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

Abstract

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