Oxidation of the metal center

Porphyrin complexes containing transition metal ions at the lower oxidation states, e.g. Mn , Fe , Co , are easily oxidized to the higher oxidation states. In the presence of strong axial ligands, such as water or pyridine, these complexes 

Despite uncertainties concerning the site of the first one-electron oxidation of Fe P, the second oxidation is thought to form a ferry 1 species, 0 = Fe P, which can oxidize or hydroxylate various organic compounds. In parallel with this chemistry of iron porphyrins, the oxidation of ruthenium and osmium porphyrins was studied by radiation chemical methods. 

Ru -porphyrins are generally stabilized with a CO molecule as an axial ligand. Thus the radiolytic oxidation of (CO)Ru OEP and (CO)Ru TPP in aerated CH2CI2 solutions was found to lead first to the radical cation (CO)Ru P- and this species reacts with CF (k = 1 x 10 L mol s ) to form 

Radiolytic oxidation of (CO)Os P in CH2CI2 formed Os P products with various axial ligands, with no indication of a radical cation. Oxidation in alkaline solutions led directly to Os P products, the Os P being an unstable intermediate in this case. 

Radiolytic oxidation of Cr -porphyrins in alkaline solutions also led to the formation of oxo-Cr -porphyrins. Further irradiation led to production of oxo-Cr -porphyrins. Pulse radiolysis studies indicated that Cr P under all conditions is oxidized first to the 7r-radical cation, Cr P-. This radical cation is unstable in aqueous solutions and undergoes disproportionation, but in CH2CI2 it is stabilized by the HCl (produced by fixe radiolysis). The radical cation was further oxidized by irradiation and the product was suggested to be a dication, Cr p2, also stabilized by the HCl. Furthermore, it was found that addition of base to Cr P converts this species into Cr P, and the process can be reversed by the addition of acid. 

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Incorporation of a chiral phosphane allowed resolution of the complex 6 which was obtained in enantiomerically pure form. Reaction of 6 with 2,2-dimethylpropanal provided the adduct 7 as the sole observable aldol product13. Oxidation of the metal center of 7 with ferric chloride induced decomplexation via reductive elimination, to provide the enantiomerically pure cy-clobutanone 8. 

Problems of dilution and pH sensitivity have also been encountered in the synthesis of cts-[Pt(S-R2SO)(olefin)Cl2] complexes (85), where deoxygenation of the sulfoxide with concomitant oxidation of the metal center occurs at low pH. The reactions of [M(Ph2PCH2CH2PPh2)Cl2] (M = Pd, Pt) with one equivalent of silver perchlorate in the presence of Me2SO yield either the O-Me O complex or its deoxygenation product, depending upon reaction conditions. A sequence of reactions, Eq. (27), has been proposed (143). 

As mentioned above, the electroactive NPs discussed here differ from typical semiconductor NPs or capped noble-metal NPs (e.g., Au-monolayer-protected dusters) in an important way. Specifically, the reduction or oxidation of the metal centers in... 

The de- and re-aromatization of the pyridine moiety of the pincer ligand appears to be crucial for this process. This is also the key underlying feature in the oxidative addition of H2 by complex 19 in an apparent iridium(111) oxidation state, which results in the formation of the dihydride complex 20 (Scheme 12.9). Similarly, the addition of CO to the iridium(I) complex 18 formally results in oxidation of the metal center and provides the iridium(III) complex 21 (Equation 12.9). 

For instance, complex 246+ exchanges up to a total of nine electrons. On reduction, it shows two monoelectronic and one bielectronic processes involving the bipyridinium units, and three monoelectronic processes concerning the bpy moieties (Fig. 13.20). On oxidation, two monoelectronic processes are observed the first one, being reversible, is assigned to the oxidation of the metal center and the second one, not fully reversible, to the oxidation of the alongside DMN unit of the macrocycle interlocked with the cyclophane. 

