Synthesis, structure and properties of aqueous transition metal(II) Schiff base complexes

Abstract

In chapter 1, two chiral and one racemic fluorescent dimeric zinc complex {$[Zn_2(\emph{slys}H)_2Cl_2]\cdot(CH_3OH)_2\cdot(H_2O)_3 (\bf{1})$, $[Zn_2(\emph{slys}H)_2 (NO_3)_2]\cdot(H_2O)_3 (\bf{2})$, $[Zn_2(\emph{rslys}H)_2(\mu-OAc)_2] (\bf{3})$} are compared herein. Presently, $\bf{3}$ has been synthesized by an one-pot reaction in which the D-/L- lysine-based Schiff base ligand [$\emph{slys}$ = 6-amino-2-{(2-hydroxybenzylidene)amino}hexanoate] is generated $\emph{in situ}$. These compounds have been characterized by single-crystal X-ray diffraction and by various spectroscopic techniques. The structures of previously reported compounds $\bf{1}$ and $\bf{2}$ are closely related in which two zinc centers possess two phenolate-bridges and an axial chloride or nitrate ligand. The novel compound $\bf{3}$ 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 ($\bf{(1 ? 3)}$) are highly blue fluorescent, either as solids or dissolved species. Under neutral and aqueous conditions (pH 7.4; 0.01 M HEPES buffer, $H_2O$:MeOH = 9:1), the quantum yields are $\Phi;_F$ = 0.17 ($\bf{1}$), 0.21 ($\bf{2}$) ($\lambda_{ex}$ = 352 nm, $\lambda_{em}$ = 452 nm), $Φ_F$ = 0.16 ($\bf{3}$) ($$\lambda$_{ex}$ = 354 nm, $\lambda_{em}$ 452 nm). Complex $\bf{3}$ metal ion detection capability (i.e., $Na^+$,$K^+$, $Ca^{2+}$, $Mg^{2+}$, $Co^{2+}$, $Ni^{2+}$, $Mn^{2+}$, $Cu^{2+}$, $Pb^{2+}$, $Ag^+$, $Hg^{2+}$, and $Cd^{2+}$) was gauged in aqueous media at physiological pH facilitated by the similarity of its starting photophysical properties with those of $\bf{1}$ and $\bf{2}$. These dizinc species were all found to act as highly selective fluorescent ON-OFF probes for $Cu^{2+}$ through the direct displacement of two zinc ions in aqueous media at physiological pH range. The phenolate-bridged compounds $\bf{1}$ and $\bf{2}$, however, showed bette...