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

This is the authors' abstract. We don't add key points for this paper.

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 ( iPr NHC) 2 Mg(Me)(Br) ( 1 ) and Na[BAr F 4 ] ( iPr NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Ar F = 3,5-bis(trifluoromethyl)phenyl) in chlorobenzene yields exclusively the bis(NHC)-stabilized dication [{( iPr NHC) 2 Mg} 2 (μ-Me) 2 ][(BAr F 4 ) 2 ] ( 2 ). If the reaction is performed in ethereal or nonpolar arene solvents, 2 undergoes Schlenk-type rearrangements to tris(NHC)-stabilized cations [( iPr NHC) 3 Mg(Me)][BAr F 4 ] ( 3[BAr F 4 ] ) and [( iPr NHC) 3 Mg(Br)][BAr F 4 ] ( 4[BAr F 4 ] ). These monomeric cations 3[A] and 4[A] (A = BAr F 4, BPh 4 ) 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 ( Me NHC) ligand and ( Me NHC) 2 MgBr 2 ( 7 ) resulted in two geometrically unique cations of the type [( Me NHC) 3 MgBr][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).