Chemistry with weakly-coordinating fluorinated alkoxyaluminate anions: Gas phase cations in condensed phases?

Many very simple and fundamentally important cations were initially observed in the gas phase, e.g. inside a mass spectrometer or even in space. The question arises, how to stabilize and study these gas phase cations in condensed phases. In this review, our work on the use of weakly-coordinating anions (WCAs) of type [Al(ORF)4]− and [(RFO)3Al–F–Al(ORF)3]− to investigate structural and chemical properties of such gas phase cations is highlighted. Ag+ complexes with various weakly basic ligands such S8, P4S3, P4, C2H4 or P3N3Cl6 as well as reactive binary P–X cations and carbocations are reviewed. This section is followed by a theoretical investigation that substantiates our claim that gas phase conditions may be approximated in condensed phases by the use of large WCAs. Hence, we carefully analyze the conditions necessary to stabilize salts of such delicate cations in solution as well as the solid state. To account for this ability of WCAs, we coined the expression pseudo gas phase conditions in condensed phases. Using a combination of quantum chemically calculated gas phase enthalpies and estimated lattice potential enthalpies in suitable Born–Fajans–Haber cycles, the explanation as well as the prediction of whether or not a certain compound may be formed is now possible. Some examples for such an approach are given.