Selective Colorimetric Detection of Hg2+ and Mg2+ with Crown Ether Substituted N‐Aryl‐9‐aminobenzo[b]quinolizinium Derivatives

Abstract 1,4‐Dioxa‐7,13‐dithia‐10‐azacyclopentadecane (AT215C5) or 1,4,7,10‐tetraoxa‐13‐azacyclopentadecane (A15C5) were attached as metal‐ion‐binding receptor units at the ortho or para positions of the 9‐amino‐N‐phenylbenzo[b]quinolizinium chromophore. The addition of Hg2+ or Mg2+ to the para isomers of the AT215C5–quinolizinium or A15C5–quinolizinium conjugate, respectively, led to a blueshift of the absorption maxima of each compound because of the reduced donor ability of the complexed amino group. In contrast, the addition of Hg2+ to a solution of the ortho‐AT215C5–quinolizinium conjugate in H2O/MeOH mixtures induced a significant redshift (ca. 50 nm) of the absorption maximum and enabled the photometric discrimination between Hg2+ and competing thiophilic cations, such as Ag+ or Pb2+, because the latter, as well as other competing metal ions, did not induce such an effect. It is proposed that the Hg2+‐induced redshift originates from the complexation of Hg2+ by the thiaazacrown ether followed by deprotonation of the secondary 9‐amino substituent of the aminobenzoquinolizinium unit. The resulting amide functionality increases the donor–acceptor interplay leading to the redshifted absorption and also coordinates to Hg2+ to form a lariat ether type complex. A similar effect was observed upon the addition of Mg2+ to the 9‐amino‐N‐phenylbenzo[b]quinolizinium derivative with the A15C5 unit in the ortho position; however, this effect was only operative in aprotic solvents, e.g. CH3CN, and with less than 1 mol‐equiv. of Mg2+.