The perfluoroadamantoxy aluminate as an ideal weakly coordinating anion? – synthesis and first applications

Abstract

Weakly coordinating anions (WCAs) facilitate the stabilization and isolation of highly reactive and almost “naked” cations. Alkoxyaluminate-based WCAs such as [Al(OC(CF₃)₃)₄]⁻ ([pf]⁻) are widely used due to their synthetic accessibility and their high stability. However, small cations are still able to coordinate the oxygen atoms of the [pf]⁻ anion or even to abstract an alkoxy ligand. The novel WCA [Al(OC₁₀F₁₅)₄]⁻ ([pfAd]⁻; OC₁₀F₁₅ = perfluoro-1-adamantoxy) is characterized by a very rigid core framework, thus indicating a higher stability towards fluoride-ion abstraction (DFT calculations) and providing hope to generate less disordered crystal structures. The [pfAd]⁻ anion was generated by the reaction of the highly acidic alcohol perfluoro-1-adamantanol C₁₀F₁₅OH with LiAlH₄ in o-DFB. Li[pfAd] could not be synthesized free of impurities (and still contains unreacted alcohol). Yet, starting from contaminated Li[pfAd], the very useful pure salts Ag[pfAd], [Ph₃C][pfAd] and [H(OEt₂)₂][pfAd] could be synthesized. The salts were characterized by NMR spectroscopy, single-crystal X-ray diffraction and IR spectroscopy. Additionally, [NO][pfAd] could be synthesized containing alcohol impurities but nonetheless enabled the synthesis of the salt P₉⁺[pfAd]⁻. The synthesis of Tl[pfAd] in a mixture of H₂O/acetone/o-DFB demonstrated the water stability of the [pfAd]⁻ anion.