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

A monocationic ZnII 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 L2– 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 L2– 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 CO32–, HCO3–, NO3–, F−, Cl−, Br−, I−, HSO4–, SO42–, 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).