Selective sensing of dihydrogen phosphate anion by a fluorescent tetranuclear pentacoordinated zinc(ii) complex

Abstract

A cresolic oxygen bridging ligand, 2,6-bis{[(2-hydroxybenzyl)(2-hydroxyethyl)amino]methyl}-4-methylphenol (L), has been synthesized and characterized. The coordination of Zn^II ions with L gives a novel tetranuclear complex, [Zn^II₄(L−3H)₂](ClO₄)₂·3.5H₂O (I), which crystallizes in a triclinic system with space group P, a = 12.261(5) Å, b = 13.887(6) Å, c = 20.738(8) Å, α = 89.996(7)°, β = 88.163(7)° and γ = 85.016(7)°. The cationic core of I is formed by four Zn^II cations bridged by two cresolic and four phenolic oxygen atoms from two ligands. All four Zn^II centers are pentacoordinated and adopt a distorted square pyramid geometry. ESMS and ¹H NMR data indicate that I is stable in solution, and the fluorescence measurement demonstrates that it has strong fluorescence at λ_ex = 298 nm. The fluorescent complex can selectively sense the dihydrogen phosphate anion in methanol, and the binding phenomenon can be monitored via UV-vis absorption changes and fluorescence quenching effects. The potential binding mode of I with H₂PO₄⁻ has been studied by ESMS and ¹H NMR spectroscopy.