Dioxygen species characteristics

Table 3 lists some relevant observed frequencies for diatomic molecules, ions and radicals the effect of the charge on the frequency in the series of the dioxygen species is worthy of note and also the vibrational frequencies characteristic of hydrogen halides which generally polymerize in the condensed phases. The HX stretching bands are shifted markedly to lower frequencies upon polymerization. 

Later reports (58) have questioned whether the earlier report (55) was correct in concluding that the planar cobalt(II) complex of salen was formed in zeolite Y. The characteristics of the supposedly zeolite-entrapped [Con(salen)] are apparently not as similar to the same species in solution as previously reported. For example, planar [Con(salen)] and its adducts with axially disposed bases are generally ESR-detect-able low-spin complexes (59), and cyclic voltammetry of the entrapped complex revealed a Co3+/Co2+ redox transition that is absent in solution (60). These data, and more recent work (58), indicate that, in the zeolite Y environment, [Con(salen)] is probably not a planar system. Further, the role of pyridine in the observed reactivity with dioxygen is unclear, since, once the pyridine ligand is bound to the cobalt center, it is doubtful that the complex could actually even fit in the zeolite Y cage. The lack of planarity may account for the differences in properties between [Con(salen)] entrapped in zeolite Y and its properties in solution. 

The dioxygen complex shows oxygen transfer reactions characteristic of species containing coordinated peroxide (42), and, for example, we have isolated a sulfato complex from the reaction with S02 and a carbonato complex from a reaction with C02 in the presence of excess phosphine, and we have detected a peroxycarbonato species from the reaction with C02 (44). A reaction with H2 simply replaces the 02 to yield H2IrCl(PCy3)2, Complex 7 (44). 

Although the basic structure of these synthetic species is very similar to that of the natural corrinoids their reactivity shows several differences thus [Co(A2ODC)] + does not form an adduct with dioxygen or give organometallic derivatives, which are characteristic features of the chemistry of Vitamin B12 and related compounds [13]. This failure has been attributed to a different electronic structure, as confirmed by the very different optical spectra of the two systems (Fig. 32). 

The characteristic feature that becomes apparent is nature s use of high-valent metal-oxo species that form during dioxygen activation. Such species are aggressive oxidants. Hence, their creation is tightly coupled to the presence of substrate in a confined space, which is absolutely critical for the reactions to proceed in a controlled manner. 

The methyliron(lll) corrphycene complex has been prepared and is stable in the absence of dioxygen the methylcobalt(lll) complex is stable even in the presence of dioxygen. 153 Radiol)ftic reduction occurs at the metal center to produce Fe(ll) and Co(l) corrphycenes, which are also stable upon further radiolytic reduction in neutral solutions they yield species with chlorin-like spectral signatures. In alkaline solutions, species with phlorin-anion-like characteristics are produced. 153 ... 

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