Oxidizing agents dioxygen

Metal ions play an important role in several of these oxidative reactions as well as in biological dioxygen metabolism. As an example, copper(II) acetate and hydrogen peroxide have been used to produce a stable oxidizing agent, hydroperoxy copper(II) compound. The same oxidation system is also obtained from copper(II) nitrate and hydrogen peroxide (Eq. 1) [103] but requires the neutralization of ensuing nitric acid by potassium bicarbonate to maintain a pH 5. 

Nature uses dioxygen and two electrons to generate the active oxidizing agent, but we have shown that one such intermediate generated electrochemically with a model system (lacking the thiolate, however) is not an oxidant. 

The initiating action of ozone on hydrocarbon oxidation was demonstrated in the case of oxidation of paraffin wax [110] and isodecane [111]. The results of these experiments were described in a monograph [109]. The detailed kinetic study of cyclohexane and cumene oxidation by a mixture of dioxygen and ozone was performed by Komissarov [112]. Ozone is known to be a very active oxidizing agent [113 116]. Ozone reacts with C—H bonds of hydrocarbons and other organic compounds with free radical formation, which was proved by different experimental methods. 

Dioxygen is a formidable oxidizing agent with relatively high standard redox potential under acidic conditions (2) ... 

The kinetic results reported by Jameson and Blackburn (11,12) for the copper catalyzed autoxidation of ascorbic acid are substantially different from those of Taqui Khan and Martell (6). The former could not reproduce the spontaneous oxidation in the absence of added catalysts when they used extremely pure reagents. These results imply that ascorbic acid is inert toward oxidation by dioxygen and earlier reports on spontaneous oxidation are artifacts due to catalytic impurities. In support of these considerations, it is worthwhile noting that trace amounts of transition metal ions, in particular Cu(II), may cause irreproducibilities in experimental work with ascorbic acid (13). While this problem can be eliminated by masking the metal ion(s), the masking agent needs to be selected carefully since it could become involved in side reactions in a given system. 

In the light of the electrochemical data it seems reasonable to propose that in cases where the substituent provides effective steric protection of the central metal, dioxygen can act as a simple (outer-sphere) M(I)-to-M(II) oxidizing agent (reversible one-electron oxidation). In contrast, in those cases where the substituent offers insufficient steric protection, dioxygen can coordinate to the metal by oxidative addition forming the peroxo compound M(III)-022 (irreversible two-electron oxidation). 

The rhodium complexes cis- and irans-[Rh(eda)2(N02 )X]+, where X = N02, or ON , photoreact (182) with dioxygen in aqueous solution to form monomeric [Rh(eda)2(H20)(02)]2+ and dimeric complexes. These complexes act as one-electron oxidizing agents, oxidizing 1 to I2 and Fe11 to Fe111. 

In the early 1960s Neil Bartlett was studying the properties ot platinum hex-alluoride. an extremely powerful oxidizing agent. In fact, by merely mixing dioxygen with plaiinum hexafluoride, it is possible to remove one electron from the oxygen molecule and isolate the product ... 

Binary Compounds. Three fluorides, PtF4 [13455-15-7], PtF5 [37782-184-8], and platinum hexafluoride [136934)5-5], PtF., are well documented. The last is a powerful oxidizing agent and can oxidize dioxygen and xenon (235). Two chlorides exist, platinum dichloride [10025-65-7],... 

Aldehyde oxidase catalyzes the oxidation of aldehydes to carboxylic acids by dioxygen, but also catalyzes the hydroxylation of pyrimidines. Despite its rather broad specificity for substrates, it may well be that aldehyde oxidase should be regarded primarily as a pyrimidine hydroxylase. Thus, xanthine oxidase and aldehyde oxidase catalyze the hydroxylation of purines and pyrimidines respectively. The oxygen incorporated into the product comes from water, not 02. The dioxygen serves as the electron acceptor and other oxidizing agents may be used. 

As mentioned above, it is not necessary for the metal ion itself to be an oxidising agent, and co-ordinated thiolates also undergo rapid oxidation by dioxygen. Very often, in these cases, only catalytic amounts of metal ion are required for the oxidation of the thiols by dioxygen. Examples are seen in the oxidation of mercaptoacetic acid or cysteine in the presence of metal ions (Fig. 9-12). This reaction has obvious implications for the effects of transition metal ions upon proteins containing cysteine residues. 

The trinuclear Fc3(CO)i2 is best prepared in aqueous solution by oxidation of [FeFl(CO)4] , the latter being obtained from Fe(CO)s in alkaline solution (see equations 12 and 13). As the oxidizing agent for [FeH(CO)4] , dioxygen, copper(II), or Mn02 can be used. ... 

Apart from the above-mentioned theoretical applications, there is also the possibility of practical applications. The octacyano complexes of Mo(V) and W(V), for example, find excellent use in the oxidation of organic substances which require a mild oxidizing agent. It was also recently observed that derivatives of the dioxotetracyano complexes of Mo(IV) and W(IV) can take up dioxygen, which leads to the activation of the O2 molecule. The complexes are thus candidates as catalysts in oxidation reactions with dioxygen as well as oxygen-transfer reactions. 

The major problem encountered in earlier works is the lack of an efficient, stable, and fast-acting catalyst capable of mediating the oxidation of water to dioxygen by a strong oxidizing agent (Eq. 8),... 

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