Theoretical Study of Metal−Ligand Interaction in Sm(III), Eu(III), and Tb(III) Complexes of Coumarin-3-Carboxylic Acid in the Gas Phase and Solution

The interaction of lanthanide(III) cations (Ln(III) = Sm(III), Eu(III), and Tb(III)) with the deprotonated form of the coumarin-3-carboxylic acid (cca^-) has been investigated by density functional theory (DFT/B3LYP) and confirmed by reference MP2 and CCSD(T) computations. Solvent effects on the geometries and stabilities of the Ln(III) complexes were computed using a combination of water clusters and a continuum solvation model. The following two series of systems were considered:  (i) Ln(cca)²⁺, Ln(cca)₂⁺, Ln(cca)₃ and (ii) Ln(cca)(H₂O)₂Cl₂, Ln(cca)₂(H₂O)₂Cl, Ln(cca)₃. The strength and character of the Ln(III)−cca^- bidentate bonding were characterized by calculated Ln−O bond lengths, binding energies, ligand deformation energies, energy partitioning analysis, σ-donation contributions, and natural population analyses. The energy decomposition calculations predicted predominant electrostatic interaction terms to the Ln−cca bonding (ionic character) and showed variations of the orbital interaction term (covalent contributions) for the Ln−cca complexes studied. Electron distribution analysis suggested that the covalent contribution comes mainly from the interaction with the carboxylate moiety of cca^-.