Reversible formation of tetraphenylpentalene, a room temperature stable antiaromatic hydrocarbon.

1,3,4,6-Tetraphenylpentalene (PhPn) has been synthesised by chemical oxidation of the corresponding pentalenide complex Mg[PhPn] with iodine. PhPn is a rare example of a room-temperature stable hydrocarbon that is antiaromatic by Hückel's rule and has been fully characterised by NMR and UV-vis spectroscopy, mass spectrometry as well as single-crystal X-ray diffraction. Quantum chemical studies including nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations showed the existence of an 8π antiaromatic core decorated with four independent 6π aromatic substituents. The formation of PhPn is reversible and it can be reduced back to the 10π aromatic PhPn with potassium.