A crystalline doubly oxidized carbene

Abstract

The chemistry of carbon is governed by the octet rule, which refers to its tendency to have eight electrons in its valence shell. However, a few exceptions do exist, for example, the trityl radical (Ph₃C∙) (ref. ¹) and carbocation (Ph₃C⁺) (ref. ²) with seven and six valence electrons, respectively, and carbenes (R₂C:)-two-coordinate octet-defying species with formally six valence electrons³. Carbenes are now powerful tools in chemistry, and have even found applications in material and medicinal sciences⁴. Can we undress the carbene further by removing its non-bonding electrons? Here we describe the synthesis of a crystalline doubly oxidized carbene (R₂C²⁺), through a two-electron oxidation/oxide-ion abstraction sequence from an electron-rich carbene⁵. Despite a cumulenic structure and strong delocalization of the positive charges, the dicoordinate carbon centre maintains significant electrophilicity, and possesses two accessible vacant orbitals. A two-electron reduction/deprotonation sequence regenerates the parent carbene, fully consistent with its description as a doubly oxidized carbene. 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.