Dioxygen complexes equilibria

However, somewhat to our surprise, mononuclear Tp -Pr- Co 02 proved unstable in solution at ambient temperature. Its decomposition produced [(Tp P M Co)2(ll-0)2] as a transient intermediate and ultimately proceed to the hydroxide. Significantly, no additional metal complex was needed to initiate Ae decomposition, and the reaction took place even in the presence of dioxygen. It is highly unlikely that the substitution of an isopropyl group for a tert-butyl substituent would change the electronic nature of the mononuclear dioxygen complex sufficiently to shift the O2 dissociation equilibrium. Thus there must be another pathway, which is open to Tp P > Co-02, but closed to the sterically more encumbered Tp Co-O2. 

Dioxygen complexes are often formed reversibly and can be characterized by equilibrium constants. In catalytic systems O2 usually enters the catalytic cycle via equilibria ofthe types (1) and (2) ... 

The three rate constants for Eq. (98) correspond to the acid-catalyzed, the acid-independent and the hydrolytic paths of the dimer-monomer equilibrium, respectively, and were evaluated independently (107). The results clearly demonstrate that the complexity of the kinetic processes is due to the interplay of the hydrolytic and the complex-formation steps and is not a consequence of electron transfer reactions. In fact, the first-order decomposition of the FeS03 complex is the only redox step which contributes to the overall kinetic profiles, because subsequent reactions with the sulfite ion radical and other intermediates are considerably faster. The presence of dioxygen did not affect the kinetic traces when a large excess of the metal ion is present, confirming that either the formation of the SO5 radical (Eq. (91)) is suppressed by reaction (101), or the reactions of Fe(II) with SO and HSO5 are preferred over those of HSO3 as was predicted by Warneck and Ziajka (86). Recently, first-order formation of iron(II) was confirmed in this system (108), which supports the first possibility cited, though the other alternative can also be feasible under certain circumstances. 

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. 

Introduction of mesityl groups at the porphyrin ring can prevent the formation of the dimeric products and the reaction with dioxygen now leads to ruthenium(VI)-dioxo complexes of TMP (tetramesitylporphyrin) [35], The tram-Ru(VI)02-TM P species can catalyse the epoxidation of alkenes as well as whole range of other oxidation reactions. After transfer of one oxygen atom to an organic substrate Ru(IV)0-TMP is formed, which disproportionates to an equilibrium of Ru02 and llu ). 

In continuing low-temperature kinetic studies of halo-Cu(I)-amine reactions with 02, Davies et al. have observed and partially characterized per-oxo-copper(II) complexes for L2Cu2C12 (L = teed) [96,97]. At room temperature, complete reduction of dioxygen occurs to give green, dinuclear LCu(Cl,0,Cl)CuL, but at lower temperatures (i.e., <-26°C) two forms of a tetra-nuclear mixed-valence peroxo Cu(II) complex exist in equilibrium. In particular, one of the forms is associated with 380 (e = 1600 M l cm-1) and 650 (e = 650) nm electronic spectral absorptions and a resonance Raman Vo 0 band at 822 cm-1. 

Equilibrium data for the oxygen absorption have been deduced, and it is claimed that, for example, [MnI2(PBu3)] will reversibly absorb and desorb completely one mole of dioxygen per mole of complex for more than 400 times, if the reaction is carried out in solution at — 20 °C. In the solid state only the thiocyanato compounds of formulation [Mn(NCS)2(PR3)] (where R = alkyl) reversibly bind oxygen.192... 

FIGURE 14. Structure of E coli amine oxidase active site in the equilibrium turnover complex. Interactions and labelling as in Figure 12 except that the reduced form of the cofactor is labelled TPR and carries a nitrogen at the 5-position. The aldehyde product, labelled Prod, is retained in the channel to the enzyme surface. Water W4 is positioned for nucleophihc attack on C5 of the cofactor and W2 is retained as a link in a proton shuttle from 04 to dioxygen. 

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