Lewis acidic alkaline earth metal complexes with a perfluorinated diphenylamide ligand

Abstract

Alkaline earth metal (Ae) chemistry with the anion [N(C₆F₅)₂]⁻ has been explored. Deprotonation of the amine (C₆F₅)₂NH, abbreviated in here as N^FH, with 0.5 equivalent of AeN′′₂ (N′′ = N(SiMe₃)₂) is fast and gave, dependent on the solvent, the complexes AeN^F₂, AeN^F₂·(THF)₂ and AeN^F₂·(Et₂O)₂ (Ae = Mg, Ca, Sr). Using a 1/1 ratio, mixed amide complexes were obtained: N^FAeN′′ (Ae = Mg, Ca, Sr). Crystal structures of the monomers AeN^F₂·(THF)₂ (Ae = Mg, Ca, Sr) and AeN^F₂·(Et₂O)₂ (Ae = Mg, Ca) are presented and compared with those of AeN′′₂·(THF)₂. In addition, crystal structures of the homoleptic dimer (MgN^F₂)₂ and the heteroleptic dimers (N^FAeN′′)₂ (Ae = Mg, Ca, Sr) are discussed. All structures are strongly influenced by very short Ae⋯F contacts down to circa 2.11 Å (Mg), 2.50 Å (Ca) and 2.73 Å (Sr). AIM analysis illustrates that, although Ae⋯F contacts are short, there is no bond-critical-point along this axis, indicating an essentially electrostatic interaction. The monomeric complexes feature strong C₆F₅⋯C₆F₅ π-stacking, resulting in unusually acute N^F–Ae–N^F angles as small as 95°. Heteroleptic (N^FAeN′′)₂ complexes retain their dimeric structure in C₆D₆ solution and there is no indication of ligand scrambling by the Schlenk equilibrium, suggesting that an electron withdrawing ligand may stabilize heteroleptic complexes. According to DFT calculations, the heteroleptic arrangement is 70 kJ mol⁻¹ more stable than the homoleptic dimers. The Lewis acidity of MgN^F₂ has been quantified with the Gutmann–Beckett method and by calculation of the Fluoride-Ion-Affinity. The latter calculations show that the Lewis acidity of MgN^F₂ and CaN^F₂ is comparable to that of B(C₆F₅)₃. Dimeric (MgN^F₂)₂ fully abstracts Et₃PO from Et₃PO·B(C₆F₅)₃ and may have potential in Lewis acid catalysis.