In analogy to 4-(p-substituted benzoyl)-N-methylpyridinium cations (1-Xs), the title species (2-Xs, X = -OCH3, -CH3, -H, -Br, -COCH3. -NO2) undergo two reversible, well-separated (DeltaE(1/2) greater than or equal to 650 mV) one-electron reductions. The effect of substitution on the E(1/2)s of 2-Xs is much weaker than the effect of the same substituents on 1-Xs: the Hammett p-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 pi- vs. sigma-electrons, and to the fact that in 1-Xs the electronic properties of the substituents are transmitted through the pi-system, while in 2-Xs there is a -CH2- 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-NO2 undergoes reduction before the second-electron reduction of the 4-benzoylpyridinium system. Introducing the recently derived substituent constant of the -NO2- (.) group (sigma(p-NO2)-. -&(z.rad) = -0.97) yields an excellent correlation for the third-electron reduction of 2-NO2 (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 -NO2-.. (C) 2003 Elsevier Ltd. All rights reserved.