The Tris(9-borabicyclo[3.3.1]nonyl)silylium Cation: A Suggestion for a Weakly Coordinated Silylium Cation in Solution

Abstract

The tris(9-borabicyclo[3.3.1]nonyl)silylium cation, Si(9-BBN)3+ (3), was investigated by ab initio quantum chemical methods, where, in particular, its coordination ability with benzene was studied. Dialkylboryl substitution leads to one of the thermodynamically most stable silylium ions investigated so far, which accordingly show a reduced tendency to coordinate with weakly nucleophilic solvents, such as benzene. The complex Si(9-BBN)3−C6H6+ (6) has a complex binding energy of just 4 kcal/mol and a Si−C bond length of 2.57 Å at HF/6-31G(d), which indicates that 6 is among the weakest silylium-cation−solvent complex ever investigated. Results suggest that silylium cations with bulky dialkylboryl substituents retain, to a large degree, their electronic character in solution and, therefore, should be interesting targets for experimental work. The nonpolar character of the 9-BBN substituents should enable usage of weakly coordinating hydrocarbon solvents, such as cyclohexane. If, in addition, weakly coordinating anions, such as the dodecamethylcarba-closo-dodecaborate anion, are utilized, it should be possible to synthesize silylium cations which are close to completely uncoordinated.