A Combined Experimental and Computational Study on the Reaction of Fluoroarenes with Mg–Mg, Mg–Zn, Mg–Al and Al–Zn Bonds
Created: 2017-10-09 13:59
Last modified: 2018-01-23 09:31
ABSTRACT: 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 SN2-like pathway in which one Mg center acts as a nucleophile and the other an electrophile. The experimentally determined Gibbs activation energy for the addition of 1 to C6F6 G‡298K(experiment) = 21.3 kcal mol-1 is modelled well by DFT with the B97X functional, G‡298K(DFT) = 25.7 kcal mol-1. The transition state for C–F activation involves a polarization of the Mg–Mg bond and significant negative charge localization on the fluoroarene moiety. This transition state is augmented by stabilizing closed-shell Mg···Fortho 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 novel diketiminate stabilized complexes containing Zn–Mg, Zn–Zn–Zn, Zn–Al and Mg–Al bonds has been prepared. Reactions of these new M–M containing complexes with perfluoroarenes were conducted and modelled by DFT. The data show that 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···Fortho interactions which in turn lower the energy of the transition states for C–F bond activation.