Unravelling the formation pathway and energetic landscape of lanthanide cages based on bis-β-diketonato ligands

Abstract

A systematic investigation was conducted to unravel the formation process of lanthanide cages based on bis-β-diketonato ligands. By employing the diamagnetic La³⁺ ion, NMR spectroscopy coupled to ESI-MS analyses revealed the consecutive and competitive formation of four different species: [La₂L]⁴⁺, [La₂L₂]²⁺, [La₂L₃], [La₂L₄]^2-. Stepwise and overall stability constants were derived. Further insights on the energetics of the equilibrium between the two most stable species the triple-stranded [La₂L₃] and the quadruple-stranded [La₂L₄]^2- cage have been conducted through variable temperature analyses, indicating that the interconversion is exergonic, endothermic and mainly entropy driven. DFT thermochemical calculations including also the explicit coordinated solvent allowed to better evaluate the role of the enthalpic and entropic factors on the step-by-step ligand association. This study focuses on the self-assembly mechanisms of triple- and quadruple-stranded lanthanide cages and on their solution behaviour, particularly concerning equilibrium and cage interconversion.