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 ß-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 ⇄ (BDI)­Mg+·arene. For the most strongly bound mesitylene ligand, a decoalescence of the 1H NMR resonances was reached at −20 °C, each species giving separate signals. From the temperature dependency of this equilibrium, the following thermodynamic parameters have been deduced: ΔH0 = −6.9 kcal·mol–1 and ΔS0 = −28.2 cal·mol–1·K–1. DFT calculations reveal that the Mg···arene bonding is mainly electrostatic of nature with only little charge transfer from arene to Mg2+ (ca. 0.05 e) and a slight polarization of π-electron density toward the metal. Substitution of benzene in (BDI)­Mg+·C6H6 for mesitylene is exothermic by −11.7 kcal/mol; however, including the B­(C6F5)4– counterion in the calculation gave an energy gain of −2.2 kcal/mol. This clearly demonstrates that weakly coordinating anions can affect these Mg···arene interactions substantially.