Protonation of the dianion of tetraphenylethylene by alcohols and water in acetonitrile. Case of mixed first-order and second-order kinetics in the proton donor

Abstract

The electrochemical reduction of tetraphenylethylene has been studied in dipolar aprotic solvents containing tetraethylammonium perchlorate. While in hexamethylphosphoramide and N,N-dimethylformamide the radical anion is more stable than the dianion, in acetonitrile the dianion is directly produced at the electrode in a reversible two-electron step. Voltammetric measurements in acetonitrile indicate that the disproportionation equilibrium constant of radical anions is large (>3 × 10²), suggesting that ion-pair interactions play a leading role in the overall reduction.

The protonation of the dianion by alcohols and water (ROH proton donors) has been studied. Kinetic measurements have led to a rate law in which the pseudo-first-order rate constant can be expressed as the sum of two terms, i.e. k_obs=a[ROH]+b[ROH]². The reactivity sequence of ROH proton donors is MeOH > EtOH > H₂O > PrⁱOH > Bu^tOH and MeOH > EtOH, H₂O > PrⁱOH > Bu^tOH for the first- and second-order contributions, respectively. Analysis of kinetic data and solute–solute–solvent interactions has led to a protonation mechanism based on the existence of a hydrogen-bonded complex between the ion-paired dianion and the ROH proton donors. The role of a second ROH species in the actual proton transfer is discussed.