The redox chemistry of 4-benzoyl-N-methylpyridinium cations in acetonitrile with and without proton donors: The role of hydrogen bonding

In anhydrous CH3CN, 4-benzoyl-N-methylpyridinium cations undergo two revers ible, well-separated (DeltaE(1/2) similar to 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 remain s unaffected while the second wave is shifted closer to the first. Both vol tammetric and spectroelectrochemical evidence suggest that the positive shi ft of the second wave is dde to hydrogen bonding between the two-electron r educed form of the ketone and the proton donors. While the one-electron red uction product is stable both in the presence and in the absence of the wea k-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-benzoylpyridi ne and 4-(alpha -hydroxybenzyl)pyridine as the two main terminal products. In the presence of stronger acids, such as acetic acid, the second wave mer ges quickly with the first, producing an irreversible two-electron reductio n wave. The only terminal product in this case is the quaternized 4-(alpha -hydroxybenzyl)-N-methylpyridinium cation. Experimental evidence points tow ard 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.