DFT Study of Americium and Europium Complexation with 2,9-Bis(1,2,4-triazin-3-yl)-1,10-Phenanthroline Ligand: The Influence of the Counteranions–Nitrate Versus Perchlorate

2,9-Bis(1,2,4-triazin-3-yl)-1,10-phenanthroline (BTPhen)-based ligands show great promise in the separation of trivalent lanthanides and actinides. Experimental studies have shown that americium forms stronger complexes with the BTPhen ligands than europium; most theoretical studies have so far failed to reproduce these results. In the current study, three different metal forms (the naked cation, its nitrate or perchlorate salts and tetraaqua solvated salts) were used to study different complexation reactions. It was shown that in the case of naked cations and salts, europium forms the most stable complex with the 2,9-bis(1,2-triazin-3-yl)-1,10-phenanthroline ligand in all of the reactions compared. However, europium is also more strongly interacting (compared to americium) with anions and water molecules in the tetraaquatrinitrato or tetraaquatriperchlorato complexes. That shifts the energies of reactions like Am(NO3)3·4H2O + [Eu(H2O)4BTPhen]3+ = [Am(H2O)4BTPhen]3+ + Eu(NO3)3·4H2O in favor of the americium being complexed with BTPhen and europium with anions and water. Therefore, the americium complexes with BTPhen become the more stable form, in an agreement with the experimental studies. Comparison of counterion influence (nitrate vs. perchlorate) indicates that bigger preference for americium over europium complexation corresponds to the nitrate complexes and stems mainly from the fact that in M(NO3)3(H2O)4 europium is stabilized more than in M(ClO4)3(H2O)4.