Metal-centered oxidation process

Larger dendrimers based on a Ru(bpy)2+ core and containing up to 54 peripheral methylester units (12) have recently been obtained [29a]. Both the metal-centered oxidation and ligand-centered reduction processes become less reversible on increasing dendrimer size [29b]. 

The hexanuclear Ru6 species has four outer and two inner metal centers oxidation active. Both in acetonitrile at room temperature ( 1/2 at + 1.52 V) and in liquid S02 at low temperature ( 1/2 at + 1.46 V), an oxidation process involving the practically simultaneous one-electron oxidation of the four outer Ru(II) centers is evidenced (Fig. 5.9 and Table 5.1). This confirms that the electronic interaction between metal centers that are not directly connected via a bridging ligand is negligible from an electrochemical viewpoint in the metal-polypyridine dendrimers. At more positive potentials, only recordable in liquid S02 at low temperature (Fig. 5.9), a bielectronic process, related to the simultaneous one-electron oxidation of the two inner metal centers at + 2.11 V, is found. This result was at a first sight surprising, since the redox... 

These carboxylate ions are weaker counterions than CP and Pp, so that the corresponding dendrimers tend to ionize more easily and to give clearer signals in MALDI-TOF mass spectra. As to the electrochemical properties, their cyclic voltammetries show a reversible metal centered oxidation and two reversible ligand-centered reduction processes at potential values very similar to those of the corresponding dendrimers with CP counterions. Therefore, the [Ru(tpy)2]2+ complexes are electrochemically equivalent and can efficiently store charges. 

Concerning the [Ru(bpy)(5)2]2+ complex (Table 2), it is interesting to note that the metal-centered oxidation is shifted to a more positive potential value in comparison to the other complexes, as expected because of the presence of two ligands easier to reduce than bpy. On reduction, this complex shows three monoelectronic and reversible processes (Table 2) that, on the basis of the electron-acceptor abilities of the two different ligands present in the complex, can be assigned, the first two, to the reduction of the two ligands 5, and, the third one to the reduction of the bpy ligand. 

The vast majority of the coordination compounds of Os that have been prepared are in the oxidation states 11 and III. Moreover, many of these compounds show reversible or well defined Os / couples in which the electronic and redox properties at the metal are controlled by the a-donor, 7r-acceptor, and r-donor properties of the ligands. Indeed, the study of the redox behavior in Os / and Ru / species, metal ions in which octahedral coordination is almost universally retained in both redox partners, has been central in recent developments to parameterize metal centered redox processes as a function of ligand donor and acceptor capacity. The chemistries of Os and Os are, therefore, intimately linked, and have been extended to studies of important mixed valence Os / binuclear and polynuclear species (see Mixed Valence Compounds). For the purposes of brevity and convenience, this section will deal with Os and Os complexes together. The extensive literature on Os / complexes has been developed with a very wide range of donor ligands a comprehensive assessment of this work is beyond the scope of this article, and the reader is directed to published comprehensive reviews. " ... 

Oxidative-addition and reductive elimination reactions of transition metal complexes are crucial to many homogeneously catalyzed reactions and are important for bond formation. Reactant molecules such as H2 and Oj undergo oxidative addition to many transition metal centers. Oxidative addition of carbon-hydrogen bonds has been an active area of research. Reductive elimination is the process whereby products are eliminated from transition metal centers. 

Rate Expression of the Proton-Catalyzed Dissolution of Oxide Minerals. Surface protonation may accelerate detachment of reduced surface metal centers. The process is similar to the acceleration of detachment of nonreduced surface metal centers by additional protonation of their nearest-neighbor hydroxo and oxo groups. Therefore we briefly discuss the theory of the proton-catalyzed dissolution of oxide minerals (8). 

