Cage Complexes of Carbenium and Silylium Cations with an Aromatic Base. Is the η⁶ Coordination Type Realizable?

The cage cations [E­((CH₂)₃)₃C₆R₃]⁺ (E = C (1), Si (2); R = H (a), Li (b), F (c)) were studied theoretically using the MP2/6-311++G­(d,p) approach. The migration of the electrophilic center E and a proton around the perimeter of the arene ring in the cations [E­((CH₂)₃)₃C₆H₃]⁺ was considered. Our results are indicative of the π type of E←arene coordination in the intramolecular complexes [E­((CH₂)₃)₃C₆R₃]⁺. The cation [Si­((CH₂)₃)₃C₆H₃]⁺ (2a), which corresponds to a peak (M – CH₃)⁺ at m/z 229 in the mass spectrum of methylsilacyclophane, exists in a firmly established η¹π form, as opposed to the case for the known complexes of the trivalent silicon atom with aromatic bases. The species [C­((CH₂)₃)₃C₆F₃]⁺ (1c), as well as 4a, obtained by connecting the equatorial carbon atoms in [Si­((CH₂)₃)₃C₆H₃]⁺ by a methylene bridge, are the first representatives of stable “face” complexes of carbenium and silylium cations with a benzene ring. The process of deprotonation of the complexes [C­((CH₂)₃)₃C₆H₃]⁺, [Si­((CH₂)₃)₃C₆H₃]⁺, and 4a was found to be energetically unfavorable, even in the presence of a strong base such as NEt₃. The effect of the counterions BX₄^– (X = F, C₆F₅) and the polarity of solvents on the structure of the above cations was investigated using the examples of toluene and DMSO.