Proton-Coupled Electron Transfer and Substituent Effects in Catechol-Based Deep Eutectic Solvents: Gross and Fine Tuning of Redox Activity

The 1,2-diol moiety in a variety of substituted catechols allows formation of room temperature ionic melts in a 2:1 ratio with choline chloride or choline dihydrogen citrate. These deep eutectic solvents were 4.3–6.6 M in redox active catechols. Substituents on 3- and 4-substituted catechols shift both E° and pKa such that Hammett parameters predict the observed Ep for oxidation in square wave voltammetry. The proton acceptor for the proton-coupled oxidation shifts the observed Ep more strongly than the substituents within the substituents and acceptors reported here. The shift is predicted well by the pKa of the conjugate acid of the proton acceptor, i.e., water in aqueous solutions or chloride or dihydrogen citrate in the DESs in this study. Together, the substituent and the proton acceptor allow gross and fine-tuning of the oxidation potential for catechol over 750 mV, the first demonstration of control of the thermodynamics of proton-coupled electron transfer in deep eutectic solvents. Changing the substituents on the HBD affords fine control in tens of millivolts, while changing the base strength of the anion of the organic salt affords gross control across hundreds of millivolts.