The role of aromaticity in determining the molecular structure and reactivity of (endohedral metallo)fullerenes

Abstract

The encapsulation of metal clusters in endohedral metallofullerenes (EMFs) takes place in cages that in most cases are far from being the most stable isomer in the corresponding hollow fullerenes. There exist several possible explanations for the choice of the hosting cages in EMFs, although the final reasons are actually not totally well understood. Moreover, the reactivity and regioselectivity of (endohedral metallo)fullerenes have in the past decade been shown to be generally dependent on a number of factors, such as the size of the fullerene cage, the type of cluster that is being encapsulated, and the number of electrons that are transferred formally from the cluster to the fullerene cage. Different rationalizations of the observed trends had been proposed, based on bond lengths, pyramidalization angles, shape and energies of (un)occupied orbitals, deformation energies of the cages, or separation distances between the pentagon rings. Recently, in our group we proposed that the quest for the maximum aromaticity (maximum aromaticity criterion) determines the most suitable hosting carbon cage for a given metallic cluster (i.e. EMF stabilization), including those cases where the IPR rule is not fulfilled. Moreover, we suggested that local aromaticity plays a determining role in the reactivity of EMFs, which can be used as a criterion for understanding and predicting the regioselectivity of different reactions such as Diels-Alder cycloadditions or Bingel-Hirsch reactions. This review highlights different aspects of the aromaticity of fullerenes and EMFs, starting from how this can be measured and ending by how it can be used to rationalize and predict their molecular structure and reactivity.

Quantifying aromaticity with electron delocalisation measures

“…10 Three years later, the synthesis of the first organic derivatives of closododecaborate and closo-decaborate by the group of Muetterties 11 was the beginning of closo-borane chemistry and three-dimensional aromaticity, the type of aromaticity that characterises fullerenes. 12 The identification 13,14 of the planar triplet ground states of C5H5 + and C5Cl5 + in the late 60's provided experimental support for the existence of triplet aromaticity as predicted by Baird. 15 In 1982…”

Rationalizing the relative abundances of trimetallic nitride template-based endohedral metallofullerenes from aromaticity measures

“…12 In 2013, with the goal of understanding the chemical origin of the stability of anionic fullerene cages, we reported an extensive study showing that, for a given C 2 n – m cage, the most stable isomers are the ones whose total aromaticity is maximized, independently of the number of APPs they have. 13,14 We formulated the maximum aromaticity criterion that gave an explanation for the isolated pentagon rule (IPR) violation in EMFs. In this study, aromaticity measurements were carried out in terms of the Additive Local Aromaticity (ALA) index.…”

“…In this study, aromaticity measurements were carried out in terms of the Additive Local Aromaticity (ALA) index. 13,14 We used the HOMA geometric descriptor of aromaticity to reduce the computational cost. Still, the HOMA-based calculations provided aromaticity trends similar to those obtained with more reliable, but computationally more expensive electronic multicenter indices.…”

Kinetic Stability of Non-IPR Fullerene Molecular Ions

“…16,17 Many non-IPR fullerenes have been isolated and characterized in the form of metallofullerenes. [18][19][20][21][22][23] Kinetic stability of metallofullerenes could be predicted from the min BRE for the negatively charged carbon cage. [24][25][26][27] We reported that carbon cages in all IPR and some non-IPR metallofullerenes so far isolated are kinetically stable with min BREs > -0.100 |β|.…”

“…43 In 2014, they made experimental and computational reinvestigation of recently pointed out that the originally proposed Sc 2 C 2 @C 2v (6073)-C 68 is more congruent not only with the size-shape complementality rule but also with the maximum aromaticity rule. [21][22][23] Considering that the tetraanion of C 2v (6073)-C 68 is the most aromatic isomer of the C 68 tetraanion, 21 it must possibly be the kinetically most stable isomer of C 68 4-. An X-ray single crystal structure may be required to solve these conflicting observations.…”