Synthesis, structure and reactivity of terphenyl-substituted germylium-ylidene cations

The first examples of isolable two-coordinate terphenyl-substituted germylium-ylidenes are reported. These systems, of composition [(NHC)(Ar)Ge]⁺, exploit a range of aryl and NHC substituents (Ar^Mes, Ar^Trip; IMe₄, IPrMe) and are isolated by employing weakly coordinating borate or aluminate counter-ions. The use of the smallest combination of substituents used in this study (IMe₄ and Ar^Mes) results in the formation of a dicationic dimer, [(IMe₄)(Ar^Mes)Ge=Ge(Ar^Mes)(IMe₄)]²⁺, which can also be viewed as an imidazolium-functionalized digermene. Increasing the steric loading (at either the NHC or terphenyl ligand) leads to the formation of the monomeric germylium-ylidene cations [(IMe₄)(Ar^Trip)Ge]⁺, [(IPrMe)(Ar^Mes)Ge]⁺, and [(IPrMe)(Ar^Trip)Ge]⁺, stabilized in the solid state to varying degrees by secondary interactions with the peripheral arene π systems or C–H bonds. These three systems are characterized by widening C–Ge–C angles (101.7(1), 105.0(2) and 107.2(1)°, respectively) as the steric profiles of the germanium-bound substituents increase. In the case of dication [(IMe₄)(Ar^Mes)Ge=Ge(Ar^Mes)(IMe₄)]²⁺, its ability to act as a functional source of the corresponding monomeric germylium-ylidene, [(IMe₄)(Ar^Mes)Ge]⁺, is demonstrated through its reaction with PhSiH₃ in bromobenzene solution, which generates the monomeric Ge^IV species [(IMe₄)(Ar^Mes)Ge(H)(SiH₂Ph)]⁺ via Si–H oxidative addition at germanium.