Ruthenium oxidation-reduction potentials

Charton (J52) has also applied the extended Hammett equation to the oxidation-reduction potentials of 5-substituted phenanthroline complexes of iron in various acidic media (95, 97, 651) and of bis-5- and 4,7-substituted phenanthroline complexes of copper in 50% dioxane (404). Thus, one should expect an overall similarity between the variations in pAa, stability constant, and oxidation-reduction potential data for the various ligands. The variations in a and )3 values found for various substitution positions and the tautomerism in the LH+ ions show that the correlation need not be good. A similar point may also be made about the comparison of data for the transoid bipyridylium ions and their cis complexes. Plots of A versus pA for various systems (95, 404) show a linear dependence to differing extents. As would be expected, the data for analogous complexes of iron (28), ruthenium (214, 217, 531), and osmium (111, 218, 220) show very good correlation. The assumption (152) that the effects of substituents are additive is borne out by these potential data, where the changes in potential on methyl substitution are additive (97). 

With the aim of mimicking, on a basic level, the photoinduced electron-transfer process from WOC to P680+ in the reaction center of PSII, ruthenium polypyridyl complexes were used (182-187) as photosensitizers as shown in Fig. 19. These compounds are particularly suitable since their photophysical and photochemical properties are well known. For example, the reduction potential [Rum(bpy)3]3+/-[Run(bpy)3]2+ (bpy = 2,2 -bipyridine) of 1.26 V vs NHE is sufficiently positive to affect the oxidation of phenols (tyrosine). As traps for the photochemically mobilized electron, viologens or [Co(NH3)5C1]2+ were used. 

Figure 7.9 Electrons travel from the electrode to the ruthenium centers through the bipyridinium dications (a), when the gold voltage (V versus Ag/AgCl) matches the reduction potential of the bipyridinium dications, but cannot return from the ruthenium centers to the electrode (b), even when the voltage is lowered below the oxidation potential of the ruthenium centers.

To illustrate the tuning aspects of the MLCT transitions in ruthenium polypyridyl complexes, let us begin by considering the well-known ruthenium mT-bipyridine complex (1). Complex 1 shows strong visible band at 466 nm, due to charge-transfer transition from metal t2g (HOMO) orbitals to tt orbitals (LUMO) of the ligand. The Ru(II)/(III) oxidation potential is at 1.3 V, and the ligand-based reduction potential is at -1.5 V versus SCE [36]. From spectro chemical and electrochemical studies of polypyridyl complexes of ruthenium, it has been con-... 

The porphyrin complexes of ruthenium and osmium display a rich oxidation-reduction chemistry. Oxidation states +2, +3 +4, and + 6 are well documented. The scope of states that can be realised at the metal is restricted by the fact that the tetrapyrrole ligands (P)2 themselves can be oxidized or reduced to radicals (P )-1 or (P )-3, respectively, at potentials about + 0.7 or - 2.0 V. 

Ruthenium complexes are excellent reagents for protein modification and electron-transfer studies. Ru +-aquo complexes readily react with surface His residues on proteins to form stable derivatives [20, 21]. Low-spin pseudo-octahedral Ru-complexes exhibit small structural changes upon redox cycling between the Ru + and Ru + formal oxidation states [3, 22]. Hence, the inner-sphere barriers to electron transfer (Ai) are small. With the appropriate choice of ligand, the Ru + + reduction potential can be varied from <0.0 to >1.5 V versus NHE [23]. Ru-bpy complexes bound to Lys and Cys residues have been employed to great advantage in studies of protein-protein ET reactions. The kinetics of electron transfer in cytochrome 65/cytochrome c [24], cytochrome c/cytochrome c peroxidase [12], and cytochrome c/cytochrome c oxidase [25] complexes have been measured with the aid of laser-initiated ET from a Ru-bpy label. 

A more detailed picture of electron transfer processes involving Ru(bipy) 3 was provided by a study of the quenching of this excited species by a series of nitrobenzene derivatives (245). The dependence of the quenching rate constant on the reduction potential of the quencher is shown in Fig. 11. The observed dependence is in excellent agreement with that expected for quenching which involves oxidation of the excited ruthenium complex. 

A.iii. Ruthenium Reagents. Ruthenium compounds are powerful oxidizing agents that are capable of cleaving alkenes. In Table 3.1, ruthenium tetroxide (RuOq) showed a reduction potential of 0.59 V in the following reaction ... 

The most numerous thermodynamic data on iron, ruthenium and osmium diimine complexes, are the oxidation-reduction (redox) potentials of the MeLi /MeLi (and similar) couples ). The relation... 

The electrochemical signature in acid medium of ruthenium nanoparticles supported onto carbon shown in Fig. 14.11a resembles that of ruthenium s well-defined surface (0001) [80]. Indeed, surface oxide formation starts at ca. 0.2 V/RHE. The cathodic scan shows a broad peak between 0.2 and 0.4 V attributed to the surface oxide reduction. The irreversibility of this peak is enhanced when a more positive anodic potential is explored. In the alkaline counterpart (pH 13) (Fig. 14.11c), this material clearly shows a well-resolved hydrogen desorption peak centered at 0.1 V. Here, one can observe the interaction of OH species with the ruthenium surface atoms in the region of 0.3 V/RHE. The reverse scan also reveals the reduction of the ruthenium oxide species centered at ca. 0.4 V. Hence, the signature of Ru nanoparticles looks similar in both media. However, comparison of parts (a) and... 

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