Trinuclear europium complexes with a tripodal ligand: formation kinetics in acetonitrile

Here we report on the characterizations (mass spectrometry and NMR) and kinetic investigations related to the formation of trinuclear complexes [Eu₃L]⁹⁺ in metal excess. The NMR studies of the mixture containing the tripodal ligand L and Eu(III) involved the identification and attribution of signals at variable temperature. Kinetic spectrophotometric studies were performed with a stopped flow device at low temperature in metal excess. Two pathways were investigated: (i) a direct complexation of L; (ii) a conversion of the tetranuclear edifice [Eu₄L₄]¹²⁺. Kinetic measurements show that the building of the trimetallic species involves two main steps. In the first one, Eu(III) ions are rapidly bound to each ligand strand. The second step is related to a slow rearrangement within the complex. The conversion of the tetranuclear complex occurs in globally fast two successive steps, but these are slower than a direct binding of Eu(III) to L, which suggests a rate limiting attack of the compact tetranuclear edifice. Although this study focuses on the formation kinetics between Ln(III) and tritopic ligands in metal excess, far from the stoichiometry of tetrahedral helicates, it provides a valuable insight into the mechanism of complexation under pseudo-first order conditions.