Cationic Magnesium π–Arene Complexes

Reaction of the trityl cation in [Ph3C+][B(C6F5)4–] with the n-butyl anion in (BDI)MgnBu led to s-hydride abstraction and formation of Ph3CH, 1-butene, and [(BDI)Mg+][B(C6F5)4–] (1) (BDI = CH[C(CH3)N-Dipp]2; Dipp = 2,6-diisopropylphenyl). The “naked” Mg center in 1 is weakly bound to B(C6F5)4– through two Mg···F interactions. Addition of arenes to 1 gave strongly bound cationic magnesium π–arene complexes (BDI)Mg+·arene in good yields arene = benzene (94%), toluene (74%), m-xylene (82%), and mesitylene (63%). 1,2,4,5-Tetramethylbenzene is too bulky to give a coordination complex. Crystal structures of these π-arene complexes show η3–arene–Mg interactions for the smaller arenes (benzene, toluene and m-xylene). In each case, the coordination sphere was filled by an additional Mg···F interaction. For mesitylene, η6-coordination was found, leaving no space at the metal for supplementary Mg···F interaction. Dissolved in C6D5Br, all arene complexes are in association–dissociation equilibrium: (BDI)Mg+ + arene ⇄...