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Updated: Dec 26, 2025

Characterizing Lewis Pairs Using Titration Coupled with In Situ Infrared Spectroscopy
Published on: February 20, 2020
Soft interactions with hard Lewis acids: generation of mono- and dicationic alkaline-earth metal arene-complexes by
Marcel Schorpp1, Ingo Krossing1
1Institut für Anorganische und Analytische Chemie , Freiburger Materialforschungszentrum (FMF) , Universität Freiburg , Albertstr. 21 , 79104 Freiburg , Germany .
Abstract:
The synthesis of the first unsupported dicationic arene complexes of calcium and strontium [(η6-HMB)AE(oDFB)4]2+ is reported (HMB = hexamethylbenzene; AE = alkaline earth metal; oDFB = ortho-difluorobenzene). They were prepared by direct oxidation of the elemental metals employing the ligand-forming radical cation salt [HMB][WCA] as an oxidant (WCA = [Al(ORF)4] or [μF-{Al(ORF)3}2]; RF = C(CF3)3). In addition, monocationic η6-HMB complexes of calcium, strontium and barium supported by coordination of the monodentate anion [F-Al(ORF)3]- are reported. In all examples, almost undistorted η6-HMB coordination is observed with rather short M-arenecentroid distances approaching those observed with the isoelectronic but negatively charged pentamethylcyclopentadienyl ligand. The structure and bonding, thermodynamic stability and Lewis acidity (fluoride/hydride ion affinities, FIA/HIA) of the generated complexes were assessed by DFT methods. It followed that the gaseous dications [(η6-HMB)AE(oDFB)4]2+ are extremely hard Lewis acids that retain FIAs close to superacidity in solution.
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