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.

Stabilizing Unusual Oxidation States of Lanthanides in Molecular Complexes: Synthesis, Properties, and Reactivity

Aurélien René Willauer

The chemistry of divalent lanthanides has generated increasing interest in the past years due to their redox properties and unique reaction pathways. Notably, molecular complexes of low-valent lanthanides have been shown to be suitable one-electron reductants that are able to activate inert small molecules such as heteroallenes and dinitrogen in mild conditions. Lanthanide(II) complexes usually undergo mono-electron transfer, but multi-electron transfer can be achieved through multiple one-electron processes in polymetallic complexes. The first part of this thesis describes attempts to expand the redox chemistry of lanthanide complexes and lays the basis for a wider understanding of the chemical and reductive properties of low-valent polynuclear lanthanide complexes. Thus, Chapter 2, 3, and 4, reports the synthesis and characterization of different polynuclear lanthanides(II) complexes of various siloxide ligands. In Chapter 2, the outcome of carbon dioxide reduction (oxalate versus carbonate) by dinuclear Ln(II) tris(tert-butoxy)siloxide complexes, depending on the solvent polarity and on the nature of the Ln ions, was investigated. Furthermore, unprecedented reduction products could be isolated from the reaction with CS2, providing the first example of acetylenedithiolate ligand formation from metal reduction of carbon disulphide. In all cases, it has been shown that cooperative binding of the substrate plays an important role in the outcome of the reaction. Chapter 3 describes a rational route to the first metallasilsesquioxane of a dinuclear divalent lanthanide which can selectively effect the reductive disproportionation of CO2 and the two-electron reduction of azobenzene. In an attempt to confer increased reductive reactivity to Eu(II) ions, Chapter 4 describes the use of the tetraphenyldisiloxanediolate ligand to form polynuclear complexes of Eu(II) that can initiate for the first time carbon dioxide activation by Eu(II) ions in an isolated complex.Differently, until 2019, the +IV oxidation state of lanthanide in molecular complexes remained limited to a single lanthanide ion, cerium. It is only recently that the terbium ion was shown to be able to access the +IV oxidation state in molecular complexes. However, given the broad applications of cerium(IV), the discovery of other molecular complexes of Ln(IV) is of great interest for chemists. Chapter 5 describes the synthesis of the third example of a Tb(IV) complex displaying an open coordination sphere and the very first report of a molecular complex of praseodymium in the +IV oxidation state. The novel Ln(IV) complexes were characterized by UV-Visible, NMR, EPR and IR spectroscopy, magnetometry, single-crystal X-ray diffraction analysis, cyclic voltammetry, and computational studies. In particular, this provided an opportunity to measure and study the luminescent spectra of Pr(IV) for the first time, which upon binding to the siloxide ligand results in a large shift of the ligand triplet state. Furthermore, these complexes have been shown to be appropriate starting materials for the synthesis of Ln(IV) phosphine oxide adducts. Preliminary studies directed towards the isolation of Nd(IV) molecular complexes are discussed.

EPFL2022

Stabilizing Unusual Oxidation States of Lanthanides in Molecular Complexes: Synthesis, Properties, and Reactivity

Aurélien René Willauer

EPFL2022