A benzene diradical dianion, which is stabilized by two gadolinium ions on either side, is the first simple molecule that is Baird aromatic in the ground state. Baird aromaticity typically only exists in high energy systems with excited states.
A simple way to identify simple aromatic molecules is by their electron count. If they have 4n + 2 π electrons they are aromatic, if it is 4n they are antiaromatic. However, this rule – Hückel’s rule – only applies to molecules in the singlet state, ie they cannot have unpaired electrons. For compounds with two unpaired electrons – a triplet state – the rules are reversed: compounds with 4n π electrons become aromatic and those with 4n + 2 π electrons become antiaromatic. This is known as the Baird flavor.
For almost all molecules, their singlet state is their ground state – molecular oxygen is a notable exception. Triplets are usually reserved for excited states, which can be achieved, for example, by light activation.
But a team of chemists from the USA and the Netherlands has now produced a benzene that is an aromatic Baird triplet in its ground state. The diradical dianion of benzene sits between two gadolinium 3+ ions, surrounded by several large substituents. Gadolinium is paramagnetic and can enter into a magnetic exchange coupling that stabilizes the triplet state of the benzene diradical to the point where it becomes the ground state of the molecule. Calculations showed that the ring current of triplet benzene is about 30% greater than that of regular singlet benzene.
While there have been some examples of Baird aromaticity in the ground state in macrocyclic and bicyclic systems, triplet benzene is the first simple monocyclic molecule with this property.