Isolation and Structure Elucidation of the Heterocumulene Anions [NCC-L]^- (L=CO, CS, N₂)

Cumulenes are molecules characterized by a series of consecutive double bonds. They serve as important reagents and intermediates in the synthesis of polymers and a wide variety of functionalized compounds, including various heterocycles. Understanding the properties of cumulenes and developing synthetic routes to these often highly reactive species is essential for unlocking new applications. Here, we report the synthesis and isolation of the cyanodiazomethanide [NCCNN]^- and cyanothioketenyl anion [NCCCS]^-. These 5-atomic anions exhibit unexpected stabilities but distinct structural differences. Despite the explosive nature of diazoacetonitrile, the [NCCNN]^- anion was sufficiently stable at 0 °C to allow for first reactivity studies and its structure elucidation revealing a bent structure. The thioketenyl anion is stable at room-temperature and can be accessed from the cyanoketenyl anion [NCCCO]^- via a [2+2] cycloaddition and cycloreversion sequence with COS elimination. Comparative structural, spectroscopic and computational studies including those on the cyanoketenyl anion [NCCCO]^-, demonstrate that the degree of bending of these heterocumulene anions [NCC-L]^- can be explained by a transition in the bonding situation from a cumulene structure to an anionic carbone, with the strongly π accepting CS ligand leading to a linear structure of the thioketenyl anion.