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

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journal contribution
posted on 12.11.2012, 00:00 by Valery F. Sidorkin, Evgeniya P. Doronina, Elena F. Belogolova
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.