Probing the difference in covalence by enthalpy measurements: a new heterocyclic N-donor ligand for actinide/lanthanide separationElectronic supplementary information (ESI) available: Ligand analysis; spectrophotometric titration of Eu/BQPhen; calorimetric titration of Nd/BQPhen; isomers of BQPhen; calculated free energy of species; solvent extraction of 241Am and 152Eu. See DOI: 10.1039/c5dt00679a

• Main Authors: Yang, Yanqiu, Liu, Jun, Yang, Liang, Li, Kun, Zhang, Huabei, Luo, Shunzhong, Rao, Linfeng
• Format: Article
• Language: English
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c5dt00679a

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 ML 2 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. BQPhen, a new tetra-dentate nitrogen-donor ligand, forms stronger complexes with Am 3+ than Eu 3+ or Nd 3+ , due to a more exothermic enthalpy of complexation for the Am 3+ complexes, implying a higher degree of covalence in the BQPhen complexes with Am 3+ than Eu 3+ or Nd 3+ .