Dioxygen bound, reactivity

While most superoxo complexes—in contrast to peroxo compounds— have been assigned a bent, end-on coordination mode [9], the superoxide ligand of Tp Cr(02)Ph was suggested to exhibit the more unusual side-on (r] ) coordination [10]. The reactivity of the complex did not allow for the determination of its molecular structure however, close analogs could be isolated, crystallized and structurally characterized by X-ray diffraction. For example, the reaction of [Tp Cr(pz H)]BARF (pz H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis(3,5-bis(trifiuoromethyl)phenyl)borate) with O2 produced the stable dioxygen complex [Tp Cr(pz H)( ] -02)]BARF (Scheme 3, bottom), which featured a side-on bound superoxide ligand (do-o = 1.327(5) A, vo-o = 1072 cm ) [11]. Other structurally characterized... 

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 reaction of C02 with Ir(CH3)CO(02)[P(p-tolyl)3]2 also results in the formation of a peroxycarbonate complex (191) via external attack by carbon dioxide. In this case, however, only gaseous carbon dioxide is required, rather than the more strenuous conditions of liquid C02. This same complex reacts with gaseous carbon monoxide to form the carbonate complex. Labeling experiments demonstrate that the coordinated CO does not participate in the reaction External attack by the added CO is responsible for the reaction (191). Coordinated CO has been shown to react with bound dioxygen, as is seen in Scheme 16. In this case, the chelating triphos ligand obviously has a significant effect on the reactivity (189). 

On semiconductors many different states of adsorbed dioxygen can be visualized, O2 (bridged), bound di-radical—O—0-, terminally and laterally n-bonded O2, O2", Oz, 0, 04 , O, 0 , 0, etc., some of which have been detected by electron spin resonance. The reactivities of few of these have been investigated. 

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