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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.

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In this work, the tissue/blood partition coefficients of seven human tissues were calculated using a nonlinear regression analysis. The dataset contained 80 structurally diverse compounds distributing into the brain, kidney, muscle, lung, liver, heart, and fat, whose acidic and basic properties were also considered by introducing the three possible forms of the compound in the human body (neutral, cationic, and anionic forms). A total of 248 data points were there in the training set (eq 5: r = 0.877, s = 0.352; eq 6: r = 0.869, s = 0.362) and 49 data points in the testing set (eq 5: r = 0.844, s = 0.342; eq 6: r = 0.860, s = 0.311). It was also concluded that the same state (neutral, cation, and anion) of a compound has essentially identical partition coefficients between the same tissue composition and the blood in these tissues. Only the different content of the three tissue compositions (lipid, protein, and water) lead to the different partition coefficient in different tissues, which offered a significant conclusion for the drug's distribution research.