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

Abstract

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@C2n (M = Y, Tb; 2n = 76, 82) and Sc3CN@C2n (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@C2n and Sc3CN@C2n 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.

Ln3@C80+ (Ln = lanthanide): a new class of stable metallofullerene cations with multicenter metal–metal bonding in the sub-nanometer confined space

“…[1][2][3][4] Among these metallofullerenes, the C 80 -containing ones are attractive compounds, not only because of the high symmetry of I h -C 80 but also for its ability to accommodate two metal atoms or a mixed metal/nonmetal cluster. [5][6][7][8][9][10][11][12][13] On one hand, the outer C 80 fullerene cage provides a subnanometer confined space to capture and stabilize the originally unstable cluster with novel chemical bonding types. For instance, a two-center one-electron metal-metal bond has been discovered in M 2 @C 80 (M = Y or lanthanides).…”

“…1–4 Among these metallofullerenes, the C 80 -containing ones are attractive compounds, not only because of the high symmetry of I h - C 80 but also for its ability to accommodate two metal atoms or a mixed metal/nonmetal cluster. 5–13…”

Locating the hydrogen atoms in endohedral clusterfullerenes by density functional theory

“…22 Its three nearest H-Sc distances of 2.01, 2.04, and 2.06 Å are much shorter than that of H-C (2.20 Å), thus confirming its m 3 -H coordination manner. Its C-N bond length of 1.47 Å is longer than that of the CRN triple bond in MCN@C 2n (1.16-1.18 Å) and CQN double bond in Sc 3 CN@C 2n (1.24-1.28 Å) at the same level of theory, 28 suggesting the formation of a C-N single bond. In addition, the C atom is situated much closer to the Sc 4 tetrahedron center than the N atom.…”

“…26,27 We recently proposed that the charge state of free CN anion can assist in quickly determining the exact C/N position: the atom with a more negative charge preferentially coordinates to the metal cation due to the enhanced electrostatic attraction. 28 Fig. S3 (ESI †) summarizes the calculated NPA charges of the C and N atoms in isolated CN qÀ (q = 0-6) anions.…”

“…26,27 Our recent density functional theory (DFT) calculations further indicate that their relative positions can be understood by the energy of isolated metal clusters, charge state of the CN unit (À1 or À3), cluster-cage interactions, and hyperconjugation effect. 28 Thus, we are also curious regarding the exact location of the troublesome H atom in the above HCFs. Although the structures of all HCFs were proposed in the original studies, [19][20][21][22] these studies mainly excluded the possibility of an exohedrally bonded hydrogen to the cage surface, and failed to consider the endohedral H position.…”