The selective sensing of anions by natural and synthetic receptors is an important area of supramolecular chemistry. [1] Anions play significant roles in biological and environmental processes, therefore the development of new chemosensors for anions is an important topic. Anion-sensing receptors incorporate into their structures groups that are capable of interaction with anions, and sensing subunits in which spectroscopic [2] or electrochemical [3] features change upon anion binding. Chemosensors with changes in their spectroscopic behavior have either fluorogenic [4] or chromogenic [5] signalling subunits. Chromogenic reagents are especially attractive because the anion determination can be carried out by the naked eye, without the use of expensive equipment. Chromogenic reagents for the selective detection of inorganic and organic anions have been reported. [6] However, it is still a challenge to find chromogenic receptors for the selective sensing of poorly coordinating anions such as nitrate. As far as we know, the only example of nitrate sensing by color change, by use of receptors coupled to dyes, was reported recently, and it involved a competitive assay between nitrate and methyl red or resorufin in binding a polyamide cage, in dichloromethane:methanol 50:50 v/v. The system, however, is not specific and addition of bromide or perchlorate also produced color changes, although to a lesser extent. [7] We now report a new and specific chromogenic reagent for nitrate using a pnitrophenylazobenzene group as a dye and a mercuric complex as an anion-binding site. The system shows a selective change of color in acetonitrile but can also be applied to the selective determination of nitrate in water.
The aza-oxa-thia macrocycle (see Scheme 1) was obtained by the cyclization of 3,6-dioxa-1,8-octanedithiol and dimesilated N,N-diethanolphenylamine in acetonitrile/K 2 CO 3 at reflux, [8] under high dilution conditions. The macrocycle was obtained in a 40 % yield. The macrocycle was coupled with the azonium salt of p-nitroaniline in HCl to obtain L 1 as a redorange powder (80 % yield). 1 H NMR and 13 C NMR spectroscopy, mass spectrometry, and elemental analysis are consistent with the proposed formulation.
The visible spectrum of the ligand L 1 in acetonitrile is characterized by an intense band centered at 490 nm (e 26 000 m À1 cm À1 ), which is responsible for the orange color of the solutions and is caused by a charge transfer from the have potential applications as a pigment dispersant, [10, or in the separation and purification of proteins. [7,