Arenium ion

An arenium ion in organic chemistry is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution. For historic reasons this complex is also called a Wheland intermediate or a sigma complex or σ-complex. The smallest arenium ion is benzenium ion C₆H₇⁺, which is protonated benzene.

Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring. The arenium ion is no longer an aromatic species; however it is relatively stable due to delocalization: the positive charge is delocalized over 3 carbon atoms via the pi system, as depicted on the following resonance structures:

A complexed electrophile can contribute to the stability of arenium ions.

A benzenium ion can be isolated as a stable compound when benzene is protonated by the carborane superacid H(CB₁₁H(CH₃)₅Br₆). The benzenium salt is crystalline with thermal stability up to 150 °C. Bond lengths deduced from X-ray crystallography are consistent with a cyclohexadienyl cation structure.

In one study a methylene arenium ion is stabilized by metal complexation:

In this reaction sequence the R-Pd(II)-Br starting complex 1 stabilized by TMEDA is converted by dppe to metal complex 2. Electrophilic attack of methyl triflate forms methylene arenium ion 3 with (based on X-ray crystallography) positive charge located in aromatic para position and with the methylene group 6° out of the plane of the ring. Reaction first with water and then with triethylamine hydrolyzes the ether group.