Rare earth metal complexes based on a tripodal ligand and their application as highly selective “turn-on” fluorescent probe for Fe³⁺ ion

A C₃-symmetric tripodal ligand, 1,1′,1″-(nitrilotri-2,1-ethanediyl)tris-2(1H)-pyridinone (ntetp), has been designed and synthesized. Two families of isostructural rare earth coordination complexes have therefore been constructed based on this ligand, namely [M(ntetp)(NO₃)₃]_n [M = Y(1), La(2), Ce(3), Pr(4), Sm(5), Gd(6), Dy(7), Ho(8)] and [M(ntetp)Cl₃]_n [M = Tb(9), Er(10), Yb(11), Lu(12)]. Single-crystal analyses reveal that both series of complexes bear one-dimensional chain structures, with the metal centers taking nine-coordinate geometry in 1–8 and distorted octahedral geometry in 9–12. Solutions of the selected representative complexes 1 and 10 show highly selective fluorescent enhancement toward Fe³⁺ ion, comparing to non-significant response toward Ca²⁺, Co²⁺, Zn²⁺, Cr³⁺, Mn²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Li⁺, Na⁺, Al³⁺, K⁺, Ba²⁺ and Mg²⁺ ions. Also, the fluorescent enhancement induced by Fe³⁺ ion is not affected by the above-mentioned ions, demonstrating promising capabilities of complexes 1 and 10 as “turn-on” fluorescent probe.