N,N′-dimethyl-2,7-diazapyrenium: a redox-dependent receptor for aromatic carboxylates

Abstract

Studies are presented showing that N,N′-dimethyl-2,7-diazapyrenium, DAP 2+, acts as a redox-dependent receptor for aromatic carboxylates in aqueous solution. Cyclic voltammetric studies of DAP 2+ by itself indicate that its one-electron reduction to DAP + is only quasi-reversible in both aqueous and acetonitrile solutions due to the slow oxidation of DAP + back to DAP 2+. In aqueous solution, the oxidation is very sensitive to other conditions such as pH and the presence of small amounts of iodide. However, in pH 7 phosphate buffer, fairly reversible voltammetry is observed at slow scan rates. Under these conditions, addition of excess aromatic carboxylate results in large negative shifts in the E 1/2 of DAP 2+/+. Similar changes are observed in the 1H NMR spectrum of DAP 2+ in the presence of the different carboxylates, indicating that the potential shift is due to strong interactions between the carboxylates and DAP 2+. Upon reduction, the favorable coulombic interactions between the anionic carboxylates and the cationic pyrenium are weakened, resulting in the observed negative shifts in E 1/2. Although the perturbation of electrostatic interactions causes the potential shifts, the magnitude of the shifts correlates more with the strength of π-stacking interactions between the carboxylates and DAP 2+. Those carboxylates whose π system can come into maximum close contact with the DAP 2+ π system give the largest potential shifts.