A DFT study of the affinity of lanthanide and actinide ions for sulfur-donor and nitrogen-donor ligands in aqueous solution Robert D. Hancock a,⇑ , Libero J. Bartolotti b,⇑ a Department of Chemistry and Biochemistry, University of North Carolina at Wilmington, Wilmington, NC 28403, United States b Department of Chemistry, East Carolina University, Greenville, NC 27858, United States article info Article history: Received 23 May 2012 Received in revised form 18 October 2012 Accepted 19 October 2012 Available online 19 November 2012 Keywords: DFT on metal complexes Lanthanides Actinides f-Block elements Sulfur donor ligands Nitrogen donor ligands abstract 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 DG(DFT) at 298 K for the gas-phase reactions: [M(H 2 O) 6 ] 3+ (g) + L(g) ? M(H 2 O) 5 L] 3+ (g)+H 2 O(g), where L is NH 3 and M is a variety of Ln(III), An(III) ions, and UO 2 2+ and NpO 2 2+ are reported, as well as for the corresponding reactions for the nona-aqua ions for Ln(III) and An(III) ions. Also reported are DG(DFT) for the reactions for L = H 2 S, 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). DG(DFT) for formation of NH 3 complexes in the gas-phase correlates well with DG(aq) values for formation of the cor- responding complexes in aqueous solution. The log K 1 (NH 3 ) values were predicted by the equation: log K 1 = E a E b + C a C b (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 bond- ing respectively. Correlations involving DG(DFT) for the reactions for L = H 2 S allow for prediction of log K 1 (R 2 S) for complexes of thioethers (R = CH 2 CH 2 OH). The importance of predicted log K 1 (NH 3 ) values lies in the fact that log K 1 for polydentate N-donor ligands correlates with log K 1 (NH 3 ) for a series of metal ions, so that log K 1 (NH 3 ) 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 K 1 with log K 1 (NH 3 ) 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 K 1 (NH 3 ). The low slope of, for example, log K 1 (terpyridyl) versus log K 1 (NH 3 ) 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. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction There has been an ongoing interest in separating actinide (An) ions such as Am(III) and Cm(III) from lanthanide(III) (Ln) ions in the treatment of nuclear waste [1]. Gd, Sm, and Eu have large neu- tron-capture cross-sections, so that further treatment of recovered Am and Cm in nuclear reactors is inhibited by the presence of these Ln(III) ions. Ac(III) is of interest [2] because of use of the a-emitter 225 Ac (t ½ = 10.0 d) in chemotherapy. The 225 Ac is attached to an antibody selective for the targeted type of cancer cell via an appro- priate ligand, and the cancer cells are destroyed by a-radiation. Po- tential solvent extractants for separation of An(III) from Ln(III) ions have relied on the somewhat greater covalence [3,4] of An–L (L = li- gand) than of Ln–L bonding in selecting potential functional groups. This greater An–L covalence has been exploited largely based on aromatic polypyridyl-type ligands. A selectivity ratio for An(III) over Ln(III) ions of up to about 10 3 has been found for N-donor ligands such as: BTP [5–10], TPEN [11,12] 4,7-diphenyl-phen [13], BTB [14,15], BTphen [16], BTTP [17], TPTZ [18–21], and ODP [22] (see Fig. 1 for key to ligand abbreviations). Sulfur donors as an approach to more covalent M–L bonding to achieve Am(III)/Ln(III) selectivity has mainly involved dithiophosphinic acids [23–29] such as L1 in Fig. 1. Accompanying these developments have been theoretical studies of Am(III) and Ln(III) M–L bonding [15,20,30–33]. 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.10.010 ⇑ Corresponding authors. E-mail addresses: hancockr@uncw.edu (R.D. Hancock), BARTOLOTTIL@ecu.edu (L.J. Bartolotti). Inorganica Chimica Acta 396 (2013) 101–107 Contents lists available at SciVerse ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica