Small endohedral metallofullerenes: exploration of the structure and growth mechanism in the Ti@C(2n) (2n = 26–50) family

The formation of the smallest fullerene, C(28), was recently reported using gas phase experiments combined with high-resolution FT-ICR mass spectrometry. An internally located group IV metal stabilizes the highly strained non-IPR C(28) cage by charge transfer (IPR = isolated pentagon rule). Ti@C(44) also appeared as a prominent peak in the mass spectra, and U@C(28) was demonstrated to form by a bottom-up growth mechanism. We report here a computational analysis using standard DFT calculations and Car–Parrinello MD simulations for the family of the titled compounds, aiming to identify the optimal cage for each endohedral fullerene and to unravel key aspects of the intriguing growth mechanisms of fullerenes. We show that all the optimal isomers from C(26) to C(50) are linked by a simple C(2) insertion, with the exception of a few carbon cages that require an additional C(2) rearrangement. The ingestion of a C(2) 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@C(28) and Ti@C(44) can be explained by the special electronic properties of these cages and their higher relative stabilities with respect to C(2) reactivity. We further verify that extrusion of C atoms from an already closed fullerene is much more energetically demanding than forming the fullerene by a bottom-up mechanism. Independent of the formation mechanism, the present investigations strongly support that, among all the possible isomers, the most stable, smaller non-IPR carbon cages are formed, a conclusion that is also valid for medium and large cages.