The energetic and structural effects of steric crowding in phosphate and dithiophosphinate complexes of lanthanide...

Metal-ligand binding strength and selectivity result from antagonistic metal-ligand M-L attractions and ligand-ligand L-L repulsions. On the basis of quantum-mechanical (QM) calculations on lanthanide complexes, we show that this interplay determines the binding affinities in the gas phase. In the series of [ML3] complexes (M = La, Eu, and Yb) with negatively charged phosphoryl ligands L- = (MeO)2PO2- and Me2PS2-, the binding energies follow the order Yb3+ > Eu3+ > La3- for a given ligand, and (MeO)2PO2- > Me2PS2- for a given cation. However, adding a neutral LH ligand to [ML3] changes the order to Eu3+ > Yb3+ > La3+ for the oxygen ligand and La3+ > Eu3- > Yb3+ for the sulfur ligand, indicating that steric strain in the first coordination sphere is largest for the smallest cation and for sulfur binding sites. We investigated the question of additional hydration of the [ML3LH] complexes in aqueous solution by molecular dynamics (MD) simulations, using two sets of atomic charges. It was found that pairwise additive potentials overestimate the coordination and hydration numbers of the cations, while adding polarization energy terms for the ligands yields better agreement between QM and MD results and supports the concept of steric strain in the first coordination sphere.