The Effect Of The Framework Structure Of Nitrogen And Oxygen Mixed Ligands On The Coordination Chemistry Of Lanthanum-actinide Solutions

In the closed nuclear fuel cycle,"Partition and Transmutation"(P-T)is the most advanced strategy for spent fuel reprocessing at present.The separation of lanthanide and subactinide elements must be carried out Before transmutation.In recent years,various kinds of extractants(containing nitrogen,oxygen,sulfur and phosphorus atoms for the separation of trivalent lanthanides and actinides have been developed.The aromatic nitrogen and oxygen mixed ligands are the research focus of extractant.The extraction and separation properties of aromatic nitrogen oxycarboxylic acid ligands and their amide derivatives have been studied extensively,mainly from amide group substituent of extractant and actinide extraction ability and selectivity.However,the relationship between ligand structure and actinide coordination chemistry is rarely reported.In this work,we first study the complexation of U(?)with PyridaZine-3-carboxylate(PZ).Thermodynamic parameters(stability constants and enthalpies of complexation)for binary U(?)/pyridazine-3-carboxylate complexes have been determined by spectrophotometry and calorimetry.The structures of a 1:2 and a 1:3 complex were identified by Single crystal X-ray diffraction.However,the thermodynamic data indicate that the complexation of U(?)with PZ is exclusively entropy driven,while U(?)/picolinate(PA)complexation is driven by both the enthalpy and entropy.In comparison with picolinate,the nitrogen atom on the 3rd position of pyridazine weakens the basicity the N atom participating in coordination with U(?).As a result,the binding strength of pyridazine-3-carboxylate to U(?)is weaker than picolinate.In this work,the complexation of 2,2'-dipyridyl-6,6'-dicarboxylate(BiPDA)and its structural anagogues 3,3'-dimethyl-2,2'-bipyridine-6,6'-dicarboxylate and 1,10-phenanthroline-2,9-dicarboxylate(DmBiPDA and PhenDA)with UO22+ and lanthanides in mixed solvent of dimethyl sulphoxide(DMSO)and 20%(V/V)water were studied.The stability constants of three ligands and UO22+ complexes were obtained by potentiometric titration and ultraviolet spectroscopic titration.In DMSO/20%H2O,there are of the same donor pre-organization between BiPDA and PhenDA,and the complexation of U(?)with them is prodominantly determined by the basicity of their coordination donors.So the complex UO2BiPDA is stable than UO2PhenDA in the mixed solvent DMSO/20%H2O.By analyzing the bond lensth and bond angle of UO2(BiPDA)and UO2(PhenDA),it suggests that the complexation of U(?)with BiPDA is stronger than that with PhenDA,in agreement with the changing trend of their stability constants in solutions.Donor preorganization improves ligand's corrdination ability with metallic ions.The exploration of two diamidic ligands,including N.N.N'.N'-tetramethyl-[2,2'-bipyridine]-6.6'-dicarboxamide(BiPDAm)and N.N,N',N'-tetramethyl-1,10-phenanthroline-2.9-dicarboxamide(PhenDAm)in the complexation with Lanthanides(?)and Uranium(?)in homogeneous conditions was studied.Spectrophotometric experiments indicate that the binding strength of the two ligands follows this trend.BiPDAm<PhenDAm.the rigid structure of phenanthroline makes two aromatic nitrogen atoms have a good pre-orgaruzation.For ligands with similar donor preorganization.the increased amplitude is larger for ligand with one more O-donor than which with one more N-donor.Compared with the tridentate ligands(BiPCAm and PhenCAm),the tetradentate ligands(BiPDAm and PhenDAm)add an oxygen coordination atom.The complexation of metallic ions with the tetradentate ligands is stronger than that with the tridentate ligands.1:2Nd(?)-PhenDAm complex were obtained,[Nd(C18H18N402)2(C104)3(CH3OH)]·2H2O..In the unit cell,two PhenDAm ligands are bound to Nd3+ through two nitrogen atoms of the phenanthroline moiety together with two oxygen atoms of the amide moieties.The crystal datas agree with the speciation calculated with the thermodynamic constants determined by UV-visible spectrophotometry in this work.The above results have important reference value for the designing and selecting new ligands of actinide separation.