Hydrogen-bonding and protonation effects in electrochemistry of quinones in aprotic solvents

Hydrogen-bonding and protonation are fundamental factors controlling potentials and mechanisms in the reduction of quinones. These are studied systematically in benzonitrile, acetonitrile, and dimethylsulfoxide solutions by cyclic voltammetry of a series of quinones of increasing basicity (chloranil to duroquinone), in the presence of hydroxylic additives of increasing hydrogen-bonding power (tert-butyl alcohol to hexafluoro-2-propanol) or acidity (trifluoroacetic acid). Electrochemical effects are demonstrated over the complete interaction range, from hydrogen bonding of reduced dianions to protonation of unreduced quinones. With increasing concentrations of additives, three clearly different types of electrochemical behavior are observed for weakly (I), moderately (III), and strongly (III) interacting quinone-additive pairs, as follows: (1) Two well-separated reduction waves, corresponding to formation of quinone mono- and dianions, shift positively, with no loss of reversibility. The second wave is smaller, shifts more strongly, and finally merges with the first. The relative heights of the waves remain constant. (II) The positive shift is accompanied by increasing height of the first peak and broadening and irreversibility of the second wave. (III) one or even two, more positively shifted, new prior waves appear, together with a new anodic wave. In interpreting these phenomena, the role of hydrogen-bonding is clearly distinguished from protonation on the basis of pK(a) values of relevant species, effects of solvent variation, magnitude of potential shifts, and the onset of irreversibility. Type I behavior is attributed to stabilization by hydrogen-bonding of mono- and dianion reduction products; the number of bonds per quinone ion and bonding equilibrium constants are estimated from the shifts in peak potentials with additive concentration. These results are supported by simulating the experimental cyclic voltammograms using these parameters. Type III behavior is assigned to initial hydrogen-bonding or protonation of the quinones. Type II is attributed to a reduction mechanism involving disproportionation of primary radicals, assisted by hydrogen-bonding or protonation of the dianion.