Undiscovered Effect of C↔N Interchange Inside the Metal Carbonitride Clusterfullerenes: A Density Functional Theory...

Putting different metal clusters into the fullerene cages form the so-called "endohedral clusterfullerenes" (ECFs), among which all the carbonitride ECFs feature a common NC unit coordinating with either one or three metal atoms. Unfortunately, their internal N and C atoms are difficult to be distinguished experimentally, resulting in the fact that the exact structure and bonding nature of the encased metal cluster still remain unclear thus far. In this work, density functional theory calculations were performed for several representative carbonitride ECFs: MNC@C₂ (M = Y, Tb; 2n = 76, 82) and Sc₃CN@C₂ (2n = 78, 80). For the first time, we focused on the C ↔ N interchange inside the cages and its effect on the chemical bonding of the trapped clusters. Computational results reveal that the two types of ECFs energetically favor the N and C atoms at the cluster center, respectively. The preference can be interpreted by the difference in several aspects, such as the energy of isolated clusters, charge states of (CN)^-/3-, and cluster-cage interactions, as well as hyperconjugation of the internal clusters. The detailed wave function analyses indicate that MNC@C₂ and Sc₃CN@C₂ bear a C≡N triple bond and a C═N double bond, respectively, regardless of the NC orientation. Compared with its slightly influence on the bonding patterns of the encaged MNC clusters, the C ↔ N interchange dramatically affects that of the Sc₃CN units involving two-center two-electron (2c-2e) bonds, undiscovered three-center two-electron (3c-2e), and four-center two-electron (4c-2e) bonds.