Antiaromaticity-aromaticity transition of cyclo[16]carbon upon metal encapsulation.

In contrast to aromatic compounds with particular stability, antiaromatic compounds are usually less stable due to their high reactivity and unfavorable formation energies. Cyclo[16]carbon (C) is a carbon ring molecule with a dual antiaromatic character. In this study, we demonstrate that C can be transformed into highly aromatic molecules upon metal encapsulation. The geometrical characteristics, electronic properties and thermodynamic stability of MC compounds (M = Ca, Sc, Ti, V, Ce, U) are fully investigated from a theoretical perspective. Based on natural population analysis, atom-in-molecules theory and localized molecular orbital analysis, the nature of the metal-carbon interaction in the MC compounds is investigated. It has been proved that the bonding between Ca and C corresponds to a typical ionic interaction, while other metal atoms form polar covalent bonds with C. By analyzing the frontier molecular orbitals and magnetic response of MC, we have found that all the encapsulated metal atoms donate two electrons to the in-plane π orbitals via either electron transfer or orbital hybridization, which makes the in-plane π orbitals completely satisfy the 4n + 2 (n = 4) Hückel aromaticity rule. The U atom formally transfers four electrons to the carbon ring in total, two to the in-plane π orbitals and two to the out-of-plane π orbitals, which results in the remarkable dual aromaticity feature of UC. The transformation of aromaticity can be utilized to develop new strategies for the synthesis of novel carbon ring molecules.