In order to understand fluorescence quenching of anthyl units in Rh(I) and Ir(I) complexes of the type shown in Fig. 26, the redox chemistry of these complexes has been investigated (91). There are two main redox processes observed with cyclic voltammetry in THF a reversible antryl centered reduction and an irreversible metal centered oxidation in all cases. The observation of irreversible oxidation waves in THF indicates that the electrode generated cationic species are not stable at room temperature. Apparently, however, the use of the solvent l,l,l,3,3,3-hexafluoropropan-2-ol (HFP) somewhat stabilizes these species. Oxidation of the M(I) species with thalium(III) trifluoroacetate in... 

Porphyrin radical cations were also produced by 7-irradiation in frozen organic media at 77 K. In hydrocarbon solvents, both the radical anion and the radical cation were formed from the porphyrin, but addition of a small amount of CCI4 eliminated the radical anion. Other experiments were carried out in chlorinated hydrocarbons. In this manner, the optical absorption spectra and the ESR spectra were recorded for the radical cations of chlorophyll, Pb TPP, Cu OEP, and V OTEP. The observed spectra led to the conclusion that in all these metalloporphyrins the species observed is the radical cation, i.e. the unpaired spin is predominantly on the porphyrin ring. Other metalloporphyrins, however, can be oxidized or reduced at the metal center. Such process are discussed below. 

The (postulated) hydrometallation pathway of hydroalkoxylation (or hydration) of olefins (Scheme 2b) relies on O-H bond activation by oxidative addition of RO-H to metal centers, a process that is studied eagerly in the organometallic chemistry community [3, 24]. However, the insertion of olefins into the M-H bonds of metal... 

The results presented in the following three reports are on systems in which the electron transfer step to Ce(IV) is rapid and the subsequent chemistry is complex. In Holecek et al. (1979) the first step in the ceric oxidation of ferrocene produces unstable ferricenium cations which subsequently decompose with oxidation of the ligand. In Soria et al. (1980) and Chum and Helene (1980) the initial electron transfer process results in both metal-centered oxidation of the tris(2-pyridinial-a-methyl-(methylimine))-Fe(II) complex as well as oxidation of the a-methyl group to an aldehyde group, with no change in the oxidation state of iron. 

Polymetallorotaxanes 7.24 (M = Zn" or Cu ) have been prepared by electropolymerization, which involved anodic oxidation of the pre-assembled metallorotaxane precursors (Scheme 7.2) [48]. Importantly, studies of these materials have allowed an evaluation of the individual contributions of the organic backbone and the metal-centered redox process to the overall conductivity measured on interdigitated microelectrodes. The Zn and Cu polymers behave quite differently. The Zn polymer behaves in a similar fashion to the metal-free material 7.25, whereas the matching of the polymer and Cu-centered redox potentials in 7.24 (M=Cu ) leads to enhancement of the communication between these two units resistance drops by a factor of 10 for the Cu polymer 7.24 relative to metal-free 7.25. In a further development in this general area, two-step electropolymerizations have been used to generate three-stranded conducting ladder polymetallorotaxanes 49]. 

Hydroxyl radicals possess week electrophilic properties as indicated by the order of reactivity of substituted benzenes and distribution of phenolic isomers, although the latter depends on the reaction conditions [28, 29]. The Fenton hydroxylation in aqueous solution reveals small (<5%) values of the NIH shift (i.e., migration of hydrogen atom from the site of hydroxylation to the adjacent carbon [30]). The reaction in CH CN demonstrated remarkably high shift values (30-40%) [31], which is typical of enzymatic processes [30]. Sawyer and coworkers proposed that the change in solvent might favor a mechanistic shift from HO to a metal-centered oxidant [32]. 

Similar Ru(II) polypyridine units were connected to dirhodium(ll) tetracarboxylate platforms to form the supramolecular assemblies 80-82 (Figures 9.37 and 9.38) [166], exhibiting severed metal-centered oxidation and ligand-centered reduction processes, and intense LC and MLCT bands centered on the Ru(II) units. Efficient energy transfer from MLCT (Ru-based) to the lowest-energy... 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

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