345911-44-6Relevant articles and documents
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
Leventis, Nicholas,Rawaswdeh, Abdel-Monem M.,Zhang, Guohui,Elder, Ian A.,Sotiriou-Leventis, Chariklia
, p. 7501 - 7510 (2007/10/03)
In anhydrous CH3CN a series of nine 4-(4-substituted-benzoyl)-N-methylpyridinium cations (substituent: -OCH3, -CH3, -H, -SCH3, -Br, -C≡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 E1/2 values of both waves and the substituent constants σp-X. The reaction constants for the two 1-e reductions are ρ1 = 2.60 and ρ2 = 3.31. The lower ρ1 value is associated with neutralization of the pyridinium ring, and the higher ρ2 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 σp-NO2- = -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., CH3-OH) 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.
The redox chemistry of 4-benzoyi-N′-methyipyridinium cations in acetonitrile with and without proton donors: The role of hydrogen bonding
Leventis, Nicholas,Elder, Ian A.,Gao, Xuerong,Bohannan, Eric W.,Sotiriou-Leventis, Chariklia,Rawashdeh, Abdel Monem M.,Overschmidt, Travis J.,Gaston, Kimberly R.
, p. 3663 - 3674 (2007/10/03)
In anhydrous CH3CN, 4-benzoyl-W-methyIpyridinium cations undergo two reversible, well-separated (ΔE1/2 ～ 0.6 V) one-electron reductions in analogy to quinones and viologens. If the solvent contains weak protic acids, such as water or alcohols, the first cyclic voltammetric wave remains unaffected while the second wave is shifted closer to the first. Both voltammetric and spectroelectrochemical evidence suggest that the positive shift of the second wave is due to hydrogen bonding between the two-electron reduced form of the ketone and the proton donors. While the one-electron reduction product is stable both in the presence and in the absence of the weak-acid proton donors, the two-electron reduction wave is reversible only in the time scale of cyclic voltammetry. Interestingly, at longer times, the hydrogen bonded adduct reacts further giving nonquaternized 4-benzoylpyridine and 4-(a-hydroxybenzyl)pyridine as the two main terminal products. In the presence of stronger acids, such as acetic acid, the second wave merges quickly with the first, producing an irreversible two-electron reduction wave. The only terminal product in this case is the quatemized 4-(α-hydroxybenzyl)-N-methyIpyridinium cation. Experimental evidence points toward a common mechanism for the formation of the nonquaternized products in the presence of weaker acids and the quaternized product in the presence of CH3CO2H.
