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 Ln3+/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.