Investigating lanthanide complex stability through kinetic and thermodynamic competition with metal ions and DTPA

Understanding the stability of lanthanide(III) complexes is essential in applications and when determining molecular properties. Here, luminescence spectroscopy was used to examine the stability of europium complexes when challenged with biologically and chemically relevant metal ions (Y³⁺, Sr²⁺, K⁺, Na⁺, La³⁺, Lu³⁺). We benchmark three ligand systems against well-characterized Eu·DOTA and Eu·DTPA complexes and check for human error by challenging with DTPA. Where thermodynamic <i>i.e.</i> equilibrium data informs on complex stability, the kinetic traces show how fast europium is released from different ligands. The luminescence spectroscopy data was validated with ¹H-NMR for selected ions and ligands. Our findings validated previous studies by showing that (i) Eu·DOTA and Eu·DTPA favors trivalent rare earth ions, (ii) kryptofix 2.2.2 and macropa favors softer divalent ions, (iii) only Eu·DOTA is kinetically inert, and (iv) Eu·DOTA remains unchanged for months when challenged by ions and DTPA in excess. The data affords the conclusion that only Eu·DOTA is stable in the presence of competing ions and ligands.