Probing the difference in covalence by enthalpy measurements: a new heterocyclic N-donor ligand for actinide/lanthanide separation

Abstract

Complexation of Am(III), Nd(III), and Eu(III) with a new heterocyclic nitrogen-donor ligand, 2,9-di(quinazolin-2-yl)-1,10-phenanthroline (denoted as BQPhen in this paper), was studied by thermodynamic measurements and theoretical computations. The stability constants of two successive complexes in dimethylformamide, ML(3+) and ML2(3+) where M stands for Nd, Eu, or Am while L stands for the BQPhen ligand, were determined by absorption spectrophotometry. The enthalpy of complexation was determined by microcalorimetry. Results show that BQPhen forms ten times stronger complexes with Am(III) than Eu(III) or Nd(III) under identical conditions, suggesting that BQPhen could be used as an efficient extractant for the separations of trivalent actinides from lanthanides. The higher binding strength of BQPhen towards Am(III) than Nd(III) or Eu(III) is mainly due to the more favourable enthalpy of complexation for Am(III)/BQPhen complexes, implying a higher degree of covalence in the Am(III)/BQPhen complexes than the lanthanide(III)/BQPhen complexes. The thermodynamic trend was corroborated with computational results and validated by solvent extraction experiments that demonstrated BQPhen preferably extracted Am(III) more than Eu(III), with a separation factor of about 10. Discussions have been made to compare BQPhen with other phenanthroline derivatives such as CyMe4-BTPhen, a bis-triazine-phenanthroline derivative that was reported in the literature. Data suggest that, under identical conditions, BQPhen would form stronger complexes with Am(III), Eu(III), and Nd(III) than CyMe4-BTPhen.

Topological Study of Bonding in Aquo and Bis(triazinyl)pyridine Complexes of Trivalent Lanthanides and Actinides: Does Covalency Imply Stability?

“…As well as being a complementary method, there are also many primarily computational studies of Ln(III) and An(III) complexes in the literature, both for ligands such as BTP, BTBP and BTPhen and other nitrogen donor ligands [24][25][26][27][28][29][30][31][32][33][34][35][36][37] . Key to several of these studies are the differences in energies of the Ln(III) and An(III) complexes and, despite the large separation factors exhibited by these ligands, these energetic differences amount to only hundredths to tenths of an eV.…”

“…Key to several of these studies are the differences in energies of the Ln(III) and An(III) complexes and, despite the large separation factors exhibited by these ligands, these energetic differences amount to only hundredths to tenths of an eV. [24][25][26]33 ] 3+ structure with tricapped trigonal prism geometry for the early and mid-series lanthanides that gradually becomes 8-coordinate, albeit as a dynamic equilibrium 38 . The same trends in coordination were observed for actinide (III) hydration 38 .…”

A New Tetradentate Mixed Aza-Thioether Macrocycle and Its Complexation Behavior towards Fe(II), Ni(II) and Cu(II) Ions

“…The new mixed aza-thioether macrocyclic ligand [13]ane(phenN 2 )S 2 was prepared from cyclization between 2,9-bis(chloromethyl)-1,10-phenanthroline and 1,3-propanedithiol in the presence of Cs 2 CO 3 as a base and source of template cation under high dilution conditions (Scheme 2). The 2,9-bis(chloromethyl)-1,10-phenanthroline can be synthesized in accordance with previously reported literature methods from 2,9-dimethyl-1,l0-phenanthroline as depicted in Scheme 2 [48][49][50][51][52]. The formation of this air-stable macrocycle was characterized by mass spectroscopy together with 1 H and 13 C-NMR spectroscopy.…”

Gas-Phase Complexes of Americium and Lanthanides with a Bis-triazinyl Pyridine: Reactivity and Bonding of Archetypes for F-Element Separations

“…21 Studies of gas-phase metal-BTP complexes have focused on trivalent lanthanides. [22][23][24] Density functional theory (DFT) has provided insights into structures and bonding of [M(BTP)3] 3+ complexes, 11,13,20,[25][26][27][28][29][30][31][32][33][34] but the origins of BTP selectivity remain unresolved.…”