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

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-II (4)(L - 3H)(2)](ClO4)(2) center dot 3.5H(2)O (I), which crystallizes in a triclinic system with space group P (1) over bar, a = 12.261(5) angstrom, b = 13.887(6) angstrom, c = 20.738(8) angstrom, alpha = 89.996(7)degrees, beta = 88.163(7)degrees and gamma = 85.016(7)degrees. 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 H-1 NMR data indicate that I is stable in solution, and the fluorescence measurement demonstrates that it has strong fluorescence at lambda(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 H-1 NMR spectroscopy.