N-Heterocyclic Carbene-Stabilized Germa-acylium Ion: Reactivity and Utility in Catalytic CO₂ Functionalizations

The first acceptor-free heavier germanium analogue of an acylium ion, [RGe(O)(NHC)₂]X (R = ^MesTer = 2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃; NHC = IMe₄ = 1,3,4,5-tetramethylimidazol-2-ylidene; X = (Cl or BArF = {(3,5-(CF₃)₂C₆H₅)₄B}), was isolated by reacting [RGe(NHC)₂]X with N₂O. Conversion of the germa-acylium ion to the first solely donor-stabilized germanium ester [(NHC)RGe(O)(OSiPh₃)] and corresponding heavier analogues ([RGe(S)(NHC)₂]X and [RGe(Se)(NHC)₂]X) demonstrated its classical acylium-like behavior. The polarized terminal GeO bond in the germa-acylium ion was utilized to activate CO₂ and silane, with the former found to be an example of reversible activation of CO₂, thus mimicking the behavior of transition metal oxides. Furthermore, its transition-metal-like nature is demonstrated as it was found to be an active catalyst in both CO₂ hydrosilylation and reductive N-functionalization of amines using CO₂ as the C₁ source. Mechanistic studies were undertaken both experimentally and computationally, which revealed that the reaction proceeds via an N-heterocyclic carbene (NHC) siloxygermylene [(NHC)RGe(OSiHPh₂)].