Structure, Stability, and Cluster-Cage Interactions in Nitride Clusterfullerenes M₃N@C_2n (M = Sc, Y; 2n = 68−98):  a Density Functional Theory Study

Abstract

Extensive semiempirical calculations of the hexaanions of IPR (isolated pentagon rule) and non-IPR isomers of C(68)-C(88) and IPR isomers of C(90)-C(98) followed by DFT calculations of the lowest energy structures were performed to find the carbon cages that can provide the most stable isomers of M(3)N@C(2n) clusterfullerenes (M = Sc, Y) with Y as a model for rare earth ions. DFT calculations of isomers of M(3)N@C(2n) (M = Sc, Y; 2n = 68-98) based on the most stable C(2n)(6-) cages were also performed. The lowest energy isomers found by this methodology for Sc(3)N@C(68), Sc(3)N@C(78), Sc(3)N@C(80), Y(3)N@C(78), Y(3)N@C(80), Y(3)N@C(84), Y(3)N@C(86), and Y(3)N@C(88) are those that have been shown to exist by single-crystal X-ray studies as Sc(3)N@C(2n) (2n = 68, 78, 80), Dy(3)N@C(80), and Tb(3)N@C(2n) (2n = 80, 84, 86, 88) clusterfullerenes. Reassignment of the carbon cage of Sc(2)@C(76) to the non-IPR Cs: 17490 isomer is also proposed. The stability of nitride clusterfullerenes was found to correlate well with the stability of the empty 6-fold charged cages. However, the dimensions of the cage in terms of its ability to encapsulate M(3)N clusters were also found to be an important factor, especially for the medium size cages and the large Y(3)N cluster. In some cases the most stable structures are based on the different cage isomers for Sc(3)N and Y(3)N clusters. Up to the cage size of C(84), non-IPR isomers of C(2n)(6-) and M(3)N@C(2n) were found to compete with or to be even more stable than IPR isomers. However, the number of adjacent pentagon pairs in the most stable non-IPR isomers decreases as cage size increases: the most stable M(3)N@C(2n) isomers have three such pairs for 2n = 68-72, two pairs for n = 74-80, and only one pair for n = 82, 84. For C(86) and C(88) the lowest energy IPR isomers are much more stable than any non-IPR isomer. The trends in the stability of the fullerene isomers and the cluster-cage binding energies are discussed, and general rules for stability of clusterfullerenes are established. Finally, the high yield of M(3)N@C(80) (Ih) clusterfullerenes for any metal is explained by the exceptional stability of the C(80)(6-) (Ih: 31924) cage, rationalized by the optimum distribution of the pentagons leading to the minimization of the steric strain, and structural similarities of C(80) (Ih: 31924) with the lowest energy non-IPR isomers of C(760(6-), C(78)(6-), C(82)(6-), and C(84)(6-) pointed out.

Electronic Structure and Redox Properties of Metal Nitride Endohedral Fullerenes M3N@C2n (M=Sc, Y, La, and Gd; 2n=80, 84, 88, 92, 96)

“…Therefore, we conclude that the cluster is able to rotate inside the I h -C 80 cage, in good agreement with experiments [38] and with previous theoretical results. [64,71] The most stable orientational isomer is very similar to that found by Popov and Dunsch (rotation of 21.58 with respect to the isomer with C 3v symmetry). [69] For the neutral Sc 3 N@D 5h -C 80 , free rotation of the cluster was also predicted, in good agreement with experiment, [72] since several orientational isomers show relative energies smaller than 4 kcal mol…”

“…[63] In recent work, Popov and Dunsch have also proposed cage isomers for large MNEFs (from C 90 to C 98 ) based on the stability of the empty fullerenes in their hexa-A C H T U N G T R E N N U N G anionic state C 2n 6À . [64] In most cases the cage structures predicted by Popov and Dunsch and by our group match fairly well.…”

Bottom-up formation of endohedral mono-metallofullerenes is directed by charge transfer

“…4a and 49 , to discern which Ca@C 2n cages exhibit enhanced stability. We find that the relative energies of empty cage dianions and Ca@C 2n follow the same trends and, therefore, the ionic model is applicable to Ca-encapsulated EMFs.…”

Trimetallic Nitride Endohedral Fullerenes: Experimental and Theoretical Evidence for the M3N6+@C2n6− model

“…Based on the electronic structure of the free fullerene cage, Poblet et al [14] proposed isomer 86 of C 92 (see Figure 3) with symmetry T as the most suitable isomer to encapsulate a MN cluster. Nevertheless, Popov and Dunsch found isomer 85 (Y 3 N@C 92 ) with D 3 symmetry more stable than isomer 86 by about 14 kcal mol À1 when the internal cluster is Y 3 N [15] . If the metal is lanthanum, the difference in the stability between the two isomers is 7.7 kcal mol À1 , isomer 85 again being the most stable species.…”

“…In most of the cases, the cage structures predicted by both groups coincide. [15] Herein we report the synthesis, electrochemistry, and theoretical characterization of the large MNEFs M 3 N@C 2n family (M = La, Ce, and Pr; n = 46 and 48). The combined electrochemical and computational analyses of the series from M 3 N@C 80 to M 3 N@C 96 verify the ionic model M 3 N 6+ @ C 2n 6À for MNEFs.…”