) with... 

A full discussion of the reactions of molecular oxygen involved in dioxygen adducts would require a separate review. Moreover, the reactions of co-ordinated molecular oxygen are discussed elsewhere l-8) dioxygen adducts of biological systems (7) and synthetic group VIII metal complexes 1,3) being of particular interest. 

Complex B) can be reduced in acid aqueous medium by Fe " to complex A), the process results in the evolution of molecular oxygen 74), this being ascribed to the decomposition of A). In analogous reductions using Cr ", and Eu ", a common intermediate was detected (75), which has been assigned the stmcture... 

The reversibility of the carrier was tested by cyclic voltammetry. The scan of the solvent and supporting electrolyte is shown in Fig. 13, with and without dissolved oxygen. The oxygen reduction occurs at about — 0.43 V. (vs. SCE). The scan with the complex added, but the solution free of dissolved oxygen is shown as Fig. 14. The carrier is seen to be reduced at about 0.04 V, well within the window of the solvent and electrolyte, and well before reduction of molecular oxygen. 

The development of such a reaction proceeding under mild conditions is a technological challenge constituting one of the key points for the finalizing of efficient and low cost fuel cells. The catalytic properties of macrocyclic complexes like porphyrins and phthalocyanines for the reduction of molecular oxygen have been well known for four decades350,351 and numerous papers are devoted to this area. Here only some relevant and recent work in this field is described. 

The electrocatalysts for oxygen reduction were prepared as follows. These complex compounds were inoculated onto the carbon (AG-3, BET area near 800 m2/g) by means of adsorption from dimethylformamide solutions. The portion of complex compound weighed so as to achieve 3% of Co content was mixed with the carbon, then 5 ml of dimethylformamide per 1 g of the carbon were added and the mixture was cured at room temperature for 24 hours. Series of samples obtained were thermally treated (pyrolyzed), and the resulting grafted carbons were tested as electrode materials in the reaction of molecular oxygen reduction. 

It is practical to place a washing bottle or scrubber in the gas line just before the manifold. The aqueous solution in this bottle contains a reductant for traces of molecular oxygen and at the same time wets the gas which will minimize a concentrating effect on the sample by drying. A practical solution is 1 mM zinc acetate, 1 pM TMP (meso-tctra(/V-methyl-4-pyridyl)porphinc-tetra-tosylate), 100 mM Na2EDTA, 100 mM Tris-HCl buffer at pH 10. The porphyrin complexates the Zn2+ and forms a light-sensitive compound that can be excited by near UV light from an 18 watt TL-tube. 

Cyclic ylide complexes are readily oxidized not only by aggressive reagents like elemental halogens but also by milder oxidants like disulfides, or even mercaptanes in the presence of molecular oxygen, as shown for a reaction with pyridine-2-thiol in air (Equation (34)).182 A similar reaction is known to take place with thiuramdi-dulfides.2,180... 

Although there are indeed only few reported methods of direct activation of molecular oxygen via transition metals, there are many reports of indirect oxidation. The majority of this research is based on palladium-based oxidation as summarized in equation 32. The palladium complex catalyzed oxidation reactions have been reviewed previously186 and also only very recently187 and in this book the palladium catalyzed oxidation of dienes and polyenes will be discussed separately and therefore will not be discussed... 

Copper is part of several essential enzymes including tyrosinase (melanin production), dopamine beta-hydroxylase (catecholamine production), copper-zinc superoxide dismutase (free radical detoxification), and cytochrome oxidase and ceruloplasmin (iron conversion) (Aaseth and Norseth 1986). All terrestrial animals contain copper as a constituent of cytochrome c oxidase, monophenol oxidase, plasma monoamine oxidase, and copper protein complexes (Schroeder et al. 1966). Excess copper causes a variety of toxic effects, including altered permeability of cellular membranes. The primary target for free cupric ions in the cellular membranes are thiol groups that reduce cupric (Cu+2) to cuprous (Cu+1) upon simultaneous oxidation to disulfides in the membrane. Cuprous ions are reoxidized to Cu+2 in the presence of molecular oxygen molecular oxygen is thereby converted to the toxic superoxide radical O2, which induces lipoperoxidation (Aaseth and Norseth 1986). 

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