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

Abstract

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). 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 1,3,4,5tetramethylimidazol-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).

Comparison of Magnesium and Zinc in Cationic π-Arene and Halobenzene Complexes

“…A large excess of a weak Lewis base can replace slightly stronger bases. A similar conclusion was reached for comparable complexes with the ( tBu BDI)Mg + cation; however, in these cases a clear indication for ( tBu BDI)Mg + •••FPh complexation was found by observation of a concentration-dependent 19 F downfield shift for the PhF ligand. 34 The absence of evidence for complex formation between ( tBu BDI)Zn + and PhF is in agreement with the aforementioned mismatch between the soft Zn Lewis acid and the hard F atom, which also explained the preference for Zn•••(π)PhF bonding.…”

“…Signals for B(C 6 F 5 ) 4 were not detected. 19 − ] (27.3 mg, 27.3 μmol, 1 equiv) was added, and after the mixture was stirred for 2 min a color change from redbrown to colorless was observed. Subsequently toluene (1 mL) was added and the reaction mixture was layered with hexane (1 mL) for crystallization.…”

“…Crystallographic details including ORTEP plots, 1 H, 11 B, 19 F, and 13 C NMR spectra, and details of the DFT calculations (PDF)…”

“…Me 2 ) ppm. Signals for B(C 6 F 5 ) 4 were not detected 19. F NMR (C 6 D 5 Br, 565 MHz, 298 K): δ −165.4 (t, 8F, meta aryl-F), −161.6 (t, 4F, para aryl-F), −131.0 (d, 8F, ortho aryl-F) ppm.…”

Lewis Acidic Cationic Strontium and Barium Complexes

“…[21][22][23][24][25][26][27][28][29] Therefore, we developed synthetic methods to access highly Lewis acidic, cationic Ae complexes which allow for intermolecular Ae-alkene interactions. While most cationic Ae metal complexes are stabilized by strong Lewis bases or strongly chelating, multidentate neutral or anionic ligands, [30][31][32][33][34][35][36][37][38][39][40][41] our interest is directed to "naked", Lewis base-free, cationic complexes. We reported cationic β-diketiminate Ae metal complexes, (BDI)Ae + , that in the presence of the weakly coordinating anion (WCA) B(C 6 F 5 ) 4 À are highly Lewis acidic and strongly bind to neutral arenes (I); BDI = HC[C(Me)N(DIPP)] 2 , DIPP = 2,6-diisopropylphenyl (Scheme 1).…”