(Invited) The Growth of Endohedral Metallofullerenes
Authors: Abella Laura , Rodriguez-Fortea Antonio , Poblet Josep M. 
Published: February 2020

Since the discovery of fullerenes, tremendous advances have been made in the field of nanoscience. However, the formation of these molecules still remains as a mystery. Several hypothetical models have been established to elucidate their formation from graphite or amorphous carbon, but none of them are strictly convincing. During the last few years, many experiments of fullerene growth have been developed. Thus, it allowed us to study their formation accurately. Herein, we present the growth mechanism studies of Ti@C2n (2n = 26-50) and Sc3N@C2n (2n = 68-80).

The experimental detection of the new family of endohedral metallofullerenes Ti@C2n (2n = 26-50) was the starting point of this study. The high-resolution FT-ICR-MS mass spectrometry showed that Ti@C26 was the smallest detected cage, and Ti@C28 and Ti@C44 present the highest intensity and stability. Once all the Ti@C2n cages were computed, we started to analyze their formation step by step. A computationally study using DFT calculations and Car-Parrinello Molecular Dynamics simulations show that all the optimal isomers from C26 to C50 are linked by a simple C2 insertion, with the exception of a few carbon cages that require an additional C2 rearrangement. The ingestion of a C2 unit is always an exergonic/exothermic process that can occur through a rather simple mechanism, with the most energetically demanding step corresponding to the closure of the carbon cage. The large formation abundance observed in mass spectra for Ti@C28 and Ti@C44 can be explained by the special electronic properties of these cages and their higher relative stabilities with respect to C2 reactivity.

Plasma synthesis techniques are used to construct many forms of carbon materials and compounds, and in particular, nitride clusterfullerenes, M3N@C2n (M = metal, C2n = even numbered cage), which are among the most intensively studied form of molecular nanocarbon. The chemical processes that result in the synthesis of nitride clusterfullerene compounds, however, are unknown because in situ analyses are not possible by conventional arc plasma discharge techniques. Very recently, Prof. Dunk investigated metallic nitride clusterfullerene self-assembly for the first time by laser vaporization of metal- and nitrogen-doped graphite.The Sc3N cluster initially nucleates formation of smaller cages, and thereafter, larger species primarily self-assemble through carbon insertion reactions. In contrast to mono- metallofullerenes, the Sc3N cluster is too large to nucleate cage sizes of C60 or smaller. Therefore, the smallest carbon cages (C2n ≤ 60) are bypassed during clusterfullerene formation and thus high-value, medium-sized cages are more efficiently synthesized. The influence of cluster and cage-size effects is elucidated by molecular behavior analysis of distinct clusterfullerenes under representative physicochemical synthesis conditions. The small, pentalene-containing Sc3N@D
3-C68 compound is explicitly demonstrated to transform through a bottom-up mechanism into Sc3N@C80.

The reaction products formed after exposure of isomerically pure, pre-existing Sc3N@D
3-C68 to carbon plasma in a low pressure He atmosphere, showed the bottom-up growth into larger Sc3N@C2n (2n = 70-94) compounds. Under the present conditions, Sc3N@C70 exhibits the largest relative abundance. However, the much larger-sized Sc3N@C80 compound was also observed and exhibits an enhanced magnitude, indicating that the very stable Sc3N@I
h-C80 clusterfullerene is formed. Pre-existing Sc3N@D
3-C68 was also studied without exposure to carbon vapor to examine its molecular behavior under the high energy conditions of synthesis. Surprisingly, Sc3N@C70, a bottom-up growth product, was the most abundant molecular reaction product, even without the presence of carbon plasma generated from graphite, suggesting that carbon insertion reactions can be favored in low carbon density, high-energy conditions. Consequently, these results provide additional evidence that the smallest Sc3N@C2n (2n = C62-C66) are not formed from larger Sc3N@C2n during self- assembly from a carbon plasma containing Sc and N.

Figure 1