M@C50 as Higher Intermediates towards Large Endohedral Metallofullerenes: Theoretical Characterization, Aromatic and Bonding Properties from Relativistic DFT Calculations

In recent years, endohedral metallofullerenes involving the C-50 cages have been observed experimentally to encapsulate several metal atoms. This is the last step in a bottom up growing mechanism to produce the most commonly observed large metallofullerenes. Nonetheless, currently, there is a lack of theoretical rationalization of such compounds. We now report, for the first time, a comprehensive theoretical study extending the experimentally known M@C-50 species to endohedral group 3 and 4 elements using dispersion-corrected density functional theory. For C-50 fullerene, isomers C-s (266) and D-5h (271) are the most energetically favorable cages to host these metals, despite being far from the ground state of neutral C-50 fullerene. Interestingly, properties of these endohedral compounds are highly comparable to those of the tri- and tetra-anions of the correspondent hollow fullerene cages. It is found that metal cage binding energies larger than -5 eV are directly related to relative abundances experimentally measured for the group 3 endohedral metallofullerenes. In addition, hypothetical group 4 metallofullerenes are also expected to be stable. Our results show that the resulting metal atoms transfer charge to the cage, to partially covalent-ionic compounds, which is the nature of the metal encapsulation within the C-50 cage, where the ionic bond character increases for the heavier elements. In all cases, highest occupied molecular orbital lowest unoccupied molecular orbital gaps smaller than 0.4 eV are found, in accordance with the high reactivity imposed by the need for further growth. Aromaticity NICS(0)(iso) indices reveal that some carbon rings close to the enclosed metal M are not fully aromatic and some are even antiaromatic, even though hollow C-50 fullerene cages are fully aromatic. Thus, the stabilities of the endohedral M@C-50 compounds are not fully ruled by the aromatic character of the carbon cage but instead by the energy characteristics of the metal-cage interactions, which are fully characterized by means of energy decomposition analyses. Such results can be useful to guide further experimental explorative synthetic efforts toward more diverse metal species encapsulated within the higher intermediate C-50 cage, which can be extended to other intermediate species already experimentally detected.