Excellent Selectivity for Actinides with a Tetradentate 2,9-Diamide-1,10-Phenanthroline Ligand in Highly Acidic Solution: A Hard–Soft Donor Combined Strategy

In
this work, we reported a phenanthroline-based tetradentate ligand
with hard–soft donors combined in the same molecule, N,N′-diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline (Et-Tol-DAPhen),
for the group separation of actinides over lanthanides. The synthesis
and solvent extraction as well as complexation behaviors of the ligand
with actinides and lanthanides are studied experimentally and theoretically.
The ligand exhibits excellent extraction ability and high selectivity
toward hexavalent, tetravalent, and trivalent actinides over lanthanides
in highly acidic solution. The chemical stoichiometry of Th­(IV) and
U­(VI) complexes with Et-Tol-DAPhen is determined to be 1:1 using X-ray
crystallography. The stability constants of some typical actinide
and lanthanide complexes of Et-Tol-DAPhen are also determined in methanol
by UV–vis spectrometry. Density functional theory (DFT) calculations
reveal that the An–N bonds of the Et-Tol-DAPhen complexes have
more covalent characters than the corresponding Eu–N bonds,
which may in turn lead to the selectivity of Et-Tol-DAPhen toward
actinides. This ligand possesses merits of both alkylamide and 2,9-bis-(5,6-dialkyl-1,2,4-triazin-3-yl)-1,10-phenanthroline
(R-BTPhen) extractants for efficient actinide extraction and the selectivity
toward minor actinides over lanthanides and hence renders huge potential
opportunities in high-level liquid waste (HLLW) partitioning.