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 an oxidant (WCA = [Al(OR^F)₄] or [μF-{Al(OR^F)₃}₂]; R^F = C(CF₃)₃). In addition, monocationic η⁶-HMB complexes of calcium, strontium and barium supported by coordination of the monodentate anion [F–Al(OR^F)₃]⁻ are reported. In all examples, almost undistorted η⁶-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 [(η⁶-HMB)AE(oDFB)₄]²⁺ are extremely hard Lewis acids that retain FIAs close to superacidity in solution.