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: ΔH
0 = −6.9 kcal·mol–1 and ΔS
0 = −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.