Electrochemically controlled hydrogen bonding. o-quinones as simple redox-dependent receptor for arylureas

9,10-Phenanthrenequinane and acenaphthenequinone are shown to act as simple redox-dependent receptors toward: aromatic ureas in CH2Cl2 and DMF. Reduct ion of the o-quinones to their radical anions greatly increases the strengt h of hydrogen banding between the quinone carbonyl oxygens and the urea N-h ydrogens. This is detected by large positive shifts in the redox potential of the quinones with: no change in electrochemical reversibility upon addit ion 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 gi;ve large shifts i n the redox potential of the quinones. The electronic character and conform ational preference of the guest are also shown to-significantly affect the magnitude of the; observed potential shift. In the presence of strong proto n donors the electrochemistry of the quinone becomes irreversible indicatin g that proton transfer has taken place. Experiments with compounds of diffe rent;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 re sponsible for the large potential shifts observed with this and similar gue sts.