Diindolylquinoxalines:  Effective Indole-Based Receptors for Phosphate Anion

Abstract

The synthesis of a new series of 2,3-diindol-3'yl quinoxalines (DIQ), as well as a comparison of their anion recognition properties to those of our previously reported pyrrole based sensors, 2,3-dipyrrol-2'yl quinoxalines (DPQ) is reported. To the best of our knowledge, this new DIQ system represents the first example of a free-standing indole-based small molecule receptor for which evidence of anion binding is available both in solution and in the solid-state. It also provides one of the few structurally characterized neutral receptor-dihydrogen phosphate complexes. This work thus serves to demonstrate the utility of indoles as an anion recognition motif.Anions are ubiquitous in biological milieus. Among the most important physiologically relevant anions is inorganic phosphate. Not only does it play a critical role in energy storage and signal transduction, 1 it recognized as essential structural element in teeth and bones. 2 On the other hand, elevated serum phosphate levels can result in calcification of tissues 3 and is a recognized problem in patients suffering from chronic kidney failure. Phosphate is also a key pollutant whose role in the eutrophication of waterways is well recognized. 4 This diversity of function has stimulated considerable current interest devoted to the recognition and sensing of phosphate anions. 5 Increasingly, efforts in this area have focused on the use of acyclic small molecule receptors, 4 including those based on amide, sulfonamide, urea, and pyrrole recognition units. While each of these motifs presents certain advantages, there is incentive to explore additional putative binding subunits that could be used to generate new receptor systems. One such motif is indole, a potential hydrogen bond donor that has yet to be exploited extensively in the area of anion sensor development. Indeed, currently only indolocarbazoles 1 6 and bis(indolyl)methane, 2 7 are known to the best of our knowledge, and, in the case of the latter system, quantitative binding studies were not fully carried out. Thus, there remains a need to explore the fundamental anion binding properties (e.g., selectivity, affinity) of indolebased receptors. Towards this end, we report here the synthesis of a new series of 2,3-diindol -3'yl quinoxalines (DIQ, 6 and 7), as well as a comparison of their anion recognition properties to those of our previously reported pyrrole based sensors, 2,3-dipyrrol-2'yl quinoxalines (DPQ, 3). 8 To the best of our knowledge, this new DIQ system, which relies on β-connectivity, represents the first example of an indole-based small molecule receptor for which evidence of anion binding is available both in solution and in the solid-state. It also provides one of the few structurally characterized neutral receptor-dihydrogen phosphate complexes. 9 sessler@mail.utexas.edu. Supporting Information Available: Details describing the synthesis and characterization of compound 6 and 7, details of fitting binding curves, and crystallographic data (CIF). This material is available free of ch...

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