A Combined Experimental and Computational Study on the Reaction of Fluoroarenes with MgâMg, MgâZn, MgâAl and AlâZn Bonds
Created: 2017-10-09 13:59
Last modified: 2018-01-23 09:31
License: Creative Commons: Public Domain Dedication 1.0
Funding: (none given)
ABSTRACT: Through a combined experimental and computational (DFT) approach, the reaction mechanism of the addition of fluoroarenes to MgâMg 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 Gâ¡298K(experiment) = 21.3 kcal mol-1 is modelled well by DFT with the ï·B97X functional, Gâ¡298K(DFT) = 25.7 kcal mol-1. The transition state for CâF activation involves a polarization of the MgâMg bond and significant negative charge localization on the fluoroarene moiety. This transition state is augmented by stabilizing closed-shell MgÂ·Â·Â·Fortho interactions that, in combination with the known trends in CâF and CâM bond strengths in fluoroarenes, provide an explanation for the experimentally determined preference for CâF 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 MâM bond (M = Mg, Zn, Al) on CâF activation has been investigated. A series of novel diketiminate stabilized complexes containing ZnâMg, ZnâZnâZn, ZnâAl and MgâAl bonds has been prepared. Reactions of these new MâM containing complexes with perfluoroarenes were conducted and modelled by DFT. The data show that CâF bond activation is dictated by the steric accessibility, and not the polarity, of the MâM bond. The more open coordination complexes lead to enhanced MgÂ·Â·Â·Fortho interactions which in turn lower the energy of the transition states for CâF bond activation.