A combined experimental and computational study on the reaction of fluoroarenes with Mg-Mg, Mg-Zn, Mg-Al and Al-Zn bonds

Through a combined experimental and computational (DFT) approach, the reaction mechanism of the addition of fluoroarenes to Mg-Mg bonds has been determined as a concerted S_NAr-like pathway in which one Mg centre acts as a nucleophile and the other an electrophile. The experimentally determined Gibbs activation energy for the addition of C₆F₆ to a Mg-Mg bond of a molecular complex, ΔG‡298 K(experiment) = 21.3 kcal mol^-1 is modelled by DFT with the ωB97X functional, ΔG‡298 K(DFT) = 25.7 kcal mol^-1. The transition state for C-F activation involves a polarisation of the Mg-Mg bond and significant negative charge localisation on the fluoroarene moiety. This transition state is augmented by stabilising closed-shell Mg⋯F_ortho interactions that, in combination with the known trends in C-F and C-M bond strengths in fluoroarenes, provide an explanation for the experimentally determined preference for C-F bond activation to occur at sites flanked by ortho-fluorine atoms. The effect of modification of both the ligand coordination sphere and the nature and polarity of the M-M bond (M = Mg, Zn, Al) on C-F activation has been investigated. A series of highly novel β-diketiminate stabilised complexes containing Zn-Mg, Zn-Zn-Zn, Zn-Al and Mg-Al bonds has been prepared, including the first crystallographic characterisation of a Mg-Al bond. Reactions of these new M-M containing complexes with perfluoroarenes were conducted and modelled by DFT. C-F bond activation is dictated by the steric accessibility, and not the polarity, of the M-M bond. The more open coordination complexes lead to enhanced Mg⋯F_ortho interactions which in turn lower the energy of the transition states for C-F bond activation.