Electrochemical reduction of 4-benzoyl- N-(4-substituted benzyl)pyridinium cations: substitution effects and linear free energy relationships

Abstract

In analogy to 4-( p-substituted benzoyl)- N-methylpyridinium cations ( 1-Xs), the title species ( 2-Xs, X=OCH 3, CH 3, H, Br, COCH 3, NO 2) undergo two reversible, well-separated (Δ E 1/2≥650 mV) one-electron reductions. The effect of substitution on the E 1/2s of 2-Xs is much weaker than the effect of the same substituents on 1-Xs: the Hammett ρ-values are 0.80 and 0.93 for the first- and second-electron reduction of 2-Xs vs. 2.3 and 3.3 for the same reductions of 1-Xs, respectively. The difference has been attributed to the different polarizability of π- vs. σ-electrons, and to the fact that in 1-Xs the electronic properties of the substituents are transmitted through the π-system, while in 2-Xs there is a CH 2 spacer. From a practical standpoint, these results suggest that the 4-benzoylpyridinium system can be used as an almost continuously tunable redox couple, whose redox potential can be course-tuned via p-benzoyl substitution and fine-tuned via p-benzyl substitution. Importantly, the nitro group of 2-NO 2 undergoes reduction before the second-electron reduction of the 4-benzoylpyridinium system. Introducing the recently derived substituent constant of the NO 2 − group ( σ p- NO 2 − − &z.rad; =−0.97) yields an excellent correlation for the third-electron reduction of 2-NO 2 (corresponding to the reduction of the carbonyl group) with the second-electron reduction of the other 2-Xs, and confirms the electron-donating properties of NO 2 − .