Complexation of Lanthanides with Glutaroimide-dioxime: Binding Strength and Coordination Modes

The complexation of lanthanides (Nd3+ and Eu3+) with glutaroimide-dioxime (H2L), a cyclic imide dioxime ligand that has been found to form stable complexes with actinides (UO22+ and NpO2+) and transition metal ions (Fe3+, Cu2+, etc.), was studied by potentiometry, absorption spectrophotometry, luminescence spectroscopy, and microcalorimetry. Lanthanides form three successive complexes, M(HL)(2+), M(HL)L, and M(HL)(2)(+) (where M stands for Nd3+/Eu3+ and HL- stands for the singly deprotonated ligand). The enthalpies of complexation, determined by microcalorimetry, show that the formation of these complexes is exothermic. The stability constants of Ln(3+)/H2L complexes are several orders of magnitude lower than that of the corresponding Fe3+/H2L complexes but are comparable with that of UO22+/H2L complexes. A structure of Eu3+/H2L complex, identified by single-crystal X-ray diffractometry, shows that the ligand coordinates to Eu3+ in a tridentate mode, via the two oxygen atoms of the oxime group and the nitrogen atom of the imide group. The relocation of protons of the oxime groups (-CH=N-OH) from the oxygen to the nitrogen atom, and the deprotonation of the imide group (-CH-NH-CH-) result in a conjugated system with delocalized electron density on the ligand (-O-N-C-N-C-N-O-) that forms strong complexes with the lanthanide ions.