Complexation of Ln 3+ lanthanide cations with phosphoryl-containing OPR 3 ligands: a quantum-mechanics study

Abstract

We report a series of ab initio quantum-mechanical calculations on M 3+ (M = La, Eu and Yb) and MCl 3 model complexes of OPR 3 ligands (R = H, Me, Et and Ph) to assess the role of R substituents and Cl − counterions on the intrinsic cation-ligand interaction energy. The calculations reveal a marked selectivity in the ligand series, as well as in the cation series. In the absence of counterions, for a given M 3+ cation, the binding sequence follows the order H < Me < Et < Ph, owing to polarization and charge-transfer effects. For a given OPR 3 ligand, the cation binding follows the sequence La 3+ < Eu 3+ < Yb 3+, as expected based on the decrease in ionic radius in this lanthanide series. Geometry optimization shows that, as the M 3+⋯OPR 3 interaction increases, the OP bond lengthens and the O⋯M 3+ distance shortens. Similar trends are observed in the R 3PO⋯MCl 3 complexes but are less pronounced, because of the ligand-anion repulsive interactions and electron transfer from Cl − to the M 3+ cation. The importance of these results in the context of designing efficient ionophores for lanthanide and actinide cations is discussed.