Tropylium cation


In organic chemistry, the tropylium ion or cycloheptatrienyl cation is an aromatic species with a formula of +. Its name derives from the molecule tropine from which cycloheptatriene was first synthesized in 1881. Salts of the tropylium cation can be stable, even with nucleophiles of moderate strength e.g., tropylium tetrafluoroborate and tropylium bromide. Its bromide and chloride salts can be made from cycloheptatriene and bromine or phosphorus pentachloride, respectively.
It is a regular heptagonal, planar, cyclic ion; as well, it has 6 π-electrons, which fulfills Hückel's rule of aromaticity. It can coordinate as a ligand to metal atoms.
The structure shown is a composite of seven resonance contributors in which each carbon atom carries part of the positive charge.
In 1891 G. Merling obtained a water-soluble bromine containing compound from a reaction of cycloheptatriene and bromine. Unlike most hydrocarbyl bromides, this compound, later named tropylium bromide, is a water-soluble solid and is insoluble in hydrocarbons, chloroform, and ether. It is purified by crystallization from hot ethanol. Reaction with aqueous silver nitrate immediately gave a precipitate of silver bromide. The structure of tropylium bromide was deduced to be a salt, C7H7+Br–, by Doering and Knox in 1954 by analysis of its infrared and ultraviolet spectra. The ionic structures of tropylium perchlorate and tropylium iodide in the solid state have been confirmed by X-ray crystallography. The bond length of the carbon-carbon bonds were found to be longer than those of benzene but still shorter than those of a typical single-bonded species like ethane.
The tropylium ion is an acid in aqueous solution as a consequence of its Lewis acidity: it first acts as a Lewis acid to form an adduct with water, which can then donate a proton to another molecule of water: C7H7+ + 2H2O ⇌ C7H7OH + H3O+. The equilibrium constant is 1.8 × 10−5, making it about as acidic in water as acetic acid.

Mass spectrometry

The tropylium ion is frequently encountered in mass spectrometry in the form of a signal at m/z = 91 and is used in mass spectrum analysis. This fragment is often found for aromatic compounds containing a benzyl unit. Upon ionization, the benzyl fragment forms a cation, which rearranges to the highly stable tropylium cation.

Reactions

The tropylium cation reacts with nucleophiles to form substituted cycloheptatrienes, for example:
Reduction by lithium aluminum hydride yields cycloheptatriene.
Reaction with a cyclopentadienide salt of sodium or lithium yields 7-cyclopentadienylcyclohepta-1,3,5-triene:
When treated with oxidising agents such as chromic acid, the tropylium cation undergoes rearrangement into benzaldehyde:
Many metal complexes of tropylium ion are known. One example is +, which is prepared by hydride abstraction from cycloheptatrienemolybdenum tricarbonyl.