Antiaromatic hydrocarbon boasts unusual stability

A rare example of an antiaromatic hydrocarbon that’s stable at room temperature has been made using a straightforward and completely reversible oxidation reaction. While annelation with other π systems is the most common way to stabilise an antiaromatic structure, 1,3,4,6-tetraphenylpentalene’s (Ph₄Pn) stability appears to come from the steric bulk of its phenyl substituents.

The researchers synthesised Ph₄Pn by oxidising Mg[Ph₄Pn] with one equivalent of iodine, which created a MgI₂ precipitate within minutes. NMR analysis of the filtrate confirmed it was a C_2h symmetric pentalene with a localised olefinic π system. The pentalene exhibits remarkable stability. Variable temperature NMR studies conducted in THF showed no significant decomposition, however, the compound did decompose when exposed to air.

XRD analysis of Ph₄Pn single crystals confirmed that its structure retains a bicyclic, co-planar pentalene core with no stacking observed in neither solid nor solution state. This means that the pentalene remains in a less stable, antiaromatic state without adopting a stacked 3D structure.

Ph₄Pn can be fully reduced back into Ph₄Pn²⁺ by treating it with a freshly prepared potassium mirror. This characteristic allows the hydrocarbon to switch between an aromatic and antiaromatic state without undergoing conformational or skeletal rearrangements.

The easy and reversible synthesis combined with the unique antiaromatic features of Ph₄Pn could see the molecule and its derivatives have applications in optoelectronics.