Self-Assembling, Chromogenic Receptors for the Recognition of Dicarboxylic Acids

The synthesis of ligands (2,9-disubstituted phenanthrolines) bearing one or two acylaminopyridine binding sites, compounds 1 and 2 respectively, is described. Each ligand can assemble on a Cu(I) template, forming two different receptors for dicarboxylic acids, Cu(1)2⁺BF₄^- and Cu(2)₂⁺BF₄^–. These orange Cu(I) complexes are shown to bind (K_a > 10⁴ M^–1) to a variety of dicarboxylic acids in chloroform, with a slight preference for the C₅-dicarboxylic acids, glutaric and N-CBz-glutamic acids, over shorter and longer substrates. Complexation is analyzed both by NMR chemical shift changes and UV–visible absorption changes. The data indicate formation of 1:1 complexes for Cu(1)₂⁺BF₄^– and 2:1 complexes for Cu(2)₂⁺BF₄^–, with the dicarboxylic acid substrate hydrogen bonding simultaneously to an acylaminopyridine binding site on each ligand. For Cu(2)₂⁺BF₄^–, the complexation event results in large shifts in the visible absorption bands and a color change from orange to red. The change in the visible absorbance, and therefore the chromogenicity, was found to be substrate dependent. The chromogenic effect is explained by a conformational change in the receptors resulting from hydrogen bond formation with the substrate.