Soft interactions with hard Lewis acids: generation of mono- and dicationic alkaline-earth metal arene-complexes by direct oxidation

The synthesis of the first unsupported dicationic arene complexes of calcium and strontium [(eta(6)-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 [mu F-{Al(ORF)(3)}(2)]; R-F = C(CF3)(3)). In addition, monocationic eta(6)-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 eta(6)-HMB coordination is observed with rather short M-arene(centroid) 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 [(eta(6)-HMB)AE(oDFB)(4)](2+) are extremely hard Lewis acids that retain FIAs close to superacidity in solution.