Eu(III) and Cm(III) Complexation by the Aminocarboxylates NTA, EDTA, and EGTA Studied with NMR, TRLFS, and ITC – An Improved Approach to More Robust Thermodynamics

The complex formation of Eu(III) and Cm(III) was studied with tetradentate, hexadentate, and octadentate coordinating ligands of the aminopolycarboxylate family, viz. nitrilotriacetate (NTA³⁻), ethylenediaminetetraacetate (EDTA⁴⁻), and ethylene glycol-bis(2-aminoethyl ether)-N,N,N′,N′-tetraacetate (EGTA⁴⁻), respectively. Based on the complexones’ pKa values obtained from ¹H nuclear magnetic resonance (NMR) spectroscopic pH titration, complex formation constants were determined by means of parallel-factor-analysis-assisted evaluation of Eu(III) and Cm(III) time-resolved laser-induced fluorescence spectroscopy (TRLFS). This was complemented by isothermal titration calorimetry (ITC), providing the enthalpy and entropy of the complex formation. This allowed us to obtain genuine species along with their molecular structures and corresponding reliable thermodynamic data. The three investigated complexones formed 1:1 complexes with both Eu(III) and Cm(III). Besides the established Eu(III)–NTA 1:1 and 1:2 complexes, we observed, for the first time, the existence of a Eu(III)–NTA 2:2 complex as of millimolar metal and ligand concentrations. Demonstrated for thermodynamic studies on Eu(III) and Cm(III) interaction with complexones, the utilized approach is commonly applicable to many other metal–ligand systems, even to high-affinity ligands.