Metal oxygen transfer from ligand

Despite of the common reaction mechanism, peroxo complexes exhibit very different reactivities - as shown by the calculated activation energies -depending on the particular structure (nature of the metal center, peroxo or hydroperoxo functionalities, type and number of ligands). We proposed a model [72, 80] that is able to qualitatively rationalize differences in the epoxidation activities of a series of structurally similar TM peroxo compounds CH3Re(02)20-L with various Lewis base ligands L. In this model the calculated activation barriers of direct oxygen transfer from a peroxo group... 

This mechanism can be illustrated by the reaction of ferrous ions with hydrogen peroxide (42), the reduction of organic peroxides by cuprous ions (63), as well as by the reduction of perchlorate ions by Ti(III) (35), V(II) (58), Eu(II) (71), The oxidation of chromous ions by bromate and nitrate ions may also be classified in this category. In the latter cases, an oxygen transfer from the ligand to the metal ion has been demonstrated (8), As analogous cases one may cite the oxidation of Cr(H20)6+2 by azide ions (15) (where it has been demonstrated that the Cr—N bond is partially retained after oxidation), and the oxidation of Cr(H20)6+2 by 0-iodo-benzoic acid (6, 8), where an iodine transfer was shown to take place. 

The two faces of the C=C double bond are prochiral, which means that opposite enantiomers result from oxygen transfer to one face rather than to the other. The binding of the alcoholic function of the substrate to titanium imposes some steric constraints so that one face of the C=C double bond is forced over the other with respect to the metal. This effect of the alcoholic function is known as secondary interaction, it is not directly involved in the actual oxygen transfer, but very important in directing it. The epoxidation reaction proceeds by oxygen transfer from the coordinated alkylperoxy ligand to the olefin via the so-called butterfly intermediate, followed by... 

Thiazyl monomer can be stabihzed by coordination to a metal, and many thionitrosyl complexes with Cr, Mo, Re, Ru, Os, Co, Rh, Ir, and Pt are known. Comparison of the spectroscopic properties and the electronic stmctures of M-NS and M NO complexes indicates that NS is a better a-donor and jr-acceptor ligand than NO. Oxygen transfer from an NO2 to an NS ligand on the same metal center occurs in ruthenium porphyrin complexes. ... 

C. Reactions Involving Oxygen Transfer from Nitrosyl Ligands to Early Transition Metals... 

As described below, the metal insertion into the salen ligand with acetic acid as solvent was determined to be fast and not the rate-limiting step of the global reaction. The activation step was then studied to determine if a pure chemical resistance limited the reaction rate or if oxygen transfer from the gas phase into the liquid mixture controlled the reaction. The kinetic rate of oxidation should be independent of the rate of oxygen introduction into the organic liquid phase under chemical control, but directly related to the rate of oxygen introduction under mass transfer limitations. 

Formation of high-valent oxometal species from (salen)metal complexes and iodosylbenzene (PhIO) and the mechanism of the oxygen transfer from these species to alkyl aryl sulfides have been investigated in detail [22]. Fluorous salen ligands 13 and 14 were synthesized and the corresponding (salen)manganese(III) complexes Mn-13 and Mn-14 were evaluated in the oxidation of alkyl aryl sulfides with PhIO under homogeneous and FB conditions, respectively [23]. 

NO disproportionation has been shown to be promoted by the Mn(II) tropocoronand complex Mn(TC-5,5) (82) (Eq. (38)), and the reaction was found to involve three equivalents of NO leading to formation of N20 and O-coordinated nitrito ligand. The electron balance is provided by oxidation of Mn(II) to Mn(III). The mononitrosyl complex Mn(TC-5,5)(NO) was proposed to react with NO to produce an unstable cis-dini-trosyl, Mn(TC-5,5)(NO)2, which is then poised to form an N-coordinated hyponitrito (0=N-N=0) ligand from which oxygen transfer occurs to another NO (82a). The intermediacy of a hyponitrito ligand parallels other proposed mechanisms for metal complex promoted NO disproportionation (5a-d). 

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