Solvents effect on redox potentials

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 chapter deals with the fundamental aspects of redox reactions in non-aque-ous solutions. In Section 4.1, we discuss solvent effects on the potentials of various types of redox couples and on reaction mechanisms. Solvent effects on redox potentials are important in connection with the electrochemical studies of such basic problems as ion solvation and electronic properties of chemical species. We then consider solvent effects on reaction kinetics, paying attention to the role of dynamical solvent properties in electron transfer processes. In Section 4.2, we deal with the potential windows in various solvents, in order to show the advantages of non-aqueous solvents as media for redox reactions. In Section 4.3, we describe some examples of practical redox titrations in non-aqueous solvents. Because many of the redox reactions are realized as electrode reactions, the subjects covered in this chapter will also appear in Part II in connection with electrochemical measurements. 

Solvent Effects on Redox Potentials and Redox Reaction Mechanisms... 

An approach to quantifying the interaction between solute and solvent and hence to solvent effects on redox potentials is that developed by Gutmann.41 Interactions between solvent and solute are treated as donor-acceptor interactions, with each solvent being characterized by two independent parameters which attempt to quantify the electron pair donor properties (donor number)... 

The solvatochromic method developed by Taft and coworkers [94] has also been used to explain the solvent effect on redox potentials. Lay [95] considered specific hydrogen bonding contributions to the thermodynamics and kinetics of electron transfer. 

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). 

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, firstly by ion-dipole or ion-induced dipole interactions and secondly by more direct interactions, such as hydrogen bonding to anions or electron pair donation to cations. The latter interactions, thus, depend on the Lewis acidity and basicity, respectively, of the solvents (Table 4.3). The redox properties of the ions at an electrode therefore depend on their being solvated, and the solvent effects on electrode potentials or polarographic half wave potentials, or similar quantities in voltammetry are manifested through the different solvation abilities of the solvents. 

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). 

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... 

Cobalt(II,III) sepulchrates have been used in the chemical education [415] and considerable number of the chemical and physicochemical studies as efficient quencher of the phosphorescence [416] and electronic excited states [417, 418], as a reductant in kinetic studies of redox reactions [419, 420], as a model for study of magnetodynamic [421], solvent [422] and pressure [423] effects on the outer-sphere electron-transfer reactions. Transfer chemical potentials (from solubility measurements) [424], electrochemical reduction potentials [425] and ligand-field parameters [426] for cobalt sepulchrates have been calculated. Solvent effect on Co chemical shift of cobalt(III) ion encapsulated in the sepulchrate cavity [427]... 

Solvent effects on the redox potential of the Fe(CN)6" -Fe(CN)6 system have been studied. A kinetic study of substitution into [(NQ4Fe(CN)(NQFe(CN)4] " by HCN, pyridine, or 4-methylpyridine (X) indicates the following mechanism ... 

Kassner RJ, Yang W (1977) Theoretical-model for effects of solvent and protein dielectric on redox potentials of iron-sulfur clusters. J Am Chem Soc 99(13) 4351-4355... 

Solvent effects on the redox potentials and electroreduction mechanisms of (TPP)CrCl have been discussed by Bas-solo and Hoffman [151, 152], as well as by Bottomley and Kadish [153]. The former research group demonstrated that (TPP)CrCl can readily coordinate with Lewis bases containing oxygen, sulfur or nitrogen donor atoms to form six-coordinate Cr(III) species of the type (TPP)CrCl(L), while the latter group indicated that the Cr(III)/Gr(II) process was accompanied by chemical reactions coupled with reversible electron transfers. The current-voltage curves led the authors to propose the electron-transfer mechanism shown in Sch. 1. 

Gritzner G, Danksagmirller K, Gutmann V (1978) Solvent effects on the redox potentials of tetraethylammonium hexacyanomanganate (III) and hexacyanoferrate (III). J Electroanal Chem 90(2) 203-210. doi 10.1016/S0022-0728(78)80054-0... 

Gutmann, V., Gritzner, G., and Danksagmiiller, K., Solvent effects on the redox potential of hexacyanoferrate (Ill)-hexacyanoferrate (II). Inorg. Chim. Acta, 17, 81-92 (1976). 

Schmidt MH, Miskelly GM, Lewis NS (1990) Effects of redox potential, steric configuration, solvent, and alkali metal cations on the binding of carbon dioxide to cobalt(I) and nickel (I) macrocycles. J Am Chem Soc 112 3420-3426... 

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