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Tris(carbene) Stabilization of Monomeric Magnesium Cations: A Neutral, Nontethered Ligand Approach

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journal contribution
posted on 20.08.2020, 11:33 by Akachukwu D. Obi, Jacob E. Walley, Nathan C. Frey, Yuen Onn Wong, Diane A. Dickie, Charles Edwin Webster, Robert J. Gilliard
Herein, we describe the syntheses and structural characterization of bis­(carbene)- and tris­(carbene)-stabilized organomagnesium cations. The reaction of the N-heterocyclic carbene (NHC) stabilized Grignard reagent (iPrNHC)2Mg­(Me)­(Br) (1) and Na­[BArF4] (iPrNHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, ArF = 3,5-bis­(trifluoromethyl)­phenyl) in chlorobenzene yields exclusively the bis­(NHC)-stabilized dication [{(iPrNHC)2Mg}2(μ-Me)2]­[(BArF4)2] (2). If the reaction is performed in ethereal or nonpolar arene solvents, 2 undergoes Schlenk-type rearrangements to tris­(NHC)-stabilized cations [(iPrNHC)3Mg­(Me)]­[BArF4] (3­[BArF4]) and [(iPrNHC)3Mg­(Br)]­[BArF4] (4­[BArF4]). These monomeric cations 3­[A] and 4­[A] (A = BArF4, BPh4) can be independently prepared as single pure products in high yields using common hydrocarbon solvents. The electronic influence of tris­(carbene) stabilization is further evidenced by an NHC-mediated ionization of magnesium bromide in the absence of abstraction reagents. The reaction between the sterically unencumbered 1,3,4,5-tetramethylimidazol-2-ylidene (MeNHC) ligand and (MeNHC)2MgBr2 (7) resulted in two geometrically unique cations of the type [(MeNHC)3MgBr]­[Br]: complex 8a bearing a weakly coordinating bromide anion resulting in a trigonal bipyramidal magnesium center, and complex 8b bearing a noncoordinating bromide anion where the magnesium atom resides in a tetrahedral coordination environment. All isolated complexes were characterized by NMR spectroscopy and single-crystal X-ray diffraction, and their bonding was investigated by density functional theory (DFT).