Acetonitrile exchange kinetics on di- and trivalent lanthanide ions

The knowledge and understanding of the mechanism as well as the rate constant of the solvent exchange reactions are fundamental, since these reactions represent an important step in complex formation reactions on metal ions. Solvent exchange reactions in aqueous or non-aqueous solutions are the simplest chemical reactions on metal cations: a solvent molecule situated in the first coordination sphere of the ion is replaced by another one, generally entering from the bulk. In this work acetonitrile was chosen as non-aqueous solvent medium. For lanthanides, CH3CN is a weakly coordinating, labile, uncharged ligand; therefore homoleptic acetonitrile lanthanide complexes could be used as essential synthetic precursors to produce a wide range of compounds under non-aqueous conditions. The presence of the anion in the first coordination sphere of the metal ion can considerably change the solvent exchange rate; therefore it is imperative that solvent exchange reactions to be studied on homoleptic metal complexes. This study has been devoted to the synthesis of homoleptic acetonitrile lanthanide complexes like [Ln(CH3CN)n][Al(OC(CF3)3)4]3 (n = 9, Ln3+ = Nd, Eu, Gd, Dy; n = 8, Ln3+ = Tm); and to the understanding of the acetonitrile exchange reaction on the lanthanide series. It has been found that in anhydrous acetonitrile nine-coordinated species are characteristic for lanthanides from the middle (Eu, Gd, Dy) and the beginning (Nd) of the Ln series, while for those from the end (Tm) eight-coordinated species are representatives. For the synthesized complexes, evidences that both in the solid state and in acetonitrile solution there is no direct metal–anion interaction are provided by: X-Ray diffraction, IR and Raman spectroscopy, conductivity and NMR techniques. The rate constants of the acetonitrile exchange reaction as well as the rotational dynamics of the homoleptic acetonitrile Nd(III), Gd(III), Dy(III), Tm(III) and Eu(II) complexes have been achieved by variable temperature and multiple field 14N NMR, 1H NMRD or EPR measurements. It has been observed that the overall labilities of the CH3CN ligands are the highest ones measured so far in non-aqueous solvents on lanthanides. It is most likely that the mechanistic pathway for the acetonitrile exchange reaction on the homoleptic nine-coordinated lanthanide ions follows a dissociative activation mode and an associative one for the eight-coordinated species.