Direct dizinc displacement approach for efficient detection of Cu(2+) in aqueous media: acetate versus phenolate bridging platforms

Abstract

Two chiral and one racemic fluorescent dimeric zinc complex {[Zn-2(slysH)(2)Cl-2]center dot(CH3OH)(2)center dot(H2O)(3) (1), [Zn-2(slysH)(2) (NO3)(2)]center dot(H2O)(3) (2), [Zn-2(rslysH)(2)(mu-OAc)(2)] (3)} are compared herein. Presently, 3 has been synthesized by a one-pot reaction in which the D-/L-lysine-based Schiff base ligand [slys = 6-amino-2-{(2-hydroxybenzylidene)amino}hexanoate] is generated in situ. These compounds have been characterized by single-crystal X-ray diffraction and by various spectroscopic techniques. The structures of previously reported compounds 1 and 2 are closely related in which two zinc centers possess two phenolate-bridges and an axial chloride or nitrate ligand. The novel compound 3 differs structurally through the presence and coordination mode of two bridging acetate groups spanning the dizinc core and formally releasing the phenolate bridging Schiff base ligands. These compounds (1-3) are highly blue fluorescent, either as solids or dissolved species. Under neutral and aqueous conditions (pH 7.4; 0.01 M HEPES buffer, H2O-MeOH = 9:1), the quantum yields are Phi(F) = 0.17 (1), 0.21 (2) (lambda(ex) = 352 nm, lambda(em) = 452 nm), Phi(F) = 0.16 (3) (lambda(ex) = 354 nm, lambda(em) = 452 nm). Complex 3 metal ion detection capability (i.e., Na+, K+, Ca2+, Mg2+, Co2+, Ni2+, Mn2+, Cu2+, Pb2+, Ag+, Hg2+, and Cd2+) was gauged in aqueous media at physiological pH facilitated by the similarity of its starting photophysical properties with those of 1 and 2. These dizinc species were all found to act as a highly selective fluorescent ON-OFF probes for Cu2+ through the direct displacement of two zinc ions in aqueous media at physiological pH range. The phenolate-bridged compounds 1 and 2, however, showed better selectivity towards Cu2+ over competitive ions, Ni2+ and Co2+ than the acetate-bridged compound 3.