Metal-centered oxidation

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 spectroscopic data and the magnetochemistry reported are, in our view, also compatible with a formulation as Mm-OR (M = Co111, Fem) and a Mnlv-OR species. Thus the generation of a coordinated iminosemiquinone is preferred over a genuine metal-centered oxidation. A closer inspection of these interesting compounds by, for example, RR spectroscopy is definitely called for. 

Electrochemistry. Compounds 15a [Fen(pz)pzEts] 15b [Fen(l-MeIm)2Ets] exhibited two reversible oxidations at 0.21 and -0.06 V (vs Ag/AgCl), which were attributed to the 2 —> 3 and 3 —> 4 metal-centered oxidations. These values are 400 mV more positive than those reported for the analogous Fe TPP, suggesting that the pz stabilizes the lower oxidation state, Fe(II) better than does the porphyrin (63). This stabilization of a basic oxidation state for the pz may be attributable to the lower energy n orbitals of the pz making it a better n acceptor than the porphyrin. 

Kruger, H.-J. and Holm, R. H. (1989) Chemical and electrochemical reactivity of nickel(II,I) thiolate complexes - examples of ligand-based oxidation and metal-centered oxidative addition. Inorg. Chem., 28, 1148-55. 

The CVs of alkynylbipyridyl-ruthenium complexes, such as [Ru(bpy)2(5-HC= C-bpy)] " and the related terpy complex, [Ru(terpy)(4-HC=C-terpy)] +, contain reversible metal-centered oxidation waves and several ligand-dependent reduction... 

Combination and rearrangement of Eqs. (16) and (23) lead to expression (24) that correlates linearly the ligand-centered reduction potential with the metal centered oxidation potential (d is the denticity of the reducible LL ligand). The slope of this line is Yl/Ym, and the intercept is a constant for the particular reducible ligand [66]. 

Relationships between redox potentials and the energy hv) of a metal-to-ligand charge transfer (M LCT) band have been well documented and expressed, for complexes [M(LL)WXYZ], in a simplified way, by Eq. (25) in which C is a constant and A (Redox) is given by Eq. (26), that is, the difference between the metal centered oxidation potential and the ligand centered reduction potential [67]. 

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. 

Some properties of Ir(III) complexes of (NANAN) ligands are listed in Table 11 [25,65,90,91] Fig. 16 displays schematic structures of the complexes. As noted above, for these non-cyclometalated complexes, the metal-centered oxidation step is difficult and is not observed in the usually explored potential range [25]. 

In either event, the decrease in the metal d orbital energy expected to accompany such metal-centered oxidations would limit the metal contribution to the HOMO in the resulting dication [107]2+, which would be anticipated to display considerable carbon character. Indeed, the third oxidation of 107 gives the open-shell cation [107]3+, for which experimental evidence strongly supports a predominantly carbon-centered radical... 

Spectroelectrochemical (EPR, UV Vis, res. Raman, and to a lesser extent IR) characterization of redox products often reveals features characteristic of reduced polypyridine ligands for ligand-localized reductions or of oxidized metal atoms for metal-centered oxidations. Stretching CO frequencies, obtained by IR spectroelectrochemistry, are an especially useful marker of a metal oxidation state in carbonyl-polypyridine complexes. 

Potentials of polypyridine-localized redox couples depend linearly on the reduction potential of free polypyridines with a slope close to unity, when measured for homologous series of structurally related complexes [26, 67, 74, 101, 151, 205, 206]. This argument should be used with caution because even the potentials of metal-centered oxidation depends on the free polypyridine reduction potential, albeit with a significantly lower slope [29, 67] (Sections 5.3.2 and 5.3.3). Nevertheless, correlations of redox potentials in a series of [M(N,N)3] complexes, N,N = bpy, 4,4 -Me2-bpy, and 5,5 -Me2-bpy, with those of free ligands were often sufficient to identify N,N-localized reductions [101, 111, 117]. 

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