Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C₆ (CH₃ )₆²

Area of Science:

• Organic Chemistry
• Physical Chemistry
• Crystallography

Background:

• The 2-norbornyl cation's structure was debated until recently proven via X-ray crystallography.
• The hexamethylbenzene dication, C6(CH3)62+, has received less structural investigation.
• Previous studies proposed a non-classical, hexacoordinated carbon structure based on NMR and reactivity, but lacked X-ray data.

Purpose of the Study:

• To determine the definitive crystal structure of the hexamethylbenzene dication.
• To provide experimental evidence for the proposed pentagonal-pyramidal structure.
• To contribute to the understanding of non-classical carbocation structures.

Main Methods:

• Preparation of the hexamethylbenzene dication salt (C6(CH3)62+(SbF6-)2·HSO3F) from hexamethyl Dewar benzene epoxide in magic acid (HSO3F/SbF5) with added HF.
• Single-crystal X-ray diffraction analysis of the synthesized salt.

Main Results:

• The crystal structure confirms a pentagonal-pyramidal geometry for the hexamethylbenzene dication.
• The apical carbon is bonded to one methyl group (1.479(3) Å) and five basal carbons (1.694(2)–1.715(3) Å).
• This provides the first direct structural evidence for this dication's unique arrangement.

Conclusions:

• The X-ray crystal structure unequivocally supports the pentagonal-pyramidal model for the hexamethylbenzene dication.
• This finding resolves structural ambiguities and advances the understanding of highly substituted carbocations.
• The study highlights the importance of X-ray crystallography in elucidating complex molecular structures in chemistry.