Synthesis, structures and optical properties of mononuclear La, Eu and Tb complexes supported by calix[4]arene ligands bearing three pendant salicylaldiminato arms

Abstract

The synthesis and complexation behaviour of a new calix[4]arene ligand H4L3 bearing three pendant salicylaldimine arms towards selected lanthanide ions has been investigated. Reaction of H4L3 with Ln(NO3)3⋅6H2O in MeOH in the presence of NEt3 provides monomeric complexes of composition [Ln(HL3)] (Ln=La3+ (4), Eu3+ (5), Tb3+ (6)). An X‐ray crystallographic analysis for 4 reveals that only two of the three ligand arms are involved in the coordination of the lanthanide ions, to give eight‐coordinated Ln3+ ions in a distorted square‐antiprismatic N2O6 environment. UV‐vis spectrophotometric titrations show that such monomeric species prevail in CH2Cl2/MeOH (1/1 v/v) solution. The apparent stability constants determined by UV‐vis spectrophotometric titration are log K=6.8(2) for 5 and log K=5.8(1) for 6. The Eu and Tb complexes luminesce only weakly at 77 K with luminescence lifetimes of 0.469 and 0.306 s.