Conversion of an NHC-Stabilized Silyliumylidene to a Dicationic Silicon(IV) Complex Capable of CO₂Activation

The synthesis and characterization of an N-heterocyclic carbene (NHC)-stabilized silyliumylidene borane adduct [mTer(IMe4)2Si→BH3][OTf] ([2][OTf]) (mTer = 2,6-(2,4,6-Me3C6H2)2C6H3; IMe4 = 1,3,4,5-tetramethyl-imidazol-2-ylidene; OTf = CF3SO3-) as well as the highly unusual hydride dication [mTer(IMe4)2SiH][OTf]2 ([3][OTf]2) are reported. While [2][OTf] is obtained by the direct reaction of silyliumylidene [mTerSi(IMe4)2][OTf] ([1][OTf]) with either Me2S·BH3 or tetrahydrofuran (THF)·BH3, usage of the amine borane Me2NH·BH3 gives protonation at silicon instead to yield the dication complex [3][OTf]2. Alternately, [3][OTf]2 can be prepared by the addition of triflic acid to [1][OTf]. The reaction outcome strongly depends on the borane source and is evidence of the unprecedented nucleophilic character of [1][OTf] as elucidated by density functional theory (DFT) calculations. [3][OTf]2 can irreversibly react with one molecule of CO2, forming the insertion product [6][OTf]2. Subsequent introduction of a second equivalent CO2 leads to a 1:1 equilibrium of single- ([6][OTf]2) and double-CO2-insertion ([7][OTf]2) products, as determined by multinuclear NMR spectroscopy, single-crystal X-ray diffraction (SC-XRD) analysis, and supported by theoretical calculations.