Dioxygen complexes reactions

AH of the commercial inorganic peroxo compounds except hydrogen peroxide are described herein, as are those commercial organic oxidation reactions that are beheved to proceed via inorganic peroxo intermediates. Ozonides and superoxides are also included, but not the dioxygen complexes of the transition metals. 

Reversible formation of a dioxygen complex by direct reaction of O2 with trani-[Ir(CO)Cl(PPh3)2] discovered by L. Vaska. 

Finally, it should be noted that there are a large number of irreversible H(G) dioxygen complexes (see Refs. 1, 77) which undergo a whole range of oxidative reactions with a variety of substrates. For instance, M(Ph3P)4, where M = Ni, Pd, Pt, react with O2 in solution as follows (194). 

Studies of coadsorption at Cu(110) and Zn(0001) where a coadsorbate, ammonia, acted as a probe of a reactive oxygen transient let to the development of the model where the kinetically hot Os transient [in the case of Cu(110)] and the molecular transient [in the case of Zn(0001)] participated in oxidation catalysis16 (see Chapters 2 and 5). At Zn(0001) dissociation of oxygen is slow and the molecular precursor forms an ammonia-dioxygen complex, the concentration of which increases with decreasing temperature and at a reaction rate which is inversely dependent on temperature. Which transient, atomic or molecular, is significant in chemical reactivity is metal dependent. 

Photooxidation of Co2(CO)8 is a complex reaction, with major products being the simple monomers Co(02)(CO) (n = 1, 2) with the dioxygen bound side-on (rj2).89 The same products arise from reaction of Co atoms with CO and 02 at 10-12 K. 

The above reaction scheme was established by a combination of uv-visible absorption and fluorescence, ir isotopic substitution, esr and kinetic measurements (37). The important point to note here is that in 02 rich Xe matrices, ground state Cu(2Sj/2) cannot avoid reactive encounters with 02 to form Cu(02)2 and Cu(02) dioxygen complexes,whereas it is proposed that the formation of CuO, Cu(03) and 03 in dilute 02/Xe matrices arises from the reaction of a long lived mobile excited state Cu(2D) with 02. On the other hand the reactions of photoexcited Ag(2P) with 02 are different (37), electron transfer being favoured to form Ag 02. ... 

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. 

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

Reviews on the activation of dioxygen by transition-metal complexes have appeared recently 9497 ). Details of the underlying reaction mechanisms could in some cases be resolved from kinetic studies employing rapid-scan and low-temperature kinetic techniques in order to detect possible reaction intermediates and to analyze complex reaction sequences. In many cases, however, detailed mechanistic insight was not available, and high-pressure experiments coupled to the construction of volume profiles were performed in efforts to fulfill this need. 

Estabrook, R.W., Hildebrandt, A.G., Baron, J., Netter, K.J. and Leibman, K. (1971) A new spectral intermediate associated with cytochrome P-450 function in liver microsomes. Biochemical and Biophysical Research Communications, 42 (1), 132-139. Pompon, D. and Coon, M.J. (1984) On the mechanism of action of cytochrome P-450. Oxidation and reduction of the ferrous dioxygen complex of liver microsomal cytochrome P-450 by cytochrome b5. Journal of Biological Chemistry, 259 (24), 15377-15385. Hildebrandt, A. and Estabrook, R.W. (1971) Evidence for the participation of cytochrome b 5 in hepatic microsomal mixed-function oxidation reactions. Archives of Biochemistry and Biophysics, 143 (1), 66-79. 

Any reaction of the type to be considered here begins with the interaction of an organometallic compound with O2. This may lead to the formation of a dioxygen complex, however fleeting its existence. Further reactions may ensue. In the following sections we summarize the available results, organized by type of transformation. We begin with the evidence for coordination of O2. 

The reaction of organometalhc compounds with O2 may produce more or less stable dioxygen complexes. An early and unambiguous example of this kind of transformation was provided in the report by van Asselt et al. of the isolation of a series of stable peroxo alkyl complexes of the type Cp Ta( -02)R (R = Me, Et, Pr, Bn, Ph) [5]. As shown in Scheme 1, O2 presumably oxidatively adds to the 16-electron fragments Cp 2TaR, which are in rapid equihbrium with the 18-electron olefin hydrides or alkylidene hydrides. 

While most superoxo complexes—in contrast to peroxo compounds— have been assigned a bent, end-on coordination mode [9], the superoxide ligand of Tp Cr(02)Ph was suggested to exhibit the more unusual side-on (r] ) coordination [10]. The reactivity of the complex did not allow for the determination of its molecular structure however, close analogs could be isolated, crystallized and structurally characterized by X-ray diffraction. For example, the reaction of [Tp Cr(pz H)]BARF (pz H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis(3,5-bis(trifiuoromethyl)phenyl)borate) with O2 produced the stable dioxygen complex [Tp Cr(pz H)( ] -02)]BARF (Scheme 3, bottom), which featured a side-on bound superoxide ligand (do-o = 1.327(5) A, vo-o = 1072 cm ) [11]. Other structurally characterized... 

The crystal structure of the dioxygen complex Tp V(0)(02)(L) has been determined, and revealed a side-on bonded O2 ligand. Based on the O - O distance of 1.379(6) A and vo-o of 960 cm it was formulated as a V(V) peroxo complex, even though these values are on the borderline between the peroxo and superoxo designations. The mechanism of this reaction is curious. Reaction with 02 showed incorporation of solely in the O2 ligand and not in the 0x0 groups. It appears that the O2 binding step must be preceded by a disproportionation (2 V(IV) V(III) + V(V)) followed by reaction of V(III) with O2. 

Dioxygen could overcome the kinetic barrier of its unpaired electrons and triplet ground state by excitation to its first excited state (xAg), in which all electrons are paired. Unfortunately, this species, referred to as singlet oxygen, is generally too reactive and too short-lived for most situations (lb, lc). However, dioxygen complexation to a transition metal can also result in activation and create stable complexes that can be studied, modified, and used in further reactions in a controlled manner (2). This latter type of activation is the subject of this chapter. 

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