Density functional calculations have been performed on Sc3N@C80 and Sc3N@C78 to examine the bonding between the scandium atoms and the fullerene cage. The encapsulation of the Sc3N unit is a strongly exothermic process that is accompanied by a formal transfer of six electrons from the scandium atoms to the fullerene cage in both complexes. In the case of Sc3N@C78, the metal ions are strongly linked to three [6:6] ring junctions of three different pyracylene patches, which are located at the midsection of the fullerene cage. This bonding restricts the Sc3N unit from freely rotating inside the cage. Geometric optimization of the structure of Sc3N@C78 indicates that the carbon cage expands to accommodate the Sc3N unit within the cage. This optimized structure has been used to re-refine the crystallographic data for {Sc3N@C78}·{Co(OEP)}·1.5(C6H6)·0.3(CHCl3). In contrast, in Sc3N@C80, the Sc3N unit is not trapped in a specific position within the inner surface of the I h cage, which is an unusual fullerene that lacks pyracylene patches. Thus, free rotation of the Sc3N group within the C80 cage is expected. Despite the electronic transfer from the Sc3N unit to the carbon cage, Sc3N@C78 and Sc3N@C80 have relatively large electron affinities and ionization potentials.

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Fullerenes containing a trimetallic nitride template (TNT) within the cage are a particularly interesting class of endohedral metallofullerenes. Not only are the cage properties modified by the presence of the incarcerated group but, almost uniquely among endohedral metallofullerenes, they are quite stable. Furthermore, they can be produced in multimilligram quantities, and these amounts should increase in the future. The electronic effect of the TNT is such that some fullerenes of sizes and symmetry that are otherwise relatively unstable become available for investigation.[1] The general formula of these TNT endohedral metallofullerenes is A 3Àn B n N@C k (n = 0-3; A,B = group III, IV, and rare-earth metals; k = 68, 78, and 80) with the archetypal examples of: Sc 3 N@C 80 , [2,3] Sc 3 N@C 68 , [4] and Sc 3 N@C 78 .[5] The structures of Sc 3 N@C 78 , Sc 3 N@C 80 , and Sc 3 N@C 68 are displayed in Figure 1.Special attention has been paid to lutetium-based TNT endohedral metallofullerenes, Lu 3Àn A n N@C 80 (n = 0-2; A = Gd, Ga, and Ho), because they may prove useful as multifunctional contrast agents for X-ray, magnetic resonance imaging, and radiopharmaceuticals.[6] The aim of this research is to use methods based on density functional theory (DFT) to answer the questions: How can the stability of the TNT endohedral metallofullerenes be predicted? Which fullerene cages between C 60 and C 84 will be capable of encapsulating TNTs? Aihara and co-workers proposed the bond resonance energy (BRE) [7] to be an indicator of the particular stabilization of free fullerene cages when they encapsulate metal units.[8] However, this method was not a predictive tool because it could not answer whether or not new cages will be capable of encapsulating TNTs.Up to now, only four carbon cages have been capable of encapsulating TNT units: D 3 -C 68 :6140, D 3h' -C 78 :5, D 5h -C 80 :6 and I h -C 80 :7. All these cages, except C 68 , satisfy the isolatedpentagon rule (IPR). But it is interesting to see that in all cases the empty IPR fullerene isomers isolated so far are different from the carbon cages found in isolable TNT endohedral metallofullerenes. The incorporation of a TNT into the fullerene results in an electron transfer from the metal atoms to the carbon cage, in other words, the formation of a stable ion pair. It should be noted that these fullerene cages are produced only when they are negatively charged by the encapsulated species. Theoretical calculations indicated that the thermodynamic stability of a fullerene molecule depends heavily on the negative charge that resides on it. [9] The bond between the nitride and the cage is markedly defined by the ionic model Sc 3 N 6+ @C k 6À (k = 68, 78, and 80).[10] From the geometric point of view, although the free Sc 3 N molecule is pyramidal, this fragment has a planar structure inside the fullerene cage. When the Sc 3 N unit is

We report here for the first time a full comparison of the exohedral reactivity of a given fullerene and its parent trinitride template endohedral metallofullerene. In particular, we study the thermodynamics and kinetics for the Diels-Alder [4 + 2] cycloaddition between 1,3-butadiene and free D3h'-C78 fullerene and between butadiene and the corresponding endohedral D3h-Sc3N@C78 derivative. The reaction is studied for all nonequivalent bonds, in both the free and the endohedral fullerenes, at the BP86/TZP//BP86/DZP level. The change in exohedral reactivity and regioselectivity when a metal cluster is encapsulated inside the cage is profound. Consequently, the Diels-Alder reaction over the free fullerene and the endohedral derivative leads to totally different cycloadducts. This is caused by the metal nitride situated inside the fullerene cage that reduces the reactivity of the free fullerene and favors the reaction over different bonds.