A characteristic reaction of CTTS excited states is photoelectron production with concomitant oxidation of the metal center.107 In the example given in equation (46),108 electrostatic repulsion of the primary photoproducts facilitates their separation and allows direct observation of the solvated electron. In other systems, photoelectron production has been inferred from the products observed in the presence of an electron scavenger such as N20 or CHC13.109... 

The structure of the l,2,3,6-f/4-OFCOT complex 64 is shown in Fig. 6 (140). The coordination geometry around iron can be considered as octahedral, with the allylic portion of the OFCOT ligand and the Fe—C a bond occupying three facial sites. The formal oxidation of the metal center is reflected in the higher values of the CO stretching frequencies (vco = 2105, 2060 cm-1) for complex 64 compared to those for the l-4-r/4-COT complex 63 (vco = 2051, 1992, 1978 cm-1) (139). 

The metal-centered reduction of iron and cobalt porphyrins [(por)Afn] yields metalloporphyrin anions [Eq. (13.13)]. The reduction potential for this reaction is 13, and is equivalent to the N- value for the oxidation of the metal-centered nucleophile [(por)uM-]. The one-electron reduction of alkyl halides yields the... 

Only one nitrido Ww complex has been documented. This was prepared by the action of Me3SiN3 followed by oxidation on dinitrogen W° complex 83 (Eq. (31)) [36]. It has been proposed that the reaction of 83 with trimethylsilylazide affords a Wn complex bearing two N3-ligands a subsequent two-electron oxidation of the metal center then affords nitrido tungsten complex 84. 

Although the addition of free radicals to metal centers, leading to one-equivalent oxidation of the metal [see Eq. (288)] is an oxidative addition, we use the term here to describe those additions of substrates to metal centers that involve overall two-equivalent changes.381 386 The reactions of alkyl halides with cobalt(II) or with iridium(I) provide examples of one-equivalent and two-equivalent oxidations of the metal center, respectively ... 

Cyclic voltametry of the Rhlu complexes [Rh(R)(salen)py] (R = Me, Pr", Pr1 salen = dianion of bis(salicylidene)ethylenediamine) displays reversible one-electron oxidations of the metal center. The ease of formation of the organometallic cation [Rh(R)(salen)py)+depends on the nature of the... 

On the other hand, polytellurides only seem to oxidize metals to the +1 or +11 state. Reaction of equimolar amounts of Te4 with M(CO)6 results in disubstitution of CO forming a cu-complex (CO)4MTe4 (M = Cr (45), W (47)47). If an excess of metal carbonyl is used in the presence of poly-telluride anion, multinuclear products can be isolated and metal-metal bonds can also form, leading to clusters. Careful manipulation of reaction conditions and choice of the polychalcogenide anion used makes possible partial oxidation of the metal centers and cluster formation. The reaction of iron carbonyls with polytelluride anions can lead to a wide array of cluster compounds, the identities of which are controlled by the stoichiometries and compositions of the starting telluride anions. For instance, reaction of [Fe(CO)5] with Te2 leads to the formation of [Fe3(CO)9(ju.3-Te)]2 (48),48 whereas its reaction with increasing amounts... 

In some cases, alkylation may be accompanied by oxidation of the metal center (equations 34 and 35). ... 

The reaction of corrole 2.82 with copper(II) acetate was also found to produce a neutral complex that is spectroscopically identical to that previously described. In this case, however, the complex that results was assigned structure 2.181, wherein oxidation of the metal center from + 2 to +3 appears to have occurred (Scheme 2.1.58). This is in marked contrast to the two previous structural assignments (i.e.. 

The reaction of [OH] with [Mn(EDTA)] (EDTA = ethylenediaminetetra-acetate) and [Mn(NTA)] (NTA = nitrilotriacetate) involves hydrogen abstraction from the ligands and not oxidation of the metal center the unstable Mn(II)-ligand radical intermediates decay via complex first- and second-order kinetics that are pH dependent and involve hydrolysis and disproportionation. The Mn(II) complexes are oxidized by [Brj], [(NCS) and Of, possibly via an inner-sphere mechanism. 

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