Peroxo dimer formation

In many systems there is a marked tendency for p-peroxo dimer formation (Scheme 67) but the 1 1 adducts may be stabilized by the use of polar solvents and by bulky groups in the ligand which hinder the dimerization step. They may also be stabilized kinetically by use of low temperatures, however bases with the ability to hydrogen bond will accelerate dimerization653 and a basicity is also important. Detailed discussions of the factors influencing stability are available.644,646... 

Firstly, compared to the same complex in free solution or as a crystalline solid, the zeolite encapsulated material displays quite remarkable stability towards autoxidation and peroxo-dimer formation(IO). For example the half-life for the oxygen adduct epr signal at room temperature in air is 4weeks when entrapped in the zeolite compared to several minutes in free solution (10) (in the crystalline Co-salen solid, peroxo-dimers are formed exclusively (10)). This is a manifestation of the extremely effective site-isolation achieved by entrapping the complexes inside the pores of the zeolite leading to elimination of the normal autoxidation mechanism. 

While peroxo dimer formation is sometimes reversible for cobalt complexes, it inevitably leads to further oxidations in case of iron complexes. Steric hindrance about the metal center would prevent dimerization processes. 

Of greatest interest are those compounds that attempt to model hemoglobin directly. Simple iron(II) porphyrins are readily autoxidized first to superoxo species, then to //-peroxo dimers and finally to /x-oxo dimers, as represented in equation (60). Bridge formation must be prevented if carrier properties are to be observed. This has been achieved by the use of low temperature and sterically hindered or immobilized iron(II) porphyrins. Irreversible oxidation is also hindered by the use of hydrophobic environments. In addition, model porphyrins should be five-coordinate to allow the ready binding of 02 this requires that one side should be protected with a hydrophobic structure. Attempts have also been made to investigate the cooperative effect by studying models in which different degrees of strain have been introduced. 

Much has been learned from synthetic complexes (Section 17-E-7). The requirements to mimic oxyhemoglobin are the formation of a 5-coordinate heme precursor having a proximal base (imidazol, pyridine, or other) and hindering pathways that would lead to irreversible formation of /i-peroxo dimers. The lifetime of the working models is increased by exclusion of acidic protons and nucleophiles from the 02 binding site and working at low temperatures. 

The proposed mechanism of oxygenation of Cr(II) porphyrins65 is identical to the mechanisms of analogous reactions with iron(II) porphyrins (Equations 4.22-4.25), and involves initial coordination of 02 to a single chromium center, followed by the formation of p-peroxo dimers of Cr(III) and their subsequent transformation into (P)... 

Metal superoxides, (L)M(n + (C - ), are most commonly prepared from (L)M" + and 02. These dioxygen adducts are rarely stable, and normally undergo subsequent reactions (formation of p-peroxo dimers is the most common decomposition pathway). Alternatively, direct reactions with inorganic superoxide (e.g., K02) may yield superoxo complexes ... 

This mechanism involves formation of binuclear complexes (u-oxo complexes) which undergo intramolecular conversion to the peroxo dimer and then hydrolysis of the... 

Necessary conditions for synthetic models to mimic oxyhemoglobins are formation of a five-coordinate heme precursor with a proximal base (e.g., imidazole, pyridine), prevention of p-peroxo/p-oxo (M-02-M/M-0-M) dimer formation upon oxygenation (i.e., reversible oxygenation), and exclusion of protons and nucleophiles. Model systems should be easily modified and allow probing of distal and proximal influences. Tetraphenyl-porphyrins (TPP) serve as the basis of numerous model complexes, because they are less susceptible to oxidation... 

These reactions can be regarded as autoxidation of Co to Co, although both steps (1) and (2) are substitution (complex formation) reactions. Tme autoxidation with loss of an electron by Co occurs on protonation of the n-peroxo dimer, leading to the formation of H2O2, as in the case of -ethylenebis(acetylacetonitiiinato)cobatt(Il). ... 

It is noteworthy that the same reaction with a copper(I) complex with HB(3,5-iPr2Pz)3 causes an instantaneous formation of a dimeric J,-Ti Ti2-peroxo complex [118-120]. Thus, the more steric bulk of HB(3-tBu-5-iPrpz)3) inhibits the secondary reaction between the putative superoxo complex and a copper(I) counterpart generating a p.- n iti -peroxo dimer. While the employment of a N3 tripodal hydrotris(pyrazolyl)borate substantiates a side-on superoxo structure, the other complex prepared with a N4 ligand adopts a terminal coordination of the superoxide as indicated in Fig. 9. 

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