Cyclo[n]carbons Form Strong N → C Dative/Covalent Bonds with Piperidine

The newly synthesized C₁₈ ring is demonstrated as the smallest all-carbon acceptor that exhibits strong electron acceptance. This study provides a quantum-chemical investigation of the electron-acceptance behavior of monocyclic carbon rings with a particular emphasis on C₁₈ through the formation of a dative bond with piperidine. The results show that C rings form strong dative bonds with piperidine, whereas the respective van der Waals (vdW) complexes are higher in energy. The main driving force is the release of angle strain of cyclo[n]carbons caused by the change in hybridization from sp to sp² associated with the formation of the dative bond. On the contrary, other sp allotropes, diynes, favorably form vdW complexes. Molecular dynamics (MD) simulations support the stability of the dative bond throughout a simulation of 20 ps. This opens up the possibility of stabilizing highly reactive C₁₈ through dative/covalent functionalization.