Multiple Substitution Effects and Three-Dimensional Nonlinear Free-Energy Relationships in the Electrochemical Reduction of the N,N'-Dibenzyl Viologen and the 4-Benzoyl-N-benzylpyridinium Cation

摘要

The simultaneous effect of two substituents on the redox potentials,E_(1/2)'s,of N,N'-di(p-substituted benzyl)-viologens (1-X,Y) and of 4-(p-substituted benzoyl)-N-(p-substituted benzyl) pyridinium cations (2-X,Y),was investigated in terms of the substituent constants,sigma-x and sigma_(p-Y).Systems 1-X,Y and 2-X,Y were chosen because they both undergo successive outer sphere one electron (1-e) reductions,and therefore there is no ambiguity regarding the mechanism or the rate-determining step.Substituents (X and Y) were chosen among the -NO_2,-Br,and -OCH_3 groups in six different combinations for l-X,Y's and nine different ones for 2-X,Y's.Response surfaces are essentially three-dimensional Hammett plots.Statistical analysis according to a quadratic model derived from the Taylor series expansion of the general extrathermodynamic relationship E_(1/2)=F(SIGMA_P-X,SIGMA_P-Y) that describes the dependence of E_(1'2)'s on the substituent constants shows that the substitution effect is not additive.Quite strong nonlinear terms appear when substituents can interact through resonance with a site involved in the electron-transfer process.This,for example,is the case for the second -e reduction of 2-X,Y's.Other significant nonlinear effects are attributed to stereoelectronic effects in combination with the high polarizability of pi-electrons.Our results have important implications on our ability to predict the effect of tuning of the redox potential of redox-active species through substitution.