Tuning the Redox Chemistry of 4-Benzoyl-N-methylpyridinium Cations through Para Substitution. Hammett Linear Free Energy Relationships and the Relative Aptitude of the Two-Electron Reduced Forms for H-Bonding

Abstract

In anhydrous CH3CN a series of nine 4-(4-substituted-benzoyl)-N-methylpyridinium cations (substituent: -OCH3, -CH3, -H, -SCH3, -Br, -Ctbd1;CH, -CHO, -NO2, and -(+)S(CH3)2) demonstrate two chemically reversible, well-separated one-electron (1-e) reductions in the same potential range as other main stream redox catalysts such as quinones and viologens. Hammett linear free energy plots yield excellent correlation between the E(1/2) values of both waves and the substituent constants sigma(p)(-)(X). The reaction constants for the two 1-e reductions are rho(1) = 2.60 and rho(2) = 3.31. The lower rho(1) value is associated with neutralization of the pyridinium ring, and the higher rho(2) value with the negative charge developing during the 2nd-e reduction. Structure-function correlations point to a purely inductive role for substitution in both 1-e reductions. The case of the 4-(4-nitrobenzoyl)-N-methylpyridinium cation is particularly noteworthy, because the 4-nitrobenzoyl moiety undergoes reduction before the 2nd reduction of the 4-benzoyl-N-methylpyridinium system. Correlation of the third wave of this compound with the 2nd-e reduction of the others yields sigma(p)(-NO)2*- = -0.97 +/- 0.02, thus placing the -NO2-* group among the strongest electron donors. Solvent deuterium isotope effects and maps of the electrostatic potential (via PM3 calculations) as a function of substitution support that 2-e reduced forms develop H-bonding with proton donors (e.g., CH3OH) via the O-atom. The average number of CH3OH molecules entering the H-bonding association increases with e-donating substituents. H-bonding shifts the 2nd reduction wave closer to the first one. This has important practical implications, because it increases the equilibrium concentration of the 2-e reduced form from disproportionation of the 1-e reduced form.

Synthesis and Electrochemistry of 2-Ethenyl and 2-Ethanyl Derivatives of 5-Nitroimidazole and Antimicrobial Activity against Giardia lamblia

“…This correlation suggests that the C2-olefin bridge provides an electronic communication between the pendant phenyl ring and the NI. Dividing the slope of this type of Hammett plot by the Nernst equation coefficient of 0.0592 V yields a dimensionless parameter that is directly comparable to the reaction parameter, ρ , obtained from the slopes of Hammett plots of other types of reactions 32. This calculation gives a ρ of 0.84 for the data in Figure 4B, indicating that the reduction of these compounds is slightly less sensitive to the substituent on the phenyl group than the acid dissociation of benzoic acid with a ρ value of 1.0.…”

Structure–property relationships based on Hammett constants in cyclometalated iridium(iii) complexes: their application to the design of a fluorine-free FIrPic-like emitter

“…43 In general, it is possible to build strong QSPR between the nature of these substituents based on their Hammett σ-values and the corresponding electrochemical properties of series of compounds, and one can observe a linear dependence of oxidation (E OX ) or reduction (E RED ) potentials with respect to the Hammett parameters. [1][2][3]5,[44][45][46][47][48] For cyclometalated iridium(III) complexes, although there are reports on the correlation between oxidation potential E OX and σ, 7,50 as well as the correlation between the redox gap (ΔE REDOX = E OX − E RED ) and absorption or emission wavelength (λ abs , λ em ), 50,51 there is apparently no general correlation between λ em and Hammett σ-values. The early work of Watts and coworkers nicely illustrates these two paradigms.…”

Palladium-Catalyzed Room-Temperature Acylative Suzuki Coupling of High-Order Aryl Borons with Carboxylic Acids

“…26 White solid: 0.128 g yield, 65%; mp 74−76 °C; 1 H NMR (400 MHz, CDCl 3 ) δ 8.98 (d, J = 1.6 Hz, 1H), 8.80 (dd, J 1 = 4.8 Hz, J 2 = 1.6 Hz, 1H), 8.11 (dd, J 1 = 8.0 Hz, J 2 = 2.0 Hz, 1H), 7.74 (d, J = 8.0 Hz, 2H), 7.45 (dd, J 1 = 7.6 Hz, J 2 = 4.8 Hz, 1H), 7.32 (d, J = 8.0 Hz, 2H), 2.46 (s, 3H); 13 p-Tolyl-4-pyridylmethanone (3sa). 27 White solid: 0.140 g yield, 71%; mp 94−95 °C; 1 H NMR (400 MHz, CDCl 3 ) δ 8.80 (dd, J 1 = 4.4 Hz, J 2 = 1.6 Hz, 2H), 7.73 (d, J = 8.0 Hz, 2H), 7.56 (dd, J 1 = 4.4 Hz, J 2 = 1.6 Hz, 2H), 7.31 (d, J = 8.0 Hz, 2H), 2.46 (s, 3H); 13…”