Soft Interactions with Hard Lewis acids: Generation of Mono- and Dicationic Alkaline-Earth Metal Arene-Complexes by Direct Oxidation.

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 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 else 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 was assessed by DFT methods. It followed that the gaseous dications [(η6-HMB)­AE(oDFB)4]2+  are extreme hard Lewis acids that retain FIAs close to superacidity in solution.