Theoretical Studies on the UO22+ and Sr2+ Complexation by Phosphoryl-Containing OPR3 Ligands: QM ab Initio Calculations in the Gas Phase and MD FEP Calculations in Aqueous Solution

We report a series of ab initio QM calculations on uranyl and Sr 2+ complexes of O PR 3 ligands (R = H Me Ph) to assess the role of substituents R and of NO 3 - counterions on the intrinsic cation−ligand interaction energy. When there are no counterions, the binding sequence of UO 2 2+ and of Sr 2+ complexes follows the order R = H < Me < Ph, due to polarization and charge-transfer effects. However, in the presence of NO 3 - counterions, the OPMe 3 and OPPh 3 complexes become of similar stability, due to the ligand−anion repulsive interactions. Complexes of OPR 3 with the spherical Sr 2+ cation are found to be less stable than those with the linear UO 2 2+ cation. In the second part of the paper we report molecular dynamics simulations in water on 1:1 and 2:1 complexes of OPR 3 with UO 2 (NO 3 ) 2 . The changes in free energies of solvation upon electronic reorganization of the ligand and UO 2 (NO 3 ) 2 induced by complexation are investigated using statistical perturbation FEP techniques and found to be nearly independent of R. The importance of these results in the context of designing efficient ionophores for uranyl cations is discussed.