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

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).