Porphyrins dioxygen adducts

Proniewicz LM, Paeng IR, Nakamoto K. 1991. Resonance raman spectra of two isomeric dioxygen adducts of iron(II) porphyrins and rr-cation radical and nonradical oxoferryl porphyrins produced in dioxygen matrixes Simultaneous observation of more than seven oxygen isotope sensitive bands J Am Chem Soc 113 3294. 

Further to section III(C), we shall see that type I dioxygen complexes can be formed in solution via intermediates of a type 11(F) structure. Consequently, sections IV( ) and IV(F) will tend to overlap in places. The dioxygen adducts of some new synthetic metal porphyrins belong to the 11(F) classification. However, a discussion of the dioxygen adducts of naturally occurring and synthetic metal porphyrins is postponed until section V. 

We saw previously that a major factor in inhibiting the bimolecular termination reaction was the presence of sufficiently bulky ligands so that a monomeric dioxygen adduct could be isolated 135). A number of synthetic metal porphyrins 239) have been prepared recently which satisfy the above requirement, and bind molecular oxygen we shall now proceed to discuss these. 

The importance of a cis effect exerted by the porphyrin ligand is more difficult to assess, but is now accepted as well (35-39). It seems that the porphyrin ligand gives the otherwise inert Fen-d6 configuration a sufficient lability to ensure a rapid equilibration of [<5] with dioxygen (31), while the imidazole moiety stabilizes the dioxygen adduct (40). 

This type of complex is derived from the mononuclear superoxo species via a further one-electron reduction of the dioxygen moiety. Cobalt is the only metal to form these complexes by reaction with dioxygen in the absence of a ligating porphyrin ring. Molybdenum and zirconium form peroxo-bridged complexes on reaction with hydrogen peroxide. In most cases the mononuclear dioxygen adducts of cobalt will react further to form the binuclear species unless specific steps are taken to prevent this. 

The reaction of iron(II) porphyrins is an area of extreme interest and importance and will be discussed separately. The second row analogues of FeI porphyrins, Ru" porphyrins, have been found to bind dioxygen reversibly153 but only UV data are available on these dioxygen adducts. 

There are three different approaches which can be employed to prevent irreversible oxidation of iron(II) porphyrins and promote dioxygen adduct formation (1) low temperatures to reduce the rate of dimerization (2) steric hindrance to prevent dimerization and (3) immobilization of the heme to prevent dimerization. 

Further work on porphyrins of the type Fe(porphyrin)L2 showed that these complexes also bind dioxygen reversibly at low tempertures.158,159,160 This was due to the kinetic stabilization of the dioxygen adduct at reduced temperatures. The presence of a covalently bound axial ligand was not important in stabilizing the dioxygen adduct relative to the p-oxo dimer. 

Steric hindrance. An obvious way of preventing p-oxo dimer formation, i.e. irreversible oxidation of iron(II) porphyrins, is to introduce bulky substituents at the porphyrin such that the resultant dioxygen adducts are unable to approach each other closely enough for dimer formation to occur. 

Iron(II) meso-tetrakis(2,4,6-trimethoxyphenyl)porphyrin and the triethoxy analogue have been synthesized by Vaska. These are different to the capped and picket fence porphyrins in that both sides of the porphyrin are sterically hindered. Dioxygen uptake by these iron(II) porphyrins at 25 °C in solution was found to be partially reversible. La Mar and co-workers166 have shown that the porphyrins Fe[T(2,4,6-OMe)3PP], Fe[T(2,4,6-OEt)3PP] and Fe[TpivPP] take up oxygen at low temperatures, in the absence of a base such as Melm or pyridine, to give dioxygen adducts of the... 

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