Anion recognition in aqueous solution by cyclic dinuclear square cage-shaped coordination complexes

Abstract

Three cyclic dinuclear complexes, namely [M2(H2L)2](ClO4)4 [M = Co2+ (1), Ni2+ (2), Zn2+ (3), H2L = (1,2)-bis-N'-(pyridin-2-ylmethylene)benzohydrazide hydrazine, C26H22N8O2], containing amide and hydrazine groups were synthesized and characterized. Each central metal ion is coordinated with two oxygen atoms and four nitrogen atoms from carbonyl, and pyridine and imine, respectively. The metal ion is six-coordinated and has a slightly deformed octahedral geometry. X-ray crystallographic analyses showed that all the three cyclic dinuclear complexes crystallize in the orthorhombic system, and belong to the C222 space group, with two molecules in each unit cell. The cyclic dinuclear molecule is linked by two H2L ligands with a Z-form-HN-NH-bridge, nearly forming a square coordination cage with edges of length around 8.4 Å. The cyclic dinuclear complexes can recognize acetate and fluoride anions in an acetonitrile solution containing 60% volume water. Recognition is governed by electrostatic interactions in cooperation with the cage structure effect with the mechanism of anion displacement reaction. The results show that the recognition of anions in acetonitrile aqueous solution is an exothermic and entropy-reducing reaction. This suggests that the enthalpy change plays an important role in the presence of highly polar water and highlights the importance of positively charged cage structure effect. A color change from light yellow to dark yellow was clearly observed for complex 3 on addition of acetate or fluoride anions in acetonitrile aqueous solution containing 60% water. Complex 3 can be used for colorimetric “naked eye” recognition of acetate or fluoride anions in acetonitrile aqueous solution. Theoretical calculations based on time dependent density functional theory (TD-DFT) show the agreement between the theoretical results and experimental data.