A Mono-Substituted Silicon(II) Cation: A Crystalline "Supersilylene"

Mono-coordinated silicon(II) cations are predicted to be reactive ambiphiles, combining the typically high Lewis acidity of silicon cations with nucleophilicity due to the presence of an electron pair at the same atomic centre. Here, a carbazole-derived scaffold was used to isolate salts with a mono-coordinated silicon(II) cation, [RSi]⁺ (R=bulky carbazolyl substituent), by halide abstraction from a base-free halosilylene, RSiI, with Ag[Al(O^t Bu^F )₄ ]. Despite the bulk of the carbazolyl moiety, the silylenylium cation [RSi]⁺ retains high reactivity. It was shown to react with an amine to form three bonds at the silicon atom in one reaction which conforms with the notion of a "supersilylene". The resulting silylium cation [RSi(H)NR'₂ ]⁺ (in the formal oxidation state Si^IV ) obtained by oxidative addition of an NH bond at [RSi]⁺ is even more acidic than the silylenylium cation (Si^II ) due to the absence of a lone pair of electrons the silicon atom.