Interaction of M3+ Lanthanide Cations with Amide, Pyridine, and Phosphoryl OPPh3 Ligands: A Quantum Mechanics Study

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Abstract
We report an ab initio quantum mechanical study on the interaction of Mn+ cations (Mn+ = La3+, Eu3+, Yb3+, Sr2+, and Na+) with model ligands L for lanthanide or actinide cations: several substituted amides, pyridines, and the phosphoryl-containing OPPh3 ligand. The interaction energies ΔE follow trends expected from the cation hardness and ligand basicity or softness in the amide series (primary < secondary-cis < secondary-trans < tertiary) as well as in the pyridine series (para-NO2 < H < Me < NMe2). Among all ligands studied, OPPh3 is clearly the best, while the (best) tertiary amide binds lanthanides slightly less than the (best) pyridine-NMe2 ligand. In the lanthanide 1:1 complexes, the energy differences ΔΔE as a function of M3+ (about 40 kcal/mol for all ligands) are less than ΔΔE in the pyridine series (up to about 90 kcal/mol) where marked polarization effects are found. The conclusions are validated by a number of methodological investigations. In addition to optimal binding features, we also investigate the directionality of ion coordination to the ligands and the effect of counterions and stoichiometry on the structural, electronic and energetic features of the complexes. The results are discussed in the context of modeling complexes of lanthanide and actinide cations and compared to those obtained with analogous Na+ and Sr2+ complexes.