Investigation of binding tendency of Eu(III) and La(III)-Schiff base complexes to selected oxy-anions and amino acids

New Eu(III) and La(III)/Schiff base complexes have been synthesized and fully characterized from the Schiff base ligand: L, (E)-N′-(pyridin-2-ylmethylene)acetohydrazide. The simple tri-aqua complex: [EuL₂(H₂O)₃]Cl₃ (EuL) emits intense red luminescence from the ⁵D₀ level of Eu(III). The binding tendency of EuL and LaL, was investigated in solution for oxy-anions such as acetate, perchlorate, sulphate, nitrate, carbonates, ethyl phosphonate, as well as some selected amino acids (AA) such as L-alanine (Ala), L-cysteine (Cys) and L-leucine (Leu). The anion selectivity is based on the anionic size and the coordination mode as bidentate chelaters. The ESI-MS confirmed the formation of 1:1 EuL-AA or EuL-oxy-anion chelates in solution. These chelates are responsible for the luminescence quenching from Eu(III). The amino acids exhibit smaller binding constants to EuL (log K = 4.08–4.98 M⁻¹) than the oxy-anions (log K = 4.65–6.54 M⁻¹), but they maintain longer luminescence lifetime from Eu(III) (0.45–0.51 ms). This work suggests that luminescence lifetime measurements of EuL may be useful in distinguishing between different types of substrates in solution.