A DFT study of the affinity of lanthanide and actinide ions for sulfur-donor and nitrogen-donor ligands in aqueous solution

DFT calculations are reported on the affinity of An(III) (An = actinide) and Ln(III) (Ln = lanthanide) ions for ligands with nitrogen-donor and sulfur-donor groups in aqueous solution, aimed at evaluating such donor groups as the basis for separating Am(III) from Ln(III) ions in the processing of nuclear waste. DFT calculations of ΔG(DFT) at 298 K for the gas-phase reactions: [M(H2O)6]3+(g) + L(g) → M(H2O)5L]3+(g) + H2O(g), where L is NH3 and M is a variety of Ln(III), An(III) ions, and UO22+ and NpO22+ are reported, as well as for the corresponding reactions for the nona-aqua ions for Ln(III) and An(III) ions. Also reported are ΔG(DFT) for the reactions for L = H2S, as representative of thioethers in aqueous solution, for M = Ln(III) and An(III) ions, as well as Al(III), Ga(III), In(III), Tl(III), Bi(III), Fe(III), and Cr(III). ΔG(DFT) for formation of NH3 complexes in the gas-phase correlates well with ΔG(aq) values for formation of the corresponding complexes in aqueous solution. The log K1(NH3) values were predicted by the equation: log K1 = Ea·Eb + Ca·Cb (R.D. Hancock, A.E. Martell, Chem. Rev. 89 (1989) 1875.), where E and C are empirical parameters representing the tendencies of the acids (‘a’) and bases (‘b’) towards ionic and covalent bonding respectively. Correlations involving ΔG(DFT) for the reactions for L = H2S allow for prediction of log K1(R2S) for complexes of thioethers (R = CH2CH2OH). The importance of predicted log K1(NH3) values lies in the fact that log K1 for polydentate N-donor ligands correlates with log K1(NH3) for a series of metal ions, so that log K1(NH3) for any metal ion is an important consideration in ligand design. In the case of saturated N-donor ligands, this is a simple linear relationship, which suggests that saturated N-donors could lead to high Am(III)/Ln(III) selectivities. For ligands that contain pyridyl-type donors, such as 1,10-phenanthroline or terpyridyl, the correlation of log K1 with log K1(NH3) is separated into two linear relations for M(II) and M(III) ions, with M(III) ions having lower affinity for pyridyl donors than do M(II) ions of similar log K1(NH3). The low slope of, for example, log K1(terpyridyl) versus log K1(NH3) for M(III) ions suggests that ligands based only on pyridyl-type donor groups may not be able to produce large Am(III)/Ln(III) selectivities. The calculations on thioether complexes also suggest that such weakly basic S-donors would produce only weak Am(III)/Ln(III) selectivity, but that more strongly basic thiol-type donors might produce larger selectivities.