A cationic tetrahedral Zn(ii) cluster based on a new salicylamide imine multidentate ligand: synthesis, structure and fluorescence sensing study

Abstract

A monocationic Zn^II tetrahedral cluster, [Zn₄L₃(μ₃-OH)]·NO₃·1.25H₂O (Zn₄L₃) based on a new salicylamide imine ligand H₂L, H₂L = 1-(2-hydroxy-3-methoxy-benzamido)-3-(2-hydroxy-3-methoxy-benzylideneamino)-propane, has been prepared. Single crystal X-ray analysis reveals that three deprotonated ligands L^2– chelate three Zn(II) centres with their salicylamide moiety in such a way that the three salicylimine groups attached to the other side of the three chelates are converged to bind another Zn(II) centre to provide a tetrahedral cluster with two phenyl groups of the same ligand L^2– being close enough to present a strong intramolecular π⋯π stacking effect. Fluorescence studies indicate that Zn₄L₃ is stable in water and exhibits highly sensitive and selective recognition of phosphates against other common anions including CO₃^2–, HCO₃^–, NO₃^–, F⁻, Cl⁻, Br⁻, I⁻, HSO₄^–, SO₄^2–, OAc⁻, BF₄^–, ClO₄^– and CF₃SO₃^– in HEPES buffer solution (pH = 7.4) + DMSO (V : V = 1 : 9). The excellent sensing capability of Zn₄L₃ for phosphate against other common anions with a low detection limit of 0.15 μM renders it a candidate probe for phosphate detection. Furthermore, the observed fluorescence quenching responses of Zn₄L₃ towards phosphates were highly reversible. The possible sensing mechanisms for phosphate detection by Zn₄L₃ was investigated by means of ¹H NMR, UV-Vis spectra and high-resolution ESI-MS spectra and the results indicate that phosphates could exclusively decompose Zn₄L₃ to release H₂L in HEPES buffer solution (pH = 7.4) + DMSO (V : V = 1 : 9).