Cage Complexes of Carbenium and Silylium Cations with an Aromatic Base. Is the η6 Coordination Type Realizable?
journal contributionposted on 12.11.2012 by Valery F. Sidorkin, Evgeniya P. Doronina, Elena F. Belogolova
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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.