Naphthoquinone derivatives as receptors for the chromogenic sensing of metal cations and anions

Abstract

The behaviour of the naphthoquinone derivatives 4-(4-dimethylamino-phenyl)-[1,2]naphthoquinone (L 1) and 4-[4-(1,4,7,10,13-pentaoxa-16-aza-cyclooctadec-16-il)-phenyl]-[1,2]naphthoquinone (L 2) as chromogenic chemosensors of metal cations and anions in acetonitrile has been studied. L 1 and L 2 are highly coloured receptors with a broad band centred at 534 (log ε = 3.80) and 532 nm (log ε = 3.86), respectively, and attributed to an intra-ligand charge transfer band involving the amine and the carbonyl groups. L 1 shows a hypsochromic shift of the absorption band in the visible zone in presence of Cu 2+, Fe 3+ and Hg 2+. Additionally, L 2 suffers bleaching induced by the metal cations Pb 2+ and Ba 2+. In both cases, the data suggested metal cation coordination through the nitrogen atom of the aniline group. 1H NMR studies in the presence of certain metals are also in agreement with this interpretation. log K for the formation of the corresponding [M(L)] 2+ complexes have been determined from nonlinear least-squares treatment of the titration profiles. Studies in the presence of anions were also carried out. In acetonitrile a chromogenic sensing of the anions HSO 4 - was observed using the metal complex Ba–L 2.