The synthesis itself is relatively straightforward. You start with an electron-rich bis(imino)carbene. First, you react the carbene with I₂ and NaOSiMe₃ to form a bis(imino)carbonyl. When you add triflate anhydride, this molecule removes the oxygen atom together with its electrons, leaving you with the doubly oxidised carbene²⁺. The nitrogenous side groups provide the C atom with the necessary electron density to make the whole thing stable.

The researchers then looked at the reactivity of the carbene²⁺. The carbene can be oxidised back to the bis(imino)carbonyl. The reaction with a neutral base such as DMAP or pyridine is also possible, as is methoxylation with dimethoxyethane, chlorination with trimethylsilyl chloride or a double reaction with hydride and C₆F₅^-.

What is special is that the double oxidation of the carbene is reversible. Normally, such a two-electron redox system is only seen in transition metals, but the authors show that this p-block element can do it as well, confirming that it is indeed a double-oxidised carbene. They now hope to use this information to isolate more doubly oxidised carbenes.

Loh, Y.K. et al. (2023) Nature, DOI: 10.1038/s41586-023-06539-x