Steric effects redox potential

The influence of the N-bonded substituents R on the half-wave potentials can be described by a Taft relation, like is found for Mo, W and Au. The small value of p points to the dominance of metal orbitals in the redox orbital (5(5). The phenyl derivates do not fit this relation, probably because of a mesomeric influence. Here, however, the n-butyl and cyclohexyl also show small deviations, probably because of steric effects. 

Among the factors which govern the thermodynamic localization of the redox potential, in addition to the previously mentioned electronic effects, there are also the steric influences.134... 

As happens for other physico-chemical techniques, one must first ask if an electrochemical investigation is able to distinguish geometric isomers of the type cisjtrans or facjmer metal complexes. In principle, this is possible since, as mentioned previously, the redox potential of an electron transfer is influenced also by steric effects. For instance, we have seen in Chapter 5 that some octahedral complexes of the scorpiand diammac display different electrochemical responses, depending on whether the two outer amino groups assume cis or trans arrangements. One must keep in mind, however, that the differences in the electrochemical response of isomeric complexes can sometimes be quite small, so may escape a first examination. 

As a general conclusion, one may state that in a homologous series of complexes a linear correlation exists between the redox potential and the spectroscopic parameters when the variation of both parameters are dictated purely by the electronic effects of the substituents. If these effects are present concomitant with steric effects induced by the substituents,... 

The redox system i (R = alkyl Iqx Viologenes was the first to be interpreted correctly (E. Weitz ). It is especially well suited for studying the effects of N-substituents because steric effects are virtually absent. In spite of the great importance of some of these quaternary salts as universal herbicides (R = CH3, paraquat only potentials Ej were known for a long period of time, since the reductions SEM/RED in aqueous medium are mostly irreversible In aprotic media, however, as in acetonitrile and DMF, E2 and Ei are ideally reversibleThis holds true for most of the investi ted substituents as can be seen from Table 1. 

Wheland RC (1976) Correlation of electrical conductivity in charge-transfer complexes with redox potentials, steric factors, and heavy atom effects. J Am Chem Soc 98 3926-3930... 

A model of a flavin-based redox enzyme was prepared.[15] Redox enzymes are often flavoproteins containing flavin cofactors flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). They mediate one- or two-electron redox processes at potentials which vary in a range of more than 500 mV. The redox properties of the flavin part must be therefore tuned by the apoenzyme to ensure the specific function of the enzyme. Influence by hydrogen bonding, aromatic stacking, dipole interactions and steric effects have been so far observed in biological systems, but coordination to metal site has never been found before. Nevertheless, the importance of such interactions for functions and structure of other biological molecules make this a conceivable scenario. 

The activity of the 4-alkylpyrocatechols (la), 3-alkylpyro-catechols (lb), 3,5-dialkylpyrocatechols (Ic), 3,6-dialkylpyro-catechols (II), 4,5-dialkylpyrocatechols (III), 3,4,6-trialkyl-pyrocatechol (TV), and 4,5- and 4,6-dialkyl-2-alkoxyphenols (V and VI) was studied in isotactic polypropylene at 180 =t 0.1° C. The relative activities Ar were correlated with substituent constants and redox potentials. In the la series the activity of 4-n-alkyl derivatives exceeded that of 4-tert-alkyl derivatives. The length of the main alkyl chain in la and lb and the steric effects in the latter exerted a specific favorable influence. In agreement with these effects, 3-tert-alkyl-5-methyl derivatives are the most active compounds in Series Ic compounds with other combinations of alkyls are weaker antioxidants as are the isomeric substances II and III or compound IV. Etherification of one hydroxyl group exerts an unfavorable effect influences of substitution in 2-alkoxyphenols (V and VI) are generally the same as in the pyrocatechols Z-ZZZ. 

First, although ionization potentials are proportional to oxidation potentials, both effectively measure unimolecular processes. Therefore, any steric effects which may hinder the formation of a bimolecular CT complex are not accounted for by simple redox potentials. It is clear that steric factors do influence rates of CT quenching 130,133,174)... 

The structures of many Nin complexes are very sensitive to steric effects these influences are also reflected in the redox potential. Complexes of the cyclam ligand (17-G-X) are particularly stable and may exist as 4-, 5-, or 6-coordinate compounds in the solid state, depending on R. If R = Me both square and sp structures exist in the same unit cell.5... 

Two perfectly reversible one-electron reduction steps are observed for Co.S ", at —0.53 V (Co +/+) and 1.205 V (Co+/ ). Comparison of the electrochemical properties of the cobalt catenate with the previously reported data for cobalt bpy or phen complexes [31, 32] shows here again a strong stabilization of the reduced states, Co and Co". Other sterically constrained polyimine ligands also lead to stabilized monovalent cobalt complexes [33-37]. Remarkable also is the drastic structural effect of the catenate on the Co +/-+ redox potential whereas bpy or phen complexes of cobalt(II) can easily be oxidized to octahedral cobalt(III) [36, 38], the redox potential value of the Co + -+ couple being close to 0 V, no oxidation peak is observed for Co.5 + prior to ligand oxidation (Ep > 1.6 V). The destabilization effect of Co " due to tetrahedral environment provided by the entwined and interlocked structure of 5 is thus very large (>1.5 V). 

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