Solvent effects redox potential

A further important feature of HMPA is its stabilizing effect on the Redox potential of [Fe(CO)4]2 by ion solvation. In less polar solvents, electron-transfer reactions take place and [Fe(CO)4]2 is oxidized to [HFe3(CO)iThis redox reaction is suppressed in HMPA. 

Azo-bridged ferrocene oligomers also show a marked dependence on the redox potentials and IT-band characteristics of the solvent, as is usual for class II mixed valence complexes 21,22). As for the conjugated ferrocene dimers, 2 and 241 the effects of solvents on the electron-exchange rates were analyzed on the basis of the Marcus-Hush theory, in which the t/max of the IT band depends on (l/Dop — 1 /Ds), where Dop and Ds are the solvent s optical and static dielectric constants, respectively (155-157). However, a detailed analysis of the solvent effect on z/max of the IT band of the azo-bridged ferrocene oligomers, 252,64+, and 642+, indicates that the i/max shift is dependent not only on the parameters in the Marcus-Hush theory but also on the nature of the solvent as donor or acceptor (92). 

It needs to be pointed out that E values may also be quite sensitive to the nature of the solvent and supporting electrolyte used for an electrochemical study. Apart from solvation effects of the non-specific type, solvent molecules may occupy coordination sites in either the starting complex or the products and hence influence redox behaviour (Fabbrizzi, 1985). Similarly, the nature of the anion present may also strongly influence the redox potential if it has ligating properties (Zeigerson etal., 1982). Because of such effects, caution needs to be exercised in attempting to compare electrochemical data which have not been obtained under similar conditions. 

The redox equilibria can be considerably shifted by the presence of additional donor units. Thus the redox potential in a donor solvent will be influenced by the presence of anions and it may be different for a metal chloride and a metal iodide. The effect becomes more pronounced if the supporting electrolyte contains anions which have donor properties. Such donor anions will compete with solvent molecules for coordination. 

The role played by the electron-donating or electron-withdrawing effects of the various ligands in rendering the oxidation process easier for [VO(salen)] compared to [VO(saloph)] or [VO(acen)] is evident. The influence of the solvent on the redox potentials is also clear. 

Consequently, it is also apparent that the solvent effect can be described on the basis of mathematical relationships between parameters which fall within the relationships defined as free energy correlations. In fact, the more parameters that are included in the mathematical treatment (multi-parameter equations), the better the description of the solvent effect that results. However, we will consider here only those parameters which take into account the solvent effect on redox potentials. 

This result confirms that in order to have an adequate treatment of the effect of solvation on the redox potential, one should make use of multiparameter equations which take into account, on a case by case basis, the acid, basic and electrostatic character of the solvent, thus allowing evaluation of their respective contributions. 

It should be mentioned that in aprotic media redox potential for the reduction of superoxide to peroxide, E 02 /0 ), is significantly catodically shifted, so that it is even more negative that the redox potential for the oxidation of superoxide to dioxygen. This is exactly the reason why the superoxide is stabilized in aprotic solvents, whereas peroxide is extremely unstable under such conditions. However, coordination of superoxide to the metal center induces effect similar to that caused by protonation, and the Oj /0 redox potential shifts anodically. Thus, upon binding to a metal cation, superoxide can be reduced in aprotic media, as well. 

The long lifetimes and high redox potentials of a range of ruthenium(II) complexes and in particular [Ru(bpy)3] " have important consequences for their use as photoactive redox catalysts. This area of research is extremely active and we now focus on the decay of the excited state of [Ru(bpy)3] + ( [Ru(bpy)3] " ) and its quenching. Braterman et al. have described the electronic absorption spectrum and structure of the emitting state of [Ru(bpy3] +, and the effects of excited state asymmetry. The effects of solvent on the absorption spectrum of [Ru(bpy)3] " have been studied. In H2O, MeCN and mixtures of these solvents, the value of e(450 nm) remains the same ((4.6 0.4) x 10 dm mol cm ). The ground state spectrum is essentially independent of... 

The EPA properties of the metal ion decrease with increasing donicity of the solvent so that the stabilizing effect of the radical anion is decreased the redox potential is shifted to more negative values by increasing solvent donicity (Fig. 2). 

If organic solvents such as ketones or alcohols are added in considerable amounts, the redox potential is shifted towards more negative values (22) since, owing to the breakdown of the outer-sphere hydration, the stabilizing effect of water is no longer available (alcohols are known to have considerably weaker EPA properties). The outer-sphere hydration structure is more readily destroyed by the increasing donicity of the solvent that replaces water in the mixture. The destructive effect is more pronounced with the reduced species than with the oxidized one since the latter is less stabilized by hydration, and the redox potentials thus become more negative. 

The redox potentials found for given concentrations of acids are related to the Ka values, which are indicative of their EPA properties. The effects become more pronounced at extremely high acid contents and increase from propionic acid to formic add. This is a bulk solvent effect (formic acid is more highly structured than propionic acid). 

PKa = 4.4, in water), less than O2 that the potential of 0 2 /H02 becomes higher than that of 02/0 2 . As a consequence, the superoxide disproportionates into O2 and HO2 , in the presence of proton sources. An evaluation of the solvent effect on the redox potential of the 02/0 2 system is not easy because of the difficulty in comparing the potential scales in various media but, obviously, assuming that the junction potential between the aqueous SCE and every solvent does not exist is far from correct [12] adopting any extrathermodynamic hypothesis would be better. The important shift in the one-electron reduction of O2 to 0 2 , almost 0.5 V, has been attributed to the solvation of 0 2 , which is much more strongly solvated by water than by the aprotic media hexamethylphosphorotriamide (HMPT) is the solvent where the 2/0 2 potential is... 

The association of sulfur and iron into simple to more complex molecular assemblies allows a great flexibility of electron transfer relays and catalysis in metalloproteins. Indeed, the array of different structures, the interactions with amino-acid residues and solvent and their effect on redox potential and spectroscopic signatures is both inspiring for chemists and electrochemists, and of paramount importance for the study of these centers in native conditions. Most of the simpler natural clusters have been synthesized and studied in the laboratory. Particularly, the multiple redox and spin states can be studied on pure synthetic samples with electrochemical and spectroscopic techniques such as EPR or Fe Mossbauer spectroscopy. More complex assembhes still resist structural... 

Catalysts and their effects on chemical reactions aid in efficiency, effectiveness and selectivity. A recent example of current research is redox and ligand exchange reactions of the oxygenation catalyst (N,N -bis(salicylidene)ethylenediaminato)co-balt(II), Co(SALEN)2 (below), and its one-electron oxidation product, Co(salen) 2-These were investigated in DMF, pyridine, and mixtures of these solvents. Solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition are important considerations (Eichhorn, 1997). The results in these solvents should be compared with other work with catalysts using more environmentally benign media (Collins et al., 1998). 

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