M3+ Lanthanide Cation Solvation by Acetonitrile:  The Role of Cation Size, Counterions, and Polarization Effects Investigated by Molecular Dynamics and Quantum Mechanical Simulations

We report a molecular dynamics (MD) study on M3+ lanthanide (La3+, Eu3+, and Yb3+) cations in dry acetonitrile solution and in M(MeCN)n3+ clusters (n = 1−15) where two classical force-field representations of the cations are compared, in conjunction with the OPLS model of acetonitrile. It is shown that a set of van der Waals cation parameters (set2) fitted from free energies of hydration overestimates the cation coordination numbers (CNs). Another set of parameters (set1), where the size of cations is scaled down by 21/6 (using the σ van der Waals value for R*) yields better results. Quantum mechanical calculations performed on M(MeCN)n3+ aggregates (n = 1−9) demonstrate the importance of charge-transfer and polarization effects. They confirm the preferred coordination number of eight for Yb3+, the Yb(MeCN)8+13+ species with one MeCN molecule in the outer coordination sphere being somewhat more stable than Yb(MeCN)93+ D3h. Adding a polarization term for the 1−6−12 OPLS acetonitrile to the force field (set2+pol) indeed markedly improves the calculated CNs. In all MD simulations, a remarkable dynamic feature is observed in the first solvation shell where the lifetime of acetonitrile molecules increases from Yb3+ to La3+, that is, inversely to the cation−solvent interaction energies and to the aqueous phase behavior. Rare-earth salts with ClO4- and F3CSO3- anions and the question of ion binding selectivity by L ligands (formation of ML33+ complexes, where L is a pyridine−dicarboxamide ligand) in acetonitrile solution are investigated by free-energy perturbation simulations, comparing the set1, set2, and set2+pol models. It is found that selectivities are markedly determined by the change in solvation-free energies of the uncomplexed cations, with pronounced counterion effects. The two simplest models (set1 or set2 without polarization) predict the correct order of complexation (Yb3+ > Eu3+ > La3+), whereas addition of polarization contribution leads to the inverse order, because of overestimation of the cation−anion interactions in the salt solutions.