Dioxygen, bonding

Figure 3.18 Metal-dioxygen bonding in platinum-dioxygen complexes.

In order to systematise the literature, the following classification is used with respect to the metal dioxygen bonding orientations found in oxygen carriers. 

Figure 2.7 Surface oxidation at Zn(0001) in an ammonia-rich NH3-02 mixture at 120, 160, 200 and 240 K compared with 02(g) at 200 K as a function of 02 exposure. An (NH3-02) complex (transient) provides a low-energy pathway to dioxygen bond cleavage. The rate of dioxygen bond cleavage is increased by a factor of up to 300 in the presence of ammonia. (Reproduced from Ref. 46).

Zn(0001) 02 C5H5N Facile route to dioxygen bond cleavage50... 

A less common reactive species is the Fe peroxo anion expected from two-electron reduction of O2 at a hemoprotein iron atom (Fig. 14, structure A). Protonation of this intermediate would yield the Fe —OOH precursor (Fig. 14, structure B) of the ferryl species. However, it is now clear that the Fe peroxo anion can directly react as a nucleophile with highly electrophilic substrates such as aldehydes. Addition of the peroxo anion to the aldehyde, followed by homolytic scission of the dioxygen bond, is now accepted as the mechanism for the carbon-carbon bond cleavage reactions catalyzed by several cytochrome P450 enzymes, including aromatase, lanosterol 14-demethylase, and sterol 17-lyase (133). A similar nucleophilic addition of the Fe peroxo anion to a carbon-nitrogen double bond has been invoked in the mechanism of the nitric oxide synthases (133). 

In the first reaction, the two-electron reduction of molecular oxygen is followed by protonation of the resulting anionic species to yield hydrogen peroxide. On the other hand, the second reaction requires cleavage of the dioxygen bond, followed ultimately by protonation of hydroxide ions to afford water this process has not been observed unless each of the oxygen atoms is able to bind to a unique metal center. 

Inspection of Table I shows that the dioxygen bond length becomes progressively larger on going from O2 to O2 and this increase is accompanied by a decrease in the dioxygen bond strength. These facts can be explained... 

INDO calculations on these complexes257 gave a net stabilization of the planar geometry over the tetrahedral one. The nickel-dioxygen bond was formed primarily through the donation and back-donation of electrons using the nu and Jt oxygen orbitals parallel to the molecular plane. 

Tsai TE, Groves JL, Wu CS (1981) Electronic structure of iron-dioxygen bond in oxy Hb A and its isolated oxy and oxy chains. J Chem Phys 74 4306-4314... 

Figure 4.5.4 Dioxygen bond activation by cytochome c oxidase (adapted from [60]).

The mechanism of MMO including the different states of the iron dimer complex has been reviewed several times [66, 67, 68]. The lowest oxidation state of the diiron complex is Fe2(II,II) which is a loosely bound, ferro-magnetically coupled dimer with a long Fe-Fe distance. This complex, termed O, reacts with O2 to form another complex P, which is normally assigned to an Fe2(III,III) peroxide complex. One or more intermediates in between O and P have been postulated [70]. In the next step, the dioxygen bond is cleaved and an unprecedented Fe2(IV,IV) complex termed Q is formed. The oxidation state assignment was made based on Mbssbauer spectroscopy [71]. Compoimd Q has been suggested to be the active oxidant that attacks methane. 

Fig. 4.78. Role of the distal histidine in the "pull" mechanism for the cleavage of the dioxygen bond and creation of the high-valent iron-oxo porphyrin it cation radical (Compound I) in peroxidases.

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