A Crystalline Doubly Oxidized Carbene

Carbon is the quintessential element of life. The chemistry of carbon is governed by the octet rule, which refers to its ten-dency to have eight electrons in its valence shell. However, a few exceptions do exist. More than a century ago, Gomberg discovered the trityl radical (Ph3C∙) , which is a stable three-coordinate carbon species with seven valence electrons. Remov-ing the non-bonding electron afforded the stable trityl carbocation (Ph3C+) with only six valence electrons. In 1988, our group reported the isolation of a stable carbene (R2C:) − a two-coordinate octet-defying species with formally six valence electrons. Carbenes are now among the most powerful tools in chemistry , and have even found applications in material and medicinal sciences , . Can we undress the carbene further by removing its non-bonding electrons? Herein, we describe the synthesis of a crystalline doubly oxidized carbene (R2C2+), through a two-electron oxidation/oxide-ion abstraction sequence from an electron-rich carbene bearing N-heterocyclic imine (NHI) substituents. Despite a cumulenic structure and strong delocalization of the positive charges, the dicoordinate carbon center maintains significant electrophilicity, and possesses the expected two accessible vacant orbitals. We show that the parent carbene can be regenerated by a two-electron reduc-tion/deprotonation sequence, confirming that the carbene and its dication are related by two electrons. This work demonstrates that the use of bulky strong electron-donor substituents can simultaneously impart electronic stabilization and steric protection to both vacant orbitals on the central carbon atom, paving the way for the isolation of a variety of doubly oxidized carbenes.