Simple Redox-Dependent Receptors for Arylureas

Abstract

9,10-Phenanthrenequinone and acenaphthenequinone are shown to act as simple redox-dependent receptors toward aromatic ureas in CH 2 Cl 2 and DMF. Reduction of the o-quinones to their radical anions greatly increases the strength of hydrogen bonding between the quinone carbonyl oxygens and the urea N-hydrogens. This is detected by large positive shifts in the redox potential of the quinones with no change in electrochemical reversibility upon addition of urea guests. Cyclic voltammetric studies with a variety of possible guests show that the effect is quite selective. Only guests with two strong hydrogen donors, such as O-H bonds or amide N-H bonds, that are capable of simultaneously interacting with both carbonyl oxygens give large shifts in the redox potential of the quinones. The electronic character and conformational preference of the guest are also shown to significantly affect the magnitude of the observed potential shift. In the presence of strong proton donors the electrochemistry of the quinone becomes irreversible indicating that proton transfer has taken place. Experiments with compounds of different acidity show that the pK a of the protonated quinone radical is about 15 on the DMSO scale, >4 pK a units smaller than that of 1,3-diphenylurea. This is further proof that hydrogen bonding and not proton transfer is responsible for the large potential shifts observed with this and similar guests.