The Halide Binding Behavior of 2-Carbamoyl-7-ureido-1H-indoles: Conformational Aspects

Indole-based anion receptors with an carboxamide unit in 2- and an urea in 7-position were prepared and found to bind halides (as well as acetate and nitrate) in chloroform solutions at room temperature. Investigations of the binding behaviour show that the receptor is selective for chloride. Surprisingly, the truncated receptor 3 without the 2-carbamoyl substituent shows the highest affinity for Cl-. Thorough H-1, C-13 and N-15 NMR investigations indicate different binding modes for acetate, nitrate and halides to the receptor 2. The observation of a major conformational change of this receptor during the binding Of the halide ions leads to an understanding of the relative binding affinities of 3 > 4 > 2 for chloride. The results of the NMR study are supported by ab initio calculations. In addition, ESI FTICR MS competition experiments of the indole 2 and the quinoline I reveal the self-aggregation of the receptors and show that halides have a higher affinity to 2 than to 1. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)