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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2023-04, Vol.52 (14), p.4355-437
• Main Authors: Billion, Andreas, Schorpp, Marcel, Feser, Rebecca, Schmitt, Manuel, Eisele, Lea, Scherer, Harald, Sonoda, Takaaki, Kawa, Hajimu, Butschke, Burkhard, Krossing, Ingo
• Format: Article
• Language: English
• Subjects: Alcohol; Anions; Cations; Impurities; Infrared spectroscopy; Ions; Lithium aluminum hydrides; NMR spectroscopy; Oxygen atoms; Perfluoro compounds; Single crystals; Spectrum analysis; Synthesis; Water stability
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d3dt00199g

Weakly coordinating anions (WCAs) facilitate the stabilization and isolation of highly reactive and almost "naked" cations. Alkoxyaluminate-based WCAs such as [Al(OC(CF 3 ) 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(OC 10 F 15 ) 4 ] − ([ pfAd ] − ; OC 10 F 15 = 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 C 10 F 15 OH with LiAlH 4 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 ], [Ph 3 C][ pfAd ] and [H(OEt 2 ) 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 P 9 + [ pfAd ] − . The synthesis of Tl[ pfAd ] in a mixture of H 2 O/acetone/ o -DFB demonstrated the water stability of the [ pfAd ] − anion. Dream WCA...? The very high fluoride ion affinity of the Lewis acid underlying the novel WCA [Al(OC 10 F 15 ) 4 ] − ([ pfAd ] − ) nurtures the hope for having found a new "dream WCA". Yet, the paths towards this WCA held some obstacles at hand...!