Interaction of M3+ lanthanide cations with amide, pyridine, and phosphorylO = PPh3 ligands: A quantum mechanics study

We report an ab initio quantum mechanical study on the interaction of Mn+ c ations (Mn+ La3+ Eu3+ Yb3+ Sr2+, and Na+) with model ligands L for lanthani de or actinide cations: several substituted amides, pyridines: and the phos phoryl-containing OPPh3 ligand. The interaction energies Delta E follow ben ds expected from the cation hardness and ligand basicity or softness in the amide series (primary ( secondary-cis < secondary-trans < tertiary) as wel l. 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 l anthanides slightly less than the (best) pyridine-NMe2 ligand. In the lanth anide 1:1 complexes, the energy differences Delta Delta E as a function of M3+ (about 40 kcal/mol for all ligands) are less than Delta Delta E 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 inv estigations. Tn addition to optimal binding features, we also investigate t he directionality of ion coordination to the ligands and the effect of coun terions and stoichiometry on the structural, electronic and:energetic featu res 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.